Bax and Bak Are Essential, Overlapping Mediators of Apoptotic Developmental Regression of the Hyaloid Vasculature and of Retinal Neurons Including Photoreceptors

Abstract

Abstract: : Purpose: Apoptosis has been implicated in ocular development and retinal degeneration, but the specific apoptotic pathways employed are incompletely understood. The purpose of this study was to characterize the roles of pro-apoptotic Bcl-2 members Bax and Bak in ocular development. Methods: Adult (>2 month old) eyes from wild-type, Bak-/-, Bax-/-, and Bax-/-Bak-/- mice were analyzed by histology and immunocytochemistry to identify persistent ocular tissue and retinal cells. Eyes from mice aged postnatal day (P) 7, during the peak of developmental apoptosis in the retina, were processed for TdT-dUTP terminal nick-end labeling (TUNEL) analysis to determine whether Bax- or double Bax/Bak- deficiency might cause a defect in developmental apoptosis. Studies with mice at P1, during the peak of ganglion cell death, are currently underway. Results: Adult Bax-/-Bak-/- eyes had statistically significant increases in inner retinal cell numbers, with an almost complete absence of TUNEL-positive cell death at P7. Some of these persistent cells in the inner retina included ectopic rod photoreceptors that normally undergo apoptosis following failure to migrate to the outer retina. These ectopic rods expressed markers of early rod differentiation: rod opsin, arrestin, and recoverin. However, they did not form ectopic outer segments or express the associated markers, ROM-1, peripherin-2, and RP1. Bax-/-Bak-/- eyes also retained persistent fetal vasculature to at least 8 months, consisting of a patent hyaloid vasculature (hyaloid artery, vasa hyaloidea propria, and tunica vasculosa lentis) but no pupillary membrane. Conclusions: Bax and Bak are essential for apoptotic regression of fetal vessels and retinal neurons and are the first apoptotic factors identified as essential for developmental photoreceptor apoptosis. Future studies will investigate the potential role of Bax and Bak in mediating pathologic photoreceptor degenerations, including retinitis pigmentosa and age-related macular degeneration, as well as eye diseases that involve persistent fetal structures, such as persistent fetal vasculature.

Keywords: apoptosis/cell death • retinal development • photoreceptors