An improved method of renal tissue engineering, by combining renal dissociation and reaggregation with a low-volume culture technique, results in development of engineered kidneys complete with loops of Henle

Production of kidney organoids arranged around single ureteric bud trees, and containing endogenous blood vessels, solely from embryonic stem cells

There is intense worldwide effort in generating kidney organoids from pluripotent stem cells, for research, for disease modelling and, perhaps, for making transplantable organs. Organoids generated from pluripotent stem cells (PSC) possess accurate micro-anatomy, but they lack higher-organization. This is a problem, especially for transplantation, as such organoids will not be able to perform their physiological functions. In this study, we develop a method for generating murine kidney organoids with improved higher-order structure, through stages using chimaeras of ex-fetu and PSC-derived cells to a system that works entirely from embryonic stem cells. These organoids have nephrons organised around a single ureteric bud tree and also make vessels, with the endothelial network approaching podocytes.

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.

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.

Combining Kidney Organoids and Genome Editing Technologies for a Better Understanding of Physiopathological Mechanisms of Renal Diseases: State of the Art

Kidney organoids derived from pluripotent stem cells became a real alternative to the use of in vitro cellular models or in vivo animal models. Indeed, the comprehension of the key steps involved during kidney embryonic development led to the establishment of protocols enabling the differentiation of pluripotent stem cells into highly complex and organized structures, composed of various renal cell types. These organoids are linked with one major application based on iPSC technology advantage: the possibility to control iPSC genome, by selecting patients with specific disease or by genome editing tools such as CRISPR/Cas9 system. This allows the generation of kidney organoïds which recapitulate important physiopathological mechanisms such as cyst formation in renal polycystic disease for example. This review will focus on studies combining these both cutting edge technologies i.e., kidney organoid differentiation and genome editing and will describe what are the main advances performed in the comprehension of physiopathological mechanisms of renal diseases, as well as discuss remaining technical barriers and perspectives in the field.

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.

Growing a new human kidney

There are 3 reasons to generate a new human kidney. The first is to learn more about the biology of the developing and mature organ. The second is to generate tissues with which to model congenital and acquired kidney diseases. In particular, growing human kidneys in this manner ultimately should help us understand the mechanisms of common chronic kidney diseases such as diabetic nephropathy and others featuring fibrosis, as well as nephrotoxicity. The third reason is to provide functional kidney tissues that can be used directly in regenerative medicine therapies. The second and third reasons to grow new human kidneys are especially compelling given the millions of persons worldwide whose lives depend on a functioning kidney transplant or long-term dialysis, as well as those with end-stage renal disease who die prematurely because they are unable to access these treatments. As shown in this review, the aim to create healthy human kidney tissues has been partially realized. Moreover, the technology shows promise in terms of modeling genetic disease. In contrast, barely the first steps have been taken toward modeling nongenetic chronic kidney diseases or using newly grown human kidney tissue for regenerative medicine therapies.

[Kidney organoids]

This review focus on kidney organoids derived from pluripotent stem cells, which become a real alternative to the use of in vitro cellular models or in vivo animals models. The comprehension of the key steps involved during kidney embryonic development led to the establishment of protocols enabling the differentiation of pluripotent stem cells into kidney organoids that are highly complex and organized structures, composed of various renal cell types. These mini-organs are endowed with major applications: the possibility to control iPSC genome (by selecting patients with specific disease or by genome editing) allows the generation of kidney organoïds which recapitulate important physiopathological mechanisms such as cyste formation in renal polycystic disease. Kidney organoids can also be used in high-throughput screening to fasten the screening of nephrotoxic/therapeutic compounds. Finally, kidney organoids have a huge interest in the context of tissue repair, which remains for now a challenging goal linked with technological barriers that need still to be overcome.

In developing mouse kidneys, orientation of loop of Henle growth is adaptive and guided by long-range cues from medullary collecting ducts

The path taken by the loop of Henle, from renal cortex to medulla and back, is critical to the ability of the kidney to concentrate urine and recover water. Unlike most developing tubules, which navigate as blind‐ended cylinders, the loop of Henle extends as a sharply bent loop, the apex of which leads the double tubes behind it in a ‘V’ shape. Here, we show that, in normal kidney development, loops of Henle extend towards the centroid of the kidney with an accuracy that increases the longer they extend. Using cultured kidney rudiments, and manipulations that rotate or remove portions of the organ, we show that loop orientation depends on long‐range cues from the medulla rather than either the orientation of the parent nephron or local cues in the cortex. The loops appear to be attracted to the most mature branch point of the collecting duct system but, if this is removed, they will head towards the most mature collecting duct branch available to them. Our results demonstrate the adaptive nature of guidance of this unusual example of a growing epithelium, and set the stage for later work devoted to understanding the molecules and mechanisms that underlie it.

