Visualization of lymphatic vessel development, growth, and function

Vegfr3-tdTomato, a reporter mouse for microscopic visualization of lymphatic vessel by multiple modalities

Lymphatic vessels are indispensable for tissue fluid homeostasis, transport of solutes and dietary lipids and immune cell trafficking. In contrast to blood vessels, which are easily visible by their erythrocyte cargo, lymphatic vessels are not readily detected in the tissue context. Their invisibility interferes with the analysis of the three-dimensional lymph vessel structure in large tissue volumes and hampers dynamic intravital studies on lymphatic function and pathofunction. An approach to overcome these limitations are mouse models, which express transgenic fluorescent proteins under the control of tissue-specific promotor elements. We introduce here the BAC-transgenic mouse reporter strain Vegfr3-tdTomato that expresses a membrane-tagged version of tdTomato under control of Flt4 regulatory elements. Vegfr3-tdTomato mice inherited the reporter in a mendelian fashion and showed selective and stable fluorescence in the lymphatic vessels of multiple organs tested, including lung, kidney, heart, diaphragm, intestine, mesentery, liver and dermis. In this model, tdTomato expression was sufficient for direct visualisation of lymphatic vessels by epifluorescence microscopy. Furthermore, lymph vessels were readily visualized using a number of microscopic modalities including confocal laser scanning, light sheet fluorescence and two-photon microscopy. Due to the early onset of VEGFR-3 expression in venous embryonic vessels and the short maturation time of tdTomato, this reporter offers an interesting alternative to Prox1-promoter driven lymphatic reporter mice for instance to study the developmental differentiation of venous to lymphatic endothelial cells.

Rapid methods for the evaluation of fluorescent reporters in tissue clearing and the segmentation of large vascular structures

Light sheet fluorescence microscopy (LSFM) of large tissue samples does not require mechanical sectioning and allows efficient visualization of spatially complex or rare structures. Therefore, LSFM has become invaluable in developmental and biomedical research. Because sample size may limit whole-mount staining, LSFM benefits from transgenic reporter organisms expressing fluorescent proteins (FPs) and, however, requires optical clearing and computational data visualization and analysis. The former often interferes with FPs, while the latter requires massive computing resources. Here, we describe 3D-polymerized cell dispersions, a rapid and straightforward method, based on recombinant FP expression in freely selectable tester cells, to evaluate and compare fluorescence retention in different tissue-clearing protocols. For the analysis of large LSFM data, which usually requires huge computing resources, we introduce a refined, interactive, hierarchical random walker approach that is capable of efficient segmentation of the vasculature in data sets even on a consumer grade PC.

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

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

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

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

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

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

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

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

The clinical significance of CCBE1 expression in human colorectal cancer

Purpose

The identification and discovery of prognostic markers for colorectal cancer (CRC) are of great clinical significance. CCBE1 is expressed in various tumors and its expression correlates with lymphangiogenesis and angiogenesis. However, the association between CCBE1 expression and CRC outcome has not been reported. The aim of this study was to investigate clinical significance of CCBE1 expression in CRC.

Patients and methods

CCBE1 expression was examined in 30 pairs of fresh CRC tissues and compared with adjacent normal (AN) tissues using quantitative real-time PCR (qRT-PCR), Western blotting and immunohistochemistry (IHC) staining. Tissue microarray immunohistochemical staining was used to study the CCBE1 expression characteristics of 204 CRC patient samples collected from January 2002 to December 2007, and the relationship of CCBE1 with clinicopathological features and prognosis of CRC was analyzed.

