Dachsous1-Fat4 Signaling Controls Endothelial Cell Polarization During Lymphatic Valve Morphogenesis-Brief Report

Meningeal lymphatic drainage: novel insights into central nervous system disease

In recent years, increasing evidence has suggested that meningeal lymphatic drainage plays a significant role in central nervous system (CNS) diseases. Studies have indicated that CNS diseases and conditions associated with meningeal lymphatic drainage dysfunction include neurodegenerative diseases, stroke, infections, traumatic brain injury, tumors, functional cranial disorders, and hydrocephalus. However, the understanding of the regulatory and damage mechanisms of meningeal lymphatics under physiological and pathological conditions is currently limited. Given the importance of a profound understanding of the interplay between meningeal lymphatic drainage and CNS diseases, this review covers seven key aspects: the development and structure of meningeal lymphatic vessels, methods for observing meningeal lymphatics, the function of meningeal lymphatics, the molecular mechanisms of meningeal lymphatic injury, the relationships between meningeal lymphatic vessels and CNS diseases, potential regulatory mechanisms of meningeal lymphatics, and conclusions and outstanding questions. We will explore the relationship between the development, structure, and function of meningeal lymphatics, review current methods for observing meningeal lymphatic vessels in both animal models and humans, and identify unresolved key points in meningeal lymphatic research. The aim of this review is to provide new directions for future research and therapeutic strategies targeting meningeal lymphatics by critically analyzing recent advancements in the field, identifying gaps in current knowledge, and proposing innovative approaches to address these gaps.

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.

Congenital Diarrhoeas and Enteropathies

Congenital diarrhoeas and enteropathies (CODE) are a heterogeneous group of disorders. Many affected infants present with catastrophic dehydration in the first few days of life, although the clinical phenotype is variable. Advances in the understanding of underlying pathomechanisms and genetic testing, as well as improved management, in particular intravenous nutrition support, have allowed affected patients to survive well beyond childhood. Awareness and understanding of these rare diseases are hence needed, both amongst paediatricians and adult physicians. In this review, we discuss the different groups of disorders based on a review of the current literature and provide a diagnostic and therapeutic approach. Many of the subtypes of CODE result in the need for prolonged or indefinite parenteral nutrition. Further research is needed to identify new CODE to improve the recognition and management of these children, which can assist in developing new targeted therapies and potentially a long-term cure.

Structure of the planar cell polarity cadherins Fat4 and Dachsous1

The atypical cadherins Fat and Dachsous are key regulators of cell growth and animal development. In contrast to classical cadherins, which form homophilic interactions to segregate cells, Fat and Dachsous cadherins form heterophilic interactions to induce cell polarity within tissues. Here, we determine the co-crystal structure of the human homologs Fat4 and Dachsous1 (Dchs1) to establish the molecular basis for Fat-Dachsous interactions. The binding domains of Fat4 and Dchs1 form an extended interface along extracellular cadherin (EC) domains 1-4 of each protein. Biophysical measurements indicate that Fat4-Dchs1 affinity is among the highest reported for cadherin superfamily members, which is attributed to an extensive network of salt bridges not present in structurally similar protocadherin homodimers. Furthermore, modeling suggests that unusual extracellular phosphorylation modifications directly modulate Fat-Dachsous binding by introducing charged contacts across the interface. Collectively, our analyses reveal how the molecular architecture of Fat4-Dchs1 enables them to form long-range, high-affinity interactions to maintain planar cell polarity.

