Membrane-tethered syntaxin-4 locally abrogates E-cadherin function and activates Smad signals, contributing to asymmetric mammary epithelial morphogenesis

A crucial stem cell plasticity regulation pathway: identification of key elements using the NCCIT human embryonic carcinoma cell line

Upon removal of stemness factors, a small subpopulation of embryonic stem cells (ESCs) spontaneously extrudes the t-SNARE protein syntaxin-4, which upregulates the cell adhesion molecule P-cadherin and induces the onset of epithelial-mesenchymal transition (EMT)-like behaviors with loss of stemness in each cell. In this study, we identified a series of molecular elements responsible for this phenomenon using several small-molecule inhibitors and the human embryonic carcinoma cell line, NCCIT. We found that the syntaxin-4-triggered morphological changes and a decrease in stemness signatures were independently induced by the activation of Rho-associated kinase (ROCK) and the abrogation of PI3K/Akt signaling. We also found that the extracellular expression of syntaxin-4 inactivated focal adhesion kinase (FAK) in association with the augmented expression of P-cadherin, and comparable controls of either of these downstream elements of syntaxin-4 accelerated both ROCK-induced F-actin stress fiber formation and P13K/Akt-suppressed loss of stemness signatures. Cells expressing P-cadherin inactivated FAK but FAK inhibition did not affect P-cadherin expression, demonstrating a causal relationship between P-cadherin and FAK in the event of syntaxin-4 induction. These results reveal a novel signaling axis in stem cells and shed new light on the crucial elements for stem cell plasticity and the maintenance of stemness.

Cooperation of membrane-translocated syntaxin4 and basement membrane for dynamic mammary epithelial morphogenesis

Mammary epithelia undergo dramatic morphogenesis after puberty. During pregnancy, luminal epithelial cells in ductal trees are arranged to form well-polarized cystic structures surrounded by a myoepithelial cell layer, an active supplier of the basement membrane (BM). Here, we identified a novel regulatory mechanism involved in this process by using a reconstituted BM-based three-dimensional culture and aggregates of a model mouse cell line, EpH4, that had either been manipulated for inducible expression of the t-SNARE protein syntaxin4 in intact or signal peptide-connected forms, or that were genetically deficient in syntaxin4. We found that cells extruded syntaxin4 upon stimulation with the lactogenic hormone prolactin, which in turn accelerated the turnover of E-cadherin. In response to extracellular expression of syntaxin4, cell populations that were less affected by the BM actively migrated and integrated into the cell layer facing the BM. Concurrently, the BM-facing cells, which were simultaneously stimulated with syntaxin4 and BM, acquired unique epithelial characteristics to undergo dramatic cellular arrangement for cyst formation. These results highlight the importance of the concerted action of extracellular syntaxin4 extruded in response to the lactogenic hormone and BM components in epithelial morphogenesis.