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Refereed/Peer-reviewed Despite a growing catalog of secreted factors critical for lymphatic network assembly, little is known about the mechanisms that modulate the expression level of these molecular cues in blood vascular endothelial cells (BECs). Here, we show that a BEC-specific transcription factor, SOX7, plays a crucial role in a non-cell-autonomous manner by modulating the transcription of angiocrine signals to pattern lymphatic vessels. While SOX7 is not expressed in lymphatic endothelial cells (LECs), the conditional loss of SOX7 function in mouse embryos causes a dysmorphic dermal lymphatic phenotype. We identify novel distant regulatory regions in mice and humans that contribute to directly repressing the transcription of a major lymphangiogenic growth factor (Vegfc) in a SOX7-dependent manner. Further, we show that SOX7 directly binds HEY1, a canonical repressor of the Notch pathway, suggesting that transcriptional repression may also be modulated by the recruitment of this protein partner at Vegfc genomic regulatory regions. Our work unveils a role for SOX7 in modulating downstream signaling events crucial for lymphatic patterning, at least in part via the transcriptional repression of VEGFC levels in the blood vascular endothelium.

Refereed/Peer-reviewed Transcriptional enhancer elements are responsible for orchestrating the temporal and spatial control over gene expression that is crucial for programming cell identity during development. Here we describe a novel enhancer element that is important for regulating the expression of Prox1 in lymphatic endothelial cells. This evolutionarily conserved enhancer is bound by key lymphatic transcriptional regulators including GATA2, FOXC2, NFATC1 and PROX1. Genome editing of the enhancer to remove five nucleotides encompassing the GATA2-binding site resulted in perinatal death of homozygous mutant mice due to profound lymphatic vascular defects. Lymphatic endothelial cells in enhancer mutant mice exhibited reduced expression of genes characteristic of lymphatic endothelial cell identity and increased expression of genes characteristic of haemogenic endothelium, and acquired the capacity to generate haematopoietic cells. These data not only reveal a transcriptional enhancer element important for regulating Prox1 expression and lymphatic endothelial cell identity but also demonstrate that the lymphatic endothelium has haemogenic capacity, ordinarily repressed by Prox1.

Refereed/Peer-reviewed Visceral adipose tissue (VAT) encases mesenteric lymphatic vessels and lymph nodes through which lymph is transported from the intestine and mesentery. Whether mesenteric lymphatics contribute to adipose tissue inflammation and metabolism and insulin resistance is unclear. Here we show that obesity is associated with profound and progressive dysfunction of the mesenteric lymphatic system in mice and humans. We find that lymph from mice and humans consuming a high-fat diet (HFD) stimulates lymphatic vessel growth, leading to the formation of highly branched mesenteric lymphatic vessels that ‘leak’ HFD-lymph into VAT and, thereby, promote insulin resistance. Mesenteric lymphatic dysfunction is regulated by cyclooxygenase (COX)-2 and vascular endothelial growth factor (VEGF)-C–VEGF receptor (R)3 signalling. Lymph-targeted inhibition of COX-2 using a glyceride prodrug approach reverses mesenteric lymphatic dysfunction, visceral obesity and inflammation and restores glycaemic control in mice. Targeting obesity-associated mesenteric lymphatic dysfunction thus represents a potential therapeutic option to treat metabolic disease.

Central conducting lymphatic anomaly (CCLA), characterized by the dysfunction of core collecting lymphatic vessels including the thoracic duct and cisterna chyli, and presenting as chylothorax, pleural effusions, chylous ascites, and lymphedema, is a severe disorder often resulting in fetal or perinatal demise. Although pathogenic variants in RAS/mitogen activated protein kinase (MAPK) signaling pathway components have been documented in some patients with CCLA, the genetic etiology of the disorder remains uncharacterized in most cases. Here, we identified biallelic pathogenic variants in

