Mature induced-pluripotent-stem-cell-derived human podocytes reconstitute kidney glomerular-capillary-wall function on a chip. Nat Biomed Eng. 2017;1. [PMID: 29038743 PMCID: PMC5639718 DOI: 10.1038/s41551-017-0069]

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.

Application of physiological shear stress to renal tubular epithelial cells

Renal tubular epithelial cells are consistently exposed to flow of glomerular filtrate that creates fluid shear stress at the apical cell surface. This biophysical stimulus regulates several critical renal epithelial cell functions, including transport, protein uptake, and barrier function. Defining the in vivo mechanical conditions in the kidney tubule is important for accurately recapitulating these conditions in vitro. Here we provide a summary of the fluid flow conditions in the kidney and how this translates into different levels of fluid shear stress down the length of the nephron. A detailed method is provided for measuring fluid flow in the proximal tubule by intravital microscopy. Devices to mimic in vivo fluid shear stress for in vitro studies are discussed, and we present two methods for culture and analysis of renal tubule epithelial cells exposed physiological levels of fluid shear stress. The first is a microfluidic device that permits application of controlled shear stress to cells cultured on porous membranes. The second is culture of renal tubule cells on an orbital shaker. Each method has advantages and disadvantages that should be considered in the context of the specific experimental objectives.

Measuring and regulating oxygen levels in microphysiological systems: design, material, and sensor considerations

As microfabrication techniques and tissue engineering methods improve, microphysiological systems (MPS) are being engineered that recapitulate complex physiological and pathophysiological states to supplement and challenge traditional animal models. Although MPS provide unique microenvironments that transcend common 2D cell culture, without proper regulation of oxygen content, MPS often fail to provide the biomimetic environment necessary to activate and investigate fundamental pathways of cellular metabolism and sub-cellular level. Oxygen exists in the human body in various concentrations and partial pressures; moreover, it fluctuates dramatically depending on fasting, exercise, and sleep patterns. Regulating oxygen content inside MPS necessitates a sensitive biological sensor to quantify oxygen content in real-time. Measuring oxygen in a microdevice is a non-trivial requirement for studies focused on understanding how oxygen impacts cellular processes, including angiogenesis and tumorigenesis. Quantifying oxygen inside a microdevice can be achieved via an array of technologies, with each method having benefits and limitations in terms of sensitivity, limits of detection, and invasiveness that must be considered and optimized. This article will review oxygen physiology in organ systems and offer comparisons of organ-specific MPS that do and do not consider oxygen microenvironments. Materials used in microphysiological models will also be analyzed in terms of their ability to control oxygen. Finally, oxygen sensor technologies are critically compared and evaluated for use in MPS.

Directed differentiation of human induced pluripotent stem cells into mature kidney podocytes and establishment of a Glomerulus Chip

Protocols have been established to direct the differentiation of human induced pluripotent stem (iPS) cells into nephron progenitor cells and organoids containing many types of kidney cells, but it has been difficult to direct the differentiation of iPS cells to form specific types of mature human kidney cells with high yield. Here, we describe a detailed protocol for the directed differentiation of human iPS cells into mature, post-mitotic kidney glomerular podocytes with high (>90%) efficiency within 26 d and under chemically defined conditions, without genetic manipulations or subpopulation selection. We also describe how these iPS cell-derived podocytes may be induced to form within a microfluidic organ-on-a-chip (Organ Chip) culture device to build a human kidney Glomerulus Chip that mimics the structure and function of the kidney glomerular capillary wall in vitro within 35 d (starting with undifferentiated iPS cells). The podocyte differentiation protocol requires skills for culturing iPS cells, and the development of a Glomerulus Chip requires some experience with building and operating microfluidic cell culture systems. This method could be useful for applications in nephrotoxicity screening, therapeutic development, and regenerative medicine, as well as mechanistic study of kidney development and disease.

Fine tuning the extracellular environment accelerates the derivation of kidney organoids from human pluripotent stem cells

The generation of organoids is one of the biggest scientific advances in regenerative medicine. Here, by lengthening the time that human pluripotent stem cells (hPSCs) were exposed to a three-dimensional microenvironment, and by applying defined renal inductive signals, we generated kidney organoids that transcriptomically matched second-trimester human fetal kidneys. We validated these results using ex vivo and in vitro assays that model renal development. Furthermore, we developed a transplantation method that utilizes the chick chorioallantoic membrane. This approach created a soft in vivo microenvironment that promoted the growth and differentiation of implanted kidney organoids, as well as providing a vascular component. The stiffness of the in ovo chorioallantoic membrane microenvironment was recapitulated in vitro by fabricating compliant hydrogels. These biomaterials promoted the efficient generation of renal vesicles and nephron structures, demonstrating that a soft environment accelerates the differentiation of hPSC-derived kidney organoids.

Directed Differentiation of Human Pluripotent Stem Cells to Podocytes under Defined Conditions

A major cause of chronic kidney disease (CKD) is glomerular disease, which can be attributed to a spectrum of podocyte disorders. Podocytes are non-proliferative, terminally differentiated cells. Thus, the limited supply of primary podocytes impedes CKD research. Differentiation of human pluripotent stem cells (hPSCs) into podocytes has the potential to produce podocytes for disease modeling, drug screening, and cell therapies. In the podocyte differentiation process described here, hPSCs are first induced to primitive streak-like cells by activating canonical Wnt signaling. Next, these cells progress to mesoderm precursors, proliferative nephron progenitors, and eventually become mature podocytes by culturing in a serum-free medium. Podocytes generated via this protocol adopt podocyte morphology, express canonical podocyte markers, and exhibit podocyte phenotypes, including albumin uptake and TGF-β1 triggered cell death. This study provides a simple, defined strategy to generate podocytes for in vitro modeling of podocyte development and disease or for cell therapies.

