Connexin30 deficiency causes instrastrial fluid-blood barrier disruption within the cochlear stria vascularis

Drosophila glia use a conserved cotransporter mechanism to regulate extracellular volume

The nervous system is protected by blood barriers that use multiple systems to control extracellular solute composition, osmotic pressure, and fluid volume. In the human nervous system, misregulation of the extracellular volume poses serious health threats. Here, we show that the glial cells that form the Drosophila blood-nerve barrier have a conserved molecular mechanism that regulates extracellular volume: the Serine/Threonine kinase Fray, which we previously showed is an ortholog of mammalian PASK/SPAK; and the Na-K-Cl cotransporter Ncc69, which we show is an ortholog of human NKCC1. In mammals, PASK/SPAK binds to NKCC1 and regulates its activity. In Drosophila, larvae mutant for Ncc69 develop a peripheral neuropathy, where fluid accumulates between glia and axons. The accumulation of fluid has no detectable impact on action potential conduction, suggesting that the role of Ncc69 is to maintain volume or osmotic homeostasis. Drosophila Ncc69 has kinetics similar to human NKCC1, and NKCC1 can rescue Ncc69, suggesting that they function in a conserved physiological mechanism. We show that fray and Ncc69 are coexpressed in nerve glia, interact in a yeast-two-hybrid assay, and have an essentially identical bulging nerve phenotype. We propose that normally functioning nerves generate extracellular solutes that are removed by Ncc69 under the control of Fray. This mechanism may perform a similar role in humans, given that NKCC1 is expressed at the blood-brain barrier.

Na+/K+-ATPase α1 identified as an abundant protein in the blood-labyrinth barrier that plays an essential role in the barrier integrity

BACKGROUND

The endothelial-blood/tissue barrier is critical for maintaining tissue homeostasis. The ear harbors a unique endothelial-blood/tissue barrier which we term "blood-labyrinth-barrier". This barrier is critical for maintaining inner ear homeostasis. Disruption of the blood-labyrinth-barrier is closely associated with a number of hearing disorders. Many proteins of the blood-brain-barrier and blood-retinal-barrier have been identified, leading to significant advances in understanding their tissue specific functions. In contrast, capillaries in the ear are small in volume and anatomically complex. This presents a challenge for protein analysis studies, which has resulted in limited knowledge of the molecular and functional components of the blood-labyrinth-barrier. In this study, we developed a novel method for isolation of the stria vascularis capillary from CBA/CaJ mouse cochlea and provided the first database of protein components in the blood-labyrinth barrier as well as evidence that the interaction of Na(+)/K(+)-ATPase α1 (ATP1A1) with protein kinase C eta (PKCη) and occludin is one of the mechanisms of loud sound-induced vascular permeability increase.

METHODOLOGY/PRINCIPAL FINDINGS

Using a mass-spectrometry, shotgun-proteomics approach combined with a novel "sandwich-dissociation" method, more than 600 proteins from isolated stria vascularis capillaries were identified from adult CBA/CaJ mouse cochlea. The ion transporter ATP1A1 was the most abundant protein in the blood-labyrinth barrier. Pharmacological inhibition of ATP1A1 activity resulted in hyperphosphorylation of tight junction proteins such as occludin which increased the blood-labyrinth-barrier permeability. PKCη directly interacted with ATP1A1 and was an essential mediator of ATP1A1-initiated occludin phosphorylation. Moreover, this identified signaling pathway was involved in the breakdown of the blood-labyrinth-barrier resulting from loud sound trauma.

CONCLUSIONS/SIGNIFICANCE

The results presented here provide a novel method for capillary isolation from the inner ear and the first database on protein components in the blood-labyrinth-barrier. Additionally, we found that ATP1A1 interaction with PKCη and occludin was involved in the integrity of the blood-labyrinth-barrier.

The human deafness-associated connexin 30 T5M mutation causes mild hearing loss and reduces biochemical coupling among cochlear non-sensory cells in knock-in mice

Mutations in the GJB2 and GJB6 genes, respectively, coding for connexin26 (Cx26) and connexin30 (Cx30) proteins, are the most common cause for prelingual non-syndromic deafness in humans. In the inner ear, Cx26 and Cx30 are expressed in different non-sensory cell types, where they largely co-localize and may form heteromeric gap junction channels. Here, we describe the generation and characterization of a mouse model for human bilateral middle/high-frequency hearing loss based on the substitution of an evolutionarily conserved threonine by a methionine residue at position 5 near the N-terminus of Cx30 (Cx30T5M). The mutation was inserted in the mouse genome by homologous recombination in mouse embryonic stem cells. Expression of the mutated Cx30T5M protein in these transgenic mice is under the control of the endogenous Cx30 promoter and was analysed via activation of the lacZ reporter gene. When probed by auditory brainstem recordings, Cx30(T5M/T5M) mice exhibited a mild, but significant increase in their hearing thresholds of about 15 dB at all frequencies. Immunolabelling with antibodies to Cx26 or Cx30 suggested normal location of these proteins in the adult inner ear, but western blot analysis showed significantly down-regulated the expression levels of Cx26 and Cx30. In the developing cochlea, electrical coupling, probed by dual patch-clamp recordings, was normal. However, transfer of the fluorescent tracer calcein between cochlear non-sensory cells was reduced, as was intercellular Ca(2+) signalling due to spontaneous ATP release from connexin hemichannels. Our findings link hearing loss to decreased biochemical coupling due to the point-mutated Cx30 in mice.

