Chapter 7 The Trabecular Meshwork and Aqueous Humor Reabsorption

Rho/Rho-associated kinase pathway in glaucoma (Review)

The Rho/ROCK pathway plays important roles in the modulation of the cytoskeletal integrity of cells, the synthesis of extracellular matrix components in the aqueous humor outflow tissue and the permeability of Schlemm's canal endothelial cells. The activation of the Rho/ROCK pathway results in trabecular meshwork (TM) contraction, and the inhibition of this pathway would provoke relaxation of TM with subsequent increase in outflow facility and, thereby, decrease intraocular pressure (IOP). ROCK inhibitors also serve as potent anti‑scarring agents via inhibition of transdifferentiation of tenon fibroblasts into myofibroblasts. Furthermore, the RhoA/ROCK pathway is involved in optic nerve neuroprotection. Inactivation of Rho/ROCK signaling increase ocular blood flow, improve retinal ganglion cell (RGC) survival and promote RGC axon regeneration. Considering the IOP modulation, potent bleb anti-scarring effect and neuroprotective properties of ROCK inhibitors, the Rho/ROCK pathway is an attractive target for anti-glaucoma therapy, and it may be used for human therapy in the near future.

[Primary open angle glaucoma. Morphological bases for the understanding of the pathogenesis and effects of antiglaucomatic substances]

The pathogenesis of glaucomatic illnesses is poorly understood. An increase in ocular pressure can be caused by an increase in the secretion of aqueous humour or a reduction in its outflow. In the elderly, outflow is reduced while at the same time less aqueous humour is produced. This balance is easily disturbed, so that age represents a risk factor for glaucoma in addition to increased ocular pressure. Therapeutic possibilities involve, on the one hand, reducing the secretion of aqueous humour, for example using, beta blockers, carbonic anhydrase inhibitors and clonidine. On the other hand, aqueous humour outflow can also be influenced by drugs. Conventional outflow is increased by the administration of miotics. The uveoscleral outflow can be increased by prostaglandin derivates. Drugs which only influence trabecular outflow are not yet available. Future therapeutic possibilities involve new aspects of the pathophysiology, e.c. the use of growth factors, free radical scavenging enzymes and choroidal blood flow.

Effects of Endothelin-1 on Human Trabecular Meshwork Cell Contraction

Trabecular meshwork (TM) cells are now considered to play an active role in the aqueous outflow mechanism, since they exhibit smooth muscle-like contractile properties. Endothelin-1 (ET-1), a potent vasoconstrictor peptide, has been proposed to play a role in the local regulation of aqueous outflow and intra-ocular pressure control. We propose an in vitro culture model as a method in the study of ET-1-induced human trabecular meshwork (HTM) cell contractility. Experiments were performed on semi-confluent HTM cells (primary cultures established from normotensive human donor eyes) at the 2nd passage with PBS as control, ET-1, sarafotoxin 6c (a selective endothelin B receptor agonist) and Y-27632 (a selective inhibitor of rho-associated kinase). The contractile status of the cells was evaluated by a morphometric analysis of the cell area, assuming that HTM cells in culture are able to reduce their area as a consequence of cytoskeletal contraction rather than regulatory volume decrease. Our data indicate that image analysis of the HTM cell area can be a reliable method in the study of TM cell contractility, since it utilizes human cells and offers versatile opportunities for the pharmacological evaluation of drugs controlling HTM cell status.

Differential gene expression in the human ciliary epithelium

The generation of expression and subtractive libraries from the ocular ciliary body and cultured ciliary epithelial cells has been instrumental in the cloning, identification and characterization of many genes which, overall reflect a representative profile of transcripts expressed in ciliary nonpigmented, ciliary pigmented and ciliary muscle cells. The cell-specific expression of some of these genes (i.e. a neurotrophic factor, a gene associated with juvenile open glaucoma, and a visual component) reveal a degree of cell differentiation with a diversity of functions and properties higher than previously thought. The protection from light-induced oxidative reactions, free radicals and detoxification, may be partially attributed to the high level of expression in the ciliary epithelium of antioxidative enzymes (i.e., glutathione S-transferase, glutathione peroxidases, selenoprotein-P). The expression of genes encoding plasma proteins (i.e., complement component C4, alpha2-macroglobulin, apolipoprotein D) is in contrast with the view that plasma proteins in aqueous humor are synthesized outside the eye (i.e., liver). The identification of neuropeptide-processing enzymes (i.e., prohormone convertases, carboxypeptidase E, peptidyl-glycine-alpha-amidating monoxigenase), neuropeptides (i.e., secretogranin II, neurotensin) and regulatory peptides (i.e., atrial natriuretic peptide and angiotensinogen) with hypertensive and hypotensive activities provide the molecular basis to support the view that the ciliary epithelium is a neuroepithelium with neuroendocrine functions. We propose a working model to demonstrate that aqueous humor and intraocular pressure are under neuroendocrine control through regulatory peptides synthesized and released by the ciliary epithelium and targeting the peptide producing cells at the inflow system by an autocrine mechanism and/or cells at the outflow system (i.e., trabecular meshwork cells) by a paracrine mechanism. Finally, we hypothesize that these mechanisms could be entrained in the light-dark cycle following the circadian rhythm of aqueous humor and intraocular pressure.

Functional morphology of the trabecular meshwork in primate eyes

The trabecular meshwork forms most of the resistance to aqueous humor outflow needed for maintenance of a pressure gradient between intraocular pressure of approximately 17 mmHg and venous pressure of approximately 10 mmHg. The composition of the extracellular material in the subendothelial or cribriform layer seems to be mainly responsible for outflow resistance. The aqueous humor pathways through the subendothelial layer can be influenced by ciliary muscle contraction and presumably also by contractile elements recently found both in trabecular meshwork and scleral spur. Pharmacologically induced disconnection of inner wall and cribriform cells leads to wash out of extracellular material through breaks of the endothelial lining of Schlemm's canal and to increase of outflow facility. In glaucomatous eyes the resistance to aqueous humor outflow is increased due to an increase in different forms of extracellular material deposited within the cribriform layer. The amount of this newly developed extracellular material is correlated with loss of axons in the optic nerve, indicating that a common factor is responsible for both changes. To investigate the effect of various factors on the biology of trabecular cells monolayer cultures derived from cribriform and corneoscleral trabecular meshwork have been established. The two cell lines can be differentiated because cribriform cells in vivo as in vitro stain for alphabeta-crystallin whereas the corneoscleral cells remain unstained. The effect of TGFbeta, a growth factor increased in aqueous humor of glaucomatous eyes and glycocorticoids on trabecular meshwork cells show typical changes in formation of extracellular matrix components and of stress proteins. Dexamethasone and oxidative damage also lead to increase of trabecular meshwork inducible glucocorticoid response (TIGR) protein. A mutation of the TIGR-gene family has recently been found in families with juvenile and chronic simple glaucoma. Future research has to clarify the significance of these genetic factors for the pathophysiology of glaucoma and the role of trabecular cell activity in this respect.