Liver tissue engineering: The future of liver therapeutics

Development of a Scalable Three-Dimensional Culture of Human Induced Pluripotent Stem Cells-Derived Liver Organoids

Human induced pluripotent stem cells (hiPSCs) represent a powerful tool for the generation of specialized cells to be used in regenerative medicine as well as hepatocellular repopulation tool to treat liver metabolic diseases such as nonalcoholic steatohepatitis (NASH). Here we describe a strategy to obtain fully functional liver organoids from hiPSCs in a scalable manner. Our approach uses a two-step process, with a first step involving the scalable formation of homogeneous and uniform-sized human embryoid bodies (hEBs), followed by the application of a four-step liver differentiation protocol for the derivation of liver organoids that possess all the features of primary human hepatocytes. This chapter will also illustrate the characterization of the liver organoids by directed biomolecular techniques.

Generation of Hepatocyte Organoids from Human iPS Cells

Human-induced pluripotent stem cells (hiPSCs) constitute a great source to generate specialized cells that can be employed in cell replacement therapy for a number of degenerative diseases. In this chapter, I describe a strategy to mass-produce fully functional hepatocyte organoids using hiPSCs interlaced with human adipose microvascular endothelial cells (HAMEC). Our unique technology employs a two-step strategy, consisting of the scalable generation of nearly spherical uniform-sized human embryoid bodies (hEBs), and the subsequent employment of a four-step hepatocyte differentiation approach for the generation of hepatocyte organoids that display all the characteristics of human primary hepatocytes. In this chapter, we also describe methodologies to characterize the hepatocyte organoids by using different biomolecular assays.

Generation of fully functional hepatocyte-like organoids from human induced pluripotent stem cells mixed with Endothelial Cells

Despite advances in stem cell research, cell transplantation therapy for liver failure is impeded by a shortage of human primary hepatocytes (HPH), along with current differentiation protocol limitations. Several studies have examined the concept of co-culture of human induced pluripotent cells (hiPSCs) with various types of supporting non-parenchymal cells to attain a higher differentiation yield and to improve hepatocyte-like cell functions both in vitro and in vivo. Co-culturing hiPSCs with human endothelial cells (hECs) is a relatively new technique that requires more detailed studies. Using our 3D human embryoid bodies (hEBs) formation technology, we interlaced Human Adipose Microvascular Endothelial Cells (HAMEC) with hiPSCs, leading to a higher differentiation yield and notable improvements across a wide range of hepatic functions. We conducted a comprehensive gene and protein secretion analysis of our HLCs coagulation factors profile, showing promising results in comparison with HPH. Furthermore, a stage-specific glycomic analysis revealed that the differentiated hepatocyte-like clusters (HLCs) resemble the glycan features of a mature tissue rather than cells in culture. We tested our HLCs in animal models, where the presence of HAMEC in the clusters showed a consistently better performance compared to the hiPSCs only group in regard to persistent albumin secretion post-transplantation.

Scalable Differentiation of Human iPSCs in a Multicellular Spheroid-based 3D Culture into Hepatocyte-like Cells through Direct Wnt/β-catenin Pathway Inhibition

Treatment of acute liver failure by cell transplantation is hindered by a shortage of human hepatocytes. Current protocols for hepatic differentiation of human induced pluripotent stem cells (hiPSCs) result in low yields, cellular heterogeneity, and limited scalability. In the present study, we have developed a novel multicellular spheroid-based hepatic differentiation protocol starting from embryoid bodies of hiPSCs (hiPSC-EBs) for robust mass production of human hepatocyte-like cells (HLCs) using two novel inhibitors of the Wnt pathway. The resultant hiPSC-EB-HLCs expressed liver-specific genes, secreted hepatic proteins such as Albumin, Alpha Fetoprotein, and Fibrinogen, metabolized ammonia, and displayed cytochrome P450 activities and functional activities typical of mature primary hepatocytes, such as LDL storage and uptake, ICG uptake and release, and glycogen storage. Cell transplantation of hiPSC-EB-HLC in a rat model of acute liver failure significantly prolonged the mean survival time and resolved the liver injury when compared to the no-transplantation control animals. The transplanted hiPSC-EB-HLCs secreted human albumin into the host plasma throughout the examination period (2 weeks). Transplantation successfully bridged the animals through the critical period for survival after acute liver failure, providing promising clues of integration and full in vivo functionality of these cells after treatment with WIF-1 and DKK-1.

