Microglia affect α-synuclein cell-to-cell transfer in a mouse model of Parkinson’s disease

Abstract

Background Cell-to-cell propagation of α-synuclein (α-syn) aggregates is thought to contribute to the pathogenesis of Parkinson’s disease (PD) and underlie the spread of α-syn neuropathology. Increased pro-inflammatory cytokine levels and activated microglia are present in PD and activated microglia can promote α-syn aggregation. However, it is unclear how microglia influence α-syn cell-to-cell transfer. Methods We developed a clinically relevant mouse model to monitor α-syn prion-like propagation between cells; we transplanted wild-type mouse embryonic midbrain neurons into a mouse striatum overexpressing human α-syn (huα-syn) following adeno-associated viral injection into the substantia nigra. In this system, we depleted or activated microglial cells and determined the effects on the transfer of huα-syn from host nigrostriatal neurons into the implanted dopaminergic neurons, using the presence of huα-syn within the grafted cells as a readout. Results First, we compared α-syn cell-to-cell transfer between host mice with a normal number of microglia to mice in which we had pharmacologically ablated 80% of the microglia from the grafted striatum. With fewer host microglia, we observed increased accumulation of huα-syn in grafted dopaminergic neurons. Second, we assessed the transfer of α-syn into grafted neurons in the context of microglia activated by one of two stimuli, lipopolysaccharide (LPS) or interleukin-4 (IL-4). LPS exposure led to a strong activation of microglial cells (as determined by microglia morphology, cytokine production and an upregulation in genes involved in the inflammatory response in the LPS-injected mice by RNA sequencing analysis). LPS-injected mice had significantly higher amounts of huα-syn in grafted neurons. In contrast, injection of IL-4 did not change the proportion of grafted dopamine neurons that contained huα-syn relative to controls. As expected, RNA sequencing analysis on striatal tissue revealed differential gene expression between LPS and IL-4-injected mice; with the genes upregulated in tissue from mice injected with LPS including several of those involved in an inflammatory response. Conclusions The absence or the hyperstimulation of microglia affected α-syn transfer in the brain. Our results suggest that under resting, non-inflammatory conditions, microglia modulate the transfer of α-syn. Pharmacological regulation of neuroinflammation could represent a future avenue for limiting the spread of PD neuropathology. Electronic supplementary material The online version of this article (10.1186/s13024-019-0335-3) contains supplementary material, which is available to authorized users.

Determinants of seeding and spreading of α-synuclein pathology in the brain

“…Since neuroinflammation has been implicated in the appearance and spread of aSYN pathology ( 32 ), the mesopontine region was examined for signs of microglial activation using an antibody for Iba1 ( 33 ). Both aSYN PFFs and aSYN monomers induced a very similar pattern of Iba1 IR at the injection site.…”

T cells limit accumulation of aggregate pathology following intrastriatal injection of α-synuclein fibrils

“…Microglia morphology was assessed as previously described [47]. Color (RGB) images of Iba-1-stained striatum, substantia nigra and frontal cortex tissues were acquired bilaterally at 60x magnification (oil immersion 1.40 N.A.)…”

“…We compared the morphology of Iba-1 immunoreactive microglia from the same anatomical level of the ipsilateral and contralateral striatum, substantia nigra and frontal cortex in mice from each experimental group. Specifically, we used a MATLAB script to define the ratio between the perimeter and surface area of Iba-1immunoreactive microglia, allowing us to calculate the hydraulic radius of each cell as an index of the activation state as previously [47]. The area:perimeter index (hydraulic radius) measures microglial activation as activated microglia are amoeboid in shape.…”

Mitochondrial Dysfunction, Oxidative Stress, and Neuroinflammation: Intertwined Roads to Neurodegeneration

“…However, a long-standing pro-inflammatory and pro-oxidant environment sustained by persistent microglia activation is detrimental as it may contribute to neurodegeneration. This is particularly relevant in the context of Alzheimer's (AD) and Parkinson's disease (PD) [7,8]. Metabolic changes in microglia have also been associated with a state of chronic low-grade inflammation observed during aging (i.e., inflamm-aging) [9,10].…”