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

Ectopic mouse TMC1 and TMC2 alone form mechanosensitive channels that are potently modulated by TMIE

The mechanotransduction (MT) channel expressed in cochlear and vestibular hair cells converts the mechanical stimulation of sound and head movements into electrochemical signals. Recently, TMC1 and TMC2 (TMC1/2) have been recognized as the pore-forming subunit of the MT channel, but TMC1/2 functional expression in heterologous cells-which is critical for unequivocally identifying them as the bona fide pore-forming subunit of the MT channel-has not been achieved because ectopic TMC1/2 become trapped in the ER. Here, we report that adding a Fyn lipidation tag to mouse TMC1/2 (mTMC1/2) drove their cell-surface expression, and, importantly, full-length mTMC1/2 expressed alone functioned as mechanosensitive channels, underscoring the view that TMC1/2 constitute the sole pore-forming subunit of the MT channel. Moreover, mouse transmembrane inner ear (TMIE) (mTMIE) protein robustly stimulated TMC1/2 channel activity by modulating their gating. Intriguingly, the N-terminal 27 residues of mTMIE were dispensable for regulating TMC1/2 in our in vitro functional assay, whereas, in striking contrast, mutating mTMIE C76C77, the predicted palmitoylation sites, eliminated mTMIE stimulation of mTMC1/2, indicating a crucial role of the palmitoyl group in regulating TMC1/2 gating. mTMC1/2+mTMIE form 18 pS and 24 pS single channels, respectively. mTMC1/2+mTMIE single channels showed biophysical and pharmacological properties similar to those of the MT channel. Our findings provide insights into several fundamental and debated aspects of the function of TMC1/2 and TMIE, and our functional assay of TMC1/2 and TMIE in heterologous cells will facilitate further functional and structural characterization of these proteins and other MT-complex components.

Mechano-electrical transduction components TMC1-CIB2 undergo a Ca2+-induced conformational change linked to hearing loss

TMC1, a unique causative gene associated with deafness, exhibits variants with autosomal dominant and recessive inheritance patterns. TMC1 codes for the transmembrane channel-like protein 1 (TMC1), a key component of the mechano-electrical transduction (MET) machinery for hearing. However, the molecular mechanism of Ca2+ regulation in MET remains unclear. Calcium and integrin-binding protein 2 (CIB2), another MET component associated with deafness, can bind with Ca2+. Our study shows that TMC1-CIB2 complex undergoes a Ca2+-induced conformational change. We identified a vertebrate-specific binding site on TMC1 that interacts with apo CIB2, linked with hearing loss. Using an ex vivo mouse organotypic cochlea model, we demonstrated that disruption of the calcium-binding site of CIB2 perturbs the MET channel conductivity. After systematically analyzing the hearing loss variants, we observed dominant mutations of TMC1 cluster around the putative ion pore or at the binding interfaces with CIB2. These findings elucidate the molecular mechanisms underlying TMC1-linked hearing loss.

LOXHD1 is indispensable for maintaining TMC1 auditory mechanosensitive channels at the site of force transmission

Hair cell bundles consist of stereocilia arranged in rows of increasing heights, connected by tip links that transmit sound-induced forces to shorter stereocilia tips. Auditory mechanotransduction channel complexes, composed of proteins TMC1/2, TMIE, CIB2, and LHFPL5, are located at the tips of shorter stereocilia. While most components can interact with the tip link in vitro, their ability to maintain the channel complexes at the tip link in vivo is uncertain. Return, using mouse models, we show that an additional component, LOXHD1, is essential for keeping TMC1-pore forming subunits at the tip link but is dispensable for TMC2. Using SUB-immunogold-SEM, we showed that TMC1 localizes near the tip link but mislocalizes without LOXHD1. LOXHD1 selectively interacts with TMC1, CIB2, LHFPL5, and tip-link protein PCDH15. Our results demonstrate that TMC1-driven mature auditory channels require LOXHD1 to stay connected to the tip link and remain functional, while TMC2-driven developmental channels do not.

