AMPK preferentially depresses retrograde transport of axonal mitochondria during localised nutrient deprivation

Mitochondrial clusters are found at regions of high energy demand, allowing cells to meet local metabolic requirements while maintaining neuronal homeostasis. AMP-activated protein kinase (AMPK), a key energy stress sensor, responds to increases in AMP/ATP ratio by activating multiple signalling cascades to overcome the energetic deficiency. In many neurological conditions, the distal axon experiences energetic stress independent of the soma. Here, we used microfluidic devices to physically isolate these two neuronal structures and to investigate whether localised AMPK signalling influenced axonal mitochondrial transport. Nucleofection of primary cortical neurons, derived from E16 mouse embryos (both sexes), with mito-GFP allowed monitoring of the transport dynamics of mitochondria within the axon, by confocal microscopy.

Pharmacological activation of AMPK at the distal axon (0.1 mM AICAR) induced a depression of the mean frequency, velocity and distance of retrograde mitochondrial transport in the adjacent axon. Anterograde mitochondrial transport was less sensitive to local AMPK stimulus, with the imbalance of bi-directional mitochondrial transport resulting in accumulation of mitochondria at the region of energetic stress signal. Mitochondria in the axon-rich white matter of the brain rely heavily on lactate as a substrate for ATP synthesis. Interestingly, localised inhibition of lactate uptake (10 nM AR-C155858) reduced mitochondrial transport in the adjacent axon in all parameters measured, similar to that observed by AICAR treatment. Co-addition of compound C restored all parameters measured to baseline levels, confirming the involvement of AMPK. This study highlights a role of AMPK signalling in the depression of axonal mitochondrial mobility during localised energetic stress.