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Ninth annual optic nerve rescue and restoration think tank: stem cells and glaucoma
July 26-27, 2002, Chicago, IL, USA
Robert Ritch
The theme of the Ninth Annual Optic Nerve Rescue and Restoration Think Tank, sponsored by The Glaucoma Foundation, New York, NY, was Stem Cells and Glaucoma, the first time an international conference has been devoted to this topic and an indicator of things to come. Stem cells (SC) derived from the central nervous system (CNS) are a new source for cells that may someday be used to repair the damaged brain, spinal cord, and retina. While SC offer great opportunities for CNS repair, many questions must be addressed before they can be used clinically. These include understanding (and control over) proliferation, migration, differentiation, and immunogenicity. At present we know very little about these key factors of SC behavior, but rapid progress is being made. The meeting fully substantiated my feeling that the future of SC in medicine is gargantuan and that the development of this field will create an as yet unconcieved revolution.
The organizers were Terete Borras, Len Levin, Julia Richards, and Robert Ritch.
J. Wayne Streilein opened the meeting with an overview. SC last for the life of the organism and can maintain their population while producing transit-amplifying daughter cells that can proceed down unique pathways of differentiation. Numerous problems remain to be overcome in harvesting and transplanting, inducing differentiation, developing function, and eliminating immune reactions.
Theodore Krupin presented an overview of the processes involved in the development of glaucomatous damage, both in the anterior segment, at the level of the trabecular meshwork, and the posterior segment, at the level of the retinal ganglion cells and the lamina cribrosa. He discussed lowering the intraocular pressure, improving ocular blood flow, and neuroprotection, as approaches or potential approaches to treatment at the present time.
Derek van der Kooy discussed retinal SC, which are found at the pigmented ciliary margin of the retina. These are easily isolated from eyebank eyes and differ from brain SC because they never form brain cells. Retinal SC grow easily and rapidly, and can produce all retinal cell types. Retinal SC may be one of the next cells to be used therapeutically, starting with photoreceptor disorders.
Scott Whittemore discussed SC in the spinal cord. SC from many sources are pluripotent and can be driven down specific lineages. In order to obtain large numbers of engrafted SC that differentiate in a desired way, we need strategies to channel cells into desired patterns.
Ryo Kubota discussed the use of gene expression analysis to determine genes involved in the maintenance of pluripotent and differentiated states.
Kwon Young presented preliminary results of adult rat hippocampal neuronal progenitor cell transplantation in a rat model of glaucoma. Some of the progenitor cells injected into the vitreous expressed MAP2, suggesting that they were starting to develop into a neuronal lineage.
Michael Young discussed SC in the mouse, and their application to human studies. Adult human neural progenitor cells grafted to diseased hosts can express mature neuronal markers, send processes to the appropriate plexiform layer, and extend neurites into the optic nerve. Local microenvironmental cues influence phenotypic differentiation of grafted cells.
Ting Xie showed how the Drosophila ovary provides an attractive model system for studying SC biology. In this system, adult SC have a limited lifespan. In order to maintain functional ovaries during the organism's lifetime, lost SC can be replaced by daughters of adjacent SC. Adherens junctions are essential for maintaining both germline and somatic SC. Cadherin-mediated cell adhesion is important for recruiting both SC types to their niches during early SC development.
Peter Hitchcock discussed the application of teleost SC to glaucoma. As a fish grows, so does its CNS, including the retina. Retinal growth is due in part to the continual generation of new neurons. Furthermore, unlike human nervous tissue, injuries to the fish retina are repaired by regenerative neurogenesis.
Iqbal Ahmad described several likely sources of neural progenitors with retinal potential that may make SC therapy for eye diseases possible. Embryonic SC-derived neural progenitors differentiate along photoreceptor lineages, and these would be the best source of retinal neurons.
Donald Sakaguchi investigated the influence of the age of the host on the fate of brain progenitor/stem cells after transplantation into developing and mature eyes of the Brazilian opossum. Transplanted cells often displayed morphologies characteristic of RGC, amacrine cells, bipolar cells, and horizontal cells. The greatest morphological integration and differentiation was observed in the youngest host eyes.
Jeffrey Rothstein discussed strategies for the SC treatment of neurodegenerative disease. A Sindbis virus model of motor neuron degeneration in rats results in permanent paralysis of the hind limbs. Glutamate kills motor neurons, mimicking the pathological cascade that occurs in ALS. Human SC produce rapid and profuse motor neuron growth when engrafted into the ventral horn, acquire immunohistochemical markers of mature neurons and astrocytes, and send axonal processes to the periphery.
Dong Feng Chen discussed the pathfinding of transplanted retinal SC. The restricted ability of neural implants to survive, migrate, and re-establish neuronal connections with the host environment has limited the success of neural transplantation. Retinas of double mutant adult knockout mice, deficient in both glial fibrillary acidic protein and vimentin, and consequently lacking intermediate filaments in reactive astrocytes, provide a permissive environment for grafted neurons to migrate and extend neurites.
Henry Edelhauser described the regions for storage (most peripheral), regeneration (paracentral), and migration in corneal endothelium. There is an area of corneal endothelial cells adjacent to Schwalbe's line that are transit-amplifying cells and slow cycling cells.
Ernst Tamm felt that the transplantation of trabecular meshwork (TM) SC to glaucomatous eyes might improve aqueous outflow. A niche for TM SC might exist under the operculum in monkeys (Schwalbe's line cells). Theoretically, TM SC might be isolated from adult TM SC or from embryonic SC.
Jack Antel noted that the lack of remyelination observed in multiple sclerosis could reflect a number of factors, including exhaustion of progenitor cells, lack of 'trophic' signals, injured axons being non-receptive to remyelination, and selective immune injury of the progenitors.
Ali Djalilian discussed immunological issues in limbal SC transplantation. Limbal SC can be transplanted by autografts in cases of unilateral disease, allografts from relatives or cadaver eyes, or cultured limbal SC. Ex-vivo expanded limbal transplantation using a 1-2 mm2 biopsy of conjunctiva can be grafted back to the donor eye.
Michal Schwartz presented the latest information on protective autoimmunity in glaucoma. Injury can induce a self-destructive process and also an immune response. These compete together. If self-destruction dominates, continuous degeneration results. If the immune response dominates but fails to stop, autoimmune disease results. If the immune response dominates and shuts off in time, protective autoimmunity results. Vaccination with self- or self-like antigens may be viewed as boosting the physiological mechanism of neuroprotection. Cop-1, which is FDA-approved in the treatment of multiple sclerosis, is a safe candidate as a vaccine for glaucoma.
Roger Beuerman presented the preliminary results of ex-vivo expanded conjunctival tissue equivalents for transplantation to the ocular surface from autologous conjunctival biopsies. These have been used in conjunctival replacement after pterygium surgery, and for the repair of a leaking, scarred filtering bleb.
The ensuing Think Tank following the presentations explored in depth the problems involved in achieving successful, functioning SC transplants, and the approaches that might be used in overcoming these difficulties. Reversal of visual damage from glaucoma will involve not only restoring retinal ganglion cells, but also achieving the proper connections and actual functional integration.
