Nancy C. Joyce

Dr. Nancy Joyce is a classic "late-bloomer". She graduated with a BA degree in Biology from Albertus Magnus College, New Haven, CT. Following graduation, she worked as a Research Assistant for the head of the Infectious Disease Unit in the Dept. of Internal Medicine and for the Chairman of the Dept. of Pathology at Yale University School of Medicine, and then for the Chief of the Dept. of Molecular Biology at the California Institute of Technology, Pasadena, CA. Each position brought with it a promotion until there was no chance for further advancement as a research technician. She was then accepted as a full-time graduate student in the Dept. of Cell Biology at Yale University School of Medicine.

Her thesis advisor was the 1974 Nobel Laureate, Dr. George E. Palade. For her thesis, Dr. Joyce studied the function of microvascular pericytes – smooth-muscle-like cells that are associated with the outer wall of the blood-carrying capillaries. Dr. Joyce received her Ph.D. in 1985 and then continued her work in Dr. Palade’s laboratory during a two-year post-doctoral fellowship. In 1987, Dr. Joyce came to Schepens Eye Research Institute as an Assistant Scientist and first worked as a co-Investigator on glaucoma with Dr. Arthur Neufeld. She is currently a Senior Scientist at Schepens and an Associate Professor in the Dept. of Ophthalmology at Harvard Medical School. Dr. Joyce’s laboratory is internationally recognized for its contributions to the field of corneal endothelial cell biology and is one of the very few, worldwide, that focuses on identifying the molecular mechanisms that regulate cell division in human corneal endothelial cells.

What is the Corneal Endothelium and What is Its Function?

The cornea is the clear, outermost layer of the eye, and is considered to be the “window” of the eye because it controls and focuses the entry of light into the eye.In fact, the cornea contributes 65-75% of the eye’s focusing power. The corneal endothelium (see the micrograph to the right of the diagram) is the single layer of cells located at the back of the cornea; it forms a barrier between the cornea and the fluid aqueous humor. If you were to look at the endothelium face-on, it would appear like a field of fried eggs, with each egg closely abutting its neighbor.

To understand the function of the endothelium, it is important to remember that the cornea does not contain blood vessels. In tissues with blood vessels, oxygen and nutrients in the blood are able to pass through the walls of the capillaries to “feed” the cells of the tissue. In the cornea, most of the oxygen comes from the air, passes into the tear film, and then into the cornea. The majority of the nutrients needed by the corneal cells are present in the aqueous humor. Normally, small amounts of aqueous humor slowly seep between the endothelial cells and enter the middle layer of corneal tissue (the stroma) where the nutrients become available to feed the cells. The endothelium contains proteins in its outer membrane that act like “pumps” which help draw fluid out of the cornea (thus balancing the flow of fluid into the cornea). Normally, the rate of fluid entering the cornea is equal to the rate of fluid “pumped” out of the cornea and this balance helps the cornea remain transparent, permitting light to pass through. If endothelial cell numbers are low, there will not be enough cells to form a barrier to fluid flow or to “pump” out the fluid. This results in fluid build-up (edema) in the cornea, causing swelling, disruption of the complex corneal structure, and permanent corneal cloudiness. Currently, the only way to restore normal vision is to remove the cloudy cornea and transplant a new cornea. There are about 40,000 corneal transplant surgeries performed annually in the U.S. and approximately 40% are due to corneal endothelial dysfunction. This is, by far, the most successful transplant surgery.

What Causes the Loss of Corneal Endothelial Cells and Can They Replace Themselves?

The corneal endothelium is a very fragile tissue. Cell numbers normally decrease with age, but there are additional causes of cell loss. The numbers can be reduced as the result of ophthalmic surgical trauma (such as cataract extraction and implantation of intraocular lenses) and of accidental trauma to the cornea, as well as the result of stress put on the cells by diseases (such as glaucoma and diabetes). Interestingly, corneal transplantation, by itself, can cause loss of endothelial cells, both immediately after surgery and over a period of several years following surgery.

Cells normally have a finite lifespan. In many tissues and organs, when cells die, other cells within the tissue divide to replace the ones that were lost. The fact that endothelial cell density (the number of cells/mm2) decreases with age strongly indicates that either these cells do not divide at all or do not divide enough to replace the lost cells. Instead of dividing as a means of cell replacement, the remaining endothelial cells simply enlarge and spread to fill the area where the dead cells were once located.  This method of “healing” the endothelium works well to a point; however, if too many cells are lost and the density falls below a certain threshold (from the average adult cell density of 2000 cells/mm2  down to 400-500 cells/mm2), there are too few cells to maintain the barrier between the cornea and aqueous humor and fluid freely enters the cornea, causing edema (swelling) and corneal blindness.

Can Corneal Endothelial Cells Divide?

