Impaired permeability to Ins(1,4,5)P3 in a mutant connexin underlies recessive hereditary deafness

Review and research gap identification in genetics causes of syndromic and nonsyndromic hearing loss in Saudi Arabia

Congenital hearing loss is one of the most common sensory disabilities worldwide. The genetic causes of hearing loss account for 50% of hearing loss. Genetic causes of hearing loss can be classified as nonsyndromic hearing loss (NSHL) or syndromic hearing loss (SHL). NSHL is defined as a partial or complete hearing loss without additional phenotypes; however, SHL, known as hearing loss, is associated with other phenotypes. Both types follow a simple Mendelian inheritance fashion. Several studies have been conducted to uncover the genetic factors contributing to NSHL and SHL in Saudi patients. However, these studies have encountered certain limitations. This review assesses and discusses the genetic factors underpinning NSHL and SHL globally, with a specific emphasis on the Saudi Arabian context. It also explores the prevalence of the most observed genetic causes of NSHL and SHL in Saudi Arabia. It also sheds light on areas where further research is needed to fully understand the genetic foundations of hearing loss in the Saudi population. This review identifies several gaps in research in NSHL and SHL and provides insights into potential research to be conducted.

Priming central sound processing circuits through induction of spontaneous activity in the cochlea before hearing onset

Sensory systems experience a period of intrinsically generated neural activity before maturation is complete and sensory transduction occurs. Here we review evidence describing the mechanisms and functions of this 'spontaneous' activity in the auditory system. Both ex vivo and in vivo studies indicate that this correlated activity is initiated by non-sensory supporting cells within the developing cochlea, which induce depolarization and burst firing of groups of nearby hair cells in the sensory epithelium, activity that is conveyed to auditory neurons that will later process similar sound features. This stereotyped neural burst firing promotes cellular maturation, synaptic refinement, acoustic sensitivity, and establishment of sound-responsive domains in the brain. While sensitive to perturbation, the developing auditory system exhibits remarkable homeostatic mechanisms to preserve periodic burst firing in deaf mice. Preservation of this early spontaneous activity in the context of deafness may enhance the efficacy of later interventions to restore hearing.

A murine model for the del(GJB6-D13S1830) deletion recapitulating the phenotype of human DFNB1 hearing impairment: generation and functional and histopathological study

Inherited hearing impairment is a remarkably heterogeneous monogenic condition, involving hundreds of genes, most of them with very small (< 1%) epidemiological contributions. The exception is GJB2, the gene encoding connexin-26 and underlying DFNB1, which is the most frequent type of autosomal recessive non-syndromic hearing impairment (ARNSHI) in most populations (up to 40% of ARNSHI cases). DFNB1 is caused by different types of pathogenic variants in GJB2, but also by large deletions that keep the gene intact but remove an upstream regulatory element that is essential for its expression. Such large deletions, found in most populations, behave as complete loss-of-function variants, usually associated with a profound hearing impairment. By using CRISPR-Cas9 genetic edition, we have generated a murine model (Dfnb1em274) that reproduces the most frequent of those deletions, del(GJB6-D13S1830). Dfnb1em274 homozygous mice are viable, bypassing the embryonic lethality of the Gjb2 knockout, and present a phenotype of profound hearing loss (> 90 dB SPL) that correlates with specific structural abnormalities in the cochlea. We show that Gjb2 expression is nearly abolished and its protein product, Cx26, is nearly absent all throughout the cochlea, unlike previous conditional knockouts in which Gjb2 ablation was not obtained in all cell types. The Dfnb1em274 model recapitulates the clinical presentation of patients harbouring the del(GJB6-D13S1830) variant and thus it is a valuable tool to study the pathological mechanisms of DFNB1 and to assay therapies for this most frequent type of human ARNSHI.

Generation of Connexin-Expressing Stable Cell Pools

Stable cell pools have the advantage of providing a definite, consistent, and reproducible transmission of a transgene of interest, compared to transient expression from a plasmid transfection. Stably expressing a transgene of interest in cells under induction is a powerful way to (switch on and) study a gene function in both in vitro and in vivo assays. Taking advantage of the ability of lentivirus (LV) to promote transgene delivery, and genomic integration and expression in both dividing and nondividing cells, a doxycycline-inducible transfer vector expressing a bicistronic transgene was developed to study the function of connexins in HeLa DH cells. Here, delving on connexin 32 (Cx32), we report how to use the backbone of this vector as a tool to generate stable pools to study the function of a gene of interest (GOI), especially with assays involving Ca2+ imaging, employing the GCaMP6s indicator. We describe a step-by-step protocol to produce the LV particle by transient transfection and the direct use of the harvested LV stock to generate stable cell pools. We further present step-by-step immunolabeling protocols to characterize the transgene protein expression by confocal microscopy using an antibody that targets an extracellular domain epitope of Cx32 in living cells, and in fixed permeabilized cells using high affinity anti-Cx32 antibodies. Using common molecular biology laboratory techniques, this protocol can be adapted to generate stable pools expressing any transgene of interest, for both in vitro and in vivo functional assays, including molecular, immune, and optical assays.

Functional Consequences of Pathogenic Variants of the GJB2 Gene (Cx26) Localized in Different Cx26 Domains

One of the most common forms of genetic deafness has been predominantly associated with pathogenic variants in the GJB2 gene, encoding transmembrane protein connexin 26 (Cx26). The Cx26 molecule consists of an N-terminal domain (NT), four transmembrane domains (TM1-TM4), two extracellular loops (EL1 and EL2), a cytoplasmic loop, and a C-terminus (CT). Pathogenic variants in the GJB2 gene, resulting in amino acid substitutions scattered across the Cx26 domains, lead to a variety of clinical outcomes, including the most common non-syndromic autosomal recessive deafness (DFNB1A), autosomal dominant deafness (DFNA3A), as well as syndromic forms combining hearing loss and skin disorders. However, for rare and poorly documented variants, information on the mode of inheritance is often lacking. Numerous in vitro studies have been conducted to elucidate the functional consequences of pathogenic GJB2 variants leading to amino acid substitutions in different domains of Cx26 protein. In this work, we summarized all available data on a mode of inheritance of pathogenic GJB2 variants leading to amino acid substitutions and reviewed published information on their functional effects, with an emphasis on their localization in certain Cx26 domains.

Age-related changes in P2Y receptor signalling in mouse cochlear supporting cells

Our sense of hearing depends on the function of a specialised class of sensory cells, the hair cells, which are found in the organ of Corti of the mammalian cochlea. The unique physiological environment in which these cells operate is maintained by a syncitium of non-sensory supporting cells, which are crucial for regulating cochlear physiology and metabolic homeostasis. Despite their importance for cochlear function, the role of these supporting cells in age-related hearing loss, the most common sensory deficit in the elderly, is poorly understood. Here, we investigated the age-related changes in the expression and function of metabotropic purinergic receptors (P2Y1 , P2Y2 and P2Y4 ) in the supporting cells of the cochlear apical coil. Purinergic signalling in supporting cells is crucial during the development of the organ of Corti and purinergic receptors are known to undergo changes in expression during ageing in several tissues. Immunolabelling and Ca2+ imaging experiments revealed a downregulation of P2Y receptor expression and a decrease of purinergic-mediated calcium responses after early postnatal stages in the supporting cells. An upregulation of P2Y receptor expression was observed in the aged cochlea when compared to 1 month-old adults. The aged mice also had significantly larger calcium responses and displayed calcium oscillations during prolonged agonist applications. We conclude that supporting cells in the aged cochlea upregulate P2Y2 and P2Y4 receptors and display purinergic-induced Ca2+ responses that mimic those observed during pre-hearing stages of development, possibly aimed at limiting or preventing further damage to the sensory epithelium. KEY POINTS: Age-related hearing loss is associated with lower hearing sensitivity and decreased ability to understand speech. We investigated age-related changes in the expression and function of metabotropic purinergic (P2Y) receptors in cochlear non-sensory supporting cells of mice displaying early-onset (C57BL/6N) and late-onset (C3H/HeJ) hearing loss. The expression of P2Y1 , P2Y2 and P2Y4 receptors in the supporting cells decreased during cochlear maturation, but that of P2Y2 and P2Y4 was upregulated in the aged cochlea. P2Y2 and P2Y4 receptors were primarily responsible for the ATP-induced Ca2+ responses in the supporting cells. The degree of purinergic expression upregulation in aged supporting cells mirrored hearing loss progression in the different mouse strains. We propose that the upregulation of purinergic-mediated signalling in the aged cochlea is subsequent to age-related changes in the hair cells and may act as a protective mechanism to limit or to avoid further damage to the sensory epithelium.