Engineering renal epithelial cells: programming and directed differentiation towards glomerular podocyte's progenitor and mature podocyte

Current knowledge of normal developmental physiology and identification of specific cell types of the kidney at molecular levels enables us to generate various cells of the kidney. The generation of renal specialized cells in vitro with its correct molecular and functional implications is the urgent need for cellular therapy in chronic kidney diseases and for organ formation. Glomerular podocytes are one of the major renal cells lose its functionality to maintain glomerular blood filtration function. In vitro, many inductions or reprogramming methods have been established for podocytes development. In these methods transcription factors, small molecules, and growth factors play the major role to remodel stem cells into podocyte progenitors and towards mature podocytes. Micro ribonucleic acids (miRNAs) have been utilizing as another strategy to generate podocyte. In this review, current protocols for in vitro glomerular podocyte differentiation have summarized emphasizing programming methods, signaling modulation, and cytoskeletal changes. Novel ideas are also pointed out, which are required for efficient optimal glomerular podocyte generation and their functional characterization in vitro with nanoarchitecture impression of the glomerular basement membrane.

Progress, obstacles, and limitations in the use of stem cells in organ-on-a-chip models

In recent years, drug development costs have soared, primarily due to the failure of preclinical animal and cell culture models, which do not directly translate to human physiology. Organ-on-a-chip (OOC) is a burgeoning technology with the potential to revolutionize disease modeling, drug discovery, and toxicology research by strengthening the relevance of culture-based models while reducing costly animal studies. Although OOC models can incorporate a variety of tissue sources, the most robust and relevant OOC models going forward will include stem cells. In this review, we will highlight the benefits of stem cells as a tissue source while considering current limitations to their complete and effective implementation into OOC models.

Modelling cancer in microfluidic human organs-on-chips

One of the problems that has slowed the development and approval of new anticancer therapies is the lack of preclinical models that can be used to identify key molecular, cellular and biophysical features of human cancer progression. This is because most in vitro cancer models fail to faithfully recapitulate the local tissue and organ microenvironment in which tumours form, which substantially contributes to the complex pathophysiology of the disease. More complex in vitro cancer models have been developed, including transwell cell cultures, spheroids and organoids grown within flexible extracellular matrix gels, which better mimic normal and cancerous tissue development than cells maintained on conventional 2D substrates. But these models still lack the tissue-tissue interfaces, organ-level structures, fluid flows and mechanical cues that cells experience within living organs, and furthermore, it is difficult to collect samples from the different tissue microcompartments. In this Review, we outline how recent developments in microfluidic cell culture technology have led to the generation of human organs-on-chips (also known as organ chips) that are now being used to model cancer cell behaviour within human-relevant tissue and organ microenvironments in vitro. Organ chips enable experimentalists to vary local cellular, molecular, chemical and biophysical parameters in a controlled manner, both individually and in precise combinations, while analysing how they contribute to human cancer formation and progression and responses to therapy. We also discuss the challenges that must be overcome to ensure that organ chip models meet the needs of cancer researchers, drug developers and clinicians interested in personalized medicine.

Organ-on-a-Chip for Cancer and Immune Organs Modeling

Bridging the gap between findings in preclinical 2D cell culture models and in vivo tissue cultures has been challenging; the simple microenvironment of 2D monolayer culture systems may not capture the cellular response to drugs accurately. Three-dimensional organotypic models have gained increasing interest due to their ability to recreate precise cellular organizations. These models facilitate investigation of the interactions between different sub-tissue level components through providing physiologically relevant microenvironments for cells in vitro. The incorporation of human-sourced tissues into these models further enables personalized prediction of drug responses. Integration of microfluidic units into the 3D models can be used to control their local environment, dynamic simulation of cell behaviors, and real-time readout of drug testing data. Cancer and immune system related diseases are severe burdens to our health care system and have created an urgent need for high-throughput, and effective drug development plans. This review focuses on recent progress in the development of "cancer-on-a-chip" and "immune organs-on-a-chip" systems designed to study disease progression and predict drug-induced responses. Future challenges and opportunities are also discussed.

Modeling organ-specific vasculature with organ-on-a-chip devices

Organ-on-a-chip devices, also known as microphysiological systems, have gained significant attention in recent years. Recent advances in tissue engineering and microfabrication have enabled these devices to provide more precise control over cellular microenvironments to mimic the tissue-level or organ-level function of the human body. These more complex tissue models can provide either an improvement in the functional expression and maturation of cells or an avenue to probe biological events and function that would otherwise be difficult to visualize and mechanistically study. This high-value information, when complimented with the existing gold-standards of cell-based assays and animal models, could potentially lead to more informed decision-making in drug development. A prevalent biological component in many organ-on-a-chip devices is an engineered vascular interface that is present in almost all organs of the human body. The vasculature and the vascular interface are particularly susceptible to biomechanical forces, they function as the conduits for inter-cellular and inter-organ interactions, and regulate drug transport. In this review, we examine the various approaches taken to model the human vasculature with an emphasis on the engineering of organ-specific vasculatures, and discuss various challenges and opportunities ahead as the field advances.

Manipulation of Nephron-Patterning Signals Enables Selective Induction of Podocytes from Human Pluripotent Stem Cells

BACKGROUND

Previous research has elucidated the signals required to induce nephron progenitor cells (NPCs) from pluripotent stem cells (PSCs), enabling the generation of kidney organoids. However, selectively controlling differentiation of NPCs to podocytes has been a challenge.

