Engineering of polarized tubular structures in a microfluidic device to study calcium phosphate stone formation

Revolutionizing nephrology research: expanding horizons with kidney-on-a-chip and beyond

Organs-on-a-chip (OoC) is a microengineered three-dimensional cell culture system developed for decades. Utilizing microfluidic technology, OoC cultivates cells on perfusable channels to construct in vitro organ models, enabling the simulation of organ-level functions under physiological and pathophysiological conditions. The superior simulation capabilities compared to traditional animal experiments and two-dimensional cell cultures, making OoC a valuable tool for in vitro research. Recently, the application of OoC has extended to the field of nephrology, where it replicates various functional units, including glomerulus-on-a-chip, proximal tubule-on-a-chip, distal tubule-on-a-chip, collecting duct-on-a-chip, and even the entire nephron-on-a-chip to precisely emulate the structure and function of nephrons. Moreover, researchers have integrated kidney models into multi-organ systems, establishing human body-on-a-chip platforms. In this review, the diverse functional kidney units-on-a-chip and their versatile applications are outlined, such as drug nephrotoxicity screening, renal development studies, and investigations into the pathophysiological mechanisms of kidney diseases. The inherent advantages and current limitations of these OoC models are also examined. Finally, the synergy of kidney-on-a-chip with other emerging biomedical technologies are explored, such as bioengineered kidney and bioprinting, and a new insight for chip-based renal replacement therapy in the future are prospected.

Confining calcium oxalate crystal growth in a carbonated apatite-coated microfluidic channel to better understand the role of Randall's plaque in kidney stone formation

Effective prevention of recurrent kidney stone disease requires the understanding of the mechanisms of its formation. Numerous in vivo observations have demonstrated that a large number of pathological calcium oxalate kidney stones develop on an apatitic calcium phosphate deposit, known as Randall's plaque. In an attempt to understand the role of the inorganic hydroxyapatite phase in the formation and habits of calcium oxalates, we confined their growth under dynamic physicochemical and flow conditions in a reversible microfluidic channel coated with hydroxyapatite. Using multi-scale characterization techniques including scanning electron and Raman microscopy, we showed the successful formation of carbonated hydroxyapatite as found in Randall's plaque. This was possible due to a new two-step flow seed-mediated growth strategy which allowed us to coat the channel with carbonated hydroxyapatite. Precipitation of calcium oxalates under laminar flow from supersaturated solutions of oxalate and calcium ions showed that the formation of crystals is a substrate and time dependent complex process where diffusion of oxalate ions to the surface of carbonated hydroxyapatite and the solubility of the latter are among the most important steps for the formation of calcium oxalate crystals. Indeed when an oxalate solution was flushed for 24 h, dissolution of the apatite layer and formation of calcium carbonate calcite crystals occurred which seems to promote calcium oxalate crystal formation. Such a growth route has never been observed in vivo in the context of kidney stones. Under our experimental conditions, our results do not show any direct promoting role of carbonated hydroxyapatite in the formation of calcium oxalate crystals, consolidating therefore the important role that macromolecules can play in the process of nucleation and growth of calcium oxalate crystals on Randall's plaque.

Stabilization of uric acid mixed crystals by melamine

Melamine stabilizes heterogeneous nucleation of calcium crystals by increasing the retention time and decreasing the rate of dissolution. Stabilization of such mixed crystals limit the efficacy of non-invasive treatment options for kidney stones. Crystalline forms of uric acid (UA) are also involved in urolithiasis or UA kidney stones; however, its interactions with contaminating melamine and the resulting effects on the retention of kidney stones remain unknown. Since melamine augments calcium crystal formation, it provides an avenue for us to understand the stability of UA-calcium phosphate (CaP) crystals. We show here that melamine facilitates UA+CaP crystal formation, resulting in greater aggregates. Moreover, melamine induced mixed crystal retention through a time-dependent manner in presence and/or absence of hydroxycitrate (crystal inhibitor), indicating its abridged effectiveness as conventional remedy. CaP was also shown to modify optical properties of UA+CaP mixed crystals. Differential staining of individual crystals revealed enhanced co-aggregation of UA and CaP. The dissolution rate of UA in presence of melamine was faster than its heterogeneous crystallization form with CaP, although the size was comparatively much smaller, suggesting disparity in regulation between UA and CaP crystallization. While melamine stabilized UA, CaP and mixed crystals in relatively physiological conditions (artificial urine), the retentions of those crystals were further augmented by melamine, even in presence of hydroxycitrate, thus reducing treatment efficacy.

