tmc2b (transmembrane channel-like 2b) is one of two zebrafish co-orthologs of vertebrate TMC2, the other being tmc2a; it lies on chromosome 5 about 5 kb from tmc1. Tmc2b is a multi-pass membrane protein of the TMC family and a pore-forming subunit of the hair-cell mechanoelectrical transduction (MET) channel, a non-selective, calcium-permeable cation channel opened by tip-link tension. GFP-tagged Tmc2b localizes to the tips of stereocilia, and its delivery to the hair bundle requires Tomt and Tmie. tmc2b is expressed only in sensory hair cells: it is the dominant Tmc transcript in lateral-line neuromasts and, in the inner ear, is expressed with tmc1 in peripheral (extrastriolar) hair cells. Tmc2b is required for mechanotransduction in most lateral-line hair cells, with Tmc2a supplying the rest, and is the main contributor to utricular (vestibular) function at low frequencies; in the saccule it contributes to hearing behind Tmc2a. A hair-cell Tmc2b transgene restores transduction, hearing and balance in larvae lacking tmc1, tmc2a and tmc2b.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005245 voltage-gated calcium channel activity | IBA GO_REF:0000033 | MODIFY | Summary: Voltage-gated calcium channel activity, propagated from the PANTHER TMC2 node (mouse Tmc2 donor). Reason: Same assessment as for tmc2a: TMC channels are gated by tip-link tension, not membrane voltage, and are non-selective, calcium-permeable cation channels. Tmc2b sits at stereocilia tips and is required for lateral-line transduction, consistent with a mechanically gated channel subunit. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH Sources checked: MGI:MGI:2151017 · Tmc2 (mouse) SOURCE BAD QuickGO shows mouse Tmc2 carries GO:0005245 by IMP/IGI from PMID:23871232, a study of calcium permeability and single-channel currents of the mechanically gated hair-cell channel, not of voltage gating (as traced in the human TMC1 review). PANTHER:PTN002761430 SUPPORTS SOURCE BUT NOT TARGET The node correctly groups vertebrate TMC2 orthologs including both zebrafish copies; only the gating mechanism in the chosen term is wrong. Proposed replacements: mechanosensitive monoatomic cation channel activity Supporting Evidence: PMID:29269857 First, by transgenesis, we showed that full-length Tmc2b fused to GFP localizes to the tips of neuromast stereocilia (Fig. 2a, b), fulfilling a key requirement of being a component of the mechanotransduction apparatus in zebrafish hair cells. PMID:39773557 Significant progress in our understanding of MET has been recently made by identifying the transmembrane channel-like proteins 1 and 2 (TMC1 and TMC2) as the pore-forming subunits of the MET channel (Kawashima et al., 2011; Pan et al., 2018). |
| GO:0005886 plasma membrane | IBA GO_REF:0000033 | ACCEPT | Summary: Plasma membrane location propagated across the TMC family. Reason: Tmc2b-GFP localizes to the tips of stereocilia, which are plasma-membrane protrusions. Correct but general. Supporting Evidence: PMID:29269857 First, by transgenesis, we showed that full-length Tmc2b fused to GFP localizes to the tips of neuromast stereocilia (Fig. 2a, b), fulfilling a key requirement of being a component of the mechanotransduction apparatus in zebrafish hair cells. |
| GO:0005886 plasma membrane | IEA GO_REF:0000002 | ACCEPT | Summary: Plasma membrane from the InterPro TMC family signature. Reason: Consistent with the IBA row and with stereocilia-tip localization of Tmc2b-GFP. |
| GO:0008381 mechanosensitive monoatomic ion channel activity | IBA GO_REF:0000033 | ACCEPT | Summary: Mechanosensitive ion channel activity asserted at the PANTHER TMC node. Reason: Core function. Tmc2b is required for mechanotransduction in most lateral-line hair cells, and a hair-cell Tmc2b transgene restores transduction in all end organs of tmc1/tmc2a/tmc2b triple mutants, showing that Tmc2b alone can support the channel. The more precise child GO:0140135 is recorded in core_functions. Supporting Evidence: PMID:29269857 94 ± 1.2% (n = 55) of control hair cells loaded fluorophore; in contrast, 35 ± 1.8% (n = 48) of tmc2b −/− hair cells loaded (Fig. 3d), confirming Tmc2b is absolutely required for a subpopulation of hair cells within a single neuromast. PMID:32371604 The results described above (1) confirm the expectation that concurrent disruption of tmc1/2a/2b leads to complete loss of MET activity and (2) reveal that exogenous Tmc2b-mEGFP can rescue auditory/vestibular deficits and restore FM labeling in all mechanosensory end organs. PMID:39773557 Significant progress in our understanding of MET has been recently made by identifying the transmembrane channel-like proteins 1 and 2 (TMC1 and TMC2) as the pore-forming subunits of the MET channel (Kawashima et al., 2011; Pan et al., 2018). |
| GO:0016020 membrane | IEA GO_REF:0000120 | ACCEPT | Summary: Membrane, from InterPro and UniProt subcellular location. Reason: Correct; less specific than plasma membrane. |
