Genetic removal of basal nitric oxide enhances contractile activity in isolated murine collecting lymphatic vessels

Characterization of the cellular components of mouse collecting lymphatic vessels reveals that lymphatic muscle cells are the innate pacemaker cells regulating lymphatic contractions

Collecting lymphatic vessels (cLVs) exhibit spontaneous contractions with a pressure-dependent frequency, but the identity of the lymphatic pacemaker cell is still debated. By analogy to pacemakers in the GI and lower urinary tracts, proposed cLV pacemaker cells include interstitial cells of Cajal like cells (ICLC) or the lymphatic muscle (LMCs) cells themselves. Here we combined immunofluorescence and scRNAseq analyses with electrophysiological methods to examine the cellular constituents of the mouse cLV wall and assess whether any cell type exhibited morphological and functional processes characteristic of pacemaker cells: a continuous if not contiguous network integrated into the electrical syncytium; spontaneous Ca2+ transients; and depolarization-induced propagated contractions. We employed inducible Cre (iCre) mouse models routinely used to target these specific cell populations including: c-kitCreER T2 to target ICLC; PdgfrβCreER T2 to target pericyte-like cells; PdgfrαCreER ™ to target CD34+ adventitial cells and ICLC; and Myh11CreER T2 to target LMCs directly. These specific inducible Cre lines were crossed to the fluorescent reporter ROSA26mT/mG, the genetically encoded Ca2+ sensor GCaMP6f, and the light-activated cation channel rhodopsin2 (ChR2). c-KitCreER T2 labeled both a sparse population of LECs and round adventitial cells that responded to the mast cell activator compound 48-80. PdgfrβCreER T2 drove recombination in both adventitial cells and LMCs, limiting its power to discriminate a pericyte-specific population. PdgfrαCreER ™ labeled a large population of interconnected, oak leaf-shaped cells primarily along the adventitial surface of the vessel. Of these cells, only LMCs consistently, but heterogeneously, displayed spontaneous Ca2+ events during the diastolic period of the contraction cycle, and whose frequency was modulated in a pressure-dependent manner. Optogenetic depolarization through the expression of ChR2 under control of Myh11CreER T2 , but not PdgfrαCreER ™ or c-KitCreER T2 , resulted in propagated contractions upon photo-stimulation. Membrane potential recordings in LMCs demonstrated that the rate of diastolic depolarization significantly correlated with contraction frequency. These findings support the conclusion that LMCs, or a subset of LMCs, are responsible for mouse cLV pacemaking.

Overview of Lymphatic Muscle Cells in Development, Physiology, and Disease

Lymphatic muscle cells (LMCs) are indispensable for proper functioning of the lymphatic system, as they provide the driving force for lymph transport. Recent studies have advanced our understanding of the molecular mechanisms that regulate LMCs, which control rhythmic contraction and vessel tone of lymphatic vessels-traits also found in cardiac and vascular smooth muscle. In this review, we discuss the molecular pathways that orchestrate LMC-mediated contractility and summarize current knowledge about their developmental origin, which may shed light on the distinct contractile characteristics of LMCs. Further, we highlight the growing evidence implicating LMC dysregulation in the pathogenesis of lymphedema and other diseases related to lymphatic vessel dysfunction. Given the limited number and efficacy of existing therapies to treat lymphedema, LMCs present a promising focus for identifying novel therapeutic targets aimed at improving lymphatic vessel contractility. Here, we discuss LMCs in health and disease, as well as therapeutic strategies aimed at targeting them to improve lymphatic vessel function.

Acute Metabolic Stress Induces Lymphatic Dysfunction Through KATP Channel Activation

Lymphatic dysfunction is an underlying component of multiple metabolic diseases, including diabetes, obesity, and metabolic syndrome. We investigated the roles of KATP channels in lymphatic contractile dysfunction in response to acute metabolic stress induced by inhibition of the mitochondrial electron transport chain. Ex vivo popliteal lymphatic vessels from mice were exposed to the electron transport chain inhibitors antimycin A and rotenone, or the oxidative phosphorylation inhibitor/protonophore, CCCP. Each inhibitor led to a significant reduction in the frequency of spontaneous lymphatic contractions and calculated pump flow, without a significant change in contraction amplitude. Contraction frequency was restored by the KATP channel inhibitor, glibenclamide. Lymphatic vessels from mice with global Kir6.1 deficiency or expressing a smooth muscle-specific dominant negative Kir6.1 channel were resistant to inhibition. Antimycin A inhibited the spontaneous action potentials generated in lymphatic muscle and this effect was reversed by glibenclamide, confirming the role of KATP channels. Antimycin A, but not rotenone or CCCP, increased dihydrorhodamine fluorescence in lymphatic muscle, indicating ROS production. Pretreatment with tiron or catalase prevented the effect of antimycin A on wild-type lymphatic vessels, consistent with its action being mediated by ROS. Our results support the conclusion that KATP channels in lymphatic muscle can be directly activated by reduced mitochondrial ATP production or ROS generation, consequent to acute metabolic stress, leading to contractile dysfunction through inhibition of the ionic pacemaker controlling spontaneous lymphatic contractions. We propose that a similar activation of KATP channels contributes to lymphatic dysfunction in metabolic disease.

Molecular pathophysiology of secondary lymphedema

Lymphedema occurs as a result of lymphatic vessel damage or obstruction, leading to the lymphatic fluid stasis, which triggers inflammation, tissue fibrosis, and adipose tissue deposition with adipocyte hypertrophy. The treatment of lymphedema is divided into conservative and surgical approaches. Among surgical treatments, methods like lymphaticovenular anastomosis and vascularized lymph node transfer are gaining attention as they focus on restoring lymphatic flow, constituting a physiologic treatment approach. Lymphatic endothelial cells form the structure of lymphatic vessels. These cells possess button-like junctions that facilitate the influx of fluid and leukocytes. Approximately 10% of interstitial fluid is connected to venous return through lymphatic capillaries. Damage to lymphatic vessels leads to lymphatic fluid stasis, resulting in the clinical condition of lymphedema through three mechanisms: Inflammation involving CD4+ T cells as the principal contributing factor, along with the effects of immune cells on the VEGF-C/VEGFR axis, consequently resulting in abnormal lymphangiogenesis; adipocyte hypertrophy and adipose tissue deposition regulated by the interaction of CCAAT/enhancer-binding protein α and peroxisome proliferator-activated receptor-γ; and tissue fibrosis initiated by the overactivity of Th2 cells, leading to the secretion of profibrotic cytokines such as IL-4, IL-13, and the growth factor TGF-β1. Surgical treatments aimed at reconstructing the lymphatic system help facilitate lymphatic fluid drainage, but their effectiveness in treating already damaged lymphatic vessels is limited. Therefore, reviewing the pathophysiology and molecular mechanisms of lymphedema is crucial to complement surgical treatments and explore novel therapeutic approaches.

Targeting lymphatic function in cardiovascular-kidney-metabolic syndrome: preclinical methods to analyze lymphatic function and therapeutic opportunities

The lymphatic vascular system spans nearly every organ in the body and serves as an important network that maintains fluid, metabolite, and immune cell homeostasis. Recently, there has been a growing interest in the role of lymphatic biology in chronic disorders outside the realm of lymphatic abnormalities, lymphedema, or oncology, such as cardiovascular-kidney-metabolic syndrome (CKM). We propose that enhancing lymphatic function pharmacologically may be a novel and effective way to improve quality of life in patients with CKM syndrome by engaging multiple pathologies at once throughout the body. Several promising therapeutic targets that enhance lymphatic function have already been reported and may have clinical benefit. However, much remains unclear of the discreet ways the lymphatic vasculature interacts with CKM pathogenesis, and translation of these therapeutic targets to clinical development is challenging. Thus, the field must improve characterization of lymphatic function in preclinical mouse models of CKM syndrome to better understand molecular mechanisms of disease and uncover effective therapies.

TRPV4-Expressing Tissue-Resident Macrophages Regulate the Function of Collecting Lymphatic Vessels via Thromboxane A2 Receptors in Lymphatic Muscle Cells

Rationale

TRPV4 channels are critical regulators of blood vascular function and have been shown to be dysregulated in many disease conditions in association with inflammation and tissue fibrosis. These are key features in the pathophysiology of lymphatic system diseases, including lymphedema and lipedema; however, the role of TRPV4 channels in the lymphatic system remains largely unexplored. TRPV4 channels are calcium permeable, non-selective cation channels that are activated by diverse stimuli, including shear stress, stretch, temperature, and cell metabolites, which may regulate lymphatic contractile function.

Objective

To characterize the expression of TRPV4 channels in collecting lymphatic vessels and to determine the extent to which these channels regulate the contractile function of lymphatics.

