Platelets mediate lymphovenous hemostasis to maintain blood-lymphatic separation throughout life

Lymphatic System Development and Function

PURPOSE OF REVIEW

This review delves into recent advancements in understanding generalized and organ-specific lymphatic development. It emphasizes the distinct characteristics and critical anomalies that can impair lymphatic function. By exploring developmental mechanisms, the review seeks to illuminate the profound impact of lymphatic malformations on overall health and disease progression.

RECENT FINDINGS

The introduction of genome sequencing, single-cell transcriptomic analysis, and advanced imaging technologies has significantly enhanced our ability to identify and characterize developmental defects within the lymphatic system. As a result, a wide range of lymphatic anomalies have been uncovered, spanning from congenital abnormalities present at birth to conditions that can become life-threatening in adulthood. Additionally, recent research highlights the heterogeneity of lymphatics, revealing organ-specific developmental pathways, unique molecular markers, and specialized physiological functions specific to each organ. A deeper understanding of the unique characteristics of lymphatic cell populations in an organ-specific context is essential for guiding future research into lymphatic disease processes. An integrated approach to translational research could revolutionize personalized medicine, where treatments are precisely tailored to individual lymphatic profiles, enhancing effectiveness and minimizing side effects.

Platelet extracellular vesicles preserve lymphatic endothelial cell integrity and enhance lymphatic vessel function

Lymphatic vessels are essential for preventing the accumulation of harmful components within peripheral tissues, including the artery wall. Various endogenous mechanisms maintain adequate lymphatic function throughout life, with platelets being essential for preserving lymphatic vessel integrity. However, since lymph lacks platelets, their impact on the lymphatic system has long been viewed as restricted to areas where lymphatics intersect with blood vessels. Nevertheless, platelets can also exert long range effects through the release of extracellular vesicles (EVs) upon activation. We observed that platelet EVs (PEVs) are present in lymph, a compartment to which they could transfer regulatory effects of platelets. Here, we report that PEVs in lymph exhibit a distinct signature enabling them to interact with lymphatic endothelial cells (LECs). In vitro experiments show that the internalization of PEVs by LECs maintains their functional integrity. Treatment with PEVs improves lymphatic contraction capacity in atherosclerosis-prone mice. We suggest that boosting lymphatic pumping with exogenous PEVs offers a novel therapeutic approach for chronic inflammatory diseases characterized by defective lymphatics.

The relationship between the secondary vascular system and the lymphatic vascular system in fish

New technologies have resulted in a better understanding of blood and lymphatic vascular heterogeneity at the cellular and molecular levels. However, we still need to learn more about the heterogeneity of the cardiovascular and lymphatic systems among different species at the anatomical and functional levels. Even the deceptively simple question of the functions of fish lymphatic vessels has yet to be conclusively answered. The most common interpretation assumes a similar dual setup of the vasculature in zebrafish and mammals: a cardiovascular circulatory system, and a lymphatic vascular system (LVS), in which the unidirectional flow is derived from surplus interstitial fluid and returned into the cardiovascular system. A competing interpretation questions the identity of the lymphatic vessels in fish as at least some of them receive their flow from arteries via specialised anastomoses, neither requiring an interstitial source for the lymphatic flow nor stipulating unidirectionality. In this alternative view, the 'fish lymphatics' are a specialised subcompartment of the cardiovascular system, called the secondary vascular system (SVS). Many of the contradictions found in the literature appear to stem from the fact that the SVS develops in part or completely from an embryonic LVS by transdifferentiation. Future research needs to establish the extent of embryonic transdifferentiation of lymphatics into SVS blood vessels. Similarly, more insight is needed into the molecular regulation of vascular development in fish. Most fish possess more than the five vascular endothelial growth factor (VEGF) genes and three VEGF receptor genes that we know from mice or humans, and the relative tolerance of fish to whole-genome and gene duplications could underlie the evolutionary diversification of the vasculature. This review discusses the key elements of the fish lymphatics versus the SVS and attempts to draw a picture coherent with the existing data, including phylogenetic knowledge.

Lymphatic failure and lymphatic interventions: Knowledge gaps and future directions for a new frontier in congenital heart disease

Lymphatic failure is a broad term that describes the lymphatic circulation's inability to adequately transport fluid and solutes out of the interstitium and into the systemic venous circulation, which can result in dysfunction and dysregulation of immune responses, dietary fat absorption, and fluid balance maintenance. Several investigations have recently elucidated the nexus between lymphatic failure and congenital heart disease, and the associated morbidity and mortality is now well-recognized. However, the precise pathophysiology and pathogenesis of lymphatic failure remains poorly understood and relatively understudied, and there are no targeted therapeutics or interventions to reliably prevent its development and progression. Thus, there is growing enthusiasm towards the development and application of novel percutaneous and surgical lymphatic interventions. Moreover, there is consensus that further investigations are needed to delineate the underlying mechanisms of lymphatic failure, which could help identify novel therapeutic targets and develop innovative procedures to improve the overall quality of life and survival of these patients. With these considerations, this review aims to provide an overview of the lymphatic circulation and its vasculature as it relates to current understandings into the pathophysiology and pathogenesis of lymphatic failure in patients with congenital heart disease, while also summarizing strategies for evaluating and managing lymphatic complications, as well as specific areas of interest for future translational and clinical research efforts.

Platelets: Orchestrators of immunity in host defense and beyond

Platelets prevent blood loss during vascular injury and contribute to thrombus formation in cardiovascular disease. Beyond these classical roles, platelets are critical for the host immune response. They guard the vasculature against pathogens via specialized receptors, intracellular signaling cascades, and effector functions. Platelets also skew inflammatory responses by instructing innate immune cells, support adaptive immunosurveillance, and influence antibody production and T cell polarization. Concomitantly, platelets contribute to tissue reconstitution and maintain vascular function after inflammatory challenges. However, dysregulated activation of these multitalented cells exacerbates immunopathology with ensuing microvascular clotting, excessive inflammation, and elevated risk of macrovascular thrombosis. This dichotomy underscores the critical importance of precisely defining and potentially modulating platelet function in immunity.

