Dominant negative connexin26 mutation R75W causing severe hearing loss influences normal programmed cell death in postnatal organ of Corti

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.

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.

Identification of gut microbes associated with feed efficiency by daily-phase feeding strategy in growing-finishing pigs

Feed efficiency is one of the most important issues for sustainable pig production. Daily-phase feeding (DPF) is a form of precision feeding that could improve feed efficiency in pigs. Gut microbiota can regulate host nutrient digestion, absorption, and metabolism. However, which key microbes may play a vital role in improving the feed efficiency during DPF remains unclear. In the present study, we used a DPF program compared to a three-phase feeding (TPF) program in growing-finishing pigs to investigate the effects of gut microbiota on feed efficiency. A total of 204 Landrace × Yorkshire pigs (75 d) were randomly assigned into 2 treatments. Each treatment was replicated 8 times with 13 to 15 pigs per replicate pen. Pigs in the TPF group were fed with a commercial feeding program that supplied fixed feed for phases I, II, and III, starting at 81, 101, and 132 d of age, respectively, and pigs in the DPF group were fed a blend of adjacent phase feed from 81 to 155 d at a gradual daily ratio and phase III feed from 155 to 180 d of age. Daily feed intake and body weight were recorded by a computerized device in the feeders. Feces and blood samples were collected from 1 pig per replicate at 155 and 180 d of age. The results showed that the DPF program remarkably improved the feed efficiency at 155 d (P < 0.001) and 180 d of age (P < 0.001), with a significant reduction of the intake of crude protein (P < 0.01), net energy (P < 0.001), crude fiber (P < 0.001), ether extract (P < 0.01), and ash (P < 0.001). The daily-phase feeding program increased the abundance of Prevotella copri (P < 0.05) and Paraprevotella clara (P < 0.05), while it decreased the abundance of Ocilibacter (P < 0.05) at 155 d of age. The results of correlation analysis indicated that the differentially abundant microbiota communities were closely associated with 20 metabolites which enriched amino acid and phenylalanine metabolism. Our results suggest that 2 key microbes may contribute to feed efficiency during daily-phase feeding strategies in pigs.

Kölliker’s organ-supporting cells and cochlear auditory development

The Kölliker’s organ is a transient cellular cluster structure in the development of the mammalian cochlea. It gradually degenerates from embryonic columnar cells to cuboidal cells in the internal sulcus at postnatal day 12 (P12)–P14, with the cochlea maturing when the degeneration of supporting cells in the Kölliker’s organ is complete, which is distinct from humans because it disappears at birth already. The supporting cells in the Kölliker’s organ play a key role during this critical period of auditory development. Spontaneous release of ATP induces an increase in intracellular Ca²⁺ levels in inner hair cells in a paracrine form via intercellular gap junction protein hemichannels. The Ca²⁺ further induces the release of the neurotransmitter glutamate from the synaptic vesicles of the inner hair cells, which subsequently excite afferent nerve fibers. In this way, the supporting cells in the Kölliker’s organ transmit temporal and spatial information relevant to cochlear development to the hair cells, promoting fine-tuned connections at the synapses in the auditory pathway, thus facilitating cochlear maturation and auditory acquisition. The Kölliker’s organ plays a crucial role in such a scenario. In this article, we review the morphological changes, biological functions, degeneration, possible trans-differentiation of cochlear hair cells, and potential molecular mechanisms of supporting cells in the Kölliker’s organ during the auditory development in mammals, as well as future research perspectives.

Reproduction and genetic diversity of Juniperus squamata along an elevational gradient in the Hengduan Mountains

