Amelioration of Tau pathology and memory deficits by targeting 5-HT7 receptor

Effect of amisulpride on the expression of serotonin receptors, neurotrophic factor BDNF and its receptors in mice with overexpression of the aggregation-prone [R406W] mutant tau protein

Serotonin 5-HT7 receptors (5-HT7R) are attracting increasing attention as important participants in the mechanisms of Alzheimer's disease and as a possible target for the treatment of various tau pathologies. In this study, we investigated the effects of amisulpride (5-HT7R inverse agonist) in C57BL/6J mice with experimentally induced expression of the gene encoding the aggregation-prone human Tau[R406W] protein in the prefrontal cortex. In these animals we examined short-term memory and the expression of genes involved in the development of tauopathy (Htr7 and Cdk5), as well as biomarkers of neurodegenerative processes - the Bdnf gene and its receptors TrkB (the Ntrk2 gene) and p75NTR (the Ngfr gene). In a short-term memory test, there was no difference in the discrimination index between mice treated with AAV-Tau[R406W] and mice treated with AAV-EGFP. Amisulpride did not affect this parameter. Administration of AAV-Tau[R406W] resulted in increased expression of the Htr7, Htr1a, and Cdk5 genes in the prefrontal cortex compared to AAV-EGFP animals. At the same time, amisulpride at the dose of 10 mg/kg in animals from the AAV-Tau[R406W] group caused a decrease in the Htr7, Htr1a genes mRNA levels compared to animals from the AAV-Tau[R406W] group treated with saline. A decrease in the expression of the Bdnf and Ntrk2 genes in the prefrontal cortex was revealed after administration of AAV-Tau[R406W]. Moreover, amisulpride at various doses (3 and 10 mg/kg) caused the same decrease in the transcription of these genes in mice without tauopathy. It is also interesting that in mice of the AAV-EGFP group, administration of amisulpride at the dose of 10 mg/kg increased the Ngfr gene mRNA level. The data obtained allow us to propose the use of amisulpride in restoring normal tau protein function. However, it should be noted that prolonged administration may result in adverse effects such as an increase in Ngfr expression and a decrease in Bdnf and Ntrk2 expression, which is probably indicative of an increase in neurodegenerative processes.

Exploiting Cell-Based Assays to Accelerate Drug Development for G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) are relevant targets for health and disease as they regulate various aspects of metabolism, proliferation, differentiation, and immune pathways. They are implicated in several disease areas, including cancer, diabetes, cardiovascular diseases, and mental disorders. It is worth noting that about a third of all marketed drugs target GPCRs, making them prime pharmacological targets for drug discovery. Numerous functional assays have been developed to assess GPCR activity and GPCR signaling in living cells. Here, we review the current literature of genetically encoded cell-based assays to measure GPCR activation and downstream signaling at different hierarchical levels of signaling, from the receptor to transcription, via transducers, effectors, and second messengers. Singleplex assay formats provide one data point per experimental condition. Typical examples are bioluminescence resonance energy transfer (BRET) assays and protease cleavage assays (e.g., Tango or split TEV). By contrast, multiplex assay formats allow for the parallel measurement of multiple receptors and pathways and typically use molecular barcodes as transcriptional reporters in barcoded assays. This enables the efficient identification of desired on-target and on-pathway effects as well as detrimental off-target and off-pathway effects. Multiplex assays are anticipated to accelerate drug discovery for GPCRs as they provide a comprehensive and broad identification of compound effects.

Structural determinants for activation of the Tau kinase CDK5 by the serotonin receptor 5-HT7R

BACKGROUND

Multiple neurodegenerative diseases are induced by the formation and deposition of protein aggregates. In particular, the microtubule-associated protein Tau leads to the development of so-called tauopathies characterized by the aggregation of hyperphosphorylated Tau within neurons. We recently showed that the constitutive activity of the serotonin receptor 7 (5-HT7R) is required for Tau hyperphosphorylation and aggregation through activation of the cyclin-dependent kinase 5 (CDK5). We also demonstrated physical interaction between 5-HT7R and CDK5 at the plasma membrane suggesting that the 5-HT7R/CDK5 complex is an integral part of the signaling network involved in Tau-mediated pathology.

METHODS

Using biochemical, microscopic, molecular biological, computational and AI-based approaches, we investigated structural requirements for the formation of 5-HT7R/CDK5 complex.