Establishment and analysis of artificial neural network diagnosis model for coagulation-related molecular subgroups in coronary artery disease

Background: Coronary artery disease (CAD) is the most common type of cardiovascular disease and cause significant morbidity and mortality. Abnormal coagulation cascade is one of the high-risk factors in CAD patients, but the molecular mechanism of coagulation in CAD is still limited. Methods: We clustered and categorized 352 CAD paitents based on the expression patterns of coagulation-related genes (CRGs), and then we explored the molecular and immunological variations across the subgroups to reveal the underlying biological characteristics of CAD patients. The feature genes between CRG-subgroups were further identified using a random forest model (RF) and least absolute shrinkage and selection operator (LASSO) regression, and an artificial neural network prediction model was constructed. Results: CAD patients could be divided into the C1 and C2 CRG-subgroups, with the C1 subgroup highly enriched in immune-related signaling pathways. The differential expressed genes between the two CRG-subgroups (DE-CRGs) were primarily enriched in signaling pathways connected to signal transduction and energy metabolism. Subsequently, 10 feature DE-CRGs were identified by RF and LASSO. We constructed a novel artificial neural network model using these 10 genes and evaluated and validated its diagnostic performance on a public dataset. Conclusion: Diverse molecular subgroups of CAD patients may each have a unique gene expression pattern. We may identify subgroups using a few feature genes, providing a theoretical basis for the precise treatment of CAD patients with different molecular subgroups.

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.

LOXHD1 is indispensable for coupling auditory mechanosensitive channels to the site of force transmission

Hearing is initiated in hair cells by the mechanical activation of ion channels in the hair bundle. The hair bundle is formed by stereocilia organized into rows of increasing heights interconnected by tip links, which convey sound-induced forces to stereocilia tips. The auditory mechanosensitive channels are complexes containing at least four protein-subunits - TMC1/2, TMIE, CIB2, and LHFPL51-16 - and are located at the tips of shorter stereocilia at a yet-undetermined distance from the lower tip link insertion point17. While multiple auditory channel subunits appear to interact with the tip link, it remains unknown whether their combined interaction alone can resist the high-frequency mechanical stimulations owing to sound. Here we show that an unanticipated additional element, LOXHD1, is indispensable for maintaining the TMC1 pore-forming channel subunits coupled to the tip link. We demonstrate that LOXHD1 is a unique element of the auditory mechanotransduction complex that selectively affects the localization of TMC1, but not its close developmental paralogue TMC2. Taking advantage of our novel immunogold scanning electron microscopy method for submembranous epitopes (SUB-immunogold-SEM), we demonstrate that TMC1 normally concentrates within 100-nm of the tip link insertion point. In LOXHD1's absence, TMC1 is instead mislocalized away from this force transmission site. Supporting this finding, we found that LOXHD1 interacts selectively in vitro with TMC1 but not with TMC2 while also binding to channel subunits CIB2 and LHFPL5 and tip-link protein PCDH15. SUB-immunogold-SEM additionally demonstrates that LOXHD1 and TMC1 are physically connected to the lower tip-link complex in situ. Our results show that the TMC1-driven mature channels require LOXHD1 to stay coupled to the tip link and remain functional, but the TMC2-driven developmental channels do not. As both tip links and TMC1 remain present in hair bundles lacking LOXHD1, it opens the possibility to reconnect them and restore hearing for this form of genetic deafness.

Novel marine natural products as effective TRPV1 channel blockers

Chronic pain management poses a formidable challenge to healthcare, exacerbated by current analgesic options' limitations and adverse effects. Transient receptor potential vanilloid 1 (TRPV1), a non-selective cation channel, has emerged as a promising target for novel analgesics. However, safety and tolerability concerns have constrained the development of TRPV1 modulators. In this study, we explored marine-derived natural products as a source of potential TRPV1 modulators using high-throughput dye-uptake assays. We identified chrexanthomycins, a family of hexacyclic xanthones, exhibited potent TRPV1 inhibitory effects, with compounds cC and cF demonstrating the most significant activity. High-resolution patch-clamp assays confirmed the direct action of these compounds on the TRPV1 channel. Furthermore, in vivo assays revealed that cC and cF effectively suppressed capsaicin-induced pain sensation in mice, comparable to the known TRPV1 inhibitor, capsazepine. Structural-activity relationship analysis highlighted the importance of specific functional groups in modulating TRPV1 activity. Our findings underscore the therapeutic potential of chrexanthomycins and pave the way for further investigations into marine-derived TRPV1 modulators for pain management.

