Whole liver engineering: A promising approach to develop functional liver surrogates

Whole Liver Derived Acellular Extracellular Matrix for Bioengineering of Liver Constructs: An Updated Review

Biomaterial templates play a critical role in establishing and bioinstructing three-dimensional cellular growth, proliferation and spatial morphogenetic processes that culminate in the development of physiologically relevant in vitro liver models. Various natural and synthetic polymeric biomaterials are currently available to construct biomimetic cell culture environments to investigate hepatic cell-matrix interactions, drug response assessment, toxicity, and disease mechanisms. One specific class of natural biomaterials consists of the decellularized liver extracellular matrix (dECM) derived from xenogeneic or allogeneic sources, which is rich in bioconstituents essential for the ultrastructural stability, function, repair, and regeneration of tissues/organs. Considering the significance of the key design blueprints of organ-specific acellular substrates for physiologically active graft reconstruction, herein we showcased the latest updates in the field of liver decellularization-recellularization technologies. Overall, this review highlights the potential of acellular matrix as a promising biomaterial in light of recent advances in the preparation of liver-specific whole organ scaffolds. The review concludes with a discussion of the challenges and future prospects of liver-specific decellularized materials in the direction of translational research.

Current and Novel Therapeutical Approaches of Classical Homocystinuria in Childhood With Special Focus on Enzyme Replacement Therapy, Liver-Directed Therapy and Gene Therapy

Classical homocystinuria is a hereditary defect of the enzyme cystathionine beta synthase, which is produced in the liver. If this enzyme fails, the synthesis pathway of cysteine from methionine is interrupted, leading to the accumulation of homocysteine in the blood plasma and homocysteine in the urine. After birth, the children are unremarkable except for the characteristic laboratory findings. Symptoms rarely appear before the second year of life. The most common symptom is a prolapse of the crystalline lens. This finding is seen in 70% of untreated 10-year-old affected individuals. As the earliest symptom, psychomotor retardation occurs in the majority of patients already during the first two years of life. Limiting factors in terms of life expectancy are thromboembolism, peripheral arterial disease, myocardial infarction, and stroke. These symptoms are due to the damage to the vessels caused by the elevated amino acid levels. About 30% suffer a thromboembolic event by the age of 20, about half by the age of 30. This review focus on present and new therapeutical approaches like the role of enzyme replacement with presentation of different novel targets in research like pegtibatinase, pegtarviliase, CDX-6512, erymethionase, chaperones, proteasome inhibitors and probiotic treatment with SYNB 1353. Furthermore, we analyze the role of liver-directed therapy with three dimensional (3D) bioprinting, liver bioengineering of liver organoids in vitro and liver transplantation. The role of different gene therapy options to treat and cure this extremely rare disease in childhood will be discussed.

High level of polarized engraftment of porcine intrahepatic cholangiocyte organoids in decellularized liver scaffolds

In Europe alone, each year 5500 people require a life-saving liver transplantation, but 18% die before receiving one due to the shortage of donor organs. Whole organ engineering, utilizing decellularized liver scaffolds repopulated with autologous cells, is an attractive alternative to increase the pool of available organs for transplantation. The development of this technology is hampered by a lack of a suitable large-animal model representative of the human physiology and a reliable and continuous cell source. We have generated porcine intrahepatic cholangiocyte organoids from adult stem cells and demonstrate that these cultures remained stable over multiple passages whilst retaining the ability to differentiate into hepatocyte- and cholangiocyte-like cells. Recellularization onto porcine scaffolds was efficient and the organoids homogeneously differentiated, even showing polarization. Our porcine intrahepatic cholangiocyte system, combined with porcine liver scaffold paves the way for developing whole liver engineering in a relevant large-animal model.

Bioartificial livers: a review of their design and manufacture

Acute liver failure (ALF) is a rapidly progressive disease with high morbidity and mortality rates. Liver transplantation and artificial liver support systems, such as artificial livers (ALs) and bioartificial livers (BALs), are the two major therapies for ALF. Compared to ALs, BALs are composed of functional hepatocytes that provide essential liver functions, including detoxification, metabolite synthesis, and biotransformation. Furthermore, BALs can potentially provide effective support as a form of bridging therapy to liver transplantation or spontaneous recovery for patients with ALF. In this review, we systematically discussed the currently available state-of-the-art designs and manufacturing processes for BAL support systems. Specifically, we classified the cell sources and bioreactors that are applied in BALs, highlighted the advanced technologies of hepatocyte culturing and bioreactor fabrication, and discussed the current challenges and future trends in developing next generation BALs for large scale clinical applications.