The bridge between transplantation and regenerative medicine: Beginning a new Banff classification of tissue engineering pathology

The science of regenerative medicine is arguably older than transplantation-the first major textbook was published in 1901-and a major regenerative medicine meeting took place in 1988, three years before the first Banff transplant pathology meeting. However, the subject of regenerative medicine/tissue engineering pathology has never received focused attention. Defining and classifying tissue engineering pathology is long overdue. In the next decades, the field of transplantation will enlarge at least tenfold, through a hybrid of tissue engineering combined with existing approaches to lessening the organ shortage. Gradually, transplantation pathologists will become tissue-(re-) engineering pathologists with enhanced skill sets to address concerns involving the use of bioengineered organs. We outline ways of categorizing abnormalities in tissue-engineered organs through traditional light microscopy or other modalities including biomarkers. We propose creating a new Banff classification of tissue engineering pathology to standardize and assess de novo bioengineered solid organs transplantable success in vivo. We recommend constructing a framework for a classification of tissue engineering pathology now with interdisciplinary consensus discussions to further develop and finalize the classification at future Banff Transplant Pathology meetings, in collaboration with the human cell atlas project. A possible nosology of pathologic abnormalities in tissue-engineered organs is suggested.

Sebinger Culture: A System Optimized for Morphological Maturation and Imaging of Cultured Mouse Metanephric Primordia

Here, we present a detailed protocol on setting up embryonic renal organ cultures using a culture method that we have optimised for anatomical maturation and imaging. Our culture method places kidney rudiments on glass in a thin film of medium, which results in very flat cultures with all tubules in the same image plane. For reasons not yet understood, this technique results in improved renal maturation compared to traditional techniques. Typically, this protocol will result in an organ formed with distinct cortical and medullary regions as well as elongated, correctly positioned loops of Henle. This article describes our method and provides detailed advice. We have published qualitative and quantitative evaluations on the performance of the technique in Sebinger et al. (2010) and Chang and Davies (2012).

Opportunities for organoids as new models of aging

The biology of aging is challenging to study, particularly in humans. As a result, model organisms are used to approximate the physiological context of aging in humans. However, the best model organisms remain expensive and time-consuming to use. More importantly, they may not reflect directly on the process of aging in people. Human cell culture provides an alternative, but many functional signs of aging occur at the level of tissues rather than cells and are therefore not readily apparent in traditional cell culture models. Organoids have the potential to effectively balance between the strengths and weaknesses of traditional models of aging. They have sufficient complexity to capture relevant signs of aging at the molecular, cellular, and tissue levels, while presenting an experimentally tractable alternative to animal studies. Organoid systems have been developed to model many human tissues and diseases. Here we provide a perspective on the potential for organoids to serve as models for aging and describe how current organoid techniques could be applied to aging research.

Asymmetric BMP4 signalling improves the realism of kidney organoids

We present a strategy for increasing the anatomical realism of organoids by applying asymmetric cues to mimic spatial information that is present in natural embryonic development, and demonstrate it using mouse kidney organoids. Existing methods for making kidney organoids in mice yield developing nephrons arranged around a symmetrical collecting duct tree that has no ureter. We use transplant experiments to demonstrate plasticity in the fate choice between collecting duct and ureter, and show that an environment rich in BMP4 promotes differentiation of early collecting ducts into uroplakin-positive, unbranched, ureter-like epithelial tubules. Further, we show that application of BMP4-releasing beads in one place in an organoid can break the symmetry of the system, causing a nearby collecting duct to develop into a uroplakin-positive, broad, unbranched, ureter-like ‘trunk’ from one end of which true collecting duct branches radiate and induce nephron development in an arrangement similar to natural kidneys. The idea of using local symmetry-breaking cues to improve the realism of organoids may have applications to organoid systems other than the kidney.