METHODS

We investigated the effects of various growth factors in cultured mouse embryonic NPCs during three distinct steps of nephron patterning: from NPC to pretubular aggregate, from the latter to epithelial renal vesicle (RV), and from RV to podocyte. We then applied the findings to human PSC-derived NPCs to establish a method for selective induction of human podocytes.

RESULTS

Mouse NPC differentiation experiments revealed that phase-specific manipulation of Wnt and Tgf-β signaling is critical for podocyte differentiation. First, optimal timing and intensity of Wnt signaling were essential for mesenchymal-to-epithelial transition and podocyte differentiation. Then, inhibition of Tgf-β signaling supported domination of the RV proximal domain. Inhibition of Tgf-β signaling in the third phase enriched the podocyte fraction by suppressing development of other nephron lineages. The resultant protocol enabled successful induction of human podocytes from PSCs with >90% purity. The induced podocytes exhibited global gene expression signatures comparable to those of adult human podocytes, had podocyte morphologic features (including foot process-like and slit diaphragm-like structures), and showed functional responsiveness to drug-induced injury.

CONCLUSIONS

Elucidation of signals that induce podocytes during the nephron-patterning process enabled us to establish a highly efficient method for selective induction of human podocytes from PSCs. These PSC-derived podocytes show molecular, morphologic, and functional characteristics of podocytes, and offer a new resource for disease modeling and nephrotoxicity testing.

A Protocol for One-Step Differentiation of Human Induced Pluripotent Stem Cells into Mature Podocytes

Within the glomerulus, podocytes are highly specialized visceral epithelial cells that are part of the glomerular filtration barrier. Human podocyte cell culture is rather challenging for primary or immortalized cells, due to the nonproliferative state of the cells. In addition, rapid dedifferentiation is often observed. Hence, iPSC-derived podocytes offer an exciting alternative to culture podocyte-like cells from different donors over prolonged time. Here we report a simple and rapid one-step protocol that drives iPSC into podocyte-like cells in 10 days.

3D organoid-derived human glomeruli for personalised podocyte disease modelling and drug screening

The podocytes within the glomeruli of the kidney maintain the filtration barrier by forming interdigitating foot processes with intervening slit diaphragms, disruption in which results in proteinuria. Studies into human podocytopathies to date have employed primary or immortalised podocyte cell lines cultured in 2D. Here we compare 3D human glomeruli sieved from induced pluripotent stem cell-derived kidney organoids with conditionally immortalised human podocyte cell lines, revealing improved podocyte-specific gene expression, maintenance in vitro of polarised protein localisation and an improved glomerular basement membrane matrisome compared to 2D cultures. Organoid-derived glomeruli retain marker expression in culture for 96 h, proving amenable to toxicity screening. In addition, 3D organoid glomeruli from a congenital nephrotic syndrome patient with compound heterozygous NPHS1 mutations reveal reduced protein levels of both NEPHRIN and PODOCIN. Hence, human iPSC-derived organoid glomeruli represent an accessible approach to the in vitro modelling of human podocytopathies and screening for podocyte toxicity.

Studies examining human podocytopathies have utilised 2D cultured primary or immortalised podocyte cell lines. Here, the authors demonstrate that 3D human glomeruli sieved from induced pluripotent stem cell-derived kidney organoids retain an improved podocyte identity in vitro facilitating disease modelling and toxicity testing.

Application of chemical reaction engineering principles to 'body-on-a-chip' systems

The combination of cell culture models with microscale technology has fostered emergence of in vitro cell-based microphysiological models, also known as organ-on-a-chip systems. Body-on-a-chip systems, which are multi-organ systems on a chip to mimic physiological relations, enable recapitulation of organ-organ interactions and potentially whole-body response to drugs, as well as serve as models of diseases. Chemical reaction engineering principles can be applied to understanding complex reactions inside the cell or human body, which can be treated as a multi-reactor system. These systems use physiologically-based pharmacokinetic (PBPK) models to guide the development of microscale systems of the body where organs or tissues are represented by living cells or tissues, and integrated into body-on-a-chip systems. Here, we provide a brief overview on the concept of chemical reaction engineering and how its principles can be applied to understanding and predicting the behavior of body-on-a-chip systems.

Programming Niche Accessibility and In Vitro Stemness with Intercellular DNA Reactions

Stem cells generally exist in low abundance and tend to lose stemness in the absence of self-renewal signals. While extracellular-matrix-mimicking techniques have been developed to support stem cell proliferation, the lack of niche cells in these synthetic systems often hampers continuous stem cell expansion and maintenance of pluripotency, which are indispensable for regenerative medicine. Here, an intercellular DNA-reaction-programmed ESPN (expansion of stem cells with pairing niches) strategy is developed for 3D culture of mammary stem cells (MaSCs). Boolean logic operations are implemented to confer DNA-programmed mechanical signaling and genetically engineered morphogen signaling by niche cells, resulting in sustained expansion of MaSCs in vitro. The creation of stem cell niches improves the proliferation of pluripotent cells by four times during one-week culture. This method thus provides a novel approach for logical regulation of stemness and proliferation of stem cells for biomedicine.

Glomerular endothelial cells versus podocytes as the cellular target in diabetic nephropathy

It usually takes several years (in some cases, decades) for predisposed individuals to move from the onset of type 1 or type 2 diabetes to the development of microalbuminuria, the first sign of diabetic nephropathy. This long, complication-free, period represents the best possible moment to start a successful preventive strategy (primary prevention) aimed to avoid or at least to postpone the increase of albumin excretion rate. Prevention is based on understanding and counteracting the initial mechanisms leading to the development of the disease and unfortunately, in case of diabetic nephropathy, most of them remain unclear. Little is also known about which, among endothelial cells and podocytes, represent the first glomerular target of the complication. Selective damage of the endothelium or of the podocyte results, as a common consequence, in an increase of albumin excretion rate. Albuminuria by itself cannot therefore be of help to solve the case. Endothelium and podocytes are involved in a continuous cross-talk and by studying the impact of diabetes on this "communication" process it should be possible to obtain some information regarding the weak component of the glomerular filter. Finally, the careful investigation of the mechanisms leading to the development podocyturia, a recently identified glomerular dysfunction associated to the pathogenesis of diabetic nephropathy, could contribute to shed some more light on the very early stages of this complication.

