Kidney spheroids recapitulate tubular organoids leading to enhanced tubulogenic potency of human kidney-derived cells

Enhancement of lacrimal gland cell function by decellularized lacrimal gland derived hydrogel

Sustainable treatment of aqueous deficient dry eye (ADDE) represents an unmet medical need and therefore requires new curative and regenerative approaches based on appropriatein vitromodels. Tissue specific hydrogels retain the individual biochemical composition of the extracellular matrix and thus promote the inherent cell´s physiological function. Hence, we created a decellularized lacrimal gland (LG) hydrogel (dLG-HG) meeting the requirements for a bioink as the basis of a LG model with potential forin vitroADDE studies. Varying hydrolysis durations were compared to obtain dLG-HG with best possible physical and ultrastructural properties while preserving the original biochemical composition. A particular focus was placed on dLG-HG´s impact on viability and functionality of LG associated cell types with relevance for a futurein vitromodel in comparison to the unspecific single component hydrogel collagen type-I (Col) and the common cell culture substrate Matrigel. Proliferation of LG epithelial cells (EpC), LG mesenchymal stem cells, and endothelial cells cultured on dLG-HG was enhanced compared to culture on Matrigel. Most importantly with respect to a functionalin vitromodel, the secretion capacity of EpC cultured on dLG-HG was higher than that of EpC cultured on Col or Matrigel. In addition to these promising cell related properties, a rapid matrix metalloproteinase-dependent biodegradation was observed, which on the one hand suggests a lively cell-matrix interaction, but on the other hand limits the cultivation period. Concluding, dLG-HG possesses decisive properties for the tissue engineering of a LGin vitromodel such as cytocompatibility and promotion of secretion, making it superior to unspecific cell culture substrates. However, deceleration of biodegradation should be addressed in future experiments.

A comprehensive review on 3D tissue models: Biofabrication technologies and preclinical applications

The limitations of traditional two-dimensional (2D) cultures and animal testing, when it comes to precisely foreseeing the toxicity and clinical effectiveness of potential drug candidates, have resulted in a notable increase in the rate of failure during the process of drug discovery and development. Three-dimensional (3D) in-vitro models have arisen as substitute platforms with the capacity to accurately depict in-vivo conditions and increasing the predictivity of clinical effects and toxicity of drug candidates. It has been found that 3D models can accurately represent complex tissue structure of human body and can be used for a wide range of disease modeling purposes. Recently, substantial progress in biomedicine, materials and engineering have been made to fabricate various 3D in-vitro models, which have been exhibited better disease progression predictivity and drug effects than convention models, suggesting a promising direction in pharmaceutics. This comprehensive review highlights the recent developments in 3D in-vitro tissue models for preclinical applications including drug screening and disease modeling targeting multiple organs and tissues, like liver, bone, gastrointestinal tract, kidney, heart, brain, and cartilage. We discuss current strategies for fabricating 3D models for specific organs with their strengths and pitfalls. We expand future considerations for establishing a physiologically-relevant microenvironment for growing 3D models and also provide readers with a perspective on intellectual property, industry, and regulatory landscape.

OCT4 induces long-lived dedifferentiated kidney progenitors poised to redifferentiate in 3D kidney spheroids

Upscaling of kidney epithelial cells is crucial for renal regenerative medicine. Nonetheless, the adult kidney lacks a distinct stem cell hierarchy, limiting the ability to long-term propagate clonal populations of primary cells that retain renal identity. Toward this goal, we tested the paradigm of shifting the balance between differentiation and stemness in the kidney by introducing a single pluripotency factor, OCT4. Here we show that ectopic expression of OCT4 in human adult kidney epithelial cells (hKEpC) induces the cells to dedifferentiate, stably proliferate, and clonally emerge over many generations. Control hKEpC dedifferentiate, assume fibroblastic morphology, and completely lose clonogenic capacity. Analysis of gene expression and histone methylation patterns revealed that OCT4 represses the HNF1B gene module, which is critical for kidney epithelial differentiation, and concomitantly activates stemness-related pathways. OCT4-hKEpC can be long-term expanded in the dedifferentiated state that is primed for renal differentiation. Thus, when expanded OCT4-hKEpC are grown as kidney spheroids (OCT4-kSPH), they reactivate the HNF1B gene signature, redifferentiate, and efficiently generate renal structures in vivo. Hence, changes occurring in the cellular state of hKEpC following OCT4 induction, long-term propagation, and 3D aggregation afford rapid scale-up technology of primary renal tissue-forming cells.

Towards a better understanding of diabetes mellitus using organoid models

Our understanding of diabetes mellitus has benefited from a combination of clinical investigations and work in model organisms and cell lines. Organoid models for a wide range of tissues are emerging as an additional tool enabling the study of diabetes mellitus. The applications for organoid models include studying human pancreatic cell development, pancreatic physiology, the response of target organs to pancreatic hormones and how glucose toxicity can affect tissues such as the blood vessels, retina, kidney and nerves. Organoids can be derived from human tissue cells or pluripotent stem cells and enable the production of human cell assemblies mimicking human organs. Many organ mimics relevant to diabetes mellitus are already available, but only a few relevant studies have been performed. We discuss the models that have been developed for the pancreas, liver, kidney, nerves and vasculature, how they complement other models, and their limitations. In addition, as diabetes mellitus is a multi-organ disease, we highlight how a merger between the organoid and bioengineering fields will provide integrative models.

Recent advances in molecular mechanisms of acute kidney injury in patients with diabetes mellitus

Several insults can lead to acute kidney injury (AKI) in native kidney and transplant patients, with diabetes critically contributing as pivotal risk factor. High glucose per se can disrupt several signaling pathways within the kidney that, if not restored, can favor the instauration of mechanisms of maladaptive repair, altering kidney homeostasis and proper function. Diabetic kidneys frequently show reduced oxygenation, vascular damage and enhanced inflammatory response, features that increase the kidney vulnerability to hypoxia. Importantly, epidemiologic data shows that previous episodes of AKI increase susceptibility to diabetic kidney disease (DKD), and that patients with DKD and history of AKI have a generally worse prognosis compared to DKD patients without AKI; it is therefore crucial to monitor diabetic patients for AKI. In the present review, we will describe the causes that contribute to increased susceptibility to AKI in diabetes, with focus on the molecular mechanisms that occur during hyperglycemia and how these mechanisms expose the different types of resident renal cells to be more vulnerable to maladaptive repair during AKI (contrast- and drug-induced AKI). Finally, we will review the list of the existing candidate biomarkers of diagnosis and prognosis of AKI in patients with diabetes.

