We identified activating KRAS mutations in the majority of tissue samples of arteriovenous malformations of the brain that we analyzed. We propose that these malformations develop as a result of KRAS-induced activation of the MAPK-ERK signaling pathway in brain endothelial cells. (Funded by the Swiss Cancer League and others.).

Activation of inflammatory pathways in the endothelium contributes to vascular diseases, including sepsis and atherosclerosis. We demonstrate that miR-146a and miR-146b are induced in endothelial cells upon exposure to pro-inflammatory cytokines. Despite the rapid transcriptional induction of the miR-146a/b loci, which is in part mediated by EGR-3, miR-146a/b induction is delayed and sustained compared to the expression of leukocyte adhesion molecules, and in fact coincides with the down-regulation of inflammatory gene expression. We demonstrate that miR-146 negatively regulates inflammation. Over-expression of miR-146a blunts endothelial activation, while knock-down of miR-146a/b in vitro or deletion of miR-146a in mice has the opposite effect. MiR-146 represses the pro-inflammatory NF-κB pathway as well as the MAP kinase pathway and downstream EGR transcription factors. Finally, we demonstrate that HuR, an RNA binding protein that promotes endothelial activation by suppressing expression of endothelial nitric oxide synthase (eNOS), is a novel miR-146 target. Thus, we uncover an important negative feedback regulatory loop that controls pro-inflammatory signalling in endothelial cells that may impact vascular inflammatory diseases.

Summary Vegf signaling specifies arterial fate during early vascular development by inducing the transcription of Delta-like 4 (Dll4), the earliest Notch ligand gene expressed in arterial precursor cells (aPCs). Dll4 expression precedes that of Notch receptors in arteries, and factors that direct its arterial-specific expression are not known. To identify the transcriptional program that initiates arterial Dll4 expression we characterized an arterial-specific and Vegf-responsive enhancer of Dll4. Our findings demonstrate that Notch signaling is not required for initiation of Dll4 expression in arteries, and suggest that Notch instead functions as a maintenance factor. Importantly, we find that Vegf signaling activates MAP kinase (MAPK)-dependent ETS factors in the arterial endothelium to drive expression of Dll4, as well as Notch4. These findings identify a Vegf/MAPK-dependent transcriptional pathway that specifies arterial identity by activating Notch signaling components, and illustrate how signaling cascades can modulate broadly expressed transcription factors to achieve tissue-specific transcriptional outputs.

Key Points• Quiescent endothelial cells secrete extracellular vesicles that can be taken up by monocytes to suppress their activation.• MiR-10a is transferred to monocytic cells and inhibits the activation of the proinflammatory nuclear factor kB pathway.The blood contains high concentrations of circulating extracellular vesicles (EVs), and their levels and contents are altered in several disease states, including cardiovascular disease. However, the function of circulating EVs, especially the microRNAs (miRNAs) that they contain, are poorly understood. We sought to determine the effect of secreted vesicles produced by quiescent endothelial cells (ECs) on monocyte inflammatory responses and to assess whether transfer of microRNAs occurs between these cells. We observed that monocytic cells cocultured (but not in contact) with ECs were refractory to inflammatory activation. Further characterization revealed that endothelium-derived EVs (EC-EVs) suppressed monocyte activation by enhancing immunomodulatory responses and diminishing proinflammatory responses. EVs isolated from mouse plasma also suppressed monocyte activation. Importantly, injection of EC-EVs in vivo repressed monocyte/macrophage activation, confirming our in vitro findings. We found that several antiinflammatory microRNAs were elevated in EC-EV-treated monocytes. In particular, miR-10a was transferred to monocytic cells from EC-EVs and could repress inflammatory signaling through the targeting of several components of the NF-kB pathway, including IRAK4. Our findings reveal that ECs secrete EVs that can modulate monocyte activation and suggest that altered EV secretion and/or microRNA content may affect vascular inflammation in the setting of cardiovascular disease. (Blood. 2015;125(20):3202-3212)

