Functional Kidney Bioengineering with Pluripotent Stem-Cell-Derived Renal Progenitor Cells and Decellularized Kidney Scaffolds

Hydrogel-exosome system in tissue engineering: A promising therapeutic strategy

Characterized by their pivotal roles in cell-to-cell communication, cell proliferation, and immune regulation during tissue repair, exosomes have emerged as a promising avenue for "cell-free therapy" in clinical applications. Hydrogels, possessing commendable biocompatibility, degradability, adjustability, and physical properties akin to biological tissues, have also found extensive utility in tissue engineering and regenerative repair. The synergistic combination of exosomes and hydrogels holds the potential not only to enhance the efficiency of exosomes but also to collaboratively advance the tissue repair process. This review has summarized the advancements made over the past decade in the research of hydrogel-exosome systems for regenerating various tissues including skin, bone, cartilage, nerves and tendons, with a focus on the methods for encapsulating and releasing exosomes within the hydrogels. It has also critically examined the gaps and limitations in current research, whilst proposed future directions and potential applications of this innovative approach.

Natural Hydrogels Support Kidney Organoid Generation and Promote In Vitro Angiogenesis

To date, strategies aiming to modulate cell to extracellular matrix (ECM) interactions during organoid derivation remain largely unexplored. Here renal decellularized ECM (dECM) hydrogels are fabricated from porcine and human renal cortex as biomaterials to enrich cell-to-ECM crosstalk during the onset of kidney organoid differentiation from human pluripotent stem cells (hPSCs). Renal dECM-derived hydrogels are used in combination with hPSC-derived renal progenitor cells to define new approaches for 2D and 3D kidney organoid differentiation, demonstrating that in the presence of these biomaterials the resulting kidney organoids exhibit renal differentiation features and the formation of an endogenous vascular component. Based on these observations, a new method to produce kidney organoids with vascular-like structures is achieved through the assembly of hPSC-derived endothelial-like organoids with kidney organoids in 3D. Major readouts of kidney differentiation and renal cell morphology are assessed exploiting these culture platforms as new models of nephrogenesis. Overall, this work shows that exploiting cell-to-ECM interactions during the onset of kidney differentiation from hPSCs facilitates and optimizes current approaches for kidney organoid derivation thereby increasing the utility of these unique cell culture platforms for personalized medicine.

Decellularized kidney extracellular matrix-based hydrogels for renal tissue engineering

Kidney regeneration is hindered by the limited pool of intrinsic reparative cells. Advanced therapies targeting renal regeneration have the potential to alleviate the clinical and financial burdens associated with kidney disease. Delivery systems for cells, extracellular vesicles, or growth factors aimed at enhancing regeneration can benefit from vehicles enabling targeted delivery and controlled release. Hydrogels, optimized to carry biological cargo while promoting regeneration, have emerged as promising candidates for this purpose. This study aims to develop a hydrogel from decellularized kidney extracellular matrix (DKECM) and explore its biocompatibility as a biomaterial for renal regeneration. The resulting hydrogel crosslinks with temperature and exhibits a high concentration of extracellular matrix. The decellularization process efficiently removes detergent residues, yielding a pathogen-free biomaterial that is non-hemolytic and devoid of α-gal epitope. Upon interaction with macrophages, the hydrogel induces differentiation into both pro-inflammatory and anti-inflammatory phenotypes, suggesting an adequate balance to promote biomaterial functionality in vivo. Renal progenitor cells encapsulated in the DKECM hydrogel demonstrate higher viability and proliferation than in commercial collagen-I hydrogels, while also expressing tubular cells and podocyte markers in long-term culture. Overall, the injectable biomaterial derived from porcine DKECM is anticipated to elicit minimal host reaction while fostering progenitor cell bioactivity, offering a potential avenue for enhancing renal regeneration in clinical settings. STATEMENT OF SIGNIFICANCE: The quest to improve treatments for kidney disease is crucial, given the challenges faced by patients on dialysis or waiting for transplants. Exciting new therapies combining biomaterials with cells can revolutionize kidney repair. In this study, researchers created a hydrogel from pig kidney. This gel could be used to deliver cells and other substances that help in kidney regeneration. Despite coming from pigs, it's safe for use in humans, with no harmful substances and reduced risk of immune reactions. Importantly, it promotes a balanced healing response in the body. This research not only advances our knowledge of kidney repair but also offers hope for more effective treatments for kidney diseases.

Current status and future prospects of decellularized kidney tissue

End-stage renal disease (ESRD) is characterized by progressive loss of kidney function, which can result in damage to various tissues and organs. Dialysis therapy and kidney transplantation are currently the only therapeutic options available for patients with ESRD. In the case of kidney transplantation, organ shortage and high organ rejection have increased the need for novel treatment modalities. Therefore, organ regeneration employing decellularization technology has emerged as a viable alternative to conventional organ transplantation. In this technology, organs are decellularized using physical, chemical, or biological means to create a natural scaffold and foundation for cell seeding. After in vivo transplantation, this scaffold can be recellularized using stem cells or adult differentiated cells, resulting in a functional organ devoid of immune response. This review focuses on the primary agents used for renal decellularization and the current status of kidney regeneration using decellularization.