Central conducting lymphatic anomaly (CCLA), characterized by the dysfunction of core collecting lymphatic vessels including the thoracic duct and cisterna chyli, and presenting as chylothorax, pleural effusions, chylous ascites, and lymphedema, is a severe disorder often resulting in fetal or perinatal demise. Although pathogenic variants in RAS/mitogen activated protein kinase (MAPK) signaling pathway components have been documented in some patients with CCLA, the genetic etiology of the disorder remains uncharacterized in most cases. Here, we identified biallelic pathogenic variants in MDFIC , encoding the MyoD family inhibitor domain containing protein, in seven individuals with CCLA from six independent families. Clinical manifestations of affected fetuses and children included nonimmune hydrops fetalis (NIHF), pleural and pericardial effusions, and lymphedema. Generation of a mouse model of human MDFIC truncation variants revealed that homozygous mutant mice died perinatally exhibiting chylothorax. The lymphatic vasculature of homozygous Mdfic mutant mice was profoundly mispatterned and exhibited major defects in lymphatic vessel valve development. Mechanistically, we determined that MDFIC controls collective cell migration, an important early event during the formation of lymphatic vessel valves, by regulating integrin β 1 activation and the interaction between lymphatic endothelial cells and their surrounding extracellular matrix. Our work identifies MDFIC variants underlying human lymphatic disease and reveals a crucial, previously unrecognized role for MDFIC in the lymphatic vasculature. Ultimately, understanding the genetic and mechanistic basis of CCLA will facilitate the development and implementation of new therapeutic approaches to effectively treat this complex disease.

SUMMARYConverging evidence indicates that extra-embryonic yolk sac is the source of both macrophages and endothelial cells in adult mouse tissues. Prevailing views are that these yolk sac-derived cells are maintained after birth by proliferative self-renewal in their differentiated states. Here we identify clonogenic, self-renewing endothelial-macrophage (EndoMac) progenitor cells in postnatal mouse aorta, heart and lung, that are independent of definitive hematopoiesis and derive from a CX3CR1+ and CSF1R+ yolk sac source. These bipotent progenitors are highly proliferative and vasculogenic, contributing to adventitial neovascularization in the aortic wall and forming perfused blood vessels after adoptive transfer into ischemic tissue. We establish a regulatory role for angiotensin II, which enhances their clonogenic, self-renewal and differentiation properties. Our findings demonstrate that tissue-resident EndoMac progenitors drive local inflammatory and vasculogenic responses by contributing to the renewal and expansion of yolk sac-derived macrophages and endothelial cells postnatally.

Despite a growing catalogue of secreted factors critical for lymphatic network assembly, little is known about the mechanisms that modulate the expression level of these molecular cues in blood vascular endothelial cells (BECs). Here, we show that a BEC-specific transcription factor, SOX7, plays a crucial role in lymphatic vessel patterning by modulating the transcription of lymphangiocrine signals. While SOX7 is not expressed in lymphatic endothelial cells (LECs), loss of SOX7 function in mouse embryos causes a dysmorphic dermal lymphatic phenotype. We identify novel distant regulatory regions in mice and humans that contribute to directly repressing the transcription of a major lymphangiogenic growth factor (Vegfc) in a SOX7-dependent manner. Further, we show that SOX7 directly binds HEY1, a canonical repressor of the Notch pathway, suggesting that transcriptional repression may also be modulated by the recruitment of this protein partner at Vegfc genomic regulatory regions. Our work unveils a role for SOX7 in modulating downstream signalling events crucial for lymphatic patterning, at least in part via the transcriptional repression of VEGFC levels in the blood vascular endothelium.

Background: Lymphatic vascular development is regulated by well-characterised signalling and transcriptional pathways. These pathways regulate lymphatic endothelial cell (LEC) migration, motility, polarity and morphogenesis. Canonical and non-canonical WNT signalling 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 signalling 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 signalling 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 signalling during the formation of lymphatic vascular networks. Refereed/Peer-reviewed

Lymphatic vascular development is regulated by well-characterised signalling and transcriptional pathways. These pathways regulate lymphatic endothelial cell (LEC) migration, motility, polarity and and morphogenesis. Canonical and non-canonical WNT signalling 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. Here we find that loss of non-canonical WNT signalling 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 signalling pathway ultimately controls LEC polarity and the morphogenesis of developing vessel networks. Our work suggests that Pkd1 acts at least in part by regulating non-canonical WNT signalling during the formation of lymphatic vascular networks.