Rationale: We previously identified somatic activating mutations in the KRAS gene in the endothelium of the majority of human sporadic brain arteriovenous malformations (bAVMs); a disorder characterized by direct connections between arteries and veins. However, whether this genetic abnormality alone is sufficient for lesion formation, as well as how active KRAS signaling contributes to AVM formation, remains unknown. Objective: To establish the first in vivo models of somatic KRAS gain of function in the endothelium in both mice and zebrafish in order to directly observe the phenotypic consequences of constitutive KRAS activity at a cellular level in vivo, and to test potential therapeutic interventions for AVMs. Methods and Results: Using both postnatal and adult mice, as well as embryonic zebrafish, we demonstrate that endothelial-specific gain of function mutations in Kras (G12D or G12V) are sufficient to induce bAVMs. Active KRAS signaling leads to altered endothelial cell morphogenesis and increased cell size, ectopic sprouting, expanded vessel lumen diameter, and direct connections between arteries and veins. Furthermore, we show that these lesions are not associated with altered endothelial growth dynamics or a lack of proper arteriovenous identity, but instead appear to feature exuberant angiogenic signaling. Finally, we demonstrate that KRAS-dependent AVMs in zebrafish are refractory to inhibition of the downstream effector PI3K, but instead require active MEK signaling. Conclusions: We demonstrate that active KRAS expression in the endothelium is sufficient for bAVM formation, even in the setting of uninjured adult vasculature. Furthermore, the finding that KRAS-dependent lesions are reversible in zebrafish suggests that MEK inhibition may represent a promising therapeutic treatment for AVM patients.

The transcriptional pathways activated downstream of vascular endothelial growth factor (VEGF) signaling during angiogenesis remain incompletely characterized. By assessing the signals responsible for induction of the Notch ligand delta-like 4 (DLL4) in endothelial cells, we find that activation of the MAPK/ERK pathway mirrors the rapid and dynamic induction of DLL4 transcription and that this pathway is required for DLL4 expression. Furthermore, VEGF/ERK signaling induces phosphorylation and activation of the ETS transcription factor ERG, a prerequisite for DLL4 induction. Transcription of DLL4 coincides with dynamic ERG-dependent recruitment of the transcriptional co-activator p300. Genome-wide gene expression profiling identified a network of VEGF-responsive and ERG-dependent genes, and ERG chromatin immunoprecipitation (ChIP)-seq revealed the presence of conserved ERG-bound putative enhancer elements near these target genes. Functional experiments performed in vitro and in vivo confirm that this network of genes requires ERK, ERG and p300 activity. Finally, genome-editing and transgenic approaches demonstrate that a highly conserved ERG-bound enhancer located upstream of HLX (which encodes a transcription factor implicated in sprouting angiogenesis) is required for its VEGFmediated induction. Collectively, these findings elucidate a novel transcriptional pathway contributing to VEGF-dependent angiogenesis.

Objective: Coronavirus disease 2019 (COVID-19) is a global pandemic involving >5 500 000 cases worldwide as of May 26, 2020. The culprit is the severe acute respiratory syndrome coronavirus-2, which invades cells by binding to angiotensin-converting enzyme 2. While the majority of patients mount an appropriate antiviral response and recover at home, others progress to respiratory distress requiring hospital admission for supplemental oxygen. In severe cases, deterioration to acute respiratory distress syndrome necessitating mechanical ventilation, development of severe thrombotic events, or cardiac injury and dysfunction occurs. In this review, we highlight what is known to date about coronavirus disease 2019 and cardiovascular risk, focusing in on the putative role of the endothelium in disease susceptibility and pathogenesis. Approach and Results: Cytokine-driven vascular leak in the lung alveolar-endothelial interface facilitates acute lung injury in the setting of viral infection. Given that the virus affects multiple organs, including the heart, it likely gains access into systemic circulation by infecting or passing from the respiratory epithelium to the endothelium for viral dissemination. Indeed, cardiovascular complications of coronavirus disease 2019 are highly prevalent and include acute cardiac injury, myocarditis, and a hypercoagulable state, all of which may be influenced by altered endothelial function. Notably, the disease course is worse in individuals with preexisting comorbidities that involve endothelial dysfunction and may be linked to elevated ACE2 (angiotensin-converting enzyme 2) expression, such as diabetes mellitus, hypertension, and cardiovascular disease. Conclusions: Rapidly emerging data on coronavirus disease 2019, together with results from studies on severe acute respiratory syndrome coronavirus-1, are providing insight into how endothelial dysfunction may contribute to the pandemic that is paralyzing the globe. This may, in turn, inform the design of biomarkers predictive of disease course, as well as therapeutics targeting pathogenic endothelial responses.

No abstract