Decellularized Human Kidney Cortex Hydrogels Enhance Kidney Microvascular Endothelial Cell Maturation and Quiescence

Human Chorionic Membrane-derived Tunable Hydrogels for Vascular Tissue Engineering Strategies

One of the foremost targets in the advancement of biomaterials to engineer vascularized tissues is not only to replicate the composition of the intended tissue but also to create thicker structures incorporating a vascular network for adequate nutrients and oxygen supply. For the first time, to the best of current knowledge, a clinically relevant biomaterial is developed, demonstrating that hydrogels made from the human decellularized extracellular matrix can exhibit robust mechanical properties (in the kPa range) and angiogenic capabilities simultaneously. These properties enable the culture and organization of human umbilical vein endothelial cells into tubular structures, maintaining their integrity for 14 days in vitro without the need for additional polymers or angiogenesis-related factors. This is achieved by repurposing the placenta chorionic membrane (CM), a medical waste with an exceptional biochemical composition, into a valuable resource for bioengineering purposes. After decellularization, the CM underwent chemical modification with methacryloyl groups, giving rise to methacrylated CM (CMMA). CMMA preserved key proteins, as well as glycosaminoglycans. The resulting hydrogels rapidly photopolymerize and have enhanced strength and customizable mechanical properties. Furthermore, they demonstrate angio-vasculogenic competence in vitro and in vivo, holding significant promise as a humanized platform for the engineering of vascularized tissues.

Research Progress in the Field of Tumor Model Construction Using Bioprinting: A Review

The development of therapeutic drugs and methods has been greatly facilitated by the emergence of tumor models. However, due to their inherent complexity, establishing a model that can fully replicate the tumor tissue situation remains extremely challenging. With the development of tissue engineering, the advancement of bioprinting technology has facilitated the upgrading of tumor models. This article focuses on the latest advancements in bioprinting, specifically highlighting the construction of 3D tumor models, and underscores the integration of these two technologies. Furthermore, it discusses the challenges and future directions of related techniques, while also emphasizing the effective recreation of the tumor microenvironment through the emergence of 3D tumor models that resemble in vitro organs, thereby accelerating the development of new anticancer therapies.

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.

ECM-derived biomaterials for regulating tissue multicellularity and maturation

Recent breakthroughs in developing human-relevant organotypic models led to the building of highly resemblant tissue constructs that hold immense potential for transplantation, drug screening, and disease modeling. Despite the progress in fine-tuning stem cell multilineage differentiation in highly controlled spatiotemporal conditions and hosting microenvironments, 3D models still experience naive and incomplete morphogenesis. In particular, existing systems and induction protocols fail to maintain stem cell long-term potency, induce high tissue-level multicellularity, or drive the maturity of stem cell-derived 3D models to levels seen in their in vivo counterparts. In this review, we highlight the use of extracellular matrix (ECM)-derived biomaterials in providing stem cell niche-mimicking microenvironment capable of preserving stem cell long-term potency and inducing spatial and region-specific differentiation. We also examine the maturation of different 3D models, including organoids, encapsulated in ECM biomaterials and provide looking-forward perspectives on employing ECM biomaterials in building more innovative, transplantable, and functional organs.

Decellularization and Their Significance for Tissue Regeneration in the Era of 3D Bioprinting

Three-dimensional bioprinting is an emerging technology that has high potential application in tissue engineering and regenerative medicine. Increasing advancement and improvement in the decellularization process have led to an increase in the demand for using a decellularized extracellular matrix (dECM) to fabricate tissue engineered products. Decellularization is the process of retaining the extracellular matrix (ECM) while the cellular components are completely removed to harvest the ECM for the regeneration of various tissues and across different sources. Post decellularization of tissues and organs, they act as natural biomaterials to provide the biochemical and structural support to establish cell communication. Selection of an effective method for decellularization is crucial, and various factors like tissue density, geometric organization, and ECM composition affect the regenerative potential which has an impact on the end product. The dECM is a versatile material which is added as an important ingredient to formulate the bioink component for constructing tissue and organs for various significant studies. Bioink consisting of dECM from various sources is used to generate tissue-specific bioink that is unique and to mimic different biometric microenvironments. At present, there are many different techniques applied for decellularization, and the process is not standardized and regulated due to broad application. This review aims to provide an overview of different decellularization procedures, and we also emphasize the different dECM-derived bioinks present in the current global market and the major clinical outcomes. We have also highlighted an overview of benefits and limitations of different decellularization methods and various characteristic validations of decellularization and dECM-derived bioinks.

The molecular mechanisms of extracellular matrix-derived hydrogel therapy in idiopathic pulmonary fibrosis models

Idiopathic Pulmonary Fibrosis (IPF) is a progressively debilitating lung condition characterized by oxidative stress, cell phenotype shifts, and excessive extracellular matrix (ECM) deposition. Recent studies have shown promising results using decellularized ECM-derived hydrogels produced through pepsin digestion in various lung injury models and even a human clinical trial for myocardial infarction. This study aimed to characterize the composition of ECM-derived hydrogels, assess their potential to prevent fibrosis in bleomycin-induced IPF models, and unravel their underlying molecular mechanisms of action. Porcine lungs were decellularized and pepsin-digested for 48 h. The hydrogel production process, including visualization of protein molecular weight distribution and hydrogel gelation, was characterized. Peptidomics analysis of ECM-derived hydrogel contained peptides from 224 proteins. Probable bioactive and cell-penetrating peptides, including collagen IV, laminin beta 2, and actin alpha 1, were identified. ECM-derived hydrogel treatment was administered as an early intervention to prevent fibrosis advancement in rat models of bleomycin-induced pulmonary fibrosis. ECM-derived hydrogel concentrations of 1 mg/mL and 2 mg/mL showed subtle but noticeable effects on reducing lung inflammation, oxidative damage, and protein markers related to fibrosis (e.g., alpha-smooth muscle actin, collagen I). Moreover, distinct changes were observed in macroscopic appearance, alveolar structure, collagen deposition, and protein expression between lungs that received ECM-derived hydrogel and control fibrotic lungs. Proteomic analyses revealed significant protein and gene expression changes related to cellular processes, pathways, and components involved in tissue remodeling, inflammation, and cytoskeleton regulation. RNA sequencing highlighted differentially expressed genes associated with various cellular processes, such as tissue remodeling, hormone secretion, cell chemotaxis, and cytoskeleton engagement. This study suggests that ECM-derived hydrogel treatment influence pathways associated with tissue repair, inflammation regulation, cytoskeleton reorganization, and cellular response to injury, potentially offering therapeutic benefits in preventing or mitigating lung fibrosis.