Methods and Results

Pressure myography on intact, isolated, and cannulated lymphatic vessels showed that pharmacological activation of TRPV4 channels with GSK1016790A (GSK101) led to contractile dysregulation. The response to GSK101 was multiphasic and included, 1) initial robust constriction that was sustained for ≥1 minute and in some instances remained for ≥4 minutes; and 2) subsequent vasodilation and partial or complete inhibition of lymphatic contractions associated with release of nitric oxide. The functional response to activation of TRPV4 channels displayed differences across lymphatics from four anatomical regions, but these differences were consistent across different species (mouse, rat, and non-human primate). Importantly, similar responses were observed following activation of TRPV4 channels in arterioles. The initial and sustained constriction was prevented with the COX inhibitor, indomethacin. We generated a controlled and spatially defined single-cell RNA sequencing (scRNAseq) dataset from intact and microdissected collecting lymphatic vessels. Our data uncovered a subset of macrophages displaying the highest expression of Trpv4 compared to other cell types within and surrounding the lymphatic vessel wall. These macrophages displayed a transcriptomic profile consistent with that of tissue-resident macrophages (TRMs), including differential expression of Lyve1 , Cd163 , Folr2 , Mrc1 , Ccl8 , Apoe , Cd209f , Cd209d , and Cd209g ; and at least half of these macrophages also expressed Timd4. This subset of macrophages also highly expressed Txa2s , which encodes the thromboxane A2 (TXA2) synthase. Inhibition of TXA2 receptors (TXA2Rs) prevented TRPV4-mediated contractile dysregulation. TXA2R activation on LMCs caused an increase in mobilization of calcium from intracellular stores through Ip3 receptors which promoted store operated calcium entry and vasoconstriction.

Conclusions

Clinical studies have linked cancer-related lymphedema with an increased infiltration of macrophages. While these macrophages have known anti-inflammatory and pro-lymphangiogenic roles, as well as promote tissue repair, our results point to detrimental effects to the pumping capacity of collecting lymphatic vessels mediated by activation of TRPV4 channels in macrophages. Pharmacological targeting of TRPV4 channels in LYVE1-expressing macrophages or pharmacological targeting of TXA2Rs may offer novel therapeutic strategies to improve lymphatic pumping function and lymph transport in lymphedema.

Pacemaking in the lymphatic system

Lymphatic collecting vessels exhibit spontaneous phasic contractions that are critical for lymph propulsion and tissue fluid homeostasis. This rhythmic activity is driven by action potentials conducted across the lymphatic muscle cell (LMC) layer to produce entrained contractions. The contraction frequency of a lymphatic collecting vessel displays exquisite mechanosensitivity, with a dynamic range from <1 to >20 contractions per minute. A myogenic pacemaker mechanism intrinsic to the LMCs was initially postulated to account for pressure-dependent chronotropy. Further interrogation into the cellular constituents of the lymphatic vessel wall identified non-muscle cell populations that shared some characteristics with interstitial cells of Cajal, which have pacemaker functions in the gastrointestinal and lower urinary tracts, thus raising the possibility of a non-muscle cell pacemaker. However, recent genetic knockout studies in mice support LMCs and a myogenic origin of the pacemaker activity. LMCs exhibit stochastic, but pressure-sensitive, sarcoplasmic reticulum calcium release (puffs and waves) from IP3R1 receptors, which couple to the calcium-activated chloride channel Anoctamin 1, causing depolarisation. The resulting electrical activity integrates across the highly coupled lymphatic muscle electrical syncytia through connexin 45 to modulate diastolic depolarisation. However, multiple other cation channels may also contribute to the ionic pacemaking cycle. Upon reaching threshold, a voltage-gated calcium channel-dependent action potential fires, resulting in a nearly synchronous calcium global calcium flash within the LMC layer to drive an entrained contraction. This review summarizes the key ion channels potentially responsible for the pressure-dependent chronotropy of lymphatic collecting vessels and various mechanisms of IP3R1 regulation that could contribute to frequency tuning.

Increased contractile activity and dilation of popliteal lymphatic vessels in the TNF-α-overexpressing TNFΔARE/+ arthritic mouse

AIMS

TNF-α acute treatment has been found to disrupt lymphatic drainage in the setting of arthritis through the NF-kB-iNOS- signaling pathway. We examined whether popliteal lymphatic vessels (pLVs) contractile activity was altered in 12- and 24- week-old females of an arthritic mouse model overexpressing TNF-α (TNFΔARE/+).

MAIN METHODS

pLVs were prepared for intravital imaging to measure lymph flow speed, and ex vivo functional responses to a stepwise increase in transmural pressure in the absence or presence of the non-selective NOS inhibitor (L-NNA) or the selective iNOS inhibitor (1400W) were compared between TNFΔARE/+ and WT mice. Total eNOS (t-eNOS) and eNOS phosphorylated at ser1177 (p-eNOS) were evaluated by western blotting.

KEY FINDINGS

In vivo imaging revealed a significantly increase in lymph flow speed in TNFΔARE/+ mice in comparison to WT at both ages. Pressure myography showed an increase in contraction frequency, diameters and fractional pump flow at both ages, whereas amplitude and ejection fraction were significantly decreased in older TNFΔARE/+ mice. Additionally, contraction frequency was increased in the presence of 1400W, and systolic diameter was abolished with L-NNA in TNFΔARE/+ mice compared to WT. Significant increases in p-eNOS expression and neutrophil recruitment (MPO activity) were observed in TNFΔARE/+ mice compared to WT.

SIGNIFICANCE

Our data reveal functional changes in pLVs, especially in advanced stage of arthritis. These alterations may be related to eNOS and iNOS response, which can affect drainage of the inflammatory content from the joints.

Lymphatic vessels and the renin-angiotensin-system

The lymphatic system is an integral part of the circulatory system and plays an important role in the fluid homeostasis of the human body. Accumulating evidence has recently suggested the involvement of lymphatic dysfunction in the pathogenesis of cardio-reno-vascular (CRV) disease. However, how the sophisticated contractile machinery of lymphatic vessels is modulated and, possibly impaired in CRV disease, remains largely unknown. In particular, little attention has been paid to the effect of the renin-angiotensin-system (RAS) on lymphatics, despite the high concentration of RAS mediators that these tissue-draining vessels are exposed to and the established role of the RAS in the development of classic microvascular dysfunction and overt CRV disease. We herein review recent studies linking RAS to lymphatic function and/or plasticity and further highlight RAS-specific signaling pathways, previously shown to drive adverse arterial remodeling and CRV organ damage that have potential for direct modulation of the lymphatic system.

Topical captopril: a promising treatment for secondary lymphedema

Transforming growth factor-beta 1 (TGF-β1)-mediated tissue fibrosis is an important regulator of lymphatic dysfunction in secondary lymphedema. However, TGF-β1 targeting can cause toxicity and autoimmune complications, limiting clinical utility. Angiotensin II (Ang II) modulates intracellular TGF-β1 signaling, and inhibition of Ang II production using angiotensin-converting enzyme (ACE) inhibitors, such as captopril, has antifibrotic efficacy in some pathological settings. Therefore, we analyzed the expression of ACE and Ang II in clinical lymphedema biopsy specimens from patients with unilateral breast cancer-related lymphedema (BCRL) and mouse models, and found that cutaneous ACE expression is increased in lymphedematous tissues. Furthermore, topical captopril decreases fibrosis, activation of intracellular TGF-β1 signaling pathways, inflammation, and swelling in mouse models of lymphedema. Captopril treatment also improves lymphatic function and immune cell trafficking by increasing collecting lymphatic pumping. Our results show that the renin-angiotensin system in the skin plays an important role in the regulation of fibrosis in lymphedema, and inhibition of this signaling pathway may hold merit for treating lymphedema.

Computational simulations of the effects of gravity on lymphatic transport

Physical forces, including mechanical stretch, fluid pressure, and shear forces alter lymphatic vessel contractions and lymph flow. Gravitational forces can affect these forces, resulting in altered lymphatic transport, but the mechanisms involved have not been studied in detail. Here, we combine a lattice Boltzmann-based fluid dynamics computational model with known lymphatic mechanobiological mechanisms to investigate the movement of fluid through a lymphatic vessel under the effects of gravity that may either oppose or assist flow. Regularly spaced, mechanical bi-leaflet valves in the vessel enforce net positive flow as the vessel walls contract autonomously in response to calcium and nitric oxide (NO) levels regulated by vessel stretch and shear stress levels. We find that large gravitational forces opposing flow can stall the contractions, leading to no net flow, but transient mechanical perturbations can re-establish pumping. In the case of gravity strongly assisting flow, the contractions also cease due to high shear stress and NO production, which dilates the vessel to allow gravity-driven flow. In the intermediate range of oppositional gravity forces, the vessel actively contracts to offset nominal gravity levels or to modestly assist the favorable hydrostatic pressure gradients.

KATP channels in lymphatic function

KATP channels function as negative regulators of active lymphatic pumping and lymph transport. This review summarizes and critiques the evidence for the expression of specific KATP channel subunits in lymphatic smooth muscle and endothelium, the roles that they play in normal lymphatic function, and their possible involvement in multiple diseases, including metabolic syndrome, lymphedema, and Cantú syndrome. For each of these topics, suggestions are made for directions for future research.

