title: A role for the intracellular protease calpain in the activation of store-operated calcium entry in human platelets.
abstract: Here, we report a novel role for the cysteine protease calpain in store-operated calcium entry. Several structurally and mechanistically unrelated inhibitors of calpain inhibited Ca2+ entry activated in human platelets by thapsigargin-evoked Ca2+ store depletion or the physiological agonist thrombin, whereas inhibitors of other cysteine proteases were without effect. The use of the cell-permeable fluorogenic calpain substrate 7-amino-4-chloromethylcoumarin, t-BOC-l-leucyl-l-methionine amide revealed rapid activation of calpain which was closely temporally correlated with Ca2+ store depletion even in the absence of a rise in cytosolic [Ca2+]. Calpain inhibition prevented the tyrosine phosphorylation of several proteins upon Ca2+ store depletion, suggesting that calpain may lie upstream of protein tyrosine phosphorylation that is known to be required for the activation of store-operated Ca2+ entry in human platelets. Earlier studies using calpain inhibitors may need reinterpretation in the light of this finding that calpain plays a role in the activation of physiological Ca2+ entry pathways.
authors.affiliation: Department of Physiology, Development and Neuroscience, Physiological Laboratory, University of Cambridge, Downing Street, Cambridge CB2 3EG, UK.
authors.forename: Alan G S
authors.initials: AG
authors.lastname: Harper
authors.forename: Stewart O
authors.initials: SO
authors.lastname: Sage
mesh_terms: D001792:Blood Platelets / Q000201:enzymology / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling; D002154:Calpain / Q000037:antagonists & inhibitors / Q000502:physiology*; D002478:Cells, Cultured; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D004151:Dipeptides / Q000494:pharmacology; D004347:Drug Interactions; D006801:Humans; D010766:Phosphorylation; D015539:Platelet Activation / Q000187:drug effects; D013324:Strontium / Q000378:metabolism; D019284:Thapsigargin / Q000494:pharmacology; D013917:Thrombin / Q000494:pharmacology; D014443:Tyrosine / Q000378:metabolism
chemical_list: D015853:Cysteine Proteinase Inhibitors; D004151:Dipeptides; D014443:Tyrosine; D019284:Thapsigargin; D013917:Thrombin; D002154:Calpain; D002118:Calcium; C058076:calpain inhibitor III; D013324:Strontium