P2X7 signaling promotes microsphere embolism-triggered microglia activation by maintaining elevation of Fas ligand

Abstract

BackgroundThe cerebral microvascular occlusion elicits microvascular injury which mimics the different degrees of stroke severity observed in patients, but the mechanisms underlying these embolic injuries are far from understood. The Fas ligand (FasL)-Fas system has been implicated in a number of pathogenic states. Here, we examined the contribution of microglia-derived FasL to brain inflammatory injury, with a focus on the potential to suppress the FasL increase by inhibition of the P2X7-FasL signaling with pharmacological or genetic approaches during ischemia.MethodsThe cerebral microvascular occlusion was induced by microsphere injection in experimental animals. Morphological changes in microglial cells were studied immunohistochemically. The biochemical analyses were used to examine the intracellular changes of P2X7/FasL signaling. The BV-2 cells and primary microglia from mice genetically deficient in P2X7 were used to further establish a linkage between microglia activation and FasL overproduction.ResultsThe FasL expression was continuously elevated and was spatiotemporally related to microglia activation following microsphere embolism. Notably, P2X7 expression concomitantly increased in microglia and presented a distribution pattern that was similar to that of FasL in ED1-positive cells at pathological process of microsphere embolism. Interestingly, FasL generation in cultured microglia cells subjected to oxygen-glucose deprivation-treated neuron-conditioned medium was prevented by the silencing of P2X7. Furthermore, FasL induced the migration of BV-2 microglia, whereas the neutralization of FasL with a blocking antibody was highly effective in inhibiting ischemia-induced microglial mobility. Similar results were observed in primary microglia from wild-type mice or mice genetically deficient in P2X7. Finally, the degrees of FasL overproduction and neuronal death were consistently reduced in P2X7−/− mice compared with wild-type littermates following microsphere embolism insult.ConclusionFasL functions as a key component of an immunoreactive response loop by recruiting microglia to the lesion sites through a P2X7-dependent mechanism. The specific modulation of P2X7/FasL signaling and aberrant microglial activation could provide therapeutic benefits in acute and subacute phase of cerebral microembolic injury.

P2X7 receptors in cerebral ischemia

“…Pannexin-1 (Px1) is robustly expressed in the brain [47] . The opening of Px1 hemichannels is regulated by activated P2X 7 Rs under pathological conditions such as ischemia, and is responsible for the massive influx of Ca 2+ from the extracellular space.…”

“…Lu et al recently used a cerebral microemboli model and an in vitro culture system to study the roles of microglial activation and P2X 7 /FasLFas signaling following ischemic insult. They proposed an immunoreactive response loop in which microglia-derived FasL exaggerates P2X 7 -mediated microglial activation and triggers a vicious cycle of neuronal cell death in the ischemic context [47] .…”

“…Nevertheless, in the later stages of cerebral ischemia, the high level of ATP stimulates microglial P2X 7 R overexpression (pore formation) [47] and leads to microglial activation, proliferation [51] and cell death. Over-activated microglia up-regulate the expression of their surface immunomodulatory proteins and release potentially neurotoxic pro-inflammatory cytokines such as IL-1β, IL-6 and TNF-α [16,47] .…”

“…Over-activated microglia up-regulate the expression of their surface immunomodulatory proteins and release potentially neurotoxic pro-inflammatory cytokines such as IL-1β, IL-6 and TNF-α [16,47] . These pro-inflammatory cytokines boost further microglial activation in an autocrine manner [16,47] and induce secondary inflammatory injury in the penumbra [22,50] which is an exaggerated and prolonged inflammatory response that results in continued neuronal cell death (not only damaged cells but also neighboring healthy cells) [10,16] .…”

The effect of lipid nanoparticle PEGylation on neuroinflammatory response in mouse brain

“…Microglia are the primary immune sentinels of the brain and respond rapidly to brain risk factors and strictly controlled events [17,18]. Ferric oxide nanoparticle exposure triggers microglial activation, recruitment and phagocytosis [19].…”