SAX-7/L1CAM and HMR-1/cadherin function redundantly in blastomere compaction and non-muscle myosin accumulation during Caenorhabditis elegans gastrulation.

Gastrulation is the first major morphogenetic movement in development and requires dynamic regulation of cell adhesion and the cytoskeleton. Caenorhabditis elegans gastrulation begins with the migration of the two endodermal precursors, Ea and Ep, from the surface of the embryo into the interior. Ea/Ep migration provides a relatively simple system to examine the intersection of cell adhesion, cell signaling, and cell movement. Ea/Ep ingression depends on correct cell fate specification and polarization, apical myosin accumulation, and Wnt activated actomyosin contraction that drives apical constriction and ingression (Lee et al., 2006; Nance et al., 2005). Here, we show that Ea/Ep ingression also requires the function of either HMR-1/cadherin or SAX-7/L1CAM. Both cadherin complex components and L1CAM are localized at all sites of cell-cell contact during gastrulation. Either system is sufficient for Ea/Ep ingression, but loss of both together leads to a failure of apical constriction and ingression. Similar results are seen with isolated blastomeres. Ea/Ep are properly specified and appear to display correct apical-basal polarity in sax-7(eq1);hmr-1(RNAi) embryos. Significantly, in sax-7(eq1);hmr-1(RNAi) embryos, Ea and Ep fail to accumulate myosin (NMY-2Colon, two colonsGFP) at their apical surfaces, but in either sax-7(eq1) or hmr-1(RNAi) embryos, apical myosin accumulation is comparable to wild type. Thus, the cadherin and L1CAM adhesion systems are redundantly required for localized myosin accumulation and hence for actomyosin contractility during gastrulation. We also show that sax-7 and hmr-1 function are redundantly required for Wnt-dependent spindle polarization during division of the ABar blastomere, indicating that these cell surface proteins redundantly regulate multiple developmental events in early embryos.

Phospholipase Cgamma activates Ras on the Golgi apparatus by means of RasGRP1.

Ras proteins regulate cellular growth and differentiation, and are mutated in 30% of cancers. We have shown recently that Ras is activated on and transmits signals from the Golgi apparatus as well as the plasma membrane but the mechanism of compartmentalized signalling was not determined. Here we show that, in response to Src-dependent activation of phospholipase Cgamma1, the Ras guanine nucleotide exchange factor RasGRP1 translocated to the Golgi where it activated Ras. Whereas Ca(2+) positively regulated Ras on the Golgi apparatus through RasGRP1, the same second messenger negatively regulated Ras on the plasma membrane by means of the Ras GTPase-activating protein CAPRI. Ras activation after T-cell receptor stimulation in Jurkat cells, rich in RasGRP1, was limited to the Golgi apparatus through the action of CAPRI, demonstrating unambiguously a physiological role for Ras on Golgi. Activation of Ras on Golgi also induced differentiation of PC12 cells, transformed fibroblasts and mediated radioresistance. Thus, activation of Ras on Golgi has important biological consequences and proceeds through a pathway distinct from the one that activates Ras on the plasma membrane.

Epidermal growth factor receptor autocrine signaling in RIE-1 cells transformed by the Ras oncogene enhances radiation resistance.

Oncogenic forms of the small GTPase Ras increase the resistance of cells to killing by ionizing radiation (IR). Although not all of the signaling pathways for radioresistance are well defined, it is now clear that Ras-dependent signaling pathways involved in radioresistance include those mediated by phosphatidylinositol 3'-kinase (PI3-K) and Raf. Nevertheless, PI3-K and Raf together are not sufficient to reconstitute all of the resistance conferred by Ras, indicating that other effectors must also contribute. We show here that Ras-driven autocrine signaling through the epidermal growth factor receptor (EGFR) also contributes to radioresistance in Ras-transformed cells. Conditioned media (CM) collected from RIE-1 rat intestinal epithelial cells expressing oncogenic Ras increased the survival of irradiated cells. Ras-CM contains elevated levels of the EGFR ligand transforming growth factor alpha (TGF-alpha). Both Ras-CM and TGF-alpha stimulated EGFR phosphorylation, and exogenous TGF-alpha mimicked the effects of Ras-CM to increase radioresistance. Blocking EGFR signaling with the EGFR/HER-2 kinase inhibitor (KI) GW572016 decreased the postradiation survival of irradiated Ras-transformed cells and normal cells but had no effect on the survival of unirradiated cells. Ras-CM and TGF-alpha also increase PI3-K activity downstream of the EGFR and increase postradiation survival, both of which are abrogated by GW572016. Thus, Ras utilizes autocrine signaling through EGFR to increase radioresistance, and the EGFR KI GW572016 acts as a radiosensitizer. The observation that Ras-transformed cells can be sensitized to killing by ionizing radiation with GW572016 demonstrates that EGFR KIs could potentially be used to radiosensitize tumors in which radioresistance is dependent on Ras-driven autocrine signaling through EGFR.