TRPA1 activation in non-sensory supporting cells contributes to regulation of cochlear sensitivity after acoustic trauma

TRPA1 channels are expressed in nociceptive neurons, where they detect noxious stimuli, and in the mammalian cochlea, where their function is unknown. Here we show that TRPA1 activation in the supporting non-sensory Hensen's cells of the mouse cochlea causes prolonged Ca2+ responses, which propagate across the organ of Corti and cause long-lasting contractions of pillar and Deiters' cells. Caged Ca2+ experiments demonstrated that, similar to Deiters' cells, pillar cells also possess Ca2+-dependent contractile machinery. TRPA1 channels are activated by endogenous products of oxidative stress and extracellular ATP. Since both these stimuli are present in vivo after acoustic trauma, TRPA1 activation after noise may affect cochlear sensitivity through supporting cell contractions. Consistently, TRPA1 deficiency results in larger but less prolonged noise-induced temporary shift of hearing thresholds, accompanied by permanent changes of latency of the auditory brainstem responses. We conclude that TRPA1 contributes to the regulation of cochlear sensitivity after acoustic trauma.

Preservation of developmental spontaneous activity enables early auditory system maturation in deaf mice

Intrinsically generated neural activity propagates through the developing auditory system to promote maturation and refinement of sound processing circuits prior to hearing onset. This early patterned activity is induced by non-sensory supporting cells in the organ of Corti, which are highly interconnected through gap junctions containing connexin 26 (Gjb2). Although loss of function mutations in Gjb2 impair cochlear development and are the most common cause of congenital deafness, it is not known if these variants disrupt spontaneous activity and the developmental trajectory of sound processing circuits in the brain. Here, we show in a new mouse model of Gjb2-mediated congenital deafness that cochlear supporting cells adjacent to inner hair cells (IHCs) unexpectedly retain intercellular coupling and the capacity to generate spontaneous activity, exhibiting only modest deficits prior to hearing onset. Supporting cells lacking Gjb2 elicited coordinated activation of IHCs, leading to coincident bursts of activity in central auditory neurons that will later process similar frequencies of sound. Despite alterations in the structure of the sensory epithelium, hair cells within the cochlea of Gjb2-deficient mice were intact and central auditory neurons could be activated within appropriate tonotopic domains by loud sounds at hearing onset, indicating that early maturation and refinement of auditory circuits was preserved. Only after cessation of spontaneous activity following hearing onset did progressive hair cell degeneration and enhanced auditory neuron excitability manifest. This preservation of cochlear spontaneous neural activity in the absence of connexin 26 may increase the effectiveness of early therapeutic interventions to restore hearing.

The pathogenesis of common Gjb2 mutations associated with human hereditary deafness in mice

Mutations in GJB2 (Gap junction protein beta 2) are the most common genetic cause of non-syndromic hereditary deafness in humans, especially the 35delG and 235delC mutations. Owing to the homozygous lethality of Gjb2 mutations in mice, there are currently no perfect mouse models carrying Gjb2 mutations derived from patients for mimicking human hereditary deafness and for unveiling the pathogenesis of the disease. Here, we successfully constructed heterozygous Gjb2+/35delG and Gjb2+/235delC mutant mice through advanced androgenic haploid embryonic stem cell (AG-haESC)-mediated semi-cloning technology, and these mice showed normal hearing at postnatal day (P) 28. A homozygous mutant mouse model, Gjb235delG/35delG, was then generated using enhanced tetraploid embryo complementation, demonstrating that GJB2 plays an indispensable role in mouse placenta development. These mice exhibited profound hearing loss similar to human patients at P14, i.e., soon after the onset of hearing. Mechanistic analyses showed that Gjb2 35delG disrupts the function and formation of intercellular gap junction channels of the cochlea rather than affecting the survival and function of hair cells. Collectively, our study provides ideal mouse models for understanding the pathogenic mechanism of DFNB1A-related hereditary deafness and opens up a new avenue for investigating the treatment of this disease.

Recent insights into gap junction biogenesis in the cochlea

In the cochlea, connexin 26 (Cx26) and connexin 30 (Cx30) co-assemble into two types of homomeric and heteromeric gap junctions between adjacent non-sensory epithelial cells. These channels provide a mechanical coupling between connected cells, and their activity is critical to maintain cochlear homeostasis. Many of the mutations in GJB2 or GJB6, which encode Cx26 and Cx30 in humans, impair the formation of membrane channels and cause autosomal syndromic and non-syndromic hearing loss. Thus, deciphering the connexin trafficking pathways in situ should represent a major step forward in understanding the pathogenic significance of many of these mutations. A growing body of evidence now suggests that Cx26/Cx30 heteromeric and Cx30 homomeric channels display distinct assembly mechanisms. Here, we review the most recent advances that have been made toward unraveling the biogenesis and stability of these gap junctions in the cochlea.

GJB2 and GJB6 gene transcripts in the human cochlea: A study using RNAscope, confocal, and super-resolution structured illumination microscopy

Background

Gap junction (GJ) proteins, connexin26 and 30, are highly prevalent in the human cochlea (HC), where they are involved in transcellular signaling, metabolic supply, and fluid homeostasis. Their genes, GJB2 and GJB6, are both located at the DFNB1 locus on chromosome 13q12. Mutations in GJB2 may cause mild to profound non-syndromic deafness. Here, we analyzed for the first time the various expressions of GJB2 and GJB6 gene transcripts in the different cell networks in the HC using the RNAscope technique.

Materials and methods

Archival paraformaldehyde-fixed sections of surgically obtained HC were used to label single mRNA oligonucleotides using the sensitive multiplex RNAscope^® technique with fluorescent-tagged probes. Positive and negative controls also included the localization of ATP1A1, ATP1A2, and KCNJ10 gene transcripts in order to validate the specificity of labeling.

Results

Confocal and super-resolution structured illumination microscopy (SR-SIM) detected single gene transcripts as brightly stained puncta. The GJB2 and GJB6 gene transcripts were distributed in the epithelial and connective tissue systems in all three cochlear turns. The largest number of GJB2 and GJB6 gene transcripts was in the outer sulcus, spiral ligament, and stria vascularis (SV). Oligonucleotides were present in the supporting cells of the organ of Corti (OC), spiral limbus fibrocytes, and the floor of the scala vestibuli. Multiplex gene data suggest that cells in the cochlear lateral wall contain either GJB2 or GJB6 gene transcripts or both. The GJB6, but not GJB2, gene transcripts were found in the intermediate cells but none were found in the marginal cells. There were no GJB2 or GJB6 gene transcripts found in the hair cells and only a few in the spiral ganglion cells.

Conclusion

Both GJB2 and GJB6 mRNA gene transcripts were localized in cells in the adult HC using RNAscope^®in situ hybridization (ISH) and high resolution microscopy. Generally, GJB6 dominated over GJB2, except in the basal cells. Results suggest that cells may contain either GJB2 or GJB6 gene transcripts or both. This may be consistent with specialized GJ plaques having separate channel permeability and gating properties. A reduction in the number of GJB2 gene transcripts was found in the basal turn. Such information may be useful for future gene therapy.

Pathological mechanisms of connexin26-related hearing loss: Potassium recycling, ATP-calcium signaling, or energy supply?

Hereditary deafness is one of the most common human birth defects. GJB2 gene mutation is the most genetic etiology. Gap junction protein 26 (connexin26, Cx26) encoded by the GJB2 gene, which is responsible for intercellular substance transfer and signal communication, plays a critical role in hearing acquisition and maintenance. The auditory character of different Connexin26 transgenic mice models can be classified into two types: profound congenital deafness and late-onset progressive hearing loss. Recent studies demonstrated that there are pathological changes including endocochlear potential reduction, active cochlear amplification impairment, cochlear developmental disorders, and so on, in connexin26 deficiency mice. Here, this review summarizes three main hypotheses to explain pathological mechanisms of connexin26-related hearing loss: potassium recycling disruption, adenosine-triphosphate-calcium signaling propagation disruption, and energy supply dysfunction. Elucidating pathological mechanisms underlying connexin26-related hearing loss can help develop new protective and therapeutic strategies for this common deafness. It is worthy of further study on the detailed cellular and molecular upstream mechanisms to modify connexin (channel) function.

