BackgroundBAMBI is a type I TGFβ receptor antagonist, whose in vivo function remains unclear, as BAMBI− /− mice lack an obvious phenotype.Methodology/Principal FindingsIdentifying BAMBI’s functions requires identification of cell-specific expression of BAMBI. By immunohistology we found BAMBI expression restricted to endothelial cells and by electron microscopy BAMBI− /− mice showed prominent and swollen endothelial cells in myocardial and glomerular capillaries. In endothelial cells over-expression of BAMBI reduced, whereas knock-down enhanced capillary growth and migration in response to TGFβ. In vivo angiogenesis was enhanced in matrigel implants and in glomerular hypertrophy after unilateral nephrectomy in BAMBI− /− compared to BAMBI+/+ mice consistent with an endothelial phenotype for BAMBI− /− mice. BAMBI’s mechanism of action in endothelial cells was examined by canonical and alternative TGFβ signaling in HUVEC with over-expression or knock-down of BAMBI. BAMBI knockdown enhanced basal and TGFβ stimulated SMAD1/5 and ERK1/2 phosphorylation, while over-expression prevented both.Conclusions/SignificanceThus we provide a first description of a vascular phenotype for BAMBI− /− mice, and provide in vitro and in vivo evidence that BAMBI contributes to endothelial and vascular homeostasis. Further, we demonstrate that in endothelial cells BAMBI interferes with alternative TGFβ signaling, most likely through the ALK 1 receptor, which may explain the phenotype observed in BAMBI− /− mice. This newly described role for BAMBI in regulating endothelial function has potential implications for understanding and treating vascular disease and tumor neo-angiogenesis.

BackgroundBAMBI (BMP and Activin Membrane Bound Inhibitor) is considered to influence TGFβ and Wnt signaling, and thereby fibrosis. Surprisingly data on cell type-specific expression of BAMBI are not available. We therefore examined the localization, gene regulation, and protein turnover of BAMBI in kidneys.Methodology/Principal FindingsBy immunofluorescence microscopy and by mRNA expression, BAMBI is restricted to endothelial cells of the glomerular and some peritubular capillaries and of arteries and veins in both murine and human kidneys. TGFβ upregulated mRNA of BAMBI in murine glomerular endothelial cells (mGEC). LPS did not downregulate mRNA for BAMBI in mGEC or in HUVECs. BAMBI mRNA had a half-life of only 60 minutes and was stabilized by cycloheximide, indicating post-transcriptional regulation due to AU-rich elements, which we identified in the 3′ untranslated sequence of both the human and murine BAMBI gene. BAMBI protein turnover was studied in HUVECs with BAMBI overexpression using a lentiviral system. Serum starvation as an inducer of autophagy caused marked BAMBI degradation, which could be totally prevented by inhibition of lysosomal and autolysosomal degradation with bafilomycin, and partially by inhibition of autophagy with 3-methyladenine, but not by proteasomal inhibitors. Rapamycin activates autophagy by inhibiting TOR, and resulted in BAMBI protein degradation. Both serum starvation and rapamycin increased the conversion of the autophagy marker LC3 from LC3-I to LC3-II and also enhanced co-staining for BAMBI and LC3 in autolysosomal vesicles.Conclusions/Significance1. BAMBI localizes to endothelial cells in the kidney and to HUVECs. 2. BAMBI mRNA is regulated by post-transcriptional mechanisms. 3. BAMBI protein is regulated by lysosomal and autolysosomal degradation. The endothelial localization and the quick turnover of BAMBI may indicate novel, yet to be defined functions of this modulator for TGFβ and Wnt protein actions in the renal vascular endothelium in health and disease.

BackgroundIntimal injury rapidly activates TGFβ and enhances vascular repair by the growth of endothelial (EC) and vascular smooth muscle cells (VSMC). The response to the TGFβ family of growth factors can be modified by BAMBI (BMP, Activin, Membrane Bound Inhibitor) acting as a non-signaling, competitive antagonist of TGFβ type I receptors such as ALK 1 and 5. In vivo the effect of BAMBI will depend on its cell-specific expression and of that of the ALK type receptors. We recently reported EC restricted BAMBI expression and genetic elimination of BAMBI resulting in an in vitro and in vivo phenotype characterized by endothelial activation and proliferation involving alternative pathway activation by TGFβ through ALK 1.Methodology/Principal FindingsTo test the hypothesis that BAMBI modulates arterial response to injury via its effects on endothelial repair and arterial wall neovascularization we used a model of femoral arterial denudation injury in wild type (WT) and BAMBI−/− mice. Arterial response was evaluated at 2 and 4 weeks after luminal endothelial denudation of femoral arteries. The BAMBI−/− genotype mice showed accelerated luminal endothelial repair at 2 weeks and a highly unusual increase in arterial wall neovascularization compared to WT mice. The exuberant intimal and medial neovessel formation with BAMBI−/− genotype was also associated with significant red blood cell extravasation. The bleeding into the neointima at 2 weeks transiently increased it’s area in the BAMBI−/−genotype despite the faster luminal endothelial repair in this group. Vascular smooth muscle cells were decreased at 2 weeks in BAMBI−/− mice, but comparable to wild type at 4 weeks.Conclusions/SignificanceThe absence of BAMBI results in a highly unusual surge in arterial wall neovascularization that surprisingly mimiks features of intra-plaque hemorrhage of advanced atheroma in a mechanical injury model. This suggests important effects of BAMBI on arterial EC homeostasis that need to be further studied in a model of inflammatory atherosclerosis.