Microglial exosomes facilitate α-synuclein transmission in Parkinson’s disease

Abstract

Accumulation of neuronal α-synuclein is a prominent feature in Parkinson’s disease. More recently, such abnormal protein aggregation has been reported to spread from cell to cell and exosomes are considered as important mediators. The focus of such research, however, has been primarily in neurons. Given the increasing recognition of the importance of non-cell autonomous-mediated neurotoxicity, it is critical to investigate the contribution of glia to α-synuclein aggregation and spread. Microglia are the primary phagocytes in the brain and have been well-documented as inducers of neuroinflammation. How and to what extent microglia and their exosomes impact α-synuclein pathology has not been well delineated. We report here that when treated with human α-synuclein preformed fibrils, exosomes containing α-synuclein released by microglia are fully capable of inducing protein aggregation in the recipient neurons. Additionally, when combined with microglial proinflammatory cytokines, these exosomes further increased protein aggregation in neurons. Inhibition of exosome synthesis in microglia reduced α-synuclein transmission. The in vivo significance of these exosomes was demonstrated by stereotaxic injection of exosomes isolated from α-synuclein preformed fibrils treated microglia into the mouse striatum. Phosphorylated α-synuclein was observed in multiple brain regions consistent with their neuronal connectivity. These animals also exhibited neurodegeneration in the nigrostriatal pathway in a time-dependent manner. Depleting microglia in vivo dramatically suppressed the transmission of α-synuclein after stereotaxic injection of preformed fibrils. Mechanistically, we report here that α-synuclein preformed fibrils impaired autophagy flux by upregulating PELI1, which in turn, resulted in degradation of LAMP2 in activated microglia. More importantly, by purifying microglia/macrophage derived exosomes in the CSF of Parkinson’s disease patients, we confirmed the presence of α-synuclein oligomer in CD11b+ exosomes, which were able to induce α-synuclein aggregation in neurons, further supporting the translational aspect of this study. Taken together, our study supports the view that microglial exosomes contribute to the progression of α-synuclein pathology and therefore, they may serve as a promising therapeutic target for Parkinson’s disease.

Prominent microglial inclusions in transgenic mouse models of α-synucleinopathy that are distinct from neuronal lesions

“…An abundance of microglial αS inclusions described here has not been reported in humans [1], albeit very recently, microglia αS inclusions in the human olfactory bulb of PD patients have been described [49]. Furthermore, seeding-prone αS species were detected in human microglial exosomes isolated from CSF of sporadic PD and MSA patients [50] and microglia are involved in the spreading of αS lesions [51,52]. These studies raise the possibility that αS aggregates in microglia in α-synucleinopathies are more common than previously thought and that they may also contribute to disease progression.…”

The Role of α-Synuclein Oligomers in Parkinson’s Disease

“…A recent study suggested that oligomerization of α-syn occurs at the pre-synapse and oligomeric α-syn has the ability to spread from cells to cells in vivo [ 75 ]. In addition to neurons, microglia were also involved in the intercellular transmission of α-syn oligomers, by secreting oligomers through exosomes and inducing α-syn accumulation in neurons [ 76 ]. While the exact mechanism of α-syn oligomers intercellular transfer is unclear, the gap junction protein connexin-32 (Cx32) was proved to preferentially ingest α-syn oligomers in neurons and oligodendrocytes [ 77 ].…”

Extracellular Vesicles in Alzheimer’s and Parkinson’s Disease: Small Entities with Large Consequences

“…Strikingly, the uptake and transfer of α-syn-containing EVs has been linked to the prion-like spreading of α-syn. EVs obtained from α-syn PFFs stimulated primary cortical neurons act as seeds to induce endogenous α-syn aggregation in recipient neurons in vitro [ 101 ]. Additionally, brain-derived EVs from DLB patients facilitate α-syn aggregation in vivo after their intracerebral injection in WT mice [ 102 ].…”

“…The stimulation of BV-2 microglia cells with α-syn results in the release of EVs that are able to induce apoptosis in recipient neurons [ 105 ]. Additionally, α-syn PFF-stimulated primary microglia secrete α-syn oligomer-containing EVs which trigger α-syn aggregation in vitro (i.e., in primary cortical neurons) and in vivo (i.e., upon intrastriatal injection in WT mice), leading to the loss of dopaminergic neurons and the occurrence of motor deficits in the latter situation [ 101 ]. Interestingly, microglia/macrophage-derived CD11b positive EVs are present in the CSF of PD and multiple system atrophy (MSA) patients and are able to induce α-syn aggregation in vitro [ 101 ].…”

“…Another study specifically investigated the microglia/macrophage-derived, CD11b positive EVs in CSF samples of PD, MSA and control patients. Although the total number of EVs and the fibrillary α-syn levels do not differ, both the total and oligomeric α-syn levels are increased in PD and MSA patients [ 101 ]. In plasma, the α-syn levels in both the total pool [ 103 , 118 ] and the L1CAM positive [ 158 , 159 ] pool of EVs are significantly elevated in PD patients compared to controls.…”