Hair bundles are specialized for ATP delivery via creatine kinase

Myosin-based nucleation of actin filaments contributes to stereocilia development critical for hearing

Assembly of actin-based stereocilia is critical for cochlear hair cells to detect sound. To tune their mechanosensivity, stereocilia form bundles composed of graded rows of ascending height, necessitating the precise control of actin polymerization. Myosin 15 (MYO15A) drives hair bundle development by delivering critical proteins to growing stereocilia that regulate actin polymerization via an unknown mechanism. Here, we show that MYO15A is itself an actin nucleation-promoting factor. Moreover, a deafness-causing mutation in the MYO15A actin-binding interface inhibits nucleation activity but still preserves some movement on filaments in vitro and partial trafficking on stereocilia in vivo. Stereocilia fail to elongate correctly in this mutant mouse, providing evidence that MYO15A-driven actin nucleation contributes to hair bundle biogenesis. Our work shows that in addition to generating force and motility, the ATPase domain of MYO15A can directly regulate actin polymerization and that disrupting this activity can promote cytoskeletal disease, such as hearing loss.

Molecular and functional profiling of cell diversity and identity in the lateral superior olive, an auditory brainstem center with ascending and descending projections

The lateral superior olive (LSO), a prominent integration center in the auditory brainstem, contains a remarkably heterogeneous population of neurons. Ascending neurons, predominantly principal neurons (pLSOs), process interaural level differences for sound localization. Descending neurons (lateral olivocochlear neurons, LOCs) provide feedback into the cochlea and are thought to protect against acoustic overload. The molecular determinants of the neuronal diversity in the LSO are largely unknown. Here, we used patch-seq analysis in mice at postnatal days P10-12 to classify developing LSO neurons according to their functional and molecular profiles. Across the entire sample (n = 86 neurons), genes involved in ATP synthesis were particularly highly expressed, confirming the energy expenditure of auditory neurons. Two clusters were identified, pLSOs and LOCs. They were distinguished by 353 differentially expressed genes (DEGs), most of which were novel for the LSO. Electrophysiological analysis confirmed the transcriptomic clustering. We focused on genes affecting neuronal input-output properties and validated some of them by immunohistochemistry, electrophysiology, and pharmacology. These genes encode proteins such as osteopontin, Kv11.3, and Kvβ3 (pLSO-specific), calcitonin-gene-related peptide (LOC-specific), or Kv7.2 and Kv7.3 (no DEGs). We identified 12 "Super DEGs" and 12 genes showing "Cluster similarity." Collectively, we provide fundamental and comprehensive insights into the molecular composition of individual ascending and descending neurons in the juvenile auditory brainstem and how this may relate to their specific functions, including developmental aspects.

SIRT3/GLUT4 signaling activation by metformin protect against cisplatin-induced ototoxicity in vitro

Cisplatin is highly effective for killing tumor cells. However, as one of its side effects, ototoxicity limits the clinical application of cisplatin. The mechanisms of cisplatin-induced ototoxicity have not been fully clarified yet. SIRT3 is a deacetylated protein mainly located in mitochondria, which regulates a variety of physiological processes in cells. The role of SIRT3 in cisplatin-induced hair cell injury has not been founded. In this study, primary cultured cochlear explants exposed to 5 μM cisplatin, as well as OC-1 cells exposed to 10 μM cisplatin, were used to establish models of cisplatin-induced ototoxicity in vitro. We found that when combined with cisplatin, metformin (75 μM) significantly up-regulated the expression of SIRT3 and alleviated cisplatin-induced apoptosis of hair cells. We regulated the expression of SIRT3 to explore the role of SIRT3 in cisplatin-induced auditory hair cell injury. Overexpression of SIRT3 promoted the survival of auditory hair cells and alleviated the apoptosis of auditory hair cells. In contrast, knockdown of SIRT3 impaired the protective effect of metformin and exacerbated cisplatin injury. In addition, we found that the protective effect of SIRT3 may be achieved by regulating GLUT4 translocation and rescuing impaired glucose uptake caused by cisplatin. Our study confirmed that upregulation of SIRT3 may antagonize cisplatin-induced ototoxicity, and provided a new perspective for the study of cisplatin-induced ototoxicity.

In vivo investigation of mitochondria in lateral line afferent neurons and hair cells

To process sensory stimuli, intense energy demands are placed on hair cells and primary afferents. Hair cells must both mechanotransduce and maintain pools of synaptic vesicles for neurotransmission. Furthermore, both hair cells and afferent neurons must continually maintain a polarized membrane to propagate sensory information. These processes are energy demanding and therefore both cell types are critically reliant on mitochondrial health and function for their activity and maintenance. Based on these demands, it is not surprising that deficits in mitochondrial health can negatively impact the auditory and vestibular systems. In this review, we reflect on how mitochondrial function and dysfunction are implicated in hair cell-mediated sensory system biology. Specifically, we focus on live imaging approaches that have been applied to study mitochondria using the zebrafish lateral-line system. We highlight the fluorescent dyes and genetically encoded biosensors that have been used to study mitochondria in lateral-line hair cells and afferent neurons. We then describe the impact this in vivo work has had on the field of mitochondrial biology as well as the relationship between mitochondria and sensory system development, function, and survival. Finally, we delineate the areas in need of further exploration. This includes in vivo analyses of mitochondrial dynamics and biogenesis, which will round out our understanding of mitochondrial biology in this sensitive sensory system.

Mitochondrial form and function in hair cells

Hair cells (HCs) are specialised sensory receptors residing in the neurosensory epithelia of inner ear sense organs. The precise morphological and physiological properties of HCs allow us to perceive sound and interact with the world around us. Mitochondria play a significant role in normal HC function and are also intricately involved in HC death. They generate ATP essential for sustaining the activity of ion pumps, Ca²⁺ transporters and the integrity of the stereociliary bundle during transduction as well as regulating cytosolic calcium homoeostasis during synaptic transmission. Advances in imaging techniques have allowed us to study mitochondrial populations throughout the HC, and how they interact with other organelles. These analyses have identified distinct mitochondrial populations between the apical and basolateral portions of the HC, in which mitochondrial morphology appears determined by the physiological processes in the different cellular compartments. Studies in HCs across species show that ototoxic agents, ageing and noise damage directly impact mitochondrial structure and function resulting in HC death. Deciphering the molecular mechanisms underlying this mitochondrial sensitivity, and how their morphology relates to their function during HC death, requires that we first understand this relationship in the context of normal HC function.

