Extranuclear sequestration of phospho-Jun N-terminal kinase and distorted villi produced by activated Rac1 in the intestinal epithelium of chimeric mice

Previously, we used a genetic mosaic system to conduct an in vivo analysis of the effects of Rad activation on the developing intestinal epithelium (S tappenbeck, T. S, and Gordon, J, I. (2000) Development 127, 2629-2642), Exp ression of a constitutively active human Rad (Rac1Leu61) in the 129/Sv-deri ved small intestinal epithelium of C57Bl/6-ROSA26 <----> 129/Sv chimeric mi ce led to precocious differentiation of some lineages with accompanying alt erations in their apical actin, We have now explored the underlying mechani sms. Rac1Leu61 leads to accumulation of the 46 kDa form of phosphorylated J un N-terminal kinase (p-Jnk) in the apical cytoplasm, but not in the nucleu s of E18,5 proliferating and differentiating intestinal epithelial cells. T he effect is cell-autonomous, selective for this mitogen-activated protein kinase family member, and accompanied by apical cytoplasmic accumulation of p21-activated kinase, c-Jun, downstream nuclear target of p-Jnk, does not show evidence of enhanced phosphorylation, providing functional evidence fo r cytoplasmic sequestration of p-Jnk in Rac1Leu61-expressing epithelium. In adult chimeras, Rad activation augments cell proliferation in crypts of Li eberkuhn, without a compensatory change in basal apoptosis and produces a d ramatic, very unusual widening of villi, These results reveal a novel in vi vo paradigm for Rad activation involving p-Jnk-mediated signaling at a dist inctive extra-nuclear site, with associated alterations in the actin cytosk eleton, They also provide a new perspective about the determinants of small intestinal villus morphogenesis.