Adaptive self-organization in the embryo: its importance to adult anatomy and to tissue engineering

The anatomy of healthy humans shows much minor variation, and twin‐studies reveal at least some of this variation cannot be explained genetically. A plausible explanation is that fine‐scale anatomy is not specified directly in a genetic programme, but emerges from self‐organizing behaviours of cells that, for example, place a new capillary where it happens to be needed to prevent local hypoxia. Self‐organizing behaviour can be identified by manipulating growing tissues (e.g. putting them under a spatial constraint) and observing an adaptive change that conserves the character of the normal tissue while altering its precise anatomy. Self‐organization can be practically useful in tissue engineering but it is limited; generally, it is good for producing realistic small‐scale anatomy but large‐scale features will be missing. This is because self‐organizing organoids miss critical symmetry‐breaking influences present in the embryo: simulating these artificially, for example, with local signal sources, makes anatomy realistic even at large scales. A growing understanding of the mechanisms of self‐organization is now allowing synthetic biologists to take their first tentative steps towards constructing artificial multicellular systems that spontaneously organize themselves into patterns, which may soon be extended into three‐dimensional shapes.

Synthetic biology meets tissue engineering

Classical tissue engineering is aimed mainly at producing anatomically and physiologically realistic replacements for normal human tissues. It is done either by encouraging cellular colonization of manufactured matrices or cellular recolonization of decellularized natural extracellular matrices from donor organs, or by allowing cells to self-organize into organs as they do during fetal life. For repair of normal bodies, this will be adequate but there are reasons for making unusual, non-evolved tissues (repair of unusual bodies, interface to electromechanical prostheses, incorporating living cells into life-support machines). Synthetic biology is aimed mainly at engineering cells so that they can perform custom functions: applying synthetic biological approaches to tissue engineering may be one way of engineering custom structures. In this article, we outline the ‘embryological cycle’ of patterning, differentiation and morphogenesis and review progress that has been made in constructing synthetic biological systems to reproduce these processes in new ways. The state-of-the-art remains a long way from making truly synthetic tissues, but there are now at least foundations for future work.

Direct Isolation and Characterization of Human Nephron Progenitors

Mature nephrons originate from a small population of uninduced nephrogenic progenitor cells (NPs) within the cap mesenchyme. These cells are characterized by the coexpression of SIX2 and CITED1. Many studies on mouse models as well as on human pluripotent stem cells have advanced our knowledge of NPs, but very little is known about this population in humans, since it is exhausted before birth and strategies for its direct isolation are still limited. Here we report an efficient protocol for direct isolation of human NPs without genetic manipulation or stepwise induction procedures. With the use of RNA‐labeling probes, we isolated SIX2⁺CITED1⁺ cells from human fetal kidney for the first time. We confirmed their nephrogenic state by gene profiling and evaluated their nephrogenic capabilities in giving rise to mature renal cells. We also evaluated the ability to culture these cells without complete loss of SIX2 and CITED1 expression over time. In addition to defining the gene profile of human NPs, this in vitro system facilitates studies of human renal development and provides a novel tool for renal regeneration and bioengineering purposes. Stem Cells Translational Medicine2017;6:419–433

Modeling renal progenitors - defining the niche

Significant recent advances in methodologies for the differentiation of pluripotent stem cells to renal progenitors as well as the definition of niche conditions for sustaining those progenitors have dramatically enhanced our understanding of their biology and developmental programing, prerequisites for establishing viable approaches to renal regeneration. In this article, we review the evolution of culture techniques and models for the study of metanephric development, describe the signaling mechanisms likely to be driving progenitor self-renewal, and discuss current efforts to generate de novo functional tissues, providing in depth protocols and niche conditions for the stabilization of the nephronic Six2+progenitor.

Concise Review: Understanding the Renal Progenitor Cell Niche In Vivo to Recapitulate Nephrogenesis In Vitro

Chronic kidney disease (CKD), defined as progressive kidney damage and a reduction of the glomerular filtration rate, can progress to end-stage renal failure (CKD5), in which kidney function is completely lost. CKD5 requires dialysis or kidney transplantation, which is limited by the shortage of donor organs. The incidence of CKD5 is increasing annually in the Western world, stimulating an urgent need for new therapies to repair injured kidneys. Many efforts are directed toward regenerative medicine, in particular using stem cells to replace nephrons lost during progression to CKD5. In the present review, we provide an overview of the native nephrogenic niche, describing the complex signals that allow survival and maintenance of undifferentiated renal stem/progenitor cells and the stimuli that promote differentiation. Recapitulating in vitro what normally happens in vivo will be beneficial to guide amplification and direct differentiation of stem cells toward functional renal cells for nephron regeneration.

