Lopez, Juan

Synaptogenesis is a requirement for cellular communication, but the specific molecular mechanisms underlying synaptogenesis are unclear. Here, we investigate and show the role of the protein Frazzled in synaptogenesis using the transheterozygous Frazzled loss-of-function (LOF) mutant in Drosophila.
Leveraging the UAS-GAL4 expression system, we drove expression of various Frazzled/DCC gene constructs in the Giant Fibers (GF) of flies and found changes to synaptogenesis and axon pathfinding.
We identified decreases in electrical synaptogenesis and distinct axon pathfinding errors in Frazzled LOF mutants. Strikingly, the expression of Frazzled intracellular domain (ICD) significantly rescues both phenotypes, while full-length Frazzled protein only partially rescues these phenotypes, prompting us to explore the role of different domains within the protein. Deleting the P1 and P2 domains of Frazzled does not rescue axon pathfinding but did partially rescue synaptogenesis while deleting the P3 domain failed to rescue either phenotype. Moreover, when we drive expression Frazzled with a point-mutated P3 domain, silencing its transcriptional activation domain, it fails to rescue both synaptogenesis and axon pathfinding.
These results strongly suggest that Frazzled regulates both synaptogenesis and axon pathfinding in the GFs and is necessary for synaptogenesis of the mixed electrochemical GF synapse. Our results provide novel insights into the molecular mechanisms governing neural circuit assembly and highlight Frazzled as a key player in axon guidance and synaptic development.

In Drosophila melanogaster, the GFS is synaptically coupled to the
Tergotrochanteral motoneurons; these neurons form a signaling pathway from the brain to
the jump muscles (Thomas and Wyman, 1983). Part of this signaling is done through gap
junctions, and placement of these gap junctions was partially shown to be regulated by the
binding of Netrin, a class of guidance molecule (Orr et al., 2014). In the present study we
investigate the role of Netrin's receptor Frazzled in the placement of gap junctions in
Drosophila at: 1) Presynaptic neurons (Giant Fibers [GF]), 2) Postsynaptic neurons
(Tergotrochanteral motoneurons [TTMn]), and 3) Presynaptic + Postsynaptic neurons
simultaneously. Effects of Frazzled were tested using Frazzled RNAi and a combination
of electrophysiological recordings and imaging of the GF-TTMn synapse. The results from
this study show that presynaptic and postsynaptic knockdown of Frazzled delayed
muscular responses and altered the anatomy of both the GF's and TTMn's.