The cornea: stasis and dynamics

The physiological roles of the cornea are to conduct external light into the eye, focus it, together with the lens, onto the retina, and to provide rigidity to the entire eyeball. Good vision thus requires maintenance of the transparency and proper refractive shape of the cornea. Although the cornea appears to be a relatively static structure, dynamic processes operate within and around the cornea at the tissue, cell, and molecular level. In this article, I review the mechanisms responsible for maintenance of corneal homeostasis as well as the development of new modes of treatment for various corneal diseases. I. The static cornea: structure and physiological functions. The cornea is derived from ectoderm, so that it can be considered as transparent skin. It is devoid of blood vessels and manifests the highest sensitivity in the entire body. The surface of the cornea is covered by tear fluid, which serves both as a lubricant and as a conduit for regulatory molecules. The cornea is also supplied with oxygen and various nutrients by the aqueous humor and a loop vascular system in addition to tear fluid. The cornea interacts with its surrounding tissues directly as well as indirectly through tear fluid or aqueous humor, with such interactions playing an important role in the regulation of corneal structure and functions. The resident cells of the cornea-epithelial cells, fibroblasts (keratocytes), and endothelial cells--also engage in mutual interactions through network systems. These interactions as well as those with infiltrated cells and regulation by nerves contribute to the maintenance of the normal structure and functions of the cornea as well as to the repair of corneal injuries. II. The dynamic cornea: maintenance of structure and functions by network systems. Developments in laser and computer technology have allowed observation of the cells and collagen fibers within the cornea. Furthermore, progress in cell and molecular biology has allowed characterization of dynamic network systems-including cell-cell and cell-extracellular matrix interactions as well as cytokines and neural factors-that contribute to the maintenance of corneal transparency and shape. III. Disruption of network systems: persistent corneal epithelial defects and corneal ulcer. Selection of the appropriate treatment for pathologic lesions of the cornea and the accompanying decrease in visual acuity requires localization of the lesion with regard to the epithelium, stroma, or endothelium of the cornea. In certain instances, however, it is not possible to determine the cause of the problem within the cornea. In such cases, the cause of the pathologic lesion and the target for treatment may lie in the surrounding tissues or environment. For example, corneal epithelial wound healing may be delayed, leading to the development of persistent epithelial defects, as a result of disruption of intercellular junctions between epithelial cells, an abnormality of the corneal basement membrane, altered concentrations of various cytokines in tear fluid, a lowered corneal sensation, or allergic reactions in the lid conjunctiva. Loss of corneal epithelial barrier function can further allow inflammatory cytokines present in tear fluid, together with infiltrated cells, to activate keratocytes and elicit excessive degradation of collagen in the stroma, thereby giving rise to corneal ulcer. IV. Development of new drugs for corneal diseases. We have attempted to apply the results of basic scientific research to the development of new drugs for corneal diseases that remain difficult to treat. The process of authorization for new drugs from the Ministry of Health, Labor, and Welfare takes more than two decades, however. The path from the bench to clinical practice is thus a long one. 1. Development of eyedrops for treatment of persistent corneal epithelial defects. We demonstrated the clinical efficacy of fibronectin eyedrops for the treatment of persistent epithelial defects of the cornea. However, the possibility of blood-borne infections has interfered with the development of serum-derived fibronectin as a drug. An automated machine for the preparation of autologous fibronectin eyedrops has therefore recently been developed. Furthermore, in seeking an alternative to fibronectin eyedrops, we are investigating the effects of a peptide corresponding to the second cell-binding domain of fibronectin on corneal epithelial wound healing. Considering that urokinase-type plasminogen activator may be expressed at the site of corneal epithelial defects and facilitates epithelial migration, the potential clinical application of annexin V, which stimulates the secretion of urokinase-type plasminogen activator for the treatment of persistent corneal epithelial defects is also now under investigation in Japan. 2. Development of eyedrops for treatment of neurotrophic keratopathy. Substance P, a neurotransmitter, stimulates corneal epithelial migration in a synergistic manner with insulin-like growth factor (IGF)--1. We have shown that eyedrops containing both the substance P-derived peptide FGLM-amide and the IGF-1--derived peptide SSSR are effective for the treatment of persistent corneal epithelial defects in individuals with diabetic keratopathy or neurotrophic keratopathy, both of which are associated with a reduction in corneal sensation. 3. Development of drugs for corneal ulcer. Treatment of corneal infection with antibiotics does not necessarily halt the process of corneal ulceration, which is characterized by excessive degradation of stromal collagen, or resolve persistent corneal epithelial defects. In addition to eyedrops for the treatment of persistent corneal epithelial defects, we have therefore also been working on the development of new drugs for the treatment of corneal ulcer. To this end, we have established an experimental system in which corneal fibroblasts are cultured in a three-dimensional collagen gel. With this system, we have shown that triptolide and steroids inhibit collagen degradation by corneal fibroblasts. Triptolide or its derivatives are thus potential drugs for the treatment of corneal ulcer and would work by acting directly on corneal fibroblasts rather than by inhibiting the secreted enzymes(matrix metalloproteinases) responsible for collagen degradation.