Nervous system maturation occurs when neurons establish synaptic connections with their targets. The molecular pathways that regulate the initiation and stabilization of synapses remain to be fully elucidated. We are using GABAergic neuromuscular junctions in C. elegans as a model system to genetically dissect the molecular pathways that regulate synapse formation. Using a GFP marker, P
unc-25 -SNT-1::GFP that is specifically localized to the presynaptic termini of 19 GABAergic motorneurons, Zhen and Jin isolated a number of new genes that affect presynaptic morphology. One of these new genes,
syd-2 encodes a member of the Liprin (LAR-type-receptor-tyrosine phosphatase-interacting proteins) protein family. SYD-2 is localized at or near the presynaptic active zone regions. In
syd-2 mutants, the complete loss of SYD-2 function resulted in moderately diffuse localization of the vesicle marker, an elongated active zone region and slightly rigid backward movement. (Zhen and Jin. Nature 401. 371-375. 1999)
syd-2 is the only C. elegans liprin gene that is expressed in the nervous system. The moderate behavioral and anatomical defects of
syd-2 null mutants suggest the presence of functionally redundant pathways that regulate synaptogenesis. To identify these redundant pathways, we performed a genetic screen for mutations that enhance
syd-2 phenotypes. Out of this screen we have isolated 8 mutants that have mild or no behavioral defects on their own, but become paralyzed in a
syd-2 null background. At least three of these mutants represent new alleles of
rpm-1 , a putative GEF gene that regulates synaptic growth, and is known to function in parallel with
syd-2 in synapse and muscle development (See abstract by Zhen, Xun and Jin, this meeting). We identified at least one new gene defined by a single allele,
hp1 , that is likely to function in parallel with
syd-2 , but is unrelated to
rpm-1 . Although both
rpm-1 ;
syd-2 and
hp1 ;
syd-2 animals show similar behavioral defects, they display different phenotypes in vesicle protein expression. While the expression of synaptic vesicle proteins is almost diminished in
rpm-1 ;
syd-2 double mutants,
hp1 ;
syd-2 animals have relatively normal levels of vesicle protein expression. However, the distribution of these proteins is completely diffuse at presynaptic termini. We are continuing our analysis of the
syd-2 enhancer mutants and are pursuing the molecular analysis of these genes.