Application of microfluidic chips in the simulation of the urinary system microenvironment

The urinary system, comprising the kidneys, ureters, bladder, and urethra, has a unique mechanical and fluid microenvironment, which is essential to the urinary system growth and development. Microfluidic models, based on micromachining and tissue engineering technology, can integrate pathophysiological characteristics, maintain cell-cell and cell-extracellular matrix interactions, and accurately simulate the vital characteristics of human tissue microenvironments. Additionally, these models facilitate improved visualization and integration and meet the requirements of the laminar flow environment of the urinary system. However, several challenges continue to impede the development of a tissue microenvironment with controllable conditions closely resemble physiological conditions. In this review, we describe the biochemical and physical microenvironment of the urinary system and explore the feasibility of microfluidic technology in simulating the urinary microenvironment and pathophysiological characteristics in vitro. Moreover, we summarize the current research progress on adapting microfluidic chips for constructing the urinary microenvironment. Finally, we discuss the current challenges and suggest directions for future development and application of microfluidic technology in constructing the urinary microenvironment in vitro.

Differential biomolecular recognition by synthetic vs. biologically-derived components in the stone-forming process using 3D microfluidics

Calcium phosphate (CaP) biomineralization is the hallmark of extra-skeletal tissue calcification and renal calcium stones. Although such a multistep process starts with CaP crystal formation, the mechanism is still poorly understood due to the complexity of the in vivo system and the lack of a suitable approach to simulate a truly in vivo-like environment. Although endogenous proteins and lipids are engaged with CaP crystals in such a biological process of stone formation, most in vitro studies use synthetic materials that can display differential bioreactivity and molecular recognition by the cellular component. Here, we used our in vitro microfluidic (MF) tubular structure, which is the first completely cylindrical platform, with renal tubular cellular microenvironments closest to the functional human kidney tubule, to understand the precise role of biological components in this process. We systematically evaluated the contribution of synthetic and biological components in the stone-forming process in the presence of dynamic microenvironmental cues that originated due to cellular pathophysiology, which are critical for the nucleation, aggregation, and growth of CaP crystals. Our results show that crystal aggregation and growth were enhanced by immunoglobulin G (IgG), which was further inhibited by etidronic acid due to the chelation of extracellular Ca²⁺. Interestingly, biogenic CaP crystals from mice urine, when applied with cell debris and non-specific protein (bovine serum albumin), exhibited a more discrete crystal growth pattern, compared to exposure to synthetic CaP crystals under similar conditions. Furthermore, proteins found on those calcium crystals from mice urine produced discriminatory effects on crystal-protein attachment. Specifically, such biogenic crystals exhibited enhanced affinity to the proteins inherent to those crystals. More importantly, a physiological comparison of crystal induction in renal tubular cells revealed that biogenic crystals are less effective at producing a sustained rise in cytosolic Ca²⁺ compared to synthetic crystals, suggesting a milder detrimental effect to downstream signaling. Finally, synthetic crystal-internalized cells induced more oxidative stress, inflammation, and cellular damage compared to the biogenic crystal-internalized cells. Together, these results suggest that the intrinsic nature of biogenically derived components are appropriate to generate the molecular recognition needed for spatiotemporal effects and are critical towards understanding the process of kidney stone formation.

