Early Response to Moderate IOP Elevation Involves ATP Release, IL-6 and the P2x7 Receptor

Abstract

Purpose: : The elevation of IOP initiates a series of changes that can eventually damage retinal ganglion cells. As intervention at these early stages may mitigate this pathology, identification of genes whose expression is changed soon after pressure elevation may be relevant. We have hypothesized that the excessive release of ATP from retinal cells is one of the earliest responses to increased IOP. The purinergic system interacts with cytokine regulation in other tissues, and elevation of IL-6 has been identified as an early response in experimental glaucoma, suggesting possible interactions. This study begins to probe the relationship between IOP elevation, IL-6 and purines.

Methods: : A model of moderate IOP elevation was adapted, whereby IOP was raised in one eye of an adult Sprague Dawley rat by cannulating the anterior chamber with sterile saline. IOP was raised to 50 mm Hg for 4 hrs and then returned to normal. Retinal blood flow was observed throughout. Retinas were obtained 24 hrs later, with subsequent RNA extraction and qPCR. In some experiments, either saline or the P2X₇ receptor antagonist A438079 was injected into the vitreal chamber before IOP elevation.

Results: : The moderate elevation in IOP did not alter expression of cFOS, Annexin A3 or BAX, suggesting this insult was insufficient to trigger apoptosis in retinal cells. Levels of NTPDase1, a marker for extracellular ATP were doubled while levels of IL-6 increased 4-fold. Changes in the levels of several purinergic receptors were also suggested. As activation of the P2X₇ receptor for ATP has been previously associated with cytokine release, the effects of P2X₇ receptor block on gene expression were examined. In the presence of antagonist A438079, levels of IL-6 rose 50-fold. The expression of other genes was not affected by A438079.

Conclusions: : These results confirm a role for IL-6 in the response to IOP rise and are consistent with the theory that excess ATP is released by retinal cells soon after moderate changes in pressure. The increased expression of IL-6 after injection of A438079 suggests the P2X₇ receptor is involved with the cytokine response to increased IOP, although the mechanisms underlying this process remain to be determined.

Keywords: ganglion cells • intraocular pressure • adenosine