GATA2 controls lymphatic endothelial cell junctional integrity and lymphovenous valve morphogenesis through miR-126

Circular RNAs increase during vascular cell differentiation and are biomarkers for vascular disease

AIMS

The role of circular RNAs (circRNAs) and their regulation in health and disease are poorly understood. Here, we systematically investigated the temporally resolved transcriptomic expression of circRNAs during differentiation of human induced pluripotent stem cells (iPSCs) into vascular endothelial cells (ECs) and smooth muscle cells (SMCs) and explored their potential as biomarkers for human vascular disease.

METHODS AND RESULTS

Using high-throughput RNA sequencing and a de novo circRNA detection pipeline, we quantified the daily levels of 31 369 circRNAs in a 2-week differentiation trajectory from human stem cells to proliferating mesoderm progenitors to quiescent, differentiated EC and SMC. We detected a significant global increase in RNA circularization, with 397 and 214 circRNAs up-regulated greater than two-fold (adjusted P < 0.05) in mature EC and SMC, compared with undifferentiated progenitor cells. This global increase in circRNAs was associated with up-regulation of host genes and their promoters and a parallel down-regulation of splicing factors. Underlying this switch, the proliferation-regulating transcription factor MYC decreased as vascular cells matured, and inhibition of MYC led to down-regulation of splicing factors such as SRSF1 and SRSF2 and changes in vascular circRNA levels. Examining the identified circRNAs in arterial tissue samples and in peripheral blood mononuclear cells (PBMCs) from patients, we found that circRNA levels decreased in atherosclerotic disease, in contrast to their increase during iPSC maturation into EC and SMC. Using machine learning, we determined that a set of circRNAs derived from COL4A1, COL4A2, HSPG2, and YPEL2 discriminated atherosclerotic from healthy tissue with an area under the receiver operating characteristic curve (AUC) of 0.79. circRNAs from HSPG2 and YPEL2 in blood PBMC samples detected atherosclerosis with an AUC of 0.73.

CONCLUSION

Time-resolved transcriptional profiling of linear and circRNA species revealed that circRNAs provide granular molecular information for disease profiling. The identified circRNAs may serve as blood biomarkers for atherosclerotic vascular disease.

Structure, function, and recombinant production of EGFL7

The secreted factor Epidermal growth factor-like protein 7 (EGFL7) is involved in angiogenesis, vasculogenesis, as well as neurogenesis. Importantly, EGFL7 is also implicated in various pathological conditions, including tumor angiogenesis in human cancers. Thus, understanding the mechanisms through which EGFL7 regulates and promotes blood vessel formation is of clear practical importance. One principle means by which EGFL7's function is investigated is via the expression and purification of the recombinant protein. This mini-review describes three methods used to produce recombinant EGFL7 protein. First, a brief overview of EGFL7's genetics, structure, and function is provided. This is followed by an examination of the advantages and disadvantages of three common expression systems used in the production of recombinant EGFL7; (i) Escherichia coli (E. coli), (ii) human embryonic kidney (HEK) 293 cells or other mammalian cells, and (iii) a baculovirus-based Sf9 insect cell expression system. Based on the available evidence, we conclude that the baculovirus-based Sf9 insect cell expression currently has the advantages of producing active recombinant EGFL7 in the native conformation with the presence of acceptable posttranslational modifications, while providing sufficient yield and stability for experimental purposes.

Endothelial cell transitions in zebrafish vascular development

In recent decades, developmental biologists have come to view vascular development as a series of progressive transitions. Mesoderm differentiates into endothelial cells; arteries, veins and lymphatic endothelial cells are specified from early endothelial cells; and vascular networks diversify and invade developing tissues and organs. Our understanding of this elaborate developmental process has benefitted from detailed studies using the zebrafish as a model system. Here, we review a number of key developmental transitions that occur in zebrafish during the formation of the blood and lymphatic vessel networks.

Connexin-45 is expressed in mouse lymphatic endothelium and required for lymphatic valve function

The expression and functional relevance of the gap junction molecule connexin-45 (Cx45; GJC1) in lymphatic endothelium were not previously known. We found that Cx45 was expressed widely in the endothelium of murine lymphatics, in both valve and nonvalve regions. Cell-specific deletion of Cx45, driven by a constitutive Cre line (Lyve1-Cre) or an inducible Cre line (Prox1-CreERT2), compromised the function of lymphatic valves, as assessed by physiological tests (back leak and closure) of isolated, single-valve vessel segments. The defects were comparable to those previously reported for loss of Cx43, and as with Cx43, deletion of Cx45 resulted in shortening or increased asymmetry of lymphatic valve leaflets, providing an explanation for the compromised valve function. In contrast with Cx43, lymphatic endothelial cell-specific (LEC-specific) deletion of Cx45 did not alter the number of valves in mesenteric or dermal lymphatic networks or the expression patterns of the canonical valve-associated proteins PROX1, ITGA9, or CLAUDIN5. Constitutive deletion of Cx45 from LECs resulted in increased backflow of injected tracer in popliteal networks in vivo and compromised the integrity of the LEC permeability barrier in a subset of collecting vessels. These findings provide evidence for an unexpected role of Cx45 in the development and maintenance of lymphatic valves.