SIGNIFICANCE

Kidneys perform a plethora of functions essential for life. When their main effector, the nephron, is irreversibly compromised, the only therapeutic choices available are artificial replacement (dialysis) or renal transplantation. Research focusing on alternative treatments includes the use of stem cells. These are immature cells with the potential to mature into renal cells, which could be used to regenerate the kidney. To achieve this aim, many problems must be overcome, such as where to take these cells from, how to obtain enough cells to deliver to patients, and, finally, how to mature stem cells into the cell types normally present in the kidney. In the present report, these questions are discussed. By knowing the factors directing the proliferation and differentiation of renal stem cells normally present in developing kidney, this knowledge can applied to other types of stem cells in the laboratory and use them in the clinic as therapy for the kidney.

Generating a self-organizing kidney from pluripotent cells

PURPOSE OF REVIEW

Recent studies on the directed differentiation of human pluripotent stem cells report tissue self-organization in vitro such that multiple component cell types arise in concert and arrange with respect to each, thereby recapitulating the morphogenetic events typical for that organ. Such self-organization has generated pituitary, optic cup, liver, brain, intestine, stomach and now kidney. Here, we will describe the cell types present within the self-organizing kidney, how these signal to each other to form a kidney organoid and the potential applications of kidney organoids.

RECENT FINDINGS

Protocols for the directed differentiation of human pluripotent cells focus on recapitulating the developmental steps required during embryogenesis. In the case of the kidney, this has involved mesodermal differentiation through posterior primitive streak and intermediate mesoderm. Recent studies have observed the simultaneous formation of both ureteric epithelium and nephron progenitors in vitro. These component cell types signal to each other to initiate nephron formation as would occur during development.

SUMMARY

The generation of kidney organoids is a major advance in nephrology. Such organoids may be useful for disease modelling and drug screening. Ultimately, our capacity to generate organoids may extend to the development of tissues for transplantation.

Self-organized Kidney Rudiments: Prospects for Better in vitro Nephrotoxicity Assays

Kidneys are essential to life but vulnerable to a range of toxicants, including therapeutic drugs and their metabolites. Indeed, nephrotoxicity is often a limiting factor in both drug use and drug development. Most toxicants damage kidneys by one of four mechanisms: damage to the membrane and its junctions, oxidative stress and free radical generation, activation of inflammatory processes, and interference with vascular regulation. Traditionally, animal models were used in preclinical screening for nephrotoxicity, but these can be poorly predictive of human reactions. Animal screens have been joined by simple single-cell–type in vitro assays using primary or immortalized human cells, particularly proximal tubule cells as these are especially vulnerable to toxicants. Recent research, aimed mainly at engineering new kidneys for transplant purposes, has resulted in a method for constructing anatomically realistic mini-kidneys from renogenic stem cells. So far, this has been done only using renogenic stem cells obtained directly from mouse embryos but, in principle, it should be possible to make them from renogenically directed human-induced pluripotent cells. If this can be done, the resulting human-based mini-kidneys would be a promising system for detecting some types of nephrotoxicity and for developing nephroprotective drugs.

Maintenance of Mouse Nephron Progenitor Cells in Aggregates with Gamma-Secretase Inhibitor

Knowledge on how to maintain and expand nephron progenitor cells (NPC) in vitro is important to provide a potentially valuable source for kidney replacement therapies. In our present study, we examined the possibility of optimizing NPC maintenance in the "re-aggregate" system. We found that Six2-expressing (Six2(+))-NPC could be maintained in aggregates reconstituted with dispersed cells from E12.5 mouse embryonic kidneys for at least up to 21 days in culture. The maintenance of Six2(+)-NPC required the presence of ureteric bud cells. The number of Six2(+)-NPC increased by more than 20-fold at day 21, but plateaued after day 14. In an attempt to further sustain NPC proliferation by passage subculture, we found that the new (P1) aggregates reconstituted from the original (P0) aggregates failed to maintain NPC. However, based on the similarity between P1 aggregates and aggregates derived from E15.5 embryonic kidneys, we suspected that the differentiated NPC in P1 aggregates may interfere with NPC maintenance. In support of this notion, we found that preventing NPC differentiation by DAPT, a γ-secretase inhibitor that inhibits Notch signaling pathway, was effective to maintain and expand Six2(+)-NPC in P1 aggregates by up to 65-fold. The Six2(+)-NPC in P1 aggregates retained their potential to epithelialize upon exposure to Wnt signal. In conclusion, we demonstrated in our present study that the "re-aggregation" system can be useful for in vitro maintenance of NPC when combined with γ-secretase inhibitor.

