TGFβ signaling is required for sprouting lymphangiogenesis during lymphatic network development in the skin

Zmiz1 is a novel regulator of lymphatic endothelial cell gene expression and function

Zinc Finger MIZ-Type Containing 1 (Zmiz1), also known as ZIMP10 or RAI17, is a transcription cofactor and member of the Protein Inhibitor of Activated STAT (PIAS) family of proteins. Zmiz1 is critical for a variety of biological processes including vascular development. However, its role in the lymphatic vasculature is unknown. In this study, we utilized human dermal lymphatic endothelial cells (HDLECs) and an inducible, lymphatic endothelial cell (LEC)-specific Zmiz1 knockout mouse model to investigate the role of Zmiz1 in LECs. Transcriptional profiling of ZMIZ1-deficient HDLECs revealed downregulation of genes crucial for lymphatic vessel development. Additionally, our findings demonstrated that loss of Zmiz1 results in reduced expression of proliferation and migration genes in HDLECs and reduced proliferation and migration in vitro. We also presented evidence that Zmiz1 regulates Prox1 expression in vitro and in vivo by modulating chromatin accessibility at Prox1 regulatory regions. Furthermore, we observed that loss of Zmiz1 in mesenteric lymphatic vessels significantly reduced valve density. Collectively, our results highlight a novel role of Zmiz1 in LECs and as a transcriptional regulator of Prox1, shedding light on a previously unknown regulatory factor in lymphatic vascular biology.

Microfluidic-Based Reconstitution of Functional Lymphatic Microvasculature: Elucidating the Role of Lymphatics in Health and Disease

The knowledge of the blood microvasculature and its functional role in health and disease has grown significantly attributable to decades of research and numerous advances in cell biology and tissue engineering; however, the lymphatics (the secondary vascular system) has not garnered similar attention, in part due to a lack of relevant in vitro models that mimic its pathophysiological functions. Here, a microfluidic-based approach is adopted to achieve precise control over the biological transport of growth factors and interstitial flow that drive the in vivo growth of lymphatic capillaries (lymphangiogenesis). The engineered on-chip lymphatics with in vivo-like morphology exhibit tissue-scale functionality with drainage rates of interstitial proteins and molecules comparable to in vivo standards. Computational and scaling analyses of the underlying transport phenomena elucidate the critical role of the three-dimensional geometry and lymphatic endothelium in recapitulating physiological drainage. Finally, the engineered on-chip lymphatics enabled studies of lymphatic-immune interactions that revealed inflammation-driven responses by the lymphatics to recruit immune cells via chemotactic signals similar to in vivo, pathological events. This on-chip lymphatics platform permits the interrogation of various lymphatic biological functions, as well as screening of lymphatic-based therapies such as interstitial absorption of protein therapeutics and lymphatic immunomodulation for cancer therapy.

Lymphatic vasculature in ovarian cancer

Ovarian cancer (OVCA) is the second most common gynecological cancer and one of the leading causes of cancer related mortality among women. Recent studies suggest that among ovarian cancer patients at least 70% of the cases experience the involvement of lymph nodes and metastases through lymphatic vascular network. However, the impact of lymphatic system in the growth, spread and the evolution of ovarian cancer, its contribution towards the landscape of ovarian tissue resident immune cells and their metabolic responses is still a major knowledge gap. In this review first we present the epidemiological aspect of the OVCA, the lymphatic architecture of the ovary, we discuss the role of lymphatic circulation in regulation of ovarian tumor microenvironment, metabolic basis of the upregulation of lymphangiogenesis which is often observed during progression of ovarian metastasis and ascites development. Further we describe the implication of several mediators which influence both lymphatic vasculature as well as ovarian tumor microenvironment and conclude with several therapeutic strategies for targeting lymphatic vasculature in ovarian cancer progression in present day.

The Impact of Stem/Progenitor Cells on Lymphangiogenesis in Vascular Disease

Lymphatic vessels, as the main tube network of fluid drainage and leukocyte transfer, are responsible for the maintenance of homeostasis and pathological repairment. Recently, by using genetic lineage tracing and single-cell RNA sequencing techniques, significant cognitive progress has been made about the impact of stem/progenitor cells during lymphangiogenesis. In the embryonic stage, the lymphatic network is primarily formed through self-proliferation and polarized-sprouting from the lymph sacs. However, the assembly of lymphatic stem/progenitor cells also guarantees the sustained growth of lymphvasculogenesis to obtain the entire function. In addition, there are abundant sources of stem/progenitor cells in postnatal tissues, including circulating progenitors, mesenchymal stem cells, and adipose tissue stem cells, which can directly differentiate into lymphatic endothelial cells and participate in lymphangiogenesis. Specifically, recent reports indicated a novel function of lymphangiogenesis in transplant arteriosclerosis and atherosclerosis. In the present review, we summarized the latest evidence about the diversity and incorporation of stem/progenitor cells in lymphatic vasculature during both the embryonic and postnatal stages, with emphasis on the impact of lymphangiogenesis in the development of vascular diseases to provide a rational guidance for future research.

TGF-β signaling in lymphatic vascular vessel formation and maintenance

Transforming growth factor (TGF)-β and its family members, including bone morphogenetic proteins (BMPs), nodal proteins, and activins, are implicated in the development and maintenance of various organs. Here, we review its role in the lymphatic vascular system (the secondary vascular system in vertebrates), which plays a crucial role in various physiological and pathological processes, participating in the maintenance of the normal tissue fluid balance, immune cell trafficking, and fatty acid absorption in the gut. The lymphatic system is associated with pathogenesis in multiple diseases, including lymphedema, inflammatory diseases, and tumor metastasis. Lymphatic vessels are composed of lymphatic endothelial cells, which differentiate from blood vascular endothelial cells (BECs). Although TGF-β family signaling is essential for maintaining blood vessel function, little is known about the role of TGF-β in lymphatic homeostasis. Recently, we reported that endothelial-specific depletion of TGF-β signaling affects lymphatic function. These reports suggest that TGF-β signaling in lymphatic endothelial cells maintains the structure of lymphatic vessels and lymphatic homeostasis, and promotes tumor lymphatic metastasis. Suppression of TGF-β signaling in lymphatic endothelial cells may therefore be effective in inhibiting cancer metastasis. We highlight recent advances in understanding the roles of TGF-β signaling in the formation and maintenance of the lymphatic system.

