Cygnets: in vivo characterization of novel cGMP indicators and in vivo imaging of intracellular cGMP.

The second messenger cyclic guanosine 5'-monophosphate (cGMP) plays a key role in the control and regulation of a steadily increasing number of diverse physiological processes. As the appreciation of the importance of understanding the cGMP signaling pathway has grown, so has the awareness of the limited techniques with which to study the rapid intracellular cGMP kinetics. We have previously demonstrated the construction of cygnets, cGMP indicators using energy transfer comprised of cyan and yellow variants of green fluorescent protein flanked by conformationally sensitive cGMP receptor portion taken from the cGMP-dependent protein kinase. Here, we report that cGMP binds to Cygnet-2.1, utilizing ECFP and Citrine, with an apparent equilibrium-binding constant of 600 nM causing a total fluorescence intensity ratio change of 45%. In contrast, cAMP could elicit a maximal 10% change in fluorescence resonance energy transfer (FRET) ratio, demonstrating an approx 500-fold selectivity for cGMP. When expressed in vascular smooth muscle cells, cygnets demonstrated even cytosolic distribution and nuclear exclusion. Cultured rat aortic smooth muscle cells, which exhibit a noncontractile, synthetic phenotype typically seen in response to atherosclerosis or vascular injury, responded to natriuretic peptide (BNP)-mediated activation of the particulate guanylyl cyclase. In conclusion, cygnets have facilitated the temporal resolution and evaluation of the contributions of cyclases and phosphodiesterases in determining overall cGMP accumulation, and the visualization of novel spatial dynamics that will contribute to more fully understanding the role of cGMP in the mediation of smooth muscle relaxation.