Nitric Oxide: A Potential Mediator of Retinal Ganglion Cell Damage in Glaucoma

In the glaucomatous optic nerve head, excessive nitric oxide (NO) may be responsible, at least in part, for degeneration of axons of retinal ganglion cells. We have demonstrated the apparent up-regulation and induction of certain isoforms of nitric oxide synthase (NOS), the enzyme that synthesizes NO, in astrocytes in the prelaminar and lamina cribrosa regions of optic nerve heads of patients with glaucoma. Evidence of NO toxicity, histochemical staining for nitrotyrosine, is present in these damaged optic nerve heads. In rats with experimentally induced, moderately elevated intraocular pressure, the isoforms of NOS were also identified.     

The interaction between intracranial pressure,intraocular pressure and lamina cribrosal compression in glaucoma

This review examines some of the biomechanical consequences associated with the opposing intraocular and intracranial forces. These forces compress the lamina cribrosa and are a potential source of glaucomatous pathology. A difference between them creates a displacement force on the lamina cribrosa. Increasing intraocular pressure and/or decreasing intracranial pressure will increase the trans‐lamina cribrosa pressure difference and the risk of its posterior displacement, canal expansion and the formation of pathological cupping. Both intraocular pressure and intracranial pressure can be elevated during a Valsalva manoeuvre with associated increases in both anterior and posterior lamina cribrosa loading as well as its compression. Any resulting thinning of or damage to the lamina cribrosa and/or retinal ganglion cell axons and/or astrocyte and glial cells attached to the matrix of the lamina cribrosa and/or reduction in blood flow to the lamina cribrosa may contribute to glaucomatous neuropathy. Thinning of the lamina cribrosa reduces its stiffness and increases the risk of its posterior displacement. Optic nerve head posterior displacement warrants medical or surgical lowering of intraocular pressure; however, compared to intraocular pressure, the trans‐lamina cribrosa pressure difference may be more important in pressure‐related pathology of the optic nerve head region. Similarly important could be increased compression loading of the lamina cribrosa. Reducing participation in activities which elevate intraocular and intracranial pressure will decrease lamina cribrosa compression exposure and may contribute to glaucoma management and may have prognostic significance for glaucoma suspects.     

Aging and ocular tissue stiffness in glaucoma

Glaucoma is a progressive and chronic neurodegenerative disorder characterized by damage to the inner layers of the retina and deformation of the optic nerve head. The degeneration of retinal ganglion cells and their axons results in an irreversible loss of vision and is correlated with increasing age. Extracellular matrix changes related to natural aging generate a stiffer extracellular environment throughout the body. Altered age-associated ocular tissue stiffening plays a major role in a significant number of ophthalmic pathologies. In glaucoma, both the trabecular meshwork and the optic nerve head undergo extensive extracellular matrix remodeling, characterized by fibrotic changes associated with cellular and molecular events (including myofibroblast activation) that drive further tissue fibrosis and stiffening. Here, we review the literature concerning the role of age-related ocular stiffening in the trabecular meshwork, lamina cribrosa, sclera, cornea, retina, and Bruch membrane/choroid and discuss their potential role in glaucoma progression. Because both trabecular meshwork and lamina cribrosa cells are mechanosensitive, we then describe molecular mechanisms underlying tissue stiffening and cell mechanotransduction and how these cellular activities can drive further fibrotic changes within ocular tissues. An improved understanding of the interplay between age-related tissue stiffening and biological responses in the trabecular meshwork and optic nerve head could potentially lead to novel therapeutic strategies for glaucoma treatment.     

Glaucoma and neuroinflammation: An overview

Glaucoma is an optic neuropathy characterized by well-defined optic disc morphological changes (i.e., cup enlargement, neuroretinal border thinning, and notching, papillary vessel modifications) consequent to retinal ganglion cell loss, axonal degeneration, and lamina cribrosa remodeling. These modifications tend to be progressive and are the main cause of functional damage in glaucoma. Despite the latest findings about the pathophysiology of the disease, the exact trigger mechanisms and the mechanism of degeneration of retinal ganglion cells and their axons have not been completely elucidated. Neuroinflammation may play a role in both the development and the progression of the disease as a result of its effects on retinal environment and retinal ganglion cells. We summarize the latest findings about neuroinflammation in glaucoma and examine the connection between risk factors, neuroinflammation, and retinal ganglion cell degeneration.     

