Imaging mitochondrial motility within live hippocampal neurons: alterations due to amyloid-β, hyperglycaemia or in vitro aging

Appropriate mitochondrial activity plays a vital role in neuronal function, from efficient ATP provision to localised Ca2+-buffering that modulates synaptic activity. Mitochondrial dysfunction is associated with many neurodegenerative diseases. Ca2+ overload disrupts mitochondrial integrity, disabling ATP production, releasing accumulated Ca2+ and disrupting redox/antioxidant balance, contributing to neuronal death. Chronically-neurotoxic environments (such as a build-up of the Alzheimer’s-associated amyloid-β peptide or hyperglycaemia) may disrupt mitochondrial turnover or motility, ultimately contributing to cellular decline. Questions remain, as to the temporal sequence, interplay and relative importance of such changes in mitochondrial physiology to neurodegeneration.
Using epifluorescence imaging of primary hippocampal cells, we showed that chronic exposure to amyloid-β1-42 (500 nM, 1 h), decreased mitochondrial motility with no gross change in mitochondrial morphology or membrane potential. Acutely switching mature neurons (3 weeks in vitro) from the commonly-used supra-physiological glucose media (25 mM) into physiological glucose (3 mM) increased mitochondrial motility; whereas immature neurons (1 week) displayed the same, high level of mitochondrial motility in either hyperglycaemic or physiological glucose. Repeatedly imaging the same individual cells throughout in vitro maturation revealed a decrease in mitochondrial motility plus shift in morphology: from a heterogeneous population containing many branched and extended mitochondria (≥20 μm) in young neurons, towards a more-homogeneous population largely comprising small organelles dispersed along processes. These results indicate that an inhibition of mitochondrial motility may be an early event preceding Alzheimer’s-associated neurodegeneration, which may also be exacerbated by diabetic-like hyperglycaemia but may also be influenced, or indeed masked, by differences in cell age.