In vivo maturation of functional renal organoids formed from embryonic cell suspensions

Development and intra-renal delivery of renal progenitor organoids for effective integration in vivo

Renal progenitor organoids have been proposed as a source of tissue for kidney regeneration; however, their clinical translatability has not been demonstrated due to an inability to mass-produce comprehensive renal progenitor organoids and the lack of an effective intra-renal delivery platform that facilitates rapid integration into functionally meaningful sites. This study addresses these shortcomings. Human-induced pluripotent stem cells were differentiated into renal progenitor cells using an established protocol and aggregated using a novel assembly method to produce high yields of organoids. Organoids were encapsulated in collagen-based scaffolds for in vitro study and in vivo implantation into mouse renal cortex. In vitro, the organoids demonstrated sustained cell viability and renal structure maturation over time. In vivo delivered organoids showed rapid integration into host renal parenchyma while showing tubular and glomerular-like structure development and maturity markers. This proof-of-concept study presents many promising results, providing a system of renal organoid formation and delivery that may support the development of clinically translatable therapies and the advancement of in vitro renal organoid studies.

From ex ovo to in vitro: xenotransplantation and vascularization of mouse embryonic kidneys in a microfluidic chip

Organoids are emerging as a powerful tool to investigate complex biological structures in vitro. Vascularization of organoids is crucial to recapitulate the morphology and function of the represented human organ, especially in the case of the kidney, whose primary function of blood filtration is closely associated with blood circulation. Current in vitro microfluidic approaches have only provided initial vascularization of kidney organoids, whereas in vivo transplantation to animal models is problematic due to ethical problems, with the exception of xenotransplantation onto a chicken chorioallantoic membrane (CAM). Although CAM can serve as a good environment for vascularization, it can only be used for a fixed length of time, limited by development of the embryo. Here, we propose a novel lab on a chip design that allows organoids of different origin to be cultured and vascularized on a CAM, as well as to be transferred to in vitro conditions when required. Mouse embryonic kidneys cultured on the CAM showed enhanced vascularization by intrinsic endothelial cells, and made connections with the chicken vasculature, as evidenced by blood flowing through them. After the chips were transferred to in vitro conditions, the vasculature inside the organoids was successfully maintained. To our knowledge, this is the first demonstration of the combination of in vivo and in vitro approaches applied to microfluidic chip design.

Generation of human-pig chimeric renal organoids using iPSC technology

Porcine organs and human induced pluripotent stem cell (iPSC)-derived organoids as alternative organs for human transplantation have garnered attention, but both face technical challenges. Interspecies chimeric organ production using human iPSCs shows promise in overcoming these challenges. Our group successfully generated chimeric renal organoids using human iPSC-derived nephron progenitor cells (NPCs) and fetal mouse kidneys. However, the current technology is limited to rodents. Therefore, this study focused on producing human-pig chimeric renal organoids, as pigs are the most promising species for xenotransplantation. Modification of existing culture systems enables continuous renal development in both species, resulting in the successful creation of human-pig chimeric renal organoids. Moreover, this method can be applied to generate humanized xenogeneic kidneys for future clinical applications. This study provides evidence that optimizing culture conditions enables the early-stage kidney development beyond species barriers, thus laying the foundation for accelerating research on humanized xenogeneic kidney fabrication for clinical purposes.

Measurement of adhesion and traction of cells at high yield reveals an energetic ratchet operating during nephron condensation

Developmental biology-inspired strategies for tissue-building have extraordinary promise for regenerative medicine, spurring interest in the relationship between cell biophysical properties and morphological transitions. However, mapping gene or protein expression data to cell biophysical properties to physical morphogenesis remains challenging with current techniques. Here, we present multiplexed adhesion and traction of cells at high yield (MATCHY). MATCHY advances the multiplexing and throughput capabilities of existing traction force and cell-cell adhesion assays using microfabrication and a semiautomated computation scheme with machine learning-driven cell segmentation. Both biophysical assays are coupled with serial downstream immunofluorescence to extract cell type/signaling state information. MATCHY is especially suited to complex primary tissue-, organoid-, or biopsy-derived cell mixtures since it does not rely on a priori knowledge of cell surface markers, cell sorting, or use of lineage-specific reporter animals. We first validate MATCHY on canine kidney epithelial cells engineered for rearranged during transfection (RET) tyrosine kinase expression and quantify a relationship between downstream signaling and cell traction. We then use MATCHY to create a biophysical atlas of mouse embryonic kidney primary cells and identify distinct biophysical states along the nephron differentiation trajectory. Our data complement expression-level knowledge of adhesion molecule changes that accompany nephron differentiation with quantitative biophysical information. These data reveal an "energetic ratchet" that accounts for spatial trends in nephron progenitor cell condensation as they differentiate into early nephron structures, which we validate through agent-based computational simulation. MATCHY offers semiautomated cell biophysical characterization at >10,000-cell throughput, an advance benefiting fundamental studies and new synthetic tissue strategies for regenerative medicine.

How Stem and Progenitor Cells Can Affect Renal Diseases

Stem and progenitor cells have been observed to contribute to regenerative processes in acute renal failure and chronic kidney disease. Recent research has delved into the intricate mechanisms by which stem and progenitor cells exert their influence on kidney diseases. Understanding how these cells integrate with the existing renal architecture and their response to injury could pave the way for innovative treatment strategies aimed at promoting kidney repair and regeneration. Overall, the role of stem and progenitor cells in kidney diseases is multifaceted, with their ability to contribute to tissue regeneration, immune modulation, and the maintenance of renal homeostasis. Here, we review the studies that we have available today about the involvement of stem and progenitor cells both in regenerative therapies and in the causes of renal diseases, as well as in natural healing mechanisms, taking into account the main kidney disorders, such as IgA nephropathy, lupus nephritis, diabetic nephropathy, C3 glomerulopathy, focal segmental glomerulosclerosis, idiopathic membranous nephropathy, anti-glomerular basement membrane glomerulonephritis, and ANCA-associated crescentic glomerulonephritis. Moreover, based on the comprehensive data available in the framework of the specific kidney diseases on stem cells and renal progenitors, we hypothesize a possible role of adult renal progenitors in exacerbating or recovering the illness.

