Oligomerization and multimerization are critical for angiopoietin-1 to bind and phosphorylate Tie2

Insights to Ang/Tie signaling pathway: another rosy dawn for treating retinal and choroidal vascular diseases

Retinal neurovascular unit (NVU) is a multi-cellular structure that consists of the functional coupling between neural tissue and vascular system. Disrupted NVU will result in the occurrence of retinal and choroidal vascular diseases, which are characterized by the development of neovascularization, increased vascular permeability, and inflammation. This pathological entity mainly includes neovascular age-related macular degeneration (neovascular-AMD), diabetic retinopathy (DR) retinal vein occlusion (RVO), and retinopathy of prematurity (ROP). Emerging evidences suggest that the angopoietin/tyrosine kinase with immunoglobulin and epidermal growth factor homology domains (Ang/Tie) signaling pathway is essential for the development of retinal and choroidal vascular. Tie receptors and their downstream pathways play a key role in modulating the vascular development, vascular stability, remodeling and angiogenesis. Angiopoietin 1 (Ang1) is a natural agonist of Tie2 receptor, which can promote vascular stability. On the other hand, angiopoietin 2 (Ang2) is an antagonist of Tie2 receptor that causes vascular instability. Currently, agents targeting the Ang/Tie signaling pathway have been used to inhibit neovascularization and vascular leakage in neovascular-AMD and DR animal models. Particularly, the AKB-9778 and Faricimab have shown promising efficacy in improving visual acuity in patients with neovascular-AMD and DR. These experimental and clinical evidences suggest that activation of Ang/Tie signaling pathway can inhibit the vascular permeability, neovascularization, thereby maintaining the normal function and structure of NVU. This review seeks to introduce the versatile functions and elucidate the modulatory mechanisms of Ang/Tie signaling pathway. Recent pharmacologic therapies targeting this pathway are also elaborated and summarized. Further translation of these findings may afford a new therapeutic strategy from bench to bedside.

Platelet Angiopoietin-1 Protects Against Murine Models of Tumor Metastasis

BACKGROUND

In addition to their fundamental roles in preserving vascular integrity, platelets also contribute to tumor angiogenesis and metastasis. However, despite being a reservoir for angiogenic and metastatic cytokines, platelets also harbor negative regulators of tumor progression. Angpt1 (angiopoietin-1) is a cytokine essential for developmental angiogenesis that also protects against tumor cell metastasis through an undefined mechanism. Although activated platelets release Angpt1 from α-granules into circulation, the contributions of platelet Angpt1 to tumor growth, angiogenesis, and metastasis have not been investigated.

METHODS

Using cytokine arrays and ELISAs, we first compared platelet Angpt1 levels in breast and melanoma mouse tumor models to tumor-free controls. We then assessed tumor growth and metastasis in mice lacking megakaryocyte and platelet Angpt1 (Angpt1Plt KO). The spontaneous metastasis of mammary-injected tumor cells to the lungs was quantified using RT-PCR (reverse transcription-polymerase chain reaction). The lung colonization of intravenously injected tumor cells and tumor cell extravasation were determined using fluorescent microscopy and flow cytometry.

RESULTS

Platelet Angpt1 is selectively upregulated in the PyMT (polyoma middle tumor antigen) breast cancer mouse model, and platelets are the principal source of Angpt1 in blood circulation. While primary tumor growth and angiogenesis were unaffected, Angpt1Plt KO mice had both increased spontaneous lung metastasis and tumor cell lung colonization following mammary or intravenous injection, respectively. Although platelet Angpt1 did not affect initial tumor cell entrapment in the lungs, Angpt1Plt KO mice had increased tumor cell retention and extravasation. Serum from Angpt1Plt KO mice increased endothelial permeability and reduced VE (vascular endothelial)-cadherin expression at endothelial junctions compared with serum from control mice (Angpt1WT).

CONCLUSIONS

Platelets provide an intravascular source of Angpt1 that restrains tumor metastasis by preserving the lung microvasculature to limit tumor cell extravasation.

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.

An overview of the molecular and clinical significance of the angiopoietin system in leukemia

The angiogenesis efficacy in solid tumors and hematological malignancies has been identified for more than twenty years. Although the exact role of angiogenesis in leukemia as a common hematological malignancy has not yet been extensively studied, its effect is demonstrated on the initiation and maintenance of a favorable microenvironment for leukemia cell proliferation. The angiopoietin family is a defined molecular mediator for angiogenesis, which contributes to vascular permeability and angiogenesis initiation. They participate in the angiogenesis process by binding to tyrosine kinase receptors (Tie) on endothelial cells. Considering the role of angiogenesis in leukemia development and the crucial effects of the Ang-Tie system in angiogenesis regulation, many studies have focused on the correlation between the Ang-Tie system and leukemia diagnosis, monitoring, and treatment. In this study, we reviewed the Ang-Tie system's potential diagnostic and therapeutic effects in different types of leukemia in the gene expression level analysis approach. The angiopoietin family context-dependent manner prevents us from defining its actual function in leukemia, emphasizing the need for more comprehensive studies.

Endothelial progenitor cells in the host defense response

Extensive injury of endothelial cells in blood vasculature, especially in the microcirculatory system, frequently occurs in hosts suffering from sepsis and the accompanied systemic inflammation. Pathological factors, including toxic components derived from invading microbes, oxidative stress associated with tissue ischemia/reperfusion, and vessel active mediators generated during the inflammatory response, are known to play important roles in mediating endothelial injury. Collapse of microcirculation and tissue edema developed from the failure of endothelial barrier function in vital organ systems, including the lung, brain, and kidney, are detrimental, which often predict fatal outcomes. The host body possesses a substantial capacity for maintaining vascular homeostasis and repairing endothelial damage. Bone marrow and vascular wall niches house endothelial progenitor cells (EPCs). In response to septic challenges, EPCs in their niche environment are rapidly activated for proliferation and angiogenic differentiation. In the meantime, release of EPCs from their niches into the blood stream and homing of these vascular precursors to tissue sites of injury are markedly increased. The recruited EPCs actively participate in host defense against endothelial injury and repair of damage in blood vasculature via direct differentiation into endothelial cells for re-endothelialization as well as production of vessel active mediators to exert paracrine and autocrine effects on angiogenesis/vasculogenesis. In recent years, investigations on significance of EPCs in host defense and molecular signaling mechanisms underlying regulation of the EPC response have achieved substantial progress, which promotes exploration of vascular precursor cell-based approaches for effective prevention and treatment of sepsis-induced vascular injury as well as vital organ system failure.

The Role of Angiopoietins in Neovascular Diabetes-Related Retinal Diseases

Diabetic retinopathy is a devastating and frequent complication of poorly controlled diabetes, whose pathogenesis is still only partially understood. Advances in basic research over the last two decades have led to the discovery of angiopoietins, proteins that strongly influence the growth and integrity of blood vessels in many vascular beds, with particular importance in the retina. Angiopoietin 1 (Ang1), produced mostly by pericytes and platelets, and angiopoietin 2 (Ang2), produced mainly by endothelial cells, bind to the same receptor (Tie2), but exert opposing effects on target cells. Ang1 maintains the stability of the mature vasculature, while Ang2 promotes vessel wall destabilization and disruption of the connections between endothelial cells and pericytes. Human retinal endothelial cells exposed to Ang2 show reduced membrane expression of the adhesion molecule VE-cadherin, and patients with proliferative diabetic retinopathy or diabetic macular edema have markedly increased vitreal concentrations of Ang2. Faricimab, a bi-specific antibody simultaneously directed against Ang2 and VEGF, has shown promising results in clinical trials among patients with diabetic retinopathy, and other agents targeting the angiopoietin system are currently in development.

