β-arrestin 2 regulates Aβ generation and γ-secretase activity in Alzheimer's disease

We are MedChem: The Frontiers in Medicinal Chemistry 2024

The Frontiers in Medicinal Chemistry (FiMC) is the largest international Medicinal Chemistry conference in Germany and took place from March 17th to 20th 2024 in Munich. Co-organized by the Division of Medicinal Chemistry of the German Chemical Society (Gesellschaft Deutscher Chemiker; GDCh) and the Division of Pharmaceutical and Medicinal Chemistry of the German Pharmaceutical Society (Deutsche Pharmazeutische Gesellschaft; DPhG), and supported by a local organizing committee from the Ludwigs-Maximilians-University Munich headed by Daniel Merk, the meeting brought together approximately 225 participants from 20 countries. The outstanding program of the four-day conference included 40 lectures by leading scientists from industry and academia as well as early career investigators. Moreover, 100 posters were presented in two highly interactive poster sessions.

GPCR-mediated natural products and compounds: Potential therapeutic targets for the treatment of neurological diseases

G protein-coupled receptors (GPCRs), widely expressed in the human central nervous system (CNS), perform numerous physiological functions and play a significant role in the pathogenesis of diseases. Consequently, identifying key therapeutic GPCRs targets for CNS-related diseases is garnering immense interest in research labs and pharmaceutical companies. However, using GPCRs drugs for treating neurodegenerative diseases has limitations, including side effects and uncertain effective time frame. Recognizing the rich history of herbal treatments for neurological disorders like stroke, Alzheimer's disease (AD), and Parkinson's disease (PD), modern pharmacological research is now focusing on the understanding of the efficacy of traditional Chinese medicinal herbs and compounds in modulating GPCRs and treatment of neurodegenerative conditions. This paper will offer a comprehensive, critical review of how certain natural products and compounds target GPCRs to treat neurological diseases. Conducting an in-depth study of herbal remedies and their efficacies against CNS-related disorders through GPCRs targeting will augment our strategies for treating neurological disorders. This will not only broaden our understanding of effective therapeutic methodologies but also identify the root causes of altered GPCRs signaling in the context of pathophysiological mechanisms in neurological diseases. Moreover, it would be informative for the creation of safer and more effective GPCR-mediated drugs, thereby establishing a foundation for future treatment of various neurological diseases.

β-arrestin2 is indispensable for the antidepressant effects of fluoxetine via inhibiting astrocytic pyroptosis in chronic mild stress mouse model for depression

β-arrestin2 is a versatile protein for signaling transduction in brain physiology and pathology. Herein, we investigated the involvement of β-arrestin2 in pharmacological effects of fluoxetine for depression. A chronic mild stress (CMS) model was established using wild-type (WT) and β-arrestin2-/- mice. Behavioral results demonstrated that CMS mice showed increased immobility time in the tail suspension test and forced swimming test, elevated concentrations of pro-inflammatory factors in peripheral blood, increased expression of pyroptosis-related proteins, and increased co-labeling of glial fibrillary acidic protein and Caspase1 p10 in the hippocampus compared to the CON group. Treatment with fluoxetine (FLX) ameliorated these conditions. However, compared with the β-arrestin2-/- CMS group, these results of the β-arrestin2-/- CMS + FLX group showed no significant changes. These results suggested that the above effects of FLX could be eliminated by knocking out β-arrestin2. Mass spectrometry implying that FLX promoted the binding of β-arrestin2 to the NLRP2 inflammasome of depressed mice. Subsequently, the results of the cellular experiments suggested that the 5HT2B receptor antagonist may attenuate L-kynurenine + ATP-induced cell pyroptosis by attenuating NLRP2 binding to β-arrestin2. We further found that the lack of β-arrestin2 eliminated the anti-pyroptosis effect of fluoxetine. In conclusion, β-arrestin2 is an essential protein for fluoxetine to alleviate pyroptosis in the hippocampal astrocytes of CMS mice. Mechanistically, we found that the 5-HT2BR-β-arrestin2-NLRP2 axis is vital for maintaining the antidepressant effects of fluoxetine.

Small Animal Models to Study Herpes Simplex Virus Infections

Herpes simplex virus type 1 (HSV-1) and herpes simplex virus type 2 (HSV-2) are two of the most prevalent human viruses worldwide. They are known to cause a variety of diseases including genital herpes, meningitis, encephalitis, cold sores and herpes stromal keratitis. The seropositive rate for HSV-1 is around 90%, whereas for HSV-2 it remains around 20-25% for the general adult population. The infections caused by these viruses remain difficult to study because a large proportion of infected individuals are asymptomatic. Furthermore, given the neurotropic characteristics of the virus, studies aimed at understanding the complex pathogenesis in humans is difficult. As a result, animal models have been developed to understand several characteristics of HSV biology, pathogenesis, disease and host responses to infection. These models are also commonly used as the first evaluation of new drugs and vaccines. There are several well-established animal models to study infection with HSV, including mice, guinea pigs and rabbits. Variables within the animal models depend on the species of animal, route of infection, viral strain, dosage, etc. This review aims at summarizing the most commonly used animal models to study HSV pathogenesis and therapies.

Exploring orphan GPCRs in neurodegenerative diseases

Neurodegenerative disorders represent a significant and growing health burden worldwide. Unfortunately, limited therapeutic options are currently available despite ongoing efforts. Over the past decades, research efforts have increasingly focused on understanding the molecular mechanisms underlying these devastating conditions. Orphan receptors, a class of receptors with no known endogenous ligands, emerge as promising druggable targets for diverse diseases. This review aims to direct attention to a subgroup of orphan GPCRs, in particular class A orphans that have roles in neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease, Huntington's disease, and Multiple sclerosis. We highlight the diverse roles orphan receptors play in regulating critical cellular processes such as synaptic transmission, neuronal survival and neuro-inflammation. Moreover, we discuss the therapeutic potential of targeting orphan receptors for the treatment of neurodegenerative disorders, emphasizing recent advances in drug discovery and preclinical studies. Finally, we outline future directions and challenges in orphan receptor research.

ERNEST COST action overview on the (patho)physiology of GPCRs and orphan GPCRs in the nervous system

G protein-coupled receptors (GPCRs) are a large family of cell surface receptors that play a critical role in nervous system function by transmitting signals between cells and their environment. They are involved in many, if not all, nervous system processes, and their dysfunction has been linked to various neurological disorders representing important drug targets. This overview emphasises the GPCRs of the nervous system, which are the research focus of the members of ERNEST COST action (CA18133) working group 'Biological roles of signal transduction'. First, the (patho)physiological role of the nervous system GPCRs in the modulation of synapse function is discussed. We then debate the (patho)physiology and pharmacology of opioid, acetylcholine, chemokine, melatonin and adhesion GPCRs in the nervous system. Finally, we address the orphan GPCRs, their implication in the nervous system function and disease, and the challenges that need to be addressed to deorphanize them.

Adrenergic receptors blockade alleviates dexamethasone-induced neurotoxicity in adult male Wistar rats: Distinct effects on β-arrestin2 expression and molecular markers of neural injury

BACKGROUND

Dexamethasone-induced neurotoxicity has been previously reported. However, the molecular mechanisms are still not completely understood.

OBJECTIVES

The current work aimed to investigate the modulatory effects of α- and β-adrenergic receptors on dexamethasone-induced neurotoxicity in rats focused on changes in β-arrestin2 and molecular markers of neural injury in cerebral cortex.