Refereed/Peer-reviewed GATA2 deficiency syndrome (G2DS) is a rare autosomal dominant genetic disease predisposing to a range of symptoms, of which myeloid malignancy and immunodeficiency including recurrent infections are most common. In the last decade since it was first reported, there have been over 480 individuals identified carrying a pathogenic or likely pathogenic germline GATA2 variant with symptoms of G2DS, with 240 of these confirmed to be familial and 24 de novo. For those that develop myeloid malignancy (75% of all carriers with G2DS disease symptoms), the median age of onset is 17 years (range 0-78 years) and myelodysplastic syndrome is the first diagnosis in 75% of these cases with acute myeloid leukemia in a further 9%. All variant types appear to predispose to myeloid malignancy and immunodeficiency. Apart from lymphedema in which haploinsufficiency seems necessary, the mutational requirements of the other less common G2DS phenotypes is still unclear. These predominantly loss-of-function variants impact GATA2 expression and function in numerous ways including perturbations to DNA binding, protein structure, protein:protein interactions, and gene transcription, splicing, and expression. In this review, we provide the first expert-curated ACMG/AMP classification with codes of published variants compatible for use in clinical or diagnostic settings.

The lymphatic vasculature is an integral component of the cardiovascular system. It is essential to maintain tissue fluid homeostasis, direct immune cell trafficking and absorb dietary lipids from the digestive tract. Major advances in our understanding of the genetic and cellular events important for constructing the lymphatic vasculature during development have recently been made. These include the identification of novel sources of lymphatic endothelial progenitor cells, the recognition of lymphatic endothelial cell specialisation and heterogeneity, and discovery of novel genes and signalling pathways underpinning developmental lymphangiogenesis. Here, we review these advances and discuss how they inform our understanding of lymphatic network formation, function and dysfunction.

Leukocyte homing driven by the chemokine CCL21 is pivotal for adaptive immunity because it controls dendritic cell (DC) and T cell migration through CCR7. ACKR4 scavenges CCL21 and has been shown to play an essential role in DC trafficking at the steady state and during immune responses to tumors and cutaneous inflammation. However, the mechanism by which ACKR4 regulates peripheral DC migration is unknown, and the extent to which it regulates CCL21 in steady-state skin and lymph nodes (LNs) is contested. Specifically, our previous findings that CCL21 levels are increased in LNs of ACKR4-deficient mice [I. Comerford et al., Blood 116, 4130–4140 (2010)] were refuted [M. H. Ulvmar et al., Nat. Immunol. 15, 623–630 (2014)], and no differences in CCL21 levels in steady-state skin of ACKR4-deficient mice were reported despite compromised CCR7-dependent DC egress in these animals [S. A. Bryce et al., J. Immunol. 196, 3341–3353 (2016)]. Here, we resolve these issues and reveal that two forms of CCL21, full-length immobilized and cleaved soluble CCL21, exist in steady-state barrier tissues, and both are regulated by ACKR4. Without ACKR4, extracellular CCL21 gradients in barrier sites are saturated and nonfunctional, DCs cannot home directly to lymphatic vessels, and excess soluble CCL21 from peripheral tissues pollutes downstream LNs. The results identify the mechanism by which ACKR4 controls DC migration in barrier tissues and reveal a complex mode of CCL21 regulation in vivo, which enhances understanding of functional chemokine gradient formation. Refereed/Peer-reviewed

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 FAT4mutations underlie a human lymphedema syndrome usc Refereed/Peer-reviewed

Mammals have two specialized vascular circulatory systems, the blood vasculature and the lymphatic vasculature. The lymphatic vasculature provides 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.

Visceral adipose tissue (VAT) encases mesenteric lymphatic vessels and lymph nodes through which lymph is transported from the intestine and mesentery. Whether mesenteric lymphatics contribute to adipose tissue inflammation and metabolism and insulin resistance is unclear. Here we show that obesity is associated with profound and progressive dysfunction of the mesenteric lymphatic system in mice and humans. We find that lymph from mice and humans consuming a high-fat diet (HFD) stimulates lymphatic vessel growth, leading to the formation of highly branched mesenteric lymphatic vessels that 'leak' HFD-lymph into VAT and, thereby, promote insulin resistance. Mesenteric lymphatic dysfunction is regulated by cyclooxygenase (COX)-2 and vascular endothelial growth factor (VEGF)-C-VEGF receptor (R)3 signalling. Lymph-targeted inhibition of COX-2 using a glyceride prodrug approach reverses mesenteric lymphatic dysfunction, visceral obesity and inflammation and restores glycaemic control in mice. Targeting obesity-associated mesenteric lymphatic dysfunction thus represents a potential therapeutic option to treat metabolic 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. Refereed/Peer-reviewed