Axon deviation in the human lamina cribrosa

AIMS—To examine the course taken by individual retinal ganglion cell axons through the human lamina cribrosa.
METHODS—Retinal ganglion cell axons were labelled using the retrograde tracer horseradish peroxidase applied directly to the optic nerve in two normal human eyes removed during the course of treatment for extraocular disease.
RESULTS—A majority of axons took a direct course through the lamina cribrosa but a significant minority, in the range 8-12%, deviated to pass between the cribrosal plates in both central and peripheral parts of the optic disc.
CONCLUSIONS—It is postulated that these axons would be selectively vulnerable to compression of the lamina cribrosa in diseases such as glaucoma in which the intraocular pressure is increased.

Keywords: retina; optic nerve; glaucoma; lamina cribrosa     

Chronic experimental glaucoma in primates. II. Effect of extended intraocular pressure elevation on optic nerve head and axonal transport

Intraocular pressure (IOP) elevations lasting from 2 to 42 days were produced in 13 primate eyes by anterior chamber injections of autologous, fixed red blood cells. The retina, optic nerve head, and optic nerves were studied by electron microscopy, and ganglion cell rapid axonal transport was examined after IOP elevations for various durations. Transport of axonal material was blocked at the scleral lamina cribrosa by IOP elevations to 50 mm Hg. With IOP elevation for less than 1 week, return to normal IOP restored normal transport in some axons. However, in other axons IOP elevation for less than 1 week intiated ganglion cell degeneration. The process of cellular death involved a rapid ascending degeneration from nerve head to brain, followed 3 to 4 weeks later by descending degeneration of the ganglion cell body and its attached axon. Axons of the superior and inferior optic nerve head and nerve seem to be damaged more extensively than those in the nasal and temporal optic nerve. Two to four days after IOP elevation, axons of the superficial optic nerve head were swollen with accumulating axonal material, leading to histologic disk edema. In those eyes with IOP elevation longer than 1 week, the loss of anterior disk nerve fibers combined with posterior and lateral movement of the lamina cribrosa lead to an increase in optic disk cupping. Astrocytes and capillaries of the optic nerve head seem to tolerate elevated IOP well and were relatively spared.     

Use of a mathematical model to estimate stress and strain during elevated pressure induced lamina cribrosa deformation.

BACKGROUND: High intraocular pressure (IOP), which is generally associated with glaucoma, causes lamina cribrosa retrodisplacement and deformation. Shear stress and strain resulting from lamina cribrosa deformation have been implicated in tissue remodeling, changes in retinal astrocyte function and retinal ganglion cell (RGC) death observed in vivo during glaucoma. METHODS: A mathematical model was developed to describe the lamina cribrosa exposed to elevated intraocular pressure (IOP). The model is based on the bending theory of plates, incorporates anatomical properties of the lamina cribrosa, and provides estimates of its biomechanical properties. The model relates IOP, the parameter normally correlated with glaucoma, and lamina cribrosa retrodisplacement to stress and strain experienced by cells, parameters that may be more closely associated with cell injury. RESULTS: We estimate that shear strains of 0.05 occur at the edge of a 200 microm thick lamina cribrosa at an IOP of 25 mm Hg. We estimate greater lamina cribrosa deformation and higher shear stress and strain for thinner lamina cribrosa and lamina cribrosa of larger radii. CONCLUSION: These results may provide better estimates of the stress and strain experienced by cells in the lamina cribrosa and may further our understanding of the forces that contribute to optic nerve degeneration during glaucoma.     

