In my last post regarding CNS myelin I left out some pretty important basic information about neuronal messaging. The most important elements in the CNS regarding action potential firing are potassium and sodium that when dissolved in water become charged ‘ions’. The axon membrane can separate charges between the outer and inner layers to store electrical energy which is called axon capacitance which helps propagate electrical signals along the axon. A neuron at rest has a more negative charge on the inside in relation to the outside because the neuronal membrane is semi permeable and only lets certain ions pass through. Potassium ions (K+) can enter readily, but sodium ions (Na+) have a harder time. The sodium potassium pump (Na+K+ATPase) also takes 3 Na+ out of the neuron for every 2 K+ it puts in, a very energy intensive process that uses energy from ATP hydrolysis. There is about a 70 millivolt less charge inside the cell full of K+ relative to outside the cell with high Na+ concentrations for a -70 millivolt resting membrane potential. Action potentials need their differential concentrations across the neuronal membrane to occur. So, when the action potential is initiated in the axon initial segment, the sodium channels open and Na+ rushes across the membrane into the neuron causing it to depolarize and then the potassium channels open just as the sodium channels close and K+ rushes across to the outside for repolarization. The resting memory potential goes toward 0 millivolts, but usually fires at about -55 millivolts. During the big influx of Na+ into the neuron, calcium ions (Ca2+) also pass into the neuron that must be removed after repolarization to avoid toxic calcium overload. The Na+/Ca2+ exchanger (NCX) uses the Na+ electrochemical gradient to remove one Ca2+ across the cell membrane for every 3 Na+ it adds in. At the same time the Na+K+ATPase is pumping Na+ out, exemplifying its importance and why it uses so much energy. In situations with decreased energy, the NCX can reverse and bring calcium into the cell causing calcium overload which activates intra-axonal calpains that starts the axonal degeneration process that breaks down cytoskeletal elements like neurofilaments and microtubules.
Anyhow, in the myelinated axon, the ion channels are separated from the internode by myelin structures to keep the Na+ channels clustered to the nodes and K+ channels to the juxtaparanode to regenerate the action potential there to ping it along to the next nodes. However, in demyelination, the axon has to adaptively respond to hopefully restore some conduction through the demyelinated segment. Axon segments further down the axon can still be myelinated with preserved conduction once the signal makes it through the demyelinated segment depending on damage extent and remyelination. Axonal adaptations are much more effective in shorter axonal segments with smaller diameters and are thought to be important to RRMS recovery. Gene-expression in demyelinated neurons see wide-ranging expression changes in axonal transport, synaptic stability, inhibitory neurotransmission, and cell stress pathway activation, thus virtually all aspects of their cellular function are altered following demyelination.
Without myelin, the ion channels are no longer held in place to cluster at the nodes and spread out along the axon. This means the entire axon membrane now has to depolarize to regenerate the axon potential. So, conduction in a complete myelinated axon is like throwing a rock into the water that skips 10 times, while an unmyelinated axon is like bending down and picking up a rock and throwing it in and then repeating this 10 times. Action potential propagation in unmyelinated axons needs more Na+ influx to overcome the higher capacitance to change potential. Thus, the sodium channels that are now spread along the axon have to increase expression, and they also increase the Nav1.2 sodium channel type. It creates a more intermittent Na+ current than the Nav 1.6 channel seen in myelinated axons and usually just expresses in unmyelinated axons. The increased Na+ expression accumulates even more Na+ in the axon that has to be removed for repolarization by increasing Na+K+ATPase operation. This increased energy demand is met by the neuron producing larger sized mitochondria in higher quantities that it then has to move out into the axon. These are along with mRNA, proteins, and other organelles produced in the neuronal cell body that must also transport down the axon creating quite a bottleneck. Mitochondria in the axon include mobile and stationary types, with stationary mitochondria congregating at high metabolic demand areas and increasing anchoring protein syntaphilin to lock in there. As these mitochondria and other cargoes have to transfer along the axon near or through inflammatory areas, they can be subject to damage. Though both mitochondria anterograde and retrograde transport deficits are seen in MS models, anterograde transport is worse and precedes axon blebbing and degeneration. Inflammatory mediators like ROS and RNS increase in the axon due to the increased mitochondria and inflammation that impair axon transport. Glutamate excitotoxicity and the pro-inflammatory cytokine TNFα also affect axon transport by causing histone deacetylase 1 (HDAC1) to relocate to the axon. It interacts with kinesins, which move axon cargo, blocking kinesin interaction with mitochondria. As kinesins need ATP energy to move cargo along the axon, a feedback loop can be set up where the axon doesn’t have enough energy to move mitochondria to meet local energy needs, hurting energy production even more further down the axon. This is seen in chronic MS with decreased mitochondria in distal axons.
