Insights into the regulation of 5-HT2A serotonin receptors by scaffolding proteins and kinases

A proximity proteomics pipeline with improved reproducibility and throughput

Proximity labeling (PL) via biotinylation coupled with mass spectrometry (MS) captures spatial proteomes in cells. Large-scale processing requires a workflow minimizing hands-on time and enhancing quantitative reproducibility. We introduced a scalable PL pipeline integrating automated enrichment of biotinylated proteins in a 96-well plate format. Combining this with optimized quantitative MS based on data-independent acquisition (DIA), we increased sample throughput and improved protein identification and quantification reproducibility. We applied this pipeline to delineate subcellular proteomes across various compartments. Using the 5HT2A serotonin receptor as a model, we studied temporal changes of proximal interaction networks induced by receptor activation. In addition, we modified the pipeline for reduced sample input to accommodate CRISPR-based gene knockout, assessing dynamics of the 5HT2A network in response to perturbation of selected interactors. This PL approach is universally applicable to PL proteomics using biotinylation-based PL enzymes, enhancing throughput and reproducibility of standard protocols.

Molecular glues as potential GPCR therapeutics

"Molecular Glues" are defined as small molecules that can either be endogenous or synthetic which promote interactions between proteins at their interface. Allosteric modulators, specifically GPCR allosteric modulators, can promote both the association and the dissociation of a given receptor's transducer but accomplishes this "at a distance" from the interface. However, recent structures of GPCR G protein complexes in the presence of allosteric modulators indicate that some GPCR allosteric modulators can act as "molecular glues" interacting with both the receptor and the transducer at the interface biasing transducer signaling in both a positive and negative manner depending on the transducer. Given these phenomena we discuss the implications for this class of allosteric modulators to be used as molecular tools and for future drug development.

A proximity proteomics pipeline with improved reproducibility and throughput

Proximity labeling (PL) through biotinylation coupled with mass spectrometry (MS) has emerged as a powerful technique for capturing spatial proteomes within living cells. Large-scale sample processing for proximity proteomics requires a workflow that minimizes hands-on time while enhancing quantitative reproducibility. Here, we present a scalable PL pipeline integrating automated enrichment of biotinylated proteins in a 96-well plate format. By combining this pipeline with an optimized quantitative MS acquisition method based on data-independent acquisition (DIA), we not only significantly increased sample throughput but also improved the reproducibility of protein identification and quantification. We applied this pipeline to delineate subcellular proteomes across various cellular compartments, including endosomes, late endosomes/lysosomes, the Golgi apparatus, and the plasma membrane. Moreover, employing 5HT2A serotonin receptor as a model, we investigated temporal changes of proximal interaction networks induced by the receptor's activation with serotonin. Finally, to demonstrate the applicability of our PL pipeline across multiple experimental conditions, we further modified the PL pipeline for reduced sample input amounts to accommodate CRISPR-based gene knockout, and assessed the dynamics of the 5HT2A network in response to the perturbation of selected proximal interactors. Importantly, the presented PL approach is universally applicable to PL proteomics using biotinylation-based PL enzymes, increasing both throughput and reproducibility of standard protocols.

Clostridium butyricum Alleviates DEHP Plasticizer-Induced Learning and Memory Impairment in Mice via Gut-Brain Axis

Di-(2-ethylhexyl) phthalate (DEHP) plasticizer, a well-known environmental and food pollutant, has neurotoxicity. However, it is unknown whether DEHP leads to learning and memory impairment through gut-brain axis and whether Clostridium butyricum can alleviate this impairment. Here, C57BL/6 mice were exposed to DEHP and treated with C. butyricum. Learning and memory abilities were evaluated through the Morris water maze. The levels of synaptic proteins, inflammatory cytokines, and 5-hydroxytryptamine (5-HT) were detected by immunohistochemistry or ELISA. Gut microbiota were analyzed through 16S rRNA sequencing. C. butyricum alleviated DEHP-induced learning and memory impairment and restored synaptic proteins. It significantly relieved DEHP-induced inflammation and recovered 5-HT levels. C. butyricum recovered the richness of the gut microbiota decreased by DEHP, with the Bifidobacterium genus increasing the most. Overall, C. butyricum alleviated DEHP-induced learning and memory impairment due to reduced inflammation and increased 5-HT secretion, which was partly attributed to the recovery of gut microbiota.

Psychedelics as Transformative Therapeutics

Over the past decade, psychedelic compounds have emerged as potentially transformative therapeutics for a variety of intractable neuropsychiatric conditions. However, historically most of the basic science has utilized these compounds as probes to interrogate various endogenous neurotransmitter systems-mainly the serotonin 5-HT_2A receptor. With the renewed interest in utilizing these compounds as therapeutics and the explosion in clinical trials, psychedelics have been purported to treat many neuropsychiatric disorders, including depression, cluster headaches, migraines, anxiety, and obsessive-compulsive disorder. It is therefore imperative to understand the biology and pharmacology behind their therapeutic mechanisms as well as expose any potential pitfalls in their widespread use as treatments. This review covers the latest advances in understanding the biological mechanisms, the newest efforts in drug discovery, and potential pitfalls when it comes to utilizing this class of compounds as emerging therapeutics.

Dynamics and Patterning of 5-Hydroxytryptamine 2 Subtype Receptors in JC Polyomavirus Entry

The organization and dynamics of plasma membrane receptors are a critical link in virus-receptor interactions, which finetune signaling efficiency and determine cellular responses during infection. Characterizing the mechanisms responsible for the active rearrangement and clustering of receptors may aid in developing novel strategies for the therapeutic treatment of viruses. Virus-receptor interactions are poorly understood at the nanoscale, yet they present an attractive target for the design of drugs and for the illumination of viral infection and pathogenesis. This study utilizes super-resolution microscopy and related techniques, which surpass traditional microscopy resolution limitations, to provide both a spatial and temporal assessment of the interactions of human JC polyomavirus (JCPyV) with 5-hydroxytrypamine 2 receptors (5-HT₂Rs) subtypes during viral entry. JCPyV causes asymptomatic kidney infection in the majority of the population and can cause fatal brain disease, and progressive multifocal leukoencephalopathy (PML), in immunocompromised individuals. Using Fluorescence Photoactivation Localization Microscopy (FPALM), the colocalization of JCPyV with 5-HT₂ receptor subtypes (5-HT_2A, 5-HT_2B, and 5-HT_2C) during viral attachment and viral entry was analyzed. JCPyV was found to significantly enhance the clustering of 5-HT₂ receptors during entry. Cluster analysis of infected cells reveals changes in 5-HT₂ receptor cluster attributes, and radial distribution function (RDF) analyses suggest a significant increase in the aggregation of JCPyV particles colocalized with 5-HT₂ receptor clusters in JCPyV-infected samples. These findings provide novel insights into receptor patterning during viral entry and highlight improved technologies for the future development of therapies for JCPyV infection as well as therapies for diseases involving 5-HT₂ receptors.