Active role of glia-like supporting cells in the organ of Corti: Membrane proteins and their roles in hearing

The organ of Corti, located in the cochlea in the inner ear, is one of the major sensory organs involved in hearing. The organ of Corti consists of hair cells, glia-like supporting cells, and the cochlear nerve, which work in harmony to receive sound from the outer ear and transmit auditory signals to the cochlear nucleus in the auditory ascending pathway. In this process, maintenance of the endocochlear potential, with a high potassium gradient and clearance of electrolytes and biochemicals in the inner ear, is critical for normal sound transduction. There is an emerging need for a thorough understanding of each cell type involved in this process to understand the sophisticated mechanisms of the organ of Corti. Hair cells have long been thought to be active, playing a primary role in the cochlea in actively detecting and transmitting signals. In contrast, supporting cells are thought to be silent and function to support hair cells. However, growing lines of evidence regarding the membrane proteins that mediate ionic movement in supporting cells have demonstrated that supporting cells are not silent, but actively play important roles in normal signal transduction. In this review, we summarize studies that characterize diverse membrane proteins according to the supporting cell subtypes involved in cochlear physiology and hearing. This review contributes to a better understanding of supporting cell functions and facilitates the development of potential therapeutic tools for hearing loss.

Connexin Mutations and Hereditary Diseases

Inherited diseases caused by connexin mutations are found in multiple organs and include hereditary deafness, congenital cataract, congenital heart diseases, hereditary skin diseases, and X-linked Charcot–Marie–Tooth disease (CMT1X). A large number of knockout and knock-in animal models have been used to study the pathology and pathogenesis of diseases of different organs. Because the structures of different connexins are highly homologous and the functions of gap junctions formed by these connexins are similar, connexin-related hereditary diseases may share the same pathogenic mechanism. Here, we analyze the similarities and differences of the pathology and pathogenesis in animal models and find that connexin mutations in gap junction genes expressed in the ear, eye, heart, skin, and peripheral nerves can affect cellular proliferation and differentiation of corresponding organs. Additionally, some dominant mutations (e.g., Cx43 p.Gly60Ser, Cx32 p.Arg75Trp, Cx32 p.Asn175Asp, and Cx32 p.Arg142Trp) are identified as gain-of-function variants in vivo, which may play a vital role in the onset of dominant inherited diseases. Specifically, patients with these dominant mutations receive no benefits from gene therapy. Finally, the complete loss of gap junctional function or altered channel function including permeability (ions, adenosine triphosphate (ATP), Inositol 1,4,5-trisphosphate (IP3), Ca²⁺, glucose, miRNA) and electric activity are also identified in vivo or in vitro.

Failure Of Hearing Acquisition in Mice With Reduced Expression of Connexin 26 Correlates With the Abnormal Phasing of Apoptosis Relative to Autophagy and Defective ATP-Dependent Ca2+ Signaling in Kölliker’s Organ

Mutations in the GJB2 gene that encodes connexin 26 (Cx26) are the predominant cause of prelingual hereditary deafness, and the most frequently encountered variants cause complete loss of protein function. To investigate how Cx26 deficiency induces deafness, we examined the levels of apoptosis and autophagy in Gjb2^loxP/loxP; ROSA26^CreER mice injected with tamoxifen on the day of birth. After weaning, these mice exhibited severe hearing impairment and reduced Cx26 expression in the cochlear duct. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) positive cells were observed in apical, middle, and basal turns of Kölliker’s organ at postnatal (P) day 1 (P1), associated with increased expression levels of cleaved caspase 3, but decreased levels of autophagy-related proteins LC3-II, P62, and Beclin1. In Kölliker’s organ cells with decreased Cx26 expression, we also found significantly reduced levels of intracellular ATP and hampered Ca²⁺ responses evoked by extracellular ATP application. These results offer novel insight into the mechanisms that prevent hearing acquisition in mouse models of non-syndromic hearing impairment due to Cx26 loss of function.

ATP and ACh Evoked Calcium Transients in the Neonatal Mouse Cochlear and Vestibular Sensory Epithelia

Hair cells in the mammalian inner ear sensory epithelia are surrounded by supporting cells which are essential for function of cochlear and vestibular systems. In mice, support cells exhibit spontaneous intracellular Ca²⁺ transients in both auditory and vestibular organs during the first postnatal week before the onset of hearing. We recorded long lasting (>200 ms) Ca²⁺ transients in cochlear and vestibular support cells in neonatal mice using the genetic calcium indicator GCaMP5. Both cochlear and vestibular support cells exhibited spontaneous intracellular Ca²⁺ transients (GCaMP5 ΔF/F), in some cases propagating as waves from the apical (endolymph facing) to the basolateral surface with a speed of ∼25 μm per second, consistent with inositol trisphosphate dependent calcium induced calcium release (CICR). Acetylcholine evoked Ca²⁺ transients were observed in both inner border cells in the cochlea and vestibular support cells, with a larger change in GCaMP5 fluorescence in the vestibular support cells. Adenosine triphosphate evoked robust Ca²⁺ transients predominantly in the cochlear support cells that included Hensen’s cells, Deiters’ cells, inner hair cells, inner phalangeal cells and inner border cells. A Ca²⁺ event initiated in one inner border cells propagated in some instances longitudinally to neighboring inner border cells with an intercellular speed of ∼2 μm per second, and decayed after propagating along ∼3 cells. Similar intercellular propagation was not observed in the radial direction from inner border cell to inner sulcus cells, and was not observed between adjacent vestibular support cells.

Natural Course of Residual Hearing with Reference to GJB2 and SLC26A4 Genotypes: Clinical Implications for Hearing Rehabilitation

BACKGROUND

Understanding the characteristics of residual hearing at low frequencies and its natural course in relation to molecular genetic etiology may be important in developing rehabilitation strategies. Thus, we aimed to explore the characteristics and natural course of residual hearing at low frequencies associated with the two most frequent deafness genes: GJB2 and SLC26A4.

METHODS

Initially, 53 GJB2 and 65 SLC26A4 subjects were enrolled, respectively. Only those whose audiograms exhibited hearing thresholds ≤70 dB at 250 and 500 Hz, and who had at least 1-year follow-up period between the first and last audiograms, were included. Collectively, the clinical characteristics of 14 ears from eight subjects with GJB2 variants, and 31 ears from 22 subjects with SLC26A4 variants fulfilled the strict criteria. In this study, a dropout rate refers to an incidence of dropping out of the cohort by cochlear implant surgery due to severe hearing deterioration.

RESULTS

Among the ears with complete serial audiogram data set, significant residual hearing at low frequencies at the time of inclusion was observed in 18.8% of those with GJB2 variants (15 out of 80 ears) and 42.6% of those with SLC26A4 variants (46 out of 108 ears), revealing a difference between two deafness genes. Subsequently, ears with SLC26A4 variants (11 of 46 ears, 23.9%) turned out to have a higher dropout rate for cochlear implantation due to hearing deterioration within the first year than those with GJB2 variants (1 of 15, 6.7%), albeit with no statistical significance. Throughout the follow-up period (mean: 37.2 ± 6.8, range: 12 to 80 months), deterioration of residual hearing at low frequencies at 250 Hz (dB HL/y) and 500 Hz (dB HL/y) of those with GJB2 variants exhibited 3.1 ± 1.3 (range: 0 to 15) and 5.2 ± 1.6 (range: 0 to 20), respectively, suggesting the deterioration of residual hearing in GJB2 variants was rather slow and gradual. Specifically, GJB2 p.Leu79Cysfs*3 show less remarkable residual hearing at low frequencies, but then a relatively stable nature. In contrast, SLC26A4 variants demonstrated a significantly higher dropout rate due to severe hearing deterioration requiring cochlear implantation compared with the GJB2 variants. This trend was observed not only in the first-year follow-up period but also in the follow-up periods thereafter. The p.His723Arg;c.919-2A>G genotype of SLC26A4, in particular, was associated with a high propensity for sudden hearing deterioration, as indicated by the dropout rate, which was as high as 46.2% for cochlear implantation due to hearing deterioration during the first year follow-up period. Furthermore, the dropout rate for cochlear implantation was observed in 7.1% of those with GJB2 variants (one out of 14 ears) and 30.3% of those with SLC26A4 variants (10 out of 33 ears) throughout the entire follow-up period.

CONCLUSIONS

Our results suggest that there is a difference with respect to the progressive nature of residual hearing at low frequencies between the two most common genes responsible for hearing loss, which may provide clinical implications of having individualized rehabilitation and timely intervention.

