Lymphangiogenesis: Origin, Specification, and Cell Fate Determination

A pluripotent stem cell model of Emberger syndrome reveals reduced lymphatic endothelial differentiation

Emberger syndrome (ES), an autosomal dominant disorder characterized by congenital deafness, primary lymphedema, and predisposition to myeloid malignancies, is caused by mutations in the GATA2 gene. Although primary lymphedema is an important hallmark of ES, the pathophysiology remains unclear due to the lack of a suitable experimental model. In this study, we isolated induced pluripotent stem cells (iPSCs) from two patients with ES (i.e., ES-iPSCs) and analyzed their in vitro lymphatic differentiation potential via the mesodermal progenitor stage. KDR+ CD34+ early mesodermal progenitors generated from either ES-iPSCs or wild-type iPSCs during a 6-days serum- and feeder-free culture supplemented with bone morphogenetic protein 4 and vascular endothelial growth factor (VEGF) had almost equivalent developmental potential. However, upon co-culture with OP9 stromal cells, KDR+ CD34+ cells derived from ES-iPSCs developed into CD31+ lymphatic vessel endothelial hyaluronan receptor-1+ VEGF receptor 3+ lymphatic endothelial cells less efficiently than KDR+ CD34+ cells derived from wild-type iPSCs. Thus, patient-derived iPSCs recapitulate impairments at an early stage of lymphangiogenesis, making them a useful experimental tool for dissecting the pathophysiology of primary lymphedema in ES and developing potential therapeutic approaches.

Meningeal lymphatic drainage: novel insights into central nervous system disease

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

Lymphatic endothelial mTORC1 instructs metabolic and developmental signaling during lymphangiogenesis

The lymphatic vasculature comprises lymphatic capillaries and collecting vessels. To support lymphatic development, lymphatic endothelial cells (LECs) utilize nutrients to fuel lymphangiogenic processes. Meanwhile, LECs maintain constant prospero homeobox 1 (PROX1) expression critical for lymphatic specification. However, molecular mechanisms orchestrating nutrient metabolism while sustaining PROX1 levels in LECs remain unclear. Here, we show that loss of RAPTOR, an indispensable mechanistic target of rapamycin complex 1 (mTORC1) component, downregulates PROX1 and impairs lymphatic capillary growth and differentiation of collecting lymphatics in mice. Mechanistically, mTORC1 inhibition in mouse and human LECs causes Myc reduction, which decreases hexokinase 2 (HK2) and glutaminase (GLS), inhibiting glycolysis and glutaminolysis. Myc or HK2/GLS ablation impedes lymphatic capillary and collecting vessel formation. Interestingly, mTORC1 regulation of PROX1 is independent of Myc-HK2/GLS signaling. Moreover, genetic interaction analysis indicates that Myc and PROX1 play crucial roles in mTORC1-regulated lymphatic development. Collectively, our findings identify mTORC1 as a key regulator of metabolic programs and PROX1 expression during lymphangiogenesis.

Treatment of Pathological Lymphangiogenesis via Circular RNA-Mediated Cholesterol Metabolism Remodeling

Purpose

Lymphangiogenesis plays important roles in the pathogenesis of human diseases, including inflammatory ocular diseases, metabolic diseases, and cancers, by affecting lipid metabolism and immune homeostasis. Despite growing evidence showing that circular RNAs (circRNAs) act as the regulators of inflammatory and metabolic pathways, their roles in lymphatic dysfunction remain unclear. This study aims to explore the involvement of circRNA-KIF6 (cKIF6) in pathological lymphangiogenesis and elucidate the underlying mechanism.

Methods

The cKIF6 expression was evaluated in mouse-sutured corneas and lymphatic endothelial cells (LECs) isolated from juvenile foreskin under inflammatory conditions. Functional assays, including viability, proliferation, migration, and tube formation, were conducted on LECs after cKIF6 silencing. Lymphangiogenesis was evaluated using mouse-sutured cornea and Matrigel plug models. Mechanistic studies explored the role of cKIF6 as a molecular sponge for miR-582 and its downstream effect on methylsterol monooxygenase 1 (MSMO1).

Results

The cKIF6 expression was significantly elevated in mouse-sutured corneas and inflamed LECs. Silencing cKIF6 impaired LEC viability, proliferation, migration, and tube formation, leading to reduced lymphangiogenesis in both in vitro and in vivo models. Mechanistically, cKIF6 acted as a miR-582 sponge, resulting in elevated MSMO1 expression and increased cholesterol content in LECs. The augmented proliferation, migration, and tube formation abilities of cKIF6-overexpressing LECs were attenuated by the inhibitor of cholesterol biosynthesis.

Conclusions

The cKIF6 regulates lymphangiogenesis by targeting cholesterol metabolism, making it a promising therapeutic target for lymphangiogenesis-related diseases.

Meningeal lymphatic vessel crosstalk with central nervous system immune cells in aging and neurodegenerative diseases

Meningeal lymphatic vessels form a relationship between the nervous system and periphery, which is relevant in both health and disease. Meningeal lymphatic vessels not only play a key role in the drainage of brain metabolites but also contribute to antigen delivery and immune cell activation. The advent of novel genomic technologies has enabled rapid progress in the characterization of myeloid and lymphoid cells and their interactions with meningeal lymphatic vessels within the central nervous system. In this review, we provide an overview of the multifaceted roles of meningeal lymphatic vessels within the context of the central nervous system immune network, highlighting recent discoveries on the immunological niche provided by meningeal lymphatic vessels. Furthermore, we delve into the mechanisms of crosstalk between meningeal lymphatic vessels and immune cells in the central nervous system under both homeostatic conditions and neurodegenerative diseases, discussing how these interactions shape the pathological outcomes. Regulation of meningeal lymphatic vessel function and structure can influence lymphatic drainage, cerebrospinal fluid-borne immune modulators, and immune cell populations in aging and neurodegenerative disorders, thereby playing a key role in shaping meningeal and brain parenchyma immunity.

Dynamics of Endothelial Cell Diversity and Plasticity in Health and Disease

Endothelial cells (ECs) are vital structural units of the cardiovascular system possessing two principal distinctive properties: heterogeneity and plasticity. Endothelial heterogeneity is defined by differences in tissue-specific endothelial phenotypes and their high predisposition to modification along the length of the vascular bed. This aspect of heterogeneity is closely associated with plasticity, the ability of ECs to adapt to environmental cues through the mobilization of genetic, molecular, and structural alterations. The specific endothelial cytoarchitectonics facilitate a quick structural cell reorganization and, furthermore, easy adaptation to the extrinsic and intrinsic environmental stimuli, known as the epigenetic landscape. ECs, as universally distributed and ubiquitous cells of the human body, play a role that extends far beyond their structural function in the cardiovascular system. They play a crucial role in terms of barrier function, cell-to-cell communication, and a myriad of physiological and pathologic processes. These include development, ontogenesis, disease initiation, and progression, as well as growth, regeneration, and repair. Despite substantial progress in the understanding of endothelial cell biology, the role of ECs in healthy conditions and pathologies remains a fascinating area of exploration. This review aims to summarize knowledge and concepts in endothelial biology. It focuses on the development and functional characteristics of endothelial cells in health and pathological conditions, with a particular emphasis on endothelial phenotypic and functional heterogeneity.

