Vascular endothelial growth factor receptor-2 promotes the development of the lymphatic vasculature

Lymphatic endothelial cell-specific NRAS p.Q61R mutant embryos show abnormal lymphatic vessel morphogenesis

Generalized lymphatic anomaly (GLA) and kaposiform lymphangiomatosis (KLA) are rare congenital disorders that arise through anomalous embryogenesis of the lymphatic system. A somatic activating NRAS p.Q61R variant has been recently detected in GLA and KLA tissues, suggesting that the NRAS p.Q61R variant plays an important role in the development of these diseases. To address this role, we studied the effect of the NRAS p.Q61R variant in lymphatic endothelial cells (LECs) on the structure of the lymphatics during embryonic and postnatal lymphangiogenesis applying inducible, LEC-specific NRAS p.Q61R variant in mice. Lox-stop-Lox NrasQ61R mice were crossed with Prox1-CreERT2 mice expressing tamoxifen-inducible Cre recombinase specifically in LECs. Whole-mount immunostaining of embryonic back skin using an antibody against the LEC surface marker VEGFR3 showed considerably greater lymphatic vessel width in LEC-specific NRAS p.Q61R mutant embryos than in littermate controls. These mutant embryos also showed a significant reduction in the number of lymphatic vessel branches. Furthermore, immunofluorescence staining of whole-mount embryonic back skin using an antibody against the LEC-specific nuclear marker Prox1 showed a large increase in the number of LECs in LEC-specific NRAS p.Q61R mutants. In contrast, postnatal induction of the NRAS p.Q61R variant in LECs did not cause abnormal lymphatic vessel morphogenesis. These results suggest that the NRAS p.Q61R variant in LECs plays a role in development of lymphatic anomalies. While this model does not directly reflect the human pathology of GLA and KLA, there are overlapping features, suggesting that further study of this model may help in studying GLA and KLA mechanisms.

Single-Cell RNA Sequencing Identifies Response of Renal Lymphatic Endothelial Cells to Acute Kidney Injury

SIGNIFICANCE STATEMENT

The renal lymphatic vasculature and the lymphatic endothelial cells that make up this network play important immunomodulatory roles during inflammation. How lymphatics respond to AKI may affect AKI outcomes. The authors used single-cell RNA sequencing to characterize mouse renal lymphatic endothelial cells in quiescent and cisplatin-injured kidneys. Lymphatic endothelial cell gene expression changes were confirmed in ischemia-reperfusion injury and in cultured lymphatic endothelial cells, validating renal lymphatic endothelial cells single-cell RNA sequencing data. This study is the first to describe renal lymphatic endothelial cell heterogeneity and uncovers molecular pathways demonstrating lymphatic endothelial cells regulate the local immune response to AKI. These findings provide insights into previously unidentified molecular pathways for lymphatic endothelial cells and roles that may serve as potential therapeutic targets in limiting the progression of AKI.

BACKGROUND

The inflammatory response to AKI likely dictates future kidney health. Lymphatic vessels are responsible for maintaining tissue homeostasis through transport and immunomodulatory roles. Owing to the relative sparsity of lymphatic endothelial cells in the kidney, past sequencing efforts have not characterized these cells and their response to AKI.

METHODS

Here, we characterized murine renal lymphatic endothelial cell subpopulations by single-cell RNA sequencing and investigated their changes in cisplatin AKI 72 hours postinjury. Data were processed using the Seurat package. We validated our findings by quantitative PCR in lymphatic endothelial cells isolated from both cisplatin-injured and ischemia-reperfusion injury, by immunofluorescence, and confirmation in in vitro human lymphatic endothelial cells.

RESULTS

We have identified renal lymphatic endothelial cells and their lymphatic vascular roles that have yet to be characterized in previous studies. We report unique gene changes mapped across control and cisplatin-injured conditions. After AKI, renal lymphatic endothelial cells alter genes involved in endothelial cell apoptosis and vasculogenic processes as well as immunoregulatory signaling and metabolism. Differences between injury models were also identified with renal lymphatic endothelial cells further demonstrating changed gene expression between cisplatin and ischemia-reperfusion injury models, indicating the renal lymphatic endothelial cell response is both specific to where they lie in the lymphatic vasculature and the kidney injury type.

CONCLUSIONS

In this study, we uncover lymphatic vessel structural features of captured populations and injury-induced genetic changes. We further determine that lymphatic endothelial cell gene expression is altered between injury models. How lymphatic endothelial cells respond to AKI may therefore be key in regulating future kidney disease progression.

Toward Characterizing Lymphatic Vasculature in the Mammary Gland During Normal Development and Tumor-Associated Remodeling

Lymphatic vasculature has been shown to promote metastatic spread of breast cancer. Lymphatic vasculature, which is made up of larger collecting vessels and smaller capillaries, has specialized cell junctions that facilitate cell intravasation. Normally, these junctions are designed to collect immune cells and other cellular components for immune surveillance by lymph nodes, but they are also utilized by cancer cells to facilitate metastasis. Although lymphatic development overall in the body has been well-characterized, there has been little focus on how the lymphatic network changes in the mammary gland during stages of remodeling such as pregnancy, lactation, and postpartum involution. In this review, we aim to define the currently known lymphangiogenic factors and lymphatic remodeling events during mammary gland morphogenesis. Furthermore, we juxtapose mammary gland pubertal development and postpartum involution to show similarities of pro-lymphangiogenic signaling as well as other molecular signals for epithelial cell survival that are critical in these morphogenic stages. The similar mechanisms include involvement of M2-polarized macrophages that contribute to matrix remodeling and vasculogenesis; signal transducer and activator of transcription (STAT) survival and proliferation signaling; and cyclooxygenase 2 (COX2)/Prostaglandin E2 (PGE2) signaling to promote ductal and lymphatic expansion. Investigation and characterization of lymphangiogenesis in the normal mammary gland can provide insight to targetable mechanisms for lymphangiogenesis and lymphatic spread of tumor cells in breast cancer.

Hyperactive KRAS/MAPK signaling disrupts normal lymphatic vessel architecture and function

Complex lymphatic anomalies (CLAs) are sporadically occurring diseases caused by the maldevelopment of lymphatic vessels. We and others recently reported that somatic activating mutations in KRAS can cause CLAs. However, the mechanisms by which activating KRAS mutations cause CLAs are poorly understood. Here, we show that KRASG12D expression in lymphatic endothelial cells (LECs) during embryonic development impairs the formation of lymphovenous valves and causes the enlargement of lymphatic vessels. We demonstrate that KRASG12D expression in primary human LECs induces cell spindling, proliferation, and migration. It also increases AKT and ERK1/2 phosphorylation and decreases the expression of genes that regulate the maturation of lymphatic vessels. We show that MEK1/2 inhibition with the FDA-approved drug trametinib suppresses KRASG12D-induced morphological changes, proliferation, and migration. Trametinib also decreases ERK1/2 phosphorylation and increases the expression of genes that regulate the maturation of lymphatic vessels. We also show that trametinib and Cre-mediated expression of a dominant-negative form of MEK1 (Map2k1 K97M) suppresses KRASG12D-induced lymphatic vessel hyperplasia in embryos. Last, we demonstrate that conditional knockout of wild-type Kras in LECs does not affect the formation or function of lymphatic vessels. Together, our data indicate that KRAS/MAPK signaling must be tightly regulated during embryonic development for the proper development of lymphatic vessels and further support the testing of MEK1/2 inhibitors for treating CLAs.

