Ординатура / Офтальмология / Английские материалы / Mechanisms of the Glaucomas_Shields, Tombran-Tink, Barnstable_2008

REFERENCES

1.Hernandez, M. R. & Pena, J. D. (1997). The optic nerve head in glaucomatous optic neuropathy. Arch Ophthalmol 115, 389–95.

2.Susanna, R., Jr. (1983). The lamina cribrosa and visual field defects in open-angle glaucoma.

Can J Ophthalmol 18, 124–6.

3.Quigley, H. A. & Addicks, E. M. (1981). Regional differences in the structure of the lamina cribrosa and their relation to glaucomatous optic nerve damage. Arch Ophthalmol 99, 137–43.

4.Emery, J. M., Landis, D., Paton, D., Boniuk, M. & Craig, J. M. (1974). The lamina cribrosa in normal and glaucomatous human eyes. Trans Am Acad Ophthalmol Otolaryngol 78, OP290–7.

5.Airaksinen, P. J., Mustonen, E. & Alanko, H. I. (1981). Optic disc haemorrhages precede retinal nerve fibre layer defects in ocular hypertension. Acta Ophthalmol (Copenh) 59, 627–41.

6.Drance, S. M., Fairclough, M., Butler, D. M. & Kottler, M. S. (1977). The importance of disc hemorrhage in the prognosis of chronic open angle glaucoma. Arch Ophthalmol 95, 226–8.

7.Anderson, D. R. (1983). What happens to the optic disc and retina in glaucoma? Ophthalmology 90, 766–70.

8.Levin, L. A. (2001). Relevance of the site of injury of glaucoma to neuroprotective strategies. Surv Ophthalmol 45, S243–9.

9.Boden, C., Sample, P. A., Boehm, A. G., Vasile, C., Akinepalli, R. & Weinreb, R. N. (2002). The structure-function relationship in eyes with glaucomatous visual field loss that crosses the horizontal meridian. Arch Ophthalmol 120, 907–12.

10.Quigley, H. A. & Addicks, E. M. (1980). Chronic experimental glaucoma in primates. II. Effect of extended intraocular pressure elevation on optic nerve head and axonal transport.

Invest Ophthalmol Vis Sci 19, 137–52.

11.Quigley, H. A., Addicks, E. M., Green, W. R. & Maumenee, A. E. (1981). Optic nerve damage in human glaucoma. II. The site of injury and susceptibility to damage. Arch Ophthalmol 99, 635–49.

12.Cragg, B. G. (1970). What is the signal for chromatolysis? Brain Res 23, 1–21.

13.Thanos, S., Pavlidis, C., Mey, J. & Thiel, H. J. (1992). Specific transcellular staining of microglia in the adult rat after traumatic degeneration of carbocyanine-filled retinal ganglion cells. Exp Eye Res 55, 101–17.

14.Watson, W. E. (1974). Cellular responses to axotomy and to related procedures. Br Med Bull 30, 112–5.

15.Grafstein, B. & Ingoglia, N. A. (1982). Intracranial transection of the optic nerve in adult mice: Preliminary observations. Exp Neurol 76, 318–30.

16.Schnitzer, J. & Scherer, J. (1990). Microglial cell responses in the rabbit retina following transection of the optic nerve. J Comp Neurol 302, 779–91.

17.Mansour-Robaey, S., Clarke, D. B., Wang, Y. C., Bray, G. M. & Aguayo, A. J. (1994). Effects of ocular injury and administration of brain-derived neurotrophic factor on survival and regrowth of axotomized retinal ganglion cells. Proc Natl Acad Sci USA 91, 1632–6.

18.Cui, Q. & Harvey, A. R. (1995). At least two mechanisms are involved in the death of retinal ganglion cells following target ablation in neonatal rats. J Neurosci 15, 8143–55.

19.Yoles, E., Muller, S. & Schwartz, M. (1997). NMDA-receptor antagonist protects neurons from secondary degeneration after partial optic nerve crush. J Neurotrauma 14, 665–75.

