Venous identity requires BMP signalling through ALK3

Pathophysiology in Brain Arteriovenous Malformations: Focus on Endothelial Dysfunctions and Endothelial-to-Mesenchymal Transition

Brain arteriovenous malformations (bAVMs) substantially increase the risk for intracerebral hemorrhage (ICH), which is associated with significant morbidity and mortality. However, the treatment options for bAVMs are severely limited, primarily relying on invasive methods that carry their own risks for intraoperative hemorrhage or even death. Currently, there are no pharmaceutical agents shown to treat this condition, primarily due to a poor understanding of bAVM pathophysiology. For the last decade, bAVM research has made significant advances, including the identification of novel genetic mutations and relevant signaling in bAVM development. However, bAVM pathophysiology is still largely unclear. Further investigation is required to understand the detailed cellular and molecular mechanisms involved, which will enable the development of safer and more effective treatment options. Endothelial cells (ECs), the cells that line the vascular lumen, are integral to the pathogenesis of bAVMs. Understanding the fundamental role of ECs in pathological conditions is crucial to unraveling bAVM pathophysiology. This review focuses on the current knowledge of bAVM-relevant signaling pathways and dysfunctions in ECs, particularly the endothelial-to-mesenchymal transition (EndMT).

ACVR1/ALK2-p21 signaling axis modulates proliferation of the venous endothelium in the retinal vasculature

The proliferation of the endothelium is a highly coordinated process to ensure the emergence, expansion, and homeostasis of the vasculature. While Bone Morphogenetic Protein (BMP) signaling fine-tunes the behaviors of endothelium in health and disease, how BMP signaling influences the proliferation of endothelium and therefore, modulates angiogenesis remains largely unknown. Here, we evaluated the role of Activin A Type I Receptor (ACVR1/ALK2), a key BMP receptor in the endothelium, in modulating the proliferation of endothelial cells. We show that ACVR1/ALK2 is a key modulator for the proliferation of endothelium in the retinal vessels. Loss of endothelial ALK2 leads to a significant reduction in endothelial proliferation and results in fewer branches/endothelial cells in the retinal vessels. Interestingly, venous endothelium appears to be more susceptible to ALK2 deletion. Mechanistically, ACVR1/ALK2 inhibits the expression of CDKN1A/p21, a critical negative regulator of cell cycle progression, in a SMAD1/5-dependent manner, thereby enabling the venous endothelium to undergo active proliferation by suppressing CDKN1A/p21. Taken together, our findings show that BMP signaling mediated by ACVR1/ALK2 provides a critical yet previously underappreciated input to modulate the proliferation of venous endothelium, thereby fine-tuning the context of angiogenesis in health and disease.

Cracking the code of the cardiovascular enigma: hPSC-derived endothelial cells unveil the secrets of endothelial dysfunction

Endothelial dysfunction is a central contributor to the development of most cardiovascular diseases and is characterised by the reduced synthesis or bioavailability of the vasodilator nitric oxide together with other abnormalities such as inflammation, senescence, and oxidative stress. The use of patient-specific and genome-edited human pluripotent stem cell-derived endothelial cells (hPSC-ECs) has shed novel insights into the role of endothelial dysfunction in cardiovascular diseases with strong genetic components such as genetic cardiomyopathies and pulmonary arterial hypertension. However, their utility in studying complex multifactorial diseases such as atherosclerosis, metabolic syndrome and heart failure poses notable challenges. In this review, we provide an overview of the different methods used to generate and characterise hPSC-ECs before comprehensively assessing their effectiveness in cardiovascular disease modelling and high-throughput drug screening. Furthermore, we explore current obstacles that will need to be overcome to unleash the full potential of hPSC-ECs in facilitating patient-specific precision medicine. Addressing these challenges holds great promise in advancing our understanding of intricate cardiovascular diseases and in tailoring personalised therapeutic strategies.

Parenchymal cues define Vegfa-driven venous angiogenesis by activating a sprouting competent venous endothelial subtype

Formation of organo-typical vascular networks requires cross-talk between differentiating parenchymal cells and developing blood vessels. Here we identify a Vegfa driven venous sprouting process involving parenchymal to vein cross-talk regulating venous endothelial Vegfa signaling strength and subsequent formation of a specialized angiogenic cell, prefabricated with an intact lumen and pericyte coverage, termed L-Tip cell. L-Tip cell selection in the venous domain requires genetic interaction between vascular Aplnra and Kdrl in a subset of venous endothelial cells and exposure to parenchymal derived Vegfa and Apelin. Parenchymal Esm1 controls the spatial positioning of venous sprouting by fine-tuning local Vegfa availability. These findings may provide a conceptual framework for understanding how Vegfa generates organo-typical vascular networks based on the selection of competent endothelial cells, induced via spatio-temporal control of endothelial Kdrl signaling strength involving multiple parenchymal derived cues generated in a tissue dependent metabolic context.

Eph-ephrin signaling couples endothelial cell sorting and arterial specification

Cell segregation allows the compartmentalization of cells with similar fates during morphogenesis, which can be enhanced by cell fate plasticity in response to local molecular and biomechanical cues. Endothelial tip cells in the growing retina, which lead vessel sprouts, give rise to arterial endothelial cells and thereby mediate arterial growth. Here, we have combined cell type-specific and inducible mouse genetics, flow experiments in vitro, single-cell RNA sequencing and biochemistry to show that the balance between ephrin-B2 and its receptor EphB4 is critical for arterial specification, cell sorting and arteriovenous patterning. At the molecular level, elevated ephrin-B2 function after loss of EphB4 enhances signaling responses by the Notch pathway, VEGF and the transcription factor Dach1, which is influenced by endothelial shear stress. Our findings reveal how Eph-ephrin interactions integrate cell segregation and arteriovenous specification in the vasculature, which has potential relevance for human vascular malformations caused by EPHB4 mutations.

Fundamental considerations for designing endothelialized in vitro models of thrombosis

Endothelialized in vitro models for cardiovascular disease have contributed greatly to our current understanding of the complex molecular mechanisms underlying thrombosis. To further elucidate these mechanisms, it is important to consider which fundamental aspects to incorporate into an in vitro model. In this review, we will focus on the design of in vitro endothelialized models of thrombosis. Expanding our understanding of the relation and interplay between the different pathways involved will rely in part on complex models that incorporate endothelial cells, blood, the extracellular matrix, and flow. Importantly, the use of tissue-specific endothelial cells will help in understanding the heterogeneity in thrombotic responses between different vascular beds. The dynamic and complex responses of endothelial cells to different shear rates underlines the importance of incorporating appropriate shear in in vitro models. Alterations in vascular extracellular matrix composition, availability of bioactive molecules, and gradients in concentration and composition of these molecules can all regulate the function of both endothelial cells and perivascular cells. Factors modulating these elements in in vitro models should therefore be considered carefully depending on the research question at hand. As the complexity of in vitro models increases, so can the variability. A bottom-up approach to designing such models will remain an important tool for researchers studying thrombosis. As new techniques are continuously being developed and new pathways are brought to light, research question-dependent considerations will have to be made regarding what aspects of thrombosis to include in in vitro models.

