Agonist-specific transactivation of phosphoinositide 3-kinase signaling pathway mediated by the dopamine D2 receptor

Cytoprotective effect of melatonin against MPP+ toxicity in SH-SY5Y cells: Role sharing of two types of antioxidative activities of melatonin

Melatonin is a neurohormone that is not only a regulator of circadian cycles, but also a potent antioxidant. Parkinson's disease (PD) is a major neurodegenerative disease that may result from oxidative stress as a part of its pathogenic cascade. Therefore, antioxidants, including melatonin, have attracted attention as potential candidates for neuroprotection against PD-related neurotoxicity. In this study, we report that melatonin has 2 types of antioxidant mechanisms of neuroprotection in an experimental cellular PD model using 1-Methyl-4-phenylpyridinium ion (MPP+) in human neuroblastoma SH-SY5Y cells. The first mechanism is a classical antioxidative mechanism through the direct action of melatonin, which reduces lipid hydroperoxide and 8-OHdG. The second mechanism is an indirect antioxidative effect via the melatonin receptor (Mel-R)/PI3K/Akt/Nrf2 cascade. Melatonin and Mel-R agonist activated PI3K/Akt signaling and Nrf2. Both Mel-R antagonist and the PI3K inhibitor blocked transcription induced by Nrf2 and the cytoprotective effect of melatonin. Interestingly, the antioxidative effect due to the Nrf2-related mechanism contributed mainly to a decrease of protein carbonyl, but not to lipid hydroperoxide and 8-OHdG. Mel-R agonist also showed a similar effect. Our results elucidate the mechanism of melatonin's powerful antioxidative effect and suggest the application of melatonin therapy to PD-related cytotoxicity.

G Protein-Dependent Activation of the PKA-Erk1/2 Pathway by the Striatal Dopamine D1/D3 Receptor Heteromer Involves Beta-Arrestin and the Tyrosine Phosphatase Shp-2

The heteromer composed of dopamine D1 and D3 receptors (D1R-D3R) has been defined as a structure able to trigger Erk1/2 and Akt signaling in a G protein-independent, beta-arrestin 1-dependent way that is physiologically expressed in the ventral striatum and is likely involved in the control of locomotor activity. Indeed, abnormal levels of D1R-D3R heteromer in the dorsal striatum have been correlated with the development of L-DOPA-induced dyskinesia (LID) in Parkinson's disease patients, a motor complication associated with striatal D1R signaling, thus requiring Gs protein and PKA activity to activate Erk1/2. Therefore, to clarify the role of the D1R/D3R heteromer in LID, we investigated the signaling pathway induced by the heteromer using transfected cells and primary mouse striatal neurons. Collectively, we found that in both the cell models, D1R/D3R heteromer-induced activation of Erk1/2 exclusively required the D1R molecular effectors, such as Gs protein and PKA, with the contribution of the phosphatase Shp-2 and beta-arrestins, indicating that heterodimerization with the D3R abolishes the specific D3R-mediated signaling but strongly allows D1R signals. Therefore, while in physiological conditions the D1R/D3R heteromer could represent a mechanism that strengthens the D1R activity, its pathological expression may contribute to the abnormal PKA-Shp-2-Erk1/2 pathway connected with LID.

Dopamine, Immunity, and Disease

The neurotransmitter dopamine is a key factor in central nervous system (CNS) function, regulating many processes including reward, movement, and cognition. Dopamine also regulates critical functions in peripheral organs, such as blood pressure, renal activity, and intestinal motility. Beyond these functions, a growing body of evidence indicates that dopamine is an important immunoregulatory factor. Most types of immune cells express dopamine receptors and other dopaminergic proteins, and many immune cells take up, produce, store, and/or release dopamine, suggesting that dopaminergic immunomodulation is important for immune function. Targeting these pathways could be a promising avenue for the treatment of inflammation and disease, but despite increasing research in this area, data on the specific effects of dopamine on many immune cells and disease processes remain inconsistent and poorly understood. Therefore, this review integrates the current knowledge of the role of dopamine in immune cell function and inflammatory signaling across systems. We also discuss the current understanding of dopaminergic regulation of immune signaling in the CNS and peripheral tissues, highlighting the role of dopaminergic immunomodulation in diseases such as Parkinson's disease, several neuropsychiatric conditions, neurologic human immunodeficiency virus, inflammatory bowel disease, rheumatoid arthritis, and others. Careful consideration is given to the influence of experimental design on results, and we note a number of areas in need of further research. Overall, this review integrates our knowledge of dopaminergic immunology at the cellular, tissue, and disease level and prompts the development of therapeutics and strategies targeted toward ameliorating disease through dopaminergic regulation of immunity. SIGNIFICANCE STATEMENT: Canonically, dopamine is recognized as a neurotransmitter involved in the regulation of movement, cognition, and reward. However, dopamine also acts as an immune modulator in the central nervous system and periphery. This review comprehensively assesses the current knowledge of dopaminergic immunomodulation and the role of dopamine in disease pathogenesis at the cellular and tissue level. This will provide broad access to this information across fields, identify areas in need of further investigation, and drive the development of dopaminergic therapeutic strategies.

