Calcium-modulating cyclophilin ligand desensitizes hormone-evoked calcium release

The Ca(2+)-modulating cyclophilin ligand (CAML) protein causes stimulation of transcription factors via activation of a store-operated Ca(2+) entry pathway. Since CAML is widely expressed in mammalian tissues, it may be an important regulator of Ca(2+) store function. In the present study, we investigated the consequence of CAML overexpression on Ca(2+) signaling using rapid confocal imaging of Fluo3-loaded NIH3T3 fibroblasts. Control and CAML-expressing cells gave concentration-dependent responses to the Ca(2+) mobilizing agonist ATP. CAML expression reduced the sensitivity of the cells so that higher concentrations of ATP were needed to achieve global Ca(2+) waves. The amplitudes of Ca(2+) waves were significantly reduced in CAML expressing cells, consistent with earlier suggestions that CAML causes depletion of internal Ca(2+) stores. With low ATP concentrations, only local Ca(2+) release events were observed. CAML did not affect the characteristics of these local Ca(2+) signals, suggesting that it does not directly affect Ca(2+) release channels.