Biphasic reinforcement of nascent adhesions by vinculin.

Vinculin is an integral component of integrin adhesions, where it functions as a molecular clutch coupling intracellular contraction to the extracellular matrix. Quantitating its contribution to the reinforcement of newly forming adhesions, however, requires ultrasensitive cell force assays covering short time and low force ranges. Here, we have combined atomic force microscopy-based single-cell force spectroscopy (SCFS) and optical tweezers force spectroscopy to investigate the role of vinculin in reinforcement of individual nascent adhesions during the first 5 min of cell contact with fibronectin or vitronectin. At minimal adhesion times (5-10 s), mouse embryonic fibroblast (MEF) wildtype (wt) and vinculin knock-out (vin(-/-) ) cells develop comparable adhesion forces on the scale of several individual integrin-ligand bonds, confirming that vinculin is dispensable for adhesion initiation. In contrast, after 60 to 120 s, adhesion strength and traction reinforce quickly in wt cells, while remaining low in vin(-/-) cells. Re-expression of full-length vinculin or a constitutively active vinculin mutant (vinT12) in MEF vin(-/-) cells restored adhesion and traction with the same efficiency, while vinculin with a mutated talin-binding head region (vinA50I) or missing the actin-binding tail-domain (vin880) was ineffective. Integrating total internal reflection fluorescence imaging into the SCFS setup furthermore enabled us to correlate vinculin-green fluorescent protein (GFP) recruitment to nascent adhesion sites with the built-up of vinculin-dependent adhesion forces directly. Vinculin recruitment and cell adhesion reinforcement followed synchronous biphasic patterns, suggesting vinculin recruitment, but not activation, as the rate-limiting step for adhesion reinforcement. Combining sensitive SCFS with fluorescence microscopy thus provides insight into the temporal sequence of vinculin-dependent mechanical reinforcement in nascent integrin adhesions.

Def-6, a novel regulator of small GTPases in podocytes, acts downstream of atypical protein kinase C (aPKC) λ/ι.

The atypical protein kinase C (aPKC) isotypes PKCλ/ι and PKCζ are both expressed in podocytes; however, little is known about differences in their function. Previous studies in mice have demonstrated that podocyte-specific loss of PKCλ/ι leads to a severe glomerular phenotype, whereas mice deficient in PKCζ develop no renal phenotype. We analyzed various effects caused by PKCλ/ι and PKCζ deficiency in cultured murine podocytes. In contrast to PKCζ-deficient podocytes, PKCλ/ι-deficient podocytes exhibited a severe actin cytoskeletal phenotype, reduced cell size, decreased number of focal adhesions, and increased activation of small GTPases. Comparative microarray analysis revealed that the guanine nucleotide exchange factor Def-6 was specifically up-regulated in PKCλ/ι-deficient podocytes. In vivo Def-6 expression is significantly increased in podocytes of PKCλ/ι-deficient mice. Cultured PKCλ/ι-deficient podocytes exhibited an enhanced membrane association of Def-6, indicating enhanced activation. Overexpression of aPKCλ/ι in PKCλ/ι-deficient podocytes could reduce the membrane-associated expression of Def-6 and rescue the actin phenotype. In the present study, PKCλ/ι was identified as an important factor for actin cytoskeletal regulation in podocytes and Def-6 as a specific downstream target of PKCλ/ι that regulates the activity of small GTPases and subsequently the actin cytoskeleton of podocytes.

Deletion of the cell adhesion adaptor protein vinculin disturbs the localization of GFAP in Bergmann glial cells.

Astrocytes operate in close spatial relationship to other cells including neurons. Structural interaction is controlled by a dynamic interplay between actin-based cell motility and contact formation via cell-cell and cell-extracellular matrix adhesions. A central player in the control of cell adhesion is the cytoskeletal adaptor protein Vinculin. Incorporation of Vinculin affects mechanical properties and turnover of cell adhesion sites. To study the in vivo function of Vinculin in astrocytes, a mouse line with astrocyte specific and inducible deletion of vinculin was generated. Deletion of vinculin decreased the expression of the glial acidic fibrillary protein (GFAP) in Bergmann glial cells in the cerebellum. In addition, localization of GFAP to Bergmann glial endfeet was disturbed, indicating a role for vinculin in controlling its expression and localization. In contrast, vimentin expression, morphology, activation state and polarity of the targeted cells as well as the localization of the extracellular matrix protein laminin was not compromised. Furthermore, stab wound lesions were performed in the cerebellar cortex. In both wildtype and vinculin knockout mice GFAP expression was upregulated in Bergmann glial cells of the lesioned area with no differences observed between genotypes in expression and localization of GFAP. These results propose a selective requirement for vinculin in cellular events related to cell adhesion in vivo. As in vitro data suggested a major role for vinculin in the control of the cytoskeletal connection affecting mechanical stability and cell motility, our data add a note of caution to the extrapolation of in vitro data to in vivo function.

The vinculin-DeltaIn20/21 mouse: characteristics of a constitutive, actin-binding deficient splice variant of vinculin.

BACKGROUND: The cytoskeletal adaptor protein vinculin plays a fundamental role in cell contact regulation and affects central aspects of cell motility, which are essential to both embryonal development and tissue homeostasis. Functional regulation of this evolutionarily conserved and ubiquitously expressed protein is dominated by a high-affinity, autoinhibitory head-to-tail interaction that spatially restricts ligand interactions to cell adhesion sites and, furthermore, limits the residency time of vinculin at these sites. To date, no mutants of the vinculin protein have been characterized in animal models. METHODOLOGY/PRINCIPAL FINDINGS: Here, we investigate vinculin-DeltaEx20, a splice variant of the protein lacking the 68 amino acids encoded by exon 20 of the vinculin gene VCL. Vinculin-DeltaEx20 was found to be expressed alongside with wild type protein in a knock-in mouse model with a deletion of introns 20 and 21 (VCL-DeltaIn20/21 allele) and shows defective head-to-tail interaction. Homozygous VCL-DeltaIn20/21 embryos die around embryonal day E12.5 showing cranial neural tube defects and exencephaly. In mouse embryonic fibroblasts and upon ectopic expression, vinculin-DeltaEx20 reveals characteristics of constitutive head binding activity. Interestingly, the impact of vinculin-DeltaEx20 on cell contact induction and stabilization, a hallmark of the vinculin head domain, is only moderate, thus allowing invasion and motility of cells in three-dimensional collagen matrices. Lacking both F-actin interaction sites of the tail, the vinculin-DeltaEx20 variant unveils vinculin's dynamic binding to cell adhesions independent of a cytoskeletal association, and thus differs from head-to-tail binding deficient mutants such as vinculin-T12, in which activated F-actin binding locks the protein variant to cell contact sites. CONCLUSIONS/SIGNIFICANCE: Vinculin-DeltaEx20 is an active variant supporting adhesion site stabilization without an enhanced mechanical coupling. Its presence in a transgenic animal reveals the potential of splice variants in the vinculin gene to alter vinculin function in vivo. Correct control of vinculin is necessary for embryonic development.