Dissecting endothelial cell heterogeneity with new tools

The formation of a blood vessel network is crucial for organ development and regeneration. Over the past three decades, the central molecular mechanisms governing blood vessel growth have been extensively studied. Recent evidence indicates that vascular endothelial cells-the specialized cells lining the inner surface of blood vessels-exhibit significant heterogeneity to meet the specific needs of different organs. This review focuses on the current understanding of endothelial cell heterogeneity, which includes both intra-organ and inter-organ heterogeneity. Intra-organ heterogeneity encompasses arterio-venous and tip-stalk endothelial cell specialization, while inter-organ heterogeneity refers to organ-specific transcriptomic profiles and functions. Advances in single-cell RNA sequencing (scRNA-seq) have enabled the identification of new endothelial subpopulations and the comparison of gene expression patterns across different subsets of endothelial cells. Integrating scRNA-seq with other high-throughput sequencing technologies promises to deepen our understanding of endothelial cell heterogeneity at the epigenetic level and in a spatially resolved context. To further explore human endothelial cell heterogeneity, vascular organoids offer powerful tools for studying gene function in three-dimensional culture systems and for investigating endothelial-tissue interactions using human cells. Developing organ-specific vascular organoids presents unique opportunities to unravel inter-organ endothelial cell heterogeneity and its implications for human disease. Emerging technologies, such as scRNA-seq and vascular organoids, are poised to transform our understanding of endothelial cell heterogeneity and pave the way for innovative therapeutic strategies to address human vascular diseases.

Endothelial endogenous CSE/H2S inhibits endothelial pyroptosis by activating sirtuin1 to attenuate LPS-induced acute lung injury

Endothelial pyroptosis, a pro-inflammatory programmed cell death, promotes endothelial inflammation and is a pivotal process in the initial stage of acute lung injury (ALI). Hydrogen sulfide (H2S), a gasotransmitter primarily dependent on cystathionine γ-lyase (CSE) in the cardiovascular and respiratory systems, plays a protective role during ALI. Nonetheless, the modulatory role and precise molecular mechanism of endothelial endogenous CSE/H2S in the pathogenesis of ALI remain elusive. Herein, we prepared an ALI mouse model using intratracheal administration of LPS (5 mg/kg), and lung injury was assessed by evaluating pulmonary edema, inflammatory response, and endothelial pyroptosis. In this model, H2S production from pulmonary tissues declined in a time-dependent manner, accompanied by a compensatory elevation of CSE protein levels. Treatment with the H2S donor (NaHS) attenuated pulmonary edema, inflammatory cell infiltration, endothelial pyroptosis, and reduced serum levels of tumor necrosis factor-alpha (TNF-α), interleukin-1β (IL-1β), and interleukin-6 (IL-6). Meanwhile, the inducible deletion of CSE in endothelial cells exacerbated these changes. The blocking effect of CSE/H2S on endothelial pyroptosis (evidenced by caspase-11 activation and GSDMD-NT formation) was also confirmed in cultured pulmonary microvascular endothelial cells (PMECs). Mechanistically, H2S-mediated regulation of sirtuin-1 (SIRT1) expression and activation (via sulfhydration) contributed to the modulatory process. Collectively, we uncovered that endothelial endogenous CSE/H2S alleviates endothelial pyroptosis by activating SIRT1, thereby preventing LPS-induced acute lung injury.

Targeted siRNA Delivery Against RUNX1 Via tFNA: Inhibiting Retinal Neovascularization and Restoring Vessels Through Dll4/Notch1 Signaling

Purpose

To assess the efficacy of tetrahedral framework nucleic acids (tFNAs) as a delivery system for small interfering RNA (siRNA) targeting RUNX1 (siRUNX1) in inhibiting retinal neovascularization (RNV) and restoring vascular integrity via the Dll4/Notch1 signaling pathway.

Methods

tFNAs and tFNAs-siRUNX1 were synthesized using annealing of single-stranded DNAs and characterized by PAGE and high-performance capillary electrophoresis. Human umbilical vein endothelial cells were treated under hypoxic conditions with tFNAs-siRUNX1, and cellular uptake was evaluated using fluorescence microscopy and flow cytometry. Angiogenesis was assessed through EdU proliferation, tube formation, and wound-healing assays. In vivo experiments used oxygen-induced retinopathy (OIR) and laser-induced choroidal neovascularization (CNV) models in mice, with subsequent imaging by optical coherence tomography (OCT) and fundus fluorescence angiography. Gene and protein expression were analyzed by RT-PCR and Western blotting, focusing on the Dll4/Notch1 pathway and apoptosis markers.

Results

tFNAs-siRUNX1 effectively inhibited endothelial cell proliferation, migration, and tube formation in vitro. In OIR and CNV models, it reduced neovascularization, nonperfusion areas, and vascular leakage. The mechanism involved modulation of the Dll4/Notch1 pathway, with decreased Dll4, Notch1, and Hes1 and increased Nts expression. tFNAs-siRUNX1 also reduced endothelial cell apoptosis via the Bcl-2/Bax pathway.

Conclusions

tFNAs-siRUNX1 is a promising delivery system for targeting RNV, inhibiting neovascularization, and restoring retinal vascular integrity, providing a potential therapeutic alternative to anti-VEGF treatments.

From bench to bedside: murine models of inherited and sporadic brain arteriovenous malformations

Brain arteriovenous malformations are abnormal vascular structures in which an artery shunts high pressure blood directly to a vein without an intervening capillary bed. These lesions become highly remodeled over time and are prone to rupture. Historically, brain arteriovenous malformations have been challenging to treat, using primarily surgical approaches. Over the past few decades, the genetic causes of these malformations have been uncovered. These can be divided into (1) familial forms, such as loss of function mutations in TGF-β (BMP9/10) components in hereditary hemorrhagic telangiectasia, or (2) sporadic forms, resulting from somatic gain of function mutations in genes involved in the RAS-MAPK signaling pathway. Leveraging these genetic discoveries, preclinical mouse models have been developed to uncover the mechanisms underlying abnormal vessel formation, and thus revealing potential therapeutic targets. Impressively, initial preclinical studies suggest that pharmacological treatments disrupting these aberrant pathways may ameliorate the abnormal pathologic vessel remodeling and inflammatory and hemorrhagic nature of these high-flow vascular anomalies. Intriguingly, these studies also suggest uncontrolled angiogenic signaling may be a major driver in bAVM pathogenesis. This comprehensive review describes the genetics underlying both inherited and sporadic bAVM and details the state of the field regarding murine models of bAVM, highlighting emerging therapeutic targets that may transform our approach to treating these devastating lesions.

Bone marrow niches orchestrate stem-cell hierarchy and immune tolerance

Stem cells reside in specialized microenvironments, termed niches, at several different locations in tissues1-3. The differential functions of heterogeneous stem cells and niches are important given the increasing clinical applications of stem-cell transplantation and immunotherapy. Whether hierarchical structures among stem cells at distinct niches exist and further control aspects of immune tolerance is unknown. Here we describe previously unknown new hierarchical arrangements in haematopoietic stem cells (HSCs) and bone marrow niches that dictate both regenerative potential and immune privilege. High-level nitric oxide-generating (NOhi) HSCs are refractory to immune attack and exhibit delayed albeit robust long-term reconstitution. Such highly immune-privileged, primitive NOhi HSCs co-localize with distinctive capillaries characterized by primary ciliated endothelium and high levels of the immune-checkpoint molecule CD200. These capillaries regulate the regenerative functions of NOhi HSCs through the ciliary protein IFT20 together with CD200, endothelial nitric oxide synthase and autophagy signals, which further mediate immunoprotection. Notably, previously described niche constituents, sinusoidal cells and type-H vessels2-10 co-localize with less immune-privileged and less potent NOlow HSCs. Together, we identify highly immune-privileged, late-rising primitive HSCs and characterize their immunoprotective niches comprising specialized vascular domains. Our results indicate that the niche orchestrates hierarchy in stem cells and immune tolerance, and highlight future immunotherapeutic targets.

