Mechanical characterization of native and sugar-modified decellularized kidneys

Transglutaminase-mediated stiffening of the glomerular basement membrane mitigates pressure-induced reductions in molecular sieving coefficient by reducing compression

Proteinuria, the presence of high molecular weight proteins in the urine, is a primary indicator of chronic kidney disease. Proteinuria results from increased molecular permeability of the glomerular filtration barrier combined with saturation or defects in tubular protein reabsorption. Any solute that passes into the glomerular filtrate traverses the glomerular endothelium, the glomerular basement membrane, and the podocyte slit diaphragm. Damage to any layer of the filter has reciprocal effects on other layers to increase glomerular permeability. The GBM is thought to act as a compressible ultrafilter that has increased molecular selectivity with increased pressure due to compression that reduced the porosity of the GBM with increased pressure. In multiple forms of chronic kidney disease, crosslinking enzymes are upregulated and may act to increase GBM stiffness. Here we show that enzymatically crosslinking porcine GBM with transglutaminase increases the stiffness of the GBM and mitigates pressure-dependent reductions in molecular sieving coefficient. This was modeled mathematically using a modified membrane transport model accounting for GBM compression. Changes in the mechanical properties of the GBM may contribute to proteinuria through pressure-dependent effects on GBM porosity.

Peroxidasin is required for full viability in development and for maintenance of tissue mechanics in adults

Basement membranes are thin strong sheets of extracellular matrix. They provide mechanical and biochemical support to epithelia, muscles, nerves, and blood vessels, among other tissues. The mechanical properties of basement membranes are conferred in part by Collagen IV (Col4), an abundant protein of basement membranes that forms an extensive two-dimensional network through head-to-head and tail-to-tail interactions. After the Col4 network is assembled into a basement membrane, it is crosslinked by the matrix-resident enzyme Peroxidasin to form a large covalent polymer. Peroxidasin and Col4 crosslinking are highly conserved throughout the animal kingdom, indicating they are important, but homozygous mutant mice have mild phenotypes. To explore the role of Peroxidasin, we analyzed mutants in Drosophila, including a new CRISPR-generated catalytic null, and found that homozygotes were mostly lethal with 13 % viable escapers. Mouse mutants also show semi-lethality, with Mendelian analysis demonstrating ∼50 % lethality and ∼50 % escapers. Despite the strong mutations, the homozygous fly and mouse escapers had low but detectable levels of Col4 crosslinking, indicating the existence of inefficient alternative crosslinking mechanisms, probably responsible for the viable escapers. Fly mutant phenotypes are consistent with decreased basement membrane stiffness. Interestingly, we found that even after basement membranes are assembled and crosslinked in wild-type animals, continuing Peroxidasin activity is required in adults to maintain tissue stiffness over time. These results suggest that Peroxidasin crosslinking may be more important than previously appreciated.

Polyethylene glycol crosslinked decellularized single liver lobe scaffolds with vascular endothelial growth factor promotes angiogenesis in vivo

BACKGROUND

Improving the mechanical properties and angiogenesis of acellular scaffolds before transplantation is an important challenge facing the development of acellular liver grafts. The present study aimed to evaluate the cytotoxicity and angiogenesis of polyethylene glycol (PEG) crosslinked decellularized single liver lobe scaffolds (DLSs), and establish its suitability as a graft for long-term liver tissue engineering.

METHODS

Using mercaptoacrylate produced by the Michael addition reaction, DLSs were first modified using N-succinimidyl S-acetylthioacetate (SATA), followed by cross-linking with PEG as well as vascular endothelial growth factor (VEGF). The optimal concentration of agents and time of the individual steps were identified in this procedure through biomechanical testing and morphological analysis. Subsequently, human umbilical vein endothelial cells (HUVECs) were seeded on the PEG crosslinked scaffolds to detect the proliferation and viability of cells. The scaffolds were then transplanted into the subcutaneous tissue of Sprague-Dawley rats to evaluate angiogenesis. In addition, the average number of blood vessels was evaluated in the grafts with or without PEG at days 7, 14, and 21 after implantation.

