Tie-1 and tie-2 define another class of putative receptor tyrosine kinase genes expressed in early embryonic vascular system

Promoting Angiogenesis/Osteogenesis by a New Copper/Magnesium Hydroxide Hybrid Nanoparticle: In Vitro and In Vivo Investigation

In this study, a new hybrid nanoparticle composed of magnesium hydroxide and copper oxide (Mg(OH)2/CuO) with an optimized ratio of magnesium (Mg) to copper (Cu) was designed and incorporated into a 3D-printed scaffold made of polycaprolactone (PCL) and gelatin. These hybrid nanostructures (MCNs) were prepared using a green, solvent-free method. Their topography, surface morphology, and structural properties were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The fabricated 3D-printed PCL/Gelatin/MCN scaffolds were investigated in vitro and in vivo. Cell viability tests on murine calvarial preosteoblasts (MC3T3-E1) and human umbilical vein endothelial cells (HUVECs) demonstrated that the scaffolds could induce proper cell proliferation. Additionally, the angiogenic and osteogenic properties of the constructs were evaluated using alkaline phosphatase (ALP) activity, osteogenesis-related, and angiogenesis-related gene expression tests. The in vivo study was conducted using a rat calvarial defect model, which confirmed the superior angiogenic and osteogenic properties of the PCL/gelatin/MCN scaffolds compared to PCL/Gelatin and PCL/Gelatin/Mg(OH)2 scaffolds. Overall, the PCL/Gelatin/MCN scaffolds showed promising potential for bone regeneration, particularly for critical-sized defects where proper angiogenesis is essential for tissue reconstruction.

Adaptive patterning of vascular network during avian skin development: Mesenchymal plasticity and dermal vasculogenesis

A vasculature network supplies blood to feather buds in the developing skin. Does the vasculature network during early skin development form by sequential sprouting from the central vasculature or does local vasculogenesis occur first that then connect with the central vascular tree? Using transgenic Japanese quail Tg(TIE1p.H2B-eYFP), we observe that vascular progenitor cells appear after feather primordia formation. The vasculature then radiates out from each bud and connects with primordial vessels from neighboring buds. Later they connect with the central vasculature. Epithelial-mesenchymal recombination shows local vasculature is patterned by the epithelium, which expresses FGF2 and VEGF. Perturbing noggin expression leads to abnormal vascularization. To study endothelial origin, we compare transcriptomes of TIE1p.H2B-eYFP+ cells collected from the skin and aorta. Endothelial cells from the skin more closely resemble skin dermal cells than those from the aorta. The results show developing chicken skin vasculature is assembled by (1) physiological vasculogenesis from the peripheral tissue, and (2) subsequently connects with the central vasculature. The work implies mesenchymal plasticity and convergent differentiation play significant roles in development, and such processes may be re-activated during adult regeneration. SUMMARY STATEMENT: We show the vasculature network in the chicken skin is assembled using existing feather buds as the template, and endothelia are derived from local bud dermis and central vasculature.

Increasing the diagnostic yield of childhood glaucoma cases recruited into the 100,000 Genomes Project

Childhood glaucoma (CG) encompasses a heterogeneous group of genetic eye disorders that is responsible for approximately 5% of childhood blindness worldwide. Understanding the molecular aetiology is key to improving diagnosis, prognosis and unlocking the potential for optimising clinical management. In this study, we investigated 86 CG cases from 78 unrelated families of diverse ethnic backgrounds, recruited into the Genomics England 100,000 Genomes Project (GE100KGP) rare disease cohort, to improve the genetic diagnostic yield. Using the Genomics England/Genomic Medicine Centres (GE/GMC) diagnostic pipeline, 13 unrelated families were solved (13/78, 17%). Further interrogation using an expanded gene panel yielded a molecular diagnosis in 7 more unrelated families (7/78, 9%). This analysis effectively raises the total number of solved CG families in the GE100KGP to 26% (20/78 families). Twenty-five percent (5/20) of the solved families had primary congenital glaucoma (PCG), while 75% (15/20) had secondary CG; 53% of this group had non-acquired ocular anomalies (including iris hypoplasia, megalocornea, ectopia pupillae, retinal dystrophy, and refractive errors) and 47% had non-acquired systemic diseases such as cardiac abnormalities, hearing impairment, and developmental delay. CYP1B1 was the most frequently implicated gene, accounting for 55% (11/20) of the solved families. We identified two novel likely pathogenic variants in the TEK gene, in addition to one novel pathogenic copy number variant (CNV) in FOXC1. Variants that passed undetected in the GE100KGP diagnostic pipeline were likely due to limitations of the tiering process, the use of smaller gene panels during analysis, and the prioritisation of coding SNVs and indels over larger structural variants, CNVs, and non-coding variants.

Beyond VEGF: Angiopoietin-Tie Signaling Pathway in Diabetic Retinopathy

Complications from diabetic retinopathy such as diabetic macular edema (DME) and proliferative diabetic retinopathy (PDR) constitute leading causes of preventable vision loss in working-age patients. Since vascular endothelial growth factor (VEGF) plays a major role in the pathogenesis of these complications, VEGF inhibitors have been the cornerstone of their treatment. Anti-VEGF monotherapy is an effective but burdensome treatment for DME. However, due to the intensive and burdensome treatment, most patients in routine clinical practice are undertreated, and therefore, their outcomes are compromised. Even in adequately treated patients, persistent DME is reported anywhere from 30% to 60% depending on the drug used. PDR is currently treated by anti-VEGF, panretinal photocoagulation (PRP) or a combination of both. Similarly, a number of eyes, despite these treatments, continue to progress to tractional retinal detachment and vitreous hemorrhage. Clearly there are other molecular pathways other than VEGF involved in the pathogenesis of DME and PDR. One of these pathways is the angiopoietin-Tie signaling pathway. Angiopoietin 1 (Ang1) plays a major role in maintaining vascular quiescence and stability. It acts as a molecular brake against vascular destabilization and inflammation that is usually promoted by angiopoietin 2 (Ang2). Several pathological conditions including chronic hyperglycemia lead to Ang2 upregulation. Recent regulatory approval of the bi-specific antibody, faricimab, may improve long term outcomes in DME. It targets both the Ang/Tie and VEGF pathways. The YOSEMITE and RHINE were multicenter, double-masked, randomized non-inferiority phase 3 clinical trials that compared faricimab to aflibercept in eyes with center-involved DME. At 12 months of follow-up, faricimab demonstrated non-inferior vision gains, improved anatomic outcomes and a potential for extended dosing when compared to aflibercept. The 2-year results of the YOSEMITE and RHINE trials demonstrated that the anatomic and functional results obtained at the 1 year follow-up were maintained. Short term outcomes of previously treated and treatment-naive eyes with DME that were treated with faricimab during routine clinical practice suggest a beneficial effect of faricimab over other agents. Targeting of Ang2 has been reported by several other means including VE-PTP inhibitors, integrin binding peptide and surrobodies.

METTL3-Mediated RNA m6A Modification Regulates the Angiogenic Behaviors of Retinal Endothelial Cells by Methylating MMP2 and TIE2

Purpose

N6-methyladenosine (m6A) is a commonly occurring modification of mRNAs, catalyzed by a complex containing methyltransferase like 3 (METTL3). Our research aims to explore how METTL3-dependent m6A modification affects the functions of retinal endothelial cells (RECs).

Methods

An oxygen-induced retinopathy (OIR) mouse model was established, and RECs were isolated using magnetic beads method. Human retinal microvascular endothelial cells (HRMECs) were treated with normoxia (21% O2) or hypoxia (1% O2). Dot blot assay determined m6A modification levels. Quantitative RT-PCR and Western blot detected the mRNA and protein expression levels of the target candidates, respectively. Genes were knocked down by small interfering RNA transfection. Matrigel-based angiogenesis and transwell assays evaluated the abilities of endothelial tube formation and migration, respectively. Methylated RNA immunoprecipitation-qPCR determined the levels of m6A modification in the target genes.

