Platelets store laminins 411/421 and 511/521 in compartments distinct from α- or dense granules and secrete these proteins via microvesicles.
BACKGROUND
Blood platelets secrete upon activation of laminins 411/421 and 511/521, large adhesive proteins mainly found in the basement membranes of blood vessels and other tissues. At present, the subcellular localization and secretion mechanisms of platelet laminins are largely unknown.


OBJECTIVE
Our aim was to compare the subcellular localization of laminins 411/421 and 511/521 and specific granule markers in platelets. We also elucidated the role of microvesicles and exosomes in laminin release in platelet activation.


METHODS
We studied laminin and granule marker protein localization in platelets by using immunofluorescence confocal microscopy and immunoelectron microscopy. Microvesicles and exosomes were separated from material released from platelets on activation by thrombin. The expression of laminins in microvesicles and exosomes was studied by using SDS-PAGE and Western blotting as well as by flow cytometric analysis. The exosomes were immunoprecipitated with magnetic microbeads coated with anti-CD63 antibodies.


CONCLUSIONS
We demonstrate that laminins 411/421 and 511/521 are present in compartments of platelets that do not express α-granule, dense granule, or lysosome marker proteins. Moreover, laminins secreted by activated platelets are mostly found in microvesicles shed from the plasma membrane, while their presence in simultaneously released exosomes is minimum.
Institute of Molecular and Cell Biology, University of Tartu, Tartu, Estonia.
M
M
Pook
L
L
Tamming
K
K
Padari
T
T
Tiido
T
T
Maimets
M
M
Patarroyo
E
E
Juronen
V
V
Jaks
S
S
Ingerpuu
D001485:Basement Membrane / Q000378:metabolism; D001792:Blood Platelets / Q000166:cytology / Q000378:metabolism*; D002448:Cell Adhesion; D003594:Cytoplasmic Granules / Q000378:metabolism*; D055354:Exosomes / Q000378:metabolism; D005109:Extracellular Matrix / Q000378:metabolism; D005434:Flow Cytometry; D006801:Humans; D007797:Laminin / Q000378:metabolism*; D018613:Microscopy, Confocal; D008856:Microscopy, Fluorescence; D019007:P-Selectin / Q000378:metabolism; D015539:Platelet Activation; D040201:Platelet Membrane Glycoprotein IIb / Q000378:metabolism; D060149:Tetraspanin 30 / Q000378:metabolism
C501369:LAMA4 protein, human; C478221:LAMB1 protein, human; D007797:Laminin; D019007:P-Selectin; D040201:Platelet Membrane Glycoprotein IIb; D060149:Tetraspanin 30; C119013:laminin 10; C097043:laminin alpha5; C089141:laminin gamma 1
blood platelets; cell-derived microparticles; exosomes; laminin; platelet activation