Has2 is the naked mole rat's hyaluronan synthase 2: a seven-pass integral membrane glycosyltransferase (EC 2.4.1.212, GT2/NodC-HAS family) that alternately transfers GlcNAc from UDP-N-acetyl-alpha-D-glucosamine and GlcUA from UDP-alpha-D-glucuronate onto a growing hyaluronan chain. Like other HAS enzymes it works at the inner face of the plasma membrane and extrudes the finished polysaccharide directly through the membrane into the extracellular matrix, so the enzyme is both the biosynthetic machine and the export channel; the protein is additionally found along its trafficking route through the endoplasmic reticulum membrane, Golgi, endolysosomes and extracellular vesicles. Of the three mammalian HAS isoenzymes, HAS2 is the one that makes the long polymers, and its product is the structural backbone of soft-tissue extracellular matrix, governing tissue hydration and viscoelasticity and signalling through the CD44 receptor. In mouse, Has2 is essential for embryonic cardiovascular morphogenesis (vasculogenesis, atrioventricular canal formation and the endocardial-cushion epithelial-to-mesenchymal transition), and in rat, medullary interstitial hyaluronan participates in renal water handling. The naked mole rat protein carries two substitutions of otherwise mammal-invariant asparagines to serine in the cytoplasmic catalytic loop, one of which is shared with other African mole-rats; the gene is otherwise under strong purifying selection across mammals. Naked mole rat fibroblasts express Has2 at elevated levels and secrete hyaluronan of unusually high molecular mass, and hyaluronan accumulates in the animal's tissues because slow hyaluronidase-mediated turnover compounds the elevated synthesis. Expressing the naked mole rat cDNA in human HEK293 cells is sufficient to make those cells secrete high-molecular-mass hyaluronan, and expressing the gene transgenically in mice raises tissue hyaluronan amount and mass, lowers cancer incidence and extends lifespan and healthspan. Removing the enzyme from naked mole rat cells abolishes high-molecular-mass hyaluronan and renders them transformable, making Has2 the biosynthetic origin of the species' hyaluronan-based cancer resistance. Reported absolute polymer sizes differ between laboratories (6-12 MDa versus a maximum near 2.5 MDa), but the qualitative finding that the naked mole rat product is larger than that of comparator mammals, and has unusual gelling and cytoprotective material properties, is reproduced.
Definition: The chemical reactions and pathways resulting in the formation of hyaluronan polymers of high molecular mass, conventionally taken to be greater than 1 MDa (more than roughly 2,500 disaccharide repeats). Distinguished from hyaluronan biosynthetic process generally by the length of the polymer produced, which determines the biological activity of the product: long-chain hyaluronan is anti-inflammatory and cytoprotective and represses mitogenic signalling, whereas short-chain hyaluronan is pro-inflammatory and angiogenic.
Justification: Polymer length is the biologically decisive variable in hyaluronan biology, and it is not currently expressible in GO. Verified against QuickGO: the ontology contains exactly eight hyaluronan terms (GO:0030212, GO:0030213, GO:0030214, GO:0050501, GO:0005540, GO:1900125, GO:1900126, GO:1900127); GO:0030213 has no is_a children at all, only its three regulation children; and none of the eight carries a secondaryIds entry, so no such term has been created and merged away either. The distinction is not naked-mole-rat-specific special pleading. It separates the three mammalian HAS isoenzymes from one another as a general rule - HAS1 and HAS3 make short polymers, HAS2 makes long ones - so a term at this level would let curators say what actually differs between three paralogues that currently receive identical annotations. It is also the property that carries the biological effect, which has been measured directly: cytoprotection scales with polymer length rather than with hyaluronan abundance. The naked mole rat is simply the extreme case that makes the gap conspicuous, since the entire reason this gene is studied is the length of its product, and that is the one thing its GO annotations cannot currently say. One caveat for the ontology editors. An alternative placement would be a molecular-function child of GO:0050501, since processivity is an intrinsic property of the enzyme. I propose the process term instead because GO molecular-function terms for glycosyltransferases are defined by the reaction catalysed, and the reaction is identical whatever the product length; and because the accumulation of long hyaluronan in tissue reflects the balance of synthesis and degradation, which is a process-level property. A defined molecular-mass threshold would also need agreeing, and the literature is not uniform - "high molecular mass" is variously used for greater than 1 MDa and for greater than 2 MDa, with "very-high-molecular-mass" reserved for greater than 6.1 MDa.
