Intersectional targeting of defined neural circuits by adeno-associated virus vectors

Abstract

The mammalian nervous system is a complex network of interconnected cells. We review emerging techniques that use the axonal transport of adeno-associated virus (AAV) vectors to dissect neural circuits. These intersectional approaches specifically target AAV-mediated gene expression to discrete neuron populations based on their axonal connectivity, including: (a) neurons with one defined output, (b) neurons with one defined input, (c) neurons with one defined input and one defined output, and (d) neurons with two defined inputs or outputs. The number of labeled neurons can be directly controlled to trace axonal projections and examine cellular morphology. These approaches can precisely target the expression of fluorescent reporters, optogenetic ion channels, chemogenetic receptors, disease-associated proteins, and other factors to defined neural circuits in mammals ranging from mice to macaques, and thereby provide a powerful new means to understand the structure and function of the nervous system.

Keywords: AAV; anterograde; axonal transport; intersectional; neural circuits; retrograde.

Figure 1.. Intersectional Targeting of Defined Neural Circuits.

(A) Targeting gene expression to neurons with one defined output by retrograde transport of AAV2-retro-hSyn1-Cre. (B) Targeting gene expression to neurons with one defined input by anterograde trans-synaptic transport of AAV1-hSyn1-Cre. (C) Targeting gene expression to neurons with one defined input and one defined output by anterograde trans-synaptic transport of AAV1-hSyn1-Cre and retrograde transport of Cre-dependent AAV2-retro. (D) Targeting gene expression to neurons with two defined inputs by anterograde trans-synaptic transport of both AAV1-hSyn1-Cre and AAV1-hSyn1-Flp.

Figure 2.. Controlling the Number of Labeled Neurons.

The number of labeled neurons can be directly controlled by local injection of a Cre-dependent AAV-CIAO-Flp vector, which acts as a gate between anterograde trans-synaptic delivery of AAV1-hSyn1-Cre and a Flp-dependent vector such as AAV-FlpDIO-tdTomato. Because only cells that receive AAV-CIAO-Flp can activate gene expression, reducing the dose of this vector reduces the number of labeled cells without altering the strength of gene expression.