An arabinogalactan isolated from Cynanchum atratum promotes lymphangiogenesis and lymphatic vessel remodeling to alleviate secondary lymphedema

Secondary lymphedema is a chronic and incurable disease lacking satisfactory therapeutic drugs. It primarily results from lymphatic vessel dysfunction resulting from factors such as tumor-related surgery, injury, or infection. Promoting lymphangiogenesis and lymphatic vessel remodeling is crucial for restoring tissue fluid drainage and treating secondary lymphedema. In this study, we discovered that the oral administration of a type-II arabinogalactan (CAPW-1, molecular weight: 64 kDa) significantly promoted lymphangiogenesis and alleviated edema in mice with secondary lymphedema. Notably, the tail diameter of the CAPW-1200 group considerably decreased in comparison to that of the lymphedema group, with an average diameter difference reaching 0.98 mm on day 14. CAPW-1 treatment also reduced the average thickness of the subcutaneous area in the CAPW-1200 group to 0.37 mm (compared with 0.73 mm in the lymphedema group). It also facilitated the return of injected indocyanine green (ICG) from the tail tip to the sciatic lymph nodes, indicating that CAPW-1 promoted lymphatic vessel remodeling at the injury site. In addition, CAPW-1 enhanced the proliferation and migration of lymphatic endothelial cells. This phenomenon was associated with the activation of the toll-like receptor 4 (TLR4)/nuclear factor-κB (NF-κB) signaling pathway, thereby promoting the expression of vascular endothelial growth factor-C (VEGF-C), which can be abolished using a TLR4 antagonist. Despite these findings, CAPW-1 did not alleviate the symptoms of lymphedema or restore lymphatic drainage in VEGFR3flox/flox/Prox1-CreERT2 mice. In summary, CAPW-1 alleviates secondary lymphedema by promoting lymphangiogenesis and lymphatic vessel remodeling through the activation of the TLR4/NF-κB/VEGF-C signaling pathway, indicating its potential as a therapeutic lymphangiogenesis agent for patients with secondary lymphedema.

Age-related changes in meningeal lymphatic function are closely associated with vascular endothelial growth factor-C expression

Meningeal lymphatic vessels (MLVs) have crucial roles in removing metabolic waste and toxic proteins from the brain and transporting them to the periphery. Aged mice show impaired meningeal lymphatic function. Nevertheless, as the disease progresses, and significant pathological changes manifest in the brain, treating the condition becomes increasingly challenging. Therefore, investigating the alterations in the structure and function of MLVs in the early stages of aging is critical for preventing age-related central nervous system degenerative diseases. We detected the structure and function of MLVs in young, middle-aged, and aged mice. Middle-aged mice, compared with young and aged mice, showed enhanced meningeal lymphatic function along with MLV expansion and performed better in the Y maze test. Moreover, age-related changes in meningeal lymphatic function were closely associated with vascular endothelial growth factor-C (VEGF-C) expression in the brain cortex. Our data suggested that the cerebral cortex may serve as a target for VEGF-C supplementation to ameliorate meningeal lymphatic dysfunction, thus providing a new strategy for preventing age-related central nervous system diseases.

The contribution of the meningeal immune interface to neuroinflammation in traumatic brain injury

Traumatic brain injury (TBI) is a major cause of disability and mortality worldwide, particularly among the elderly, yet our mechanistic understanding of what renders the post-traumatic brain vulnerable to poor outcomes, and susceptible to neurological disease, is incomplete. It is well established that dysregulated and sustained immune responses elicit negative consequences after TBI; however, our understanding of the neuroimmune interface that facilitates crosstalk between central and peripheral immune reservoirs is in its infancy. The meninges serve as the interface between the brain and the immune system, facilitating important bi-directional roles in both healthy and disease settings. It has been previously shown that disruption of this system exacerbates neuroinflammation in age-related neurodegenerative disorders such as Alzheimer's disease; however, we have an incomplete understanding of how the meningeal compartment influences immune responses after TBI. In this manuscript, we will offer a detailed overview of the holistic nature of neuroinflammatory responses in TBI, including hallmark features observed across clinical and animal models. We will highlight the structure and function of the meningeal lymphatic system, including its role in immuno-surveillance and immune responses within the meninges and the brain. We will provide a comprehensive update on our current knowledge of meningeal-derived responses across the spectrum of TBI, and identify new avenues for neuroimmune modulation within the neurotrauma field.

Angpt1 binding to Tie1 regulates the signaling required for lymphatic vessel development in zebrafish

Development of the vascular system is regulated by multiple signaling pathways mediated by receptor tyrosine kinases. Among them, angiopoietin (Ang)/Tie signaling regulates lymphatic and blood vessel development in mammals. Of the two Tie receptors, Tie2 is well known as a key mediator of Ang/Tie signaling, but, unexpectedly, recent studies have revealed that the Tie2 locus has been lost in many vertebrate species, whereas the Tie1 gene is more commonly present. However, Tie1-driven signaling pathways, including ligands and cellular functions, are not well understood. Here, we performed comprehensive mutant analyses of angiopoietins and Tie receptors in zebrafish and found that only angpt1 and tie1 mutants show defects in trunk lymphatic vessel development. Among zebrafish angiopoietins, only Angpt1 binds to Tie1 as a ligand. We indirectly monitored Ang1/Tie1 signaling and detected Tie1 activation in sprouting endothelial cells, where Tie1 inhibits nuclear import of EGFP-Foxo1a. Angpt1/Tie1 signaling functions in endothelial cell migration and proliferation, and in lymphatic specification during early lymphangiogenesis, at least in part by modulating Vegfc/Vegfr3 signaling. Thus, we show that Angpt1/Tie1 signaling constitutes an essential signaling pathway for lymphatic development in zebrafish.

