3. General Mechanisms of Nitro-Oxidative Stress in the Optic Nerve
3.1. Generation of Reactive Oxygen and Nitrogen Species
Mitochondria are vital intracellular organelles responsible for essential chemical reactions that produce energy substrates
[70][71]. In addition to their various cellular functions, such as modulating intracellular calcium levels, synthesizing nucleotides, lipids, and amino acids, and regulating apoptosis, mitochondria also generate ROS
[70][72][73][74]. ROS, at basal levels, serve as critical mediators of signaling pathways, including hypoxic and inflammatory pathways
[44][70][75][76]. The fundamental function of mitochondria is to regulate oxygen metabolism and produce energy in the form of ATP
[70][71][77]. The electron transport chain (ETC) within the inner mitochondrial membrane plays a central role in this process
[78]. Despite the efficiency of oxidative phosphorylation, electron leaks can occur, leading to the direct interaction of electron carriers with molecular oxygen (O
2) in the mitochondrial matrix. This interaction results in the donation of electrons and the generation of superoxide (O
2•−)
[71][77][79][80]. While mitochondria are recognized as the main source of ROS in the cell, other significant sources include the enzymatic activities of nitric oxide synthase (NOS) and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX)
[3][79][81][82]. NOS generates nitric oxide (NO), while NOX (comprising seven isoforms: NOX1, -2, -3, -4, -5, DUOX1, -2) transfers electrons from cytosolic NADPH to molecular O
2, generating O
2•− [83][84].
NO is a free radical that plays a pivotal role in various physiological functions
[85]. It serves as a regulator of vascular tone
[86][87][88]. Additionally, NO acts as a signaling molecule in neurotransmission and as a regulator of gene transcription
[89][90][91][92][93][94]. The production of NO is facilitated by the activity of NOS, an enzyme that has three isoforms: neuronal NOS (nNOS or NOS I); inducible NOS (iNOS or NOS II); and endothelial NOS (eNOS or NOS III)
[85][95]. NO rapidly and spontaneously reacts with O
2•− through a “diffusion-limited reaction”
[96][97]. As a result, a highly damaging RNS termed peroxynitrite (ONOO
−) is generated
[85][96][97]. Peroxynitrite contributes to the pathogenesis of diverse retinal disorders, being also newly proposed as a critical factor in the pathogenesis of glaucoma
[98][99][100].
3.2. Oxidative Damage and Antioxidant Defense Systems
ROS and RNS play a physiological role in cellular responses to hypoxia, cell proliferation, cell death, inflammation, or infection
[44][76]. Immune cells, such as phagocytes, produce ROS, which provide reactions necessary for an appropriate killing of pathogens
[101][102]. Due to endogenous or exogenous trigger factors, the balance between pro- and antioxidant systems can be critically undermined, resulting in nitro-oxidative stress. In this context, radicals begin to compete for paired electrons with intracellular substrates
[103], creating oxidative damage. Oxidative injuries are recognized to be a crucial player in the pathogenesis of a variety of pathologies, including ocular diseases
[36][104][105][106][107]. At the biomolecular level, three general forms of injuries caused by reactive species can be distinguished: DNA lesions
[108][109], protein alterations
[110][111], and lipid peroxidation
[96][112]. The consequences of DNA damage are modifications in the expression of proteins and the altered regulation of fundamental activities, like oxidative phosphorylation, according to the vicious cycle theory
[113][114][115]. In this context, mitochondrial ROS also induce activation of the nod-like receptor family pyrin domain-containing 3 (Nlrp3) inflammasome, a key factor in pyroptotic cell death during inflammation
[116].
Antioxidant systems are responsible for defending cells and tissues from the damaging impact of reactive species, which are constantly produced as a “by-product” of oxidative phosphorylation but also serve, at basal levels, physiological functions
[76]. Enzymatic antioxidants comprise SOD, catalase (CAT), glutathione peroxidase (GPX), glutathione-S-transferase (GST), heme oxygenase (HO), peroxiredoxin, and thioredoxin
[103][117][118][119][120][121][122].
Nonenzymatic antioxidants can be classified into direct and indirect agents. Direct antioxidants react with ROS or RNS, “being sacrificed in the process of their antioxidant actions”
[123][124]. Free radical scavengers are, for example, glutathione (GSH)
[125], carotenoids
[126], vitamin C (ascorbic acid)
[127], and vitamin E (α-tocopherol)
[128]. Alternatively, indirect antioxidants are molecules, such as vitamin C, that upregulate antioxidant proteins, for example, via the nuclear factor erythroid-2-related factor 2 (Nrf2)
[123][127] or molecules, like α-lipoic acid
[129]. Examples of antioxidant compounds adsorbed with the food are resveratrol and betulinic acid
[98][130][131].
3.3. Oxidative Stress in Retinal Ganglion Cells
The retina belongs to the metabolically most active organs in the human body
[132] and requires a relatively large amount of energy substrates
[133], which makes it particularly vulnerable to energy insufficiency
[134]. Oxygen supply is essential for retinal function
[135], and its consumption occurs very rapidly, like in the brain
[136][137][138]. Hence, conditions that can modify the supply of molecules, such as O
2, necessary for the production of energy substrates, like ATP, may rapidly generate significant damage in RGCs due to their susceptibility to oxygen deficiency (
Figure 2). Thus, an appropriate blood supply via retrobulbar and retinal vessels is crucial for the proper function of RGCs. Studies in retrobulbar blood vessels reported that ROS blunted endothelial function partially by reducing the contribution of the NOS pathway to endothelium-dependent vasodilation
[139]. Likewise, moderately elevated IOP induced endothelial dysfunction in retinal arterioles together with RGC loss
[140][141]. Zadeh et al. found in apolipoprotein E (ApoE)-deficient mice that hypercholesterolemia caused oxidative stress and endothelial dysfunction in retinal arterioles but did neither lead to increased ROS levels in the RGC layer nor to loss of RGCs, indicative of compensatory effects
[142]. In contrast, a study in pigs reported that after only 12 min of ocular ischemia and 20 h of reperfusion, endothelial dysfunction, retinal edema, and RGC loss occurred
[143]. ROS generation due to ischemia/reperfusion (I/R) injury is reported to be caused by diverse enzymes involved in the regulation of oxidative metabolism, such as NOX2, xanthine oxidase (XO), uncoupled eNOS, and by ETC dysfunction
[143][144][145][146]. Hyperglycemia was also described to be a cause of endothelial dysfunction and oxidative stress in the retina
[147][148][149] via the involvement of NOX2 due to the activation of the receptor of an advanced glycation end product (RAGE)-, mitogen-activated protein kinase (MAPK)-, polyol-, protein kinase C (PKC)-, renin–angiotensin system (RAS) signaling pathways
[150][151][152][153][154][155].
Figure 2. Model representing the ROS impact on the retina and on the optic nerve. ROS: reactive oxidative species; NOX2: NADPH oxidase type 2; XO: xanthine oxidase; eNOS: endothelial nitric oxide synthase; RGC: retinal ganglion cell; RNFL: retinal nerve fiber layer; RPE: retinal pigment epithelium; BM: Brunch’s membrane. Up arrows mean increase or upregulation.