Single-cell RNA sequencing identifies response of renal lymphatic endothelial cells to acute kidney injury

The inflammatory response to acute kidney injury (AKI) likely dictates future renal health. Lymphatic vessels are responsible for maintaining tissue homeostasis through transport and immunomodulatory roles. Due to the relative sparsity of lymphatic endothelial cells (LECs) in the kidney, past sequencing efforts have not characterized these cells and their response to AKI. Here we characterized murine renal LEC subpopulations by single-cell RNA sequencing and investigated their changes in cisplatin AKI. We validated our findings by qPCR in LECs isolated from both cisplatin-injured and ischemia reperfusion injury, by immunofluorescence, and confirmation in in vitro human LECs. We have identified renal LECs and their lymphatic vascular roles that have yet to be characterized in previous studies. We report unique gene changes mapped across control and cisplatin injured conditions. Following AKI, renal LECs alter genes involved endothelial cell apoptosis and vasculogenic processes as well as immunoregulatory signaling and metabolism. Differences between injury models are also identified with renal LECs further demonstrating changed gene expression between cisplatin and ischemia reperfusion injury models, indicating the renal LEC response is both specific to where they lie in the lymphatic vasculature and the renal injury type. How LECs respond to AKI may therefore be key in regulating future kidney disease progression.

Enzymatically Crosslinked Collagen as a Versatile Matrix for In Vitro and In Vivo Co-Engineering of Blood and Lymphatic Vasculature

Adequate vascularization is required for the successful translation of many in vitro engineered tissues. This study presents a novel collagen derivative that harbors multiple recognition peptides for orthogonal enzymatic crosslinking based on sortase A (SrtA) and Factor XIII (FXIII). SrtA-mediated crosslinking enables the rapid co-engineering of human blood and lymphatic microcapillaries and mesoscale capillaries in bulk hydrogels. Whereas tuning of gel stiffness determines the extent of neovascularization, the relative number of blood and lymphatic capillaries recapitulates the ratio of blood and lymphatic endothelial cells originally seeded into the hydrogel. Bioengineered capillaries readily form luminal structures and exhibit typical maturation markers both in vitro and in vivo. The secondary crosslinking enzyme Factor XIII is used for in situ tethering of the VEGF mimetic QK peptide to collagen. This approach supports the formation of blood and lymphatic capillaries in the absence of exogenous VEGF. Orthogonal enzymatic crosslinking is further used to bioengineer hydrogels with spatially defined polymer compositions with pro- and anti-angiogenic properties. Finally, macroporous scaffolds based on secondary crosslinking of microgels enable vascularization independent from supporting fibroblasts. Overall, this work demonstrates for the first time the co-engineering of mature micro- and meso-sized blood and lymphatic capillaries using a highly versatile collagen derivative.

Modeling lymphangiogenesis: Pairing in vitro and in vivo metrics

Lymphangiogenesis is the mechanism by which the lymphatic system develops and expands new vessels facilitating fluid drainage and immune cell trafficking. Models to study lymphangiogenesis are necessary for a better understanding of the underlying mechanisms and to identify or test new therapeutic agents that target lymphangiogenesis. Across the lymphatic literature, multiple models have been developed to study lymphangiogenesis in vitro and in vivo. In vitro, lymphangiogenesis can be modeled with varying complexity, from monolayers to hydrogels to explants, with common metrics for characterizing proliferation, migration, and sprouting of lymphatic endothelial cells (LECs) and vessels. In comparison, in vivo models of lymphangiogenesis often use genetically modified zebrafish and mice, with in situ mouse models in the ear, cornea, hind leg, and tail. In vivo metrics, such as activation of LECs, number of new lymphatic vessels, and sprouting, mirror those most used in vitro, with the addition of lymphatic vessel hyperplasia and drainage. The impacts of lymphangiogenesis vary by context of tissue and pathology. Therapeutic targeting of lymphangiogenesis can have paradoxical effects depending on the pathology including lymphedema, cancer, organ transplant, and inflammation. In this review, we describe and compare lymphangiogenic outcomes and metrics between in vitro and in vivo studies, specifically reviewing only those publications in which both testing formats are used. We find that in vitro studies correlate well with in vivo in wound healing and development, but not in the reproductive tract or the complex tumor microenvironment. Considerations for improving in vitro models are to increase complexity with perfusable microfluidic devices, co-cultures with tissue-specific support cells, the inclusion of fluid flow, and pairing in vitro models of differing complexities. We believe that these changes would strengthen the correlation between in vitro and in vivo outcomes, giving more insight into lymphangiogenesis in healthy and pathological states.

Sinusoidal and lymphatic vessel growth is controlled by reciprocal VEGF-C-CDH5 inhibition

Sinusoids are specialized, low pressure blood vessels in the liver, bone marrow, and spleen required for definitive hematopoiesis. Unlike other blood endothelial cells (ECs), sinusoidal ECs express high levels of VEGFR3. VEGFR3 and its ligand VEGF-C are known to support lymphatic growth, but their function in sinusoidal vessels is unknown. In this study, we define a reciprocal VEGF-C/VEGFR3-CDH5 (VE-cadherin) signaling axis that controls growth of both sinusoidal and lymphatic vessels. Loss of VEGF-C or VEGFR3 resulted in cutaneous edema, reduced fetal liver size, and bloodless bone marrow due to impaired lymphatic and sinusoidal vessel growth. Mice with membrane-retained VE-cadherin conferred identical lymphatic and sinusoidal defects, suggesting that VE-cadherin opposes VEGF-C/VEGFR3 signaling. In developing mice, loss of VE-cadherin rescued defects in sinusoidal and lymphatic growth caused by loss of VEGFR3 but not loss of VEGF-C, findings explained by potentiated VEGF-C/VEGFR2 signaling in VEGFR3-deficient lymphatic ECs. Mechanistically, VEGF-C/VEGFR3 signaling induces VE-cadherin endocytosis and loss of function via SRC-mediated phosphorylation, while VE-cadherin prevents VEGFR3 endocytosis required for optimal receptor signaling. These findings establish an essential role for VEGF-C/VEGFR3 signaling during sinusoidal vascular growth, identify VE-cadherin as a powerful negative regulator of VEGF-C signaling that acts through both VEGFR3 and VEGFR2 receptors, and suggest that negative regulation of VE-cadherin is required for effective VEGF-C/VEGFR3 signaling during growth of sinusoidal and lymphatic vessels. Manipulation of this reciprocal negative regulatory mechanism, e.g. by reducing VE-cadherin function, may be used to stimulate therapeutic sinusoidal or lymphatic vessel growth.

The ambivalent nature of the relationship between lymphatics and cancer

Do lymphatic vessels support cancer cells? Or are they vessels that help suppress cancer development? It is known that the lymphatic system is a vehicle for tumor metastasis and that the lymphangiogenic regulator VEGF-C supports the tumor. One such role of VEGF-C is the suppression of the immune response to cancer. The lymphatic system has also been correlated with an increase in interstitial fluid pressure of the tumor microenvironment. On the other hand, lymphatic vessels facilitate immune surveillance to mount an immune response against tumors with the support of VEGF-C. Furthermore, the activation of lymphatic fluid drainage may prove to filter and decrease tumor interstitial fluid pressure. In this review, we provide an overview of the dynamic between lymphatics, cancer, and tumor fluid pressure to suggest that lymphatic vessels may be used as an antitumor therapy due to their capabilities of immune surveillance and fluid pressure drainage. The application of this potential may help to prevent tumor proliferation or increase the efficacy of drugs that target cancer.

