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41 Thioredoxins 1 and 2 Protect RGC

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Fig. 41.2 The effect of TRX1 and TRX2 overexpression on RGC survival after ONT (a and b) and IOP elevation (c). (a and b). TRX1 and TRX2 overexpression increased RGC survival by approximately 35 and 135% 1 and 2 weeks after ONT, respectively (n = 4–7; P < 0.05, P < 0.001). (c). Approximately 45 and 37% of RGC that were destined to die due to elevated IOP were preserved by TRX1 and TRX2 overexpression, respectively (n = 5–14; P < 0.05, P < 0.005)

were expressing TRX1-EGFP or TRX2-EGFP, respectively. RGCs constituted approximately 70% of all transfected cells in the GCL. The RGC protective effect of TRX1 and TRX2 overexpression was evaluated 5 weeks after IOP elevation. At this time point the loss of RGCs in non-transfected retinas was approximately 27% compared to the control. RGC loss in EGFP-TRX1 or EGFP-TRX2 transfected retinas was approximately 15 and 17%, respectively, compared to the non-transfected or pEGFP-transfected control eyes (Fig. 41.2c).

41.4 Discussion

RGC degeneration after ONT and during glaucomatous neurodegeneration was associated with oxidative damage due to increased ROS levels. ROS have direct neurotoxic effects on RGCs and also contribute to secondary degeneration by affecting glial function (Thanos et al. 1993). The current study was initiated with the aim to determine the role of TRX proteins, important regulators of the cellular redox state, in RGC protection against ONTand elevated IOP-induced oxidative injury.

TRX cytoprotective effect was first analyzed in RGC-5 cells treated with glutamate and BSO. Although overexpression of both TRX1 and TRX2 had a cytoprotective effect against oxidative stress induced by these agents, the effect of TRX1 was more potent than by TRX2. Based on data observed in RGC-5 cells, we analyzed the effect of these proteins on RGC survival after ONT and IOP elevation. ONT shifts the cellular redox status toward oxidation, which may lead to cell death by affecting mitochondrial functions or caspase activation (Nguyen et al. 2003). The survival of RGCs was shown to depend on redox state and stimulated by ROS scavengers (Geiger et al. 2002). In our study, TRX1 and TRX2 overexpression increased RGC survival by approximately 35 and 135% 1 and 2 weeks after axotomy, respectively. More pronounced effect of TRX proteins at two compared to

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1 week after ONT could be explained by attenuation of secondary events associated with increased oxidative damage: RGCs dying early after axotomy may damage neighboring RGCs or lead to activation of microglial cells, which in turn could contribute to secondary RGC degeneration. Induced expression of nitric oxide (NO) synthase by injured RGCs and glial cells and subsequent NO toxicity associated with cellular oxidation has been implicated in RGC death after ONT (Koeberle and Ball 1999).

TRX overexpression also supported RGC survival after IOP elevation. TRX1 and TRX2 preserved approximately 45 and 37% of cells, respectively, that were destined to die due to high IOP. We believe that the observed neuroprotective effect of TRX1 and TRX2 could be even higher considering relatively low efficiency of ELP-mediated RGC transfection with TRX-expression constructs. Approximately 30% of transfected cells were non-RGCs, including glial cells and since oxidative stress-induced dysfunction of glial cells has been proposed to play a role in secondary neuronal damage in glaucoma (Tezel and Wax 2003), TRX overexpression may decrease the impact of oxidative stress in these cells and thus contribute to RGC survival.

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