Elevation plays a crucial factor in the distribution of plants, as environmental conditions become increasingly harsh at higher elevations. Previous studies have mainly focused on the effects of large-scale elevational gradients on plants, with little attention on the impact of smaller-scale gradients. In this study we used 14 microsatellite loci to survey the genetic structure of 332 Juniperus squamata plants along elevation gradient from two sites in the Hengduan Mountains. We found that the genetic structure (single, clonal, mosaic) of J. squamata shrubs is affected by differences in elevational gradients of only 150 m. Shrubs in the mid-elevation plots rarely have a clonal or mosaic structure compared to shrubs in lower- or higher-elevation plots. Human activity can significantly affect genetic structure, as well as reproductive strategy and genetic diversity. Sub-populations at mid-elevations had the highest yield of seed cones, lower levels of asexual reproduction and higher levels of genetic diversity. This may be due to the trade-off between elevational stress and anthropogenic disturbance at mid-elevation since there is greater elevational stress at higher-elevations and greater intensity of anthropogenic disturbance at lower-elevations. Our findings provide new insights into the finer scale genetic structure of alpine shrubs, which may improve the conservation and management of shrublands, a major vegetation type on the Hengduan Mountains and the Qinghai-Tibet Plateau.

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.

Confirmation of COL4A6 variants in X-linked nonsyndromic hearing loss and its clinical implications

Hearing loss (HL) is one of the most common sensory defects, of which X-linked nonsyndromic hearing loss (NSHL) accounts for only 1-2%. While a COL4A6 variant has been reported in a single Hungarian family with NSHL associated with inner ear malformation, causative role of COL4A6 variants and their phenotypic consequences in NSHL remain elusive. Here we report two families in which we identified a male member with X-linked HL. Each has inherited a rare hemizygous COL4A6 variant from their respective mothers, NM_001287758.1: c.3272 G > C (p.Gly1091Ala) and c.951 + 1 G > C. An in vitro minigene splicing assay revealed that c.951 + 1 G > T leads to skipping of exon 15, strongly suggesting a pathogenic role for this variant in the HL phenotype. The p.Gly1091Ala variant is classified as a variant of unknown significance based on the variant interpretation guidelines. This report provides evidence for variants in the COL4A6 gene resulting in X-linked NSHL. It highlights the importance of in-depth genetic studies in all family members in addition to the proband, especially in multiplex families, to determine the precise etiology of HL.

Lineage-tracing and translatomic analysis of damage-inducible mitotic cochlear progenitors identifies candidate genes regulating regeneration

Cochlear supporting cells (SCs) are glia-like cells critical for hearing function. In the neonatal cochlea, the greater epithelial ridge (GER) is a mitotically quiescent and transient organ, which has been shown to nonmitotically regenerate SCs. Here, we ablated Lgr5+ SCs using Lgr5-DTR mice and found mitotic regeneration of SCs by GER cells in vivo. With lineage tracing, we show that the GER houses progenitor cells that robustly divide and migrate into the organ of Corti to replenish ablated SCs. Regenerated SCs display coordinated calcium transients, markers of the SC subtype inner phalangeal cells, and survive in the mature cochlea. Via RiboTag, RNA-sequencing, and gene clustering algorithms, we reveal 11 distinct gene clusters comprising markers of the quiescent and damaged GER, and damage-responsive genes driving cell migration and mitotic regeneration. Together, our study characterizes GER cells as mitotic progenitors with regenerative potential and unveils their quiescent and damaged translatomes.

Single-Cell RNA Sequencing Analysis Reveals Greater Epithelial Ridge Cells Degeneration During Postnatal Development of Cochlea in Rats

Greater epithelial ridge cells, a transient neonatal cell group in the cochlear duct, which plays a crucial role in the functional maturation of hair cell, structural development of tectorial membrane, and refinement of audio localization before hearing. Greater epithelial ridge cells are methodologically homogeneous, while whether different cell subtypes are existence in this intriguing region and the degeneration mechanism during postnatal cochlear development are poorly understood. In the present study, single-cell RNA sequencing was performed on the cochlear duct of postnatal rats at day 1 (P1) and day 7 (P7) to identify subsets of greater epithelial ridge cell and progression. Gene ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis were used to examine genes enriched biological processes in these clusters. We identified a total of 26 clusters at P1 and P7 rats and found that the cell number of five cell clusters decreased significantly, while four clusters had similar gene expression patterns and biological properties. The genes of these four cell populations were mainly enriched in Ribosome and P13K-Akt signal pathway. Among them, Rps16, Rpsa, Col4a2, Col6a2, Ctsk, and Jun are particularly interesting as their expression might contribute to the greater epithelial ridge cells degeneration. In conclusion, our study provides an important reference resource of greater epithelial ridge cells landscape and mechanism insights for further understanding greater epithelial ridge cells degeneration during postnatal rat cochlear development.

