Carefully Constructing Circuits in the Developing CNS: How DSCAM Intricately Modulates Visual Circuit Assembly in vivo

The wiring of functional neural circuits during embryonic development requires coordinated organization between developing axon and dendritic arbors, a process that is dependent on an array of molecular guidance cues including neurotrophins, chemoattractants, repulsive cues, cell adhesion molecules, etc. These molecules work in concert to direct and establish precise, topographically organized synaptic connections which are all necessary for the formation of specific neuronal circuits and, ultimately, for proper brain function. For my dissertation, I explore the role of Down Syndrome Cell Adhesion Molecule (DSCAM), a multifaceted cell surface protein implicated in Down Syndrome and Autism. DSCAM is most prominently known for shaping the self-avoidance pattern of dendritic arbors in Drosophila neurons and mouse retinal cells, but how it modulates the interconnectivity between axons and dendrites of “central” visual neuronal circuits in developing vertebrates remains unclear. To examine how DSCAM mediates pre- and postsynaptic circuit development, we utilized the Xenopus laevis retinotectal circuit as an accessible model to study events in real time and within the in-tact animal brain in vivo. This model also provides us a unique temporal and spatial understanding of how central circuits are dynamically shaped. DSCAM gene expression was altered using a targeted knockdown approach aimed at individual RGCs and tectal neurons; subsequent changes in the morphology of RGC axons and dendritic arbors of tectal neurons were observed through confocal imaging. As RGC axons innervate the tectum, retinal axons remained relatively simple when DSCAM expression was decreased. Conversely, downregulating DSCAM in tectal neurons exhibited abnormally increased dendritic growth and branching rates while also inducing dendrites to take on convoluted directional pathways. Changes in dendritic morphology of tectal neurons by DSCAM knockdown corresponded with functional deficits in visually guided behavior of freely swimming tadpoles. Together, our observations implicate DSCAM in the control of both pre- and postsynaptic structural and functional connectivity in the developing retinotectal circuit, where it primarily acts as a neuronal brake to limit and guide postsynaptic dendrite growth of tectal neurons while it also facilitates arborization of presynaptic RGC axons cell autonomously. I also observe how DSCAM coordinates the topography of retinal axons as bundles of axons project and arborize into the tectum. Additionally, I demonstrate that DSCAM plays a role in a subset of retinotectal synapse, by stabilizing their connections in the developing tadpole central nervous system. Together, my observations implicate DSCAM in the control of both pre- and postsynaptic structural and functional connectivity in the developing retinotectal circuit. These crucial developmental instructions mediated by DSCAM are necessary for normal visual system function