A role of platelet C-type lectin-like receptor-2 and its ligand podoplanin in vascular biology

PURPOSE OF REVIEW

Platelets are essential for hemostasis and are also vital in lymphatic and lung development and the maintenance of vascular integrity. Platelet activation receptor C-type lectin-like receptor 2 (CLEC-2) and its endogenous ligand podoplanin (PDPN) in lymphatic endothelial cells (LECs) and other cells regulate these processes. This review aims to comprehensively summarize the roles of platelet CLEC-2 and PDPN. This review also focuses on discussing the underlying mechanisms by which platelet CLEC-2 and PDPN mediate blood/lymphatic separation.

FINDINGS

CLEC-2/PDPN-induced platelet activation in the primary lymph sacs, developmental lymphovenous junctions, neonatal mesentery, and the site of tumor lymphangiogenesis prevents blood/lymphatic vessel misconnection. Further, CLEC-2/PDPN-induced platelet activation is essential for lung development. Mice deficient in CLEC-2 or PDPN show blood-filled lymphatics, lung malformations, and cerebrovascular abnormalities. CLEC-2 deletion in steady-state adult mice did not result in blood/lymphatic vessel mixing. In adulthood, CLEC-2 maintains vascular integrity and that of high endothelial venules in lymph nodes. CLEC-2 deletion in adulthood results in hemorrhage under inflammatory conditions, and hemolymph nodes.

SUMMARY

The platelet CLEC-2/LEC PDPN interaction prevents blood/lymphatic vessel mixing at active remodeling sites of the blood/lymphatic system, but not in steady-state adult mice. This interaction also regulates vascular integrity when vascular permeability increases before and after birth.

Lymphatic vessel: origin, heterogeneity, biological functions, and therapeutic targets

Lymphatic vessels, comprising the secondary circulatory system in human body, play a multifaceted role in maintaining homeostasis among various tissues and organs. They are tasked with a serious of responsibilities, including the regulation of lymph absorption and transport, the orchestration of immune surveillance and responses. Lymphatic vessel development undergoes a series of sophisticated regulatory signaling pathways governing heterogeneous-origin cell populations stepwise to assemble into the highly specialized lymphatic vessel networks. Lymphangiogenesis, as defined by new lymphatic vessels sprouting from preexisting lymphatic vessels/embryonic veins, is the main developmental mechanism underlying the formation and expansion of lymphatic vessel networks in an embryo. However, abnormal lymphangiogenesis could be observed in many pathological conditions and has a close relationship with the development and progression of various diseases. Mechanistic studies have revealed a set of lymphangiogenic factors and cascades that may serve as the potential targets for regulating abnormal lymphangiogenesis, to further modulate the progression of diseases. Actually, an increasing number of clinical trials have demonstrated the promising interventions and showed the feasibility of currently available treatments for future clinical translation. Targeting lymphangiogenic promoters or inhibitors not only directly regulates abnormal lymphangiogenesis, but improves the efficacy of diverse treatments. In conclusion, we present a comprehensive overview of lymphatic vessel development and physiological functions, and describe the critical involvement of abnormal lymphangiogenesis in multiple diseases. Moreover, we summarize the targeting therapeutic values of abnormal lymphangiogenesis, providing novel perspectives for treatment strategy of multiple human diseases.

Synthetic hyaluronic acid coating preserves the phenotypes of lymphatic endothelial cells

Lymphatic endothelial cells (LECs) play a critical role in the formation and maintenance of the lymphatic vasculature, which is essential for the immune system, fluid balance, and tissue repair. However, LECs are often difficult to study in vivo and in vitro models that accurately mimic their behaviors and phenotypes are limited. In particular, LECs have been shown to lose their lymphatic markers over time while being cultured in vitro, which reflect their plasticity and heterogeneity in vivo. Since LECs uniquely express lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1), we hypothesized that surface coating with hyaluronic acid (HA) can preserve LEC phenotypes and functionalities. Dopamine conjugated hyaluronic acid (HA-DP) was synthesized with 42% degree of substitution to enable surface modification and conjugation onto standard tissue culture plates. Compared to fibronectin coating and tissue culture plate controls, surface coating with HA-DP was able to preserve lymphatic markers, such as prospero homeobox protein 1 (Prox1), podoplanin (PDPN), and LYVE-1 over several passages in vitro. LECs cultured on HA-DP expressed lower levels of focal adhesion kinase (FAK) and YAP/TAZ, which may be responsible for the maintenance of the lymphatic characteristics. Collectively, the HA-DP coating may provide a novel method for culturing human LECs in vitro toward more representative studies in basic lymphatic biology and lymphatic regeneration.

Lymphatic Dysfunction Models an Autoimmune Emphysema Phenotype of Chronic Obstructive Pulmonary Disease

Chronic Obstructive Pulmonary Disease (COPD) is a heterogeneous disease that is characterized by many clinical phenotypes. One such phenotype of COPD is defined by emphysema, pathogenic lung tertiary lymphoid organs (TLOs), and autoantibody production. We have previously shown that lymphatic dysfunction can cause lung TLO formation and lung injury in mice. We now sought to uncover whether underlying lymphatic dysfunction may be a driver of lung injury in cigarette smoke (CS)-induced COPD. We found that lung TLOs in mice with lymphatic dysfunction produce autoantibodies and are associated with a lymphatic endothelial cell subtype that expresses antigen presentation genes. Mice with underlying lymphatic dysfunction develop increased emphysema after CS exposure, with increased size and activation of TLOs. CS further increased autoantibody production in mice with lymphatic dysfunction. B-cell blockade prevented TLO formation and decreased lung injury after CS in mice with lymphatic dysfunction. Using tissue from human COPD patients, we also found evidence of a lymphatic gene signature that was specific to patients with emphysema and prominent TLOs compared to COPD patients without emphysema. Taken together, these data suggest that lymphatic dysfunction may underlie lung injury in a subset of COPD patients with an autoimmune emphysema phenotype.

