Valproate promotes survival of retinal ganglion cells in a rat model of optic nerve crush

Abstract

Valproate (VPA) is an anticonvulsant and mood-stabilizing drug. It is a broad-spectrum histone deacetylase inhibitor with neuroprotective effects. We investigated whether VPA reduces retinal neuronal death induced by optic nerve crush (ONC). To evaluate further VPA-mediated neuroprotection on retinal ganglion cells (RGCs), another histone deacetylase (HDAC) inhibitor, sodium butyrate (SB) was compared with VPA. Adult male Wistar rats were subjected to ONC injury. VPA and SB were administered subcutaneously 1day prior to ONC until sacrifice 14days later. RGC density was counted using hematoxylin and eosin (H&E) staining of the retinal section and retrograde labeling with FluoroGold. Retinal function was evaluated by electroretinography (ERG) after ONC. Immunofluorescence of activated caspase-3 in ganglion cell layer (GCL) and the detection of bcl-2 mRNA expression in the retina were used to evaluate apoptosis of retinal cells. In addition, brain-derived neurotrophic factor (BDNF) in retinas was measured using an enzyme-linked immunosorbent assay (ELISA) and real-time polymerase chain reaction (PCR). Western blot was used to analyze histone H3 acetylation, the protein kinase B (Akt) and extracellular signal-regulated kinase (Erk) phosphorylation levels, and tropomyosin-related kinase B (TrkB) levels. The transcriptional activation of the BDNF gene was analyzed by measuring the levels of acetylation or methylation of histone H3 using chromatin immunoprecipitation assay. The RGC density in the VPA and SB treated-groups were significantly higher as compared with those of the corresponding vehicle group following ONC. VPA and SB suppressed reductions in a- and b-wave amplitudes of the ERG and attenuated the activation of caspase-3 in the RGCs, which was accompanied by upregulation in Akt and Erk phosphorylation in the retina. Furthermore, VPA upregulated levels of bcl-2, BDNF, TrkB in the retina post-injury. VPA and SB treatment resulted in the hyperacetylation of histone H3K14, attenuated histone H3K9 hypermethylation in the BDNF promoter, and promoted transcriptional activity. These results demonstrate that VPA appears to protect RGCs from ONC by inhibiting neuronal apoptosis possibly via the activation of BDNF-TrkB signaling and HDAC inhibition.