Development of lymphatic vasculature and morphological characterization in rat kidney

Moving toward a better understanding of renal lymphatics: challenges and opportunities

The development of lymphatic-specific markers has enabled detailed visualization of the lymphatic vascular network that has greatly enhanced our ability to explore this often-overlooked system. Lymphatics remove fluid, solutes, macromolecules, and cells from the interstitium and return them to circulation. The kidneys have lymphatics. As in other organs, the kidney lymphatic vessels are highly sensitive to changes in the local microenvironment. The sensitivity to its milieu may be especially relevant in kidneys because they are central in regulating fluid homeostasis and clearance of metabolites delivered into and eliminated from the renal interstitial compartment. Numerous physiologic conditions and diseases modify the renal interstitial volume, pressure, and composition that can, in turn, influence the growth and function of the renal lymphatics. The impact of the renal microenvironment is further heightened by the fact that kidneys are encapsulated. This review considers the development, structure, and function of the renal lymphatic vessels and explores how factors within the kidney interstitial compartment modify their structure and functionality. Moreover, although currently there are no pharmaceutical agents that specifically target the lymphatic network, we highlight several medications currently used in children with kidney disease and hypertension that have significant but underappreciated effects on lymphatics.

VEGFC Overexpression in Kidney Progenitor Cells Is a Model of Renal Lymphangiectasia-Brief Report

BACKGROUND

Lymphangiogenesis is believed to be a protective response in the setting of multiple forms of kidney injury and mitigates the progression of interstitial fibrosis. To augment this protective response, promoting kidney lymphangiogenesis is being investigated as a potential treatment to slow the progression of kidney disease. As injury-related lymphangiogenesis is driven by signaling from the receptor VEGFR3 (vascular endothelial growth factor receptor 3) in response to the cognate growth factor VEGF (vascular endothelial growth factor)-C released by tubular epithelial cells, this signaling pathway is a candidate for future kidney therapeutics. However, the consequences to kidney development and function to targeting this signaling pathway remain poorly defined.

METHODS

We generated a new mouse model expressing Vegfc under regulation of the nephron progenitor Six2Cre driver strain (Six2Vegfc). Mice underwent a detailed phenotypic evaluation. Whole kidneys were processed for histology and 3-dimensional imaging.

RESULTS

Six2Vegfc mice had reduced body weight and kidney function compared with littermate controls. Six2Vegfc kidneys demonstrated large peripelvic fluid-filled lesions with distortion of the pelvicalcyceal system which progressed in severity with age. Three-dimensional imaging showed a 3-fold increase in total cortical vascular density. Histology confirmed a substantial increase in LYVE1+ (lymphatic vessel endothelial hyaluronan receptor-1)/PDPN+ (podoplanin)/VEGFR3+ lymphatic capillaries extending alongside EMCN+ (endomucin) peritubular capillaries. There was no change in EMCN+ peritubular capillary density.

CONCLUSIONS

Kidney lymphatic density was robustly increased in the Six2Vegfc mice. There were no changes in peritubular blood capillary density despite these endothelial cells also expressing VEGFR3. The model resulted in malformation of the lymphatic hilar plexus, resulting in severe hydronephrosis that resembled a human condition termed renal lymphangiectasia. This study defines the vascular consequences of augmenting VEGFC signaling during kidney development and provides new insight into human renal lymphatic malformations.

VEGF-C overexpression in kidney progenitor cells is a model of renal lymphangiectasia

Background

Lymphangiogenesis is believed to be a protective response in the setting of multiple forms of kidney injury and mitigates the progression of interstitial fibrosis. To augment this protective response, promoting kidney lymphangiogenesis is being investigated as a potential treatment to slow the progression of kidney disease.As injury related lymphangiogenesis is driven by signaling from the receptor VEGFR-3 in response to the cognate growth factor VEGF-C released by tubular epithelial cells, this signaling pathway is a candidate for future kidney therapeutics. However, the consequences to kidney development and function to targeting this signaling pathway remains poorly defined.