METHODS

Male Wistar rats were subcutaneously injected with dexamethasone (10 mg/kg/day) for 7 days to induce neural injury in the cerebral cortex. The experiment involved 5 groups: control, dexamethasone, carvedilol, propranolol, and doxazosin. In the last 3 groups, drugs were given 2 hours before dexamethasone injection. At the end of experiment, brain samples were collected for measurement of brain derived neurotrophic factor (BDNF), glial fibrillary acidic protein (GFAP), kinase activity of protein kinase B (Akt), diacylglycerol (DAG), α-smooth muscle actin (α-SMA), Smad3, β-amyloid and phospho-tau protein levels in addition to histopathological examination of brain tissue using hematoxylin-eosin, Nissl, and Sirius red stains. Moreover, β-arrestin2 levels in the cerebral cortex were measured using immunohistochemical examination.

RESULTS

Dexamethasone slightly reduced brain weight and significantly decreased BDNF, Akt kinase activity and β-arrestin2 but markedly induced degeneration of cortical neurons and significantly increased GFAP, DAG, α-SMA, Smad3, β-amyloid and phospho-tau protein levels compared to controls. Carvedilol, propranolol, and doxazosin reversed all dexamethasone-induced molecular changes and slightly ameliorated the histopathological changes. Carvedilol significantly increased brain weight and β-arrestin2 levels compared to dexamethasone, propranolol, and doxazosin groups.

CONCLUSION

blocking α- and/or β-adrenergic receptors alleviate dexamethasone-induced neurotoxicity despite their distinct effects on β-arrestin2 levels in the cerebral cortex.

Distinct roles of the extracellular surface residues of glucagon-like peptide-1 receptor in β-arrestin 1/2 signaling

Glucagon-like peptide-1 receptor (GLP-1R) is a prime drug target for type 2 diabetes and obesity. The ligand initiated GLP-1R interaction with G protein has been well studied, but not with β-arrestin 1/2. Therefore, bioluminescence resonance energy transfer (BRET), mutagenesis and an operational model were used to evaluate the roles of 85 extracellular surface residues on GLP-1R in β-arrestin 1/2 recruitment triggered by three representative GLP-1R agonists (GLP-1, exendin-4 and oxyntomodulin). Residues selectively regulated β-arrestin 1/2 recruitment for diverse ligands, and β-arrestin isoforms were identified. Mutation of residues K130-S136, L142 and Y145 on the transmembrane helix 1 (TM1)-extracellular domain (ECD) linker decreased β-arrestin 1 recruitment but increased β-arrestin 2 recruitment. Other extracellular loop (ECL) mutations, including P137A, Q211A, D222A and M303A selectively affected β-arrestin 1 recruitment while D215A, L217A, Q221A, S223A, Y289A, S301A, F381A and I382A involved more in β-arrestin 2 recruitment for the ligands. Oxyntomodulin engaged more broadly with GLP-1R extracellular surface to drive β-arrestin 1/2 recruitment than GLP-1 and exendin-4; I147, W214 and L218 involved in β-arrestin 1 recruitment, while L141, D215, L218, D293 and F381 in β-arrestin 2 recruitment for oxyntomodulin particularly. Additionally, the non-conserved residues on β-arrestin 1/2 C-domains contributed to interaction with GLP-1R. Further proteomic profiling of GLP-1R stably expressed cell line upon ligand stimulation with or without β-arrestin 1/2 overexpression demonstrated both commonly and biasedly regulated proteins and pathways associated with cognate ligands and β-arrestins. Our study offers valuable information about ligand induced β-arrestin recruitment mediated by GLP-1R and consequent intracellular signaling events.

New precision medicine avenues to the prevention of Alzheimer's disease from insights into the structure and function of γ-secretases

Two phase-III clinical trials with anti-amyloid peptide antibodies have met their primary goal, i.e. slowing of Alzheimer's disease (AD) progression. However, antibody therapy may not be the optimal therapeutic modality for AD prevention, as we will discuss in the context of the earlier small molecules described as "γ-secretase modulators" (GSM). We review here the structure, function, and pathobiology of γ-secretases, with a focus on how mutations in presenilin genes result in early-onset AD. Significant progress has been made in generating compounds that act in a manner opposite to pathogenic presenilin mutations: they stabilize the proteinase-substrate complex, thereby increasing the processivity of substrate cleavage and altering the size spectrum of Aβ peptides produced. We propose the term "γ-secretase allosteric stabilizers" (GSAS) to distinguish these compounds from the rather heterogenous class of GSM. The GSAS represent, in theory, a precision medicine approach to the prevention of amyloid deposition, as they specifically target a discrete aspect in a complex cell biological signalling mechanism that initiates the pathological processes leading to Alzheimer's disease.

The multifaceted functions of β-arrestins and their therapeutic potential in neurodegenerative diseases

Arrestins are multifunctional proteins that regulate G-protein-coupled receptor (GPCR) desensitization, signaling, and internalization. The arrestin family consists of four subtypes: visual arrestin1, β-arrestin1, β-arrestin2, and visual arrestin-4. Recent studies have revealed the multifunctional roles of β-arrestins beyond GPCR signaling, including scaffolding and adapter functions, and physically interacting with non-GPCR receptors. Increasing evidence suggests that β-arrestins are involved in the pathogenesis of a variety of neurodegenerative diseases, including Alzheimer's disease (AD), frontotemporal dementia (FTD), and Parkinson's disease (PD). β-arrestins physically interact with γ-secretase, leading to increased production and accumulation of amyloid-beta in AD. Furthermore, β-arrestin oligomers inhibit the autophagy cargo receptor p62/SQSTM1, resulting in tau accumulation and aggregation in FTD. In PD, β-arrestins are upregulated in postmortem brain tissue and an MPTP model, and the β2AR regulates SNCA gene expression. In this review, we aim to provide an overview of β-arrestin1 and β-arrestin2, and describe their physiological functions and roles in neurodegenerative diseases. The multifaceted roles of β-arrestins and their involvement in neurodegenerative diseases suggest that they may serve as promising therapeutic targets.

Activation of GPR3-β-arrestin2-PKM2 pathway in Kupffer cells stimulates glycolysis and inhibits obesity and liver pathogenesis

Kupffer cells are liver resident macrophages and play critical role in fatty liver disease, yet the underlying mechanisms remain unclear. Here, we show that activation of G-protein coupled receptor 3 (GPR3) in Kupffer cells stimulates glycolysis and protects mice from obesity and fatty liver disease. GPR3 activation induces a rapid increase in glycolysis via formation of complexes between β-arrestin2 and key glycolytic enzymes as well as sustained increase in glycolysis through transcription of glycolytic genes. In mice, GPR3 activation in Kupffer cells results in enhanced glycolysis, reduced inflammation and inhibition of high-fat diet induced obesity and liver pathogenesis. In human fatty liver biopsies, GPR3 activation increases expression of glycolytic genes and reduces expression of inflammatory genes in a population of disease-associated macrophages. These findings identify GPR3 activation as a pivotal mechanism for metabolic reprogramming of Kupffer cells and as a potential approach for treating fatty liver disease.

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.

Neurocognitive Effect of Biased µ-Opioid Receptor Agonist Oliceridine, a Utility Function Analysis and Comparison with Morphine

BACKGROUND

Oliceridine (Olinvyk) is a μ-opioid receptor agonist that in contrast to conventional opioids preferentially engages the G-protein-coupled signaling pathway. This study was designed to determine the utility function of oliceridine versus morphine based on neurocognitive tests and cold pressor test.

METHODS

The study had a randomized, double-blind, placebo-controlled, partial block three-way crossover design. Experiments were performed in 20 male and female volunteers. The subjects received intravenous oliceridine (1 or 3 mg; cohorts of 10 subjects/dose), morphine (5 or 10 mg; cohorts of 10 subjects/dose), or placebo on three separate occasions. Before and after dosing, neurocognitive tests, cold pressor test, and plasma drug concentrations were obtained at regular intervals. Population pharmacokinetic-pharmacodynamic analyses served as the basis for construction of a utility function, which is an objective function of probability of benefit minus probability of harm. Antinociception served as the measure of benefit, and slowing of saccadic peak velocity and increased body sway as the measures of neurocognitive harm.

