Adult-onset hearing loss is very common, but we know little about the underlying molecular pathogenesis impeding the development of therapies. We took a genetic approach to identify new molecules involved in hearing loss by screening a large cohort of newly generated mouse mutants using a sensitive electrophysiological test, the auditory brainstem response (ABR). We review here the findings from this screen. Thirty-eight unexpected genes associated with raised thresholds were detected from our unbiased sample of 1,211 genes tested, suggesting extreme genetic heterogeneity. A wide range of auditory pathophysiologies was found, and some mutant lines showed normal development followed by deterioration of responses, revealing new molecular pathways involved in progressive hearing loss. Several of the genes were associated with the range of hearing thresholds in the human population and one, SPNS2 , was involved in childhood deafness. The new pathways required for maintenance of hearing discovered by this screen present new therapeutic opportunities.

The developmental and physiological complexity of the auditory system is likely reflected in the underlying set of genes involved in auditory function. In humans, over 150 non-syndromic loci have been identified, and there are more than 400 human genetic syndromes with a hearing loss component. Over 100 non-syndromic hearing loss genes have been identified in mouse and human, but we remain ignorant of the full extent of the genetic landscape involved in auditory dysfunction. As part of the International Mouse Phenotyping Consortium, we undertook a hearing loss screen in a cohort of 3006 mouse knockout strains. In total, we identify 67 candidate hearing loss genes. We detect known hearing loss genes, but the vast majority, 52, of the candidate genes were novel. Our analysis reveals a large and unexplored genetic landscape involved with auditory function.

The family of WD40-repeat (WDR) proteins is one of the largest in eukaryotes, but little is known about their function in brain development. Among 26 WDR genes assessed, we found 7 displaying a major impact in neuronal morphology when inactivated in mice. Remarkably, all seven genes showed corpus callosum defects, including thicker (Atg16l1, Coro1c, Dmxl2, and Herc1), thinner (Kif21b and Wdr89), or absent corpus callosum (Wdr47), revealing a common role for WDR genes in brain connectivity. We focused on the poorly studied WDR47 protein sharing structural homology with LIS1, which causes lissencephaly. In a dosage-dependent manner, mice lacking Wdr47 showed lethality, extensive fiber defects, microcephaly, thinner cortices, and sensory motor gating abnormalities. We showed that WDR47 shares functional characteristics with LIS1 and participates in key microtubule-mediated processes, including neural stem cell proliferation, radial migration, and growth cone dynamics. In absence of WDR47, the exhaustion of late cortical progenitors and the consequent decrease of neurogenesis together with the impaired survival of late-born neurons are likely yielding to the worsening of the microcephaly phenotype postnatally. Interestingly, the WDR47-specific C-terminal to LisH (CTLH) domain was associated with functions in autophagy described in mammals. Silencing WDR47 in hypothalamic GT1-7 neuronal cells and yeast models independently recapitulated these findings, showing conserved mechanisms. Finally, our data identified superior cervical ganglion-10 (SCG10) as an interacting partner of WDR47. Taken together, these results provide a starting point for studying the implications of WDR proteins in neuronal regulation of microtubules and autophagy.WD40-repeat proteins | corpus callosum agenesis | microcephaly | neurogenesis | autophagy T he function of WD40-repeat (WDR)-containing proteins, one of the largest eukaryotic protein families, is largely unknown. Their importance is, however, evident based on their highly conserved repeating units from bacteria to mammals (1), commonly made of seven repetitive blades of 40 amino acids that end with a tryptophan-aspartic acid dipeptide at the C terminus.As shown by crystallography studies, including the crystal structure of the beta gamma dimer of the G-protein transducin (2), a classical WDR protein, all WDR proteins are predicted to fold into a circularized beta-propeller structure, serving as a rigid platform (or scaffold) for protein-protein interactions by providing many stable and symmetrical surfaces (3, 4). One reason why WDR domains may have been less studied than other common domains, such as kinases or PDZ or SH3 domains (3), is that no WDR domain has yet been found with catalytic activity (3), but this does not mean that the scaffold domains are less important. To the contrary, their serving as a platform for multiple enzymatic reactions and signaling events is highly significant (5).In recent years, human genetic studies have also begun to recognize the importance of WDR gen...

