Functional repair after ischemic injury through high efficiency in situ astrocyte-to-neuron conversion

Mammalian brains have largely lost internal neural regeneration capability except for a few discrete neurogenic niches. After brain injury, the cerebral cortex is especially difficult to repair due to its extremely low rate of adult neurogenesis. Previous studies have converted glial cells into neurons, but the total number of neurons generated is rather limited, casting doubt about its therapeutic potential. Here, we demonstrate that high-efficiency neuroregeneration can be achieved in adult mammalian brains by making use of an engineered AAV Cre-FLEX system to convert a large number of reactive astrocytes into functional neurons. Specifically, using a combination of GFAP::Cre and FLEX-NeuroD1 AAV system, we were able to regenerate enough new neurons from astrocytes to cover about 40% of the neurons lost from an ischemic injury (400 NeuN+ new neurons/mm2), compared to previously reported an average of <1% of cortical neurons (2-8 NeuN+ neurons/mm2) in an ischemic-injured adult mammalian cortex. Importantly, this in situ astrocyte-to-neuron conversion process also improved survival of injured pre-existing neurons, (additional 400 neurons/mm2), leading to a repaired motor cortex with layered cortical structures. Moreover, NeuroD1-converted neurons not only form functional neural circuits but also rescue motor and memory deficits after ischemic injury. Our results establish the proof-of-principle that a highly efficient in situ astrocyte-to-neuron conversion approach provides a novel treatment for neurological disorders that are in need of new neurons.