Prestin and Motility of the Cochlear Outer Hair Cell.

Choi,  Chul Hee

Review

Hearing Sciences, Auditory and vestibular disorders

Korean Journal of Audiology 2011;15(3):101-106.

Prestin and Motility of the Cochlear Outer Hair Cell.

Department of Audiology and Speech-Language Pathology and Research Institute of Biomimetic Sensory Control, College of Medical Sciences, Catholic University of Daegu, Gyeongsan, Korea. cchoi@cu.ac.kr

Abstract

The main objective of this study is to describe the role and function of prestin on cochlear amplification based on the relationship of electromotility and prestin in the outer hair cells (OHCs). After the finding of cochlear active process or amplification, OHCs have been received a lot of attention as a source of the cochlear amplification. In response to acoustic signals, the OHCs produce the receptor potentials resulting in changes in the length of the OHCs called electromotility. The electromotility originates within the lateral wall of the OHCs and relates to the unique structures of the OHCs. The OHC electromotility depends on particles of the lateral plasma membrane due to an area motor in the lateral plasma membrane. Recently, it has been reported that the electromotility requires a voltage-dependent membrane based motor protein, prestin. Prestin means fast in Italian. The presence of prestin is essential for cochlear amplification and electromotility. Prestin is a member of solute carrier 26 anion transporter family. Prestin is associated with the unique structure of the lateral wall of the OHCs. Prestin forms motor complexes with other proteins and lipids of the lateral wall sensing the transmembrane potential and generating force by changing its surface area. Recently, prestin knockout mice have been used to prove the presence of prestin. Prestin is required for electromotility of the OHCs and for cochlear amplification in normal hearing because targeted depletion of prestin in mice leads to loss of OHC electromotility and loss of hearing sensitivity up to 60 dB. In addition, recent studies have shown that the loss of cochlear amplification after intense noise exposure can result from damage to prestin and prestin involves in the process of aminoglycoside-induced apoptosis in OHCs. These show that prestin plays an important role in transducing apoptosis signals in response to antibiotics. Therefore, the presence of prestin is mandatory for cochlear active process and amplification in normal hearing.

Keywords: Outer hair cells (OHCs);Electromotility;Prestin;Cochlear amplification;OHC length;Noise-induced hearing loss;Ototoxicity

Address for correspondence : Chul-Hee Choi, PhD, Department of Audiology and Speech-Language Pathology and Research Institute of Biomimetic Sensory Control, College of Medical Sciences, Catholic University of Daegu, Hayang-eup, 13-3 Hayang-ro, Gyeongsan 712-702, Korea
Tel : +82-53-850-3185, Fax : +82-53-850-3383, E-mail ; cchoi@cu.ac.kr

Introduction

Hearing sensitivity in normal mammals is drastically improved by about 40 dB by a cochlear amplifier, which is involved with mechano-electrical transduction (MET) located within the stereocilia on the apex of the cells modulating receptor potentials in response to hair cell bundle deflection,^1,^2,^3,^4,^5,^6,⁷⁾ and electromotility placed in the outer hair cell (OHC) lateral wall triggering changes in the OHC length in response to the receptor potentials.^4,^8,^9,¹⁰⁾ While MET has been observed in both inner and outer hair cells of non-mammals and mammals, electromotility has been only observed in OHC of mammals. Recently, it has reported that OHC electromotility requires a voltage-dependent membrane based motor protein, prestin which is a member of solute carrier (SLC) 26 anion transporter family.^11,¹²⁾ That is, the presence of prestin is essential for the OHC electromotility and for cochlear amplification. Therefore, this paper deals with the recent studies identifying the relationship between prestin and electromotility, the recent findings of the structure and function of prestin, and the recent prestin-related topics.

OHC Electromotility and Cochlear Amplification

When acoustic signals reach the ear, they evoke the motion of the basilar membrane containing a single row of inner hair cells and three rows of OHC, which deflects the hair cell bundle located in the stereocilia by opening of the transduction channels.^13,^14,¹⁵⁾ When the hair cell bundle deflects to the tallest stereocilia, the transduction channels open and allow K+ and Ca2+ ions into the OHC, which leads to an increase of the hair cell receptor potential resulting in depolarization.¹⁶⁾ When the hair cell bundle deflects to the opposite direction, the channels close and outflow the ions out of the OHC, which leads to a decrease of the hair cell receptor potential resulting in hyperpolarization. The hair cell receptor potential drives to change the length of the OHC soma.⁴⁾ Depolarization causes the OHC to contract along its longitudinal axis while hyperpolarization causes it to expand. The modulation of the length of OHC leads to the change of the basilar membrane vibration. In summary, the increased OHC receptor shortens the OHC (thick) and moves up the basilar membrane (BM) while the decreased OHC receptor elongates (thin) the cell and moves down the BM.⁴⁾ This motility of the OHC is unique to only mammals. Therefore, in mammals, the OHC motility is necessary for cochlear amplification. Fig. 1 show the feedback loop which is involved with cochlear amplification in terms of the mechano-electrical transduction, electromotility, and efferent feedback from the brain.¹⁷⁾

