Selective and Rapid Uptake of Adeno-Associated Virus Type 2 in Brain

Recombinant adeno-associated virus (AAV) vectors effectively transfer and express foreign genes in the brain. The transferred genes, however, are selectively expressed in neurons, and the cause of this specificity is not understood. To address this question, wild-type AAV-2 capsids were covalently labeled with the fluorophore, Cy3, and infused into the inferior colliculus or the hippocampus. Using antibodies to identify neurons (NeuN), astrocytes (GFAP), or oligodendrocytes (OX-42), clear neuron-specific uptake of the virus was observed as early as 6 min after the start of the infusion. By 30 min postinfusion, AAV particles were present in the nucleus of neurons, yet in both the inferior colliculus and hippocampus, a subset of neurons did not take up the virus particles. No AAV particles were found in astrocytes 1.5 min or 24 hr after virus infusion. Interestingly, 1 hr postinfusion, no AAV particles were found in microglia, yet by 24 hr postinfusion, a punctate pattern of AAV particles was found in microglia. To test whether virus uptake correlated with vector-transduced cells, an rAAV-CMV-GFP virus was infused. By 3 days postinfusion, GFP was localized to neuronal populations with no expression in astrocytes or microglia, similar to that of fluorescent virus uptake. These findings demonstrate that in brain, AAV particles rapidly bind and enter primarily neurons with a pattern similar to that of in vivo vector transduction. In addition, these studies indicate that viral binding and uptake, independent of promoter tropism, can explain the specificity of AAV brain transduction. Thus, this first description of AAV kinetic disposition in vivo should facilitate targeted application of this vector for human brain gene therapy. To investigate in vivo observations of selective neural gene expression after AAV infection, wild-type AAV-2 capsids were covalently labeled with a fluorophore and infused into the inferior colliculus or the hippocampus. Using antibodies to identify neurons, astrocytes, or oligodendrocytes, clear neuron-specific uptake of the virus was observed as early as 6 min after the start of the infusion, and by 30 min AAV particles were clearly present in the nucleus of neurons. In both the inferior colliculus and hippocampus, however, a subset of neurons did not take up the virus particles, and no AAV particles were found in astrocytes. Three days after AAV-CMV-GFP infusion, gene product was localized to neuronal populations similar to that of fluorescent virus uptake, but not in astrocytes or microglia. Thus, the binding and uptake of AAV provide a promoter-independent explanation for the observed in vivo neural tropism.