Methods

We generated a new mouse model expressing Vegf-C under regulation of the nephron progenitor Six2Cre driver strain (Six2Vegf-C). Mice underwent a detailed phenotypic evaluation. Whole kidneys were processed for histology and micro computed tomography 3-dimensional imaging.

Results

Six2Vegf-C mice had reduced body weight and kidney function compared to littermate controls. Six2Vegf-C kidneys demonstrated large peripelvic fluid filled lesions with distortion of the pelvicalcyceal system which progressed in severity with age. 3D imaging showed a 3-fold increase in total cortical vascular density. Histology confirmed a substantial increase in LYVE1+/PDPN+/VEGFR3+ lymphatic capillaries extending alongside EMCN+ peritubular capillaries. There was no change in EMCN+ peritubular capillary density.

Conclusions

Kidney lymphangiogenesis was robustly induced in the Six2Vegf-C mice. There were no changes in peritubular blood capillary density despite these endothelial cells also expressing VEGFR-3. The model resulted in a severe cystic kidney phenotype that resembled a human condition termed renal lymphangiectasia. This study defines the vascular consequences of augmenting VEGF-C signaling during kidney development and provides new insight into a mimicker of human cystic kidney 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.

The lymphatics in kidney health and disease

The mammalian vascular system consists of two networks: the blood vascular system and the lymphatic vascular system. Throughout the body, the lymphatic system contributes to homeostatic mechanisms by draining extravasated interstitial fluid and facilitating the trafficking and activation of immune cells. In the kidney, lymphatic vessels exist mainly in the kidney cortex. In the medulla, the ascending vasa recta represent a hybrid lymphatic-like vessel that performs lymphatic-like roles in interstitial fluid reabsorption. Although the lymphatic network is mainly derived from the venous system, evidence supports the existence of lymphatic beds that are of non-venous origin. Following their development and maturation, lymphatic vessel density remains relatively stable; however, these vessels undergo dynamic functional changes to meet tissue demands. Additionally, new lymphatic growth, or lymphangiogenesis, can be induced by pathological conditions such as tissue injury, interstitial fluid overload, hyperglycaemia and inflammation. Lymphangiogenesis is also associated with conditions such as polycystic kidney disease, hypertension, ultrafiltration failure and transplant rejection. Although lymphangiogenesis has protective functions in clearing accumulated fluid and immune cells, the kidney lymphatics may also propagate an inflammatory feedback loop, exacerbating inflammation and fibrosis. Greater understanding of lymphatic biology, including the developmental origin and function of the lymphatics and their response to pathogenic stimuli, may aid the development of new therapeutic agents that target the lymphatic system.

Beyond a Passive Conduit: Implications of Lymphatic Biology for Kidney Diseases

The kidney contains a network of lymphatic vessels that clear fluid, small molecules, and cells from the renal interstitium. Through modulating immune responses and via crosstalk with surrounding renal cells, lymphatic vessels have been implicated in the progression and maintenance of kidney disease. In this Review, we provide an overview of the development, structure, and function of lymphatic vessels in the healthy adult kidney. We then highlight the contributions of lymphatic vessels to multiple forms of renal pathology, emphasizing CKD, transplant rejection, and polycystic kidney disease and discuss strategies to target renal lymphatics using genetic and pharmacologic approaches. Overall, we argue the case for lymphatics playing a fundamental role in renal physiology and pathology and treatments modulating these vessels having therapeutic potential across the spectrum of kidney disease.