Whole genome sequencing and inheritance-based variant filtering as a tool for unraveling missing heritability in pediatric cancer

Survival rates for pediatric cancer have significantly increased the past decades, now exceeding 70-80% for most cancer types. The cause of cancer in children and adolescents remains largely unknown and a genetic susceptibility is considered in up to 10% of the cases, but most likely this is an underestimation. Families with multiple pediatric cancer patients are rare and strongly suggestive for an underlying predisposition to cancer. The absence of identifiable mutations in known cancer predisposing genes in such families could indicate undiscovered heritability. To discover candidate susceptibility variants, whole genome sequencing was performed on germline DNA of a family with two children affected by Burkitt lymphoma. Using an inheritance-based filtering approach, 18 correctly segregating coding variants were prioritized without a biased focus on specific genes or variants. Two variants in FAT4 and DCHS2 were highlighted, both involved in the Hippo signaling pathway, which controls tissue growth and stem cell activity. Similarly, a set of nine non-coding variants was prioritized, which might contribute, in differing degrees, to the increased cancer risk within this family. In conclusion, inheritance-based whole genome sequencing in selected families or cases is a valuable approach to prioritize variants and, thus, to further unravel genetic predisposition in childhood cancer.

Molecular Mechanisms Driving Lymphedema and Other Lymphatic Anomalies

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

Low Efficacy of Genetic Tests for the Diagnosis of Primary Lymphedema Prompts Novel Insights into the Underlying Molecular Pathways

Lymphedema is a chronic inflammatory disorder caused by ineffective fluid uptake by the lymphatic system, with effects mainly on the lower limbs. Lymphedema is either primary, when caused by genetic mutations, or secondary, when it follows injury, infection, or surgery. In this study, we aim to assess to what extent the current genetic tests detect genetic variants of lymphedema, and to identify the major molecular pathways that underlie this rather unknown disease. We recruited 147 individuals with a clinical diagnosis of primary lymphedema and used established genetic tests on their blood or saliva specimens. Only 11 of these were positive, while other probands were either negative (63) or inconclusive (73). The low efficacy of such tests calls for greater insight into the underlying mechanisms to increase accuracy. For this purpose, we built a molecular pathways diagram based on a literature analysis (OMIM, Kegg, PubMed, Scopus) of candidate and diagnostic genes. The PI3K/AKT and the RAS/MAPK pathways emerged as primary candidates responsible for lymphedema diagnosis, while the Rho/ROCK pathway appeared less critical. The results of this study suggest the most important pathways involved in the pathogenesis of lymphedema, and outline the most promising diagnostic and candidate genes to diagnose this disease.

DCHS1, Lix1L, and the Septin Cytoskeleton: Molecular and Developmental Etiology of Mitral Valve Prolapse

Mitral valve prolapse (MVP) is a common cardiac valve disease that often progresses to serious secondary complications requiring surgery. MVP manifests as extracellular matrix disorganization and biomechanically incompetent tissues in the adult setting. However, MVP has recently been shown to have a developmental basis, as multiple causal genes expressed during embryonic development have been identified. Disease phenotypes have been observed in mouse models with human MVP mutations as early as birth. This study focuses on the developmental function of DCHS1, one of the first genes to be shown as causal in multiple families with non-syndromic MVP. By using various biochemical techniques as well as mouse and cell culture models, we demonstrate a unique link between DCHS1-based cell adhesions and the septin-actin cytoskeleton through interactions with cytoplasmic protein Lix1-Like (LIX1L). This DCHS1-LIX1L-SEPT9 axis interacts with and promotes filamentous actin organization to direct cell-ECM alignment and valve tissue shape.

CDH5, a Possible New Candidate Gene for Genetic Testing of Lymphedema

Background: Expressed by endothelial cells, CDH5 is a cadherin involved in vascular morphogenesis and in the maintenance of vascular integrity and lymphatic function. The main purpose of our study was to identify distinct variants of the CDH5 gene that could be associated with lymphatic malformations and predisposition for lymphedema. Methods and Results: We performed Next Generation Sequencing of the CDH5 gene in 235 Italian patients diagnosed with lymphedema but who tested negative for variants in known lymphedema genes. We detected six different variants in CDH5 five missense and one nonsense. We also tested available family members of the probands. For family members who carried the same variant as the proband, we performed lymphoscintigraphy to detect any lymphatic system abnormalities. Variants were modeled in silico. The results showed that CDH5 variants may contribute to the onset of lymphedema, although further in vitro studies are needed to confirm this hypothesis. Conclusions: Based on our findings, we propose CDH5 as a new gene that could be screened in patients with lymphedema to gather additional evidence.

