Grhl2 deficiency impairs otic development and hearing ability in a zebrafish model of the progressive dominant hearing loss DFNA28

Association between genetic variations in GRHL2 and noise-induced hearing loss in Chinese high intensity noise exposed workers: a case-control analysis

The grainyhead like 2 (GRHL2) is a transcription factor, and the role among noise exposed workers is not well established. We tested whether GRHL2 polymorphisms are associated with the risk of noise-induced hearing loss (NIHL) in Chinese high intensity noise exposed workers. We genotyped six polymorphisms of GRHL2 gene (i.e., rs611419, rs3779617, rs3735713, rs3735714, rs3735715, and rs6989650) of 340 NIHL cases and 356 control subjects who exposed to noise higher than 85 dB (A) [Lex, 8 h=time-weighted average of levels of noise exposure (Lex) for a nominal 8 h working day] in a Chinese population. Compared with rs611419 AA genotype, the AT/TT genotypes conferred protection against NIHL [adjusted odds ratio (OR)=0.71, 95% confidence interval (CI)=0.52-0.98]. No altered NIHL risk was associated with the other five polymorphisms. In the combined analyses, we found that the combined genotypes with three to eight variant alleles were associated with an decrease risk of NIHL compared with those with zero to two variant alleles, and the decrease risk was more pronounced among subgroups of exposure time>20 yr (0.31, 0.16-0.62) and drinkers (0.51, 0.29-0.90). Polymorphisms of GRHL2 may positively contribute to the etiology of NIHL.

Semicircular canal morphogenesis in the zebrafish inner ear requires the function of gpr126 (lauscher), an adhesion class G protein-coupled receptor gene

Morphogenesis of the semicircular canal ducts in the vertebrate inner ear is a dramatic example of epithelial remodelling in the embryo, and failure of normal canal development results in vestibular dysfunction. In zebrafish and Xenopus, semicircular canal ducts develop when projections of epithelium, driven by extracellular matrix production, push into the otic vesicle and fuse to form pillars. We show that in the zebrafish, extracellular matrix gene expression is high during projection outgrowth and then rapidly downregulated after fusion. Enzymatic disruption of hyaluronan in the projections leads to their collapse and a failure to form pillars: as a result, the ears swell. We have cloned a zebrafish mutant, lauscher (lau), identified by its swollen ear phenotype. The primary defect in the ear is abnormal projection outgrowth and a failure of fusion to form the semicircular canal pillars. Otic expression of extracellular matrix components is highly disrupted: several genes fail to become downregulated and remain expressed at abnormally high levels into late larval stages. The lau mutations disrupt gpr126, an adhesion class G protein-coupled receptor gene. Expression of gpr126 is similar to that of sox10, an ear and neural crest marker, and is partially dependent on sox10 activity. Fusion of canal projections and downregulation of otic versican expression in a hypomorphic lau allele can be restored by cAMP agonists. We propose that Gpr126 acts through a cAMP-mediated pathway to control the outgrowth and adhesion of canal projections in the zebrafish ear via the regulation of extracellular matrix gene expression.

Claudins in teleost fishes

Teleost fishes are a large and diverse animal group that represent close to 50% of all described vertebrate species. This review consolidates what is known about the claudin (Cldn) family of tight junction (TJ) proteins in teleosts. Cldns are transmembrane proteins of the vertebrate epithelial/endothelial TJ complex that largely determine TJ permeability. Cldns achieve this by expressing barrier or pore forming properties and by exhibiting distinct tissue distribution patterns. So far, ~63 genes encoding for Cldn TJ proteins have been reported in 16 teleost species. Collectively, cldns (or Cldns) are found in a broad array of teleost fish tissues, but select genes exhibit restricted expression patterns. Evidence to date strongly supports the view that Cldns play a vital role in the embryonic development of teleost fishes and in the physiology of tissues and organ systems studied thus far.