Impact of Hemostasis on the Lymphatic System in Development and Disease

Lymphatic vessels form a systemic network that maintains interstitial fluid homeostasis and regulates immune responses and is strictly separated from the circulatory system. During embryonic development, lymphatic endothelial cells originate from blood vascular endothelial cells in the cardinal veins and form lymph sacs. Platelets are critical for separating lymph sacs from the cardinal veins through interactions between CLEC-2 (C-type lectin-like receptor-2) and PDPN (podoplanin) in lymphatic endothelial cells. Therefore, deficiencies of these genes cause blood-filled lymphatic vessels, leading to abnormal lymphatic vessel maturation. The junction between the thoracic duct and the subclavian vein has valves and forms physiological thrombi dependent on CLEC-2/PDPN signaling to prevent blood backflow into the thoracic duct. In addition, platelets regulate lymphangiogenesis and maintain blood/lymphatic separation in pathological conditions, such as wound healing and inflammatory diseases. More recently, it was reported that the entire hemostatic system is involved in lymphangiogenesis. Thus, the hemostatic system plays a crucial role in the establishment, maintenance, and rearrangement of lymphatic networks and contributes to body fluid homeostasis, which suggests that the hemostatic system is a potential target for treating lymphatic disorders. This review comprehensively summarizes the role of the hemostatic system in lymphangiogenesis and lymphatic vessel function and discusses challenges and future perspectives.

Reduction in disialyl-T antigen levels in mice deficient for both St6galnac3 and St6galnac4 results in blood filling of lymph nodes

Sialic acid (SA) is present at the terminal ends of carbohydrate chains in glycoproteins and glycolipids and is involved in various biological phenomena. The biological function of the disialyl-T (SAα2-3Galβ1-3(SAα2-6)GalNAcα1-O-Ser/Thr) structure is largely unknown. To elucidate the role of disialyl-T structure and determine the key enzyme from the N-acetylgalactosaminide α2,6-sialyltransferase (St6galnac) family involved in its in vivo synthesis, we generated St6galnac3- and St6galnac4-deficient mice. Both single-knockout mice developed normally without any prominent phenotypic abnormalities. However, the St6galnac3::St6galnact4 double knockout (DKO) mice showed spontaneous hemorrhage of the lymph nodes (LN). To identify the cause of bleeding in the LN, we examined podoplanin, which modifies the disialyl-T structures. The protein expression of podoplanin in the LN of DKO mice was similar to that in wild-type mice. However, the reactivity of MALII lectin, which recognizes disialyl-T, in podoplanin immunoprecipitated from DKO LN was completely abolished. Moreover, the expression of vascular endothelial cadherin was reduced on the cell surface of high endothelial venule (HEV) in the LN, suggesting that hemorrhage was caused by the structural disruption of HEV. These results suggest that podoplanin possesses disialyl-T structure in mice LN and that both St6galnac3 and St6galnac4 are required for disialyl-T synthesis.

Comprehensive analysis identifies CLEC1B as a potential prognostic biomarker in hepatocellular carcinoma

BACKGROUND

C-type lectin domain family 1 member B (CLEC1B, encoding the CLEC-2 protein), a member of the C-type lectin superfamily, is a type II transmembrane receptor involved in platelet activation, angiogenesis, and immune and inflammatory responses. However, data regarding its function and clinical prognostic value in hepatocellular carcinoma (HCC) remain scarce.

METHODS

The expression of CLEC1B was explored using The Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO) databases. RT-qPCR, western blot, and immunohistochemistry assays were employed to validate the downregulation of CLEC1B. Univariate Cox regression and survival analyses were used to evaluate the prognostic value of CLEC1B. Gene Set Enrichment Analysis (GSEA) was conducted to investigate the potential association between cancer hallmarks and CLEC1B expression. The TISIDB database was applied to search for the correlation between immune cell infiltration levels and CLEC1B expression. The association between CLEC1B and immunomodulators was conducted by Spearman correlation analysis based on the Sangerbox platform. Annexin V-FITC/PI apoptosis kit was used for the detection of cell apoptosis.

RESULTS

The expression of CLEC1B was low in various tumors and exhibited a promising clinical prognostic value for HCC patients. The expression level of CLEC1B was tightly associated with the infiltration of various immune cells in the HCC tumor microenvironment (TME) and positively correlated with a bulk of immunomodulators. In addition, CLEC1B and its related genes or interacting proteins are implicated in multiple immune-related processes and signaling pathways. Moreover, overexpression of CLEC1B significantly influenced the treatment effects of sorafenib on HCC cells.

CONCLUSIONS

Our results reveal that CLEC1B could serve as a potential prognostic biomarker and may be a novel immunoregulator for HCC. However, its function in immune regulation should be further explored.

A biologically active lipid, thromboxane, as a regulator of angiogenesis and lymphangiogenesis

Thromboxane (TX) and prostaglandins are metabolites of arachidonic acid, a twenty-carbon unsaturated fatty acid, and have a variety of actions that are exerted via specific receptors. Angiogenesis is defined as the formation of new blood vessels from pre-existing vascular beds and is a critical component of pathological conditions, including inflammation and cancer. Lymphatic vessels play crucial roles in the regulation of interstitial fluid, immune surveillance, and the absorption of dietary fat from the intestine; and they are also involved in the pathogenesis of various diseases. Similar to angiogenesis, lymphangiogenesis, the formation of new lymphatic vessels, is a critical component of pathological conditions. The TP-dependent accumulation of platelets in microvessels has been reported to enhance angiogenesis under pathological conditions. Although the roles of some growth factors and cytokines in angiogenesis and lymphangiogenesis have been well characterized, accumulating evidence suggests that TX induces the production of proangiogenic and prolymphangiogenic factors through the activation of adenylate cyclase, and upregulates angiogenesis and lymphangiogenesis under disease conditions. In this review, we discuss the role of TX as a regulator of angiogenesis and lymphangiogenesis, and its emerging importance as a therapeutic target.

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

INTRODUCTION

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSION

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

Platelets and (Lymph)angiogenesis

The formation of new blood and lymphatic vessels is essential for both the development of multicellular organisms and (patho)physiological processes like wound repair and tumor growth. In the 1990s, circulating blood platelets were first postulated to regulate tumor angiogenesis by interacting with the endothelium and releasing angiogenic regulators from specialized α granules. Since then, many studies have validated the contributions of platelets to tumor angiogenesis, while uncovering novel roles for platelets in other angiogenic processes like wound resolution and retinal vascular disease. Although the majority of (lymph)angiogenesis occurs during development, platelets appear necessary for lymphatic but not vascular growth, implying their particular importance in pathological cases of adult angiogenesis. Future work is required to determine whether drugs targeting platelet production or function offer a clinically relevant tool to limit detrimental angiogenesis.

