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Int. J. Mol. Sci. 2020, 21, 2370 2 of 20 neurological function including retinal degeneration, optic nerve injury, traumatic brain injury and spinal cord injury [4–8]. Various studies have focused on PBM effects such as cell proliferation and migration, oxidative stress, apoptosis and inflammatory processes in different disease models [9]. Although PBM shows positive effects, the underlying cellular and molecular mechanisms are only poorly described, especially concerning biological reactions. Most studies focused on cytochrome c oxidase (CCO) serving as primary photoacceptor and being the key target and effector for PBM, leading to enhanced energy metabolism [10–12]. CCO, complex IV in the mitochondrial respiratory chain, functions as a chromophore, which contains heme and copper centers to absorb light in the red to near-infrared spectral range. In recent years, there has been an increasing interest in using PBM for therapy of retinal diseases. Protective functions were reported for different neuronal and nonneuronal retinal cell types, i.e., photoreceptors, the retinal pigment epithelium (RPE), retinal ganglion cells and Müller cells [4,13–20]. Besides studying individual cell types, several studies focus on effects in different diseases models such as age-related macular degeneration (AMD) and retinopathy models for possible transfer into therapy [18,21–26]. Most of these retina-related studies concentrate especially on the beneficial effects of 670 nm red light on retinal function, while other wavelengths are only rarely studied [4,27]. Results support the hypothesis of CCO as the primary target of the mitochondrial respiratory chain leading to enhanced retinal ATP production in AMD models [21,24,25,28] and increased mitochondrial membrane potential, shown in murine RPE [26]. In addition to mitochondrial components, retina-related studies demonstrate positive RL effects concerning retinal inflammation and cell stress in models of AMD. In particular, tumor necrosis factor-α, calcitonin, C3 and 4-hydroxy-2-nonenal (4HNE) were reduced in the outer retina and C3b, C3d and amyloid-beta expression decreased in Bruchs membrane [18,21,26]. Moreover, reduced neovascularization and reduced photoreceptor cell death in oxygen-induced retinopathy models were reported [18,21–23]. Additionally, the protection of the photoreceptor ultrastructure combined with an improved retinal function verified by electroretinogram were demonstrated in an animal model with methanol induced mitochondrial dysfunction [6]. Only very few publications studied the ability of wavelengths other than 670 nm to be supportive for retinal injuries. Giacci et al. analyzed effects of 670 nm and 830 nm on a retinal degeneration model, showing reduced cell death and 8-hydroxyguanosine immunoreactivity after 670 nm pretreatment, but no changes in 830 nm treated animals [4]. However, Ivandic et al. observed positive effects of 780 nm light treatment; they reported improved visual acuity, reduced edema and blebbing in wet AMD [27]. The addressed RL and NIRL studies indicate that PBM can offer a non-invasive approach that is easy to deliver, to prevent or slow down the progress of retinal pathology. Focusing on collective PBM effects is a demanding question since a large number of studies use different irradiation sources, different wavelengths, durations and frequencies. Therefore, we decided to analyze two different wavelengths (670, 810 nm) with different irradiation sources (LED and diode laser) to reach a higher consensus and comparability. The purpose of the present study was to examine whether PBM has beneficial effects in light-induced photoreceptor damage to achieve a better understanding of the underlying cellular and molecular mechanisms of RL and NIRL action. To verify beneficial effects on photoreceptors, the oxidative cell stress was analyzed by measuring reactive oxygen species (ROS), lipid peroxidation and mitochondria-related apoptosis. Due to the recommended action of PBM on mitochondrial function on oxidative cell stress and cell viability, the participation of RL and NIRL action on respiratory complexes was analyzed. Currently, there is limited knowledge concerning the influence of PBM on respiratory chain complexes, mainly focusing on complex IV, leading to improved cell survival. Therefore, the additional analysis of other complexes might explain the connections of the RL and NIRL action more conclusively. Moreover, the RL and NIRL triggered regulatory effects based on mRNA alterations were examined, which might explain these observations. To examine the positive effects of PBM, an established ex vivo retina tissue cultivation system was used [29,30]. With regard to our study, an ex vivo application, especially whole eyeballs cultivation, isPDF Image | Photobiomodulation Mediates Neuroprotection against Retinal
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