The neuron specific RNA-binding proteins NOVA1 and NOVA2 are highly homologous alternative splicing regulators. NOVA proteins regulate at least 700 alternative splicing events in vivo, yet relatively little is known about the biologic consequences of NOVA action and in particular about functional differences between NOVA1 and NOVA2. Transcriptome-wide searches for isoform-specific functions, using NOVA1 and NOVA2 specific HITS-CLIP and RNA-seq data from mouse cortex lacking either NOVA isoform, reveals that NOVA2 uniquely regulates alternative splicing events of a series of axon guidance related genes during cortical development. Corresponding axonal pathfinding defects were specific to NOVA2 deficiency: Nova2-/- but not Nova1-/- mice had agenesis of the corpus callosum, and axonal outgrowth defects specific to ventral motoneuron axons and efferent innervation of the cochlea. Thus we have discovered that NOVA2 uniquely regulates alternative splicing of a coordinate set of transcripts encoding key components in cortical, brainstem and spinal axon guidance/outgrowth pathways during neural differentiation, with severe functional consequences in vivo.DOI: http://dx.doi.org/10.7554/eLife.14371.001

Hair cells, the sensory receptors of the internal ear, subserve different functions in various receptor organs: they detect oscillatory stimuli in the auditory system, but transduce constant and step stimuli in the vestibular and lateral-line systems. We show that a hair cell's function can be controlled experimentally by adjusting its mechanical load. By making bundles from a single organ operate as any of four distinct types of signal detector, we demonstrate that altering only a few key parameters can fundamentally change a sensory cell's role. The motions of a single hair bundle can resemble those of a bundle from the amphibian vestibular system, the reptilian auditory system, or the mammalian auditory system, demonstrating an essential similarity of bundles across species and receptor organs.auditory system | dynamical system | hair cell | Hopf bifurcation | vestibular system T he hair bundles of vertebrates are mechanosensory organelles responsible for detecting sounds in the auditory system, linear and angular accelerations in the vestibular system, and water movements and pressure gradients in the lateral-line system of fishes and amphibians (1, 2). In each instance an appropriate stimulus deflects the bundles, depolarizing the hair cells from which they emerge (3). Hair bundles are not simply passive detectors, however, for they actively amplify their responses to mechanical stimulation (4, 5). Hair-bundle motility contributes to an active process that endows the auditory system with the ability to detect stimuli with energies near that of thermal fluctuations (6), to distinguish tones that differ by less than 0.2% in frequency (7), and to accommodate inputs varying over a millionfold range in amplitude (4,(8)(9)(10)(11).Auditory, vestibular, and lateral-line organs respond to distinct patterns of mechanical input. The mechanical properties and environments of hair bundles differ correspondingly between different organisms, among receptor organs, and with the tonotopic position along individual auditory organs (12-15). A mathematical model predicts that the response of a hair bundle is regulated both by its intrinsic mechanical characteristics and by its mechanical load (16). Motivated by this proposition, we investigated how the load stiffness and constant force imposed on a bundle control its dynamics and response to external perturbations. ResultsMapping the Hair Bundle's Experimental State Diagram. The hair bundle's state diagram characterizes its behavior for different combinations of two control parameters: load stiffness, or force per unit displacement, and constant force. These control parameters describe the mechanical load imposed on a hair bundle within a sensory organ. A theoretical model of hair-bundle dynamics predicts the qualitative structure of the state diagram ( Fig. 1 A-C). To test this prediction experimentally, we varied the mechanical load imposed on an individual hair bundle and monitored its displacement. The load was delivered to an individual hair bundle by attaching the tip o...

Gap detection or gap pre-pulse inhibition of the acoustic startle (GPIAS) has been successfully used in rat and guinea pig models of tinnitus, yet this system has been proven to have low efficacy in CBA mice, with low basal GPIAS and subtle tinnitus-like effects. Here, we tested five mouse strains (CBA, BalbC, CD-1, C57BL/6 and 129sv) for pre-pulse inhibition (PPI) and gap detection with varying interstimulus intervals (ISI) and found that mice from a CBA genetic background had the poorest capacities of suppressing the startle response in the presence of a pre-pulse or a gap. CD-1 mice displayed variable responses throughout all ISI. Interestingly, C57BL/6, 129sv and BalbC showed efficient suppression with either pre-pulses or gaps with shorter ISI. The glutamate aspartate transporter (GLAST) is expressed in support cells from the cochlea and buffers the excess of glutamate. We hypothesized that loss of GLAST function could sensitize the ear to tinnitus-inducing agents, such as salicylate. Using shorter ISI to obtain a greater dynamic range to assess tinnitus-like effects, we found that disruption of gap detection by salicylate was exacerbated across various intensities of a 32-kHz narrow band noise gap carrier in GLAST knockout (KO) mice when compared to their wild-type (WT) littermates. Auditory brainstem responses (ABR) and distortion-product otoacoustic emission (DPOAE) were performed to evaluate the effects on hearing functions. Salicylate caused greater auditory threshold shifts (near 15 dB) in GLAST KO mice than in WT mice across all tested frequencies, despite similarly reduced DPOAE. Despite these changes, inhibition using broad-band gap carriers and 32 kHz pre-pulses were not affected. Our study suggests that GLAST deficiency could become a useful experimental model to decipher the mechanisms underlying drug-induced tinnitus. Future studies addressing the neurological correlates of tinnitus in this model could provide additional insights into the mechanisms of tinnitus.