Improving hemocompatibility of decellularized liver scaffold using Custodiol solution

Organ decellularization is one of the most promising approaches of tissue engineering to overcome the shortage of organs available for transplantation. However, there are key hurdles that still hinder its clinical application, and the lack of hemocompatibility of decellularized materials is a central one. In this work, we demonstrate that Custodiol (HTK solution), a common solution used in organ transplantation, increased the hemocompatibility of acellular scaffolds obtained from rat livers. We showed that Custodiol inhibited ex vivo, in vitro, and in vivo blood coagulation to such extent that allowed successful transplantation of whole-liver scaffolds into recipient animals. Scaffolds previously perfused with Custodiol showed no signs of platelet aggregation and maintained in vitro and in vivo cellular compatibility. Proteomic analysis revealed that proteins related to platelet aggregation were reduced in Custodiol samples while control samples were enriched with thrombogenicity-related proteins. We also identified distinct components that could potentially be involved with this anti-thrombogenic effect and thus require further investigation. Therefore, Custodiol perfusion emerge as a promising strategy to reduce the thrombogenicity of decellularized biomaterials and could benefit several applications of whole-organ tissue engineering.

Liver regeneration biology: Implications for liver tumour therapies

The liver has remarkable regenerative potential, with the capacity to regenerate after 75% hepatectomy in humans and up to 90% hepatectomy in some rodent models, enabling it to meet the challenge of diverse injury types, including physical trauma, infection, inflammatory processes, direct toxicity, and immunological insults. Current understanding of liver regeneration is based largely on animal research, historically in large animals, and more recently in rodents and zebrafish, which provide powerful genetic manipulation experimental tools. Whilst immensely valuable, these models have limitations in extrapolation to the human situation. In vitro models have evolved from 2-dimensional culture to complex 3 dimensional organoids, but also have shortcomings in replicating the complex hepatic micro-anatomical and physiological milieu. The process of liver regeneration is only partially understood and characterized by layers of complexity. Liver regeneration is triggered and controlled by a multitude of mitogens acting in autocrine, paracrine, and endocrine ways, with much redundancy and cross-talk between biochemical pathways. The regenerative response is variable, involving both hypertrophy and true proliferative hyperplasia, which is itself variable, including both cellular phenotypic fidelity and cellular trans-differentiation, according to the type of injury. Complex interactions occur between parenchymal and non-parenchymal cells, and regeneration is affected by the status of the liver parenchyma, with differences between healthy and diseased liver. Finally, the process of termination of liver regeneration is even less well understood than its triggers. The complexity of liver regeneration biology combined with limited understanding has restricted specific clinical interventions to enhance liver regeneration. Moreover, manipulating the fundamental biochemical pathways involved would require cautious assessment, for fear of unintended consequences. Nevertheless, current knowledge provides guiding principles for strategies to optimise liver regeneration potential.

A bioartificial liver support system integrated with a DLM/GelMA-based bioengineered whole liver for prevention of hepatic encephalopathy via enhanced ammonia reduction

Although bioartificial liver support systems (BLSSs) play an essential role in maintaining partial liver functions and detoxification for liver failure patients, hepatocytes are unanimously seeded in biomaterials, which lack the hierarchal structures and mechanical cues of native liver tissues. To address this challenge, we developed a new BLSS by combining a decellularized liver matrix (DLM)/GelMA-based bioengineered whole liver and a perfusion-based, oxygenated bioreactor. The novel bioengineered whole liver was fabricated by integrating photocrosslinkable gelatin (GelMA) and hepatocytes into a DLM. The combination of GelMA and the DLM not only provided a biomimetic extracellular microenvironment (ECM) for enhanced cell immobilization and growth with elevated hepatic functions (e.g., albumin secretion and CYP activities), but also provided biomechanical support to maintain the native structure of the liver. In addition, the perfusion-based, oxygenated bioreactor helped deliver oxygen to the interior tissues of the bioengineered liver, which was of importance for long-term culture. Most importantly, this new bioengineered whole liver decreased ammonia concentration by 45%, whereas direct seeding of hepatocytes in a naked DLM showed no significant reduction. Thus, the developed BLSS integrated with the DLM/GelMA-based bioengineered whole liver can potentially help elevate liver functions and prevent HE in liver failure patients while waiting for liver transplantation.