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.

Decellularized ECM hydrogels: prior use considerations, applications, and opportunities in tissue engineering and biofabrication

Tissue development, wound healing, pathogenesis, regeneration, and homeostasis rely upon coordinated and dynamic spatial and temporal remodeling of extracellular matrix (ECM) molecules. ECM reorganization and normal physiological tissue function, require the establishment and maintenance of biological, chemical, and mechanical feedback mechanisms directed by cell-matrix interactions. To replicate the physical and biological environment provided by the ECM in vivo, methods have been developed to decellularize and solubilize tissues which yield organ and tissue-specific bioactive hydrogels. While these biomaterials retain several important traits of the native ECM, the decellularizing process, and subsequent sterilization, and solubilization result in fragmented, cleaved, or partially denatured macromolecules. The final product has decreased viscosity, moduli, and yield strength, when compared to the source tissue, limiting the compatibility of isolated decellularized ECM (dECM) hydrogels with fabrication methods such as extrusion bioprinting. This review describes the physical and bioactive characteristics of dECM hydrogels and their role as biomaterials for biofabrication. In this work, critical variables when selecting the appropriate tissue source and extraction methods are identified. Common manual and automated fabrication techniques compatible with dECM hydrogels are described and compared. Fabrication and post-manufacturing challenges presented by the dECM hydrogels decreased mechanical and structural stability are discussed as well as circumvention strategies. We further highlight and provide examples of the use of dECM hydrogels in tissue engineering and their role in fabricating complex in vitro 3D microenvironments.

Injectable extracellular matrix hydrogels contribute to native cell infiltration in a rat partial nephrectomy model

Decellularized extracellular matrix (dECM) hydrogels have cytocompatibility, and are currently being investigated for application in soft tissues as a material that promotes native cell infiltration and tissue reconstruction. A dECM hydrogel has broad potential for application in organs with complex structures or various tissue injury models. In this study, we investigated the practical application of a dECM hydrogel by injecting a kidney-derived dECM hydrogel into a rat partial nephrectomy model. The prepared dECM hydrogel was adjustable in viscosity to allow holding at the excision site, and after gelation, had an elastic modulus similar to that of kidney tissue. In addition, the migration of renal epithelial cells and vascular endothelial cells embedded in dECM hydrogels was observed in vitro. Four weeks after injection of the dECM hydrogel to the partial excision site of the kidneys, infiltration of renal tubular constituent cells and native cells with high proliferative activity, as well as angiogenesis, were observed inside the injected areas. This study is the first to show that dECM hydrogels can be applied to the kidney, one of the most complex structural organs and that they can function as a scaffold to induce angiogenesis and infiltration of organ-specific renal tubular constituent cells, providing fundamental insights for further application of dECM hydrogels.

Kidney Decellularized Extracellular Matrix Enhanced the Vascularization and Maturation of Human Kidney Organoids

Kidney organoids derived from human pluripotent stem cells (hPSCs) have extensive potential for disease modelling and regenerative medicine. However, the limited vascularization and immaturity of kidney organoids have been still remained to overcome. Extracellular matrix (ECM) can provide mechanical support and a biochemical microenvironment for cell growth and differentiation. Here in vitro methods using a kidney decellularized extracellular matrix (dECM) hydrogel to culture hPSC-derived kidney organoids, which have extensive vascular network and their own endothelial cells, are reported. Single-cell transcriptomics reveal that the vascularized kidney organoids cultured using the kidney dECM have more mature patterns of glomerular development and higher similarity to human kidney than those cultured without the kidney dECM. Differentiation of α-galactosidase A (GLA)-knock-out hPSCs generated using CRISPR/Cas9 into kidney organoids by the culture method using kidney dECM efficiently recapitulate Fabry nephropathy with vasculopathy. Transplantation of kidney organoids with kidney dECM into kidney of mouse accelerates the recruitment of endothelial cells from the host mouse kidney and maintains vascular integrity with the more organized slit diaphragm-like structures than those without kidney dECM. The kidney dECM methodology for inducing extensive vascularization and maturation of kidney organoids can be applied to studies for kidney development, disease modeling, and regenerative medicine.

Advances in hydrogel-based vascularized tissues for tissue repair and drug screening

The construction of biomimetic vasculatures within the artificial tissue models or organs is highly required for conveying nutrients, oxygen, and waste products, for improving the survival of engineered tissues in vitro. In recent times, the remarkable progress in utilizing hydrogels and understanding vascular biology have enabled the creation of three-dimensional (3D) tissues and organs composed of highly complex vascular systems. In this review, we give an emphasis on the utilization of hydrogels and their advantages in the vascularization of tissues. Initially, the significance of vascular elements and the regeneration mechanisms of vascularization, including angiogenesis and vasculogenesis, are briefly introduced. Further, we highlight the importance and advantages of hydrogels as artificial microenvironments in fabricating vascularized tissues or organs, in terms of tunable physical properties, high similarity in physiological environments, and alternative shaping mechanisms, among others. Furthermore, we discuss the utilization of such hydrogels-based vascularized tissues in various applications, including tissue regeneration, drug screening, and organ-on-chips. Finally, we put forward the key challenges, including multifunctionalities of hydrogels, selection of suitable cell phenotype, sophisticated engineering techniques, and clinical translation behind the development of the tissues with complex vasculatures towards their future development.

Liver scaffolds obtained by decellularization: A transplant perspective in liver bioengineering

Liver transplantation is the only definitive treatment for many diseases that affect this organ, however, its quantity and viability are reduced. The study of liver scaffolds based on an extracellular matrix is a tissue bioengineering strategy with great application in regenerative medicine. Collectively, recent studies suggest that liver scaffold transplantation may assist in reestablishing hepatic function in preclinical diseased animals, which represents a great potential for application as a treatment for patients with liver disease in the future. This review focuses on useful strategies to promote liver scaffold transplantation and the main open questions about this context. We outline the current knowledge about ex vivo bioengineered liver transplantation, including the surgical techniques, recipient survival time, scaffold preparation before transplantation, and liver disease models. We also highlight the current limitations and future directions regarding in vivo bioengineering techniques.

