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Alpha-synuclein (α-syn) is a soluble protein found in neuronal and neuroglial cells. It typically assumes a random coil structure, but in several neurodegenerative disorders is found in mutant, insoluble aggregates. These have been shown to contribute to Lewy Body Dementia and Parkinson’s disease, as well as a condition, called multiple system atrophy. Collectively, these are referred to as synucleinopathies.
Unfortunately, much is still unknown about α-syn, both in health and disease, and scientists are uncertain of its function. Research shows that it is localized in presynaptic terminals and has indicated a role in dopamine transmission. Disordered dopamine signaling is also a characteristic of Parkinson’s disease and mutations in the gene encoding α-syn have been identified in familial cases of Parkinson’s disease. However, the α-syn pathology is present in a diverse number of brain regions, suggesting that not only dopaminergic neurons are affected.
In a recent study, researchers studied mice with three different levels of α-syn expression to better understand the role of the protein.
A new model of synucleinopathy
The researchers studied three groups of mice at age five months: 1) α-syn KO mouse, with reduced α-syn expression (n=22); 2) wild-type mouse, with physiologic α-syn expression (n=24); and 3) M83 mouse model, with pathologic α-syn expression (n=21). The mice underwent MRI and microPET imaging using five radiotracers to examine different aspects of glucose metabolism, dopamine, and serotonin neurotransmission. Within the following month, the mice were sacrificed, and the researchers carried out post-mortem analysis of α-syn pathology using Western blot and thioflavin-S staining.
The M83 mouse model was developed by the researchers in an attempt to better mimic the early-stage natural history of synucleinopathies. M83 transgenic mice, which express a mutant form of α-syn, do not usually exhibit disease until after eight months. The researchers took brain homogenate from existing M83 mice that were displaying motor impairment and inoculated the brains of 6-8-week-old transgenic mice. They have previously shown that this leads to accelerated and reliable development of α-syn pathology. At the time the researchers performed their experiments, the mice were not yet displaying any neurologic or motor symptoms.
The researchers performed MRI imaging to accurately delineate brain structures and these were coregistered with the PET images. They analyzed the data volume of interest analysis in which 20 areas of interest were predefined from the MRI. They also used a voxel-based analysis to provide more precise data on the location of changes in neurotransmission.
The team’s study focused on dopaminergic and serotoninergic pathways in the brain and, accordingly, they used tracers for dopamine (D2) receptors and dopamine transporter, serotonin (5-HT1) receptors, and serotonin transporters.
A translational approach
The findings showed that the three mouse models had differing levels of D2-receptor expression in several key brain regions, which the team believe confirms the purported role of α-syn in dopamine neurotransmission. However, there was no difference in DAT density between the three groups, which was confirmed in the post-mortem analyses. The researchers say that, together, their results suggest that the presence of physiologic or pathologic α-syn affects function (dopamine receptor density), rather than neuroanatomy (DAT density, which reflects dopaminergic neural cell density).
The effects on the serotoninergic system were similar, with the early stage mouse model displaying overexpression of serotonin receptors, but no change in serotonin transporter. However, physiologic expression of α-syn (i.e. wild-type) was not associated with serotonin receptor expression, indicating that this is a “pathophysiological signature” in synucleinopathy.
The researchers say that by focusing on the early stages of synucleinopathy, before the disease manifests research could help to identify markers that could be used to detect disease early and could be translated into clinical practice. They add that theirs was the first study of its type to explore the three different levels of α-syn expression in an animal model, and could lead to clinical research using the same radiotracers to better understand these diseases.
“Our results highlight the interest of in vivo PET neuroimaging with the same radiotracers for the exploration of neurotransmission reactivity in the synucleinopathy models,” the authors write in CNS Neuroscience & Therapeutics. “...[T]his PET translational approach encourages designing clinical studies associating the same radiopharmaceuticals to explore early stages of neurodegenerative pathologies, such as Parkinson’s disease.”
Flexible small-animal imaging
The researchers carried out their MRI study using a Bruker BioSpec system. The BioSpec series is a modular, multi-purpose system for small animal imaging and molecular MRI. Thanks to this innovative design, it can be adapted to virtually any application in small-animal imaging.
It features UltraShield Refrigerated USR magnet technology at field strengths from 4.7 to 21 Tesla and bore sizes range between 11 and 40 cm. Sensitivity is further enhanced by the revolutionary MRI CryoProbe technology, which delivers an increase in signal-to-noise ratio by a factor of 2.5 over standard room temperature RF-coils in routine in vivo MRI applications. This helps to reduce the scanning time, reducing anesthesia time and stress on the animals, while enhancing productivity.
Levigoureux E, Bouillot C, Baron T, et al. PET imaging of the influence of physiological and pathological α-synuclein on dopaminergic and serotonergic neurotransmission in mouse models. CNS Neurosci Ther 2019; 25: 57-68. doi: DOI: 10.1111/cns.12978.