Neuronal and extraneuronal expression and regulation of the human alpha5 nicotinic receptor subunit gene

The mRNA encoding the human α5 nicotinic subunit was detected in several structures of the nervous system but appeared to be mainly expressed in cerebellum, thalamus, and the autonomic ganglia. For the first time, the α5 transcript was also detected in several nonneuronal tissues, with maximal expressions being found throughout the gastrointestinal tract, thymus, and testis. Many other extraneuronal sites expressed α5, but there were also nonexpressing organs, such as the liver, spleen, and kidney. To understand the transcriptional mechanisms controlling such a diversified expression of α5 in neuronal and nonneuronal cells, we isolated the 5'-regulatory region of the human gene and characterized its properties. Here we identify the α5 core promoter and demonstrate that the DNA regions surrounding it contain elements (with positive or negative activities) that work in a tissue- specific fashion. In particular, the segment specifying the 5'-untranslated region in neuronal cells has most of the properties of an enhancer because it activates a heterologous promoter in a position- and orientation-independent fashion. We therefore conclude that the expression of α5 relies on a highly complex promoter that uses distinct regulatory elements to comply with the different functional and developmental requirements of the various tissues and organs.