Confirmation of GRHL2 as the gene for the DFNA28 locus

More than 10 years ago, a c.1609_1610insC mutation in the grainyhead-like 2 (GRHL2) gene was identified in a large family with nonsyndromic sensorineural hearing loss, so far presenting the only evidence for GRHL2 being an autosomal-dominant deafness gene (DFNA28). Here, we report on a second large family, in which post-lingual hearing loss with a highly variable age of onset and progression segregated with a heterozygous non-classical splice site mutation in GRHL2. The c.1258-1G>A mutation disrupts the acceptor recognition sequence of intron 9, creating a new AG splice site, which is shifted by only one nucleotide in the 3′ direction. cDNA analysis confirmed a p.Gly420Glufs*111 frameshift mutation in exon 10. © 2013 Wiley Periodicals, Inc.

CaMK-II activation is essential for zebrafish inner ear development and acts through Delta-Notch signaling

Zebrafish inner ear development is characterized by the crystallization of otoliths onto immotile kinocilia that protrude from sensory "hair" cells. The stereotypical formation of these sensory structures is dependent on the expression of key patterning genes and on Ca2+ signals. One potential target of Ca2+ signaling in the inner ear is the type II Ca2+/calmodulin-dependent protein kinase (CaMK-II), which is preferentially activated in hair cells, with intense activation at the base of kinocilia. In zebrafish, CaMK-II is encoded by seven genes; the expression of one of these genes (camk2g1) is enriched in hair cells. The suppression of camk2g1 expression by antisense morpholino oligonucleotides or inhibition of CaMK-II activation by the pharmacological antagonist, KN-93, results in aberrant otolith formation without preventing cilia formation. In fact, CaMK-II suppression results in additional ciliated hair cells and altered levels of Delta-Notch signaling members. DeltaA and deltaD transcripts are increased and DeltaD protein accumulates in hair cells of CaMK-II morphants, indicative of defective recycling and/or exocytosis. Our findings indicate that CaMK-II plays a critical role in the developing ear, influencing cell differentiation through extranuclear effects on Delta-Notch signaling. Continued expression and activation of CaMK-II in maculae and cristae in older embryos suggests continued roles in auditory sensory maturation and transduction.

Claudins and the modulation of tight junction permeability

Claudins are tight junction membrane proteins that are expressed in epithelia and endothelia and form paracellular barriers and pores that determine tight junction permeability. This review summarizes our current knowledge of this large protein family and discusses recent advances in our understanding of their structure and physiological functions.

Visual input modulates audiomotor function via hypothalamic dopaminergic neurons through a cooperative mechanism

Visual cues often modulate auditory signal processing, leading to improved sound detection. However, the synaptic and circuit mechanism underlying this cross-modal modulation remains poorly understood. Using larval zebrafish, we first established a cross-modal behavioral paradigm in which a preceding flash enhances sound-evoked escape behavior, which is known to be executed through auditory afferents (VIII(th) nerves) and command-like neurons (Mauthner cells). In vivo recording revealed that the visual enhancement of auditory escape is achieved by increasing sound-evoked Mauthner cell responses. This increase in Mauthner cell responses is accounted for by the increase in the signal-to-noise ratio of sound-evoked VIII(th) nerve spiking and efficacy of VIII(th) nerve-Mauthner cell synapses. Furthermore, the visual enhancement of Mauthner cell response and escape behavior requires light-responsive dopaminergic neurons in the caudal hypothalamus and D1 dopamine receptor activation. Our findings illustrate a cooperative neural mechanism for visual modulation of audiomotor processing that involves dopaminergic neuromodulation.

