Initial anterior-posterior (a-p) polarity in C. elegans is established at fertilization. A set of asymmetric cell divisions follow during which cell-fate determinants become localized differentially in anterior and posterior cells. As development proceeds a Wnt signal, MOM-2, functions to maintain and to propagate a common ap cellular polarity. At the 4-cell stage, Wnt signaling acts in parallel with signaling mediated by the tyrosine kinase pp60 Src to control both cell fate and cell division orientation in the endoderm precursor cell named EMS. The Wnt and Src pathways converge on several common components including WRM-1 (a beta-catenin related protein), LIT-1(a Serfrhr Kinase), and POP-1 an HMG-domain transcription factor. Recent genetic studies indicate that coordination of the cell-division axis with the polarized axis of the EMS cell depends on signaling mediated by the major cell-cycle regulator CDK-1 and its 13 kd binding partner CKS-1. Our findings suggest a model in which CDK-1 phosphorylates WRM-1 during mitosis to unmask cues that direct the rotation of the EMS spindle onto the a-p axis of the cell. The activated WRM-1 is then free to transduce to the nucleus of the posterior EMS daughter, called E, where it functions to down regulate and/or to promote nuclear export of the endoderm repressor, POP-1. The proposed studies will use cell-biological as well as biochemical methods to test the above model for endoderm induction. These studies will focus on the mechanisms of WRM-1 regulation by the Wnt, Src, and CDK-1 pathways. Protein interaction screens, suppressor-genetic screens, and conditional forward genetic screens will identify additional components of each signaling pathway. The localization of WRM-1 protein will be examined both in wild-type strains and in strains with defects in each signaling pathway, including strains that express specific mutant forms of WRM-1 itself. Proper control of cellular polarity is essential for the development and homeostasis of tissues in humans, and defects in this process are implicated in numerous forms of cancer. The significance of this work lies in the opportunity it provides to study important regulators of cellular polarity within a relatively simple and well characterized genetic model system.