The Joyce laboratory is one of the few, worldwide, that studies human corneal endothelial cells and their ability to divide (their proliferative capacity). Studies from this lab clearly indicate that endothelial cells retain the ability to divide, even though they do not normally proliferate in vivo (in the living eye). If a wound is made in the endothelium of a donor cornea (from a cadaver) and this cornea is placed in culture medium containing factors known to stimulate cell division, the cells at the wound edge will migrate into the wound bed and divide, eventually “healing” the wound.

Endothelial cells can also be gently removed from the cornea and placed in culture medium containing growth factor. These cells will divide, filling the culture dish. So, yes, corneal endothelial cells are intrinsically capable of dividing and proliferating.

Are There Potential New Methods to Treat or Prevent Endothelial Cell Loss?

Worldwide, there is a need to find new methods to prevent the loss of corneal endothelial cells and/or to increase endothelial cell density in patients who suffer from corneal blindness due to low endothelial cell counts. One way to increase endothelial cell density is to use molecular methods to induce a limited amount of cell division directly in patients. A method to prevent endothelial cell loss resulting from corneal transplantation is to induce division in cells in the donor cornea prior to transplantation. Another method is to culture endothelial cells on a biomembrane and then transplant this construct to the cornea of patients suffering from corneal endothelial dysfunction. In addition, it may be possible to culture stem-like cells so that they exhibit characteristics of normal corneal endothelium, thus providing a ready source of cells for transplant on a biodegradable membrane. All these potential methods require basic information regarding the proliferative capacity of corneal endothelial cells.  The overall goal of the Joyce laboratory is to develop methods to prevent or treat corneal blindness caused by low endothelial cell density by transiently stimulating proliferation in these cells. To reach this goal, we have been intensively studying the molecular mechanisms that regulate corneal endothelial cell division.

What Prevents Corneal Endothelial Cells from Dividing?

The Joyce lab has found several conditions that appear to work together to inhibit corneal endothelial cells from dividing in vivo. One is “contact inhibition”: When cells come into contact with one another, molecular signals resulting from this contact cause the synthesis of proteins that inhibit cells from completing the steps necessary for the cell to duplicate its DNA and form daughter cells. If cell-cell contacts are broken and the proper growth factors are present, the cells will divide. Another mechanism that inhibits endothelial cells from dividing is the presence of transforming growth factor-beta2 (TGF-_2). This is a protein present in the aqueous humor, a fluid that is in contact with these cells. TGF-_2 suppresses the entry of corneal endothelial cells into the S-phase of the cell cycle (the part of the cell-division cycle in which DNA is duplicated), thus inhibiting cell division.

Of the factors that contribute to inhibition of endothelial cell division, perhaps the most interesting and important is age. We have found that the ability of corneal endothelial cells to divide decreases with donor age. Fewer cells from older donors (>50 years old) will divide in response to growth factors compared with cells from young donors (<30 years old). Cells from older donors also take longer to enter and complete the division process. In addition, endothelial cells from older donors express more of the proteins known to inhibit cells from entering the cell cycle upon growth factor stimulation.

Why Be Concerned About Age-Related Differences in Proliferative Capacity?

There are two major reasons why it is important to understand the effect of age on the ability of corneal endothelial cells to divide. One is that the majority of donor corneas used for transplantation are from individuals greater than 50 years old. This means that most donor corneas contain endothelial cells with a decreased ability to divide. The other reason is that the majority of patients suffering from corneal endothelial dysfunction are older than 50. This means that we need to learn how to most effectively stimulate division of these cells in order to increase cell density in patients at risk for corneal blindness due to endothelial cell loss or dysfunction.

Is Decreased Ability to Divide Due to Increased Oxidative DNA Damage?

The characteristics exhibited by central corneal endothelial cells from older individuals are very similar to characteristics of cells that have entered senescence – a healthy life-stage in which cells are inhibited from dividing, to prevent tumor formation. In searching the literature for an explanation for the age-related decrease in corneal endothelial cell proliferative capacity, the Joyce lab found that oxidative DNA damage from high metabolic activity and/or from light can induce cellular senescence and decrease proliferative capacity. This type of damage is already considered a potential cause of dysfunction in the ocular lens and in retinal pigment epithelial cells. The “pump” activity of the corneal endothelium requires that the cells be highly metabolically active and may thus cause increased oxidative stress. In addition, since light passes through the cornea, including the endothelial cells, it is possible that light also contributes to an age-dependent accumulation of oxidative DNA damage. The lab recently used an antibody to detect oxidized DNA damage in endothelial cells of donor corneas. Less oxidative DNA damage was observed in cells from young donors. These results have led the Joyce lab to hypothesize that there is a gradual, age-related increase in oxidative DNA damage that is responsible for the decreased proliferative capacity exhibited by corneal endothelial cells from older donors. Recently, the lab developed a unique model to rapidly “age” young human corneal endothelial cells, and cause the type of DNA damage observed in the endothelium of older individuals.  From this model, they hope to further test their hypothesis and to develop antioxidant treatments to prevent oxidative DNA damage and thus retain the capacity of corneal endothelial cells from older individual to divide.