Receptor Interaction Profiles of 4-Alkoxy-3,5-Dimethoxy-Phenethylamines (Mescaline Derivatives) and Related Amphetamines

3,4,5-Trimethoxyphenethylamine (mescaline) is a psychedelic alkaloid found in peyote cactus. Related 4-alkoxy-3,5-dimethoxy-substituted phenethylamines (scalines) and amphetamines (3C-scalines) are reported to induce similarly potent psychedelic effects and are therefore potential novel therapeutics for psychedelic-assisted therapy. Herein, several pharmacologically uninvestigated scalines and 3C-scalines were examined at key monoamine targets in vitro. Binding affinity at human serotonergic 5-HT_1A, 5-HT_2A, and 5-HT_2C, adrenergic α_1A and α_2A, and dopaminergic D₂ receptors, rat and mouse trace amine-associated receptor 1 (TAAR1), and human monoamine transporters were assessed using target specific transfected cells. Furthermore, activation of human 5-HT_2A and 5-HT_2B receptors, and TAAR1 was examined. Generally, scalines and 3C-scalines bound with weak to moderately high affinity to the 5-HT_2A receptor (K_i = 150–12,000 nM). 3C-scalines showed a marginal preference for the 5-HT_2A vs the 5-HT_2C and 5-HT_1A receptors whereas no preference was observed for the scalines. Extending the 4-alkoxy substituent increased 5-HT_2A and 5-HT_2C receptors binding affinities, and enhanced activation potency and efficacy at the 5-HT_2A but not at the 5-HT_2B receptor. Introduction of fluorinated 4-alkoxy substituents generally increased 5-HT_2A and 5-HT_2C receptors binding affinities and increased the activation potency and efficacy at the 5-HT_2A and 5-HT_2B receptors. Overall, no potent affinity was observed at non-serotonergic targets. As observed for other psychedelics, scalines and 3C-scalines interacted with the 5-HT_2A and 5-HT_2C receptors and bound with higher affinities (up to 63-fold and 34-fold increase, respectively) when compared to mescaline.

Bimodal modulation of short-term motor memory via dynamic sodium pumps in a vertebrate spinal cord

Dynamic neuronal Na⁺/K⁺ pumps normally only respond to intense action potential firing owing to their low affinity for intracellular Na⁺. Recruitment of these Na⁺ pumps produces a post-activity ultraslow afterhyperpolarization (usAHP) up to ∼10 mV in amplitude and ∼60 s in duration, which influences neuronal properties and future network output. In spinal motor networks, the usAHP underlies short-term motor memory (STMM), reducing the intensity and duration of locomotor network output in a manner dependent on the interval between locomotor bouts. In contrast to tonically active Na⁺ pumps that help set and maintain the resting membrane potential, dynamic Na⁺ pumps are selectively antagonized by low concentrations of ouabain, which, we show, blocks both the usAHP and STMM. We examined whether dynamic Na⁺ pumps and STMM can be influenced by neuromodulators, focusing on 5-HT and nitric oxide. Bath-applied 5-HT alone had no significant effect on the usAHP or STMM. However, this is due to the simultaneous activation of two distinct 5-HT receptor subtypes (5-HT7 and 5-HT2a) that have opposing facilitatory and suppressive influences, respectively, on these two features of the locomotor system. Nitric oxide modulation exerts a potent inhibitory effect that can completely block the usAHP and erase STMM. Using selective blockers of 5-HT7 and 5-HT2a receptors and a nitric oxide scavenger, PTIO, we further provide evidence that the two modulators constitute an endogenous control system that determines how the spinal network self-regulates the intensity of locomotor output in light of recent past experience.

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Short-term memory in a spinal locomotor network is controlled by dynamic Na⁺ pumps

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Na⁺ pumps mediate an underlying ultraslow AHP modulated by 5-HT receptors and NO

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5-HT7Rs increase and 5-HT2aRs and NO decrease the usAHP and short-term motor memory

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Endogenous 5-HT and NO regulate the usAHP and short-term motor memory

The Role of Phospholipase C in GABAergic Inhibition and Its Relevance to Epilepsy

Epilepsy is characterized by recurrent seizures due to abnormal hyperexcitation of neurons. Recent studies have suggested that the imbalance of excitation and inhibition (E/I) in the central nervous system is closely implicated in the etiology of epilepsy. In the brain, GABA is a major inhibitory neurotransmitter and plays a pivotal role in maintaining E/I balance. As such, altered GABAergic inhibition can lead to severe E/I imbalance, consequently resulting in excessive and hypersynchronous neuronal activity as in epilepsy. Phospholipase C (PLC) is a key enzyme in the intracellular signaling pathway and regulates various neuronal functions including neuronal development, synaptic transmission, and plasticity in the brain. Accumulating evidence suggests that neuronal PLC is critically involved in multiple aspects of GABAergic functions. Therefore, a better understanding of mechanisms by which neuronal PLC regulates GABAergic inhibition is necessary for revealing an unrecognized linkage between PLC and epilepsy and developing more effective treatments for epilepsy. Here we review the function of PLC in GABAergic inhibition in the brain and discuss a pathophysiological relationship between PLC and epilepsy.

GRK2 mediates β-arrestin interactions with 5-HT2 receptors for JC polyomavirus endocytosis

JC polyomavirus (JCPyV) infects the majority of the population, establishing a lifelong, asymptomatic infection in the kidney of healthy individuals. People that become severely immunocompromised may experience JCPyV reactivation, which can cause progressive multifocal leukoencephalopathy (PML), a neurodegenerative disease. Due to a lack of therapeutic options, PML results in fatality or significant debilitation among affected individuals. Cellular internalization of JCPyV is mediated by serotonin 5-hydroxytryptamine subfamily 2 receptors (5-HT₂Rs) via clathrin-mediated endocytosis. The JCPyV entry process requires the clathrin-scaffolding proteins β-arrestin, adaptor protein 2 (AP2), and dynamin. Further, a β-arrestin interacting domain, the Ala-Ser-Lys (ASK) motif, within the C-terminus of 5-HT_2AR is important for JCPyV internalization and infection. Interestingly, 5-HT₂R subtypes A, B, and C equally support JCPyV entry and infection, and all subtypes contain an ASK motif, suggesting a conserved mechanism for viral entry. However, the role of the 5-HT₂R ASK motifs and the activation of β-arrestin-associated proteins during internalization has not been fully elucidated. Through mutagenesis, the ASK motifs within 5-HT_2BR and 5-HT_2CR were identified as critical for JCPyV internalization and infectivity. Further, utilizing biochemical pulldown techniques, mutagenesis of the ASK motifs in 5-HT_2BR and 5-HT_2CR resulted in reduced β-arrestin binding. Utilizing small-molecule chemical inhibitors and RNA interference, G-protein receptor kinase 2 (GRK2) was determined to be required for JCPyV internalization and infection by mediating interactions between β-arrestin and the ASK motif of 5-HT₂Rs. These findings demonstrate that GRK2 and β-arrestin interactions with 5-HT₂Rs are critical for JCPyV entry by clathrin-mediated endocytosis and resultant infection.IMPORTANCE As intracellular parasites, viruses require a host cell to replicate and cause disease. Therefore, virus-host interactions contribute to viral pathogenesis. JC polyomavirus (JCPyV) infects most of the population, establishing a lifelong asymptomatic infection within the kidney. Under conditions of severe immunosuppression JCPyV may spread to the central nervous system, causing the fatal demyelinating disease progressive multifocal leukoencephalopathy (PML). Individuals living with HIV or undergoing immunomodulatory therapies are at risk for developing PML. The mechanisms of how JCPyV uses specific receptors on the surface of host cells to initiate internalization and infection is a poorly understood process. We have further identified cellular proteins involved in JCPyV internalization and infection and elucidated their specific interactions that are responsible for activation of receptors. Collectively, these findings illuminate how viruses usurp cellular receptors during infection, contributing to current development efforts for therapeutic options for the treatment or prevention of PML.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Biased signaling in platelet G-protein coupled receptors