Results

The m6A modification levels were significantly upregulated in the retinas and RECs of OIR mice. Exposure to hypoxia significantly elevated both METTL3 expression and m6A modification levels in HRMECs. METTL3 knockdown curtailed endothelial tube formation and migration in vitro under both normoxic and hypoxic conditions. Concurrently, this knockdown in HRMECs resulted in reduced m6A modification levels of MMP2 and TIE2 transcripts, subsequently leading to a decrease in their respective protein expressions. Notably, knockdown of MMP2 and TIE2 also markedly inhibited the angiogenic activities of HRMECs.

Conclusions

METTL3-mediated m6A modification promotes the angiogenic behaviors of RECs by targeting MMP2 and TIE2, suggesting its significance in retinal angiogenesis and METTL3 as a potential therapeutic target.

Macrophage-mediated extracellular matrix remodeling controls host Staphylococcus aureus susceptibility in the skin

Hypodermis is the predominant site of Staphylococcus aureus infections that cause cellulitis. Given the importance of macrophages in tissue remodeling, we examined the hypodermal macrophages (HDMs) and their impact on host susceptibility to infection. Bulk and single-cell transcriptomics uncovered HDM subsets with CCR2-dichotomy. HDM homeostasis required the fibroblast-derived growth factor CSF1, ablation of which abrogated HDMs from the hypodermal adventitia. Loss of CCR2- HDMs resulted in accumulation of the extracellular matrix component, hyaluronic acid (HA). HDM-mediated HA clearance required sensing by the HA receptor, LYVE-1. Cell-autonomous IGF1 was required for accessibility of AP-1 transcription factor motifs that controlled LYVE-1 expression. Remarkably, loss of HDMs or IGF1 limited Staphylococcus aureus expansion via HA and conferred protection against cellulitis. Our findings reveal a function for macrophages in the regulation of HA with an impact on infection outcomes, which may be harnessed to limit the establishment of infection in the hypodermal niche.

Plasma angiopoietin-2 and its association with heart failure in patients with atrial fibrillation

AIMS

Several biomarkers are associated with clinical outcomes in patients with atrial fibrillation (AF), but a causal relationship has not been established. This study aimed to evaluate angiopoietin-2, a novel candidate biomarker of endothelial inflammation and vascular remodelling, in patients with AF.

METHODS AND RESULTS

Angiopoietin-2 was measured in plasma obtained from patients with AF treated with aspirin monotherapy (exploration cohort, n = 2987) or with oral anticoagulation (validation cohort, n = 13 079). Regression models were built to assess the associations between angiopoietin-2, clinical characteristics, and outcomes. In both cohorts, plasma angiopoietin-2 was independently associated with AF on the baseline electrocardiogram and persistent/permanent AF, age, history of heart failure, female sex, tobacco use/smoking, body mass index, renal dysfunction, diabetes, and N-terminal pro-B-type natriuretic peptide (NT-proBNP). Angiopoietin-2 was independently associated with subsequent hospitalization for heart failure after adjusting for age, creatinine, and clinical characteristics in the exploration cohort [c-index 0.79, 95% confidence interval (CI) 0.75-0.82; third vs. first quartile, hazard ratio (HR) 1.74, 95% CI 1.26-2.41] and in the validation cohort (c-index 0.76, 95% CI 0.74-0.78; HR 1.58, 95% CI 1.37-1.82). In both cohorts, the association persisted when also adjusting for NT-proBNP (P ≤ 0.001). In full multivariable models also adjusted for NT-proBNP, angiopoietin-2 did not show statistically significant associations with ischaemic stroke, cardiovascular and all-cause death, or major bleeding that were consistent across the two cohorts.

CONCLUSIONS

In patients with AF, plasma levels of angiopoietin-2 were independently associated with subsequent hospitalization for heart failure and provided incremental prognostic value to clinical risk factors and NT-proBNP.

The role of eIF4F-driven mRNA translation in regulating the tumour microenvironment

Cells can rapidly adjust their proteomes in dynamic environments by regulating mRNA translation. There is mounting evidence that dysregulation of mRNA translation supports the survival and adaptation of cancer cells, which has stimulated clinical interest in targeting elements of the translation machinery and, in particular, components of the eukaryotic initiation factor 4F (eIF4F) complex such as eIF4E. However, the effect of targeting mRNA translation on infiltrating immune cells and stromal cells in the tumour microenvironment (TME) has, until recently, remained unexplored. In this Perspective article, we discuss how eIF4F-sensitive mRNA translation controls the phenotypes of key non-transformed cells in the TME, with an emphasis on the underlying therapeutic implications of targeting eIF4F in cancer. As eIF4F-targeting agents are in clinical trials, we propose that a broader understanding of their effect on gene expression in the TME will reveal unappreciated therapeutic vulnerabilities that could be used to improve the efficacy of existing cancer therapies.

In vitro inhibition of cancer angiogenesis and migration by a nanobody that targets the orphan receptor Tie1

The human signaling molecules Tie1 and Tie2 receptor tyrosine kinases (RTKs) play important pathophysiological roles in many diseases, including different cancers. The activity of Tie1 is mediated mainly through the downstream angiopoietin-1 (Ang1)-dependent activation of Tie2, rendering both Tie 1 and the Tie1/Tie2/Ang1 axis attractive putative targets for therapeutic intervention. However, the development of inhibitors that target Tie1 and an understanding of their effect on Tie2 and on the Tie1/Tie2/Ang1 axis remain unfulfilled tasks, due, largely, to the facts that Tie1 is an orphan receptor and is difficult to produce and use in the quantities required for immune antibody library screens. In a search for a selective inhibitor of this orphan receptor, we sought to exploit the advantages (e.g., small size that allows binding to hidden epitopes) of non-immune nanobodies and to simultaneously overcome their limitations (i.e., low expression and stability). We thus performed expression, stability, and affinity screens of yeast-surface-displayed naïve and predesigned synthetic (non-immune) nanobody libraries against the Tie1 extracellular domain. The screens yielded a nanobody with high expression and good affinity and specificity for Tie1, thereby yielding preferential binding for Tie1 over Tie2. The stability, selectivity, potency, and therapeutic potential of this synthetic nanobody were profiled using in vitro and cell-based assays. The nanobody triggered Tie1-dependent inhibition of RTK (Tie2, Akt, and Fak) phosphorylation and angiogenesis in endothelial cells, as well as suppression of human glioblastoma cell viability and migration. This study opens the way to developing nanobodies as therapeutics for different cancers associated with Tie1 activation.

Role of the Endothelium in Neonatal Diseases

In both fetal and neonatal physiologic and pathologic processes in most organs, endothelial cells are known to play critical roles. Although the endothelium is one of the most ubiquitous cell type in the body, the tight adherence to the blood vessel wall has made it difficult to study their diverse function and structure. In this article, we have reviewed endothelial cell origins and explored their heterogeneity in terms of structure, function, developmental changes, and their role in inflammatory and infectious diseases. We have also attempted to evaluate the untapped therapeutic potentials of endothelial cells in neonatal disease. This article comprises various peer-reviewed studies, including ours, and an extensive database literature search from EMBASE, PubMed, and Scopus.

Gabrb3 endothelial cell-specific knockout mice display abnormal blood flow, hypertension, and behavioral dysfunction

Our recent studies uncovered a novel GABA signaling pathway in embryonic forebrain endothelial cells that works independently from neuronal GABA signaling and revealed that disruptions in endothelial GABA_A receptor-GABA signaling from early embryonic stages can directly contribute to the origin of psychiatric disorders. In the GABA_A receptor β3 subunit endothelial cell conditional knockout (Gabrb3^ECKO) mice, the β3 subunit is deleted selectively from endothelial cells, therefore endothelial GABA_A receptors become inactivated and dysfunctional. There is a reduction in vessel densities and increased vessel morphology in the Gabrb3^ECKO telencephalon that persists in the adult neocortex. Gabrb3^ECKO mice show behavioral deficits such as impaired reciprocal social interactions, communication deficits, heightened anxiety, and depression. Here, we characterize the functional changes in Gabrb3^ECKO mice by evaluating cortical blood flow, examine the consequences of loss of endothelial Gabrb3 on cardiac tissue, and define more in-depth altered behaviors. Red blood cell velocity and blood flow were increased in the cortical microcirculation of the Gabrb3^ECKO mice. The Gabrb3^ECKO mice had a reduction in vessel densities in the heart, similar to the brain; exhibited wavy, myocardial fibers, with elongated 'worm-like' nuclei in their cardiac histology, and developed hypertension. Additional alterations in behavioral function were observed in the Gabrb3^ECKO mice such as increased spontaneous exploratory activity and rearing in an open field, reduced short term memory, decreased ambulatory activity in CLAMS testing, and altered prepulse inhibition to startle, an important biomarker of psychiatric diseases such as schizophrenia. Our results imply that vascular Gabrb3 is a key player in the brain as well as the heart, and its loss in both organs can lead to concurrent development of psychiatric and cardiac dysfunction.