Tissues rely on stem cells (SCs) for homeostasis and wound repair. SCs reside in specialized microenvironments (niches) whose complexities and roles in orchestrating tissue growth are still unfolding. Here, we identify lymphatic capillaries as critical SC-niche components. In skin, lymphatics form intimate networks around hair follicle (HF) SCs. When HFs regenerate, lymphatic-SC connections become dynamic. Using a mouse model, we unravel a secretome switch in SCs that controls lymphatic behavior. Resting SCs express angiopoietin-like protein 7 (Angptl7), promoting lymphatic drainage. Activated SCs switch to Angptl4, triggering transient lymphatic dissociation and reduced drainage. When lymphatics are perturbed or the secretome switch is disrupted, HFs cycle precociously and tissue regeneration becomes asynchronous. In unearthing lymphatic capillaries as a critical SC-niche element, we have learned how SCs coordinate their activity across a tissue.

The importance of lymphatic vessels in a myriad of human diseases is rapidly gaining recognition; lymphatic vessel dysfunction is a feature of disorders including congenital lymphatic anomalies, primary lymphoedema and obesity, while improved lymphatic vessel function increases the efficacy of immunotherapy for cancer and neurological disease and promotes cardiac repair following myocardial infarction. Understanding how the growth and function of lymphatic vessels is precisely regulated therefore stands to inform the development of novel therapeutics applicable to a wide range of human diseases. Lymphatic vascular development is initiated during embryogene-sis following establishment of the major blood vessels and the onset of blood flow. Lymphatic endothelial progenitor cells arise from a combination of venous and non‐venous sources to generate the initial lymphatic vascular structures in the vertebrate embryo, which are then further ramified and remodelled to elaborate an extensive lymphatic vascular network. Signalling mediated via vascular endothelial growth factor (VEGF) family members and vascular endothelial growth factor receptor (VEGFR) tyrosine kinases is crucial for development of both the blood and lymphatic vascular networks, though distinct components are utilised to different degrees in each vascular compart-ment. Although much is known about the regulation of VEGFA/VEGFR2 signalling in the blood vasculature, less is understood regarding the mechanisms by which VEGFC/VEGFD/VEGFR3 signalling is regulated during lymphatic vascular development. This review will focus on recent ad-vances in our understanding of the cellular and molecular mechanisms regulating VEGFA‐, VEGFC‐ and VEGFD‐mediated signalling via VEGFRs which are important for driving the con-struction of lymphatic vessels during development and disease. Refereed/Peer-reviewed

Lymphatic vascular development involves specification of lymphatic endothelial progenitors that subsequently undergo sprouting, proliferation and tissue growth to form a complex second vasculature. The Hippo pathway and effectors Yap and Taz control organ growth and regulate morphogenesis and cellular proliferation. Yap and Taz control angiogenesis but a role in lymphangiogenesis remains to be fully elucidated. Here we show that YAP displays dynamic changes in lymphatic progenitors and Yap1 is essential for lymphatic vascular development in zebrafish. Maternal and Zygotic (MZ) yap1 mutants show normal specification of lymphatic progenitors, abnormal cellular sprouting and reduced numbers of lymphatic progenitors emerging from the cardinal vein during lymphangiogenesis. Furthermore, Yap1 is indispensable for Vegfc-induced proliferation in a transgenic model of Vegfc overexpression. Paracrine Vegfc-signalling ultimately increases nuclear YAP in lymphatic progenitors to control lymphatic development. We thus identify a role for Yap in lymphangiogenesis, acting downstream of Vegfc to promote expansion of this vascular lineage. Refereed/Peer-reviewed

The correct assignment of cell fate within fields of multipotent progenitors is essential for accurate tissue diversification. The first lymphatic vessels arise from pre-existing veins after venous endothelial cells become specified as lymphatic progenitors. Prox1 specifies lymphatic fate and labels these progenitors; however, the mechanisms restricting Prox1 expression and limiting the progenitor pool remain unknown. We identified a zebrafish mutant that displayed premature, expanded, and prolonged lymphatic specification. The gene responsible encodes the regulator of alternative splicing, Nova2. In zebrafish and human endothelial cells, Nova2 selectively regulates pre-mRNA splicing for components of signaling pathways and phosphoproteins. Nova2-deficient endothelial cells display increased Mapk/Erk signaling, and Prox1 expression is dynamically controlled by Erk signaling. We identify a mechanism whereby Nova2-regulated splicing constrains Erk signaling, thus limiting lymphatic progenitor cell specification. This identifies the capacity of a factor that tunes mRNA splicing to control assignment of cell fate during vascular differentiation. Lymphatics arise from progenitor cells that are specified along embryonic cardinal veins. How progenitor number is limited remains unknown. In this issue of Developmental Cell, Baek et al. show that Nova2 regulates a splicing program to limit vascular Flt4/Mapk/Erk signaling upstream of the master regulator of lymphatic fate, Prox1. Refereed/Peer-reviewed

