Differential effects of allosteric M(1) muscarinic acetylcholine receptor agonists on receptor activation, arrestin 3 recruitment, and receptor downregulation

A M1 muscarinic acetylcholine receptor positive allosteric modulator improves pathology and cognitive deficits in female APPswe/PSEN1ΔE9 mice

BACKGROUND AND PURPOSE

Alzheimer's disease (AD) is a neurodegenerative disease characterized by progressive cognitive decline and women account for 60% of diagnosed cases. Beta-amyloid (Aβ) oligomers is considered the principal neurotoxic species in AD brains. The M1 muscarinic acetylcholine receptor (M1 mAChR) plays a key role in memory and learning. M1 mAChR agonists show pro-cognitive activity but cause many adverse off-target effects. A new orally bioavailable M1 mAChR positive allosteric modulator (PAM), VU0486846, is devoid of direct agonist activity or adverse effects but was not tested for disease-modifying efficacy in female AD mice.

EXPERIMENTAL APPROACH

Nine-month-old female APPswe/PSEN1ΔE9 (APPswe) and wildtype mice were treated with VU0486846 in drinking water (10mg/kg/day) for 4 or 8 weeks. Cognitive function of mice was assessed after treatment and brains were harvested for biochemical and immunohistochemical assessment.

KEY RESULTS

VU0486846 improved cognitive function of APPswe mice when tested in novel object recognition and Morris water maze. This was paralleled by a significant reduction in Aβ oligomers and plaques and neuronal loss in hippocampus. VU0486846 reduced Aβ oligomer production in APPswe mice by increasing M1 mAChR expression and shifting the processing of amyloid precursor protein from amyloidogenic cleavage to non-amyloidogenic cleavage. Specifically, VU0486846 reduced the expression of β-secretase 1 (BACE1), whereas it enhanced the expression of the α-secretase ADAM10 in APPswe hippocampus.

CONCLUSION AND IMPLICATIONS

Using M1 mAChR PAMs can be a viable disease-modifying approach that should be exploited clinically to slow AD in women.

Functionally selective and biased agonists of muscarinic receptors

Disruption of cholinergic signalling via muscarinic receptors is associated with various pathologies, like Alzheimer's disease or schizophrenia. Selective muscarinic agonists possess therapeutic potential in the treatment of diabetes, pain or Sjögren's syndrome. The orthosteric binding site of all subtypes of the muscarinic receptor is structurally identical, making the development of affinity-based selective agonists virtually impossible. Some agonists, however, are functionally selective; they activate only a subset of receptors or signalling pathways. Others may stabilise specific conformations of the receptor leading to non-uniform modulation of individual signalling pathways (biased agonists). Functionally selective and biased agonists represent a promising approach for selective activation of individual subtypes of muscarinic receptors. In this work we review chemical structures, receptor binding and agonist-specific conformations of currently known functionally selective and biased muscarinic agonists in the context of their intricate intracellular signalling. Further, we take a perspective on the possible use of biased agonists for tissue and organ-specific activation of muscarinic receptors.

Fine Tuning Muscarinic Acetylcholine Receptor Signaling Through Allostery and Bias

The M1 and M4 muscarinic acetylcholine receptors (mAChRs) are highly pursued drug targets for neurological diseases, in particular for Alzheimer's disease and schizophrenia. Due to high sequence homology, selective targeting of any of the M1-M5 mAChRs through the endogenous ligand binding site has been notoriously difficult to achieve. With the discovery of highly subtype selective mAChR positive allosteric modulators in the new millennium, selectivity through targeting an allosteric binding site has opened new avenues for drug discovery programs. However, some hurdles remain to be overcome for these promising new drug candidates to progress into the clinic. One challenge is the potential for on-target side effects, such as for the M1 mAChR where over-activation of the receptor by orthosteric or allosteric ligands can be detrimental. Therefore, in addition to receptor subtype selectivity, a drug candidate may need to exhibit a biased signaling profile to avoid such on-target adverse effects. Indeed, recent studies in mice suggest that allosteric modulators for the M1 mAChR that bias signaling toward specific pathways may be therapeutically important. This review brings together details on the signaling pathways activated by the M1 and M4 mAChRs, evidence of biased agonism at these receptors, and highlights pathways that may be important for developing new subtype selective allosteric ligands to achieve therapeutic benefit.

