Primary Human Hepatocytes Repopulate Livers of Mice After In Vitro Culturing and Lentiviral-Mediated Gene Transfer

Strategies to capitalize on cell spheroid therapeutic potential for tissue repair and disease modeling

Cell therapies offer a tailorable, personalized treatment for use in tissue engineering to address defects arising from trauma, inefficient wound repair, or congenital malformation. However, most cell therapies have achieved limited success to date. Typically injected in solution as monodispersed cells, transplanted cells exhibit rapid cell death or insufficient retention at the site, thereby limiting their intended effects to only a few days. Spheroids, which are dense, three-dimensional (3D) aggregates of cells, enhance the beneficial effects of cell therapies by increasing and prolonging cell-cell and cell-matrix signaling. The use of spheroids is currently under investigation for many cell types. Among cells under evaluation, spheroids formed of mesenchymal stromal cells (MSCs) are particularly promising. MSC spheroids not only exhibit increased cell survival and retained differentiation, but they also secrete a potent secretome that promotes angiogenesis, reduces inflammation, and attracts endogenous host cells to promote tissue regeneration and repair. However, the clinical translation of spheroids has lagged behind promising preclinical outcomes due to hurdles in their formation, instruction, and use that have yet to be overcome. This review will describe the current state of preclinical spheroid research and highlight two key examples of spheroid use in clinically relevant disease modeling. It will highlight techniques used to instruct the phenotype and function of spheroids, describe current limitations to their use, and offer suggestions for the effective translation of cell spheroids for therapeutic treatments.

A Preliminary Investigation of Embedding In Vitro HepaRG Spheroids into Recombinant Human Collagen Type I for the Promotion of Liver Differentiation

Background: In vitro three-dimensional (3D) hepatic spheroid culture has shown great promise in toxicity testing because it better mimics the cell–cell and cell–matrix interactions found in in vivo conditions than that of the traditional two-dimensional (2D) culture. Despite embedding HepaRG spheroids with collagen type I (collagen I) extracellular matrix (ECM) revealed a much better differentiation capability, almost all the collagen utilized in in vitro hepatocytes cultures is animal-derived collagen that may limit its use in human toxicity testing. Method: Here, a preliminary investigation of HepaRG cells cultured in different dimensionalities and with the addition of ECM was performed. Comparisons of conventional 2D culture with 3D spheroid culture were performed based on their functional or structural differences over 7 days. Rat tail collagen (rtCollagen) I and recombinant human collagen (rhCollagen) I were investigated for their ability in promoting HepaRG spheroid differentiation. Results: An immunofluorescence analysis of the hepatocyte-specific functional protein albumin suggested that HepaRG spheroids demonstrated better hepatic function than spheroids from 2D culture, and the function of HepaRG spheroids improved in a time-dependent manner. The fluorescence intensities per unit area of spheroids formed by 1000 cells on days 7 and 10 were 25.41 and 45.38, respectively, whereas almost undetectable fluorescence was obtained with 2D cells. In addition, the embedding of HepaRG spheroids into rtCollagen and rhCollagen I showed that HepaRG differentiation can be accelerated relative to the differentiation of spheroids grown in suspension, demonstrating the great promise of HepaRG spheroids. Conclusions: The culture conditions established in this study provide a potentially novel alternative for promoting the differentiation of HepaRG spheroids into mature hepatocytes through a collagen-embedded in vitro liver spheroid model. This culture method is envisioned to provide insights for future drug toxicology.

Novel vectors and approaches for gene therapy in liver diseases

Gene therapy is becoming an increasingly valuable tool to treat many genetic diseases with no or limited treatment options. This is the case for hundreds of monogenic metabolic disorders of hepatic origin, for which liver transplantation remains the only cure. Furthermore, the liver contains 10-15% of the body's total blood volume, making it ideal for use as a factory to secrete proteins into the circulation. In recent decades, an expanding toolbox has become available for liver-directed gene delivery. Although viral vectors have long been the preferred approach to target hepatocytes, an increasing number of non-viral vectors are emerging as highly efficient vehicles for the delivery of genetic material. Herein, we review advances in gene delivery vectors targeting the liver and more specifically hepatocytes, covering strategies based on gene addition and gene editing, as well as the exciting results obtained with the use of RNA as a therapeutic molecule. Moreover, we will briefly summarise some of the limitations of current liver-directed gene therapy approaches and potential ways of overcoming them.

