Two-photon brightness of azobenzene photoswitches designed for glutamate receptor optogenetics

Abstract

Mammalian neurotransmitter-gated receptors can be conjugated to photoswitchable tethered ligands (PTLs) to enable photoactivation, or photoantagonism, while preserving normal function at neuronal synapses. "MAG" PTLs for ionotropic and metabotropic glutamate receptors (GluRs) are based on an azobenzene photoswitch that is optimally switched into the liganding state by blue or near-UV light, wavelengths that penetrate poorly into the brain. To facilitate deep-tissue photoactivation with near-infrared light, we measured the efficacy of two-photon (2P) excitation for two MAG molecules using nonlinear spectroscopy. Based on quantitative characterization, we find a recently designed second generation PTL, , to have a favorable 2P absorbance peak at 850 nm, enabling efficient 2P activation of the GluK2 kainate receptor, LiGluR. We also achieve 2P photoactivation of a metabotropic receptor, LimGluR3, with a new mGluR-specific PTL, D-MAG0 460 . 2P photoswitching is efficiently achieved using digital holography to shape illumination over single somata of cultured neurons. Simultaneous Ca 2+ -imaging reports on 2P photoswitching in multiple cells with high temporal resolution. The combination of electrophysiology or Ca 2+ imaging with 2P activation by optical wavefront shaping should make second generation PTL-controlled receptors suitable for studies of intact neural circuits.optogenetics | pharmacology | multiphoton | photoswitch | azobenzene M odern neurobiology relies heavily on optical microscopy to observe, and, increasingly, to manipulate (1, 2), biological processes in live tissue. Among these methods, 2-photon-excited fluorescence microscopy (2PM) with near-infrared (NIR) light has emerged as an important technique for extending optical microscopy to highly scattering tissue (3, 4). Remarkably, barely 25 years after the first 2P-excited fluorescence image was published (5), 2PM is now performed in awake, behaving animals (4, 6-8). Naturally, optical manipulations in the brain can benefit from the many advantages of 2PM (9). In particular, the inherent spatial confinement of 2P absorption is critical for optical manipulation of individual cells (10) in the intact mammalian brain, where it is difficult to control the spatial extent of gene expression or confine soluble reagents. However, 2P-excited optogenetics is not yet as widespread in adoption as 2PM, being widely perceived to require sophisticated optical techniques (11-13) or reagent concentrations that compromise pharmacological specificity (2, 14).The rapid time to adoption of 2PM owes at least some credit to the availability of spectroscopic data on the 2P-excited efficacy, or brightness, of synthetic and genetically encoded fluorophores (15-17). Brightness, defined for fluorophores as the product of absorption cross-section and fluorescence quantum yield, gives the experimenter an objective metric to assess fluorescent reporters and identify appropriate optical parameters, such as the optimal excitation wavelength and range of light intensities (18). By ...

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“…For these reasons, we turned to the photoswitchable tethered ligand (PTL) approach in which, a cysteine residue on the outer, solvent facing surface of the LBD covalently anchors a “ligand on a string,” which can be rapidly and reversibly photoisomerized to place the ligand in the orthosteric binding pocket and thereby agonize or antagonize only that subunit. This approach has been used in a variety of neurotransmitter receptors (Volgraf et al, 2006; Reiner et al, 2015; Lin et al, 2015), including mGluRs (Levitz et al, 2013; Carroll et al, 2015). …”

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