Ovarian extracellular matrix-based hydrogel for human ovarian follicle survival in vivo: A pilot work

To successfully assemble a bio-engineered ovary, we need to create a three-dimensional matrix able to accommodate isolated follicles and cells. The goal of this study was to develop an extracellular matrix hydrogel (oECM) derived from decellularized bovine ovaries able to support, in combination with alginate, human ovarian follicle survival and growth in vitro. Two different hydrogels (oECM1, oECM2) were produced and compared in terms of decellularization efficiency (dsDNA), ECM preservation (collagen and glycosaminoglycan levels), ultrastructure, rigidity, and cytotoxicity. oECM2 showed significantly less dsDNA, greater retention of glycosaminoglycans and better rigidity than oECM1. Isolated human ovarian follicles were then encapsulated in four selected hydrogel combinations: (1) 100% oECM2, (2) 90% oECM2 + 10% alginate, (3) 75% oECM2 + 25% alginate, and (4) 100% alginate. After 1 week of in vitro culture, follicle recovery rate, viability, and growth were analyzed. On day 7 of in vitro culture, follicle recovery rates were 0%, 23%, 65%, 82% in groups 1-4, respectively, rising proportionally with increased alginate content. However, there was no difference in follicle viability or growth between groups 2 and 3 and controls (group 4). In conclusion, since pure alginate cannot be used to graft preantral follicles due to its poor revascularization and degradation after grafting, oECM2 hydrogel combined with alginate may provide a new and promising alternative to graft isolated human follicles in a bio-engineered ovary.

Patterned vascularization in a directional ice-templated scaffold of decellularized matrix

Vascularization is fundamental for large-scale tissue engineering. Most of the current vascularization strategies including microfluidics and three-dimensional (3D) printing aim to precisely fabricate microchannels for individual microvessels. However, few studies have examined the remodeling capacity of the microvessels in the engineered constructs, which is important for transplantation in vivo. Here we present a method for patterning microvessels in a directional ice-templated scaffold of decellularized porcine kidney extracellular matrix. The aligned microchannels made by directional ice templating allowed for fast and efficient cell seeding. The pure decellularized matrix without any fixatives or cross-linkers maximized the potential of tissue remodeling. Dramatical microvascular remodeling happened in the scaffold in 2 weeks, from small primary microvessel segments to long patterned microvessels. The majority of the microvessels were aligned in parallel and interconnected with each other to form a network. This method is compatible with other engineering techniques, such as microfluidics and 3D printing, and multiple cell types can be co-cultured to make complex vascularized tissue and organ models.

Bioengineering Strategies To Develop Podocyte Culture Systems

Unraveling the complex behavior of healthy and disease podocytes by analyzing the changes in their unique arrangement of foot processes, slit diaphragm and the 3D morphology is a long-standing goal in kidney-glomerular research. The complexities surrounding the podocytes' accessibility in animal models and growing evidence of differences between humans and animal systems have compelled researchers to look for alternate approaches to study podocyte behaviors. With the advent of bioengineered models, an increasingly powerful and diverse set of tools is available to develop novel podocyte culture systems. This review discusses the pertinence of various culture models of podocytes to study podocyte mechanisms in both normal physiology and disease conditions. While no one in vitro system comprehensively recapitulates podocytes' in vivo architecture, we emphasize how the existing systems can be exploited to answer targeted questions on podocyte structure and function. We highlight the distinct advantages and limitations of using these models to study podocyte behaviors and screen therapeutics. Finally, we discuss various considerations and potential engineering strategies for developing next-generation complex 3D culture models for studying podocyte behaviors in vitro.

Decellularized kidney extracellular matrix bioinks recapitulate renal 3D microenvironmentin vitro

Decellularized extracellular matrices (ECMs) are able to provide the necessary and specific cues for remodeling and maturation of tissue-specific cells. Nevertheless, their use for typical biofabrication applications requires chemical modification or mixing with other polymers, mainly due to the limited viscoelastic properties. In this study, we hypothesize that a bioink exclusively based on decellularized kidney ECM (dKECM) could be used to bioprint renal progenitor cells. To address these aims, porcine kidneys were decellularized, lyophilized and digested to yield a viscous solution. Then, the bioprinting process was optimized using an agarose microparticle support bath containing transglutaminase for enzymatic crosslinking of the dKECM. This methodology was highly effective to obtain constructs with good printing resolution and high structural integrity. Moreover, the encapsulation of primary renal progenitor cells resulted in high cell viability, with creation of 3D complex structures over time. More importantly, this tissue-specific matrix was also able to influence cellular growth and differentiation over time. Taken together, these results demonstrate that unmodified dKECM bioinks have great potential for bioengineering renal tissue analogs with promising translational applications and/or forin vitromodel systems. Ultimately, this strategy may have greater implications on the biomedical field for the development of bioengineered substitutes using decellularized matrices from other tissues.

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.

3D Mapping Reveals a Complex and Transient Interstitial Matrix During Murine Kidney Development

BACKGROUND

The extracellular matrix (ECM) is a network of proteins and glycosaminoglycans that provides structural and biochemical cues to cells. In the kidney, the ECM is critical for nephrogenesis; however, the dynamics of ECM composition and how it relates to 3D structure during development is unknown.

METHODS

Using embryonic day 14.5 (E14.5), E18.5, postnatal day 3 (P3), and adult kidneys, we fractionated proteins based on differential solubilities, performed liquid chromatography-tandem mass spectrometry, and identified changes in ECM protein content (matrisome). Decellularized kidneys were stained for ECM proteins and imaged in 3D using confocal microscopy.

RESULTS

We observed an increase in interstitial ECM that connects the stromal mesenchyme to the basement membrane (TNXB, COL6A1, COL6A2, COL6A3) between the embryo and adult, and a transient elevation of interstitial matrix proteins (COL5A2, COL12A1, COL26A1, ELN, EMID1, FBN1, LTBP4, THSD4) at perinatal time points. Basement membrane proteins critical for metanephric induction (FRAS1, FREM2) were highest in abundance in the embryo, whereas proteins necessary for integrity of the glomerular basement membrane (COL4A3, COL4A4, COL4A5, LAMB2) were more abundant in the adult. 3D visualization revealed a complex interstitial matrix that dramatically changed over development, including the perinatal formation of fibrillar structures that appear to support the medullary rays.

CONCLUSION

By correlating 3D ECM spatiotemporal organization with global protein abundance, we revealed novel changes in the interstitial matrix during kidney development. This new information regarding the ECM in developing kidneys offers the potential to inform the design of regenerative scaffolds that can guide nephrogenesis in vitro.

