Capsid-engineered AAV vector overcomes a key intracellular barrier and efficiently transduces spiral ganglion neurons in adult mice.

Hearing loss, the most prevalent sensory disorder, can result from pathology in various cochlear cell types. To develop effective gene therapy-based treatment strategies, vectors must be adapted to meet cell type-specific requirements. Here, we report a novel adeno-associated virus (AAV) capsid variant, AAV.MPI, which transduces spiral ganglion neurons (SGNs) in mature mouse cochleae with remarkable efficacy, even at low doses. This unique feature is mediated by a heptamer peptide, identified by a phage display peptide library screen, and inserted into the GH12/GH13 loop in each of the 60 subunits forming the mature AAV2 capsid. In addition to its change in vector tropism, AAV.MPI showed higher transgene expression efficacy. In line with this, uncoating assays revealed a more efficient release of vector genomes from AAV.MPI compared to AAV2 capsids, both in vitro and in vivo. Uncoating of AAV.MPI-in contrast to AAV2-likely occurs independently of cell cycle progression. Exploring AAV.MPI's SGN-tropism, SGN degeneration in deafened mice was prevented by vector-mediated overexpression of brain-derived neurotrophic factor. In summary, we present AAV.MPI as a novel AAV capsid variant adapted for cochlear gene therapy targeting SGNs. We also highlight vector uncoating as a key limiting post-entry step in auditory cell transduction.