Connexin 26: required for normal auditory function

Elevated auditory brainstem response thresholds in mice with Connexin36 gene ablation

CONCLUSION

Expression of connexin36 (Cx36) and electrical synapses formed by Cx36-containing gap junctions contribute to normal auditory brainstem response thresholds in mice.

OBJECTIVES

Electrical synaptic transmission mediated by gap junctions has not been intensively studied in the auditory system. This study used transgenic mice with knockout of the gene coding for the major protein that forms neuronal gap junctions in mammalian brain (Cx36) to evaluate the role of Cx36 in murine hearing.

METHODS

Auditory brainstem response (ABR) thresholds and distortion product otoacoustic emissions (DPOAEs) were measured in 26 wild-type and 26 Cx36 knockout mice. ABR thresholds were used to assess auditory brainstem function at four frequencies. DPOAEs were delivered for seven frequency pairs to assess cochlear function.

RESULTS

The magnitudes of the 2f1-f2 distortion products were not different between Cx36 knockout and wild-type mice, suggesting similar cochlear function in the two groups. ABR thresholds were significantly elevated in the Cx36 knockout compared with the wild-type groups, suggesting impaired function in the auditory brainstem. The results suggest that electrical synapses formed by Cx36-containing gap junctions contribute to auditory sound processing and function at the level of the brainstem, not the cochlea. These findings may be important for understanding human auditory pathology.

Connexin26 hemichannels with a mutation that causes KID syndrome in humans lack sensitivity to CO2

Mutations in connexin26 (Cx26) underlie a range of serious human pathologies. Previously we have shown that Cx26 hemichannels are directly opened by CO₂ (Meigh et al., 2013). However the effects of human disease-causing mutations on the CO₂ sensitivity of Cx26 are entirely unknown. Here, we report the first connection between the CO₂ sensitivity of Cx26 and human pathology, by demonstrating that Cx26 hemichannels with the mutation A88V, linked to Keratitis-Ichthyosis-Deafness syndrome, are both CO₂ insensitive and associated with disordered breathing in humans.

DOI:http://dx.doi.org/10.7554/eLife.04249.001

Identification and genotype/phenotype correlation of mutations in a large German cohort with hearing loss

The prevalence of hearing impairment is estimated as approximately 1 on 1,000 newborn children. To assess a higher mutation detection rate in individuals with hearing loss a three-step mutation screening program consisting of GJB2 in first line, then GJB1, GJB3 and GJB6 (second step) and if tested negative or heterozygote, testing of GJA1, GJB4, SLC26A4 and PJVK (third) was performed. Audiograms were derived from all patients to characterize audiological features of GJB2 mutations especially. In 59 patients (31.3%) of the 188 probands, the hearing impairment was due to GJB2 mutations, 45 (23.9%) of these being homozygous for 35delG mutation and 14 (7.4%) compound heterozygous for GJB2 mutations in the coding region of exon 2 whereas no significant sequence variation was found in exon 1. In 22 (11.7%) additional patients a single recessive mutation in GJB2, GJB3, GJB6 and SLC26A4 without a second mutation on the other allele was identified, making genetic counseling difficult. Our study showed significant difference in hearing loss degree in the patients with GJB2-mutations. Forty-five (45.5%) GJB2-cases were identified in 99 individuals diagnosed with severe to profound hearing loss, 14 (17.7%) GJB2-cases were identified in 79 individuals with moderate deafness whereas no clear GJB2 mutation was found in 10 patients with mild hearing loss (p < 0.001). Revealing a high variability of hearing levels in identical genotypes (even intrafamilial), a significant genotype-phenotype correlation could not be established. Based on the identified mutations spectrum and frequencies, speaking mostly of GJB2, a step by step screening for mutations can be devised and in addition may lead to a better stratification of patients for specific therapeutical approaches.

Pannexins, distant relatives of the connexin family with specific cellular functions?