Direct Tie2 Agonists Stabilize Vasculature for the Treatment of Diabetic Macular Edema

Purpose

Diabetic macular edema (DME) is the leading cause of vision loss and blindness among working-age adults. Although current intravitreal anti-vascular endothelial growth factor (VEGF) therapies improve vision for many patients with DME, approximately half do not achieve the visual acuity required to drive. We therefore sought additional approaches to resolve edema and improve vision for these patients.

Methods

We explored direct agonists of Tie2, a receptor known to stabilize vasculature and prevent leakage. We identified a multivalent PEG–Fab conjugate, Tie2.1-hexamer, that oligomerizes Tie2 and drives receptor activation and characterized its activities in vitro and in vivo.

Results

Tie2.1-hexamer normalized and stabilized intercellular junctions of stressed endothelial cell monolayers in vitro, suppressed vascular leak in mice under conditions where anti-VEGF alone was ineffective, and demonstrated extended ocular exposure and robust pharmacodynamic responses in non-human primates.

Conclusions

Tie2.1-hexamer directly activates the Tie2 pathway, reduces vascular leak, and is persistent within the vitreal humor.

Translational Relevance

Our study presents a promising potential therapeutic for the treatment of DME.

Lymphangiogenesis requires Ang2/Tie/PI3K signaling for VEGFR3 cell surface expression

Vascular endothelial growth factor C (VEGF-C) induces lymphangiogenesis via VEGF receptor 3 (VEGFR3), which is encoded by the most frequently mutated gene in human primary lymphedema. Angiopoietins (Angs) and their Tie receptors regulate lymphatic vessel development, and mutations of the ANGPT2 gene were recently found in human primary lymphedema. However, the mechanistic basis of Ang2 activity in lymphangiogenesis is not fully understood. Here, we used gene deletion, blocking Abs, transgene induction, and gene transfer to study how Ang2, its Tie2 receptor, and Tie1 regulate lymphatic vessels. We discovered that VEGF-C–induced Ang2 secretion from lymphatic endothelial cells (LECs) was involved in full Akt activation downstream of phosphoinositide 3 kinase (PI3K). Neonatal deletion of genes encoding the Tie receptors or Ang2 in LECs, or administration of an Ang2-blocking Ab decreased VEGFR3 presentation on LECs and inhibited lymphangiogenesis. A similar effect was observed in LECs upon deletion of the PI3K catalytic p110α subunit or with small-molecule inhibition of a constitutively active PI3K located downstream of Ang2. Deletion of Tie receptors or blockade of Ang2 decreased VEGF-C–induced lymphangiogenesis also in adult mice. Our results reveal an important crosstalk between the VEGF-C and Ang signaling pathways and suggest new avenues for therapeutic manipulation of lymphangiogenesis by targeting Ang2/Tie/PI3K signaling.

Systems biology of angiogenesis signaling: Computational models and omics

Angiogenesis is a highly regulated multiscale process that involves a plethora of cells, their cellular signal transduction, activation, proliferation, differentiation, as well as their intercellular communication. The coordinated execution and integration of such complex signaling programs is critical for physiological angiogenesis to take place in normal growth, development, exercise, and wound healing, while its dysregulation is critically linked to many major human diseases such as cancer, cardiovascular diseases, and ocular disorders; it is also crucial in regenerative medicine. Although huge efforts have been devoted to drug development for these diseases by investigation of angiogenesis‐targeted therapies, only a few therapeutics and targets have proved effective in humans due to the innate multiscale complexity and nonlinearity in the process of angiogenic signaling. As a promising approach that can help better address this challenge, systems biology modeling allows the integration of knowledge across studies and scales and provides a powerful means to mechanistically elucidate and connect the individual molecular and cellular signaling components that function in concert to regulate angiogenesis. In this review, we summarize and discuss how systems biology modeling studies, at the pathway‐, cell‐, tissue‐, and whole body‐levels, have advanced our understanding of signaling in angiogenesis and thereby delivered new translational insights for human diseases.

This article is categorized under:

Cardiovascular Diseases > Computational Models

Cancer > Computational Models

Reviewing Schwannoma-Hemangioma Composite Tumors With Their Tumorigenetic Molecular Pathways and Associated Syndromic Manifestations

Schwannomas are common peripheral nerve sheath tumors. Cavernous hemangiomas are vascular tumors that can affect any organ system. The coexistence of cavernous hemangioma with peripheral nervous system neoplasms is a rare occurrence. So far, 37 cases have been documented, and they have been divided into two categories: conjoined association (neoplasms discovered within the tumor tissue) and discrete association (neoplasms discovered outside the tumor tissue, thus placing neoplasms and tumors in close proximity but in different locations). Schwannomas and neurofibromas are the most prevalent tumors linked to cavernous hemangiomas that have been documented. The author provides a comprehensive review of all such cases published in the past with an emphasis on the implications of their tumorigenetic molecular pathways and syndromic manifestations.

Signaling pathways and intervention therapies in sepsis

Sepsis is defined as life-threatening organ dysfunction caused by dysregulated host systemic inflammatory and immune response to infection. Over decades, advanced understanding of host-microorganism interaction has gradually unmasked the genuine nature of sepsis, guiding toward new definition and novel therapeutic approaches. Diverse clinical manifestations and outcomes among infectious patients have suggested the heterogeneity of immunopathology, while systemic inflammatory responses and deteriorating organ function observed in critically ill patients imply the extensively hyperactivated cascades by the host defense system. From focusing on microorganism pathogenicity, research interests have turned toward the molecular basis of host responses. Though progress has been made regarding recognition and management of clinical sepsis, incidence and mortality rate remain high. Furthermore, clinical trials of therapeutics have failed to obtain promising results. As far as we know, there was no systematic review addressing sepsis-related molecular signaling pathways and intervention therapy in literature. Increasing studies have succeeded to confirm novel functions of involved signaling pathways and comment on efficacy of intervention therapies amid sepsis. However, few of these studies attempt to elucidate the underlining mechanism in progression of sepsis, while other failed to integrate preliminary findings and describe in a broader view. This review focuses on the important signaling pathways, potential molecular mechanism, and pathway-associated therapy in sepsis. Host-derived molecules interacting with activated cells possess pivotal role for sepsis pathogenesis by dynamic regulation of signaling pathways. Cross-talk and functions of these molecules are also discussed in detail. Lastly, potential novel therapeutic strategies precisely targeting on signaling pathways and molecules are mentioned.

Structural insights into the clustering and activation of Tie2 receptor mediated by Tie2 agonistic antibody

Angiopoietin (Angpt)-Tie receptor 2 (Tie2) plays key roles in vascular development and homeostasis as well as pathological vascular remodeling. Therefore, Tie2-agonistic antibody and engineered Angpt1 variants have been developed as potential therapeutics for ischemic and inflammatory vascular diseases. However, their underlying mechanisms for Tie2 clustering and activation remain elusive and the poor manufacturability and stability of Angpt1 variants limit their clinical application. Here, we develop a human Tie2-agonistic antibody (hTAAB), which targets the membrane proximal fibronectin type III domain of Tie2 distinct from the Angpt-binding site. Our Tie2/hTAAB complex structures reveal that hTAAB tethers the preformed Tie2 homodimers into polygonal assemblies through specific binding to Tie2 Fn3 domain. Notably, the polygonal Tie2 clustering induced by hTAAB is critical for Tie2 activation and are resistant to antagonism by Angpt2. Our results provide insight into the molecular mechanism of Tie2 clustering and activation mediated by hTAAB, and the structure-based humanization of hTAAB creates a potential clinical application.