Novel integrative methodology for engineering large liver tissue equivalents based on three-dimensional scaffold fabrication and cellular aggregate assembly

A novel engineering methodology for organizing a large liver tissue equivalent was established by intergrating both 'top down' and 'bottom up' approaches. A three-dimensional (3D) scaffold was engineered comprising 43 culture chambers (volume: 11.63 cm(3)) assembled in a symmetrical pattern on 3 layers, a design which enables further scaling up of the device to a clinically significant size (volume: 500 cm(3)). In addition, an inter-connected flow channel network was designed and proved to homogenously deliver culture medium to each chamber with the same pressure drop. After fabrication using nylon-12 and a selective laser sintering process, co-cultured cellular aggregates of human hepatoma Hep G2 and TMNK-1 cells were loosely packed into the culture chambers with biodegradable poly-L-lactic acid fibre pieces for 9 days of perfusion culture. The device enabled increased hepatic function and well-maintained cell viability, demonstrating the importance of an independent medium flow supply for cell growth and function provided by the current 3D scaffold. This integrative methodology from the macro- to the micro-scale provides an efficient way of arranging engineered liver tissue with improved mass transfer, making it possible to further scale up to a construct with clinically relevant size while maintaining high per-volume-based physiological function in the near future.

Titanium Dioxide Nanoparticles Trigger Loss of Function and Perturbation of Mitochondrial Dynamics in Primary Hepatocytes

Titanium dioxide (TiO2) nanoparticles are one of the most highly manufactured and employed nanomaterials in the world with applications in copious industrial and consumer products. The liver is a major accumulation site for many nanoparticles, including TiO2, directly through intentional exposure or indirectly through unintentional ingestion via water, food or animals and increased environmental contamination. Growing concerns over the current usage of TiO2 coupled with the lack of mechanistic understanding of its potential health risk is the motivation for this study. Here we determined the toxic effect of three different TiO2 nanoparticles (commercially available rutile, anatase and P25) on primary rat hepatocytes. Specifically, we evaluated events related to hepatocyte functions and mitochondrial dynamics: (1) urea and albumin synthesis using colorimetric and ELISA assays, respectively; (2) redox signaling mechanisms by measuring reactive oxygen species (ROS) production, manganese superoxide dismutase (MnSOD) activity and mitochondrial membrane potential (MMP); (3) OPA1 and Mfn-1 expression that mediates the mitochondrial dynamics by PCR; and (4) mitochondrial morphology by MitoTracker Green FM staining. All three TiO2 nanoparticles induced a significant loss (p < 0.05) in hepatocyte functions even at concentrations as low as 50 ppm with commercially used P25 causing maximum damage. TiO2 nanoparticles induced a strong oxidative stress in primary hepatocytes. TiO2 nanoparticles exposure also resulted in morphological changes in mitochondria and substantial loss in the fusion process, thus impairing the mitochondrial dynamics. Although this study demonstrated that TiO2 nanoparticles exposure resulted in substantial damage to primary hepatocytes, more in vitro and in vivo studies are required to determine the complete toxicological mechanism in primary hepatocytes and subsequently liver function.

Vascularized subcutaneous human liver tissue from engineered hepatocyte/fibroblast sheets in mice

Subcutaneous liver tissue engineering is an attractive and minimally invasive approach used to curative treat hepatic failure and inherited liver diseases. However, graft failure occurs frequently due to insufficient infiltration of blood vessels (neoangiogenesis), while the maintenance of hepatocyte phenotype and function requires in vivo development of the complex cellular organization of the hepatic lobule. Here we describe a subcutaneous human liver construction allowing for rapidly vascularized grafts by transplanting engineered cellular sheets consisting of human primary hepatocytes adhered onto a fibroblast layer. The engineered hepatocyte/fibroblast sheets (EHFSs) showed superior expression levels of vascularization-associated growth factors (vascular endothelial growth factor, transforming growth factor beta 1, and hepatocyte growth factor) in vitro. EHFSs developed into vascularized subcutaneous human liver tissues contained glycogen stores, synthesized coagulation factor IX, and showed significantly higher synthesis rates of liver-specific proteins (albumin and alpha 1 anti-trypsin) in vivo than tissues from hepatocyte-only sheets. The present study describes a new approach for vascularized human liver organogenesis under mouse skin. This approach could prove valuable for establishing novel cell therapies for liver diseases.