Bioactive food components and their inhibitory actions in multiple platelet pathways

In addition to hemostasis and thrombosis, blood platelets are involved in various processes such as inflammation, infection, immunobiology, cancer metastasis, wound repair and angiogenesis. Platelets' hemostatic and non-hemostatic functions are mediated by the expression of various membrane receptors and the release of proteins, ions and other mediators. Therefore, specific activities of platelets responsible for the non-hemostatic disease are to be inhibited while leaving the platelet's hemostatic function unaffected. Platelets' anti-aggregatory property has been used as a primary criterion for antiplatelet drugs/bioactives; however, their non-hemostatic activities are not well known. This review describes the hemostatic and non-hemostatic function of human blood platelets and the modulatory effects of bioactive food components. PRACTICAL APPLICATIONS: In this review, we have discussed the antiplatelet effects of several food components. These bioactive compounds inhibit both hemostatic and non-hemostatic pathways involving blood platelet. Platelets have emerged as critical biological factors of normal and pathologic vascular healing and other diseases such as cancers and inflammatory and immune disorders. The challenge for therapeutic intervention in these disorders will be to find drugs and bioactive compounds that preferentially block specific sites implicated in emerging roles of platelets' complicated contribution to inflammation, tumour growth, or other disorders while leaving at least some of their hemostatic function intact.

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.

Lymphoedema conditions disrupt endothelial barrier function in vitro

Lymphatic vessel contractions generate net antegrade pulsatile lymph flow. By contrast, impaired lymphatic vessels are often associated with lymphoedema and altered lymph flow. The effect of lymphoedema on the lymph flow field and endothelium is not completely known. Here, we characterized the lymphatic flow field of a platelet-specific receptor C-type lectin-like receptor 2 (CLEC2) deficient lymphoedema mouse model. In regions of lymphoedema, collecting vessels were significantly distended, vessel contractility was greatly diminished and pulsatile lymph flow was replaced by quasi-steady flow. In vitro exposure of human dermal lymphatic endothelial cells (LECs) to lymphoedema-like quasi-steady flow conditions increased intercellular gap formation and permeability in comparison to normal pulsatile lymph flow. In the absence of flow, LECs exposed to steady pressure (SP) increased intercellular gap formation in contrast with pulsatile pressure (PP). The absence of pulsatility in steady fluid flow and SP conditions without flow-induced upregulation of myosin light chain (MLCs) regulatory subunits 9 and 12B mRNA expression and phosphorylation of MLCs, in contrast with pulsatile flow and PP without flow. These studies reveal that the loss of pulsatility, which can occur with lymphoedema, causes LEC contraction and an increase in intercellular gap formation mediated by MLC phosphorylation.

Kaposiform Lymphangiomatosis: Pathologic Aspects in 43 Patients

Kaposiform lymphangiomatosis is an uncommon generalized lymphatic anomaly with distinctive clinical, radiologic, histopathologic, and molecular findings. Herein, we document the pathology in 43 patients evaluated by the Boston Children's Hospital Vascular Anomalies Center from 1999 to 2020. The most frequent presentations were respiratory difficulty, hemostatic abnormalities, and a soft tissue mass. Imaging commonly revealed involvement of some combination of mediastinal, pulmonary, pleural, and pericardial compartments and most often included spleen and skeleton. Histopathology was characterized by dilated, redundant, and abnormally configured lymphatic channels typically accompanied by dispersed clusters of variably canalized, and often hemosiderotic, spindled lymphatic endothelial cells that were immunopositive for D2-40, PROX1, and CD31. An activating lesional NRAS variant was documented in 9 of 10 patients. The clinical course was typically aggressive, marked by hemorrhage, thrombocytopenia, diminished fibrinogen levels, and a mortality rate of 21%.

Antithrombotic Effects of Fostamatinib in Combination with Conventional Antiplatelet Drugs

New antithrombotic medications with less effect on haemostasis are needed for the long-term treatment of acute coronary syndromes (ACS). The platelet receptor glycoprotein VI (GPVI) is critical in atherothrombosis, mediating platelet activation at atherosclerotic plaque. The inhibition of spleen tyrosine kinase (Syk) has been shown to block GPVI-mediated platelet function. The aim of our study was to investigate if the Syk inhibitor fostamatinib could be repurposed as an antiplatelet drug, either alone or in combination with conventional antiplatelet therapy. The effect of the active metabolite of fostamatinib (R406) was assessed on platelet activation and function induced by atherosclerotic plaque and a range of agonists in the presence and absence of the commonly used antiplatelet agents aspirin and ticagrelor. The effects were determined ex vivo using blood from healthy volunteers and aspirin- and ticagrelor-treated patients with ACS. Fostamatinib was also assessed in murine models of thrombosis. R406 mildly inhibited platelet responses induced by atherosclerotic plaque homogenate, likely due to GPVI inhibition. The anti-GPVI effects of R406 were amplified by the commonly-used antiplatelet medications aspirin and ticagrelor; however, the effects of R406 were concentration-dependent and diminished in the presence of plasma proteins, which may explain why fostamatinib did not significantly inhibit thrombosis in murine models. For the first time, we demonstrate that the Syk inhibitor R406 provides mild inhibition of platelet responses induced by atherosclerotic plaque and that this is mildly amplified by aspirin and ticagrelor.

Roles of Podoplanin in Malignant Progression of Tumor

Podoplanin (PDPN) is a cell-surface mucin-like glycoprotein that plays a critical role in tumor development and normal development of the lung, kidney, and lymphatic vascular systems. PDPN is overexpressed in several tumors and is involved in their malignancy. PDPN induces platelet aggregation through binding to platelet receptor C-type lectin-like receptor 2. Furthermore, PDPN modulates signal transductions that regulate cell proliferation, differentiation, migration, invasion, epithelial-to-mesenchymal transition, and stemness, all of which are crucial for the malignant progression of tumor. In the tumor microenvironment (TME), PDPN expression is upregulated in the tumor stroma, including cancer-associated fibroblasts (CAFs) and immune cells. CAFs play significant roles in the extracellular matrix remodeling and the development of immunosuppressive TME. Additionally, PDPN functions as a co-inhibitory molecule on T cells, indicating its involvement with immune evasion. In this review, we describe the mechanistic basis and diverse roles of PDPN in the malignant progression of tumors and discuss the possibility of the clinical application of PDPN-targeted cancer therapy, including cancer-specific monoclonal antibodies, and chimeric antigen receptor T technologies.