Lymphatic Vascular Permeability

Blood vessels have a regulated permeability to fluid and solutes, which allows for the delivery of nutrients and signaling molecules to all cells in the body, a process essential to life. The lymphatic vasculature is the second network of vessels in the body, making up part of the immune system, yet is not typically thought of as having a permeability to fluid and solute. However, the major function of the lymphatic vasculature is to regulate tissue fluid balance to prevent edema, so lymphatic vessels must be permeable to absorb and transport fluid efficiently. Only recently were lymphatic vessels discovered to be permeable, which has had many functional implications. In this review, we will provide an overview of what is known about lymphatic vascular permeability, discuss the biophysical and signaling mechanisms regulating lymphatic permeability, and examine the disease relevance of this new property of lymphatic vessels.

Persistent popliteal lymphatic muscle cell coverage defects despite amelioration of arthritis and recovery of popliteal lymphatic vessel function in TNF-Tg mice following anti-TNF therapy

While rheumatoid arthritis patients and tumor necrosis factor transgenic (TNF-Tg) mice with inflammatory-erosive arthritis display lymphatic drainage deficits, the mechanisms responsible remain unknown. As ultrastructural studies of joint-draining popliteal lymphatic vessels (PLVs) in TNF-Tg mice revealed evidence of lymphatic muscle cell (LMC) damage, we aimed to evaluate PLV-LMC coverage in TNF-Tg mice. We tested the hypothesis that alpha smooth muscle actin (αSMA)+ PLV-LMC coverage decreases with severe inflammatory-erosive arthritis, and is recovered by anti-TNF therapy facilitated by increased PLV-LMC turnover during amelioration of joint disease. TNF-Tg mice with established disease received anti-TNF monoclonal antibody (mAb) or placebo IgG isotype control mAb therapy (n = 5) for 6-weeks, while wild-type (WT) littermates (n = 8) received vehicle (PBS). Bromodeoxyuridine (BrdU) was also administered daily during the treatment period to monitor PLV-LMC turnover. Effective anti-TNF therapy was confirmed by longitudinal assessment of popliteal lymph node (PLN) volume via ultrasound, PLV contraction frequency via near-infrared imaging of indocyanine green, and ankle bone volumes via micro-computed tomography (micro-CT). Terminal knee micro-CT, and ankle and knee histology were also performed. PLVs were immunostained for αSMA and BrdU to evaluate PLV-LMC coverage and turnover, respectively, via whole-mount fluorescent microscopy. Anti-TNF therapy reduced PLN volume, increased talus and patella bone volumes, and reduced tarsal and knee synovial areas compared to placebo treated TNF-Tg mice (p < 0.05), as expected. Anti-TNF therapy also increased PLV contraction frequency at 3-weeks (from 0.81 ± 1.0 to 3.2 ± 2.0 contractions per minute, p < 0.05). However, both anti-TNF and placebo treated TNF-Tg mice exhibited significantly reduced αSMA+ PLV-LMC coverage compared to WT (p < 0.05). There was no correlation of αSMA+ PLV-LMC coverage restoration with amelioration of inflammatory-erosive arthritis. Similarly, there was no difference in PLV-LMC turnover measured by BrdU labeling between WT, TNF-Tg placebo, and TNF-Tg anti-TNF groups with an average of < 1% BrdU+ PLV-LMCs incorporated per week. Taken together these results demonstrate that PLV-LMC turnover in adult mice is limited, and that recovery of PLV function during amelioration of inflammatory-erosive arthritis occurs without restoration of αSMA+ LMC coverage. Future studies are warranted to investigate the direct and indirect effects of chronic TNF exposure, and the role of proximal inflammatory cells on PLV contractility.

Effect of Human Synovial Fluid From Osteoarthritis Patients and Healthy Individuals on Lymphatic Contractile Activity

The lymphatic system has been proposed to play a crucial role in preventing the development and progression of osteoarthritis (OA). As OA develops and progresses, inflammatory cytokines and degradation by-products of joint tissues build up in the synovial fluid (SF) providing a feedback system to exacerbate disease. The lymphatic system plays a critical role in resolving inflammation and maintaining overall joint homeostasis; however, there is some evidence that the lymphatics can become dysfunctional during OA. We hypothesized that the functional mechanics of lymphatic vessels (LVs) draining the joint could be directly compromised due to factors within SF derived from osteoarthritis patients (OASF). Here, we utilized OASF and SF derived from healthy (non-OA) individuals (healthy SF (HSF)) to investigate potential effects of SF entering the draining lymph on migration of lymphatic endothelial cells (LECs) in vitro, and lymphatic contractile activity of rat femoral LVs (RFLVs) ex vivo. Dilutions of both OASF and HSF containing serum resulted in a similar LEC migratory response to the physiologically endothelial basal medium-treated LECs (endothelial basal medium containing serum) in vitro. Ex vivo, OASF and HSF treatments were administered within the lumen of isolated LVs under controlled pressures. OASF treatment transiently enhanced the RFLVs tonic contractions while phasic contractions were significantly reduced after 1 h of treatment and complete ceased after overnight treatment. HSF treatment on the other hand displayed a gradual decrease in lymphatic contractile activity (both tonic and phasic contractions). The observed variations after SF treatments suggest that the pump function of lymphatic vessel draining the joint could be directly compromised in OA and thus might present a new therapeutic target.

Current Understanding of Pathological Mechanisms of Lymphedema

Significance: Lymphedema is a common disease that affects hundreds of millions of people worldwide with significant financial and social burdens. Despite increasing prevalence and associated morbidities, the mainstay treatment of lymphedema is largely palliative without an effective cure due to incomplete understanding of the disease. Recent Advances: Recent studies have described key histological and pathological processes that contribute to the progression of lymphedema, including lymphatic stasis, inflammation, adipose tissue deposition, and fibrosis. This review aims to highlight cellular and molecular mechanisms involved in each of these pathological processes. Critical Issues: Despite recent advances in the understanding of the pathophysiology of lymphedema, cellular and molecular mechanisms underlying the disease remains elusive due to its complex nature. Future Directions: Additional research is needed to gain a better insight into the cellular and molecular mechanisms underlying the pathophysiology of lymphedema, which will guide the development of therapeutic strategies that target specific pathology of the disease.

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.

Lymphatic Collecting Vessel: New Perspectives on Mechanisms of Contractile Regulation and Potential Lymphatic Contractile Pathways to Target in Obesity and Metabolic Diseases

Obesity and metabolic syndrome pose a significant risk for developing cardiovascular disease and remain a critical healthcare challenge. Given the lymphatic system's role as a nexus for lipid absorption, immune cell trafficking, interstitial fluid and macromolecule homeostasis maintenance, the impact of obesity and metabolic disease on lymphatic function is a burgeoning field in lymphatic research. Work over the past decade has progressed from the association of an obese phenotype with Prox1 haploinsufficiency and the identification of obesity as a risk factor for lymphedema to consistent findings of lymphatic collecting vessel dysfunction across multiple metabolic disease models and organisms and characterization of obesity-induced lymphedema in the morbidly obese. Critically, recent findings have suggested that restoration of lymphatic function can also ameliorate obesity and insulin resistance, positing lymphatic targeted therapies as relevant pharmacological interventions. There remain, however, significant gaps in our understanding of lymphatic collecting vessel function, particularly the mechanisms that regulate the spontaneous contractile activity required for active lymph propulsion and lymph return in humans. In this article, we will review the current findings on lymphatic architecture and collecting vessel function, including recent advances in the ionic basis of lymphatic muscle contractile activity. We will then discuss lymphatic dysfunction observed with metabolic disruption and potential pathways to target with pharmacological approaches to improve lymphatic collecting vessel function.

Lymphatic Valve Dysfunction in Western Diet-Fed Mice: New Insights Into Obesity-Induced Lymphedema

A two-way connection between obesity and lymphatic dysfunction has now been established. Clinical studies have demonstrated that obesity significantly increases the risk for developing secondary lymphedema. Using animal-models, obesity and metabolic syndrome have been linked to different aspects of lymphatic structural abnormalities and lymphatic dysfunction, including impaired contractility, impaired flow-mediated responses, impaired fluid transport, as well as increased permeability, and abnormal dendritic cell migration among others. Dysfunction of lymphatic valves is a main form of lymphatic dysfunction, known to result in severe edematous phenotypes; however, the extent of lymphatic valve deficiency in secondary lymphedema, including obesity-induced lymphedema, remains unknown. Therefore, the aims of the present study were 1) to determine whether western diet-induced obesity results in lymphatic valve dysfunction, and 2) to determine whether lymphatic valve dysfunction in western diet-induced obesity results from the diet itself, or as a consequence of the metabolic alterations induced by the diet. First, we quantitatively assessed and compared valve function in isolated popliteal and mesenteric collecting lymphatic vessels from control and western diet-induced obese C57BL/6J (WT) mice. Feeding a western diet for 14 weeks induced obesity and elevated plasma glucose and cholesterol levels when compared to controls. The function of lymphatic valves in popliteal lymphatics was not affected by diet-induced obesity; however, significant back-leak of pressure was observed in mesenteric lymphatic valves. Dysfunctional, leaky valves from obese animals also required significantly higher adverse pressure to trigger valve closure. Importantly, when subjected to treatment with a western diet, globally deficient PAI-1 mice were significantly protected against metabolic dysfunction and displayed fully functional, competent mesenteric lymphatic valves. In conclusion, our findings show for the first time that, in association with the metabolic alterations induced by the western diet, lymphatic valve dysfunction can be a critical component of obesity-induced lymphedema.