Kidney-on-a-chip: untapped opportunities

The organs-on-a-chip technology has shown strong promise in mimicking the complexity of native tissues in vitro and ex vivo, and recently significant advances have been made in applying this technology to studies of the kidney and its diseases. Individual components of the nephron, including the glomerulus, proximal tubule, and distal tubule/medullary collecting duct, have been successfully mimicked using organs-on-a-chip technology and yielding strong promises in advancing the field of ex vivo drug toxicity testing and augmenting renal replacement therapies. Although these models show promise over 2-dimensional cell systems in recapitulating important nephron features in vitro, nephron functions, such as tubular secretion, intracellular metabolism, and renin and vitamin D production, as well as prostaglandin synthesis are still poorly recapitulated in on-chip models. Moreover, construction of multiple-renal-components-on-a-chip models, in which various structures and cells of the renal system interact with each other, has remained a challenge. Overall, on-chip models show promise in advancing models of normal and pathological renal physiology, in predicting nephrotoxicity, and in advancing treatment of chronic kidney diseases.

Organs-on-a-Chip Module: A Review from the Development and Applications Perspective

In recent years, ever-increasing scientific knowledge and modern high-tech advancements in micro- and nano-scales fabrication technologies have impacted significantly on various scientific fields. A micro-level approach so-called "microfluidic technology" has rapidly evolved as a powerful tool for numerous applications with special reference to bioengineering and biomedical engineering research. Therefore, a transformative effect has been felt, for instance, in biological sample handling, analyte sensing cell-based assay, tissue engineering, molecular diagnostics, and drug screening, etc. Besides such huge multi-functional potentialities, microfluidic technology also offers the opportunity to mimic different organs to address the complexity of animal-based testing models effectively. The combination of fluid physics along with three-dimensional (3-D) cell compartmentalization has sustained popularity as organ-on-a-chip. In this context, simple humanoid model systems which are important for a wide range of research fields rely on the development of a microfluidic system. The basic idea is to provide an artificial testing subject that resembles the human body in every aspect. For instance, drug testing in the pharma industry is crucial to assure proper function. Development of microfluidic-based technology bridges the gap between in vitro and in vivo models offering new approaches to research in medicine, biology, and pharmacology, among others. This is also because microfluidic-based 3-D niche has enormous potential to accommodate cells/tissues to create a physiologically relevant environment, thus, bridge/fill in the gap between extensively studied animal models and human-based clinical trials. This review highlights principles, fabrication techniques, and recent progress of organs-on-chip research. Herein, we also point out some opportunities for microfluidic technology in the future research which is still infancy to accurately design, address and mimic the in vivo niche.

Scalable Fabrication of Stretchable, Dual Channel, Microfluidic Organ Chips

A significant number of lead compounds fail in the pharmaceutical pipeline because animal studies often fail to predict clinical responses in human patients. Human Organ-on-a-Chip (Organ Chip) microfluidic cell culture devices, which provide an experimental in vitro platform to assess efficacy, toxicity, and pharmacokinetic (PK) profiles in humans, may be better predictors of therapeutic efficacy and safety in the clinic compared to animal studies. These devices may be used to model the function of virtually any organ type and can be fluidically linked through common endothelium-lined microchannels to perform in vitro studies on human organ-level and whole body-level physiology without having to conduct experiments on people. These Organ Chips consist of two perfused microfluidic channels separated by a permeable elastomeric membrane with organ-specific parenchymal cells on one side and microvascular endothelium on the other, which can be cyclically stretched to provide organ-specific mechanical cues (e.g., breathing motions in lung). This protocol details the fabrication of flexible, dual channel, Organ Chips through casting of parts using 3D printed molds, enabling combination of multiple casting and post-processing steps. Porous poly (dimethyl siloxane) (PDMS) membranes are cast with micrometer sized through-holes using silicon pillar arrays under compression. Fabrication and assembly of Organ Chips involves equipment and steps that can be implemented outside of a traditional cleanroom. This protocol provides researchers with access to Organ Chip technology for in vitro organ- and body-level studies in drug discovery, safety and efficacy testing, as well as mechanistic studies of fundamental biological processes.

Recent Progress in the Development of Microfluidic Vascular Models

The blood vessel is part of the circulatory system, and systemic circulation provides the blood supply to all tissues. Arteries are pathways through which the blood is carried, and the capillaries have a key role in material exchange to maintain the tissue environment. Blood vessels have structures appropriate for their functions, and their sizes and cell types are different. In this review, we introduced recent studies of the microfluidic vascular models. The model structures are classified mainly as poly(dimethylsiloxane) and hydrogel microchannels and self-assembled networks. Basic phenomena and functions were realized in vascular models, including fluid shear stress, cell strain, interstitial flow, endothelial permeation, angiogenesis, and thrombosis. In some models, endothelial cells were co-cultured with smooth muscle cells, pericytes, and fibroblasts in an extracellular matrix. Examples of vascular models involving the brain, lung, liver, kidney, placenta, and cancer were also introduced.