New Structural Perspectives in G Protein-Coupled Receptor-Mediated Src Family Kinase Activation

Src family kinases (SFKs) are key regulators of cell proliferation, differentiation, and survival. The expression of these non-receptor tyrosine kinases is strongly correlated with cancer development and tumor progression. Thus, this family of proteins serves as an attractive drug target. The activation of SFKs can occur via multiple signaling pathways, yet many of them are poorly understood. Here, we summarize the current knowledge on G protein-coupled receptor (GPCR)-mediated regulation of SFKs, which is of considerable interest because GPCRs are among the most widely used pharmaceutical targets. This type of activation can occur through a direct interaction between the two proteins or be allosterically regulated by arrestins and G proteins. We postulate that a rearrangement of binding motifs within the active conformation of arrestin-3 mediates Src regulation by comparison of available crystal structures. Therefore, we hypothesize a potentially different activation mechanism compared to arrestin-2. Furthermore, we discuss the probable direct regulation of SFK by GPCRs and investigate the intracellular domains of exemplary GPCRs with conserved polyproline binding motifs that might serve as scaffolding domains to allow such a direct interaction. Large intracellular domains in GPCRs are often understudied and, in general, not much is known of their contribution to different signaling pathways. The suggested direct interaction between a GPCR and a SFK could allow for a potential immediate allosteric regulation of SFKs by GPCRs and thereby unravel a novel mechanism of SFK signaling. This overview will help to identify new GPCR-SFK interactions, which could serve to explain biological functions or be used to modulate downstream effectors.

Ouabain-induced activation of phospholipase C zeta and its contributions to bovine sperm capacitation

The sperm-derived oocyte activating factor, phospholipase C zeta (PLC ζ), is the only PLC isoform reported in cattle. The objectives were to (1) localize PLC ζ in fresh and capacitated bovine sperm and (2) investigate the activation of PLC ζ during bull sperm capacitation and contributions of PLC activity to this process. We confirmed interaction of testis-specific isoform of Na/K-ATPase (ATP1A4) with PLC ζ (immunolocalization and immunoprecipitation) and tyrosine phosphorylation (immunoprecipitation) of PLC ζ (a post-translational protein modification commonly involved in activation of PLC in somatic cells) during capacitation. Furthermore, incubation of sperm under capacitating conditions upregulated PLC-mediated hyperactivated motility, tyrosine phosphoprotein content, acrosome reaction, and F-actin formation (flow cytometry), implying that PLC activity is enhanced during capacitation and contributing to these capacitation processes. In conclusion, we inferred that PLC ζ is activated during capacitation by tyrosine phosphorylation through a mechanism involving ATP1A4, contributing to capacitation-associated biochemical events.

Tumor suppressor DRD2 facilitates M1 macrophages and restricts NF-κB signaling to trigger pyroptosis in breast cancer

Rationale: Breast cancer (BrCa) is the most common cancer worldwide, and the 5-year relative survival rate has declined in patients diagnosed at stage IV. Advanced BrCa is considered as incurable, which still lack effective treatment strategies. Identifying and characterizing new tumor suppression genes is important to establish effective prognostic biomarkers or therapeutic targets for late-stage BrCa.

Methods: RNA-seq was applied in BrCa tissues and normal breast tissues. Through analyzing differentially expressed genes, DRD2 was selected for further analysis. And expression and promoter methylation status of DRD2 were also determined. DRD2 functions were analyzed by various cell biology assays in vitro. Subcutaneous tumor model was used to explore DRD2 effects in vivo. A co-cultivated system was constructed to investigate interactions of DRD2 and macrophages in vitro. WB, IHC, IF, TUNEL, qRT-PCR, Co-IP, Antibody Array, and Mass Spectrum analysis were further applied to determine the detailed mechanism.

Results: In BrCa, DRD2 was found to be downregulated due to promoter methylation. Higher expression of DRD2 positively correlated with longer survival times especially in HER2-positive patients. DRD2 also promoted BrCa cells sensitivity to Paclitaxel. Ectopic expression of DRD2 significantly inhibited BrCa tumorigenesis. DRD2 also induced apoptosis as well as necroptosis in vitro and in vivo. DRD2 restricted NF-κB signaling pathway activation through interacting with β-arrestin2, DDX5 and eEF1A2. Interestingly, DRD2 also regulated microenvironment as it facilitated M1 polarization of macrophages, and triggered GSDME-executed pyroptosis.

Conclusion: Collectively, this study novelly manifests the role of DRD2 in suppressing BrCa tumorigenesis, predicting prognosis and treatment response. And this study further reveals the critical role of DRD2 in educating M1 macrophages, restricting NF-κB signaling pathway and triggering different processes of programmed cell death in BrCa. Taking together, those findings represent a predictive and therapeutic target for BrCa.