Early detection of an O 2 deficit in the bloodstream is essential to initiate corrective changes in the breathing pattern of mammals. Carotid bodies serve an essential role in this respect; their type I cells depolarize when O 2 levels fall, causing voltage-gated Ca 2؉ entry. Subsequent neurosecretion elicits increased afferent chemosensory fiber discharge to induce appropriate changes in respiratory function (1). Although depolarization of type I cells by hypoxia is known to arise from K ؉ channel inhibition, the identity of the signaling pathway has been contested, and the coupling mechanism is unknown (2). We tested the hypothesis that AMP-activated protein kinase (AMPK) is the effector of hypoxic chemotransduction. AMPK is co-localized at the plasma membrane of type I cells with O 2 -sensitive K ؉ channels. In isolated type I cells, activation of AMPK using 5-aminoimidazole-4-carboxamide riboside (AICAR) inhibited O 2 -sensitive K ؉ currents (carried by large conductance Ca 2؉ -activated (BK Ca ) channels and TASK (tandem pore, acidsensing potassium channel)-like channels, leading to plasma membrane depolarization, Ca 2؉ influx, and increased chemosensory fiber discharge. Conversely, the AMPK antagonist compound C reversed the effects of hypoxia and AICAR on type I cell and carotid body activation. These results suggest that AMPK activation is both sufficient and necessary for the effects of hypoxia. Furthermore, AMPK activation inhibited currents carried by recombinant BK Ca channels, whereas purified AMPK phosphorylated the ␣ subunit of the channel in immunoprecipitates, an effect that was stimulated by AMP and inhibited by compound C. Our findings demonstrate a central role for AMPK in stimulus-response coupling by hypoxia and identify for the first time a link between metabolic stress and ion channel regulation in an O 2 -sensing system.Chronic and intermittent deficits in O 2 supply to the body precipitate a variety of pathologies including dementia (3) and pulmonary hypertension (4). To develop effective therapies, it is necessary to understand the homeostatic mechanisms that monitor O 2 supply to the body and elicit corrective changes in respiratory and circulatory function to maintain O 2 levels. O 2 -sensitive ion channels, which were first identified in the carotid body type I cell, play a pivotal role in this respect and have now been reported in a diverse range of highly specialized O 2 -sensing tissues (5). Within the carotid body, clusters of type I cells lie in presynaptic contact with afferent sensory fibers, whose discharge increases in proportion to the degree of systemic arterial O 2 deficit, providing information concerning blood O 2 levels to the central respiratory centers (1, 2). This occurs subsequent to hypoxic inhibition of type I cell K ϩ channels, membrane depolarization, voltage-gated Ca 2ϩ influx (6), and consequent neurotransmitter release. For many years, there has existed compelling evidence that mitochondria serve an important role in O 2 sensing by type I cells (2, 7). Inde...

Measurements of auditory evoked potentials can be used to determine reliably an audiometric representation of hearing sensitivity in mice. In a high-throughput phenotyping screen of mice carrying targeted mutations of single genes, the auditory brainstem response (ABR) is used to gain an estimate of hearing threshold for broadband click stimuli and pure tone frequencies ranging from 6 to 30 kHz. Comparison of thresholds obtained in mutant and wild-type mice give a means to determine mild, moderate, and severe hearing impairment. This gives a clear advantage over using a "clickbox" test to assess hearing by observations of the Preyer reflex. The ABR screen has identified several mutant lines with mild and moderate hearing loss, which appear to demonstrate normal Preyer responses. The ABR technique also allows frequency-selective hearing loss to be identified. Curr. Protoc. Mouse Biol. 1:279-287 © 2011 by John Wiley & Sons, Inc.