Intercellular communication (IC) is mediated by gap junctions (GJs) and hemichannels, which consist of proteins. This has been particularly well documented for the connexin (Cx) family. Initially, Cxs were thought to be the only proteins capable of GJ formation in vertebrates. About 10 years ago, however, a new GJ-forming protein family related to invertebrate innexins (Inxs) was discovered in vertebrates, and named the pannexin (Panx) family. Panxs, which are structurally similar to Cxs, but evolutionarily distinct, have been shown to be co-expressed with Cxs in vertebrates. Both protein families show distinct properties and have their own particular function. Identification of the mechanisms that control Panx channel gating is a major challenge for future work. In this review, we focus on the specific properties and role of Panxs in normal and pathological conditions.

Neuronal connexin expression in the cochlear nucleus of big brown bats

We present immunohistochemical data describing the presence and distribution of connexins, structural component of gap junctions, in the cochlear nuclei of adult big brown bats (Eptesicus fuscus). Echolocating big brown bats show microsecond scale echo-delay sensitivity that requires accurate synchronization of neuronal responses to the timing of echoes. Midbrain and auditory cortical neuronal response timing is similar to that observed in other non-echolocating mammals, suggesting that lower auditory processing nuclei may have specialized mechanisms for obtaining the required temporal hyperacuity. Our data shows that connexin 36, a gap junction protein specific to neurons, is most densely expressed in the bat's cochlear nuclear complex, the medullary region that receives and processes first-order afferents from the auditory nerve. Cx36 expression is absent in the cochlear nucleus of normal mice, which have high-frequency hearing sensitivity similar to big brown bats. Glial connexins, Cx26 and Cx43, expressed in astrocytes and several inner ear structures, are also found in the bat cochlear nucleus complex, associated with major fiber tracts in and around the cochlear nuclei. The extensive presence of neuronally-associated Cx36 in brainstem auditory structures of adult bats suggests a possible role for gap junctions in mediating echo-delay hyperacuity.

Nuclear and mitochondrial genes mutated in nonsyndromic impaired hearing

Half of the cases with congenital impaired hearing are hereditary (HIH). HIH may occur as part of a multisystem disease (syndromic HIH) or as disorder restricted to the ear and vestibular system (nonsyndromic HIH). Since nonsyndromic HIH is almost exclusively caused by cochlear defects, affected patients suffer from sensorineural hearing loss. One percent of the total human genes, i.e. 300-500, are estimated to cause syndromic and nonsyndromic HIH. Of these, approximately 120 genes have been cloned thus far, approximately 80 for syndromic HIH and 42 for nonsyndromic HIH. In the majority of the cases, HIH manifests before (prelingual), and rarely after (postlingual) development of speech. Prelingual, nonsyndromic HIH follows an autosomal recessive trait (75-80%), an autosomal dominant trait (10-20%), an X-chromosomal, recessive trait (1-5%), or is maternally inherited (0-20%). Postlingual nonsyndromic HIH usually follows an autosomal dominant trait. Of the 41 mutated genes that cause nonsyndromic HIH, 15 cause autosomal dominant HIH, 15 autosomal recessive HIH, 6 both autosomal dominant and recessive HIH, 2 X-linked HIH, and 3 maternally inherited HIH. Mutations in a single gene may not only cause autosomal dominant, nonsyndromic HIH, but also autosomal recessive, nonsyndromic HIH (GJB2, GJB6, MYO6, MYO7A, TECTA, TMC1), and even syndromic HIH (CDH23, COL11A2, DPP1, DSPP, GJB2, GJB3, GJB6, MYO7A, MYH9, PCDH15, POU3F4, SLC26A4, USH1C, WFS1). Different mutations in the same gene may cause variable phenotypes within a family and between families. Most cases of recessive HIH result from mutations in a single locus, but an increasing number of disorders is recognized, in which mutations in two different genes (GJB2/GJB6, TECTA/KCNQ4), or two different mutations in a single allele (GJB2) are involved. This overview focuses on recent advances in the genetic background of nonsyndromic HIH.