Physiology assays in human kidney organoids

Kidney organoids derived from human pluripotent stem cells constitute a novel model of disease, development, and regenerative therapy. Organoids are human, experimentally accessible, high throughput, and enable reconstitution of tissue-scale biology in a petri dish. Although gene expression patterns in organoid cells have been analyzed extensively, less is known about the functionality of these structures. Here, we review assays of physiological function in human kidney organoids, including best practices for quality control, and future applications. Tubular structures in organoids accumulate specific molecules through active transport, including dextran and organic anions, and swell with fluid in response to cAMP stimulation. When engrafted into animal models in vivo, organoids form vascularized glomerulus-like structures capable of size-selective filtration. Organoids exhibit metabolic, endocrine, injury, and infection phenotypes, although their specificity is not yet fully clear. To properly interpret organoid physiology assays, it is important to incorporate appropriate negative and positive controls, statistical methods, data presentation, molecular mechanisms, and clinical data sets. Improvements in organoid perfusion, patterning, and maturation are needed to enable branching morphogenesis, urine production, and renal replacement. Reconstituting renal physiology with kidney organoids is a new field with potential to provide fresh insights into classical phenomena.

Generation of human tonsil epithelial organoids as an ex vivo model for SARS-CoV-2 infection

The palatine tonsils (hereinafter referred to as "tonsils") serve as a reservoir for viral infections and play roles in the immune system's first line of defense. The aims of this study were to establish tonsil epithelial cell-derived organoids and examine their feasibility as an ex vivo model for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. The tonsil organoids successfully recapitulated the key characteristics of the tonsil epithelium, including cellular composition, histologic properties, and biomarker distribution. Notably, the basal layer cells of the organoids express molecules essential for SARS-CoV-2 entry, such as angiotensin-converting enzyme 2 (ACE2), transmembrane serine protease 2 (TMPRSS2) and furin, being susceptible to the viral infection. Changes in the gene expression profile in tonsil organoids revealed that 395 genes associated with oncostatin M signaling and lipid metabolism were highly upregulated within 72 h after SARS-CoV-2 infection. Notably, remdesivir suppressed the viral RNA copy number in organoid culture supernatants and intracellular viral protein levels in a dose-dependent manner. Here, we suggest that tonsil epithelial organoids could provide a preclinical and translational research platform for investigating SARS-CoV-2 infectivity and transmissibility or for evaluating antiviral candidates.

Kidney Injury in COVID-19: Epidemiology, Molecular Mechanisms, and Potential Therapeutic Targets

As of December 2021, SARS-CoV-2 had caused over 250 million infections and 5 million deaths worldwide. Furthermore, despite the development of highly effective vaccines, novel variants of SARS-CoV-2 continue to sustain the pandemic, and the search for effective therapies for COVID-19 remains as urgent as ever. Though the primary manifestation of COVID-19 is pneumonia, the disease can affect multiple organs, including the kidneys, with acute kidney injury (AKI) being among the most common extrapulmonary manifestations of severe COVID-19. In this article, we start by reflecting on the epidemiology of kidney disease in COVID-19, which overwhelmingly demonstrates that AKI is common in COVID-19 and is strongly associated with poor outcomes. We also present emerging data showing that COVID-19 may result in long-term renal impairment and delve into the ongoing debate about whether AKI in COVID-19 is mediated by direct viral injury. Next, we focus on the molecular pathogenesis of SARS-CoV-2 infection by both reviewing previously published data and presenting some novel data on the mechanisms of cellular viral entry. Finally, we relate these molecular mechanisms to a series of therapies currently under investigation and propose additional novel therapeutic targets for COVID-19.

Analysis of the Physico-Chemical, Mechanical and Biological Properties of Crosslinked Type-I Collagen from Horse Tendon: Towards the Development of Ideal Scaffolding Material for Urethral Regeneration

Urethral stenosis is a pathological condition that consists in the narrowing of the urethral lumen because of the formation of scar tissue. Unfortunately, none of the current surgical approaches represent an optimal solution because of the high stricture recurrence rate. In this context, we preliminarily explored the potential of an insoluble type-I collagen from horse tendon as scaffolding material for the development of innovative devices for the regeneration of injured urethral tracts. Non-porous collagen-based substrates were produced and optimized, in terms of crosslinking density of the macromolecular structure, to either provide mechanical properties compliant with the urinary tract physiological stress and better sustain tissue regeneration. The effect of the adopted crosslinking strategy on the protein integrity and on the substrate physical-chemical, mechanical and biological properties was investigated in comparison with a decellularized matrix from porcine small intestinal submucosa (SIS patch), an extensively used xenograft licensed for clinical use in urology. The optimized production protocols allowed the preservation of the type I collagen native structure and the realization of a substrate with appealing end-use properties. The biological response, preliminarily investigated by immunofluorescence experiments on human adult renal stem/progenitor cells until 28 days, showed the formation of a stem-cell monolayer within 14 days and the onset of spheroids within 28 days. These results suggested the great potential of the collagen-based material for the development of scaffolds for urethral plate regeneration and for in vitro cellular studies.

Two new applications in the study of angiogenesis the CAM assay: Acellular scaffolds and organoids

The chick embryo chorioallantoic membrane (CAM) is a rich vascularized extraembryonic membrane that is commonly used as an in vivo experimental model to study molecules with angiogenic and anti-angiogenic activity, tumor growth and metastasis. Among other applications of the CAM assay, more recently this assay has been used for the study of acellular scaffolds and of organoids, and of their angiogenic capacity. The aim of this review article is to summarize the literature data concerning these two new applications of the CAM assay and to underline the advantages of this assay.

Human Kidney Spheroids and Monolayers Provide Insights into SARS-CoV-2 Renal Interactions

BACKGROUND

Although coronavirus disease 2019 (COVID-19) causes significan t morbidity, mainly from pulmonary involvement, extrapulmonary symptoms are also major componen ts of the disease. Kidney disease, usually presenting as AKI, is particularly severe among patients with COVID-19. It is unknown, however, whether such injury results from direct kidney infection with COVID-19's causative virus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), or from indirect mechanisms.

METHODS

Using ex vivo cell models, we sought to analyze SARS-CoV-2 interactions with kidney tubular cells and assess direct tubular injury. These models comprised primary human kidney epithelial cells (derived from nephrectomies) and grown as either proliferating monolayers or quiescent three-dimensional kidney spheroids.

RESULTS

We demonstrated that viral entry molecules and high baseline levels of type 1 IFN-related molecules were present in monolayers and kidney spheroids. Although both models support viral infection and replication, they did not exhibit a cytopathic effect and cell death, outcomes that were strongly present in SARS-CoV-2-infected controls (African green monkey kidney clone E6 [Vero E6] cultures). A comparison of monolayer and spheroid cultures demonstrated higher infectivity and replication of SARS-CoV-2 in actively proliferating monolayers, although the spheroid cultures exhibited high er levels of ACE2. Monolayers exhibited elevation of some tubular injury molecules-including molecules related to fibrosis (COL1A1 and STAT6) and dedifferentiation (SNAI2)-and a loss of cell identity, evident by reduction in megalin (LRP2). The three-dimensional spheroids were less prone to such injury.

