Pdgfrb-Cre targets lymphatic endothelial cells of both venous and non-venous origins

Vascular architecture regulates mesenchymal stromal cell heterogeneity via P53-PDGF signaling in the mouse incisor

Mesenchymal stem cells (MSCs) reside in niches to maintain tissue homeostasis and contribute to repair and regeneration. Although the physiological functions of blood and lymphatic vasculature are well studied, their regulation of MSCs as niche components remains largely unknown. Using adult mouse incisors as a model, we uncover the role of Trp53 in regulating vascular composition through THBS2 to maintain mesenchymal tissue homeostasis. Loss of Trp53 in GLI1+ progeny increases arteries and decreases other vessel types. Platelet-derived growth factors from arteries deposit in the MSC region and interact with PDGFRA and PDGFRB. Significantly, PDGFRA+ and PDGFRB+ cells differentially contribute to defined cell lineages in the adult mouse incisor. Collectively, our results highlight Trp53's importance in regulating the vascular niche for MSCs. They also shed light on how different arterial cells provide unique cues to regulate MSC subpopulations and maintain their heterogeneity. Furthermore, they provide mechanistic insight into MSC-vasculature crosstalk.

Identification of distinct vascular mural cell populations during zebrafish embryonic development

BACKGROUND

Mural cells are an essential perivascular cell population that associate with blood vessels and contribute to vascular stabilization and tone. In the embryonic zebrafish vasculature, pdgfrb and tagln are commonly used as markers for identifying pericytes and vascular smooth muscle cells. However, the overlapping and distinct expression patterns of these markers in tandem have not been fully described.

RESULTS

Here, we used the Tg(pdgfrb:Gal4FF; UAS:RFP) and Tg(tagln:NLS-EGFP) transgenic lines to identify single- and double-positive perivascular cell populations on the cranial, axial, and intersegmental vessels between 1 and 5 days postfertilization. From this comparative analysis, we discovered two novel regions of tagln-positive cell populations that have the potential to function as mural cell precursors. Specifically, we found that the hypochord-a reportedly transient structure-contributes to tagln-positive cells along the dorsal aorta. We also identified a unique mural cell progenitor population that resides along the midline between the neural tube and notochord and contributes to intersegmental vessel mural cell coverage.

CONCLUSION

Together, our findings highlight the variability and versatility of tracking both pdgfrb and tagln expression in mural cells of the developing zebrafish embryo and reveal unexpected embryonic cell populations that express pdgfrb and tagln.

Alkali injury-induced pathological lymphangiogenesis in the iris facilitates the infiltration of T cells and ocular inflammation

Inflammatory lymphangiogenesis is intimately linked to immune regulation and tissue homeostasis. However, current evidence has suggested that classic lymphatic vessels are physiologically absent in intraocular structures. Here, we show that neolymphatic vessels were induced in the iris after corneal alkali injury (CAI) in a VEGFR3-dependent manner. Cre-loxP-based lineage tracing revealed that these lymphatic endothelial cells (LECs) originate from existing Prox1+ lymphatic vessels. Notably, the ablation of iridial lymphangiogenesis via conditional deletion of VEGFR3 alleviated the ocular inflammatory response and pathological T cell infiltration. Our findings demonstrate that iridial neolymphatics actively participate in pathological immune responses following injury and suggest intraocular lymphangiogenesis as a valuable therapeutic target for the treatment of ocular inflammation.

