Mutant Protein Aggregates from Humans Aid the Spread of ALS

Post by Sarah Hill

What's the science?

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease in which motor neurons progressively die, causing muscles to atrophy. During ALS onset the protein superoxide dismutase 1 (SOD1) misfolds and begins to aggregate, forming a 'seed' within the motor neuron, which subsequently activates SOD1 aggregation in neighboring motor neurons. One theory is that SOD1 spreads in a prion-like manner (replication of the aggregates based on the initial seed). Previously, Bergh and colleagues found that two structurally distinct strains of SOD1 aggregates, termed A and B, form in the spinal cord of mice expressing mutant SOD1, inducing accumulation of aggregates and ALS-like disease. Many genetic mutations contributing to SOD1 misfolding have been identified in ALS, however, It is still unknown which SOD1 mutations result in the formation of aggregates and how these aggregates may differ for various mutations. This week in Acta Neuropathologica, Ekhtiari Bidhendi and colleagues report that seeds prepared from a human ALS patient carrying the SOD1 G127X mutation, a mutation which promotes misfolding, triggers prion-like SOD1 misfolding and aggregation in the spinal cord of transgenic mice.

How did they do it?

Human ALS patients carrying the G127X mutation express low amounts of aggregated SOD1, making it difficult to study the structure of the protein. To get around this, the authors engineered mice to express the human G127X mutation. They then prepared aggregate seeds from spinal cord tissue from the G127X mice, as well as from an ALS patient carrying the G127X mutation. Control seeds were similarly prepared from control mice (without mutation) and a human control patient. Next, the authors implanted either the G127X seeds or the control seeds into the spinal cord of mice expressing mutant human SOD1 and monitored the animals for progression of ALS-like symptoms. Once the animals reached the terminal disease stage, spinal cord and brain tissue were harvested and either used for immunohistochemistry or prepared into extracts for Western blotting. Finally, to determine the structure of the induced SOD1 aggregates, the authors carried out binary epitope-mapping on the isolated tissue extracts, a technique they previously developed to infer aggregate structure based on how well SOD1 binds to specific antibodies in various folding states.

What did they find?

Mice inoculated with mouse- and human-derived G127X aggregates experienced early onset of an ALS-like motor neuron disease, with symptoms including shortened lifespan, weight loss, and decreased number of motor neurons. Interestingly, the onset of G127X-induced motor neuron disease preceded the normal disease onset observed in control-inoculated and non-inoculated mice. Immunohistochemistry confirmed these results, with SOD1 aggregation appearing in G127X-inoculated animals significantly sooner than in control- and non-inoculated groups. The distribution of G127X aggregates was relatively even throughout the spinal cord segments, while control seeds resulted in an uneven aggregate distribution. The varying seed types also resulted in differences in location of symptom onset, with a majority of G127X-inoculated mice first exhibiting hind leg paralysis and control-inoculated mice exhibiting paralysis of hind legs, for legs, or both. Based on the binary epitope-mapping assay, the authors concluded that the G127X mutation led to formation of A-strain SOD1 aggregates in the spinal cord of inoculated animals: the aggregates from mice had a structure similar to that of A-strain seeds. The structure of the induced SOD1 aggregates replicated that of the original implanted mutant seed for all types of seeds, suggesting that transmission of SOD1 occurred in a prion-like manner. Taken together, these results suggest that motor neuron disease induced by the G127X mutation occurs through the prion-like spread of A-strain SOD1 aggregates.     

What's the impact?

A current challenge in ALS research is understanding whether different strains of protein aggregates exist and how each aggregate strain is transmitted from cell to cell. We now know that the ALS-inducing SOD1 mutation (G127X) results in the prion-like spread of SOD1 aggregates. The binary epitope-mapping protocol developed previously by the authors is widely applicable to other neurodegenerative diseases, such as Alzheimer's and Parkinson's, and provides a framework for pinpointing differences in the structure and transmission of aggregate strains.  

Ekhtiari Bidhendi et al., Mutant superoxide dismutase aggregates from human spinal cord transmit amyotrophic lateral sclerosis. Acta Neuropathologica (2018).Access the original scientific publication here.