Kidney organoids: steps towards better organization and function

Kidney organoids - 3D representations of kidneys made either from pluripotent or tissue stem cells - have been available for well over a decade. Their application could confer notable benefits over longstanding in vivo approaches with the potential for clinically aligned human cells and reduced ethical burdens. They been used, at a proof-of-concept level, in development in disease modeling (including with patient-derived stem cells), and in screening drugs for efficacy/toxicity. They differ from real kidneys: they represent only foetal-stage tissue, in their simplest forms they lack organ-scale anatomical organization, they lack a properly arranged vascular system, and include non-renal cells. Cell specificity may be improved by better techniques for differentiation and/or sorting. Sequential assembly techniques that mimic the sequence of natural development, and localized sources of differentiation-inducing signals, improve organ-scale anatomy. Organotypic vascularization remains a challenge: capillaries are easy, but the large vessels that should serve them are absent from organoids and, even in cultured real kidneys, these large vessels do not survive without blood flow. Transplantation of organoids into hosts results in their being vascularized (though probably not organotypically) and in some renal function. It will be important to transplant more advanced organoids, with a urine exit, in the near future to assess function more stringently. Transplantation of human foetal kidneys, followed by nephrectomy of host kidneys, keeps rats alive for many weeks, raising hope that, if organoids can be produced even to the limited size and complexity of foetal kidneys, they may one day be useful in renal replacement.

Advancing Organoid Engineering for Tissue Regeneration and Biofunctional Reconstruction

Tissue damage and functional abnormalities in organs have become a considerable clinical challenge. Organoids are often applied as disease models and in drug discovery and screening. Indeed, several studies have shown that organoids are an important strategy for achieving tissue repair and biofunction reconstruction. In contrast to established stem cell therapies, organoids have high clinical relevance. However, conventional approaches have limited the application of organoids in clinical regenerative medicine. Engineered organoids might have the capacity to overcome these challenges. Bioengineering-a multidisciplinary field that applies engineering principles to biomedicine-has bridged the gap between engineering and medicine to promote human health. More specifically, bioengineering principles have been applied to organoids to accelerate their clinical translation. In this review, beginning with the basic concepts of organoids, we describe strategies for cultivating engineered organoids and discuss the multiple engineering modes to create conditions for breakthroughs in organoid research. Subsequently, studies on the application of engineered organoids in biofunction reconstruction and tissue repair are presented. Finally, we highlight the limitations and challenges hindering the utilization of engineered organoids in clinical applications. Future research will focus on cultivating engineered organoids using advanced bioengineering tools for personalized tissue repair and biofunction reconstruction.

Measurement of adhesion and traction of cells at high yield (MATCHY) reveals an energetic ratchet driving nephron condensation

Engineering of embryonic strategies for tissue-building has extraordinary promise for regenerative medicine. This has led to a resurgence in interest in the relationship between cell biophysical properties and morphological transitions. However, mapping gene or protein expression data to cell biophysical properties to physical morphogenesis remains challenging with current techniques. Here we present MATCHY (multiplexed adhesion and traction of cells at high yield). MATCHY advances the multiplexing and throughput capabilities of existing traction force and cell-cell adhesion assays using microfabrication and an automated computation scheme with machine learning-driven cell segmentation. Both biophysical assays are coupled with serial downstream immunofluorescence to extract cell type/signaling state information. MATCHY is especially suited to complex primary tissue-, organoid-, or biopsy-derived cell mixtures since it does not rely on a priori knowledge of cell surface markers, cell sorting, or use of lineage-specific reporter animals. We first validate MATCHY on canine kidney epithelial cells engineered for RET tyrosine kinase expression and quantify a relationship between downstream signaling and cell traction. We go on to create a biophysical atlas of primary cells dissociated from the mouse embryonic kidney and use MATCHY to identify distinct biophysical states along the nephron differentiation trajectory. Our data complement expression-level knowledge of adhesion molecule changes that accompany nephron differentiation with quantitative biophysical information. These data reveal an 'energetic ratchet' that explains spatial nephron progenitor cell condensation from the niche as they differentiate, which we validate through agent-based computational simulation. MATCHY offers automated cell biophysical characterization at >104-cell throughput, a highly enabling advance for fundamental studies and new synthetic tissue design strategies for regenerative medicine.

In Vivo Vascularization Chamber for the Implantation of Embryonic Kidneys

A major obstacle to the implantation of ex vivo engineered tissues is the incorporation of functional vascular supply to support the growth of new tissue and to minimize ischemic injury. Existing prevascularization systems, such as arteriovenous (AV) loop-based systems, require microsurgery, limiting their use to larger animals. We aimed to develop an implantable device that can be prevascularized to enable vascularization of tissues in small rodents, and test its application on the vascularization of embryonic kidneys. Implanting the chamber between the abdominal aorta and the inferior vena cava, we detected endothelial cells and vascular networks after 48 h of implantation. Loading the chamber with collagen I (C), Matrigel (M), or Matrigel + vascular endothelial growth factor) (MV) had a strong influence on vascularization speed: Chambers loaded with C took 7 days to vascularize, 4 days for chambers with M, and 2 days for chambers with MV. Implantation of E12.5 mouse embryonic kidneys into prevascularized chambers (C, MV) was followed with significant growth and ureteric branching over 22 days. In contrast, the growth of kidneys in non-prevascularized chambers was stunted. We concluded that our prevascularized chamber is a valuable tool for vascularizing implanted tissues and tissue-engineered constructs. Further optimization will be necessary to control the directional growth of vascular endothelial cells within the chamber and the vascularization grade. Impact Statement Vascularization of engineered tissue, or organoids, constructs is a major hurdle in tissue engineering. Failure of vascularization is associated with prolonged ischemia time and potential tissue damage due to hypoxic effects. The method presented, demonstrates the use of a novel chamber that allows rapid vascularization of native and engineered tissues. We hope that this technology helps to stimulate research in the field of tissue vascularization and enables researchers to generate larger engineered vascularized tissues.

Modeling Liver Development and Disease in a Dish

Historically, biological research has relied primarily on animal models. While this led to the understanding of numerous human biological processes, inherent species-specific differences make it difficult to answer certain liver-related developmental and disease-specific questions. The advent of 3D organoid models that are either derived from pluripotent stem cells or generated from healthy or diseased tissue-derived stem cells have made it possible to recapitulate the biological aspects of human organs. Organoid technology has been instrumental in understanding the disease mechanism and complements animal models. This review underscores the advances in organoid technology and specifically how liver organoids are used to better understand human-specific biological processes in development and disease. We also discuss advances made in the application of organoid models in drug screening and personalized medicine.