Lymphatic vessels in cancer

The lymphatic system, composed of initial and collecting lymphatic vessels as well as lymph nodes that are present in almost every tissue of the human body, acts as an essential transport system for fluids, biomolecules and cells between peripheral tissues and the central circulation. Consequently, it is required for normal body physiology but is also involved in the pathogenesis of various diseases, most notably cancer. The important role of tumor-associated lymphatic vessels and lymphangiogenesis in the formation of lymph node metastasis has been elucidated during the last two decades, whereas the underlying mechanisms and the relation between lymphatic and peripheral organ dissemination of cancer cells are incompletely understood. Lymphatic vessels are also important for tumor-host communication, relaying molecular information from a primary or metastatic tumor to regional lymph nodes and the circulatory system. Beyond antigen transport, lymphatic endothelial cells, particularly those residing in lymph node sinuses, have recently been recognized as direct regulators of tumor immunity and immunotherapy responsiveness, presenting tumor antigens and expressing several immune-modulatory signals including PD-L1. In this review, we summarize recent discoveries in this rapidly evolving field and highlight strategies and challenges of therapeutic targeting of lymphatic vessels or specific lymphatic functions in cancer patients.

Infection-induced lymphatic zippering restricts fluid transport and viral dissemination from skin

Lymphatic vessels are often considered passive conduits that flush antigenic material, pathogens, and cells to draining lymph nodes. Recent evidence, however, suggests that lymphatic vessels actively regulate diverse processes from antigen transport to leukocyte trafficking and dietary lipid absorption. Here we tested the hypothesis that infection-induced changes in lymphatic transport actively contribute to innate host defense. We demonstrate that cutaneous vaccinia virus infection by scarification activates dermal lymphatic capillary junction tightening (zippering) and lymph node lymphangiogenesis, which are associated with reduced fluid transport and cutaneous viral sequestration. Lymphatic-specific deletion of VEGFR2 prevented infection-induced lymphatic capillary zippering, increased fluid flux out of tissue, and allowed lymphatic dissemination of virus. Further, a reduction in dendritic cell migration to lymph nodes in the absence of lymphatic VEGFR2 associated with reduced antiviral CD8+ T cell expansion. These data indicate that VEGFR2-driven lymphatic remodeling is a context-dependent, active mechanism of innate host defense that limits viral dissemination and facilitates protective, antiviral CD8+ T cell responses.

Interplay of vascular endothelial growth factor receptors in organ-specific vessel maintenance

Vascular endothelial growth factors (VEGFs) and their receptors (VEGFRs) are quintessential for the development and maintenance of blood and lymphatic vessels. However, genetic interactions between the VEGFRs are poorly understood. VEGFR2 is the dominant receptor that is required for the growth and survival of the endothelium, whereas deletion of VEGFR1 or VEGFR3 was reported to induce vasculature overgrowth. Here we show that vascular regression induced by VEGFR2 deletion in postnatal and adult mice is aggravated by additional deletion of VEGFR1 or VEGFR3 in the intestine, kidney, and pancreas, but not in the liver or kidney glomeruli. In the adult mice, hepatic and intestinal vessels regressed within a few days after gene deletion, whereas vessels in skin and retina remained stable for at least four weeks. Our results show changes in endothelial transcriptomes and organ-specific vessel maintenance mechanisms that are dependent on VEGFR signaling pathways and reveal previously unknown functions of VEGFR1 and VEGFR3 in endothelial cells.

Pazopanib-induced chylothorax in a patient with renal cell carcinoma

Pazopanib is an oral multi-kinase inhibitor approved for the treatment of advanced renal cell carcinoma (RCC). It is an anti-angiogenic agent, which blocks the activation signaling pathways of tyrosine kinases and prevents the activities of primarily vascular endothelial growth factor receptors (VEGFR)-2 and VEGFR-3, which are important in lymphangiogenesis. Herein, we report a patient with advanced RCC who developed asymptomatic left-sided chylothorax under pazopanib therapy. Chylothorax developed in the 16th month and gradually increased until it was diagnosed by thoracentesis in the 22nd month. The development of chylothorax was attributed to pazopanib therapy after ruling out all possible traumatic and nontraumatic etiologies. The 'Adverse Drug Reaction Probability Scale' revealed a total score of 6, which fell into 'probable' category. Chylothorax regressed significantly 5 weeks after the discontinuation of pazopanib therapy.

The role of lacteal integrity and junction transformation in obesity: A promising therapeutic target?

Lacteals are the central lymphatic vessels in the villi of the small intestine and perform nutrient absorption, especially dietary lipids, and the transportation of antigen and antigen-presenting cells. Remodeling, proliferation, and cell-cell junctions of lymphatic endothelial cells (LECs) in lacteals are the basis of the maintenance of lacteal integrity and dietary lipid absorption. Normal lipid absorption in the diet depends on sound lacteal development and proliferation, especially integrity maintenance, namely, maintaining the appropriate proportion of button-like and zipper-like junctions. Maintaining the integrity and transforming button-to-zipper junctions in lacteals are strongly connected with obesity, which could be regulated by intestinal flora and molecular signalings, such as vascular endothelial growth factor C-vascular endothelial growth receptor 3 (VEGFC-VEGFR3) signaling, Hippo signaling, Notch signaling, angiopoietin-TIE signaling, VEGF-A/VEGFR2 signaling, and PROX1. This manuscript reviews the molecular mechanism of development, integrity maintenance, and junction transformation in lacteal related to obesity.

Isolating and characterizing lymphatic endothelial progenitor cells for potential therapeutic lymphangiogenic applications

Lymphatic dysfunction is associated with the progression of several vascular disorders, though currently, there are limited strategies to promote new lymphatic vasculature (i.e., lymphangiogenesis) to restore lost lymphatic function. One promising approach to stimulate lymphangiogenesis involves delivering endothelial progenitor cells (EPCs), which are naturally involved in de novo blood vessel formation and have recently been identified to include a lymphatic subpopulation. However, the contribution of lymphatic EPCs in lymphangiogenesis is not clear and challenges with maintaining the activity of transplanted EPCs remain. Thus, the objective of this study was to isolate lymphatic EPCs from human umbilical cord blood and characterize their role in the initial stages of blood or lymphatic vasculature formation. Furthermore, this study also tested the applicability of alginate hydrogels to deliver lymphatic EPCs for a possible therapeutic application. We postulated and confirmed that blood and lymphatic EPC colonies could be isolated from human umbilical cord blood. Additionally, EPC populations responded to either angiogenic or lymphangiogenic growth factors and could stimulate their respective mature endothelial cells in vasculature models in vitro. Finally, lymphatic EPCs maintained their ability to promote lymphatic sprouts after prolonged interactions with the alginate hydrogel microenvironment. These results suggest EPCs have both a blood and a lymphatic population that have specific roles in promoting revascularization and highlight the potential of alginate hydrogels for the delivery of lymphatic EPCs. STATEMENT OF SIGNIFICANCE: Despite the potential therapeutic benefit of promoting lymphatic vasculature, lymphangiogenesis remains understudied. One appealing strategy for promoting lymphangiogenesis involves delivering lymphatic endothelial progenitor cells (EPCs), which are a subpopulation of EPCs involved in de novo vessel formation. Here, we investigate the role of isolated blood and lymphatic EPC subpopulations in promoting the early stages of vascularization and the utility of alginate hydrogels to deliver lymphatic EPCs. We determined that EPCs had two populations that expressed either blood or lymphatic markers, could stimulate their respective mature vasculature in tissue constructs and that alginate hydrogels maintained the therapeutic potential of lymphatic EPCs. We anticipate this work could support promising biomaterial applications of EPCs to promote revascularization, which could have many therapeutic applications.