Heritable epigenetic diversity for conservation and utilization of epigenetic germplasm resources of clonal East African Highland banana (EAHB) accessions

KEY MESSAGE

Genetically identical East African Highland banana (EAHB) clones are epigenetically diverse with heritable epialleles that can contribute to morphological diversity. Heritable epigenetic variation can contribute to agronomic traits in crops and should be considered in germplasm conservation. Despite the genetic uniformity arising from a genetic bottleneck of one ancestral clone, followed by subsequent vegetative propagation, East African Highland bananas (EAHBs) display significant phenotypic diversity potentially arising from somatic mutations, heritable epialleles and/or genotype-by-environment interactions. Here, we use DNA methylation profiling across EAHB accessions representing most of the primary EAHB genepool to demonstrate that the genetically uniform EAHB genepool harbours significant epigenetic diversity. By analysing 724 polymorphic DNA methylation sites by methylation-sensitive AFLP across 90 EAHB cultivars, we could differentiate the EAHB varieties according to their regions (Kenya and Uganda). In contrast, there was minimal association of DNA methylation variation with the five morphological groups that are used to classify EAHBs. We further analysed DNA methylation patterns in parent-offspring cohort, which were maintained in offspring generated by sexual (seed) and asexual (vegetative) propagation, with higher levels of altered DNA methylation observed in vegetatively generated offspring. Our results indicate that the phenotypic diversity of near-isogenic EAHBs is mirrored by considerable DNA methylation variation, which is transmitted between generations by both vegetative reproduction and seed reproduction. Genetically uniform vegetatively propagated crops such as EAHBs harbour considerable heritable epigenetic variation, where heritable epialleles could arise in offspring and contribute to functional traits. This study provides a basis for developing strategies for conservation of epigenetic resources and for integration of epimarkers into crop breeding programmes.

GJB2 Mutations Linked to Hearing Loss Exhibit Differential Trafficking and Functional Defects as Revealed in Cochlear-Relevant Cells

GJB2 gene (that encodes Cx26) mutations are causal of hearing loss highlighting the importance of Cx26-based channel signaling amongst the supporting cells in the organ of Corti. While the majority of these GJB2 mutations are inherited in an autosomal recessive manner, others are inherited in an autosomal dominant manner and lead to syndromic hearing loss as well as skin diseases. To assess if common or divergent mechanisms are at the root of GJB2-linked hearing loss, we expressed several mutants in cochlear-relevant HEI-OC1 cells derived from the developing organ of Corti. Since supporting cells of the mature mammalian organ of Corti have negligible Cx43, but HEI-OC1 cells are rich in Cx43, we first used CRISPR-Cas9 to ablate endogenous Cx43, thus establishing a connexin-deficient platform for controlled reintroduction of hearing-relevant connexins and Cx26 mutants. We found three distinct outcomes and cellular phenotypes when hearing loss-linked Cx26 mutants were expressed in cochlear-relevant cells. The dominant syndromic Cx26 mutant N54K had trafficking defects and did not fully prevent wild-type Cx26 gap junction plaque formation but surprisingly formed gap junctions when co-expressed with Cx30. In contrast, the dominant syndromic S183F mutant formed gap junctions incapable of transferring dye and, as expected, co-localized in the same gap junctions as wild-type Cx26 and Cx30, but also gained the capacity to intermix with Cx43 within gap junctions. Both recessive non-syndromic Cx26 mutants (R32H and R184P) were retained in intracellular vesicles including early endosomes and did not co-localize with Cx30. As might be predicted, none of the Cx26 mutants prevented Cx43 gap junction plaque formation in Cx43-rich HEI-OC1 cells while Cx43-ablation had little effect on the expression of reference genes linked to auditory cell differentiation. We conclude from our studies in cochlear-relevant cells that the selected Cx26 mutants likely evoke hearing loss via three unique connexin defects that are independent of Cx43 status.