Resident macrophages in the cochlear blood-labyrinth barrier and their renewal via migration of bone-marrow-derived cells

A large population of perivascular cells was found to be present in the area of the blood-labyrinth barrier in the stria vascularis of normal adult cochlea. The cells were identified as perivascular resident macrophages (PVMs), as they were positive for several macrophage surface molecules including F4/80, CD68, and CD11b. The macrophages, which were closely associated with microvessels and structurally intertwined with endothelial cells and pericytes, constitutively expressed scavenger receptor classes A(1) and B(1) and accumulated blood-borne proteins such as horseradish peroxidase and acetylated low-density lipoprotein. The PVMs were demonstrated to proliferate slowly, as evidenced by the absence of 5-bromo-2-deoxyuridine (BrdU)-positive PVMs at 3-14 days in normal mice injected with BrdU. However, in irradiated mice, the majority of the PVMs turned over via bone-marrow-cell migration within a 10-month time-frame. The existence of PVMs in the vascular wall of the blood-labyrinth barrier might therefore serve as a source for progenitor cells for postnatal vasculogenesis and might contribute to the repair of damaged vessels in the context of a local inflammatory response.

Competitive antagonism of fluorescent gentamicin uptake in the cochlea

Aminoglycosides enter inner ear hair cells via apical endocytosis, or mechanoelectrical transduction channels, implying that, in vivo, aminoglycosides enter hair cells from endolymph prior to exerting their cytotoxic effect. If so, circulating aminoglycosides likely cross the strial blood-labyrinth barrier and enter marginal cells prior to clearance into endolymph. We characterized the competitive antagonism of unconjugated aminoglycosides on the uptake of fluorescent gentamicin (GTTR) in the stria vascularis and kidney cells at an early time point. In mice, uptake of GTTR by kidney proximal tubule cells was competitively antagonized by gentamicin at all doses, but only weakly by kanamycin (mimicking in vitro data). GTTR fluorescence was approximately 100-fold greater in proximal tubule cells than in the stria vascularis. Furthermore, only high molar ratios of aminoglycosides significantly reduced strial uptake of GTTR. Thus, gentamicin antagonism of GTTR uptake is more efficacious in proximal tubules than in the stria vascularis. Competitive antagonism of GTTR uptake is indicative of specific cell-regulatable uptake mechanisms (e.g., ion channels, transporters) in the kidney. Strial uptake mechanisms have lower specific affinity for gentamicin, and/or density (compared to the kidney), yet may be critical to transport gentamicin across the strial blood-labyrinth barrier into marginal cells.

Serum homocysteine and folate concentrations are associated with prevalent age-related hearing loss

Elevated total serum homocysteine (tHcy) concentrations associated with vitamin B-12 or folate deficiencies may adversely affect blood flow to the cochlea, leading to age-related hearing loss (presbycusis). However, only 2 small cross-sectional studies have assessed the link between folate, vitamin B-12, or tHcy and presbycusis. We aimed to determine both the cross-sectional and longitudinal association between serum concentrations of folate, vitamin B-12, or tHcy and risk of age-related hearing loss. The Blue Mountains Hearing Study is a population-based survey of age-related hearing loss (1997-1999 to 2002-2004). Presbycusis was measured in 2956 participants (aged >or=50 y) and was defined as the pure-tone average of frequencies 0.5, 1.0, 2.0, and 4.0 kHz >25 dB hearing level (HL). Serum concentrations of folate, vitamin B-12, and tHcy were determined from blood samples. Participants with elevated tHcy (>20 micromol/L) concentrations had a 64% increased likelihood of prevalent hearing loss (>25 dB HL) [multivariate-adjusted odds ratio (OR) 1.64; 95% CI, 1.06-2.53]. Low serum folate levels (<11 nmol/L) increased the odds of prevalent mild hearing loss (>25-40 dB HL), multivariate-adjusted [OR 1.37 (CI 1.04-1.81)]. Serum vitamin B-12, however, was not significantly associated with prevalent hearing loss. Serum folate, vitamin B-12, and tHcy concentrations were also not significantly associated with an increased risk of incident hearing loss. Serum concentrations of tHcy and folate were associated with age-related hearing loss cross-sectionally, but no temporal links were observed, which could be due to insufficient study power. Further, large prospective studies will be required in the future to assess these associations.

ATP-mediated cell-cell signaling in the organ of Corti: the role of connexin channels