RESULTS

The PEG crosslinked DLS maintained their three-dimensional structure and were more translucent after decellularization than native DLS, which presented a denser and more porous network structure. The results for Young's modulus proved that the mechanical properties of 0.5 PEG crosslinked DLS were the best and close to that of native livers. The PEG-VEGF-DLS could better promote cell proliferation and differentiation of HUVECs compared with the groups without PEG cross-linking. Importantly, the average density of blood vessels was higher in the PEG-VEGF-DLS than that in other groups at days 7, 14, and 21 after implantation in vivo.

CONCLUSIONS

The PEG crosslinked DLS with VEGF could improve the biomechanical properties of native DLS, and most importantly, their lack of cytotoxicity provides a new route to promote the proliferation of cells in vitro and angiogenesis in vivo in liver tissue engineering.

Analysis of Drosophila and mouse mutants reveals that Peroxidasin is required for tissue mechanics and full viability

Basement membranes are thin strong sheets of extracellular matrix. They provide mechanical and biochemical support to epithelia, muscles, nerves, and blood vessels, among other tissues. The mechanical properties of basement membranes are conferred in part by Collagen IV (Col4), an abundant protein of basement membrane that forms an extensive two-dimensional network through head-to-head and tail-to-tail interactions. After the Col4 network is assembled into a basement membrane, it is crosslinked by the matrix-resident enzyme Peroxidasin to form a large covalent polymer. Peroxidasin and Col4 crosslinking are highly conserved, indicating they are essential, but homozygous mutant mice have mild phenotypes. To explore the role of Peroxidasin, we analyzed mutants in Drosophila, including a newly generated catalytic null, and found that homozygotes were mostly lethal with 13% viable escapers. A Mendelian analysis of mouse mutants shows a similar pattern, with homozygotes displaying ~50% lethality and ~50% escapers. Despite the strong mutations, the homozygous escapers had low but detectable levels of Col4 crosslinking, indicating that inefficient alternative mechanisms exist and that are probably responsible for the viable escapers. Further, fly mutants have phenotypes consistent with a decrease in stiffness. Interestingly, we found that even after adult basement membranes are assembled and crosslinked, Peroxidasin is still required to maintain stiffness. These results suggest that Peroxidasin crosslinking may be more important than previously appreciated.

A kidney proximal tubule model to evaluate effects of basement membrane stiffening on renal tubular epithelial cells

The kidney tubule consists of a single layer of epithelial cells supported by the tubular basement membrane (TBM), a thin layer of specialized extracellular matrix (ECM). The mechanical properties of the ECM are important for regulating a wide range of cell functions including proliferation, differentiation and cell survival. Increased ECM stiffness plays a role in promoting multiple pathological conditions including cancer, fibrosis and heart disease. How changes in TBM mechanics regulate tubular epithelial cell behavior is not fully understood. Here we introduce a cell culture system that utilizes in vivo-derived TBM to investigate cell-matrix interactions in kidney proximal tubule cells. Basement membrane mechanics was controlled using genipin, a biocompatibility crosslinker. Genipin modification resulted in a dose-dependent increase in matrix stiffness. Crosslinking had a marginal but statistically significant impact on the diffusive molecular transport properties of the TBM, likely due to a reduction in pore size. Both native and genipin-modified TBM substrates supported tubular epithelial cell growth. Cells were able to attach and proliferate to form confluent monolayers. Tubular epithelial cells polarized and assembled organized cell-cell junctions. Genipin modification had minimal impact on cell viability and proliferation. Genipin stiffened TBM increased gene expression of pro-fibrotic cytokines and altered gene expression for N-cadherin, a proximal tubular epithelial specific cell-cell junction marker. This work introduces a new cell culture model for cell-basement membrane mechanobiology studies that utilizes in vivo-derived basement membrane. We also demonstrate that TBM stiffening affects tubular epithelial cell function through altered gene expression of cell-specific differentiation markers and induced increased expression of pro-fibrotic growth factors.

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

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

Decellularization in Tissue Engineering and Regenerative Medicine: Evaluation, Modification, and Application Methods

