The structural and functional interrelationships of muscarinic acetylcholine receptor subtypes

Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function

Acetylcholine (ACh), the major parasympathetic neurotransmitter, is released by intrapancreatic nerve endings during the preabsorptive and absorptive phases of feeding. In beta-cells, ACh binds to muscarinic M(3) receptors and exerts complex effects, which culminate in an increase of glucose (nutrient)-induced insulin secretion. Activation of PLC generates diacylglycerol. Activation of PLA(2) produces arachidonic acid and lysophosphatidylcholine. These phospholipid-derived messengers, particularly diacylglycerol, activate PKC, thereby increasing the efficiency of free cytosolic Ca(2+) concentration ([Ca(2+)](c)) on exocytosis of insulin granules. IP3, also produced by PLC, causes a rapid elevation of [Ca(2+)](c) by mobilizing Ca(2+) from the endoplasmic reticulum; the resulting fall in Ca(2+) in the organelle produces a small capacitative Ca(2+) entry. ACh also depolarizes the plasma membrane of beta-cells by a Na(+)- dependent mechanism. When the plasma membrane is already depolarized by secretagogues such as glucose, this additional depolarization induces a sustained increase in [Ca(2+)](c). Surprisingly, ACh can also inhibit voltage-dependent Ca(2+) channels and stimulate Ca(2+) efflux when [Ca(2+)](c) is elevated. However, under physiological conditions, the net effect of ACh on [Ca(2+)](c) is always positive. The insulinotropic effect of ACh results from two mechanisms: one involves a rise in [Ca(2+)](c) and the other involves a marked, PKC-mediated increase in the efficiency of Ca(2+) on exocytosis. The paper also discusses the mechanisms explaining the glucose dependence of the effects of ACh on insulin release.

Depolarization affects the binding properties of muscarinic acetylcholine receptors and their interaction with proteins of the exocytic apparatus

Membrane depolarization is the signal that triggers release of neurotransmitter from nerve terminals. As a result of depolarization, voltage-dependent Ca(2+) channels open, level of intracellular Ca(2+) increases. and release of neurotransmitter commences. Previous study had shown that in rat brain synaptosomes, muscarinic acetylcholine (ACh) receptors (mAChRs) interact with soluble NSF attachment protein receptor proteins of the exocytic machinery in a voltage-dependent manner. It was suggested that this interaction might control the rapid, synchronous release of acetylcholine. The present study investigates the mechanism for such a voltage-dependent interaction. Here we show that depolarization shifts mAChRs, specifically the m2 receptor subtype, to a low affinity state toward its agonists. At resting potential, mAChRs are in a high affinity state (K(d) of approximately 20 nM) and they shift to a low affinity state (K(d) of tens of microM) upon membrane depolarization. In addition, interaction between m2 receptor subtype and the exocytic machinery increases with receptor occupancy. Both phenomena are independent of Ca(2+) influx. We propose that these results may explain control of ACh release from nerve terminals. At resting potential the exocytic machinery is clamped due to its interaction with the occupied mAChR and depolarization relieves this interaction. This, together with Ca(2+) influx, enables release of ACh to commence.

Muscarinic acetylcholine receptors in the hippocampus, neocortex and amygdala: a review of immunocytochemical localization in relation to learning and memory

Immunocytochemical mapping studies employing the extensively used monoclonal anti-muscarinic acetylcholine receptor (mAChR) antibody M35 are reviewed. We focus on three neuronal muscarinic cholinoceptive substrates, which are target regions of the cholinergic basal forebrain system intimately involved in cognitive functions: the hippocampus; neocortex; and amygdala. The distribution and neurochemistry of mAChR-immunoreactive cells as well as behaviorally induced alterations in mAChR-immunoreactivity (ir) are described in detail. M35+ neurons are viewed as cells actively engaged in neuronal functions in which the cholinergic system is typically involved. Phosphorylation and subsequent internalization of muscarinic receptors determine the immunocytochemical outcome, and hence M35 as a tool to visualize muscarinic receptors is less suitable for detection of the entire pool of mAChRs in the central nervous system (CNS). Instead, M35 is sensitive to and capable of detecting alterations in the physiological condition of muscarinic receptors. Therefore, M35 is an excellent tool to localize alterations in cellular cholinoceptivity in the CNS. M35-ir is not only determined by acetylcholine (ACh), but by any substance that changes the phosphorylation/internalization state of the mAChR. An important consequence of this proposition is that other neurotransmitters than ACh (especially glutamate) can regulate M35-ir and the cholinoceptive state of a neuron, and hence the functional properties of a neuron. One of the primary objectives of this review is to provide a synthesis of our data and literature data on mAChR-ir. We propose a hypothesis for the role of muscarinic receptors in learning and memory in terms of modulation between learning and recall states of brain areas at the postsynaptic level as studied by way of immunocytochemistry employing the monoclonal antibody M35.

Growth factors and cancer

Signaling pathways that mediate the normal functions of growth factors are commonly subverted in cancer. Oncogenes identified by a variety of approaches have been shown to function at critical steps in mitogenic signaling. Progression through the cell cycle requires the coordinated actions of members of two complementary classes of growth factors, and oncogenes appear to replace the actions of one set of these growth factors. Growth factors can also influence normal cell differentiation, and constitutive activation of growth-promoting pathways in cancer cells can modulate the cell phenotype as well. Paracrine actions of growth factors and cytokines may also influence the stepwise series of genetic events that lead to malignancy. New approaches for cancer therapy are being developed that intervene at various steps in growth factor signaling pathways.

