TRPV4 channels' dominant role in the temperature modulation of intrinsic contractility and lymph flow of rat diaphragmatic lymphatics

Using noninvasive imaging to assess manual lymphatic drainage on lymphatic/venous responses in a spaceflight analog

This retrospective case series (clinicaltrials.gov NCT06405282) used noninvasive imaging devices (NIID) to assess the effect of manual lymphatic drainage (MLD) on dermal/venous fluid distribution, perfusion, and temperature alterations of the head, neck, upper torso, and legs while in the 6-degree head-down tilt validated spaceflight analog. A lymphatic fluid scanner measured tissue dielectric constant levels. Near-infrared spectroscopy assessed perfusion, by measuring tissue oxygenation saturation. Long-wave infrared thermography measured tissue temperature gradients. Fifteen healthy, university students participated. NIID assessments were taken 1 minute after assuming the HDT position and then every 30 minutes, with MLD administered from 180 to 195 minutes. Subjects returned to the sitting position and were assessed at post-225 min NIID demonstrated significant changes from baseline (p < 0.01), although these changes at areas of interest varied. MLD had a reverse effect on all variables. NIID assessment supported the potential use of MLD to mitigate fluid shifts during a spaceflight analog.

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.

Flow-dependent regulation of rat mesenteric lymphatic vessel contractile response requires activation of endothelial TRPV4 channels

OBJECTIVES

The objective of our study is to evaluate the involvement of the transient receptor potential vanilloid 4 (TRPV4) in the alteration of lymphatic pumping in response to flow and determine the signaling pathways involved.

METHODS

We used immunofluorescence imaging and western blotting to assess TRPV4 expression in rat mesenteric lymphatic vessels. We examined inhibition of TRPV4 with HC067047, nitric oxide synthase (NOS) with L-NNA and cyclooxygenases (COXs) with indomethacin on the contractile response of pressurized lymphatic vessels to flow changes induced by a stepwise increase in pressure gradients, and the functionality of endothelial TRPV4 channels by measuring the intracellular Ca2+ response of primary lymphatic endothelial cell cultures to the selective agonist GSK1016790A.

RESULTS

TRPV4 protein was expressed in both the endothelial and the smooth muscle layer of rat mesenteric lymphatics with high endothelial expression around the valve sites. When maintained under constant transmural pressure, most lymphatic vessels displayed a decrease in contraction frequency under conditions of flow and this effect was ablated through inhibition of NOS, COX or TRPV4.

CONCLUSIONS

Our findings demonstrate a critical role for TRPV4 in the decrease in contraction frequency induced in lymphatic vessels by increases in flow rate via the production and action of nitric oxide and dilatory prostanoids.

Fluid Osmolarity Modulates the Rate of Spontaneous Contraction of Lymphatic Vessels and Lymph Flow by Means of a Cooperation between TRPV and VRAC Channels

Lymphatic vessels are capable of sustaining lymph formation and propulsion via an intrinsic mechanism based on the spontaneous contraction of the lymphatic muscle in the wall of lymphatic collectors. Exposure to a hyper- or hypo-osmolar environment can deeply affect the intrinsic contraction rate and therefore alter lymph flow. In this work, we aimed at defining the putative receptors underlying such a response. Functional experiments were conducted in ex vivo rat diaphragmatic specimens containing spontaneously contracting lymphatic vessels that were exposed to either hyper- or hypo-osmolar solutions. Lymphatics were challenged with blockers to TRPV4, TRPV1, and VRAC channels, known to respond to changes in osmolarity and/or cell swelling and expressed by lymphatic vessels. Results show that the normal response to a hyperosmolar environment is a steady decrease in the contraction rate and lymph flow and can be prevented by blocking TRPV1 channels with capsazepine. The response to a hyposmolar environment consists of an early phase of an increase in the contraction rate, followed by a decrease. The early phase is abolished by blocking VRACs with DCPIB, while blocking TRPV4 mainly resulted in a delay of the early response. Overall, our data suggest that the cooperation of the three channels can shape the response of lymphatic vessels in terms of contraction frequency and lymph flow, with a prominent role of TRPV1 and VRACs.

