Novel allosteric effects of amiodarone at the muscarinic M5 receptor

Allosteric modulators of M1 muscarinic receptors enhance acetylcholine efficacy and decrease locomotor activity and turning behaviors in zebrafish

Allosteric modulation of muscarinic acetylcholine receptors (mAChR) has been identified as a potential strategy for regulating cholinergic signaling in the treatment of various neurological disorders. Most positive allosteric modulators (PAMs) of mAChR enhance agonist affinity and potency, while very few PAMs (e.g., amiodarone) selectively enhance G protein coupling efficacy. The key structural features of amiodarone responsible for enhancement of mAChR efficacy were examined in CHO cells expressing M1 receptors. Subsequent incorporation of these structural features into previously identified allosteric modulators of potency (i.e., n-benzyl isatins) generated ligands that demonstrated similar or better enhancement of mAChR efficacy, lower in vivo toxicity, and higher allosteric binding affinity relative to amiodarone. Notable ligands include 8a, c which respectively demonstrated the strongest binding affinity and the most robust enhancement of mAChR efficacy as calculated from an allosteric operational model. Amiodarone derivatives and hybrid ligands were additionally screened in wildtype zebrafish (Danio rerio) to provide preliminary in vivo toxicity data as well as to observe effects on locomotor and turning behaviors relative to other mAChR PAMs. Several compounds, including 8a, c, reduced locomotor activity and increased measures of turning behaviors in zebrafish, suggesting that allosteric modulation of muscarinic receptor efficacy might be useful in the treatment of repetitive behaviors associated with autism spectrum disorder (ASD) and other neuropsychiatric disorders.

Hybrid Allosteric Modulators of M1 Muscarinic Receptors Enhance Acetylcholine Efficacy and Decrease Locomotor Activity and Turning Behaviors in Zebrafish

Allosteric modulation of muscarinic acetylcholine receptors (mAChR) has been identified as a potential strategy for regulating cholinergic signaling in the treatment of various neurological disorders. Most positive allosteric modulators (PAMs) of mAChR enhance agonist affinity and potency, while very few PAMs selectively enhance G-protein coupling efficacy (e.g., amiodarone). The key structural features of amiodarone responsible for enhancement of mAChR efficacy were examined in CHO cells expressing M1 receptors. Subsequent incorporation of these structural features into previously identified allosteric modulators of potency (i.e., n-benzyl isatins) generated hybrid ligands that demonstrated similar or better enhancement of mAChR efficacy, lower in vivo toxicity, and higher allosteric binding affinity relative to amiodarone. Notable hybrid ligands include 8a and 8b which respectively demonstrated the strongest binding affinity and the most robust enhancement of mAChR efficacy as calculated from an allosteric operational model. Amiodarone derivatives and hybrid ligands were additionally screened in wildtype zebrafish (Danio rerio) to provide preliminary in vivo toxicity data as well as to observe effects on locomotor and turning behaviors relative to other mAChR PAMs. Several compounds, including 8a and 8c, reduced locomotor activity and increased measures of turning behaviors in zebrafish, suggesting that allosteric modulation of muscarinic receptor efficacy might be useful in the treatment of repetitive behaviors associated with autism spectrum disorder (ASD) and other neuropsychiatric disorders.

