β1 integrin−mediated spreading and outside-in signaling is independent of kindlin-integrin interaction. (A) Spreading of thrombin-stimulated platelets on soluble collagen in the absence or presence of 0.75 mM Mn²⁺ and on fibrinogen. Representative pictures are shown 45 minutes after platelet seeding (scale bar represents 5 μm). (B) Spreading area of mutant platelets on soluble collagen is shown relative to wild-type (WT) platelets at 45 minutes (bars represent mean values ± SEM; significance level are indicated; *P < .05; **P < .01). (C) Integrin outside-in signaling was evaluated by FAK autophosphorylation at Tyr379. Washed platelets from wild-type (WT), β1 knockout (KO), and TTAA mice as well as from kindlin-3−deficient mice (K3^(−/−)) were stimulated with 5 μg/mL fibrillar collagen and seeded on soluble collagen-coated surfaces in the absence or presence of Mn²⁺. To analyze basal FAK phosphorylation, unstimulated platelets were seeded on BSA. Cells were lysed after 30 minutes and subjected to immunoblotting for FAK Y397 phosphorylation and total FAK.