Frazzled/DCC Regulates Gap Junction Formation at a <i>Drosophila</i> Giant Synapse

Loss-of-function (LOF) Frazzled/DCC mutants disrupt synaptogenesis in the Giant Fiber (GF) System of <i>Drosophila</i> We observed weaker physiology in LOF male and female specimens, characterized by longer latencies and reduced response frequencies between the GFs and the motor neurons. These physiological phenotypes are linked to a loss of gap junctions in the GFs, specifically the loss of the shaking-B(neural+16) isoform of innexin in the presynaptic terminal. We present evidence of Frazzled's role in gap junction regulation by utilizing the UAS-GAL4 system in <i>Drosophila</i> to rescue mutant phenotypes. Expression of various UAS-Frazzled constructs in a Frazzled LOF background was used to dissect the role of different parts of the Frazzled receptor in the assembly of electrical synapses. Expressing Frazzled's intracellular domain in Frazzled LOF mutants rescued axon pathfinding and synaptogenesis. This is supported by the complementary result that Frazzled fails to rescue synaptic function when the transcriptional activation domain is disrupted, as shown by the deletion of the highly conserved intracellular P3 domain or by a construct with a point mutation in the highly conserved P3 domain known to be required for transcriptional activation. A computational model clarifies the role of gap junctions and the function of the GF System. The present work shows how various domains of a guidance molecule regulate synaptogenesis through the regulation of synaptic components.