LHFPL5 is a key element in force transmission from the tip link to the hair cell mechanotransducer channel

During auditory transduction, sound-evoked vibrations of the hair cell stereociliary bundles open mechanotransducer (MET) ion channels via tip links extending from one stereocilium to its neighbor. How tension in the tip link is delivered to the channel is not fully understood. The MET channel comprises a pore-forming subunit, transmembrane channel-like protein (TMC1 or TMC2), aided by several accessory proteins, including LHFPL5 (lipoma HMGIC fusion partner-like 5). We investigated the role of LHFPL5 in transduction by comparing MET channel activation in outer hair cells of Lhfpl5-/- knockout mice with those in Lhfpl5+/- heterozygotes. The 10 to 90 percent working range of transduction in Tmc1+/+; Lhfpl5+/- was 52 nm, from which the single-channel gating force, Z, was evaluated as 0.34 pN. However, in Tmc1+/+; Lhfpl5-/- mice, the working range increased to 123 nm and Z more than halved to 0.13 pN, indicating reduced sensitivity. Tip link tension is thought to activate the channel via a gating spring, whose stiffness is inferred from the stiffness change on tip link destruction. The gating stiffness was ~40 percent of the total bundle stiffness in wild type but was virtually abolished in Lhfpl5-/-, implicating LHFPL5 as a principal component of the gating spring. The mutation Tmc1 p.D569N reduced the LHFPL5 immunolabeling in the stereocilia and like Lhfpl5-/- doubled the MET working range, but other deafness mutations had no effect on the dynamic range. We conclude that tip-link tension is transmitted to the channel primarily via LHFPL5; residual activation without LHFPL5 may occur by direct interaction between PCDH15 and TMC1.

The tetraspan LHFPL5 is critical to establish maximal force sensitivity of the mechanotransduction channel of cochlear hair cells

The mechanoelectrical transduction (MET) channel of cochlear hair cells is gated by the tip link, but the mechanisms that establish the exquisite force sensitivity of this MET channel are not known. Here, we show that the tetraspan lipoma HMGIC fusion partner-like 5 (LHFPL5) directly couples the tip link to the MET channel. Disruption of these interactions severely perturbs MET. Notably, the N-terminal cytoplasmic domain of LHFPL5 binds to an amphipathic helix in TMC1, a critical gating domain conserved between different MET channels. Mutations in the amphipathic helix of TMC1 or in the N-terminus of LHFPL5 that perturb interactions of LHFPL5 with the amphipathic helix affect channel responses to mechanical force. We conclude that LHFPL5 couples the tip link to the MET channel and that channel gating depends on a structural element in TMC1 that is evolutionarily conserved between MET channels. Overall, our findings support a tether model for transduction channel gating by the tip link.

The conductance and organization of the TMC1-containing mechanotransducer channel complex in auditory hair cells

We studied the role of TMC1 as the central component of the hair cell mechanotransducer (MET) channel by characterizing transduction in mice harboring mutations in the pore region. All Tmc1 mutations reduced the Ca²⁺ influx into the hair bundle. Two mutations (Tmc1 p.D528N or Tmc1 p.E520Q) also decreased channel conductance and two (Tmc1 p. D569N or Tmc1 p.W554L) lowered expression. These mutations endorse TMC1 as the pore of the MET channel. The MET channel also contains accessory subunits, LHFPL5 and TMIE. MET currents were small in Lhfpl5 or Tmie knockout mice. Nevertheless, MET channels could still be activated by hair bundle displacement; single-channel conductance was unaffected in Lhfpl5^−/− but reduced in Tmie^−/−, suggesting TMIE likely contributes to the pore.