Single molecule studies of the native hair cell mechanosensory transduction complex

Hearing and balance rely on the conversion of a mechanical stimulus into an electrical signal, a process known as mechanosensory transduction (MT). In vertebrates, this process is accomplished by an MT complex that is located in hair cells of the inner ear. While the past three decades of research have identified many subunits that are important for MT and revealed interactions between these subunits, the composition and organization of a functional complex remains unknown. The major challenge associated with studying the MT complex is its extremely low abundance in hair cells; current estimates of MT complex quantity range from 3-60 attomoles per cochlea or utricle, well below the detection limit of most biochemical assays that are used to characterize macromolecular complexes. Here we describe the optimization of two single molecule assays, single molecule pull-down (SiMPull) and single molecule array (SiMoA), to study the composition and quantity of native mouse MT complexes. We demonstrate that these assays are capable of detecting and quantifying low attomoles of the native MT subunits protocadherin-15 (PCDH15) and lipoma HMGIC fusion partner-like protein 5 (LHFPL5). Our results illuminate the stoichiometry of PCDH15- and LHFPL5-containing complexes and establish SiMPull and SiMoA as productive methods for probing the abundance, composition, and arrangement of subunits in the native MT complex.

Novel autosomal dominant TMC1 variants linked to hearing loss: insight into protein-lipid interactions

BACKGROUND

TMC1, which encodes transmembrane channel-like protein 1, forms the mechanoelectrical transduction (MET) channel in auditory hair cells, necessary for auditory function. TMC1 variants are known to cause autosomal dominant (DFNA36) and autosomal recessive (DFNB7/11) non-syndromic hearing loss, but only a handful of TMC1 variants underlying DFNA36 have been reported, hampering analysis of genotype-phenotype correlations.

METHODS

In this study, we retrospectively reviewed 338 probands in an in-house database of genetic hearing loss, evaluating the clinical phenotypes and genotypes of novel TMC1 variants associated with DFNA36. To analyze the structural impact of these variants, we generated two structural models of human TMC1, utilizing the Cryo-EM structure of C. elegans TMC1 as a template and AlphaFold protein structure database. Specifically, the lipid bilayer-embedded protein database was used to construct membrane-embedded models of TMC1. We then examined the effect of TMC1 variants on intramolecular interactions and predicted their potential pathogenicity.

RESULTS

We identified two novel TMC1 variants related to DFNA36 (c.1256T > C:p.Phe419Ser and c.1444T > C:p.Trp482Arg). The affected subjects had bilateral, moderate, late-onset, progressive sensorineural hearing loss with a down-sloping configuration. The Phe419 residue located in the transmembrane domain 4 of TMC1 faces outward towards the channel pore and is in close proximity to the hydrophobic tail of the lipid bilayer. The non-polar-to-polar variant (p.Phe419Ser) alters the hydrophobicity in the membrane, compromising protein-lipid interactions. On the other hand, the Trp482 residue located in the extracellular linker region between transmembrane domains 5 and 6 is anchored to the membrane interfaces via its aromatic rings, mediating several molecular interactions that stabilize the structure of TMC1. This type of aromatic ring-based anchoring is also observed in homologous transmembrane proteins such as OSCA1.2. Conversely, the substitution of Trp with Arg (Trp482Arg) disrupts the cation-π interaction with phospholipids located in the outer leaflet of the phospholipid bilayer, destabilizing protein-lipid interactions. Additionally, Trp482Arg collapses the CH-π interaction between Trp482 and Pro511, possibly reducing the overall stability of the protein. In parallel with the molecular modeling, the two mutants degraded significantly faster compared to the wild-type protein, compromising protein stability.