Three-dimensional culture in a microgravity bioreactor improves the engraftment efficiency of hepatic tissue constructs in mice

Tissue-engineered liver using primary hepatocytes has been considered a valuable new therapeutic modality as an alternative to whole organ liver transplantation for different liver diseases. The development of clinically feasible liver tissue engineering approaches, however, has been hampered by the poor engraftment efficiency of hepatocytes. We developed a three-dimensional (3D) culture system using a microgravity bioreactor (MB), biodegradable scaffolds and growth-factor-reduced Matrigel to construct a tissue-engineered liver for transplantation into the peritoneal cavity of non-obese diabetic severe combined immunodeficient mice. The number of viable cells in the hepatic tissue constructs was stably maintained in the 3D MB culture system. Hematoxylin-eosin staining and zonula occludens-1 expression revealed that neonatal mouse liver cells were reorganized to form tissue-like structures during MB culture. Significantly upregulated hepatic functions (albumin secretion, urea production and cytochrome P450 activity) were observed in the MB culture group. Post-transplantation analysis indicated that the engraftment efficiency of the hepatic tissue constructs prepared in MB cultures was higher than that of those prepared in the static cultures. Higher level of hepatic function in the implants was confirmed by the expression of albumin. These findings suggest that 3D MB culture systems may offer an improved method for creating tissue-engineered liver because of the higher engraftment efficiency and the reduction of the initial cell function loss.

Determining the optimal decellularization and sterilization protocol for preparing a tissue scaffold of a human-sized liver tissue

Attaining a well-qualified whole decellularized organ applicable for an enduring and successful transplantation, decellularization protocols should be organ specific in terms of decellularizing agents and time of tissue exposure. Since a bioscaffold resulting from a large solid organ should have the potential to preserve its three-dimensional architecture and consistency for at least several months in the site of transplantation, evaluating the mechanical properties of the bioscaffold is mandatory before transplantation. In the current study, we compared five different decellularization protocols and also two main decellularization techniques (perfusion vs. diffusion) to decellularize the sheep liver, which is similar to the human liver in terms of size and anatomy. Moreover, we assessed the retaining of vascular network by dye injection and angiography. We also determined the most proper sterilization method by comparing six different sterilization methods. The mechanical properties of the scaffolds were assessed by applying tensile strength, suture retention, and compressive strength tests. The perfusion technique showed better results compared to the diffusion technique. The protocol containing ammonium hydroxide and triton X-100 was the most proper decellularization protocol leading to completely decellularized livers along with intact vascular network. Furthermore, we noted that application of streptokinase in washing step facilitates decellularization. Our results also showed that a combination of two sterilization methods is necessary for complete sterilization of a sheep liver and peracetic acid or ethylene oxide+gamma irradiation was associated with the best outcome. Determining the most appropriate decellularization and sterilization method for each organ along with assessing the mechanical properties of the resulting bioscaffold are principal steps before fabricating efficient artificial organs in the foreseeable future.

Human hepatocyte propagation system in the mouse livers: functional maintenance of the production of coagulation and anticoagulation factors

We previously reported that cell-based therapies using isolated hepatocytes including hepatocyte transplantation and liver tissue engineering approaches provide therapeutic benefits to hemophilia. For clinical application of these approaches, it is important to establish an active hepatocyte proliferation system that enables providing a sufficient number of hepatocytes. We also reported that human hepatocytes, which were transplanted into the liver of urokinase-type plasminogen activator transgenic severe combined immunodeficiency (uPA/SCID) mice, were able to proliferate while retaining their ability to produce coagulation factor IX. The objective of this study was to explore the functionalities of other coagulation and anticoagulation factors of the propagated human hepatocytes in uPA/SCID mice. Human hepatocytes were transplanted into the liver of uPA/SCID mice, and the propagation status of human hepatocytes in the mice was monitored by the increase in serum human albumin levels and immunohistochemical evaluation on the liver sections. Using uPA/SCID livers with various stages of human hepatocyte propagation, we analyzed the gene expression levels of coagulation factors (prothrombin, factor VII, factor X, and factor VIII) and anticoagulation factors (protein C and protein S) by real-time polymerase chain reaction (PCR) using human-specific primers. As a result, the total amount of raw messenger RNA expression levels increased in all genes analyzed according to the progress of hepatocyte propagation and proliferation. Except for factor VIII, the gene expression levels of the highly repopulated uPA/SCID mouse livers with human hepatocyte showed higher levels than those of normal human livers, indicating that propagated human hepatocytes in the uPA/SCID system possess full functions to produce most of the coagulation-related factors. The current work demonstrated that human hepatocytes can be propagated in experimental animals while maintaining normal gene expression levels of coagulation-related factors. It could be speculated that the propagated cells serve as a cell source for the treatment of various types of coagulation factor deficiencies.