Hypoxia regulate developmental coronary angiogenesis potentially through VEGF-R2- and SOX17-mediated signaling

BACKGROUND

The development of coronary vessels in embryonic mouse heart involves various progenitor populations, including sinus venosus (SV), endocardium, and proepicardium. ELA/APJ signaling is known to regulate coronary growth from the SV, whereas VEGF-A/VEGF-R2 signaling controls growth from the endocardium. Previous studies suggest hypoxia might regulate coronary growth, but its specific downstream pathways are unclear. In this study, we further investigated the role of hypoxia and have identified SOX17- and VEGF-R2-mediated signaling as the potential downstream pathways in its regulation of developmental coronary angiogenesis.

RESULTS

HIF-1α stabilization by knocking out von Hippel Lindau (VHL) protein in the myocardium (cKO) disrupted normal coronary angiogenesis in embryonic mouse hearts, resembling patterns of accelerated coronary growth. VEGF-R2 expression was increased in coronary endothelial cells under hypoxia in vitro and in VHL cKO hearts in vivo. Similarly, SOX17 expression was increased in the VHL cKO hearts, while its knockout in the endocardium disrupted normal coronary growth.

CONCLUSION

These findings provide further evidence that hypoxia regulates developmental coronary growth potentially through VEGF-R2 and SOX17 pathways, shedding light on mechanisms of coronary vessel development.

Differential bone and vessel type formation at superior and dura periosteum during cranial bone defect repair

The cranial mesenchyme, originating from both neural crest and mesoderm, imparts remarkable regional specificity and complexity to postnatal calvarial tissue. While the distinct embryonic origins of the superior and dura periosteum of the cranial parietal bone have been described, the extent of their respective contributions to bone and vessel formation during adult bone defect repair remains superficially explored. Utilizing transgenic mouse models in conjunction with high-resolution multiphoton laser scanning microscopy (MPLSM), we have separately evaluated bone and vessel formation in the superior and dura periosteum before and after injury, as well as following intermittent treatment of recombinant peptide of human parathyroid hormone (rhPTH), Teriparatide. Our results show that new bone formation along the dura surface is three times greater than that along the superior periosteal surface following injury, regardless of Teriparatide treatment. Targeted deletion of PTH receptor PTH1R via SMA-CreER and Col 1a (2.3)-CreER results in selective reduction of bone formation, suggesting different progenitor cell pools in the adult superior and dura periosteum. Consistently, analyses of microvasculature show higher vessel density and better organized arterial-venous vessel network associated with a 10-fold more osteoblast clusters at dura periosteum as compared to superior periosteum. Intermittent rhPTH treatment further enhances the arterial vessel ratio at dura periosteum and type H vessel formation in cortical bone marrow space. Taken together, our study demonstrates a site-dependent coordinated osteogenic and angiogenic response, which is determined by regional osteogenic progenitor pool as well as the coupling blood vessel network at the site of cranial defect repair.

Angiogenesis in the mature mouse cortex is governed in a regional- and Notch1-dependent manner

Cerebral angiogenesis is well appreciated in development and after injury, but the extent to which it occurs across cortical regions in normal adult mice and the underlying mechanisms are incompletely understood. Using in vivo imaging, we show that angiogenesis in anterior-medial cortical regions (retrosplenial and sensorimotor cortex) was exceptionally rare. By contrast, angiogenesis was significantly elevated in posterior-lateral regions such as visual cortex, primarily within 200 μm of the cortical surface. There was no effect of sex on angiogenesis rates, nor were there regional differences in vessel pruning (for either sex). To understand the mechanisms, we surveyed gene expression and found that Notch-related genes were enriched in ultra-stable retrosplenial cortex. Using endothelial-specific knockdown of Notch1, cerebral angiogenesis was significantly increased along with genes implicated in angiogenesis (Apln, Angpt2, Cdkn1a). Our study shows that angiogenesis is regionally dependent and that manipulations of Notch1 could unlock the angiogenic potential of the mature vasculature.

Resolving the design principles that control postnatal vascular growth and scaling

After birth, tissues grow continuously until reaching adult size, with each organ exhibiting unique cellular dynamics, growth patterns, and (stem or non-stem) cell sources. Using a suite of experimental and computational multiscale approaches, we found that aortic expansion is guided by specific biological principles and scales with the vertebral column rather than animal body weight. Expansion proceeds via two distinct waves of arterial cell proliferation along blood flow that are spatially stochastic, yet temporally coordinated. Each wave exhibits unique cell cycle kinetics and properties, with the first wave exhibiting cell cycle durations as fast as 6 hours. Single-cell RNA sequencing showed changes in fatty acid metabolism concomitant with an increase in cell size. Mathematical modeling and experiments indicated endothelial cell extrusion is essential for homeostatic aortic growth and balancing excess proliferation. In a genetic model of achondroplasia, the aorta achieves proper scaling through enhanced cell extrusion while maintaining normal proliferation dynamics. Collectively, these results provide a blueprint of the principles that orchestrate aortic growth which depends entirely on differentiated cell proliferation rather than resident stem cells.

Arterial endothelial deletion of hereditary hemorrhagic telangiectasia 2/ Alk1 causes epistaxis and cerebral microhemorrhage with aberrant arteries and defective smooth muscle coverage

Hereditary Hemorrhagic Telangiectasia (HHT) is an autosomal dominant vascular disorder with manifestations including severe nose bleeding and microhemorrhage in brains. Despite being the second most common inherited bleeding disorder, the pathophysiological mechanism underlying HHT-associated hemorrhage is poorly understood. HHT pathogenesis is thought to follow a Knudsonian two-hit model, requiring a second somatic mutation for lesion formation. Mutations in activin receptor-like kinase 1 ( ALK1 ) gene cause HHT type 2. We hypothesize that somatic mutation of Alk1 in arterial endothelial cells (AECs) leads to arterial defects and hemorrhage. Here, we mutated Alk1 in AECs in postnatal mice using Bmx(PAC)-Cre ERT2 and found that somatic arterial endothelial mutation of Alk1 was sufficient to induce spontaneous epistaxis and multifocal cerebral microhemorrhage. This bleeding occurred in the presence of tortuous and enlarged blood vessels, loss of arterial molecular marker Efnb2 , disorganization of vascular smooth muscle, and impaired vasoregulation. Our data suggest that arterial endothelial deletion of Alk1 leading to reduced arterial identity and disrupted vascular smooth muscle cell coverage is a plausible molecular mechanism for HHT-associated severe epistaxis. This work provides the first evidence that somatic Alk1 mutation in AECs can cause hemorrhagic vascular lesions, offering a novel preclinical model critically needed for studying HHT-associated epistaxis, and delineating an arterial mechanism to HHT pathophysiology.