Platelets are small megakaryocyte-derived, anucleate, disk-like structures that play an outsized role in human health and disease. Both a decrease in the number of platelets and a variety of platelet function disorders result in petechiae or bleeding that can be life threatening. Conversely, the inappropriate activation of platelets, within diseased blood vessels, remains the leading cause of death and morbidity by affecting heart attacks and stroke. The fine balance of the platelet state in healthy individuals is controlled by a number of receptor-mediated signaling pathways that allow the platelet to rapidly respond and maintain haemostasis. G-protein coupled receptors (GPCRs) are particularly important regulators of platelet function. Here we focus on the major platelet-expressed GPCRs and discuss the roles of downstream signaling pathways (e.g., different G-protein subtypes or β-arrestin) in regulating the different phases of the platelet activation. Further, we consider the potential for selectively targeting signaling pathways that may contribute to platelet responses in disease through development of biased agonists. Such selective targeting of GPCR-mediated signaling pathways by drugs, often referred to as biased signaling, holds promise in delivering therapeutic interventions that do not present significant side effects, especially in finely balanced physiological systems such as platelet activation in haemostasis.

Mapping the physiological and molecular markers of stress and SSRI antidepressant treatment in S100a10 corticostriatal neurons

In mood disorders, psychomotor and sensory abnormalities are prevalent, disabling, and intertwined with emotional and cognitive symptoms. Corticostriatal neurons in motor and somatosensory cortex are implicated in these symptoms, yet mechanisms of their vulnerability are unknown. Here, we demonstrate that S100a10 corticostriatal neurons exhibit distinct serotonin responses and have increased excitability, compared with S100a10-negative neurons. We reveal that prolonged social isolation disrupts the specific serotonin response which gets restored by chronic antidepressant treatment. We identify cell-type-specific transcriptional signatures in S100a10 neurons that contribute to serotonin responses and strongly associate with psychomotor and somatosensory function. Our studies provide a strong framework to understand the pathogenesis and create new avenues for the treatment of mood disorders.

Pharmacotherapy with sertraline rescues brain development and behavior in a mouse model of CDKL5 deficiency disorder

Mutations in the X-linked cyclin-dependent kinase-like 5 (CDKL5) gene cause a severe neurodevelopmental disorder, CDKL5 deficiency disorder (CDD). CDKL5 is fundamental for correct brain development and function, but the molecular mechanisms underlying aberrant neurologic dysfunction in CDD are incompletely understood. Here we show a dysregulation of hippocampal and cortical serotonergic (5-HT) receptor expression in heterozygous Cdkl5 knockout (KO) female mice, suggesting that impaired 5-HT neurotransmission contributes to CDD. We demonstrate that targeting impaired 5-HT signaling via the selective serotonin reuptake inhibitor (SSRI) sertraline rescues CDD-related neurodevelopmental and behavioral defects in heterozygous Cdkl5 KO female mice. In particular, chronic treatment with sertraline normalized locomotion, stereotypic and autistic-like features, and spatial memory in Cdkl5 KO mice. These positive behavioral effects were accompanied by restored neuronal survival, dendritic development and synaptic connectivity. At a molecular level, sertraline increased brain-derived neurotrophic factor (BDNF) expression and restored abnormal phosphorylation levels of tyrosine kinase B (TrkB) and its downstream target the extracellular signal-regulated kinase (ERK1/2). Since sertraline is an FDA-approved drug with an extensive safety and tolerability data package, even for children, our findings suggest that sertraline may improve neurodevelopment in children with CDD. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

JC Polyomavirus Entry by Clathrin-Mediated Endocytosis Is Driven by β-Arrestin

JC polyomavirus (JCPyV) establishes a persistent, lifelong, asymptomatic infection within the kidney of the majority of the human population. Under conditions of severe immunosuppression or immune modulation, JCPyV can reactivate in the central nervous system (CNS) and cause progressive multifocal leukoencephalopathy (PML), a fatal demyelinating disease. Initiation of infection is mediated through viral attachment to α2,6-sialic acid-containing lactoseries tetrasaccharide c (LSTc) on the surface of host cells. JCPyV internalization is dependent on serotonin 5-hydroxytryptamine subfamily 2 receptors (5-HT₂Rs), and entry is thought to occur by clathrin-mediated endocytosis (CME). However, the JCPyV entry process and the cellular factors involved in viral internalization remain poorly understood. Treatment of cells with small-molecule chemical inhibitors and RNA interference of 5-HT₂R endocytic machinery, including β-arrestin, clathrin, AP2, and dynamin, significantly reduced JCPyV infection. However, infectivity of the polyomavirus simian virus 40 (SV40) was not affected by CME-specific treatments. Inhibition of clathrin or β-arrestin specifically reduced JCPyV internalization but did not affect viral attachment. Furthermore, mutagenesis of a β-arrestin binding domain (Ala-Ser-Lys) within the intracellular C terminus of 5-HT_2AR severely diminished internalization and infection, suggesting that β-arrestin interactions with 5-HT_2AR are critical for JCPyV infection and entry. These conclusions illuminate key host factors that regulate clathrin-mediated endocytosis of JCPyV, which is necessary for viral internalization and productive infection.IMPORTANCE Viruses usurp cellular factors to invade host cells. Activation and utilization of these proteins upon initiation of viral infection are therefore required for productive infection and resultant viral disease. The majority of healthy individuals are asymptomatically infected by JC polyomavirus (JCPyV), but if the host immune system is compromised, JCPyV can cause progressive multifocal leukoencephalopathy (PML), a rare, fatal, demyelinating disease. Individuals infected with HIV or taking prolonged immunomodulatory therapies have a heightened risk for developing PML. The cellular proteins and pathways utilized by JCPyV to mediate viral entry are poorly understood. Our findings further characterize how JCPyV utilizes the clathrin-mediated endocytosis pathway to invade host cells. We have identified specific components of this pathway that are necessary for the viral entry process and infection. Collectively, the conclusions increase our understanding of JCPyV infection and pathogenesis and may contribute to the future development of novel therapeutic strategies for PML.

Serotonergic gene polymorphisms (5-HTTLPR, 5HTR1A, 5HTR2A), and population differences in aggression: traditional (Hadza and Datoga) and industrial (Russians) populations compared

Background

Current knowledge on genetic basis of aggressive behavior is still contradictory. This may be due to the fact that the majority of studies targeting associations between candidate genes and aggression are conducted on industrial societies and mainly dealing with various types of psychopathology and disorders. Because of that, our study was carried on healthy adult individuals of both sex (n = 853).

Methods

Three populations were examined: two traditional (Hadza and Datoga) and one industrial (Russians), and the association of aggression with the following polymorphisms 5-HTTLPR, rs6295 (5HTR1A gene), and rs6311 (5HTR2A gene) were tested. Aggression was measured as total self-ratings on Buss-Perry Aggression Questionnaire.