The lymphatic vasculature is an integral component of the immune system. Lymphatic vessels are a key highway via which immune cells are trafficked, serving not simply as a passive route of transport, but to actively shape and coordinate immune responses. Reciprocally, immune cells provide signals that impact the growth, development, and activity of the lymphatic vasculature. In addition to immune cell trafficking, lymphatic vessels are crucial for fluid homeostasis and lipid absorption. The field of lymphatic vascular research is rapidly expanding, fuelled by rapidly advancing technology that has enabled the manipulation and imaging of lymphatic vessels, together with an increasing recognition of the involvement of lymphatic vessels in a myriad of human pathologies. In this review we provide an overview of the genetic pathways and cellular processes important for development and maturation of the lymphatic vasculature, discuss recent work revealing important roles for the lymphatic vasculature in directing immune cell traffic and coordinating immune responses and highlight the involvement of lymphatic vessels in a range of pathological settings. Refereed/Peer-reviewed

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

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

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

The adrenal medulla is composed of neuroendocrine chromaffin cells that secrete adrenaline into the systemic circulation to maintain physiological homeostasis and enable the autonomic stress response. How chromaffin cell precursors colonise the adrenal medulla and how they become connected to central nervous system-derived preganglionic sympathetic neurons remain largely unknown. By combining lineage tracing, gene expression studies, genetic ablation and the analysis of mouse mutants, we demonstrate that preganglionic axons direct chromaffin cell precursors into the adrenal primordia. We further show that preganglionic axons and chromaffin cell precursors require class 3 semaphorin (SEMA3) signalling through neuropilins (NRP) to target the adrenal medulla. Thus, SEMA3 proteins serve as guidance cues to control formation of the adrenal neuroendocrine system by establishing appropriate connections between preganglionic neurons and adrenal chromaffin cells that regulate the autonomic stress response., Summary: A new role is revealed for semaphorin/neuropilin signalling in guiding preganglionic sympathetic axons and chromaffin cell precursors into the adrenal primordia, ensuring correct regulation of the autonomic stress response.

Lymphatic vessels serve crucial roles in the regulation of tissue fluid homeostasis, dietary lipid absorption and immune cell trafficking. Defects in lymphatic vessel morphogenesis and function have been associated with lymphedema, obesity, hypertension and tumour metastasis. Morphogenetic events important for construction of the lymphatic vasculature during development include the specification and emergence of lymphatic endothelial progenitor cells, their differentiation and assembly into interconnected vessels and vascular remodeling, ultimately giving rise to a functional vascular network. Despite the embryonic origins of lymphatic endothelial progenitor cells being long debated, work performed over the last decade had overwhelmingly supported at least a great majority of progenitor cells arising from the venous vasculature. Here, we review the most recent advances in the field of lymphatic vessel morphogenesis, with a focus on studies that have identified novel sources of embryonic lymphatic endothelial progenitor cells, together with the cellular mechanisms by which lymphatic vessels are initially assembled.

Heterozygous germline mutations in the zinc finger transcription factor GATA2 have recently been shown to underlie a range of clinical phenotypes, including Emberger syndrome, a disorder characterized by lymphedema and predisposition to myelodysplastic syndrome/acute myeloid leukemia (MDS/AML). Despite well-defined roles in hematopoiesis, the functionsof GATA2 in the lymphatic vasculature and the mechanisms by which GATA2 mutations result in lymphedema have not been characterized. Here, we have provided a molecular explanation for lymphedema predisposition in a subset of patients with germline GATA2 mutations. Specifically, we demonstrated that Emberger-associated GATA2 missense mutations result in complete loss of GATA2 function, with respect to the capacity to regulate the transcription of genes that are important for lymphatic vessel valve development. We identified a putative enhancer element upstream of the key lymphatic transcriptional regulator PROX1 that is bound by GATA2, and the transcription factors FOXC2 and NFATC1. Emberger GATA2missense mutants had a profoundly reduced capacity to bind this element. Conditional Gata2 deletion in mice revealed that GATA2 is required for both development and maintenance of lymphovenous and lymphatic vessel valves. Together, our data unveil essential roles for GATA2 in the lymphatic vasculature and explain why a select catalogue of human GATA2 mutations results in lymphedema Refereed/Peer-reviewed

