The death of motor neurons is one of the main pathological hallmarks of ALS, and the disease often starts at a small muscle and propagates to other muscles. Muscle denervation appears during the early stages of ALS pathogenesis, and it can be observed by electromyography. This denervation is the result of motor neuron degeneration, probably with a series of pathogenetic factors converging to create a toxic microenvironment. Yet some scientists are not so sure motor neurons die, they think they might simply enter a sort of frozen state to mitigate a stressful situation. This article belongs to a tiny circle as it tells that it's possible to partially reverse the disease and it presents a good mechanism of action, whereas most articles are extremely vague about ALS etiology.
Indeed, it is known that plastic events, such as synaptic plasticity, axonal sprouting, and morphological changes, within the spared motor neuron population can be responsible for compensatory adaptation after the loss of function caused by the neurotoxic removal of a spinal motor neuron subset. These spontaneous plastic changes are known to take place also in ALS models, but their ability to sustain motor function is transient and incapable of counteracting disease progression. Therefore, a therapeutic approach to manage the disease (but not cure it) should be capable of both improving plastic changes and supporting neuroprotection to slow down motor neuron degeneration.
In the authors' view, the use of a simplified in vivo model of motor neuron degeneration would help in the step-by-step dissection of ALS pathogenesis.
The authors used a specific toxin, CTB-Sap, to selectively kill certain motor neurons in the spinal cord by injecting a compound into muscles. CTB-Sap is a compound made by combining cholera toxin-B (which binds to neurons) with saporin (a toxin that kills cells). When injected into muscles, this compound is taken up by the synapses of the lower motor neurons that control those muscles. After the motor neurons take up CTB-Sap, it travels backward (retrograde) along the neuron to the cell body in the spinal cord. The saporin then kills the neuron.
It is a valuable tool for studying compensatory plastic changes, including synaptic plasticity, axonal sprouting, and other morphological and functional adaptations. The authors think that in ALS animal models, when motor neuron degeneration occurs progressively, the remaining cells may try to compensate for the motor deficits. It's when the progressive loss of motor neurons exceeds the compensatory capacity of the surviving cells, that the first signs of the disease appear.
Despite intensive research, it remains poorly understood why motor neurons are specifically targeted in ALS. As motor neurons and the skeletal muscle they control consume enormous amounts of energy, mitochondria use aerobic respiration to generate adenosine triphosphate (ATP), which is used throughout the cell as a source of chemical energy.
Mitochondria are sort of microbes symbiotes of cells and like other microbes, they can divide or even fusion depending on the needs of the host cell. Several genes encode fission and fusion proteins: MFN1, MFN2, OPA1, DRP1 (Dynamin-Related Protein 1): This protein is essential for mitochondrial fission. MFF (Mitochondrial Fission Factor).
Several publications have associated abnormal mitochondrial dynamics with excessive mitochondrial fission predominantly mediated by the hyperactivation of the dynamin-related protein 1 (DRP1). This cytosolic GTPase, is recruited to the outer mitochondrial membrane, where it assembles into a ring-like structure around the mitochondria, causing constriction and subsequent division. High levels of DRP1 trigger mitochondrial damage which causes insufficient ATP production, indeed fission and fusion events consume a lot of energy.
To prove that an abnormal fission (division) of mitochondria causes a motor neuron disease, it's necessary to show that in such a case inhibiting mitochondria restores some muscle function. Previous studies have proven that spontaneous motor recovery is possible sometimes after toxin administration. Yet these kinds of plastic changes are not enough to counteract the functional effects of the progressive motoneuron degeneration. The authors wanted to use a mitochondrial division inhibitor to prove that it's the mitochondrial division that is a cause of motor neuron disease.
Mdivi-1, a cell-permeable quinazolinone, is an inhibitor of DRP1, so it is capable of inhibiting the fission process by directly decreasing the GTPase enzymatic activity of DRP1. This results in neuroprotection in animal models of Alzheimer’s disease, Parkinson’s disease, and multiple sclerosis. In an attempt to determine the therapeutic impact of Mdivi-1 after motor neuron loss, the scientists used the already established mouse CTB-Sap model which is characterized by up-regulation of DRP1, together with increased mitochondrial fission. They wanted to investigate if the administration of Mdivi-1 could be neuroprotective on damaged or stressed motor neurons, and whether it may promote spinal cord (SC) plasticity.
The drug was administered to the mouse model, a localized removal of spinal motor neurons was induced by injection of CTB-Sap in the calf muscle and it moved from the muscle to the lower motor neuron retrogradly. This simple model of selective motor neuron depletion allows the scientists to focus on the functional and molecular mechanisms of neuroplastic changes upon motor neuron removal.
As expected, a few days after CTB-Sap injection in the right calf muscle, all animals started to display an evident decline in the motor activity of the right back leg that reached a maximum during the first two weeks after the lesion. The observation of limb motion during free exploration of an open field revealed frequent curling of toes, loss of support, and foot-dragging. Motor deficits were accompanied (and caused by) the partial loss of motor neurons innervating the calf muscle and located in the lumbar region of the spinal cord, and the muscle denervation is confirmed by the presence of spontaneous electromyography activity in anesthetized mice.
This functional decline was followed by a spontaneous partial recovery during the experimental period and, as the scientists hypothesized, Mdivi-1 treatment was capable of reducing the early back leg deficit despite the presence of a slightly toxic effect of the drug, as demonstrated by the loss of body weight. The grid walk test confirmed the beneficial effects of treatment in the preservation of motor performance, although a spontaneous recovery (but slower) was seen also in untreated animals.
The beneficial effects of the Mdivi-1 drug were probably limited to some aspects of the motor activity, such as motor coordination, as suggested by clinical scoring and grid walk test results, whereas gait analysis was not able to efficiently reveal the effects of treatment.
However, the observed effects of treatment on motor coordination cannot be explained only by its action on the affected muscle, and a detailed mechanistic study of mitochondrial dynamics should include, for instance, the spinal cord, cerebellum, motor cortex, basal ganglia, and also some general aspects of metabolism.
The phenomena of motor neurons attempting to form new connections and adapt to new conditions in the tissue microenvironment in response to tissue damage or neuronal loss have been well documented in the literature. This process may increase in soma size and dendritic complexity of surviving motor neurons, which might be attributable to their active hunt for new synapses and increased synaptic efficacy. Therefore, the observed increase in motor neurons’ size only in Mdivi-1-treated mice may be proof of neuronal adaptation, promoted by the known activity of the drug onto mitochondrial dynamics, and likely involving motor neuron itself but also the whole sensorimotor spinal cord circuitry and supraspinal pathways.
The results of the present study have confirmed that the CTB-Sap model is a valid tool for research in motor neuron diseases, proving that compensatory plastic changes may take place after the removal of a spinal motor neuron subset. Moreover, it seems likely that treating this animal model with a drug known to inhibit mitochondrial fission may increase this intrinsic plastic capability and protect motor neurons from degeneration.