FK506-binding protein (FKBP12) regulates ryanodine receptor-evoked Ca2+ release in colonic but not aortic smooth muscle

In smooth muscle, the ryanodine receptor (RyR) mediates Ca(2+) release from the sarcoplasmic reticulum (SR) Ca(2+) store. Release may be regulated by the RyR accessory FK506-binding protein (FKBP12) either directly, as a result of FKBP12 binding to RyR, or indirectly via modulation of the activity of the phosphatase calcineurin or kinase mTOR. Here we report that RyR-mediated Ca(2+) release is modulated by FKBP12 in colonic but not aortic myocytes. Neither calcineurin nor mTOR are required for FKBP12 modulation of Ca(2+) release in colonic myocytes to occur. In colonic myocytes, co-immunoprecipitation techniques established that FKBP12 and calcineurin each associated with the RyR2 receptor isoform (the main isoform in this tissue). Single colonic myocytes were voltage clamped in the whole cell configuration and cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) increases evoked by the RyR activator caffeine. Under these conditions FK506, which displaces FKBP12 (to inhibit calcineurin) and rapamycin, which displaces FKBP12 (to inhibit mTOR), each increased the [Ca(2+)](c) rise evoked by caffeine. Notwithstanding, neither mTOR nor calcineurin are required to potentiate caffeine-evoked Ca(2+) increases evoked by each drug. Thus, the mTOR and phosphatidylinositol 3-kinase inhibitor, LY294002, which directly inhibits mTOR without removing FKBP12 from RyR, did not alter caffeine-evoked [Ca(2+)](c) transients. Nor did inhibition of calcineurin by cypermethrin, okadaic acid or calcineurin inhibitory peptide block the FK506-induced increase in RyR-mediated Ca(2+) release. In aorta, although RyR3 (the main isoform), FKBP12 and calcineurin were each present, RyR-mediated Ca(2+) release was unaffected by either FK506, rapamycin or the calcineurin inhibitors cypermethrin and okadaic acid in single voltage clamped aortic myocytes. Presumably failure of FKBP12 to associate with RyR3 resulted in the immunosuppressant drugs (FK506 and rapamycin) being unable to alter the activity of RyR. The effects of these drugs are therefore, apparently dependent on an association of FKBP12 with RyR. Together, removal of FKBP12 from RyR augmented Ca(2+) release via the channel in colonic myocytes. Neither calcineurin nor mTOR are required for the FK506- or rapamycin-induced potentiation of RyR Ca(2+) release to occur. The results indicate that FKBP12 directly inhibits RyR channel activity in colonic myocytes but not in aorta.     

Caerulein-induced intracellular pancreatic zymogen activation is dependent on calcineurin

Aberrant cytosolic Ca(2+) flux in pancreatic acinar cells is critical to the pathological pancreatic zymogen activation observed in acute pancreatitis, but the downstream effectors are not known. In this study, we examined the role of Ca(2+)-activated protein phosphatase 2B (or calcineurin) in zymogen activation. Isolated pancreatic acinar cells were stimulated with supraphysiological caerulein (100 nM) with or without the calcineurin inhibitors FK506 or cell-permeable calcineurin inhibitory peptide (CiP). Chymotrypsin activity was measured as a marker of zymogen activation, and the percent amylase secretion was used as a measure of enzyme secretion. Cytosolic Ca(2+) changes were recorded in acinar cells loaded with the intermediate Ca(2+)-affinity dye fluo-5F using a scanning confocal microscope. A 50% reduction in chymotrypsin activity was observed after pretreatment with 1 microM FK506 or 10 microM CiP. These pretreatments did not affect amylase secretion or the rise in cytosolic Ca(2+) after caerulein stimulation. These findings suggest that calcineurin mediates caerulein-induced intra-acinar zymogen activation but not enzyme secretion or the initial caerulein-induced cytosolic Ca(2+) signal.