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theses

The formation of a complicate nervous system requires precise navigation and elaboration for growth cone and dendrites to connect to their target. During development, guidance molecules and receptors control the majority of the circuitry events, including promotion or inhibition of neurite growth. Semphorin 3A, a secreted Class 3 Semaphorin member, is well known for its chemorepellent function on growth cone mediated by the Neuropilin1/PlexinA4 holoreceptor complex. Recently, it has been shown to have opposite cellular responses in promoting dendrite growth and branching in mouse cortical pyramidal neurons (1, 2). However, the mechanism underlying how Semaphorin 3A/Neuropilin1/Plexin-A4 signaling regulates dendrite elaboration is unclear. Here, I have shown the importance and function of three distinct domains in the cytoplasmic region of the signaling transducing receptor of Semaphorin 3A, Plexin-A4, in regulating cortical neuron dendritic morphology. Both the C1 and H/RBD domains were found to be sufficient to trigger cortical pyramidal neuron dendrite elaboration, while the C2 domain was not necessary for dendrites growth and branching. Using biochemical and molecular methods in combination with in vitro assays, I found and demonstrated that the Rho-GEF, FARP2 associates with PlexinA4 and mediates dendritic elaboration in primary cortical neurons following the ligand, Semaphorin 3A activation of the signaling pathway. In addition, I demonstrated that Plexin-A4 extracellular domains could interact with its co-receptor Neuropilin1, independent of the ligand Semaphorin 3A. Therefore, the extracellular Semaphorin domain may play a role in preventing Plexin-A4 activation, consistent with previous studies (3, 4). Previously, another member of the Type A Plexin receptor, Plexin-A3, was shown to play a key role in inhibiting cortical neuron dendritic spine morphogenesis in vivo (2). However, the mechanism of how Plexin-A3 signals to restrain spine formation is unknown. I have generated an array of Plexin-A3 cytoplasmic deletion mutant constructs, analogous to the Plexin-A4 deletion mutants used for this study for future studies in investigating cortical neuron dendritic spine morphology. These molecular tools that I have developed will be useful for researchers to investigate the intracellular signaling mechanisms of Semaphorin signaling in regulating neuron morphology. Taken together, my findings provide new insights to Plexin-A4 signaling, in particular, highlighting distinct intracellular domains and downstream effectors required for promoting dendritic morphology.

ETD_5962

M.S.

Includes bibliographical references

Includes vita

by Sheng-Shiang (Anson) Peng

doi:10.7282/T3TF0001

ETD graduate

The author owns the copyright to this work.