F-domain valency determines outcome of signaling through the angiopoietin pathway

Angiopoietins 1 and 2 (Ang1 and Ang2) regulate angiogenesis through their similar F-domains by activating Tie2 receptors on endothelial cells. Despite the similarity in the underlying receptor-binding interaction, the two angiopoietins have opposite effects: Ang1 induces phosphorylation of AKT, strengthens cell-cell junctions, and enhances endothelial cell survival while Ang2 can antagonize these effects, depending on cellular context. To investigate the molecular basis for the opposing effects, we examined the phenotypes of a series of computationally designed protein scaffolds presenting the Ang1 F-domain in a wide range of valencies and geometries. We find two broad phenotypic classes distinguished by the number of presented F-domains: Scaffolds presenting 3 or 4 F-domains have Ang2-like activity, upregulating pFAK and pERK but not pAKT, while scaffolds presenting 6, 8, 12, 30, or 60 F-domains have Ang1-like activity, upregulating pAKT and inducing migration and vascular stability. The scaffolds with 6 or more F-domains display super-agonist activity, producing stronger phenotypes at lower concentrations than Ang1. Tie2 super-agonist nanoparticles reduced blood extravasation and improved blood-brain barrier integrity four days after a controlled cortical impact injury.

A systems biology model of junctional localization and downstream signaling of the Ang-Tie signaling pathway

The Ang–Tie signaling pathway is an important vascular signaling pathway regulating vascular growth and stability. Dysregulation in the pathway is associated with vascular dysfunction and numerous diseases that involve abnormal vascular permeability and endothelial cell inflammation. The understanding of the molecular mechanisms of the Ang–Tie pathway has been limited due to the complex reaction network formed by the ligands, receptors, and molecular regulatory mechanisms. In this study, we developed a mechanistic computational model of the Ang–Tie signaling pathway validated against experimental data. The model captures and reproduces the experimentally observed junctional localization and downstream signaling of the Ang–Tie signaling axis, as well as the time-dependent role of receptor Tie1. The model predicts that Tie1 modulates Tie2’s response to the context-dependent agonist Ang2 by junctional interactions. Furthermore, modulation of Tie1’s junctional localization, inhibition of Tie2 extracellular domain cleavage, and inhibition of VE-PTP are identified as potential molecular strategies for potentiating Ang2’s agonistic activity and rescuing Tie2 signaling in inflammatory endothelial cells.

Ang2 inhibitors and Tie2 activators: potential therapeutics in perioperative treatment of early stage cancer

Anti-angiogenic drugs targeting the VEGF pathway are most effective in advanced metastatic disease settings of certain types of cancers, whereas they have been unsuccessful as adjuvant therapies of micrometastatic disease in numerous phase III trials involving early-stage (resectable) cancers. Newer investigational anti-angiogenic drugs have been designed to inhibit the Angiopoietin (Ang)-Tie pathway. Acting through Tie2 receptors, the Ang1 ligand is a gatekeeper of endothelial quiescence. Ang2 is a dynamically expressed pro-angiogenic destabilizer of endothelium, and its upregulation is associated with poor prognosis in cancer. Besides using Ang2 blockers as inhibitors of tumor angiogenesis, little attention has been paid to their use as stabilizers of blood vessels to suppress tumor cell extravasation and metastasis. In clinical trials, Ang2 blockers have shown limited efficacy in advanced metastatic disease settings. This review summarizes preclinical evidence suggesting the potential utility of Ang2 inhibitors or Tie2 activators as neoadjuvant or adjuvant therapies in the prevention or treatment of early-stage micrometastatic disease. We further discuss the rationale and potential of combining these strategies with immunotherapy, including immune checkpoint targeting antibodies.

Characterization of ANGPT2 mutations associated with primary lymphedema

Primary lymphedema is caused by developmental and functional defects of the lymphatic vascular system that result in accumulation of protein-rich fluid in tissues, resulting in edema. The 28 currently known genes causing primary lymphedema can explain <30% of cases. Angiopoietin 1 (ANGPT1) and ANGPT2 function via the TIE1-TIE2 (tyrosine kinase with immunoglobulin-like and epidermal growth factor-like domains 1 and 2) receptor complex and α5β1 integrin to form an endothelial cell signaling pathway that is critical for blood and lymphatic vessel formation and remodeling during embryonic development, as well as for homeostasis of the mature vasculature. By screening a cohort of 543 individuals affected by primary lymphedema, we identified one heterozygous de novo ANGPT2 whole-gene deletion and four heterozygous ANGPT2 missense mutations. Functional analyses revealed three missense mutations that resulted in decreased ANGPT2 secretion and inhibited the secretion of wild-type (WT)-ANGPT2, suggesting that they have a dominant-negative effect on ANGPT2 signaling. WT-ANGPT2 and soluble mutants T299M and N304K activated TIE1 and TIE2 in an autocrine assay in human lymphatic endothelial cells. Molecular modeling and biophysical studies showed that amino-terminally truncated ANGPT subunits formed asymmetrical homodimers that bound TIE2 in a 2:1 ratio. The T299M mutant, located in the dimerization interphase, showed reduced integrin α5 binding, and its expression in mouse skin promoted hyperplasia and dilation of cutaneous lymphatic vessels. These results demonstrate that primary lymphedema can be associated with ANGPT2 mutations and provide insights into TIE1 and TIE2 activation mechanisms.

Intrinsic differences in the mechanisms of Tie2 binding to angiopoietins exploited by directed evolution to create an Ang2-selective ligand trap

Angiopoietins Ang1 and Ang2 are secreted ligands for the endothelial receptor tyrosine kinase Tie2 essential for vascular development and maintenance. Ang1 acts as an agonist to maintain normal vessel function, whereas Ang2 acts as a Tie2 antagonist. Ang2 is increased in macular edema, sepsis, and other conditions, in which it blocks Ang1-mediated signaling, causing vascular dysfunction and contributing to disease pathology. Therefore, Ang2 is an attractive therapeutic target. Previously, we reported a Tie2 ectodomain variant that selectively binds Ang2 and acts as soluble ligand trap to sequester Ang2; however, the mechanism of Ang2-binding selectivity is unknown. In the present study, we used directed protein evolution to enhance Ang2-binding affinity of this Tie2 ectodomain trap. We examined contributions of individual residues in the ligand-binding interface of Tie2 to Ang1 and Ang2 binding. Surprisingly, different combinations of Tie2 residues were found to bind each ligand, with hydrophobic residues binding both ligands and polar residues contributing selectively to either Ang1 or Ang2 binding. Our analysis also identified a single Tie2 residue, His168, with a pivotal role in both Ang1 and Ang2 binding, enabling competition between binding ligands. In summary, this study reports an enhanced-affinity Ang2-selective ligand trap with potential for therapeutic development and reveals the mechanism behind its selectivity. It also provides the first analysis of contributions of individual Tie2 residues to Ang1 and Ang2 binding and identifies selectivity-determining residues that could be targeted in the future design of small molecule and other inhibitors of Ang2 for the treatment of vascular dysfunction.