Tannic acid acts as an agonist of the dopamine D2L receptor, regulates immune responses, and ameliorates experimentally induced colitis in mice

Tannic acid (TA) is an herbal polyphenol containing a galloyl group that has been prescribed to treat gastroenteritis, diarrhea, and irritable bowel syndrome. TA has anti-inflammatory, anti-cancer, and anti-viral properties; however, the molecular mechanisms of these potential therapeutic effects are still largely unknown. Here, we examined the ability of TA to induce anti-inflammatory responses. TA was found to be an agonist of the dopamine D2L receptor. TA reduced interferon (IFN)-γ and interleukin (IL)-1β secretion but upregulated tumor necrosis factor α and IL-10 secretion from lipopolysaccharide (LPS)-stimulated mouse splenocytes. TA also reduced IFN-γ secretion but enhanced IL-10 secretion from anti-cluster of differentiation (CD) 3/CD28 antibody-stimulated splenocytes. An immune subset study confirmed that TA regulated cytokine secretion by various types of immune cells in the context of stimulation with LPS or anti-CD3/CD28 antibodies. Administration of TA to mice with experimentally induced colitis strikingly suppressed weight loss, colon shrinkage, and IL-17 secretion from mesenteric lymph node lymphocytes in response to CD3/CD28 stimulation. These data suggest that TA suppresses inflammatory responses in colitis by regulating cytokine secretion by immune cells in the colon.

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Tannic acid is an agonist of the dopamine D2L receptor.

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Tannic acid suppresses IFN-γ secretion by LPS-stimulated splenocytes.

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Tannic acid modulates anti-CD3/CD28 antibody-stimulated cytokine levels in CD4⁺ T cells.

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Tannic acid ameliorates dextran sodium salt (DSS)-induced colitis in C57BL/6 mice.

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Tannic acid reduces production of IL-17 in DSS-induced colitis.

Fluoxetine improves behavioural deficits induced by chronic alcohol treatment by alleviating RNA editing of 5-HT2C receptors

The alcoholism and major depressive disorder are common comorbidity, with alcohol-induced depressive symptoms being eased by selective serotonin re-uptake inhibitors (SSRIs), although the mechanisms underlying pathology and therapy are poorly understood. Chronic alcohol consumption affects the activity of serotonin 2C receptors (5-HT_2CR) by regulating adenosine deaminases acting on RNA (ADARs) in neurons. Astrogliopathic changes contribute to alcohol addiction, while decreased release of ATP from astrocytes can trigger depressive-like behaviours in mice. In this study, we discovered that chronic alcohol treatment increased editing of RNA of 5-HT_2CR via up-regulating the expression of ADAR2, consequently reducing the release of ATP from astrocytes induced by 5-HT_2CR agonist, MK212. Moreover, SSRI antidepressant fluoxetine decreased the expression of ADAR2 through the transactivation of EGFR/PI3K/AKT/cFos signalling pathway. The increased release of astroglial ATP by MK212 which was suppressed by chronic alcohol consumption, and reduction in ADAR2 activity eliminated the RNA editing of 5-HT_2CR increased by alcohol in vitro and recovered the release of ATP from astrocytes induced by MK212. Meanwhile, fluoxetine improved the behavioural and motor symptoms induced by alcohol addiction and decreased the alcohol intake. Our study suggests that the astrocytic 5-HT_2CR contribute to alcohol addiction; fluoxetine thus can be used to alleviate depression, treat alcohol addiction and improve motor coordination.

Dopamine regulates cytokine secretion during innate and adaptive immune responses

Dopamine (DA) is synthesized by various immune cells. DA receptors (DARs), which comprise five isoforms, are expressed on the surface of these cells. Therefore, it is likely that DA plays a role in regulating innate and adaptive responses. However, the underlying molecular mechanism(s) is largely unknown. Here, we found that, during innate immune responses, DA suppressed secretion of IFN-γ, TNF-α and IL-1β, but promoted secretion of IL-10 and CXCL1 by lipopolysaccharide (LPS)-stimulated mouse splenocytes, suggesting that DA regulates cytokine secretion. Immune subset studies indicated that DA suppressed secretion of IFN-γ, TNF-α and IL-1β by NK cells, as well as secretion of TNF-α by neutrophils and monocytes; however, DA up-regulated IL-10 secretion by neutrophils, monocytes, B cells, macrophages (Mφs) and dendritic cells within the splenocyte population. In addition, DA up-regulated secretion of CXCL1 by LPS-stimulated NK cells and Mφs. Meanwhile, treatment with DAR agonists or antagonists suppressed secretion of inflammatory cytokines from LPS-stimulated splenocytes. Pre-treatment of LPS-stimulated splenocytes with the PI3K inhibitor wortmannin reversed DA-mediated suppression of IFN-γ secretion, indicating that DA regulates IFN-γ secretion via the inositol 1,4,5-trisphosphate signaling pathway in these cells. Administration of DA and LPS to mice immunized with chicken ovalbumin (OVA) increased secretion of IL-5 by mouse lung lymphocytes, suggesting that DA promotes OVA-specific Th2-mediated immune responses by these cells. Taken together, these findings indicate that DA regulates cytokine secretion during innate and adaptive immune responses.

