Thrombosis can be initiated when activated platelets adhere to injured blood vessels via the interaction of subendothelial
collagen with its platelet receptor,
glycoprotein (GP) VI. Here we observed that incubation of platelets with
convulxin,
collagen, or
collagen-related peptide (CRP) resulted in GPVI signaling-dependent loss of surface GPVI and the appearance of an approximately 55-kDa soluble fragment of GPVI as revealed by immunoblotting.
Ethylenediaminetetraacetic acid (
EDTA) or
GM6001 (a
metalloproteinase inhibitor with broad specificity) prevented this loss. In other receptor systems,
calmodulin binding to membrane-proximal cytoplasmic sequences regulates
metalloproteinase-mediated ectodomain shedding. In this regard, we have previously shown that
calmodulin binds to a positively charged, membrane-proximal sequence within the cytoplasmic tail of GPVI. Incubation of platelets with
calmodulin inhibitor
W7 (150 microM) resulted in a time-dependent loss of GPVI from the platelet surface. Both
EDTA and
GM6001 prevented this loss. Surface plasmon resonance demonstrated that
W7 specifically blocked the association of
calmodulin with an immobilized synthetic
peptide corresponding to the
calmodulin-binding sequence of GPVI. These findings suggest that disruption of
calmodulin binding to
receptor cytoplasmic tails by agonist binding to the receptor triggers
metalloproteinase-mediated loss of GPVI from the platelet surface. This process may represent a potential mechanism to regulate GPVI-dependent platelet adhesion.