Long-Term Expression and Safety of AAV1-Mediated PI3Kδ Overexpression in the Adult Rat Cortex

Mature CNS neurons are incapable of sufficiently regenerating their axons following spinal cord injury (SCI). This is largely due to developmental changes in epigenetic control leading to suppression of axon growth transcriptomic profile, leading to a shift towards synapse function support. Recently, manipulating the PI3K/Akt/mTOR pathway through PI3K{delta} overexpression in cortical neurons enhanced axonal regeneration of corticospinal tract axons, which was accompanied by functional recovery monitored for up to 16 weeks. However, PI3K is more widely known for its role as an oncogene, and since overexpression is achieved by the use of AAVs, valid safety concerns are raised as it is unknown what the long-term consequences of sustained PI3K{delta} expression in the brain are, which may be necessary to achieve complete re-establishment of the motor pathway. In this study, AAV1-hSYN-PIK3CD was injected into the motor cortex of rats, which survived for 1 year. Comparison with uninjected control animals reveal stable PI3K{delta} expression and sustained pathway activation through increased pS6. PI3K{delta}-treated animals show absence of tumour formation, neural soma hypertrophy, glial cell activation, or haematological or biochemical abnormalities. Thus, long-term neuronal PI3K{delta} expression appears to be well tolerated and may provide a safe and durable strategy to promote functional repair following SCI.