Structural and functional diversity of mitochondria in vestibular/cochlear hair cells and vestibular calyx afferents

Mitochondria supply energy in the form of ATP to drive a plethora of cellular processes. In heart and liver cells, mitochondria occupy over 20% of the cellular volume and the major need for ATP is easily identifiable - i.e., to drive cross-bridge recycling in cardiac cells or biosynthetic machinery in liver cells. In vestibular and cochlear hair cells the overall cellular mitochondrial volume is much less, and mitochondria structure varies dramatically in different regions of the cell. The regional demands for ATP and cellular forces that govern mitochondrial structure and localization are not well understood. Below we review our current understanding of the heterogeneity of form and function in hair cell mitochondria. A particular focus of this review will be on regional specialization in vestibular hair cells, where large mitochondria are found beneath the cuticular plate in close association with the striated organelle. Recent findings on the role of mitochondria in hair cell death and aging are covered along with potential therapeutic approaches. Potential avenues for future research are discussed, including the need for integrated computational modeling of mitochondrial function in hair cells and the vestibular afferent calyx.

Brain type of creatine kinase induces doxorubicin resistance via TGF-β signaling in MDA-MB-231 breast cancer cells

Brain type of creatine kinase (CKB) regulates energy homeostasis by reversibly transferring phosphate groups between phosphocreatine and ATP at sites of high energy demand. Several types of cancer cells exhibit upregulated CKB expression, but the function of CKB in cancer cells remains unclear. In this study, we investigated the function of CKB in breast cancer by overexpressing CKB in MDA-MB-231 cells. The overexpression of CKB did not affect cell growth rate, cell cycle distribution, ATP level or key mediators of aerobic glycolysis and lactate dehydrogenase isoform levels. Meanwhile, CKB overexpression did increase resistance to doxorubicin. TGF-β-induced Smad phosphorylation and Smad-dependent transcriptional activity were significantly up-regulated by CKB expression without changes in inhibitory Smad protein levels. Moreover, treatment with TGF-β considerably enhanced cell viability during doxorubicin treatment and decreased doxorubicin-induced apoptosis in CKB-expressing MDA-MB-231 cells compared to control cells. These results suggest that CKB attenuates doxorubicin-induced apoptosis and potentiates resistance to doxorubicin by enhancing TGF-β signaling in MDA-MB-231 cells.

Structural basis for tunable control of actin dynamics by myosin-15 in mechanosensory stereocilia

The motor protein myosin-15 is necessary for the development and maintenance of mechanosensory stereocilia, and mutations in myosin-15 cause hereditary deafness. In addition to transporting actin regulatory machinery to stereocilia tips, myosin-15 directly nucleates actin filament ("F-actin") assembly, which is disrupted by a progressive hearing loss mutation (p.D1647G, "jordan"). Here, we present cryo-electron microscopy structures of myosin-15 bound to F-actin, providing a framework for interpreting the impacts of deafness mutations on motor activity and actin nucleation. Rigor myosin-15 evokes conformational changes in F-actin yet maintains flexibility in actin's D-loop, which mediates inter-subunit contacts, while the jordan mutant locks the D-loop in a single conformation. Adenosine diphosphate-bound myosin-15 also locks the D-loop, which correspondingly blunts actin-polymerization stimulation. We propose myosin-15 enhances polymerization by bridging actin protomers, regulating nucleation efficiency by modulating actin's structural plasticity in a myosin nucleotide state-dependent manner. This tunable regulation of actin polymerization could be harnessed to precisely control stereocilium height.

Bioinformatic Analysis of the Perilymph Proteome to Generate a Human Protein Atlas

The high complexity of the cellular architecture of the human inner ear and the inaccessibility for tissue biopsy hampers cellular and molecular analysis of inner ear disease. Sampling and analysis of perilymph may present an opportunity for improved diagnostics and understanding of human inner ear pathology. Analysis of the perilymph proteome from patients undergoing cochlear implantation was carried out revealing a multitude of proteins and patterns of protein composition that may enable characterisation of patients into subgroups. Based on existing data and databases, single proteins that are not present in the blood circulation were related to cells within the cochlea to allow prediction of which cells contribute to the individual perilymph proteome of the patients. Based on the results, we propose a human atlas of the cochlea. Finally, druggable targets within the perilymph proteome were identified. Understanding and modulating the human perilymph proteome will enable novel avenues to improve diagnosis and treatment of inner ear diseases.

Hearing Loss in Neurological Disorders

Sensorineural hearing loss (SNHL) affects approximately 466 million people worldwide, which is projected to reach 900 million by 2050. Its histological characteristics are lesions in cochlear hair cells, supporting cells, and auditory nerve endings. Neurological disorders cover a wide range of diseases affecting the nervous system, including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), autism spectrum disorder (ASD), etc. Many studies have revealed that neurological disorders manifest with hearing loss, in addition to typical nervous symptoms. The prevalence, manifestations, and neuropathological mechanisms underlying vary among different diseases. In this review, we discuss the relevant literature, from clinical trials to research mice models, to provide an overview of auditory dysfunctions in the most common neurological disorders, particularly those associated with hearing loss, and to explain their underlying pathological and molecular mechanisms.

Recent progress in developing fluorescent probes for imaging cell metabolites

Cellular metabolites play a crucial role in promoting and regulating cellular activities, but it has been difficult to monitor these cellular metabolites in living cells and in real time. Over the past decades, iterative development and improvements of fluorescent probes have been made, resulting in the effective monitoring of metabolites. In this review, we highlight recent progress in the use of fluorescent probes for tracking some key metabolites, such as adenosine triphosphate, cyclic adenosine monophosphate, cyclic guanosine 5'-monophosphate, Nicotinamide adenine dinucleotide (NADH), reactive oxygen species, sugar, carbon monoxide, and nitric oxide for both whole cell and subcellular imaging.

