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]

Human Organs-on-Chips for Virology

While conventional in vitro culture systems and animal models have been used to study the pathogenesis of viral infections and to facilitate development of vaccines and therapeutics for viral diseases, models that can accurately recapitulate human responses to infection are still lacking. Human organ-on-a-chip (Organ Chip) microfluidic culture devices that recapitulate tissue–tissue interfaces, fluid flows, mechanical cues, and organ-level physiology have been developed to narrow the gap between in vitro experimental models and human pathophysiology. Here, we describe how recent developments in Organ Chips have enabled re-creation of complex pathophysiological features of human viral infections in vitro.

Microfluidic Organ Chip culture devices are emerging alternatives to conventional in vitro and animal models due to their ability to replicate many structural and functional features of human physiology and disease states.

Recent innovations demonstrate that Organ Chip technology is a promising strategy for virology studies where there have been successes in reproducing various viral disease phenotypes.

Organ Chips have enabled investigation of many aspects of viral infection, including virus–host interactions, viral therapy-resistance evolution, and development of new antiviral therapeutics, as well as underlying pathogenesis.

As Organ Chip-based assays provide accessibility to study virus-induced diseases in real time and at high resolution, they can open new avenues to uncover viral pathogenesis in a human-relevant environment and may eventually enable development of novel therapeutics and vaccines.

Stem cells: a potential treatment option for kidney diseases

The prevalence of kidney diseases is emerging as a public health problem. Stem cells (SCs), currently considered as a promising tool for therapeutic application, have aroused considerable interest and expectations. With self-renewal capabilities and great potential for proliferation and differentiation, stem cell therapy opens new avenues for the development of renal function and structural repair in kidney diseases. Mounting evidence suggests that stem cells exert a therapeutic effect mainly by replacing damaged tissues and paracrine pathways. The benefits of various types of SCs in acute kidney disease and chronic kidney disease have been demonstrated in preclinical studies, and preliminary results of clinical trials present its safety and tolerability. This review will focus on the stem cell-based therapy approaches for the treatment of kidney diseases, including various cell sources used, possible mechanisms involved, and outcomes that are generated so far, along with prospects and challenges in clinical application.

h-FIBER: Microfluidic Topographical Hollow Fiber for Studies of Glomerular Filtration Barrier

Kidney-on-a-chip devices may revolutionize the discovery of new therapies. However, fabricating a 3D glomerulus remains a challenge, due to a requirement for a microscale soft material with complex topography to support cell culture in a native configuration. Here, we describe the use of microfluidic spinning to recapitulate complex concave and convex topographies over multiple length scales, required for biofabrication of a biomimetic 3D glomerulus. We produced a microfluidic extruded topographic hollow fiber (h-FIBER), consisting of a vessel-like perfusable tubular channel for endothelial cell cultivation, and a glomerulus-like knot with microconvex topography on its surface for podocyte cultivation. Meter long h-FIBERs were produced in microfluidics within minutes, followed by chemically induced inflation for generation of topographical cues on the 3D scaffold surface. The h-FIBERs were assembled into a hot-embossed plastic 96-well plate. Long-term perfusion, podocyte barrier formation, endothelialization, and permeability tests were easily performed by a standard pipetting technique on the platform. Following long-term culture (1 month), a functional filtration barrier, measured by the transfer of albumin from the blood vessel side to the ultrafiltrate side, suggested the establishment of an engineered glomerulus.

The Importance of Mechanical Forces for in vitro Endothelial Cell Biology

Blood and lymphatic vessels are lined by endothelial cells which constantly interact with their luminal and abluminal extracellular environments. These interactions confer physical forces on the endothelium, such as shear stress, stretch and stiffness, to mediate biological responses. These physical forces are often altered during disease, driving abnormal endothelial cell behavior and pathology. Therefore, it is critical that we understand the mechanisms by which endothelial cells respond to physical forces. Traditionally, endothelial cells in culture are grown in the absence of flow on stiff substrates such as plastic or glass. These cells are not subjected to the physical forces that endothelial cells endure in vivo, thus the results of these experiments often do not mimic those observed in the body. The field of vascular biology now realize that an intricate analysis of endothelial signaling mechanisms requires complex in vitro systems to mimic in vivo conditions. Here, we will review what is known about the mechanical forces that guide endothelial cell behavior and then discuss the advancements in endothelial cell culture models designed to better mimic the in vivo vascular microenvironment. A wider application of these technologies will provide more biologically relevant information from cultured cells which will be reproducible to conditions found in the body.

Evolution of Biochip Technology: A Review from Lab-on-a-Chip to Organ-on-a-Chip

Following the advancements in microfluidics and lab-on-a-chip (LOC) technologies, a novel biomedical application for microfluidic based devices has emerged in recent years and microengineered cell culture platforms have been created. These micro-devices, known as organ-on-a-chip (OOC) platforms mimic the in vivo like microenvironment of living organs and offer more physiologically relevant in vitro models of human organs. Consequently, the concept of OOC has gained great attention from researchers in the field worldwide to offer powerful tools for biomedical researches including disease modeling, drug development, etc. This review highlights the background of biochip development. Herein, we focus on applications of LOC devices as a versatile tool for POC applications. We also review current progress in OOC platforms towards body-on-a-chip, and we provide concluding remarks and future perspectives for OOC platforms for POC applications.

Clinically Relevant Tissue Scale Responses as New Readouts from Organs-on-a-Chip for Precision Medicine

Organs-on-chips (OOC) are widely seen as being the next generation in vitro models able to accurately recreate the biochemical-physical cues of the cellular microenvironment found in vivo. In addition, they make it possible to examine tissue-scale functional properties of multicellular systems dynamically and in a highly controlled manner. Here we summarize some of the most remarkable examples of OOC technology's ability to extract clinically relevant tissue-level information. The review is organized around the types of OOC outputs that can be measured from the cultured tissues and transferred to clinically meaningful information. First, the creation of functional tissues-on-chip is discussed, followed by the presentation of tissue-level readouts specific to OOC, such as morphological changes, vessel formation and function, tissue properties, and metabolic functions. In each case, the clinical relevance of the extracted information is highlighted.