Renal tissue engineering for regenerative medicine using polymers and hydrogels

Chronic Kidney Disease (CKD) is a growing worldwide problem, leading to end-stage renal disease (ESRD). Current treatments for ESRD include haemodialysis and kidney transplantation, but both are deemed inadequate since haemodialysis does not address all other kidney functions, and there is a shortage of suitable donor organs for transplantation. Research in kidney tissue engineering has been initiated to take a regenerative medicine approach as a potential treatment alternative, either to develop effective cell therapy for reconstruction or engineer a functioning bioartificial kidney. Currently, renal tissue engineering encompasses various materials, mainly polymers and hydrogels, which have been chosen to recreate the sophisticated kidney architecture. It is essential to address the chemical and mechanical aspects of the materials to ensure they can support cell development to restore functionality and feasibility. This paper reviews the types of polymers and hydrogels that have been used in kidney tissue engineering applications, both natural and synthetic, focusing on the processing and formulation used in creating bioactive substrates and how these biomaterials affect the cell biology of the kidney cells used.

Tissue engineering of decellularized pancreas scaffolds for regenerative medicine in diabetes

Diabetes mellitus is a global disease requiring long-term treatment and monitoring. At present, pancreas or islet transplantation is the only reliable treatment for achieving stable euglycemia in Type I diabetes patients. However, the shortage of viable pancreata for transplantation limits the use of this therapy for the majority of patients. Organ decellularization and recellularization is emerging as a promising solution to overcome the shortage of viable organs for transplantation by providing a potential alternative source of donor organs. Several studies on decellularization and recellularization of rodent, porcine, and human pancreata have been performed, and show promise for generating usable decellularized pancreas scaffolds for subsequent recellularization and transplantation. In this state-of-the-art review, we provide an overview of the latest advances in pancreas decellularization, recellularization, and revascularization. We also discuss clinical considerations such as potential transplantation sites, donor source, and immune considerations. We conclude with an outlook on the remaining work that needs to be done in order to realize the goal of using this technology to create bioengineered pancreata for transplantation in diabetes patients. STATEMENT OF SIGNIFICANCE: Pancreas or islet transplantation is a means of providing insulin-independence in diabetes patients. However, due to the shortage of viable pancreata, whole-organ decellularization and recellularization is emerging as a promising solution to overcome organ shortage for transplantation. Several studies on decellularization and recellularization of rodent, porcine, and human pancreata have shown promise for generating usable decellularized pancreas scaffolds for subsequent recellularization and transplantation. In this state-of-the-art review, we highlight the latest advances in pancreas decellularization, recellularization, and revascularization. We also discuss clinical considerations such as potential transplantation sites, donor source, and immune considerations. We conclude with future work that needs to be done in order to realize clinical translation of bioengineered pancreata for transplantation in diabetes patients.

Angiogenesis and Re-endothelialization in decellularized scaffolds: Recent advances and current challenges in tissue engineering

Decellularization of tissues and organs has recently become a promising approach in tissue engineering and regenerative medicine to circumvent the challenges of organ donation and complications of transplantations. However, one main obstacle to reaching this goal is acellular vasculature angiogenesis and endothelialization. Achieving an intact and functional vascular structure as a vital pathway for supplying oxygen and nutrients remains the decisive challenge in the decellularization/re-endothelialization procedure. In order to better understand and overcome this issue, complete and appropriate knowledge of endothelialization and its determining variables is required. Decellularization methods and their effectiveness, biological and mechanical characteristics of acellular scaffolds, artificial and biological bioreactors, and their possible applications, extracellular matrix surface modification, and different types of utilized cells are factors affecting endothelialization consequences. This review focuses on the characteristics of endothelialization and how to optimize them, as well as discussing recent developments in the process of re-endothelialization.

Role of extracellular matrix components and structure in new renal models in vitro

The extracellular matrix (ECM), a complex set of fibrillar proteins and proteoglycans, supports the renal parenchyma and provides biomechanical and biochemical cues critical for spatial-temporal patterning of cell development and acquisition of specialized functions. As in vitro models progress towards biomimicry, more attention is paid to reproducing ECM-mediated stimuli. ECM's role in in vitro models of renal function and disease used to investigate kidney injury and regeneration is discussed. Availability, affordability, and lot-to-lot consistency are the main factors determining the selection of materials to recreate ECM in vitro. While simpler components can be synthesized in vitro, others must be isolated from animal or human tissues, either as single isolated components or as complex mixtures, such as Matrigel or decellularized formulations. Synthetic polymeric materials with dynamic and instructive capacities are also being explored for cell mechanical support to overcome the issues with natural products. ECM components can be used as simple 2D coatings or complex 3D scaffolds combining natural and synthetic materials. The goal is to recreate the biochemical signals provided by glycosaminoglycans and other signaling molecules, together with the stiffness, elasticity, segmentation, and dimensionality of the original kidney tissue, to support the specialized functions of glomerular, tubular, and vascular compartments. ECM mimicking also plays a central role in recent developments aiming to reproduce renal tissue in vitro or even in therapeutical strategies to regenerate renal function. Bioprinting of renal tubules, recellularization of kidney ECM scaffolds, and development of kidney organoids are examples. Future solutions will probably combine these technologies.

Three-Dimensional Bioprinting of Decellularized Extracellular Matrix-Based Bioinks for Tissue Engineering

Three-dimensional (3D) bioprinting is one of the most promising additive manufacturing technologies for fabricating various biomimetic architectures of tissues and organs. In this context, the bioink, a critical element for biofabrication, is a mixture of biomaterials and living cells used in 3D printing to create cell-laden structures. Recently, decellularized extracellular matrix (dECM)-based bioinks derived from natural tissues have garnered enormous attention from researchers due to their unique and complex biochemical properties. This review initially presents the details of the natural ECM and its role in cell growth and metabolism. Further, we briefly emphasize the commonly used decellularization treatment procedures and subsequent evaluations for the quality control of the dECM. In addition, we summarize some of the common bioink preparation strategies, the 3D bioprinting approaches, and the applicability of 3D-printed dECM bioinks to tissue engineering. Finally, we present some of the challenges in this field and the prospects for future development.

