Selective Lineage Specification by Mab-19 during Caenorhabditis Elegans Male Peripheral Sense Organ Development

The action of the gene mab-19 is required for specification of a subset of Caenorhabditis elegans male peripheral sense organ (ray) lineages. Two mab-19 alleles, isolated in screens for ray developmental mutations, resulted in males that lacked the three most posterior rays. Cell lineage alterations of male-specific divisions of the most posterior lateral hypodermal (seam) blast cell, T, resulted in the ray loss phenotype in mab-19 mutant animals. Postembryonic seam lineage defects were limited to male-specific T descendent cell divisions. Embryonic lethality resulted when either mab-19 mutation was placed over a chromosomal deficiency encompassing the mab-19 locus. The earliest detectable defect was aberrant hypodermal cell movements during morphogenesis. From these data, it is inferred that both mab-19 alleles described are hypomorphs, and further reduction of mab-19 function results in embryos that are unable to complete morphogenesis. Thus, mab-19 may play a larger role in developmental regulation of hypodermal cell fate, including sensory ray development in males. Body morphology mutations, passage through the dauer stage, and heat or CdCl(2) treatment suppressed mab-19 male phenotypes. A model is presented in which all three types of suppression result in a physiological stress response, which in turn leads to correction of the mab-19 defect.     

Expression of the homeotic gene mab-5 during Caenorhabditis elegans embryogenesis.

mab-5 is a member of a complex of homeobox-containing genes evolutionarily related to the Antennapedia and bithorax complexes of Drosophila melanogaster. Like the homeotic genes in Drosophila, mab-5 is required in a particular region along the anterior-posterior body axis, and acts during postembryonic development to give cells in this region their characteristic identities. We have used a mab-5-lacZ fusion integrated into the C. elegans genome to study the posterior-specific expression of mab-5 during embryogenesis. The mab-5-lacZ fusion was expressed in the posterior of the embryo by 180 minutes after the first cleavage, indicating that the mechanisms responsible for the position-specific expression of mab-5-lacZ act at a relatively early stage of embryogenesis. In embryos homozygous for mutations in the par genes, which disrupt segregation of factors during early cleavages, expression of mab-5-lacZ was no longer localized to the posterior. This suggests that posterior-specific expression of mab-5 depends on the appropriate segregation of developmental factors during early embryogenesis. After extrusion of any blastomere of the four-cell embryo, descendants of the remaining three cells could still express the mab-5-lacZ fusion. In these partial embryos, however, the fusion was often expressed in cells scattered throughout the embryo, suggesting that cell-cell interactions and/or proper positioning of early blastomeres are required for mab-5 expression to be localized to the posterior.     

mab-20 encodes Semaphorin-2a and is required to prevent ectopic cell contacts during epidermal morphogenesis in Caenorhabditis elegans

The Semaphorins are a family of secreted and transmembrane proteins known to elicit growth cone repulsion and collapse. We made and characterized a putative null mutant of the C. elegans gene semaphorin-2a (Ce-sema-2a). This mutant failed to complement mutants of mab-20 (Baird, S. E., Fitch, D. H., Kassem, I. A. A. and Emmons, S. W. (1991) Development 113, 515-526). In addition to low-frequency axon guidance errors, mab-20 mutants have unexpected defects in epidermal morphogenesis. Errant epidermal cell migrations affect epidermal enclosure of the embryo, body shape and sensory rays of the male tail. These phenotypic traits are explained by the formation of inappropriate contacts between cells of similar type and suggest that Ce-Sema-2a may normally prevent formation or stabilization of ectopic adhesive contacts between these cells.     

mab-7 encodes a novel transmembrane protein that orchestrates sensory ray morphogenesis in C. elegans

The tapered sensory rays of the male Caenorhabditis elegans are important for successful male/hermaphrodite copulation. A group of ram (ray morphology abnormal) genes encoding modifying enzymes and transmembrane protein have been reported as key regulators controlling ray morphogenesis. Here we report the characterization of another component essential for this morphogenetic process encoded by mab-7. This gene is active in the hypodermis, structural cells, the body seam and several head neurons. It encodes a novel protein with a hydrophobic region at the N-terminus, an EGF-like motif, an ShKT motif and a long C-terminal tail. All these domains are shown to be critical to MAB-7 activity except the EGF-like domain, which appears to be regulatory and dispensable. MAB-7 is shown to be a type II membrane protein, tethered on the cell surface by the N-terminal transmembrane domain with the remainder of the protein exposed to the extracellular matrix. Since ectopic mab-7 expression in any ray cell or even in touch neurons of non-ray lineage can rescue the mutant phenotype, mab-7 is probably acting non-autonomously. It may facilitate intercellular communication among ray cells to augment normal ray morphogenesis.     

