Elevation of IOP triggers responses from cytokines IL-6 and IL-1β; involvement of both optic nerve head astrocytes and retinal ganglion cells

Abstract

Purpose: Increasing evidence suggests that cytokines contribute to a local inflammatory response in glaucoma. We and others have previously shown that transient elevation of IOP with the “controlled elevation of IOP” (CEI) model increases retinal levels of IL-6 mRNA. In this study, we asked whether an increase in protein levels can accompany this rise, whether IL-1β is also involved, whether cytokines are elevated during chronic IOP elevation, and probed the cellular source of these cytokines.

Methods: In the CEI model of acute IOP elevation, pressure was increased in rats by canulation of the anterior segment; IOP was raised to 50 mm Hg for 4 hrs and tissue extracted 24 hrs later. The Tg-MYOCY437H mouse was used as a model of chronic glaucoma; IOP in 8 month old Tg-MYOC mice was 15.7 ± 0.5 mmHg compared to 11.8 ± 1 mmHg in littermate controls. Rat optic nerve head astrocytes were cultured onto a silicone substrate and subjected to 5% strain using a custom designed apparatus. Rat retinal ganglion cells (RGCs) were isolated using the immunopanning method and exposed to an analogous stretch. mRNA levels were compared using qPCR and protein was quantified from immunoblots, or for RGCs with an antibody cytokine array.

Results: Elevation of IOP to 50 mm Hg for 4 hrs elevated IL-6 proteins in the retina, consistent with the rise in mRNA. Levels of IL-1β mRNA were also elevated, and the increase in mature 17 kD IL-1β protein levels with CEI approached significance. mRNA for IL-1β was increased in 8 month old Tg-MYOC mice. In isolated rat optic nerve astrocytes, IL-6 was released from astrocytes by stretch. Stretch also increased mRNA for IL-6 and IL-1β in astrocytes. Stretch of isolated RGCs led to the release of both IL-6 and IL-1β.

Conclusions: Upregulation and release of the cytokines IL-6 and IL-1β may be a general response to mechanical strain in the posterior eye. Cytokine levels in the retinal were increased in vivo by transient elevation of IOP and by chronic elevation with the Tg-MYOC model, and released from ganglion cells. Cytokines in optic nerve astrocytes also increased in response to stretch. The cellular mechanisms linking mechanical strain to cytokine release are being investigated.

Keywords: 695 retinal degenerations: cell biology • 531 ganglion cells • 490 cytokines/chemokines