A novel class of peptides with facilitating action on neuronal nicotinic receptors of rat chromaffin cells in vitro: functional and molecular dynamics studies

Prefrontal neuromodulation by nicotinic receptors for cognitive processes

RATIONALE

The prefrontal cortex (PFC) mediates executive functions, a set of control processes that optimize performance on cognitive tasks. It enables appropriate decision-making and mediates adapted behaviors, all processes impaired in psychiatric or degenerative disorders. Key players of normal functioning of the PFC are neurotransmitter (NT) systems arising from subcortical nuclei and targeting PFC subareas and, also, neuronal nicotinic acetylcholine receptors (nAChRs). These ion channels, located on multiple cell compartments in all brain areas, mediate direct cholinergic transmission and modulate the release of NTs that cross onto PFC neurons or interneurons.

OBJECTIVE

We compiled current knowledge concerning the role of nAChRs in NT release, focusing on the PFC. We point out plausible mechanisms of interaction among PFC circuits implicated in executive functions and emphasized the role of β2-containing nAChRs, the high-affinity receptors for acetylcholine (ACh). These receptors are more directly implicated in behavioral flexibility either when located on PFC neurons or in the monoaminergic or cholinergic systems targeting the PFC.

RESULTS

We shed light on potentially crucial roles played by nAChRs in complex interactions between local and afferent NTs. We show how they could act on cognition via PFC networks.

CONCLUSIONS

nAChRs are crucial for decision-making, during integration of emotional and motivational features, both mediated by different NT pathways in the PFC. We review the knowledge recently gained on cognitive functions in mice and our current understanding of PFC NT modulation. The combination of these data is expected to provide new hypotheses concerning the role of AChRs in cognitive processes.

Positive and negative modulation of nicotinic receptors

Nicotinic acetylcholine receptors (AChRs) are one of the best characterized ion channels from the Cys-loop receptor superfamily. The study of acetylcholine binding proteins and prokaryotic ion channels from different species has been paramount for the understanding of the structure-function relationship of the Cys-loop receptor superfamily. AChR function can be modulated by different ligand types. The neurotransmitter ACh and other agonists trigger conformational changes in the receptor, finally opening the intrinsic cation channel. The so-called gating process couples ligand binding, located at the extracellular portion, to the opening of the ion channel, located at the transmembrane region. After agonist activation, in the prolonged presence of agonists, the AChR becomes desensitized. Competitive antagonists overlap the agonist-binding sites inhibiting the pharmacological action of agonists. Positive allosteric modulators (PAMs) do not bind to the orthostetic binding sites but allosterically enhance the activity elicited by agonists by increasing the gating process (type I) and/or by decreasing desensitization (type II). Instead, negative allosteric modulators (NAMs) produce the opposite effects. Interestingly, this negative effect is similar to that found for another class of allosteric drugs, that is, noncompetitive antagonists (NCAs). However, the main difference between both categories of drugs is based on their distinct binding site locations. Although both NAMs and NCAs do not bind to the agonist sites, NACs bind to sites located in the ion channel, whereas NAMs bind to nonluminal sites. However, this classification is less clear for NAMs interacting at the extracellular-transmembrane interface where the ion channel mouth might be involved. Interestingly, PAMs and NAMs might be developed as potential medications for the treatment of several diseases involving AChRs, including dementia-, skin-, and immunological-related diseases, drug addiction, and cancer. More exciting is the potential combination of specific agonists with specific PAMs. However, we are still in the beginning of understanding how these compounds act and how these drugs can be used therapeutically.

Cholinergic receptor pathways involved in apoptosis, cell proliferation and neuronal differentiation

Acetylcholine (ACh) has been shown to modulate neuronal differentiation during early development. Both muscarinic and nicotinic acetylcholine receptors (AChRs) regulate a wide variety of physiological responses, including apoptosis, cellular proliferation and neuronal differentiation. However, the intracellular mechanisms underlying these effects of AChR signaling are not fully understood. It is known that activation of AChRs increase cellular proliferation and neurogenesis and that regulation of intracellular calcium through AChRs may underlie the many functions of ACh. Intriguingly, activation of diverse signaling molecules such as Ras-mitogen-activated protein kinase, phosphatidylinositol 3-kinase-Akt, protein kinase C and c-Src is modulated by AChRs. Here we discuss the roles of ACh in neuronal differentiation, cell proliferation and apoptosis. We also discuss the pathways involved in these processes, as well as the effects of novel endogenous AChRs agonists and strategies to enhance neuronal-differentiation of stem and neural progenitor cells. Further understanding of the intracellular mechanisms underlying AChR signaling may provide insights for novel therapeutic strategies, as abnormal AChR activity is present in many diseases.

