M1 muscarinic acetylcholine receptor interacts with BACE1 and regulates its proteosomal degradation

α7 nicotinic acetylcholine receptors are necessary for basal forebrain activation to increase expression of the nerve growth factor receptor TrkA

Activation of the basal forebrain leads to increases in the expression of the nerve growth factor receptor, Tropomyosin receptor kinase A (TrkA) and decreases in expression of the beta amyloid cleavage enzyme 1 (BACE1) in the cerebral cortex of both sexes of 5xFAD mice. The studies described in this report were designed to determine if these changes were dependent on acetylcholine receptors. Mice were stimulated unilaterally in the basal forebrain for two weeks. Animals were administered a cholinergic antagonist, or saline, 30 minutes prior to stimulation. Animals administered saline exhibited significant increases in TrkA expression and decreases in BACE1 in the stimulated hemisphere relative to the unstimulated. While both nonselective nicotinic and muscarinic acetylcholine receptor blockade attenuated the BACE1 decline, only the nicotinic receptor antagonism blocked the TrkA increase. Next, we applied selective nicotinic antagonists, and the α7 antagonist blocked the TrkA increases, but the α4β2 antagonist did not. BACE1 declines were not blocked by either intervention. Mice with a loxP conditional knockout of the gene for the α7 nicotinic receptor were also employed in these studies. Animals were either stimulated bilaterally for two weeks, or left unstimulated. With or without stimulation, the expression of TrkA receptors was lower in the cortical region with the α7 nicotinic receptor knockdown. We thus conclude that α7 nicotinic receptor activation is necessary for normal expression of TrkA and increases caused by basal forebrain activation, while BACE1 declines caused by stimulation have dependency on a broader array of receptor subtypes.

Investigation of Potential Drug Targets Involved in Inflammation Contributing to Alzheimer's Disease Progression

Alzheimer's disease has become a major public health issue. While extensive research has been conducted in the last few decades, few drugs have been approved by the FDA to treat Alzheimer's disease. There is still an urgent need for understanding the disease pathogenesis, as well as identifying new drug targets for further drug discovery. Alzheimer's disease is known to arise from a build-up of amyloid beta (Aβ) plaques as well as tangles of tau proteins. Along similar lines to Alzheimer's disease, inflammation in the brain is known to stem from the degeneration of tissue and build-up of insoluble materials. A minireview was conducted in this work assessing the genes, proteins, reactions, and pathways that link brain inflammation and Alzheimer's disease. Existing tools in Systems Biology were implemented to build protein interaction networks, mainly for the classical complement pathway and G protein-coupled receptors (GPCRs), to rank the protein targets according to their interactions. The top 10 protein targets were mainly from the classical complement pathway. With the consideration of existing clinical trials and crystal structures, proteins C5AR1 and GARBG1 were identified as the best targets for further drug discovery, through computational approaches like ligand-protein docking techniques.

Benincasa hispida alleviates amyloid pathology by inhibition of Keap1/Nrf2-axis: Emphasis on oxidative and inflammatory stress involved in Alzheimer's disease model

OBJECTIVES

Alzheimer's disease is a progressive neurodegenerative disorder with cognitive and memory impairment. Benincasa hispida is being used in the treatment of various neurological diseases in Ayurveda system of medicine. The objective of the study was to investigate the effect of Benincasa hispida fruit extract in the Alzheimer's disease rats.

METHODS

Benincasa hispida fruits extract was administered orally for 16 weeks at doses of 250 and 500-mg/kg/day. The cognitive deficits were examined by behavioural tests like Morris water maze test, Y-maze and rota-rod test. Biochemical and neurochemical analysis of Acetylcholine, dopamine, serotonin levels and anti-oxidant, anti-inflammatory markers were evaluated and the mRNA expression of Keap/Nrf2 axis was analysed by RT-PCR.

RESULTS

Aluminum chloride (AlCl₃) induction altered the behavioural profile and produced significant alterations in the cortical and hippocampal regions of the brain and the treatment with Benincasa hispida extract at doses of 250-mg/kg/day (p<0.05) and 500mg/kg/day (p<0.05) alleviated the acetylcholine, dopamine and serotonin neurotransmitter levels. The antioxidant enzyme markers such as superoxide dismutase (SOD), Catalase (CAT), glutathione (GSH) were increased and the oxidative stress marker malondialdehyde(MDA) was decreased. The inflammatory cytokine levels of TNF-α, IL-1β were decreased in Alzheimer's disease induced rats. We further estimated Keap/Nrf2/HO-1 genes these anti-oxidant genes were upregulated(p < 0.001) in treatment groups. Further, the neuroprotective activity of Benincasa was further confirmed by histopathological studies of hippocampal CA3 fields.

