Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0015232 heme transmembrane transporter activity	IBA GO_REF:0000033	KEEP AS NON CORE	Summary: heme transmembrane transporter activity (GO:0015232) is retained cautiously as a historical/contested inferred context but is not the current core function. The heme-import model derives from 2010 hemin-binding and heme-analog uptake assays in heterologous systems, whereas 2024 structural and physiological work de-orphanizes FLVCR2/MFSD7c as a choline/ethanolamine transporter and notes that FLVCR2-mediated heme uptake has not been confirmed. Reason: UniProt now emphasizes choline transport at the BBB and describes heme transport only as an additional/by-similarity activity. The falcon deep research synthesis treats heme transport as an older, less secure hypothesis superseded by direct choline-transport evidence; it is retained as non-core pending direct zebrafish testing. Supporting Evidence: file:DANRE/flvcr2a/flvcr2a-uniprot.txt ethanolamine (By similarity) file:DANRE/flvcr2a/flvcr2a-uniprot.txt heme b transporter (By similarity) file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md Earlier literature proposed heme import by FLVCR2 based on hemin-binding and heme-analog uptake assays file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md The 2024 Nature paper explicitly notes that FLVCR2-mediated heme uptake “has not been confirmed”
GO:0016020 membrane	IBA GO_REF:0000033	REMOVE	Summary: membrane (GO:0016020) is too broad and should not be modified across GO aspects. Reason: Specific membrane locations are reviewed separately; this generic CC annotation should be retired rather than replaced with MF/BP transport terms. Supporting Evidence: file:DANRE/flvcr2a/flvcr2a-uniprot.txt Choline uniporter that specifically mediates choline uptake file:DANRE/flvcr2a/flvcr2a-uniprot.txt Reaction=choline(out) = choline(in) file:DANRE/flvcr2a/flvcr2a-uniprot.txt Cell membrane PMID:38302740 MFSD7c is a choline transporter at the blood-brain barrier
GO:0020037 heme binding	IBA GO_REF:0000033	KEEP AS NON CORE	Summary: heme binding (GO:0020037) is retained cautiously as a historical/contested inferred heme context but is not the current core function. Historical support comes from 2010 hemin-agarose binding assays for FLVCR2; 2024 work reframes the FLVCR2/MFSD7c family around choline/ethanolamine transport. Reason: UniProt now emphasizes choline transport at the BBB and describes heme transport only as an additional/by-similarity activity; the falcon synthesis treats heme binding/transport as a legacy hypothesis not confirmed in current mechanistic work. Supporting Evidence: file:DANRE/flvcr2a/flvcr2a-uniprot.txt ethanolamine (By similarity) file:DANRE/flvcr2a/flvcr2a-uniprot.txt heme b transporter (By similarity) file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md Earlier literature proposed heme import by FLVCR2 based on hemin-binding and heme-analog uptake assays
GO:0097037 heme export	IBA GO_REF:0000033	KEEP AS NON CORE	Summary: heme export (GO:0097037) is retained cautiously as a historical/contested inferred heme context but is not the current core function. 2024 mechanistic work concludes FLVCR2-mediated heme uptake has not been confirmed and prioritizes choline/ethanolamine as the primary transported substrates. Reason: UniProt now emphasizes choline transport at the BBB and describes heme transport only as an additional/by-similarity activity; heme export is retained as a legacy hypothesis pending direct zebrafish testing. Supporting Evidence: file:DANRE/flvcr2a/flvcr2a-uniprot.txt ethanolamine (By similarity) file:DANRE/flvcr2a/flvcr2a-uniprot.txt heme b transporter (By similarity) file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md The 2024 Nature paper explicitly notes that FLVCR2-mediated heme uptake “has not been confirmed”
GO:0005789 endoplasmic reticulum membrane	IEA GO_REF:0000044	KEEP AS NON CORE	Summary: endoplasmic reticulum membrane (GO:0005789) is supported as an inferred membrane location but is not the primary functional site for the choline transporter annotation. Reason: The core choline transporter role is best tied to plasma membrane/BBB context. Supporting Evidence: file:DANRE/flvcr2a/flvcr2a-uniprot.txt Endoplasmic reticulum membrane file:DANRE/flvcr2a/flvcr2a-uniprot.txt Mitochondrion membrane file:DANRE/flvcr2a/flvcr2a-uniprot.txt Cell membrane
GO:0005886 plasma membrane	IEA GO_REF:0000044	ACCEPT	Summary: plasma membrane (GO:0005886) is supported for Flvcr2a/MFSD7c. Reason: Current evidence supports MFSD7c as a choline transporter at the blood-brain barrier/plasma membrane. Supporting Evidence: file:DANRE/flvcr2a/flvcr2a-uniprot.txt Choline uniporter that specifically mediates choline uptake file:DANRE/flvcr2a/flvcr2a-uniprot.txt Reaction=choline(out) = choline(in) file:DANRE/flvcr2a/flvcr2a-uniprot.txt Cell membrane PMID:38302740 MFSD7c is a choline transporter at the blood-brain barrier
GO:0015220 choline transmembrane transporter activity	IEA GO_REF:0000117	ACCEPT	Summary: choline transmembrane transporter activity (GO:0015220) is supported for Flvcr2a/MFSD7c. Reason: Current evidence supports MFSD7c as a choline transporter at the blood-brain barrier/plasma membrane. Supporting Evidence: file:DANRE/flvcr2a/flvcr2a-uniprot.txt Choline uniporter that specifically mediates choline uptake file:DANRE/flvcr2a/flvcr2a-uniprot.txt Reaction=choline(out) = choline(in) file:DANRE/flvcr2a/flvcr2a-uniprot.txt Cell membrane PMID:38302740 MFSD7c is a choline transporter at the blood-brain barrier
GO:0015871 choline transport	IEA GO_REF:0000108	ACCEPT	Summary: choline transport (GO:0015871) is supported for Flvcr2a/MFSD7c. Reason: Current evidence supports MFSD7c as a choline transporter at the blood-brain barrier/plasma membrane. Supporting Evidence: file:DANRE/flvcr2a/flvcr2a-uniprot.txt Choline uniporter that specifically mediates choline uptake file:DANRE/flvcr2a/flvcr2a-uniprot.txt Reaction=choline(out) = choline(in) file:DANRE/flvcr2a/flvcr2a-uniprot.txt Cell membrane PMID:38302740 MFSD7c is a choline transporter at the blood-brain barrier
GO:0022857 transmembrane transporter activity	IEA GO_REF:0000002	MODIFY	Summary: transmembrane transporter activity (GO:0022857) is too broad for Flvcr2a. The specific function is choline transmembrane transporter activity, with mechanistic evidence that the FLVCR2/MFSD7c family operates as a facilitative uniporter driving downhill transport independent of sodium or pH gradients, with substrate selectivity mediated by conserved aromatic (cation-pi) residues. Reason: The supported molecular function should be the specific choline transmembrane transporter activity; 2024 structural/transport work defines the FLVCR2 family as uniporters rather than coupled/ATP-driven pumps. Proposed replacements: choline transmembrane transporter activity Supporting Evidence: file:DANRE/flvcr2a/flvcr2a-uniprot.txt Choline uniporter that specifically mediates choline uptake file:DANRE/flvcr2a/flvcr2a-uniprot.txt Reaction=choline(out) = choline(in) file:DANRE/flvcr2a/flvcr2a-uniprot.txt Cell membrane PMID:38302740 MFSD7c is a choline transporter at the blood-brain barrier file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md both operate as uniporters file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md downhill transport independent of sodium or pH gradients
GO:0031966 mitochondrial membrane	IEA GO_REF:0000044	KEEP AS NON CORE	Summary: mitochondrial membrane (GO:0031966) is supported as an inferred membrane location but is not the primary functional site for the choline transporter annotation. Reason: The core choline transporter role is best tied to plasma membrane/BBB context. Supporting Evidence: file:DANRE/flvcr2a/flvcr2a-uniprot.txt Endoplasmic reticulum membrane file:DANRE/flvcr2a/flvcr2a-uniprot.txt Mitochondrion membrane file:DANRE/flvcr2a/flvcr2a-uniprot.txt Cell membrane
GO:0034229 ethanolamine transport	IEA GO_REF:0000108	KEEP AS NON CORE	Summary: ethanolamine transport (GO:0034229) is supported by similarity as a secondary/conditional transported substrate. In MFSD7c-focused assays ethanolamine uptake was not increased by MFSD7c alone but became significant when ethanolamine kinase (ETNK1) was co-expressed (metabolic trapping); human FLVCR2 structural work independently supports choline and ethanolamine as the transported solutes. Reason: The best-supported zebrafish/core role is choline transport; ethanolamine transport is retained as non-core substrate context, supported but more conditional than choline in MFSD7c-specific data. Supporting Evidence: file:DANRE/flvcr2a/flvcr2a-uniprot.txt ethanolamine (By similarity) file:DANRE/flvcr2a/flvcr2a-uniprot.txt heme b transporter (By similarity) file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md MFSD7c did not increase ethanolamine uptake alone, but ethanolamine transport became significant when ethanolamine kinase (ETNK1) was co-expressed
GO:0055085 transmembrane transport	IEA GO_REF:0000002	MODIFY	Summary: transmembrane transport (GO:0055085) is too broad for Flvcr2a. Reason: The supported biological process should be the specific choline transport process. Proposed replacements: choline transport Supporting Evidence: file:DANRE/flvcr2a/flvcr2a-uniprot.txt Choline uniporter that specifically mediates choline uptake file:DANRE/flvcr2a/flvcr2a-uniprot.txt Reaction=choline(out) = choline(in) file:DANRE/flvcr2a/flvcr2a-uniprot.txt Cell membrane PMID:38302740 MFSD7c is a choline transporter at the blood-brain barrier
GO:0005789 endoplasmic reticulum membrane	ISS GO_REF:0000024	KEEP AS NON CORE	Summary: endoplasmic reticulum membrane (GO:0005789) is supported as an inferred membrane location but is not the primary functional site for the choline transporter annotation. Reason: The core choline transporter role is best tied to plasma membrane/BBB context. Supporting Evidence: file:DANRE/flvcr2a/flvcr2a-uniprot.txt Endoplasmic reticulum membrane file:DANRE/flvcr2a/flvcr2a-uniprot.txt Mitochondrion membrane file:DANRE/flvcr2a/flvcr2a-uniprot.txt Cell membrane
GO:0005886 plasma membrane	ISS GO_REF:0000024	ACCEPT	Summary: plasma membrane (GO:0005886) is supported for Flvcr2a/MFSD7c and is the functional site of choline transport. Transport assays for human/mouse MFSD7c rely on plasma-membrane localization, and a transport-deficient disease mutant (S203Y) retained normal plasma-membrane localization, indicating its defect reflects functional impairment rather than mislocalization. Reason: Current evidence supports MFSD7c as a choline transporter at the blood-brain barrier/plasma membrane, with plasma-membrane localization confirmed in heterologous transport assays. Supporting Evidence: file:DANRE/flvcr2a/flvcr2a-uniprot.txt Choline uniporter that specifically mediates choline uptake file:DANRE/flvcr2a/flvcr2a-uniprot.txt Reaction=choline(out) = choline(in) file:DANRE/flvcr2a/flvcr2a-uniprot.txt Cell membrane PMID:38302740 MFSD7c is a choline transporter at the blood-brain barrier file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md a transport-deficient mutant (S203Y) was stated not to have defective plasma membrane localization
GO:0031966 mitochondrial membrane	ISS GO_REF:0000024	KEEP AS NON CORE	Summary: mitochondrial membrane (GO:0031966) is supported as an inferred membrane location but is not the primary functional site for the choline transporter annotation. Reason: The core choline transporter role is best tied to plasma membrane/BBB context. Supporting Evidence: file:DANRE/flvcr2a/flvcr2a-uniprot.txt Endoplasmic reticulum membrane file:DANRE/flvcr2a/flvcr2a-uniprot.txt Mitochondrion membrane file:DANRE/flvcr2a/flvcr2a-uniprot.txt Cell membrane
GO:0034228 ethanolamine transmembrane transporter activity	ISS GO_REF:0000024	KEEP AS NON CORE	Summary: ethanolamine transmembrane transporter activity (GO:0034228) is supported by similarity as a secondary transported substrate. A 2024 Nature study concludes that human FLVCR1 and FLVCR2 mediate cellular transport of choline and ethanolamine, supporting ethanolamine as a bona fide FLVCR2-family substrate while choline remains the best-supported activity for MFSD7c. Reason: The best-supported zebrafish/core role is choline transport; ethanolamine transport is retained as non-core substrate context, strongly supported for human FLVCR2 in structural/transport assays. Supporting Evidence: file:DANRE/flvcr2a/flvcr2a-uniprot.txt ethanolamine (By similarity) file:DANRE/flvcr2a/flvcr2a-uniprot.txt heme b transporter (By similarity) file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md A 2024 Nature study concludes that human FLVCR1 and FLVCR2 mediate cellular transport of choline and ethanolamine
GO:0015220 choline transmembrane transporter activity	IDA PMID:38302740 MFSD7c functions as a transporter of choline at the blood-br...	ACCEPT	Summary: choline transmembrane transporter activity (GO:0015220) is the core molecular function of Flvcr2a/MFSD7c. Critically, the zebrafish orthologs themselves were directly tested: the two zebrafish MFSD7c isoforms (DaMfsd7c_a, DaMfsd7c_b) show choline transport activity in heterologous assays, providing direct species-relevant support rather than inference alone. 2024 mechanistic work establishes the transporter operates as a facilitative uniporter mediating concentration-driven (downhill) choline movement. Reason: Current evidence supports MFSD7c as a choline transporter at the blood-brain barrier/plasma membrane, with direct transport activity demonstrated for the zebrafish orthologs. Supporting Evidence: file:DANRE/flvcr2a/flvcr2a-uniprot.txt Choline uniporter that specifically mediates choline uptake file:DANRE/flvcr2a/flvcr2a-uniprot.txt Reaction=choline(out) = choline(in) file:DANRE/flvcr2a/flvcr2a-uniprot.txt Cell membrane PMID:38302740 MFSD7c is a choline transporter at the blood-brain barrier file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md zebrafish MFSD7c isoforms “DaMfsd7c_a” and “DaMfsd7c_b” show choline transport activity in heterologous assays file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md zebrafish isoforms a and b also showed choline transport activity
GO:0150104 transport across blood-brain barrier	ISS GO_REF:0000024	ACCEPT	Summary: transport across blood-brain barrier (GO:0150104) is supported for Flvcr2a/MFSD7c. In mammals, endothelial-specific Mfsd7c knockout reduces brain uptake of injected radiolabeled choline while peripheral organ signals remain comparable, and the transporter is expressed in CNS endothelial/BBB cells. For zebrafish this is a conservation-based (ISS) inference; direct in vivo zebrafish BBB choline-flux data are not yet available. Reason: Current evidence supports MFSD7c as a choline transporter at the blood-brain barrier/plasma membrane, consistent with mammalian endothelial knockout reducing brain choline import. Retained as a conservation-based inference for zebrafish. Supporting Evidence: file:DANRE/flvcr2a/flvcr2a-uniprot.txt Choline uniporter that specifically mediates choline uptake file:DANRE/flvcr2a/flvcr2a-uniprot.txt Reaction=choline(out) = choline(in) file:DANRE/flvcr2a/flvcr2a-uniprot.txt Cell membrane PMID:38302740 MFSD7c is a choline transporter at the blood-brain barrier file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md loss of MFSD7c reduces brain uptake of injected radiolabeled choline while leaving peripheral organ signals comparable

