Project Summary:

A systematic forward genetic Drosophila screen for electroretinogram mutants lacking synaptic transients has identified fuseless (fusl), encoding an 8-pass transmembrane protein with homology to a family of transporters, which colocalizes with horse radish peroxidase (HRP) at the presynaptic membrane. Null fusl mutants display a >75% reduction in evoked synaptic transmission, which is rescued by presynaptic expression of a fusl transgene, whereas both the frequency and amplitude of spontaneous fusion events is increased >2-fold. Synaptic architecture is comparable to controls in fusl mutants, but presynaptic active zones are reduced ~2-fold and the density of vesicles clustered and docked at active zones is strikingly elevated. Cycling vesicle exocytosis is greatly reduced in fusl mutants, and the calcium-dependence of neurotransmitter release and calcium-dependent facilitation are both dramatically compromised, which is likely explained by the significant reduction in expression of the calcium channel pore (1 subunit) seen in fuseless mutants throughout development, and the rescue of the neurotransmitter release defects using transgenically expressed cation channels in high calcium saline. The Fusl mutation is rescued with the overexpression of the Fuseless protein using the elav-Gene Switch system during development, indicating a developmental role for fuseless. The mechanistic defect linking these mutant phenotypes is impaired calcium dynamics underlying vesicle exocytosis. These data indicate that Fusl regulates calcium channel expression and active zone domains required for efficient synaptic vesicle exocytosis mediating neurotransmission.

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