CONCLUSIONS

This results expand the genetic spectrum of disease-causing TMC1 variants related to DFNA36 and provide insight into TMC1 transmembrane protein-lipid interactions.

Nanobead-based single-molecule pulldown for single cells

Investigation of cell-to-cell variability holds critical physiological and clinical implications. Thus, numerous new techniques have been developed for studying cell-to-cell variability, and these single-cell techniques can also be used to investigate rare cells. Moreover, for studying protein-protein interactions (PPIs) in single cells, several techniques have been developed based on the principle of the single-molecule pulldown (SiMPull) assay. However, the applicability of these single-cell SiMPull (sc-SiMPull) techniques is limited because of their high technical barrier and special requirements for target cells and molecules. Here, we report a highly innovative nanobead-based approach for sc-SiMPull that is based on our recently developed microbead-based, improved version of SiMPull for cell populations. In our sc-SiMPull method, single cells are captured in microwells and lysed in situ, after which commercially available, pre-surface-functionalized magnetic nanobeads are placed in the microwells to specifically capture proteins of interest together with their binding partners from cell extracts; subsequently, the PPIs are examined under a microscope at the single-molecule level. Relative to previously published methods, nanobead-based sc-SiMPull is considerably faster, easier to use, more reproducible, and more versatile for distinct cell types and protein molecules, and yet provides similar sensitivity and signal-to-background ratio. These crucial features should enable universal application of our method to the study of PPIs in single cells.

Calcium and Integrin-binding protein 2 (CIB2) controls force sensitivity of the mechanotransducer channels in cochlear outer hair cells

Calcium and Integrin-Binding Protein 2 (CIB2) is an essential subunit of the mechano-electrical transduction (MET) complex in mammalian auditory hair cells. CIB2 binds to pore-forming subunits of the MET channel, TMC1/2 and is required for their transport and/or retention at the tips of mechanosensory stereocilia. Since genetic ablation of CIB2 results in complete loss of MET currents, the exact role of CIB2 in the MET complex remains elusive. Here, we generated a new mouse strain with deafness-causing p.R186W mutation in Cib2 and recorded small but still measurable MET currents in the cochlear outer hair cells. We found that R186W variant causes increase of the resting open probability of MET channels, steeper MET current dependence on hair bundle deflection (I-X curve), loss of fast adaptation, and increased leftward shifts of I-X curves upon hair cell depolarization. Combined with AlphaFold2 prediction that R186W disrupts one of the multiple interacting sites between CIB2 and TMC1/2, our data suggest that CIB2 mechanically constraints TMC1/2 conformations to ensure proper force sensitivity and dynamic range of the MET channels. Using a custom piezo-driven stiff probe deflecting the hair bundles in less than 10 µs, we also found that R186W variant slows down the activation of MET channels. This phenomenon, however, is unlikely to be due to direct effect on MET channels, since we also observed R186W-evoked disruption of the electron-dense material at the tips of mechanotransducing stereocilia and the loss of membrane-shaping BAIAP2L2 protein from the same location. We concluded that R186W variant of CIB2 disrupts force sensitivity of the MET channels and force transmission to these channels.

Mechanoelectrical transduction-related genetic forms of hearing loss

Hair cells of the mammalian cochlea are specialized mechanosensory cells that convert mechanical stimuli into electrical signals to initiate the neuronal responses that lead to the perception of sound. The mechanoelectrical transduction (MET) machinery of cochlear hair cells is a multimeric protein complex that consists of the pore forming subunits of the MET channel and several essential accessory subunits that are crucial to regulate channel function and render the channel mechanically sensitive. Mutations have been discovered in the genes that encode all known components of the MET machinery. These mutations cause hearing loss with or without vestibular dysfunction. Some mutations also affect other tissues such as the retina. In this brief review, we will summarize gene mutations that affect the MET machinery of hair cells and how the study of the affected genes has illuminated our understanding of the physiological role of the encoded proteins.

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.