Genipin-crosslinked decellularized scaffold induces regeneration of defective rat kidneys

Objective: The purpose of this study was to improve the performance of decellularized renal scaffolds by the genipin crosslinking method to facilitate the regeneration of tissues and cells and provide better conditions for the regeneration and repair of defective kidneys. Methods: SD rats were randomly divided into three groups: normal group, uncrosslinked scaffold group and genipin-crosslinked scaffold group. Hematoxylin eosin, Masson and immunofluorescence staining was used to observe the histomorphological characteristics of the kidneys in each group. The preservation of the renal vascular structure in the three groups was observed by vascular casting. A collagenase degradation assay was used to detect the antidegradation ability of the kidney in the three groups. CCK8 assays were used to test the in vitro biocompatibility of the scaffolds. The lower 1/3 of the rat left kidney was excised, and the defect was filled with decellularized renal scaffolds to observe the effect of scaffold implantation on the regenerative ability of the defective kidney. Results: Histological images showed that the genipin-crosslinked scaffold did not destroy the structure of the scaffold, and the collagen fibers in the scaffold was more regular, and the outline of the glomerulus was clearer than uncrosslinked scaffold. The results of casting showed that the vascular structure of genipin-crosslinked scaffold was still intact. The anti-degradation ability test showed that the anti-degradation ability of genipin-crosslinked scaffold was significantly higher than that of the uncrosslinked scaffold. Cell culture experiments showed that the genipin-crosslinked scaffold had no cytotoxicity and promoted cell proliferation to some extent. In vivo scaffold transplantation experiments further demonstrated that the genipin-crosslinked scaffold had better anti-degradation and anti-inflammatory ability. Conclusion: Genipin-crosslinked rat kidney scaffold complemented kidney defects in rats can enhance scaffold-induced kidney regeneration and repair.

Promotion of Cyst Formation from a Renal Stem Cell Line Using Organ-Specific Extracellular Matrix Gel Format Culture System

Researchers have long awaited the technology to develop an in vitro kidney model. Here, we establish a rapid fabricating technique for kidney-like tissues (cysts) using a combination of an organ-derived extracellular matrix (ECM) gel format culture system and a renal stem cell line (CHK-Q cells). CHK-Q cells, which are spontaneously immortalized from the renal stem cells of the Chinese hamster, formed renal cyst-like structures in a type-I collagen gel sandwich culture on day 1 of culture. The cysts fused together and expanded while maintaining three-dimensional structures. The expression of genes related to kidney development and maturation was increased compared with that in a traditional monolayer. Under the kidney-derived ECM (K-ECM) gel format culture system, cyst formation and maturation were induced rapidly. Gene expressions involved in cell polarities, especially for important material transporters (typical markers Slc5a1 and Kcnj1), were restored. K-ECM composition was an important trigger for CHK-Q cells to promote kidney-like tissue formation and maturation. We have established a renal cyst model which rapidly expressed mature kidney features via the combination of K-ECM gel format culture system and CHK-Q cells.

Decellularization of porcine kidney with submicellar concentrations of SDS results in the retention of ECM proteins required for the adhesion and maintenance of human adult renal epithelial cells

When decellularizing kidneys, it is important to maintain the integrity of the acellular extracellular matrix (ECM), including associated adhesion proteins and growth factors that allow recellularized cells to adhere and migrate according to ECM specificity. Kidney decellularization requires the ionic detergent sodium dodecyl sulfate (SDS); however, this results in a loss of ECM proteins important for cell adherence, migration, and growth, particularly glycosaminoglycan (GAG)-associated proteins. Here, we demonstrate that using submicellar concentrations of SDS results in a greater retention of structural proteins, GAGs, growth factors, and cytokines. When porcine kidney ECM scaffolds were recellularized using human adult primary renal epithelial cells (RECs), the ECM promoted cell survival and the uniform distribution of cells throughout the ECM. Cells maintained the expression of mature renal epithelial markers but did not organize on the ECM, indicating that mature cells are unable to migrate to specific locations on ECM scaffolds.

Biomimetic models of the glomerulus

The use of biomimetic models of the glomerulus has the potential to improve our understanding of the pathogenesis of kidney diseases and to enable progress in therapeutics. Current in vitro models comprise organ-on-a-chip, scaffold-based and organoid approaches. Glomerulus-on-a-chip designs mimic components of glomerular microfluidic flow but lack the inherent complexity of the glomerular filtration barrier. Scaffold-based 3D culture systems and organoids provide greater microenvironmental complexity but do not replicate fluid flows and dynamic responses to fluidic stimuli. As the available models do not accurately model the structure or filtration function of the glomerulus, their applications are limited. An optimal approach to glomerular modelling is yet to be developed, but the field will probably benefit from advances in biofabrication techniques. In particular, 3D bioprinting technologies could enable the fabrication of constructs that recapitulate the complex structure of the glomerulus and the glomerular filtration barrier. The next generation of in vitro glomerular models must be suitable for high(er)-content or/and high(er)-throughput screening to enable continuous and systematic monitoring. Moreover, coupling of glomerular or kidney models with those of other organs is a promising approach to enable modelling of partial or full-body responses to drugs and prediction of therapeutic outcomes.

