Liisa Kanninen, 2016
Abstract
"Standard two-dimensional (2D) in vitro cell culture systems do not mimic the complexity found in the liver as three-dimensional (3D) cell-cell and cell-matrix interactions are missing. Although the concept of cell culturing was established over 100 years ago the currently used culture techniques are not yet ideal. In the field of pharmacy especially, the need of physiologically-relevant models to characterize biotransformation pathways during drug development is urgent. Hepatocytes, the main cell type of the liver, are essential components in these in vitro models. Liver cell lines and derivation of hepatocyte-like cells from stem cells are alternative sources to primary isolations for obtaining hepatocytes.
In the liver, hepatocytes are in continuous interaction with other cells and surrounding extracellular matrix (ECM). Moreover, liver functions are strictly dependent on correct tissue architecture. One approach to improve the standard cell culture systems is to mimic the hepatocytes’ natural microenvironment and organization by culturing the cells within biomaterial matrices. Matrix-based culture systems for hepatocytes have been developed from natural, synthetic and hybrid biomaterials and the cells can be grown in 2D or 3D configuration. The aim of this thesis was to find new defined culture matrices for in vitro hepatic differentiation.
First, we studied two biomaterials, nanofibrillar cellulose (NFC) hydrogel and hyaluronic acid-gelatin (HG) hydrogel, to construct functional liver 3D organoids. Both of the studied hydrogels supported 3D spheroid organization of human liver progenitor HepaRG cells and their functional polarization. The 3D culture systems promoted hepatic differentiation of progenitor cells faster than the standard 2D culture. However, the 3D hydrogels did not enhance hepatocyte-like properties if the HepaRG cells were pre-differentiated to hepatocyte-like cells in advance. Subsequently, we showed that NFC hydrogel culture can be combined with highresolution imaging since the intact spheroids can be enzymatically released from the matrix. This was not possible with the HG hydrogel. We demonstrated that silica bioreplication preserved the 3D spheroid structure with its fine details and cellular antigens and allowed detailed morphological analysis of the spheroids cultured in NFC hydrogel.
Next, we developed a xeno-free matrix for hepatic specification of human pluripotent stem cell-derived definite endoderm (DE) cells using a three-step approach. We first proved our hypothesis that a liver progenitor-like matrix, HepaRG-derived acellular matrix (ACM), supports hepatic lineage differentiation of DE cells. Then, we characterized the ECM proteins secreted by HepaRG cells, and finally we showed that the identified proteins, laminin-511 and laminin-521, can replicate the effect of HepaRG-ACM. The human pluripotent stem cell-derived hepatic cells expressed mature hepatocyte-like functions but the phenotype of the cells was eventually closer to fetal hepatocytes than mature cells. Thus, hepatic maturation should be further studied. In conclusion, this thesis describes new biomaterials for hepatic differentiation, a protocol to form 3D spheroids and to transfer intact spheroids to high-resolution imaging, and that the described threestep approach can guide the identification of new defined matrices."
Liisa Kanninen, Bioinspired matrices for in vitro hepatic differentiation, University of Helsinki, Finland, 2016.