CONCLUSIONS

SARS-CoV-2 can infect kidney cells without a cytopathic effect. AKI-induced cellular proliferation may potentially intensify infectivity and tubular damage by SARS-CoV-2, suggesting that early intervention in AKI is warranted to help minimize kidney infection.

Bioengineered Kidney Models: Methods and Functional Assessments

Investigations into bioengineering kidneys have been extensively conducted owing to their potential for preclinical assays and regenerative medicine. Various approaches and methods have been developed to improve the structure and function of bioengineered kidneys. Assessments of functional properties confirm the adequacy of bioengineered kidneys for multipurpose translational applications. This review is to summarize the studies performed in kidney bioengineering in the past decade. We identified 84 original articles from PubMed and Mendeley with keywords of kidney organoid or kidney tissue engineering. Those were categorized into 5 groups based on their approach: de-/recellularization of kidney, reaggregation of kidney cells, kidney organoids, kidney in scaffolds, and kidney-on-a-chip. These models were physiologically assessed by filtration, tubular reabsorption/secretion, hormone production, and nephrotoxicity. We found that bioengineered kidney models have been developed from simple cell cultures to multicellular systems to recapitulate kidney function and diseases. Meanwhile, only about 50% of these studies conducted functional assessments on their kidney models. Factors including cell composition and organization are likely to alter the applicability of physiological assessments in bioengineered kidneys. Combined with recent technologies, physiological assessments importantly contribute to the improvement of the bioengineered kidney model toward repairing and refunctioning the damaged kidney.

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.

Tumor Spheroids of an Aggressive Form of Central Neurocytoma Have Transit-Amplifying Progenitor Characteristics with Enhanced EGFR and Tumor Stem Cell Signaling

Central neurocytoma (CN) has been known as a benign neuronal tumor. In rare cases, CN undergoes malignant transformation to glioblastomas (GBM). Here we examined its cellular origin by characterizing differentiation potential and gene expression of CN-spheroids. First, we demonstrate that both CN tissue and cultured primary cells recapitulate the hierarchal cellular composition of subventricular zone (SVZ), which is comprised of neural stem cells (NSCs), transit amplifying progenitors (TAPs), and neuroblasts. We then derived spheroids from CN which displayed EGFR+/MASH+ TAP and BLBP+ radial glial cell (RGC) characteristic, and mitotic neurogenesis and gliogenesis by single spheroids were observed with cycling multipotential cells. CN-spheroids expressed increased levels of pluripotency and tumor stem cell genes such as KLF4 and TPD5L1, when compared to their differentiated cells and human NSCs. Importantly, Gene Set Enrichment Analysis showed that gene sets of GBM-Spheroids, EGFR Signaling, and Packaging of Telomere Ends are enriched in CN-spheroids in comparison with their differentiated cells. We speculate that CN tumor stem cells have TAP and RGC characteristics, and upregulation of EGFR signaling as well as downregulation of eph-ephrin signaling have critical roles in tumorigenesis of CN. And their ephemeral nature of TAPs destined to neuroblasts, might reflect benign nature of CN.

Ex Vivo Expanded 3D Human Kidney Spheres Engraft Long Term and Repair Chronic Renal Injury in Mice

End-stage renal disease is a worldwide epidemic requiring renal replacement therapy. Harvesting tissue from failing kidneys and autotransplantation of tissue progenitors could theoretically delay the need for dialysis. Here we use healthy and end-stage human adult kidneys to robustly expand proliferative kidney epithelial cells and establish 3D kidney epithelial cultures termed "nephrospheres." Formation of nephrospheres reestablishes renal identity and function in primary cultures. Transplantation into NOD/SCID mice shows that nephrospheres restore self-organogenetic properties lost in monolayer cultures, allowing long-term engraftment as tubular structures, potentially adding nephron segments and demonstrating self-organization as critical to survival. Furthermore, long-term tubular engraftment of nephrospheres is functionally beneficial in murine models of chronic kidney disease. Remarkably, nephrospheres inhibit pro-fibrotic collagen production in cultured fibroblasts via paracrine modulation, while transplanted nephrospheres induce transcriptional signatures of proliferation and release from quiescence, suggesting re-activation of endogenous repair. These data support the use of human nephrospheres for renal cell therapy.

Scalable fabrication of renal spheroids and nephron-like tubules by bioprinting and controlled self-assembly of epithelial cells

Scalable fabrication concepts of 3D kidney tissue models are required to enable their application in pharmaceutical high-throughput screenings. Yet the reconstruction of complex tissue structures remains technologically challenging. We present a novel concept reducing the fabrication demands, by using controlled cellular self-assembly to achieve higher tissue complexities from significantly simplified construct designs. We used drop-on-demand bioprinting to fabricate locally confined patterns of renal epithelial cells embedded in a hydrogel matrix. These patterns provide defined local cell densities (cell count variance < 11 %) with high viability (92 ± 2 %). Based on these patterns, controlled self-assembly leads to the formation of renal spheroids and nephron-like tubules with a predefined size and spatial localization. With this, we fabricated scalable arrays of hollow epithelial spheroids. The spheroid sizes correlated with the initial cell count per unit and could be stepwise adjusted, ranging from Ø = 84, 104, 120 to 131 µm in diameter (size variance < 9 %). Furthermore, we fabricated scalable line-shaped patterns, which self-assembled to hollow cellular tubules (Ø = 105 ± 22 µm). These showed a continuous lumen with prescribed orientation, lined by an epithelial monolayer with tight junctions. Additionally, upregulated expression of kidney-specific functional genes compared to 2D cell monolayers indicated increased tissue functionality, as revealed by mRNA sequencing. Furthermore, our concept enabled the fabrication of hybrid tubules, which consisted of arranged subsections of different cell types, combining murine and human epithelial cells. Finally, we integrated the self-assembled fabrication into a microfluidic chip and achieved fluidic access to the lumen at the terminal sites of the tubules. With this, we realized flow conditions with a wall shear stress of 0.05 ± 0.02 dyne/cm² driven by hydrostatic pressure for scalable dynamic culture towards a nephron-on-chip model.

