Increased platelet Ca2+ channel Orai1 expression upon platelet activation and in patients with acute myocardial infarction

The serum- & glucocorticoid-inducible kinase in the regulation of platelet function

The serum- and glucocorticoid-inducible kinase 1 (SGK1) is expressed in megakaryocytes and circulating platelets. In megakaryocytes, SGK1 activates transcription factor nuclear factor kappa-B (NF-κB), which in turn stimulates expression of Orai1, a Ca(2+) channel protein accomplishing store-operated Ca(2+) enrty (SOCE). SGK1 enhances SOCE and several Ca(2+) -sensitive platelet functions, including degranulation, integrin αII b β3 activation, phosphatidylserine exposure, aggregation and thrombus formation. As shown in other cell types, stimulators of SGK1 expression include ischaemia, oxidative stress, hyperglycaemia, advanced glycation end products (AGEs) and a variety of hormones such as glucocorticoids, mineralocorticoids, transforming growth factor beta (TGFβ), interleukin 6 (IL-6), platelet-derived growth factor (PDGF), thrombin and endothelin. Thus, SGK1-sensitive Ca(2+) signalling may contribute to altered platelet function in several clinical conditions including inflammation, metabolic syndrome, diabetes mellitus and chronic renal failure. Nevertheless, further studies are needed defining the contribution of altered SGK1 expression and activity to physiology and pathophysiology of platelets.

Orai1-induced store-operated Ca(2+) entry enhances phospholipase activity and modulates canonical transient receptor potential channel 6 function in murine platelets

BACKGROUND

Orai1, the major store-operated Ca(2+) entry (SOCE) channel in platelets, is not only critical for enhancing diverse signaling pathways, but may also regulate receptor-operated Ca(2+) entry (ROCE). Dynamic coupling of the Orai1 signalosome to canonical transient receptor potential channels (TRPCs) has been suggested as an essential step in the activation of SOCE and ROCE. However, the functional significance of the biochemical interaction between Orai and TRPC isoforms remains controversial.

OBJECTIVE

We aimed to elucidate the role of Orai1 in diacylglycerol (DAG)-mediated ROCE.

METHODS

Trpc6(-/-) , Orai1(-/-) and Orai1(-/-) /Trpc6(-/-) mice were generated, and their platelets were analyzed.

RESULTS

Thapsigargin (TG)-induced SOCE was further reduced in Orai1(-/-) /Trpc6(-/-) platelets as compared with Orai1(-/-) platelets, thus revealing that TG-induced signaling pathways can activate TRPC6. Thapsigargin-induced SOCE leads to enhanced phospholipase C and D activity in wild-type platelets. The activity of both enzymes was significantly reduced in Orai1(-/-) platelets upon TG stimulation, whereas receptor-induced phospholipase activity was not affected. Furthermore, TG-induced and glycoprotein VI-mediated thromboxane A2 release was strongly dependent on Orai1-mediated SOCE.

CONCLUSION

The regulation of TRPC6 activity can occur independently of the physical interaction with Orai1. TRPC6 operates in crosstalk with Orai1 through Orai1-induced DAG production via phospholipase activation. Orai1-induced DAG production and thromboxane release amplify the second phase of Ca(2+) signaling in platelets.

Regulation of STIM1/Orai1-dependent Ca2+ signalling in platelets

Platelet secretion and aggregation as well as thrombus formation of blood platelets critically depend on increase of cytosolic Ca2+ concentration ([Ca2+]i) mainly resulting from intracellular Ca2+ release followed by store operated Ca2+ entry (SOCE) through Ca2+ release activated channels (CRAC). SOCE is in part accomplished by the pore forming unit Orai and its regulator stromal interaction molecule (STIM). Orai1 and STIM1 transcription is stimulated by NF-κB (nuclear factor kappa B). Serum- and glucocorticoid-inducible kinase 1 (SGK1) up-regulates NF-κB-activity in megakaryocytes and thus Orai1-expression and SOCE in platelets. SGK1 is thus a powerful regulator of platelet Ca2+-signalling and thrombus formation and presumably participates in the regulation of platelet activation by a variety of hormones as well as clinical conditions (e.g. type 2 diabetes or metabolic syndrome) associated with platelet hyperaggregability and increased risk of thromboocclusive events. SOCE in platelets is further regulated by scaffolding protein Homer and chaperone protein cyclophilin A (CyPA). Additional potential regulators of Orai1/STIM1 and thus SOCE in platelets include AMP activated kinase (AMPK), protein kinase A (PKA), reactive oxygen species, lipid rafts, pH and mitochondrial Ca2+ buffering. Future studies are required defining the significance of those mechanisms for platelet Orai1 abundance and function, for SOCE into platelets and for platelet function in cardiovascular diseases.