Myosin IIa is critical for cAMP-mediated endothelial secretion of von Willebrand factor

Abstract

Nonmuscle myosin II has been implicated in regulation of von Willebrand factor (VWF) release from endothelial Weibel-Palade bodies (WPBs), but the specific role of myosin IIa isoform is poorly defined. Here, we report that myosin IIa is expressed both in primary human endothelial cells and intact mouse vessels, essential for cyclic adenosine monophosphate (cAMP)-mediated endothelial VWF secretion. Downregulation of myosin IIa by shRNAs significantly suppressed both forskolin- and epinephrine-induced VWF secretion. Endothelium-specific myosin IIa knockout mice exhibited impaired epinephrine-stimulated VWF release, prolonged bleeding time, and thrombosis. Further study showed that in resting cells, myosin IIa deficiency disrupted the peripheral localization of Rab27-positive WPBs along stress fibers; on stimulation by cAMP agonists, myosin IIa in synergy with zyxin promotes the formation of a functional actin framework, which is derived from preexisting cortical actin filaments, around WPBs, facilitating fusion and subsequent exocytosis. In summary, our findings not only identify new functions of myosin IIa in regulation of WPB positioning and the interaction between preexisting cortical actin filaments and exocytosing vesicles before fusion but also reveal myosin IIa as a physiological regulator of endothelial VWF secretion in stress-induced hemostasis and thrombosis.

Tuning the endothelial response: differential release of exocytic cargos from Weibel‐Palade bodies

“…Our research complements recent research confirming that VWF release is boosted by an actomyosin ring [25,26]. However, there are differences in the findings.…”

“…We and others demonstrated that VWF release is boosted by a contractile actomyosin ring forming around the fused WPB to squeeze out content [9,25,26], but whether this boost affects platelet and leukocyte recruitment to endothelial cells was undetermined. We first analyzed the effect of actomyosin ring inhibition on VWF string formation ( Fig.…”

“…We concluded that rings form de novo after fusion [9], whereas Han et al report actin remodeling before fusion from a pre-existing framework. We also differ on whether WPB localization is generally affected by myosin II inhibition [9,26]. The different conclusions might reflect which cell surface was imaged (apical vs. basal) or spinning disk vs. custom microscopy [9,25].…”

Exocytosis of Weibel–Palade bodies: how to unpack a vascular emergency kit

“…The actin cytoskeleton plays opposing roles in the release of WPBs. On the one hand, it is necessary for peripheral distribution of WPBs, which is important for releasability and depends on myosin IIa . On the other hand, actin acts as a barrier , and, by anchoring WPBs to the actin cytoskeleton, MyRIP acts as a brake during exocytosis .…”

“…Then, shortly after opening of the fusion pore, an actomyosin ring is assembled coating the distal end of the fusing granule, a process that involves de novo actin polymerization . Inhibition of the myosin IIa‐dependent contractility of the actomyosin ring prevented VWF release from fusing WPBs, while not affecting the secretory fate of P‐selectin (but not in ), leading to the hypothesis that contraction of such rings provides the necessary physical force to squeeze VWF out of the fusing granule. It is currently unclear what initiates actomyosin ring formation on the postfusion WPB, but the involvement of the tension sensor zyxin suggests that changes in local (membrane) tension, such as on a swelling granule, may be an important factor in this process.…”