P58. The biphasic function of Wnt signaling during neural differentiation of mouse embryonic stem cells

Abstract

Activation of Wnt signaling enhances self-renewal of mouse embryonic and neural stem/progenitor cells. However, contrary to these results, undifferentiated ES cells shows very low level of endogenous Wnt signaling and ectopic activation of Wnt signaling blocks neural differentiation. Therefore it is not clear whether or when endogenous Wnt/β-catenin signaling is necessary for self-renewal or neural differentiation of ES cells. When we examined the expression profile of Wnt signaling components, expressions of most Wnts are very low in undifferentiated ES cells while some of Wnts are increased during differentiation. We generated stable ES cell lines that probe Wnt signaling activity and these cells showed that Wnt signaling was very low in undifferentiated ES cells while increased during embryonic body formation or neural differentiation. However, conditioned media from differentiated ES cells did not induce higher reporter activity than that of undifferentiated ES cells, which suggests that ES cells may change the sensitivity to Wnt signaling during differentiation whereas the levels of secreted Wnts are same. Changes in the composition of Tcfs or Lef1 in ES cells at different conditions may be a possible mechanism. In addition, we observed that activation of Wnt/β-catenin signaling at different time periods resulted in differential effects on neural differentiation of 46C ES cells that show increased Sox1 promoter driven GFP during neural differentiation; activation of Wnt signaling by the treatment with BIO (a GSK3β inhibitor) in early or late period blocks or enhances neural differentiation, respectively. Consistent with these results, inactivation of Wnt/β-catenin signaling in late period by the treatment with endo-IWR1 (an Axin stabilizer) blocked neural differentiation. Overall, our data suggest that Wnt/β-catenin signaling plays biphasic roles during neural differentiation of mouse embryonic stem cells. Supported by the National Research Foundation of Korea (2006-2004046) and Brain Korea 21 program.