Defining Ocular Diseases

Basic Cellular and Molecular Determinants

Thyroid Eye Disease: A Role for TSHR Autoantigen

Thyroid eye disease (TED) is an autoimmune disorder associated with Graves’ disease (GD). This work by Crisp et al. (p. 3249) adds to the weight of evidence indicating that the autoantigen in GD, the thyrotropin receptor (TSHR), also plays a central role in TED. The TSHR is expressed at various stages of adipocyte differentiation, but is upregulated and is not restricted to or aberrant in TED orbits. The work highlights the importance of the type of autoimmune response to the TSHR in the development of TED. This issue, and also the effect of TSHR agonists in driving adipose hypertrophy, will be addressed in the future. 

Cone-Rod Dystrophy: A Synapse-Specific HRG4 Mutation

HRG4 is the first synaptic protein shown to be enriched specifically in the rod and cone photoreceptor ribbon synapses. A mutation in its Caenorhabditis elegans orthologue, UNC119, causes abnormal coordination and feeding, thought to be due to defective chemosensation. In this issue, Kobayashi et al. (p. 3268) report a heterozygous truncation mutation in HRG4 in a patient with late-onset cone-rod dystrophy. The authors have confirmed the pathogenicity of this mutation by constructing a transgenic mouse model expressing the same mutation. The mice develop a late-onset photoreceptor degeneration characterized by b-wave depression and prominent synaptic degeneration. This is the first example of a retinopathy-causing protein localized to the synaptic end of the photoreceptors. 

ADC: A New Crystallin Mutation

Crystallins are the major water-soluble proteins of the vertebrate ocular lens, and it is thought that lens refractivity and transparency are due to the unique spatial arrangements of these proteins within the lens fiber cells. Specific mutations in humans have been found in five different crystallin genes in families with autosomal dominant cataract (ADC), underscoring the functional importance of these proteins with regard to lens transparency. Bateman et al. (p. 3278) report the discovery of a second splice-site junction mutation in the βA1-crystallin gene in a Brazilian family with a history of ADCs. 

Corneal Opacity: Sequellae in Lumican-Null Mice

Mice deficient in lumican, a major corneal proteoglycan, manifest corneal opacification. In the current study, backscattered light was measured by in vivo confocal microscopy (CMTF) as an index of corneal transparency. Chakravarti et al. (p. 3365) found a significant increase in backscattered light in the posterior stroma of lumican-null mice. Electron microscopic examination of the cornea indicated a marked loss of collagen fibril organization and thicker, laterally-fused fibrils in the posterior stroma. The authors also show that lumican is normally present at high levels in the posterior stroma. It is suggested that an underlying cause for loss of transparency in the lumican-deficient mice may be the loss of regulated growth of collagen fibrils in the posterior stroma. CMTF also revealed a 40% reduction in stromal thickness of thelumican-null mice. Thus, lumican mutations may also contribute to cornea plana as has been shown recently for keraotocan, a lumican-like proteoglycan. 

Corneal Grafts: A Mediator of Rejection?

Although most corneal grafts are successful, approximately 10% will undergo immune rejection. It is widely believed that organ graft rejection is mediated by cytotoxic T lymphocytes (CTLs), which directly kill individual cells of the transplant. However, Hegde and Niederkorn (p. 3341) report that CD8⁺ and perforin knockout mice are incapable of generating CTLs, yet reject 100% of their corneal transplants. These results suggest that CD4⁺ T-cell-mediated delayed-type hypersensitivity is the mediator of corneal allograft rejection. Identifying the mechanisms of corneal graft rejection will pave the way for targeted therapeutic strategies to enhance the success of keratoplasties in high-risk patients. 

Glaucoma: A Müller Cell Etiology?

Glutamate and nitric oxide may be important excitotoxins causing retinal ganglion cell (RGC) death associated with glaucoma. To investigate whether Müller glial cells can protect RGCs from these excitotoxins, purified RGCs were cultured in the presence or absence of purified Müller cells. RGC death induced by glutamate was blocked by Müller cells which removed the amino acid through their glutamate transporters. When in direct contact with RGCs, Müller cells also provided protection from the toxic effects of NO. These findings by Kawasaki et al. (p. 3444) suggest that functional disorders of Müller glial cells might be one class of etiologies of glaucoma. 

RPE Immune Privilege: Monolayers and CD95

New concepts in treating ocular diseases such as age-related macular degeneration include transplantation of retinal pigment epithelium (RPE). Wenkel and Streilein (p. 3467) demonstrate that allografts of murine neonatal RPE as intact sheets not only survive long term when implanted intraocularly, but when transplanted to a nonprivileged site, such as the space beneath the kidney capsule. The authors further demonstrate that constitutive expression of CD95 ligand (Fas ligand) on the transplanted RPE confers upon the graft its resistance to immune rejection. These results indicate that RPE is inherently immune-privileged as a tissue, and suggest that the integrity of the RPE graft as a monolayer may be required for its privileged status. 

