Research Letters

Multiple-system atrophy: a new α-synuclein disease?

Multiple system atrophy (MSA) is a progressive and sporadic neurodegenerative disorder characterized by the histological appearance of glial cytoplasmic inclusions primarily composed of α-synuclein. Recently, complement-mediated neuroinflammation has been proposed as a key factor in the pathogenesis of numerous neurodegenerative disorders. We conducted immunohistochemical/immunofluorescent assays targeting a number of complements to explore the role of complements in MSA pathogenesis using brain samples from deceased patients and controls. Complement deposition was notably increased in the cerebral vasculature and myelin sheath in the MSA brains. Furthermore, fibrinogen leakage resulting from the disruption of the blood–brain barrier (BBB) was observed, along with the presence of C1q-positive microglia clusters surrounding the MSA brain vessels. These immunohistochemical/immunofluorescent findings suggest that complement activation and BBB disruption play critical roles in MSA progression.

α-synuclein, β-synuclein, and γ-synuclein are abundantly expressed proteins in the vertebrate nervous system. α-synuclein functions in neurotransmitter release by binding to and clustering synaptic vesicles and chaperoning SNARE-complex assembly. Pathologically, aggregates originating from soluble pools of α-synuclein are deposited into Lewy bodies in Parkinson’s disease and related synucleinopathies. The functions of β-synuclein and γ-synuclein in presynaptic terminals remain poorly studied. Using in vitro liposome binding studies, circular dichroism spectroscopy, immunoprecipitation, and fluorescence resonance energy transfer (FRET) experiments on isolated synaptic vesicles in combination with subcellular fractionation of brains from synuclein mouse models, we show that β-synuclein and γ-synuclein have a reduced affinity toward synaptic vesicles compared with α-synuclein, and that heteromerization of β-synuclein or γ-synuclein with α-synuclein results in reduced synaptic vesicle binding of α-synuclein in a concentration-dependent manner. Our data suggest that β-synuclein and γ-synuclein are modulators of synaptic vesicle binding of α-synuclein and thereby reduce α-synuclein’s physiological activity at the neuronal synapse.

Granulovacuolar degeneration (GVD) is characterized by the presence of membrane-delineated vacuoles harboring a dense core, the so-called GVD body (GVB). GVBs are most commonly observed in the pyramidal cells of hippocampus, predominantly in cornu ammonis 1 in Alzheimer’s disease (AD), besides several other diseases with tau accumulation. GVBs are small and round organelles in neuronal cells, which are surrounded by membranes and have granules. GVBs are double-membraned and positive for endosomal markers, indicating that GVBs are the structures of these autophagic and endocytic pathways. However, GVBs are not exactly the same as these organelles; GVBs are also positive for the markers of organelles that emerge under stress, including stress granules, necrosome, and unfolded protein response (UPR). Thus, GVD might be a condition in which the intracellular degradation system via autophagy malfunctions due to tau accumulation, and abnormal proteins cannot be degraded. Structures composed of cores and the surrounding outer layer were observed not only in the neurons but also in the glial cells in multiple system atrophy (MSA) brains. The GVB-like structures in oligodendroglia in MSA were also detected by the markers of GVBs in neurons and expressed α-synuclein in the early stages of glial cytoplasmic inclusions. Thus, GVD is a state wherein autophagy is impaired by aggregating proteins such as tau and α-synuclein, leading to cell death or inclusion body formation in neurodegeneration.

Differential diagnosis of Parkinson-plus patients (PSP, CBD, MSA) and Parkinson's disease (PD) patients is often not straightforward, particularly in atypical cases or at the initial stages of the diseases. Classic CSF biomarkers (amyloid-beta – Aβ₄₂, tau protein – τ_T and phosphorylated tau protein – τ_P-181) are established biomarkers in the diagnosis of Alzheimer's disease (AD). CSF a-synuclein (α-syn) has emerged as a promising biomarker in patients with Parkinsonism. The aim of this study was to analyze the CSF biochemical profile of patients with Parkinsonism.

We analyzed the CSF biomarker profile (Aβ₄₂, τ_T, τ_P-181, α-syn) and all relevant ratios in 68 patients with Parkinsonism (19 PSP, 15 MSA, 17 CBD, 17 PD) and 18 controls, diagnosed by latest established diagnostic criteria.

CBD patients exhibited elevated τ_T and decreased Aβ₄₂ compared to the other groups. Five CBD, one PSP patient and one control had a typical AD CSF profile. After exclusion of these patients, the τ_T/Aβ₄₂ ratio was significantly elevated in MSA patients compared to PD patients and provided excellent specificity and adequate sensitivity in their differential diagnosis.

CSF biochemical profile analysis is important in distinguishing AD patients with a CBS phenotype from non-AD CBS patients. The τ_T/Aβ₄₂ ratio is useful in the differential diagnosis of MSA from PD.

Axonal dystrophy has been described as an early pathological feature of neurodegenerative disorders including Alzheimer's disease and amyotrophic lateral sclerosis. Axonal inclusions have also been reported to occur in several neurodegenerative lysosomal storage disorders including Mucopolysaccharidosis type IIIA (MPS IIIA; Sanfilippo syndrome). This disorder results from a mutation in the gene encoding the lysosomal sulphatase sulphamidase, and as a consequence heparan sulphate accumulates, accompanied by secondarily-stored gangliosides. The precise basis of symptom generation in MPS IIIA has not been elucidated, however axonal dystrophy may conceivably lead to impaired vesicular trafficking, neuronal dysfunction and/or death. We have utilised a faithful murine model of MPS IIIA to determine the spatio-temporal profile of neuronal inclusion formation and determine the effect of restoring normal lysosomal function. Dopaminergic (tyrosine hydroxylase-positive), cholinergic (choline acetyltransferase-positive) and GABAergic (glutamic acid decarboxylase_65/67-positive) neurons were found to exhibit axonal dystrophy in MPS IIIA mouse brain. Axonal lesions present by ~ seven weeks of age were Rab5-positive but lysosomal integral membrane protein-2 negative, suggesting early endosomal involvement. By 9–12-weeks of age, immunoreactivity for the autophagosome-related proteins LC3 and p62 and the proteasomal subunit 19S was noted in the spheroidal structures, together with wildtype α-synuclein, phosphorylated Thr-181 Tau and amyloid precursor protein, indicative of impaired axonal trafficking. Sulphamidase replacement reduced but did not abrogate the axonal lesions. Therefore, if axonal dystrophy impairs neuronal activity and ultimately, neuronal function, its incomplete resolution warrants further investigation.