Identification of tissue sections from decellularized liver scaffolds for repopulation experiments

BACKGROUND

Biological organ engineering is a novel experimental approach to generate functional liver grafts by decellularization and repopulation. Currently, healthy organs of small or large animals and human organs with preexisting liver diseases are used to optimize decellularization and repopulation.However, the effects of morphological changes on allo- and xenogeneic cell-scaffold interactions during repopulation procedure, e.g., using scaffold-sections, are unknown. We present a sequential morphological workflow to identify murine liver scaffold-sections with well-preserved microarchitecture.

METHODS

Native livers (CONT, n = 9) and livers with experimentally induced pathologies (hepatics steatosis: STEA, n = 7; hepatic fibrosis induced by bile duct ligation: BDL, n = 9; nodular regenerative hyperplasia induced by 90% partial hepatectomy: PH, n = 8) were decellularized using SDS and Triton X-100 to generate cell-free scaffolds. Scaffold-sections were assessed using a sequential morphological workflow consisting of macroscopic, microscopic and morphological evaluation: (1) The scaffold was evaluated by a macroscopic decellularization score. (2) Regions without visible tissue remnants were localized for sampling and histological processing. Subsequent microscopical examination served to identify tissue samples without cell remnants. (3) Only cell-free tissue sections were subjected to detailed liver-specific morphological assessment using a histological and immunohistochemical decellularization score.

RESULTS

Decellularization was feasible in 33 livers, which were subjected to the sequential morphological workflow. In 11 of 33 scaffolds we achieved a good macroscopic decellularization result (CONT: 3 scaffolds; STEA: 3 scaffolds; BDL: 3 scaffolds; PH: 2 scaffolds). The microscopic assessment resulted in the selection of 88 cell-free tissue sections (CONT: 15 sections; STEA: 38 sections; BDL: 30 sections; PH: 5 sections). In 27 of those sections we obtained a good histological decellularization result (CONT: 3 sections; STEA: 6 sections; BDL: 17 sections; PH: 1 section). All experimental groups contained sections with a good immunohistochemical decellularization result (CONT: 6 sections; STEA: 5 sections; BDL: 4 sections; PH: 1 section).

DISCUSSION

Decellularization was possible in all experimental groups, irrespectively of the underlying morphological alteration. Furthermore, our proposed sequential morphological workflow was suitable to detect tissue sections with well-preserved hepatic microarchitecture, as needed for further repopulation experiments.

Portal venous repopulation of decellularised rat liver scaffolds with syngeneic bone marrow stem cells

Liver transplantation is the only life-saving treatment for end-stage liver failure but is limited by the organ shortage and consequences of immunosuppression. Repopulation of decellularised scaffolds with recipient cells provides a theoretical solution, allowing reliable and timely organ sourcing without the need for immunosuppression. Recellularisation of the vasculature of decellularised liver scaffolds was investigated as an essential prerequisite to the survival of other parenchymal components. Liver decellularisation was carried out by portal vein perfusion using a detergent-based solution. Decellularised scaffolds were placed in a sterile perfusion apparatus consisting of a sealed organ chamber, functioning at 37°C in normal atmospheric conditions. The scaffold was perfused via portal vein with culture medium. A total of 10⁷ primary cultured bone marrow stem cells, selected by plastic adherence, were infused into the scaffold, after which repopulated scaffolds were perfused for up to 30 days. The cultured stem cells were assessed for key marker expression using fluorescence-activated cell sorting (FACS), and recellularised scaffolds were analysed by light, electron and immunofluorescence microscopy. Stem cells were engrafted in portal, sinusoidal and hepatic vein compartments, with cell alignment reminiscent of endothelium. Cell surface marker expression altered following engraftment, from haematopoietic to endothelial phenotype, and engrafted cells expressed sinusoidal endothelial endocytic receptors (mannose, Fc and stabilin receptors). These results represent one step towards complete recellularisation of the liver vasculature and progress towards the objective of generating transplantable neo-organs.

Evaluation methods as quality control in the generation of decellularized peripheral nerve allografts

Nowadays, the high incidence of peripheral nerve injuries and the low success ratio of surgical treatments are driving research to the generation of novel alternatives to repair critical nerve defects. In this sense, tissue engineering has emerged as a possible alternative with special attention to decellularization techniques. Tissue decellularization offers the possibility to obtain a cell-free, natural extracellular matrix (ECM), characterized by an adequate 3D organization and proper molecular composition to repair different tissues or organs, including peripheral nerves. One major problem, however, is that there are no standard quality control methods to evaluate decellularized tissues. Therefore, in this review, a brief description of current strategies for peripheral nerve repair is given, followed by an overview of different decellularization methods used for peripheral nerves. Furthermore, we extensively discuss the available and currently used methods to demonstrate the success of tissue decellularization in terms of the cell removal, preservation of essential ECM molecules and maintenance or modification of biomechanical properties. Finally, orientative guidelines for the evaluation of decellularized peripheral nerve allografts are proposed.