Single-cell proteomics reveals changes in expression during hair-cell development

Hearing and balance rely on small sensory hair cells that reside in the inner ear. To explore dynamic changes in the abundant proteins present in differentiating hair cells, we used nanoliter-scale shotgun mass spectrometry of single cells, each ~1 picoliter, from utricles of embryonic day 15 chickens. We identified unique constellations of proteins or protein groups from presumptive hair cells and from progenitor cells. The single-cell proteomes enabled the de novo reconstruction of a developmental trajectory using protein expression levels, revealing proteins that greatly increased in expression during differentiation of hair cells (e.g., OCM, CRABP1, GPX2, AK1, GSTO1) and those that decreased during differentiation (e.g., TMSB4X, AGR3). Complementary single-cell transcriptome profiling showed corresponding changes in mRNA during maturation of hair cells. Single-cell proteomics data thus can be mined to reveal features of cellular development that may be missed with transcriptomics.

Synaptic mitochondria regulate hair-cell synapse size and function

Sensory hair cells in the ear utilize specialized ribbon synapses. These synapses are defined by electron-dense presynaptic structures called ribbons, composed primarily of the structural protein Ribeye. Previous work has shown that voltage-gated influx of Ca2+ through CaV1.3 channels is critical for hair-cell synapse function and can impede ribbon formation. We show that in mature zebrafish hair cells, evoked presynaptic-Ca2+ influx through CaV1.3 channels initiates mitochondrial-Ca2+ (mito-Ca2+) uptake adjacent to ribbons. Block of mito-Ca2+ uptake in mature cells depresses presynaptic-Ca2+ influx and impacts synapse integrity. In developing zebrafish hair cells, mito-Ca2+ uptake coincides with spontaneous rises in presynaptic-Ca2+ influx. Spontaneous mito-Ca2+ loading lowers cellular NAD+/NADH redox and downregulates ribbon size. Direct application of NAD+ or NADH increases or decreases ribbon size respectively, possibly acting through the NAD(H)-binding domain on Ribeye. Our results present a mechanism where presynaptic- and mito-Ca2+ couple to confer proper presynaptic function and formation.

Effects of Ageing on the Mitochondrial Genome in Rat Vestibular Organs

Background:

Deterioration in vestibular function occurs with ageing and is linked to age-related falls. Sensory hair cells located in the inner ear vestibular labyrinth are critical to vestibular function. Vestibular hair cells rely predominantly on oxidative phosphorylation (OXPHOS) for ener-gy production and contain numerous mitochondria. Mitochondrial DNA (mtDNA) mutations and perturbed energy production are associated with the ageing process.

Objective:

We investigated the effects of ageing on mtDNA in vestibular hair and support cells, and vestibular organ gene expression, to better understand mechanisms of age-related vestibular deficits.

Methods:

Vestibular hair and supporting cell layers were microdissected from young and old rats, and mtDNA was quantified by qPCR. Additionally, vestibular organ gene expression was analysed by microarray and gene set enrichment analyses.

Results:

In contrast to most other studies, we found no evidence of age-related mtDNA deletion mu-tations. However, we found an increase in abundance of major arc genes near the mtDNA control re-gion. There was also a marked age-related reduction in mtDNA copy number in both cell types. Ves-tibular organ gene expression, gene set enrichment analysis showed the OXPHOS pathway was down regulated in old animals.

Conclusion:

Given the importance of mtDNA to mitochondrial OXPHOS and hair cell function, our findings suggest the vestibular organs are potentially on the brink of an energy crisis in old animals

LMO7 deficiency reveals the significance of the cuticular plate for hearing function

Sensory hair cells, the mechanoreceptors of the auditory and vestibular systems, harbor two specialized elaborations of the apical surface, the hair bundle and the cuticular plate. In contrast to the extensively studied mechanosensory hair bundle, the cuticular plate is not as well understood. It is believed to provide a rigid foundation for stereocilia motion, but specifics about its function, especially the significance of its integrity for long-term maintenance of hair cell mechanotransduction, are not known. We discovered that a hair cell protein called LIM only protein 7 (LMO7) is specifically localized in the cuticular plate and the cell junction. Lmo7 KO mice suffer multiple cuticular plate deficiencies, including reduced filamentous actin density and abnormal stereociliar rootlets. In addition to the cuticular plate defects, older Lmo7 KO mice develop abnormalities in inner hair cell stereocilia. Together, these defects affect cochlear tuning and sensitivity and give rise to late-onset progressive hearing loss.

Natural Products Containing 'Rare' Organophosphorus Functional Groups

Phosphorous-containing molecules are essential constituents of all living cells. While the phosphate functional group is very common in small molecule natural products, nucleic acids, and as chemical modification in protein and peptides, phosphorous can form P⁻N (phosphoramidate), P⁻S (phosphorothioate), and P⁻C (e.g., phosphonate and phosphinate) linkages. While rare, these moieties play critical roles in many processes and in all forms of life. In this review we thoroughly categorize P⁻N, P⁻S, and P⁻C natural organophosphorus compounds. Information on biological source, biological activity, and biosynthesis is included, if known. This review also summarizes the role of phosphorylation on unusual amino acids in proteins (N- and S-phosphorylation) and reviews the natural phosphorothioate (P⁻S) and phosphoramidate (P⁻N) modifications of DNA and nucleotides with an emphasis on their role in the metabolism of the cell. We challenge the commonly held notion that nonphosphate organophosphorus functional groups are an oddity of biochemistry, with no central role in the metabolism of the cell. We postulate that the extent of utilization of some phosphorus groups by life, especially those containing P⁻N bonds, is likely severely underestimated and has been largely overlooked, mainly due to the technological limitations in their detection and analysis.

Molecular Composition of Vestibular Hair Bundles

The vertebrate hair bundle, responsible for transduction of mechanical signals into receptor potentials in sensory hair cells, is an evolutionary masterpiece. Composed of actin-filled stereocilia of precisely regulated length, width, and number, the structure of the hair bundle is optimized for sensing auditory and vestibular stimuli. Recent developments in identifying the lipids and proteins constituting the hair bundle, obtained through genetics, biochemistry, and imaging, now permit a description of the consensus composition of vestibular bundles of mouse, rat, and chick.