Transport of organic anions and cations in murine embryonic kidney development and in serially-reaggregated engineered kidneys

Recent advances in renal tissue engineering have shown that dissociated, early renogenic tissue from the developing embryo can self-assemble into morphologically accurate kidney-like organs arranged around a central collecting duct tree. In order for such self-assembled kidneys to be useful therapeutically or as models for drug screening, it is necessary to demonstrate that they are functional. One of the main functional characteristics of mature kidneys is transport of organic anions and cations into and out of the proximal tubule. Here, we show that the transport function of embryonic kidneys allowed to develop in culture follows a developmental time-course that is comparable to embryonic kidney development in vivo. We also demonstrate that serially-reaggregated engineered kidneys can transport organic anions and cations through specific uptake and efflux channels. These results support the physiological relevance of kidneys grown in culture, a commonly used model for kidney development and research, and suggest that serially-reaggregated kidneys self-assembled from separated cells have some functional characteristics of intact kidneys.

Cell-cell interactions driving kidney morphogenesis

The mammalian kidney forms via cell-cell interactions between an epithelial outgrowth of the nephric duct and the surrounding nephrogenic mesenchyme. Initial morphogenetic events include ureteric bud branching to form the collecting duct (CD) tree and mesenchymal-to-epithelial transitions to form the nephrons, requiring reciprocal induction between adjacent mesenchyme and epithelial cells. Within the tips of the branching ureteric epithelium, cells respond to mesenchyme-derived trophic factors by proliferation, migration, and mitosis-associated cell dispersal. Self-inhibition signals from one tip to another play a role in branch patterning. The position, survival, and fate of the nephrogenic mesenchyme are regulated by ECM and secreted signals from adjacent tip and stroma. Signals from the ureteric tip promote mesenchyme self-renewal and trigger nephron formation. Subsequent fusion to the CDs, nephron segmentation and maturation, and formation of a patent glomerular basement membrane also require specialized cell-cell interactions. Differential cadherin, laminin, nectin, and integrin expression, as well as intracellular kinesin and actin-mediated regulation of cell shape and adhesion, underlies these cell-cell interactions. Indeed, the capacity for the kidney to form via self-organization has now been established both via the recapitulation of expected morphogenetic interactions after complete dissociation and reassociation of cellular components during development as well as the in vitro formation of 3D kidney organoids from human pluripotent stem cells. As we understand more about how the many cell-cell interactions required for kidney formation operate, this enables the prospect of bioengineering replacement structures based on these self-organizing properties.

Prospect for kidney bioengineering: shortcomings of the status quo

INTRODUCTION

Dialysis and renal transplantation are the only two therapeutic options offered to patients affected by end-stage kidney disease; however, neither treatment can be considered definitive. In fact, dialysis is able to replace only the filtration function of the kidney without substituting its endocrine and metabolic roles, and dramatically impacts on patient's quality of life. On the other hand, kidney transplantation is severely limited by the shortage of transplantable organs, the need for immunosuppressive therapies and a narrow half-life. Regenerative medicine approaches are promising tools aiming to improve this condition.

AREAS COVERED

Cell therapies, bioartificial kidney, organ bioengineering, 3D printer and kidney-on-chip represent the most appealing areas of research for the treatment of end-stage kidney failure. The scope of this review is to summarize the state of the art, limits and directions of each branch.

EXPERT OPINION

In the future, these emerging technologies could provide definitive, curative and theoretically infinite options for the treatment of end-stage kidney disease. Progress in stem cells-based therapies, decellularization techniques and the more recent scientific know-how for the use of the 3D printer and kidney-on-chip could lead to a perfect cellular-based therapy, the futuristic creation of a bioengineered kidney in the lab or to a valid bioartificial alternative.

Node retraction during patterning of the urinary collecting duct system

This report presents a novel mechanism for remodelling a branched epithelial tree. The mouse renal collecting duct develops by growth and repeated branching of an initially unbranched ureteric bud: this mechanism initially produces an almost fractal form with young branches connected to the centre of the kidney via a sequence of nodes (branch points) distributed widely throughout the developing organ. The collecting ducts of a mature kidney have a different form: from the nephrons in the renal cortex, long, straight lengths of collecting duct run almost parallel to one another through the renal medulla, and open together to the renal pelvis. Here we present time-lapse studies of E11.5 kidneys growing in culture: after about 5 days, the collecting duct trees show evidence of ‘node retraction’, in which the node of a ‘Y’-shaped branch moves downwards, shortening the stalk of the ‘Y’, lengthening its arms and narrowing their divergence angle so that the ‘Y’ becomes a ‘V’. Computer simulation suggests that node retraction can transform a spread tree, like that of an early kidney, into one with long, almost-parallel medullary rays similar to those seen in a mature real kidney.

