Microarray analysis of retinal endothelial tip cells identifies CXCR4 as a mediator of tip cell morphology and branching

Molecular hydrogen promotes retinal vascular regeneration and attenuates neovascularization and neuroglial dysfunction in oxygen-induced retinopathy mice

BACKGROUND

Retinopathy of Prematurity (ROP) is a proliferative retinal vascular disease occurring in the retina of premature infants and is the main cause of childhood blindness. Nowadays anti-VEGF and retinal photocoagulation are mainstream treatments for ROP, but they develop a variety of complications. Hydrogen (H2) is widely considered as a useful neuroprotective and antioxidative therapeutic method for hypoxic-ischemic disease without toxic effects. However, whether H2 provides physiological angiogenesis promotion, neovascularization suppression and glial protection in the progression of ROP is largely unknown.This study aims to investigate the effects of H2 on retinal angiogenesis, neovascularization and neuroglial dysfunction in the retinas of oxygen-induced retinopathy (OIR) mice.

METHODS

In this study, mice that were seven days old and either wild-type (WT) or Nrf2-deficient (Nrf2-/-) were exposed to 75% oxygen for 5 days and then returned to normal air conditions. Different stages of hydrogen gas (H2) inhalation were administered. Vascular obliteration, neovascularization, and blood vessel leakage were analyzed and compared. To count the number of neovascularization endothelial nuclei, routine HE staining of retinal sections was conducted. Immunohistochemistry was performed using DyLight 594 labeled GSL I-isolectin B4 (IB4), as well as primary antibodies against proliferating cell nuclear antigen (PCNA), glial fibrillary acidic protein (GFAP), and Iba-1. Western blots were used to measure the expression of NF-E2-related factor 2 (Nrf2), vascular endothelial growth factor (VEGF), Notch1, Dll4, and HIF-1α. Additionally, the expression of target genes such as NQO1, HO-1, Notch1, Hey1, Hey2, and Dll4 was measured. Human umbilical vein endothelial cells (HUVECs) treated with H2 under hypoxia were used as an in vitro model. RT-PCR was used to evaluate the mRNA expression of Nrf2, Notch/Dll4, and the target genes. The expression of reactive oxygen species (ROS) was observed using immunofluorescence staining.

RESULTS

Our results indicate that 3-4% H2 does not disturb retinal physiological angiogenesis, but ameliorates vaso-obliteration and neovascularization in OIR mice. Moreover, H2 prevents the decreased density and reverses the morphologic and functional changes in retinal astrocytes caused by oxygen-induced injury. In addition, H2 inhalation reduces microglial activation, especially in the area of neovascularization in OIR mice. H2 plays a protective role in vascular regeneration by promoting Nrf2 activation and suppressing the Dll4-induced Notch signaling pathway in vivo. Also, H2 promotes the proliferation of HUVECs under hypoxia by negatively regulating the Dll4/Notch pathway and reducing ROS levels through Nrf2 pathway aligning with our findings in vivo.Moreover, the retinal oxygen-sensing mechanisms (HIF-1α/VEGF) are also involved in hydrogen-mediated retinal revascularization and neovascularization suppression.

CONCLUSIONS

Collectively, our results indicate that H2 could be a promising therapeutic agent for POR treatment and that its beneficial effect in human ROP might involve the activation of the Nrf2-Notch axis as well as HIF-1α/VEGF pathways.

New Insights in ATP Synthesis as Therapeutic Target in Cancer and Angiogenic Ocular Diseases

Lactate and ATP formation by aerobic glycolysis, the Warburg effect, is considered a hallmark of cancer. During angiogenesis in non-cancerous tissue, proliferating stalk endothelial cells (ECs) also produce lactate and ATP by aerobic glycolysis. In fact, all proliferating cells, both non-cancer and cancer cells, need lactate for the biosynthesis of building blocks for cell growth and tissue expansion. Moreover, both non-proliferating cancer stem cells in tumors and leader tip ECs during angiogenesis rely on glycolysis for pyruvate production, which is used for ATP synthesis in mitochondria through oxidative phosphorylation (OXPHOS). Therefore, aerobic glycolysis is not a specific hallmark of cancer but rather a hallmark of proliferating cells and limits its utility in cancer therapy. However, local treatment of angiogenic eye conditions with inhibitors of glycolysis may be a safe therapeutic option that warrants experimental investigation. Most types of cells in the eye such as photoreceptors and pericytes use OXPHOS for ATP production, whereas proliferating angiogenic stalk ECs rely on glycolysis for lactate and ATP production. (J Histochem Cytochem XX.XXX-XXX, XXXX).

FOXO1 stimulates tip cell-enriched gene expression in endothelial cells

Forkhead box O (FOXO) family proteins are expressed in various cells, and play crucial roles in cellular metabolism, apoptosis, and aging. FOXO1-null mice exhibit embryonic lethality due to impaired endothelial cell (EC) maturation and vascular remodeling. However, FOXO1-mediated genome-wide regulation in ECs remains unclear. Here, we demonstrate that VEGF dynamically regulates FOXO1 cytosol-nucleus translocation. FOXO1 re-localizes to the nucleus via PP2A phosphatase. RNA-seq combined with FOXO1 overexpression/knockdown in ECs demonstrated that FOXO1 governs the VEGF-responsive tip cell-enriched genes, and further inhibits DLL4-NOTCH signaling. Endogenous FOXO1 ChIP-seq revealed that FOXO1 binds to the EC-unique tip-enriched genes with co-enrichment of EC master regulators, and the condensed chromatin region as a pioneer factor. We identified new promoter/enhancer regions of the VEGF-responsive tip cell genes regulated by FOXO1: ESM1 and ANGPT2. This is the first study to identify cell type-specific FOXO1 functions, including VEGF-mediated tip cell definition in primary cultured ECs.

VEGF signaling: Role in angiogenesis and beyond

Angiogenesis is a crucial process for tissue development, repair, and tumor survival. Vascular endothelial growth factor (VEGF) is a key driver secreted by cancer cells, promoting neovascularization. While VEGF's role in angiogenesis is well-documented, its influence on the other aspects in tumor microenvironemt is less discussed. This review elaborates on VEGF's impact on intercellular interactions within the tumor microenvironment, including how VEGF affects pericyte proliferation and migration and mediates interactions between tumor-associated macrophages and cancer cells, resulting in PDL-1-mediated immunosuppression and Nrf2-mediated epithelial-mesenchymal transition. The review discusses VEGF's involvement in intra-organelle crosstalk, tumor metabolism, stemness, and epithelial-mesenchymal transition. It also provides insights into current anti-VEGF therapies and their limitations in cancer treatment. Overall, this review aims to provide a thorough overview of the current state of knowledge concerning VEGF signaling and its impact, not only on angiogenesis but also on various other oncogenic processes.

Integration of Pan-Cancer Single-Cell and Spatial Transcriptomics Reveals Stromal Cell Features and Therapeutic Targets in Tumor Microenvironment

Stromal cells are physiologically essential components of the tumor microenvironment (TME) that mediates tumor development and therapeutic resistance. Development of a logical and unified system for stromal cell type identification and characterization of corresponding functional properties could help design antitumor strategies that target stromal cells. Here, we performed a pan-cancer analysis of 214,972 nonimmune stromal cells using single-cell RNA sequencing from 258 patients across 16 cancer types and analyzed spatial transcriptomics from 16 patients across seven cancer types, including six patients receiving anti-PD-1 treatment. This analysis uncovered distinct features of 39 stromal subsets across cancer types, including various functional modules, spatial locations, and clinical and therapeutic relevance. Tumor-associated PGF+ endothelial tip cells with elevated epithelial-mesenchymal transition features were enriched in immune-depleted TME and associated with poor prognosis. Fibrogenic and vascular pericytes (PC) derived from FABP4+ progenitors were two distinct tumor-associated PC subpopulations that strongly interacted with PGF+ tips, resulting in excess extracellular matrix (ECM) abundance and dysfunctional vasculature. Importantly, ECM-related cancer-associated fibroblasts enriched at the tumor boundary acted as a barrier to exclude immune cells, interacted with malignant cells to promote tumor progression, and regulated exhausted CD8+ T cells via immune checkpoint ligand-receptors (e.g., LGALS9/TIM-3) to promote immune escape. In addition, an interactive web-based tool (http://www.scpanstroma.yelab.site/) was developed for accessing, visualizing, and analyzing stromal data. Taken together, this study provides a systematic view of the highly heterogeneous stromal populations across cancer types and suggests future avenues for designing therapies to overcome the tumor-promoting functions of stromal cells.