Genomic organization, intragenic tandem duplication, and expression analysis of chicken TGFBR2 gene

Transforming growth factor beta receptor Ⅱ (TGFBR2), a core member of the transforming growth factor-β (TGF-β) signaling pathway. To date, chicken TGFBR2 (cTGFBR2) genomic structure has not been fully explored. Here, the complete sequences of cTGFBR2 transcript isoforms were determined by 5′ and 3′ rapid amplification of cDNA ends (5′ & 3′ RACE) and reverse transcription polymerase chain reaction (RT-PCR); the tissue expression profiling of cTGFBR2 transcript isoforms was performed using quantitative real-time polymerase chain reaction (qRT-PCR). The results showed that cTGFBR2 gene produced 3 transcript isoforms though alternative transcription initiation, splicing, and polyadenylation, which were designated as cTGFBR2-1, cTGFBR2-2, and cTGFBR2-3, respectively. These 3 cTGFBR2 transcript isoforms encoded 3 protein isoforms: cTGFBR2-1, cTGFBR2-2, and cTGFBR2-3. Duplication analysis revealed that, unlike other animal species, cTGFBR2 gene harbored a 5.5-kb intragenic tandem duplication. Tissue expression profiling in the 4-wk-old Arbor Acres (AA) broiler chickens showed that cTGFBR2-1 was ubiquitously expressed, with high expression in abdominal fat, subcutaneous fat, lung, gizzard, and muscle; cTGFBR2-2 was highly expressed in heart, kidney, gizzard, and muscle; cTGFBR2-3 was weakly expressed in all the tested chicken tissues. Tissue expression profiling in the 7-wk-old broiler chickens of the fat and lean lines of Northeast Agricultural University broiler lines divergently selected for abdominal fat content (NEAUHLF) showed that cTGFBR2-1 was significantly differentially expressed in all the tested tissues except heart, cTGFBR2-2 was significantly differentially expressed in all the tested tissues except subcutaneous fat and liver, and cTGFBR2-3 was significantly differentially expressed in all the tested tissues between the lean and fat lines. Intriguingly, in the fat line, the 3 cTGFBR2 transcript isoforms were expressed to varying degrees in all the 3 tested fat tissues, while in the lean line, only cTGFBR2-1 was expressed in all the 3 tested fat tissues. This is the first report of intragenic tandem duplication within TGFBR2 gene. Our findings pave the way for further studies on the functions and regulation of cTGFBR2 gene.

Fetal nuchal edema and developmental anomalies caused by gene mutations in mice

Fetal nuchal edema, a subcutaneous accumulation of extracellular fluid in the fetal neck, is detected as increased nuchal translucency (NT) by ultrasonography in the first trimester of pregnancy. It has been demonstrated that increased NT is associated with chromosomal anomalies and genetic syndromes accompanied with fetal malformations such as defective lymphatic vascular development, cardiac anomalies, anemia, and a wide range of other fetal anomalies. However, in many clinical cases of increased NT, causative genes, pathogenesis and prognosis have not been elucidated in humans. On the other hand, a large number of gene mutations have been reported to induce fetal nuchal edema in mouse models. Here, we review the relationship between the gene mutants causing fetal nuchal edema with defective lymphatic vascular development, cardiac anomalies, anemia and blood vascular endothelial barrier anomalies in mice. Moreover, we discuss how studies using gene mutant mouse models will be useful in developing diagnostic method and predicting prognosis.

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.

Endothelial Microparticle-Mediated Transfer of microRNA-19b Inhibits the Function and Distribution of Lymphatic Vessels in Atherosclerotic Mice

In recent years, the function of the lymphatic system in atherosclerosis has attracted attention due to its role in immune cell trafficking, cholesterol removal from the periphery, and regulation of the inflammatory response. However, knowledge of the mechanisms regulating lymphangiogenesis and lymphatic function in the pathogenesis of atherosclerosis is limited. Endothelial microparticles carrying circulating microRNA (miRNA)s are known to mediate cell–cell communication, and our previous research showed that miRNA-19b in EMPs (EMP^miR-19b) was significantly increased in circulation and atherosclerotic vessels, and this increase in EMP^miR-19b promoted atherosclerosis. The present study investigated whether atherogenic EMP^miR-19b influences pathological changes of the lymphatic system in atherosclerosis. We first verified increased miR-19b levels and loss of lymphatic system function in atherosclerotic mice. Atherogenic western diet-fed ApoE^-/- mice were injected with phosphate-buffered saline, EMPs carrying control miRNA (EMP^control), or EMP^miR-19b intravenously. The function and distribution of the lymphatic system was assessed via confocal microscopy, Evans blue staining, and pathological analysis. The results showed that lymphatic system dysfunction existed in the early stage of atherosclerosis, and the observed pathological changes persisted at the later stage, companied by an increased microRNA-19b level. In ApoE^-/- mice systemically treated with EMP^miR-19b, the distribution, transport function, and permeability of the lymphatic system were significantly inhibited. In vitro experiments showed that miRNA-19b may damage the lymphatic system by inhibiting lymphatic endothelial cell migration and tube formation, and a possible mechanism is the inhibition of transforming growth factor beta receptor type II (TGF-βRII) expression in lymphatic endothelial cells by miRNA-19b. Together, our findings demonstrate that atherogenic EMP^miR-19b may destroy lymphatic system function in atherosclerotic mice by downregulating TGF-βRII expression.

Endothelial-specific depletion of TGF-β signaling affects lymphatic function

BACKGROUND

Transforming growth factor (TGF)-β is a multifunctional cytokine involved in cell differentiation, cell proliferation, and tissue homeostasis. Although TGF-β signaling is essential for maintaining blood vessel functions, little is known about the role of TGF-β in lymphatic homeostasis.

METHODS

To delineate the role of TGF-β signaling in lymphatic vessels, TβRII^fl/fl mice were crossed with Prox1-Cre^ERT2 mice to generate TβRII^fl/fl; Prox1-Cre^ERT2 mice. The TβRII gene in the lymphatic endothelial cells (LECs) of the conditional knockout TβRII^iΔLEC mice was selectively deleted using tamoxifen. The effects of TβRII gene deletion on embryonic lymphangiogenesis, postnatal lymphatic structure and drainage function, tumor lymphangiogenesis, and lymphatic tumor metastasis were investigated.