Cloning and developmental expression analysis of ltd-1, the Caenorhabditis elegans homologue of the mouse kyphoscoliosis (ky) gene

We have characterized the developmental expression pattern of the Caenorhabditis elegans homologue of the mouse ky gene. The Ky protein has a putative key function in muscle development and has homologues in invertebrates, fungi and a cyanobacterium. The C. elegans Ky homologue gene has been named ltd-1 for LIM and transglutaminase domains gene. The LTD-1::GFP construct is expressed in developing hypodermal cells from the twofold stage embryo through adulthood. These data define the ltd-1 gene as a novel marker for C. elegans epithelial cell development.     

Posterior pattern formation in C. elegans involves position-specific expression of a gene containing a homeobox

During postembryonic development in C. elegans, posterior-specific pattern formation requires the gene mab-5. Within the posterior body region, mab-5 activity controls epidermal, neuronal, and mesodermal cell differentiation, and also the direction of cell migration. Here, we show that mab-5 RNA is localized in the posterior body region, indicating that mab-5 activity is targeted to posterior cells, at least in part, by a mechanism that operates at the level of mab-5 RNA synthesis or stabilization. We also show that mab-5 contains a homeobox similar to that of the Drosophila Antennapedia gene. This suggests that mab-5 influences cell differentiation and cell migration by regulating gene expression, and clearly demonstrates that genes containing homeoboxes influence global aspects of pattern formation in organisms other than Drosophila.     

Characterization of seven genes affecting Caenorhabditis elegans hindgut development.

We have identified and characterized 12 mutations in seven genes that affect the development of the Caenorhabditis elegans hindgut. We find that the mutations can disrupt the postembryonic development of the male-specific blast cells within the hindgut, the hindgut morphology in both males and hermaphrodites, and in some cases, the expression of a hindgut marker in hermaphrodite animals. Mutations in several of the genes also affect viability. On the basis of their mutant phenotypes, we propose that the genes fall into four distinct classes: (1) egl-5 is required for regional identity of the tail; (2) sem-4 is required for a variety of ectodermal and mesodermal cell types, including cells in the hindgut; (3) two genes, lin-49 and lin-59, affect development of many cells, including hindgut; and (4) three genes, mab-9, egl-38, and lin-48, are required for patterning fates within the hindgut, making certain hindgut cells different from others. We also describe a new allele of the Pax gene egl-38 that is temperature sensitive and affects the conserved beta-hairpin of the EGL-38 paired domain. Our results suggest that a combination of different factors contribute to normal C. elegans hindgut development.     

mab-3, a gene required for sex-specific yolk protein expression and a male-specific lineage in C. elegans

The gene mab-3 appears to regulate a subset of sex-specific events in C. elegans male development. Mutations in mab-3 have no apparent effect on hermaphrodites, but cause synthesis of yolk proteins and a limited lineage alteration in males. We infer that mab-3 has at least two distinct male-specific functions. First, mab-3 activity prevents yolk protein production by males, without affecting stage or tissue specificity of expression. Second, mab-3 activity is required for expression of the male V ray cell lineage. Epistasis analysis is most consistent with a model in which mab-3 is controlled by tra-1, the last switch gene known to act in the somatic sex determination pathway. We discuss how genes such as mab-3 might generate sexual dimorphism.     

