Summary Intense noise damages the cochlear organ of Corti, particularly the outer hair cells (OHCs)[1], however this epithelium is not innervated by nociceptors of somatosensory ganglia, which detect damage elsewhere in the body. The only sensory neurons innervating the organ of Corti originate from the spiral ganglion, roughly 95% of which innervate exclusively inner hair cells (IHCs)[2-4]. Upon sound stimulation, IHCs release glutamate to activate AMPA-type receptors on these myelinated type-I neurons, which carry the neuronal signals to the cochlear nucleus. The remaining spiral ganglion cells (type-IIs) are unmyelinated and contact OHCs[2-4]. Their function is unknown. Using immunoreactivity to cFos, we documented neuronal activation in the brainstem of Vglut3−/− mice, in which the canonical auditory pathway (activation of type-I afferents by glutamate released from inner hair cells) is silenced[5, 6]. In these deaf mice, we found responses to noxious noise, that damages hair cells, but not to innocuous noise, in neurons of the cochlear nucleus, but not in the vestibular or trigeminal nuclei. This response originates in the cochlea and not in other areas also stimulated by intense noise (middle ear and vestibule) as it was absent in CD1 mice with selective cochlear degeneration but normal vestibular and somatosensory function. These data imply the existence of an alternative neuronal pathway from cochlea to brainstem that is activated by tissue-damaging noise and does not require glutamate release from IHCs. This detection of noise-induced tissue damage, possibly by type-II cochlear afferents, represents a novel form of sensation we term auditory nociception.

The mammalian cochlea contains two types of mechanosensory hair cells (HCs) that play different and critical roles in hearing. Inner hair cells (IHCs), with an elaborate presynaptic apparatus, signal to cochlear neurons and communicate sound information to the brain. Outer hair cells (OHCs) mechanically amplify sound-induced vibrations, enabling enhanced sensitivity to sound and sharp tuning. Cochlear HCs are solely generated during development and their death, most often of OHCs, is the main cause of deafness. OHCs and IHCs, together with supporting cells, originate embryonically from the prosensory region of the otocyst, but how HCs differentiate into two different types is unknown 1 – 3 . Here we show that Insm1 , which encodes a zinc finger protein transiently expressed in nascent OHCs, consolidates their fate by preventing trans-differentiation into IHCs. In the absence of INSM1 many HCs born embryonically as OHCs switch fates to become mature IHCs. In order to identify the genetic mechanisms by which Insm1 operates, we compared transcriptomes of immature IHCs vs OHCs, as well as OHCs with and without INSM1. OHCs lacking INSM1 upregulate a set of genes, most of which are normally preferentially expressed by IHCs. The homeotic cell transformation of OHCs without INSM1 into IHCs reveals for the first time a mechanism by which these neighboring mechanosensory cells begin to differ: INSM1 represses a core set of early IHC-enriched genes in embryonic OHCs and makes them unresponsive to an IHC-inducing gradient, so that they proceed to mature as OHCs. Without INSM1, some of the OHCs upregulating these few IHC-enriched transcripts trans-differentiate into IHCs, revealing the first candidate genes for IHC-specific differentiation.

Chromosomal instability (CIN) is a hallmark of many cancers. Restricting the localization of centromeric histone H3 variant CENP-A to centromeres prevents CIN. CENP-A overexpression (OE) and mislocalization have been observed in cancers and correlate with poor prognosis; however, the molecular consequences of CENP-A OE on CIN and aneuploidy have not been defined. Here, we show that CENP-A OE leads to its mislocalization and CIN with lagging chromosomes and micronuclei in pseudodiploid DLD1 cells and xenograft mouse model. CIN is due to reduced localization of proteins to the kinetochore, resulting in defects in kinetochore integrity and unstable kinetochore–microtubule attachments. CENP-A OE contributes to reduced expression of cell adhesion genes and higher invasion of DLD1 cells. We show that CENP-A OE contributes to aneuploidy with karyotypic heterogeneity in human cells and xenograft mouse model. In summary, our results provide a molecular link between CENP-A OE and aneuploidy, and suggest that karyotypic heterogeneity may contribute to the aggressive phenotype of CENP-A–overexpressing cancers.

Histone chaperones modulate the stability of histones beginning from histone synthesis, through incorporation into DNA, and during recycling during transcription and replication. Following histone removal from DNA, chaperones regulate histone storage and degradation. Here, we demonstrate that UBR7 is a histone H3.1 chaperone that modulates the supply of pre-existing postnucleosomal histone complexes. We demonstrate that UBR7 binds to post-nucleosomal H3K4me3 and H3K9me3 histones via its UBR box and PHD. UBR7 binds to the non-nucleosomal histone chaperone NASP. In the absence of UBR7, the pool of NASP-bound postnucleosomal histones accumulate and chromatin is depleted of H3K4me3-modified histones. We propose that the interaction of UBR7 with NASP and histones opposes the histone storage functions of NASP and that UBR7 promotes reincorporation of postnucleosomal H3 complexes.

Myelination is a cellular adaptation allowing rapid conduction along axons. We have investigated peripheral axons of the zebrafish maxillary barbel (ZMB), an optically clear sensory appendage. Each barbel carries taste buds, solitary chemosensory cells, and epithelial nerve endings, all of which regenerate after amputation (LeClair and Topczewski [2010] PLoS One 5:e8737). The ZMB contains axons from the facial nerve; however, myelination within the barbel itself has not been established. Transcripts of myelin basic protein (mbp) are expressed in normal and regenerating adult barbels, indicating activity in both maintenance and repair. Myelin was confirmed in situ by using toluidine blue, an anti-MBP antibody, and transmission electron microscopy (TEM). The adult ZMB contains ~180 small-diameter axons (<2 μm), approximately 60% of which are myelinated. Developmental myelination was observed via whole-mount immunohistochemistry 4-6 weeks postfertilization, showing myelin sheaths lagging behind growing axons. Early-regenerating axons (10 days postsurgery), having no or few myelin layers, were disorganized within a fibroblast-rich collagenous scar. Twenty-eight days postsurgery, barbel axons had grown out several millimeters and were organized with compact myelin sheaths. Fiber types and axon areas were similar between normal and regenerated tissue; within 4 weeks, regenerating axons restored ~85% of normal myelin thickness. Regenerating barbels express multiple promyelinating transcription factors (sox10, oct6 = pou3f1; krox20a/b = egr2a/b) typical of Schwann cells. These observations extend our understanding of the zebrafish peripheral nervous system within a little-studied sensory appendage. The accessible ZMB provides a novel context for studying axon regeneration, Schwann cell migration, and remyelination in a model vertebrate.