Representative tertiary amines were linked to the 8-cyano-7-hydroxyquinolinyl (CyHQ) photoremovable protecting group (PPG) to create photoactivatable forms suitable for use in studying cell physiology. The photoactivation of tamoxifen and 4-hydroxytamoxifen, which can be used to activate Cre recombinase and CRISPR-Cas9 gene editing, demonstrated that highly efficient release of bioactive molecules could be achieved through one- and two-photon excitation (1PE and 2PE). CyHQ-protected anilines underwent a photoaza-Claisen rearrangement instead of releasing amines. Time-resolved spectroscopic studies revealed that photorelease of the tertiary amines was extremely fast, occurring from a singlet excited state of CyHQ on the 70 ps time scale.

Photoremovable protecting groups (PPGs) are powerful tools for physiological studies, harnessing light as an on/off switch to provide tight spatio-temporal control over the release of biological effectors through two-photon excitation (2PE) in tissue culture and whole-animal studies. We carried out a series of systematic structural modifications to the (8-cyano-7-hydroxyquinolin-2-yl)­methyl (CyHQ) chromophore to conduct an SAR study with the aim of enhancing its photochemical properties, especially its two-photon uncaging action cross section (δu). The best results were obtained when substituents were added at the C4 position, which improved δu for release of acetate up to 7-fold, while retaining all the other excellent properties of the CyHQ PPG, including high quantum yield (Φu), low susceptibility to spontaneous hydrolysis in the dark, and good aqueous solubility. Hammett correlation analysis suggested that photolysis efficiency is favored by electron-rich substituents at C4, giving important insights into the mechanism of the photolysis reaction. The four best CyHQ derivatives were used to mediate the efficient release of homopiperonylic acid in high yield under simulated physiological conditions. Our efforts have led to the development of 2PE-sensitive PPGs with remarkable δu values (up to 2.64 GM), excellent quantum yields (up to 0.88), and high-yielding effector release (up to 92%).

Kinetic analysis of dopamine receptor activation and inactivation and the study of dopamine-dependent signaling requires precise simulation of the presynaptic release of the neurotransmitter dopamine and tight temporal control over the release of dopamine receptor antagonists. The 8-cyano-7-hydroxyquinolinyl (CyHQ) photoremovable protecting group was conjugated to dopamine and the dopamine receptor antagonist sulpiride to generate “caged” versions of these neuromodulators (CyHQ-O-DA and CyHQ-sulpiride, respectively) that could release their payloads with 365 or 405 nm light or through 2-photon excitation (2PE) at 740 nm. These compounds are stable under physiological conditions in the dark, yet photolyze rapidly and cleanly to yield dopamine or sulpiride and the caging remnant CyHQ–OH. CyHQ-O-DA mediated the light activation of dopamine-1 (D1) receptors on the breast cancer cell line MDA-MB-231 in culture. In mouse brain slice from the substantia nigra pars compacta, localized flash photolysis of CyHQ-O-DA accurately mimicked the natural presynaptic release of dopamine and activation of dopamine-2 (D2) receptors, causing a robust, concentration-dependent, and repeatable G protein-coupled inwardly rectifying potassium channel-mediated outward current in whole-cell voltage clamp recordings that was amplified by cocaine and blocked by sulpiride. Photolysis of CyHQ-sulpiride rapidly blocked synaptic activity, enabling measurement of the unbinding rates of dopamine and quinpirole, a D2 receptor agonist. These tools will enable more detailed study of dopamine receptors, their interactions with other GPCRs, and the physiology of dopamine signaling in the brain.

A dopamine D2 receptor mutation was recently identified in a family with a novel hyperkinetic movement disorder. Compared to the wild type D2 receptor, the novel allelic variant D2-I 212 F activates a Gα i1 β 1 γ 2 heterotrimer with higher potency and modestly enhanced basal activity in human embryonic kidney (HEK) 293 cells and has decreased capacity to recruit arrestin3. We now report that omitting overexpressed G protein-coupled receptor kinase-2 (GRK2) decreased the potency and efficacy of quinpirole for arrestin recruitment. The relative efficacy of quinpirole for arrestin recruitment to D2-I 212 F compared to D2-WT was considerably lower without overexpressed GRK2 than with added GRK2. D2-I 212 F exhibited higher basal activation of Gα oA than Gα i1 but little or no increase in the potency of quinpirole relative to D2-WT. Other signs of D2-I 212 F constitutive activity for G protein-mediated signaling, in addition to basal activation of Gα i/o , were enhanced basal inhibition of forskolin-stimulated cyclic AMP accumulation that was reversed by the inverse agonists sulpiride and spiperone and a ∼4-fold increase in the apparent affinity of D2-I 212 F for quinpirole, determined from competition binding assays. In mouse midbrain slices, inhibition of tonic current by the inverse agonist sulpiride in dopamine neurons expressing D2-I 212 F was consistent with our hypothesis of enhanced constitutive activity and sensitivity to dopamine relative to D2-WT. Molecular dynamics simulations with D2 receptor models suggested that an ionic lock between the cytoplasmic ends of the third and sixth α-helices that constrains many G protein-coupled receptors in an inactive conformation spontaneously breaks in D2-I 212 F. Overall, these results confirm that D2-I 212 F is a constitutively active and signaling-biased D2 receptor mutant and also suggest that the effect of the likely pathogenic variant in a given brain region will depend on the nature of G protein and GRK expression.

The direct release of dialkylanilines was achieved by controlling the outcome of a photorearrangement reaction promoted by the (8-cyano-7-hydroxyquinolin-2-yl)­methyl (CyHQ) photoremovable protecting group. The substrate scope was investigated to obtain structure–activity relationships and to propose a reaction mechanism. Introducing a methyl substituent at the 2-methyl position of the CyHQ core enabled the bypass of the photorearrangement and significantly improved the aniline release efficiency. We successfully applied the strategy to the photoactivation of mifepristone (RU-486), an antiprogestin drug that is also used to induce the LexPR gene expression system in zebrafish and the gene-switch regulatory system based on the pGL-VP chimeric regulator in mammals.