Neuropilin-1 regulates renin synthesis in juxtaglomerular cells

Renin is the key enzyme of the systemic renin-angiotensin-aldosterone system, which plays an essential role in regulating blood pressure and maintaining electrolyte and extracellular volume homeostasis. Renin is mainly produced and secreted by specialized juxtaglomerular (JG) cells in the kidney. In the present study, we report for the first time that the conserved transmembrane receptor neuropilin-1 (NRP1) participates in the development of JG cells and plays a key role in renin production. We used the myelin protein zero-Cre (P0-Cre) to abrogate Nrp1 constitutively in P0-Cre lineage-labelled cells of the kidney. We found that the P0-Cre precursor cells differentiate into renin-producing JG cells. We employed a lineage-tracing strategy combined with RNAscope quantification and metabolic studies to reveal a cell-autonomous role for NRP1 in JG cell function. Nrp1-deficient animals displayed abnormal levels of tissue renin expression and failed to adapt properly to a homeostatic challenge to sodium balance. These findings provide new insights into cell fate decisions and cellular plasticity operating in P0-Cre-expressing precursors and identify NRP1 as a novel key regulator of JG cell maturation. KEY POINTS: Renin is a centrepiece of the renin-angiotensin-aldosterone system and is produced by specialized juxtaglomerular cells (JG) of the kidney. Neuropilin-1 (NRP1) is a conserved membrane-bound receptor that regulates vascular and neuronal development, cancer aggressiveness and fibrosis progression. We used conditional mutagenesis and lineage tracing to show that NRP1 is expressed in JG cells where it regulates their function. Cell-specific Nrp1 knockout mice present with renin paucity in JG cells and struggle to adapt to a homeostatic challenge to sodium balance. The results support the versatility of renin-producing cells in the kidney and may open new avenues for therapeutic approaches.

Tissue-specific Cre driver mice to study vascular diseases

Vascular diseases, including atherosclerosis and abdominal aneurysms, are the primary cause of mortality and morbidity among the elderly worldwide. The life quality of patients is significantly compromised due to inadequate therapeutic approaches and limited drug targets. To expand our comprehension of vascular diseases, gene knockout (KO) mice, especially conditional knockout (cKO) mice, are widely used for investigating gene function and mechanisms of action. The Cre-loxP system is the most common method for generating cKO mice. Numerous Cre driver mice have been established to study the main cell types that compose blood vessels, including endothelial cells, smooth muscle cells, and fibroblasts. Here, we first discuss the characteristics of each layer of the arterial wall. Next, we provide an overview of the representative Cre driver mice utilized for each of the major cell types in the vessel wall and their most recent applications in vascular biology. We then go over Cre toxicity and discuss the practical methods for minimizing Cre interference in experimental outcomes. Finally, we look into the future of tissue-specific Cre drivers by introducing the revolutionary single-cell RNA sequencing and dual recombinase system.

Stretch regulates alveologenesis and homeostasis via mesenchymal Gαq/11-mediated TGFβ2 activation

Alveolar development and repair require tight spatiotemporal regulation of numerous signalling pathways that are influenced by chemical and mechanical stimuli. Mesenchymal cells play key roles in numerous developmental processes. Transforming growth factor-β (TGFβ) is essential for alveologenesis and lung repair, and the G protein α subunits Gαq and Gα11 (Gαq/11) transmit mechanical and chemical signals to activate TGFβ in epithelial cells. To understand the role of mesenchymal Gαq/11 in lung development, we generated constitutive (Pdgfrb-Cre+/-;Gnaqfl/fl;Gna11-/-) and inducible (Pdgfrb-Cre/ERT2+/-;Gnaqfl/fl;Gna11-/-) mesenchymal Gαq/11 deleted mice. Mice with constitutive Gαq/11 gene deletion exhibited abnormal alveolar development, with suppressed myofibroblast differentiation, altered mesenchymal cell synthetic function, and reduced lung TGFβ2 deposition, as well as kidney abnormalities. Tamoxifen-induced mesenchymal Gαq/11 gene deletion in adult mice resulted in emphysema associated with reduced TGFβ2 and elastin deposition. Cyclical mechanical stretch-induced TGFβ activation required Gαq/11 signalling and serine protease activity, but was independent of integrins, suggesting an isoform-specific role for TGFβ2 in this model. These data highlight a previously undescribed mechanism of cyclical stretch-induced Gαq/11-dependent TGFβ2 signalling in mesenchymal cells, which is imperative for normal alveologenesis and maintenance of lung homeostasis.