Multiple aspects of lymphatic dysfunction in an ApoE -/- mouse model of hypercholesterolemia

Introduction: Rodent models of cardiovascular disease have uncovered various types of lymphatic vessel dysfunction that occur in association with atherosclerosis, type II diabetes and obesity. Previously, we presented in vivo evidence for impaired lymphatic drainage in apolipoprotein E null (ApoE ^-/- ) mice fed a high fat diet (HFD). Whether this impairment relates to the dysfunction of collecting lymphatics remains an open question. The ApoE ^-/- mouse is a well-established model of cardiovascular disease, in which a diet rich in fat and cholesterol on an ApoE deficient background accelerates the development of hypercholesteremia, atherosclerotic plaques and inflammation of the skin and other tissues. Here, we investigated various aspects of lymphatic function using ex vivo tests of collecting lymphatic vessels from ApoE ^+/+ or ApoE ^-/- mice fed a HFD. Methods: Popliteal collectors were excised from either strain and studied under defined conditions in which we could quantify changes in lymphatic contractile strength, lymph pump output, secondary valve function, and collecting vessel permeability. Results: Our results show that all these aspects of lymphatic vessel function are altered in deleterious ways in this model of hypercholesterolemia. Discussion: These findings extend previous in vivo observations suggesting significant dysfunction of lymphatic endothelial cells and smooth muscle cells from collecting vessels in association with a HFD on an ApoE-deficient background. An implication of our study is that collecting vessel dysfunction in this context may negatively impact the removal of cholesterol by the lymphatic system from the skin and the arterial wall and thereby exacerbate the progression and/or severity of atherosclerosis and associated inflammation.

Off-Target Effect of Lovastatin Disrupts Dietary Lipid Uptake and Dissemination through Pro-Drug Inhibition of the Mesenteric Lymphatic Smooth Muscle Cell Contractile Apparatus

Previously published, off-target effects of statins on skeletal smooth muscle function have linked structural characteristics within this drug class to myopathic effects. However, the effect of these drugs on lymphatic vascular smooth muscle cell function, and by proxy dietary cholesterol uptake, by the intestinal lymphatic network has not been investigated. Several of the most widely prescribed statins (Atorvastatin, Pravastatin, Lovastatin, and Simvastatin) were tested for their in-situ effects on smooth muscle contractility in rat mesenteric collecting lymphatic vessels. Lovastatin and Simvastatin had a concentration-dependent effect of initially increasing vessel contraction frequency before flatlining the vessel, a phenomenon which was found to be a lactone-ring dependent phenomenon and could be ameliorated through use of Lovastatin- or Simvastatin-hydroxyacid (HA). Simvastatin treatment further resulted in mitochondrial depolymerization within primary-isolated rat lymphatic smooth muscle cells (LMCs) while Lovastatin was found to be acting in a mitochondrial-independent manner, increasing the function of RhoKinase. Lovastatin’s effect on RhoKinase was investigated through pharmacological testing and in vitro analysis of increased MLC and MYPT1 phosphorylation within primary isolated LMCs. Finally, acute in vivo treatment of rats with Lovastatin, but not Lovastatin-HA, resulted in a significantly decreased dietary lipid absorption in vivo through induced disfunction of mesenteric lymph uptake and trafficking.

Large-conductance calcium-activated K+ channels, rather than KATP channels, mediate the inhibitory effects of nitric oxide on mouse lymphatic pumping

BACKGROUND AND PURPOSE

K_ATP channels are negative regulators of lymphatic vessel excitability and contractility and are proposed to be targets for immune cell products that inhibit lymph transport. Previous studies in rat and guinea pig mesenteric lymphatics found that NO-mediated inhibition of lymphatic contraction was prevented or reversed by the K_ATP channel inhibitor, glibenclamide. We revisited this hypothesis using mouse lymphatic vessels and K_ATP channel knockout mice.

EXPERIMENTAL APPROACH

Mouse popliteal lymphatics were isolated, and contractility was assessed using pressure myography. K⁺ channel expression was determined by PCR analysis of FACS-purified lymphatic smooth muscle cells.

KEY RESULTS

The NO-producing agonist, ACh, and the NO donor, NONOate, both produced dose-dependent inhibition of spontaneous lymphatic contractions that were blocked by the soluble GC inhibitor, ODQ, or the PKG inhibitor, Rp-8-Br-PET-cGMPS. Surprisingly, the inhibitory effects of both were preserved in K_ir 6.1^-/- vessels, suggesting that K_ATP channels did not mediate NO-induced responses. We hypothesized a role for BK channels, given their prominence in arterial smooth muscle. Indeed, BK channels were expressed in mouse lymphatic smooth muscle and NS11021 (a BK channel activator) caused dilation and reduced contraction frequency, whereas iberiotoxin and penitrem A (BK channel inhibitors) produced right-ward shifts in NONOate concentration-response curves.

CONCLUSION AND IMPLICATIONS

Inhibition of mouse lymphatic contractions by NO primarily involves activation of BK channels, rather than K_ATP channels. Thus, BK channels are a potential target for therapeutic reversal of lymph pump inhibition by NO generated by immune cell activation of iNOS in chronic lymphoedema.

Ex vivo Demonstration of Functional Deficiencies in Popliteal Lymphatic Vessels From TNF-Transgenic Mice With Inflammatory Arthritis

Background: Recent studies demonstrated lymphangiogenesis and expansion of draining lymph nodes during chronic inflammatory arthritis, and lymphatic dysfunction associated with collapse of draining lymph nodes in rheumatoid arthritis (RA) patients and TNF-transgenic (TNF-Tg) mice experiencing arthritic flare. As the intrinsic differences between lymphatic vessels afferent to healthy, expanding, and collapsed draining lymph nodes are unknown, we characterized the ex vivo behavior of popliteal lymphatic vessels (PLVs) from WT and TNF-Tg mice. We also interrogated the mechanisms of lymphatic dysfunction through inhibition of nitric oxide synthase (NOS). Methods: Popliteal lymph nodes (PLNs) in TNF-Tg mice were phenotyped as Expanding or Collapsed by in vivo ultrasound and age-matched to WT littermate controls. The PLVs were harvested and cannulated for ex vivo functional analysis over a relatively wide range of hydrostatic pressures (0.5-10 cmH2O) to quantify the end diastolic diameter (EDD), tone, amplitude (AMP), ejection fraction (EF), contraction frequency (FREQ), and fractional pump flow (FPF) with or without NOS inhibitors Data were analyzed using repeated measures two-way ANOVA with Bonferroni's post hoc test. Results: Real time videos of the cannulated PLVs demonstrated the predicted phenotypes of robust vs. weak contractions of the WT vs. TNF-Tg PLV, respectively. Quantitative analyses confirmed that TNF-Tg PLVs had significantly decreased AMP, EF, and FPF vs. WT (p < 0.05). EF and FPF were recovered by NOS inhibition, while the reduction in AMP was NOS independent. No differences in EDD, tone, or FREQ were observed between WT and TNF-Tg PLVs, nor between Expanding vs. Collapsed PLVs. Conclusion: These findings support the concept that chronic inflammatory arthritis leads to NOS dependent and independent draining lymphatic vessel dysfunction that exacerbates disease, and may trigger arthritic flare due to decreased egress of inflammatory cells and soluble factors from affected joints.

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.

Dichotomous effects on lymphatic transport with loss of caveolae in mice

AIM

Fluid and macromolecule transport from the interstitium into and through lymphatic vessels is necessary for tissue homeostasis. While lymphatic capillary structure suggests that passive, paracellular transport would be the predominant route of macromolecule entry, active caveolae-mediated transcellular transport has been identified in lymphatic endothelial cells (LECs) in vitro. Caveolae also mediate a wide array of endothelial cell processes, including nitric oxide regulation. Thus, how does the lack of caveolae impact "lymphatic function"?

METHODS

Various aspects of lymphatic transport were measured in mice constitutively lacking caveolin-1 ("CavKO"), the protein required for caveolae formation in endothelial cells, and in mice with a LEC-specific Cav1 gene deletion (Lyve1-Cre x Cav1^flox/flox ; "LyCav") and ex vivo in their vessels and cells.

