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Page 136 Chalatsa et al. Neuroimmunol Neuroinflammation 2020;7:132-40 I http://dx.doi.org/10.20517/2347-8659.2020.01

ratio of plasma CSF-derived exosomal DJ-1 to total DJ-1 were significantly higher in patients with PD,
[4]
compared with controls . Several factors have been shown to affect the release of α-synuclein through
2+
2+
exosomes such as the activity of Glucocerebrosidase enzyme (GCase), ion homeostasis, such as Zn , Ca
and Mn , as well as neuronal activity and neurotransmitter release. The heterozygous mutations in the
2+
GBA1 gene are considered as an important risk factor for PD. In this regard, it has been demonstrated that
GCase overexpression leads to a decrease of exosome secretion while chronic pharmacological inhibition
of GCase activity in vivo profoundly increased exosomes levels, as well as exosome-associated α-synuclein
oligomers . In addition, decreased GCase activity has been demonstrated in brain samples with increased
[73]
[75]
[74]
α-synuclein levels and in CSF from sporadic PD patients . More recently, a study by Cerri et al. , showed
that exosomes from PD patients contain a greater amount of α-synuclein compared to healthy subjects
whereas no differences were found in plasma total α-synuclein levels. Importantly, the authors showed a
significant inverse correlation between GCase activity and this ratio in PD patients.

Notably, exosomes being a snapshot of the intracellular milieu, comprise a great source of bioactive
molecules, including various RNA species. In a study conducted by Gui and co-workers, where exosomal
miRNAs were isolated from the CSF of PD patients, 16 and 11 exosomal miRNAs were found upregulated
[76]
and down- regulated, respectively, in PD patients compared to controls . Validated hits were found to be
miR-1 and miR-19b-3p, significantly reduced in PD CSF exosomes, in contrast to miR-153, miR-409-3p,
[76]
miR-10a-5p, and let-7g-3p which were found to be increased . This evidence highlights the potential
diagnostic value of CSF exosomal RNA in the assessment of PD.

Although the role of exosome-associated α-synuclein as a potential biomarker remains relatively
controversial, there are specific parameters that should be taken into account. Firstly, with regards to the
source of exosomes, including plasma, saliva, CSF, there are certain protocols for their acquisition, followed
by even more meticulous procedures for exosome isolation and purification. Differences during the
aforementioned protocols may account for deviations between groups. In the same context, total exosome
isolation may mask differences that could be found in exosomes of specific cellular origin. For example,
Tomlinson and colleagues , following an unbiased proteomic approach, did not find any significant
[72]
increase of α-synuclein in total exosomes isolated by ultracentrifugation from the serum of PD patients.
[70]
On the contrary, Shi et al. showed a significant α-synuclein enrichment in neuron-specific exosomes,
isolated from PD patients’ plasma by immunoprecipitation of the neuronal adhesion molecule L1CAM.
More importantly, the notion that α-synuclein exists and exerts its detrimental effects in different strains,
leading to different aggregates that cause as many distinct synucleinopathies, i.e., PD, DLB, MSA has been
cemented [77,78] . Given the fact that exosomal cargo mirrors the state of the cell from which it originates,
exosome-associated α-synuclein may reflect the dynamic nature of α-synuclein species. To this end, it is of
a pivotal importance to develop techniques that could allow detection and quantification of all the different
α-synuclein conformers. Overall, although in its infancy, the study of exosome-associated α-synuclein as a
potential biomarker is quite promising, yet, it requires more combinatorial approaches.

CONCLUSION
Over the last 10 years there has been considerable amount of research effort placed in the evaluation of
neuronal α-synuclein as a diagnostic or, even, a prognostic biomarker for PD and related synucleinopathies.
The majority of studies have indicated that CSF α-synuclein could be useful for the diagnosis of
synucleinopathy that could also aid the distinguishment of PD patients from patients with other
neurodegenerative conditions. However, its utility as a biomarker is hampered by the lack of a universally
validated assay of high diagnostic accuracy. To this end, following the strict recently established standard
operating protocols for CSF collection and storage and correlating the measurement of monomeric
α-synuclein with the oligomeric or phosphorylated forms would greatly improve the diagnostic value of the
assessment of α-synuclein in CSF. To further ameliorate the specificity of α-synuclein measurement, recent