Mechanisms and cell lineages in lymphatic vascular development

Engineering blood and lymphatic microvascular networks

The human vasculature plays a crucial role in the blood supply of nearly all organs as well as the drainage of the interstitial fluid. Consequently, if these physiological systems go awry, pathological changes might occur. Hence, the regeneration of existing vessels, as well as approaches to engineer artificial blood and lymphatic structures represent current challenges within the field of vascular research. In this review, we provide an overview of both the vascular blood circulation and the long-time neglected but equally important lymphatic system, with regard to their organotypic vasculature. We summarize the current knowledge within the field of vascular tissue engineering focusing on the design of co-culture systems, thereby mainly discussing suitable cell types, scaffold design and disease models. This review will mainly focus on addressing those subjects concerning atherosclerosis. Moreover, current technological approaches such as vascular organ-on-a-chip models and microfluidic devices will be discussed.

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

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

Ontogenesis of the Mouse Ocular Surface Lymphatic Vascular Network

Purpose

Ocular lymphatic vessels play major physiological role in eye homeostasis and their dysfunction can contribute to the progression of several eye diseases. In this study, we characterized their spatiotemporal development and the cellular mechanisms occurring during their ontogenesis in the mouse eye.

Methods

Whole mount immunofluorescent staining and imaging by standard or lightsheet fluorescence microscopy were performed on late embryonic and early postnatal eye mouse samples.

Results

We observed that the ocular surface lymphatic vascular network develops at the early postnatal stages (between P0 and P5) from two nascent trunks arising at the nasal side on both sides of the nictitating membrane. These nascent vessels further branch and encircle the whole eye surface by sprouting lymphangiogenesis. In addition, we got evidence for the existence of a transient lymphvasculogenesis process generating lymphatic vessel fragments that will mostly formed the corneolimbal lymphatic vasculature which further connect to the conjunctival lymphatic network. Our results also support that CD206-positive macrophages can transdifferentiate and then integrate into the lymphatic neovessels.

Conclusions

Several complementary cellular processes participate in the development of the lymphatic ocular surface vasculature. This knowledge paves the way for the design of new therapeutic strategies to interfere with ocular lymphatic vessel formation when needed.

The modes of angiogenesis: an updated perspective

Following the process of vasculogenesis during development, angiogenesis generates new vascular structures through a variety of different mechanisms or modes. These different modes of angiogenesis involve, for example, increasing microvasculature density by sprouting of endothelial cells, splitting of vessels to increase vascular surface area by intussusceptive angiogenesis, fusion of capillaries to increase blood flow by coalescent angiogenesis, and the recruitment of non-endothelial cells by vasculogenic mimicry. The recent reporting on coalescent angiogenesis as a new mode of vessel formation warrants a brief overview of angiogenesis mechanisms to provide a more complete picture. The journal Angiogenesis is devoted to the delineation of the different modes and mechanisms that collectively dictate blood vessel formation, inhibition, and function in health and disease.

Understanding the development, pathogenesis, and injury response of meningeal lymphatic networks through the use of animal models

Meningeal lymphatic vessels (MLVs) help maintain central nervous system (CNS) homeostasis via their ability to facilitate macromolecule waste clearance and neuroimmune trafficking. Although these vessels were overlooked for centuries, they have now been characterized in humans, non-human primates, and rodents. Recent studies in mice have explored the stereotyped growth and expansion of MLVs in dura mater, the various transcriptional, signaling, and environmental factors regulating their development and long-term maintenance, and the pathological changes these vessels undergo in injury, disease, or with aging. Key insights gained from these studies have also been leveraged to develop therapeutic approaches that help augment or restore MLV functions to improve brain health and cognition. Here, we review fundamental processes that control the development of peripheral lymphatic networks and how these might apply to the growth and expansion of MLVs in their unique meningeal environment. We also emphasize key findings in injury and disease models that may reveal additional insights into the plasticity of these vessels throughout the lifespan. Finally, we highlight unanswered questions and future areas of study that can further reveal the exciting therapeutic potential of meningeal lymphatics.