Hematopoietic versus leukemic stem cell quiescence: Challenges and therapeutic opportunities

Hematopoietic stem cells (HSC) are responsible for the production of mature blood cells. To ensure that the HSC pool does not get exhausted over the lifetime of an individual, most HSCs are in a state of quiescence with only a small proportion of HSCs dividing at any one time. HSC quiescence is carefully controlled by both intrinsic and extrinsic, niche-driven mechanisms. In acute myeloid leukemia (AML), the leukemic cells overtake the hematopoietic bone marrow niche where they acquire a quiescent state. These dormant AML cells are resistant to chemotherapeutics. Because they can re-establish the disease after therapy, they are often termed as quiescent leukemic stem cells (LSC) or leukemia-initiating cells. While advancements are being made to target particular driver mutations in AML, there is less focus on how to tackle the drug resistance of quiescent LSCs. This review summarises the current knowledge on the biochemical characteristics of quiescent HSCs and LSCs, the intracellular signaling pathways and the niche-driven mechanisms that control quiescence and the key differences between HSC- and LSC-quiescence that may be exploited for therapy.

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.

New soluble angiopoietin analog of Hepta-ANG1 prevents pathological vascular leakage

Vascular leak is a key driver of organ injury in diseases, and strategies that reduce enhanced permeability and vascular inflammation are promising therapeutic targets. Activation of the angiopoietin-1 (ANG1)-Tie2 tyrosine kinase signaling pathway is an important regulator of vascular quiescence. Here we describe the design and construction of a new soluble ANG1 mimetic that is a potent activator of endothelial Tie2 in vitro and in vivo. Using a chimeric fusion strategy, we replaced the extracellular matrix (ECM) binding and oligomerization domain of ANG1 with a heptameric scaffold derived from the C-terminus of serum complement protein C4-binding protein α. We refer to this new fusion protein biologic as Hepta-ANG1, which forms a stable heptamer and induces Tie2 phosphorylation in cultured cells, and in the lung following intravenous injection of mice. Injection of Hepta-ANG1 ameliorates vascular endothelial growth factor- and lipopolysaccharide-induced vascular leakage, in keeping with the known functions of Angpt1-Tie2 in maintaining quiescent vascular stability. The new Hepta-ANG1 fusion is easy to produce and displays remarkable stability with high multimericity that can potently activate Tie2. It could be a new candidate ANG1 mimetic therapy for treatments of inflammatory vascular leak, such as acute respiratory distress syndrome and sepsis.

The Amino-Terminal Oligomerization Domain of Angiopoietin-2 Affects Vascular Remodeling, Mammary Gland Tumor Growth, and Lung Metastasis in Mice

Angiopoietin-2 (ANGPT2) is a context-dependent TIE2 agonistic or antagonistic ligand that induces diverse responses in cancer. Blocking ANGPT2 provides a promising strategy for inhibiting tumor growth and metastasis, yet variable effects of targeting ANGPT2 have complicated drug development. ANGPT2₄₄₃ is a naturally occurring, lower oligomeric protein isoform whose expression is increased in cancer. Here, we use a knock-in mouse line (mice expressing Angpt2₄₄₃), a genetic model for breast cancer and metastasis (MMTV-PyMT), a syngeneic melanoma lung colonization model (B16F10), and orthotopic injection of E0771 breast cancer cells to show that alternative forms increase the diversity of Angpt2 function. In a mouse retina model of angiogenesis, expression of Angpt2₄₄₃ caused impaired venous development, suggesting enhanced function as a competitive antagonist for Tie2. In mammary gland tumor models, Angpt2₄₄₃ differentially affected primary tumor growth and vascularization; these varying effects were associated with Angpt2 protein localization in the endothelium or in the stromal extracellular matrix as well as the frequency of Tie2-positive tumor blood vessels. In the presence of metastatic cells, Angpt2₄₄₃ promoted destabilization of pulmonary vasculature and lung metastasis. In vitro, ANGPT2₄₄₃ was susceptible to proteolytical cleavage, resulting in a monomeric ligand (ANGPT2_DAP) that inhibited ANGPT1- or ANGPT4-induced TIE2 activation but did not bind to alternative ANGPT2 receptor α5β1 integrin. Collectively, these data reveal novel roles for the ANGPT2 N-terminal domain in blood vessel remodeling, tumor growth, metastasis, integrin binding, and proteolytic regulation. SIGNIFICANCE: This study identifies the role of the N-terminal oligomerization domain of angiopoietin-2 in vascular remodeling and lung metastasis and provides new insights into mechanisms underlying the versatile functions of angiopoietin-2 in cancer.See related commentary by Kamiyama and Augustin, p. 35.

Tumor Development and Angiogenesis in Adult Brain Tumor: Glioblastoma

Angiogenesis is the growth of new capillaries from the preexisting blood vessels. Glioblastoma (GBM) tumors are highly vascularized tumors, and glioma growth depends on the formation of new blood vessels. Angiogenesis is a complex process involving proliferation, migration, and differentiation of vascular endothelial cells (ECs) under the stimulation of specific signals. It is controlled by the balance between its promoting and inhibiting factors. Various angiogenic factors and genes have been identified that stimulate glioma angiogenesis. Therefore, attention has been directed to anti-angiogenesis therapy in which glioma proliferation is inhibited by inhibiting the formation of new tumor vessels using angiogenesis inhibitory factors and drugs. Here, in this review, we highlight and summarize the various molecular mediators that regulate GBM angiogenesis with focus on recent clinical research on the potential of exploiting angiogenic pathways as a strategy in the treatment of GBM patients.

The Angiopoietin-Tie2 Pathway in Critical Illness

Lethal features of sepsis and acute respiratory distress syndrome (ARDS) relate to the health of small blood vessels. For example, alveolar infiltration with proteinaceous fluid is often driven by breach of the microvascular barrier. Spontaneous thrombus formation within inflamed microvessels exacerbates organ ischemia, and in its final stages, erupts into overt disseminated intravascular coagulation. Disruption of an endothelial signaling axis, the Angiopoietin-Tie2 pathway, may mediate the abrupt transition from microvascular integrity to pathologic disruption. This review summarizes preclinical and clinical results that implicate the Tie2 pathway as a promising target to restore microvascular health in sepsis and ARDS.

Targeted IgMs agonize ocular targets with extended vitreal exposure

Treatment of ocular disease is hindered by the presence of the blood-retinal barrier, which restricts access of systemic drugs to the eye. Intravitreal injections bypass this barrier, delivering high concentrations of drug to the targeted tissue. However, the recommended dosing interval for approved biologics is typically 6-12 weeks, and frequent travel to the physician's office poses a substantial burden for elderly patients with poor vision. Real-world data suggest that many patients are under-treated. Here, we investigate IgMs as a novel platform for treating ocular disease. We show that IgMs are well-suited to ocular administration due to moderate viscosity, long ocular exposure, and rapid systemic clearance. The complement-dependent cytotoxicity of IgMs can be readily removed with a P436G mutation, reducing safety liabilities. Furthermore, dodecavalent binding of IgM hexamers can potently activate pathways implicated in the treatment of progressive blindness, including the Tie2 receptor tyrosine kinase signaling pathway for the treatment of diabetic macular edema, or the death receptor 4 tumor necrosis family receptor pathway for the treatment of wet age-related macular degeneration. Collectively, these data demonstrate the promise of IgMs as therapeutic agonists for treating progressive blindness.