Altered gating properties of functional Cx26 mutants associated with recessive non-syndromic hearing loss

Connexins (Cx) form gap junctions that allow the exchange of small metabolites and ions. In the inner ear, Cx26 is the major gap junction protein and mutations in the Cx26-encoding gene, GJB2, are the most frequent cause of autosomal recessive non-syndromic hearing loss (DFNB1). We have functionally analyzed five Cx26 mutations associated with DFNB1, comprising the following single amino-acid substitutions: T8M, R143W, V153I, N206S and L214P. Coupling of cells expressing wild-type or mutant Cx26 was measured in the paired Xenopus oocyte assay. We found that the R143W, V153I and L214P mutations were unable to form functional channels. In contrast, the T8M and N206S mutants did electrically couple cells, though their voltage gating properties were different from wild-type Cx26 channels. The electrical coupling of oocytes expressing the T8M and N206S mutants suggest that these channels may retain high permeability to potassium ions. Therefore, deafness associated with Cx26 mutations may not only depend on reduced potassium re-circulation in the inner ear. Instead, abnormalities in the exchange of other metabolites through the cochlear gap junction network may also produce deafness.

Functional Domain Mapping and Selective Trans-dominant Effects Exhibited by Cx26 Disease-causing Mutations

Mutations in Cx26 are a major cause of autosomal dominant and recessive forms of sensorineural deafness. Some mutations in Cx26 are associated not only with deafness but also with skin disease. We examined the subcellular localization and function of two green fluorescent protein (GFP)-tagged Cx26 point mutants that exhibit both phenotypes, G59A-GFP and D66H-GFP. D66H-GFP was retained within the brefeldin A-insensitive trans-Golgi network, whereas a population of G59A-GFP was transported to the cell surface. Neither G59A nor D66H formed gap junctions that were permeable to small fluorescent dyes, suggesting they are loss-of-function mutations. When co-expressed with wild-type Cx26, both G59A and D66H exerted dominant-negative effects on Cx26 function. G59A also exerted a trans-dominant negative effect on co-expressed wild type Cx32 and Cx43, whereas D66H exerted a trans-dominant negative effect on Cx43 but not Cx32. We propose that the severity of the skin disease is dependent on the specific nature of the Cx26 mutation and the trans-dominant selectivity of the Cx26 mutants on co-expressed connexins. Additional systematic mutations at residue D66, in which the overall charge of this motif was altered, suggested that the first extracellular loop is critical for Cx26 transport to the cell surface as well as function of the resulting gap junction channels.

Gap junctions in the inner ear: comparison of distribution patterns in different vertebrates and assessement of connexin composition in mammals

The distribution and size of gap junctions (GJ) in the sensory epithelia of the inner ear have been examined in a reptile (gecko), birds (chicken and owl), and mammals (mouse, guinea pig, gerbil, and bat), and the connexin composition of GJs in the mammalian inner ear has been assessed. Freeze fracture revealed a common pattern of GJ distribution in auditory and vestibular sensory epithelia in the different vertebrate classes. In all these tissues, GJs are numerous, often occupying more than 25% of the plasma membrane area of supporting cells and sometimes composed of more than 100,000 channels. Screening for 12 members of the connexin family in the mammalian inner ear by RT-PCR, Western blotting, and immunohistochemistry revealed four connexin isotypes, cx26, cx30, cx31, and cx43, in the cochlea and three, cx26, cx30, and cx43, in the vestibular organs. With antibodies characterised for their specificity, cx26 and cx30 colocalised in supporting cells of the organ of Corti, in the basal cell region of the stria vascularis, and in type 1 fibrocytes of the spiral ligament. No other connexin was detected in these regions. Cx31 was localised among type 2 fibrocytes below the spiral prominence, a region where cx30 was not expressed and cx26 expression appeared to be low. Cx43 was detected only in the region of "tension fibrocytes" lining the inner aspect of the otic capsule. This suggests separate functional compartments in the cochlea. In addition to cx26 and cx30, cx43 was detected in supporting cells of the vestibular sensory epithelia. Where cx26 and cx30 were colocalised, double immunogold labelling of thin sections showed both cx26 and cx30 evenly distributed in individual GJ plaques, a pattern consistent with the presence of heteromeric connexons. Coimmunoprecipitation of cochlear membrane proteins solubilised with a procedure that preserves the oligomeric structure of connexons confirmed the presence of heteromeric cx26/cx30 connexons. Heteromeric cx26/cx30 connexons may be unique to the inner ear, which could be one factor underlying the non-syndromic character of the deafness caused by mutations in cx26.