The ebb and flow of cardiac lymphatics: a tidal wave of new discoveries

The heart is imbued with a vast lymphatic network that is responsible for fluid homeostasis and immune cell trafficking. Disturbances in the forces that regulate microvascular fluid movement can result in myocardial edema, which has profibrotic and proinflammatory consequences and contributes to cardiovascular dysfunction. This review explores the complex relationship between cardiac lymphatics, myocardial edema, and cardiac disease. It covers the revised paradigm of microvascular forces and fluid movement around the capillary as well as the arsenal of preclinical tools and animal models used to model myocardial edema and cardiac disease. Clinical studies of myocardial edema and their prognostic significance are examined in parallel to the recent elegant animal studies discerning the pathophysiological role and therapeutic potential of cardiac lymphatics in different cardiovascular disease models. This review highlights the outstanding questions of interest to both basic scientists and clinicians regarding the roles of cardiac lymphatics in health and disease.

PDGFRβ+ cells play a dual role as hematopoietic precursors and niche cells during mouse ontogeny

Hematopoietic stem cell (HSC) generation in the aorta-gonad-mesonephros region requires HSC specification signals from the surrounding microenvironment. In zebrafish, PDGF-B/PDGFRβ signaling controls hematopoietic stem/progenitor cell (HSPC) generation and is required in the HSC specification niche. Little is known about murine HSPC specification in vivo and whether PDGF-B/PDGFRβ is involved. Here, we show that PDGFRβ is expressed in distinct perivascular stromal cell layers surrounding the mid-gestation dorsal aorta, and its deletion impairs hematopoiesis. We demonstrate that PDGFRβ⁺ cells play a dual role in murine hematopoiesis. They act in the aortic niche to support HSPCs, and in addition, PDGFRβ⁺ embryonic precursors give rise to a subset of HSPCs that persist into adulthood. These findings provide crucial information for the controlled production of HSPCs in vitro.

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PDGFRβ deletion affects hematopoietic development in the AGM in vivo

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The transcriptome and hematopoietic support of the PDGFRβ-KO niche are altered

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The osteogenic gene profile and differentiation of KO AGM MSCs are affected

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PDGFRβ⁺ early embryonic precursors contribute to EC and HSPC lineages in vivo

Emerging roles for CNS fibroblasts in health, injury and disease

Recent transcriptomic, histological and functional studies have begun to shine light on the fibroblasts present in the meninges, choroid plexus and perivascular spaces of the brain and spinal cord. Although the origins and functions of CNS fibroblasts are still being described, it is clear that they represent a distinct cell population, or populations, that have likely been confused with other cell types on the basis of the expression of overlapping cellular markers. Recent work has revealed that fibroblasts play crucial roles in fibrotic scar formation in the CNS after injury and inflammation, which have also been attributed to other perivascular cell types such as pericytes and vascular smooth muscle cells. In this Review, we describe the current knowledge of the location and identity of CNS perivascular cell types, with a particular focus on CNS fibroblasts, including their origin, subtypes, roles in health and disease, and future areas for study.

Lymphatic Clearance of Immune Cells in Cardiovascular Disease

Recent advances in our understanding of the lymphatic system, its function, development, and role in pathophysiology have changed our views on its importance. Historically thought to be solely involved in the transport of tissue fluid, lipids, and immune cells, the lymphatic system displays great heterogeneity and plasticity and is actively involved in immune cell regulation. Interference in any of these processes can be deleterious, both at the developmental and adult level. Preclinical studies into the cardiac lymphatic system have shown that invoking lymphangiogenesis and enhancing immune cell trafficking in ischaemic hearts can reduce myocardial oedema, reduce inflammation, and improve cardiac outcome. Understanding how immune cells and the lymphatic endothelium interact is also vital to understanding how the lymphatic vascular network can be manipulated to improve immune cell clearance. In this Review, we examine the different types of immune cells involved in fibrotic repair following myocardial infarction. We also discuss the development and function of the cardiac lymphatic vasculature and how some immune cells interact with the lymphatic endothelium in the heart. Finally, we establish how promoting lymphangiogenesis is now a prime therapeutic target for reducing immune cell persistence, inflammation, and oedema to restore heart function in ischaemic heart disease.