Human kidney clonal proliferation disclose lineage-restricted precursor characteristics

In-vivo single cell clonal analysis in the adult mouse kidney has previously shown lineage-restricted clonal proliferation within varying nephron segments as a mechanism responsible for cell replacement and local regeneration. To analyze ex-vivo clonal growth, we now preformed limiting dilution to generate genuine clonal cultures from one single human renal epithelial cell, which can give rise to up to 3.4 * 10⁶ cells, and analyzed their characteristics using transcriptomics. A comparison between clonal cultures revealed restriction to either proximal or distal kidney sub-lineages with distinct cellular and molecular characteristics; rapidly amplifying de-differentiated clones and a stably proliferating cuboidal epithelial-appearing clones, respectively. Furthermore, each showed distinct molecular features including cell-cycle, epithelial-mesenchymal transition, oxidative phosphorylation, BMP signaling pathway and cell surface markers. In addition, analysis of clonal versus bulk cultures show early clones to be more quiescent, with elevated expression of renal developmental genes and overall reduction in renal identity markers, but with an overlapping expression of nephron segment identifiers and multiple identity. Thus, ex-vivo clonal growth mimics the in-vivo situation displaying lineage-restricted precursor characteristics of mature renal cells. These data suggest that for reconstruction of varying renal lineages with human adult kidney based organoid technology and kidney regeneration ex-vivo, use of multiple heterogeneous precursors is warranted.

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.

Inhibiting WNT and NOTCH in renal cancer stem cells and the implications for human patients

Current treatments for clear cell renal cell cancer (ccRCC) are insufficient because two-thirds of patients with metastases progress within two years. Here we report the identification and characterization of a cancer stem cell (CSC) population in ccRCC. CSCs are quantitatively correlated with tumor aggressiveness and metastasis. Transcriptional profiling and single cell sequencing reveal that these CSCs exhibit an activation of WNT and NOTCH signaling. A significant obstacle to the development of rational treatments has been the discrepancy between model systems and the in vivo situation of patients. To address this, we use CSCs to establish non-adherent sphere cultures, 3D tumor organoids, and xenografts. Treatment with WNT and NOTCH inhibitors blocks the proliferation and self-renewal of CSCs in sphere cultures and organoids, and impairs tumor growth in patient-derived xenografts in mice. These findings suggest that our approach is a promising route towards the development of personalized treatments for individual patients.

Cancer stem cells are thought to be largely resistant to treatment and can be responsible for tumour recurrence. Here, using renal cancer organoids, self-renewing sphere cultures and PDX from patients, the authors show that the proliferation of stem cells within organoids, PDX and spheres can be blocked by the concomitant inhibition of the NOTCH and WNT pathways.

A critical review of adverse effects to the kidney: mechanisms, data sources, and in silico tools to assist prediction

INTRODUCTION

The kidney is a major target for toxicity elicited by pharmaceuticals and environmental pollutants. Standard testing which often does not investigate underlying mechanisms has proven not to be an adequate hazard assessment approach. As such, there is an opportunity for the application of computational approaches that utilize multiscale data based on the Adverse Outcome Pathway (AOP) paradigm, coupled with an understanding of the chemistry underpinning the molecular initiating event (MIE) to provide a deep understanding of how structural fragments of molecules relate to specific mechanisms of nephrotoxicity. Aims covered: The aim of this investigation was to review the current scientific landscape related to computational methods, including mechanistic data, AOPs, publicly available knowledge bases and current in silico models, for the assessment of pharmaceuticals and other chemicals with regard to their potential to elicit nephrotoxicity. A list of over 250 nephrotoxicants enriched with, where possible, mechanistic and AOP-derived understanding was compiled. Expert opinion: Whilst little mechanistic evidence has been translated into AOPs, this review identified a number of data sources of in vitro, in vivo, and human data that may assist in the development of in silico models which in turn may shed light on the interrelationships between nephrotoxicity mechanisms.

Regenerative medicine in kidney disease: where we stand and where to go

The kidney is a complex organ with more than 20 types of specialized cells that play an important role in maintaining the body's homeostasis. The epithelial tubular cell is formed during embryonic development and has little proliferative capacity under physiological conditions, but after acute injury the kidney does have regenerative capacity. However, after repetitive or severe lesions, it may undergo a maladaptation process that predisposes it to chronic kidney injury. Regenerative medicine includes various repair and regeneration techniques, and these have gained increasing attention in the scientific literature. In the future, not only will these techniques contribute to the repair and regeneration of the human kidney, but probably also to the construction of an entire organ. New mechanisms studied for kidney regeneration and repair include circulating stem cells as mesenchymal stromal/stem cells and their paracrine mechanisms of action; renal progenitor stem cells; the leading role of tubular epithelial cells in the tubular repair process; the study of zebrafish larvae to understand the process of nephron development, kidney scaffold and its repopulation; and, finally, the development of organoids. This review elucidates where we are in terms of current scientific knowledge regarding these mechanisms and the promises of future scientific perspectives.

Role of CD133 Molecule in Wnt Response and Renal Repair

Renal repair after injury is dependent on clonal expansion of proliferation-competent cells. In the human kidney, the expression of CD133 characterizes a population of resident scattered cells with resistance to damage and ability to proliferate. However, the biological function of the CD133 molecule is unknown. By RNA sequencing, we found that cells undergoing cisplatin damage lost the CD133 signature and acquired metanephric mesenchymal and regenerative genes such as SNAIL1, KLF4, SOX9, and WNT3. CD133 was reacquired in the recovery phase. In CD133-Kd cells, lack of CD133 limited cell proliferation after injury and was specifically correlated with deregulation of Wnt signaling and E-cadherin pathway. By immunoprecipitation, CD133 appeared to form a complex with E-cadherin and β-catenin. In parallel, CD133-Kd cells showed lower β-catenin levels in basal condition and after Wnt pathway activation and reduced TCF/LEF promoter activation in respect to CD133⁺ cells. Finally, the lack of CD133 impaired generation of nephrospheres while favoring senescence. These data indicate that CD133 may act as a permissive factor for β-catenin signaling, preventing its degradation in the cytoplasm. Therefore, CD133 itself appears to play a functional role in renal tubular repair through maintenance of proliferative response and control of senescence. Stem Cells Translational Medicine 2018;7:283-294.