| GO:0034220 monoatomic ion transmembrane transport | IEA GO_REF:0000108 | MODIFY | Summary: Ion transmembrane transport inferred from the mechanosensitive ion channel activity. Reason: Correct in kind but under-specific: the MET channel conducts cations. Same modification as for tmc2a and human TMC1. Proposed replacements: monoatomic cation transmembrane transport |
| GO:0042472 inner ear morphogenesis | IEP PMID:25114259 Tip-link protein protocadherin 15 interacts with transmembra... | REMOVE | Summary: Inner ear morphogenesis, inferred from tmc2b expression in the ear. Reason: Expression in otic hair cells does not show a role in shaping the ear. tmc2b is expressed only in hair cells, the protein is a transduction channel subunit, and hair bundles and other MET components are intact in tmc triple mutants. Supporting Evidence: PMID:25114259 In contrast to the broad expression pattern of pcdh15a in the larval brain and sensory hair cells ( 5 ), expression of tmc1 , tmc2a , and tmc2b genes was restricted to hair cells of the inner ear and lateral line organ PMID:32371604 Together, these results suggest that hair bundles are intact and central components of the MET complex are properly localized in tmc triple mutants. |
| GO:0048839 inner ear development | IEP PMID:25114259 Tip-link protein protocadherin 15 interacts with transmembra... | REMOVE | Summary: Inner ear development, inferred from tmc2b expression in the ear. Reason: Same as inner ear morphogenesis: an expression pattern, not evidence of a developmental role; tmc mutant hair bundles form normally. Supporting Evidence: PMID:25114259 In contrast to the broad expression pattern of pcdh15a in the larval brain and sensory hair cells ( 5 ), expression of tmc1 , tmc2a , and tmc2b genes was restricted to hair cells of the inner ear and lateral line organ PMID:32371604 Together, these results suggest that hair bundles are intact and central components of the MET complex are properly localized in tmc triple mutants. |
| GO:0048884 neuromast development | IEP PMID:25114259 Tip-link protein protocadherin 15 interacts with transmembra... | REMOVE | Summary: Neuromast development, inferred from tmc2b expression in neuromast hair cells. Reason: The annotation rests on expression. tmc2b mutants have normal hair bundles; their posterior neuromasts have about one-third fewer hair cells, a reduction also seen in other transduction mutants and most simply read as a consequence of lost transduction rather than a role of the channel subunit in building the organ. Supporting Evidence: PMID:25114259 In the lateral-line neuromasts, only tmc2b was robustly detected by in situ hybridization PMID:29269857 In mutants, no defects in hair bundle morphology (Supplementary Fig. 2a–d) or hearing (Supplementary Fig. 2f, g) were observed. PMID:29269857 However, posterior neuromasts of mutants had a mild reduction in hair cell numbers (approximately one-third decrease, 6 dpf; Supplementary Fig. 2c–e; Supplementary Fig. 3 ). |
| GO:0050910 detection of mechanical stimulus involved in sensory perception of sound | IBA GO_REF:0000033 | ACCEPT | Summary: Detection of mechanical stimulus involved in sensory perception of sound, from the PANTHER TMC2 node. Reason: Tmc2b contributes to hearing, although less than Tmc2a: tmc2b single mutants hear, but tmc2a/tmc2b double mutants have lower saccular microphonics than tmc2a single mutants, and in the absence of Tmc2a most of the remaining current is carried by Tmc2b. A Tmc2b transgene restores hearing in triple mutants. Supporting Evidence: PMID:32371604 These results suggest that Tmc2b makes some contribution to macular hair-cell activity. PMID:32167554 However, in the absence of Tmc2a, most of the remaining current is carried by Tmc2b. PMID:32371604 The results described above (1) confirm the expectation that concurrent disruption of tmc1/2a/2b leads to complete loss of MET activity and (2) reveal that exogenous Tmc2b-mEGFP can rescue auditory/vestibular deficits and restore FM labeling in all mechanosensory end organs. |
| GO:0050910 detection of mechanical stimulus involved in sensory perception of sound | IGI PMID:29269857 A molecular basis for water motion detection by the mechanos... | MODIFY | Summary: Sound-detection term from the tmc2a/tmc2b double mutant of Chou et al. 2017. Reason: The double mutant was assayed in the lateral line in this paper, where transduction was abolished; the lateral line detects water motion, not sound. The finding belongs under the general mechanosensory detection term (same change as on the tmc2a row). Hearing is covered by the IBA row. Proposed replacements: detection of mechanical stimulus involved in sensory perception Supporting Evidence: PMID:29269857 Stimulus-evoked microphonic potentials were not detectable in neuromasts with A–P orientations in the PLL of tmc2a −/− /tmc2b −/− larvae (Fig. 8a, b) at 6 dpf, yet robust signatures were apparent in controls. |