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.

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.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

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.

How do the Fat-Dachsous and core planar polarity pathways act together and independently to coordinate polarized cell behaviours?

Planar polarity describes the coordinated polarization of cells within the plane of a tissue. This is controlled by two main pathways in Drosophila: the Frizzled-dependent core planar polarity pathway and the Fat–Dachsous pathway. Components of both of these pathways become asymmetrically localized within cells in response to long-range upstream cues, and form intercellular complexes that link polarity between neighbouring cells. This review examines if and when the two pathways are coupled, focusing on the Drosophila wing, eye and abdomen. There is strong evidence that the pathways are molecularly coupled in tissues that express a specific isoform of the core protein Prickle, namely Spiny-legs. However, in other contexts, the linkages between the pathways are indirect. We discuss how the two pathways act together and independently to mediate a diverse range of effects on polarization of cell structures and behaviours.

Atypical cadherin FAT4 orchestrates lymphatic endothelial cell polarity in response to flow

The atypical cadherin FAT4 has established roles in the regulation of planar cell polarity and Hippo pathway signaling that are cell context dependent. The recent identification of FAT4 mutations in Hennekam syndrome, features of which include lymphedema, lymphangiectasia, and mental retardation, uncovered an important role for FAT4 in the lymphatic vasculature. Hennekam syndrome is also caused by mutations in collagen and calcium binding EGF domains 1 (CCBE1) and ADAM metallopeptidase with thrombospondin type 1 motif 3 (ADAMTS3), encoding a matrix protein and protease, respectively, that regulate activity of the key prolymphangiogenic VEGF-C/VEGFR3 signaling axis by facilitating the proteolytic cleavage and activation of VEGF-C. The fact that FAT4, CCBE1, and ADAMTS3 mutations underlie Hennekam syndrome suggested that all 3 genes might function in a common pathway. We identified FAT4 as a target gene of GATA-binding protein 2 (GATA2), a key transcriptional regulator of lymphatic vascular development and, in particular, lymphatic vessel valve development. Here, we demonstrate that FAT4 functions in a lymphatic endothelial cell-autonomous manner to control cell polarity in response to flow and is required for lymphatic vessel morphogenesis throughout development. Our data reveal a crucial role for FAT4 in lymphangiogenesis and shed light on the mechanistic basis by which FAT4 mutations underlie a human lymphedema syndrome.

Development and aging of the lymphatic vascular system

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

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.

Lymphatic Valves and Lymph Flow in Cancer-Related Lymphedema

Lymphedema is a complex disease caused by the accumulation of fluid in the tissues resulting from a dysfunctional or damaged lymphatic vasculature. In developed countries, lymphedema most commonly occurs as a result of cancer treatment. Initially, impaired lymph flow causes edema, but over time this results in inflammation, fibrotic and fatty tissue deposition, limited mobility, and bacterial infections that can lead to sepsis. While chronically impaired lymph flow is generally believed to be the instigating factor, little is known about what pathophysiological changes occur in the lymphatic vessels to inhibit lymph flow. Lymphatic vessels not only regulate lymph flow through a variety of physiologic mechanisms, but also respond to lymph flow itself. One of the fascinating ways that lymphatic vessels respond to flow is by growing bicuspid valves that close to prevent the backward movement of lymph. However, lymphatic valves have not been investigated in cancer-related lymphedema patients, even though the mutations that cause congenital lymphedema regulate genes involved in valve development. Here, we review current knowledge of the regulation of lymphatic function and development by lymph flow, including newly identified genetic regulators of lymphatic valves, and provide evidence for lymphatic valve involvement in cancer-related lymphedema.

