Summary
The combined action of collagen and thrombin induces the formation of COAT platelets,
which are characterised by a coat of procoagulant and adhesive molecules on their
surface. Although recent work has started to highlight their clinical relevance, the
exact mechanisms regulating the formation of procoagulant COAT platelets remain unclear.
Therefore, we employed flow cytometry in order to visualise in real time surface and
intracellular events following simultaneous platelet activation with convulxin and
thrombin. After a rapid initial response pattern characterised by the homogenous activation
of the fibrinogen receptor glycoprotein IIb/IIIa in all platelets, starting with a
delay of about 2 minutes an increasing fraction transforms to procoagulant COAT platelets.
Their surface is characterised by progressive loss of PAC-1 binding, expression of
negative phospholipids and retention of α-granule von Willebrand factor. Intracellular
events in procoagulant COAT platelets are a marked increase of free calcium into the
low micromolar range, concomitantly with early depolarisation of the mitochondrial
membrane and activation of caspase-3, while non-COAT platelets keep the intracellular
free calcium in the nanomolar range and maintain an intact mitochondrial membrane.
We show for the first time that the flow-cytometrically distinct fractions of COAT
and non-COAT platelets differentially phosphorylate two signalling proteins, PKCα
and p38MAPK, which may be involved in the regulation of the different calcium fluxes
observed in COAT versus non-COAT platelets. This study demonstrates the utility of
concomitant cellular and signalling evaluation using flow cytometry in order to further
dissect the mechanisms underlying the dichotomous platelet response observed after
collagen/thrombin stimulation.
Supplementary Material to this article is available online at www.thrombosis-online.com.
Keywords
Platelets - procoagulant - COAT - surface - intracellular signalling - calcium - phosphorylation
- apoptosis - flow cytometry