# Site‐specific noninvasive delivery of retrograde viral vectors to the brain

## TL;DR

This study shows that a modified virus can be noninvasively delivered to the brain to target specific neuron connections, offering a new way to treat brain disorders with high precision.

## Contribution

The study demonstrates the first noninvasive delivery of AAV9.retro using FUS-BBBO for retrograde transduction of neuronal projections.

## Key findings

- AAV9.retro can transduce neuronal projections noninvasively with FUS-BBBO as effectively as direct injection.
- AAV9.retro outperforms AAV8 in retrograde transduction and matches its transduction at the delivery site.
- Noninvasive delivery may offer improved specificity compared to invasive methods.

## Abstract

Neuronal activity underlies the brain function. Different behaviors, physiological processes, and disorders depend on which neurons are active at a given moment. Treating brain disorders without side effects will require exclusive control of disease‐relevant neurons. Traditionally, small molecule drugs could control a subset of neurons that express a molecularly specific receptor. Local noninvasive therapies such as delivery of neuromodulatory agents with focused ultrasound blood–brain barrier opening (FUS‐BBBO) also added spatial precision, allowing one to control specific brain regions without surgery. However, the final characteristic of neurons, which other neurons they connect to, remains underexplored as a therapeutic target. If targeting neurons based on their connectivity was possible noninvasively, it would open the doors to broadly deployable precise therapies that can target selected subgroups of neurons within a brain region. Such delivery could be achieved with retrograde‐tracing adeno‐associated viral vectors (AAVs). For noninvasive delivery with FUS‐BBBO, AAV9 has emerged as the most promising serotype. However, its retrograde‐tracing version, the AAV9.retro, has not been evaluated for FUS‐BBBO delivery. Here, we show that following such noninvasive delivery, AAV9.retro can safely transduce neuronal projections with comparable efficiency to a direct intracranial injection. Compared to AAV8, a naturally occurring vector with low retrograde transduction, AAV9.retro offers superior retrograde transduction and comparable transduction at the site of delivery. Overall, we show that AAV9.retro is a valuable FUS‐BBBO gene delivery vector, while also highlighting the surprising possibility of improved specificity of transduction of projections compared to invasive delivery.