We have shown that dominant, gain-of-function mutations in
egl-19 and
unc-58 disrupt the outgrowth of the HSN axons. The HSNs, which are located laterally and just posterior to the vulva, each send an axon ventrally that enters the ventral nerve cord. In
egl-19(
n2368dm) mutants, 23% of HSN axons grow laterally along the body wall instead of ventrally. In
unc-58(
e665dm) mutants, 45% of HSN axons grow laterally instead of ventrally. Recessive alleles of either gene do not cause pathfinding defects.
egl-19 encodes an alpha-1 subunit of a voltage-gated Ca2+ channel (1). Intracellular Ca2+ levels are known to be important for the directed motility of growth cones.
unc-58 may also encode an ion channel. Some time ago, Gary Ruvkun found that the paralysis of
unc-58(dm) mutants could be rescued by growing the animals on alpha-endosulfan, a drug that blocks GABA-gated Cl- channels in insects. We found that alpha-endosulfan also rescues the HSN axon pathfinding defect caused by dominant
unc-58 mutations. Recently, a gene, that is similar to vertebrate glycine-gated Cl- channels, has been identified in the region to which
unc-58 maps genetically. We are in the process of determining whether this sequence is altered in
unc-58 mutants. To address the question of whether these dominant mutations might disrupt hsn pathfinding by a common mechanism, double mutants were constructed that carry the
unc-58(
e665dm) mutation and recessive mutations in either
egl-19 or
unc-36.
unc-36 encodes a homolog of the alpha-2 subunit of voltage gated Ca2+ channels and has been shown to interact genetically with
egl-19 (2). Recessive mutations in both
egl-19 and
unc-36 suppressed the HSN pathfinding defects of
unc-58(
e665dm) mutants; only 7% (n=114) of
egl-19(
n582);
unc-58(
e665dm) and 6% (n=110) of
unc-36(
n251);
unc-58(
e665dm) HSNs were misdirected. The observation that these mutations interact genetically supports the idea that altering Ca2+ levels can affect axonal pathfinding. However, the observation that a mutation in
egl-19, which encodes the principle subunit of a Ca2+ channel, and mutations in
unc-58, which might encode a Cl- channel, both cause a similar defect in ventrally directed growth of HSN suggests that this step in HSN outgrowth might be particularly vulnerable to changes in membrane potential or intracellular Ca2+ levels. The suppression of the HSN pathfinding defect displayed by
unc-58 gain-of-function mutations by loss-of-function mutations in Ca2+ channel subunit genes suggests that
unc-58 might cause its effect by influencing the intracellular Ca2+ levels. Literature Cited: (1) L. Lobel, M. Grabner, H. Glossmann, and B. Horvitz, WBG 13(1):46. (2) L. Lobel, R. Lee, L. Avery, and B. Horvitz, WBG 13(2):71.