Podoplanin promotes tumor growth, platelet aggregation, and venous thrombosis in murine models of ovarian cancer

BACKGROUND

Podoplanin (PDPN) is a sialylated membrane glycoprotein that binds to C-type lectin-like receptor 2 on platelets resulting in platelet activation. PDPN is expressed on lymphatic endothelial cells, perivascular fibroblasts/pericytes, cancer cells, cancer-associated fibroblasts, and tumor stromal cells. PDPN's expression on malignant epithelial cells plays a role in metastasis. Furthermore, the expression of PDPN in brain tumors (high-grade gliomas) was found to correlate with an increased risk of venous thrombosis.

OBJECTIVE

We examined the expression of PDPN and its role in tumor progression and venous thrombosis in ovarian cancer.

METHODS

We used mouse models of ovarian cancer and venous thrombosis.

RESULTS

Ovarian cancer cells express PDPN and release PDPN-rich extracellular vesicles (EVs), and cisplatin and topotecan (chemotherapies commonly used in ovarian cancer) increase the expression of podoplanin in cancer cells. The expression of PDPN in ovarian cancer cells promotes tumor growth in a murine model of ovarian cancer and that knockdown of PDPN gene expression results in smaller primary tumors. Both PDPN-expressing ovarian cancer cells and their EVs cause platelet aggregation. In a mouse model of venous thrombosis, PDPN-expressing EVs released from HeyA8 ovarian cancer cells produce more frequent thrombosis than PDPN-negative EVs derived from PDPN-knockdown HeyA8 cells. Blood clots induced by PDPN-positive EVs contain more platelets than those in blood clots induced by PDPN-negative EVs.

CONCLUSIONS

In summary, our findings demonstrate that the expression of PDPN by ovarian cancer cells promotes tumor growth and venous thrombosis in mice.

Human Dectin-1 is O-glycosylated and serves as a ligand for C-type lectin receptor CLEC-2

C-type lectin receptors (CLRs) elicit immune responses upon recognition of glycoconjugates present on pathogens and self-components. While Dectin-1 is the best-characterized CLR recognizing β-glucan on pathogens, the endogenous targets of Dectin-1 are not fully understood. Herein, we report that human Dectin-1 is a ligand for CLEC-2, another CLR expressed on platelets. Biochemical analyses revealed that Dectin-1 is a mucin-like protein as its stalk region is highly O-glycosylated. A sialylated core 1 glycan attached to the EDxxT motif of human Dectin-1, which is absent in mouse Dectin-1, provides a ligand moiety for CLEC-2. Strikingly, the expression of human Dectin-1 in mice rescued the lethality and lymphatic defect resulting from a deficiency of Podoplanin, a known CLEC-2 ligand. This finding is the first example of an innate immune receptor also functioning as a physiological ligand to regulate ontogeny upon glycosylation.

Coagulation in Lymphatic System

The lymphatic system maintains homeostasis of the internal environment between the cells in tissues and the blood circulation. The coagulation state of lymph is determined by conditions of coagulation factors and lymphatic vessels. Internal obliteration, external compression or abnormally increased lymphatic pressure may predispose to localized lymphatic coagulation. In physiological conditions, an imbalance of antithrombin and thrombokinase reduces lymphatic thrombosis. However, the release of factor X by lymphatic endothelium injury may trigger coagulation casacade, causing blockage of lymphatic vessels and lymphedema. Heterogeneity of lymphatic vessels in various tissues may lead to distinct levels and patterns of coagulation in specific lymphatic vessels. The quantitative and qualitative measurement of clotting characteristic reveals longer time for clotting to occur in the lymph than in the blood. Cancer, infections, amyloidosis and lymph node dissection may trigger thrombosis in the lymphatic vessels. In contrast to venous or arterial thrombosis, lymphatic thrombosis has rarely been reported, and its actual prevalence is likely underestimated. In this review, we summarize the mechanisms of coagulation in lymphatic system, and discuss the lymphatic thrombosis-related diseases.

Platelets: Implications in Aortic Valve Stenosis and Bioprosthetic Valve Dysfunction From Pathophysiology to Clinical Care

Aortic stenosis (AS) is the most common heart valve disease requiring surgery in developed countries, with a rising global burden associated with aging populations. The predominant cause of AS is believed to be driven by calcific degeneration of the aortic valve and a growing body of evidence suggests that platelets play a major role in this disease pathophysiology. Furthermore, platelets are a player in bioprosthetic valve dysfunction caused by their role in leaflet thrombosis and thickening. This review presents the molecular function of platelets in the context of recent and rapidly evolving understanding the role of platelets in AS, both of the native aortic valve and bioprosthetic valves, where there remain concerns about the effects of subclinical leaflet thrombosis on long-term prosthesis durability. This review also presents the role of antiplatelet and anticoagulation therapies on modulating the impact of platelets on native and bioprosthetic aortic valves, highlighting the need for further studies to determine whether these therapies are protective and may increase the life span of surgical and transcatheter aortic valve implants. By linking molecular mechanisms through which platelets drive disease of native and bioprosthetic aortic valves with studies evaluating the clinical impact of antiplatelet and antithrombotic therapies, we aim to bridge the gaps between our basic science understanding of platelet biology and their role in patients with AS and ensuing preventive and therapeutic implications.