Results

Distributions of allelic frequencies of 5-HTTLPR and 5HTR1A polymorphisms were significantly different among the three populations. Consequently, the association analyses for these two candidate genes were carried out separately for each population, while for the 5HTR2A polymorphism, it was conducted on the pooled data that made possible to introduce ethnic factor in the ANOVA model. The traditional biometrical approach revealed no sex differences in total aggression in all three samples. The three-way ANOVA (μ + 5-HTTLPR + 5HTR1A + 5HTR2A +ε) with measures of self-reported total aggression as dependent variable revealed significant effect of the second serotonin receptor gene polymorphism for the Hadza sample. For the Datoga, the interaction effect between 5-HTTLPR and 5HTR1A was significant. No significant effects of the used polymorphisms were obtained for Russians. The results of two-way ANOVA with ethnicity and the 5HTR2A polymorphism as main effects and their interactions revealed the highly significant effect of ethnicity, 5HTR2A polymorphism, and their interaction on total aggression.

Conclusions

Our data provided obvious confirmation for the necessity to consider the population origin, as well as cultural background of tested individuals, while searching for associations between genes and behavior, and demonstrated the role of cultural attitudes towards the use of in-group aggression. Our data partly explained the reasons for disagreement in results of different teams, searching for candidate-gene associations with behavior without considerations of culturally desirable norms. Previous studies suggested that the 5HTR2A gene polymorphism associates with aggression and criminality. Our data extended these findings, demonstrating the role of rs6311 (5HTR2A gene) in aggression in adult healthy men and women from our samples. We found that G-allele carriers were rated higher on total aggression.

ERK Is a Critical Regulator of JC Polyomavirus Infection

The human JC polyomavirus (JCPyV) infects the majority of the population worldwide and presents as an asymptomatic, persistent infection in the kidneys. In individuals who are immunocompromised, JCPyV can become reactivated and cause a lytic infection in the central nervous system resulting in the fatal, demyelinating disease progressive multifocal leukoencephalopathy (PML). Infection is initiated by interactions between the capsid protein viral protein 1 (VP1) and the α2,6-linked sialic acid on lactoseries tetrasaccharide c (LSTc), while JCPyV internalization is facilitated by 5-hydroxytryptamine 2 receptors (5-HT₂Rs). The mechanisms by which the serotonin receptors mediate virus entry and the signaling cascades required to drive viral infection remain poorly understood. JCPyV was previously shown to induce phosphorylation of extracellular signal-regulated kinase (ERK), a downstream target of the mitogen-activated protein kinase (MAPK) pathway, upon virus entry. However, it remained unclear whether ERK activation was required for JCPyV infection. Both ERK-specific small interfering RNA (siRNA) and ERK inhibitor treatments resulted in significantly diminished JCPyV infection in both kidney and glial cells yet had no effect on the infectivity of the polyomavirus simian virus 40 (SV40). Experiments characterizing the role of ERK during steps in the viral life cycle indicate that ERK activation is required for viral transcription, as demonstrated by a significant reduction in production of large T antigen (TAg), a key viral protein associated with the initiation of viral transcription and viral replication. These findings delineate the role of the MAPK-ERK signaling pathway in JCPyV infection, elucidating how the virus reprograms the host cell to promote viral pathogenesis.IMPORTANCE Viral infection is dependent upon host cell factors, including the activation of cellular signaling pathways. These interactions between viruses and host cells are necessary for infection and play an important role in viral disease outcomes. The focus of this study was to determine how the human JC polyomavirus (JCPyV), a virus that resides in the kidney of the majority of the population and can cause the fatal, demyelinating disease progressive multifocal leukoencephalopathy (PML) in the brains of immunosuppressed individuals, usurps a cellular signaling pathway to promote its own infectious life cycle. We demonstrated that the activation of extracellular signal-regulated kinase (ERK), a component of the mitogen-activated protein kinase (MAPK) pathway, promotes JCPyV transcription, which is required for viral infection. Our findings demonstrate that the MAPK-ERK signaling pathway is a key determinant of JCPyV infection, elucidating new information regarding the signal reprogramming of host cells by a pathogenic virus.

Phospholipid flippase ATP11C is endocytosed and downregulated following Ca2+-mediated protein kinase C activation

We and others showed that ATP11A and ATP11C, members of the P4-ATPase family, translocate phosphatidylserine (PS) and phosphatidylethanolamine from the exoplasmic to the cytoplasmic leaflets at the plasma membrane. PS exposure on the outer leaflet of the plasma membrane in activated platelets, erythrocytes, and apoptotic cells was proposed to require the inhibition of PS-flippases, as well as activation of scramblases. Although ATP11A and ATP11C are cleaved by caspases in apoptotic cells, it remains unclear how PS-flippase activity is regulated in non-apoptotic cells. Here we report that the PS-flippase ATP11C, but not ATP11A, is sequestered from the plasma membrane via clathrin-mediated endocytosis upon Ca²⁺-mediated PKC activation. Importantly, we show that a characteristic di-leucine motif (SVRPLL) in the C-terminal cytoplasmic region of ATP11C becomes functional upon PKC activation. Moreover endocytosis of ATP11C is induced by Ca²⁺-signaling via Gq-coupled receptors. Our data provide the first evidence for signal-dependent regulation of mammalian P4-ATPase.

ATP11C is a flippase that uses ATP hydrolysis to translocate phospholipids at the plasma membrane. Here, the authors show that the activation of Ca2+-dependent protein kinase C increases ATP11C endocytosis thus downregulating phospholipid translocation.

A Specific Nutrient Combination Attenuates the Reduced Expression of PSD-95 in the Proximal Dendrites of Hippocampal Cell Body Layers in a Mouse Model of Phenylketonuria

The inherited metabolic disease phenylketonuria (PKU) is characterized by increased concentrations of phenylalanine in the blood and brain, and as a consequence neurotransmitter metabolism, white matter, and synapse functioning are affected. A specific nutrient combination (SNC) has been shown to improve synapse formation, morphology and function. This could become an interesting new nutritional approach for PKU. To assess whether treatment with SNC can affect synapses, we treated PKU mice with SNC or an isocaloric control diet and wild-type (WT) mice with an isocaloric control for 12 weeks, starting at postnatal day 31. Immunostaining for post-synaptic density protein 95 (PSD-95), a post-synaptic density marker, was carried out in the hippocampus, striatum and prefrontal cortex. Compared to WT mice on normal chow without SNC, PKU mice on the isocaloric control showed a significant reduction in PSD-95 expression in the hippocampus, specifically in the granular cell layer of the dentate gyrus, with a similar trend seen in the cornus ammonis 1 (CA1) and cornus ammonis 3 (CA3) pyramidal cell layer. No differences were found in the striatum or prefrontal cortex. PKU mice on a diet supplemented with SNC showed improved expression of PSD-95 in the hippocampus. This study gives the first indication that SNC supplementation has a positive effect on hippocampal synaptic deficits in PKU mice.

Insights into Serotonin Receptor Trafficking: Cell Membrane Targeting and Internalization

Serotonin receptors (5-HTRs) mediate both central and peripheral control on numerous physiological functions such as sleep/wake cycle, thermoregulation, food intake, nociception, locomotion, sexual behavior, gastrointestinal motility, blood coagulation, and cardiovascular homeostasis. Six families of the G-protein-coupled receptors comprise most of serotonin receptors besides the conserved 5-HT3R Cys-loop type which belongs to the family of Cys-loop ligand-gated cation channel receptors. Many of these receptors are targets of pharmaceutical drugs, justifying the importance for elucidating their coupling, signaling and functioning. Recently, special interest has been focused on their trafficking inside cell lines or neurons in conjunction with their interaction with partner proteins. In this review, we describe the trafficking of 5-HTRs including their internalization, desensitization, or addressing to the plasma membrane depending on specific mechanisms which are peculiar for each class of serotonin receptor.

