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Jeff Hardin

Publications and source records attributed to Jeff Hardin.

12 recordsLinked to original sources
PubMed2006

Wnt/Frizzled signaling controls C. elegans gastrulation by activating actomyosin contractility.

BACKGROUND: Embryonic patterning mechanisms regulate the cytoskeletal machinery that drives morphogenesis, but there are few cases where links between patterning mechanisms and morphogenesis are well understood. We have used a combination of genetics, in vivo imaging, and cell manipulations to identify such links in C. elegans gastrulation. Gastrulation in C. elegans begins with the internalization of endodermal precursor cells in a process that depends on apical constriction of ingressing cells. RESULTS: We show that ingression of the endodermal precursor cells is regulated by pathways, including a Wnt-Frizzled signaling pathway, that specify endodermal cell fate. We find that Wnt signaling has a role in gastrulation in addition to its earlier roles in regulating endodermal cell fate and cell-cycle timing. In the absence of Wnt signaling, endodermal precursor cells polarize and enrich myosin II apically but fail to contract their apical surfaces. We show that a regulatory myosin light chain normally becomes phosphorylated on the apical side of ingressing cells at a conserved site that can lead to myosin-filament formation and contraction of actomyosin networks and that this phosphorylation depends on Wnt signaling. CONCLUSIONS: We conclude that Wnt signaling regulates C. elegans gastrulation through regulatory myosin light-chain phosphorylation, which results in the contraction of the apical surface of ingressing cells. These findings forge new links between cell-fate specification and morphogenesis, and they represent a novel mechanism by which Wnt signaling can regulate morphogenesis.

Actomyosin↗
PubMed2006

mig-5/Dsh controls cell fate determination and cell migration in C. elegans.

Cell fate determination and cell migration are two essential events in the development of an organism. We identify mig-5, a Dishevelled family member, as a gene that regulates several cell fate decisions and cell migrations that are important during C. elegans embryonic and larval development. In offspring from mig-5 mutants, cell migrations are defective during hypodermal morphogenesis, QL neuroblast migration, and the gonad arm migration led by the distal tip cells (DTCs). In addition to abnormal migration, DTC fate is affected, resulting in either an absent or an extra DTC. The cell fates of the anchor cell in hermaphrodites and the linker cells in the male gonad are also defective, often resulting in the cells adopting the fates of their sister lineage. Moreover, 2 degrees vulval precursor cells occasionally adopt the 3 degrees vulval cell fate, resulting in a deformed vulva, and the P12 hypodermal precursor often differentiates into a second P11 cell. These defects demonstrate that MIG-5 is essential in determining proper cell fate and cell migration throughout C. elegans development.

Adaptor Proteins, Signal Transducing↗
PubMed2006

Wnt-dependent spindle polarization in the early C. elegans embryo.

Correct orientation of the mitotic spindle is crucial for the proper segregation of localized determinants and the correct spatial organization of cells in early embryos. The cues dividing cells use to orient their mitotic spindles are currently the subject of intensive investigation in a number of model systems. One of the cues that cells use during spindle orientation is provided by components of the Wnt signaling pathway. Because of its stereotypical cleavage divisions, the availability of Wnt pathway mutants and the ability to perform RNAi, and because cell-cell interactions can be studied in vitro, the C. elegans embryo continues to be a useful system for identifying specific cell-cell interactions in which Wnt-dependent signals polarize the mitotic spindle. This review discusses the evidence for involvement of Wnt signaling during spindle orientation in several contexts in the early C. elegans embryo, a process that involves upstream Wnt effectors but does not involve downstream nuclear effectors of Wnt signaling, and places this Wnt spindle orientation pathway in the larger context of other known modulators of spindle orientation in animal embryos.

Animals↗
PubMed2006

A homologue of snail is expressed transiently in subsets of mesenchyme cells in the sea urchin embryo and is down-regulated in axis-deficient embryos.

