The discoid shape of the resting blood platelet is maintained by its marginal microtubule band. Structural studies have concluded that this band is composed of a single microtubule coiled 8-12 times around the cell periphery. To understand the dynamics of the microtubule coil, we took advantage of EB1 and EB3, proteins that highlight the ends of growing microtubules. Immunofluorescence microscopy with anti-EB1 revealed clear staining of numerous (8.7 +/− 2.0, range 4–12) comet-like dashes in the microtubule coil, suggesting the presence of several microtubule plus ends. Consistent with this observation, rhodamine-tubulin added to permeabilized platelets incorporates at multiple (7.9 +/−1.9) points throughout the microtubule coil. To visualize microtubule dynamics in platelets, we retrovirally directed megakaryocytes to express the microtubule plus-end marker EB3-GFP and isolated platelets released in these cultures. Fluorescence time-lapse microscopy of EB3-GFP-expressing resting platelets revealed multiple microtubule plus ends that grew in both clockwise and counterclockwise directions. Antibodies that recognize tyrosinated tubulin, which preferentially label newly assembled microtubules and not stable microtubules, stain the microtubule coil. These results indicate that resting platelets contain a bipolar array of microtubules that undergoes continuous assembly. When EB3-GFP-expressing platelets are activated with thrombin, the number of polymerizing microtubules increases dramatically and the microtubules grow into filopodia. Collectively, these results suggest that the marginal band of the resting blood platelet is highly dynamic, bipolar, and contains multiple microtubule plus ends. These ends are amplified in platelet activation and point towards the active edges of the cells and the tips of filopodia.

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