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Work in our laboratory centers
around the retina and particularly the retinal pigment epithelium, a layer of
pigmented cells at the back of the eye behind the neural retina. These RPE cells
perform a variety of functions including inhibition of light scattering (as a
result of their pigmentation) and endocytosis and breakdown of disk membranes
that are shed by the photoreceptor cells on a diurnal basis. The RPE cells
(together with the endothelial cells of the retinal capillaries) form the
blood-retinal barrier and thereby control access of blood-borne molecules to the
neural retina. Thus, they participate in the nourishment and hormonal control of
neural retina cells. RPE cells also participate in retinol metabolism, that is
in the visual cycle. They generate 11-cis-retinaldehyde that is used in the
formation of rhodopsin in the photoreceptors. As a result of light absorption,
transretinaldehyde is formed by bleaching of rhodopsin. This is reduced to
all-trans-retinol which is then moved back to the RPE cells where 11-cis-retinol
is regenerated. RPE cells are critical to the functioning of the eye and are
involved in many eye diseases.
Proliferative vitreoretinopathy
(PVR) is a disease of older people and is becoming increasingly common as the
population ages. It occurs as a result of trauma to the retina in which there is
a breach of the blood-retinal barrier and a tear in the neural retina. Along
with age-related macular degeneration and diabetic retinopathy, PVR is one of
the commoner causes of visual loss in older people in western countries.
We are investigating the changes that occur in human RPE cells as they undergo
epithelial-mesenchymal transformation (EMT). In PVR, RPE cells come in contact
with the vitreous humor via a tear in the neural retina. Normally, the RPE cells
are separated from the vitreous by the neural retina. If this tear occurs at the
same time as some inflammatory response, such as occurs with a breach in the
blood-retinal barrier, the RPE cells change their behavior. Under normal
circumstances, RPE cells do not divide or move during an individual's lifetime
but when the neural retina tear and the inflammation occur as a result of eye
trauma, the cells detach from their underlying basement membrane and move
through the neural retina to the vitreous. Here they divide and undergo an EMT
so that they become fibroblast-like. These cells secrete a new extracellular
matrix which may be attached to the vitreous side of the retina. This is called
an epiretinal membrane (ERM). Fibroblastic cells often exhibit a "wound-healing
response" in which they pull on their extracellular matrix. When this happens in
an ERM attached to the neural retina, the retina can be pulled away from the
back of the eye. This retinal detachment can lead to blindness. Many of the
changes that occur in the EMT exhibited by RPE cells in PVR are very reminiscent
of a similar EMT that occurs when epithelial cells become transformed in cancer.
Again, the cells become fibroblast-like, proliferate, secrete a new
extracellular matrix and invade other tissues. We are investigating the changes
in RPE cells when they become transformed and how this process is controlled. To
determine changes in gene expression during transformation, we are using two
approaches:
i) A candidate gene approach. In this the expression of genes (and their
proteins) that are suspected to be involved in transformation (such as adhesion
proteins and growth factors) is investigated.
ii) An "ignorance-based" approach in which we do not make any assumptions of
what genes may be involved in transformation. To do this "gene arrays" are used
to measure the expression of 20,000 genes simultaneously. Genes that show
altered expression during the transformation of RPE cells are then identified
and their role in transformation determined.
Our current investigations utilize a simple in vitro model of PVR in
which cultured human RPE cells are exposed to vitreous humor in their culture
medium. Cells treated in this way undergo an EMT and alter gene expression in
many ways that would be expected of the transition from an epithelial to a
fibroblastic state.
We hope that these investigations, by showing how RPE cells change their
behavior in PVR, may lead to new therapeutic approaches to this disease. |
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Retinal pigment epithelial cells stained for
alpha5 integrin (green) and focal adhesion kinase (red)
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