Serotonin 2A receptor clustering in peripheral lymphocytes is altered in major depression and may be a biomarker of therapeutic efficacy

BACKGROUND

In a previous report, we showed that the clustering of serotonin (5HT) transporter (SERT) protein on cell membranes of peripheral lymphocytes predicts responsivity to antidepressant medication in two subpopulations of naïve depression patients (Rivera-Baltanas et al., J Affect Disord, 2012, 137, 46-55). In this study, we extended this idea to 5-HT2A receptor clusters in a similar patient population.

METHODS

We collected blood samples from a subset of patients from our previous study on SERT clustering (20 untreated and newly diagnosed depression patients, and 20 matched control subjects). Blood samples were collected at the time of diagnosis and after 8 weeks of pharmacological treatment and at analogous times in control subjects. We used the Hamilton scale to quantify the level of depression in patients both before and after treatment. We then used immunocytochemistry to assess 5-HT2A receptor clusters in lymphocytes at the same time points.

RESULTS

We found that both the size and number of 5-HT2A receptor clusters were increased in naïve depression patients compared to control subjects. Interestingly, there were individual differences in the distribution of 5-HT2A receptor cluster size that allowed us to differentiate the depression patients into two subgroups: a D-I group and a D-II group. After 8 weeks of pharmacological treatment, patients in both groups showed an improvement of symptoms, but patients in the D-II group had a much better outcome with many of them showing remission of symptoms. Furthermore, although treatment decreased cluster number and size in both D-I and D-II groups, only the D-II patients showed an increase in the number of clusters within the modal peak. Importantly, the same patients that belonged in the D-I or D-II groups in the present report were also assigned to the same groups in our previous study on SERT clustering.

LIMITATIONS

The data should be replicated within a proper clinical trial.

CONCLUSIONS

5-HT2A receptor clusters in peripheral lymphocytes are altered in major depression, partially reversed by antidepressant treatment, and may be considered a putative biomarker of therapeutic efficacy in major depression.

Serotonin acts through 5-HT1 and 5-HT2 receptors to exert biphasic actions on GnRH neuron excitability in the mouse

The effect of serotonin (5-HT) on the electrical excitability of GnRH neurons was examined using gramicidin perforated-patch electrophysiology in transgenic GnRH-green fluorescent protein mice. In diestrous female, the predominant effect of 5-HT was inhibition (70%) with 50% of these cells also exhibiting a late-onset excitation. Responses were dose dependent (EC(50) = 1.2μM) and persisted in the presence of amino acid receptor antagonists and tetrodotoxin, indicating a predominant postsynaptic action of 5-HT. Studies in neonatal, juvenile, peripubertal, and adult mice revealed that 5-HT exerted less potent responses from GnRH neurons with advancing postnatal age in both sexes. In adult male mice, 5-HT exerted less potent hyperpolarizing responses with more excitations compared with females. In addition, adult proestrous female GnRH neurons exhibited reduced inhibition and a complete absence of biphasic hyperpolarization-excitation responses. Studies using 5-HT receptor antagonists demonstrated that the activation of 5-HT(1A) receptors mediated the inhibitory responses, whereas the excitation was mediated by the activation of 5-HT(2A) receptors. The 5-HT-mediated hyperpolarization involved both potassium channels and adenylate cyclase activation, whereas the 5-HT excitation was dependent on protein kinase C. The effects of exogenous 5-HT were replicated using fluoxetine, which enhances endogenous 5-HT levels. These studies demonstrate that 5-HT exerts a biphasic action on most GnRH neurons whereby a fast 5HT(1A)-mediated inhibition occurs alongside a slow 5-HT(2A) excitation. The balance of 5-HT-evoked inhibition vs excitation is developmentally regulated, sexually differentiated, and variable across the estrous cycle and may play a role in regulation of hypothalamic-pituitary-gonadal axis throughout postnatal development.

Disruption of 5-HT2A receptor-PDZ protein interactions alleviates mechanical hypersensitivity in carrageenan-induced inflammation in rats

Despite common pathophysiological mechanisms, inflammatory and neuropathic pain do not respond equally to the analgesic effect of antidepressants, except for selective serotonin reuptake inhibitors (SSRIs), which show a limited efficacy in both conditions. We previously demonstrated that an interfering peptide (TAT-2ASCV) disrupting the interaction between 5-HT2A receptors and its associated PDZ proteins (e.g. PSD-95) reveals a 5-HT2A receptor-mediated anti-hyperalgesic effect and enhances the efficacy of fluoxetine (a SSRI) in diabetic neuropathic pain conditions in rats. Here, we have examined whether the same strategy would be useful to treat inflammatory pain. Sub-chronic inflammatory pain was induced by injecting λ-carrageenan (100 µl, 2%) into the left hind paw of the rat. Mechanical hyperalgesia was assessed after acute treatment with TAT-2ASCV or/and fluoxetine (SSRI) 2.5 h after λ-carrageenan injection. Possible changes in the level of 5-HT2A receptors and its associated PDZ protein PSD-95 upon inflammation induction were quantified by Western blotting in dorsal horn spinal cord. Administration of TAT-2ASCV peptide (100 ng/rat, intrathecally) but not fluoxetine (10 mg/kg, intraperitoneally) relieves mechanical hyperalgesia (paw pressure test) in inflamed rats. This anti-hyperalgesic effect involves spinal 5-HT2A receptors and GABAergic interneurons as it is abolished by a 5-HT2A antagonist (M100907, 150 ng/rat, intrathecally) and a GABAA antagonist, (bicuculline, 3 µg/rat, intrathecally). We also found a decreased expression of 5-HT2A receptors in the dorsal spinal cord of inflamed animals which could not be rescued by TAT-2ASCV injection, while the amount of PSD-95 was not affected by inflammatory pain. Finally, the coadministration of fluoxetine does not further enhance the anti-hyperalgesic effect of TAT-2ASCV peptide. This study reveals a role of the interactions between 5-HT2A receptors and PDZ proteins in the pathophysiological pathways of inflammatory pain and opens new perspectives in its control thanks to molecules disrupting 5-HT2A receptor/PDZ protein interactions.

Multicomponent assembly processes for the synthesis of diverse yohimbine and corynanthe alkaloid analogues

A strategy involving a Mannich-type multicomponent assembly process followed by a 1,3-dipolar cycloaddition has been developed for the rapid and efficient construction of parent heterocyclic scaffolds bearing indole and isoxazolidine rings. These key intermediates were then readily elaborated using well-established protocols for refunctionalization and cross-coupling to access a diverse 180-member library of novel pentacyclic and tetracyclic compounds related to the Yohimbine and Corynanthe alkaloids. Several other new multicomponent assembly processes were developed to access dihydro-β-carboline-fused benzodiazepines, pyrimidinediones, and rutaecarpine derivatives.

