Uptake of plasma fibrinogen into the alpha granules of human megakaryocytes and platelets.

Abstract

The origin of platelet a-granule fibrinogen (Fg), whether from endogeneous synthesis or exogeneous derivation, remains unknown. Although Fg biosynthesis by megakaryocytes (MK) has been suggested, recent studies have demonstrated that certain a-granular proteins originate primarily from plasma. To study the origin of a-granule Fg, platelet-associated Fg was measured by ELISA and Western blotting, and localized by immunofluorescence and immunoelectron microscopy in a patient with symptomatic congenital afibrinogenemia before and after replacement therapy with cryoprecipitate. a-Granule Fg was detected in the majority of platelets as early as 24 h postinfusion, suggesting that direct platelet uptake was occurring. Platelet Fg reached a maximum value of 42.5% of normal values at 3 d postinfusion and was localized in the a-granules, while plasma levels followed a typical half-life profile. Significant a-granule Fg was still detectable at 13 d postinfusion, with plasma Fg virtually absent. Studies on cultured CFU-MKs from the patient also confirmed that MKs can incorporate exogeneous Fg into a-granules. These results indicate that platelet a-granule Fg can be derived from the circulating plasma pool and that Fg uptake can occur in both platelets and MKs.

Arf6 controls platelet spreading and clot retraction via integrin αIIbβ3 trafficking

“…Although recognized, for ;30 years, as a process to load granules with Fg, 18,36 our data imply more acute functions for endocytosis in clot formation. A refined role for such platelet endocytosis is still unclear; however, one could envision the endocytic trafficking of integrins being involved in modulating interplatelet connections in a growing thrombus.…”

“…15,16 Also, it mediates plasma Fg endocytosis and storage into a-granules by megakaryocytes and platelets. 7,17,18 Although unknown in platelets, Arf6 clearly affects integrin trafficking in other cells: both endocytosis and recycling. [19][20][21] In cancer cells, active and inactive b 1 integrins traffic via different endocytic recycling routes, although their endocytosis equally requires both clathrin and dynamin.…”

Requirement of VPS33B, a member of the Sec1/Munc18 protein family, in megakaryocyte and platelet -granule biogenesis

“…Abnormal VWF processing in megakaryocte ␣-granules but not in endothelial cells is also seen in the GPS. 54,55 The plasma-derived ␣-granule protein fibrinogen, which is normally acquired by megakaryocytes and platelets through ␣ IIb ␤ 3 -dependent endocytosis, [56][57][58] was also not detected in ARC platelets despite its normal presence in plasma. A deficiency of both megakaryocyte-synthesized (␤-TG, PF4, VWF, TSP-1) 37 and endocytosed (fibrinogen) soluble ␣-granule proteins further suggests that ARC megakaryocytes are unable to capture and package proteins into ␣-granule compartments, analogous to what is seen in the GPS.…”

Gray platelet syndrome. Dissociation between abnormal sorting in megakaryocyte alpha-granules and normal sorting in Weibel-Palade bodies of endothelial cells.

“…One of these differences concerns adhesive proteins such as fibronectin, thrombospondin, and vWF, which are all retained in a-granules (22), whereas in endothelial cells only the vWF is stored in WPB, while thrombospondin and fibronectin are secreted constitutively (35). Likewise, WPB usually originate from a single biosynthetic pathway (36), whereas a-granules are formed by two general routes: endogenous synthesis and an endocytic pathway (37,38), which provides yet another difference between these organelles. We can conclude that the anomaly observed in megakaryocytes and platelets of patients with the GPS is specific to this cell type.…”