Connexin 26 (Cx26) and connexin 30 (Cx30) form hemichannels that release ATP from the endolymphatic surface of cochlear supporting and epithelial cells and also form gap junction (GJ) channels that allow the concomitant intercellular diffusion of Ca(2+) mobilizing second messengers. Released ATP in turn activates G-protein coupled P2Y(2) and P2Y(4) receptors, PLC-dependent generation of IP(3), release of Ca(2+) from intracellular stores, instigating the regenerative propagation of intercellular Ca(2+) signals (ICS). The range of ICS propagation is sensitive to the concentration of extracellular divalent cations and activity of ectonucleotidases. Here, the expression patterns of Cx26 and Cx30 were characterized in postnatal cochlear tissues obtained from mice aged between P5 and P6. The expression gradient along the longitudinal axis of the cochlea, decreasing from the basal to the apical cochlear turn (CT), was more pronounced in outer sulcus (OS) cells than in inner sulcus (IS) cells. GJ-mediated dye coupling was maximal in OS cells of the basal CT, inhibited by the nonselective connexin channel blocker carbenoxolone (CBX) and absent in hair cells. Photostimulating OS cells with caged inositol (3,4,5) tri-phosphate (IP(3)) resulted in transfer of ICS in the lateral direction, from OS cells to IS cells across the hair cell region (HCR) of medial and basal CTs. ICS transfer in the opposite (medial) direction, from IS cells photostimulated with caged IP(3) to OS cells, occurred mostly in the basal CT. In addition, OS cells displayed impressive rhythmic activity with oscillations of cytosolic free Ca(2+) concentration ([Ca(2+)](i)) coordinated by the propagation of Ca(2+) wavefronts sweeping repeatedly through the same tissue area along the coiling axis of the cochlea. Oscillations evoked by uncaging IP(3) or by applying ATP differed greatly, by as much as one order of magnitude, in frequency and waveform rise time. ICS evoked by direct application of ATP propagated along convoluted cellular paths in the OS, which often branched and changed dynamically over time. Potential implications of these findings are discussed in the context of developmental regulation and cochlear pathophysiology.

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (doi:10.1007/s11302-010-9192-9) contains supplementary material, which is available to authorized users.

The role of potassium recirculation in cochlear amplification

PURPOSE OF REVIEW

Normal cochlear function depends on maintaining the correct ionic environment for the sensory hair cells. Here we review recent literature on the cellular distribution of potassium transport-related molecules in the cochlea.

RECENT FINDINGS

Transgenic animal models have identified novel molecules essential for normal hearing and support the idea that potassium is recycled in the cochlea. The findings indicate that extracellular potassium released by outer hair cells into the space of Nuel is taken up by supporting cells, that the gap junction system in the organ of Corti is involved in potassium handling in the cochlea, that the gap junction system in stria vascularis is essential for the generation of the endocochlear potential, and that computational models can assist in the interpretation of the systems biology of hearing and integrate the molecular, electrical, and mechanical networks of the cochlear partition. Such models suggest that outer hair cell electromotility can amplify over a much broader frequency range than expected from isolated cell studies.

SUMMARY

These new findings clarify the role of endolymphatic potassium in normal cochlear function. They also help current understanding of the mechanisms of certain forms of hereditary hearing loss.

How is the highly positive endocochlear potential formed? The specific architecture of the stria vascularis and the roles of the ion-transport apparatus

Cochlear endolymph, an extracellular solution containing 150 mM K(+), exhibits a positive potential of +80 mV. This is called the endocochlear potential (EP) and is essential for audition. The mechanism responsible for formation of the EP has been an enigma for the half century since its first measurement. A key element is the stria vascularis, which displays a characteristic tissue structure and expresses multiple ion-transport apparatus. The stria comprises two epithelial layers: a layer of marginal cells and one composed of intermediate and basal cells. Between the two layers lies an extracellular space termed the intrastrial space (IS), which is thus surrounded by the apical membranes of intermediate cells and the basolateral membranes of marginal cells. The fluid in the IS exhibits a low concentration of K(+) and a positive potential similar to the EP. We have demonstrated that the IS is electrically isolated from the neighboring extracellular fluids, perilymph, and endolymph, which allows the IS to sustain its positive potential. This IS potential is generated by K(+) diffusion across the apical membranes of intermediate cells, where inwardly rectifying Kir4.1 channels are localized. The low K(+) concentration in the IS, which is mandatory for the large K(+)-diffusion potential, is maintained by Na(+),K(+)-ATPases and Na(+),K(+),2Cl(-)-cotransporters expressed at the basolateral membranes of marginal cells. An additional K(+)-diffusion potential formed by KCNQ1/KCNE1-K(+) channels at the apical membranes of marginal cells also contributes to the EP. Therefore, the EP depends on an electrically isolated space and two K(+)-diffusion potentials in the stria vascularis.

Hearing loss: mechanisms revealed by genetics and cell biology

Hearing loss (HL), or deafness in its most severe form, affects an estimated 28 and 22.5 million Americans and Europeans, respectively. The numbers are higher in regions such as India and the Middle East, where consanguinity contributes to larger numbers of recessively inherited hearing impairment (HI). As a result of work-related difficulties, educational and developmental delays, and social stigmas and exclusion, the economic impact of HL is very high. At the other end of the spectrum, a rich deaf culture, particularly for individuals whose parents and even grandparents were deaf, is a social movement that believes that deafness is a difference in human experience rather than a disability. This review attempts to cover the remarkable progress made in the field of the genetics of HL over the past 20 years. Mutations in a significant number of genes have been discovered over the years that contribute to clinically heterogeneous forms of HL, enabling genetic counseling and prediction of progression of HL. Cell biological assays, protein localization in the inner ear, and detailed analysis of spontaneous and transgenic mouse models have provided an incredibly rich resource for elucidating mechanisms of hereditary hearing loss (HHL). This knowledge is providing answers for the families with HL, who contribute a great deal to the research being performed worldwide.

Potassium ion movement in the inner ear: insights from genetic disease and mouse models

Sensory transduction in the cochlea and vestibular labyrinth depends on fluid movements that deflect the hair bundles of mechanosensitive hair cells. Mechanosensitive transducer channels at the tip of the hair cell stereocilia allow K(+) to flow into cells. This unusual process relies on ionic gradients unique to the inner ear. Linking genes to deafness in humans and mice has been instrumental in identifying the ion transport machinery important for hearing and balance. Morphological analysis is difficult in patients, but mouse models have helped to investigate phenotypes at different developmental time points. This review focuses on cellular ion transport mechanisms in the stria vascularis that generate the major electrochemical gradients for sensory transduction.