Corneal Lymphangiogenesis: Current Pathophysiological Understandings and Its Functional Role in Ocular Surface Disease

The cornea is a transparent and avascular tissue that plays a central role in light refraction and provides a physical barrier to the external environment. Corneal avascularity is a unique histological feature that distinguishes it from the other parts of the body. Functionally, corneal immune privilege critically relies on corneal avascularity. Corneal lymphangiogenesis is now recognized as a general pathological feature in many pathologies, including dry eye disease (DED), corneal allograft rejection, ocular allergy, bacterial and viral keratitis, and transient corneal edema. Currently, sizable data from clinical and basic research have accumulated on the pathogenesis and functional role of ocular lymphangiogenesis. However, because of the invisibility of lymphatic vessels, ocular lymphangiogenesis has not been studied as much as hemangiogenesis. We reviewed the basic mechanisms of lymphangiogenesis and summarized recent advances in the pathogenesis of ocular lymphangiogenesis, focusing on corneal allograft rejection and DED. In addition, we discuss future directions for lymphangiogenesis research.

The lymphatics in kidney health and disease

The mammalian vascular system consists of two networks: the blood vascular system and the lymphatic vascular system. Throughout the body, the lymphatic system contributes to homeostatic mechanisms by draining extravasated interstitial fluid and facilitating the trafficking and activation of immune cells. In the kidney, lymphatic vessels exist mainly in the kidney cortex. In the medulla, the ascending vasa recta represent a hybrid lymphatic-like vessel that performs lymphatic-like roles in interstitial fluid reabsorption. Although the lymphatic network is mainly derived from the venous system, evidence supports the existence of lymphatic beds that are of non-venous origin. Following their development and maturation, lymphatic vessel density remains relatively stable; however, these vessels undergo dynamic functional changes to meet tissue demands. Additionally, new lymphatic growth, or lymphangiogenesis, can be induced by pathological conditions such as tissue injury, interstitial fluid overload, hyperglycaemia and inflammation. Lymphangiogenesis is also associated with conditions such as polycystic kidney disease, hypertension, ultrafiltration failure and transplant rejection. Although lymphangiogenesis has protective functions in clearing accumulated fluid and immune cells, the kidney lymphatics may also propagate an inflammatory feedback loop, exacerbating inflammation and fibrosis. Greater understanding of lymphatic biology, including the developmental origin and function of the lymphatics and their response to pathogenic stimuli, may aid the development of new therapeutic agents that target the lymphatic system.

Emerging roles of PLCγ1 in endothelial biology

Phospholipase C γ1 (PLCγ1) is a member of the PLC family that functions as signal transducer by hydrolyzing membrane lipid to generate second messengers. The unique protein structure of PLCγ1 confers a critical role as a direct effector of VEGFR2 and signaling mediated by other receptor tyrosine kinases. The distinct vascular phenotypes in PLCγ1-deficient animal models and the gain-of-function mutations of PLCγ1 found in human endothelial cancers point to a major physiological role of PLCγ1 in the endothelial system. In this review, we discuss aspects of physiological and molecular function centering around PLCγ1 in the context of endothelial cells and provide a perspective for future investigation.

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.

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.

Acquired lymphangioma circumscriptum in high-grade penile intraepithelial neoplasia

Lymphangioma circumscriptum (LC) is a rare, benign vascular malformation induced by abnormal lymphatic vessels of the skin. LC might be either congenital or acquired, and is predominantly located on the trunk, buttock, axillary region, or thighs. Penile LC is rare. This case report describes a patient with acquired LC associated with high-grade penile intraepithelial neoplasia induced by human papillomavirus type 66. As the patient had multifocal lesions on the glans penis and prepuce we decided to perform circumcision, followed by electrocoagulation of the lesions on the glans. Electrocautery should be considered as a first choice for treatment of LC located at surgically challenging regions such as the glans penis.

Evolutionary Differences in the Vegf/Vegfr Code Reveal Organotypic Roles for the Endothelial Cell Receptor Kdr in Developmental Lymphangiogenesis

Lymphatic vascular development establishes embryonic and adult tissue fluid balance and is integral in disease. In diverse vertebrate organs, lymphatic vessels display organotypic function and develop in an organ-specific manner. In all settings, developmental lymphangiogenesis is considered driven by vascular endothelial growth factor (VEGF) receptor-3 (VEGFR3), whereas a role for VEGFR2 remains to be fully explored. Here, we define the zebrafish Vegf/Vegfr code in receptor binding studies. We find that while Vegfd directs craniofacial lymphangiogenesis, it binds Kdr (a VEGFR2 homolog) but surprisingly, unlike in mammals, does not bind Flt4 (VEGFR3). Epistatic analyses and characterization of a kdr mutant confirm receptor-binding analyses, demonstrating that Kdr is indispensible for rostral craniofacial lymphangiogenesis, but not caudal trunk lymphangiogenesis, in which Flt4 is central. We further demonstrate an unexpected yet essential role for Kdr in inducing lymphatic endothelial cell fate. This work reveals evolutionary divergence in the Vegf/Vegfr code that uncovers spatially restricted mechanisms of developmental lymphangiogenesis.

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.

Systems Biology Will Direct Vascular-Targeted Therapy for Obesity

Healthy adipose tissue expansion and metabolism during weight gain require coordinated angiogenesis and lymphangiogenesis. These vascular growth processes rely on the vascular endothelial growth factor (VEGF) family of ligands and receptors (VEGFRs). Several studies have shown that controlling vascular growth by regulating VEGF:VEGFR signaling can be beneficial for treating obesity; however, dysregulated angiogenesis and lymphangiogenesis are associated with several chronic tissue inflammation symptoms, including hypoxia, immune cell accumulation, and fibrosis, leading to obesity-related metabolic disorders. An ideal obesity treatment should minimize adipose tissue expansion and the advent of adverse metabolic consequences, which could be achieved by normalizing VEGF:VEGFR signaling. Toward this goal, a systematic investigation of the interdependency of vascular and metabolic systems in obesity and tools to predict personalized treatment ranges are necessary to improve patient outcomes through vascular-targeted therapies. Systems biology can identify the critical VEGF:VEGFR signaling mechanisms that can be targeted to regress adipose tissue expansion and can predict the metabolic consequences of different vascular-targeted approaches. Establishing a predictive, biologically faithful platform requires appropriate computational models and quantitative tissue-specific data. Here, we discuss the involvement of VEGF:VEGFR signaling in angiogenesis, lymphangiogenesis, adipogenesis, and macrophage specification – key mechanisms that regulate adipose tissue expansion and metabolism. We then provide useful computational approaches for simulating these mechanisms, and detail quantitative techniques for acquiring tissue-specific parameters. Systems biology, through computational models and quantitative data, will enable an accurate representation of obese adipose tissue that can be used to direct the development of vascular-targeted therapies for obesity and associated metabolic disorders.

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.

Lymphatics in bone arise from pre-existing lymphatics

Bones do not normally have lymphatics. However, individuals with generalized lymphatic anomaly (GLA) or Gorham-Stout disease (GSD) develop ectopic lymphatics in bone. Despite growing interest in the development of tissue-specific lymphatics, the cellular origin of bone lymphatic endothelial cells (bLECs) is not known and the development of bone lymphatics has not been fully characterized. Here, we describe the development of bone lymphatics in mouse models of GLA and GSD. Through lineage-tracing experiments, we show that bLECs arise from pre-existing Prox1-positive LECs. We show that bone lymphatics develop in a stepwise manner where regional lymphatics grow, breach the periosteum and then invade bone. We also show that the development of bone lymphatics is impaired in mice that lack osteoclasts. Last, we show that rapamycin can suppress the growth of bone lymphatics in our models of GLA and GSD. In summary, we show that bLECs can arise from pre-existing LECs and that rapamycin can prevent the growth of bone lymphatics.

