Cellular pathology in ALS is characterized by a build-up of free radicals and increased oxidative stress. Studies of cells in certain ALS animal models and also in ALS patients have shown an increase in oxidative damage from free radicals, and apparently, the more the disease progresses, the greater the amount of oxidative damage. The most widely studied familial ALS gene is SOD1, an enzyme that uses copper and zinc to produce hydrogen peroxide (H2O2) from toxic superoxide anion radicals ().1 SOD1 is a potent antioxidant that is essential for defending cells against damage from superoxide radicals. Multiple SOD1 variants are causal for ALS and many theories exist to account for the disease-causing effects of these mutations. There theories include: preferential localization of SOD1 to the cytoplasm from the nucleus resulting in nuclear oxidative damage2, SOD1 aggregation in the mitochondria resulting in bioenergetic deficits3, increased production of the hydroxyl radical from hydrogen peroxide4, and proteasome destabilization5. SOD1 neurons undergoing endoplasmic reticulum (ER) stress are also more susceptible to death in vivo6 and astrocytes with the SOD1 mutation can cause death of motor neurons by a mitochondrial dependent mechanism.7 There is much controversy over proving that oxidative process is what truly causes the neuronal insults characteristic of ALS, and although oxidative damage does in fact occur in the disease, some researchers believe it is a secondary, downstream effect of the disease.
Abnormal Glial Interactions
While ALS only attacks motor neurons, ALS is much more than a motor neuron disease. Cells in the spinal cord known as microglia can become activated during ALS. These cells are also involved in inflammation and appear to accelerate disease progression. Astrocytes and microglia, cells entrusted to keep motor neurons healthy and free from infection have been shown to produce toxic substances that damage motor neurons, fueling the progression of ALS. Macrophages and certain subtypes T-cells infiltrate the nervous system potentially unleashing a storm of cytokines of their own that further contributes to the disease. Oligodendrocytes appear to lose their ability to provide energy-rich lactate to motor neurons, and the lack of this critical metabolite may be contributing to their denervation and death. Evidence for the role of glial interactions and inflammation in ALS progression come from in vivo models of the disease and from patients. The ALS SOD1 mouse models show, over the course of disease progression, a shift in microglial activation state from a trophic neuroprotective role (M2) into a more deleterious anti-microbicidal role (M1), which is toxic to neurons.8 Multiple studies have also highlighted that glia in their inflammatory state are especially toxic to motor neurons, and especially to those harboring ALS mutations.9
It has been demonstrated that in ALS misfolded proteins accumulate and aggregate causing an unnatural clumping of intracellular proteins. A major pathological hallmark of ALS is abnormal accumulation of misfolded oligomers or protein inclusions containing TDP-43, FUS or SOD1 protein.10 Aggregates of these proteins have been isolated from both familial ALS and sporadic ALS patients, suggesting they are important features of the disease.
Additionally, dipeptide repeats formed in C9orf72 cases, the most common genetic cause of ALS, also result in significant UPR induction resulting in UPR mediated neuronal death12. C9orf72, TDP-43, SOD1 and FUS mutant cases show significant induction of unfolded proteins which mediate UPR activation and cell death30. Also, deubiquitinase inhibitors targeted at proteins such as Usp14 have shown promise likely due to the degradation of aberrant misfolded proteins which would otherwise be detected by the UPR.31,32 Moreover, hyperexcitability, an exciting topic in ALS research as well as the target of retigibine, has been shown to induce the UPR which may produce a vicious cycle resulting in cellular death13,14.
Mitochondria are small organelles that act as the powerhouses of every cell. They supply energy crucial to cell survival. Neuronal cells are highly dependent on mitochondria, and mitochondrial dysfunction is associated with a variety of neurodegenerative diseases. Defects in mitochondrial function in ALS are many, including defective bioenergetics, impaired morphology, impaired trafficking, and abnormal calcium homeostasis. Studies of neurons in ALS patients have been shown to be hypometabolic, suggesting widespread mitochondrial dysfunction.11 Furthermore, many studies have shown mitochondrial morphology abnormalities in SOD1-fALS patients, both from patient biopsy and post-mortem tissue.11 Mitochondria are targets of SOD1 toxicity, with mutant SOD1 mice developing progressive bioenergetics abnormalities in the CNS13, impaired ATP production14 and defective calcium uptake.15
Mitochondrial movement throughout motor neurons – a normal occurrence – has been demonstrated to become erratic in ALS.12 Drugs that inhibit apoptosis have shown some success in ALS mice – they delay disease onset and progress. These studies and many others suggest mitochondrial dysfunction is present and drives ALS progression.
For more information on ALS visit the ALS Association website at www.alsa.org.
- Tanzi et al. Mutations in Cu/Zn superoxide dismutase gene are associated with familial amyotrophic lateral sclerosis. 1993.
- Sau D et al. Mutation of SOD1 in ALS: a gain of a loss of function. Mol. Gen. 2007.
- Vijayvergiya et al. Mutant Superoxide Dismutase 1 (SOD1) Forms Aggregates in the brain Mitochondrial Matrix of Amyotrophic Lateral Sclerosis Mice. J. Neurosci.
- Yim MB et al. Copper, zinc superoxide dismutase catalyzes hydroxyl radical production from hydrogen peroxide. 1990.
- Mulligan VK et al. Early Steps in oxidation-induced SOD1 misfolding: implications for non-amyloid protein aggregation in familial ALS. Mol. Biol. 2012.
- Saxena, S et al. A role for motoneuron subtype–selective ER stress in disease manifestations of FALS mice. Nature Neuroscience.
- Cassina, P et al. Mitochondrial Dysfunction in SOD1G93A-Bearing Astrocytes Promotes Motor Neuron Degeneration: Prevention by Mitochondrial-Targeted Antioxidants. Neurosci. 2008.
- Lewis, CA et al. The Neuroinflammatory Response in ALS: The Roles of Microglia and T Cells. Neurology Research International.
- Di Giogio, FD et al. Non-cell autonomous effect of glia on motor neurons in an embryonic stem cell-based ALS model. Nature Neuroscience.
- Matus, S et al. ER dysfunction and protein folding stress in ALS. International J. of Cell Bio. 2013.
- Sasaki, S et al. Mitochondrial alterations in the spinal cord of patients with sporadic amyotrophic lateral sclerosis. J. Neuropathol. Exp. Neurol. 66, 10–16.
- Sheng, Z. et al. Mitochondrial transport in neurons: impact on synaptic homeostasis and neurodegeneration. Rev. Neuroscience. 2012.
- Martin, L.J et al. Mitochondrial pathobiology in ALS. Bioenerg. Biomembr. 43, 569–579.
- Mattiazzi, M., et al. Mutated human SOD1 causes dysfunction of oxidative phosphorylation in mitochondria of transgenic mice. J. Biol. Chem. 277, 29626–29633.
- Parone, P.A., et al. Enhancing mitochondrial calcium buffering capacity reduces aggregation of misfolded SOD1 and motor neuron cell death without extending survival in mouse models of inherited amyotrophic lateral sclerosis. Neurosci. 33, 4657–4671.
- Blokhuis, AM et al. Protein aggregation in amyotrophic lateral sclerosis. Acta Neuropathologica. 2013.
- Atkin, G et al. Ubiquitin pathways in neurodegenerative disease. Front Mol. Neurosci.
- Lee, BH et al. Enhancement of proteasome activity by a small-molecule inhibitor of USP14. 2010.