Lymphatic vessels are an integral component of the cardiovascular system, serving important roles in fluid homeostasis, lipid absorption, and immune cell trafficking. Defining the mechanisms by which the lymphatic vasculature is constructed and remodeled into a functional vascular network not only provides answers to fascinating biological questions, but is fundamental to understanding how lymphatic vessel growth and development goes awry in human pathologies. While long recognized as dysfunctional in lymphedema and exploited as a route of tumor metastasis, recent work has highlighted important roles for lymphatic vessels in modulating immune responses, regulating salt-sensitive hypertension and important for lung inflation at birth. Substantial progress in our understanding of the signaling pathways important for development and morphogenesis of the lymphatic vasculature has been made in recent years. Here, we review advances in our knowledge of the best characterized of these signaling pathways, that involving the vascular endothelial growth factor (VEGF) family members VEGF-C and VEGF-D, together with their receptors VEGFR2 and VEGFR3. Recent work has defined multiple levels at which signal transduction by means of this key axis is regulated; these include control of ligand processing and bioavailability, modulation of receptor activation by interacting proteins, and regulation of receptor endocytosis and trafficking. Developmental Dynamics 244:323–331, 2015. © 2014 Wiley Periodicals, Inc.

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

DEPDC5 mutations have recently been shown to cause epilepsy in humans. Evidence from in vitro studies has implicated DEPDC5 as a negative regulator of mTORC1 during amino acid insufficiency as part of the GATOR1 complex. To investigate the role of DEPDC5 in vivo we generated a null mouse model using targeted CRISPR mutagenesis. Depdc5 homozygotes display severe phenotypic defects between 12.5-15.5 dpc, including hypotrophy, anaemia, oedema, and cranial dysmorphology as well as blood and lymphatic vascular defects. mTORC1 hyperactivity was observed in the brain of knockout embryos and in fibroblasts and neurospheres isolated from knockout embryos and cultured in nutrient deprived conditions. Heterozygous mice appeared to be normal and we found no evidence of increased susceptibility to seizures or tumorigenesis. Together, these data support mTORC1 hyperactivation as the likely pathogenic mechanism that underpins DEPDC5 loss of function in humans and highlights the potential utility of mTORC1 inhibitors in the treatment of DEPDC5-associated epilepsy.

The integration of multiple morphogenic signalling pathways and transcription factor networks is essential to mediate neural crest (NC) cell induction, delamination, survival, stem-cell properties, fate choice and differentiation. Although the transcriptional control of NC development is well documented in mammals, the role of post-transcriptional modifications, and in particular ubiquitination, has not been explored. Here we report an essential role for the ubiquitin ligase Nedd4 in cranial NC cell development. Our analysis of Nedd4−/− embryos identified profound deficiency of cranial NC cells in the absence of structural defects in the neural tube. Nedd4 is expressed in migrating cranial NC cells and was found to positively regulate expression of the NC transcription factors Sox9, Sox10 and FoxD3. We found that in the absence of these factors, a subset of cranial NC cells undergo apoptosis. In accordance with a lack of cranial NC cells, Nedd4−/− embryos have deficiency of the trigeminal ganglia, NC derived bone and malformation of the craniofacial skeleton. Our analyses therefore uncover an essential role for Nedd4 in a subset of cranial NC cells and highlight E3 ubiquitin ligases as a likely point of convergence for multiple NC signalling pathways and transcription factor networks. Refereed/Peer-reviewed