Renal stem cell reprogramming: Prospects in regenerative medicine

Stem cell therapy is a promising future enterprise for renal replacement in patients with acute and chronic kidney disease, conditions which affect millions worldwide and currently require patients to undergo lifelong medical treatments through dialysis and/or organ transplant. Reprogramming differentiated renal cells harvested from the patient back into a pluripotent state would decrease the risk of tissue rejection and provide a virtually unlimited supply of cells for regenerative medicine treatments, making it an exciting area of current research in nephrology. Among the major hurdles that need to be overcome before stem cell therapy for the kidney can be applied in a clinical setting are ensuring the fidelity and relative safety of the reprogrammed cells, as well as achieving feasible efficiency in the reprogramming processes that are utilized. Further, improved knowledge about the genetic control of renal lineage development is vital to identifying predictable and efficient reprogramming approaches, such as the expression of key modulators or the regulation of gene activity through small molecule mimetics. Here, we discuss several recent advances in induced pluripotent stem cell technologies. We also explore strategies that have been successful in renal progenitor generation, and explore what these methods might mean for the development of cell-based regenerative therapies for kidney disease.

Engineered renal tissue as a potential platform for pharmacokinetic and nephrotoxicity testing

Pharmacology and regenerative medicine interact in two ways. One is the use of drugs to promote tissue regeneration. The other, less obvious but with great potential, is the use of techniques developed for regenerative medicine to engineer realistic human organoids for drug screening. This review focuses on testing for nephrotoxicity, often a problem with drugs and poorly predicted in animals. Current human-based screens mainly use proximal tubule cells growing in 2D monolayers. Realism might be improved by collagen-based culture systems that encourage proximal tubule cells to grow as tubules. More realistic would be a recently developed technique for engineering functioning 'mini-kidneys' from suspensions of stem cells, a technique that works in mouse but that could also be applied to humans.

Experimental renal progenitor cells: repairing and recreating kidneys?

Strategies to facilitate repair or generate new nephrons are exciting prospects for acute and chronic human renal disease. Repair of kidney injury involves not just local mechanisms but also mobilisation of progenitor/stem cells from intrarenal niches, including papillary, tubular and glomerular locations. Diverse markers characterise these unique cells, often including CD24 and CD133. Extrarenal stem cells may also contribute to repair, with proposed roles in secreting growth factors, transfer of microvesicles and exosomes and immune modulation. Creating new nephrons from stem cells is beginning to look feasible in mice in which kidneys can be dissociated into single cells and will then generate mature renal structures when recombined. The next step is to identify the correct human markers for progenitor cells from the fetus or mature kidney with similar potential to form new kidneys. Intriguingly, development can continue in vivo: whole foetal kidneys and recombined organs engraft, develop a blood supply and grow when xenotransplanted, and there are new advances in decellularised scaffolds to promote differentiation. This is an exciting time for human kidney repair and regeneration. Many of the approaches and techniques are in their infancy and based on animal rather than human work, but there is a rapid pace of discovery, and we predict that therapies based on advances in this field will come into clinical practice in the next decade.

Engineering kidneys from simple cell suspensions: an exercise in self-organization

Increasing numbers of people approaching and living with end-stage renal disease and failure of the supply of transplantable kidneys to keep pace has created an urgent need for alternative sources of new organs. One possibility is tissue engineering of new organs from stem cells. Adult kidneys are arguably too large and anatomically complex for direct construction, but engineering immature kidneys, transplanting them, and allowing them to mature within the host may be more feasible. In this review, we describe a technique that begins with a suspension of renogenic stem cells and promotes these cells' self-organization into organ rudiments very similar to foetal kidneys, with a collecting duct tree, nephrons, corticomedullary zonation and extended loops of Henle. The engineered rudiments vascularize when transplanted to appropriate vessel-rich sites in bird eggs or adult animals, and show preliminary evidence for physiological function. We hope that this approach might one day be the basis of a clinically useful technique for renal replacement therapy.