Functional analysis of mutation and partial duplication of α7 nicotinic receptor subunit gene : implication in schizophrenia

1. The α7 nicotinic receptor subunit (CHRNA7) gene has been considered as a susceptible gene for schizophrenia. In this study, we found a novel mutation in CHRNA7 gene in a Japanese schizophrenic patient. Since the mutation results in amino acid substitution (G423S) in the receptor cytoplasmic loop, we investigated functional differences between wild-type (WT) and the α7-G423S mutant receptor expressed in Xenopus oocytes. In the presence of a protein kinase C (PKC) activator, stimulation with α7 agonist induces phosphorylation and inhibition of the α7-G423S mutant receptor, but not WT receptor. These results demonstrate that the G423S mutation promotes the receptor desensitization by a protein kinase C-dependent mechanism.2. The CHRNA7 gene is partially duplicated and forms a hybrid (CHRFAM7A) with a novel gene FAM7A. In this study, we found that the cDNA of CHRFAM7A gene was translated into protein (dup-α7) in Xenopus oocytes and human SH-SY5Y cells, and that the protein was coimmunoprecipitated with α7 receptor subunit. Moreover, electrophysiological and biochemical studies demonstrated that the expression of dup-α7 protein caused the reduction of α7 receptor activity. These findings suggest the existence of a novel mechanism for regulating α7 receptor activity via dup-α7 protein.3. We examined effects of several alkaloids from amphibians and ascidians on neuronal nicotinic receptors, and found that the alkaloid (+)-205B is a patent and selective blocker of α7 nicotinic receptor.4. We found that chronic nicotine stimulation caused enhanced expression of gonadotropin releasing hormone in hypothalamic GT1-7 cells via t nicotinic receptor, and upregulation of α7 nicotinic receptor in vascular smooth muscle cells.These results suggest that the G423S mutation in CHRNA7 and the dup-α7 protein excess perturb the function of α7 nicotinic receptor and serve as predisposing factors for central cholinergic disorders, such as schizophrenia.