Regulatory Effects of the Actin-binding Proteins Moesin and MyosinII on Synaptic Activity at the Drosophila Neuromuscular Junction

The nervous system is made up of specialized cells which receive and respond to environmental stimuli. Intercellular communication in the nervous system is achieved predominantly through chemical synaptic transmission. Within the chemical synapse, the actin cytoskeleton plays a major role in regulating synaptic activities, although the extent and clarity in our understanding of these processes is still limited. Using the genetically pliable model, Drosophila melanogaster, this thesis begins to unravel contributions of actin binding proteins to synaptic development and physiology at the larval neuromuscular junction (NMJ). Two actin binding proteins, Moesin and Nonmuscle Myosin II (NMMII) were selected for study based on previous studies implicating them in synaptic development. Combining genetics, fluorescent imaging and electrophysiological recordings this thesis unveils previously unidentified functions for Moesin and NMMII in morphology and physiology of the Drosophila NMJ. Moesin was found to help restrain synaptic growth but did not affect synaptic physiology. By correlating morphological and electrophysiological measurements in Moesin mutants, it was determined that physiology and morphology can be independently regulated at the NMJ. NMMII was used to investigate a role for actin binding proteins in physiology at the Drosophila NMJ. By using the fluorescent imaging technique, FRAP, this becomes the first research to implicate NMMII in unstimulated synaptic vesicle mobility. FRAP indicated that vesicle mobility was highly dependent on the expression level of NMMII. Electrophysiological analysis of NMMII indicated distinct mechanisms for spontaneous and evoked vesicle release. NMMII expression exhibited a positive correlation with basal synaptic transmission and was important in mobilizing vesicles for synaptic potentiation. In addition, NMMII was found to be involved in a high frequency dependent low frequency depression. This work begins to identify how vesicles traverse within boutons and suggests differential mechanisms of synaptic release, both of which are partially dependent of NMMII expression. Studying Moesin and NMMII have revealed a complex interplay between the actin cytoskeleton and synaptic function and together this research furthers our understanding of how the actin cytoskeleton regulates synaptic activity.Ph