Angiopoietin-Tie Signaling Pathway in Endothelial Cells: A Computational Model

The angiopoietin-Tie signaling pathway is an important vascular signaling pathway involved in angiogenesis, vascular stability, and quiescence. Dysregulation in the pathway is linked to the impairments in vascular function associated with many diseases, including cancer, ocular diseases, systemic inflammation, and cardiovascular diseases. The present study uses a computational signaling pathway model validated against experimental data to quantitatively study various mechanistic aspects of the angiopoietin-Tie signaling pathway, including receptor activation, trafficking, turnover, and molecular mechanisms of its regulation. The model provides mechanistic insights into the controversial role of Ang2 and its regulators vascular endothelial protein tyrosine phosphatase (VE-PTP) and Tie1 and predicts synergistic effects of inhibition of VE-PTP, Tie1, and Tie2 cleavage on enhancing the vascular protective actions of Tie2.

Targeting the vascular-specific phosphatase PTPRB protects against retinal ganglion cell loss in a pre-clinical model of glaucoma

Elevated intraocular pressure (IOP) due to insufficient aqueous humor outflow through the trabecular meshwork and Schlemm’s canal (SC) is the most important risk factor for glaucoma, a leading cause of blindness worldwide. We previously reported loss of function mutations in the receptor tyrosine kinase TEK or its ligand ANGPT1 cause primary congenital glaucoma in humans and mice due to failure of SC development. Here, we describe a novel approach to enhance canal formation in these animals by deleting a single allele of the gene encoding the phosphatase PTPRB during development. Compared to Tek haploinsufficient mice, which exhibit elevated IOP and loss of retinal ganglion cells, Tek^+/-;Ptprb^+/- mice have elevated TEK phosphorylation, which allows normal SC development and prevents ocular hypertension and RGC loss. These studies provide evidence that PTPRB is an important regulator of TEK signaling in the aqueous humor outflow pathway and identify a new therapeutic target for treatment of glaucoma.

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.

Role of endotoxemia in causing renal dysfunction in cirrhosis

Renal failure is a challenging problem in patients with cirrhosis since mortality increases with worsening renal function, hence the inclusion of serum creatinine in calculating the Model for End-Stage Liver Disease score for liver transplant evaluation. Among the various causes, infection is the leading etiology of mortality associated with cirrhosis. Bacterial infection frequently precipitates renal failure in patients with cirrhosis with the reported prevalence around 34%. Patients with cirrhosis are at increased risk of infections due to impaired immunity and increased gut permeability leading to bacterial translocation in the setting of portal hypertension. One of the most feared complications of severely decompensated liver and renal failure is hepatorenal syndrome, of which liver transplant may be the only available treatment. Furthermore, in those with spontaneous bacterial peritonitis and urinary tract infection, progressive renal failure occurs despite resolution of infection. Thus, the effects of endotoxemia on renal function in cirrhosis have become a major focus of research. The mechanisms of the damaging effects of endotoxin on renal function are complex but, in essence, involve dysregulated inflammation, circulatory dysfunction, poor clearance of endotoxin burden, as well as vasomotor nephropathy. In this article, we will review the mechanisms of endotoxemia-induced renal dysfunction in the setting of cirrhosis through the effects on renal blood flow, renal vascular endothelium, glomerular filtration rate, and tubular function.

Fibulin-7 is overexpressed in glioblastomas and modulates glioblastoma neovascularization through interaction with angiopoietin-1

Glioblastoma (GBM) is pathologically characterized by highly malignant neoplastic cells, focal necrosis and aberrant blood vessels composed of disorganized endothelial cells and pericytes. The recent cancer microarray database revealed upregulation of fibulin-7 (Fbln7), a member of the fibulin family, but provided no information on the tissue localization or biological function. In the present study, we demonstrated that Fbln7 is markedly overexpressed by the GBM tissue among astrocytic tumors, and immunolocalized mainly to endothelial cells and pericytes of the glomeruloid and hypertrophied microvessels. The production of Fbln7 by endothelial cells and pericytes was confirmed in cultured human umbilical vein endothelial cells (HUVEC) and human brain vascular pericytes (HBVP) and vascular endothelial growth factor (VEGF) stimulated the Fbln7 expression in HUVEC. Fbln7 bound to angiopoietin-1, but not angiopoietin-2 or Tie2 receptor, through interaction between the N-terminal portions of Fbln7 and angiopoietin-1, and it blocked phosphorylation of Tie2 receptor in HUVEC. In a coculture assay using HUVEC and HBVP, multilayered and irregular-shaped tube-like structures of HUVEC were induced by treatment with a high concentration of VEGF. This was accompanied by Fbln7 overproduction by HUVEC and angiopoietin-1 expression by HBVP. The production of aberrant VEGF-induced tube-like structures was attenuated by treatment with antibody or synthetic peptides specific to the Fbln7 N-terminal domain or knockdown of Fbln7. These data demonstrate that Fbln7 is overexpressed by endothelial cells and pericytes of the abnormal microvessels in GBM, and suggest that Fbln7 may contribute to the aberrant vessel formation by modulation of the angiopoietin-1/angiopoietin-2-Tie2 axis.

A collagen IV-derived peptide disrupts α5β1 integrin and potentiates Ang2/Tie2 signaling

The angiopoietin (Ang)/Tie2 signaling pathway is essential for maintaining vascular homeostasis, and its dysregulation is associated with several diseases. Interactions between Tie2 and α5β1 integrin have emerged as part of this control; however, the mechanism is incompletely understood. AXT107, a collagen IV-derived peptide, has strong antipermeability activity and has enabled the elucidation of this previously undetermined mechanism. Previously, AXT107 was shown to inhibit VEGFR2 and other growth factor signaling via receptor tyrosine kinase association with specific integrins. AXT107 disrupts α5β1 and stimulates the relocation of Tie2 and α5 to cell junctions. In the presence of Ang2 and AXT107, junctional Tie2 is activated, downstream survival signals are upregulated, F-actin is rearranged to strengthen junctions, and, as a result, endothelial junctional permeability is reduced. These data suggest that α5β1 sequesters Tie2 in nonjunctional locations in endothelial cell membranes and that AXT107-induced disruption of α5β1 promotes clustering of Tie2 at junctions and converts Ang2 into a strong agonist, similar to responses observed when Ang1 levels greatly exceed those of Ang2. The potentiation of Tie2 activation by Ang2 even extended to mouse models in which AXT107 induced Tie2 phosphorylation in a model of hypoxia and inhibited vascular leakage in an Ang2-overexpression transgenic model and an LPS-induced inflammation model. Because Ang2 levels are very high in ischemic diseases, such as diabetic macular edema, neovascular age-related macular degeneration, uveitis, and cancer, targeting α5β1 with AXT107 provides a potentially more effective approach to treat these diseases.

Angiopoietin 1 influences ischemic reperfusion renal injury via modulating endothelium survival and regeneration

Background

Damage to the endothelium due to ischemia reperfusion injury (IRI) leads to a disruption of the microvasculature, which could be influenced by angiopoietin 1 via its effects on endothelium. We investigated the physiological and therapeutic roles of angiopoietin 1 in renal IRI using angiopoietin 1 knockout and over-expression mice.

Methods

Renal IRI was induced by clamping the right renal artery seven days after left uninephrectomy for 25 min followed by reperfusion. A whole body angiopoietin 1 knockout was achieved by induction with tamoxifen. The renal tubule over-expression of angiopoietin 1 was induced by doxycycline.