Single-nuclei multiomic analyses identify human cardiac lymphatic endothelial cells associated with coronary arteries in the epicardium

Cardiac lymphatic vessels play important roles in fluid homeostasis, inflammation, disease, and regeneration of the heart. The developing cardiac lymphatics in human fetal hearts are closely associated with coronary arteries, similar to those in zebrafish hearts. We identify a population of cardiac lymphatic endothelial cells (LECs) that reside in the epicardium. Single-nuclei multiomic analysis of the human fetal heart reveals the plasticity and heterogeneity of the cardiac endothelium. Furthermore, we find that VEGFC is highly expressed in arterial endothelial cells and epicardium-derived cells, providing a molecular basis for the arterial association of cardiac lymphatic development. Using a cell-type-specific integrative analysis, we identify a population of cardiac lymphatic endothelial cells marked by the PROX1 and the lymphangiocrine RELN and enriched in binding motifs of erythroblast transformation specific (ETS) variant (ETV) transcription factors. We report the in vivo molecular characterization of human cardiac lymphatics and provide a valuable resource to understand fetal heart development.

Pathological angiogenesis: mechanisms and therapeutic strategies

In multicellular organisms, angiogenesis, the formation of new blood vessels from pre-existing ones, is an essential process for growth and development. Different mechanisms such as vasculogenesis, sprouting, intussusceptive, and coalescent angiogenesis, as well as vessel co-option, vasculogenic mimicry and lymphangiogenesis, underlie the formation of new vasculature. In many pathological conditions, such as cancer, atherosclerosis, arthritis, psoriasis, endometriosis, obesity and SARS-CoV-2(COVID-19), developmental angiogenic processes are recapitulated, but are often done so without the normal feedback mechanisms that regulate the ordinary spatial and temporal patterns of blood vessel formation. Thus, pathological angiogenesis presents new challenges yet new opportunities for the design of vascular-directed therapies. Here, we provide an overview of recent insights into blood vessel development and highlight novel therapeutic strategies that promote or inhibit the process of angiogenesis to stabilize, reverse, or even halt disease progression. In our review, we will also explore several additional aspects (the angiogenic switch, hypoxia, angiocrine signals, endothelial plasticity, vessel normalization, and endothelial cell anergy) that operate in parallel to canonical angiogenesis mechanisms and speculate how these processes may also be targeted with anti-angiogenic or vascular-directed therapies.

The lymphatic vasculature in lung function and respiratory disease

The lymphatic vasculature maintains tissue homeostasis via fluid drainage in the form of lymph and immune surveillance due to migration of leukocytes through the lymphatics to the draining lymph nodes. Lymphatic endothelial cells (LECs) form the lymphatic vessels and lymph node sinuses and are key players in shaping immune responses and tolerance. In the healthy lung, the vast majority of lymphatic vessels are found along the bronchovascular structures, in the interlobular septa, and in the subpleural space. Previous studies in both mice and humans have shown that the lymphatics are necessary for lung function from the neonatal period through adulthood. Furthermore, changes in the lymphatic vasculature are observed in nearly all respiratory diseases in which they have been analyzed. Recent work has pointed to a causative role for lymphatic dysfunction in the initiation and progression of lung disease, indicating that these vessels may be active players in pathologic processes in the lung. However, the mechanisms by which defects in lung lymphatic function are pathogenic are understudied, leaving many unanswered questions. A more comprehensive understanding of the mechanistic role of morphological, functional, and molecular changes in the lung lymphatic endothelium in respiratory diseases is a promising area of research that is likely to lead to novel therapeutic targets. In this review, we will discuss our current knowledge of the structure and function of the lung lymphatics and the role of these vessels in lung homeostasis and respiratory disease.

Current Status of Lymphangiogenesis: Molecular Mechanism, Immune Tolerance, and Application Prospect

The lymphatic system is a channel for fluid transport and cell migration, but it has always been controversial in promoting and suppressing cancer. VEGFC/VEGFR3 signaling has long been recognized as a major molecular driver of lymphangiogenesis. However, many studies have shown that the neural network of lymphatic signaling is complex. Lymphatic vessels have been found to play an essential role in the immune regulation of tumor metastasis and cardiac repair. This review describes the effects of lipid metabolism, extracellular vesicles, and flow shear forces on lymphangiogenesis. Moreover, the pro-tumor immune tolerance function of lymphatic vessels is discussed, and the tasks of meningeal lymphatic vessels and cardiac lymphatic vessels in diseases are further discussed. Finally, the value of conversion therapy targeting the lymphatic system is introduced from the perspective of immunotherapy and pro-lymphatic biomaterials for lymphangiogenesis.