Spatiotemporal dynamics and heterogeneity of renal lymphatics in mammalian development and cystic kidney disease

Heterogeneity of lymphatic vessels during embryogenesis is critical for organ-specific lymphatic function. Little is known about lymphatics in the developing kidney, despite their established roles in pathology of the mature organ. We performed three-dimensional imaging to characterize lymphatic vessel formation in the mammalian embryonic kidney at single-cell resolution. In mouse, we visually and quantitatively assessed the development of kidney lymphatic vessels, remodeling from a ring-like anastomosis under the nascent renal pelvis; a site of VEGF-C expression, to form a patent vascular plexus. We identified a heterogenous population of lymphatic endothelial cell clusters in mouse and human embryonic kidneys. Exogenous VEGF-C expanded the lymphatic population in explanted mouse embryonic kidneys. Finally, we characterized complex kidney lymphatic abnormalities in a genetic mouse model of polycystic kidney disease. Our study provides novel insights into the development of kidney lymphatic vasculature; a system which likely has fundamental roles in renal development, physiology and disease.

In most organs in the body, fluid tends to build up in the spaces between cells, especially if the organs become inflamed. Each organ has a ‘waste disposal system’; a set of specialized tubes called lymphatic vessels, to clear away this excess fluid and keep a check on inflammation. Defects in these tubes have been linked to a wide range of diseases including heart attacks, obesity, dementia and cancer.

The kidneys are responsible for filtering blood and balancing many of the body’s chemical processes. Polycystic kidney disease (PKD) is the most common genetic kidney disorder and it results in cysts filled with fluid building up in the kidney. The growth of cysts in PKD may be due to a problem with the lymphatic vessels. However, compared to other organs, how lymphatic vessels first form within the kidney and what they do is not well understood.

Now, Jafree et al. have used three-dimensional imaging to study how lymphatic vessels form in the kidneys of mice and humans. The experiments showed that lymphatic vessels first appear when mouse kidneys are about half developed, and start to grow rapidly when the kidneys are thought to begin filtering blood. Clusters of cells that may help lymphatic vessels to grow were also found hidden deep within the kidneys of mouse embryos. Treating the kidneys with a factor that stimulates the growth of lymphatic vessels increased the numbers of these clusters. Jafree et al. found similar clusters of cells in human kidneys, suggesting that lymphatic vessels in the kidneys of different mammals may develop in the same way.

Further experiments showed that the lymphatic vessels of kidneys in mice with PKD become distorted early on in the disease, when cysts are still small and before the mice develop symptoms. In the future, identifying drugs that target kidney lymphatic vessels may lead to more effective treatments for patients with PKD and other kidney diseases.

A novel role of HIF-1α/PROX-1/LYVE-1 axis on tissue regeneration after renal ischaemia/reperfusion in mice

Context: Renal ischaemia reperfusion (I/R) is a common clinical condition with a high morbidity and mortality rate. To date, I/R-induced renal injury remains an ineffective treatment. Objective: We hypothesis that angiogenesis and lymphangiogenesis markers, prospero homeobox-1 (PROX-1) and lymphatic endothelial hyaluronan receptor-1 (LYVE-1), are critical during I/R. Material and methods: Kunming mice were subjected to I/R and observed for the following eight consecutive days. Pathology analysis and protein distribution were detected by H&E staining, immunohistochemistry and immunofluorescence confocal analysis. Results: After I/R treatment, renal pathology was changed. HIF-1α was induced in the early stage and colocalisation with PROX-1 mainly in the renal tubular region, whereas PROX-1 and LYVE-1 were colocalised in the glomerulus of the endothelial region. Conclusions: In this study, we revealed HIF-1α/PROX-1/LVYE-1 axis dynamic changes in different regions after I/R and demonstrated for the first time it activates during I/R repair.

Angiotensin inhibition in the developing kidney; tubulointerstitial effect

BACKGROUND

Renin-angiotensin system (RAS) blockade during nephrogenesis causes a broad range of renal mal-development. Here, we hypothesized that disruption of renal lymphangiogenesis may contribute to tubulointerstitial alterations after RAS blockade during kidney maturation.

