Current topics of physiology and pharmacology in the lymphatic system

Anti-proliferation evaluation of new derivatives of indole-6-carboxylate ester as receptor tyrosine kinase inhibitors

Aim: The main goal was to create two new groups of indole derivatives, hydrazine-1-carbothioamide (4a and 4b) and oxadiazole (5, and 6a-e) that target EGFR (4a, 4b, 5) or VEGFR-2 (6a-e). Materials & methods: The new derivatives were characterized using various spectroscopic techniques. Docking studies were used to investigate the binding patterns to EGFR/VEGFR-2, and the anti-proliferative properties were tested in vitro. Results: Compounds 4a (targeting EGFR) and 6c (targeting VEGFR-2) were the most effective cytotoxic agents, arresting cancer cells in the G2/M phase and inducing the extrinsic apoptosis pathway. Conclusion: The results of this study show that compounds 4a and 6c are promising cytotoxic compounds that inhibit the tyrosine kinase activity of EGFR and VEGFR-2, respectively.

Banxia Xiexin decoction promotes gastric lymphatic pumping by regulating lymphatic smooth muscle cell contraction and energy metabolism in a stress-induced gastric ulceration rat model

ETHNOPHARMACOLOGICAL RELEVANCE

The traditional Chinese medicine (TCM) formula Banxia Xiexin decoction (BXD) has definite therapeutic effect in treating stress-induced gastric ulceration (SIGU) and many other gastrointestinal diseases, but its effect on gastric lymphatic pumping (GLP) remains unclear.

AIM OF THE STUDY

Elucidating the role of GLP in SIGU and BXD treatment, and exploring the molecular mechanisms of GLP regulation.

MATERIALS AND METHODS

In vivo GLP imaging were performed on SIGU rat model, and the lymphatic dynamic parameters were evaluated. Gastric antrum tissues and serum were collected for macroscopic, histopathological and ulcerative parameters analysis. Gastric lymphatic vessel (GLV) tissues were collected for RNA-Seq assays. Differentially expressed genes (DEGs) were screened from RNA-Seq result and submitted for transcriptomic analysis. Key DEGs and their derivative proteins were measured by qRT-PCR and WB.

RESULTS

GLP was significantly suppressed in SIGU rats. BXD could recover GLP, ameliorate stomach lymphostasis, and alleviate the ulcerative damage. Transcriptome analysis of GLV showed the top up-DEGs were concentrated in smooth muscle contraction signaling pathway, while the top the down-DEGs were concentrated in energy metabolism pathways especially fatty acid degradation pathway, which indicated BXD can promote lymphatic smooth muscle contraction, regulate energy metabolism, and reduce fatty acid degradation. The most possible target of these mechanisms was the lymphatic smooth muscle cells (LSMCs) which drove the GLP. This speculation was further validated by the qRT-PCR and WB assessments for the level of key genes and proteins.

CONCLUSIONS

By activating the smooth muscle contraction signaling pathway, restoring energy supply, modulating energy metabolism program and reducing fatty acid degradation, BXD effectively recovered GLP, mitigated the accumulation of inflammatory cytokines and metabolic wastes in the stomach, which importantly contributes to its efficacy in treating SIGU.

Revolutionizing lymph node metastasis imaging: the role of drug delivery systems and future perspectives

The deployment of imaging examinations has evolved into a robust approach for the diagnosis of lymph node metastasis (LNM). The advancement of technology, coupled with the introduction of innovative imaging drugs, has led to the incorporation of an increasingly diverse array of imaging techniques into clinical practice. Nonetheless, conventional methods of administering imaging agents persist in presenting certain drawbacks and side effects. The employment of controlled drug delivery systems (DDSs) as a conduit for transporting imaging agents offers a promising solution to ameliorate these limitations intrinsic to metastatic lymph node (LN) imaging, thereby augmenting diagnostic precision. Within the scope of this review, we elucidate the historical context of LN imaging and encapsulate the frequently employed DDSs in conjunction with a variety of imaging techniques, specifically for metastatic LN imaging. Moreover, we engage in a discourse on the conceptualization and practical application of fusing diagnosis and treatment by employing DDSs. Finally, we venture into prospective applications of DDSs in the realm of LNM imaging and share our perspective on the potential trajectory of DDS development.

Signaling mechanisms underlying lymphatic vessel dysfunction in skin aging and possible anti-aging strategies

Aging-related skin diseases are gradually increasing due to the imbalance of cutaneous homeostasis in the aging population. Skin aging-induced inflammation promotes systemic inflammation and may lead to whole-body aging. Lymphatic vessels play an important role in maintaining fluid and homeostasis balance. In intrinsically aged skin, the number of lymphatic vessels decrease and their functions decline, which is related to the reduced adhesion junctions between lymphatic endothelial cells, particularly VE-cadherin. VEGFC/VEGFR-3 signal pathway plays an important role in remodeling and expansion of lymphatic vessels; the downregulation of this pathway contributes to the dysfunction of lymphatic vessels. Meanwhile, we proposed some additional mechanisms. Decline of the pumping activity of lymphatic vessels might be related to age-related changes in extracellular matrix, ROS increase, and eNOS/iNOS disturbances. In extrinsically aged skin, the hyperpermeability of lymphatic vessels results from a decrease in endothelial-specific tight junction molecules, upregulation of VEGF-A, and downregulation of the VEGFC/VEGFR-3 signaling pathway. Furthermore, some of the Phyto therapeutics could attenuate skin aging by modulating the lymphatic vessels. This review summarized the lymphatic vessel dysfunction in skin aging and anti-aging strategies based on lymphatic vessel modulation.

Lymphatic-draining nanoparticles deliver Bay K8644 payload to lymphatic vessels and enhance their pumping function

Dysfunction of collecting lymphatic vessel pumping is associated with an array of pathologies. S-(-)-Bay K8644 (BayK), a small-molecule agonist of L-type calcium channels, improves vessel contractility ex vivo but has been left unexplored in vivo because of poor lymphatic access and risk of deleterious off-target effects. When formulated within lymph-draining nanoparticles (NPs), BayK acutely improved lymphatic vessel function, effects not seen from treatment with BayK in its free form. By preventing rapid drug access to the circulation, NP formulation also reduced BayK's dose-limiting side effects. When applied to a mouse model of lymphedema, treatment with BayK formulated in lymph-draining NPs, but not free BayK, improved pumping pressure generated by intact lymphatic vessels and tissue remodeling associated with the pathology. This work reveals the utility of a lymph-targeting NP platform to pharmacologically enhance lymphatic pumping in vivo and highlights a promising approach to treating lymphatic dysfunction.