Liver tissue engineering utilizing hepatocytes propagated in mouse livers in vivo

Recent advances in tissue engineering technologies have highlighted the ability to create functional liver systems using isolated hepatocytes in vivo. Considering the serious shortage of donor livers that can be used for hepatocyte isolation, it has remained imperative to establish a hepatocyte propagation protocol to provide highly efficient cell recovery allowing for subsequent tissue engineering procedures. Donor primary hepatocytes were isolated from human α-1 antitrypsin (hA1AT) transgenic mice and were transplanted into the recipient liver of urokinase-type plasminogen activator-severe combined immunodeficiency (uPA/SCID) mice. Transplanted donor hepatocytes actively proliferated within the recipient liver of the uPA/SCID mice. At week 8 or later, full repopulation of the uPA/SCID livers with the transplanted hA1AT hepatocytes were confirmed by blood examination and histological assessment. Proliferated hA1AT hepatocytes were recovered from the recipient uPA/SCID mice, and we generated hepatocyte sheets using these recovered hepatocytes for subsequent transplantation into the subcutaneous space of mice. Stable persistency of the subcutaneously engineered liver tissues was confirmed for up to 90 days, which was the length of our present study. These new data demonstrate the feasibility in propagating murine hepatocytes prior to the development of hepatic cells and bioengineered liver systems. The ability to regenerate and expand hepatocytes has potential clinical value whereby procurement of small amounts of tissue could be expanded to sufficient quantities prior to their use in hepatocyte transplantation or other hepatocyte-based therapies.

Cell Therapy Using Adipose-Derived Stem Cells for Chronic Liver Injury in Mice

The present study investigated whether transplantation of autologous adipose-derived stem cells (ASCs) administered into the systemic circulation of a mouse with chronic liver injury provides therapeutic efficacy in the absence of any undesirable side effects. The ASCs used were isolated from mice with the same genetic background as the recipient mice and expanded in vitro. For the induction of chronic liver injury, mice were repetitively administered twice a week with CCl₄, a well-known hepatotoxin, for a period of 4 weeks. One day after the eighth dose of CCl₄, ASC transplantation was performed by tail vein injection and subsequently followed by two additional doses of CCl₄ administration. The recipient mice were divided into four groups (vehicle control, 1.5×10³, 1.5×10⁴, and 1.5×10⁵ ASCs per mouse). One day after the final CCl₄ administration, all mice were sacrificed to assess serum markers and liver histology. The level of serum markers for liver injury and hepatic function did not differ among the four groups. Similarly, no difference was observed in the liver histology between groups. Cell transplantation with ASCs in our model of chronic liver failure did not result in any observable side effects, but from our results, a single application of ASCs seems to be ineffective in improving liver injury.

