Severe bleeding in mice embryos lacking Kindlin-3 (Kind3^-/-).

From Nature Medicine, published online 17 February 2008 (doi:10.1038/nm1722).

Platelets aggregate at injury sites to seal damaged blood vessels and stop bleeding. Platelet integrins must be activated to ensure firm adhesion and aggregation, switching from a low- to a high-affinity state upon contact with a wounded vessel. In Nature Medicine, Reinhard Fässler and colleagues report that the Kindlin-3 protein activates platelet integrin in vivo by binding to its cytoplasmic tail.

Kindlin-3 (Kind3) is exclusively expressed in haematopoietic cells and localizes to integrin adhesion sites. Reinhard Fässler and colleagues observed that mice lacking Kindlin-3 (Kind3^-/-) died within one week of birth and showed severe bleeding due to impaired platelet function. The authors also found that loss of Kindlin-3 offered protection from vessel occlusion after mesenteric arteriole injury, which normally leads to thrombosis.

Platelet receptors interact with collagen and von Willebrand factor (vWF) at sites of blood vessel injury. Together with locally released thrombin and soluble platelet agonists, the platelet receptors trigger integrin _IIb3 activation and platelet aggregation. The authors showed that Kind3^-/- platelets failed to aggregate upon exposure to platelet agonists, but could still change from a discoid to a spherical shape, suggesting a specific defect in _IIb3-dependent aggregation rather than a general impairment of signalling. Antibody staining showed that integrin _IIb3 was no longer activated, which was confirmed by the inability of stimulated Kind3^-/- platelets to bind fibrinogen.

Integrin _IIb3 normally interacts with collagen-bound vWF, whereas integrin ₂₁ binds directly to collagen fibres. Kind3^-/- platelets were unable to stably adhere to collagen fibres, which, together with a loss of staining for activated ₁ integrin-specific antibodies, suggested that neither _IIb3 nor ₂₁ integrins were activated.

The cytoskeletal protein Talin interacts with the tail of integrin b through its phosphotyrosine-binding (PTB) domain - a final step in integrin _IIb3 activation that is common irrespective of the upstream signalling pathway. Similar to Talin, the presence of a region of Kindlin-3 containing a PTB domain is sufficient for integrin binding. However, unlike Talin, Kindlin-3 directly bound ₁ and ₃ tails at their distal rather than their proximal membrane residues.

Together the findings of Reinhard Fässler and colleagues highlight an essential role for Kindlin-3 in platelet integrin activation during haemostasis and thrombosis, which changes the current view of Talin being sufficient for integrin activation. As Kindlin-3 can activate both _IIb3 and ₂₁ integrins, it may be a general regulator of integrin function similar to Talin; however, the relative roles of Kindlin-3 and Talin, and the hierarchy of their binding to integrins remain to be determined.

Author: Kim Baumann