Lymphatic Programing and Specialization in Hybrid Vessels

Building on a large body of existing blood vascular research, advances in lymphatic research have helped kindle broader investigations into vascular diversity and endothelial plasticity. While the endothelium of blood and lymphatic vessels can be distinguished by a variety of molecular markers, the endothelia of uniquely diverse vascular beds can possess distinctly heterogeneous or hybrid expression patterns. These expression patterns can then provide further insight on the development of these vessels and how they perform their specialized function. In this review we examine five highly specialized hybrid vessel beds that adopt partial lymphatic programing for their specialized vascular functions: the high endothelial venules of secondary lymphoid organs, the liver sinusoid, the Schlemm’s canal of the eye, the renal ascending vasa recta, and the remodeled placental spiral artery. We summarize the morphology and endothelial expression pattern of these vessels, compare them to each other, and interrogate their specialized functions within the broader blood and lymphatic vascular systems.

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.

Simultaneous chylous ascites and chylothorax during ramucirumab plus docetaxel chemotherapy in a patient with non-small lung cell cancer

A 69-year-old woman was diagnosed as having non-small cell lung cancer (adenocarcinoma, T1aN3M1b). She had no history of surgery or abdominal trauma. She was treated with ramucirumab (10 mg/kg) plus docetaxel (60 mg/m²) intravenously (RAM + DTX) every 3 weeks. Although an enhanced CT examination showed a partial tumor response after eight courses of RAM + DTX, she gradually began to experience abdominal fullness with severe peripheral pitching edema. Her body weight increased by 18 kg in 2 months and RAM + DTX chemotherapy was discontinued. An enhanced CT examination showed a large amount of ascites and pleural effusion, with no obstructions of the central vein or lymphatic ducts. The ascites were white and milky in appearance and contained 527 mg/dL of triglyceride. In addition, her pleural effusion was also white and milky in appearance. No further increases in ascites and pleural effusion were observed thereafter. Four months after her last RAM + DTX chemotherapy, she continued to exhibit a partial response and no increases in ascites or pleural effusion were present. The chylous effusion might have been caused by the RAM + DTX chemotherapy.

Application of microscale culture technologies for studying lymphatic vessel biology

Immense progress in microscale engineering technologies has significantly expanded the capabilities of in vitro cell culture systems for reconstituting physiological microenvironments that are mediated by biomolecular gradients, fluid transport, and mechanical forces. Here, we examine the innovative approaches based on microfabricated vessels for studying lymphatic biology. To help understand the necessary design requirements for microfluidic models, we first summarize lymphatic vessel structure and function. Next, we provide an overview of the molecular and biomechanical mediators of lymphatic vessel function. Then we discuss the past achievements and new opportunities for microfluidic culture models to a broad range of applications pertaining to lymphatic vessel physiology. We emphasize the unique attributes of microfluidic systems that enable the recapitulation of multiple physicochemical cues in vitro for studying lymphatic pathophysiology. Current challenges and future outlooks of microscale technology for studying lymphatics are also discussed. Collectively, we make the assertion that further progress in the development of microscale models will continue to enrich our mechanistic understanding of lymphatic biology and physiology to help realize the promise of the lymphatic vasculature as a therapeutic target for a broad spectrum of diseases.

Abnormal lymphatic vessel development is associated with decreased decidual regulatory T cells in severe preeclampsia

PROBLEM

The lymphatic vasculature controls leukocytes trafficking and limits the adaptive immune response. In previous models of preeclampsia (PE), defective immune function caused by disruption of lymphangiogenesis was shown to be involved in the disease pathophysiology. Especially, the dysfunction of regulatory T cells (Treg) at the maternal-fetal interface may be one of the causes of severe PE. In particular, activation of Tregs to obtain immune tolerance requires adequate antigen presentation through the lymphatic system. We hypothesized that impaired lymphangiogenesis and imbalanced Tregs at the maternal-fetal interface are associated with the pathophysiology of severe PE. However, the current research addressing this hypothesis is limited. Therefore, to compare differences in lymphangiogenesis in severe PE and normal conditions, we aimed to examine the location of lymphatics at the maternal-fetal interface and to investigate the association between lymphangiogenesis and Tregs in severe PE.

METHOD OF STUDY

We obtained entire uterus from normal pregnant mice. Placental and fetal membranes, including decidua, were obtained from 10 pregnant women with severe PE and 10 gestational age-matched controls. Immunohistochemistry for LYVE1 was used to localize the distribution of lymphatic vessels and CD4, CD25, and FOXP3 for Treg.

RESULTS

LYVE1-positive vessels were present in the uterine wall of mice. LYVE1-positive lymphatic vessels were localized on the human decidua. Tubular lymphatics were abundant in the control decidua, but significantly reduced in severe PE. Furthermore, lymphatic vessel density correlated with the number of decidual Tregs.

CONCLUSION

Abnormal decidual lymphangiogenesis is associated with reduced numbers of decidual Tregs in severe PE.

Increased tumor vascularization is associated with the amount of immune competent PD-1 positive cells in testicular germ cell tumors

Testicular germ cell cancer in a metastatic state is curable with a cisplatin-based first line chemotherapy. However, 10-15% of these patients are resistant to first line chemotherapy and are thus left with only palliative options. Immunotherapies and inhibition of angiogenesis used in multiple types of cancer; however, the molecular context of angiogenesis and immune checkpoints in the development and progression of testicular cancers is still unknown. Therefore, the present study performed tissue micro array based analysis of 84 patients with immunohistochemistry of programmed cell death protein 1 (PD-1), programmed cell death ligand 1 (PD-L1) and vascular endothelial growth factor receptor 2 (VEGFR2) of testicular cancer and corresponding normal appearing testis tissue, matching the results with clinical data. The results demonstrated that PD-L1 was significantly upregulated in testicular tumors and that PD-1 positive cells significantly infiltrated the testicular tumor when compared with normal testicular tissue. VEGFR2 was significantly upregulated in testicular cancer. It was indicated that PD-1 expressing cytotoxic cells may require pathologic tumor vessels to pass the blood-testis-barrier in order to migrate into the tumor. Notably, when matching the clinical data for PD-1, PD-L1 and VEGFR2 there were no differences in expression in the different International Germ Cell Cancer Collaborative Group stages of non-seminoma. These data suggested that the anti-PD-1/PD-L1 immunotherapy and the anti-angiogenic therapy, sequentially or in combination, may be a promising option in the treatment of testicular cancer.

Cytokines regulating lymphangiogenesis

Lymphatic vessels are established by differentiation of lymphendothelial progenitors during embryogenesis. Lymphangiogenesis, the formation of new lymphatic vessels from pre-existing ones is rare in the healthy adult but takes place during pathological conditions such as inflammation, tissue repair and tumor growth. Conditions of dysfunctional lymphatics exist after surgical interventions or in certain genetic diseases. A key lymphangiogenic stimulator is vascular endothelial growth factor-C (VEGFC) acting on VEGF receptor-3 (VEGFR3) expressed on lymphendothelial cells. Other cytokines may act directly to regulate lymphangiogenesis positively or negatively, or indirectly by inducing expression of VEGFC. This review describes different known lymphangiogenic cytokines, their mechanism of action and role in lymphangiogenesis in health and disease.