Advanced biomedical applications based on emerging 3D cell culturing platforms

It is of great value to develop reliable in vitro models for cell biology and toxicology. However, ethical issues and the decreasing number of donors restrict the further use of traditional animal models in various fields, including the emerging fields of tissue engineering and regenerative medicine. The huge gap created by the restrictions in animal models has pushed the development of the increasingly recognized three-dimensional (3D) cell culture, which enables cells to closely simulate authentic cellular behaviour such as close cell-to-cell interactions and can achieve higher functionality. Furthermore, 3D cell culturing is superior to the traditional 2D cell culture, which has obvious limitations and cannot closely mimic the structure and architecture of tissues. In this study, we review several methods used to form 3D multicellular spheroids. The extracellular microenvironment of 3D spheroids plays a role in many aspects of biological sciences, including cell signalling, cell growth, cancer cell generation, and anti-cancer drugs. More recently, they have been explored as basic construction units for tissue and organ engineering. We review this field with a focus on the previous research in different areas using spheroid models, emphasizing aqueous two-phase system (ATPS)-based techniques. Multi-cellular spheroids have great potential in the study of biological systems and can closely mimic the in vivo environment. New technologies to form and analyse spheroids such as the aqueous two-phase system and magnetic levitation are rapidly overcoming the technical limitations of spheroids and expanding their applications in tissue engineering and regenerative medicine.

Expansion, in vivo-ex vivo cycling, and genetic manipulation of primary human hepatocytes

The ability to study human liver disease is limited by available hepatocyte models. Primary human hepatocytes (PHH) and xenograft models suffer from limited availability, donor-to-donor variability, and high cost. Here we report two transformative advances. First, the alkaloid retrorsine improves humanization of the murine liver, which allows routine production of highly humanized mice and high-quality mouse-passaged PHH. Second, the ability to genetically modify PHH cultures and retransplant to create highly humanized mice with genetically altered grafts. When combined, these two advances open new frontiers for creating disease-specific PHH models and for performing genetic and other screens in PHH.

Primary human hepatocytes (PHHs) are an essential tool for modeling drug metabolism and liver disease. However, variable plating efficiencies, short lifespan in culture, and resistance to genetic manipulation have limited their use. Here, we show that the pyrrolizidine alkaloid retrorsine improves PHH repopulation of chimeric mice on average 10-fold and rescues the ability of even poorly plateable donor hepatocytes to provide cells for subsequent ex vivo cultures. These mouse-passaged (mp) PHH cultures overcome the marked donor-to-donor variability of cryopreserved PHH and remain functional for months as demonstrated by metabolic assays and infection with hepatitis B virus and Plasmodium falciparum. mpPHH can be efficiently genetically modified in culture, mobilized, and then recultured as spheroids or retransplanted to create highly humanized mice that carry a genetically altered hepatocyte graft. Together, these advances provide flexible tools for the study of human liver disease and evaluation of hepatocyte-targeted gene therapy approaches.

Multi-color RGB marking enables clonality assessment of liver tumors in a murine xenograft model

We recently introduced red-green-blue (RGB) marking for clonal cell tracking based on individual color-coding. Here, we applied RGB marking to study clonal development of liver tumors. Immortalized, non-tumorigenic human fetal hepatocytes expressing the human telomerase reverse transcriptase (FH-hTERT) were RGB-marked by simultaneous transduction with lentiviral vectors encoding mCherry, Venus, and Cerulean. Multi-color fluorescence microscopy was used to analyze growth characteristics of RGB-marked FH-hTERT in vitro and in vivo after transplantation into livers of immunodeficient mice with endogenous liver damage (uPA/SCID). After initially polyclonal engraftment we observed oligoclonal regenerative nodules derived from transplanted RGB-marked FH-hTERT. Some mice developed monochromatic invasive liver tumors; their clonal origin was confirmed both on the molecular level, based on specific lentiviral-vector insertion sites, and by serial transplantation of one tumor. Vector insertions in proximity to the proto-oncogene MCF2 and the transcription factor MITF resulted in strong upregulation of mRNA expression in the respective tumors. Notably, upregulated MCF2 and MITF expression was also observed in 21% and 33% of 24 human hepatocellular carcinomas analyzed. In conclusion, liver repopulation with RGB-marked FH-hTERT is a useful tool to study clonal progression of liver tumors caused by insertional mutagenesis in vivo and will help identifying genes involved in liver cancer.