METHODS

Newborn rat pups were treated with enalapril (30 mg/kg/day) or vehicle for 7 days after birth. Lymphangiogenesis was assessed via immunostaining and/or immunoblots for vascular endothelial growth factor (VEGF)-C, VEGF receptor (VEGFR)-3, Podoplanin, and Ki-67. The intrarenal expression of fibroblast growth factor (FGF)-1, FGF-2, FGF receptor (R)-1, α-smooth muscle actin (α-SMA), and fibroblast-specific protein (FSP)-1 was also determined. Sirius Red staining was performed to evaluate interstitial collagen deposition.

RESULTS

On postnatal day 8, renal lymphangiogenesis was disrupted by neonatal enalapril treatment. The expression of podoplanin and Ki-67 decreased in enalapril-treated kidneys. While the expression of VEGF-C was decreased, the levels of VEGFR-3 receptor increased following enalapril treatment. Enalapril treatment also reduced the renal expression of FGF-1, FGF-2, and FGFR-1. Enalapril-treated kidneys exhibited profibrogenic properties with increased expression of α-SMA and FSP-1 and enhanced deposition of interstitial collagen.

CONCLUSION

Enalapril treatment during postnatal renal maturation can disrupt renal lymphangiogenesis along with tubulointerstitial changes, which may result in a pro-fibrotic environment in the developing rat kidney.

Renal Lymphatics: Anatomy, Physiology, and Clinical Implications

Renal lymphatics are abundant in the cortex of the normal kidney but have been largely neglected in discussions around renal diseases. They originate in the substance of the renal lobule as blind-ended initial capillaries, and can either follow the main arteries and veins toward the hilum, or penetrate the capsule to join capsular lymphatics. There are no valves present in interlobular lymphatics, which allows lymph formed in the cortex to exit the kidney in either direction. There are very few lymphatics present in the medulla. Lymph is formed from interstitial fluid in the cortex, and is largely composed of capillary filtrate, but also contains fluid reabsorbed from the tubules. The two main factors that contribute to renal lymph formation are interstitial fluid volume and intra-renal venous pressure. Renal lymphatic dysfunction, defined as a failure of renal lymphatics to adequately drain interstitial fluid, can occur by several mechanisms. Renal lymphatic inflow may be overwhelmed in the setting of raised venous pressure (e.g., cardiac failure) or increased capillary permeability (e.g., systemic inflammatory response syndrome). Similarly, renal lymphatic outflow, at the level of the terminal thoracic duct, may be impaired by raised central venous pressures. Renal lymphatic dysfunction, from any cause, results in renal interstitial edema. Beyond a certain point of edema, intra-renal collecting lymphatics may collapse, further impairing lymphatic drainage. Additionally, in an edematous, tense kidney, lymphatic vessels exiting the kidney via the capsule may become blocked at the exit point. The reciprocal negative influences between renal lymphatic dysfunction and renal interstitial edema are expected to decrease renal function due to pressure changes within the encapsulated kidney, and this mechanism may be important in several common renal conditions.

Attenuated Lymphatic Proliferation Ameliorates Diabetic Nephropathy and High-Fat Diet-Induced Renal Lipotoxicity

Lymphangiogenesis occurs in response to renal injury and is correlated with interstitial fibrosis. Diabetes- and high-fat diet (HFD)-induced intrarenal lipotoxicity and their relationships with lymphangiogenesis are not established. We used PPARα agonist, fenofibrate, to unravel the linkage between lipotoxicity and lymphangiogenesis. Eight-week-old male C57BLKS/J db/db mice and HFD Spontaneously hypertensive rats (SHRs) were fed fenofibrate for 12 weeks. HK-2 and RAW264.7 cells were used to investigate their lymphangiogenic capacity in relation to lipotoxicity. Fenofibrate improved intrarenal lipotoxicity by increasing expression of PPARα and phosphorylation of AMPK. Lymphatic proliferation was attenuated; expression of lymphatic endothelial hyaluronan receptor-1 (LYVE-1), podoplanin, vascular endothelial growth factor-C (VEGF-C), and vascular endothelial growth factor receptor-3 (VEGFR-3) was decreased. In parallel, extent of tubulointerstitial fibrosis, apoptosis and inflammatory cell infiltration was reduced. In HK2 cells, palmitate- and high glucose-induced over expression of lymphatic makers was diminished by fenofibrate via activation of PPARα-AMPK-pACC signaling. Enhanced expression of M1 phenotype in RAW264.7 cells correlated with increased lymphatic growth. A causal relationship between lipotoxicity and lymphatic proliferation with a cellular link to macrophage activation can be speculated; pro-inflammatory M1 type macrophage is involved in the development of lymphangiogenesis through stimulation of VEGF-C and by its transdifferentiation into lymphatic endothelial cells.