Specialized post-arterial capillaries facilitate adult bone remodelling

The vasculature of the skeletal system is crucial for bone formation, homoeostasis and fracture repair, yet the diversity and specialization of bone-associated vessels remain poorly understood. Here we identify a specialized type of post-arterial capillary, termed type R, involved in bone remodelling. Type R capillaries emerge during adolescence around trabecular bone, possess a distinct morphology and molecular profile, and are associated with osteoprogenitors and bone-resorbing osteoclasts. Endothelial cell-specific overexpression of the transcription factor DACH1 in postnatal mice induces a strong increase in arteries and type R capillaries, leading to local metabolic changes and enabling trabecular bone formation in normally highly hypoxic areas of the diaphysis. Indicating potential clinical relevance of type R capillaries, these vessels respond to anti-osteoporosis treatments and emerge during ageing inside porous structures that are known to weaken compact bone. Our work outlines fundamental principles of vessel specialization in the developing, adult and ageing skeletal system.

ATN-161 alleviates caerulein-induced pancreatitis

Pancreatitis is a common gastrointestinal disorder that causes hospitalization with significant morbidity and mortality. The mechanistic pathophysiology of pancreatitis is complicated, limiting the discovery of pharmacological intervention methods. Here, we show that the administration of ATN-161, an antagonist of Integrin-α5, significantly mitigates the pathological condition of acute pancreatitis induced by caerulein. We find that CK19-positive pancreatic ductal cells align parallel to blood vessels in the pancreas. In the caerulein-induced acute pancreatitis model, the newly emergent CK19-positive cells are highly vascularized, with a significant increase in vascular density and endothelial cell number. Single-cell RNA sequencing analysis shows that ductal and endothelial cells are intimate interacting partners, suggesting the existence of a ductal-endothelial interface in the pancreas. Pancreatitis dramatically reduces the crosstalk in the ductal-endothelial interface but promotes the Spp-1/Integrin-α5 signaling. Blocking this signaling with ATN-161 significantly reduces acinar-to-ductal metaplasia, pathological angiogenesis, and restores other abnormal defects induced by caerulein. Our work reveals the therapeutic potential of ATN-161 as an uncharacterized pharmacological method to alleviate the symptoms of pancreatitis.

Zonation and ligand and dose dependence of sphingosine 1-phosphate receptor-1 signalling in blood and lymphatic vasculature

AIMS

Circulating levels of sphingosine 1-phosphate (S1P), an HDL-associated ligand for the endothelial cell (EC) protective S1P receptor-1 (S1PR1), are reduced in disease states associated with endothelial dysfunction. Yet, as S1PR1 has high affinity for S1P and can be activated by ligand-independent mechanisms and EC autonomous S1P production, it is unclear if relative reductions in circulating S1P can cause endothelial dysfunction. It is also unclear how EC S1PR1 insufficiency, whether induced by deficiency in circulating ligand or by S1PR1-directed immunosuppressive therapy, affects different vascular subsets.

METHODS AND RESULTS

We here fine map the zonation of S1PR1 signalling in the murine blood and lymphatic vasculature, superimpose cell-type-specific and relative deficiencies in S1P production to define ligand source and dose dependence, and correlate receptor engagement to essential functions. In naïve blood vessels, despite broad expression, EC S1PR1 engagement was restricted to resistance-size arteries, lung capillaries, and a subset of high-endothelial venules (HEVs). Similar zonation was observed for albumin extravasation in EC S1PR1-deficient mice, and brain extravasation was reproduced with arterial EC-selective S1pr1 deletion. In lymphatic ECs, S1PR1 engagement was high in collecting vessels and lymph nodes and low in blind-ended capillaries that drain tissue fluids. While EC S1P production sustained S1PR1 signalling in lymphatics and HEV, haematopoietic cells provided ∼90% of plasma S1P and sustained signalling in resistance arteries and lung capillaries. S1PR1 signalling and endothelial function were both surprisingly sensitive to reductions in plasma S1P with apparent saturation around 50% of normal levels. S1PR1 engagement did not depend on sex or age but modestly increased in arteries in hypertension and diabetes. Sphingosine kinase (Sphk)-2 deficiency also increased S1PR1 engagement selectively in arteries, which could be attributed to Sphk1-dependent S1P release from perivascular macrophages.

CONCLUSION

This study highlights vessel subtype-specific S1PR1 functions and mechanisms of engagement and supports the relevance of S1P as circulating biomarker for endothelial function.

Studying the Pulmonary Endothelium in Health and Disease: An Official American Thoracic Society Workshop Report

Lung endothelium resides at the interface between the circulation and the underlying tissue, where it senses biochemical and mechanical properties of both the blood as it flows through the vascular circuit and the vessel wall. The endothelium performs the bidirectional signaling between the blood and tissue compartments that is necessary to maintain homeostasis while physically separating both, facilitating a tightly regulated exchange of water, solutes, cells, and signals. Disruption in endothelial function contributes to vascular disease, which can manifest in discrete vascular locations along the artery-to-capillary-to-vein axis. Although our understanding of mechanisms that contribute to endothelial cell injury and repair in acute and chronic vascular disease have advanced, pathophysiological mechanisms that underlie site-specific vascular disease remain incompletely understood. In an effort to improve the translatability of mechanistic studies of the endothelium, the American Thoracic Society convened a workshop to optimize rigor, reproducibility, and translation of discovery to advance our understanding of endothelial cell function in health and disease.

A systems view of the vascular endothelium in health and disease

The dysfunction of blood-vessel-lining endothelial cells is a major cause of mortality. Although endothelial cells, being present in all organs as a single-cell layer, are often conceived as a rather inert cell population, the vascular endothelium as a whole should be considered a highly dynamic and interactive systemically disseminated organ. We present here a holistic view of the field of vascular research and review the diverse functions of blood-vessel-lining endothelial cells during the life cycle of the vasculature, namely responsive and relaying functions of the vascular endothelium and the responsive roles as instructive gatekeepers of organ function. Emerging translational perspectives in regenerative medicine, preventive medicine, and aging research are developed. Collectively, this review is aimed at promoting disciplinary coherence in the field of angioscience for a broader appreciation of the importance of the vasculature for organ function, systemic health, and healthy aging.

Intramyocardial Sprouting Tip Cells Specify Coronary Arterialization

BACKGROUND

The elaborate patterning of coronary arteries critically supports the high metabolic activity of the beating heart. How coronary endothelial cells coordinate hierarchical vascular remodeling and achieve arteriovenous specification remains largely unknown. Understanding the molecular and cellular cues that pattern coronary arteries is crucial to develop innovative therapeutic strategies that restore functional perfusion within the ischemic heart.

METHODS

Single-cell transcriptomics and histological validation were used to delineate heterogeneous transcriptional states of the developing and mature coronary endothelium with a focus on sprouting endothelium and arterial cell specification. Genetic lineage tracing and high-resolution 3-dimensional imaging were used to characterize the origin and mechanisms of coronary angiogenic sprouting, as well as to fate-map selective endothelial lineages. Integration of single-cell transcriptomic data from ischemic adult mouse hearts and human embryonic data served to assess the conservation of transcriptional states across development, disease, and species.

RESULTS

We discover that coronary arteries originate from cells that have previously transitioned through a specific tip cell phenotype. We identify nonoverlapping intramyocardial and subepicardial tip cell populations with differential gene expression profiles and regulatory pathways. Esm1-lineage tracing confirmed that intramyocardial tip cells selectively contribute to coronary arteries and endocardial tunnels, but not veins. Notably, prearterial cells are detected from development stages to adulthood, increasingly in response to ischemic injury, and in human embryos, suggesting that tip cell-to-artery specification is a conserved mechanism.

CONCLUSIONS

A tip cell-to-artery specification mechanism drives arterialization of the intramyocardial plexus and endocardial tunnels throughout life and is reactivated upon ischemic injury. Differential sprouting programs govern the formation and specification of the venous and arterial coronary plexus.

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).