Parent term: hyaluronan biosynthetic process
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000139 Golgi membrane | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Golgi membrane localization mapped from the UniProt SubCell keyword "Golgi apparatus membrane". Real but peripheral: the Golgi is a transit compartment on the route to the plasma membrane, where catalysis actually occurs. Reason: The UniProt SUBCELLULAR LOCATION block that generated this row is itself entirely ECO:0000250 by similarity to human Q92819, and describes the Golgi as one waypoint on a trafficking itinerary running from the endoplasmic reticulum through the Golgi to the plasma membrane and thence to endolysosomes or extracellular vesicles. There is no naked mole rat evidence bearing on it either way, and no reason to doubt it - HAS2 is a polytopic membrane protein and must traverse the secretory pathway. Retained, but the functional site is the plasma membrane, where the enzyme polymerises hyaluronan on the cytoplasmic face and extrudes it outward. Supporting Evidence: PMID:33846452 Three HA synthase isoforms (HAS1, HAS2, and HAS3), present in all mammals, synthesize HA at different rates, directly from the inner aspect of the plasma membrane into the extracellular matrix, and with different average sizes. |
| GO:0005764 lysosome | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Lysosomal localization mapped from the UniProt SubCell keyword. This reflects the degradative endpoint of HAS2 trafficking rather than a site of hyaluronan synthesis. Reason: UniProt describes HAS2 as travelling from the plasma membrane back to endosomes and lysosomes, and the human ortholog is additionally turned over by autophagy; the lysosome is therefore a genuine but terminal destination, associated with removing the enzyme rather than with it doing anything. No naked mole rat data address it. Kept because it is not wrong, but it can carry no weight in a functional summary, and it should not be confused with the separate and well-supported fact that hyaluronan itself is catabolised lysosomally by other proteins. The autophagic-turnover point is taken from the human-ortholog Affinage record as a conserved-mechanism lead only (it cites PMID:32084457 for an ATG9A-dependent route); no claim is made that it has been observed in this species. Supporting Evidence: PMID:39009271 Naked mole-rats (NMRs) accumulate abundant high-molecular weight hyaluronan (HA) in their tissues, suggesting decreased HA degradation. |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Endoplasmic reticulum membrane localization mapped from the UniProt SubCell keyword; the site of co-translational insertion of this seven-pass membrane protein and the start of its trafficking route, not a site of hyaluronan output. Reason: The UniProt entry annotates seven transmembrane helices, so insertion into and residence in the endoplasmic reticulum membrane is a necessary consequence of the protein's topology. In the human ortholog, phosphorylation-site mutants that cannot traffic onward are retained in the endoplasmic reticulum and make no hyaluronan, which is precisely the point: residence there is compatible with the enzyme being inactive, so this is a biosynthetic waypoint and not a functional location. The Thr-110 phosphorylation result is a human-ortholog observation taken from the Affinage record as a conserved-mechanism lead (it cites PMID:30394292); no equivalent experiment has been done in the naked mole rat. Retained as non-core. |
| GO:0005886 plasma membrane | IEA GO_REF:0000120 | ACCEPT | Summary: Plasma membrane localization, supported here by two independent WITH/FROM sources (UniProt SubCell SL-0039 and PANTHER ancestral node PTN000530842). This is the site where HAS2 does its work. Reason: This is the functional location of the enzyme, not merely a compartment it visits. All three mammalian HAS isoenzymes polymerise hyaluronan at the inner face of the plasma membrane and extrude the growing chain directly through the membrane into the extracellular matrix, so the enzyme is simultaneously the synthase and the export channel. The dual grounding is notable: the SubCell keyword and the PANTHER ancestral node agree, meaning the placement is supported both by the curated human record and by the phylogenetic distribution of the family. Naked mole rat fibroblasts secrete hyaluronan into the medium, which requires a plasma-membrane-resident synthase. Supporting Evidence: PMID:33846452 Three HA synthase isoforms (HAS1, HAS2, and HAS3), present in all mammals, synthesize HA at different rates, directly from the inner aspect of the plasma membrane into the extracellular matrix, and with different average sizes. PMID:23783513 naked mole-rat fibroblasts secrete extremely high-molecular-mass hyaluronan (HA), which is over five times larger than human or mouse HA |