T-495, a novel low cooperative M1 receptor positive allosteric modulator, improves memory deficits associated with cholinergic dysfunction and is characterized by low gastrointestinal side effect risk

M₁ muscarinic acetylcholine receptor (M₁ R) activation can be a new therapeutic approach for the treatment of cognitive deficits associated with cholinergic hypofunction. However, M₁ R activation causes gastrointestinal (GI) side effects in animals. We previously found that an M₁ R positive allosteric modulator (PAM) with lower cooperativity (α-value) has a limited impact on ileum contraction and can produce a wider margin between cognitive improvement and GI side effects. In fact, TAK-071, a novel M₁ R PAM with low cooperativity (α-value of 199), improved scopolamine-induced cognitive deficits with a wider margin against GI side effects than a high cooperative M₁ R PAM, T-662 (α-value of 1786), in rats. Here, we describe the pharmacological characteristics of a novel low cooperative M₁ R PAM T-495 (α-value of 170), using the clinically tested higher cooperative M₁ R PAM MK-7622 (α-value of 511) as a control. In rats, T-495 caused diarrhea at a 100-fold higher dose than that required for the improvement of scopolamine-induced memory deficits. Contrastingly, MK-7622 showed memory improvement and induction of diarrhea at an equal dose. Combination of T-495, but not of MK-7622, and donepezil at each sub-effective dose improved scopolamine-induced memory deficits. Additionally, in mice with reduced acetylcholine levels in the forebrain via overexpression of A53T α-synuclein (ie, a mouse model of dementia with Lewy bodies and Parkinson's disease with dementia), T-495, like donepezil, reversed the memory deficits in the contextual fear conditioning test and Y-maze task. Thus, low cooperative M₁ R PAMs are promising agents for the treatment of memory deficits associated with cholinergic dysfunction.

Anti-inflammatory and antioxidant effects of muscarinic acetylcholine receptor (mAChR) activation in the rat hippocampus

Recently we found that acute treatment with Oxotremorine (Oxo), a non-selective mAChRs agonist, up-regulates heat shock proteins and activates their transcription factor heat shock factor 1 in the rat hippocampus. Here we aimed to investigate: a) if acute treatment with Oxo may regulate pro-inflammatory or anti-inflammatory cytokines and oxidative stress in the rat hippocampus; b) if chronic restraint stress (CRS) induces inflammatory or oxidative alterations in the hippocampus and whether such alterations may be affected by chronic treatment with Oxo. In the acute experiment, rats were injected with single dose of Oxo (0.4 mg/kg) and sacrificed at 24 h, 48 h and 72 h. In the CRS experiment, the rats were exposed for 21 days to the CRS and then were treated with Oxo (0.2 mg/kg) for further 10 days. The acute Oxo treatment showed an ability to significantly reduce reactive oxygen species (ROS), singlet oxygen (¹O₂), pro-inflammatory cytokines levels (IL-1β and IL-6) and phosphorylated NF-κB-p65. Acute Oxo treatment also increased superoxide dismutase (SOD)-2 protein levels and stimulated SOD activity. No differences were detected in the anti-inflammatory cytokine levels, including IL-10 and TGF-β1. In the group of rats exposed to the CRS were found increased hippocampal IL-1β and IL-6 levels, together with a reduction of SOD activity level. These changes produced by CRS were counteracted by chronic Oxo treatment. In contrast, the upregulation of ROS and ¹O₂ levels in the CRS group was not counteracted by chronic Oxo treatment. The results revealed a hippocampal anti-inflammatory and antioxidant effect of Oxo treatment in both basal conditions and anti-inflammatory in the CRS rat model.

Selective attenuation of Ether-a-go-go related K+ currents by endogenous acetylcholine reduces spike-frequency adaptation and network correlation

Most neurons do not simply convert inputs into firing rates. Instead, moment-to-moment firing rates reflect interactions between synaptic inputs and intrinsic currents. Few studies investigated how intrinsic currents function together to modulate output discharges and which of the currents attenuated by synthetic cholinergic ligands are actually modulated by endogenous acetylcholine (ACh). In this study we optogenetically stimulated cholinergic fibers in rat neocortex and find that ACh enhances excitability by reducing Ether-à-go-go Related Gene (ERG) K+ current. We find ERG mediates the late phase of spike-frequency adaptation in pyramidal cells and is recruited later than both SK and M currents. Attenuation of ERG during coincident depolarization and ACh release leads to reduced late phase spike-frequency adaptation and persistent firing. In neuronal ensembles, attenuating ERG enhanced signal-to-noise ratios and reduced signal correlation, suggesting that these two hallmarks of cholinergic function in vivo may result from modulation of intrinsic properties.