Transmembrane channel-like protein 1 (TMC1) is thought to form the ion-conducting pore of the mechanoelectrical transducer (MET) channel in auditory hair cells. Using single-channel analysis and ionic permeability measurements, we characterized six missense mutations in the purported pore region of mouse TMC1. All mutations reduced the Ca²⁺ permeability of the MET channel, triggering hair cell apoptosis and deafness. In addition, Tmc1 p.E520Q and Tmc1 p.D528N reduced channel conductance, whereas Tmc1 p.W554L and Tmc1 p.D569N lowered channel expression without affecting the conductance. Tmc1 p.M412K and Tmc1 p.T416K reduced only the Ca²⁺ permeability. The consequences of these mutations endorse TMC1 as the pore of the MET channel. The accessory subunits, LHFPL5 and TMIE, are thought to be involved in targeting TMC1 to the tips of the stereocilia. We found sufficient expression of TMC1 in outer hair cells of Lhfpl5 and Tmie knockout mice to determine the properties of the channels, which could still be gated by hair bundle displacement. Single-channel conductance was unaffected in Lhfpl5^−/− but was reduced in Tmie^−/−, implying TMIE very likely contributes to the pore. Both the working range and half-saturation point of the residual MET current in Lhfpl5^−/− were substantially increased, suggesting that LHFPL5 is part of the mechanical coupling between the tip-link and the MET channel. Based on counts of numbers of stereocilia per bundle, we estimate that each PCDH15 and LHFPL5 monomer may contact two channels irrespective of location.

Putting the Pieces Together: the Hair Cell Transduction Complex

Identification of the components of the mechanosensory transduction complex in hair cells has been a major research interest for many auditory and vestibular scientists and has attracted attention from outside the field. The past two decades have witnessed a number of significant advances with emergence of compelling evidence implicating at least a dozen distinct molecular components of the transduction machinery. Yet, how the pieces of this ensemble fit together and function in harmony to enable the senses of hearing and balance has not been clarified. The goal of this review is to summarize a 2021 symposium presented at the annual mid-winter meeting of the Association for Research in Otolaryngology. The symposium brought together the latest insights from within and beyond the field to examine individual components of the transduction complex and how these elements interact at molecular, structural, and biophysical levels to gate mechanosensitive channels and initiate sensory transduction in the inner ear. The review includes a brief historical background to set the stage for topics to follow that focus on structure, properties, and interactions of proteins such as CDH23, PCDH15, LHFPL5, TMIE, TMC1/2, and CIB2/3. We aim to present the diversity of ideas in this field and highlight emerging theories and concepts. This review will not only provide readers with a deeper appreciation of the components of the transduction apparatus and how they function together, but also bring to light areas of broad agreement, areas of scientific controversy, and opportunities for future scientific discovery.

Deafness mutation D572N of TMC1 destabilizes TMC1 expression by disrupting LHFPL5 binding

Transmembrane channel-like protein 1 (TMC1) and lipoma HMGIC fusion partner-like 5 (LHFPL5) are recognized as two critical components of the mechanotransduction complex in inner-ear hair cells. However, the physical and functional interactions of TMC1 and LHFPL5 remain largely unexplored. We examined the interaction between TMC1 and LHFPL5 by using multiple approaches, including our recently developed ultrasensitive microbead-based single-molecule pulldown (SiMPull) assay. We demonstrate that LHFPL5 physically interacts with and stabilizes TMC1 in both heterologous expression systems and in the soma and hair bundle of hair cells. Moreover, the semidominant deafness mutation D572N in human TMC1 (D569N in mouse TMC1) severely disrupted LHFPL5 binding and destabilized TMC1 expression. Thus, our findings reveal previously unrecognized physical and functional interactions of TMC1 and LHFPL5 and provide insights into the molecular mechanism by which the D572N mutation causes deafness. Notably, these findings identify a missing link in the currently known physical organization of the mechanotransduction macromolecular complex. Furthermore, this study has demonstrated the power of the microbead-based SiMPull assay for biochemical investigation of rare cells such as hair cells.