RESULTS

The oliceridine and morphine C50 values, i.e., the effect-site concentrations causing 50% effect, were as follows: antinociception, 13 ± 2 and 23 ± 7 ng/ml; saccadic peak velocity, 90 ± 14 and 54 ± 15 ng/ml; and body sway, 10 ± 2 and 5.6 ± 0.8 ng/ml, respectively. The ratio oliceridine/morphine of the therapeutic indices, C50(benefit)/C50(harm), were 0.34 (95% CI, 0.17 to 0.7; P < 0.01) for saccadic peak velocity and 0.33 (0.16 to 0.50; P < 0.01) for body sway. The oliceridine utility was positive across the effect-site concentration 5 to 77 ng/ml, indicative of a greater probability of benefit than harm. The morphine utility was not significantly different from 0 from 0 to 100 ng/ml. Over the concentration range 15 to 50 ng/ml, the oliceridine utility was superior to that of morphine (P < 0.01). Similar observations were made for body sway.

CONCLUSIONS

These data indicate that over the clinical concentration range, oliceridine is an analgesic with a favorable safety profile over morphine when considering analgesia and neurocognitive function.

EDITOR’S PERSPECTIVE

Development of Fluorescent AF64394 Analogues Enables Real-Time Binding Studies for the Orphan Class A GPCR GPR3

The orphan G protein-coupled receptor (oGPCR) GPR3 represents a potential drug target for the treatment of Alzheimer's disease and metabolic disorders. However, the limited toolbox of pharmacological assays hampers the development of advanced ligands. Here, we developed a signaling pathway-independent readout of compound-GPR3 interaction. Starting from computational binding pose predictions of the most potent GPR3 ligand, we designed a series of fluorescent AF64394 analogues and assessed their suitability for BRET-based binding studies. The most potent ligand, 45 (UR-MB-355), bound to GPR3 and closely related receptors, GPR6 and GPR12, with similar submicromolar affinities. Furthermore, we found that 45 engages GPR3 in a distinct mode compared to AF64394, and coincubation studies with the GPR3 agonist diphenyleneiodonium chloride revealed allosteric modulation of 45 binding. These insights provide new cues for the pharmacological manipulation of GPR3 activity. This novel binding assay will foster the development of future drugs acting through these pharmacologically attractive oGPCRs.

The development of diphenyleneiodonium analogs as GPR3 agonists

G protein-coupled receptor 3 (GPR3) is an orphan receptor potentially involved in many important physiological processes such as drug abuse, neuropathic pain, and anxiety and depression related disorders. Pharmacological studies of GPR3 have been limited due to the restricted number of known agonists and inverse agonists for this constitutively active receptor. In this medicinal chemistry study, we report the discovery of GPR3 agonists based off the diphenyleneiodonium (DPI) scaffold. The most potent full agonist was the 3-trifluoromethoxy analog (32) with an EC50 of 260 nM and 90% efficacy compared to DPI. Investigation of a homology model of GPR3 from multiple sequence alignment resulted in the finding of a binding site rich in potential π-π and π-cation interactions stabilizing DPI-scaffold agonists. MMGBSA free energy analysis showed a good correlation with trends in observed EC50s. DPI analogs retained the same high receptor selectivity for GPR3 over GPR6 and GPR12 as observed with DPI. Collectively, the DPI analog series shows that order of magnitude improvements in potency with the scaffold were attainable; however, attempts to replace the iodonium ion to make the scaffold more druggable failed.

Characterization of behavioral changes in T-maze alternation from dopamine D1 agonists with different receptor coupling mechanisms

RATIONALE

Dopamine D1 receptor agonists have been shown to improve working memory, but often have a non-monotonic (inverted-U) dose-response curve. One hypothesis is that this may reflect dose-dependent differential engagement of D1 signaling pathways, a mechanism termed functional selectivity or signaling bias.

OBJECTIVES AND METHODS

To test this hypothesis, we compared two D1 ligands with different signaling biases in a rodent T-maze alternation task. Both tested ligands (2-methyldihydrexidine and CY208243) have high intrinsic activity at cAMP signaling, but the former also has markedly higher intrinsic activity at D1-mediated recruitment of β-arrestin. The spatial working memory was assessed via the alternation behavior in the T-maze where the alternate choice rate quantified the quality of the memory and the duration prior to making a choice represented the decision latency.

RESULTS

Both D1 drugs changed the alternate rate and the choice latency in a dose-dependent manner, albeit with important differences. 2-Methyldihydrexidine was somewhat less potent but caused a more homogeneous improvement than CY208243 in spatial working memory. The maximum changes in the alternate rate and the choice latency tended to occur at different doses for both drugs.

CONCLUSIONS

These data suggest that D1 signaling bias in these two pathways (cAMP vs β-arrestin) has complex effects on cognitive processes as assessed by T-maze alternation. Understanding these mechanisms should allow the identification or discovery of D1 agonists that can provide superior cognitive enhancement.

Astrocytic AT1R deficiency ameliorates Aβ-induced cognitive deficits and synaptotoxicity through β-arrestin2 signaling

Alzheimer's disease (AD) seriously influences human health, and there is no effective treatment to prevent or cure AD. Recent studies have shown that angiotensin II type 1 receptor (AT1R) blockers significantly reduce the prevalence of AD, while the precise role and mechanism of AT1R in AD remain obscure. In this study, for the first time, we identified that astrocytic but not neuronal AT1R levels were significantly increased in AD model rats and found that astrocyte-specific knockout of AT1R significantly ameliorated amyloid β (Aβ)-induced cognitive deficits and synaptotoxicity. Pretreating astrocytes with an AT1R blocker also alleviated Aβ-induced synaptotoxicity in the coculture system of hippocampal neurons and astrocytes. Moreover, AT1R could directly bind to Aβ1-42 and activate the astrocytic β-arrestin2 pathway in a biased manner, and biased inhibition of the astrocytic AT1R/β-arrestin2 pathway relieved Aβ-induced neurotoxicity. Furthermore, we demonstrated that astrocytic AT1R/β-arrestin2 pathway-mediated synaptotoxicity was associated with the aggregation of autophagosomes, which triggered the disordered degradation of Aβ. Our findings reveal a novel molecular mechanism of astrocytic AT1R in Aβ-induced neurodegeneration and might contribute to establishing new targets for AD prevention and therapy.

Suvorexant Acutely Decreases Tau Phosphorylation and Aβ in the Human CNS

OBJECTIVE

In Alzheimer's disease, hyperphosphorylated tau is associated with formation of insoluble paired helical filaments that aggregate as neurofibrillary tau tangles and are associated with neuronal loss and cognitive symptoms. Dual orexin receptor antagonists decrease soluble amyloid-β levels and amyloid plaques in mouse models overexpressing amyloid-β, but have not been reported to affect tau phosphorylation. In this randomized controlled trial, we tested the acute effect of suvorexant, a dual orexin receptor antagonist, on amyloid-β, tau, and phospho-tau.

METHODS

Thirty-eight cognitively unimpaired participants aged 45 to 65 years were randomized to placebo (N = 13), suvorexant 10 mg (N = 13), and suvorexant 20 mg (N = 12). Six milliliters of cerebrospinal fluid were collected via an indwelling lumbar catheter every 2 hours for 36 hours starting at 20:00. Participants received placebo or suvorexant at 21:00. All samples were processed and measured for multiple forms of amyloid-β, tau, and phospho-tau via immunoprecipitation and liquid chromatography-mass spectrometry.

