Activation of the G_q-signaling cascade induces the production of inositol-1,4,5-trisphosphate (IP₃) and diacylglycerol by phospholipase C. This signaling pathway is involved in many fundamental cellular processes such as cell differentiation and modulation of cardiac pacemaking. We aimed to investigate this using light-induced G_q-signaling by optogenetics.

Therefore we overexpressed in HL-1 cardiomyocytes the optogenetic protein optoa₁-AR, a chimeric rhodopsin/a₁-adrenoreceptor that enables light-induced activation of the G_q-protein (Nature 2009; 458:1025–9). Illumination of these cells with blue light induced an increase of frequency of Ca²⁺ transients and of diastolic Ca²⁺ levels. Both of these effects could be blocked by the IP₃-receptor blocker 2-aminoethoxydiphenylborate or the phospholipase C blocker U-73122.

Unfortunately, the high light-intensities required for activation of optoa₁-AR induced adverse side effects in long term experiments. Therefore, we have alternatively used melanopsin, a light-sensitive G_q-coupled protein (Nature 2005;433:745–9). Melanopsin expressing HEK 293 cells increased cytosolic Ca²⁺ and produced IP₃ upon illumination. Importantly, we found that activation of melanopsin required lower light-intensities (~1 mW/mm²) compared to optoa₁-AR (~7 mW/mm²). To study the function of melanopsin in cardiomyocytes we transfected HL-1 cells. Preliminary experiments showed that illumination of melanopsin positive cells increased their spontaneous electrical activity.

In order to determine a potential role of light-induced G_q-signaling in cell differentiation, we generated stable transfected embryonic stem cell lines with optoa₁-AR or melanopsin expression under control of the chicken b-actin promoter. Undifferentiated stem cells from both cell lines showed light-induced elevation of cytosolic Ca²⁺ and could be used in further experiments for modulation of cell differentiation by light.

Thus, optoa₁-AR and melanopsin are optogenetic tools for light-induced G_q-signaling and will be useful to investigate cardiac function and cell differentiation in vitro and in vivo at high spatio-temporal precision.