RESULTS

We demonstrated that 5-HT7R domains responsible for coupling to Gs proteins are not involved in receptor interaction with CDK5. We also created a structural model of the 5-HT7R/CDK5 complex and refined the interaction interface. The model predicted two conserved phenylalanine residues, F278 and F281, within the third intracellular loop of 5-HT7R to be potentially important for complex formation. While site-directed mutagenesis of these residues did not influence Gs protein-mediated receptor signaling, replacement of both phenylalanines by alanine residues significantly reduced 5-HT7R/CDK5 interaction and receptor-mediated CDK5 activation, leading to reduced Tau hyperphosphorylation and aggregation. Molecular dynamics simulations of 5-HT7R/CDK5 complex for wild-type and receptor mutants confirmed binding interface stability of the initial model.

CONCLUSIONS

Our results provide a structural basis for the development of novel drugs targeting the 5-HT7R/CDK5 interaction interface for the selective treatment of Tau-related disorders, including frontotemporal dementia and Alzheimer's disease.

Serotonin Receptors in Myocardial Infarction: Friend or Foe?

Acute myocardial infarction (AMI) is one of the leading causes of death worldwide and treatment costs pose a major burden on the global health care system. Despite the variety of treatment options, individual recovery can be still poor and the mortality rate, especially in the first few years after the event, remains high. Therefore, intense research is currently focused on identifying novel target molecules to improve the outcome following AMI. One of the potentially interesting targets is the serotonergic system (5-HT system), not at least because of its connection to mental disorders. It is known that patients suffering from AMI have an increased risk of developing depression and vice versa. This implicates that the 5-HT system can be affected in response to AMI and might thus represent a target structure for patients' treatment. This review aims to highlight the importance of the 5-HT system after AMI by describing the role of individual serotonin receptors (5-HTR) in the regulation of physiological and pathophysiological responses. It particularly focuses on the signaling pathways of the serotonin receptors 1, 2, 4, and 7, which are expressed in the cardiovascular system, during disease onset, and the following remodeling process. This overview also emphasizes the importance of the 5-HT system in AMI etiology and highlights 5-HTRs as potential treatment targets.

The brain serotonin system in autism

Autism spectrum disorders (ASDs) are among the most common neurodevelopmental diseases. These disorders are characterized by lack of social interaction, by repetitive behavior, and often anxiety and learning disabilities. The brain serotonin (5-HT) system is known to be crucially implicated in a wide range of physiological functions and in the control of different kinds of normal and pathological behavior. A growing number of studies indicate the involvement of the brain 5-HT system in the mechanisms underlying both ASD development and ASD-related behavioral disorders. There are some review papers describing the role of separate key players of the 5-HT system in an ASD and/or autistic-like behavior. In this review, we summarize existing data on the participation of all members of the brain 5-HT system, namely, 5-HT transporter, tryptophan hydroxylase 2, MAOA, and 5-HT receptors, in autism in human and various animal models. Additionally, we describe the most recent studies involving modern techniques for in vivo regulation of gene expression that are aimed at identifying exact roles of 5-HT receptors, MAOA, and 5-HT transporter in the mechanisms underlying autistic-like behavior. Altogether, results of multiple research articles show that the brain 5-HT system intimately partakes in the control of some types of ASD-related behavior, and that specific changes in a function of a certain 5-HT receptor, transporter, and/or enzyme may normalize this aberrant behavior. These data give hope that some of clinically used 5-HT-related drugs have potential for ASD treatment.

Amisulpride as a potential disease-modifying drug in the treatment of tauopathies

INTRODUCTION

Hyperphosphorylation and aggregation of the microtubule-associated protein tau cause the development of tauopathies, such as Alzheimer's disease and frontotemporal dementia (FTD). We recently uncovered a causal link between constitutive serotonin receptor 7 (5-HT7R) activity and pathological tau aggregation. Here, we evaluated 5-HT7R inverse agonists as novel drugs in the treatment of tauopathies.

METHODS

Based on structural homology, we screened multiple approved drugs for their inverse agonism toward 5-HT7R. Therapeutic potential was validated using biochemical, pharmacological, microscopic, and behavioral approaches in different cellular models including tau aggregation cell line HEK293 tau bimolecular fluorescence complementation, primary mouse neurons, and human induced pluripotent stem cell-derived neurons carrying an FTD-associated tau mutation as well as in two mouse models of tauopathy.

