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Chalatsa et al. Neuroimmunol Neuroinflammation 2020;7:132-40 I http://dx.doi.org/10.20517/2347-8659.2020.01 Page 133

Keywords: Cerebrospinal fluid, α-synuclein, Parkinson’s disease, biomarker, exosomes, synucleinopathies

INTRODUCTION
The formation of large inclusions mostly containing protein aggregates is a common pathological hallmark
in a wide spectrum of neurodegenerative disorders such as Alzheimer’s Disease (AD) and Huntington
[1]
Disease . Particularly three distinct neurological conditions, Parkinson’s Disease (PD) including
Parkinson’s disease dementia, Dementia with Lewy Bodies (DLB) and Multiple System Atrophy (MSA),
are characterized by the aberrant accumulation of the presynaptic protein α-synuclein. In PD and DLB,
α-synuclein deposits are found either in the cytoplasm of neurons, where they are called lewy bodies
[2]
(LB), or in the neuronal terminals, where they are called Lewy neurites whereas in MSA, α-synuclein
deposition occurs in glial cells. α-Synuclein is also genetically linked with the development of PD since
specific point mutations or multiplications (duplications, triplications) of the SNCA gene encoding
[3-8]
for α-synuclein result in the familial forms of PD . The genetic association of α-synuclein with PD
is further strengthened by all the genome-wide association studies performed so far which indicate a
strong correlation of PD with variations in the SNCA gene [9,10] . These biochemical and genetic linkages
of α-synuclein with pathology as well as the observation that the protein is present in biological fluids or
peripheral tissues led to the assumption that α-synuclein could serve as a potential candidate biomarker for
[11]
PD diagnosis and also aid the differential diagnosis between the synucleinopathies .
A plethora of studies have assessed the absolute quantification of α-synuclein levels as a marker of
synucleinopathy with the ultimate aim to discriminate PD patients from healthy subjects or other unrelated
neurological controls. In this regard, understanding the structural biology of α-synuclein is critical; the
protein is highly modified at the post-translational level and has the ability to adopt different conformations
depending on the surrounding milieu. From all the modifications that have been reported so far,
phosphorylation is considered most closely related to PD pathology since almost 90% of α-synuclein in LB
appears to be hyper-phosphorylated . In addition, the assembly into multiple-sized oligomers has been
[12]
[13]
considered an early event in the pathological process of aggregate formation . As such, different forms
of α-synuclein, i.e., monomeric, oligomeric and phosphorylated, have been targeted in order to increase
[14]
diagnostic accuracy . The measurements have been performed in bodily fluids [cerebrospinal fluid (CSF),
blood plasma or serum, saliva], isolated secreted vesicles (exosomes) and peripheral tissues (skin, olfactory
or gut mucosa, salivary gland) using a variety of analytical approaches depending on the nature of the
biological sample and the form of α-synuclein detected with each assay .
[15]

In comparison with the other biological fluids, the assessment of α-synuclein in the CSF has provided the
[16]
most consistent results in terms of analytical validation by different laboratories . CSF α-synuclein is
mostly detected by means of immunoassays that use specific antibodies to target the different α-synuclein
forms. Even though the absolute concentrations can vary from study to study, the results obtained so
far are supported by several meta-analysis studies suggesting that CSF α-synuclein could serve as a
[17]
potential marker of synucleinopathy . In this review, we aim to discuss the results from the assessment of
α-synuclein in CSF and exosomes and explain the factors responsible for the variability among the different
studies.

MEASUREMENT OF CSF α-SYNUCLEIN
Being primarily produced by the choroid plexus within the ventricles of the central nervous system, CSF
is an established biological fluid to study neurodegenerative disorders since it is expected to mirror brain
microenvironment. The quantification of total α-synuclein, as well as its oligomeric and phosphorylated
[19]
forms, can be measured in CSF using different techniques, such as ELISA , xMAP technology ,
[18]