The Role of Interleukin-6 Signalling Molecules in Murine Models of Photoreceptor Degeneration

We previously reported that in inherited photoreceptor degenerations (IPDs), the mutant photoreceptors (PRs) are at a constant risk of death (Pacione, Szego et al, 2003). Using microarrays to identify genes that may mediate the constant risk, I identified 145 differentially expressed transcripts in the Rds+/- mouse model of IPD at a time when 90% of the PRs were alive. A major finding was the up-regulation, quantified by qPCR, of four components of a putative IL-6 cytokine signaling pathway: Oncostatin M (Osm) (2-fold increased), Oncostatin M receptor (Osmr)(2.6-fold increased), Stat-3 (2.3-fold increased), C/EBP delta(3.2-fold increased). Similarly, I found increases in the cognate proteins Osmr (3-fold), Stat-3 (2.6-fold), and the phosphorylated, transcriptionally active form of Stat-3, pStat-3 (5.8-fold)(all p<0.01). Other Il-6 cytokine signaling molecules were largely unchanged, but the mRNA of leukemia inhibitory factor (Lif), was increased (3.0-fold). Comparable increases of most transcripts were also present in the Rd1-/- and mutant rhodopsin P347S transgenic (P347S) IPD models. The increases in cytokine signaling molecules occurred predominantly in Müller glia, although C/EBP delta transcript was increased in PRs. Because exogenous IL-6 cytokine treatment slows PR death in IPDs, I asked whether the endogenous increases in IL-6 pathway proteins in IPD retinas were a survival response, and generated IPD models with Osmr, Lif or C/EBP delta loss-of-function (LOF) mutations. Osmr LOF decreased PR survival in the retinas of Rds+/-;Osmr-/- mice, which had 12.5% fewer PRs than those of Rds+/-;Osmr+/+ mice (n=9, p<0.05) at 4 month of age, and Tg-RHO(P347S);Osmr-/- mice had 13.5% fewer PRs (n=6, p<0.01) at 31 days of age. Unexpectedly, Osmr LOF had no effect on pStat3 levels in Rds+/-;Osmr-/- retinas, indicating that retinal Stat3 activation may be predominantly regulated by other molecules. In contrast to the Osmr LOF, Lif or C/EBP delta LOF unexpectedly increased mutant PR survival. Rd1-/-;Lif -/- mice at 13 days had 14% more PRs than Rd1-/-;Lif+/+ mice (n=6, p<0.003) and a 1.7 fold decrease in pStat-3 (n=4, p<.05). Similarly, 8 month-old Rds+/-; C/EBP delta-/- mice had 18% more PRs than Rds+/-; C/EBP delta+/+ mice (n=5, p