Summary
Activated platelets release large lipid-protein complexes termed microparticles. These
platelet microparticles (PMP) are composed of vesicular fragments of the plasma membrane
and α-granules. PMP facilitate coagulation, promote platelet and leukocyte adhesion
to the subendothelial matrix, support angiogenesis and stimulate vascular smooth muscle
proliferation. Objectives: PMP were separated into 4 size classes to facilitate identification of active protein
and lipid components. PMP were obtained from activated human platelets and separated
into 4 size classes by gel filtration chromatography. Proteins were identified using
2-dimensional, liquid chromatography tandem mass spectrometry. Functional effects
on platelets were determined using the PFA-100→ and on endothelial cells by measuring
transendothelial cell electrical resistance. PMP size classes differed significantly
in their contents of plasma membrane receptors and adhesion molecules, chemokines,
growth factors and protease inhibitors. The two smallest size classes (3 and 4) inhibited
collagen/adenosine-diphosphate-mediated platelet thrombus formation, while fractions
2 and 4 stimulated barrier formation by endothelial cells. Heat denaturation blocked
the effect of fraction 4 on endothelial cell function, but not fraction 2 implying
that the active component in fraction 4 is a protein and in fraction 2 is a heat-stable
protein or lipid but not sphingosine-1-phosphate. Proteomic and functional analysis
of PMP size fractions has shown that PMP can be separated into different size classes
that differ in protein components, protein/lipid ratio, and functional effects on
platelets and endothelial cells. This analysis will facilitate identification of active
components in the PMP and clarify their involvement in diseases such as atherosclerosis
and cancer.
Keywords
Blood platelets - microparticles - proteomics - platelet activation - transendothelial
cell electrical resistance - sphingosine-1-phosphate