516

(Pineda et al., 2007) and Parkinson’s disease. In a mouse model of Parkinson’s disease, the engraftment of neural stem cells engineered to express GDNF was found to ameliorate the degeneration of dopaminergic neurons upon subsequent exposure to the toxin 6-hydroxydopamine (Akerud et al., 2001). Furthermore, this neuroprotection resulted in a significant alleviation of the behavioral impairment caused by the substantia nigra lesion. In addition to demonstrating neuroprotection, this study confirmed that the engineered stem cells provided a stable, local delivery system with neurotrophic factor production for up to 4 months after transplantation.

While these techniques are yet to be applied to RGC degeneration in glaucoma, it is clear that effects observed in diverse regions of the CNS are likely to be highly transferable to the retina. Furthermore, the retina is more accessible than other regions of the CNS, making it an ideal model for the development of these potential therapies.

Endogenous stem cells

It is now understood that endogenous pools of neural stem cells proliferate in response to brain injury, particularly in response to stroke (Felling and Levison, 2003; Tai and Svendsen, 2004). Furthermore, it has been demonstrated that this proliferation may be enhanced via the exogenous application of growth factors (Nakatomi et al., 2002; Tureyen et al., 2005; Ninomiya et al., 2006) and drugs (Zheng and Chen, 2007). Previous research has primarily focused on the role that enhanced neurogenesis plays in recovery following stroke injury. However, while proliferation of stem cells is upregulated following ischemic stroke injury, only a percentage of the newly generated cells differentiate into neurons and survive in the long term (Naylor et al., 2005). Given that transplantation of undifferentiated neural precursor cells can provide neuroprotection in various neuropathological conditions, it seems entirely conceivable that a similar role may be played by the proliferation of endogenous neural stem cells. However, to date, this concept has received no direct investigation and the contribution of

endogenous stem cell proliferation to neuroprotection in disease remains to be elucidated.

Key challenges

Given that many of the cell-based therapies we have discussed are reliant on transplantation, one of the key hurdles to the development of such clinical treatments is finding an acceptable source of stem cells, or their derivatives, for this purpose. This problem has a number of facets, perhaps the most well-publicized of which is the ethical acceptability of using stem cells derived from human embryonic or fetal tissue for clinical therapies — an emotive issue that is unlikely to be resolved anytime soon. Furthermore, we need to consider the safety of using such cells in patients. Most ES cells are cultured in media that contain essential products currently derived from animal sources. Contamination of the therapeutic product by these ingredients is a potential problem, posing a risk of pathogens crossing the species barrier. This may also be exacerbated in patients receiving transplants, as such people are likely to require chronic administration of immune suppression drugs in order to prevent graft rejection. Graft rejection is, of course, a problem in its own right, although the immune-privileged status of the CNS may alleviate this, at least in part. Another safety concern is the propensity of stem cells in general, and ES cells in particular, to generate tumors following transplantation. Finally, if we are to use modified stem cells as a local delivery system, the safety of the transfection system used to manipulate cells in vitro prior to transplantation will need to be verified.

Much of this chapter has considered experiments and findings from investigation within the CNS in general, rather than the eye specifically. This is because very little work examining stem cell-mediated neuroprotection in the retina, and more specifically glaucoma, has been published to date. If these methods are to be translated to a glaucoma therapy, many technical issues will need to be resolved. These include how to deliver stem cells to the retina so their neuroprotection will be most effective. Most grafts are delivered to the

brain via injection, which may also work very well for the retina given its accessibility. Within the eye, this method raises the question of whether to deliver the cells intravitreally, close to stressed RGCs in glaucoma, or subretinally, where they may be nourished by the choroidal blood supply. Cells may also be transferred to the host using an artificial scaffold, which has the advantage of protecting the cells during transfer from the in vitro to the in vivo environment. Alternatively, many investigators are now delivering stem cells systemically, via infusion into blood vessels, which may also work for the retina, provided that the cells can cross the blood-retinal barrier and migrate to the site of injury.

A unique consideration for cell transplantation in the eye is whether the introduction of new cells into the retina, or into the vitreous, will impact negatively on vision. Unlike slow-release, biodegradable delivery systems, transplanted cells may have the ability to survive for very long periods, perhaps indefinitely, in vivo and are unlikely to be easily removed once engrafted. Indeed, with a chronic progressive disease like glaucoma, a stem cell-based neuroprotective strategy would need to exhibit very long survival and function to ameliorate visual field loss over the life of the patient. Thus, another hurdle to the development of such therapies will be ensuring transplanted cells survive. In addition, the retina appears to be more resistant to the integration of transplanted cells compared to the brain. Therefore, further research is needed in order to discover how we may manipulate the retinal environment to encourage the integration of engrafted stem cells.

