816B Poster - 13. Neural development and physiology
Friday April 08, 2:00 PM - 4:00 PM

Authors:
Yu-Chieh David Chen; Claude Desplan

Affiliation: New York University, Department of Biology, New York, NY

Keywords:
c. synaptogenesis; e. synaptic function and organization

How vast numbers of neurons are specified into correct cell fates and connected with their proper targets during development represents a fascinating area of developmental neuroscience. Mechanisms of stochastic and deterministic cell specification programs to achieve neuronal diversity have been extensively studied. A number of axon guidance molecules have also been identified. However, little is known about the coordination between neuronal specification and specific connectivity patterns, especially when two synaptic partners undergo two different modes of cell specification (stochastic vs. deterministic). In the fly retina, pale (p) and yellow (y) subtypes of color photoreceptors (R7 and R8) are stochastically specified, whereas their synaptic partners in the optic lobe are produced through highly deterministic programs. How do stochastically determined p vs. y R7 and R8 find their respective targets that are deterministically specified in the optic lobes?

By focusing on p/y R7 and their main downstream target p/y Dm8 neurons, we and others have recently identified a pair of DIPs and Dprs, members of an interacting network of immunoglobulin superfamily proteins, as critical regulators of the synaptic connection between yR7 that expresses Dpr11 and yDm8 that expresses DIPγ. We thus hypothesize that different pairs of cell adhesion molecules can mediate the matching of other synaptic partners. We aimed to uncover such molecular code for the pairing of pR7 and pDm8. It is unlikely to be achieved via another DIP-Dpr pair since our recent single-cell RNA sequencing (scRNAseq) data showed no other DIPs expressed in Dm8s. To identify candidate molecule(s) that mediate pR7-pDm8 connectivity, we bioinformatically split our recent scRNAseq transcriptomic profile of the Dm8 cluster into DIPγ⁺ (yDm8s) and DIPγ^- (pDm8s) neurons during development. We found that Beat-IIIc, a member of Beat family proteins, is specifically expressed in pDm8s cells during the developmental stage of synaptic partner pairing. Members of the Beat family and Side proteins form a receptor-ligand system to control axonal targeting. Since there are 14 Beat and 8 Side members, we aim to survey the expression pattern of all the Sides and look for their expression in pR7s. Preliminary experiments suggest Side-V is a promising candidate as it is only expressed in a subset (presumably p) of R7 terminals. We have generated CRISPR mutant alleles of these candidates and will be presenting our functional analyses of these candidate molecules in regulating synaptic partner matching. Overall, our work has uncovered novel molecular mechanisms regulating synaptic pairing and probes the fundamental principles underlying the propagation of stochastic cell fate choices during circuit assembly.