Research Progress, Challenges, and Breakthroughs of Organoids as Disease Models

Traditional cell lines and xenograft models have been widely recognized and used in research. As a new research model, organoids have made significant progress and development in the past 10 years. Compared with traditional models, organoids have more advantages and have been applied in cancer research, genetic diseases, infectious diseases, and regenerative medicine. This review presented the advantages and disadvantages of organoids in physiological development, pathological mechanism, drug screening, and organ transplantation. Further, this review summarized the current situation of vascularization, immune microenvironment, and hydrogel, which are the main influencing factors of organoids, and pointed out the future directions of development.

Kidney bioengineering by using decellularized kidney scaffold and renal progenitor cells

Patients with end-stage renal disease often need dialysis to maintain their lives because of donor organ shortage. The creation of a transplantable graft to permanently replace kidney function would overcome the organ shortage problem and the morbidity associated with immunosuppression. In the present study, we decellularized rat kidneys by the perfusion of detergent, yielding acellular scaffolds with the vascular, uretic, as well as cortical and medullary architecture. To regenerate the functional organ, we seeded tubular epithelial cells and mouse kidney progenitor cells from the ureter together with endothelial cells and mouse kidney progenitor cells from the renal artery. The renal constructs from seeded cells were cultured in a whole-organ bioreactor. After 3 months of organ culture, the seeded cells formed renal tubules, grew in the glomeruli, and some mouse kidney progenitor cells were also scattered in the interstitium. We tested the function of the bioengineered kidney with standardized perfusate in vitro. The bioengineered kidney not only produced urine but also reabsorbed albumin, glucose, and calcium. We conclude that seeded cell-based bioengineering of kidneys with physiological secreting and reabsorbing properties is possible and holds therapeutic promise.

Bioengineering platforms for cell therapeutics derived from pluripotent and direct reprogramming

Pluripotent and direct reprogramming technologies hold great potential for tissue repair and restoration of tissue and organ function. The implementation of induced pluripotent stem cells and directly reprogrammed cells in biomedical research has resulted in a significant leap forward in the highly promising area of regenerative medicine. While these therapeutic strategies are promising, there are several obstacles to overcome prior to the introduction of these therapies into clinical settings. Bioengineering technologies, such as biomaterials, bioprinting, microfluidic devices, and biostimulatory systems, can enhance cell viability, differentiation, and function, in turn the efficacy of cell therapeutics generated via pluripotent and direct reprogramming. Therefore, cellular reprogramming technologies, in combination with tissue-engineering platforms, are poised to overcome current bottlenecks associated with cell-based therapies and create new ways of producing engineered tissue substitutes.

Renal Regeneration: The Role of Extracellular Matrix and Current ECM-Based Tissue Engineered Strategies

Natural extracellular matrices (ECM) are currently being studied as an alternative source for organ transplantation or as new solutions to treat kidney injuries, which can evolve to end-stage renal disease, a life devastating condition. This paper provides an overview on the current knowledge in kidney ECM and its usefulness on future investigations. The composition and structure of kidney ECM is herein associated with its intrinsic capacity of remodeling and repair after insult. Moreover, it provides a deeper insight on altered ECM components during disease. The use of decellularized kidney matrices is discussed in the second part of the review, with emphasis on how these matrices contribute to tissue-specific differentiation of embryonic, pluripotent, and other stem cells. The evolution on the field toward different uses of xenogeneic ECM as a biological scaffold material is discussed, namely the major outcomes on whole kidney recellularization and its in vivo implantation. At last, the recent literature on the use of processed kidney decellularized ECM to produce diverse biomaterial substrates, such as hydrogels, membranes, and bioinks are reviewed, with emphasis on future perspectives of its translation into the clinic.

Bioengineered Kidney Models: Methods and Functional Assessments

Investigations into bioengineering kidneys have been extensively conducted owing to their potential for preclinical assays and regenerative medicine. Various approaches and methods have been developed to improve the structure and function of bioengineered kidneys. Assessments of functional properties confirm the adequacy of bioengineered kidneys for multipurpose translational applications. This review is to summarize the studies performed in kidney bioengineering in the past decade. We identified 84 original articles from PubMed and Mendeley with keywords of kidney organoid or kidney tissue engineering. Those were categorized into 5 groups based on their approach: de-/recellularization of kidney, reaggregation of kidney cells, kidney organoids, kidney in scaffolds, and kidney-on-a-chip. These models were physiologically assessed by filtration, tubular reabsorption/secretion, hormone production, and nephrotoxicity. We found that bioengineered kidney models have been developed from simple cell cultures to multicellular systems to recapitulate kidney function and diseases. Meanwhile, only about 50% of these studies conducted functional assessments on their kidney models. Factors including cell composition and organization are likely to alter the applicability of physiological assessments in bioengineered kidneys. Combined with recent technologies, physiological assessments importantly contribute to the improvement of the bioengineered kidney model toward repairing and refunctioning the damaged kidney.

The role of vascular niche and endothelial cells in organogenesis and regeneration

The term vascular niche indicate the physical and biochemical microenvironment around blood vessel where endothelial cells, pericytes, and smooth muscle cells organize themselves to form blood vessels and release molecules involved in the recruitment of hematopoietic stem cells, endothelial progenitor cells and mesenchymal stem cells. The vascular niche creates a permissive environment that enables different cell types to realize their developmental or regenerative programs. In this context, the proximity between the endothelium and the new-forming cellular components of organs suggests an essential role of endothelial cells in the organs maturation. Dynamic interactions between specific organ endothelial cells and different cellular conponents are crucial for different organ morphogenesis and function. Conversely, organs provide cues shaping vascular network structure.