Decorin suppresses tumor lymphangiogenesis: A mechanism to curtail cancer progression

The complex interplay between malignant cells and the cellular and molecular components of the tumor stroma is a key aspect of cancer growth and development. These tumor-host interactions are often affected by soluble bioactive molecules such as proteoglycans. Decorin, an archetypical small leucine-rich proteoglycan primarily expressed by stromal cells, affects cancer growth in its soluble form by interacting with several receptor tyrosine kinases (RTK). Overall, decorin leads to a context-dependent and protracted cessation of oncogenic RTK activity by attenuating their ability to drive a prosurvival program and to sustain a proangiogenic network. Through an unbiased transcriptomic analysis using deep RNAseq, we identified that decorin down-regulated a cluster of tumor-associated genes involved in lymphatic vessel (LV) development when systemically delivered to mice harboring breast carcinoma allografts. We found that Lyve1 and Podoplanin, two established markers of LVs, were markedly suppressed at both the mRNA and protein levels, and this suppression correlated with a significant reduction in tumor LVs. We further identified that soluble decorin, but not its homologous proteoglycan biglycan, inhibited LV sprouting in an ex vivo 3D model of lymphangiogenesis. Mechanistically, we found that decorin interacted with vascular endothelial growth factor receptor 3 (VEGFR3), the main lymphatic RTK, and its activity was required for the decorin-mediated block of lymphangiogenesis. Finally, we identified that Lyve1 was in part degraded via decorin-evoked autophagy in a nutrient- and energy-independent manner. These findings implicate decorin as a biological factor with antilymphangiogenic activity and provide a potential therapeutic agent for curtailing breast cancer growth and metastasis.

PDGFRα+ITGA11+ fibroblasts foster early-stage cancer lymphovascular invasion and lymphatic metastasis via ITGA11-SELE interplay

Cancer-associated fibroblasts (CAFs) exhibit considerable heterogeneity in advanced cancers; however, the functional annotation and mechanism of CAFs in early-stage cancers remain elusive. Utilizing single-cell RNA sequencing and spatial transcriptomic, we identify a previously unknown PDGFRα+ITGA11+ CAF subset in early-stage bladder cancer (BCa). Multicenter clinical analysis of a 910-case cohort confirms that PDGFRα+ITGA11+ CAFs are associated with lymphovascular invasion (LVI) and poor prognosis in early-stage BCa. These CAFs facilitate LVI and lymph node (LN) metastasis in early-stage BCa, as evidenced in a PDGFRα+ITGA11+ CAFs-specific deficient mouse model. Mechanistically, PDGFRα+ITGA11+ CAFs promote lymphangiogenesis via recognizing ITGA11 surface receptor SELE on lymphatic endothelial cells to activate SRC-p-VEGFR3-MAPK pathway. Further, CHI3L1 from PDGFRα+ITGA11+ CAFs aligns the surrounding matrix to assist cancer cell intravasation, fostering early-stage BCa LVI and LN metastasis. Collectively, our study reveals the crucial role of PDGFRα+ITGA11+ CAFs in shaping metastatic landscape, informing the treatment of early-stage BCa LVI.

Impaired meningeal lymphatic drainage in Listeria monocytogenes infection

Previous studies have demonstrated an association between lymphatic vessels and diseases caused by bacterial infections. Listeria monocytogenes (LM) bacterial infection can affect multiple organs, including the intestine, brain, liver and spleen, which can be fatal. However, the impacts of LM infection on morphological and functional changes of lymphatic vessels remain unexplored. In this study, we found that LM infection not only induces meningeal and mesenteric lymphangiogenesis in mice, but also impairs meningeal lymphatic vessels (MLVs)-mediated macromolecules drainage. Interestingly, we found that the genes associated with lymphatic vessel development and function, such as Gata2 and Foxc2, were downregulated, suggesting that LM infection may affect cellular polarization and valve development. On the other hand, photodynamic ablation of MLVs exacerbated inflammation and bacterial load in the brain of mice with LM infection. Overall, our findings indicate that LM infection induces lymphangiogenesis and may affect cell polarization, cavity formation, and valve development during lymphangiogenesis, ultimately impairing MLVs drainage.

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.

An HGF-dependent positive feedback loop between bladder cancer cells and fibroblasts mediates lymphangiogenesis and lymphatic metastasis

BACKGROUND

Cancer-associated fibroblasts (CAFs) play a vital role in facilitating tumor progression through extensive reciprocal interplay with cancer cells. Tumor-derived extracellular vesicles (EVs) are the critical mediators involved in the crosstalk between cancer cells and stromal cells, contributing to the metastasis of cancers. Yet, the biological mechanisms of tumor-derived EVs in triggering CAFs phenotype to stimulate the lymph node (LN) metastasis of bladder cancer (BCa) are largely unknown. Here, we aimed to explore the effects and molecular mechanisms of tumor-derived EV-mediated CAFs phenotype in regulating BCa LN metastasis.

METHODS

The high-throughput sequencing was utilized to identify the crucial long non-coding RNA (lncRNA) associated with CAF enrichment in BCa. The functional role of the transition of fibroblasts to CAFs induced by LINC00665-mediated EVs was investigated through the in vitro and in vivo assays. Chromatin isolation by RNA purification assays, fluorescence resonance energy transfer assays, cytokine profiling and patient-derived xenograft (PDX) model were performed to explore the underlying mechanism of LINC00665 in the LN metastasis of BCa.

RESULTS

We found that CAFs are widely enriched in the tumor microenvironment of BCa, which correlated with BCa lymphangiogenesis and LN metastasis. We then identified a CAF-associated long non-coding RNA, LINC00665, which acted as a crucial mediator of CAF infiltration in BCa. Clinically, LINC00665 was associated with LN metastasis and poor prognosis in patients with BCa. Mechanistically, LINC00665 transcriptionally upregulated RAB27B expression and induced H3K4me3 modification on the promoter of RAB27B through the recruitment of hnRNPL. Moreover, RAB27B-induced EVs secretion endowed fibroblasts with the CAF phenotype, which reciprocally induced LINC00665 overexpression to form a RAB27B-HGF-c-Myc positive feedback loop, enhancing the lymphangiogenesis and LN metastasis of BCa. Importantly, we demonstrated that blocking EV-transmitted LINC00665 or HGF broke this loop and impaired BCa lymphangiogenesis in a PDX model.

CONCLUSION

Our study uncovers a precise mechanism that LINC00665 sustains BCa LN metastasis by inducing a RAB27B-HGF-c-Myc positive feedback loop between BCa cells and fibroblasts, suggesting that LINC00665 could be a promising therapeutic target for patients with LN metastatic BCa.

Unraveling the lymphatic system in the spinal cord meninges: a critical element in protecting the central nervous system

The lymphatic vasculature plays a crucial role in fluid clearance and immune responses in peripheral organs by connecting them to distal lymph nodes. Recently, attention has been drawn to the lymphatic vessel network surrounding the brain's border tissue (Aspelund et al. in J Exp Med 212:991-999, 2015. https://doi.org/10.1084/jem.20142290 ; Louveau et al. in Nat Neurosci 21:1380-1391, 2018. https://doi.org/10.1038/s41593-018-0227-9 ), which guides immune cells in mediating protection against tumors (Song et al. in Nature 577:689-694, 2020. https://doi.org/10.1038/s41586-019-1912-x ) and pathogens Li et al. (Nat Neurosci 25:577-587, 2022. https://doi.org/10.1038/s41593-022-01063-z ) while also contributing to autoimmunity (Louveau et al. 2018) and neurodegeneration (Da Mesquita et al. in Nature 560:185-191, 2018. https://doi.org/10.1038/s41586-018-0368-8 ). New studies have highlighted the integral involvement of meningeal lymphatic vessels in neuropathology. However, our limited understanding of spinal cord meningeal lymphatics and immunity hinders efforts to protect and heal the spinal cord from infections, injury, and other immune-mediated diseases. This review aims to provide a comprehensive overview of the state of spinal cord meningeal immunity, highlighting its unique immunologically relevant anatomy, discussing immune cells and lymphatic vasculature, and exploring the potential impact of injuries and inflammatory disorders on this intricate environment.