Usefulness of pyruvate dehydrogenase-E1α expression to determine SUVmax cut-off value of [18F]FDG-PET for predicting lymph node metastasis in lung cancer

A more accurate cut-off value of maximum standardized uptake value (SUVmax) in [¹⁸F]fluorodeoxyglucose positron emission tomography/computed tomography ([¹⁸F]FDG-PET/CT) is necessary to improve preoperative nodal staging in patients with lung cancer. Overall, 223 patients with lung cancer who had undergone [¹⁸F]FDG-PET/CT within 2 months before surgery were enrolled. The expression of glucose transporter-1, pyruvate kinase-M2, pyruvate dehydrogenase-E1α (PDH-E1α), and carbonic anhydrase-9 was evaluated by immunohistochemistry. Clinicopathological background was retrospectively investigated. According to PDH-E1α expression in primary lesion, a significant difference (p = 0.021) in SUVmax of metastatic lymph nodes (3.0 with PDH-positive vs 4.5 with PDH-negative) was found, but not of other enzymes. When the cut-off value of SUVmax was set to 2.5, the sensitivity and specificity were 0.529 and 0.562, respectively, and the positive and negative predictive values were 0.505 and 0.586, respectively. However, when the cut-off value of SUVmax was set according to PDH-E1α expression (2.7 with PDH-positive and 3.2 with PDH-negative), the sensitivity and specificity were 0.441 and 0.868, respectively, and the positive and negative predictive values were 0.738 and 0.648, respectively. The SUVmax cut-off value for metastatic lymph nodes depends on PDH-E1α expression in primary lung cancer. The new SUVmax cut-off value according to PDH-E1α expression showed higher specificity for [¹⁸F]FDG-PET in the diagnosis of lymph node metastasis.

Reactive Oxygen Species in Regulating Lymphangiogenesis and Lymphatic Function

The lymphatic system is pivotal for immunosurveillance and the maintenance of tissue homeostasis. Lymphangiogenesis, the formation of new lymphatic vessels from pre-existing vessels, has both physiological and pathological roles. Recent advances in the molecular mechanisms regulating lymphangiogenesis have opened a new area of research on reparative lymphangiogenesis for the treatment of various pathological disorders comprising neurological disorders, cardiac repair, autoimmune disease, obesity, atherosclerosis, etc. Reactive oxygen species (ROS) produced by the various cell types serve as signaling molecules in several cellular mechanisms and regulate various aspects of growth-factor-mediated responses, including lymphangiogenesis. The ROS, including superoxide anion, hydrogen peroxide, and nitric oxide, play both beneficial and detrimental roles depending upon their levels and cellular microenvironment. Low ROS levels are essential for lymphangiogenesis. On the contrary, oxidative stress due to enhanced ROS generation and/or reduced levels of antioxidants suppresses lymphangiogenesis via promoting lymphatic endothelial cell apoptosis and death. In this review article, we provide an overview of types and sources of ROS, discuss the role of ROS in governing lymphangiogenesis and lymphatic function, and summarize the role of lymphatics in various diseases.

Induction of memory-like CD8+ T cells and CD4+ T cells from human naive T cells in culture

Memory T cells are crucial players in vertebrate adaptive immunity but their development is incompletely understood. Here, we describe a method to produce human memory-like T cells from naive human T cells in culture. Using commercially available human T-cell differentiation kits, both purified naive CD8⁺ T cells and purified naive CD4⁺ T cells were activated via T-cell receptor signaling and appropriate cytokines for several days in culture. All the T-cell activators were then removed from the medium and the cultures were continued in hypoxic condition (1% O₂ atmosphere) for several more days; during this period, most of the cells died, but some survived in a quiescent state for a month. The survivors had small round cell bodies, expressed differentiation markers characteristic of memory T cells and restarted proliferation when the T-cell activators were added back. We could also induce memory-like T cells from naive human T cells without hypoxia, if we froze the activated T cells or prepared the naive T cells from chilled filter buffy coats.

Estimation of the Lymph Flow Through Thoracic Duct in Human Subjects Using the Urine Osmolarity: Applicable for Evaluating the Effectiveness of Manual Lymph Drainage

Currently, there are many methods to evaluate the effectiveness of manual lymph drainage in the treatment of lymphedema, that is, limb volume measurement, bio-electrical impedance measurement, computer tomography, and ultrasound imaging. However, it is difficult for these methods to accurately address the lymph flow generated by manual lymph drainage. Therefore, we aimed at developing a concise and accurate method to measure the lymph flow through the thoracic duct in human subjects, which is applicable for evaluating the effectiveness of manual lymph drainage. In the present mini-review, we demonstrate the developed method in detail and its scientific evidence for the effectiveness obtained with animal and human clinical experiments. In rat in vivo experiments, intragastric administration of distilled water significantly increased mesenteric flow, which was transported via the cisterna chyli and then the thoracic duct. The manual massage on the cisterna chyli in the anesthetized rabbit significantly accelerated the lymph flow through the thoracic duct, resulting in marked hemodilution. Abdominal respiration in the supine position in human subjects produced similar hemodilution, with a marked decrease in the concentration of vasopressin in the blood. On this basis, we developed a new method to accurately measure the lymph flow through the thoracic duct by using changes in the concentration of vasopressin in the blood. In addition, with changes in urine osmolarity depending on the concentration of vasopressin in the blood, we developed a more concise and noninvasive method for evaluating the lymph flow through the thoracic duct in human subjects. These methods may be applicable for evaluating the effectiveness for the manual lymph drainage in the patients with lymphedema.

From digestion and absorption to innate immunity and health care: water and food intake may contribute to IL-22 in ILC3-dependent mucosal immunity in the jejunum

In this review, with our current studies we demonstrated medical evidence that water and food intake are useful for IL-22-related mucosal immunity-dependent maintenance of health care. The traditional Japanese health care practices recommend daily consumption of suitable volume of water. However, immunological mechanisms that support of the traditional practices are still unsolved. We focused on type 3 innate lymphoid cells (ILC3s), because the ILC3s are mainly housed in the lamina propria of the jejunum. IL-22 released from the ILC3 is transported through mesenteric lymph in collaboration with the albumin-mediated movement of consumed water. Thus, water intake-mediated upregulation of IL-22-dependent mucosal immunity contributes to the traditional Japanese health care practices. We also reviewed current studies that food intake-mediated increase in VIP-dependent neuronal activity in the small intestine and the food intake included with tryptophan-derived metabolites may accelerate the IL-22 in ILC3s-dependent mucosal immunity and then contribute in keeping health care.