Notch signaling regulates UNC5B to suppress endothelial proliferation, migration, junction activity, and retinal plexus branching

Notch signaling guides vascular development and function by regulating diverse endothelial cell behaviors, including migration, proliferation, vascular density, endothelial junctions, and polarization in response to flow. Notch proteins form transcriptional activation complexes that regulate endothelial gene expression, but few of the downstream effectors that enable these phenotypic changes have been characterized in endothelial cells, limiting our understanding of vascular Notch activities. Using an unbiased screen of translated mRNA rapidly regulated by Notch signaling, we identified novel in vivo targets of Notch signaling in neonatal mouse brain endothelium, including UNC5B, a member of the netrin family of angiogenic-regulatory receptors. Endothelial Notch signaling rapidly upregulates UNC5B in multiple endothelial cell types. Loss or gain of UNC5B recapitulated specific Notch-regulated phenotypes. UNC5B expression inhibited endothelial migration and proliferation and was required for stabilization of endothelial junctions in response to shear stress. Loss of UNC5B partially or wholly blocked the ability of Notch activation to regulate these endothelial cell behaviors. In the developing mouse retina, endothelial-specific loss of UNC5B led to excessive vascularization, including increased vascular outgrowth, density, and branchpoint count. These data indicate that Notch signaling upregulates UNC5B as an effector protein to control specific endothelial cell behaviors and inhibit angiogenic growth.

Endothelial Cell Flow-Mediated Quiescence Is Temporally Regulated and Utilizes the Cell Cycle Inhibitor p27

BACKGROUND

Endothelial cells regulate their cell cycle as blood vessels remodel and transition to quiescence downstream of blood flow-induced mechanotransduction. Laminar blood flow leads to quiescence, but how flow-mediated quiescence is established and maintained is poorly understood.

METHODS

Primary human endothelial cells were exposed to laminar flow regimens and gene expression manipulations, and quiescence depth was analyzed via time-to-cell cycle reentry after flow cessation. Mouse and zebrafish endothelial expression patterns were examined via scRNA-seq (single-cell RNA sequencing) analysis, and mutant or morphant fish lacking p27 were analyzed for endothelial cell cycle regulation and in vivo cellular behaviors.

RESULTS

Arterial flow-exposed endothelial cells had a distinct transcriptome, and they first entered a deep quiescence, then transitioned to shallow quiescence under homeostatic maintenance conditions. In contrast, venous flow-exposed endothelial cells entered deep quiescence early that did not change with homeostasis. The cell cycle inhibitor p27 (CDKN1B) was required to establish endothelial flow-mediated quiescence, and expression levels positively correlated with quiescence depth. p27 loss in vivo led to endothelial cell cycle upregulation and ectopic sprouting, consistent with loss of quiescence. HES1 and ID3, transcriptional repressors of p27 upregulated by arterial flow, were required for quiescence depth changes and the reduced p27 levels associated with shallow quiescence.

CONCLUSIONS

Endothelial cell flow-mediated quiescence has unique properties and temporal regulation of quiescence depth that depends on the flow stimulus. These findings are consistent with a model whereby flow-mediated endothelial cell quiescence depth is temporally regulated downstream of p27 transcriptional regulation by HES1 and ID3. The findings are important in understanding endothelial cell quiescence misregulation that leads to vascular dysfunction and disease.

Identification of New Markers of Angiogenic Sprouting Using Transcriptomics: New Role for RND3

BACKGROUND

New blood vessel formation requires endothelial cells to transition from a quiescent to an invasive phenotype. Transcriptional changes are vital for this switch, but a comprehensive genome-wide approach focused exclusively on endothelial cell sprout initiation has not been reported.

METHODS

Using a model of human endothelial cell sprout initiation, we developed a protocol to physically separate cells that initiate the process of new blood vessel formation (invading cells) from noninvading cells. We used this model to perform multiple transcriptomics analyses from independent donors to monitor endothelial gene expression changes.

RESULTS

Single-cell population analyses, single-cell cluster analyses, and bulk RNA sequencing revealed common transcriptomic changes associated with invading cells. We also found that collagenase digestion used to isolate single cells upregulated the Fos proto-oncogene transcription factor. Exclusion of Fos proto-oncogene expressing cells revealed a gene signature consistent with activation of signal transduction, morphogenesis, and immune responses. Many of the genes were previously shown to regulate angiogenesis and included multiple tip cell markers. Upregulation of SNAI1 (snail family transcriptional repressor 1), PTGS2 (prostaglandin synthase 2), and JUNB (JunB proto-oncogene) protein expression was confirmed in invading cells, and silencing JunB and SNAI1 significantly reduced invasion responses. Separate studies investigated rounding 3, also known as RhoE, which has not yet been implicated in angiogenesis. Silencing rounding 3 reduced endothelial invasion distance as well as filopodia length, fitting with a pathfinding role for rounding 3 via regulation of filopodial extensions. Analysis of in vivo retinal angiogenesis in Rnd3 heterozygous mice confirmed a decrease in filopodial length compared with wild-type littermates.

CONCLUSIONS

Validation of multiple genes, including rounding 3, revealed a functional role for this gene signature early in the angiogenic process. This study expands the list of genes associated with the acquisition of a tip cell phenotype during endothelial cell sprout initiation.

Impact of heterozygous ALK1 mutations on the transcriptomic response to BMP9 and BMP10 in endothelial cells from hereditary hemorrhagic telangiectasia and pulmonary arterial hypertension donors

Heterozygous activin receptor-like kinase 1 (ALK1) mutations are associated with two vascular diseases: hereditary hemorrhagic telangiectasia (HHT) and more rarely pulmonary arterial hypertension (PAH). Here, we aimed to understand the impact of ALK1 mutations on BMP9 and BMP10 transcriptomic responses in endothelial cells. Endothelial colony-forming cells (ECFCs) and microvascular endothelial cells (HMVECs) carrying loss of function ALK1 mutations were isolated from newborn HHT and adult PAH donors, respectively. RNA-sequencing was performed on each type of cells compared to controls following an 18 h stimulation with BMP9 or BMP10. In control ECFCs, BMP9 and BMP10 stimulations induced similar transcriptomic responses with around 800 differentially expressed genes (DEGs). ALK1-mutated ECFCs unexpectedly revealed highly similar transcriptomic profiles to controls, both at the baseline and upon stimulation, and normal activation of Smad1/5 that could not be explained by a compensation in cell-surface ALK1 level. Conversely, PAH HMVECs revealed strong transcriptional dysregulations compared to controls with > 1200 DEGs at the baseline. Consequently, because our study involved two variables, ALK1 genotype and BMP stimulation, we performed two-factor differential expression analysis and identified 44 BMP9-dysregulated genes in mutated HMVECs, but none in ECFCs. Yet, the impaired regulation of at least one hit, namely lunatic fringe (LFNG), was validated by RT-qPCR in three different ALK1-mutated endothelial models. In conclusion, ALK1 heterozygosity only modified the BMP9/BMP10 regulation of few genes, including LFNG involved in NOTCH signaling. Future studies will uncover whether dysregulations in such hits are enough to promote HHT/PAH pathogenesis, making them potential therapeutic targets, or if second hits are necessary.