Angiopoietin-2-induced lymphatic endothelial cell migration drives lymphangiogenesis via the β1 integrin-RhoA-formin axis

Lymphangiogenesis is an essential physiological process but also a determining factor in vascular-related pathological conditions. Angiopoietin-2 (Ang2) plays an important role in lymphatic vascular development and function and its upregulation has been reported in several vascular-related diseases, including cancer. Given the established role of the small GTPase RhoA on cytoskeleton-dependent endothelial functions, we investigated the relationship between RhoA and Ang2-induced cellular activities. This study shows that Ang2-driven human dermal lymphatic endothelial cell migration depends on RhoA. We demonstrate that Ang2-induced migration is independent of the Tie receptors, but dependent on β1 integrin-mediated RhoA activation with knockdown, pharmacological approaches, and protein sequencing experiments. Although the key proteins downstream of RhoA, Rho kinase (ROCK) and myosin light chain, were activated, blockade of ROCK did not abrogate the Ang2-driven migratory effect. However, formins, an alternative target of RhoA, were identified as key players, and especially FHOD1. The Ang2-RhoA relationship was explored in vivo, where lymphatic endothelial RhoA deficiency blocked Ang2-induced lymphangiogenesis, highlighting RhoA as an important target for anti-lymphangiogenic treatments.

Retinal vein occlusion: drug targets and therapeutic implications

INTRODUCTION

The pathogenesis of retinal vein occlusion (RVO) is extremely complex and includes several mediators. These mediators represent potential drug targets that can be used in the development of intravitreal drugs.

AREAS COVERED

PubMed/MEDLINE databases were accessed between April-May 2021 to find the most relevant scientific papers regarding drug targets and therapeutic implications in RVO, focusing on current therapeutic options and potential cornerstones of future advances in treatment.

EXPERT OPINION

Before the introduction of intravitreal therapies, the visual outcome following a diagnosis of RVO was extremely poor. Anti-VEGF and corticosteroid treatments have radically changed RVO prognosis, helping to preserve patients' visual function and their quality of life. According to current clinical data, anti-VEGF and corticosteroid drugs are associated with both pros and cons; the present recommendation is to employ anti-VEGF molecules as a first-line treatment. Advances in our understanding of the biomolecular characteristics of RVO offer a solid basis for the development of new therapeutic targets and treatments.

The Hematopoietic Bone Marrow Niche Ecosystem

The bone marrow (BM) microenvironment, also called the BM niche, is essential for the maintenance of fully functional blood cell formation (hematopoiesis) throughout life. Under physiologic conditions the niche protects hematopoietic stem cells (HSCs) from sustained or overstimulation. Acute or chronic stress deregulates hematopoiesis and some of these alterations occur indirectly via the niche. Effects on niche cells include skewing of its cellular composition, specific localization and molecular signals that differentially regulate the function of HSCs and their progeny. Importantly, while acute insults display only transient effects, repeated or chronic insults lead to sustained alterations of the niche, resulting in HSC deregulation. We here describe how changes in BM niche composition (ecosystem) and structure (remodeling) modulate activation of HSCs in situ. Current knowledge has revealed that upon chronic stimulation, BM remodeling is more extensive and otherwise quiescent HSCs may be lost due to diminished cellular maintenance processes, such as autophagy, ER stress response, and DNA repair. Features of aging in the BM ecology may be the consequence of intermittent stress responses, ultimately resulting in the degeneration of the supportive stem cell microenvironment. Both chronic stress and aging impair the functionality of HSCs and increase the overall susceptibility to development of diseases, including malignant transformation. To understand functional degeneration, an important prerequisite is to define distinguishing features of unperturbed niche homeostasis in different settings. A unique setting in this respect is xenotransplantation, in which human cells depend on niche factors produced by other species, some of which we will review. These insights should help to assess deviations from the steady state to actively protect and improve recovery of the niche ecosystem in situ to optimally sustain healthy hematopoiesis in experimental and clinical settings.

Role of receptor tyrosine kinases mediated signal transduction pathways in tumor growth and angiogenesis-New insight and futuristic vision

In the past two decades, significant progress has been made in the past two decades towards the understanding of the basic mechanisms underlying cancer growth and angiogenesis. In this context, receptor tyrosine kinases (RTKs) play a pivotal role in cell proliferation, differentiation, growth, motility, invasion, and angiogenesis, all of which contribute to tumor growth and progression. Mutations in RTKs lead to abnormal signal transductions in several pathways such as Ras-Raf, MEK-MAPK, PI3K-AKT and mTOR pathways, affecting a wide range of biological functions including cell proliferation, survival, migration and vascular permeability. Increasing evidence demonstrates that multiple kinases are involved in angiogenesis including RTKs such as vascular endothelial growth factor, platelet derived growth factor, epidermal growth factor, insulin-like growth factor-1, macrophage colony-stimulating factor, nerve growth factor, fibroblast growth factor, Hepatocyte Growth factor, Tie 1 & 2, Tek, Flt-3, Flt-4 and Eph receptors. Overactivation of RTKs and its downstream regulation is implicated in tumor initiation and angiogenesis, representing one of the hallmarks of cancer. This review discusses the role of RTKs, PI3K, and mTOR, their involvement, and their implication in pro-oncogenic cellular processes and angiogenesis with effective approaches and newly approved drugs to inhibit their unrestrained action.

Comprehensive phenotyping of endothelial cells using flow cytometry 2: Human

In vascular research, clinical samples and samples from animal models are often used together to foster translation of preclinical findings to humans. General concepts of endothelia and murine-specific endothelial phenotypes were discussed in part 1 of this two part series. Here, in part 2, we present a comprehensive overview of human-specific endothelial phenotypes. Pan-endothelial cell markers, organ specific endothelial antigens, and flow cytometric immunophenotyping of blood-borne endothelial cells are reviewed.

Comprehensive phenotyping of endothelial cells using flow cytometry 1: Murine

The endothelium forms a selective barrier between circulating blood or lymph and surrounding tissue. Endothelial cells play an essential role in vessel homeostasis, and identification of these cells is critical in vascular biology research. However, characteristics of endothelial cells differ depending on the location and type of blood or lymph vessel. Endothelial cell subsets are numerous and often identified using different flow cytometric markers, making immunophenotyping these cells complex. In part 1 of this two part review series, we present a comprehensive overview of markers for the flow cytometric identification and phenotyping of murine endothelial subsets. These subsets can be distinguished using a panel of cell surface and intracellular markers shared by all endothelial cells in combination with additional markers of specialized endothelial cell types. This review can be used to determine the best markers for identifying and phenotyping desired murine endothelial cell subsets.

Illuminating the Regenerative Properties of Stem Cells In Vivo with Bioluminescence Imaging

Preclinical animal studies are essential to the development of safe and effective stem cell therapies. Bioluminescence imaging (BLI) is a powerful tool in animal studies that enables the real-time longitudinal monitoring of stem cells in vivo to elucidate their regenerative properties. This review describes the application of BLI in preclinical stem cell research to address critical challenges in producing successful stem cell therapeutics. These challenges include stem cell survival, proliferation, homing, stress response, and differentiation. The applications presented here utilize bioluminescence to investigate a variety of stem and progenitor cells in several different in vivo models of disease and implantation. An overview of luciferase reporters is provided, along with the advantages and disadvantages of BLI. Additionally, BLI is compared to other preclinical imaging modalities and potential future applications of this technology are discussed in emerging areas of stem cell research.

