Modulation of metabotropic and ionotropic functions of the nicotinic α7 receptor by the intracellular domain

Abstract

The α7 receptor is a nicotinic receptor present in the nervous system and in non-neuronal cells. It has been demonstrated that α7 not only mediates fast synaptic transmission in neurons, but also regulates inflammatory responses in immune cell, neurite growth and neuronal protection, as well as cancer cell proliferation. The concept of α7 as a dual metabotropic/ionotropic receptor is attracting increasing attention. A key role in this dual nature is played by the receptor intracellular domain (ICD), which contains sites for phosphorylation and intracellular signaling. To explore the relationship between metabotropic and ionotropic activities we expressed α7 in mammalian cells and performed single-channel recordings to determine the channel properties and western blot to determine signaling pathways triggered by α7 activation. Single-channel recordings of human α7 from cells exposed to inhibitors of Src family kinases showed increased open durations and frequency of opening events. The effects were recapitulated using a receptor carrying mutations of the two ICD tyrosine residues, thus indicating that phosphorylation modulates receptor ionotropic activity. Cells exposed to the specific α7 agonist, PNU-282987, showed an increase of ERK1/2 phosphorylation, which was abolished by exposure to a tyrosine kinase inhibitor. PNU-282987 was not able to trigger ERK phosphorylation neither from cells expressing the double mutant receptor lacking tyrosine residues nor from cells co-expressing α7 and the ICD domain. Finally, the exposure of cells coexpressing α7 and ß2 adrenergic receptors to nicotine (α7 agonist) and isoproterenol (ß2 agonist) decreased phosphorylation of CREB, a known effector of the ß2 adrenergic receptor. This study indicates that the phosphorylated state of α7-ICD plays a role in the dual metabotropic/ionotropic receptor responses. It also opens doors for future studies exploring the role of the ICD as a modulator of the crosstalk between α7 and G-protein coupled receptors.