Involvement of Macroautophagy in Multiple System Atrophy and Protein Aggregate Formation in Oligodendrocytes

Abstract

α-Synuclein-containing glial cytoplasmic inclusions (GCIs) originating in oligodendrocytes are the characteristic hallmark for neuropathological diagnosis of multiple system atrophy (MSA). α-Synuclein can be degraded either by the proteasomal machinery or by autophagy, a lysosomal pathway which involves the formation of autophagosomes. The autophagosome takes up polyubiquitinated proteins via the autophagosomal protein LC3 and the ubiquitin binding protein p62. In the present study, neuropathological examination of seven MSA cases revealed that LC3-immunoreactivity is found to be associated with α-synuclein-positive GCIs. These are also prominently stained by antibodies against p62 and ubiquitin, indicating that the autophagic pathway is upregulated during pathogenesis, which might be due to a persistent downregulation of proteasomal activity. To further address this question in a cellular context, we have investigated whether proteasomal inhibition in cultured rat brain oligodendrocytes promotes the recruitment of LC3 and p62 to protein aggregates. The data show that the autophagic marker LC3-II is upregulated and LC3 is recruited to the growing protein aggregates in cultured oligodendrocytes when the proteasome is impaired. However, aggregated proteins remain in the oligodendroglial cytoplasm and cannot be cleared efficiently. In conclusion, autophagy and the ubiquitin proteasome system are closely connected, and the presence of LC3-positive vesicles in GCIs indicates that macroautophagy participates in MSA pathogenesis.

The α-Tubulin Deacetylase HDAC6 in Aggresome Formation and Autophagy

“…In MSA GCIs were positively labeled by antibodies against HDAC6; they also displayed immunoreactivity against p62, the 20S proteasome and gamma-tubulin, indicating that GCIs share common features with aggresomes (Chiba et al, 2012). Data from our laboratory further depicted that GCIs in MSA brains contain the autophagy marker LC3, which sustains the idea that autophagy is involved in GCI formation and oligodendroglial cell death (Schwarz et al, 2012). In cultured oligodendrocytes we further showed that LC3 and p62 are recruited to the aggresomes after proteasomal inhibition and that these aggregates resemble GCIs.…”

Oligodendroglia and Myelin in Neurodegenerative Diseases: More Than Just Bystanders?

“…Triggered by increased proteolytic stress, oligodendroglial alpha-synuclein accumulation was enhanced, and myelin dysfunction and neuronal cell death associated with an altered motor phenotype were detected supporting the increased vulnerability of OLGs in MSA [195]. An early involvement of proteolytic failure in OLGs during MSA pathogenesis is further supported by in vitro evidences for altered autophagic and proteasomal functionality in OLGs upon alpha-synuclein expression [196]. In line, by enhancing oxidative modifications of alpha-synuclein using the mitochondrial inhibitor 3-nitropropionic acid (3NP), neuropathology and neurological deficits exacerbated in both the PLP- and the MBP-driven transgenic MSA model implying increased susceptibility of alpha-synuclein-bearing OLGs toward oxidative stress [187, 197].…”

The neuropathology of multiple system atrophy and its therapeutic implications

“…Free extracellular oligomeric α-syn is taken up by oligodendrocytes by clathrin-dependent endocytosis (Kisos et al, 2012; Konno et al, 2012), and endocytic vesicles containing α-syn are then directed to lysosomal degradation; however, cytosolic α-syn might also be degraded by other mechanisms such as UPR and proteolysis (Hoozemans et al, 2007; Xilouri et al, 2013). Impairments in clearance mechanisms such as autophagy have already been described in MSA and other synucleino-pathies (Lynch-Day et al, 2012; Schwarz et al, 2012). …”

“…However, it is unclear why oligodendrocytes preferentially uptake and/or fail to clear α-syn in MSA brains, a neuropathological event not widely observed in other synucleinopathies. One hypothesis is that the incorporation of extracellular, misfolded α-syn may impair the endogenous clearance machinery of the oligodendrocyte, progressively leading to α-syn accumulation (Pukass and Richter-Landsberg, 2015; Schwarz et al, 2012). Another option is that a dysfunction in the clearance machinery is a prerequisite for α-syn uptake and/or accumulation in oligodendrocytes.…”