Mitochondrial Proteolipid Complexes of Creatine Kinase

Isoforms of creatine kinase (CK) generate and use phosphocreatine, a concentrated and highly diffusible cellular "high energy" intermediate, for the main purpose of energy buffering and transfer in order to maintain cellular energy homeostasis. The mitochondrial CK isoform (mtCK) localizes to the mitochondrial intermembrane and cristae space, where it assembles into peripherally membrane-bound, large cuboidal homooctamers. These are part of proteolipid complexes wherein mtCK directly interacts with cardiolipin and other anionic phospholipids, as well as with the VDAC channel in the outer membrane. This leads to a stabilization and cross-linking of inner and outer mitochondrial membrane, forming so-called contact sites. Also the adenine nucleotide translocator of the inner membrane can be recruited into these proteolipid complexes, probably mediated by cardiolipin. The complexes have functions mainly in energy transfer to the cytosol and stimulation of oxidative phosphorylation, but also in restraining formation of reactive oxygen species and apoptosis. In vitro evidence indicates a putative role of mtCK in mitochondrial phospholipid distribution, and most recently a role in thermogenesis has been proposed. This review summarizes the essential structural and functional data of these mtCK complexes and describes in more detail the more recent advances in phospholipid interaction, thermogenesis, cancer and evolution of mtCK.

Genome-wide identification of directed gene networks using large-scale population genomics data

Identification of causal drivers behind regulatory gene networks is crucial in understanding gene function. Here, we develop a method for the large-scale inference of gene-gene interactions in observational population genomics data that are both directed (using local genetic instruments as causal anchors, akin to Mendelian Randomization) and specific (by controlling for linkage disequilibrium and pleiotropy). Analysis of genotype and whole-blood RNA-sequencing data from 3072 individuals identified 49 genes as drivers of downstream transcriptional changes (Wald P < 7 × 10-10), among which transcription factors were overrepresented (Fisher's P = 3.3 × 10-7). Our analysis suggests new gene functions and targets, including for SENP7 (zinc-finger genes involved in retroviral repression) and BCL2A1 (target genes possibly involved in auditory dysfunction). Our work highlights the utility of population genomics data in deriving directed gene expression networks. A resource of trans-effects for all 6600 genes with a genetic instrument can be explored individually using a web-based browser.

The advantage of channeling nucleotides for very processive functions

Nucleoside triphosphate (NTP)s, like ATP (adenosine 5'-triphosphate) and GTP (guanosine 5'-triphosphate), have long been considered sufficiently concentrated and diffusible to fuel all cellular ATPases (adenosine triphosphatases) and GTPases (guanosine triphosphatases) in an energetically healthy cell without becoming limiting for function. However, increasing evidence for the importance of local ATP and GTP pools, synthesised in close proximity to ATP- or GTP-consuming reactions, has fundamentally challenged our view of energy metabolism. It has become evident that cellular energy metabolism occurs in many specialised 'microcompartments', where energy in the form of NTPs is transferred preferentially from NTP-generating modules directly to NTP-consuming modules. Such energy channeling occurs when diffusion through the cytosol is limited, where these modules are physically close and, in particular, if the NTP-consuming reaction has a very high turnover, i.e. is very processive. Here, we summarise the evidence for these conclusions and describe new insights into the physiological importance and molecular mechanisms of energy channeling gained from recent studies. In particular, we describe the role of glycolytic enzymes for axonal vesicle transport and nucleoside diphosphate kinases for the functions of dynamins and dynamin-related GTPases.

The advantage of channeling nucleotides for very processive functions

Nucleoside triphosphate (NTP)s, like ATP (adenosine 5'-triphosphate) and GTP (guanosine 5'-triphosphate), have long been considered sufficiently concentrated and diffusible to fuel all cellular ATPases (adenosine triphosphatases) and GTPases (guanosine triphosphatases) in an energetically healthy cell without becoming limiting for function. However, increasing evidence for the importance of local ATP and GTP pools, synthesised in close proximity to ATP- or GTP-consuming reactions, has fundamentally challenged our view of energy metabolism. It has become evident that cellular energy metabolism occurs in many specialised 'microcompartments', where energy in the form of NTPs is transferred preferentially from NTP-generating modules directly to NTP-consuming modules. Such energy channeling occurs when diffusion through the cytosol is limited, where these modules are physically close and, in particular, if the NTP-consuming reaction has a very high turnover, i.e. is very processive. Here, we summarise the evidence for these conclusions and describe new insights into the physiological importance and molecular mechanisms of energy channeling gained from recent studies. In particular, we describe the role of glycolytic enzymes for axonal vesicle transport and nucleoside diphosphate kinases for the functions of dynamins and dynamin-related GTPases.

Development and regeneration of vestibular hair cells in mammals

Vestibular sensation is essential for gaze stabilization, balance, and perception of gravity. The vestibular receptors in mammals, Type I and Type II hair cells, are located in five small organs in the inner ear. Damage to hair cells and their innervating neurons can cause crippling symptoms such as vertigo, visual field oscillation, and imbalance. In adult rodents, some Type II hair cells are regenerated and become re-innervated after damage, presenting opportunities for restoring vestibular function after hair cell damage. This article reviews features of vestibular sensory cells in mammals, including their basic properties, how they develop, and how they are replaced after damage. We discuss molecules that control vestibular hair cell regeneration and highlight areas in which our understanding of development and regeneration needs to be deepened.

Analysis of the Proteome of Hair-Cell Stereocilia by Mass Spectrometry

Characterization of proteins that mediate mechanotransduction by hair cells, the sensory cells of the inner ear, is hampered by the scarcity of these cells and their sensory organelle, the hair bundle. Mass spectrometry, with its high sensitivity and identification precision, is the ideal method for determining which proteins are present in bundles and what proteins they interact with. We describe here the isolation of mouse hair bundles, as well as preparation of bundle protein samples for mass spectrometry. We also describe protocols for data-dependent (shotgun) and parallel reaction monitoring (targeted) mass spectrometry that allow us to identify and quantify proteins of the hair bundle. These sensitive methods are particularly useful for comparing proteomes of wild-type mice and mice with deafness mutations affecting hair-bundle proteins.

The Transcriptomics to Proteomics of Hair Cell Regeneration: Looking for a Hair Cell in a Haystack

Mature mammals exhibit very limited capacity for regeneration of auditory hair cells, while all non-mammalian vertebrates examined can regenerate them. In an effort to find therapeutic targets for deafness and balance disorders, scientists have examined gene expression patterns in auditory tissues under different developmental and experimental conditions. Microarray technology has allowed the large-scale study of gene expression profiles (transcriptomics) at whole-genome levels, but since mRNA expression does not necessarily correlate with protein expression, other methods, such as microRNA analysis and proteomics, are needed to better understand the process of hair cell regeneration. These technologies and some of the results of them are discussed in this review. Although there is a considerable amount of variability found between studies owing to different species, tissues and treatments, there is some concordance between cellular pathways important for hair cell regeneration. Since gene expression and proteomics data is now commonly submitted to centralized online databases, meta-analyses of these data may provide a better picture of pathways that are common to the process of hair cell regeneration and lead to potential therapeutics. Indeed, some of the proteins found to be regulated in the inner ear of animal models (e.g., IGF-1) have now gone through human clinical trials.