The emerging importance of lymphatics in health and disease: an NIH workshop report

The lymphatic system (LS) is composed of lymphoid organs and a network of vessels that transport interstitial fluid, antigens, lipids, cholesterol, immune cells, and other materials in the body. Abnormal development or malfunction of the LS has been shown to play a key role in the pathophysiology of many disease states. Thus, improved understanding of the anatomical and molecular characteristics of the LS may provide approaches for disease prevention or treatment. Recent advances harnessing single-cell technologies, clinical imaging, discovery of biomarkers, and computational tools have led to the development of strategies to study the LS. This Review summarizes the outcomes of the NIH workshop entitled "Yet to be Charted: Lymphatic System in Health and Disease," held in September 2022, with emphasis on major areas for advancement. International experts showcased the current state of knowledge regarding the LS and highlighted remaining challenges and opportunities to advance the field.

Decorin suppresses tumor lymphangiogenesis: A mechanism to curtail cancer progression

The complex interplay between malignant cells and the cellular and molecular components of the tumor stroma is a key aspect of cancer growth and development. These tumor-host interactions are often affected by soluble bioactive molecules such as proteoglycans. Decorin, an archetypical small leucine-rich proteoglycan primarily expressed by stromal cells, affects cancer growth in its soluble form by interacting with several receptor tyrosine kinases (RTK). Overall, decorin leads to a context-dependent and protracted cessation of oncogenic RTK activity by attenuating their ability to drive a pro-survival program and to sustain a pro-angiogenic network. Through an unbiased transcriptomic analysis using deep RNAseq, we discovered that decorin downregulated a cluster of tumor-associated genes involved in lymphatic vessel development when systemically delivered to mice harboring breast carcinoma allografts. We found that Lyve1 and Podoplanin, two established markers of lymphatic vessels, were markedly suppressed at both the mRNA and protein levels and this suppression correlated with a significant reduction in tumor lymphatic vessels. We further discovered that soluble decorin, but not its homologous proteoglycan biglycan, inhibited lymphatic vessel sprouting in an ex vivo 3D model of lymphangiogenesis. Mechanistically, we found that decorin interacted with VEGFR3, the main lymphatic RTK, and its activity was required for the decorin-mediated block of lymphangiogenesis. Finally, we discovered that Lyve1 was in part degraded via decorin-evoked autophagy in a nutrient- and energy-independent manner. These findings implicate decorin as a new biological factor with anti-lymphangiogenic activity and provide a potential therapeutic agent for curtailing breast cancer growth and metastasis.

The tissue-specific transcriptional landscape underlines the involvement of endothelial cells in health and disease

Endothelial cells (ECs) that line vascular and lymphatic vessels are being increasingly recognized as important to organ function in health and disease. ECs participate not only in the trafficking of gases, metabolites, and cells between the bloodstream and tissues but also in the angiocrine-based induction of heterogeneous parenchymal cells, which are unique to their specific tissue functions. The molecular mechanisms regulating EC heterogeneity between and within different tissues are modeled during embryogenesis and become fully established in adults. Any changes in adult tissue homeostasis induced by aging, stress conditions, and various noxae may reshape EC heterogeneity and induce specific transcriptional features that condition a functional phenotype. Heterogeneity is sustained via specific genetic programs organized through the combinatory effects of a discrete number of transcription factors (TFs) that, at the single tissue-level, constitute dynamic networks that are post-transcriptionally and epigenetically regulated. This review is focused on outlining the TF-based networks involved in EC specialization and physiological and pathological stressors thought to modify their architecture.

Regulating Lymphatic Vasculature in Fibrosis: Understanding the Biology to Improve the Modeling

Fibrosis occurs in many chronic diseases with lymphatic vascular insufficiency (e.g., kidney disease, tumors, and lymphedema). New lymphatic capillary growth can be triggered by fibrosis-related tissue stiffening and soluble factors, but questions remain for how related biomechanical, biophysical, and biochemical cues affect lymphatic vascular growth and function. The current preclinical standard for studying lymphatics is animal modeling, but in vitro and in vivo outcomes often do not align. In vitro models can also be limited in their ability to separate vascular growth and function as individual outcomes, and fibrosis is not traditionally included in model design. Tissue engineering provides an opportunity to address in vitro limitations and mimic microenvironmental features that impact lymphatic vasculature. This review discusses fibrosis-related lymphatic vascular growth and function in disease and the current state of in vitro lymphatic vascular models while highlighting relevant knowledge gaps. Additional insights into the future of in vitro lymphatic vascular models demonstrate how prioritizing fibrosis alongside lymphatics will help capture the complexity and dynamics of lymphatics in disease. Overall, this review aims to emphasize that an advanced understanding of lymphatics within a fibrotic disease-enabled through more accurate preclinical modeling-will significantly impact therapeutic development toward restoring lymphatic vessel growth and function in patients.

Overview of the meningeal lymphatic vessels in aging and central nervous system disorders

In the aging process and central nervous system (CNS) diseases, the functions of the meningeal lymphatic vessels (MLVs) are impaired. Alterations in MLVs have been observed in aging-related neurodegenerative diseases, brain tumors, and even cerebrovascular disease. These findings reveal a new perspective on aging and CNS disorders and provide a promising therapeutic target. Additionally, recent neuropathological studies have shown that MLVs exchange soluble components between the cerebrospinal fluid (CSF) and interstitial fluid (ISF) and drain metabolites, cellular debris, misfolded proteins, and immune cells from the CSF into the deep cervical lymph nodes (dCLNs), directly connecting the brain with the peripheral circulation. Impairment and dysfunction of meningeal lymphatics can lead to the accumulation of toxic proteins in the brain, exacerbating the progression of neurological disorders. However, for many CNS diseases, the causal relationship between MLVs and neuropathological changes is not fully clear. Here, after a brief historical retrospection, we review recent discoveries about the hallmarks of MLVs and their roles in the aging and CNS diseases, as well as potential therapeutic targets for the treatment of neurologic diseases.

