Endogenous origin of glomerular endothelial and mesangial cells in grafts of embryonic kidneys

Origins and Molecular Mechanisms Underlying Renal Vascular Development

Kidneys play a crucial role in maintaining homeostasis within the body, and this function is intricately linked to the vascular structures within them. For vascular cells in the kidney to mature and function effectively, a well-coordinated spatial alignment between the nephrons and complex network of blood vessels is essential. This arrangement ensures efficient blood filtration and regulation of the electrolyte balance, blood pressure, and fluid levels. Additionally, the kidneys are vital in regulating the acid-base balance and producing hormones involved in erythropoiesis and blood pressure control. This article focuses on the vascular development of the kidneys, summarizing the current understanding of the origin and formation of the renal vasculature, and the key molecules involved. A comprehensive review of existing studies has been conducted to elucidate the cellular and molecular mechanisms governing renal vascular development. Specific molecules play a critical role in the development of renal vasculature, contributing to the spatial alignment between nephrons and blood vessels. By elucidating the cellular and molecular mechanisms involved in renal vascular development, this study aims to advance renal regenerative medicine and offer potential avenues for therapeutic interventions in kidney disease.

The anterior chamber of the eye technology and its anatomical, optical, and immunological bases

The anterior chamber of the eye (ACE) is distinct in its anatomy, optics, and immunology. This guarantees that the eye perceives visual information in the context of physiology even when encountering adverse incidents like inflammation. In addition, this endows the ACE with the special nursery bed iris enriched in vasculatures and nerves. The ACE constitutes a confined space enclosing an oxygen/nutrient-rich, immune-privileged, and less stressful milieu as well as an optically transparent medium. Therefore, aside from visual perception, the ACE unexpectedly serves as an excellent transplantation site for different body parts and a unique platform for noninvasive, longitudinal, and intravital microimaging of different grafts. On the basis of these merits, the ACE technology has evolved from the prototypical through the conventional to the advanced version. Studies using this technology as a versatile biomedical research platform have led to a diverse range of basic knowledge and in-depth understanding of a variety of cells, tissues, and organs as well as artificial biomaterials, pharmaceuticals, and abiotic substances. Remarkably, the technology turns in vivo dynamic imaging of the morphological characteristics, organotypic features, developmental fates, and specific functions of intracameral grafts into reality under physiological and pathological conditions. Here we review the anatomical, optical, and immunological bases as well as technical details of the ACE technology. Moreover, we discuss major achievements obtained and potential prospective avenues for this technology.

β-Catenin in the kidney stroma modulates pathways and genes to regulate kidney development

BACKGROUND

Kidney development is regulated by cellular interactions between the ureteric epithelium, mesenchyme, and stroma. Previous studies demonstrate essential roles for stromal β-catenin in kidney development. However, how stromal β-catenin regulates kidney development is not known. We hypothesize that stromal β-catenin modulates pathways and genes that facilitate communications with neighboring cell populations to regulate kidney development.

RESULTS

We isolated purified stromal cells with wild type, deficient, and overexpressed β-catenin by fluorescence-activated cell sorting and conducted RNA Sequencing. A Gene Ontology network analysis demonstrated that stromal β-catenin modulates key kidney developmental processes, including branching morphogenesis, nephrogenesis and vascular formation. Specific stromal β-catenin candidate target genes that may mediate these effects included secreted, cell-surface and transcriptional factors that regulate branching morphogenesis and nephrogenesis (Wnts, Bmp, Fgfr, Tcf/Lef) and secreted vascular guidance cues (Angpt1, VEGF, Sema3a). We validated established β-catenin targets including Lef1 and novel candidate β-catenin targets including Sema3e which have unknown roles in kidney development.

CONCLUSIONS

These studies advance our understanding of gene and biological pathway dysregulation in the context of stromal β-catenin misexpression during kidney development. Our findings suggest that during normal kidney development, stromal β-catenin may regulate secreted and cell-surface proteins to communicate with adjacent cell populations.

Serial intravital imaging captures dynamic and functional endothelial remodeling with single-cell resolution

Endothelial cells are important in the maintenance of healthy blood vessels and in the development of vascular diseases. However, the origin and dynamics of endothelial precursors and remodeling at the single-cell level have been difficult to study in vivo owing to technical limitations. Therefore, we aimed to develop a direct visual approach to track the fate and function of single endothelial cells over several days and weeks in the same vascular bed in vivo using multiphoton microscopy (MPM) of transgenic Cdh5-Confetti mice and the kidney glomerulus as a model. Individual cells of the vascular endothelial lineage were identified and tracked owing to their unique color combination, based on the random expression of cyan/green/yellow/red fluorescent proteins. Experimental hypertension, hyperglycemia, and laser-induced endothelial cell ablation rapidly increased the number of new glomerular endothelial cells that appeared in clusters of the same color, suggesting clonal cell remodeling by local precursors at the vascular pole. Furthermore, intravital MPM allowed the detection of distinct structural and functional alterations of proliferating endothelial cells. No circulating Cdh5-Confetti⁺ cells were found in the renal cortex. Moreover, the heart, lung, and kidneys showed more significant clonal endothelial cell expansion compared with the brain, pancreas, liver, and spleen. In summary, we have demonstrated that serial MPM of Cdh5-Confetti mice in vivo is a powerful technical advance to study endothelial remodeling and repair in the kidney and other organs under physiological and disease conditions.

Mixing Cells for Vascularized Kidney Regeneration

The worldwide rise in prevalence of chronic kidney disease (CKD) demands innovative bio-medical solutions for millions of kidney patients. Kidney regenerative medicine aims to replenish tissue which is lost due to a common pathological pathway of fibrosis/inflammation and rejuvenate remaining tissue to maintain sufficient kidney function. To this end, cellular therapy strategies devised so far utilize kidney tissue-forming cells (KTFCs) from various cell sources, fetal, adult, and pluripotent stem-cells (PSCs). However, to increase engraftment and potency of the transplanted cells in a harsh hypoxic diseased environment, it is of importance to co-transplant KTFCs with vessel forming cells (VFCs). VFCs, consisting of endothelial cells (ECs) and mesenchymal stem-cells (MSCs), synergize to generate stable blood vessels, facilitating the vascularization of self-organizing KTFCs into renovascular units. In this paper, we review the different sources of KTFCs and VFCs which can be mixed, and report recent advances made in the field of kidney regeneration with emphasis on generation of vascularized kidney tissue by cell transplantation.