Conclusion

Stem cell transplantation may be a promising approach to human glaucoma treatment if barriers related to the control of differentiation, integration, and long-term survival of grafted cells can be overcome, and if safety and efficacy can be demonstrated. In the short term, however, glaucoma models provide a very useful system in which to explore the neuroprotective potential of cellular

517

transplantation where both degenerating and transplanted cells can be directly visualized.

Acknowledgments

The funding for our work in this field has been provided by the Gates Cambridge Trust, Fight for Sight, the Glaucoma Research Foundation, and the GSK Clinical Fellowship Program.

References

Abdouh, M. and Bernier, G. (2006) In vivo reactivation of a quiescent cell population located in the ocular ciliary body of adult mammals. Exp. Eye Res., 83(1): 153–164.

Akerud, P., Canals, J.M., Snyder, E.Y. and Arenas, E. (2001) Neuroprotection through delivery of glial cell line-derived neurotrophic factor by neural stem cells in a mouse model of Parkinson’s disease. J. Neurosci., 21(20): 8108–8118.

Arnhold, S., Absenger, Y., Klein, H., Addicks, K. and Schraermeyer, U. (2006) Transplantation of bone marrowderived mesenchymal stem cells rescue photoreceptor cells in the dystrophic retina of the rhodopsin knockout mouse. Graefes. Arch. Clin. Exp. Ophthalmol., 245(3): 414–422.

Asami, M., Sun, G., Yamaguchi, M. and Kosaka, M. (2007) Multipotent cells from mammalian iris pigment epithelium. Dev. Biol., 304(1): 433–446.

Cordeiro, M.F., Guo, L., Luong, V., Harding, G., Wang, W., Jones, H.E., Moss, S.E., Sillito, A.M. and Fitzke, F.W. (2004) Real-time imaging of single nerve cell apoptosis in retinal neurodegeneration. Proc. Natl. Acad. Sci. U.S.A., 101(36): 13352–13356.

Corti, S., Locatelli, F., Papadimitriou, D., Del Bo, R., Nizzardo, M., Nardini, M., Donadoni, C., Salani, S., Fortunato, F., Strazzer, S., Bresolin, N. and Comi, G.P. (2007) Neural stem cells LewisX+ CXCR4+ modify disease progression in an amyotrophic lateral sclerosis model. Brain, 130(Pt 5): 1289–1305.

518

Felling, R.J. and Levison, S.W. (2003) Enhanced neurogenesis following stroke. J. Neurosci. Res., 73(3): 277–283.

Gupta, N. and Yucel, Y.H. (2007) Glaucoma as a neurodegenerative disease. Curr. Opin. Ophthalmol., 18(2): 110–114.

Haas, S., Weidner, N. and Winkler, J. (2005) Adult stem cell therapy in stroke. Curr. Opin. Neurol., 18(1): 59–64.

Hitchcock, P., Ochocinska, M., Sieh, A. and Otteson, D. (2004) Persistent and injury-induced neurogenesis in the vertebrate retina. Prog. Retin. Eye Res., 23(2): 183–194.

Horita, Y., Honmou, O., Harada, K., Houkin, K., Hamada, H. and Kocsis, J.D. (2006) Intravenous administration of glial cell line-derived neurotrophic factor gene-modified human mesenchymal stem cells protects against injury in a cerebral ischemia model in the adult rat. J. Neurosci. Res., 84(7): 1495–1504.

Krabbe, C., Zimmer, J. and Meyer, M. (2005) Neural transdifferentiation of mesenchymal stem cells: a critical review. APMIS, 113(11–12): 831–844.

Kumar, D.M. and Agarwal, N. (2007) Oxidative stress in glaucoma: a burden of evidence. J. Glaucoma, 16(3): 334–343.

Lawrence, J.M., Sauve, Y., Keegan, D.J., Coffey, P.J., Hetherington, L., Girman, S., Whiteley, S.J., Kwan, A.S., Pheby, T. and Lund, R.D. (2000) Schwann cell grafting into the retina of the dystrophic RCS rat limits functional deterioration. Invest. Ophthalmol. Vis. Sci., 41(2): 518–528.

Lawrence, J.M., Singhal, S., Bhatia, B., Keegan, D.J., Reh, T.A., Luthert, P.J., Khaw, P.T. and Limb, G.A. (2007) MIOM1 cells and similar Mu¨ller glial cell lines derived from adult human retina exhibit neural stem cell characteristics. Stem Cells, 25(8): 2033–2043.

Lee, H.J., Kim, K.S., Park, I.H. and Kim, S.U. (2007) Human neural stem cells over-expressing VEGF provide neuroprotection, angiogenesis and functional recovery in mouse stroke model. PLoS ONE, 2(1): p. e156.

Liu, H., Honmou, O., Harada, K., Nakamura, K., Houkin, K., Hamada, H. and Kocsis, J.D. (2006) Neuroprotection by PlGF gene-modified human mesenchymal stem cells after cerebral ischaemia. Brain, 129(Pt 10): 2734–2745.