Multitargeting nature of muscarinic orthosteric agonists and antagonists

Muscarinic receptors (mAChRs) are typical members of the G protein-coupled receptor (GPCR) family and exist in five subtypes from M1 to M5. Muscarinic receptor subtypes do not sufficiently differ in affinity to orthosteric antagonists or agonists; therefore, the analysis of receptor subtypes is complicated, and misinterpretations can occur. Usually, when researchers mainly specialized in CNS and peripheral functions aim to study mAChR involvement in behavior, learning, spinal locomotor networks, biological rhythms, cardiovascular physiology, bronchoconstriction, gastrointestinal tract functions, schizophrenia, and Parkinson's disease, they use orthosteric ligands and they do not use allosteric ligands. Moreover, they usually rely on manufacturers' claims that could be misleading. This review aimed to call the attention of researchers not deeply focused on mAChR pharmacology to this fact. Importantly, limited selective binding is not only a property of mAChRs but is a general attribute of most neurotransmitter receptors. In this review, we want to give an overview of the most common off-targets for established mAChR ligands. In this context, an important point is a mention the tremendous knowledge gap on off-targets for novel compounds compared to very well-established ligands. Therefore, we will summarize reported affinities and give an outline of strategies to investigate the subtype's function, thereby avoiding ambiguous results. Despite that, the multitargeting nature of drugs acting also on mAChR could be an advantage when treating such diseases as schizophrenia. Antipsychotics are a perfect example of a multitargeting advantage in treatment. A promising strategy is the use of allosteric ligands, although some of these ligands have also been shown to exhibit limited selectivity. Another new direction in the development of muscarinic selective ligands is functionally selective and biased agonists. The possible selective ligands, usually allosteric, will also be listed. To overcome the limited selectivity of orthosteric ligands, the recommended process is to carefully examine the presence of respective subtypes in specific tissues via knockout studies, carefully apply "specific" agonists/antagonists at appropriate concentrations and then calculate the probability of a specific subtype involvement in specific functions. This could help interested researchers aiming to study the central nervous system functions mediated by the muscarinic receptor.

Identification of a Novel Allosteric Site at the M5 Muscarinic Acetylcholine Receptor

The M5 muscarinic acetylcholine receptor (mAChR) has emerged as an exciting therapeutic target for the treatment of addiction and behavioral disorders. This has been in part due to promising preclinical studies with the M5 mAChR selective negative allosteric modulator (NAM), ML375. The binding site of ML375 remains unknown, however, making it difficult to develop improved M5 mAChR selective modulators. To determine the possible location of the ML375 binding site, we used radioligand binding and functional assays to show that ML375 does not interact with the well-characterized "common" mAChR allosteric site located in the receptor's extracellular vestibule, nor a previously proposed second allosteric site recognized by the modulator, amiodarone. Molecular docking was used to predict potential allosteric sites within the transmembrane (TM) domain of the M5 mAChR. These predicted sites were assessed using M5-M2 mAChR receptor chimeras and further targeted with site-directed mutagenesis, which enabled the identification of a putative binding site for ML375 at the interface of TMs 2-4. Collectively, these results identify a third allosteric site at the M5 mAChR and highlight the ability of allosteric modulators to selectively target highly conserved proteins.

Antiparasitic Properties of Cardiovascular Agents against Human Intravascular Parasite Schistosoma mansoni

The intravascular parasitic worm Schistosoma mansoni is a causative agent of schistosomiasis, a disease of great global public health significance. Praziquantel is the only drug available to treat schistosomiasis and there is an urgent demand for new anthelmintic agents. Adopting a phenotypic drug screening strategy, here, we evaluated the antiparasitic properties of 46 commercially available cardiovascular drugs against S. mansoni. From these screenings, we found that amiodarone, telmisartan, propafenone, methyldopa, and doxazosin affected the viability of schistosomes in vitro, with effective concentrations of 50% (EC₅₀) and 90% (EC₉₀) values ranging from 8 to 50 µM. These results were further supported by scanning electron microscopy analysis. Subsequently, the most effective drug (amiodarone) was further tested in a murine model of schistosomiasis for both early and chronic S. mansoni infections using a single oral dose of 400 mg/kg or 100 mg/kg daily for five consecutive days. Amiodarone had a low efficacy in chronic infection, with the worm and egg burden reduction ranging from 10 to 30%. In contrast, amiodarone caused a significant reduction in worm and egg burden in early infection (>50%). Comparatively, treatment with amiodarone is more effective in early infection than praziquantel, demonstrating the potential role of this cardiovascular drug as an antischistosomal agent.