20.Stys, P. K., Ransom, B. R., Waxman, S. G. & Davis, P. K. (1990). Role of extracellular calcium in anoxic injury of mammalian central white matter. Proc Natl Acad Sci USA 87, 4212–6.

21.Kiryu-Seo, S., Sasaki, M., Yokohama, H., Nakagomi, S., Hirayama, T., Aoki, S., Wada, K. & Kiyama, H. (2000). Damage-induced neuronal endopeptidase (DINE) is a unique metallopeptidase expressed in response to neuronal damage and activates superoxide scavengers.

Proc Natl Acad Sci USA 97, 4345–50.

22.Kikuchi, M., Tenneti, L. & Lipton, S. A. (2000). Role of p38 mitogen-activated protein kinase in axotomy-induced apoptosis of rat retinal ganglion cells. J Neurosci 20, 5037–44.

23.Aguayo, A. J., Clarke, D. B., Jelsma, T. N., Kittlerova, P., Friedman, H. C. & Bray, G. M. (1996). Effects of neurotrophins on the survival and regrowth of injured retinal neurons.

Ciba Foundation Symp 196, 135–44.

24.Moretto, G., Xu, R. Y., Walker, D. G. & Kim, S. U. (1994). Co-expression of mRNA for neurotrophic factors in human neurons and glial cells in culture. J Neuropathol Exp Neurol 53, 78–85.

25.Lambert, W., Agarwal, R., Howe, W., Clark, A. F. & Wordinger, R. J. (2001). Neurotrophin and neurotrophin receptor expression by cells of the human lamina cribrosa. Invest Ophthalmol Vis Sci 42, 2315–23.

26.Reynolds, A. J., Bartlett, S. E. & Hendry, I. A. (2000). Molecular mechanisms regulating the retrograde axonal transport of neurotrophins. Brain Res Brain Res Rev 33, 169–78.

27.Meyer-Franke, A., Kaplan, M. R., Pfrieger, F. W. & Barres, B. A. (1995). Characterization of the signaling interactions that promote the survival and growth of developing retinal ganglion cells in culture. Neuron 15, 805–19.

28.Peinado-Ramon, P., Salvador, M., Villegas-Perez, M. P. & Vidal-Sanz, M. (1996). Effects of axotomy and intraocular administration of NT-4, NT-3, and brain-derived neurotrophic factor on the survival of adult rat retinal ganglion cells. A quantitative in vivo study. Invest Ophthalmol Vis Sci 37, 489–500.

29.Chen, H. & Weber, A. J. (2001). BDNF enhances retinal ganglion cell survival in cats with optic nerve damage. Invest Ophthalmol Vis Sci 42, 966–74.

30.Ko, M. L., Hu, D. N., Ritch, R., Sharma, S. C. & Chen, C. F. (2001). Patterns of retinal ganglion cell survival after brain-derived neurotrophic factor administration in hypertensive eyes of rats. Neurosci Lett 305, 139–42.

31.Martin, K. R., Quigley, H. A., Zack, D. J., Levkovitch-Verbin, H., Kielczewski, J., Valenta, D., Baumrind, L., Pease, M. E., Klein, R. L. & 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, 4357–65.

32.Wang, N., Zeng, M., Ruan, Y., Wu, H., Chen, J., Fan, Z. & Zhen, H. (2002). Protection of retinal ganglion cells against glaucomatous neuropathy by neurotrophin-producing, genetically modified neural progenitor cells in a rat model. Chin Med J (Engl) 115, 1394–400.

33.McKinnon, S. J., Lehman, D. M., Tahzib, N. G., Ransom, N. L., Reitsamer, H. A., Liston, P., LaCasse, E., Li, Q., Korneluk, R. G. & Hauswirth, W. W. (2002). Baculoviral IAP repeat- containing-4 protects optic nerve axons in a rat glaucoma model. Mol Ther 5, 780–7.

