mua-3, a gene required for mechanical tissue integrity in Caenorhabditis elegans, encodes a novel transmembrane protein of epithelial attachment complexes.

Normal locomotion of the nematode Caenorhabditis elegans requires transmission of contractile force through a series of mechanical linkages from the myofibrillar lattice of the body wall muscles, across an intervening extracellular matrix and epithelium (the hypodermis) to the cuticle. Mutations in mua-3 cause a separation of the hypodermis from the cuticle, suggesting this gene is required for maintaining hypodermal-cuticle attachment as the animal grows in size postembryonically. mua-3 encodes a predicted 3,767 amino acid protein with a large extracellular domain, a single transmembrane helix, and a smaller cytoplasmic domain. The extracellular domain contains four distinct protein modules: 5 low density lipoprotein type A, 52 epidermal growth factor, 1 von Willebrand factor A, and 2 sea urchin-enterokinase-agrin modules. MUA-3 localizes to the hypodermal hemidesmosomes and to other sites of mechanically robust transepithelial attachments, including the rectum, vulva, mechanosensory neurons, and excretory duct/pore. In addition, it is shown that MUA-3 colocalizes with cytoplasmic intermediate filaments (IFs) at these sites. Thus, MUA-3 appears to be a protein that links the IF cytoskeleton of nematode epithelia to the cuticle at sites of mechanical stress.

Conservation and novelty in the evolution of cell adhesion and extracellular matrix genes.

New proteins and modules have been invented throughout evolution. Gene "birth dates" in Caenorhabditis elegans range from the origins of cellular life through adaptation to a soil habitat. Possibly half are "metazoan" genes, having arisen sometime between the yeast-metazoan and nematode-chordate separations. These include basement membrane and cell adhesion molecules implicated in tissue organization. By contrast, epithelial surfaces facing the environment have specialized components invented within the nematode lineage. Moreover, interstitial matrices were likely elaborated within the vertebrate lineage. A strategy for concerted evolution of new gene families, as well as conservation of adaptive genes, may underlie the differences between heterochromatin and euchromatin.

Fragile skeletal muscle attachments in dystrophic mutants of Caenorhabditis elegans: isolation and characterization of the mua genes.

Over 30 Caenorhabditis elegans mutants were identified with normal muscle differentiation and initial locomotion followed by catastrophic detachment of skeletal muscles from the body wall. Reducing the strength of muscle contraction in these mutants with a myosin gene mutation suppresses muscle detachment. These dystrophic mutants identify a novel class of genes required for growth and maintenance of functional muscle attachments, not exceptional alleles of genes required for muscle differentiation and contractility. Nine new genes, named mua, and two previously published loci, unc-23 and vab-10, cause fragile musscle attachments. The primary sites of muscle detachment, including the plane of tissue separation, are characteristic for each gene. We suggest these genes identify feedback mechanisms whereby local strain regulates the extent of myofibril contraction and the placement of new muscle attachments in functioning muscles. Finally, we draw some comparisons to vertebrate skin fragility diseases and muscular dystrophies.

Feedback control of sex determination by dosage compensation revealed through Caenorhabditis elegans sdc-3 mutations.

In Caenorhabditis elegans, sex determination and dosage compensation are coordinately controlled through a group of genes that respond to the primary sex determination signal. Here we describe a new gene, sdc-3, that also controls these processes. In contrast to previously described genes, the sex determination and dosage compensation activities of sdc-3 are separately mutable, indicating that they function independently. Paradoxically, the sdc-3 null phenotype fails to reveal the role of sdc-3 in sex determination: sdc-3 null mutations that lack both activities disrupt dosage compensation but cause no overt sexual transformation. We demonstrate that the dosage compensation defect of sdc-3 null alleles suppresses their sex determination defect. This self-suppression phenomenon provides a striking example of how a disruption in dosage compensation can affect sexual fate. We propose that the suppression occurs via a feedback mechanism that acts at an early regulatory step in the sex determination pathway to promote proper sexual identity.

Genes that implement the hermaphrodite mode of dosage compensation in Caenorhabditis elegans.

We report a genetic characterization of several essential components of the dosage compensation process in Caenorhabditis elegans. Mutations in the genes dpy-26, dpy-27, dpy-28, and the newly identified gene dpy-29 disrupt dosage compensation, resulting in elevated X-linked gene expression in XX animals and an incompletely penetrant maternal-effect XX-specific lethality. These dpy mutations appear to cause XX animals to express each set of X-linked genes at a level appropriate for XO animals. XO dpy animals are essentially wild type. Both the viability and the level of X-linked gene expression in XX animals carrying mutations in two or more dpy genes are the same as in animals carrying only a single mutation, consistent with the view that these genes act together in a single process (dosage compensation). To define a potential time of action for the gene dpd-28 we performed reciprocal temperature-shift experiments with a heat sensitive allele. The temperature-sensitive period for lethality begins 5 hr after fertilization at the 300-cell stage and extends to about 9 hr, a point well beyond the end of cell proliferation. This temperature-sensitive period suggests that dosage compensation is functioning in XX animals by mid-embryogenesis, when many zygotically transcribed genes are active. While mutations in the dpy genes have no effect on the sexual phenotype of otherwise wild-type XX or XO animals, they do have a slight feminizing effect on animals whose sex-determination process is already genetically perturbed. The opposite directions of the feminizing effects on sex determination and the masculinizing effects on dosage compensation caused by the dpy mutations are inconsistent with the wild-type dpy genes acting to coordinately control both processes. Instead, the feminizing effects are most likely an indirect consequence of disruptions in dosage compensation caused by the dpy mutations. Based on the cumulative evidence, the likely mechanism of dosage compensation in C. elegans involves reducing X-linked gene expression in XX animals to equal that in XO animals via the action of the dpy genes.

xol-1: a gene that controls the male modes of both sex determination and X chromosome dosage compensation in C. elegans.

Loss-of-function mutations in the X-linked gene xol-1 cause the feminization and death of XO animals (normally males) by shifting the sex determination and dosage compensation pathways toward their hermaphrodite modes. XO-specific lethality most likely results from the reduction in X chromosome expression caused by xol-1 mutations. Mutations in genes required for the hermaphrodite mode of dosage compensation suppress lethality but not feminization, and restore X chromosome expression to nearly wild-type levels. Mutations in genes that control the hermaphrodite modes of both sex determination and dosage compensation fully suppress both defects. These interactions suggest that xol-1 is the earliest-acting gene in the known hierarchy controlling the male/hermaphrodite decision and is perhaps the gene nearest the primary sex-determining signal. We propose that the wild-type xol-1 gene product promotes male development by ensuring that genes (or gene products) directing hermaphrodite sex determination and dosage compensation are inactive in XO animals. Interestingly, in addition to feminizing XO animals, xol-1 mutations further masculinize XX animals already partially masculinized.