RESULTS

Deficiency of LEC-specific TGF-β signaling in embryos, where lymphangiogenesis is active, caused dorsal edema with dilated lymphatic vessels at E13.5. Postnatal mice in which lymphatic vessels had already been formed displayed dilation and increased bifurcator of lymphatic vessels after tamoxifen administration. Similar dilation was also observed in tumor lymphatic vessels. The drainage of FITC-dextran, which was subcutaneously injected into the soles of the feet of the mice, was reduced in TβRII^iΔLEC mice. Furthermore, Lewis lung carcinoma cells constitutively expressing GFP (LLC-GFP) transplanted into the footpads of the mice showed reduced patellar lymph node metastasis.

CONCLUSION

These data suggest that TGF-β signaling in LECs maintains the structure of lymphatic vessels and lymphatic homeostasis, in addition to promoting tumor lymphatic metastasis. Therefore, suppression of TGF-β signaling in LECs might be effective in inhibiting cancer metastasis.

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.

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.

Unique functions for Notch4 in murine embryonic lymphangiogenesis

In mice, embryonic dermal lymphatic development is well understood and used to study gene functions in lymphangiogenesis. Notch signaling is an evolutionarily conserved pathway that modulates cell fate decisions, which has been shown to both inhibit and promote dermal lymphangiogenesis. Here, we demonstrate distinct roles for Notch4 signaling versus canonical Notch signaling in embryonic dermal lymphangiogenesis. Actively growing embryonic dermal lymphatics expressed NOTCH1, NOTCH4, and DLL4 which correlated with Notch activity. In lymphatic endothelial cells (LECs), DLL4 activation of Notch induced a subset of Notch effectors and lymphatic genes, which were distinctly regulated by Notch1 and Notch4 activation. Treatment of LECs with VEGF-A or VEGF-C upregulated Dll4 transcripts and differentially and temporally regulated the expression of Notch1 and Hes/Hey genes. Mice nullizygous for Notch4 had an increase in the closure of the lymphangiogenic fronts which correlated with reduced vessel caliber in the maturing lymphatic plexus at E14.5 and reduced branching at E16.5. Activation of Notch4 suppressed LEC migration in a wounding assay significantly more than Notch1, suggesting a dominant role for Notch4 in regulating LEC migration. Unlike Notch4 nulls, inhibition of canonical Notch signaling by expressing a dominant negative form of MAML1 (DNMAML) in Prox1+ LECs led to increased lymphatic density consistent with an increase in LEC proliferation, described for the loss of LEC Notch1. Moreover, loss of Notch4 did not affect LEC canonical Notch signaling. Thus, we propose that Notch4 signaling and canonical Notch signaling have distinct functions in the coordination of embryonic dermal lymphangiogenesis.

Analysis of Angiogenesis in Mouse Embryonic Dorsal Skin by Whole-Mount Fluorescent Staining

The blood vascular system is a tree-like hierarchical branching structure and needs to function even before fully established. Abnormal formation of blood vessels results in embryonic lethality and also contributes to the pathogenesis of a number of human diseases, including cancer metastasis. To understand the molecular events associated with blood vessel formation, we established a fluorescence staining-based protocol on mouse embryonic skin. We harvested mouse embryonic skin and performed whole-mount staining. The reconstructed three-dimensional vascular structure provided detailed information on angiogenesis.

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.

Pkd1 and Wnt5a genetically interact to control lymphatic vascular morphogenesis in mice

BACKGROUND

Lymphatic vascular development is regulated by well-characterized signaling and transcriptional pathways. These pathways regulate lymphatic endothelial cell (LEC) migration, motility, polarity, and morphogenesis. Canonical and non-canonical WNT signaling pathways are known to control LEC polarity and development of lymphatic vessels and valves. PKD1, encoding Polycystin-1, is the most commonly mutated gene in polycystic kidney disease but has also been shown to be essential in lymphatic vascular morphogenesis. The mechanism by which Pkd1 acts during lymphangiogenesis remains unclear.

RESULTS

Here we find that loss of non-canonical WNT signaling components Wnt5a and Ryk phenocopy lymphatic defects seen in Pkd1 knockout mice. To investigate genetic interaction, we generated Pkd1;Wnt5a double knockout mice. Loss of Wnt5a suppressed phenotypes seen in the lymphatic vasculature of Pkd1^-/- mice and Pkd1 deletion suppressed phenotypes observed in Wnt5a^-/- mice. Thus, we report mutually suppressive roles for Pkd1 and Wnt5a, with developing lymphatic networks restored to a more wild type state in double mutant mice. This genetic interaction between Pkd1 and the non-canonical WNT signaling pathway ultimately controls LEC polarity and the morphogenesis of developing vessel networks.

CONCLUSION

Our work suggests that Pkd1 acts at least in part by regulating non-canonical WNT signaling during the formation of lymphatic vascular networks.

The BMP Pathway in Blood Vessel and Lymphatic Vessel Biology

Bone morphogenetic proteins (BMPs) were originally identified as the active components in bone extracts that can induce ectopic bone formation. In recent decades, their key role has broadly expanded beyond bone physiology and pathology. Nowadays, the BMP pathway is considered an important player in vascular signaling. Indeed, mutations in genes encoding different components of the BMP pathway cause various severe vascular diseases. Their signaling contributes to the morphological, functional and molecular heterogeneity among endothelial cells in different vessel types such as arteries, veins, lymphatic vessels and capillaries within different organs. The BMP pathway is a remarkably fine-tuned pathway. As a result, its signaling output in the vessel wall critically depends on the cellular context, which includes flow hemodynamics, interplay with other vascular signaling cascades and the interaction of endothelial cells with peri-endothelial cells and the surrounding matrix. In this review, the emerging role of BMP signaling in lymphatic vessel biology will be highlighted within the framework of BMP signaling in the circulatory vasculature.

Whole-genome resequencing reveals loci with allelic transmission ratio distortion in F1 chicken population

Allelic transmission ratio distortion (TRD) is the significant deviation from the expected ratio under Mendelian inheritance theory, which may be resulted from multiple disrupted biological processes, including germline selection, meiotic drive, gametic competition, imprint error, and embryo lethality. However, it is less known that whether or what extent the allelic TRD is present in farm animals. In this study, whole-genome resequencing technology was applied to reveal TRD loci in chicken by constructing a full-sib F₁ hybrid population. Through the whole-genome resequencing data of two parents (30 ×) and 38 offspring (5 ×), we detected a total of 2850 TRD SNPs (p-adj < 0.05) located within 400 genes showing TRD, and all of them were unevenly distributed on macrochromosomes and microchromosomes. Our findings suggested that TRD in the chicken chromosome 16 might play an important role in chicken immunity and disease resistance and the MYH1F with significant TRD and allele-specific expression could play a key role in the fast muscle development. In addition, functional enrichment analyses revealed that many genes (e.g., TGFBR2, TGFBR3, NOTCH1, and NCOA1) with TRD were found in the significantly enriched biological process and InterPro terms in relation to embryonic lethality and germline selection. Our results suggested that TRD is considerably prevalent in the chicken genome and has functional implications.