RESULTS

In each model, lymphatic architecture was largely unchanged. The lymphatic conductance, or initial tissue uptake, was significantly higher in both CavKO mice and LyCav mice by quantitative microlymphangiography and the permeability to 70 kDa dextran was significantly increased in monolayers of LECs isolated from CavKO mice. Conversely, transport within the lymphatic system to the sentinel node was significantly reduced in anaesthetized CavKO and LyCav mice. Isolated, cannulated collecting vessel studies identified significantly reduced phasic contractility when lymphatic endothelium lacks caveolae. Inhibition of nitric oxide synthase was able to partially restore ex vivo vessel contractility.

CONCLUSION

Macromolecule transport across lymphatics is increased with loss of caveolae, yet phasic contractility reduced, resulting in reduced overall lymphatic transport function. These studies identify lymphatic caveolar biology as a key regulator of active lymphatic transport functions.

Foxo1 deletion promotes the growth of new lymphatic valves

Patients with congenital lymphedema suffer from tissue swelling in part due to mutations in genes regulating lymphatic valve development. Lymphatic valve leaflets grow and are maintained throughout life in response to oscillatory shear stress (OSS), which regulates gene transcription in lymphatic endothelial cells (LECs). Here, we identified the first transcription factor, Foxo1, that repressed lymphatic valve formation by inhibiting the expression of valve-forming genes. We showed that both embryonic and postnatal ablation of Foxo1 in LECs induced additional valve formation in postnatal and adult mice in multiple tissues. Our quantitative analyses revealed that after deletion, the total number of valves in the mesentery was significantly (P < 0.01) increased in the Foxo1LEC-KO mice compared with Foxo1fl/fl controls. In addition, our quantitative real-time PCR (RT-PCR) data from cultured LECs showed that many valve-forming genes were significantly (P < 0.01) upregulated upon knockdown of FOXO1. To confirm our findings in vivo, rescue experiments showed that Foxc2+/- mice, a model of lymphedema-distichiasis, had 50% fewer lymphatic valves and that the remaining valves exhibited backleak. Both valve number and function were completely restored to control levels upon Foxo1 deletion. These findings established FOXO1 as a clinically relevant target to stimulate de novo lymphatic valve formation and rescue defective valves in congenital lymphedema.

Imaging technology of the lymphatic system

The lymphatic system plays critical roles in tissue fluid homeostasis and immunity and has been implicated in the development of many different pathologies, ranging from lymphedema, the spread of cancer to chronic inflammation. In this review, we first summarize the state-of-the-art of lymphatic imaging in the clinic and the advantages and disadvantages of these existing techniques. We then detail recent progress on imaging technology, including advancements in tracer design and injection methods, that have allowed visualization of lymphatic vessels with excellent spatial and temporal resolution in preclinical models. Finally, we describe the different approaches to quantifying lymphatic function that are being developed and discuss some emerging topics for lymphatic imaging in the clinic. Continued advancements in lymphatic imaging technology will be critical for the optimization of diagnostic methods for lymphatic disorders and the evaluation of novel therapies targeting the lymphatic system.

Altered Lymphatic Vessel Anatomy and Markedly Diminished Lymph Clearance in Affected Hands of Patients With Active Rheumatoid Arthritis

OBJECTIVE

To assess differences between lymphatic function in the affected hands of rheumatoid arthritis (RA) patients with active synovitis and that of healthy controls, using indocyanine green (ICG) dye and near-infrared (NIR) imaging.

METHODS

NIR imaging of the hands of 8 patients with active RA and 13 healthy controls was performed following web space injection of 0.1 ml of 100 μM ICG. The percentage of ICG retention in the web spaces was determined by NIR imaging at baseline and at 7 days (±1 day) after the initial injections; image analysis provided contraction frequency. ICG+ lymphatic vessel (LV) length and branching architecture were assessed.

RESULTS

Retention of ICG in RA hands was higher compared to controls (P < 0.01). The average contraction frequency of ICG+ LVs in RA patients and in controls did not differ (mean ± SD 0.53 ± 0.39 contractions/minute versus 0.51 ± 0.35 contractions/minute). Total ICG+ LV length in RA hands was lower compared to healthy controls (58.3 ± 15.0 cm versus 71.4 ± 16.1 cm; P < 0.001), concomitant with a decrease in the number of ICG+ basilic LVs in the hands of RA patients (P < 0.05).

CONCLUSION

Lymphatic drainage in the hands of RA patients with active disease was reduced compared to controls. This reduction was associated with a decrease in total length of ICG+ LVs on the dorsal surface of the hands, which continued to contract at a similar rate to that observed in controls. These findings provide a plausible mechanism for exacerbation of synovitis and joint damage, specifically the accumulation and retention of inflammatory cells and catabolic factors in RA joints due to impaired efferent lymphatic flow. NIR/ICG imaging of RA hands is feasible and warrants formal investigation as a primary outcome measure for arthritis disease severity and/or persistence in future clinical trials.

Effects of Elevated Downstream Pressure and the Role of Smooth Muscle Cell Coupling through Connexin45 on Lymphatic Pacemaking

Lymphatic vessels rely on spontaneous lymphatic muscle cell (LMC) contractions and one-way intraluminal valves to efficiently pump lymph and return it into the bloodstream. Intraluminal pressure is known to regulate the contractile function of lymphatics, with pressure elevation leading to increased contraction frequency and decreased amplitude. Contractions are normally initiated by a dominant pacemaker and are highly entrained among strongly coupled LMCs. Previously, we found that connexin45 is the major connexin isoform mediating LMC-LMC electrical coupling. Lymphatics from mice lacking smooth muscle connexin45 display uncoordinated, impaired contractions. Here, we utilized this connexin45-deficient model, pressure myography, and recently developed, novel analytical tools to assess the effects of elevated downstream pressure on the number, location, and frequency of lymphatic pacemakers. Our results show that, in vessels from healthy controls, an increase in downstream pressure resulted in the recruitment/development of new pacemakers and increased contractile frequency while a dominant pacemaker continued to be observed. In contrast, vessels from connexin45-deficient mice displayed significantly more pacemakers, but none were dominant; this worsened with elevated downstream pressure. These results suggest a potential protective mechanism through which the lymphatic vasculature adapts to transient increases in downstream pressure, but which may not be sustained in scenarios with chronic elevated downstream pressure.

Characterization of rat tail lymphatic contractility and biomechanics: incorporating nitric oxide-mediated vasoregulation

The lymphatic system transports lymph from the interstitial space back to the great veins via a series of orchestrated contractions of chains of lymphangions. Biomechanical models of lymph transport, validated with ex vivo or in vivo experimental results, have proved useful in revealing novel insight into lymphatic pumping; however, a need remains to characterize the contributions of vasoregulatory compounds in these modelling tools. Nitric oxide (NO) is a key mediator of lymphatic pumping. We quantified the active contractile and passive biaxial biomechanical response of rat tail collecting lymphatics and changes in the contractile response to the exogenous NO administration and integrated these findings into a biomechanical model. The passive mechanical response was characterized with a three-fibre family model. Nonlinear regression and non-parametric bootstrapping were used to identify best-fit material parameters to passive cylindrical biaxial mechanical data, assessing uniqueness and parameter confidence intervals; this model yielded a good fit (R² = 0.90). Exogenous delivery of NO via sodium nitroprusside (SNP) elicited a dose-dependent suppression of contractions; the amplitude of contractions decreased by 30% and the contraction frequency decreased by 70%. Contractile function was characterized with a modified Rachev-Hayashi model, introducing a parameter that is related to SNP concentration; the model provided a good fit (R² = 0.89) to changes in contractile responses to varying concentrations of SNP. These results demonstrated the significant role of NO in lymphatic pumping and provide a predictive biomechanical model to integrate the combined effect of mechanical loading and NO on lymphatic contractility and mechanical response.

Regulation of lymphatic function and injury by nitrosative stress in obese mice

Objective

Obesity results in lymphatic dysfunction, but the cellular mechanisms that mediate this effect remain largely unknown. Previous studies in obese mice have shown that inducible nitric oxide synthase-expressing (iNOS⁺) inflammatory cells accumulate around lymphatic vessels. In the current study, we therefore tested the hypothesis that increased expression of iNOS results in nitrosative stress and injury to the lymphatic endothelial cells (LECs). In addition, we tested the hypothesis that lymphatic injury, independent of obesity, can modulate glucose and lipid metabolism.

Methods

We compared the metabolic changes and lymphatic function of wild-type and iNOS knockout mice fed a normal chow or high-fat diet for 16 weeks. To corroborate our in vivo findings, we analyzed the effects of reactive nitrogen species on isolated LECs. Finally, using a genetically engineered mouse model that allows partial ablation of the lymphatic system, we studied the effects of acute lymphatic injury on glucose and lipid metabolism in lean mice.

