Gipc3 mutations associated with audiogenic seizures and sensorineural hearing loss in mouse and human

Sensorineural hearing loss affects the quality of life and communication of millions of people, but the underlying molecular mechanisms remain elusive. Here, we identify mutations in Gipc3 underlying progressive sensorineural hearing loss (age-related hearing loss 5, ahl5) and audiogenic seizures (juvenile audiogenic monogenic seizure 1, jams1) in mice and autosomal recessive deafness DFNB15 and DFNB95 in humans. Gipc3 localizes to inner ear sensory hair cells and spiral ganglion. A missense mutation in the PDZ domain has an attenuating effect on mechanotransduction and the acquisition of mature inner hair cell potassium currents. Magnitude and temporal progression of wave I amplitude of afferent neurons correlate with susceptibility and resistance to audiogenic seizures. The Gipc3343A allele disrupts the structure of the stereocilia bundle and affects long-term function of auditory hair cells and spiral ganglion neurons. Our study suggests a pivotal role of Gipc3 in acoustic signal acquisition and propagation in cochlear hair cells

Role for a Novel Usher Protein Complex in Hair Cell Synaptic Maturation

The molecular mechanisms underlying hair cell synaptic maturation are not well understood. Cadherin-23 (CDH23), protocadherin-15 (PCDH15) and the very large G-protein coupled receptor 1 (VLGR1) have been implicated in the development of cochlear hair cell stereocilia, while clarin-1 has been suggested to also play a role in synaptogenesis. Mutations in CDH23, PCDH15, VLGR1 and clarin-1 cause Usher syndrome, characterized by congenital deafness, vestibular dysfunction and retinitis pigmentosa. Here we show developmental expression of these Usher proteins in afferent spiral ganglion neurons and hair cell synapses. We identify a novel synaptic Usher complex comprised of clarin-1 and specific isoforms of CDH23, PCDH15 and VLGR1. To establish the in vivo relevance of this complex, we performed morphological and quantitative analysis of the neuronal fibers and their synapses in the Clrn1−/− mouse, which was generated by incomplete deletion of the gene. These mice showed a delay in neuronal/synaptic maturation by both immunostaining and electron microscopy. Analysis of the ribbon synapses in Ames waltzerav3J mice also suggests a delay in hair cell synaptogenesis. Collectively, these results show that, in addition to the well documented role for Usher proteins in stereocilia development, Usher protein complexes comprised of specific protein isoforms likely function in synaptic maturation as well

Genetic approaches to studying mouse models of human seizure disorders.

In conclusion, we have discussed a reverse genetics approach to studying seizure disorders in mice (Fig. 1), employing a targeted mutagenesis method to exploit the genetic defects identified in human epilepsy families. After detailed characterization of the nature of the human mutation and the mouse counterpart gene, a targeting vector containing the human disease allele is created. The endogenous mouse gene is replaced by the human disease allele through homologous recombination in ES cells, leading to the generation of chimeric animals. Mice carrying one copy or both copies of the human mutation can be bred to study the phenotypic effect of heterozygous and homozygous mutations. At this stage, one may want to split the newly created mice into two groups. One group will go through seizure phenotyping tests, while the other group will be used to generate disease allele-carrying mice on a different genetic background. Phenotypic characterization of mice on different inbred strains includes behavioral monitoring and EEG analysis looking for the occurrence of spontaneous seizures, as well as routine cage examination looking for handling-provoked seizure and ECT- and PTZ- induced seizure paradigms looking for sensitivity to these stimuli. A complete evaluation of the seizure phenotype at the whole-animal level establishes the relevance of the mouse model to the human condition. Further investigation including imaging, electrophysiology and AED response in these mouse models will shed light on the mechanistic basis of the convulsive disorder. Current epilepsy research in mouse genetics offers promise for understanding the molecular mechanisms that underlie epileptogenesis in humans. A large-scale forward genetic effort to create novel mouse mutants with seizure phenotypes by in vivo chemical mutagenesis with ethyl-nitroso urea (ENU) is underway at the Jackson Laboratory (http://www.jax.org/nmf/). Genetic mapping and isolation of the affected genes in these seizure-prone models will provide additional molecular pathways involved in seizures. The mutant mice generated through both forward and reverse genetic approaches will be a valuable resource for the biomedical community to study epilepsy at the molecular level and to characterize the pathological consequences of seizures in the whole organism