Renin Cells, the Kidney, and Hypertension

Renin cells are essential for survival perfected throughout evolution to ensure normal development and defend the organism against a variety of homeostatic threats. During embryonic and early postnatal life, they are progenitors that participate in the morphogenesis of the renal arterial tree. In adult life, they are capable of regenerating injured glomeruli, control blood pressure, fluid-electrolyte balance, tissue perfusion, and in turn, the delivery of oxygen and nutrients to cells. Throughout life, renin cell descendants retain the plasticity or memory to regain the renin phenotype when homeostasis is threatened. To perform all of these functions and maintain well-being, renin cells must regulate their identity and fate. Here, we review the major mechanisms that control the differentiation and fate of renin cells, the chromatin events that control the memory of the renin phenotype, and the major pathways that determine their plasticity. We also examine how chronic stimulation of renin cells alters their fate leading to the development of a severe and concentric hypertrophy of the intrarenal arteries and arterioles. Lastly, we provide examples of additional changes in renin cell fate that contribute to equally severe kidney disorders.

The Renal Extracellular Matrix as a Supportive Scaffold for Kidney Tissue Engineering: Progress and Future Considerations

During the past decades, diverse methods have been used toward renal tissue engineering in order to replace renal function. The goals of all these techniques included the recapitulation of renal filtration, re-absorptive, and secretary functions, and replacement of endocrine/metabolic activities. It is also imperative to develop a reliable, up scalable, and timely manufacturing process. Decellularization of the kidney with intact ECM is crucial for in-vivo compatibility and targeted clinical application. Contemporarily there is an increasing interest and research in the field of regenerative medicine including stem cell therapy and tissue bioengineering in search for new and reproducible sources of kidneys. In this chapter, we sought to determine the most effective method of renal decellularization and recellularization with emphasis on biologic composition and support of stem cell growth. Current barriers and limitations of bioengineered strategies will be also discussed, and strategies to overcome these are suggested.

Stem cell-derived kidney organoids: engineering the vasculature

Kidney organoids can be generated from human pluripotent stem cells (PSCs) using protocols that resemble the embryonic development of the kidney. The renal structures thus generated offer great potential for disease modeling, drug screening, and possibly future therapeutic application. At the same time, use of these PSC-derived organoids is hampered by lack of maturation and off-target differentiation. Here, we review the main protocols for the generation of kidney organoids from human-induced PSCs, discussing their advantages and limitations. In particular, we will focus on the vascularization of the kidney organoids, which appears to be one of the critical factors to achieve maturation and functionality of the organoids.

Host Lymphotoxin-β Receptor Signaling Is Crucial for Angiogenesis of Metanephric Tissue Transplanted into Lymphoid Sites

The mouse lymph node (LN) can provide a niche to grow metanephric kidney to maturity. Here, we show that signaling through the lymphotoxin-β receptor (LTβR) is critical for kidney organogenesis both in the LN and the omentum. By transplanting kidney rudiments either in the LNs of mice undergoing LTβR antagonist treatment or in the omenta of Ltbr knockout (Ltbr-/-) mice, the host LTβR signals were found to be crucial for obtaining a well-vascularized kidney graft. Indeed, defective LTβR signaling correlated with decreased expression of endothelial and angiogenic markers in kidney grafts as well as structural alterations. Because the number of glomerular endothelial cells expressing the LTβR target nuclear factor κB-inducing kinase (NIK) decreased in the absence of a functional LTβR, it was speculated that an LTβR/NIK axis mediated the angiogenetic signals required for successful ectopic kidney organogenesis, given the established role of NIK in neovascularization. However, the transplantation of kidney rudiments in omenta of Nik-/- mice revealed that NIK is dispensable for ectopic kidney vascular integration and maturation. Finally, defective LTβR signaling impaired compensatory glomerular adaptation to renal mass reduction, indicating that kidney regeneration approaches, besides whole kidney reconstruction, might benefit from the presence of LTβR signals.

Reconstitution of the embryonic kidney identifies a donor cell contribution to the renal vasculature upon transplantation

The kidney possesses a highly organised vasculature that is required for its filtration function. While recent advances in stem cell biology have enabled the in vitro generation of kidney tissues, at least partially, recapitulation of the complicated vascular architecture remains a huge challenge. Herein we develop a method to reconstitute both the kidney and its vascular architecture in vitro, using dissociated and sorted mouse embryonic kidney cells. Upon transplantation, arteriolar networks were re-established that ran through the interstitial space between branching ureteric buds and eventually entered glomeruli. Using this system, we found that donor-derived endothelial cells significantly contributed to the arterioles and glomerular capillaries formed after transplantation. Unexpectedly, the near-complete depletion of canonical endothelial cells from the donor embryonic kidney suggested the existence of unidentified donor-derived endothelial precursors that were negative for canonical endothelial markers, but still contributed significantly to the vasculature in the transplants. Thus, our protocol will serve as a useful platform for identification of renal endothelial precursors and induction of these precursors from pluripotent stem cells.

Regulation of Renal Differentiation by Trophic Factors

Classically, trophic factors are considered as proteins which support neurons in their growth, survival, and differentiation. However, most neurotrophic factors also have important functions outside of the nervous system. Especially essential renal growth and differentiation regulators are glial cell line-derived neurotrophic factor (GDNF), bone morphogenetic proteins (BMPs), and fibroblast growth factors (FGFs). Here we discuss how trophic factor-induced signaling contributes to the control of ureteric bud (UB) branching morphogenesis and to maintenance and differentiation of nephrogenic mesenchyme in embryonic kidney. The review includes recent advances in trophic factor functions during the guidance of branching morphogenesis and self-renewal versus differentiation decisions, both of which dictate the control of kidney size and nephron number. Creative utilization of current information may help better recapitulate renal differentiation in vitro, but it is obvious that significantly more basic knowledge is needed for development of regeneration-based renal therapies.