Tannic acid attenuates vascular calcification-induced proximal tubular cells damage through paracrine signaling

Vascular calcification is common in chronic kidney disease; however, the extent to which such condition can affect the renal microvasculature and the neighboring cell types is unclear. Our induced-calcification model in renal proximal tubular (PT) cells exhibited endoplasmic reticulum (ER) stress and oxidative damage, leading to apoptosis. Here, we utilized such calcification in mouse vascular smooth muscle (MOVAS-1) cells as a vascular calcification model, because it exhibited reactive oxygen species (ROS) generation, ER and oxidative stress, inflammatory, and apoptotic gene expressions. To demonstrate whether the vascular calcification condition can dictate the function of the adjacent PT cell layer, we utilized a Transwell multilayer culture system by combining those MOVAS-1 cells in the bottom chamber and polarized PT cells in the upper chamber to show the dimensional cross-signaling effect. Interestingly, calcification of MOVAS-1 cells, in this co-culture, induced H2O2 and lactate dehydrogenase (LDH) release leading to store-operated Ca2+ entry, ROS generation, and activation of oxidative, inflammatory, and apoptotic gene expressions in PT cells through paracrine signaling. Interestingly, application of tannic acid (TA) to either calcified MOVAS-1 or uncalcified PT cells diminished such detrimental pathway activation. Furthermore, the TA-mediated protection was much higher in the PT cells when applied on the calcified MOVAS-1 cells, and the delayed the pathological effects in neighboring PT cells can well be via paracrine signaling. Together, these results provide evidence of vascular calcification-induced PT cell damage, and the protective role of TA in preventing such pathological consequences, which can potentially be used as a nephroprotective remedy.

Microfluidic lumen-based systems for advancing tubular organ modeling

Microfluidic lumen-based systems are microscale models that recapitulate the anatomy and physiology of tubular organs. These technologies can mimic human pathophysiology and predict drug response, having profound implications for drug discovery and development. Herein, we review progress in the development of microfluidic lumen-based models from the 2000s to the present. The core of the review discusses models for mimicking blood vessels, the respiratory tract, the gastrointestinal tract, renal tubules, and liver sinusoids, and their application to modeling organ-specific diseases. We also highlight emerging application areas, such as the lymphatic system, and close the review discussing potential future directions.

Multiplex bioimaging of single-cell spatial profiles for precision cancer diagnostics and therapeutics

Cancers exhibit functional and structural diversity in distinct patients. In this mass, normal and malignant cells create tumor microenvironment that is heterogeneous among patients. A residue from primary tumors leaks into the bloodstream as cell clusters and single cells, providing clues about disease progression and therapeutic response. The complexity of these hierarchical microenvironments needs to be elucidated. Although tumors comprise ample cell types, the standard clinical technique is still the histology that is limited to a single marker. Multiplexed imaging technologies open new directions in pathology. Spatially resolved proteomic, genomic, and metabolic profiles of human cancers are now possible at the single-cell level. This perspective discusses spatial bioimaging methods to decipher the cascade of microenvironments in solid and liquid biopsies. A unique synthesis of top-down and bottom-up analysis methods is presented. Spatial multi-omics profiles can be tailored to precision oncology through artificial intelligence. Data-driven patient profiling enables personalized medicine and beyond.

Kidney-based in vitro models for drug-induced toxicity testing

The kidney is frequently involved in adverse effects caused by exposure to foreign compounds, including drugs. An early prediction of those effects is crucial for allowing novel, safe drugs entering the market. Yet, in current pharmacotherapy, drug-induced nephrotoxicity accounts for up to 25% of the reported serious adverse effects, of which one-third is attributed to antimicrobials use. Adverse drug effects can be due to direct toxicity, for instance as a result of kidney-specific determinants, or indirectly by, e.g., vascular effects or crystals deposition. Currently used in vitro assays do not adequately predict in vivo observed effects, predominantly due to an inadequate preservation of the organs' microenvironment in the models applied. The kidney is highly complex, composed of a filter unit and a tubular segment, together containing over 20 different cell types. The tubular epithelium is highly polarized, and the maintenance of this polarity is critical for optimal functioning and response to environmental signals. Cell polarity is dependent on communication between cells, which includes paracrine and autocrine signals, as well as biomechanic and chemotactic processes. These processes all influence kidney cell proliferation, migration, and differentiation. For drug disposition studies, this microenvironment is essential for prediction of toxic responses. This review provides an overview of drug-induced injuries to the kidney, details on relevant and translational biomarkers, and advances in 3D cultures of human renal cells, including organoids and kidney-on-a-chip platforms.

