| Summary: | Inherited photoreceptor degenerations (IPDs) are the most common monogenic cause of blindness in humans. To discover genes that may influence the risk of death in IPDs, microarray studies were used, and ET-2 was identified as the most differentially expressed transcript. ET-2 mRNA was 32-fold (p< 0.004), 70-fold (p< 0.009) and 72-fold (p<0.0009) increased in the Rds+/- , Tg(RHO P347S) and Rd1-/- mouse models of IPD, respectively, and the ET-2 peptide was minimally three-fold upregulated in Rds+/- retinas. The increased ET-2 transcript was detected solely in the photoreceptors (PRs) of Rds+/- and Tg(RHO P347S) retinas, but not in wild-type (wt) retinas by in situ hybridization. To determine the biological role of ET-2 in IPDs, mouse IPD models were crossed to ET-2-null mice. At age 40 and 15 days, ET-2-/-; Tg(RHO P347S) and ET-2-/-;Rd1-/- retinas showed, respectively, a 41% (n=6;p<0.003) and 49% rescue of PR degeneration (n=5;p<0.007). Unexpectedly, however, the PRs in ET-2-/- Rd1-/-retinal explants were not rescued (n=6;p>0.05), suggesting that the rescue observed in vivo might be due to extraocular mechanisms. Additionally, the expression of ET-2 mRNA from an rAAV5-CBA-ET-2 vector in ET-2-/-; Rd1-/- retinas did not restore PR degeneration (n=6;p>0.05). A survey of the extraocular phenotypes of ET-2 null mice showed them to be hypoxic owing to aberrant lung development, with a loss of normal alveolarization of the lung. Erythropoietin (EPO) levels were 11-fold elevated in the serum of ET-2 null mice (n=7;p<0.05) and retinal vascular endothelial growth factor (VEGF) was increased 4-fold (n=4;p<0.05). To examine the role of hypoxia in PR degeneration and to exclude increased EPO levels as the sole factor accounting for the rescue of mutant PRs in ET-2-/-; Tg(RHO P347S) and ET-2-/-;Rd1-/- mice in vivo, the effect of hypoxia on PR death in Rd1-/- retinal explants was examined. Rd1-/- explants cultured in 6% O2 from PN10 to PN17 showed a 32% rescue of PR death (n=5;p<0.05). Although ET-2 may mediate PR death through a direct role in mutant PRs, the PR rescue observed in ET-2-/-; Tg(RHO P347S) and ET-2-/-;Rd1-/-retinas may also result from systemic hypoxia due to poor lung function in ET-2-/- animals.
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