The heterochronic genes in C. elegans regulate the timing of developmental events, and the study of developmental timing in worms has led to the discovery of genes and pathways that play conserved roles in timing. The C. elegans period homolog
lin-42 is a key player in the heterochronic pathway. In other organisms, period is a critical part of the circadian clock, which synchronizes gene expression and physiology to the light-dark cycle. LIN-42 shares multiple domains of homology with this family of proteins, in particular the PAS domain, which mediates protein interactions. Also like period,
lin-42 mRNA and protein levels cycle. In C. elegans,
lin-42 expression oscillates with the larval cycle rather than the light-dark cycle, as period expression does. The
lin-42 genomic locus is complex, and produces four isoforms. Pre-existing alleles of
lin-42 all left one isoform intact; therefore, the phenotype of
lin-42(0) animals was unknown. Using Mos-Tc1 technology, we engineered a
lin-42 allele with a 10.2 kb deletion that removes the coding region of each isoform. We have found that
lin-42(0) mutants have similar phenotypes as previously characterized
lin-42(lf) alleles; however, phenotypes are more severe in
lin-42(0).
lin-42(lf) and
lin-42(0) animals have precocious heterochronic defects; for example, the hypodermal seam cells terminally differentiate one stage too early in these mutants. Also like
lin-42(lf), null mutants have a molting defect. In
lin-42(lf) mutants, entry into the molt is delayed and the duration of molts is often extended. In
lin-42(0), this phenotype is more severe. These animals not only delay but often fail to complete ecdysis, which results in early larval lethality and marked developmental delay in animals that survive. Our analyses have revealed some new clues of the role of
lin-42 in larval development. From genetic studies,
lin-42 acts in parallel with
lin-28 and in opposition to
daf-12 and
let-7-family miRNAs to prevent the L2-to-L3 transition from occurring too early. Furthermore, qRT-PCR analyses indicate that
lin-42 is required to ensure that the miRNA
let-7 is not expressed too early. Recently, several reports have shown that
daf-12 and
lin-28 act in opposition to regulate the accumulation of
let-7. Interestingly, while
daf-12 regulates
let-7 paralogous miRNAs as well,
lin-28 and
lin-42 specifically regulate
let-7. Also, reports illustrate that
let-7, like
lin-42, regulates molting. Further work is needed to elucidate how
lin-42 regulates
let-7 accumulation, and how this network controls developmental timing and molting. We are pursuing a combination of biochemical, molecular and genetic analyses to further this goal.