| GO:0031982 vesicle | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Root-level vesicle term mapped from the UniProt SubCell keyword "Vesicle". Uninformative as it stands, and the informative part of the same claim is carried by the separate GO:1903561 extracellular vesicle annotation. Reason: This is the cellular-component analogue of annotating "protein binding": it asserts only that the protein is in some vesicle, which is true of most membrane proteins. I have not replaced it with a more specific child because the UniProt note it derives from covers two genuinely different vesicle populations - intracellular transport and endosomal vesicles on the trafficking route, and extracellular vesicles into which HAS2 is exported - so any single specific replacement would over-commit. Retained as non-core; the specific, informative claim is already annotated separately as extracellular vesicle. |
| GO:0050501 hyaluronan synthase activity | IEA GO_REF:0000120 | ACCEPT | Summary: The core molecular function. Automatically assigned via ARBA, an explicit RHEA:12528 / EC:2.4.1.212 reaction mapping, and the PANTHER ancestral node - and independently demonstrated for the naked mole rat protein itself by heterologous expression. Reason: This row is unusually well grounded for an IEA. Its WITH/FROM carries a direct reaction mapping (RHEA:12528, EC:2.4.1.212) that matches the two CATALYTIC ACTIVITY reactions in the UniProt entry, so the enzymatic identity does not rest on family membership alone. More importantly, the annotation is not merely an inference in this species: expressing the cloned naked mole rat HAS2 cDNA in human HEK293 cells made those cells secrete high-molecular-mass hyaluronan, which is a direct assay of the naked mole rat gene product's activity in a heterologous host, and shRNA knockdown of HAS2 in naked mole rat fibroblasts abolished hyaluronan production. The evidence code understates the evidence: this annotation is eligible for an IDA row against PMID:23783513, which would give the flagship naked mole rat gene its first PMID-backed GO annotation. Supporting Evidence: PMID:23783513 Indeed, when the cDNA for the naked mole-rat HAS2 was overexpressed in human HEK293 cells, they began secreting HMW-HA PMID:23783513 We then generated H-Ras V12 and SV40 LT expressing naked mole-rat cells, in which HMW-HA was abolished by either integrating shRNA targeting HAS2 |
| GO:0085029 extracellular matrix assembly | IEA GO_REF:0000118 | ACCEPT | Summary: TreeGrafter propagation from PANTHER ancestral node PTN000530842. Sound: hyaluronan is a principal structural component of soft-tissue extracellular matrix and HAS2 extrudes it straight into that compartment. Reason: Placing extracellular matrix assembly at the ancestral hyaluronan synthase node is a defensible phylogenetic judgement - the defining activity of every characterised member of this family is to extrude a matrix polysaccharide across the plasma membrane, so the process is inseparable from the molecular function rather than being a distant downstream consequence. In the naked mole rat the claim is directly corroborated: the hyaluronan this enzyme makes accumulates in tissue extracellular matrix and forms self-assembling, spontaneously gelling structures. This is a core process for the gene, not a peripheral one. Supporting Evidence: PMID:23783513 Naked mole-rat skin, heart, brain and kidney were highly enriched for HA PMID:31036852 Unlike HA that is commercially available, NMR HA readily forms robust gels without the need for chemical cross-linking. |
