Blockade of microglial adenosine A2A receptor impacts inflammatory mechanisms, reduces ARPE-19 cell dysfunction and prevents photoreceptor loss in vitro

Abstract

Age-related macular degeneration (AMD) is characterized by pathological changes in the retinal pigment epithelium (RPE) and loss of photoreceptors. Growing evidence has demonstrated that reactive microglial cells trigger RPE dysfunction and loss of photoreceptors, and inflammasome pathways and complement activation contribute to AMD pathogenesis. We and others have previously shown that adenosine A2A receptor (A2AR) blockade prevents microglia-mediated neuroinflammatory processes and mediates protection to the retina. However, it is still unknown whether blocking A2AR in microglia protects against the pathological features of AMD. Herein, we show that an A2AR antagonist, SCH58261, prevents the upregulation of the expression of pro-inflammatory mediators and the alterations in the complement system triggered by an inflammatory challenge in human microglial cells. Furthermore, blockade of A2AR in microglia decreases the inflammatory response, as well as complement and inflammasome activation, in ARPE-19 cells exposed to conditioned medium of activated microglia. Finally, we also show that blocking A2AR in human microglia increases the clearance of apoptotic photoreceptors. This study opens the possibility of using selective A2AR antagonists in therapy for AMD, by modulating the interplay between microglia, RPE and photoreceptors.

Modulation of three key innate immune pathways for the most common retinal degenerative diseases

“…Subsequent activation of MAP kinases including extracellular signal‐regulated kinase (ERK) 1/2 and IkappaB kinase (IKK) then induces altered gene expression (Kyriakis & Avruch, ; Schulte & Fredholm, ; Chio et al, ; Dang et al, ). Recently, we showed that A2AR antagonism also limits complement and inflammasome activation (Madeira et al, ). The exposure of human microglia to RPE cell debris induced activation of the complement cascade which is strongly associated with the pathogenesis of AMD (Zipfel & Skerka, ; Schick et al, ).…”

“…Under pathological conditions, immune cells secrete neuraminidases which cleave sialic acid residues on neurons (Amith et al, ; Pshezhetsky & Hinek, ; Nomura et al, ). Desialylated neurons are consequently opsonized by complement component C1q, which is produced and secreted by microglia (Linnartz et al, ; Madeira et al, ). Indeed, soluble sialic acid residues accumulate in serum, and C1q is found in the retina during early stages of AMD (van der Schaft et al, ; Goswami et al, ).…”

“…However, in case of persistent insult, chronic over‐activation of the inflammatory response can lead to devastating tissue remodeling (Chen & Xu, ). Pro‐inflammatory factors such as reactive oxygen species (ROS), TNF‐α, and CCL2 as well as complement activators such as C1q are released into the cytosol by overly active microglia (Scholz et al, ; Madeira et al, ). Furthermore, microglia over‐express the anaphylatoxin receptors C5aR and C3aR, while complement inhibitors such as CFH and CFI are downregulated (Zipfel & Skerka, ; Guillonneau et al, ; Madeira et al, ).…”

“…Furthermore, microglia over‐express the anaphylatoxin receptors C5aR and C3aR, while complement inhibitors such as CFH and CFI are downregulated (Zipfel & Skerka, ; Guillonneau et al, ; Madeira et al, ). Complement factors also act as enhanced triggers for inflammasome assembly, which leads to the activation of the pro‐inflammatory cytokines pro‐IL‐1β and pro‐IL‐18 (Nebel et al, ; Madeira et al, ). The resulting chronic inflammatory response is associated with a decline in RPE function and structure, breach of the blood–retina barrier (BRB), new vessel formation, and recruitment of choroidal macrophages (Donoso et al, ; Liu et al, ; Sato et al, ).…”

“…Pro-inflammatory factors such as reactive oxygen species (ROS), TNF-a, and CCL2 as well as complement activators such as C1q are released into the cytosol by overly active microglia (Scholz et al, 2015a;Madeira et al, 2018). Furthermore, microglia overexpress the anaphylatoxin receptors C5aR and C3aR, while complement inhibitors such as CFH and CFI are downregulated (Zipfel & Skerka, 2009;Guillonneau et al, 2017;Madeira et al, 2018). Complement factors also act as enhanced triggers for inflammasome assembly, which leads to the activation of the pro-inflammatory cytokines pro-IL-1b and pro-IL-18 (Nebel et al, 2017;Madeira et al, 2018).…”