Successful management of lymphangiosarcoma in a puppy using a tyrosine kinase inhibitor

A puppy was diagnosed with lymphangiosarcoma associated with lymphedema based on lymphography and histopathology. The lesions resolved after toceranib therapy, and the dog remains in remission 1 year later. This is the first report of a successful outcome following oral toceranib as first-line therapy for lymphangiosarcoma in a dog.

Corneal lymphangiogenesis facilitates ocular surface inflammation and cell trafficking in dry eye disease

PURPOSE

While the normal cornea has limited innervation by the lymphatic system, chronic immune-inflammatory disorders such as dry eye (DE) can induce lymphangiogenesis in the ocular surface. Using a conditional knock-down murine model, Lyve-1^Cre;VEGFR2^flox mice, this study investigated the role of lymphangiogenesis in the pathophysiology of DE.

METHODS

DE was induced in both wild type (WT) B6 and Lyve-1^Cre;VEGFR2^flox mice. Tissue immunostaining and volumetric gross measurements were used to assess changes in the ocular surface, skin, and lymph nodes (LNs). The expression of lymphangiogenic factors (TNF-α, IL-6/-8/-12/-17, VEGF-C/-D, IFN-γ, VEGFR-2/-3, Lyve-1, and podoplanin) and the frequency of immune cells (CD4, CD11b, and CD207) on the ocular surface and lacrimal glands were quantified by real-time polymerase chain reaction and flow cytometry.

RESULTS

Compared to WT mice, there were fewer lymphatic vessels and a reduction in lymphangiogenic markers in the ocular surface and skin of Lyve-1^Cre;VEGFR2^flox mice. After DE induction, mRNA levels of TNF-α, IL-8, and IFN-γ were significantly reduced in Lyve-1^Cre;VEGFR2^flox mice compared to WT mice (p < .01). Surprisingly, the LNs from Lyve-1^Cre;VEGFR2^flox mice with DE were significantly smaller and populated by fewer dendritic cells and effector T cells than those from WT mice (p < .001). Furthermore, immunostaining showed corneal nerves in the DE-induced Lyve-1^Cre;VEGFR2^flox mice were notably intact like in the naïve condition.

CONCLUSIONS

Inhibition of lymphangiogenesis in the cornea effectively attenuates not only the inflammatory response including trafficking of immune cells but also preserves corneal nerves under desiccating stress. Corneal lymphangiogenesis might be a contributing factor in deterioration on the ocular surface homeostasis.

Transcription Factors Regulating Embryonic Development of Pulmonary Vasculature

Lung morphogenesis is a highly orchestrated process beginning with the appearance of lung buds on approximately embryonic day 9.5 in the mouse. Endodermally derived epithelial cells of the primitive lung buds undergo branching morphogenesis to generate the tree-like network of epithelial-lined tubules. The pulmonary vasculature develops in close proximity to epithelial progenitor cells in a process that is regulated by interactions between the developing epithelium and underlying mesenchyme. Studies in transgenic and knockout mouse models demonstrate that normal lung morphogenesis requires coordinated interactions between cells lining the tubules, which end in peripheral saccules, juxtaposed to an extensive network of capillaries. Multiple growth factors, microRNAs, transcription factors, and their associated signaling cascades regulate cellular proliferation, migration, survival, and differentiation during formation of the peripheral lung. Dysregulation of signaling events caused by gene mutations, teratogens, or premature birth causes severe congenital and acquired lung diseases in which normal alveolar architecture and the pulmonary capillary network are disrupted. Herein, we review scientific progress regarding signaling and transcriptional mechanisms regulating the development of pulmonary vasculature during lung morphogenesis.

The Ets2 Repressor Factor (Erf) Is Required for Effective Primitive and Definitive Hematopoiesis

Erf is a gene for a ubiquitously expressed Ets DNA-binding domain-containing transcriptional repressor. Erf haploinsufficiency causes craniosynostosis in humans and mice, while its absence in mice leads to failed chorioallantoic fusion and death at embryonic day 10.5 (E10.5). In this study, we show that Erf is required in all three waves of embryonic hematopoiesis. Mice lacking Erf in the embryo proper exhibited severe anemia and died around embryonic day 14.5. Erf epiblast-specific knockout embryos had reduced numbers of circulating blood cells from E9.5 onwards, with the development of severe anemia by E14.5. Elimination of Erf resulted in both reduced and more immature primitive erythroblasts at E9.5 to E10.5. Reduced definitive erythroid colony-forming activity was found in the bloodstream of E10.5 embryos and in the fetal liver at E11.5 to E13.5. Finally, elimination of Erf resulted in impaired repopulation ability, indicating that Erf is necessary for hematopoietic stem cell maintenance or differentiation. We conclude that Erf is required for both primitive and erythromyeloid progenitor waves of hematopoietic stem cell (HSC)-independent hematopoiesis as well as for the normal function of HSCs.

Carbohydrate-binding protein CLEC14A regulates VEGFR-2- and VEGFR-3-dependent signals during angiogenesis and lymphangiogenesis

Controlled angiogenesis and lymphangiogenesis are essential for tissue development, function, and repair. However, aberrant neovascularization is an essential pathogenic mechanism in many human diseases, including diseases involving tumor growth and survival. Here, we have demonstrated that mice deficient in C-type lectin family 14 member A (CLEC14A) display enhanced angiogenic sprouting and hemorrhage as well as enlarged jugular lymph sacs and lymphatic vessels. CLEC14A formed a complex with VEGFR-3 in endothelial cells (ECs), and CLEC14A KO resulted in a marked reduction in VEGFR-3 that was concomitant with increases in VEGFR-2 expression and downstream signaling. Implanted tumor growth was profoundly reduced in CLEC14A-KO mice compared with that seen in WT littermates, but tumor-bearing CLEC14A-KO mice died sooner. Tumors in CLEC14A-KO mice had increased numbers of nonfunctional blood vessels and severe hemorrhaging. Blockade of VEGFR-2 signaling suppressed these vascular abnormalities and enhanced the survival of tumor-bearing CLEC14A-KO mice. We conclude that CLEC14A acts in vascular homeostasis by fine-tuning VEGFR-2 and VEGFR-3 signaling in ECs, suggesting its relevance in the pathogenesis of angiogenesis-related human disorders.

Defective lymphatic valve development and chylothorax in mice with a lymphatic-specific deletion of Connexin43

Lymphatic valves (LVs) are cusped luminal structures that permit the movement of lymph in only one direction and are therefore critical for proper lymphatic vessel function. Congenital valve aplasia or agenesis can, in some cases, be a direct cause of lymphatic disease. Knowledge about the molecular mechanisms operating during the development and maintenance of LVs may thus aid in the establishment of novel therapeutic approaches to treat lymphatic disorders. In this study, we examined the role of Connexin43 (Cx43), a gap junction protein expressed in lymphatic endothelial cells (LECs), during valve development. Mouse embryos with a null mutation in Cx43 (Gja1) were previously shown to completely lack mesenteric LVs at embryonic day 18. However, interpreting the phenotype of Cx43^-/- mice was complicated by the fact that global deletion of Cx43 causes perinatal death due to heart defects during embryogenesis. We have now generated a mouse model (Cx43^∆LEC) with a lymphatic-specific ablation of Cx43 and show that the absence of Cx43 in LECs causes a delay (rather than a complete block) in LV initiation, an increase in immature valves with incomplete leaflet elongation, a reduction in the total number of valves, and altered lymphatic capillary patterning. The physiological consequences of these lymphatic changes were leaky valves, insufficient lymph transport and reflux, and a high incidence of lethal chylothorax. These results demonstrate that the expression of Cx43 is specifically required in LECs for normal development of LVs.