Morphological, Mechanical and Hydrodynamic Aspects of Diaphragmatic Lymphatics

The diaphragmatic lymphatic vascular network has unique anatomical characteristics. Studying the morphology and distribution of the lymphatic network in the mouse diaphragm by fluorescence-immunohistochemistry using LYVE-1 (a lymphatic endothelial marker) revealed LYVE1⁺ structures on both sides of the diaphragm-both in its the muscular and tendinous portion, but with different vessel density and configurations. On the pleural side, most LYVE1⁺ configurations are vessel-like with scanty stomata, while the peritoneal side is characterized by abundant LYVE1⁺ flattened lacy-ladder shaped structures with several stomata-like pores, particularly in the muscular portion. Such a complex, three-dimensional organization is enriched, at the peripheral rim of the muscular diaphragm, with spontaneously contracting lymphatic vessel segments able to prompt contractile waves to adjacent collecting lymphatics. This review aims at describing how the external tissue forces developing in the diaphragm, along with cyclic cardiogenic and respiratory swings, interplay with the spontaneous contraction of lymphatic vessel segments at the peripheral diaphragmatic rim to simultaneously set and modulate lymph flow from the pleural and peritoneal cavities. These details may provide useful in understanding the role of diaphragmatic lymphatics not only in physiological but, more so, in pathophysiological circumstances such as in dialysis, metastasis or infection.

Lymphatics - not just a chylomicron conduit

PURPOSE OF REVIEW

Lymphatics are known to have active, regulated pumping by smooth muscle cells that enhance lymph flow, but whether active regulation of lymphatic pumping contributes significantly to the rate of appearance of chylomicrons (CMs) in the blood circulation (i.e., CM production rate) is not currently known. In this review, we highlight some of the potential mechanisms by which lymphatics may regulate CM production.

RECENT FINDINGS

Recent data from our lab and others are beginning to provide clues that suggest a more active role of lymphatics in regulating CM appearance in the circulation through various mechanisms. Potential contributors include apolipoproteins, glucose, glucagon-like peptide-2, and vascular endothelial growth factor-C, but there are likely to be many more.

SUMMARY

The digested products of dietary fats absorbed by the small intestine are re-esterified and packaged by enterocytes into large, triglyceride-rich CM particles or stored temporarily in intracellular cytoplasmic lipid droplets. Secreted CMs traverse the lamina propria and are transported via lymphatics and then the blood circulation to liver and extrahepatic tissues, where they are stored or metabolized as a rich energy source. Although indirect data suggest a relationship between lymphatic pumping and CM production, this concept requires more experimental evidence before we can be sure that lymphatic pumping contributes significantly to the rate of CM appearance in the blood circulation.

Draining the Pleural Space: Lymphatic Vessels Facing the Most Challenging Task

Lymphatic vessels exploit the mechanical stresses of their surroundings together with intrinsic rhythmic contractions to drain lymph from interstitial spaces and serosal cavities to eventually empty into the blood venous stream. This task is more difficult when the liquid to be drained has a very subatmospheric pressure, as it occurs in the pleural cavity. This peculiar space must maintain a very low fluid volume at negative hydraulic pressure in order to guarantee a proper mechanical coupling between the chest wall and lungs. To better understand the potential for liquid drainage, the key parameter to be considered is the difference in hydraulic pressure between the pleural space and the lymphatic lumen. In this review we collected old and new findings from in vivo direct measurements of hydraulic pressures in anaesthetized animals with the aim to better frame the complex physiology of diaphragmatic and intercostal lymphatics which drain liquid from the pleural cavity.

Interplay between Gut Lymphatic Vessels and Microbiota

Lymphatic vessels play a distinctive role in draining fluid, molecules and even cells from interstitial and serosal spaces back to the blood circulation. Lymph vessels of the gut, and especially those located in the villi (called lacteals), not only serve this primary function, but are also responsible for the transport of lipid moieties absorbed by the intestinal mucosa and serve as a second line of defence against possible bacterial infections. Here, we briefly review the current knowledge of the general mechanisms allowing lymph drainage and propulsion and will focus on the most recent findings on the mutual relationship between lacteals and intestinal microbiota.

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.

