Cyclic AMP is a key regulator of M1 to M2a phenotypic conversion of microglia in the presence of Th2 cytokines

Abstract

BackgroundMicroglia and macrophages play a central role in neuroinflammation. Pro-inflammatory cytokines trigger their conversion to a classically activated (M1) phenotype, sustaining inflammation and producing a cytotoxic environment. Conversely, anti-inflammatory cytokines polarize the cells towards an alternatively activated (M2), tissue reparative phenotype. Elucidation of the signal transduction pathways involved in M1 to M2 phenotypic conversion may provide insight into how the innate immune response can be harnessed during distinct phases of disease or injury to mediate neuroprotection and neurorepair.MethodsMicroglial cells (cell line and primary) were subjected to combined cyclic adenosine monophosphate (cyclic AMP) and IL-4, or either alone, in the presence of pro-inflammatory mediators, lipopolysaccharide (LPS), or tumor necrosis factor-α (TNF-α). Their effects on the expression of characteristic markers for M1 and M2 microglia were assessed. Similarly, the M1 and M2 phenotypes of microglia and macrophages within the lesion site were then evaluated following a contusive spinal cord injury (SCI) to the thoracic (T8) spinal cord of rats and mice when the agents were administered systemically.ResultsIt was demonstrated that cyclic AMP functions synergistically with IL-4 to promote M1 to M2 conversion of microglia in culture. The combination of cyclic AMP and IL-4, but neither alone, induced an Arg-1+/iNOS−cell phenotype with concomitant expression of other M2-specific markers including TG2 and RELM-α. M2-converted microglia showed ameliorated production of pro-inflammatory cytokines (TNF-α and IP-10) and reactive oxygen species, with no alteration in phagocytic properties. M2a conversion required protein kinase A (PKA), but not the exchange protein directly activated by cyclic AMP (EPAC). Systemic delivery of cyclic AMP and IL-4 after experimental SCI also promoted a significant M1 to M2a phenotypic change in microglia and macrophage population dynamics in the lesion.ConclusionsUsing primary microglia, microglial cell lines, and experimental models of CNS injury, we demonstrate that cyclic AMP levels are a critical determinant in M1–M2 polarization. High levels of cyclic AMP promoted an Arg-1+ M2a phenotype when microglia were activated with pro-inflammatory stimuli and Th2 cytokines. Th2 cytokines or cyclic AMP independently did not promote these changes. Phenotypic conversion of microglia provides a powerful new therapeutic approach for altering the balance of cytotoxic to reparative microglia in a diversity of neurological diseases and injury.

Early Methylprednisolone Treatment Can Stabilize the Blood-Optic Nerve Barrier in a Rat Model of Anterior Ischemic Optic Neuropathy (rAION)

“…This may exacerbate the area of the initial injury in the nerve, resulting in cytotoxic environments. 33,34 As a quick response within 24 hours after central nervous system (CNS) injury, the oligodendrocyte precursor cells exhibiting a reactive morphology would cause negative effects due to the influx of solutes and macromolecules from the disrupted BBB after microinjections of lipopolysaccharide for inducing CNS injury in the rat model. 35,36 In the experiment involving compression injuries to the spinal cord, the inhibition of the persistent activation of macrophages resulted in neuroprotective effect on neuronal death associated with local microenvironmental changes through the production of TNF-a, IL-1, and IL-6.…”

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“…Moreover, in the stress condition, the fluoxetine-induced reduction in the whole hippocampus levels of IFNɣ, a main cytokine leading to pro-inflammatory microglial response, was concordant with the increased expression of anti-inflammatory markers in CD11b+ cells. Among these, iNOS, which is deeply involved and upregulated in the inflammatory response (Ghosh et al, 2016), was markedly reduced upon fluoxetine treatment. In the enriched condition, the fluoxetine-induced increase of TLR4 and CD14 levels in the whole hippocampus is in line with the expression of pro-inflammatory-related genes in CD11b+ cells.…”

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“…Microglia could be activated into the classic activated state (M1) or the alternative activated state (M2), the former being cytotoxic, the latter being neurotrophic [19] . The elucidation of the signal transduction pathways involved in the phenotypic conversion of M1 to M2 may therefore provide insights into how the innate immune response can be harnessed during distinct phases of disease or injury to mediate neuroprotection and neurorepair [20] .…”