Neurabin scaffolding of adenosine receptor and RGS4 regulates anti-seizure effect of endogenous adenosine

Endogenous adenosine is an essential protective agent against neural damage by various insults to the brain. However, the therapeutic potential of adenosine receptor-directed ligands for neuroprotection is offset by side effects in peripheral tissues and organs. An increase in adenosine receptor responsiveness to endogenous adenosine would enhance neuroprotection while avoiding the confounding effects of exogenous ligands. Here we report novel regulation of adenosine-evoked responses by a neural tissue-specific protein, neurabin. Neurabin attenuated adenosine A(1) receptor (A1R) signaling by assembling a complex between the A1R and the regulator of G-protein signaling 4 (RGS4), a protein known to turn off G-protein signaling. Inactivation of the neurabin gene enhanced A1R signaling and promoted the protective effect of adenosine against excitotoxic seizure and neuronal death in mice. Furthermore, administration of a small molecule inhibitor of RGS4 significantly attenuated seizure severity in mice. Notably, the dose of kainate capable of inducing an ∼50% rate of death in wild-type (WT) mice did not affect neurabin-null mice or WT mice cotreated with an RGS4 inhibitor. The enhanced anti-seizure and neuroprotective effect achieved by disruption of the A1R/neurabin/RGS4 complex is elicited by the on-site and on-demand release of endogenous adenosine, and does not require administration of A1R ligands. These data identify neurabin-RGS4 as a novel tissue-selective regulatory mechanism for fine-tuning adenosine receptor function in the nervous system. Moreover, these findings implicate the A1R/neurabin/RGS4 complex as a valid therapeutic target for specifically manipulating the neuroprotective effects of endogenous adenosine.

Recent Patents on Emerging Therapeutics for the Treatment of Glaucoma, Age Related Macular Degeneration and Uveitis

Advancements in the field and rising interest among pharmaceutical researchers have led to the development of new molecules with enhanced therapeutic activity. Design of new drugs which can target a particular pathway and/or explore novel targets is of immense interest to ocular pharmacologists worldwide. Delivery of suitable pharmacologically active agents at proper dose (within the therapeutic window) to the target tissues without any toxicity to the healthy ocular tissues still remain an elusive task. Moreover, the presence of static and dynamic barriers to drug absorption including the corneal epithelium (lipophilic), corneal and scleral stroma (hydrophilic), conjunctival lymphatics, choroidal vasculature and the blood-ocular barriers also pose a significant challenge for achieving therapeutic drug concentrations at the target site. Although many agents are currently available, new compounds are being introduced for treating various ocular diseases. Deeper understanding of the etiology and complex mechanisms associated with the disease condition would aid in the development of potential therapeutic candidates. Novel small molecules as well as complex biotechnology derived macromolecules with superior efficacy, safety and tolerability are being developed. Therefore, this review article provides an overview of existing drugs, treatment options, advances in emerging therapeutics and related recent patents for the treatment of ocular disorders such as glaucoma, age related macular degeneration (AMD) and uveitis.

Glycogen Synthase Kinase-3 is an Intermediate Modulator of Serotonin Neurotransmission

Serotonin is a neurotransmitter with broad functions in brain development, neuronal activity, and behaviors; and serotonin is the prominent drug target in several major neuropsychiatric diseases. The multiple actions of serotonin are mediated by diverse serotonin receptor subtypes and associated signaling pathways. However, the key signaling components that mediate specific function of serotonin neurotransmission have not been fully identified. This review will provide evidence from biochemical, pharmacological, and animal behavioral studies showing that serotonin regulates the activation states of brain glycogen synthase kinase-3 (GSK3) via type 1 and type 2 serotonin receptors. In return, GSK3 directly interacts with serotonin receptors in a highly selective manner, with a prominent effect on modulating serotonin 1B receptor activity. Therefore, GSK3 acts as an intermediate modulator in the serotonin neurotransmission system, and balanced GSK3 activity is essential for serotonin-regulated brain function and behaviors. Particularly important, several classes of serotonin-modulating drugs, such as antidepressants and atypical antipsychotics, regulate GSK3 by inhibiting its activity in brain, which reinforces the importance of GSK3 as a potential therapeutic target in neuropsychiatric diseases associated with abnormal serotonin function.

Synthesis and evaluation of bidentate ligands designed to interact with PDZ domains

We designed bidentate ligands to target PDZ domains through two binding sites: site S0, delimited by the GLGF loop, and site S1, a zone situated around loop β(B)/β(C). A molecular docking study allowed us to design a generic S0 binder, to which was attached a variable size linker, itself linked to an amino acid aimed to interact with the S1 site of PDZ domains. A series of 15 novel bidentate ligands was prepared in 6-11 steps in good overall yield (24-43%). Some of these ligands showed an inhibitory activity against serotonin 5-HT2A receptor/PSD-95 interaction. This was assessed by pull-down assay using a synthetic decapeptide corresponding to the C-terminal residues of the receptor as a bait.

Novel ocular antihypertensive compounds in clinical trials

Introduction:

Glaucoma is a multifactorial disease characterized by progressive optic nerve injury and visual field defects. Elevated intraocular pressure (IOP) is the most widely recognized risk factor for the onset and progression of open-angle glaucoma, and IOP-lowering medications comprise the primary treatment strategy. IOP elevation in glaucoma is associated with diminished or obstructed aqueous humor outflow. Pharmacotherapy reduces IOP by suppressing aqueous inflow and/or increasing aqueous outflow.

Purpose:

This review focuses on novel non-FDA approved ocular antihypertensive compounds being investigated for IOP reduction in ocular hypertensive and glaucoma patients in active clinical trials within approximately the past 2 years.

Methods:

The mode of IOP reduction, pharmacology, efficacy, and safety of these new agents were assessed. Relevant drug efficacy and safety trials were identified from searches of various scientific literature databases and clinical trial registries. Compounds with no specified drug class, insufficient background information, reformulations, and fixed-combinations of marketed drugs were not considered.

Results:

The investigational agents identified comprise those that act on the same targets of established drug classes approved by the FDA (ie, prostaglandin analogs and β-adrenergic blockers) as well as agents belonging to novel drug classes with unique mechanisms of action. Novel targets and compounds evaluated in clinical trials include an actin polymerization inhibitor (ie, latrunculin), Rho-associated protein kinase inhibitors, adenosine receptor analogs, an angiotensin II type 1 receptor antagonist, cannabinoid receptor agonists, and a serotonin receptor antagonist.

Conclusion:

The clinical value of novel compounds for the treatment of glaucoma will depend ultimately on demonstrating favorable efficacy and benefit-to-risk ratios relative to currently approved prostaglandin analogs and β-blockers and/or having complementary modes of action.

Strategies to discover unexpected targets for drugs active at G protein-coupled receptors

G protein-coupled receptors (GPCRs) are an evolutionarily conserved family of signaling molecules comprising approximately 2% of the human genome; this receptor family remains a central focus in basic pharmacology studies and drug discovery efforts. Detailed studies of drug action at GPCRs over the past decade have revealed existing and novel ligands that exhibit polypharmacology-that is, drugs with activity at more than one receptor target for which they were designed. These "off-target" drug actions can be a liability that causes adverse side effects; however, in several cases, drugs with less selectivity demonstrate better clinical efficacy. Here we review physical screening and cheminformatic approaches that define drug activity at the GPCR receptorome. In many cases, such profiling has revealed unexpected targets that explain therapeutic actions as well as off-targets underlying drug side effects. Such drug-receptor profiling has also provided new insights into mechanisms of action of existing drugs and has suggested directions for future drug development.