Osteochondral Regeneration with a Scaffold-Free Three-Dimensional Construct of Adipose Tissue-Derived Mesenchymal Stromal Cells in Pigs

Osteochondral lesion is a major joint disease in humans. Therefore, this study was designed to investigate the regeneration of articular cartilage and subchondral bone, using three-dimensional constructs of autologous adipose tissue-derived mesenchymal stromal cells without any biocompatible scaffolds. Mesenchymal stromal cells were harvested by liposuction from seven pigs, isolated enzymatically, and expanded until construct creation. The pig models had two osteochondral defects (cylindrical defects with a diameter of 5.2 mm and a depth of 5 mm) in one of their patello-femoral grooves. A columnar structure consisting of approximately 770 spheroids of 5 × 10⁴ autologous mesenchymal stromal cells were implanted into one of the defects (implanted defect), while the other defect was not implanted (control). The defects were evaluated pathologically at 6 months (in three pigs) and 12 months (in five pigs) after implantation. At 6 months after surgery, histopathology revealed active endochondral ossification underneath the plump fibrocartilage in the implanted defects, but a deficiency of fibrocartilaginous coverage in the controls. At 12 months after surgery, the fibrocartilage was transforming into hyaline cartilage as thick as the surrounding normal cartilage and the subchondral bone was thickening in the implanted defects. The histological averages of the implanted sites were significantly higher than those in the control sites at both 6 and 12 months after surgery. The implantation of a scaffold-free three-dimensional construct of autologous mesenchymal stromal cells into an osteochondral defect can induce regeneration of hyaline cartilage and subchondral bone structures over a period of 12 months.

Development and characterization of cholangioids from normal and diseased human cholangiocytes as an in vitro model to study primary sclerosing cholangitis

Primary sclerosing cholangitis (PSC) is an incurable, fibroinflammatory biliary disease for which there is no effective pharmacotherapy. We recently reported cholangiocyte senescence as an important phenotype in PSC while others showed that portal macrophages accumulate in PSC. Unfortunately, our ability to explore cholangiocyte senescence and macrophage accumulation has been hampered by limited in vitro models. Thus, our aim was to develop and characterize a three-dimensional (3D) model of normal and diseased bile ducts (cholangioids) starting with normal human cholangiocytes (NHC), senescent NHC (NHC-sen), and cholangiocytes from PSC patients. In 3D culture, NHCs formed spheroids of ~5000 cells with a central lumen of ~150 μm. By confocal microscopy and western blot, cholangioids retained expression of cholangiocyte proteins (cytokeratin 7/19) and markers of epithelial polarity (secretin receptor and GM130). Cholangioids are functionally active, and upon secretin stimulation, luminal size increased by ~80%. Cholangioids exposed to hydrogen peroxide exhibited cellular senescence and the senescence-associated secretory phenotype (SASP; increased IL-6, p21, SA-β-Gal, yH2A.x and p16 expression). Furthermore, cholangioids derived from NHC-sen or PSC patients were smaller and had slower growth than the controls. When co-cultured with THP-1 macrophages, the number of macrophages associated with NHC-sen or PSC cholangioids was five- to seven-fold greater compared to co-culture with non-senescent NHC. We observed that NHC-sen and PSC cholangioids release greater number of extracellular vesicles (EVs) compared to controls. Moreover, conditioned media from NHC-sen cholangioids resulted in an ~2-fold increase in macrophage migration. In summary, we developed a method to generate normal and diseased cholangioids, characterized them morphologically and functionally, showed that they can be induced to senescence and SASP, and demonstrated both EV release and macrophage attraction. This novel model mimics several features of PSC, and thus will be useful for studying the pathogenesis of PSC and potentially identifying new therapeutic targets.

Nephrosphere-Derived Cells Are Induced to Multilineage Differentiation when Cultured on Human Decellularized Kidney Scaffolds

In end-stage chronic kidney disease, the option of organ transplantation is limited because of the scarce availability of kidneys. The combination of stem cell research, regenerative medicine, and tissue engineering seems a promising approach to produce new transplantable kidneys. Currently, the possibility to repopulate naturally obtained scaffolds with cells of different sources is advancing. Our aim was to test, for the first time, whether the nephrosphere (NS) cells, composed by renal stem/progenitor-like cells, were able to repopulate different nephron portions of renal extracellular matrix scaffolds obtained after decellularization of human renal tissue slices. Our decellularization protocol enabled us to obtain a completely acellular renal scaffold while maintaining the extracellular matrix structure and composition in terms of collagen IV, laminin, and fibronectin. NS cells, cultured on decellularized renal scaffolds with basal medium, differentiated into proximal and distal tubules as well as endothelium, as highlighted by histology and by the specific expression of epithelial cytokeratin 8.18, proximal tubular CD10, distal tubular cytokeratin 7, and endothelial von Willebrand factor markers. Endothelial medium promoted the differentiation toward the endothelium, whereas epithelial medium promoted the differentiation toward the epithelium. NS cells seem to be a good tool for scaffold repopulation, paving the way for experimental investigations focused on whole-kidney reconstruction.

How to grow a kidney: patient-specific kidney organoids come of age

The notion of regrowing a patient's kidney in a dish has fascinated researchers for decades and has spurred visions of revolutionary clinical applications. Recently, this option has come closer to reality. Key technologies have been developed to generate patient-specific pluripotent stem cells and to edit their genome. Several laboratories have devised protocols to differentiate patient-specific pluripotent stem cells into kidney cells or into in vitro organoids that resemble the kidney with respect to cell types, tissue architecture and disease pathology. This was possible because of rapidly expanding knowledge regarding the cellular and molecular basis of embryonic kidney development. Generating kidney cells or organoids from patient-specific stem cells may prove to be clinically useful in several ways. First, patient-specific kidney cells or organoids could be used to predict an individual's response to stressors, toxins or medications and thereby develop personalized treatment decisions. Second, patient-specific stem cells harbour the individual's genetic defects. This may potentially enable genetic rescue attempts to establish the significance of a genetic defect in a stem cell-derived organoid or it may allow testing of patient-specific targeted therapies for kidney disease in vitro. From a tissue engineering perspective, patient-specific kidney organoids might provide a key advance towards engineering immunocompatible transplantable kidneys. This review article summarizes recent developments in the field and discusses its current limitations and future perspectives.

Inhibin-A and Decorin Secreted by Human Adult Renal Stem/Progenitor Cells Through the TLR2 Engagement Induce Renal Tubular Cell Regeneration

Acute kidney injury (AKI) is a public health problem worldwide. Several therapeutic strategies have been made to accelerate recovery and improve renal survival. Recent studies have shown that human adult renal progenitor cells (ARPCs) participate in kidney repair processes, and may be used as a possible treatment to promote regeneration in acute kidney injury. Here, we show that human tubular ARPCs (tARPCs) protect physically injured or chemically damaged renal proximal tubular epithelial cells (RPTECs) by preventing cisplatin-induced apoptosis and enhancing proliferation of survived cells. tARPCs without toll-like receptor 2 (TLR2) expression or TLR2 blocking completely abrogated this regenerative effect. Only tARPCs, and not glomerular ARPCs, were able to induce tubular cell regeneration process and it occurred only after damage detection. Moreover, we have found that ARPCs secreted inhibin-A and decorin following the RPTEC damage and that these secreted factors were directly involved in cell regeneration process. Polysaccharide synthetic vesicles containing these molecules were constructed and co-cultured with cisplatin damaged RPTECs. These synthetic vesicles were not only incorporated into the cells, but they were also able to induce a substantial increase in cell number and viability. The findings of this study increase the knowledge of renal repair processes and may be the first step in the development of new specific therapeutic strategies for renal repair.

