We isolated a gain-of-function mutation,
ju89 (also named
syd-5),that alters the size and morphology of GABAergic synapses in C. elegans. We mapped
ju89 to chromosome I between
hp6 and Y67A6 SNPs and were able to rescue the synaptic defects of
ju89 with either cosmid F26E12 or the fosmid H01I7. We further defined the minimal rescuing region to a 4.5kb subclone that contains the C.elegans alpha-tubulin gene
tba-1 (F26E4.8).
ju89 is a missense mutation in the C-terminus of TBA-1, a region involved in binding of microtubules to MAPs and motor proteins.
tba-1 is one of two alpha-tubulin genes in the C. elegans genome and is expressed throughout the C. elegans nervous system during development (Fukushige et al., 1995). Null mutations in
tba-1 have not been identified, however, RNAi experiments (Fraser et al., 2000) suggest complete loss of function of
tba-1may result in severe embryonic defects or lethality. By contrast,
ju89 mutant animals are viable and have no apparent defects in axon extension or pathfinding of the GABAergic neurons. Instead,
ju89 mutant worms exhibit defects in synapse shape and size. The synaptic vesicle marker SNB::GFP is expressed as abnormally small or large puncta in
ju89 worms and is also diffused weakly along neuron processes and commissures. A similar pattern of reduced and oversized synapses is observed in the mutants using antisera to the presynaptic active zone proteins SYD-2 and UNC-10. These defects suggest the
ju89 lesion does not disrupt microtubule assembly, but is likely to affect specific aspects of microtubule function or regulation that are essential for synapse growth or stabilization. EM analysis of synapse ultrastructure of
ju89 mutants and mapping and characterization of suppressors of
ju89 are in progress.