Results

The mesenteric lymphatic vessels of obese wild-type animals were dilated, leaky, and surrounded by iNOS⁺ inflammatory cells with resulting increased accumulation of reactive nitrogen species when compared with lean wild-type or obese iNOS knockout animals. These changes in obese wild-type mice were associated with systemic glucose and lipid abnormalities, as well as decreased mesenteric LEC expression of lymphatic-specific genes, including vascular endothelial growth factor receptor 3 (VEGFR-3) and antioxidant genes as compared with lean wild-type or obese iNOS knockout animals. In vitro experiments demonstrated that isolated LECs were more sensitive to reactive nitrogen species than blood endothelial cells, and that this sensitivity was ameliorated by antioxidant therapies. Finally, using mice in which the lymphatics were specifically ablated using diphtheria toxin, we found that the interaction between metabolic abnormalities caused by obesity and lymphatic dysfunction is bidirectional. Targeted partial ablation of mesenteric lymphatic channels of lean mice resulted in increased accumulation of iNOS⁺ inflammatory cells and increased reactive nitrogen species. Lymphatic ablation also caused marked abnormalities in insulin sensitivity, serum glucose and insulin concentrations, expression of insulin-sensitive genes, lipid metabolism, and significantly increased systemic and mesenteric white adipose tissue (M-WAT) inflammatory responses.

Conclusions

Our studies suggest that increased iNOS production in obese animals plays a key role in regulating lymphatic injury by increasing nitrosative stress. In addition, our studies suggest that obesity-induced lymphatic injury may amplify metabolic abnormalities by increasing systemic and local inflammatory responses and regulating insulin sensitivity. These findings suggest that manipulation of the lymphatic system may represent a novel means of treating metabolic abnormalities associated with obesity.

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Increased iNOS+ cells around mesenteric lymphatics of high fat diet-induced obese mice.

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iNOS knockout mice are protected from obesity-induced lymphatic dysfunction.

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Lymphatic endothelial cells are highly sensitive to nitrosative stress.

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Nitrosative stress causes lymphatic gene regulation.

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Lymphatic injury alone enhances iNOS+ cells and causes insulin resistance and dyslipidemia.

Anti-Inflammatory and Cytotoxicity Effects of Cudrania tricuspidata Fruits Vinegar in a Co-Culture System with RAW264.7 Macrophages and 3T3-L1 Adipocytes

Vinegar has been found to have in vitro improvement effect on inflammatory biomarkers, and clinically used to improve inflammation and obesity-related diseases. This study was designed to analyze in vitro anti-inflammatory effects of Cudrania tricuspidata fruits vinegar (CTFV) in a co-culture system with macrophages and adipocytes. We analyzed the physicochemical properties and polyphenolic ingredients of CTFV, and investigated in vitro anti-inflammatory effects of CTFV in a co-culture system with macrophages and adipocytes. The cells were cultured in the presence of CTFV for 24 h in contact with each other, then, harvested. The levels of monocyte chemoattractant protein (MCP)-1, tumour necrosis factor (TNF)-α, inducible nitric oxide synthase (iNOS), nitric oxide (NO), and interleukin (IL)-6 were evaluated by using the Griess reagent, western blot, or enzyme-linked immunosorbent assay assays. We found that increasing levels for NO, iNOS, TNF-α, IL-6 and MCP-1 were caused by LPS treatment and co-culture using the contact method, whereas CTFV efficaciously attenuated inflammatory response by improving inflammatory parameters including NO, iNOS, TNF-α, IL-6 and MCP-1. The present study indicates that CTFV might provide a nutraceutical product or functional food resource for improving inflammation processed via the interaction of adipocytes and macrophages.

Lymphatic Valves and Lymph Flow in Cancer-Related Lymphedema

Lymphedema is a complex disease caused by the accumulation of fluid in the tissues resulting from a dysfunctional or damaged lymphatic vasculature. In developed countries, lymphedema most commonly occurs as a result of cancer treatment. Initially, impaired lymph flow causes edema, but over time this results in inflammation, fibrotic and fatty tissue deposition, limited mobility, and bacterial infections that can lead to sepsis. While chronically impaired lymph flow is generally believed to be the instigating factor, little is known about what pathophysiological changes occur in the lymphatic vessels to inhibit lymph flow. Lymphatic vessels not only regulate lymph flow through a variety of physiologic mechanisms, but also respond to lymph flow itself. One of the fascinating ways that lymphatic vessels respond to flow is by growing bicuspid valves that close to prevent the backward movement of lymph. However, lymphatic valves have not been investigated in cancer-related lymphedema patients, even though the mutations that cause congenital lymphedema regulate genes involved in valve development. Here, we review current knowledge of the regulation of lymphatic function and development by lymph flow, including newly identified genetic regulators of lymphatic valves, and provide evidence for lymphatic valve involvement in cancer-related lymphedema.

A Distinct Role of the Autonomic Nervous System in Modulating the Function of Lymphatic Vessels under Physiological and Tumor-Draining Conditions

Lymphatic vessels (LVs) are important in the regulation of tissue fluid homeostasis and the pathogenesis of tumor progression. We investigated the innervation of LVs and the response to agonists and antagonists of the autonomic nervous system in vivo. While skin-draining collecting LVs express muscarinic, α₁- and β₂-adrenergic receptors on lymphatic endothelial cells and smooth muscle cells, intestinal lacteals express only β-adrenergic receptors and muscarinic receptors on their smooth muscle cells. Quantitative in vivo near-infrared imaging of the exposed flank-collecting LV revealed that muscarinic and α₁-adrenergic agonists increased LV contractility, whereas activation of β₂-adrenergic receptors inhibited contractility and initiated nitric oxide (NO)-dependent vasodilation. Tumor-draining LVs were expanded and showed a higher innervation density and contractility that was reduced by treatment with atropine, phentolamine, and, most potently, isoproterenol. These findings likely have clinical implications given the impact of lymphatic fluid drainage on intratumoral fluid pressure and thus drug delivery.

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Murine lymphatic vessels are innervated in an organ-specific manner

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α₁-adrenergic and muscarinic agents enhance lymphatic contractility in vivo

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β₂-adrenergic agents reduce lymphatic contractility

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Tumor-draining lymphatic vessels have increased innervation and contractility

Simplified method to quantify valve back-leak uncovers severe mesenteric lymphatic valve dysfunction in mice deficient in connexins 43 and 37

KEY POINTS

Lymphatic valve defects are one of the major causes of lymph transport dysfunction; however, there are no accessible methods for quantitatively assessing valve function. This report describes a novel technique for quantifying lymphatic valve back-leak. Postnatal endothelial-specific deletion of connexin 43 (Cx43) in connexin 37 null (Cx37-/- ) mice results in rapid regression of valve leaflets and severe valve dysfunction. This method can also be used for assessing the function of venous and lymphatic valves from various species, including humans.

ABSTRACT

The lymphatic system relies on robust, spontaneous contractions of collecting lymphatic vessels and one-way secondary lymphatic valves to efficiently move lymph forward. Secondary valves prevent reflux and allow for the generation of propulsive pressure during each contraction cycle. Lymphatic valve defects are one of the major causes of lymph transport dysfunction. Genetic mutations in multiple genes have been associated with the development of primary lymphoedema in humans; and many of the same mutations in mice result in valve defects that subsequently lead to chylous ascites or chylothorax. At present the only experimental technique for the quantitative assessment of lymphatic valve function utilizes the servo-null micropressure system, which is highly accurate and precise, but relatively inaccessible and difficult to use. We developed a novel, simplified alternative method for quantifying valve function and determining the degree of pressure back-leak through an intact valve in pressurized, single-valve segments of isolated lymphatic vessels. With this diameter-based method, the competence of each lymphatic valve is challenged over a physiological range of pressures (e.g. 0.5-10cmH2 O) and pressure back-leak is extrapolated from calibrated, pressure-driven changes in diameter upstream from the valve. Using mesenteric lymphatic vessels from C57BL/6J, Ub-CreERT2 ;Rasa1fx/fx , Foxc2Cre/+ , Lyve1-Cre;Cx43fx/fx , and Prox1-CreERT2 ;Cx43fx/fx ;Cx37-/- mice, we tested our method on lymphatic valves displaying a wide range of dysfunction, from fully competent to completely incompetent. Our results were validated by simultaneous direct measurement of pressure back-leak using a servo-null micropressure system. Our diameter-based technique can be used to quantify valve function in isolated lymphatic valves from a variety of species. This method also revealed that haplodeficiency in Foxc2 (Foxc2Cre/+ ) is not sufficient to cause significant valve dysfunction; however, postnatal endothelial-specific deletion of Cx43 in Cx37-/- mice results in rapid regression of valve leaflets and severe valve dysfunction.