Functional Evaluation of a Rare Variant c.516G>C (p.Trp172Cys) in the GJB2 (Connexin 26) Gene Associated with Nonsyndromic Hearing Loss

Mutations in the GJB2 gene encoding transmembrane protein connexin 26 (Cx26) are the most common cause for hearing loss worldwide. Cx26 plays a crucial role in the ionic and metabolic homeostasis in the inner ear, indispensable for normal hearing process. Different pathogenic mutations in the GJB2 gene can affect all stages of the Cx26 life cycle and result in nonsyndromic autosomal recessive (DFNB1) or dominant (DFNA3) deafness and syndromes associating hearing loss with skin disorders. This study aims to elucidate the functional consequences of a rare GJB2 variant c.516G>C (p.Trp172Cys) found with high frequency in deaf patients from indigenous populations of Southern Siberia (Russia). The substitution c.516G>C leads to the replacement of tryptophan at a conserved amino acid position 172 with cysteine (p.Trp172Cys) in the second extracellular loop of Cx26 protein. We analyzed the subcellular localization of mutant Cx26-p.Trp172Cys protein by immunocytochemistry and the hemichannels permeability by dye loading assay. The GJB2 knockout HeLa cell line has been generated using CRISPR/Cas9 genome editing tool. Subsequently, the HeLa transgenic cell lines stably expressing different GJB2 variants (wild type and mutations associated with hearing loss) were established based on knockout cells and used for comparative functional analysis. The impaired trafficking of mutant Cx26-p.Trp172Cys protein to the plasma membrane and reduced hemichannels permeability support the pathogenic effect of the c.516G>C (p.Trp172Cys) variant and its association with nonsyndromic hearing loss. Our data contribute to a better understanding of the role of mutations in the second extracellular loop of Cx26 protein in pathogenesis of deafness.

Hereditary Hearing Impairment with Cutaneous Abnormalities

Syndromic hereditary hearing impairment (HHI) is a clinically and etiologically diverse condition that has a profound influence on affected individuals and their families. As cutaneous findings are more apparent than hearing-related symptoms to clinicians and, more importantly, to caregivers of affected infants and young individuals, establishing a correlation map of skin manifestations and their underlying genetic causes is key to early identification and diagnosis of syndromic HHI. In this article, we performed a comprehensive PubMed database search on syndromic HHI with cutaneous abnormalities, and reviewed a total of 260 relevant publications. Our in-depth analyses revealed that the cutaneous manifestations associated with HHI could be classified into three categories: pigment, hyperkeratosis/nail, and connective tissue disorders, with each category involving distinct molecular pathogenesis mechanisms. This outline could help clinicians and researchers build a clear atlas regarding the phenotypic features and pathogenetic mechanisms of syndromic HHI with cutaneous abnormalities, and facilitate clinical and molecular diagnoses of these conditions.

Tricellular adherens junctions provide a cell surface delivery platform for connexin 26/30 oligomers in the cochlea

In the cochlea, connexins 26 (Cx26) and 30 (Cx30) largely co-assemble into heteromeric gap junctions, which connect adjacent non-sensory epithelial cells. These channels are believed to ensure the rapid removal of K⁺ away from the base of sensory hair cells, resulting in K⁺ recycling back to the endolymph to maintain cochlear homeostasis. Many of the mutations in GJB2 and GJB6, which encode CX26 and CX30, impair the formation of membrane channels and cause autosomal hearing loss in humans. Although recent advances have been made, several important questions remain about connexin trafficking and gap junction biogenesis. Here we show that tricellular adherens junctions present at the crossroad between adjacent gap junction plaques, provide an unexpected cell surface delivery platform for Cx26/Cx30 oligomers. Using an in situ proximity ligation assay, we detected the presence of non-junctional Cx26/Cx30 oligomers within lipid raft-enriched tricellular junction sites. In addition, we observed that cadherin homophilic interactions are critically involved in microtubule-mediated trafficking of Cx26/Cx30 oligomers to the cell surface. Overall, our results unveil an unexpected role for tricellular junctions in the trafficking and assembly of membrane channels.

Lens Connexin Channels Show Differential Permeability to Signaling Molecules

Gap junction channels mediate the direct intercellular passage of small ions as well as larger solutes such as second messengers. A family of proteins called connexins make up the subunits of gap junction channels in chordate animals. Each individual connexin forms channels that exhibit distinct permeability to molecules that influence cellular signaling, such as calcium ions, cyclic nucleotides, or inositol phosphates. In this review, we examine the permeability of connexin channels containing Cx43, Cx46, and Cx50 to signaling molecules and attempt to relate the observed differences in permeability to possible in vivo consequences that were revealed by studies of transgenic animals where these connexin genes have been manipulated. Taken together, these data suggest that differences in the permeability of individual connexin channels to larger solutes like 3',5'-cyclic adenosine monophosphate (cAMP) and inositol 1,4,5-trisphosphate (IP₃) could play a role in regulating epithelial cell division, differentiation, and homeostasis in organs like the ocular lens.

Gene therapy for genetic mutations affecting non-sensory cells in the cochlea

Congenital hearing loss (HL) affects about 1 in every 500 infants. Among those affected more than half are caused by genetic mutations. According to the cellular sites affected by mutations in the cochlea, deafness genes could be classified into three major groups: those affecting the function of hair cells and synapses, cochlear supporting cells, and cells in the stria vascularis (SV) as well as in the lateral wall. The second and third groups account for more than half of all sensorineural hearing loss (SNHL) cases caused by genetic mutations. Current major treatment options for SNHL patients are hearing aids and cochlear implants (CIs). Hearing aids can only help patients with moderate to severe HL. Resolution of CIs is still improving and these devices are quite expensive especially when lifetime rehabilitation and maintenance costs are included. Tremendous efforts have been made to find novel treatments that are expected to restore hearing with higher-resolution and more natural quality, and to have a significantly lower cost over the lifetime of uses. Gene therapy studies have made impressive progresses in preclinical trials. This review focuses on deafness genes that affect supporting cells and cells in the SV of the cochlea. We will discuss recent progresses and remaining challenges for gene therapies targeting mutations in deafness genes belonging to this category.

Organ-on-chip model shows that ATP release through connexin hemichannels drives spontaneous Ca2+ signaling in non-sensory cells of the greater epithelial ridge in the developing cochlea

Prior work supports the hypothesis that ATP release through connexin hemichannels drives spontaneous Ca2+ signaling in non-sensory cells of the greater epithelial ridge (GER) in the developing cochlea; however, direct proof is lacking. To address this issue, we plated cochlear organotypic cultures (COCs) and whole cell-based biosensors with nM ATP sensitivity (ATP-WCBs) at the bottom and top of an ad hoc designed transparent microfluidic chamber, respectively. By performing dual multiphoton Ca2+ imaging, we monitored the propagation of intercellular Ca2+ waves in the GER of COCs and ATP-dependent Ca2+ responses in overlying ATP-WCBs. Ca2+ signals in both COCs and ATP-WCBs were inhibited by supplementing the extracellular medium with ATP diphosphohydrolase (apyrase). Spontaneous Ca2+ signals were strongly depressed in the presence of Gjb6-/- COCs, in which connexin 30 (Cx30) is absent and connexin 26 (Cx26) is strongly downregulated. In contrast, spontaneous Ca2+ signals were not affected by replacement of Panx1-/- with Panx1+/+ COCs in the microfluidic chamber. Similar results were obtained by estimating ATP release from COCs using a classical luciferin-luciferase bioluminescence assay. Therefore, connexin hemichannels and not pannexin 1 channels mediate the release of ATP that is responsible for Ca2+ wave propagation in the developing mouse cochlea. The technological advances presented here have the potential to shed light on a plethora of unrelated open issues that involve paracrine signaling in physiology and pathology and cannot be addressed with standard methods.

MITF variants cause nonsyndromic sensorineural hearing loss with autosomal recessive inheritance

MITF is a known gene underlying autosomal dominant hearing loss, Waardenburg syndrome (WS). Biallelic MITF mutations have been found associated with a rare hearing loss syndrome consisting eye abnormalities and albinism; and a more severe type of WS whose heterozygous parents were affected with classic WS in both cases. The aims of this study were to identify a new candidate gene causing autosomal recessive nonsyndromic hearing loss (ARNSHL) and confirm its causation by finding additional families affected with the candidate gene and supporting evidences from functional analyses. By using whole exome sequencing, we identified a homozygous c.1022G>A: p.Arg341His variant of MITF, which co-segregated with the hearing loss in five affected children of a consanguineous hearing couple. Targeted exome sequencing in a cohort of 130 NSHL individuals, using our in-house gene panel revealed a second family with c.1021C>T: p.Arg341Cys MITF variant. Functional studies confirmed that the Arg341His and Arg341Cys alleles yielded a normal sized MITF protein, with aberrant cytosolic localization as supported by the molecular model and the reporter assay. In conclusion, we demonstrate MITF as a new cause of ARNSHL, with heterozygous individuals free of symptoms. MITF should be included in clinical testing for NSHL, though it is rare.