Renal proximal tubular epithelial cells: review of isolation, characterization, and culturing techniques

The kidney is a complex organ, comprised primarily of glomerular, tubular, mesangial, and endothelial cells, and podocytes. The fact that renal cells are terminally differentiated at 34 weeks of gestation is the main obstacle in regeneration and treatment of acute kidney injury or chronic kidney disease. Furthermore, the number of chronic kidney disease patients is ever increasing and with it the medical community should aim to improve existing and develop new methods of renal replacement therapy. On the other hand, as polypharmacy is on the rise, thought should be given into developing new ways of testing drug safety. A possible way to tackle these issues is with isolation and culture of renal cells. Several protocols are currently described to isolate the desired cells, of which the most isolated are the proximal tubular epithelial cells. They play a major role in water homeostasis, acid-base control, reabsorption of compounds, and secretion of xenobiotics and endogenous metabolites. When exposed to ischemic, toxic, septic, or obstructive conditions their death results in what we clinically perceive as acute kidney injury. Additionally, due to renal cells' limited regenerative potential, the profibrotic environment inevitably leads to chronic kidney disease. In this review we will focus on human proximal tubular epithelial cells. We will cover human kidney culture models, cell sources, isolation, culture, immortalization, and characterization subdivided into morphological, phenotypical, and functional characterization.

Cell derived extracellular matrix-rich biomimetic substrate supports podocyte proliferation, differentiation and maintenance of native phenotype

Current technologies and available scaffold materials do not support long-term cell viability, differentiation and maintenance of podocytes, the ultra-specialized kidney resident cells that are responsible for the filtration of the blood. We developed a new platform which imitates the native kidney microenvironment by decellularizing fibroblasts grown on surfaces with macromolecular crowding. Human immortalized podocytes cultured on this platform displayed superior viability and metabolic activity up to 28 days compared to podocytes cultured on tissue culture plastic surfaces. The new platform displayed a softer surface and an abundance of growth factors and associated molecules. More importantly it enabled podocytes to display molecules responsible for their structure and function and a superior development of intercellular connections/interdigitations, consistent with maturation. The new platform can be used to study podocyte biology, test drug toxicity and determine whether sera from patients with podocytopathies are involved in the expression of glomerular pathology.

Differentiating Induced Pluripotent Stem Cells into Renal Cells: A New Approach to Treat Kidney Diseases

Renal disease is a major issue for global public health. Despite some progress in supportive care, the mortality rates among patients with this condition remain alarmingly high. Studies in pursuit of innovative strategies to treat renal diseases, especially stimulating kidney regeneration, have been developed. In this field, stem cell-based therapy has been a promising area. Induced pluripotent stem cell-derived renal cells (iPSC-RCs) represent an interesting source of cells for treating kidney diseases. Advances in regenerative medicine using iPSC-RCs and their application to the kidney are discussed in this review. Furthermore, the way differentiation protocols of induced pluripotent stem cells into renal cells may also be applied for the generation of kidney organoids is also described, contributing to studies in renal development, kidney diseases, and drug toxicity tests. The translation of the differentiation methodologies into animal model studies and the safety and feasibility of renal differentiated cells as a treatment for kidney injury are also highlighted. Although only few studies were published in this field, the results seem promising and support the use of iPSC-RCs as a potential therapy in the future.

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.

Decellularization of Extracellular Matrix from Equine Skeletal Muscle

Equine represents an attractive animal model for musculoskeletal tissue diseases, exhibiting much similarity to the injuries that occur in humans. Cell therapy and tissue bioengineering have been widely used as a therapeutic alternative by regenerative medicine in musculoskeletal diseases. Thus, the aim of this study was to produce an acellular biomaterial of equine skeletal muscle and to evaluate its effectiveness in supporting the in vitro culture of equine induced pluripotency stem cells (iPSCs). Biceps femoris samples were frozen at -20°C for 4 days and incubated in 1% sodium dodecyl sulfate (SDS), 5 mM EDTA + 50 mM Tris and 1% Triton X-100; the effectiveness of the decellularization was evaluated by the absence of remnant nuclei (histological and 4',6-diamidino-2-phenylindole [DAPI] analysis), preservation of extracellular matrix (ECM) proteins (immunofluorescence and immunohistochemistry) and organization of ECM ultrastructure (scanning electron microscopy). Decellularized samples were recellularized with iPSCs at the concentration of 50,000 cells/cm² and cultured in vitro for 9 days, and the presence of the cells in the biomaterial was evaluated by histological analysis and presence of nuclei. Decellularized biomaterial showed absence of remnant nuclei and muscle fibers, as well as the preservation of ECM architecture, vascular network and proteins, laminin, fibronectin, elastin, collagen III and IV. After cellularization, iPSC nuclei were present at 9 days after incubation, indicating the decellularized biomaterial-supported iPSC survival. It is concluded that the ECM biomaterial produced from the decellularized equine skeletal muscle has potential for iPSC adhesion, representing a promising biomaterial for regenerative medicine in the therapy of musculoskeletal diseases.

Decellularization of canine kidney for three-dimensional organ regeneration

BACKGROUND AND AIM

Kidney regeneration is required for dogs with end-stage renal failure. Decellularization is one of the bioengineering techniques, which involves the removal of all tissue cells and cellular components and conservation of the extracellular matrix (ECM). Studies in rats have shown that decellularized kidney has regenerative potential; however, there are no reports on renal decellularization in dogs. Here, we showed the decellularization of the canine kidney.

MATERIALS AND METHODS

The renal artery of the cadaveric canine kidney was cannulated and the whole kidney was frozen at -80°C. After completely thawing, it was perfused with physiological saline and sodium dodecyl sulfate (0.5%, 6 h) through the cannulated renal artery to achieve decellularization. To assess the efficiency of the decellularization protocol, histological and immunohistochemical analysis of decellularized kidney was performed.