| GO:0050910 detection of mechanical stimulus involved in sensory perception of sound | IMP PMID:29269857 A molecular basis for water motion detection by the mechanos... | MODIFY | Summary: Sound-detection term from the tmc2b single mutant of Chou et al. 2017. Reason: The paper reports that tmc2b single mutants have no hearing defect; the phenotype is loss of transduction in most lateral-line hair cells. The mechanosensory role is real, but the sound-perception child is contradicted for this genotype, so the row should move to the general detection term. Proposed replacements: detection of mechanical stimulus involved in sensory perception Supporting Evidence: PMID:29269857 In mutants, no defects in hair bundle morphology (Supplementary Fig. 2a–d) or hearing (Supplementary Fig. 2f, g) were observed. PMID:29269857 94 ± 1.2% (n = 55) of control hair cells loaded fluorophore; in contrast, 35 ± 1.8% (n = 48) of tmc2b −/− hair cells loaded (Fig. 3d), confirming Tmc2b is absolutely required for a subpopulation of hair cells within a single neuromast. |
| GO:0060005 vestibular reflex | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Vestibular reflex, from the PANTHER TMC2 node. Reason: Organism-level reflex downstream of utricular transduction. Both double-mutant combinations that reduce vestibular-induced eye movements include the tmc2b allele, and Tmc2b suffices for low-frequency responses; correct, but a consequence of the channel function. Supporting Evidence: PMID:32371604 The two double-mutant combinations that gave rise to a deficit included the tmc2b sa8817 allele, suggesting that Tmc2b is the primary contributor to vestibular end organ function. PMID:37952940 In contrast, tmc1/2a mutants were comparable to WT siblings, suggesting that tmc2b was sufficient to mediate responses at sub-Hz frequencies. |
| GO:0070588 calcium ion transmembrane transport | IEA GO_REF:0000108 | ACCEPT | Summary: Calcium ion transmembrane transport inferred from the channel activity terms. Reason: Calcium entry through the MET channel follows from the channel activity; Tmc2b-only neuromast bundles show mechanically evoked calcium signals. Supporting Evidence: PMID:36905930 This idea is supported by our results, as we show that the calcium signals in hair bundles expressing both Tmc2b and Tmc2a are larger compared to those expressing just Tmc2b (Figure 6). |
| GO:0032426 stereocilium tip | IDA PMID:29269857 A molecular basis for water motion detection by the mechanos... | NEW | Summary: Full-length Tmc2b-GFP localizes to the tips of stereocilia in neuromast hair cells, and Tmc2b-GFP is present in inner-ear hair bundles. Reason: Direct localization of the full-length tagged protein, reproduced by several groups, with the hair-bundle signal lost in tomt and tmie mutants that block its trafficking. This is the site where the channel works and is more informative than the plasma membrane rows. No equivalent data exist for Tmc2a (its transgene did not express), so the term is proposed for tmc2b only. Supporting Evidence: PMID:29269857 First, by transgenesis, we showed that full-length Tmc2b fused to GFP localizes to the tips of neuromast stereocilia (Fig. 2a, b), fulfilling a key requirement of being a component of the mechanotransduction apparatus in zebrafish hair cells. PMID:28534737 We find that GFP-tagged Tmc1 and Tmc2b fail to localize to the hair bundle in mercury mutants, and that Tomt can rescue this trafficking defect. PMID:30726219 In tmie zebrafish mutants, we observed that GFP-tagged Tmc1 and Tmc2b, which are subunits of the MET channel, fail to target to the hair bundle. |
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Download this section (compressed HTML)Q: Can Tmc2a substitute for Tmc2b in the lateral-line hair cells that depend absolutely on Tmc2b (e.g. anterior-facing cells of posterior neuromasts) if it is expressed there? This would decide whether the pair split by expression only.
Q: What sets the Tmc2b-independent residual transduction in neuromasts IO4, SO1 and SO3, given that tmc2a is barely detectable in the lateral line by in situ hybridization?
Q: Is the reduced neuromast hair-cell number in tmc2b mutants a general consequence of lost transduction, or a Tmc2b-specific role in hair-cell maintenance?
Experiment: Express Tmc2a under the tmc2b regulatory region (or a lateral-line-specific driver) in tmc2b mutants and score dye uptake and microphonics in posterior neuromasts.
Hypothesis: Tmc2a and Tmc2b proteins are interchangeable and the pair differs only in expression.
Type: cross-rescue
Experiment: Compare MET current kinetics and adaptation in extrastriolar hair cells of tmc1/tmc2a double mutants (Tmc2b only) and striolar cells of tmc1/tmc2b double mutants (Tmc2a only).
Hypothesis: The utricular low-frequency role of Tmc2b reflects its extrastriolar expression rather than channel properties.
Type: electrophysiology
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