TRPV1 Modulates Pressure–Induced Interleukin–6 Release by Retinal Microglia

Abstract

Purpose: : The pathogenesis of glaucoma is likely to involve the release of inflammatory cytokines by retinal glia. In previous work, we demonstrate that elevated pressure induces activation of nuclear factor kappa B (NFΚB) and a proteosome–dependent increase in the release of interleukin–6 (IL–6) by retinal microglia. Here we examine the role of calcium (Ca²⁺) channels, in particular transient receptor potential (TRP) channels, in upstream modulation of pressure–induced NFΚB activation and IL–6 release in microglia.

Methods: : We utilized PCR and in situ hybridization to examine the expression of TRP channels from six subfamilies in whole rat retina and in microglia purified from rat retina by immunomagnetic separation. We compared TRP expression in microglia cultures exposed to ambient or elevated pressure using PCR and immunocytochemistry. We used immunocytochemistry to examine activation of NFΚB and ELISA to quantify IL–6 in microglia cultures exposed to ambient or elevated pressure, both with and without EGTA chelation of extracellular Ca²⁺ and broad antagonism of Ca²⁺ channels with ruthenium red. Based on the robust expression of the TRP vanilloid receptor 1 (TRPV1), we also examined the effect of the TRPV1–specific antagonist iodo–resiniferatoxin (IDX).

Results: : Although eight TRP subunits were expressed in whole rat retina, microglia expressed only three representing the TRPVand melastatin–related (TRPM) subfamilies. Elevated pressure did not alter TRP gene expression in microglia cultures, but appeared to decrease protein expression on the cell surface. Exposure to 24 hours of elevated pressure induced nuclear translocation of NFΚB and a 2–fold increase in the release of IL–6 by microglia. Ca²⁺ chelation inhibited NFΚB translocation and decreased IL–6 release 7–fold. Ruthenium red and IDX also altered pressure–induced release of IL–6 and activation of NFΚB in a complex manner that was dose–dependent.

Conclusions: : These studies reveal a Ca²⁺–dependent mechanism for pressure–induced activation of NFΚB and subsequent release of IL–6 by microglia that may be modulated by TRP channels, in particular TRPV1.

Keywords: microglia • cytokines/chemokines • calcium