Results

In the normal mice, the renal expression of angiopoietin 1 increased 7 days to 14 days after IRI. The angiopoietin 1 knockout caused a delay in the recovery of renal function, less tubular regeneration and more residual tubular necrosis. The endothelial density was lower and the VE-cadherin protein loss was greater in the knockout mice. The over-expression of angiopoietin 1 attenuated the tubular necrosis and renal function impairment 1 and 3 days after IRI. The loss of the endothelium was ameliorated in the over-expression mice. This protective effect was associated with the up-regulation of the gene expression of epidermal growth factor, hepatocyte growth factor, and insulin like growth factor-1 and less tubular apoptosis. The over-expression of angiopoietin 1 stimulated tumor necrosis factor-α, C-C chemokine receptor type 2 and CX3C chemokine receptor 1 inflammatory gene expression, but did not influence macrophage infiltration.

Conclusions

Altogether, the augmentation and downregulation of angiopoietin 1 attenuated renal damage and impaired renal recovery, respectively, by influencing the survival/regeneration of the endothelium. The manipulation of angiopoietin 1 represents a novel therapeutic approach for the treatment of ischemic kidney injury.

Electronic supplementary material

The online version of this article (10.1186/s10020-019-0072-7) contains supplementary material, which is available to authorized users.

Anti-angiogenic Targets: Angiopoietin and Angiopoietin Receptors

Tumor blood vessel formation (angiogenesis) is essential for tumor growth and metastasis. Two main endothelial ligand–receptor pathways regulating angiogenesis are vascular endothelial growth factor (VEGF) receptor and angiopoietin-TIE receptor pathways. The angiopoietin-TIE pathway is required for the remodeling and maturation of the blood and lymphatic vessels during embryonic development after VEGF and VEGF-C mediated development of the primary vascular plexus. Angiopoietin-1 (ANGPT1) stabilizes the vasculature after angiogenic processes, via tyrosine kinase with immunoglobulin-like and EGF-like domains 2 (TIE2) activation. In contrast, ANGPT2 is upregulated at sites of vascular remodeling. ANGPT2 is secreted by activated endothelial cells in inflammation, promoting vascular destabilization. ANGPT2 has been found to be expressed in many human cancers. Intriguingly, in preclinical models inhibition of ANGPT2 has provided promising results in preventing tumor angiogenesis, tumor growth, and metastasis, making it an attractive candidate to target in tumors. However, until now the first ANGPT2 targeting therapies have been less effective in clinical trials than in experimental models. Additionally, in preclinical models combined therapy against ANGPT2 and VEGF or immune checkpoint inhibitors has been superior to monotherapies, and these pathways are also targeted in early clinical trials. In order to improve current anti-angiogenic therapies and successfully exploit ANGPT2 as a target for cancer treatment, the biology of the angiopoietin-TIE pathway needs to be profoundly clarified.

Development of an Orthogonal Tie2 Ligand Resistant to Inhibition by Ang2

Angiopoietin-1 (Ang1) is a vascular protective ligand that acts through the receptor tyrosine kinase Tie2 to enhance endothelial survival and quiescence. In sepsis, diabetic retinopathy, and a range of other diseases, Ang2, an antagonist of Tie2, increases markedly. This antagonist suppresses Ang1 protective effects leading to vascular destabilization, inflammation, and endothelial death. Administration of recombinant Ang1 can counter Ang2 antagonism and restore vascular function. However, recombinant Ang1 is needed at sufficiently high concentrations to block Ang2, and the protein is difficult to produce, requires mammalian expression systems, and is prone to aggregation. Here we present an engineered synthetic Tie2 ligand that is not antagonized by Ang2 but is easy to produce and more robust than Ang1. Using a peptide phage display, we isolated a heptameric sequence that binds Tie2-ectodomain and fused this to the coiled:coil domain of cartilage oligomeric matrix protein. This pentameric protein is 60 kDa in size, expressed in E. coli, and facile to purify. The protein, designated TSL1, binds to Tie2-ectodomain in vitro and on the cell surface. TSL1 inhibits endothelial apoptosis. Crucially, TSL1 binds at a site on Tie2 distinct from the angiopoietin-binding site and is resistant to antagonism by Ang2. This engineered ligand has several advantages over recombinant Ang1 for potential therapeutic applications. The study also highlights the value of orthogonal ligands for regulating cellular receptors without being subject to antagonism or modulation by endogenous ligands.

Targeting the Tie2-αvβ3 integrin axis with bi-specific reagents for the inhibition of angiogenesis

BACKGROUND

Increased activity of the receptor tyrosine kinase Tie2 has been implicated in the promotion of pathological angiogenesis. This activity is mainly mediated through angiopoietin (Ang)1- and Ang2-dependent activation of integrins by Tie2, rendering the Ang/Tie2/integrin axis an attractive putative target for cancer therapeutics.

RESULTS

To target this axis, we developed single domain, non-immunoglobulin high-affinity bi-specific protein inhibitors against both Tie2 and αvβ3 integrin. We have previously engineered the Ang2-binding domain of Tie2 (Ang2-BD) as a Tie2 inhibitor. Here, we engineered an exposed loop in Ang2-BD to generate variants that include an integrin-binding Arg-Gly-Asp (RGD) motif and used flow cytometry screening of a yeast-displayed Ang2-BD RGD loop library to identify the integrin antagonists. The bi-specific antagonists targeting both Tie2 and αvβ3 integrin inhibited adhesion and proliferation of endothelial cells cultured together with the αvβ3 integrin ligand vitronectin, as well as endothelial cell invasion and tube formation. The bi-specific reagents inhibited downstream signaling by Tie2 intracellularly in response to its agonist Ang1 more effectively than the wild-type Ang2 BD that binds Tie2 alone.

CONCLUSIONS

Collectively, this study-the first to describe inhibitors targeting all the known functions resulting from Tie2/integrin αvβ3 cross-talk-has created new tools for studying Tie2- and integrin αvβ3-dependent molecular pathways and provides the basis for the rational and combinatorial engineering of ligand-Tie2 and ligand-integrin αvβ3 receptor interactions. Given the roles of these pathways in cancer angiogenesis and metastasis, this proof of principle study paves the route to create novel Tie2/integrin αvβ3-targeting proteins for clinical use as imaging and therapeutic agents.

Angiopoietin 2 Levels in the Risk Stratification and Mortality Outcome Prediction of Sepsis-Associated Coagulopathy

It has been well established that angiopoietin 2 (Ang-2), a glycoprotein involved in activation of the endothelium, plays an integral role in the pathophysiology of sepsis and many other inflammatory conditions. However, the role of Ang-2 in sepsis-associated coagulopathy (SAC) specifically has not been defined. The aim of this study was to measure Ang-2 plasma levels in patients with sepsis and suspected disseminated intravascular coagulation (DIC) in order to demonstrate its predictive value in SAC severity determination and 28-day mortality outcome. Plasma samples were collected from 102 patients with sepsis and suspected DIC at intensive care unit (ICU) admission. The Ang-2 plasma levels were quantified using a sandwich enzyme-linked immunosorbent assay method. The International Society on Thrombosis and Haemostasis DIC scoring system was used to compare the accuracy of Ang-2 levels versus clinical illness severity scores in predicting SAC severity. Mean Ang-2 levels in patients with sepsis and DIC were significantly higher in comparison to healthy controls (P < 0.0001), and median Ang-2 levels showed a downward trend over time (P = 0.0008). Baseline Ang-2 levels and clinical illness severity scores were higher with increasing severity of disease, and Ang-2 was a better predictor of DIC severity than clinical illness scores. This study demonstrates that Ang-2 levels are significantly upregulated in SAC, and this biomarker can be used to risk stratify patients with sepsis into non-overt DIC and overt DIC. Furthermore, the Ang-2 level at ICU admission in a patient with sepsis and suspected DIC may provide a predictive biomarker for mortality outcome.