Inhibition of native and recombinant nicotinic acetylcholine receptors by the myristoylated alanine-rich C kinase substrate peptide

A variety of peptide ligands are known to inhibit the function of neuronal nicotinic acetylcholine receptors (nAChRs), including small toxins and brain-derived peptides such as beta-amyloid(1-42) and synthetic apolipoproteinE peptides. The myristoylated alanine-rich C kinase substrate (MARCKS) protein is a major substrate of protein kinase C and is highly expressed in the developing and adult brain. The ability of a 25-amino acid synthetic MARCKS peptide, derived from the effector domain (ED), to modulate nAChR activity was tested. To determine the effects of the MARCKS ED peptide on nAChR function, receptors were expressed in Xenopus laevis oocytes, and two-electrode voltage-clamp experiments were performed. The MARCKS ED peptide completely inhibited acetylcholine (ACh)-evoked responses from alpha7 nAChRs in a dose-dependent manner, yielding an IC(50) value of 16 nM. Inhibition of ACh-induced responses was both activity- and voltage-independent. The MARCKS ED peptide was unable to block alpha-bungarotoxin binding. A MARCKS ED peptide in which four serine residues were replaced with aspartate residues was unable to inhibit alpha7 nAChR-mediated currents. The MARCKS ED peptide inhibited ACh-induced alpha4beta2 and alpha2beta2 responses, although with decreased potency. The effects of the MARCKS ED peptide on native nAChRs were tested using acutely isolated rat hippocampal slices. In hippocampal interneurons, the MARCKS ED peptide was able to block native alpha7 nAChRs in a dose-dependent manner. The MARCKS ED peptide represents a novel antagonist of neuronal nAChRs that has considerable utility as a research tool.

Nicotinic acetylcholine receptors of adrenal chromaffin cells

In the adrenal medulla, acetylcholine released by the sympathetic splanchnic nerves activates neuronal-type nicotinic acetylcholine receptors (nAChRs) on the membrane of chromaffin cells which liberate catecholamines into the bloodstream in preparation for the fight and flight reactions. On adrenal chromaffin cells the main class of nAChRs is a pentameric assembly of alpha3 and beta4 subunits that forms ion channels which produce membrane depolarization by increasing Na+, K+ and Ca2+ permeability. Homomeric alpha7 nicotinic receptors are expressed in a species-dependent manner and do not contribute to catecholamine secretion. Chromaffin cell nAChRs rapidly activate and desensitize with full recovery on washout. nAChR activity is subjected to various types of dynamic regulation. It is allosterically modulated by the endogenous neuropeptide substance P that stabilizes receptors in their desensitized state, thus depressing their responsiveness. The full-length peptide CGRP acts as a negative allosteric modulator by inhibiting responses without changing desensitization, whereas its N-terminal fragments act as positive allosteric modulators to transiently enhance nAChR function. nAChR expression increases when cells are chronically exposed to either selective antagonists or agonists such as nicotine, a protocol mimicking the condition of chronic heavy smokers. In this case, large upregulation of nAChRs occurs even though most of the extra nAChRs remain inside the cells, creating a mismatch between the increase in total nAChRs and increase in functional nAChRs on the cell surface. These findings highlight the plastic properties of cholinergic neurotransmission in the adrenal medulla to provide robust mechanisms for adapting catecholamine release to acute and chronic changes in sympathetic activity.

Class II G protein-coupled receptors and their ligands in neuronal function and protection

G protein-coupled receptors (GPCRs) play pivotal roles in regulating the function and plasticity of neuronal circuits in the nervous system. Among the myriad of GPCRs expressed in neural cells, class II GPCRs which couples predominantly to the Gs-adenylate cyclase-cAMP signaling pathway, have recently received considerable attention for their involvement in regulating neuronal survival. Neuropeptides that activate class II GPCRs include secretin, glucagon-like peptides (GLP-1 and GLP-2), growth hormone-releasing hormone (GHRH), pituitary adenylate cyclase activating peptide (PACAP), corticotropin-releasing hormone (CRH), vasoactive intestinal peptide (VIP), parathyroid hormone (PTH), and calcitonin-related peptides. Studies of patients and animal and cell culture models, have revealed possible roles for class II GPCRs signaling in the pathogenesis of several prominent neurodegenerative conditions including stroke, Alzheimer's, Parkinson's, and Huntington's diseases. Many of the peptides that activate class II GPCRs promote neuron survival by increasing the resistance of the cells to oxidative, metabolic, and excitotoxic injury. A better understanding of the cellular and molecular mechanisms by which class II GPCRs signaling modulates neuronal survival and plasticity will likely lead to novel therapeutic interventions for neurodegenerative disorders.

