Telomere Lengths in POAG Trabecular Meshwork Cells

To find a cure for primary open angle glaucoma it is essential to understand what causes the disease in the first place. The high eye pressure thought to cause blinding damage in glaucoma is caused by a decreased outflow of aqueous humor through the outflow filter. The clear fluid that inflates the eye is called the aqueous humor. It is constantly produced by tissues within the eye, so it must constantly also leave the eye, or pressure would rise dramatically. The outflow filter is called the trabecular meshwork. The trabecular meshwork is a sieve-like structure that circles just outside the edge of the iris, inside the eye. The aqueous humor is filtered by the trabecular meshwork as it passes out of the eye. The trabecular meshwork is coated with a lining of flat cells. These cells are called the trabecular meshwork endothelial cells, or TM cells for short. The TM cells can engulf small particles and digest them, but they also are very important in keeping the sieve-like structure of the meshwork open. By forming a protective layer around the strands of the sieve, they keep the structure from being dissolved by the aqueous humor. At the same time, the lining keeps the strands from sticking together, which is what partially dissolved strands without a lining would do. For these reasons TM cells are important for maintaining the outflow sieve open and flowing freely. In open angle glaucoma, it is known that there are more cells lacking from the meshwork strands than in normal adults. We all lose cells as we age, but in glaucoma the loss takes place earlier in life, or at a greater rate. Our grant proposal is geared toward finding our why the cells are lost from the strands. It is our belief that such a premature loss would predispose the strands to be degraded and to stick together, which would cause the sieve to compress and flow. to be impeded. Impeding flow of aqueous humor as it leaves the eye would cause the pressure to rise. Excessive loss of cells from the trabecular meshwork could be caused from greater than usual cell death, or by a failure to replace cells that are lost through normal aging. It is likely to be a combination of both factors, but in general cell division should be able to keep pace with normal loss. That is what happens in most areas of the body that experience physical wear and tear, such as the skin and the cells that line the intestine. Although this assumption will need to be tested in the future, it is my feeling that the explanation for fewer meshwork cells in glaucoma lies in the lack of replacement of cells that are lost, or replacement that is too slow. Although some investigators have stated that the TM might not have any cell division in adults, we have data to show that their data was not sufficient to support such a statement, and have many good reasons to think some cell division does go on in normal eyes. If there is cell division in normal TM, too little cell division in glaucoma TM is likely to play a role in the decreased cell number in glaucoma TM. For this reason our lab has been trying to study cell division in TM cells from patients with glaucoma. We have been growing TM cells in special fluids that give excellent results in growing TM cells from young eyes, and good results in older adults. But glaucoma TM cells do not grow very well. They often divide a few times, and then stop. We are trying to find out why. Cells that cannot divide in culture are often found to be lacking a specific part of their chromosomes. Chromosomes are the part of every cell that contain the genes. They are made up of DNA. The chromosomes of every cell in the body is the same, except for the very ends of each one. These ends, the telomeres, start out all the same. But each time a cell divides it loses a little bit of its telomere. Some cells have the ability to add DNA back to their telomeres, but most cells do not. In cells that do not make more telomere, the more they divide, the shorter the telomere becomes. Unfortunately, if a cell runs out of telomere, it can no longer divide. In this way the telomere acts as a marker of how many divisions a cell has left, and the body is assured that daughter cells are not missing any genes. The hypothesis that is being tested in our proposed study is that TM cells from patients with glaucoma stop dividing because they have run out of telomere. We will test the hypothesis by (1) measuring the telomere lengths in cultures of glaucoma and normal TM cells, (2) measuring the amount of the enzyme that can make new telomeres in cells, and (3) by adding back the enzyme that makes telomeres to cells that have stopped dividing, and seeing if that revives cell division. If glaucoma cells are lacking in telomeres, then we may be able to add back the telomere making enzyme (telomerase) to stimulate TM cells to grow again. This might or might not be sufficient to recover trabecular meshwork function, but it should be a step toward preventing gradual damage in patients that are identified early in life. With new genetic information available, earlier and earlier detection of glaucoma seems likely. On the other hand, if glaucoma TM cells are not short on telomeres, we will have learned something valuable about TM cell growth control, and new directions can be explored.