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.

Molecular Regulation of Sprouting Angiogenesis

The term angiogenesis arose in the 18th century. Several studies over the next 100 years laid the groundwork for initial studies performed by the Folkman laboratory, which were at first met with some opposition. Once overcome, the angiogenesis field has flourished due to studies on tumor angiogenesis and various developmental models that can be genetically manipulated, including mice and zebrafish. In addition, new discoveries have been aided by the ability to isolate primary endothelial cells, which has allowed dissection of various steps within angiogenesis. This review will summarize the molecular events that control angiogenesis downstream of biochemical factors such as growth factors, cytokines, chemokines, hypoxia-inducible factors (HIFs), and lipids. These and other stimuli have been linked to regulation of junctional molecules and cell surface receptors. In addition, the contribution of cytoskeletal elements and regulatory proteins has revealed an intricate role for mobilization of actin, microtubules, and intermediate filaments in response to cues that activate the endothelium. Activating stimuli also affect various focal adhesion proteins, scaffold proteins, intracellular kinases, and second messengers. Finally, metalloproteinases, which facilitate matrix degradation and the formation of new blood vessels, are discussed, along with our knowledge of crosstalk between the various subclasses of these molecules throughout the text. Compr Physiol 8:153-235, 2018.

Amelioration of sepsis by TIE2 activation-induced vascular protection

Protection of endothelial integrity has been recognized as a frontline approach to alleviating sepsis progression, yet no effective agent for preserving endothelial integrity is available. Using an unusual anti-angiopoietin 2 (ANG2) antibody, ABTAA (ANG2-binding and TIE2-activating antibody), we show that activation of the endothelial receptor TIE2 protects the vasculature from septic damage and provides survival benefit in three sepsis mouse models. Upon binding to ANG2, ABTAA triggers clustering of ANG2, assembling an ABTAA/ANG2 complex that can subsequently bind and activate TIE2. Compared with a conventional ANG2-blocking antibody, ABTAA was highly effective in augmenting survival from sepsis by strengthening the endothelial glycocalyx, reducing cytokine storms, vascular leakage, and rarefaction, and mitigating organ damage. Together, our data advance the role of TIE2 activation in ameliorating sepsis progression and open a potential therapeutic avenue for sepsis to address the lack of sepsis-specific treatment.

Angiopoietin-1 is required for Schlemm's canal development in mice and humans

Primary congenital glaucoma (PCG) is a leading cause of blindness in children worldwide and is caused by developmental defects in 2 aqueous humor outflow structures, Schlemm's canal (SC) and the trabecular meshwork. We previously identified loss-of-function mutations in the angiopoietin (ANGPT) receptor TEK in families with PCG and showed that ANGPT/TEK signaling is essential for SC development. Here, we describe roles for the major ANGPT ligands in the development of the aqueous outflow pathway. We determined that ANGPT1 is essential for SC development, and that Angpt1-knockout mice form a severely hypomorphic canal with elevated intraocular pressure. By contrast, ANGPT2 was dispensable, although mice deficient in both Angpt1 and Angpt2 completely lacked SC, indicating that ANGPT2 compensates for the loss of ANGPT1. In addition, we identified 3 human subjects with rare ANGPT1 variants within an international cohort of 284 PCG patients. Loss of function in 2 of the 3 patient alleles was observed by functional analysis of ANGPT1 variants in a combined in silico, in vitro, and in vivo approach, supporting a causative role for ANGPT1 in disease. By linking ANGPT1 with PCG, these results highlight the importance of ANGPT/TEK signaling in glaucoma pathogenesis and identify a candidate target for therapeutic development.

Engineering a monomeric variant of macrophage colony-stimulating factor (M-CSF) that antagonizes the c-FMS receptor

Enhanced activation of the signaling pathways that mediate the differentiation of mononuclear monocytes into osteoclasts is an underlying cause of several bone diseases and bone metastasis. In particular, dysregulation and overexpression of macrophage colony-stimulating factor (M-CSF) and its c-FMS tyrosine kinase receptor, proteins that are essential for osteoclast differentiation, are known to promote bone metastasis and osteoporosis, making both the ligand and its receptor attractive targets for therapeutic intervention. With this aim in mind, our starting point was the previously held concept that the potential of the M-CSF_C31S mutant as a therapeutic is derived from its inability to dimerize and hence to act as an agonist. The current study showed, however, that dimerization is not abolished in M-CSF_C31S and that the protein retains agonistic activity toward osteoclasts. To design an M-CSF mutant with diminished dimerization capabilities, we solved the crystal structure of the M-CSF_C31S dimer complex and used structure-based energy calculations to identify the residues responsible for its dimeric form. We then used that analysis to develop M-CSF_C31S,M27R, a ligand-based, high-affinity antagonist for c-FMS that retained its binding ability but prevented the ligand dimerization that leads to receptor dimerization and activation. The monomeric properties of M-CSF_C31S,M27R were validated using dynamic light scattering and small-angle X-ray scattering analyses. It was shown that this mutant is a functional inhibitor of M-CSF-dependent c-FMS activation and osteoclast differentiation in vitro Our study, therefore, provided insights into the sequence-structure-function relationships of the M-CSF/c-FMS interaction and of ligand/receptor tyrosine kinase interactions in general.

Regulation of Angiopoietin Signalling by Soluble Tie2 Ectodomain and Engineered Ligand Trap

Angiopoietin-1 (Angpt1) is a glycoprotein ligand important for maintaining the vascular system. It signals via a receptor tyrosine kinase expressed on the surface on endothelial cells, Tie2. This receptor can undergo regulated ectodomain cleavage that releases the ligand-binding domain (sTie2) into the circulation. The concentration of sTie2 is increased in a range of conditions, including peripheral arterial disease and myocardial infarction, where it has been suggested to bind and block Angpt1 resulting in vascular dysfunction. Here we use a joint mathematical modelling and experimental approach to assess the potential impact of sTie2 on the ability of Angpt1 to signal. We find that the concentrations of sTie2 relative to Angpt1 required to suppress signalling by the ligand are more than ten–fold higher than those ever seen in normal or disease conditions. In contrast to the endogenous sTie2, an engineered form of sTie2, which presents dimeric ligand binding sites, inhibits Angpt1 signalling at seventy-fold lower concentrations. While loss of Tie2 ectodomain can suppress Angpt1 signalling locally in the cells in which the receptor is lost, our study shows that the resulting increase in circulating sTie2 is unlikely to affect Angpt1 activity elsewhere in the body.

Endothelial dysfunction during long-term follow-up in children with STEC hemolytic-uremic syndrome

BACKGROUND

Shiga-toxin-producing Escherichia coli (STEC)-associated hemolytic-uremic syndrome (HUS) is a major cause of acute kidney injury (AKI), especially in children. Its long-term outcome with respect to endothelial damage remains largely elusive.

METHODS

This was a cross-sectional study in 26 children who had suffered from STEC-HUS in the past and achieved a complete recovery of renal function (eGFR >90 ml/min/1.73 m²). Skin microcirculation after local heating was assessed by laser Doppler fluximetry, carotid-femoral pulse wave velocity (PWV), carotid intima media thickness (cIMT), 24-h ambulatory blood pressure, and angiopoietin (Ang) 1 and 2 serum levels after a median follow-up period of 6.1 years. The results were compared to those of healthy controls.