Loss of zygotic NUP107 protein causes missing of pharyngeal skeleton and other tissue defects with impaired nuclear pore function in zebrafish embryos

The Nup107-160 multiprotein subcomplex is essential for the assembly of nuclear pore complexes. The developmental functions of individual constituents of this subcomplex in vertebrates remain elusive. In particular, it is unknown whether Nup107 plays an important role in development of vertebrate embryos. Zebrafish nup107 is maternally expressed and its zygotic expression becomes prominent in the head region and the intestine from 24 h postfertilization (hpf) onward. In this study, we generate a zebrafish mutant line, nup107(tsu068Gt), in which the nup107 locus is disrupted by an insertion of Tol2 transposon element in the first intron and as a result it fails to produce normal transcripts. Homozygous nup107(tsu068Gt) mutant embryos exhibit tissue-specific defects after 3 days postfertilization (dpf), including loss of the pharyngeal skeletons, degeneration of the intestine, absence of the swim bladder, and smaller eyes. These mutants die at 5-6 days. Extensive apoptosis occurs in the affected tissues, which is partially dependent on p53 apoptotic pathways. In cells of the defective tissues, FG-repeat nucleoporins are disturbed and nuclear pore number is reduced, leading to impaired translocation of mRNAs from the nucleus to the cytoplasm. Our findings shed new light on developmental function of Nup107 in vertebrates.

Midbrain-hindbrain boundary patterning and morphogenesis are regulated by diverse grainy head-like 2-dependent pathways

The isthmic organiser located at the midbrain-hindbrain boundary (MHB) is the crucial developmental signalling centre responsible for patterning mesencephalic and metencephalic regions of the vertebrate brain. Formation and maintenance of the MHB is characterised by a hierarchical program of gene expression initiated by fibroblast growth factor 8 (Fgf8), coupled with cellular morphogenesis, culminating in the formation of the tectal-isthmo-cerebellar structures. Here, we show in zebrafish that one orthologue of the transcription factor grainy head-like 2 (Grhl2), zebrafish grhl2b plays a central role in both MHB maintenance and folding by regulating two distinct, non-linear pathways. Loss of grhl2b expression induces neural apoptosis and extinction of MHB markers, which are rescued by re-expression of engrailed 2a (eng2a), an evolutionarily conserved target of the Grhl family. Co-injection of sub-phenotypic doses of grhl2b and eng2a morpholinos reproduces the apoptosis and MHB marker loss, but fails to substantially disrupt formation of the isthmic constriction. By contrast, a novel direct grhl2b target, spec1, identified by phylogenetic analysis and confirmed by ChIP, functionally cooperates with grhl2b to induce MHB morphogenesis, but plays no role in apoptosis or maintenance of MHB markers. Collectively, these data show that MHB maintenance and morphogenesis are dissociable events regulated by grhl2b through diverse transcriptional targets.

Grainy head and its target genes in epithelial morphogenesis and wound healing

The Grainy head (Grh) family of transcription factors is characterized by a unique DNA-binding domain that binds to a conserved consensus sequence. Nematodes and flies have a single grh gene, whereas mice and humans have evolved three genes encoding Grainy head-like (Grhl) factors. We review the biological function of Grh in different animals and the mechanisms modulating its activity. grh and grhl genes play a remarkably conserved role in epithelial organ development and extracellular barrier repair after tissue damage. Recent studies in flies and vertebrates suggest that Grh factors may be primary determinants of cell adhesion and epithelial tissue formation. Grh proteins can dimerize and act as activators or repressors in different developmental contexts. In flies, tissue-specific, alternative splicing generates different Grh isoforms with different DNA-binding specificities and functions. Grh activity is also modulated by receptor tyrosine kinases: it is phosphorylated by extracellular signal regulated kinase, and this phosphorylation is selectively required for epidermal barrier repair. Two mechanisms have been proposed to explain the repressive function of Grh on target gene transcription. First, Grh can target the Polycomb silencing complex to specific response elements. Second, it can directly compete for DNA binding with transcriptional activators. Understanding the molecular mechanisms of gene regulation by Grh factors is likely to elucidate phylogenetically conserved mechanisms of epithelial cell morphogenesis and regeneration upon tissue damage.