Amyotrophic lateral sclerosis is a non-cell autonomous disease, and motor neuron degeneration is modulated by intracellular and intercellular damages. Or at least this is what tells some scientists, indeed there is an abundance of proposal for ALS etiology and no consensus.
Another dissension point between ALS scientists is if the disease starts in the brain, or in a muscle. The former is the mainstream hypothesis. Both camps have proven again and again that their proposal was the right one.
A third mystery is that scientists almost never bothered to explore the most obvious manifestation of ALS: The muscle wasting.
With the amelioration of tools' performance, scientist's attention is turning to extra cellular vesicles.
Extra cellular vesicles
In the Central Nervous System (CNS), intercellular crosstalk happens among neurons, between neurons and glia or cells of the innate immune system, through different modalities, involving the release into the extracellular space of molecules such as neurotransmitters, neurotrophic factors, metabolites, and mutant proteins encapsulated or not in vesicles.
C9orf72, which presents aberrant hexanucleotide (GGGGCC) expansion in the non-coding region in ALS patients, regulates vesicle trafficking. Other proteins such as SOD1, TDP-43 or FUS are found in vesicles in ALS.
Where we discuss of muscles
Although much less studied than for motor neurons, abnormalities have been also described in skeletal muscle from ALS patients.
Accumulation of misfolded mutant proteins is observed in skeletal muscle.
In line with the pivotal role of defective mitochondrial respiratory chain and oxidative stress in ALS skeletal muscle, increasing levels of PGC‐1α, a transcription coactivator that promotes mitochondrial biogenesis, can improve muscle function even at late stages of the disease.
Skeletal muscle is a major site of glucose storage in the form of glycogen, which is transformed into ATP through glycolysis. The dysfunction of fast‐twitch type IIb myofibres in ALS is consistent with glucose intolerance and insulin resistance reported in ALS patients.
Myofibres from transgenic mice over expressing wild‐type TDP‐43 show impaired insulin‐mediated glucose uptake.
Does muscles kill motor neurons? In ALS, muscles are supposed to die from inactivity as motor neurons do not anymore activate them. A publication on MedRxiv proposes that it is actually the other way round: Muscles kill motor neurons. After all it is well known that many ALS patients were having intense sport activities. And an ALS-like phenotype was observed in mice when exogenous human mutant SOD1 expression was restricted to the skeletal muscle.
The authors of the pre-print, Laura Le Gall, Stephanie Duguez, Pierre Francois Pradat and colleagues, recall that pathological proteins have been identified in circulating extracellular vesicles of sporadic ALS patients. So they hypothesized that muscle vesicles may be involved in ALS pathology.
An accumulation of multivesicular bodies was observed in muscle biopsies of 27 sporadic ALS patients.
Study of muscle biopsies and biopsy-derived denervation-naïve differentiated muscle stem cells (myotubes) revealed a consistent disease signature in ALS myotubes, including intracellular accumulation of exosome-like vesicles and disruption of RNA-processing.
Compared to vesicles from healthy control myotubes, when administered to healthy motor neurons the vesicles of ALS myotubes induced shortened, less branched neurites, cell death, and disrupted localization of RNA and RNA-processing proteins.
This article may revolutionize the understanding of ALS' etiology.
