One of the hallmarks of type 2 diabetes is that pancreatic beta cells fail to release sufficient amounts of insulin in the presence of elevated blood glucose levels. Insulin secretion is modulated by many hormones and neurotransmitters including acetylcholine, the major neurotransmitter of the peripheral parasympathetic nervous system. The physiological role of muscarinic acetylcholine receptors expressed by pancreatic beta cells remains unclear at present. Here, we demonstrate that mutant mice selectively lacking the M3 muscarinic acetylcholine receptor subtype in pancreatic beta cells display impaired glucose tolerance and greatly reduced insulin release. In contrast, transgenic mice selectively overexpressing M3 receptors in pancreatic beta cells show a profound increase in glucose tolerance and insulin release. Moreover, these mutant mice are resistant to diet-induced glucose intolerance and hyperglycemia. These findings indicate that beta cell M3 muscarinic receptors play a key role in maintaining proper insulin release and glucose homeostasis.
Identification of the specific muscarinic acetylcholine receptor (mAChR) subtypes mediating stimulation of salivary secretion is of considerable clinical interest. Recent pharmacological and molecular genetic studies have yielded somewhat confusing and partially contradictory results regarding the involvement of individual mAChRs in this activity. In the present study, we re-examined the roles of M 1 and M 3 mAChRs in muscarinic agonist-mediated stimulation of salivary secretion by using M 1 and M 3 receptor single-knockout (KO) mice and newly generated M 1 /M 3 receptor double-KO mice. When applied at a low dose (1 mg/kg, s.c.), the muscarinic agonist pilocarpine showed significantly reduced secretory activity in both M 1 and M 3 receptor single-KO mice. However, when applied at higher doses, pilocarpine induced only modestly reduced (5 mg/kg, s.c.) or unchanged (15 mg/kg, s.c.) salivation responses, respectively, in M 1 and M 3 receptor single-KO mice, indicating that the presence of either M 1 or M 3 receptors is sufficient to mediate robust salivary output. Quantitative reverse transcriptase-polymerase chain reaction studies with salivary gland tissue showed that the inactivation of the M 1 or M 3 mAChR genes did not lead to significantly altered mRNA levels of the remaining mAChR subtypes. Strikingly, the sialagogue activity of pilocarpine was abolished in M 1 /M 3 receptor double-KO mice. However, salivary glands from M 1 /M 3 receptor double-KO mice remained responsive to stimulation by the -adrenergic receptor agonist, (S)-isoproterenol. Taken together these studies support the concept that a mixture of M 1 and M 3 receptors mediates cholinergic stimulation of salivary flow.
Most mitochondrial proteins are synthesized with an N-terminal signal sequence that targets these proteins to various compartments within the mitochondria. Signal sequences have been shown to be functional by fusing them to a nonmitochondrial passenger protein and observing import. In many cases, a signal sequence has been fused to passenger proteins, such as dihydrofolate reductase, and import occurred. There are, though, several unexplained instances in which a signal sequence was attached to a passenger protein and import was not observed. In this study, the N-terminal 23 residues of the matrix enzyme rhodanese could import several passenger proteins but were unable to import the mature form of mitochondrial aldehyde dehydrogenase (mALDH). However, if these same 23 residues were fused to the middle portion of mALDH, import was recovered, suggesting that the rhodanese signal sequence and N terminus of mALDH were incompatible for import. Circular dichroism data indicated that a peptide corresponding to the region of fusion between rhodanese and mALDH had less structure than corresponding peptides from imported fusion proteins, suggesting that mALDH may alter the helix in the rhodanese signal sequence, thus preventing import.
Muscarinic acetylcholine receptors (mAChRs) expressed by pancreatic acinar cells play an important role in mediating acetylcholine-dependent stimulation of digestive enzyme secretion. To examine the potential roles of M 1 and M 3 mAChRs in this activity, we used M 1 and M 3 receptor single knockout (KO) and M 1 /M 3 receptor double KO mice as novel experimental tools. Specifically, we examined the ability of the muscarinic agonist carbachol to stimulate amylase secretion in vitro, using dispersed pancreatic acini prepared from wild-type and mAChR mutant mice. Quantitative reverse transcription-polymerase chain reaction studies using RNA prepared from mouse pancreatic acini showed that deletion of the M 1 or M 3 mAChR genes did not lead to significantly altered mRNA levels of the remaining mAChR subtypes. Moreover, immunoprecipitation studies with M 1 and M 3 mAChR-selective antisera demonstrated that both mAChR subtypes are expressed by mouse pancreatic acini. Strikingly, carbachol-induced stimulation of amylase secretion was significantly impaired in acinar preparations from both M 1 and M 3 receptor single KO mice and abolished in acinar preparations from M 1 /M 3 receptor double KO mice. However, another pancreatic secretagogue, bombesin, retained its ability to fully stimulate amylase secretion in acinar preparations from M 1 /M 3 receptor double KO mice. Together, these studies support the concept that cholinergic stimulation of pancreatic amylase secretion is mediated by a mixture of M 1 and M 3 mAChRs and that other mAChR subtypes do not make a significant contribution to this activity. These findings clarify the long-standing question regarding the molecular nature of the mAChR subtypes mediating the secretion of digestive enzymes from the exocrine pancreas.
Previous studies have shown that β-cell M(3) muscarinic acetylcholine receptors (M3Rs) play a key role in maintaining blood glucose homeostasis by enhancing glucose-dependent insulin release. In this study, we tested the hypothesis that long-term, persistent activation of β-cell M3Rs can improve glucose tolerance and ameliorate the metabolic deficits associated with the consumption of a high-fat diet. To achieve the selective and persistent activation of β-cell M3Rs in vivo, we generated transgenic mice that expressed the Q490L mutant M3R in their pancreatic β-cells (β-M3-Q490L Tg mice). The Q490L point mutation is known to render the M3R constitutively active. The metabolic phenotypes of the transgenic mice were examined in several in vitro and in vivo metabolic tests. In the presence of 15 mm glucose and the absence of M3R ligands, isolated perifused islets prepared from β-M3-Q490L Tg mice released considerably more insulin than wild-type control islets. This effect could be completely blocked by incubation of the transgenic islets with atropine (10 μm), an inverse muscarinic agonist, indicating that the Q490L mutant M3R exhibited ligand-independent signaling (constitutive activity) in mouse β-cells. In vivo studies showed that β-M3-Q490L Tg mice displayed greatly improved glucose tolerance and increased serum insulin levels as well as resistance to diet-induced glucose intolerance and hyperglycemia. These results suggest that chronic activation of β-cell M3Rs may represent a useful approach to boost insulin output in the long-term treatment of type 2 diabetes.
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