Cholinergic System and Its Therapeutic Importance in Inflammation and Autoimmunity

Neurological and immunological signals constitute an extensive regulatory network in our body that maintains physiology and homeostasis. The cholinergic system plays a significant role in neuroimmune communication, transmitting information regarding the peripheral immune status to the central nervous system (CNS) and vice versa. The cholinergic system includes the neurotransmitter\ molecule, acetylcholine (ACh), cholinergic receptors (AChRs), choline acetyltransferase (ChAT) enzyme, and acetylcholinesterase (AChE) enzyme. These molecules are involved in regulating immune response and playing a crucial role in maintaining homeostasis. Most innate and adaptive immune cells respond to neuronal inputs by releasing or expressing these molecules on their surfaces. Dysregulation of this neuroimmune communication may lead to several inflammatory and autoimmune diseases. Several agonists, antagonists, and inhibitors have been developed to target the cholinergic system to control inflammation in different tissues. This review discusses how various molecules of the neuronal and non-neuronal cholinergic system (NNCS) interact with the immune cells. What are the agonists and antagonists that alter the cholinergic system, and how are these molecules modulate inflammation and immunity. Understanding the various functions of pharmacological molecules could help in designing better strategies to control inflammation and autoimmunity.

Emerging Roles of SRSF3 as a Therapeutic Target for Cancer

Ser/Arg-rich (SR) proteins are RNA-binding proteins known as constitutive and alternative splicing (AS) regulators that regulate multiple aspects of the gene expression program. Ser/Arg-rich splicing factor 3 (SRSF3) is the smallest member of the SR protein family, and its level is controlled by multiple factors and involves complex mechanisms in eukaryote cells, whereas the aberrant expression of SRSF3 is associated with many human diseases, including cancer. Here, we review state-of-the-art research on SRSF3 in terms of its function, expression, and misregulation in human cancers. We emphasize the negative consequences of the overexpression of the SRSF3 oncogene in cancers, the pathways underlying SRSF3-mediated transformation, and implications of potential anticancer drugs by downregulation of SRSF3 expression for cancer therapy. Cumulative research on SRSF3 provides critical insight into its essential part in maintaining cellular processes, offering potential new targets for anti-cancer therapy.

The Muscarinic Acetylcholine Receptor M5: Therapeutic Implications and Allosteric Modulation

The muscarinic acetylcholine receptor (mAChR) subtype 5 (M₅) was the most recent mAChR to be cloned and has since emerged as a potential therapeutic target for a number of indications. Early studies with knockout animals have provided clues to the receptor's role in physiological processes related to Alzheimer's disease, schizophrenia, and addiction, and until recently, useful subtype-selective tools to further probe the pharmacology of M₅ have remained elusive. Small-molecule allosteric modulators have since gained traction as a means by which to selectively examine muscarinic pharmacology. This review highlights the discovery and optimization of M₅ positive allosteric modulators (PAMs) and negative allosteric modulators (NAMs).

Toward an understanding of the structural basis of allostery in muscarinic acetylcholine receptors

Burger et al. summarize our mechanistic understanding of allostery in the prototypical GPCR, the muscarinic acetylcholine receptor.

Recent breakthroughs and developments in structural biology have led to a spate of crystal structures for G protein–coupled receptors (GPCRs). This is the case for the muscarinic acetylcholine receptors (mAChRs) where inactive-state structures for four of the five subtypes and two active-state structures for one subtype are available. These mAChR crystal structures have provided new insights into receptor mechanisms, dynamics, and allosteric modulation. This is highly relevant to the mAChRs given that these receptors are an exemplar model system for the study of GPCR allostery. Allosteric mechanisms of the mAChRs are predominantly consistent with a two-state model, albeit with some notable recent exceptions. Herein, we discuss the mechanisms for positive and negative allosteric modulation at the mAChRs and compare and contrast these to evidence offered by pharmacological, biochemical, and computational approaches. This analysis provides insight into the fundamental pharmacological properties exhibited by GPCR allosteric modulators, such as enhanced subtype selectivity, probe dependence, and biased modulation while highlighting the current challenges that remain. Though complex, enhanced molecular understanding of allosteric mechanisms will have considerable influence on our understanding of GPCR activation and signaling and development of therapeutic interventions.