SIGNIFICANCE

Comprehensive characterization of tumor-associated nonimmune stromal cells provides a robust resource for dissecting tumor microenvironment complexity and guiding stroma-targeted therapy development across multiple human cancer types.

Role of cell rearrangement and related signaling pathways in the dynamic process of tip cell selection

Angiogenesis is a complex, highly-coordinated and multi-step process of new blood vessel formation from pre-existing blood vessels. When initiated, the sprouting process is spearheaded by the specialized endothelial cells (ECs) known as tip cells, which guide the organization of accompanying stalk cells and determine the function and morphology of the finally-formed blood vessels. Recent studies indicate that the orchestration and coordination of angiogenesis involve dynamic tip cell selection, which is the competitive selection of cells to lead the angiogenic sprouts. Therefore, this review attempt to summarize the underlying mechanisms involved in tip cell specification in a dynamic manner to enable readers to gain a systemic and overall understanding of tip cell formation, involving cooperative interaction of cell rearrangement with Notch and YAP/TAZ signaling. Various mechanical and chemical signaling cues are integrated to ensure the right number of cells at the right place during angiogenesis, thereby precisely orchestrating morphogenic functions that ensure correct patterning of blood vessels. Video Abstract.

Role of Novel Inflammatory Factors in Central Retinal Vein Occlusion with Macular Edema

Background and Objectives: To investigate associations among the aqueous humor levels of novel inflammatory factors, including FMS-related tyrosine kinase 3 ligand (Flt-3L), fractalkine, CXC chemokine ligand 16 (CXCL-16), and endocan-1; the severity of macular edema in central retinal vein occlusion (CRVO); and the prognosis of CRVO with macular edema after antivascular endothelial growth factor (VEGF) therapy. Materials and Methods: Aqueous humor was obtained during anti-VEGF treatment with intravitreal ranibizumab injection (IRI) in patients with CRVO and macular edema (n = 19) and during cataract surgery in patients with cataracts (controls, n = 20), and the levels of VEGF and novel inflammatory factors were measured. Macular edema was evaluated by central macular thickness (CMT) and neurosensory retinal thickness (TNeuro), and improvement was evaluated by calculating the percentage change in CMT and TNeuro from before to 1 month after IRI. Results: The levels of VEGF and the novel inflammatory factors were significantly higher in the CRVO group, and the levels of Flt-3L, CXCL-16, and endocan-1 were significantly correlated with each other and with the aqueous flare value. Baseline levels of Flt-3L, CXCL-16, and endocan-1 had a significantly negative correlation with the change in CMT, and the baseline level of CXCL-16 was significantly negatively correlated with the change in TNeuro. Conclusions: Relations among novel inflammatory factors should be further investigated. These findings may help improve understanding of macular edema in CRVO patients and aid the development of new treatments targeting novel inflammatory factors.

Serum endocan levels in children with rheumatic aortic insufficiency: can it differentiate bicuspid aortic valve disease from rheumatic heart disease?

AIM

In this study, it was aimed to examine the serum endocan levels in patients with rheumatic aortic regurgitation and to investigate whether it has a value in differentiating it from aortic regurgitation due to bicuspid aortic valve.

METHODS

Blood samples were collected from patients with rheumatic aortic regurgitation (Group 1), incidentally diagnosed patients with borderline or definite rheumatic aortic regurgitation (Group 2), children with bicuspid aortic valve accompanied by aortic regurgitation (Group 3) and healthy children (Group 4) of similar age.

RESULTS

There were 12 children in Group 1, 13 in Group 2, 25 in Group 3, and 25 in Group 4. Groups were similar in terms of age (p = 0.291). There was no statistically significant difference between median serum endocan levels of Group 1 and Group 2 (p = 0.624), and Group 3 and Group 4 (p = 0.443). Despite that, the median serum endocan levels of Group 1 and Group 2 were significantly higher than that of both Group 3 and Group 4 (p = 0.000 for all).

CONCLUSIONS

Our results indicate that serum endocan level can be used to differentiate rheumatic aortic regurgitation from non-rheumatic aortic regurgitation. It is thought that the prognostic role of this marker should be confirmed in long-term, prospective studies with larger samples.

Fibrillin-1 regulates endothelial sprouting during angiogenesis

Fibrillin-1 is an extracellular matrix protein that assembles into microfibrils which provide critical functions in large blood vessels and other tissues. Mutations in the fibrillin-1 gene are associated with cardiovascular, ocular, and skeletal abnormalities in Marfan syndrome. Here, we reveal that fibrillin-1 is critical for angiogenesis which is compromised by a typical Marfan mutation. In the mouse retina vascularization model, fibrillin-1 is present in the extracellular matrix at the angiogenic front where it colocalizes with microfibril-associated glycoprotein-1, MAGP1. In Fbn1C1041G/+ mice, a model of Marfan syndrome, MAGP1 deposition is reduced, endothelial sprouting is decreased, and tip cell identity is impaired. Cell culture experiments confirmed that fibrillin-1 deficiency alters vascular endothelial growth factor-A/Notch and Smad signaling which regulate the acquisition of endothelial tip cell/stalk cell phenotypes, and we showed that modulation of MAGP1 expression impacts these pathways. Supplying the growing vasculature of Fbn1C1041G/+ mice with a recombinant C-terminal fragment of fibrillin-1 corrects all defects. Mass spectrometry analyses showed that the fibrillin-1 fragment alters the expression of various proteins including ADAMTS1, a tip cell metalloprotease and matrix-modifying enzyme. Our data establish that fibrillin-1 is a dynamic signaling platform in the regulation of cell specification and matrix remodeling at the angiogenic front and that mutant fibrillin-1-induced defects can be rescued pharmacologically using a C-terminal fragment of the protein. These findings, identify fibrillin-1, MAGP1, and ADAMTS1 in the regulation of endothelial sprouting, and contribute to our understanding of how angiogenesis is regulated. This knowledge may have critical implications for people with Marfan syndrome.

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.

Shaping the brain vasculature in development and disease in the single-cell era

The CNS critically relies on the formation and proper function of its vasculature during development, adult homeostasis and disease. Angiogenesis - the formation of new blood vessels - is highly active during brain development, enters almost complete quiescence in the healthy adult brain and is reactivated in vascular-dependent brain pathologies such as brain vascular malformations and brain tumours. Despite major advances in the understanding of the cellular and molecular mechanisms driving angiogenesis in peripheral tissues, developmental signalling pathways orchestrating angiogenic processes in the healthy and the diseased CNS remain incompletely understood. Molecular signalling pathways of the 'neurovascular link' defining common mechanisms of nerve and vessel wiring have emerged as crucial regulators of peripheral vascular growth, but their relevance for angiogenesis in brain development and disease remains largely unexplored. Here we review the current knowledge of general and CNS-specific mechanisms of angiogenesis during brain development and in brain vascular malformations and brain tumours, including how key molecular signalling pathways are reactivated in vascular-dependent diseases. We also discuss how these topics can be studied in the single-cell multi-omics era.

DACH1 inhibits the proliferation and migration of papillary thyroid carcinoma

DACH1 is an important component of the retinal determinate gene network (RDGN), which regulates the expression of target genes by directly binding or interacting with other factors. DACH1 shows inhibitory effects in most tumors, but its role in papillary thyroid carcinoma is unclear and warrants further investigation. We assessed the expression of DACH1 in different tissues and correlation with immune infiltration by The Cancer Genome Atlas (TCGA) and Tumor Immune Estimation Resource (TIMMER2.0 databases). The effects of DACH1 on the proliferation and migration of TPC-1 and Bcpap cells were assessed by cell viability assay, colony formation assay, wound healing assay, transwell migration assay, and flow cytometry. Finally, the effects of DACH1 on CXCL8, CXCL10, and CXCL12 expression in Nthy-ori-3-1, TPC-1 and Bcpap cells were assessed by enzyme-linked immunosorbent assay kit and real-time polymerase chain reaction, respectively. The results showed that DACH1 was differentially expressed in different tumors and tissues. Basal expression of DACH1 was lower in thyroid and papillary thyroid carcinoma than in other normal tissues and corresponding tumors, and positively correlated with CD8⁺ T cell infiltration. In Nthy-ori-3-1, TPC-1 and Bcpap cells, overexpression of DACH1 inhibited cell migration and proliferation, and the opposite results was obtained by knocking down DACH1 using small interfering RNA. We also demonstrated that DACH1 regulated chemokines CXCL8, CXCL10, and CXCL12, thereby modulating tumor immunity.