RPE Immune Privilege: Deviant APC Behavior?

Retinal pigment epithelium (RPE) is implicated in the maintenance of the immune privilege of the eye. In the present study, Willermain et al. (p. 3485) have investigated the potential of RPE cells to function as antigen-presenting cells. The authors found that besides expressing a high level of HLA-DR molecules, IFNγ-activated RPE cells also express the costimulatory molecules CD40. However, they never express B7.1 or B7.2, nor secrete IL-12, even under CD40 stimulation. Accordingly, RPE cells do not activate allogenic lymphocytes, but even inhibit T-cell proliferation, partlythrough induction of apoptosis. Altogether, the results suggest that RPE cells could thus be implicated more in a deviant antigen presentation, supporting their role in peripheral tolerance. 

PSC: Differentiation and ΔFosB

The mechanisms involved in the formation of posterior subcapsular cataract (PSC) have been difficult to investigate because of the limited number of good models for the disease. Now, Kelz et al. (p. 3523) report the development of PSC as a result of the expression of ΔFosB. The work clearly shows that the newly developing lens fibers arising after birth fail to overlap and form posterior sutures causing morphological disruption, loss of transparency, and eventual rupture of the lens. Of particular interest is the observation that fibers which differentiate abnormally on the posterior side appear normal on the anterior side forming a typical Y suture. WhileΔ FosB is expressed in the retina, there is no detected pathology until a much later time than is required for PSC development. The work suggests that anterior and posterior lens fiber differentiation may be controlled differently. 

AR Cataract: First Report of a Molecular Defect

At least 14 genetic loci have been previously linked to autosomal dominant congenital cataract, but none to recessive cataract. Pras et al. (p. 3511) performed linkage analysis between these 14 loci and 2 inbred families with autosomal recessive congenital cataract. In one of the families, three polymorphic chromosome 21 markers, close to CRYAA, segregated with the disease phenotype. Sequencing of the CRYAA gene in this family revealed a nonsense mutation (W9X). The other family did not show linkage to any of the 14 loci. This study constitutes the first description of a molecular defect underlying autosomal recessive cataract, and provides proof of their genetic heterogeneity. 

Cataract: Aging and TGFβ

Aging is strongly associated with cataract, the leading cause of blindness worldwide. Here, Hales et al. (p. 3544) present the results of a study examining the effects of aging on susceptibility to TGFβ-induced cataract in cultured rat lenses. Lenses from weanling, adult, and senile rats develop anterior subcapsular cataracts when exposed to TGFβ. Lenses from adult and senile animals were generally more adversely affected by TGFβ. The results suggest that an acute or chronic elevation of TGFβ activity, and/or an age-related increase in lens cell susceptibility to TGFβ, may be one of the triggering factors in the etiology of certain forms of cataract. 

Diabetic Retinopathy: Vascular Permeability Origin?

Increased retinal vascular permeability is widely recognized as a hallmark of diabetic retinopathy. Barber et al. (p. 3561) show by immunohistochemistry in whole retinas that diabetes reduces the content of the tight junction protein, occludin, and induces its redistribution within endothelial cells. These changes are accompanied by increased expression of glial fibrillary acidic protein in Müller cells and a concomitant decrease in the astrocytes, which can be reversed by treatment with insulin. The authors propose that diabetes increases vascular permeability by altering retinal glial cells, thus changing the way that they regulate vascular endothelial cell tight junctions. 

Diabetic Retinopathy: A Leptin Link?

The cytokine leptin regulates appetite, body mass, and insulin activity, is angiogenic, and promotes wound healing. These latter two properties led Gariano et al. (p. 3576) to hypothesize that leptin is involved in retinal diseases with fibrovascular proliferation. The authors found elevated leptin levels in the vitreous of patients with proliferative diabetic retinopathy, and detected leptin and its receptor in epiretinal fibrovascular tissue from eyes with diabetic retinopathy and retinal detachment. In addition to its metabolic actions in diabetes mellitus and weight regulation, leptin may participate in two key components of diabetic retinal disease: angiogenesis and fibrosis.  

Clinical Classifications and Methodologies

Corneal Dystrophy: Bowman’s vs. Epithelial Origin?

Reis-Bücklers’ and Thiel-Behnke corneal dystrophies are the two major forms of the Bowman’s layer dystrophies. The identification of mutations in the BIGH3 (or transforming growth factor, β-induced [TGFBI]) gene on chromosome 5q31 has facilitated a re-evaluation and clarification of classification. Ridgway et al. (p. 3286) have undertaken a detailed analysis of families with both Bowman’s layer dystrophies which suggests that there is a strong genotype:phenotype correlation. The histopathological and ultrastructural appearances that the authors report, together with clinical correlations, suggest that these are not disorders of Bowman’s layer, but are of epithelial origin. 

