Effects of Nitric Oxide on G-Protein Signaling

A recent multicenter clinical trial found that two types of glaucoma drugs, a2-adrenergics and ß-blockers, are almost equally effective in glaucoma treatment. Unfortunately, how these drugs work is not fully understood. Investigations in many laboratories suggested that both of these drugs reduce the activity of an enzyme, called adenylate cyclase, in the ciliary epithelial cells that produce aqueous humor in the front portion of the eye. Excessive production or decreased drainage of this fluid (aqueous humor) is thought to result in glaucoma. Adrenergic glaucoma drugs regulate adenylate cyclase activity through other proteins which serve as mediators. These mediator proteins are called G-proteins. There are many different G-proteins and they are grouped into 4 different classes. One class, called Gi, mediates effects of a2-adrenergic drugs and another class, called Gs, mediates the effects of ß-blockers. If these mediator proteins function normally, the drugs remain effective, but if their function is interfered with the drugs would not work. In recent years a new kind of chemical was discovered in the body. It is called Nitric Oxide and it is shown to be important in regulating blood pressure and in fighting bacterial infections. Some researchers have suggested a therapeutic value of nitric oxide in glaucoma. This is based on the observation that it reduces intraocular pressure. Others did not find a similar effect. Whether and how nitric oxide works therapeutically in glaucoma remains to be firmly established. Recent experiments in our laboratory showed that nitric oxide regulates modification of a Gi protein which mediates the effects of a2-adrenergic drugs. This modification, which is simply the attachment of a chemical called ADP-ribose, occurs in the body when infected by bacteria called Bordetella Pertussis (which cause whooping cough). Our findings suggest that nitric oxide promotes a similar modification. Bacterial modification makes the Gi-protein inactive. We therefore suspect that nitric oxide would similarly inactivate Gi in the ciliary body and thus, make the a2-adrenergic drugs ineffective in glaucoma treatment. We propose to test this hypothesis by isolating cells from the tissue that produces aqueous humor and studying the properties of their G-proteins. In particular, we will determine if other G-proteins are also modified, what consequences this modification has on the G-protein(s), and whether it interferes with the action of a2-adrenergic drugs on adenylate cyclase activity. Other drugs which work through Gi, as well as other proteins which are regulated by a2-adrenergic drugs through Gi, will also be investigated. The results from this investigation would be important for three reasons: first, they would help understand how nitric oxide works~ secondly, they would reveal if nitric oxide drugs would make a2-adrenergic drugs ineffective in glaucoma treatment~ and thirdly, by exploring the effects of a2-adrenergic drugs on effector proteins besides adenylate cyclase, they would help understand the mechanism of a2-adrenergic action in the treatment of glaucoma. The results could help in designing drugs specifically targeted at influencing certain proteins in the ciliary epithelial cells.