Proplatelet formation: IIb or not IIb?

Comment on Larson and Watson, page 1509

Findings by Larson and Watson suggest a role for platelet GPIIb in murine proplatelet formation through a mechanism that is independent of classic outside-in integrin signaling networks.

Phillips and Agin¹  established the currently accepted Roman numeral nomenclature for platelet surface proteins in 1977, and none is more storied than platelet glycoprotein IIb (GPIIb). Also called integrin α_IIb, GPIIb plays a critical role in hemostasis through its function as the primary platelet fibrinogen receptor, has been a popular and successful target for antiplatelet drugs, and its absence leads to the inherited bleeding disorder Glanzmann thrombasthenia. Larson and Watson suggest that α_IIb may also play an important role in the initiation of proplatelet formation.

Growing evidence suggests that platelets are generated from megakaryocytes through the formation of cytoplasmic extensions called proplatelets. Mature platelets are released from the ends of the proplatelets, and proplatelet extensions undergo extensive branching that increases the number of ends available for platelet release.²  Recent reports have shown that proplatelet extensions have a complex and highly ordered structure that allows them to function much like a railway system transporting platelet contents from the body of the megakaryocytes to the distal, maturing platelets.³  Despite such elegant cell biology, we remain largely uninformed regarding what triggers and regulates proplatelet formation.

Using simple in vitro proplatelet assays, Larson and Watson observed that fibrinogen-coated surfaces promote megakaryocyte proplatelet formation, whereas laminin-, vitro-nectin-, and collagen-coated surfaces do not. Proplatelet formation on fibrinogen-coated surfaces was significantly reduced in the presence of an α_IIbβ₃ antagonist or blocking antibody, and they were not detectable in megakaryocytes from α_IIb^-/- mice, implicating a critical role for α_IIb in fibrinogen-stimulated proplatelet formation. Supporting these in vitro studies, TPO injection into α_IIb^-/- mice yielded a blunted rise in circulating platelets relative to wild-type mice. Buoyed by recent reports implicating the importance of regional bone marrow compartments in the release of platelets from megakaryocytes,⁴  the authors used immunostaining to localize bone marrow fibrinogen. Interestingly, they found that it largely colocalized with sinusoidal endothelium, the site where platelets are thought to be released into circulation and the location where megakaryocytes apparently localize during SDF-1-stimulated platelet release.⁴ 

While α_IIb is one member of the integrin α_IIbβ₃ heterodimer, perturbations of classic integrin outside-in signaling did not impair fibrinogen-induced proplatelet formation. In fact, the Src family kinase inhibitor PP2 potentiated proplatelet formation on fibrinogen-coated surfaces, PLCγ2^-/- murine megakaryocytes formed proplatelets on fibrinogen-coated surfaces more efficiently than did wild-type megakaryocytes, and inhibition of intracellular Ca⁺⁺ elevation did not diminish fibrinogen-induced proplatelet formation, yet all of these maneuvers are expected to impair classic integrin outside-in signaling.

Larson and Watson remind us that developing hematopoietic cells, and not just stem cells, have an environment that plays a functional role in the biology of the cell, and, like most interesting work, their report raises more questions than it answers. What is the biologic significance of their findings? Why do patients with Glanzmann thrombasthenia have normal platelet counts? Is the fibrinogen-α_IIb interaction important for human proplatelet formation? Investigating these and related mechanistic questions should help us understand what triggers platelet formation from human megakaryocytes. ▪