Advances in biomimetic hydrogels for organoid culture

An organoid is a 3-dimensional (3D) cell culture system that mimics the structural and functional characteristics of organs, and it has promising applications in regenerative medicine, precision drug screening and personalised therapy. However, current culture techniques of organoids usually use mouse tumour-derived scaffolds (Matrigel) or other animal-derived decellularised extracellular matrices as culture systems with poorly defined components and undefined chemical and physical properties, which limit the growth of organoids and the reproducibility of culture conditions. In contrast, some synthetic culture materials have emerged in recent years with well-defined compositions, and flexible adjustment and optimisation of physical and chemical properties, which can effectively support organoid growth and development and prolong survival time of organoid in vitro. In this review, we will introduce the challenge of animal-derived decellularised extracellular matrices in organoid culture, and summarise the categories of biomimetic hydrogels currently used for organoid culture, and then discuss the future opportunities and perspectives in the development of advanced hydrogels in organoids. We hope that this review can promote academic communication in the field of organoid research and provide some assistance in advancing the development of organoid cultivation technology.

Single-cell RNA sequencing and kidney organoid differentiation

Since 2015, Japanese researchers have made great progress in developing a method to differentiate human pluripotent stem cells (hPSCs) into kidney organoids. Protocols have been established to produce increasingly complex three-dimensional (3D) structures, which are used as a human kidney disease model and adapted for high-throughput screening. During this period, single-cell RNA sequencing (scRNA-seq) technology was developed to perform a comprehensive analysis at the single-cell level. We have performed a comprehensive analysis using scRNA-seq to define how kidney organoids can be applied to understand kidney development and pathology. The structure of kidney organoids is complex and contains many cell types of varying maturity. Since only a few proteins and mRNAs can be identified by immunostaining and other techniques, we performed scRNA-seq, which is an unbiased technology that can comprehensively categorize all cell types present in organoids. The aim of this study is to review the problems of kidney organoids based on scRNA-seq and the efforts to address the problems and predict future applications with this powerful technique.

Strategies for Improving Vascularization in Kidney Organoids: A Review of Current Trends

Kidney organoids possess the potential to revolutionize the treatment of renal diseases. However, their growth and maturation are impeded by insufficient growth of blood vessels. Through a PubMed search, we have identified 34 studies that attempted to address this challenge. Researchers are exploring various approaches including animal transplantation, organ-on-chips, and extracellular matrices (ECMs). The most prevalent method to promote the maturation and vascularization of organoids involves transplanting them into animals for in vivo culture, creating an optimal environment for organoid growth and the development of a chimeric vessel network between the host and organoids. Organ-on-chip technology permits the in vitro culture of organoids, enabling researchers to manipulate the microenvironment and investigate the key factors that influence organoid development. Lastly, ECMs have been discovered to aid the formation of blood vessels during organoid differentiation. ECMs from animal tissue have been particularly successful, although the underlying mechanisms require further research. Future research building upon these recent studies may enable the generation of functional kidney tissues for replacement therapies.

Give Them Vasculature and Immune Cells: How to Fill the Gap of Organoids

Valid and relevant models are critical for research to have biological relevance or to proceed in the right path. As well-established two-dimensional cell cultures lack niches and cues and rodent models differ in species, three-dimensional organoids emerged as a powerful platform for research. Cultured in vitro from stem cells, organoids are heterogeneous in cells and closely resemble the in vivo settings. Organoids also recapitulate the unique human features if cultured from a human source and are subjected to genetic modification. However, one type of organoid possesses only a limited selection of cells. In particular, the absence of vasculature and immune cells restricts the organoids from nutrition, cues, or critical interactions, undermining the validity of organoids as physiological or pathological models. To fill the current gap, there is an urgent need to provide organoids with vasculature and immune cells. In this paper, we review the methods to generate physiological and pathological organoid models and summarize ways to vascularize or immunize them. Our discussion continues with some advantages and disadvantages of each method and some emerging solutions to current problems.

Kidney Disease and Viral Infection in COVID-19: Why Are Kidney Organoid and Biopsy Studies Not in Agreement?

CONTEXT

The clinical course of coronavirus disease-19 (COVID-19) can be complicated by acute kidney injury and proteinuria. Kidney cells express receptors for SARS-CoV-2, the virus responsible for COVID-19. Direct infection of the kidney parenchyma by SARS-CoV-2 has been proposed as the cause of renal dysfunction in COVID-19. Subject of Review: Kidney organoids derived from human embryonic stem cells or induced pluripotent cells can be reproducibly infected by SARS-CoV-2 in vitro and used to study therapeutics. However, kidney biopsy studies of COVID-19 patients with renal dysfunction have shown no evidence of viral infection. Second Opinion: Kidney organoids are susceptible to SARS-CoV-2 infection, which is probably facilitated by their limited architectural complexity and maturation compared to the intact organ and by the in vitro culture conditions. Conversely, kidneys in COVID-19 patients appear resistant to infection and may be injured through indirect mechanisms mediated by the host response to the respiratory viral infection, genetic susceptibility to the immune response, physiological disturbances, and therapies. More studies are needed to better understand why kidney organoids are more susceptible than mature kidneys to SARS-CoV-2 infection and further characterize the mechanisms of kidney injury in COVID-19.

Patient-derived podocyte spheroids reveal new insights into the etiopathogenesis of Alport syndrome

Alport syndrome (AS) is a rare disease characterized by defective glomerular basement membranes, caused by mutations in COL4A3, COL4A4, and COL4A5, which synthesize collagen type IV. Patients present with progressive proteinuria, hematuria and podocyte loss. There is currently no cure for Alport syndrome, and this is mainly due to its complex and variable pathogenesis, as well as the lack of models that can faithfully mimic the human phenotype. Here we have developed a novel human culture model of Alport syndrome and used it to study the effects of different mutations on podocyte development and biology. First, we established a differentiation protocol that allowed us to generate podocyte spheroids from patient-derived human induced pluripotent stem cells (hiPSCs). We have then carried out discovery proteomics and demonstrated that a total of 178 proteins were differentially expressed between Alport (AS1 and AS3) and control (LT) podocytes. GO analysis indicated alterations in several metabolic pathways, such as oxidative phosphorylation, RNA maturation, chromatin condensation, and proliferation. Although functional assays showed no changes in lactate production and mitochondrial potential compared to healthy controls, immunofluorescence and electron microscopy analysis showed key morphological changes related to the phenotypical maturation of Alport podocytes. Moreover, the studied mutations led to persistent proliferation, increased reactive oxygen species (ROS) production and the concomitant expression of peroxisome proliferator-activated receptors α and γ (PPARα and PPARγ) in podocytes. These data on patient-derived podocytes provide evidence that collagen mutations, in addition to playing a central role in the defective development of the glomerular filtration barrier, cause significant alterations in podocyte development and metabolism very early in development, even before the formation of the filtering apparatus. In conclusion, our study provides a new methodological platform for the differentiation of podocytes and to study human podocytopathies in a personalized manner, and reveals new insights into the etiopathogenesis and pathobiology of Alport syndrome.