Advances in Auditory and Vestibular Medicine

Auditory and Vestibular medicine is becoming more accepted as a specialty of its own, Medical NeurOtology. Recent advances in the field have been instrumental in the understanding of the scientific foundations, pathophysiology, clinical approach and management of patients with hearing and vestibular disorders. This paper will review these advances.

Melanin precursors prevent premature age-related and noise-induced hearing loss in albino mice

Strial melanocytes are required for normal development and correct functioning of the cochlea. Hearing deficits have been reported in albino individuals from different species, although melanin appears to be not essential for normal auditory function. We have analyzed the auditory brainstem responses (ABR) of two transgenic mice: YRT2, carrying the entire mouse tyrosinase (Tyr) gene expression-domain and undistinguishable from wild-type pigmented animals; and TyrTH, non-pigmented but ectopically expressing tyrosine hydroxylase (Th) in melanocytes, which generate the precursor metabolite, L-DOPA, but not melanin. We show that young albino mice present a higher prevalence of profound sensorineural deafness and a poorer recovery of auditory thresholds after noise-exposure than transgenic mice. Hearing loss was associated with absence of cochlear melanin or its precursor metabolites and latencies of the central auditory pathway were unaltered. In summary, albino mice show impaired hearing responses during ageing and after noise damage when compared to YRT2 and TyrTH transgenic mice, which do not show the albino-associated ABR alterations. These results demonstrate that melanin precursors, such as L-DOPA, have a protective role in the mammalian cochlea in age-related and noise-induced hearing loss.

Differential passage of gadolinium through the mouse inner ear barriers evaluated with 4.7T MRI

Magnetic resonance imaging (MRI), supplemented by contrast agents, is a powerful tool that can be used to visualise the structures of the inner ear in vivo and assess some aspects of physiology, such as the permeability of agents through membranes. The mouse is an excellent animal species for investigating human diseases, including hearing loss but detailed MRI studies with contrast have not been reported. In this work, we aimed to demonstrate the limits of MR imaging resolution of the fine inner ear structures in the mouse and to explore the permeability of the intracochlear barriers to gadolinium-tetra-azacyclo-dodecane-tetra-acetic acid (Gd-DOTA) administered by intravenous injection (IV) or intratympanic (IT) routes. Twenty-three female FVB mice were imaged with a 4.7-T MR scanner using both 2D and high resolution 3D sequences. Inner ear region of interest (ROI) signal intensities and perilymph volumes were evaluated. Finer structures were studied using 3D acquisition and reconstruction techniques and comparisons were made to similarly oriented histological sections that were examined by light microscopy. Gd-DOTA enhancement occurred in the perilymphatic compartment and highlighted the contiguous inner ear structures, but enhancement did not appear within the endolymph. The dynamic uptake of Gd-DOTA in the perilymphatic compartments reached an initial plateau 80min after IV administration and continued to slightly increase to a maximum level by 100min. The perilymph volume demonstrated by Gd-DOTA uptake was statistically significantly larger in the IV group (1.72mm(3)) than in the IT group (1.28mm(3)) (p<0.05).

Development of form and function in the mammalian cochlea

The cochlea possesses specialized features to receive sound signals and to resolve and convert the frequency and intensity components within each signal for auditory perception. It consists of precisely patterned and polarized sensory cells adorned with a highly specialized mechanotransduction apparatus for sensitivity and adaptation, and discrete nonsensory cellular networks for biochemical and mechanical support to drive an integrated cellular response and mechanotransduction. This review summarizes recent discoveries about the roles of FGF, Notch, and Hedgehog signaling and transcriptional factors in the differentiation and patterning of the auditory sensory organ, the Usher complex, and the planar cell polarity pathway in the formation and polarization of mechanotransduction component hair bundles, and the contribution of nonsensory cell networks in the stria vascularis and the sensory region toward the maturation of the mammalian cochlea.

Matrix imbalance by inducing expression of metalloproteinase and oxidative stress in cochlea of hyperhomocysteinemic mice

Clinical study reports hearing loss in patients with low folic acid (FA) and elevated homocysteine (Hcy). We hypothesize that elevated Hcy induces imbalance in matrix turnover and oxidative stress in cochlea. Cystathione beta-synthase heterozygous knockout mice were used as model for hyperhomocysteinemia. Matrix remodeling induced by Hcy resulted from elevated MMP-2, -9, and -14. MMP-2 and -9 showed elevated gelatinase activity in CBS (+/-) cochlea. Tissue inhibitors of matrix metalloproteinase were significantly lower in CBS (+/-) cochlea. The expression analyses for MMPs and TIMPs were equally represented at protein and mRNA levels. Cochlea of CBS mice showed following structural changes; (1) detachment of tectorial membrane lying on hair cells (2) thinner s. vascularis (3) large fibroblast in spiral ligament. Hcy induced higher protein nitrotyrosination and cytosolic NADPHoxidase subunit p22(phox) in cochlea. It is thus suggested that Hcy induced matrix imbalance, structural changes and oxidative stress in cochlea.