Microfluidic Cell Trapping for Single-Cell Analysis

The single-cell capture microfluidic chip has many advantages, including low cost, high throughput, easy manufacturing, integration, non-toxicity and good stability. Because of these characteristics, the cell capture microfluidic chip is increasingly becoming an important carrier on the study of life science and pharmaceutical analysis. Important promises of single-cell analysis are the paring, fusion, disruption and analysis of intracellular components for capturing a single cell. The capture, which is based on the fluid dynamics method in the field of micro fluidic chips is an important way to achieve and realize the operations mentioned above. The aim of this study was to compare the ability of three fluid dynamics-based microfluidic chip structures to capture cells. The effects of cell growth and distribution after being captured by different structural chips and the subsequent observation and analysis of single cells on the chip were compared. It can be seen from the experimental results that the microfluidic chip structure most suitable for single-cell capture is a U-shaped structure. It enables single-cell capture as well as long-term continuous culture and the single-cell observation of captured cells. Compared to the U-shaped structure, the cells captured by the microcavity structure easily overlapped during the culture process and affected the subsequent analysis of single cells. The flow shortcut structure can also be used to capture and observe single cells, however, the shearing force of the fluid caused by the chip structure is likely to cause deformation of the cultured cells. By comparing the cell capture efficiency of the three chips, the reagent loss during the culture process and the cell growth state of the captured cells, we are provided with a theoretical support for the design of a single-cell capture microfluidic chip and a reference for the study of single-cell capture in the future.

Melamine promotes calcium crystal formation in three-dimensional microfluidic device

Melamine, which induces proximal tubular (PT) cell damage has a greater nephrotoxic effect when combined with cyanuric and uric acids; however, it is unknown whether such effect can stimulate calcium phosphate (CaP)/calcium oxalate (CaOx) stone formation. Here, we show that melamine acts as an inducer of CaP, CaOx and CaP + CaOx (mixed) crystal formations in a time and concentration-dependent manner by stabilizing those crystals and further co-aggregating with melamine. To explore the physiological relevance of such melamine-augmented calcium crystal formation, we used 2-dimensional (2D) and 3D microfluidic (MF) device, embedded with PT cells, which also resembled the effect of melamine-stimulated CaP, CaOx and mixed crystal formation. Significantly, addition of preformed CaP and/or CaOx crystal in the presence of melamine, further potentiated those crystal formations in 3D MFs, which helped the growth and aggregation of mixed crystals. Our data show that the mechanism of such predisposition of stone formation could be largely due to co-crystallization between melamine and CaP/CaOx and pronounced effect on induction of stone-forming pathway activation in 3D MF. Taken together, melamine-induced CaP and/or CaOx crystal formation ex-vivo will help us in understanding the larger role of melamine as an environmental toxicant in producing the pathology in similar cellular microenvironments.

Organs-on-a-chip

In the last few years, considerable attention has been given to in vitro models in an attempt to reduce the use of animals and to decrease the rate of preclinical failure associated with the development of new drugs. Simple two-dimensional cultures grown in a dish are now frequently replaced by organotypic cultures with three-dimensional (3-D) architecture, which enables interactions between cells, promoting their differentiation and increasing their in vivo likeness. Microengineering now enables the incorporation of small devices into 3-D culture models to reproduce the complex microenvironment of the modeled organ, often referred to as organs-on-a-chip (OoCs). This review describes various OoCs developed to mimic liver, brain, kidney, and lung tissues. Current challenges encountered in attempts to recreate the in vivo environment are described, as well as some examples of OoCs. Finally, attention is given to the ongoing evolution of OoCs with the aim of solving one of the major limitations in that they can only represent a single organ. Multi-organ-on-a-chip (MOC) systems mimic organ interactions observed in the human body and aim to provide the features of compound uptake, metabolism, and excretion, while simultaneously allowing for insights into biological effects. MOCs might therefore represent a new paradigm in drug development, providing a better understanding of dose responses and mechanisms of toxicity, enabling the detection of drug resistance and supporting the evaluation of pharmacokinetic–pharmacodynamics parameters.