Advances in our understanding of genetic kidney disease using kidney organoids

A significant proportion of kidney disease presenting in childhood is likely genetic in origin with a growing number of genes implicated in its development. However, many children may have changes in previously undescribed or unrecognised genes. The recent development of methods for generating human kidney organoids from human pluripotent stem cells has the potential to substantially change the rate of diagnosis and the development of new treatments for some forms of genetic kidney disease. In this review, we discuss how accurately a kidney organoid models the human kidney, identifying the strengths and weaknesses of these potentially patient-derived models of renal disease.

Integrating Biophysics in Toxicology

Integration of biophysical stimulation in test systems is established in diverse branches of biomedical sciences including toxicology. This is largely motivated by the need to create novel experimental setups capable of reproducing more closely in vivo physiological conditions. Indeed, we face the need to increase predictive power and experimental output, albeit reducing the use of animals in toxicity testing. In vivo, mechanical stimulation is essential for cellular homeostasis. In vitro, diverse strategies can be used to model this crucial component. The compliance of the extracellular matrix can be tuned by modifying the stiffness or through the deformation of substrates hosting the cells via static or dynamic strain. Moreover, cells can be cultivated under shear stress deriving from the movement of the extracellular fluids. In turn, introduction of physical cues in the cell culture environment modulates differentiation, functional properties, and metabolic competence, thus influencing cellular capability to cope with toxic insults. This review summarizes the state of the art of integration of biophysical stimuli in model systems for toxicity testing, discusses future challenges, and provides perspectives for the further advancement of in vitro cytotoxicity studies.

Generating Kidney from Stem Cells

Human kidney tissue can now be generated via the directed differentiation of human pluripotent stem cells. This advance is anticipated to facilitate the modeling of human kidney diseases, provide platforms for nephrotoxicity screening, enable cellular therapy, and potentially generate tissue for renal replacement. All such applications will rely upon the accuracy and reliability of the model and the capacity for stem cell-derived kidney tissue to recapitulate both normal and diseased states. In this review, we discuss the models available, how well they recapitulate the human kidney, and how far we are from application of these cells for use in cellular therapies.

Development of a Novel Hanging Drop Platform for Engineering Controllable 3D Microenvironments

Conventional biomedical research is mostly performed by utilizing a two-dimensional monolayer culture, which fails to recapitulate the three-dimensional (3D) organization and microenvironment of native tissues. To overcome this limitation, several methods are developed to fabricate microtissues with the desired 3D microenvironment. However, they tend to be time-consuming, labor-intensive, or costly, thus hindering the application of 3D microtissues as models in a wide variety of research fields. In the present study, we have developed a pressure-assisted network for droplet accumulation (PANDA) system, an easy-to-use chip that comprises a multichannel fluidic system and a hanging drop cell culture module for uniform 3D microtissue formation. This system can control the desired artificial niches for modulating the fate of the stem cells to form the different sizes of microtissue by adjusting the seeding density. Furthermore, a large number of highly consistent 3D glomerulus-like heterogeneous microtissues that are composed of kidney glomerular podocytes and mesenchymal stem cells have been formed successfully. These data suggest that the developed PANDA system can be employed as a rapid and economical platform to fabricate microtissues with tunable 3D microenvironment and cellular heterogeneity, thus can be employed as tissue-mimicking models in various biomedical research.

Nanofabrication of Isoporous Membranes for Cell Fractionation

Cell fractionations and other biological separations frequently require several steps. They could be much more effectively done by filtration, if isoporous membranes would be available with high pore density, and sharp pore size distribution in the micro- and nanoscale. We propose a combination of two scalable methods, photolithography and dry reactive ion etching, to fabricate a series of polyester membranes with isopores of size 0.7 to 50 μm and high pore density with a demonstrated total area of 38.5 cm². The membranes have pore sizes in the micro- and submicro-range, and pore density 10-fold higher than track-etched analogues, which are the only commercially available isoporous polymeric films. Permeances of 220,000 L m⁻² h⁻¹bar⁻¹ were measured with pore size 787 nm. The method does not require organic solvents and can be applied to many homopolymeric materials. The pore reduction from 2 to 0.7 μm was obtained by adding a step of chemical vapor deposition. The isoporous system was successfully demonstrated for the organelle fractionation of Arabidopsis homogenates and could be potentially extended to other biological fractionations.

Biofabrication Strategies and Engineered In Vitro Systems for Vascular Mechanobiology

The vascular system is integral for maintaining organ-specific functions and homeostasis. Dysregulation in vascular architecture and function can lead to various chronic or acute disorders. Investigation of the role of the vascular system in health and disease has been accelerated through the development of tissue-engineered constructs and microphysiological on-chip platforms. These in vitro systems permit studies of biochemical regulation of vascular networks and parenchymal tissue and provide mechanistic insights into the biophysical and hemodynamic forces acting in organ-specific niches. Detailed understanding of these forces and the mechanotransductory pathways involved is necessary to develop preventative and therapeutic strategies targeting the vascular system. This review describes vascular structure and function, the role of hemodynamic forces in maintaining vascular homeostasis, and measurement approaches for cell and tissue level mechanical properties influencing vascular phenomena. State-of-the-art techniques for fabricating in vitro microvascular systems, with varying degrees of biological and engineering complexity, are summarized. Finally, the role of vascular mechanobiology in organ-specific niches and pathophysiological states, and efforts to recapitulate these events using in vitro microphysiological systems, are explored. It is hoped that this review will help readers appreciate the important, but understudied, role of vascular-parenchymal mechanotransduction in health and disease toward developing mechanotherapeutics for treatment strategies.

Application of microfluidic technology in cancer research and therapy

Cancer is a heterogeneous disease that requires a multimodal approach to diagnose, manage and treat. A better understanding of the disease biology can lead to identification of novel diagnostic/prognostic biomarkers and the discovery of the novel therapeutics with the goal of improving patient outcomes. Employing advanced technologies can facilitate this, enabling better diagnostic and treatment for cancer patients. In this regard, microfluidic technology has emerged as a promising tool in the studies of cancer, including single cancer cell analysis, modeling angiogenesis and metastasis, drug screening and liquid biopsy. Microfluidic technologies have opened new ways to study tumors in the preclinical and clinical settings. In this chapter, we highlight novel application of this technology in area of fundamental, translational and clinical cancer research.

