A ROLE FOR MAP1 IN SYNAPTIC FUNCTION REVEALED THROUGH INTERACTIONS WITH PRESYNAPTIC CALCIUM CHANNELS IN DROSOPHILA

Open Access
Author:
Zou, Beiyan
Graduate Program:
Genetics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 27, 2007
Committee Members:
  • Douglas Cavener, Committee Chair
  • Gong Chen, Committee Member
  • Kyung An Han, Committee Member
  • Zhi Chun Lai, Committee Member
  • Richard W Ordway, Committee Member
Keywords:
  • synaptic transmission
  • MAP1
  • presynaptic calcium channel
  • cacophony
  • futsch
  • Drosophila
Abstract:
Synaptic transmission is a fundamental aspect of neural function. This process typically occurs at chemical synapses, where presynaptic release of chemical neurotransmitters leads to excitation or inhibition of the postsynaptic cell. Neurotransmitter release is triggered by calcium influx through presynaptic voltage-gated calcium channels. In Drosophila, the molecular basis of presynaptic calcium channel function has been defined through our analysis of a temperature-sensitive (TS) paralytic mutant of the calcium channel alpha1 subunit gene, cacophony (cac). This mutant, referred to as cacTS2, has also served as a starting point for further genetic analysis. To broaden our understanding of the functions and interactions of cac-encoded calcium channels, we have conducted a screen for genetic modifiers of cacTS2. Of ten mutations recovered, four were intragenic and six were extragenic. Analysis of intragenic modifiers has broadened our understanding of intramolecular interactions within the calcium channel alpha1 subunit. Here we report characterization of three extragenic enhancers of cacTS2, which map to a single locus designated as e(cac)A. Genetic analysis, sequencing analysis, complementation tests, immunoblotting and immunocytochemistry have shown that e(cac)A is futsch, the homolog of microtubule-associated protein 1 (MAP1) in Drosophila. Electrophysiological analysis of synaptic transmission has revealed that futsch mutations enhance the synaptic phenotype of cacTS2. Furthermore, immunocytochemical analysis indicates that the localization of presynaptic calcium channels, microtubules and actin as well as the distribution of synaptic vesicles is normal in futsch mutants, suggesting a role for dMAP1/FUTSCH in synaptic function. Accordingly, dMAP1/FUTSCH exhibits a punctate distribution along microtubules within presynaptic boutons and extending to active zones. Taken together, our findings have implicated novel molecular mechanisms of synaptic transmission, in which a specialized form of presynaptic microtubule-based cytoskeleton may function in synaptic transmission through interactions with presynaptic calcium channels. A part of this work has revealed posttranslational proteolytic processing of a dMAP1/FUTSCH protein precursor as demonstrated previously for mammalian MAP1s. A ~20kD fragment of dMAP1/FUTSCH was specifically detected using a newly developed anti-LCf antibody and this was shown to be a dMAP1/FUTSCH light chain by colocalization with the heavy chain at the neuromuscular synapses and co-immunoprecipitation of the heavy and light chains. These findings demonstrated a structural conservation among MAP1 proteins. Further molecular and functional characterization is expected to define the in vivo roles of MAP1 in synaptic transmission and the underlying mechanisms.