Orange/far-red hybrid voltage indicators with reduced phototoxicity enable reliable long-term imaging in neurons and cardiomyocytes.

Hybrid voltage indicators (HVIs) are chemogenetic sensors that combines the superior photophysical properties of organic dyes and the genetic targetability of protein sensors to report transient membrane voltage changes. They exhibit boosted sensitivity in excitable cells such as neurons and cardiomyocytes. However, the voltage signals recorded during long-term imaging are severely diminished or distorted due to phototoxicity and photobleaching issues. To capture stable electrophysiological activities over a long time, we employ cyanine dyes conjugated with a cyclooctatetraene (COT) molecule as the fluorescence reporter of HVI. The resulting orange-emitting HVI-COT-Cy3 enables high-fidelity voltage imaging for up to 30 min in cultured primary neurons with a sensitivity of ~ -30% ΔF/F0 per action potential (AP). It also maximally preserves the signal of individual APs in cardiomyocytes. The far-red-emitting HVI-COT-Cy5 allows two-color voltage/calcium imaging with GCaMP6s in neurons and cardiomyocytes for 15 min. We leverage the HVI-COT series with reduced phototoxicity and photobleaching to evaluate the impact of drug candidates on the electrophysiology of excitable cells.

ß2-Adrenergic Stimulation Compartmentalizes ß1 Signaling Into Nanoscale Local Domains by Targeting the C-Terminus of ß1-Adrenoceptors.

RATIONALE: ßARs (ß-adrenergic receptors) are prototypical GPCRs (G protein-coupled receptors) that play a pivotal role in sympathetic regulation. In heart cells, ß1AR signaling mediates a global response, including both l-type Ca2+ channels in the sarcolemma/T tubules and RyRs (ryanodine receptors) in the SR (sarcoplasmic reticulum). In contrast, ß2AR mediates local signaling with little effect on the function of SR proteins. OBJECTIVE: To investigate the signaling relationship between ß1ARs and ß2ARs. METHOD AND RESULTS: Using whole-cell patch-clamp analyses combined with confocal Ca2+ imaging, we found that the activation of compartmentalized ß2AR signaling was able to convert the ß1AR signaling from global to local mode, preventing ß1ARs from phosphorylating RyRs that were only nanometers away from sarcolemma/T tubules. This offside compartmentalization was eliminated by selective inhibition of ß2AR, GRK2 (GPCR kinase-2), ßarr1 (ß-arrestin-1), and phosphodiesterase-4. A knockin rat model harboring mutations of the last 3 serine residues of the ß1AR C terminus, a component of the putative ßarr1 binding site and GRK2 phosphorylation site, eliminated the offside compartmentalization conferred by ß2AR activation. CONCLUSIONS: ß2AR stimulation compartmentalizes ß1AR signaling into nanoscale local domains in a phosphodiesterase-4-dependent manner by targeting the C terminus of ß1ARs. This finding reveals a fundamental negative feed-forward mechanism that serves to avoid the cytotoxicity of circulating catecholamine and to sharpen the transient ß1AR response of sympathetic excitation.