isual impairment and blindness present a major medical and public health problem. According to a recent report released by the Prevent Blindness America organization, the costs associated with adult vision problems are estimated at $51.4 billion a year in the U.S. More than 10 million people in this country are already blind or visually impaired, and 50,000 more will become blind each year. A leading cause of visual impairment is age-related retinal degeneration. As people today are generally healthier and live longer than before, and baby boomers are getting older (the U.S. Census Bureau estimates the number of persons aged 65 years and over will increase from 35 million in 2000 to 70 million in 2030), age-associated vision problems will inevitably increase their share of demands on the already burdened health system. Because of the current lack of effective treatment of degenerative retinal diseases, stem cell-based therapy provides a promising approach that may restore and sustain retinal function and prevent blindness, thus holding great hope for many people.
The retina is a very thin layer of light-sensitive neural tissue lining at the back of the eyeball. It is composed of six classes of neurons and one type of glial cell that are interconnected in a highly organized laminar structure (Fig. 1A). The rod and cone photoreceptor cells reside in the outer nuclear layer; the horizontal, bipolar, and amacrine interneurons plus the Müller glial cells in the inner nuclear layer; and the ganglion and displaced amacrine cells in the ganglion cell layer. The major function of the retina is to convert light signals detected by photoreceptors into electrical pulses, which are then sent to the brain through the optic nerve derived from projecting axons of the ganglion cells. Any loss and damage of the various retinal cell types would disrupt the normal transmission of nerve signals and lead to impaired vision. Age-related macular degeneration (AMD), a leading cause of vision loss in Americans over age 65, results from gradual degeneration of the central part of the retina called the macula, which allows us to see in fine detail. Thus AMD causes blurry central vision that may progress to form blind spots, which may affect any of the common daily activities, such as reading and driving, of independent living. Retinitis pigmentosa is a group of hereditary retinal disorders that cause the retina to deteriorate over time, producing gradual degeneration of the light-sensing rods and cones, resulting in vision loss and blindness. Glaucoma leads to vision loss by causing degeneration of ganglion cells, another retinal cell type. These cells are the sole output neurons in the retina and form the optic nerve essential for conveying light signals to the higher visual system. At present, some of the degenerative retinal disorders can be treated by medication or surgery with limited success. However, there is no cure yet.
Stem cells are characterized by self-renewal and the multipotentiality to make all of the cell types of the tissue of their origin. These properties make them better candidates for cell replacement treatment for a multitude of degenerative diseases including retinal degeneration. Efforts have been made in the past several years to derive mammalian retinal stem cells from embryonic stem (ES) cells and postnatal and adult eyes. When treated with proper growth factors and signaling molecules, both human and mouse ES cells can be induced to form retinal progenitor cells (Fig. 1D). These cells are able to differentiate into different cell types, including ganglion, amacrine, photoreceptor, bipolar and horizontal cells (Fig. 1E,F), and integrate into normal and degenerating mouse retinas, albeit at very low frequency. Despite the lack of regenerative capacity by the adult mammalian retina, retinal stem cells have also been isolated from the pigmented ciliary margin of postnatal and adult mouse and human eyes. These adult retinal stem cells display properties similar to those of ES-derived retinal progenitor cells.
A major obstacle to developing a stem cell-based therapy for degenerative retinal disorders is the poor integration and differentiation of retinal stem cells transplanted into recipient retinas. One area of our research focuses on identifying both intrinsic and extrinsic regulatory factors that play a role in determining and specifying various retinal cell types. These factors may someday be used to direct the differentiation of retinal stem cells toward desired cell types lost in recipient patients, hence achieving controlled retinal regeneration. Thus far, we have identified and characterized a number of transcription factors (e.g. Brn3b, Math5, Foxn4 and Ptf1a), which are DNA-binding proteins regulating RNA expression, involved in retinal development. Brn3b and Math5 are required for ganglion cell development – fate specification and differentiation depend on Brn3b while Math5 confers the progenitors with the competence of ganglion cell production. Genetic ablation of both genes in mice results in a great ganglion cell loss, whereas their overexpression in retinal progenitors promotes ganglion differentiation in mouse and chick models. Foxn4 is required by retinal progenitors to establish the competence state for the generation of amacrine and horizontal cells and Ptf1a for fate determination of these two interneuron types. Targeted inactivation of Foxn4 in mice causes the loss of most amacrine cells and the elimination of horizontal cells, while its overexpression strongly promotes an amacrine cell fate (Fig. 1B,C). This type of research on the basic mechanisms of retinal cell development is expected to identify candidate molecules useful for stimulating the differentiation and integration of transplanted retinal stem cells.
The ultimate outcome of many retinal degenerative diseases is the degeneration of all retinal cell types even though the degeneration may be initiated only from photoreceptor or ganglion cells. Therefore, depending on the progression of degeneration, it may well be necessary to consider regeneration of all neuron types in retinal cell replacement treatment. We will next apply our current knowledge about retinal developmental regulators to direct differentiation of inner retinal cell types from mouse stem cells. We plan to test the ability of various intrinsic and extrinsic factors, such as Brn3b and Foxn4, to induce the differentiation of ganglion, amacrine and horizontal cells from embryonic and adult retinal stem cells. These stem cells will be treated with signaling molecules or expressed with various transcription factors to facilitate the differentiation of stem cells toward a particular type such as ganglion cells. These “sensitized” retinal stem cells will then be transplanted into mouse models of retinal degeneration to investigate their potential for differentiation and integration. The knowledge gained from these studies promises to improve stem cell-based therapies for successful future treatment of degenerative retinal diseases such as glaucoma, thereby fulfilling the hope of millions of the visually impaired to see a bright and colorful world.
Mengqing Xiang is a professor at the Center for Advanced Biotechnology and Medicine and Department of Pediatrics, UMDNJ-Robert Wood Johnson Medical School. He earned his PhD degree from the University of Texas M.D. Anderson Cancer Center and conducted his postdoctoral studies at the Johns Hopkins University School of Medicine. His work on the molecular mechanisms of neural development and diseases has been funded by various federal, state and private agencies including the National Institutes of Health, March of Dimes Birth Defects Foundation, Alexandrine and Alexander L. Sinsheimer Fund, New Jersey Commission on Spinal Cord Research, and New Jersey Commission on Science and Technology.