RESULTS

The ratio of phosphorylated-tau-threonine-181 to unphosphorylated-tau-threonine-181, a measure of phosphorylation at this tau phosphosite, decreased ~10% to 15% in participants treated with suvorexant 20 mg compared to placebo. However, phosphorylation at tau-serine-202 and tau-threonine-217 were not decreased by suvorexant. Suvorexant decreased amyloid-β ~10% to 20% compared to placebo starting 5 hours after drug administration.

INTERPRETATION

In this study, suvorexant acutely decreased tau phosphorylation and amyloid-β concentrations in the central nervous system. Suvorexant is approved by the US Food and Drug Administration to treatment insomnia and may have potential as a repurposed drug for the prevention of Alzheimer's disease, however, future studies with chronic treatment are needed. ANN NEUROL 2023;94:27-40.

Sex-dependent effects of acute stress on amyloid-β in male and female mice

The risk of developing Alzheimer's disease is mediated by a combination of genetics and environmental factors, such as stress, sleep abnormalities and traumatic brain injury. Women are at a higher risk of developing Alzheimer's disease than men, even when controlling for differences in lifespan. Women are also more likely to report high levels of stress than men. Sex differences in response to stress may play a role in the increased risk of Alzheimer's disease in women. In this study, we use in vivo microdialysis to measure levels of Aβ in response to acute stress in male and female mice. We show that Aβ levels are altered differently between female and male mice (APP/PS1 and wild-type) in response to stress, with females showing significantly increased levels of Aβ while most males do not show a significant change. This response is mediated through β-arrestin involvement in Corticotrophin Releasing Factor receptor signalling pathway differences in male and female mice as male mice lacking β-arrestin show increase in Aβ in response to stress similar to females.

β-arrestin1 regulates astrocytic reactivity via Drp1-dependent mitochondrial fission: implications in postoperative delirium

Postoperative delirium (POD) is a frequent and debilitating complication, especially amongst high risk procedures, such as orthopedic surgery. This kind of neurocognitive disorder negatively affects cognitive domains, such as memory, awareness, attention, and concentration after surgery; however, its pathophysiology remains unknown. Multiple lines of evidence supporting the occurrence of inflammatory events have come forward from studies in human patients' brain and bio-fluids (CSF and serum), as well as in animal models for POD. β-arrestins are downstream molecules of guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs). As versatile proteins, they regulate numerous pathophysiological processes of inflammatory diseases by scaffolding with inflammation-linked partners. Here we report that β-arrestin1, one type of β-arrestins, decreases significantly in the reactive astrocytes of a mouse model for POD. Using β-arrestin1 knockout (KO) mice, we find aggravating effect of β-arrestin1 deficiency on the cognitive dysfunctions and inflammatory phenotype of astrocytes in POD model mice. We conduct the in vitro experiments to investigate the regulatory roles of β-arrestin1 and demonstrate that β-arrestin1 in astrocytes interacts with the dynamin-related protein 1 (Drp1) to regulate mitochondrial fusion/fission process. β-arrestin1 deletion cancels the combination of β-arrestin1 and cellular Drp1, thus promoting the translocation of Drp1 to mitochondrial membrane to provoke the mitochondrial fragments and the subsequent mitochondrial malfunctions. Using β-arrestin1-biased agonist, cognitive dysfunctions of POD mice and pathogenic activation of astrocytes in the POD-linked brain region are reduced. We, therefore, conclude that β-arrestin1 is a promising target for the understanding of POD pathology and development of POD therapeutics.

The role of opioid receptors in modulating Alzheimer's Disease

Alzheimer's disease (AD) is a complex neurological disorder characterized by accumulation of amyloid plaques and neurofibrillary tangles. Long term investigation of AD pathogenesis suggests that β-site amyloid precursor protein [APP] cleaving enzyme 1 (BACE1) and γ-secretase enzymes promote the amyloidogenic pathway and produce toxic Aβ peptides that are predisposed to aggregate in the brain. Hence, the targeted inhibition of BACE1/γ-secretase expression and function is a promising approach for AD therapy. Several reports have suggested that the opioid family of G-protein coupled receptors modulate the etiology of AD progression. It has also been found that changes in the signaling pathways of opioid receptors increased the expression of BACE1 and γ-secretase, and is strongly correlated with abnormal production of Aβ and pathogenesis of AD. Thus, the opioid receptor family is a promising candidate for targeted drug development to treat AD. In this review, we outline the involvement and mechanisms of opioid receptor signaling modulation in Alzheimer's Disease progression.

Deletion of Arrb2 Down-regulates Autophagy in the Mouse Hippocampus via Akt-mTOR Pathway Activation

The cytoplasmic multifunctional adaptor protein β-arrestin 2 (Arrb2) is involved in the occurrence of various nervous system diseases, such as Alzheimer's disease and Parkinson's disease. Previous laboratory studies have shown that the expression and function of the Arrb2 gene was increased in valproic acid-induced autistic mice models. However, few reports have examined the possible role of Arrb2 in the pathogenesis of autism spectrum disorder. Therefore, Arrb2-deficient (Arrb2^-/-) mice were further studied to uncover the physiological function of Arrb2 in the nervous system. In this study, we found that Arrb2^-/- mice had normal behavioral characteristics compared with wild-type mice. The autophagy marker protein LC3B was decreased in the hippocampus of Arrb2^-/- mice compared to wild-type mice. Western blot analysis revealed that deletion of Arrb2 caused hyperactivation of Akt-mTOR signaling in the hippocampus. In addition, abnormal mitochondrial dysfunction was observed in Arrb2^-/- hippocampal neurons, which was characterized by a reduction in mitochondrial membrane potential and adenosine triphosphate production and an increase in reactive oxygen species levels. Therefore, this study elucidates the interaction between Arrb2 and the Akt-mTOR signaling pathway and provides insights into the role of Arrb2 in hippocampal neuron autophagy.

Interactions between β-arrestin proteins and the cytoskeletal system, and their relevance to neurodegenerative disorders

β-arrestins, which have multiple cellular functions, were initially described as proteins that desensitize rhodopsin and other G protein-coupled receptors. The cytoskeletal system plays a role in various cellular processes, including intracellular transport, cell division, organization of organelles, and cell cycle. The interactome of β-arrestins includes the major proteins of the three main cytoskeletal systems: tubulins for microtubules, actins for the actin filaments, and vimentin for intermediate filaments. β-arrestins bind to microtubules and regulate their activity by recruiting signaling proteins and interacting with assembly proteins that regulate the actin cytoskeleton and the intermediate filaments. Altered regulation of the cytoskeletal system plays an essential role in the development of Alzheimer's, Parkinson's and other neurodegenerative diseases. Thus, β-arrestins, which interact with the cytoskeleton, were implicated in the pathogenesis progression of these diseases and are potential targets for the treatment of neurodegenerative disorders in the future.