Lymphatic Contractile Function: A Comprehensive Review of Drug Effects and Potential Clinical Application

BACKGROUND

The lymphatic system and the cardiovascular system work together to maintain body fluid homeostasis. Despite that, the lymphatic system has been relatively neglected as a potential drug target and a source of adverse effects from cardiovascular drugs. Like the heart, the lymphatic vessels undergo phasic contractions to promote lymph flow against a pressure gradient. Dysfunction or failure of the lymphatic pump results in fluid imbalance and tissue oedema. While this can due to drug effects, it is also a feature of breast cancer-associated lymphoedema, chronic venous insufficiency, congestive heart failure and acute systemic inflammation. There are currently no specific drug treatments for lymphatic pump dysfunction in clinical use despite the wealth of data from pre-clinical studies.

AIM

To identify (1) drugs with direct effects on lymphatic tonic and phasic contractions with potential for clinical application, and (2) drugs in current clinical use that have a positive or negative side effect on lymphatic function.

METHODS

We comprehensively reviewed all studies that tested the direct effect of a drug on the contractile function of lymphatic vessels.

RESULTS

Of the 208 drugs identified from 193 studies, about a quarter had only stimulatory effects on lymphatic tone, contraction frequency and/or contraction amplitude. Of FDA-approved drugs, there were 14 that increased lymphatic phasic contractile function. The most frequently used class of drug with inhibitory effects on lymphatic pump function were the calcium channels blockers.

CONCLUSION

This review highlights the opportunity for specific drug treatments of lymphatic dysfunction in various disease states and for avoiding adverse drug effects on lymphatic contractile function.

Water intake releases serotonin from enterochromaffin cells in rat jejunal villi

The present study aims to investigate the roles of water intake in serotonin production and release in rat jejunum. We evaluated the changes in concentrations of serotonin in the portal vein and mesenteric lymph vessel induced by the intragastric administration of distilled water. The density of granules in enterochromaffin cells and the immunoreactivity of serotonin in the jejunal villi were investigated before and after water intake. The effects of intravenous administration of serotonin and/or ketanserin on mesenteric lymph flow and concentrations of albumin and IL-22 in the lymph were also addressed. Water intake increased serotonin concentration in the portal vein, but not in the mesenteric lymph vessel. The flux of serotonin through the portal vein was significantly larger than that through the mesenteric lymph vessel. Water intake decreased the density of granules in the enterochromaffin cells and increased the immunoreactivity of serotonin in the jejunal villi. The intravenous administration of serotonin increased significantly mesenteric lymph flow and the concentrations of albumin and IL-22; both were significantly reduced by the intravenous pretreatment with ketanserin. We showed that serotonin released from enterochromaffin cells by water intake was mainly transported through the portal vein. Additionally, serotonin in blood was found to increase mesenteric lymph formation with permeant albumin in the jejunal villi via the activation of 5-HT2 receptor.

Water intake accelerates ATP release from myofibroblast cells in rats: ATP-mediated podoplanin-dependent control for physiological function and immunity

We previously demonstrated that water intake increased mesenteric lymph flow and the total flux of IL-22 in rat jejunum. The drained water and the higher permeability of albumin in the jejunal microcirculation contributed to increase the lymph flow and IL-22 transport via the activation of great bulk flow in the jejunal villi. To address the effects of water intake-mediated great bulk flow-dependent mechanical force on jejunal physiological function and immunological regulation of innate lymphoid cells (ILC)-3, we examined the effects of shear stress stimulation on cultured rat myofibroblast cells. Next, we investigated the effects of water intake on podoplanin and IL-22 expressions in cultured human intestinal epithelial cells and rat in vivo jejunal preparations, respectively. Shear stress stimulation of the myofibroblast cells induced ATP release via an activation of cell surface F₁/F₀ ATP synthase. ATP produced podoplanin expression in the intestinal epithelial cells. Water intake accelerated immunohistochemical expressions of podoplanin and IL-22 in the interepithelial layers and lamina propria of the jejunum. ATP dose-dependently increased IL-22 mRNA expression in ILC-3, which are housed in the lamina propria. Water intake also increased immunohistochemical and mRNA expressions of ecto-nucleoside triphosphate diphosphohydrolases 2 and 5 in jejunal villi. In conclusion, water intake-mediated shear stress stimulation-dependent ATP release from myofibroblast cells maintains higher tissue colloid osmotic pressure in the jejunal microcirculation through podoplanin upregulation in the interepithelial layers. ATP induces IL-22 mRNA expression in ILC-3 in jejunal villi, which may contribute to regulation of mucosal immunity in small intestine.NEW & NOTEWORTHY We investigated effects of shear stress stimulation on cultured myofibroblast cells and water intake on podoplanin and IL-22 expressions in rat jejunal villi. The stimulation induced ATP release from the cells. Water intake accelerated podoplanin and IL-22 expression levels. ATP increased IL-22 mRNA expression in innate lymphoid cells (ILC)-3. Hence, water intake maintains higher osmotic pressure in the jejunal villi through ATP release and podoplanin upregulation. Water intake may regulate the mucosal immunity.

The Role of Lymphatics in Cholestasis: A Comprehensive Review

Cholestatic liver disease affects millions of people worldwide and stems from a plethora of causes such as immune dysfunction, genetics, cancerous growths, and lifestyle choices. While not considered a classical lymphatic organ, the liver plays a vital role in the lymph system producing up to half of the body's lymph per day. The lymphatic system is critical to the health of an organism with its networks of vessels that provide drainage for lymphatic fluid and routes for surveilling immune cells. Cholestasis results in an increase of inflammatory cytokines, growth factors, and inflammatory infiltrate. Left unchecked, further disease progression will include collagen deposition which impedes both the hepatic and lymphatic ducts, eventually resulting in an increase in hepatic decompensation, increasing portal pressures, and accumulation of fluid within the abdominal cavity (ascites). Despite the documented interplay between these vital systems, little is known about the effect of liver disease on the lymph system and its biological response. This review looks at the current cholestatic literature from the perspective of the lymphatic system and summarizes what is known about the role of the lymph system in liver pathogenesis during hepatic injury and remodeling, immune-modulating events, or variations in interstitial pressures.

Mathematical Modelling of the Structure and Function of the Lymphatic System

This paper presents current knowledge about the structure and function of the lymphatic system. Mathematical models of lymph flow in the single lymphangion, the series of lymphangions, the lymph nodes, and the whole lymphatic system are considered. The main results and further perspectives are discussed.