Diverse deafness mechanisms of connexin mutations revealed by studies using in vitro approaches and mouse models

Mutations in connexins (Cxs), the constitutive protein subunits of gap junction (GJ) intercellular channels, are one of the most common human genetic defects that cause severe prelingual non-syndromic hearing impairments. Many subtypes of Cxs (e.g., Cxs 26, 29, 30, 31, 43) and pannexins (Panxs) are expressed in the cochlea where they contribute to the formation of a GJ-based intercellular communication network. Cx26 and Cx30 are the predominant cochlear Cxs and they co-assemble in most GJ plaques to form hybrid GJs. The cellular localization of specific Cx subtypes provides a basis for understanding the molecular structure of GJs and hemichannels in the cochlea. Information about the interactions among the various co-assembled Cx partners is critical to appreciate the functional consequences of various types of genetic mutations. In vitro studies of reconstituted GJs in cell lines have yielded surprisingly heterogeneous mechanisms of dysfunction caused by various Cx mutations. Availability of multiple lines of Cx-mutant mouse models has provided some insight into the pathogenesis processes in the cochlea of deaf mice. Here we summarize recent advances in understanding the structure and function of cochlear GJs and give a critical review of current findings obtained from both in vitro studies and mouse models on the mechanisms of Cx mutations that lead to cell death in the cochlea and hearing loss.

Identification and characterization of pannexin expression in the mammalian cochlea

The gap junction in vertebrates is encoded by the connexin gene family. Recently, a new gene family termed pannexin (Panx) has been identified in vertebrates and found to encode gap junctional proteins as well. To date, three pannexin isoforms (Panx1, 2, and 3) have been cloned from mouse and human genomes. In this study, expression of pannexins in the mouse and rat cochlea was investigated. Polymerase chain reaction and Western blot analysis showed that all three pannexin isoforms were expressed in the cochlea. Immunofluorescent staining showed that Panx1 expression was extensive. In the organ of Corti, Panx1 labeling was found in supporting cells, including pillar cells, Hensen cells, Claudius cells, and Boettcher cells. Both surface plaque-like punctate labeling and diffuse-cytoplasmic labeling were visible. However, the labeling was weak and rare in Deiters cells. No labeling was found in the hair cells. Intense labeling for Panx1 was also observed in the interdental cells in the spiral limbus, the inner and outer sulcus cells, and the type II fibrocytes in the spiral prominence and central region in the cochlear lateral wall. In addition, Panx1 labeling was detectable in Reissner's membrane and strial blood vessel cells. Panx2 labeling was restricted to the basal cells in the stria vascularis and was also detectable in the spiral ganglion neurons. However, no overlapping labeling for Panx1 and Panx2 was observed. Finally, Panx3 labeling was exclusively observed in the cochlear bone. Thus, Panx1, 2, and 3 are abundantly expressed in the mammalian cochlea and demonstrate distinct cellular distributions. Like connexins, they may play an important role in hearing.

Cx36 makes channels coupling human pancreatic beta-cells, and correlates with insulin expression

Previous studies have documented that the insulin-producing beta-cells of laboratory rodents are coupled by gap junction channels made solely of the connexin36 (Cx36) protein, and have shown that loss of this protein desynchronizes beta-cells, leading to secretory defects reminiscent of those observed in type 2 diabetes. Since human islets differ in several respects from those of laboratory rodents, we have now screened human pancreas, and islets isolated thereof, for expression of a variety of connexin genes, tested whether the cognate proteins form functional channels for islet cell exchanges, and assessed whether this expression changes with beta-cell function in islets of control and type 2 diabetics. Here, we show that (i) different connexin isoforms are differentially distributed in the exocrine and endocrine parts of the human pancreas; (ii) human islets express at the transcript level different connexin isoforms; (iii) the membrane of beta-cells harbors detectable levels of gap junctions made of Cx36; (iv) this protein is concentrated in lipid raft domains of the beta-cell membrane where it forms gap junctions; (v) Cx36 channels allow for the preferential exchange of cationic molecules between human beta-cells; (vi) the levels of Cx36 mRNA correlated with the expression of the insulin gene in the islets of both control and type 2 diabetics. The data show that Cx36 is a native protein of human pancreatic islets, which mediates the coupling of the insulin-producing beta-cells, and contributes to control beta-cell function by modulating gene expression.

Gap-junction channels dysfunction in deafness and hearing loss

Gap-junction channels connect the cytoplasm of adjacent cells, allowing the diffusion of ions and small metabolites. They are formed at the appositional plasma membranes by a family of related proteins named connexins. Mutations in connexins 26, 31, 30, 32, and 43 have been associated with nonsyndromic or syndromic deafness. The majority of these mutations are inherited in an autosomal recessive manner, but a few of them have been associated with dominantly inherited hearing loss. Mutations in the connexin26 gene (GJB2) are the most common cause of genetic deafness. This review summarizes the most relevant and recent information about different mutations in connexin genes found in human patients, with emphasis on GJB2. The possible effects of the mutations on channel expression and function are discussed, in addition to their possible physiologic consequences for inner ear physiology. Finally, we propose that connexin channels (gap junctions and hemichannels) may be targets for age-related hearing loss induced by oxidative damage.

Gap junctions and connexins in the inner ear: their roles in homeostasis and deafness

PURPOSE OF REVIEW

Mutations in GJB2 and GJB6, the genes encoding the gap-junction proteins connexin 26 and connexin 30, are the most common cause of autosomal recessive nonsyndromic deafness in many populations across the world. In this review, we discuss current ideas about the roles of gap junctions in the inner ear and the implications of connexin mutations on auditory function.