Core Functions

flvcr2a/MFSD7c functions as a plasma-membrane choline transporter at the blood-brain barrier.

Molecular Function:

choline transmembrane transporter activity

Directly Involved In:

choline transport transport across blood-brain barrier

Cellular Locations:

plasma membrane

Supporting Evidence:

file:DANRE/flvcr2a/flvcr2a-uniprot.txt
Choline uniporter that specifically mediates choline uptake
file:DANRE/flvcr2a/flvcr2a-uniprot.txt
Reaction=choline(out) = choline(in)
file:DANRE/flvcr2a/flvcr2a-uniprot.txt
Cell membrane
PMID:38302740
MFSD7c is a choline transporter at the blood-brain barrier
file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
zebrafish isoforms a and b also showed choline transport activity
file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
both operate as **uniporters**

References

GO_REF:0000002

Gene Ontology annotation through association of InterPro records with GO terms

Flvcr2a matches InterPro signatures IPR011701 and IPR020846 (major facilitator superfamily / MFS domain); the InterPro2GO mapping assigns transmembrane transporter activity (GO:0022857) and transmembrane transport (GO:0055085).

GO_REF:0000024

Manual transfer of experimentally-verified manual GO annotation data to orthologs by curator judgment of sequence similarity

Endoplasmic reticulum membrane (GO:0005789), plasma membrane (GO:0005886), mitochondrial membrane (GO:0031966), ethanolamine transmembrane transporter activity (GO:0034228), and transport across blood-brain barrier (GO:0150104) were transferred to zebrafish Flvcr2a by curator-judged sequence similarity to its mammalian orthologs (human FLVCR2 UniProtKB:Q9UPI3 and mouse Mfsd7c UniProtKB:Q91X85).

GO_REF:0000033

Annotation inferences using phylogenetic trees

Phylogenetic inference within PANTHER families PTN000858905 and PTN000858822 supports heme transmembrane transporter activity (GO:0015232), heme binding (GO:0020037), heme export (GO:0097037), and membrane localization (GO:0016020).

GO_REF:0000044

Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt

The UniProt Swiss-Prot subcellular location keywords SL-0097 (Endoplasmic reticulum membrane), SL-0039 (Cell membrane), and SL-0171 (Mitochondrion membrane) map Flvcr2a to endoplasmic reticulum membrane (GO:0005789), plasma membrane (GO:0005886), and mitochondrial membrane (GO:0031966).

GO_REF:0000108

Automatic assignment of GO terms using logical inference, based on inter-ontology links

Logical inference over inter-ontology links derives choline transport (GO:0015871) from choline transmembrane transporter activity (GO:0015220) and ethanolamine transport (GO:0034229) from ethanolamine transmembrane transporter activity (GO:0034228).

GO_REF:0000117

Electronic Gene Ontology annotations created by ARBA machine learning models

The ARBA machine-learning model ARBA00027949 predicts choline transmembrane transporter activity (GO:0015220) for Flvcr2a.

PMID:38302740

MFSD7c functions as a transporter of choline at the blood-brain barrier.

Mfsd7c-knockout mouse and cell-based assays established MFSD7c (Flvcr2) as a facilitative, electrogenic choline transporter at the blood-brain barrier whose choline transport function is conserved across vertebrates; Fowler syndrome missense mutations abolish or reduce this activity.

"MFSD7c is a choline transporter at the blood-brain barrier"

file:DANRE/flvcr2a/flvcr2a-uniprot.txt

UniProtKB entry A0A0R4ILB2 for Danio rerio flvcr2a

UniProt describes Flvcr2a/MFSD7c as a choline uniporter at the blood-brain barrier.

"Choline uniporter that specifically mediates choline uptake"

file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md

Falcon deep research report (Edison Scientific Literature) for Danio rerio flvcr2a (mfsd7c-a)

The zebrafish orthologs were directly tested: the two zebrafish MFSD7c isoforms (DaMfsd7c_a and DaMfsd7c_b) show choline transport activity in heterologous assays, providing direct species-relevant support for choline transport rather than inference from mammalian orthologs alone.

"zebrafish MFSD7c isoforms “DaMfsd7c_a” and “DaMfsd7c_b” show choline transport activity in heterologous assays"
2024 structural/functional work in humans establishes that FLVCR1 and FLVCR2 mediate choline and ethanolamine transport as facilitative uniporters performing downhill transport independent of sodium or pH gradients, with substrate selectivity mediated by conserved aromatic (cation-pi) residues.