Computational Drug Repositioning Identifies Statins as Modifiers of Prognostic Genetic Expression Signatures and Metastatic Behavior in Melanoma

Despite advances in melanoma treatment, more than 70% of patients with distant metastasis die within 5 years. Proactive treatment of early melanoma to prevent metastasis could save lives and reduce overall healthcare costs. Currently, there are no treatments specifically designed to prevent early melanoma from progressing to metastasis. We used the Connectivity Map to conduct an in silico drug screen and identified 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors (statins) as a drug class that might prevent melanoma metastasis. To confirm the in vitro effect of statins, RNA sequencing was completed on A375 cells after treatment with fluvastatin to describe changes in the melanoma transcriptome. Statins induced differential expression in genes associated with metastasis and are used in commercially available prognostic tests for melanoma metastasis. Finally, we completed a chart review of 475 patients with melanoma. Patients taking statins were less likely to have metastasis at the time of melanoma diagnosis in both univariate and multivariate analyses (24.7% taking statins vs. 37.6% not taking statins, absolute risk reduction = 12.9%, P = 0.038). These findings suggest that statins might be useful as a treatment to prevent melanoma metastasis. Prospective trials are required to verify our findings and to determine the mechanism of metastasis prevention.

Loss of Primary Cilia Protein IFT20 Dysregulates Lymphatic Vessel Patterning in Development and Inflammation

Microenvironmental signals produced during development or inflammation stimulate lymphatic endothelial cells to undergo lymphangiogenesis, in which they sprout, proliferate, and migrate to expand the vascular network. Many cell types detect changes in extracellular conditions via primary cilia, microtubule-based cellular protrusions that house specialized membrane receptors and signaling complexes. Primary cilia are critical for receipt of extracellular cues from both ligand-receptor pathways and physical forces such as fluid shear stress. Here, we report the presence of primary cilia on immortalized mouse and primary adult human dermal lymphatic endothelial cells in vitro and on both luminal and abluminal domains of mouse corneal, skin, and mesenteric lymphatic vessels in vivo. The purpose of this study was to determine the effects of disrupting primary cilia on lymphatic vessel patterning during development and inflammation. Intraflagellar transport protein 20 (IFT20) is part of the transport machinery required for ciliary assembly and function. To disrupt primary ciliary signaling, we generated global and lymphatic endothelium-specific IFT20 knockout mouse models and used immunofluorescence microscopy to quantify changes in lymphatic vessel patterning at E16.5 and in adult suture-mediated corneal lymphangiogenesis. Loss of IFT20 during development resulted in edema, increased and more variable lymphatic vessel caliber and branching, as well as red blood cell-filled lymphatics. We used a corneal suture model to determine ciliation status of lymphatic vessels during acute, recurrent, and tumor-associated inflammatory reactions and wound healing. Primary cilia were present on corneal lymphatics during all of the mechanistically distinct lymphatic patterning events of the model and assembled on lymphatic endothelial cells residing at the limbus, stalk, and vessel tip. Lymphatic-specific deletion of IFT20 cell-autonomously exacerbated acute corneal lymphangiogenesis resulting in increased lymphatic vessel density and branching. These data are the first functional studies of primary cilia on lymphatic endothelial cells and reveal a new dimension in regulation of lymphatic vascular biology.

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.

Stromal regulation of tumor-associated lymphatics

Recent advances have identified a growing array of roles played by lymphatics in the tumor microenvironment, from providing a route of metastasis to immune modulation. The tumor microenvironment represents an exceptionally complex, dynamic niche comprised of a diverse mixture of cancer cells and normal host cells termed the stroma. This review discusses our current understanding of stromal elements and how they regulate lymphatic growth and functional properties in the tumor context.

Emerging Roles for Neuropilin-2 in Cardiovascular Disease

Cardiovascular disease, the leading cause of death worldwide, is predominantly associated with atherosclerosis. Atherosclerosis is a chronic inflammatory disease characterised by the narrowing of large to medium-sized arteries due to a build-up of plaque. Atherosclerotic plaque is comprised of lipids, extracellular matrix, and several cell types, including endothelial, immune, and vascular smooth muscle cells. Such narrowing of the blood vessels can itself restrict blood flow to vital organs but most severe clinical complications, including heart attacks and strokes, occur when lesions rupture, triggering the blood to clot and obstructing blood flow further down the vascular tree. To circumvent such obstructions, percutaneous coronary intervention or bypass grafts are often required; however, re-occlusion of the treated artery frequently occurs. Neuropilins (NRPs), a multifunctional family of cell surface co-receptors, are expressed by endothelial, immune, and vascular smooth muscle cells and are regulators of numerous signalling pathways within the vasculature. Here, we review recent studies implicating NRP2 in the development of occlusive vascular diseases and discuss how NRP2 could be targeted for therapeutic intervention.

A homozygous variant in growth and differentiation factor 2 (GDF2) may cause lymphatic dysplasia with hydrothorax and nonimmune hydrops fetalis

The etiology of nonimmune hydrops fetalis is extensive and includes genetic disorders. We describe a term-born female neonate with late onset extensive nonimmune hydrops, that is, polyhydramnios, edema, and congenital bilateral chylothorax. This newborn was successfully treated with repetitive thoracocentesis, total parenteral feeding, octreotide intravenously and finally surgical pleurodesis and corticosteroids. A genetic cause seemed plausible as the maternal history revealed a fatal nonimmune hydrops fetalis. A homozygous truncating variant in GDF2 (c.451C>T, p.(Arg151*)) was detected with exome sequencing. Genetic analysis of tissue obtained from the deceased fetal sibling revealed the same homozygous variant. The parents and two healthy siblings were heterozygous for the GDF2 variant. Skin and lung biopsies in the index patient, as well as the revised lung biopsy of the deceased fetal sibling, showed lymphatic dysplasia and lymphangiectasia. To the best of our knowledge, this is the first report of an association between a homozygous variant in GDF2 with lymphatic dysplasia, hydrothorax and nonimmune hydrops fetalis.

