Reducing retinal oxidative metabolism to arrest progression of ischemic retinopathies

Abstract

Purpose : Photoreceptors have very high energy needs even under basal condition, thus showing very high oxygen (O2) consumption rates (OCR); these are maintained when O2 delivery to the retina is compromised (e.g. ischemia), leading to a prominent metabolic mismatch that fosters development of strong and deleterious hypoxic responses favoring disease progression. Here, we assess whether reducing retinal oxidative metabolism (by using purine metabolites) could prevent progression of ischemic retinopathies in the OIR model.

Methods : OIR was induced in Bl6/C57 mice and intravitreal injections with inosine or PBS were performed at P12. A Seahorse flux analyzer was used to measure O2 consumption rates in vaso-obliterated (VO) and vascularized retinal areas. Retinal flatmounts were analyzed for pre-retinal neovascularization and VO, and RT-PCR was performed to assess levels of pro-angiogenic and pro-inflammatory cytokines in retinal lysates, at P17. ERG and TUNEL analyses were conducted to assess neuronal cell apoptosis and retinal function at P30.

Results : Inosine injections induced a 15% reduction in basal OCR selectively in VO areas and reduced development of ischemia-driven pathological retinal features in P17 OIR mice by (1) reducing the VO (ischemic) areas by 62.5% (p<0.001), (2) inhibiting pre-retinal neovascularization by 39% (p<0.001) and (3) attenuating the retinal pro-inflammatory response, when compared with PBS controls. Moreover, ERG analyses revealed improved photopic function in inosine-treated eyes at P30.
TUNEL staining excluded increases in retinal cell apoptosis in inosine-injected eyes. OCR were unchanged between inosine and controls in peripheral, vascularized areas of the OIR retina, suggesting that inosine may induce a selective metabolic effect in regions undergoing ischemic stress to allow for higher tolerance to hypoxia locally. Furthermore, maximal respiratory capacity was not compromised by inosine, suggesting that mitochondrial fitness is not affected.

We found a 3.6 fold upregulation in inosine serum levels in long-term diabetic patients who do not develop late-stage diabetic retinopathy suggesting that inosine may play a protective role in ischemic retinopathies.

Conclusions : Reducing retinal oxidative metabolism to a functional minimum may be a useful therapeutic strategy in ischemic retinopathies by better matching metabolic supply and demand.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.