CONCLUSIONS

The findings of the current study indicates Benincasa hispida as a possible neuroprotective alternative for Alzheimer's disease.

Therapeutic Potential of Multifunctional Derivatives of Cholinesterase Inhibitors

The aim of this work is to review tacrine analogues from the last three years, which were not included in the latest review work, donepezil and galantamine hybrids from 2015 and rivastigmine derivatives from 2014. In this account, we summarize the efforts toward the development and characterization of non-toxic inhibitors of cholinesterases based on mentioned drugs with various interesting additional properties such as antioxidant, decreasing β-amyloid plaque aggregation, nitric oxide production, pro-inflammatory cytokines release, monoamine oxidase-B activity, cytotoxicity and oxidative stress in vitro and in animal model that classify these hybrids as potential multifunctional therapeutic agents for Alzheimer's disease. Moreover, herein, we have described the cholinergic hypothesis, mechanisms of neurodegeneration and current pharmacotherapy of Alzheimer's disease based on the restoration of cholinergic function through blocking enzymes that break down acetylcholine.

Role of GPCR signaling and calcium dysregulation in Alzheimer's disease

Alzheimer's disease (AD), a late onset neurodegenerative disorder is characterized by the loss of memory, disordered cognitive function, caused by accumulation of amyloid-β (Aβ) peptide and neurofibrillary tangles (NFTs) in the neocortex and hippocampal brain area. Extensive research has been done on the findings of the disease etiology or pathological causes of aggregation of Aβ and hyperphosphorylation of tau protein without much promising results. Recently, calcium dysregulation has been reported to play an important role in the pathophysiology of AD. Calcium ion acts as one of the major secondary messengers, regulates many signaling pathways involved in cell survival, proliferation, differentiation, transcription and apoptosis. Calcium signaling is one of the major signaling pathways involved in the formation of memory, generation of energy and other physiological functions. It also can modulate function of many proteins upon binding. Dysregulation in calcium homeostasis leads to many physiological changes leading to neurodegenerative diseases including AD. In AD, GPCRs generate secondary messengers which regulate calcium homeostasis inside the cell and is reported to be disturbed in the pathological condition. Calcium channels and receptors present on the plasma membrane and intracellular organelle maintain calcium homeostasis through different signaling mechanisms. In this review, we have summarized the different calcium channels and receptors involved in calcium dysregulation which in turn play a critical role in the pathogenesis of AD. Understanding the role of calcium channels and GPCRs to maintain calcium homeostasis is an attempt to develop effective AD treatments.

A gender dimorphism in up-regulation of BACE1 gene expression in schizophrenia

Schizophrenia has long been considered as a devastating brain disorder in which both genetic and environmental factors are involved. The BACE1 gene is one of the most important susceptibility genes for this disorder. However, the changes in BACE1 expression in schizophrenic patients compared with healthy subjects have not been evaluated yet. In this case-control study, we examined BACE1 expression in a group of 50 patients with schizophrenia and 50 healthy controls. The level of BACE1 gene expression was measured using Real-Time PCR. Substantial increase in gene expression was detected in the patients compared with normal individuals (P = 0.001). Furthermore, a gender dimorphism was observed in BACE1 gene expression in the patients in a way that the male patients manifested a statistically significant higher levels of BACE1 expression (P = 0.002). BACE1 might be implicated in the pathogenesis of schizophrenia. Besides, BACE1 physiology may be gender -based at some levels. Our findings warrant an investigation of BACE1 gene in a larger number of cases and controls.

The concept of hybrid molecules of tacrine and benzyl quinolone carboxylic acid (BQCA) as multifunctional agents for Alzheimer's disease

Novel tacrine-benzyl quinolone carboxylic acid (tacrine-BQCA) hybrids were designed based on multi-target directed ligands (MTLDs) paradigm, synthesized and evaluated in vitro as inhibitors of human acetylcholinesterase (hAChE) and human butyrylcholinesterase (hBChE). Tacrine moiety is represented herein as 7-methoxytacrine, 6-chlorotacrine or unsubstituted tacrine forming three different families of seven members, i.e. 21 compounds in overall. Introducing BQCA, a positive modulator of M1 muscarinic acetylcholine receptors (mAChRs), the action of novel compounds on M1 mAChRs was evaluated via Fluo-4 NW assay on the Chinese hamster ovarian (CHO-M1WT2) cell line. All the novel tacrine-BQCA hybrids were able to block the action of hAChE and hBChE in micromolar to nanomolar range. The hAChE kinetic profile of 5p was found to be mixed-type which is consistent with our docking experiments. Moreover, selected ligands were assessed for their potential hepatotoxicity on HepG2 cell line and presumable permeation through the blood-brain barrier by PAMPA assay. Expected agonistic profile towards M1 mAChRs delivered by BQCA moiety was not confirmed. From all the hybrids, 5o can be highlighted as non-selective cholinesterase inhibitor (hAChE IC₅₀ = 74.5 nM; hBChE IC₅₀ = 83.3 nM) with micromolar antagonistic activity towards M1 mAChR (IC₅₀ = 4.23 μM). A non-selective pattern of cholinesterase inhibition is likely to be valuable during the onset as well as later stages of AD.

