Beta-amyloid inhibits integrated mitochondrial respiration and key enzyme activities

β-Amyloid Fragment 25–35 Causes Mitochondrial Dysfunction in Primary Cortical Neurons

Beta-amyloid deposition and compromised energy metabolism both occur in vulnerable brain regions in Alzheimer's disease. It is not known whether beta-amyloid is the cause of impairment of energy metabolism, nor whether impaired energy metabolism is specific to neurons. Our results, using primary neuronal cultures, show that 24-h incubation with A beta(25-35) caused a generalized decrease in the specific activity of mitochondrial enzymes per milligram of cellular protein, induced mitochondrial swelling, and decreased total mitochondrial number. Incubation with A beta(25-35) decreased ATP concentration to 58% of control in neurons and 71% of control in astrocytes. Levels of reduced glutathione were also lowered by A beta(25-35) in both neurons (from 5.1 to 2.9 nmol/mg protein) and astrocytes (from 25.2 to 14.9 nmol/mg protein). We conclude that 24-h treatment with extracellular A beta(25-35) causes mitochondrial dysfunction in both astrocytes and neurons, the latter being more seriously affected. In astrocytes mitochondrial impairment was confined to complex I inhibition, whereas in neurons a generalized loss of mitochondria was seen.

Mitochondria, Ca2+ and neurodegenerative disease

Mitochondria play a central role in cell biology not only as producers of ATP, but also in the sequestration of Ca(2+) and the generation of free radicals. They are also repositories of several proteins which regulate apoptosis. Perturbations in the normal functions of mitochondria will inevitably disturb cell function, may sensitise cells to neurotoxic insults and may initiate cell death. Neuronal Ca(2+) overload, such as follows excessive stimulation of Ca(2+) permeant excitatory amino acid receptors, can cause cell death. Recent evidence suggests that the accumulation of Ca(2+) into mitochondria during episodes of cellular Ca(2+) overload initiates a cascade of events that culminate in cell death. Cell death appears to require not only mitochondrial Ca(2+) overload, but rather a combination of raised intramitochondrial Ca(2+) concentration with increased production of nitric oxide and possibly other oxyradical species. Cell death may proceed through either necrotic or apoptotic mechanisms, depending on the rate of consumption and depletion of ATP. Evidence is also accumulating to suggest that more subtle alterations in mitochondrial function may serve as predisposing factors in the pathogenesis of a number of neurodegenerative disorders.