Intracellular cyclophilin A is an important Ca2+ regulator in platelets and critically involved in arterial thrombus formation

Medizinische Klinik III, Department of Cardiology and Cardiovascular Diseases, Eberhard Karls University, Tübingen, Germany.
Blood (Impact Factor: 10.45). 06/2012; 120(6):1317-26. DOI: 10.1182/blood-2011-12-398438
Source: PubMed


Platelet adhesion and aggregation play a critical role in primary hemostasis. Uncontrolled platelet activation leads to pathologic thrombus formation and organ failure. The decisive central step for different processes of platelet activation is the increase in cytosolic Ca(2+) activity ([Ca(2+)](i)). Activation-dependent depletion of intracellular Ca(2+) stores triggers Ca(2+) entry from the extracellular space. Stromal interaction molecule 1 (STIM1) has been identified as a Ca(2+) sensor that regulates store-operated Ca(2+) entry through activation of the pore-forming subunit Orai1, the major store-operated Ca(2+) entry channel in platelets. In the present study, we show for the first time that the chaperone protein cyclophilin A (CyPA) acts as a Ca(2+) modulator in platelets. CyPA deficiency strongly blunted activation-induced Ca(2+) mobilization from intracellular stores and Ca(2+) influx from the extracellular compartment and thus impaired platelet activation substantially. Furthermore, the phosphorylation of the Ca(2+) sensor STIM1 was abrogated upon CyPA deficiency, as shown by immunoprecipitation studies. In a mouse model of arterial thrombosis, CyPA-deficient mice were protected against arterial thrombosis, whereas bleeding time was not affected. The results of the present study identified CyPA as an important Ca(2+) regulator in platelets, a critical mechanism for arterial thrombosis.

    • "CypA was first recognized as a cell receptor for the potent immunosuppressive drug cyclosporin A (CsA) which upon binding, reduces T cell activation (Liu et al., 1992). CypA has also demonstrated to be a powerful regulator of Ca 2+ in activated platelets (Elvers et al., 2012). Furthermore , CypA may play a role in the tick immune response to pathogens (Maeda et al., 2013). "

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    • "The MEK-ERK pathway was early suggested as being involved in the activation of SOCE in human platelets, probably as a downstream effector of Ras proteins,66 and it has recently been demonstrated that constitutive dephosphorylation of STIM1 at ERK1/2 target sites impairs platelet adhesion to fibrinogen,67 supporting a physiological role for STIM1 phosphorylation by this signaling pathway. "
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    ABSTRACT: Calcium ion (Ca(2+)) concentration plays a key role in cell signaling in eukaryotic cells. At the cellular level, Ca(2+) directly participates in such diverse cellular events as adhesion and migration, differentiation, contraction, secretion, synaptic transmission, fertilization, and cell death. As a consequence of these diverse actions, the cytosolic concentration of free Ca(2+) is tightly regulated by the coordinated activity of Ca(2+) channels, Ca(2+) pumps, and Ca(2+)-binding proteins. Although many of these regulators have been studied in depth, other proteins have been described recently, and naturally far less is known about their contribution to cell physiology. Within this last group of proteins, STIM1 has emerged as a major contributor to Ca(2+) signaling by means of its activity as Ca(2+) channel regulator. STIM1 is a protein resident mainly, but not exclusively, in the endoplasmic reticulum (ER), and activates a set of plasma membrane Ca(2+) channels termed store-operated calcium channels (SOCs) when the concentration of free Ca(2+) within the ER drops transiently as a result of Ca(2+) release from this compartment. Knowledge regarding the molecular architecture of STIM1 has grown considerably during the last years, and several structural domains within STIM1 have been reported to be required for the specific molecular interactions with other important players in Ca(2+) signaling, such as Ca(2+) channels and microtubules. Within the modulators of STIM1, phosphorylation has been shown to both activate and inactivate STIM1-dependent Ca(2+) entry depending on the cell type, cell cycle phase, and the specific residue that becomes modified. Here we shall review current knowledge regarding the modulation of STIM1 by phosphorylation.
    Full-text · Article · Nov 2013 · Communicative & integrative biology
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    • "Thus, our results provide a new potential mediator, STIM1, in those signaling pathways where MAPK and Ca 2+ entry are involved. This would be the case for platelet activation (Elvers et al., 2012) and myoblast differentiation (Lee et al., 2012), although the hypothesis has not been fully confirmed in those cell types due to the lack of phosphospecific antibodies against those sites. Because our work has focused on the molecular mechanism underlying the regulation of STIM1 activity by phosphorylation, the involvement of STIM1 phosphorylation in signaling pathways and physiological events mediated by ERK1/2 will require further study in the future. "
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    ABSTRACT: STIM1 (stromal interaction molecule 1) is a key regulator of store-operated calcium entry (SOCE). Upon depletion of Ca(2+) concentration within the endoplasmic reticulum (ER), STIM1 relocalizes at ER-plasma membrane junctions, activating store-operated calcium channels (SOCs). Although one knows the molecular details for STIM1-SOCs binding, the regulation of SOCE remains largely unknown. A detailed list of phosphoresidues within the STIM1 sequence has been reported. However, the molecular pathways controlling this phosphorylation and its function are still under study. Using phospho-specific antibodies, it is demonstrated here that ERK1/2 mediates STIM1 phosphorylation at Ser575, Ser608, and Ser621 during Ca(2+) store depletion, and that Ca(2+) entry and store refilling restore phosphorylation to basal levels. This phosphorylation occurs in parallel to the dissociation from end-binding protein 1 (EB1), a regulator of growing microtubule ends. While Ser to Ala mutation of residues 575, 608, and 621 showed a constitutive binding to EB1 even after Ca(2+) store depletion, Ser to Glu mutation of these residues, to mimic the phosphorylation profile attained after store depletion, triggered full dissociation from EB1. Given that wild-type STIM1 and STIM1(S575E/S608E/S621E) activate SOCE similarly, a model is proposed to explain how ERK1/2-mediated phosphorylation of STIM1 regulates SOCE. This regulation is based on the phosphorylation of STIM1 to trigger dissociation from EB1 during Ca(2+) store depletion, an event that is fully reverted by Ca(2+) entry and store refilling.
    Full-text · Article · May 2013 · Journal of Cell Science
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