Disease-Modifying Effects of M1 Muscarinic Acetylcholine Receptor Activation in an Alzheimer's Disease Mouse Model

Alzheimer's disease (AD) is the leading cause of dementia worldwide, and currently no disease-modifying therapy is available to slow or prevent AD, underscoring the urgent need for neuroprotective therapies. Selective M₁ muscarinic acetylcholine receptor (mAChR) activation is an attractive mechanism for AD therapy since M₁ mediates key effects on memory, cognition, and behavior and has potential for disease-modifying effects on Aβ formation and tau phosphorylation. To validate M₁ as a neuroprotective treatment target for AD, the M₁-selective agonist, VU0364572, was chronically dosed to 5XFAD mice from a young age preceding Aβ pathology (2 months) to an age where these mice are known to display memory impairments (6 months). Chronic M₁ activation prevented mice from becoming memory-impaired, as measured by Morris water maze (MWM) testing at 6 months of age. Additionally, M₁ activation significantly reduced levels of soluble and insoluble Aβ_40,42 in the cortex and hippocampus of these animals, as measured by ELISA and immunohistochemistry. Moreover, soluble hippocampal Aβ₄₂ levels were strongly correlated with MWM memory impairments and M₁ activation with VU0364572 abolished this correlation. Finally, VU0364572 significantly decreased oligomeric (oAβ) levels in the cortex, suggesting one mechanism whereby VU0364572 may be exerting its neuroprotective effects is by reducing the available oAβ pool in the brain. These findings suggest that chronic M₁ activation has neuroprotective potential for preventing memory impairments and reducing neuropathology in AD. M₁ activation therefore represents a promising avenue for preventative treatment, as well as a promising opportunity to combine symptomatic and disease-modifying effects for early AD treatment.

Ethylatropine Bromide as a Peripherally Restricted Muscarinic Antagonist

Quaternary ammonium analogues of atropine that are unable to cross the blood-brain barrier are used to alleviate peripheral muscarinic toxicity in animal models of epilepsy produced by systemic administration of pilocarpine or diisopropylfluorophosphate (DFP). Currently, methylatropine is the most popular and potent of these quaternary derivatives; however, it is expensive and produced in limited quantity. Here, we propose the use of ethylatropine bromide as an alternative to methylatropine. The synthesis of ethylatropine bromide is simple, inexpensive and has low environmental impact. We demonstrate the efficacy of ethylatropine bromide to antagonize the carbachol induced rise in intracellular calcium in a calcium mobilization assay, and its ability to prevent pilocarpine-induced total fluid secretions in mice without blocking pilocarpine-induced seizures. The ease of synthesis, cost effectiveness, and efficacy makes ethylatropine bromide a desirable alternative to methylatropine as a peripherally restricted acetylcholine receptor antagonist.

Effects of Selective M1 Muscarinic Receptor Activation on Hippocampal Spatial Representations and Neuronal Oscillations

The muscarinic M₁ acetylcholine receptor is a key target for drugs aimed at treating cognitive dysfunction, including the memory impairment in Alzheimer's disease. The overall question of the current study was to ask how systemic administration of the bitopic M₁ agonist VU0364572, the M₁ positive allosteric modulator BQCA, and the acetylcholinesterase inhibitor donepezil (current standard of care for Alzheimer's disease), would impact spatial memory-related hippocampal function in rats. Hippocampal pyramidal neuron spiking and local field potentials were recorded from regions CA1 and CA3 as rats freely foraged in a recording enclosure. To assess the relative stability versus flexibility of the rats' spatial representations, the walls of the recording enclosure were reshaped in 15-m intervals. As compared to the control condition, systemic administration of VU0364572 increased spatial correlations of CA1 and CA3 pyramidal neuron spiking across all enclosure shape comparisons, whereas BQCA and donepezil appeared to decrease these spatial correlations. Further, both VU0364572 and BQCA increased intrahippocampal synchrony as measured by CA3-CA1 field-field coherence in frequency ranges that tended to align with the prominence of those oscillations for the behavioral state (i.e., theta during locomotion and slow gamma during stationary moments). The results indicated that VU0364572 and BQCA influenced hippocampal function differently but in ways that might both be beneficial for treating memory dysfunction.