Endothelial SMAD1/5 signaling couples angiogenesis to osteogenesis in juvenile bone

Skeletal development depends on coordinated angiogenesis and osteogenesis. Bone morphogenetic proteins direct bone formation in part by activating SMAD1/5 signaling in osteoblasts. However, the role of SMAD1/5 in skeletal endothelium is unknown. Here, we found that endothelial cell-conditional SMAD1/5 depletion in juvenile mice caused metaphyseal and diaphyseal hypervascularity, resulting in altered trabecular and cortical bone formation. SMAD1/5 depletion induced excessive sprouting and disrupting the morphology of the metaphyseal vessels, with impaired anastomotic loop formation at the chondro-osseous junction. Endothelial SMAD1/5 depletion impaired growth plate resorption and, upon long-term depletion, abrogated osteoprogenitor recruitment to the primary spongiosa. Finally, in the diaphysis, endothelial SMAD1/5 activity was necessary to maintain the sinusoidal phenotype, with SMAD1/5 depletion inducing formation of large vascular loops and elevated vascular permeability. Together, endothelial SMAD1/5 activity sustains skeletal vascular morphogenesis and function and coordinates growth plate remodeling and osteoprogenitor recruitment dynamics in juvenile mouse bone.

Building human artery and vein endothelial cells from pluripotent stem cells, and enduring mysteries surrounding arteriovenous development

Owing to their manifold roles in health and disease, there have been intense efforts to synthetically generate blood vessels in vitro from human pluripotent stem cells (hPSCs). However, there are multiple types of blood vessel, including arteries and veins, which are molecularly and functionally different. How can we specifically generate either arterial or venous endothelial cells (ECs) from hPSCs in vitro? Here, we summarize how arterial or venous ECs arise during embryonic development. VEGF and NOTCH arbitrate the bifurcation of arterial vs. venous ECs in vivo. While manipulating these two signaling pathways biases hPSC differentiation towards arterial and venous identities, efficiently generating these two subtypes of ECs has remained challenging until recently. Numerous questions remain to be fully addressed. What is the complete identity, timing and combination of extracellular signals that specify arterial vs. venous identities? How do these extracellular signals intersect with fluid flow to modulate arteriovenous fate? What is a unified definition for endothelial progenitors or angioblasts, and when do arterial vs. venous potentials segregate? How can we regulate hPSC-derived arterial and venous ECs in vitro, and generate organ-specific ECs? In turn, answers to these questions could avail the production of arterial and venous ECs from hPSCs, accelerating vascular research, tissue engineering, and regenerative medicine.

Molecular and Spatial Signatures of Mouse Embryonic Endothelial Cells at Single-Cell Resolution

BACKGROUND

Mature endothelial cells (ECs) are heterogeneous, with subtypes defined by tissue origin and position within the vascular bed (ie, artery, capillary, vein, and lymphatic). How this heterogeneity is established during the development of the vascular system, especially arteriovenous specification of ECs, remains incompletely characterized.

METHODS

We used droplet-based single-cell RNA sequencing and multiplexed error-robust fluorescence in situ hybridization to define EC and EC progenitor subtypes from E9.5, E12.5, and E15.5 mouse embryos. We used trajectory inference to analyze the specification of arterial ECs (aECs) and venous ECs (vECs) from EC progenitors. Network analysis identified candidate transcriptional regulators of arteriovenous differentiation, which we tested by CRISPR (clustered regularly interspaced short palindromic repeats) loss of function in human-induced pluripotent stem cells undergoing directed differentiation to aECs or vECs (human-induced pluripotent stem cell-aECs or human-induced pluripotent stem cell-vECs).

RESULTS

From the single-cell transcriptomes of 7682 E9.5 to E15.5 ECs, we identified 19 EC subtypes, including Etv2+Bnip3+ EC progenitors. Spatial transcriptomic analysis of 15 448 ECs provided orthogonal validation of these EC subtypes and established their spatial distribution. Most embryonic ECs were grouped by their vascular-bed types, while ECs from the brain, heart, liver, and lung were grouped by their tissue origins. Arterial (Eln, Dkk2, Vegfc, and Egfl8), venous (Fam174b and Clec14a), and capillary (Kcne3) marker genes were identified. Compared with aECs, embryonic vECs and capillary ECs shared fewer markers than their adult counterparts. Early capillary ECs with venous characteristics functioned as a branch point for differentiation of aEC and vEC lineages.

CONCLUSIONS

Our results provide a spatiotemporal map of embryonic EC heterogeneity at single-cell resolution and demonstrate that the diversity of ECs in the embryo arises from both tissue origin and vascular-bed position. Developing aECs and vECs share common venous-featured capillary precursors and are regulated by distinct transcriptional regulatory networks.

Transcriptional regulators of arterial and venous identity in the developing mammalian embryo

The complex and hierarchical vascular network of arteries, veins, and capillaries features considerable endothelial heterogeneity, yet the regulatory pathways directing arteriovenous specification, differentiation, and identity are still not fully understood. Recent advances in analysis of endothelial-specific gene-regulatory elements, single-cell RNA sequencing, and cell lineage tracing have both emphasized the importance of transcriptional regulation in this process and shed considerable light on the mechanism and regulation of specification within the endothelium. In this review, we discuss recent advances in our understanding of how endothelial cells acquire arterial and venous identity and the role different transcription factors play in this process.

rasa1-related arteriovenous malformation is driven by aberrant venous signalling

Arteriovenous malformations (AVMs) develop where abnormal endothelial signalling allows direct connections between arteries and veins. Mutations in RASA1, a Ras GTPase activating protein, lead to AVMs in humans and, as we show, in zebrafish rasa1 mutants. rasa1 mutants develop cavernous AVMs that subsume part of the dorsal aorta and multiple veins in the caudal venous plexus (CVP) - a venous vascular bed. The AVMs progressively enlarge and fill with slow-flowing blood. We show that the AVM results in both higher minimum and maximum flow velocities, resulting in increased pulsatility in the aorta and decreased pulsatility in the vein. These hemodynamic changes correlate with reduced expression of the flow-responsive transcription factor klf2a. Remodelling of the CVP is impaired with an excess of intraluminal pillars, which is a sign of incomplete intussusceptive angiogenesis. Mechanistically, we show that the AVM arises from ectopic activation of MEK/ERK in the vein of rasa1 mutants, and that cell size is also increased in the vein. Blocking MEK/ERK signalling prevents AVM initiation in mutants. Alterations in venous MEK/ERK therefore drive the initiation of rasa1 AVMs.