Heparin-based hydrogels induce human renal tubulogenesis in vitro

Dialysis or kidney transplantation is the only therapeutic option for end stage renal disease. Accordingly, there is a large unmet clinical need for new causative therapeutic treatments. Obtaining robust models that mimic the complex nature of the human kidney is a critical step in the development of new therapeutic strategies. Here we establish a synthetic in vitro human renal tubulogenesis model based on a tunable glycosaminoglycan-hydrogel platform. In this system, renal tubulogenesis can be modulated by the adjustment of hydrogel mechanics and degradability, growth factor signaling, and the presence of insoluble adhesion cues, potentially providing new insights for regenerative therapy. Different hydrogel properties were systematically investigated for their ability to regulate renal tubulogenesis. Hydrogels based on heparin and matrix metalloproteinase cleavable peptide linker units were found to induce the morphogenesis of single human proximal tubule epithelial cells into physiologically sized tubule structures. The generated tubules display polarization markers, extracellular matrix components, and organic anion transport functions of the in vivo renal proximal tubule and respond to nephrotoxins comparable to the human clinical response. The established hydrogel-based human renal tubulogenesis model is thus considered highly valuable for renal regenerative medicine and personalized nephrotoxicity studies.

STATEMENT OF SIGNIFICANCE

The only cure for end stage kidney disease is kidney transplantation. Hence, there is a huge need for reliable human kidney models to study renal regeneration and establish alternative treatments. Here we show the development and application of an in vitro human renal tubulogenesis model using heparin-based hydrogels. To the best of our knowledge, this is the first system where human renal tubulogenesis can be monitored from single cells to physiologically sized tubule structures in a tunable hydrogel system. To validate the efficacy of our model as a drug toxicity platform, a chemotherapy drug was incubated with the model, resulting in a drug response similar to human clinical pathology. The established model could have wide applications in the field of nephrotoxicity and renal regenerative medicine and offer a reliable alternative to animal models.

Modeling Physiological Events in 2D vs. 3D Cell Culture

Cell culture has become an indispensable tool to help uncover fundamental biophysical and biomolecular mechanisms by which cells assemble into tissues and organs, how these tissues function, and how that function becomes disrupted in disease. Cell culture is now widely used in biomedical research, tissue engineering, regenerative medicine, and industrial practices. Although flat, two-dimensional (2D) cell culture has predominated, recent research has shifted toward culture using three-dimensional (3D) structures, and more realistic biochemical and biomechanical microenvironments. Nevertheless, in 3D cell culture, many challenges remain, including the tissue-tissue interface, the mechanical microenvironment, and the spatiotemporal distributions of oxygen, nutrients, and metabolic wastes. Here, we review 2D and 3D cell culture methods, discuss advantages and limitations of these techniques in modeling physiologically and pathologically relevant processes, and suggest directions for future research.

Comparing adult renal stem cell identification, characterization and applications

Despite growing interest and effort, a consensus has yet to be reached in regards to the identification of adult renal stem cells. Organ complexity and low turnover of renal cells has made stem cell identification difficult and lead to the investigation of multiple possible populations. In this review, we summarize the work that has been done toward finding and characterizing an adult renal stem cell population. In addition to giving a general overview of what has been done, we aim to highlight the variation in methods and outcomes. The methods used to locate potential stem cell populations can vary widely, but even within the relatively standard practice of BrdU labeling of slowly dividing cells, there are significant differences in protocols and results. Additional diversity exists in cell marker profiles and apparent differentiation potential seen in potential stem cell sources. Cataloging the variety of methods and outcomes seen so far may help to streamline future investigation and stear the field toward consensus. But even without firmly defined populations, the application of renal stem cells holds tantalizing potential. Populations of highly proliferative, multipotent cells of renal origin show the ability to engraft in injured kidneys, mitigate functional loss and occasionally show the ability to generate nephrons de novo. The progress toward regenerative medicine applications is also summarized.

Modelling kidney disease with CRISPR-mutant kidney organoids derived from human pluripotent epiblast spheroids

Human-pluripotent-stem-cell-derived kidney cells (hPSC-KCs) have important potential for disease modelling and regeneration. Whether the hPSC-KCs can reconstitute tissue-specific phenotypes is currently unknown. Here we show that hPSC-KCs self-organize into kidney organoids that functionally recapitulate tissue-specific epithelial physiology, including disease phenotypes after genome editing. In three-dimensional cultures, epiblast-stage hPSCs form spheroids surrounding hollow, amniotic-like cavities. GSK3β inhibition differentiates spheroids into segmented, nephron-like kidney organoids containing cell populations with characteristics of proximal tubules, podocytes and endothelium. Tubules accumulate dextran and methotrexate transport cargoes, and express kidney injury molecule-1 after nephrotoxic chemical injury. CRISPR/Cas9 knockout of podocalyxin causes junctional organization defects in podocyte-like cells. Knockout of the polycystic kidney disease genes PKD1 or PKD2 induces cyst formation from kidney tubules. All of these functional phenotypes are distinct from effects in epiblast spheroids, indicating that they are tissue specific. Our findings establish a reproducible, versatile three-dimensional framework for human epithelial disease modelling and regenerative medicine applications.

Generating organized kidney tissues from human pluripotent stem cell is a major challenge. Here, Freedman et al. describe a differentiation system forming spheroids and tubular structures, characteristic of these kidney structures, and using CRISPR/Cas9, delete PKD1/2, to model polycystic kidney disease.

Potential Reparative Role of Resident Adult Renal Stem/Progenitor Cells in Acute Kidney Injury

Human kidney is particularly susceptible to ischemia and toxins with consequential tubular necrosis and activation of inflammatory processes. This process can lead to the acute renal injury, and even if the kidney has a great capacity for regeneration after tubular damage, in several circumstances, the normal renal repair program may not be sufficient to achieve a successful regeneration. Resident adult renal stem/progenitor cells could participate in this repair process and have the potentiality to enhance the renal regenerative mechanism. This could be achieved both directly, by means of their capacity to differentiate and integrate into the renal tissues, and by means of paracrine factors able to induce or improve the renal repair or regeneration. Recent genetic fate-tracing studies indicated that tubular damage is instead repaired by proliferative duplication of epithelial cells, acquiring a transient progenitor phenotype and by fate-restricted clonal cell progeny emerging from different nephron segments. In this review, we discuss about the properties and the reparative characteristics of high regenerative CD133(+)/CD24(+) cells, with a view to a future application of these cells for the treatment of acute renal injury.

