IgSF protein interactions drive specificity in circuit wiring and synaptic elaboration

Project Summary
In this application, we examine the molecular mechanisms that instruct neural wiring and axon terminal
elaboration. We focus on the Drosophila neuromuscular system due to its invariant connectivity, limited
synaptic partners, and accessibility. Given that this ‘simple’ circuit has been studied for over four decades, it is
somewhat surprising that fundamental questions still exist as to how motor neurons choose their appropriate
muscle targets and how each motor neuron develops a unique, yet stereotyped, axon terminal structure that
underlies synaptic function. Conceptually, both of these developmental processes rely on specificity cues to
guide synaptic partner matching (role 1) and synaptic elaboration at each axon terminal (role 2). In support of
the first role, we previously discovered two interacting cell surface proteins (CSPs), DIP-α and Dpr10, that are
required for wiring a motor neuron to a subset of muscles. In support of the second role, these CSPs continue
to be expressed after connectivity, implying additional functions in synaptic development. Our central
hypothesis is that combinatorial Dpr-DIP interactions, in addition to specifying synaptic connections, also
participate in determining the structure and function of specified synapses. Insights into circuit development
arose in a prior collaboration where we characterized the ‘Dpr-ome’, the set of interactions between two
families of the immunoglobulin superfamily, the Dprs and DIPs. These 32 proteins bind to one another in
unique combinations, and our preliminary data reveal unique expression patterns in the Drosophila larval
neuromuscular circuit. Additionally, our data support a combinatorial Dpr-DIP interaction model that leverages
cis/trans interactions to instruct highly specific synaptic partnerships. We also reveal a novel signaling pathway
that underlies local synaptic elaboration. Given our findings and genetic reagents, we are in a unique position
not only to compare axon branch-specific identification tags but also to ask if synaptic elaboration of
neighboring axon terminals can be independently regulated. In the first aim, we capitalize on the Dpr-ome and
the expression of 6 DIPs in multi-innervating motor neurons to perform single-cell genetic manipulations and
examine how combinatorial Dpr-DIP codes instruct connectivity. In addition, we generate affinity variants to
reveal a coordinated cis/trans interaction model that enhances specificity. In the second aim, we utilize
functional and genetic approaches to understand how co-innervating inputs develop unique morphological and
functional properties. We identify a novel crosstalk signaling pathway between axon arbors that locally sculpts
NMJ size. Overall, these studies will uncover fundamental developmental programs required for neural circuit
wiring and axon terminal elaboration, with emphasis on how CSP codes modulate each of these processes.

Grant Number: 5R01NS123439-05
NIH Institute/Center: NIH
Principal Investigator: Robert Carrillo