Multi-lineage Human iPSC-Derived Platforms for Disease Modeling and Drug Discovery

Human induced pluripotent stem cells (hiPSCs) provide a powerful platform for disease modeling and have unlocked new possibilities for understanding the mechanisms governing human biology, physiology, and genetics. However, hiPSC-derivatives have traditionally been utilized in two-dimensional monocultures, in contrast to the multi-systemic interactions that influence cells in the body. We will discuss recent advances in generating more complex hiPSC-based systems using three-dimensional organoids, tissue-engineering, microfluidic organ-chips, and humanized animal systems. While hiPSC differentiation still requires optimization, these next-generation multi-lineage technologies can augment the biomedical researcher's toolkit and enable more realistic models of human tissue function.

The myriad possibility of kidney organoids

Purpose of review

Human kidney development and the mechanisms of many kidney diseases are incompletely understood partly due to the lack of appropriate models. Kidney organoids derived from human pluripotent stem cells (hPSCs) are a new and rapidly developing in-vitro system covering the window of early nephrogenesis and having the capacity for disease modelling. The application of global analytic tools such as RNA sequencing and proteomics is providing new and unexpected insights into kidney organoids with relevance for development and disease. In this review, we focus on the most significant advances in the field over the last 2 years.

Recent findings

There have been several protocol modifications for the differentiation of hPSCs into kidney organoids, including the additional step of implantation into mice. These changes have improved the vascularization and maturity of the major cell types in the organoids, increased the production scale, and reduced the cost and labour intensity of culturing organoids. Single-cell RNA sequencing and global proteomics of kidney organoids have provided important insights into the multiple cell populations, origin of cells, and regulatory relationships between genes. There has been an increase in research using patient-derived induced pluripotent stem cells (iPSCs), or combining gene editing with iPSC-derived kidney organoids as a novel disease-modelling platform for improving our understanding of disease mechanisms, drug testing and discovery, and the potential for personalized therapy. Finally, there has been progress in culturing hPSCs-derived kidney cells in microfluidic kidney-on-a-chip devices and this may provide a means of further improving the maturity of kidney organoids.

Summary

The review summarizes the latest progress on kidney organoids including differentiation protocols, analysis tools, and applications. Despite some limitations, hPSC-derived kidney organoids are authentic and practical models for investigating kidney development and disease and progressing understanding about tissue regeneration, drug screening, and disease modelling.

Microfluidic Platforms toward Rational Material Fabrication for Biomedical Applications

The emergence of micro/nanomaterials in recent decades has brought promising alternative approaches in various biomedicine-related fields such as pharmaceutics, diagnostics, and therapeutics. These micro/nanomaterials for specific biomedical applications shall possess tailored properties and functionalities that are closely correlated to their geometries, structures, and compositions, therefore placing extremely high demands for manufacturing techniques. Owing to the superior capabilities in manipulating fluids and droplets at microscale, microfluidics has offered robust and versatile platform technologies enabling rational design and fabrication of micro/nanomaterials with precisely controlled geometries, structures and compositions in high throughput manners, making them excellent candidates for a variety of biomedical applications. This review briefly summarizes the progress of microfluidics in the fabrication of various micro/nanomaterials ranging from 0D (particles), 1D (fibers) to 2D/3D (film and bulk materials) materials with controllable geometries, structures, and compositions. The applications of these microfluidic-based materials in the fields of diagnostics, drug delivery, organs-on-chips, tissue engineering, and stimuli-responsive biodevices are introduced. Finally, an outlook is discussed on the future direction of microfluidic platforms for generating materials with superior properties and on-demand functionalities. The integration of new materials and techniques with microfluidics will pave new avenues for preparing advanced micro/nanomaterials with enhanced performance for biomedical applications.

Application of microphysiological systems in biopharmaceutical research and development

Within the last 10 years, several tissue microphysiological systems (MPS) have been developed and characterized for retention of morphologic characteristics and specific gene/protein expression profiles from their natural in vivo state. Once developed, their utility is typically further tested by comparing responses to known toxic small-molecule pharmaceuticals in efforts to develop strategies for further toxicity testing of compounds under development. More recently, application of this technology in biopharmaceutical (large molecules) development is beginning to be more appreciated. In this review, we describe some of the advances made for tissue-specific MPS and outline the advantages and challenges of applying and further developing MPS technology in preclinical biopharmaceutical research.

Recent advances in nanotechnology-based drug delivery systems for the kidney

The application of nanotechnology in medicine has the potential to make a great impact on human health, ranging from prevention to diagnosis and treatment of disease. The kidneys are the main organ of the human urinary system, responsible for filtering the blood, and concentrating metabolic waste into urine by means of the renal glomerulus. The glomerular filtration apparatus presents a barrier against therapeutic agents based on charge and/or molecular size. Therefore, drug delivery to the kidneys faces significant difficulties resulting in treatment failure in several renal disorders. Accordingly, different strategies have recently being explored for enhancing the delivery of therapeutic agents across the filtration barrier of the glomerulus. Nanosystems with different physicochemical properties, including size, shape, surface, charge, and possessing biological features such as high cellular internalization, low cytotoxicity, controllable pharmacokinetics and biodistribution, have shown promising results for renal therapy. Different types of nanoparticles (NPs) have been used to deliver drugs to the kidney. In this review, we discuss nanotechnology-based drug delivery approaches for acute kidney injury, chronic kidney disease, renal fibrosis, renovascular hypertension and kidney cancer.

