Neuroinflammation, associated with the onset and progression of neurodegenerative events, is predominantly orchestrated by microglia, the resident immune cells of the central nervous system. Microglia-mediated neuroinflammation, thus, results from dysregulation of its physiological functions, which are tightly controlled by several signalling pathways, including growth factor signalling. Despite the involvement of Platelet-Derived Growth Factor Receptor (PDGFR) signalling and microglia in physiologic and pathologic angiogenesis, this signalling remains uncharacterised in microglia. Thus, the role of PDGFR signalling in microglia-mediated neuroinflammatory responses have been investigated using BV-2 microglial cell line.
The expression of PDGFs and their receptors were first characterised in microglia using quantitative polymerase chain reaction, immunocytochemistry, and western blot. Microglia functions, PDGF receptor’s role and intracellular signal transduction pathways were investigated using recombinant human PDGF ligands.
Resting BV-2 cells expressed Pdgfa, Pdgfb, Pdgfc, Pdgfrα and Pdgfrβ genes. In Lipopolysaccharide (LPS) activated cells, Pdgfb and Pdgfrβ were significantly upregulated and sustained at protein levels, Pdgfa and Pdgfrα were not significantly altered, while Pdgfc was significantly downregulated. PDGF induced an amoeboid-like phenotype with concomitant downregulation of the homeostatic gene, P2Y purinoceptor-12 (P2ry12); however, migratory and phagocytic capacities were not significantly affected. Also, reactive oxygen species (ROS) levels were significantly decreased with an upregulation of the anti-oxidant genes, glutathione reductase (Gsr) and superoxide dismutase 2 (Sod2), though, the main ROS generating enzymes, Nicotinamide adenine dinucleotide phosphate (NADPH) oxidases, Nox1 and Nox2, were significantly upregulated. PDGF induced an inflammatory response in BV-2 cells; however, decreasing neurotoxicity effects on 661W photoreceptor cells cultured in microglia conditioned medium. Pharmacological inhibition and RNAi mediated silencing of Pdgfrβ gene reduced both LPS and PDGF-induced inflammatory responses while variable results were seen for Pdgfrα gene. Using small-molecule kinase inhibitors, a role for phosphatidylinositol-3-kinase/Akt (PI3K/Akt), extracellular-signal-regulated kinase -1/2 (ERK1/2) and c-Jun N-terminal kinase (JNK) pathways were defined in PDGF-induced inflammatory response in BV-2 cells.
These results demonstrate a fundamental regulatory role of autocrine PDGFR signalling in microglia-mediated responses in resting and activated state