Machine Learning-based Virtual Screening and Its Applications to Alzheimer's Drug Discovery: A Review

BACKGROUND

Virtual Screening (VS) has emerged as an important tool in the drug development process, as it conducts efficient in silico searches over millions of compounds, ultimately increasing yields of potential drug leads. As a subset of Artificial Intelligence (AI), Machine Learning (ML) is a powerful way of conducting VS for drug leads. ML for VS generally involves assembling a filtered training set of compounds, comprised of known actives and inactives. After training the model, it is validated and, if sufficiently accurate, used on previously unseen databases to screen for novel compounds with desired drug target binding activity.

OBJECTIVE

The study aims to review ML-based methods used for VS and applications to Alzheimer's Disease (AD) drug discovery.

METHODS

To update the current knowledge on ML for VS, we review thorough backgrounds, explanations, and VS applications of the following ML techniques: Naïve Bayes (NB), k-Nearest Neighbors (kNN), Support Vector Machines (SVM), Random Forests (RF), and Artificial Neural Networks (ANN).

RESULTS

All techniques have found success in VS, but the future of VS is likely to lean more largely toward the use of neural networks - and more specifically, Convolutional Neural Networks (CNN), which are a subset of ANN that utilize convolution. We additionally conceptualize a work flow for conducting ML-based VS for potential therapeutics for AD, a complex neurodegenerative disease with no known cure and prevention. This both serves as an example of how to apply the concepts introduced earlier in the review and as a potential workflow for future implementation.

CONCLUSION

Different ML techniques are powerful tools for VS, and they have advantages and disadvantages albeit. ML-based VS can be applied to AD drug development.

Acute Effects of Muscarinic M1 Receptor Modulation on AβPP Metabolism and Amyloid-β Levels in vivo: A Microdialysis Study

Indirect modulation of cholinergic activity by cholinesterase inhibition is currently a widely established symptomatic treatment for Alzheimer's disease (AD). Selective activation of certain muscarinic receptor subtypes has emerged as an alternative cholinergic-based amyloid-lowering strategy for AD, as selective muscarinic M1 receptor agonists can reduce amyloid-β (Aβ) production by shifting endoproteolytic amyloid-β protein precursor (AβPP) processing toward non-amyloidogenic pathways. In this study, we addressed the hypothesis that acute stimulation of muscarinic M1 receptors can inhibit Aβ production in awake and freely moving AβPP transgenic mice. By combining intracerebral microdialysis with retrodialysis, we determined hippocampal Aβ concentrations during simultaneous pharmacological modulation of brain M1 receptor function. Infusion with a M1 receptor agonist AF102B resulted in a rapid reduction of interstitial fluid (ISF) Aβ levels while treatment with the M1 antagonist dicyclomine increased ISF Aβ levels reaching significance within 120 minutes of treatment. The reduction in Aβ levels was associated with PKCα and ERK activation resulting in increased levels of the α-secretase ADAM17 and a shift in AβPP processing toward the non-amyloidogenic processing pathway. In contrast, treatment with the M1 receptor antagonist dicyclomine caused a decrease in levels of phosphorylated ERK that was independent of PKCα, and led to an elevation of β-secretase levels associated with increased amyloidogenic AβPP processing. The results of this study demonstrate rapid effects of in vivo M1 receptor modulation on the ISF pool of Aβ and suggest that intracerebral microdialysis with retrodialysis is a useful technical approach for monitoring acute treatment effects of muscarinic receptor modulators on AβPP/Aβ metabolism.

G Protein-Coupled Receptors (GPCRs) in Alzheimer's Disease: A Focus on BACE1 Related GPCRs

The G protein coupled receptors (GPCRs) have been considered as one of the largest families of validated drug targets, which involve in almost overall physiological functions and pathological processes. Meanwhile, Alzheimer’s disease (AD), the most common type of dementia, affects thinking, learning, memory and behavior of elderly people, that has become the hotspot nowadays for its increasing risks and incurability. The above fields have been intensively studied, and the link between the two has been demonstrated, whereas the way how GPCRs perturb AD progress are yet to be further explored given their complexities. In this review, we summarized recent progress regarding the GPCRs interacted with β-site APP cleaving enzyme 1 (BACE1), a key secretase in AD pathogenesis. Then we discussed the current findings on the regulatory roles of GPCRs on BACE1, and the possibility for pharmaceutical treatment of AD patients by the allosteric modulators and biased ligands of GPCRs. We hope this review can provide new insights into the understanding of mechanistic link between GPCRs and BACE1, and highlight the potential of GPCRs as therapeutic target for AD.

