Sialic Acid Loss Regulates Platelet Survival and Integrity

It becomes increasingly apparent that, besides the intrinsic apoptotic machinery, surface glycan modifications regulate platelet survival. Platelets with reduced α2,3-linked sialic acid during sepsis due to S. pneumoniae infection, after cold storage, or in mice lacking the sialyltransferase ST3GalIV are cleared by the hepatic Ashwell-Morell receptor (AMR, ASGPR1/2). Platelet survival in Asgr2^-/- mice was increased by ∼35% when compared to that of WT mice, which results in a ∼50% increase in circulating platelet counts, despite a loss of surface sialic acid. We reasoned that sialidase activity increases on the surface of circulating platelets as they age, a process that would facilitate sialic acid hydrolysis and removal from the circulation. To test this hypothesis, we directly injected the sialidase inhibitor 2-deoxy-2,3-dehydro-N-acetylneuraminic acid (DANA) into WT mice and determined endogenous platelet circulatory times. Platelet survival was prolonged by ∼30% (T_1/2 of 62.0 ± 2.7 h) in DANA-treated mice, compared to that of mock-treated mice (T_1/2 of 47.5 ± 4.3 h). DANA injections decreased terminal sialic acid loss on circulating platelets by ∼40% by day 2, compared to control platelets, as evidenced by binding of RCA-I lectin that specifically recognizes terminal β1-4 galactose moieties exposed by sialic acid removal. Freshly isolated, resting platelets from Asgr2^-/- mice (AMR-platelets) were significantly smaller in size (22%) and had increased sialidase Neu1 (∼5 fold), but not Neu3 surface expression, when compared to WT platelets or St3gal4^-/- platelets, as measured by flow cytometry. We next investigated if AMR-platelets age/deteriorate faster upon in vitro storage. Platelets were isolated from WT, Asgr2^-/- and St3gal4^-/- mice and stored for 24hrs at room temperature, and sialidase expression (Neu1 and Neu3) as well as microvesiculation were measured by flow cytometry. Although significant Neu1 and Neu3 surface expression increase was measured on platelets from all phenotype after storage, Neu1 and Neu3 surface expression was significantly higher in AMR-platelets (∼2 and 4 fold, respectively) when compared to WT and St3gal4^-/- platelets. AMR-platelets, but not St3gal4^-/- platelets microvesiculated upon storage, consistent with a faster deterioration of aged AMR-platelets. We next injected into WT and Asgr2^-/- mice the BH3 mimetic, ABT-737, which binds and inhibits the pro-apoptotic Bcl-2, Bcl-xL and Bcl-w. After injection of ABT-737, platelets in the Asgr2^-/- mouse were cleared more efficiently (∼20%) from the circulation when compared to those in WT mice. Collectively, our data show that blood borne sialidases contribute to loss of sialic acid during circulation to regulate platelet survival. Our data also suggest that platelet glycan degradation, i.e. sialic acid loss, may trigger the intrinsic apoptotic machinery in platelets, linking glycan degradation and intrinsic apoptotic machinery in the clearance mechanisms regulating platelet survival.