Connexin43 and connexin50 channels exhibit different permeability to the second messenger inositol triphosphate

Gap junction channels made of different connexins have distinct permeability to second messengers, which could affect many cell processes, including lens epithelial cell division. Here, we have compared the permeability of IP₃ and Ca²⁺ through channels made from two connexins, Cx43 and Cx50, that are highly expressed in vertebrate lens epithelial cells. Solute transfer was measured while simultaneously monitoring junctional conductance via dual whole-cell/perforated patch clamp. HeLa cells expressing Cx43 or Cx50 were loaded with Fluo-8, and IP₃ or Ca²⁺ were delivered via patch pipette to one cell of a pair, or to a monolayer while fluorescence intensity changes were recorded. Cx43 channels were permeable to IP₃ and Ca²⁺. Conversely, Cx50 channels were impermeable to IP₃, while exhibiting high permeation of Ca²⁺. Reduced Cx50 permeability to IP₃ could play a role in regulating cell division and homeostasis in the lens.

Structure and Function of Cochlear Gap Junctions and Implications for the Translation of Cochlear Gene Therapies

Connexins (Cxs) are ubiquitous membrane proteins that are found throughout vertebrate organs, acting as building blocks of the gap junctions (GJs) known to play vital roles in the normal function of many organs. Mutations in Cx genes (particularly GJB2, which encodes Cx26) cause approximately half of all cases of congenital hearing loss in newborns. Great progress has been made in understanding GJ function and the molecular mechanisms for the role of Cxs in the cochlea. Data reveal that multiple types of Cxs work together to ensure normal development and function of the cochlea. These findings include many aspects not proposed in the classic K+ recycling theory, such as the formation of normal cochlear morphology (e.g., the opening of the tunnel of Corti), the fine-tuning of the innervation of nerve fibers to the hair cells (HCs), the maturation of the ribbon synapses, and the initiation of the endocochlear potential (EP). New data, especially those collected from targeted modification of major Cx genes in the mouse cochlea, have demonstrated that Cx26 plays an essential role in the postnatal maturation of the cochlea. Studies also show that Cx26 and Cx30 assume very different roles in the EP generation, given that only Cx26 is required for normal hearing. This article will review our current understanding of the molecular structure, cellular distribution, and major functions of cochlear GJs. Potential implications of the knowledge of cochlear GJs on the design and implementation of translational studies of cochlear gene therapies for Cx mutations are also discussed.

Lens Connexin Channels Have Differential Permeability to the Second Messenger cAMP

Purpose

Gap junction channels exhibit connexin specific biophysical properties, including the selective intercellular passage of larger solutes, such as second messengers. Here, we have examined the cyclic nucleotide permeability of the lens connexins, which could influence events like epithelial cell division and differentiation.

Methods

We compared the cAMP permeability through channels composed of Cx43, Cx46, or Cx50 using simultaneous measurements of junctional conductance and intercellular transfer. For cAMP detection, the recipient cells were transfected with a cAMP sensor gene, the cyclic nucleotide-modulated channel from sea urchin sperm (SpIH). cAMP was introduced via patch pipette into the cell of the pair that did not express SpIH. SpIH-derived currents were recorded from the other cell of a pair that expressed SpIH. cAMP permeability was also directly visualized in transfected cells using a chemically modified fluorescent form of the molecule.

Results

cAMP transfer was observed for homotypic Cx43 channels over a wide range of junctional conductance. Homotypic Cx46 channels also transferred cAMP, but permeability was reduced compared with Cx43. In contrast, homotypic Cx50 channels exhibited extremely low permeability to cAMP, when compared with either Cx43, or Cx46.

Conclusions

These data show that channels made from Cx43 and Cx46 result in the intercellular delivery of cAMP in sufficient quantity to activate cyclic nucleotide-modulated channels. The data also suggest that the greatly reduced cAMP permeability of Cx50 channels could play a role in the regulation of cell division in the lens.

Inner Ear Connexin Channels: Roles in Development and Maintenance of Cochlear Function

Connexin 26 and connexin 30 are the prevailing isoforms in the epithelial and connective tissue gap junction systems of the developing and mature cochlea. The most frequently encountered variants of the genes that encode these connexins, which are transcriptionally coregulated, determine complete loss of protein function and are the predominant cause of prelingual hereditary deafness. Reducing connexin 26 expression by Cre/loxP recombination in the inner ear of adult mice results in a decreased endocochlear potential, increased hearing thresholds, and loss of >90% of outer hair cells, indicating that this connexin is essential for maintenance of cochlear function. In the developing cochlea, connexins are necessary for intercellular calcium signaling activity. Ribbon synapses and basolateral membrane currents fail to mature in inner hair cells of mice that are born with reduced connexin expression, even though hair cells do not express any connexin. In contrast, pannexin 1, an alternative mediator of intercellular signaling, is dispensable for hearing acquisition and auditory function.

A Human-Derived Monoclonal Antibody Targeting Extracellular Connexin Domain Selectively Modulates Hemichannel Function

Connexin hemichannels, which are plasma membrane hexameric channels (connexons) composed of connexin protein protomers, have been implicated in a host of physiological processes and pathological conditions. A number of single point pathological mutations impart a “leaky” character to the affected hemichannels, i.e., make them more active or hyperactive, suggesting that normal physiological condition could be recovered using selective hemichannel inhibitors. Recently, a human-derived monoclonal antibody named abEC1.1 has been shown to inhibit both wild type and hyperactive hemichannels composed of human (h) connexin 26 (hCx26) subunits. The aims of this work were (1) to characterize further the ability of abEC1.1 to selectively modulate connexin hemichannel function and (2) to assess its in vitro stability in view of future translational applications. In silico analysis of abEC1.1 interaction with the hCx26 hemichannel identified critically important extracellular domain amino acids that are conserved in connexin 30 (hCx30) and connexin 32 (hCx32). Patch clamp experiments performed in HeLa DH cells confirmed the inhibition efficiency of abEC1.1 was comparable for hCx26, hCx30 and hCx32 hemichannels. Of note, even a single amino acid difference in the putative binding region reduced drastically the inhibitory effects of the antibody on all the other tested hemichannels, namely hCx30.2/31.3, hCx30.3, hCx31, hCx31.1, hCx37, hCx43 and hCx45. Plasma membrane channels composed of pannexin 1 were not affected by abEC1.1. Finally, size exclusion chromatography assays showed the antibody does not aggregate appreciably in vitro. Altogether, these results indicate abEC1.1 is a promising tool for further translational studies.

Spontaneous and Acetylcholine Evoked Calcium Transients in the Developing Mouse Utricle

Spontaneous calcium transients are present during early postnatal development in the mouse retina and cochlea, and play an important role in maturation of the sensory organs and neural circuits in the central nervous system (CNS). It is not known whether similar calcium transients occur during postnatal development in the vestibular sensory organs. Here we demonstrate spontaneous intracellular calcium transients in sensory hair cells (HCs) and supporting cells (SCs) in the murine utricular macula during the first two postnatal weeks. Calcium transients were monitored using a genetically encoded calcium indicator, GCaMP5G (G5), at 100 ms-frame⁻¹ in excised utricle sensory epithelia, including HCs, SCs, and neurons. The reporter line expressed G5 and tdTomato (tdT) in a Gad2-Cre dependent manner within a subset of utricular HCs, SCs and neurons. Kinetics of the G5 reporter limited temporal resolution to calcium events lasting longer than 200 ms. Spontaneous calcium transients lasting 1-2 s were observed in the expressing population of HCs at birth and slower spontaneous transients lasting 10-30 s appeared in SCs by P3. Beginning at P5, calcium transients could be modulated by application of the efferent neurotransmitter acetylcholine (ACh). In mature mice, calcium transients in the utricular macula occurred spontaneously, had a duration 1-2 s, and could be modulated by the exogenous application of acetylcholine (ACh) or muscarine. Long-lasting calcium transients evoked by ACh in mature mice were blocked by atropine, consistent with previous reports describing the role of muscarinic receptors expressed in calyx bearing afferents in efferent control of vestibular sensation. Large spontaneous and ACh evoked transients were reversibly blocked by the inositol trisphosphate receptor (IP₃R) antagonist aminoethoxydiphenyl borate (2-APB). Results demonstrate long-lasting calcium transients are present in the utricular macula during the first postnatal week, and that responses to ACh mature over this same time period.

