Epithelial cells perform their physiological functions by organizing into three-dimensional tissue structures. One of our laboratory focuses is on a protein called epimorphin, the temporal extracellular projection of which has been shown to control the overall structure of tissue architectures. Epimorphin lacks a signal peptide for secretion and the major population remains in the cytoplasmic surface of the membrane, where it functions as a membrane fusion mediator t-SNARE protein. Then, how this molecule translocates across the membrane and gets accessible to the target cells to elicit their morphogenic responses? By the detailed analyses of epitope-tagged epimorphin and its related family members, we found this year that 1) cellular damage or calcium influx leads to extracellular projection of a secretory complex containing epimorphin, annexin II and extravesicular domain of synaptotagmin, 2) extracellularly presented epimorphin is specifically cleaved between E245 and H246 in the SNARE domain and released toward the target cells, and 3) secreted epimorphin is captured by integrin cell surface receptors to activate focal adhesion kinase in the target cells. These results suggest a new model for consequence of epimorphin's extracellular action (Fig. 1). We are now trying to establish the functional relationship between epimorphin's intracellular membrane fusion function and extracellular morphoregulatory function (that apparently distinct roles are encoded in a single molecule should have a biological significance). Also, we will analyze the effect of extracellularly presented epimorphin on the multicellular arrangement and functional differentiation of keratinocyte in 3-D cultures using HaCaT model. (by Hirai, Y.)
Another research project currently working on is the analysis of regulatory mechanisms of tight junction (TJ) formation in keratinocyte. Recently, we found that the localization of ZO-1, one of the components of TJ, changes dramatically in HaCaT, human epidermal keratinocyte cell line, when this cell line is cultured with JNK inhibitor. Moreover claudin-4, another components of TJ, was newly phosphorylated during this process. To understand the biological significance of this claudin-4 phosphorylation, mutant claudin-4 proteins in which putative phosphorylated amino acid was substituted to alanine were introduced into HaCaT and examined the effects on TJ formation. Then, one of mutant claudin-4 molecule S195A in which 195th serine is substituted to alanine, showed dominant-negative effect on TJ formation. These results strongly suggest that TJ formation of HaCaT is regulated by the phosphorylation of claudin-4. Now, we are looking for the kinase which phosphorylates 195th serine of claudin-4 in HaCaT.　　(by Aono, S)