The N- or C-terminal domains of DSH-2 can activate the C. elegans Wnt/beta-catenin asymmetry pathway.

Dishevelleds are modular proteins that lie at the crossroads of divergent Wnt signaling pathways. The DIX domain of dishevelleds modulates a beta-catenin destruction complex, and thereby mediates cell fate decisions through differential activation of Tcf transcription factors. The DEP domain of dishevelleds mediates planar polarity of cells within a sheet through regulation of actin modulators. In Caenorhabditis elegans asymmetric cell fate decisions are regulated by asymmetric localization of signaling components in a pathway termed the Wnt/beta-catenin asymmetry pathway. Which domain(s) of Disheveled regulate this pathway is unknown. We show that C. elegans embryos from dsh-2(or302) mutant mothers fail to successfully undergo morphogenesis, but transgenes containing either the DIX or the DEP domain of DSH-2 are sufficient to rescue the mutant phenotype. Embryos lacking zygotic function of SYS-1/beta-catenin, WRM-1/beta-catenin, or POP-1/Tcf show defects similar to dsh-2 mutants, including a loss of asymmetry in some cell fate decisions. Removal of two dishevelleds (dsh-2 and mig-5) leads to a global loss of POP-1 asymmetry, which can be rescued by addition of transgenes containing either the DIX or DEP domain of DSH-2. These results indicate that either the DIX or DEP domain of DSH-2 is capable of activating the Wnt/beta-catenin asymmetry pathway and regulating anterior-posterior fate decisions required for proper morphogenesis.

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.

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.

Mutation of the H-helix in antithrombin decreases heparin stimulation of protease inhibition.

Blood clotting proceeds through the sequential proteolytic activation of a series of serine proteases, culminating in thrombin cleaving fibrinogen into fibrin. The serine protease inhibitors (serpins) antithrombin (AT) and protein C inhibitor (PCI) both inhibit thrombin in a heparin-accelerated reaction. Heparin binds to the positively charged D-helix of AT and H-helix of PCI. The H-helix of AT is negatively charged, and it was mutated to contain neutral or positively charged residues to see if they contributed to heparin stimulation or protease specificity in AT. To assess the impact of the H-helix mutations on heparin stimulation in the absence of the known heparin-binding site, negative charges were also introduced in the D-helix of AT. AT with both positively charged H- and D-helices showed decreases in heparin stimulation of thrombin and factor Xa inhibition by 10- and 5-fold respectively, a decrease in affinity for heparin sepharose, and a shift in the heparin template curve. In the absence of a positively charged D-helix, changing the H-helix from neutral to positively charged increased heparin stimulation of thrombin inhibition 21-fold, increased heparin affinity and restored a normal maximal heparin concentration for inhibition.

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.

An agar mount for observation of Caenorhabditis elegans embryos.

The Caenorhabditis elegans embryo is particularly amenable to microscopy and embryological studies because of its short developmental time, transparent shell, and nonpigmented cells. The agar mount described in this protocol is an easy way to prepare live C. elegans embryos for microscopic visualization. The mount slightly embeds the embryo in agar to hold it in place. The mount also slightly compresses the embryo to provide consistent orientation such that every embryo will be positioned with either its right side or its left side facing the objective. Other techniques can result in random orientations that complicate analysis and make identification of individual blastomeres more challenging.

Acquisition of 4D DIC microscopic data to determine cell contacts in Caenorhabditis elegans embryos.

The Caenorhabditis elegans embryo is particularly amenable to microscopy and embryological studies because of its short developmental time, transparent shell, and nonpigmented cells. Acquisition of stacks of images throughout the thickness of the embryo over time is a crucial method for identifying the positions and contacts between cells. Such four-dimensional (4D) microscopy is a routine tool in laboratories that study early C. elegans development. Differential interference contrast (DIC) microscopy is the focus here because of its broad availability, common use for C. elegans imaging, and wide applicability to microscopic analysis of embryos of other organisms. This protocol describes the use of a custom script within µManager's Beanshell scripting language. The script is helpful for reducing the number of shutter open/close events during 4D acquisition.

Analysis of 4D DIC microscopic data to determine cell contacts in Caenorhabditis elegans embryos.