Localized Prox1 Regulates Aortic Valve Endothelial Cell Diversity and Extracellular Matrix Stratification in Mice

BACKGROUND

Specialized valve endothelial cell (VEC) populations are localized oriented to blood flow in developing aortic and mitral valves, but their roles in valve development and disease are unknown. In the aortic valve (AoV), a population of VECs on the fibrosa side expresses the transcription factor Prox1 together with genes found in lymphatic ECs. In this study, we examine Prox1's role in regulating a lymphatic-like gene network and promoting VEC diversity required for the development of the stratified trilaminar extracellular matrix (ECM) of murine AoV leaflets.

METHODS

To determine whether disruption of Prox1 localization affects heart valve development, we generated mice (NFATc1enCre Prox1 gain-of-function) in which Prox1 is overexpressed on the ventricularis side of the AoV beginning in embryonic development. To identify potential targets of Prox1, we performed cleavage under targets and release using nuclease on wild-type and NFATc1enCre Prox1 gain-of-function AoVs with validation by colocalization in vivo using RNA in situ hybridization in NFATc1enCre Prox1 gain-of-function AoVs. Natural induction of Prox1 and target gene expression was evaluated in myxomatous AoVs in a mouse model of Marfan syndrome (Fbn1C1039G/+).

RESULTS

The overexpression of Prox1 is sufficient to cause enlargement of AoVs by postnatal day (P)0, as well as a decrease in ventricularis-specific gene expression and disorganized interstitial ECM layers at P7. We identified potential targets of Prox1 known to play roles in lymphatic ECs including Flt1, Efnb2, Egfl7, and Cx37. Ectopic Prox1 colocalized with induced Flt1, Efnb2, and Cx37 expression in NFATc1enCre Prox1 gain-of-function AoVs. Moreover, in Marfan syndrome myxomatous AoVs, endogenous Prox1, and its identified targets, were ectopically induced in ventricularis side VECs.

CONCLUSIONS

Our results support a role for Prox1 in localized lymphatic-like gene expression on the fibrosa side of the AoV. Furthermore, localized VEC specialization is required for development of the stratified trilaminar ECM critical for AoV function and is dysregulated in congenitally malformed valves.

Single-nucleus multiregion transcriptomic analysis of brain vasculature in Alzheimer's disease

Cerebrovascular dysregulation is a hallmark of Alzheimer's disease (AD), but the changes that occur in specific cell types have not been fully characterized. Here, we profile single-nucleus transcriptomes in the human cerebrovasculature in six brain regions from 220 individuals with AD and 208 age-matched controls. We annotate 22,514 cerebrovascular cells, including 11 subtypes of endothelial, pericyte, smooth muscle, perivascular fibroblast and ependymal cells. We identify 2,676 differentially expressed genes in AD, including downregulation of PDGFRB in pericytes, and of ABCB1 and ATP10A in endothelial cells, and validate the downregulation of SLC6A1 and upregulation of APOD, INSR and COL4A1 in postmortem AD brain tissues. We detect vasculature, glial and neuronal coexpressed gene modules, suggesting coordinated neurovascular unit dysregulation in AD. Integration with AD genetics reveals 125 AD differentially expressed genes directly linked to AD-associated genetic variants. Lastly, we show that APOE4 genotype-associated differences are significantly enriched among AD-associated genes in capillary and venule endothelial cells, as well as subsets of pericytes and fibroblasts.

GATA2 regulates blood/lymph separation in a platelet-dependent and lymphovenous valve-independent manner

INTRODUCTION

Lymphatic vessels collect interstitial fluid, immune cells, and digested lipids and return these bodily fluids to blood through two pairs of lymphovenous valves (LVVs). Like other cardiovascular valves LVVs prevent the backflow of blood into the lymphatic vessels. In addition to LVVs, platelets are necessary to prevent the entry of blood into the lymphatic vessels. Platelet thrombi are observed at LVVs suggesting that LVVs and platelets function in synergy to regulate blood/lymphatic separation.

OBJECTIVES

The primary objective of this work is to determine whether platelets can regulate blood/lymph separation independently of LVVs.

METHODS

The transcription factor GATA2 is necessary for the development of both LVVs and hematopoietic stem cells. Using various endothelial- and hematopoietic cell expressed Cre-lines, we conditionally deleted Gata2. We hypothesized that this strategy would identify the tissue- and time-specific roles of GATA2 and reveal whether platelets and LVVs can independently regulate blood/lymph separation.

RESULTS

Lymphatic vasculature-specific deletion of Gata2 results in the absence of LVVs without compromising blood/lymph separation. In contrast, deletion of GATA2 from both lymphatic vasculature and hematopoietic cells results in the absence of LVVs, reduced number of platelets and blood-filled lymphatic vasculature.

CONCLUSION

GATA2 promotes blood/lymph separation through platelets. Furthermore, LVVs are the only known sites of interaction between blood and lymphatic vessels. The fact that blood is able to enter the lymphatic vessels of mice lacking LVVs and platelets indicates that under these circumstances the lymphatic and blood vessels are connected at yet to be identified sites.