A pharmaceutical industry perspective on microphysiological kidney systems for evaluation of safety for new therapies

The human kidney contains approximately one million nephrons. As the functional unit of the kidney, the nephron affords an opportunity to approximate the kidney at a microphysiological scale. Recent emergence of physiologically accurate human tissue models has radically advanced the possibilities of mimicking organ biology and multi-organ combinations in vitro. Anatomically, the nephron is one of the most complex, sequentially integrated microfluidic units in the body making the miniaturized microfluidic systems excellent candidates for capturing the kidney biology in vitro. While these models are promising, there are a number of considerations for practical implementation into a drug development paradigm. Opportunities for pharmaceutical industry applications of new MPS models often start with drug safety testing. As such, the intent of this article is to focus on safety and ADME applications. This article reviews biological functions of the kidney and options for characterizing known roles in nephrotoxicity. The concept of "context-of-use" is introduced as a framework for describing and verifying the specific features of an MPS platform for use in drug development. Overall, we present a perspective on key attributes of microphysiological kidney models, which the pharmaceutical industry could leverage to improve confident safety and ADME evaluations of experimental therapies.

Robotic fluidic coupling and interrogation of multiple vascularized organ chips

Organ chips can recapitulate organ-level (patho)physiology, yet pharmacokinetic and pharmacodynamic analyses require multi-organ systems linked by vascular perfusion. Here, we describe an 'interrogator' that employs liquid-handling robotics, custom software and an integrated mobile microscope for the automated culture, perfusion, medium addition, fluidic linking, sample collection and in situ microscopy imaging of up to ten organ chips inside a standard tissue-culture incubator. The robotic interrogator maintained the viability and organ-specific functions of eight vascularized, two-channel organ chips (intestine, liver, kidney, heart, lung, skin, blood-brain barrier and brain) for 3 weeks in culture when intermittently fluidically coupled via a common blood substitute through their reservoirs of medium and endothelium-lined vascular channels. We used the robotic interrogator and a physiological multicompartmental reduced-order model of the experimental system to quantitatively predict the distribution of an inulin tracer perfused through the multi-organ human-body-on-chips. The automated culture system enables the imaging of cells in the organ chips and the repeated sampling of both the vascular and interstitial compartments without compromising fluidic coupling.

The tooth on-a-chip: a microphysiologic model system mimicking the biologic interface of the tooth with biomaterials

The tooth has a unique configuration with respect to biomaterials that are used for its treatment. Cells inside of the dental pulp interface indirectly with biomaterials via a calcified permeable membrane, formed by the dentin matrix and several thousands of dentinal tubules (∼2 μm in diameter). Although the cytotoxic response of the dental pulp to biomaterials has been extensively studied, there is a shortage of in vitro model systems that mimic the dentin-pulp interface and enable an improved understanding of the morphologic, metabolic and functional influence of biomaterials on live dental pulp cells. To address this shortage, here we developed an organ-on-a-chip model system which integrates cells cultured directly on a dentin wall within a microfluidic device that replicates some of the architecture and dynamics of the dentin-pulp interface. The tooth-on-a-chip is made out of molded polydimethylsiloxane (PDMS) with a design consisting of two chambers separated by a dentin fragment. To characterize pulp cell responses to dental materials on-chip, stem cells from the apical papilla (SCAPs) were cultured in odontogenic medium and seeded onto the dentin surface, and observed using live-cell microscopy. Next, to evaluate the tooth-on-a-chip as a platform for materials testing, standard dental materials used clinically (2-hydroxyethylmethacrylate - HEMA, phosphoric acid - PA, and Adper-Scotchbond - SB) were tested for cytotoxicity, cell morphology, and metabolic activity on-chip, and compared against standardized off-chip controls. All dental materials had cytotoxic effects in both on-chip and off-chip systems in the following order: HEMA > SB > PA (p < 0.05), and cells presented consistently higher metabolic activity on-chip than off-chip (p < 0.05). Furthermore, the tooth-on-a-chip enabled real-time tracking of gelatinolytic activity in a model hybrid layer (HL) formed in the microdevice, which suggests that dental pulp cells may contribute to the proteolytic activity in the HL more than endogenous proteases. In conclusion, the tooth-on-a-chip is a novel platform that replicates near-physiologic conditions of the pulp-dentin interface and enables live-cell imaging to study dental pulp cell response to biomaterials.

Microphysiological Systems: Next Generation Systems for Assessing Toxicity and Therapeutic Effects of Nanomaterials

Microphysiological systems, also known as organ-on-a-chip platforms, show promise for the development of new testing methods that can be more accurate than both conventional two-dimensional cultures and costly animal studies. The development of more intricate microphysiological systems can help to better mimic the human physiology and highlight the systemic effects of different drugs and materials. Nanomaterials are among a technologically important class of materials used for diagnostic, therapeutic, and monitoring purposes; all of which and can be tested using new organ-on-a-chip systems. In addition, the toxicity of nanomaterials which have entered the body from ambient air or diet can have deleterious effects on various body systems. This in turn can be studied in newly developed microphysiological systems. While organ-on-a-chip models can be useful, they cannot pick up secondary and systemic toxicity. Thus, the utilization of multi-organ-on-a-chip systems for advancing nanotechnology will largely be reflected in the future of drug development, toxicology studies and precision medicine. Various aspects of related studies, current challenges, and future perspectives are discussed in this paper.

Organoids: An invaluable tool in pharmacology

Advances in stem cell cultures and human-induced pluripotent stem cells have inculcated interests in a rapidly evolving concept – ”organoids.” These are three-dimensional (3D) structures mimicking some of the phenomena of the real organs at anatomical, multicellular, and functional levels in vitro. Organoids have been proven to be better than two-dimensional cell culture in replicating the functionality, architectural, and geometrical features of tissues in vivo. Recent advancements have led to the generation of models for organ development and disease, finding applications in the drug discovery, screening of novel compounds, and personalized medicine. Since organoids follow the same natural pathway as the normal tissue or pathology, they can be used to study the expression of various genotypes and phenotypic variations across different species. In the light of these advancements, organoids are now being merged with bioengineering to come up with even better and reliable models to predict the disease progression and effectiveness of precision medicines, few of its important applications. This article discusses the various aspects of this emerging concept along with its uses, both in the present times and near future, with a special focus on pharmacological applications.

