A novel association between platelet filamin A and soluble N-ethylmaleimide sensitive factor attachment proteins regulates granule secretion

Abstract

Background and ObjectiveThe molecular mechanisms that underpin platelet granule secretion remain poorly defined. Filamin A (FLNA) is an actin-crosslinking and signaling scaffold protein whose role in granule exocytosis has not been explored despite evidence that FLNA gene mutations confer platelet defects in humans. Methods and ResultsUsing platelets from platelet-specific conditional Flna-knockout mice, we showed that the loss of FLNA confers a severe defect in alpha (α)- and dense (δ)-granule exocytosis, as measured based on the release of platelet factor 4 (aka CXCL4) and adenosine triphosphate (ATP), respectively. This defect was observed following activation of both immunoreceptor tyrosine-based activation motif (ITAM) signaling by collagen-related peptide (CRP) and G protein–coupled receptor (GPCR) signaling by thrombin and the thromboxane mimetic U46619. CRP–induced spikes in intracellular calcium [Ca2+]i were impaired in FLNA-null platelets relative to controls, confirming that FLNA regulates ITAM-driven proximal signaling. In contrast, GPCR-mediated spikes in [Ca2+]i in response to thrombin and U46619 were unaffected by FLNA. Normal platelet secretion requires complexing of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins synaptosomal-associated protein 23 (SNAP23) and syntaxin-11 (STX11). We determined that FLNA coimmunoprecipitates with both SNAP23 and STX11 upon platelet stimulation. ConclusionFLNA regulates GPCR-driven platelet granule secretion and associates with SNAP23 and STX11 in an activation-dependent manner.