A novel evaluation system for whole-organ-engineered liver graft by ex vivo application to a highly reproducible hepatic failure rat model

In recent years, studies on liver graft construction using the decellularized liver as a template for transplantation therapy have attracted much attention. However, the therapeutic effect of constructed liver grafts in hepatic failure has not been evaluated. Therefore, we aimed to develop a novel evaluation system demonstrating the curative effect of a constructed liver graft in animals with hepatic failure. First, we developed a highly reproducible rat model of hepatic failure by combining 80% partial hepatectomy with warm ischemia. In this model, severity could be controlled by the warm ischemic period. We also constructed a liver graft by recellularization of decellularized liver, and confirmed the ammonia metabolic function in the graft in vitro as one of the most important functions for recovery from hepatic failure. The graft was then applied to our developed hepatic failure rat model using a blood extracorporeal circulation system. In this application, the graft metabolized the ammonia in the blood of animals with hepatic failure and was thus suggested to be effective for the treatment of hepatic failure. In summary, a novel evaluation system for whole-organ-engineered liver graft as a preliminary stage of transplantation was developed. This system was expected to provide much information about the curative effect of a constructed liver graft.

Hepatocyte culture on 3D porous scaffolds of PCL/PMCL

The development of three-dimensional (3D) porous scaffolds for soft tissue engineering mainly focused on manipulation of scaffold properties with cell behaviors. By emulsion freeze-drying method, four types of porous scaffolds were prepared from amorphous poly(4-methy-ε-caprolactone) (PMCL) and semi-crystalline poly(ε-caprolactone) (PCL) at different weight ratios, named as PMCL0, PMCL30, PMCL50 and PMCL70, respectively. Visual observation on cross-sectional images of the scaffolds appeared as sponge-like materials with three-dimensional and highly porous morphologies. However, the pore size, porosity and wettability of blends were not decreased linearly with increasing amorphous PMCL. Distinguished from PMCL30 or PMCL70, PMCL50 preserved intact PCL crystals distributed in amorphous matrix, resulting in the lowest Young's modulus (E) and relatively high wettability. From in vitro cell culture, it was observed that PMCL50 scaffold supported human induced hepatocytes (hiHeps) proliferation and function preservation best among all scaffolds. hiHeps on PMCL50 inclined to adopt fibroblastic morphology, whereas formed spheroidal morphology on PMCL0. It was suggested that our bare scaffolds with tailored properties have shown remarkable capability towards hiHep proliferation and function expression.

Decellularization of Whole Human Liver Grafts Using Controlled Perfusion for Transplantable Organ Bioscaffolds

Liver transplantation is the only effective treatment for end-stage liver disease, but absolute donor shortage remains a limiting factor. Recent advances in tissue engineering focus on generation of native extracellular matrix (ECM) by decellularized complete livers in animal models. Although proof of concept has been reported for human livers, this study aims to perform whole liver decellularization in a clinically relevant series using controlled machine perfusion. In this study, we describe a mild nondestructive decellularization protocol, effective in 11 discarded human whole liver grafts to generate constructs that reliably maintain hepatic architecture and ECM components using machine perfusion, while completely removing cellular DNA and RNA. The decellularization process preserved the ultrastructural ECM components confirmed by histology, electron microscopy, and proteomic analysis. Anatomical characteristics of the native microvascular network and biliary drainage of the liver were confirmed by contrast computed tomography scanning. Decellularized vascular matrix remained suitable for normal suturing and no major histocompatibility complex molecules were detected, suggesting absence of allo-reactivity when used for transplantation. After extensive washing, decellularized scaffolds were nontoxic for cells after reseeding human mesenchymal stromal or umbilical vein endothelial endothelium cells. Indeed, evidence of effective recellularization of the vascular lining was obtained. In conclusion, we established an effective method to generate clinically applicable liver scaffolds from human discarded whole liver grafts and show proof of concept that reseeding of normal human cells in the scaffold is feasible. This supports new opportunities for bioengineering of transplantable grafts in the future.