Molecular mechanisms regulating ocular apoptosis in zebrafish gdf6a mutants

Purpose. To characterize the molecular mechanisms underlying retinal apoptosis induced by loss of Gdf6, a TGFβ ligand. Methods. The role of Gdf6 in regulating apoptosis was studied using a zebrafish gdf6a^-/- mutant, which encodes a truncated, nonfunctional protein. To investigate whether intrinsic or extrinsic apoptotic mechanisms were involved, morpholino antisense oligonucleotides targeting baxa, baxb, and p53 were employed. Caspase-3 immunohistochemistry (IHC) was performed to assay apoptosis. Pharmacologic inhibition (using SB203580) and IHC were used to investigate the role of p38 mitogen activated protein (MAP) kinase activation in gdf6a^-/--induced apoptosis. To assess the role of Gdf6a in transcriptional regulation of TGFβ signal transducers, in situ hybridization (ISH) was performed using probes to smad1, 5, 7, and 8. Results. Results indicate maximal ocular apoptosis occurs 28 hours post fertilization (hpf) in gdf6a^-/- mutants that is mediated independently of p53 by intrinsic mechanisms involving Bax proteins. Also, gdf6a^-/- mutants exhibit markedly increased p38 MAP kinase activation that can be inhibited to significantly reduce retinal apoptosis. A reduction in retinal smad1 expression was also noted in gdf6a^-/- mutants. Conclusions. gdf6a^-/--induced apoptosis is characterized by the involvement of intrinsic apoptotic pathways, p38 MAP kinases, and dysregulated smad expression. Modulation of key mediators can inhibit retinal apoptosis offering potential avenues of therapy. However, the efficacy of pharmacomodulation in improvement of visual function needs to be further examined.