Pathological abnormalities associated with amyotrophic lateral sclerosis (ALS) have been identified in a mouse model using a new imaging technique developed at the University of Birmingham. Using the method, known as native ambient mass spectrometry imaging (MSI), the University of Birmingham scientists, working in collaboration with a team at the University of Sheffield, identified a metal deficiency in the SOD1 protein, showing that it accumulates in specific regions of the brain and spinal cord in the ALS mouse model.
The researchers say the technology will help to better understand changes in the brain that lead to ALS and could eventually yield insights that will help with the development of new treatments. They reported on their studies in Nature Communications, in a paper titled “Mass spectrometry imaging of SOD1 protein-metal complexes in SOD1G93A transgenic mice implicates demetalation with pathology.” In their paper the team noted, “Further work will focus on the application of native ambient mass spectrometry imaging of the various hSOD1 species over the time-course of the development of pathology in the model, expansion to additional models and human cases, and continued development of the imaging technology.”
ALS is a muscle wasting condition caused by messages from the brain’s motor neurones not reaching the muscles, causing them to weaken. Around 5,000 people in the U.K. have the disorder at any one time and currently there is no cure. Mutations in the gene coding the metalloenzyme superoxide dismutase 1 (SOD1) are responsible for ~20% of familial ALS, the authors noted.
Under physiological conditions human SOD1 (hSOD1) matures to form a non-covalently bound homodimer that incorporates one zinc ion, one copper ion and one intramolecular disulfide bond per subunit, the authors explained. “The canonical view is that mutations in SOD1 destabilize the native structure of the protein, resulting in monomerization and aggregation of the protein. This pathway results in a toxic gain of function that causes degeneration of motor neurons.”
The university of Birmingham researchers have developed a technique that enables them to examine specific proteins in their native state, directly from brain and spinal cord tissue samples. Called native ambient mass spectrometry, the tool enables the structure of proteins to be studied in relation to their location within the tissue in greater detail than ever before. “Native ambient MSI is a label-free molecular imaging technique with the unique capability to identify and map the distribution of endogenous protein complexes within tissue sections, including metal-bound proteins and membrane proteins,” the investigators stated.
Using the technology in collaboration with colleagues at the University of Sheffield, the researchers were able to identify a metal deficiency in SOD1 and show that it accumulates in specific regions of the brain and spinal cord in the human SOD1G93A mouse model. This is the first time that detailed molecular imaging has been able to show how versions of the protein with missing metal ions accumulate in the affected mice. “Central nervous system tissue sections from transgenic mice expressing human SOD1 were imaged by native ambient MSI,” the team stated. “The disease model, hSOD1G93A, showed abundance of metal-deficient hSOD1G93A complexes in motor-associated structures of the CNS. The distribution indicates a relationship between the cellular environment of the affected motor regions and hSOD1G93A existing in a demetalated state.”
Lead researcher Helen Cooper, PhD, at Birmingham’s School of Biosciences, said: “This approach is the first to show that this form of SOD1 correlates with the pathology of motor neurone disease. It’s a very early step towards finding treatments for MND and is also an exciting new route for understanding the molecular basis of other diseases in unprecedented detail.” The authors further commented, “The techniques developed here show that demetalation is a key pathological change observed for hSOD1G93A and may help to define whether this mechanism contributes to a broader population of sporadic ALS.”
Added Richard Mead, PhD, at the Sheffield Institute for Translational Neuroscience, “We were very excited to apply this fantastic methodology which Helen’s team have developed to gain new insights into the biology of MND and we look forward to using the technology further to explore why motor neurons die and find new interventions for those affected by MND.”
The next steps for the researchers will be to test to see if the same imbalances are present in human tissue samples, and to try to treat the imbalance in the mice using available drug compounds. The team wrote, “Overall, these results have implications for understanding the role of SOD1 toxic gain of function in ALS, which is particularly relevant in the context of therapeutics which reduce mutant SOD1 levels in ALS patient CNS …”
