**1. Introduction**

Glaucoma derives from an increase of the intra-ocular pressure (IOP) due to accumulation of the aqueous humor which causes degenerative events at the level of the retina and the optic nerve. This results in a progressive damage of the optic nerve that is paralleled by the gradual loss of retinal ganglion cells (RGC). The pathology causes increasing eyesight deterioration particularly in the peripheral areas of the visual field. The optic nerve papilla becomes paler and shows an augmented excavation as compared with a normal physiological situation. The increase of the IOP is to be ascribed, in the majority of cases, to an alteration of the ocular hydrodynamics: in particular the normal efflux of aqueous humor from the anterior chamber of the eye is severely hindered. The drainage system is located in the limbal regions or in the sclero-corneal junction. The inner surface presents a hollow (depression) known as inner scleral spur which is filled by the trabecular meshwork and the canal of Schlemm. Primary open angle glaucoma is caused by the failure of drainage from the trabecular meshwork, while the primary closed angle glaucoma consists in a modification of the iris-corneal angle. It is commonly accepted that glaucoma is the second cause of blindness in the world; as a matter of fact it has been estimated that 68 millions of patients are affected by this pathology and out of them, about 7 millions suffer complete bilateral blindness as a consequence of the glaucoma. The onset of the disease may occur at any age, also at childhood, but it is significantly more frequent in elderly people. Glaucoma is generally categorized in five different groups; two of them are the above mentioned open and closed angle primary glaucoma which are also the most widespread ones. A broad variety of pathological conditions may induce, as secondary

© 2013 Calandrella et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

event, the obstruction of the drainage system of the drainage angle which results in glaucoma. The primary open angle, which represents more than 60% of the cases, is a chronic condition. The outflow angle is not altered; the aqueous humor produced by the ciliary body reaches the trabecular meshwork, but its drainage is not efficient. This is possibly due to the decrease of diffusion towards the Schlemm's canal which causes a continuing increase of the IOP ending in the progressive degeneration of the optic nerve. Among the secondary factors contributing to the insurgence of glaucoma one should take into account: age (above 70), myopia and ethnic origin since the African populations seem to be more prone to develop the disease.

reduction of neurotrophins required for the survival of the RGC [5]. Furthermore, a reduction of the axon-plasma transport and the accumulation of toxic level of neurotransmitters have been observed; also, an increase of nitric oxide and endothelins as well as remodeling of the extra-cellular matrix has been monitored. Studies validate, on the other hand, the theory of the ischemic model, *i,e,* the vascular model of ischemia, as a main cause of the increased mechanical

Experimental Glaucoma After Oxidative Stress and Modulation of the Consequent Apoptotic Events in a Rat Model

http://dx.doi.org/10.5772/54628

17

The ischemic hypothesis postulates that the high intraocular pressure and the deformation of the *lamina cribrosa* may generate a compression of the blood vessels at retina and/or optic nerve level with a subsequent ischemic damage. In the pathological ischemic condition a temporary interruption of blood perfusion occurs and this determines a lack of oxygen, glucose and trophic substances in general. In patients with normal pressure and open angle glaucoma it was reported a decrease of the blood flow at the head of the optic nerve and an increase of hemagglutination. In addition, in this type of glaucoma an alteration of endothelin-mediated blood flow occurs. This protein is expressed in the endothelial cells and constricts blood vessels thus raising the blood pressure; its action is mainly exerted on the smooth muscles of the blood vessels [6]. The raise of the IOP plays a crucial role in the etiology of the disease, however the observation of glaucoma patients with normal pressure values suggests that diverse factors

The glaucoma neuropathy may be also due to an insufficient vascular perfusion of the optic nerve which causes an ischemic damage to this organ. The ischemia thus generated, ends in an oxidative stress at RGC level and causes apoptotic death. This phenomenon happens because when re-perfusion initiates, the presence of oxygen in the tissue exposed to ischemia, induces the formation of radical oxygen species (ROS). When the concentration of ROS is too high, the anti-oxidant systems of the cell become unable to inactivate them, due to a deficient homeostasis, thus the free radicals are no longer neutralized and may cause cell death either via apoptosis or necrosis. In conclusion both types of stress, the mechanical and the ischemic