Thiol-ene cross-linked alginate hydrogel encapsulation modulates the extracellular matrix of kidney organoids by reducing abnormal type 1a1 collagen deposition

Differentiated kidney organoids from induced pluripotent stem cells hold promise as a treatment for patients with kidney diseases. Before these organoids can be translated to the clinic, shortcomings regarding their cellular and extracellular compositions, and their developmental plateau need to be overcome. We performed a proteomic analysis on kidney organoids cultured for a prolonged culture time and we found a specific change in the extracellular matrix composition with increased expression of types 1a1, 2 and 6a1 collagen. Such an excessive accumulation of specific collagen types is a hallmark of renal fibrosis that causes a life-threatening pathological condition by compromising key functions of the human kidney. Here we hypothesized the need for a three-dimensional environment to grow the kidney organoids, which could better mimic the in vivo surroundings of the developing kidney than standard culture on an air-liquid interface. Encapsulating organoids for four days in a soft, thiol-ene cross-linked alginate hydrogel resulted in decreased type 1a1 collagen expression. Furthermore, the encapsulation did not result in any changes of organoid structural morphology. Using a biomaterial to modulate collagen expression allows for a prolonged kidney organoid culture in vitro and a reduction of abnormal type 1a1 collagen expression bringing kidney organoids closer to clinical application.

Enhancing Kidney Vasculature in Tissue Engineering-Current Trends and Approaches: A Review

Chronic kidney diseases are a leading cause of fatalities around the world. As the most sought-after organ for transplantation, the kidney is of immense importance in the field of tissue engineering. The primary obstacle to the development of clinically relevant tissue engineered kidneys is precise vascularization due to the organ's large size and complexity. Current attempts at whole-kidney tissue engineering include the repopulation of decellularized kidney extracellular matrices or vascular corrosion casts, but these approaches do not eliminate the need for a donor organ. Stem cell-based approaches, such as kidney organoids vascularized in microphysiological systems, aim to construct a kidney without the need for organ donation. These organ-on-a-chip models show complex, functioning kidney structures, albeit at a small scale. Novel methodologies for developing engineered scaffolds will allow for improved differentiation of kidney stem cells and organoids into larger kidney grafts with clinical applications. While currently, kidney tissue engineering remains mostly limited to individual renal structures or small organoids, further developments in vascularization techniques, with technologies such as organoids in microfluidic systems, could potentially open doors for a large-scale growth of whole engineered kidneys for transplantation.

Protective Effects of Extracellular Matrix-Derived Hydrogels in Idiopathic Pulmonary Fibrosis

Idiopathic pulmonary fibrosis (IPF) is a chronic progressive disease with significant gas exchange impairment owing to exaggerated extracellular matrix (ECM) deposition and myofibroblast activation. IPF has no cure, and although nintedanib and pirfenidone are two approved medications for symptom management, the total treatment cost is exuberant and prohibitive to a global uninsured patient population. New therapeutic alternatives with moderate costs are needed to treat IPF. ECM hydrogels derived from decellularized lungs are cost-effective therapeutic candidates to treat pulmonary fibrosis because of their reported antioxidant properties. Oxidative stress contributes to IPF pathophysiology by damaging macromolecules, interfering with tissue remodeling, and contributing to myofibroblast activation. Thus, preventing oxidative stress has beneficial outcomes in IPF. For this purpose, this review describes ECM hydrogel's properties to regulate oxidative stress and tissue remodeling in IPF.

Relation between micro- and nanostructure features and biological properties of the decellularized rat liver

Organ decellularization is one of the promising technologies of regenerative medicine, which allows obtaining cell-free extracellular matrix (ECM), which provide preservation of the composition, architecture, vascular network and biological activity of the ECM. The method of decellularization opens up wide prospects for its practical application not only in the field of creating full-scale bioengineered structures, but also in the manufacture of vessels, microcarriers, hydrogels, and coatings. The main goal of our work was the investigation of structure and biological properties of lyophilized decellularized Wistar rat liver fragments (LDLFs), as well as we assessed the regenerative potential of the obtained ECM. We obtained decellularized liver of a Wistar rat, the vascular network and the main components of the ECM of tissue were preserved. H&E staining of histological sections confirmed the removal of cells. DNA content of ECM is equal to 0.7% of native tissue DNA content. Utilizing scanning probe nanotomogrphy method, we showed sinuous, rough topography and highly nanoporous structure of ECM, which provide high level of mouse 3T3 fibroblast and Hep-G₂cells biocompatibility. Obtained LDLF had a high regenerative potential, which we studied in an experimental model of a full-thickness rat skin wound healing: we observed the acceleration of wound healing by 2.2 times in comparison with the control.

Organ-on-a-chip systems for vascular biology

Organ-on-a-chip (OOC) platforms involve the miniaturization of cell culture systems and enable a variety of novel experimental approaches. These range from modeling the independent effects of biophysical forces on cells to screening novel drugs in multi-organ microphysiological systems, all within microscale devices. As in living systems, the incorporation of vascular structure is a key feature common to almost all organ-on-a-chip systems. In this review we highlight recent advances in organ-on-a-chip technologies with a focus on the vasculature. We first present the developmental process of the blood vessels through which vascular cells assemble into networks and remodel to form complex vascular beds under flow. We then review self-assembled vascular models and flow systems for the study of vascular development and biology as well as pre-patterned vascular models for the generation of perfusable microvessels for modeling vascular and tissue function. We finally conclude with a perspective on developing future OOC approaches for studying different aspects of vascular biology. We highlight the fit for purpose selection of OOC models towards either simple but powerful testbeds for therapeutic development, or complex vasculature to accurately replicate human physiology for specific disease modeling and tissue regeneration.

