The alpha-7 nicotinic acetylcholine receptor agonist GTS-21 engages the glucagon-like peptide-1 incretin hormone axis to lower levels of blood glucose in db/db mice

Nicotinic Signaling Stimulates Glucagon Secretion in Mouse and Human Pancreatic α-Cells

Smoking is widely regarded as a risk factor for type 2 diabetes because nicotine contributes to insulin resistance by desensitizing the insulin receptors in muscle, liver, or fat. Little is known, however, about the immediate regulation of islet hormonal output by nicotine, an agonist of ionotropic cholinergic receptors. We investigated this by imaging cytosolic Ca2+ dynamics in mouse and human islets using confocal microscopy and measuring glucagon secretion in response to the alkaloid from isolated mouse islets. Nicotine acutely stimulated cytosolic Ca2+ in glucagon-secreting α-cells but not in insulin-secreting β-cells. The 2.8- ± 0.5-fold (P < 0.05) increase in Ca2+, observed in >70% of α-cells, correlated well with a 2.5- ± 0.3-fold stimulation of glucagon secretion. Nicotine-induced elevation of cytosolic Ca2+ relied on influx from the extracellular compartment rather than release of the cation from intracellular depots. Metabotropic cholinergic signaling, monitored at the level of intracellular diacylglycerol, was limited to 69% of α-cells versus 94% of β-cells. We conclude that parasympathetic regulation of pancreatic islet hormone release uses different signaling pathways in β-cells (metabotropic) and α-cells (metabotropic and ionotropic), resulting in the fine-tuning of acetylcholine-induced glucagon exocytosis. Sustained nicotinic stimulation is, therefore, likely to attenuate insulin sensitivity by increasing glucagon release.

ARTICLE HIGHLIGHTS

Alterations in Neuronal Nicotinic Acetylcholine Receptors in the Pathogenesis of Various Cognitive Impairments

Cognitive impairment is a typical symptom of both neurodegenerative and certain other diseases. In connection with these different pathologies, the etiology and neurological and metabolic changes associated with cognitive impairment must differ. Until these characteristics and differences are understood in greater detail, pharmacological treatment of the different forms of cognitive impairment remains suboptimal. Neurotransmitter receptors, including neuronal nicotinic acetylcholine receptors (nAChRs), dopamine receptors, and glutamine receptors, play key roles in the functions and metabolisms of the brain. Among these, the role of nAChRs in the development of cognitive impairment has attracted more and more attention. The present review summarizes what is presently known concerning the structure, distribution, metabolism, and function of nAChRs, as well as their involvement in major cognitive disorders such as Alzheimer's disease, Parkinson's disease, vascular dementia, schizophrenia, and diabetes mellitus. As will be discussed, the relevant scientific literature reveals clearly that the α4β2 and α7 nAChR subtypes and/or subunits of the receptors play major roles in maintaining cognitive function and in neuroprotection of the brain. Accordingly, focusing on these as targets of drug therapy can be expected to lead to breakthroughs in the treatment of cognitive disorders such as AD and schizophrenia.

α7nAChR Activation Combined with Endothelial Progenitor Cell Transplantation Attenuates Lung Injury in Diabetic Rats with Sepsis through the NF-κB Pathway

Chronic diabetes mellitus compromises the vascular system, which causes organ injury, including in the lung. Due to the strong compensatory ability of the lung, patients always exhibit subclinical symptoms. Once sepsis occurs, the degree of lung injury is more severe under hyperglycemic conditions. The α7 nicotinic acetylcholine receptor (α7nAChR) plays an important role in regulating inflammation and metabolism and can improve endothelial progenitor cell (EPC) functions. In the present study, lung injury caused by sepsis was compared between diabetic rats and normal rats. We also examined whether α7nAChR activation combined with EPC transplantation could ameliorate lung injury in diabetic sepsis rats. A type 2 diabetic model was induced in rats via a high-fat diet and streptozotocin. Then, a rat model of septic lung injury was established by intraperitoneal injection combined with endotracheal instillation of LPS. The oxygenation indices, wet-to-dry ratios, and histopathological scores of the lungs were tested after PNU282987 treatment and EPC transplantation. IL-6, IL-8, TNF-α, and IL-10 levels were measured. Caspase-3, Bax, Bcl-2, and phosphorylated NF-κB (p-NF-κB) levels were determined by blotting. Sepsis causes obvious lung injury, which is exacerbated by diabetic conditions. α7nAChR activation and endothelial progenitor cell transplantation reduced lung injury in diabetic sepsis rats, alleviating inflammation and decreasing apoptosis. This treatment was more effective when PNU282987 and endothelial progenitor cells were administered together. p-NF-κB levels decreased following treatment with PNU282987 and EPCs. In conclusion, α7nAChR activation combined with EPC transplantation can alleviate lung injury in diabetic sepsis rats through the NF-κB signaling pathway.