Development of the renal vasculature

The kidney vasculature has a unique and complex architecture that is central for the kidney to exert its multiple and essential physiological functions with the ultimate goal of maintaining homeostasis. An appropriate development and coordinated assembly of the different vascular cell types and their association with the corresponding nephrons is crucial for the generation of a functioning kidney. In this review we provide an overview of the renal vascular anatomy, histology, and current knowledge of the embryological origin and molecular pathways involved in its development. Understanding the cellular and molecular mechanisms involved in renal vascular development is the first step to advance the field of regenerative medicine.

Emerging Concepts in Organ-Specific Lymphatic Vessels and Metabolic Regulation of Lymphatic Development

The lymphatic system has been less well characterized than the blood vascular system; however, work in recent years has uncovered novel regulators and non-venous lineages that contribute to lymphatic formation in various organs. Further, the identification of organ-specific lymphatic beds underscores their potential interaction with organ development and function, and highlights the possibility of targeting these organ-specific lymphatics beds in disease. This review focuses on newly described metabolic and epigenetic regulators of lymphangiogenesis and the interplay between lymphatic development and function in a number of major organ systems.

Peroxisome proliferator-activated receptor alpha agonist suppresses neovascularization by reducing both vascular endothelial growth factor and angiopoietin-2 in corneal alkali burn

We investigated the effect of a peroxisome proliferator-activated receptor alpha (PPARα) agonist ophthalmic solution in wound healing using a rat corneal alkali burn model. After instillation of a selective agonist of PPARα, fenofibrate, onto the burned cornea, PPARα-positive cells were observed in vascular endothelial cells, and there was upregulation of mRNA of PPARα in corneal stroma. Fenofibrate suppressed expression of neutrophils and macrophages during the early phase, and development of neovascularization and myofibroblast generation during the late phase. Fenofibrate reduced not only mRNA expression of vascular endothelial growth factor-A but also angiopoietin-1 and angiopoietin-2. Furthermore, fenofibrate suppressed scar formation by reducing type III collagen expression. These data suggest that a PPARα agonist ophthalmic solution might be a new strategy for treating corneal wounds through not only anti-inflammatory effects but also by preventing neovascularization.

Refuting the hypothesis that semaphorin-3f/neuropilin-2 exclude blood vessels from the cap mesenchyme in the developing kidney

BACKGROUND

During murine kidney development, new cortical blood vessels form and pattern in cycles that coincide with cycles of collecting duct branching and the accompanying splitting of the cap mesenchyme (nephron progenitor cell populations that "cap" collecting duct ends). At no point in the patterning cycle do blood vessels enter the cap mesenchyme. We hypothesized that the exclusion of blood vessels from the cap mesenchyme may be controlled, at least in part, by an anti-angiogenic signal expressed by the cap mesenchyme cells.

RESULTS

We show that semaphorin-3f (Sema3f), a known anti-angiogenic factor, is expressed in cap mesenchymal cells and its receptor, neuropilin-2 (Nrp2), is expressed by newly forming blood vessels in the cortex of the developing kidney. We hypothesized that Sema3f/Nrp2 signaling excludes vessels from the cap mesenchyme. Genetic ablation of Sema3f and of Nrp2, however, failed to result in vessels invading the cap mesenchyme.

