Proplatelet formation is selectively inhibited by collagen type I through Syk-independent GPVI signaling

Abstract

Collagen receptors GPVI (also known as GP6) and integrin α2β1 are highly expressed on blood platelets and megakaryocytes, their immediate precursors. After vessel injury, subendothelial collagen becomes exposed and induces platelet activation to prevent blood loss. Collagen types I and IV are thought to have opposite effects on platelet biogenesis, directing proplatelet formation (PPF) towards the blood vessels to prevent premature release within the marrow cavity. We used megakaryocytes lacking collagen receptors or treated megakaryocytes with blocking antibodies, and could demonstrate that collagen-I-mediated inhibition of PPF is specifically controlled by GPVI. Other collagen types competed for binding and diminished the inhibitory signal, which was entirely dependent on receptor-proximal Src family kinases, whereas Syk and LAT were dispensable. Adhesion assays indicate that megakaryocyte binding to collagens is mediated by α2β1, and that collagen IV at the vascular niche might displace collagen I from megakaryocytes and thus contribute to prevention of premature platelet release into the marrow cavity and thereby directionally promote PPF at the vasculature.

Thrombopoiesis is spatially regulated by the bone marrow vasculature

“…To this end, we confirmed that the bone-like structures at the outer linings of femur or sternum sections are positive for collagen I 21 (Supplementary Fig. 3A), and partly positive for osteocalcin (Supplementary Fig.…”

Heparan sulfates are critical regulators of the inhibitory megakaryocyte-platelet receptor G6b-B

“…Femurs of mice aged 6–12 weeks were sectioned and stained as described previously (Kawamoto, 2003; Semeniak et al, 2016). In brief, megakaryocytes were stained with anti-GPIX antibody (emfret analytics, Eibelstadt, Germany), CD105 (eBioscicnce) was used as an endothelial cell marker.…”

Matrix Mechanosensation in the Erythroid and Megakaryocytic Lineages

“…MKs express two main receptors for collagen, integrin-α2β1 and glycoprotein VI [47]; however, MK expression of leukocyte-associated immunoglobulin-like 1 (LAIR1) and discoidin domain receptor (DDR1) that also bind collagen have been described in the literature [48,49]. Early MK progenitors (CFU-MK) have a lower affinity for adhering to collagen compared to later MK progenitors [50], which is interesting as integrin-α2β1 and glycoprotein VI expression occurs early in MK development [50,51]. It has been established that collagen I inhibits proplatelet formation (PPF) in order to prevent the release of immature platelets in the bone marrow, while collagen IV and laminin support PPF in the sinusoids [52].…”