Gata3 is a critical regulator of cochlear wiring.

Spiral ganglion neurons (SGNs) play a key role in hearing by rapidly and faithfully transmitting signals from the cochlea to the brain. Identification of the transcriptional networks that ensure the proper specification and wiring of SGNs during development will lay the foundation for efforts to rewire a damaged cochlea. Here, we show that the transcription factor Gata3, which is expressed in SGNs throughout their development, is essential for formation of the intricately patterned connections in the cochlea. We generated conditional knock-out mice in which Gata3 is deleted after SGNs are specified. Cochlear wiring is severely disrupted in these animals, with premature extension of neurites that follow highly abnormal trajectories toward their targets, as shown using in vitro neurite outgrowth assays together with time-lapse imaging of whole embryonic cochleae. Expression profiling of mutant neurons revealed a broad shift in gene expression toward a more differentiated state, concomitant with minor changes in SGN identity. Thus, Gata3 appears to serve as an "intermediate regulator" that guides SGNs through differentiation and preserves the auditory fate. As the first auditory-specific regulator of SGN development, Gata3 provides a useful molecular entry point for efforts to engineer SGNs for the restoration of hearing.

Spatiotemporal fate map of neurogenin1 (Neurog1) lineages in the mouse central nervous system.

Neurog1 (Ngn1, Neurod3, neurogenin1) is a basic helix-loop-helix (bHLH) transcription factor essential for neuronal differentiation and subtype specification during embryogenesis. Due to the transient expression of Neurog1 and extensive migration of neuronal precursors, it has been challenging to understand the full complement of Neurog1 lineage cells throughout the central nervous system (CNS). Here we labeled and followed Neurog1 lineages using inducible Cre-flox recombination systems with Neurog1-Cre and Neurog1-CreER(T2) BAC (bacterial artificial chromosome) transgenic mice. Neurog1 lineage cells are restricted to neuronal fates and contribute to diverse but discrete populations in each brain region. In the forebrain, Neurog1 lineages include mitral cells and glutamatergic interneurons in the olfactory bulb, pyramidal and granule neurons in the hippocampus, and pyramidal cells in the cortex. In addition, most of the thalamus, but not the hypothalamus, arises from Neurog1 progenitors. Although Neurog1 lineages are largely restricted to glutamatergic neurons, there are multiple exceptions including Purkinje cells and other GABAergic neurons in the cerebellum. This study provides the first overview of the spatiotemporal fate map of Neurog1 lineages in the CNS.

Auditory neurons make stereotyped wiring decisions before maturation of their targets.

Cochlear ganglion neurons communicate sound information from cochlear hair cells to auditory brainstem neurons through precisely wired circuits. Understanding auditory circuit assembly is a significant challenge because of the small size of the otic vesicle and difficulties labeling and imaging embryonic neurons. We used genetic fate mapping in the mouse to visualize the morphologies of individual cochlear ganglion neurons throughout development, from their origin in the Neurogenin1-positive neurogenic domain in the otic vesicle to the formation of connections with targets in the cochlea and in the cochlear nucleus. We found that auditory neurons with different patterns of connectivity arise from discrete populations of Neurogenin1-positive precursors that make stereotyped wiring decisions depending on when and where they are born. Auditory precursors are segregated from vestibular precursors early in neurogenesis. Within this population, cochlear ganglion neurons with type I and type II morphologies are apparent before birth and develop within common pools of precursors. The peripheral projections are initially complex and branched and then become simple and straight after reaching the edge of the sensory epithelium. Subsequently, a small number of projections attain obvious type II morphologies, beginning at embryonic day 16.5 (E16.5), when hair cells begin to differentiate. Centrally, cochlear ganglion axons are topographically organized in the auditory brainstem as early as E15.5, when the cochlear nucleus is still immature. These findings suggest that Neurogenin1 precursors possess intrinsic programs of differentiation that direct early auditory circuit assembly events before the maturation of presynaptic and postsynaptic target cells.

Cross-regulation of Ngn1 and Math1 coordinates the production of neurons and sensory hair cells during inner ear development.

Temporal and spatial coordination of multiple cell fate decisions is essential for proper organogenesis. Here, we define gene interactions that transform the neurogenic epithelium of the developing inner ear into specialized mechanosensory receptors. By Cre-loxP fate mapping, we show that vestibular sensory hair cells derive from a previously neurogenic region of the inner ear. The related bHLH genes Ngn1 (Neurog1) and Math1 (Atoh1) are required, respectively, for neural and sensory epithelial development in this system. Our analysis of mouse mutants indicates that a mutual antagonism between Ngn1 and Math1 regulates the transition from neurogenesis to sensory cell production during ear development. Furthermore, we provide evidence that the transition to sensory cell production involves distinct autoregulatory behaviors of Ngn1 (negative) and Math1 (positive). We propose that Ngn1, as well as promoting neurogenesis, maintains an uncommitted progenitor cell population through Notch-mediated lateral inhibition, and Math1 irreversibly commits these progenitors to a hair-cell fate.

The Zuker collection: a resource for the analysis of autosomal gene function in Drosophila melanogaster.

The majority of genes of multicellular organisms encode proteins with functions that are not required for viability but contribute to important physiological functions such as behavior and reproduction. It is estimated that 75% of the genes of Drosophila melanogaster are nonessential. Here we report on a strategy used to establish a large collection of stocks that is suitable for the recovery of mutations in such genes. From approximately 72,000 F(3) cultures segregating for autosomes heavily treated with ethyl methanesulfonate (EMS), approximately 12,000 lines in which the treated second or third chromosome survived in homozygous condition were selected. The dose of EMS induced an estimated rate of 1.2-1.5 x 10(-3) mutations/gene and predicts five to six nonessential gene mutations per chromosome and seven to nine alleles per locus in the samples of 6000 second chromosomes and 6000 third chromosomes. Due to mosaic mutations induced in the initial exposure to the mutagen, many of the lines are segregating or are now fixed for lethal mutations on the mutagenized chromosome. The features of this collection, known as the Zuker collection, make it a valuable resource for forward and reverse genetic screens for mutations affecting a wide array of biological functions.