Elevated [Ca2+]i Levels Occur with Decreased Calpain Activity in Aged Fibroblasts and Their Reversal by Energy-Rich Compounds: New Paradigm for Alzheimer's Disease Prevention

Elevated intracellular Ca2+ levels in the aging brain are widely thought to hyperactivate Ca2+ signaling and Ca2+-dependent enzymes, leading to neuronal death through an excitatory mechanism in Alzheimer's disease (AD). This "Ca2+ overload" hypothesis has been questioned by our theoretical analyses. To better understand the relationship between the "level" and functionality of Ca2+ in aging, in this study we simultaneously measured intracellular Ca2+ transients and calpain activity in cultured human fibroblasts. We found that Ca2+ transitions elicited by bradykinin were indeed overstayed or elevated in levels in old cells but, remarkably, calpain activity was decreased compared to young cells. Also, treating young cells with the energy inhibitor rotenone or with H2O2 recapitulated the Ca2+ overstay and calpain inactivation found in old cells. More importantly, treating old cells with high-energy compounds such as phosphoenol pyruvate or phosphocreatine, which boosted cellular ATP content, reduced the Ca2+ overstay and re-activated calpain. Moreover, Ca2+ levels and calpain activity were dramatically raised in the dying cells killed by detergent. Finally, Ca2+ oscillations induced by low dose of bradykinin in old cells exhibited lower spike frequency, but higher overall levels. Collectively, these results suggest that (a) Ca2+ overload in old cells arises from an inefficient Ca2+ handling system compromised by age-related energy depletion and oxidative stress; and (b) despite elevated levels, the functionality of Ca2+ signaling has diminished in old cells. Thus, the study reinforces the concept that tonic promotion of bioenergetics and Ca2+ signaling function is a reasonable and new paradigm to protect the aging brain cells to prevent AD.