Microglial glutamate release evoked by α‐synuclein aggregates is prevented by dopamine

Abstract

When activated, microglial cells have the potential not only to secrete typical proinflammatory mediators but also to release the neurotransmitter glutamate in amounts that may promote excitotoxicity. Here, we wished to determine the potential of the Parkinson's disease (PD) protein α‐Synuclein (αS) to stimulate glutamate release using cultures of purified microglial cells. We established that glutamate release was robustly increased when microglial cultures were treated with fibrillary aggregates of αS but not with the native monomeric protein. Promotion of microglial glutamate release by αS aggregates (αSa) required concomitant engagement of TLR2 and P2X7 receptors. Downstream to cell surface receptors, the release process was mediated by activation of a signaling cascade sequentially involving phosphoinositide 3‐kinase (PI3K) and NADPH oxidase, a superoxide‐producing enzyme. Inhibition of the Xc‐ antiporter, a plasma membrane exchange system that imports extracellular l‐cystine and exports intracellular glutamate, prevented the release of glutamate induced by αSa, indicating that system Xc‐ was the final effector element in the release process downstream to NADPH oxidase activation. Of interest, the stimulation of glutamate release by αSa was abrogated by dopamine through an antioxidant effect requiring D1 dopamine receptor activation and PI3K inhibition. Altogether, present data suggest that the activation of microglial cells by αSa may possibly result in a toxic build‐up of extracellular glutamate contributing to excitotoxic stress in PD. The deficit in dopamine that characterizes this disorder may further aggravate this process in a vicious circle mechanism.

Unilateral intranigral administration of β-sitosterol β-D-glucoside triggers pathological α-synuclein spreading and bilateral nigrostriatal dopaminergic neurodegeneration in the rat

“…This result is in agreement with recent findings that microglia and monocytes can take up free and exosome-associated α-synuclein oligomers resulting from age-dependent defects [6]. Another cell death mechanism of α-synuclein is excitotoxicity [17], and there is evidence that the exposure to BSSG triggers excitotoxicity mediated by the NMDA receptor in rat neocortex slices [85]. Therefore, the BSSG-induced pathological αsynuclein aggregates could cause the death of dopaminergic neurons by activating excitotoxicity.…”

“…β-Gal staining, a senescence marker [17], coincided with TH (+) cells since day 30 in the SNpc of both sides, and its area density was significantly higher (p < 0.05) than in the controls (Fig. 8a,b).…”

CMT-3 targets different α-synuclein aggregates mitigating their toxic and inflammogenic effects

“…The fibrillary state of α-synuclein contributes to neurodegeneration through different mechanisms 11 , 12 , 14 , 15 . Therefore, we studied the effect of CMT-3 and DOX on disassembled α-synuclein preformed fibrils (αS PFF ).…”

“…Oligomeric α-synuclein species have been shown to elicit toxic effects by different mechanisms, such as alteration of membrane permeability with concomitant calcium influx 6 , mitochondrial damage 7 , lysosomal leakage 8 , microtubule disruption 9 , and interference with axonal transport 10 . Fibrillar species, on the other hand provoke neurotoxicity mainly by triggering inflammatory processes 11 , but also by catalyzing their own propagation 12 , destabilizing proteostasis networks 13 , 14 , and affecting integrity of cytosolic organelles 15 . Considering that oxidative stress and pro-inflammatory cytokines also promote the toxic aggregation of α-synuclein 16 , all these processes are suggested to integrate a vicious cycle that results in neuronal death, with subsequent spreading of toxic species into neighboring healthy neurons 17 .…”

Microglial NMDA receptors drive pro‐inflammatory responses via PARP‐1/TRMP2 signaling

“…Consistent with our previous findings, showing that the ionotropic function of NMDARs is necessary for NMDA to trigger pro‐inflammatory microglia responses, NMDA treatment‐induced TRPM2 currents were prevented by APV, 7‐CK and MK‐801 (Figure f). In addition to NMDARs, microglia also express ionotropic glutamate receptors of the AMPA subtype, which could potentially participate in the activation of TRPM2 as a results of treatment‐induced elevation in the extracellular concentration of glutamate released from microglia in a paracrine/autocrine manner (Barger, Goodwin, Porter, & Beggs, ; Dos‐Santos‐Pereira et al, ; Noda, Nakanishi, & Akaike, ; Socodato et al, ; Takaki et al, ). Ruling out this mechanism, NMDA treatment‐induced TRPM2 currents were not prevented when AMPARs were blocked by CNQX (20 μM; Figure f).…”