For some time it appeared that the inclusions found in the cytosol of neurons in degenerative disease were causing the disease, likely due to disruption of movement of organelles and proteins up and down the axons. But evidence is now suggesting that may not be the case. With TDP-43, it appears that depletion of it from the nucleus is the cause of disease. With SOD1, an unknown toxic gain of function is still theorized. However, a recent study reported that mutant SOD1 interacts with TDP-43 where normal SOD1 did not. Conflicting previous studies have found and not found misfolded SOD1 inclusions in the motor neuron cytoplasm of sporadic ALS patients, though a study using novel antibodies specific for denatured SOD1 reported small inclusions in all tested SALS patients. SOD1 is a highly complex protein and such are easily misfolded. Usually this is no problem as either chaperones refold the protein or intracellular autophagy destroys the errant protein. But age and stress can cause autophagy to decrease in effectiveness, possibly leaving errant proteins in the cell where they can do damage. So assuming that mutant SOD1 is present either by genetic mutation or routine misfolding not corrected or cleared by the cell, and assuming interactions between mutant SOD1 and TDP-43 where TDP-43 is depleted from the nucleus, it could be held that mutant or misfolded SOD1 causes disease through depletion of nuclear TDP-43.
Activation of the glial cells (astrocytes, microglia, etc.) is a driving force in ALS progression. Experiments where mutant SOD1 was limited solely to the glia demonstrated the ability to drive disease on their own. In the case of inherited forms of ALS where particular mutated genes produce mutated forms of proteins throughout the body it is easy to imagine disease spreading rapidly once initiated. But what drives the more common sporadic forms? A clue might be found by looking at prion disease. In fact, a recent study shows that Huntington's Disease may very well spread this way, and it may be applicable to other neurodegenerative diseases. Indeed another study found that extracellular SOD1 can induce microglia to release pro-inflammatory cytokines and free radicals which promote motorneuron damage. A more recent study showed that introduced mutant SOD1 can induce disease. The biotech company Amorfix makes antibodies against extracellular mutant SOD1 and is now in trials to use these antibodies as a vaccine against ALS.
So with mutant or misfolded SOD1 we have multiple paths for disease as well as a likely pathway for spread of disease. Each of these questions are comparatively easy to test and seemingly easy to intercept in the extracellular space. I look forward to more studies to further illuminate these questions.