Amiodarone promotes cancer cell death through elevated truncated SRSF3 and downregulation of miR-224

Amiodarone is a widely used class III antiarrhythmic agent which prolongs the action potential and refractory period by blockage of several types of myocardial potassium channels. Emerging evidence suggests that amiodarone sensitize tumor cells in response to chemotherapy. Nevertheless, little is known about the underlying molecular mechanism. To gain further insight, we demonstrated that amiodarone accumulated the population of a premature termination codon-containing isoform of serine and arginine rich splicing factor 3 (SRSF3-PTC) without increasing alternative spliced p53 beta isoform. Amiodarone enhanced reactive oxygen species production and increased cell apoptosis, whereas reduced DNA damage. Moreover, amiodarone suppressed miR-224 and increased its target COX-2 expression. Taken together, our results suggested amiodarone caused cancer cell death might be through increased SRSF3-PTC and miR-224 reduction in a p53-independent manner.

Outline of Therapeutic Interventions With Muscarinic Receptor-Mediated Transmission

Muscarinc receptor-mediated signaling takes part in many physiological functions ranging from complex higher nervous activity to vegetative responses. Specificity of action of the natural muscarinic agonist acetylcholine is effected by action on five muscarinic receptor subtypes with particular tissue and cellular localization, and coupling preference with different G-proteins and their signaling pathways. In addition to physiological roles it is also implicated in pathologic events like promotion of carcinoma cells growth, early pathogenesis of neurodegenerative diseases in the central nervous system like Alzheimer´s disease and Parkinson´s disease, schizophrenia, intoxications resulting in drug addiction, or overactive bladder in the periphery. All of these disturbances demonstrate involvement of specific muscarinic receptor subtypes and point to the importance to develop selective pharmacotherapeutic interventions. Because of the high homology of the orthosteric binding site of muscarinic receptor subtypes there is virtually no subtype selective agonist that binds to this site. Activation of specific receptor subtypes may be achieved by developing allosteric modulators of acetylcholine binding, since ectopic binding domains on the receptor are less conserved compared to the orthosteric site. Potentiation of the effects of acetylcholine by allosteric modulators would be beneficial in cases where acetylcholine release is reduced due to pathological conditions. When presynaptic function is severly compromised, the utilization of ectopic agonists can be a thinkable solution.

Identification of the antiarrhythmic drugs amiodarone and lorcainide as potent H3 histamine receptor inverse agonists

Use of molecular pharmacology to reprofile older drugs discovered before the advent of recombinant technologies is a fruitful method to elucidate mechanisms of drug action, expand understanding of structure-activity relationships between drugs and receptors, and in some cases, repurpose approved drugs. The H3 histamine receptor is a G-protein-coupled receptor (GPCR) primarily expressed in the central nervous system where among many things it modulates cognitive processes, nociception, feeding and drinking behavior, and sleep/wakefulness. In binding assays and functional screens of the H3 histamine receptor, the antiarrhythmic drugs lorcainide and amiodarone were identified as potent, selective antagonists/inverse agonists of human and rat H3 histamine receptors, with relatively little or no activity at over 20 other monoamine GPCRs, including H1, H2, and H4 receptors. Potent antagonism of H3 receptors was unique to amiodarone and lorcainide of 20 antiarrhythmic drugs tested, representing six pharmacological classes. These results expand the pharmacophore of H3 histamine receptor antagonist/inverse agonists and may explain, in part, the effects of lorcainide on sleep in humans.

Oxidative stress disruption of receptor-mediated calcium signaling mechanisms

Background

Oxidative stress increases the cytosolic content of calcium in the cytoplasm through a combination of effects on calcium pumps, exchangers, channels and binding proteins. In this study, oxidative stress was produced by exposure to tert-butyl hydroperoxide (tBHP); cell viability was assessed using a dye reduction assay; receptor binding was characterized using [³H]N-methylscopolamine ([³H]MS); and cytosolic and luminal endoplasmic reticulum (ER) calcium concentrations ([Ca²⁺]_i and [Ca²⁺]_L, respectively) were measured by fluorescent imaging.