Selective blockade of spinal D2DR by levo-corydalmine attenuates morphine tolerance via suppressing PI3K/Akt-MAPK signaling in a MOR-dependent manner

Morphine tolerance remains a challenge in the management of chronic pain in the clinic. As shown in our previous study, the dopamine D2 receptor (D2DR) expressed in spinal cord neurons might be involved in morphine tolerance, but the underlying mechanisms remain to be elucidated. In the present study, selective spinal D2DR blockade attenuated morphine tolerance in mice by inhibiting phosphatidylinositol 3 kinase (PI3K)/serine–threonine kinase (Akt)-mitogen activated protein kinase (MAPK) signaling in a μ opioid receptor (MOR)-dependent manner. Levo-corydalmine (l-CDL), which exhibited micromolar affinity for D2DR in D2/CHO-K1 cell lines in this report and effectively alleviated bone cancer pain in our previous study, attenuated morphine tolerance in rats with chronic bone cancer pain at nonanalgesic doses. Furthermore, the intrathecal administration of l-CDL obviously attenuated morphine tolerance, and the effect was reversed by a D2DR agonist in mice. Spinal D2DR inhibition and l-CDL also inhibited tolerance induced by the MOR agonist DAMGO. l-CDL and a D2DR small interfering RNA (siRNA) decreased the increase in levels of phosphorylated Akt and MAPK in the spinal cord; these changes were abolished by a PI3K inhibitor. In addition, the activated Akt and MAPK proteins in mice exhibiting morphine tolerance were inhibited by a MOR antagonist. Intrathecal administration of a PI3K inhibitor also attenuated DAMGO-induced tolerance. Based on these results, l-CDL antagonized spinal D2DR to attenuate morphine tolerance by inhibiting PI3K/Akt-dependent MAPK phosphorylation through MOR. These findings provide insights into a more versatile treatment for morphine tolerance.

By blocking dopamine receptors located in the spinal cord, a compound found in a traditional Chinese herbal medicine may help mitigate tolerance to morphine, a common problem among cancer patients who regularly take the opioid painkiller. A team led by Ji-Hua Liu and Bo-Yang Yu from China Pharmaceutical University in Nanjing had previously showed that inhibiting dopamine D2 receptors in spinal neurons prevented mice from developing morphine tolerance, but it wasn’t clear why. They have now demonstrated that blocking D2 receptors prevents the relay of cellular signals from morphine-binding “μ-opioid” receptors to mediators of drug tolerance. Levo-corydalmine, a compound isolated from the Asian Corydalis plant, binds and inhibits D2 receptors. When administered directly into the spinal cords of mice and rats, it blocked downstream signaling, reducing morphine tolerance.

Dsg2 via Src-mediated transactivation shapes EGFR signaling towards cell adhesion

Rapidly renewing epithelial tissues such as the intestinal epithelium require precise tuning of intercellular adhesion and proliferation to preserve barrier integrity. Here, we provide evidence that desmoglein 2 (Dsg2), an adhesion molecule of desmosomes, controls cell adhesion and proliferation via epidermal growth factor receptor (EGFR) signaling. Dsg2 is required for EGFR localization at intercellular junctions as well as for Src-mediated EGFR activation. Src binds to EGFR and is required for localization of EGFR and Dsg2 to cell-cell contacts. EGFR is critical for cell adhesion and barrier recovery. In line with this, Dsg2-deficient enterocytes display impaired barrier properties and increased cell proliferation. Mechanistically, Dsg2 directly interacts with EGFR and undergoes heterotypic-binding events on the surface of living enterocytes via its extracellular domain as revealed by atomic force microscopy. Thus, our study reveals a new mechanism by which Dsg2 via Src shapes EGFR function towards cell adhesion.

Biphasic Regulation of Caveolin-1 Gene Expression by Fluoxetine in Astrocytes: Opposite Effects of PI3K/AKT and MAPK/ERK Signaling Pathways on c-fos

Previously, we reported that fluoxetine acts on 5-HT_2B receptor and induces epidermal growth factor receptor (EGFR) transactivation in astrocytes. Recently, we have found that chronic treatment with fluoxetine regulates Caveolin-1 (Cav-1)/PTEN/PI3K/AKT/glycogen synthase kinase 3β (GSK-3β) signaling pathway and glycogen content in primary cultures of astrocytes with bi-phasic concentration dependence. At low concentrations fluoxetine down-regulates Cav-1 gene expression, decreases membrane content of PTEN, increases PI3K activity and increases phosphorylation of GSK-3β and increases its activity; at high concentrations fluoxetine acts on PTEN/PI3K/AKT/GSK-3β in an inverse fashion. Here, we present the data indicating that acute treatment with fluoxetine at lower concentrations down-regulates c-Fos gene expression via PI3K/AKT signaling pathway; in contrast at higher concentrations fluoxetine up-regulates c-Fos gene expression via MAPK/extracellular-regulated kinase (ERK) signaling pathway. However, acute treatment with fluoxetine has no effect on Cav-1 protein content. Similarly, chronic effects of fluoxetine on Cav-1 gene expression are suppressed by inhibitor of PI3K at lower concentrations, but by inhibitor of MAPK at higher concentrations, indicating that the mechanism underlying bi-phasic regulation of Cav-1 gene expression by fluoxetine is opposing effects of PI3K/AKT and MAPK/ERK signal pathways on c-Fos gene expression. The effects of fluoxetine on Cav-1 gene expression at both lower and higher concentrations are abolished by AG1478, an inhibitor of EGFR, indicating the involvement of 5-HT_2B receptor induced EGFR transactivation as we reported previously. However, PP1, an inhibitor of Src only abolished the effect by lower concentrations, suggesting the relevance of Src with PI3K/AKT signal pathway during activation of EGFR.