Mutations in SOX18, VEGFC and Vascular Endothelial Growth Factor 3 underlie the hereditary lymphatic disorders hypotrichosis-lymphedema-telangiectasia (HLT), Milroy-like lymphedema and Milroy disease, respectively. Genes responsible for hereditary lymphedema are key regulators of lymphatic vascular development in the embryo. To identify novel modulators of lymphangiogenesis, we used a mouse model of HLT (Ragged Opossum) and performed gene expression profiling of aberrant dermal lymphatic vessels. Expression studies and functional analysis in zebrafish and mice revealed one candidate, ArfGAP with RhoGAP domain, Ankyrin repeat and PH domain 3 (ARAP3), which is down-regulated in HLT mouse lymphatic vessels and necessary for lymphatic vascular development in mice and zebrafish. We position this known regulator of cell behaviour during migration as a mediator of the cellular response to Vegfc signalling in lymphatic endothelial cells in vitro and in vivo. Our data refine common mechanisms that are likely to contribute during both development and the pathogenesis of lymphatic vascular disorders. Refereed/Peer-reviewed

Impaired cardiac muscle growth and aberrant myocyte arrangement underlie congenital heart disease and cardiomyopathy. We show that cardiac-specific inactivation of the murine homeobox transcription factor Prox1 results in the disruption of expression and localisation of sarcomeric proteins, gross myofibril disarray and growth-retarded hearts. Furthermore, we demonstrate that Prox1 is required for direct transcriptional regulation of the genes encoding the structural proteins α-actinin, N-RAP and zyxin, which collectively function to maintain an actin-α-actinin interaction as the fundamental association of the sarcomere. Aspects of abnormal heart development and the manifestation of a subset of muscular-based disease have previously been attributed to mutations in key structural proteins. Our study reveals an essential requirement for direct transcriptional regulation of sarcomere integrity, in the context of enabling foetal cardiomyocyte hypertrophy, maintenance of contractile function and progression towards inherited or acquired myopathic disease. Refereed/Peer-reviewed

Mature megakaryocytes depend on the function of Bcl-x(L), a member of the Bcl-2 family of prosurvival proteins, to proceed safely through the process of platelet shedding. Despite this, loss of Bcl-x(L) does not prevent the growth and maturation of megakaryocytes, suggesting redundancy with other prosurvival proteins. We therefore generated mice with a megakaryocyte-specific deletion of Mcl-1, which is known to be expressed in megakaryocytes. Megakaryopoiesis, platelet production, and platelet lifespan were unperturbed in Mcl-1(Pf4Δ/Pf4Δ) animals. However, treatment with ABT-737, a BH3 mimetic compound that inhibits the prosurvival proteins Bcl-2, Bcl-x(L), and Bcl-w resulted in the complete ablation of megakaryocytes and platelets. Genetic deletion of both Mcl-1 and Bcl-x(L) in megakaryocytes resulted in preweaning lethality. Megakaryopoiesis in Bcl-x(Pf4Δ/Pf4Δ) Mcl-1(Pf4Δ/Pf4Δ) embryos was severely compromised, and these animals exhibited ectopic bleeding. Our studies indicate that the combination of Bcl-x(L) and Mcl-1 is essential for the viability of the megakaryocyte lineage. Sylvia and Charles Viertel Foundation Leukemia & Lymphoma Society Refereed/Peer-reviewed

Recent work has established that heterozygous germline GATA2 mutations predispose carriers to familial myelodysplastic syndrome (MDS)/acute myeloid leukemia (AML), “MonoMAC” syndrome, and DCML deficiency. Here, we describe a previously unreported MDS family carrying a missense GATA2 mutation (p.Thr354Met), one patient with MDS/AML carrying a frameshift GATA2 mutation (p.Leu332Thrfs*53), another with MDS harboring a GATA2 splice site mutation, and 3 patients exhibiting MDS or MDS/AML who have large deletions encompassing the GATA2 locus. Intriguingly, 2 MDS/AML or “MonoMAC” syndrome patients with GATA2 deletions and one with a frameshift mutation also have primary lymphedema. Primary lymphedema occurs as a result of aberrations in the development and/or function of lymphatic vessels, spurring us to investigate whether GATA2 plays a role in the lymphatic vasculature. We demonstrate here that GATA2 protein is present at high levels in lymphatic vessel valves and that GATA2 controls the expression of genes important for programming lymphatic valve development. Our data expand the phenotypes associated with germline GATA2 mutations to include predisposition to primary lymphedema and suggest that complete haploinsufficiency or loss of function of GATA2, rather than missense mutations, is the key predisposing factor for lymphedema onset. Moreover, we reveal a crucial role for GATA2 in lymphatic vascular development.