Galantamine improves glycemic control and diabetic nephropathy in Leprdb/db mice

Galantamine, a centrally acting acetylcholinesterase inhibitor, has been shown to attenuate inflammation and insulin resistance in patients with metabolic syndrome. We investigated the effects of galantamine on glycemic control and development of diabetic nephropathy (DN) in Leprdb/db mice. Galantamine significantly reduced food intake, body weight, blood glucose and HbA1c levels. Insulin resistance (HOMA-IR, QUICKI), HOMA-β and elevations in plasma inflammatory cytokine levels (TNF-α, IL-6 and HMGB-1) were all attenuated by galantamine. Galantamine also ameliorated diabetes-induced kidney injury as evidenced by improvements in renal function (BUN, creatinine, albuminuria), histologic injury and apoptosis. Improved glycemic control and nephropathy were associated with increased circulating GLP-1, decreased renal P-38 MAPK and caspase-1 activation and reduced SGLT-2 expression. These findings provide insights into the mechanisms by which galantamine improves glycemic control and attenuates DN in the Leprdb/db mouse model.

Neuroimmune nexus in the pathophysiology and therapy of inflammatory disorders: role of α7 nicotinic acetylcholine receptors

The α7-nicotinic acetylcholine receptor (α7nAChR) is a key protein in the cholinergic anti-inflammatory pathway (CAP) that links the nervous and immune systems. Initially, the pathway was discovered based on the observation that vagal nerve stimulation (VNS) reduced the systemic inflammatory response in septic animals. Subsequent studies form a foundation for the leading hypothesis about the central role of the spleen in CAP activation. VNS evokes noradrenergic stimulation of ACh release from T cells in the spleen, which in turn activates α7nAChRs on the surface of macrophages. α7nAChR-mediated signaling in macrophages reduces inflammatory cytokine secretion and modifies apoptosis, proliferation, and macrophage polarization, eventually reducing the systemic inflammatory response. A protective role of the CAP has been demonstrated in preclinical studies for multiple diseases including sepsis, metabolic disease, cardiovascular diseases, arthritis, Crohn's disease, ulcerative colitis, endometriosis, and potentially COVID-19, sparking interest in using bioelectronic and pharmacological approaches to target α7nAChRs for treating inflammatory conditions in patients. Despite a keen interest, many aspects of the cholinergic pathway are still unknown. α7nAChRs are expressed on many other subsets of immune cells that can affect the development of inflammation differently. There are also other sources of ACh that modify immune cell functions. How the interplay of ACh and α7nAChR on different cells and in various tissues contributes to the anti-inflammatory responses requires additional study. This review provides an update on basic and translational studies of the CAP in inflammatory diseases, the relevant pharmacology of α7nAChR-activated drugs and raises some questions that require further investigation.

α7 nicotinic acetylcholine receptor agonist improved brain injury and impaired glucose metabolism in a rat model of ischemic stroke