Structure and functional expression of a human interleukin-8 receptor

Interleukin-8 (IL-8) is a member of a family of pro-inflammatory cytokines. Although the best characterized activities of IL-8 include the chemoattraction and activation of neutrophils, other members of this family have a wide range of specific actions including the chemotaxis and activation of monocytes, the selective chemotaxis of memory T cells, the inhibition of hematopoietic stem cell proliferation, and the induction of neutrophil infiltration in vivo. A complementary DNA encoding the IL-8 receptor from human neutrophils has now been isolated. The amino acid sequence shows that the receptor is a member of the superfamily of receptors that couple to guanine nucleotide binding proteins (G proteins). The sequence is 29% identical to that of receptors for the other neutrophil chemoattractants, fMet-Leu-Phe and C5a. Mammalian cells transfected with the IL-8 receptor cDNA clone bind IL-8 with high affinity and respond specifically to IL-8 by transiently mobilizing calcium. The IL-8 receptor may be part of a subfamily of related G protein-coupled receptors that transduce signals for the IL-8 family of pro-inflammatory cytokines.

Carbachol and bradykinin elevate cyclic AMP and rapidly deplete ATP in cultured rat sympathetic neurons

The agonists carbachol (CCh) and bradykinin (BK) and 54 mM KCl (high K+) were among the most potent stimulants of cyclic AMP (cAMP) production in cultured rat sympathetic neurons, measured with the use of a high-fidelity assay developed for small samples. The rise in cAMP evoked by CCh (through muscarinic receptors), BK, and high K+ was inhibited in Ca2(+)-depleted medium (1.3 mM Ca2+ and 2 mM BAPTA or EGTA), which also prevented the sustained rise in [Ca2+]i evoked by each of these stimuli, showing that elevation of cAMP requires extracellular Ca2+ and, possibly, Ca2+ influx. Preliminary results obtained with the novel calmodulin inhibitor CGS 9343B, which blocked the elevation of cAMP, and with the cyclogenase inhibitor indomethacin, which partially blocked the actions of the agonists but not those of high K+, suggest that calmodulin and arachidonate metabolites may be two components of the signaling pathway. In addition to their effects on cAMP metabolism, CCh, muscarine, and BK, but not nicotine, caused a 30-40% decrease in ATP levels. This effect was much greater than that evoked by high K+ and was largely inhibited by CGS 9343B but slightly enhanced in the Ca(+)-depleted medium, showing that agonists are still active in the absence of [Ca2+]o. Thus, agonists that activate phosphoinositide metabolism can also increase cAMP production and substantially deplete cells of ATP. These novel actions may have to be taken into account when the mechanisms by which such agonists regulate cell function are being considered.

Ligand binding and G protein coupling of muscarinic receptors in airway smooth muscle

Ligand binding properties of muscarinic receptors were examined in membranes and isolated cells prepared from bovine trachea. The binding of the muscarinic antagonist [3H]quinuclidinyl benzilate (QNB) to both membranes and isolated cells was saturable, reversible, and of high affinity [dissociation constant (KD) = 100-200 pM]. The binding constants of three selective antagonists, pirenzepine, AF-DX 116, and 4-DAMP, were examined, and the results indicate that the smooth muscle cells contain at least two receptor subtypes. The majority of receptors exhibit binding constants for these selective antagonists similar to those of the M2-subtype. AF-DX 116 binding curves indicated the presence of a small population of receptors with binding constants similar to those reported for the M3-subtype. These findings suggest that the smooth muscle cells may contain both M2- and M3-receptors and are in agreement with evidence of the presence of mRNAs coding for these two subtypes in tracheal extracts (A. Maeda, T. Kubo, M. Mishina, and S. Numa. FEBS Lett. 239: 339-342, 1988). [3H]QNB displacement curves of the muscarinic agonist oxotremorine were best described as a sum of binding to high- and low-affinity sites with KD values of 3.8 nM and 2.2 microM. Guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) shifted the high-affinity sites to low affinity, suggesting that the high-affinity sites may represent receptors coupled to G proteins. Pertussis toxin catalyzed the ADP ribosylation of a 40- to 41-kDa protein band present in the membranes but had no significant effect on high-affinity agonist binding, suggesting that most of the receptors are coupled to G proteins in a toxin-insensitive manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular mechanisms of regulation of neuronal muscarinic receptor sensitivity

Like other neurotransmitter receptors, muscarinic acetylcholine receptors are subject to regulation by the state of receptor activation. Prolonged increases in the concentration of muscarinic agonists result in a decrease in receptor density and loss of receptor sensitivity, both in vivo and in vitro. On the other hand, when the receptor is deprived of acetylcholine for a long duration in vivo, the receptor becomes more sensitive in responding to muscarinic agonists. However, it has been more difficult to demonstrate increases in receptor concentration that accompany this supersensitive state. The purpose of this review is to provide current information related to the characteristics of muscarinic receptor regulation and the molecular mechanisms underlying this phenomenon, regarding both the density of receptors and their transduction mechanisms. Furthermore, possible feedback regulatory roles of different second messenger signals are discussed. Particular emphasis is dedicated to molecular mechanisms of regulation of neuronal muscarinic receptors.