In recent years, the utilization of stem cell therapy to regenerate cardiac tissue has been proposed as a possible strategy to treat cardiac damage (Gnecchi et al., 2012, Aguirre et al., 2013; Sanganalmath and Bolli, 2013). Although encouraging results have been obtained in experimental models, the efficiency of cardiac regeneration is very poor and one of the major barriers to progress in the area of cell therapy for damaged heart is represented by the limited capacity of cells to differentiate into mature cardiomyocytes (CMC) (Laflamme and Murry, 2011). Cell manipulation and transfection represent versatile tools in this context (Melo et al., 2005; Dzau et al., 2005). Murine P19 embryonal carcinoma cells are a well-established cell line capable of differentiating in vitro into spontaneously beating CMC. This cell system with its limited cell culture requirements, protocol reproducibility and ease in uptake and subsequent expression of ectopic genetic materials render it ideal for the study of the cardiac differentiation process. P19 cells have been successfully used to gain important insights into the early molecular processes of CMC differentiation (van der Heyden and Defize, 2003; van der Heyden et al., 2003). P19 cells can also be maintained in an undifferentiated state in a monolayer culture when grown in adherence; this condition allows the enrichment of large cell numbers useful for cardiac differentiation protocols (McBurney, 1993). On the other hand, when cultured in bacterial dishes, P19 cells will grow in suspension and generate embryoid bodies (EB). When exposed to dimethyl sulfoxide (DMSO), EB differentiate into spontaneously beating cells, which can be defined as CMC. This definition is based on their gene and protein expression and their electrophysiological properties (Wobus et al., 1994; van der Heyden et al., 2003). In our laboratory, we used this in vitro model to verify whether the over-expression of a defined combination of miRNA can synergistically induce effective cardiac differentiation (Pisano et al., 2015). We used miRNA1, miRNA133 and miRNA499 alone or in combination. Here, we describe how we transiently transfect P19 cells to over-express a single or a combination of miRNA precursors (pre-miRNA).
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