Endothelin-2-Mediated Protection of Mutant Photoreceptors in Inherited Photoreceptor Degeneration

Abstract

Expression of the Endothelin-2 (Edn2) mRNA is greatly increased in the photoreceptors (PRs) of mouse models of inherited PR degeneration (IPD). To examine the role of Edn2 in mutant PR survival, we generated Edn2−/− mice carrying homozygous Pde6brd1 alleles or the Tg(RHO P347S) transgene. In the Edn2−/− background, PR survival increased 110% in Pde6brd1/rd1 mice at post-natal (PN) day 15, and 60% in Tg(RHO P347S) mice at PN40. In contrast, PR survival was not increased in retinal explants of Pde6brd1/rd1; Edn2−/− mice. This finding, together with systemic abnormalities in Edn2−/− mice, suggested that the increased survival of mutant PRs in the Edn2−/− background resulted at least partly from the systemic EDN2 loss of function. To examine directly the role of EDN2 in mutant PRs, we used a scAAV5-Edn2 cDNA vector to restore Edn2 expression in Pde6brd1/rd1; Edn2−/− PRs and observed an 18% increase in PR survival at PN14. Importantly, PR survival was also increased after injection of scAAV5-Edn2 into Pde6brd1/rd1 retinas, by 31% at PN15. Together, these findings suggest that increased Edn2 expression is protective to mutant PRs. To begin to elucidate Edn2-mediated mechanisms that contribute to PR survival, we used microarray analysis and identified a cohort of 20 genes with >4-fold increased expression in Tg(RHO P347S) retinas, including Fgf2. Notably, increased expression of the FGF2 protein in Tg(RHO P347S) PRs was ablated in Tg(RHO P347S); Edn2−/− retinas. Our findings indicate that the increased expression of PR Edn2 increases PR survival, and suggest that the Edn2-dependent increase in PR expression of FGF2 may contribute to the augmented survival.

Understanding Cone Photoreceptor Cell Death in Achromatopsia

“…They suggest that activation of STAT3 signaling pathways is achieved via a 28-fold upregulation of endothelin 2 ( Edn2) which is secreted in response to photoreceptor stress. Once again there are parallels between these findings and studies in the rd1 mouse which have shown retinal upregulation of STAT3 (Samardzija et al 2006) and Edn2 (Bramall et al 2013). This suggests that Stat3 signaling and Edn2 activation act as a potent cell survival response but do not however explain by which step of the active degeneration process they are triggered by.…”

Endothelins Inhibit Osmotic Swelling of Rat Retinal Glial and Bipolar Cells by Activation of Growth Factor Signaling

“…ET-2 released from injured photoreceptors may activate Müller cells which subsequently release bFGF, a survival factor of photoreceptors [19,31]. However, ET-2 may also induce bFGF expression in photoreceptors [2]. We found that, in contrast to ET-1, ET-2 osmotic swelling of bipolar cell somata in retinal slices is an indirect effect, likely mediated by ET-1-induced release of swelling-inhibitory factors from Müller cells.…”

“…ET-2 released from injured photoreceptors may activate Müller cells; activated Müller cells produce and release basic fibroblast growth factor (bFGF) which supports the survival of photoreceptors [19,31]. In addition, ET-2 may induce bFGF expression in photoreceptors [2]. In response to photoreceptor injury, Müller cells also produce leukemia inhibitory factor that induces ET-2 in photoreceptors [19].…”

STAT3 promotes survival of mutant photoreceptors in inherited photoreceptor degeneration models

“…For example, the expression of both endothelin-2 and FGF2 is strongly increased in the retina of many IPD and light-damaged models (3,4,7,13), and both have been shown to increase mutant PR survival (4,57,58). Moreover, FGF-2 expression in light-damaged and IPD retinas is regulated by LIF (13,24), and pSTAT3…”

“…Despite this diversity of genetic etiology, disease progression may be regulated by shared pathways that either promote or resist photoreceptor (PR) death (3). The identification of responses to a PR mutation that are common to many if not most IPDs include the increased expression of endothelin 2 (3,4), accumulation of reactive oxygen species (ROS) (1,5), increased expression of GFAP (3,6,7), and exponential death kinetics (8,9). Some of these common events may have no influence on PR death, but others may represent general pathogenetic or survival responses that influence the rate of PR death, or even dictate it.…”