Biomaterials for Bioprinting Microvasculature

Microvasculature functions at the tissue and cell level, regulating local mass exchange of oxygen and nutrient-rich blood. While there has been considerable success in the biofabrication of large- and small-vessel replacements, functional microvasculature has been particularly challenging to engineer due to its size and complexity. Recently, three-dimensional bioprinting has expanded the possibilities of fabricating sophisticated microvascular systems by enabling precise spatiotemporal placement of cells and biomaterials based on computer-aided design. However, there are still significant challenges facing the development of printable biomaterials that promote robust formation and controlled 3D organization of microvascular networks. This review provides a thorough examination and critical evaluation of contemporary biomaterials and their specific roles in bioprinting microvasculature. We first provide an overview of bioprinting methods and techniques that enable the fabrication of microvessels. We then offer an in-depth critical analysis on the use of hydrogel bioinks for printing microvascularized constructs within the framework of current bioprinting modalities. We end with a review of recent applications of bioprinted microvasculature for disease modeling, drug testing, and tissue engineering, and conclude with an outlook on the challenges facing the evolution of biomaterials design for bioprinting microvasculature with physiological complexity.

The Tie2 signaling pathway in retinal vascular diseases: a novel therapeutic target in the eye

Background

Retinal vascular diseases such as neovascular age-related macular degeneration, diabetic retinopathy and/or diabetic macular edema, and retinal vein occlusion with macular edema—share several key pathophysiologic aspects including neovascularization, vascular permeability, and inflammation. The role of vascular endothelial growth factor (VEGF) in these processes, and the therapeutic benefits of VEGF inhibition, have been well characterized. Anti-VEGF therapy is highly effective for many patients but is not uniformly effective in all patients and imposes a significant treatment burden. More recently, the role of the Tie2 signaling pathway in the pathophysiology of retinal vascular diseases has been investigated, and the Tie2 pathway represents a novel therapeutic target for these conditions.

Areas covered

The index review describes the Tie2 pathway and its complementary role to the VEGF pathway in the angiogenesis cascade and will summarize studies of molecules in development to therapeutically modulate the Tie2 pathway in retinal vascular diseases.

Conclusions

Activation of the Tie2 pathway leads to downstream signaling that promotes vascular health and stability and decreases vascular permeability and inflammation. AXT107 is a collagen IV–derived synthetic peptide with a dual mechanism of action that involves suppression of VEGF signaling and activation of the Tie2 pathway; these actions are accomplished by AXT107 binding to and disrupting different integrin, leading to blockade of the VEGF receptor and rearrangement of cellular Tie2 rendering it susceptible to Ang2 agonism. Other Tie2 agonist compounds are also in development, including faricimab and razuprotafib. Tie2 activation only modestly impacts angiogenesis on its own but significantly potentiates VEGF suppression. Co-regulation of the VEGF and Tie2 signaling pathways has the potential to improve functional and structural outcomes in eyes with retinal vascular diseases.

Tek (Tie2) is not required for cardiovascular development in zebrafish

Angiopoietin/TIE signalling plays a major role in blood and lymphatic vessel development. In mouse, Tek (previously known as Tie2) mutants die prenatally due to a severely underdeveloped cardiovascular system. In contrast, in zebrafish, previous studies have reported that although embryos injected with tek morpholinos (MOs) exhibit severe vascular defects, tek mutants display no obvious vascular malformations. To further investigate the function of zebrafish Tek, we generated a panel of loss-of-function tek mutants, including RNA-less alleles, an allele lacking the MO-binding site, an in-frame deletion allele and a premature termination codon-containing allele. Our data show that all these mutants survive to adulthood with no obvious cardiovascular defects. MO injections into tek mutants lacking the MO-binding site or the entire tek locus cause similar vascular defects to those observed in MO-injected +/+ siblings, indicating off-target effects of the MOs. Surprisingly, comprehensive phylogenetic profiling and synteny analyses reveal that Tek was lost in the largest teleost clade, suggesting a lineage-specific shift in the function of TEK during vertebrate evolution. Altogether, these data show that Tek is dispensable for zebrafish development, and probably dispensable in most teleost species.

Distinct Roles of VEGFA and ANGPT2 in Lung Adenocarcinoma and Squamous Cell Carcinoma

Background: Vascular endothelial growth factor A (VEGFA) and angiopoietin 2 (ANGPT2) are key mediators in angiogenesis. The expression and clinical significance of VEGFA and ANGPT2 have been investigated in lung cancer, but the results are controversial. The specific roles of VEGFA and ANGPT2 in adenocarcinoma (ADC) and squamous cell carcinoma (SQC) are still not fully understood. To characterize it, we conducted the current study.

Materials and Methods: The relationships between clinic-pathological characteristics and the protein expressions of VEGFA and ANGPT2 were analyzed on tissue microarrays by immunohistochemistry (IHC) staining. Then public databases were used to evaluate the association of VEGFA and ANGPT2 mRNA expressions with clinic-pathological parameters and prognosis. Cobalt chloride (CoCl₂) was adopted to mimic a hypoxic microenvironment and western blot was used to detect the expression of hypoxia inducible factor-1α (HIF-1α), VEGFA and ANGPT2 in lung cancer cell lines.

Results: IHC staining revealed that the expressions of VEGFA and ANGPT2 were enriched in lung cancer tissues compared with normal tissues. Additionally, both VEGFA and ANGPT2 protein levels were significantly associated with the tumor size and lymph node metastasis only in ADC, not SQC. More importantly, increased VEGFA and ANGPT2 protein levels were negatively correlated with overall survival (OS) of ADC individuals. Meta-analyses of 22 gene expression omnibus (GEO) databases of lung cancer implicated that patients with higher VEGFA and ANGPT2 mRNA expressions tended to have advanced stage in ADC rather than SQC. Kaplan-Meier plot analyses further verified that high levels of VEGFA and ANGPT2 mRNA were associated with poor survival only in ADC. Moreover, the combination of VEGFA and ANGPT2 could more precisely predict prognosis in ADC. In hypoxia-mimicking conditions, induced expression of HIF-1α unregulated VEGFA and ANGPT2 proteins abundance.

Conclusion: Our results showed hypoxia upregulated the protein levels of VEGFA and ANGPT2 in lung cancer cell lines, and the roles of VEGFA and ANGPT2 were distinct in ADC and SQC. Combined detections of VEGFA and ANGPT2 may be valuable prognostic biomarkers for ADC and double block of VEGFA and ANGPT2 may improve therapeutic outcome.

The Angiopoietin ligands and Tie receptors: potential diagnostic biomarkers of vascular disease

The Angiopoietin-1 (Angpt1)/Tie2 signaling pathway is important in regulating vascular function. Angpt1-induced Tie2 activation promotes vascular endothelial cell survival and reduces vascular leakage. Angiopoietin-2 (Angpt2), a weak agonist/antagonist of Tie2, opposes and regulates Angpt1 action. The Tie family of receptor tyrosine kinases, Tie2 and Tie1, exist as either homo-or heterodimers. The molecular complex between the receptors is also crucial in controlling Angpt1 signaling; hence, the molecular balance between Angpt1:Angpt2 and Tie2:Tie1 is important in determining endothelial integrity and vascular stability. This review presents evidence of the change observed in the Angiopoietin/Tie molecules in various pathophysiological conditions and discusses the potential clinical applications of these molecules in vascular complications.

Loss of reticulocalbin 2 lowers blood pressure and restrains ANG II-induced hypertension in vivo

Hypertension affects over 1 billion people worldwide and increases the risk for heart failure, stroke, and chronic kidney disease. Despite high prevalence and devastating impact, its etiology still remains poorly understood for most hypertensive cases. Rcn2, which encodes reticulocalbin 2, is a candidate gene for atherosclerosis that we have previously reported in mice. Here, we identified Rcn2 as a novel regulator of blood pressure in mice. Rcn2 was abundantly expressed in the endothelium and adventitia of normal arteries and was dramatically upregulated in the medial layer of the artery undergoing structural remodeling. Deletion of Rcn2 lowered basal blood pressure and attenuated ANG II-induced hypertension in C57BL/6 mice. siRNA knockdown of Rcn2 dramatically increased production of the nitric oxide (NO) breakdown products nitrite and nitrate by endothelial cells but not by smooth muscle cells. Isolated carotid arteries from Rcn2-/- mice showed an increased sensitivity to the ACh-induced NO-mediated relaxant response compared with arteries of Rcn2+/+ mice. Analysis of a recent meta-data set showed associations of genetic variants near RCN2 with blood pressure in humans. These data suggest that Rcn2 regulates blood pressure and contributes to hypertension through actions on endothelial NO synthase.