Lu, P., Jones, L.L., Snyder, E.Y. and Tuszynski, M.H. (2003) Neural stem cells constitutively secrete neurotrophic factors and promote extensive host axonal growth after spinal cord injury. Exp. Neurol., 181(2): 115–129.

Lucas, S.M., Rothwell, N.J. and Gibson, R.M. (2006) The role of inflammation in CNS injury and disease. Br. J. Pharmacol., 147(Suppl 1): S232–S240.

Maass, A., von Leithner, P.L., Luong, V., Guo, L., Salt, T.E., Fitzke, F.W. and Cordeiro, M.F. (2007) Assessment of rat and mouse RGC apoptosis imaging in vivo with different scanning laser ophthalmoscopes. Curr. Eye Res., 32(10): 851–861.

MacNeil, A., Pearson, R.A., MacLaren, R.E., Smith, A.J., Sowden, J.C. and Ali, R.R. (2007) Comparative analysis of progenitor cells isolated from the iris, pars plana, and ciliary body of the adult porcine eye. Stem. Cells, 25(10): 2430–2438.

Madhavan, L., Ourednik, V. and Ourednik, J. (2008) Neural stem/progenitor cells initiate the formation of cellular

networks that provide neuroprotection by growth factormodulated antioxidant expression. Stem Cells, 26(1): 254–265.

Maragakis, N.J., Rao, M.S., Llado, J., Wong, V., Xue, H., Pardo, A., Herring, J., Kerr, D., Coccia, C. and Rothstein, J.D. (2005) Glial restricted precursors protect against chronic glutamate neurotoxicity of motor neurons in vitro. Glia, 50(2): 145–159.

Martin, K.R., Quigley, H.A., Valenta, D., Kielczewski, J. and Pease, M.E. (2006) Optic nerve dynein motor protein distribution changes with intraocular pressure elevation in a rat model of glaucoma. Exp. Eye Res., 83(2): 255–262.

Martin, K.R., Quigley, H.A., Zack, D.J., Levkovitch-Verbin, H., Kielczewski, J., Valenta, D., Baumrind, L., Pease, M.E., Klein, R.L. and Hauswirth, W.W. (2003) Gene therapy with brain-derived neurotrophic factor as a protection: retinal ganglion cells in a rat glaucoma model. Invest. Ophthalmol. Vis. Sci., 44(10): 4357–4365.

Martino, G. and Pluchino, S. (2006) The therapeutic potential of neural stem cells. Nat. Rev. Neurosci., 7(5): 395–406.

McFarland, H.F. and Martin, R. (2007) Multiple sclerosis: a complicated picture of autoimmunity. Nat. Immunol., 8(9): 913–919.

Meyer, J.S., Katz, M.L., Maruniak, J.A. and Kirk, M.D. (2006) Embryonic stem cell-derived neural progenitors incorporate into degenerating retina and enhance survival of host photoreceptors. Stem Cells, 24(2): 274–283.

Nakatomi, H., Kuriu, T., Okabe, S., Yamamoto, S., Hatano, O., Kawahara, N., Tamura, A., Kirino, T. and Nakafuku, M. (2002) Regeneration of hippocampal pyramidal neurons after ischemic brain injury by recruitment of endogenous neural progenitors. Cell, 110(4): 429–441.

Naylor, M., Bowen, K.K., Sailor, K.A., Dempsey, R.J. and Vemuganti, R. (2005) Preconditioning-induced ischemic tolerance stimulates growth factor expression and neurogenesis in adult rat hippocampus. Neurochem. Int., 47(8): 565–572.

Nickells, R.W. (2007) From ocular hypertension to ganglion cell death: a theoretical sequence of events leading to glaucoma. Can. J. Ophthalmol., 42(2): 278–287.

Ninomiya, M., Yamashita, T., Araki, N., Okano, H. and Sawamoto, K. (2006) Enhanced neurogenesis in the ischemic striatum following EGF-induced expansion of transit-ampli- fying cells in the subventricular zone. Neurosci. Lett., 403 (1–2): 63–67.

Nomura, T., Honmou, O., Harada, K., Houkin, K., Hamada, H. and Kocsis, J.D. (2005) I.V. infusion of brain-derived neurotrophic factor gene-modified human mesenchymal stem cells protects against injury in a cerebral ischemia model in adult rat. Neuroscience, 136(1): 161–169.

Otteson, D.C. and Hitchcock, P.F. (2003) Stem cells in the teleost retina: persistent neurogenesis and injury-induced regeneration. Vision Res., 43(8): 927–936.

Passweg, J. and Tyndall, A. (2007) Autologous stem cell transplantation in autoimmune diseases. Semin. Hematol., 44(4): 278–285.

