Sphingosine 1-phosphate activates the MAP3K1-JNK pathway to promote epithelial movement and morphogenesis

Abstract MAP 3 kinase 1 (MAP3K1) plays an essential role in embryonic eyelid development. It regulates epithelial morphogenesis through the spatial-temporal activation of Jun N-terminal kinases (JNKs), resulting in forward progression of the embryonic eyelid epithelial cells to enable eyelid closure. The developmental signals that activate the MAP3K1-JNK pathway are still unknown, mainly due to the lack of suitable keratinocyte lines to elucidate the mechanisms of pathway regulation. To address this deficiency, we developed a straightforward method for long-term culture of mouse keratinocytes in feeder-free conditions using Ca 2+ -free media. Cells grown under these conditions displayed characteristic basal epithelial morphology and keratin 14 expression, but did not form tight- or adherens-junctions. Increased extracellular Ca 2+ levels restored the formation of cell-cell junctions. Using keratinocyte lines derived from wild type and Map3k1 -deficient mice, we found that sphingosine 1-phosphate (S1P) activated the JNK-c-JUN pathways in a manner dependent on MAP3K1 kinase activity and that this MAP3K1-mediated signaling led to epithelial cell migration. The in vivo roles of this pathway were examined through crossing of genetic mutant mice. Loss-of-function of the S1P receptor (S1pr) 2/ 3 became haploinsufficient only when combined with Map3k1 and Jnk1 mutations such that the compound mutants displayed eyelid closure defects, suggesting these gene products cooperated in eye morphogenesis. Results of this work establish the S1PR-MAP3K1-JNK pathway as a crucial signaling mechanism for epithelial cell movement and morphogenesis.