Presenilins, Deranged Calcium Homeostasis, Synaptic Loss and Dysfunction in Alzheimer's Disease

Alzheimer's disease (AD) is the most common age-related neurodegenerative disorder affecting millions of people. Synaptic dysfunction and physical loss of synapses are responsible for memory impairments in AD. The molecular mechanisms responsible for synaptic loss in AD are not understood. The main risk factor for sporadic AD (SAD) is advanced age. Missense mutations in presenilin (PS) proteins and in amyloid precursor protein (APP) are responsible for majority of rare familial AD (FAD) cases. Increased production of A 42 amyloidogenic peptide occurs in SAD and FAD. Synaptotoxic effects of A 42 may be linked to synaptic loss in AD. FAD mutations in PS proteins disrupt endoplasmic reticulum (ER) calcium (Ca²⁺) leak function of PSs and result in increased Ca²⁺ levels in neuronal ER. Similar increases in neuronal ER Ca²⁺ levels occur in aging neurons. Increased neuronal ER Ca²⁺ levels lead to a compensatory upregulation of ER Ca²⁺ release channels, the ryanodine receptors (RyanR), and downregulation of the synaptic store-operated Ca²⁺ entry (SOC) pathway. In this review we propose a hypothesis that excessive Ca²⁺ release from the ER and insufficient SOC Ca²⁺ entry lead to destabilization and eventual elimination of mature mushroom spines in PS-FAD neurons and in aging SAD neurons. The proposed Ca²⁺-dependent spine destabilization mechanism may act in parallel or synergistically with A 42 synaptotoxicity mechanisms. The proposed model may help to establish a cause-and-effect connection between abnormal Ca²⁺ and amyloid homeostasis and synaptic loss in AD.