A complex interaction between neural and glial cells exists during the differentiation and the life of the nervous system. As a matter of fact, neuroglia cells maintain the normal functions of the nervous system since they control the extra cellular environment, block the toxic agents and supply the trophic resources and, last but not least, provide a structural support to the neurons. In glaucoma, astrocytes play a very important role as far as the re-modeling of the *lamina cribrosa* is concerning. Actually, they may also have a role also in the onset of the disease. Studies conducted on human glaucoma have, in fact, evidenced that the disorganization at astrocyte level in the anterior areas of the optic nerve, is associated to hypertrophy and overexpression of the glial fibrillary acidic protein (GFAP) which also occurs in astrocyte cultures subjected to high hydrostatic pressure. Following ischemic episodes, traumas or neurodegenerative disorders, the phenotype of the astrocyte cells and microglia, activates the production of cytokines, ROS, nitric oxide and tumor necrosis factor α (TNF-α); all these molecules are mediators involved in the tissue damage [2]. In a similar way, glial cells located

compression and subsequent oxidative stress at cell level.

act synergistically to the insurgence of the pathology.

one, can contribute to the establishment of the disease [2].

**1.1. Cellular targets of the ocular hypertension**

In the primary closed angle glaucoma which occurs in about 10% of patients, a closure of the filtration angle in the eye is observed and this is occasionally due to the trabecular obstruction by the iris. The mode of insurgence of this type of glaucoma, unlike other forms, is very rapid and is therefore also known as acute glaucoma. In this condition one of the main risk factors is also associated to familial and/or ethnic factors. As a matter of fact, East asian populations, the Chinese one in particular, show a significant aptitude towards this pathology, other risk factors being the patient's age (above 50 years of age the incidence of the pathology increases) and hypermetropia. To date a decisive therapy for neither open nor closed angle glaucoma is available, however some treatments exist allowing the slowing, and in some cases the arrest, of the progression of the disease.

Secondary glaucoma may develop as a consequence of other pathologies such as inflammation, cataract, traumas, pigments released from the iris and, finally, tumors. In this situation the eye activates its defense producing the hyper-secretion of aqueous humor thus leading to ocular hypertension. One of the main characteristics of glaucoma is the increased excavation of the optic disk which extends towards its margins. Even though some studies support the idea that the pathology may start at retinal level, some indications exist that the early lesions occur at the level of the head of the optic nerve, in particular on the *lamina cribrosa*. Investigations demonstrate that the death of RGC occurs by apoptosis [1, 2]; the activation of this process, most likely, causes a reduction of the number of axons forming the optic nerve and this would evolve to the clinical signs consisting in the characteristic increase of papilla excavation which results in a reduction of the optic visual field. The ganglion retinal cells are the first target of the damage, mainly those found in the temporal region of the retina where the *lamina cribro‐ sa* is thinner and thus gives an inefficient structural support to the RGC axons [3].

Hypotheses on the mechanisms of cell degeneration are diverse, the mechanical stress and the ischemic model being two of the most corroborated ones. The mechanical stress theory purports that the increase of the IOP within the anterior chamber causes a direct hyperpressure at the retina-vitreous interface. This mechanical stress would directly trigger cell death by physical compression. According to this theory the mechanical insult causes modi‐ fications of the cell function: with respect to this, it has been reported that this type of insult may alter gene expression in organs such as the heart and the endothelial vessels. Furthermore, by the activation of transduction pathways, different functional responses are induced in retinal cells and astrocytes [4]. This IOP-induced mechanical stress could also inhibit the retrograde transport along the ganglion cell axons. Regarding this particular point, it has been observed a block in the axonal transport at the level of the *lamina cribrosa* followed by a drastic reduction of neurotrophins required for the survival of the RGC [5]. Furthermore, a reduction of the axon-plasma transport and the accumulation of toxic level of neurotransmitters have been observed; also, an increase of nitric oxide and endothelins as well as remodeling of the extra-cellular matrix has been monitored. Studies validate, on the other hand, the theory of the ischemic model, *i,e,* the vascular model of ischemia, as a main cause of the increased mechanical compression and subsequent oxidative stress at cell level.

