Programmed cell death (PCD) during development of the mouse retina involves activation of the mitochondrial pathway. Previous work has shown that the multidomain Bcl-2 family
proteins Bax and Bak are fundamentally involved in this process. To induce mitochondrial membrane permeabilization, Bax and Bak require that prosurvival members of the family be inactivated by binding of "BH3-only" members. We showed previously that the BH3-only
protein BimEL is highly expressed during postnatal
retinal development but decreases dramatically thereafter. The purpose of this study was to investigate a possible role for Bim, in
retinal development and degeneration, upstream of Bax and Bak. Bim-/- mice analyzed for defective
retinal development exhibit an increase in
retinal thickness and a delay in PCD, thereby confirming a role for Bim. We also demonstrate that in response to certain death stimuli, bim+/+
retinal explants upregulate BimEL leading to
caspase activation and cell death, whereas bim-/- explants are resistant to apoptosis. Finally, we analyzed Bim expression in the
retinal degeneration (rd) mouse, an in vivo model of
retinal degeneration. Bim
isoforms, which decrease during development, are not reexpressed during
retinal degeneration and ultimately photoreceptor cells die by a
caspase-independent mechanism. Thus, we conclude that in cases in which BimEL is reexpressed during pathological cell death, developmental cell death pathways are reactivated. However, the absence of BimEL expression correlates with
caspase-independent death in the rd model.