G protein-biased GPR3 signaling ameliorates amyloid pathology in a preclinical Alzheimer's disease mouse model

Biased G protein-coupled receptor (GPCR) ligands, which preferentially activate G protein or β-arrestin signaling pathways, are leading to the development of drugs with superior efficacy and reduced side effects in heart disease, pain management, and neuropsychiatric disorders. Although GPCRs are implicated in the pathophysiology of Alzheimer's disease (AD), biased GPCR signaling is a largely unexplored area of investigation in AD. Our previous work demonstrated that GPR3-mediated β-arrestin signaling modulates amyloid-β (Aβ) generation in vitro and that Gpr3 deficiency ameliorates Aβ pathology in vivo. However, Gpr3-deficient mice display several adverse phenotypes, including elevated anxiety-like behavior, reduced fertility, and memory impairment, which are potentially associated with impaired G protein signaling. Here, we generated a G protein-biased GPR3 mouse model to investigate the physiological and pathophysiological consequences of selective elimination of GPR3-mediated β-arrestin signaling in vivo. In contrast to Gpr3-deficient mice, G protein-biased GPR3 mice do not display elevated anxiety levels, reduced fertility, or cognitive impairment. We further determined that G protein-biased signaling reduces soluble Aβ levels and leads to a decrease in the area and compaction of amyloid plaques in the preclinical AppNL-G-F AD mouse model. The changes in amyloid pathology are accompanied by robust microglial and astrocytic hypertrophy, which suggest a protective glial response that may limit amyloid plaque development in G protein-biased GPR3 AD mice. Collectively, these studies indicate that GPR3-mediated G protein and β-arrestin signaling produce discrete and separable effects and provide proof of concept for the development of safer GPCR-targeting therapeutics with more directed pharmacological action for AD.

Plasma Exo-miRNAs Correlated with AD-Related Factors of Chinese Individuals Involved in Aβ Accumulation and Cognition Decline

Numerous studies have investigated the risk factors of Alzheimer’s disease (AD); however, AD-risk factors related miRNAs were rarely reported. In this study, AD-risk factor related miRNAs of 105 Chinese individuals (45 AD patients and 60 cognitively normal controls) were investigated. The results showed that Hsa-miR-185-5p, Hsa-miR-20a-5p, and Hsa-miR-497-5p were related to AD and education, Hsa-miR-185-5p, Hsa-miR-181c-5p, Hsa-miR-664a-3p, Hsa-miR-27a-3p, Hsa-miR-451a, and Hsa-miR-320a were related to AD and depression. Target prediction of above miRNAs showed that these miRNAs were involved in the generation and clearance of amyloid-beta (Aβ), important molecules related to cognition, and disease-activated microglia response to AD. It is worth noting that Hsa-miR-185-5p was related to both education and depression, whose decreased expression pattern in AD patients was alleviated by education and enhanced by depression, and participates in Aβ generation and accumulation. Our results indicated that certain education and depression factors can contribute to AD progression by modulating miRNA expression, implying that preventive interventions might alter AD progression in Chinese patients.

GPCR kinases generate an APH1A phosphorylation barcode to regulate amyloid-β generation

Emerging evidence suggests that G protein-coupled receptor (GPCR) kinases (GRKs) are associated with the pathophysiology of Alzheimer's disease (AD). However, GRKs have not been directly implicated in regulation of the amyloid-β (Aβ) pathogenic cascade in AD. Here, we determine that GRKs phosphorylate a non-canonical substrate, anterior pharynx-defective 1A (APH1A), an integral component of the γ-secretase complex. Significantly, we show that GRKs generate distinct phosphorylation barcodes in intracellular loop 2 (ICL2) and the C terminus of APH1A, which differentially regulate recruitment of the scaffolding protein β-arrestin 2 (βarr2) to APH1A and γ-secretase-mediated Aβ generation. Further molecular dynamics simulation studies reveal an interaction between the βarr2 finger loop domain and ICL2 and ICL3 of APH1A, similar to a GPCR-β-arrestin complex, which regulates γ-secretase activity. Collectively, these studies provide insight into the molecular and structural determinants of the APH1A-βarr2 interaction that critically regulate Aβ generation.

Understanding How Physical Exercise Improves Alzheimer’s Disease: Cholinergic and Monoaminergic Systems

Alzheimer’s disease (AD) is an age-related neurodegenerative disorder, characterized by the accumulation of proteinaceous aggregates and neurofibrillary lesions composed of β-amyloid (Aβ) peptide and hyperphosphorylated microtubule-associated protein tau, respectively. It has long been known that dysregulation of cholinergic and monoaminergic (i.e., dopaminergic, serotoninergic, and noradrenergic) systems is involved in the pathogenesis of AD. Abnormalities in neuronal activity, neurotransmitter signaling input, and receptor function exaggerate Aβ deposition and tau hyperphosphorylation. Maintenance of normal neurotransmission is essential to halt AD progression. Most neurotransmitters and neurotransmitter-related drugs modulate the pathology of AD and improve cognitive function through G protein-coupled receptors (GPCRs). Exercise therapies provide an important alternative or adjunctive intervention for AD. Cumulative evidence indicates that exercise can prevent multiple pathological features found in AD and improve cognitive function through delaying the degeneration of cholinergic and monoaminergic neurons; increasing levels of acetylcholine, norepinephrine, serotonin, and dopamine; and modulating the activity of certain neurotransmitter-related GPCRs. Emerging insights into the mechanistic links among exercise, the neurotransmitter system, and AD highlight the potential of this intervention as a therapeutic approach for AD.

Aberrant role of pyruvate kinase M2 in the regulation of gamma-secretase and memory deficits in Alzheimer's disease

Toxic amyloid beta (Aβ) species cause synaptic dysfunction and neurotoxicity in Alzheimer's disease (AD). As of yet, however, there are no reported regulators for gamma-secretase, which links a risky environment to amyloid accumulation in AD. Here, we report that pyruvate kinase M2 (PKM2) is a positive regulator of gamma-secretase under hypoxia. From a genome-wide functional screen, we identify PKM2 as a gamma-secretase activator that is highly expressed in the brains of both patients and murine models with AD. PKM2 regulates Aβ production and the amount of active gamma-secretase complex by changing the gene expression of aph-1 homolog. Hypoxia induces PKM2 expression, thereby promoting gamma-secretase activity. Moreover, transgenic expression of PKM2 in 3xTg AD model mice enhances hippocampal production of Aβ and exacerbates the impairment of spatial and recognition memory. Taken together, these findings indicate that PKM2 is an important gamma-secretase regulator that promotes Aβ production and memory impairment under hypoxia.

β-arrestin1 promotes tauopathy by transducing GPCR signaling, disrupting microtubules and autophagy

GPCRs regulator, β-arrestin1, is increased in FTLD-tau patients, is required for β2-adrenergic receptor and metabotropic glutamate receptor 2-induced tau phosphorylation, promotes tau aggregation by impairing autophagy, and destabilizes microtubule dynamics, whereas genetic reduction in β-arrestin1 mitigates tauopathy and cognitive impairments.

G protein–coupled receptors (GPCRs) have been shown to play integral roles in Alzheimer’s disease pathogenesis. However, it is unclear how diverse GPCRs similarly affect Aβ and tau pathogenesis. GPCRs share a common mechanism of action via the β-arrestin scaffolding signaling complexes, which not only serve to desensitize GPCRs by internalization, but also mediate multiple downstream signaling events. As signaling via the GPCRs, β2-adrenergic receptor (β2AR), and metabotropic glutamate receptor 2 (mGluR2) promotes hyperphosphorylation of tau, we hypothesized that β-arrestin1 represents a point of convergence for such pathogenic activities. Here, we report that β-arrestins are not only essential for β2AR and mGluR2-mediated increase in pathogenic tau but also show that β-arrestin1 levels are increased in brains of Frontotemporal lobar degeneration (FTLD-tau) patients. Increased β-arrestin1 in turn drives the accumulation of pathogenic tau, whereas reduced ARRB1 alleviates tauopathy and rescues impaired synaptic plasticity and cognitive impairments in PS19 mice. Biochemical and cellular studies show that β-arrestin1 drives tauopathy by destabilizing microtubules and impeding p62/SQSTM1 autophagy flux by interfering with p62 body formation, which promotes pathogenic tau accumulation.