RESULTS

Antipsychotic drug amisulpride is a potent 5-HT7R inverse agonist. Amisulpride ameliorated tau hyperphosphorylation and aggregation in vitro. It further reduced tau pathology and abrogated memory impairment in mice.

DISCUSSION

Amisulpride may be a disease-modifying drug for tauopathies.

Serotonin Receptors as a Potential Target in the Treatment of Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia worldwide that has an increasing impact on aging societies. Besides its critical role in the control of various physiological functions and behavior, brain serotonin (5-HT) system is involved in the regulation of migration, proliferation, differentiation, maturation, and programmed death of neurons. At the same time, a growing body of evidence indicates the involvement of 5-HT neurotransmission in the formation of insoluble aggregates of β-amyloid and tau protein, the main histopathological signs of AD. The review describes the role of various 5-HT receptors and intracellular signaling cascades induced by them in the pathological processes leading to the development of AD, first of all, in protein aggregation. Changes in the functioning of certain types of 5-HT receptors or associated intracellular signaling mediators prevent accumulation of β-amyloid plaques and tau protein neurofibrillary tangles. Based on the experimental data, it can be suggested that the use of 5-HT receptors as new drug targets will not only improve cognitive performance in AD, but will be also important in treating the causes of AD-related dementia.

Functional Dimerization of Serotonin Receptors: Role in Health and Depressive Disorders

Understanding the neurobiological underpinnings of depressive disorder constitutes a pressing challenge in the fields of psychiatry and neurobiology. Depression represents one of the most prevalent forms of mental and behavioral disorders globally. Alterations in dimerization capacity can influence the functional characteristics of serotonin receptors and may constitute a contributing factor to the onset of depressive disorders. The objective of this review is to consolidate the current understanding of interactions within the 5-HT receptor family and between 5-HT receptors and members of other receptor families. Furthermore, it aims to elucidate the role of such complexes in depressive disorders and delineate the mechanisms through which antidepressants exert their effects.

Mitochondrial Interaction with Serotonin in Neurobiology and Its Implication in Alzheimer's Disease

Alzheimer's disease (AD) is a lethal neurodegenerative disorder characterized by severe brain pathologies and progressive cognitive decline. While the exact cause of this disease remains unknown, emerging evidence suggests that dysregulation of neurotransmitters contributes to the development of AD pathology and symptoms. Serotonin, a critical neurotransmitter in the brain, plays a pivotal role in regulating various brain processes and is implicated in neurological and psychiatric disorders, including AD. Recent studies have shed light on the interplay between mitochondrial function and serotonin regulation in brain physiology. In AD, there is a deficiency of serotonin, along with impairments in mitochondrial function, particularly in serotoninergic neurons. Additionally, altered activity of mitochondrial enzymes, such as monoamine oxidase, may contribute to serotonin dysregulation in AD. Understanding the intricate relationship between mitochondria and serotonin provides valuable insights into the underlying mechanisms of AD and identifies potential therapeutic targets to restore serotonin homeostasis and alleviate AD symptoms. This review summarizes the recent advancements in unraveling the connection between brain mitochondria and serotonin, emphasizing their significance in AD pathogenesis and underscoring the importance of further research in this area. Elucidating the role of mitochondria in serotonin dysfunction will promote the development of therapeutic strategies for the treatment and prevention of this neurodegenerative disorder.

Amisulpride Decreases Tau Protein Hyperphosphorylation in the Brain of OXYS Rats

AIM

In this study, OXYS rats of three ages (1, 3, and 6 months), a proven model of Alzheimer's disease (AD), at various stages of disease progression were used to thoroughly study the effects of amisulpride on behavior and tau protein phosphorylation.

BACKGROUND

With the growing number of patients with AD, the problem of finding a cure is very acute. Neurodegeneration in AD has various causes, one of which is hyperphosphorylation of tau protein.

OBJECTIVE

This study aimed to investigate whether amisulpride would affect pathological tau phosphorylation in AD.

METHODS

We assessed the influence of chronic administration of amisulpride (3 weeks, 3 mg/kg per day, intraperitoneally)-a 5-HT7 receptor inverse agonist-on behavior and tau hyperphosphorylation in OXYS rats (at ages of 1, 3, and 6 months).

