A Perfusion Bioreactor for Longitudinal Monitoring of Bioengineered Liver Constructs

The native liver as inspiration to create superior in vitro hepatic models

Drug induced liver injury (DILI) is one of the major reasons of drug withdrawal during the different phases of drug development. The later in the drug development a drug is discovered to be toxic, the higher the economical as well as the ethical impact will be. In vitro models for early detection of drug liver toxicity are under constant development, however to date a superior model of the liver is still lacking. Ideally, a highly reliable model should be established to maintain the different hepatic cell functionalities to the greatest extent possible, during a period of time long enough to allow for tracking of the toxicity of compounds. In the case of DILI, toxicity can appear even after months of exposure. To reach this goal, an in vitro model should be developed that mimics the in vivo liver environment, function and response to external stimuli. The different approaches for the development of liver models currently used in the field of tissue engineering will be described in this review. Combining different technologies, leading to optimal materials, cells and 3D-constructs will ultimately lead to an ideal superior model that fully recapitulates the liver.

Advances in Recellularization of Decellularized Liver Grafts with Different Liver (Stem) Cells: Towards Clinical Applications

Liver transplantation is currently the only curative therapy for patients with acute or chronic liver failure. However, a dramatic gap between the number of available liver grafts and the number of patients on the transplantation waiting list emphasizes the need for valid liver substitutes. Whole-organ engineering is an emerging field of tissue engineering and regenerative medicine. It aims to generate transplantable and functional organs to support patients on transplantation waiting lists until a graft becomes available. It comprises two base technologies developed in the last decade; (1) organ decellularization to generate a three-dimensional (3D) extracellular matrix scaffold of an organ, and (2) scaffold recellularization to repopulate both the parenchymal and vascular compartments of a decellularized organ. In this review article, recent advancements in both technologies, in relation to liver whole-organ engineering, are presented. We address the potential sources of hepatocytes and non-parenchymal liver cells for repopulation studies, and the role of stem-cell-derived liver progeny is discussed. In addition, different cell seeding strategies, possible graft modifications, and methods used to evaluate the functionality of recellularized liver grafts are outlined. Based on the knowledge gathered from recent transplantation studies, future directions are summarized.

Hep3Gel: A Shape-Shifting Extracellular Matrix-Based, Three-Dimensional Liver Model Adaptable to Different Culture Systems

Drug-induced hepatotoxicity is a leading cause of clinical trial withdrawal. Therefore, in vitro modeling the hepatic behavior and functionalities is not only crucial to better understand physiological and pathological processes but also to support drug development with reliable high-throughput platforms. Different physiological and pathological models are currently under development and are commonly implemented both within platforms for standard 2D cultures and within tailor-made chambers. This paper introduces Hep3Gel: a hybrid alginate-extracellular matrix (ECM) hydrogel to produce 3D in vitro models of the liver, aiming to reproduce the hepatic chemomechanical niche, with the possibility of adapting its shape to different manufacturing techniques. The ECM, extracted and powdered from porcine livers by a specifically set-up procedure, preserved its crucial biological macromolecules and was embedded within alginate hydrogels prior to crosslinking. The viscoelastic behavior of Hep3Gel was tuned, reproducing the properties of a physiological organ, according to the available knowledge about hepatic biomechanics. By finely tuning the crosslinking kinetics of Hep3Gel, its dualistic nature can be exploited either by self-spreading or adapting its shape to different culture supports or retaining the imposed fiber shape during an extrusion-based 3D-bioprinting process, thus being a shape-shifter hydrogel. The self-spreading ability of Hep3Gel was characterized by combining empirical and numerical procedures, while its use as a bioink was experimentally characterized through rheological a priori printability evaluations and 3D printing tests. The effect of the addition of the ECM was evident after 4 days, doubling the survival rate of cells embedded within control hydrogels. This study represents a proof of concept of the applicability of Hep3Gel as a tool to develop 3D in vitro models of the liver.