Direct differentiation of human pluripotent stem cells into vascular network along with supporting mural cells

During embryonic development, endothelial cells (ECs) undergo vasculogenesis to form a primitive plexus and assemble into networks comprised of mural cell-stabilized vessels with molecularly distinct artery and vein signatures. This organized vasculature is established prior to the initiation of blood flow and depends on a sequence of complex signaling events elucidated primarily in animal models, but less studied and understood in humans. Here, we have developed a simple vascular differentiation protocol for human pluripotent stem cells that generates ECs, pericytes, and smooth muscle cells simultaneously. When this protocol is applied in a 3D hydrogel, we demonstrate that it recapitulates the dynamic processes of early human vessel formation, including acquisition of distinct arterial and venous fates, resulting in a vasculogenesis angiogenesis model plexus (VAMP). The VAMP captures the major stages of vasculogenesis, angiogenesis, and vascular network formation and is a simple, rapid, scalable model system for studying early human vascular development in vitro.

Mesenchymal Stromal Cells Facilitate Tip Cell Fusion Downstream of BMP-Mediated Venous Angiogenesis-Brief Report

BACKGROUND

The goal of this study was to identify and characterize cell-cell interactions that facilitate endothelial tip cell fusion downstream of BMP (bone morphogenic protein)-mediated venous plexus formation.

METHODS

High resolution and time-lapse imaging of transgenic reporter lines and loss-of-function studies were carried out to study the involvement of mesenchymal stromal cells during venous angiogenesis.

RESULTS

BMP-responsive stromal cells facilitate timely and precise fusion of venous tip cells during developmental angiogenesis.

CONCLUSIONS

Stromal cells are required for anastomosis of venous tip cells in the embryonic caudal hematopoietic tissue.

BMPR1A promotes ID2-ZEB1 interaction to suppress excessive endothelial to mesenchymal transition

AIMS

Components of bone morphogenetic protein (BMP) signalling have been implicated in both pathogenesis of pulmonary arterial hypertension (PAH) and endothelial-mesenchymal transition (EndoMT). In particular, the importance of BMP type 2 receptor in these processes has been extensively analysed. However, the contribution of BMP type 1 receptors (BMPR1s) to the onset of PAH and EndoMT remains poorly understood. BMPR1A, one of BMPR1s, was recently implicated in the pathogenesis of PAH, and was found to be down-regulated in the lungs of PAH patients, neither the downstream mechanism nor its contribution to EndoMT has been described. Therefore, we aim to delineate the role of endothelial BMPR1A in modulating EndoMT and pathogenesis of PAH.

METHODS AND RESULTS

We find that BMPR1A knockdown in endothelial cells (ECs) induces hallmarks of EndoMT, and deletion of endothelial Bmpr1a in adult mice (Bmpr1aiECKO) leads to development of PAH-like symptoms due to excessive EndoMT. By lineage tracing, we show that endothelial-derived smooth muscle cells are increased in endothelial Bmpr1a-deleted mice. Mechanistically, we identify ZEB1 as a primary target for BMPR1A in this setting; upon BMPR1A activation, ID2 physically interacts and sequesters ZEB1 to attenuate transcription of Tgfbr2, which in turn lowers the responses of ECs towards transforming growth factor beta (TGFβ) stimulation and prevents excessive EndoMT. In Bmpr1aiECKO mice, administering endothelial targeting lipid nanoparticles containing siRNA against Tgfbr2 effectively ameliorate PAH, reiterating the importance of BMPR1A-ID2/ZEB1-TGFBR2 axis in modulating progression of EndoMT and pathogenesis of PAH.

CONCLUSIONS

We demonstrate that BMPR1A is key to maintain endothelial identity and to prevent excessive EndoMT. We identify BMPR1A-induced interaction between ID2 and ZEB1 is the key regulatory step for onset of EndoMT and pathogenesis of PAH. Our findings indicate that BMPR1A-ID2/ZEB1-TGFBR2 signalling axis could serve as a potential novel therapeutic target for PAH and other EndoMT-related vascular disorders.

The tissue-specific transcriptional landscape underlines the involvement of endothelial cells in health and disease

Endothelial cells (ECs) that line vascular and lymphatic vessels are being increasingly recognized as important to organ function in health and disease. ECs participate not only in the trafficking of gases, metabolites, and cells between the bloodstream and tissues but also in the angiocrine-based induction of heterogeneous parenchymal cells, which are unique to their specific tissue functions. The molecular mechanisms regulating EC heterogeneity between and within different tissues are modeled during embryogenesis and become fully established in adults. Any changes in adult tissue homeostasis induced by aging, stress conditions, and various noxae may reshape EC heterogeneity and induce specific transcriptional features that condition a functional phenotype. Heterogeneity is sustained via specific genetic programs organized through the combinatory effects of a discrete number of transcription factors (TFs) that, at the single tissue-level, constitute dynamic networks that are post-transcriptionally and epigenetically regulated. This review is focused on outlining the TF-based networks involved in EC specialization and physiological and pathological stressors thought to modify their architecture.

Temporally and regionally distinct morphogenetic processes govern zebrafish caudal fin blood vessel network expansion

Blood vessels form elaborate networks that depend on tissue-specific signalling pathways and anatomical structures to guide their growth. However, it is not clear which morphogenetic principles organize the stepwise assembly of the vasculature. We therefore performed a longitudinal analysis of zebrafish caudal fin vascular assembly, revealing the existence of temporally and spatially distinct morphogenetic processes. Initially, vein-derived endothelial cells (ECs) generated arteries in a reiterative process requiring vascular endothelial growth factor (Vegf), Notch and cxcr4a signalling. Subsequently, veins produced veins in more proximal fin regions, transforming pre-existing artery-vein loops into a three-vessel pattern consisting of an artery and two veins. A distinct set of vascular plexuses formed at the base of the fin. They differed in their diameter, flow magnitude and marker gene expression. At later stages, intussusceptive angiogenesis occurred from veins in distal fin regions. In proximal fin regions, we observed new vein sprouts crossing the inter-ray tissue through sprouting angiogenesis. Together, our results reveal a surprising diversity among the mechanisms generating the mature fin vasculature and suggest that these might be driven by separate local cues.