Buttons and Zippers: Endothelial Junctions in Lymphatic Vessels

Button-like junctions are discontinuous contacts at the border of oak-leaf-shaped endothelial cells of initial lymphatic vessels. These junctions are distinctively different from continuous zipper-like junctions that create the endothelial barrier in collecting lymphatics and blood vessels. Button junctions are point contacts, spaced about 3 µm apart, that border valve-like openings where fluid and immune cells enter lymphatics. In intestinal villi, openings between button junctions in lacteals also serve as entry routes for chylomicrons. Like zipper junctions that join endothelial cells, buttons consist of adherens junction proteins (VE-cadherin) and tight junction proteins (claudin-5, occludin, and others). Buttons in lymphatics form from zipper junctions during embryonic development, can convert into zippers in disease or after experimental genetic or pharmacological manipulation, and can revert back to buttons with treatment. Multiple signaling pathways and local microenvironmental factors have been found to contribute to button junction plasticity and could serve as therapeutic targets in pathological conditions ranging from pulmonary edema to obesity.

Molecular Mechanisms Driving Lymphedema and Other Lymphatic Anomalies

Lymphatic vasculature regulates fluid homeostasis by absorbing interstitial fluid and returning it to blood. Lymphatic vasculature is also critical for lipid absorption and inflammatory response. Lymphatic vasculature is composed of lymphatic capillaries, collecting lymphatic vessels, lymphatic valves, and lymphovenous valves. Defects in any of these structures could lead to lymphatic anomalies such as lymphedema, cystic lymphatic malformation, and Gorham-Stout disease. Basic research has led to a deeper understanding of the stepwise development of the lymphatic vasculature. VEGF-C and shear stress signaling pathways have evolved as critical regulators of lymphatic vascular development. Loss-of-function and gain-of-function mutations in genes that are involved in these signaling pathways are associated with lymphatic anomalies. Importantly, drugs that target these molecules are showing outstanding efficacy in treating certain lymphatic anomalies. In this article, we summarize these exciting developments and highlight the future challenges.

Myeloid cell tropism enables MHC-E-restricted CD8+ T cell priming and vaccine efficacy by the RhCMV/SIV vaccine

The strain 68-1 rhesus cytomegalovirus (RhCMV)-based vaccine for simian immunodeficiency virus (SIV) can stringently protect rhesus macaques (RMs) from SIV challenge by arresting viral replication early in primary infection. This vaccine elicits unconventional SIV-specific CD8+ T cells that recognize epitopes presented by major histocompatibility complex (MHC)-II and MHC-E instead of MHC-Ia. Although RhCMV/SIV vaccines based on strains that only elicit MHC-II- and/or MHC-Ia-restricted CD8+ T cells do not protect against SIV, it remains unclear whether MHC-E-restricted T cells are directly responsible for protection and whether these responses can be separated from the MHC-II-restricted component. Using host microRNA (miR)-mediated vector tropism restriction, we show that the priming of MHC-II and MHC-E epitope-targeted responses depended on vector infection of different nonoverlapping cell types in RMs. Selective inhibition of RhCMV infection in myeloid cells with miR-142-mediated tropism restriction eliminated MHC-E epitope-targeted CD8+ T cell priming, yielding an exclusively MHC-II epitope-targeted response. Inhibition with the endothelial cell-selective miR-126 eliminated MHC-II epitope-targeted CD8+ T cell priming, yielding an exclusively MHC-E epitope-targeted response. Dual miR-142 + miR-126-mediated tropism restriction reverted CD8+ T cell responses back to conventional MHC-Ia epitope targeting. Although the magnitude and differentiation state of these CD8+ T cell responses were generally similar, only the vectors programmed to elicit MHC-E-restricted CD8+ T cell responses provided protection against SIV challenge, directly demonstrating the essential role of these responses in RhCMV/SIV vaccine efficacy.

Transcription Factor Control of Lymphatic Quiescence and Maturation of Lymphatic Neovessels in Development and Physiology

The lymphatic system is a vascular system comprising modified lymphatic endothelial cells, lymph nodes and other lymphoid organs. The system has diverse, but critical functions in both physiology and pathology, and forms an interface between the blood vascular and immune system. It is increasingly evident that remodelling of the lymphatic system occurs alongside remodelling of the blood microvascular system, which is now considered a hallmark of most pathological conditions as well as being critical for normal development. Much attention has focussed on how the blood endothelium undergoes phenotypic switching in development and disease, resulting in over two decades of research to probe the mechanisms underlying the resulting heterogeneity. The lymphatic system has received less attention, and consequently there are fewer descriptions of functional and molecular heterogeneity, but differential transcription factor activity is likely an important control mechanism. Here we introduce and discuss significant transcription factors of relevance to coordinating cellular responses during lymphatic remodelling as the lymphatic endothelium dynamically changes from quiescence to actively remodelling.