GPR3 accelerates neurite outgrowth and neuronal polarity formation via PI3 kinase-mediating signaling pathway in cultured primary neurons

During neuronal development, immature neurons extend neurites and subsequently polarize to form an axon and dendrites. We have previously reported that G protein-coupled receptor 3 (GPR3) levels increase during neuronal development, and that GPR3 has functions in neurite outgrowth and neuronal differentiation in cerebellar granular neurons. Moreover, GPR3 is transported and concentrated at the tips of neurite, thereby contributing to the local activation of protein kinase A (PKA). However, the signaling pathways for GPR3-mediated neurite outgrowth and its subsequent effects on neuronal polarization have not yet been elucidated. We therefore analyzed the signaling pathways related to GPR3-mediated neurite outgrowth, and also focused on the possible roles of GPR3 in axon polarization. We demonstrated that, in cerebellar granular neurons, GPR3-mediated neurite outgrowth was mediated by multiple signaling pathways, including those of PKA, extracellular signal-regulated kinases (ERKs), and most strongly phosphatidylinositol 3-kinase (PI3K). In addition, the GPR3-mediated activation of neurite outgrowth was associated with G protein-coupled receptor kinase 2 (GRK2)-mediated signaling and phosphorylation of the C-terminus serine/threonine residues of GPR3, which affected downstream protein kinase B (Akt) signaling. We further demonstrated that GPR3 was transiently increased early in the development of rodent hippocampal neurons. It was subsequently concentrated at the tip of the longest neurite, and was thus associated with accelerated polarity formation in a PI3K-dependent manner in rat hippocampal neurons. In addition, GPR3 knockout in mouse hippocampal neurons led to delayed neuronal polarity formation, thereby affecting the dephosphorylation of collapsing response mediator protein 2 (CRMP2), which is downstream of the PI3K signaling pathway. Taken together, these findings suggest that the intrinsic expression of GPR3 in differentiated neurons constitutively activates PI3K-mediated signaling pathway predominantly, thus accelerating neurite outgrowth and further augmenting polarity formation in primary cultured neurons.

Functional Selectivity of Dopamine D1 Receptor Signaling: Retrospect and Prospect

Research progress on dopamine D₁ receptors indicates that signaling no longer is limited to G protein-dependent cyclic adenosine monophosphate phosphorylation but also includes G protein-independent β-arrestin-related mitogen-activated protein kinase activation, regulation of ion channels, phospholipase C activation, and possibly more. This review summarizes recent studies revealing the complexity of D₁ signaling and its clinical implications, and suggests functional selectivity as a promising strategy for drug discovery to magnify the merit of D₁ signaling. Functional selectivity/biased receptor signaling has become a major research front because of its potential to improve therapeutics through precise targeting. Retrospective pharmacological review indicated that many D₁ ligands have some degree of mild functional selectivity, and novel compounds with extreme bias at D₁ signaling were reported recently. Behavioral and neurophysiological studies inspired new methods to investigate functional selectivity and gave insight into the biased signaling of several drugs. Results from recent clinical trials also supported D₁ functional selectivity signaling as a promising strategy for discovery and development of better therapeutics.

Motor Stereotypic Behavior Was Associated With Immune Response in Macaques: Insight From Transcriptome and Gut Microbiota Analysis

Motor stereotypic behaviors (MSBs) are common in captive rhesus macaques (Macaca mulatta) and human with psychiatric diseases. However, large gaps remain in our understanding of the molecular mechanisms that mediate this behavior and whether there are similarities between human and non-human primates that exhibit this behavior, especially at gene expression and gut microbiota levels. The present study combined behavior, blood transcriptome, and gut microbiota data of two groups of captive macaques to explore this issue (i.e., MSB macaques with high MSB exhibition and those with low: control macaques). Observation data showed that MSB macaques spent the most time on MSB (33.95%), while the CONTROL macaques allocated more time to active (30.99%) and general behavior (30.0%), and only 0.97% of their time for MSB. Blood transcriptome analysis revealed 382 differentially expressed genes between the two groups, with 339 upregulated genes significantly enriched in inflammation/immune response-related pathway. We also identified upregulated pro-inflammatory genes TNFRSF1A, IL1R1, and IL6R. Protein–protein interaction network analysis screened nine hub genes that were all related to innate immune response, and our transcriptomic results were highly similar to findings in human psychiatric disorders. We found that there were significant differences in the beta-diversity of gut microbiota between MSB and CONTROL macaques. Of which Phascolarctobacterium, the producer of short chain fatty acids (SCFAs), was less abundant in MSB macaques. Meanwhile, PICRUSTs predicted that SCFAs intermediates biosynthesis and metabolic pathways were significantly downregulated in MSB macaques. Together, our study revealed that the behavioral, gene expression levels, and gut microbiota composition in MSB macaques was different to controls, and MSB was closely linked with inflammation and immune response. This work provides valuable information for future in-depth investigation of MSB and human psychiatric diseases.

G-protein coupled receptor, PI3K and Rho signaling pathways regulate the cascades of Tau and amyloid-β in Alzheimer's disease

Alzheimer's disease is a progressive neurodegenerative disease characterized by the presence of amyloid-β plaques in the extracellular environment and aggregates of Tau protein that forms neurofibrillary tangles (NFTs) in neuronal cells. Along with these pathological proteins, the disease shows neuroinflammation, neuronal death, impairment in the immune function of microglia and synaptic loss, which are mediated by several important signaling pathways. The PI3K/Akt-mediated survival-signaling pathway is activated by many receptors such as G-protein coupled receptors (GPCRs), triggering receptor expressed on myeloid cells 2 (TREM2), and lysophosphatidic acid (LPA) receptor. The signaling pathway not only increases the survival of neurons but also regulates inflammation, phagocytosis, cellular protection, Tau phosphorylation and Aβ secretion as well. In this review, we focused on receptors, which activate PI3K/Akt pathway and its potential to treat Alzheimer's disease. Among several membrane receptors, GPCRs are the major drug targets for therapy, and GPCR signaling pathways are altered during Alzheimer's disease. Several GPCRs are involved in the pathogenic progression, phosphorylation of Tau protein by activation of various cellular kinases and are involved in the amyloidogenic pathway of amyloid-β synthesis. Apart from various GPCR signaling pathways, GPCR regulating/ interacting proteins are involved in the pathogenesis of Alzheimer's disease. These include several small GTPases, Ras homolog enriched in brain, GPCR associated sorting proteins, β-arrestins, etc., that play a critical role in disease progression and has been elaborated in this review.

Opposing functions of β-arrestin 1 and 2 in Parkinson's disease via microglia inflammation and Nprl3

Although β-arrestins (ARRBs) regulate diverse physiological and pathophysiological processes, their functions and regulation in Parkinson's disease (PD) remain poorly defined. In this study, we show that the expression of β-arrestin 1 (ARRB1) and β-arrestin 2 (ARRB2) is reciprocally regulated in PD mouse models, particularly in microglia. ARRB1 ablation ameliorates, whereas ARRB2 knockout aggravates, the pathological features of PD, including dopaminergic neuron loss, neuroinflammation and microglia activation in vivo, and microglia-mediated neuron damage in vitro. We also demonstrate that ARRB1 and ARRB2 produce adverse effects on inflammation and activation of the inflammatory STAT1 and NF-κB pathways in primary cultures of microglia and macrophages and that two ARRBs competitively interact with the activated form of p65, a component of the NF-κB pathway. We further find that ARRB1 and ARRB2 differentially regulate the expression of nitrogen permease regulator-like 3 (Nprl3), a functionally poorly characterized protein, as revealed by RNA sequencing, and that in the gain- and loss-of-function studies, Nprl3 mediates the functions of both ARRBs in microglia inflammatory responses. Collectively, these data demonstrate that two closely related ARRBs exert opposite functions in microglia-mediated inflammation and the pathogenesis of PD which are mediated at least in part through Nprl3 and provide novel insights into the understanding of the functional divergence of ARRBs in PD.

