Neurofibrillary tangles, which contain abnormally hyperphosphorylated forms of
tau protein, are one of the neuropathological hallmarks of
Alzheimer's disease (AD). This altered phosphorylation state of
tau protein may be due to increased
kinase activity or/and decreased
phosphatase activity. In the present study, we characterized human
calcineurin phosphatase activity in postmortem superior frontal cortex and sensorimotor cortex and measured
calcineurin phosphatase activity in samples from individuals with moderate to severe AD (n = 7) and age-matched controls (n = 5). Basal
phosphatase activity was reduced by 25% (P < 0.05) in AD frontal cortex.
Nickel-stimulated
calcineurin activity was decreased by 52% (P < 0.05) and 30% (P < 0.05) in P2 and total cell homogenate, respectively, compared to age-matched controls. No differences in
phosphatase activities were detected in the sensorimotor cortex. The decrease in
nickel-stimulated
calcineurin phosphatase activity in frontal lobe correlated with the neurofibrillary tangle pathology (total cell homogenate, r = -0.77, P < 0.05; P2 fraction, r = -0.76, P < 0.02), but not with diffuse or
neuritic plaques. Despite the changes in
calcineurin phosphatase activity in the superior frontal cortex,
calcineurin protein levels determined by immunoblot were similar in control and AD cases. In addition, no changes in
calcineurin regulatory
proteins (
cyclophilin A and
FKBP12) levels were observed. These studies suggest that decrease of
calcineurin activity may play a role in paired-helical filament formation and/or stabilization, and the decrease of activity was not accompanied by a decrease of
calcineurin protein expression.