Particulate kidney extracellular matrix: bioactivity and proteomic analysis of a novel scaffold from porcine origin

Decellularized matrices are attractive substrates, being able to retain growth factors and proteins present in the native tissue. Several biomaterials can be produced by processing these matrices. However, new substrates capable of being injected that reverse local kidney injuries are currently scarce. Herein, we hypothesized that the decellularized particulate kidney porcine ECM (pKECM) could support renal progenitor cell cultures for posterior implantation. Briefly, kidneys are cut into pieces, decellularized by immersion on detergent solutions, lyophilized and reduced into particles. Then, ECM particles are analyzed for nuclear material remaining by DNA quantification and histological examination, molecular conformation by FITR and structural morphology by SEM. Protein extraction is also optimized for posterior identification and quantification by mass spectrometry. The results obtained confirm the collagenous structure and composition of the ECM, the effective removal of nucleic material and the preservation of ECM proteins with great similarity to human kidneys. Human renal progenitor cells (hRPCs) are seeded in different ratios with pKECM, on 3D suspensions. The conducted assays for cell viability, proliferation and distribution over 7 days of culture suggest that these matrices as biocompatible and bioactive substrates for hRPCs. Also, by analyzing CD133 expression, an optimal ratio for specific phenotypic expression is revealed, demonstrating the potential of these substrates to modulate cellular behavior. The initial hypothesis of developing and characterizing a particulate ECM biomaterial as a consistent substrate for 3D cultures is successfully validated. The findings in this manuscript suggest these particles as valuable tools for regenerative nephrology by minimizing surgeries and locally reversing small injuries which can lead to chronic renal disfunction.

Multiphoton-Guided Creation of Complex Organ-Specific Microvasculature

Engineering functional human tissues in vitro is currently limited by difficulty replicating the small caliber, complex connectivity, cellularity, and 3D curvature of the native microvasculature. Multiphoton ablation has emerged as a promising technique for fabrication of microvascular structures with high resolution and full 3D control, but cellularization and perfusion of complex capillary-scale structures has remained challenging. Here, multiphoton ablation combined with guided endothelial cell growth from pre-formed microvessels is used to successfully create perfusable and cellularized organ-specific microvascular structures at anatomic scale within collagen hydrogels. Fabrication and perfusion of model 3D pulmonary and renal microvascular beds is demonstrated, as is replication and perfusion of a brain microvascular unit derived from in vivo data. Successful endothelialization and blood perfusion of a kidney-specific microvascular structure is achieved, using laser-guided angiogenesis. Finally, proof-of-concept hierarchical blood vessels and complex multicellular models are created, using multistep patterning with multiphoton ablation techniques. These successes open new doors for the creation of engineered tissues and organ-on-a-chip devices.

Retinoic Acid Benefits Glomerular Organotypic Differentiation from Adult Renal Progenitor Cells In Vitro

When in certain culture conditions, organotypic cultures are able to mimic developmental stages of an organ, generating higher-order structures containing functional subunits and progenitor niches. Despite the major advances in the area, researchers have not been able to fully recapitulate the complexity of kidney tissue. Pluripotent stem cells are extensively used in the field, but very few studies make use of adult stem cells. Herein, we describe a simple and feasible method for achieving glomerular epithelial differentiation on an organotypic model comprising human renal progenitor cells from adult kidney (hRPCs). Their glomerular differentiative potential was studied using retinoic acid (RA), a fundamental molecule for intermediate mesoderm induction on early embryogenesis. Immunofluorescence, specific cell surface markers expression and gene expression analysis confirm the glomerular differentiative potential of RA in a short-term culture. We also compared the potential of RA with a potent WNT agonist, CHIR99021, on the differentiative capacity of hRPCs. Gene expression and immunofluorescence analysis confirmed that hRPCs are more sensitive to RA stimulation when compared to CHIR9901. Endothelial cells were also included on the spheroids, resulting in a higher organizational level. The assembly potential of these cells and their selective stimulation will give new insights on adult organotypic cell culture studies and will hopefully guide more works in this important area of research.

Designing Decellularized Extracellular Matrix-Based Bioinks for 3D Bioprinting

3D bioprinting is an emerging technology to fabricate tissues and organs by precisely positioning cells into 3D structures using printable cell-laden formulations known as bioinks. Various bioinks are utilized in 3D bioprinting applications; however, developing the perfect bioink to fabricate constructs with biomimetic microenvironment and mechanical properties that are similar to native tissues is a challenging task. In recent years, decellularized extracellular matrix (dECM)-based bioinks have received an increasing attention in 3D bioprinting applications, since they are derived from native tissues and possess unique, complex tissue-specific biochemical properties. This review focuses on designing dECM-based bioinks for tissue and organ bioprinting, including commonly used decellularization and decellularized tissue characterization methods, bioink formulation and characterization, applications of dECM-based bioinks, and most recent advancements in dECM-based bioink design.

Matrigel-Free Laminin-Entactin Matrix to Induce Human Renal Proximal Tubule Structure Formation In Vitro

A successful in vitro tissue model must recapitulate the native tissue features while also being reproducible. Currently, Matrigel is the principal biomaterial used to induce the formation of proximal convoluted tubules (PCTs) in vitro, because of its similar composition and structure with the kidney tubular basement membrane and the presence of critical growth factors. However, Matrigel is not well-defined, and batch-to-batch variability is a significant issue. Here, we define a Matrigel-free method, using a laminin-entactin (L-E) matrix to support the formation of proximal tubular-like structures in vitro using immortalized human renal epithelial cells (RPTEC/TERT1) cocultured with murine fibroblast stromal cells (FOXD1^lacZ+). The matrix supports the presence of specific components of the tubular basement membrane (laminin, entactin/nidogen, and heparan sulfate proteoglycan) in addition to fibroblast growth factor 8a (FGF-8a). The matrix also induces tubulogenesis, leading to the formation of PCTs based on several key markers, including E-cadherin, aquaporin-1, and Na⁺/K⁺ ATPase. Moreover, these PCT structures displayed cell polarity and a well-defined lumen after 18 days in culture. This laminin-entactin (L-E) matrix constitutes a defined and consistent biomaterial that can be used in kidney tissue engineering for understanding in vitro proximal tubule development and for nephrotoxicity studies.

Bioprinting: From Tissue and Organ Development to in Vitro Models

Bioprinting techniques have been flourishing in the field of biofabrication with pronounced and exponential developments in the past years. Novel biomaterial inks used for the formation of bioinks have been developed, allowing the manufacturing of in vitro models and implants tested preclinically with a certain degree of success. Furthermore, incredible advances in cell biology, namely, in pluripotent stem cells, have also contributed to the latest milestones where more relevant tissues or organ-like constructs with a certain degree of functionality can already be obtained. These incredible strides have been possible with a multitude of multidisciplinary teams around the world, working to make bioprinted tissues and organs more relevant and functional. Yet, there is still a long way to go until these biofabricated constructs will be able to reach the clinics. In this review, we summarize the main bioprinting activities linking them to tissue and organ development and physiology. Most bioprinting approaches focus on mimicking fully matured tissues. Future bioprinting strategies might pursue earlier developmental stages of tissues and organs. The continuous convergence of the experts in the fields of material sciences, cell biology, engineering, and many other disciplines will gradually allow us to overcome the barriers identified on the demanding path toward manufacturing and adoption of tissue and organ replacements.