| GO:0030213 hyaluronan biosynthetic process | IEA GO_REF:0000120 | ACCEPT | Summary: The core biological process, assigned by combined automatic methods with UniPathway UPA00341 in the WITH/FROM field, and independently demonstrated in naked mole rat cells by loss of function. Reason: Directly entailed by the molecular function and independently supported in this species: shRNA knockdown of HAS2 in naked mole rat fibroblasts abolishes high-molecular-mass hyaluronan production, and transgenic expression of the naked mole rat gene in mice raises tissue hyaluronan amount and molecular mass. Both directions of manipulation therefore agree. Core. Supporting Evidence: PMID:23783513 We then generated H-Ras V12 and SV40 LT expressing naked mole-rat cells, in which HMW-HA was abolished by either integrating shRNA targeting HAS2 PMID:37612507 analysis using pulse-field gel electrophoresis showed that hyaluronan extracted from the tissues of nmrHas2 mice was more abundant and had a higher molecular mass in the muscle, heart, kidneys and small intestine |
| GO:0000139 Golgi membrane | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Curator-reviewed ISS transfer of Golgi membrane localization from human HAS2 (UniProtKB:Q92819). Duplicates the SubCell-derived IEA row and receives the same treatment. Reason: Same claim as the GO_REF:0000044 row, arrived at by a different route; both ultimately trace to the human record. The transfer itself is unobjectionable - HAS2 is under strong purifying selection across mammals, so there is no reason to expect its trafficking itinerary to differ in the naked mole rat - but the Golgi is a compartment the enzyme passes through, not one where it functions. Non-core. Supporting Evidence: PMID:25948568 we found evidence of strong purifying selection acting on the HAS2 gene across all mammals, and the NMR remains unique in its particular HAS2 sequence |
| GO:0005764 lysosome | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Curator-reviewed ISS transfer of lysosomal localization from human HAS2 (UniProtKB:Q92819); the degradative endpoint of the trafficking route. Reason: Duplicates the SubCell IEA row. Retained on the same grounds: it is a real destination in the human ortholog and there is no naked mole rat evidence against it, but it marks where the enzyme is destroyed rather than where it acts, so it cannot contribute to a functional summary. |
| GO:0005789 endoplasmic reticulum membrane | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Curator-reviewed ISS transfer of endoplasmic reticulum membrane localization from human HAS2 (UniProtKB:Q92819); the point of membrane insertion for this seven-pass protein. Reason: Duplicates the SubCell IEA row and is retained for the same reason: an obligatory biosynthetic waypoint for a polytopic membrane protein, and in the human ortholog a compartment in which the enzyme is demonstrably held inactive when onward trafficking is blocked. Not a functional location. |
| GO:0005886 plasma membrane | ISS GO_REF:0000024 | ACCEPT | Summary: Curator-reviewed ISS transfer of plasma membrane localization from human HAS2 (UniProtKB:Q92819). This is the site of catalysis and extrusion. Reason: GOA carries this claim twice from the same donor (two separate rows dated 2022 and 2025), collapsed here into one entry. The transfer is sound and the destination is the functional one: hyaluronan synthases build the polymer on the cytoplasmic face of the plasma membrane and thread it directly outward, which is also what makes secretion by naked mole rat fibroblasts into the culture medium possible. Core location. Supporting Evidence: PMID:33846452 Three HA synthase isoforms (HAS1, HAS2, and HAS3), present in all mammals, synthesize HA at different rates, directly from the inner aspect of the plasma membrane into the extracellular matrix, and with different average sizes. |
| GO:0031982 vesicle | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Curator-reviewed ISS transfer of the root-level vesicle term from human HAS2 (UniProtKB:Q92819). Uninformative, and subsumed by the specific extracellular vesicle annotation from the same donor. Reason: Duplicates the SubCell IEA row. As there, I have not proposed a specific replacement: the underlying human observation spans both intracellular trafficking vesicles and exported extracellular vesicles, so choosing one child would assert more than the source supports. Retained as an uninformative parent whose informative content is already annotated separately. |
