We established that cells from Ewing tumors form neurites in response to Wnt-3a and have begun to define the mechanisms that account for this effect. Frizzled3 (Fzd3) was identified as the primary Wnt receptor that mediates the process, which also requires Dishevelled-2 (Dvl-2), Dishevelled-3 (Dvl-3), and amino-terminal c-Jun kinase (JNK). We showed that Dickkopf-1 also promotes neurite outgrowth in these cells, apparently by facilitating Fzd3/JNK activation by endogenous Wnts. Neurite outgrowth induced by Wnt-3a was associated with Dvl-2/3 phosphorylation;both neurite formation and Dvl phosphorylation were blocked by the casein kinase I delta/epsilon (CKId/e) inhibitor, IC261. Knockdown of CKId with small interfering RNA suppressed Wnt-3a-dependent neuritogenesis, whereas knockdown of CKIe stimulated neurite formation in the absence of exogenous Wnt-3a. The contrasting effects of CKId and CKIe on neurite outgrowth might be due to the preferential localization of CKId at the centrosome, which is thought to have a critical role in neurite formation. Knockdown of the atypical PKCiota also blocked Wnt-3a-dependent neurite outgrowth. Preliminary experiments suggest that PKCiota may be regulated by CKId and/or Dvl. This work is significant not only because it provides insights about mechanisms involved in the formation of neurites. Many of the factors that participate in neurite outgrowth also have important roles in the formation of cellular extensions critical for cell migration. The information obtained in the Ewing tumor cell model is likely to enhance our understanding of cell movement in physiological contexts and metastasis. Moreover, we have determined that CKId and Dvl also are required for the formation of primary cilia. Defective primary cilia are responsible for several disorders including neural tube defects, polycystic kidney disease and situs inversus. Aberrant Wnt signaling also can elicit these abnormalities. Thus, our studies of CKId and Dvl should extend our knowledge about the ways in which Wnt signaling controls embryonic development.