Patterns of differentiation of renin lineage cells during nephrogenesis

Developmentally heterogeneous renin expressing cells serve as progenitors for mural, glomerular and tubular cells during nephrogenesis and are collectively termed renin lineage cells (RLCs). In this study, we quantified different renal vascular and tubular cell types based on specific markers, assessed proliferation, and de-novo differentiation in the RLC population. We used kidney sections of mRenCre-mT/mG mice throughout nephrogenesis. Marker positivity was evaluated in whole digitalized sections. At embryonic day 16, RLCs appeared in the developing kidney, and expression of all stained markers in RLCs was observed. The proliferation rate of RLCs did not differ from the proliferation rate of non-RLCs. The RLCs expanded mainly by de-novo differentiation (neogenesis). The fractions of RLCs originating from the stromal progenitors of the metanephric mesenchyme (renin producing cells, vascular smooth muscle cells, mesangial cells) decreased during nephrogenesis. In contrast, aquaporin 2 positive RLCs in the collecting duct system that embryonically emerges almost exclusively from the ureteric bud, expanded postpartum. The cubilin positive RLC fraction in the proximal tubule, deriving from the cap mesenchyme, remained constant. During nephrogenesis, RLCs were continuously detectable in the vascular and tubular compartments of the kidney. Therein, various patterns of RLC differentiation that depend on the embryonic origin of the cells were identified.

Endothelial ontogeny and the establishment of vascular heterogeneity

The establishment of distinct cellular identities was pivotal during the evolution of Metazoa, enabling the emergence of an array of specialized tissues with different functions. In most animals including vertebrates, cell specialization occurs in response to a combination of intrinsic (e.g., cellular ontogeny) and extrinsic (e.g., local environment) factors that drive the acquisition of unique characteristics at the single-cell level. The first functional organ system to form in vertebrates is the cardiovascular system, which is lined by a network of endothelial cells whose organ-specific characteristics have long been recognized. Recent genetic analyses at the single-cell level have revealed that heterogeneity exists not only at the organ level but also between neighboring endothelial cells. Thus, how endothelial heterogeneity is established has become a key question in vascular biology. Drawing upon evidence from multiple organ systems, here we will discuss the role that lineage history may play in establishing endothelial heterogeneity.

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.

Mechanisms and cell lineages in lymphatic vascular development

Lymphatic vessels have critical roles in both health and disease and their study is a rapidly evolving area of vascular biology. The consensus on how the first lymphatic vessels arise in the developing embryo has recently shifted. Originally, they were thought to solely derive by sprouting from veins. Since then, several studies have uncovered novel cellular mechanisms and a diversity of contributing cell lineages in the formation of organ lymphatic vasculature. Here, we review the key mechanisms and cell lineages contributing to lymphatic development, discuss the advantages and limitations of experimental techniques used for their study and highlight remaining knowledge gaps that require urgent attention. Emerging technologies should accelerate our understanding of how lymphatic vessels develop normally and how they contribute to disease.

Formation and Growth of Cardiac Lymphatics during Embryonic Development, Heart Regeneration, and Disease

The lymphatic system plays crucial roles in regulating fluid homeostasis, immune surveillance, and lipid transport. As is in most of the body's organs, the heart possesses an extensive lymphatic network. Moreover, a robust lymphangiogenic response has been shown to take place following myocardial infarction, highlighting cardiac lymphatics as potential targets for therapeutic intervention. Yet, the unique molecular properties and functions of the heart's lymphatic system have only recently begun to be addressed. In this review, we discuss the mechanisms underlying the formation and growth of cardiac lymphatics during embryonic development and describe their characteristics across species. We further summarize recent findings highlighting diverse cellular origins for cardiac lymphatic endothelial cells and how they integrate to form a single functional lymphatic network. Finally, we outline novel therapeutic avenues aimed at enhancing lymphatic vessel formation and integrity following cardiac injury, which hold great promise for promoting healing of the infarcted heart.