The Lymphatic Vasculature in the 21st Century: Novel Functional Roles in Homeostasis and Disease

Mammals have two specialized vascular circulatory systems: the blood vasculature and the lymphatic vasculature. The lymphatic vasculature is a unidirectional conduit that returns filtered interstitial arterial fluid and tissue metabolites to the blood circulation. It also plays major roles in immune cell trafficking and lipid absorption. As we discuss in this review, the molecular characterization of lymphatic vascular development and our understanding of this vasculature's role in pathophysiological conditions has greatly improved in recent years, changing conventional views about the roles of the lymphatic vasculature in health and disease. Morphological or functional defects in the lymphatic vasculature have now been uncovered in several pathological conditions. We propose that subtle asymptomatic alterations in lymphatic vascular function could underlie the variability seen in the body's response to a wide range of human diseases.

Shear stimulation of FOXC1 and FOXC2 differentially regulates cytoskeletal activity during lymphatic valve maturation

Mutations in the transcription factor FOXC2 are predominately associated with lymphedema. Herein, we demonstrate a key role for related factor FOXC1, in addition to FOXC2, in regulating cytoskeletal activity in lymphatic valves. FOXC1 is induced by laminar, but not oscillatory, shear and inducible, endothelial-specific deletion impaired postnatal lymphatic valve maturation in mice. However, deletion of Foxc2 induced valve degeneration, which is exacerbated in Foxc1; Foxc2 mutants. FOXC1 knockdown (KD) in human lymphatic endothelial cells increased focal adhesions and actin stress fibers whereas FOXC2-KD increased focal adherens and disrupted cell junctions, mediated by increased ROCK activation. ROCK inhibition rescued cytoskeletal or junctional integrity changes induced by inactivation of FOXC1 and FOXC2 invitro and vivo respectively, but only ameliorated valve degeneration in Foxc2 mutants. These results identify both FOXC1 and FOXC2 as mediators of mechanotransduction in the postnatal lymphatic vasculature and posit cytoskeletal signaling as a therapeutic target in lymphatic pathologies.

Biallelic mutation of FBXL7 suggests a novel form of Hennekam syndrome

Hennekam lymphangiectasia-lymphedema syndrome is an autosomal recessive disorder characterized by congenital lymphedema, intestinal lymphangiectasia, facial dysmorphism, and variable intellectual disability. Known disease genes include CCBE1, FAT4, and ADAMTS3. In a patient with clinically diagnosed Hennekam syndrome but without mutations or copy-number changes in the three known disease genes, we identified a homozygous single-exon deletion affecting FBXL7. Specifically, exon 3, which encodes the F-box domain and several leucine-rich repeats of FBXL7, is eliminated. Our analyses of databases representing >100,000 control individuals failed to identify biallelic loss-of-function variants in FBXL7. Published studies in Drosophila indicate Fbxl7 interacts with Fat, of which human FAT4 is an ortholog, and mutation of either gene yields similar morphological consequences. These data suggest that FBXL7 may be the fourth gene for Hennekam syndrome, acting via a shared pathway with FAT4.

Genetics of vascular malformation and therapeutic implications

PURPOSE OF REVIEW

Vascular malformations (VaMs) are a consequence of disrupted morphogenesis that may involve arterial, capillary, venous, or lymphatic endothelium alone or in a combination. VaMs can have serious health impacts, leading to life-threatening conditions sometimes. Genetic mutations affecting proliferation, migration, adhesion, differentiation, and survival of endothelial cells, as well as integrity of extracellular matrix are believed to be the pathogenesis of these disorders. Here, we present an updated review of genetic mutations and potential therapeutic targets for VaMs.