TGF-β signaling in lymphatic vascular vessel formation and maintenance

Transforming growth factor (TGF)-β and its family members, including bone morphogenetic proteins (BMPs), nodal proteins, and activins, are implicated in the development and maintenance of various organs. Here, we review its role in the lymphatic vascular system (the secondary vascular system in vertebrates), which plays a crucial role in various physiological and pathological processes, participating in the maintenance of the normal tissue fluid balance, immune cell trafficking, and fatty acid absorption in the gut. The lymphatic system is associated with pathogenesis in multiple diseases, including lymphedema, inflammatory diseases, and tumor metastasis. Lymphatic vessels are composed of lymphatic endothelial cells, which differentiate from blood vascular endothelial cells (BECs). Although TGF-β family signaling is essential for maintaining blood vessel function, little is known about the role of TGF-β in lymphatic homeostasis. Recently, we reported that endothelial-specific depletion of TGF-β signaling affects lymphatic function. These reports suggest that TGF-β signaling in lymphatic endothelial cells maintains the structure of lymphatic vessels and lymphatic homeostasis, and promotes tumor lymphatic metastasis. Suppression of TGF-β signaling in lymphatic endothelial cells may therefore be effective in inhibiting cancer metastasis. We highlight recent advances in understanding the roles of TGF-β signaling in the formation and maintenance of the lymphatic system.

Mechanical forces in lymphatic vessel development: Focus on transcriptional regulation

The lymphatic system is crucial for the maintenance of interstitial fluid and protein homeostasis. It has important roles in collecting excess plasma and interstitial fluid leaked from blood vessels, lipid absorption and transportation in the digestive system, and immune surveillance and response. The development of lymphatic vessels begins during fetal life as lymphatic endothelial progenitor cells first differentiate into lymphatic endothelial cells (LECs) by expressing the master lymphatic vascular regulator, prospero-related homeobox 1 (PROX1). The lymphatic vasculature forms a hierarchical network that consists of blind-ended and unidirectional vessels. Although much progress has been made in the elucidation of the cellular and molecular mechanisms underlying the formation of the lymphatic vascular system, the causes of lymphatic vessel abnormalities and disease are poorly understood and complicated; specifically, the mechanistic basis for transcriptional dysregulation in lymphatic vessel development remains largely unclear. In this review, we discuss the recent advances in our understanding of the molecular and cellular mechanisms of lymphatic vascular development, including LEC differentiation, lymphangiogenesis, and valve formation, and the significance of mechanical forces in lymphatic vessels, with a focus on transcriptional regulation. We also summarize the current knowledge on epigenetic mechanisms of lymphatic gene expression.

Essential Roles of Exocyst Complex Component 3-like 2 on Cardiovascular Development in Mice

Angiogenesis is a process to generate new blood vessels from pre-existing vessels and to maintain vessels, and plays critical roles in normal development and disease. However, the molecular mechanisms underlying angiogenesis are not fully understood. This study examined the roles of exocyst complex component (Exoc) 3-like 2 (Exoc3l2) during development in mice. We found that Exoc3l1, Exoc3l2, Exoc3l3 and Exoc3l4 are expressed abundantly in endothelial cells at embryonic day 8.5. The generation of Exoc3l2 knock-out (KO) mice showed that disruption of Exoc3l2 resulted in lethal in utero. Substantial numbers of Exoc3l2 KO embryos exhibited hemorrhaging. Deletion of Exoc3l2 using Tie2-Cre transgenic mice demonstrated that Exoc3l2 in hematopoietic and endothelial lineages was responsible for the phenotype. Taken together, these findings reveal that Exoc3l2 is essential for cardiovascular and brain development in mice.

Comprehensive Review of the Vascular Niche in Regulating Organ Regeneration and Fibrosis

The vasculature occupies a large area of the body, and none of the physiological activities can be carried out without blood vessels. Blood vessels are not just passive conduits and barriers for delivering blood and nutrients. Meanwhile, endothelial cells covering the vascular lumen establish vascular niches by deploying some growth factors, known as angiocrine factors, and actively participate in the regulation of a variety of physiological processes, such as organ regeneration and fibrosis and the occurrence and development of cancer. After organ injury, vascular endothelial cells regulate the repair process by secreting various angiocrine factors, triggering the proliferation and differentiation process of stem cells. Therefore, analyzing the vascular niche and exploring the factors that maintain vascular homeostasis can provide strong theoretical support for clinical treatment targeting blood vessels. Here we mainly discuss the regulatory mechanisms of the vascular niche in organ regeneration and fibrosis.

An analysis modality for vascular structures combining tissue-clearing technology and topological data analysis

The blood and lymphatic vasculature networks are not yet fully understood even in mouse because of the inherent limitations of imaging systems and quantification methods. This study aims to evaluate the usefulness of the tissue-clearing technology for visualizing blood and lymphatic vessels in adult mouse. Clear, unobstructed brain/body imaging cocktails and computational analysis (CUBIC) enables us to capture the high-resolution 3D images of organ- or area-specific vascular structures. To evaluate these 3D structural images, signals are first classified from the original captured images by machine learning at pixel base. Then, these classified target signals are subjected to topological data analysis and non-homogeneous Poisson process model to extract geometric features. Consequently, the structural difference of vasculatures is successfully evaluated in mouse disease models. In conclusion, this study demonstrates the utility of CUBIC for analysis of vascular structures and presents its feasibility as an analysis modality in combination with 3D images and mathematical frameworks.

Neuroimmune signaling at the brain borders

The central nervous system (CNS) has been viewed as an immunologically privileged site, but emerging works are uncovering a large array of neuroimmune interactions primarily occurring at its borders. CNS barriers sites host diverse population of both innate and adaptive immune cells capable of, directly and indirectly, influence the function of the residing cells of the brain parenchyma. These structures are only starting to reveal their role in controlling brain function under normal and pathological conditions and represent an underexplored therapeutic target for the treatment of brain disorders. This review will highlight the development of the CNS barriers to host neuro-immune interactions and emphasize their newly described roles in neurodevelopmental, neurological, and neurodegenerative disorders, particularly for the meninges.

KRAS mutant-driven SUMOylation controls extracellular vesicle transmission to trigger lymphangiogenesis in pancreatic cancer

Lymph node (LN) metastasis occurs frequently in pancreatic ductal adenocarcinoma (PDAC) and predicts poor prognosis for patients. The KRASG12D mutation confers an aggressive PDAC phenotype that is susceptible to lymphatic dissemination. However, the regulatory mechanism underlying KRASG12D mutation-driven LN metastasis in PDAC remains unclear. Herein, we found that PDAC with the KRASG12D mutation (KRASG12D PDAC) sustained extracellular vesicle-mediated (EV-mediated) transmission of heterogeneous nuclear ribonucleoprotein A1 (hnRNPA1) in a SUMOylation-dependent manner and promoted lymphangiogenesis and LN metastasis in vitro and in vivo. Mechanistically, hnRNPA1 bound with SUMO2 at the lysine 113 residue via KRASG12D-induced hyperactivation of SUMOylation, which enabled its interaction with TSG101 to enhance hnRNPA1 packaging and transmission via EVs. Subsequently, SUMOylation induced EV-packaged-hnRNPA1 anchoring to the adenylate- and uridylate-rich elements of PROX1 in lymphatic endothelial cells, thus stabilizing PROX1 mRNA. Importantly, impeding SUMOylation of EV-packaged hnRNPA1 dramatically inhibited LN metastasis of KRASG12D PDAC in a genetically engineered KrasG12D/+ Trp53R172H/+ Pdx-1-Cre (KPC) mouse model. Our findings highlight the mechanism by which KRAS mutant-driven SUMOylation triggers EV-packaged hnRNPA1 transmission to promote lymphangiogenesis and LN metastasis, shedding light on the potential application of hnRNPA1 as a therapeutic target in patients with KRASG12D PDAC.