Vascular endothelial cell development and diversity

Vascular endothelial cells form the inner layer of blood vessels where they have a key role in the development and maintenance of the functional circulatory system and provide paracrine support to surrounding non-vascular cells. Technical advances in the past 5 years in single-cell genomics and in in vivo genetic labelling have facilitated greater insights into endothelial cell development, plasticity and heterogeneity. These advances have also contributed to a new understanding of the timing of endothelial cell subtype differentiation and its relationship to the cell cycle. Identification of novel tissue-specific gene expression patterns in endothelial cells has led to the discovery of crucial signalling pathways and new interactions with other cell types that have key roles in both tissue maintenance and disease pathology. In this Review, we describe the latest findings in vascular endothelial cell development and diversity, which are often supported by large-scale, single-cell studies, and discuss the implications of these findings for vascular medicine. In addition, we highlight how techniques such as single-cell multimodal omics, which have become increasingly sophisticated over the past 2 years, are being utilized to study normal vascular physiology as well as functional perturbations in disease.

Krüppel-Like Factors Orchestrate Endothelial Gene Expression Through Redundant and Non-Redundant Enhancer Networks

Background Proper function of endothelial cells is critical for vascular integrity and organismal survival. Studies over the past 2 decades have identified 2 members of the KLF (Krüppel-like factor) family of proteins, KLF2 and KLF4, as nodal regulators of endothelial function. Strikingly, inducible postnatal deletion of both KLF2 and KLF4 resulted in widespread vascular leak, coagulopathy, and rapid death. Importantly, while transcriptomic studies revealed profound alterations in gene expression, the molecular mechanisms underlying these changes remain poorly understood. Here, we seek to determine mechanisms of KLF2 and KLF4 transcriptional control in multiple vascular beds to further understand their roles as critical endothelial regulators. Methods and Results We integrate chromatin occupancy and transcription studies from multiple transgenic mouse models to demonstrate that KLF2 and KLF4 have overlapping yet distinct binding patterns and transcriptional targets in heart and lung endothelium. Mechanistically, KLFs use open chromatin regions in promoters and enhancers and bind in context-specific patterns that govern transcription in microvasculature. Importantly, this occurs during homeostasis in vivo without additional exogenous stimuli. Conclusions Together, this work provides mechanistic insight behind the well-described transcriptional and functional heterogeneity seen in vascular populations, while also establishing tools into exploring microvascular endothelial dynamics in vivo.

Endothelial SMAD1/5 signaling couples angiogenesis to osteogenesis during long bone growth

Skeletal development depends on coordinated angiogenesis and osteogenesis. Bone morphogenetic proteins direct bone development by activating SMAD1/5 signaling in osteoblasts. However, the role of SMAD1/5 in skeletal endothelium is unknown. Here, we found that endothelial cell-conditional SMAD1/5 depletion in juvenile mice caused metaphyseal and diaphyseal hypervascularity, resulting in altered cancellous and cortical bone formation. SMAD1/5 depletion induced excessive sprouting, disrupting the columnar structure of the metaphyseal vessels and impaired anastomotic loop morphogenesis at the chondro-osseous junction. Endothelial SMAD1/5 depletion impaired growth plate resorption and, upon long term depletion, abrogated osteoprogenitor recruitment to the primary spongiosa. Finally, in the diaphysis, endothelial SMAD1/5 activity was necessary to maintain the sinusoidal phenotype, with SMAD1/5 depletion inducing formation of large vascular loops, featuring elevated endomucin expression, ectopic tip cell formation, and hyperpermeability. Together, endothelial SMAD1/5 activity sustains skeletal vascular morphogenesis and function and coordinates growth plate remodeling and osteoprogenitor recruitment dynamics during bone growth.

Neurovascular Interactions in the Development of the Vasculature

Vertebrates have developed a network of blood vessels and nerves throughout the body that enables them to perform complex higher-order functions and maintain homeostasis. The 16th-century anatomical text 'De humani corporis fabrica' describes the networks of blood vessels and nerves as having a branching pattern in which they are closely aligned and run parallel one to another. This close interaction between adjacent blood vessels and nerves is essential not only for organogenesis during development and repair at the time of tissue damage but also for homeostasis and functional expression of blood vessels and nerves. Furthermore, it is now evident that disruptions in neurovascular interactions contribute to the progression of various diseases including cancer. Therefore, we highlight recent advances in vascular biology research, with a particular emphasis on neurovascular interactions.

Endocardium-to-coronary artery differentiation during heart development and regeneration involves sequential roles of Bmp2 and Cxcl12/Cxcr4

Endocardial cells lining the heart lumen are coronary vessel progenitors during embryogenesis. Re-igniting this developmental process in adults could regenerate blood vessels lost during cardiac injury, but this requires additional knowledge of molecular mechanisms. Here, we use mouse genetics and scRNA-seq to identify regulators of endocardial angiogenesis and precisely assess the role of CXCL12/CXCR4 signaling. Time-specific lineage tracing demonstrated that endocardial cells differentiated into coronary endothelial cells primarily at mid-gestation. A new mouse line reporting CXCR4 activity-along with cell-specific gene deletions-demonstrated it was specifically required for artery morphogenesis rather than angiogenesis. Integrating scRNA-seq data of endocardial-derived coronary vessels from mid- and late-gestation identified a Bmp2-expressing transitioning population specific to mid-gestation. Bmp2 stimulated endocardial angiogenesis in vitro and in injured neonatal mouse hearts. Our data demonstrate how understanding the molecular mechanisms underlying endocardial angiogenesis can identify new potential therapeutic targets promoting revascularization of the injured heart.

Endothelial cell cycle state determines propensity for arterial-venous fate

During blood vessel development, endothelial cells become specified toward arterial or venous fates to generate a circulatory network that provides nutrients and oxygen to, and removes metabolic waste from, all tissues. Arterial-venous specification occurs in conjunction with suppression of endothelial cell cycle progression; however, the mechanistic role of cell cycle state is unknown. Herein, using Cdh5-CreERT2;R26FUCCI2aR reporter mice, we find that venous endothelial cells are enriched for the FUCCI-Negative state (early G1) and BMP signaling, while arterial endothelial cells are enriched for the FUCCI-Red state (late G1) and TGF-β signaling. Furthermore, early G1 state is essential for BMP4-induced venous gene expression, whereas late G1 state is essential for TGF-β1-induced arterial gene expression. Pharmacologically induced cell cycle arrest prevents arterial-venous specification defects in mice with endothelial hyperproliferation. Collectively, our results show that distinct endothelial cell cycle states provide distinct windows of opportunity for the molecular induction of arterial vs. venous fate.

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.

Epigenetic Regulation of Endothelial Cell Lineages During Zebrafish Development-New Insights From Technical Advances

Epigenetic regulation is integral in orchestrating the spatiotemporal regulation of gene expression which underlies tissue development. The emergence of new tools to assess genome-wide epigenetic modifications has enabled significant advances in the field of vascular biology in zebrafish. Zebrafish represents a powerful model to investigate the activity of cis-regulatory elements in vivo by combining technologies such as ATAC-seq, ChIP-seq and CUT&Tag with the generation of transgenic lines and live imaging to validate the activity of these regulatory elements. Recently, this approach led to the identification and characterization of key enhancers of important vascular genes, such as gata2a, notch1b and dll4. In this review we will discuss how the latest technologies in epigenetics are being used in the zebrafish to determine chromatin states and assess the function of the cis-regulatory sequences that shape the zebrafish vascular network.