"Soldier's Heart": A Genetic Basis for Elevated Cardiovascular Disease Risk Associated with Post-traumatic Stress Disorder

"Soldier's Heart," is an American Civil War term linking post-traumatic stress disorder (PTSD) with increased propensity for cardiovascular disease (CVD). We have hypothesized that there might be a quantifiable genetic basis for this linkage. To test this hypothesis we identified a comprehensive set of candidate risk genes for PTSD, and tested whether any were also independent risk genes for CVD. A functional analysis algorithm was used to identify associated signaling networks. We identified 106 PTSD studies that report one or more polymorphic variants in 87 candidate genes in 83,463 subjects and controls. The top upstream drivers for these PTSD risk genes are predicted to be the glucocorticoid receptor (NR3C1) and Tumor Necrosis Factor alpha (TNFA). We find that 37 of the PTSD candidate risk genes are also candidate independent risk genes for CVD. The association between PTSD and CVD is significant by Fisher's Exact Test (P = 3 × 10-54). We also find 15 PTSD risk genes that are independently associated with Type 2 Diabetes Mellitus (T2DM; also significant by Fisher's Exact Test (P = 1.8 × 10-16). Our findings offer quantitative evidence for a genetic link between post-traumatic stress and cardiovascular disease, Computationally, the common mechanism for this linkage between PTSD and CVD is innate immunity and NFκB-mediated inflammation.

Reappraising striatal D1- and D2-neurons in reward and aversion

The striatum has been involved in complex behaviors such as motor control, learning, decision-making, reward and aversion. The striatum is mainly composed of medium spiny neurons (MSNs), typically divided into those expressing dopamine receptor D1, forming the so-called direct pathway, and those expressing D2 receptor (indirect pathway). For decades it has been proposed that these two populations exhibit opposing control over motor output, and recently, the same dichotomy has been proposed for valenced behaviors. Whereas D1-MSNs mediate reinforcement and reward, D2-MSNs have been associated with punishment and aversion. In this review we will discuss pharmacological, genetic and optogenetic studies that indicate that there is still controversy to what concerns the role of striatal D1- and D2-MSNs in this type of behaviors, highlighting the need to reconsider the early view that they mediate solely opposing aspects of valenced behaviour.

Dysbindin-1 modifies signaling and cellular localization of recombinant, human D₃ and D₂ receptors

Dystrobrevin binding protein-1 (dysbindin-1), a candidate gene for schizophrenia, modulates cognition, synaptic plasticity and frontocortical circuitry and interacts with glutamatergic and dopaminergic transmission. Loss of dysbindin-1 modifies cellular trafficking of dopamine (DA) D2 receptors to increase cell surface expression, but its influence upon signaling has never been characterized. Further, the effects of dysbindin-1 upon closely related D3 receptors remain unexplored. Hence, we examined the impact of dysbindin-1 (isoform A) co-expression on the localization and coupling of human D2L and D3 receptors stably expressed in Chinese hamster ovary or SH-SY5Y cells lacking endogenous dysbindin-1. Dysbindin-1 co-transfection decreased cell surface expression of both D3 and D2L receptors. Further, while their affinity for DA was unchanged, dysbindin-1 reduced the magnitude and potency of DA-induced adenylate cylase recruitment/cAMP production. Dysbindin-1 also blunted the amplitude of DA-induced phosphorylation of ERK1/2 and Akt at both D2L and D3 receptors without, in contrast to cAMP, affecting the potency of DA. Interference with calveolin/clathrin-mediated processes of internalization prevented the modification by dysbindin-1 of ERK1/2 and adenylyl cyclase stimulation at D2L and D3 receptors. Finally, underpinning the specificity of the influence of dysbindin-1 on D2L and D3 receptors, dysbindin-1 did not modify recruitment of adenylyl cyclase by D1 receptors. These observations demonstrate that dysbindin-1 influences cell surface expression of D3 in addition to D2L receptors, and that it modulates activation of their signaling pathways. Accordingly, both a deficiency and an excess of dysbindin-1 may be disruptive for dopaminergic transmission, supporting its link to schizophrenia and other CNS disorders. Dysbindin-1, a candidate gene for schizophrenia, alters D2 receptors cell surface expression. We demonstrate that dysbindin-1 expression also influences cell surface levels of D3 receptors. Further, Dysbindin-1 reduces DA-induced adenylate cylase recruitment/cAMP production and modifies major signaling pathways (Akt and extracellular signal-regulated kinases1/2 (ERK1/2)) of both D2 and D3 receptors. Dysbindin-1 modulates thus D2 and D3 receptor signaling, supporting a link to schizophrenia.