The use of lineage tracing to study kidney injury and regeneration

Lineage tracing is a powerful tool to track cells in vivo and provides enhanced spatial, temporal, and kinetic resolution of the mechanisms that underlie tissue renewal and repair. The data obtained from novel mouse models engineered for lineage tracing has started to transform our understanding of the changes in cell fate that underlie renal pathophysiology, the role of stem and/or progenitor cells in kidney development, and the mechanisms of kidney regeneration. The complexity of the genetic systems that are engineered for lineage tracing requires careful analysis and interpretation. In this Review we emphasize that close attention in lineage tracing studies must be paid to the specificity of the promoter, the use of drug-controlled activation of Cre recombinase as a genetic switch, and the type of reporter that should be engineered into lineage tracing genetic constructs. We evaluate the optimal experimental conditions required to achieve the pre-specified aims of the study and discuss the novel genetic techniques that are becoming available to study putative renal progenitor cells and the mechanisms of kidney regeneration.

Differentiation capacity and maintenance of differentiated phenotypes of human mesenchymal stromal cells cultured on two distinct types of 3D polymeric scaffolds

This study shows that the classical validation of hMSC differentiation potential on 3D scaffolds might not be sufficient to ensure the maintenance of the cells functionality in the absence of differentiation inducing soluble factors.

Kidney stem cells in development, regeneration and cancer

The generation of nephrons during development depends on differentiation via a mesenchymal to epithelial transition (MET) of self-renewing, tissue-specific stem cells confined to a specific anatomic niche of the nephrogenic cortex. These cells may transform to generate oncogenic stem cells and drive pediatric renal cancer. Once nephron epithelia are formed the view of post-MET tissue renal growth and maintenance by adult tissue-specific epithelial stem cells becomes controversial. Recently, genetic lineage tracing that followed clonal evolution of single kidney cells showed that the need for new cells is constantly driven by fate-restricted unipotent clonal expansions in varying kidney segments arguing against a multipotent adult stem cell model. Lineage-restriction was similarly maintained in kidney organoids grown in culture. Importantly, kidney cells in which Wnt was activated were traced to give significant clonal progeny indicating a clonogenic hierarchy. In vivo nephron epithelia may be endowed with the capacity akin to that of unipotent epithelial stem/progenitor such that under specific stimuli can clonally expand/self renew by local proliferation of mature differentiated cells. Finding ways to ex vivo preserve and expand the observed in vivo kidney-forming capacity inherent to both the fetal and adult kidneys is crucial for taking renal regenerative medicine forward. Some of the strategies used to achieve this are sorting human fetal nephron stem/progenitor cells, growing adult nephrospheres or reprogramming differentiated kidney cells toward expandable renal progenitors.

In vivo clonal analysis reveals lineage-restricted progenitor characteristics in mammalian kidney development, maintenance, and regeneration

The mechanism and magnitude by which the mammalian kidney generates and maintains its proximal tubules, distal tubules, and collecting ducts remain controversial. Here, we use long-term in vivo genetic lineage tracing and clonal analysis of individual cells from kidneys undergoing development, maintenance, and regeneration. We show that the adult mammalian kidney undergoes continuous tubulogenesis via expansions of fate-restricted clones. Kidneys recovering from damage undergo tubulogenesis through expansions of clones with segment-specific borders, and renal spheres developing in vitro from individual cells maintain distinct, segment-specific fates. Analysis of mice derived by transfer of color-marked embryonic stem cells (ESCs) into uncolored blastocysts demonstrates that nephrons are polyclonal, developing from expansions of singly fated clones. Finally, we show that adult renal clones are derived from Wnt-responsive precursors, and their tracing in vivo generates tubules that are segment specific. Collectively, these analyses demonstrate that fate-restricted precursors functioning as unipotent progenitors continuously maintain and self-preserve the mouse kidney throughout life.

Concise review: can the intrinsic power of branching morphogenesis be used for engineering epithelial tissues and organs?

Branching morphogenesis is critical to the development of organs such as kidney, lung, mammary gland, prostate, pancreas, and salivary gland. Essentially, an epithelial bud becomes an iterative tip-stalk generator (ITSG) able to form a tree of branching ducts and/or tubules. In different organs, branching morphogenesis is governed by similar sets of genes. Epithelial branching has been recapitulated in vitro (or ex vivo) using three-dimensional cell culture and partial organ culture systems, and several such systems relevant to kidney tissue engineering are discussed here. By adapting systems like these it may be possible to harness the power inherent in the ITSG program to propagate and engineer epithelial tissues and organs. It is also possible to conceive of a universal ITSG capable of propagation that may, by recombination with organ-specific mesenchymal cells, be used for engineering many organ-like tissues similar to the organ from which the mesenchyme cells were derived, or toward which they are differentiated (from stem cells). The three-dimensional (3D) branched epithelial structure could act as a dynamic branching cellular scaffold to establish the architecture for the rest of the tissue. Another strategy-that of recombining propagated organ-specific ITSGs in 3D culture with undifferentiated mesenchymal stem cells-is also worth exploring. If feasible, such engineered tissues may be useful for the ex vivo study of drug toxicity, developmental biology, and physiology in the laboratory. Over the long term, they have potential clinical applications in the general fields of transplantation, regenerative medicine, and bioartificial medical devices to aid in the treatment of chronic kidney disease, diabetes, and other diseases.

Concise review: stem/progenitor cells for renal tissue repair: current knowledge and perspectives

The kidney is a specialized low-regenerative organ with several different types of cellular lineages; however, the identity of renal stem/progenitor cells with nephrogenic potential and their preferred niche(s) are largely unknown and debated. Most of the therapeutic approaches to kidney regeneration are based on administration of cells proven to enhance intrinsic reparative capabilities of the kidney. Endogenous or exogenous cells of different sources were tested in rodent models of ischemia-reperfusion, acute kidney injury, or chronic disease. The translation to clinics is at the moment focused on the role of mesenchymal stem cells. In addition, bioproducts from stem/progenitor cells, such as extracellular vesicles, are likely a new promising approach for reprogramming resident cells. This concise review reports the current knowledge about resident or exogenous stem/progenitor populations and their derived bioproducts demonstrating therapeutic effects in kidney regeneration upon injury. In addition, possible approaches to nephrogenesis and organ generation using organoids, decellularized kidneys, and blastocyst complementation are surveyed.