Integrating Biological Advances Into the Clinical Management of Breast Cancer Related Lymphedema

Breast cancer-related lymphedema (BCRL) occurs in a significant number of breast cancer survivors as a consequence of the axillary lymphatics' impairment after therapy (mainly axillary surgery and irradiation). Despite the recent achievements in the clinical management of these patients, BCRL is often diagnosed at its occurrence. In most cases, it remains a progressive and irreversible condition, with dramatic consequences in terms of quality of life and on sanitary costs. There are still no validated pre-surgical strategies to identify individuals that harbor an increased risk of BCRL. However, clinical, therapeutic, and tumor-specific traits are recurrent in these patients. Over the past few years, many studies have unraveled the complexity of the molecular and transcriptional events leading to the lymphatic system ontogenesis. Additionally, molecular insights are coming from the study of the germline alterations involved at variable levels in BCRL models. Regrettably, there is a substantial lack of predictive biomarkers for BCRL, given that our knowledge of its molecular milieu remains extremely puzzled. The purposes of this review were (i) to outline the biology underpinning the ontogenesis of the lymphatic system; (ii) to assess the current state of knowledge of the molecular alterations that can be involved in BCRL pathogenesis and progression; (iii) to discuss the present and short-term future perspectives in biomarker-based patients' risk stratification; and (iv) to provide practical information that can be employed to improve the quality of life of these patients.

Current understanding of lymphatic vessels in the central nervous system

Lymphangiogenesis is associated with some pathological conditions such as inflammation, tissue repair, and tumor growth. Recently, a paradigm shift occurred following the discovery of meningeal lymphatic structures in the human central nervous system (CNS); these structures may be a key drainage route for cerebrospinal fluid (CSF) into the peripheral blood and may also contribute to inflammatory reaction and immune surveillance of the CNS. Lymphatic vessels located along the dural sinuses absorb CSF from the adjacent subarachnoid space and brain interstitial fluid via the glymphatic system, which is composed of aquaporin-4 water channels expressed on perivascular astrocytic end-feet membranes. Magnetic resonance imaging (MRI) clearly visualized these lymphatic vessels in the human dura mater. The conception of some neurological disorders, such as multiple sclerosis and Alzheimer's disease, has been changed by this paradigm shift. Meningeal lymphatic vessels could be a promising therapeutic target for the prevention of neurological disorders. However, the involvement of meningeal lymphatic vessels in the pathophysiology has not been fully elucidated and is the subject of future investigations. In this article, to understand the involvement of meningeal lymphatic vessels in neurological disorders, we review the differences between lymphangiogenesis in the CNS and in other tissues during both developmental and adulthood stages, and pathological conditions that may be associated with meningeal lymphatic vessels in the CNS.

Water intake increases mesenteric lymph flow and the total flux of albumin, long-chain fatty acids, and IL-22 in rats: new concept of absorption in jejunum

The traditional Japanese health care custom recommends that a suitable volume of water is consumed. However, physiological and immunological mechanisms in support of this practice are unknown. Therefore, we conducted rat and rabbit in vivo experiments to investigate the effects of intragastric administration of distilled water on the jejunal-originated lymph flow and the concentrations and total flux of cells, albumin, long-chain fatty acids, and innate lymphoid cell 3 (ILC-3)-secreted interleukin-22 (IL-22) through mesenteric lymph vessels. The distribution and activity of ILC-3 in rat small intestine by water intake were evaluated using flow cytometry and RT-PCR. The intragastric administration of distilled water caused significant increases in rat mesenteric lymph flow and in the total flux of cells, albumin, long-chain fatty acids, and IL-22 through the lymph vessels. Intravenously injected Evans blue dye was rapidly transported into rabbit mesenteric lymph vessel and cisterna chyli. The distribution of ILC-3 and the expression of IL-22 mRNA were maximal in the lamina propria cells of the rat jejunum. No significant presence of ILC-3 in the lymph was observed in the control and under water intake conditions. In conclusion, the absorbed water in the jejunum is transported through mesenteric lymph vessels. The higher permeability of albumin in the jejunal microcirculation may play key roles in the transport of consumed water and the reservoir and transporter of long-chain fatty acids. Water intake also accelerates the transfer of IL-22 to the mesenteric lymph, which may contribute, in part, to maintaining and promoting the innate immunity in the body. NEW & NOTEWORTHY The higher permeability of albumin-mediated transport of water-soluble substances in mesenteric lymph vessels of the jejunum may have a large impact on the classic concept suggesting that water-soluble small molecules travel to the liver via the portal vein. ILC-3 is mainly housed in the lamina propria of the jejunum, especially its upper part. IL-22 released from the ILC-3 is also transported through mesenteric lymph in collaboration with the albumin-mediated movement of consumed water.

Histamine as an Endothelium-Derived Relaxing Factor in Aged Mesenteric Lymphatic Vessels

BACKGROUND

Knowledge of the mechanisms by which aging affects contracting lymphatic vessels remains incomplete; therefore, the functional role of histamine in the reaction of aged lymphatic vessels to increases in flow remains unknown.

METHODS AND RESULTS

We measured and analyzed parameters of lymphatic contractility in isolated and pressurized rat mesenteric lymphatic vessels (MLVs) obtained from 9- and 24-month Fischer-344 rats under control conditions and after pharmacological blockade of nitric oxide (NO) by Nω-Nitro-L-arginine methyl ester hydrochloride (L-NAME, 100 μM) or/and blockade of histamine production by α-methyl-DL-histidine dihydrochloride (α-MHD, 10 μM). We also quantitatively compared results of immunohistochemical labeling of the histamine-producing enzyme, histidine decarboxylase (HDC) in adult and aged MLVs. Our data provide the first demonstration of an increased functional role of histamine as an endothelial-derived relaxing factor in aged MLVs, which appears in parallel with the abolished role of NO in the reactions of these lymph vessels to increases in flow. In addition, we found an increased expression of HDC in endothelium of aged MLVs.

CONCLUSIONS

Our findings provide the basis for better understanding of the processes of aging in lymphatic vessels and for setting new important directions for investigations of the aging-associated disturbances in lymph flow and the immune response.

Intraocular Pressure-Lowering Effect of Latanoprost Is Hampered by Defective Cervical Lymphatic Drainage

Purpose

To evaluate whether defects in cervical lymphatic drainage influence the intraocular pressure (IOP)-lowering effect of latanoprost in patients with primary open-angle glaucoma (POAG) who have undergone unilateral radical neck dissection (uRND).

