Intestinal epithelial cells can absorb food through their highly specialized apical surface, the so-called brush border, which is composed of many microvilli. Two processes are crucial for proper gut function: the polarization of enterocytes as well as the regulation of their number in the tissue. The polarity protein and tumor suppressor PAR-4/LKB1 has been shown to be involved in both processes. Its ectopic activation is sufficient to induce the formation of an apical brush border with microvilli-like structures in intestinal epithelial cancer cell lines. Moreover, mutations in the
lkb1 gene are responsible for the Peutz-Jeghers syndrome in which patients develop benign intestinal polyps. However, as this master kinase acts through various signaling pathways, it is essential to better characterize its role and its downstream effectors in intestinal cells in vivo. In order to do so, we used confocal and transmission electron microscopy to observe the intestinal epithelium in C. elegans
par-4 thermosensitive mutant embryos. Surprisingly, we have found that PAR-4 loss-of-function does not inhibit the formation of microvilli in intestinal cells, even if they present mild defects. However,
par-4 mutant embryos display extra intestinal cells, which lead to striking defects of the intestinal epithelium architecture, notably the appearance of strong intestinal lumen deformations. Lineage experiments revealed that PAR-4 does not control the number of enterocytes by regulating the cell cycle, but by controlling cell fate specification during embryogenesis. While the intestine exclusively arises from the E blastomere in wild type embryos, we observed that the C lineage also gives rise to intestinal cells in
par-4 mutants. We are currently investigating the molecular mechanisms underlying this novel function of PAR-4. We are in particular testing whether PAR-4 prevents intestinal specification in the C lineage by inhibiting the transcription factors SKN-1 and/or MED-1/2. Furthermore, we are also investigating the possible link between PAR-4 and two other kinases, PAR-1 and GSK-3, which control intestinal cell number and specification of the C lineage, respectively (our observations and Schlesinger et al., Genes Dev., 1999; Maduro et al., Mol Cell, 2001). Altogether this work will allow us to characterize a novel signaling pathway by which PAR-4 regulates cell fate specification. This appears to be essential to prevent intestinal hyperplasia and lumen deformations.