RECENT FINDINGS

Increased number of genetic mutations have been discovered in vascular anomalies via targeted deep sequencing. When a genetic defect is identified, it often presents in only a small percentage of cells within the malformation. In addition, mutations within the same gene may result in different clinical phenotypes. Management of VaMs can be challenging depending on the severity and functional impairment associated. There are no standard treatment algorithms available to date for VaMs, therefore the disorder has significant unmet clinical needs. Currently, the focus of therapeutic development is to target constitutively activated intracellular signaling pathways resulted from genetic mutations.

SUMMARY

Knowledge about the genetic mutations and altered signaling pathways related to VaMs have improved our understanding about the pathogenesis of vascular anomalies and provided insights to the development of new targeted therapies.

The effects of valve leaflet mechanics on lymphatic pumping assessed using numerical simulations

The lymphatic system contains intraluminal leaflet valves that function to bias lymph flow back towards the heart. These valves are present in the collecting lymphatic vessels, which generally have lymphatic muscle cells and can spontaneously pump fluid. Recent studies have shown that the valves are open at rest, can allow some backflow, and are a source of nitric oxide (NO). To investigate how these valves function as a mechanical valve and source of vasoactive species to optimize throughput, we developed a mathematical model that explicitly includes Ca²⁺ -modulated contractions, NO production and valve structures. The 2D lattice Boltzmann model includes an initial lymphatic vessel and a collecting lymphangion embedded in a porous tissue. The lymphangion segment has mechanically-active vessel walls and is flanked by deformable valves. Vessel wall motion is passively affected by fluid pressure, while active contractions are driven by intracellular Ca²⁺ fluxes. The model reproduces NO and Ca²⁺ dynamics, valve motion and fluid drainage from tissue. We find that valve structural properties have dramatic effects on performance, and that valves with a stiffer base and flexible tips produce more stable cycling. In agreement with experimental observations, the valves are a major source of NO. Once initiated, the contractions are spontaneous and self-sustained, and the system exhibits interesting non-linear dynamics. For example, increased fluid pressure in the tissue or decreased lymph pressure at the outlet of the system produces high shear stress and high levels of NO, which inhibits contractions. On the other hand, a high outlet pressure opposes the flow, increasing the luminal pressure and the radius of the vessel, which results in strong contractions in response to mechanical stretch of the wall. We also find that the location of contraction initiation is affected by the extent of backflow through the valves.

Lymphatic and blood systems: Identical or fraternal twins?

Blood and lymphatic systems work in close collaboration to ensure their respective physiological functions. The lymphatic vessel network is being extensively studied, but has been overlooked as compared to the blood vasculature mainly due to the problematic discrimination of lymphatic vessels from the blood ones. This issue has been fortunately resolved in the past decade leading to the emergence of a huge amount of data in lymphatic biology revealing many shared features with the blood vasculature. However, this likeliness between the two vascular systems may lead to a simplistic view of lymphatics and a direct transcription of what is known for the blood system to the lymphatic one, thereby neglecting the lymphatic specificities. In this context, this review aims to clarify the main differences between the two vascular systems focusing on recently discovered lymphatic features.

Human pre-valvular endocardial cells derived from pluripotent stem cells recapitulate cardiac pathophysiological valvulogenesis

Genetically modified mice have advanced our understanding of valve development and disease. Yet, human pathophysiological valvulogenesis remains poorly understood. Here we report that, by combining single cell sequencing and in vivo approaches, a population of human pre-valvular endocardial cells (HPVCs) can be derived from pluripotent stem cells. HPVCs express gene patterns conforming to the E9.0 mouse atrio-ventricular canal (AVC) endocardium signature. HPVCs treated with BMP2, cultured on mouse AVC cushions, or transplanted into the AVC of embryonic mouse hearts, undergo endothelial-to-mesenchymal transition and express markers of valve interstitial cells of different valvular layers, demonstrating cell specificity. Extending this model to patient-specific induced pluripotent stem cells recapitulates features of mitral valve prolapse and identified dysregulation of the SHH pathway. Concurrently increased ECM secretion can be rescued by SHH inhibition, thus providing a putative therapeutic target. In summary, we report a human cell model of valvulogenesis that faithfully recapitulates valve disease in a dish.