Results

CCBE1 was highly expressed in CRC tissues compared with matched AN tissues (P=0.001). Moreover, high expression of CCBE1 was significantly associated with tumor differentiation, lymph node metastasis, vascular invasion, liver metastasis and TNM stage in CRC patients (P≤0.01). Kaplan-Meier survival analysis revealed that high CCBE1 expression, poor tumor differentiation, lymph node metastasis and vascular invasion were significantly associated (all P<0.001) with poor prognosis for patients. Furthermore, univariate and multivariate Cox analysis revealed that high CCBE1 expression, poor tumor differentiation, lymph node metastasis and vascular invasion were independent risk factors for both overall survival (OS) and disease-free survival (DFS) of CRC patients (all P<0.05). OS and DFS of 267 CRC patients from The Cancer Genome Atlas (TCGA) database showed the same trend (log-rank P=6e-04, HR [high] =2.4; log-rank P=0.0081, HR [high] =1.9).

Conclusion

High levels of CCBE1 contribute to the aggressiveness and poor prognosis of CRC. CCBE1 can serve as a novel potential biomarker to predict CRC patients' prognosis.

Lymphatic Vessels: A Potential Approach to the Treatment of Atherosclerosis?

Many basic and clinical studies have demonstrated that atherosclerosis is a chronic inflammatory disease. Although there are many factors affecting atherosclerosis, the role of lymphatic vessels in this disease has been neglected. Traditionally, lymphatic vessels have been considered to be passages for transporting interstitial fluid to the blood circulation. However, as early as the last century, researchers found that there are numerous lymphatic vessels surrounding sites of atherosclerosis; however, the relationship between lymphatic vessels and atherosclerosis is not clear. With further research, lymphatic vessels were determined to be involved in the induction and resolution of arterial inflammation and also to play a positive role in plaque cholesterol transport. There are abundant immune cells around atherosclerosis, and these immune cells not only have a significant impact on plaque formation but also affect local lymphangiogenesis (IAL). This promotion of IAL seems to relieve the progression of atherosclerosis. Therefore, research into the relationship between lymphatic vessels and atherosclerosis is of great importance for improving atherosclerosis treatment. This review highlights what is known about the relationship between lymphatic vessels and atherosclerosis, including the effect of immune cells on IAL, and reverse cholesterol transport. In addition, we present some of our views on the improvement of atherosclerosis treatment, which have significant clinical value in research.

Lymphatic endothelial progenitor cells: origins and roles in lymphangiogenesis

How are lymphatic vessels built? What are the sources of progenitor cells employed to construct lymphatic vessels during embryogenesis and in pathological situations? Are lymphatic vessels in different tissues built the same way? These questions have been highly topical and actively debated in the field of lymphangiogenesis research for more than 100 years. While embryonic veins and cells of mesenchymal origin have been recognised as sources of embryonic lymphatic endothelial cells for many years, recent advances in technology have revealed the existence of additional sources of lymphatic endothelial cells important for embryonic lymphangiogenesis. Intriguingly, distinct progenitor cell sources appear to be employed in a tissue specific manner during development. Gaining further insight into the identity of lymphatic endothelial progenitor cells and the signals that direct their assembly, both during development and in disease, has the potential to enable the design of therapeutics able to selectively target specific lymphatic vessel beds, a feature likely to prove valuable for the treatment of human disorders including cancer, lymphoedema and inflammatory disease.

Three-Dimensional Visualization of the Lymphatic Vasculature

Like the circulatory blood vessel system, the dendriform lymphatic vascular system forms a disseminated organ that is virtually indispensible for the function of most other organs. Formation and maintenance of the correct topology are essential for lymph vessel physiology and hence analysis of its three-dimensional architecture provides crucial functional information.Here we describe protocols for whole-mount immunostaining of the vessel systems in various mouse tissues, mouse fetuses, and human skin biopsies. The resulting samples are suited after flat mounting for confocal microscopy or after optical tissue clearing for light sheet microscopy. Both microscopic modalities use optical sectioning to generate image stacks from which the three-dimensional vessel structure can be digitally reconstructed. We introduce the open software package Voreen, developed at the University of Münster. Voreen has been adapted and extended for the interactive visualization of large multichannel image stacks on commodity hardware, allowing for a faithful digital representation of the spatial structure of the vessel systems in whole-mount stained tissue samples.