Complementary and Inducible creERT2 Mouse Models for Functional Evaluation of Endothelial Cell Subtypes in the Bone Marrow

In the adult bone marrow (BM), endothelial cells (ECs) are an integral component of the hematopoietic stem cell (HSC)-supportive niche, which modulates HSC activity by producing secreted and membrane-bound paracrine signals. Within the BM, distinct vascular arteriole, transitional, and sinusoidal EC subtypes display unique paracrine expression profiles and create anatomically-discrete microenvironments. However, the relative contributions of vascular endothelial subtypes in supporting hematopoiesis is unclear. Moreover, constitutive expression and off-target activity of currently available endothelial-specific and endothelial-subtype-specific murine cre lines potentially confound data analysis and interpretation. To address this, we describe two tamoxifen-inducible cre-expressing lines, Vegfr3-creERT2 and Cx40-creERT2, that efficiently label sinusoidal/transitional and arteriole endothelium respectively in adult marrow, without off-target activity in hematopoietic or perivascular cells. Utilizing an established mouse model in which cre-dependent recombination constitutively-activates MAPK signaling within adult endothelium, we identify arteriole ECs as the driver of MAPK-mediated hematopoietic dysfunction. These results define complementary tamoxifen-inducible creERT2-expressing mouse lines that label functionally-discrete and non-overlapping sinusoidal/transitional and arteriole EC populations in the adult BM, providing a robust toolset to investigate the differential contributions of vascular subtypes in maintaining hematopoietic homeostasis.

Effects of Chronic Inflammatory Activation of Murine and Human Arterial Endothelial Cells at Normal Lipoprotein and Cholesterol Levels In Vivo and In Vitro

The activation of endothelial cells is crucial for immune defense mechanisms but also plays a role in the development of atherosclerosis. We have previously shown that inflammatory stimulation of endothelial cells on top of elevated lipoprotein/cholesterol levels accelerates atherogenesis. The aim of the current study was to investigate how chronic endothelial inflammation changes the aortic transcriptome of mice at normal lipoprotein levels and to compare this to the inflammatory response of isolated endothelial cells in vitro. We applied a mouse model expressing constitutive active IκB kinase 2 (caIKK2)-the key activator of the inflammatory NF-κB pathway-specifically in arterial endothelial cells and analyzed transcriptomic changes in whole aortas, followed by pathway and network analyses. We found an upregulation of cell death and mitochondrial beta-oxidation pathways with a predicted increase in endothelial apoptosis and necrosis and a simultaneous reduction in protein synthesis genes. The highest upregulated gene was ACE2, the SARS-CoV-2 receptor, which is also an important regulator of blood pressure. Analysis of isolated human arterial and venous endothelial cells supported these findings and also revealed a reduction in DNA replication, as well as repair mechanisms, in line with the notion that chronic inflammation contributes to endothelial dysfunction.

Endothelial HIFα/PDGF-B to smooth muscle Beclin1 signaling sustains pathological muscularization in pulmonary hypertension

Mechanisms underlying maintenance of pathological vascular hypermuscularization are poorly delineated. Herein, we investigated retention of smooth muscle cells (SMCs) coating normally unmuscularized distal pulmonary arterioles in pulmonary hypertension (PH) mediated by chronic hypoxia with or without Sugen 5416, and reversal of this pathology. With hypoxia in mice or culture, lung endothelial cells (ECs) upregulated hypoxia-inducible factor 1α (HIF1-α) and HIF2-α, which induce platelet-derived growth factor B (PDGF-B), and these factors were reduced to normoxic levels with re-normoxia. Re-normoxia reversed hypoxia-induced pulmonary vascular remodeling, but with EC HIFα overexpression during re-normoxia, pathological changes persisted. Conversely, after establishment of distal muscularization and PH, EC-specific deletion of Hif1a, Hif2a, or Pdgfb induced reversal. In human idiopathic pulmonary artery hypertension, HIF1-α, HIF2-α, PDGF-B, and autophagy-mediating gene products, including Beclin1, were upregulated in pulmonary artery SMCs and/or lung lysates. Furthermore, in mice, hypoxia-induced EC-derived PDGF-B upregulated Beclin1 in distal arteriole SMCs, and after distal muscularization was established, re-normoxia, EC Pdgfb deletion, or treatment with STI571 (which inhibits PDGF receptors) downregulated SMC Beclin1 and other autophagy products. Finally, SMC-specific Becn1 deletion induced apoptosis, reversing distal muscularization and PH mediated by hypoxia with or without Sugen 5416. Thus, chronic hypoxia induction of the HIFα/PDGF-B axis in ECs is required for non-cell-autonomous Beclin1-mediated survival of pathological distal arteriole SMCs.

Sustained meningeal lymphatic vessel atrophy or expansion does not alter Alzheimer's disease-related amyloid pathology

Discovery of meningeal lymphatic vessels (LVs) in the dura mater, also known as dural LVs (dLVs) that depend on vascular endothelial growth factor C expression, has raised interest in their possible involvement in Alzheimer's disease (AD). Here we find that in the APdE9 and 5xFAD mouse models of AD, dural amyloid-β (Aβ) is confined to blood vessels and dLV morphology or function is not altered. The induction of sustained dLV atrophy or hyperplasia in the AD mice by blocking or overexpressing vascular endothelial growth factor C, impaired or improved, respectively, macromolecular cerebrospinal fluid (CSF) drainage to cervical lymph nodes. Yet, sustained manipulation of dLVs did not significantly alter the overall brain Aβ plaque load. Moreover, dLV atrophy did not alter the behavioral phenotypes of the AD mice, but it improved CSF-to-blood drainage. Our results indicate that sustained dLV manipulation does not affect Aβ deposition in the brain and that compensatory mechanisms promote CSF clearance.

Comparison of Glutathione Nanoparticles, CoEnzyme Q10, and Fish Oil for Prevention of Oxygen-Induced Retinopathy in Neonatal Rats

Notch ligands and receptors are important for cell specification and angiogenesis, but their role in oxygen-induced retinopathy (OIR) is not well studied. Delta-like ligand (DLL)-4/Notch inhibits angiogenesis, while Jagged-1/Notch promotes angiogenesis. We tested the hypothesis that early supplementation with antioxidants and/or fish oil curtails severe OIR by inducing DLL-4/Notch and reducing Jagged-1/Notch. Newborn rats were exposed to brief intermittent hypoxia (IH) during hyperoxia, during which they received daily oral supplements of (1) fish oil, (2) coenzyme Q10 (CoQ10) in olive oil (OO), (3) glutathione nanoparticles (nGSH), (4) fish oil + CoQ10, or (5) OO (controls) from birth (P0) to P14. At P14, the pups were placed in room air (RA) until P21, with no further treatment. Oxidative stress, apoptosis, ocular histopathology, and Notch signaling were assessed. Neonatal IH resulted in severe retinal damage consistent with retinopathy of prematurity (ROP). Retinal damage was associated with induced oxidative stress and Jagged-1/Notch signaling, as well as reduced DLL-4/Notch signaling. All treatments reversed these outcomes, but nGSH produced the most beneficial outcomes. Severe OIR promoted the induction of Jagged-1/Notch and curtailed DLL-4/Notch, which was an effect that could be reversed with nGSH supplementation. These findings may indicate a potential alternate pathway for ROP treatment and/or prevention.

Transcription factors regulating vasculogenesis and angiogenesis

Transcription factors (TFs) play a crucial role in regulating the dynamic and precise patterns of gene expression required for the initial specification of endothelial cells (ECs), and during endothelial growth and differentiation. While sharing many core features, ECs can be highly heterogeneous. Differential gene expression between ECs is essential to pattern the hierarchical vascular network into arteries, veins and capillaries, to drive angiogenic growth of new vessels, and to direct specialization in response to local signals. Unlike many other cell types, ECs have no single master regulator, instead relying on differing combinations of a necessarily limited repertoire of TFs to achieve tight spatial and temporal activation and repression of gene expression. Here, we will discuss the cohort of TFs known to be involved in directing gene expression during different stages of mammalian vasculogenesis and angiogenesis, with a primary focus on development.