Regenerative potential of induced pluripotent stem cells derived from patients undergoing haemodialysis in kidney regeneration

Kidney regeneration from pluripotent stem cells is receiving a lot of attention because limited treatments are currently available for chronic kidney disease (CKD). It has been shown that uremic state in CKD is toxic to somatic stem/progenitor cells, such as endothelial progenitor and mesenchymal stem cells, affecting their differentiation and angiogenic potential. Recent studies reported that specific abnormalities caused by the non-inherited disease are often retained in induced pluripotent stem cell (iPSC)-derived products obtained from patients. Thus, it is indispensable to first assess whether iPSCs derived from patients with CKD due to non-inherited disease (CKD-iPSCs) have the ability to generate kidneys. In this study, we generated iPSCs from patients undergoing haemodialysis due to diabetes nephropathy and glomerulonephritis (HD-iPSCs) as representatives of CKD-iPSCs or from healthy controls (HC-iPSCs). HD-iPSCs differentiated into nephron progenitor cells (NPCs) with similar efficiency to HC-iPSCs. Additionally, HD-iPSC-derived NPCs expressed comparable levels of NPC markers and differentiated into vascularised glomeruli upon transplantation into mice, as HC-iPSC-derived NPCs. Our results indicate the potential of HD-iPSCs as a feasible cell source for kidney regeneration. This is the first study paving the way for CKD patient-stem cell-derived kidney regeneration, emphasising the potential of CKD-iPSCs.

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.

Mouse Metanephric Mesenchymal Cell-Derived Angioblasts Undergo Vasculogenesis in Three-Dimensional Culture

In vitro models for the investigation of renal vascular development are limited. We previously showed that isolated metanephric mesenchymal (MM) and ureteric bud (UB) cells grown in three-dimensional (3D) matrices formed organoids that consisted of primitive vascular structures surrounding a polarized epithelium. Here, we examined the potential of two principal effectors of vasculogenesis, vascular endothelial growth factor A (VEGF-A), and platelet-derived growth factor B chain (PDGF-BB), to stimulate MM cell differentiation. The results showed that MM cells possess angioblast characteristics by expressing phenotypic markers for endothelial and mesenchymal cells. UB cells synthesize VEGF-A and PDGF-BB proteins and RNA, whereas the MM cells express the respective cognate receptors, supporting their role in directional induction of vasculogenesis. VEGF-A stimulated proliferation of MM cells in monolayer and in 3D sponges but did not affect MM cell migration, organization, or vasculogenesis. However, PDGF-BB stimulated MM cell proliferation, migration, and vasculogenesis in monolayer and organization of the cells into primitive capillary-like assemblies in 3D sea sponge scaffolds in vitro. A role for PDGF-BB in vasculogenesis in the 3D MM/UB co-culture system was validated by direct interference with PDGF-BB or PDGF receptor-β cell interactions to implicate PDGF-BB as a primary effector of MM cell vasculogenesis. Thus, MM cells resemble early renal angioblasts that may provide an ideal platform for the investigation of renal vasculogenesis in vitro.

Cycles of vascular plexus formation within the nephrogenic zone of the developing mouse kidney

The renal vasculature is required for blood filtration, blood pressure regulation, and pH maintenance, as well as other specialised kidney functions. Yet, despite its importance, many aspects of its development are poorly understood. To provide a detailed spatiotemporal analysis of kidney vascularisation, we collected images of embryonic mouse kidneys at various developmental time-points. Here we describe the first stages of kidney vascularisation and demonstrate that polygonal networks of vessels (endothelial plexuses) form in cycles at the periphery of the kidney. We show that kidney vascularisation initiates at E11, when vessels connected to the embryonic circulation form a ring around the ureteric bud. From E13.5, endothelial plexuses organise around populations of cap mesenchymal and ureteric bud cells in a cyclical, predictable manner. Specifically, as the ureteric bud bifurcates, endothelia form across the bifurcation site as the cap mesenchyme splits. The plexuses are vascular, carry erythrocytes, are enclosed within a basement membrane, and can always be traced back to the renal artery. Our results are a major step towards understanding how the global architecture of the renal vasculature is achieved.

Development of the Mammalian Kidney

The basic unit of kidney function is the nephron. In the mouse, around 14,000 nephrons form in a 10-day period extending into early neonatal life, while the human fetus forms the adult complement of nephrons in a 32-week period completed prior to birth. This review discusses our current understanding of mammalian nephrogenesis: the contributing cell types and the regulatory processes at play. A conceptual developmental framework has emerged for the mouse kidney. This framework is now guiding studies of human kidney development enabled in part by in vitro systems of pluripotent stem cell-seeded nephrogenesis. A near future goal will be to translate our developmental knowledge-base to the productive engineering of new kidney structures for regenerative medicine.

Hemovascular Progenitors in the Kidney Require Sphingosine-1-Phosphate Receptor 1 for Vascular Development

The close relationship between endothelial and hematopoietic precursors during early development of the vascular system suggested the possibility of a common yet elusive precursor for both cell types. Whether similar or related progenitors for endothelial and hematopoietic cells are present during organogenesis is unclear. Using inducible transgenic mice that specifically label endothelial and hematopoietic precursors, we performed fate-tracing studies combined with colony-forming assays and crosstransplantation studies. We identified a progenitor, marked by the expression of helix-loop-helix transcription factor stem cell leukemia (SCL/Tal1). During organogenesis of the kidney, SCL/Tal1(+) progenitors gave rise to endothelium and blood precursors with multipotential colony-forming capacity. Furthermore, appropriate morphogenesis of the kidney vasculature, including glomerular capillary development, arterial mural cell coating, and lymphatic vessel development, required sphingosine 1-phosphate (S1P) signaling via the G protein-coupled S1P receptor 1 in these progenitors. Overall, these results show that SCL/Tal1(+) progenitors with hemogenic capacity originate and differentiate within the early embryonic kidney by hemovasculogenesis (the concomitant formation of blood and vessels) and underscore the importance of the S1P pathway in vascular development.