Lymphangiogenesis Guidance Mechanisms and Therapeutic Implications in Pathological States of the Cornea

Corneal lymphangiogenesis is one component of the neovascularization observed in several inflammatory pathologies of the cornea including dry eye disease and corneal graft rejection. Following injury, corneal (lymph)angiogenic privilege is impaired, allowing ingrowth of blood and lymphatic vessels into the previously avascular cornea. While the mechanisms underlying pathological corneal hemangiogenesis have been well described, knowledge of the lymphangiogenesis guidance mechanisms in the cornea is relatively scarce. Various signaling pathways are involved in lymphangiogenesis guidance in general, each influencing one or multiple stages of lymphatic vessel development. Most endogenous factors that guide corneal lymphatic vessel growth or regression act via the vascular endothelial growth factor C signaling pathway, a central regulator of lymphangiogenesis. Several exogenous factors have recently been repurposed and shown to regulate corneal lymphangiogenesis, uncovering unique signaling pathways not previously known to influence lymphatic vessel guidance. A strong understanding of the relevant lymphangiogenesis guidance mechanisms can facilitate the development of targeted anti-lymphangiogenic therapeutics for corneal pathologies. In this review, we examine the current knowledge of lymphatic guidance cues, their regulation of inflammatory states in the cornea, and recently discovered anti-lymphangiogenic therapeutic modalities.

Cdh5-lineage-independent origin of dermal lymphatics shown by temporally restricted lineage tracing

Temporally restricted lineage tracing reveals a non-venous source of dermal lymphatic vessels and highlights Cre induction strategy as a critical parameter for stage-specific cell labeling.

The developmental origins of lymphatic endothelial cells (LECs) have been under intense research after a century-long debate. Although previously thought to be of solely venous endothelial origin, additional sources of LECs were recently identified in multiple tissues in mice. Here, we investigated the regional differences in the origin(s) of the dermal lymphatic vasculature by lineage tracing using the pan-endothelial Cdh5-CreER^T2 line. Tamoxifen-induced labeling of blood ECs at E9.5, before initiation of lymphatic development, traced most of the dermal LECs but with lower efficiency in the lumbar compared with the cervical skin. By contrast, when used at E9.5 but not at E11.5, 4-hydroxytamoxifen, the active metabolite of tamoxifen that provides a tighter window of Cre activity, revealed low labeling frequency of LECs, and lymphvasculogenic clusters in the lumbar skin in particular. Temporally restricted lineage tracing thus reveals contribution of LECs of Cdh5-lineage–independent origin to dermal lymphatic vasculature. Our results further highlight Cre induction strategy as a critical parameter in defining the temporal window for stage-specific lineage tracing during early developmental stages of rapid tissue differentiation.

The rising impact of angiogenesis research

While inhibiting pathological angiogenesis has been long associated with the field of oncology, recent advances in angiogenesis research have impacted the progress of disease treatment for additional non-malignant diseases or chronic conditions in the fields of ophthalmology, cardiology, and gynecology. Moreover, stimulators of angiogenesis find application in ischemic diseases, while inhibitors of angiogenesis are being used to limit blood vessel formation, but in judicious ways that modify or "reprogram" the vasculature as a reinforcement for immunotherapy. We have noticed an increasing impact, as evidenced by increases in the total number of citations, in the literature surrounding the angiogenesis field suggesting that targeting angiogenesis per se is well established as a tractable approach for therapy in diverse conditions.