Characterizing Lymphangiogenesis and Concurrent Inflammation in Adipose Tissue in Response to VEGF-D

The metabolic consequences of obesity arise from local inflammation within expanding adipose tissue. In pre-clinical studies targeting various inflammatory factors, systemic metabolism can be improved through reduced adipose inflammation. Lymphatic vessels are a critical regulator of inflammation through roles in fluid and macromolecule transport and immune cell trafficking and immunomodulation. Lymphangiogenesis, the expansion of the lymphatic network, is often a necessary step in restoring tissue homeostasis. Using Adipo-VD mice, a model of adipocyte-specific, inducible overexpression of the potent lymphangiogenic factor vascular endothelial growth factor-D (VEGF-D), we previously identified that dense de novo adipose lymphatics reduced immune accumulation and improved glucose homeostasis in obesity. On chow diet, however, Adipo-VD mice demonstrated increased adipose tissue immune cells, fibrosis, and inflammation. Here, we characterize the time course of resident macrophage accumulation and lymphangiogenesis in male and female Adipo-VD mice fed chow and high fat diets, examining multiple adipose depots over 4 months. We find that macrophage infiltration occurs early, but resolves with concurrent lymphatic expansion that begins robustly after 1 month of VEGF-D overexpression in white adipose tissue. In obesity, female Adipo-VD mice exhibit reduced lymphangiogenesis and maintain a more glycolytic metabolism compared to Adipo-VD males and their littermates. Adipose lymphatic structures appear to expand by a lymphvasculogenic mechanism involving lymphatic endothelial cell proliferation and organization with a cell source we that failed to identify; hematopoietic cells afford minimal structural contribution. While a net positive effect occurs in Adipo-VD mice, adipose tissue lymphangiogenesis demonstrates a dichotomous, and time-dependent, inflammatory tissue remodeling response.

A Second Heart Field-Derived Vasculogenic Niche Contributes to Cardiac Lymphatics

The mammalian heart contains multiple cell types that appear progressively during embryonic development. Advance in determining cardiac lineage diversification has often been limited by the unreliability of genetic tracers. Here we combine clonal analysis, genetic lineage tracing, tissue transplantation, and mutant characterization to investigate the lineage relationships between epicardium, arterial mesothelial cells (AMCs), and the coronary vasculature. We report a contribution of the second heart field (SHF) to a vasculogenic niche composed of AMCs and sub-mesothelial cells at the base of the pulmonary artery. Sub-mesothelial cells from this niche differentiate into lymphatic endothelial cells and, in close association with AMC-derived cells, contribute to and are essential for the development of ventral cardiac lymphatics. In addition, regionalized epicardial/mesothelial retinoic acid signaling regulates lymphangiogenesis, contributing to the niche properties. These results uncover a SHF vasculogenic contribution to coronary lymphatic development through a local niche at the base of the great arteries.

Cardiac lymphatics in health and disease

The lymphatic vasculature, which accompanies the blood vasculature in most organs, is indispensable in the maintenance of tissue fluid homeostasis, immune cell trafficking, and nutritional lipid uptake and transport, as well as in reverse cholesterol transport. In this Review, we discuss the physiological role of the lymphatic system in the heart in the maintenance of cardiac health and describe alterations in lymphatic structure and function that occur in cardiovascular pathology, including atherosclerosis and myocardial infarction. We also briefly discuss the role that immune cells might have in the regulation of lymphatic growth (lymphangiogenesis) and function. Finally, we provide examples of how the cardiac lymphatics can be targeted therapeutically to restore lymphatic drainage in the heart to limit myocardial oedema and chronic inflammation.