Kidney organoids: a pioneering model for kidney diseases

The kidney is a vital organ that regulates the bodily fluid and electrolyte homeostasis via tailored urinary excretion. Kidney injuries that cause severe or progressive chronic kidney disease have driven the growing population of patients with end-stage kidney disease, leading to substantial patient morbidity and mortality. This irreversible kidney damage has also created a huge socioeconomical burden on the healthcare system, highlighting the need for novel translational research models for progressive kidney diseases. Conventional research methods such as in vitro 2D cell culture or animal models do not fully recapitulate complex human kidney diseases. By contrast, directed differentiation of human induced pluripotent stem cells enables in vitro generation of patient-specific 3D kidney organoids, which can be used to model acute or chronic forms of hereditary, developmental, and metabolic kidney diseases. Furthermore, when combined with biofabrication techniques, organoids can be used as building blocks to construct vascularized kidney tissues mimicking their in vivo counterpart. By applying gene editing technology, organoid building blocks may be modified to minimize the process of immune rejection in kidney transplant recipients. In the foreseeable future, the universal kidney organoids derived from HLA-edited/deleted induced pluripotent stem cell (iPSC) lines may enable the supply of bioengineered organotypic kidney structures that are immune-compatible for the majority of the world population. Here, we summarize recent advances in kidney organoid research coupled with novel technologies such as organoids-on-chip and biofabrication of 3D kidney tissues providing convenient platforms for high-throughput drug screening, disease modelling, and therapeutic applications.

Morphometric Study of the Nutrient Foramen of the Humerus in the Population of Bihar

INTRODUCTION

 Fracture or surgical intervention of fracture of the shaft of the humerus may cause injury to the nutrient artery leading to the nonunion or delayed union of the fracture. It is important to find the number and location of the nutrient artery. So the knowledge regarding the nutrient foramen helps to protect them during any operative procedure of the shaft of the humerus. The main objective of this study is to find out the number, location, and direction of the nutrient foramen of the humerus.

MATERIALS AND METHODS

The study was conducted on 80 dried humeri of unknown gender obtained from Narayan Medical College, Sasaram, Bihar, India, and also from other medical colleges of Bihar. The number, location, and direction of nutrient foramen were observed.

RESULTS

The majority of humeri showed one nutrient foramen, which was found in 91.25%, followed by 3.75% with double foramen and 1.25% with triple foramen. Nutrient foramen was absent in 3.75% of the humerus. The majority (89.02%) of nutrient foramen was found on the anteromedial surface followed by anterolateral (9.76%) and posterior surface (1.22%). The majority of nutrient foramen was found on the middle third (86.58%) of the shaft, followed by 13.42% on the distal third. No nutrient foramen was found on the proximal third of the humerus. All nutrient foramina were directed downward.

CONCLUSION

The location of the nutrient foramen of the humerus was not constant; it may present on anteromedial, anterolateral, or posterior surfaces. Similarly, it may present on the middle or distal third of the shaft of the humerus. This study will help surgeons planning the surgical intervention of the shaft of the humerus, which will possibly reduce the chances of nonunion or delayed union.

Creating a kidney organoid-vasculature interaction model using a novel organ-on-chip system

Kidney organoids derived from human induced pluripotent stem cells (iPSCs) have proven to be a valuable tool to study kidney development and disease. However, the lack of vascularization of these organoids often leads to insufficient oxygen and nutrient supply. Vascularization has previously been achieved by implantation into animal models, however, the vasculature arises largely from animal host tissue. Our aim is to transition from an in vivo implantation model towards an in vitro model that fulfils the advantages of vascularization whilst being fully human-cell derived. Our chip system supported culturing of kidney organoids, which presented nephron structures. We also showed that organoids cultured on chip showed increased maturation of endothelial populations based on a colocalization analysis of endothelial markers. Moreover, we observed migration and proliferation of human umbilical vein endothelial cells (HUVECs) cultured in the channels of the chip inside the organoid tissue, where these HUVECs interconnected with endogenous endothelial cells and formed structures presenting an open lumen resembling vessels. Our results establish for the first-time vascularization of kidney organoids in HUVEC co-culture conditions using a microfluidic organ-on-chip. Our model therefore provides a useful insight into kidney organoid vascularization in vitro and presents a tool for further studies of kidney development and drug testing, both for research purposes and pre-clinical applications.

A scalable organoid model of human autosomal dominant polycystic kidney disease for disease mechanism and drug discovery

Human pluripotent stem-cell-derived organoids are models for human development and disease. We report a modified human kidney organoid system that generates thousands of similar organoids, each consisting of 1-2 nephron-like structures. Single-cell transcriptomic profiling and immunofluorescence validation highlighted patterned nephron-like structures utilizing similar pathways, with distinct morphogenesis, to human nephrogenesis. To examine this platform for therapeutic screening, the polycystic kidney disease genes PKD1 and PKD2 were inactivated by gene editing. PKD1 and PKD2 mutant models exhibited efficient and reproducible cyst formation. Cystic outgrowths could be propagated for months to centimeter-sized cysts. To shed new light on cystogenesis, 247 protein kinase inhibitors (PKIs) were screened in a live imaging assay identifying compounds blocking cyst formation but not overall organoid growth. Scaling and further development of the organoid platform will enable a broader capability for kidney disease modeling and high-throughput drug screens.

Introducing blood flow in kidney explants by engraftment onto the chick chorioallantoic membrane is not sufficient to induce arterial smooth muscle cell development

Kidney explant cultures are an important tool to gain insights into developmental processes, insights that can be used to develop strategies for engineering kidneys from stem cells. However, explants are not connected to a perfused vascular system. This limits their survival and limits physiological studies, for example of blood filtration, the main function of the kidney. Previous studies have shown that grafting kidneys onto avian chorioallantoic membrane (CAM) can establish perfusion and enable glomerular vascularization, but the realism and maturity of the resultant vasculature has not been examined. Here, we show that vasculature of kidney explants grafted onto CAM is very different from natural kidney vasculature, showing excessive growth of endothelial cells, absence of a hierarchical arterio-venous network and no vascular smooth muscle cell recruitment. The model therefore has serious limits.