Unwrapping the origins and roles of the renal endothelium

The renal vasculature, like all vessels, is lined by a thin layer of simple squamous epithelial cells called an endothelium. These endothelial-lined vessels can be subdivided into four major compartments: arteries, veins, capillaries and lymphatics. The renal vasculature is a highly integrated network that forms through the active processes of angiogenesis and vasculogenesis. Determination of the precise contribution of these two processes and of the molecular signaling that governs the differentiation, specification and maturation of these critical cell populations is the focus of an actively evolving field of research. Although much of the focus has concentrated on the origin of the glomerular capillaries, in this review we extend the investigation to the origins of the endothelial cells throughout the entire kidney and the signaling events that cause their distinct functional and molecular profiles. A thorough understanding of endothelial cell biology may play a critical role in a better understanding of renal vascular diseases.

The lymph node as a new site for kidney organogenesis

The shortage of organs for kidney transplantation has created the need to develop new strategies to restore renal structure and function. Given our recent finding that the lymph node (LN) can serve as an in vivo factory to generate or sustain complex structures like liver, pancreas, and thymus, we investigated whether it could also support kidney organogenesis from mouse renal embryonic tissue (metanephroi). Here we provide the first evidence that metanephroi acquired a mature phenotype upon injection into LN, and host cells likely contributed to this process. Urine-like fluid-containing cysts were observed in several grafts 12 weeks post-transplantation, indicating metanephroi transplants' ability to excrete products filtered from the blood. Importantly, the kidney graft adapted to a loss of host renal mass, speeding its development. Thus, the LN might provide a unique tool for studying the mechanisms of renal maturation, cell proliferation, and fluid secretion during cyst development. Moreover, we provide evidence that inside the LN, short-term cultured embryonic kidney cells stimulated with the Wnt agonist R-Spondin 2 gave rise to a monomorphic neuron-like cell population expressing the neuronal 200-kDa neurofilament heavy marker. This finding indicates that the LN might be used to validate the differentiation potential of candidate stem cells in regenerative nephrology.

The earliest metanephric arteriolar progenitors and their role in kidney vascular development

The development of the kidney arterioles is poorly understood. Mature arterioles contain several functionally and morphologically distinct cell types, including smooth muscle, endothelial, and juxtaglomerular cells, and they are surrounded by interconnected pericytes, fibroblasts, and other interstitial cells. We have shown that the embryonic kidney possesses all of the necessary precursors for the development of the renal arterial tree, and those precursors assemble in situ to form the kidney arterioles. However, the identity of those precursors was unclear. Within the embryonic kidney, several putative progenitors marked by the expression of either the winged-forkhead transcription factor 1 (Foxd1+ progenitor), the aspartyl-protease renin (Ren+ progenitor), and/or hemangioblasts (Scl+ progenitor) are likely to differentiate and endow most of the cells of the renal arterial tree. However, the lineage relationships and the role of these distinct progenitors in renal vascular morphogenesis have not been delineated. We, therefore, designed a series of experiments to ascertain the hierarchical lineage relationships between Foxd1+ and Ren+ progenitors and the role of these two precursors in the morphogenesis and patterning of the renal arterial tree. Results show that 1) Foxd1+ cells are the precursors for all the mural cells (renin cells, smooth muscle cells, perivascular fibroblasts, and pericytes) of the renal arterial tree and glomerular mesangium, and 2) Foxd1 per se directs the origin, number, orientation, and cellular composition of the renal vessels.

Functional genetic targeting of embryonic kidney progenitor cells ex vivo

The embryonic mammalian metanephric mesenchyme (MM) is a unique tissue because it is competent to generate the nephrons in response to Wnt signaling. An ex vivo culture in which the MM is separated from the ureteric bud (UB), the natural inducer, can be used as a classic tubule induction model for studying nephrogenesis. However, technological restrictions currently prevent using this model to study the molecular genetic details before or during tubule induction. Using nephron segment-specific markers, we now show that tubule induction in the MM ex vivo also leads to the assembly of highly segmented nephrons. This induction capacity was reconstituted when MM tissue was dissociated into a cell suspension and then reaggregated (drMM) in the presence of human recombinant bone morphogenetic protein 7/human recombinant fibroblast growth factor 2 for 24 hours before induction. Growth factor-treated drMM also recovered the capacity for organogenesis when recombined with the UB. Cell tracking and time-lapse imaging of chimeric drMM cultures indicated that the nephron is not derived from a single progenitor cell. Furthermore, viral vector-mediated transduction of green fluorescent protein was much more efficient in dissociated MM cells than in intact mesenchyme, and the nephrogenic competence of transduced drMM progenitor cells was preserved. Moreover, drMM cells transduced with viral vectors mediating Lhx1 knockdown were excluded from the nephric tubules, whereas cells transduced with control vectors were incorporated. In summary, these techniques allow reproducible cellular and molecular examinations of the mechanisms behind nephrogenesis and kidney organogenesis in an ex vivo organ culture/organoid setting.

A SAGE based approach to human glomerular endothelium: defining the transcriptome, finding a novel molecule and highlighting endothelial diversity

Background

Large scale transcript analysis of human glomerular microvascular endothelial cells (HGMEC) has never been accomplished. We designed this study to define the transcriptome of HGMEC and facilitate a better characterization of these endothelial cells with unique features. Serial analysis of gene expression (SAGE) was used for its unbiased approach to quantitative acquisition of transcripts.

Results

We generated a HGMEC SAGE library consisting of 68,987 transcript tags. Then taking advantage of large public databases and advanced bioinformatics we compared the HGMEC SAGE library with a SAGE library of non-cultured ex vivo human glomeruli (44,334 tags) which contained endothelial cells. The 823 tags common to both which would have the potential to be expressed in vivo were subsequently checked against 822,008 tags from 16 non-glomerular endothelial SAGE libraries. This resulted in 268 transcript tags differentially overexpressed in HGMEC compared to non-glomerular endothelia. These tags were filtered using a set of criteria: never before shown in kidney or any type of endothelial cell, absent in all nephron regions except the glomerulus, more highly expressed than statistically expected in HGMEC. Neurogranin, a direct target of thyroid hormone action which had been thought to be brain specific and never shown in endothelial cells before, fulfilled these criteria. Its expression in glomerular endothelium in vitro and in vivo was then verified by real-time-PCR, sequencing and immunohistochemistry.