34.Shen, S., Wiemelt, A. P., McMorris, F. A. & Barres, B. A. (1999). Retinal ganglion cells lose trophic responsiveness after axotomy. Neuron 23, 285–95.

35.Pearson, H. E. & Thompson, T. P. (1993). Atrophy and degeneration of ganglion cells in central retina following loss of postsynaptic target neurons in the dorsal lateral geniculate nucleus of the adult cat. Exp Neurol 119, 113–9.

36.Carpenter, P., Sefton, A. J., Dreher, B. & Lim, W. L. (1986). Role of target tissue in regulating the development of retinal ganglion cells in the albino rat: effects of kainate lesions in the superior colliculus. J Comp Neurol 251, 240–59.

37.Lander, H. M. (1997). An essential role for free radicals and derived species in signal transduction. FASEB J 11, 118–24.

38.Finkel, T. (1999). Signal transduction by reactive oxygen species in non-phagocytic cells.

J Leukoc Biol 65, 337–40.

39.Cooper, C. E., Patel, R. P., Brookes, P. S. & Darley-Usmar, V. M. (2002). Nanotransducers in cellular redox signaling: modification of thiols by reactive oxygen and nitrogen species.

Trends Biochem Sci 27, 489–92.

40.Levonen, A. L., Patel, R. P., Brookes, P., Go, Y. M., Jo, H., Parthasarathy, S., Anderson, P. G. & Darley-Usmar, V. M. (2001). Mechanisms of cell signaling by nitric oxide and peroxynitrite: from mitochondria to MAP kinases. Antioxid Redox Signal 3, 215–29.

41.Avshalumov, M. V., Chen, B. T., Marshall, S. P., Pena, D. M. & Rice, M. E. (2003). Glutamate-dependent inhibition of dopamine release in striatum is mediated by a new diffusible messenger, H2O2. J Neurosci 23, 2744–50.

42.Valks, D. M., Kemp, T. J. & Clerk, A. (2003). Regulation of Bcl-xL expression by H2O2 in cardiac myocytes. J Biol Chem 278, 25542–7.

43.Murrell, G. A., Francis, M. J. & Bromley, L. (1990). Modulation of fibroblast proliferation by oxygen free radicals. Biochem J 265, 659–65.

44.Knapp, L. T. & Klann, E. (2002). Potentiation of hippocampal synaptic transmission by superoxide requires the oxidative activation of protein kinase C. J Neurosci 22, 674–83.

45.Dugan, L. L., Creedon, D. J., Johnson, E. M., Jr. & Holtzman, D. M. (1997). Rapid suppression of free radical formation by nerve growth factor involves the mitogen-activated protein kinase pathway. Proc Natl Acad Sci USA 94, 4086–91.

46.Greenlund, L. J., Deckwerth, T. L. & Johnson, E. M. (1995). Superoxide dismutase delays neuronal apoptosis: a role for reactive oxygen species in programmed neuronal death. Neuron 14, 303–15.

47.Hill, I. E., Murray, C., Richard, J., Rasquinha, I. & MacManus, J. P. (2000). Despite the internucleosomal cleavage of DNA, reactive oxygen species do not produce other markers of apoptosis in cultured neurons. Exp Neurol 162, 73–88.

48.Richter-Landsberg, C. & Vollgraf, U. (1998). Mode of cell injury and death after hydrogen peroxide exposure in cultured oligodendroglia cells. Exp Cell Res 244, 218–29.

49.Luetjens, C. M., Bui, N. T., Sengpiel, B., Munstermann, G., Poppe, M., Krohn, A. J., Bauerbach, E., Krieglstein, J. & Prehn, J. H. (2000). Delayed mitochondrial dysfunction in excitotoxic neuron death: cytochrome c release and a secondary increase in superoxide production. J Neurosci 20, 5715–23.

50.Kirkland, R. A. & Franklin, J. L. (2001). Evidence for redox regulation of cytochrome C release during programmed neuronal death: antioxidant effects of protein synthesis and caspase inhibition. J Neurosci 21, 1949–63.