Noradrenergic sympathetic sprouting and cholinergic reinnervation maintains non-amyloidogenic processing of AβPP

Alzheimer's disease (AD) is characterized by amyloid-β (Aβ) plaques, hyperphosphorylated tau neurofibrillary tangles, and cholinergic dysfunction. Cholinergic degeneration can be mimicked in rats by lesioning medial septum cholinergic neurons. Hippocampal cholinergic denervation disrupts retrograde nerve growth factor (NGF) transport, leading to its accumulation, which subsequently triggers sprouting of noradrenergic sympathetic fibers from the superior cervical ganglia into hippocampus. Previously we reported that coincident with noradrenergic sprouting is the partial reinnervation of hippocampus with cholinergic fibers and the maintenance of a M1 muscarinic acetylcholine receptor (M1 mAChR) dependent long-term depression at CA3-CA1 synapses that is lost in the absence of sprouting. These findings suggest that sympathetic sprouting and the accompanying cholinergic reinnervation maintains M1 mAChR function. Importantly, noradrenergic sympathetic and cholinergic sprouting have been demonstrated in human postmortem AD hippocampus. Furthermore, M1 mAChRs are a recent focus as a therapeutic target for AD given their role in cognition and non-amyloidogenic processing of amyloid-β protein precursor (AβPP). Here we tested the hypotheses that noradrenergic sympathetic sprouting is triggered by NGF, that sprouting maintains non-amyloidogenic AβPP processing, and that sprouting is prevented by intrahippocampal Aβ42 infusion. We found that NGF stimulates sprouting, that sprouting maintains non-amyloidogenic AβPP processing, and that Aβ42 is not only toxic to central cholinergic fibers innervating hippocampus but it prevents and reverses noradrenergic sympathetic sprouting and the accompanying cholinergic reinnervation. These findings reiterate the clinical implications of sprouting as an innate compensatory mechanism and emphasize the importance of M1 mAChRs as an AD therapeutic target.

M1 muscarinic acetylcholine receptor in Alzheimer's disease

The degeneration of cholinergic neurons and cholinergic hypofunction are pathologies associated with Alzheimer's disease (AD). Muscarinic acetylcholine receptors (mAChRs) mediate acetylcholine-induced neurotransmission and five mAChR subtypes (M1-M5) have been identified. Among them, M1 mAChR is widely expressed in the central nervous system and has been implicated in many physiological and pathological brain functions. In addition, M1 mAChR is postulated to be an important therapeutic target for AD and several other neurodegenerative diseases. In this article, we review recent progress in understanding the functional involvement of M1 mAChR in AD pathology and in developing M1 mAChR agonists for AD treatment.

Mechanisms governing the reactivation-dependent destabilization of memories and their role in extinction

The extinction of learned associations has traditionally been considered to involve new learning, which competes with the original memory for control over behavior. However, a recent resurgence of interest in reactivation-dependent amnesia has revealed that the retrieval of fear-related memory (with what is essentially a brief extinction session) can result in its destabilization. This review discusses some of the cellular and molecular mechanisms that are involved in the destabilization of a memory following its reactivation and/or extinction, and investigates the evidence that extinction may involve both new learning as well as a partial destabilization-induced erasure of the original memory trace.

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.

Targeting selective activation of M(1) for the treatment of Alzheimer's disease: further chemical optimization and pharmacological characterization of the M(1) positive allosteric modulator ML169

The M(1) muscarinic acetylcholine receptor is thought to play an important role in memory and cognition, making it a potential target for the treatment of Alzheimer's disease (AD) and schizophrenia. Moreover, M(1) interacts with BACE1 and regulates its proteosomal degradation, suggesting selective M(1) activation could afford both palliative cognitive benefit as well as disease modification in AD. A key challenge in targeting the muscarinic acetylcholine receptors is achieving mAChR subtype selectivity. Our lab has previously reported the M(1) selective positive allosteric modulator ML169. Herein we describe our efforts to further optimize this lead compound by preparing analogue libraries and probing novel scaffolds. We were able to identify several analogues that possessed submicromolar potency, with our best example displaying an EC(50) of 310 nM. The new compounds maintained complete selectivity for the M(1) receptor over the other subtypes (M(2)-M(5)), displayed improved DMPK profiles, and potentiated the carbachol (CCh)-induced excitation in striatal MSNs. Selected analogues were able to potentiate CCh-mediated nonamyloidogenic APPsα release, further strengthening the concept that M(1) PAMs may afford a disease-modifying role in the treatment of AD.