A sandwiched ventricular explant assay to model mouse coronary angiogenesis ex vivo

Developing an ex vivo system that mimics in vivo developmental coronary angiogenesis provides an improved understanding of its underlying molecular and cellular mechanisms. Here, we present a sandwiched embryonic ventricular explant assay to model mouse coronary angiogenesis ex vivo. We describe steps for breeding mice, labeling endocardial cells, isolating murine hearts, dissecting left ventricles, and making sandwiched explants in Matrigel. We then detail procedures for modeling coronary angiogenesis and taking images. For complete details on the use and execution of this protocol, please refer to Lu et al. (2023)1.

Zebrafish functional xenograft vasculature platform identifies PF-502 as a durable vasculature normalization drug

Tumor vasculature often exhibits disorder and inefficiency. Vascular normalization offers potential for alleviating hypoxia and optimizing drug delivery in tumors. However, identifying effective agents is hindered by a lack of robust screening. We aimed to establish a comprehensive method using the zebrafish functional xenograft vasculature platform (zFXVP) to visualize and quantify tumor vasculature changes. Employing zFXVP, we systematically screened compounds, identifying PF-502 as a robust vascular normalization agent. Mechanistic studies showed PF-502 induces endothelial cell-cycle arrest, streamlines vasculature, and activates Notch1 signaling, enhancing stability and hemodynamics. In murine models, PF-502 exhibited pronounced vascular normalization and improved drug delivery at a sub-maximum tolerated dose. These findings highlight zFXVP's utility and suggest PF-502 as a promising adjunctive for vascular normalization in clinical settings.

Age exacerbates the effect of myopia on retinal capillaries and string vessels

The retinal vasculature supplies oxygen and nutrition to the cells and is crucial for an adequate retinal function. In myopia, excessive eye growth is associated with various anatomical changes that can lead to myopia-related complications. However, how myopia-induced ocular growth affects the integrity of the aged retinal microvasculature at the cellular level is not well understood. Here, we studied how aging interacts with myopia-induced alteration of the retinal microvasculature in fourteen marmoset retinas (Callithrix jacchus). String vessel and capillary branchpoint were imaged and quantified in all four capillary plexi of the retinal vasculature. As marmosets with lens-induced myopia aged, they developed increasing numbers of string vessels in all four vascular plexi, with increased vessel branchpoints in the parafoveal and peripapillary retina and decreased vessel branchpoints in the peripheral retina. These myopia-induced changes to the retinal microvasculature suggest an adaptive reorganization of the retinal microvascular cellular structure template with aging and during myopia development and progression.

Disentangling brain vasculature in neurogenesis and neurodegeneration using single-cell transcriptomics

The vasculature is increasingly recognized to impact brain function in health and disease across the life span. During embryonic brain development, angiogenesis and neurogenesis are tightly coupled, coordinating the proliferation, differentiation, and migration of neural and glial progenitors. In the adult brain, neurovascular interactions continue to play essential roles in maintaining brain function and homeostasis. This review focuses on recent advances that leverage single-cell transcriptomics of vascular cells to uncover their subtypes, their organization and zonation in the embryonic and adult brain, and how dysfunction in neurovascular and gliovascular interactions contributes to the pathogenesis of neurodegenerative diseases. Finally, we highlight key challenges for future research in neurovascular biology.

Mechanisms of skin vascular maturation and maintenance captured by longitudinal imaging of live mice

A functional network of blood vessels is essential for organ growth and homeostasis, yet how the vasculature matures and maintains homeostasis remains elusive in live mice. By longitudinally tracking the same neonatal endothelial cells (ECs) over days to weeks, we found that capillary plexus expansion is driven by vessel regression to optimize network perfusion. Neonatal ECs rearrange positions to evenly distribute throughout the developing plexus and become positionally stable in adulthood. Upon local ablation, adult ECs survive through a plasmalemmal self-repair response, while neonatal ECs are predisposed to die. Furthermore, adult ECs reactivate migration to assist vessel repair. Global ablation reveals coordinated maintenance of the adult vascular architecture that allows for eventual network recovery. Lastly, neonatal remodeling and adult maintenance of the skin vascular plexus are orchestrated by temporally restricted, neonatal VEGFR2 signaling. Our work sheds light on fundamental mechanisms that underlie both vascular maturation and adult homeostasis in vivo.

The role of blood flow in vessel remodeling and its regulatory mechanism during developmental angiogenesis

Vessel remodeling is essential for a functional and mature vascular network. According to the difference in endothelial cell (EC) behavior, we classified vessel remodeling into vessel pruning, vessel regression and vessel fusion. Vessel remodeling has been proven in various organs and species, such as the brain vasculature, subintestinal veins (SIVs), and caudal vein (CV) in zebrafish and yolk sac vessels, retina, and hyaloid vessels in mice. ECs and periendothelial cells (such as pericytes and astrocytes) contribute to vessel remodeling. EC junction remodeling and actin cytoskeleton dynamic rearrangement are indispensable for vessel pruning. More importantly, blood flow has a vital role in vessel remodeling. In recent studies, several mechanosensors, such as integrins, platelet endothelial cell adhesion molecule-1 (PECAM-1)/vascular endothelial cell (VE-cadherin)/vascular endothelial growth factor receptor 2 (VEGFR2) complex, and notch1, have been shown to contribute to mechanotransduction and vessel remodeling. In this review, we highlight the current knowledge of vessel remodeling in mouse and zebrafish models. We further underline the contribution of cellular behavior and periendothelial cells to vessel remodeling. Finally, we discuss the mechanosensory complex in ECs and the molecular mechanisms responsible for vessel remodeling.

Prerequisite endocardial-mesenchymal transition for murine cardiac trabecular angiogenesis

Coronary heart disease damages the trabecular myocardium, and the regeneration of trabecular vessels may alleviate ischemic injury. However, the origins and developmental mechanisms of trabecular vessels remain unknown. Here, we show that murine ventricular endocardial cells generate trabecular vessels through an "angioEMT" mechanism. Time course fate mapping defined a specific wave of trabecular vascularization by ventricular endocardial cells. Single-cell transcriptomics and immunofluorescence identified a subpopulation of ventricular endocardial cells that underwent endocardial-mesenchymal transition (EMT) before these cells generated trabecular vessels. Ex vivo pharmacological activation and in vivo genetic inactivation experiments identified an EMT signal in ventricular endocardial cells involving SNAI2-TGFB2/TGFBR3, which was a prerequisite for later trabecular-vessel formation. Additional loss- and gain-of-function genetic studies showed that VEGFA-NOTCH1 signaling regulated post-EMT trabecular angiogenesis by ventricular endocardial cells. Our finding that trabecular vessels originate from ventricular endocardial cells through a two-step angioEMT mechanism could inform better regeneration medicine for coronary heart disease.

Genetic or therapeutic neutralization of ALK1 reduces LDL transcytosis and atherosclerosis in mice

Low-density lipoprotein (LDL) accumulation in the arterial wall contributes to atherosclerosis initiation and progression1. Activin A receptor-like type 1 (ACVRL1, called activin-like kinase receptor (ALK1)) is a recently identified receptor that mediates LDL entry and transcytosis in endothelial cells (ECs)2,3. However, the role of this pathway in vivo is not yet known. In the present study, we show that genetic deletion of ALK1 in arterial ECs of mice substantially limits LDL accumulation, macrophage infiltration and atherosclerosis without affecting cholesterol or triglyceride levels. Moreover, a selective monoclonal antibody binding ALK1 efficiently blocked LDL transcytosis, but not bone morphogenetic protein-9 (BMP9) signaling, dramatically reducing plaque formation in LDL receptor knockout mice fed a high-fat diet. Thus, our results demonstrate that blocking LDL transcytosis into the endothelium may be a promising therapeutic strategy that targets the initiating event of atherosclerotic cardiovascular disease.

