Differential regulation of calcineurin isoforms in transplant patients: a new look at an old problem.

Background. Calcineurin is a ubiquitously expressed calcium-dependent phosphatase that is inhibited by the immunosuppressant drugs cyclosporine and tacrolimus. Measuring calcineurin activity in transplant patients has been complicated by a lack of consistent correlation between drug level and enzyme activity, particularly with chronic use. Data from mice lacking the CnA alpha or CnA beta isoform of the catalytic subunit of calcineurin demonstrate that loss of CnA beta results in immunosuppression, whereas loss of CnA alpha does not. As such, methods to examine activity of the CnA beta isoform may be more clinically relevant than nonspecific assays. Methods. Because the current enzyme assays are nonisoform specific, we examined association of the CnA alpha or CnA beta isoform with calmodulin (CaM), a shared binding protein that is only associated with the catalytic subunit in the presence of calcium. We then compared semiquantitated data with total calcineurin activity and immune status in a cohort of pretransplantation controls and postrenal transplantation patients. Results. We found no difference in calcineurin activity between the groups and no difference in the amount of nonisoform-specific catalytic subunit bound to CaM. However, association of CnA alpha-CaM is increased in transplant patients, whereas CnA beta-CaM is decreased. In addition, the amount of CnA beta-CaM corresponds positively with T-cell activity and negatively with tacrolimus level. Finally, CnA beta-CaM is lower in stable transplant patients compared with those with acute rejection. Conclusions. These data suggest that monitoring specifically the A isoform may be more informative than nonisoform-specific assay methods.