Worm Breeder's Gazette 14(5): 41 (February 1, 1997)

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

syd-1(ju2)II May Be Required For Type D Motorneuron Synaptic Specificity

Steven Hallam, Yishi Jin

Department of Biology, Sinsheimer Labs, University of California Santa Cruz

     In wild type L1 animals DD motorneurons form neuromuscular
junctions (NMJs) with ventral body wall muscles. Following the birth of
VD motorneurons in the late L1 stage, DDs undergo synaptic remodeling:
their ventral NMJs are eliminated while new NMJs are formed on the
dorsal side (J. White et al., (1978) Nature 271, 764-766). The nascent
VDs form NMJs with ventral body wall muscles.     
     We isolated the ju2 mutation in a screen for abnormal synaptic
patterns of the type D neurons using the [Punc-25VAMP-GFP] array as a
marker (see abstract by M. Zhen and Y. Jin this gazette). This array
carries a fusion gene of the green fluorescent protein (GFP) and the C.
elegans  vesicle associated membrane protein (VAMP) that is driven by
the unc-25 promoter. The GFP is specifically localized to the
presynaptic zones of DDs and VDs (Y. Jin and B. Horvitz, 1995
International Worm Meeting abstract 291). In ju2 mutant animals the
synaptic specificity of type D neurons is changed without alterations in
cell morphology. The ju2 mutation causes premature remodeling of
juvenile DDs such that DDs form NMJs onto the dorsal body wall muscles
in early L1 animals. In older larvae and adults the ju2  mutation causes
a reduction in the number of ventral NMJs made from VDs. ju2  mutant
animals show a ventral coiler phenotype when backing and also exhibit
mild defects in egg laying. Based on the mutant phenotype, we have named
the gene syd-1, for synapse defective.
     We mapped syd-1(ju2)  to the cluster of linkage group II. Animals
hemizygous for ju2 over a deficiency exhibit a similar ventral coiler
phenotype to ju2 homozygotes, suggesting that ju2 is likely a null
allele for syd-1. We obtained germline transformation rescue of syd-1
with a 15.6 kb minimal rescuing sequence. The minimal rescuing sequence
carries onepredicted gene with sequence homology to GTPase activating
proteins (GAPs) such as N-Chimaerin and the gene product of the break
point cluster region (BCR). We have introduced a stop codon in the
predicted gene, injection of which failed to rescue the ju2  mutant
phenotype. The GAP domain of the predicted SYD-1 bears 28% sequence
identity and 51% sequence similarity to that of BCR. The GAP domain of
BCR functions as a regulator of the small GTPases Rac and Cdc42Hs. Both
Rac and Cdc42Hs have been implicated in the clustering of integrins and
the formation of focal complexes (Nobes, C.D. and Hall A. (1995) Cell
81, 53-63). The sequence similarity between syd-1 and BCR hints at the
possibility that syd-1 may regulate the organization of the actin
cytoskeleton of type D motorneurons. 
     On Northerns we detected a 3.4 kb transcript for syd-1. We have
obtained a partial cDNA corresponding to this transcript (from Y.
Kohara) and are in the process of obtaining the complete 5 prime portion
of the message.  In addition, we have initiated the construction of GFP
reporters with which to determine the expression pattern of syd-1.
Previously we showed that the temporal regulation of DD synaptic
remodeling is under the control of the heterochronic genes lin-4 and
lin-14 (S. Hallam and Y. Jin 1996 West Coast Worm Meeting abstract 108).
We have begun to investigate potential genetic interactions between
syd-1 and lin-4 and lin-14.