Bioorthogonal chemical labeling of endogenous neurotransmitter receptors in living mouse brains.

Neurotransmitter receptors are essential components of synapses for communication between neurons in the brain. Because the spatiotemporal expression profiles and dynamics of neurotransmitter receptors involved in many functions are delicately governed in the brain, in vivo research tools with high spatiotemporal resolution for receptors in intact brains are highly desirable. Covalent labeling by chemical reaction (chemical labeling) of proteins without genetic manipulation is now a powerful method for analyzing receptors in vitro. However, selective target receptor labeling in the brain has not yet been achieved. This study shows that ligand-directed alkoxyacylimidazole (LDAI) chemistry can be used to selectively tether synthetic probes to target endogenous receptors in living mouse brains. The reactive LDAI reagents with negative charges were found to diffuse well over the whole brain and could selectively label target endogenous receptors, including AMPAR, NMDAR, mGlu1, and GABAAR. This simple and robust labeling protocol was then used for various applications: three-dimensional spatial mapping of endogenous receptors in the brains of healthy and disease-model mice; multi-color receptor imaging; and pulse-chase analysis of the receptor dynamics in postnatal mouse brains. Here, results demonstrated that bioorthogonal receptor modification in living animal brains may provide innovative molecular tools that contribute to the in-depth understanding of complicated brain functions.

Ligand-directed two-step labeling to quantify neuronal glutamate receptor trafficking.

The regulation of glutamate receptor localization is critical for development and synaptic plasticity in the central nervous system. Conventional biochemical and molecular biological approaches have been widely used to analyze glutamate receptor trafficking, especially for α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate-type glutamate receptors (AMPARs). However, conflicting findings have been reported because of a lack of useful tools for analyzing endogenous AMPARs. Here, we develop a method for the rapid and selective labeling of AMPARs with chemical probes, by combining affinity-based protein labeling and bioorthogonal click chemistry under physiological temperature in culture medium. This method allows us to quantify AMPAR distribution and trafficking, which reveals some unique features of AMPARs, such as a long lifetime and a rapid recycling in neurons. This method is also successfully expanded to selectively label N-methyl-D-aspartate-type glutamate receptors. Thus, bioorthogonal two-step labeling may be a versatile tool for investigating the physiological and pathophysiological roles of glutamate receptors in neurons.

Chemical Tools for Endogenous Protein Labeling and Profiling.

Protein analysis under biological conditions is now regarded as indispensable for understanding the structure and function of proteins, in addition to in vitro studies using purified target proteins. Because there are many molecules other than the protein-of-interest (POI) under live cell conditions, selective labeling of a POI is critical to distinguish the POI from other proteins for precise analysis. Protein labeling strategies utilizing genetically encoded tags have been used in POI modification in the complex environment of live cells. However, genetic manipulation may often induce overexpression of the POI and/or perturb the cellular context, resulting in unexpected artifacts in the protein analysis. Alternatively, recent progress in chemical biology has produced two major chemical approaches for analyzing endogenous proteins under native conditions. In this review, we summarize these techniques that utilize either protein-selective chemical labeling or proteome-directed chemical modification.