The sphingomyelin (SM) pathway, initiated by hydrolysis of plasma membrane SM to ceramide, is an ubiquitous signaling system linking diverse environmental stresses and cell surface receptors to intracellular effector pathways. To evaluate the SM pathway in C. elegans , we cloned two distinct acid SMase genes,
asm-1 and
asm-2 [Lin et al., J. Biol. Chem. 273, 14374 (1998)]. These genes are 30% identical with each other and the human gene. When expressed in COS-7 cells, ASM-1 is entirely secreted whereas ASM-2 is only 20% secreted. Further, ASM-2, but not ASM-1, is Zn 2+ -dependent. Using the Tc1 transposon insertion/deletion approach, an
asm-1 null allele (
kk1) was isolated. Asm-1
(kk1) homozygotes display no detectable phenotype. Our laboratory investigates ionizing radiation (XRT) signaling. C. elegans when irradiated with 10-400 Gy develop dose-dependent vulval malformations. Low dose XRT [lte]50 Gy induces a protruding vulval phenotype, whereas at 50 Gy a vulvaless phenotype also occurs. These vulval malformations occur exclusively after XRT in L1 or early L2, and at higher doses are manifest in 75% of adults. lf mutations in genes of the RAS-MAP kinase pathway including
ksr-1(
ku68),
mek-2(
n1989) and
sur-1(
ku1) enhanced sensitivity to XRT. Similarly,
asm-1(
kk1) animals exhibit increased XRT sensitivity. These effects appear apoptosis-independent, as
ced-3(
n717) does not affect radiosensitivity. Animals expressing an
asm-1 transgene (kkEx1) are radioresistant and even at 400 Gy display almost no vulvaless. These data indicate that the MAP kinase pathway provides a signal protecting developing vulva from XRT damage. Epistasis analysis revealed that KSR-1 is likely to be downstream of ASM-1. Subsequent investigations assessed developmental signaling through the MAP kinase pathway. Asm-1
(kk1) reduced penetrance of the Muv phenotype caused by gf
let-60(
n1046) by 50%. Furthermore,
asm-1(
kk1) synergistically enhanced the abnormal vulva phenotype caused by failure of P6.pp division in
sur-1(
ku1) . These investigations show that
asm-1 , like
ksr-1 , is not required for normal vulval development but becomes important when signaling is disturbed by mutation or XRT stress. ASM-1 appears to molecularly order downstream (or parallel) to LET-60 and upstream of KSR-1.