The Caenorhabditis elegans heterochronic gene pathway controls stage-specific transcription of collagen genes

Structure of the VAP-peptide (BmACP-6.7) gene in the silkworm, Bombyx mori and a possible regulation of its expression by BmFTZ-F1

The VAP-peptide (BmACP-6.7) is a hydrophobic peptide localized in adult cuticle of the silkworm, Bombyx mori. We isolated and characterized the VAP-peptide gene as a useful marker gene to analyze molecular mechanisms of terminal differentiation processes in the adult. The gene is composed of two exons interrupted by one intron. The 5' upstream promoter region is shown to bear a nucleotide sequence similar to the cis-element that is recognized and bound by the Bombyx mori FTZ-F1 protein (BmFTZ-F1). Expression of the BmFTZ-F1 gene preceded expression of the VAP-peptide gene and injection of 20-hydroxyecdysone suppressed the expression of both genes. An in vitro binding assay indicated direct interaction of BmFTZ-F1 with the VAP-peptide gene promoter sequence. Therefore, BmFTZ-F1 is proposed to be a possible factor regulating the stage-specific expression of the VAP-peptide gene towards adult life.

Expression and function of members of a divergent nuclear receptor family in Caenorhabditis elegans

Nuclear receptors (NRs) are a large class of ligand-regulated transcriptional modulators that have been shown to play roles in many developmental processes. The Caenorhabditis elegans genome is predicted to encode a large and divergent family of NR proteins. The functions of most of these genes are unknown. As a first step toward defining their roles, we have initiated an expression and functional survey of a subset of these genes. In this study, we demonstrate expression of 21 of 28 NR genes examined, indicating that a large fraction of the predicted genes likely encode functional gene products. We show that five genes are expressed predominantly in neuronal cells, while others are expressed in multiple cell types. Interestingly, we find that eight genes are expressed exclusively in the lateral hypodermal (seam) cells. These eight genes share a high degree of overall homology and cluster in a neighbor-joining tree derived from sequence analysis of the NRs, suggesting that they arose by gene duplication from a common ancestor. We show that overexpression of each of three members of this subfamily results in similar developmental defects, consistent with a redundant role for these genes in the function of the lateral hypodermal cells.

ELT-3: A Caenorhabditis elegans GATA factor expressed in the embryonic epidermis during morphogenesis

We have identified a gene encoding a new member of the Caenorhabditis elegans GATA transcription factor family, elt-3. The predicted ELT-3 polypeptide contains a single GATA-type zinc finger (C-X2-C-X17-C-X2-C) along with a conserved adjacent basic region. elt-3 mRNA is present in all stages of C. elegans development but is most abundant in embryos. Reporter gene analysis and antibody staining show that elt-3 is first expressed in the dorsal and ventral hypodermal cells, and in hypodermal cells of the head and tail, immediately after the final embryonic cell division that gives rise to these cells. No expression is seen in the lateral hypodermal (seam) cells. elt-3 expression is maintained at a constant level in the epidermis until the 2(1/2)-fold stage of development, after which reporter gene expression declines to a low level and endogenous protein can no longer be detected by specific antibody. A second phase of elt-3 expression in cells immediately anterior and posterior to the gut begins in pretzel-stage embryos. elt-1 and lin-26 are two genes known to be important in specification and maintenance of hypodermal cell fates. We have found that elt-1 is required for the formation of most, but not all, elt-3-expressing cells. In contrast, lin-26 function does not appear necessary for elt-3 expression. Finally, we have characterised the candidate homologue of elt-3 in the nematode Caenorhabditis briggsae. Many features of the elt-3 genomic and transcript structure are conserved between the two species, suggesting that elt-3 is likely to perform an evolutionarily significant function during development.