Kir6.1-dependent KATP channels in lymphatic smooth muscle and vessel dysfunction in mice with Kir6.1 gain-of-function

KEY POINTS

Spontaneous contractions are essential for normal lymph transport and these contractions are exquisitely sensitive to the K_ATP channel activator pinacidil. K_ATP channel Kir6.1 and SUR2B subunits are expressed in mouse lymphatic smooth muscle (LSM) and form functional K_ATP channels as verified by electrophysiological techniques. Global deletion of Kir6.1 or SUR2 subunits results in severely impaired lymphatic contractile responses to pinacidil. Smooth muscle-specific expression of Kir6.1 gain-of-function mutant (GoF) subunits results in profound lymphatic contractile dysfunction and LSM hyperpolarization that is partially rescued by the K_ATP inhibitor glibenclamide. In contrast, lymphatic endothelial-specific expression of Kir6.1 GoF has essentially no effect on lymphatic contractile function. The high sensitivity of LSM to K_ATP channel GoF offers an explanation for the lymphoedema observed in patients with Cantú syndrome, a disorder caused by gain-of-function mutations in genes encoding Kir6.1 or SUR2, and suggests that glibenclamide may be an appropriate therapeutic agent.

ABSTRACT

This study aimed to understand the functional expression of K_ATP channel subunits in distinct lymphatic cell types, and assess the consequences of altered K_ATP channel activity on lymphatic pump function. K_ATP channel subunits Kir6.1 and SUR2B were expressed in mouse lymphatic muscle by PCR, but only Kir6.1 was expressed in lymphatic endothelium. Spontaneous contractions of popliteal lymphatics from wild-type (WT) (C57BL/6J) mice, assessed by pressure myography, were very sensitive to inhibition by the SUR2-specific K_ATP channel activator pinacidil, which hyperpolarized both mouse and human lymphatic smooth muscle (LSM). In vessels from mice with deletion of Kir6.1 (Kir6.1^-/- ) or SUR2 (SUR2[STOP]) subunits, contractile parameters were not significantly different from those of WT vessels, suggesting that basal K_ATP channel activity in LSM is not an essential component of the lymphatic pacemaker, and does not exert a strong influence over contractile strength. However, these vessels were >100-fold less sensitive than WT vessels to pinacidil. Smooth muscle-specific expression of a Kir6.1 gain-of-function (GoF) subunit resulted in severely impaired lymphatic contractions and hyperpolarized LSM. Membrane potential and contractile activity was partially restored by the K_ATP channel inhibitor glibenclamide. In contrast, lymphatic endothelium-specific expression of Kir6.1 GoF subunits had negligible effects on lymphatic contraction frequency or amplitude. Our results demonstrate a high sensitivity of lymphatic contractility to K_ATP channel activators through activation of Kir6.1/SUR2-dependent channels in LSM. In addition, they offer an explanation for the lymphoedema observed in patients with Cantú syndrome, a disorder caused by gain-of-function mutations in genes encoding Kir6.1/SUR2.

Regulation of Lymphatic Function in Obesity

The lymphatic system has many functions, including macromolecules transport, fat absorption, regulation and modulation of adaptive immune responses, clearance of inflammatory cytokines, and cholesterol metabolism. Thus, it is evident that lymphatic function can play a key role in the regulation of a wide array of biologic phenomenon, and that physiologic changes that alter lymphatic function may have profound pathologic effects. Recent studies have shown that obesity can markedly impair lymphatic function. Obesity-induced pathologic changes in the lymphatic system result, at least in part, from the accumulation of inflammatory cells around lymphatic vessel leading to impaired lymphatic collecting vessel pumping capacity, leaky initial and collecting lymphatics, alterations in lymphatic endothelial cell (LEC) gene expression, and degradation of junctional proteins. These changes are important since impaired lymphatic function in obesity may contribute to the pathology of obesity in other organ systems in a feed-forward manner by increasing low-grade tissue inflammation and the accumulation of inflammatory cytokines. More importantly, recent studies have suggested that interventions that inhibit inflammatory responses, either pharmacologically or by lifestyle modifications such as aerobic exercise and weight loss, improve lymphatic function and metabolic parameters in obese mice. The purpose of this review is to summarize the pathologic effects of obesity on the lymphatic system, the cellular mechanisms that regulate these responses, the effects of impaired lymphatic function on metabolic syndrome in obesity, and the interventions that may improve lymphatic function in obesity.

Lymphatic flow modulation as adjunct therapy for septic shock

The lymphatic system is an important component of human health and is critical in maintaining microcirculatory flow and immune system homeostasis. During septic shock, increased capillary permeability results in excess filtration of intravascular fluid and solutes producing interstitial edema with subsequent hydrostatic and oncotic gradient breakdown. The accumulation of interstitial fluid results in impaired solute exchange, leukocyte signaling, and aberrancy in capillary flow. Modulation of lymphatic flow during times of interstitial volume overload such as septic shock may decrease interstitial volume resulting in improved perfusion, decreased end-organ damage, and contribute to disease resolution. Multiple studies in both humans and animals have shown nitric oxide to be a potent modulator of lymphatic function. The present study suggests a hypothetical adjunct therapy for patients with septic shock through the use of phosphodiesterase inhibitors, which may improve microcirculatory flow by decreasing interstitial fluid volume via increased lymphatic fluid drainage.

Histopathologic Features of Lymphedema: A Molecular Review

An estimated 5 million people in the United States are affected by secondary lymphedema, with most cases attributed to malignancies or malignancy-related treatments. The pathogenesis of secondary lymphedema has historically been attributed to lymphatic injury or dysfunction; however, recent studies illustrate the complexity of lymphedema as a disease process in which many of its clinical features such as inflammation, fibrosis, adipogenesis, and recurrent infections contribute to on-going lymphatic dysfunction in a vicious cycle. Investigations into the molecular underpinning of these features further our understanding of the pathophysiology of this disease and suggests new therapeutics.

T-type, but not L-type, voltage-gated calcium channels are dispensable for lymphatic pacemaking and spontaneous contractions

The spontaneous contractions of collecting lymphatic vessels provide an essential propulsive force to return lymph centrally. These contractions are driven by an intrinsic electrical pacemaker, working through an unknown underlying ionic mechanism that becomes compromised in some forms of lymphedema. In previous studies, T-type voltage-gated Ca2+ channels (VGCCs) were implicated in this pacemaking mechanism, based on the effects of the reputedly selective T-type VGCC inhibitors mibefradil and Ni2+. Our goal was to test this idea in a more definitive way using genetic knock out mice. First, we demonstrated through both PCR and immunostaining that mouse lymphatic muscle cells expressed Cav3.1 and Cav3.2 and produced functional T-type VGCC currents when patch clamped. We then employed genetic deletion strategies to selectively test the roles of each T-type VGCC isoform in the regulation of lymphatic pacemaking. Surprisingly, global deletion of either, or both, isoform(s) was without significant effect on either the frequency, amplitude, or fractional pump flow of lymphatic collectors from two different regions of the mouse, studied ex vivo. Further, both WT and Cav3.1-/-; 3.2-/- double knock-out lymphatic vessels responded similarly to mibefradil and Ni2+, which substantially reduced contraction amplitudes and slightly increased frequencies at almost all pressures in both strains: a pattern consistent with inhibition of L-type rather than T-type VGCCs. Neither T-type VGCC isoform was required for ACh-induced inhibition of contraction, a mechanism by which those channels in smooth muscle are thought to be targets of endothelium-derived nitric oxide. Sharp intracellular electrode measurements in lymphatic smooth muscle revealed only subtle, but not significant, differences in the resting membrane potential and action potential characteristics between vessels from wild-type and Cav3.1-/-; 3.2-/- double knock-out mice. In contrast, smooth-muscle specific deletion of the L-type VGCC, Cav1.2, completely abolished all lymphatic spontaneous contractions. Collectively our results suggest that, although T-type VGCCs are expressed in mouse lymphatic smooth muscle, they do not play a significant role in modulating the frequency of the ionic pacemaker or the amplitude of spontaneous contractions. We conclude that the effects of mibefradil and Ni2+ in other lymphatic preparations are largely or completely explained by off-target effects on L-type VGCCs, which are essential for controlling both the frequency and strength of spontaneous contractions.

Lymphatic remodelling in response to lymphatic injury in the hind limbs of sheep

Contractile activity in the lymphatic vasculature is essential for maintaining fluid balance within organs and tissues. However, the mechanisms by which collecting lymphatics adapt to changes in fluid load and how these adaptations influence lymphatic contractile activity are unknown. Here we report a model of lymphatic injury based on the ligation of one of two parallel lymphatic vessels in the hind limb of sheep and the evaluation of structural and functional changes in the intact, remodelling lymphatic vessel over a 42-day period. We show that the remodelled lymphatic vessel displayed increasing intrinsic contractile frequency, force generation and vessel compliance, as well as decreasing flow-mediated contractile inhibition via the enzyme endothelial nitric oxide synthase. A computational model of a chain of lymphatic contractile segments incorporating these adaptations predicted increases in the flow-generation capacity of the remodelled vessel at the expense of normal mitochondrial function and elevated oxidative stress within the lymphatic muscle. Our findings may inform interventions for mitigating lymphatic muscle fatigue in patients with dysfunctional lymphatics.