Cell Shape Regulation Based on Hepatocyte Sheet Engineering Technologies

The de novo engineering of a uniform hepatocyte sheet in vitro is considered as a novel approach for liver-directed therapeutics. Hepatocytes can be cultured on a temperature-responsive culture dishes coated with poly( N-isopropylacrylamide) (PIPAAm). Following multiple days of culturing, the hepatocytes can be easily harvested as a uniform sheet by decreasing temperature from 37°C to 20°C. By modifying the sheet harvesting protocol, we have noticed that two different forms of the hepatocyte sheets, “extended” and “shrinking,” were obtained. This study describes the methods for harvesting the two different forms of sheets, and their cellular structure and hepatocyte-specific functions. To obtain an “extended sheet” form, a cluster of hepatocytes covered with a support membrane was harvested by the temperature reduction. For the “shrinking sheet” form, the hepatocyte sheet was floated after reducing the culture temperature, and the floating process allowed the sheet to shrink spontaneously. Histological analysis revealed that the hepatocytes in the extended sheet form were predominantly flat, whereas the shrinking sheet contained cuboidal shaped hepatocytes. The preservation of hepatocyte-specific ultrastructures was confirmed in both types of sheets. To investigate hepatocyte-specific functionality, the harvested hepatocyte sheets were recultured on Matrigel-coated dishes. Assessment of protein production levels and chemical metabolizing activities showed the similar functionalities for each form. In contrast, the recalculation of these values per sheet versus per square centimeter of sheet surface demonstrated that the function of the shrinking sheet was significantly higher than that of the extended sheets. This study demonstrated that the hepatocyte sheets created on the PIPAAm dish could spontaneously shrink in size, but retain their hepatocyte functionality. This type of hepatocyte sheet could be utilized for the engineering of liver tissue in limited areas that are unable to give adequate transplant space.

Engineering of implantable liver tissues

In this chapter, from the engineering point of view, we introduce the results from our group and related research on three typical configurations of engineered liver tissues; cell sheet-based tissues, sheet-like macroporous scaffold-based tissues, and tissues based on special scaffolds that comprise a flow channel network. The former two do not necessitate in vitro prevascularization and are thus promising in actual human clinical trials for liver diseases that can be recovered by relatively smaller tissue mass. The third approach can implant a much larger mass but is still not yet feasible. In all cases, oxygen supply is the key engineering factor. For the first configuration, direct oxygen supply using an oxygen-permeable polydimethylsiloxane membrane enables various liver cells to exhibit distinct behaviors, complete double layers of mature hepatocytes and fibroblasts, spontaneous thick tissue formation of hepatocarcinoma cells and fetal hepatocytes. Actual oxygen concentration at the cell level can be strictly controlled in this culture system. Using this property, we found that initially low then subsequently high oxygen concentrations were favorable to growth and maturation of fetal cells. For the second configuration, combination of poly-L: -lactic acid 3D scaffolds and appropriate growth factor cocktails provides a suitable microenvironment for the maturation of cells in vitro but the cell growth is limited to a certain distance from the inner surfaces of the macropores. However, implantation to the mesentery leaves of animals allows the cells again to proliferate and pack the remaining spaces of the macroporous structure, suggesting the high feasibility of 3D culture of hepatocyte progenitors for liver tissue-based therapies. For the third configuration, we proposed a design criterion concerning the dimensions of flow channels based on oxygen diffusion and consumption around the channel. Due to the current limitation in the resolution of 3D microfabrication processes, final cell densities were less than one-tenth of those of in vivo liver tissues; cells preferentially grew along the surfaces of the channels and this fact suggested the necessity of improved 3D fabrication technologies with higher resolution. In any case, suitable oxygen supply, meeting the cellular demand at physiological concentrations, was the most important factor that should be considered in engineering liver tissues. This enables cells to utilize aerobic respiration that produces almost 20 times more ATP from the same glucose consumption than anaerobic respiration (glycolysis). This also allows the cells to exhibit their maximum reorganization capability that cannot be observed in conventional anaerobic conditions.

Hepatocyte transplantation through the hepatic vein: a new route of cell transplantation to the liver

The efficiency of hepatocyte transplantation into the liver varies with the method of administration. This study investigated whether retrograde infusion via the hepatic vein provides a sufficient number of donor cells for the liver. Donor hepatocytes were isolated from dipeptidyl peptidase IV (DPPIV(+)) rats and transplanted into DPPIV(-) rat livers either by antegrade portal vein infusion or retrograde hepatic vein infusion. Hepatocyte engraftment ratios and localization were evaluated by histological DPPIV enzymatic staining at 1 week and 8 weeks after the transplantation. No significant differences in engraftment efficiency were observed at either 1 week or 8 weeks after transplantation by either route. However, the localization of the transplanted hepatocytes differed with the administration route. Portal vein infusion resulted in predominantly periportal engraftment, whereas hepatic vein infusion led to pericentral zone engraftment. Immunohistochemical analysis showed that the transplanted hepatocytes engrafted in the pericentral zone after retrograde infusion displayed intense CYP2E1 staining similar to the surrounding native hepatocytes. CYP2E1 staining was further enhanced by administration of isosafrole, an inducing agent for various cytochrome P450 enzymes, including CYP2E1. This study demonstrates a novel approach of transplanting hepatocytes into the liver through retrograde hepatic vein infusion as the means to target cell implantation to the pericentral zone.