RESULTS

The results of hematoxylin and eosin (H and E) staining revealed that the decellularized canine kidney had no apparent cellular components. In addition, 4',6-diamidino-2-phenylindole (DAPI) staining showed no visible nuclear components within the whole decellularized kidney. Therefore, both H and E and DAPI staining showed decellularization of the canine kidney. Our decellularization protocol also preserved the basement membrane of glomerulus, shown by periodic acid methenamine silver, periodic acid-Schiff, fibronectin, and collagen type IV stain.

CONCLUSION

Our decellularization protocol could eliminate cellular components and remaining native ECM structures of canine kidney. These results could promote further research into canine kidney regeneration, which may be the first small step to regenerate the canine kidney waiting for renal transplantation.

Adult Tissue Extracellular Matrix Determines Tissue Specification of Human iPSC-Derived Embryonic Stage Mesodermal Precursor Cells

The selection of pluripotent stem cell (PSC)-derived cells for tissue modeling and cell therapy will be influenced by their response to the tissue environment, including the extracellular matrix (ECM). Whether and how instructive memory is imprinted in adult ECM and able to impact on the tissue specific determination of human PSC-derived developmentally fetal mesodermal precursor (P-meso) cells is investigated. Decellularized ECM (dECM) is generated from human heart, kidney, and lung tissues and recellularized with P-meso cells in a medium not containing any differentiation inducing components. While P-meso cells on kidney dECM differentiate exclusively into nephronal cells, only beating clusters containing mature and immature cardiac cells form on heart dECM. No tissue-specific differentiation of P-meso cells is observed on endoderm-derived lung dECM. P-meso-derived endothelial cells, however, are found on all dECM preparations independent of tissue origin. Clearance of heparan-sulfate proteoglycans (HSPG) from dECM abolishes induction of tissue-specific differentiation. It is concluded that HSPG-bound factors on adult tissue-derived ECM are essential and sufficient to induce tissue-specific specification of uncommitted fetal stage precursor cells.

Vascular reconstruction: A major challenge in developing a functional whole solid organ graft from decellularized organs

Bioengineering a functional organ holds great potential to overcome the current gap between the organ need and shortage of available organs. Whole organ decellularization allows the removal of cells from large-scale organs, leaving behind extracellular matrices containing different growth factors, structural proteins, and a vascular network with a bare surface. Successful application of decellularized tissues as transplantable organs is hampered by the inability to completely reline the vasculature by endothelial cells (ECs), leading to blood coagulation, loss of vascular patency, and subsequent death of reseeded cells. Therefore, an intact, continuous layer of endothelium is essential to maintain proper functioning of the vascular system, which includes the transfer of nutrients to surrounding tissues and protecting other types of cells from shear stress. Here, we aimed to summarize the available cell sources that can be used for reendothelialization in addition to different trials performed by researchers to reconstruct vascularization of decellularized solid organs. Additionally, different techniques for enhancing reendothelialization and the methods used for evaluating reendothelialization efficiency along with the future prospective applications of this field are discussed. STATEMENT OF SIGNIFICANCE: Despite the great progress in whole organ decellularization, reconstruction of vasculature within the engineered constructs is still a major roadblock. Reconstructed endothelium acts as a multifunctional barrier of vessels, which can reduce thrombosis and help delivering of oxygen and nutrients throughout the whole organ. Successful reendothelialization can be achieved through reseeding of appropriate cell types on the naked vasculature with or without modification of its surface. Here, we present the current research milestones that so far established to reconstruct the vascular network in addition to the methods used for evaluating the efficiency of reendotheilization. Thus, this review is quite significant and will aid the researchers to know where we stand toward biofabricating a transplantable organ from decellularizd extracellular matrix.

Naturally-Derived Biomaterials for Tissue Engineering Applications

Naturally-derived biomaterials have been used for decades in multiple regenerative medicine applications. From the simplest cell microcarriers made of collagen or alginate, to highly complex decellularized whole-organ scaffolds, these biomaterials represent a class of substances that is usually first in choice at the time of electing a functional and useful biomaterial. Hence, in this chapter we describe the several naturally-derived biomaterials used in tissue engineering applications and their classification, based on composition. We will also describe some of the present uses of the generated tissues like drug discovery, developmental biology, bioprinting and transplantation.

Engineering the vasculature of decellularized rat kidney scaffolds using human induced pluripotent stem cell-derived endothelial cells

Generating new kidneys using tissue engineering technologies is an innovative strategy for overcoming the shortage of donor organs for transplantation. Here we report how to efficiently engineer the kidney vasculature of decellularized rat kidney scaffolds by using human induced pluripotent stem cell (hiPSCs)-derived endothelial cells (hiPSC-ECs). In vitro, hiPSC-ECs responded to flow stress by acquiring an alignment orientation, and attached to and proliferated on the acellular kidney sections, maintaining their phenotype. The hiPSC-ECs were able to self-organize into chimeric kidney organoids to form vessel-like structures. Ex vivo infusion of hiPSC-ECs through the renal artery and vein of acellular kidneys resulted in the uniform distribution of the cells in all the vasculature compartments, from glomerular capillaries to peritubular capillaries and small vessels. Ultrastructural analysis of repopulated scaffolds through transmission and scanning electron microscopy demonstrated the presence of continuously distributed cells along the vessel wall, which was also confirmed by 3D reconstruction of z-stack images showing the continuity of endothelial cell coverage inside the vessels. Notably, the detection of fenestrae in the endothelium of glomerular capillaries but not in the vascular capillaries was clear evidence of site-specific endothelial cell specialisation.