Cyclooxygenase-1 and Cyclooxygenase-2 in the Human Optic Nerve Head

To investigate the hypothesis that eicosanoids act as cellular mediators in the optic nerve head of normals and of patients with glaucoma, we have determined the presence of the two cyclooxygenase (COX) isoforms in human tissue. Histological sections of optic nerve heads were studied by immunohistochemistry. Age matched normal donors were compared with eyes from glaucoma patients with moderate to severe nerve damage. Polyclonal antibodies to human COX-1 and COX-2 were localized with immunoperoxidase staining. Specific antibodies for vascular endothelia and microglia were also co-localized. In normal and glaucomatous eyes, COX-1 was localized exclusively to the prelaminar and lamina cribrosa regions of the optic nerve head. No staining for COX-1 was observed in the nerve fiber layer or the myelinated optic nerve. COX-1 was associated with the astrocytes of the glial columns and the cribriform plates, but not with the endothelia lining the capillaries. In glaucoma, more astrocytes appeared to be stained with antibody to COX-1 than in normals and staining was intensely perinuclear. There was no staining for COX-2 in normal tissue. A few COX-2 positive cells were found in the prelaminar, lamina cribrosa and postlaminar regions of the glaucomatous optic nerves. Positive staining for COX-2 was not associated with microglia. COX-1 is constitutively present in astrocytes that are localized exclusively to the prelaminar and lamina cribrosa regions of the human optic nerve head. Eicosanoids, synthesized by COX-1 in this tissue, may have a homeostatic and a neuroprotective role related to the axons of the retinal ganglion cells. The sparse presence of COX-2 in glaucomatous tissue probably reflects the lack of inflammation associated with glaucomatous optic neuropathy.     

The pathogenic role of transforming growth factor-β2 in glaucomatous damage to the optic nerve head

In patients with primary open angle glaucoma (POAG), the optic nerve head (ONH) shows characteristic cupping correlated with visual field defects. The progressive optic neuropathy is characterized by irreversible loss of retinal ganglion cells (RGC). The critical risk factor for axonal damage at the ONH is an elevated intraocular pressure (IOP). The increase in IOP correlates with axonal loss in the ONH, which might be due to an impaired axoplasmatic flow leading to the loss of RGCs. Damage to the optic nerve is thought to occur in the lamina cribrosa (LC) region of the ONH, which is composed of characteristic sieve-like connective tissue cribriform plates through which RGC axons exit the eye. The cupping of the optic disc, and the compression and excavation of LC are characteristic signs of glaucomatous ONH remodelling. In ONH of POAG patients a disorganized distribution and deposition of elastic fibers and a typical pronounced thickening of the connective tissue septae surrounding the optic nerve fibers is found.Transforming growth factor (TGF)-β2 could be one of the pathogenic factors responsible for the structural alterations in POAG patients as the TGF-β2 levels in the ONH of glaucomatous eyes are elevated as well as in the aqueous homour. TGF-β2 leads to an increased synthesis of extracellular matrix (ECM) molecules mediated by connective tissue growth factor and to an impaired ECM degradation in cultured ONH astrocytes. Bone morphogenetic protein (BMP)-4 effectively antagonizes the effects of TGF-β2 on matrix deposition. The BMP antagonist gremlin blocks this inhibition, allowing TGF-β2 stimulation of ECM synthesis. Overall, the ECM in the ONH is kept in balance in the OHN by factors that augment or block the activity of TGF-β2.     

Biomechanics of the sclera and effects on intraocular pressure

Accumulating evidence indicates that glaucoma is a multifactorial neurodegenerative disease characterized by the loss of retinal ganglion cells (RGC), resulting in gradual and progressive permanent loss of vision. Reducing intraocular pressure (IOP) remains the only proven method for preventing and delaying the progression of glaucomatous visual impairment. However, the specific role of IOP in optic nerve injury remains controversial, and little is known about the biomechanical mechanism by which elevated IOP leads to the loss of RGC. Published studies suggest that the biomechanical properties of the sclera and scleral lamina cribrosa determine the biomechanical changes of optic nerve head, and play an important role in the pathologic process of loss of RGC and optic nerve damage. This review focuses on the current understanding of biomechanics of sclera in glaucoma and provides an overview of the possible interactions between the sclera and IOP. Treatments and interventions aimed at the sclera are also discussed.     

Expression of myocilin/TIGR in normal and glaucomatous primate optic nerves

Myocilin/TIGR was the first molecule discovered to be linked with primary open angle glaucoma (POAG), a blinding disease characterized by progressive loss of retinal ganglion cells. Mutations in myocilin/TIGR have been associated with age of disease onset and severity. The function of myocilin/TIGR and its role in glaucoma is unknown. Myocilin/TIGR has been studied in the trabecular meshwork to determine a role in regulation of intraocular pressure. The site of damage to the axons of the retinal ganglion cells is the optic nerve head (ONH). The myocilin/TIGR expression was examined in fetal through adult human optic nerve as well as in POAG. Myocilin/TIGR was expressed in the myelinated optic nerve of children and normal adults but not in the fetal optic nerve before myelination. Also examined was the expression in monkeys with experimental glaucoma. The results demonstrate that optic nerve head astrocytes constitutively express myocilin/TIGR in vivo in primates. Nevertheless, myocilin/TIGR is apparently reduced in glaucomatous ONH. The colocalization of myocilin/TIGR to the myelin suggests a role of myocilin/TIGR in the myelinated optic nerve.     