The Caenorhabditis elegans embryo is particularly amenable to microscopy and embryological studies because of its short developmental time, transparent shell, and nonpigmented cells. Identification of cell contacts is important for understanding how cells within the embryo can be polarized by their neighbors. This protocol describes a technique for identifying cell membranes and potential contacts between different blastomeres in the embryo and for following those contacts through development. This protocol involves manual segmentation of the membrane of each blastomere from four-dimensional (4D) differential interference contrast (DIC) data sets. Although this technique is described for use with C. elegans embryos, it will work with any 4D DIC data set. The recent development of a pleckstrin homology (PH)-domain tagged::GFP expressed in C. elegans embryos simplifies this analysis; however, many organisms lack appropriate green fluorescent protein (GFP) transgenes. The use of the GFP transgene improves on laser-mediated techniques for labeling embryonic cell membranes with fluorescent dye. Such methods require that a laser be used to carefully permeabilize the eggshell for entry of the dye into the embryo. The technique described in this protocol requires only easily obtainable 4D DIC data sets.

Laser killing of blastomeres in Caenorhabditis elegans.

The Caenorhabditis elegans embryo is particularly amenable to microscopy and embryological studies because of its short developmental time, transparent shell, and nonpigmented cells. Within the embryo, contacts between cells often establish the polarization of neighboring cells. Blastomere isolation and recombination experiments have led to a wealth of understanding of the events in the four-cell C. elegans embryo. However, identifying individual blastomeres after isolation at stages past the four-cell stage is limited. In addition, removal of blastomeres from their native surroundings can interfere with many cell contacts besides the contacts of interest. An alternative approach for studying cell interactions within the C. elegans embryo is to use laser ablation of individual cells. Laser ablation can be used to kill one of two cells in contact with each other to understand what happens when a cell no longer signals to its neighbor. Additionally, killing a cell that is between two cells that will eventually contact each other can result in the corpse of the cell forming a steric barrier between the cells, preventing the contact. This protocol describes laser ablation of embryos mounted on an agar mount.

Interdisciplinary training in mathematical biology through team-based undergraduate research and courses.

Inspired by BIO2010 and leveraging institutional and external funding, Truman State University built an undergraduate program in mathematical biology with high-quality, faculty-mentored interdisciplinary research experiences at its core. These experiences taught faculty and students to bridge the epistemological gap between the mathematical and life sciences. Together they created the infrastructure that currently supports several interdisciplinary courses, an innovative minor degree, and long-term interdepartmental research collaborations. This article describes how the program was built with support from the National Science Foundation's Interdisciplinary Training for Undergraduates in Biology and Mathematics program, and it shares lessons learned that will help other undergraduate institutions build their own program.

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.

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.

Thrombomodulin enhances the reactivity of thrombin with protein C inhibitor by providing both a binding site for the serpin and allosterically modulating the activity of thrombin.

Thrombomodulin (TM), or its epidermal growth factor-like domains 456 (TM456), enhances the catalytic efficiency of thrombin toward both protein C and protein C inhibitor (PCI) by 2-3 orders of magnitude. Structural and mutagenesis data have indicated that the interaction of basic residues of the heparin-binding exosite of protein C with the acidic residues of TM4 is partially responsible for the efficient activation of the substrate by the thrombin-TM456 complex. Similar to protein C, PCI has a basic exosite (H-helix) that constitutes the heparin-binding site of the serpin. To determine whether TM accelerates the reactivity of thrombin with PCI by providing a binding site for the H-helix of the serpin, an antithrombin (AT) mutant was constructed in which the H-helix of the serpin was replaced with the same region of PCI (AT-PCIH-helix). Unlike PCI, the H-helix of AT is negatively charged. It was discovered that TM456 slightly (<2-fold) impaired the reactivity of AT with thrombin; however, it enhanced the reactivity of AT-PCIH-helix with the protease by an order of magnitude. Further studies revealed that the substitution of Arg35 of thrombin with an Ala also resulted in an order of magnitude enhancement in reactivity of the protease with both PCI and AT-PCIH-helix independent of TM. We conclude that TM enhances the reactivity of PCI with thrombin by providing both a binding site for the serpin and a conformational modulation of the extended binding pocket of thrombin.