Bax expression impacts postnatal retinal vascular development and hyperoxia sensitivity

Apoptosis plays prominent roles during organ development, maturation and homeostasis. In the retina, Bcl-2 family members function through the intrinsic cell death pathway with vital roles during vascular development and hyperoxia-mediated vessel obliteration during oxygen induced ischemic retinopathy (OIR). Bim, a BH3 only protein Bcl-2 family member, binds and activates Bax and/or Bak to facilitate apoptosis. In some systems deletion of both Bax and Bak are required to prevent cell loss, such as regression of ocular hyaloid vasculature. We previously showed Bim expression significantly impacts normal retinal vascular development and sensitivity to hyperoxia. Mice deficient in Bim (Bim^−/−) show increased retinal vascular density and are protected from hyperoxia mediated vessel obliteration. Since Bim activates Bax, here we determined the impact lack of Bax expression has on these processes. Compared to Bax^+/+ mice, retinas from Bax^−/− mice had significantly increased numbers of retinal endothelial cells and pericytes. We also demonstrated that hyperoxia-mediated vessel obliteration during OIR was significantly decreased in the absence of Bax. Although the increased endothelial cell numbers were comparable to that of Bim^−/− mice, the increased numbers of pericytes were not to the extent noted in Bim^−/− mice. These changes were supported by partial protection of retinal vessels from hyperoxia in Bax^−/− mice compared to that noted in Bim^−/− mice. Thus, Bim-Bax driven pathway is sufficient to remove excess endothelial cells but not pericytes during postnatal retinal vascularization and hyperoxia-mediated vessel obliteration. Thus, additional Bim-mediated pathway(s) are required for removal of pericytes and hyperoxia-mediated vessel obliteration.