α-Synuclein overexpression increases the tropism of pre-formed fibrils and MSA-patient derived seeds towards oligodendroglia

Multiple system atrophy (MSA) is a rare, fatal neurodegenerative disease, pathologically characterized by glial cytoplasmic inclusions (GCIs) containing α-synuclein (αSyn). The causes leading to aberrant αSyn accumulation in oligodendroglia remain unknown. Recent evidence of increased αSyn mRNA in GCI-bearing oligodendrocytes suggests that oligodendroglia αSyn expression may be a key determinant of GCI formation. To ascertain this, we generated a new transgenic (TG) mouse line overexpressing human (h-) αSyn through the myelin basic protein promoter (MBP-hSNCA mice). Immunofluorescence combined with machine learning-based classification confirmed age-independent oligodendroglial αSyn localization, with no evidence of aggregated and pathological αSyn, as assessed by a FRET-based assay, biochemical fractionation, and the seed amplification assay (SAA). Upon intrastriatal inoculation of hαSyn preformed fibrils (hPFFs), the fraction of oligodendroglial αSyn positive insoluble aggregates increased over time in TG compared to wildtype (WT) mice, reaching significance six to nine months after inoculation. This was accompanied by an increase in brain αSyn seeding potency. As proof of concept, we investigated whether preferential oligodendroglia vulnerability is influenced by intrinsic fibril potency by intrastriatally injecting TG and WT mice with αSyn fibrils amplified from 2 MSA or 1 PD patient's brains. Six months post inoculation, MSA-inoculated brains retained the MSA-characteristic seeding profile and exhibited higher seeding potency compared to PD-inoculated mice. TG mice showed significantly more widespread phosphorylated αSyn (pSyn) pathology than WT, independent of whether they were seeded with MSA- or PD-fibrils. However, oligodendroglial inclusions were significantly more abundant in TG mice injected with MSA-derived αSyn. In conclusion, our results indicate that intracellular αSyn expression facilitates the development of oligodendroglial pathology, supporting the hypothesis that increased αSyn expression may represent a rate-limiting step in GCI formation.