Apolipoprotein E-derived peptides block alpha7 neuronal nicotinic acetylcholine receptors expressed in xenopus oocytes

For decades, the pathology of Alzheimer's disease has been associated with dysfunction of cholinergic signaling; however, the cellular mechanisms by which nicotinic acetylcholine receptor (nAChR) function is impaired in Alzheimer's disease are as yet unknown. The most significant genetic risk factor for the development of Alzheimer's disease is inheritance of the epsilon4 allele of apolipoprotein E (apoE). Recent data have demonstrated the ability of apoE-derived peptides to inhibit nAChRs in rat hippocampus. In the current study, the functional interaction between nAChRs and apoE-derived peptides was investigated in Xenopus oocytes expressing selected nAChRs. Both a 17-amino acid peptide fragment, apoE(133-149), and an eight-amino acid peptide, apoE(141-148), were able to maximally block acetylcholine (ACh)-mediated peak current responses for homomeric alpha7 nAChRs. ApoE peptide inhibition was dose-dependent and voltage- and activity-independent. The current findings suggest that apoE peptides are noncompetitive for acetylcholine and do not block functional alpha-bungarotoxin binding. ApoE peptides had a significantly decreased ability to inhibit ACh-mediated peak current responses for alpha4beta2 and alpha2beta2 nAChRs. Amino acid substitutions in the apoE peptide sequence suggest that the arginines are critical for peptide blockade of the alpha7 nAChR. The current data suggest that apoE fragments can disrupt nAChR signaling through a direct blockade of alpha7 nAChRs. These results may be useful in elucidating the mechanisms underlying memory loss and cognitive decline seen in Alzheimer's disease as well as aid in the development of novel therapeutics using apoE-derived peptides.

A structural model of agonist binding to the alpha3beta4 neuronal nicotinic receptor

(alpha3)2(beta4)3 is the most abundant type of neuronal nicotinic ACh receptor (nAChR) mediating cholinergic actions on the autonomic nervous system. Studies to refine or devise drugs selectively acting on (alpha3)2(beta4)3 receptors would benefit from a detailed description of the hitherto unclear agonist-binding domain. The present study reports a three-dimensional model for the ligand-binding domain (LBD) of this receptor either in its unoccupied or agonist-bound conformation. The receptor model was based on the structure of the acetylcholine-binding protein (AChBP), and was obtained using molecular modelling techniques. ACh, nicotine and cytisine (full agonists), muscarine (a selective agonist of muscarinic ACh receptors) and the allosteric modulator eserine were docked into the binding pockets of the receptor model. Simulated agonist-receptor complexes were compared with the agonist-binding complex of the AChBP, as well as of the (alpha4)2(beta2)3 type of nAChR, which is the commonest in the brain. Agonist docking identified discrete amino-acid residues of the beta subunits important for pharmacological selectivity of nAChRs. The predicted affinity of muscarine for the nAChR was low, suggesting the present model to be suitable for effective discrimination of nicotinic agonist binding versus nonselective cholinergic binding. Furthermore, the current model outlined a potential binding site for the allosteric modulator eserine, the site of action of which has remained elusive. The present LBD model of the receptor in its free state provides a novel structural framework to interpret experimental observations and a useful template for future investigations to develop (alpha3)2(beta4)3-selective ligands.

Modulation of neuronal nicotinic receptor function by the neuropeptides CGRP and substance P on autonomic nerve cells

1. One classical example of how neuropeptides can affect the function of ligand-gated receptors is the modulation of neuronal nicotinic receptors (nAChRs) by substance P. The present review updates current understanding of this action by substance P and compares it with other neuropeptides more recently found to modulate nAChRs in the autonomic nervous system. 2. Calcitonin gene-related peptide (CGRP) and its N-terminal fragments have been shown to exert complex inhibitory as well facilitatory actions on nAChRs. Fragments such as CGRP(1-4), CGRP(1-5) and CGRP(1-6) rapidly and reversibly enhance agonist sensitivity of nAChRs without directly activating those receptors. Longer fragments or the full-length peptide potently inhibit responses mediated by nAChRs via an apparently competitive-type antagonism. This phenomenon differs from the substance P-induced block, which is agonist use-dependent and preferential towards large nicotinic responses. 3. It is argued that the full-length peptides CGRP and substance P might play distinct roles in the activity-dependent modulation of cholinergic neurotransmission, by inhibiting background noise in the case of CGRP or by reducing excessive excitation in the case of substance P. Hence, multiple neuropeptide mechanisms may represent a wide array of fine-tuning processes to regulate nicotinic synaptic transmission. 4. The availability of novel CGRP derivatives with a strong enhancing action on nAChRs may offer new leads for the drug design targeted for potentiation of nAChRs in the autonomic nervous system as well as in the brain, a subject of interest to counteract the deficit of the nAChR function associated with neurodegenerative diseases like Alzheimer's and Parkinson's diseases.

Cholinergic nicotinic receptors: competitive ligands, allosteric modulators, and their potential applications

Discovery of the important role played by nicotinic acetylcholine receptors (nAChRs) in several CNS disorders has called attention to these membrane proteins and to ligands able to modulate their functions. The existence of different subtypes at multiple levels has complicated the understanding of this receptor's physiological role, but at the same time has increased the efforts to discover selective compounds in order to improve the pharmacological characterization of this kind of receptor and to make the possible therapeutical use of its modulators safer. This review focuses on the structure of new ligands for nAChRs, agonists, antagonists and allosteric modulators, and on their possible applications.