Curvature facilitates podocyte culture in a biomimetic platform

Most kidney diseases begin with abnormalities in glomerular podocytes, motivating the need for podocyte models to study pathophysiological mechanisms and new treatment options. However, podocytes cultured in vitro face a limited ability to maintain appreciable extents of differentiation hallmarks, raising concerns over the relevance of study results. Many key properties such as nephrin expression and morphology reach plateaus that are far from the in vivo levels. Here, we demonstrate that a biomimetic topography, consisting of microhemispheres arrayed over the cell culture substrate, promotes podocyte differentiation in vitro. We define new methods for fabricating microscale curvature on various substrates, including a thin porous membrane. By growing podocytes on our topographic substrates, we found that these biophysical cues augmented nephrin gene expression, supported full-size nephrin protein expression, encouraged structural arrangement of F-actin and nephrin within the cell, and promoted process formation and even interdigitation compared to the flat substrates. Furthermore, the topography facilitated nephrin localization on curved structures while nuclei lay in the valleys between them. The improved differentiation was also evidenced by tracking barrier function to albumin over time using our custom topomembranes. Overall, our work presents accessible methods for incorporating microcurvature on various common substrates, and demonstrates the importance of biophysical stimulation in supporting higher-fidelity podocyte cultivation in vitro.

Modeling Hematological Diseases and Cancer With Patient-Specific Induced Pluripotent Stem Cells

The advent of induced pluripotent stem cells (iPSCs) together with recent advances in genome editing, microphysiological systems, tissue engineering and xenograft models present new opportunities for the investigation of hematological diseases and cancer in a patient-specific context. Here we review the progress in the field and discuss the advantages, limitations, and challenges of iPSC-based malignancy modeling. We will also discuss the use of iPSCs and its derivatives as cellular sources for drug target identification, drug development and evaluation of pharmacological responses.

From mechanobiology to developmentally inspired engineering

The field of mechanobiology emerged based on the recognition of the central role that physical forces play in development and physiology. In this article, which is based on a lecture I presented at the 2018 Royal Society meeting on Mechanics of Development, I review work from my laboratory carried out over the 40 years which helped to birth this field. I will also describe how we are leveraging the fundamental design principles that govern mechanoregulation to develop new experimental tools and organ-engineering approaches as well as novel mechanotherapeutics.This article is part of the Theo Murphy meeting issue 'Mechanics of development'.

Recapitulating kidney development: Progress and challenges

Decades of research into the molecular and cellular regulation of kidney morphogenesis in rodent models, particularly the mouse, has provided both an atlas of the mammalian kidney and a roadmap for recreating kidney cell types with potential applications for the treatment of kidney disease. With advances in both our capacity to maintain nephron progenitors in culture, reprogram to kidney cell types and direct the differentiation of human pluripotent stem cells to kidney endpoints, renal regeneration via cellular therapy or tissue engineering may be possible. Human kidney models also have potential for disease modelling and drug screening. Such applications will rely upon the accuracy of the model at the cellular level and the capacity for stem-cell derived kidney tissue to recapitulate both normal and diseased kidney tissue. In this review, we will discuss the available cell sources, how well they model the human kidney and how far we are from application either as models or for tissue engineering.

Differentiation of human iPSCs into functional podocytes

Podocytes play a critical role in glomerular barrier function, both in health and disease. However, in vivo terminally differentiated podocytes are difficult to be maintained in in vitro culture. Induced pluripotent stem cells (iPSCs) offer the unique possibility for directed differentiation into mature podocytes. The current differentiation protocol to generate iPSC-derived podocyte-like cells provides a robust and reproducible method to obtain podocyte-like cells after 10 days that can be employed in in vitro research and biomedical engineering. Previous published protocols were improved by testing varying differentiation media, growth factors, seeding densities, and time course conditions. Modifications were made to optimize and simplify the one-step differentiation procedure. In contrast to earlier protocols, adherent cells for differentiation were used, the use of fetal bovine serum (FBS) was reduced to a minimum, and thus ß-mercaptoethanol could be omitted. The plating densities of iPSC stocks as well as the seeding densities for differentiation cultures turned out to be a crucial parameter for differentiation results. Conditionally immortalized human podocytes served as reference controls. iPSC-derived podocyte-like cells showed a typical podocyte-specific morphology and distinct expression of podocyte markers synaptopodin, podocin, nephrin and WT-1 after 10 days of differentiation as assessed by immunofluorescence staining or Western blot analysis. qPCR results showed a downregulation of pluripotency markers Oct4 and Sox-2 and a 9-fold upregulation of the podocyte marker synaptopodin during the time course of differentiation. Cultured podocytes exhibited endocytotic uptake of albumin. In toxicological assays, matured podocytes clearly responded to doxorubicin (Adriamycin^™) with morphological alterations and a reduction in cell viability after 48 h of incubation.

Proteinuric Kidney Diseases: A Podocyte's Slit Diaphragm and Cytoskeleton Approach

Proteinuric kidney diseases are a group of disorders with diverse pathological mechanisms associated with significant losses of protein in the urine. The glomerular filtration barrier (GFB), comprised of the three important layers, the fenestrated glomerular endothelium, the glomerular basement membrane (GBM), and the podocyte, dictates that disruption of any one of these structures should lead to proteinuric disease. Podocytes, in particular, have long been considered as the final gatekeeper of the GFB. This specialized visceral epithelial cell contains a complex framework of cytoskeletons forming foot processes and mediate important cell signaling to maintain podocyte health. In this review, we will focus on slit diaphragm proteins such as nephrin, podocin, TRPC6/5, as well as cytoskeletal proteins Rho/small GTPases and synaptopodin and their respective roles in participating in the pathogenesis of proteinuric kidney diseases. Furthermore, we will summarize the potential therapeutic options targeting the podocyte to treat this group of kidney diseases.