PDZD7-MYO7A complex identified in enriched stereocilia membranes

While more than 70 genes have been linked to deafness, most of which are expressed in mechanosensory hair cells of the inner ear, a challenge has been to link these genes into molecular pathways. One example is Myo7a (myosin VIIA), in which deafness mutations affect the development and function of the mechanically sensitive stereocilia of hair cells. We describe here a procedure for the isolation of low-abundance protein complexes from stereocilia membrane fractions. Using this procedure, combined with identification and quantitation of proteins with mass spectrometry, we demonstrate that MYO7A forms a complex with PDZD7, a paralog of USH1C and DFNB31. MYO7A and PDZD7 interact in tissue-culture cells, and co-localize to the ankle-link region of stereocilia in wild-type but not Myo7a mutant mice. Our data thus describe a new paradigm for the interrogation of low-abundance protein complexes in hair cell stereocilia and establish an unanticipated link between MYO7A and PDZD7.

Imaging Adenosine Triphosphate (ATP)

Adenosine triphosphate (ATP) is a universal mediator of metabolism and signaling across unicellular and multicellular species. There is a fundamental interdependence between the dynamics of ATP and the physiology that occurs inside and outside the cell. Characterizing and understanding ATP dynamics provide valuable mechanistic insight into processes that range from neurotransmission to the chemotaxis of immune cells. Therefore, we require the methodology to interrogate both temporal and spatial components of ATP dynamics from the subcellular to the organismal levels in live specimens. Over the last several decades, a number of molecular probes that are specific to ATP have been developed. These probes have been combined with imaging approaches, particularly optical microscopy, to enable qualitative and quantitative detection of this critical molecule. In this review, we survey current examples of technologies available for visualizing ATP in living cells, and identify areas where new tools and approaches are needed to expand our capabilities.

Noisy galvanic vestibular stimulation enhances spatial memory in cognitive impairment-induced by intracerebroventricular-streptozotocin administration

There are several anatomical connections between vestibular system and brain areas construct spatial memory. Since subliminal noisy galvanic vestibular stimulation (GVS) has been demonstrated to enhance some types of memory, we speculated that application of noisy GVS may improve spatial memory in a rat model of intracerebroventricular streptozotocin (ICV-STZ)-induced cognitive impairment. Moreover, we attempted to determine the effect of repeated exposure to GVS on spatial memory performance. The spatial memory was assessed using Morris water maze test. The groups received 1 (ICV-STZ/GVS-I) or 5 (ICV-STZ/GVS-II) sessions, each lasting 30 min, of low amplitude noisy GVS, or no GVS at all (Control, ICV-saline, ICV-STZ/noGVS). Hippocampal morphological changes investigated with cresyl violet staining and the immediate early gene product c-Fos, as a neuronal activity marker, was measured. Hippocampal c-Fos positive cells increased in both GVS stimulated groups. We observed significantly improved spatial performance only in ICV-STZ/GVS-II group. Histological evaluation showed normal density in ICV-STZ/GVS-II group whereas degeneration observed in ICV-STZ/GVS-I group similar to ICV-STZ/noGVS. The results showed the improvement of memory impairment after repeated exposure to GVS. This effect may be due in part to frequent activation of the vestibular neurons and the hippocampal regions connected to them. Our current study suggests the potential role of GVS as a practical method to combat cognitive decline induced by sporadic Alzheimer disease.

Creatine kinase in cell cycle regulation and cancer

The phosphocreatine-creatine kinase (CK) shuttle system is increasingly recognized as a fundamental mechanism for ATP homeostasis in both excitable and non-excitable cells. Many intracellular processes are ATP dependent. Cell division is a process requiring a rapid rate of energy turnover. Cell cycle regulation is also a key point to understanding the mechanisms underlying cancer progression. It has been known for about 40 years that aberrant CK levels are associated with various cancers and for over 30 years that CK is involved in mitosis regulation. However, the underlying molecular mechanisms have not been investigated sufficiently until recently. By maintaining ATP at sites of high-energy demand, CK can regulate cell cycle progression by affecting the intracellular energy status as well as by influencing signaling pathways that are essential to activate cell division and cytoskeleton reorganization. Aberrant CK levels may impair cell viability under normal or stressed conditions and induce cell death. The involvement of CK in cell cycle regulation and cellular energy metabolism makes it a potential diagnostic biomarker and therapeutic target in cancer. To understand the multiple physiological/pathological functions of CK, it is necessary to identify CK-binding partners and regulators including proteins, non-coding RNAs and participating endogenous small molecular weight chemical compounds. This review will focus on molecular mechanisms of CK in cell cycle regulation and cancer progression. It will also discuss the implications of recent mechanistic studies, the emerging problems and future challenges of the multifunctional enzyme CK.

Mitochondrial fission protein Drp1 regulates mitochondrial transport and dendritic arborization in cerebellar Purkinje cells

Mitochondria dynamically change their shape by repeated fission and fusion in response to physiological and pathological conditions. Recent studies have uncovered significant roles of mitochondrial fission and fusion in neuronal functions, such as neurotransmission and spine formation. However, the contribution of mitochondrial fission to the development of dendrites remains controversial. We analyzed the function of the mitochondrial fission GTPase Drp1 in dendritic arborization in cerebellar Purkinje cells. Overexpression of a dominant-negative mutant of Drp1 in postmitotic Purkinje cells enlarged and clustered mitochondria, which failed to exit from the soma into the dendrites. The emerging dendrites lacking mitochondrial transport remained short and unstable in culture and in vivo. The dominant-negative Drp1 affected neither the basal respiratory function of mitochondria nor the survival of Purkinje cells. Enhanced ATP supply by creatine treatment, but not reduced ROS production by antioxidant treatment, restored the hypomorphic dendrites caused by inhibition of Drp1 function. Collectively, our results suggest that Drp1 is required for dendritic distribution of mitochondria and thereby regulates energy supply in growing dendritic branches in developing Purkinje cells.