Ganglion cell death in glaucoma: pathology recapitulates ontogeny

I review some improvements in our knowledge about the death of retinal ganglion cells in glaucoma during the last 20 years. These include the realisation that glaucoma damage precedes its detection by perimetry, the fact that the lamina cribrosa is a major site of axonal injury to ganglion cells, and the association between regional structure of optic nerve head connective tissue and the pattern of glaucoma damage. The selective susceptibility of larger retinal ganglion cells and its functional significance are described.
Apoptosis is the mode of cell death in at least some ganglion cells in experimental glaucoma. This supports a theory that retrograde axonal transport failure leads to loss of trophic factor influence on ganglion cells, causing them to initiate their own suicide. As a consequence of this theory, two therapeutic avenues are suggested for prevention of glaucoma injury and cell death: delivery of trophic factors and manipulation of ganglion cell genetic expression of controlling influences over programmed cell death.     

A biomechanical paradigm for axonal insult within the optic nerve head in aging and glaucoma

This article is dedicated to Rosario Hernandez for her warm support of my own work and her genuine enthusiasm for the work of her colleagues throughout her career. I first met Rosario as a research fellow in Harry Quigley’s laboratory between 1991 and 1993. Along with Harry, John Morrison, Elaine Johnson, Abe Clark, Colm O’Brien and many others, Rosario’s work has provided lamina cribrosa astrocyte cellular mechanisms that are biomechanically plausible and in so doing provided credibility to early notions of the optic nerve head (ONH) as a biomechanical structure.We owe a large intellectual debt to Rosario for her dogged persistence in the characterization of the ONH astrocyte and lamina cribrosacyte in age and disease. Two questions run through her work and remain of central importance today. First, how do astrocytes respond to and alter the biomechanical environment of the ONH and the physiologic stresses created therein? Second, how do these physiologic demands on the astrocyte influence their ability to deliver the support to retinal ganglion cell axon transport and flow against the translaminar pressure gradient?The purpose of this article is to summarize what is known about the biomechanical determinants of retinal ganglion cell axon physiology within the ONH in the optic neuropathy of aging and Glaucoma. My goal is to provide a biomechanical framework for this discussion. This framework assumes that the ONH astrocytes and glia fundamentally support and influence both the lamina cribrosa extracellular matrix and retinal ganglion cell axon physiology. Rosario Hernandez was one of the first investigators to recognize the implications of this unique circumstance. Many of the ideas contained herein have been initially presented within or derived from her work (Hernandez, M.R., 2000. The optic nerve head in glaucoma: role of astrocytes in tissue remodeling. Prog Retin Eye Res. 19, 297–321.; Hernandez, M.R., Pena, J.D., 1997. The optic nerve head in glaucomatous optic neuropathy. Arch Ophthalmol. 115, 389–395.).     

Elevated intraocular pressure and optic nerve injury models in the rat

Models of experimentally elevated intraocular pressure in rats provide valuable opportunities to discover and study mechanisms of pressure-induced optic nerve damage. The structure and vasculature of the rat optic nerve head have several anatomic similarities and differences from the primate that allow useful comparisons and insights into human glaucoma. Specifically, the ultrastructural relationship between astrocytes, retinal ganglion cell axons and the connective tissues of the optic nerve head appear quite similar to the primate, and have a high potential for revealing cellular mechanisms of axonal injury. Three widely used models of creating elevated IOP in rats exist. However, they are not all equivalent and appear to differ in the relationship they exhibit between the level of pressure and extent of optic nerve damage. This indicates that these models may differ in the mechanisms by which they produce elevated eye pressure. All of these models are amenable to a variety of methods for evaluating damage. These include objective and subjective histologic assessment of the optic nerve, counting cells in the retinal ganglion cell layer of the retina and the use of retinal whole mounts to count retinal ganglion cells that have been back-labeled with dye applied to the superior colliculus. In the decade since their introduction, these versatile models have provided important insights into mechanisms of pressure-induced optic nerve damage using sensitive molecular biology techniques. They have also allowed the evaluation of several potential strategies for neuroprotection in glaucoma, ranging from currently available drugs to gene transfer studies.     

