Genetic analysis of synaptogyrin function in the synaptic vesicle cycle

Abstract

Neuronal communication relies on the continual replenishment of synaptic vesicles that are primed for neurotransmitter release in response to action potentials. A vast array of proteins is required to mediate synaptic vesicle biogenesis, trafficking, docking, exocytosis, and endocytosis. Synaptogyrin and synaptophysin are abundant and evolutionarily conserved synaptic vesicles proteins that were identified over twenty years ago, yet their exact function in the synaptic vesicle cycle remains unknown. To further elucidate the role of these proteins, we have generated and characterized a synaptogyrin null mutant in Drosophila, whose genome encodes a single synaptogyrin isoform and lacks a synaptophysin homolog. Here we demonstrate that Drosophila synaptogyrin is abundantly expressed in neurons, where it localizes to the presynaptic terminal of the larval neuromuscular junction (NMJ). Drosophila lacking synaptogyrin are viable and fertile and have no overt deficits in motor function or courtship behavior. Ultrastructural analysis of mutant larvae revealed an increase in average synaptic vesicle diameter as well as enhanced variability in the size of synaptic vesicles. In addition, the resolution of endocytic cisternae into synaptic vesicles in response to robust exocytosis is defective in synaptogyrin mutants. While basal synaptic transmission at the larval NMJ is unaffected, synaptogyrin mutants do display increased facilitation during high-frequency stimulation, indicating that synaptic vesicle exocytosis is abnormally regulated during strong stimulation conditions. These results suggest that, while not required for neurotransmission, Drosophila synaptogyrin nevertheless modulates synaptic vesicle exo-endocytosis, especially during elevated rates of synaptic vesicle fusion.