Date of Award
Doctor of Philosophy (PhD)
Neurotransmission is based on a series of events termed the synaptic vesicle cycle. Several molecular components contributing to the synaptic vesicle cycle are known, including the cytoskeletal protein actin and the synaptic vesicle-associated phosphoprotein synapsin. The studies presented in this thesis were designed to investigate the role of the interaction between synapsin and actin in an intact model of synaptic transmission, the lamprey reticulospinal synapse. To establish the role of actin in the presynaptic terminal, several actin-directed compounds were injected into reticulospinal axons. Axons were maintained at rest or stimulated under various conditions, fixed, and the ultrastructure of the terminal was studied by electron microscopy. Unexpectedly, an actin-based network was revealed that appears to participate in the translocation of synaptic vesicles from endocytic regions of the plasma membrane back to the vesicle cluster. The prevailing hypothesis of synapsin function states that it interacts with actin to maintain organization of the vesicle cluster. In the present experiments, actin-modifying compounds did not visibly affect the vesicle cluster organization. Therefore, to investigate the potential relevance of the interaction between synapsin and actin to the novel actin-based translocation of vesicles, the localization of synapsin was determined in synapses fixed after several conditions of activity, using post-embedding techniques. In active synapses, synapsin was found on synaptic vesicles throughout the vesicle cluster, supporting a role for synapsin in mediating the dynamics of synaptic vesicle distal and proximal to the active zone. Synapsin was also present on several elements of the actin-based translocation network, including: the plasma membrane area where filaments are localized, the actin matrix itself and vesicles enmeshed in the matrix. Consistent with all data obtained, a novel hypothesis is presented, namely that the interaction of synapsin and actin plays an important role in the translocation of locally recycled synaptic vesicles in the presynaptic nerve terminal, facilitating the continuous supply of vesicles necessary to sustain neurotransmitter release. This hypothesis provides that synapsin serves as a direct functional link between the endocytic and exocytic pathways, as suggested by its binding partners that are known to participate in signal transduction, the endocytic pathway and the regulation of actin dynamics.
Bloom, Ona E., "Molecular Anatomy of a Living Nerve Terminal: Actin and Synapsin in the Synaptic Vesicle Cycle" (2001). Student Theses and Dissertations. 323.