Calcium signaling and genetic rare diseases: An auditory perspective

Deafness is a highly heterogeneous disorder which stems, for 50%, from genetic origins. Sensory transduction relies mainly on sensory hair cells of the cochlea, in the inner ear. Calcium is key for the function of these cells and acts as a fundamental signal transduction. Its homeostasis depends on three factors: the calcium influx, through the mechanotransduction channel at the apical pole of the hair cell as well as the voltage-gated calcium channel at the base of the cells; the calcium buffering via Ca²⁺-binding proteins in the cytoplasm, but also in organelles such as mitochondria and the reticulum endoplasmic mitochondria-associated membranes with specialized proteins; and the calcium extrusion through the Ca-ATPase pump, located all over the plasma membrane. In addition, the synaptic transmission to the central nervous system is also controlled by calcium. Genetic studies of inherited deafness have tremendously helped understand the underlying molecular pathways of calcium signaling. In this review, we discuss these different factors in light of the associated genetic diseases (syndromic and non-syndromic deafness) and the causative genes.

Multiple plasma membrane reporters discern LHFPL5 region that blocks trafficking to the plasma membrane

The mechano-electrical transduction (MET) channel of the inner ear receptor cells, termed hair cells, is a protein complex that enables our senses of hearing and balance. Hair cell MET requires an elaborate interplay of multiple proteins that form the MET channel. One of the MET complex components is the transmembrane protein LHFPL5, which is required for hair cell MET and hearing. LHFPL5 is thought to form a multi-protein complex with other MET channel proteins, such as PCDH15, TMIE, and TMC1. Despite localizing to the plasma membrane of stereocilia, the mechanosensing organelles of hair cells, LHFPL5 requires its binding partner within the MET complex, PCDH15, to localize to the stereocilia tips in hair cells and to the plasma membrane in heterologous cells. Using the Aquaporin 3-tGFP reporter (AGR) for plasma membrane localization, we found that a region within extracellular loop 1, which interacts with PCDH15, precludes the trafficking of AGR reporter to the plasma membrane in heterologous cell lines. Our results suggest that the presence of protein partners may mask endoplasmic reticulum retention regions or enable the proper folding and trafficking of the MET complex components, to facilitate expression of the MET complex at the stereocilia membrane.

Sensing sound: Cellular specializations and molecular force sensors

Organisms of all phyla express mechanosensitive ion channels with a wide range of physiological functions. In recent years, several classes of mechanically gated ion channels have been identified. Some of these ion channels are intrinsically mechanosensitive. Others depend on accessory proteins to regulate their response to mechanical force. The mechanotransduction machinery of cochlear hair cells provides a particularly striking example of a complex force-sensing machine. This molecular ensemble is embedded into a specialized cellular compartment that is crucial for its function. Notably, mechanotransduction channels of cochlear hair cells are not only critical for auditory perception. They also shape their cellular environment and regulate the development of auditory circuitry. Here, we summarize recent discoveries that have shed light on the composition of the mechanotransduction machinery of cochlear hair cells and how this machinery contributes to the development and function of the auditory system.

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.

Mechanotransduction in mammalian sensory hair cells

In the inner ear, the auditory and vestibular systems detect and translate sensory information regarding sound and balance. The sensory cells that transform mechanical input into an electrical signal in these systems are called hair cells. A specialized organelle on the apical surface of hair cells called the hair bundle detects mechanical signals. Displacement of the hair bundle causes mechanotransduction channels to open. The morphology and organization of the hair bundle, as well as the properties and characteristics of the mechanotransduction process, differ between the different hair cell types in the auditory and vestibular systems. These differences likely contribute to maximizing the transduction of specific signals in each system. This review will discuss the molecules essential for mechanotransduction and the properties of the mechanotransduction process, focusing our attention on recent data and differences between the auditory and vestibular systems.