Bioengineering Organs for Blood Detoxification

For patients with severe kidney or liver failure the best solution is currently organ transplantation. However, not all patients are eligible for transplantation and due to limited organ availability, most patients are currently treated with therapies using artificial kidney and artificial liver devices. These therapies, despite their relative success in preserving the patients' life, have important limitations since they can only replace part of the natural kidney or liver functions. As blood detoxification (and other functions) in these highly perfused organs is achieved by specialized cells, it seems relevant to review the approaches leading to bioengineered organs fulfilling most of the native organ functions. There, the culture of cells of specific phenotypes on adapted scaffolds that can be perfused takes place. In this review paper, first the functions of kidney and liver organs are briefly described. Then artificial kidney/liver devices, bioartificial kidney devices, and bioartificial liver devices are focused on, as well as biohybrid constructs obtained by decellularization and recellularization of animal organs. For all organs, a thorough overview of the literature is given and the perspectives for their application in the clinic are discussed.

Small-Sized Cationic miRi-PCNPs Selectively Target the Kidneys for High-Efficiency Antifibrosis Treatment

Small-sized cationic miRi (microRNA-21 inhibitor)-PCNPs (low molecular weight chitosan (LMWC)-modified polylactide-co-glycoside (PLGA) nanoparticles (PLNPs)) with special kidney-targeting and high-efficiency antifibrosis treatment are fabricated through coupling miRi, PLGA, and LMWC. In the miRi-PCNPs, easily degraded miRi is encapsulated in PCNPs and thus prevented from degradation by nuclease. Cytotoxicity, immunotoxicity, and systemic toxicity assays and in vitro and ex vivo fluorescence imaging suggest that PCNPs possess excellent biocompatibility, higher cellular uptake efficiency, and selective kidney-targeting capacity. Western blotting, pathological staining, and real-time polymerase chain reaction analyses show that the therapeutic effect of miRi-PCNPs on kidney fibrosis is much higher than that of miRi, which is mainly through suppressing transforming growth factor beta-1/drosophila mothers against decapentaplegic protein 3 (TGF-β1/Smad3) and extracellular signal-regulated kinases/mitogen-activated protein kinase signaling pathway by inhibiting the expression of microRNA-21. For example, the tubule damage index and tubulointerstitial fibrosis area in the miRi-PCNPs group are ≈2.5-fold lower than those in the saline and bare miRi groups. The miRi-PCNPs with special kidney-targeting and high-efficiency antifibrosis treatment may represent a promising strategy for designing and developing a therapeutic treatment for kidney fibrosis.

Tissue and organ decellularization in regenerative medicine

The advancement and improvement in decellularization methods can be attributed to the increasing demand for tissues and organs for transplantation. Decellularized tissues and organs, which are free of cells and genetic materials while retaining the complex ultrastructure of the extracellular matrix (ECM), can serve as scaffolds to subsequently embed cells for transplantation. They have the potential to mimic the native physiology of the targeted anatomic site. ECM from different tissues and organs harvested from various sources have been applied. Many techniques are currently involved in the decellularization process, which come along with their own advantages and disadvantages. This review focuses on recent developments in decellularization methods, the importance and nature of detergents used for decellularization, as well as on the role of the ECM either as merely a physical support or as a scaffold in retaining and providing cues for cell survival, differentiation and homeostasis. In addition, application, status, and perspectives on commercialization of bioproducts derived from decellularized tissues and organs are addressed. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 34:1494-1505, 2018.

Applications of kidney organoids derived from human pluripotent stem cells

The establishment of protocols to differentiate kidney organoids from human pluripotent stem cells provides potential applications of kidney organoids in regenerative medicine. Modeling of renal diseases, drug screening, nephrotoxicity testing of compounds, and regenerative therapy are attractive applications. Although much progress still remains to be made in the development of kidney organoids, recent advances in clustered regularly interspaced short palindromic repeat (CRISPR)-CRISPR-associated system 9 (Cas9) genome editing and three-dimensional bioprinting technologies have contributed to the application of kidney organoids in clinical fields. In this section, we review recent advances in the applications of kidney organoids to kidney disease modelling, drug screening, nephrotoxicity testing, and regenerative therapy.

Whole Organ Tissue Vascularization: Engineering the Tree to Develop the Fruits

Tissue engineering aims to regenerate and recapitulate a tissue or organ that has lost its function. So far successful clinical translation has been limited to hollow organs in which rudimental vascularization can be achieved by inserting the graft into flaps of the omentum or muscle fascia. This technique used to stimulate vascularization of the graft takes advantage of angiogenesis from existing vascular networks. Vascularization of the engineered graft is a fundamental requirement in the process of engineering more complex organs, as it is crucial for the efficient delivery of nutrients and oxygen following in-vivo implantation. To achieve vascularization of the organ many different techniques have been investigated and exploited. The most promising results have been obtained by seeding endothelial cells directly into decellularized scaffolds, taking advantage of the channels remaining from the pre-existing vascular network. Currently, the main hurdle we need to overcome is achieving a fully functional vascular endothelium, stable over a long time period of time, which is engineered using a cell source that is clinically suitable and can generate, in vitro, a yield of cells suitable for the engineering of human sized organs. This review will give an overview of the approaches that have recently been investigated to address the issue of vascularization in the field of tissue engineering of whole organs, and will highlight the current caveats and hurdles that should be addressed in the future.

