Nicotine is one of the most common and devastating drugs of abuse. Although smoking has declined in this country over the last twenty years, it has not decreased in the mentally ill, particularly in schizophrenia where more than 80% of these patients smoke. Nicotinic receptors are the first point of contact with the drug, making regulation of this receptor family an important first target for study. The a7 nicotinic receptor (a7*) has been implicated in tobacco use and deficits in cognition and sensory processing in the mental illness schizophrenia. The gene for this receptor, CHRNA7, was selected as the most promising target in drug development for cognitive deficits in this disease. Surface expression of the a7* receptor is low in postmortem brain of schizophrenic patients, but we have found adequate levels of both mRNA and protein for the receptor in schizophrenic smokers. This suggests that the assembly and trafficking of the receptor may be aberrant in schizophrenics. This proposal will investigate two mechanisms for the low levels of a7* surface receptors: 1) expression of a dominant negative splice variant, and 2) expression of RIC3, a protein associated with a7* assembly. Our data indicate that two polymorphisms in exon9/intron 9 may be associated with the presence of a splice variant missing exon 9. The variants are in linkage disequilibrium, representing a single mutant allele. The polymorphisms are associated with both schizophrenia and smoking in schizophrenia. The splice variant, missing exon 9, results in a frame change and premature termination. The shorter peptide acts as a dominant negative in assembly with the full-length subunit in an oocyte model. Our hypothesis is that the exon9/intron9 mutation affects RNA splicing and expression of the splice variant missing exon9, which blocks assembly of a competent receptor and surface binding. An alternative hypothesis involves aberrant expression of RIC3, a protein that facilitates assembly. The completion of this work will determine whether the hypothesized mechanisms are operative in the low levels of binding seen in postmortem brain of schizophrenic subjects. Lay statement: Tobacco products are one of the most common and devastating drugs of abuse, and are heavily used in the mentally ill. Underlying low levels of surface receptors may contribute to a biological need for these patients to smoke. This proposal will study mechanisms for cell surface expression of a nicotinic receptor that responds to the nicotine in tobacco.
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