Methods

We enrolled (1) bilateral POAG patients who had started (bilateral) latanoprost (0.005%) monotherapy prior to their uRND and (2) treatment-naïve, bilateral glaucoma suspects (GSs) who had undergone the same surgery. We compared the eyes ipsilateral to the uRND with their fellow eyes in terms of the changes in IOP between the baseline (prior to the uRND) and the follow-up visits (1, 3, and 6 months after the uRND).

Results

The study involved 22 eyes of 11 POAG patients and 14 eyes of 7 GSs. In the POAG patients, IOP had increased significantly after surgery in the eyes ipsilateral to the uRND (from 14.7±1.4mmHg to 17.1±2.2mmHg; P = 0.007). Interestingly, in the eyes contralateral to the uRND, IOP had not changed significantly after surgery (from 14.2±1.8mmHg to 14.4±2.0mmHg; P = 0.826). In GSs, the eyes ipsilateral to the uRND did not differ significantly from their fellow eyes in terms of post-operative IOP change (ipsilateral value: 0.3±0.5mmHg, fellow eyes: -0.1±0.7mmHg; P = 0.242).

Conclusion

In the POAG patients, IOP had increased significantly in the eyes ipsilateral to the uRND. However, it had not changed significantly in the eyes contralateral to the surgery or in the eyes of the GSs. These findings suggest that, latanoprost works, at least in part, by enhancing outflow from the aqueous humor via the uveolymphatic pathway.

Lymphatic drainage from the eye: A new target for therapy

Lowering intraocular pressure (IOP) has been central to glaucoma care for over a century. In order to prevent sight loss from disease, there has been considerable focus on medical and surgical methods to improve fluid drainage from the eye. In spite of this, our understanding of exactly how aqueous humor leaves the eye is not complete. Recently, lymphatic vessels have been discovered in the human uvea, with studies showing lymphatic fluid outflow in several models, in addition to evidence for their pharmacological enhancement. The presence of a lymphatic outflow system points to an exciting, expanded understanding of how fluid and particulate materials such as proteins move out of the eye, and how IOP may be regulated. We coin the term "uveolymphatic pathway"-to reflect a comprehensive and compelling new target for glaucoma and an exciting opportunity for future investigations to better understand the eye in health and disease.

Lipid-coated ZnO nanoparticles as lymphatic-targeted drug carriers: study on cell-specific toxicity in vitro and lymphatic targeting in vivo

Lipid coated ZnO nanoparticles (LZnO NPs) were developed as a novel lymphatic drug delivery system. High lymphotropism and tumour cells selective toxicity ensure the nanoparticles being a promising candidate for treatment of tumor metastasis.

Proposed new lymphology combined with lymphatic physiology, innate immunology, and oncology

As one of the lymphatic functions, it is well known that the transport and drainage of hydrophilic substances including plasma protein through the lymphatic system play pivotal roles in maintaining the homeostasis of the internal environment between the cells in tissues in collaboration with the exchange of the substances through the blood capillaries and venules. The physiological functions of the lymphatic system have been studied by many investigations of microcirculation, i.e., Yoffey and Courtice, Ruszunyak et al., Földie and Casley-Smigh et al., Roddie, Schmid-Schönbein et al., and Ohhashi et al. On the other hand, it is also well known that the initial clinical signs of primary diseases such as inflammation, tumors, and circulatory disorders including infarction and thrombosis appear as functional abnormalities of the internal environment in tissues. These abnormalities of the functions are strongly related to immunological defense reactions around the internal environment and abnormal actions of the transport and drainage of the lymphatic system. Taking into consideration the current inspired findings in lymphatic physiology, innate immunology, and oncology, we have proposed a new lymphology combined with new knowledge of the three above-mentioned academic fields from a defense mechanism points of view. In this review, we would like to demonstrate comprehensively our latest studies related to the possibility of establishing a new lymphology, hoping the readers will evaluate this possibility.

Primary and secondary lymphatic valve development: molecular, functional and mechanical insights

Fluid homeostasis in vertebrates critically relies on the lymphatic system forming a hierarchical network of lymphatic capillaries and collecting lymphatics, for the efficient drainage and transport of extravasated fluid back to the cardiovascular system. Blind-ended lymphatic capillaries employ specialized junctions and anchoring filaments to encourage a unidirectional flow of the interstitial fluid into the initial lymphatic vessels, whereas collecting lymphatics are responsible for the active propulsion of the lymph to the venous circulation via the combined action of lymphatic muscle cells and intraluminal valves. Here we describe recent findings on molecular and physical factors regulating the development and maturation of these two types of valves and examine their role in tissue-fluid homeostasis.

Mechanosensing in developing lymphatic vessels

The lymphatic vasculature is responsible for fluid homeostasis, transport of immune cells, inflammatory molecules, and dietary lipids. It is composed of a network of lymphatic capillaries that drain into collecting lymphatic vessels and ultimately bring fluid back to the blood circulation. Lymphatic endothelial cells (LECs) that line lymphatic capillaries present loose overlapping intercellular junctions and anchoring filaments that support fluid drainage. When interstitial fluid accumulates within tissues, the extracellular matrix (ECM) swells and pulls the anchoring filaments. This results in opening of the LEC junctions and permits interstitial fluid uptake. The absorbed fluid is then transported within collecting lymphatic vessels, which exhibit intraluminal valves that prevent lymph backflow and smooth muscle cells that sequentially contract to propel lymph.Mechanotransduction involves translation of mechanical stimuli into biological responses. LECs have been shown to sense and respond to changes in ECM stiffness, fluid pressure-induced cell stretch, and fluid flow-induced shear stress. How these signals influence LEC function and lymphatic vessel growth can be investigated by using different mechanotransduction assays in vitro and to some extent in vivo.In this chapter, we will focus on the mechanical forces that regulate lymphatic vessel expansion during embryonic development and possibly secondary lymphedema. In mouse embryos, it has been recently shown that the amount of interstitial fluid determines the extent of lymphatic vessel expansion via a mechanosensory complex formed by β1 integrin and vascular endothelial growth factor receptor-3 (VEGFR3). This model might as well apply to secondary lymphedema.