AF710B, a Novel M1/σ1 Agonist with Therapeutic Efficacy in Animal Models of Alzheimer’s Disease

We previously developed orthosteric M1 muscarinic agonists (e.g. AF102B, AF267B and AF292), which act as cognitive enhancers and potential disease modifiers. We now report on a novel compound, AF710B, a highly potent and selective allosteric M1 muscarinic and σ1 receptor agonist. AF710B exhibits an allosteric agonistic profile on the M1 muscarinic receptor; very low concentrations of AF710B significantly potentiated the binding and efficacy of carbachol on M1 receptors and their downstream effects (p-ERK1/2, p-CREB). AF710B (1-30 µg/kg, p.o.) was a potent and safe cognitive enhancer in rats treated with the M1 antagonist trihexyphenidyl (passive avoidance impairment). These effects of AF710B involve σ1 receptor activation. In agreement with its antiamnesic properties, AF710B (at 30 nM), via activation of M1 and a possible involvement of σ1 receptors, rescued mushroom synapse loss in PS1-KI and APP-KI neuronal cultures, while AF267B (1 µM) was less potent in PS1-KI and ineffective in APP-KI models, respectively. In female 3xTg-AD mice, AF710B (10 µg/kg, i.p./daily/2 months) (i) mitigated cognitive impairments in the Morris water maze; (ii) decreased BACE1, GSK3β activity, p25/CDK5, neuroinflammation, soluble and insoluble Aβ40, Aβ42, plaques and tau pathologies. AF710B differs from conventional σ1 and M1 muscarinic (orthosteric, allosteric or bitopic) agonists. These results highlight AF710B as a potential treatment for Alzheimer's disease (e.g. improving cognitive deficits, synaptic loss, amyloid and tau pathologies, and neuroinflammation) with a superior profile over a plethora of other therapeutic strategies.

Further optimization of the M1 PAM VU0453595: Discovery of novel heterobicyclic core motifs with improved CNS penetration

This Letter describes the continued chemical optimization of the VU0453595 series of M1 positive allosteric modulators (PAMs). By surveying alternative 5,6- and 6,6-heterobicylic cores for the 6,7-dihydro-5H-pyrrolo[3,4-b]pyridine-5-one core of VU453595, we found new cores that engendered not only comparable or improved M1 PAM potency, but significantly improved CNS distribution (Kps 0.3-3.1). Moreover, this campaign provided fundamentally distinct M1 PAM chemotypes, greatly expanding the available structural diversity for this valuable CNS target, devoid of hydrogen-bond donors.

Allosteric modulation of the M1 muscarinic acetylcholine receptor: improving cognition and a potential treatment for schizophrenia and Alzheimer's disease

Allosteric modulation of AMPA, NR2B, mGlu2, mGlu5 and M1, targeting glutamatergic dysfunction, represents a significant area of research for the treatment of schizophrenia. Of these targets, clinical promise has been demonstrated using muscarinic activators for the treatment of Alzheimer's disease (AD) and schizophrenia. These diseases have inspired researchers to determine the effects of modulating cholinergic transmission in the forebrain, which is primarily regulated by one of five subtypes of muscarinic acetylcholine receptor (mAChR), a subfamily of G-protein-coupled receptors (GPCRs). Of these five subtypes, M1 is highly expressed in brain regions responsible for learning, cognition and memory. Xanomeline, an orthosteric muscarinic agonist with modest selectivity, was one of the first compounds that displayed improvements in behavioral disturbances in AD patients and efficacy in schizophrenics. Since these initial clinical results, many scientists, including those in our laboratories, have strived to elucidate the role of M1 with compounds that display improved selectivity for this receptor by targeting allosteric modes of receptor activation. A survey of selected compounds in this area will be presented.