Potential role of transforming growth factor-beta 1/Smad signaling in secondary lymphedema after cancer surgery

Secondary lymphedema often develops after cancer surgery, and over 250 million patients suffer from this complication. A major symptom of secondary lymphedema is swelling with fibrosis, which lowers the patient's quality of life, even if cancer does not recur. Nonetheless, the pathophysiology of secondary lymphedema remains unclear, with therapeutic approaches limited to physical or surgical therapy. There is no effective pharmacological therapy for secondary lymphedema. Notably, the lack of animal models that accurately mimic human secondary lymphedema has hindered pathophysiological investigations of the disease. Here, we developed a novel rat hindlimb model of secondary lymphedema and showed that our rat model mimics human secondary lymphedema from early to late stages in terms of cell proliferation, lymphatic fluid accumulation, and skin fibrosis. Using our animal model, we investigated the disease progression and found that transforming growth factor‐beta 1 (TGFB1) was produced by macrophages in the acute phase and by fibroblasts in the chronic phase of the disease. TGFB1 promoted the transition of fibroblasts into myofibroblasts and accelerated collagen synthesis, resulting in fibrosis, which further indicates that myofibroblasts and TGFB1/Smad signaling play key roles in fibrotic diseases. Furthermore, the presence of myofibroblasts in skin samples from lymphedema patients after cancer surgery emphasizes the role of these cells in promoting fibrosis. Suppression of myofibroblast‐dependent TGFB1 production may therefore represent an effective pharmacological treatment for inhibiting skin fibrosis in human secondary lymphedema after cancer surgery.

Lymphatic MAFB regulates vascular patterning during developmental and pathological lymphangiogenesis

MAFB is a transcription factor involved in the terminal differentiation of several cell types, including macrophages and keratinocytes. MAFB is also expressed in lymphatic endothelial cells (LECs) and is upregulated by VEGF-C/VEGFR-3 signaling. Recent studies have revealed that MAFB regulates several genes involved in lymphatic differentiation and that global Mafb knockout mice show defects in patterning of lymphatic vessels during embryogenesis. However, it has remained unknown whether this effect is LEC-intrinsic and whether MAFB might also be involved in postnatal lymphangiogenesis. We established conditional, lymphatic-specific Mafb knockout mice and found comparable lymphatic patterning defects during embryogenesis as in the global MAFB knockout. Lymphatic MAFB deficiency resulted in increased lymphatic branching in the diaphragm at P7, but had no major effect on lymphatic patterning or function in healthy adult mice. By contrast, tumor-induced lymphangiogenesis was enhanced in mice lacking lymphatic MAFB. Together, these data reveal that LEC-expressed MAFB is involved in lymphatic vascular morphogenesis during embryonic and postnatal development as well as in pathological conditions. Therefore, MAFB could represent a target for therapeutic modulation of lymphangiogenesis.

EW-7197, a Transforming Growth Factor-Beta Type I Receptor Kinase Inhibitor, Ameliorates Acquired Lymphedema in a Mouse Tail Model

Background: Acquired lymphedema is a common consequence of cancer surgery. Fibrosis is one of the main causes of chronic lymphedema since it hinders lymphatic regeneration and this causes a significant decrease in lymphatic flow and accumulation of excessive protein-rich fluid. The transforming growth factor-β1 (TGF-β1) signaling pathway is known in a process of wound repair and fibrosis. In our study, the purpose was to evaluate the efficacy of EW-7197, a peroral TGF-β type I receptor kinase inhibitor, in treating acquired lymphedema. Methods and Results: For lymphedema mouse tail model, we used 10- to 12-week-old female C57BL/6 mice. The skin was circumferentially excised, making a circular band followed by cauterization of lymphatic collecting vessels. Two groups were made in this study: control and treatment. The treatment group (n = 12) received a solution consisting of 0.1 mL of artificial gastric juice and 20 mg/kg EW-7197 by gavage once daily. For evaluation, tail diameter measurement, fluorescence lymphography, and immunofluorescence images were used. EW-7197 treatment ameliorates acquired lymphedema in a mouse tail model by increasing lymphangiogenesis and interstitial flow of the lymphatics by inhibition of the fibrosis. The differences in maximal tail thicknesses between the control and treatment groups were statistically significant from 2 to 4 weeks after surgery. The treatment group showed a greater number of lymphatic vessels at the surgery site than the control group. The treatment group also showed more FITC coverage area at the surgery site. Conclusion: EW-7197 treatment ameliorates acquired lymphedema in a mouse tail model by increasing lymphangiogenesis and interstitial flow.

FOXO1 regulates developmental lymphangiogenesis by upregulating CXCR4 in the mouse-tail dermis

Lymphangiogenesis plays important roles in normal fetal development and postnatal growth. However, its molecular regulation remains unclear. Here, we have examined the function of forkhead box protein O1 (FOXO1) transcription factor, a known angiogenic factor, in developmental dermal lymphangiogenesis using endothelial cell-specific FOXO1-deficient mice. FOXO1-deficient mice showed disconnected and dilated lymphatic vessels accompanied with increased proliferation and decreased apoptosis in the lymphatic capillaries. Comprehensive DNA microarray analysis of the causes of in vivo phenotypes in FOXO1-deficient mice revealed that the gene encoding C-X-C chemokine receptor 4 (CXCR4) was the most drastically downregulated in FOXO1-deficient primary lymphatic endothelial cells (LECs). CXCR4 was expressed in developing dermal lymphatic capillaries in wild-type mice but not in FOXO1-deficient dermal lymphatic capillaries. Furthermore, FOXO1 suppression impaired migration toward the exogenous CXCR4 ligand, C-X-C chemokine ligand 12 (CXCL12), and coordinated proliferation in LECs. These results suggest that FOXO1 serves an essential role in normal developmental lymphangiogenesis by promoting LEC migration toward CXCL12 and by regulating their proliferative activity. This study provides valuable insights into the molecular mechanisms underlying developmental lymphangiogenesis.

Somatic activating mutations in PIK3CA cause generalized lymphatic anomaly

Generalized lymphatic anomaly (GLA) is a vascular disorder characterized by diffuse or multifocal lymphatic malformations (LMs). Here, Rodriguez-Laguna et al. report that somatic activating PIK3CA mutations can cause GLA, and we provide preclinical and clinical evidence to support the use of rapamycin for the treatment of GLA.

