Isoform-specific functions of protein kinase C:
the platelet paradigm
M.T. Harper and A.W. Poole1
Department of Physiology and Pharmacology, University of Bristol, School of Medical Sciences, University Walk, Bristol BS8 1TD, U.K.
Platelets are central to haemostasis and thrombosis. Many key steps in platelet activation and aggregation
are regulated by members of the PKC (protein kinase C) family. Multiple isoforms of PKC are expressed
in platelets, and evidence is emerging that different isoforms play distinct roles in the platelet activation
process. This may, in part, be regulated by isoform-specific interactions between PKC family members and
other intracellular signalling molecules, such as tyrosine kinases, or the actin cytoskeleton regulator, VASP
(vasodilator-stimulated phosphoprotein). The contributions of individual PKC isoforms can be addressed
directly in platelets from knockout mouse models, which are providing key insights into the physiological
function of PKC isoform diversity and can be a valuable complimentary approach to more commonly
used pharmacological analyses. Using knockout mouse models, recent reports have demonstrated the
importance of PKCβ and PKCθ in integrin-dependent platelet spreading, and also a novel role for PKCδ in
regulating filopodial formation, highlighting the utility of such models to investigate the functions of specific
PKC isoforms in a physiological process that is significant to our understanding of cardiovascular disease.
Platelets play a central role in physiological blood clotting
(haemostasis) after vascular damage. Reduced platelet
function can lead to prolonged bleeding times, as is seen
in Bernard–Soulier syndrome or von Willebrand’s disease.
However, inappropriate or excessive activation of platelets
also has important pathological implications. A sufficiently
large thrombus (intravascular clot) can occlude blood
vessels, restricting blood flow. Fragmentation of a thrombus
by shear stress or the fibrinolytic system can release
circulating emboli that may become lodged in small vessels
elsewhere in the body. Occlusion of vessels in the cerebral,
coronary or pulmonary vascular beds may result in stroke,
myocardial infarction or pulmonary oedema respectively,
the consequences of which are often fatal: cardiovascular and
cerebrovascular diseases are the leading cause of death in the
developed world . A central aim of platelet research is to
identify the pathways that are crucial for haemostasis, and
those that tend to lead to thrombosis.
Many intracellular signalling pathways are involved in
platelet activation, to which a rise in [Ca2+]i (intracellular
Ca2+concentration) and activation of PKC (protein kinase
C) isoforms are central. Damage to the vessel wall exposes
Key words: conventional protein kinase C (cPKC), filopodium, mouse model, platelet, rottlerin,
vasodilator-stimulated phosphoprotein (VASP).
Abbreviations used: BTK, Bruton’s tyrosine kinase; [Ca2+]i, intracellular Ca2+concentration; DAG,
diacylglycerol; GP, glycoprotein; IP3, inositol trisphosphate; PKA, protein kinase A; PKC, protein
kinase C; aPKC, atypical PKC; cPKC, conventional PKC; nPKC, novel PKC; PLC, phospholipase C;
RACK1, receptor for activated C-kinase; TxA2, thromboxane A2; VASP, vasodilator-stimulated
phosphoprotein; vWF, von Willebrand factor.
1To whom correspondence should be addressed (email email@example.com).
components of the extracellular matrix, in particular
subendothelial collagen, to flowing blood. Platelets adhere to
activates PLCγ (phospholipase Cγ), resulting in generation
of IP3(inositol trisphosphate) and DAG (diacylglycerol) .
IP3induces release of Ca2+from intracellular stores, leading
to an increase in [Ca2+]i, followed by Ca2+entry across the
in [Ca2+]i. The initial platelet responses are amplified by
agonists released from the dense granules, such as ADP
and ATP, and by TxA2 (thromboxane A2) synthesized
from membrane arachidonate. α-Granule contents are
also secreted, which include proteins that are important in
and also proteins that are important in thrombin generation.
Activation of the fibrinogen receptor, integrin αIIbβ3,
represents a final common pathway for both soluble
and subendothelial agonists. This is known as ‘inside-out’
vWF/fibrinogen surface if they also have activated αIIbβ3.
Ligand binding to αIIbβ3 initiates ‘outside-in signalling’,
which is required for platelet responses such as spreading
. Outside-in signalling is also involved in presentation of a
procoagulant surface in a Ca2+-dependent manner that forms
and results in a large increase in thrombin generation. All of
these responses are tightly regulated by PKC.
PKC in platelet activation
PKCs form a family of related serine/threonine kinases that
are part of the AGC-type kinase (protein kinase A/protein
C ?The Authors Journal compilation
C ?2007 Biochemical Society