The cochlea resolves sound frequencies through molecular, structural, and functional specializations established along its tonotopic axis. Although transient morphogen gradients establish positional information during embryogenesis, the mechanisms that maintain and refine this information as cochlear function matures remain poorly understood. Here, we identify the chromatin modifier HMGA2 as a candidate molecular link between opposing hedgehog (HH) and retinoic acid (RA) signals and long-term auditory function. In vivo manipulation of RA and HH signaling demonstrated that RA suppresses, whereas HH activation promotes, Hmga2 expression, establishing its apex-to-base decreasing gradient. Conditional deletion of Hmga2 did not affect cochlear length, hair cell numbers, mechanotransduction, or Prestin localization but caused a delayed and persistent low-frequency hearing deficit. Longitudinal auditory measurements revealed two distinct functional phenotypes that may contribute to this deficit: non-tonotopic DPOAE abnormalities consistent with altered cochlear micromechanics and a late, low-frequency reduction in ABR P1 amplitude consistent with impaired inner hair cell-afferent signaling. Together, these findings identify HMGA2 as a molecular link between embryonic morphogen signaling and long-term low-frequency auditory function and suggest that persistent HMGA2-dependent regulation contributes to the maintenance of low-frequency auditory function.
Within the neuronal classes of the retina, amacrine cells (ACs) exhibit the greatest neuronal diversity in morphology and function. We show that the selective expression of the transcription factor Gbx2 is required for dendritic stratification of an individual AC subtype in the mouse retina. We identify Robo1 and Robo2 as downstream effectors that when deleted, phenocopy the dendritic misprojections seen in Gbx2 mutants. Slit1 and Slit2, the ligands of Robo receptors, are localized to the upper layers of the inner plexiform layer where we observe the dendritic misprojections in both Gbx2 and Robo1/2 mutants. We show that Robo receptors also are required for the proper dendritic stratification of additional AC subtypes, such as Vglut3+ ACs. These results show both that Gbx2 regulates the terminal differentiation of a single AC subtype and identify Slit-Robo signaling as a developmental mechanism for dendrite stratification in the retina.
It has long been thought that neurons are the only cells to utilize ion flux to temporally regulate secretion for cell-to-cell signaling, but we discovered that ectoderm and mesenchymal cells are excitable, undergo voltage-dependent calcium waves, and harness calcium influx to control secretion of essential morphogens, bone morphogenetic proteins (BMPs). Furthermore, altering ion flux during morphogenesis results in craniofacial and limb abnormalities in organisms as diverse as insects and mammals, suggesting that the role for electrical signaling is conserved. In neurons, calcium activates the SNARE complex to drive fusion of secretory vesicles to temporally regulate secretion. Our discoveries inspired the hypothesis that calcium induces the SNARE complex to control BMP secretion. Here, we use pharmacological inhibition of the SNARE complex (Botulinum toxin, BoNT-C) and genetic tools in Drosophila to test this hypothesis in the wing primordia (wing disc). To test whether the SNARE complex is important in epithelial cells for BMP secretion, we apply BoNT-C to the wing disc and measure BMP secretion and calcium activity. We found that BoNT-C inhibits BMP secretion and increases endogenous calcium activity in the epithelial cells of the wing disc, as it does in neurons. Furthermore, expression of BoNT-C in the BMP-producing cells significantly reduces measures of downstream signaling. These data open the possibility that the SNARE complex may control the secretion of other developmental morphogens. Furthermore, our data suggest that pharmacological inhibition of the SNARE complex may not be specific to neuronal targets.
Metamorphosis can provide an opportunity for animals with complex life cycles to transition between differing environments and still maintain optimal functionality across ontogeny. The structure and function of the amphibian visual system generally remodels to overcome differences in media (water, air) and fluctuations of available light. This in turn causes a shift in the selective pressures acting on the visual system at each life stage. Few comparative studies have evaluated how metamorphosis drives the evolution and expression of visual system genes across closely related species that occupy disparate habitats. Salamanders offer an excellent model to study visual system adaptations because of their diverse life cycles (biphasic and paedomorphic) and drastic habitat transitions (surface and cave-adapted). We sequenced whole-eye transcriptomes of larvae and adults across 13 lineages of Brook Salamanders to test whether there is a relationship between the expression of opsin and phototransduction genes, and lineages that differ in life cycle mode. Our study revealed that numerous differences in gene expression were lineage-specific. However, there was significant upregulation of rhodopsin and several rod-specific phototransduction genes in biphasic adults, whereas paedomorphic salamanders retained juvenilized expression patterns similar to larvae. The paedomorphic troglomorph species, Eurycea latitans exhibited aberrant expression patterns of rhodopsin and some rod-related genes. This study has implications for how metamorphosis allows salamanders to adaptively decouple their visual system in order to optimize to disparate larval and adult environments.