Hair-bundle proteomes of avian and mammalian inner-ear utricles

Examination of multiple proteomics datasets within or between species increases the reliability of protein identification. We report here proteomes of inner-ear hair bundles from three species (chick, mouse, and rat), which were collected on LTQ or LTQ Velos ion-trap mass spectrometers; the constituent proteins were quantified using MS2 intensities, which are the summed intensities of all peptide fragmentation spectra matched to a protein. The data are available via ProteomeXchange with identifiers PXD002410 (chick LTQ), PXD002414 (chick Velos), PXD002415 (mouse Velos), and PXD002416 (rat LTQ). The two chick bundle datasets compared favourably to a third, already-described chick bundle dataset, which was quantified using MS1 peak intensities, the summed intensities of peptides identified by high-resolution mass spectrometry (PXD000104; updated analysis in PXD002445). The mouse bundle dataset described here was comparable to a different mouse bundle dataset quantified using MS1 intensities (PXD002167). These six datasets will be useful for identifying the core proteome of vestibular hair bundles.

Proteomics studies in inner ear disorders: pathophysiology and biomarkers

Although proteomics has been exploited in a wide range of diseases for identification of biomarkers and pathophysiological mechanisms, there are still biomedical disciplines such as otology where proteomics platforms are underused due to technical challenges and/or complex features of the disease. Thus, in the past few years, healthcare and scientific agencies have advocated the development and adoption of proteomic technologies in otological research. However, few studies have been conducted and limited literature is available in this area. Here, we present the state of the art of proteomics in otology, discussing the substantial evidence from recent experimental models and clinical studies in inner-ear conditions. We also delineate a series of critical issues including minute size of the inner ear, delicacy and poor accessibility of tissue that researchers face while undertaking otology proteomics research. Furthermore, we provide perspective to enhance the impact and lead to the clinical implementation of these proteomics-based strategies.

The proteome of mouse vestibular hair bundles over development

Development of the vertebrate hair bundle is a precisely orchestrated event that culminates in production of a tightly ordered arrangement of actin-rich stereocilia and a single axonemal kinocilium. To understand how the protein composition of the bundle changes during development, we isolated bundles from young (postnatal days P4-P6) and mature (P21-P25) mouse utricles using the twist-off method, then characterized their constituent proteins using liquid-chromatography tandem mass spectrometry with data-dependent acquisition. Using MaxQuant and label-free quantitation, we measured relative abundances of proteins in both bundles and in the whole utricle; comparison of protein abundance between the two fractions allows calculation of enrichment in bundles. These data, which are available via ProteomeXchange with identifier PXD002167, will be useful for examining the proteins present in mammalian vestibular bundles and how their concentrations change over development.

Synergistic action of dendritic mitochondria and creatine kinase maintains ATP homeostasis and actin dynamics in growing neuronal dendrites

The distribution of mitochondria within mature, differentiated neurons is clearly adapted to their regional physiological needs and can be perturbed under various pathological conditions, but the function of mitochondria in developing neurons has been less well studied. We have studied mitochondrial distribution within developing mouse cerebellar Purkinje cells and have found that active delivery of mitochondria into their dendrites is a prerequisite for proper dendritic outgrowth. Even when mitochondria in the Purkinje cell bodies are functioning normally, interrupting the transport of mitochondria into their dendrites severely disturbs dendritic growth. Additionally, we find that the growth of atrophic dendrites lacking mitochondria can be rescued by activating ATP-phosphocreatine exchange mediated by creatine kinase (CK). Conversely, inhibiting cytosolic CKs decreases dendritic ATP levels and also disrupts dendrite development. Mechanistically, this energy depletion appears to perturb normal actin dynamics and enhance the aggregation of cofilin within growing dendrites, reminiscent of what occurs in neurons overexpressing the dephosphorylated form of cofilin. These results suggest that local ATP synthesis by dendritic mitochondria and ATP-phosphocreatine exchange act synergistically to sustain the cytoskeletal dynamics necessary for dendritic development.

A short splice form of Xin-actin binding repeat containing 2 (XIRP2) lacking the Xin repeats is required for maintenance of stereocilia morphology and hearing function

Approximately one-third of known deafness genes encode proteins located in the hair bundle, the sensory hair cell's mechanoreceptive organelle. In previous studies, we used mass spectrometry to characterize the hair bundle's proteome, resulting in the discovery of novel bundle proteins. One such protein is Xin-actin binding repeat containing 2 (XIRP2), an actin-cross-linking protein previously reported to be specifically expressed in striated muscle. Because mutations in other actin-cross-linkers result in hearing loss, we investigated the role of XIRP2 in hearing function. In the inner ear, XIRP2 is specifically expressed in hair cells, colocalizing with actin-rich structures in bundles, the underlying cuticular plate, and the circumferential actin belt. Analysis using peptide mass spectrometry revealed that the bundle harbors a previously uncharacterized XIRP2 splice variant, suggesting XIRP2's role in the hair cell differs significantly from that reported in myocytes. To determine the role of XIRP2 in hearing, we applied clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9-mediated genome-editing technology to induce targeted mutations into the mouse Xirp2 gene, resulting in the elimination of XIRP2 protein expression in the inner ear. Functional analysis of hearing in the resulting Xirp2-null mice revealed high-frequency hearing loss, and ultrastructural scanning electron microscopy analyses of hair cells demonstrated stereocilia degeneration in these mice. We thus conclude that XIRP2 is required for long-term maintenance of hair cell stereocilia, and that its dysfunction causes hearing loss in the mouse.

Creatine kinase B is necessary to limit myoblast fusion during myogenesis

Myoblast fusion is critical for proper muscle growth and regeneration. During myoblast fusion, the localization of some molecules is spatially restricted; however, the exact reason for such localization is unknown. Creatine kinase B (CKB), which replenishes local ATP pools, localizes near the ends of cultured primary mouse myotubes. To gain insights into the function of CKB, we performed a yeast two-hybrid screen to identify CKB-interacting proteins. We identified molecules with a broad diversity of roles, including actin polymerization, intracellular protein trafficking, and alternative splicing, as well as sarcomeric components. In-depth studies of α-skeletal actin and α-cardiac actin, two predominant muscle actin isoforms, demonstrated their biochemical interaction and partial colocalization with CKB near the ends of myotubes in vitro. In contrast to other cell types, specific knockdown of CKB did not grossly affect actin polymerization in myotubes, suggesting other muscle-specific roles for CKB. Interestingly, knockdown of CKB resulted in significantly increased myoblast fusion and myotube size in vitro, whereas knockdown of creatine kinase M had no effect on these myogenic parameters. Our results suggest that localized CKB plays a key role in myotube formation by limiting myoblast fusion during myogenesis.