51.Mannick, J. B., Schonhoff, C., Papeta, N., Ghafourifar, P., Szibor, M., Fang, K. & Gaston, B. (2001). S-Nitrosylation of mitochondrial caspases. J Cell Biol 154, 1111–6.

52.Sugawara, T., Noshita, N., Lewen, A., Gasche, Y., Ferrand-Drake, M., Fujimura, M., Morita-Fujimura, Y. & Chan, P. H. (2002). Overexpression of copper/zinc superoxide dismutase in transgenic rats protects vulnerable neurons against ischemic damage by blocking the mitochondrial pathway of caspase activation. J Neurosci 22, 209–17.

53.Levin, L. A., Clark, J. A. & Johns, L. K. (1996). Effect of lipid peroxidation inhibition on retinal ganglion cell death. Invest Ophthalmol Vis Sci 37, 2744–9.

54.Kortuem, K., Geiger, L. K. & Levin, L. A. (2000). Differential susceptibility of retinal ganglion cells to reactive oxygen species. Invest Ophthalmol Vis Sci 41, 3176–82.

55.Geiger, L. K., Kortuem, K. R., Alexejun, C. & Levin, L. A. (2002). Reduced redox state allows prolonged survival of axotomized neonatal retinal ganglion cells. Neuroscience 109, 635–42.

56.Lieven, C. J., Schlieve, C. R., Hoegger, M. J. & Levin, L. A. (2006). Retinal ganglion cell axotomy induces an increase in intracellular superoxide anion. Invest Ophthalmol Vis Sci 47, 1477–85.

57.Schlieve, C. R., Lieven, C. J. & Levin, L. A. (2006). Biochemical activity of reactive oxygen species scavengers do not predict retinal ganglion cell survival. Invest Ophthalmol Vis Sci 47, 3878–86.

58.Castagne, V. & Clarke, P. G. (1996). Axotomy-induced retinal ganglion cell death in development: its timecourse and its diminution by antioxidants. Proc R Soc Lond B Biol Sci 263, 1193–7.

59.Castagne, V., Lefevre, K., Natero, R., Clarke, P. G. & Bedker, D. A. (1999). An optimal redox status for the survival of axotomized ganglion cells in the developing retina. Neuroscience 93, 313–20.

60.Park, C. & Raines, R. T. (2001). Adjacent cysteine residues as a redox switch. Protein Eng 14, 939–42.

61.Carugo, O., Cemazar, M., Zahariev, S., Hudaky, I., Gaspari, Z., Perczel, A. & Pongor, S. (2003). Vicinal disulfide turns. Protein Eng 16, 637–9.

62.Kim, J. R., Kwon, K. S., Yoon, H. W., Lee, S. R. & Rhee, S. G. (2002). Oxidation of proteinaceous cysteine residues by dopamine-derived H2O2 in PC12 cells. Arch Biochem Biophys 397, 414–23.

63.Hecht, D. & Zick, Y. (1992). Selective inhibition of protein tyrosine phosphatase activities by H2O2 and vanadate in vitro. Biochem Biophys Res Commun 188, 773–9.

64.Hausladen, A. & Fridovich, I. (1994). Superoxide and peroxynitrite inactivate aconitases, but nitric oxide does not. J Biol Chem 269, 29405–8.

65.Chiarugi, P., Fiaschi, T., Taddei, M. L., Talini, D., Giannoni, E., Raugei, G. & Ramponi, G. (2001). Two vicinal cysteines confer a peculiar redox regulation to low molecular weight protein tyrosine phosphatase in response to platelet-derived growth factor receptor stimulation. J Biol Chem 276, 33478–87.

66.Lipton, S. A., Choi, Y. B., Pan, Z. H., Lei, S. Z., Chen, H. S., Sucher, N. J., Loscalzo, J., Singel, D. J. & Stamler, J. S. (1993). A redox-based mechanism for the neuroprotective and neurodestructive effects of nitric oxide and related nitroso-compounds. Nature 364, 626–32.