Piezo1-Regulated Mechanotransduction Controls Flow-Activated Lymphatic Expansion

BACKGROUND

Mutations in PIEZO1 (Piezo type mechanosensitive ion channel component 1) cause human lymphatic malformations. We have previously uncovered an ORAI1 (ORAI calcium release-activated calcium modulator 1)-mediated mechanotransduction pathway that triggers lymphatic sprouting through Notch downregulation in response to fluid flow. However, the identity of its upstream mechanosensor remains unknown. This study aimed to identify and characterize the molecular sensor that translates the flow-mediated external signal to the Orai1-regulated lymphatic expansion.

METHODS

Various mutant mouse models, cellular, biochemical, and molecular biology tools, and a mouse tail lymphedema model were employed to elucidate the role of Piezo1 in flow-induced lymphatic growth and regeneration.

RESULTS

Piezo1 was found to be abundantly expressed in lymphatic endothelial cells. Piezo1 knockdown in cultured lymphatic endothelial cells inhibited the laminar flow-induced calcium influx and abrogated the flow-mediated regulation of the Orai1 downstream genes, such as KLF2 (Krüppel-like factor 2), DTX1 (Deltex E3 ubiquitin ligase 1), DTX3L (Deltex E3 ubiquitin ligase 3L,) and NOTCH1 (Notch receptor 1), which are involved in lymphatic sprouting. Conversely, stimulation of Piezo1 activated the Orai1-regulated mechanotransduction in the absence of fluid flow. Piezo1-mediated mechanotransduction was significantly blocked by Orai1 inhibition, establishing the epistatic relationship between Piezo1 and Orai1. Lymphatic-specific conditional Piezo1 knockout largely phenocopied sprouting defects shown in Orai1- or Klf2- knockout lymphatics during embryo development. Postnatal deletion of Piezo1 induced lymphatic regression in adults. Ectopic Dtx3L expression rescued the lymphatic defects caused by Piezo1 knockout, affirming that the Piezo1 promotes lymphatic sprouting through Notch downregulation. Consistently, transgenic Piezo1 expression or pharmacological Piezo1 activation enhanced lymphatic sprouting. Finally, we assessed a potential therapeutic value of Piezo1 activation in lymphatic regeneration and found that a Piezo1 agonist, Yoda1, effectively suppressed postsurgical lymphedema development.

CONCLUSIONS

Piezo1 is an upstream mechanosensor for the lymphatic mechanotransduction pathway and regulates lymphatic growth in response to external physical stimuli. Piezo1 activation presents a novel therapeutic opportunity for preventing postsurgical lymphedema. The Piezo1-regulated lymphangiogenesis mechanism offers a molecular basis for Piezo1-associated lymphatic malformation in humans.

Translational Value of Tumor-Associated Lymphangiogenesis in Cholangiocarcinoma

The prognosis of cholangiocarcinoma remains poor in spite of the advances in immunotherapy and molecular profiling, which has led to the identification of several targetable genetic alterations. Surgical procedures, including both liver resection and liver transplantation, still represent the treatment with the best curative potential, though the outcomes are significantly compromised by the early development of lymph node metastases. Progression of lymphatic metastasis from the primary tumor to tumor-draining lymph nodes is mediated by tumor-associated lymphangiogenesis, a topic largely overlooked until recently. Recent findings highlight tumor-associated lymphangiogenesis as paradigmatic of the role played by the tumor microenvironment in sustaining cholangiocarcinoma invasiveness and progression. This study reviews the current knowledge about the intercellular signaling and molecular mechanism of tumor-associated lymphangiogenesis in cholangiocarcinoma in the hope of identifying novel therapeutic targets to halt a process that often limits the success of the few available treatments.

Lymph Leakage Promotes Immunosuppression by Enhancing Anti-Inflammatory Macrophage Polarization

Lymphatic vasculature is a network of capillaries and vessels capable of draining extracellular fluid back to blood circulation and to facilitate immune cell migration. Although the role of the lymphatic vasculature as coordinator of fluid homeostasis has been extensively studied, the consequences of abnormal lymphatic vasculature function and impaired lymph drainage have been mostly unexplored. Here, by using the Prox1+/- mice with defective lymphatic vasculature and lymphatic leakage, we provide evidence showing that lymph leakage induces an immunosuppressive environment by promoting anti-inflammatory M2 macrophage polarization in different inflammatory conditions. In fact, by using a mouse model of tail lymphedema where lymphatic vessels are thermal ablated leading to lymph accumulation, an increasing number of anti-inflammatory M2 macrophages are found in the lymphedematous tissue. Moreover, RNA-seq analysis from different human tumors shows that reduced lymphatic signature, a hallmark of lymphatic dysfunction, is associated with increased M2 and reduced M1 macrophage signatures, impacting the survival of the patients. In summary, we show that lymphatic vascular leakage promotes an immunosuppressive environment by enhancing anti-inflammatory macrophage differentiation, with relevance in clinical conditions such as inflammatory bowel diseases or cancer.

The meningeal lymphatic vasculature in neuroinflammation

The lymphatic vasculature is a unidirectional network of lymphatic endothelial cells, whose main role is to maintain fluid homeostasis along with the absorption of dietary fat in the gastrointestinal organs and management and coordination of immune cell trafficking into lymph nodes during homeostasis and under inflammatory conditions. In homeostatic conditions, immune cells, such as dendritic cells, macrophages, or T cells can enter into the lymphatic vasculature and move easily through the lymph reaching secondary lymph nodes where immune cell activation or peripheral tolerance can be modulated. However, under inflammatory conditions such as pathogen infection, increased permeabilization of lymphatic vessels allows faster immune cell migration into inflamed tissues following a chemokine gradient, facilitating pathogen clearance and the resolution of inflammation. Interestingly, since the re-discovery of lymphatic vasculature in the central nervous system, known as the meningeal lymphatic vasculature, the role of these lymphatics as a key player in several neurological disorders has been described, with emphasis on the neurodegenerative process. Alternatively, less has been discussed about meningeal lymphatics and its role in neuroinflammation. In this review, we discuss current knowledge about the anatomy and function of the meningeal lymphatic vasculature and specifically analyze its contribution to different neuroinflammatory processes, highlighting the potential therapeutic target of meningeal lymphatic vasculature in these pathological conditions.

