952. Effects of rAAV-BDNF Expression in the Striatum of the R6/1 Mouse

Abstract

Huntington's disease (HD) is characterized by a late-stage selective loss of medium-size spiny neurons (MSNs) in the basal ganglia; however, a disruption in striatal neuronal function precedes cell loss and motor abnormalities. In vivo and in vitro studies have demonstrated that brain-derived neurotrophic factor (BDNF) expression is critical for the proper differentiation, function and long-term survival of MSNs. To determine whether over-expression of BDNF is beneficial in models of HD, we designed a gene transfer strategy involving the use of recombinant-adeno associated virus (rAAV) expressing recombinant BDNF. R6/1 mice received unilateral intrastriatal injections of rAAV2-CBA-BDNF. Five-weeks post-surgery, analysis of striatal tissue revealed robust localized expression of the BDNF transgene and a diffuse distribution of BDNF protein throughout the striatum demonstrating that BDNF was secreted. We performed mRNA profiling on a subset of striatal-specific HD-affected transcripts and found significant recovery (10-25%) in the levels of DARPP-32, ppENK and CB1 transcripts. However, we observed a high level of mortality (>20%) as well as behaviors indicative of a disruption in normal striatal function. We hypothesized that the high levels of BDNF accounted for this toxicity. To test this hypothesis, we re-designed the rAAV expression system to express lower levels of BDNF in the striatum. The new rAAV5-TR-BDNF vector resulted in 100-fold less BDNF compared to the rAAV2-CBA-BDNF vector in the striatum. Although the rate of mortality was reduced, long-term expression of low levels of BDNF in the striatum of both R6/1 and wild-type mice led to abnormal behaviors and death. This demonstrates that long-term expression of striatal BDNF can be detrimental to the animal. The interpretation of these results is unclear at this time because the animals appeared healthy until their sudden death. The mode of delivery may also be suboptimal since BDNF is not normally expressed in MSNs but reaches striatum via the corticostriatal pathway.