The ischemic hypothesis postulates that the high intraocular pressure and the deformation of the *lamina cribrosa* may generate a compression of the blood vessels at retina and/or optic nerve level with a subsequent ischemic damage. In the pathological ischemic condition a temporary interruption of blood perfusion occurs and this determines a lack of oxygen, glucose and trophic substances in general. In patients with normal pressure and open angle glaucoma it was reported a decrease of the blood flow at the head of the optic nerve and an increase of hemagglutination. In addition, in this type of glaucoma an alteration of endothelin-mediated blood flow occurs. This protein is expressed in the endothelial cells and constricts blood vessels thus raising the blood pressure; its action is mainly exerted on the smooth muscles of the blood vessels [6]. The raise of the IOP plays a crucial role in the etiology of the disease, however the observation of glaucoma patients with normal pressure values suggests that diverse factors act synergistically to the insurgence of the pathology.

The glaucoma neuropathy may be also due to an insufficient vascular perfusion of the optic nerve which causes an ischemic damage to this organ. The ischemia thus generated, ends in an oxidative stress at RGC level and causes apoptotic death. This phenomenon happens because when re-perfusion initiates, the presence of oxygen in the tissue exposed to ischemia, induces the formation of radical oxygen species (ROS). When the concentration of ROS is too high, the anti-oxidant systems of the cell become unable to inactivate them, due to a deficient homeostasis, thus the free radicals are no longer neutralized and may cause cell death either via apoptosis or necrosis. In conclusion both types of stress, the mechanical and the ischemic one, can contribute to the establishment of the disease [2].

#### **1.1. Cellular targets of the ocular hypertension**

event, the obstruction of the drainage system of the drainage angle which results in glaucoma. The primary open angle, which represents more than 60% of the cases, is a chronic condition. The outflow angle is not altered; the aqueous humor produced by the ciliary body reaches the trabecular meshwork, but its drainage is not efficient. This is possibly due to the decrease of diffusion towards the Schlemm's canal which causes a continuing increase of the IOP ending in the progressive degeneration of the optic nerve. Among the secondary factors contributing to the insurgence of glaucoma one should take into account: age (above 70), myopia and ethnic

In the primary closed angle glaucoma which occurs in about 10% of patients, a closure of the filtration angle in the eye is observed and this is occasionally due to the trabecular obstruction by the iris. The mode of insurgence of this type of glaucoma, unlike other forms, is very rapid and is therefore also known as acute glaucoma. In this condition one of the main risk factors is also associated to familial and/or ethnic factors. As a matter of fact, East asian populations, the Chinese one in particular, show a significant aptitude towards this pathology, other risk factors being the patient's age (above 50 years of age the incidence of the pathology increases) and hypermetropia. To date a decisive therapy for neither open nor closed angle glaucoma is available, however some treatments exist allowing the slowing, and in some cases the arrest,

Secondary glaucoma may develop as a consequence of other pathologies such as inflammation, cataract, traumas, pigments released from the iris and, finally, tumors. In this situation the eye activates its defense producing the hyper-secretion of aqueous humor thus leading to ocular hypertension. One of the main characteristics of glaucoma is the increased excavation of the optic disk which extends towards its margins. Even though some studies support the idea that the pathology may start at retinal level, some indications exist that the early lesions occur at the level of the head of the optic nerve, in particular on the *lamina cribrosa*. Investigations demonstrate that the death of RGC occurs by apoptosis [1, 2]; the activation of this process, most likely, causes a reduction of the number of axons forming the optic nerve and this would evolve to the clinical signs consisting in the characteristic increase of papilla excavation which results in a reduction of the optic visual field. The ganglion retinal cells are the first target of the damage, mainly those found in the temporal region of the retina where the *lamina cribro‐*

*sa* is thinner and thus gives an inefficient structural support to the RGC axons [3].

Hypotheses on the mechanisms of cell degeneration are diverse, the mechanical stress and the ischemic model being two of the most corroborated ones. The mechanical stress theory purports that the increase of the IOP within the anterior chamber causes a direct hyperpressure at the retina-vitreous interface. This mechanical stress would directly trigger cell death by physical compression. According to this theory the mechanical insult causes modi‐ fications of the cell function: with respect to this, it has been reported that this type of insult may alter gene expression in organs such as the heart and the endothelial vessels. Furthermore, by the activation of transduction pathways, different functional responses are induced in retinal cells and astrocytes [4]. This IOP-induced mechanical stress could also inhibit the retrograde transport along the ganglion cell axons. Regarding this particular point, it has been observed a block in the axonal transport at the level of the *lamina cribrosa* followed by a drastic

origin since the African populations seem to be more prone to develop the disease.

of the progression of the disease.