Summary: The chorioallantoic membrane assay has been previously used to verify the vascularization potential of engineered organoids. We identified severe limitations of this assay, such as a lack of arterial maturity.

Beneficial Impact of Interspecies Chimeric Renal Organoids Against a Xenogeneic Immune Response

Background

Animal fetal kidneys have the potential to be used as scaffolds for organ regeneration. We generated interspecies chimeric renal organoids by adding heterologous rat renal progenitor cells to single cells from mouse fetal kidneys and applying the renal development mechanism of mouse fetuses to rat renal progenitor cells to examine whether rat renal progenitor cells can differentiate into renal tissues of the three progenitor cell lineages of kidneys between different species. Furthermore, we investigated whether chimeric renal organoids with an increased proportion of recipient cells reduce xenogeneic rejection.

Methods

C57BL/6JJmsSlc mice (B6 mice) and Sprague-Dawley-Tg (CAG-EGFP) rat (GFP rats) fetuses were used as donors, and mature male NOD/Shi-scid, IL-2RγKO Jic mice (NOG mice) and Sprague-Dawley rats (SD rats) were used as recipients. First, fetal kidneys were removed from E13.5 B6 mice or E15.5 GFP rats and enzymatically dissociated into single cells. These cells were then mixed in equal proportions to produce chimeric renal organoids in vitro. The chimeric organoids were transplanted under the renal capsule of NOG mice, and maturation of the renal tissues in the organoids was observed histologically. Furthermore, chimeric organoids were prepared by changing the ratio of cells derived from mouse and rat fetal kidneys and transplanted under the renal capsule of SD rats subjected to mild immunosuppression to pathologically analyze the strength of the xenogeneic immune response.

Results

The cap mesenchyme was reconstructed in vitro, and nephron progenitor cells and ureteric buds were mosaically comprised GFP-negative mouse and GFP-positive rat cells. In the in vivo environment of immunodeficient mice, chimeric renal organoids mosaically differentiated and matured into renal tissues of three lineages. Chimeric renal organoids with high rates of recipient rat cells showed milder rejection than complete xenograft organoids. The vessels of recipient rats entered from the periphery of the transplanted chimeric renal organoids, which might reduce their immunogenicity.

Conclusion

Interspecies chimeric renal organoids may differentiate into mature renal tissues of each renal progenitor cell lineage. Furthermore, they may reduce transplant rejection compared with xenograft organoids.

Mesenchymal Stem/Stromal Cells in Organ Transplantation

Organ transplantation is essential and crucial for saving and enhancing the lives of individuals suffering from end-stage organ failure. Major challenges in the medical field include the shortage of organ donors, high rates of organ rejection, and long wait times. To address the current limitations and shortcomings, cellular therapy approaches have been developed using mesenchymal stem/stromal cells (MSC). MSC have been isolated from various sources, have the ability to differentiate to important cell lineages, have anti-inflammatory and immunomodulatory properties, allow immunosuppressive drug minimization, and induce immune tolerance towards the transplanted organ. Additionally, rapid advances in the fields of tissue engineering and regenerative medicine have emerged that focus on either generating new organs and organ sources or maximizing the availability of existing organs. This review gives an overview of the various properties of MSC that have enabled its use as a cellular therapy for organ preservation and transplant. We also highlight emerging fields of tissue engineering and regenerative medicine along with their multiple sub-disciplines, underlining recent advances, widespread clinical applications, and potential impact on the future of tissue and organ transplantation.

Tissue Culture Models of AKI: From Tubule Cells to Human Kidney Organoids

AKI affects approximately 13.3 million people around the world each year, causing CKD and/or mortality. The mammalian kidney cannot generate new nephrons after postnatal renal damage and regenerative therapies for AKI are not available. Human kidney tissue culture systems can complement animal models of AKI and/or address some of their limitations. Donor-derived somatic cells, such as renal tubule epithelial cells or cell lines (RPTEC/hTERT, ciPTEC, HK-2, Nki-2, and CIHP-1), have been used for decades to permit drug toxicity screening and studies into potential AKI mechanisms. However, tubule cell lines do not fully recapitulate tubular epithelial cell properties in situ when grown under classic tissue culture conditions. Improving tissue culture models of AKI would increase our understanding of the mechanisms, leading to new therapeutics. Human pluripotent stem cells (hPSCs) can be differentiated into kidney organoids and various renal cell types. Injury to human kidney organoids results in renal cell-type crosstalk and upregulation of kidney injury biomarkers that are difficult to induce in primary tubule cell cultures. However, current protocols produce kidney organoids that are not mature and contain off-target cell types. Promising bioengineering techniques, such as bioprinting and "kidney-on-a-chip" methods, as applied to kidney nephrotoxicity modeling advantages and limitations are discussed. This review explores the mechanisms and detection of AKI in tissue culture, with an emphasis on bioengineered approaches such as human kidney organoid models.

Advances in tissue engineering technology for kidney regeneration and construction

Tissue engineering is a highly interdisciplinary research field aiming at repairing, replacing, and regenerating the defective tissues. Over several decades of research, a variety of methods have been developed. The technical methods can be categorized into scaffold-based and scaffold-free strategies. In this mini review, the discussion will be focused on the technical methods of tissue engineering for kidney regeneration and construction.

Post-translational modifications by SIRT3 de-2-hydroxyisobutyrylase activity regulate glycolysis and enable nephrogenesis

Abnormal kidney development leads to lower nephron number, predisposing to renal diseases in adulthood. In embryonic kidneys, nephron endowment is dictated by the availability of nephron progenitors, whose self-renewal and differentiation require a relatively repressed chromatin state. More recently, NAD⁺-dependent deacetylase sirtuins (SIRTs) have emerged as possible regulators that link epigenetic processes to the metabolism. Here, we discovered a novel role for the NAD⁺-dependent deacylase SIRT3 in kidney development. In the embryonic kidney, SIRT3 was highly expressed only as a short isoform, with nuclear and extra-nuclear localisation. The nuclear SIRT3 did not act as deacetylase but exerted de-2-hydroxyisobutyrylase activity on lysine residues of histone proteins. Extra-nuclear SIRT3 regulated lysine 2-hydroxyisobutyrylation (Khib) levels of phosphofructokinase (PFK) and Sirt3 deficiency increased PFK Khib levels, inducing a glycolysis boost. This altered Khib landscape in Sirt3^−/− metanephroi was associated with decreased nephron progenitors, impaired nephrogenesis and a reduced number of nephrons. These data describe an unprecedented role of SIRT3 in controlling early renal development through the regulation of epigenetics and metabolic processes.