"A 2024 Nature study concludes that human **FLVCR1 and FLVCR2 mediate cellular transport of choline and ethanolamine** and that both operate as **uniporters**"
The historical heme-import model (from 2010 hemin-binding and heme-analog uptake assays) is reframed by 2024 mechanistic work, which explicitly states that FLVCR2-mediated heme uptake has not been confirmed and prioritizes choline/ethanolamine as the primary substrates; heme transport should be treated as a contested legacy hypothesis.

"The 2024 Nature paper explicitly notes that **FLVCR2-mediated heme uptake “has not been confirmed”**"
In mammals MFSD7c is required for BBB choline handling: endothelial-specific knockout reduces brain uptake of injected radiolabeled choline while peripheral organ signals remain comparable, and loss of MFSD7c/FLVCR2 causes neurovascular developmental defects (brain angiogenic sprouting) relevant to Fowler syndrome.

"loss of MFSD7c reduces brain uptake of injected radiolabeled choline while leaving peripheral organ signals comparable"

Suggested Experiments

Experiment: Express zebrafish flvcr2a in a cultured cell line and measure radiolabeled choline uptake and single-cell patch-clamp currents to test for facilitative, electrogenic choline transport.

Hypothesis: Zebrafish flvcr2a mediates facilitative, electrogenic choline uptake, like its mammalian ortholog MFSD7c.

Experiment: Generate a flvcr2a zebrafish mutant and perform metabolomic profiling of choline-related metabolites in brain versus other tissues, and assess cerebral vasculature.

Hypothesis: Loss of flvcr2a disrupts brain choline homeostasis in zebrafish, modeling Fowler syndrome.

Experiment: Test recombinant flvcr2a in a heme b transport/export assay and compare its activity with that of a bona fide heme transporter.

Hypothesis: Zebrafish flvcr2a does not contribute substantial heme transport in vivo despite FLVCR-family homology.

Deep Research

Falcon

(flvcr2a-deep-research-falcon.md)

this with annotations you find in gene/protein databases, but these can be outdated or inaccurate. Falcon Edison Scientific Literature 18 citations 1 artifacts 2026-05-30T12:05:20.234344

Edison artifact artifact-00 (text/markdown)

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Comprehensive research report: Zebrafish flvcr2a (mfsd7c-a) — functional annotation and current evidence

0) Executive summary (evidence-weighted)

Zebrafish flvcr2a (syn. mfsd7c-a; UniProt: A0A0R4ILB2) encodes a predicted Major Facilitator Superfamily (MFS) / SLC49 (FLVCR1/2-like) multipass membrane transporter. The strongest current mechanistic and physiological evidence across vertebrates supports facilitative (uniporter-like), concentration-driven transport of choline (and, context-dependently, ethanolamine) across the plasma membrane, with particular relevance to blood–brain barrier (BBB)/CNS endothelial choline handling and neurovascular development (nguyen2024mfsd7cfunctionsas pages 3-4, nguyen2024mfsd7cfunctionsas pages 7-8, ri2024molecularmechanismof pages 7-7).

A key zebrafish-relevant experimental point is that zebrafish MFSD7c isoforms “DaMfsd7c_a” and “DaMfsd7c_b” show choline transport activity in heterologous assays, supporting conservation of substrate specificity in Danio rerio and directly connecting the vertebrate FLVCR2/MFSD7C functional model to zebrafish (nguyen2024mfsd7cfunctionsas pages 3-4).

Earlier literature proposed heme import by FLVCR2 based on hemin-binding and heme-analog uptake assays (duffy2010thefowlersyndromeassociated pages 2-3), and recent heme-focused reviews still cite that work (belot2024updateonheme pages 19-20). However, 2023–2024 structural/functional work in humans and 2024 BBB physiology work in mice strongly “de-orphanize” FLVCR2/MFSD7C as a choline/ethanolamine transporter, and explicitly note that a physiological role for FLVCR2-mediated heme uptake is not confirmed (ri2024molecularmechanismof pages 7-7, nguyen2024mfsd7cfunctionsas pages 3-4).

1) Target identity verification (mandatory)

Gene/protein in scope:
- Organism: Danio rerio (zebrafish). (User-provided UniProt context)
- Gene symbol: flvcr2a; synonym mfsd7c-a. (User-provided UniProt context)
- Protein family/domains: MFS transporter; FLVCR1/2 (SLC49)-like transporter family. This matches vertebrate MFSD7C/FLVCR2 literature describing FLVCR2/MFSD7C as an MFS-domain membrane transporter (kalailingam2020deficiencyofmfsd7c pages 1-2, ri2024molecularmechanismof pages 7-7).
- Ambiguity check: The major risk is confusing zebrafish flvcr2a with mammalian FLVCR2. Here, the linkage is supported because a 2024 Cell Research study explicitly tested zebrafish orthologs/isoforms (DaMfsd7c_a, DaMfsd7c_b) for transport activity, confirming the zebrafish entity is a bona fide FLVCR2/MFSD7C-family transporter (nguyen2024mfsd7cfunctionsas pages 3-4).

2) Key concepts and definitions (current understanding)

2.1 MFS (Major Facilitator Superfamily) transporters

MFS proteins are secondary transporters that often perform facilitated diffusion (uniport) or coupled transport across membranes, using alternating-access conformational changes. For FLVCR2-family proteins, the current leading model is uniport (“downhill”) transport for specific organic cations (choline/ethanolamine) rather than ATP-driven pumping (ri2024molecularmechanismof pages 7-7).

2.2 What “de-orphanizing” FLVCR2 means

FLVCR2/MFSD7C had long-standing uncertainty about its physiological substrate(s) (often discussed as heme). 2023–2024 work provides direct biochemical transport evidence and structures with bound substrates to identify its transported solutes (ri2024molecularmechanismof pages 7-7, nguyen2024mfsd7cfunctionsas pages 3-4).

2.3 Biological role of choline and ethanolamine transport

Choline and ethanolamine are precursors for phosphatidylcholine (PC) and phosphatidylethanolamine (PE) (Kennedy pathway), central to membrane biogenesis. BBB transport/recycling of choline is therefore plausibly important for neurodevelopmental lipid metabolism. In vivo BBB data for MFSD7C show that altering endothelial MFSD7C changes brain choline flux and choline metabolite levels (nguyen2024mfsd7cfunctionsas pages 7-8).

3) Molecular function: substrate specificity and directionality

3.1 Best-supported transported substrate(s): choline (primary) and ethanolamine (secondary/context-dependent)

Choline transport (strong evidence):
- In HEK293 cells, overexpression of human or mouse MFSD7c increases radiolabeled choline uptake by ~1.5–2-fold (nguyen2024mfsd7cfunctionsas pages 3-4).
- Import is concentration- and time-dependent (nguyen2024mfsd7cfunctionsas pages 3-4).
- Zebrafish isoforms DaMfsd7c_a and DaMfsd7c_b exhibit choline transport activity, directly supporting conservation in zebrafish (nguyen2024mfsd7cfunctionsas pages 3-4).

Ethanolamine transport (supported but more conditional in MFSD7C-focused work):
- MFSD7c did not increase ethanolamine uptake alone, but ethanolamine transport became significant when ethanolamine kinase (ETNK1) was co-expressed—consistent with metabolic “sink” effects and facilitative transport (nguyen2024mfsd7cfunctionsas pages 7-8).

Independent 2024 mechanistic evidence in humans:
- A 2024 Nature study concludes that human FLVCR1 and FLVCR2 mediate cellular transport of choline and ethanolamine and that both operate as uniporters (ri2024molecularmechanismof pages 7-7). The authors conclude downhill transport independent of sodium or pH gradients, with selectivity mediated by conserved aromatic residues (cation–π interactions) (ri2024molecularmechanismof pages 7-7).

3.2 Directionality: facilitative, gradient-driven; can support bidirectional flux

MFSD7c displays properties of a facilitative transporter and can mediate release/export of intracellular choline as well as import, consistent with an equilibrative uniporter/channel-like mechanism rather than an obligate importer (nguyen2024mfsd7cfunctionsas pages 7-8). In a BBB context, the authors propose MFSD7c can act as a choline exporter from brain parenchyma-derived pools into endothelial cells (recycling) (nguyen2024mfsd7cfunctionsas pages 7-8).

3.3 Quantitative transport parameters (recent data)

In the Cell Research 2024 study, when MFSD7c was co-expressed with choline kinase A (ChKA) (lowering intracellular free choline), choline uptake increased ~9-fold and the inferred kinetics were Km ~100 µM and Vmax ~0.117 µmol/well/60 min (nguyen2024mfsd7cfunctionsas pages 7-8). A disease-associated mutant (S203Y) showed severely impaired kinetics (Km ~1197 µM; Vmax ~0.039 µmol/well/60 min) (nguyen2024mfsd7cfunctionsas pages 7-8).

4) Subcellular localization and tissue context

4.1 Plasma membrane localization

Both the human preprint (2023) and the 2024 Nature study report plasma-membrane localization in HEK293 assays used for transport measurements (ri2023structuralandmechanistic pages 1-4, ri2024molecularmechanismof pages 7-7). In the Cell Research 2024 MFSD7c study, a transport-deficient mutant (S203Y) was stated not to have defective plasma membrane localization, supporting that transport phenotypes reflect functional impairment rather than mislocalization (nguyen2024mfsd7cfunctionsas pages 3-4).

4.2 BBB/CNS endothelial expression and neurovascular context

Multiple mouse studies identify MFSD7c/FLVCR2 as a CNS endothelial/BBB-expressed gene, and knockout produces neurovascular developmental phenotypes (kalailingam2020deficiencyofmfsd7c pages 1-2, santander2020lackofflvcr2 pages 1-3). In endothelial-conditional knockout mice, loss of MFSD7c reduces brain uptake of injected radiolabeled choline while leaving peripheral organ signals comparable (nguyen2024mfsd7cfunctionsas pages 7-8).

Quantitative BBB-related statistic: In the choline-injection experiment (supraphysiological dose), the brain accounted for ~2–3% of total radioactive signal, and this brain signal was significantly reduced upon endothelial MFSD7c knockout (nguyen2024mfsd7cfunctionsas pages 7-8).

5) Biological processes, pathways, and phenotypes informing zebrafish annotation

Because direct in vivo zebrafish phenotypes for flvcr2a were not retrieved in the accessible corpus, the most defensible zebrafish annotation is built from: (i) direct zebrafish ortholog transport activity, and (ii) conserved vertebrate BBB/endothelial roles.

