Reduction of calcineurin enzymatic activity in Alzheimer's disease: Correlation with neuropathologic changes

Neurofibrillary tangles (NFT), neuritic plaques, and dystrophic neurites are the classic neuropathologic hallmarks of Alzheimer's disease (AD), all of which contain to varying degrees abnormally and/or hyperphosphorylated forms of the microtubule-associated protein tau. Protein phosphatase 2B (calcineurin) dephosphorylates tau isolated from AD-brains to control levels in vitro as well as regulates tau phosphorylation and function in vivo. It has been hypothesized that the changes in tau phosphorylation observed in AD may be due to increases in kinase activity and/or decreases in phosphatase activity. In order to investigate the latter possibility, we examined calcineurin enzyme activity using the substrate para-nitrophenylphosphate (pNPP) in postmortem brain samples from individuals with moderate to severe AD (n=8) and age-matched controls (n=7). The stimulation of calcineurin activity by manganese chloride (1 mM) was reduced by 60% (p<0.01) in whole-cell homogenates prepared from AD temporal cortex (Brodmann area 38). On the other hand, in P2 membrane fractions, the stimulation of calcineurin activity by manganese chloride as well as nickel chloride (1 mM) was reduced by 37% (p<0.05) and 79% (p<0.01), respectively. The manganese stimulated calcineurin activity in the temporal cortex inversely correlated with both the number of NFT (r=-0.60, p<0.02) and neuritic/core plaques (r=-0.63, p<0.02) in whole-cell homogenates, but only with NFT (r=-0.61, p<0.02) in P2 membrane fractions. The nickel-stimulated calcineurin activity did not correlate with neuropathology measures in either whole-cell or P2 membrane fractions. In striate visual cortex (Brodmann area 17), an area relatively unaffected in AD, neither whole-cell nor P2 membrane calcineurin activity were significantly altered. To our knowledge, this is the first report of a reduction in calcineurin phosphatase activity in AD which correlates with the neuropathological features in a region-, subcellular fraction-, and divalent cation-specific manner.