Cell surface F1/FO ATP synthase contributes to interstitial flow-mediated development of the acidic microenvironment in tumor tissues

To address pivotal roles of cell surface F1/FO ATP synthase in the development of acidic microenvironment in tumor tissues, we investigated effects of shear stress stimulation on the cultured human breast cancer cells, MDA-MB-231 and MDA-MB-157, or human melanoma cells, SK-Mel-1. Shear stress stimulation (0.5-5.0 dyn/cm(2)), the levels of which are similar to those produced by the interstitial flow, induced strength-dependent corelease of ATP and H(+) from the cells, which triggered CO2 gas excretion. In contrast, the same level of shear stress stimulation did not induce significant ATP release and CO2 gas excretion from the control human mammary epithelial cells (HMEC). Marked immunocytochemical and mRNA expression of cell surface F1/FO ATP synthase, vacuolar-ATPase (V-ATPase), carbonic anhydrase type IX, and ectonucleoside triphosphate diphosphohydrolase (ENTPDase) 3 were detected in MDA-MB-231 cells, but little or no expression on the HMEC. Pretreatment with cell surface F1/FO ATP synthase inhibitors, but not cell surface V-ATPase inhibitors, caused a significant reduction of the shear stress stimulation-mediated ATP release and CO2 gas excretion from MDA-MB-231 cells. The ENTPDase activity in the shear stress-loaded MDA-MB-231 cell culture medium supernatant increased significantly in a time-dependent manner. In addition, MDA-MB-231 cells displayed strong staining for purinergic 2Y1 (P2Y1) receptors on their surfaces, and the receptors partially colocalized with ENTPDase 3. These findings suggest that cell surface F1/FO ATP synthase, but not V-ATPase, may play key roles in the development of interstitial flow-mediated acidic microenvironment in tumor tissues through the shear stress stimulation-induced ATP and H(+) corelease and CO2 gas production.

Aged lymphatic contractility: recent answers and new questions

Abstract An overview is presented of recent findings related to biology of aging of the lymph transport system. The authors discuss recently obtained data on the aging-associated alterations of lymphatic contractility in thoracic duct and mesenteric lymphatic vessels; on comparisons of function of aged mesenteric lymphatic vessels in situ versus isolated specimens and important conclusions which arose from these studies; on aging-associated changes in functional status of mast cells located close to aged mesenteric lymphatic vessels; on evidence of presence of oxidative stress in aged lymphatic vessels and changes in arrangement of muscle cells in their walls. The authors conclude that future continuation of the research efforts in this area is necessary and will be able to provide not only novel fundamental knowledge on the biology of lymphatic aging, but also will create solid foundation for the subsequent developments of lymphatic-oriented therapeutic interventions in many diseases of the elderly.

Cyclic guanosine monophosphate and the dependent protein kinase regulate lymphatic contractility in rat thoracic duct

  We have previously demonstrated a principal role for nitric oxide (NO) in the endothelium/shear-dependent regulation of contractility in rat thoracic duct (TD). In this study we tested the hypothesis that cyclic guanosine monophosphate (cGMP) and the dependent protein kinase (PKG) are central to the intrinsic and extrinsic flow-dependent modulation of lymphatic contractility. Lymphatic diameters and indices of pumping in isolated, cannulated and pressurized segments of rat TD were measured. The influences of increased transmural pressure (1-5 cmH2O) and imposed flow (1-5 cm H2O transaxial pressure gradients) on lymphatic function were studied before and after: (1) inhibition of guanylate cyclase (GC) with and without a NO donor; (2) application of stable cGMP analogue; and (3) inhibition of the cGMP activation of PKG. Additionally, Western blotting and immunofluorescent tissue staining were used to analyse the PKG isoforms expressed in TD. We found that the GC inhibitor ODQ induced changes in TD contractility similar to NO synthase blockade and prevented the relaxation induced by the NO donor S-nitroso-N-acetylpenicillamine. The cGMP analogue, 8-(4-Chlorophenylthio)-guanosine 3,5-cyclic monophosphate sodium salt (8pCPTcGMP), mimicked the extrinsic flow-induced relaxation in a dose-dependent manner, whereas treatment with the cGMP/PKG inhibitor, guanosine 3,5-cyclic monophosphorothioate, 8-(4-chlorophenylthio)-, Rp-isomer, triethylammonium salt (Rp-8-Br-PETcGMPS), eliminated intrinsic flow-dependent relaxation, and largely inhibited extrinsic flow-dependent relaxation. Western blotting demonstrated that both PKG-Iα and -Iβ isoforms are found in TD, with ∼10 times greater expression of the PKG-Iα protein in TD compared with the aorta and vena cava. The PKG-Iβ isoform expressed equally in TD and vena cava, both being ∼2 times higher than that in the aorta. Immunofluorescent labelling of PKG-Iα protein in the wall of rat thoracic duct confirmed its localization inside TD muscle cells. These findings demonstrate that cGMP is critical to the flow-dependent regulation of TD contractility; they also indicate an important involvement of PKG, especially PKG-Iα in these processes and identifies PKG protein as a potential therapeutic target.

Hypoxia and hypoxia-inducible factors as regulators of T cell development, differentiation, and function

Oxygen is a molecule that is central to cellular respiration and viability, yet there are multiple physiologic and pathological contexts in which cells experience conditions of insufficient oxygen availability, a state known as hypoxia. Given the metabolic challenges of a low oxygen environment, hypoxia elicits a range of adaptive responses at the cellular, tissue, and systemic level to promote continued survival and function. Within this context, T lymphocytes are a highly migratory cell type of the adaptive immune system that frequently encounters a wide range of oxygen tensions in both health and disease. It is now clear that oxygen availability regulates T cell differentiation and function, a response orchestrated in large part by the hypoxia-inducible factor transcription factors. Here, we discuss the physiologic scope of hypoxia and hypoxic signaling, the contribution of these pathways in regulating T cell biology, and current gaps in our understanding. Finally, we discuss how emerging therapies that modulate the hypoxic response may offer new modalities to alter T cell function and the outcome of acute and chronic pathologies.

Current topics of immunohistochemical and biological properties of human lymphatic endothelial cells

We examined the immunohistochemical properties of selective lymph vessel markers such as LYVE-1, podoplanin, Prox-1, and VEGF R3, as well as NO synthase (NOS) and cyclo-oxygenase (COX) in two kinds of human lymphatic endothelial cell isolated from collecting and initial lymph vessels. The constitutively expressed genes in the two kinds of lymphatic endothelial cell were also evaluated using oligonucleotide microarray analysis and RT-PCR. We also investigated the effects of the oxygen concentration in culture conditions on the proliferative activities of the two kinds of human lymphatic endothelial cell. Immunoreactivity to LYVE-1 and the RT-PCR expression level of LYVE-1 mRNA in endothelial cells of initial lymph vessels were stronger than those of collecting lymph vessels. Immunoreactivity to ecNOS, iNOS, COX1, and COX2 was also found to be significantly higher than in collecting lymph vessels. In contrast, an increase in the O2 concentration ranging from 5% to 21% caused a significant reduction in the proliferative activity of endothelial cells in collecting lymph vessels. In conclusion, these findings suggest that there exists a marked heterogeneity in the immunohistochemical, genomic, and proliferative activity of human lymphatic endothelial cells between initial and collecting lymph vessels. (*English Translation of J Jpn Coll Angiol, 2008, 48: 125-130.).

