Signaling mechanisms that control presynaptic morphogenesis at the Drosophila neuromuscular junction

The development of the pretsynaptic terminal requires the assembly of many adhesion, receptor, scaffold and signaling molecules. Previous work from our lab has focused on the relationship between the receptor tyrosine phosphatase LAR, the heparan sulfate proteoglycans Syndecan and Dallylike and the scaffold protein Liprin-α in promoting presynaptic differentiation. In this current study, we expand this work to identify downstream effectors known for their direct interaction with actin cytoskeleton. Thus, we define the LAR signaling pathway that links cell surface cues at the presynaptic terminal to actin cytoskeleton during presynaptic development. LAR and Syndecan binding occurs at the cell surface. Liprin-α localizes to the synaptic terminal through its cytoplasmic interaction with LAR. However, the functional relationship between the three molecules LAR, Syndecan and Liprin-α has never been explored. We demonstrate that LAR, Syndecan and Liprin-α depend on each other for activity and, thus, each serves as a limiting factor during presynaptic differentiation. While a hierarchical relationship between the three molecules was not established genetically, these results are indicative of a complex. Liprin-α has been linked to multiple proteins necessary for presynaptic differentiation. However, at the presynaptic terminal LAR cytoplasmic interactions are limited to the Liprin-α partnership. To expand the LAR signaling pathway, we focused on two LAR substrates as possible downstream effectors: the Abelson tyrosine kinase and Enabled. The importance of actin remodeling during presynaptic differentiation has been demonstrated through pharmacological applications. Interestingly, both Abelson (Abl) tyrosine kinase and Enabled proteins are characterized by their association with cytoskeleton structure. We use genetics and microscopy to demonstrate Abl requirement for presynaptic growth and establish Abl as a modulator of active zone assembly. Further, we show that Abl function is required for LAR, Syndecan and Liprin-α activity, placing Abl downstream of these molecules. Enabled is a substrate of both LAR and Abl. We establish a role for Enabled in both synaptic growth and active zone assembly. Further, epistasis reveals the requirement for Enabled activity in the LAR pathway, placing Enabled function downstream of LAR, Syndecan and Liprin-α. Taken together, the LAR signal transduction pathway functions to modulate actin dynamics necessary for proper synaptic development.