Gene-teratogen interactions influence the penetrance of birth defects by altering Hedgehog signaling strength

Birth defects result from interactions between genetic and environmental factors, but the mechanisms remain poorly understood. We find that mutations and teratogens interact in predictable ways to cause birth defects by changing target cell sensitivity to Hedgehog (Hh) ligands. These interactions converge on a membrane protein complex, the MMM complex, that promotes degradation of the Hh transducer Smoothened (SMO). Deficiency of the MMM component MOSMO results in elevated SMO and increased Hh signaling, causing multiple birth defects. In utero exposure to a teratogen that directly inhibits SMO reduces the penetrance and expressivity of birth defects in Mosmo-/- embryos. Additionally, tissues that develop normally in Mosmo-/- embryos are refractory to the teratogen. Thus, changes in the abundance of the protein target of a teratogen can change birth defect outcomes by quantitative shifts in Hh signaling. Consequently, small molecules that re-calibrate signaling strength could be harnessed to rescue structural birth defects.

Analyzing protein-protein interactions in rare cells using microbead-based single-molecule pulldown assay

For studying protein-protein interactions (PPIs) in general, a powerful and commonly used technique is conventional coimmunoprecipitation (co-IP/pulldown) followed by western blotting. However, the technique does not provide precise information regarding the kinetics and stoichiometry of PPIs. Another drawback is that the sensitivity of conventional co-IP is not suitable for examining PPIs in rare cells such as sensory hair cells, circulating tumor cells, embryonic stem cells, and subsets of immune cells. The current single-molecule pulldown (SiMPull) assay can potentially be used for studying PPIs in rare cells but its wide application is hindered by the high technical barrier and time consumption. We report an innovative, agarose microbead-based approach for SiMPull. We used commercially available, pre-surface-functionalized agarose microbeads to capture the protein of interest together with its binding partners specifically from cell extracts and observed these interactions under a microscope at the single-molecule level. Relative to the original method, microbead-based SiMPull is considerably faster, easier to use, and more reproducible and yet provides similar sensitivity and signal-to-background ratio; specifically, with the new method, sample-preparation time is substantially decreased (from ∼10 to ∼3 h). These crucial features should facilitate wide application of the powerful and versatile SiMPull method in common biological and clinical laboratories. Notably, by exploiting the simplicity and ultrahigh sensitivity of microbead-based SiMPull, we used the method in the study of rare auditory hair cells and γδ T cells for the first time.

Mechanisms in cochlear hair cell mechano-electrical transduction for acquisition of sound frequency and intensity

Sound signals are acquired and digitized in the cochlea by the hair cells that further transmit the coded information to the central auditory pathways. Any defect in hair cell function may induce problems in the auditory system and hearing-based brain function. In the past 2 decades, our understanding of auditory transduction has been substantially deepened because of advances in molecular, structural, and functional studies. Results from these experiments can be perfectly embedded in the previously established profile from anatomical, histological, genetic, and biophysical research. This review aims to summarize the progress on the molecular and cellular mechanisms of the mechano-electrical transduction (MET) channel in the cochlear hair cells, which is involved in the acquisition of sound frequency and intensity-the two major parameters of an acoustic cue. We also discuss recent studies on TMC1, the molecule likely to form the MET channel pore.

Opposing roles of E3 ligases TRIM23 and TRIM21 in regulation of ion channel ANO1 protein levels

Anoctamin-1 (ANO1) (TMEM16A) is a calcium-activated chloride channel that plays critical roles in diverse physiological processes, such as sensory transduction and epithelial secretion. ANO1 levels have been shown to be altered under physiological and pathological conditions, although the molecular mechanisms that control ANO1 protein levels remain unclear. The ubiquitin–proteasome system is known to regulate the levels of numerous ion channels, but little information is available regarding whether and how ubiquitination regulates levels of ANO1. Here, we showed that two E3 ligases, TRIM23 and TRIM21, physically interact with the C terminus of ANO1. In vitro and in vivo assays demonstrated that whereas TRIM23 ubiquitinated ANO1 leading to its stabilization, TRIM21 ubiquitinated ANO1 and induced its degradation. Notably, ANO1 regulation by TRIM23 and TRIM21 is involved in chemical-induced pain sensation, salivary secretion, and heart-rate control in mice, and TRIM23 also mediates ANO1 upregulation induced by epidermal growth factor treatment. Our results suggest that these two antagonistic E3 ligases act together to control ANO1 expression and function. Our findings reveal a previously unrecognized mechanism for regulating ANO1 protein levels and identify a potential molecular link between ANO1 regulation, epidermal growth factor, and other signaling pathways.