Metabolic gene NR4A1 as a potential therapeutic target for non-smoking female non-small cell lung cancer patients

BACKGROUND

Although cigarette smoking is considered one of the key risk factors for lung cancer, 15% of male patients and 53% of female patients with lung cancer are non-smokers. Metabolic changes are critical features of cancer. Therapeutic target identification from a metabolic perspective in non-small cell lung cancer (NSCLC) tissue of female non-smokers has long been ignored.

RESULTS

Based on microarray data retrieved from Affymetrix expression arrays E-GEOD-19804, we found that the downregulated genes in non-smoking female NSCLC patients tended to participate in protein/amino acid and lipid metabolism, while upregulated genes were more involved in protein/amino acid and carbohydrate metabolism. Combining nutrient metabolic co-expression, protein-protein interaction network construction and overall survival assessment, we identified NR4A1 and TIE1 as potential therapeutic targets for NSCLC in female non-smokers. To accelerate the drug development for non-smoking female NSCLC patients, we identified nilotinib as a potential agonist targeting NR4A1 encoded protein by molecular docking and molecular dynamic stimulation. We also show that nilotinib inhibited proliferation and induced senescence of cells in non-smoking female NSCLC patients in vitro.

CONCLUSIONS

These results not only uncover nutrient metabolic characteristics in non-smoking female NSCLC patients, but also provide a new paradigm for identifying new targets and drugs for novel therapy for such patients.

Endothelial siRNA delivery in nonhuman primates using ionizable low-molecular weight polymeric nanoparticles

Dysfunctional endothelial cells contribute to the pathophysiology of many diseases, including vascular disease, stroke, hypertension, atherosclerosis, organ failure, diabetes, retinopathy, and cancer. Toward the goal of creating a new RNA-based therapy to correct aberrant endothelial cell gene expression in humans, efficient gene silencing in the endothelium of nonhuman primates was achieved by delivering small interfering RNA (siRNA) with 7C1, a low-molecular weight, ionizable polymer that forms nanoparticles. After a single intravenous administration of 1 mg of siRNA per kilogram of animal, 7C1 nanoparticles delivering Tie2 siRNA caused Tie2 mRNA levels to decrease by approximately 80% in the endothelium of the lung. Significant decreases in Tie2 mRNA were also found in the heart, retina, kidney, pancreas, and bone. Blood chemistry and liver function analysis before and after treatment all showed protein and enzyme concentrations within the normal reference ranges. Furthermore, after controlling for siRNA-specific effects, no significant increases in inflammatory cytokine concentrations were found in the serum. Similarly, no gross lesions or significant underlying pathologies were observed after histological examination of nonhuman primate tissues. This study is the first demonstration of endothelial gene silencing in multiple nonhuman primate organs using systemically administered siRNA nanoparticles and highlights the potential of this approach for the treatment of disease in humans.

Utilizing combinatorial engineering to develop Tie2 targeting antagonistic angiopoetin-2 ligands as candidates for anti-angiogenesis therapy

In many human cancers, the receptor tyrosine kinase (RTK) Tie2 plays important roles in mediating proliferation, survival, migration and angiogenesis. Thus, molecules that could potently inhibit activation of the Tie2 receptor would have a significant impact on cancer therapy. Nevertheless, attempts to develop Tie2-targeted inhibitors have met with little success, and there is currently no FDA-approved therapeutic selectively targeting Tie2. We used a combinatorial protein engineering approach to develop a new generation of angiopoietin (Ang)2-derived Tie2 antagonists as potential cancer therapeutics and as tools to study angiogenesis. The construct for designing a yeast surface display (YSD) library of potential antagonists was an Ang2 binding domain (Ang2-BD) that retains Tie2 binding ability but prevents ligand multimerization and receptor dimerization and activation. This mutant library was then screened by quantitative high-throughput flow cytometric sorting to identify Ang2-BD variants with increased expression, stability and affinity to Tie2. The selected variants were recombinantly expressed and showed high affinity to soluble and cellular Tie2 and strongly inhibited both Tie2 phosphorylation and endothelial capillary tube formation and cell invasion compared to the parental Ang2-BD. The significance of the study lies in the insight it provides into the sequence-structure-function relationships and mechanism of action of the antagonistic Ang mutants. The approach of using a natural protein ligand as a molecular scaffold for engineering high-affinity agents can be applied to other ligands to create functional protein antagonists against additional biomedical targets.

Context-dependent functions of angiopoietin 2 are determined by the endothelial phosphatase VEPTP

The angiopoietin (ANGPT)-TIE2/TEK signaling pathway is essential for blood and lymphatic vascular homeostasis. ANGPT1 is a potent TIE2 activator, whereas ANGPT2 functions as a context-dependent agonist/antagonist. In disease, ANGPT2-mediated inhibition of TIE2 in blood vessels is linked to vascular leak, inflammation, and metastasis. Using conditional knockout studies in mice, we show TIE2 is predominantly activated by ANGPT1 in the cardiovascular system and by ANGPT2 in the lymphatic vasculature. Mechanisms underlying opposing actions of ANGPT2 in blood vs. lymphatic endothelium are poorly understood. Here we show the endothelial-specific phosphatase VEPTP (vascular endothelial protein tyrosine phosphatase) determines TIE2 response to ANGPT2. VEPTP is absent from lymphatic endothelium in mouse in vivo, permitting ANGPT2/TIE2-mediated lymphangiogenesis. Inhibition of VEPTP converts ANGPT2 into a potent TIE2 activator in blood endothelium. Our data support a model whereby VEPTP functions as a rheostat to modulate ANGPT2 ligand effect on TIE2.

Progressive loss of retinal blood vessels in a live model of retinitis pigmentosa

OBJECTIVE

To assess retinal blood vessels in a live retinitis pigmentosa (RP) model with rd1 mutation and green fluorescent protein (GFP) expressed in vascular endothelium.

METHODS

Homozygous (hm) Tie2-GFP mice with rd1 mutation and known retinal degeneration were crossed with wild-type CD1 mice to generate control heterozygous (ht) Tie2-GFP mice. The retinas of 16 live hm mice were evaluated at 2 weeks and 3, 5, and 8 months of age, and compared with age-matched control ht and CD1 mice by optical coherence tomography (OCT) and confocal scanning laser ophthalmoscopy (cSLO). Fluorescence intensity was measured and compared between strains at 3, 5, and 8 months. In vivo findings were validated by immunostaining with collagen IV and isolectin histopathology.

RESULTS

All hm Tie2-GFP mice showed progressive outer retinal degeneration by OCT. Loss of small branches of blood vessels and then larger main vessels was seen by cSLO. Retinal tissue and vessels were preserved in control ht mice. At all ages, measurements of fluorescence intensity were reduced in hm compared with ht mice (p < 0.001). In all strains, intensity at 8 months was reduced compared with 3 months (p < 0.001) and 5 months (p = 0.021). Histopathological studies confirmed in vivo findings and revealed a pattern of blood vessel regression in the deep plexus, followed by intermediate and superficial retinal plexuses.

CONCLUSIONS

This is the first evidence of progressive loss of retinal blood vessels in a live mouse model of RP. These findings may be highly relevant to understanding retinal degeneration in RP to prevent blindness.

Angiopoietin-Tie signalling in the cardiovascular and lymphatic systems

Endothelial cells that form the inner layer of blood and lymphatic vessels are important regulators of vascular functions and centrally involved in the pathogenesis of vascular diseases. In addition to the vascular endothelial growth factor (VEGF) receptor pathway, the angiopoietin (Ang)-Tie system is a second endothelial cell specific ligand-receptor signalling system necessary for embryonic cardiovascular and lymphatic development. The Ang-Tie system also regulates postnatal angiogenesis, vessel remodelling, vascular permeability and inflammation to maintain vascular homoeostasis in adult physiology. This system is implicated in numerous diseases where the vasculature has an important contribution, such as cancer, sepsis, diabetes, atherosclerosis and ocular diseases. Furthermore, mutations in the TIE2 signalling pathway cause defects in vascular morphogenesis, resulting in venous malformations and primary congenital glaucoma. Here, we review recent advances in the understanding of the Ang-Tie signalling system, including cross-talk with the vascular endothelial protein tyrosine phosphatase (VE-PTP) and the integrin cell adhesion receptors, focusing on the Ang-Tie system in vascular development and pathogenesis of vascular diseases.