光相干断层扫描检测筛板结构的临床应用及新进展

筛板是位于视旁深部,由胶原纤维构成的筛束和穿行有神经、血管及神经胶质细胞的筛孔组成的复杂结构,为视网膜神经节细胞轴突穿出眼球时提供结构和营养支持。眼压引起机械应力的直接作用以及筛板变形和重塑导致轴浆运输、血运障碍,共同导致视网膜神经节细胞轴突损伤进而死亡。因此,筛板被认为是青光眼病变的始发部位。光相干断层扫描技术的发展...     

细胞外基质重塑在原发性开角型青光眼的作用及研究进展

青光眼是一组以特征性视神经萎缩和视野缺损为共同特征的疾病,病理性眼压增高是其主要危险因素。视网膜神经节细胞( retinal ganglion cells, RGCs)凋亡及其轴突丢失是青光眼的主要病理特征。细胞外基质( extracellular matrix, ECM)含量和成分的变化对小梁网构型、视乳头筛板结构、RGCs凋亡起着决定性作用。青光眼患者小梁网及房水中转化生长因子-β2( transforming growth factor-β2,TGF-β2)增加,引起ECM分泌增加和堆积导致眼压升高;高眼压引起视神经乳头ECM成分的改变,引起神经营养因子剥夺,导致RGCs凋亡;同时,高眼压引起视网膜基质金属蛋白酶类-9(matrix metalloproteinase-9,MMPs-9)活性增加,层连黏蛋白的减少又将导致 RGCs 凋亡的增加。因此,研究ECM和青光眼的关系至关重要,可能为原发性开角型青光眼发病机制及治疗提供新的方向。     

应用OCT探究眼压与筛板参数的关系

筛板的变形与血流减少一直被视作青光眼视神经轴突损伤的首发因素.病理性眼压升高与青光眼的发生发展间有紧密关系.通过OCT技术衡量筛板及周边参数随眼压变化来研究青光眼发病机制受到了广泛关注.研究表明筛板深度(LCD)、筛板前表面厚度(PTT)、筛板曲率指数(LCCI)以及视盘血管密度等参数均与眼压具有相关性.眼压升高可对筛...     

Role of cerebrospinal fluid pressure in the pathogenesis of glaucoma

The pathogenesis of normal (intraocular) pressure glaucoma has remained unclear so far. As hospital‐based studies showed an association of normal‐pressure glaucoma with low systemic blood pressure, particularly at night, and with vasospastic symptoms, it has been hypothesized that a vascular factor may play a primary role in the pathogenesis of normal‐pressure glaucoma. That assumption may, however, be contradicted by the morphology of the optic nerve head. Eyes with normal‐pressure glaucoma and glaucomatous eyes with high‐intraocular pressure can show a strikingly similar appearance of the optic nerve head, including a loss of neuroretinal rim, a deepening of the optic cup, and an enlargement of parapapillary atrophy. These features, however, are not found in any (other) vascular optic neuropathy, with the exception of an enlargement and deepening of the optic cup in arteritic anterior ischaemic optic neuropathy. One may additionally take into account (i) that it is the trans‐lamina cribrosa pressure difference (and not the trans‐corneal pressure difference, i.e. the so‐called intraocular pressure) which is of importance for the physiology and pathophysiology of the optic nerve head; (ii) that studies have shown that the anatomy of the optic nerve head including the intraocular pressure, the anatomy and biomechanics of the lamina cribrosa and peripapillary sclera, retrobulbar orbital cerebrospinal fluid pressure and the retrobulbar optic nerve tissue pressure may be of importance for the pathogenesis of the highly myopic type of chronic open‐angle glaucoma; (iii) that studies have suggested a physiological association between the pressure in all three fluid filled compartments, i.e. the systemic arterial blood pressure, the cerebrospinal fluid pressure and the intraocular pressure; (iv) that an experimental investigation suggested that a low cerebrospinal fluid pressure may play a role in the pathogenesis of normal (intraocular) pressure glaucoma; and (v) that recent clinical studies reported that patients with normal (intraocular) pressure glaucoma had significantly lower cerebrospinal fluid pressure and a higher trans‐lamina cribrosa pressure difference when compared to normal subjects. One may, therefore, postulate that a low cerebrospinal fluid pressure may be associated with normal (intraocular) pressure glaucoma. A low systemic blood pressure, particularly at night, could physiologically be associated with a low cerebrospinal fluid pressure, which leads to an abnormally high trans‐lamina cribrosa pressure difference and as such to a similar situation as if the cerebrospinal fluid pressure is normal and the intraocular pressure is elevated. This model could explain why patients with normal (intraocular) pressure glaucoma tend to have a low systemic blood pressure, and why eyes with normal (intraocular) pressure glaucoma and eyes with high‐pressure glaucoma, in contrast to eyes with a direct vascular optic neuropathy, show profound similarities in the appearance of the optic nerve head.     