5.1 Choline metabolism and BBB choline handling

The 2024 Cell Research study integrates metabolomics and isotope tracing to argue MFSD7c regulates brain choline levels and participates in LPC-derived choline recycling at the BBB (nguyen2024mfsd7cfunctionsas pages 7-8, nguyen2024mfsd7cfunctionsas pages 3-4). These processes connect mechanistically to membrane lipid homeostasis (PC synthesis) and neurodevelopment.

5.2 Neurovascular development / angiogenesis (Fowler syndrome-relevant biology)

Two independent 2020 JCI papers provide strong phenotype evidence that FLVCR2/MFSD7c is required for normal brain angiogenic sprouting:
- Global Mfsd7c knockout embryos display inhibited CNS vessel growth (ventricular/subventricular zones), vessel tip dilation/fusion (glomeruloid), hypoxia, neuronal death, and microcephaly/ventriculomegaly (kalailingam2020deficiencyofmfsd7c pages 1-2).
- Flvcr2 inactivation impairs brain angiogenesis while reportedly not disrupting BBB integrity, indicating uncoupling of vessel morphogenesis from BBB formation (santander2020lackofflvcr2 pages 1-3).

These data support a working model: in vertebrates, FLVCR2/MFSD7C-mediated metabolite transport (now strongly linked to choline) is intertwined with CNS endothelial metabolic state and angiogenic signaling programs (kalailingam2020deficiencyofmfsd7c pages 1-2, nguyen2024mfsd7cfunctionsas pages 7-8).

6) Recent developments and latest research (prioritized 2023–2024)

6.1 2024: Physiological de-orphaning at the BBB and vertebrate conservation

Nguyen et al. (Cell Research, Feb 2024) identify MFSD7c as a BBB choline transporter using metabolomics (>520 metabolites), electrophysiology (electrogenic transport), and isotope-tracing paradigms; importantly, they show zebrafish MFSD7c isoforms have choline transport activity, directly informing zebrafish functional annotation (nguyen2024mfsd7cfunctionsas pages 3-4, nguyen2024mfsd7cfunctionsas pages 7-8). URL: https://doi.org/10.1038/s41422-023-00923-y (nguyen2024mfsd7cfunctionsas pages 3-4).

6.2 2024: Structural mechanism and substrate-binding chemistry

Ri et al. (Nature, Jun 2024) present cryo-EM and functional transport assays establishing that human FLVCR2 transports choline and ethanolamine and operates as a uniporter. The study defines binding-pocket residues (conserved aromatic side chains and cation–π interactions) and emphasizes that physiological specialization between FLVCR paralogs still requires confirmation in vivo (ri2024molecularmechanismof pages 7-7). URL: https://doi.org/10.1038/s41586-024-07444-7 (ri2024molecularmechanismof pages 7-7).

6.3 2023: Preprint precursor to the 2024 Nature mechanism

A 2023 bioRxiv preprint reported choline/ethanolamine transport and plasma membrane localization for human FLVCR1/2, providing early support for the 2024 model (ri2023structuralandmechanistic pages 1-4). URL: https://doi.org/10.1101/2023.09.15.557925 (ri2023structuralandmechanistic pages 1-4).

7) Reconciling the historical “heme transporter” model with the 2024 choline model

7.1 Evidence supporting heme import (historical)

Duffy et al. (Molecular and Cellular Biology, Nov 2010) reported hemin-agarose binding by FLVCR2 and functional uptake assays (e.g., ZnMP uptake; Xenopus oocyte [55Fe]hemin uptake). Hemin competition reduced hemin-agarose binding by ~23% (25 µM) and 36% (50 µM) (duffy2010thefowlersyndromeassociated pages 2-3). URL: https://doi.org/10.1128/MCB.00690-10 (duffy2010thefowlersyndromeassociated pages 2-3).

7.2 Current interpretation (2024)

The 2024 Nature paper explicitly notes that FLVCR2-mediated heme uptake “has not been confirmed” and that prior biochemical evidence for heme roles was not definitive; it concludes choline and ethanolamine are the primary substrates supported by multiple lines of evidence (ri2024molecularmechanismof pages 7-7).

Implication for zebrafish flvcr2a annotation: annotate choline transporter (facilitative/uniporter) as the primary molecular function, and treat heme transport as a historical/contested hypothesis that would require direct zebrafish testing under physiological conditions.

8) Current applications and real-world implementations

8.1 Disease mechanism models: Fowler syndrome / congenital neurovascular disorders

Mouse genetic models (global or endothelial loss) are used to define how FLVCR2/MFSD7C loss causes microcephaly-associated vasculopathy and hydrocephalus-like phenotypes, providing a translational framework for vertebrate developmental vascular biology (kalailingam2020deficiencyofmfsd7c pages 1-2, santander2020lackofflvcr2 pages 1-3).

8.2 BBB metabolite flux measurements and pathway-level implementation

The 2024 BBB-focused work provides implementable experimental frameworks:
- Radiotracer choline injection with organ distribution measurements to quantify BBB import defects (nguyen2024mfsd7cfunctionsas pages 7-8).
- Stable-isotope tracing to follow LPC-derived choline and infer recycling/export steps at the BBB (nguyen2024mfsd7cfunctionsas pages 7-8).

These approaches can be adapted to zebrafish (e.g., tracer exposures in larvae, endothelial reporters, CRISPR mutants) to directly test whether flvcr2a supports choline flux in CNS endothelium.

8.3 Structure-guided variant interpretation and mutagenesis

The 2024 Nature structural mechanism identifies conserved binding-pocket features (e.g., aromatic residues mediating cation–π interactions), enabling structure-guided testing of zebrafish residues and variants for substrate binding/transport hypotheses (ri2024molecularmechanismof pages 7-7).

9) Expert opinions, cautions, and open questions (authoritative sources)

Ri et al. caution that assigning distinct physiological roles of FLVCR1 vs FLVCR2 “at physiological conditions” requires in vivo confirmation and emphasize potential ligand promiscuity typical of SLC transporters (ri2024molecularmechanismof pages 7-7).
Kalailingam et al. highlight that prior heme importer assays for MFSD7c lacked strong physiological support and prioritize in vivo roles in CNS endothelium/angiogenesis (kalailingam2020deficiencyofmfsd7c pages 1-2).

10) Statistics and quantitative data points (recent and relevant)

Key extracted quantitative points:
- Zebrafish relevance: zebrafish DaMfsd7c_a and DaMfsd7c_b show choline transport activity in heterologous assays (qualitative, but direct) (nguyen2024mfsd7cfunctionsas pages 3-4).
- Transport amplification by metabolic sink: ~9-fold increased uptake with ChKA co-expression (nguyen2024mfsd7cfunctionsas pages 7-8).
- Kinetics (with ChKA co-expression): Km 100 µM; Vmax 0.117 µmol/well/60 min (nguyen2024mfsd7cfunctionsas pages 7-8).
- BBB radiotracer distribution: brain ~2–3% of total signal; reduced in endothelial KO (nguyen2024mfsd7cfunctionsas pages 7-8).
- Heme-binding competition (historical): hemin reduced FLVCR2 pulldown binding by 23% (25 µM) and 36% (50 µM) (duffy2010thefowlersyndromeassociated pages 2-3).
- Structural cavity volumes: FLVCR2 ~579 Å³ vs FLVCR1 ~513 Å³ (ri2024molecularmechanismof pages 7-7).

11) Practical functional annotation for zebrafish flvcr2a (recommended wording)

Primary molecular function (most supported):
- Facilitative transporter (uniporter-like) for choline across the plasma membrane; can support bidirectional flux depending on concentration gradients and intracellular metabolic trapping. (nguyen2024mfsd7cfunctionsas pages 7-8, ri2024molecularmechanismof pages 7-7, nguyen2024mfsd7cfunctionsas pages 3-4)

Secondary/conditional function (supported but less direct):
- Ethanolamine transport is plausible and strongly supported for human FLVCR2 in structural/transport assays; MFSD7c-mediated ethanolamine flux may require metabolic trapping (ETNK1) in some contexts. (ri2024molecularmechanismof pages 7-7, nguyen2024mfsd7cfunctionsas pages 7-8)

Cellular component/localization (most supported):
- Plasma membrane; enriched/functional in CNS endothelial (BBB) contexts in mammals; zebrafish localization remains to be measured directly. (nguyen2024mfsd7cfunctionsas pages 3-4, kalailingam2020deficiencyofmfsd7c pages 1-2)

Biological process (inferred and cross-vertebrate supported):
- Choline homeostasis at the BBB; contribution to brain choline availability/recycling and lipid precursor metabolism; neurovascular development/angiogenesis. (nguyen2024mfsd7cfunctionsas pages 7-8, kalailingam2020deficiencyofmfsd7c pages 1-2, santander2020lackofflvcr2 pages 1-3)

Legacy/contested hypothesis (should be labeled as such):
- Heme import has historical experimental support in heterologous systems but is not confirmed as the primary physiological function in newer mechanistic work. (duffy2010thefowlersyndromeassociated pages 2-3, ri2024molecularmechanismof pages 7-7)

12) Evidence limitations specific to Danio rerio flvcr2a

Within the currently retrieved corpus, there is no direct in vivo zebrafish mutant phenotype, tissue expression map, or subcellular localization imaging for flvcr2a. The strongest zebrafish-specific evidence is conserved choline transport activity of zebrafish MFSD7c isoforms in a heterologous assay (nguyen2024mfsd7cfunctionsas pages 3-4). Therefore, zebrafish annotations beyond molecular function (e.g., exact larval phenotypes, spatial expression domains) should be treated as hypotheses derived from vertebrate conservation until validated experimentally in zebrafish.

