A Viral Toolbox of Genetically Encoded Fluorescent Synaptic Tags

Fibronectin intrabodies generated with mRNA display (FingRs) are a recently developed tool for labeling excitatory or inhibitory synapses, with the benefit of not altering endogenous synaptic protein expression levels or synaptic transmission. Here, we generated a viral vector FingR toolbox that allows for multi-color, neuron-type-specific labeling of excitatory or inhibitory synapses in multiple brain regions. We screened various fluorophores, FingR fusion configurations, and transcriptional control regulations in adeno-associated virus (AAV) and retrovirus vector designs. We report the development of a red FingR variant and demonstrated dual labeling of excitatory and inhibitory synapses in the same cells. Furthermore, we developed cre-inducible FingR AAV variants and demonstrated their utility, finding that the density of inhibitory synapses in aspiny striatal cholinergic interneurons remained unchanged in response to dopamine depletion. Finally, we generated FingR retroviral vectors, which enabled us to track the development of excitatory and inhibitory synapses in hippocampal adult-born granule cells.

A MicroRNA-Based Gene-Targeting Tool for Virally Labeling Interneurons in the Rodent Cortex

More specific and broadly applicable viral gene-targeting tools for labeling neuron subtypes are needed to advance neuroscience research, especially in rodent transgenic disease models and genetically intractable species. Here, we develop a viral vector that restricts transgene expression to GABAergic interneurons in the rodent neocortex by exploiting endogenous microRNA regulation. Our interneuron-targeting, microRNA-guided neuron tag, "GABA mAGNET," achieves >95% interneuron selective labeling in the mouse cortex, including in a murine model of autism and also some preferential labeling of interneurons in the rat brain. We demonstrate an application of our GABA mAGNET by performing simultaneous, in vivo optogenetic control of two distinct neuron subtypes. This interneuron labeling tool highlights the potential of microRNA-based viral gene targeting to specific neuron subtypes.