Enhanced caffeine-induced Ca²⁺ release in the 3xTg-AD mouse model of Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent form of dementia among the elderly and is a complex disorder that involves altered proteolysis, oxidative stress and disruption of ion homeostasis. Animal models have proven useful in studying the impact of mutant AD-related genes on other cellular signaling pathways, such as Ca²⁺ signaling. Along these lines, disturbances of intracellular Ca²⁺ ([Ca²⁺]_i) homeostasis are an early event in the pathogenesis of AD. Here, we have employed microfluorimetric measurements of [Ca²⁺]_i to investigate disturbances in Ca²⁺ homeostasis in primary cortical neurons from a triple transgenic mouse model of Alzheimer's disease (3xTg-AD). Application of caffeine to mutant presenilin-1 knock-in neurons (PS1_KI) and 3xTg-AD neurons evoked a peak rise of [Ca²⁺]_i that was significantly greater than those observed in non-transgenic neurons, although all groups had similar decay rates of their Ca²⁺ transient. This finding suggests that Ca ²⁺ stores are greater in both PS1_KI and 3xTg-AD neurons as calculated by the integral of the caffeine-induced Ca²⁺ transient signal. Western blot analysis failed to identify changes in the levels of several Ca²⁺ binding proteins (SERCA-2B, calbindin, calsenilin and calreticulin) implicated in the pathogenesis of AD. However, ryanodine receptor expression in both PS1_KI and 3xTg-AD cortex was significantly increased. Our results suggest that the enhanced Ca²⁺ response to caffeine observed in both PS1_KI and 3xTg-AD neurons may not be attributable to an alteration of endoplasmic reticulum store size, but to the increased steady-state levels of the ryanodine receptor.