Recent advance in lymph dynamic analysis in lymphatics and lymph nodes

Lymphatics are a unidirectional transport system that carries fluid from the interstitial space and back into the blood stream. Initial lymphatics take up not only fluid but also high-molecular-weight substances, such as plasma proteins and hyaluronan; immune cells, such as lymphocytes, macrophages, and dendritic cells; and colloidal particles, such as carbon particles, bacteria, and tattoo dye. Interstitially injected colloidal particles are known to accumulate in the regional lymph nodes. This phenomenon is applied to find sentinel lymph nodes in cancer patients. Lymph flow rate and composition are influenced by interstitial fluid, lymphatic pump activity, and intra-lymphatic pressure. Lymph composition is changed during its flow downstream. In this review, the main focus is on the mechanisms of lymph formation at the initial lymphatics and lymph transport through the collecting lymphatics and lymph nodes. (*English Translation of J Jpn Coll Angiol, 2008, 48: 113-123.).

Lymphatic vessels in health and disease

The lymphatic vasculature plays vital roles in tissue fluid balance, immune defense, metabolism, and cancer metastasis. In adults, lymphatic vessel formation and remodeling occur primarily during inflammation, development of the corpus luteum, wound healing, and tumor growth. Unlike the blood circulation, where unidirectional flow is sustained by the pumping actions of the heart, pumping actions intrinsic to the lymphatic vessels themselves are important drivers of lymphatic flow. This review summarizes critical components that control lymphatic physiology.

Evidence of increased oxidative stress in aged mesenteric lymphatic vessels

BACKGROUND

We have previously shown that aging is associated with weakened rat mesenteric lymphatic vessel (MLV) contractility. However, the specific mechanisms contributing to this aging-associated contractile degeneration remain unknown. Aging is often associated with elevations in oxidative stress, and reactive oxygen species (ROS) have been shown to reduce the contractility of MLV. Thus in the present study, we sought to assess whether aging is associated with increased levels of oxidative stress and oxidative damage in MLV.

METHODS AND RESULTS

MLV were isolated from 9-mo- and 24-mo-old Fischer-344 rats and subjected to the following experimental techniques: measurement of total superoxide dismutase (SOD) activity; estimation of lipid peroxidation levels via measurement of thiobarbituric acid reactive substances (TBARS); detection of superoxide and mitochondrial ROS in live MLV; Western blot analysis, and immunohistochemical labeling of the SOD isoforms and nitro-tyrosine proteins. We found that aging is associated with increased levels of cellular superoxide and mitochondrial ROS concomitant with a reduction in Cu/Zn-SOD protein expression and total SOD enzymatic activity in MLV. This increase in oxidative stress and decrease in antioxidant activity was associated with evidence of increased lipid (as indicated by TBARS) and protein (as indicated by nitro-tyrosine labeling) oxidative damage.

CONCLUSIONS

Thus for the first time, we demonstrate that aging-associated increases in oxidative stress and oxidative damage is indeed present in the walls of MLV and may contribute to the aging-associated lymphatic pump dysfunction we previously reported.

Impaired lymphatic contraction associated with immunosuppression

To trigger an effective immune response, antigen and antigen-presenting cells travel to the lymph nodes via collecting lymphatic vessels. However, our understanding of the regulation of collecting lymphatic vessel function and lymph transport is limited. To dissect the molecular control of lymphatic function, we developed a unique mouse model that allows intravital imaging of autonomous lymphatic vessel contraction. Using this method, we demonstrated that endothelial nitric oxide synthase (eNOS) in lymphatic endothelial cells is required for robust lymphatic contractions under physiological conditions. By contrast, under inflammatory conditions, inducible NOS (iNOS)-expressing CD11b(+)Gr-1(+) cells attenuate lymphatic contraction. This inhibition of lymphatic contraction was associated with a reduction in the response to antigen in a model of immune-induced multiple sclerosis. These results suggest the suppression of lymphatic function by the CD11b(+)Gr-1(+) cells as a potential mechanism of self-protection from autoreactive responses during on-going inflammation. The central role for nitric oxide also suggests that other diseases such as cancer and infection may also mediate lymphatic contraction and thus immune response. Our unique method allows the study of lymphatic function and its molecular regulation during inflammation, lymphedema, and lymphatic metastasis.

Mesenteric lymph flow in adult and aged rats

The objective of study was to evaluate the aging-associated changes, contractile characteristics of mesenteric lymphatic vessels (MLV), and lymph flow in vivo in male 9- and 24-mo-old Fischer-344 rats. Lymphatic diameter, contraction amplitude, contraction frequency, and fractional pump flow, lymph flow velocity, wall shear stress, and minute active wall shear stress load were determined in MLV in vivo before and after N(ω)-nitro-L-arginine methyl ester hydrochloride (L-NAME) application at 100 μM. The active pumping of the aged rat MLV in vivo was found to be severely depleted, predominantly through the aging-associated decrease in lymphatic contractile frequency. Such changes correlate with enlargement of aged MLV, which experienced much lower minute active shear stress load than adult vessels. At the same time, pumping in aged MLV in vivo may be rapidly increased back to levels of adult vessels predominantly through the increase in contraction frequency induced by nitric oxide (NO) elimination. Findings support the idea that in aged tissues surrounding the aged MLV, the additional source of some yet unlinked lymphatic contraction-stimulatory metabolites is counterbalanced or blocked by NO release. The comparative analysis of the control data obtained from experiments with both adult and aged MLV in vivo and from isolated vessel-based studies clearly demonstrated that ex vivo isolated lymphatic vessels exhibit identical contractile characteristics to lymphatic vessels in vivo.

Gastrointestinal lymphatics in health and disease

Lymphatics perform essential transport and immune regulatory functions to maintain homeostasis in the gastrointestinal (GI) system. Although blood and lymphatic vessels function as parallel and integrated systems, our understanding of lymphatic structure, regulation and functioning lags far behind that of the blood vascular system. This chapter reviews lymphatic flow, differences in lymphangiogenic and hemangiogenic factors, lymphatic fate determinants and structural features, and examines how altered molecular signaling influences lymphatic function in organs of the GI system. Innate errors in lymphatic development frequently disturb GI functioning and physiology. Expansion of lymphatics, a prominent feature of GI inflammation, may also play an important role in tissue restitution following injury. Destruction or dysregulation of lymphatics, following injury, surgery or chronic inflammation also exacerbates GI disease activity. Understanding the physiological roles played by GI lymphatics is essential to elucidating their underlying contributions to forms of congenital and acquired forms of GI pathology, and will provide novel approaches for therapy.

