JCB: Report

Mammalian platelets are small cell fragments with neither nu­ cleus nor microtubule organizing center. They circulate in the blood stream to survey vascular integrity. In their quiescent state, they have a flat, discoid morphology caused by a micro­ tubular ring structure, called the marginal band (MB), in the pe­ riphery of resting platelets (White and Rao, 1998). Endothelial damage will induce platelet activation, which is composed of several successive events (Hartwig, 2006). Within seconds, they adopt a 3D, sphere­shaped morphology, extend filopodia, se­ crete the content of their granules, and spread on the subendo­ thelial layer or aggregate with surrounding platelets to form a hemostatic plug. In the activated platelet, a smaller microtubule ring structure is observed, and it has been suggested that acto­ myosin tension leads to the spherical shape and induces the cen­ tripetal contraction of the MB (White and Burris, 1984; Johnson et al., 2007). So far, microtubule motor proteins have not been implicated in the maintenance of the resting MB, the shape change, or the MB reorganization during platelet activation (Patel et al., 2005; Hartwig, 2006). MB microtubules in resting platelets are heavily acety­ lated (Patel­Hett et al., 2008). In general, prominent microtubule acetylation is observed when microtubule motor actions are im­ portant either for cellular transport processes (Reed et al., 2006; Dompierre et al., 2007) or for microtubule movements, as for instance, in mitotic spindles, primary cilia, or flagella (Piperno et al., 1987; Schatten et al., 1988). We therefore hypothesized that microtubule motor proteins might be functionally impor­ tant in platelets.