Vegfr3-tdTomato, a reporter mouse for microscopic visualization of lymphatic vessel by multiple modalities

Clinical Relevance of Animal Models of Lymphatic Dysfunction and Lymphedema

Lymphedema is a chronic progressive condition, and treatment options are limited to physical therapy or surgical intervention, underscoring the need to develop preventative strategies. To do so, we must first understand the underlying mechanisms that contribute to the development of clinical lymphedema, which can be caused by a myriad of factors, including genetic mutations, infectious agents, and cancer treatments. Animal models are essential to study the pathogenesis of clinical lymphedema and to develop therapeutic interventions. Many animal models mimic the various aspects of lymphatic dysfunction and lymphedema seen in humans, and some species better represent different aspects or causes of lymphedema. However, no single model perfectly recapitulates human disease in a cost- and time-efficient manner; therefore, findings should be verified in multiple models and multiple species. In doing so, researchers will increase the likelihood of collecting rigorous, reliable data that could be effectively and efficiently translated into the clinic. This review explores genetic, infectious, and surgical animal models of lymphatic dysfunction and lymphedema and describes how these models can be used to understand clinical forms of lymphedema. Collectively, this information can provide valuable insight for the translational study of lymphatic diseases.

Ontogenesis of the Mouse Ocular Surface Lymphatic Vascular Network

Purpose

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

Methods

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

Results

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

Conclusions

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

Three-dimensional spatial quantitative analysis of cardiac lymphatics in the mouse heart

OBJECTIVE

Three-dimensional (3D) microscopy and image data analysis are necessary for studying the morphology of cardiac lymphatic vessels (LyVs) and their association with other cell types. We aimed to develop a methodology for 3D multiplexed lightsheet microscopy and highly sensitive and quantitative image analysis to identify pathological remodeling in the 3D morphology of LyVs in young adult mouse hearts with familial hypertrophic cardiomyopathy (HCM).

METHODS

We developed a 3D lightsheet microscopy workflow providing a quick turn-around (as few as 5-6 days), multiplex fluorescence detection, and preservation of LyV structure and epitope markers. Hearts from non-transgenic and transgenic (TG) HCM mice were arrested in diastole, retrograde perfused, immunolabeled, optically cleared, and imaged. We built an image-processing pipeline to quantify LyV morphological parameters at the chamber and branch levels.

RESULTS

Chamber-specific pathological alterations of LyVs were identified, and significant changes were seen in the right atrium (RA). TG hearts had a higher volume percent of ER-TR7+ fibroblasts and reticular fibers. In the RA, we found associations between ER-TR7+ volume percent and both LyV segment density and median diameter.

CONCLUSIONS

This workflow and study enabled multi-scale analysis of pathological changes in cardiac LyVs of young adult mice, inviting ideas for research on LyVs in cardiac disease.

Three-dimensional mapping of hepatic lymphatic vessels and transcriptome profiling of lymphatic endothelial cells in healthy and diseased livers

Rationale: Hepatic lymphatics are essential for liver homeostasis and immune function. However, the 3D structure and spatial distribution of hepatic lymphatic vessels (LVs) need to be confirmed. Moreover, the molecular information of hepatic lymphatic endothelial cells (LyECs) needs to be further studied. The bottleneck is the lack of specific markers or labeling methods for hepatic lymphatic endothelial cells (LyECs) Methods: Here, we proposed a method for the spatiotemporal sequential injection of antibodies (STSI-Ab) to selectively label hepatic LyECs in vivo. In addition, we also developed an efficient hepatic LyEC sorting method and performed deep transcriptome sequencing on hepatic LyECs. Results: The STSI-Ab method achieved selective labeling of the mouse hepatic lymphatic network. Three-dimensional fluorescence imaging results of the STSI-Ab mouse liver lobe clearly showed that hepatic LVs entangled with the portal vein but were not present in the central vein. The imaging data inspired a novel hepatic lobule structure model with an added set of LVs in the portal area. Furthermore, deep transcriptome sequencing of isolated hepatic LyECs and Masson's trichrome staining results suggested that hepatic LyECs might be an important source of collagen fibers deposited in the portal area during the process of liver fibrosis and bile duct ligation (BDL). Conclusions: We proposed an STSI-Ab method for selectively labeling hepatic LVs, distinguishing the hepatic LVs from other vessels, and mapping their 3D structure. This study opens an avenue for understanding hepatic lymphatic structure and it will be very beneficial to the study of hepatic LyEC functions.

The ebb and flow of cardiac lymphatics: a tidal wave of new discoveries

The heart is imbued with a vast lymphatic network that is responsible for fluid homeostasis and immune cell trafficking. Disturbances in the forces that regulate microvascular fluid movement can result in myocardial edema, which has profibrotic and proinflammatory consequences and contributes to cardiovascular dysfunction. This review explores the complex relationship between cardiac lymphatics, myocardial edema, and cardiac disease. It covers the revised paradigm of microvascular forces and fluid movement around the capillary as well as the arsenal of preclinical tools and animal models used to model myocardial edema and cardiac disease. Clinical studies of myocardial edema and their prognostic significance are examined in parallel to the recent elegant animal studies discerning the pathophysiological role and therapeutic potential of cardiac lymphatics in different cardiovascular disease models. This review highlights the outstanding questions of interest to both basic scientists and clinicians regarding the roles of cardiac lymphatics in health and disease.

Lymphangiogenesis and Lymphatic Barrier Dysfunction in Renal Fibrosis

As an integral part of the vascular system, the lymphatic vasculature is essential for tissue fluid homeostasis, nutritional lipid assimilation and immune regulation. The composition of the lymphatic vasculature includes fluid-absorbing initial lymphatic vessels (LVs), transporting collecting vessels and anti-regurgitation valves. Although, in recent decades, research has drastically enlightened our view of LVs, investigations of initial LVs, also known as lymphatic capillaries, have been stagnant due to technical limitations. In the kidney, the lymphatic vasculature mainly presents in the cortex, keeping the local balance of fluid, solutes and immune cells. The contribution of renal LVs to various forms of pathology, especially chronic kidney diseases, has been addressed in previous studies, however with diverging and inconclusive results. In this review, we discuss the most recent advances in the proliferation and permeability of lymphatic capillaries as well as their influencing factors. Novel technologies to visualize and measure LVs function are described. Then, we highlight the role of the lymphatic network in renal fibrosis and the crosstalk between kidney and other organs, such as gut and heart.

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.