Interaction of the α7-nicotinic subunit with its human-specific duplicated dupα7 isoform in mammalian cells: Relevance in human inflammatory responses

Abstract

The α7 nicotinic receptor subunit and its partially duplicated human-specific dupα7 isoform are coexpressed in neuronal and non-neuronal cells. In these cells, α7 subunits form homopentameric α7 nicotinic acetylcholine receptors (α7-nAChRs) implicated in numerous pathologies. In immune cells, α7-nAChRs are essential for vagal control of inflammatory response in sepsis. Recent studies show that the dupα7 subunit is a dominant-negative regulator of α7-nAChR activity in Xenopus oocytes. However, its biological significance in mammalian cells, particularly immune cells, remains unexplored, as the duplicated form is indistinguishable from the original subunit in standard tests. Here, using immunocytochemistry, confocal microscopy, coimmunoprecipitation, FRET, flow cytometry, and ELISA, we addressed this challenge in GH4C1 rat pituitary cells and RAW264.7 murine macrophages transfected with epitope- and fluorescent protein-tagged α7 or dupα7. We used quantitative RT-PCR of dupα7 gene expression levels in peripheral blood mononuclear cells (PBMCs) from patients with sepsis to analyze its relationship with PBMC α7 mRNA levels and with serum concentrations of inflammatory markers. We found that a physical interaction between dupα7 and α7 subunits in both cell lines generates heteromeric nAChRs that remain mainly trapped in the endoplasmic reticulum. The dupα7 sequestration of α7 subunits reduced membrane expression of functional α7-nAChRs, attenuating their anti-inflammatory capacity in lipopolysaccharide-stimulated macrophages. Moreover, the PBMC's dupα7 levels correlated inversely with their α7 levels and directly with the magnitude of the patients' inflammatory state. These results indicate that dupα7 probably reduces human vagal anti-inflammatory responses and suggest its involvement in other α7-nAChR-mediated pathophysiological processes.

Keywords: GH4C1 cells; RAW264.7 macrophages; dupα7 nicotinic subunit; human sepsis; inflammation; macrophage; nicotinic acetylcholine receptors (nAChR); protein assembly; sepsis; α-nicotinic subunit.