Transplantation of a fetal liver cell-loaded hyaluronic acid sponge onto the mesentery recovers a Wilson's disease model rat

An auxiliary liver represents a promising alternative for liver transplantation. The use of a large amount of mature hepatocytes, however, despite their high function, is limited in a clinical setting. Here, we propose a novel transplantation system that dramatically improved a diseased animal by incorporating fetal liver cells (FLCs) as a cell source, the mesentery as a transplantation site and a hyaluronic acid (HA) sponge as a cell scaffold. We transplanted wild-type Long Evans Agouti rat FLCs embedded in HA sponges onto the mesentery of Long Evans Cinnamon (LEC) rats, an animal model for Wilson's disease. The FLC-loaded HA sponges successfully grafted and consequently prevented jaundice. Accordingly, the treated animals showed a significant reduction in blood copper concentration, which consequently led to significant decreases in serum total bilirubin and direct bilirubin, and to a significant increase in albumin productivity. Furthermore, haematoxylin and eosin staining of the host livers demonstrated that fibrosis at the periportal area was moderated in the treated animals. In conclusion, we transplanted FLC-loaded HA sponges onto the mesenteric blood vessels, leading to thick, liver-like tissue possessing blood vessels, and the liver tissue engineered thus exhibited a remarkable therapeutic effect on the copper metabolism deficiency of LEC rats.

Functional life-long maintenance of engineered liver tissue in mice following transplantation under the kidney capsule

The ability to engineer biologically active cells and tissue matrices with long-term functional maintenance has been a principal focus for investigators in the field of hepatocyte transplantation and liver tissue engineering. The present study was designed to determine the efficacy and temporal persistence of functional engineered liver tissue following transplantation under the kidney capsule of a normal mouse. Hepatocytes were isolated from human alpha-1 antitrypsin (hA1AT) transgenic mouse livers. Hepatocytes were subsequently transplanted under the kidney capsule space in combination with extracellular matrix components (Matrigel) for engineering liver tissues. The primary outcome of interest was to assess the level of engineering liver tissue function over the experimental period, which was 450 days. Long-term survival by the engineered liver tissue was confirmed by measuring the serum level of hA1AT in the recipient mice throughout the experimental period. In addition, administration of chemical compounds at day 450 resulted in the ability of the engineered liver tissue to metabolize exogenously circulating compounds and induce drug-metabolizing enzyme production. Moreover, we were able to document that the engineered tissues could retain their native regenerative potential similar to that of naïve livers. Overall, these results demonstrated that liver tissues could be engineered at a heterologous site while stably maintaining its functionality for nearly the life span of a normal mouse.

Engineering Liver Tissues under the Kidney Capsule Site Provides Therapeutic Effects to Hemophilia B Mice

Recent advances in liver tissue engineering have encouraged further investigation into the evaluation of therapeutic benefits based on animal disease models. In the present study, liver tissues were engineered in coagulation factor IX knockout (FIX-KO) mice, a mouse model of hemophilia B, to determine if the tissue engineering approach would provide therapeutic benefits. Primary hepatocytes were isolated from the liver of wild-type mice and suspended in a mixture of culture medium and extracellular matrix components. The hepatocyte suspension was injected into the space under the bilateral kidney capsules of the FIX-KO mice to engineer liver tissues. The plasma FIX activities (FIX:C) of the untreated FIX-KO mice were undetectable at any time point. In contrast, the liver tissue engineered FIX-KO mice achieved 1.5–2.5% of plasma FIX activities (FIX:C) and this elevated FIX:C level persisted throughout the 90 day experimental period. Significant FIX mRNA expression levels were found in the engineered liver tissues at levels similar to the wild-type livers. The present study demonstrates that liver tissue engineering could provide therapeutic benefits in the treatment of hemophilia B.