Engineering mesenchymal stem cell sheets on functional polyelectrolyte multilayer substrates

Abstract

Cell sheet engineering is a novel and effective means of forming tissue substitutes in regenerative medicine. This technology allows for the manufacturing of cell sheets for either direct transplantation or the assembly of three-dimensionally organized tissue-mimicking structures. In the last decade, cell sheet engineering emerged as a promising alternative to injected cell delivery and biomaterial vehicles. Cell sheets manufactured on thermosensitive polymer coatings are already successfully employed in clinical applications such as human corneal reconstruction and treatment of human myocardial dysfunction. The main goal of my thesis was to develop an alternative, electrochemically responsive surface coating for the site-specific release of cell sheets by applying of micro-electrical currents. In context of my work, polyelectrolyte multilayer (PEM)-based platforms should be used for the synthesis of cell sheets composed of human stem / progenitor and end-differentiated cells. In the first part of my thesis, I determined the critical parameters for the isolation and long-term cultivation of human term placenta-derived mesenchymal stem cells (PD-MSCs). My results revealed that the isolated cells were of maternal origin and possessed very high proliferation and very low mortality rates up to passage twenty. The phenotypic profile of PD-MSC-isolates fulfilled the recently defined minimal criteria for the determination of mesenchymal stem cells (MSCs) and remained constant throughout continuous sub-confluent culture. The plasticity of PD-MSCs allowed their differentiation into adipogenic, chondrogenic, and osteogenic lineages as well as endothelium. In the second part of my thesis, I designed and optimized a PEM-based platform for the growth and intact peeling of cell sheets generated from different human cell types. Therefore, I established PEM substrates of a constant thickness with variations in rigidity. As a preliminary experiment, I used the assembly composed from nine layer-pairs of positively-charged poly-L-lysine (PLL) and negatively-charged hyaluronic acid (HA). The use of such polymers allowed for the construction of soft coatings and has been previously described as an attractive system for culturing different cell types. Moreover, the coating stiffness was adjusted by modulating the heterobifunctional crosslinkers present. For the assembly of stiff coatings, I used a film comprised of positively-charged poly-(allylamine)-hydrochloride (PAH) and negatively-charged poly-(styrene)-sulfonate (PSS). The potential of such coatings in cell sheet formation was previously demonstrated by endothelial cell cultivation. I demonstrated that the stiffness of the PEM substrate plays a critical role in the adhesion, spreading, and growth of human cells. Although the soft substrate supported the adhesion of the control murine muscle cell line C2C12, it only provided a minimal support for the adhesion of human cell types such as placenta-derived MSCs (PD-MSCs), adipose tissue-derived MSCs (AT-MSCs), human muscle cells (HMCs), human umbilical cord endothelial cells (HUVECs), and human late outgrowth endothelial cells (OECs). In contrast, the semi-stiff and stiff substrates were deemed suitable in culturing all of the tested human cell types and allowed for the complete mesodermal differentiation of stem cell sheets generated from AT-MSCs and PD-MSCs. In the final part of my thesis, I established the optimal conditions for the generation and detachment of PD-MSC sheets. In order to find a PEM architecture that minimally affected the cell sheet growth, I decided to utilize stiff (PAH/PSS) coatings. Such coatings supported the successful formation of PD-MSC sheets without any chemical modification. They also allowed for cell sheet detachment mediated either electrochemically by applying the micro-electrical current about 1.8 mV or by decreasing the environmental pH to 4. The viable PD-MSC sheets that detached from the (PAH/PSS) platform were able to adhere on the fresh TCPS substrates and could be successfully differentiated towards adipogenesis and osteogenesis. In summary, my thesis explored and implemented a PEM-based platform as substrates for cell sheet engineering and differentiation. Comparable with costly classical thermosensitive platforms, economically priced PEM based platforms allowed formation of cell sheets from different human stem / progenitor and adult cell types during a very short time period. Moreover, charge and pH-mediated peeling of cell sheets supported by PEM platforms are not only faster, but easier to automatize than thermosensitive platforms as well. My results lead me to believe that PEM based platforms indeed have potential as substrates for cell sheet engineering. They can be used for manufacturing single stem cell sheets geared for direct transplantation or for the generation of tissue-like structures destined for drug screening applications.