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

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

The effect of respiration and body position on terminal thoracic duct diameter and the lymphovenous junction: An exploratory ultrasound study

The thoracic duct (TD) drains most of the body's lymph back to the venous system via its lymphovenous junction (LVJ), playing a pivotal role in fluid homeostasis, fat absorption and the systemic immune response. The respiratory cycle is thought to assist with lymph flow, but the precise mechanism underpinning terminal TD lymph flow into the central veins is not well understood. The aim of this study was to use ultrasonography (US) to explore the relationship between terminal TD lymph flow, the respiratory cycle, and gravity. The left supraclavicular fossa was scanned in healthy non-fasted volunteers using high-resolution (13-5 MHz) US to identify the terminal TD and the presence of a lymphovenous valve (LVV). The TD's internal diameter was measured in relation to respiration (inspiration vs. expiration) and body positioning (supine vs. Trendelenburg). The terminal TD was visualized in 20/33 (61%) healthy volunteers. An LVV was visualized in only 4/20 (20%) cases. The mean terminal TD diameter in the supine position was 1.7 mm (range 0.8-3.1 mm); this increased in full inspiration (mean 1.8 mm, range 0.9-3.2 mm, p < 0.05), and in the Trendelenburg position (mean 1.8 mm, range 1.2-3.1 mm, p < 0.05). The smallest mean terminal TD diameter occurred in full expiration (1.6 mm, range 0.7-3.1 mm, p < 0.05). Respiration and gravity impact the terminal TD diameter. Due to the challenges of visualizing the TD and LVJ, other techniques such as dynamic magnetic resonance imaging will be required to fully understand the factors governing TD lymph flow.

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

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

The heterogeneity of megakaryocytes and platelets and implications for ex vivo platelet generation

Platelets, the chief effector of hemostasis, are small anucleate blood cells generated from megakaryocytes (MKs), and the defects in platelet production or function lead to a variety of bleeding complications. Emerging evidence indicates that MKs and platelets are much more diverse than previously appreciated and involved in many physiological and pathological processes besides hemostasis, such as innate and adaptive immune responses, angiogenesis, and tumor metastasis, while the ontogenic variations in MK and platelet function have also become a focus in the field. However, whether MKs and platelets fulfill these distinct functions by utilizing distinct subpopulations remains poorly understood. New studies aimed at deciphering the MK transcriptome at the single‐cell level have provided some key insights into the functional heterogeneity of MKs. In this review, we will discuss some of the recent discoveries of functional and developmental heterogeneity of MKs and its potential link to the heterogeneity of platelets. We will also discuss the implications of these findings while focusing on the ex vivo generation of platelets from human pluripotent stem cells. The improved understanding of the heterogeneity underlying human MK development and platelet production should open new avenues for future platelet regeneration and clinical treatment of related diseases.

Ticagrelor prevents tumor metastasis via inhibiting cell proliferation and promoting platelet apoptosis

Tumor cells can activate platelets, which in turn facilitate tumor cell survival and dissemination. Platelets inhibition or blocking platelet-tumor cell interactions has become a strategy to suppress tumor progression. In this study, we investigated the effect of ticagrelor, a new antiplatelet drug, on tumor cell proliferation and metastasis. Our results show that ticagrelor not only inhibits the proliferation, migration, and invasion of B16F10 and Lewis lung carcinoma cells but also induces platelet apoptosis. In addition, we find that apoptosis of the platelet cells is dose dependent. Further, the result of in-vivo experiments proved that ticagrelor treatment decreased the tumor metastasis. The results of this study demonstrate that ticagrelor may be a potential anti-tumor agent for tumor metastasis.

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

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

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.

Hypercholesterolemia and Lymphatic Defects: The Chicken or the Egg?

Lymphatic vessels are necessary for maintaining tissue fluid balance, trafficking of immune cells, and transport of dietary lipids. Growing evidence suggest that lymphatic functions are limited under hypercholesterolemic conditions, which is closely related to atherosclerotic development involving the coronary and other large arteries. Indeed, ablation of lymphatic systems by Chy-mutation as well as depletion of lymphangiogenic factors, including vascular endothelial growth factor-C and -D, in mice perturbs lipoprotein composition to augment hypercholesterolemia. Several investigations have reported that periarterial microlymphatics were attracted by atheroma-derived lymphangiogenic factors, which facilitated lymphatic invasion into the intima of atherosclerotic lesions, thereby modifying immune cell trafficking. In contrast to the lipomodulatory and immunomodulatory roles of the lymphatic systems, the critical drivers of lymphangiogenesis and the details of lymphatic insults under hypercholesterolemic conditions have not been fully elucidated. Interestingly, cholesterol-lowering trials enable hypercholesterolemic prevention of lymphatic drainage in mice; however, a causal relationship between hypercholesterolemia and lymphatic defects remains elusive. In this review, the contribution of aberrant lymphangiogenesis and lymphatic cholesterol transport to hypercholesterolemic atherosclerosis was highlighted. The causal relationship between hypercholesterolemia and lymphatic insults as well as the current achievements in the field were discussed.

Updated Understanding of Platelets in Thrombosis and Hemostasis: The Roles of Integrin PSI Domains and their Potential as Therapeutic Targets

Platelets are small blood cells known primarily for their ability to adhere and aggregate at injured vessels to arrest bleeding. However, when triggered under pathological conditions, the same adaptive mechanism of platelet adhesion and aggregation may cause thrombosis, a primary cause of heart attack and stroke. Over recent decades, research has made considerable progress in uncovering the intricate and dynamic interactions that regulate these processes. Integrins are heterodimeric cell surface receptors expressed on all metazoan cells that facilitate cell adhesion, movement, and signaling, to drive biological and pathological processes such as thrombosis and hemostasis. Recently, our group discovered that the plexin-semaphorin-integrin (PSI) domains of the integrin β subunits exert endogenous thiol isomerase activity derived from their two highly conserved CXXC active site motifs. Given the importance of redox reactions in integrin activation and its location in the knee region, this PSI domain activity may be critically involved in facilitating the interconversions between integrin conformations. Our monoclonal antibodies against the β3 PSI domain inhibited its thiol isomerase activity and proportionally attenuated fibrinogen binding and platelet aggregation. Notably, these antibodies inhibited thrombosis without significantly impairing hemostasis or causing platelet clearance. In this review, we will update mechanisms of thrombosis and hemostasis, including platelet versatilities and immune-mediated thrombocytopenia, discuss critical contributions of the newly discovered PSI domain thiol isomerase activity, and its potential as a novel target for anti-thrombotic therapies and beyond.

