Enhanced axonal response of mitochondria to demyelination offers neuroprotection: implications for multiple sclerosis

Abstract

Axonal loss is the key pathological substrate of neurological disability in demyelinating disorders, including multiple sclerosis (MS). However, the consequences of demyelination on neuronal and axonal biology are poorly understood. The abundance of mitochondria in demyelinated axons in MS raises the possibility that increased mitochondrial content serves as a compensatory response to demyelination. Here, we show that upon demyelination mitochondria move from the neuronal cell body to the demyelinated axon, increasing axonal mitochondrial content, which we term the axonal response of mitochondria to demyelination (ARMD). However, following demyelination axons degenerate before the homeostatic ARMD reaches its peak. Enhancement of ARMD, by targeting mitochondrial biogenesis and mitochondrial transport from the cell body to axon, protects acutely demyelinated axons from degeneration. To determine the relevance of ARMD to disease state, we examined MS autopsy tissue and found a positive correlation between mitochondrial content in demyelinated dorsal column axons and cytochrome c oxidase (complex IV) deficiency in dorsal root ganglia (DRG) neuronal cell bodies. We experimentally demyelinated DRG neuron-specific complex IV deficient mice, as established disease models do not recapitulate complex IV deficiency in neurons, and found that these mice are able to demonstrate ARMD, despite the mitochondrial perturbation. Enhancement of mitochondrial dynamics in complex IV deficient neurons protects the axon upon demyelination. Consequently, increased mobilisation of mitochondria from the neuronal cell body to the axon is a novel neuroprotective strategy for the vulnerable, acutely demyelinated axon. We propose that promoting ARMD is likely to be a crucial preceding step for implementing potential regenerative strategies for demyelinating disorders.

Mitochondrial bioenergetic deficits in C9orf72 amyotrophic lateral sclerosis motor neurons cause dysfunctional axonal homeostasis

“…The successful manipulation of a key pathway involved in mitochondrial energy metabolism adds to the emerging spinal cord injury literature that has shown that restoring cellular energetics and axonal transport promotes axonal outgrowth and regeneration [ 42 ]. Our data suggest that targeting this tractable pathway may thus also be highly relevant to neurodegeneration [ 56 , 75 ], accepting the possibility of there being other competing mechanisms driving the dysfunctional axonal homeostasis observed in C9orf72 MNs.…”

“…Briefly, FLAG-PGC1α-6xHIS (originated from Addgene [67637]) and P2A-eGFP (originated from pDONR-P2A-eGFP), were shuttled into a lentiviral backbone pLenti6-cppt-delta CMV-DEST-opre, to generate a FLAG-PGC1α-6xHIS-P2A-eGFP lentiviral construct. PGC1α overexpression lentivirus was generated as previously described [ 63 , 76 ] and transduced as previously described by our group [ 56 ] in human iPSC-derived MNs at a multiplicity of infection (MOI) of 10. This achieved a mean transduction efficiency of circa 70% MNs.…”

“…MNs, plated on laminin/Matrigel ® /fibronectin-coated µ-slide 8-well ibidi dishes (80826, ibidi GmbH) at a density of 50,000–70,000 cells per well were sparsely transduced with lentivirus expressing mitoDsRed2 at platedown using an MOI of 0.5, optimised to visualise 1–2 labelled cells per field of view, as previously described by our group [ 37 , 56 ]. Fluorescence live-cell imaging of mitochondrial axonal transport was performed at day 21 post platedown at 63X magnification (Plan-Apochrat 1.40 NA oil DIC M27 objective, Carl Zeiss) using an Axio Observer Z1 inverted motorised microscope (Carl Zeiss) equipped with a Cy3 FL filter-set (Carl Zeiss), Zen 2011 z-stack, time lapse and Definite Focus modules (Carl Zeiss), and an S1 Environmental System (Carl Zeiss) incubation chamber maintained at 37 °C and 5% CO 2 .…”

Neuronal hibernation following hippocampal demyelination

“…Failure to exchange axonal Na + for extracellular K + will activate the Na + /Ca 2+ exchanger and dramatically increase axoplasmic concentrations of Ca 2+ , which can induce a virtual axonal hypoxia and cause axonal degeneration [ 91 ]. While the axon eventually compensates for this increased energy demand by increasing the volume of axoplasmic mitochondria [ 92 – 95 ], it may be initially vulnerable to the increased ionic exchange associated with loss of myelin. We consider dendritic silencing to be a neuroprotective response that reduces axonal conduction and helps prevent degeneration of demyelinated axons.…”

Advances in microfluidic in vitro systems for neurological disease modeling

“…More recently Licht‐Mayer et al used an axonal guidance device as a model of demyelination using a co‐culture of neurons with oligodendrocyte precursor cells and inducing demyelination by exposing the axonal compartment to lysolecithin, (which is similarly used in animal models of demyelination). Licht‐Mayer et al used this system to help uncover a novel process in which mitochondria move from the neuronal cell body to the demyelinated axon, increasing axonal mitochondrial content, and that enhancing this process protects acutely demyelinated axons from degeneration (Licht‐Mayer et al., 2020). Understanding such mechanisms of protection may have profound future implications for treatment of MS.…”