Sphingolipids connected
to corneal disease

Published May 01 2017

Keratoconus is a corneal disease Keratoconus is a corneal disease. IMAGE COURTESY OF NAWAJES A. MANDAL

Keratoconus is a progressive eye disease that results in the thinning, bulging and scarring of the cornea, which is the clear layer at the front of the eye. The disease typically begins at puberty and stops around age 30, with significant vision impairment potentially occurring at later stages. Keratoconus affects about one in 2,000 people worldwide, and about 25 percent of patients undergo corneal transplantation to treat advanced symptoms. The underlying cause of keratoconus is unknown, but a recent report in the Journal of Lipid Research elucidates a potential role for fats known as sphingolipids and suggests a way to treat the disease.

Nawajes A. Mandal of the University of Tennessee Health Sciences Center, a senior author on the JLR report, says, “Keratoconus is a very challenging disease to work with because we do not know how or where the disease is initiated, how it progresses and how to stop it.” There is no animal model for the disease, so studies rely on cells from normal and keratoconus corneas.

Sphingolipids are specialized fat molecules important in cell-membrane structure, cellular processes and molecular signaling. They also are involved in disease mechanisms, such as tissue inflammation and fibrosis; they are known to interact with other profibrotic, proinflammatory molecules in the cell. Given that keratoconus is a fibrotic disease that involves dense protein deposition on the cornea, there is a significant possibility that sphingolipids are involved.

Mandal, his fellow senior author, Dimitrios Karamichos, and colleagues first sought to determine whether sphingolipid levels were altered in keratoconus corneal cells compared with normal corneal cells. They observed that two sphingolipids, ceramide and sphingosine-1-phosphate, were more abundant in the keratoconus cells, suggesting they may contribute to the disease.

Another class of molecules involved in corneal healing and keratoconus are the proteins known as transforming growth factors, with TGF-β being the main factor. Mandal and colleagues previously had investigated TGF-β in keratoconus but now wanted to determine if the growth factor was connected to sphingolipids. The investigators found that three types of TGF-β, known as 1, 2 and 3, differentially regulated sphingolipid levels in keratoconus and normal cells. Moreover, the investigators discovered that TGF-β treatment also changed the expression of sphingolipid-production genes in keratoconus and normal cells.

At this point in the research, several molecular players that change in keratoconus had been identified. The results, however, did not yet show that these molecules were potentially causative in keratoconus. The final set of experiments performed by Mandal, Karamichos and colleagues demonstrated just that.

The researchers treated normal cells with ceramide and sphingosine-1-phosphate. The treatment caused the cells to become more like keratoconus cells by expressing higher levels of profibrotic genes. Finally, when ceramide synthesis was blocked in keratoconus cells using a potent inhibitory drug, the cells had lower levels of profibrotic gene expression and became more like normal cells.

These results open the door to possible treatments for keratoconus based on the sphingolipid pathway. Mandal explains, “These findings are very exciting, as we are not only discovering a novel mechanism of human keratoconus development but also generating some clues about how keratoconus could be reversed by manipulating sphingolipids and thus leading to development of novel therapeutics.”

Stefan Lukianov Stefan Lukianov is a Ph.D. candidate at Harvard Medical School.