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.

Aging-related modifications to G protein-coupled receptor signaling diversity

Aging is a highly complex molecular process, affecting nearly all tissue systems in humans and is the highest risk factor in developing neurodegenerative disorders such as Alzheimer's and Parkinson's disease, cardiovascular disease and Type 2 diabetes mellitus. The intense complexity of the aging process creates an incentive to develop more specific drugs that attenuate or even reverse some of the features of premature aging. As our current pharmacopeia is dominated by therapeutics that target members of the G protein-coupled receptor (GPCR) superfamily it may be prudent to search for effective anti-aging therapeutics in this fertile domain. Since the first demonstration of GPCR-based β-arrestin signaling, it has become clear that an enhanced appreciation of GPCR signaling diversity may facilitate the creation of therapeutics with selective signaling activities. Such 'biased' ligand signaling profiles can be effectively investigated using both standard molecular biological techniques as well as high-dimensionality data analyses. Through a more nuanced appreciation of the quantitative nature across the multiple dimensions of signaling bias that drugs possess, researchers may be able to further refine the efficacy of GPCR modulators to impact the complex aberrations that constitute the aging process. Identifying novel effector profiles could expand the effective pharmacopeia and assist in the design of precision medicines. This review discusses potential non-G protein effectors, and specifically their potential therapeutic suitability in aging and age-related disorders.

Cofilin, a Master Node Regulating Cytoskeletal Pathogenesis in Alzheimer's Disease

 The defining pathological hallmarks of Alzheimer’s disease (AD) are proteinopathies marked by the amyloid-β (Aβ) peptide and hyperphosphorylated tau. In addition, Hirano bodies and cofilin-actin rods are extensively found in AD brains, both of which are associated with the actin cytoskeleton. The actin-binding protein cofilin known for its actin filament severing, depolymerizing, nucleating, and bundling activities has emerged as a significant player in AD pathogenesis. In this review, we discuss the regulation of cofilin by multiple signaling events impinging on LIM kinase-1 (LIMK1) and/or Slingshot homolog-1 (SSH1) downstream of Aβ. Such pathophysiological signaling pathways impact actin dynamics to regulate synaptic integrity, mitochondrial translocation of cofilin to promote neurotoxicity, and formation of cofilin-actin pathology. Other intracellular signaling proteins, such as β-arrestin, RanBP9, Chronophin, PLD1, and 14-3-3 also impinge on the regulation of cofilin downstream of Aβ. Finally, we discuss the role of activated cofilin as a bridge between actin and microtubule dynamics by displacing tau from microtubules, thereby destabilizing tau-induced microtubule assembly, missorting tau, and promoting tauopathy.

Identification of a β-Arrestin 2 Mutation Related to Autism by Whole-Exome Sequencing

Autism spectrum disorder (ASD) is a complex neurological disease characterized by impaired social communication and interaction skills, rigid behavior, decreased interest, and repetitive activities. The disease has a high degree of genetic heterogeneity, and the genetic cause of ASD in many autistic individuals is currently unclear. In this study, we report a patient with ASD whose clinical features included social interaction disorder, communication disorder, and repetitive behavior. We examined the patient's genetic variation using whole-exome sequencing technology and found new de novo mutations. After analysis and evaluation, ARRB2 was identified as a candidate gene. To study the potential contribution of the ARRB2 gene to the human brain development and function, we first evaluated the expression profile of this gene in different brain regions and developmental stages. Then, we used weighted gene coexpression network analysis to analyze the associations between ARRB2 and ASD risk genes. Additionally, the spatial conformation and stability of the ARRB2 wild type and mutant proteins were examined by simulations. Then, we further established a mouse model of ASD. The results showed abnormal ARRB2 expression in the mouse ASD model. Our study showed that ARRB2 may be a risk gene for ASD, but the contribution of de novo ARRB2 mutations to ASD is unclear. This information will provide references for the etiology of ASD and aid in the mechanism-based drug development and treatment.

Detailed neuronal distribution of GPR3 and its co-expression with EF-hand calcium-binding proteins in the mouse central nervous system

The G-protein coupled receptor 3 (GPR3), a member of the class A rhodopsin-type GPR family, constitutively activates Gαs proteins without any ligands. Although there have been several reports concerning the functions of GPR3 in neurons, the physiological roles of GPR3 have not been fully elucidated. To address this issue, we analyzed GPR3 distribution in detail using fluorescence-based X-gal staining in heterozygous GPR3 knockout/LacZ knock-in mice, and further investigated the types of GPR3-expressing neurons using fluorescent double labeling with various EF-hand Ca²⁺-binding proteins. In addition to the previously reported GPR3-expressing areas, we identified GPR3 expression in the basal ganglia and in many nuclei of the cranial nerves, in regions related to olfactory, auditory, emotional, and motor functions. In addition, GPR3 was not only observed in excitatory neurons in layer V of the cerebral cortex, the CA2 region of the hippocampus, and the lateral nucleus of the thalamus, but also in γ-aminobutyric acid (GABA)-ergic interneurons in the cortex, hippocampus, thalamus, striatum, and cerebellum. GPR3 was frequently co-expressed with neuronal Ca²⁺-binding protein 2 (NECAB2) in neurons in various regions of the central nervous system, especially in the hippocampal CA2, medial habenular nucleus, lateral thalamic nucleus, dorsolateral striatum, brainstem, and spinal cord anterior horn. Furthermore, GPR3 also co-localized with NECAB2 at the tips of neurites in differentiated PC12 cells. These results suggest that GPR3 and NECAB2 are highly co-expressed in specific neurons, and that GPR3 may modulate Ca²⁺ signaling by interacting with NECAB2 in specific areas of the central nervous system.

β-Arrestin2 is a critical component of the GPCR-eNOS signalosome

Endothelial cell nitric oxide (NO) synthase (eNOS), the enzyme responsible for synthesis of NO in endothelial cells, is regulated by complex posttranslational mechanisms. Sinusoidal portal hypertension, a disorder characterized by liver sinusoidal endothelial cell (SEC) injury with resultant reduced eNOS activity and NO production within the liver, has been associated with defects in eNOS protein-protein interactions and posttranslational modifications. We and others have previously identified novel eNOS interactors, including G protein-coupled receptor (GPCR) kinase interactor 1 (GIT1), which we found to play an unexpected stimulatory role in GPCR-mediated eNOS signaling. Here we report that β-arrestin 2 (β-Arr2), a canonical GPCR signaling partner, localizes in SECs with eNOS in a GIT1/eNOS/NO signaling module. Most importantly, we show that β-Arr2 stimulates eNOS activity, and that β-Arr2 expression is reduced and formation of the GIT1/eNOS/NO signaling module is interrupted during liver injury. In β-Arr2-deficient mice, bile duct ligation injury (BDL) led to significantly reduced eNOS activity and to a dramatic increase in portal hypertension compared to BDL in wild-type mice. Overexpression of β-Arr2 in injured or β-Arr2-deficient SECs rescued eNOS function by increasing eNOS complex formation and NO production. We also found that β-Arr2-mediated GIT1/eNOS complex formation is dependent on Erk1/2 and Src, two kinases known to interact with and be activated by β-Arr2 in response to GCPR activation. Our data emphasize that β-Arr2 is an integral component of the GIT1/eNOS/NO signaling pathway and have implications for the pathogenesis of sinusoidal portal hypertension.