Dopamine D2 receptors' effects on renal inflammation are mediated by regulation of PP2A function

Lack or downregulation of the dopamine D2 receptor (D2R) results in increased renal expression of injury markers and proinflammatory factors that is independent of a blood pressure increase. This study aimed to determine the mechanisms involved in the regulation of renal inflammation by D2Rs. Silencing D2Rs in mouse renal proximal tubule cells increased the expression of the proinflammatory TNF-α, monocyte chemoattractant protein-1 (MCP-1), and IL-6. D2R downregulation also increased Akt phosphorylation and activity, and glycogen synthase kinase-3β (GSK3β) phosphorylation and cyclin D1 expression, downstream targets of Akt; however. phosphatidylinositol 3-kinase (PI3K) activity was not affected. Conversely, D2R stimulation decreased Akt and GSK3β phosphorylation and cyclin D1 expression. Increased phospho-Akt, in the absence of increased PI3K activity, may result from decreased Akt dephosphorylation. Inhibition of protein phosphatase 2A (PP2A) with okadaic acid reproduced the effects of D2R downregulation on Akt, GSK3β, and cyclin D1. The PP2A catalytic subunit and regulatory subunit PPP2R2C coimmunoprecipitated with the D2R. Basal phosphatase activity and the expression of PPP2R2C were decreased by D2R silencing that also blunted the increase in phosphatase activity induced by D2R stimulation. Similarly, silencing PPP2R2C also increased the phosphorylation of Akt and GSK3β. Moreover, downregulation of PPP2R2C resulted in increased expression of TNF-α, MCP-1, and IL-6, indicating that decreased phosphatase activity may be responsible for the D2R effect on inflammatory factors. Indeed, the increase in NF-κB reporter activity induced by D2R silencing was blunted by increasing PP2A activity with protamine. Our results show that D2R controls renal inflammation, at least in part, by modulation of the Akt pathway through effects on PP2A activity/expression.

Catecholaminergic based therapies for functional recovery after TBI

Among the many pathophysiologic consequences of traumatic brain injury are changes in catecholamines, including dopamine, epinephrine, and norepinephrine. In the context of TBI, dopamine is the one most extensively studied, though some research exploring epinephrine and norepinephrine have also been published. The purpose of this review is to summarize the evidence surrounding use of drugs that target the catecholaminergic system on pathophysiological and functional outcomes of TBI using published evidence from pre-clinical and clinical brain injury studies. Evidence of the effects of specific drugs that target catecholamines as agonists or antagonists will be discussed. Taken together, available evidence suggests that therapies targeting the catecholaminergic system may attenuate functional deficits after TBI. Notably, it is fairly common for TBI patients to be treated with catecholamine agonists for either physiological symptoms of TBI (e.g. altered cerebral perfusion pressures) or a co-occuring condition (e.g. shock), or cognitive symptoms (e.g. attentional and arousal deficits). Previous clinical trials are limited by methodological limitations, failure to replicate findings, challenges translating therapies to clinical practice, the complexity or lack of specificity of catecholamine receptors, as well as potentially counfounding effects of personal and genetic factors. Overall, there is a need for additional research evidence, along with a need for systematic dissemination of important study details and results as outlined in the common data elements published by the National Institute of Neurological Diseases and Stroke. Ultimately, a better understanding of catecholamines in the context of TBI may lead to therapeutic advancements. This article is part of a Special Issue entitled SI:Brain injury and recovery.

A Selective Role for Lmo4 in Cue-Reward Learning

The ability to use environmental cues to predict rewarding events is essential to survival. The basolateral amygdala (BLA) plays a central role in such forms of associative learning. Aberrant cue-reward learning is thought to underlie many psychopathologies, including addiction, so understanding the underlying molecular mechanisms can inform strategies for intervention. The transcriptional regulator LIM-only 4 (LMO4) is highly expressed in pyramidal neurons of the BLA, where it plays an important role in fear learning. Because the BLA also contributes to cue-reward learning, we investigated the role of BLA LMO4 in this process using Lmo4-deficient mice and RNA interference. Lmo4-deficient mice showed a selective deficit in conditioned reinforcement. Knockdown of LMO4 in the BLA, but not in the nucleus accumbens, recapitulated this deficit in wild-type mice. Molecular and electrophysiological studies identified a deficit in dopamine D2 receptor signaling in the BLA of Lmo4-deficient mice. These results reveal a novel, LMO4-dependent transcriptional program within the BLA that is essential to cue-reward learning.

Single or combined treatment with L-DOPA and quinpirole differentially modulate expression and phosphorylation of key regulatory kinases in neuroblastoma cells

In the past decades, the clinical use of dopamine agonists has expanded from adjunct therapy in patients with a deteriorating response to L-3,4-dihydroxyphenylalanine (L-DOPA) to monotherapy for the treatment of early PD. Dopamine agonists provide their antiparkinsonian benefit through stimulation of brain postsynaptic type 2 dopamine receptors that exert their effect through classical cAMP-dependent mechanisms, as well as cAMP-independent cellular signaling cascades, including the Akt/glycogen synthase kinase 3 (GSK3) pathway. Alterations of Akt/GSK3 have been observed and may contribute to the neurodegenerative processes and the development of L-DOPA-induced dyskinesia. The effects L-DOPA and quinpirole, a dopamine agonist, on the two key regulatory kinases, Akt and GSK3, were evaluated in neuroblastoma cell line. L-DOPA and dopamine agonist dose-dependently and differentially modulated Akt and GSK3 expression and phosphorylation when added alone or combined. The combined treatment inverted or potentiated the modulatory properties of the single compound. The drug- and concentration-dependent balance of dopamine receptor stimulation over auto-oxidation may distinctively modulate GSK3 isoforms and Akt. Our results indicate that particular attention must be given to drug concentration and combination when multiple therapies are applied for the clinical treatment of PD patients.

