Nerve growth factor (NGF) gene therapy rescues and stimulates cholinergic neurons, which degenerate in Alzheimer's disease (AD). In a recent clinical trial for AD, intraparenchymal adeno-associated virus serotype 2 (AAV2)-NGF delivery was safe but did not improve cognition. Before concluding that NGF gene therapy is ineffective, it must be shown that AAV2-NGF successfully engaged the target cholinergic neurons of the basal forebrain. In this study, patients with clinically diagnosed early- to middle-stage AD received a total dose of 2 × 10¹¹ vector genomes of AAV2-NGF by stereotactic injection of the nucleus basalis of Meynert. After a mean survival of 4.0 years, AAV2-NGF targeting, spread, and expression were assessed by immunolabeling of NGF and the low-affinity NGF receptor p75 at 15 delivery sites in 3 autopsied patients. NGF gene expression persisted for at least 7 years at sites of AAV2-NGF injection. However, the mean distance of AAV2-NGF spread was only 0.96 ± 0.34 mm. NGF did not directly reach cholinergic neurons at any of the 15 injection sites due to limited spread and inaccurate stereotactic targeting. Because AAV2-NGF did not directly engage the target cholinergic neurons, we cannot conclude that growth factor gene therapy is ineffective for AD. Upcoming clinical trials for AD will utilize real-time magnetic resonance imaging guidance and convection-enhanced delivery to improve the targeting and spread of growth factor gene delivery.

Loss of midbrain dopamine neurons causes the cardinal symptoms of Parkinson's disease. However, not all dopamine neurons are equally vulnerable and a better understanding of the cell-type specific properties relating to selective dopamine neuron degeneration is needed. Most midbrain dopamine neurons express the vesicular glutamate transporter VGLUT2 during development and a subset continue to express low levels of VGLUT2 in adulthood, enabling the co-release of glutamate. Moreover, VGLUT2 expression in dopamine neurons can be neuroprotective since its genetic disruption was shown to sensitize dopamine neurons to neurotoxins. Here, we show that in response to toxic insult, and in two distinct models of alpha-synuclein stress, VGLUT2 dopamine neurons were resilient to degeneration. Dopamine neurons expressing VGLUT2 were enriched whether or not insult induced dopamine neuron loss, suggesting that while VGLUT2 dopamine neurons are more resilient, VGLUT2 expression can also be transcriptionally upregulated by injury. Finally, we observed that VGLUT2 expression was enhanced in surviving dopamine neurons from post-mortem Parkinson's disease individuals. These data indicate that emergence of a glutamatergic identity in dopamine neurons may be part of a neuroprotective response in Parkinson's disease.

Brain-derived neurotrophic factor (BDNF) gene delivery to the entorhinal cortex is a candidate for treatment of Alzheimer's disease (AD) to reduce neurodegeneration that is associated with memory loss. Accurate targeting of the entorhinal cortex in AD is complex due to the deep and atrophic state of this brain region. Using MRI-guided methods with convection-enhanced delivery, we were able to accurately and consistently target AAV2-BDNF delivery to the entorhinal cortex of non-human primates; 86 ± 3% of transduced cells in the targeted regions co-localized with the neuronal marker NeuN. The volume of AAV2-BDNF (3 × 10⁸ vg/µl) infusion linearly correlated with the number of BDNF labeled cells and the volume (mm³) of BDNF immunoreactivity in the entorhinal cortex. BDNF is normally trafficked to the hippocampus from the entorhinal cortex; in these experiments, we also found that BDNF immunoreactivity was elevated in the hippocampus following therapeutic BDNF vector delivery to the entorhinal cortex, achieving growth factor distribution through key memory circuits. These findings indicate that MRI-guided infusion of AAV2-BDNF to the entorhinal cortex of the non-human primate results in safe and accurate targeting and distribution of BDNF to both the entorhinal cortex and the hippocampus. These methods are adaptable to human clinical trials.