α7 Nicotinic receptor gene delivery into mouse hippocampal neurons leads to functional receptor expression, improved spatial memory-related performance, and tau hyperphosphorylation

Brain α7 nicotinic receptors have become therapeutic targets for Alzheimer's disease (AD) based on their memory-enhancing and neuroprotective actions. This study investigated the feasibility of increasing neuronal α7 receptor functions using a gene delivery approach based on neuron-selective recombinant adeno-associated virus (rAAV)-derived vectors. In order to determine whether α7 receptor-mediated cytotoxicity was dependent on receptor density, rat α7 nicotinic receptors were expressed at high concentrations in GH4C1 cells as measured with nicotine-displaceable [3H]methyllycaconitine (MLA) binding. The potency of GTS-21 (an α7 receptor agonist) to induce cell loss was similar in these cells to that seen in pheochromocytoma (PC12) cells expressing nine-times-lower receptor levels, suggesting that cytotoxicity was more dependent on agonist concentration than receptor density. Hippocampal transduction with rat α7 nicotinic receptors increased [3H]MLA binding in this region in wild type and α7 receptor-knockout (KO) mice without apparent cytotoxicity. No difference was observed in Kd values for MLA binding between endogenous and transgenic receptors. Single cell recordings demonstrated that dentate granule cells that normally have no α7 receptor response did so following α7 receptor gene delivery in wild type mice. Recovery of α7 function was also observed in stratum oriens and stratum radiatum neurons of KO mice following gene delivery. Wild type mice exhibited improved acquisition performance in the Morris water task 1 month after bilateral hippocampal transductions with the rat α7 receptor gene compared with green fluorescent protein-transduced controls. However, both groups reached similar training levels and there was no difference in subsequent probe performance. Finally, this gene delivery approach was used to test whether α7 receptors affect tau-phosphorylation. Chronic (i.e. 2 month but not 2 week) expression of high levels of α7 receptors in hippocampus increased AT8 staining characteristic of hyperphosphorylated tau in that region, indicating that endogenous agonist-mediated receptor activation may be able to modulate this process.