Decellularized kidney matrix as functional material for whole organ tissue engineering

Renal transplantation is currently the most effective treatment for end-stage renal disease, which represents one of the major current public health problems. However, the number of available donor kidneys is drastically insufficient to meet the demand, causing prolonged waiting lists. For this reason, tissue engineering offers great potential to increase the pool of donated organs for kidney transplantation, by way of seeding cells on supporting scaffolding material. Biological scaffolds are prepared by removing cellular components from the donor organs using a decellularization process with detergents, enzymes or other cell lysing solutions. Extracellular matrix which makes up the scaffold is critical to directing the cell attachment and to creating a suitable environment for cell survival, proliferation and differentiation. Researchers are now studying whole intact scaffolds produced from the kidneys of animals or humans without adversely affecting extracellular matrix, biological activity and mechanical integrity. The process of recellularization includes cell seeding strategies and the choice of the cell source to repopulate the scaffold. This is the most difficult phase, due to the complexity of the kidney. Indeed, no studies have provided sufficient results of complete renal scaffold repopulation and differentiation. This review summarizes the research that has been conducted to obtain decellularized kidney scaffolds and to repopulate the scaffolds, evaluating the best cell sources, the cell seeding methods and the cell differentiation in kidney scaffolds.

Nephrosphere-Derived Cells Are Induced to Multilineage Differentiation when Cultured on Human Decellularized Kidney Scaffolds

In end-stage chronic kidney disease, the option of organ transplantation is limited because of the scarce availability of kidneys. The combination of stem cell research, regenerative medicine, and tissue engineering seems a promising approach to produce new transplantable kidneys. Currently, the possibility to repopulate naturally obtained scaffolds with cells of different sources is advancing. Our aim was to test, for the first time, whether the nephrosphere (NS) cells, composed by renal stem/progenitor-like cells, were able to repopulate different nephron portions of renal extracellular matrix scaffolds obtained after decellularization of human renal tissue slices. Our decellularization protocol enabled us to obtain a completely acellular renal scaffold while maintaining the extracellular matrix structure and composition in terms of collagen IV, laminin, and fibronectin. NS cells, cultured on decellularized renal scaffolds with basal medium, differentiated into proximal and distal tubules as well as endothelium, as highlighted by histology and by the specific expression of epithelial cytokeratin 8.18, proximal tubular CD10, distal tubular cytokeratin 7, and endothelial von Willebrand factor markers. Endothelial medium promoted the differentiation toward the endothelium, whereas epithelial medium promoted the differentiation toward the epithelium. NS cells seem to be a good tool for scaffold repopulation, paving the way for experimental investigations focused on whole-kidney reconstruction.

Obstacles in Renal Regenerative Medicine: Metabolic and Epigenetic Parallels Between Cellular Reprogramming and Kidney Cancer Oncogenesis

CONTEXT

Regenerative medicine has recently presented a revolutionary solution to end-stage kidney disease. Reprogramming patients' own cells generates induced pluripotent stem cells that are subsequently differentiated to "kidney organoid," a structure that is anatomically and functionally similar to the kidney. This approach holds the promise of a transplantable, immunocompetent, and functional kidney that could be produced in vitro. However, caution must be taken due to the molecular-level similarities between induced pluripotent stem cells and renal cell carcinomas. As such, if cell reprogramming is not tightly controlled, it can lead to carcinogenic changes.

OBJECTIVE

Based on recent next-generation sequencing results and other supporting data, we identified three major molecular attributes of renal cell carcinoma: metabolic alterations, epigenetic changes, and miRNA-based alterations. Strikingly, these variations are mirrored in induced pluripotent stem cells, which are the main cell source of renal regenerative medicine. Our objective was to discuss the shared metabolic, epigenetic and miRNA-regulated characteristics and to abridge their significance in renal regenerative medicine.

EVIDENCE ACQUISITION

English-language literature was retrieved through PubMed.

EVIDENCE SYNTHESIS

Authors collected the published evidence and evaluated the content based on independent literature findings. Articles were filtered to include only highly relevant, recent publications that presented reproducible results by authorities of the field.

CONCLUSIONS

The kidney represents a unique metabolic environment that could be hijacked by induced pluripotent stem cells or by partially differentiated cells for oncogenic transformation. Future differentiation protocols must produce kidney organoids that are fully engaged in filtration function.

PATIENT SUMMARY

A new technology can produce mini-kidneys or kidney organoids. This review discusses some of the challenges this technology has to face, including its high oncogenic potential. Understanding these similarities will lead to the safe creation of new functional kidney units in patients with kidney failure.

Stem cell-derived kidney cells and organoids: Recent breakthroughs and emerging applications

The global rise in the numbers of kidney patients and the shortage in transplantable organs have led to an increasing interest in kidney-specific regenerative therapies, renal disease modelling and bioartificial kidneys. Sources for large quantities of high-quality renal cells and tissues would be required, also for applications in in vitro platforms for compound safety and efficacy screening. Stem cell-based approaches for the generation of renal-like cells and tissues would be most attractive, but such methods were not available until recently. This situation has drastically changed since 2013, and various protocols for the generation of renal-like cells and precursors from pluripotent stem cells (PSC) have been established. The most recent breakthroughs were related to the establishment of various protocols for the generation of PSC-derived kidney organoids. In combination with recent advances in genome editing, bioprinting and the establishment of predictive renal screening platforms this results in exciting new possibilities. This review will give a comprehensive overview over current PSC-based protocols for the generation of renal-like cells, precursors and organoids, and their current and potential applications in regenerative medicine, compound screening, disease modelling and bioartificial organs.