ISL1 is necessary for auditory neuron development and contributes toward tonotopic organization.

A cardinal feature of the auditory pathway is frequency selectivity, represented in a tonotopic map from the cochlea to the cortex. The molecular determinants of the auditory frequency map are unknown. Here, we discovered that the transcription factor ISL1 regulates the molecular and cellular features of auditory neurons, including the formation of the spiral ganglion and peripheral and central processes that shape the tonotopic representation of the auditory map. We selectively knocked out Isl1 in auditory neurons using Neurod1Cre strategies. In the absence of Isl1, spiral ganglion neurons migrate into the central cochlea and beyond, and the cochlear wiring is profoundly reduced and disrupted. The central axons of Isl1 mutants lose their topographic projections and segregation at the cochlear nucleus. Transcriptome analysis of spiral ganglion neurons shows that Isl1 regulates neurogenesis, axonogenesis, migration, neurotransmission-related machinery, and synaptic communication patterns. We show that peripheral disorganization in the cochlea affects the physiological properties of hearing in the midbrain and auditory behavior. Surprisingly, auditory processing features are preserved despite the significant hearing impairment, revealing central auditory pathway resilience and plasticity in Isl1 mutant mice. Mutant mice have a reduced acoustic startle reflex, altered prepulse inhibition, and characteristics of compensatory neural hyperactivity centrally. Our findings show that ISL1 is one of the obligatory factors required to sculpt auditory structural and functional tonotopic maps. Still, upon Isl1 deletion, the ensuing central plasticity of the auditory pathway does not suffice to overcome developmentally induced peripheral dysfunction of the cochlea.

Early ear neuronal development, but not olfactory or lens development, can proceed without SOX2.

SOX2 is essential for maintaining neurosensory stem cell properties, although its involvement in the early neurosensory development of cranial placodes remains unclear. To address this, we used Foxg1-Cre to conditionally delete Sox2 during eye, ear, and olfactory placode development. Foxg1-Cre mediated early deletion of Sox2 eradicates all olfactory placode development, and disrupts retinal development and invagination of the lens placode. In contrast to the lens and olfactory placodes, the ear placode invaginates and delaminates NEUROD1 positive neurons. Furthermore, we show that SOX2 is not necessary for early ear neurogenesis, since the early inner ear ganglion is formed with near normal central projections to the hindbrain and peripheral projections to the undifferentiated sensory epithelia of E11.5-12.5 ears. However, later stages of ear neurosensory development, in particular, the late forming auditory system, critically depend on the presence of SOX2. Our data establish distinct differences for SOX2 requirements among placodal sensory organs with similarities between olfactory and lens but not ear placode development, consistent with the unique neurosensory development and molecular properties of the ear.

Neurod1 Is Essential for the Primary Tonotopic Organization and Related Auditory Information Processing in the Midbrain.

Hearing depends on extracting frequency, intensity, and temporal properties from sound to generate an auditory map for acoustical signal processing. How physiology intersects with molecular specification to fine tune the developing properties of the auditory system that enable these aspects remains unclear. We made a novel conditional deletion model that eliminates the transcription factor NEUROD1 exclusively in the ear. These mice (both sexes) develop a truncated frequency range with no neuroanatomically recognizable mapping of spiral ganglion neurons onto distinct locations in the cochlea nor a cochleotopic map presenting topographically discrete projections to the cochlear nuclei. The disorganized primary cochleotopic map alters tuning properties of the inferior colliculus units, which display abnormal frequency, intensity, and temporal sound coding. At the behavioral level, animals show alterations in the acoustic startle response, consistent with altered neuroanatomical and physiological properties. We demonstrate that absence of the primary afferent topology during embryonic development leads to dysfunctional tonotopy of the auditory system. Such effects have never been investigated in other sensory systems because of the lack of comparable single gene mutation models.SIGNIFICANCE STATEMENT All sensory systems form a topographical map of neuronal projections from peripheral sensory organs to the brain. Neuronal projections in the auditory pathway are cochleotopically organized, providing a tonotopic map of sound frequencies. Primary sensory maps typically arise by molecular cues, requiring physiological refinements. Past work has demonstrated physiologic plasticity in many senses without ever molecularly undoing the specific mapping of an entire primary sensory projection. We genetically manipulated primary auditory neurons to generate a scrambled cochleotopic projection. Eliminating tonotopic representation to auditory nuclei demonstrates the inability of physiological processes to restore a tonotopic presentation of sound in the midbrain. Our data provide the first insights into the limits of physiology-mediated brainstem plasticity during the development of the auditory system.

Combined Atoh1 and Neurod1 Deletion Reveals Autonomous Growth of Auditory Nerve Fibers.