Reduced postnatal expression of cochlear Connexin26 induces hearing loss and affects the developmental status of pillar cells in a dose-dependent manner

Mutations in the GJB2 gene (which encodes Connexin26 (Cx26)) are the most common cause of non-syndromic deafness. Previous studies showed that an extensive knockout of the Gjb2 gene in cochlear epithelium can cause severe deafness, significant hair cell (HC) loss and failure of pillar cells (a type of supporting cell, PCs) to differentiate in mice. This study aimed to establish different mouse models with gradient reductions of cochlear Cx26 expression and to investigate the effect of different reduced levels of cochlear Cx26 expression on hearing and development of PCs. According to the reduction in the levels of cochlear Cx26, these models were named high knockdown (KD), middle KD and low KD group. In the low KD group, the mice showed normal hearing and well-developed PCs. In the high KD group, up to 90 percent of supporting cells (SCs) lost Cx26 expression. These mice exhibited severe deafness, rapid hair cell degeneration and juvenile PCs. In the middle KD group, nearly half of SCs lost Cx26 expression. However, these mice showed a moderate deafness and a late-onset hair cell loss. Moreover, nearly all the PCs in mice of this group were in a partially differentiated state. These results indicated that reduction of postnatal expression of cochlear Cx26 induces hearing loss in a dose-dependent manner. Null Cx26 in a few SCs affects the developmental status of PCs and the hair cell degeneration pattern. The abnormal developmental status of PCs may be a potential cause of Gjb2-related hearing loss.

Manipulation of crossover frequency and distribution for plant breeding

The crossovers (COs) that occur during meiotic recombination lead to genetic diversity upon which natural and artificial selection can act. The potential of tinkering with the mechanisms of meiotic recombination to increase the amount of genetic diversity accessible for breeders has been under the research spotlight for years. A wide variety of approaches have been proposed to increase CO frequency, alter CO distribution and induce COs between non-homologous chromosomal regions. For most of these approaches, translational biology will be crucial for demonstrating how these strategies can be of practical use in plant breeding. In this review, we describe how tinkering with meiotic recombination could benefit plant breeding and give concrete examples of how these strategies could be implemented into breeding programs.

Chronic prenatal hypoxia impairs cochlear development, a mechanism involving connexin26 expression and promoter methylation

Chronic prenatal hypoxia is a damaging to fetal development and may have various consequences, including hearing loss. Connexin 26 (Cx26) is one of the major protein subunits required for gap junction formation, and has an important role in maintaining homeostasis in the cochlea and normal hearing. Cx26 mutation and expression abnormality are closely associated with inherited nonsyndromic deafness, but the association between Cx26 and prenatal hypoxia is less established. The present study aimed to examine Cx26 expression and aberrant methylation the Cx26 promoter region in the cochlea from rats exposed to chronic prenatal hypoxia. Hematoxylin and eosin staining demonstrated that the number of hair cells in the organ of Corti were less in the hypoxia group. Reverse transcription-quantitative polymerase chain reaction and western blot analysis revealed that protein and mRNA levels of Cx26 were decreased in the hypoxia group compared with the control group. Further bisulfite sequencing analysis revealed that prenatal hypoxia significantly increased the methylation status of the promoter region of the Cx26 gene. These results demonstrate that chronic prenatal hypoxia caused hearing impairment, and suggest that promoter region hypermethylation and expression downregulation of Cx26 underlie the mechanism of action.

Dynamics and genetics of a disease-driven species decline to near extinction: lessons for conservation

Amphibian chytridiomycosis has caused precipitous declines in hundreds of species worldwide. By tracking mountain chicken (Leptodactylus fallax) populations before, during and after the emergence of chytridiomycosis, we quantified the real-time species level impacts of this disease. We report a range-wide species decline amongst the fastest ever recorded, with a loss of over 85% of the population in fewer than 18 months on Dominica and near extinction on Montserrat. Genetic diversity declined in the wild, but emergency measures to establish a captive assurance population captured a representative sample of genetic diversity from Montserrat. If the Convention on Biological Diversity’s targets are to be met, it is important to evaluate the reasons why they appear consistently unattainable. The emergence of chytridiomycosis in the mountain chicken was predictable, but the decline could not be prevented. There is an urgent need to build mitigation capacity where amphibians are at risk from chytridiomycosis.

