MTRNR2L8 is a nuclear human locus on chromosome 11 that resembles the humanin small open reading frame of the mitochondrial large-subunit rRNA gene MT-RNR2, one of thirteen such MT-RNR2-like sequences in the human genome that are generally interpreted as nuclear insertions of mitochondrial DNA. Its predicted product, humanin-like 8 (HN8), is a 24-residue peptide (MAPRGFSCLLLSTSEIDLPVKRRA) that is the closest of the whole family to humanin, matching it at 23 of 24 positions and differing only by Leu12->Ser; every residue humanin requires for neuroprotective activity or for secretion is retained. The locus is transcribed: RT-PCR detects it in human tissues with high levels in heart, kidney and testis and none in skeletal muscle, its transcript is down-regulated 6 hours and up-regulated 24 hours after staurosporine treatment of cultured cells, its myocardial transcript rises after one hour of hyperoxia in patients undergoing coronary artery bypass grafting, and its promoter is hypomethylated in large-artery atherosclerotic stroke. Whether a peptide is actually made from this transcript, and what it does, is unestablished: no HN8 peptide has been detected in human tissue or fluid, no molecular activity has been measured for it, and no binding partner or receptor interaction has been reported. The humanin peptide from which it is named is cytoprotective and antiapoptotic, binds BAX, BID, BIM and IGFBP3 intracellularly and signals through FPRL1/FPR2 and a CNTFR-alpha/WSX-1/gp130 complex when secreted, but those properties were measured on the mitochondrially encoded peptide and on synthetic analogues of it, not on this gene product.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005576 extracellular region | IEA GO_REF:0000044 | MARK AS OVER ANNOTATED | Summary: Electronic mapping of UniProt's Secreted subcellular-location line for P0CJ75. That line is itself annotated ECO:0000250 by similarity to humanin (UniProtKB:Q8IVG9), so the row carries no observation made on MTRNR2L8. Reason: MTRNR2L8 is known only as a transcript (UniProt protein existence level 2); no peptide from this locus has been detected in any compartment, by mass spectrometry or otherwise. UniProt's Secreted and Cytoplasm lines for P0CJ75 are themselves ECO:0000250 by-similarity statements transferred from humanin (file:human/MTRNR2L8/MTRNR2L8-uniprot.txt), so both the ISS row (donor UniProtKB:Q8IVG9) and the subcellular-location IEA row state the same humanin claim, once directly and once via the controlled-vocabulary mapping. Secretion of humanin is not a generic property of the sequence: it depends on Leu9-Leu11 and on Pro19-Val20 [PMID:35372353], and Bodzioch et al. proposed on the alignment that position 19 is what decides whether a nuclear isoform is secreted at all [PMID:19477263]. In MTRNR2L8 both of those segments, Leu9-Leu11 and Pro19-Val20, are intact - HN8 differs from humanin at Leu12 alone - so unlike the loci that have lost Pro19 there is no residue-level argument against secretion here. The claim may well be right if the peptide exists, so I mark it as an over-annotation rather than removing it: what is wrong is that it is asserted of this locus on the strength of humanin alone. Supporting Evidence: PMID:35372353 HN was found to be an extracellularly secreted factor whereby two amino acid structures, Leu9-Leu11, and Pro19-Va120, appear to be essential for the secretion of HN peptide PMID:19477263 Proline vs serine in position 19 may determine whether the peptide is secreted or not, while threonine in position 13 may be important for cell surface receptor binding. file:human/MTRNR2L8/MTRNR2L8-uniprot.txt SUBCELLULAR LOCATION: Secreted |
| GO:0005576 extracellular region | ISS GO_REF:0000024 | MARK AS OVER ANNOTATED | Summary: Curator transfer of humanin's secreted location to MTRNR2L8 on sequence similarity. Humanin's own extracellular annotations are IDA/EXP from several studies, so the donor side is sound; the question is only whether the transfer is safe. Reason: MTRNR2L8 is known only as a transcript (UniProt protein existence level 2); no peptide from this locus has been detected in any compartment, by mass spectrometry or otherwise. Secretion of humanin depends on Leu9-Leu11 and on Pro19-Val20 [PMID:35372353], and both segments are intact in HN8, which differs from humanin at Leu12 alone; the transfer therefore does not fail on the residues. What it lacks is a premise: an ISS asserts that the donor observation holds for the target, and