Bioengineered Kidney Models: Methods and Functional Assessments

Navigating the kidney organoid: insights into assessment and enhancement of nephron function

Kidney organoids are three-dimensional structures generated from pluripotent stem cells (PSCs) that are capable of recapitulating the major structures of mammalian kidneys. As this technology is expected to be a promising tool for studying renal biology, drug discovery, and regenerative medicine, the functional capacity of kidney organoids has emerged as a critical question in the field. Kidney organoids produced using several protocols harbor key structures of native kidneys. Here, we review the current state, recent advances, and future challenges in the functional characterization of kidney organoids, strategies to accelerate and enhance kidney organoid functions, and access to PSC resources to advance organoid research. The strategies to construct physiologically relevant kidney organoids include the use of organ-on-a-chip technologies that integrate fluid circulation and improve organoid maturation. These approaches result in increased expression of the major tubular transporters and elements of mechanosensory signaling pathways suggestive of improved functionality. Nevertheless, continuous efforts remain crucial to create kidney tissue that more faithfully replicates physiological conditions for future applications in kidney regeneration medicine and their ethical use in patient care.NEW & NOTEWORTHY Kidney organoids are three-dimensional structures derived from stem cells, mimicking the major components of mammalian kidneys. Although they show great promise, their functional capacity has become a critical question. This review explores the advancements and challenges in evaluating and enhancing kidney organoid function, including the use of organ-on-chip technologies, multiomics data, and in vivo transplantation. Integrating these approaches to further enhance their physiological relevance will continue to advance disease modeling and regenerative medicine applications.

Kidney organoids: a pioneering model for kidney diseases

The kidney is a vital organ that regulates the bodily fluid and electrolyte homeostasis via tailored urinary excretion. Kidney injuries that cause severe or progressive chronic kidney disease have driven the growing population of patients with end-stage kidney disease, leading to substantial patient morbidity and mortality. This irreversible kidney damage has also created a huge socioeconomical burden on the healthcare system, highlighting the need for novel translational research models for progressive kidney diseases. Conventional research methods such as in vitro 2D cell culture or animal models do not fully recapitulate complex human kidney diseases. By contrast, directed differentiation of human induced pluripotent stem cells enables in vitro generation of patient-specific 3D kidney organoids, which can be used to model acute or chronic forms of hereditary, developmental, and metabolic kidney diseases. Furthermore, when combined with biofabrication techniques, organoids can be used as building blocks to construct vascularized kidney tissues mimicking their in vivo counterpart. By applying gene editing technology, organoid building blocks may be modified to minimize the process of immune rejection in kidney transplant recipients. In the foreseeable future, the universal kidney organoids derived from HLA-edited/deleted induced pluripotent stem cell (iPSC) lines may enable the supply of bioengineered organotypic kidney structures that are immune-compatible for the majority of the world population. Here, we summarize recent advances in kidney organoid research coupled with novel technologies such as organoids-on-chip and biofabrication of 3D kidney tissues providing convenient platforms for high-throughput drug screening, disease modelling, and therapeutic applications.

Plant vs. Kidney: Evaluating Nephrotoxicity of Botanicals with the Latest Toxicological Tools

Botanicals can cause nephrotoxicity via numerous mechanisms, including disrupting renal blood flow, damaging compartments along the nephron, and obstructing urinary flow. While uncommon, there are various reports of botanical-induced nephrotoxicity in the literature, such as from aristolochia (Aristolochia spp.) and rhubarb (Rheum spp.). However, at present, it is a challenge to assess the toxic potential of botanicals because their chemical composition is variable due to factors such as growing conditions and extraction techniques. Therefore, selecting a single representative sample for an in vivo study is difficult. Given the increasing use of botanicals as dietary supplements and herbal medicine, new approach methodologies (NAMs) are needed to evaluate the potential for renal toxicity to ensure public safety. Such approaches include in vitro models that use layers of physiological complexity to emulate the in vivo microenvironment, enhance the functional viability and differentiation of cell cultures, and improve sensitivity to nephrotoxic insults. Furthermore, computational tools such as physiologically based pharmacokinetic (PBPK) modeling can add confidence to these tools by simulating absorption, distribution, metabolism, and excretion. The development and implementation of NAMs for renal toxicity testing will allow specific mechanistic data to be generated, leading to a better understanding of the nephrotoxic potential of botanicals.

Kidney-on-a-Chip: Mechanical Stimulation and Sensor Integration

Bioengineered in vitro models of the kidney offer unprecedented opportunities to better mimic the in vivo microenvironment. Kidney-on-a-chip technology reproduces 2D or 3D features which can replicate features of the tissue architecture, composition, and dynamic mechanical forces experienced by cells in vivo. Kidney cells are exposed to mechanical stimuli such as substrate stiffness, shear stress, compression, and stretch, which regulate multiple cellular functions. Incorporating mechanical stimuli in kidney-on-a-chip is critically important for recapitulating the physiological or pathological microenvironment. This review will explore approaches to applying mechanical stimuli to different cell types using kidney-on-a-chip models and how these systems are used to study kidney physiology, model disease, and screen for drug toxicity. We further discuss sensor integration into kidney-on-a-chip for monitoring cellular responses to mechanical or other pathological stimuli. We discuss the advantages, limitations, and challenges associated with incorporating mechanical stimuli in kidney-on-a-chip models for a variety of applications. Overall, this review aims to highlight the importance of mechanical stimuli and sensor integration in the design and implementation of kidney-on-a-chip devices.

Live functional assays reveal longitudinal maturation of transepithelial transport in kidney organoids

Kidney organoids derived from hPSCs have opened new opportunities to develop kidney models for preclinical studies and immunocompatible kidney tissues for regeneration. Organoids resemble native nephrons that consist of filtration units and tubules, yet little is known about the functional capacity of these organoid structures. Transcriptomic analyses provide insight into maturation and transporter activities that represent kidney functions. However, functional assays in organoids are necessary to demonstrate the activity of these transport proteins in live tissues. The three-dimensional (3D) architecture adds complexity to real-time assays in kidney organoids. Here, we develop a functional assay using live imaging to assess transepithelial transport of rhodamine 123 (Rh123), a fluorescent substrate of P-glycoprotein (P-gp), in organoids affixed to coverslip culture plates for accurate real-time observation. The identity of organoid structures was probed using Lotus Tetragonolobus Lectin (LTL), which binds to glycoproteins present on the surface of proximal tubules. Within 20 min of the addition of Rh123 to culture media, Rh123 accumulated in the tubular lumen of organoids. Basolateral-to-apical accumulation of the dye/marker was reduced by pharmacologic inhibition of MDR1 or OCT2, and OCT2 inhibition reduced the Rh123 uptake. The magnitude of Rh123 transport was maturation-dependent, consistent with MDR1 expression levels assessed by RNA-seq and immunohistochemistry. Specifically, organoids on day 21 exhibit less accumulation of Rh123 in the lumen unlike later-stage organoids from day 30 of differentiation. Our work establishes a live functional assessment in 3D kidney organoids, enabling the functional phenotyping of organoids in health and disease.