The LIN-29 transcription factor is required for proper morphogenesis of the Caenorhabditis elegans male tail

The Caenorhabditis elegans gene lin-29 encodes a zinc-finger transcription factor that is required for hypodermal cell terminal differentiation and proper vulva morphogenesis. Here we demonstrate that lin-29 is also required in males for productive mating. We show that lin-29 males can perform the early mating behaviors including response to hermaphrodite contact and vulva location, but they do not perform the subsequent steps of vulva attachment via spicule insertion and sperm transfer. Consistent with this observation, we found that lin-29 mutant spicules are on average 43% shorter than wild-type spicules while other male mating structures appear unaltered. In lin-29 mutants, spicule development goes awry after the generation of spicule cells, when spicule morphogenesis occurs in wild-type males. We show that LIN-29 accumulates in many cells of the wild-type male tail, including those that form the spicules. We demonstrate, through analysis of genetic mosaics, that the formation of wild-type-length spicules requires lin-29(+) in the AB.p lineage, the lineage that gives rise to the spicules and other male copulatory structures. Our mosaic analysis also reveals a role for lin-29(+) in the P1 lineage, which mainly produces sex muscles, cells of the somatic gonad, and body wall muscles.

Identification of heterochronic mutants in Caenorhabditis elegans. Temporal misexpression of a collagen::green fluorescent protein fusion gene

The heterochronic genes lin-4, lin-14, lin-28, and lin-29 specify the timing of lateral hypodermal seam cell terminal differentiation in Caenorhabditis elegans. We devised a screen to identify additional genes involved in this developmental timing mechanism based on identification of mutants that exhibit temporal misexpression from the col-19 promoter, a downstream target of the heterochronic gene pathway. We fused the col-19 promoter to the green fluorescent protein gene (gfp) and demonstrated that hypodermal expression of the fusion gene is adult-specific in wild-type animals and temporally regulated by the heterochronic gene pathway. We generated a transgenic strain in which the col-19::gfp fusion construct is not expressed because of mutation of lin-4, which prevents seam cell terminal differentiation. We have identified and characterized 26 mutations that restore col-19::gfp expression in the lin-4 mutant background. Most of the mutations also restore other aspects of the seam cell terminal differentiation program that are defective in lin-4 mutant animals. Twelve mutations are alleles of three previously identified genes known to be required for proper timing of hypodermal terminal differentiation. Among these are four new alleles of lin-42, a heterochronic gene for which a single allele had been described previously. Two mutations define a new gene, lin-58. When separated from lin-4, the lin-58 mutations cause precocious seam cell terminal differentiation and thus define a new member of the heterochronic gene pathway.

CHR3: a Caenorhabditis elegans orphan nuclear hormone receptor required for proper epidermal development and molting

CHR3 is a Caenorhabditis elegans orphan nuclear hormone receptor highly homologous to Drosophila DHR3, an ecdysone-inducible gene product involved in metamorphosis. Related vertebrate factors include RORalpha/RZRalpha, RZRbeta and RevErb. Gel-shift studies show that CHR3 can bind the DR5-type hormone response sequence. CHR3 is a nuclear protein present in all blastomeres during early embryogenesis. During morphogenesis, both CHR3 protein and zygotically active reporter genes are detectable in epidermal cells and their precursors. Inhibition of the gene encoding CHR3 results in several larval defects associated with abnormal epidermal cell function, including molting and body size regulation, suggesting that CHR3 is an essential epidermal factor required for proper postembryonic development.

Temporal pattern formation by heterochronic genes

Heterochrony describes the phylogenetic variation in the relative timing of major developmental events. Such heterochronic variation has been noted across phylogeny, including closely related species, suggesting that particular genetic loci control global aspects of developmental timing, and that variation at those loci may play important roles in evolutionary change. Genetic analyses of heterochronic mutations in the nematode Caenorhabditis elegans reveal that control of temporal patterning is analogous to the dedicated genetic pathways that control the patterning of the spatial axes in Drosophila and other metazoans. These pathways generate graded or binary levels of regulatory factors that pattern particular axes of the developing animal. C. elegans heterochronic genes constitute a regulatory cascade that both generates a temporal decrease in the level of the LIN-14 and LIN-28 proteins and responds to the changes in these gene activities to coordinate the temporal sequence of many cell fates as the animal develops. The temporal regulation of lin-14 and lin-28 gene activities is posttranscriptional and mediated by the antisense RNA product of the lin-4 gene. Hormonal control of developmental timing is a common theme throughout phylogeny. Heterochronic genes that involve hormonal signaling have been identified in vertebrates as well as C. elegans.