The evolving cardiac lymphatic vasculature in development, repair and regeneration

The lymphatic vasculature has an essential role in maintaining normal fluid balance in tissues and modulating the inflammatory response to injury or pathogens. Disruption of normal development or function of lymphatic vessels can have severe consequences. In the heart, reduced lymphatic function can lead to myocardial oedema and persistent inflammation. Macrophages, which are phagocytic cells of the innate immune system, contribute to cardiac development and to fibrotic repair and regeneration of cardiac tissue after myocardial infarction. In this Review, we discuss the cardiac lymphatic vasculature with a focus on developments over the past 5 years arising from the study of mammalian and zebrafish model organisms. In addition, we examine the interplay between the cardiac lymphatics and macrophages during fibrotic repair and regeneration after myocardial infarction. Finally, we discuss the therapeutic potential of targeting the cardiac lymphatic network to regulate immune cell content and alleviate inflammation in patients with ischaemic heart disease.

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.

Role of Podoplanin-Positive Cells in Cardiac Fibrosis and Angiogenesis After Ischemia

New insights into the cellular and extra-cellular composition of scar tissue after myocardial infarction (MI) have been identified. Recently, a heterogeneous podoplanin-expressing cell population has been associated with fibrogenic and inflammatory responses and lymphatic vessel growth during scar formation. Podoplanin is a mucin-like transmembrane glycoprotein that plays an important role in heart development, cell motility, tumorigenesis, and metastasis. In the adult mouse heart, podoplanin is expressed only by cardiac lymphatic endothelial cells; after MI, it is acquired with an unexpected heterogeneity by PDGFRα-, PDGFRβ-, and CD34-positive cells. Podoplanin may therefore represent a sign of activation of a cohort of progenitor cells during different phases of post-ischemic myocardial wound repair. Podoplanin binds to C-type lectin-like receptor 2 (CLEC-2) which is exclusively expressed by platelets and a variety of immune cells. CLEC-2 is upregulated in CD11b^high cells, including monocytes and macrophages, following inflammatory stimuli. We recently published that inhibition of the interaction between podoplanin-expressing cells and podoplanin-binding cells using podoplanin-neutralizing antibodies reduces but does not fully suppress inflammation post-MI while improving heart function and scar composition after ischemic injury. These data support an emerging and alternative mechanism of interactome in the heart that, when neutralized, leads to altered inflammatory response and preservation of cardiac function and structure. The overarching objective of this review is to assimilate and discuss the available evidence on the functional role of podoplanin-positive cells on cardiac fibrosis and remodeling. A detailed characterization of cell-to-cell interactions and paracrine signals between podoplanin-expressing cells and the other type of cells that compose the heart tissue is needed to open a new line of investigation extending beyond the known function of these cells. This review attempts to discuss the role and biology of podoplanin-positive cells in the context of cardiac injury, repair, and remodeling.

Roles of Thromboxane Receptor Signaling in Enhancement of Lipopolysaccharide-Induced Lymphangiogenesis and Lymphatic Drainage Function in Diaphragm

[Figure: see text].

Lymphovenous shunts: from development to clinical applications

The lymphatic system is a vast network of vessels that functions to return excess fluid from the interstitial space to the blood stream. Lymphovenous shunts are anastomoses, either natural or surgical, that connect the lymphatic and venous systems. Connections between the thoracic duct and venous system or between the right lymphatic duct and venous system are prime examples of anatomic lymphovenous shunts. Lymphovenous shunts are also present peripherally in tissues such as lymph nodes. Furthermore, pathologic lymphovenous shunts are observed in conditions such as lymphedema, malignancy, and lymphovenous malformations. Surgically, lymphovenous shunts may be constructed as an approach to treat lymphedema. Here, we discuss anatomic and surgical lymphovenous shunts in the context of normal development and disease. This perspective is intended to give an understanding of the role of lymphovenous shunts in health and disease and to show how they can be leveraged to treat disease surgically.

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.

Blood and lymphatic systems are segregated by the FLCN tumor suppressor

Blood and lymphatic vessels structurally bear a strong resemblance but never share a lumen, thus maintaining their distinct functions. Although lymphatic vessels initially arise from embryonic veins, the molecular mechanism that maintains separation of these two systems has not been elucidated. Here, we show that genetic deficiency of Folliculin, a tumor suppressor, leads to misconnection of blood and lymphatic vessels in mice and humans. Absence of Folliculin results in the appearance of lymphatic-biased venous endothelial cells caused by ectopic expression of Prox1, a master transcription factor for lymphatic specification. Mechanistically, this phenotype is ascribed to nuclear translocation of the basic helix-loop-helix transcription factor Transcription Factor E3 (TFE3), binding to a regulatory element of Prox1, thereby enhancing its venous expression. Overall, these data demonstrate that Folliculin acts as a gatekeeper that maintains separation of blood and lymphatic vessels by limiting the plasticity of committed endothelial cells.

Blood and lymphatic vessels bear a strong resemblance but do not share a lumen, thus maintaining their distinct functions. Here, the authors describe that Folliculin, a tumor suppressor, prevents the fusion of these vessels during development by limiting the plasticity of venous and lymphatic endothelial cells.

The Lymphovenous Junction of the Thoracic Duct: A Systematic Review of its Structural and Functional Anatomy

Background: The lymphovenous junction (LVJ) of the thoracic duct (TD) is the principle outlet of the lymphatic system. Interest in this junction is growing as its role in lymphatic outflow obstruction is being realized, and as minimally invasive procedures for accessing the terminal TD become more common. Despite the growing clinical significance of the LVJ, its precise form and function remain unclear. The aim of this article was to systematically review the literature surrounding the structure and function of the LVJ and its associated lymphovenous valve (LVV). Methods and Results: A systematic review of the structure and function of the LVJ and LVV was undertaken using the MEDLINE, Scopus, and Google Scholar databases. Human and animal studies up to November 2019, with no language or past date restriction, were included. Forty-six relevant articles were reviewed. The LVJ shows marked anatomical variation. A valve is frequently absent at the LVJ, but when present it displays numerous distinct morphologies. These include bicuspid semilunar, ostial, and flap-like structure. Other factors, such as functional platelet plugs, or the tangential/intramural course of the terminal TD across the vein wall, may work to prevent blood from entering the lymphatic system. Conclusions: The form and function of the LVJ remain unclear. Dedicated studies of this area in vivo are required to elucidate how this part of the body functions in both health and disease.