Shear stress switches the association of endothelial enhancers from ETV/ETS to KLF transcription factor binding sites

Endothelial cells (ECs) lining blood vessels are exposed to mechanical forces, such as shear stress. These forces control many aspects of EC biology, including vascular tone, cell migration and proliferation. Despite a good understanding of the genes responding to shear stress, our insight into the transcriptional regulation of these genes is much more limited. Here, we set out to study alterations in the chromatin landscape of human umbilical vein endothelial cells (HUVEC) exposed to laminar shear stress. To do so, we performed ChIP-Seq for H3K27 acetylation, indicative of active enhancer elements and ATAC-Seq to mark regions of open chromatin in addition to RNA-Seq on HUVEC exposed to 6 h of laminar shear stress. Our results show a correlation of gained and lost enhancers with up and downregulated genes, respectively. DNA motif analysis revealed an over-representation of KLF transcription factor (TF) binding sites in gained enhancers, while lost enhancers contained more ETV/ETS motifs. We validated a subset of flow responsive enhancers using luciferase-based reporter constructs and CRISPR-Cas9 mediated genome editing. Lastly, we characterized the shear stress response in ECs of zebrafish embryos using RNA-Seq. Our results lay the groundwork for the exploration of shear stress responsive elements in controlling EC biology.

Mapping the developing human cardiac endothelium at single-cell resolution identifies MECOM as a regulator of arteriovenous gene expression

AIMS

Coronary vasculature formation is a critical event during cardiac development, essential for heart function throughout perinatal and adult life. However, current understanding of coronary vascular development has largely been derived from transgenic mouse models. The aim of this study was to characterize the transcriptome of the human foetal cardiac endothelium using single-cell RNA sequencing (scRNA-seq) to provide critical new insights into the cellular heterogeneity and transcriptional dynamics that underpin endothelial specification within the vasculature of the developing heart.

METHODS AND RESULTS

We acquired scRNA-seq data of over 10 000 foetal cardiac endothelial cells (ECs), revealing divergent EC subtypes including endocardial, capillary, venous, arterial, and lymphatic populations. Gene regulatory network analyses predicted roles for SMAD1 and MECOM in determining the identity of capillary and arterial populations, respectively. Trajectory inference analysis suggested an endocardial contribution to the coronary vasculature and subsequent arterialization of capillary endothelium accompanied by increasing MECOM expression. Comparative analysis of equivalent data from murine cardiac development demonstrated that transcriptional signatures defining endothelial subpopulations are largely conserved between human and mouse. Comprehensive characterization of the transcriptional response to MECOM knockdown in human embryonic stem cell-derived EC (hESC-EC) demonstrated an increase in the expression of non-arterial markers, including those enriched in venous EC.

CONCLUSIONS

scRNA-seq of the human foetal cardiac endothelium identified distinct EC populations. A predicted endocardial contribution to the developing coronary vasculature was identified, as well as subsequent arterial specification of capillary EC. Loss of MECOM in hESC-EC increased expression of non-arterial markers, suggesting a role in maintaining arterial EC identity.

Finding and Verifying Enhancers for Endothelial-Expressed Genes

Identification and analysis of enhancers for endothelial-expressed genes can provide crucial information regarding their upstream transcriptional regulators. However, enhancer identification can be challenging, particularly for people with limited access or experience of bioinformatics, and transgenic analysis of enhancer activity patterns can be prohibitively expensive. Here we describe how to use publicly available datasets displayed on the UCSC Genome Browser to identify putative endothelial enhancers for mammalian genes. Furthermore, we detail how to utilize mosaic Tol2-mediated transgenesis in zebrafish to verify whether a putative enhancer is capable of directing endothelial-specific patterns of gene expression.

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.

A census of the lung: CellCards from LungMAP

The human lung plays vital roles in respiration, host defense, and basic physiology. Recent technological advancements such as single-cell RNA sequencing and genetic lineage tracing have revealed novel cell types and enriched functional properties of existing cell types in lung. The time has come to take a new census. Initiated by members of the NHLBI-funded LungMAP Consortium and aided by experts in the lung biology community, we synthesized current data into a comprehensive and practical cellular census of the lung. Identities of cell types in the normal lung are captured in individual cell cards with delineation of function, markers, developmental lineages, heterogeneity, regenerative potential, disease links, and key experimental tools. This publication will serve as the starting point of a live, up-to-date guide for lung research at https://www.lungmap.net/cell-cards/. We hope that Lung CellCards will promote the community-wide effort to establish, maintain, and restore respiratory health.

Shear Stress Alterations Activate BMP4/pSMAD5 Signaling and Induce Endothelial Mesenchymal Transition in Varicose Veins

Chronic venous diseases, including varicose veins, are characterized by hemodynamic disturbances due to valve defects, venous insufficiency, and orthostatism. Veins are physiologically low shear stress systems, and how altered hemodynamics drives focal endothelial dysfunction and causes venous remodeling is unknown. Here we demonstrate the occurrence of endothelial to mesenchymal transition (EndMT) in human varicose veins. Moreover, the BMP4-pSMAD5 pathway was robustly upregulated in varicose veins. In vitro flow-based assays using human vein, endothelial cells cultured in microfluidic chambers show that even minimal disturbances in shear stress as may occur in early stages of venous insufficiency induce BMP4-pSMAD5-based phenotype switching. Furthermore, low shear stress at uniform laminar pattern does not induce EndMT in venous endothelial cells. Targeting the BMP4-pSMAD5 pathway with small molecule inhibitor LDN193189 reduced SNAI1/2 expression in venous endothelial cells exposed to disturbed flow. TGFβ inhibitor SB505124 was less efficient in inhibiting EndMT in venous endothelial cells exposed to disturbed flow. We conclude that disturbed shear stress, even in the absence of any oscillatory flow, induces EndMT in varicose veins via activation of BMP4/pSMAD5-SNAI1/2 signaling. The present findings serve as a rationale for the possible use of small molecular mechanotherapeutics in the management of varicose veins.

Endothelial Heterogeneity in Development and Wound Healing

The vasculature is comprised of endothelial cells that are heterogeneous in nature. From tissue resident progenitors to mature differentiated endothelial cells, the diversity of these populations allows for the formation, maintenance, and regeneration of the vascular system in development and disease, particularly during situations of wound healing. Additionally, the de-differentiation and plasticity of different endothelial cells, especially their capacity to undergo endothelial to mesenchymal transition, has also garnered significant interest due to its implication in disease progression, with emphasis on scarring and fibrosis. In this review, we will pinpoint the seminal discoveries defining the phenotype and mechanisms of endothelial heterogeneity in development and disease, with a specific focus only on wound healing.