Conditional Deletion of Bmal1 Accentuates Microvascular and Macrovascular Injury

The brain and muscle aryl hydrocarbon receptor nuclear translocator-like protein (BMAL)-1 constitutes a major transcriptional regulator of the circadian clock. Here, we explored the impact of conditional deletion of Bmal1 in endothelium and hematopoietic cells in murine models of microvascular and macrovascular injury. We used two models of Bmal1fx/fx;Tek-Cre mice, a retinal ischemia/reperfusion model and a neointimal hyperplasia model of the femoral artery. Eyes were enumerated for acellular capillaries and were stained for oxidative damage markers using nitrotyrosine immunohistochemistry. LSK (lineage-negative, stem cell antigen-1-positive, c-Kit-positive) cells were quantified and proliferation assessed. Hematopoiesis is influenced by innervation to the bone marrow, which we assessed using IHC analysis. The number of acellular capillaries increased threefold, and nitrotyrosine staining increased 1.5-fold, in the retinas of Bmal1fx/fx;Tek-Cre mice. The number of LSK cells from the Bmal1fx/fx;Tek-Cre mice decreased by 1.5-fold and was accompanied by a profound decrease in proliferative potential. Bmal1fx/fx;Tek-Cre mice also exhibited evidence of bone marrow denervation, demonstrating a loss of neurofilament-200 staining. Injured femoral arteries showed a 20% increase in neointimal hyperplasia compared with similarly injured wild-type controls. Our study highlights the importance of the circadian clock in maintaining vascular homeostasis and demonstrates that specific deletion of BMAL1 in endothelial and hematopoietic cells results in phenotypic features similar to those of diabetes.

Elevated expression of Tie1 is accompanied by acquisition of cancer stemness properties in colorectal cancer

The Tie receptors 1 and 2 (Tie1/2) play crucial roles in embryonic angiogenesis. Recent studies suggest enhanced expression of Tie1 in several types of cancer and negative correlations between Tie1 levels and clinical outcome. These observations suggest important functions of Tie1 not only for vascular formation but also in tumorigenesis. Ligands for Tie2, that is angiopoietins 1-4, have been identified, but not for Tie1. To determine the molecular functions of Tie1, its detailed characterization in tumors would be helpful. Herein, we report that Tie1 is up-regulated in colorectal cancer. Detailed analysis using tumor-bearing models and immunohistochemistry combined with Flow cytometric analysis and cell sorting (FACS) revealed that Tie1 protein was expressed in a small population of malignant tumor cells. Intriguingly, Tie1 expression was observed and could be maintained only in vivo. Further analysis using sphere-formation culture revealed that Tie1-positive cells are enriched within the population of tumor cells with cancer stemness properties. Indeed, Tie1-positive tumor cells derived from a murine model overexpressed Lgr5, a typical stemness marker for colorectal cancer. Our results provide a novel insight into Tie1 function in tumorigenesis and suggest clinical applications to target cancer stem cells.

Tie1: an orphan receptor provides context for angiopoietin-2/Tie2 signaling

Angiopoietin-1/Tie2 (ANG1/Tie2) signaling is well documented as regulating angiogenesis and vessel maturation. This pathway is complicated by involvement of the orphan receptor Tie1, which has been implicated as both a positive and negative regulator of ANG1/Tie2 signaling, and ANG2, which can serve as both a Tie2 agonist and antagonist, depending on the context. Two papers in this issue of the JCI provide new insight into this complicated pathway. Korhonen et al. reveal that Tie1 acts to modulate the effects of ANG1 and ANG2 on Tie2 in vitro and in vivo. Kim et al. demonstrate that ANG2 acts as a Tie2 agonist in non-pathological conditions, whereas in the setting of inflammation, ANG2 functions as a Tie2 antagonist and promotes vascular dysfunction. Both studies indicate that inflammation promotes cleavage of the ectodomain of Tie1 and that this cleavage event corresponds with the switch of ANG2 from a Tie2 agonist to an antagonist. The results of these studies lay the groundwork for future strategies to therapeutically exploit this pathway in diseases characterized by adverse vascular remodeling and increased permeability.

Suppression of KSHV-induced angiopoietin-2 inhibits angiogenesis, infiltration of inflammatory cells, and tumor growth

Kaposi's sarcoma (KS) is a highly angiogenic and inflammatory neoplasia. The angiogenic and inflammatory cytokine angiopoietin-2 (Ang-2) is strongly expressed in KS due to Kaposi's sarcoma-associated herpesvirus (KSHV) infection. In the present study, we determined how Ang-2 contributes to development of KS by using telomerase-immortalized human umbilical vein endothelial cells (TIVE) as a model, which become malignantly transformed and express increased levels of Ang-2 following KSHV infection. Ang-2 released from TIVE-KSHV cells induces tyrosine phosphorylation of Tie-2 receptor from both human and mouse endothelial cells and promotes angiogenesis in nude mice. Functional inhibition or expressional "knock-down" of Ang-2 in these cells blocks angiogenesis and inhibits tumor growth. Ang-2 suppression also reduces the numbers of infiltrating monocytes/macrophages in tumors. In transwell-based cell migration assays, Ang-2 indeed enhances migration of human monocytes in a dose-dependent manner. These results underscore a pivotal role of KSHV-induced Ang-2 in KS tumor development by promoting both angiogenesis and inflammation. Our data also suggest that selective drug targeting of Ang-2 may be used for treatment of KS.

Biodegradable Mg-Cu alloys with enhanced osteogenesis, angiogenesis, and long-lasting antibacterial effects

A series of biodegradable Mg-Cu alloys is designed to induce osteogenesis, stimulate angiogenesis, and provide long-lasting antibacterial performance at the same time. The Mg-Cu alloys with precipitated Mg2Cu intermetallic phases exhibit accelerated degradation in the physiological environment due to galvanic corrosion and the alkaline environment combined with Cu release endows the Mg-Cu alloys with prolonged antibacterial effects. In addition to no cytotoxicity towards HUVECs and MC3T3-E1 cells, the Mg-Cu alloys, particularly Mg-0.03Cu, enhance the cell viability, alkaline phosphatase activity, matrix mineralization, collagen secretion, osteogenesis-related gene and protein expressions of MC3T3-E1 cells, cell proliferation, migration, endothelial tubule forming, angiogenesis-related gene, and protein expressions of HUVECs compared to pure Mg. The favorable osteogenesis and angiogenesis are believed to arise from the release of bioactive Mg and Cu ions into the biological environment and the biodegradable Mg-Cu alloys with osteogenesis, angiogenesis, and long-term antibacterial ability are very promising in orthopedic applications.

Tie2 Regulates Tumor Metastasis of Oral Squamous Cell Carcinomas

The endothelial-specific receptor, tyrosine kinase with immunoglobulin-like loops and epidermal growth factor homology domains-2 (Tie2) is a member of the tyrosine kinase family and is ubiquitous in normal tissues; however, little is known about the mechanisms and roles of Tie2 in oral squamous cell carcinomas (OSCCs). In the current study, we investigated the expression status of Tie2 in OSCCs by quantitative reverse transcriptase-polymerase chain reaction, immunoblotting, and immunohistochemistry and the functional mechanisms of Tie2 using its overexpressed OSCC (oeTie2) cells and Tie2 blocking by its antibody. We found that Tie2 expression was down-regulated significantly (p < 0.05) in OSCCs compared with normal counterparts in vitro and in vivo. Interestingly, oeTie2 cells showed higher cellular adhesion (p < 0.05) and lower cellular invasion (p < 0.05) compared with control cells; whereas there was similar cellular proliferation in both transfectants. Furthermore, cellular adhesion was inhibited and invasion was activated by Tie2 function-blocking antibody (p < 0.05), indicating that Tie2 directly regulates cellular adhesion and invasion. As expected, among the clinical variables analyzed, Tie2-positivity in patients with OSCC was correlated closely with negative lymph node metastasis. These results suggested for the first time that Tie2 plays an important role in tumor metastasis and may be a potential biomarker for OSCC metastasis.