Spiking Pattern of the Mouse Developing Inner Hair Cells Is Mostly Invariant Along the Tonotopic Axis

During development, the sensory cells of the cochlea, the inner hair cells (IHCs), fire spontaneous calcium action potentials. This activity at the pre-hearing stage allows the IHCs to autonomously excite the auditory nerve fibers and hence, represents an efficient mechanism to shape the tonotopic organization along the ascending auditory pathway. Using calcium imaging, we show that the activity in the developing cochlea consists of calcium waves that propagate across the supporting and sensory cells. Both basal and apical IHCs were characterized by similar spontaneous calcium transients interspaced with silent periods, consistent with bursts of action potentials recorded in patch-clamp. In addition, adjacent auditory hair cells tend to have a synchronized [Ca²⁺]_i activity, irrespective of their location along the base-to-apex gradient of the cochlea. Finally, we show that the mechanical ablation of the inner phalangeal cells (IPCs), a class of supporting cells, reduces the synchronized [Ca²⁺]_i activity between neighboring sensory cells. These findings support the hypothesis that the tonotopic map refinement in higher auditory centers would depend on the synchronization of a discrete number of auditory sensory cells.

Calcium interactions with Cx26 hemmichannel: Spatial association between MD simulations biding sites and variant pathogenicity

Connexinophaties are a collective of diseases related to connexin channels and hemichannels. In particular many Cx26 alterations are strongly associated to human deafness. Calcium plays an important role on this structures regulation. Here, using calcium as a probe, extensive atomistic Molecular Dynamics simulations were performed on the Cx26 hemichannel embedded in a lipid bilayer. Exploring different initial conditions and calcium concentration, simulation reached ∼4 μs. Several analysis were carried out in order to reveal the calcium distribution and localization, such as electron density profiles, density maps and distance time evolution, which is directly associated to the interaction energy. Specific amino acid interactions with calcium and their stability were capture within this context. Few of these sites such as, GLU42, GLU47, GLY45 and ASP50, were already suggested in the literature. Besides, we identified novel calcium biding sites: ASP2, ASP117, ASP159, GLU114, GLU119, GLU120 and VAL226. To the best of our knowledge, this is the first time that these sites are reported within this context. Furthermore, since various pathologies involving the Cx26 hemichannel are associated with pathogenic variants in the corresponding CJB2 gene, using ClinVar, we were able to spatially associate the 3D positions of the identified calcium binding sites within the framework of this work with reported pathogenic variants in the CJB2 gene. This study presents a first step on finding associations between molecular features and pathological variants of the Cx26 hemichannel.

Specific mutations in the permease domain of septal protein SepJ differentially affect functions related to multicellularity in the filamentous cyanobacterium Anabaena

Filamentous, heterocyst-forming cyanobacteria are multicellular organisms in which growth requires the activity of two interdependent cell types that exchange nutrients and regulators. Vegetative cells provide heterocysts with reduced carbon, and heterocysts provide vegetative cells with fixed nitrogen. Additionally, heterocyst differentiation from vegetative cells is regulated by inhibitors of differentiation produced by prospective heterocysts and heterocysts. Proteinaceous structures known as septal junctions join the cells in the filament. The SepJ protein is involved in formation of septal junctions in the model heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120. SepJ bears extra-membrane and membrane (permease) domains and is located at the cell poles in the intercellular septa of the filament. Here we created Anabaena mutants that produce SepJ proteins altered in the permease domain. Some of these mutant SepJ proteins did not provide functions needed for Anabaena to form long filaments and (in some cases) differentiate heterocysts, identifying amino acids and amino acid stretches that are important for the structure or function of the protein. Some other mutant SepJ proteins fulfilled filamentation and heterocyst differentiation functions but failed to provide normal communication function assessed via the intercellular transfer of the fluorescent marker calcein. These mutant SepJ proteins bore mutations in amino acids located at the cytoplasmic face of the permease, which could affect access of the fluorescent marker to the septal junctions. Overall, the data are consistent with the idea that SepJ carries out multiple roles in the multicellular function of the Anabaena filament.

Connexins and Disease

Inherited or acquired alterations in the structure and function of connexin proteins have long been associated with disease. In the present work, we review current knowledge on the role of connexins in diseases associated with the heart, nervous system, cochlea, and skin, as well as cancer and pleiotropic syndromes such as oculodentodigital dysplasia (ODDD). Although incomplete by virtue of space and the extent of the topic, this review emphasizes the fact that connexin function is not only associated with gap junction channel formation. As such, both canonical and noncanonical functions of connexins are fundamental components in the pathophysiology of multiple connexin related disorders, many of them highly debilitating and life threatening. Improved understanding of connexin biology has the potential to advance our understanding of mechanisms, diagnosis, and treatment of disease.

Decreased disulphide/thiol ratio in patients with autosomal recessive non-syndromic hearing loss

INTRODUCTION

Oxidative stress plays a key role in the formation of age-related, noise-induced and drug-induced hearing loss. Thiols are organic compounds which can react with free radicals to protect against tissue and cell damage caused by reactive oxygen. There are no studies in literature on the association between autosomal recessive non-syndromic hearing loss(ARNSHL) including GJB2 and non-GJB2 mutations and thiol-disulphide balance. In this study, we aim to assess whether thiol-disulphide balance is disrupted in patients with ARNSHL.

METHODS

Thirty-one ARNSHL patients and thirty-one healthy controls were included in this study. Patients whose parents were first degree cousins and who had at least two congenital hearing loss in the same family were included in the study. Audiological tests included air - bone pure tone audiometry and auditory brain stem response. GJB2 gene analysis was performed using sanger sequence method. Tests of thiol/disulphide homeostasis were conducted using the automated spectrophotometric method. We first investigated whether there was a significant difference between ARNSHL patients and healthy controls. Then, in order to determine the differential effect of the GJB2 gene mutations and non-GJB2 gene mutations on the thiol-disulphide balance, subjects were divided into three groups: Group 1 included patients with GJB2 mutations; Group 2 included patients with non-GJB2 mutations; Group 3 included healthy subjects.

RESULTS

Patients with ARNSHL had significantly higher native thiol (411.6 ± 54.3 μmol/l vs. 368.0 ± 64.3 μmol/l, p = 0.006), total thiol levels (440.3 ± 56.2 μmol/l vs. 402.4 ± 65.9 μmol/l, p = 0.018), and lower disulphide levels (14.3 ± 5.7 μmol/l) vs. (17.1 ± 4.9 μmol/l), (p = 0.043) compared to the control group. Moreover, disulphide /native thiol (p < 0.001) and disulphide/total thiol (p < 0.001) were also detected lower in the ARNSHL group compared to the control group. Thiol-disulphide hemostasis parameters between all three groups showed that the native thiol and total thiol were increased in the Group 1 and Group 2. The disulphide levels decreased in Group 1 and 2, although not statistically significant.

CONCLUSION

It was shown that thiol levels increased and disulphide levels decreased in patients with autosomal recessive non-syndromic hearing loss. It also may suggest that there is a reverse association between ARNSHL and oxidative stress. Further studies are needed on whether or not ARNSHL cause oxidative stress limited to the inner ear and cochlea.

A Cell Junctional Protein Network Associated with Connexin-26

GJB2 mutations are the leading cause of non-syndromic inherited hearing loss. GJB2 encodes connexin-26 (CX26), which is a connexin (CX) family protein expressed in cochlea, skin, liver, and brain, displaying short cytoplasmic N-termini and C-termini. We searched for CX26 C-terminus binding partners by affinity capture and identified 12 unique proteins associated with cell junctions or cytoskeleton (CGN, DAAM1, FLNB, GAPDH, HOMER2, MAP7, MAPRE2 (EB2), JUP, PTK2B, RAI14, TJP1, and VCL) by using mass spectrometry. We show that, similar to other CX family members, CX26 co-fractionates with TJP1, VCL, and EB2 (EB1 paralogue) as well as the membrane-associated protein ASS1. The adaptor protein CGN (cingulin) co-immuno-precipitates with CX26, ASS1, and TJP1. In addition, CGN co-immunoprecipitation with CX30, CX31, and CX43 indicates that CX association is independent on the CX C-terminus length or sequence. CX26, CGN, FLNB, and DAMM1 were shown to distribute to the organ of Corti and hepatocyte plasma membrane. In the mouse liver, CX26 and TJP1 co-localized at the plasma membrane. In conclusion, CX26 associates with components of other membrane junctions that integrate with the cytoskeleton.