Summary table of key evidence

Claim (function/substrate/localization/role)	Species/system	Evidence type	Key quantitative/statistical details	Main takeaway for zebrafish flvcr2a annotation	Citation (include DOI URL and year)
MFSD7c/FLVCR2 is a choline transporter, and zebrafish orthologs are active	Human and mouse MFSD7c in HEK293; zebrafish DaMfsd7c_a and DaMfsd7c_b; medaka; frog	Radioactive choline transport assays across vertebrate orthologs	Overexpression of hMFSD7c or mMfsd7c increased intracellular [3H]-choline by ~1.5- to 2-fold; zebrafish isoforms a and b also showed choline transport activity	Direct cross-species evidence supports annotating zebrafish flvcr2a/MFSD7C-a as a choline transporter rather than an uncharacterized orphan transporter	Nguyen et al., 2024, Cell Research, https://doi.org/10.1038/s41422-023-00923-y (nguyen2024mfsd7cfunctionsas pages 3-4)
MFSD7c is a facilitative plasma-membrane choline transporter with BBB relevance	Human MFSD7c in HEK293; mouse embryos/BBB	Transport assay; metabolomics; physiological inference	Choline uptake was concentration- and time-dependent; fetal brain metabolomics covered >520 metabolites; Mfsd7c loss increased brain choline while reducing CDP-choline; heme and bilirubin in fetal brains were comparable between WT and KO	For zebrafish flvcr2a, the strongest current annotation is plasma-membrane choline transport linked to choline metabolism, not a primary heme-transport role	Nguyen et al., 2024, Cell Research, https://doi.org/10.1038/s41422-023-00923-y (nguyen2024mfsd7cfunctionsas pages 3-4)
MFSD7c behaves as a facilitative/electrogenic transporter and can mediate bidirectional choline movement	Human MFSD7c in HEK293	Transport assay; patch clamp; release assay	Co-expression with choline kinase A increased choline uptake ~9-fold; apparent Km for choline import under this condition was 100 µM and Vmax 0.117 µmol/well/60 min; expression was required for intracellular choline release; choline import increased membrane potential	Zebrafish flvcr2a is best inferred to function as a gradient-driven choline transporter/uniporter-like protein capable of import and export depending on context	Nguyen et al., 2024, Cell Research, https://doi.org/10.1038/s41422-023-00923-y (nguyen2024mfsd7cfunctionsas pages 4-7, nguyen2024mfsd7cfunctionsas pages 7-8)
Ethanolamine can also be transported, but less strongly supported than choline in MFSD7c study	Human MFSD7c in HEK293	Transport assay with metabolic trapping	Ethanolamine uptake was not increased by MFSD7c alone but became significant with ETNK1 co-expression; L-carnitine increase was slight and considered a weak ligand	Zebrafish flvcr2a may transport ethanolamine, but choline is the best-supported substrate from MFSD7c-specific vertebrate data	Nguyen et al., 2024, Cell Research, https://doi.org/10.1038/s41422-023-00923-y (nguyen2024mfsd7cfunctionsas pages 4-7)
MFSD7c localizes to plasma membrane and is expressed in CNS endothelial cells/BBB	Mouse embryos/adult brain; human/mouse cell systems	Immunohistochemistry; localization controls; functional cell assays	S203Y reduced transport without loss of plasma-membrane localization; prior and current data place MFSD7c in CNS endothelial cells and plasma membrane	For zebrafish flvcr2a, the likely cellular location is plasma membrane of endothelial/BBB-like cells rather than an exclusively mitochondrial compartment	Nguyen et al., 2024, Cell Research, https://doi.org/10.1038/s41422-023-00923-y (nguyen2024mfsd7cfunctionsas pages 3-4); Kalailingam et al., 2020, JCI, https://doi.org/10.1172/JCI136727 (kalailingam2020deficiencyofmfsd7c pages 1-2)
MFSD7c is required for BBB choline handling in vivo	Endothelial-specific Mfsd7c knockout mice	Conditional knockout; radiotracer uptake; stable-isotope tracing	Brain radioactive choline signal was significantly reduced in endothelial KO mice, whereas peripheral organs were comparable; brain accounted for ~2%–3% of total radioactive signal; endogenous and labeled choline accumulated in KO brain	Zebrafish flvcr2a likely contributes to endothelial/brain choline flux and could influence brain phospholipid precursor supply during development	Nguyen et al., 2024, Cell Research, https://doi.org/10.1038/s41422-023-00923-y (nguyen2024mfsd7cfunctionsas pages 7-8)
MFSD7c is implicated in export/recycling of LPC-derived choline at the BBB	Endothelial-specific Mfsd7c knockout mice	Stable-isotope lipid tracing	After LPC-d49 tracing, deuterated choline-d13 accumulated in KO brains, supporting a defect in choline export/recycling from brain parenchyma to endothelium	Zebrafish flvcr2a annotation should mention a role in choline recycling/homeostasis, not just uptake	Nguyen et al., 2024, Cell Research, https://doi.org/10.1038/s41422-023-00923-y (nguyen2024mfsd7cfunctionsas pages 7-8)
Human FLVCR2 is structurally and biochemically a choline/ethanolamine transporter	Human FLVCR2 in HEK293 and cryo-EM structural studies	Radioligand transport assays; cryo-EM; mutagenesis; MD simulation	FLVCR1 cavity ~513 Å3, FLVCR2 cavity ~579 Å3; conserved W102/F324/Y325 in FLVCR2 coordinate ligand; authors conclude FLVCR1/2 are uniporters enabling downhill transport independent of sodium or pH gradients	Because zebrafish flvcr2a belongs to the same conserved FLVCR2/MFS family, current best inference is choline/ethanolamine uniport activity with conserved aromatic binding chemistry	Ri et al., 2024, Nature, https://doi.org/10.1038/s41586-024-07444-7 (ri2024molecularmechanismof pages 7-7)
The 2024 FLVCR2 model revises the earlier heme-import hypothesis	Human FLVCR1/FLVCR2	Expert synthesis within primary structural paper	Authors explicitly state that FLVCR2-mediated heme uptake has not been confirmed and that choline and ethanolamine are the primary transport substrates	For zebrafish flvcr2a, annotation should prioritize choline/ethanolamine transport and treat heme transport as an older, less secure model	Ri et al., 2024, Nature, https://doi.org/10.1038/s41586-024-07444-7 (ri2024molecularmechanismof pages 7-7)
Earlier literature supported FLVCR2 as a cell-surface heme importer	Human FLVCR2 in CHO cells, TE671 cells, Xenopus oocytes	Hemin-agarose binding; ZnMP uptake; [55Fe]hemin uptake; siRNA knockdown	Free hemin reduced hemin-agarose binding by ~23% (25 µM) and ~36% (50 µM); FY981 Env reduced ZnMP uptake by ~40%; FLVCR2 knockdown disrupted heme uptake	Heme-import activity remains part of the historical literature for FLVCR2-family proteins, but for zebrafish flvcr2a it should be considered secondary/contested versus newer choline data	Duffy et al., 2010, Mol Cell Biol, https://doi.org/10.1128/MCB.00690-10 (duffy2010thefowlersyndromeassociated pages 2-3)
Loss of MFSD7c causes CNS vascular-development defects relevant to Fowler syndrome	Global Mfsd7c knockout mice; comparison to human FLVCR2 disease	Knockout phenotype; histology; transcriptomics	KO caused late-gestation lethality; impaired angiogenic growth in ventricular/subventricular zones; dilated/fused vascular tips, glomeruloid vessels, hypoxia, neuronal cell death, reduced cortical layers, enlarged ventricles, microcephaly	For zebrafish flvcr2a, likely biological roles include CNS endothelial function, angiogenic brain vascular development, and neurodevelopmental support through metabolite transport	Kalailingam et al., 2020, JCI, https://doi.org/10.1172/JCI136727 (kalailingam2020deficiencyofmfsd7c pages 1-2)

Table: This table compiles the strongest retrieved evidence relevant to zebrafish flvcr2a/MFSD7C-a, emphasizing direct vertebrate transport data, family-level structural mechanisms, and knockout phenotypes. It is useful for assigning a cautious, evidence-weighted functional annotation that prioritizes choline transport and BBB/CNS vascular roles while noting the older contested heme-import model.