Motility and Unique Structure of OHC

The change of OHC length in response to electrical stimulation was first observed by Brownell, et al.⁸⁾ who reported that depolarization by current leads to the OHC contraction and hyperpolarization by current leads to the OHC elongation. The change of the OHC length produced by electrical stimulation has been termed electromotility. No other cell type in the mammalian cochlea displays electromotility. Therefore, electromotility is an only characteristic displayed exclusively by mammalian OHCs. The basic mechanisms of the OHC electomotility have been sought in terms of the OHC forces and the OHC stiffness because the axial stiffness of the OHC is itself voltage dependent.¹⁸⁾
The OHC electromotility originates within the lateral wall of the OHC.^19,²⁰⁾ The anatomic structure of the OHC is essential for the motility. The OHCs are very sensitive to the osmotic strength of the external solution because the OHCs swell in response to the hypo-osmotic solution while they shrink in response to the hyper-osmotic solution.^21,^22,^23,²⁴⁾ Because of the cylindrical structure, swelling leads to the OHC contraction while volume shrinking leads to the cell elongation. Therefore, the change of osmolality produces changes in turgor pressure of the OHC.
The structure of the OHCs in vertebrate cells is very unique because of a hydrostat with a pressured fluid core and a mechanically reinforced lateral wall maintaining their cylindrical shape. The OHCs have the lateral wall with three sublaminate layers containing a plasma membrane, a cytoskeleton, and a membrane organelle called subsurface cisternae. Fig. 2 shows the lateral wall with three sublaminate layers. The plasma membrane is a phospholipid bilayer holding many particles between the inner and outer leaflets which is recently known as prestin.^18,²⁵⁾ The cytoskeleton contains parallel actin filaments crosslinked with spectrin, associated with protein 4.1 and pillars of unknown composition tethering the actin filaments to the plasma membrane.^26,²⁷⁾ The subsurface cisternae is an intracellular organelle, similar to endoplasmic reticulum or Golgi apparatus, that lines the inside of the cytoskeleton.^27,²⁸⁾
The OHC motility depends on particles of the lateral plasma membrane due to an area motor in the lateral plasma membrane, a structure in the plane of the lateral membrane that can be switched between the contraction and elongation states by a change of membrane polarization.²⁹⁾ The particles of the lateral plasma membrane are 8-10 mm in diameter with high densities ranging from about 3,000 µm-2 to 8,000 µm-2, which representing several different membrane proteins.³⁰⁾ Furthermore, the OHC motility results from an actuator because the OHC switches between the contraction and elongation states when there is external source of energy.³¹⁾ The fact that there is no internal source of energy bought a new concept of piezoelectricity in modeling OHCs, which produces electrical forces needed for electromotility at high frequencies.³²⁾

Pharmacology of OHC Motility

There are many agents which can modulate and affect the OHC motility. Modifiers of OHC motility can alter the voltage dependence of the Q-V curves.³³⁾ A number of metal ions of the lanthanide series such as gadolinium (Gd3+), lutetium (Lu3+), and lanthanum (La3+) can reduce electromotility by blocking mechanosensitive channels at micromolar concentrations.^34,³⁵⁾ A cationic peptide toxin, GsMTx4 isolated from a tarantula venom is effective against stretch-activated channels and affect the membrane motor of OHCs.³⁶⁾ Salicylate and aspirin can destroy or reduce the OHC electromotility.^37,³⁸⁾ Protein reactive agents such as specific sulfhydryl reagents, p-chloromercuriphenylsulfonic acid and p-hydroxymercuriphenylsulfonic acid inhibit longitudinal OHC movements.³⁹⁾ All reagents such as N-ethylmaleimide, dithiothreitol, and diamide showed no significant effects on the electrical properties of the OHC.³⁰⁾ However, diamide leads to changes in the mechanical properties of the OHC. Agents affecting the lipid environment of the motor such as chlorpromazine inhibit the OHC membrane voltage without affecting its magnitude and inhibits cochlear function measured in DPOAE and compound action potential in guinea pig.^24,⁴⁰⁾ Okadaic acid leads to a hyperpolarizing shift in the nonlinear capacitance while trifluoperazine and W-7 produced a depolarizing shift.⁴¹⁾ 2,3-butanedione monoxime affects nonlinear capacitance by targeting a site on the motor protein for kinases. Quinine (a well known ototoxic drug) affects in vivo electromotility of OHCs at low concentration and changes the cochlear amplifier via an effect on electro-mechanical transduction (EMT).³⁸⁾