Back and forth: History of and new insights on the vertebrate lymphatic valve

Lymphatic valves develop from pre‐existing endothelial cells through a step‐wise process involving complex changes in cell shape and orientation, along with extracellular matrix interactions, to form two intraluminal leaflets. Once formed, valves prevent back‐flow within the lymphatic system to ensure drainage of interstitial fluid back into the circulatory system, thereby serving a critical role in maintaining fluid homeostasis. Despite the extensive anatomical characterization of lymphatic systems across numerous genus and species dating back several hundred years, valves were largely thought to be phylogenetically restricted to mammals. Accordingly, most insights into molecular and genetic mechanisms involved in lymphatic valve development have derived from mouse knockouts, as well as rare diseases in humans. However, we have recently used a combination of imaging and genetic analysis in the zebrafish to demonstrate that valves are a conserved feature of the teleost lymphatic system. Here, we provide a historical overview of comparative lymphatic valve anatomy together with recent efforts to define molecular pathways that contribute to lymphatic valve morphogenesis. Finally, we integrate our findings in zebrafish with previous work and highlight the benefits that this model provides for investigating lymphatic valve development.

Whole-mount Immunohistochemistry to Visualize Mouse Embryonic Dermal Lymphatic Vasculature

Lymphatic vessels are abundant in the skin where they regulate interstitial fluid uptake and immune surveillance. Defects in dermal lymphatic vessels, such as fewer vessels and abnormal lymphatic vessel coverage with mural cells, are frequently associated with lymphedema and other lymphatic disorders. Whole-mount immunohistochemistry allows the visualization of dermal lymphatic vessels and identifies morphogenetic defects. Most dermal lymphatic vessels start growing during embryogenesis from lymph sacs that are located close to the axilla towards the dorsal and ventral midlines. Here, we present an approach that we have developed to permeabilize, immunolabel, clear, and visualize the lymphatic vessels. These simple and inexpensive techniques reproducibly generate images of dermal lymphatic vessels with great clarity.

Development and physiological functions of the lymphatic system: insights from human genetic studies of primary lymphedema

Primary lymphedema is a long-term (chronic) condition characterized by tissue lymph retention and swelling that can affect any part of the body, although it usually develops in the arms or legs. Due to the relevant contribution of the lymphatic system to human physiology, while this review mainly focuses on the clinical and physiological aspects related to the regulation of fluid homeostasis and edema, clinicians need to know that the impact of lymphatic dysfunction with a genetic origin can be wide ranging. Lymphatic dysfunction can affect immune function so leading to infection; it can influence cancer development and spread, and it can determine fat transport so impacting on nutrition and obesity. Genetic studies and the development of imaging techniques for the assessment of lymphatic function have enabled the recognition of primary lymphedema as a heterogenic condition in terms of genetic causes and disease mechanisms. In this review, the known biological functions of several genes crucial to the development and function of the lymphatic system are used as a basis for understanding normal lymphatic biology. The disease conditions originating from mutations in these genes are discussed together with a detailed clinical description of the phenotype and the up-to-date knowledge in terms of disease mechanisms acquired from in vitro and in vivo research models.

Generation and characterization of cardiac valve endothelial-like cells from human pluripotent stem cells

The cardiac valvular endothelial cells (VECs) are an ideal cell source that could be used for making the valve organoids. However, few studies have been focused on the derivation of this important cell type. Here we describe a two-step chemically defined xeno-free method for generating VEC-like cells from human pluripotent stem cells (hPSCs). HPSCs were specified to KDR⁺/ISL1⁺ multipotent cardiac progenitors (CPCs), followed by differentiation into valve endothelial-like cells (VELs) via an intermediate endocardial cushion cell (ECC) type. Mechanistically, administration of TGFb1 and BMP4 may specify VEC fate by activating the NOTCH/WNT signaling pathways and previously unidentified targets such as ATF3 and KLF family of transcription factors. When seeded onto the surface of the de-cellularized porcine aortic valve (DCV) matrix scaffolds, hPSC-derived VELs exhibit superior proliferative and clonogenic potential than the primary VECs and human aortic endothelial cells (HAEC). Our results show that hPSC-derived valvular cells could be efficiently generated from hPSCs, which might be used as seed cells for construction of valve organoids or next generation tissue engineered heart valves.

Cheng et al. provide a detailed characterization of the differentiation of human pluripotent stem cells to valve endothelial cells and their function. Their results show that the valve endothelial-like cells express key markers for valve endothelial cells, exhibiting proliferative and clonogenic potential.

Beyond PROX1: transcriptional, epigenetic, and noncoding RNA regulation of lymphatic identity and function

The lymphatic vascular system acts as the major transportation highway of tissue fluids, and its activation or impairment is associated with a wide range of diseases. There has been increasing interest in understanding the mechanisms that control lymphatic vessel formation (lymphangiogenesis) and function in development and disease. Here, we discuss recent insights into new players whose identification has contributed to deciphering the lymphatic regulatory code. We reveal how lymphatic endothelial cells, the building blocks of lymphatic vessels, utilize their transcriptional, post-transcriptional, and epigenetic portfolio to commit to and maintain their vascular lineage identity and function, with a particular focus on development.

Treatment-induced arteriolar revascularization and miR-126 enhancement in bone marrow niche protect leukemic stem cells in AML

BACKGROUND

During acute myeloid leukemia (AML) growth, the bone marrow (BM) niche acquires significant vascular changes that can be offset by therapeutic blast cytoreduction. The molecular mechanisms of this vascular plasticity remain to be fully elucidated. Herein, we report on the changes that occur in the vascular compartment of the FLT3-ITD+ AML BM niche pre and post treatment and their impact on leukemic stem cells (LSCs).