The deployment of cell lineages that form the mammalian heart

The function of the mammalian heart depends on the interplay between different cardiac cell types. The deployment of these cells, with precise spatiotemporal regulation, is also important during development to establish the heart structure. In this Review, we discuss the diverse origins of cardiac cell types and the lineage relationships between cells of a given type that contribute to different parts of the heart. The emerging lineage tree shows the progression of cell fate diversification, with patterning cues preceding cell type segregation, as well as points of convergence, with overlapping lineages contributing to a given tissue. Several cell lineage markers have been identified. However, caution is required with genetic-tracing experiments in comparison with clonal analyses. Genetic studies on cell populations provided insights into the mechanisms for lineage decisions. In the past 3 years, results of single-cell transcriptomics are beginning to reveal cell heterogeneity and early developmental trajectories. Equating this information with the in vivo location of cells and their lineage history is a current challenge. Characterization of the progenitor cells that form the heart and of the gene regulatory networks that control their deployment is of major importance for understanding the origin of congenital heart malformations and for producing cardiac tissue for use in regenerative medicine.

Development of mouse models of angiosarcoma driven by p53

Angiosarcomas are a rare group of tumours which have poor prognosis and limited treatment options. The development of new therapies has been hampered by a lack of good preclinical models. Here, we describe the development of an autochthonous mouse model of angiosarcoma driven by loss of p53 in VE-cadherin-expressing endothelial cells. Using Cdh5-Cre to drive recombination in adult endothelial cells, mice developed angiosarcomas with 100% penetrance upon homozygous deletion of Trp53 with a median lifespan of 325 days. In contrast, expression of the R172H mutant p53 resulted in formation of thymic lymphomas with a more rapid onset (median lifespan 151 days). We also used Pdgfrb-Cre-expressing mice, allowing us to target predominantly pericytes, as these have been reported as the cell of origin for a number of soft tissue sarcomas. Pdgfrb-Cre also results in low levels of recombination in venous blood endothelial cells in multiple tissues during development. Upon deletion of Trp53 in Pdgfrb-Cre-expressing mice (Pdgfrb-Cre,Trp53fl/fl mice), 65% developed lymphomas and 21% developed pleomorphic undifferentiated soft tissue sarcomas. None developed angiosarcomas. In contrast, 75% of Pdgfrb-Cre,Trp53R172H/R172H mice developed angiosarcomas, with 60% of these mice also developing lymphomas. The median lifespan of the Pdgfrb-Cre,Trp53R172H/R172H mice was 151 days. Re-implantation of angiosarcoma tumour fragments from Cdh5-Cre, Trp53fl/fl mice provided a more consistent and rapid model of angiosarcoma than the two spontaneous models. The ability to passage tumour fragments through the mouse provides a novel model which is amenable to preclinical studies and will help the development of potential new therapies for angiosarcoma.

The Cdh5-CreERT2 transgene causes conditional Shb gene deletion in hematopoietic cells with consequences for immune cell responses to tumors

The tamoxifen-responsive conditional Cdh5-CreERT2 is commonly used for endothelial cell specific conditional deletion of loxP-flanked gene sequences. To address the role of endothelial cell Shb gene for B16F10 melanoma immune responses, tamoxifen-injected Cdh5-CreERT2/WT and Cdh5-CreERT2/Shb^flox/flox mice received subcutaneous tumor cell injections. We observed a decrease of tumor myeloid cell Shb mRNA in the tamoxifen treated Cdh5-CreERT2/Shb^flox/flox mice, which was not present when the mice had undergone a preceding bone marrow transplantation using wild type bone marrow. Differences in CD4+/FoxP3+ Tregs were similarly abolished by a preceding bone marrow transplantation. In ROSA26-mTmG mice, Cdh5-CreERT2 caused detectable floxing in certain bone marrow populations and in spleen cells. Floxing in bone marrow could be detected two months after tamoxifen treatment. In the spleen, however, floxing was undetectable two months after tamoxifen treatment, suggesting that Cdh5-CreERT2 is operating in a non-renewable population of hematopoietic cells in this organ. These data suggest that conditional gene deletion in hematopoietic cells is a potential confounder in experiments attempting to assess the role of endothelial specific effects. A cautious approach to achieve an endothelial-specific phenotype would be to adopt a strategy that includes a preceding bone marrow transplantation.

