Genetic strategies to access activated neurons

Abstract

A major goal of modern neuroscience is to understand how ensembles of neurons participate in neural circuits underlying behavior. The recent explosion of genetically-encoded circuit analysis tools has allowed neuroscientists to characterize molecularly-defined neuronal types with unprecedented detail. However, since neurons defined by molecular expression can be functionally heterogeneous, targeting circuit analysis tools to neurons based on their activity is critical to elucidating the neural basis of behavior. Here we review genetic strategies to access activated neurons and characterize their functional properties, molecular profiles, connectivity, and causal roles in sensory-coding, memory, and valence-encoding. We also discuss future possibilities for improving these strategies and using them to screen brain-wide activity patterns underlying adaptive and maladaptive behaviors.

Figure 1

Summary of genetic strategies to visualize or manipulate previously activated neurons. (A) Fos and Arc promoters drive fluorescent proteins or optogenetic tools in both transgenic animals and viruses. Typically, peak labeling occurs a few hours after an experience and effector proteins last less than a day. (B) In Fos-tTA transgenic mice, neural activity in the absence of Dox leads to expression of TRE-conditional effectors including LacZ, chemogenetic tools, and ontogenetic tools for days following an experience. (C) In TRAP and ArcCreER^T2 transgenic mice, Fos and Arc promoters drive CreER^T2 to achieve permanent expression of Cre-dependent effector proteins in previously activated neurons. (D) In CANE, Fos drives TVA. Injection of EnvA-pseudotyped lentivirus delivers Cre to recently activated neurons, resulting in permanent expression of Cre-dependent effector genes. (E) In E-SARE viruses, a synthetic promoter drives effectors, or CreER^T2 to achieve permanent expression of Cre-dependent effector proteins in transiently activated neurons with high S/N. (F) In RAM viruses, a synthetic promoter drives destabilized tTA (d2tTA) which can drive expression of TRE-dependent effectors when activity occurs in the absence of Dox. (G) CaMPARI integrates a green fluorescent Ca²⁺ indicator with a photoconvertible protein. With the intersection of neural activity and user-delivered UV light, one can visualize recently activated neurons in red until the photoconverted protein is turned over. (H) TRIC uses Ca²⁺ to bring together DNA-binding (DBD) and activation (AD) domains of a transcription activator to drive effector genes to visualize or manipulate previously activated neurons. TRIC operates on a slower timescale, optimal for measuring slow changes in physiological states.