VIPAR, a quantitative approach to 3D histopathology applied to lymphatic malformations

BACKGROUND

Lack of investigatory and diagnostic tools has been a major contributing factor to the failure to mechanistically understand lymphedema and other lymphatic disorders in order to develop effective drug and surgical therapies. One difficulty has been understanding the true changes in lymph vessel pathology from standard 2D tissue sections.

METHODS

VIPAR (volume information-based histopathological analysis by 3D reconstruction and data extraction), a light-sheet microscopy-based approach for the analysis of tissue biopsies, is based on digital reconstruction and visualization of microscopic image stacks. VIPAR allows semiautomated segmentation of the vasculature and subsequent nonbiased extraction of characteristic vessel shape and connectivity parameters. We applied VIPAR to analyze biopsies from healthy lymphedematous and lymphangiomatous skin.

RESULTS

Digital 3D reconstruction provided a directly visually interpretable, comprehensive representation of the lymphatic and blood vessels in the analyzed tissue volumes. The most conspicuous features were disrupted lymphatic vessels in lymphedematous skin and a hyperplasia (4.36-fold lymphatic vessel volume increase) in the lymphangiomatous skin. Both abnormalities were detected by the connectivity analysis based on extracted vessel shape and structure data. The quantitative evaluation of extracted data revealed a significant reduction of lymphatic segment length (51.3% and 54.2%) and straightness (89.2% and 83.7%) for lymphedematous and lymphangiomatous skin, respectively. Blood vessel length was significantly increased in the lymphangiomatous sample (239.3%).

CONCLUSION

VIPAR is a volume-based tissue reconstruction data extraction and analysis approach that successfully distinguished healthy from lymphedematous and lymphangiomatous skin. Its application is not limited to the vascular systems or skin.

FUNDING

Max Planck Society, DFG (SFB 656), and Cells-in-Motion Cluster of Excellence EXC 1003.

E2F8 is a Potential Therapeutic Target for Hepatocellular Carcinoma

E2F transcriptional factors are widely expressed in a number of tissues and organs, possessing many regulatory functions related to cellular proliferation, differentiation, DNA repair, cell-cycle and cell apoptosis. E2F8 is a recently identified member of the E2F family with a duplicated DNA-binding domain feature discriminated from E2F1-6, controlling gene expression in a dimerization partner-independent manner. It is indispensable for angiogenesis, lymphangiogenesis and embryonic development. Although E2F8 and E2F7 perform complementary and overlapping functions in many cell metabolisms, E2F8, but not E2F7, overexpresses remarkably in hepatocellular carcinoma (HCC) to facilitate the HCC occurrence and development via activating a E2F1/ Cyclin D1 signaling pathway to regulate the G1- to S-phase transition of cell cycle progression or transcriptionally suppressing CDK1 to induce hepatocyte polyploidization. It also involves closely a variety of cellular physiological functions and pathological processes, which may bring a new breakthrough for the treatment of certain diseases, especially the HCC. Here, we summarize the latest progress of E2F8 on its relevant functions and mechanisms as well as potential application.

Integration of CD45-positive leukocytes into newly forming lymphatics of adult mice

The embryonic origin of lymphatic endothelial cells (LECs) has been a matter of controversy since more than a century. However, recent studies in mice have supported the concept that embryonic lymphangiogenesis is a complex process consisting of growth of lymphatics from specific venous segments as well as the integration of lymphangioblasts into the lymphatic networks. Similarly, the mechanisms of adult lymphangiogenesis are poorly understood and have rarely been studied. We have recently shown that endothelial progenitor cells isolated from the lung of adult mice have the capacity to form both blood vessels and lymphatics when grafted with Matrigel plugs into the skin of syngeneic mice. Here, we followed up on these experiments and studied the behavior of host leukocytes during lymphangiogenesis in the Matrigel plugs. We observed a striking co-localization of CD45(+) leukocytes with the developing lymphatics. Numerous CD45(+) cells expressed the LEC marker podoplanin and were obviously integrated into the lining of lymphatic capillaries. This indicates that, similar to inflammation-induced lymphangiogenesis in man, circulating CD45(+) cells of adult mice are capable of initiating lymphangiogenesis and of adopting a lymphvasculogenic cellular differentiation program. The data are discussed in the context of embryonic and inflammation-induced lymphangiogenesis.

