John Hartwig, Ph.D.

Professor of Medicine

Brigham & Women’s Hospital
Harvard Medical School
One Blackfan Circle
Karp Family Research Building, 6th Floor
Boston, MA 02115

Tel 617 355 9006
Fax 617 355 9016
hartwig@rics.bwh.harvard.edu

> Selected Papers


Research Overview
Our laboratory is focused on defining the: (1) structure and mechanics of platelet shape change and its cytoskeletal dynamics; (2) signal pathways that regulate actin assembly; (3) mechanical events that lead to the formation of blood platelets and; (4) mechanisms that remove damaged and senile platelets from blood. Platelets, which circulate as small discs in blood, are shed from the ends of megakaryocyte proplatelets. Platelet formation and release from the proplatelet ends is driven by the sliding of cytoplasmic microtubule within bundles that run into and loop at the ends of the proplatelets. Platelet discoid shape is maintained by a cortical microtubule ring that resides beneath the plasma membrane on the edges of the disc face. This ring is the residue of the microtubule bundles of the proplatelet. When vascular damage is detected, platelets rapidly first convert into spidery shapes having long filopods and subsequently adhere tightly to the damaged surface by extending circumferential lamellae to efficiently cover the damaged surface. Platelet shape change is driven by actin rearrangements that first requires the disruption of cortical actin filaments used in the resting cell to interconnect the spectrin-based skeleton and support the plasma membrane of the resting cell, followed by a robust actin assembly reaction that fills filopods and lamellae with filaments. Disassembly of the membrane skeleton is mediated by phosphorylation and the dissociation of adducin from ends of spectrin and the barbed ends of actin filaments, and by calcium activation of gelsolin, which binds to and severs cortical actin filaments. Regulation of the actin assembly reaction is driven primarily by the small GTPase, rac1, which recruits the essential actin regulatory proteins and activates lipid kinases to synthesize polyphosphoinositides, which spatially collect actin regulatory proteins at the plasma membrane and regulate their activities. Key molecules in the actin assembly reaction are gelsolin, cofilin, and the Arp2/3 complex. The relationship of the upstream WASP family of Arp2/3 regulators to platelet function remains poorly understood. WASP confers survival advantage to blood platelets, as most circulating platelets from female mice carriers for WASP deficiency express WASP and platelets lacking WASP, or its constitutive binding partner WIP, are rapidly removed from the circulation of wild-type mice. Although actin responses in WASP null platelets are normal, other known regulators of Arp2/3 complex function, such as the WAVE proteins and cortactin, may contribute to platelet actin assembly downstream of Rac. Lack of WASP expression may alter the surface of platelets to target them for phagocytic clearance, independent of its known role in Arp2/3 complex activation. Senile, damaged, and diseased platelets are removed in the spleen and liver by phagocytes by mechanisms we are defining.