New treatment options for hearing loss

Hearing loss is the most common form of sensory impairment in humans and affects more than 40 million people in the United States alone. No drug-based therapy has been approved by the Food and Drug Administration, and treatment mostly relies on devices such as hearing aids and cochlear implants. Over recent years, more than 100 genetic loci have been linked to hearing loss and many of the affected genes have been identified. This understanding of the genetic pathways that regulate auditory function has revealed new targets for pharmacological treatment of the disease. Moreover, approaches that are based on stem cells and gene therapy, which may have the potential to restore or maintain auditory function, are beginning to emerge.

Whole cell, label free protein quantitation with data independent acquisition: quantitation at the MS2 level

Label free quantitation by measurement of peptide fragment signal intensity (MS2 quantitation) is a technique that has seen limited use due to the stochastic nature of data dependent acquisition (DDA). However, data independent acquisition has the potential to make large scale MS2 quantitation a more viable technique. In this study we used an implementation of data independent acquisition--SWATH--to perform label free protein quantitation in a model bacterium Clostridium stercorarium. Four tryptic digests analyzed by SWATH were probed by an ion library containing information on peptide mass and retention time obtained from DDA experiments. Application of this ion library to SWATH data quantified 1030 proteins with at least two peptides quantified (∼ 40% of predicted proteins in the C. stercorarium genome) in each replicate. Quantitative results obtained were very consistent between biological replicates (R(2) ∼ 0.960). Protein quantitation by summation of peptide fragment signal intensities was also highly consistent between biological replicates (R(2) ∼ 0.930), indicating that this approach may have increased viability compared to recent applications in label free protein quantitation. SWATH based quantitation was able to consistently detect differences in relative protein quantity and it provided coverage for a number of proteins that were missed in some samples by DDA analysis.

ATP hydrolysis is critically required for function of CaV1.3 channels in cochlear inner hair cells via fueling Ca2+ clearance

Sound encoding is mediated by Ca(2+) influx-evoked release of glutamate at the ribbon synapse of inner hair cells. Here we studied the role of ATP in this process focusing on Ca(2+) current through CaV1.3 channels and Ca(2+) homeostasis in mouse inner hair cells. Patch-clamp recordings and Ca(2+) imaging demonstrate that hydrolyzable ATP is essential to maintain synaptic Ca(2+) influx in inner hair cells via fueling Ca(2+)-ATPases to avoid an increase in cytosolic [Ca(2+)] and subsequent Ca(2+)/calmodulin-dependent inactivation of CaV1.3 channels.

Genetics of auditory mechano-electrical transduction

The hair bundles of cochlear hair cells play a central role in the auditory mechano-electrical transduction (MET) process. The identification of MET components and of associated molecular complexes by biochemical approaches is impeded by the very small number of hair cells within the cochlea. In contrast, human and mouse genetics have proven to be particularly powerful. The study of inherited forms of deafness led to the discovery of several essential proteins of the MET machinery, which are currently used as entry points to decipher the associated molecular networks. Notably, MET relies not only on the MET machinery but also on several elements ensuring the proper sound-induced oscillation of the hair bundle or the ionic environment necessary to drive the MET current. Here, we review the most significant advances in the molecular bases of the MET process that emerged from the genetics of hearing.

Regulation of brain-type creatine kinase by AMP-activated protein kinase: interaction, phosphorylation and ER localization

AMP-activated protein kinase (AMPK) and cytosolic brain-type creatine kinase (BCK) cooperate under energy stress to compensate for loss of adenosine triphosphate (ATP) by either stimulating ATP-generating and inhibiting ATP-consuming pathways, or by direct ATP regeneration from phosphocreatine, respectively. Here we report on AMPK-dependent phosphorylation of BCK from different species identified by in vitro screening for AMPK substrates in mouse brain. Mass spectrometry, protein sequencing, and site-directed mutagenesis identified Ser6 as a relevant residue with one site phosphorylated per BCK dimer. Yeast two-hybrid analysis revealed interaction of active AMPK specifically with non-phosphorylated BCK. Pharmacological activation of AMPK mimicking energy stress led to BCK phosphorylation in astrocytes and fibroblasts, as evidenced with a highly specific phospho-Ser6 antibody. BCK phosphorylation at Ser6 did not affect its enzymatic activity, but led to the appearance of the phosphorylated enzyme at the endoplasmic reticulum (ER), close to the ER calcium pump, a location known for muscle-type cytosolic creatine kinase (CK) to support Ca²⁺-pumping.

Bottom-up and shotgun proteomics to identify a comprehensive cochlear proteome

Proteomics is a commonly used approach that can provide insights into complex biological systems. The cochlear sensory epithelium contains receptors that transduce the mechanical energy of sound into an electro-chemical energy processed by the peripheral and central nervous systems. Several proteomic techniques have been developed to study the cochlear inner ear, such as two-dimensional difference gel electrophoresis (2D-DIGE), antibody microarray, and mass spectrometry (MS). MS is the most comprehensive and versatile tool in proteomics and in conjunction with separation methods can provide an in-depth proteome of biological samples. Separation methods combined with MS has the ability to enrich protein samples, detect low molecular weight and hydrophobic proteins, and identify low abundant proteins by reducing the proteome dynamic range. Different digestion strategies can be applied to whole lysate or to fractionated protein lysate to enhance peptide and protein sequence coverage. Utilization of different separation techniques, including strong cation exchange (SCX), reversed-phase (RP), and gel-eluted liquid fraction entrapment electrophoresis (GELFrEE) can be applied to reduce sample complexity prior to MS analysis for protein identification.

A terrified-sound stress induced proteomic changes in adult male rat hippocampus

In this study, we investigated the biochemical mechanisms in the adult rat hippocampus underlying the relationship between a terrified-sound induced psychological stress and spatial learning. Adult male rats were exposed to a terrified-sound stress, and the Morris water maze (MWM) has been used to evaluate changes in spatial learning and memory. The protein expression profile of the hippocampus was examined using two-dimensional gel electrophoresis (2DE), matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, and bioinformatics analysis. The data from the MWM tests suggested that a terrified-sound stress improved spatial learning. The proteomic analysis revealed that the expression of 52 proteins was down-regulated, while that of 35 proteins were up-regulated, in the hippocampus of the stressed rats. We identified and validated six of the most significant differentially expressed proteins that demonstrated the greatest stress-induced changes. Our study provides the first evidence that a terrified-sound stress improves spatial learning in rats, and that the enhanced spatial learning coincides with changes in protein expression in rat hippocampus.

