Neurodevelopmental Roles of Semaphorin6A/PlexinA2 Signaling in Zebrafish
A multitude of complex cellular changes are required throughout development in order for a single cell to transform into a fully functioning organism. Cellular events including proliferation, migration, and differentiation have to be carefully controlled in order for development to proceed correctly. In order to study such dynamic processes, in vivo models are often utilized. Using the zebrafish (Danio rerio) as a model system, we have investigated the role of an axon guidance signaling pair, Semaphorin6A (Sema6A) and PlexinA2 (PlxnA2), in neurodevelopment.
A previous investigation into the developmental expression patterns of sema6A and plxnA2 in zebrafish, revealed overlapping expression in the developing eye. At this early stage, the cells in the optic vesicles are undifferentiated retinal precursor cells (RPCs) and therefore do not require Sema/Plxn signaling for their canonical axon guidance role. To understand what the function of this early expression was, we knocked down both sema6a and plxna2 and observed 1) a loss of cohesion of RPCs within optic vesicles, and 2) a decrease in RPC proliferation (Ebert et al., 2014). Because these phenotypes were seen at an early stage and given that many developmental processes are dependent on genetic regulation, we hypothesized that Sema6A/PlxnA2 signaling could be regulating transcription of downstream target genes. To investigate this, we performed a microarray experiment and uncovered 58 differentially regulated genes (Emerson et al., 2017a). Prior to our study, it was not known that Sema/Plxn signaling led to changes in gene transcription.
In an effort to understand the contribution of identified candidate genes to early sema6A/plxnA2 knockdown phenotypes, candidate genes with predicted functions in proliferation and migration were investigated. First, we show that rasl11b is important for regulation of RPC proliferation in the developing optic vesicles. Second, we show that shootin-1 is important in optic vesicle migration, retinal pigmented epithelium formation and optic tract patterning. Furthermore, PlxnA2 regulation of shootin-1 levels is important in sensory and motor axon patterning and branching in the peripheral nervous system.
Belonging to a large family of proteins with the ability to cross talk, Semas and Plxns rely on spatially and temporally differential expression patterns to perform their tissue-specific roles. Here, we used in situ hybridization to comprehensively uncover the neuronal expression patterns of the PlxnA family in the early developing zebrafish (Emerson et al., 2017b). In addition, we present for the first time that zebrafish have two genes for PlxnA1, A1a and A1b, which show divergent expression patterns.
Semas and Plxns are critical for many aspects of development and together, this body of work provides further insight into the downstream signaling mechanisms and roles of these essential developmental signaling proteins.