One of the earliest observations in ALS is a weakening of the energy-producing organelles of the cell called mitochondria. The mitochondria are found to only output about half of the energy as normal while producing more waste in the form of Reactive Oxygen Species (ROS). Think of your car engine horribly out of tune where you don’t get your normal acceleration and nasty black smoke pours out of your tailpipe which causes you to fail smog inspection, only this smoke causes your car to rust away from the inside out. In addition to reduced energy output, mitochondria in neurodegenerative diseases also don’t repair themselves well and have, in previous research, suffered from axonal transport problems which appeared linked to the “die-back” of the axon and detachment from the neuromuscular junction (NMJ).
However, a recent study that used live imaging of mouse motor neurons showed that axonal transport was independent of mitochondrial density and axonal degeneration. In mice with different SOD1 mutations, axonal transport was affected differently (or not at all). The motor neurons of the mice bred to have the SOD1 G93A mutation, which are used in the majority of studies because they are the model most understood, showed extensive axonal transport problems early on while G85R motor neurons showed very little even up to end-stage. Even mice bred to overexpress (multiple copies of the gene) human wild-type (non-mutant) SOD1 showed axonal transport issues without motor neuron degeneration. The “code” for the mutations refer to nucleotide transpositions at numbered locations in the SOD1 gene that creates the SOD1 protein. Below you can see movies of transport in the various types of neurons relative to controls.
WT-SOD1 (human, overexpressed)
I have previously posted about the role of mitochondria in ALS and discussed various implications. I have also posted about the possibility of a role of SOD1 in both FALS and SALS as well as the possibility of SOD1 as a prion in the propagation of ALS. In all the mutant SOD1, mitochondria show dysfunction early. Mitochondria also have “communication” with the Schwann cells at the NMJ which is an early event in the destruction of the NMJ and commencement of the axonal die-back.
Nothing is as simple as we would like, and ALS exemplifies this axiom. What would seem apparently causal might merely suggest another, as yet undiscovered, mechanism which may underlie other related observed effects. And sometimes what appears to be a related effect may just be something separate. Regardless of any doubt cast on the role of axonal transport in ALS, the mitochondrial dysfunction with which it is associated still appears strongly implicated. The cause of that remains under investigation.