| GO:0050501 hyaluronan synthase activity | ISS GO_REF:0000024 | ACCEPT | Summary: The core molecular function, transferred by curator-reviewed ISS from all three named orthologs - human Q92819, mouse P70312 and rat O35776 (three GOA rows collapsed into this entry). Independently demonstrated on the naked mole rat protein itself. Reason: The ortholog projection is about as safe as such projections get: the enzyme is under strong purifying selection across mammals, three separate curators independently transferred the same activity from three species, and the UniProt entry carries the matching RHEA reactions and EC number. But the annotation does not need to rest on projection at all. Expressing the cloned naked mole rat cDNA in human HEK293 cells made those cells secrete high-molecular-mass hyaluronan - a direct assay of this gene product's activity. The two Asn-to-Ser substitutions in the catalytic loop are a quantitative refinement of processivity, not a loss of activity; if anything they make the enzyme better at what the term describes. Accepted as core, with the recommendation that GOA add an IDA row against PMID:23783513 alongside the ISS transfers, since the direct assay exists and is already cited by UniProt for FUNCTION. Supporting Evidence: PMID:23783513 Indeed, when the cDNA for the naked mole-rat HAS2 was overexpressed in human HEK293 cells, they began secreting HMW-HA PMID:23783513 The conserved regions carrying Asparagine to Serine substitutions correspond to the cytoplasmic loop containing the enzyme’s active site. PMID:25948568 we found evidence of strong purifying selection acting on the HAS2 gene across all mammals, and the NMR remains unique in its particular HAS2 sequence |
| GO:0005794 Golgi apparatus | ISS GO_REF:0000024 | MODIFY | Summary: Curator-reviewed ISS transfer of the whole-organelle Golgi term from human HAS2 (UniProtKB:Q92819). Less informative than the Golgi membrane annotation already present from the same donor. Reason: HAS2 is not a Golgi lumenal or peripheral protein: the UniProt entry annotates seven transmembrane helices and describes the location as "Golgi apparatus membrane". The membrane term is therefore the accurate one, and GO:0000139 Golgi membrane is already annotated from this same donor by both ISS and SubCell IEA. This row adds only a less precise restatement, so it should be replaced by the membrane term rather than kept alongside it. This is a precision fix, not a change of biological claim, and either way the Golgi remains a transit compartment rather than a functional site. Proposed replacements: Golgi membrane |
| GO:1903561 extracellular vesicle | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Curator-reviewed ISS transfer from human HAS2 (UniProtKB:Q92819): HAS2 protein leaves the cell on extracellular vesicles. Plausible and specific, but with no naked mole rat data and no established functional role. Reason: This is the informative half of the vague "vesicle" annotation, and it is a real reported property of the human ortholog. It is kept because sequence-based transfer of a trafficking route is reasonable for a protein under strong purifying selection. It is not core because nothing establishes that vesicle-borne HAS2 is catalytically productive - hyaluronan synthesis requires access to cytosolic UDP-sugars, which an exported vesicle does not obviously provide - and because no naked mole rat experiment has looked for it. |
| GO:0000271 polysaccharide biosynthetic process | ISS GO_REF:0000024 | MODIFY | Summary: Curator-reviewed ISS transfer from mouse Has2 (UniProtKB:P70312). Biologically true but placed on a parallel, less informative ontology branch from the hyaluronan-specific term that is also annotated from the same donor. Reason: Hyaluronan is literally a polysaccharide, so this is not wrong. But GO deliberately routes glycosaminoglycan biosynthesis away from generic polysaccharide biosynthesis: checked against QuickGO, GO:0000271 sits beneath GO:0016051 carbohydrate biosynthetic process and GO:0005976 polysaccharide metabolic process, whereas GO:0030213 sits beneath GO:0006024 glycosaminoglycan biosynthetic process and GO:1901137 carbohydrate derivative biosynthetic process. GO:0000271 is therefore not an ancestor of GO:0030213 and this row is not simply a redundant parent - it is a second, cross-branch statement of the same biology at lower resolution. Since GO:0030213 is already annotated from this same mouse donor, replacing rather than retaining is the right fix. Proposed replacements: hyaluronan biosynthetic process |