METHODS

BM vasculature was evaluated in FLT3-ITD+ AML models (MllPTD/WT/Flt3ITD/ITD mouse and patient-derived xenograft) by 3D confocal imaging of long bones, calvarium vascular permeability assays, and flow cytometry analysis. Cytokine levels were measured by Luminex assay and miR-126 levels evaluated by Q-RT-PCR and miRNA staining. Wild-type (wt) and MllPTD/WT/Flt3ITD/ITD mice with endothelial cell (EC) miR-126 knockout or overexpression served as controls. The impact of treatment-induced BM vascular changes on LSC activity was evaluated by secondary transplantation of BM cells after administration of tyrosine kinase inhibitors (TKIs) to MllPTD/WT/Flt3ITD/ITD mice with/without either EC miR-126 KO or co-treatment with tumor necrosis factor alpha (TNFα) or anti-miR-126 miRisten.

RESULTS

In the normal BM niche, CD31+Sca-1high ECs lining arterioles have miR-126 levels higher than CD31+Sca-1low ECs lining sinusoids. We noted that during FLT3-ITD+ AML growth, the BM niche lost arterioles and gained sinusoids. These changes were mediated by TNFα, a cytokine produced by AML blasts, which induced EC miR-126 downregulation and caused depletion of CD31+Sca-1high ECs and gain in CD31+Sca-1low ECs. Loss of miR-126high ECs led to a decreased EC miR-126 supply to LSCs, which then entered the cell cycle and promoted leukemia growth. Accordingly, antileukemic treatment with TKI decreased the BM blast-produced TNFα and increased miR-126high ECs and the EC miR-126 supply to LSCs. High miR-126 levels safeguarded LSCs, as shown by more severe disease in secondary transplanted mice. Conversely, EC miR-126 deprivation via genetic or pharmacological EC miR-126 knock-down prevented treatment-induced BM miR-126high EC expansion and in turn LSC protection.

CONCLUSIONS

Treatment-induced CD31+Sca-1high EC re-vascularization of the leukemic BM niche may represent a LSC extrinsic mechanism of treatment resistance that can be overcome with therapeutic EC miR-126 deprivation.

Biochemical and mechanical signals in the lymphatic vasculature

Lymphatic vasculature is an integral part of the cardiovascular system where it maintains interstitial fluid balance. Additionally, lymphatic vasculature regulates lipid assimilation and inflammatory response. Lymphatic vasculature is composed of lymphatic capillaries, collecting lymphatic vessels and valves that function in synergy to absorb and transport fluid against gravitational and pressure gradients. Defects in lymphatic vessels or valves leads to fluid accumulation in tissues (lymphedema), chylous ascites, chylothorax, metabolic disorders and inflammation. The past three decades of research has identified numerous molecules that are necessary for the stepwise development of lymphatic vasculature. However, approaches to treat lymphatic disorders are still limited to massages and compression bandages. Hence, better understanding of the mechanisms that regulate lymphatic vascular development and function is urgently needed to develop efficient therapies. Recent research has linked mechanical signals such as shear stress and matrix stiffness with biochemical pathways that regulate lymphatic vessel growth, patterning and maturation and valve formation. The goal of this review article is to highlight these innovative developments and speculate on unanswered questions.

A novel role of Hippo-Yap/TAZ signaling pathway in lymphatic vascular development

The lymphatic vasculature plays important role in regulating fluid homeostasis, intestinal lipid absorption, and immune surveillance in humans. Malfunction of lymphatic vasculature leads to several human diseases. Understanding the fundamental mechanism in lymphatic vascular development not only expand our knowledge, but also provide a new therapeutic insight. Recently, Hippo-YAP/TAZ signaling pathway, a key mechanism of organ size and tissue homeostasis, has emerged as a critical player that regulate lymphatic specification, sprouting, and maturation. In this review, we discuss the mechanistic regulation and pathophysiological significant of Hippo pathway in lymphatic vascular development.

GATA2 participates in the recanalization of lymphatic vessels after surgical lymph node extirpation

Lymphatic recanalization failure after lymphadenectomy constitutes a major risk of lymphedema in cancer surgery. It has been reported that GATA2, a zinc finger transcription factor, is expressed in lymphatic endothelial cells and is involved in the development of fetal lymphatic vessels. GATA3, another member of the GATA family of transcription factors, is required for the differentiation of lymphoid tissue inducer (LTi) cells and is essential for lymph node formation. However, how GATA2 and GATA3 function in recanalization after the surgical extirpation of lymphatic vessels has not been elucidated. Employing a new model of lymphatic recanalization, we examined the lymphatic reconnection process in Gata2 heterozygous deficient (Gata2^+/- ) and Gata3 heterozygous deficient (Gata3^+/- ) mice. We found that lymphatic recanalization was significantly impaired in Gata2^+/- mice, while Gata3^+/- mice rarely showed such abnormalities. Notably, the perturbed lymphatic recanalization in the Gata2^+/- mice was partially restored by crossing with the Gata3^+/- mice. Our results demonstrate for the first time that GATA2 participates in the regeneration of damaged lymphatic vessels and the unexpected suppressive activity of GATA3 against lymphatic recanalization processes.