The terminal differentiation factor LIN-29 is required for proper vulval morphogenesis and egg laying in Caenorhabditis elegans

Caenorhabditis elegans vulval development culminates during exit from the L4-to-adult molt with the formation of an opening through the adult hypodermis and cuticle that is used for egg laying and mating. Vulva formation requires the heterochronic gene lin-29, which triggers hypodermal cell terminal differentiation during the final molt. lin-29 mutants are unable to lay eggs or mate because no vulval opening forms; instead, a protrusion forms at the site of the vulva. We demonstrate through analysis of genetic mosaics that lin-29 is absolutely required in a small subset of lateral hypodermal seam cells, adjacent to the vulva, for wild-type vulva formation and egg laying. However, lin-29 function is not strictly limited to the lateral hypodermis. First, LIN-29 accumulates in many non-hypodermal cells with known roles in vulva formation or egg laying. Second, animals homozygous for one lin-29 allele, ga94, have the vulval defect and cannot lay eggs, despite having a terminally differentiated adult lateral hypodermis. Finally, vulval morphogenesis and egg laying requires lin-29 activity within the EMS lineage, a lineage that does not generate hypodermal cells.

Stage-specific accumulation of the terminal differentiation factor LIN-29 during Caenorhabditis elegans development

The Caenorhabditis elegans gene lin-29 is required for the terminal differentiation of the lateral hypodermal seam cells during the larval-to-adult molt. We find that lin-29 protein accumulates in the nuclei of these cells, consistent with its predicted role as a zinc finger transcription factor. The earliest detectable LIN-29 accumulation in seam cell nuclei is during the last larval stage (L4), following the final seam cell division, which occurs during the L3-to-L4 molt. LIN-29 accumulates in all hypodermal nuclei during the L4 stage. The time of LIN-29 appearance in the hypodermis is controlled by the heterochronic gene pathway: LIN-29 accumulates in the hypodermis abnormally early, during the third larval stage, in loss-of-function lin-14, lin-28 and lin-42 mutants, and fails to accumulate in hypodermis of lin-4 mutants. LIN-29 also accumulates stage-specifically in the nuclei of a variety of non-hypodermal cells during development. Its accumulation is dependent upon the upstream heterochronic genes in some, but not all, of these non-hypodermal cells.

Heterochronic genes control cell cycle progress and developmental competence of C. elegans vulva precursor cells

Heterochronic genes control the timing of vulval development in the C. elegans hermaphrodite. lin-14 or lin-28 loss-of-function mutations cause the vulval precursor cells (VPCs) to enter S phase and to divide one larval stage earlier than in the wild type. A precocious vulva is formed by essentially normal cell lineage patterns, governed by the same intercellular signals as in the wild type. Mutations that prevent the normal developmental down-regulation of lin-14, activity delay or block VPC division and prevent vulval differentiation. A genetic pathway that includes lin-4, lin-14, and lin-28 controls when VPCs complete G1 and also controls when VPCs acquire the competence to respond to the intercellular patterning signals and express vulval fates.

The heterochronic gene lin-29 encodes a zinc finger protein that controls a terminal differentiation event in Caenorhabditis elegans

A hierarchy of heterochronic genes, lin-4, lin-14, lin-28 and lin-29, temporally restricts terminal differentiation of Caenorhabditis elegans hypodermal seam cells to the final molt. This terminal differentiation event involves cell cycle exit, cell fusion and the differential regulation of genes expressed in the larval versus adult hypodermis. lin-29 is the most downstream gene in the developmental timing pathway and thus it is the most direct known regulator of these diverse processes. We show that lin-29 encodes a protein with five zinc fingers of the (Cys)2-(His)2 class and thus likely controls these processes by regulating transcription in a stage-specific manner. Consistent with this role, a lin-29 fusion protein binds in vitro to the 5′ regulatory sequences necessary in vivo for expression of col-19, a collagen gene expressed in the adult hypodermis. lin-29 mRNA is detected in the first larval stage and increases in abundance through subsequent larval stages until the final molt, when lin-29 activity is required for terminal differentiation.