Enzymes and Biochemical Pathways in Ocular Tissues: Keys to Disease Causes and Treatments

Fas and AMD

Loss of retinal pigment epithelial (RPE) cells is a pivotal event in the development of age-related macular degeneration (AMD). Jiang et al. (p. 645) show that a functional Fas-mediated apoptotic pathway is present in cultured human RPE cells and can be activated with sFasL or by upregulation of FasL and Fas expression with an oxidant. Antioxidants block oxidant-induced FasL and Fas expression and associated apoptosis. The data demonstrate a novel mechanism for the death of RPE cells in AMD and suggest that a regulation of Fas-mediated apoptosis could be a new therapeutic strategy for this disease. 

MMPs and Pterygia

This study by Di Girolamo et al. (p. 671) identifies types of connective tissue degrading enzymes, matrix metalloproteinases (MMPs) and their inhibitors, in pterygia and in cultured pterygium epithelial cells. These proteases may be actively involved in the extensive tissue remodeling such as the dissolution of BowmanÆs layer, angiogenesis, cell migration, and tissue invasion associated with pterygia. UV may also be in the development of pterygia, as UV can upregulate the expression of proinflammatory cytokines, which in turn induce MMPs. Future therapeutical approaches may include MMP inhibitors. 

MAP Kinase and Wound Healing

The cornea is highly innervated, and impaired nerve function results in epithelial defects. Here, You et al. (p. 692) show that genes for several members of the neurotrophin family such as NGF, NT-3, NT-4, and BDNF as well as TGF-β family member GDNF, are expressed in human corneal epithelium and stroma. Also, receptors TrkA, TrkB, and TrkC are present. Both NGF and GDNF increased epithelial proliferation by inducing the MAP kinase pathway. The results suggest that several neurotrophins and GDNF are expressed in corneal cells to carry out regulatory functions and that these cytokines might be useful for treating impaired corneal wound healing. 

Myocilin/TIGR and OAG

Myocilin/TIGR has recently been associated with GLC1A-linked juvenile-onset primary open-angle glaucoma (POAG) and some forms of typical late-onset POAG. It has been speculated that altered myocilin/TIGR may obstruct aqueous humor outflow through the trabecular meshwork. Karali et al. (p. 729) show now that myocilin/TIGR is localized extra- or intracellularly in numerous ocular tissues, including cornea, sclera, vitreous humor, ciliary body, iris, and retina. A distinct finding is the presence of myocilin/TIGR in optic nerve axons and lamina cribrosa astrocytes, which indicates that the trabecular meshwork might not be the only target of abnormal myocilin/TIGR in GLC1A-linked open-angle glaucoma. 

TrkB and OAG

While apoptosis is now known to occur in glaucoma, the pathway that leads to cell death is not elucidated. It is established that retinal ganglion cells are partially dependent upon brain-derived neurotrophic factor (BDNF), that BDNF and its receptor (TrkB) are present in ganglion cells, and that the receptor-BDNF complex moves in axonal transport. Pease et al. (p. 764) found that experimental glaucoma, both acute and chronic, in rats and monkeys leads to the blockade of axonally transported TrkB receptors at the optic nerve head. In addition, radiolabeled BDNF injected into the superior colliculus during acute experimental glaucoma failed to reach ganglion cells in the eye. BDNF deprivation may represent an important step in glaucoma pathogenesis. 

Fas and EAU

To investigate the role of apoptosis in immunopathogenic mechanisms of experimental autoimmune uveoretinitis (EAU), apoptotic cells and expression of Fas and FasL were examined in rats immunized with S-antigen. The study by Sueda et al. (p. 799) demonstrated that: 1) apoptotic cells were present in the eye with EAU and their kinetics were in accord with disease activities; 2) Fas and FasL were expressed on infiltrating cells in the eye with EAU; 3) FasL was expressed on resident cells of the ciliary body; and 4) the kinetics of FasL expression corresponded with the kinetics of apoptotic cells. These findings indicate that Fas/FasL-mediated apoptosis takes place in the eye with EAU and plays a role in the immunopathogenic mechanisms to downregulate the disease. 

PI-3K and Lens Fibers

Phosphatidylinositol 3-kinase (PI-3K) is an important signal transduction enzyme that plays a role in a wide array of cellular functions such as proliferation, differentiation, and cell survival. In this study, Chandrasekher and Bazan (p. 844) describe the phosphorylation of PI-3K leading to its activation in response to insulin and IGF-1 stimulation in lens epithelial cells. The enzyme activity is also sustained in fully differentiated mature fiber cells. These findings open the possibilities that PI-3K could be a signal mediator in proliferation and differentiation processes promoted by various growth factors in lens epithelial cells and could have a function in the normal maintenance of fiber cells. 

