Retinal diseases, such as hereditary retinitis pigmentosa and age-related macular degeneration, are characterized by the progressive loss of photoreceptors. Histone deacetylase 6 (HDAC6) is considered as a stress surveillance factor and a potential target for neuroprotection and regeneration. Overexpression of HDAC6 has been connected to neurodegenerative disorders, and its suppression may provide protection. Here we show that HDAC6 is constitutively present in the mouse retina, and in the cone-like mouse cell line 661W. In 661W cells HDAC6 inhibition by the specific inhibitor tubastatin A (TST) led to the acetylation of α-tubulin, which is a major substrate for HDAC6. After oxidative stress, exerted by hydrogen peroxide, TST promoted cell survival and the upregulation of heat-shock proteins HSP70 and HSP25 by activation of heat-shock transcription factor 1. Furthermore, in response to oxidative stress the redox regulatory protein peroxiredoxin 1 (Prx1) was modulated in 661W cells by HDAC6 inhibition. The peroxide reducing activity of Prx1 is dependent on its acetylation, which is mediated by HDAC6. Pre-incubation with TST prevented the inactivation of Prx1 and its preserved activity may exert protective effects in photoreceptor cells. To determine whether TST treatment has a therapeutic effect on visual function, the dyeucd6 zebrafish model of inherited sight loss was utilized. Zebrafish have developed as a suitable model system for pharmacological testing. In vivo application of TST caused the hyperacetylation of α-tubulin, indicating that HDAC6 is active in this model. Furthermore, TST was sufficient to rescue visual function and retinal morphology. Hence, HDAC6 inhibition and the regulation of peroxiredoxin activity may play a significant role in protecting retinal cells and in particular photoreceptors, which are exposed to high levels of reactive oxygen species derived from oxidative stress-induced injuries.

Controversially, histone deacetylase inhibitors (HDACi) are in clinical trial for the treatment of inherited retinal degeneration. Utilizing the zebrafish dye ucd6 model, we determined if treatment with HDACi can rescue cone photoreceptor-mediated visual function. dye exhibit defective visual behaviour and retinal morphology including ciliary marginal zone (CMZ) cell death and decreased photoreceptor outer segment (OS) length, as well as gross morphological defects including hypopigmentation and pericardial oedema. HDACi treatment of dye results in significantly improved optokinetic (OKR) (~43 fold, p < 0.001) and visualmotor (VMR) (~3 fold, p < 0.05) responses. HDACi treatment rescued gross morphological defects and reduced CMZ cell death by 80%. Proteomic analysis of dye eye extracts suggested BDNF-TrkB and Akt signaling as mediators of HDACi rescue in our dataset. Co-treatment with the TrkB antagonist ANA-12 blocked HDACi rescue of visual function and associated Akt phosphorylation. Notably, sole treatment with a BDNF mimetic, 7,8-dihydroxyflavone hydrate, significantly rescued dye visual function (~58 fold increase in OKR, p < 0.001, ~3 fold increase in VMR, p < 0.05). In summary, HDACi and a BDNF mimetic are sufficient to rescue retinal cell death and visual function in a vertebrate model of inherited blindness.

This review discusses the therapeutic potential of brain-derived neurotrophic factor (BDNF) for retinal degeneration. BDNF, nerve growth factor (NGF), neurotrophin 3 (NT-3) and NT-4/NT-5 belong to the neurotrophin family. These neuronal modulators activate a common receptor and a specific tropomyosin-related kinase (Trk) receptor. BDNF was identified as a photoreceptor protectant in models of retinal degeneration as early as 1992. However, development of effective therapeutics that exploit this pathway has been difficult due to challenges in sustaining therapeutic levels in the retina.

Previous studies report that retinitis pigmentosa (RP) patients treated with the histone deacetylase inhibitor (HDACi) valproic acid (VPA) present with improved visual fields and delayed vision loss. However, other studies report poor efficacy and safety of HDACi in other cohorts of retinal degeneration patients. Furthermore, the molecular mechanisms by which HDACi can improve visual function is unknown, albeit HDACi can attenuate pro-apoptotic stimuli and induce expression of neuroprotective factors. Thus, further analysis of HDACi is warranted in pre-clinical models of retinal degeneration including zebrafish. Analysis of HDAC expression in developing zebrafish reveals diverse temporal expression patterns during development and maturation of visual function.