Microglia/macrophages get close up next to axons in inflammatory demyelination, and produce ROS and RNS that can damage mitochondria. The reactive species nitric oxide (NO) isn’t inherently cytotoxic, but negatively affects mitochondria complex IV which reduces mitochondria respiration. Basically, NO disrupts neuron energy production, compromising the axon’s ability to remove excess sodium, which can cause NCX reversal.
Mitochondria DNA (mtDNA) doesn’t have protective histones and some DNA repair enzymes making it open to damage, which is seen in MS. Chronic MS accumulates respiratory deficient neurons with high rates of mtDNA deletions, which amplify over time via clonal expansion. Mitochondria injury and respiratory chain dysfunction free more electrons that can react with oxygen to induce more ROS-mediated damage. This feedback loop increasingly damages neuron energy production.
Damaged mitochondria are removed by mitophagy, which requires mitochondria membrane fusion with the lysosome in the soma or to some extent in the axon. As stationary mitochondria in the axon are anchored into high energy demand areas, any that are damaged through inflammation or chronic stress can’t be removed by this process. Therefore, chronic demyelination in SPMS needs an increased energy supply, but instead accumulates damaged mitochondria that can’t be removed.
Nonetheless, these axon adaptations help restore conduction through very short demyelinated segments, but it ends up in slow, ectopic, energy-intensive action potential propagation. These sections are more excitable and create trains of ectopic impulses that can cause spasms, tingling, numbness, pain, and zingers seen in MS. These adaptations are also a short term fix as the unmyelinated axon is open to damage from a multitude of factors without OL support and remyelination. As glucose metabolism decreases with aging, the ability of OLs to provide glycolysis byproducts via MCTs and metabolites, proteins, lipids, mRNAs, and miRNAs via EVs is all the more important. However, remyelinated axons also see increased mitochondria, so metabolic support isn’t fully restored and a higher energetic burden is put on neurons. Remyelinated myelin sheaths are thinner and internodes are shorter than those made during development that may cause persistent motor deficits seen even with remyelination. However, depending on its extent, it can decrease axon damage and restore ion channel clustering for action potential propagation to restore saltatory conduction.
Duncan GJ, Simkins TJ and Emery B (2021) Neuron-Oligodendrocyte Interactions in the Structure and Integrity of Axons. Front. Cell Dev. Biol. 9:653101. https://doi.org/10.3389/fcell.2021.653101
Catherine Lubetzki, Bruno Stankoff, Chapter 4 – Demyelination in multiple sclerosis, Editor(s): Douglas S. Goodin, Handbook of Clinical Neurology, Elsevier, Volume 122, 2014, Pages 89-99, ISSN 0072-9752, ISBN 9780444520012, https://doi.org/10.1016/B978-0-444-52001-2.00004-2.
Licht-Mayer, S., Campbell, G.R., Canizares, M. et al. Enhanced axonal response of mitochondria to demyelination offers neuroprotection: implications for multiple sclerosis. Acta Neuropathol 140, 143–167 (2020). https://doi.org/10.1007/s00401-020-02179-x