ARRB1 ameliorates liver ischaemia/reperfusion injury via antagonizing TRAF6-mediated Lysine 6-linked polyubiquitination of ASK1 in hepatocytes

Hepatic ischaemia/reperfusion (I/R) injury is a major clinical problem during liver surgical procedures, which usually lead to early transplantation failure and higher organ rejection rate, and current effective therapeutic strategies are still limited. Therefore, in‐depth exploring of the molecular mechanisms underlying liver I/R injury is key to the development of new therapeutic methods. β‐arrestins are multifunctional proteins serving as important signalling scaffolds in numerous physiopathological processes, including liver‐specific diseases. However, the role and underlying mechanism of β‐arrestins in hepatic I/R injury remain largely unknown. Here, we showed that only ARRB1, but not ARRB2, was down‐regulated during liver I/R injury. Hepatocyte‐specific overexpression of ARRB1 significantly ameliorated liver damage, as demonstrated by decreases in serum aminotransferases, hepatocellular necrosis and apoptosis, infiltrating inflammatory cells and secretion of pro‐inflammatory cytokines relative to control mice, whereas experiments with ARRB1 knockout mice gotten opposite effects. Mechanistically, ARRB1 directly interacts with ASK1 in hepatocytes and inhibits its TRAF6‐mediated Lysine 6‐linked polyubiquitination, which then prevents the activation of ASK1 and its downstream signalling pathway during hepatic I/R injury. In addition, inhibition of ASK1 remarkably abolished the disruptive effect result from ARRB1 deficiency in liver I/R injury in vivo, indicating that ASK1 was required for ARRB1 function in hepatic I/R injury. In conclusion, we proposed that ARRB1 is a novel protective regulator during liver I/R injury, and modulation of the regulatory axis between ARRB1 and ASK1 could be a novel therapeutic strategy to prevent this pathological process.

Interaction between Angiotensin Receptor and β-Adrenergic Receptor Regulates the Production of Amyloid β-Protein

Alzheimer's disease (AD) is characterized by the formation of extracellular amyloid plaques containing the amyloid β-protein (Aβ) within the parenchyma of the brain. Aβ is considered to be the key pathogenic factor of AD. Recently, we showed that Angiotensin II type 1 receptor (AT₁R), which regulates blood pressure, is involved in Aβ production, and that telmisartan (Telm), which is an angiotensin II receptor blocker (ARB), increased Aβ production via AT₁R. However, the precise mechanism underlying how AT₁R is involved in Aβ production is unknown. Interestingly, AT₁R, a G protein-coupled receptor, was strongly suggested to be involved in signal transduction by heterodimerization with β₂-adrenergic receptor (β₂-AR), which is also shown to be involved in Aβ generation. Therefore, in this study, we aimed to clarify whether the interaction between AT₁R and β₂-AR is involved in the regulation of Aβ production. To address this, we analyzed whether the increase in Aβ production by Telm treatment is affected by β-AR antagonist using fibroblasts overexpressing amyloid precursor protein (APP). We found that the increase in Aβ production by Telm treatment was decreased by the treatment of β₂-AR selective antagonist ICI-118551 more strongly than the treatment of β₁-AR selective antagonists. Furthermore, deficiency of AT₁R abolished the effect of β₂-AR antagonist on the stimulation of Aβ production caused by Telm. Taken together, the interaction between AT₁R and β₂-AR is likely to be involved in Aβ production.

β-Arrestin2 oligomers impair the clearance of pathological tau and increase tau aggregates

Multiple G protein-coupled receptors (GPCRs) are targets in the treatment of dementia, and the arrestins are common to their signaling. β-Arrestin2 was significantly increased in brains of patients with frontotemporal lobar degeneration (FTLD-tau), a disease second to Alzheimer's as a cause of dementia. Genetic loss and overexpression experiments using genetically encoded reporters and defined mutant constructs in vitro, and in cell lines, primary neurons, and tau P301S mice crossed with β-arrestin2-/- mice, show that β-arrestin2 stabilizes pathogenic tau and promotes tau aggregation. Cell and mouse models of FTLD showed this to be maladaptive, fueling a positive feedback cycle of enhanced neuronal tau via non-GPCR mechanisms. Genetic ablation of β-arrestin2 markedly ablates tau pathology and rescues synaptic plasticity defects in tau P301S transgenic mice. Atomic force microscopy and cellular studies revealed that oligomerized, but not monomeric, β-arrestin2 increases tau by inhibiting self-interaction of the autophagy cargo receptor p62/SQSTM1, impeding p62 autophagy flux. Hence, reduction of oligomerized β-arrestin2 with virus encoding β-arrestin2 mutants acting as dominant-negatives markedly reduces tau-laden neurofibrillary tangles in FTLD mice in vivo. Reducing β-arrestin2 oligomeric status represents a new strategy to alleviate tau pathology in FTLD and related tauopathies.

Oligomers Are Promising Targets for Drug Development in the Treatment of Proteinopathies

Currently, there is no effective treatment of proteinopathies, as well as their diagnosis in the early stages of the disease until the first clinical symptoms appear. The proposed model of fibrillation of the Aβ peptide and its fragments not only describes molecular rearrangements, but also offers models of processes that occur during the formation of amyloid aggregates. Since this model is also characteristic of other proteins and peptides, a new potential target for drug development in the treatment of Alzheimer’s disease (AD) and other proteinopathies is proposed on the basis of this model. In our opinion, it is oligomers that are promising targets for innovative developments in the treatment of these diseases.

Extended Human G-Protein Coupled Receptor Network: Cell-Type-Specific Analysis of G-Protein Coupled Receptor Signaling Pathways

G-protein coupled receptors (GPCRs) mediate crucial physiological functions in humans, have been implicated in an array of diseases, and are therefore prime drug targets. GPCRs signal via a multitude of pathways, mainly through G-proteins and β-arrestins, to regulate effectors responsible for cellular responses. The limited number of transducers results in different GPCRs exerting control on the same pathway, while the availability of signaling proteins in a cell defines the result of GPCR activation. The aim of this study was to construct the extended human GPCR network (hGPCRnet) and examine the effect that cell-type specificity has on GPCR signaling pathways. To achieve this, protein-protein interaction data between GPCRs, G-protein coupled receptor kinases (GRKs), Gα subunits, β-arrestins, and effectors were combined with protein expression data in cell types. This resulted in the hGPCRnet, a very large interconnected network, and similar cell-type-specific networks in which, distinct GPCR signaling pathways were formed. Finally, a user friendly web application, hGPCRnet ( http://bioinformatics.biol.uoa.gr/hGPCRnet ), was created to allow for the visualization and exploration of these networks and of GPCR signaling pathways. This work, and the resulting application, can be useful in further studies of GPCR function and pharmacology.

Hedgehog Signal and Genetic Disorders

The hedgehog (Hh) family comprises sonic hedgehog (Shh), Indian hedgehog (Ihh), and desert hedgehog (Dhh), which are versatile signaling molecules involved in a wide spectrum of biological events including cell differentiation, proliferation, and survival; establishment of the vertebrate body plan; and aging. These molecules play critical roles from embryogenesis to adult stages; therefore, alterations such as abnormal expression or mutations of the genes involved and their downstream factors cause a variety of genetic disorders at different stages. The Hh family involves many signaling mediators and functions through complex mechanisms, and achieving a comprehensive understanding of the entire signaling system is challenging. This review discusses the signaling mediators of the Hh pathway and their functions at the cellular and organismal levels. We first focus on the roles of Hh signaling mediators in signal transduction at the cellular level and the networks formed by these factors. Then, we analyze the spatiotemporal pattern of expression of Hh pathway molecules in tissues and organs, and describe the phenotypes of mutant mice. Finally, we discuss the genetic disorders caused by malfunction of Hh signaling-related molecules in humans.