An expression and function analysis of the CXCR4/SDF-1 signalling axis during pituitary gland development

The chemokine SDF-1 (CXCL12) and its receptor CXCR4 control several processes during embryonic development such as the regulation of stem cell proliferation, differentiation, and migration. However, the role of this pathway in the formation of the pituitary gland is not understood. We sought to characterise the expression patterns of CXCR4, SDF-1 and CXCR7 at different stages of pituitary gland development. Our expression profiling revealed that SDF-1 is expressed in progenitor-rich regions of the pituitary anterior lobe, that CXCR4 and CXCR7 have opposite expression domains and that CXCR4 expression is conserved between mice and human embryos. We then assessed the importance of this signalling pathway in the development and function of the murine pituitary gland through conditional deletion of CXCR4 in embryonic pituitary progenitors. Successful and specific ablation of CXCR4 expression in embryonic pituitary progenitors did not lead to observable embryonic nor postnatal defects but allowed the identification of stromal CXCR4+ cells not derived from HESX1+ progenitors. Further analysis of constitutive SDF-1, CXCR7 and CXCR4 mutants of the pathway indicates that CXCR4 expression in HESX1+ cells and their descendants is not essential for normal pituitary development in mice.

Long noncoding RNA GATA2-AS1 augments endothelial Hypoxia Inducible Factor 1-α induction and regulates hypoxic signaling

Vascular endothelial cells form the inner cellular lining of blood vessels and have myriad physiologic functions including angiogenesis and response to hypoxia. We recently identified a set of endothelial cell (EC)-enriched long noncoding RNAs (lncRNAs) in differentiated human primary cell types and described the role of the STEEL lncRNA in angiogenic patterning. We sought to further understand the role of EC-enriched lncRNAs in physiologic adaptation of the vascular endothelium. In this work, we describe an abundant, cytoplasmic, and EC-enriched lncRNA, GATA2-AS1, that is divergently transcribed from the EC-enriched developmental regulator, GATA2. While GATA2-AS1 is largely co-expressed with GATA2 in ECs, GATA2-AS1 and GATA2 appear to be complementary rather than synergistic as they have mostly distinct target genes. Common single nucleotide variants (SNVs) in GATA2-AS1 exons are associated with early onset coronary artery disease (CAD) and decreased expression of GATA2-AS1 in endothelial cell lines. In most cells, HIF1-α is central to the transcriptional response to hypoxia, while in ECs, both HIF1-α and HIF2-α are required to coordinate an acute and chronic response respectively. In this setting, GATA2-AS1 contributes to the "HIF switch" and augments HIF1-α induction in acute hypoxia to regulate HIF1-α/ HIF2-α balance. In hypoxia, GATA2-AS1 orchestrates HIF1-α-dependent induction of the glycolytic pathway, and HIF1-α-independent maintenance of mitochondrial biogenesis. Similarly, GATA2-AS1 coordinates both metabolism and "tip/stalk" cell signaling to regulate angiogenesis in hypoxic ECs. Furthermore, we find that GATA2-AS1 expression patterns are perturbed in atherosclerotic disease. Together, these results define a role for GATA2-AS1 in the EC-specific response to hypoxia.

Single-cell analysis of multiple cancer types reveals differences in endothelial cells between tumors and normal tissues

Endothelial cells (ECs) play an important role in tumor progression. Currently, the main target of anti-angiogenic therapy is the vascular endothelial growth factor (VEGF) pathway. Some patients do benefit from anti-VEGF/VEGFR therapy; however, a large number of patients do not have response or acquire drug resistance after treatment. Moreover, anti-VEGF/VEGFR therapy may lead to nephrotoxicity and cardiovascular-related side effects due to its action on normal ECs. Therefore, it is necessary to identify targets that are specific to tumor ECs and could be applied to various cancer types. We integrated single-cell RNA sequencing data from six cancer types and constructed a multi-cancer EC atlas to decode the characteristic of tumor ECs. We found that tip-like ECs mainly exist in tumor tissues but barely exist in normal tissues. Tip-like ECs are involved in the promotion of tumor angiogenesis and inhibition on anti-tumor immune responses. Moreover, tumor cells, myeloid cells, and pericytes are the main sources of pro-angiogenic factors. High proportion of tip-like ECs is associated with poor prognosis in multiple cancer types. We also identified that prostate-specific membrane antigen (PSMA) is a specific marker for tip-like ECs in all the cancer types we studied. In summary, we demonstrate that tip-like ECs are the main differential EC subcluster between tumors and normal tissues. Tip-like ECs may promote tumor progression through promoting angiogenesis while inhibiting anti-tumor immune responses. PSMA was a specific marker for tip-like ECs, which could be used as a potential target for the diagnosis and treatment of non-prostate cancers.

Transcriptomic analysis of 3D vasculature-on-a-chip reveals paracrine factors affecting vasculature growth and maturation

In vitro models of vasculature are of great importance for modelling vascular physiology and pathology. However, there is usually a lack of proper spatial patterning of interacting heterotypic cells in conventional vasculature dish models, which might confound results between contact and non-contact interactions. We use a microfluidic platform with structurally defined separation between human microvasculature and fibroblasts to probe their dynamic, paracrine interactions. We also develop a novel, versatile technique to retrieve cells embedded in extracellular matrix from the microfluidic device for downstream transcriptomic analysis, and uncover growth factor and cytokine expression profiles associated with improved vasculature growth. Paired receptor-ligand analysis further reveals paracrine signaling molecules that could be supplemented into the medium for vasculatures models where fibroblast coculture is undesirable or infeasible. These findings also provide deeper insights into the molecular cues for more physiologically relevant vascular mimicry and vascularized organoid model for clinical applications such as drug screening and disease modeling.

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.

Pericyte-to-endothelial cell signaling via vitronectin-integrin regulates blood-CNS barrier

Endothelial cells of blood vessels of the central nervous system (CNS) constitute blood-CNS barriers. Barrier properties are not intrinsic to these cells; rather they are induced and maintained by CNS microenvironment. Notably, the abluminal surfaces of CNS capillaries are ensheathed by pericytes and astrocytes. However, extrinsic factors from these perivascular cells that regulate barrier integrity are largely unknown. Here, we establish vitronectin, an extracellular matrix protein secreted by CNS pericytes, as a regulator of blood-CNS barrier function via interactions with its integrin receptor, α5, in endothelial cells. Genetic ablation of vitronectin or mutating vitronectin to prevent integrin binding, as well as endothelial-specific deletion of integrin α5, causes barrier leakage in mice. Furthermore, vitronectin-integrin α5 signaling maintains barrier integrity by actively inhibiting transcytosis in endothelial cells. These results demonstrate that signaling from perivascular cells to endothelial cells via ligand-receptor interactions is a key mechanism to regulate barrier permeability.

Cellular metabolic adaptations in rheumatoid arthritis and their therapeutic implications

Activation of endothelium and immune cells is fundamental to the initiation of autoimmune diseases such as rheumatoid arthritis (RA), and it results in trans-endothelial cell migration and synovial fibroblast proliferation, leading to joint destruction. In RA, the synovial microvasculature is highly dysregulated, resulting in inefficient oxygen perfusion to the synovium, which, along with the high metabolic demands of activated immune and stromal cells, leads to a profoundly hypoxic microenvironment. In inflamed joints, infiltrating immune cells and synovial resident cells have great requirements for energy and nutrients, and they adapt their metabolic profiles to generate sufficient energy to support their highly activated inflammatory states. This shift in metabolic capacity of synovial cells enables them to produce the essential building blocks to support their proliferation, activation and invasiveness. Furthermore, it results in the accumulation of metabolic intermediates and alteration of redox-sensitive pathways, affecting signalling pathways that further potentiate the inflammatory response. Importantly, the inflamed synovium is a multicellular tissue, with cells differing in their metabolic requirements depending on complex cell-cell interactions, nutrient supply, metabolic intermediates and transcriptional regulation. Therefore, understanding the complex interplay between metabolic and inflammatory pathways in synovial cells in RA will provide insight into the underlying mechanisms of disease pathogenesis.

