Microtubule-Driven Conformational Changes in Platelet Morphogenesis

Abstract

Blood platelets form from mature megakaryocytes in a complicated multi-stage process that remains poorly understood. During the final stages of platelet formation, it is hypothesized that platelet-precursors, called preplatelets, undergo a microtuble-driven structural transformation from spherical to bar-bell shaped, before eventually separating into platelets. Initially, a microtubule band forms at the equator of the preplatelet. As sliding microtubules extend the contour length of the band, the preplatelet membrane deforms to accommodate the longer band. Eventually, the extensional force generated by the microtubules is overpowered by the force required to bend the preplatelet cortex, and the microtubule band is forced to twists upon itself to continue elongating without further increasing the bending energy of the membrane. The conformation shift of the microtubule is accompanied by a transformation in the shape of the preplatelet from a spheroid to an elongated bar-bell. In addition, the BAR-domain-possessing protein CIP4 has recently been found to serve a multi-functional role during platelet formation by inducing membrane curvature, altering membrane mechanical properties, and recruiting the cleaving protein dynamin to regions of high curvature. However the effects of CIP4 on the microtubule-platelet membrane system during microtubule elongation remain unknown. To elucidate this interplay, we employ a combination of variational techniques and Monte Carlo simulations. Using these methods, we calculate the equilibrium shapes of the microtubule-membrane system as a function of microtubule-band length, and investigate how these conformations depend on the extensional force generated by the microtubules, the mechanical properties of the membrane, and the concentration CIP4. The results are used to outline conditions leading to thrombocytopenia (reduced platelet count) and thrombocytosis (elevated platelet count). Finally, the extension of the model to other stages of platelet formation is discussed.