Identification of human nephron progenitors capable of generation of kidney structures and functional repair of chronic renal disease

Identification of tissue-specific renal stem/progenitor cells with nephrogenic potential is a critical step in developing cell-based therapies for renal disease. In the human kidney, stem/progenitor cells are induced into the nephrogenic pathway to form nephrons until the 34 week of gestation, and no equivalent cell types can be traced in the adult kidney. Human nephron progenitor cells (hNPCs) have yet to be isolated. Here we show that growth of human foetal kidneys in serum-free defined conditions and prospective isolation of NCAM1(+) cells selects for nephron lineage that includes the SIX2-positive cap mesenchyme cells identifying a mitotically active population with in vitro clonogenic and stem/progenitor properties. After transplantation in the chick embryo, these cells-but not differentiated counterparts-efficiently formed various nephron tubule types. hNPCs engrafted and integrated in diseased murine kidneys and treatment of renal failure in the 5/6 nephrectomy kidney injury model had beneficial effects on renal function halting disease progression. These findings constitute the first definition of an intrinsic nephron precursor population, with major potential for cell-based therapeutic strategies and modelling of kidney disease.

Reactivation of NCAM1 defines a subpopulation of human adult kidney epithelial cells with clonogenic and stem/progenitor properties

The nephron is composed of a monolayer of epithelial cells that make up its various compartments. In development, these cells begin as mesenchyme. NCAM1, abundant in the mesenchyme and early nephron lineage, ceases to express in mature kidney epithelia. We show that, once placed in culture and released from quiescence, adult human kidney epithelial cells (hKEpCs), uniformly positive for CD24/CD133, re-express NCAM1 in a specific cell subset that attains a stem/progenitor state. Immunosorted NCAM1(+) cells overexpressed early nephron progenitor markers (PAX2, SALL1, SIX2, WT1) and acquired a mesenchymal fate, indicated by high vimentim and reduced E-cadherin levels. Gene expression and microarray analysis disclosed both a proximal tubular origin of these cells and molecules regulating epithelial-mesenchymal transition. NCAM1(+) cells generated clonal progeny when cultured in the presence of fetal kidney conditioned medium, differentiated along mesenchymal lineages but retained the unique propensity to generate epithelial kidney spheres and produce epithelial renal tissue on single-cell grafting in chick CAM and mouse. Depletion of NCAM1(+) cells from hKEpCs abrogated stemness traits in vitro. Eliminating these cells during the regenerative response that follows glycerol-induced acute tubular necrosis worsened peak renal injury in vivo. Thus, higher clone-forming and developmental capacities characterize a distinct subset of adult kidney-derived cells. The ability to influence an endogenous regenerative response via NCAM1 targeting may lead to novel therapeutics for renal diseases.

PKH(high) cells within clonal human nephrospheres provide a purified adult renal stem cell population

The existence and identification of adult renal stem cells is a controversial issue. In this study, renal stem cells were identified from cultures of clonal human nephrospheres. The cultured nephrospheres exhibited the activation of stem cell pathways and contained cells at different levels of maturation. In each nephrosphere the presence of 1.12-1.25 cells mirroring stem cell properties was calculated. The nephrosphere cells were able to generate three-dimensional tubular structures in 3D cultures and in vivo. In clonal human nephrospheres a PKH(high) phenotype was isolated using PKH26 epifluorescence, which can identify quiescent cells within the nephrospheres. The PKH(high) cells, capable of self-renewal and of generating a differentiated epithelial, endothelial and podocytic progeny, can also survive in vivo maintaining the undifferentiated status. The PKH(high) status, together with a CD133(+)/CD24(-) phenotype, identified a homogeneous cell population displaying in vitro self-renewal and multipotency capacity. The resident adult renal stem cell population isolated from nephrospheres can be used for the study of mechanisms that regulate self-renewal and differentiation in adult renal tissue as well as in renal pathological conditions.

Chromatin-modifying agents reactivate embryonic renal stem/progenitor genes in human adult kidney epithelial cells but abrogate dedifferentiation and stemness

Recent studies have suggested that epigenetic modulation with chromatin-modifying agents can induce stemness and dedifferentiation and increase developmental plasticity. For instance, valproic acid (VPA), a histone deacetylase (HDAC) inhibitor, has been shown to promote self-renewal/expansion of hematopoietic stem cells and facilitate the generation of induced pluripotent stem cells (iPSCs). Previously, we observed that downregulation of embryonic renal stem/progenitor genes in the adult kidney was associated, at least in part, with epigenetic silencing. Therefore, we hypothesized that VPA may alter the expression of these genes and reprogram mature human adult kidney epithelial cells (hKEpCs) to a stem/progenitor-like state. Here, using quantitative RT-PCR and flow cytometry [fluorescence-activated cell sorting (FACS)] analysis, we show in VPA-treated primary cultures of human adult and fetal kidney significant reinduction of the renal stem/progenitor markers SIX2, OSR1, SALL1, NCAM, and PSA-NCAM. Robust SIX2 mRNA re-expression was confirmed at the protein level by western blot and was associated with epigenetic changes of the histones at multiple sites of the SIX2 promoter leading to gene activation, significantly increased acetylation of histones H4, and methylation of lysine 4 on H3. Furthermore, we could demonstrate synergistic effects of VPA and Wnt antagonists on SIX2 and also OSR1 reinduction. Nevertheless, VPA resulted in upregulation of E-CADHERIN and reduction in VIMENTIN, preventing the skewing of hKEpCs towards a more replicative mesenchymal state required for clonogenic expansion and acquisition of stem cell characters, altogether inducing cell senescence at early passages. These results demonstrating that chromatin-modifying agents prevent dedifferentiation of hKEpCs have important clinical implications as they may limit ex-vivo self-renewal/expansion and possibly the in vivo renal regenerative capacity initiated by dedifferentiation.

Renal hypodysplasia associates with a WNT4 variant that causes aberrant canonical WNT signaling

Abnormal differentiation of the renal stem/progenitor pool into kidney tissue can lead to renal hypodysplasia (RHD), but the underlying causes of RHD are not well understood. In this multicenter study, we identified 20 Israeli pedigrees with isolated familial, nonsyndromic RHD and screened for mutations in candidate genes involved in kidney development, including PAX2, HNF1B, EYA1, SIX1, SIX2, SALL1, GDNF, WNT4, and WT1. In addition to previously reported RHD-causing genes, we found that two affected brothers were heterozygous for a missense variant in the WNT4 gene. Functional analysis of this variant revealed both antagonistic and agonistic canonical WNT stimuli, dependent on cell type. In HEK293 cells, WNT4 inhibited WNT3A induced canonical activation, and the WNT4 variant significantly enhanced this inhibition of the canonical WNT pathway. In contrast, in primary cultures of human fetal kidney cells, which maintain WNT activation and more closely represent WNT signaling in renal progenitors during nephrogenesis, this mutation caused significant loss of function, resulting in diminished canonical WNT/β-catenin signaling. In conclusion, heterozygous WNT4 variants are likely to play a causative role in renal hypodysplasia.