Emerging Kidney Models to Investigate Metabolism, Transport, and Toxicity of Drugs and Xenobiotics

The kidney is a major clearance organ of the body and is responsible for the elimination of many xenobiotics and prescription drugs. With its multitude of uptake and efflux transporters and metabolizing enzymes, the proximal tubule cell (PTC) in the nephron plays a key role in the disposition of xenobiotics and is also a primary site for toxicity. In this minireview, we first provide an overview of the major transporters and metabolizing enzymes in the PTCs responsible for biotransformation and disposition of drugs. Next, we discuss different cell sources that have been used to model PTCs in vitro, their pros and cons, and their characterization. As current technology is inadequate to evaluate reliably drug disposition and toxicity in the kidney, we then discuss recent advancements in kidney microphysiological systems (MPS) and the need to develop robust in vitro platforms that could be routinely used by pharmaceutical companies to screen compounds. Finally, we discuss the new and exciting field of stem cell-derived kidney models as potential cell sources for future kidney MPS. Given the push from both regulatory agencies and pharmaceutical companies to use more predictive "human-like" in vitro systems in the early stages of drug development to reduce attrition, these emerging models have the potential to be a game changer and may revolutionize how renal disposition and kidney toxicity in drug discovery are evaluated in the future.

A CRISP(e)R view on kidney organoids allows generation of an induced pluripotent stem cell-derived kidney model for drug discovery

Development of physiologically relevant cellular models with strong translatability to human pathophysiology is critical for identification and validation of novel therapeutic targets. Herein we describe a detailed protocol for generation of an advanced 3-dimensional kidney cellular model using induced pluripotent stem cells, where differentiation and maturation of kidney progenitors and podocytes can be monitored in live cells due to CRISPR/Cas9-mediated fluorescent tagging of kidney lineage markers (SIX2 and NPHS1). Utilizing these cell lines, we have refined the previously published procedures to generate a new, higher throughput protocol suitable for drug discovery. Using paraffin-embedded sectioning and whole-mount immunostaining, we demonstrated that organoids grown in suspension culture express key markers of kidney biology (WT1, ECAD, LTL, nephrin) and vasculature (CD31) within renal cortical structures with microvilli, tight junctions and podocyte foot processes visualized by electron microscopy. Additionally, the organoids resemble the adult kidney transcriptomics profile, thereby strengthening the translatability of our in vitro model. Thus, development of human nephron-like structures in vitro fills a major gap in our ability to assess the effect of potential treatment on key kidney structures, opening up a wide range of possibilities to improve clinical translation.

Screening of Drug-Transporter Interactions in a 3D Microfluidic Renal Proximal Tubule on a Chip

Drug-transporter interactions could impact renal drug clearance and should ideally be detected in early stages of drug development to avoid toxicity-related withdrawals in later stages. This requires reliable and robust assays for which current high-throughput screenings have, however, poor predictability. Kidney-on-a-chip platforms have the potential to improve predictability, but often lack compatibility with high-content detection platforms. Here, we combined conditionally immortalized proximal tubule epithelial cells overexpressing organic anion transporter 1 (ciPTEC-OAT1) with the microfluidic titer plate OrganoPlate to develop a screenings assay for renal drug-transporter interactions. In this platform, apical localization of F-actin and intracellular tight-junction protein zonula occludens-1 (ZO-1) indicated appropriate cell polarization. Gene expression levels of the drug transporters organic anion transporter 1 (OAT1; SLC22A6), organic cation transporter 2 (OCT2; SLC22A2), P-glycoprotein (P-gp; ABCB1), and multidrug resistance-associated protein 2 and 4 (MRP2/4; ABCC2/4) were similar levels to 2D static cultures. Functionality of the efflux transporters P-gp and MRP2/4 was studied as proof-of-concept for 3D assays using calcein-AM and 5-chloromethylfluorescein-diacetate (CMFDA), respectively. Confocal imaging demonstrated a 4.4 ± 0.2-fold increase in calcein accumulation upon P-gp inhibition using PSC833. For MRP2/4, a 3.0 ± 0.2-fold increased accumulation of glutathione-methylfluorescein (GS-MF) was observed upon inhibition with a combination of PSC833, MK571, and KO143. Semi-quantitative image processing methods for P-gp and MRP2/4 was demonstrated with corresponding Z'-factors of 0.1 ± 0.3 and 0.4 ± 0.1, respectively. In conclusion, we demonstrate a 3D microfluidic PTEC model valuable for screening of drug-transporter interactions that further allows multiplexing of endpoint read-outs for drug-transporter interactions and toxicity.

Human Microphysiological Systems and Organoids as in Vitro Models for Toxicological Studies

Organoids and microphysiological systems represent two current approaches to reproduce organ function in vitro. These systems can potentially provide unbiased assays of function which are needed to understand the mechanism of action of environmental toxins. Culture models that replicate organ function and interactions among cell types and tissues move beyond existing screens that target individual pathways and provide a means to assay context-dependent function. The current state of organoid cultures and microphysiological systems is reviewed and applications discussed. While few studies have examined environmental pollutants, studies with drugs demonstrate the power of these systems to assess toxicity as well as mechanism of action. Strengths and limitations of organoids and microphysiological systems are reviewed and challenges are identified to produce suitable high capacity functional assays.

Regenerating the kidney using human pluripotent stem cells and renal progenitors

INTRODUCTION

Chronic kidney disease is a major health-care problem worldwide and its cost is becoming no longer affordable. Indeed, restoring damaged renal structures or building a new kidney represents an ambitious and ideal alternative to renal replacement therapy. Streams of research have explored the possible application of pluripotent stem cells (SCs) (embryonic SCs and induced pluripotent SCs) in different strategies aimed at regenerate functioning nephrons and at understanding the mechanisms of kidney regeneration.

