The fate of stem cell differentiation into a specific lineage of cells and the long-term proliferation of stem cells, especially human induced pluripotent (iPS) cells and human embryonic stem (ES) cells, is known to be regulated by inhibitors, growth factors, and/or small bioactive molecules in culture media. Recently, the physical cues of the biomaterials, particularly the stiffness of cell culture biomaterials, have been recognized to be an important factor guiding the fate of stem cell proliferation and differentiation1,2,3,4,5,6. Therefore, several researchers have started to investigate the fate of stem cells, which are cultured on hydrogels, on differentiation, mainly using polyacrylamide hydrogels with varying stiffness.
The stiffness of biomaterials can control focal adhesions, cell morphology, cell phenotype, and stem cell adhesion, especially in two-dimensional (2-D) cultivation1,2,3,5. Mechano-sensing of biomaterials by stem cells is generally controlled by focal adhesion signaling via integrin receptors. NMMIIA, nonmuscle myosin IIA-dependent contractility of the cytoskeleton of actin plays a critical role in the mechanosensing process of stem cells in 2-D cell cultivation systems3,4,5,7,8,9,10,11.
Engler and his colleagues developed an interesting notion that adult stem cells, such as bone marrow stem (BMS) cells cultivated on cell culture biomaterials with a similar stiffness to that of specific tissues, tend to differentiate into cells originated from specific tissues5. BMS cells incubated on 2-D soft polyacrylamide hydrogels coated with collagen type I (with a stiffness comparable to that of brain tissues) in expansion media were spontaneously induced to differentiate into early neuron lineages, whereas BMS cells cultured on hydrogels with a stiffness similar to that of muscle or collagenous bone tissues were found to induce differentiation into early lineages of myocytes and osteoblasts, respectively, on 2-D polyacrylamide hydrogels3,5. Many researchers have investigated the stem cell fate of differentiation cultured on polyacrylamide hydrogels immobilized with collagen type I12,13,14,15,16,17,18,19,20,21. However, it should be mentioned that some contradictory reports1,18,22,23,24 exist for the well-known idea suggested by Engler et al.5 This is because Engler's idea5 was developed solely on polyacrylamide hydrogels and their results have originated from specific characteristics of the biomaterial (polyacrylamide), and not solely from the physical cue (stiffness) of the biomaterial. Therefore, it is important to develop another type of hydrogel, of which the stiffness can be controlled by crosslinking of the hydrogels. For this purpose, bioinert hydrogels were developed, which were prepared from polyvinyl alcohol-co-itaconic acid (P-IA) with a different stiffness, which was controlled by the crosslinking degree with a changing crosslinking time25,26,27,28,29,30,31,32. The stem cells can be cultivated on nonmodified P-IA hydrogels, as well as P-IA hydrogels grafted with extracellular matrices (ECMs) and oligopeptides. In a previous study25, human hematopoietic stem cells (hHSCs) from umbilical cord blood were cultivated on P-IA hydrogels with different stiffness values ranging from a 3 kPa to 30 kPa storage modulus where fibronectin or an oligopeptide derived from fibronectin (CS1, EILDVPST) was grafted onto the P-IA hydrogels. High ex vivo fold expansion of hHSCs was observed in the P-IA hydrogels grafted with CS1 or fibronectin, which displayed an intermediate stiffness ranging from 12 kPa to 30 kPa25.
Human iPS and ES cells cannot be cultivated on conventional tissue culture polystyrene (TCP) dishes33,34 because human ES and iPS cells require specific binding to ECMs, such as vitronectin or laminin to maintain their pluripotency during long-term culture. Therefore, several structures of oligopeptide-grafted P-IA hydrogels with optimal stiffness characteristics were designed and prepared in formations of a single chain, a single chain with a joint segment, a dual chain with a joint segment, and a branched-type chain32. Oligopeptide sequences were selected from integrin- and glycosaminoglycan-binding domains of ECMs. The P-IA hydrogels grafted with vitronectin-derived oligopeptides with a dual chain or joint segment, which have a storage modulus at approximately 25 kPa, supported the long-term culture of human ES and iPS cells for over 12 passages under xeno-free and chemical defined conditions32. The joint segment and dual chain with cell adhesion molecules on the hydrogels facilitated the proliferation and pluripotency of human ES and iPS cells32. Here, a protocol for preparing P-IA hydrogels (with a storage modulus from 10 kPa to 30 kPa, which was measured under wet conditions in the air) grafted with and without oligopeptides or ECMs is described. How to culture and passage several stem cells (including amniotic fluid stem cells, adipose-derived stem cells, human ES cells, and human iPS cells) is shown.