Ear development requires the transcription factors ATOH1 for hair cell differentiation and NEUROD1 for sensory neuron development. In addition, NEUROD1 negatively regulates Atoh1 gene expression. As we previously showed that deletion of the Neurod1 gene in the cochlea results in axon guidance defects and excessive peripheral innervation of the sensory epithelium, we hypothesized that some of the innervation defects may be a result of abnormalities in NEUROD1 and ATOH1 interactions. To characterize the interdependency of ATOH1 and NEUROD1 in inner ear development, we generated a new Atoh1/Neurod1 double null conditional deletion mutant. Through careful comparison of the effects of single Atoh1 or Neurod1 gene deletion with combined double Atoh1 and Neurod1 deletion, we demonstrate that NEUROD1-ATOH1 interactions are not important for the Neurod1 null innervation phenotype. We report that neurons lacking Neurod1 can innervate the flat epithelium without any sensory hair cells or supporting cells left after Atoh1 deletion, indicating that neurons with Neurod1 deletion do not require the presence of hair cells for axon growth. Moreover, transcriptome analysis identified genes encoding axon guidance and neurite growth molecules that are dysregulated in the Neurod1 deletion mutant. Taken together, we demonstrate that much of the projections of NEUROD1-deprived inner ear sensory neurons are regulated cell-autonomously.

Incomplete and delayed Sox2 deletion defines residual ear neurosensory development and maintenance.

The role of Sox2 in neurosensory development is not yet fully understood. Using mice with conditional Islet1-cre mediated deletion of Sox2, we explored the function of Sox2 in neurosensory development in a model with limited cell type diversification, the inner ear. In Sox2 conditional mutants, neurons initially appear to form normally, whereas late- differentiating neurons of the cochlear apex never form. Variable numbers of hair cells differentiate in the utricle, saccule, and cochlear base but sensory epithelium formation is completely absent in the apex and all three cristae of the semicircular canal ampullae. Hair cells differentiate only in sensory epithelia known or proposed to have a lineage relationship of neurons and hair cells. All initially formed neurons lacking hair cell targets die by apoptosis days after they project toward non-existing epithelia. Therefore, late neuronal development depends directly on Sox2 for differentiation and on the survival of hair cells, possibly derived from common neurosensory precursors.

Overexpression of v-myb oncogene or c-myb proto-oncogene in insect cells: characterization of newly induced nucleolus-like structures accumulating Myb protein.

The oncoprotein v-Myb induces myeloid leukemia and its cellular counterpart c-Myb is involved in the regulation of hematopoiesis. Although intensively studied, their precise subcellular localization is not known. In order to expand our knowledge in this respect, we used an artificial system overexpressing these proteins. We investigated the subcellular localization of Myb proteins in cultured non-synchronized insect cells transfected with recombinant baculoviruses overexpressing either v-myb oncogene or c-myb proto-oncogene. The cell expressing Myb proteins underwent extensive nuclear changes and exhibited distinct nuclear structures resembling nucleoli. The bulk of v-Myb and c-Myb proteins accumulated in such nucleolus-like structures which, according to the nucleolar nomenclature, we classified to three types: compact of enlarged size (type I), large ring-shaped (type II) and with nucleolonemas (type III). We investigated these structures for the presence of important nucleolar macromolecules in order to establish whether they were compatible with the function in the production of ribosomes. Strikingly, our results indicated that the different forms of these structures did not represent genuine nucleoli. They rather reflected progressive changes, induced by the virus infection and high expression of v-myb genes, accompanied by the formation of these prominent nucleolus-like structures highly enriched in Myb protein. Gradual changes in number of individual nucleolus-like forms during infection, increasing amount of Myb protein and DNA localized in them together with decreasing amount of RNA and their different interaction with viral particles indicate that the nucleolus-like structure of type I is a precursor of the type II and finally of the type III.

Direct comparison of enoxaparin and nadroparin in a rabbit model of arterial thrombosis prevention.

INTRODUCTION: The aim of the study was to evaluate and compare the efficacy of standard unfractionated heparin (UFH) and low-molecular weight heparins (LMWH's). MATERIALS AND METHODS: We modified a previously published rabbit model of arterial thrombosis prevention [1,2] to compare unfractionated heparin and two different doses of two low-molecular weight heparin fragments--nadroparin and enoxaparin. Thrombosis in the distal aorta was triggered by vessel wall injury and critical stenosis. Blood flow in the damaged arterial segment was monitored by a flow probe placed distal to the constrictor. The primary endpoints of the study were: (1) cumulative flow, (2) time to occlusion and (3) residual clot weight. Thirty six animals were split into 6 groups with six animals in each group. Control groups were given saline or heparin and four more groups were used to compare LMWH's at 2 different doses. RESULTS: In our study, all treatments were superior to the saline control group (alpha