From remodeling to quiescence: The transformation of the vascular network

The vascular system is essential for embryogenesis, healing, and homeostasis. Dysfunction or deregulated blood vessel function contributes to multiple diseases, including diabetic retinopathy, cancer, hypertension, or vascular malformations. A balance between the formation of new blood vessels, vascular remodeling, and vessel quiescence is fundamental for tissue growth and function. Whilst the major mechanisms contributing to the formation of new blood vessels have been well explored in recent years, vascular remodeling and quiescence remain poorly understood. In this review, we highlight the cellular and molecular mechanisms responsible for vessel remodeling and quiescence during angiogenesis. We further underline how impaired remodeling and/or destabilization of vessel networks can contribute to vascular pathologies. Finally, we speculate how addressing the molecular mechanisms of vascular remodeling and stabilization could help to treat vascular-related disorders.

ETS-dependent enhancers for endothelial-specific expression of serum/glucocorticoid-regulated kinase 1 during mouse embryo development

Serum/glucocorticoid-regulated kinase 1 (SGK1) is predominantly expressed in endothelial cells of mouse embryos, and Sgk1 null mice show embryonic lethality due to impaired vascular formation. However, how the SGK1 expression is controlled in developing vasculature remains unknown. In this study, we first identified a proximal endothelial enhancer through lacZ reporter mouse analyses. The mouse Sgk1 proximal enhancer was narrowed down to the 5' region of the major transcription initiation site, while a human corresponding region possessed relatively weak activity. We then searched for distal enhancer candidates using in silico analyses of publicly available databases for DNase accessibility, RNA polymerase association and chromatin modification. A region approximately 500 kb distant from the human SGK1 gene was conserved in the mouse, and the mouse and human genomic fragments drove transcription restricted to embryonic endothelial cells. Minimal fragments of both proximal and distal enhancers had consensus binding elements for the ETS transcription factors, which were essential for the responsiveness to ERG, FLI1 and ETS1 proteins in luciferase assays and the endothelial lacZ reporter expression in mouse embryos. These results suggest that endothelial SGK1 expression in embryonic vasculature is maintained through at least two ETS-regulated enhancers located in the proximal and distal regions.

The Role of BMP Signaling in Endothelial Heterogeneity

Bone morphogenetic proteins (BMPs), which compose the largest group of the transforming growth factor-β (TGF-ß) superfamily, have been implied to play a crucial role in diverse physiological processes. The most intriguing feature of BMP signaling is that it elicits heterogeneous responses from cells with equivalent identity, thus permitting highly context-dependent signaling outcomes. In endothelial cells (ECs), which are increasingly perceived as a highly heterogeneous population of cells with respect to their morphology, function, as well as molecular characteristics, BMP signaling has shown to elicit diverse and often opposite effects, illustrating the innate complexity of signaling responses. In this review, we provide a concise yet comprehensive overview of how outcomes of BMP signaling are modulated in a context-dependent manner with an emphasis on the underlying molecular mechanisms and summarize how these regulations of the BMP signaling promote endothelial heterogeneity.

Endothelial ontogeny and the establishment of vascular heterogeneity

The establishment of distinct cellular identities was pivotal during the evolution of Metazoa, enabling the emergence of an array of specialized tissues with different functions. In most animals including vertebrates, cell specialization occurs in response to a combination of intrinsic (e.g., cellular ontogeny) and extrinsic (e.g., local environment) factors that drive the acquisition of unique characteristics at the single-cell level. The first functional organ system to form in vertebrates is the cardiovascular system, which is lined by a network of endothelial cells whose organ-specific characteristics have long been recognized. Recent genetic analyses at the single-cell level have revealed that heterogeneity exists not only at the organ level but also between neighboring endothelial cells. Thus, how endothelial heterogeneity is established has become a key question in vascular biology. Drawing upon evidence from multiple organ systems, here we will discuss the role that lineage history may play in establishing endothelial heterogeneity.

The next generation of endothelial differentiation: Tissue-specific ECs

Endothelial cells (ECs) sense and respond to fluid flow and regulate immune cell trafficking in all organs. Despite sharing the same mesodermal origin, ECs exhibit heterogeneous tissue-specific characteristics. Human pluripotent stem cells (hPSCs) can potentially be harnessed to capture this heterogeneity and further elucidate endothelium behavior to satisfy the need for increased accuracy and breadth of disease models and therapeutics. Here, we review current strategies for hPSC differentiation to blood vascular ECs and their maturation into continuous, fenestrated, and sinusoidal tissues. We then discuss the contribution of hPSC-derived ECs to recent advances in organoid development and organ-on-chip approaches.

Association between erythrocyte dynamics and vessel remodelling in developmental vascular networks

Sprouting angiogenesis is an essential vascularization mechanism consisting of sprouting and remodelling. The remodelling phase is driven by rearrangements of endothelial cells (ECs) within the post-sprouting vascular plexus. Prior work has uncovered how ECs polarize and migrate in response to flow-induced wall shear stress (WSS). However, the question of how the presence of erythrocytes (widely known as red blood cells (RBCs)) and their impact on haemodynamics affect vascular remodelling remains unanswered. Here, we devise a computational framework to model cellular blood flow in developmental mouse retina. We demonstrate a previously unreported highly heterogeneous distribution of RBCs in primitive vasculature. Furthermore, we report a strong association between vessel regression and RBC hypoperfusion, and identify plasma skimming as the driving mechanism. Live imaging in a developmental zebrafish model confirms this association. Taken together, our results indicate that RBC dynamics are fundamental to establishing the regional WSS differences driving vascular remodelling via their ability to modulate effective viscosity.

Mechanisms and cell lineages in lymphatic vascular development

Lymphatic vessels have critical roles in both health and disease and their study is a rapidly evolving area of vascular biology. The consensus on how the first lymphatic vessels arise in the developing embryo has recently shifted. Originally, they were thought to solely derive by sprouting from veins. Since then, several studies have uncovered novel cellular mechanisms and a diversity of contributing cell lineages in the formation of organ lymphatic vasculature. Here, we review the key mechanisms and cell lineages contributing to lymphatic development, discuss the advantages and limitations of experimental techniques used for their study and highlight remaining knowledge gaps that require urgent attention. Emerging technologies should accelerate our understanding of how lymphatic vessels develop normally and how they contribute to disease.

