Transmembrane semaphorin control of neural circuit assembly in the mammalian retina

Precise patterns of neuronal connectivity are established during development in the nervous system, which allows for the generation of functional neuronal circuitry and subsequent human behaviors. Throughout the nervous system, synaptic connections among distinct neuronal cell types are organized within laminae. Understanding developmental mechanisms that generate specific patterns of synaptic connectivity within laminae is fundamental to unraveling the nervous system wiring and function.;My thesis focused on elucidating molecular cues that control mammalian retinal circuit assembly underlying visual perception. I addressed how the semaphorin family of guidance cues and their conventional receptors, neuropilins and plexins, participate in retinal circuit assembly. To define the physiological roles played by semaphorins in retinal circuit formation, I used a mouse genetics approach and performed phenotypic assessment of mutant mice lacking each neuropilin and plexin by immunohistochemistry. The phenotypic analysis of these mutant mice defined a critical role for transmembrane Sema6A and its receptor PlexinA4 signaling in the development of laminar stratification within the inner plexiform layer (IPL). Mice with null mutations in PlexinA4 or Sema6A exhibit defects in stereotypic lamina-specific neurite arborization of dopaminergic amacrine cells, M1-type melanopsin-expressing retinal ganglion cells, and also calbindin-positive cells in the IPL. Moreover, my analysis of these mutant mice revealed a crucial role for this specific ligand-receptor pair in the stratification of horizontal cell neurites within the outer plexiform layer.;My study also demonstrates crucial roles for transmembrane class5 semaphorins, Sema5A and Sema5B, in segregating neuronal processes from multiple classes of retinal ganglion cells, amacrine and bipolar cells within the IPL. These neurite targeting defects result in visual function abnormalities, including alterations in select ERG and RGC responses. Moreover, my results show that Sema5A and Sema5B inhibit retinal neurite outgrowth through the PlexinA1 and PlexinA3 receptors both in vitro and in vivo. These findings define a set of ligands and receptors required for the establishment of inner retinal neuron laminar stratification and retinal function.;In conclusion, my thesis identified critical roles for distinct transmembrane semaphorins in establishing laminar organization in the mammalian retina, which provides novel insights into our understanding of vertebrate retinal development and circuit assembly.