16 Glaucoma - Basic and Clinical Aspects

A complex interaction between neural and glial cells exists during the differentiation and the life of the nervous system. As a matter of fact, neuroglia cells maintain the normal functions of the nervous system since they control the extra cellular environment, block the toxic agents and supply the trophic resources and, last but not least, provide a structural support to the neurons. In glaucoma, astrocytes play a very important role as far as the re-modeling of the *lamina cribrosa* is concerning. Actually, they may also have a role also in the onset of the disease. Studies conducted on human glaucoma have, in fact, evidenced that the disorganization at astrocyte level in the anterior areas of the optic nerve, is associated to hypertrophy and overexpression of the glial fibrillary acidic protein (GFAP) which also occurs in astrocyte cultures subjected to high hydrostatic pressure. Following ischemic episodes, traumas or neurodegenerative disorders, the phenotype of the astrocyte cells and microglia, activates the production of cytokines, ROS, nitric oxide and tumor necrosis factor α (TNF-α); all these molecules are mediators involved in the tissue damage [2]. In a similar way, glial cells located in the retina and in the head of the optic nerve may carry out their normal physiological role as supporters of the cell bodies and their relative axons of the ganglion cells; on the contrary they may have a noxious role towards the same structures in pathological conditions.

Apart from the elevated intraocular pressure, other risk factors such as genetic background, decreased corneal thickness, age and vascular dys-regulation may play an important role in the insurgence of glaucoma [32 - 39]. However, even if these factors may determine a risk to develop the disease, it remains difficult to establish a cause/effect relationship to develop this pathology: actually, one should consider that a high intraocular pressure is common among open-angle patients but many individuals showing this sign eventually will not develop glaucoma [40]. A further apparently paradoxical phenomenon is that a significant number of glaucoma patients progressively lose vision even though they react positively to drugs lowering the IOP [41 - 44]. In conclusion the cause of RGC in glaucoma still remains to be fully elucidated. Certainly the understanding of the apoptic death in RGC determined by the pathology is to be ascribed to the high complexity and the multifactorial character of the disease. The development of new neuroprotective therapies, even though will give a scant contribution to the elucidation of the molecular and cellular mechanisms underlying the disease, will certainly help to slow the development and progression of the pathology in

Experimental Glaucoma After Oxidative Stress and Modulation of the Consequent Apoptotic Events in a Rat Model

http://dx.doi.org/10.5772/54628

19

The association of ophthalmologic diseases to a mitochondrial etiology is assuming an increasingly interest: many authors consider, as a matter of fact, that the pathologies originate from impaired mitochondrial function, oxidative stress and enhanced apoptotic death. The mitochondrial role in the development of primary congenital glaucoma, characterized by trabecular dysgenesis, has been recently suggested. The formation of the trabecular meshwork during development is thought to have particular sensitivity to oxidative stress induced damage. Mitochondrial DNA (mtDNA) mutations, in particular, are emerging as causative agents of ophthalmologic disorders affecting mostly the optic nerve and the retina as well as the extra-ocular muscles. Also in these cases antioxidant therapy represents a good tool to treat these ophthalmologic conditions. Mitochondrial dysfunction is suggested, for example, to play an important role in age related macular degeneration, glaucoma and diabetes dependant retinopathy. Some biomarkers have been identified in the mitochondrial oxidative stress response: for instance, prohibitins also known as PHB may have diverse functions and are also involved in mitochondrial structure and functionality. These proteins present a ring-like structure with 16–20 alternating Phb1 and Phb2 subunits in the inner mitochondrial membrane [45]. The precise molecular function of the PHB molecular complex is not clear even though it has been hypothesized that they may have a role as chaperone for respiration chain proteins or as providers of a scaffold for the optimal mitochondrial morphology and function. Prohib‐ itins have been demonstrated to stimulate cell proliferation both in plants and mammals such as rodents. As far as tissue re-modeling is concerned, the proteins of the matrix metallopro‐ teinase (MMP) family could be a useful tool in gene therapy aimed at the protection/rescue of the RGCs. Therefore PHB and MMP could constitute an effective biomarker and/or a thera‐

glaucoma patients.

**1.3. Mitochondrial malfunctions and ophthalmogical diseases**

peutic target for ophthalmologic pathologies. (For a recent review see [46]).