Mimicking the Kidney: A Key Role in Organ-on-Chip Development

Pharmaceutical drug screening and research into diseases call for significant improvement in the effectiveness of current in vitro models. Better models would reduce the likelihood of costly failures at later drug development stages, while limiting or possibly even avoiding the use of animal models. In this regard, promising advances have recently been made by the so-called "organ-on-chip" (OOC) technology. By combining cell culture with microfluidics, biomedical researchers have started to develop microengineered models of the functional units of human organs. With the capacity to mimic physiological microenvironments and vascular perfusion, OOC devices allow the reproduction of tissue- and organ-level functions. When considering drug testing, nephrotoxicity is a major cause of attrition during pre-clinical, clinical, and post-approval stages. Renal toxicity accounts for 19% of total dropouts during phase III drug evaluation-more than half the drugs abandoned because of safety concerns. Mimicking the functional unit of the kidney, namely the nephron, is therefore a crucial objective. Here we provide an extensive review of the studies focused on the development of a nephron-on-chip device.

Calcium oxalate precipitation by diffusion using laminar microfluidics: toward a biomimetic model of pathological microcalcifications

The effect of mixing calcium and oxalate precursors by diffusion at miscible liquid interfaces on calcium oxalate crystalline phases, and in physiological conditions (concentrations and flow rates), is studied using a microfluidic channel. This channel has similar dimensions as the collection duct in human kidneys and serves as a biomimetic model in order to understand the formation of pathological microcalcifications.

A Role for 3D Printing in Kidney-on-a-Chip Platforms

The advancement of "kidney-on-a-chip" platforms - submillimeter-scale fluidic systems designed to recapitulate renal functions in vitro - directly impacts a wide range of biomedical fields, including drug screening, cell and tissue engineering, toxicity testing, and disease modelling. To fabricate kidney-on-a-chip technologies, researchers have primarily adapted traditional micromachining techniques that are rooted in the integrated circuit industry; hence the term, "chip." A significant challenge, however, is that such methods are inherently monolithic, which limits one's ability to accurately recreate the geometric and architectural complexity of the kidney in vivo. Better reproduction of the anatomical complexity of the kidney will allow for more instructive modelling of physiological and pathophysiological events. Emerging additive manufacturing or "three-dimensional (3D) printing" techniques could provide a promising alternative to conventional methodologies. In this article, we discuss recent progress in the development of both kidney-on-a-chip platforms and state-of-the-art submillimeter-scale 3D printing methods, with a focus on biophysical and architectural capabilities. Lastly, we examine the potential for 3D printing-based approaches to extend the efficacy of kidney-on-a-chip systems.

Kidney bioengineering in regenerative medicine: An emerging therapy for kidney disease

The prevalence of end-stage renal disease is emerging as a serious worldwide public health problem because of the shortage of donor organs and the need to take lifelong immunosuppressive medication in patients who receive a transplanted kidney. Recently, tissue bioengineering of decellularization and recellularization scaffolds has emerged as a novel strategy for organ regeneration, and we review the critical technologies supporting these methods. We present a summary of factors associated with experimental protocols that may shed light on the future development of kidney bioengineering and we discuss the cell sources and bioreactor techniques applied to the recellularization process. Finally, we review some artificial renal engineering technologies and their future prospects, such as kidney on a chip and the application of three-dimensional and four-dimensional printing in kidney tissue engineering.