Renal engineering: strategies to address the problem of the ureter

Current techniques for making renal organoids generate tissues that show function when transplanted into a host, but they have no ureter through which urine can drain. There are at least 4 possible strategies for adding a ureter: connecting to ta host ureter; inducing an engineered kidney to make a ureter; making a stem-cell derived ureter; and replacement of only damaged cortex and outer medulla, using remaining host calyces, pelvis and ureter. Here we review progress: local BMP4 can induce a collecting duct tubule to become a ureter; a urothelial tube can be produced directly from pluripotent cells, and connect to the collecting duct system of a renal organoid; it is possible to graft ES cell-derived ureters into host kidney rudiments and see connection, smooth muscle development and spontaneous contraction, but this has not yet been achieved with all components being derived from ES cells. Remaining problems are discussed.

Connection of ES Cell-derived Collecting Ducts and Ureter-like Structures to Host Kidneys in Culture

Work toward renal generation generally aims either to introduce suspensions of stem cells into kidneys in the hope that they will rebuild damaged tissue, or to construct complete new kidneys from stem cells with the aim of transplanting the engineered organs. In principle, there might be a third approach; to engineer renal tissue 'modules' in vitro and to use them to replace sections of damaged host kidney. This approach would require the urine collecting system or ureter of the new tissues to connect to those of the host. In this report, we demonstrate a method that allows collecting duct trees or ureters, engineered from ES cells, to connect to the collecting duct system or ureter, respectively, of fetal kidneys in culture.

Modelling Renal Filtration and Reabsorption Processes in a Human Glomerulus and Proximal Tubule Microphysiological System

Kidney microphysiological systems (MPS) serve as potentially valuable preclinical instruments in probing mechanisms of renal clearance and osmoregulation. Current kidney MPS models target regions of the nephron, such as the glomerulus and proximal tubule (PCT), but fail to incorporate multiple filtration and absorption interfaces. Here, we describe a novel, partially open glomerulus and PCT microdevice that integrates filtration and absorption in a single MPS. The system equalizes pressure on each side of the PCT that operates with one side "closed" by recirculating into the bloodstream, and the other "opened" by exiting as primary filtrate. This design precisely controls the internal fluid dynamics and prevents loss of all fluid to the open side. Through this feature, an in vitro human glomerulus and proximal tubule MPS was constructed to filter human serum albumin and reabsorb glucose for seven days of operation. For proof-of-concept experiments, three human-derived cell types-conditionally immortalized human podocytes (CIHP-1), human umbilical vein endothelial cells (HUVECs), and human proximal tubule cells (HK-2)-were adapted into a common serum-free medium prior to being seeded into the three-component MPS (T-junction splitter, glomerular housing unit, and parallel proximal tubule barrier model). This system was optimized geometrically (tubing length, tubing internal diameter, and inlet flow rate) using in silico computational modeling. The prototype tri-culture MPS successfully filtered blood serum protein and generated albumin filtration in a physiologically realistic manner, while the device cultured only with proximal tubule cells did not. This glomerulus and proximal convoluted tubule MPS is a potential prototype for the human kidney used in both human-relevant testing and examining pharmacokinetic interactions.

Human iPSC-derived neural crest stem cells can produce EPO and induce erythropoiesis in anemic mice

Inadequate production of erythropoietin (EPO) leads to anemia. Although erythropoiesis-stimulating agents can be used to treat anemia, these approaches are limited by high costs, adverse effects, and the need for frequent injections. Developing methods for the generation and transplantation of EPO-producing cells would allow for the design of personalized and complication-free therapeutic solutions. In mice, the first EPO source are neural crest cells (NCCs), which ultimately migrate to the fetal kidney to differentiate into EPO-producing fibroblasts. In humans however, it remains unknown whether NCCs can produce EPO in response to hypoxia. Here, we developed a new protocol to differentiate human induced pluripotent stem cells (hiPSCs) into NCCs and showed that cthese cells can produce functional EPO that can induce human CD34⁺ hematopoietic progenitor differentiation into erythroblasts in vitro. Moreover, we showed that hiPSC-derived NCCs can be embedded in clinical-grade atelocollagen scaffolds and subcutaneously transplanted into anemic mice to produce human EPO, accelerate hematocrit recovery, and induce erythropoiesis in the spleen. Our findings provide unprecedented evidence of the ability of human NCCs to produce functional EPO in response to hypoxia, and proof-of-concept for the potential clinical use of NCC-containing scaffolds as cell therapy for renal and non-renal anemia.

Organoids in Tissue Transplantation

Improvements in stem cell-based research and genetic modification tools enable stem cell-based tissue regeneration applications in clinical therapies. Although inadequate cell numbers in culture, invasive isolation procedures, and poor survival rates after transplantation remain as major challenges, cell-based therapies are useful tools for tissue regeneration.Organoids hold a great promise for tissue regeneration, organ and disease modeling, drug testing, development, and genetic profiling studies. Establishment of 3D cell culture systems eliminates the disadvantages of 2D models in terms of cell adaptation and tissue structure and function. Organoids possess the capacity to mimic the specific features of tissue architecture, cell-type composition, and the functionality of real organs while preserving the advantages of simplified and easily accessible cell culture models. Thus, organoid technology might emerge as an alternative to cell and tissue transplantation. Although transplantation of various organoids in animal models has been demonstrated, liöitations related to vascularized structure formation, cell viability and functionality remain as obstacles in organoid-based transplantation therapies. Clinical applications of organoid-based transplantations might be possible in the near future, when limitations related to cell viability and tissue integration are solved. In this review, the literature was analyzed and discussed to explore the current status of organoid-based transplantation studies.

Advances in removing mass transport limitations for more physiologically relevant in vitro 3D cell constructs

Spheroids and organoids are promising models for biomedical applications ranging from human disease modeling to drug discovery. A main goal of these 3D cell-based platforms is to recapitulate important physiological parameters of their in vivo organ counterparts. One way to achieve improved biomimetic architectures and functions is to culture cells at higher density and larger total numbers. However, poor nutrient and waste transport lead to low stability, survival, and functionality over extended periods of time, presenting outstanding challenges in this field. Fortunately, important improvements in culture strategies have enhanced the survival and function of cells within engineered microtissues/organs. Here, we first discuss the challenges of growing large spheroids/organoids with a focus on mass transport limitations, then highlight recent tools and methodologies that are available for producing and sustaining functional 3D in vitro models. This information points toward the fact that there is a critical need for the continued development of novel cell culture strategies that address mass transport in a physiologically relevant human setting to generate long-lasting and large-sized spheroids/organoids.