Zebrafish facial lymphatics develop through sequential addition of venous and non-venous progenitors

Lymphatic vessels are known to be derived from veins; however, recent lineage-tracing experiments propose that specific lymphatic networks may originate from both venous and non-venous sources. Despite this, direct evidence of a non-venous lymphatic progenitor is missing. Here, we show that the zebrafish facial lymphatic network is derived from three distinct progenitor populations that add sequentially to the developing facial lymphatic through a relay-like mechanism. We show that while two facial lymphatic progenitor populations are venous in origin, the third population, termed the ventral aorta lymphangioblast (VA-L), does not sprout from a vessel; instead, it arises from a migratory angioblast cell near the ventral aorta that initially lacks both venous and lymphatic markers, and contributes to the facial lymphatics and the hypobranchial artery. We propose that sequential addition of venous and non-venous progenitors allows the facial lymphatics to form in an area that is relatively devoid of veins. Overall, this study provides conclusive, live imaging-based evidence of a non-venous lymphatic progenitor and demonstrates that the origin and development of lymphatic vessels is context-dependent.

Functions and Regeneration of Mature Cardiac Lymphatic Vessels in Atherosclerosis, Myocardial Infarction, and Heart Failure

Cardiac lymphatic vessels play a vital role in maintaining cardiac homeostasis both under physiological and pathological conditions. Clearer illustration of the anatomy of cardiac lymphatics has been achieved by fluorescence exhibition comparing to dye injection. Besides, identification of specific lymphatic markers in recent decades paves the way for researches in development and regeneration of cardiac lymphatics, such as VEGF-C/VEGFR-3, EphB4/ephrin-B2, Prox-1, Podoplanin, and Lyve-1. Knocking out each of these markers in mice model also reveals the signaling pathways instructing the formation of cardiac lymphatics. In the major cardiovascular disease series of atherosclerosis, myocardial infarction (MI), and heart failure, cardiac lymphatics regulate the transportation of extravasated proteins and lipids, inflammatory and immune responses, as well as fluid balance. Elementary intervention methods, such as lymphatic factor protein injection VEGF-C, are applied in murine MI models to restore or enhance functions of lymphatic vessels, achieving improvements in cardiac function. Also, data from our laboratory showed that intramyocardial EphB4 injection also improved lymphatic regeneration in mouse MI model. Therefore, we believe that enhancing functions and regeneration of mature cardiac lymphatic vessels in cardiovascular diseases is of great potential therapeutic meaning in the future.

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

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

Lymphatic endothelial progenitor cells: origins and roles in lymphangiogenesis

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

Genetic Lineage Tracing of Lymphatic Endothelial Cells in Mice

Lineage tracing allows for identification of all progeny produced by a single cell or groups of cells and can thus be used to assess developmental fate of cells. Here we focus on one of the most widely used lineage tracing approaches that utilize the Cre/loxP system for site-specific genetic recombination in studying the developmental origins of lymphatic endothelial cells (LECs) in the mouse embryo. We discuss general considerations for a successful Cre/loxP based lineage tracing experiment and provide information about strains that are available for genetic lineage tracing of LECs. A protocol for lineage tracing analysis of the lymphatic vasculature by whole-mount immunofluorescence in two embryonic tissues, the skin and the mesentery, is also provided.

Mechanotransduction in Blood and Lymphatic Vascular Development and Disease

The blood and lymphatic vasculatures are hierarchical networks of vessels, which constantly transport fluids and, therefore, are exposed to a variety of mechanical forces. Considering the role of mechanotransduction is key for fully understanding how these vascular systems develop, function, and how vascular pathologies evolve. During embryonic development, for example, initiation of blood flow is essential for early vascular remodeling, and increased interstitial fluid pressure as well as initiation of lymph flow is needed for proper development and maturation of the lymphatic vasculature. In this review, we introduce specific mechanical forces that affect both the blood and lymphatic vasculatures, including longitudinal and circumferential stretch, as well as shear stress. In addition, we provide an overview of the role of mechanotransduction during atherosclerosis and secondary lymphedema, which both trigger tissue fibrosis.