Mechanosensation and Mechanotransduction by Lymphatic Endothelial Cells Act as Important Regulators of Lymphatic Development and Function

Our understanding of the function and development of the lymphatic system is expanding rapidly due to the identification of specific molecular markers and the availability of novel genetic approaches. In connection, it has been demonstrated that mechanical forces contribute to the endothelial cell fate commitment and play a critical role in influencing lymphatic endothelial cell shape and alignment by promoting sprouting, development, maturation of the lymphatic network, and coordinating lymphatic valve morphogenesis and the stabilization of lymphatic valves. However, the mechanosignaling and mechanotransduction pathways involved in these processes are poorly understood. Here, we provide an overview of the impact of mechanical forces on lymphatics and summarize the current understanding of the molecular mechanisms involved in the mechanosensation and mechanotransduction by lymphatic endothelial cells. We also discuss how these mechanosensitive pathways affect endothelial cell fate and regulate lymphatic development and function. A better understanding of these mechanisms may provide a deeper insight into the pathophysiology of various diseases associated with impaired lymphatic function, such as lymphedema and may eventually lead to the discovery of novel therapeutic targets for these conditions.

Development and aging of the lymphatic vascular system

The lymphatic vasculature has a pivotal role in regulating body fluid homeostasis, immune surveillance and dietary fat absorption. The increasing number of in vitro and in vivo studies in the last decades has shed light on the processes of lymphatic vascular development and function. Here, we will discuss the current progress in lymphatic vascular biology such as the mechanisms of lymphangiogenesis, lymphatic vascular maturation and maintenance and the emerging mechanisms of lymphatic vascular aging.

YAP and TAZ maintain PROX1 expression in the developing lymphatic and lymphovenous valves in response to VEGF-C signaling

Lymphatic vasculature is an integral part of digestive, immune and circulatory systems. The homeobox transcription factor PROX1 is necessary for the development of lymphatic vessels, lymphatic valves (LVs) and lymphovenous valves (LVVs). We and others previously reported a feedback loop between PROX1 and vascular endothelial growth factor-C (VEGF-C) signaling. PROX1 promotes the expression of the VEGF-C receptor VEGFR3 in lymphatic endothelial cells (LECs). In turn, VEGF-C signaling maintains PROX1 expression in LECs. However, the mechanisms of PROX1/VEGF-C feedback loop remain poorly understood. Whether VEGF-C signaling is necessary for LV and LVV development is also unknown. Here, we report for the first time that VEGF-C signaling is necessary for valve morphogenesis. We have also discovered that the transcriptional co-activators YAP and TAZ are required to maintain PROX1 expression in LVs and LVVs in response to VEGF-C signaling. Deletion of Yap and Taz in the lymphatic vasculature of mouse embryos did not affect the formation of LVs or LVVs, but resulted in the degeneration of these structures. Our results have identified VEGF-C, YAP and TAZ as a crucial molecular pathway in valve development.

Network patterning, morphogenesis and growth in lymphatic vascular development

The lymphatic vasculature is a vital component of the vertebrate vascular system that mediates tissue fluid homeostasis, lipid uptake and immune surveillance. The development of the lymphatic vasculature starts in the early vertebrate embryo, when a subset of blood vascular endothelial cells of the cardinal veins acquires lymphatic endothelial cell fate. These cells sprout from the veins, migrate, proliferate and organize to give rise to a highly structured and unique vascular network. Cellular cross-talk, cell-cell communication and the interpretation of signals from surrounding tissues are all essential for coordinating these processes. In this chapter, we highlight new findings and review research progress with a particular focus on LEC migration and guidance, expansion of the LEC lineage, network remodeling and morphogenesis of the lymphatic vasculature.

The Roles of Non-Coding RNAs in Tumor-Associated Lymphangiogenesis

Simple Summary

The lymphatic system plays key roles in the bodies’ defence against disease, including cancer. The expansion of this system is termed lymphangiogenesis and it is orchestrated by factors and conditions within the microenvironment. One approach to prevent cancer progression is by interfering with these microenvironment factors that promote this process and that facilitate the spread of cancer cells to distant organs. One of these factors are non-coding RNAs. This review will summarize recent findings of the distinct roles played by non-coding RNAs in the lymphatic system within normal tissues and tumours. Understanding the mechanisms involved in this process can provide new avenues for therapeutic intervention for inhibiting the spread of cancer.

