HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY Mechanisms and implications of platelet discoid shape

The platelet marginal band consists of a single peripheral microtubule (MT) that is wound in 8 to 12 coils and maintains discoid cell shape. About 90 % of -tubu-lin in the marginal band is of the diver-gent, megakaryocyte (MK)/platelet-restricted 1 isoform. 1-tubulin–null mice show reduced proplatelet forma-tion, thrombocytopenia, and platelet spherocytosis. Here, we show that struc-tural abnormalities in resting 1-tubu-lin/ platelets include frequent kinks and breaks in the marginal band. Platelets derived from mice lacking the transcrip-tion factor GATA1 show similar defects, probably as a direct consequence of ab-sent 1-tubulin. 1-tubulin/ platelets have normal ratios of -tubulin isotypes but the marginal band is half the normal thickness, which is sufficient to maintain elliptical cell shape. Thus, a threshold 50 % or less of the normal amount of 1-tubulin is required to preserve mar-ginal band integrity and cell shape. 1-tubulin/ platelets have normal size and contents and show no defects in seroto-nin release or aggregation. Accordingly, the apparently isolated spherocytosis al-lows investigation of the role of discoid platelet shape in hemostasis. On agonist stimulation, the disorganized MTs in 1-tubulin/ platelets fail to condense into central rings and instead are dispersed in short bundles and linear arrays. Neverthe-less, intravital microscopy and flow cham-ber studies demonstrate full functionality of these spherocytic platelets under physiologic shear conditions. Together, these findings highlight the essential re-quirements of the MK/platelet-restricted 1-tubulin isoform in platelet structure and suggest that spherocytosis does not impair many aspects of platelet function