To be or not to be: endothelial cell plasticity in development, repair, and disease

Endothelial cells display an extraordinary plasticity both during development and throughout adult life. During early development, endothelial cells assume arterial, venous, or lymphatic identity, while selected endothelial cells undergo additional fate changes to become hematopoietic progenitor, cardiac valve, and other cell types. Adult endothelial cells are some of the longest-lived cells in the body and their participation as stable components of the vascular wall is critical for the proper function of both the circulatory and lymphatic systems, yet these cells also display a remarkable capacity to undergo changes in their differentiated identity during injury, disease, and even normal physiological changes in the vasculature. Here, we discuss how endothelial cells become specified during development as arterial, venous, or lymphatic endothelial cells or convert into hematopoietic stem and progenitor cells or cardiac valve cells. We compare findings from in vitro and in vivo studies with a focus on the zebrafish as a valuable model for exploring the signaling pathways and environmental cues that drive these transitions. We also discuss how endothelial plasticity can aid in revascularization and repair of tissue after damage- but may have detrimental consequences under disease conditions. By better understanding endothelial plasticity and the mechanisms underlying endothelial fate transitions, we can begin to explore new therapeutic avenues.

Cerebrovascular development: mechanisms and experimental approaches

The cerebral vasculature plays a central role in human health and disease and possesses several unique anatomic, functional and molecular characteristics. Despite their importance, the mechanisms that determine cerebrovascular development are less well studied than other vascular territories. This is in part due to limitations of existing models and techniques for visualisation and manipulation of the cerebral vasculature. In this review we summarise the experimental approaches used to study the cerebral vessels and the mechanisms that contribute to their development.

Endothelial Extracellular Vesicles: From Keepers of Health to Messengers of Disease

Endothelium has a rich vesicular network that allows the exchange of macromolecules between blood and parenchymal cells. This feature of endothelial cells, along with their polarized secretory machinery, makes them the second major contributor, after platelets, to the particulate secretome in circulation. Extracellular vesicles (EVs) produced by the endothelial cells mirror the remarkable molecular heterogeneity of their parent cells. Cargo molecules carried by EVs were shown to contribute to the physiological functions of endothelium and may support the plasticity and adaptation of endothelial cells in a paracrine manner. Endothelium-derived vesicles can also contribute to the pathogenesis of cardiovascular disease or can serve as prognostic or diagnostic biomarkers. Finally, endothelium-derived EVs can be used as therapeutic tools to target endothelium for drug delivery or target stromal cells via the endothelial cells. In this review we revisit the recent evidence on the heterogeneity and plasticity of endothelial cells and their EVs. We discuss the role of endothelial EVs in the maintenance of vascular homeostasis along with their contributions to endothelial adaptation and dysfunction. Finally, we evaluate the potential of endothelial EVs as disease biomarkers and their leverage as therapeutic tools.

Vascular endothelial cell specification in health and disease

There are two vascular networks in mammals that coordinately function as the main supply and drainage systems of the body. The blood vasculature carries oxygen, nutrients, circulating cells, and soluble factors to and from every tissue. The lymphatic vasculature maintains interstitial fluid homeostasis, transports hematopoietic cells for immune surveillance, and absorbs fat from the gastrointestinal tract. These vascular systems consist of highly organized networks of specialized vessels including arteries, veins, capillaries, and lymphatic vessels that exhibit different structures and cellular composition enabling distinct functions. All vessels are composed of an inner layer of endothelial cells that are in direct contact with the circulating fluid; therefore, they are the first responders to circulating factors. However, endothelial cells are not homogenous; rather, they are a heterogenous population of specialized cells perfectly designed for the physiological demands of the vessel they constitute. This review provides an overview of the current knowledge of the specification of arterial, venous, capillary, and lymphatic endothelial cell identities during vascular development. We also discuss how the dysregulation of these processes can lead to vascular malformations, and therapeutic approaches that have been developed for their treatment.

Endothelial struts enable the generation of large lumenized blood vessels de novo

De novo blood vessel formation occurs through coalescence of endothelial cells (ECs) into a cord-like structure, followed by lumenization either through cell-^1-3 or cord-hollowing^4-7. Vessels generated in this manner are restricted in diameter to one or two ECs, and these models fail to explain how vasculogenesis can form large-diameter vessels. Here, we describe a model for large vessel formation that does not require a cord-like structure or a hollowing step. In this model, ECs coalesce into a network of struts in the future lumen of the vessel, a process dependent upon bone morphogenetic protein signalling. The vessel wall forms around this network and consists initially of only a few patches of ECs. To withstand external forces and to maintain the shape of the vessel, strut formation traps erythrocytes into compartments to form a rigid structure. Struts gradually prune and ECs from struts migrate into and become part of the vessel wall. Experimental severing of struts resulted in vessel collapse, disturbed blood flow and remodelling defects, demonstrating that struts enable the patency of large vessels during their formation.

Zebrafish Vascular Development: General and Tissue-Specific Regulation

Circulation is required for the delivery of oxygen and nutrition to tissues and organs, as well as waste collection. Therefore, the heart and vessels develop first during embryogenesis. The circulatory system consists of the heart, blood vessels, and blood cells, which originate from the mesoderm. The gene expression pattern required for blood vessel development is predetermined by the hierarchical and sequential regulation of genes for the differentiation of mesodermal cells. Herein, we review how blood vessels form distinctly in different tissues or organs of zebrafish and how vessel formation is universally or tissue-specifically regulated by signal transduction pathways and blood flow. In addition, the unsolved issues of mutual contacts and interplay of circulatory organs during embryogenesis are discussed.

Early events in endothelial flow sensing

Responses of vascular and lymphatic endothelial cells (ECs) to fluid shear stress (FSS) from blood or lymphatic fluid flow govern the development, physiology, and diseases of these structures. Extensive research has characterized the signaling, gene expression and cytoskeletal pathways that mediate effects on EC phenotype and vascular morphogenesis. But the primary mechanisms by which ECs transduce the weak forces from flow into biochemical signals are less well understood. This review covers recent advances in our understanding of the immediate mechanisms of FSS mechanotransduction, integrating results from different disciplines, addressing their roles in development, physiology and disease, and suggesting important questions for future work.

ETS factors are required but not sufficient for specific patterns of enhancer activity in different endothelial subtypes

Correct vascular differentiation requires distinct patterns of gene expression in different subtypes of endothelial cells. Members of the ETS transcription factor family are essential for the transcriptional activation of arterial and angiogenesis-specific gene regulatory elements, leading to the hypothesis that they play lineage-defining roles in arterial and angiogenic differentiation directly downstream of VEGFA signalling. However, an alternative explanation is that ETS binding at enhancers and promoters is a general requirement for activation of many endothelial genes regardless of expression pattern, with subtype-specificity provided by additional factors. Here we use analysis of Ephb4 and Coup-TFII (Nr2f2) vein-specific enhancers to demonstrate that ETS factors are equally essential for vein, arterial and angiogenic-specific enhancer activity patterns. Further, we show that ETS factor binding at these vein-specific enhancers is enriched by VEGFA signalling, similar to that seen at arterial and angiogenic enhancers. However, while arterial and angiogenic enhancers can be activated by VEGFA in vivo, the Ephb4 and Coup-TFII venous enhancers are not, suggesting that the specificity of VEGFA-induced arterial and angiogenic enhancer activity occurs via non-ETS transcription factors. These results support a model in which ETS factors are not the primary regulators of specific patterns of gene expression in different endothelial subtypes.

