Nidogen(entactin) can interact with many other basement membrane molecules including laminin, perlecan, and collagens type IV and XVIII. The single C. elegans nidogen gene,
nid-1, encodes highly conserved laminin-binding and perlecan/collagen IV-binding domains. A null mutation,
nid-1(0), shows no alteration in perlecan, collagen IV or laminin localization. The
nid-1(0) mutant is viable and fertile. Since loss of either perlecan, collagen or laminin causes lethality, these results show that nidogen can not be essential for their function or for general assembly of basement membranes.
nid-1 mutants have defects in axonal positioning and synaptic organizatio and function, but how NID-1 affects these processes is unclear. As one approach to better understand NID-1 functions, we have screened for mutations that have enhanced phenotypes in the
nid-1(0) versus wild-type genetic backgrounds. Such mutations may identify genes that function with or in parallel to
nid-1. The screen has produced mutations in the guidance receptors
sax-3 and
unc-40 and the Rac GTPase
mig-2. Each double mutant with
nid-1(0) produces 5080% Pun (pharynx unattached) phenotype, arresting as late embryos or L1 larvae. The pharynx and arcade cells appear to develop normally, but there is a subsequent failure of adhesion between
hyp1 and the arcade. In addition to the synthetic Pun phenotype, the
nid-1(0) background also enhances the neural guidance defects of
sax-3,
unc-40,
mig-2(lof) and
mig-2(gof) mutations.
nid-1(0) enhances
sax-3 partial loss-of-function alleles for both the Pun and neural guidance phenotypes, suggesting that it could directly influence SAX-3 function.We have tested other known downstream effectors of
sax-3 and
unc-40 (
unc-34,
unc-73,
ced-10,
ced-2,
ced-5 and
ced-12) and find that they all produce the synthetic Pun phenotype in the
nid-1(0) background. The penetrance of Pun ranges from 40-80%, with the strongest effect being with
unc-34. These results indicate that the effectors downstream from SAX-3 and UNC-40 that influence the actin cytoskeleton are critical for
hyp1/arcade adhesion in the absence of NID-1. We had earlier found that mutations in the matrix receptor dystroglycan,
dgn-1, also cause the synthetic Pun phenotype in the
nid-1(0)background. In total, these results indicate that NID-1 functions with SAX-3, UNC-40 and DGN-1 in
hyp1/arcade adhesion and growth cone guidance.