Discs Large (DLG), a Drosophila member of the Membrane Associated Guanylated Kinases (MAGUKs) family, exists at the pre- and post-synaptic terminals of Drosophila larval Type I neuromuscular junctions (NMJs). To understand the in vivo roles of MAGUKs as synaptic organizers, Drosophila Type I synapses were used as a model system. Analysis of dlg mutants revealed that DLG is important for proper development of synaptic structures. Mutations in dlg result in abnormal growth of the postsynaptic membrane, the subsynaptic reticulum (SSR). Expression of transgenic DLG at synaptic terminals in dlg mutant larvae rescued this SSR phenotype. In addition DLG is important for the proper localization of synaptic components via interaction between PDZ domains 1 and 2 of DLG and S/TXV motifs in specific synaptic proteins such as Fasciclin II (FasII). For example, the localization of FasII, a Drosophila structural homologue of neuronal cell adhesion molecule (N-CAM), at Type I synapses depends on DLG. A severe hypomorph dlg mutant allele (dlg x1-2) showed reduced FasII localization at Type I synapses. Furthermore ultrastructural defects in dlgx1-2 (increased number of active zones at pre-synaptic terminals) resembled abnormalities found in FasII hypomorph mutant boutons. This observation suggests that DLG and FasII are components of the same signal transduction pathway which regulates the development of pre-synaptic structure. Coimmunoprecipitation of body wall muscle extracts, ELISA, and yeast two-hybrid assay have demonstrated a direct interaction between DLG and FasII. These results suggest the role of DLG as a synaptic organizer which regulates the proper localization of synaptic components. Both DLG binding partners, Shaker channels and FasII, regulate or are regulated by neuronal activity. Therefore we attempted to determine if DLG function could also be regulated by synaptic activity. In particular we investigated the functions of Ca2+/Calmodulin-dependent kinase II (CaMKII), an enzyme whose kinase activity is regulated by synaptic activity, and which partially colocalizes with DLG at NMJs and exists in the same protein complex as DLG. We found that increased CaMKII activity phenocopied the defects of dlg mutant synapses and induced the dissociation of DLG from the synaptic membrane. Decreased CaMKII activity induced the opposite effects. In vitro, CaMKII phosphorylated DLG fragments with a stoichiometry close to one. Furthermore expression of transgenic site-directed mutations of dlg that blocked or mimicked phosphorylation had effects similar to those observed upon inhibiting or constitutively activating CaMKII. We conclude that CaMKII-dependent DLG phosphorylation regulates the association of DLG with the synaptic complex during development and plasticity, thus providing a link between synaptic activity and structure.
| Identifer | oai:union.ndltd.org:UMASS/oai:scholarworks.umass.edu:dissertations-3363 |
| Date | 01 January 2000 |
| Creators | Koh, Young-Ho |
| Publisher | ScholarWorks@UMass Amherst |
| Source Sets | University of Massachusetts, Amherst |
| Language | English |
| Detected Language | English |
| Type | text |
| Source | Doctoral Dissertations Available from Proquest |
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