CONCLUSIONS

Despite complementary expression patterns, our data suggest that Sema3f and Nrp2 are dispensable for the exclusion of vessels from the cap mesenchyme during kidney development. These results should provoke additional experiments to ascertain the biological significance of Sema3f/Nrp2 expression in the developing kidney. Developmental Dynamics 246:1047-1056, 2017. © 2017 Wiley Periodicals, Inc.

The First Scube3 Mutant Mouse Line with Pleiotropic Phenotypic Alterations

The vertebrate Scube (Signal peptide, CUB, and EGF-like domain-containing protein) family consists of three independent members, Scube1–3, which encode secreted cell surface-associated membrane glycoproteins. Limited information about the general function of this gene family is available, and their roles during adulthood. Here, we present the first Scube3 mutant mouse line (Scube3^N294K/N294K), which clearly shows phenotypic alterations by carrying a missense mutation in exon 8, and thus contributes to our understanding of SCUBE3 functions. We performed a detailed phenotypic characterization in the German Mouse Clinic (GMC). Scube3^N294K/N294K mutants showed morphological abnormalities of the skeleton, alterations of parameters relevant for bone metabolism, changes in renal function, and hearing impairments. These findings correlate with characteristics of the rare metabolic bone disorder Paget disease of bone (PDB), associated with the chromosomal region of human SCUBE3. In addition, alterations in energy metabolism, behavior, and neurological functions were detected in Scube3^N294K/N294K mice. The Scube3^N294K/N294K mutant mouse line may serve as a new model for further studying the effect of impaired SCUBE3 gene function.

Vascular endothelial growth factor signaling is necessary for expansion of medullary microvessels during postnatal kidney development

Postnatal inhibition or deletion of angiotensin II (ANG II) AT1 receptors impairs renal medullary mircrovascular development through a mechanism that may include vascular endothelial growth factor (VEGF). The present study was designed to test if VEGF/VEGF receptor signaling is necessary for the development of the renal medullary microcirculation. Endothelial cell-specific immunolabeling of kidney sections from rats showed immature vascular bundles at postnatal day (P) 10 with subsequent expansion of bundles until P21. Medullary VEGF protein abundance coincided with vasa recta bundle formation. In human fetal kidney tissue, immature vascular bundles appeared early in the third trimester (GA27-28) and expanded in size until term. Rat pups treated with the VEGF receptor-2 (VEGFR2) inhibitor vandetanib (100 mg·kg−1·day−1) from P7 to P12 or P10 to P16 displayed growth retardation and proteinuria. Stereological quantification showed a significant reduction in total length (386 ± 13 vs. 219 ± 16 m), surface area, and volume of medullary microvessels. Vascular bundle architecture was unaffected. ANG II-AT1A/1B−/− mice kidneys displayed poorly defined vasa recta bundles whereas mice with collecting duct principal cell-specific AT1A deletion displayed no medullary microvascular phenotype. In conclusion, VEGFR2 signaling during postnatal development is necessary for expansion of the renal medullary microcirculation but not structural patterning of the vasa recta bundles, which occurs through an AT1-mediated mechanism.

Lymphangiogenesis in renal diseases: passive bystander or active participant?

Lymphatic vessels (LVs) are involved in a number of physiological and pathophysiological processes such as fluid homoeostasis, immune surveillance, and resolution of inflammation and wound healing. Lymphangiogenesis, the outgrowth of existing LVs and the formation of new ones, has received increasing attention over the past decade on account of its prominence in organ physiology and pathology, which has been enabled by the development of specific tools to study lymph vessel functions. Several studies have been devoted to renal lymphatic vasculature and lymphangiogenesis in kidney diseases, such as chronic renal transplant dysfunction, primary renal fibrotic disorders, proteinuria, diabetic nephropathy and renal inflammation. This review describes the most recent findings on lymphangiogenesis, with a specific focus on renal lymphangiogenesis and its impact on renal diseases. We suggest renal lymphatics as a possible target for therapeutic interventions in renal medicine to dampen tubulointerstitial tissue remodelling and improve renal functioning.