here the target peptide has never been shown to be made at all. HN8 is the closest of the thirteen nuclear loci to humanin (23/24 identity), so if any of them is secreted this is the best candidate - which makes this row a reasonable hypothesis and a poor annotation. Marked over-annotated rather than removed. Propagation Review Root cause: PROPAGATION BAD Failure modes: WRONG ORTHOLOG OR PARALOG Sources checked: UniProtKB:Q8IVG9 · MT-RNR2 SUPPORTS SOURCE BUT NOT TARGET Humanin, the mitochondrially encoded peptide. Its secreted location is directly observed and the source annotation is sound. What does not transfer is the premise: this nuclear MT-RNR2-like sequence is a paralogous genomic copy rather than an ortholog of a protein-coding gene, and no peptide product of it has been observed, let alone observed outside the cell. Supporting Evidence: PMID:35372353 HN was found to be an extracellularly secreted factor whereby two amino acid structures, Leu9-Leu11, and Pro19-Va120, appear to be essential for the secretion of HN peptide PMID:35372353 It is predicted that these various isoforms might contribute to differential neuroprotective effects and receptor binding, but their individual roles remain to be investigated. |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | MARK AS OVER ANNOTATED | Summary: Electronic mapping of UniProt's Cytoplasm line, which is itself a by-similarity statement transferred from humanin. Reason: MTRNR2L8 is known only as a transcript (UniProt protein existence level 2); no peptide from this locus has been detected in any compartment. UniProt's Secreted and Cytoplasm lines for P0CJ75 are themselves ECO:0000250 by-similarity statements transferred from humanin (file:human/MTRNR2L8/MTRNR2L8-uniprot.txt), so both the ISS row (donor UniProtKB:Q8IVG9) and the subcellular-location IEA row state the same humanin claim, once directly and once via the controlled-vocabulary mapping. A translated 24-residue peptide would plausibly be cytosolic, which is why this row is the least objectionable of the four localisation rows; but it is asserted from humanin, not observed here, and it adds nothing to the record of this locus. Supporting Evidence: PMID:35372353 It is predicted that these various isoforms might contribute to differential neuroprotective effects and receptor binding, but their individual roles remain to be investigated. file:human/MTRNR2L8/MTRNR2L8-uniprot.txt Cytoplasm {ECO:0000250|UniProtKB:Q8IVG9} |
| GO:0005737 cytoplasm | ISS GO_REF:0000024 | MARK AS OVER ANNOTATED | Summary: Curator transfer of humanin's cytoplasmic location on sequence similarity; humanin's own cytoplasm annotations are IDA/EXP. Reason: The same humanin claim as the paired IEA row. A translated 24-residue peptide would plausibly be cytosolic, and HN8 differs from humanin at a single position, so nothing in the sequence argues against it. But no HN8 peptide has been observed in any compartment, so the row states a plausible inference about the humanin sequence rather than an observation about this gene product. Propagation Review Root cause: PROPAGATION BAD Failure modes: WRONG ORTHOLOG OR PARALOG Sources checked: UniProtKB:Q8IVG9 · MT-RNR2 SUPPORTS SOURCE BUT NOT TARGET Humanin has direct cytoplasmic localisation evidence, so the source is sound; the transfer assumes a peptide product of this nuclear locus that has never been detected. Supporting Evidence: PMID:35372353 It is predicted that these various isoforms might contribute to differential neuroprotective effects and receptor binding, but their individual roles remain to be investigated. file:human/MTRNR2L8/MTRNR2L8-uniprot.txt Cytoplasm {ECO:0000250|UniProtKB:Q8IVG9} |
| GO:0048019 receptor antagonist activity | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: Phylogenetic propagation of humanin's receptor antagonist activity to MTRNR2L8 from the humanin-family node PANTHER:PTN002141596. The grounding experiment is humanin competing with amyloid-beta at FPRL1 [PMID:15153530]; nothing comparable has been done with HN8. Reason: The IBD behind this row places the activity at the humanin-family node, and the placement is reasonable for the mitochondrial peptide: humanin is a genuine FPRL1 ligand and the antagonism of amyloid-beta at that receptor is directly observed [PMID:15153530]. Arguing with the row means asking whether MTRNR2L8 sits inside the clade that inherited the function, and the difficulty is that the question is not really phylogenetic. The