Contribution of Endothelial Laminin-Binding Integrins to Cellular Processes Associated with Angiogenesis

Endothelial cells engage extracellular matrix and basement membrane components through integrin-mediated adhesion to promote angiogenesis. Angiogenesis involves the sprouting of endothelial cells from pre-existing vessels, their migration into surrounding tissue, the upregulation of angiogenesis-associated genes, and the formation of new endothelial tubes. To determine whether the endothelial laminin-binding integrins, α6β4, and α3β1 contribute to these processes, we employed RNAi technology in organotypic angiogenesis assays, as well in migration assays, in vitro. The endothelial depletion of either α6β4 or α3β1 inhibited endothelial sprouting, indicating that these integrins have non-redundant roles in this process. Interestingly, these phenotypes were accompanied by overlapping and distinct changes in the expression of angiogenesis-associated genes. Lastly, depletion of α6β4, but not α3β1, inhibited migration. Taken together, these results suggest that laminin-binding integrins regulate processes associated with angiogenesis by distinct and overlapping mechanisms.

eNOS controls angiogenic sprouting and retinal neovascularization through the regulation of endothelial cell polarity

The roles of nitric oxide (NO) and endothelial NO synthase (eNOS) in the regulation of angiogenesis are well documented. However, the involvement of eNOS in the sprouting of endothelial tip-cells at the vascular front during sprouting angiogenesis remains poorly defined. In this study, we show that downregulation of eNOS markedly inhibits VEGF-stimulated migration of endothelial cells but increases their polarization, as evidenced by the reorientation of the Golgi in migrating monolayers and by the fewer filopodia on tip cells at ends of sprouts in endothelial cell spheroids. The effect of eNOS inhibition on EC polarization was prevented in Par3-depleted cells. Importantly, downregulation of eNOS increased the expression of polarity genes, such as PARD3B, PARD6A, PARD6B, PKCΖ, TJP3, and CRB1 in endothelial cells. In retinas of eNOS knockout mice, vascular development is retarded with decreased vessel density and vascular branching. Furthermore, tip cells at the extremities of the vascular front have a marked reduction in the number of filopodia per cell and are more oriented. In a model of oxygen-induced retinopathy (OIR), eNOS deficient mice are protected during the initial vaso-obliterative phase, have reduced pathological neovascularization, and retinal endothelial tip cells have fewer filopodia. Single-cell RNA sequencing of endothelial cells from OIR retinas revealed enrichment of genes related to cell polarity in the endothelial tip-cell subtype of eNOS deficient mice. These results indicate that inhibition of eNOS alters the polarity program of endothelial cells, which increases cell polarization, regulates sprouting angiogenesis and normalizes pathological neovascularization during retinopathy.

Proteoglycan Endocan: A multifaceted therapeutic target in Cancer

Endocan is known to be a circulating dermatan sulfate proteoglycan that regulates endothelial cell function. Dysregulation of endocan expression is observed not only in the tumor vasculature but also in cancer cells. Accumulating evidence has revealed that disordered endocan facilitates cancer progression via enhancing cancer cell proliferation, cell mobility, and cancer stemness properties. Recently, various interacting proteins and diverse subcellular localizations of endocan were identified in cancer cells. Herein, we summarize the application of endocan in cancer diagnoses and prognoses using serum and tumor specimens. We further discuss that the aberrant molecular characteristics of endocan may be due to the mislocalization of endocan in cancer cells. Defining the specific cellular roles of endocan will provide a promising diagnostic factor and therapeutic target for cancer patients.

Pro-angiogenic approach for skeletal muscle regeneration

The angiogenesis process is a phenomenon in which numerous molecules participate in the stimulation of the new vessels' formation from pre-existing vessels. Angiogenesis is a crucial step in tissue regeneration and recovery of organ and tissue function. Muscle diseases affect millions of people worldwide overcome the ability of skeletal muscle to self-repair. Pro-angiogenic therapies are key in skeletal muscle regeneration where both myogenesis and angiogenesis occur. These therapies have been based on mesenchymal stem cells (MSCs), exosomes, microRNAs (miRs) and delivery of biological factors. The use of different calls of biomaterials is another approach, including ceramics, composites, and polymers. Natural polymers are use due its bioactivity and biocompatibility in addition to its use as scaffolds and in drug delivery systems. One of these polymers is the natural rubber latex (NRL) which is biocompatible, bioactive, versatile, low-costing, and capable of promoting tissue regeneration and angiogenesis. In this review, the advances in the field of pro-angiogenic therapies are discussed.

The histone acetyltransferase HBO1 promotes efficient tip cell sprouting during angiogenesis

Blood vessel growth and remodelling are essential during embryonic development and disease pathogenesis. The diversity of endothelial cells (ECs) is transcriptionally evident and ECs undergo dynamic changes in gene expression during vessel growth and remodelling. Here, we investigated the role of the histone acetyltransferase HBO1 (KAT7), which is important for activating genes during development and for histone H3 lysine 14 acetylation (H3K14ac). Loss of HBO1 and H3K14ac impaired developmental sprouting angiogenesis and reduced pathological EC overgrowth in the retinal endothelium. Single-cell RNA sequencing of retinal ECs revealed an increased abundance of tip cells in Hbo1-deficient retinas, which led to EC overcrowding in the retinal sprouting front and prevented efficient tip cell migration. We found that H3K14ac was highly abundant in the endothelial genome in both intra- and intergenic regions, suggesting that HBO1 acts as a genome organiser that promotes efficient tip cell behaviour necessary for sprouting angiogenesis. This article has an associated 'The people behind the papers' interview.

Selective and Marked Blockade of Endothelial Sprouting Behavior Using Paclitaxel and Related Pharmacologic Agents

Whether alterations in the microtubule cytoskeleton affect the ability of endothelial cells (ECs) to sprout and form branching networks of tubes was investigated in this study. Bioassays of human EC tubulogenesis, where both sprouting behavior and lumen formation can be rigorously evaluated, were used to demonstrate that addition of the microtubule-stabilizing drugs, paclitaxel, docetaxel, ixabepilone, and epothilone B, completely interferes with EC tip cells and sprouting behavior, while allowing for EC lumen formation. In bioassays mimicking vasculogenesis using single or aggregated ECs, these drugs induce ring-like lumens from single cells or cyst-like spherical lumens from multicellular aggregates with no evidence of EC sprouting behavior. Remarkably, treatment of these cultures with a low dose of the microtubule-destabilizing drug, vinblastine, led to an identical result, with complete blockade of EC sprouting, but allowing for EC lumen formation. Administration of paclitaxel in vivo markedly interfered with angiogenic sprouting behavior in developing mouse retina, providing corroboration. These findings reveal novel biological activities for pharmacologic agents that are widely utilized in multidrug chemotherapeutic regimens for the treatment of human malignant cancers. Overall, this work demonstrates that manipulation of microtubule stability selectively interferes with the ability of ECs to sprout, a necessary step to initiate and form branched capillary tube networks.

Dynamic Expression of Membrane Type 1-Matrix Metalloproteinase (Mt1-mmp/Mmp14) in the Mouse Embryo

MT1-MMP/MMP14 belongs to a subgroup of the matrix metalloproteinases family that presents a transmembrane domain, with a cytosolic tail and the catalytic site exposed to the extracellular space. Deficient mice for this enzyme result in early postnatal death and display severe defects in skeletal, muscle and lung development. By using a transgenic line expressing the LacZ reporter under the control of the endogenous Mt1-mmp promoter, we reported a dynamic spatiotemporal expression pattern for Mt1-mmp from early embryonic to perinatal stages during cardiovascular development and brain formation. Thus, Mt1-mmp shows expression in the endocardium of the heart and the truncus arteriosus by E8.5, and is also strongly detected during vascular system development as well as in endothelial cells. In the brain, LacZ reporter expression was detected in the olfactory bulb, the rostral cerebral cortex and the caudal mesencephalic tectum. LacZ-positive cells were observed in neural progenitors of the spinal cord, neural crest cells and the intersomitic region. In the limb, Mt1-mmp expression was restricted to blood vessels, cartilage primordium and muscles. Detection of the enzyme was confirmed by Western blot and immunohistochemical analysis. We suggest novel functions for this metalloproteinase in angiogenesis, endocardial formation and vascularization during organogenesis. Moreover, Mt1-mmp expression revealed that the enzyme may contribute to heart, muscle and brain throughout development.