Conclusions

Our results represent an extensive molecular characterization of HGMEC beyond a mere database, underline the endothelial heterogeneity, and propose neurogranin as a potential link in the kidney-thyroid axis.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-725) contains supplementary material, which is available to authorized users.

Patterning the renal vascular bed

The renal vascular bed has a stereotypic architecture that is essential for the kidney's role in excreting metabolic waste and regulating the volume and composition of body fluids. The kidney's excretory functions are dependent on the delivery of the majority of renal blood flow to the glomerular capillaries, which filter plasma removing from it metabolic waste, as well as vast quantities of solutes and fluids. The renal tubules reabsorb from the glomerular filtrate solutes and fluids required for homeostasis, while the post-glomerular capillary beds return these essential substances back into the systemic circulation. Thus, the kidney's regulatory functions are dependent on the close proximity or alignment of the post-glomerular capillary beds with the renal tubules. This review will focus on our current knowledge of the mechanisms controlling the embryonic development of the renal vasculature. An understanding of this process is critical for developing novel therapies to prevent vessel rarefaction and will be essential for engineering renal tissues suitable for restoring kidney function to the ever-increasing population of patients with end stage renal disease.

Renal blood flow and oxygenation drive nephron progenitor differentiation

During kidney development, the vasculature develops via both angiogenesis (branching from major vessels) and vasculogenesis (de novo vessel formation). The formation and perfusion of renal blood vessels are vastly understudied. In the present study, we investigated the regulatory role of renal blood flow and O2 concentration on nephron progenitor differentiation during ontogeny. To elucidate the presence of blood flow, ultrasound-guided intracardiac microinjection was performed, and FITC-tagged tomato lectin was perfused through the embryo. Kidneys were costained for the vasculature, ureteric epithelium, nephron progenitors, and nephron structures. We also analyzed nephron differentiation in normoxia compared with hypoxia. At embryonic day 13.5 (E13.5), the major vascular branches were perfused; however, smaller-caliber peripheral vessels remained unperfused. By E15.5, peripheral vessels started to be perfused as well as glomeruli. While the interior kidney vessels were perfused, the peripheral vessels (nephrogenic zone) remained unperfused. Directly adjacent and internal to the nephrogenic zone, we found differentiated nephron structures surrounded and infiltrated by perfused vessels. Furthermore, we determined that at low O2 concentration, little nephron progenitor differentiation was observed; at higher O2 concentrations, more differentiation of the nephron progenitors was induced. The formation of the developing renal vessels occurs before the onset of blood flow. Furthermore, renal blood flow and oxygenation are critical for nephron progenitor differentiation.

Ablation of the renal stroma defines its critical role in nephron progenitor and vasculature patterning

The renal stroma is an embryonic cell population located in the cortex that provides a structural framework as well as a source of endothelial progenitors for the developing kidney. The exact role of the renal stroma in normal kidney development hasn't been clearly defined. However, previous studies have shown that the genetic deletion of Foxd1, a renal stroma specific gene, leads to severe kidney malformations confirming the importance of stroma in normal kidney development. This study further investigates the role of renal stroma by ablating Foxd1-derived stroma cells themselves and observing the response of the remaining cell populations. A Foxd1cre (renal stroma specific) mouse was crossed with a diphtheria toxin mouse (DTA) to specifically induce apoptosis in stromal cells. Histological examination of kidneys at embryonic day 13.5–18.5 showed a lack of stromal tissue, mispatterning of renal structures, and dysplastic and/or fused horseshoe kidneys. Immunofluorescence staining of nephron progenitors, vasculature, ureteric epithelium, differentiated nephron progenitors, and vascular supportive cells revealed that mutants had thickened nephron progenitor caps, cortical regions devoid of nephron progenitors, aberrant vessel patterning and thickening, ureteric branching defects and migration of differentiated nephron structures into the medulla. The similarities between the renal deformities caused by Foxd1 genetic knockout and Foxd1DTA mouse models reveal the importance of Foxd1 in mediating and maintaining the functional integrity of the renal stroma.

In vivo imaging of kidney glomeruli transplanted into the anterior chamber of the mouse eye

Multiphoton microscopy enables live imaging of the renal glomerulus. However, repeated in vivo imaging of the same glomerulus over extended periods of time and the study of glomerular function independent of parietal epithelial and proximal tubular cell effects has not been possible so far. Here, we report a novel approach for non-invasive imaging of acapsular glomeruli transplanted into the anterior chamber of the mouse eye. After microinjection, glomeruli were capable of engrafting on the highly vascularized iris. Glomerular structure was preserved, as demonstrated by podocyte specific expression of cyan fluorescent protein and by electron microscopy. Injection of fluorescence-labeled dextrans of various molecular weights allowed visualization of glomerular filtration and revealed leakage of 70 kDa dextran in an inducible model of proteinuria. Our findings demonstrate functionality and long-term survival of glomeruli devoid of Bowman's capsule and provide a novel approach for non-invasive longitudinal in vivo study of glomerular physiology and pathophysiology.

Endothelial Progenitors Exist within the Kidney and Lung Mesenchyme

The renal endothelium has been debated as arising from resident hemangioblast precursors that transdifferentiate from the nephrogenic mesenchyme (vasculogenesis) and/or from invading vessels (angiogenesis). While the Foxd1-positive renal cortical stroma has been shown to differentiate into cells that support the vasculature in the kidney (including vascular smooth muscle and pericytes) it has not been considered as a source of endothelial cell progenitors. In addition, it is unclear if Foxd1-positive mesenchymal cells in other organs such as the lung have the potential to form endothelium. This study examines the potential for Foxd1-positive cells of the kidney and lung to give rise to endothelial progenitors. We utilized immunofluorescence (IF) and fluorescence-activated cell sorting (FACS) to co-label Foxd1-expressing cells (including permanently lineage-tagged cells) with endothelial markers in embryonic and postnatal mice. We also cultured FACsorted Foxd1-positive cells, performed in vitro endothelial cell tubulogenesis assays and examined for endocytosis of acetylated low-density lipoprotein (Ac-LDL), a functional assay for endothelial cells. Immunofluorescence and FACS revealed that a subset of Foxd1-positive cells from kidney and lung co-expressed endothelial cell markers throughout embryogenesis. In vitro, cultured embryonic Foxd1-positive cells were able to differentiate into tubular networks that expressed endothelial cell markers and were able to endocytose Ac-LDL. IF and FACS in both the kidney and lung revealed that lineage-tagged Foxd1-positive cells gave rise to a significant portion of the endothelium in postnatal mice. In the kidney, the stromal-derived cells gave rise to a portion of the peritubular capillary endothelium, but not of the glomerular or large vessel endothelium. These findings reveal the heterogeneity of endothelial cell lineages; moreover, Foxd1-positive mesenchymal cells of the developing kidney and lung are a source of endothelial progenitors that are likely critical to patterning the vasculature.

