Platelet alpha-granules: basic biology and clinical correlates

Abstract

alpha-Granules are essential to normal platelet activity. These unusual secretory granules derive their cargo from both regulated secretory and endocytotic pathways in megakaryocytes. Rare, inheritable defects of alpha-granule formation in mice and man have enabled identification of proteins that mediate cargo trafficking and alpha-granule formation. In platelets, alpha-granules fuse with the plasma membrane upon activation, releasing their cargo and increasing platelet surface area. The mechanisms that control alpha-granule membrane fusion have begun to be elucidated at the molecular level. SNAREs and SNARE accessory proteins that control alpha-granule secretion have been identified. Proteomic studies demonstrate that hundreds of bioactive proteins are released from alpha-granules. This breadth of proteins implies a versatile functionality. While initially known primarily for their participation in thrombosis and hemostasis, the role of alpha-granules in inflammation, atherosclerosis, antimicrobial host defense, wound healing, angiogenesis, and malignancy has become increasingly appreciated as the function of platelets in the pathophysiology of these processes has been defined. This review will consider the formation, release, and physiologic roles of alpha-granules with special emphasis on work performed over the last decade.

Conflict of interest statement

Conflict of interest statement

The authors have no conflicts of interest to declare.

Figures

Figure 1. Working model of α–granule formation in megakaryocytes

α–Granule cargo derives from budding of the trans-Golgi network (TGN) and endocytosis of the plasma membrane. Both processes are clathrin-mediated. Receptor-mediated endocytosis is depicted in this figure; however, pinocytosis of α–granule cargo can also occur. Vesicles can subsequently be delivered to multivesicular bodies (MVBs), where sorting of vesicles occurs. It is possible that vesicles may also be delivered directly to α–granules. Some vesicles within MVBs contain exosomes. MVBs can mature to become α–granules.

Figure 2. Model of transport of α–granules during platelet formation

Platelet α–granules are transported along microtubules from the megakaryocyte cell body through long pseudopodial extensions termed proplatelets. Platelets form as bulges along the length of these extensions. α–Granules are maintained in the nascent platelets by coiled microtubules. The insert demonstrates subpopulations of α–granules containing distinct cargos being transported along a proplatelet. α–Granules containing fibrinogen are shown in green, while those containing vWf are shown in red. (Insert from Italiano et al., Blood, 111:1227–1233.)

Figure 3. Absence of α–granules in platelets from patients harboring a mutation in VPS33B

Thin-section transmission electron micrographs of platelets (A) from a fetus with a mutation in VPS33B and (B) platelets from an unaffected fetus. Abundant α–granules indicated with white arrows in control platelets are lacking in platelets with mutant VSP33B. Bar, 500 nm. (Adapted from Lo et al., Blood, 106:4159–4166).

Figure 4. Role of SNAREs α–granule membrane fusion

A) The primary vSNARE mediating platelet α–granule secretion is VAMP-8, with VAMP-3 and perhaps VAMP-2 serving subordinate functions. Platelet tSNAREs include syntaxins and SNAP-23. Syntaxin-2 and 4 participate in α–granule release. Coiled-coil domains (bolded) within vSNAREs (blue) and tSNAREs (orange) interact, forming a twisted 4-helical bundle. B) Interaction of the coiled-coil domains brings the opposing membranes of the granule and target membrane into close apposition. C) Binding of vSNAREs and tSNAREs generates energy required for membrane fusion. Pore formation with release of granule contents subsequently ensues.

Figure 5. Hypothetical model of atherogenesis triggered by platelets

Activated platelets roll along the endothelial monolayer via GPIbα/P-selectin or PSGL-1/P-selectin. Thereafter, platelets firmly adhere to vascular endothelium via β3 integrins, release proinflammatory compounds (IL-1β, CD40L), and induce a proatherogenic phenotype of ECs (chemotaxis, MCP-1; adhesion, ICAM-1). Subsequently, adherent platelets recruit circulating leukocytes, bind them, and inflame them by receptor interactions and paracrine pathways, thereby initiating leukocyte transmigration and foam cell formation. Thus, platelets provide the inflammatory basis for plaque formation before physically occluding the vessel by thrombosis upon plaque rupture. (Adapted from Gawaz et al., J. Clin. Invest., 115:3378).

Figure 6. Degranulated platelets are unable to prevent thrombocytopenia-induced tumor bleeding

At day 8 after tumor cell implantation, mice were injected with either the control IgG (Control) or the platelet-depleting IgG (Depleted). A subset of mice was transfused 30 min before the induction of thrombocytopenia with tyrode buffer (no transfusion) or 7 × 108 of either resting or activated platelets and s.c. Lewis lung carcinoma cells were photographed 18 h later. Bar, 5 mm. (Adapted from Ho-Tin-Noe et al., Cancer Res, 68:6851–6858)