67.Crossthwaite, A. J., Hasan, S. & Williams, R. J. (2002). Hydrogen peroxide-mediated phosphorylation of ERK1/2, Akt/PKB and JNK in cortical neurones: dependence on Ca(2+) and PI3-kinase. J Neurochem 80, 24–35.

68.Kim, S. O., Merchant, K., Nudelman, R., Beyer, W. F., Jr., Keng, T., DeAngelo, J., Hausladen, A. & Stamler, J. S. (2002). OxyR: a molecular code for redox-related signaling. Cell 109, 383–96.

69.Azevedo, D., Tacnet, F., Delaunay, A., Rodrigues-Pousada, C. & Toledano, M. B. (2003). Two redox centers within Yap1 for H(2)O(2) and thiol-reactive chemicals signaling. Free Radic Biol Med 35, 889–900.

70.Peshenko, I. V. & Shichi, H. (2001). Oxidation of active center cysteine of bovine 1-Cys peroxiredoxin to the cysteine sulfenic acid form by peroxide and peroxynitrite. Free Radic Biol Med 31, 292–303.

71.Krezel, A., Lesniak, W., Jezowska-Bojczuk, M., Mlynarz, P., Brasun, J., Kozlowski, H. & Bal, W. (2001). Coordination of heavy metals by dithiothreitol, a commonly used thiol group protectant. J Inorg Biochem 84, 77–88.

72.Ruegg, U. T. & Rudinger, J. (1977). Reductive cleavage of cystine disulfides with tributylphosphine. Methods Enzymol 47, 111–6.

73.Burns, J. A., Butler, J. C., Moran, J. & Whitesides, G. M. (1991). Selective reduction of disulfides by tris(2-carboxyethyl)phosphine. J Org Chem 56, 2648–50.

74.Rhee, S. S. & Burke, D. H. (2004). Tris(2-carboxyethyl)phosphine stabilization of RNA: comparison with dithiothreitol for use with nucleic acid and thiophosphoryl chemistry.

Anal Biochem 325, 137–43.

75.Getz, E. B., Xiao, M., Chakrabarty, T., Cooke, R. & Selvin, P. R. (1999). A comparison between the sulfhydryl reductants tris(2-carboxyethyl)phosphine and dithiothreitol for use in protein biochemistry. Anal Biochem 273, 73–80.

76.Han, J. C. & Han, G. Y. (1994). A procedure for quantitative determination of tris(2-carboxyethyl)phosphine, an odorless reducing agent more stable and effective than dithiothreitol. Anal Biochem 220, 5–10.

77.Krezel, A., Latajka, R., Bujacz, G. D. & Bal, W. (2003). Coordination properties of tris(2-carboxyethyl)phosphine, a newly introduced thiol reductant, and its oxide. Inorg Chem 42, 1994–2003.

78.Swanson, K. I., Schlieve, C. R., Lieven, C. J. & Levin, L. A. (2005). Neuroprotective effect of sulfhydryl reduction in a rat optic nerve crush model. Invest Ophthalmol Vis Sci 46, 3737–41.

79.Klöcker, N., Zerfowski, M., Gellrich, N. C. & Bahr, M. (2001). Morphological and functional analysis of an incomplete CNS fiber tract lesion: Graded crush of the rat optic nerve. J Neurosci Methods 110, 147–53.

80.Schlieve, C. R., Tam, A., Nilsson, B. L., Lieven, C. J., Raines, R. T. & Levin, L. A. (2006). Synthesis and characterization of a novel class of reducing agents that are highly neuroprotective for retinal ganglion cells. Exp Eye Res 83, 1252–9.

that deprivation of neurotrophins, mainly brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), secreted by targets in the brain, leads to apoptotic death of RGCs during development (9–12) (see Fig. 1). In contrast, the role of endogenous target-derived factors in the maintenance of adult RGCs in glaucoma is less clear. It has been proposed that blockade of axonal transport during IOP elevation leads to neurotrophin deprivation and neuronal death (6). Indirect evidence for this idea is provided by the observation that both anterograde and retrograde axonal transport in the optic nerve is blocked in experimental glaucoma (13–15). In addition, increased immunolabeling for BDNF and its receptor TrkB was found behind the optic nerve head following IOP increase in a glaucoma rat model (16).

