Aging-associated shifts in functional status of mast cells located by adult and aged mesenteric lymphatic vessels

The position- and lymphatic lumen-controlled tissue chambers to study live lymphatic vessels and surrounding tissues ex vivo

BACKGROUND

Until now, there has been no tool available to provide lymphatic researchers the ability to perform experiments in tissue explants containing lymphatic vessels under tissue position- and lymphatic lumen-controlled conditions.

METHODS AND RESULTS

In this article we provide technical details and description of the method of using the newly developed and implemented the position- and lymphatic lumen-controlled tissue chambers to study live lymphatic vessels and surrounding tissues ex vivo. In this study, we, for the first time, performed detailed comparative analysis of the contractile and pumping activity of rat mesenteric lymphatic vessels (MLVs) situated within tissue explants mounted in new tissue chambers and isolated, cannulated, and pressurized rat MLVs maintained in isolated vessel setups. We found no significant differences of the effects of both transmural pressure- and wall shear stress sensitivities of MLVs in tissue chambers and isolated MLVs.

CONCLUSIONS

We conclude that this new experimental tool, a position- and lymphatic lumen-controlled tissue chamber, allows precise investigation of lymphatic function of MLVs interacting with elements of the tissue microenvironment. This method provides an important new set of experimental tools to investigate lymphatic function.

VEGF-C induces lymphangiogenesis and angiogenesis in the rat mesentery culture model

OBJECTIVE

Lymphatic and blood microvascular systems are critical for tissue function. Insights into the coordination of both systems can be gained by investigating the relationships between lymphangiogenesis and angiogenesis. Recently, our laboratory established the rat mesentery culture model as a novel tool to investigate multicellular interactions during angiogenesis in an intact microvascular network scenario. The objective of this study was to determine whether the rat mesentery culture model can be used to study lymphangiogenesis.

METHODS

Mesenteric tissue windows were harvested from adult male Wistar rats and cultured for three or five days in either serum-free MEM or MEM supplemented with VEGF-C. Tissues were immunolabeled for PECAM and LYVE-1 to identify blood and lymphatic endothelial cells, respectively. Tissues selected randomly from those containing vascular networks were quantified for angiogenesis and lymphangiogenesis.

RESULTS

VEGF-C treatment resulted in an increase in the density of blood vessel sprouting compared to controls by day 3. By day 5, lymphatic sprouting was increased compared to controls.

CONCLUSIONS

These results are consistent with in vivo findings that lymphangiogenesis lags angiogenesis after chronic stimulation and establish a tool for investigating the interrelationships between lymphangiogenesis and angiogenesis in a multisystem microvascular environment.

Bridging the divide between pathogenesis and detection in lymphedema

While our understanding of the lymphatic system has improved substantially in the past few decades, the translation of this knowledge into improved healthcare solutions for patients suffering from secondary lymphedema has been severely limited. The challenge facing clinicians is two-fold. First, there is no reliable, affordable, diagnostic capable of detecting the disease before symptoms of the lymphedema develop and the efficacy of treatment options becomes limited. Second, our understanding of the disease pathogenesis, its risk factors, and the underlying physiologic mechanisms is still in its infancy. These two challenges go hand in hand as limited diagnostic options have hindered our ability to understand lymphedema progression, and the lack of known underlying mechanisms involved in the disease prohibits the development of new diagnostic targets. This review serves to discuss the recent developments in clinical and lab research settings of both lymphedema diagnostic technologies and our understanding of the mechanisms driving disease risk and progression. We will show how these two lines of research are synergistically working with the ultimate goal of improving patient outcomes for those suffering from this horrible disease, identifying key areas of further research that are warranted to move the field forward and provide clinical relief for this neglected patient population.

Emerging trends in the pathophysiology of lymphatic contractile function

Lymphatic contractile dysfunction is central to a number of pathologies that affect millions of people worldwide. Due to its critical role in the process of inflammation, a dysfunctional lymphatic system also compromises the immune response, further exacerbating a number of inflammation related diseases. Despite the critical physiological functions accomplished by the transport of lymph, a complete understanding of the contractile machinery of the lymphatic system lags far behind that of the blood vasculature. However, there has been a surge of recent research focusing on different mechanisms that underlie both physiological and pathophysiological aspects of lymphatic contractile function. This review summarizes those emerging paradigms that shed some novel insights into the contractile physiology of the lymphatics in normal as well as different disease states. In addition, this review emphasizes the recent progress made in our understanding of various contractile parameters and regulatory elements that contribute to the normal functioning of the lymphatics.

Mast cell-directed recruitment of MHC class II positive cells and eosinophils towards mesenteric lymphatic vessels in adulthood and elderly

BACKGROUND

Aging impairs mesenteric lymph flow, which is crucial for fluid and macromolecule homeostasis, fat absorption, and immune function. Previously, we demonstrated that mast cells (MCs) line mesenteric lymphatic vessels (MLVs) with a greater degree of basal activation of MCs in aged mesentery. The number of intact MCs available to react acutely to inflammatory stimuli was decreased with age. However, the role of mast cells in recruiting other immune cells towards MLVs and its aging-associated alterations has not been explored before in great detail.

METHODS AND RESULTS

In this study we treated live mesenteric tissue isolated from Sprague Dawley (SD) rats, as well as adult 9-mo and aged 24-mo Fischer-344 (F-344) rats for 2 hours with MC activators (48/80 and Substance P) and performed whole mount IHC and vital dye staining of the mesenteric segments containing MLVs to identify immune cell recruitment towards MLVs after mast cell (MC) activation. Number of major histocompatibility complex (MHC) class II positive APCs and eosinophils near MLVs was counted and compared between treatments and ages.