Nephrotic syndrome in a dish: recent developments in modeling in vitro

Nephrotic syndrome is a heterogeneous disease, and one of the most frequent glomerular disorders among children. Depending on the etiology, it may result in end-stage renal disease and the need for renal replacement therapy. A dysfunctional glomerular filtration barrier, comprising of endothelial cells, the glomerular basement membrane and podocytes, characterizes nephrotic syndrome. Podocytes are often the primary target cells in the pathogenesis, in which not only the podocyte function but also their crosstalk with other glomerular cell types can be disturbed due to a myriad of factors. The pathophysiology of nephrotic syndrome is highly complex and studying molecular mechanisms in vitro requires state-of-the-art cell-based models resembling the in vivo situation and preferably a fully functional glomerular filtration barrier. Current advances in stem cell biology and microfluidic platforms have heralded a new era of three-dimensional (3D) cultures that might have the potential to recapitulate the glomerular filtration barrier in vitro. Here, we highlight the molecular basis of nephrotic syndrome and discuss requirements to accurately study nephrotic syndrome in vitro, including an overview of specific podocyte markers, cutting-edge stem cell organoids, and the implementation of microfluidic platforms. The development of (patho) physiologically relevant glomerular models will accelerate the identification of molecular targets involved in nephrotic syndrome and may be the harbinger of a new era of therapeutic avenues.

Bioprinting a 3D vascular construct for engineering a vessel-on-a-chip

The organ-on-a-chip model mimics the structural and functional features of human tissues or organs and has great importance in translational research. For vessel-on-a-chip model, conventional fabrication techniques are unable to efficiently imitate the intimal-medial unit of the vessel wall. Bioprinting technology, which can precisely control the organization of cells, biomolecules, and the extracellular matrix, has the potential to fabricate three-dimensional (3D) tissue constructs with spatial heterogeneity. In this study, we applied the gelatin-methacryloyl-based bioprinting technology to print 3D construct containing endothelial cells (ECs) and smooth muscle cells (SMCs) on a microfluidic chip. Compared with traditional culture system, EC-SMC coculturing chip model upregulated αSMA and SM22 protein expression of the SMC to a greater degree and maintains the contractile phenotype of the SMC, which mimics the natural vascular microenvironment. This strategy enabled us to establish an in vitro vascular model for studies of the physiologic and pathologic process in vascular wall.

Induced pluripotent stem cells for therapy personalization in pediatric patients: Focus on drug-induced adverse events

Adverse drug reactions (ADRs) are major clinical problems, particularly in special populations such as pediatric patients. Indeed, ADRs may be caused by a plethora of different drugs leading, in some cases, to hospitalization, disability or even death. In addition, pediatric patients may respond differently to drugs with respect to adults and may be prone to developing different kinds of ADRs, leading, in some cases, to more severe consequences. To improve the comprehension, and thus the prevention, of ADRs, the set-up of sensitive and personalized assays is urgently needed. Important progress is represented by the possibility of setting up groundbreaking patient-specific assays. This goal has been powerfully achieved using induced pluripotent stem cells (iPSCs). Due to their genetic and physiological species-specific differences and their ability to be differentiated ideally into all tissues of the human body, this model may be accurate in predicting drug toxicity, especially when this toxicity is related to individual genetic differences. This review is an up-to-date summary of the employment of iPSCs as a model to study ADRs, with particular attention to drugs used in the pediatric field. We especially focused on the intestinal, hepatic, pancreatic, renal, cardiac, and neuronal levels, also discussing progress in organoids creation. The latter are three-dimensional in vitro culture systems derived from pluripotent or adult stem cells simulating the architecture and functionality of native organs such as the intestine, liver, pancreas, kidney, heart, and brain. Based on the existing knowledge, these models are powerful and promising tools in multiple clinical applications including toxicity screening, disease modeling, personalized and regenerative medicine.

TRPC Channels in Proteinuric Kidney Diseases

Over a decade ago, mutations in the gene encoding TRPC6 (transient receptor potential cation channel, subfamily C, member 6) were linked to development of familial forms of nephrosis. Since this discovery, TRPC6 has been implicated in the pathophysiology of non-genetic forms of kidney disease including focal segmental glomerulosclerosis (FSGS), diabetic nephropathy, immune-mediated kidney diseases, and renal fibrosis. On the basis of these findings, TRPC6 has become an important target for the development of therapeutic agents to treat diverse kidney diseases. Although TRPC6 has been a major focus for drug discovery, more recent studies suggest that other TRPC family members play a role in the pathogenesis of glomerular disease processes and chronic kidney disease (CKD). This review highlights the data implicating TRPC6 and other TRPC family members in both genetic and non-genetic forms of kidney disease, focusing on TRPC3, TRPC5, and TRPC6 in a cell type (glomerular podocytes) that plays a key role in proteinuric kidney diseases.

Emerging In Vitro Systems to Screen and Predict Drug-Induced Kidney Toxicity

Drug attrition related to kidney toxicity remains a challenge in drug discovery and development. In vitro models established over the past 2 decades to supplement in vivo studies have improved the throughput capacity of toxicity evaluation, but usually suffer from low predictive value. To achieve a paradigm shift in the prediction of drug-induced kidney toxicity, two aspects are fundamental: increased physiological relevance of the kidney model, and use of appropriate toxicity end points. Recent studies have suggested that increasing the physiological relevance of kidney models can improve their sensitivity to drug-induced damage. Here, we discuss how advanced culture models, including modified cell lines, induced pluripotent stem cells, kidney organoid cultures, and microfluidic devices enhance in vivo similarity. To this end, culture models aim to increase the proximal tubule epithelial phenotype, reconstitute multiple tissue compartments and extracellular matrix, allow exposure to fluid shear stress, and enable interaction between multiple cell types. Applying computation-aided end points and novel biomarkers to advanced culture models will further improve sensitivity and clinical relevance of in vitro drug-induced toxicity prediction. Implemented at the right stage of drug discovery and development and coupled to high-content evaluation techniques, these models have the potential to reduce attrition and aid the selection of candidate drugs with an appropriate safety profile.