Decellularization systems and devices: State-of-the-art

Extracellular matrix (ECM) is a natural biomaterial scaffold that provides biochemical and structural support to its surrounding cells, forming tissue and respective organs. These ECM proteins can be extracted from organs and tissues through decellularization, which is the process of removing cellular content and nuclear material from the organs to obtain decellularized ECM (dECM). dECM is a versatile and functional biomaterial that can be used as the base component of bioinks for rebuilding tissue and organs. Intact dECM of whole organs can be used as a scaffold for recellularization with human stem cells to produce a functioning organ. As decellularization is a relatively new lab process, the associated technologies and devices are largely non-standardized and only available in small, lab-specific scales. Additionally, there is a lack of standardized protocols to analyze the quality and consistency of harvested dECM for medical applications. This review discusses the relevant decellularization systems and devices currently available to facilitate further development of this process for larger scales with the intention to commercialize dECM materials. STATEMENT OF SIGNIFICANCE: Extracellular matrix (ECM) is a natural cocktail of biomaterials that provides biochemical and structural support to its surrounding cells. ECM proteins are extracted from organs and tissues through decellularization. Being a versatile and functional biomaterial, decellularized extracellular matrix (dECM) is being used as base component of bioinks/hydrogels for rebuilding of tissue and organ constructs. Decellularization is a relatively new lab process with associated technologies/devices being largely non-standardized and only available in lab-specific scales. We discuss categories of decellularization systems and devices for the first time being used in academic and commercial settings. We highlight inherent challenges with the current systems and suggest possible solutions. We comment on further development of these processes for large-scale and commercial applications of dECM.

Co-cultures of renal progenitors and endothelial cells on kidney decellularized matrices replicate the renal tubular environment in vitro

AIM

Herein we propose creating a bilayer tubular kidney in-vitro model. It is hypothesized that membranes composed of decellularized porcine kidney extracellular matrix are valid substitutes of the tubular basement membrane by mimicking the physiological relevance of the in vivo environment and disease phenotypes.

METHODS

Extracellular matrix was obtained from decellularized porcine kidneys. After processing by lyophilization and milling, it was dissolved in an organic solvent and blended with poly(caprolactone). Porous membranes were obtained by electrospinning and seeded with human primary renal progenitor cells to evaluate phenotypic alterations. To create a bilayer model of the in vivo tubule, the same cells were differentiated into epithelial tubular cells and co-cultured with endothelial cells in opposite sites.

RESULTS

Our results demonstrate increasing metabolic activity, proliferation and total protein content of renal progenitors over time. We confirmed the expression of several genes encoding epithelial transport proteins and we could also detect tubular-specific proteins by immunofluorescence stainings. Functional and transport assays were performed trough the bilayer by quantifying both human serum albumin uptake and inulin leakage. Furthermore, we validated the chemical modulation of nephrotoxicity on this epithelium-endothelium model by cisplatin exposure.

CONCLUSION

The use of decellularized matrices in combination with primary renal cells was shown to be a valuable tool for modelling renal function and disease in vitro. We successfully validated our hypothesis by replicating the physiological conditions of an in vitro tubular bilayer model. The developed system may contribute significantly for the future investigation of advanced therapies for kidney diseases.

Decellularized Extracellular Matrix-based Bioinks for Engineering Tissue- and Organ-specific Microenvironments

Biomaterials-based biofabrication methods have gained much attention in recent years. Among them, 3D cell printing is a pioneering technology to facilitate the recapitulation of unique features of complex human tissues and organs with high process flexibility and versatility. Bioinks, combinations of printable hydrogel and cells, can be utilized to create 3D cell-printed constructs. The bioactive cues of bioinks directly trigger cells to induce tissue morphogenesis. Among the various printable hydrogels, the tissue- and organ-specific decellularized extracellular matrix (dECM) can exert synergistic effects in supporting various cells at any component by facilitating specific physiological properties. In this review, we aim to discuss a new paradigm of dECM-based bioinks able to recapitulate the inherent microenvironmental niche in 3D cell-printed constructs. This review can serve as a toolbox for biomedical engineers who want to understand the beneficial characteristics of the dECM-based bioinks and a basic set of fundamental criteria for printing functional human tissues and organs.

Decellularized tissues as platforms for in vitro modeling of healthy and diseased tissues

Biomedical engineers are at the forefront of developing novel treatments to improve human health, however, many products fail to translate to clinical implementation. In vivo pre-clinical animal models, although the current best approximation of complex disease conditions, are limited by reproducibility, ethical concerns, and poor accurate prediction of human response. Hence, there is a need to develop physiologically relevant, low cost, scalable, and reproducible in vitro platforms to provide reliable means for testing drugs, biomaterials, and tissue engineered products for successful clinical translation. One emerging approach of developing physiologically relevant in vitro models utilizes decellularized tissues/organs as biomaterial platforms for 2D and 3D models of healthy and diseased tissue. Decellularization is a process that removes cellular content and produces tissue-specific extracellular matrix scaffolds that can more accurately recapitulate an organ/tissue's native microenvironment compared to other natural or synthetic materials. Decellularized tissues hold enormous potential for in vitro modeling of various disease phenotypes and tissue responses to drugs or external conditions such as aging, toxin exposure, or even implantation. In this review, we highlight the need for in vitro models, the advantages and limitations of implementing decellularized tissues, and considerations of the decellularization process. We discuss current research efforts towards applying decellularized tissues as platforms to generate in vitro models of healthy and diseased tissues, and where we foresee the field progressing. A variety of organs/tissues are discussed, including brain, heart, kidney, large intestine, liver, lung, skeletal muscle, skin, and tongue. STATEMENT OF SIGNIFICANCE: Many biomedical products fail to reach clinical translation due to animal model limitations. Development of physiologically relevant in vitro models can provide a more economic, scalable, and reproducible means of testing drugs/therapeutics for successful clinical translation. The use of decellularized tissues as platforms for in vitro models holds promise, as these scaffolds can effectively replicate native tissue complexity, but is not widely explored. This review discusses the need for in vitro models, the promise of decellularized tissues as biomaterial substrates, and the current research applying decellularized tissues towards the creation of in vitro models. Further, this review provides insights into the current limitations and future of such in vitro models.