Reduced Lymphatic Function Predisposes to Calcium Channel Blocker Edema: A Randomized Placebo-Controlled Clinical Trial

Background: The current belief is that the calcium channel blocker (CCB)-induced edema is due to a preferential arterial over venous dilatation leading to increased fluid filtration. We challenged this conviction by measuring the lymphatic removal of interstitial fluid during chronic systemic treatment with the CCB, amlodipine. Lymphatic vessels could potentially be an off-target effect of the drugs and play a role in CCB edema. Methods and Results: Sixteen healthy postmenopausal women completed a 12-week double-blinded randomized placebo-controlled crossover trial. Lymphatic function was assessed by near-infrared fluorescence imaging. The lymphatic function during amlodipine treatment compared with placebo did not show any difference in pumping pressure (53.9 ± 13.9 mmHg vs. 54.7 ± 9.4 mmHg, p = 0.829), contraction frequency (0.4 ± 0.2/min vs. 0.4 ± 0.3/min, p = 0.932), refill time (440 ± 438 seconds vs. 442 ± 419 seconds, p = 0.990), or propagation velocity of lymph packets (18 ± 10 mm/s vs. 15 ± 7 mm/s, p = 0.124). However, the subjects who developed edema during CCB treatment had a 20% lower baseline lymphatic pumping pressure (48.9 ± 4.4 mmHg, n = 7) than the subjects not affected by treatment (59.1 ± 1.2 mmHg, n = 9, p = 0.025). Contraction frequency, refill time, and lymph packet velocity showed no differences in baseline values between the two groups. Conclusion: Our results suggest that CCB does not directly impair lymphatic function. However, our results show that a reduced lymphatic function predisposes to CCB edema, which may explain why some patients develop edema during treatment.

The effects of valve leaflet mechanics on lymphatic pumping assessed using numerical simulations

The lymphatic system contains intraluminal leaflet valves that function to bias lymph flow back towards the heart. These valves are present in the collecting lymphatic vessels, which generally have lymphatic muscle cells and can spontaneously pump fluid. Recent studies have shown that the valves are open at rest, can allow some backflow, and are a source of nitric oxide (NO). To investigate how these valves function as a mechanical valve and source of vasoactive species to optimize throughput, we developed a mathematical model that explicitly includes Ca²⁺ -modulated contractions, NO production and valve structures. The 2D lattice Boltzmann model includes an initial lymphatic vessel and a collecting lymphangion embedded in a porous tissue. The lymphangion segment has mechanically-active vessel walls and is flanked by deformable valves. Vessel wall motion is passively affected by fluid pressure, while active contractions are driven by intracellular Ca²⁺ fluxes. The model reproduces NO and Ca²⁺ dynamics, valve motion and fluid drainage from tissue. We find that valve structural properties have dramatic effects on performance, and that valves with a stiffer base and flexible tips produce more stable cycling. In agreement with experimental observations, the valves are a major source of NO. Once initiated, the contractions are spontaneous and self-sustained, and the system exhibits interesting non-linear dynamics. For example, increased fluid pressure in the tissue or decreased lymph pressure at the outlet of the system produces high shear stress and high levels of NO, which inhibits contractions. On the other hand, a high outlet pressure opposes the flow, increasing the luminal pressure and the radius of the vessel, which results in strong contractions in response to mechanical stretch of the wall. We also find that the location of contraction initiation is affected by the extent of backflow through the valves.

Lymphatic system identification, pathophysiology and therapy in the cardiovascular diseases

The mammalian circulatory system comprises both the cardiovascular system and the lymphatic system. In contrast to the closed, high-pressure and circular blood vascular circulation, the lymphatic system forms an open, low-pressure and unidirectional transit network from the extracellular space to the venous system. It plays a key role in regulating tissue fluid homeostasis, absorption of gastrointestinal lipids, and immune surveillance throughout the body. Despite the critical physiological functions of the lymphatic system, a complete understanding of the lymphatic vessels lags far behind that of the blood vasculatures due to the challenge of their visualization. During the last 20 years, discoveries of underlying genes responsible for lymphatic vessel biology, combined with state-of-the-art lymphatic function imaging and quantification techniques, have established the importance of the lymphatic vasculature in the pathogenesis of cardiovascular diseases including lymphedema, obesity and metabolic diseases, dyslipidemia, hypertension, inflammation, atherosclerosis and myocardial infraction. In this review, we highlight the most recent advances in the field of lymphatic vessel biology, with an emphasis on the new identification techniques of lymphatic system, pathophysiological mechanisms of atherosclerosis and myocardial infarction, and new therapeutic perspectives of lymphangiogenesis.

Ano1 mediates pressure-sensitive contraction frequency changes in mouse lymphatic collecting vessels

Lymphatic collecting vessels exhibit spontaneous contractions with a pressure-dependent contraction frequency. The initiation of contraction has been proposed to be mediated by the activity of a Ca2+-activated Cl- channel (CaCC). Here, we show that the canonical CaCC Anoctamin 1 (Ano1, TMEM16a) plays an important role in lymphatic smooth muscle pacemaking. We find that isolated murine lymphatic muscle cells express Ano1, and demonstrate functional CaCC currents that can be inhibited by the Ano1 inhibitor benzbromarone. These currents are absent in lymphatic muscle cells from Cre transgenic mouse lines targeted for Ano1 genetic deletion in smooth muscle. We additionally show that loss of functional Ano1 in murine inguinal-axillary lymphatic vessels, whether through genetic manipulation or pharmacological inhibition, results in an impairment of the pressure-frequency relationship that is attributable to a hyperpolarized resting membrane potential and a significantly depressed diastolic depolarization rate preceding each action potential. These changes are accompanied by alterations in action potential shape and duration, and a reduced duration but increased amplitude of the action potential-induced global "Ca2+ flashes" that precede lymphatic contractions. These findings suggest that an excitatory Cl- current provided by Ano1 is critical for mediating the pressure-sensitive contractile response and is a major component of the murine lymphatic action potential.

The Intestinal Lymphatic System: Functions and Metabolic Implications

The lymphatic system of the gut plays important roles in the transport of dietary lipids, as well as in immunosurveillance and removal of interstitial fluid. Historically, despite its crucial functions in intestinal homeostasis, the lymphatic system has been poorly studied. In the last 2 decades, identification of specific molecular mediators of lymphatic endothelial cells (LECs) growth together with novel genetic approaches and intravital imaging techniques, have advanced our understanding of the mechanisms regulating intestinal lymphatic physiology in health and disease. As its metabolic implications are gaining recognition, intestinal lymphatic biology is currently experiencing a surge in interest. This review describes current knowledge related to molecular control of intestinal lymphatic vessel structure and function. We discuss regulation of chylomicron entry into lymphatic vessels by vascular endothelial growth factors (VEGFs), hormones, transcription factors and the specific signaling pathways involved. The information covered supports the emerging role of intestinal lymphatics in etiology of the metabolic syndrome and their potential as a therapeutic target.

The Intestinal Lymphatic System: Functions and Metabolic Implications

The lymphatic system of the gut plays important roles in the transport of dietary lipids, as well as in immunosurveillance and removal of interstitial fluid. Historically, despite its crucial functions in intestinal homeostasis, the lymphatic system has been poorly studied. In the last 2 decades, identification of specific molecular mediators of lymphatic endothelial cells (LECs) growth together with novel genetic approaches and intravital imaging techniques, have advanced our understanding of the mechanisms regulating intestinal lymphatic physiology in health and disease. As its metabolic implications are gaining recognition, intestinal lymphatic biology is currently experiencing a surge in interest. This review describes current knowledge related to molecular control of intestinal lymphatic vessel structure and function. We discuss regulation of chylomicron entry into lymphatic vessels by vascular endothelial growth factors (VEGFs), hormones, transcription factors and the specific signaling pathways involved. The information covered supports the emerging role of intestinal lymphatics in etiology of the metabolic syndrome and their potential as a therapeutic target.

The Intestinal Lymphatic System: Functions and Metabolic Implications

The lymphatic system of the gut plays important roles in the transport of dietary lipids, as well as in immunosurveillance and removal of interstitial fluid. Historically, despite its crucial functions in intestinal homeostasis, the lymphatic system has been poorly studied. In the last 2 decades, identification of specific molecular mediators of lymphatic endothelial cells (LECs) growth together with novel genetic approaches and intravital imaging techniques, have advanced our understanding of the mechanisms regulating intestinal lymphatic physiology in health and disease. As its metabolic implications are gaining recognition, intestinal lymphatic biology is currently experiencing a surge in interest. This review describes current knowledge related to molecular control of intestinal lymphatic vessel structure and function. We discuss regulation of chylomicron entry into lymphatic vessels by vascular endothelial growth factors (VEGFs), hormones, transcription factors and the specific signaling pathways involved. The information covered supports the emerging role of intestinal lymphatics in etiology of the metabolic syndrome and their potential as a therapeutic target.