References

(nguyen2024mfsd7cfunctionsas pages 3-4): Xuan Thi Anh Nguyen, Thanh Nha Uyen Le, Toan Q. Nguyen, Hoa Thi Thuy Ha, Anna Artati, Nancy C. P. Leong, Dat T. Nguyen, Pei Yen Lim, Adelia Vicanatalita Susanto, Qianhui Huang, Ling Fam, Lo Ngah Leong, Isabelle Bonne, Angela Lee, Jorge L. Granadillo, Catherine Gooch, Dejie Yu, Hua Huang, Tuck Wah Soong, Matthew Wook Chang, Markus R. Wenk, Jerzy Adamski, Amaury Cazenave-Gassiot, and Long N. Nguyen. Mfsd7c functions as a transporter of choline at the blood–brain barrier. Cell Research, 34:245-257, Feb 2024. URL: https://doi.org/10.1038/s41422-023-00923-y, doi:10.1038/s41422-023-00923-y. This article has 27 citations and is from a domain leading peer-reviewed journal.
(nguyen2024mfsd7cfunctionsas pages 7-8): Xuan Thi Anh Nguyen, Thanh Nha Uyen Le, Toan Q. Nguyen, Hoa Thi Thuy Ha, Anna Artati, Nancy C. P. Leong, Dat T. Nguyen, Pei Yen Lim, Adelia Vicanatalita Susanto, Qianhui Huang, Ling Fam, Lo Ngah Leong, Isabelle Bonne, Angela Lee, Jorge L. Granadillo, Catherine Gooch, Dejie Yu, Hua Huang, Tuck Wah Soong, Matthew Wook Chang, Markus R. Wenk, Jerzy Adamski, Amaury Cazenave-Gassiot, and Long N. Nguyen. Mfsd7c functions as a transporter of choline at the blood–brain barrier. Cell Research, 34:245-257, Feb 2024. URL: https://doi.org/10.1038/s41422-023-00923-y, doi:10.1038/s41422-023-00923-y. This article has 27 citations and is from a domain leading peer-reviewed journal.
(ri2024molecularmechanismof pages 7-7): Keiken Ri, Tsai-Hsuan Weng, Ainara Claveras Cabezudo, Wiebke Jösting, Yu Zhang, Andre Bazzone, Nancy C. P. Leong, Sonja Welsch, Raymond T. Doty, Gonca Gursu, Tiffany Jia Ying Lim, Sarah Luise Schmidt, Janis L. Abkowitz, Gerhard Hummer, Di Wu, Long N. Nguyen, and Schara Safarian. Molecular mechanism of choline and ethanolamine transport in humans. Nature, 630:501-508, May 2024. URL: https://doi.org/10.1038/s41586-024-07444-7, doi:10.1038/s41586-024-07444-7. This article has 36 citations and is from a highest quality peer-reviewed journal.
(duffy2010thefowlersyndromeassociated pages 2-3): Simon P. Duffy, Jennifer Shing, Punit Saraon, Lloyd C. Berger, Maribeth V. Eiden, Andrew Wilde, and Chetankumar S. Tailor. The fowler syndrome-associated protein flvcr2 is an importer of heme. Nov 2010. URL: https://doi.org/10.1128/mcb.00690-10, doi:10.1128/mcb.00690-10. This article has 160 citations and is from a domain leading peer-reviewed journal.
(belot2024updateonheme pages 19-20): Audrey Belot, Herve Puy, Iqbal Hamza, and Herbert L. Bonkovsky. Update on heme biosynthesis, tissue‐specific regulation, heme transport, relation to iron metabolism and cellular energy. Liver International, 44:2235-2250, Jun 2024. URL: https://doi.org/10.1111/liv.15965, doi:10.1111/liv.15965. This article has 53 citations and is from a peer-reviewed journal.
(kalailingam2020deficiencyofmfsd7c pages 1-2): Pazhanichamy Kalailingam, Kai Qi Wang, Xiu Ru Toh, Toan Q. Nguyen, Madhuvanthi Chandrakanthan, Zafrul Hasan, Clair Habib, Aharon Schif, Francesca Clementina Radio, Bruno Dallapiccola, Karin Weiss, and Long N. Nguyen. Deficiency of mfsd7c results in microcephaly-associated vasculopathy in fowler syndrome. Journal of Clinical Investigation, Jun 2020. URL: https://doi.org/10.1172/jci136727, doi:10.1172/jci136727. This article has 32 citations and is from a highest quality peer-reviewed journal.
(ri2023structuralandmechanistic pages 1-4): Keiken Ri, Tsai-Hsuan Weng, Ainara Claveras Cabezudo, Wiebke Jösting, Zhang Yu, Andre Bazzone, Nancy C.P. Leong, Sonja Welsch, Raymond T. Doty, Gonca Gursu, Tiffany Jia Ying Lim, Sarah Luise Schmidt, Janis L. Abkowitz, Gerhard Hummer, Di Wu, Long N Nguyen, and Schara Safarian. Structural and mechanistic insights into human choline and ethanolamine transport. bioRxiv, Dec 2023. URL: https://doi.org/10.1101/2023.09.15.557925, doi:10.1101/2023.09.15.557925. This article has 4 citations.
(santander2020lackofflvcr2 pages 1-3): Nicolas Santander, Carlos O. Lizama, Eman Meky, Gabriel L. McKinsey, Bongnam Jung, Dean Sheppard, Christer Betsholtz, and Thomas D. Arnold. Lack of flvcr2 impairs brain angiogenesis without affecting the blood-brain barrier. Journal of Clinical Investigation, Jun 2020. URL: https://doi.org/10.1172/jci136578, doi:10.1172/jci136578. This article has 33 citations and is from a highest quality peer-reviewed journal.
(nguyen2024mfsd7cfunctionsas pages 4-7): Xuan Thi Anh Nguyen, Thanh Nha Uyen Le, Toan Q. Nguyen, Hoa Thi Thuy Ha, Anna Artati, Nancy C. P. Leong, Dat T. Nguyen, Pei Yen Lim, Adelia Vicanatalita Susanto, Qianhui Huang, Ling Fam, Lo Ngah Leong, Isabelle Bonne, Angela Lee, Jorge L. Granadillo, Catherine Gooch, Dejie Yu, Hua Huang, Tuck Wah Soong, Matthew Wook Chang, Markus R. Wenk, Jerzy Adamski, Amaury Cazenave-Gassiot, and Long N. Nguyen. Mfsd7c functions as a transporter of choline at the blood–brain barrier. Cell Research, 34:245-257, Feb 2024. URL: https://doi.org/10.1038/s41422-023-00923-y, doi:10.1038/s41422-023-00923-y. This article has 27 citations and is from a domain leading peer-reviewed journal.

Artifacts

Edison artifact artifact-00

Citations

duffy2010thefowlersyndromeassociated pages 2-3
belot2024updateonheme pages 19-20
ri2024molecularmechanismof pages 7-7
ri2023structuralandmechanistic pages 1-4
55Fe
3H
https://doi.org/10.1038/s41422-023-00923-y
https://doi.org/10.1038/s41586-024-07444-7
https://doi.org/10.1101/2023.09.15.557925
https://doi.org/10.1128/MCB.00690-10
https://doi.org/10.1172/JCI136727
https://doi.org/10.1038/s41422-023-00923-y,
https://doi.org/10.1038/s41586-024-07444-7,
https://doi.org/10.1128/mcb.00690-10,
https://doi.org/10.1111/liv.15965,
https://doi.org/10.1172/jci136727,
https://doi.org/10.1101/2023.09.15.557925,
https://doi.org/10.1172/jci136578,

📚 Additional Documentation

Notes

(flvcr2a-notes.md)

flvcr2a review notes

Curated in DANRE batch 04. Flvcr2a should no longer be reviewed as primarily a heme transporter: the current UniProt entry and 2024 paper support MFSD7c as a choline transporter at the BBB PMID:38302740. I retained ethanolamine/heme transport as non-core inferred capabilities.
Follow-up: changed generic membrane CC from cross-aspect MODIFY to REMOVE; specific membrane locations remain reviewed separately.