Deformation of the Outer Hair Cells and the Accumulation of Caveolin-2 in Connexin 26 Deficient Mice

Background

Mutations in GJB2, which encodes connexin 26 (Cx26), a cochlear gap junction protein, represent a major cause of pre-lingual, non-syndromic deafness. The degeneration of the organ of Corti observed in Cx26 mutant—associated deafness is thought to be a secondary pathology of hearing loss. Here we focused on abnormal development of the organ of Corti followed by degeneration including outer hair cell (OHC) loss.

Methods

We investigated the crucial factors involved in late-onset degeneration and loss of OHC by ultrastructural observation, immunohistochemistry and protein analysis in our Cx26-deficient mice (Cx26^f/fP0Cre).

Results

In ultrastructural observations of Cx26^f/fP0Cre mice, OHCs changed shape irregularly, and several folds or notches were observed in the plasma membrane. Furthermore, the mutant OHCs had a flat surface compared with the characteristic wavy surface structure of OHCs of normal mice. Protein analysis revealed an increased protein level of caveolin-2 (CAV2) in Cx26^f/fP0Cre mouse cochlea. In immunohistochemistry, a remarkable accumulation of CAV2 was observed in Cx26^f/fP0Cre mice. In particular, this accumulation of CAV2 was mainly observed around OHCs, and furthermore this accumulation was observed around the shrunken site of OHCs with an abnormal hourglass-like shape.

Conclusions

The deformation of OHCs and the accumulation of CAV2 in the organ of Corti may play a crucial role in the progression of, or secondary OHC loss in, GJB2-associated deafness. Investigation of these molecular pathways, including those involving CAV2, may contribute to the elucidation of a new pathogenic mechanism of GJB2-associated deafness and identify effective targets for new therapies.

Inner ear cell therapy targeting hereditary deafness by activation of stem cell homing factors

Congenital deafness affects about 1 in 1000 children and more than half of them have a genetic background such as Connexin26 (CX26) gene mutation. Inner ear cell therapy for sensorineural hearing loss has been expected to be an effective therapy for hereditary deafness. Previously, we developed a novel strategy for inner ear cell therapy using bone marrow mesenchymal stem cells as a supplement for cochlear fibrocytes functioning for cochlear ion transport. For cell therapy targeting hereditary deafness, a more effective cell delivery system to induce the stem cells into cochlear tissue is required, because gene mutations affect all cochlear cells cochlear cells expressing genes such as GJB2 encoding CX26. Stem cell homing is one of the crucial mechanisms to be activated for efficient cell delivery to the cochlear tissue. In our study, monocyte chemotactic protein-1, stromal cell-derived factor-1 and their receptors were found to be a key regulator for stem cell recruitment to the cochlear tissue. Thus, the activation of stem cell homing may be an efficient strategy for hearing recovery in hereditary deafness.

Apparent phenotypic anticipation in autosomal dominant connexin 26 deafness

BACKGROUND

Connexin 26 (GJB2) mutations are associated with various types of hearing loss, either without associated symptoms or with skin disease, constituting a form of syndromic hearing loss. These mutations can lead to deafness in either a recessive or a dominant autosomal form of inheritance.

METHODS

Ascertainment of a Jewish Ashkenazi family with nonsyndromic hearing loss led to the construction of a pedigree for a four-generation family, with hearing loss detected in three successive generations. The entire coding region of the GJB2 gene was amplified and sequenced by Sanger sequencing.

RESULTS

Audiological analysis revealed that the age of onset and severity of hearing loss were earlier and more severe, respectively, in each successive generation of an Ashkenazi Jewish family. A mutation, c.224G>A, leading to missense p.Arg75Gln was detected only in the affected members of the family.

CONCLUSIONS

The entire coding region of GJB2 should be checked in hearing-impaired patients by Sanger sequencing, rather than examination only of the two most prevalent mutations, regardless of mode of inheritance or ethnicity. Furthermore, predictions regarding phenotype based on genotype can be difficult to make due to clinical variability in multigenerational families, as demonstrated in the family presented in this study.