Frazzled/DCC regulates gap junction formation at a Drosophila giant synapse through transcription

Loss of function Frazzled/DCC mutants demonstrate that the gene regulates synaptogenesis in the Giant Fiber System of Drosophila. In frazzled loss of function (LOF) mutants, we observe weaker physiology, 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. A GF biophysical computational model is provided to test the role of gap junctions and the function of the Giant Fiber System. We present evidence of Frazzled’s role in gap junction regulation by utilizing the UAS-GAL4 system in Drosophila 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. Driving Frazzled’s intracellular domain in Frazzled LOF mutants rescued axon pathfinding and synaptogenesis. This is supported by the fact that Frazzled fails to rescue the synaptic function when its 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. The present work is the first to show how a guidance molecule regulates synaptogenesis through transcriptional regulation of synaptic components.