Functional analysis of connexin-26 mutants associated with hereditary recessive deafness

The physiological importance of connexin-26 (Cx26) gap junctions in regulating auditory function is indicated by the finding that autosomal recessive DFNB1 deafness is associated with mutations of the Cx26 gene. To investigate the pathogenic role of Cx26 mutation in recessive hearing loss, four putative DFNB1 Cx26 mutants (V84L, V95M, R127H, and R143W) were stably expressed in N2A cells, a communication-deficient cell line. In N2A cells expressing (R127H) Cx26 gap junctions, macroscopic junctional conductance and ability of transferring neurobiotin between transfected cells were greatly reduced. Despite the formation of defective junctional channels, immunoreactivity of (R127H) Cx26 was mainly localized in the cell membrane and prominent in the region of cell-cell contact. Mutant (V84L), (V95M), or (R143W) Cx26 protein formed gap junctions with a junctional conductance similar to that of wild-type Cx26 junctional channels. (V84L), (V95M), or (R143W) Cx26 gap junctions also permitted neurobiotin transfer between pairs of transfected N2A cells. The present study suggests that (R127H) mutation associated with hereditary sensorineural deafness results in the formation of defective Cx26 gap junctions, which may lead to the malfunction of cochlear gap junctions and hearing loss. Further studies are required to determine the exact mechanism by which mutant (V84L), (V95M), and (R143W) Cx26 proteins, which are capable of forming functional homotypic junctional channels in N2A cells, cause the cochlear dysfunction and sensorineural deafness.

Otoacoustic emissions and brainstem evoked potentials in compound carriers of connexin 26 mutations

This study compares the effects of mutations in the gap junction protein connexin 26 (Cx26), on outer hair cells (OHCs), inner hair cells (IHCs) and auditory nerve/brainstem among carriers of these mutations. One hundred and twenty eight individuals, from a village with widespread consanguinity and congenital deafness, due to three Cx26 mutations, were selected among relatives of deaf persons, and divided into non-carriers, carriers of one mutation, homozygous to one mutation, or compound heterozygous carriers of two different mutations. Distortion product otoacoustic emissions (DPOAEs), auditory brainstem responses (ABRs) and audiometric evaluation were compared in these genetic groups. Hearing loss among homozygotes and compound heterozygotes was comparable and ranged from mild to profound. Most ABRs from these groups showed no responses or partial responses (peaks III, V) with prolonged latencies, but some individuals had all peaks at normal latencies. DPOAEs were absent, except sporadic responses. Carriers of one mutation had significantly smaller DPOAEs compared to non-carriers, although normal pure tone audiograms and ABRs were found in these groups. In conclusion, based on DPOAEs, Cx26 mutations may impact OHC function among carriers of one or two Cx26 mutations. IHC/nerve impairment among homozygotes and compound heterozygotes is variable. OHCs may be more susceptible to Cx26 mutations compared to IHCs and the auditory nerve and brainstem pathway activated by them.

Autoantibodies to inner ear and endothelial antigens in Cogan's syndrome

BACKGROUND

Cogan's syndrome is a chronic inflammatory disease of unknown origin, characterised by sensorineural hearing loss, episcleritis, and vasculitis. An autoimmune origin has been suggested but not proven. Our aim was to establish whether or not an autoimmune process is the cause of the disease.

METHODS

We used pooled IgG immunoglobulins derived from eight patients with Cogan's syndrome to screen a random peptide library to identify disease relevant autoantigen peptides. Among the identified peptides, one was recognised by all the patients' sera. Antibodies against peptides were affinity purified from patients' sera and used to characterise the autoantigen, to stain human cochlea, and to transfer the features of Cogan's disease into animals.

