Calcium dysregulation and membrane disruption as a ubiquitous neurotoxic mechanism of soluble amyloid oligomers

Increasing evidence suggests that amyloid pepticles associated with a variety of degenerative diseases induce neurotoxicity in their intermediate oligomeric state, rather than as monomers or fibrils. To test this hypothesis and investigate the possible involvement of Ca²⁺ signaling disruptions in amyloid-induced cytotoxicity, we made homogeneous preparations of disease-related amyloids (Aβ, prion, islet amyloid polypeptide, polyglutamine, and lysozyme) in various aggregation states and tested their actions on fluo-3-loaded SH-SY5Y cells. Application of oligomeric forms of all amyloids tested (0.6-6 μg ml^-1) rapidly (∼5s) elevated intracellular Ca²⁺, whereas equivalent amounts of monomers and fibrils did not. Ca²⁺ signals evoked by Aβ42 oligomers persisted after depletion of intracellular Ca²⁺ stores, and small signals remained in Ca²⁺-free medium, indicating contributions from both extracellular and intracellular Ca²⁺ sources. The increased membrane permeability to Ca²⁺ cannot be attributed to activation of endogenous Ca²⁺ channels, because responses were unaffected by the potent Ca²⁺-channel blocker cobalt (20 μm). Instead, observations that Aβ42 and other oligomers caused rapid cellular leakage of anionic fluorescent dyes point to a generalized increase in membrane permeability. The resulting unregulated flux of ions and molecules may provide a common mechanism for oligomer-mediated toxicity in many amyloidogenic diseases, with dysregulation of Ca²⁺ ions playing a crucial role because of their strong trans-membrane concentration gradient and involvement in cell dysfunction and death.