Modulation of endothelium function by fatty acids

The endothelium acts as the barrier that prevents circulating lipids such as lipoproteins and fatty acids into the arterial wall; it also regulates normal functioning in the circulatory system by balancing vasodilation and vasoconstriction, modulating the several responses and signals. Plasma lipids can interact with endothelium via different mechanisms and produce different phenotypes. Increased plasma-free fatty acids (FFAs) levels are associated with the pathogenesis of atherosclerosis and cardiovascular diseases (CVD). Because of the multi-dimensional roles of plasma FFAs in mediating endothelial dysfunction, increased FFA level is now considered an essential link in the onset of endothelial dysfunction in CVD. FFA-mediated endothelial dysfunction involves several mechanisms, including dysregulated production of nitric oxide and cytokines, metaflammation, oxidative stress, inflammation, activation of the renin-angiotensin system, and apoptosis. Therefore, modulation of FFA-mediated pathways involved in endothelial dysfunction may prevent the complications associated with CVD risk. This review presents details as to how endothelium is affected by FFAs involving several metabolic pathways.

Matrix stiffness modulates tip cell formation through the p-PXN-Rac1-YAP signaling axis

Endothelial tip cell outgrowth of blood-vessel sprouts marks the initiation of angiogenesis which is critical in physiological and pathophysiological procedures. However, how mechanical characteristics of extracellular matrix (ECM) modulates tip cell formation has been largely neglected. In this study, we found enhanced CD31 expression in the stiffening outer layer of hepatocellular carcinoma than in surrounding soft tissues. Stiffened matrix promoted sprouting from endothelial cell (EC) spheroids and upregulated expressions of tip cell-enriched genes in vitro. Moreover, tip cells showed increased cellular stiffness, more actin cytoskeleton organization and enhanced YAP nuclear transfer than stalk and phalanx ECs. We further uncovered that substrate stiffness regulates FAK and Paxillin phosphorylation in focal adhesion of ECs promoting Rac1 transition from inactive to active state. YAP is subsequently activated and translocated into nucleus, leading to increased tip cell specification. p-Paxillin can also loosen the intercellular connection which also facilitates tip cell specification. Collectively our present study shows that matrix stiffness modulates tip cell formation through p-PXN-Rac1-YAP signaling axis, shedding light on the role of mechanotransduction in tip cell formation. This is of special significance in biomaterial design and treatment of some pathological situations.

•

Mechanotransduction is implicated in angiogenesis and tip cell formation.

•

Tip cells showed different mechanical properties from stalk and phalanx ECs.

•

Paxillin, Rac1 and YAP might be novel treatment targets for some diseases.

•

Material stiffness affects tip cell specification.

Matricellular Protein SPARCL1 Regulates Blood Vessel Integrity and Antagonizes Inflammatory Bowel Disease

BACKGROUND

The understanding of vascular plasticity is key to defining the role of blood vessels in physiologic and pathogenic processes. In the present study, the impact of the vascular quiescence marker SPARCL1 on angiogenesis, capillary morphogenesis, and vessel integrity was evaluated.

METHODS

Angiogenesis was studied using the metatarsal test, an ex vivo model of sprouting angiogenesis. In addition, acute and chronic dextran sodium sulfate colitis models with SPARCL1 knockout mice were applied.

RESULTS

This approach indicated that SPARCL1 inhibits angiogenesis and supports vessel morphogenesis and integrity. Evidence was provided that SPARCL1-mediated stabilization of vessel integrity counteracts vessel permeability and inflammation in acute and chronic dextran sodium sulfate colitis models. Structure-function analyses of purified SPARCL1 identified the acidic domain of the protein necessary for its anti-angiogenic activity.

CONCLUSIONS

Our findings inaugurate SPARCL1 as a blood vessel-derived anti-angiogenic molecule required for vessel morphogenesis and integrity. SPARCL1 opens new perspectives as a vascular marker of susceptibility to colitis and as a therapeutic molecule to support blood vessel stability in this disease.

Specialized endothelial tip cells guide neuroretina vascularization and blood-retina-barrier formation

Endothelial tip cells guiding tissue vascularization are primary targets for angiogenic therapies. Whether tip cells require differential signals to develop their complex branching patterns remained unknown. Here, we show that diving tip cells invading the mouse neuroretina (D-tip cells) are distinct from tip cells guiding the superficial retinal vascular plexus (S-tip cells). D-tip cells have a unique transcriptional signature, including high TGF-β signaling, and they begin to acquire blood-retina barrier properties. Endothelial deletion of TGF-β receptor I (Alk5) inhibits D-tip cell identity acquisition and deep vascular plexus formation. Loss of endothelial ALK5, but not of the canonical SMAD effectors, leads to aberrant contractile pericyte differentiation and hemorrhagic vascular malformations. Oxygen-induced retinopathy vasculature exhibits S-like tip cells, and Alk5 deletion impedes retina revascularization. Our data reveal stage-specific tip cell heterogeneity as a requirement for retinal vascular development and suggest that non-canonical-TGF-β signaling could improve retinal revascularization and neural function in ischemic retinopathy.

APLN/APLNR Signaling Controls Key Pathological Parameters of Glioblastoma

Simple Summary

The neurovascular peptide Apelin and its receptor APLNR are upregulated during glioblastoma pathology. Here we summarize their role in the brain tumor microenvironment composed of neurons, astrocytes, and the vascular and immune systems. Targeting APLN/APLNR signaling promises to unfold multimodal actions in future GBM therapy, acting as an anti-angiogenic and an anti-invasive treatment, and offering the possibility to reduce neurological symptoms and increase overall survival simultaneously.

Abstract

Glioblastoma (GBM) is the most common and aggressive primary brain tumor in adults. GBM-expansion depends on a dense vascular network and, coherently, GBMs are highly angiogenic. However, new intratumoral blood vessels are often aberrant with consequences for blood-flow and vascular barrier function. Hence, the delivery of chemotherapeutics into GBM can be compromised. Furthermore, leaky vessels support edema-formation, which can result in severe neurological deficits. The secreted signaling peptide Apelin (APLN) plays an important role in the formation of GBM blood vessels. Both APLN and the Apelin receptor (APLNR) are upregulated in GBM cells and control tumor cell invasiveness. Here we summarize the current evidence on the role of APLN/APLNR signaling during brain tumor pathology. We show that targeting APLN/APLNR can induce anti-angiogenic effects in GBM and simultaneously blunt GBM cell infiltration. In addition, we discuss how manipulation of APLN/APLNR signaling in GBM leads to the normalization of tumor vessels and thereby supports chemotherapy, reduces edema, and improves anti-tumorigenic immune reactions. Hence, therapeutic targeting of APLN/APLNR signaling offers an interesting option to address different pathological hallmarks of GBM.

Network-Based Analysis Reveals Gene Signature in Tip Cells and Stalk Cells

BACKGROUND

Angiogenesis occurs during various physiological or pathological processes such as wound healing and tumor growth. Differentiation of vascular endothelial cells into tip cells and stalk cells initiates formation of new blood vessels. Tip cells and stalk cells are endothelial cells with different biological characteristics and functions.

OBJECTIVE

The aim of this study was to determine the mechanisms of angiogenesis by exploring differences in gene expression of tip cells and stalk cells.

METHODS

Raw data were retrieved from NCBI Gene Expression Omnibus (GSE19284). Data were reanalyzed using bioinformatics methods that employ robust statistical methods, including identification of differentially expressed genes (DEGs) between stalk and tip cells, weighted gene correlation network analysis (WGCNA), gene ontology and pathway enrichment analysis using DAVID tools, integration of protein-protein interaction (PPI) networks and screening of hub genes. DEGs of stalk and tip cells were grouped as dataset A. Gene modules associated with differentiation of stalk and tip cells screened by WGCNA were named dataset B. Further, we retrieved existing markers of angiogenesis from previous experimental studies on tip and stalk cells which we named dataset C. Intersection of datasets A, B and C was used as a candidate gene. Subsequently, we verified the results applying quantitative polymerase chain reaction (Q -PCR) to our clinical specimen. In general, the Q-PCR results coincide with the majority of the expression profile.

RESULTS

We identified five candidate genes, including ESM1，CXCR4，JAG1，FLT1 and PTK2 and two pathways, including Rap1 signaling pathway and PI3K-Akt signaling pathway in vascular endothelial cells that differentiate into tip cells and stalk cells using bioinformatic analysis.