There are few human models that can recapitulate valve development in vitro. Here, the authors derive human pre-valvular endocardial cells (HPVCs) from iPSCs and show they can recapitulate early valvulogenesis, and patient derived HPVCs have features of mitral valve prolapse and identified SHH dysregulation.

How many cadherins do human endothelial cells express?

The vasculature is the paradigm of a compartment generated by parallel cellular barriers that aims to transport oxygen, nutrients and immune cells in complex organisms. Vascular barrier dysfunction leads to fatal acute and chronic inflammatory diseases. The endothelial barrier lines the inner side of vessels and is the main regulator of vascular permeability. Cadherins comprise a superfamily of 114 calcium-dependent adhesion proteins that contain conserved cadherin motifs and form cell-cell junctions in metazoans. In mature human endothelial cells, only VE (vascular endothelial)-cadherin and N (neural)-cadherin have been investigated in detail. Although both cadherins are essential for regulating endothelial permeability, no comprehensive expression studies to identify which other family members could play a relevant role in endothelial cells has so far been performed. Here, we have reviewed gene and protein expression databases to analyze cadherin expression in mature human endothelium and found that at least 24 cadherin superfamily members are significantly expressed. Based on data obtained from other cell types, organisms and experimental models, we discuss their potential functions, many of them unrelated to the formation of endothelial cell-cell junctions. The expression of this new set of endothelial cadherins highlights the important but still poorly defined roles of planar cell polarity, the Hippo pathway and mitochondria metabolism in human vascular homeostasis.

Mechanically activated ion channel PIEZO1 is required for lymphatic valve formation

PIEZOs are mechanically activated cation channels. Recently, loss-of-function mutations of human PIEZO1 were found among patients with familial lymphedema, suggesting a requirement of PIEZO1 in the lymphatic system. In this paper, utilizing mouse models lacking PIEZO1 in endothelial cells, we show that this ion channel is required for the formation of lymphatic valves, a key structure for proper circulation of lymph in the body. The requirement of PIEZO1 in valve formation provides mechanistic insight on how PIEZO1 variants cause lymphatic dysfunction in patients. This study also extends the relevance of PIEZOs beyond acute signaling molecules (e.g., touch sensation) and highlights the importance of these ion channels in controlling morphological/structural specification during development.

PIEZO1 is a cation channel that is activated by mechanical forces such as fluid shear stress or membrane stretch. PIEZO1 loss-of-function mutations in patients are associated with congenital lymphedema with pleural effusion. However, the mechanistic link between PIEZO1 function and the development or function of the lymphatic system is currently unknown. Here, we analyzed two mouse lines lacking PIEZO1 in endothelial cells (via Tie2Cre or Lyve1Cre) and found that they exhibited pleural effusion and died postnatally. Strikingly, the number of lymphatic valves was dramatically reduced in these mice. Lymphatic valves are essential for ensuring proper circulation of lymph. Mechanical forces have been implicated in the development of lymphatic vasculature and valve formation, but the identity of mechanosensors involved is unknown. Expression of FOXC2 and NFATc1, transcription factors known to be required for lymphatic valve development, appeared normal in Tie2Cre;Piezo1^cKO mice. However, the process of protrusion in the valve leaflets, which is associated with collective cell migration, actin polymerization, and remodeling of cell–cell junctions, was impaired in Tie2Cre;Piezo1^cKO mice. Consistent with these genetic findings, activation of PIEZO1 by Yoda1 in cultured lymphatic endothelial cells induced active remodeling of actomyosin and VE-cadherin⁺ cell–cell adhesion sites. Our analysis provides evidence that mechanically activated ion channel PIEZO1 is a key regulator of lymphatic valve formation.