CONCLUSIONS

With greater density of MCs near MLVs, we for the first time demonstrated that mesenteric MC activation by compound 48/80 and Substance P resulted in recruitment of MHC class II positive cells and eosinophils towards MLVs. This effect was reduced in cromolyn-injected rats, thus confirming that MCs are necessary for such recruitment. The immune cell presence near MLVs after MC activation was reduced in aged tissues. We link these findings to our previous report of lesser number of intact MCs available for initiating an acute immune response in aged mesentery. Cumulatively, these findings serve as the first step in study of the aging-associated mechanisms that link MCs, lymphatic vessels, and disordered immune function in the elderly.

Electrophysiological properties of rat mesenteric lymphatic vessels and their regulation by stretch

BACKGROUND

In mammals, lymph is propelled centrally primarily via the phasic contractions of collecting lymphatic vessels, known as lymphatic pumping. Electrophysiological studies conducted in guinea pig and sheep mesenteric lymphatic vessels indicate that contractions are initiated in the lymphatic muscle by nifedipine-sensitive action potentials (APs). Lymphatic pumping is highly sensitive to luminal fluid loading and the mechanical properties of this stretch-induced pumping have been consistently studied, in particular in rat mesenteric lymphatic vessels. However, membrane potential (Vm) and the electrophysiological events underlying stretch-induced lymphatic pumping have not been investigated in the rat. The aim of this study was thus to examine the properties of rat mesenteric lymphatic muscle Vm under resting conditions and to assess changes in Vm caused by distension.

METHODS AND RESULTS

Lymphatic muscle Vm was measured with sharp intracellular microelectrodes either in unstretched conditions or under isometric tension provided by a wire-myograph. In unstretched vessels, Vm was -48 ± 2 mV (n=30). APs (amplitude ∼25 mV) were observed at a frequency of ∼8/min and were abolished by nifedipine. Under isometric tension, Vm was less polarized (-36 ± 1 mV, n=23), even at minimum tension. Increase in tension led to increase in contraction strength and contraction/AP frequency, while Vm was slightly hyperpolarized and AP amplitude not markedly altered.

CONCLUSIONS

In our experimental conditions, rat lymphatic muscle has electrophysiological characteristics similar to that in other species. It responds to an increase in isometric tension with an increase in AP frequency, but resting Vm is not significantly affected.

Vascular hyperpermeability and aging

Vascular hyperpermeability, the excessive leakage of fluid and proteins from blood vessels to the interstitial space, commonly occurs in traumatic and ischemic injuries. This hyperpermeability causes tissue vasogenic edema, which often leads to multiple organ failure resulting in patient death. Vascular hyperpermeability occurs most readily in small blood vessels as their more delicate physical constitution makes them an easy target for barrier dysfunction. A single layer of endothelial cells, linked to one another by cell adhesion molecules, covers the interior surface of each blood vessel. The cell adhesion molecules play a key role in maintaining barrier functions like the regulation of permeability. Aging is a major risk factor for microvascular dysfunction and hyperpermeability. Apart from age-related remodeling of the vascular wall, endothelial barrier integrity and function declines with the advancement of age. Studies that address the physiological and molecular basis of vascular permeability regulation in aging are currently very limited. There have been many cellular and molecular mechanisms proposed to explain aging-related endothelial dysfunction but their true relationship to barrier dysfunction and hyperpermeability is not clearly known. Among the several mechanisms that promote vascular dysfunction and hyperpermeability, the following are considered major contributors: oxidative stress, inflammation, and the activation of apoptotic signaling pathways. In this review we highlighted (a) the physiological, cellular and molecular changes that occur in the vascular system as a product of aging; (b) the potential mechanisms by which aging leads to barrier dysfunction and vascular hyperpermeability in the peripheral and the blood-brain barrier; (c) the mechanisms by which the age-related increases in oxidative stress, inflammatory markers and apoptotic signaling etc. cause endothelial dysfunction and their relationship to hyperpermeability; and (d) the relationship between aging, vascular permeability and traumatic injuries.

The effects of inflammatory cytokines on lymphatic endothelial barrier function

Proper lymphatic function is necessary for the transport of fluids, macromolecules, antigens and immune cells out of the interstitium. The lymphatic endothelium plays important roles in the modulation of lymphatic contractile activity and lymph transport, but it's role as a barrier between the lymph and interstitial compartments is less well understood. Alterations in lymphatic function have long been associated with edema and inflammation although the integrity of the lymphatic endothelial barrier during inflammation is not well-defined. In this paper we evaluated the integrity of the lymphatic barrier in response to inflammatory stimuli commonly associated with increased blood endothelial permeability. We utilized in vitro assays of lymphatic endothelial cell (LEC) monolayer barrier function after treatment with different inflammatory cytokines and signaling molecules including TNF-α, IL-6, IL-1β, IFN-γ and LPS. Moderate increases in an index of monolayer barrier dysfunction were noted with all treatments (20-60 % increase) except IFN-γ which caused a greater than 2.5-fold increase. Cytokine-induced barrier dysfunction was blocked or reduced by the addition of LNAME, except for IL-1β and LPS treatments, suggesting a regulatory role for nitric oxide. The decreased LEC barrier was associated with modulation of both intercellular adhesion and intracellular cytoskeletal activation. Cytokine treatments reduced the expression of VE-cadherin and increased scavenging of β-catenin in the LECs and this was partially reversed by LNAME. Likewise the phosphorylation of myosin light chain 20 at the regulatory serine 19 site, which accompanied the elevated monolayer barrier dysfunction in response to cytokine treatment, was also blunted by LNAME application. This suggests that the lymphatic barrier is regulated during inflammation and that certain inflammatory signals may induce large increases in permeability.

Aged lymphatic contractility: recent answers and new questions

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