Loss of Primary Cilia Protein IFT20 Dysregulates Lymphatic Vessel Patterning in Development and Inflammation

Microenvironmental signals produced during development or inflammation stimulate lymphatic endothelial cells to undergo lymphangiogenesis, in which they sprout, proliferate, and migrate to expand the vascular network. Many cell types detect changes in extracellular conditions via primary cilia, microtubule-based cellular protrusions that house specialized membrane receptors and signaling complexes. Primary cilia are critical for receipt of extracellular cues from both ligand-receptor pathways and physical forces such as fluid shear stress. Here, we report the presence of primary cilia on immortalized mouse and primary adult human dermal lymphatic endothelial cells in vitro and on both luminal and abluminal domains of mouse corneal, skin, and mesenteric lymphatic vessels in vivo. The purpose of this study was to determine the effects of disrupting primary cilia on lymphatic vessel patterning during development and inflammation. Intraflagellar transport protein 20 (IFT20) is part of the transport machinery required for ciliary assembly and function. To disrupt primary ciliary signaling, we generated global and lymphatic endothelium-specific IFT20 knockout mouse models and used immunofluorescence microscopy to quantify changes in lymphatic vessel patterning at E16.5 and in adult suture-mediated corneal lymphangiogenesis. Loss of IFT20 during development resulted in edema, increased and more variable lymphatic vessel caliber and branching, as well as red blood cell-filled lymphatics. We used a corneal suture model to determine ciliation status of lymphatic vessels during acute, recurrent, and tumor-associated inflammatory reactions and wound healing. Primary cilia were present on corneal lymphatics during all of the mechanistically distinct lymphatic patterning events of the model and assembled on lymphatic endothelial cells residing at the limbus, stalk, and vessel tip. Lymphatic-specific deletion of IFT20 cell-autonomously exacerbated acute corneal lymphangiogenesis resulting in increased lymphatic vessel density and branching. These data are the first functional studies of primary cilia on lymphatic endothelial cells and reveal a new dimension in regulation of lymphatic vascular biology.

Lymphatic Vessels and Their Surroundings: How Local Physical Factors Affect Lymph Flow

Simple Summary

Lymphatic vessels are responsible for the drainage of liquids, solutes, and cells from interstitial spaces and serosal cavities. Their task is fundamental in order to avoid fluid accumulation leading to tissue swelling and edema. The lymphatic system does not possess a central pump, instead lymph is propelled against an overall hydraulic pressure gradient from interstitial spaces to central veins thanks to two pumping mechanisms, which rely on extrinsic forces or the intrinsic rhythmic contractility of lymphatic muscle cells embedded in vessel walls. This latter mechanism can very rapidly adapt to subtle changes in the microenvironment due to hydraulic pressure, lymph flow-induced wall shear stress, liquid osmolarity, and local tissue temperature. Thus, endothelial and lymphatic muscle cells possess mechanosensors that sense these stimuli and promote a change in contraction frequency and amplitude to modulate lymph flow accordingly. In this review, we will focus on the known physical parameters that can modulate lymph flow and on their putative cellular and molecular mechanisms of transduction.

Abstract

Lymphatic vessels drain and propel lymph by exploiting external forces that surrounding tissues exert upon vessel walls (extrinsic mechanism) and by using active, rhythmic contractions of lymphatic muscle cells embedded in the vessel wall of collecting lymphatics (intrinsic mechanism). The latter mechanism is the major source of the hydraulic pressure gradient where scant extrinsic forces are generated in the microenvironment surrounding lymphatic vessels. It is mainly involved in generating pressure gradients between the interstitial spaces and the vessel lumen and between adjacent lymphatic vessels segments. Intrinsic pumping can very rapidly adapt to ambient physical stimuli such as hydraulic pressure, lymph flow-derived shear stress, fluid osmolarity, and temperature. This adaptation induces a variable lymph flow, which can precisely follow the local tissue state in terms of fluid and solutes removal. Several cellular systems are known to be sensitive to osmolarity, temperature, stretch, and shear stress, and some of them have been found either in lymphatic endothelial cells or lymphatic muscle. In this review, we will focus on how known physical stimuli affect intrinsic contractility and thus lymph flow and describe the most likely cellular mechanisms that mediate this phenomenon.