Multi-Layered hypertrophied MEE formation by microtubule disruption via GEF-H1/RhoA/ROCK signaling pathway

Background: Formation of the secondary palate is complex and disturbance during palatal fusion may result in cleft palate. The processes of adhesion, intercalation, and disappearance of medial edge epithelia (MEE) are characterized by morphological changes requiring dynamic cytoskeletal rearrangement. Microtubules are one of the cytoskeletal elements involved in maintenance of cell morphology. Microtubule‐disrupting drugs have been reported to cause craniofacial malformations including cleft palate. The mechanisms underlying the failure of palatal fusion remain poorly understood. We evaluated the effect of nocodazole (NDZ), a drug that disrupts microtubules, on palatal fusion in organ culture. Results: NDZ caused failure of palatal fusion due to the induction of a multi‐layered hypertrophied MEE in the mid‐region of the secondary palatal shelves. Microtubule disruption increased RhoA activity and stress fiber formation. Pharmacological inhibition of the RhoA/ROCK pathway blocked multi‐layered MEE formation. Partial prevention of hypertrophied MEE was observed with Y27632 and cytochalasin, but not with blebbistatin. NDZ induced re‐localization of GEF‐H1 into cytoplasm from cell–cell junctions. Conclusions: The present study provided evidence that the GEF‐H1/RhoA/ROCK pathway plays a pivotal role in linking microtubule disassembly to the remodeling of the actin cytoskeleton, which resulted in a multi‐layered hypertrophied MEE and failure of palatal fusion. Developmental Dynamics 241:1169–1182, 2012. © 2012 Wiley Periodicals, Inc. Key findings: Microtubule disassembly causes failure of palatal fusion due to multi‐layered hypertrophied MEE formation in the middle region of the palatal shelves. Nocodazole activates RhoA/ROCK signaling pathway and stress fiber formation in MEE Multi‐layered hypertrophied MEE formation is not dependent on TGF‐β/Smad signaling. GEF‐H1/RhoA/ROCK signaling pathway is involved in multi‐layred hypertrophied MEE formation.