Activation of the Macrophage α7 Nicotinic Acetylcholine Receptor and Control of Inflammation

Abstract

Inflammatory responses to stimuli are essential body defenses against foreign threats. However, uncontrolled inflammation may result in serious health problems, which can be life-threatening. The α7 nicotinic acetylcholine receptor, a ligand-gated ion channel expressed in the nervous and immune systems, has an essential role in the control of inflammation. Activation of the macrophage α7 receptor by acetylcholine, nicotine, or other agonists, selectively inhibits production of pro-inflammatory cytokines while leaving anti-inflammatory cytokines undisturbed. The neural control of this regulation pathway was discovered recently and it was named the cholinergic anti-inflammatory pathway (CAP). When afferent vagus nerve terminals are activated by cytokines or other pro-inflammatory stimuli, the message travels through the afferent vagus nerve, resulting in action potentials traveling down efferent vagus nerve fibers in a process that eventually leads to macrophage α7 activation by acetylcholine and inhibition of pro-inflammatory cytokines production. The mechanism by which activation of α7 in macrophages regulates pro-inflammatory responses is subject of intense research, and important insights have thus been made. The results suggest that activation of the macrophage α7 controls inflammation by inhibiting NF-κB nuclear translocation, and activating the JAK2/STAT3 pathway among other suggested pathways. While the α7 is well characterized as a ligand-gated ion channel in neurons, whole-cell patch clamp experiments suggest that α7's ion channel activity, defined as the translocation of ions across the membrane in response to ligands, is absent in leukocytes, and therefore, ion channel activity is generally assumed not to be required for the operation of the CAP. In this perspective, we briefly review macrophage α7 activation as it relates to the control of inflammation, and broaden the current view by providing single-channel currents as evidence that the α7 expressed in macrophages retains its ion translocation activity despite the absence of whole-cell currents. Whether this ion-translocating activity is relevant for the proper operation of the CAP or other important physiological processes remains obscure.

Fig. 1

The cholinergic anti-inflammatory reflex. Invasion of pathogens and tissue damage produce inflammatory responses that are regulated in part by the cholinergic anti-inflammatory reflex. Endotoxins or pro-inflammatory cytokines signal through vagus nerve afferent fibers to the brain wherein the message is processed, resulting in an anti-inflammatory signal transmitted through vagus nerve efferent fibers to the celiac plexus. The splenic nerve then activates T-cells that express choline acetyltransferase (ChAT⁺ T-cells) in the spleen, which travel to macrophages and secrete ACh activating their α7 receptors. The efferent arm of the anti-inflammatory reflex is known as the cholinergic anti-inflammatory pathway (CAP). Activation of the α7 inhibits NF-κB by interfering with I-κB phosphorylation and NF-κB transcriptional activity (red lines). Activation of the α7 has been proposed to inhibit STAT3 phosphorylation, leaving uSTAT3 available to bind NF-κB subunits p50 and p65, and avoiding their nuclear translocation (light green arrows). On the other hand, it has been reported that the anti-inflammatory effect that results from activation of α7 is due to activation of the Jak2/STAT3 signaling pathway (yellow arrows). In a separate work, activated STAT3 was shown to induce the production of tristetraprolin (TTP), which is suggested as mediator of the anti-inflammatory effect (blue arrow). An additional proposed mechanism for the cholinergic regulation of pro-inflammatory responses comprise the upregulation of heme oxygenase-1 (HO-1) mediated by the activation of the phosphoinositol-3-kinase (PI3K)/Akt/Nrf-2 pathway (purple arrows)

Fig. 2

Patch clamp characterization of the α7 in MDMs. a Patch clamp experiments performed in the cell-attached configuration, using ACh (30 μM) as agonist demonstrate that the α7 expressed in MDMs function as ion channels. b and c Exposure to selective α7 positive allosteric modulator PNU-120596 (10 μM) results in prolonged openings confirming that currents are indeed from α7. c. This trace shows how PNU-120596 first increases the frequency of openings consistent with previous observations by daCosta et al. 2011. All experiments were performed at a holding potential of −100 mV. d. Patch clamp experiments performed in the cell-attached configuration, and using choline (15 μM) as agonist further demonstrates that the α7 expressed in MDMs behaves as a ligand-gated ion channel