PAPC couples the segmentation clock to somite morphogenesis by regulating N-cadherin dependent adhesion

Vertebrate segmentation is characterized by the periodic formation of epithelial somites from the mesenchymal presomitic mesoderm (PSM). How the rhythmic signaling pulse delivered by the Segmentation Clock is translated into the periodic morphogenesis of somites remains poorly understood. Here, we focused on the role of Paraxial protocadherin (PAPC/Pcdh8) in this process. We showed that in chicken and mouse embryos, PAPC expression is tightly regulated by the Clock and Wavefront system in the posterior PSM. We observed that PAPC exhibits a striking complementary pattern to N-Cadherin (CDH2), marking the interface of the future somite boundary in the anterior PSM. Gain and loss of function of PAPC in chicken embryos disrupt somite segmentation by altering the CDH2-dependent epithelialization of PSM cells. Our data suggest that clathrin mediated endocytosis is increased in PAPC expressing cells, subsequently affecting CDH2 internalization in the anterior compartment of the future somite. This in turn generates a differential adhesion interface, allowing formation of the acellular fissure that defines the somite boundary. Thus periodic expression of PAPC in the anterior PSM triggers rhythmic endocytosis of CDH2, allowing for segmental de-adhesion and individualization of somites.