| GO:0085029 extracellular matrix assembly | ISS GO_REF:0000024 | ACCEPT | Summary: Curator-reviewed ISS transfer from mouse Has2 (UniProtKB:P70312), duplicating the TreeGrafter row. A core process, corroborated in the naked mole rat. Reason: Two independent routes - curator ISS from mouse and PANTHER ancestral-node propagation - converge on the same process, which is exactly what one expects for an activity whose product is a defining constituent of extracellular matrix. Naked mole rat tissues are enriched for this product and the polymer forms self-assembling gel-like matrix structures, so the process is realised in this species and not merely inferred. Supporting Evidence: PMID:23783513 Naked mole-rat skin, heart, brain and kidney were highly enriched for HA PMID:31036852 In common with mouse HA, NMR HA forms a range of assemblies corresponding to a wide distribution of molecular weights. |
| GO:0001570 vasculogenesis | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Curator-reviewed ISS transfer from mouse Has2 (UniProtKB:P70312), grounded in mouse knockout embryology. Very likely conserved, but a pleiotropic developmental role rather than what this enzyme is for in the naked mole rat. Reason: Retained rather than removed on two positive grounds. First, HAS2 is under strong purifying selection across all mammals, so an ancestral, embryonically essential role is very unlikely to have been lost in this lineage. Second, the naked mole rat's divergent hyaluronan phenotype is explicitly postnatal - high-molecular-mass hyaluronan begins to accumulate after birth because it is incompatible with the rapid proliferation of embryogenesis - so the embryonic behaviour of the enzyme is precisely the part that is not expected to differ. Marked non-core because it is a downstream developmental requirement of matrix hyaluronan, one of several, and does not describe the enzyme's own activity or its salient role in this species. Supporting Evidence: PMID:25948568 we found evidence of strong purifying selection acting on the HAS2 gene across all mammals, and the NMR remains unique in its particular HAS2 sequence PMID:37612507 In the naked mole-rat, HMM-HA begins to accumulate postnatally |
| GO:0030213 hyaluronan biosynthetic process | ISS GO_REF:0000024 | ACCEPT | Summary: The core biological process, transferred by curator-reviewed ISS from both mouse Has2 (UniProtKB:P70312) and rat Has2 (UniProtKB:O35776) - two GOA rows collapsed into this entry - and directly demonstrated in naked mole rat cells. Reason: Two independent curators transferred this from two rodent orthologs, and in this species it is supported by manipulation in both directions: knocking HAS2 down in naked mole rat fibroblasts abolishes high-molecular-mass hyaluronan, and expressing the naked mole rat gene transgenically in mice raises hyaluronan amount and molecular mass across several tissues. Among the three HAS isoenzymes this is the one that makes the long polymers, and it is the one the naked mole rat overexpresses. Core. Supporting Evidence: PMID:23783513 We then generated H-Ras V12 and SV40 LT expressing naked mole-rat cells, in which HMW-HA was abolished by either integrating shRNA targeting HAS2 PMID:37612507 Has2 mainly produces HMM-HA and shows higher expression in naked mole-rats compared with in mice and humans PMID:38052795 NMR and DMR had dramatically higher HAS2 expression. |
| GO:0036302 atrioventricular canal development | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Curator-reviewed ISS transfer from mouse Has2 (UniProtKB:P70312), grounded in mouse cardiac knockout phenotypes. Conserved developmental role, not a core function here. Reason: Same reasoning as vasculogenesis: strong purifying selection on HAS2 across mammals argues the ancestral cardiac morphogenetic requirement is intact, and the naked mole rat's own hyaluronan divergence is postnatal, so there is no positive reason to think the embryonic role differs. No naked mole rat embryology on this gene exists, so this is retained on conservation grounds rather than on evidence, and it describes a tissue-level developmental requirement for matrix hyaluronan rather than an activity of the protein. Non-core. Supporting Evidence: PMID:37612507 In the naked mole-rat, HMM-HA begins to accumulate postnatally |