RESULTS

Chronic administration of amisulpride dramatically decreased tau phosphorylation in the frontal cortex and hippocampus of 3-month-old OXYS rats. Additionally, in 1- and 3-month-old rats' hippocampi, amisulpride diminished the mRNA level of the Cdk5 gene encoding one of the main tau kinases involved in the 5-HT7 receptor-induced effect on tau phosphorylation.

CONCLUSION

Thus, We found that chronic administration of amisulpride could reduce pathological tau hyperphosphorylation while reducing anxiety. We propose amisulpride to have therapeutic potential against AD and that it can be the most effective in the early stages of the disease.

Serotonin Receptor 5-HT2A Regulates TrkB Receptor Function in Heteroreceptor Complexes

Serotonin receptor 5-HT_2A and tropomyosin receptor kinase B (TrkB) strongly contribute to neuroplasticity regulation and are implicated in numerous neuronal disorders. Here, we demonstrate a physical interaction between 5-HT_2A and TrkB in vitro and in vivo using co-immunoprecipitation and biophysical and biochemical approaches. Heterodimerization decreased TrkB autophosphorylation, preventing its activation with agonist 7,8-DHF, even with low 5-HT_2A receptor expression. A blockade of 5-HT_2A receptor with the preferential antagonist ketanserin prevented the receptor-mediated downregulation of TrkB phosphorylation without restoring the TrkB response to its agonist 7,8-DHF in vitro. In adult mice, intraperitoneal ketanserin injection increased basal TrkB phosphorylation in the frontal cortex and hippocampus, which is in accordance with our findings demonstrating the prevalence of 5-HT_2A–TrkB heteroreceptor complexes in these brain regions. An expression analysis revealed strong developmental regulation of 5-HT_2A and TrkB expressions in the cortex, hippocampus, and especially the striatum, demonstrating that the balance between TrkB and 5-HT_2A may shift in certain brain regions during postnatal development. Our data reveal the functional role of 5-HT_2A–TrkB receptor heterodimerization and suggest that the regulated expression of 5-HT_2A and TrkB is a molecular mechanism for the brain-region-specific modulation of TrkB functions during development and under pathophysiological conditions.

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.

Activation of the 5-HT7 receptor and MMP-9 signaling module in the hippocampal CA1 region is necessary for the development of depressive-like behavior

Major depressive disorder is a complex disease resulting from aberrant synaptic plasticity that may be caused by abnormal serotonergic signaling. Using a combination of behavioral, biochemical, and imaging methods, we analyze 5-HT7R/MMP-9 signaling and dendritic spine plasticity in the hippocampus in mice treated with the selective 5-HT7R agonist (LP-211) and in a model of chronic unpredictable stress (CUS)-induced depressive-like behavior. We show that acute 5-HT7R activation induces depressive-like behavior in mice in an MMP-9-dependent manner and that post mortem brain samples from human individuals with depression reveal increased MMP-9 enzymatic activity in the hippocampus. Both pharmacological activation of 5-HT7R and modulation of its downstream effectors as a result of CUS lead to dendritic spine elongation and decreased spine density in this region. Overall, the 5-HT7R/MMP-9 pathway is specifically activated in the CA1 subregion of the hippocampus during chronic stress and is crucial for inducing depressive-like behavior.

Novel and atypical pathways for serotonin signaling

Serotonin (5-HT) appeared billions of years before 5-HT receptors and synapses. It is thus not surprising that 5-HT can control biological processes independently of its receptors. One example is serotonylation, which consists of covalent binding of 5-HT to the primary amine of glutamine. Over the past 20 years, serotonylation has been involved in the regulation of many signaling mechanisms. One of the most striking examples is the recent evidence that serotonylation of histone H3 constitutes an epigenetic mark. However, the pathophysiological role of histone H3 serotonylation remains to be discovered. All but one of the 5-HT receptors are G-protein-coupled receptors (GPCRs). The signaling pathways they control are finely tuned, and new, unexpected regulatory mechanisms are being uncovered continuously. Some 5-HT receptors (5-HT_2C, 5-HT₄, 5-HT₆, and 5-HT₇) signal through mechanisms that require neither G-proteins nor β-arrestins, the two classical and almost universal GPCR signal transducers. 5-HT₆ receptors are constitutively activated via their association with intracellular GPCR-interacting proteins (GIPs), including neurofibromin 1, cyclin-dependent kinase 5 (Cdk5), and G-protein-regulated inducer of neurite outgrowth 1 (GPRIN1). Interactions of 5-HT₆ receptor with Cdk5 and GPRIN1 are not concomitant but occur sequentially and play a key role in dendritic tree morphogenesis. Furthermore, 5-HT₆ receptor-mediated G-protein signaling in neurons is different in the cell body and primary cilium, where it is modulated by smoothened receptor activation. Finally, 5-HT_2A receptors form heteromers with mGlu₂ metabotropic glutamate receptors. This heteromerization results in a specific phosphorylation of mGlu₂ receptor on a serine residue (Ser⁸⁴³) upon agonist stimulation of 5-HT_2A or mGlu₂ receptor. mGlu₂ receptor phosphorylation on Ser⁸⁴³ is an essential step in engagement of G_i/o signaling not only upon mGlu₂ receptor activation but also following 5-HT_2A receptor activation, and thus represents a key molecular event underlying functional crosstalk between both receptors.

