Bcl-2 Expression in Pericytes and Astrocytes Impacts Vascular Development and Homeostasis.

B-cell lymphoma 2 (Bcl-2) protein is the founding member of a group of proteins known to modulate apoptosis. Its discovery set the stage for identification of family members with either pro- or anti-apoptotic properties. Expression of Bcl-2 plays an important role during angiogenesis by influencing not only vascular cell survival, but also migration and adhesion. Although apoptosis and migration are postulated to have roles during vascular remodeling and regression, the contribution of Bcl-2 continues to emerge. We previously noted that the impaired retinal vascularization and an inability to undergo pathologic neovascularization observed in mice globally lacking Bcl-2 did not occur when mice lacked the expression of Bcl-2 only in endothelial cells. To further examine the effect of Bcl-2 expression during vascularization of the retina, we assessed its contribution in pericytes or astrocytes by generating mice with a conditional Bcl-2 allele (Bcl-2Flox/Flox) and Pdgfrb-cre (Bcl-2PC mice) or Gfap-cre (Bcl-2AC mice). Bcl-2PC and Bcl-2AC mice demonstrated increased retinal vascular cell apoptosis, reduced numbers of pericytes and endothelial cells and fewer arteries and veins in the retina. Bcl-2PC mice also demonstrated delayed advancement of the superficial retinal vascular layer and aberrant vascularization of the deep vascular plexus and central retina. Although pathologic neovascularization in oxygen-induced ischemic retinopathy (OIR) was not affected by lack of expression of Bcl-2 in either pericytes or astrocytes, laser-induced choroidal neovascularization (CNV) was significantly reduced in Bcl-2PC mice compared to littermate controls. Together these studies begin to reveal how cell autonomous modulation of apoptosis in vascular cells impacts development and homeostasis.

Bim expression in endothelial cells and pericytes is essential for regression of the fetal ocular vasculature.

Apoptosis plays a central role in developmental and pathological angiogenesis and vessel regression. Bim is a pro-apoptotic Bcl-2 family member that plays a prominent role in both developmental and pathological ocular vessel regression, and neovascularization. Endothelial cells (EC) and pericytes (PC) each play unique roles during vascular development, maintenance and regression. We recently showed that germline deletion of Bim results in persistent hyaloid vasculature, increased retinal vascular density and prevents retinal vessel regression in response to hyperoxia. To determine whether retinal vascular regression is attributable to Bim expression in EC or PC we generated mice carrying a conditional Bim allele (BimFlox/Flox) and VE-cadherin-cre (BimEC mice) or Pdgfrb-cre (BimPC mice). BimEC and BimPC mice demonstrated attenuated hyaloid vessel regression and postnatal retinal vascular remodeling. We also observed decreased retinal vascular apoptosis and proliferation. Unlike global Bim -/- mice, mice conditionally lacking Bim in EC or PC underwent hyperoxia-mediated vessel obliteration and subsequent retinal neovascularization during oxygen-induced ischemic retinopathy similar to control littermates. Thus, understanding the cell autonomous role Bim plays in the retinal vascular homeostasis will give us new insight into how to modulate pathological retinal neovascularization and vessel regression to preserve vision.