Ras mediates radioresistance through both phosphatidylinositol 3-kinase-dependent and Raf-dependent but mitogen-activated protein kinase/extracellular signal-regulated kinase kinase-independent signaling pathways.

Cells transformed by the oncogenic small GTPase, Ras, display a radioresistant phenotype in response to ionizing radiation (IR). To determine the mechanisms by which Ras mediates radioresistance in epithelial cells, we assessed the importance of three major survival pathways that can be activated by Ras [phosphatidylinositol 3-kinase (PI3-K)>Akt, nuclear factor kappaB (NF-kappaB), and Raf>mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK)>extracellular signal-regulated kinase] as necessary or sufficient for Ras-mediated radioresistance in matched pairs of RIE-1 rat intestinal epithelial cells expressing oncogenic Ras or empty vector (RIE-Ras and RIE-vector). Inhibiting PI3-K with LY294002 sensitized RIE-1 cells to IR in a dose-dependent manner, indicating that PI3-K is necessary for radioresistance, whereas inhibition of NF-kappaB with the super-repressor IkappaBalpha had little effect on survival. Expression of either the constitutively active catalytic subunit of PI3-K, p110alpha-CAAX, or the Ras effector domain mutant 12V/40C, which retains binding to PI3-K but is impaired in binding to other Ras effectors, was sufficient to confer partial radioresistance. Expression of either a constitutively active form of the serine/threonine kinase Raf-1 or the Ras effector domain mutant 12V/35S, which retains binding to Raf but is impaired in binding to other Ras effectors, was also sufficient to confer partial radioresistance. Surprisingly, however, even complete inhibition of MEK activity by using U0126 resulted in no change in post-IR survival whatsoever. Thus, whereas Raf contributes to Ras-mediated radioresistance, this is accomplished through a MEK-independent pathway. Taken together, these results indicate that multiple pathways, including both PI3-K-dependent and Raf-dependent but MEK-independent signaling, are required for Ras-mediated radioresistance in epithelial cells. Finally, we demonstrate that Ras-mediated radioresistance can be uncoupled from Ras-mediated transformation, in that PI3-K is required for radioresistance but not transformation, whereas MEK and NF-kappaB are required for transformation but not radioresistance in RIE-1 epithelial cells.

Studying human disease genes in Caenorhabditis elegans: a molecular genetics laboratory project.

Scientists routinely integrate information from various channels to explore topics under study. We designed a 4-wk undergraduate laboratory module that used a multifaceted approach to study a question in molecular genetics. Specifically, students investigated whether Caenorhabditis elegans can be a useful model system for studying genes associated with human disease. In a large-enrollment, sophomore-level laboratory course, groups of three to four students were assigned a gene associated with either breast cancer (brc-1), Wilson disease (cua-1), ovarian dysgenesis (fshr-1), or colon cancer (mlh-1). Students compared observable phenotypes of wild-type C. elegans and C. elegans with a homozygous deletion in the assigned gene. They confirmed the genetic deletion with nested polymerase chain reaction and performed a bioinformatics analysis to predict how the deletion would affect the encoded mRNA and protein. Students also performed RNA interference (RNAi) against their assigned gene and evaluated whether RNAi caused a phenotype similar to that of the genetic deletion. As a capstone activity, students prepared scientific posters in which they presented their data, evaluated whether C. elegans was a useful model system for studying their assigned genes, and proposed future directions. Assessment showed gains in understanding genotype versus phenotype, RNAi, common bioinformatics tools, and the utility of model organisms.