📄 View Raw YAML

id: A0A0R4ILB2
gene_symbol: flvcr2a
product_type: PROTEIN
status: DRAFT
taxon:
  id: NCBITaxon:7955
  label: Danio rerio
description: flvcr2a encodes zebrafish MFSD7c/FLVCR2A, a multi-pass membrane transporter now supported as a choline transporter
  at the blood-brain barrier. The core function is choline transmembrane transport, with ethanolamine/heme transport and ER/mitochondrial
  membrane annotations retained as supported but non-core or inferred contexts.
existing_annotations:
- term:
    id: GO:0015232
    label: heme transmembrane transporter activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: |-
      heme transmembrane transporter activity (GO:0015232) is retained cautiously as a historical/contested
      inferred context but is not the current core function. The heme-import model derives from 2010 hemin-binding
      and heme-analog uptake assays in heterologous systems, whereas 2024 structural and physiological work
      de-orphanizes FLVCR2/MFSD7c as a choline/ethanolamine transporter and notes that FLVCR2-mediated heme
      uptake has not been confirmed.
    action: KEEP_AS_NON_CORE
    reason: UniProt now emphasizes choline transport at the BBB and describes heme transport only as an additional/by-similarity
      activity. The falcon deep research synthesis treats heme transport as an older, less secure hypothesis superseded
      by direct choline-transport evidence; it is retained as non-core pending direct zebrafish testing.
    additional_reference_ids:
    - file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
    supported_by:
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: ethanolamine (By similarity)
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: heme b transporter (By similarity)
    - reference_id: file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
      supporting_text: |-
        Earlier literature proposed **heme import** by FLVCR2 based on hemin-binding and heme-analog uptake assays
    - reference_id: file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
      supporting_text: |-
        The 2024 Nature paper explicitly notes that **FLVCR2-mediated heme uptake “has not been confirmed”**
- term:
    id: GO:0016020
    label: membrane
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: membrane (GO:0016020) is too broad and should not be modified across GO aspects.
    action: REMOVE
    reason: Specific membrane locations are reviewed separately; this generic CC annotation should be retired rather than
      replaced with MF/BP transport terms.
    supported_by:
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Choline uniporter that specifically mediates choline uptake
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Reaction=choline(out) = choline(in)
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Cell membrane
    - reference_id: PMID:38302740
      supporting_text: MFSD7c is a choline transporter at the blood-brain barrier
- term:
    id: GO:0020037
    label: heme binding
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: |-
      heme binding (GO:0020037) is retained cautiously as a historical/contested inferred heme context but is not the
      current core function. Historical support comes from 2010 hemin-agarose binding assays for FLVCR2; 2024 work
      reframes the FLVCR2/MFSD7c family around choline/ethanolamine transport.
    action: KEEP_AS_NON_CORE
    reason: UniProt now emphasizes choline transport at the BBB and describes heme transport only as an additional/by-similarity
      activity; the falcon synthesis treats heme binding/transport as a legacy hypothesis not confirmed in current mechanistic work.
    additional_reference_ids:
    - file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
    supported_by:
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: ethanolamine (By similarity)
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: heme b transporter (By similarity)
    - reference_id: file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
      supporting_text: |-
        Earlier literature proposed **heme import** by FLVCR2 based on hemin-binding and heme-analog uptake assays
- term:
    id: GO:0097037
    label: heme export
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: |-
      heme export (GO:0097037) is retained cautiously as a historical/contested inferred heme context but is not the
      current core function. 2024 mechanistic work concludes FLVCR2-mediated heme uptake has not been confirmed and
      prioritizes choline/ethanolamine as the primary transported substrates.
    action: KEEP_AS_NON_CORE
    reason: UniProt now emphasizes choline transport at the BBB and describes heme transport only as an additional/by-similarity
      activity; heme export is retained as a legacy hypothesis pending direct zebrafish testing.
    additional_reference_ids:
    - file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
    supported_by:
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: ethanolamine (By similarity)
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: heme b transporter (By similarity)
    - reference_id: file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
      supporting_text: |-
        The 2024 Nature paper explicitly notes that **FLVCR2-mediated heme uptake “has not been confirmed”**
- term:
    id: GO:0005789
    label: endoplasmic reticulum membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  review:
    summary: endoplasmic reticulum membrane (GO:0005789) is supported as an inferred membrane location but is not the primary
      functional site for the choline transporter annotation.
    action: KEEP_AS_NON_CORE
    reason: The core choline transporter role is best tied to plasma membrane/BBB context.
    supported_by:
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Endoplasmic reticulum membrane
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Mitochondrion membrane
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Cell membrane
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  review:
    summary: plasma membrane (GO:0005886) is supported for Flvcr2a/MFSD7c.
    action: ACCEPT
    reason: Current evidence supports MFSD7c as a choline transporter at the blood-brain barrier/plasma membrane.
    supported_by:
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Choline uniporter that specifically mediates choline uptake
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Reaction=choline(out) = choline(in)
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Cell membrane
    - reference_id: PMID:38302740
      supporting_text: MFSD7c is a choline transporter at the blood-brain barrier
- term:
    id: GO:0015220
    label: choline transmembrane transporter activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  review:
    summary: choline transmembrane transporter activity (GO:0015220) is supported for Flvcr2a/MFSD7c.
    action: ACCEPT
    reason: Current evidence supports MFSD7c as a choline transporter at the blood-brain barrier/plasma membrane.
    supported_by:
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Choline uniporter that specifically mediates choline uptake
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Reaction=choline(out) = choline(in)
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Cell membrane
    - reference_id: PMID:38302740
      supporting_text: MFSD7c is a choline transporter at the blood-brain barrier
- term:
    id: GO:0015871
    label: choline transport
  evidence_type: IEA
  original_reference_id: GO_REF:0000108
  review:
    summary: choline transport (GO:0015871) is supported for Flvcr2a/MFSD7c.
    action: ACCEPT
    reason: Current evidence supports MFSD7c as a choline transporter at the blood-brain barrier/plasma membrane.
    supported_by:
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Choline uniporter that specifically mediates choline uptake
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Reaction=choline(out) = choline(in)
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Cell membrane
    - reference_id: PMID:38302740
      supporting_text: MFSD7c is a choline transporter at the blood-brain barrier
- term:
    id: GO:0022857
    label: transmembrane transporter activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  review:
    summary: |-
      transmembrane transporter activity (GO:0022857) is too broad for Flvcr2a. The specific function is choline
      transmembrane transporter activity, with mechanistic evidence that the FLVCR2/MFSD7c family operates as a
      facilitative uniporter driving downhill transport independent of sodium or pH gradients, with substrate
      selectivity mediated by conserved aromatic (cation-pi) residues.
    action: MODIFY
    reason: The supported molecular function should be the specific choline transmembrane transporter activity;
      2024 structural/transport work defines the FLVCR2 family as uniporters rather than coupled/ATP-driven pumps.
    proposed_replacement_terms:
    - id: GO:0015220
      label: choline transmembrane transporter activity
    additional_reference_ids:
    - file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
    supported_by:
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Choline uniporter that specifically mediates choline uptake
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Reaction=choline(out) = choline(in)
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Cell membrane
    - reference_id: PMID:38302740
      supporting_text: MFSD7c is a choline transporter at the blood-brain barrier
    - reference_id: file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
      supporting_text: |-
        both operate as **uniporters**
    - reference_id: file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
      supporting_text: |-
        downhill transport independent of sodium or pH gradients
- term:
    id: GO:0031966
    label: mitochondrial membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  review:
    summary: mitochondrial membrane (GO:0031966) is supported as an inferred membrane location but is not the primary functional
      site for the choline transporter annotation.
    action: KEEP_AS_NON_CORE
    reason: The core choline transporter role is best tied to plasma membrane/BBB context.
    supported_by:
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Endoplasmic reticulum membrane
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Mitochondrion membrane
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Cell membrane
- term:
    id: GO:0034229
    label: ethanolamine transport
  evidence_type: IEA
  original_reference_id: GO_REF:0000108
  review:
    summary: |-
      ethanolamine transport (GO:0034229) is supported by similarity as a secondary/conditional transported substrate.
      In MFSD7c-focused assays ethanolamine uptake was not increased by MFSD7c alone but became significant when
      ethanolamine kinase (ETNK1) was co-expressed (metabolic trapping); human FLVCR2 structural work independently
      supports choline and ethanolamine as the transported solutes.
    action: KEEP_AS_NON_CORE
    reason: The best-supported zebrafish/core role is choline transport; ethanolamine transport is retained as non-core
      substrate context, supported but more conditional than choline in MFSD7c-specific data.
    additional_reference_ids:
    - file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
    supported_by:
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: ethanolamine (By similarity)
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: heme b transporter (By similarity)
    - reference_id: file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
      supporting_text: |-
        MFSD7c did not increase ethanolamine uptake alone, but ethanolamine transport became significant when **ethanolamine kinase (ETNK1)** was co-expressed
- term:
    id: GO:0055085
    label: transmembrane transport
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  review:
    summary: transmembrane transport (GO:0055085) is too broad for Flvcr2a.
    action: MODIFY
    reason: The supported biological process should be the specific choline transport process.
    proposed_replacement_terms:
    - id: GO:0015871
      label: choline transport
    supported_by:
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Choline uniporter that specifically mediates choline uptake
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Reaction=choline(out) = choline(in)
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Cell membrane
    - reference_id: PMID:38302740
      supporting_text: MFSD7c is a choline transporter at the blood-brain barrier
- term:
    id: GO:0005789
    label: endoplasmic reticulum membrane
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  review:
    summary: endoplasmic reticulum membrane (GO:0005789) is supported as an inferred membrane location but is not the primary
      functional site for the choline transporter annotation.
    action: KEEP_AS_NON_CORE
    reason: The core choline transporter role is best tied to plasma membrane/BBB context.
    supported_by:
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Endoplasmic reticulum membrane
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Mitochondrion membrane
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Cell membrane
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  review:
    summary: |-
      plasma membrane (GO:0005886) is supported for Flvcr2a/MFSD7c and is the functional site of choline transport.
      Transport assays for human/mouse MFSD7c rely on plasma-membrane localization, and a transport-deficient disease
      mutant (S203Y) retained normal plasma-membrane localization, indicating its defect reflects functional impairment
      rather than mislocalization.
    action: ACCEPT
    reason: Current evidence supports MFSD7c as a choline transporter at the blood-brain barrier/plasma membrane,
      with plasma-membrane localization confirmed in heterologous transport assays.
    additional_reference_ids:
    - file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
    supported_by:
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Choline uniporter that specifically mediates choline uptake
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Reaction=choline(out) = choline(in)
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Cell membrane
    - reference_id: PMID:38302740
      supporting_text: MFSD7c is a choline transporter at the blood-brain barrier
    - reference_id: file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
      supporting_text: |-
        a transport-deficient mutant (S203Y) was stated not to have defective plasma membrane localization
- term:
    id: GO:0031966
    label: mitochondrial membrane
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  review:
    summary: mitochondrial membrane (GO:0031966) is supported as an inferred membrane location but is not the primary functional
      site for the choline transporter annotation.
    action: KEEP_AS_NON_CORE
    reason: The core choline transporter role is best tied to plasma membrane/BBB context.
    supported_by:
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Endoplasmic reticulum membrane
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Mitochondrion membrane
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Cell membrane
- term:
    id: GO:0034228
    label: ethanolamine transmembrane transporter activity
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  review:
    summary: |-
      ethanolamine transmembrane transporter activity (GO:0034228) is supported by similarity as a secondary transported
      substrate. A 2024 Nature study concludes that human FLVCR1 and FLVCR2 mediate cellular transport of choline and
      ethanolamine, supporting ethanolamine as a bona fide FLVCR2-family substrate while choline remains the best-supported
      activity for MFSD7c.
    action: KEEP_AS_NON_CORE
    reason: The best-supported zebrafish/core role is choline transport; ethanolamine transport is retained as non-core
      substrate context, strongly supported for human FLVCR2 in structural/transport assays.
    additional_reference_ids:
    - file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
    supported_by:
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: ethanolamine (By similarity)
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: heme b transporter (By similarity)
    - reference_id: file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
      supporting_text: |-
        A 2024 Nature study concludes that human **FLVCR1 and FLVCR2 mediate cellular transport of choline and ethanolamine**
- term:
    id: GO:0015220
    label: choline transmembrane transporter activity
  evidence_type: IDA
  original_reference_id: PMID:38302740
  review:
    summary: |-
      choline transmembrane transporter activity (GO:0015220) is the core molecular function of Flvcr2a/MFSD7c.
      Critically, the zebrafish orthologs themselves were directly tested: the two zebrafish MFSD7c isoforms
      (DaMfsd7c_a, DaMfsd7c_b) show choline transport activity in heterologous assays, providing direct
      species-relevant support rather than inference alone. 2024 mechanistic work establishes the transporter
      operates as a facilitative uniporter mediating concentration-driven (downhill) choline movement.
    action: ACCEPT
    reason: Current evidence supports MFSD7c as a choline transporter at the blood-brain barrier/plasma membrane,
      with direct transport activity demonstrated for the zebrafish orthologs.
    additional_reference_ids:
    - file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
    supported_by:
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Choline uniporter that specifically mediates choline uptake
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Reaction=choline(out) = choline(in)
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Cell membrane
    - reference_id: PMID:38302740
      supporting_text: MFSD7c is a choline transporter at the blood-brain barrier
    - reference_id: file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
      supporting_text: |-
        zebrafish MFSD7c isoforms “DaMfsd7c_a” and “DaMfsd7c_b” show choline transport activity in heterologous assays
    - reference_id: file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
      supporting_text: |-
        zebrafish isoforms a and b also showed choline transport activity
- term:
    id: GO:0150104
    label: transport across blood-brain barrier
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  review:
    summary: |-
      transport across blood-brain barrier (GO:0150104) is supported for Flvcr2a/MFSD7c. In mammals, endothelial-specific
      Mfsd7c knockout reduces brain uptake of injected radiolabeled choline while peripheral organ signals remain
      comparable, and the transporter is expressed in CNS endothelial/BBB cells. For zebrafish this is a conservation-based
      (ISS) inference; direct in vivo zebrafish BBB choline-flux data are not yet available.
    action: ACCEPT
    reason: Current evidence supports MFSD7c as a choline transporter at the blood-brain barrier/plasma membrane,
      consistent with mammalian endothelial knockout reducing brain choline import. Retained as a conservation-based
      inference for zebrafish.
    additional_reference_ids:
    - file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
    supported_by:
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Choline uniporter that specifically mediates choline uptake
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Reaction=choline(out) = choline(in)
    - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
      supporting_text: Cell membrane
    - reference_id: PMID:38302740
      supporting_text: MFSD7c is a choline transporter at the blood-brain barrier
    - reference_id: file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
      supporting_text: |-
        loss of MFSD7c reduces brain uptake of injected radiolabeled choline while leaving peripheral organ signals comparable
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO terms
  findings:
  - statement: Flvcr2a matches InterPro signatures IPR011701 and IPR020846 (major
      facilitator superfamily / MFS domain); the InterPro2GO mapping assigns transmembrane
      transporter activity (GO:0022857) and transmembrane transport (GO:0055085).
- id: GO_REF:0000024
  title: Manual transfer of experimentally-verified manual GO annotation data to orthologs by curator judgment of sequence
    similarity
  findings:
  - statement: Endoplasmic reticulum membrane (GO:0005789), plasma membrane (GO:0005886),
      mitochondrial membrane (GO:0031966), ethanolamine transmembrane transporter
      activity (GO:0034228), and transport across blood-brain barrier (GO:0150104)
      were transferred to zebrafish Flvcr2a by curator-judged sequence similarity
      to its mammalian orthologs (human FLVCR2 UniProtKB:Q9UPI3 and mouse Mfsd7c
      UniProtKB:Q91X85).
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings:
  - statement: Phylogenetic inference within PANTHER families PTN000858905 and PTN000858822
      supports heme transmembrane transporter activity (GO:0015232), heme binding
      (GO:0020037), heme export (GO:0097037), and membrane localization (GO:0016020).
- id: GO_REF:0000044
  title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative
    changes to GO terms applied by UniProt
  findings:
  - statement: The UniProt Swiss-Prot subcellular location keywords SL-0097 (Endoplasmic
      reticulum membrane), SL-0039 (Cell membrane), and SL-0171 (Mitochondrion membrane)
      map Flvcr2a to endoplasmic reticulum membrane (GO:0005789), plasma membrane
      (GO:0005886), and mitochondrial membrane (GO:0031966).
- id: GO_REF:0000108
  title: Automatic assignment of GO terms using logical inference, based on inter-ontology links
  findings:
  - statement: Logical inference over inter-ontology links derives choline transport
      (GO:0015871) from choline transmembrane transporter activity (GO:0015220) and
      ethanolamine transport (GO:0034229) from ethanolamine transmembrane transporter
      activity (GO:0034228).
- id: GO_REF:0000117
  title: Electronic Gene Ontology annotations created by ARBA machine learning models
  findings:
  - statement: The ARBA machine-learning model ARBA00027949 predicts choline transmembrane
      transporter activity (GO:0015220) for Flvcr2a.
- id: PMID:38302740
  title: MFSD7c functions as a transporter of choline at the blood-brain barrier.
  findings:
  - statement: Mfsd7c-knockout mouse and cell-based assays established MFSD7c (Flvcr2)
      as a facilitative, electrogenic choline transporter at the blood-brain barrier
      whose choline transport function is conserved across vertebrates; Fowler syndrome
      missense mutations abolish or reduce this activity.
    supporting_text: MFSD7c is a choline transporter at the blood-brain barrier
- id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
  title: UniProtKB entry A0A0R4ILB2 for Danio rerio flvcr2a
  findings:
  - statement: UniProt describes Flvcr2a/MFSD7c as a choline uniporter at the blood-brain barrier.
    supporting_text: Choline uniporter that specifically mediates choline uptake
- id: file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
  title: Falcon deep research report (Edison Scientific Literature) for Danio rerio flvcr2a (mfsd7c-a)
  findings:
  - statement: |-
      The zebrafish orthologs were directly tested: the two zebrafish MFSD7c isoforms (DaMfsd7c_a and DaMfsd7c_b)
      show choline transport activity in heterologous assays, providing direct species-relevant support for choline
      transport rather than inference from mammalian orthologs alone.
    supporting_text: |-
      zebrafish MFSD7c isoforms “DaMfsd7c_a” and “DaMfsd7c_b” show choline transport activity in heterologous assays
  - statement: |-
      2024 structural/functional work in humans establishes that FLVCR1 and FLVCR2 mediate choline and ethanolamine
      transport as facilitative uniporters performing downhill transport independent of sodium or pH gradients, with
      substrate selectivity mediated by conserved aromatic (cation-pi) residues.
    supporting_text: |-
      A 2024 Nature study concludes that human **FLVCR1 and FLVCR2 mediate cellular transport of choline and ethanolamine** and that both operate as **uniporters**
  - statement: |-
      The historical heme-import model (from 2010 hemin-binding and heme-analog uptake assays) is reframed by 2024
      mechanistic work, which explicitly states that FLVCR2-mediated heme uptake has not been confirmed and prioritizes
      choline/ethanolamine as the primary substrates; heme transport should be treated as a contested legacy hypothesis.
    supporting_text: |-
      The 2024 Nature paper explicitly notes that **FLVCR2-mediated heme uptake “has not been confirmed”**
  - statement: |-
      In mammals MFSD7c is required for BBB choline handling: endothelial-specific knockout reduces brain uptake of
      injected radiolabeled choline while peripheral organ signals remain comparable, and loss of MFSD7c/FLVCR2 causes
      neurovascular developmental defects (brain angiogenic sprouting) relevant to Fowler syndrome.
    supporting_text: |-
      loss of MFSD7c reduces brain uptake of injected radiolabeled choline while leaving peripheral organ signals comparable
core_functions:
- description: flvcr2a/MFSD7c functions as a plasma-membrane choline transporter at the blood-brain barrier.
  molecular_function:
    id: GO:0015220
    label: choline transmembrane transporter activity
  directly_involved_in:
  - id: GO:0015871
    label: choline transport
  - id: GO:0150104
    label: transport across blood-brain barrier
  locations:
  - id: GO:0005886
    label: plasma membrane
  supported_by:
  - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
    supporting_text: Choline uniporter that specifically mediates choline uptake
  - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
    supporting_text: Reaction=choline(out) = choline(in)
  - reference_id: file:DANRE/flvcr2a/flvcr2a-uniprot.txt
    supporting_text: Cell membrane
  - reference_id: PMID:38302740
    supporting_text: MFSD7c is a choline transporter at the blood-brain barrier
  - reference_id: file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
    supporting_text: |-
      zebrafish isoforms a and b also showed choline transport activity
  - reference_id: file:DANRE/flvcr2a/flvcr2a-deep-research-falcon.md
    supporting_text: |-
      both operate as **uniporters**
suggested_questions:
- question: Does zebrafish flvcr2a transport choline at the blood-brain barrier with
    the same facilitative, electrogenic mechanism demonstrated for mammalian MFSD7c,
    and is it required for brain choline homeostasis in zebrafish?
- question: Is the heme-transport activity inferred from FLVCR-family orthologs functionally
    relevant for zebrafish flvcr2a, or has this paralog specialized for choline and
    ethanolamine transport?
suggested_experiments:
- hypothesis: Zebrafish flvcr2a mediates facilitative, electrogenic choline uptake,
    like its mammalian ortholog MFSD7c.
  description: Express zebrafish flvcr2a in a cultured cell line and measure radiolabeled
    choline uptake and single-cell patch-clamp currents to test for facilitative,
    electrogenic choline transport.
- hypothesis: Loss of flvcr2a disrupts brain choline homeostasis in zebrafish, modeling
    Fowler syndrome.
  description: Generate a flvcr2a zebrafish mutant and perform metabolomic profiling
    of choline-related metabolites in brain versus other tissues, and assess cerebral
    vasculature.
- hypothesis: Zebrafish flvcr2a does not contribute substantial heme transport in
    vivo despite FLVCR-family homology.
  description: Test recombinant flvcr2a in a heme b transport/export assay and compare
    its activity with that of a bona fide heme transporter.