Reproduction of different tissues using scaffolds and materials is a major element in regenerative medicine. The regeneration of whole organs with decellularized extracellular matrix (dECM) has remained a goal despite the use of these materials for different purposes. Recently, decellularization techniques have been widely used in producing scaffolds that are appropriate for regenerating damaged organs and may be able to overcome the shortage of donor organs. Decellularized ECM offers several advantages over synthetic compounds, including the preserved natural microenvironment features. Different decellularization methods have been developed, each of which is appropriate for removing cells from specific tissues under certain conditions. A variety of methods have been advanced for evaluating the decellularization process in terms of cell removal efficiency, tissue ultrastructure preservation, toxicity, biocompatibility, biodegradability, and mechanical resistance in order to enhance the efficacy of decellularization methods. Modification techniques improve the characteristics of decellularized scaffolds, making them available for the regeneration of damaged tissues. Moreover, modification of scaffolds makes them appropriate options for drug delivery, disease modeling, and improving stem cells growth and proliferation. However, considering different challenges in the way of decellularization methods and application of decellularized scaffolds, this field is constantly developing and progressively moving forward. This review has outlined recent decellularization and sterilization strategies, evaluation tests for efficient decellularization, materials processing, application, and challenges and future outlooks of decellularization in regenerative medicine and tissue engineering.

Recent advancement of decellularization extracellular matrix for tissue engineering and biomedical application

BACKGROUND

Decellularized extracellular matrixs (dECMs) derived from organs and tissues have emerged as a promising tool, as they encompass the characteristics of an ideal tissue scaffold: complex composition, vascular networks and unique tissue-specific architecture. Consequently, their use has propagated throughout tissue engineering and regenerative medicine. dECM can be easily obtained from various tissues/organs by appropriate decellularization protocolsand is entitled to provide necessary cues to cells homing.

METHODS

In this review, we describe the decellularization and sterilization methods that are commonly used in recent research, the effects of these methods upon biologic scaffold material are discussed. Also, we summarize the recent developments of recellularization and vascularization techniques in regeneration medicine. Additionally, dECM preservation methods is mentioned, which provides the basis for the establishment of organ bank.

RESULTS

Biomedical applications and the status of current research developments relating to dECM biomaterials are outlined, including transplantation in vivo, disease models and drug screening, organoid, 3D bioprinting, tissue reconstruction and rehabilitation and cell transplantation and culture. Finally, critical challenges and future developing technologies are discussed.

CONCLUSIONS

With the development of tissue engineering and regenerative medicine, dECM will have broader applications in the field of biomedicine in the near future.

Heparin modification improves the re-endothelialization and angiogenesis of decellularized kidney scaffolds through antithrombosis and anti-inflammation in vivo

Background

Constructing tissue-engineered kidneys using decellularized kidney scaffolds (DKS) has attracted widespread attention as it is expected to be the key to solving the shortage of donor kidneys. However, thrombosis and the host inflammatory response are unfavorable factors that hider the re-endothelialization and vascularization of the decellularized scaffolds.

Methods

Heparin was immobilized into the DKS using the method of 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide/N-hydroxysuccinimide (EDC/NHS) activation. Fourier-transform infrared (FTIR) spectra were used to verify the heparinization of DKS. Human umbilical vein endothelial cells (HUVECs) were seeded and cultured in the DKS, then the sliced scaffolds were transplanted subcutaneously into nude mouse. Scanning electron microscopy and a series of histochemical stains including hematoxylin and eosin (H&E), elastic Verhöeff-Van Gieson (EVG), Sirius red, Masson’s trichrome, and toluidine blue (TB) staining were used for morphological characterization. The qRT-PCR analysis, immunohistochemistry (IHC), and immunofluorescence (IF) staining were used to determine the expression of related molecular markers.

Results

The rat DKS completely retained the extracellular matrix and heparinized modification. The H&E staining results showed there were more HUVECs covering the internal surfaces of tubular structures in the HEP-DKS group compared with the DKS group. The IF analysis results revealed that CD31, Ki67, and CD206 had higher positive rates in HUVECs in the HEP-DKS group compared to the DKS group. Both groups of scaffolds showed blood vessel formation via H&E staining, and there were more blood vessels in the HEP-DKS group compared with the native DKS group (P<0.05). The qRT-PCR results showed that the levels of IL-1β, IL-6, and TNF-α in the HEP-DKS group were significantly lower than those of the native DKS group, while the expression level of IL-10 was significantly higher than that in the native DKS group (P<0.05).

Conclusions

Heparin modification improves the re-endothelialization and vascular regeneration of the DKS through anticoagulation in vitro and in vivo. The anti-inflammatory effect of heparin on the transplanted host was initially confirmed, and it is considered that this effect may play a non-negligible role in promoting DKS re-endothelialization and angiogenesis. Heparinized DKS is therefore a promising candidate for kidney tissue engineering.