Minireview: Dopaminergic regulation of insulin secretion from the pancreatic islet

Exogenous dopamine inhibits insulin secretion from pancreatic β-cells, but the lack of dopaminergic neurons in pancreatic islets has led to controversy regarding the importance of this effect. Recent data, however, suggest a plausible physiologic role for dopamine in the regulation of insulin secretion. We review the literature underlying our current understanding of dopaminergic signaling that can down-regulate glucose-stimulated insulin secretion from pancreatic islets. In this negative feedback loop, dopamine is synthesized in the β-cells from circulating L-dopa, serves as an autocrine signal that is cosecreted with insulin, and causes a tonic inhibition on glucose-stimulated insulin secretion. On the whole animal scale, L-dopa is produced by cells in the gastrointestinal tract, and its concentration in the blood plasma increases following a mixed meal. By reviewing the outcome of certain types of bariatric surgery that result in rapid amelioration of glucose tolerance, we hypothesize that dopamine serves as an "antiincretin" signal that counterbalances the stimulatory effect of glucagon-like peptide 1.

Cellular functions regulated by phosphorylation of EGFR on Tyr845

The Src gene product (Src) and the epidermal growth factor receptor (EGFR) are prototypes of oncogene products and function primarily as a cytoplasmic non-receptor tyrosine kinase and a transmembrane receptor tyrosine kinase, respectively. The identification of Src and EGFR, and the subsequent extensive investigations of these proteins have long provided cutting edge research in cancer and other molecular and cellular biological studies. In 1995, we reported that the human epidermoid carcinoma cells, A431, contain a small fraction of Src and EGFR in which these two kinase were in physical association with each other, and that Src phosphorylates EGFR on tyrosine 845 (Y845) in the Src-EGFR complex. Y845 of EGFR is located in the activation segment of the kinase domain, where many protein kinases contain kinase-activating autophosphorylation sites (e.g., cAMP-dependent protein kinase, Src family kinases, transmembrane receptor type tyrosine kinases) or trans-phosphorylation sites (e.g., cyclin-dependent protein kinase, mitogen-activated protein kinase, Akt protein kinase). A number of studies have demonstrated that Y845 phosphorylation serves an important role in cancer as well as normal cells. Here we compile the experimental facts involving Src phosphorylation of EGFR on Y845, by which cell proliferation, cell cycle control, mitochondrial regulation of cell metabolism, gamete activation and other cellular functions are regulated. We also discuss the physiological relevance, as well as structural insights of the Y845 phosphorylation.

Receptor-Heteromer Investigation Technology and its application using BRET

Receptor heteromerization has the potential to alter every facet of receptor functioning, leading to new pharmacological profiles with increased signaling diversity and regulation from that of the monomeric receptor, or indeed receptor homomer. An understanding of the molecular consequences of receptor heteromerization will provide new insights into the physiology and pathology mediated by receptors, expanding the possibilities for pharmacological discovery. Particularly advantageous approaches to investigate novel heteromer pharmacology utilize cell-based assay technologies that assess ligand-dependent functional responses specific to the receptor heteromer. Importantly, this allows for differentiation of heteromer-specific pharmacology from pharmacology associated with the co-expressed receptor monomers and homomers. The Receptor-Heteromer Investigation Technology (Receptor-HIT) successfully employs a proximity-based reporter system, such as bioluminescence resonance energy transfer (BRET), in a configuration that enables determination of such heteromer-specific pharmacology. Therefore, Receptor-HIT provides a simple, robust and versatile approach for investigating the elusive "biochemical fingerprint" of receptor heteromers.

Sperm epidermal growth factor receptor (EGFR) mediates α7 acetylcholine receptor (AChR) activation to promote fertilization

To attain fertilization the spermatozoon binds to the egg zona pellucida (ZP) via sperm receptor(s) and undergoes an acrosome reaction (AR). Several sperm receptors have been described in the literature; however, the identity of this receptor is not yet certain. In this study, we suggest that the α7 nicotinic acetylcholine receptor (α7nAChR) might be a sperm receptor activated by ZP to induce epidermal growth factor receptor (EGFR)-mediated AR. We found that isolated ZP or α7 agonists induced the AR in sperm from WT but not α7-null spermatozoa, and the induced AR was inhibited by α7 or EGFR antagonists. Moreover, α7-null sperm showed very little binding to the egg, and microfluidic affinity in vitro assay clearly showed that α7nAChR, as well as EGFR, interacted with ZP3. Induction of EGFR activation and the AR by an α7 agonist was inhibited by a Src family kinase (SFK) inhibitor. In conclusion we suggest that activation of α7 by ZP leads to SFK-dependent EGFR activation, Ca(2+) influx, and the acrosome reaction.

