The use of fluorescence correlation spectroscopy to monitor cell surface beta 2-adrenoceptors at low expression levels in human embryonic stem cell-derived cardiomyocytes and fibroblasts

The importance of cell phenotype in determining the molecular mechanisms underlying beta(2)-adrenoceptor (beta 2AR) function has been noted previously when comparing responses in primary cells and recombinant model cell lines. Here, we have generated haplotype-specific SNAP-tagged beta 2AR human embryonic stem (ES) cell lines and applied fluorescence correlation spectroscopy (FCS) to study cell surface receptors in progenitor cells and in differentiated fibroblasts and cardiomyocytes. FCS was able to quantify SNAP-tagged beta 2AR number and diffusion in both ES-derived cardiomyocytes and CRISPR/Cas9 genome-edited HEK293T cells, where the expression level was too low to detect using standard confocal microscopy. These studies demonstrate the power of FCS in investigating cell surface beta 2ARs at the very low expression levels often seen in endogenously expressing cells. Furthermore, the use of ES cell technology in combination with FCS allowed us to demonstrate that cell surface beta 2ARs internalize in response to formoterol-stimulation in ES progenitor cells but not following their differentiation into ES-derived fibroblasts. This indicates that the process of agonist-induced receptor internalization is strongly influenced by cell phenotype and this may have important implications for drug treatment with long-acting beta 2AR agonists.