Vital staining of cardiac myocytes during embryonic stem cell cardiogenesis in vitro

Mouse embryonic stem (ES) cells differentiate in vitro into a variety of cell types, including spontaneously contracting cardiac myocytes. The primary aim of this work was to use vital stain techniques for real-time detection of developing cardiac myocytes in ES cell differentiation cultures. The -440 to +6 human cardiac α-actin promoter was used to direct expression of the Escherichia coil reporter gene lacZ (pHCAct-lacZ) into ES cell-derived cardiac myocytes during cardiogenesis in vitro. Undifferentiated ES cells were electroporated with HCActlacZ together with a plasmid containing the neomycin gene under the direction of the phosphoglycerate kinase promoter, and stable transformants were selected in G418. Individual clones were screened for activation of lacZ gene expression in cardiac myocytes developing in vitro. Results showed that expression of the HCActlacZ reporter construct was activated very early during the ES cell differentiation program, at a time point before the appearance of spontaneous contractile activity. The earliest detection was at day 6 of differentiation, when ≃25% of the differentiation cultures expressed the reporter construct, with expression increasing to ≃70% at day 9 and continuing throughout the duration of spontaneous contractile activity exhibited by the ES cell- derived cardiac myocytes. Indirect immunofluorescence assays provide evidence that expression was restricted to the cardiac myocytes in culture. In the present study, we show vital staining of transgene expression in living cardiac myocytes using lipophilic fluorogenic β-galactopyranoside substrates for real-time detection of the reporter gene during continuous contraction of the ES cell myocytes in vitro. The vital stain approach used in the present study will permit the identification of differentiating ES cells that are committed to the cardiac lineage for analysis of gene expression at early time points of ES cell cardiogenesis and, in addition, will aid in selecting genetically modified ES cell cardiac myocytes for use in functional studies.