AREAS COVERED

In this review, we will focus on the main potential applications of human pluripotent SCs to kidney regeneration, including those leading to rebuilding new kidneys or part of them (organoids, scaffolds, biological microdevices) as well as those aimed at understanding the pathophysiological mechanisms of renal disease and regenerative processes (modeling of kidney disease, genome editing). Moreover, we will discuss the role of endogenous renal progenitors cells in order to understand and promote kidney regeneration, as an attractive alternative to pluripotent SCs.

EXPERT OPINION

Opportunities and pitfalls of all these strategies will be underlined, finally leading to the conclusion that a deeper knowledge of the biology of pluripotent SCs is mandatory, in order to allow us to hypothesize their clinical application.

Engineered Kidney Tubules for Modeling Patient-Specific Diseases and Drug Discovery

The lack of engineering systems able to faithfully reproduce complex kidney structures in vitro has made it difficult to efficiently model kidney diseases and development. Using polydimethylsiloxane (PDMS) scaffolds and a kidney-derived cell line we developed a system to rapidly engineer custom-made 3D tubules with typical renal epithelial properties. This system was successfully employed to engineer patient-specific tubules, to model polycystic kidney disease (PKD) and test drug efficacy, and to identify a potential new pharmacological treatment. By optimizing our system we constructed functional ureteric bud (UB)-like tubules from human induced pluripotent stem cells (iPSCs), and identified a combination of growth factors that induces budding morphogenesis like embryonic kidneys do. Finally, we applied this assay to investigate budding defects in UB-like tubules derived from a patient with a PAX2 mutation. Our system enables the modeling of human kidney disease and development, drug testing and discovery, and lays the groundwork for engineering anatomically correct kidney tissues in vitro and developing personalized medicine applications.

Use of porous membranes in tissue barrier and co-culture models

Porous membranes enable the partitioning of cellular microenvironments in vitro, while still allowing physical and biochemical crosstalk between cells, a feature that is often necessary for recapitulating physiological functions. This article provides an overview of the different membranes used in tissue barrier and cellular co-culture models with a focus on experimental design and control of these systems. Specifically, we discuss how the structural, mechanical, chemical, and even the optical and transport properties of different membranes bestow specific advantages and disadvantages through the context of physiological relevance. This review also explores how membrane pore properties affect perfusion and solute permeability by developing an analytical framework to guide the design and use of tissue barrier or co-culture models. Ultimately, this review offers insight into the important aspects one must consider when using porous membranes in tissue barrier and lab-on-a-chip applications.

Advances in predictive in vitro models of drug-induced nephrotoxicity

In vitro screens for nephrotoxicity are currently poorly predictive of toxicity in humans. Although the functional proteins that are expressed by nephron tubules and mediate drug susceptibility are well known, current in vitro cellular models poorly replicate both the morphology and the function of kidney tubules and therefore fail to demonstrate injury responses to drugs that would be nephrotoxic in vivo. Advances in protocols to enable the directed differentiation of pluripotent stem cells into multiple renal cell types and the development of microfluidic and 3D culture systems have opened a range of potential new platforms for evaluating drug nephrotoxicity. Many of the new in vitro culture systems have been characterized by the expression and function of transporters, enzymes, and other functional proteins that are expressed by the kidney and have been implicated in drug-induced renal injury. In vitro platforms that express these proteins and exhibit molecular biomarkers that have been used as readouts of injury demonstrate improved functional maturity compared with static 2D cultures and represent an opportunity to model injury to renal cell types that have hitherto received little attention. As nephrotoxicity screening platforms become more physiologically relevant, they will facilitate the development of safer drugs and improved clinical management of nephrotoxicants.

Nanotechnological foundations of a «new» Nephrology

After contextualising the generic frameworks of nanotechnology and nanomedicine, the 2disciplines are discussed in the field of Nephrology. The potential downside to nanonephrology is the renal clearance of nanoparticles, the use of which is ever-increasing both for nanomedicinal purposes and in nanofoods. The positive impact of nanotechnology in Nephrology is centred on the development of renal nanodiagnostics for basic renal function studies, the early diagnosis of acute kidney injury, reliable and simple follow-up of chronic kidney disease and the improvement of magnetic resonance imaging. Renal drug nanotherapies comprise an important and dual-faceted area: The protection of drugs and nephrotoxic agents (e.g. antibiotics, antiretrovirals, contrast media, etc.) on the one hand, and the development of new kidney disease medications on the other. Renal 'nanotheranostics' is a promising but little-studied area. The impact of nanostructured supports on renal tissue regeneration is also discussed. The article concludes with a brief analysis of the various nanonephrology perspectives.

Developmentally inspired human 'organs on chips'

Although initially developed to replace animal testing in drug development, human 'organ on a chip' (organ chip) microfluidic culture technology offers a new tool for studying tissue development and pathophysiology, which has brought us one step closer to carrying out human experimentation in vitro In this Spotlight article, I discuss the central role that developmental biology played in the early stages of organ-chip technology, and how these models have led to new insights into human physiology and disease mechanisms. Advantages and disadvantages of the organ-chip approach relative to organoids and other human cell cultures are also discussed.

