Aim: 

Cyclosporin A has long been used as a calcineurin inhibitor that impairs a number of signalling pathways derived from phosphorylation/dephosphorylation unbalance and calcineurin down regulation; meanwhile the role of cyclophilins, which are the specific targets of cyclosporine A, remains largely unconsidered (Adams et al. 2005). We have now explored the role of cyclophilins in intracellular Ca²⁺ homeostasis in human platelets.

Methods: 

Intracellular Ca²⁺ homeostasis was determined by spectrofluorimetry. Protein-protein interactions were analyzed by coimmunoprecipitation and western blotting.

Results: 

Cyclophilins inhibition, by platelet incubation with 50 mM cyclosporin A for 5 or 30 min, significantly reduced SERCA2b activity without affecting other Ca²⁺-ATPases like SERCA3a or the plasma membrane Ca²⁺-ATPase (PMCA). SERCA2b inhibition was found to be independent of calcineurin as observed by platelet treatment with a more specific calcineurin inhibitor, cypermethrin (Enan & Matsumura 1992) which did not modify Ca²⁺ reuptake by SERCA2b. Coimmunoprecipitation experiments revealed that cyclophilins interacted with SERCA2b, an event altered by cyclosporin A.

Conclusion: 

Cyclophilins play a relevant role in Ca²⁺ signalling in human platelets mediated by the regulation of SERCA2b activity, a role that has often misinterpreted as mediated by calcineurin.

References: Adams, B., Musiyenko, A., Kumar, R. & Barik, S. 2005. A novel class of dual-family immunophilins. J Biol Chem 280, 24308-24314.

Enan, E. & Matsumura, F. 1992. Specific inhibition of calcineurin by type II synthetic pyrethroid insecticides. Biochem Pharmacol 43, 1777-1784.

P.C.R. and the present study are supported by MEC through "Ramon y Cajal program" (RYC2007-00349) and MEC grant (BFU2007-60104).