A Tie2-driven BAC-TRAP transgenic line for in vivo endothelial gene profiling

Recent technological innovations including bacterial artificial chromosome-based translating ribosome affinity purification (BAC-TRAP) have greatly facilitated analysis of cell type-specific gene expression in vivo, especially in the nervous system. To better study endothelial gene expression in vivo, we have generated a BAC-TRAP transgenic mouse line where the L10a ribosomal subunit is tagged with EGFP and placed under the control of the endothelium-specific Tie2 (Tek) promoter. We show that transgene expression in this line is widely, but specifically, detected in endothelial cells in several brain regions throughout pre- and postnatal development, as well as in other organs. We also show that this line results in highly significant enrichment of endothelium-specific mRNAs from brain tissues at different stages. This BAC-TRAP line therefore provides a useful genetic tool for in vivo endothelial gene profiling under various developmental, physiological, and pathological conditions. genesis 54:136-145, 2016. © 2016 Wiley Periodicals, Inc.

Temporal expression in rats of receptor tyrosine kinase Tie2 during early wound healing after tooth extraction

We examined the role of Tie2 in regulating wound healing after tooth extraction. Wistar rats underwent maxillary incisor tooth extraction, and immunodetection techniques were used to determine Tie2 expression in the healing wound. The wound was initially filled with blood coagulum containing densely aggregated erythrocytes, leukocytes, fibrin, and endothelial progenitor cells, indicating that blood vessel formation started in the socket. Tie2 was detected on monocytic cell membranes. On day 3, fibroblastic cells proliferated in the coagulum, small vessels appeared by day 5, and new bone formed in the vessel-rich area. Robust woven bone trabeculae were present around vessels by day 7, and woven bone and osteoclast-like giant cells were present on day 10. Woven bone surrounded sinusoidal capillary-like vessels. Full-length (140-160 kDa) Tie2 was not detected at any time, although Tie2 fragments were present in the healing wound. N-terminus- and C-terminus-specific Tie2 antibodies detected 40-kDa and 60-kDa fragments or 70-kDa and 50-kDa fragments, respectively. The levels of these fragments decreased during the first 3 days and started to increase by day 5-10. The Tie2 extracellular domain initially inhibited angiogenesis, and its degradation relieved inhibition of new vessel formation. The onset of vessel formation in the wound may be induced by scattered endothelial progenitor cells.

Plasma angiopoietin-2 outperforms other markers of endothelial injury in prognosticating pediatric ARDS mortality

Angiopoietin-2 (Ang-2) is a key mediator of pulmonary vascular permeability. This study tested the association between plasma Ang-2 and mortality in pediatric acute respiratory distress syndrome (ARDS), with stratification for prior hematopoietic cellular transplantation (HCT), given the severe, yet poorly understood, ARDS phenotype of this subgroup. We enrolled 259 children <18 years of age with ARDS; 25 had prior HCT. Plasma Ang-2, von Willebrand Factor antigen (vWF), and vascular endothelial growth factor (VEGF) were measured on ARDS days 1 and 3 and correlated with patient outcomes. Day 1 and day 3 Ang-2 levels were associated with mortality independent of age, sex, race, and P/F ratio [odds ratio (OR) 3.7, 95% CI 1.1-11.5, P = 0.027; and OR 10.2, 95% confidence interval (CI) 2.2-46.5, P = 0.003, for each log10 increase in Ang-2]. vWF was associated with mortality (P = 0.027), but VEGF was not. The association between day 1 Ang-2 and mortality was independent of levels of both vWF and VEGF (OR 3.6, 95% CI 1.1-12.1, P = 0.039, for each log10 increase in Ang-2). 45% of the cohort had a rising Ang-2 between ARDS day 1 and 3 (adjusted mortality OR 3.3, 95% CI 1.2-9.2, P = 0.026). HCT patients with a rising Ang-2 had 70% mortality compared with 13% mortality for those without (OR 16.3, 95% CI 1.3-197.8, P = 0.028). Elevated plasma levels of Ang-2 were associated with mortality independent of vWF and VEGF. A rising Ang-2 between days 1 and 3 was strongly associated with mortality, particularly in pediatric HCT patients, suggesting vulnerability to ongoing endothelial damage.

Gingipains from the Periodontal Pathogen Porphyromonas gingivalis Play a Significant Role in Regulation of Angiopoietin 1 and Angiopoietin 2 in Human Aortic Smooth Muscle Cells

Angiopoietin 1 (Angpt1) and angiopoietin 2 (Angpt2) are the ligands of tyrosine kinase (Tie) receptors, and they play important roles in vessel formation and the development of inflammatory diseases, such as atherosclerosis. Porphyromonas gingivalis is a Gram-negative periodontal bacterium that is thought to contribute to the progression of cardiovascular disease. The aim of this study was to investigate the role of P. gingivalis infection in the modulation of Angpt1 and Angpt2 in human aortic smooth muscle cells (AoSMCs). We exposed AoSMCs to wild-type (W50 and 381), gingipain mutant (E8 and K1A), and fimbrial mutant (DPG-3 and KRX-178) P. gingivalis strains and to different concentrations of tumor necrosis factor (TNF). The atherosclerosis risk factor TNF was used as a positive control in this study. We found that P. gingivalis (wild type, K1A, DPG3, and KRX178) and TNF upregulated the expression of Angpt2 and its transcription factor ETS1, respectively, in AoSMCs. In contrast, Angpt1 was inhibited by P. gingivalis and TNF. However, the RgpAB mutant E8 had no effect on the expression of Angpt1, Angpt2, or ETS1 in AoSMCs. The results also showed that ETS1 is critical for P. gingivalis induction of Angpt2. Exposure to Angpt2 protein enhanced the migration of AoSMCs but had no effect on proliferation. This study demonstrates that gingipains are crucial to the ability of P. gingivalis to markedly increase the expressed Angpt2/Angpt1 ratio in AoSMCs, which determines the regulatory role of angiopoietins in angiogenesis and their involvement in the development of atherosclerosis. These findings further support the association between periodontitis and cardiovascular disease.

Rapamycin improves TIE2-mutated venous malformation in murine model and human subjects

Venous malformations (VMs) are composed of ectatic veins with scarce smooth muscle cell coverage. Activating mutations in the endothelial cell tyrosine kinase receptor TIE2 are a common cause of these lesions. VMs cause deformity, pain, and local intravascular coagulopathy, and they expand with time. Targeted pharmacological therapies are not available for this condition. Here, we generated a model of VMs by injecting HUVECs expressing the most frequent VM-causing TIE2 mutation, TIE2-L914F, into immune-deficient mice. TIE2-L914F-expressing HUVECs formed VMs with ectatic blood-filled channels that enlarged over time. We tested both rapamycin and a TIE2 tyrosine kinase inhibitor (TIE2-TKI) for their effects on murine VM expansion and for their ability to inhibit mutant TIE2 signaling. Rapamycin prevented VM growth, while TIE2-TKI had no effect. In cultured TIE2-L914F-expressing HUVECs, rapamycin effectively reduced mutant TIE2-induced AKT signaling and, though TIE2-TKI did target the WT receptor, it only weakly suppressed mutant-induced AKT signaling. In a prospective clinical pilot study, we analyzed the effects of rapamycin in 6 patients with difficult-to-treat venous anomalies. Rapamycin reduced pain, bleeding, lesion size, functional and esthetic impairment, and intravascular coagulopathy. This study provides a VM model that allows evaluation of potential therapeutic strategies and demonstrates that rapamycin provides clinical improvement in patients with venous malformation.