flvcr2a

Gene Description

Existing Annotations Review

Core Functions

flvcr2a/MFSD7c functions as a plasma-membrane choline transporter at the blood-brain barrier.

References

Suggested Questions for Experts

Suggested Experiments

Deep Research

Falcon

Comprehensive research report: Zebrafish flvcr2a (mfsd7c-a) — functional annotation and current evidence

0) Executive summary (evidence-weighted)

1) Target identity verification (mandatory)

2) Key concepts and definitions (current understanding)

2.1 MFS (Major Facilitator Superfamily) transporters

2.2 What “de-orphanizing” FLVCR2 means

2.3 Biological role of choline and ethanolamine transport

3) Molecular function: substrate specificity and directionality

3.1 Best-supported transported substrate(s): choline (primary) and ethanolamine (secondary/context-dependent)

3.2 Directionality: facilitative, gradient-driven; can support bidirectional flux

3.3 Quantitative transport parameters (recent data)

4) Subcellular localization and tissue context

4.1 Plasma membrane localization

4.2 BBB/CNS endothelial expression and neurovascular context

5) Biological processes, pathways, and phenotypes informing zebrafish annotation

5.1 Choline metabolism and BBB choline handling

5.2 Neurovascular development / angiogenesis (Fowler syndrome-relevant biology)

6) Recent developments and latest research (prioritized 2023–2024)

6.1 2024: Physiological de-orphaning at the BBB and vertebrate conservation

6.2 2024: Structural mechanism and substrate-binding chemistry

6.3 2023: Preprint precursor to the 2024 Nature mechanism

7) Reconciling the historical “heme transporter” model with the 2024 choline model

7.1 Evidence supporting heme import (historical)

7.2 Current interpretation (2024)

8) Current applications and real-world implementations

8.1 Disease mechanism models: Fowler syndrome / congenital neurovascular disorders

8.2 BBB metabolite flux measurements and pathway-level implementation

8.3 Structure-guided variant interpretation and mutagenesis

9) Expert opinions, cautions, and open questions (authoritative sources)

10) Statistics and quantitative data points (recent and relevant)

11) Practical functional annotation for zebrafish flvcr2a (recommended wording)

12) Evidence limitations specific to Danio rerio flvcr2a

Summary table of key evidence

Artifacts

Citations

📚 Additional Documentation

Notes

flvcr2a review notes

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