rhomboid mediates specification of blue- and green-sensitive R8 photoreceptor cells in Drosophila.

Color vision is based on the differential color sensitivity of retinal photoreceptors, however the developmental programs that control photoreceptor cell differentiation and specify color sensitivity are poorly understood. In Drosophila there is growing evidence that the color sensitivity of the R8 cell within an individual ommatidium is regulated by an inductive signal from the adjacent R7 cell. We previously examined the retinal patterning defect in Scutoid mutants, which results from a disruption of rhomboid expression. Here we show that loss of rhomboid blocks the induction of Rh5 expression and misexpression of rhomboid leads to the inappropriate induction of Rh5. These effects are specific to rhomboid, because its paralogue roughoid is neither required nor sufficient for the induction of Rh5 expression. We show that rhomboid is required cell-autonomously within the R8 photoreceptor cells and nonautonomously elsewhere in the eye for Rh5 induction. Interestingly, we found that the Epidermal growth factor receptor is also required for Rh5 induction, and its activation is sufficient to rescue the loss of Rh5 induction in a rhomboid mutant. This suggests that rhomboid may function in R8 cells to activate Epidermal growth factor receptor signaling in R7 cells and promote their differentiation to a signaling competent state.

A role for chemokine signaling in neural crest cell migration and craniofacial development.

Neural crest cells (NCCs) are a unique population of multipotent cells that migrate along defined pathways throughout the embryo and give rise to many diverse cell types including pigment cells, craniofacial cartilage and the peripheral nervous system (PNS). Aberrant migration of NCCs results in a wide variety of congenital birth defects including craniofacial abnormalities. The chemokine Sdf1 and its receptors, Cxcr4 and Cxcr7, have been identified as key components in the regulation of cell migration in a variety of tissues. Here we describe a novel role for the zebrafish chemokine receptor Cxcr4a in the development and migration of cranial NCCs (CNCCs). We find that loss of Cxcr4a, but not Cxcr7b, results in aberrant CNCC migration defects in the neurocranium, as well as cranial ganglia dysmorphogenesis. Moreover, overexpression of either Sdf1b or Cxcr4a causes aberrant CNCC migration and results in ectopic craniofacial cartilages. We propose a model in which Sdf1b signaling from the pharyngeal arch endoderm and optic stalk to Cxcr4a expressing CNCCs is important for both the proper condensation of the CNCCs into pharyngeal arches and the subsequent patterning and morphogenesis of the neural crest derived tissues.

Prdm1a is necessary for posterior pharyngeal arch development in zebrafish.

Multiple tissue interactions and signaling within the pharyngeal arches are required for development of the craniofacial skeleton. Here, we focus on the role of the transcription factor prdm1a in the differentiation of the posterior skeleton. prdm1a is expressed in the presumptive pharyngeal arch region and later in an endodermal pouch, the otic vesicle, and pharyngeal teeth. prdm1a mutants display a reduction in pharyngeal arch markers, a loss of posterior ceratobranchial cartilages, and a reduction in most neural crest-derived dermal bones. This is likely caused by a decrease in the number of proliferating cells but not an increase in cell death. Finally, a reduction in two key developmental signaling pathways, Fgf and retinoic acid, alters prdm1a expression, suggesting that prdm1a expression is mediated by these signaling pathways to pattern the posterior craniofacial skeleton. Together, these results indicate an essential role for prdm1a in the development of the zebrafish craniofacial skeleton.

Drosophila R8 photoreceptor cell subtype specification requires hibris.