Generalized lymphatic anomaly (GLA) is a vascular disorder characterized by diffuse or multifocal lymphatic malformations (LMs). The etiology of GLA is poorly understood. We identified four distinct somatic PIK3CA variants (Glu542Lys, Gln546Lys, His1047Arg, and His1047Leu) in tissue samples from five out of nine patients with GLA. These same PIK3CA variants occur in PIK3CA-related overgrowth spectrum and cause hyperactivation of the PI3K–AKT–mTOR pathway. We found that the mTOR inhibitor, rapamycin, prevented lymphatic hyperplasia and dysfunction in mice that expressed an active form of PIK3CA (His1047Arg) in their lymphatics. We also found that rapamycin reduced pain in patients with GLA. In conclusion, we report that somatic activating PIK3CA mutations can cause GLA, and we provide preclinical and clinical evidence to support the use of rapamycin for the treatment of this disabling and deadly disease.

HHEX is a transcriptional regulator of the VEGFC/FLT4/PROX1 signaling axis during vascular development

Formation of the lymphatic system requires the coordinated expression of several key regulators: vascular endothelial growth factor C (VEGFC), its receptor FLT4, and a key transcriptional effector, PROX1. Yet, how expression of these signaling components is regulated remains poorly understood. Here, using a combination of genetic and molecular approaches, we identify the transcription factor hematopoietically expressed homeobox (HHEX) as an upstream regulator of VEGFC, FLT4, and PROX1 during angiogenic sprouting and lymphatic formation in vertebrates. By analyzing zebrafish mutants, we found that hhex is necessary for sprouting angiogenesis from the posterior cardinal vein, a process required for lymphangiogenesis. Furthermore, studies of mammalian HHEX using tissue-specific genetic deletions in mouse and knockdowns in cultured human endothelial cells reveal its highly conserved function during vascular and lymphatic development. Our findings that HHEX is essential for the regulation of the VEGFC/FLT4/PROX1 axis provide insights into the molecular regulation of lymphangiogenesis.

VEGFC, its receptor FLT4, and transcriptional effector PROX1 control formation of the lymphatic system but how is unclear. Here, the authors show that the transcription factor hematopoietically expressed homeobox (HHEX) regulates VEGFC, FLT4 and PROX1 in fish and mammals during angiogenic sprouting and lymphatic formation.

A blood capillary plexus-derived population of progenitor cells contributes to genesis of the dermal lymphatic vasculature during embryonic development

Despite the essential role of the lymphatic vasculature in tissue homeostasis and disease, knowledge of the organ-specific origins of lymphatic endothelial progenitor cells remains limited. The assumption that most murine embryonic lymphatic endothelial cells (LECs) are venous derived has recently been challenged. Here, we show that the embryonic dermal blood capillary plexus constitutes an additional, local source of LECs that contributes to the formation of the dermal lymphatic vascular network. We describe a novel mechanism whereby rare PROX1-positive endothelial cells exit the capillary plexus in a Ccbe1-dependent manner to establish discrete LEC clusters. As development proceeds, these clusters expand and further contribute to the growing lymphatic system. Lineage tracing and analyses of Gata2-deficient mice confirmed that these clusters are endothelial in origin. Furthermore, ectopic expression of Vegfc in the vasculature increased the number of PROX1-positive progenitors within the capillary bed. Our work reveals a novel source of lymphatic endothelial progenitors employed during construction of the dermal lymphatic vasculature and demonstrates that the blood vasculature is likely to remain an ongoing source of LECs during organogenesis, raising the question of whether a similar mechanism operates during pathological lymphangiogenesis.

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.

Matrix stiffness controls lymphatic vessel formation through regulation of a GATA2-dependent transcriptional program

Tissue and vessel wall stiffening alters endothelial cell properties and contributes to vascular dysfunction. However, whether extracellular matrix (ECM) stiffness impacts vascular development is not known. Here we show that matrix stiffness controls lymphatic vascular morphogenesis. Atomic force microscopy measurements in mouse embryos reveal that venous lymphatic endothelial cell (LEC) progenitors experience a decrease in substrate stiffness upon migration out of the cardinal vein, which induces a GATA2-dependent transcriptional program required to form the first lymphatic vessels. Transcriptome analysis shows that LECs grown on a soft matrix exhibit increased GATA2 expression and a GATA2-dependent upregulation of genes involved in cell migration and lymphangiogenesis, including VEGFR3. Analyses of mouse models demonstrate a cell-autonomous function of GATA2 in regulating LEC responsiveness to VEGF-C and in controlling LEC migration and sprouting in vivo. Our study thus uncovers a mechanism by which ECM stiffness dictates the migratory behavior of LECs during early lymphatic development.

TGF-β Signaling in Control of Cardiovascular Function

Genetic studies in animals and humans indicate that gene mutations that functionally perturb transforming growth factor β (TGF-β) signaling are linked to specific hereditary vascular syndromes, including Osler-Rendu-Weber disease or hereditary hemorrhagic telangiectasia and Marfan syndrome. Disturbed TGF-β signaling can also cause nonhereditary disorders like atherosclerosis and cardiac fibrosis. Accordingly, cell culture studies using endothelial cells or smooth muscle cells (SMCs), cultured alone or together in two- or three-dimensional cell culture assays, on plastic or embedded in matrix, have shown that TGF-β has a pivotal effect on endothelial and SMC proliferation, differentiation, migration, tube formation, and sprouting. Moreover, TGF-β can stimulate endothelial-to-mesenchymal transition, a process shown to be of key importance in heart valve cushion formation and in various pathological vascular processes. Here, we discuss the roles of TGF-β in vasculogenesis, angiogenesis, and lymphangiogenesis and the deregulation of TGF-β signaling in cardiovascular diseases.

Histological and Morphological Characterization of Developing Dermal Lymphatic Vessels

The capacity to visualize the lymphatic vasculature in three-dimensions has revolutionized our understanding of the morphogenetic mechanisms important for constructing the lymphatic vascular network during development. Two complementary approaches are commonly employed to assess the function of genes and signaling pathways important for development of the dermal lymphatic vasculature in the mouse embryo. The first of these is whole-mount immunostaining of embryonic skin to analyze dermal lymphatic vessel network patterning and morphology in two and three dimensions. The second is immunostaining of thin tissue sections to examine lymphatic vessel identity, lumen formation and protein localization within discrete lymphatic endothelial cells in a two-dimensional setting. Here we present detailed protocols for multicolor immunofluorescent immunostaining of embryonic dorsal skin and thin tissue cryosections. Each of these methods generates high-resolution images of the dermal lymphatic vasculature, yielding information integral to in-depth characterization of lymphatic vessel phenotypes in the developing mouse embryo.