Prestin and Motility

The presence of prestin was first identified by Dallos and colleagues who used a subtractive cloning strategy to amplify transcripts expressed in OHCs.¹¹⁾ Prestin was named after the musical notation presto meaning fast in Italian. Prestin is a member number A5 of a superfamily SLC26 of integral membrane proteins, which is a family of anion-bicarbonate transporters and characterized by a sulfate transport motif in the animo acid sequence.⁴²⁾ Prestin forms 10-12 transmembrane (TM) α-helical regions as well as cytoplasmic N- and C-termini.⁴³⁾ TM2 contains the sulfate motif defining the family while the long COOH-terminal region from amino acids 496 to 744 contains runs of both positive and negative charges as well as a sequence defined as a STAT domain. The box surrounding TM5-6 and including a phosphorylation site can be alternative structures reducing prestin from a 10- α-helix to a 12 α-helix structure. The gene expression of prestin was observed in human and various animals such as guinea pigs, rat, mice, and gerbil as well as zebrafish, eel, mosquitoes, bats, and flies. Although prestin is most closely related to SLC26A6,^43,^44,⁴⁵⁾ the human and mouse orthologues of A6 have only 78% amino acid identity while four different mammalian species such as human, mouse, rat, and gerbil have 92.7% amino acid identity.⁴⁶⁾ Prestin in mammals has several unique features differentiating from other members of the family.⁴⁵⁾ First, prestin expresses voltage-dependent charge movement and motility. Second, prestin is abundantly expressed in OHCs. Third, prestin exists as stable tetramers. Fourth, although the basic function of SLC26A members is to transport anions, it is an incomplete transporter which fails to unload a bound anion at the extracellular face of the protein⁴⁷⁾ and an electrogenic anion exchanger.¹⁸⁾
In addition, prestin is related to the unique structure of the lateral wall of the OHCs. The OHC lateral wall forming a unique trilaminate structure consisting of the plasma membrane, the cortical lattice, and the subsurface cisternae is capable of changing its length in response to transmembrane voltage change.¹⁸⁾ The electomotility results from conformational changes of membrane-bound protein molecules, prestin. Furthermore, the electromotility is affected by axial stiffness changes when the membrane potential cell is altered. The plasma membrane, the cortical lattice, and the subsurface cisternae contribute to the global axial stiffness of OHCs. The plasma membrane may be the dominant contributor to the axial stiffness of the OHCs⁴⁹⁾ while the cortical lattice may contribute to about 70% of the axial stiffness.⁵⁰⁾ The stiffness of the motor protein (prestin) is a major contributor to the global axial stiffness of OHCs.¹⁸⁾ The global stiffness of OHC consists of two components: passive and active as shown in Fig. 2. The passive components indicate longitudinal (C₁₁), circumferential (C₂₂), and mixed (C₁₂) elastic moduli of the orthotropic OHC wall while the active components include longitudinal and circumferential strain (ε_x^a and ε_θ^a).
Prestin forms motor complexes with other proteins and lipids of the lateral wall, which senses the transmembrane potential and individually generates force by changing its surface area.^34,^51,^52,^53,^54,⁵⁵⁾ The forces generated by each of the individual motors are coupled together through the lateral wall plasma membrane and cytoskeleton in order to achieve a net change in cell length.⁵⁶⁾ Because the OHC is fixed apically to the reticular lamina and basally to the cup of a Deiter's cell, electromotile shape changes can modify the vibration of the cochlear partition.^28,⁴⁷⁾ Intracellular anions (Ca²⁺ and Cl^-) can modulate prestin function and may function as the voltage sensor.^47,⁵⁷⁾ Changing the intracellular anion content inhibits electromotility and decreases the longitudinal stiffness of OHC.⁵⁸⁾