Results

Activation of M3 muscarinic receptors induced a biphasic increase in [Ca²⁺]_i: an initial, inositol trisphosphate (IP3)-mediated release of Ca²⁺ from endoplasmic reticulum (ER) stores followed by a sustained phase of Ca²⁺ entry (i.e., store-operated calcium entry; SOCE). Under non-cytotoxic conditions, tBHP increased resting [Ca²⁺]_i; a 90 minute exposure to tBHP (0.5-10 mM ) increased [Ca²⁺]_i from 26 to up to 127 nM and decreased [Ca²⁺]_L by 55%. The initial response to 10 μM carbamylcholine was depressed by tBHP in the absence, but not the presence, of extracellular calcium. SOCE, however, was depressed in both the presence and absence of extracellular calcium. Acute exposure to tBHP did not block calcium influx through open SOCE channels. Activation of SOCE following thapsigargin-induced depletion of ER calcium was depressed by tBHP exposure. In calcium-free media, tBHP depressed both SOCE and the extent of thapsigargin-induced release of Ca²⁺ from the ER. M3 receptor binding parameters (ligand affinity, guanine nucleotide sensitivity, allosteric modulation) were not affected by exposure to tBHP.

Conclusions

Oxidative stress induced by tBHP affected several aspects of M3 receptor signaling pathway in CHO cells, including resting [Ca²⁺]_i, [Ca²⁺]_L, IP3 receptor mediated release of calcium from the ER, and calcium entry through the SOCE. tBHP had little effect on M3 receptor binding or G protein coupling. Thus, oxidative stress affects multiple aspects of calcium homeostasis and calcium dependent signaling.

Overview of muscarinic receptor subtypes

The physiological role of muscarinic receptors is highly complex and, although not completely understood, has become clearer over the last decade. Recent pharmacological evidence with novel compounds, together with data from transgenic mice, suggests that all five subtypes have defined functions in the nervous system as well as mediating the non neuronal, hormonal actions of acetylcholine. Numerous novel agonists, allosteric regulators, and antagonists have now been identified with authentic subtype specificity in vitro and in vivo. These compounds provide additional pharmacological opportunities for selective subtype modulation as well as a new generation of muscarinic receptor-based therapeutics.

Experimental conditions can obscure the second high-affinity site in LeuT

Neurotransmitter:Na(+) symporters (NSSs), the targets of antidepressants and psychostimulants, recapture neurotransmitters from the synapse in a Na(+)-dependent symport mechanism. The crystal structure of the NSS homolog LeuT from Aquifex aeolicus revealed one leucine substrate in an occluded, centrally located (S1) binding site next to two Na(+) ions. Computational studies combined with binding and flux experiments identified a second substrate (S2) site and a molecular mechanism of Na(+)-substrate symport that depends upon the allosteric interaction of substrate molecules in the two high-affinity sites. Here we show that the S2 site, which has not yet been identified by crystallographic approaches, can be blocked during preparation of detergent-solubilized LeuT, thereby obscuring its crucial role in Na(+)-coupled symport. This finding points to the need for caution in selecting experimental environments in which the properties and mechanistic features of membrane proteins can be delineated.

Muscarinic agonists and antagonists in schizophrenia: recent therapeutic advances and future directions

Existing therapies for schizophrenia have limited efficacy, and significant residual positive, negative, and cognitive symptoms remain in many individuals with the disorder even after treatment with the current arsenal of antipsychotic drugs. Preclinical and clinical data suggest that selective activation of the muscarinic cholinergic system may represent novel therapeutic mechanisms for the treatment of schizophrenia. The therapeutic relevance of earlier muscarinic agonists was limited by their lack of receptor selectivity and adverse event profile arising from activation of nontarget muscarinic receptors. Recent advances in developing compounds that are selective to muscarinic receptor subtypes or activate allosteric receptor sites offer tremendous promise for therapeutic targeting of specific muscarinic receptor subtypes in schizophrenia.