PDGFRβ-P2A-CreERT2 mice: a genetic tool to target pericytes in angiogenesis

Pericytes are essential mural cells distinguished by their association with small caliber blood vessels and the presence of a basement membrane shared with endothelial cells. Pericyte interaction with the endothelium plays an important role in angiogenesis; however, very few tools are currently available that allow for the targeting of pericytes in mouse models, limiting our ability to understand their biology. We have generated a novel mouse line expressing tamoxifen-inducible Cre-recombinase under the control of the platelet-derived growth factor receptor β promoter: PDGFRβ-P2A-CreER ^T2 . We evaluated the expression of the PDGFRβ-P2A-CreER ^T2 line by crossing it with fluorescent reporter lines and analyzed reporter signal in the angiogenic retina and brain at different time points after tamoxifen administration. Reporter lines showed labeling of NG2⁺, desmin⁺, PDGFRβ⁺ perivascular cells in the retina and the brain, indicating successful targeting of pericytes; however, signal from reporter lines was also observed in a small subset of glial cells both in the retina and the brain. We also evaluated recombination in tumors and found efficient recombination in perivascular cells associated with tumor vasculature. As a proof of principle, we used our newly generated driver to delete Notch signaling in perivascular cells and observed a loss of smooth muscle cells in retinal arteries, consistent with previously published studies evaluating Notch3 null mice. We conclude that the PDGFRβ-P2A-CreER ^T2 line is a powerful new tool to target pericytes and will aid the field in gaining a deeper understanding of the role of these cells in physiological and pathological settings.

Cardiac Lymphatics - A New Avenue for Therapeutics?

Recent progress in lymphatic vessel biology and in novel imaging techniques has established the importance of the lymphatic vasculature as part of the cardiovascular system. The lymphatic vessel network regulates many physiological processes important for heart function such as fluid balance, transport of extravasated proteins, and trafficking of immune cells. Therefore, lymphangiogenic therapy could be beneficial in the treatment of cardiovascular diseases, for example by improving reverse cholesterol transport (RCT) from atherosclerotic lesions or by resolving edema and fibrosis after myocardial infarction. In this review we first describe recent findings on the development and function of cardiac lymphatic vessels, and subsequently focus on the prospects of pro- and anti-lymphangiogenic therapies in cardiovascular diseases.

Neurogenic Radial Glia-like Cells in Meninges Migrate and Differentiate into Functionally Integrated Neurons in the Neonatal Cortex

Whether new neurons are added in the postnatal cerebral cortex is still debated. Here, we report that the meninges of perinatal mice contain a population of neurogenic progenitors formed during embryonic development that migrate to the caudal cortex and differentiate into Satb2⁺ neurons in cortical layers II-IV. The resulting neurons are electrically functional and integrated into local microcircuits. Single-cell RNA sequencing identified meningeal cells with distinct transcriptome signatures characteristic of (1) neurogenic radial glia-like cells (resembling neural stem cells in the SVZ), (2) neuronal cells, and (3) a cell type with an intermediate phenotype, possibly representing radial glia-like meningeal cells differentiating to neuronal cells. Thus, we have identified a pool of embryonically derived radial glia-like cells present in the meninges that migrate and differentiate into functional neurons in the neonatal cerebral cortex.

Heterogeneity in the lymphatic vascular system and its origin

Lymphatic vessels have historically been viewed as passive conduits for fluid and immune cells, but this perspective is increasingly being revised as new functions of lymphatic vessels are revealed. Emerging evidence shows that lymphatic endothelium takes an active part in immune regulation both by antigen presentation and expression of immunomodulatory genes. In addition, lymphatic vessels play an important role in uptake of dietary fat and clearance of cholesterol from peripheral tissues, and they have been implicated in obesity and arteriosclerosis. Lymphatic vessels within different organs and in different physiological and pathological processes show a remarkable plasticity and heterogeneity, reflecting their functional specialization. In addition, lymphatic endothelial cells (LECs) of different organs were recently shown to have alternative developmental origins, which may contribute to the development of the diverse lymphatic vessel and endothelial functions seen in the adult. Here, we discuss recent developments in the understanding of heterogeneity within the lymphatic system considering the organ-specific functional and molecular specialization of LECs and their developmental origin.