Glaucoma: A New “Decision Tree”

About 150 years after Donders coined the term glaucoma simplex in The Netherlands, a proposal from this same country for an international classification system for this disorder is given on page 3321. Wolfs et al. (p. 3309) used the 97.5th and 99.5th percentiles of disk measurements in a population to delineate probable and possible glaucomatous optic neuropathy. Combining this with glaucomatous visual field defects, the authors made a decision tree for classification of definite, probable, and possible primary open-angle glaucoma. This system was applied to the Rotterdam data in order to arrive at the prevalence of POAG. 

Visual Acuity: Does the AMA “Rule”?

Since 1955 governmental agencies, clinical studies, and ophthalmic handbooks have followed the recommendation of the American Medical Association¹ for characterizing binocular visual acuity as a weighted average of the monocular visual acuities. However, Rubin et al. (p. 3327) demonstrate that this algorithm is incorrect. Rather than a weighted average, binocular acuity is actually much closer to the acuity of the better-seeing eye. Furthermore, monocular acuity is as good a predictor of visual function in everyday activities as binocular acuity. These results suggest that current standards for the evaluation of visual disability should be revised. 

¹Spaeth EB, Fralick FB. Estimation of loss of visual efficiency. A. M. A. Archives of Ophthalmology. 1955;54:462–468. 

Strabismus Surgery: Is MMC Effective?

This study was designed to further examine the tissue effects of mitomycin C (MMC) in rabbits after extraocular muscle surgery, particularly with regard to the action of this compound on scarring. Minguini et al. (p. 3399) have presented a new way to quantify the inflammatory response of the muscle-tendon-scleral interface, which provides a new animal model for measuring that inflammation. The results did not support the hypothesis that MMC would be effective in reducing postoperative scarring after strabismus procedure in rabbits. The discussion highlights the importance of using a sensitive histological method for evaluating the tissue reaction. 

Strabismic Amblyopia: A New Hyperacuity Paradigm

Grating acuity is insensitive to infantile strabismic amblyopia. Since adults with strabismic amblyopia show more profound impairment of hyperacuity than grating acuity, an infant hyperacuity test may be more sensitive to amblyopia than grating acuity. Birch et al. (p. 3410) used a hyperacuity paradigm for detection of radial deformation of circular contours and found that infants strongly prefer to look at deformed circles over nondeformed circles. Radial deformation hyperacuity matured very rapidly between 4 and 6 months of age and continued to mature more slowly throughout infancy. This new hyperacuity paradigm may provide a better method than grating acuity for evaluating infantile strabismic amblyopia. 

Visual Field: How Best to Assess Loss?

Progressive visual field loss is notoriously difficult to identify due to between-examination variability, such as long-term fluctuation (LF). Hutchings et al. (p. 3429) describe a method for calculating LF for Humphrey Field Analyzer standard threshold fields and provide estimates of its magnitude in a clinically stable glaucoma population. A framework for using the LF values is proposed that identifies the necessity for repeat visual fields and improves the evaluation of progression. The methodology may also prove useful in evaluating perimetric performance of new clinical psychophysical techniques, such as short wavelength automated perimetry. 

Lenticular UV Damage: Photochemistry vs. Photobiology

Changes in the lens after ultraviolet radiation have been related to a photochemical law, the Bunsen-Roscoe law. According to this law the exposure time and the irradiance are reciprocal for the dose. If the dose is constant, so is the damage to the lens. Ayala et al. (p. 3539) demonstrated in rats that different exposure times would induce different damage in the lens at the same dose. These results suggest that a photochemical law cannot be directly applied for explaining this photobiological phenomenon. 

Retinal Hypoxia: Dissecting ERG Changes

Retinal hypoxia may play a role in several retinal diseases. It has complex effects on retinal electrical signals, with slow RPE-generated potentials appearing more sensitive to hypoxia than inner retinal signals, such as the b-wave. A missing link has been knowledge of the effect of hypoxia on the a-wave, which is generated by photoreceptors. This study by Kang Derwent and Linsenmeier (p. 3634) used bright flashes of light to generate ERGs containing a-waves from the cat retina during graded hypoxia. The a-wave proved to be extremely resistant to hypoxia, changing in amplitude by no more than 15% (average 9%) even at arterial Po ₂ of 20 to 30 mm Hg, whereas the b-wave was often reduced by 40% or more. There appear to be mechanisms, probably including increased glycolysis, that protect the photoreceptor during short-term hypoxia. The main failure of retinal function in hypoxia is at the photoreceptor synapse or later.