A bioartificial environment for kidney epithelial cells based on a supramolecular polymer basement membrane mimic and an organotypical culture system

Renal applications in healthcare, such as renal replacement therapies and nephrotoxicity tests, could potentially benefit from bioartificial kidney membranes with fully differentiated and functional human tubular epithelial cells. A replacement of the natural environment of these cells is required to maintain and study cell functionality cell differentiation in vitro. Our approach was based on synthetic supramolecular biomaterials to mimic the natural basement membrane (BM) on which these cells grow and a bioreactor to provide the desired organotypical culture parameters. The BM mimics were constructed from ureidopyrimidinone (UPy)-functionalized polymer and bioactive peptides by electrospinning. The resultant membranes were shown to have a hierarchical fibrous BM-like structure consisting of self-assembled nanofibres within the electrospun microfibres. Human kidney-2 (HK-2) epithelial cells were cultured on the BM mimics under organotypical conditions in a custom-built bioreactor. The bioreactor facilitated in situ monitoring and functionality testing of the cultures. Cell viability and the integrity of the epithelial cell barrier were demonstrated inside the bioreactor by microscopy and transmembrane leakage of fluorescently labelled inulin, respectively. Furthermore, HK-2 cells maintained a polarized cell layer and showed modulation of both gene expression of membrane transporter proteins and metabolic activity of brush border enzymes when subjected to a continuous flow of culture medium inside the new bioreactor for 21 days. These results demonstrated that both the culture and study of renal epithelial cells was facilitated by the bioartificial in vitro environment that is formed by synthetic supramolecular BM mimics in our custom-built bioreactor. Copyright © 2015 John Wiley & Sons, Ltd.

Vascular Calcification and Stone Disease: A New Look towards the Mechanism

Calcium phosphate (CaP) crystals are formed in pathological calcification as well as during stone formation. Although there are several theories as to how these crystals can develop through the combined interactions of biochemical and biophysical factors, the exact mechanism of such mineralization is largely unknown. Based on the published scientific literature, we found that common factors can link the initial stages of stone formation and calcification in anatomically distal tissues and organs. For example, changes to the spatiotemporal conditions of the fluid flow in tubular structures may provide initial condition(s) for CaP crystal generation needed for stone formation. Additionally, recent evidence has provided a meaningful association between the active participation of proteins and transcription factors found in the bone forming (ossification) mechanism that are also involved in the early stages of kidney stone formation and arterial calcification. Our review will focus on three topics of discussion (physiological influences-calcium and phosphate concentration-and similarities to ossification, or bone formation) that may elucidate some commonality in the mechanisms of stone formation and calcification, and pave the way towards opening new avenues for further research.

Engineered in vitro disease models

The ultimate goal of most biomedical research is to gain greater insight into mechanisms of human disease or to develop new and improved therapies or diagnostics. Although great advances have been made in terms of developing disease models in animals, such as transgenic mice, many of these models fail to faithfully recapitulate the human condition. In addition, it is difficult to identify critical cellular and molecular contributors to disease or to vary them independently in whole-animal models. This challenge has attracted the interest of engineers, who have begun to collaborate with biologists to leverage recent advances in tissue engineering and microfabrication to develop novel in vitro models of disease. As these models are synthetic systems, specific molecular factors and individual cell types, including parenchymal cells, vascular cells, and immune cells, can be varied independently while simultaneously measuring system-level responses in real time. In this article, we provide some examples of these efforts, including engineered models of diseases of the heart, lung, intestine, liver, kidney, cartilage, skin and vascular, endocrine, musculoskeletal, and nervous systems, as well as models of infectious diseases and cancer. We also describe how engineered in vitro models can be combined with human inducible pluripotent stem cells to enable new insights into a broad variety of disease mechanisms, as well as provide a test bed for screening new therapies.

Prospect for kidney bioengineering: shortcomings of the status quo

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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

Tissue-engineered kidney disease models

Renal disease represents a major health problem that often results in end-stage renal failure necessitating dialysis and eventually transplantation. Historically these diseases have been studied with patient observation and screening, animal models, and two-dimensional cell culture. In this review, we focus on recent advances in tissue engineered kidney disease models that have the capacity to compensate for the limitations of traditional modalities. The cells and materials utilized to develop these models are discussed and tissue engineered models of polycystic kidney disease, drug-induced nephrotoxicity, and the glomerulus are examined in detail. The application of these models has the potential to direct future disease treatments and preclinical drug development.