CONCLUSION

Bioinformatics approaches provide new avenues for basic research in different fields such as angiogenesis. The findings of this study provide new perspectives and basis for the study of molecular mechanisms of vascular endothelial cell differentiation into stalk and tip cells. Genes and pathways identified in this study are potential biomarkers and therapeutic targets for angiogenesis in tumor.

Early Imaging and Molecular Changes with Neoadjuvant Bevacizumab in Stage II/III Breast Cancer

This prospective, phase II study evaluated novel biomarkers as predictors of response to bevacizumab in patients with breast cancer (BC), using serial imaging methods and gene expression analysis. Patients with primary stage II/III BC received bevacizumab 15 mg/kg (cycle 1; C1), then four cycles of neoadjuvant docetaxel doxorubicin, and bevacizumab every 3 weeks (C2-C5). Tumour proliferation and hypoxic status were evaluated using 18F-fluoro-3'-deoxy-3'-L-fluorothymidine (FLT)- and 18F-fluoromisonidazole (FMISO)-positron emission tomography (PET) at baseline, and during C1 and C5. Pre- and post-bevacizumab vascular changes were evaluated using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI). Molecular biomarkers were assessed using microarray analysis. A total of 70 patients were assessed for treatment efficacy. Significant decreases from baseline in tumour proliferation (FLT-PET), vascularity, and perfusion (DCE-MRI) were observed during C1 (p ≤ 0.001), independent of tumour subtype. Bevacizumab treatment did not affect hypoxic tumour status (FMISO-PET). Significant changes in the expression of 28 genes were observed after C1. Changes in vascular endothelial growth factor receptor (VEGFR)-2p levels were observed in 65 patients, with a > 20% decrease in VEGFR-2p observed in 13/65. Serial imaging techniques and molecular gene profiling identified several potentially predictive biomarkers that may predict response to neoadjuvant bevacizumab therapy in BC patients.

Advances in Chemokine Signaling Pathways as Therapeutic Targets in Glioblastoma

With a median patient survival of 15 months, glioblastoma (GBM) is still one of the deadliest malign tumors. Despite immense efforts, therapeutic regimens fail to prolong GBM patient overall survival due to various resistance mechanisms. Chemokine signaling as part of the tumor microenvironment plays a key role in gliomagenesis, proliferation, neovascularization, metastasis and tumor progression. In this review, we aimed to investigate novel therapeutic approaches targeting various chemokine axes, including CXCR2/CXCL2/IL-8, CXCR3/CXCL4/CXCL9/CXCL10, CXCR4/CXCR7/CXCL12, CXCR6/CXCL16, CCR2/CCL2, CCR5/CCL5 and CX3CR1/CX3CL1 in preclinical and clinical studies of GBM. We reviewed targeted therapies as single therapies, in combination with the standard of care, with antiangiogenic treatment as well as immunotherapy. We found that there are many antagonist-, antibody-, cell- and vaccine-based therapeutic approaches in preclinical and clinical studies. Furthermore, targeted therapies exerted their highest efficacy in combination with other established therapeutic applications. The novel chemokine-targeting therapies have mainly been examined in preclinical models. However, clinical applications are auspicious. Thus, it is crucial to broadly investigate the recently developed preclinical approaches. Promising preclinical applications should then be investigated in clinical studies to create new therapeutic regimens and to overcome therapy resistance to GBM treatment.

Targeting Endothelial Cell-Specific Molecule 1 Protein in Cancer: A Promising Therapeutic Approach

Despite the dramatic advances in cancer research in the past few years, effective therapeutic strategies are urgently needed. Endothelial cell-specific molecule 1 (ESM-1), a soluble dermatan sulfate proteoglycan, also known as endocan, serves as a diagnostic and prognostic indicator due to its aberrant expression under pathological conditions, including cancer, sepsis, kidney diseases, and cardiovascular disease. Significantly, ESM-1 can promote cancer progression and metastasis through the regulation of tumor cell proliferation, migration, invasion, and drug resistant. In addition, ESM-1 is involved in the tumor microenvironment, containing inflammation, angiogenesis, and lymph angiogenesis. This article reviews the molecular and biological characteristics of ESM-1 in cancer, the underlying mechanisms, the currently clinical and pre-clinical applications, and potential therapeutic strategies. Herein, we propose that ESM-1 is a new therapeutic target for cancer therapy.

Alterations in the spatiotemporal expression of the chemokine receptor CXCR4 in endothelial cells cause failure of hierarchical vascular branching

The C-X-C chemokine receptor CXCR4 and its ligand CXCL12 play an important role in organ-specific vascular branching morphogenesis. CXCR4 is preferentially expressed by arterial endothelial cells, and local secretion of CXCL12 determines the organotypic pattern of CXCR4⁺ arterial branching. Previous loss-of-function studies clearly demonstrated that CXCL12-CXCR4 signaling is necessary for proper arterial branching in the developing organs such as the skin and heart. To further understand the role of CXCL12-CXCR4 signaling in organ-specific vascular development, we generated a mouse model carrying the Cre recombinase-inducible Cxcr4 transgene. Endothelial cell-specific Cxcr4 gain-of-function embryos exhibited defective vascular remodeling and formation of a hierarchical vascular branching network in the developing skin and heart. Ectopic expression of CXCR4 in venous endothelial cells, but not in lymphatic endothelial cells, caused blood-filled, enlarged lymphatic vascular phenotypes, accompanied by edema. These data suggest that CXCR4 expression is tightly regulated in endothelial cells for appropriate vascular development in an organ-specific manner.

The endosomal RIN2/Rab5C machinery prevents VEGFR2 degradation to control gene expression and tip cell identity during angiogenesis

Sprouting angiogenesis is key to many pathophysiological conditions, and is strongly regulated by vascular endothelial growth factor (VEGF) signaling through VEGF receptor 2 (VEGFR2). Here we report that the early endosomal GTPase Rab5C and its activator RIN2 prevent lysosomal routing and degradation of VEGF-bound, internalized VEGFR2 in human endothelial cells. Stabilization of endosomal VEGFR2 levels by RIN2/Rab5C is crucial for VEGF signaling through the ERK and PI3-K pathways, the expression of immediate VEGF target genes, as well as specification of angiogenic 'tip' and 'stalk' cell phenotypes and cell sprouting. Using overexpression of Rab mutants, knockdown and CRISPR/Cas9-mediated gene editing, and live-cell imaging in zebrafish, we further show that endosomal stabilization of VEGFR2 levels is required for developmental angiogenesis in vivo. In contrast, the premature degradation of internalized VEGFR2 disrupts VEGF signaling, gene expression, and tip cell formation and migration. Thus, an endosomal feedforward mechanism maintains receptor signaling by preventing lysosomal degradation, which is directly linked to the induction of target genes and cell fate in collectively migrating cells during morphogenesis.

At the Bench: Pre-clinical evidence for multiple functions of CXCR4 in cancer

Signaling through chemokine receptor, C-X-C chemokine receptor type 4 (CXCR4) regulates essential processes in normal physiology, including embryogenesis, tissue repair, angiogenesis, and trafficking of immune cells. Tumors co-opt many of these fundamental processes to directly stimulate proliferation, invasion, and metastasis of cancer cells. CXCR4 signaling contributes to critical functions of stromal cells in cancer, including angiogenesis and multiple cell types in the tumor immune environment. Studies in animal models of several different types of cancers consistently demonstrate essential functions of CXCR4 in tumor initiation, local invasion, and metastasis to lymph nodes and distant organs. Data from animal models support clinical observations showing that integrated effects of CXCR4 on cancer and stromal cells correlate with metastasis and overall poor prognosis in >20 different human malignancies. Small molecules, Abs, and peptidic agents have shown anticancer efficacy in animal models, sparking ongoing efforts at clinical translation for cancer therapy. Investigators also are developing companion CXCR4-targeted imaging agents with potential to stratify patients for CXCR4-targeted therapy and monitor treatment efficacy. Here, pre-clinical studies demonstrating functions of CXCR4 in cancer are reviewed.