RECENT FINDINGS

In recent years, a complex picture of the roles of gap junctions in cochlear physiology emerged. Rather than being mere conduits for the circulation of potassium ions in the inner ear, gap junctions have been implicated in intercellular signaling among nonsensory cells and may be involved in the maintenance of the endothelial barrier in the stria vascularis. Studies of mutant channels and mouse models for connexin-related deafness have provided valuable insights into some of the mechanisms by which connexin dysfunction causes cochlear degeneration. They have also identified potential therapeutic interventions for specific connexin mutations, such as the restoration of normal connexin 26 protein levels in GJB6-associated deafness.

SUMMARY

Despite recent advances, a better understanding of the complexity of gap-junctional communication in the inner ear and the structure-function relationships of connexin proteins is required for the development of mechanism-based treatments of connexin-associated hearing loss.

Gap junction mediated intercellular metabolite transfer in the cochlea is compromised in connexin30 null mice

Connexin26 (Cx26) and connexin30 (Cx30) are two major protein subunits that co-assemble to form gap junctions (GJs) in the cochlea. Mutations in either one of them are the major cause of non-syndromic prelingual deafness in humans. Because the mechanisms of cochlear pathogenesis caused by Cx mutations are unclear, we investigated effects of Cx30 null mutation on GJ-mediated ionic and metabolic coupling in the cochlea of mice. A novel flattened cochlear preparation was used to directly assess intercellular coupling in the sensory epithelium of the cochlea. Double-electrode patch clamp recordings revealed that the absence of Cx30 did not significantly change GJ conductance among the cochlear supporting cells. The preserved electrical coupling is consistent with immunolabeling data showing extensive Cx26 GJs in the cochlea of the mutant mice. In contrast, dye diffusion assays showed that the rate and extent of intercellular transfer of multiple fluorescent dyes (including a non-metabolizable D-glucose analogue, 2-NBDG) among cochlear supporting cells were severely reduced in Cx30 null mice. Since the sensory epithelium in the cochlea is an avascular organ, GJ-facilitated intercellular transfer of nutrient and signaling molecules may play essential roles in cellular homeostasis. To test this possibility, NBDG was used as a tracer to study the contribution of GJs in transporting glucose into the cochlear sensory epithelium when delivered systemically. NBDG uptake in cochlear supporting cells was significantly reduced in Cx30 null mice. The decrease was also observed with GJ blockers or glucose competition, supporting the specificity of our tests. These data indicate that GJs facilitate efficient uptake of glucose in the supporting cells. This study provides the first direct experimental evidence showing that the transfer of metabolically-important molecules in cochlear supporting cells is dependent on the normal function of GJs, thereby suggesting a novel pathogenesis process in the cochlea for Cx-mutation-linked deafness.

Absence of strial melanin coincides with age-associated marginal cell loss and endocochlear potential decline

Cochlear stria vascularis contains melanin-producing intermediate cells that play a critical role in the production of the endocochlear potential (EP) and in maintaining the high levels of K(+) that normally exist in scala media. The melanin produced by intermediate cells can be exported to the intrastrial space, where it may be taken up by strial marginal cells and basal cells. Because melanin can act as an antioxidant and metal chelator, evidence for its role in protecting the stria and organ of Corti against noise, ototoxins, and aging has long been sought. While some evidence supports a protective role of melanin against noise and ototoxins, no evidence yet presented has demonstrated a clear role for melanin in maintaining the EP during aging. We tested this by comparing basal turn EPs and a host of cochlear cellular metrics in aging C57BL/6 (B6) mice and C57BL/6-Tyr(c-2J) mice. The latter mice carry a naturally occurring inactivating mutation of the tyrosinase locus, and produce no strial melanin. Because these two strains are coisogenic, and because pigmented B6 mice show essentially no age-related EP decline, they provide an ideal test of importance of melanin in the aging stria. Pigmented and albino B6 mice showed identical rates of hearing loss and sensory cell loss. However, after two years of age, basal turn EPs significantly diverged, with 42% of albinos showing EPs below 100 mV versus only 18% of pigmented mice. The clearest anatomical correlate of this EP difference was significantly reduced strial thickness in the albinos that was highly correlated with loss of marginal cells. Combined with findings in human temporal bones, plus recent work in BALB/c mice and gerbils, the present findings point to a common etiology in strial presbycusis whereby EP reduction is principally linked to marginal cell loss or dysfunction. For any individual, genetic background, environmental influences, and stochastic events may work together to determine whether marginal cell density or function falls below some critical level, and thus whether EP decline occurs.

Coordinated control of connexin 26 and connexin 30 at the regulatory and functional level in the inner ear

Connexin 26 (Cx26) and connexin 30 (Cx30) are encoded by two genes (GJB2 and GJB6, respectively) that are found within 50 kb in the same complex deafness locus, DFNB1. Immunocytochemistry and quantitative PCR analysis of Cx30 KO mouse cultures revealed that Cx26 is downregulated at the protein level and at the mRNA level in nonsensory cells located between outer hair cells and the stria vascularis. To explore connexin coregulation, we manipulated gene expression using the bovine adeno-associated virus. Overexpression of Cx30 in the Cx30 KO mouse by transduction with bovine adeno-associated virus restored Cx26 expression, permitted the formation of functional gap junction channels, and rescued propagating Ca(2+) signals. Ablation of Cx26 by transduction of Cx26(loxP/loxP) cultures with a Cre recombinase vector caused concurrent downregulation of Cx30 and impaired intercellular communication. The coordinated regulation of Cx26 and Cx30 expression appears to occur as a result of signaling through PLC and the NF-kappaB pathway, because activation of IP(3)-mediated Ca(2+) responses by stimulation of P2Y receptors for 20 min with 20 nM ATP increased the levels of Cx26 transcripts in Cx30 KO cultures. This effect was inhibited by expressing a stable form of the IkappaB repressor protein that prevents activation/translocation of NF-kappaB. Thus, our data reveal a Ca(2+)-dependent control in the expression of inner ear connexins implicated in hereditary deafness as well as insight into the hitherto unexplained observation that some deafness-associated DFNB1 alleles are characterized by hereditable reduction of both GJB2 and GJB6 expression.

