One critical step for establishing of the precise connections between neurons is migration of axons to the correct target territory. The ray sensory neurons in the C. elegans male form multiple synapses within the preanal ganglion. Each ray axon takes a specific pathway into target region. In a screen for mutants with defective ray axon guidance, we isolated two alleles (
bx124 and
bx127) of the previously identified gene
unc-27. In
unc-27 mutants, ray sensory axons and circumferential axons of VNC neurons appear to wander. We have found that this appearance of wandering is in fact due to a physical distortion of the pathway these axons follow. UNC-27 encodes troponin I, a component of Troponin Complex, which binds to tropomyosin and regulates muscle contraction. Mutants for
unc-27 display structural abnormalities in body wall muscle (Zengel & Epstein. Cell Motil., 1980). We find that body wall muscles are misplaced and detached from hypodermis in these mutants. Mutations in most genes encoding myofilament proteins do not affect axon circumferential migration except for
mup-2. MUP-2 encodes troponin T, another component of the Troponin Complex and is required for muscle cells positioning during development (Myers CD, et al J Cell Biol., 1996). We showed by mosaic analysis that UNC-27 expression in muscles but not in neurons or hypodermis rescued the axon wandering phenotype. Body wall muscles are tightly attached to hypodermis and cuticle by dense bodies and half-desmosome. Axons usually migrate alongside the basement membrane between muscle and hypodermis. Together, these results suggest that troponin I (or troponin complex) control axon pathway by controlling muscle positioning during development. Detachment of the sarcomere from hypodermis causes misplacement of muscles and this misplacement changes location of the substrate for the axons to migrate along. Two additional mutants are currently being characterized. In
bx130, there are many short, irregular outgrowths sprouting from the ray neuron cell body and axons form extra branches. Only ray axons are affected in
bx130; other neurons throughout the body appear to be normal, as judged by
unc-119::GFP and other subset neuron markers. We have mapped
bx130 to LGIII (-2.65) and cloning is in progress. In
bx129, ray 1-3 axons bifurcate abnormally or migrate in the wrong direction.
bx129 causes male copulation defect (cod). This suggests that in addition to some axons missing their axon targets, most or all ray neurons may be non-functional. We are now characterizing
bx129 phenotype and identifying the gene affected by
bx129.