#### **1.2. Oxidative stress and retinal ganglion cell death in glaucoma**

Oxidative stress is initiated by the imbalance between the production of ROS and their elimination by antioxidants. This phenomenon plays a key role in neuronal damage ending with neuron death which usually occurs by apoptosis. These reactive oxygen species are produced by mitochondria but can also derive from enzymatic degradation of neurotrans‐ mitters, neuroinflammatory mediators, and redox reactions [7]. Mitochondrial dysfunction can result in an increased level of ROS which is often found in neurodegenerative pathologies. Abnormal protein folding, defective ubiquitination and proteasome degradation systems may cause the production of ROS [8]. This promotes neuronal death *via* diverse molecular mecha‐ nisms including protein modification and DNA damage [9]. In any case, whether the oxidative stress triggers cell death is a component of a more complex neuro-degenerative process is yet to be elucidated [8]. Literature reports exist showing that neural damage occurs following oxidative stress in animal models of optic nerve injury and in human glaucoma. For example, DNA damage as well as protein and lipid peroxidation products, such as malonal-dihaldehyde accumulate in the trabecular meshwork and retina in animals with raised IOP [2, 10 - 16]. The high concentration of intra-cellular ROS has also been proposed as a crucial death signal after axonal injury, even though this may not directly cause a glaucoma, which would lead to RGC apoptosis [17 – 21]. Dysfunction of perfusion and reduced oxygen availability may play a role in the insurgence of an oxidative damage [22, 23]. The formation of ROS at mitochondrion level is required to activate a transcription factor known as hypoxia-inducible factor-1 alpha that induces the expression of several genes involved in the control of hypoxia, [24, 25]. Cells have a very effective protective antioxidant system including superoxide dismutase (SOD), catalase, glutathione peroxidase and glutathione reductase [26]; if this systems partially or totally fail in neutralizing the ROS in the RGCs population, the progression of glaucoma could be triggered. Evidence exists supporting this idea; as a matter of fact SOD activity is lower than normal in the trabecular meshwork of glaucoma patients [27, 28] and in the retina as monitored in experimental ocular model of hypertension [19]. A recent study *in vivo* showed a dramatic increase in RGCs after optic nerve axotomy which preceded apoptosis [19]. Reactive oxygen species alter the redox equilibrium in the cell and this produces cysteine sulfhydryl oxidation. As a consequence oxidative cross-linking leads to the formation of new disulfide bonds that result in conformational changes of the proteins and activation of apoptotic signals [29, 30]. To date many studies have shed light on the molecular events causing the death of RGC. These evidences were gathered from investigations on animal models where acute or chronic optic nerve damage was generated and in experimentally induced glaucoma. A number of cellular phenomena are involved in the apoptotic death of RGCs; just to mention some: deprivation of neurotrophic factor, loss of synaptic connectivity, oxidative stress, axonal transport failure (for an exhaustive review on this topics see [31]).

Apart from the elevated intraocular pressure, other risk factors such as genetic background, decreased corneal thickness, age and vascular dys-regulation may play an important role in the insurgence of glaucoma [32 - 39]. However, even if these factors may determine a risk to develop the disease, it remains difficult to establish a cause/effect relationship to develop this pathology: actually, one should consider that a high intraocular pressure is common among open-angle patients but many individuals showing this sign eventually will not develop glaucoma [40]. A further apparently paradoxical phenomenon is that a significant number of glaucoma patients progressively lose vision even though they react positively to drugs lowering the IOP [41 - 44]. In conclusion the cause of RGC in glaucoma still remains to be fully elucidated. Certainly the understanding of the apoptic death in RGC determined by the pathology is to be ascribed to the high complexity and the multifactorial character of the disease. The development of new neuroprotective therapies, even though will give a scant contribution to the elucidation of the molecular and cellular mechanisms underlying the disease, will certainly help to slow the development and progression of the pathology in glaucoma patients.