Allosteric modulation of the M₃ muscarinic receptor by amiodarone and N-ethylamiodarone: application of the four-ligand allosteric two-state model

We have reported previously that amiodarone interacts with muscarinic receptors via a novel allosteric site. This study presents mechanistic details on the nature of that interaction. Amiodarone enhanced the maximal level of agonist-stimulated release of arachidonic acid (AA) from Chinese hamster ovary cells that expressed M₃ muscarinic receptors; this enhancement was observed for acetylcholine and for the partial agonist pilocarpine. A similar effect of amiodarone was observed when pilocarpine was used to stimulate inositol phosphate (IP) metabolism, but not when acetylcholine was used. Subsequent studies showed that the IP response exhibited a much larger receptor reserve than the AA response, and reduction of that reserve by receptor alkylation unmasked amiodarone's enhancement of the maximal IP response to acetylcholine. Modulating the receptor reserve also revealed acetylcholine's greater affinity (K(A)) for the conformation of the receptor that mediates the AA response. The amiodarone analog N-ethylamiodarone (NEA) did not alter maximal agonist response but merely reduced agonist potency (that is, it appeared to be an antagonist). However, the action of NEA could be clearly distinguished from the action of the orthosteric antagonist NMS. Demonstration of this point was facilitated by an elaboration of Hall's allosteric two-state model; this new model represents a system composed of two ligands that compete with each other at the orthosteric site and two ligands that compete with each other at the allosteric site. In conclusion, amiodarone competes with NEA at a novel, extracellular, allosteric site to enhance the maximal stimulation evoked by acetylcholine and pilocarpine in two different responses.

Muscarinic receptors: their roles in disorders of the central nervous system and potential as therapeutic targets

Phylogenetically, acetylcholine is an ancient neurochemical. Therefore, it is not surprising that cholinergic neurons project extensively throughout the central nervous system, innervating a wide range of structures within the brain. In fact, acetylcholine is involved in processes that underpin some of our most basic central functions. Both muscarinic and nicotinic receptor families, which mediate cholinergic transmission, have been implicated in the pathophysiology of psychiatric and neurological disorders. The question that remains to be definitively answered is whether or not these receptors are viable targets for the development of future therapeutic agents.

Role of M1, M3, and M5 muscarinic acetylcholine receptors in cholinergic dilation of small arteries studied with gene-targeted mice

Acetylcholine regulates perfusion of numerous organs via changes in local blood flow involving muscarinic receptor-induced release of vasorelaxing agents from the endothelium. The purpose of the present study was to determine the role of M₁, M₃, and M₅ muscarinic acetylcholine receptors in vasodilation of small arteries using gene-targeted mice deficient in either of the three receptor subtypes (M1R(-/-), M3R(-/-), or M5R(-/-) mice, respectively). Muscarinic receptor gene expression was determined in murine cutaneous, skeletal muscle, and renal interlobar arteries using real-time PCR. Moreover, respective arteries from M1R(-/-), M3R(-/-), M5R(-/-), and wild-type mice were isolated, cannulated with micropipettes, and pressurized. Luminal diameter was measured using video microscopy. mRNA for all five muscarinic receptor subtypes was detected in all three vascular preparations from wild-type mice. However, M(3) receptor mRNA was found to be most abundant. Acetylcholine produced dose-dependent dilation in all three vascular preparations from M1R(-/-), M5R(-/-), and wild-type mice. In contrast, cholinergic dilation was virtually abolished in arteries from M3R(-/-) mice. Deletion of either M₁, M₃, or M₅ receptor genes did not affect responses to nonmuscarinic vasodilators, such as substance P and nitroprusside. These findings provide the first direct evidence that M₃ receptors mediate cholinergic vasodilation in cutaneous, skeletal muscle, and renal interlobar arteries. In contrast, neither M₁ nor M₅ receptors appear to be involved in cholinergic responses of the three vascular preparations tested.