Atomic force microscopy imaging of the structure of the motor protein prestin reconstituted into an artificial lipid bilayer

The susceptibility of cochlear outer hair cells to cyclodextrin is not related to their electromotile activity

Niemann-Pick Type C1 (NPC1) disease is a fatal neurovisceral disorder caused by dysfunction of NPC1 protein, which plays a role in intracellular cholesterol trafficking. The cholesterol-chelating agent, 2-hydroxypropyl-β-cyclodextrin (HPβCD), is currently undergoing clinical trials for treatment of this disease. Though promising in alleviating neurological symptoms, HPβCD causes irreversible hearing loss in NPC1 patients and outer hair cell (OHC) death in animal models. We recently found that HPβCD-induced OHC death can be significantly alleviated in a mouse model lacking prestin, an OHC-specific motor protein required for the high sensitivity and sharp frequency selectivity of mammalian hearing. Since cholesterol status is known to influence prestin’s electromotility, we examined how prestin contributes to HPβCD-induced OHC death in the disease context using the NPC1 knockout (KO) mouse model (NPC1-KO). We found normal expression and localization of prestin in NPC1-KO OHCs. Whole-cell patch-clamp recordings revealed a significant depolarization of the voltage-operating point of prestin in NPC1-KO mice, suggesting reduced levels of cholesterol in the lateral membrane of OHCs that lack NPC1. OHC loss and elevated thresholds were found for high frequency regions in NPC1-KO mice, whose OHCs retained their sensitivity to HPβCD. To investigate whether prestin’s electromotile function contributes to HPβCD-induced OHC death, the prestin inhibitor salicylate was co-administered with HPβCD to WT and NPC1-KO mice. Neither oral nor intraperitoneal administration of salicylate mitigated HPβCD-induced OHC loss. To further determine the contribution of prestin’s electromotile function, a mouse model expressing a virtually nonelectromotile prestin protein (499-prestin) was subjected to HPβCD treatment. 499-prestin knockin mice showed no resistance to HPβCD-induced OHC loss. As 499-prestin maintains its ability to bind cholesterol, our data imply that HPβCD-induced OHC death is ascribed to the structural role of prestin in maintaining the OHC’s lateral membrane, rather than its motor function.

Prestin Contributes to Membrane Compartmentalization and Is Required for Normal Innervation of Outer Hair Cells

Outer hair cells (OHC) act as amplifiers and their function is modified by medial olivocochlear (MOC) efferents. The unique OHC motor protein, prestin, provides the molecular basis for somatic electromotility, which is required for sensitivity and frequency selectivity, the hallmarks of mammalian hearing. Prestin proteins are the major component of the lateral membrane of mature OHCs, which separates apical and basal domains. To investigate the contribution of prestin to this unique arrangement, we compared the distribution of membrane proteins in OHCs of wildtype (WT) and prestin-knockout (KO) mice. In WT, the apical protein PMCA2 was exclusively localized to the hair bundles, while it was also found at the lateral membrane in KOs. Similarly, a basal protein KCNQ4 did not coalesce at the base of OHCs but was widely dispersed in mice lacking prestin. Since the expression levels of PMCA2 and KCNQ4 remained unchanged in KOs, the data indicate that prestin is required for the normal distribution of apical and basal membrane proteins in OHCs. Since OHC synapses predominate in the basal subnuclear region, we also examined the synaptic architecture in prestin-KO mice. Although neurite densities were not affected, MOC efferent terminals in prestin-KO mice were no longer constrained to the basal pole as in WT. This trend was evident as early as at postnatal day 12. Furthermore, terminals were often enlarged and frequently appeared as singlets when compared to the multiple clusters of individual terminals in WT. This abnormality in MOC synaptic morphology in prestin-KO mice is similar to defects in mice lacking MOC pathway proteins such as α9/α10 nicotinic acetylcholine receptors and BK channels, indicating a role for prestin in the proper establishment of MOC synapses. To investigate the contribution of prestin’s electromotility, we also examined OHCs from a mouse model that expresses non-functional prestin (499-prestin). We found no changes in PMCA2 localization and MOC synaptic morphology in OHCs from 499-prestin mice. Taken together, these results indicate that prestin, independent of its motile function, plays an important structural role in membrane compartmentalization, which is required for the formation of normal efferent-OHC synapses in mature OHCs.

Activity-dependent regulation of prestin expression in mouse outer hair cells

Prestin is a membrane protein necessary for outer hair cell (OHC) electromotility and normal hearing. Its regulatory mechanisms are unknown. Several mouse models of hearing loss demonstrate increased prestin, inspiring us to investigate how hearing loss might feedback onto OHCs. To test whether centrally mediated feedback regulates prestin, we developed a novel model of inner hair cell loss. Injection of diphtheria toxin (DT) into adult CBA mice produced significant loss of inner hair cells without affecting OHCs. Thus, DT-injected mice were deaf because they had no afferent auditory input despite OHCs continuing to receive normal auditory mechanical stimulation and having normal function. Patch-clamp experiments demonstrated no change in OHC prestin, indicating that loss of information transfer centrally did not alter prestin expression. To test whether local mechanical feedback regulates prestin, we used Tecta(C1509G) mice, where the tectorial membrane is malformed and only some OHCs are stimulated. OHCs connected to the tectorial membrane had normal prestin levels, whereas OHCs not connected to the tectorial membrane had elevated prestin levels, supporting an activity-dependent model. To test whether the endocochlear potential was necessary for prestin regulation, we studied Tecta(C1509G) mice at different developmental ages. OHCs not connected to the tectorial membrane had lower than normal prestin levels before the onset of the endocochlear potential and higher than normal prestin levels after the onset of the endocochlear potential. Taken together, these data indicate that OHC prestin levels are regulated through local feedback that requires mechanoelectrical transduction currents. This adaptation may serve to compensate for variations in the local mechanical environment.

Prestin at year 14: progress and prospect

Prestin, the motor protein of cochlear outer hair cells, was identified 14 years ago. Prestin-based outer hair cell motility is responsible for the exquisite sensitivity and frequency selectivity seen in the mammalian cochlea. Prestin is the 5th member of an eleven-member membrane transporter superfamily of SLC26A proteins. Unlike its paralogs, which are capable of transporting anions across the cell membrane, prestin primarily functions as a motor protein with unique capability of performing direct and reciprocal electromechanical conversion on microsecond time scale. Significant progress in the understanding of its structure and the molecular mechanism has been made in recent years using electrophysiological, biochemical, comparative genomics, structural bioinformatics, molecular dynamics simulation, site-directed mutagenesis and domain-swapping techniques. This article reviews recent advances of the structural and functional properties of prestin with focus on the areas that are critical but still controversial in understanding the molecular mechanism of how prestin works: The structural domains for voltage sensing and interaction with anions and for conformational change. Future research directions and potential application of prestin are also discussed. This article is part of a Special Issue entitled <Annual Reviews 2014>.

Imaging of transmembrane proteins directly incorporated within supported lipid bilayers using atomic force microscopy

Structural analysis of transmembrane proteins remains a challenge in biology, mainly due to their difficulty in being overexpressed and the required use of detergents that impair different steps of biochemistry classically used to obtain 3D crystals. In this context, we have developed a new technique for protein incorporation within supported lipid bilayers that only requires a few picomoles of protein per assay. Proteins are directly inserted into a detergent-destabilized bilayer that can be imaged in buffer with atomic force microscopy (AFM) allowing structural analysis down to sub-nanometer lateral resolution. In this chapter, we describe the main guidelines for this technique, from the choice of detergent to the requirements for AFM high-resolution imaging.