High-resolution visualization of pial surface vessels by flattened whole mount staining

Understanding development of the cerebral vasculature is essential for the central nervous system (CNS) research and therapeutic developments. Here, we developed a simple, convenient, and fast method-the flattened cortex whole mount (FCWM) technique-for imaging of pial cerebral vessels. FCWM involves dissection of the whole cerebral cortex followed by flattening, sectioning and application of CLARITY technology. Compared to conventional methods, FCWM offers several advantages including (1) high-resolution visualization of the whole cortex pial surface vessel structures and distributions; (2) precise localization of a particular blood vessel, allowing observations of a desired blood vessel during normal development or in disease settings; (3) compatibility with confocal imaging. Application of FCWM for examination of cerebral vasculature during postnatal development or in stroke settings allowed us to demonstrate that cerebral blood vessels manifest type-specific maturation and remodeling which are linked to the rate of endothelial proliferation.

The angiopoietin receptor Tie2 is atheroprotective in arterial endothelium

Leukocytes and resident cells in the arterial wall contribute to atherosclerosis, especially at sites of disturbed blood flow. Expression of endothelial Tie1 receptor tyrosine kinase is enhanced at these sites, and attenuation of its expression reduces atherosclerotic burden and decreases inflammation. However, Tie2 tyrosine kinase function in atherosclerosis is unknown. Here we provide genetic evidence from humans and from an atherosclerotic mouse model to show that TIE2 is associated with protection from coronary artery disease. We show that deletion of Tie2, or both Tie2 and Tie1, in the arterial endothelium promotes atherosclerosis by increasing Foxo1 nuclear localization, endothelial adhesion molecule expression and accumulation of immune cells. We also show that Tie2 is expressed in a subset of aortic fibroblasts, and its silencing in these cells increases expression of inflammation-related genes. Our findings indicate that unlike Tie1, the Tie2 receptor functions as the dominant endothelial angiopoietin receptor that protects from atherosclerosis.

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.

Notch Signaling in Acute Inflammation and Sepsis

Notch signaling, a highly conserved pathway in mammals, is crucial for differentiation and homeostasis of immune cells. Besides, this pathway is also directly involved in the transmission of immune signals. Notch signaling per se does not have a clear pro- or anti-inflammatory effect, but rather its impact is highly dependent on the immune cell type and the cellular environment, modulating several inflammatory conditions including sepsis, and therefore significantly impacts the course of disease. In this review, we will discuss the contribution of Notch signaling on the clinical picture of systemic inflammatory diseases, especially sepsis. Specifically, we will review its role during immune cell development and its contribution to the modulation of organ-specific immune responses. Finally, we will evaluate to what extent manipulation of the Notch signaling pathway could be a future therapeutic strategy.

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.

High-resolution imaging of the osteogenic and angiogenic interface at the site of murine cranial bone defect repair via multiphoton microscopy

The spatiotemporal blood vessel formation and specification at the osteogenic and angiogenic interface of murine cranial bone defect repair were examined utilizing a high-resolution multiphoton-based imaging platform in conjunction with advanced optical techniques that allow interrogation of the oxygen microenvironment and cellular energy metabolism in living animals. Our study demonstrates the dynamic changes of vessel types, that is, arterial, venous, and capillary vessel networks at the superior and dura periosteum of cranial bone defect, suggesting a differential coupling of the vessel type with osteoblast expansion and bone tissue deposition/remodeling during repair. Employing transgenic reporter mouse models that label distinct types of vessels at the site of repair, we further show that oxygen distributions in capillary vessels at the healing site are heterogeneous as well as time- and location-dependent. The endothelial cells coupling to osteoblasts prefer glycolysis and are less sensitive to microenvironmental oxygen changes than osteoblasts. In comparison, osteoblasts utilize relatively more OxPhos and potentially consume more oxygen at the site of repair. Taken together, our study highlights the dynamics and functional significance of blood vessel types at the site of defect repair, opening up opportunities for further delineating the oxygen and metabolic microenvironment at the interface of bone tissue regeneration.

Rab35 governs apicobasal polarity through regulation of actin dynamics during sprouting angiogenesis

In early blood vessel development, trafficking programs, such as those using Rab GTPases, are tasked with delivering vesicular cargo with high spatiotemporal accuracy. However, the function of many Rab trafficking proteins remain ill-defined in endothelial tissue; therefore, their relevance to blood vessel development is unknown. Rab35 has been shown to play an enigmatic role in cellular behaviors which differs greatly between tissue-type and organism. Importantly, Rab35 has never been characterized for its potential contribution in sprouting angiogenesis; thus, our goal was to map Rab35’s primary function in angiogenesis. Our results demonstrate that Rab35 is critical for sprout formation; in its absence, apicobasal polarity is entirely lost in vitro and in vivo. To determine mechanism, we systematically explored established Rab35 effectors and show that none are operative in endothelial cells. However, we find that Rab35 partners with DENNd1c, an evolutionarily divergent guanine exchange factor, to localize to actin. Here, Rab35 regulates actin polymerization through limiting Rac1 and RhoA activity, which is required to set up proper apicobasal polarity during sprout formation. Our findings establish that Rab35 is a potent brake of actin remodeling during blood vessel development.

The promiscuous GTPase Rab35 has been shown to be involved in many important cellular functions. In this article, Francis et al. illustrate how Rab35 acts as a critical brake to actin remodeling during sprouting angiogenesis and how it is necessary for proper blood vessel development.

Caspase-4/11 is critical for angiogenesis by repressing Notch1 signaling via inhibiting γ-secretase activity

BACKGROUND AND PURPOSE

Notch1 activation mediated by γ-secretase is critical for angiogenesis. GeneCards database predicted that Caspase-4 (CASP4, with murine ortholog CASP11) interacts with presenilin-1, the catalytic core of γ-secretase. Therefore, we investigated the role of CASP4/11 in angiogenesis.

EXPERIMENTAL APPROACH

In vivo, we studied the role of Casp11 in several angiogenesis mouse models using Casp11 wild-type and knockout mice. In vitro, we detected the effects of CASP4 on endothelial functions and Notch signaling by depleting or overexpressing CASP4 in human umbilical vein endothelial cells (HUVECs). The functional domain responsible for the binding of CASP4 and presenilin-1 was detected by mutagenesis and co-immunoprecipitation.

KEY RESULTS

Casp11 deficiency remarkably impaired adult angiogenesis in ischemic hindlimbs, melanoma xenografts and Matrigel plugs, but not the developmental angiogenesis of retina. Bone marrow transplantation revealed that the pro-angiogenic effect depended on CASP11 derived from non-hematopoietic cells. CASP4 expression was induced by inflammatory factors and CASP4 knockdown decreased cell viability, proliferation, migration and tube formation in HUVECs. Mechanistically, CASP4/11 deficiency increased Notch1 activation in vivo and in vitro, while CASP4 overexpression repressed Notch1 signaling in HUVECs. Moreover, CASP4 knockdown increased γ-secretase activity. γ-Secretase inhibitor DAPT restored the effects of CASP4 siRNA on Notch1 activation and angiogenesis in HUVECs. Notably, the catalytic activity of CASP4/11 was dispensable. Instead, CASP4 directly interacted with presenilin-1 through the caspase recruitment domain (CARD).