Rat liver extracellular matrix and perfusion bioreactor culture promote human amnion epithelial cell differentiation towards hepatocyte-like cells

Congenital and chronic liver diseases have a substantial health burden worldwide. The most effective treatment available for these patients is whole organ transplantation; however, due to the severely limited supply of donor livers and the side effects associated with the immunosuppressive regimen required to accept allograft, the mortality rate in patients with end-stage liver disease is annually rising. Stem cell-based therapy aims to provide alternative treatments by either cell transplantation or bioengineered construct transplantation. Human amnion epithelial cells (AEC) are a widely available, ethically neutral source of cells with the plasticity and potential of multipotent stem cells and immunomodulatory properties of perinatal cells. AEC have been proven to be able to achieve functional improvement towards hepatocyte-like cells, capable of rescuing animals with metabolic disorders; however, they showed limited metabolic activities in vitro. Decellularised extracellular matrix (ECM) scaffolds have gained recognition as adjunct biological support. Decellularised scaffolds maintain native ECM components and the 3D architecture instrumental of the organ, necessary to support cells' maturation and function. We combined ECM-scaffold technology with primary human AEC, which we demonstrated being equipped with essential ECM-adhesion proteins, and evaluated the effects on AEC differentiation into functional hepatocyte-like cells (HLC). This novel approach included the use of a custom 4D bioreactor to provide constant oxygenation and media perfusion to cells in 3D cultures over time. We successfully generated HLC positive for hepatic markers such as ALB, CYP3A4 and CK18. AEC-derived HLC displayed early signs of hepatocyte phenotype, secreted albumin and urea, and expressed Phase-1 and -2 enzymes. The combination of liver-specific ECM and bioreactor provides a system able to aid differentiation into HLC, indicating that the innovative perfusion ECM-scaffold technology may support the functional improvement of multipotent and pluripotent stem cells, with important repercussions in the bioengineering of constructs for transplantation.

Development of a biomimetic bioreactor for regenerative endodontics research

In the context of regenerative endodontics research with the development of biomaterials, this work aimed to develop and test a prototype biomimetic bioreactor of a human tooth. The bioreactor was designed to reproduce a shaped dental canal connected with a cavity reproducing the periapical region and irrigated through two fluidic channels intended to reproduce the apical residual vascular supply. A test biomaterial composed of polylactic acid/polycaprolactone-tannic acid (PLA/PCL-TA) was produced by electrospinning/electrospraying and calibrated to be inserted in a dental canal. This biomaterial was first used to evaluate its imbibition capacity and the oximetry inside the bioreactor. Then, Dental Pulp Stem Cells (DPSCs) were cultured on PLA/PCL-TA cones for 1-3 weeks in the bioreactor; afterward cell adhesion, proliferation, and migration were histologically assessed. Complete imbibition biomaterial was obtained in 10 min and oximetry was stable over time. In the bioreactor, DPSCs were able to adhere, proliferate and migrate onto the surface and inside the biomaterial. In conclusion, this bioreactor was used successfully to test a biomaterial intended to support pulp regeneration and constitutes a new in vitro experimental model closer to clinical reality.

Physiologically relevant microsystems to study viral infection in the human liver

Viral hepatitis is a leading cause of liver disease and mortality. Infection can occur acutely or chronically, but the mechanisms that govern the clearance of virus or lack thereof are poorly understood and merit further investigation. Though cures for viral hepatitis have been developed, they are expensive, not readily accessible in vulnerable populations and some patients may remain at an increased risk of developing hepatocellular carcinoma (HCC) even after viral clearance. To sustain infection in vitro, hepatocytes must be fully mature and remain in a differentiated state. However, primary hepatocytes rapidly dedifferentiate in conventional 2D in vitro platforms. Physiologically relevant or physiomimetic microsystems, are increasingly popular alternatives to traditional two-dimensional (2D) monocultures for in vitro studies. Physiomimetic systems reconstruct and incorporate elements of the native cellular microenvironment to improve biologic functionality in vitro. Multiple elements contribute to these models including ancillary tissue architecture, cell co-cultures, matrix proteins, chemical gradients and mechanical forces that contribute to increased viability, longevity and physiologic function for the tissue of interest. These microsystems are used in a wide variety of applications to study biological phenomena. Here, we explore the use of physiomimetic microsystems as tools for studying viral hepatitis infection in the liver and how the design of these platforms is tailored for enhanced investigation of the viral lifecycle when compared to conventional 2D cell culture models. Although liver-based physiomimetic microsystems are typically applied in the context of drug studies, the platforms developed for drug discovery purposes offer a solid foundation to support studies on viral hepatitis. Physiomimetic platforms may help prolong hepatocyte functionality in order to sustain chronic viral hepatitis infection in vitro for studying virus-host interactions for prolonged periods.