An In Vitro Co-Culture Model of Bone Marrow Mesenchymal Stromal Cells and Peripheral Blood Mononuclear Cells Promotes the Differentiation of Myeloid Angiogenic Cells and Pericyte-Like Cells

Mesenchymal stromal cells (MSCs) are known to stimulate the survival and growth of endothelial cells (ECs) by producing paracrine signals, as well as to differentiate into pericytes and thereby support blood vessel formation and stability. On the other hand, cells with an EC-like phenotype have been found within the CD14⁺ and CD34⁺ cell populations of peripheral blood (PB) mononuclear cells (MNCs). The aim of this study was to investigate the proangiogenic differentiation potential of human MSC-MNC co-cultures. Bone marrow-derived MSCs (2,500 cells/cm²) were co-cultured with MNCs (50,000 cells/cm²), which were isolated from the PB of healthy donors. MSCs and MNCs cultured alone at same cell densities were used as controls. Cells in MNC fraction and in co-cultures were isolated for CD14, CD34, and CD31 surface markers with magnetic-activated cell sorting. Co-cultures were analyzed for cell proliferation and morphology, as well as for the expression of various hematopoietic, endothelial, and pericyte markers by immunocytochemistry, quantitative PCR (qPCR), and flow cytometry. Vascular endothelial growth factor (VEGF) expression and secretion was measured with qPCR and enzyme-linked immunosorbent assay, respectively. Our results show that in co-cultures with MSCs, CD14⁺CD45⁺ MNCs differentiated into spindle-shaped, nonproliferative, EC-like, myeloid angiogenic cells (MACs) expressing CD31, but also into pericyte-like cells expressing neural/glial antigen 2 (NG2) and CD146. Functionality of the isolated MACs was demonstrated in co-cultures with human umbilical vein endothelial cells, where they supported the formation of tube-like structures. NG2⁺ cells of MNC-origin were found among both CD34^-CD14⁺ and CD34^-CD14^- cell populations, indicating the existence of different subtypes of pericyte-like cells. In addition, VEGF was shown to be secreted in MSC-MNC co-cultures, mainly by MSCs. In conclusion, MSCs were shown to possess proangiogenic capacity in MSC-MNC co-cultures as they supported the differentiation of functional MACs, as well as the differentiation of pericyte-like cells of MNC origin. This phenomenon was mediated at least partially via secreted VEGF.

Single-Cell Analysis of Blood-Brain Barrier Response to Pericyte Loss

RATIONALE

Pericytes are capillary mural cells playing a role in stabilizing newly formed blood vessels during development and tissue repair. Loss of pericytes has been described in several brain disorders, and genetically induced pericyte deficiency in the brain leads to increased macromolecular leakage across the blood-brain barrier (BBB). However, the molecular details of the endothelial response to pericyte deficiency remain elusive.

OBJECTIVE

To map the transcriptional changes in brain endothelial cells resulting from lack of pericyte contact at single-cell level and to correlate them with regional heterogeneities in BBB function and vascular phenotype.

METHODS AND RESULTS

We reveal transcriptional, morphological, and functional consequences of pericyte absence for brain endothelial cells using a combination of methodologies, including single-cell RNA sequencing, tracer analyses, and immunofluorescent detection of protein expression in pericyte-deficient adult Pdgfbret/ret mice. We find that endothelial cells without pericyte contact retain a general BBB-specific gene expression profile, however, they acquire a venous-shifted molecular pattern and become transformed regarding the expression of numerous growth factors and regulatory proteins. Adult Pdgfbret/ret brains display ongoing angiogenic sprouting without concomitant cell proliferation providing unique insights into the endothelial tip cell transcriptome. We also reveal heterogeneous modes of pericyte-deficient BBB impairment, where hotspot leakage sites display arteriolar-shifted identity and pinpoint putative BBB regulators. By testing the causal involvement of some of these using reverse genetics, we uncover a reinforcing role for angiopoietin 2 at the BBB.

CONCLUSIONS

By elucidating the complexity of endothelial response to pericyte deficiency at cellular resolution, our study provides insight into the importance of brain pericytes for endothelial arterio-venous zonation, angiogenic quiescence, and a limited set of BBB functions. The BBB-reinforcing role of ANGPT2 (angiopoietin 2) is paradoxical given its wider role as TIE2 (TEK receptor tyrosine kinase) receptor antagonist and may suggest a unique and context-dependent function of ANGPT2 in the brain.

Established, New and Emerging Concepts in Brain Vascular Development

In this review, we discuss the state of our knowledge as it relates to embryonic brain vascular patterning in model systems zebrafish and mouse. We focus on the origins of endothelial cell and the distinguishing features of brain endothelial cells compared to non-brain endothelial cells, which is revealed by single cell RNA-sequencing methodologies. We also discuss the cross talk between brain endothelial cells and neural stem cells, and their effect on each other. In terms of mechanisms, we focus exclusively on Wnt signaling and the recent developments associated with this signaling network in brain vascular patterning, and the benefits and challenges associated with strategies for targeting the brain vasculature. We end the review with a discussion on the emerging areas of meningeal lymphatics, endothelial cilia biology and novel cerebrovascular structures identified in vertebrates.

The DOCK protein family in vascular development and disease

The vascular network is established and maintained through the processes of vasculogenesis and angiogenesis, which are tightly regulated during embryonic and postnatal life. The formation of a functional vasculature requires critical cellular mechanisms, such as cell migration, proliferation and adhesion, which are dependent on the activity of small Rho GTPases, controlled in part by the dedicator of cytokinesis (DOCK) protein family. Whilst the majority of DOCK proteins are associated with neuronal development, a growing body of evidence has indicated that members of the DOCK family may have key functions in the control of vasculogenic and angiogenic processes. This is supported by the involvement of several angiogenic signalling pathways, including chemokine receptor type 4 (CXCR4), vascular endothelial growth factor (VEGF) and phosphatidylinositol 3-kinase (PI3K), in the regulation of specific DOCK proteins. This review summarises recent progress in understanding the respective roles of DOCK family proteins during vascular development. We focus on existing in vivo and in vitro models and known human disease phenotypes and highlight potential mechanisms of DOCK protein dysfunction in the pathogenesis of vascular disease.

Vascular Heterogeneity With a Special Focus on the Hepatic Microenvironment

Utilizing single-cell sequencing, recent studies were able to analyze at a greater resolution the heterogeneity of the vasculature and its complex composition in different tissues. Differing subpopulations have been detected, distinguishable only by their transcriptome. Designed to provide further insight into the heterogeneity of the functional vascular tissue, endothelial cells have been the main target of those studies. This review aims to present a synopsis of the variability of the different vascular beds, their endothelial variety, and the supporting cells that allow the vessels to serve their various purposes. Firstly, we are going to chart vascular tissue heterogeneity on a cellular level, describing endothelial diversity as well as stromal microenvironmental variety and interaction in a physiological setting. Secondly, we will summarize the current knowledge of pathological vessel formation in the context of cancer. Conventional anti-tumor therapeutic targets as well as anti-angiogenetic therapy is frequently limited by poor response of the tumor tissue. Reasons for moderate response and resistance to treatment can be found through different drivers of angiogenesis, different mechanisms of blood supply, but also in poorly understood tissue diversity. Based on this, we are comparing how pathologies alter the normal structure of vascular tissues highlighting the involved mechanisms. Lastly, illustrating the concept above, we will focus on the hepatic microenvironment, an organ of frequent metastatic spreading (e.g., from colorectal, breast, and lung cancers). We will address how the hepatic vasculature usually develops and subsequently we will describe how common liver metastases vary in their vasculature and the way they supply themselves (e.g., angiogenesis versus vessel co-option).

Defining an Upstream VEGF (Vascular Endothelial Growth Factor) Priming Signature for Downstream Factor-Induced Endothelial Cell-Pericyte Tube Network Coassembly

OBJECTIVE

In this work, we have sought to define growth factor requirements and the signaling basis for different stages of human vascular morphogenesis and maturation. Approach and Results: Using a serum-free model of endothelial cell (EC) tube morphogenesis in 3-dimensional collagen matrices that depends on a 5 growth factor combination, SCF (stem cell factor), IL (interleukin)-3, SDF (stromal-derived factor)-1α, FGF (fibroblast growth factor)-2, and insulin (factors), we demonstrate that VEGF (vascular endothelial growth factor) pretreatment of ECs for 8 hours (ie, VEGF priming) leads to marked increases in the EC response to the factors which includes; EC tip cells, EC tubulogenesis, pericyte recruitment and proliferation, and basement membrane deposition. VEGF priming requires VEGFR2, and the effect of VEGFR2 is selective to the priming response and does not affect factor-dependent tubulogenesis in the absence of priming. Key molecule and signaling requirements for VEGF priming include RhoA, Rock1 (Rho-kinase), PKCα (protein kinase C α), and PKD2 (protein kinase D2). siRNA suppression or pharmacological blockade of these molecules and signaling pathways interfere with the ability of VEGF to act as an upstream primer of downstream factor-dependent EC tube formation as well as pericyte recruitment. VEGF priming was also associated with the formation of actin stress fibers, activation of focal adhesion components, upregulation of the EC factor receptors, c-Kit, IL-3Rα, and CXCR4 (C-X-C chemokine receptor type 4), and upregulation of EC-derived PDGF (platelet-derived growth factor)-BB, PDGF-DD, and HB-EGF (heparin-binding epidermal growth factor) which collectively affect pericyte recruitment and proliferation.