Connexin 26 Immunohistochemistry in Temporal Bones With Cochlear Otosclerosis

HYPOTHESIS

Connexin-26 (Cx26) expression is diminished in the spiral ligament of subjects with hearing loss and cochlear otosclerosis (CO).

BACKGROUND

Human temporal bone (HTB) studies have demonstrated that CO is associated with hyalinization of the spiral ligament. We hypothesize that hyalinization is associated with a loss of fibrocytes with a consequent decline in Cx26 expression. Cx26 and Connexin-30 (Cx30) encode gap junction proteins expressed in supporting cells of the organ of Corti, the spiral limbus, stria vascularis, and in fibrocytes of the spiral ligament. These gap junctions are critical for potassium recycling and maintenance of the endocochlear potential. Diminished expression of these proteins would likely be associated with hearing dysfunction.

METHODS

Histopathology and clinical characteristics of 45 HTB specimens with CO and spiral ligament hyalinization were reviewed. Those with sensorineural or mixed hearing loss but normal or near-normal hair cell counts were analyzed with light microscopy, and Cx26-immunoreactive (IR) signal was qualitatively assessed.

RESULTS

H&E staining demonstrated hyalinization in the spiral ligament and loss of type II and type III fibrocytes. Cx26-IR was diminished throughout the cochlea affected with CO compared with normal controls.

CONCLUSIONS

Cx26-IR reduction in the spiral ligament of subjects with CO likely plays a role in hearing loss.

Cues to Opening Mechanisms From in Silico Electric Field Excitation of Cx26 Hemichannel and in Vitro Mutagenesis Studies in HeLa Transfectans

Connexin channels play numerous essential roles in virtually every organ by mediating solute exchange between adjacent cells, or between cytoplasm and extracellular milieu. Our understanding of the structure-function relationship of connexin channels relies on X-ray crystallographic data for human connexin 26 (hCx26) intercellular gap junction channels. Comparison of experimental data and molecular dynamics simulations suggests that the published structures represent neither fully-open nor closed configurations. To facilitate the search for alternative stable configurations, we developed a coarse grained (CG) molecular model of the hCx26 hemichannel and studied its responses to external electric fields. When challenged by a field of 0.06 V/nm, the hemichannel relaxed toward a novel configuration characterized by a widened pore and an increased bending of the second transmembrane helix (TM2) at the level of the conserved Pro87. A point mutation that inhibited such transition in our simulations impeded hemichannel opening in electrophysiology and dye uptake experiments conducted on HeLa tranfectants. These results suggest that the hCx26 hemichannel uses a global degree of freedom to transit between different configuration states, which may be shared among the whole connexin family.

DFNB1 Non-syndromic Hearing Impairment: Diversity of Mutations and Associated Phenotypes

The inner ear is a very complex sensory organ whose development and function depend on finely balanced interactions among diverse cell types. The many different kinds of inner ear supporting cells play the essential roles of providing physical and physiological support to sensory hair cells and of maintaining cochlear homeostasis. Appropriately enough, the gene most commonly mutated among subjects with hereditary hearing impairment (HI), GJB2, encodes the connexin-26 (Cx26) gap-junction channel protein that underlies both intercellular communication among supporting cells and homeostasis of the cochlear fluids, endolymph and perilymph. GJB2 lies at the DFNB1 locus on 13q12. The specific kind of HI associated with this locus is caused by recessively-inherited mutations that inactivate the two alleles of the GJB2 gene, either in homozygous or compound heterozygous states. We describe the many diverse classes of genetic alterations that result in DFNB1 HI, such as large deletions that either destroy the GJB2 gene or remove a regulatory element essential for GJB2 expression, point mutations that interfere with promoter function or splicing, and small insertions or deletions and nucleotide substitutions that target the GJB2 coding sequence. We focus on how these alterations disrupt GJB2 and Cx26 functions and on their different effects on cochlear development and physiology. We finally discuss the diversity of clinical features of DFNB1 HI as regards severity, age of onset, inner ear malformations and vestibular dysfunction, highlighting the areas where future research should be concentrated.

Mouse Panx1 Is Dispensable for Hearing Acquisition and Auditory Function

Panx1 forms plasma membrane channels in brain and several other organs, including the inner ear. Biophysical properties, activation mechanisms and modulators of Panx1 channels have been characterized in detail, however the impact of Panx1 on auditory function is unclear due to conflicts in published results. To address this issue, hearing performance and cochlear function of the Panx1−/− mouse strain, the first with a reported global ablation of Panx1, were scrutinized. Male and female homozygous (Panx1−/−), hemizygous (Panx1+/−) and their wild type (WT) siblings (Panx1+/+) were used for this study. Successful ablation of Panx1 was confirmed by RT-PCR and Western immunoblotting in the cochlea and brain of Panx1−/− mice. Furthermore, a previously validated Panx1-selective antibody revealed strong immunoreactivity in WT but not in Panx1−/− cochleae. Hearing sensitivity, outer hair cell-based “cochlear amplifier” and cochlear nerve function, analyzed by auditory brainstem response (ABR) and distortion product otoacoustic emission (DPOAE) recordings, were normal in Panx1+/− and Panx1−/− mice. In addition, we determined that global deletion of Panx1 impacts neither on connexin expression, nor on gap-junction coupling in the developing organ of Corti. Finally, spontaneous intercellular Ca²⁺ signal (ICS) activity in organotypic cochlear cultures, which is key to postnatal development of the organ of Corti and essential for hearing acquisition, was not affected by Panx1 ablation. Therefore, our results provide strong evidence that, in mice, Panx1 is dispensable for hearing acquisition and auditory function.

Altered CO2 sensitivity of connexin26 mutant hemichannels in vitro

Connexin26 (Cx26) mutations underlie human pathologies ranging from hearing loss to keratitis ichthyosis deafness (KID) syndrome. Cx26 hemichannels are directly gated by CO₂ and contribute to the chemosensory regulation of breathing. The KID syndrome mutation A88V is insensitive to CO₂, and has a dominant negative action on the CO₂ sensitivity of Cx26^WT hemichannels, and reduces respiratory drive in humans. We have now examined the effect of further human mutations of Cx26 on its sensitivity to CO₂ : Mutated Cx26 subunits, carrying one of A88S, N14K, N14Y, M34T, or V84L, were transiently expressed in HeLa cells. The CO₂-dependence of hemichannel activity, and their ability to exert dominant negative actions on cells stably expressing Cx26^WT, was quantified by a dye-loading assay. The KID syndrome mutation, N14K, abolished the sensitivity of Cx26 to CO₂ Both N14Y and N14K exerted a powerful dominant negative action on the CO₂ sensitivity of Cx26^WT None of the other mutations (all recessive) had a dominant negative action. A88S shifted the affinity of Cx26 to slightly higher levels without reducing its ability to fully open to CO₂ M34T did not change the affinity of Cx26 for CO₂ but reduced its ability to open in response to CO₂ V84L had no effect on the CO₂-sensitivity of Cx26. Some pathological mutations of Cx26 can therefore alter the CO₂ sensitivity of Cx26 hemichannels. The loss of CO₂ sensitivity could contribute to pathology and consequent reduced respiratory drive could be an unrecognized comorbidity of these pathologies.

Dissipative structures and biological rhythms

Sustained oscillations abound in biological systems. They occur at all levels of biological organization over a wide range of periods, from a fraction of a second to years, and with a variety of underlying mechanisms. They control major physiological functions, and their dysfunction is associated with a variety of physiological disorders. The goal of this review is (i) to give an overview of the main rhythms observed at the cellular and supracellular levels, (ii) to briefly describe how the study of biological rhythms unfolded in the course of time, in parallel with studies on chemical oscillations, (iii) to present the major roles of biological rhythms in the control of physiological functions, and (iv) the pathologies associated with the alteration, disappearance, or spurious occurrence of biological rhythms. Two tables present the main examples of cellular and supracellular rhythms ordered according to their period, and their role in physiology and pathophysiology. Among the rhythms discussed are neural and cardiac rhythms, metabolic oscillations such as those occurring in glycolysis in yeast, intracellular Ca⁺⁺ oscillations, cyclic AMP oscillations in Dictyostelium amoebae, the segmentation clock that controls somitogenesis, pulsatile hormone secretion, circadian rhythms which occur in all eukaryotes and some bacteria with a period close to 24 h, the oscillatory dynamics of the enzymatic network driving the cell cycle, and oscillations in transcription factors such as NF-ΚB and tumor suppressors such as p53. Ilya Prigogine's concept of dissipative structures applies to temporal oscillations and allows us to unify within a common framework the various rhythms observed at different levels of biological organization, regardless of their period and underlying mechanism.

