Specific transduction of CSMN by AAV2 upon motor cortex injection

The application of adeno-associated virus (AAV) in gene therapy has multiple advantages due to its long-term expression in the central nervous system (CNS) and low immunoreactivity in humans. Gene therapy strategies in CNS include Canavan’s disease, Alzheimer’s disease and motor neuron diseases such as amyotrophic lateral sclerosis (ALS). Targeting only the vulnerable neuron populations without affecting other neuron types within the cerebral cortex is a major obstacle for translational neuroscience. This applies to ALS, in which the corticospinal motors neurons (CSMN; a.k.a upper motor neurons) show selective vulnerability and progressive degeneration. In this study, seven different AAV serotypes that harbor the eGFP gene were tested for their ability to transduce CSMN upon direct injection into the layer V of the motor cortex. CSMN transduction was confirmed by Ctip2 immunocytochemistry and by presence of red fluorescent microsphere in the CSMN after retrograde labeling by injection into the corticospinal tract (CST). Large pyramidal neurons located in layer V showed higher tropism for AAV2-2. In an effort to increase the selective transduction of CSMN by AAV, we used capsid proteins that are engineered, and different promoters to drive the eGFP expression. Our results suggest that the choice of the promoter is critically important to enhance selectivity of gene expression in CSMN. Furthermore, specific transduction of CSMN was feasible in mouse models with progressive CSMN degeneration. Identification of AAV serotypes that transduce only a select set of neuron populations at symptomatic stage of disease is critically important to develop effective and long-term gene therapy approaches in the cerebral cortex.