Vertebrate members of the zinc finger transcription factor family related to Drosophila snail are expressed in neural crest and paraxial mesoderm along the left-right axis of the embryo. As simple deuterostomes, echinoderms are an important sister phylum for the chordates. We have identified populations of patterned, nonskeletogenic mesenchyme in the sea urchin Lytechinus variegatus by their expression of a sea urchin member of the snail family (Lv-snail). Lv-snail mRNA and protein are detectable at the midgastrula stage within the archenteron. At the late gastrula stage, a contiguous cluster of cells on the left side of the tip of the archenteron is Lv-snail-positive. At the early prism stage, two small clusters of mesenchyme cells near the presumptive arm buds are also Lv-snail-positive. At the pluteus stage, staining is detectable in isolated mesenchyme cells and the ciliated band. Based on fate mapping of secondary mesenchyme cells (SMCs) and double-label immunostaining, these patterns are consistent with expression of SNAIL by novel subsets of SMCs that are largely distinct from skeletogenic mesenchyme. In radialized embryos lacking normal bilateral symmetry, mesenchymal expression of Lv-SNAIL is abolished. These results suggest that transient expression of Lv-snail may be important for the differentiation of a subset of axially patterned nonskeletogenic mesenchyme cells and suggest conserved functions for snail family members in deuterostome development.

Animals↗
PubMed2004

The inositol 1,4,5-trisphosphate receptor regulates epidermal cell migration in Caenorhabditis elegans.

Polarized migration and spreading of epithelial sheets is important during many processes in vivo, including embryogenesis and wound healing. However, the signaling pathways that regulate epithelial migrations are poorly understood. To identify molecular components that regulate the spreading of epithelial sheets, we performed a screen for mutations that perturb epidermal cell migration during embryogenesis in Caenorhabditis elegans. We identified one mutant (jc5) as a weak mutation in itr-1, which encodes the single inositol 1,4,5-trisphosphate receptor (ITR) in C. elegans. During the migration of the embryonic epidermis, jc5 embryos display defects including misdirected migration or premature cessation of migration. Cells that halt their migration have disorganized F-actin and display reduced filopodial protrusive activity at their leading edge. Furthermore, some filopodia formed by epidermal cells in itr-1(jc5) embryos exhibit abnormally long lifetimes. Pharmacological studies with the inositol 1,4,5-trisphosphate antagonist xestospongin C phenocopy these defects, confirming that ITR function is important for proper epidermal migration. Our results provide the first molecular evidence that movements of embryonic epithelial cell sheets can be controlled by ITRs and suggest that such regulation may be a widespread mechanism for coordinating epithelial cell movements during embryogenesis.

Animals↗
PubMed2004

Sticky worms: adhesion complexes in C. elegans.

Caenorhabditis elegans is a powerful model system for investigating the establishment, regulation and function of adhesive structures in vivo. C. elegans has several adhesion complexes related to those in vertebrates. These include: (1) epithelial apical junctions, which have features of both adherens and tight junctions; (2) dense bodies, which are muscle-attachment structures similar to focal adhesions; (3) fibrous organelles, which resemble hemidesmosomes and mediate mechanical coupling between tissues; and (4) a putative dystrophin-glycoprotein complex that has potential roles in muscle function and embryogenesis. Recent work has increased our understanding of these structures and has given new insights into the functions of their vertebrate counterparts.

Adherens Junctions↗
PubMed2004

Skin tight: cell adhesion in the epidermis of Caenorhabditis elegans.

The powerful genetics, genomics and microscopy tools available for C. elegans make it well suited to studying how epithelial cells adhere to one another and the extracellular matrix, and how the integrated, simultaneous activities of multiple cell adhesion complexes function to shape an organism. Recent studies using forward and reverse genetics have shed light on how phylogenetically conserved cell adhesion complexes, such as the cadherin/catenin complex, claudins, the Discs large complex and hemidesmosome-like attachment structures, regulate epithelial cell adhesion, providing new insights into conserved cell adhesion mechanisms in higher eukaryotes.

Actins↗
PubMed2004

Multiple Wnt signaling pathways converge to orient the mitotic spindle in early C. elegans embryos.