PKC and Lens Development

The protein kinase C (PKC) family includes serine/threonine kinases which are critical for intracellular signal transduction. Berthoud et al. (p. 850) demonstrate that certain PKC isoenzymes may also have an important role in intercellular communication. The authors have shown that chicken lens cells contain several PKC isoenzymes with specific developmental patterns of expression. Treatment of lens cultures with a PKC-activating tumor promoter (12-O-tetradecanoylphorbol-13-acetate) decreased intercellular coupling and translocated PKC γ to the plasma membrane. These results suggest that hormone- or growth factor-induced activation of PKC may regulate the extent of intercellular communication in the developing lens. 

ET-1 and Hyperoxia

Hyperoxia is known to induce vasoconstriction in retinal vessels. This may be linked to the constant oxygen demand of the retina, and endothelin-1 (ET-1) may play a role. To test this idea, Dallinger et al. (p. 864) treated healthy subjects with an endothelin_A receptor antagonist and the blue field entopic technique used to determine the effect of hyperoxia on retinal blood flow. The results support the view that ET-1 participates in hyperoxia-induced vasoconstriction in the human retina. These findings may have implications for the treatment of eye diseases associated with abnormal retinal oxygenation. 

Transporters and Drug Delivery

Ueda et al. (p. 870) demonstrate in a kinetic study the possible existence of a transporter protein in the pigmented rabbit conjunctiva for absorbing organic cations such as epinephrine, dopamine, and choline in tears. This transporter protein belongs to the inside-negative cell membrane potential-dependent organic cation transporter family. With broad substrate specificity, this transporter protein may serve as a conduit for the entry of organic cation-type ophthalmic drugs to the uveal tract. Several antiglaucoma drugs, including brimonidine, dipivefrine, and carbachol, are potential substrates for the conjunctival organic cation transporter. 

NOS and Smooth Muscle

Nitric oxide (NO) is an important mediator of smooth muscle relaxation. Despite the demonstration of nitric oxide synthase (NOS) in ocular tissues, the role of endogenously produced NO in ocular physiology and pathology is still unclear. In this study, Pianka et al. (p. 880) have investigated the role of endogenously produced NO. Using specific NOS or guanylyl cyclase inhibitors and monitoring the muscle cGMP content, the authors demonstrate that endogenous NO causes an increase in tissue cGMP and thereby mediates approximately 70% of the relaxation of bovine iris sphincter. Further experiments in vivo are required to assess the importance of this pathway to normal physiology. 

TIMP-3 and Sorsby’s Dystrophy

Sorsby’s fundus dystrophy (SFD) and age-related macular degeneration (AMD)share several clinical and pathological features. After studying the pathology of eyes from a donor with SFD who had a Ser-181-Cys mutation in the TIMP-3 gene, Chong et al. (p. 898) believe that TIMP-3 accumulates in the Bruch’s membrane of SFD eyes. The authors postulate the mutant TIMP-3 is not defective in action, but has prolonged action because the mutant protein is more resistant to degradation. It is possible that normal TIMP-3 might also fail to degrade in AMD eyes by a mechanism other than mutation in the protein. 

cGMP-Gated Channels and RP

Cyclic nucleotide-gated ion channels, first characterized in rod photoreceptors, have also been identified in many other retinal cells where they play important roles in the processing of visual information. Mutations in the gene encoding the human rod cGMP-gated channel α subunit (rCNGα) have been implicated in one form of the progressive degenerative disease, retinitis pigmentosa (RP). Leconte and Barnstable (p. 917) generated transgenic mice in which the expression of rCNGα has been impaired using an antisense approach, which resulted in a slow and progressive degeneration of both photoreceptors and bipolar cells. These transgenic mice provide a new model for characterizing mechanisms of retinal degeneration and testing therapies designed to slow down their progression. 

c-fos and Apoptosis

The lack of c-fos has been shown to lead to a resistance to light-induced apoptosis of photoreceptors in mice. Kueng-Hitz et al. (p. 909) have compared the retina of c-fos ^+/+ and c-fos ^−/− mice in their ability to absorb and transduce light. The findings demonstrate an attenuated retinal function, a reduced number of rods, and a reduced rhodopsin content in c-fos ^−/− mice when comparedto c-fos ^+/+ mice. However, c-fos does not appear to be essential for the ability to absorb photons, nor for phototransduction or the function of second-order neurons. The study implies the necessity of focusing further investigations on the role of c-fos in the apoptotic cascade.