CONCLUSIONS

Overall, this study defines a signaling signature for a separable upstream VEGF priming step, which can activate ECs to respond to downstream factors that are necessary to form branching tube networks with associated mural cells.

YAP Orchestrates Heterotypic Endothelial Cell Communication via HGF/c-MET Signaling in Liver Tumorigenesis

The oncogene yes-associated protein (YAP) controls liver tumor initiation and progression via cell extrinsic functions by creating a tumor-supporting environment in conjunction with cell autonomous mechanisms. However, how YAP controls organization of the microenvironment and in particular the vascular niche, which contributes to liver disease and hepatocarcinogenesis, is poorly understood. To investigate heterotypic cell communication, we dissected murine and human liver endothelial cell (EC) populations into liver sinusoidal endothelial cells (LSEC) and continuous endothelial cells (CEC) through histomorphological and molecular characterization. In YAP^S127A-induced tumorigenesis, a gradual replacement of LSECs by CECs was associated with dynamic changes in the expression of genes involved in paracrine communication. The formation of new communication hubs connecting CECs and LSECs included the hepatocyte growth factor (Hgf)/c-Met signaling pathway. In hepatocytes and tumor cells, YAP/TEA domain transcription factor 4 (TEAD4)-dependent transcriptional induction of osteopontin (Opn) stimulated c-Met expression in EC with CEC phenotype, which sensitized these cells to the promigratory effects of LSEC-derived Hgf. In human hepatocellular carcinoma, the presence of a migration-associated tip-cell signature correlated with poor clinical outcome and the loss of LSEC marker gene expression. The occurrence of c-MET-expressing CECs in human liver cancer samples was confirmed at the single-cell level. In summary, YAP-dependent changes of the liver vascular niche comprise the formation of heterologous communication hubs in which tumor cell-derived factors modify the cross-talk between LSECs and CECs via the HGF/c-MET axis. SIGNIFICANCE: YAP-dependent changes of the liver vascular niche comprise the formation of heterologous communication hubs in which tumor cell-derived factors modify the cross-talk between EC subpopulations. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/24/5502/F1.large.jpg.

Angiogenesis in gynecological cancers and the options for anti-angiogenesis therapy

Angiogenesis is required in cancer, including gynecological cancers, for the growth of primary tumors and secondary metastases. Development of anti-angiogenesis therapy in gynecological cancers and improvement of its efficacy have been a major focus of fundamental and clinical research. However, survival benefits of current anti-angiogenic agents, such as bevacizumab, in patients with gynecological cancer, are modest. Therefore, a better understanding of angiogenesis and the tumor microenvironment in gynecological cancers is urgently needed to develop more effective anti-angiogenic therapies, either or not in combination with other therapeutic approaches. We describe the molecular aspects of (tumor) blood vessel formation and the tumor microenvironment and provide an extensive clinical overview of current anti-angiogenic therapies for gynecological cancers. We discuss the different phenotypes of angiogenic endothelial cells as potential therapeutic targets, strategies aimed at intervention in their metabolism, and approaches targeting their (inflammatory) tumor microenvironment.

Inactivation of endothelial cell phosphoinositide 3-kinase β inhibits tumor angiogenesis and tumor growth

Angiogenesis inhibitors, such as the receptor tyrosine kinase (RTK) inhibitor sunitinib, target vascular endothelial growth factor (VEGF) signaling in cancers. However, only a fraction of patients respond, and most ultimately develop resistance to current angiogenesis inhibitor therapies. Activity of alternative pro-angiogenic growth factors, acting via RTK or G-protein coupled receptors (GPCR), may mediate VEGF inhibitor resistance. The phosphoinositide 3-kinase (PI3K)β isoform is uniquely coupled to both RTK and GPCRs. We investigated the role of endothelial cell (EC) PI3Kβ in tumor angiogenesis. Pro-angiogenic GPCR ligands were expressed by patient-derived renal cell carcinomas (PD-RCC), and selective inactivation of PI3Kβ reduced PD-RCC-stimulated EC spheroid sprouting. EC-specific PI3Kβ knockout (ΕC-βKO) in mice potentiated the sunitinib-induced reduction in subcutaneous growth of LLC1 and B16F10, and lung metastasis of B16F10 tumors. Compared to single-agent sunitinib treatment, tumors in sunitinib-treated ΕC-βKO mice showed a marked decrease in microvessel density, and reduced new vessel formation. The fraction of perfused mature tumor microvessels was increased in ΕC-βKO mice suggesting immature microvessels were most sensitive to combined sunitinib and PI3Kβ inactivation. Taken together, EC PI3Kβ inactivation with sunitinib inhibition reduces microvessel turnover and decreases heterogeneity of the tumor microenvironment, hence PI3Kβ inhibition may be a useful adjuvant antiangiogenesis therapy with sunitinib.

Hypoxia-induced downregulation of Sema3a and CXCL12/CXCR4 regulate the formation of the coronary artery stem at the proper site

BACKGROUND

During the formation of the coronary artery stem, endothelial strands from the endothelial progenitor pool surrounding the conotruncus penetrate into the aortic wall. Vascular endothelial growth factors (VEGFs) as well as CXCL12/CXCR4 signaling are thought to play a role in the formation of the coronary stem. However, the mechanisms regulating how endothelial strands exclusively invade into the aorta remain unknown.

METHODS AND RESULTS

Immunohistochemistry showed that before the formation of endothelial strands, Sema3a was highly expressed in endothelial progenitors surrounding the great arteries. At the onset of/during invasion of endothelial strands into the aorta, Sema3a was downregulated and CXCR4 was upregulated in the endothelial strands. In situ hybridization showed that Cxcl12 was highly expressed in the aortic wall compared with in the pulmonary artery. Using avian embryonic hearts, we established two types of endothelial penetration assay, in which coronary endothelial strands preferentially invaded into the aorta in culture. Sema3a blocking peptide induced an excess number of endothelial strands penetrating into the pulmonary artery, whereas recombinant Sema3a inhibited the formation of endothelial strands. In cultured coronary endothelial progenitors, recombinant VEGF protein induced CXCR4-positive endothelial strands, which were capable of being attracted by CXCL12-impregnated beads. Monoazo rhodamine detected that hypoxia was predominant in aortic/subaortic region in ovo and hypoxic condition downregulated the expression of Sema3a in culture.

CONCLUSION

Results suggested that hypoxia in the aortic region downregulates the expression of Sema3a, thereby enhancing VEGF activity to induce the formation of CXCR4-positive endothelial strands, which are subsequently attracted into the Cxcl12-positive aortic wall to connect the aortic lumen.

Extracellular matrix compression temporally regulates microvascular angiogenesis

Mechanical cues influence tissue regeneration, and although vasculature is known to be mechanically sensitive, little is known about the effects of bulk extracellular matrix deformation on the nascent vessel networks found in healing tissues. Previously, we found that dynamic matrix compression in vivo potently regulated revascularization during bone tissue regeneration; however, whether matrix deformations directly regulate angiogenesis remained unknown. Here, we demonstrated that load initiation time, magnitude, and mode all regulate microvascular growth, as well as upstream angiogenic and mechanotransduction signaling pathways. Immediate load initiation inhibited angiogenesis and expression of early sprout tip cell selection genes, while delayed loading enhanced microvascular network formation and upstream signaling pathways. This research provides foundational understanding of how extracellular matrix mechanics regulate angiogenesis and has critical implications for clinical translation of new regenerative medicine therapies and physical rehabilitation strategies designed to enhance revascularization during tissue regeneration.