Platelet Hyaluronan Synthase 3 Regulates Thrombin Signaling and Adhesion to Fibrinogen Under Venous Shear

Hyaluronan (HA) is an essential glycosaminoglycan with supportive roles in hematopoiesis. HA is synthesized by hyaluronan synthases (HAS1, HAS2, and HAS3) and plays a crucial role in cellular signaling and extracellular matrix interactions. Megakaryocytes (MKs), the platelet progenitor cell, express only HAS2 and HAS3, and dysregulated metabolism of HA by MKs leads to thrombocytopenia. To unravel the contribution of HAS3 in platelet function, Using HAS1/3 knockout (dKO) mice, we demonstrate that thrombin‐mediated activation is significantly impaired, whereas collagen‐dependent activation remains intact. Functional assays indicate that platelet aggregation, integrin α IIb β3 activation, and granule secretion are reduced in HAS1/3 KO platelets. However, tail bleeding times remain normal, suggesting that primary hemostasis is not severely affected. Under flow conditions, dKO platelet adhesion to fibrinogen is deficient under venous shear, while adhesion under arterial shear is unaffected. Mechanistically, these impairments correlate with reduced phosphorylation of AKT (p‐AKT), while phosphorylation of PLCγ (p‐PLCγ) remains preserved, suggesting that expression of HAS3 selectively regulates platelet function. These findings highlight HA synthesis as a novel regulator of thrombin‐induced platelet activation and suggest that HAS enzymes may be previously unknown modulators of hemostatic and thrombotic responses.