| GO:0090500 endocardial cushion to mesenchymal transition | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Curator-reviewed ISS transfer from mouse Has2 (UniProtKB:P70312). The most specific of the three cardiac/vascular developmental terms, and the same judgement applies. Reason: Hyaluronan-rich cardiac jelly is required for endocardial cushion cells to lose polarity and become migratory mesenchyme, and Has2 is its source; the term is mechanistically apt for the mouse. It is retained for the naked mole rat because the gene is strongly constrained across mammals and because the species' divergence in hyaluronan is postnatal, but no naked mole rat evidence bears on it, and like the other developmental terms it is a downstream tissue-level consequence of matrix hyaluronan rather than a core function of the enzyme. Non-core. Supporting Evidence: PMID:25948568 we found evidence of strong purifying selection acting on the HAS2 gene across all mammals, and the NMR remains unique in its particular HAS2 sequence |
| GO:0035810 positive regulation of urine volume | ISS GO_REF:0000024 | MARK AS OVER ANNOTATED | Summary: Organ-level renal physiology transferred by ISS from rat Has2 (UniProtKB:O35776). The one naked mole rat measurement that bears on it points the other way: the renal medulla is the single tissue where this species' hyaluronan content is lower, not higher, than comparators. Reason: Sequence similarity licenses transferring what a protein does chemically; it does not license transferring an organism-level physiological outcome, which additionally depends on where the gene is expressed and on how the whole organ is tuned. The rat phenotype behind this term is a water-loading-responsive rise in medullary interstitial hyaluronan that reduces water reabsorption and permits excretion of dilute urine. In the naked mole rat, quantitative assay of cortex and medulla separately found the renal medulla to be the sole exception to this species' general hyaluronan enrichment - its medullary content is lower than in guinea pig and closer to that of desert gerbils, a lineage in which low medullary hyaluronan accompanies maximal water conservation, the opposite of increased urine volume. The animal's urine-concentrating ability is described as only moderate, and the water-loading experiment that defines the rat phenotype has never been performed in this species. That is a positive, species-specific argument that the transfer is a stretch. It is not enough for REMOVE: hyaluronan is present in the naked mole rat medulla around the vasa recta, whole-kidney staining in the original study reported hyaluronan enrichment, and transgenic expression of this gene does raise mouse kidney hyaluronan - so the underlying capability plausibly exists even if the physiological role does not transfer. Flagged as over-annotation, with the resolving experiment named in suggested_experiments. Supporting Evidence: PMID:33846452 HA content in the mouse and rat renal medulla is known to be much higher than in the cortex and to increase during water loading PMID:33846452 The only exception is the renal medulla, in which HA content appeared lower in NMR than the other species. PMID:33846452 The renal medulla average HA content we found in the NMR (150 µg/g dry weight) is closer to that of desert gerbils PMID:33846452 The NMR has no access to free water but shows only a moderate, not very high, kidney concentrating ability PMID:33846452 we do not know how the NMR renal medullary HA content would respond to water loading |
| GO:0070295 renal water absorption | ISS GO_REF:0000024 | MARK AS OVER ANNOTATED | Summary: The companion renal term, transferred by ISS from the same rat donor (UniProtKB:O35776). Flagged on the same grounds, with the added problem that it and GO:0035810 point in opposite physiological directions. Reason: Both rat-derived renal terms describe the same underlying axis - medullary interstitial hyaluronan modulating collecting-duct water permeability - read out in opposite directions depending on hydration state, which is why one gene carries both "positive regulation of urine volume" and "renal water absorption". Transferring that pair into a species by sequence similarity asserts not just that the protein is present but that the whole hydration-responsive regulatory loop is conserved, and that is exactly what has not been tested. The available naked mole rat data are unhelpful to the transfer: the renal medulla is the one tissue where this species' hyaluronan