Mammalian kidney development: principles, progress, and projections

The mammalian kidney is a vital organ with considerable cellular complexity and functional diversity. Kidney development is notable for requiring distinct but coincident tubulogenic processes involving reciprocal inductive signals between mesenchymal and epithelial progenitor compartments. Key molecular pathways mediating these interactions have been identified. Further, advances in the analysis of gene expression and gene activity, coupled with a detailed knowledge of cell origins, are enhancing our understanding of kidney morphogenesis and unraveling the normal processes of postnatal repair and identifying disease-causing mechanisms. This article focuses on recent insights into central regulatory processes governing organ assembly and renal disease, and predicts future directions for the field.

Reciprocal induction of simple organogenesis by mouse kidney progenitor cells in three-dimensional co-culture

Kidney development is regulated by a coordinated reciprocal induction of metanephric mesenchymal (MM) and ureteric bud (UB) cells. Here, established MM and UB progenitor cell lines were recombined in three-dimensional Matrigel implants in SCID mice. Differentiation potential was examined for changes in phenotype, organization, and the presence of specialized proteins using immunofluorescence and bright-field and electron microscopy. Both cell types, when grown alone, did not develop into specialized structures. When combined, the cells organized into simple organoid structures of polarized epithelia with lumens surrounded by capillary-like structures. Tracker experiments indicated the UB cells formed the tubuloid structures, and the MM cells were the source of the capillary-like cells. The epithelial cells stained positive for pancytokeratin, the junctional complex protein ZO-1, collagen type IV, as well as UB and collecting duct markers, rearranged during transfection (RET), Dolichos biflorus lectin, EndoA cytokeratin, and aquaporin 2. The surrounding cells expressed α-smooth muscle actin, vimentin, platelet endothelial cell adhesion molecule 1 (PECAM), and aquaporin 1, a marker of vasculogenesis. The epithelium exhibited apical vacuoles, microvilli, junctional complexes, and linear basement membranes. Capillary-like structures showed endothelial features with occasional pericytes. UB cell epithelialization was augmented in the presence of MM cell-derived conditioned medium, glial-derived neurotrophic factor (GDNF), hepatocyte growth factor (HGF), or fibronectin. MM cells grown in the presence of UB-derived conditioned medium failed to undergo differentiation. However, UB cell-derived conditioned medium induced MM cell migration. These studies indicate that tubulogenesis and vasculogenesis can be partially recapitulated by recombining individual MM and UB cell lineages, providing a new model system to study organogenesis ex vivo.

Development of the renal arterioles

The kidney is a highly vascularized organ that normally receives a fifth of the cardiac output. The unique spatial arrangement of the kidney vasculature with each nephron is crucial for the regulation of renal blood flow, GFR, urine concentration, and other specialized kidney functions. Thus, the proper and timely assembly of kidney vessels with their respective nephrons is a crucial morphogenetic event leading to the formation of a functioning kidney necessary for independent extrauterine life. Mechanisms that govern the development of the kidney vasculature are poorly understood. In this review, we discuss the anatomical development, embryological origin, lineage relationships, and key regulators of the kidney arterioles and postglomerular circulation. Because renal disease is associated with deterioration of the kidney microvasculature and/or the reenactment of embryonic pathways, understanding the morphogenetic events and processes that maintain the renal vasculature may open new avenues for the preservation of renal structure and function and prevent the progression of renal disease.

Development of kidney glomerular endothelial cells and their role in basement membrane assembly

Data showing that the embryonic day 12 (E12) mouse kidney contains its own pool of endothelial progenitor cells is presented. Mechanisms that regulate metanephric endothelial recruitment and differentiation, including the hypoxia-inducible transcription factors and vascular endothelial growth factor/vascular endothelial growth factor receptor signaling system, are also discussed. Finally, evidence that glomerular endothelial cells contribute importantly to assembly of the glomerular basement membrane (GBM), especially the laminin component, is reviewed. Together, this forum offers insights on blood vessel development in general, and formation of the glomerular capillary in particular, which inarguably is among the most unique vascular structures in the body.

Contribution of endothelial cells to organogenesis: a modern reappraisal of an old Aristotelian concept

It is well established that many tissue-derived factors are involved in blood vessel formation, but evidence is now emerging that endothelial cells themselves represent a crucial source of instructive signals to non-vascular tissue cells during organ development. Thus, endothelial cell signalling is currently believed to promote fundamental cues for cell fate specification, embryo patterning, organ differentiation and postnatal tissue remodelling. This review article summarizes some of the recent advances in our understanding of the role of endothelial cells as effector cells in organ formation.

A tissue-engineered model of fetal distal lung tissue

In extending our previous studies toward development of an engineered distal lung tissue construct (M. J. Mondrinos, S. Koutzaki, E. Jiwanmall, M. Li, J. P. Dechadarevian, P. I. Lelkes, and C. M. Finck. Tissue Eng 12: 717-728, 2006), we studied the effects of exogenous fibroblast growth factors FGF10, FGF7, and FGF2 on mixed populations of embryonic day 17.5 murine fetal pulmonary cells cultured in three-dimensional collagen gels. The morphogenic effects of the FGFs alone and in various combinations were assessed by whole mount immunohistochemistry and confocal microscopy. FGF10/7 significantly increased epithelial budding and proliferation; however, only FGF10 alone induced widespread budding. FGF7 alone induced dilation of epithelial structures but not widespread budding. FGF2 alone had a similar dilation, but not budding, effect in epithelial structures, and, in addition, significantly enhanced endothelial tubular morphogenesis and network formation, as well as mesenchymal proliferation. The combination of FGF10/7/2 induced robust budding of epithelial structures and the formation of uniform endothelial networks in parallel. These data suggest that appropriate combinations of exogenous FGFs chosen to target specific FGF receptor isoforms will allow for control of lung epithelial and mesenchymal cell behavior in the context of an engineered system. We propose that tissue-engineered fetal distal lung constructs could provide a potential source of tissue or cells for lung augmentation in pediatric pulmonary pathologies, such as pulmonary hypoplasia and bronchopulmonary dysplasia. In addition, engineered systems will provide alternative in vitro venues for the study of lung developmental biology and pathobiology.