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Vein-specific enhancers can contain essential ETS motifs.

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VEGFA induced an increase in ETS binding at vein, arterial and angiogenic enhancers.

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VEGFA stimulation cannot induce vein-specific enhancer activity.

P130Cas/bcar1 mediates zebrafish caudal vein plexus angiogenesis

P130CAS/BCAR1 belongs to the CAS family of adaptor proteins, with important regulatory roles in cell migration, cell cycle control, and apoptosis. Previously, we and others showed that P130CAS mediates VEGF-A and PDGF signalling in vitro, but its cardiovascular function in vivo remains relatively unexplored. We characterise here a novel deletion model of P130CAS in zebrafish. Using in vivo microscopy and transgenic vascular reporters, we observed that while bcar1−/− zebrafish showed no arterial angiogenic or heart defects during development, they strikingly failed to form the caudal vein plexus (CVP). Endothelial cells (ECs) within the CVP of bcar1−/− embryos produced fewer filopodial structures and did not detach efficiently from neighbouring cells, resulting in a significant reduction in ventral extension and overall CVP area. Mechanistically, we show that P130Cas mediates Bmp2b-induced ectopic angiogenic sprouting of ECs in the developing embryo and provide pharmacological evidence for a role of Src family kinases in CVP development.

Differentiated cancer cell-originated lactate promotes the self-renewal of cancer stem cells in patient-derived colorectal cancer organoids

Tumors harbor diverse compartments of cells with distinct metabolic properties and phenotypes, but the mechanism by which metabolic commensalism among distinct subsets of cancer cells affects tumor progression remains unclear. Colorectal cancer (CRC) has been reported to consist of cancer stem cells (CSCs) and differentiated cancer cells (non-CSCs). In the present study, organoid models were employed to show that CSCs and non-CSCs in CRC were characterized by distinct metabolic phenotypes. Treatment with either non-CSC-derived conditioned medium or exogenous lactate enhanced organoid-forming and tumor-initiating capacity of CSCs. In tumor regeneration assays with co-implanted CSCs and non-CSCs, the tumor-initiating activity was reduced when either monocarboxylate transporter (MCT)4 in non-CSCs or MCT1 in CSCs was silenced or inhibited. Mechanistically, oxiadative phosphorylation-derived reactive oxygen species in CSCs activated AKT-Wnt/β-catenin signaling, which could be induced by lactate from non-CSCs. Overall, these results suggest that CSCs and non-CSCs possess distinct metabolic profiles and, unexpectedly, non-CSC-originated lactate promotes self-renewal of CSCs and thus contributes to CRC progression. Our findings establish a rationale for developing novel therapies targeting the metabolic commensalism between different cell populations in CRC.

It Takes Two to Tango: Endothelial TGFβ/BMP Signaling Crosstalk with Mechanobiology

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor-beta (TGFβ) superfamily of cytokines. While some ligand members are potent inducers of angiogenesis, others promote vascular homeostasis. However, the precise understanding of the molecular mechanisms underlying these functions is still a growing research field. In bone, the tissue in which BMPs were first discovered, crosstalk of TGFβ/BMP signaling with mechanobiology is well understood. Likewise, the endothelium represents a tissue that is constantly exposed to multiple mechanical triggers, such as wall shear stress, elicited by blood flow or strain, and tension from the surrounding cells and to the extracellular matrix. To integrate mechanical stimuli, the cytoskeleton plays a pivotal role in the transduction of these forces in endothelial cells. Importantly, mechanical forces integrate on several levels of the TGFβ/BMP pathway, such as receptors and SMADs, but also global cell-architecture and nuclear chromatin re-organization. Here, we summarize the current literature on crosstalk mechanisms between biochemical cues elicited by TGFβ/BMP growth factors and mechanical cues, as shear stress or matrix stiffness that collectively orchestrate endothelial function. We focus on the different subcellular compartments in which the forces are sensed and integrated into the TGFβ/BMP growth factor signaling.

miR-18a Inhibits BMP4 and HIF-1α Normalizing Brain Arteriovenous Malformations

RATIONALE

Brain arteriovenous malformations (AVMs) are abnormal tangles of vessels where arteries and veins directly connect without intervening capillary nets, increasing the risk of intracerebral hemorrhage and stroke. Current treatments are highly invasive and often not feasible. Thus, effective noninvasive treatments are needed. We previously showed that AVM-brain endothelial cells (BECs) secreted higher VEGF (vascular endothelial growth factor) and lower TSP-1 (thrombospondin-1) levels than control BEC; and that microRNA-18a (miR-18a) normalized AVM-BEC function and phenotype, although its mechanism remained unclear.

OBJECTIVE

To elucidate the mechanism of action and potential clinical application of miR-18a as an effective noninvasive treatment to selectively restore the phenotype and functionality of AVM vasculature.

METHODS AND RESULTS

The molecular pathways affected by miR-18a in patient-derived BECs and AVM-BECs were determined by Western blot, RT-qPCR (quantitative reverse transcription polymerase chain reaction), ELISA, co-IP, immunostaining, knockdown and overexpression studies, flow cytometry, and luciferase reporter assays. miR-18a was shown to increase TSP-1 and decrease VEGF by reducing PAI-1 (plasminogen activator inhibitor-1/SERPINE1) levels. Furthermore, miR-18a decreased the expression of BMP4 (bone morphogenetic protein 4) and HIF-1α (hypoxia-inducible factor 1α), blocking the BMP4/ALK (activin-like kinase) 2/ALK1/ALK5 and Notch signaling pathways. As determined by Boyden chamber assays, miR-18a also reduced the abnormal AVM-BEC invasiveness, which correlated with a decrease in MMP2 (matrix metalloproteinase 2), MMP9, and ADAM10 (ADAM metallopeptidase domain 10) levels. In vivo pharmacokinetic studies showed that miR-18a reaches the brain following intravenous and intranasal administration. Intranasal co-delivery of miR-18a and NEO100, a good manufacturing practices-quality form of perillyl alcohol, improved the pharmacokinetic profile of miR-18a in the brain without affecting its pharmacological properties. Ultra-high-resolution computed tomography angiography and immunostaining studies in an Mgp^-/- AVM mouse model showed that miR-18a decreased abnormal cerebral vasculature and restored the functionality of the bone marrow, lungs, spleen, and liver.

CONCLUSIONS

miR-18a may have significant clinical value in preventing, reducing, and potentially reversing AVM.