Human adenylate kinase 2 deficiency causes a profound hematopoietic defect associated with sensorineural deafness

Reticular dysgenesis is an autosomal recessive form of human severe combined immunodeficiency characterized by an early differentiation arrest in the myeloid lineage and impaired lymphoid maturation. In addition, affected newborns have bilateral sensorineural deafness. Here we identify biallelic mutations in AK2 (adenylate kinase 2) in seven individuals affected with reticular dysgenesis. These mutations result in absent or strongly decreased protein expression. We then demonstrate that restoration of AK2 expression in the bone marrow cells of individuals with reticular dysgenesis overcomes the neutrophil differentiation arrest, underlining its specific requirement in the development of a restricted set of hematopoietic lineages. Last, we establish that AK2 is specifically expressed in the stria vascularis region of the inner ear, which provides an explanation of the sensorineural deafness in these individuals. These results identify a previously unknown mechanism involved in regulation of hematopoietic cell differentiation and in one of the most severe human immunodeficiency syndromes.

Specific expression of Gsta4 in mouse cochlear melanocytes: a novel role for hearing and melanocyte differentiation

Mammalian pigment cells produce melanin as the main pigment. Melanocytes, one of the two types of mammalian pigment cells, differentiate from the neural crest and migrate to a variety of organs during development. Melanocytes exist not only in the skin but also in other sites such as the cochlea where they are essential for hearing. Mitf(mi-bw) is one of the known recessive alleles of the mouse microphthalmia-associated transcription factor (Mitf) locus, which is essential for the development of pigment cells. Homozygous Mitf(mi-bw)/Mitf(mi-bw) mice have a completely white coat with black eyes and are deaf due to the lack of melanocytes. By comparing gene expression profiles in cochleae of wild-type and Mitf(mi-bw)/Mitf(mi-bw) mice, we now demonstrate the specific expression of glutathione S-transferase alpha 4 (Gsta4) in the stria vascularis. Gsta4 encodes one of the cytosolic glutathione S-transferases (GSTs) which participate in detoxification processes of many tissues. This gene is specifically expressed in intermediate cells of the stria vascularis, suggesting a novel function for cochlear melanocytes. Moreover, among mammalian pigment cells, expression of Gsta4 was restricted to cochlear melanocytes, suggesting that melanocytes in various tissues differentiate from one another depending on their location.

Strial microvascular pathology and age-associated endocochlear potential decline in NOD congenic mice

NOD/ShiLtJ (previously NOD/LtJ) inbred mice show polygenic autoimmune disease and are commonly used to model autoimmune-related type I diabetes, as well as Sjogren's syndrome. They also show rapidly progressing hearing loss, partly due to the combined effects of Cdh23ahl and Ahl2. Congenic NOD.NON-H2nb1/LtJ mice, which carry corrective alleles within the H2 histocompatibility gene complex, are free from diabetes and other overt signs of autoimmune disease, but still exhibit rapidly progressive hearing loss. Here we show that cochlear pathology in these congenics broadly includes hair cell and neuronal loss, plus endocochlear potential (EP) decline from initially normal values after two months of age. The EP reduction follows often dramatic degeneration of capillaries in stria vascularis, with resulting strial degeneration. The cochlear modiolus also features perivascular inclusions that resemble those in some mouse autoimmune models. We posit that cochlear hair cell/neural and strial pathology arise independently. While sensory cell loss may be closely tied to Cdh23ahl and Ahl2, the strial microvascular pathology and modiolar anomalies we observe may arise from alleles on the NOD background related to immune function. Age-associated EP decline in NOD.NON-H2nb1 mice may model forms of strial age-related hearing loss caused principally by microvascular disease. The remarkable strial capillary loss in these mice may also be useful for studying the relation between strial vascular insufficiency and strial function.

The endocochlear potential depends on two K+ diffusion potentials and an electrical barrier in the stria vascularis of the inner ear

An endocochlear potential (EP) of +80 mV is essential for audition. Although the regulation of K(+) concentration ([K(+)]) in various compartments of the cochlear stria vascularis seems crucial for the formation of the EP, the mechanism remains uncertain. We have used multibarreled electrodes to measure the potential, [K(+)], and input resistance in each compartment of the stria vascularis. The stria faces two fluids, perilymph and endolymph, and contains an extracelluar compartment, the intrastrial space (IS), surrounded by two epithelial layers, the marginal cell (MC) layer and that composed of intermediate and basal cells. Fluid in the IS exhibits a low [K(+)] and a positive potential, called the intrastrial potential (ISP). We found that the input resistance of the IS was high, indicating this space is electrically isolated from the neighboring extracellular fluids. This arrangement is indispensable for maintaining positive ISP. Inhibiting the K(+) transporters of the stria by anoxia, ouabain, or bumetanide caused the [K(+)] of the IS to increase and the intracellular [K(+)] of MCs to decrease, reducing both the ISP and the EP. Calculations indicate that the ISP represents the K(+) diffusion potential across the apical membranes of intermediate cells through Ba(2+)-sensitive K(+) channels. The K(+) diffusion potential across the apical membranes of MCs also contributes to the EP. Because the EP depends on two K(+) diffusion potentials and an electrical barrier in the stria vascularis, interference with any of these elements can interrupt hearing.