Human-Origin iPSC-Based Recellularization of Decellularized Whole Rat Livers

End-stage liver diseases lead to mortality of millions of patients, as the only treatment available is liver transplantation and donor scarcity means that patients have to wait long periods before receiving a new liver. In order to minimize donor organ scarcity, a promising bioengineering approach is to decellularize livers that do not qualify for transplantation. Through decellularization, these organs can be used as scaffolds for developing new functional organs. In this process, the original cells of the organ are removed and ideally should be replaced by patient-specific cells to eliminate the risk of immune rejection. Induced pluripotent stem cells (iPSCs) are ideal candidates for developing patient-specific organs, yet the maturity and functionality of iPSC-derived cells do not match those of primary cells. In this study, we introduced iPSCs into decellularized rat liver scaffolds prior to the start of differentiation into hepatic lineages to maximize the exposure of iPSCs to native liver matrices. Through exposure to the unique composition and native 3D organization of the liver microenvironment, as well as the more efficient perfusion culture throughout the differentiation process, iPSC differentiation into hepatocyte-like cells was enhanced. The resulting cells showed significantly higher expression of mature hepatocyte markers, including important CYP450 enzymes, along with lower expression of fetal markers, such as AFP. Importantly, the gene expression profile throughout the different stages of differentiation was more similar to native development. Our study shows that the native 3D liver microenvironment has a pivotal role to play in the development of human-origin hepatocyte-like cells with more mature characteristics.

Engineering complexity in human tissue models of cancer

Major progress in the understanding and treatment of cancer have tremendously improved our knowledge of this complex disease and improved the length and quality of patients' lives. Still, major challenges remain, in particular with respect to cancer metastasis which still escapes effective treatment and remains responsible for 90% of cancer related deaths. In recent years, the advances in cancer cell biology, oncology and tissue engineering converged into the engineered human tissue models of cancer that are increasingly recapitulating many aspects of cancer progression and response to drugs, in a patient-specific context. The complexity and biological fidelity of these models, as well as the specific questions they aim to investigate, vary in a very broad range. When selecting and designing these experimental models, the fundamental question is "how simple is complex enough" to accomplish a specific goal of cancer research. Here we review the state of the art in developing and using the human tissue models in cancer research and developmental drug screening. We describe the main classes of models providing different levels of biological fidelity and complexity, discuss their advantages and limitations, and propose a framework for designing an appropriate model for a given study. We close by outlining some of the current needs, opportunities and challenges in this rapidly evolving field.

Recent Advances in Liver Engineering With Decellularized Scaffold

Liver transplantation is currently the only effective treatment for patients with end-stage liver disease; however, donor liver scarcity is a notable concern. As a result, extensive endeavors have been made to diversify the source of donor livers. For example, the use of a decellularized scaffold in liver engineering has gained considerable attention in recent years. The decellularized scaffold preserves the original orchestral structure and bioactive chemicals of the liver, and has the potential to create a de novo liver that is fit for transplantation after recellularization. The structure of the liver and hepatic extracellular matrix, decellularization, recellularization, and recent developments are discussed in this review. Additionally, the criteria for assessment and major obstacles in using a decellularized scaffold are covered in detail.

Advances in 3D cell culture for liver preclinical studies

The 3D cell culture model is an indispensable tool in the study of liver biology in the field of health and disease and the development of clinically relevant products for liver therapies. The 3D culture model captures critical factors of the microenvironmental niche required by hepatocytes for exhibiting optimal phenotypes, thus enabling the pursuit of a range of preclinical studies that are not entirely feasible in conventional 2D cell models. In this review, we highlight the major attributes associated with and the components needed for the development of a functional 3D liver culture model for a range of applications.