Cancer Modeling-on-a-Chip with Future Artificial Intelligence Integration

Cancer is one of the leading causes of death worldwide, despite the large efforts to improve the understanding of cancer biology and development of treatments. The attempts to improve cancer treatment are limited by the complexity of the local milieu in which cancer cells exist. The tumor microenvironment (TME) consists of a diverse population of tumor cells and stromal cells with immune constituents, microvasculature, extracellular matrix components, and gradients of oxygen, nutrients, and growth factors. The TME is not recapitulated in traditional models used in cancer investigation, limiting the translation of preliminary findings to clinical practice. Advances in 3D cell culture, tissue engineering, and microfluidics have led to the development of "cancer-on-a-chip" platforms that expand the ability to model the TME in vitro and allow for high-throughput analysis. The advances in the development of cancer-on-a-chip platforms, implications for drug development, challenges to leveraging this technology for improved cancer treatment, and future integration with artificial intelligence for improved predictive drug screening models are discussed.

Effective reconstruction of functional organotypic kidney spheroid for in vitro nephrotoxicity studies

Stable and reproducible kidney cellular models could accelerate our understanding of diseases, help therapeutics development, and improve nephrotoxicity screenings. Generation of a reproducible in vitro kidney models has been challenging owing to the cellular heterogeneity and structural complexity of the kidney. We generated mixed immortalized cell lines that stably maintained their characteristic expression of renal epithelial progenitor markers for the different lineages of kidney cellular compartments via the BMP7 signaling pathway from a mouse and a human whole kidney. These cells were used to generate functional and matured kidney spheroids containing multiple renal lineages, such as the proximal tubule, loop of Henle, distal tubules, and podocytes, using extracellular matrix and physiological force, named spheroid-forming unit (SFU). They expressed all apical and basolateral transporters that are important for drug metabolism and displayed key functional aspects of the proximal tubule, including protein endocytosis and increased gamma-glutamyltransferase activity, and cyclic AMP responded to external cues, such as parathyroid hormone. Following exposure, cells fluxed and took up drugs via proximal tubule-specific apical or basolateral transporters, and displayed increased cell death and expression of renal injury marker. Here, we developed a new differentiation method to generate kidney spheroids that structurally recapitulate important features of the kidney effectively and reproducibly using mixed immortalized renal cells, and showed their application for renal toxicity studies.

Tetrafluoroethylene-Propylene Elastomer for Fabrication of Microfluidic Organs-on-Chips Resistant to Drug Absorption

Organs-on-chips are microfluidic devices typically fabricated from polydimethylsiloxane (PDMS). Since PDMS has many attractive properties including high optical clarity and compliance, PDMS is very useful for cell culture applications; however, PDMS possesses a significant drawback in that small hydrophobic molecules are strongly absorbed. This drawback hinders widespread use of PDMS-based devices for drug discovery and development. Here, we describe a microfluidic cell culture system made of a tetrafluoroethylene-propylene (FEPM) elastomer. We demonstrated that FEPM does not absorb small hydrophobic compounds including rhodamine B and three types of drugs, nifedipine, coumarin, and Bay K8644, whereas PDMS absorbs them strongly. The device consists of two FEPM layers of microchannels separated by a thin collagen vitrigel membrane. Since FEPM is flexible and biocompatible, this microfluidic device can be used to culture cells while applying mechanical strain. When human umbilical vein endothelial cells (HUVECs) were subjected to cyclic strain (~10%) for 4 h in this device, HUVECs reoriented and aligned perpendicularly in response to the cyclic stretch. Moreover, we demonstrated that this device can be used to replicate the epithelial–endothelial interface as well as to provide physiological mechanical strain and fluid flow. This method offers a robust platform to produce organs-on-chips for drug discovery and development.

Microfluidics-Based Biomaterials and Biodevices

The rapid development of microfluidics technology has promoted new innovations in materials science, particularly by interacting with biological systems, based on precise manipulation of fluids and cells within microscale confinements. This article reviews the latest advances in microfluidics-based biomaterials and biodevices, highlighting some burgeoning areas such as functional biomaterials, cell manipulations, and flexible biodevices. These areas are interconnected not only in their basic principles, in that they all employ microfluidics to control the makeup and morphology of materials, but also unify at the ultimate goals in human healthcare. The challenges and future development trends in biological application are also presented.

Transplantation of Mouse Induced Pluripotent Stem Cell-Derived Podocytes in a Mouse Model of Membranous Nephropathy Attenuates Proteinuria

Injury to podocytes is a principle cause of initiation and progression of both immune and non-immune mediated glomerular diseases that result in proteinuria and decreased function of the kidney. Current advances in regenerative medicine shed light on the therapeutic potential of cell-based strategies for treatment of such disorders. Thus, there is hope that generation and transplantation of podocytes from induced pluripotent stem cells (iPSCs), could potentially be used as a curative treatment for glomerulonephritis caused by podocytes injury and loss. Despite several reports on the generation of iPSC-derived podocytes, there are rare reports about successful use of these cells in animal models. In this study, we first generated a model of anti-podocyte antibody-induced heavy proteinuria that resembled human membranous nephropathy and was characterized by the presence of sub-epithelial immune deposits and podocytes loss. Thereafter, we showed that transplantation of functional iPSC-derived podocytes following podocytes depletion results in recruitment of iPSC-derived podocytes within the damaged glomerulus, and leads to attenuation of proteinuria and histological alterations. These results provided evidence that application of iPSCs-derived renal cells could be a possible therapeutic strategy to favorably influence glomerular diseases outcomes.