Lymphangiogenesis guidance by paracrine and pericellular factors

This review by Vaahtomeri et al. discusses the mechanisms by which the lymphatic vasculature network is formed, remodeled, and adapted to physiological and pathological challenges. It describes how the lymphatic vasculature network is controlled by an intricate balance of growth factors and biomechanical cues.

Lymphatic vessels are important for tissue fluid homeostasis, lipid absorption, and immune cell trafficking and are involved in the pathogenesis of several human diseases. The mechanisms by which the lymphatic vasculature network is formed, remodeled, and adapted to physiological and pathological challenges are controlled by an intricate balance of growth factor and biomechanical cues. These transduce signals for the readjustment of gene expression and lymphatic endothelial migration, proliferation, and differentiation. In this review, we describe several of these cues and how they are integrated for the generation of functional lymphatic vessel networks.

Arf6 in lymphatic endothelial cells regulates lymphangiogenesis by controlling directional cell migration

The small GTPase Arf6 plays pivotal roles in a wide variety of cellular events such as endocytosis, exocytosis, and actin cytoskeleton reorganization. However, the physiological functions of Arf6 at the whole animal level have not yet been thoroughly understood. Here, we show that Arf6 regulates developmental and tumor lymphangiogenesis in mice. Lymphatic endothelial cell (LEC)-specific Arf6 conditional knockout (LEC-Arf6 cKO) mouse embryos exhibit severe skin edema and impairment in the formation of lymphatic vessel network at the mid-gestation stage. Knockdown of Arf6 in human LECs inhibits in vitro capillary tube formation and directed cell migration induced by vascular endothelial growth factor-C (VEGF-C) by inhibiting VEGF-C-induced internalization of β1 integrin. Finally, we found that LEC-Arf6 cKO mice transplanted with B16 melanoma cells attenuated tumor lymphangiogenesis and progression. Collectively, these results demonstrate that Arf6 in LECs plays a crucial role in physiological and pathological lymphangiogenesis.

Myeloid Wnt ligands are required for normal development of dermal lymphatic vasculature

Resident tissue myeloid cells play a role in many aspects of physiology including development of the vascular systems. In the blood vasculature, myeloid cells use VEGFC to promote angiogenesis and can use Wnt ligands to control vascular branching and to promote vascular regression. Here we show that myeloid cells also regulate development of the dermal lymphatic vasculature using Wnt ligands. Using myeloid-specific deletion of the WNT transporter Wntless we show that myeloid Wnt ligands are active at two distinct stages of development of the dermal lymphatics. As lymphatic progenitors are emigrating from the cardinal vein and intersomitic vessels, myeloid Wnt ligands regulate both their numbers and migration distance. Later in lymphatic development, myeloid Wnt ligands regulate proliferation of lymphatic endothelial cells (LEC) and thus control lymphatic vessel caliber. Myeloid-specific deletion of WNT co-receptor Lrp5 or Wnt5a gain-of-function also produce elevated caliber in dermal lymphatic capillaries. These data thus suggest that myeloid cells produce Wnt ligands to regulate lymphatic development and use Wnt pathway co-receptors to regulate the balance of Wnt ligand activity during the macrophage-LEC interaction.

Vascular heterogeneity and specialization in development and disease

Blood and lymphatic vessels pervade almost all body tissues and have numerous essential roles in physiology and disease. The inner lining of these networks is formed by a single layer of endothelial cells, which is specialized according to the needs of the tissue that it supplies. Whereas the general mechanisms of blood and lymphatic vessel development are being defined with increasing molecular precision, studies of the processes of endothelial specialization remain mostly descriptive. Recent insights from genetic animal models illuminate how endothelial cells interact with each other and with their tissue environment, providing paradigms for vessel type- and organ-specific endothelial differentiation. Delineating these governing principles will be crucial for understanding how tissues develop and maintain, and how their function becomes abnormal in disease.

FGF-dependent metabolic control of vascular development

Blood and lymphatic vasculatures are intimately involved in tissue oxygenation and fluid homeostasis maintenance. Assembly of these vascular networks involves sprouting, migration and proliferation of endothelial cells. Recent studies have suggested that changes in cellular metabolism are important to these processes. Although much is known about vascular endothelial growth factor (VEGF)-dependent regulation of vascular development and metabolism, little is understood about the role of fibroblast growth factors (FGFs) in this context. Here we identify FGF receptor (FGFR) signalling as a critical regulator of vascular development. This is achieved by FGF-dependent control of c-MYC (MYC) expression that, in turn, regulates expression of the glycolytic enzyme hexokinase 2 (HK2). A decrease in HK2 levels in the absence of FGF signalling inputs results in decreased glycolysis, leading to impaired endothelial cell proliferation and migration. Pan-endothelial- and lymphatic-specific Hk2 knockouts phenocopy blood and/or lymphatic vascular defects seen in Fgfr1/Fgfr3 double mutant mice, while HK2 overexpression partly rescues the defects caused by suppression of FGF signalling. Thus, FGF-dependent regulation of endothelial glycolysis is a pivotal process in developmental and adult vascular growth and development.

Fluorescence and Bioluminescence Imaging of Angiogenesis in Flk1-Nano-lantern Transgenic Mice

Angiogenesis is important for normal development as well as for tumour growth. However, the molecular and cellular mechanisms underlying angiogenesis are not fully understood, partly because of the lack of a good animal model for imaging. Here, we report the generation of a novel transgenic (Tg) mouse that expresses a bioluminescent reporter protein, Nano-lantern, under the control of Fetal liver kinase 1 (Flk1). Flk1-Nano-lantern BAC Tg mice recapitulated endogenous Flk1 expression in endothelial cells and lymphatic endothelial cells during development and tumour growth. Importantly, bioluminescence imaging of endothelial cells from the aortic rings of Flk1-Nano-lantern BAC Tg mice enabled us to observe endothelial sprouting for 18 hr without any detectable phototoxicity. Furthermore, Flk1-Nano-lantern BAC Tg mice achieved time-lapse luminescence imaging of tumour angiogenesis in freely moving mice with implanted tumours. Thus, this transgenic mouse line contributes a unique model to study angiogenesis within both physiological and pathological contexts.