The lhfpl5 Ohnologs lhfpl5a and lhfpl5b Are Required for Mechanotransduction in Distinct Populations of Sensory Hair Cells in Zebrafish

Hair cells sense and transmit auditory, vestibular, and hydrodynamic information by converting mechanical stimuli into electrical signals. This process of mechano-electrical transduction (MET) requires a mechanically gated channel localized in the apical stereocilia of hair cells. In mice, lipoma HMGIC fusion partner-like 5 (LHFPL5) acts as an auxiliary subunit of the MET channel whose primary role is to correctly localize PCDH15 and TMC1 to the mechanotransduction complex. Zebrafish have two lhfpl5 genes (lhfpl5a and lhfpl5b), but their individual contributions to MET channel assembly and function have not been analyzed. Here we show that the zebrafish lhfpl5 genes are expressed in discrete populations of hair cells: lhfpl5a expression is restricted to auditory and vestibular hair cells in the inner ear, while lhfpl5b expression is specific to hair cells of the lateral line organ. Consequently, lhfpl5a mutants exhibit defects in auditory and vestibular function, while disruption of lhfpl5b affects hair cells only in the lateral line neuromasts. In contrast to previous reports in mice, localization of Tmc1 does not depend upon Lhfpl5 function in either the inner ear or lateral line organ. In both lhfpl5a and lhfpl5b mutants, GFP-tagged Tmc1 and Tmc2b proteins still localize to the stereocilia of hair cells. Using a stably integrated GFP-Lhfpl5a transgene, we show that the tip link cadherins Pcdh15a and Cdh23, along with the Myo7aa motor protein, are required for correct Lhfpl5a localization at the tips of stereocilia. Our work corroborates the evolutionarily conserved co-dependence between Lhfpl5 and Pcdh15, but also reveals novel requirements for Cdh23 and Myo7aa to correctly localize Lhfpl5a. In addition, our data suggest that targeting of Tmc1 and Tmc2b proteins to stereocilia in zebrafish hair cells occurs independently of Lhfpl5 proteins.

Novel Mutations in KCNQ4, LHFPL5 and COCH Genes in Iranian Families with Hearing Impairment

BACKGROUND

Hearing loss (HL) is the most common sensory deficit in humans, and genetic factors contribute to about half of the cases. With 112 causative genes identified so far and a disproportionate share of the genes within different ethnic groups, HL has proven to be quite heterogeneous.

METHODS

Twenty Iranian families having at least 2 children with hereditary HL were initially verified to be GJB2-negative and were then subjected to whole exome sequencing (WES). Sanger sequencing was used to confirm segregation of the variant identified in each family.

RESULTS

In 3 families, WES revealed 3 novel variants in KCNQ4, LHFPL5 and COCH genes. The KCNQ4 gene (DFNA2A) encodes a potassium channel (KV7.4) and the heterozygous variant identified (c.1647C>G, p.F549L) resulted in the substitution of Phe549 residing in the KV7.4 cytoplasmic region. The homozygous variant (c.34A>T, p.K12X) was identified in the LHFPL5 gene (DFNB67) which encodes a transmembrane protein, and another variant in a homozygous state (c.116T>A, p.L39X) was identified in the COCH gene which encodes a secretory protein. Pathogenic variants in the COCH gene are associated with late onset autosomal dominant hearing loss (DFNA9) but the affected individuals displayed early onset HL with a recessive mode of inheritance.

CONCLUSION

The 16% contribution of GJB2 to HL in the Iranian population necessitates the discovery of the remaining causal factors. This study is the first to report KCNQ4 and COCH related HL in the Iranian population and the second study, globally, to report HL due to biallelic inactivation of the COCH gene.

LHFPL5 mutation: A rare cause of non-syndromic autosomal recessive hearing loss

Hearing loss is a debilitating disorder that impairs language acquisition, resulting in disability in children and potential isolation in adulthood. Its onset can have a genetic basis, though environmental factors, which are often preventable, can also cause the condition. The genetic forms are highly heterogeneous, and early detection is necessary to arrange appropriate patient support. Here we report the molecular basis of hereditary hearing loss in a consanguineous family with multiple affected members from Oman. Combining homozygosity mapping with whole exome sequencing identified a novel homozygous nucleotide substitution c.575T > C in the lipoma HMGIC fusion partner-like 5 gene (LHFPL5), that converted the 192^nd amino acid residue in the protein from a leucine to a proline, p.(Leu192Pro). Sanger sequencing confirmed segregation with the disease phenotype as expected for a recessive condition and the variant was absent in 123,490 subjects from various disease-specific and population genetic studies as well as 150 unrelated individuals and 35 deaf patients of Omani ethnicity. This study, which describes a novel LHFPL5 mutation in a family of Omani origin with hereditary hearing loss, supports previous clinical descriptions of the condition and contributes to the genetic spectrum of mutations in this form of deafness.