Microfluidic three-dimensional cell culture of stem cells for high-throughput analysis

Although the recent advances in stem cell engineering have gained a great deal of attention due to their high potential in clinical research, the applicability of stem cells for preclinical screening in the drug discovery process is still challenging due to difficulties in controlling the stem cell microenvironment and the limited availability of high-throughput systems. Recently, researchers have been actively developing and evaluating three-dimensional (3D) cell culture-based platforms using microfluidic technologies, such as organ-on-a-chip and organoid-on-a-chip platforms, and they have achieved promising breakthroughs in stem cell engineering. In this review, we start with a comprehensive discussion on the importance of microfluidic 3D cell culture techniques in stem cell research and their technical strategies in the field of drug discovery. In a subsequent section, we discuss microfluidic 3D cell culture techniques for high-throughput analysis for use in stem cell research. In addition, some potential and practical applications of organ-on-a-chip or organoid-on-a-chip platforms using stem cells as drug screening and disease models are highlighted.

Methods of Delivering Mechanical Stimuli to Organ-on-a-Chip

Recent advances in integrating microengineering and tissue engineering have enabled the creation of promising microengineered physiological models, known as organ-on-a-chip (OOC), for experimental medicine and pharmaceutical research. OOCs have been used to recapitulate the physiologically critical features of specific human tissues and organs and their interactions. Application of chemical and mechanical stimuli is critical for tissue development and behavior, and they were also applied to OOC systems. Mechanical stimuli applied to tissues and organs are quite complex in vivo, which have not adequately recapitulated in OOCs. Due to the recent advancement of microengineering, more complicated and physiologically relevant mechanical stimuli are being introduced to OOC systems, and this is the right time to assess the published literature on this topic, especially focusing on the technical details of device design and equipment used. We first discuss the different types of mechanical stimuli applied to OOC systems: shear flow, compression, and stretch/strain. This is followed by the examples of mechanical stimuli-incorporated OOC systems. Finally, we discuss the potential OOC systems where various types of mechanical stimuli can be applied to a single OOC device, as a better, physiologically relevant recapitulation model, towards studying and evaluating experimental medicine, human disease modeling, drug development, and toxicology.

Kidney organoids: accurate models or fortunate accidents

There are now many reports of human kidney organoids generated via the directed differentiation of human pluripotent stem cells (PSCs) based on an existing understanding of mammalian kidney organogenesis. Such kidney organoids potentially represent tractable tools for the study of normal human development and disease with improvements in scale, structure, and functional maturation potentially providing future options for renal regeneration. The utility of such organotypic models, however, will ultimately be determined by their developmental accuracy. While initially inferred from mouse models, recent transcriptional analyses of human fetal kidney have provided greater insight into nephrogenesis. In this review, we discuss how well human kidney organoids model the human fetal kidney and how the remaining differences challenge their utility.

A Technology Roadmap for Innovative Approaches to Kidney Replacement Therapies: A Catalyst for Change

The number of patients dialyzed for ESKD exceeds 500,000 in the United States and more than 2.6 million people worldwide, with the expectation that the worldwide number will double by 2030. The human cost of health and societal financial cost of ESKD is substantial. Dialytic therapy is associated with an unacceptably high morbidity and mortality rate and poor quality of life. Although innovation in many areas of science has been transformative, there has been little innovation in dialysis or alternatives for kidney replacement therapy (KRT) since its introduction approximately 70 years ago. Advances in kidney biology, stem cells and kidney cell differentiation protocols, biomaterials, sensors, nano/microtechnology, sorbents and engineering, and interdisciplinary approaches and collaborations can lead to disruptive innovation. The Kidney Health Initiative, a public-private partnership between the American Society of Nephrology and the US Food and Drug Administration, has convened a multidisciplinary group to create a technology roadmap for innovative approaches to KRT to address patients' needs. The Roadmap is a living document. It identifies the design criteria that must be considered to replace the myriad functions of the kidney, as well as scientific, technical, regulatory, and payor milestones required to commercialize and provide patient access to KRT alternatives. Various embodiments of potential solutions are discussed, but the Roadmap is agnostic to any particular solution set. System enablers are identified, including vascular access, biomaterial development, biologic and immunologic modulation, function, and safety monitoring. Important Roadmap supporting activities include regulatory alignment and innovative financial incentives and payment pathways. The Roadmap provides estimated timelines for replacement of specific kidney functions so that approaches can be conceptualized in ways that are actionable and attract talented innovators from multiple disciplines. The Roadmap has been used to guide the selection of KidneyX prizes for innovation in KRT.

Plasma-induced covalent immobilization and patterning of bioactive species in microfluidic devices

Here, we present a straightforward technique to create bio-functional microfluidic channels using CO2 plasma to induce both carboxylic and hydroxyl groups onto the channel surface. Consequently, not only does the surface allow for irreversible covalent bonding to an oxygen plasma treated PDMS for microfluidic device fabrication, but it also provides functionality for biomolecular immobilization. Furthermore, we demonstrate integration of this technique with microcontact printing to covalently micropattern functional biomolecules inside microfluidic channels. The bio-functionality and efficacy of the microcontact printed antibodies is demonstrated for both bioassays as well as patterning and culturing different cell lines. Results show that the introduced method can be an excellent candidate for cell culture studies in microfluidics. With the new printing method, full cell confluency (∼400 cells per mm2) was achieved after incubation for only 1 day, which is significantly greater than other conventional cell culture techniques inside microfluidic devices. As a proof of concept, we demonstrated the endothelial cells functionality by stimulating von Willebrand Factor secretion under shear stress. This is done via perfusion of histamine through the channel and performing immunofluorescence labeling to observe the inflammatory response of the cells. The developed method eliminates the need for wet chemistry and significantly simplifies producing bio-functional chips which can be used for biosensing, organs-on-chips and tissue engineering applications.

A Toolbox to Characterize Human Induced Pluripotent Stem Cell-Derived Kidney Cell Types and Organoids

BACKGROUND

The generation of reporter lines for cell identity, lineage, and physiologic state has provided a powerful tool in advancing the dissection of mouse kidney morphogenesis at a molecular level. Although use of this approach is not an option for studying human development in vivo, its application in human induced pluripotent stem cells (iPSCs) is now feasible.

METHODS

We used CRISPR/Cas9 gene editing to generate ten fluorescence reporter iPSC lines designed to identify nephron progenitors, podocytes, proximal and distal nephron, and ureteric epithelium. Directed differentiation to kidney organoids was performed according to published protocols. Using immunofluorescence and live confocal microscopy, flow cytometry, and cell sorting techniques, we investigated organoid patterning and reporter expression characteristics.

