Mapping and kinetics of microglia/neuron cell-to-cell contacts in the 6-OHDA murine model of Parkinson's disease

Abstract

As neuroinflammatory processes are involved in the pathogenesis of Parkinson's disease (PD), we provide several key data describing the time-course of microglial accumulation in relation with behavioral alterations and neurodegeneration in a murine model of PD induced by intrastriatal injection of 6-hydroxydopamine (6-OHDA). Our study argues for a major role of microglia which accumulation is somehow early and transient in spite of the neuronal loss progression. Moreover, we observed less 6-OHDA-induced neurodegeneration associated with less inflammatory reaction in DAP-12 Knock-In mice. The direct cell-to-cell contacts that may support physical interactions between microglia and altered dopaminergic neurons are ill-defined, while it is currently hypothesized that microglia support an immune-mediated amplification of neurodegeneration by establishing a molecular cross talk with neurons. Indeed, we sought to map microglia/neuron appositions in substantia nigra (SN) of 6-OHDA injected C57Bl/6 mice and CX3CR1/(GFP/+) mice. Confocal immunofluorescence analyses followed by 3D reconstitutions reveal close appositions between the soma of TH+ neurons and microglial cell bodies and ramifications. Interestingly, some microglial ramifications penetrated TH(+) somas and about 40% of GFP(+) microglial cells in the injured SN harbored TH(+) intracytoplasmic inclusions. These results suggest a direct cross talk between neurons and microglia that may exert a microphagocytic activity toward TH+ neurons. Altogether, these results obtained in a murine PD model may participate in the understanding of microglial cells' function in neurodegenerative diseases.

Ontogeny and Functions of Central Nervous System Macrophages

“…It was also shown that CD200-CD200R blocking exacerbates neurodegeneration in a rat model (76). Moreover, CX3CR1 GFP/+ microglia have cell-to-cell contact with neurons and some microglia penetrate neuronal somata (77). These results suggest possible microglial involvement in the pathogenesis of PD.…”

The role of Fyn kinase in mediating microglial pro-inflammatory responses in Parkinson's disease

“…To determine whether Fyn plays a role in microglial-dopaminergic neuron contact formation, we adopted the 6-OHDA mouse model. The formation of gliapses was described recently in the 6-OHDA model (Virgone-Carlotta et al, 2013). Our results from high magnification confocal analysis revealed the formation of gliapses was almost completely blocked in 6-OHDA injected Fyn -/-mice (Fig.…”

“…The formation of these contacts was rapidly followed by microglial phagocytosis of neurons, and inhibiting the formation of these contacts proved to be neuroprotective in the MPTP model (Barcia et al, 2012). Similar results were obtained using the 6-OHDA model (Virgone-Carlotta et al, 2013). The Fyndependent formation of microglial-neuronal contacts was an area we explored using the 6-OHDA model .…”

“…Despite its usage, which began in the 1960s, evidence of neuroinflammation in this model was only recently demonstrated with unequivocal clarity Virgone-Carlotta et al, 2013).…”

“…However, when they encounter a potential pathogen, a dead or dying neuron or neurotoxic stress, they switch to an 'activated' phenotype, producing pro-inflammatory cytokines and chemokines, reactive nigtrogen species, and reactive oxygen species. Activated microglia may also directly contribute to cell death by phagocytizing dopaminergic neurons (Barcia et al, 2012;Virgone-Carlotta et al, 2013). Thus, the pathophysiology of PD is accompanied by a sustained pro-inflammatory microglial response that contributes to neuron death, thereby exacerbating disease progression.…”