Optical highlighter molecules in neurobiology

Expanding the Genetic Code for Neuronal Studies

Genetic code expansion is one of the most powerful technologies in protein engineering. In addition to the 20 canonical amino acids, the expanded genetic code is supplemented by unnatural amino acids, which have artificial side chains that can be introduced into target proteins in vitro and in vivo. A wide range of chemical groups have been incorporated co-translationally into proteins in single cells and multicellular organisms by using genetic code expansion. Incorporated unnatural amino acids have been used for novel structure-function relationship studies, bioorthogonal labelling of proteins in cellulo for microscopy and in vivo for tissue-specific proteomics, the introduction of post-translational modifications and optical control of protein function, to name a few examples. In this Minireview, the development of genetic code expansion technology is briefly introduced, then its applications in neurobiology are discussed, with a focus on studies using mammalian cells and mice as model organisms.

Utilizing GCaMP transgenic mice to monitor endogenous Gq/11-coupled receptors

The family of GCaMPs are engineered proteins that contain Ca²⁺ binding motifs within a circularly permutated variant of the Aequorea Victoria green fluorescent protein (cp-GFP). The rapidly advancing field of utilizing GCaMP reporter constructs represents a major step forward in our ability to monitor intracellular Ca²⁺ dynamics. With the use of these genetically encoded Ca²⁺ sensors, investigators have studied activation of endogenous G_q types of G protein-coupled receptors (GPCRs) and subsequent rises in intracellular calcium. Escalations in intracellular Ca²⁺ from GPCR activation can be faithfully monitored in space and time as an increase in fluorescent emission from these proteins. Further, transgenic mice are now commercially available that express GCaMPs in a Cre recombinase dependent fashion. These GCaMP reporter mice can be bred to distinct Cre recombinase driver mice to direct expression of this sensor in unique populations of cells. Concerning the central nervous system (CNS), sources of calcium influx, including those arising from G_q activation can be observed in targeted cell types like neurons or astrocytes. This powerful genetic method allows simultaneous monitoring of the activity of dozens of cells upon activation of endogenous G_q-coupled GPCRs. Therefore, in combination with pharmacological tools, this strategy of monitoring GPCR activation is amenable to analysis of orthosteric and allosteric ligands of G_q-coupled receptors in their endogenous environments.