CONCLUSIONS AND IMPLICATIONS

These findings reveal a critical role of CASP4/11 in adult angiogenesis and make this molecule a promising therapeutic target for angiogenesis-related diseases in the future.

Flow goes forward and cells step backward: endothelial migration

Systemic and pulmonary circulations constitute a complex organ that serves multiple important biological functions. Consequently, any pathological processing affecting the vasculature can have profound systemic ramifications. Endothelial and smooth muscle are the two principal cell types composing blood vessels. Critically, endothelial proliferation and migration are central to the formation and expansion of the vasculature both during embryonic development and in adult tissues. Endothelial populations are quite heterogeneous and are both vasculature type- and organ-specific. There are profound molecular, functional, and phenotypic differences between arterial, venular and capillary endothelial cells and endothelial cells in different organs. Given this endothelial cell population diversity, it has been challenging to determine the origin of endothelial cells responsible for the angiogenic expansion of the vasculature. Recent technical advances, such as precise cell fate mapping, time-lapse imaging, genome editing, and single-cell RNA sequencing, have shed new light on the role of venous endothelial cells in angiogenesis under both normal and pathological conditions. Emerging data indicate that venous endothelial cells are unique in their ability to serve as the primary source of endothelial cellular mass during both developmental and pathological angiogenesis. Here, we review recent studies that have improved our understanding of angiogenesis and suggest an updated model of this process.

Cells that line the inside of veins possess a unique ability to grow new blood vessels and a better understanding of these cells could lead to new treatments for cancer, autoimmunity and other diseases associated with abnormal blood vessel formation. Michael Simons and colleagues from Yale University School of Medicine in New Haven, USA, review the attributes of venous endothelial cells, such as their unique ability to proliferate and migrate against blood flow, and then to form new intricate networks of minute blood vessels, in response to appropriate signals. The authors discuss emerging evidence implicating these cells in a variety of diseases, and suggest that drugs aimed at modulating the molecular function or migratory activities of venous endothelial cells could be used to correct abnormal blood vessel expansion.

The onset of PI3K-related vascular malformations occurs during angiogenesis and is prevented by the AKT inhibitor miransertib

Low‐flow vascular malformations are congenital overgrowths composed of abnormal blood vessels potentially causing pain, bleeding and obstruction of different organs. These diseases are caused by oncogenic mutations in the endothelium, which result in overactivation of the PI3K/AKT pathway. Lack of robust in vivo preclinical data has prevented the development and translation into clinical trials of specific molecular therapies for these diseases. Here, we demonstrate that the Pik3ca^H1047R activating mutation in endothelial cells triggers a transcriptome rewiring that leads to enhanced cell proliferation. We describe a new reproducible preclinical in vivo model of PI3K‐driven vascular malformations using the postnatal mouse retina. We show that active angiogenesis is required for the pathogenesis of vascular malformations caused by activating Pik3ca mutations. Using this model, we demonstrate that the AKT inhibitor miransertib both prevents and induces the regression of PI3K‐driven vascular malformations. We confirmed the efficacy of miransertib in isolated human endothelial cells with genotypes spanning most of human low‐flow vascular malformations.

Notch Signaling in Vascular Endothelial and Mural Cell Communications

The Notch signaling pathway is a highly versatile and evolutionarily conserved mechanism with an important role in cell fate determination. Notch signaling plays a vital role in vascular development, regulating several fundamental processes such as angiogenesis, arterial/venous differentiation, and mural cell investment. Aberrant Notch signaling can result in severe vascular phenotypes as observed in cerebral autosomal-dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) and Alagille syndrome. It is known that vascular endothelial cells and mural cells interact to regulate vessel formation, cell maturation, and stability of the vascular network. Defective endothelial-mural cell interactions are a common phenotype in diseases characterized by impaired vascular integrity. Further refinement of the role of Notch signaling in the vascular junctions will be critical to attempts to modulate Notch in the context of human vascular disease. In this review, we aim to consolidate and summarize our current understanding of Notch signaling in the vascular endothelial and mural cells during development and in the adult vasculature.

Colony stimulating factor-1 producing endothelial cells and mesenchymal stromal cells maintain monocytes within a perivascular bone marrow niche

Macrophage colony stimulating factor-1 (CSF-1) plays a critical role in maintaining myeloid lineage cells. However, congenital global deficiency of CSF-1 (Csf1^op/op) causes severe musculoskeletal defects that may indirectly affect hematopoiesis. Indeed, we show here that osteolineage-derived Csf1 prevented developmental abnormalities but had no effect on monopoiesis in adulthood. However, ubiquitous deletion of Csf1 conditionally in adulthood decreased monocyte survival, differentiation, and migration, independent of its effects on bone development. Bone histology revealed that monocytes reside near sinusoidal endothelial cells (ECs) and leptin receptor (Lepr)-expressing perivascular mesenchymal stromal cells (MSCs). Targeted deletion of Csf1 from sinusoidal ECs selectively reduced Ly6C^- monocytes, whereas combined depletion of Csf1 from ECs and MSCs further decreased Ly6C^hi cells. Moreover, EC-derived CSF-1 facilitated recovery of Ly6C^- monocytes and protected mice from weight loss following induction of polymicrobial sepsis. Thus, monocytes are supported by distinct cellular sources of CSF-1 within a perivascular BM niche.

Loss of vascular endothelial notch signaling promotes spontaneous formation of tertiary lymphoid structures

Tertiary lymphoid structures (TLS) are lymph node-like immune cell clusters that emerge during chronic inflammation in non-lymphoid organs like the kidney, but their origin remains not well understood. Here we show, using conditional deletion strategies of the canonical Notch signaling mediator Rbpj, that loss of endothelial Notch signaling in adult mice induces the spontaneous formation of bona fide TLS in the kidney, liver and lung, based on molecular, cellular and structural criteria. These TLS form in a stereotypical manner around parenchymal arteries, while secondary lymphoid structures remained largely unchanged. This effect is mediated by endothelium of blood vessels, but not lymphatics, since a lymphatic endothelial-specific targeting strategy did not result in TLS formation, and involves loss of arterial specification and concomitant acquisition of a high endothelial cell phenotype, as shown by transcriptional analysis of kidney endothelial cells. This indicates a so far unrecognized role for vascular endothelial cells and Notch signaling in TLS initiation.

Loss of canonical Notch signaling in vascular endothelial cells induces spontaneous formation of proto-typical tertiary lymphoid structures in mouse kidney, liver and lungs, which form around central arteries that acquire a high endothelial cell signature

A High-Efficiency AAV for Endothelial Cell Transduction Throughout the Central Nervous System

Endothelial cells have a crucial role in nervous system function, and mounting evidence points to endothelial impairment as a major contributor to a wide range of neurological diseases. However, tools to genetically interrogate these cells in vivo remain limited. Here, we describe AAV-BI30, a capsid that specifically and efficiently transduces endothelial cells throughout the central nervous system. At relatively low systemic doses, this vector transduces the majority of arterial, capillary, and venous endothelial cells in the brain, retina, and spinal cord vasculature of adult C57BL/6 mice. Furthermore, we show that AAV-BI30 robustly transduces endothelial cells in multiple mouse strains and rats in vivo and human brain microvascular endothelial cells in vitro. Finally, we demonstrate AAV-BI30's capacity to achieve efficient and endothelial-specific Cre-mediated gene manipulation in the central nervous system. This combination of attributes makes AAV-BI30 uniquely well-suited to address outstanding research questions in neurovascular biology and aid the development of therapeutics to remediate endothelial dysfunction in disease.