Thymocytes in Lyve1-CRE/S1pr1f/f Mice Accumulate in the Thymus due to Cell-Intrinsic Loss of Sphingosine-1-Phosphate Receptor Expression

T cell emigration from the thymus is essential for immunological homeostasis. While stromal cell-produced sphingosine-1-phosphate (S1P) has been shown to promote thymocyte egress via the S1P receptor, S1PR1, the significance of S1P/S1PR1 signaling in the thymic stromal cells that surround T cells remains unclear. To address this issue, we developed conditional knockout mice (Lyve1-CRE/S1pr1^f/f mice) in which S1pr1 was selectively targeted in cells expressing the lymphatic endothelial cell marker, Lyve1. In these mice, T cells were significantly reduced in secondary lymphoid tissues, and CD62L⁺ mature CD4 and CD8 single-positive (SP) T cells accumulated in the medulla failed to undergo thymus egress. Using a Lyve1 reporter strain in which Lyve1 lineage cells expressed tdTomato fluorescent protein, we unexpectedly found that a considerable proportion of the thymocytes were fluorescently labeled, indicating that they belonged to the Lyve1 lineage. The CD4 and CD8 SP thymocytes in Lyve1-CRE/S1pr1^f/f mice exhibited an egress-competent phenotype (HSA^low, CD62L^high, and Qa-2^high), but were CD69^high and lacked S1PR1 expression. In addition, CD4 SP thymocytes from these mice were unable to migrate to the periphery after their intrathymic injection into wild-type (WT) mice. In contrast, WT T cells could migrate to the periphery in both WT and Lyve1-CRE/S1pr1^f/f thymuses. These results demonstrated that thymocyte egress is mediated by T cell-expressed, but not stromal cell-expressed, S1PR1 and caution against using the Lyve1-CRE system for selectively gene deletion in lymphatic endothelial cells.

Tbx1: Transcriptional and Developmental Functions

Recent data have paved the way to mechanistic studies into the role of Tbx1 during development. Tbx1 is haploinsufficient and is involved in an important genetic disorder. The gene encodes a T-box transcription factor that is expressed from approximately E7.5 in mouse embryos and continues to be expressed in a highly dynamic manner. It is neither a strong transcriptional activator nor a strong repressor, but it regulates a large number of genes through epigenetic modifications. Here, we review recent literature concerning mechanisms of gene regulation by Tbx1 and its role in mammalian development, with a special focus on the cardiac, vascular, and central nervous systems.

Regulation of lymphangiogenesis in the diaphragm by macrophages and VEGFR-3 signaling

Lymphatic vessels play important roles in fluid drainage and in immune responses, as well as in pathological processes including cancer progression and inflammation. While the molecular regulation of the earliest lymphatic vessel differentiation and development has been investigated in much detail, less is known about the control and timing of lymphatic vessel maturation in different organs, which often occurs postnatally. We investigated the time course of lymphatic vessel development on the pleural side of the diaphragmatic muscle in mice, the so-called submesothelial initial diaphragmatic lymphatic plexus. We found that this lymphatic network develops largely after birth and that it can serve as a reliable and easily quantifiable model to study physiological lymphangiogenesis in vivo. Lymphangiogenic growth in this tissue was highly dependent on vascular endothelial growth factor receptor (VEGFR)-3 signaling, whereas VEGFR-1 and -2 signaling was dispensable. During diaphragm development, macrophages appeared first in a linearly arranged pattern, followed by ingrowth of lymphatic vessels along these patterned lines. Surprisingly, ablation of macrophages in colony-stimulating factor-1 receptor (Csf1r)-deficient mice and by treatment with a CSF-1R-blocking antibody did not inhibit the general lymphatic vessel development in the diaphragm but specifically promoted branch formation of lymphatic sprouts. In agreement with these findings, incubation of cultured lymphatic endothelial cells with conditioned medium from P7 diaphragmatic macrophages significantly reduced LEC sprouting. These results indicate that the postnatal diaphragm provides a suitable model for studies of physiological lymphangiogenic growth and maturation, and for the identification of modulators of lymphatic vessel growth.

Pias1 is essential for erythroid and vascular development in the mouse embryo

The protein inhibitor of activated STAT-1 (PIAS1) is one of the few known SUMO E3 ligases. PIAS1 has been implicated in several biological processes including repression of innate immunity and DNA repair. However, PIAS1 function during development and tissue differentiation has not been studied. Here, we report that Pias1 is required for proper embryonic development. Approximately 90% of Pias1 null embryos die in utero between E10.5 and E12.5. We found significant apoptosis within the yolk sac (YS) blood vessels and concomitant loss of red blood cells (RBCs) resulting in profound anemia. In addition, Pias1 loss impairs YS angiogenesis and results in defective capillary plexus formation and blood vessel occlusions. Moreover, heart development is impaired as a result of loss of myocardium muscle mass. Accordingly, we found that Pias1 expression in primary myoblasts enhances the induction of cardiac muscle genes MyoD, Myogenin and Myomaker. PIAS1 protein regulation of cardiac gene transcription is dependent on transcription factors Myocardin and Gata-4. Finally, endothelial cell specific inactivation of Pias1 in vivo impairs YS erythrogenesis, angiogenesis and recapitulates loss of myocardium muscle mass. However, these defects are not sufficient to recapitulate the lethal phenotype of Pias1 null embryos. These findings highlight Pias1 as an essential gene for YS erythropoiesis and vasculogenesis in vivo.

CHD4-regulated plasmin activation impacts lymphovenous hemostasis and hepatic vascular integrity

The chromatin-remodeling enzyme CHD4 maintains vascular integrity at mid-gestation; however, it is unknown whether this enzyme contributes to later blood vessel or lymphatic vessel development. Here, we addressed this issue in mice harboring a deletion of Chd4 specifically in cells that express lymphatic vessel endothelial hyaluronan receptor 1 (LYVE1), which include lymphatic endothelial cells (LECs) and liver sinusoidal endothelial cells. Chd4 mutant embryos died before birth and exhibited severe edema, blood-filled lymphatics, and liver hemorrhage. CHD4-deficient embryos developed normal lymphovenous (LV) valves, which regulate the return of lymph to the blood circulation; however, these valves lacked the fibrin-rich thrombi that prevent blood from entering the lymphatic system. Transcripts of the urokinase plasminogen activator receptor (uPAR), which facilitates activation of the fibrin-degrading protease plasmin, were upregulated in Chd4 mutant LYVE1+ cells, and plasmin activity was elevated near the LV valves. Genetic reduction of the uPAR ligand urokinase prevented degradation of fibrin-rich thrombi at the LV valves and largely resolved the blood-filled lymphatics in Chd4 mutants. Urokinase reduction also ameliorated liver hemorrhage and prolonged embryonic survival by reducing plasmin-mediated extracellular matrix degradation around sinusoidal blood vessels. These results highlight the susceptibility of LV thrombi and liver sinusoidal vessels to plasmin-mediated damage and demonstrate the importance of CHD4 in regulating embryonic plasmin activation after mid-gestation.