New Tmc1 Deafness Mutations Impact Mechanotransduction in Auditory Hair Cells

Transmembrane channel-like protein isoform 1 (TMC1) is a major component of the mechano-electrical transducer (MET) channel in cochlear hair cells and is subject to numerous mutations causing deafness. We report a new dominant human deafness mutation, TMC1 p.T422K, and have characterized the homologous mouse mutant, Tmc1 p.T416K, which caused deafness and outer hair cell (OHC) loss by the fourth postnatal week. MET channels showed decreased Ca²⁺ permeability and resting open probability, but no change in single-channel conductance or expression. Three adjacent deafness mutations are TMC1 p.L416R, p.G417R, and p.M418K, the last homologous to the mouse Beethoven that exhibits similar channel effects. All substitute a positive for a neutral residue, which could produce charge screening in the channel pore or influence binding of an accessory subunit. Channel properties were compared in mice of both sexes between dominant (Tmc1 p.T416K, Tmc1 p.D569N) and recessive (Tmc1 p.W554L, Tmc1 p.D528N) mutations of residues near the putative pore of the channel. Tmc1 p.W554L and p.D569N exhibit reduced maximum current with no effect on single-channel conductance, implying a smaller number of channels transported to the stereociliary tips; this may stem from impaired TMC1 binding to LHFPL5. Tmc1 p.D528N, located in the pore's narrowest region, uniquely caused large reductions in MET channel conductance and block by dihydrostreptomycin (DHS). For Tmc1 p.T416K and Tmc1 p.D528N, transduction loss occurred between P15 and P20. We propose two mechanisms linking channel mutations and deafness: decreased Ca²⁺ permeability, common to all mutants, and decreased resting open probability in low Ca²⁺, confined to dominant mutations.

SIGNIFICANCE STATEMENT Transmembrane channel-like protein isoform 1 (TMC1) is thought to be a major component of the mechanotransducer channel in auditory hair cells, but the protein organization and channel structure are still uncertain. We made four mouse lines harboring Tmc1 point mutations that alter channel properties, causing hair cell degeneration and deafness. These include a mouse homolog of a new human deafness mutation pT416K that decreased channel Ca²⁺ permeability by introducing a positively-charged amino acid in the putative pore. All mutations are consistent with the channel structure predicted from modeling, but only one, p.D528N near the external face of the pore, substantially reduced channel conductance and Ca²⁺ permeability and virtually abolished block by dihydrostreptomycin (DHS), strongly endorsing its siting within the pore.

Alternative Splicing of Three Genes Encoding Mechanotransduction-Complex Proteins in Auditory Hair Cells

The mechanotransduction (MT) complex in auditory hair cells converts the mechanical stimulation of sound waves into neural signals. Recently, the MT complex has been suggested to contain at least four distinct integral membrane proteins: protocadherin 15 (PCDH15), transmembrane channel-like protein 1 (TMC1), lipoma HMGIC fusion partner-like 5 (LHFPL5), and transmembrane inner ear protein (TMIE). However, the composition, function, and regulation of the MT-complex proteins remain incompletely investigated. Here, we report previously undescribed splicing isoforms of TMC1, LHFPL5, and TMIE. We identified four alternative splicing events for the genes encoding these three proteins by analyzing RNA-seq libraries of auditory hair cells from adult mice [over postnatal day (P)28], and we then verified the alternative splicing events by using RT-PCR and Sanger sequencing. Moreover, we examined the tissue-specific distribution, developmental expression patterns, and tonotopic gradient of the splicing isoforms by performing semiquantitative and quantitative real-time PCR (qRT-PCR), and we found that the alternative splicing of TMC1 and LHFPL5 is cochlear-specific and occurs in both neonatal and adult mouse cochleae. Our findings not only reveal the potential complexity of the MT-complex composition, but also provide critical insights for guiding future research on the function, regulation, and trafficking of TMC1, LHFPL5, and TMIE and on the clinical diagnosis of hearing loss related to aberrant splicing of these three key genes in hearing.