Modulation of monoamine receptors by adaptor proteins and lipid rafts: role in some effects of centrally acting drugs and therapeutic agents

The monoamines and their cognate receptors are widespread in the central nervous system and are vital for normal brain function. Dysfunction in these systems underlies several psychiatric and neurological disease states, and consequently monoamines are targets of a host of pharmacotherapies. This review provides an overview on how monoamine receptors are regulated by adaptor proteins and lipid rafts with emphasis on interactions in nerve cells. Monoamine receptors have prominent intracellular loops that provide binding sites for adaptor proteins. Receptor function is further modulated by cholesterol and submembranous microdomains termed lipid rafts. These interactions determine several facets of G protein-coupled receptor (GPCR) function including trafficking, localization, and signaling. Possible roles of adaptor proteins and lipid rafts in disease states and in mediating actions of drugs and therapeutic agents are also discussed.

Brain serotonergic circuitries

Brain serotonergic circuitries interact with other neurotransmitter systems on a multitude of different molecular levels. In humans, as in other mammalian species, serotonin (5-HT) plays a modulatory role in almost every physiological function. Furthermore, serotonergic dysfunction is thought to be implicated in several psychiatric and neurodegenerative disorders. We describe the neuroanatomy and neurochemistry of brain serotonergic circuitries. The contribution of emergent in vivo imaging methods to the regional localization of binding site receptors and certain aspects of their functional connectivity in correlation to behavior is also discussed. 5-HT cell bodies, mainly localized in the raphe nuclei, send axons to almost every brain region. It is argued that the specificity of the local chemocommunication between 5-HT and other neuronal elements mainly depends on mechanisms regulating the extracellular concentration of 5-HT the diversity of high-affinity membrane receptors, and their specific transduction modalities.

Irving Page Lecture: 5-HT(2A) serotonin receptor biology: interacting proteins, kinases and paradoxical regulation

5-Hydroxytryptamine(2A) (5-HT(2A)) serotonin receptors are important pharmacological targets for a large number of central nervous system and peripheral serotonergic medications. In this review article I summarize work mainly from my lab regarding serotonin receptor anatomy, pharmacology, signaling and regulation. I highlight the role of serotonin receptor interacting proteins and the emerging paradigm of G-protein coupled receptor functional selectivity.

Hallucinogenic 5-hydroxytryptamine 2A receptor agonist effects in senescence-accelerated mice

Many neuropharmacological agents modulate the activity and conformation of heptahelical G protein-coupled receptors and activate ligand-specific signaling pathways. The hallucinogenic chemical 1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane (DOI), a serotonin receptor 2A (5-HT2AR) agonist, evokes extracellular signal-regulated kinase 1/2 (ERK1/2) signaling and head-twitch behavior. We previously reported that the senescence accelerated-prone mouse 6 (SAMP6) exhibited altered emotional behavior and increased levels of a serotonin-biosynthesizing enzyme compared to the senescence accelerated-resistant mouse 1 (SAMR1); however, the mechanism underlying the relationship between specific receptor signaling and behavioral phenotypes was unclear. In this study, we performed head-twitch tests and examined the total and phosphorylated levels of ERK1/2 and cAMP-responsive element-binding protein (CREB) in the bilateral somatosensory cortex to assess the differences between SAMP6 and SAMR1 using DOI. Although DOI dose-dependently increased the head-twitch response in both strains, the responses of SAMP6 given 0.3 and 1.0 mg/kg DOI were significantly greater than those of SAMR1 given DOI at the same doses. Although no dose-dependent increase in total ERK1/2 and total CREB expression was detected in response to DOI, the levels of phospho-ERK1/2 and -CREB increased in both strains. The phospho-ERK1/2 and -CREB levels in SAMP6 given 0.3 and 1.0 mg/kg DOI were significantly higher than those in SAMR1 given DOI at the same doses. These results indicate that SAMP6 increases DOI-dependent ERK1/2-CREB signaling leading to more head-twitch responses than SAMR1, and that SAMP6 could provide a useful model for examining the relationship between 5-HT2AR regulatory signaling and behavioral phenotypes.

Measuring endogenous 5-HT release by emission tomography: promises and pitfalls

Molecular in vivo neuroimaging techniques can be used to measure regional changes in endogenous neurotransmitters, evoked by challenges that alter synaptic neurotransmitter concentration. This technique has most successfully been applied to the study of endogenous dopamine release using positron emission tomography, but has not yet been adequately extended to other neurotransmitter systems. This review focuses on how the technique has been applied to the study of the 5-hydroxytryptamine (5-HT) system. The principles behind visualising fluctuations in neurotransmitters are introduced, with reference to the dopaminergic system. Studies that aim to image acute, endogenous 5-HT release or depletion at 5-HT receptor targets are summarised, with particular attention to studies in humans. Radiotracers targeting the 5-HT(1A), 5-HT(2A), and 5-HT(4) receptors and the serotonin reuptake transporter have been explored for their sensitivity to 5-HT fluctuations, but with mixed outcomes; tracers for these targets cannot reliably image endogenous 5-HT in humans. Shortcomings in our basic knowledge of the mechanisms underlying changes in binding potential are addressed, and suggestions are made as to how the selection of targets, radiotracers, challenge paradigms, and experimental design might be optimised to improve our chances of successfully imaging endogenous neurotransmitters in the future.

Disrupting 5-HT(2A) receptor/PDZ protein interactions reduces hyperalgesia and enhances SSRI efficacy in neuropathic pain

Antidepressants are one of the first-line treatments for neuropathic pain. Despite the influence of serotonin (5-hydroxytryptamine, 5-HT) in pain modulation, selective serotonin reuptake inhibitors (SSRIs) are less effective than tricyclic antidepressants. Here, we show, in diabetic neuropathic rats, an alteration of the antihyperalgesic effect induced by stimulation of 5-HT(2A) receptors, which are known to mediate SSRI-induced analgesia. 5-HT(2A) receptor density was not changed in the spinal cord of diabetic rats, whereas postsynaptic density protein-95 (PSD-95), one of the PSD-95/disc large suppressor/zonula occludens-1 (PDZ) domain containing proteins interacting with these receptors, was upregulated. Intrathecal injection of a cell-penetrating peptidyl mimetic of the 5-HT(2A) receptor C-terminus, which disrupts 5-HT(2A) receptor-PDZ protein interactions, induced an antihyperalgesic effect in diabetic rats, which results from activation of 5-HT(2A) receptors by endogenous 5-HT. The peptide also enhanced antihyperalgesia induced by the SSRI fluoxetine. Its effects likely resulted from an increase in receptor responsiveness, because it revealed functional 5-HT(2A) receptor-operated Ca(2+) responses in neurons, an effect mimicked by knockdown of PSD-95. Hence, 5-HT(2A) receptor/PDZ protein interactions might contribute to the resistance to SSRI-induced analgesia in painful diabetic neuropathy. Disruption of these interactions might be a valuable strategy to design novel treatments for neuropathic pain and to increase the effectiveness of SSRIs.