RESULTS

Each iPSC reporter line formed well patterned kidney organoids. All reporter lines showed congruence of endogenous gene and protein expression, enabling isolation and characterization of kidney cell types of interest. We also demonstrated successful application of reporter lines for time-lapse imaging and mouse transplantation experiments.

CONCLUSIONS

We generated, validated, and applied a suite of fluorescence iPSC reporter lines for the study of morphogenesis within human kidney organoids. This fluorescent iPSC reporter toolbox enables the visualization and isolation of key populations in forming kidney organoids, facilitating a range of applications, including cellular isolation, time-lapse imaging, protocol optimization, and lineage-tracing approaches. These tools offer promise for enhancing our understanding of this model system and its correspondence with human kidney morphogenesis.

A glomerulus-on-a-chip to recapitulate the human glomerular filtration barrier

In this work we model the glomerular filtration barrier, the structure responsible for filtering the blood and preventing the loss of proteins, using human podocytes and glomerular endothelial cells seeded into microfluidic chips. In long-term cultures, cells maintain their morphology, form capillary-like structures and express slit diaphragm proteins. This system recapitulates functions and structure of the glomerulus, including permselectivity. When exposed to sera from patients with anti-podocyte autoantibodies, the chips show albuminuria proportional to patients' proteinuria, phenomenon not observed with sera from healthy controls or individuals with primary podocyte defects. We also show its applicability for renal disease modeling and drug testing. A total of 2000 independent chips were analyzed, supporting high reproducibility and validation of the system for high-throughput screening of therapeutic compounds. The study of the patho-physiology of the glomerulus and identification of therapeutic targets are also feasible using this chip.

Human kidney organoids: progress and remaining challenges

Kidney organoids are regarded as important tools with which to study the development of the normal and diseased human kidney. Since the first reports of human pluripotent stem cell-derived kidney organoids 5 years ago, kidney organoids have been successfully used to model glomerular and tubular diseases. In parallel, advances in single-cell RNA sequencing have led to identification of a variety of cell types in the organoids, and have shown these to be similar to, but more immature than, human kidney cells in vivo. Protocols for the in vitro expansion of stem cell-derived nephron progenitor cells (NPCs), as well as those for the selective induction of specific lineages, especially glomerular podocytes, have also been reported. Although most current organoids are based on the induction of NPCs, an induction protocol for ureteric buds (collecting duct precursors) has also been developed, and approaches to generate more complex kidney structures may soon be possible. Maturation of organoids is a major challenge, and more detailed analysis of the developing kidney at a single cell level is needed. Eventually, organotypic kidney structures equipped with nephrons, collecting ducts, ureters, stroma and vascular flow are required to generate transplantable kidneys; such attempts are in progress.

Perfusion culture maintained with an air-liquid interface to stimulate epithelial cell organization in renal organoids in vitro

Background

Organoids derived from induced pluripotent stem (iPS) or embryonic stem (ES) cells have been evaluated as in vitro models of development and disease. However, maintaining these cells under long-term static culture conditions is difficult because of nutrition shortages and waste accumulation. To overcome these issues, perfusion culture systems are required for organoid technology. A system with a stable microenvironment, nutrient availability, and waste removal will accelerate organoid generation. The aim of this study was to develop a novel perfusion system for renal organoids by maintaining the air-liquid interface with a device fabricated using a 3D printer.

Results

Our results revealed slow flow at the organoid cultivation area based on microbead movement on the membrane, which depended on the perfusion rate under the membrane. Moreover, the perfused culture medium below the organoids via a porous membrane diffused throughout the organoids, maintaining the air-liquid interface. The diffusion rates within organoids were increased according to the flow rate of the culture medium under the membrane. The perfused culture medium also stimulated cytoskeletal and basement membrane re-organization associated with promotion tubular formation under 2.5 μL/min flow culture. In contrast, tubules in organoids were diminished at a flow rate of 10 μL/min.

Conclusions

Our liquid-air interface perfusion system accelerated organization of the renal organoids. These results suggest that suitable perfusion conditions can accelerate organization of epithelial cells and tissues in renal organoids in vitro.

Induced pluripotent stem cells in disease modelling and drug discovery

The derivation of induced pluripotent stem cells (iPSCs) over a decade ago sparked widespread enthusiasm for the development of new models of human disease, enhanced platforms for drug discovery and more widespread use of autologous cell-based therapy. Early studies using directed differentiation of iPSCs frequently uncovered cell-level phenotypes in monogenic diseases, but translation to tissue-level and organ-level diseases has required development of more complex, 3D, multicellular systems. Organoids and human-rodent chimaeras more accurately mirror the diverse cellular ecosystems of complex tissues and are being applied to iPSC disease models to recapitulate the pathobiology of a broad spectrum of human maladies, including infectious diseases, genetic disorders and cancer.

Physiologically Based Pharmacokinetic and Pharmacodynamic Analysis Enabled by Microfluidically Linked Organs-on-Chips

Physiologically based pharmacokinetic (PBPK) modeling and simulation approaches are beginning to be integrated into drug development and approval processes because they enable key pharmacokinetic (PK) parameters to be predicted from in vitro data. However, these approaches are hampered by many limitations, including an inability to incorporate organ-specific differentials in drug clearance, distribution, and absorption that result from differences in cell uptake, transport, and metabolism. Moreover, such approaches are generally unable to provide insight into pharmacodynamic (PD) parameters. Recent development of microfluidic Organ-on-a-Chip (Organ Chip) cell culture devices that recapitulate tissue-tissue interfaces, vascular perfusion, and organ-level functionality offer the ability to overcome these limitations when multiple Organ Chips are linked via their endothelium-lined vascular channels. Here, we discuss successes and challenges in the use of existing culture models and vascularized Organ Chips for PBPK and PD modeling of human drug responses, as well as in vitro to in vivo extrapolation (IVIVE) of these results, and how these approaches might advance drug development and regulatory review processes in the future.