| 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 (pqac-00000019) |
| 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 (pqac-00000019) |
| 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 (pqac-00000017, pqac-00000018) |
| 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 (pqac-00000017) |
| 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 (pqac-00000019); Kalailingam et al., 2020, JCI, https://doi.org/10.1172/JCI136727 (pqac-00000021) |
| 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 (pqac-00000018) |
| 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 (pqac-00000018) |
| 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 (pqac-00000016) |
| 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 (pqac-00000016) |
| 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 (pqac-00000020) |
| 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 (pqac-00000021) |


*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.*