How cells integrate the input of multiple polarizing signals during division is poorly understood. We demonstrate that two distinct Caenorhabditis elegans Wnt pathways contribute to the polarization of the ABar blastomere by differentially regulating its duplicated centrosomes. Contact with the C blastomere orients the ABar spindle through a nontranscriptional Wnt spindle alignment pathway, while a Wnt/beta-catenin pathway controls the timing of ABar spindle rotation. The three C. elegans Dishevelled homologs contribute to these processes in different ways, suggesting that functional distinctions may exist among them. We also find that CKI (KIN-19) plays a role not only in the Wnt/beta-catenin pathway, but also in the Wnt spindle orientation pathway as well. Based on these findings, we establish a model for the coordination of cell-cell interactions and distinct Wnt signaling pathways that ensures the robust timing and orientation of spindle rotation during a developmentally regulated cell division event.

Alleles↗
PubMed2004

Models of morphogenesis: the mechanisms and mechanics of cell rearrangement.

The directional rearrangement of cells is a key mechanism for reshaping embryos. Despite substantial recent progress in understanding the basic signal transduction pathways that allow cells to orient themselves in space, the extrinsic cues that activate these pathways are just beginning to be understood. Even less-well understood are the physical mechanisms cells use to change position, especially when those cells are epithelial, and how mechanical forces within the embryo affect those movements. Recent studies are providing clues regarding how this fundamental process occurs with such remarkable reliability.

Animals↗
PubMed2003

The Caenorhabditis elegans p120 catenin homologue, JAC-1, modulates cadherin-catenin function during epidermal morphogenesis.

The cadherin-catenin complex is essential for tissue morphogenesis during animal development. In cultured mammalian cells, p120 catenin (p120ctn) is an important regulator of cadherin-catenin complex function. However, information on the role of p120ctn family members in cadherin-dependent events in vivo is limited. We have examined the role of the single Caenorhabditis elegans p120ctn homologue JAC-1 (juxtamembrane domain [JMD]-associated catenin) during epidermal morphogenesis. Similar to other p120ctn family members, JAC-1 binds the JMD of the classical cadherin HMR-1, and GFP-tagged JAC-1 localizes to adherens junctions in an HMR-1-dependent manner. Surprisingly, depleting JAC-1 expression using RNA interference (RNAi) does not result in any obvious defects in embryonic or postembryonic development. However, jac-1(RNAi) does increase the severity and penetrance of morphogenetic defects caused by a hypomorphic mutation in the hmp-1/alpha-catenin gene. In these hmp-1 mutants, jac-1 depletion causes failure of the embryo to elongate into a worm-like shape, a process that involves contraction of the epidermis. Associated with failed elongation is the detachment of actin bundles from epidermal adherens junctions and failure to maintain cadherin in adherens junctions. These results suggest that JAC-1 acts as a positive modulator of cadherin function in C. elegans.

Actin Cytoskeleton↗
PubMed2003

The cell junction protein VAB-9 regulates adhesion and epidermal morphology in C. elegans.

Epithelial cell junctions are essential for cell polarity, adhesion and morphogenesis. We have analysed VAB-9, a cell junction protein in Caenorhabditis elegans. VAB-9 is a predicted four-pass integral membrane protein that has greatest similarity to BCMP1 (brain cell membrane protein 1, a member of the PMP22/EMP/Claudin family of cell junction proteins) and localizes to the adherens junction domain of C. elegans apical junctions. Here, we show that VAB-9 requires HMR-1/cadherin for localization to the cell membrane, and both HMP-1/alpha-catenin and HMP-2/beta-catenin for maintaining its distribution at the cell junction. In vab-9 mutants, morphological defects correlate with disorganization of F-actin at the adherens junction; however, localization of the cadherin-catenin complex and epithelial polarity is normal. These results suggest that VAB-9 regulates interactions between the cytoskeleton and the adherens junction downstream of or parallel to alpha-catenin and/or beta-catenin. Mutations in vab-9 enhance adhesion defects through functional loss of the cell junction genes apical junction molecule 1 (ajm-1) and discs large 1 (dlg-1), suggesting that VAB-9 is involved in cell adhesion. Thus, VAB-9 represents the first characterized tetraspan adherens junction protein in C. elegans and defines a new family of such proteins in higher eukaryotes.

Actin Cytoskeleton↗