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.

Clinical Application of Human Induced Pluripotent Stem Cell-Derived Organoids as an Alternative to Organ Transplantation

Transplantation is essential and crucial for individuals suffering from end-stage organ failure diseases. However, there are still many challenges regarding these procedures, such as high rates of organ rejection, shortage of organ donors, and long waiting lines. Thus, investments and efforts to develop laboratory-grown organs have increased over the past years, and with the recent progress in regenerative medicine, growing organs in vitro might be a reality within the next decades. One of the many different strategies to address this issue relies on organoid technology, a miniaturized and simplified version of an organ. Here, we address recent progress on organoid research, focusing on transplantation of intestine, retina, kidney, liver, pancreas, brain, lung, and heart organoids. Also, we discuss the main outcomes after organoid transplantation, common challenges faced by these promising regenerative medicine approaches, and future perspectives on the field.

Kidney Organoids as Disease Models: Strengths, Weaknesses and Perspectives

Chronic kidney disease is a major global health problem, as it affects 10% of the global population and kills millions of patients every year. It is therefore of the utmost importance to develop models that can help us to understand the pathogenesis of CKD and improve our therapeutic strategies. The discovery of human induced pluripotent stem cells (hiPSCs) and, more recently, the development of methods for the generation of 3D organoids, have opened the way for modeling human kidney development and disease in vitro, and testing new drugs directly on human tissue. In this review we will discuss the most recent advances in the field of kidney organoids for modeling disease, as well as the prospective applications of these models for drug screening. We will also emphasize the impact of CRISPR/cas9 genome engineering on the field, point out the current limitations of the existing organoid technologies, and discuss a set of technical developments that may help to overcome limitations and facilitate the incorporation of these exciting tools into basic biomedical research.

Regenerated nephrons in kidney cortices ameliorate exacerbated serum creatinine levels in rats with adriamycin nephropathy

Kidney regeneration could be classified into 2 groups: kidney generation and kidney repair. We have attempted in vivo nephron generation for kidney repair, as a therapy for chronic renal failure (CRF), by exploiting cellular interactions via conditioned media. In the previous report, we demonstrated the generation of rich nephrons in rat intact kidney cortices through percapsular injection of mesenchymal stem cell (MSC)-differentiated tubular epithelial cells (TECs) after pretreatment of 3-dimensional culture using a small amount of gel complex and condensed medium. In this study, to verify the amelioration of serum creatinine (sCr) levels by regenerated nephrons in rats with CRF, we first created damaged kidneys through systemic administration of adriamycin, and implanted the pretreated MSC-differentiated TECs into unilateral kidney cortices 2 weeks after adriamycin administration (A-2W, that is I-0W). After recovery of acute kidney injury, the control rats without cell implantation showed re-exacerbation of sCr levels, resulting in death within A-12W. Alternatively, the cell-implanted rats had a formation of mature nephrons in I-3W, and showed significant amelioration of sCr levels in I-7W. As a result, these rats could live until euthanization in I-12W or I-16W, indicating the utility of cell injection therapy into a kidney (K-CIT) for CRF. We expect that our K-CIT or the refined methods will be applied to patients with CRF.

Towards an in vitro model of the glomerular barrier unit with an innovative bioassembly method

BACKGROUND

The development of an artificial glomerular unit may be pivotal for renal pathophysiology studies at a multicellular scale. Using a tissue engineering approach, we aimed to reproduce in part the specific glomerular barrier architecture by manufacturing a glomerular microfibre (Mf).

METHODS

Immortalized human glomerular cell lines of endothelial cells (GEnCs) and podocytes were used. Cells and a three-dimensional (3D) matrix were characterized by immunofluorescence with confocal analysis, Western blot and polymerase chain reaction. Optical and electron microscopy were used to study Mf and cell shapes. We also analysed cell viability and cell metabolism within the 3D construct at 14 days.

RESULTS

Using the Mf manufacturing method, we repeatedly obtained a cellularized Mf sorting human glomerular cells in 3D. Around a central structure made of collagen I, we obtained an internal layer composed of GEnC, a newly formed glomerular basement membrane rich in α5 collagen IV and an external layer of podocytes. The cell concentration, optimal seeding time and role of physical stresses were modulated to obtain the Mf. Cell viability and expression of specific proteins (nephrin, synaptopodin, vascular endothelial growth factor receptor 2 (VEGFR2) and von Willebrandt factor (vWF)) were maintained for 19 days in the Mf system. Mf ultrastructure, observed with EM, had similarities with the human glomerular barrier.

CONCLUSION

In summary, with our 3D bio-engineered glomerular fibre, GEnC and podocytes produced a glomerular basement membrane. In the future, this glomerular Mf will allow us to study cell interactions in a 3D system and increase our knowledge of glomerular pathophysiology.

Successful Introduction of Human Renovascular Units into the Mammalian Kidney

BACKGROUND

Cell-based therapies aimed at replenishing renal parenchyma have been proposed as an approach for treating CKD. However, pathogenic mechanisms involved in CKD such as renal hypoxia result in loss of kidney function and limit engraftment and therapeutic effects of renal epithelial progenitors. Jointly administering vessel-forming cells (human mesenchymal stromal cells [MSCs] and endothelial colony-forming cells [ECFCs]) may potentially result in in vivo formation of vascular networks.

METHODS

We administered renal tubule-forming cells derived from human adult and fetal kidneys (previously shown to exert a functional effect in CKD mice) into mice, alongside MSCs and ECFCs. We then assessed whether this would result in generation of "renovascular units" comprising both vessels and tubules with potential interaction.

RESULTS

Directly injecting vessel-forming cells and renal tubule-forming cells into the subcutaneous and subrenal capsular space resulted in self-organization of donor-derived vascular networks that connected to host vasculature, alongside renal tubules comprising tubular epithelia of different nephron segments. Vessels derived from MSCs and ECFCs augmented in vivo tubulogenesis by the renal tubule-forming cells. In vitro coculture experiments showed that MSCs and ECFCs induced self-renewal and genes associated with mesenchymal-epithelial transition in renal tubule-forming cells, indicating paracrine effects. Notably, after renal injury, renal tubule-forming cells and vessel-forming cells infused into the renal artery did not penetrate the renal vascular network to generate vessels; only administering them into the kidney parenchyma resulted in similar generation of human renovascular units in vivo.