High prevalence of congenital deafness on Reunion Island is due to a founder variant of LHFPL5

Reunion Island is a French oversea department in the Indian Ocean with 1.6/1000, an estimated prevalence of deafness that is almost double as compared to the mainland France. Twelve children having isolated bilateral prelingual profound deafness along with motor delay attributed to vestibular areflexia were enrolled. Their mean walking age was 19 months. Electroretinography and temporal bone CT-scans were normal in all cases. A novel homozygous frameshift lipoma HMGIC fusion partner-like 5 (LHFPL5) variant c.185delT p.(Phe62Serfs*23) was identified using whole-exome sequencing. It was found in seven families. Four patients from two different families from both Reunion Island and mainland France, were compound heterozygous: c.185delT p.(Phe62Serfs*23) and c.472C > T p.(Arg158Trp). The phenotype observed in our patients completely mimics the hurry-scurry (hscy) murine Tmhs knock-out model. The recurrent occurrence of same LHFPL5 variant in Reunion Island is attributed to common ancestor couple born in 1693.

Mechanically Gated Ion Channels in Mammalian Hair Cells

Hair cells in the inner ear convert mechanical stimuli provided by sound waves and head movements into electrical signal. Several mechanically evoked ionic currents with different properties have been recorded in hair cells. The search for the proteins that form the underlying ion channels is still in progress. The mechanoelectrical transduction (MET) channel near the tips of stereociliary in hair cells, which is responsible for sensory transduction, has been studied most extensively. Several components of the sensory mechanotransduction machinery in stereocilia have been identified, including the multi-transmembrane proteins tetraspan membrane protein in hair cell stereocilia (TMHS)/LHFPL5, transmembrane inner ear (TMIE) and transmembrane channel-like proteins 1 and 2 (TMC1/2). However, there remains considerable uncertainty regarding the molecules that form the channel pore. In addition to the sensory MET channel, hair cells express the mechanically gated ion channel PIEZO2, which is localized near the base of stereocilia and not essential for sensory transduction. The function of PIEZO2 in hair cells is not entirely clear but it might have a role in damage sensing and repair processes. Additional stretch-activated channels of unknown molecular identity and function have been found to localize at the basolateral membrane of hair cells. Here, we review current knowledge regarding the different mechanically gated ion channels in hair cells and discuss open questions concerning their molecular composition and function.

Molecular Identity of the Mechanotransduction Channel in Hair Cells: Not Quiet There Yet

Hair cells in the mammalian cochlea are specialized mechanosensory cells that convert sound-induced vibrations into electrochemical signals. The molecular composition of the mechanotransduction channel underlying auditory perception has been difficult to define. The study of genes that are linked to inherited forms of deafness has recently provided tantalizing clues. Current findings indicate that the mechanotransduction channel in hair cells is a complex molecular machine. Four different proteins (TMHS/LHFPL5, TMIE, TMC1, and TMC2) have so far been linked to the transduction channel, but which proteins contribute to the channel pore still needs to be determined. Current evidence also suggests that the channel complex may contain additional, yet to be identified components.