Pease, M.E., McKinnon, S.J., Quigley, H.A., Kerrigan-Baum- rind, L.A. and Zack, D.J. (2000) Obstructed axonal transport of BDNF and its receptor TrkB in experimental glaucoma. Invest. Ophthalmol. Vis. Sci., 41(3): 764–774.

Pineda, J.R., Rubio, N., Akerud, P., Urban, N., Badimon, L., Arenas, E., Alberch, J., Blanco, J. and Canals, J.M. (2007) Neuroprotection by GDNF-secreting stem cells in a Huntington’s disease model: optical neuroimage tracking of brain-grafted cells. Gene Ther., 14(2): 118–128.

Pluchino, S., Zanotti, L., Rossi, B., Brambilla, E., Ottoboni, L., Salani, G., Martinello, M., Cattalini, A., Bergami, A., Furlan, R., Comi, G., Constantin, G. and Martino, G. (2005) Neurosphere-derived multipotent precursors promote neuroprotection by an immunomodulatory mechanism. Nature, 436(7048): 266–271.

Quigley, H.A. and Broman, A.T. (2006) The number of people with glaucoma worldwide in 2010 and 2020. Br. J. Ophthalmol., 90(3): 262–267.

Schraermeyer, U., Kayatz, P., Thumann, G., Luther, T.T., Szurman, P., Kociok, N. and Bartz-Schmidt, K.U. (2000) Transplantation of iris pigment epithelium into the choroid slows down the degeneration of photoreceptors in the RCS rat. Graefes. Arch. Clin. Exp. Ophthalmol., 238(12): 979–984.

Schraermeyer, U., Thumann, G., Luther, T., Kociok, N., Armhold, S., Kruttwig, K., Andressen, C., Addicks, K. and Bartz-Schmidt, K.U. (2001) Subretinally transplanted embryonic stem cells rescue photoreceptor cells from degeneration in the RCS rats. Cell Transplant., 10(8): 673–680.

Tai, Y.T. and Svendsen, C.N. (2004) Stem cells as a potential treatment of neurological disorders. Curr. Opin. Pharmacol., 4(1): 98–104.

Takahashi, K., Tanabe, K., Ohnuki, M., Narita, M., Ichisaka, T., Tomoda, K. and Yamanaka, S. (2007) Induction of pluripotent stem cells from adult human fibroblasts by defined factors. Cell, 131(5): 861–872.

519

Tureyen, K., Vemuganti, R., Bowen, K.K., Sailor, K.A. and Dempsey, R.J. (2005) EGF and FGF-2 infusion increases post-ischemic neural progenitor cell proliferation in the adult rat brain. Neurosurgery, 57(6): 1254–1263.

Xu, H., Sta Iglesia, D.D., Kielczewski, J.L., Valenta, D.F., Pease, M.E., Zack, D.J. and Quigley, H.A. (2007) Characteristics of progenitor cells derived from adult ciliary body in mouse, rat, and human eyes. Invest. Ophthalmol. Vis. Sci., 48(4): 1674–1682.

Yasuhara, T., Matsukawa, N., Hara, K., Yu, G., Xu, L., Maki, M., Kim, S.U. and Borlongan, C.V. (2006) Transplantation of human neural stem cells exerts neuroprotection in a rat model of Parkinson’s disease. J. Neurosci., 26(48): 12497–12511.

Yu, J., Vodyanik, M.A., Smuga-Otto, K., Antosiewicz-Bourget, J., Frane, J.L., Tian, S., Nie, J., Jonsdottir, G.A., Ruotti, V., Stewart, R., Slukvin, I.I. and Thomson, J.A. (2007) Induced pluripotent stem cell lines derived from human somatic cells. Science, 318(5858): 1917–1920.

Yu, S., Tanabe, T., Dezawa, M., Ishikawa, H. and Yoshimura, N. (2006) Effects of bone marrow stromal cell injection in an experimental glaucoma model. Biochem. Biophys. Res. Commun., 344(4): 1071–1079.

Zhang, Y., Klassen, H.J., Tucker, B.A., Perez, M.T. and Young, M.J. (2007) CNS progenitor cells promote a permissive environment for neurite outgrowth via a matrix metalloproteinase-2-dependent mechanism. J. Neurosci., 27(17): 4499–4506.

Zheng, Z. and Chen, B. (2007) Effects of Pravastatin on neuroprotection and neurogenesis after cerebral ischemia in rats. Neurosci. Bull., 23(4): 189–197.

Zhu, W., Mao, Y., Zhao, Y., Zhou, L.F., Wang, Y., Zhu, J.H., Zhu, Y. and Yang, G.Y. (2005) Transplantation of vascular endothelial growth factor-transfected neural stem cells into the rat brain provides neuroprotection after transient focal cerebral ischemia. Neurosurgery, 57(2): 325–333.