Roles of PDGF in animal development

Recent advances in genetic manipulation have greatly expanded our understanding of cellular responses to platelet-derived growth factors (PDGFs) during animal development. In addition to driving mesenchymal proliferation, PDGFs have been shown to direct the migration, differentiation and function of a variety of specialized mesenchymal and migratory cell types, both during development and in the adult animal. Furthermore, the availability of genomic sequence data has facilitated the identification of novel PDGF and PDGF receptor (PDGFR) family members in C. elegans, Drosophila, Xenopus, zebrafish and mouse. Early data from these different systems suggest that some functions of PDGFs have been evolutionarily conserved.

Microvessel formation from mouse aorta is stimulated in vitro by secreted VEGF and extracts from metanephroi

We have demonstrated that during culture under 5% O(2,) the addition of recombinant human VEGF or FGF2 to mouse embryonic aorta explants (thoracic level to lateral vessels supplying the mesonephros and metanephros) stimulates microvessel formation. Here we show that microvessel formation is also stimulated by addition to explants of supernatants obtained from metanephroi grown in serum-free organ culture or of metanephroi extracts. Supernatants and extracts from metanephroi grown under hypoxic conditions are more stimulatory than supernatants/extracts from metanephroi grown in room air. VEGF and FGF2 can be detected by using immunohistochemistry in developing nephrons in the cultured renal anlagen. Metanephroi supernatants contain more VEGF if renal anlagen are grown under hypoxic conditions than if they are grown in room air. Metanephros supernatant-stimulated microvessel formation is completely inhibited by soluble sFlt-1 fusion protein or anti-VEGF antibodies (alphaVEGF). Extract-stimulated microvessel formation is inhibited by alphaVEGF or anti-FGF2 antibodies, or both. We conclude that metanephroi produce growth factors including VEGF and FGF that enhance microvessel formation from embryonic thoracic aorta in vitro.

Microvascular assembly and cell invasion in chick mesonephros grafted onto chorioallantoic membrane

Embryonic tissues, in common with other tissues, including tumours, tend to develop a substantial vasculature when transplanted onto the chorioallantoic membrane (CAM). Studies conducted to date have not examined in any detail the identity of vessels that supply these grafts, although it is known that the survival of transplanted tissues depends on their ability to connect with CAM vessels supplying oxygen and nutrients. We grafted the mesonephros, a challenging model for studies in vascular development, when it was fully developed (HH35). We used reciprocal chick-quail transplantations in order to study the arterial and venous connections and to analyse the cell invasion from the CAM to the organ, whose degeneration in normal conditions is rapid. The revascularization of the grafted mesonephros was produced by the formation of peripheral anastomoses between the graft and previous host vasculatures. The assembly of graft and CAM blood vessels occurred between relatively large arteries or veins, resulting in chimeric vessels of varying morphology depending on their arterial or venous status. Grafts showed an increased angiogenesis from their original vasculature, suggesting that the normal vascular degeneration of the mesonephros was partially inhibited. Three types of isolated host haemangioblast were identified in the mesonephros: migrating angioblast-like cells, indicating vasculogenesis, undifferentiated haematopoietic cells and macrophages, which might have been involved in the angiogenesis. Tomato lectin was found to bind activated macrophages in avian embryos.

Xenotransplantation of developing kidneys

The number of kidney transplants performed per year is limited by the availability of donor organs. One novel solution to this shortage envisions "growing" new kidneys in situ via xenotransplantation of renal anlagen. We have shown that developing metanephroi transplanted into the omentum of animal hosts undergo differentiation and growth, become vascularized by blood vessels of host origin, and exhibit excretory function. Metanephroi can be stored for up to 3 days in vitro before transplantation with no impairment in growth or function postimplantation. Metanephroi can be transplanted across both concordant (rat --> mouse) and discordant/highly disparate (pig --> rodent) xenogeneic barriers. This review summarizes experimental data relating to the transplantation of developing kidneys.

Effect of platelet-derived growth factor isoforms in rat metanephric mesenchymal cells

Platelet-derived growth factor (PDGF) B-chain or PDGF beta-receptor-deficient mice lack mesangial cells. To explore potential mechanisms for failure of PDGF A-chain to rescue mesangial cell phenotype, we investigated the biological effects and signaling pathways of PDGF AA and PDGF BB in metanephric mesenchymal (MM) cells isolated from rat kidney. PDGF AA caused modest cell migration but had no effect on DNA synthesis, unlike PDGF BB, which potently stimulated migration and DNA synthesis. PDGF AA and PDGF BB significantly increased the activities of phosphatidylinositol 3-kinase (PI 3-K) and mitogen-activated protein kinase (MAPK). PDGF BB was more potent than PDGF AA in activating PI 3-K or MAPK in these cells. Pretreatment of MM cells with the MAPK kinase (MEK) inhibitor PD-098059 abrogated PDGF BB-induced DNA synthesis, whereas the PI 3-K inhibitor wortmannin had a very modest inhibitory effect on DNA synthesis (approximately Delta20%). On the other hand, wortmannin completely blocked PDGF AA- and PDGF BB-induced migration, whereas PD-098059 had a modest inhibitory effect on cell migration. These data demonstrate that activation of MAPK is necessary for the mitogenic effect of PDGF BB, whereas PI 3-K is required for the chemotactic effect of PDGF AA and PDGF BB. Although PDGF AA stimulates PI 3-K and MAPK activity, it is not mitogenic and only modestly chemotactic. Collectively, the data may have implications related to the failure of PDGF AA to rescue mesangial cell phenotype in PDGF B-chain or PDGF-beta-receptor deficiency.

