Regionally distinct GFAP promoter expression plays a role in off-target neuron expression following AAV5 transduction

Astrocyte to neuron reprogramming has been performed using viral delivery of neurogenic transcription factors in GFAP expressing cells. Recent reports of off-target expression in cortical neurons following adeno-associated virus (AAV) transduction to deliver the neurogenic factors have confounded our understanding of the efficacy of direct cellular reprogramming. To shed light on potential mechanisms that may underlie the neuronal off-target expression of GFAP promoter driven expression of neurogenic factors in neurons, two regionally distinct cortices were compared-the motor cortex (MC) and medial prefrontal cortex (mPFC)-and investigated: (1) the regional tropism and astrocyte transduction with an AAV5-GFAP vector, (2) the expression of Gfap in MC and mPFC neurons; and (3) material transfer between astrocytes and neurons. Using a Cre-based system (AAV5-hGFAP-Cre; Rosa26R-tdTomato reporter mice), regional differences were observed in tdTomato expression between the MC and mPFC. Interestingly, this correlated with the presence of a greater expression of Gfap mRNA in neurons in the mPFC. Additionally, intercellular material transfer of Cre and tdTomato was observed between astrocytes and neurons in both regions, albeit at very low frequencies. Our study highlights regionally distinct variation in neurons that warrants consideration when designing genetic constructs for gene therapies targeting astrocytes including astrocyte to neuron reprogramming.