Ginkgo biloba delays light-induced photoreceptor degeneration through antioxidant and antiapoptotic properties

Abstract

Intense exposure to artificial bright light increases the risk of retinal damage resulting in blurred vision and blindness. Long-term exposure to bright light elevates oxidative stress-induced apoptosis, which results in photoreceptor cell degeneration. However, to the best of our knowledge, the molecular mechanism associated with light-induced retinopathy remains unclear. In the present study, the mechanisms involved in light-induced oxidative stress and apoptosis were investigated along with the protective effects of Ginkgo biloba (EGb 761) in photoreceptor cell degeneration. EGb 761 was administered to mice at a dose of 50 or 100 mg/kg for 7 days prior to exposure to bright light (5,000 lux for 24 h). Furthermore, photoreceptor cell disorders were evaluated using electroretinogram (ERG) and H&E staining analyses. The expression levels of antioxidant genes and proteins ERK, thioredoxin (Trx) and nuclear factor erythroid 2-related factor 2 (Nrf-2) and the induction of apoptosis cytochrome c (Cyc), cleaved caspase-3 and Bax, were determined by reverse transcription-quantitative PCR and western blotting. ERG and histological analysis revealed that exposure to bright light induced functional and morphological changes to the photoreceptor cells. Exposure to bright light increased the levels of Cyc, cleaved caspase-3 and Bax, and decreased the levels of phosphorylated (p-) Erk, Nrf-2 and thioredoxin (Trx). However, treatment of mice with EGb 761 increased the expression levels of antiapoptotic (Bcl-2) and antioxidant (p-Erk, Trx and Nrf-2) proteins and decreased the expression levels of the apoptotic genes (Cyc, cleaved caspase-3 and Bax). Based on these findings, the present study suggested that prolonged exposure to light induces photoreceptor cell degeneration, where EGb 761 treatment may serve a therapeutic effect on the development of photoreceptor cell degeneration.

Keywords: Ginkgo biloba; apoptosis; light damage; oxidative stress; retinal degeneration.

Figure 1

EGb-761 protects the retinal function measured by electroretinogram. (A) Amplitudes of a-wave and b-wave exposed to 5,000 lux of light for 24 h with/without EGb-761 pre-treatment. Statistical analysis of the (B) a-wave and (C) b-wave. The data are presented as the mean ± SD (n=3 in each group). ^**P<0.01, ^***P<0.001, ^****P<0.0001. EGb-761, L + HG (100 mg/kg); L + LG (50 mg/kg). L, light, LG, Low Ginkgo; HG, high Ginkgo.

Figure 2

Protective effect of EGb-761 in light-induced photoreceptor degeneration. (A) Effect of EGb-761 on the morphology of the retina in light-damaged mice (magnification, x400). (B) Spider graph of ONL thickness in the light-induced retinal degeneration groups. (C) Thickness of the ONL in light-treated mice. The data are presented as the mean ± SD (n=3 in each group) ^****P<0.0001. EGb-761, L + HG (100 mg/kg); L + LG (50 mg/kg); ONL, outer nuclear layer; RPE, retinal pigment epithelium; OS, IS, INL, inner nuclear layer; GCL, ganglion cell layer.

Figure 3

EGb 761 treatment increases the protein levels of Trx, Nrf-2 and p-Erk in light-induced mouse retina degeneration. Protein expression levels of (A) Nrf-2, p-Erk and (B) Trx were detected by western blotting. Semi-quantification of (C) Nrf-2, (D) p-Erk and (E) Trx. The data are presented as the mean ± SD (n=4 in each group). ^*P<0.05, ^**P<0.01. L + HG (100 mg/kg); L + LG (50 mg/kg); Nrf-2, nuclear factor erythroid 2-related factor 2; p-, phosphorylated; Trx, thioredoxin.

Figure 4

EGb 761 treatment decreases the expression levels of Cyc, Bax, Bak-1 and cleaved caspase-3, and increases the expression levels of Bcl2 in light-induced mouse retina degeneration. The expression levels of (A) Cyc and (B) Bak-1 were detected using reverse transcription-quantitative PCR. (C) Expression levels of cleaved caspase-3, Bax and Bcl2 were detected using western blotting. Semi-quantification of (D) cleaved caspase-3, (E) Bax and (F) Bcl2. The data are presented as the mean ± SD (n=4 in each group). ^*P<0.05, ^**P<0.01, ^***P<0.001. Bak-1, BCL2 antagonist/killer 1; Cyc, cytochrome c; L + HG (100 mg/kg); L + LG (50 mg/kg).

Figure 5

Schematic summary of the protective effects of EGb 761 against light-induced photoreceptor degeneration. Cyc, cytochrome c; Nrf-2, nuclear factor erythroid 2-related factor 2; ROS, reactive oxygen species; Trx, thioredoxin.