Local sources of neurotrophins within the retina may also play a prominent role in adult RGC survival (see Fig. 2). Evidence for this premise comes from studies where RGC target tissue was ablated in adult animals and neuronal loss was not detected until several months after the lesion (17–19). This finding suggests that retina-derived neurotrophic factors, acting in a paracrine or autocrine fashion, support the survival

Target source in the brain

Neurotrophic supply

Neurotrophic deprivation

Fig. 1. In the “neurotrophic factor hypothesis,” neurons compete for limited amounts of target-derived neurotrophic factors that are required for survival. Neurons that successfully compete for these factors survive, whereas less-competitive neurons die. In glaucoma, neurotrophic deprivation caused by obstruction of the axonal transport may be involved in retinal ganglion cell (RGC) death.

RPE

PS

ONL

OPL

INL

IPL

GCL

Fig. 2. Local sources of endogenous neurotrophic factors within the retina may play a prominent role in adult retinal ganglion cell (RGC) survival. Diffusible factors secreted by supporting Müller glia or retinal pigment epithelium (RPE) may act in paracrine fashion to promote the survival of injured RGCs. Factors secreted by retinal neurons, including RGCs, may act in autocrine or paracrine fashion to stimulate neuronal survival after injury. PS: photoreceptor segments; ONL: outer nuclear layer; OPL: outer plexiform layer; INL: inner nuclear layer; IPL: inner plexiform layer; GCL: ganglion cell layer.

of RGCs that have been disconnected from their targets. Thus, it has been proposed that target-derived factors may act to compensate for a deficiency in local trophic support (20). Of interest, neurotrophin receptor TrkB gene expression is markedly downregulated in RGCs after axotomy or chronic IOP elevation (21,22) indicating that injured RGCs may have impaired ability to respond to neurotrophins produced locally within the retina.

Collectively, these results suggest that deficiencies in neurotrophic factors and/or their receptors, derived either from the target or from the retina, may play a role in RGC death in glaucoma. Consequently, exogenous administration of neurotrophic factors has been extensively studied as a potential strategy to prevent or delay the death of injured RGCs. In this chapter, we will focus on neurotrophic factors with demonstrated efficacy for keeping RGCs alive in different models of optic nerve injury, including chronic IOP elevation. In addition, the intracellular signaling pathways that mediate neurotrophic-factor RGC survival and/or regeneration, as well as novel compounds that lead to highly specific activation of neuroprotective signaling will be discussed.

THE NEUROTROPHINS

Neurotrophins are diffusible trophic molecules that exert a potent survival effect on adult central nervous system (CNS) neurons undergoing degeneration induced by a broad variety of stimuli. They are a family of small, secreted peptides that include NGF, BDNF, neurotrophin-3 (NT-3), and neurotrophin-4/5 (NT-4/5) in

mammals (23,24). Neurotrophin-6 (NT-6) and neurotrophin-7 (NT-7) are two family members that have been identified exclusively in fish (25,26). In addition to survival, neurotrophins mediate several key cellular responses in the developing and mature CNS including proliferation, differentiation, axon growth, as well as dendrite and synapse formation (23,27,28).

The biological effects of neurotrophins are mediated by cell surface receptors of the tropomyosin-related kinase (Trk) family comprising TrkA, the receptor for NGF; TrkB, the receptor for BDNF and NT-4; and TrkC, the receptor for NT-3 (29–31). These receptors share in common a large extracellular ligand-binding domain, a single transmembrane domain, and a cytoplasmic tyrosine kinase catalytic domain (see Fig. 3). In addition, the p75 receptor (p75NTR), related to the tumor necrosis factor receptor, binds all the neurotrophins with equal affinity (32,33). Both receptors can be present on the same cell population; however, activation of Trk receptors has been typically associated with cell survival, whereas p75NTR receptor can stimulate both survival and apoptotic signals (34,35). Developing and adult RGCs have been shown to express high levels of the BDNF receptor TrkB (36–38). Importantly, adult RGCs do not seem to express p75NTR (39,40), and a newly identified receptor named TROY has been shown to mediate some of the functions originally attributed to p75NTR (41).

