FRETting Mice Shed Light on Cardiac Adrenergic Signaling
See related article, pages 1084–1091
β-adrenergic receptor (β-AR) signaling in cardiac myocytes influences contractile and relaxation states in the heart. Classically, following hormone activation, β-AR preferentially couples with Gs, which in turn activates adenylyl cyclase and cAMP production. The predominant effector of cAMP, cAMP-dependent protein kinase (PKA), then phosphorylates many proteins important for cardiac function such as L-type calcium channels and phospholamban in the sarcoplasmic reticulum membrane. However, studies in myocytes have shown not all receptors that transduce signals via cAMP generate the same functional effects.1 These observations have led to the concept of cAMP compartmentation, which attributes the functional specificity and differential regulation to intricate spatial and temporal control of signaling molecules in the cAMP pathway. In this issue of Circulation Research, Nikolaev et al2 take another look at the differentially regulated β1 and β2-adrenergic signaling by using FRET-based cAMP imaging in adult cardiomyocytes, a method that is well suited for revealing the spatiotemporal complexity in cAMP signaling.
Fluorescence ratio imaging of cAMP in living cells was first introduced 15 years ago with the development of a bimolecular indicator using fluorescent dye-tagged regulatory and catalytic subunits of PKA.3 This class of cAMP indicators has evolved to become genetically encodable with the use of green fluorescent protein (GFP) variants4,5 and in recent years, unimolecular with the use of single cAMP binding domain-containing proteins and protein fragments.6–8 Nikolaev et al add to our molecular toolbox with the development of yet another FRET-based cAMP sensor with new characteristics. This sensor uses the cAMP binding domain of the hyperpolarization-activated cyclic nucleotide gated channel 2 (HCN2) as opposed to the binding domains from PKA or exchange proteins directly activated by cAMP (Epac), used in other FRET sensors. The resulting biosensor, HCN2-camps, maintains …