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.