Vagus nerve stimulation through the action of acetylcholine can modulate inflammatory responses and metabolism. α7 Nicotinic Acetylcholine Receptor (α7nAChR) is a key component in the biological functions of acetylcholine. To further explore the health benefits of vagus nerve stimulation, this study aimed to investigate whether α7nAChR agonists offer beneficial effects against poststroke inflammatory and metabolic changes and to identify the underlying mechanisms in a rat model of stroke established by permanent cerebral ischemia. We found evidence showing that pretreatment with α7nAChR agonist, GTS-21, improved poststroke brain infarction size, impaired motor coordination, brain apoptotic caspase 3 activation, dysregulated glucose metabolism, and glutathione reduction. In ischemic cortical tissues and gastrocnemius muscles with GTS-21 pretreatment, macrophages/microglia M1 polarization-associated Tumor Necrosis Factor-α (TNF-α) mRNA, Cluster of Differentiation 68 (CD68) protein, and Inducible Nitric Oxide Synthase (iNOS) protein expression were reduced, while expression of anti-inflammatory cytokine IL-4 mRNA, and levels of M2 polarization-associated CD163 mRNA and protein were increased. In the gastrocnemius muscles, stroke rats showed a reduction in both glutathione content and Akt Serine 473 phosphorylation, as well as an elevation in Insulin Receptor Substrate-1 Serine 307 phosphorylation and Dynamin-Related Protein 1 Serine 616 phosphorylation. GTS-21 reversed poststroke changes in the gastrocnemius muscles. Overall, our findings, provide further evidence supporting the neuroprotective benefits of α7nAChR agonists, and indicate that they may potentially exert anti-inflammatory and metabolic effects peripherally in the skeletal muscle in an acute ischemic stroke animal model.

GTS-21, a selective alpha7 nicotinic acetylcholine receptor agonist, ameliorates diabetic nephropathy in Leprdb/db mice

Diabetic nephropathy (DN) is a serious complicating factor in human type 2 diabetes mellitus (T2DM), and it commonly results in end-stage renal disease (ESRD) that requires kidney dialysis. Here, we report that the α7 nicotinic acetylcholine receptor (α7nAChR) agonist GTS-21 exerts a novel anti-inflammatory action to ameliorate DN, as studied using an inbred strain of Leprdb/db mice in which hyperglycemia and obesity co-exist owing to defective leptin receptor (Lepr) signaling. For this analysis, GTS-21 was administered to 10-12 week-old male and female mice as a 4 mg/kg intraperitoneal injection, twice-a-day, for 8 weeks. Kidney function and injury owing to DN were monitored by determination of plasma levels of BUN, creatinine, KIM-1 and NGAL. Histologic analysis of glomerular hypertrophy and mesangial matrix expansion were also used to assess DN in these mice. Concurrently, renal inflammation was assessed by measuring IL-6 and HMGB1, while also quantifying renal cell apoptosis, and apoptotic signaling pathways. We found that Leprdb/db mice exhibited increased markers of BUN, creatinine, NGAL, KIM-1, IL-6, cytochrome C, and HMGB-1. These abnormalities were also accompanied by histologic kidney injury (mesangial matrix expansion and apoptosis). Remarkably, all such pathologies were significantly reduced by GTS-21. Collectively, our results provide new evidence that the α7nAChR agonist GTS-21 has the ability to attenuate diabetes-induced kidney injury. Additional studies are warranted to further investigate the involvement of the vagal cholinergic anti-inflammatory reflex pathway (CAP) in ameliorating diabetic nephropathy.

The alpha-7 nicotinic acetylcholine receptor agonist GTS-21 does not affect food intake in rats

Obesity is a prevalent disease, but effective treatments remain limited. Agonists of the alpha-7 nicotinic acetylcholine receptor (α7nAChR) promote negative energy balance in mice, but these effects are not well-studied in rats. We tested the hypothesis that the α7nAChR agonist GTS-21 would decrease food intake and body weight in adult male Sprague Dawley rats. Contrary to our hypothesis, acute systemic administration of GTS-21 produced no significant effects on chow or high-fat diet (HFD) intake. Acute intracerebroventricular (ICV) GTS-21 also had no impact on chow intake, and actually increased body weight at the highest dose tested. Previous studies suggest that GTS-21 engages the food intake-suppressive glucagon-like peptide-1 (GLP-1) system in mice. As there are known species differences in GLP-1 physiology between mice and rats, we tested the ability of GTS-21 to elicit GLP-1 secretion in rats. Our results showed that plasma levels of total GLP-1 in rats were not significantly altered by peripheral GTS-21 injection. These results represent an advance in understanding how α7nAChR activation impacts energy balance control in rodents and suggest that there may be important differences between rats and mice in the ability of GTS-21/α7nAChR activation to increase secretion of GLP-1.