Design and Characterization of a Human Monoclonal Antibody that Modulates Mutant Connexin 26 Hemichannels Implicated in Deafness and Skin Disorders

Background: Mutations leading to changes in properties, regulation, or expression of connexin-made channels have been implicated in 28 distinct human hereditary diseases. Eight of these result from variants of connexin 26 (Cx26), a protein critically involved in cell-cell signaling in the inner ear and skin. Lack of non-toxic drugs with defined mechanisms of action poses a serious obstacle to therapeutic interventions for diseases caused by mutant connexins. In particular, molecules that specifically modulate connexin hemichannel function without affecting gap junction channels are considered of primary importance for the study of connexin hemichannel role in physiological as well as pathological conditions. Monoclonal antibodies developed in the last three decades have become the most important class of therapeutic biologicals. Recombinant methods permit rapid selection and improvement of monoclonal antibodies from libraries with large diversity.

Methods: By screening a combinatorial library of human single-chain fragment variable (scFv) antibodies expressed in phage, we identified a candidate that binds an extracellular epitope of Cx26. We characterized antibody action using a variety of biochemical and biophysical assays in HeLa cells, organotypic cultures of mouse cochlea and human keratinocyte-derived cells.

Results: We determined that the antibody is a remarkably efficient, non-toxic, and completely reversible inhibitor of hemichannels formed by connexin 26 and does not affect direct cell-cell communication via gap junction channels. Importantly, we also demonstrate that the antibody efficiently inhibits hyperative mutant Cx26 hemichannels implicated in autosomal dominant non-syndromic hearing impairment accompanied by keratitis and hystrix-like ichthyosis-deafness (KID/HID) syndrome. We solved the crystal structure of the antibody, identified residues that are critical for binding and used molecular dynamics to uncover its mechanism of action.

Conclusions: Although further studies will be necessary to validate the effect of the antibody in vivo, the methodology described here can be extended to select antibodies against hemichannels composed by other connexin isoforms and, consequently, to target other pathologies associated with hyperactive hemichannels. Our study highlights the potential of this approach and identifies connexins as therapeutic targets addressable by screening phage display libraries expressing human randomized antibodies.

Connexin-Mediated Signaling in Nonsensory Cells Is Crucial for the Development of Sensory Inner Hair Cells in the Mouse Cochlea

Mutations in the genes encoding for gap junction proteins connexin 26 (Cx26) and connexin 30 (Cx30) have been linked to syndromic and nonsyndromic hearing loss in mice and humans. The release of ATP from connexin hemichannels in cochlear nonsensory cells has been proposed to be the main trigger for action potential activity in immature sensory inner hair cells (IHCs), which is crucial for the refinement of the developing auditory circuitry. Using connexin knock-out mice, we show that IHCs fire spontaneous action potentials even in the absence of ATP-dependent intercellular Ca²⁺ signaling in the nonsensory cells. However, this signaling from nonsensory cells was able to increase the intrinsic IHC firing frequency. We also found that connexin expression is key to IHC functional maturation. In Cx26 conditional knock-out mice (Cx26^Sox10-Cre), the maturation of IHCs, which normally occurs at approximately postnatal day 12, was partially prevented. Although Cx30 has been shown not to be required for hearing in young adult mice, IHCs from Cx30 knock-out mice exhibited a comprehensive brake in their development, such that their basolateral membrane currents and synaptic machinery retain a prehearing phenotype. We propose that IHC functional differentiation into mature sensory receptors is initiated in the prehearing cochlea provided that the expression of either connexin reaches a threshold level. As such, connexins regulate one of the most crucial functional refinements in the mammalian cochlea, the disruption of which contributes to the deafness phenotype observed in mice and DFNB1 patients.

SIGNIFICANCE STATEMENT The correct development and function of the mammalian cochlea relies not only on the sensory hair cells, but also on the surrounding nonsensory cells. Although the nonsensory cells have been largely implicated in the general homeostasis in the mature cochlea, their involvement in the initial functional differentiation of the sensory inner hair cells is less clear. Using mutant mouse models for the most common form of congenital deafness in humans, which are knock-outs for the gap-junction channels connexin 26 and connexin 30 genes, we show that defects in nonsensory cells prevented the functional maturation of inner hair cells. In connexin knock-outs, inner hair cells remained stuck at a prehearing stage of development and, as such, are unable to process sound information.

Molecular composition and distribution of gap junctions in the sensory epithelium of the human cochlea-a super-resolution structured illumination microscopy (SR-SIM) study

BACKGROUND

Mutations in the GJB2 gene, which encodes the Connexin26 (Cx26) protein, are the most common cause of childhood hearing loss in American and European populations. The cochlea contains a gap junction (GJ) network in the sensory epithelium and two connective tissue networks in the lateral wall and spiral limbus. The syncytia contain the GJ proteins beta 2 (GJB2/Cx26) and beta 6 (GJB6/Cx30). Our knowledge of their expression in humans is insufficient due to the limited availability of tissue. Here, we sought to establish the molecular arrangement of GJs in the epithelial network of the human cochlea using surgically obtained samples.

METHODS

We analyzed Cx26 and Cx30 expression in GJ networks in well-preserved adult human auditory sensory epithelium using confocal, electron, and super-resolution structured illumination microscopy (SR-SIM).

RESULTS

Cx30 plaques (<5 μm) dominated, while Cx26 plaques were subtle and appeared as 'mini-junctions' (2-300 nm). 3-D volume rendering of Z-stacks and orthogonal projections from single optical sections suggested that the GJs are homomeric/homotypic and consist of assemblies of identical GJs composed of either Cx26 or Cx30. Occasionally, the two protein types were co-expressed, suggesting functional cooperation.

CONCLUSIONS

Establishing the molecular composition and distribution of the GJ networks in the human cochlea may increase our understanding of the pathophysiology of Cx-related hearing loss. This information may also assist in developing future strategies to treat genetic hearing loss.

Hypothesis of K+-Recycling Defect Is Not a Primary Deafness Mechanism for Cx26 (GJB2) Deficiency

K⁺-recycling defect is a long-standing hypothesis for deafness mechanism of Connexin26 (Cx26, GJB2) mutations, which cause the most common hereditary deafness and are responsible for >50% of nonsyndromic hearing loss. The hypothesis states that Cx26 deficiency may disrupt inner ear gap junctions and compromise sinking and recycling of expelled K⁺ ions after hair cell excitation, causing accumulation of K⁺-ions in the extracellular space around hair cells producing K⁺-toxicity, which eventually induces hair cell degeneration and hearing loss. However, this hypothesis has never been directly evidenced, even though it has been widely referred to. Recently, more and more experiments demonstrate that this hypothesis may not be a deafness mechanism underlying Cx26 deficiency. In this review article, we summarized recent advances on the K⁺-recycling and mechanisms underlying Cx26 deficiency induced hearing loss. The mechanisms underlying K⁺-sinking, which is the first step for K⁺-recycling in the cochlea, and Cx26 deficiency induced cochlear developmental disorders, which are responsible for Cx26 deficiency induced congenital deafness and associated with disruption of permeability of inner ear gap junctional channels to miRNAs, are also summarized and discussed.

Ca2+ signaling, apoptosis and autophagy in the developing cochlea: Milestones to hearing acquisition

In mammals, the sense of hearing arises through a complex sequence of morphogenetic events that drive the sculpting of the auditory sensory epithelium into its terminally functional three-dimensional shape. While the majority of the underlying mechanisms remain unknown, it has become increasingly clear that Ca²⁺ signaling is at center stage and plays numerous fundamental roles both in the sensory hair cells and in the matrix of non-sensory, epithelial and supporting cells, which embed them and are tightly interconnected by a dense network of gap junctions formed by connexin 26 (Cx26) and connexin 30 (Cx30) protein subunits. In this review, we discuss the intricate interplay between Ca²⁺ signaling, connexin expression and function, apoptosis and autophagy in the crucial steps that lead to hearing acquisition.

Human diseases associated with connexin mutations

Gap junctions and hemichannels comprised of connexins impact many cellular processes. Significant advances in our understanding of the functional role of these channels have been made by the identification of a host of genetic diseases caused by connexin mutations. Prominent features of connexin disorders are the inability of other connexins expressed in the same cell type to compensate for the mutated one, and the ability of connexin mutants to dominantly influence the activity of other wild-type connexins. Functional studies have begun to identify some of the underlying mechanisms whereby connexin channel mutation contributes to the disease state. Detailed mechanistic understanding of these functional differences will help to facilitate new pathophysiology driven therapies for the diverse array of connexin genetic disorders. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.