FINDINGS

We identified an immunodominant peptide that shows similarity with autoantigens such as SSA/Ro and with the reovirus III major core protein lambda 1. The peptide sequence shows similarity also with the cell-density enhanced protein tyrosine phosphatase-1 (DEP-1/CD148), which is expressed on the sensory epithelia of the inner ear and on endothelial cells. IgG antibodies against the peptide, purified from the patients' sera, recognised autoantigens and DEP-1/CD148 protein, bound human cochlea, and inhibited proliferation of cells expressing DEP-1/CD148. The same antibodies bound connexin 26, gene mutations of which lead to congenital inner-ear deafness. Furthermore, these antibodies were able to induce the features of Cogan's disease in mice.

INTERPRETATION

Our results indicate that Cogan's syndrome is an autoimmune disease, characterised by the presence of autoantibodies able to induce tissue damage on binding of cell-surface molecules present on the sensory epithelia of the inner ear and on endothelial cells.

Effectiveness of sequencing connexin 26 (GJB2) in cases of familial or sporadic childhood deafness referred for molecular diagnostic testing

PURPOSE

Hearing loss is a common congenital disorder that is frequently associated with mutations in the GJB2 gene encoding the connexin 26 protein (Cx26). We sought to evaluate the effectiveness of direct DNA sequencing for detection of Cx26 mutations as a clinical diagnostic test.

METHODS

We designed a clinical assay using a three-step polymerase chain reaction (PCR)-based DNA sequencing strategy to detect all possible mutations in the open reading frame and flanking sequences of Cx26. The results of the first 324 cases of childhood deafness referred for diagnostic testing were analyzed.

RESULTS

A total of 127 of the 324 (39.2%) cases had at least one mutant Cx26 allele (36.1% of sporadic cases, 70% of familial cases). Of these 127 case, 57 (44.8%) were homozygotes or compound heterozygotes. Thirty-four different mutations were identified, including 10 novel mutations, 6 of which (T8M, K15T, R32L, M93I, N206S, and 511-512insAACG) may be pathogenic. We also provide new evidence on the pathogenicity or nonpathogenicity of 12 previously reported mutations, and clarify the confusing nomenclature of the 313-326del14 mutation.

CONCLUSION

A simple and rigorous method for efficient PCR-based sequence analysis of Cx26 is a sensitive clinical assay for evaluating deaf children. Its widespread use is likely to identify additional pathogenic mutations and lead to a better understanding of the clinical significance of previously identified mutations.

Better speech performance in cochlear implant patients with GJB2-related deafness

OBJECTIVE

We applied mutation screening in seven cochlear implant users to identify those persons with GJB2-related deafness to determine whether etiology of deafness was predictive of speech performance after implantation.

METHODS

Direct sequence of GJB2 was conducted over seven cochlear implant users with prelingual hearing impairment and their speech, language and cognitive performance was examined.

RESULTS

The three persons with GJB2-related deafness had a mean vocabulary of 1243 words compared to a mean vocabulary of 195 words in the four children with GJB2-unrelated deafness, although the number of patients examined here was limited. The developmental quotient (DQ) of cognitive ability also was higher in those children with GJB2-related deafness.

CONCLUSIONS

These preliminary results suggest that better speech performance after cochlear implantation may be observed in persons with GJB2-related deafness. In the future, detailed phenotypic studies and mutation screening for non-syndromic hearing loss may play an important role in the preoperative assessment of prelingually-deafened children.

The effects of a connexin 26 mutation--35delG--on oto-acoustic emissions and brainstem evoked potentials: homozygotes and carriers