Abstract

Lymphatic vessels are regarded as the ”forgotten” circulation. Despite this, growing evidence has shown significant roles for the lymphatic circulation in normal and pathological conditions in humans, including cancers. The dissemination of tumor cells to other organs is often mediated by lymphatic vessels that serve as a conduit and is often referred to as tumor-associated lymphangiogenesis. Some of the most well-studied lymphangiogenic factors that govern tumor lymphangiogenesis are the vascular endothelial growth factor (VEGF-C/D and VEGFR-2/3), neuroplilin-2 (NRP2), fibroblast growth factor (FGF), and hepatocyte growth factor (HGF), to name a few. However, recent findings have illustrated that non-coding RNAs are significantly involved in regulating gene expression in most biological processes, including lymphangiogenesis. In this review, we focus on the regulation of growth factors and non-coding RNAs (ncRNAs) in the lymphatic development in normal and cancer physiology. Then, we discuss the lymphangiogenic factors that necessitate tumor-associated lymphangiogenesis, with regards to ncRNAs in various types of cancer. Understanding the different roles of ncRNAs in regulating lymphatic vasculature in normal and cancer conditions may pave the way towards the development of ncRNA-based anti-lymphangiogenic therapy.

EphrinB2-EphB4 signalling provides Rho-mediated homeostatic control of lymphatic endothelial cell junction integrity

Endothelial integrity is vital for homeostasis and adjusted to tissue demands. Although fluid uptake by lymphatic capillaries is a critical attribute of the lymphatic vasculature, the barrier function of collecting lymphatic vessels is also important by ensuring efficient fluid drainage as well as lymph node delivery of antigens and immune cells. Here, we identified the transmembrane ligand EphrinB2 and its receptor EphB4 as critical homeostatic regulators of collecting lymphatic vessel integrity. Conditional gene deletion in mice revealed that EphrinB2/EphB4 signalling is dispensable for blood endothelial barrier function, but required for stabilization of lymphatic endothelial cell (LEC) junctions in different organs of juvenile and adult mice. Studies in primary human LECs further showed that basal EphrinB2/EphB4 signalling controls junctional localisation of the tight junction protein CLDN5 and junction stability via Rac1/Rho-mediated regulation of cytoskeletal contractility. EphrinB2/EphB4 signalling therefore provides a potential therapeutic target to selectively modulate lymphatic vessel permeability and function.

Lymph vessels are thin walled tubes that, similar to blood vessels, carry white blood cells, fluids and waste. Unlike veins and arteries, however, lymph vessels do not carry red blood cells and their main function is to remove excess fluid from tissues. The cells that line vessels in the body are called endothelial cells, and they are tightly linked together by proteins to control what goes into and comes out of the vessels. The chemical, physical and mechanical signals that control the junctions between endothelial cells are often the same in different vessel types, but their effects can vary.

The endothelial cells of both blood and lymph vessels have two interacting proteins on their membrane known as EphrinB2 and its receptor, EphB4. When these two proteins interact, the EphB4 receptor becomes activated, which leads to changes in the junctions that link endothelial cells together. Frye et al. examined the role of EphrinB2 and EphB4 in the lymphatic system of mice. When either EphrinB2 or EphB4 are genetically removed in newborn or adult mice, lymph vessels become disrupted, but no significant effect is observed on blood vessels. The reason for the different responses in blood and lymph vessels is unknown.

The results further showed that lymphatic endothelial cells need EphB4 and EphrinB2 to be constantly interacting to maintain the integrity of the lymph vessels. Further examination of human endothelial cells grown in the laboratory revealed that this constant signalling controls the internal protein scaffold that determines a cell’s shape and integrity. Changes in the internal scaffold affect the organization of the junctions that link neighboring lymphatic endothelial cells together.

The loss of signalling between EphrinB2 and EphB4 in lymph vessels reflects the increase in vessel leakage seen in response to bacterial infections and in some genetic conditions such as lymphoedema. Finding ways to control the signalling between these two proteins could help treat these conditions by developing drugs that improve endothelial cell integrity in lymph vessels.

Light sheet microscopy-based 3-dimensional histopathology of the lymphatic vasculature in Emberger syndrome

Introduction Lymphovascular diseases represent a heterogenous group of inherited and sporadic disorders and refer to a range of possible underlying pathologies and pathogenesis.

Emberger Syndrome, an inherited form of lymphedema, is characterized by bilateral lower limb lymphedema, however, upper limbs do not show any signs of swelling.

To identify disease-associated histopathological alterations in patients with Emberger Syndrome and to elucidate potential histological differences between the lymphatic vasculature of upper and lower limbs, a detailed knowledge on the 3-dimensional tissue and vessel architecture is essential. However, the current gold standard in 2-dimensional histology provides only very limited spatial information.

Material and methods To elucidate the underlying vascular pathology in Emberger Syndrome on the cellular level, we applied the 3-dimensional visualization and analysis approach VIPAR (volume information-based histopathological analysis by 3D reconstruction and data extraction) to entire wholemount immunofluorescence-stained human tissue samples. VIPAR is a light sheet microscopy-based imaging technique, which allows 3-dimensional reconstruction of entire tissue biopsies followed by automated and semi-automated analysis of vascular parameters in 3-dimensional space.

Results Using VIPAR we could show that in Emberger Syndrome the dermal lymphatic vasculature is intact and non-disrupted.

However, lower limbs showed an hypoplastic lymphatic vasculature with absence of lymphatic valves in pre-collecting and collecting vessels. In contrast to the lower limbs, the lymphatic vasculature of the upper limbs showed no morphological alterations of lymphatic vessels and lymphatic valves compared to healthy controls.

