Bradykinin induces formation of vesicle-like structures containing vinculin and PtdIns(4,5)P2 in renal papillary collecting duct cells

The inhibition of sphingomyelin synthase 1 activity induces collecting duct cells to lose their epithelial phenotype

Epithelial tissue requires that cells attach to each other and to the extracellular matrix by the assembly of adherens junctions (AJ) and focal adhesions (FA) respectively. We have previously shown that, in renal papillary collecting duct (CD) cells, both AJ and FA are located in sphingomyelin (SM)-enriched plasma membrane microdomains. In the present work, we investigated the involvement of SM metabolism in the preservation of the epithelial cell phenotype and tissue organization. To this end, primary cultures of renal papillary CD cells were performed. Cultured cells preserved the fully differentiated epithelial phenotype as reflected by the presence of primary cilia. Cells were then incubated for 24h with increasing concentrations of D609, a SM synthase (SMS) inhibitor. Knock-down experiments silencing SMS 1 and 2 were also performed. By combining biochemical and immunofluorescence studies, we found experimental evidences suggesting that, in CD cells, SMS 1 activity is essential for the preservation of cell-cell adhesion structures and therefore for the maintenance of CD tissue/tubular organization. The inhibition of SMS 1 activity induced CD cells to lose their epithelial phenotype and to undergo an epithelial-mesenchymal transition (EMT) process.

A Structural Model for Vinculin Insertion into PIP2-Containing Membranes and the Effect of Insertion on Vinculin Activation and Localization

Vinculin, a scaffolding protein that localizes to focal adhesions (FAs) and adherens junctions, links the actin cytoskeleton to the adhesive super-structure. While vinculin binds to a number of cytoskeletal proteins, it can also associate with phosphatidylinositol 4,5-bisphosphate (PIP₂) to drive membrane association. To generate a structural model for PIP₂-dependent interaction of vinculin with the lipid bilayer, we conducted lipid-association, nuclear magnetic resonance, and computational modeling experiments. We find that two basic patches on the vinculin tail drive membrane association: the basic collar specifically recognizes PIP₂, while the basic ladder drives association with the lipid bilayer. Vinculin mutants with defects in PIP₂-dependent liposome association were then expressed in vinculin knockout murine embryonic fibroblasts. Results from these analyses indicate that PIP₂ binding is not required for localization of vinculin to FAs or FA strengthening, but is required for vinculin activation and turnover at FAs to promote its association with the force transduction FA nanodomain.

Mechanisms and Functions of Vinculin Interactions with Phospholipids at Cell Adhesion Sites

The cytoskeletal protein vinculin is a major regulator of cell adhesion and attaches to the cell surface by binding to specific phospholipids. Structural, biochemical, and biological studies provided much insight into how vinculin binds to membranes, what components it recognizes, and how lipid binding is regulated. Here we discuss the roles and mechanisms of phospholipids in regulating the structure and function of vinculin and of its muscle-specific metavinculin splice variant. A full appreciation of these processes is necessary for understanding how vinculin regulates cell motility, migration, and wound healing, and for understanding of its role in cancer and cardiovascular diseases.

Physiologically induced restructuring of focal adhesions causes mobilization of vinculin by a vesicular endocytic recycling pathway

In epithelial cells, vinculin is enriched in cell adhesion structures but is in equilibrium with a large cytosolic pool. It is accepted that when cells adhere to the extracellular matrix, a part of the soluble cytosolic pool of vinculin is recruited to specialized sites on the plasma membrane called focal adhesions (FAs) by binding to plasma membrane phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2). We have previously shown that bradykinin (BK) induces both a reversible dissipation of vinculin from FAs, by the phospholipase C (PLC)-mediated hydrolysis of PtdIns(4,5)P2, and the concomitant internalization of vinculin. Here, by using an immunomagnetic method, we isolated vinculin-containing vesicles induced by BK stimulation. By analyzing the presence of proteins involved in vesicle traffic, we suggest that vinculin can be delivered in the site of FA reassembly by a vesicular endocytic recycling pathway. We also observed the formation of vesicle-like structures containing vinculin in the cytosol of cells treated with lipid membrane-affecting agents, which caused dissipation of FAs due to their deleterious effect on membrane microdomains where FAs are inserted. However, these vesicles did not contain markers of the recycling endosomal compartment. Vinculin localization in vesicles has not been reported before, and this finding challenges the prevailing model of vinculin distribution in the cytosol. We conclude that the endocytic recycling pathway of vinculin could represent a physiological mechanism to reuse the internalized vinculin to reassembly new FAs, which occurs after long time of BK stimulation, but not after treatment with membrane-affecting agents.