Serotonin 5-HT7 receptors require cyclin-dependent kinase 5 to rescue hippocampal synaptic plasticity in a mouse model of Fragile X Syndrome

Fragile X Syndrome is a genetic form of intellectual disability associated with autism, epilepsy and mood disorders. Electrophysiology studies in Fmr1 knockout (KO) mice, a murine model of Fragile X Syndrome, have demonstrated alterations of synaptic plasticity, with exaggerated long-term depression induced by activation of metabotropic glutamate receptors (mGluR-LTD) in Fmr1 KO hippocampus. We have previously demonstrated that activation of serotonin 5-HT7 receptors reverses mGluR-LTD in the hippocampus of wild-type and Fmr1 KO mice, thus correcting a synaptic dysfunction typically observed in this disease model. Here we show that pharmacological inhibition of cyclin-dependent kinase 5 (Cdk5, a signaling molecule recently shown to be a modulator of brain synaptic plasticity) enhanced mGluR-LTD in wild-type hippocampal neurons, which became comparable to exaggerated mGluR-LTD observed in Fmr1 KO neurons. Furthermore, Cdk5 inhibition prevented 5-HT7 receptor-mediated reversal of mGluR-LTD both in wild-type and in Fmr1 KO neurons. Our results show that Cdk5 modulates hippocampal synaptic plasticity. 5-HT7 receptors require Cdk5 to modulate synaptic plasticity in wild-type and rescue abnormal plasticity in Fmr1 KO neurons, pointing out Cdk5 as a possible novel target in Fragile X Syndrome.

Functional predication of differentially expressed circRNAs/lncRNAs in the prefrontal cortex of Nrf2-knockout mice

In the central nervous system, nuclear factor erythroid-2-related factor 2 (Nrf2) protects neurons from oxidant injury, thereby ameliorating neurodegeneration. We explored the key circular RNAs (circRNAs) and long non-coding RNAs (lncRNAs) involved in Nrf2-induced neuroprotection. We used microarrays to examine the circRNAs (DEcircRNAs), lncRNAs (DElncRNAs) and mRNAs (DEmRNAs) differentially expressed between Nrf2 (+/+) and Nrf2 (-/-) mice and identified DEcircRNA/DElncRNA-miRNA-DEmRNA interaction networks. In total, 197 DEcircRNAs, 685 DElncRNAs and 356 DEmRNAs were identified in prefrontal cortical tissues from Nrf2 (-/-) mice. The expression patterns of selected DEcircRNAs (except for mmu_circ_0003404) and DElncRNAs in qRT-PCR analyses were generally consistent with the microarray analysis results. Functional annotation of the DEmRNAs in the DEcircRNA/DElncRNA-miRNA-DEmRNA networks indicated that five non-coding RNAs (mmu_circ_0000233, ENSMUST00000204847, NONMMUT024778, NONMMUT132160 and NONMMUT132168) may contribute to Nrf2 activity, with the help of mmu_circ_0015035 and NONMMUT127961. The results also revealed that four non-coding RNAs (cicRNA.20127, mmu_circ_0012936, ENSMUST00000194077 and NONMMUT109267) may influence glutathione metabolism. Additionally, 44 DEcircRNAs and 7 DElncRNAs were found to possess coding potential. These findings provide clues to the molecular pathways through which Nrf2 protects neurons.