Oxidative and bioenergetic balance as key determinants of neural viability in the retina: insights from retinal neurodegenerative diseases

The study of the mechanisms regulating retinal neuron viability can unravel the role of key determinants for neuronal survival potentially suitable as therapeutic targets. The elevated metabolism of the retina may increase retinal neuron susceptibility to metabolic stressors, which can damage retinal cells by altering mitochondrial activity and unbalancing the bioenergetic and oxidative status. Many studies have reported an early dysregulation of bioenergetic and oxidative mechanisms prior to cell death in the pathogenesis of many retinal neurodegenerative disorders induced by different type of stressors, suggesting the key role of the maintenance of metabolic and oxidative processes as a general mechanism for retinal survival and a potential target for new therapies. Therefore, this thesis is focused on providing further insights into the role of the bioenergetic and oxidative balance as determinants of retinal neuron viability and suggesting possible novel non-invasive approaches for neuroprotection in different retinal disorders. In particular, the effects of a positive modulation of cell bioenergetic and oxidative status have been assessed in models of retinal neurodegenerative diseases by the non-invasive administration of natural antioxidant and ATP-boosting molecules. In particular, the effects of a positive modulation in the oxidative balance by the dietary administration of an antioxidant compound including cyanidin-3-glucoside (C3G), verbascoside and zinc, was first tested in a model of streptozotocin (STZ)-induced diabetic retinopathy (DR), where oxidative stress occurs as a result of a hyperactivated mitochondrial activity promoted by hyperglycemia. In this model, the counteraction of the oxidative stress and its related inflammation by the compound was effective in reducing the hyperglycemia-induced vascular and neuronal alterations, including BRB dysfunction, retinal cell degeneration and visual loss. Thus, these findings demonstrate the beneficial regulation of oxidative balance in counteracting the high glucose-induced neurovascular alterations, suggesting novel molecules for the treatment of DR. Oxidative stress can also result from mitochondrial hypoactivity, as occurs under light damage (LD). Since the oxidative stress is a shared pathological event both in LD and DR, a similar antioxidant strategy and similar molecules used in the STZ model were used to test the beneficial effects of the oxidative stress counteraction in a model of LD. In particular, the dietary supplementation of antioxidant molecules, including lutein and C3G, administered either alone or in combination, prevented the LD-driven oxidative stress and inflammation, protecting photoreceptors from degeneration and functional impairment. In addition, the protective efficacy of lutein and C3G was also confirmed in a more complex antioxidant multicomponent formula with verbascoside and zinc, confirming the potential of these molecules as novel treatment option for the oxidative stress-related retinal disorders. Considering that an antioxidant strategy was effective in reducing retinal cell degeneration under reduced mitochondrial activity promoted by LD, it was investigated whether the modulation of the oxidative balance was sufficient to counteract the impairment of retinal cell viability in a multifactorial context. In this respect, glaucoma was used as a model of multifactorial disease, representing a retinal ganglion cell (RGC) specific disorder where metabolic disturbances and reduced mitochondrial activity concur with other pathophysiological events to determine RGC degeneration. The efficacy of antioxidant molecules included in the spearmint extract (SPE) was investigated in a model of methylcellulose (MCE)-induced glaucoma, where the elevated intraocular pressure (IOP) reproduces a glaucomatous stress. The administration of SPE in this model resulted in a dose-dependent improvement of RGC activity, density and trophism, showing an antioxidant and anti-inflammatory effect evidenced by the reduction of oxidative stress- and inflammation-related markers, together with an increase in the retinal antioxidant defense. These findings demonstrate the capacity of SPE to counteract glaucoma-related oxidative stress and improve RGC viability and function, thus suggesting a novel complementary strategy for the management of the disease. The protective effect associated with the positive regulation of RGC redox balance under glaucomatous stress suggested the potential to target the upstream mitochondrial and metabolic mechanisms to improve RGC viability under metabolic stress. RGCs were therefore used as a model to investigate whether the positive improvement of cell bioenergetics improves RGC health under metabolic stress by using pyrroloquinoline quinone (PQQ), which has been identified as a metabolic regulator in non-neuronal systems. The metabolic capacity of PQQ was confirmed in a retinal context by assessing the increase in ATP production in in vitro and in vivo RGC-related tissues. Moreover, the administration of PQQ was associated with moderate influence on molecular and morphological changes in RGC mitochondria and altered metabolic profile of RGC-related tissues, suggesting possible mechanisms by which PQQ may exert its metabolic effect. Given its ATP-boosting activity, the neuroprotective efficacy of PQQ was investigated in different models of RGC stress where bioenergetic capacity has been compromised. PQQ neuroprotection was first tested in an ex vivo model of retinal axotomy, where the administration of PQQ partially prevented the RGC loss observed in this model. In addition, the neuroprotective properties of PQQ have been confirmed in an in vivo model of rotenone-induced RGC degeneration, where the treatment with PQQ prevented RGC death. Therefore, PQQ may be used as a novel compound providing bioenergetic support to improve RGC resiliency under metabolic stress. Taken together, the data reported in this thesis provide further insights on the role of the bioenergetic and oxidative balance as determinants of retinal neuron viability and suggest possible novel non-invasive complementary approaches for neuroprotection in different retinal disorders.