Mouse models for human hereditary deafness

Hearing impairment is a frequent condition in humans. Identification of the causative genes for the early onset forms of isolated deafness began 15 years ago and has been very fruitful. To date, approximately 50 causative genes have been identified. Yet, limited information regarding the underlying pathogenic mechanisms can be derived from hearing tests in deaf patients. This chapter describes the success of mouse models in the elucidation of some pathophysiological processes in the auditory sensory organ, the cochlea. These models have revealed a variety of defective structures and functions at the origin of deafness genetic forms. This is illustrated by three different examples: (1) the DFNB9 deafness form, a synaptopathy of the cochlear sensory cells where otoferlin is defective; (2) the Usher syndrome, in which deafness is related to abnormal development of the hair bundle, the mechanoreceptive structure of the sensory cells to sound; (3) the DFNB1 deafness form, which is the most common form of inherited deafness in Caucasian populations, mainly caused by connexin-26 defects that alter gap junction communication between nonsensory cochlear cells.

Islet-cell-to-cell communication as basis for normal insulin secretion

The emergence of pancreatic islets has necessitated the development of a signalling system for the intra- and inter-islet coordination of beta cells. With evolution, this system has evolved into a complex regulatory network of partially cross-talking pathways, whereby individual cells sense the state of activity of their neighbours and, accordingly, regulate their own level of functioning. A consistent feature of this network in vertebrates is the expression of connexin (Cx)-36-made cell-to-cell channels, which cluster at gap junction domains of the cell membrane, and which adjacent beta cells use to share cytoplasmic ions and small metabolites within individual islets. This chapter reviews what is known about Cx36, and the mechanism whereby this beta-cell connexin significantly regulates insulin secretion. It further outlines other less established functions of the protein and evaluates its potential relevance for the development of novel therapeutic approaches to diabetes.

Connexin40 regulates renin production and blood pressure

Renin secretion is regulated by coordinated signaling between the various cells of the juxtaglomerular apparatus. The renin-secreting cells (RSC), which play a major role in the control of blood pressure, are coupled to each other and to endothelial cells by Connexin40 (Cx40)-containing channels. In this study, we show that Cx40 knockout (Cx40-/-) mice, but not their heterozygous littermates, are hypertensive due to the increase in the number of RSC, renin biosynthesis, and plasma renin. Treatment with the angiotensin II receptor AT1 antagonist candesartan or the angiotensin II-converting enzyme inhibitor ramipril reduced the blood pressure of the Cx40-/- mice to the same levels seen in wild-type (WT) mice. The elevated blood pressure of the knockout mice was not affected by clipping one renal artery (2K1C, renin-dependent model of hypertension) or after a high salt diet. Under these conditions, however, Cx40-/- mice showed an altered production and release of renin. The renin mRNA ratio between the clipped and the non-clipped kidney was lower in the knockout than in the WT 2K1C mice. This indicates that the response to a change in blood pressure was altered. The RSC of the Cx40-/- mice did not have a compensatory increase in the levels of either Cx43 or Cx37. Our data show that renin secretion is dependent on Cx40 and suggest the Cx40-/- mice may be a genetic model of renin-dependent hypertension.

Functional significance of channels and transporters expressed in the inner ear and kidney

A number of ion channels and transporters are expressed in both the inner ear and kidney. In the inner ear, K+cycling and endolymphatic K+, Na+, Ca2+, and pH homeostasis are critical for normal organ function. Ion channels and transporters involved in K+cycling include K+channels, Na+-2Cl−-K+cotransporter, Na+/K+-ATPase, Cl−channels, connexins, and K+/Cl−cotransporters. Furthermore, endolymphatic Na+and Ca2+homeostasis depends on Ca2+-ATPase, Ca2+channels, Na+channels, and a purinergic receptor channel. Endolymphatic pH homeostasis involves H+-ATPase and Cl−/HCO3−exchangers including pendrin. Defective connexins (GJB2 and GJB6), pendrin (SLC26A4), K+channels (KCNJ10, KCNQ1, KCNE1, and KCNMA1), Na+-2Cl−-K+cotransporter (SLC12A2), K+/Cl−cotransporters (KCC3 and KCC4), Cl−channels (BSND and CLCNKA + CLCNKB), and H+-ATPase (ATP6V1B1 and ATPV0A4) cause hearing loss. All these channels and transporters are also expressed in the kidney and support renal tubular transport or signaling. The hearing loss may thus be paralleled by various renal phenotypes including a subtle decrease of proximal Na+-coupled transport (KCNE1/KCNQ1), impaired K+secretion (KCNMA1), limited HCO3−elimination (SLC26A4), NaCl wasting (BSND and CLCNKB), renal tubular acidosis (ATP6V1B1, ATPV0A4, and KCC4), or impaired urinary concentration (CLCNKA). Thus, defects of channels and transporters expressed in the kidney and inner ear result in simultaneous dysfunctions of these seemingly unrelated organs.