Neuronal NOS localises to human airway cilia

•We sought to localise neuronal NOS (nNOS) in human airway epithelium.•nNOS immunoreactivity localised to cilia of healthy bronchial and nasal polyps tissues.•nNOS immunoreactivity was specifically blocked by nNOS peptide fragment.•nNOS immunoreactivity localised to cilia of healthy human epithelial cells differentiated at an air–liquid interface.•nNOS immunofluorescence labelling co-localised with β-tubulin in proximal portion of cilia.•We report a novel localisation of nNOS at the proximal portion of cilia in airway epithelium.

BackgroundAirway NO synthase (NOS) isoenzymes are responsible for rapid and localised nitric oxide (NO) production and are expressed in airway epithelium. We sought to determine the localisation of neuronal NOS (nNOS) in airway epithelium due to the paucity of evidence.Methods and resultsSections of healthy human bronchial tissue in glycol methacrylate resin and human nasal polyps in paraffin wax were immunohistochemically labelled and reproducibly demonstrated nNOS immunoreactivity, particularly at the proximal portion of cilia; this immunoreactivity was blocked by a specific nNOS peptide fragment. Healthy human epithelial cells differentiated at an air–liquid interface (ALI) confirmed the presence of all three NOS isoenzymes by immunofluorescence labelling. Only nNOS immunoreactivity was specific to the ciliary axonemeand co-localised with the cilia marker β-tubulin in the proximal part of the ciliary axoneme.ConclusionsWe report a novel localisation of nNOS at the proximal portion of cilia in airway epithelium and conclude that its independent and local regulation of NO levels is crucial for normal cilia function.