#### **1.3. Mitochondrial malfunctions and ophthalmogical diseases**

in the retina and in the head of the optic nerve may carry out their normal physiological role as supporters of the cell bodies and their relative axons of the ganglion cells; on the contrary

Oxidative stress is initiated by the imbalance between the production of ROS and their elimination by antioxidants. This phenomenon plays a key role in neuronal damage ending with neuron death which usually occurs by apoptosis. These reactive oxygen species are produced by mitochondria but can also derive from enzymatic degradation of neurotrans‐ mitters, neuroinflammatory mediators, and redox reactions [7]. Mitochondrial dysfunction can result in an increased level of ROS which is often found in neurodegenerative pathologies. Abnormal protein folding, defective ubiquitination and proteasome degradation systems may cause the production of ROS [8]. This promotes neuronal death *via* diverse molecular mecha‐ nisms including protein modification and DNA damage [9]. In any case, whether the oxidative stress triggers cell death is a component of a more complex neuro-degenerative process is yet to be elucidated [8]. Literature reports exist showing that neural damage occurs following oxidative stress in animal models of optic nerve injury and in human glaucoma. For example, DNA damage as well as protein and lipid peroxidation products, such as malonal-dihaldehyde accumulate in the trabecular meshwork and retina in animals with raised IOP [2, 10 - 16]. The high concentration of intra-cellular ROS has also been proposed as a crucial death signal after axonal injury, even though this may not directly cause a glaucoma, which would lead to RGC apoptosis [17 – 21]. Dysfunction of perfusion and reduced oxygen availability may play a role in the insurgence of an oxidative damage [22, 23]. The formation of ROS at mitochondrion level is required to activate a transcription factor known as hypoxia-inducible factor-1 alpha that induces the expression of several genes involved in the control of hypoxia, [24, 25]. Cells have a very effective protective antioxidant system including superoxide dismutase (SOD), catalase, glutathione peroxidase and glutathione reductase [26]; if this systems partially or totally fail in neutralizing the ROS in the RGCs population, the progression of glaucoma could be triggered. Evidence exists supporting this idea; as a matter of fact SOD activity is lower than normal in the trabecular meshwork of glaucoma patients [27, 28] and in the retina as monitored in experimental ocular model of hypertension [19]. A recent study *in vivo* showed a dramatic increase in RGCs after optic nerve axotomy which preceded apoptosis [19]. Reactive oxygen species alter the redox equilibrium in the cell and this produces cysteine sulfhydryl oxidation. As a consequence oxidative cross-linking leads to the formation of new disulfide bonds that result in conformational changes of the proteins and activation of apoptotic signals [29, 30]. To date many studies have shed light on the molecular events causing the death of RGC. These evidences were gathered from investigations on animal models where acute or chronic optic nerve damage was generated and in experimentally induced glaucoma. A number of cellular phenomena are involved in the apoptotic death of RGCs; just to mention some: deprivation of neurotrophic factor, loss of synaptic connectivity, oxidative stress, axonal transport failure (for

they may have a noxious role towards the same structures in pathological conditions.

**1.2. Oxidative stress and retinal ganglion cell death in glaucoma**

18 Glaucoma - Basic and Clinical Aspects

an exhaustive review on this topics see [31]).

The association of ophthalmologic diseases to a mitochondrial etiology is assuming an increasingly interest: many authors consider, as a matter of fact, that the pathologies originate from impaired mitochondrial function, oxidative stress and enhanced apoptotic death. The mitochondrial role in the development of primary congenital glaucoma, characterized by trabecular dysgenesis, has been recently suggested. The formation of the trabecular meshwork during development is thought to have particular sensitivity to oxidative stress induced damage. Mitochondrial DNA (mtDNA) mutations, in particular, are emerging as causative agents of ophthalmologic disorders affecting mostly the optic nerve and the retina as well as the extra-ocular muscles. Also in these cases antioxidant therapy represents a good tool to treat these ophthalmologic conditions. Mitochondrial dysfunction is suggested, for example, to play an important role in age related macular degeneration, glaucoma and diabetes dependant retinopathy. Some biomarkers have been identified in the mitochondrial oxidative stress response: for instance, prohibitins also known as PHB may have diverse functions and are also involved in mitochondrial structure and functionality. These proteins present a ring-like structure with 16–20 alternating Phb1 and Phb2 subunits in the inner mitochondrial membrane [45]. The precise molecular function of the PHB molecular complex is not clear even though it has been hypothesized that they may have a role as chaperone for respiration chain proteins or as providers of a scaffold for the optimal mitochondrial morphology and function. Prohib‐ itins have been demonstrated to stimulate cell proliferation both in plants and mammals such as rodents. As far as tissue re-modeling is concerned, the proteins of the matrix metallopro‐ teinase (MMP) family could be a useful tool in gene therapy aimed at the protection/rescue of the RGCs. Therefore PHB and MMP could constitute an effective biomarker and/or a thera‐ peutic target for ophthalmologic pathologies. (For a recent review see [46]).
