Structural Differences Seen in Toxic Proteins Marking Parkinson’s, Multiple System Atrophy
Assessing the shape of alpha-synuclein aggregates in the brain helps to distinguish between Parkinson’s disease (PD) and another progressive neurodegenerative disorder known as multiple system atrophy (MSA), a study suggests.
The study, “Discriminating α-synuclein strains in Parkinson’s disease and multiple system atrophy,” was published in Nature.
Parkinson’s and MSA share many symptoms, particularly in earlier disease stages. This often leads to misdiagnoses, which pose an obstacle for early treatment.
Both PD and MSA are also characterized by the buildup of the protein alpha-synuclein in the brain, which is toxic to brain cells.
Researchers analyzed alpha-synuclein aggregates in cerebrospinal fluid (that which surrounds the brain and spinal cord) taken from 94 people with Parkinson’s, 75 with MSA, and 56 individuals with other neurological diseases serving as controls.
They used a technique called Protein Misfolding Cyclic Amplification (PMCA). This assay involves cyclic chemical reactions that make more of the protein — similar to a ‘copying’ technique called polymerase chain reaction (PCR) used with DNA.
Fluorescent dyes can then be used to gain insights into the structure of the proteins. Basically, the dyes bind to the proteins differently depending on their shape, which affects how they glow.
Several notable differences were evident between the fluorescence profiles of alpha-synuclein aggregates taken from people with MSA and those with PD. In MSA samples, fluorescence tended to increase quickly to a plateau of less than 1,800 fluorescent units. Parkinson’s samples, in contrast, tended to increase more slowly but reach a higher plateau — between 2,000 and 8,000 fluorescent units. Control samples did not show any fluorescence over the background levels.
Based on the results of this test, 85/88 (96.6%) PD samples and 61/65 (93.8%) MSA samples could be correctly identified as such. Some samples did not form aggregates at all, resulting in no fluorescent reading.
“Combining all samples, we correctly distinguished PD from MSA in 146 of the 153 samples analyzed—an overall sensitivity of 95.4%,” the researchers wrote.
Importantly, these same differences in fluorescent profiles were also observed in alpha-synuclein in brain tissue samples from people who had died with either disease. This suggests structural differences are inherent in the alpha-synuclein aggregates that form in the brains of people with Parkinson’s and with MSA.
Further analysis verified this finding. The researchers used a number of molecular tests to tease out the differences in shapes between the two types of alpha-synuclein. They also performed assays to confirm that this result was due to the shape of the protein, not the amount of protein in different samples or other factors.
Their work demonstrated that different fluorescent dyes bound to PD or MSA alpha-synuclein in different ways, indicative of their structural differences. The researchers also used cryo-electron microscopy to more directly assess the ‘shape’ of the proteins.
“[Alpha-synuclein] filaments from patients with MSA are predominantly twisted, whereas those from patients with PD are mostly straight,” the researchers wrote.
“It is important for physicians to have an objective way to differentiate between PD and MSA in order to provide patients with the best care. Currently, the only way to differentiate them is to wait and see how the disease progresses, with MSA advancing much more rapidly than PD,” Claudio Soto, PhD, the study’s senior author and a neurology professor with The University of Texas Health Science Center at Houston (UTHealth), said in a press release.
“By the time people show progressed symptoms of MSA, a substantial amount of brain cells are already damaged or dead, and they can’t be brought back. It has been difficult to develop treatment for both diseases because of the high rates of misdiagnosis, so we needed to find a way to distinguish between the two at the onset of early symptoms,” Soto added.
These structural differences likely have important biological consequences, which the researchers assessed by exposing cells in dishes to alpha-synuclein aggregates derived from people with MSA or PD. The MSA aggregates were significantly more toxic to the cells, possibly as a consequence of their different structures.
“Our results demonstrate that α-syn [alpha synuclein] aggregates exist as distinct conformational strains with different biochemical and structural properties, which will help to improve our understanding of the pathogenesis of these diseases,” the researchers concluded.
“These data may enable the development of a biochemical test for the specific diagnosis of different disorders that involve the misfolding of α-syn, with potential future applications in clinical trials and personalized medicine,” they added.