WNT-3a modulates platelet function by regulating small GTPase activity.

Here we provide evidence that WNT-3a modulates platelet function by regulating the activity of four key GTPase proteins: Rap1, Cdc42, Rac1 and RhoA. We observe WNT-3a to differentially regulate small GTPase activity in platelets, promoting the GDP-bound form of Rap1b to inhibit integrin-α(IIb)β(3) adhesion, while concomitantly increasing Cdc42 and Rac1-GTP levels thereby disrupting normal platelet spreading. We demonstrate that Daam-1 interacts with Dishevelled upon platelet activation, which correlates with increased RhoA-GTP levels.

Synaptotagmin-like protein 1 interacts with the GTPase-activating protein Rap1GAP2 and regulates dense granule secretion in platelets.

The small guanine-nucleotide-binding protein Rap1 plays a key role in platelet aggregation and hemostasis, and we recently identified Rap1GAP2 as the only GTPase-activating protein of Rap1 in platelets. In search of Rap1GAP2-associated proteins, we performed yeast-2-hybrid screening and found synaptotagmin-like protein 1 (Slp1) as a new binding partner. We confirmed the interaction of Rap1GAP2 and Slp1 in transfected COS-1 and HeLa cells and at endogenous level in human platelets.

Cyclic nucleotide-dependent protein kinases inhibit binding of 14-3-3 to the GTPase-activating protein Rap1GAP2 in platelets.

GTPase-activating proteins are required to terminate signaling by Rap1, a small guanine nucleotide-binding protein that controls integrin activity and cell adhesion. Recently, we identified Rap1GAP2, a GTPase-activating protein of Rap1 in platelets. Here we show that 14-3-3 proteins interact with phosphorylated serine 9 at the N terminus of Rap1GAP2.

Phosphodiesterase 2A forms a complex with the co-chaperone XAP2 and regulates nuclear translocation of the aryl hydrocarbon receptor.

Phosphodiesterase type 2A (PDE2A) hydrolyzes cyclic nucleotides cAMP and cGMP, thus efficiently controlling cNMP-dependent signaling pathways. PDE2A is composed of an amino-terminal region, two regulatory GAF domains, and a catalytic domain. Cyclic nucleotide hydrolysis is known to be activated by cGMP binding to GAF-B; however, other mechanisms may operate to fine-tune local cyclic nucleotide levels.

Rap1GAP2 is a new GTPase-activating protein of Rap1 expressed in human platelets.

The Ras-like guanine-nucleotide-binding protein Rap1 controls integrin alpha(IIb)beta3 activity and platelet aggregation. Recently, we have found that Rap1 activation can be blocked by the nitric oxide/cyclic guanosine monophosphate (NO/cGMP) signaling pathway by type 1 cGMP-dependent protein kinase (cGKI). In search of possible targets of NO/cGMP/cGKI, we studied the expression of Rap1-specific GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs) in platelets.