Regulation of G protein-coupled receptors by allosteric ligands

Topographically distinct, druggable, allosteric sites may be present on all G protein-coupled receptors (GPCRs). As such, targeting these sites with synthetic small molecules offers an attractive approach to develop receptor-subtype selective chemical leads for the development of novel therapies. A crucial part of drug development is to understand the acute and chronic effects of such allosteric modulators at their corresponding GPCR target. Key regulatory processes including cell-surface delivery, endocytosis, recycling, and down-regulation tightly control the number of receptors at the surface of the cell. As many GPCR therapeutics will be administered chronically, understanding how such ligands modulate these regulatory pathways forms an essential part of the characterization of novel GPCR ligands. This is true for both orthosteric and allosteric ligands. In this Review, we summarize our current understanding of GPCR regulatory processes with a particular focus on the effects and implications of allosteric targeting of GPCRs.

Distinct roles of β-arrestin 1 and β-arrestin 2 in ORG27569-induced biased signaling and internalization of the cannabinoid receptor 1 (CB1)

The cannabinoid receptor 1 (CB1) is a G protein-coupled receptor primarily expressed in brain tissue that has been implicated in several disease states. CB1 allosteric compounds, such as ORG27569, offer enormous potential as drugs over orthosteric ligands, but their mechanistic, structural, and downstream effects upon receptor binding have not been established. Previously, we showed that ORG27569 enhances agonist binding affinity to CB1 but inhibits G protein-dependent agonist signaling efficacy in HEK293 cells and rat brain expressing the CB1 receptor (Ahn, K. H., Mahmoud, M. M., and Kendall, D. A. (2012) J. Biol. Chem. 287, 12070-12082). Here, we identify the mediators of CB1 receptor internalization and ORG27569-induced G protein-independent signaling. Using siRNA technology, we elucidate an ORG27569-induced signaling mechanism for CB1 wherein β-arrestin 1 mediates short term signaling to ERK1/2 with a peak at 5 min and other upstream kinase components including MEK1/2 and c-Src. Consistent with these findings, we demonstrate co-localization of CB1-GFP with red fluorescent protein-β-arrestin 1 upon ORG27569 treatment using confocal microscopy. In contrast, we show the critical role of β-arrestin 2 in CB1 receptor internalization upon treatment with CP55940 (agonist) or treatment with ORG27569. These results demonstrate for the first time the involvement of β-arrestin in CB1-biased signaling by a CB1 allosteric modulator and also define the differential role of the two β-arrestin isoforms in CB1 signaling and internalization.

Sphingosine-1-phosphate and its receptors: structure, signaling, and influence

The sphingosine-1-phosphate (S1P) receptor signaling system has biological and medical importance and is the first lipid G protein-coupled receptor (GPCR) structure to be solved to 2.8-Å resolution. S1P binds to five high-affinity GPCRs generating multiple downstream signals that play essential roles in vascular development and endothelial integrity, control of cardiac rhythm, and routine oral treatment of multiple sclerosis. Genetics, chemistry, and now structural biology have advanced this integrated biochemical system. The S1P receptors have a novel N-terminal fold that occludes access to the binding pocket from the extracellular environment as well as orthosteric and bitopic ligands with very different physicochemical properties. S1P receptors and metabolizing enzymes have been deleted, inducibly deleted, and knocked in as tagged or altered receptors in mice. An array of genetic models allows analysis of integrated receptor function in vivo. We can now directly understand causal relationships among protein expression, signal, and control points in physiology and pathology.

Fluorescent derivatives of AC-42 to probe bitopic orthosteric/allosteric binding mechanisms on muscarinic M1 receptors

Two fluorescent derivatives of the M1 muscarinic selective agonist AC-42 were synthesized by coupling the lissamine rhodamine B fluorophore (in ortho and para positions) to AC42-NH(2). This precursor, prepared according to an original seven-step procedure, was included in the study together with the LRB fluorophore (alone or linked to an alkyl chain). All these compounds are antagonists, but examination of their ability to inhibit or modulate orthosteric [(3)H]NMS binding revealed that para-LRB-AC42 shared several properties with AC-42. Carefully designed experiments allowed para-LRB-AC42 to be used as a FRET tracer on EGFP-fused M1 receptors. Under equilibrium binding conditions, orthosteric ligands, AC-42, and the allosteric modulator gallamine behaved as competitors of para-LRB-AC42 binding whereas other allosteric compounds such as WIN 51,708 and N-desmethylclozapine were noncompetitive inhibitors. Finally, molecular modeling studies focused on putative orthosteric/allosteric bitopic poses for AC-42 and para-LRB-AC42 in a 3D model of the human M1 receptor.

