Transcriptomic analysis identifies novel targets for individual bone morphogenetic protein type 1 receptors in endothelial cells

Bone Morphogenetic Protein-4 Impairs Retinal Endothelial Cell Barrier, a Potential Role in Diabetic Retinopathy

Bone Morphogenetic Protein 4 (BMP4) is a secreted growth factor of the Transforming Growth Factor beta (TGFβ) superfamily. The goal of this study was to test whether BMP4 contributes to the pathogenesis of diabetic retinopathy (DR). Immunofluorescence of BMP4 and the vascular marker isolectin-B4 was conducted on retinal sections of diabetic and non-diabetic human and experimental mice. We used Akita mice as a model for type-1 diabetes. Proteins were extracted from the retina of postmortem human eyes and 6-month diabetic Akita mice and age-matched control. BMP4 levels were measured by Western blot (WB). Human retinal endothelial cells (HRECs) were used as an in vitro model. HRECs were treated with BMP4 (50 ng/mL) for 48 h. The levels of phospho-smad 1/5/9 and phospho-p38 were measured by WB. BMP4-treated and control HRECs were also immunostained with anti-Zo-1. We also used electric cell-substrate impedance sensing (ECIS) to calculate the transcellular electrical resistance (TER) under BMP4 treatment in the presence and absence of noggin (200 ng/mL), LDN193189 (200 nM), LDN212854 (200 nM) or inhibitors of vascular endothelial growth factor receptor 2 (VEGFR2; SU5416, 10 μM), p38 (SB202190, 10 μM), ERK (U0126, 10 μM) and ER stress (Phenylbutyric acid or PBA, 30 μmol/L). The impact of BMP4 on matrix metalloproteinases (MMP2 and MMP9) was also evaluated using specific ELISA kits. Immunofluorescence of human and mouse eyes showed increased BMP4 immunoreactivity, mainly localized in the retinal vessels of diabetic humans and mice compared to the control. Western blots of retinal proteins showed a significant increase in BMP4 expression in diabetic humans and mice compared to the control groups (p < 0.05). HRECs treated with BMP4 showed a marked increase in phospho-smad 1/5/9 (p = 0.039) and phospho-p38 (p = 0.013). Immunofluorescence of Zo-1 showed that BMP4-treated cells exhibited significant barrier disruption. ECIS also showed a marked decrease in TER of HRECs by BMP4 treatment compared to vehicle-treated HRECs (p < 0.001). Noggin, LDN193189, LDN212854, and inhibitors of p38 and VEGFR2 significantly mitigated the effects of BMP4 on the TER of HRECs. Our finding provides important insights regarding the role of BMP4 as a potential player in retinal endothelial cell dysfunction in diabetic retinopathy and could be a novel target to preserve the blood-retinal barrier during diabetes.

Environmental and intrinsic modulations of venous differentiation

Endothelial cells in veins differ in morphology, function and gene expression from those in arteries and lymphatics. Understanding how venous and arterial identities are induced during development is required to understand how arterio-venous malformations occur, and to improve the outcome of vein grafts in surgery by promoting arterialization of veins. To identify factors that promote venous endothelial cell fate in vivo, we isolated veins from quail embryos, at different developmental stages, that were grafted into the coelom of chick embryos. Endothelial cells migrated out from the grafted vein and their colonization of host veins and/or arteries was quantified. We show that venous fate is promoted by sympathetic vessel innervation at embryonic day 11. Removal of sympathetic innervation decreased vein colonization, while norepinephrine enhanced venous colonization. BMP treatment or inhibition of ERK enhanced venous fate, revealing environmental neurotransmitter and BMP signaling and intrinsic ERK inhibition as actors in venous fate acquisition. We also identify the BMP antagonist Noggin as a potent mediator of venous arterialization.

The versatility and paradox of BMP signaling in endothelial cell behaviors and blood vessel function

Blood vessels expand via sprouting angiogenesis, and this process involves numerous endothelial cell behaviors, such as collective migration, proliferation, cell–cell junction rearrangements, and anastomosis and lumen formation. Subsequently, blood vessels remodel to form a hierarchical network that circulates blood and delivers oxygen and nutrients to tissue. During this time, endothelial cells become quiescent and form a barrier between blood and tissues that regulates transport of liquids and solutes. Bone morphogenetic protein (BMP) signaling regulates both proangiogenic and homeostatic endothelial cell behaviors as blood vessels form and mature. Almost 30 years ago, human pedigrees linked BMP signaling to diseases associated with blood vessel hemorrhage and shunts, and recent work greatly expanded our knowledge of the players and the effects of vascular BMP signaling. Despite these gains, there remain paradoxes and questions, especially with respect to how and where the different and opposing BMP signaling outputs are regulated. This review examines endothelial cell BMP signaling in vitro and in vivo and discusses the paradox of BMP signals that both destabilize and stabilize endothelial cell behaviors.

Ubiquitin-conjugating enzyme V variant 1 enables cellular responses toward fibroblast growth factor signaling in endothelium

Ubiquitination has been shown to provide an essential regulatory role in modulating the duration and amplitude of the signaling activity in angiogenesis. While successive enzymatic reactions mediated by three distinct types of enzymes commonly known as E1, E2, and E3 are required for ubiquitination, the role of E3s which govern the final step of ubiquitination has been extensively analyzed in angiogenesis. In contrast, the role of E2s, which determine the context and functional consequences of ubiquitination, remains largely unknown with respect to angiogenesis. To better elucidate the role of E2s in modulating endothelial behaviors during angiogenesis, we first systematically analyze the expression pattern of E2s in endothelial cells (ECs) using previously published scRNA-seq data and identify ubiquitin-conjugating enzyme variant 1 (UBE2V1), an unconventional E2 without innate catalytic activity, as one of the most abundantly expressed E2s in ECs. While ubiquitously expressed in diverse cell types, abrogation of UBE2V1 significantly impairs proliferation and viability of human umbilical vein endothelial cells (HUVECs) without affecting other cell types, suggesting that UBE2V1 is likely to possess nonredundant functions in ECs. Consistent with this idea, UBE2V1 appears to be critical for morphogenesis and migration of ECs during angiogenesis. Interestingly, we find that UBE2V1 is essential for fibroblast growth factor 2 (FGF2)-induced angiogenesis, but appears to have minor effects on vascular endothelial growth factor-A-induced angiogenesis in vitro as well as in vivo. Therefore, it seems that UBE2V1 could enable ECs to distinguish two related yet distinct angiogenic cues. Mechanistically, we show that UBE2V1 promotes ubiquitination of MEK kinase 1, a key mediator of FGF2 signaling, to enhance phosphorylation of extracellular signal-regulated kinase 1/2 in HUVECs. Taken together, our results illustrate the unique role of UBE2V1 as a key modulator for angiogenic behaviors in ECs.