Prestin Knockout Mice

The fact that prestin is required for electromotility of the outer hair cell and for the cochlear amplifier in normal hearing was demonstrated by prestin knockout mice. Targeted deletion of prestin in mice leads to loss of OHC electromotility in vitro as well as 40-60 dB loss of cochlear sensitivity in vivo without destroying mechano-electrical transduction of OHCs.¹²⁾ In heterozygous mutant mice, the electromotility was halved and the cochlear thresholds increased two times compared to those in wild-type controls. In addition, the length of the OHC body decreased up to 60% of that of wild type controls. These results strongly suggest that prestin is a true motor protein and responsible for the cochlear amplification.
After measuring the cochlear functions of the prestin knockout mice with compound action potential, otoacoustic emissions (OAEs), and cochlear microphonic, another study demonstrated that there are elevated auditory thresholds and high thresholds of OAE. These results suggest that loss of OHC motor function affects frequency selectivity. Therefore, prestin is required for cochlear amplifier of normal hearing.⁵⁹⁾ To overcome the contraction of OHC found in heterozygous mutant mice developed by Liberman and colleagues,¹²⁾ Dallos, et al.⁶⁰⁾ created a 499 prestin knockin mouse with normal length of OHC. After obtaining both in vitro and in vivo experimental data, they concluded that prestin-based OHC motility is necessary for mammalian cochlear amplification.

Roles of Prestin in Noise-Induced Hearing Loss and Ototoxicity

The effects of an intense noise exposure (110 dB SPL at 10-20 kHz for 4 h) on the OHC motor protein (prestin) and structural proteins in the OHC membrane skeleton was investigated using gene expression and measurements of cochlear function.⁹⁾ It was reported that prestin gene expression was significantly up-regulated after an intense noise exposure with peak at 3 days after noise exposure and about 70% of the peak level at 7 days after exposure and similar levels to the control group at 28 days after exposure. The gene expression up-regulation reflects damage in the OHC lateral wall because prestin exists in the lateral membrane. Other gene expressions of both β-actin and β-spectrin were significantly up-regulated after noise exposure reflecting damage to actin and spectrin. Damage of these proteins may change stiffness of the OHC membrane cytoskeleton and affect electromotility of the OHC. Therefore, the loss of cochlear amplification after noise exposure may result from damage to OHC motor proteins and damage to the OHC membrane cytoskeleton.
Recently, it was reported that prestin may be involved in the process of aminoglycoside-induced apoptosis in OHCs.⁶¹⁾ The study showed that prestin is capable of acting as a mediator of antibiotic-induced apoptosis because of its anion-transporting capacity, which is independent of its voltage-sensing capacity. In addition, it was reported that prestin significantly enhanced the kanamycin-induced apoptosis. The effect of prestin on apoptosis depends on extracellular chloride because chloride influx may be involved in mediating the apoptosis-enhancing effect of prestin. Due to the voltage-sensing capacity, prestin can sense the influx of cations through ionophore kanamycin and result in conformational change facilitating chloride transport. On the other hand, the exogenous expression of prestin may activate endogenous chloride uptake pathways upon kanamycin treatment. Therefore, prestin is capable of playing an important role in transducing apoptotic signals in response to antibiotics.

Conclusion

After the identification of cochlear amplifier, the OHC has received a great deal of attention due to its unique features of the cells. The OHC is involved in two transduction processes: MET and EMT, called electromotility. The MET is located within the stereocilia on the apex of the OHC in both non-mammalian and mammalian species whereas the EMT originates in the lateral wall of the OHC in mammals. However, although many studies have demonstrated the role of the OHC as a cochlear amplifier for several decades, the basic mechanisms behind the cochlear amplification still remains in question and the results of many experiments of the cochlear amplification are not consistent.
Recent discovery of the motor protein, prestin of the OHC displays an important landmark of the origin of cochlear amplifier. However, in spite of research efforts identifying the exact location of cochlear amplifier, a clear and complete understanding of prestin is still not possible. Based on research literatures, we dealt with OHC electromotility and cochlear amplification, motility and unique structure of OHC, prestin and motility, prestin knockout mice, and roles of prestin in noise-induced hearing loss and ototoxicity. However, a lot of problems of prestin and cochlear amplification need to be solved. These problems include the function of prestin in different species, its means of voltage sensing, the nature of its conformational change, its roles in a variety of hearing loss, and its interaction with other structures. Many of these questions may be answered when a structural secret of the prestin is found.

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