Isolation of human lymphatic endothelial cells by multi-parameter fluorescence-activated cell sorting

Lymphatic system disorders such as primary lymphedema, lymphatic malformations and lymphatic tumors are rare conditions that cause significant morbidity but little is known about their biology. Isolating highly pure human lymphatic endothelial cells (LECs) from diseased and healthy tissue would facilitate studies of the lymphatic endothelium at genetic, molecular and cellular levels. It is anticipated that these investigations may reveal targets for new therapies that may change the clinical management of these conditions. A protocol describing the isolation of human foreskin LECs and lymphatic malformation lymphatic endothelial cells (LM LECs) is presented. To obtain a single cell suspension tissue was minced and enzymatically treated using dispase II and collagenase II. The resulting single cell suspension was then labelled with antibodies to cluster of differentiation (CD) markers CD34, CD31, Vascular Endothelial Growth Factor-3 (VEGFR-3) and PODOPLANIN. Stained viable cells were sorted on a fluorescently activated cell sorter (FACS) to separate the CD34(Low)CD31(Pos)VEGFR-3(Pos)PODOPLANIN(Pos) LM LEC population from other endothelial and non-endothelial cells. The sorted LM LECs were cultured and expanded on fibronectin-coated flasks for further experimental use.

A tale of two models: mouse and zebrafish as complementary models for lymphatic studies

Lymphatic vessels provide essential roles in maintaining fluid homeostasis and lipid absorption. Dysfunctions of the lymphatic vessels lead to debilitating pathological conditions, collectively known as lymphedema. In addition, lymphatic vessels are a critical moderator for the onset and progression of diverse human diseases including metastatic cancer and obesity. Despite their clinical importance, there is no currently effective pharmacological therapy to regulate functions of lymphatic vessels. Recent efforts to manipulate the Vascular Endothelial Growth Factor-C (VEGFC) pathway, which is arguably the most important signaling pathway regulating lymphatic endothelial cells, to alleviate lymphedema yielded largely mixed results, necessitating identification of new targetable signaling pathways for therapeutic intervention for lymphedema. Zebrafish, a relatively new model system to investigate lymphatic biology, appears to be an ideal model to identify novel therapeutic targets for lymphatic biology. In this review, we will provide an overview of our current understanding of the lymphatic vessels in vertebrates, and discuss zebrafish as a promising in vivo model to study lymphatic vessels.

Blood and lymphatic vascular tube formation in mouse

The blood and lymphatic vasculatures are essential for nutrient delivery, gas exchange and fluid homeostasis in all tissues of higher vertebrates. They are composed of a hierarchical network of vessels, which are lined by vascular or lymphatic endothelial cells. For blood vascular lumen formation to occur, endothelial cell cords polarize creating apposing apical cell surfaces, which repulse each other and give rise to a small intercellular lumen. Following cell shape changes, the vascular lumen expands. Various junctional proteins, polarity complexes, extracellular matrix binding and actin remodelling molecules are required for blood vascular lumen formation. In contrast, little is known regarding the molecular mechanisms leading to lymphatic vascular tube formation. Current models agree that lymphatic vessels share a blood vessel origin, but they differ in identifying the mechanism by which a lymphatic lumen is formed. A ballooning mechanism was proposed, in which lymph sacs are connected via their lumen to the cardinal veins. Alternatively, a mechanism involving budding of streams of lymphatic endothelial cells from either the cardinal veins or both the cardinal veins and the intersomitic vessels, and subsequent assembly and lumenisation was recently described. Here, we discuss what is currently known about the molecular and cellular machinery that guides blood and lymphatic vascular tube formation in mouse.