The intricate patterns of synaptic connections in a nervous system underlie nearly all aspects of neuronal function, but how each neuron distinguishes its appropriate synaptic targets from other non-target cells during synapse formation is not well understood. Recent studies have shown that in the absence of members of the Ig Superfamily, SYG-1 or SYG-2 protein, HSNL axon fails to recognize its normal targets and forms morphologically normal synapse onto abnormal target cells. The observation of presynaptic specialization being formed onto abnormal target cells in syg-1 /syg-2 mutant animals leads us to hypothesize that synaptic target choices of HSNL are flexible and maybe even reversible and the competition between the potential targets determines the preference of target selection, which results in synaptic specificity.In order to understand molecular mechanism of how SYG-1/SYG-2 dominates the synaptic competition and determines target specificity in HSNL synaptogenesis, we took two strategies:1. Identify proteins that interact with the cytoplasmic domain of SYG-1 using yeast two-hybrid approach. From this screen, we identified a set of SYG-1 potential binding partners, including a PDZ domain containing protein, which plays a role in selecting and transporting transmembrane proteins to the cell surface and an SCF ubiquitin ligase complex component, which has been shown interacting with presynaptic protein. Currently, we are examining the expression pattern and loss of function phenotype of SYG-1 interacting molecules, and the progress will be presented at the meeting.2. Enhancer/suppressor screen to identify genes involved in synaptic choice competition. In syg-1/syg-2 mutant animals, HSNL neuron still forms synapses but onto abnormal targets. In order to understand the molecular mechanisms underlying synapse formation at those abnormal targets we have carried out a direct genetic screen for second-site mutations with abnormal (enhancer) or wild type (suppressor) pattern of synaptic vesicle marker in a syg-1 mutant background. From a pilot screen, we isolated the wy43 mutation. Homozygous wy43 mutants appear wild type. In wy4; syg-1 double mutants the ectopic synaptic vesicle clustering of syg-1 is suppressed. We mapped wy43 to chromosome I. We are currently performing fine mapping and germline transformation to determine the identity of wy43. Through these approaches, we hope to understand how SYG-1 and SYG-2 determine synaptic choice by interacting with other synaptic specificity molecules.

Affiliation:
- Biological Sciences, Stanford University, Stanford, CA.