C. elegans POP-1/TCF functions in a canonical Wnt pathway that controls cell migration and in a noncanonical Wnt pathway that controls cell polarity

In Caenorhabditis elegans, Wnt signaling pathways are important in controlling cell polarity and cell migrations. In the embryo, a novel Wnt pathway functions through a (beta)-catenin homolog, WRM-1, to downregulate the levels of POP-1/Tcf in the posterior daughter of the EMS blastomere. The level of POP-1 is also lower in the posterior daughters of many anteroposterior asymmetric cell divisions during development. I have found that this is the case for of a pair of postembryonic blast cells in the tail. In wild-type animals, the level of POP-1 is lower in the posterior daughters of the two T cells, TL and TR. Furthermore, in lin-44/Wnt mutants, in which the polarities of the T cell divisions are frequently reversed, the level of POP-1 is frequently lower in the anterior daughters of the T cells. I have used a novel RNA-mediated interference technique to interfere specifically with pop-1 zygotic function and have determined that pop-1 is required for wild-type T cell polarity. Surprisingly, none of the three C. elegans (beta)-catenin homologs appeared to function with POP-1 to control T cell polarity. Wnt signaling by EGL-20/Wnt controls the migration of the descendants of the QL neuroblast by regulating the expression the Hox gene mab-5. Interfering with pop-1 zygotic function caused defects in the migration of the QL descendants that mimicked the defects in egl-20/Wnt mutants and blocked the expression of mab-5. This suggests that POP-1 functions in the canonical Wnt pathway to control QL descendant migration and in novel Wnt pathways to control EMS and T cell polarities.     

The ERK MAP kinase cascade mediates tail swelling and a protective response to rectal infection in C. elegans

The nematode Caenorhabditis elegans is proving to be an attractive model organism for investigating innate immune responses to infection. Among the known pathogens of C. elegans is the bacterium Microbacterium nematophilum, which adheres to the nematode rectum and postanal cuticle, inducing swelling of the underlying hypodermal tissue and causing mild constipation. We find that on infection by M. nematophilum, an extracellular signal-regulated kinase (ERK) mitogen-activated protein (MAP) kinase cascade mediates tail swelling and protects C. elegans from severe constipation, which would otherwise arrest development and cause sterility. Involvement in pathogen defense represents a new role for ERK MAP kinase signaling in this organism.     

The Caenorhabditis elegans pumilio homolog, puf-9, is required for the 3'UTR-mediated repression of the let-7 microRNA target gene, hbl-1

The Puf family of RNA-binding proteins directs cell fates by regulating gene expression at the level of translation and RNA stability. Here, we report that the Caenorhabditis elegans pumilio homolog, puf-9, controls the differentiation of epidermal stem cells at the larval-to-adult transition. Genetic analysis reveals that loss-of-function mutations in puf-9 enhance the lethality and heterochronic phenotypes caused by mutations in the let-7 microRNA (miRNA), while suppressing the heterochronic phenotypes of lin-41, a let-7 target and homolog of Drosophila Brat. puf-9 interacts with another known temporal regulator hbl-1, the Caenorhabditis elegans ortholog of hunchback. We present evidence demonstrating that puf-9 is required for the 3'UTR-mediated regulation of hbl-1, in both the hypodermis and the ventral nerve cord. Finally, we show that this regulation is dependent on a region of the hbl-1 3'UTR that contains putative Puf family binding sites as well as binding sites for the let-7 miRNA family, suggesting that puf-9 and let-7 may mediate hypodermal seam cell differentiation by regulating common targets.     

A new submicroscopic deletion that refines the 9p region for sex reversal

Male to female sex reversal has been described in patients with deletions of chromosome 9p, and a region critical for sex reversal has been localized to p24.3, at the tip of the chromosome (TD9). It was proposed that the sex reversal may arise by haploinsufficiency for a gene localized to the minimum deletion. The 9p24.3 genes DMRT1 and DMRT2 are the favorite TD9 candidates to date, in virtue of their sequence similarity to doublesex and mab-3, sexual regulators in Drosophila and Caenorhabditis elegans, respectively. The hypothesis of sex reversal by combined haploinsufficiency for the two genes was put forward to explain the lack of mutations in either gene in XY sex-reversed females. Here we describe a XY sex-reversed patient carrying a novel 9p deletion that extends over less than 700 kb of genomic DNA. This region defines the smallest interval for sex reversal found to date. DMRT1 and DMRT2 map outside this region. Our data do not support the hypothesis of combined haploinsufficiency for DMRT1 and DMRT2. Nevertheless, DMRT1 localizes very close to the deletion breakpoint and has a pattern of expression compatible with a role in sex determination. It therefore remains a candidate gene for 9p sex reversal.