Distinct expression of splice variants of neuronal nitric oxide synthase in the human gastrointestinal tract

Background & Aims: Changes of neuronal nitric oxide synthase (nNOS) expression have been linked to several human gastrointestinal disorders such as achalasia, diabetic gastroparesis, and hypertrophic pyloric stenosis. They could be caused by differential transcriptional control or alternative splicing generating different nNOS proteins. The aims of this study were to characterize 5'-splice variants, promoter usage, and site-specific expression of nNOS in the human gastrointestinal tract. Methods: 5'-Splice variants were characterized by immunoblotting, reverse-transcription polymerase chain reaction, 5'-rapid amplification of complementary DNA ends, and Southern blotting; Genomic analysis was performed by rapid amplification of genomic ends, followed by reporter gene assays. Results: Six different 5'-splice variants of nNOS-messenger RNA were identified showing specific expressions at various sites of the human gastrointestinal tract. Three variants encode for nNOS alpha, which has a specific N-terminal PDZ/GLGF domain and interaction sites for regulatory proteins. Two variants encode for nNOS beta and 1 for nNOS gamma, which both lack the protein-binding domains of nNOS alpha, In addition to 2 known first exons, a novel first exon of human nNOS with a separate functionally active downstream promoter and multiple binding sites for transcription factors was identified and characterized. Conclusions: Six 5'-mRNA splice variants of nNOS encoding 3 different nNOS proteins are expressed in the human gut. The differential expression of these proteins could be implicated in different biological functions.