Bioengineering strategies to accelerate stem cell therapeutics

Although only a few stem cell-based therapies are currently available to patients, stem cells hold tremendous regenerative potential, and several exciting clinical applications are on the horizon. Biomaterials with tuneable mechanical and biochemical properties can preserve stem cell function in culture, enhance survival of transplanted cells and guide tissue regeneration. Rapid progress with three-dimensional hydrogel culture platforms provides the opportunity to grow patient-specific organoids, and has led to the discovery of drugs that stimulate endogenous tissue-specific stem cells and enabled screens for drugs to treat disease. Therefore, bioengineering technologies are poised to overcome current bottlenecks and revolutionize the field of regenerative medicine.

The presence of human mesenchymal stem cells of renal origin in amniotic fluid increases with gestational time

Background

Established therapies for managing kidney dysfunction such as kidney dialysis and transplantation are limited due to the shortage of compatible donated organs and high costs. Stem cell-based therapies are currently under investigation as an alternative treatment option. As amniotic fluid is composed of fetal urine harboring mesenchymal stem cells (AF-MSCs), we hypothesized that third-trimester amniotic fluid could be a novel source of renal progenitor and differentiated cells.

Methods

Human third-trimester amniotic fluid cells (AFCs) were isolated and cultured in distinct media. These cells were characterized as renal progenitor cells with respect to cell morphology, cell surface marker expression, transcriptome and differentiation into chondrocytes, osteoblasts and adipocytes. To test for renal function, a comparative albumin endocytosis assay was performed using AF-MSCs and commercially available renal cells derived from kidney biopsies. Comparative transcriptome analyses of first, second and third trimester-derived AF-MSCs were conducted to monitor expression of renal-related genes.

Results

Regardless of the media used, AFCs showed expression of pluripotency-associated markers such as SSEA4, TRA-1-60, TRA-1-81 and C-Kit. They also express the mesenchymal marker Vimentin. Immunophenotyping confirmed that third-trimester AFCs are bona fide MSCs. AF-MSCs expressed the master renal progenitor markers SIX2 and CITED1, in addition to typical renal proteins such as PODXL, LHX1, BRN1 and PAX8. Albumin endocytosis assays demonstrated the functionality of AF-MSCs as renal cells. Additionally, upregulated expression of BMP7 and downregulation of WT1, CD133, SIX2 and C-Kit were observed upon activation of WNT signaling by treatment with the GSK-3 inhibitor CHIR99201. Transcriptome analysis and semiquantitative PCR revealed increasing expression levels of renal-specific genes (e.g., SALL1, HNF4B, SIX2) with gestational time. Moreover, AF-MSCs shared more genes with human kidney cells than with native MSCs and gene ontology terms revealed involvement of biological processes associated with kidney morphogenesis.

Conclusions

Third-trimester amniotic fluid contains AF-MSCs of renal origin and this novel source of kidney progenitors may have enormous future potentials for disease modeling, renal repair and drug screening.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-0864-7) contains supplementary material, which is available to authorized users.

Current developments and applications of microfluidic technology toward clinical translation of nanomedicines

Nanoparticulate drug delivery systems hold great potential for the therapy of many diseases, especially cancer. However, the translation of nanoparticulate drug delivery systems from academic research to industrial and clinical practice has been slow. This slow translation can be ascribed to the high batch-to-batch variations and insufficient production rate of the conventional preparation methods, and the lack of technologies for rapid screening of nanoparticulate drug delivery systems with high correlation to the in vivo tests. These issues can be addressed by the microfluidic technologies. For example, microfluidics can not only produce nanoparticles in a well-controlled, reproducible, and high-throughput manner, but also create 3D environments with continuous flow to mimic the physiological and/or pathological processes. This review provides an overview of the microfluidic devices developed to prepare nanoparticulate drug delivery systems, including drug nanosuspensions, polymer nanoparticles, polyplexes, structured nanoparticles and theranostic nanoparticles. We also highlight the recent advances of microfluidic systems in fabricating the increasingly realistic models of the in vivo milieu for rapid screening of nanoparticles. Overall, the microfluidic technologies offer a promise approach to accelerate the clinical translation of nanoparticulate drug delivery systems.

Modeling human diseases with induced pluripotent stem cells: from 2D to 3D and beyond

The advent of human induced pluripotent stem cells (iPSCs) presents unprecedented opportunities to model human diseases. Differentiated cells derived from iPSCs in two-dimensional (2D) monolayers have proven to be a relatively simple tool for exploring disease pathogenesis and underlying mechanisms. In this Spotlight article, we discuss the progress and limitations of the current 2D iPSC disease-modeling platform, as well as recent advancements in the development of human iPSC models that mimic in vivo tissues and organs at the three-dimensional (3D) level. Recent bioengineering approaches have begun to combine different 3D organoid types into a single '4D multi-organ system'. We summarize the advantages of this approach and speculate on the future role of 4D multi-organ systems in human disease modeling.

Mechanical challenges and cytoskeletal impairments in focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is a histologically defined form of kidney injury typically mediated by podocyte dysfunction. Podocytes rely on their intricate actin-based cytoskeleton to maintain the glomerular filtration barrier in the face of mechanical challenges resulting from pulsatile blood flow and filtration of this blood flow. This review summarizes the mechanical challenges faced by podocytes in the form of stretch and shear stress, both of which may play a role in the progression of podocyte dysfunction and detachment. It also reviews how podocytes respond to these mechanical challenges in dynamic fashion through rearranging their cytoskeleton, triggering various biochemical pathways, and, in some disease states, altering their morphology in the form of foot process effacement. Furthermore, this review highlights the growing body of evidence identifying several mutations of important cytoskeleton proteins as causes of FSGS. Lastly, it synthesizes the above evidence to show that a better understanding of how these mutations leave podocytes vulnerable to the mechanical challenges they face is essential to better understanding the mechanisms by which they lead to disease. The review concludes with future research directions to fill this gap and some novel techniques with which to pursue these directions.