Human Liver-Derived Extracellular Matrix for the Culture of Distinct Human Primary Liver Cells

The lack of robust methods to preserve, purify and in vitro maintain the phenotype of the human liver’s highly specialized parenchymal and non-parenchymal cell types importantly hampers their exploitation for the development of research and clinical applications. There is in this regard a growing interest in the use of tissue-specific extracellular matrix (ECM) to provide cells with an in vitro environment that more closely resembles that of the native tissue. In the present study, we have developed a method that allows for the isolation and downstream application of the human liver’s main cell types from cryopreserved material. We also isolated and solubilized human liver ECM (HL-ECM), analyzed its peptidomic and proteomic composition by mass spectrometry and evaluated its interest for the culture of distinct primary human liver cells. Our analysis of the HL-ECM revealed proteomic diversity, type 1 collagen abundance and partial loss of integrity following solubilization. Solubilized HL-ECM was evaluated either as a coating or as a medium supplement for the culture of human primary hepatocytes, hepatic stellate cells and liver sinusoidal endothelial cells. Whereas the solubilized HL-ECM was suitable for cell culture, its impact on the phenotype and/or functionality of the human liver cells was limited. Our study provides a first detailed characterization of solubilized HL-ECM and a first report of its influence on the culture of distinct human primary liver cells.

Graft immaturity and safety concerns in transplanted human kidney organoids

For chronic kidney disease, regeneration of lost nephrons with human kidney organoids derived from induced pluripotent stem (iPS) cells is proposed to be an attractive potential therapeutic option. It remains unclear, however, whether organoids transplanted into kidneys in vivo would be safe or functional. Here, we purified kidney organoids and transplanted them beneath the kidney capsules of immunodeficient mice to test their safety and maturity. Kidney organoid grafts survived for months after transplantation and became vascularized from host mouse endothelial cells. Nephron-like structures in grafts appeared more mature than kidney organoids in vitro, but remained immature compared with the neighboring mouse kidney tissue. Ultrastructural analysis revealed filtration barrier-like structures, capillary lumens, and tubules with brush border in the transplanted kidney organoids, which were more mature than those of the kidney organoids in vitro but not as organized as adult mammalian kidneys. Immaturity was a common feature of three separate differentiation protocols by immunofluorescence analysis and single cell RNA sequencing. Stroma of transplanted kidney organoid grafts were filled with vimentin-positive mesenchymal cells, and chondrogenesis, cystogenesis, and stromal expansion were observed in the long term. Transcription profiles showed that long-term maintenance after kidney organoid transplantation induced transcriptomic reprogramming with prominent suppression of cell-cycle-related genes and upregulation of extracellular matrix organization. Our data suggest that kidney organoids derived from iPS cells may be transplantable but strategies to improve nephron differentiation and purity are required before they can be applied in humans as a therapeutic option.

ECM in Differentiation: A Review of Matrix Structure, Composition and Mechanical Properties

Stem cell regenerative potential owing to the capacity to self-renew as well as differentiate into other cell types is a promising avenue in regenerative medicine. Stem cell niche not only provides physical scaffolding but also possess instructional capacity as it provides a milieu of biophysical and biochemical cues. Extracellular matrix (ECM) has been identified as a major dictator of stem cell lineage, thus understanding the structure of in vivo ECM pertaining to specific tissue differentiation will aid in devising in vitro strategies to improve the differentiation efficiency. In this review, we summarize details about the native architecture, composition and mechanical properties of in vivo ECM of the early embryonic stages and the later adult stages. Native ECM from adult tissues categorized on their origin from respective germ layers are discussed while engineering techniques employed to facilitate differentiation of stem cells into particular lineages are noted. Overall, we emphasize that in vitro strategies need to integrate tissue specific ECM biophysical cues for developing accurate artificial environments for optimizing stem cell differentiation.

3D Bioprinting: from Benches to Translational Applications

Over the last decades, the fabrication of 3D tissues has become commonplace in tissue engineering and regenerative medicine. However, conventional 3D biofabrication techniques such as scaffolding, microengineering, and fiber and cell sheet engineering are limited in their capacity to fabricate complex tissue constructs with the required precision and controllability that is needed to replicate biologically relevant tissues. To this end, 3D bioprinting offers great versatility to fabricate biomimetic, volumetric tissues that are structurally and functionally relevant. It enables precise control of the composition, spatial distribution, and architecture of resulting constructs facilitating the recapitulation of the delicate shapes and structures of targeted organs and tissues. This Review systematically covers the history of bioprinting and the most recent advances in instrumentation and methods. It then focuses on the requirements for bioinks and cells to achieve optimal fabrication of biomimetic constructs. Next, emerging evolutions and future directions of bioprinting are discussed, such as freeform, high-resolution, multimaterial, and 4D bioprinting. Finally, the translational potential of bioprinting and bioprinted tissues of various categories are presented and the Review is concluded by exemplifying commercially available bioprinting platforms.

Poly(ethylene glycol)-crosslinked gelatin hydrogel substrates with conjugated bioactive peptides influence endothelial cell behavior

The basement membrane is a specialized extracellular matrix substrate responsible for support and maintenance of epithelial and endothelial structures. Engineered basement membrane-like hydrogel systems have the potential to advance understanding of cell-cell and cell-matrix interactions by allowing precise tuning of the substrate or matrix biochemical and biophysical properties. In this investigation, we developed tunable hydrogel substrates with conjugated bioactive peptides to modulate cell binding and growth factor signaling by endothelial cells. Hydrogels were formed by employing a poly(ethylene glycol) crosslinker to covalently crosslink gelatin polymers and simultaneously conjugate laminin-derived YIGSR peptides or vascular endothelial growth factor (VEGF)-mimetic QK peptides to the gelatin. Rheological characterization revealed rapid formation of hydrogels with similar stiffnesses across tested formulations, and swelling analysis demonstrated dependency on peptide and crosslinker concentrations in hydrogels. Levels of phosphorylated VEGF Receptor 2 in cells cultured on hydrogel substrates revealed that while human umbilical vein endothelial cells (HUVECs) responded to both soluble and conjugated forms of the QK peptide, conditionally-immortalized human glomerular endothelial cells (GEnCs) only responded to the conjugated presentation of the peptide. Furthermore, whereas HUVECs exhibited greatest upregulation in gene expression when cultured on YIGSR- and QK-conjugated hydrogel substrates after 5 days, GEnCs exhibited greatest upregulation when cultured on Matrigel control substrates at the same time point. These results indicate that conjugation of bioactive peptides to these hydrogel substrates significantly influenced endothelial cell behavior in cultures but with differential responses between HUVECs and GEnCs.