Novel fused pyrrole heterocyclic ring systems as structure analogs of LE 300: Synthesis and pharmacological evaluation as serotonin 5-HT(2A), dopamine and histamine H(1) receptor ligands

LE 300 represents a structurally novel type of antagonists acting preferentially at the dopamine D(1)/D(5 )receptors and the serotonin 5-HT(2A )receptor. This compound consists of a ten-membered central azecine ring fused to an indole ring on one side and a benzene moiety on the other side. To estimate the importance of the indole and / or phenyl moieties in this highly active benz-indolo-azecine, both rings were removed and replaced with a 1H-pyrrole counterpart. Accordingly, some new analogs of LE 300 namely, pyrrolo[2,3-g]indolizine, pyrrolo[3,2-a]quinolizine rings and their corresponding dimethylpyrrolo[2,3-d]azonine, and dimethylpyrrolo[2,3-d]azecine were synthesized to be evaluated for their activity at the 5-HT(2A) and dopamine D(1), D(2L), D(4), D(5) receptors in relation to LE 300. In addition, their activity at the H(1)-histamine receptors was also determined. The results suggested that the rigid pyrrolo[2,3-g]indolizine 7 and pyrrolo[3,2-a]quinolizine 8 analogs lacked biological activity in the adopted three bioassays. However, their corresponding flexible pyrrolo[2,3-d]azonine 11 and pyrrolo[2,3-d]azecine 12 derivatives revealed weak partial agonistic activity and weak antagonistic potency at the serotonin 5-HT(2A )and histamine H(1 )receptors, respectively. Meanwhile, they showed no affinity to any of the four utilized dopamine receptors. Variation in ring size did not contribute to a significant influence on the three tested bioactivities. Removal of the hydrophobic moiety (phenyl ring) and replacement of the indole moiety with a 1H-pyrrole counterpart led to a dramatic alteration in the profile of activity of such azecine-type compounds.

Brain serotonergic circuitries

Brain serotonergic circuitries interact with other neurotransmitter systems on a multitude of different molecular levels. In humans, as in other mammalian species, serotonin (5-HT) plays a modulatory role in almost every physiological function. Furthermore, serotonergic dysfunction is thought to be implicated in several psychiatric and neurodegenerative disorders. We describe the neuroanatomy and neurochemistry of brain serotonergic circuitries. The contribution of emergent in vivo imaging methods to the regional localization of binding site receptors and certain aspects of their functional connectivity in correlation to behavior is also discussed. 5-HT cell bodies, mainly localized in the raphe nuclei, send axons to almost every brain region. It is argued that the specificity of the local chemocommunication between 5-HT and other neuronal elements mainly depends on mechanisms regulating the extracellular concentration of 5-HT the diversity of high-affinity membrane receptors, and their specific transduction modalities.

Genetic deletion of p90 ribosomal S6 kinase 2 alters patterns of 5-hydroxytryptamine 2A serotonin receptor functional selectivity

The concept of functional selectivity has now thoroughly supplanted the previously entrenched notion of intrinsic efficacy by explaining how agonists and antagonists exhibit a range of efficacies for distinct receptor-mediated responses. It is noteworthy that functional selectivity accommodates significant changes in efficacy resulting from differential expression of G protein-coupled receptor modifying proteins (i.e., "conditional efficacy")-a phenomenon with profound implications for drug discovery. We have uncovered a novel regulatory mechanism whereby p90 ribosomal S6 kinase 2 (RSK2) interacts with 5-hydroxytryptamine(2A) (5-HT(2A)) serotonin receptors and attenuates receptor signaling via direct receptor phosphorylation (Proc Natl Acad Sci U S A 103:4717-4722, 2006; J Biol Chem 284:5557-5573, 2009). This discovery, together with the mounting evidence for conditional efficacy, suggested to us that 5-HT(2A) agonist signaling might be disproportionately affected by alterations in RSK2 expression. To test this hypothesis, we evaluated a chemically diverse set of 5-HT(2A) agonists at three readouts of 5-HT(2A) receptor activation in both wild-type (WT) and RSK2 knock-out (KO) mouse embryonic fibroblasts (MEFs). Here we report that 5-HT(2A) receptor agonist efficacies were significantly and variably augmented in RSK2 KO MEFs compared with WT MEFs. As a result, relative agonist efficacies were significantly altered, and even reversed, between WT and RSK2 KO MEFs for a single effector readout. This study provides the first evidence that deletion of a single kinase can elicit profound changes in patterns of agonist functional selectivity.

Rapid modulation of spine morphology by the 5-HT2A serotonin receptor through kalirin-7 signaling

The 5-HT(2A) serotonin receptor is the most abundant serotonin receptor subtype in the cortex and is predominantly expressed in pyramidal neurons. The 5-HT(2A) receptor is a target of several hallucinogens, antipsychotics, anxiolytics, and antidepressants, and it has been associated with several psychiatric disorders, conditions that are also associated with aberrations in dendritic spine morphogenesis. However, the role of 5-HT(2A) receptors in regulating dendritic spine morphogenesis in cortical neurons is unknown. Here we show that the 5-HT(2A) receptor is present in a subset of spines, in addition to dendritic shafts. It colocalizes with PSD-95 and with multiple PDZ protein-1 (MUPP1) in a subset of dendritic spines of rat cortical pyramidal neurons. MUPP1 is enriched in postsynaptic density (PSD) fractions, is targeted to spines in pyramidal neurons, and enhances the localization of 5-HT(2A) receptors to the cell periphery. 5-HT(2A) receptor activation by the 5-HT(2) receptor agonist DOI induced a transient increase in dendritic spine size, as well as phosphorylation of p21-activated kinase (PAK) in cultured cortical neurons. PAK is a downstream target of the neuronal Rac guanine nucleotide exchange factor (RacGEF) kalirin-7 that is important for spine remodeling. Kalirin-7 regulates dendritic spine morphogenesis in neurons but its role in neuromodulator signaling has not been investigated. We show that peptide interference that prevents the localization of kalirin-7 to the postsynaptic density disrupts DOI-induced PAK phosphorylation and spine morphogenesis. These results suggest a potential role for serotonin signaling in modulating spine morphology and kalirin-7's function at cortical synapses.

Ribosomal S6 kinase 2 directly phosphorylates the 5-hydroxytryptamine 2A (5-HT2A) serotonin receptor, thereby modulating 5-HT2A signaling

The 5-hydroxytryptamine 2A (5-HT(2A)) receptor is a member of the G protein-coupled receptor superfamily (GPCR) and plays a key role in transducing a variety of cellular signals elicited by 5-hydroxytryptamine in both peripheral and central tissues. Despite its broad physiological importance, our current understanding of 5-HT(2A) receptor regulation is incomplete. We recently reported the novel finding that the multifunctional ERK effector ribosomal S6 kinase 2 (RSK2) physically interacts with the 5-HT(2A) receptor third intracellular (i3) loop and modulates receptor signaling (Sheffler, D. J., Kroeze, W. K., Garcia, B. G., Deutch, A. Y., Hufeisen, S. J., Leahy, P., Bruning, J. C., and Roth, B. L. (2006) Proc. Natl. Acad. Sci. U. S. A. 103, 4717-4722). We report here that RSK2 directly phosphorylates the 5-HT(2A) receptor i3 loop at the conserved residue Ser-314, thereby modulating 5-HT(2A) receptor signaling. Furthermore, these studies led to the discovery that RSK2 is required for epidermal growth factor-mediated heterologous desensitization of the 5-HT(2A) receptor. We arrived at these conclusions via multiple lines of evidence, including in vitro kinase experiments, tandem mass spectrometry, and site-directed mutagenesis. Our findings were further validated using phospho-specific Western blot analysis, metabolic labeling studies, and whole-cell signaling experiments. These results support a novel regulatory mechanism in which a downstream effector of the ERK/MAPK pathway directly interacts with, phosphorylates, and modulates signaling of the 5-HT(2A) serotonin receptor. To our knowledge, these findings are the first to demonstrate that a downstream member of the ERK/MAPK cascade phosphorylates a GPCR as well as mediates cross-talk between a growth factor and a GPCR.