Connexin37 and Connexin43 deficiencies in mice disrupt lymphatic valve development and result in lymphatic disorders including lymphedema and chylothorax

Intraluminal valves are required for the proper function of lymphatic collecting vessels and large lymphatic trunks like the thoracic duct. Despite recent progress in the study of lymphvasculogenesis and lymphangiogenesis, the molecular mechanisms controlling the morphogenesis of lymphatic valves remain poorly understood. Here, we report that gap junction proteins, or connexins (Cxs), are required for lymphatic valvulogenesis. Cx37 and Cx43 are expressed early in mouse lymphatic development in the jugular lymph sacs, and later in development these Cxs become enriched and differentially expressed by lymphatic endothelial cells on the upstream and downstream sides of the valves. Specific deficiencies of Cx37 and Cx43 alone or in combination result in defective valve formation in lymphatic collecting vessels, lymphedema, and chylothorax. We also show that Cx37 regulates jugular lymph sac size and that both Cx37 and Cx43 are required for normal thoracic duct development, including valve formation. Another Cx family member, Cx47, whose human analog is mutated in some families with lymphedema, is also highly enriched in a subset of endothelial cells in lymphatic valves. Mechanistically, we present data from Foxc2-/- embryos suggesting that Cx37 may be a target of regulation by Foxc2, a transcription factor that is mutated in human lymphedema-distichiasis syndrome. These results show that at least three Cxs are expressed in the developing lymphatic vasculature and, when defective, are associated with clinically manifest lymphatic disorders in mice and man.

Methods for lymphatic vessel culture and gene transfection

OBJECTIVE

To develop the techniques needed for the specific gene/protein targeting transfection experiments in isolated lymphatic vessels, we completed two major tasks: 1) optimize the experimental conditions to maintain the viability of isolated rat lymphatic vessels in culture for sufficiently long periods of time to permit knockdown or overexpression of selected proteins/genes and 2) develop effective transfection protocols for lymphatic muscle and endothelial cells in intact lymphatic vessels without nonspecific impairment of lymphatic contractile function due to the transfection protocol itself.

METHODS

Experimental protocols were developed for the maintenance of isolated lymphatic vessels under nonpressurized and pressurized conditions for 3-12 days in culture and for adenoviral gene transfection of the lymphatic muscle and endothelial cells.

RESULTS

The data demonstrate the effectiveness of the newly developed experimental protocols for the maintenance of isolated rat mesenteric lymphatic vessels and thoracic duct in culture up to 3-12 days without significant impairment of the parameters of their pumping and effective adenoviral/GFP transfection of lymphatic endothelial and muscle cells in isolated rat mesenteric lymphatic vessels.

CONCLUSIONS

These experimental techniques will extend the set of the modern experimental tools available to researchers investigating the physiology of lymphatic function.

Shear stress-induced ATP-mediated endothelial constitutive nitric oxide synthase expression in human lymphatic endothelial cells

To clarify the roles of lymphatic endothelial cells (LEC) in the regulation of endothelial constitutive nitric oxide synthase (ecNOS) expression, we examined the effects of shear stress on ecNOS immunohistochemical staining and mRNA and protein expression in human LEC as well as on ATP release from these cells. Shear stress at 0.5 or 1.0 dyn/cm2 increased ecNOS immunohistochemical staining and ecNOS mRNA and protein expression in cultured LEC. The same strength of shear stress produced a significant release of ATP from the LEC. Exogenous ATP ranging in concentration from 10−9 to 10−6 M produced a significant increase in ecNOS immunohistochemical expression in a dose-dependent manner. The increase in ecNOS expression mediated by 10−6M ATP was significantly reduced by 10−5 M suramin. Suramin (10−5 M) caused a significant reduction in the shear stress-mediated increases in ecNOS immunohistochemical staining and mRNA expression. The shear stress-mediated increases in ecNOS expression were significantly reduced by 3 mM tetraethylammonium, 10−4 M apamin, 10−9 M iberiotoxin, 10−5 M 2-aminoethoxydephenyl borate, or 10−5M xestospongin C, but not 10−5 M glybenclamide or 10−5 M nifedipine. The shear stress-mediated increases in ecNOS expression were significantly potentiated by pinacidil or NS1619 in a dose-dependent manner. The immunohistochemical expression of small- (SKCa) and big-conductance (BKCa) Ca2+-activated K+ channels was confirmed on the surfaces of human LEC. These findings suggest that shear stress produces a significant release of ATP from LEC, which activates the purinergic P2X/2Y receptor, thereby facilitating ecNOS mRNA and protein expression through inositol 1,4,5-trisphosphate-mediated release of intracellular Ca2+ ions and the activation of Ca2+-activated K+ channels in LEC.

Lymphatic endothelial cells, lymphedematous lymphangiogenesis, and molecular control of edema formation

Lymphedema, defined as the abnormal accumulation of protein-rich fluid in soft tissues, results from the dysfunction of lymphatic system, an imbalance between lymph formation and its absorption into the initial lymphatics. Primary lymphedema occurs rarely on idiopathic or developmental abnormalities, especially hypoplasia or aplasia of lymphatics. Secondary lymphedema commonly develops when lymph transport is impaired due to lymphatic damage or resection of lymph nodes in surgery, infection, and radiation. Lymphatic endothelial cells (LECs) actively participate in the phenotypic consequences of a deranged lymphangiogenesis relating to tissue fluid accumulation in the pathogenesis of lymphedema. Recent insights into molecular genetic bases have shown an updated genotype-phenotype correlation between lymphangiogenesis, lymphatic function, and lymphedema. FOXC2, EphrinB2, VEGFR-3, VEGF-C, angiopoietin-2, Prox-1 and podoplanin have proved to be important factors of the genetic cascade linking to hereditary lymphedema, and embryonic and postnatal lymphatic development. FOXC2 may have a key role in regulating interactions between LECs and smooth muscle cells, and in the morphogenesis of lymphatic valves. Reduced VEGFR-3 tyrosine kinase activity and subsequent failure in transducing sufficient physiological VEGF-C/-D signals may affect LEC function and structure in the intercellular junctions and peri-lymphatic components. Identification of genetic markers in humans and animal models would facilitate the management of environmental factors influencing the expression and severity of lymphedema, and provide a basis for developing novel targeted therapies for the disease.