Molecular Structure of the Hair Cell Mechanoelectrical Transduction Complex

Abstract

Cochlear hair cells employ mechanically gated ion channels located in stereocilia that open in response to sound wave-induced motion of the basilar membrane, converting mechanical stimulation to graded changes in hair cell membrane potential. Membrane potential changes in hair cells cause neurotransmitter release from hair cells that initiate electrical signals in the nerve terminals of afferent fibers from spiral ganglion neurons. These signals are then propagated within the central nervous system (CNS) to mediate the sensation of hearing. Recent studies show that the mechanoelectrical transduction (MET) machinery of hair cells is formed by an ensemble of proteins. Candidate components forming the MET channel have been identified, but none alone fulfills all criteria necessary to define them as pore-forming subunits of the MET channel. We will review here recent findings on the identification and function of proteins that are components of the MET machinery in hair cells and consider remaining open questions.

Figure 1.

Cochlear hair cell anatomy. (A) Hair cells contain actin-rich stereocilia embedded in the cuticular plate at their apical surface. Stereocilia are interconnected via extracellular linkages including tip links, ankle links, and horizontal top connectors. The mechanoelectrical transduction (MET) channel that mediates auditory processing is located at the tips of shorter stereocilia at the lower end of tip links. (B) The MET channel pore is nonselective for cations with ion fluxes in hair cells composed predominantly of Ca²⁺ and K⁺. Upon normal stimulation, ions flow from the extracellular space containing endolymph into the hair cell. (C) The lower end of the tip link, composed of protocadherin 15 (PCDH15), is inserted into the membrane at the tip of the stereocilia in close proximity to the MET channel pore. It is currently unknown whether the tip link binds directly to the channel pore, is connected via linker protein to the pore, or transmits force to the channel pore by impacting the local lipid environment.

Figure 2.

Proteins with predicted membrane topology similar to lipoma high mobility group IC fusion partner-like 5 (LHFPL5). (A) LHFPL5/tetraspan membrane protein of hair cell stereocilia (TMHS) contains four predicted transmembrane domains with cytoplasmic amino- and carboxy-terminal domains. (B) ORAI1 (Prakriya et al. 2006), the pore-forming domain of the calcium-release activated calcium channel (CRAC), has a similar topology to LHFPL5. (C) LRRC8A (Qiu et al. 2014), the pore-forming subunit of the volume-regulated anion channel (VRAC), contains four transmembrane domains with cytoplasmic amino- and carboxy-terminal domains. (D) CACNG2/Stargazin (Twomey et al. 2016) is an accessory subunit of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) receptors with similar topology to LHFPL5. Lengths of domains are to relative scale. Diagrams are depicted as extracellular and intracellular at the plasma membrane, but for endoplasmic reticulum (ER) proteins, “extracellular” = ER lumen. (Figures generated based on predicted topologies from Uniprot.)

Figure 3.

Proteins with predicted membrane topology similar to transmembrane inner ear (TMIE). (A) Full-length TMIE (Zhao et al. 2014) contains two transmembrane domains with intracellular amino and carboxyl termini. (B) After cleavage of a predicted amino-terminal signal sequence, TMIE contains a single transmembrane domain with an extracellular amino terminus and an intracellular carboxyl terminus. It is unknown which version of TMIE predominates in vivo, or whether both full-length and cleaved versions are present. (C) MEC-4 (Lai et al. 1996), the pore-forming subunit of the Caenorhabditis elegans light-touch mechanosensory channel, contains two transmembrane domains with cytoplasmic amino- and carboxy-terminal domains similar to full-length TMIE. (D) MscL (Chang et al. 1998), a mechanosensory channel expressed in prokaryotes has similar topology to full-length TMIE. Lengths of domains are to relative scale. Diagrams are depicted as extracellular and intracellular at the plasma membrane, but for endoplasmic reticulum (ER) proteins, “extracellular” = ER lumen. (Figures generated based on predicted topologies from Uniprot.)

Figure 4.

Proteins with predicted membrane topology similar to transmembrane channel-like proteins 1 and 2 (TMC1/2). (A) Full-length TMC1 (Labay et al. 2010) contains six predicted transmembrane domains with intracellular amino and carboxyl termini. (B) Full-length TMC2 (Kurima et al. 2003) contains six predicted transmembrane domains with intracellular amino and carboxyl termini. (C) KCNA1 (Long et al. 2005), the pore-forming subunit of the Shaker K⁺ channel Kv1.2, contains six transmembrane domains, a membrane reentrant loop forming the pore between TM5 and TM6 and cytoplasmic amino- and carboxy-terminal domains. Some alternate topologies for TMC1/2 predict a possible pore reentrant loop between TM4 and TM5 (Labay et al. 2010). (D) ZnT-1 (Palmiter and Huang 2004), a zinc transporter that regulates transport of zinc between the endoplasmic reticulum (ER) and the cytoplasm, has a similar predicted topology to TMC1/2. Lengths of domains are to relative scale. Diagrams are depicted as extracellular and intracellular at the plasma membrane, but for ER proteins, “extracellular” = ER lumen. E.C., extracellular; I.C., intracellular. (Figures generated based on predicted topologies from Uniprot.)

Figure 5.

Molecules that contribute to mechanoelectrical transduction (MET) in cochlear hair cells. CDH23 and PCDH15 interact to form the tip link, which connects the tip of shorter row stereocilia to the side of taller row stereocilia. Harmonin, SANS, and MYOVIIA all localize to the upper tip-link density and interact in a complex with CDH23 to regulate MET and tip-link tension. LHFPL5, TMIE, and TMC1/2 all localize to the tips of stereocilia at the lower end of the tip link and are essential for MET function. PIP₂ and CIB2 localize to the tips of stereocilia and directly contribute to MET function. CLRN1, MYOXVa, and Whirlin localize to the tips of stereocilia and impact MET function, but these effects may be secondary to their effects on stereocilia morphology. TOMT localizes exclusively to the cell body but is essential for MET function by regulating TMC1/2 localization to stereocilia.