Accurate label-free protein quantitation with high- and low-resolution mass spectrometers

Label-free quantitation of proteins analyzed by tandem mass spectrometry uses either integrated peak intensity from the parent-ion mass analysis (MS1) or features from fragment-ion analysis (MS2), such as spectral counts or summed fragment-ion intensity. We directly compared MS1 and MS2 quantitation by analyzing human protein standards diluted into Escherichia coli extracts on an Orbitrap mass spectrometer. We found that summed MS2 intensities were nearly as accurate as integrated MS1 intensities, and both outperformed MS2 spectral counting in accuracy and linearity. We compared these results to those obtained from two low-resolution ion-trap mass spectrometers; summed MS2 intensities from LTQ and LTQ Velos instruments were similar in accuracy to those from the Orbitrap. Data from all three instruments are available via ProteomeXchange with identifier PXD000602. Abundance measurements using MS1 or MS2 intensities had limitations, however. While measured protein concentration was on average well-correlated with the known concentration, there was considerable protein-to-protein variation. Moreover, not all human proteins diluted to a mole fraction of 10(-3) or lower were detected, with a strong falloff below 10(-4) mole fraction. These results show that MS1 and MS2 intensities are simple measures of protein abundance that are on average accurate but should be limited to quantitation of proteins of intermediate to higher fractional abundance.

Correlation of actin crosslinker and capper expression levels with stereocilia growth phases

During development of the chick cochlea, actin crosslinkers and barbed-end cappers presumably influence growth and remodeling of the actin paracrystal of hair cell stereocilia. We used mass spectrometry to identify and quantify major actin-associated proteins of the cochlear sensory epithelium from E14 to E21, when stereocilia widen and lengthen. Tight actin crosslinkers (i.e. fascins, plastins, and espin) are expressed dynamically during cochlear epithelium development between E7 and E21, with FSCN2 replacing FSCN1 and plastins remaining low in abundance. Capping protein, a barbed-end actin capper, is located at stereocilia tips; it is abundant during growth phase II, when stereocilia have ceased elongating and are increasing in diameter. Capping protein levels then decline during growth phase III, when stereocilia reinitiate barbed-end elongation. Although actin crosslinkers are readily detected by electron microscopy in developing chick cochlea stereocilia, quantitative mass spectrometry of stereocilia isolated from E21 chick cochlea indicated that tight crosslinkers are present there in stoichiometric ratios relative to actin that are much lower than their ratios for vestibular stereocilia. These results demonstrate the value of quantitation of global protein expression in chick cochlea during stereocilia development.

In-depth proteomic analysis of mouse cochlear sensory epithelium by mass spectrometry

Proteomic analysis of sensory organs such as the cochlea is challenging due to its small size and difficulties with membrane protein isolation. Mass spectrometry in conjunction with separation methods can provide a more comprehensive proteome, because of the ability to enrich protein samples, detect hydrophobic proteins, and identify low abundant proteins by reducing the proteome dynamic range. GELFrEE as well as different separation and digestion techniques were combined with FASP and nanoLC-MS/MS to obtain an in-depth proteome analysis of cochlear sensory epithelium from 30-day-old mice. Digestion with LysC/trypsin followed by SCX fractionation and multiple nanoLC-MS/MS analyses identified 3773 proteins with a 1% FDR. Of these, 694 protein IDs were in the plasmalemma. Protein IDs obtained by combining outcomes from GELFrEE/LysC/trypsin with GELFrEE/trypsin/trypsin generated 2779 proteins, of which 606 additional proteins were identified using the GELFrEE/LysC/trypsin approach. Combining results from the different techniques resulted in a total of 4620 IDs, including a number of previously unreported proteins. GO analyses showed high expression of binding and catalytic proteins as well as proteins associated with metabolism. The results show that the application of multiple techniques is needed to provide an exhaustive proteome of the cochlear sensory epithelium that includes many membrane proteins. The mass spectrometry proteomics data have been deposited to the ProteomeXchange Consortium with the data set identifier PXD000231.

Knockdown of creatine kinase B inhibits ovarian cancer progression by decreasing glycolysis

Creatine kinase plays a key role in the energy homeostasis of vertebrate cells. Creatine kinase B (CKB), a cytosolic isoform of creatine kinase, shows upregulated expression in a variety of cancers. In this research, we confirmed that some ovarian cancer tissues had elevated CKB expression at the protein level. The functions of CKB in ovarian cancer progression were investigated in the ovarian cancer cell line Skov3, which has a high CKB expression. It was found that CKB knockdown inhibited Skov3 cell proliferation and induced apoptosis under hypoxia or hypoglycemia conditions. CKB depletion also sensitized Skov3 to chemotherapeutic agents. Furthermore, the CKB knockdown reduced glucose consumption and lactate production, and increased ROS production and oxygen consumption. This suggested that CKB knockdown decreased cytosolic glycolysis and resulted in a tumor suppressive metabolic state in Skov3 cells. Consequently, we found that the knockdown of CKB induced G2 arrest in cell cycle by elevating p21 expression and affected the PI3K/Akt and AMPK pathways. These findings provide new insights in the role of CKB in cancer cell survival and tumor progression. Our results also suggest that CKB depletion/inhibition in combination with chemotherapeutic agents might have synergistic effects in ovarian cancer therapy.

Sound-induced length changes in outer hair cell stereocilia

Hearing relies on mechanical stimulation of stereocilia bundles on the sensory cells of the inner ear. When sound hits the ear, these stereocilia pivot about a neck-like taper near their base. More than three decades of research have established that sideways deflection of stereocilia is essential for converting mechanical stimuli into electrical signals. Here we show that mammalian outer hair cell stereocilia not only move sideways but also change length during sound stimulation. Currents that enter stereocilia through mechanically sensitive ion channels control the magnitude of both length changes and bundle deflections in a reciprocal manner: the smaller the length change, the larger is the bundle deflection. Thus, the transduction current is important for maintaining the resting mechanical properties of stereocilia. Hair cell stimulation is most effective when bundles are in a state that ensures minimal length change.