content falls below comparators, medullary localization around the vasa recta looks like that of mouse and guinea pig with no enrichment, and no water-loading challenge has been done. As with GO:0035810 this falls short of contradiction, so it is marked over-annotated rather than removed. Supporting Evidence: PMID:33846452 The role of this interstitial HA is likely to reduce water reabsorption and allow the excretion of diluted urine PMID:33846452 In the kidney, our results show a very similar HA localization between NMR, GP, and mice tissues PMID:33846452 The only exception is the renal medulla, in which HA content appeared lower in NMR than the other species. PMID:33846452 we do not know how the NMR renal medullary HA content would respond to water loading |
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Download this section (compressed HTML)Q: Are the two Asn-to-Ser substitutions in the cytoplasmic catalytic loop actually causal for longer hyaluronan, or is the naked mole rat phenotype carried by elevated Has2 expression plus slow hyaluronidase-mediated turnover? The consensus review states the mechanistic link is unclear, and the only matched-expression transfection evidence that HAS2 sequence sets polymer length comes from blind mole rat, not naked mole rat, HAS2.
Q: What is the actual maximum molecular mass of naked mole rat hyaluronan? Two groups using different isolation and sizing methods report 6-12 MDa and a maximum near 2.5 MDa, and the discrepancy has been attributed to isolation procedure without being resolved.
Q: Does hyaluronan in the naked mole rat renal medulla behave as it does in rat - rising on water loading and reducing water reabsorption - or has this species, whose medullary hyaluronan content resembles that of desert gerbils, uncoupled Has2 from renal water handling?
Q: Should GO be able to express hyaluronan polymer length, and if so at process or molecular-function level? The three HAS paralogues differ chiefly in the length of what they make, and no current term captures that.
Q: Does Has2 exported on extracellular vesicles retain catalytic activity, given that hyaluronan synthesis consumes cytosolic UDP-sugars? If not, the extracellular vesicle annotation records a fate rather than a functional location.
Experiment: Express naked mole rat, mouse and human HAS2 at matched levels in the same hyaluronidase-low host cell line, together with naked mole rat constructs in which each Asn-to-Ser substitution is individually reverted, and size the secreted hyaluronan by a single method with markers extending well beyond 6 MDa. This is the experiment that would settle whether the substitutions are causal, and running all constructs in one host removes the host-cell hyaluronidase confound that the consensus review blames for the variable sizes seen previously.
Experiment: Reconstitute purified recombinant naked mole rat and mouse HAS2 into proteoliposomes with defined UDP-sugar concentrations and measure the polymer-size distribution directly. This separates intrinsic processivity from cellular context - expression level, UDP-sugar supply and degradation - which no cell-based assay can do.
Experiment: Perform a water-loading and water-restriction protocol in naked mole rats with quantitative measurement of renal medullary hyaluronan, Has2 expression in medullary interstitial cells, and urine osmolality and volume. This is the decisive test of the two rat-derived renal annotations flagged as over-annotated: if medullary hyaluronan rises on water loading as it does in rat, the transfers are vindicated; if it does not, they should be removed.
Experiment: Run a standardised cross-laboratory hyaluronan sizing exercise on aliquots of the same naked mole rat skin and fibroblast-medium samples, with each laboratory using its own extraction protocol and a shared set of size markers, to determine whether the 6-12 MDa versus 2.5 MDa discrepancy is a property of the samples or of the isolation procedure.
Experiment: Quantify hyaluronan mass and size in naked mole rat embryos and neonates, and compare with cardiac and vascular morphogenesis, to test directly whether the postnatal onset of high-molecular-mass hyaluronan means that Has2 behaves like the conserved mammalian enzyme during the developmental window in which the mouse knockout phenotypes arise.
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