NGF BDNF NT-4 NT-3

p75

Fig. 3. The biological effects of neurotrophins are mediated by Trk receptors: TrkA is the receptor for NGF; TrkB is the receptor for brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4); and TrkC is the receptor for NT-3. The p75 receptor (p75NTR), related to the tumor necrosis factor receptor, binds all the neurotrophins with equal affinity.

Brain-Derived Neurotrophic Factor

Originally discovered and isolated from pig brain in 1982 by Yves Barde and collaborators (42), BDNF was initially found to promote robust survival of primary sensory neurons (43,44). The first evidence of the neuroprotective effect of BDNF on central neurons came from studies demonstrating BDNF-induced survival of cultured RGCs (45). Since then, it has become clear that BDNF is the most potent neurotrophin for developing (46,47) and adult RGCs following optic nerve injury (48–50). BDNF is strongly expressed in the superior colliculus (51,52) and is retrogradely transported by RGC axons to the retina (46,53). Within the retina, BDNF is mainly produced by cells in the ganglion cell layer and by some cells of the inner nuclear layer (37,54,55).

Many groups have investigated the effect of intraocular injection of BDNF in models of RGC death. For example, injection of BDNF protein in the superior colliculus has been shown to markedly reduce developmental RGC death (46). In the adult rat and cat visual system, a single intravitreal injection of BDNF promoted robust survival of RGCs after axotomy (48,56–60). Similarly, intraocular delivery of BDNF protein (61) or gene transfer using adeno-associated virus (AAV) led to RGC neuroprotection in experimental glaucoma induced by chronic IOP elevation (62).

Long-term studies of RGC survival have demonstrated that the effect of exogenous BDNF is temporary: it delays, but does not prevent, the onset of RGC death (48,58). Administration of BDNF by repeated intravitreal injections or osmotic minipumps failed to extend the time-course of RGC protection (48,63). Moreover, delivery of BDNF by adenovirus-infected Müller cells provided a sustained source of neurotrophin but resulted in transient RGC rescue (58). Why might this happen? The biological effect of BDNF is exerted upon binding to its cognate receptor TrkB. There is now evidence that the levels of expression of these receptors in RGCs vary following injury, in some cases limiting the intrinsic capacity of these neurons to respond to neurotrophic factor stimulation. In vitro studies using highly purified RGCs have shown that TrkB internalization is likely to contribute to loss of trophic responsiveness after axotomy (64,65). In vivo, TrkB mRNA and protein levels are markedly downregulated in adult RGCs following axotomy close to the eye (66,67) and in experimental glaucoma (22). Together, these results suggest that reduced TrkB expression in injured RGCs contributes to their desensitization to exogenous, and possibly endogenous, BDNF. Consistent with this, a strategy that involved upregulation of TrkB in RGCs by AAV-mediated gene transfer markedly enhanced the duration and level of BDNFinduced neuroprotection after optic nerve axotomy (66). It would be relevant to assess whether such neuroprotective strategy is effective in experimental glaucoma.

Neuroprotective Signaling Pathways Downstream

of the BDNF/TrkB Complex

There are several important limitations to applying exogenous factors as neuroprotective therapy for RGCs. The survival effect of diffusible factors is transient, as discussed above, and it may also be non-specific. BDNF, for example, will likely affect other retinal cells that express the receptor TrkB, including amacrine cells, Müller glia, and cone photoreceptors (37,68,69). This pleiotropic, non-specific effect may also be applicable to other neurotrophic factors. In addition, exogenous neurotrophic factors