Rescue of Hearing by Gene Delivery to Inner-Ear Hair Cells Using Exosome-Associated AAV

Adeno-associated virus (AAV) is a safe and effective vector for gene therapy for retinal disorders. Gene therapy for hearing disorders is not as advanced, in part because gene delivery to sensory hair cells of the inner ear is inefficient. Although AAV transduces the inner hair cells of the mouse cochlea, outer hair cells remain refractory to transduction. Here, we demonstrate that a vector, exosome-associated AAV (exo-AAV), is a potent carrier of transgenes to all inner ear hair cells. Exo-AAV1-GFP is more efficient than conventional AAV1-GFP, both in mouse cochlear explants in vitro and with direct cochlear injection in vivo. Exo-AAV shows no toxicity in vivo, as assayed by tests of auditory and vestibular function. Finally, exo-AAV1 gene therapy partially rescues hearing in a mouse model of hereditary deafness (lipoma HMGIC fusion partner-like 5/tetraspan membrane protein of hair cell stereocilia [Lhfpl5/Tmhs^-/-]). Exo-AAV is a powerful gene delivery system for hair cell research and may be useful for gene therapy for deafness.

Mutations in eight small DFNB genes are not a frequent cause of non-syndromic hereditary hearing loss in Czech patients

OBJECTIVES

To evaluate the contribution of eight small NSHL-AR (non-syndromic deafness, autosomal recessive) genes to hereditary hearing loss in Czech patients.

PATIENTS AND METHODS

Unrelated Czech patients, adults and children, diagnosed with pre-lingual hereditary hearing loss with at least one similarly affected deaf sibling and with previously excluded mutations in the GJB2 gene were investigated by Sanger sequencing of the selected eight small NSHL-AR associated genes (CABP2 - 51 patients, CIB2 - 45 patients, PJVK/DFNB59 - 53 patients, GJB3 - 46 patients, ILDR1 - 48 patients, LHFPL5 - 66 patients, LRTOMT - 60 patients, TMIE - 64 patients).

RESULTS

Mutations were detected in the LHFPL5 (DFNB67) gene. The patient is heterozygote for two already described pathogenic variants (p.Tyr127Cys, p.Thr165Met). In five samples, five rare heterozygous variants (two novel) predicted as pathogenic were detected in genes CABP2, ILDR1, LHFPL5 and LRTOMT.

CONCLUSION

Mutations in eight small NSHL-AR genes are not a frequent cause of hereditary hearing loss in the Czech Republic. This diagnostic approach permitted the clarification of HL in only one patient - two heterozygous mutations were detected in LHFPL5 gene for the first time in Central Europe. As the use of panel base MPS certainly improves the diagnostic yield, future studies should rather profit from that diagnostic strategy.

Subunit determination of the conductance of hair-cell mechanotransducer channels

Cochlear hair cells convert sound stimuli into electrical signals by gating of mechanically sensitive ion channels in their stereociliary (hair) bundle. The molecular identity of this ion channel is still unclear, but its properties are modulated by accessory proteins. Two such proteins are transmembrane channel-like protein isoform 1 (TMC1) and tetraspan membrane protein of hair cell stereocilia (TMHS, also known as lipoma HMGIC fusion partner-like 5, LHFPL5), both thought to be integral components of the mechanotransduction machinery. Here we show that, in mice harboring an Lhfpl5 null mutation, the unitary conductance of outer hair cell mechanotransducer (MT) channels was reduced relative to wild type, and the tonotopic gradient in conductance, where channels from the cochlear base are nearly twice as conducting as those at the apex, was almost absent. The macroscopic MT current in these mutants was attenuated and the tonotopic gradient in amplitude was also lost, although the current was not completely extinguished. The consequences of Lhfpl5 mutation mirror those due to Tmc1 mutation, suggesting a part of the MT-channel conferring a large and tonotopically variable conductance is similarly disrupted in the absence of Lhfpl5 or Tmc1. Immunolabelling demonstrated TMC1 throughout the stereociliary bundles in wild type but not in Lhfpl5 mutants, implying the channel effect of Lhfpl5 mutations stems from down-regulation of TMC1. Both LHFPL5 and TMC1 were shown to interact with protocadherin-15, a component of the tip link, which applies force to the MT channel. We propose that titration of the TMC1 content of the MT channel sets the gradient in unitary conductance along the cochlea.