Induction of HLA-DR expression in human lamina cribrosa astrocytes by cytokines and simulated ischemia

PURPOSE: Recent evidence strongly suggests that activated immunity occurs during the neurodegenerative process of glaucomatous optic neuropathy. Although activation of lamina cribrosa astrocytes has been identified in glaucomatous optic nerve head, their role on the activated immune responses seen in glaucoma patients is unknown. Here, the authors aimed to study the potential role of lamina cribrosa astrocytes as a component of activated immune responses seen in glaucoma patients. METHODS: Expression of HLA-DR in optic nerve head astrocytes was studied using immunohistochemistry in postmortem eyes of patients with glaucoma and normal donors. Serum cytokine levels of patients with glaucoma and control subjects were measured using enzyme-linked, immunosorbent assay. In addition, in vitro experiments were performed using astrocyte cultures derived from human optic nerve head or fetal human brain. The cultured astrocytes were incubated under selected stress conditions such as exposure to cytokines, IFN-gamma and IL-10, or simulated ischemia for up to 48 hours. The expression of HLA-DR was studied in these cells using flow cytometry and immunocytochemistry. RESULTS: Immunohistochemistry demonstrated an upregulation of the HLA-DR expression in the optic nerve head astrocytes in glaucoma. In addition, serum levels of IL-10 was higher in the patients with normal pressure glaucoma compared to age-matched control subjects (P: = 0.001). Regarding in vitro experiments, unlike brain astrocytes, the percentage of cells expressing HLA-DR was approximately 3 times higher in the cultures of optic nerve head astrocytes exposed to simulated ischemia compared to cultures incubated under normal conditions (P: = 0.09). Incubation with IFN-gamma induced HLA-DR expression in brain and lamina cribrosa astrocytes, up to 25-fold, (P < 0.001) either in the absence or presence of simulated ischemia. Induction of HLA-DR expression by IL-10 was approximately 6 times higher in lamina cribrosa astrocytes incubated under simulated ischemia compared to that incubated under normal condition (P: = 0.004) and was not prominent in brain astrocytes. CONCLUSIONS: These findings suggest that optic nerve head astrocytes function as antigen-presenting cells and that their immunogenic capacity is more sensitive to ischemia than brain astrocytes. Taken together, these findings provide novel evidence that regulation of immunogenic capacity of optic nerve head astrocytes by cytokines or ischemic stress may have a role during the neurodegeneration process in patients with glaucoma.     

Glaucoma as primary optic neuropathy

The appearance of the new concept of glaucomatous optic neuropathy directed to idea that high intraocular pressure and vascular insufficiency in the optic nerve head are only risk factors in the appearance of glaucoma. Glaucomatous optic neuropathy is the consequence of progressive loss of the retinal ganglion cells whose axons comprise the optic nerve. A great number of neurotrophic agents are under investigation with the view of preventing the dead of retinal ganglion cells. Induction of cell rescue mechanisms may be an alternate and efficient strategy for neuroprotection. The paper presents both actual data about the pathophysiological mechanisms of glaucoma (neurotrophin deprivation, excitotoxicity caused by glutamate, "ischemic cascade", apoptosis of retinal ganglion cells) and neuroprotective and neuroregenerative therapy and antiapoptotic agents.