The bright future of nanotechnology in lymphatic system imaging and imaging-guided surgery

Lymphatic system is identified the second vascular system after the blood circulation in mammalian species, however the research on lymphatic system has long been hampered by the lack of comprehensive imaging modality. Nanomaterials have shown the potential to enhance the quality of lymphatic imaging due to the unparalleled advantages such as the specific passive targeting and efficient co-delivery of cocktail to peripheral lymphatic system, ease molecular engineering for precise active targeting and prolonged retention in the lymphatic system of interest. Multimodal lymphatic imaging based on nanotechnology provides a complementary means to understand the kinetics of lymphoid tissues and quantify its function. In this review, we introduce the established approaches of lymphatic imaging used in clinic and summarize their strengths and weaknesses, and list the critical influence factors on lymphatic imaging. Meanwhile, the recent developments in the field of pre-clinical lymphatic imaging are discussed to shed new lights on the design of new imaging agents, the improvement of delivery methods and imaging-guided surgery strategies.

Lymphatic endothelial cell fate specification in the mammalian embryo: An historical perspective

Development of the mammalian lymphatic vasculature is a stepwise process requiring the specification of lymphatic endothelial cell progenitors in the embryonic veins, and their subsequent budding to give rise to most of the mature lymphatic vasculature. In mice, formation of the lymphatic vascular network starts inside the cardinal vein at around E9.5 when a subpopulation of venous endothelial cells gets committed into the lymphatic lineage by their acquisition of Prox1 expression. Identification of critical genes regulating lymphatic development facilitated the detailed cellular and molecular characterization of some of the cellular and molecular mechanisms regulating the early steps leading to the formation of the mammalian lymphatic vasculature. A better understanding of basic aspects of early lymphatic development, and the availability of novel tools and animal models has been instrumental in the identification of important novel functional roles of this vasculature network.

Endothelial-specific depletion of TGF-β signaling affects lymphatic function

BACKGROUND

Transforming growth factor (TGF)-β is a multifunctional cytokine involved in cell differentiation, cell proliferation, and tissue homeostasis. Although TGF-β signaling is essential for maintaining blood vessel functions, little is known about the role of TGF-β in lymphatic homeostasis.

METHODS

To delineate the role of TGF-β signaling in lymphatic vessels, TβRII^fl/fl mice were crossed with Prox1-Cre^ERT2 mice to generate TβRII^fl/fl; Prox1-Cre^ERT2 mice. The TβRII gene in the lymphatic endothelial cells (LECs) of the conditional knockout TβRII^iΔLEC mice was selectively deleted using tamoxifen. The effects of TβRII gene deletion on embryonic lymphangiogenesis, postnatal lymphatic structure and drainage function, tumor lymphangiogenesis, and lymphatic tumor metastasis were investigated.

RESULTS

Deficiency of LEC-specific TGF-β signaling in embryos, where lymphangiogenesis is active, caused dorsal edema with dilated lymphatic vessels at E13.5. Postnatal mice in which lymphatic vessels had already been formed displayed dilation and increased bifurcator of lymphatic vessels after tamoxifen administration. Similar dilation was also observed in tumor lymphatic vessels. The drainage of FITC-dextran, which was subcutaneously injected into the soles of the feet of the mice, was reduced in TβRII^iΔLEC mice. Furthermore, Lewis lung carcinoma cells constitutively expressing GFP (LLC-GFP) transplanted into the footpads of the mice showed reduced patellar lymph node metastasis.

CONCLUSION

These data suggest that TGF-β signaling in LECs maintains the structure of lymphatic vessels and lymphatic homeostasis, in addition to promoting tumor lymphatic metastasis. Therefore, suppression of TGF-β signaling in LECs might be effective in inhibiting cancer metastasis.

Single-Cell RNA Sequencing Reveals Heterogeneity and Functional Diversity of Lymphatic Endothelial Cells

Lymphatic endothelial cells (LECs) line the lymphatic vasculature and play a central role in the immune response. LECs have abilities to regulate immune transport, to promote immune cell survival, and to cross present antigens to dendritic cells. Single-cell RNA sequencing (scRNA) technology has accelerated new discoveries in the field of lymphatic vascular biology. This review will summarize these new findings in regard to embryonic development, LEC heterogeneity with associated functional diversity, and interactions with other cells. Depending on the organ, location in the lymphatic vascular tree, and micro-environmental conditions, LECs feature unique properties and tasks. Furthermore, adjacent stromal cells need the support of LECs for fulfilling their tasks in the immune response, such as immune cell transport and antigen presentation. Although aberrant lymphatic vasculature has been observed in a number of chronic inflammatory diseases, the knowledge on LEC heterogeneity and functional diversity in these diseases is limited. Combining scRNA sequencing data with imaging and more in-depth functional experiments will advance our knowledge of LECs in health and disease. Building the case, the LEC could be put forward as a new therapeutic target in chronic inflammatory diseases, counterweighting the current immune-cell focused therapies.

The Role of Lymphangiogenesis in Coronary Atherosclerosis

Lymphatic circulation, a one-way channel system independent of blood circulation, collects interstitial fluid in a blind-end way. Existing widely in various organs and tissues, lymphatic vessels play important roles in maintaining tissue fluid homeostasis, regulating immune function, and promoting lipid transport. Recent studies have shown clear evidence that lymphangiogenesis has a strong mutual effect on coronary atherosclerosis (AS). In this study, we focus on this topic, especially in the aspects of relevant ligand/receptor, inflammation, and adipose metabolism. For the moment, however, the role of lymphangiogenesis and remodeling in coronary AS still remains controversial. The studies of our group and accumulating published evidence show that the pathological remodeling of lymphatic vessels in coronary AS may have a negative effect, but normal functional lymphangiogenesis is probably beneficial to the regression of coronary AS. Thus, the conclusion of this review is that lymphatic vessel function rather than its quantity determines its influence in AS, which needs more evidence to support.

The lymphatics in kidney health and disease

The mammalian vascular system consists of two networks: the blood vascular system and the lymphatic vascular system. Throughout the body, the lymphatic system contributes to homeostatic mechanisms by draining extravasated interstitial fluid and facilitating the trafficking and activation of immune cells. In the kidney, lymphatic vessels exist mainly in the kidney cortex. In the medulla, the ascending vasa recta represent a hybrid lymphatic-like vessel that performs lymphatic-like roles in interstitial fluid reabsorption. Although the lymphatic network is mainly derived from the venous system, evidence supports the existence of lymphatic beds that are of non-venous origin. Following their development and maturation, lymphatic vessel density remains relatively stable; however, these vessels undergo dynamic functional changes to meet tissue demands. Additionally, new lymphatic growth, or lymphangiogenesis, can be induced by pathological conditions such as tissue injury, interstitial fluid overload, hyperglycaemia and inflammation. Lymphangiogenesis is also associated with conditions such as polycystic kidney disease, hypertension, ultrafiltration failure and transplant rejection. Although lymphangiogenesis has protective functions in clearing accumulated fluid and immune cells, the kidney lymphatics may also propagate an inflammatory feedback loop, exacerbating inflammation and fibrosis. Greater understanding of lymphatic biology, including the developmental origin and function of the lymphatics and their response to pathogenic stimuli, may aid the development of new therapeutic agents that target the lymphatic system.