Dermal lymphatic dilation in a mouse model of alopecia areata

Mouse models of various types of inflammatory skin disease are often accompanied by increased dermal angiogenesis. The C3H/HeJ inbred strain spontaneously develops alopecia areata (AA), a cell mediated autoimmune disorder that can be controllably expanded using full thickness skin grafts to young unaffected mice. This provides a reproducible and progressive model for AA in which the vascularization of the skin can be examined. Mice receiving skin grafts from AA or normal mice were evaluated at 5, 10, 15, and 20 weeks after engraftment. Lymphatics are often overlooked as they are small slit-like structures above the hair follicle that resemble artifact-like separation of collagen bundles with some fixatives. Lymphatics are easily detected using lymphatic vessel endothelial hyaluronan receptor 1 (LYVE1) by immunohistochemistry to label their endothelial cells. Using LYVE1, there were no changes in distribution or numbers of lymphatics although they were more prominent (dilated) in the mice with AA. Lyve1 transcripts were not significantly upregulated except at 10 weeks after skin grafting when clinical signs of AA first become apparent. Other genes involved with vascular growth and dilation or movement of immune cells were dysregulated, mostly upregulated. These findings emphasize aspects of AA not commonly considered and provide potential targets for therapeutic intervention.

Primary Tumor-Secreted Lymphangiogenic Factors Induce Pre-Metastatic Lymphvascular Niche Formation at Sentinel Lymph Nodes in Oral Squamous Cell Carcinoma

OBJECTIVES

The objectives of this study were to evaluate the formation of lymphvascular niches in lymph nodes of patients with oral squamous cell carcinoma (OSCC), and investigate the roles of lymphangiogenic and angiogenic factors, such as vascular endothelial growth factor (VEGF)-A, VEGF-C, and VEGF-D, expressed in the primary tumors.

MATERIALS AND METHODS

Forty-four patients with previously untreated clinically late T2 or T3 OSCC of cN0 were evaluated for primary tumors and 166 sentinel lymph nodes (SLNs). Primary tumors were immunohistochemically analyzed for expressions of VEGFs. Densities of lymphatic vessels (LVDpodoplanin) and high endothelial venules (HEVD) in the SLNs were also calculated using antibodies for each marker, podoplanin and MECA-79, respectively.

RESULTS

In 25 patients, all lymph nodes were metastasis-negative, whereas, in 19 patients, metastasis was positive for at least one lymph node (either at SLN, non-SLN, or nodal recurrence). From the analyses of 140 SLNs without metastasis, LVDpodoplanin in 50 SLNs of metastasis-positive cases was significantly higher than that in 90 SLNs of metastasis-negative cases (p = 0.0025). HEVD was not associated with lymph node metastasis. The patients with VEGF-A-High or VEGF-D-High tumors had significantly higher LVDpodoplanin than patients with their Low counterparts (p = 0.0233 and p = 0.0209, respectively). In cases with lymph node metastasis, the VEGF-D-expression score was significantly higher than in those without lymph node metastasis (p = 0.0006).

CONCLUSIONS

These results suggest that lymph node lymphangiogenesis occurs before metastasis in OSCC. VEGF-A and VEGF-D play critical roles in this process. VEGF-D is a potential predictive marker of positive lymph node metastasis in cN0 patients.

Lymphangiogenesis and Inflammation-Looking for the "Missing Pieces" of the Puzzle

Several papers about lymphangiogenesis and inflammation focused on the detailed and complicated descriptions of the molecular pathways accompanying both non-tumor and tumor inflammatory-induced lymphatic vessel development. Many authors are tempted to present inflammatory-induced lymphangiogenesis in pathologic conditions neglecting the role of inflammatory cells during embryonic lymphatic vessel development. Some of the inflammatory cells are largely characterized in inflammatory-induced lymphangiogenesis, while others as mast cells, eosinophils, or plasma cells are less studied. No phenotypic characterization of inflammation-activated lymphatic endothelial cell is available in this moment. Another paradox is related to the existence of few papers regarding lymphangiogenesis inside lymphoid organs and for their related pathology. There are still several "missing pieces of such a big puzzle" of lymphangiogenesis and inflammation, with a direct impact on the ineffectiveness of the anti-inflammatory therapy as lymphangiogenesis inhibitors. The present paper will focus on the controversial issues of lymphangiogenesis and inflammation.

VEGF-trap aflibercept significantly improves long-term graft survival in high-risk corneal transplantation

BACKGROUND

Graft failure because of immune rejection remains a significant problem in organ transplantation, and lymphatic and blood vessels are important components of the afferent and efferent arms of the host alloimmune response, respectively. We compare the effect of antihemangiogenic and antilymphangiogenic therapies on alloimmunity and graft survival in a murine model of high-risk corneal transplantation.

METHODS

Orthotopic corneal transplantation was performed in hemevascularized and lymph-vascularized high-risk host beds, and graft recipients received subconjunctival vascular endothelial growth factor (VEGF)-trap, anti-VEGF-C, sVEGFR-3, or no treatment, beginning at the time of surgery. Fourteen days after transplantation, graft hemeangiogenesis and lymphangiogenesis were evaluated by immunohistochemistry. The frequencies of Th1 cells in regional lymphoid tissue and graft-infiltrating immune cells were evaluated by flow cytometry. Long-term allograft survival was compared using Kaplan-Meier curves.

RESULTS

VEGF-trap significantly decreased graft hemangiogenesis as compared to the control group and was most effective in reducing the frequency of graft-infiltrating immune cells. Anti-VEGF-C and sVEGFR3 significantly decreased graft lymphangiogenesis and lymphoid Th1 cell frequencies as compared to control. VEGF-trap (72%), anti-VEGF-C (25%), and sVEGFR-3 (11%) all significantly improved in the 8-week graft survival compared to control (0%), although VEGF-trap was significantly more effective than both anti-VEGF-C (P < 0.05) and sVEGFR-3 (P < 0.05).

CONCLUSION

In a clinically relevant model of high-risk corneal transplantation in which blood and lymphatic vessels are present and treatment begins at the time of transplantation, VEGF-trap is significantly more effective in improving long-term graft survival as compared to anti-VEGF-C and sVEGFR-3, but all approaches improve survival when compared to untreated control.

Molecular controls of lymphatic VEGFR3 signaling

OBJECTIVES

Vascular endothelial growth factor receptor 3 (VEGFR3) plays important roles both in lymphangiogenesis and angiogenesis. On stimulation by its ligand VEGF-C, VEGFR3 is able to form both homodimers as well as heterodimers with VEGFR2 and activates several downstream signal pathways, including extracellular signal-regulated kinases (ERK)1/2 and protein kinase B (AKT). Despite certain similarities with VEGFR2, molecular features of VEGFR3 signaling are still largely unknown.

APPROACH AND RESULTS

Human dermal lymphatic endothelial cells were used to examine VEGF-C-driven activation of signaling. Compared with VEGF-A activation of VEGFR2, VEGF-C-induced VEGFR3 activation led to a more extensive AKT activation, whereas activation of ERK1/2 displayed a distinctly different kinetics. Furthermore, VEGF-C, but not VEGF-A, induced formation of VEGFR3/VEGFR2 complexes. Silencing VEGFR2 or its partner neuropilin 1 specifically abolished VEGF-C-induced AKT but not ERK activation, whereas silencing of neuropilin 2 had little effect on either signaling pathway. Finally, suppression of vascular endothelial phosphotyrosine phosphatase but not other phosphotyrosine phosphatases enhanced VEGF-C-induced activation of both ERK and AKT pathways. Functionally, both ERK and AKT pathways are important for lymphatic endothelial cells migration.

CONCLUSIONS

VEGF-C activates AKT signaling via formation of VEGFR3/VEGFR2 complex, whereas ERK is activated by VEGFR3 homodimer. Neuropilin 1 and vascular endothelial phosphotyrosine phosphatase are involved in regulation of VEGFR3 signaling.