The purpose of this study was to examine whether outer hair cells (OHCs), inner hair cells and the brainstem auditory pathway are impaired due to a mutation in a gap junction protein, connexin 26 (Cx26), 35delG. Fifty-six individuals, from a village with widespread consanguinity and profound, non-syndromic congenital deafness, due to 35delG mutation, were selected among relatives of deaf people. The individuals were either non-carriers (n=20), heterozygous (n=20) or homozygous (n=16) for the mutation. Distortion product oto-acoustic emissions (DPOAEs) and auditory brainstem evoked potentials (ABEPs) in mutation non-carriers, in heterozygotes (carriers) and in subjects homozygous for the mutation were compared in addition to audiometric evaluation. Most deaf homozygotes had no DPOAEs, except some sporadic responses at 1000, 8000 and 10000 Hz. This was also observed in audiometry which showed profound hearing loss in most cases. Two cases were unique: one had moderate to severe hearing loss and the other had severe to profound hearing loss. A significant difference was found between non-carriers and carriers of 35delG: non-carriers had larger DPOAE responses than heterozygotes at all frequencies. The prevalence of responses got lower with higher frequencies in both groups, but between 6000 and 10000 Hz 50-70% of the carriers had no DPOAE responses, compared to 30-60% of non-carriers. In both groups responses diminished with age, but no significant interaction was found between age and the genetic group. ABEPs among homozygotes were variable: in most homozygotes ABEPs were absent or partial (waves III, V) with prolonged latencies, but two subjects had ABEPs within normal limits, in one ear. ABEPs were normal with no differences between carriers and non-carriers. We suggest that OHC function is affected by the 35delG mutation in Cx26. In addition, the hearing of carriers of this mutation may be impaired at very high frequencies (8000-10000 Hz), which are not assessed in routine audiometry or ABEP testing.

OCP1, an F-box protein, co-localizes with OCP2/SKP1 in the cochlear epithelial gap junction region

Immunohistochemical data indicate that OCP1 co-localizes exactly with OCP2 in the epithelial gap junction region of the guinea pig organ of Corti (OC). Despite the abundance of OCP1 in the OC, gaining access to its coding sequence -- and, in particular, the 5' end of the coding sequence -- proved unexpectedly challenging. The putative full-length OCP1 cDNA -- 1180 nucleotides in length -- includes a 67 nucleotide 5' leader sequence, 300 codons (including initiation and termination signals), and a 216 nucleotide 3' untranslated region. The cDNA encodes a protein having a predicted molecular weight of 33,700. The inferred amino acid sequence harbors an F-box motif spanning residues 52--91, consistent with a role for OCP1 and OCP2 in the proteasome-mediated degradation of select OC proteins. Although OCP1 displays extensive homology to an F-box protein recently cloned from rat brain (NFB42), clustered sequence non-identities indicate that the two proteins are transcribed from distinct genes. The presumptive human OCP1 gene was identified in the human genome databank. Located on chromosome 1p35, the inferred translation product exhibits 94% identity with the guinea pig OCP1 coding sequence.

Genetics and molecular biology of deafness. Update

This article discusses the latest research in the molecular biology and genetics of hearing impairment and its importance to otolaryngologists. Recent research has led to the discovery of many of the genes and gene products that are responsible for hereditary hearing impairment. State mandated screening of newborn infants for hearing loss ensures that a large number of hearing-impaired children will be detected at a very early age. Additionally, these children often will be referred to the otolaryngologist for evaluation of the hearing impairment. It is the otolaryngologist who must gather a detailed family history and perform a thorough physical examination to fully assess the cause of the hearing impairment. In taking the family history, it is important to note that the diagnosis of a hereditary hearing impairment often involves the evaluation of a large-sized family that has a history of hearing disorders. A history of an affected individual in a small family does not necessarily support a diagnosis of hearing impairment in later affected offspring because of the small sample size. Often, a hearing impairment that is part of a syndrome may not be detected because the physical findings associated with a syndrome are subtle in a young infant. For example, the white forelock seen in patients with Waardenburg's syndrome type I cannot be visualized in the infant who lacks hair. Additionally, some patients with syndromic hearing impairment do not present with physical findings, but rather they exhibit abnormal laboratory studies. Additional points to remember include the following: As infectious iatrogenic causes of hearing impairment decrease, the relative incidence of hereditary hearing impairment will increase. Hereditary hearing impairment can present as an isolated finding, or in association with a number of anomalies recognizable as a syndrome. The study of genetics and molecular biology has led to the identification of genes associated with hearing impairment and will allow for future screening and possible therapy for the hearing-impaired. The screening of newborns for hearing impairment using the techniques of molecular biologists and geneticists will result in early identification and appropriate intervention for those at risk for hereditary hearing impairment. An understanding of the syndromic and nonsyndromic causes of hereditary hearing impairment can help the otolaryngologist make a diagnosis and provide appropriate audiologic and educational management to the patient.