Rapid Morphologic and Molecular Activation of Microglial Cells by Stimulation of the P2X7 Receptor Correlates with Neuron Loss

Abstract

Background: The endogenous signals leading to microglial activation represent central components of neuroinflammatory cascades. Given ATP release accompanies mechanical strain to neural tissue, and the P2X7R for ATP is expressed on microglial cells, we examined the morphological and molecular consequences of P2X7R stimulation in vivo and in vitro in detail to enhance understanding of the response. Methods: IL-1β release was determined with ELISA. Expression of mRNA used qPCR. ATP release was determined with the luciferin/luciferase assay while fura-2 indicated cytoplasmic calcium. Microglial migration used Boyden chambers. Morphological changes were quantified from Iba1-immunostained cells. Results: Sholl analysis of Iba1-stained cells showed retraction of microglial ramifications one day after injection of P2X7R agonist BzATP into mouse retinae. Mean branch length also decreased, while cell body size and expression of Nos2, Tnfa, Arg1, Chil3 increased. BzATP induced similar morphological changes in ex vivo tissue isolated from Cx3CR1-GFP mice, suggesting cell recruitment was unnecessary. Primary microglial cultures were developed to investigate the autonomous nature of the response. Isolated microglial cells expressed P2X7R, while increased intracellular Ca 2+ triggered by BzATP and blocked by antagonist A839977 confirmed functional expression. BzATP induced process retraction and cell body enlargement within minutes in isolated microglial cells, and increased expression of Nos2 and Arg1 . BzATP both increased expression of IL-1β, and triggered a substantial release, suggesting P2X7R both primes and activates the NLRP3 inflammasome. ATP increased microglial migration, but this required P2Y12R, not P2X7R involvement. As ATP release often accompanies mechanical strain, responses to intraocular pressure elevation were determined. Transient elevation increased ATP release and led to microglial process retraction, cell body enlargement and gene upregulation resembling the responses to BzATP injection. These pressure-dependent changes to microglia were reduced in P2X7R -/- mice. Critically, the loss of retinal ganglion cell neurons accompanying increased pressure was correlated with microglial activation in C57Bl/6J, but not P2X7R -/- mice.Conclusions: P2X7R stimulation induced morphological and molecular markers of activation in retinal microglial cells in vivo and in vitro , affecting IL-1β release and rapid process retraction but not cell migration. Parallel responses accompanied transient pressure elevation, suggesting ATP release and P2X7R stimulation contribute to the microglial response to rising pressure.