Discussion Based on the 3-dimensional histopathological analysis we were able to perform a detailed phenotyping of lymphatic vessels in the upper and lower limb in Emberger Syndrome and to identify the underlying vascular pathology. In addition, we could show vascular alteration between the upper and lower limbs indicating a vascular heterogeneity of dermal lymph vessels causing the lower limb lymphedema.

S1PR1 regulates the quiescence of lymphatic vessels by inhibiting laminar shear stress-dependent VEGF-C signaling

During the growth of lymphatic vessels (lymphangiogenesis), lymphatic endothelial cells (LECs) at the growing front sprout by forming filopodia. Those tip cells are not exposed to circulating lymph, as they are not lumenized. In contrast, LECs that trail the growing front are exposed to shear stress, become quiescent, and remodel into stable vessels. The mechanisms that coordinate the opposed activities of lymphatic sprouting and maturation remain poorly understood. Here, we show that the canonical tip cell marker Delta-like 4 (DLL4) promotes sprouting lymphangiogenesis by enhancing VEGF-C/VEGF receptor 3 (VEGFR3) signaling. However, in lumenized lymphatic vessels, laminar shear stress (LSS) inhibits the expression of DLL4, as well as additional tip cell markers. Paradoxically, LSS also upregulates VEGF-C/VEGFR3 signaling in LECs, but sphingosine 1-phosphate receptor 1 (S1PR1) activity antagonizes LSS-mediated VEGF-C signaling to promote lymphatic vascular quiescence. Correspondingly, S1pr1 loss in LECs induced lymphatic vascular hypersprouting and hyperbranching, which could be rescued by reducing Vegfr3 gene dosage in vivo. In addition, S1PR1 regulates lymphatic vessel maturation by inhibiting RhoA activity to promote membrane localization of the tight junction molecule claudin-5. Our findings suggest a potentially new paradigm in which LSS induces quiescence and promotes the survival of LECs by downregulating DLL4 and enhancing VEGF-C signaling, respectively. S1PR1 dampens LSS/VEGF-C signaling, thereby preventing sprouting from quiescent lymphatic vessels. These results also highlight the distinct roles that S1PR1 and DLL4 play in LECs when compared with their known roles in the blood vasculature.

Lymphatic Endothelial Cell Junctions: Molecular Regulation in Physiology and Diseases

Lymphatic endothelial cells (LECs) lining lymphatic vessels develop specialized cell-cell junctions that are crucial for the maintenance of vessel integrity and proper lymphatic vascular functions. Successful lymphatic drainage requires a division of labor between lymphatic capillaries that take up lymph via open "button-like" junctions, and collectors that transport lymph to veins, which have tight "zipper-like" junctions that prevent lymph leakage. In recent years, progress has been made in the understanding of these specialized junctions, as a result of the application of state-of-the-art imaging tools and novel transgenic animal models. In this review, we discuss lymphatic development and mechanisms governing junction remodeling between button and zipper-like states in LECs. Understanding lymphatic junction remodeling is important in order to unravel lymphatic drainage regulation in obesity and inflammatory diseases and may pave the way towards future novel therapeutic interventions.

An integrated hypothesis for miR-126 in vascular disease

microRNA miR-126 was among the early discovered miRNAs that are expressed specifically in the vasculature and have critical functions in vascular development. Recent studies have started to unveil potentially important function of miR-126 in vascular diseases, including atherosclerosis, coronary artery disease, stroke and diabetic vasculopathy. The action of miR-126 reflects its function in angiogenesis and inflammation. The expression of miR-126 is downregulated in a variety of vascular diseases, and miR-126 overexpression appears to beneficial for most vascular disease models. In the minireview, we summarize the historic and current research regarding miR-126 function and mechanisms in the vascular system, its link to long noncoding RNAs (lncRNA), as well as the potential of miR-126-based therapeutics for vascular diseases. To explain the seemingly conflicting function of miR-126 from different studies, an integrated hypothesis is proposed that miR-126 has strand- and cell type-specific functions in angiogenesis and inflammation, making it beneficial in many different vascular disease models.

GATA2 controls lymphatic endothelial cell junctional integrity and lymphovenous valve morphogenesis through miR-126

Mutations in the transcription factor GATA2 cause lymphedema. GATA2 is necessary for the development of lymphatic valves and lymphovenous valves, and for the patterning of lymphatic vessels. Here, we report that GATA2 is not necessary for valvular endothelial cell (VEC) differentiation. Instead, GATA2 is required for VEC maintenance and morphogenesis. GATA2 is also necessary for the expression of the cell junction molecules VE-cadherin and claudin 5 in lymphatic vessels. We identified miR-126 as a target of GATA2, and miR-126-/- embryos recapitulate the phenotypes of mice lacking GATA2. Primary human lymphatic endothelial cells (HLECs) lacking GATA2 (HLECΔGATA2) have altered expression of claudin 5 and VE-cadherin, and blocking miR-126 activity in HLECs phenocopies these changes in expression. Importantly, overexpression of miR-126 in HLECΔGATA2 significantly rescues the cell junction defects. Thus, our work defines a new mechanism of GATA2 activity and uncovers miR-126 as a novel regulator of mammalian lymphatic vascular development.