MOLECULAR MECHANISMS UNDERLYING THE ASSEMBLY AND FUNCTION OF VISUAL SYSTEM CIRCUITRY

The establishment of neuronal connectivity requires a series of highly orchestrated events including neuronal migration, axon guidance, dendritic patterning, synapse formation and elimination, and circuit remodeling. Understanding the developmental programs and molecular machinery that underlie these processes is essential in order to unravel how the nervous system is assembled and how it functions. The central aim of my thesis was to define the molecular and cellular mechanisms that govern the establishment of neuronal connectivity in the visual system, with a focus on murine retina circuitry assembly and retinorecipient targeting. My experiments first addressed the roles played by class 6 transmembrane semaphorins and their receptors in the functional assembly of retinal direction-selective circuits and in the development of the accessory optic system. Through extensive phenotypic analyses, genetic labeling and manipulation, and functional assays I uncovered the roles played by Sema6A and its receptor PlexA2 in defining On versus Off direction-selective pathways. In addition, I discovered that PlexA2 and PlexA4 act in concert to mediate Sema6A-dependent development and function of the accessory optic visual system. These findings provide novel insight into how a guidance cue, in this case Sema6A, serves as molecular determinant to distinguish structurally similar but functionally distinct retinal circuits, and how the same guidance molecules are utilized multiple times, at times through distinct mechanisms, to regulate different aspects of retinal development. In collaboration with Dr. Martín Riccomagno, a postdoctoral fellow in the Kolodkin laboratory, my work next investigated the molecular mechanisms that sculpt single-cell ganglion cell layer (GCL) formation. Through extensive genetic analyses and in vivo manipulation experiments, I found that Cas family adaptor proteins are essential for correct cell body lamination of retinal ganglion cell (RGCs) during development. These findings for the first time reveal in vivo roles played by vertebrate Cas proteins during neural development, shedding light on the basic principle of how the nervous system is organized. Finally, my thesis work provides a foundation for future studies devoted to uncovering the entire spectrum of molecules and signaling events required for retinal cell migration, dendritic stratification, axon guidance, and retinorecipient targeting. My functional studies and observations in the mammalian retina will also enrich our knowledge with respect to the basic principles underlying neural circuit assembly, thereby advancing our understanding of the establishment of neuronal connectivity across the entire nervous system