nuclear MT-RNR2-like loci are copies of a mitochondrial rRNA segment, so descent in this tree tracks genomic sequence rather than a protein-coding lineage, and membership of the node does not establish that the ORF is translated, that the peptide is stable, or that it reaches a receptor. On residues, HN8 gives the row less trouble than any other member: it differs from humanin at Leu12 alone, retaining Pro3, Ser7, Cys8, Leu9-Leu11, Thr13, Ser14 and Pro19, including position 13, which Bodzioch et al. singled out as the one that may matter for cell-surface receptor binding [PMID:19477263]. Humanin Leu12->Ala abolishes neuroprotective activity, but synthetic HN5 carries the same Leu12->Ser change and remains cytoprotective [PMID:19477263], so this is a caution and not a refutation. A second, term-level point applies to every member of this node: receptor antagonist activity describes humanin's competition with amyloid-beta at FPRL1, which is a property measured in a specific assay, whereas humanin also carries receptor ligand activity by IDA - so even for the donor the term captures one facet of the interaction. The node placement is defensible and the row may yet prove right, so this is an over-annotation rather than an error: it states a molecular activity for a gene product that has never been shown to exist. Propagation Review Root cause: PROPAGATION BAD Failure modes: WRONG ORTHOLOG OR PARALOG Sources checked: PANTHER:PTN002141596 SUPPORTS SOURCE BUT NOT TARGET Humanin-family ancestral node. The assertion is correctly placed for the mitochondrially encoded peptide, which has the experimental grounding; its mechanical descent to nuclear MT-RNR2-like copies carries no evidence that those copies yield a peptide with receptor-level activity. UniProtKB:Q8IVG9 · MT-RNR2 SUPPORTS SOURCE BUT NOT TARGET Humanin, with IDA for this term from the FPRL1 competition experiments [PMID:15153530]. Sound at source; all assays used synthetic humanin. Supporting Evidence: PMID:15153530 Humanin shares human FPRL1 and mouse FPR2 with Abeta(42) and suggest that Humanin may exert its neuroprotective effects by competitively inhibiting the access of FPRL1 to Abeta(42). PMID:19477263 Proline vs serine in position 19 may determine whether the peptide is secreted or not, while threonine in position 13 may be important for cell surface receptor binding. PMID:35372353 It is predicted that these various isoforms might contribute to differential neuroprotective effects and receptor binding, but their individual roles remain to be investigated. |
| GO:1900118 negative regulation of execution phase of apoptosis | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: Phylogenetic propagation of negative regulation of the execution phase of apoptosis to MTRNR2L8 from the humanin-family node. The co-listed donor is UniProtKB:P0CJ72 (MTRNR2L5), whose own evidence for this term is an IMP from an assay on synthetic HN5 peptide [PMID:19477263]. Reason: The antiapoptotic function is the best-supported property of this family, and the IBD placement is sensible: humanin blocks BAX, BID and BIM and is antiapoptotic in many systems, and synthetic HN5 peptide - the product of one of these very nuclear loci - is cytoprotective against staurosporine-induced apoptosis [PMID:19477263]. That last point cuts against a blanket dismissal, and it carries further for HN8 than for most of the family: HN8 differs from humanin only at Leu12, the very position at which synthetic HN5 also diverges and remains active, so every residue humanin needs for cytoprotection is present. What the row still lacks is anything specific to this locus. Its transcript is detectable and staurosporine-responsive [PMID:19477263], its myocardial level rises after hyperoxia [PMID:26769839] and its promoter is hypomethylated in atherosclerotic stroke [PMID:31084332] - but a regulated transcript is not a peptide: no HN8 peptide has been observed, no apoptosis assay has used it, and the systematic human-genetics lookup across all thirteen loci found no association with disease and no expression QTL that would argue for a functional product [PMID:31753007]. I mark the row as an over-annotation: plausible, arguably the most plausible instance of it in the family, and still an assertion about the humanin sequence rather than about this gene product. Propagation Review Root cause: PROPAGATION BAD Failure modes: WRONG ORTHOLOG OR PARALOG Sources checked: PANTHER:PTN002141596 SUPPORTS SOURCE BUT NOT TARGET