Bidirectional Crosstalk between Lymphatic Endothelial Cell and T Cell and Its Implications in Tumor Immunity

Lymphatic vessels have been traditionally considered as passive transporters of fluid and lipids. However, it is apparent from recent literature that the function of lymphatic vessels is not only restricted to fluid balance homeostasis but also extends to regulation of immune cell trafficking, antigen presentation, tolerance, and immunity, all which may impact the progression of inflammatory responses and diseases such as cancer. The lymphatic system and the immune system are intimately connected, and there is emergent evidence for a crosstalk between T cell and lymphatic endothelial cell (LEC). This review describes how LECs in lymph nodes can affect multiple functional properties of T cells and the impact of these LEC-driven effects on adaptive immunity and, conversely, how T cells can modulate LEC growth. The significance of such crosstalk between T cells and LECs in cancer will also be discussed.

Activin receptor-like kinases: a diverse family playing an important role in cancer

The role and function of the members of the TGFβ superfamily has been a substantial area of research focus for the last several decades. During that time, it has become apparent that aberrations in TGFβ family signaling, whether through the BMP, Activin, or TGFβ arms of the pathway, can result in tumorigenesis or contribute to its progression. Downstream signaling regulates cellular growth under normal physiological conditions yet induces diverse processes during carcinogenesis, ranging from epithelial- to-mesenchymal transition to cell migration and invasion to angiogenesis. Due to these observations, the question has been raised how to utilize and target components of these signaling pathways in cancer therapy. Given that these cascades include both ligands and receptors, there are multiple levels at which to interfere. Activin receptor-like kinases (ALKs) are a group of seven type I receptors responsible for TGFβ family signal transduction and are utilized by many ligands within the superfamily. The challenge lies in specifically targeting the often-overlapping functional effects of BMP, Activin, or TGFβ signaling during cancer progression. This review focuses on the characteristic function of the individual receptors within each subfamily and their recognized roles in cancer. We next explore the clinical utility of therapeutically targeting ALKs as some have shown partial responses in Phase I clinical trials but disappointing outcomes when used in Phase II studies. Finally, we discuss the challenges and future directions of this body of work.

Transforming growth factor β as regulator of cancer stemness and metastasis

Key elements of cancer progression towards metastasis are the biological actions of cancer stem cells and stromal cells in the tumour microenvironment. Cross-communication between tumour and stromal cells is mediated by secreted cytokines, one of which, the transforming growth factor β (TGFβ), regulates essentially every cell within the malignant tissue. In this article, we focus on the actions of TGFβ on cancer stem cells, cancer-associated fibroblasts and immune cells that assist the overall process of metastatic dissemination. We aim at illustrating intricate connections made by various cells in the tumour tissue and which depend on the action of TGFβ.

Lymphatic System in Cardiovascular Medicine

The mammalian circulatory system comprises both the cardiovascular system and the lymphatic system. In contrast to the blood vascular circulation, the lymphatic system forms a unidirectional transit pathway from the extracellular space to the venous system. It actively regulates tissue fluid homeostasis, absorption of gastrointestinal lipids, and trafficking of antigen-presenting cells and lymphocytes to lymphoid organs and on to the systemic circulation. The cardinal manifestation of lymphatic malfunction is lymphedema. Recent research has implicated the lymphatic system in the pathogenesis of cardiovascular diseases including obesity and metabolic disease, dyslipidemia, inflammation, atherosclerosis, hypertension, and myocardial infarction. Here, we review the most recent advances in the field of lymphatic vascular biology, with a focus on cardiovascular disease.

Combined targeting of TGF-β, EGFR and HER2 suppresses lymphangiogenesis and metastasis in a pancreatic cancer model

Pancreatic ductal adenocarcinomas (PDACs) are aggressive with frequent lymphatic spread. By analysis of data from The Cancer Genome Atlas, we determined that ~35% of PDACs have a pro-angiogenic gene signature. We now show that the same PDACs exhibit increased expression of lymphangiogenic genes and lymphatic endothelial cell (LEC) markers, and that LEC abundance in human PDACs correlates with endothelial cell microvessel density. Lymphangiogenic genes and LECs are also elevated in murine PDACs arising in the KRC (mutated Kras; deleted RB) and KIC (mutated Kras; deleted INK4a) genetic models. Moreover, pancreatic cancer cells (PCCs) derived from KRC tumors express and secrete high levels of lymphangiogenic factors, including the EGF receptor ligand, amphiregulin. Importantly, TGF-β1 increases lymphangiogenic genes and amphiregulin expression in KRC PCCs but not in murine PCCs that lack SMAD4, and combinatorial targeting of the TGF-β type I receptor (TβRI) with LY2157299 and EGFR/HER2 with lapatinib suppresses tumor growth and metastasis in a syngeneic orthotopic model, and attenuates tumor lymphangiogenesis and angiogenesis while reducing lymphangiogenic genes and amphiregulin and enhancing apoptosis. Therefore, this combination could be beneficial in PDACs with lymphangiogenic or angiogenic gene signatures.

Distinct iris gene expression profiles of primary angle closure glaucoma and primary open angle glaucoma and their interaction with ocular biometric parameters

Background

This study aimed to evaluate differences in iris gene expression profiles between primary angle closure glaucoma (PACG) and primary open angle glaucoma (POAG) and their interaction with biometric characteristics.

Design

Prospective study.

Participants

Thirty‐five subjects with PACG and thirty‐three subjects with POAG who required trabeculectomy were enrolled at the Singapore National Eye Centre, Singapore.

Methods

Iris specimens, obtained by iridectomy, were analysed by real‐time polymerase chain reaction for expression of type I collagen, vascular endothelial growth factor (VEGF)‐A, ‐B and ‐C, as well as VEGF receptors (VEGFRs) 1 and 2. Anterior segment optical coherence tomography (ASOCT) imaging for biometric parameters, including anterior chamber depth (ACD), anterior chamber volume (ACV) and lens vault (LV), was also performed pre‐operatively.

Main Outcome Measures

Relative mRNA levels between PACG and POAG irises, biometric measurements, discriminant analyses using genes and biometric parameters.

Results

COL1A1, VEGFB, VEGFC and VEGFR2 mRNA expression was higher in PACG compared to POAG irises. LV, ACD and ACV were significantly different between the two subgroups. Discriminant analyses based on gene expression, biometric parameters or a combination of both gene expression and biometrics (LV and ACV), correctly classified 94.1%, 85.3% and 94.1% of the original PACG and POAG cases, respectively. The discriminant function combining genes and biometrics demonstrated the highest accuracy in cross‐validated classification of the two glaucoma subtypes.

Conclusions

Distinct iris gene expression supports the pathophysiological differences that exist between PACG and POAG. Biometric parameters can combine with iris gene expression to more accurately define PACG from POAG.