RESULTS

All patients were normotensive, mean eGFR was 102 (range 91-154) ml/min/1.73 m², and 13 of the 26 patients showed albuminuria. Endothelial dysfunction was present in 13 patients, and the mean serum Ang2/Ang1 ratio was increased compared to healthy children (each p < 0.05). In contrast, mean values for PWV and cIMT in the patients did not differ from those of the controls. Endothelial dysfunction was significantly associated with younger age at STEC-HUS manifestation, time after HUS, and presence of albuminuria.

CONCLUSION

The results of this study highlight the need for long-term follow-up of STEC-HUS patients even after complete recovery of eGFR and lack of hypertension with respect to microvascular damage.

Therapeutic targeting of the angiopoietin-TIE pathway

The endothelial angiopoietin (ANG)-TIE growth factor receptor pathway regulates vascular permeability and pathological vascular remodelling during inflammation, tumour angiogenesis and metastasis. Drugs that target the ANG-TIE pathway are in clinical development for oncological and ophthalmological applications. The aim is to complement current vascular endothelial growth factor (VEGF)-based anti-angiogenic therapies in cancer, wet age-related macular degeneration and macular oedema. The unique function of the ANG-TIE pathway in vascular stabilization also renders this pathway an attractive target in sepsis, organ transplantation, atherosclerosis and vascular complications of diabetes. This Review covers key aspects of the function of the ANG-TIE pathway in vascular disease and describes the recent development of novel therapeutics that target this pathway.

Plastic roles of pericytes in the blood-retinal barrier

The blood-retinal barrier (BRB) consists of tightly interconnected capillary endothelial cells covered with pericytes and glia, but the role of the pericytes in BRB regulation is not fully understood. Here, we show that platelet-derived growth factor (PDGF)-B/PDGF receptor beta (PDGFRβ) signalling is critical in formation and maturation of BRB through active recruitment of pericytes onto growing retinal vessels. Impaired pericyte recruitment to the vessels shows multiple vascular hallmarks of diabetic retinopathy (DR) due to BRB disruption. However, PDGF-B/PDGFRβ signalling is expendable for maintaining BRB integrity in adult mice. Although selective pericyte loss in stable adult retinal vessels surprisingly does not cause BRB disintegration, it sensitizes retinal vascular endothelial cells (ECs) to VEGF-A, leading to upregulation of angiopoietin-2 (Ang2) in ECs through FOXO1 activation and triggering a positive feedback that resembles the pathogenesis of DR. Accordingly, either blocking Ang2 or activating Tie2 greatly attenuates BRB breakdown, suggesting potential therapeutic approaches to reduce retinal damages upon DR progression.

The Angiopoietin-Tie2 Signaling Axis in Systemic Inflammation

Systemic inflammation is a hallmark of commonly encountered diseases ranging from bacterial sepsis to sterile syndromes such as major trauma. Derangements in the host vasculature contribute to the cardinal manifestations of sepsis in profound ways. Recent studies of control pathways regulating the vascular endothelium have illuminated how this single cell layer toggles between quiescence and activation to affect the development of shock and multiorgan dysfunction. This article focuses on one such control pathway, the Tie2 receptor and its ligands the angiopoietins, to describe a growing body of genetic, biochemical, mechanistic, and human studies that implicate Tie2 as a critical switch. In health, activated Tie2 maintains the endothelium in a quiescent state characterized by dynamic barrier function and antiadhesion against circulating leukocytes. In sepsis and related diseases, expression of the angiopoietins becomes markedly imbalanced and Tie2 signaling is greatly attenuated. These rapid molecular changes potentiate pathophysiologic responses throughout the body, resulting in injurious vascular leakage and organ inflammation. The Tie2 axis, therefore, may be a promising avenue for future translational studies.

Dimerization of Tie2 mediated by its membrane-proximal FNIII domains

Tie1 and Tie2, members of the tyrosine kinase family with immunoglobulin and EGF homology domains, are receptor tyrosine kinases found primarily in endothelial cells with key roles in development and maintenance of the vasculature and in angiogenesis. They are attractive targets for therapeutic intervention in tumor angiogenesis, inflammation, and sepsis. Tie2 is regulated directly by the multimeric angiopoietin (Ang) ligands, with Ang1 being its primary activator. Structural studies have shown how Angs bind to the Tie2 ligand-binding region, but do not explain Tie2 activation and suggest a passive role for the Tie2 extracellular region (ECR) in ligand-induced receptor dimerization. Here we show that the Tie2 ECR forms strong dimers even in the absence of bound ligand. Dimerization is mediated by membrane-proximal fibronectin type III (FNIII) domains that were omitted in previous structural studies. We describe a 2.5-Å resolution X-ray crystal structure of the membrane-proximal three Tie2 FNIII domains, Tie2(FNIIIa-c), revealing two possible dimerization modes that primarily involve the third FNIII domain, FNIIIc. Mutating these dimer interfaces implicates one of them (dimer 1) in soluble Tie2 (sTie2) dimerization in solution but suggests that both could play a role in Ang1-induced Tie2 activation, possibly modulated by Tie1. Through small-angle X-ray scattering studies of sTie2 dimers in solution and modeling based on crystal structures, we suggest that Ang1 binding may cross-link Tie2 dimers into higher-order oligomers, potentially explaining how Tie2 is differentially clustered following ligand engagement in different cellular contexts. Our results also firmly implicate FNIII domain-mediated interactions in Tie2 activation, identifying a potential Achilles' heel for therapeutic inhibition.

Structural basis of Tie2 activation and Tie2/Tie1 heterodimerization

The endothelial cell (EC)-specific receptor tyrosine kinases Tie1 and Tie2 are necessary for the remodeling and maturation of blood and lymphatic vessels. Angiopoietin-1 (Ang1) growth factor is a Tie2 agonist, whereas Ang2 functions as a context-dependent agonist/antagonist. The orphan receptor Tie1 modulates Tie2 activation, which is induced by association of angiopoietins with Tie2 in cis and across EC-EC junctions in trans Except for the binding of the C-terminal angiopoietin domains to the Tie2 ligand-binding domain, the mechanisms for Tie2 activation are poorly understood. We report here the structural basis of Ang1-induced Tie2 dimerization in cis and provide mechanistic insights on Ang2 antagonism, Tie1/Tie2 heterodimerization, and Tie2 clustering. We find that Ang1-induced Tie2 dimerization and activation occurs via the formation of an intermolecular β-sheet between the membrane-proximal (third) Fibronectin type III domains (Fn3) of Tie2. The structures of Tie2 and Tie1 Fn3 domains are similar and compatible with Tie2/Tie1 heterodimerization by the same mechanism. Mutagenesis of the key interaction residues of Tie2 and Tie1 Fn3 domains decreased Ang1-induced Tie2 phosphorylation and increased the basal phosphorylation of Tie1, respectively. Furthermore, the Tie2 structures revealed additional interactions between the Fn 2 (Fn2) domains that coincide with a mutation of Tie2 in primary congenital glaucoma that leads to defective Tie2 clustering and junctional localization. Mutagenesis of the Fn2-Fn2 interface increased the basal phosphorylation of Tie2, suggesting that the Fn2 interactions are essential in preformed Tie2 oligomerization. The interactions of the membrane-proximal domains could provide new targets for modulation of Tie receptor activity.