Despite observations of a link between lymphatic vessels and lipids that date as far back as 300, a link between lymphatic vessels and adipose tissue has only recently been recognized. This review will summarize documented evidence that supports a close relationship between lymphatic vessels and adipose tissue biology. Lymphatic vessels mediate lipid absorption and transport, share an intimate spatial association with adipose tissue, and regulate the traffic of immune cells that rely on specialized adipose tissue depots as a reservior of energy deployed to fight infection. Important links between inflammation and adipose tissue biology will also be discussed in this article, as will recent evidence connecting lymphatic vascular dysfunction with the onset of obesity. There seems little doubt that future research in this topical field will ensure that the link between lymphatic vascular function and adipose tissue is firmly established.

The ability to target gene deletion to a specific cellular compartment via the Cre/loxP system has been a powerful tool in the analysis of broadly expressed genes. Here, we report the generation of a transgenic mouse line in which expression of Cre-recombinase is under the regulatory control of the VE-Cadherin promoter. Temporal distribution and activity of the enzyme was evaluated with two independent Cre reporter lines. Histological analysis was performed throughout development and in the adult. Recombination of lox P sites with subsequent expression of beta-galactosidase or GFP was detected as early as E7.5 in endothelial cells of the yolk sac. Progressive staining of the embryonic vasculature was noted from E8.5-13.5; however, more contiguous reporter expression was only seen by E14.5 onward in all endothelial compartments including arteries, veins, and capillaries. In addition, we found Cre activity in lymphatic endothelial cells. Unlike other endothelial-specific Cre mice, this model showed expression in the adult quiescent vasculature. Furthermore, the constitutive nature of the VE-Cadherin promoter in the adult can be advantageous for analysis of gene deletion in pathological settings.

Within the vascular system, the mucin-type transmembrane glycoprotein T1alpha/podoplanin is predominantly expressed by lymphatic endothelium, and recent studies have shown that it is regulated by the lymphatic-specific homeobox gene Prox1. In this study, we examined the role of T1alpha/podoplanin in vascular development and the effects of gene disruption in mice. T1alpha/podoplanin is first expressed at around E11.0 in Prox1-positive lymphatic progenitor cells, with predominant localization in the luminal plasma membrane of lymphatic endothelial cells during later development. T1alpha/podoplanin(-/-) mice die at birth due to respiratory failure and have defects in lymphatic, but not blood vessel pattern formation. These defects are associated with diminished lymphatic transport, congenital lymphedema and dilation of lymphatic vessels. T1alpha/podoplanin is also expressed in the basal epidermis of newborn wild-type mice, but gene disruption did not alter epidermal differentiation. Studies in cultured endothelial cells indicate that T1alpha/podoplanin promotes cell adhesion, migration and tube formation, whereas small interfering RNA-mediated inhibition of T1alpha/podoplanin expression decreased lymphatic endothelial cell adhesion. These data identify T1alpha/podoplanin as a novel critical player that regulates different key aspects of lymphatic vasculature formation.

In mammals, the lymphatic vascular system develops by budding of lymphatic progenitor endothelial cells from embryonic veins to form a distinct network of draining vessels with important functions in the immune response and in cancer metastasis. However, the lineage-specific molecular characteristics of blood vascular versus lymphatic endothelium have remained poorly defined. We isolated lymphatic endothelial cells (LECs) and blood vascular endothelial cells (BVECs) by immunomagnetic isolation directly from human skin. Cultured LECs but not BVECs expressed the lymphatic markers Prox1 and LYVE-1 and formed LYVE-1-positive vascular tubes after implantation in vivo. Transcriptional profiling studies revealed increased expression of several extracellular matrix and adhesion molecules in BVECs, including versican, collagens, laminin, and N-cadherin, and of the growth factor receptors endoglin and vascular endothelial growth factor receptor-1/Flt-1. Differential immunostains of human skin confirmed the blood vessel-specific expression of these genes. During embryonic development, endoglin expression was gradually down-regulated on lymphatic endothelium whereas vascular endothelial growth factor receptor-1 was absent from lymphatics. We also identified several genes with specific expression in LECs. These results demonstrate that some lineage-specific genes are only expressed during distinct developmental stages and they identify new molecular markers for blood vascular and lymphatic endothelium with important implications for future studies of vascular development and function.