Cell differentiation and cell fate determination in sensory systems are essential for stimulus discrimination and coding of environmental stimuli. Color vision is based on the differential color sensitivity of retinal photoreceptors, however the developmental programs that control photoreceptor cell differentiation and specify color sensitivity are poorly understood. In Drosophila melanogaster, there is evidence that the color sensitivity of different photoreceptors in the compound eye is regulated by inductive signals between cells, but the exact nature of these signals and how they are propagated remains unknown. We conducted a genetic screen to identify additional regulators of this process and identified a novel mutation in the hibris gene, which encodes an irre cell recognition module protein (IRM). These immunoglobulin super family cell adhesion molecules include human KIRREL and nephrin (NPHS1). hibris is expressed dynamically in the developing Drosophila melanogaster eye and loss-of-function mutations give rise to a diverse range of mutant phenotypes including disruption of the specification of R8 photoreceptor cell diversity. We demonstrate that hibris is required within the retina, and that hibris over-expression is sufficient to disrupt normal photoreceptor cell patterning. These findings suggest an additional layer of complexity in the signaling process that produces paired expression of opsin genes in adjacent R7 and R8 photoreceptor cells.

Subacute developmental exposure of zebrafish to the organophosphate pesticide metabolite, chlorpyrifos-oxon, results in defects in Rohon-Beard sensory neuron development.

Organophosphate pesticides (OPs) are environmental toxicants known to inhibit the catalytic activity of acetylcholinesterase (AChE) resulting in hypercholinergic toxicity symptoms. In developing embryos, OPs have been hypothesized to affect both cholinergic and non-cholinergic pathways. In order to understand the neurological pathways affected by OP exposure during embryogenesis, we developed a subacute model of OP developmental exposure in zebrafish by exposing embryos to a dose of the OP metabolite chlorpyrifos-oxon (CPO) that is non-lethal and significantly inhibited AChE enzymatic activity compared to control embryos (43% at 1 day post-fertilization (dpf) and 11% at 2dpf). Phenotypic analysis of CPO-exposed embryos demonstrated that embryonic growth, as analyzed by gross morphology, was normal in 85% of treated embryos. Muscle fiber formation was similar to control embryos as analyzed by birefringence, and nicotinic acetylcholine receptor (nAChR) cluster formation was quantitatively similar to control embryos as analyzed by α-bungarotoxin staining. These results indicate that partial AChE activity during the early days of zebrafish development is sufficient for general development, muscle fiber, and nAChR development. Rohon-Beard (RB) sensory neurons exhibited aberrant peripheral axon extension and gene expression profiling suggests that several genes responsible for RB neurogenesis are down-regulated. Stability of CPO in egg water at 28.5 °C was determined by HPLC-UV-MS analysis which revealed that the CPO concentration used in our studies hydrolyzes in egg water with a half-life of 1 day. The result that developmental CPO exposure affected RB neurogenesis without affecting muscle fiber or nAChR cluster formation demonstrates that zebrafish are a strong model system for characterizing subtle neurological pathologies resulting from environmental toxicants.

Disruption of photoreceptor cell patterning in the Drosophila Scutoid mutant.

Cell fate determination in many systems is based upon inductive events driven by cell-cell interactions. Inductive signaling regulates many aspects of Drosophila compound eye development. Accumulating evidence suggests that the color sensitivity of the R8 photoreceptor cell within an individual ommatidium is regulated by an inductive signal from the adjacent R7 photoreceptor cell. This signal is thought to control an induced versus default cell-fate switch that coordinates the visual pigment expression and color sensitivities of adjacent R7 and R8 photoreceptor cells. Here we describe a disruption in R7 and R8 cell patterning in Scutoid mutants that is due to inappropriate signals from Rh4-expressing R7 cells inducing Rh5 expression in adjacent R8 cells. This dominant phenotype results from the misexpression of the transcriptional repressor snail, which with the co-repressor C-terminal-Binding-Protein represses rhomboid expression in the developing eye. We show that loss of rhomboid suppresses the Scutoid phenotype. However in contrast to the loss of rhomboid alone, which entirely blocks the normal inductive signal from the R7 to the R8 photoreceptor cell, Scutoid rhomboid double mutants display normal Rh5 and Rh6 expression. Our detailed analysis of this unusual dominant gain-of-function neomorphic phenotype suggests that the induction of Rh5 expression in Scutoid mutants is partially rhomboid independent.