Network patterning, morphogenesis and growth in lymphatic vascular development

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

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

Simple Summary

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

Abstract

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

Regulation of Platelet Production and Life Span: Role of Bcl-xL and Potential Implications for Human Platelet Diseases

Blood platelets have important roles in haemostasis, where they quickly stop bleeding in response to vascular damage. They have also recognised functions in thrombosis, immunity, antimicrobal defense, cancer growth and metastasis, tumour angiogenesis, lymphangiogenesis, inflammatory diseases, wound healing, liver regeneration and neurodegeneration. Their brief life span in circulation is strictly controlled by intrinsic apoptosis, where the prosurvival Bcl-2 family protein, Bcl-xL, has a major role. Blood platelets are produced by large polyploid precursor cells, megakaryocytes, residing mainly in the bone marrow. Together with Mcl-1, Bcl-xL regulates megakaryocyte survival. This review describes megakaryocyte maturation and survival, platelet production, platelet life span and diseases of abnormal platelet number with a focus on the role of Bcl-xL during these processes.

Platelet factor 4 is a biomarker for lymphatic-promoted disorders

Genetic or acquired defects of the lymphatic vasculature often result in disfiguring, disabling, and, occasionally, life-threatening clinical consequences. Advanced forms of lymphedema are readily diagnosed clinically, but more subtle presentations often require invasive imaging or other technologies for a conclusive diagnosis. On the other hand, lipedema, a chronic lymphatic microvascular disease with pathological accumulation of subcutaneous adipose tissue, is often misdiagnosed as obesity or lymphedema; currently there are no biomarkers or imaging criteria available for a conclusive diagnosis. Recent evidence suggests that otherwise-asymptomatic defective lymphatic vasculature likely contributes to an array of other pathologies, including obesity, inflammatory bowel disease, and neurological disorders. Accordingly, identification of biomarkers of lymphatic malfunction will provide a valuable resource for the diagnosis and clinical differentiation of lymphedema, lipedema, obesity, and other potential lymphatic pathologies. In this paper, we profiled and compared blood plasma exosomes isolated from mouse models and from human subjects with and without symptomatic lymphatic pathologies. We identified platelet factor 4 (PF4/CXCL4) as a biomarker that could be used to diagnose lymphatic vasculature dysfunction. Furthermore, we determined that PF4 levels in circulating blood plasma exosomes were also elevated in patients with lipedema, supporting current claims arguing that at least some of the underlying attributes of this disease are also the consequence of lymphatic defects.

Extracellular Vesicles as Potential Prognostic Markers of Lymphatic Dysfunction

Despite significant efforts made to treat cardiovascular disease (CVD), more than half of cardiovascular events still occur in asymptomatic subjects devoid of traditional risk factors. These observations underscore the need for the identification of new biomarkers for the prevention of atherosclerosis, the main underlying cause of CVD. Extracellular vesicles (EVs) and lymphatic vessel function are emerging targets in this context. EVs are small vesicles released by cells upon activation or death that are present in several biological tissues and fluids, including blood and lymph. They interact with surrounding cells to transfer their cargo, and the complexity of their biological content makes these EVs potential key players in several chronic inflammatory settings. Many studies focused on the interaction of EVs with the most well-known players of atherosclerosis such as the vascular endothelium, smooth muscle cells and monocytes. However, the fate of EVs within the lymphatic network, a crucial route in the mobilization of cholesterol out the artery wall, is not known. In this review, we aim to bring forward evidence that EVs could be at the interplay between lymphatic function and atherosclerosis by summarizing the recent findings on the characterization of EVs in this setting.

Platelet CLEC-2 and lung development

In this article, the State of the Art lecture "Platelet CLEC-2 and Lung Development" presented at the ISTH congress 2019 is reviewed. During embryonic development, blood cells are often considered as porters of nutrition and oxygen but not as active influencers of cell differentiation. However, recent studies revealed that platelets actively facilitate cell differentiation by releasing biological substances during development. C-type lectin-like receptor 2 (CLEC-2) has been identified as a receptor for the platelet-activating snake venom rhodocytin. An endogenous ligand of CLEC-2 is the membrane protein podoplanin (PDPN), which is expressed on the surface of certain types of tumor cells and lymphatic endothelial cells (LECs). Deletion of CLEC-2 from platelets in mice results in death just after birth due to lung malformation and blood/lymphatic vessel separation. During development, lymphatic vessels are derived from cardinal veins. At this stage, platelets are activated by binding of CLEC-2 to LEC PDPN and release trandforming growth factor-β (TGF-β). This cytokine inhibits LEC migration and proliferation, facilitating blood/lymphatic vessel separation. TGF-β released upon platelet-expressed CLEC-2/LEC PDPN also facilitates differentiation of lung mesothelial cells into alveolar duct myofibroblasts (adMYFs) in the developing lung. AdMYFs generate elastic fibers inside the lung, so that the lung can be properly inflated. Thus, platelets act as an ultimate natural drug delivery system that enables biological substances to be specifically delivered to the target at high concentrations by receptor/ligand interactions during development.

Integrating Biological Advances Into the Clinical Management of Breast Cancer Related Lymphedema

Breast cancer-related lymphedema (BCRL) occurs in a significant number of breast cancer survivors as a consequence of the axillary lymphatics' impairment after therapy (mainly axillary surgery and irradiation). Despite the recent achievements in the clinical management of these patients, BCRL is often diagnosed at its occurrence. In most cases, it remains a progressive and irreversible condition, with dramatic consequences in terms of quality of life and on sanitary costs. There are still no validated pre-surgical strategies to identify individuals that harbor an increased risk of BCRL. However, clinical, therapeutic, and tumor-specific traits are recurrent in these patients. Over the past few years, many studies have unraveled the complexity of the molecular and transcriptional events leading to the lymphatic system ontogenesis. Additionally, molecular insights are coming from the study of the germline alterations involved at variable levels in BCRL models. Regrettably, there is a substantial lack of predictive biomarkers for BCRL, given that our knowledge of its molecular milieu remains extremely puzzled. The purposes of this review were (i) to outline the biology underpinning the ontogenesis of the lymphatic system; (ii) to assess the current state of knowledge of the molecular alterations that can be involved in BCRL pathogenesis and progression; (iii) to discuss the present and short-term future perspectives in biomarker-based patients' risk stratification; and (iv) to provide practical information that can be employed to improve the quality of life of these patients.