Cdc42 is required for cytoskeletal support of endothelial cell adhesion during blood vessel formation in mice

The Rho family of small GTPases has been shown to be required in endothelial cells (ECs) during blood vessel formation. However, the underlying cellular events controlled by different GTPases remain unclear. Here, we assess the cellular mechanisms by which Cdc42 regulates mammalian vascular morphogenesis and maintenance. In vivo deletion of Cdc42 in embryonic ECs (Cdc42(Tie2KO)) results in blocked lumen formation and endothelial tearing, leading to lethality of mutant embryos by E9-10 due to failed blood circulation. Similarly, inducible deletion of Cdc42 (Cdc42(Cad5KO)) at mid-gestation blocks angiogenic tubulogenesis. By contrast, deletion of Cdc42 in postnatal retinal vessels leads to aberrant vascular remodeling and sprouting, as well as markedly reduced filopodia formation. We find that Cdc42 is essential for organization of EC adhesion, as its loss results in disorganized cell-cell junctions and reduced focal adhesions. Endothelial polarity is also rapidly lost upon Cdc42 deletion, as seen by failed localization of apical podocalyxin (PODXL) and basal actin. We link observed failures to a defect in F-actin organization, both in vitro and in vivo, which secondarily impairs EC adhesion and polarity. We also identify Cdc42 effectors Pak2/4 and N-WASP, as well as the actomyosin machinery, to be crucial for EC actin organization. This work supports the notion of Cdc42 as a central regulator of the cellular machinery in ECs that drives blood vessel formation.

Immunohistochemical Expression of Angiogenic Factors by Neoplastic Epithelial Cells Is Associated With Canine Prostatic Carcinogenesis

The dog is the only species, other than humans, in which spontaneous prostatic cancer occurs; therefore, dogs are a valuable model for the study of factors that regulate tumor progression. Angiogenesis is important in the development and spread of a variety of cancers, including prostate cancer. To better define the role of cancer epithelial cells in prostate cancer neovascularization, immunohistochemical staining for angiogenic factors (vascular endothelial growth factor [VEGF], platelet endothelial cell adhesion molecule-1 [PECAM-1], Tie-2, and fibroblast growth factor-2 [FGF-2]) was performed on formalin-fixed, paraffin-embedded tissues from 10 normal prostates, 15 hyperplastic prostates, and 11 prostatic carcinomas from dogs. Normal and hyperplastic epithelial cells were negative for PECAM-1, VEGF, and Tie-2, while the same markers were expressed with a variable intensity of cytoplasmic staining by neoplastic cells. Mild to moderate FGF-2 staining was detected in all normal prostates with less than 10% of positive cells, mainly distributed in the basal layer. The percentage of FGF-2-positive hyperplastic cells was variable, with both basal and secretory cells exhibiting a perinuclear to diffuse cytoplasmic staining. The mean number of positive cells and the intensity of staining were higher in prostatic carcinomas than normal and hyperplastic prostates. Moreover, microvessel density analyzed on PECAM-1-stained slides was increased in prostate cancer compared with normal and hyperplastic prostates. Therefore, prostatic neoplastic cells are capable of simultaneous expression of various angiogenic factors and may increase tumor proliferation and angiogenesis in a paracrine and autocrine fashion.

Dendrimer-Inspired Nanomaterials for the in Vivo Delivery of siRNA to Lung Vasculature

Targeted RNA delivery to lung endothelial cells has the potential to treat conditions that involve inflammation, such as chronic asthma and obstructive pulmonary disease. To this end, chemically modified dendrimer nanomaterials were synthesized and optimized for targeted small interfering RNA (siRNA) delivery to lung vasculature. Using a combinatorial approach, the free amines on multigenerational poly(amido amine) and poly(propylenimine) dendrimers were substituted with alkyl chains of increasing length. The top performing materials from in vivo screens were found to primarily target Tie2-expressing lung endothelial cells. At high doses, the dendrimer-lipid derivatives did not cause chronic increases in proinflammatory cytokines, and animals did not suffer weight loss due to toxicity. We believe these materials have potential as agents for the pulmonary delivery of RNA therapeutics.

Angiopoietins promote ovarian cancer progression by establishing a procancer microenvironment

Despite decades of research, the survival rate of ovarian cancer patients is largely unchanged. Current chemotherapeutic drugs are effective only transiently because patients with advanced disease eventually develop resistance. Thus, there is a pressing need for identifying novel therapeutic targets in ovarian cancer. Mounting evidence suggests that angiopoietins (Angpts) may play an essential role in cancer progression; however, the expression profiles and biological effects of Angpts on ovarian cancer remain largely unknown. Here, we show that, compared with their normal counterparts, expressions of Angpt1, Angpt2, and Angpt4 are increased in ovarian cancer cells and tissues and that human ovarian cancer cells also express the Angpt receptor Tie-2-receptor tyrosine kinase. We show that increased expression of Angpt1, Angpt2, or Angpt4 promotes intraperitoneal growth of ovarian cancers and shortens survival of the experimental mice. We further show, for the first time, that Angpts promote accumulation of cancer-associated fibroblasts and tumor angiogenesis in the ovarian cancer microenvironment, as well as enhance ovarian cancer cell proliferation and invasion in vivo. In addition, we establish a novel function of Angpts in promoting proliferation and invasion and inducing Tie-2 and extracellular signal-regulated kinase 1/2 activation in ovarian cancer-associated fibroblasts. Taken together, these data suggest that the Angpt-Tie-2 functional axis is an important player in ovarian cancer progression and an attractive target for ovarian cancer therapy.

The TIE Receptor Family

The endothelial TIE1 and TIE2 receptor tyrosine kinases form a distinct subfamily characterized by their unique extracellular domains. Together with the angiopoietin growth factors (ANGPT1, ANGPT2, ANGPT4, also abbreviated as ANG), the TIE receptors form an endothelial specific signaling pathway with important functions in the regulation of lymphatic and cardiovascular development and vascular homeostasis. Angiopoietins exist in multimeric forms that activate the TIE receptors via unique mechanism. In endothelial cell–cell contacts, angiopoietins induce the formation of homomeric in trans TIE receptor complexes extending across the cell junctions, whereas matrix-bound angiopoietin-1 (ANG1) activates the TIE receptors in a cis configuration. In comparison to the vascular endothelial growth factor receptors, the TIE receptors undergo little ubiquitin-mediated degradation after activation, whereas TIE2 signaling is negatively regulated by the vascular endothelial protein tyrosine phosphatase, VE-PTP. ANG1 activation of TIE2 supports vascular stabilization, whereas angiopoietin-2 (ANG2), a context-dependent weak TIE2 agonist/antagonist, promotes pathological tumor angiogenesis, vascular permeability, and inflammation. Recently, ANG2 has been found to mediate some of its vascular destabilizing and angiogenic functions via integrin signalling. The circulating levels of ANG2 are increased in cancer, and in several human diseases associated with inflammation and vascular leak, for example, in sepsis. Blocking of ANG2 has emerged as a potential novel therapeutic strategy for these diseases. In addition, preclinical results demonstrate that genetic TIE1 deletion in mice inhibits the vascularization and growth of tumor isografts and protects from atherosclerosis, with little effect on normal vascular homeostasis in adult mice. The ability of the ANG-TIE pathway to control vessel stability and angiogenesis makes it an interesting vascular target for the treatment of the various diseases.

Angiopoietin-2: a multifaceted cytokine that functions in both angiogenesis and inflammation

Angiogenesis and inflammation are two highly linked processes. In the last decade, several factors with dual function in both of these major pathways have been identified. This review focuses on angiopoietin-2 (Ang-2), an important proangiogenic factor that has more recently been implicated in mediating inflammatory processes as well. Ang-2 is upregulated in multiple inflammatory diseases and has been implicated in the direct control of inflammation-related signaling pathways. As a consequence of its multiple roles, designs for therapeutic targeting of Ang-2 should consider the dual function of this factor in regulating angiogenesis and inflammation.

Ionizable amphiphilic dendrimer-based nanomaterials with alkyl-chain-substituted amines for tunable siRNA delivery to the liver endothelium in vivo

A library of dendrimers was synthesized and optimized for targeted small interfering RNA (siRNA) delivery to different cell subpopulations within the liver. Using a combinatorial approach, a library of these nanoparticle-forming materials was produced wherein the free amines on multigenerational poly(amido amine) and poly(propylenimine) dendrimers were substituted with alkyl chains of increasing length, and evaluated for their ability to deliver siRNA to liver cell subpopulations. Interestingly, two lead delivery materials could be formulated in a manner to alter their tissue tropism within the liver-with formulations from the same material capable of preferentially delivering siRNA to 1) endothelial cells, 2) endothelial cells and hepatocytes, or 3) endothelial cells, hepatocytes, and tumor cells in vivo. The ability to broaden or narrow the cellular destination of siRNA within the liver may provide a useful tool to address a range of liver diseases.