CONCLUSIONS

Combined cell therapy of vessel-forming cells and renal tubule-forming cells aimed at alleviating renal hypoxia and enhancing tubulogenesis holds promise as the basis for new renal regenerative therapies.

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.

Regenerative medicine therapies: lessons from the kidney

We focus on three strategies for renal regenerative medicine; administering cells to replace damaged tissue, promoting endogenous regeneration, and growing stem cell-derived organs. Mouse kidney regeneration can be promoted by stem cells injected into the circulation which do not become new kidney tissue but seem to secrete regeneration-promoting humoral factors. This argues against direct replacement but encourages developing pharmacological stimulators of endogenous regeneration. Simple ‘kidneys’ have been made from stem cells, but there is a large gap between what has been achieved and a useful transplantable organ. Most current work aims to stimulate endogenous regeneration or to grow new organs but much remains to be done; misplaced hype about short-term prospects of regenerative medicine helps neither researchers nor patients.

Recapitulating kidney development in vitro by priming and differentiating mouse embryonic stem cells in monolayers

In order to harness the potential of pluripotent stem cells, we need to understand how to differentiate them to our target cell types. Here, we developed a protocol to differentiate mouse embryonic stem cells (ESCs) to renal progenitors in a step-wise manner. Microarrays were used to track the transcriptional changes at each stage of differentiation and we observed that genes associated with metanephros, ureteric bud, and blood vessel development were significantly upregulated as the cells differentiated towards renal progenitors. Priming the ESCs and optimizing seeding cell density and growth factor concentrations helped improve differentiation efficiency. Organoids were used to determine the developmental potential of the renal progenitor cells. Aggregated renal progenitors gave rise to organoids consisting of LTL+/E-cadherin+ proximal tubules, cytokeratin+ ureteric bud-derived tubules, and extracellular matrix proteins secreted by the cells themselves. Over-expression of key kidney developmental genes, Pax2, Six1, Eya1, and Hox11 paralogs, during differentiation did not improve differentiation efficiency. Altogether, we developed a protocol to differentiate mouse ESCs in a manner that recapitulates embryonic kidney development and showed that precise gene regulation is essential for proper differentiation to occur.

The emergence of regenerative medicine in organ transplantation: 1st European Cell Therapy and Organ Regeneration Section meeting

Regenerative medicine is emerging as a novel field in organ transplantation. In September 2019, the European Cell Therapy and Organ Regeneration Section (ECTORS) of the European Society for Organ Transplantation (ESOT) held its first meeting to discuss the state‐of‐the‐art of regenerative medicine in organ transplantation. The present article highlights the key areas of interest and major advances in this multidisciplinary field in organ regeneration and discusses its implications for the future of organ transplantation.

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.

Endothelium structure and function in kidney health and disease

The kidney harbours different types of endothelia, each with specific structural and functional characteristics. The glomerular endothelium, which is highly fenestrated and covered by a rich glycocalyx, participates in the sieving properties of the glomerular filtration barrier and in the maintenance of podocyte structure. The microvascular endothelium in peritubular capillaries, which is also fenestrated, transports reabsorbed components and participates in epithelial cell function. The endothelium of large and small vessels supports the renal vasculature. These renal endothelia are protected by regulators of thrombosis, inflammation and complement, but endothelial injury (for example, induced by toxins, antibodies, immune cells or inflammatory cytokines) or defects in factors that provide endothelial protection (for example, regulators of complement or angiogenesis) can lead to acute or chronic renal injury. Moreover, renal endothelial cells can transition towards a mesenchymal phenotype, favouring renal fibrosis and the development of chronic kidney disease. Thus, the renal endothelium is both a target and a driver of kidney and systemic cardiovascular complications. Emerging therapeutic strategies that target the renal endothelium may lead to improved outcomes for both rare and common renal diseases.

Single cell census of human kidney organoids shows reproducibility and diminished off-target cells after transplantation

Human iPSC-derived kidney organoids have the potential to revolutionize discovery, but assessing their consistency and reproducibility across iPSC lines, and reducing the generation of off-target cells remain an open challenge. Here, we profile four human iPSC lines for a total of 450,118 single cells to show how organoid composition and development are comparable to human fetal and adult kidneys. Although cell classes are largely reproducible across time points, protocols, and replicates, we detect variability in cell proportions between different iPSC lines, largely due to off-target cells. To address this, we analyze organoids transplanted under the mouse kidney capsule and find diminished off-target cells. Our work shows how single cell RNA-seq (scRNA-seq) can score organoids for reproducibility, faithfulness and quality, that kidney organoids derived from different iPSC lines are comparable surrogates for human kidney, and that transplantation enhances their formation by diminishing off-target cells.

Single cell census of human kidney organoids shows reproducibility and diminished off-target cells after transplantation

Human iPSC-derived kidney organoids have the potential to revolutionize discovery, but assessing their consistency and reproducibility across iPSC lines, and reducing the generation of off-target cells remain an open challenge. Here, we profile four human iPSC lines for a total of 450,118 single cells to show how organoid composition and development are comparable to human fetal and adult kidneys. Although cell classes are largely reproducible across time points, protocols, and replicates, we detect variability in cell proportions between different iPSC lines, largely due to off-target cells. To address this, we analyze organoids transplanted under the mouse kidney capsule and find diminished off-target cells. Our work shows how single cell RNA-seq (scRNA-seq) can score organoids for reproducibility, faithfulness and quality, that kidney organoids derived from different iPSC lines are comparable surrogates for human kidney, and that transplantation enhances their formation by diminishing off-target cells.

Lymphotoxin-Beta Receptor Signaling Is Crucial for the Vascularization of Transplanted Metanephros

This commentary highlights the article by Francipane et al that studied the molecular signals supporting kidney vascularization in host lymphoid sites and omenta.

Organizing Organoids: Stem Cells Branch Out

In this issue of Cell Stem Cell, Taguchi and Nishinakamura (2017) describe a carefully optimized method for making a branch-competent ureteric bud, a tissue fundamental to kidney development, from mouse embryonic stem cells and human induced pluripotent stem cells. The work illuminates embryology and has important implications for making more realistic kidney organoids.