Introduction
The study of genetic bleeding disorders provided the first link between platelet functions
and specific membrane glycoproteins. Two examples are well known and have been the
subject of numerous reviews. First, Glanzmann’s thrombasthenia is a bleeding disorder
caused by a defect of platelet aggregation in which the glycoprotein αIIbβ3 (GP IIb-IIIa)
is either lacking or is expressed but is defective.1 We now know that αIIbβ3 exists on the surface of unstimulated platelets in an inactive
form but, through a process known as “inside-out” signaling, responds to platelet
stimulation to become a receptor for soluble fibrinogen and von Willebrand factor
(vWF) to mediate platelet aggregation. αIIbβ3 is also known to bind immobilized fibrinogen
and, through a process known as “outside-in” signaling, to induce platelet stimulation.2 A second example is Bernard-Soulier syndrome, a bleeding disorder caused by the failure
of platelets to bind to subendothelial matrices due to the lack of or defective GP
Ib-IX-V.3 It is now known that GP Ib-IX-V binds to vWF to mediate the adhesion of unstimulated
platelets to injured blood vessel walls.4,5 GP Ib-IX-V interactions also induce platelet stimulation, a process mediated by signaling
through GP Ib-IX-V.6 The mechanisms responsible for the binding of adhesive proteins to αIIbβ3 and GP
Ib-IX-V are beginning to be understood and, as such, targets for therapeutic intervention
have been identified. Three parenteral αIIbβ3 antagonists have demonstrated a therapeutic
benefit in large-scale clinical trials of acute coronary syndromes, including unstable
angina, non Q-wave myocardial infarction, and percutaneous intervention, and are now
commercially available.7 Many orally available αIIbβ3 antagonists are presently in clinical trials. Although
GP Ib antagonists have not been pursued as aggressively, animal studies have shown
that they do have a proven antithrombotic benefit.8 Despite these advances in the understanding of glycoprotein ligand binding and development
of therapeutic antagonists of adhesive protein receptors, the mechanisms responsible
for transducing signals through these receptors have remained elusive.
It is now established that signal transduction reactions through αIIbβ3 and GP Ib-IX-V
are not only involved in platelet aggregation to cause vessel occlusions, but also
that glycoprotein signaling affects thrombus growth and stability, as well as the
biology and perhaps the pathology of the vessels in which aggregates occur. In one
example, platelet-derived growth factor (PDGF), secreted in response to αIIbβ3 signaling
from the α-granules of aggregated platelets, is a primary smooth muscle cell mitogen
and is believed to be involved not only in the response to vascular injury but also
in atherosclerotic lesion progression.9,10 In another example, CD 154 (previously termed CD40 ligand) redistributes from α-granule
membranes to the surface of aggregated platelets in response to αIIbβ3 signaling.11 CD 154 is an important inflammatory mediator that induces the release of cytokines
from endothelial and smooth muscle cells, initiates vascular inflammation, and participates
in atherosclerotic lesion progression.12 A third example involves the assembly of prothrombinase and factor Xase on the surface
of aggregated platelets, enabling platelet thrombi to be procoagulant and accounting
for the apparent anticoagulant activity of αIIbβ3 antagonists.13,14 In addition, platelet aggregates also display fibrinogen and vWF bound to platelet
membrane glycoproteins that function to recruit additional platelets and, therefore,
enhance thrombus growth.15 More recent data also indicate that platelet aggregation induces de novo protein
synthesis.16,17 These and other events are secondary to the initial adhesion and aggregation reactions
of platelets and are consequences of signaling reactions induced by the adhesion and
aggregation receptors. Thus, characterization of the membrane glycoprotein signal
transduction pathways has become essential, not only to understand platelet function,
but also to determine whether there are additional ways by which platelet-mediated
pathologies can be regulated.
Platelet membrane glycoprotein signaling reactions either do not occur in nucleated
cells normally used for transfection studies or are insufficiently characterized.
Accordingly, the use of genetics to study mechanisms of platelet adhesive protein
receptor signaling has been limited. The advent of technologies that facilitate genetic
manipulations in the mouse genome has produced new ways to define protein function
and determine the structure-function relationships of individual proteins and is proving
of value in unraveling signal transduction pathways in platelets. Although one should
always be cautious in extrapolating data from mouse to human platelets (as demonstrated
by the PAR receptors, see below), it is impressive that much of what has been learned
about platelets appears to apply to both mouse and human. Indeed, this review summarizes
the status of genetic manipulations of the mouse genome that have contributed to our
understanding of platelet membrane adhesion receptor signaling in platelets.