Clinical Operational Tolerance and Immunosuppression Minimization in Kidney Transplantation: Where Do We Stand?

BACKGROUND

The 20th century represents a breakthrough in the transplantation era, since the first kidney transplantation between identical twins was performed. This was the first case of tolerance, since the recipient did not need immunosuppression. However, as transplantation became possible, an immunosuppression-free status became the ultimate goal, since the first tolerance case was a clear exception from the hard reality nowadays represented by rejection.

METHODS

A plethora of studies was described over the past decades to understand the molecular mechanisms responsible for rejection. This review focuses on the most relevant studies found in the literature where renal tolerance cases are claimed. Contrasting, and at the same time, encouraging outcomes are herein discussed and a glimpse on the main renal biomarkers analyzed in this field is provided.

RESULTS

The activation of the immune system has been shown to play a central role in organ failure, but also it seems to induce a tolerance status when an allograft is performed, despite tolerance is still rare to register. Although there are still overwhelming challenges to overcome and various immune pathways remain arcane; the immunosuppression minimization might be more attainable than previously believed.

CONCLUSION

. Multiple biomarkers and tolerance mechanisms suspected to be involved in renal transplantation have been investigated to understand their real role, with still no clear answers on the topic. Thus, the actual knowledge provided necessarily leads to more in-depth investigations, although many questions in the past have been answered, there are still many issues on renal tolerance that need to be addressed.

Tissue-Engineered Grafts from Human Decellularized Extracellular Matrices: A Systematic Review and Future Perspectives

Tissue engineering and regenerative medicine involve many different artificial and biologic materials, frequently integrated in composite scaffolds, which can be repopulated with various cell types. One of the most promising scaffolds is decellularized allogeneic extracellular matrix (ECM) then recellularized by autologous or stem cells, in order to develop fully personalized clinical approaches. Decellularization protocols have to efficiently remove immunogenic cellular materials, maintaining the nonimmunogenic ECM, which is endowed with specific inductive/differentiating actions due to its architecture and bioactive factors. In the present paper, we review the available literature about the development of grafts from decellularized human tissues/organs. Human tissues may be obtained not only from surgery but also from cadavers, suggesting possible development of Human Tissue BioBanks from body donation programs. Many human tissues/organs have been decellularized for tissue engineering purposes, such as cartilage, bone, skeletal muscle, tendons, adipose tissue, heart, vessels, lung, dental pulp, intestine, liver, pancreas, kidney, gonads, uterus, childbirth products, cornea, and peripheral nerves. In vitro recellularizations have been reported with various cell types and procedures (seeding, injection, and perfusion). Conversely, studies about in vivo behaviour are poorly represented. Actually, the future challenge will be the development of human grafts to be implanted fully restored in all their structural/functional aspects.

Diverse preparation methods for small intestinal submucosa (SIS): Decellularization, components, and structure

The native extracellular matrix (ECM) biomaterial derived from small intestinal submucosa (SIS) is widely applied in tissue engineering for tissue repair and regeneration. SIS ECM is obtained through physical and chemical methods to remove the intrinsic cells, which would otherwise cause adverse immune reactions when the SIS ECM is implanted into the host body. Several research teams have reported diverse SIS decellularization methods. However, there was no consensus on the criteria to be used for the decellularization methods for SIS and further research on the mechanism action of SIS is needed for comprehensive detection of the biological composition. In this present study, we used three reported methods to prepare SIS and compared their effects on decellularization and the remaining biological components, microstructure and cytocompatibility. SIS prepared by the three kinds of decellularization methods all achieved the recommended criteria, had good biocompatibility and retained most active components. Nevertheless, regardless of which decellularization method was used, the microstructure and bioactive components of the prepared SIS were damaged in varying degrees. We recommend that researchers need to select a decellularization method that would be appropriate to use according to their research purposes. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 689-697, 2019.

Reconstructing the Human Renal Vascular-Tubular Unit In Vitro

Engineered human kidney-on-a-chip platforms show tremendous promise for disease modeling and drug screening. Outstanding challenges exist, however, in reconstructing the complex architecture, cellular make-up, and matrix composition necessary for the proper modeling of kidney function. Herein, the first fully tunable human kidney-on-a-chip platform is reported that allows the reconstruction of the native architecture of the renal endothelial-epithelial exchange interface using entirely cell-remodelable matrix and patient-derived kidney cells. This platform consists of a double-layer human renal vascular-tubular unit (hRVTU) enabled by a thin collagen membrane that replicates the kidney exchange interface. It is shown that endothelial and epithelial cells lining their respective lumens remodel the membrane in culture into a ≈1 µm thick exchange interface composed of native basement membrane proteins. This interface displays sufficient mechanical integrity for media flow and blood perfusion. As a proof of principle, it is demonstrated that the hRVTU performs kidney-specific functions including reabsorption of albumin and glucose from the epithelial channel. By incorporating multiple cell populations from single donors, it is demonstrated that the hRVTU may have utility for future precision medicine applications. The success of the system provides new opportunities for the next generation of organ-on-a-chip models.

Heart valve tissue-derived hydrogels: Preparation and characterization of mitral valve chordae, aortic valve, and mitral valve gels

Heart valve (HV) diseases are among the leading causes of death and continue to threaten public health worldwide. The current clinical options for HV replacement include mechanical and biological prostheses. However, an ongoing problem with current HV prostheses is their failure to integrate with the host tissue and their inability grow and remodel within the body. Tissue engineered heart valves (TEHVs) are a promising solution to these problems, as they are able to grow and remodel somatically with the rest of the body. Recently, decellularized HVs have demonstrated great potential as valve replacements because they are tissue specific, but recellularization is still a challenge due to the dense HV extracellular matrix (ECM) network. In this proof-of-concept work, we decellularized porcine mitral valve chordae, aortic valve leaflets, and mitral valve leaflets and processed them into injectable hydrogels that could accommodate any geometry. While the three valvular ECMs contained various amounts of collagen, they displayed similar glycosaminoglycan contents. The hydrogels had similar nanofibrous structures and gelation kinetics with various compressive strengths. When encapsulated with NIH 3 T3 fibroblasts, all the hydrogels supported cell survivals up to 7 days. Decellularized HV ECM hydrogels may show promising potential HV tissue engineering applications. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1732-1740, 2019.