The catalytic subunit of protein phosphatase 1 gamma regulates thrombin-induced murine platelet alpha(IIb)beta(3) function.
BACKGROUND
Hemostasis and thrombosis are regulated by agonist-induced activation of platelet integrin alpha(IIb)beta(3). Integrin activation, in turn is mediated by cellular signaling via protein kinases and protein phosphatases. Although the catalytic subunit of protein phosphatase 1 (PP1c) interacts with alpha(IIb)beta(3), the role of PP1c in platelet reactivity is unclear.


RESULTS
Using gamma isoform of PP1c deficient mice (PP1cgamma(-/-)), we show that the platelets have moderately decreased soluble fibrinogen binding and aggregation to low concentrations of thrombin or protease-activated receptor 4 (PAR4)-activating peptide but not to adenosine diphosphate (ADP), collagen or collagen-related peptide (CRP). Thrombin-stimulated PP1cgamma(-/-) platelets showed decreased alpha(IIb)beta(3) activation despite comparable levels of alpha(IIb)beta(3), PAR3, PAR4 expression and normal granule secretion. Functions regulated by outside-in integrin alpha(IIb)beta(3) signaling like adhesion to immobilized fibrinogen and clot retraction were not altered in PP1cgamma(-/-) platelets. Thrombus formation induced by a light/dye injury in the cremaster muscle venules was significantly delayed in PP1cgamma(-/-) mice. Phosphorylation of glycogen synthase kinase (GSK3)beta-serine 9 that promotes platelet function, was reduced in thrombin-stimulated PP1cgamma(-/-) platelets by an AKT independent mechanism. Inhibition of GSK3beta partially abolished the difference in fibrinogen binding between thrombin-stimulated wild type and PP1cgamma(-/-) platelets.


CONCLUSIONS
These studies illustrate a role for PP1cgamma in maintaining GSK3beta-serine9 phosphorylation downstream of thrombin signaling and promoting thrombus formation via fibrinogen binding and platelet aggregation.
Gushiken
Francisca C
FC
Department of Medicine, Baylor College of Medicine, Houston, Texas, USA.
Hyojeong
Han
H
Pradhan
Subhashree
S
Langlois
Kimberly W
KW
Alrehani
Nawaf
N
Cruz
Miguel A
MA
Rumbaut
Rolando E
RE
Vijayan
K Vinod
KV
D000818:Animals; D001792:Blood Platelets / Q000187:drug effects* / Q000201:enzymology / Q000378:metabolism*; D020134:Catalytic Domain; D004195:Disease Models, Animal; D005340:Fibrinogen / Q000378:metabolism; D038362:Glycogen Synthase Kinase 3 / Q000378:metabolism; D000071679:Glycogen Synthase Kinase 3 beta; D007527:Isoenzymes / Q000378:metabolism; D051379:Mice; D010768:Phosphoserine / Q000378:metabolism; D010974:Platelet Aggregation / Q000187:drug effects; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000378:metabolism*; D054645:Protein Phosphatase 1 / Q000172:deficiency / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D018179:Receptors, Thrombin / Q000378:metabolism; D022142:Secretory Vesicles / Q000187:drug effects / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D013917:Thrombin / Q000494:pharmacology*; D013927:Thrombosis / Q000139:chemically induced / Q000201:enzymology / Q000473:pathology
D007527:Isoenzymes; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; D018179:Receptors, Thrombin; D010768:Phosphoserine; D005340:Fibrinogen; D000071679:Glycogen Synthase Kinase 3 beta; C000605507:Gsk3b protein, mouse; D051057:Proto-Oncogene Proteins c-akt; D038362:Glycogen Synthase Kinase 3; D054645:Protein Phosphatase 1; D013917:Thrombin