Platelet actin nodules are podosome-like structures dependent on Wiskott-Aldrich Syndrome protein and ARP2/3 complex

The actin nodule is a novel F-actin structure present in platelets during early spreading. However, only limited detail is known regarding nodule organisation and function. Here we use electron-microscopy, SIM and dSTORM super-resolution, and live-cell TIRF microscopy to characterise the structural organisation and signalling pathways associated with nodule formation. Nodules are composed of up to 4 actin-rich structures linked together by actin bundles. They are enriched in the adhesion-related proteins talin and vinculin, have a central core of tyrosine phosphorylated proteins and are depleted of integrins at the plasma membrane. Nodule formation is dependent on Wiskott-Aldrich Syndrome protein (WASp) and the ARP2/3 complex. WASp-/-mouse blood displays impaired platelet aggregate formation at arteriolar shear rates. We propose actin nodules are platelet podosome-related structures required for platelet-platelet interaction and their absence contributes to the bleeding diathesis of Wiskott-Aldrich Syndrome.