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

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

Insights Into Unveiling a Potential Role of Tertiary Lymphoid Structures in Metastasis

Tertiary lymphoid structures (TLSs) develop in non-lymphatic tissue in chronic inflammation and cancer. TLS can mature to lymph node (LN) like structures with germinal centers and associated vasculature. TLS neogenesis in cancer is highly varied and tissue dependent. The role of TLS in adaptive antitumor immunity is of great interest. However, data also show that TLS can play a role in cancer metastasis. The importance of lymphatics in cancer distant metastasis is clear yet the precise detail of how various immunosurveillance mechanisms interplay within TLS and/or draining LN is still under investigation. As part of the tumor lymphatics, TLS vasculature can provide alternative routes for the establishment of the pre-metastatic niche and cancer dissemination. The nature of the cytokine and chemokine signature at the heart of TLS induction can be key in determining the success of antitumor immunity or in promoting cancer invasiveness. Understanding the biochemical and biomechanical factors underlying TLS formation and the resulting impact on the primary tumor will be key in deciphering cancer metastasis and in the development of the next generation of cancer immunotherapeutics.

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

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

Hyaluronan and Its Receptors: Key Mediators of Immune Cell Entry and Trafficking in the Lymphatic System

Entry to the afferent lymphatics marks the first committed step for immune cell migration from tissues to draining lymph nodes both for the generation of immune responses and for timely resolution of tissue inflammation. This critical process occurs primarily at specialised discontinuous junctions in initial lymphatic capillaries, directed by chemokines released from lymphatic endothelium and orchestrated by adhesion between lymphatic receptors and their immune cell ligands. Prominent amongst the latter is the large glycosaminoglycan hyaluronan (HA) that can form a bulky glycocalyx on the surface of certain tissue-migrating leucocytes and whose engagement with its key lymphatic receptor LYVE-1 mediates docking and entry of dendritic cells to afferent lymphatics. Here we outline the latest insights into the molecular mechanisms by which the HA glycocalyx together with LYVE-1 and the related leucocyte receptor CD44 co-operate in immune cell entry, and how the process is facilitated by the unusual character of LYVE-1 • HA-binding interactions. In addition, we describe how pro-inflammatory breakdown products of HA may also contribute to lymphatic entry by transducing signals through LYVE-1 for lymphangiogenesis and increased junctional permeability. Lastly, we outline some future perspectives and highlight the LYVE-1 • HA axis as a potential target for immunotherapy.

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.

The physiological and pathological functions of VEGFR3 in cardiac and lymphatic development and related diseases

Vascular endothelial growth factor receptors (VEGFRs) are part of the evolutionarily conserved VEGF signalling pathways that regulate the development and maintenance of the body's cardiovascular and lymphovascular systems. VEGFR3, encoded by the FLT4 gene, has an indispensable and well-characterized function in development and establishment of the lymphatic system. Autosomal dominant VEGFR3 mutations, that prevent the receptor functioning as a homodimer, cause one of the major forms of hereditary primary lymphoedema; Milroy disease. Recently, we and others have shown that FLT4 variants, distinct to those observed in Milroy disease cases, predispose individuals to Tetralogy of Fallot, the most common cyanotic congenital heart disease, demonstrating a novel function for VEGFR3 in early cardiac development. Here, we examine the familiar and emerging roles of VEGFR3 in the development of both lymphovascular and cardiovascular systems, respectively, compare how distinct genetic variants in FLT4 lead to two disparate human conditions, and highlight the research still required to fully understand this multifaceted receptor.

Tumor-derived exosomal BCYRN1 activates WNT5A/VEGF-C/VEGFR3 feedforward loop to drive lymphatic metastasis of bladder cancer

BACKGROUND

Patients with lymph node (LN) metastatic bladder cancer (BCa) present with extremely poor prognosis. BCa-derived exosomes function as crucial bioactive cargo carriers to mediate the signal transduction in tumor microenvironment triggering tumor metastasis. However, the mechanisms underlying exosome-mediated LN metastasis in BCa are unclear.

METHODS

We conducted the high-throughput sequencing to explore the expression profile of long noncoding RNA (lncRNA) in urinary exosomes (urinary-EXO) from patients with BCa and further evaluated the clinical relevance of exosomal lncRNA BCYRN1 in a larger 210-case cohort. The functional role of exosomal BCYRN1 was evaluated through the migration and tube formation assays in vitro and the footpad-popliteal LN metastasis model in vivo. RNA pull-down assays, luciferase assays, and actinomycin assays were conducted to detect the regulatory mechanism of exosomal BCYRN1.

RESULTS

LncRNA BCYRN1 was substantially upregulated in urinary-EXO from patients with BCa, and associated with the LN metastasis of BCa. We demonstrated that exosomal BCYRN1 markedly promoted tube formation and migration of human lymphatic endothelial cells (HLECs) in vitro and lymphangiogenesis and LN metastasis of BCa in vivo. Mechanistically, BCYRN1 epigenetically upregulated WNT5A expression by inducing hnRNPA1-associated H3K4 trimethylation in WNT5A promoter, which activated Wnt/β-catenin signaling to facilitate the secretion of VEGF-C in BCa. Moreover, exosomal BCYRN1 was transmitted to HLECs to stabilize the VEGFR3 mRNA and thus formed an hnRNPA1/WNT5A/VEGFR3 feedforward regulatory loop, ultimately promoting the lymphatic metastasis of BCa. Importantly, blocking VEGFR3 with specific inhibitor, SAR131675 significantly impaired exosomal BCYRN1-induced the LN metastasis in vivo. Clinically, exosomal BCYRN1 was positively associated with the shorter survival of BCa patients and identified as a poor prognostic factor of patients.

CONCLUSION

Our results uncover a novel mechanism by which exosomal BCYRN1 synergistically enhances VEGF-C/VEGFR3 signaling-induced lymphatic metastasis of BCa, indicating that BCYRN1 may serve as an encouraging therapeutic target for patients with BCa.

A novel role of Hippo-Yap/TAZ signaling pathway in lymphatic vascular development

The lymphatic vasculature plays important role in regulating fluid homeostasis, intestinal lipid absorption, and immune surveillance in humans. Malfunction of lymphatic vasculature leads to several human diseases. Understanding the fundamental mechanism in lymphatic vascular development not only expand our knowledge, but also provide a new therapeutic insight. Recently, Hippo-YAP/TAZ signaling pathway, a key mechanism of organ size and tissue homeostasis, has emerged as a critical player that regulate lymphatic specification, sprouting, and maturation. In this review, we discuss the mechanistic regulation and pathophysiological significant of Hippo pathway in lymphatic vascular development.