Allosteric modulator ORG27569 induces CB1 cannabinoid receptor high affinity agonist binding state, receptor internalization, and Gi protein-independent ERK1/2 kinase activation

The cannabinoid receptor 1 (CB1), a member of the class A G protein-coupled receptor family, is expressed in brain tissue where agonist stimulation primarily activates the pertussis toxin-sensitive inhibitory G protein (G(i)). Ligands such as CP55940 ((1R,3R,4R)-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-4-(3- hydroxypropyl)cyclohexan-1-ol) and Δ(9)-tetrahydrocannabinol are orthosteric agonists for the receptor, bind the conventional binding pocket, and trigger G(i)-mediated effects including inhibition of adenylate cyclase. ORG27569 (5-chloro-3-ethyl-1H-indole-2-carboxylic acid [2-(4-piperidin-1-yl-phenyl)ethyl]amide) has been identified as an allosteric modulator that displays positive cooperativity for CP55940 binding to CB1 yet acts as an antagonist of G protein coupling. To examine this apparent conundrum, we used the wild-type CB1 and two mutants, T210A and T210I (D'Antona, A. M., Ahn, K. H., and Kendall, D. A. (2006) Biochemistry 45, 5606-5617), which collectively cover a spectrum of receptor states from inactive to partially active to more fully constitutively active. Using these receptors, we demonstrated that ORG27569 induces a CB1 receptor state that is characterized by enhanced agonist affinity and decreased inverse agonist affinity consistent with an active conformation. Also consistent with this conformation, the impact of ORG27569 binding was most dramatic on the inactive T210A receptor and less pronounced on the already active T210I receptor. Although ORG27569 antagonized CP55940-induced guanosine 5'-3-O-(thio)triphosphate binding, which is indicative of G protein coupling inhibition in a concentration-dependent manner, the ORG27569-induced conformational change of the CB1 receptor led to cellular internalization and downstream activation of ERK signaling, providing the first case of allosteric ligand-biased signaling via CB1. ORG27569-induced ERK phosphorylation persisted even after pertussis toxin treatment to abrogate G(i) and occurs in HEK293 and neuronal cells.

Muscarinic receptor trafficking

Knowledge of the mechanisms responsible for the trafficking of neurotransmitter receptors away from the cell surface is of obvious importance in understanding what regulates their expression and function. This chapter will focus on the mechanisms responsible for the internalization and degradation of muscarinic receptors. There are both receptor subtype-specific and cell-type specific differences in muscarinic receptor trafficking. Studies on muscarinic receptor trafficking both in cells in culture and in vivo will be described, and the potential physiological consequences of this trafficking will be discussed.

Therapeutic potential of β-arrestin- and G protein-biased agonists

Members of the seven-transmembrane receptor (7TMR), or G protein-coupled receptor (GPCR), superfamily represent some of the most successful targets of modern drug therapy, with proven efficacy in the treatment of a broad range of human conditions and disease processes. It is now appreciated that β-arrestins, once viewed simply as negative regulators of traditional 7TMR-stimulated G protein signaling, act as multifunctional adapter proteins that regulate 7TMR desensitization and trafficking and promote distinct intracellular signals in their own right. Moreover, several 7TMR biased agonists, which selectively activate these divergent signaling pathways, have been identified. Here we highlight the diversity of G protein- and β-arrestin-mediated functions and the therapeutic potential of selective targeting of these in disease states.

Orthosteric- and allosteric-induced ligand-directed trafficking at GPCRs

Many orthosteric agonists differentially activate downstream effectors of GPCRs. Such defined induction of signaling has strongly supported the hypothesis termed 'ligand-directed trafficking of receptor signaling' (LDTRS). More recently, subtype-selective GPCR activators, such as allosteric agonists and positive allosteric modulators, have also exhibited the capacity to activate specific signaling pathways. Based on this finding, it may be possible to achieve ligand-specific receptor active states that optimize the biological responses specific to GPCRs. This review discusses recent studies in which both orthosteric and allosteric compounds have been demonstrated to induce LDTRS.