Transplantation of metanephroi after preservation in vitro

To determine whether transplanted metanephroi grow, differentiate, and function in hosts after preservation in vitro, we implanted metanephroi from embryonic day 15 (E15) Sprague-Dawley rat embryos into the omentum of nonimmunosuppressed uninephrectomized Sprague-Dawley (host) rats. Metanephroi were either implanted directly or suspended in ice-cold University of Wisconsin (UW) preservation solution with or without added growth factors for 3 days before implantation. The size and extent of tissue differentiation preimplantation of E15 metanephroi implanted directly were not distinguishable from the size and differentiation of metanephroi preserved for 3 days. In contrast, E16 metanephroi were larger than E15 metanephroi preserved for 3 days. E16 metanephroi or E13 metanephroi grown in organ culture for 3 days contained more differentiated nephron structures than those in E15 metanephroi preserved for 3 days. By 4 wk posttransplantation, metanephroi that had been preserved for 3 days had grown and differentiated such that glomeruli, proximal and distal tubules, and collecting ducts with normal structure had developed. At 12 wk posttransplantation, inulin clearances of preserved metanephroi were comparable to those of metanephroi that had been implanted directly. Addition of growth factors to the UW solution enhanced inulin clearances. Here we show for the first time that functional kidneys develop from metanephroi transplanted from rat embryos to adult rats after as long as 3 days of preservation in vitro.

Embryonic origin and lineage of juxtaglomerular cells

To define the embryonic origin and lineage of the juxtaglomerular (JG) cell, transplantation of embryonic kidneys between genetically marked and wild-type mice; labeling studies for renin, smooth muscle, and endothelial cells at different developmental stages; and single cell RT-PCR for renin and other cell identity markers in prevascular kidneys were performed. From embryonic kidney day 12 to day 15 (E12 to E15), renin cells did not yet express smooth muscle or endothelial markers. At E16 renin cells acquired smooth muscle but not endothelial markers, indicating that these cells are not related to the endothelial lineage, and that the smooth muscle phenotype is a later event in the differentiation of the JG cell. Prevascular genetically labeled E12 mouse kidneys transplanted into the anterior chamber of the eye or under the kidney capsule of adult mice demonstrated that renin cell progenitors originating within the metanephric blastema differentiated in situ to JG cells. We conclude that JG cells originate from the metanephric mesenchyme rather than from an extrarenal source. We propose that renin cells are less differentiated than (and have the capability to give rise to) smooth muscle cells of the renal arterioles.

Transplantation of rat metanephroi into mice

To determine whether transplanted metanephroi grow and differentiate after implantation into the omentum in hosts of a different species, we implanted metanephroi from embryonic day 15 (E15) rat embryos into uninephrectomized mice (hosts). Some host mice received human CTLA4Ig (hCTLA4Ig), anti-CD45RB, and anti-CD154 (tolerance-inducing agents). E15 metanephroi contained only metanephric blastema, segments of ureteric bud, and primitive nephrons with no glomeruli. Rat metanephroi did not grow or differentiate in mice that received no tolerance-inducing agents. However, by 2 wk posttransplantation in mice that received hCTLA4Ig, anti-CD45RB, and anti-CD154, metanephroi from E15 rats had enlarged, become vascularized, and formed mature tubules and glomeruli. Rat metanephroi contained cells that stained specifically for mouse CD31, a marker for sprouting endothelial cells. Some rat glomerular capillary loops stained positively for mouse CD31. Here, we show that chimeric kidneys develop from metanephroi transplanted rat→mouse and that glomeruli are vascularized, at least in part, by host vessels.

Transplantation of metanephroi across the major histocompatibility complex in rats

To determine whether transplanted metanephroi grow, differentiate, and function in hosts that differ in major histocompatibility complex loci (RT1 loci in rats) from donors in a defined way, we implanted metanephroi from embryonic day (E) 15 PVG (RT1(c)) rat embryos into the omentum of nonimmunosupressed uninephrectomized PVG-RT1(avl) (host) rats. By 4 wk posttransplantation, metanephroi had grown and differentiated such that glomeruli, proximal and distal tubules, and collecting ducts had normal structure and ultrastructure. At 12 wk posttransplantation, weights of metanephroi were 54 +/- 8 mg. Inulin clearances were 0.9 +/- 0.3 microl. min(-1). 100 g rat wt(-1). In vitro, splenocytes from PVG rats stimulated the proliferation of cells originating from both PVG-RT1(avl) rats in which a transplant had been performed and PVG-RT1(avl) rats with no transplant. Full-thickness PVG-RT1(avl) skin engrafted normally on PVG-RT1(avl) rats in which PVG metanephroi had been previously implanted and metanephroi retained a normal appearance. In contrast, skin from PVG rats sloughed, and the tubular architecture of metanephroi was obliterated by a mononuclear cell infiltrate consistent with acute rejection. Here we show for the first time that functional chimeric kidneys develop from metanephroi transplanted across the MHC into nonimmunosupressed hosts and provide evidence that a state of peripheral immune tolerance secondary to T cell "ignorance" permits their survival.

The molecular basis of embryonic kidney development

The development of the mature mammalian kidney begins with the invasion of metanephric mesenchyme by ureteric bud. Mesenchymal cells near the bud become induced and convert to an epithelium which goes on to generate the functional filtering unit of the kidney, the nephron. The collecting duct system is elaborated by the branching ureter, the growth of which is dependent upon signals from the metanephric mesenchyme. The process of reciprocal induction between ureter and mesenchyme is repeated many times over during development and is the key step in generating the overall architecture of the kidney. Genetic studies in mice have allowed researchers to begin to unravel the molecular signals that govern these early events. These experiments have revealed that a number of essential gene products are required for distinct steps in kidney organogenesis. Here we review and summarize the developmental role played by some of these molecules, especially certain transcription factors and growth factors and their receptors. Although the factors involved are far from completely known a rough framework of a molecular cascade which governs embryonic kidney development is beginning to emerge.