AN IMPORTANT STIMULATORY ROLE FOR THE cGMP-DEPENDENT PROTEIN KINASE II
IN PLATELET ACTIVATION, IN VIVO THROMBOSIS AND HAEMOSTASIS
Abstract
Background and Purpose: The intracellular second messenger cGMP mediates
signals by activating two types of cGMP-dependent protein kinases (PKG),
PKG I and PKG II, differentially expressed in different cells. In
platelets, cGMP mediates biphasic signals that stimulate and inhibit
platelet activation, and the downstream signaling of cGMP is mediated by
PKG I, the only PKG known to be expressed in platelets. However,
functional defects of PKG I knockout platelets did not fully explain the
roles of cGMP and the effect of PKG inhibitors on platelet activation.
Experimental Approach: To determine if PKG II is present in platelets
and plays a role in platelet activation, we performed RT-PCR and
isolation of PKG II protein using cGMP-conjugated beads. We further
determined platelet aggregation and ATP release in vitro, and
FeCl3-injured carotid artery thrombosis as well as tail bleeding time in
vivo. Key Results: PKG II is expressed in platelets and plays an
important role in selectively stimulating platelet activation but not in
the negative regulatory role of cGMP. Collagen-induced platelet
aggregation and ATP secretion were reduced in PKG II-deficient mice but
not PKG I-deficient mice. In contrast, low-dose thrombin-induced
platelet activation depended on PKG I but not PKG II. Tail bleeding time
and FeCl3-induced artery thrombus formation were significantly prolonged
in PKG II knockout mice. Conclusion and Implication: PKG II-mediated
cGMP signals are important in platelet activation, thrombosis and
haemostasis in vitro and in vivo.