Proper migration of lymphatic endothelial cells requires survival and guidance cues from arterial mural cells

The migration of lymphatic endothelial cells (LECs) is key for the development of the complex and vast lymphatic vascular network that pervades most tissues in an organism. In zebrafish, arterial intersegmental vessels together with chemokines have been shown to promote lymphatic cell migration from the horizontal myoseptum (HM). We observed that emergence of mural cells around the intersegmental arteries coincides with lymphatic departure from HM which raised the possibility that arterial mural cells promote LEC migration. Our live imaging and cell ablation experiments revealed that LECs migrate slower and fail to establish the lymphatic vascular network in the absence of arterial mural cells. We determined that mural cells are a source for the C-X-C motif chemokine 12 (Cxcl12a and Cxcl12b), vascular endothelial growth factor C (Vegfc) and collagen and calcium-binding EGF domain-containing protein 1 (Ccbe1). We showed that chemokine and growth factor signalling function cooperatively to induce robust LEC migration. Specifically, Vegfc-Vegfr3 signalling, but not chemokines, induces extracellular signal-regulated kinase (ERK) activation in LECs, and has an additional pro-survival role in LECs during the migration. Together, the identification of mural cells as a source for signals that guide LEC migration and survival will be important in the future design for rebuilding lymphatic vessels in disease contexts.

Angiopoietin-2-induced lymphatic endothelial cell migration drives lymphangiogenesis via the β1 integrin-RhoA-formin axis

Lymphangiogenesis is an essential physiological process but also a determining factor in vascular-related pathological conditions. Angiopoietin-2 (Ang2) plays an important role in lymphatic vascular development and function and its upregulation has been reported in several vascular-related diseases, including cancer. Given the established role of the small GTPase RhoA on cytoskeleton-dependent endothelial functions, we investigated the relationship between RhoA and Ang2-induced cellular activities. This study shows that Ang2-driven human dermal lymphatic endothelial cell migration depends on RhoA. We demonstrate that Ang2-induced migration is independent of the Tie receptors, but dependent on β1 integrin-mediated RhoA activation with knockdown, pharmacological approaches, and protein sequencing experiments. Although the key proteins downstream of RhoA, Rho kinase (ROCK) and myosin light chain, were activated, blockade of ROCK did not abrogate the Ang2-driven migratory effect. However, formins, an alternative target of RhoA, were identified as key players, and especially FHOD1. The Ang2-RhoA relationship was explored in vivo, where lymphatic endothelial RhoA deficiency blocked Ang2-induced lymphangiogenesis, highlighting RhoA as an important target for anti-lymphangiogenic treatments.

Homeostatic maintenance of the lymphatic vasculature

The lymphatic vasculature is emerging as a multifaceted regulator of tissue homeostasis and regeneration. Lymphatic vessels drain fluid, macromolecules, and immune cells from peripheral tissues to lymph nodes (LNs) and the systemic circulation. Their recently uncovered functions extend beyond drainage and include direct modulation of adaptive immunity and paracrine regulation of organ growth. The developmental mechanisms controlling lymphatic vessel growth have been described with increasing precision. It is less clear how the essential functional features of lymphatic vessels are established and maintained. We discuss the mechanisms that maintain lymphatic vessel integrity in adult tissues and control vessel repair and regeneration. This knowledge is crucial for understanding the pathological vessel changes that contribute to disease, and provides an opportunity for therapy development.

Lymphatics in Eye Fluid Homeostasis: Minor Contributors or Significant Actors?

Lymphatic vessels exert major effects on the maintenance of interstitial fluid homeostasis, immune cell trafficking, lipid absorption, tumor progression and metastasis. Recently, novel functional roles for the lymphatic vasculature have emerged, which can be associated with pathological situations. Among them, lymphatics have been proposed to participate in eye aqueous humor drainage, with potential consequences on intraocular pressure, a main risk factor for progression of glaucoma disease. In this review, after the description of eye fluid dynamics, we provide an update on the data concerning the distribution of ocular lymphatics. Particular attention is given to the results of investigations allowing the three dimensional visualization of the ocular surface vasculature, and to the molecular mechanisms that have been characterized to regulate ocular lymphatic vessel development. The studies concerning the potential role of lymphatics in aqueous humor outflow are reported and discussed. We also considered the novel studies mentioning the existence of an ocular glymphatic system which may have, in connection with lymphatics, important repercussions in retinal clearance and in diseases affecting the eye posterior segment. Some remaining unsolved questions and new directions to explore are proposed to improve the knowledge about both lymphatic and glymphatic system interactions with eye fluid homeostasis.