Humanin-family node. The ancestral antiapoptotic assertion is well placed for humanin and for the synthetic HN5 peptide, but descent in this tree is descent of a mitochondrial rRNA segment copied into the nucleus, and says nothing about whether this copy is translated. UniProtKB:P0CJ72 · MTRNR2L5 SUPPORTS SOURCE BUT NOT TARGET MTRNR2L5/HN5, the only nuclear humanin-like sequence with a functional assay. Its synthetic peptide is cytoprotective [PMID:19477263], which supports the node; it does not by itself establish that a sibling locus yields a peptide at all. Supporting Evidence: PMID:19477263 Cytoprotection against the STS-induced apoptosis conferred by the polymorphic HN5 variant, in which threonine in position 13 is replaced with isoleucine, is reduced compared to the wild type HN5 peptide. PMID:19477263 At least ten of these nuclear genes are expressed in human tissues, and respond to staurosporine (STS) and beta-carotene. PMID:31753007 None of the found associations were statistically significant after correction for multiple testing. |
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Download this section (compressed HTML)Q: Is a peptide produced from the MTRNR2L8 locus at all? Everything annotated to this gene follows from the assumption that it is, and the assumption rests on an intact open reading frame plus RT-PCR detection of the transcript [PMID:19477263]. MTRNR2L8 is the strongest test case in the family, because its predicted peptide differs from humanin at a single position and its transcript is both abundant in heart and kidney and regulated by hyperoxia [PMID:26769839] and by promoter methylation [PMID:31084332].
Suggested experts: Alan Saghatelian, Pinchas Cohen, Jonathan Wren
Q: Can circulating humanin be apportioned between the mitochondrial gene and this locus? HN8 matches humanin at 23 of 24 positions, with the single difference at Leu12, so an immunoassay for humanin cannot be assumed to exclude HN8, and the clinical literature on serum humanin may be measuring a mixture. A single tryptic or chymotryptic peptide spanning position 12 would discriminate the two by mass spectrometry.
Suggested experts: Pinchas Cohen
Q: How should GO treat a family in which one member has all the experimental evidence and twelve nuclear copies have none? The present solution propagates humanin's activity to every copy through PANTHER:PTN002141596, which produces thirteen gene products annotated as receptor antagonists and apoptosis regulators on the strength of one mitochondrial peptide. A node restricted to MT-RNR2 plus MTRNR2L5, the two sequences with functional data, would say what is actually known.
Q: Does the MTRNR2L8 promoter methylation signal in atherosclerotic stroke [PMID:31084332] act through a peptide, through the transcript, or through something unrelated to this locus at all, such as a regulatory element shared with a neighbouring gene on chromosome 11? The answer determines whether the locus is a gene with a product or a methylation-sensitive marker that happens to carry an open reading frame.
Experiment: Targeted (PRM) mass spectrometry of human heart, kidney and testis, and of plasma, for a peptide spanning position 12 of HN8, the only position at which it differs from humanin. This is the one experiment that would convert the entire annotation set on this gene from inference to observation, and because HN8 is the closest sequence to humanin it would simultaneously calibrate how specific existing humanin measurements are.
Experiment: Ribosome profiling or ORF-capture of the MTRNR2L8 transcript in heart or kidney, where it is most abundant, to test whether the predicted ORF is translated rather than merely transcribed.
Experiment: Synthesise the HN8 peptide and run it side by side with humanin in the assays that define humanin activity - staurosporine- and amyloid-beta-induced death, FPRL1 competition, BAX binding - as was done for HN5 alone [PMID:19477263]. Since HN8 differs from humanin only at Leu12, the comparison also isolates the contribution of that residue.
Experiment: CRISPR deletion of the MTRNR2L8 locus in a cardiomyocyte or renal cell line that expresses it, with apoptosis and hyperoxia challenge, to test whether the locus contributes to cytoprotection as a gene rather than as a synthetic peptide. The thirteen near-identical loci make guide design the hard part, and off-target cutting at the paralogs is the control that matters.
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