Extranuclear sequestration of phospho-Jun N-terminal kinase and distorted villi produced by activated Rac1 in the...

Previously, we used a genetic mosaic system to conduct an in vivo analysis of the effects of Rac1 activation on the developing intestinal epithelium ( Stappenbeck, T. S. and Gordon, J. I. (2000) Development 127, 2629-2642). Expression of a constitutively active human Rac1 (Rac1Leu61) in the 129/Sv-derived small intestinal epithelium of C57Bl/6-ROSA26<-->129/Sv chimeric mice led to precocious differentiation of some lineages with accompanying alterations in their apical actin. We have now explored the underlying mechanisms. Rac1Leu61 leads to accumulation of the 46 kDa form of phosphorylated Jun N-terminal kinase (p-Jnk) in the apical cytoplasm, but not in the nucleus of E18.5 proliferating and differentiating intestinal epithelial cells. The 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, a downstream nuclear target of p-Jnk, does not show evidence of enhanced phosphorylation, providing functional evidence for cytoplasmic sequestration of p-Jnk in Rac1Leu61-expressing epithelium. In adult chimeras, Rac1 activation augments cell proliferation in crypts of Lieberkühn, without a compensatory change in basal apoptosis and produces a dramatic, very unusual widening of villi. These results reveal a novel in vivo paradigm for Rac1 activation involving p-Jnk-mediated signaling at a distinctive extra-nuclear site, with associated alterations in the actin cytoskeleton. They also provide a new perspective about the determinants of small intestinal villus morphogenesis.