Angiotensin II induces epithelial-to-mesenchymal transition in renal epithelial cells through reactive oxygen species/Src/caveolin-mediated activation of an epidermal growth factor receptor-extracellular signal-regulated kinase signaling pathway

Chronic activation of the renin-angiotensin system plays a deleterious role in progressive kidney damage, and the renal proximal tubule is known to play an important role in tubulointerstitial fibrosis; however, the underlying molecular mechanism is unclear. Here we report that in the proximal tubule-like LLCPKcl4 cells expressing angiotensin II (Ang II) type 1 receptor, Ang II induced changes in cell morphology and expression of epithelial-to-mesenchymal transition (EMT) markers, which were inhibited by the miotogen-activated protein (MAP) kinase/extracellular signal-regulated kinase (ERK)-activating kinase (MEK) inhibitor PD98059 or the Src kinase inhibitor PP2. Ang II-stimulated phosphorylation of caveolin-1 (Cav) at Y14 and epidermal growth factor receptor (EGFR) at Y845 and induced association of these phosphoproteins in caveolin-enriched lipid rafts, thereby leading to prolonged EGFR-ERK signaling that was inhibited by Nox4 small interfering RNA (siRNA) and Src siRNA. Two different antioxidants not only inhibited phosphorylation of Src at Y416 but also blocked the EGFR-ERK signaling. Moreover, erlotinib (the EGFR tyrosine kinase inhibitor), EGFR siRNA, and Cav siRNA all inhibited both prolonged EGFR-ERK signaling and phenotypic changes induced by Ang II. Thus, this report provides the first evidence that reactive oxygen species (ROS)/Src-dependent activation of persistent Cav-EGFR-ERK signaling mediates renal tubular cell dedifferentiation and identifies a novel molecular mechanism that may be involved in progressive renal injury caused by chronic exposure to Ang II.

Neurotransmitter receptors and cognitive dysfunction in Alzheimer's disease and Parkinson's disease

Cognitive dysfunction is one of the most typical characteristics in various neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease (advanced stage). Although several mechanisms like neuronal apoptosis and inflammatory responses have been recognized to be involved in the pathogenesis of cognitive dysfunction in these diseases, recent studies on neurodegeneration and cognitive dysfunction have demonstrated a significant impact of receptor modulation on cognitive changes. The pathological alterations in various receptors appear to contribute to cognitive impairment and/or deterioration with correlation to diversified mechanisms. This article recapitulates the present understandings and concepts underlying the modulation of different receptors in human beings and various experimental models of Alzheimer's disease and Parkinson's disease as well as a conceptual update on the underlying mechanisms. Specific roles of serotonin, adrenaline, acetylcholine, dopamine receptors, and N-methyl-D-aspartate receptors in Alzheimer's disease and Parkinson's disease will be interactively discussed. Complex mechanisms involved in their signaling pathways in the cognitive dysfunction associated with the neurodegenerative diseases will also be addressed. Substantial evidence has suggested that those receptors are crucial neuroregulators contributing to cognitive pathology and complicated correlations exist between those receptors and the expression of cognitive capacities. The pathological alterations in the receptors would, therefore, contribute to cognitive impairments and/or deterioration in Alzheimer's disease and Parkinson's disease. Future research may shed light on new clues for the treatment of cognitive dysfunction in neurodegenerative diseases by targeting specific alterations in these receptors and their signal transduction pathways in the frontal-striatal, fronto-striato-thalamic, and mesolimbic circuitries.

EGFR signaling promotes TGFβ-dependent renal fibrosis

The mechanisms by which angiotensin II (Ang II) promotes renal fibrosis remain incompletely understood. Ang II both stimulates TGFβ signaling and activates the EGF receptor (EGFR), but the relative contribution of these pathways to renal fibrogenesis is unknown. Using a murine model with EGFR-deficient proximal tubules, we demonstrate that upstream activation of EGFR-dependent ERK signaling is critical for mediating sustained TGFβ expression in renal fibrosis. Persistent activation of the Ang II receptor stimulated ROS-dependent phosphorylation of Src, leading to sustained EGFR-dependent signaling for TGFβ expression. Either genetic or pharmacologic inhibition of EGFR significantly decreased TGFβ-mediated fibrogenesis. We conclude that TGFβ-mediated tissue fibrosis relies on a persistent feed-forward mechanism of EGFR/ERK activation through an unexpected signaling pathway, highlighting EGFR as a potential therapeutic target for modulating tissue fibrogenesis.

Src family kinases mediate epidermal growth factor receptor signaling from lipid rafts in breast cancer cells

Activation of the epidermal growth factor receptor (EGFR) regulates cellular proliferation, survival, and migration of breast cancer cells. In particular, EGFR recruits signaling proteins to the cell membrane leading to their phosphorylation and activation. However, EGFR also localizes to other cellular structures, including endosomes, mitochondrion, and nuclei. Recently, we demonstrated that lipid raft localization of EGFR in triple-negative breast cancer cell lines promotes EGFR protein-dependent, EGFR kinase-independent activation of Akt. Here, we further define the mechanism by which lipid rafts regulate EGFR signaling to Akt. Specifically, we show that the non-receptor tyrosine kinase c-Src co-localizes and co-associates with EGFR and lipid rafts. Breast cancer cells resistant to treatment with EGFR inhibitors, were also resistant to treatment with Src family kinase (SFK) inhibitors; however, the combination of EGFR and SFK inhibitors synergistically decreases cell viability. We found that this decrease in cell viability observed with EGFR and SFK inhibitor co-treatment correlates with loss of Akt phosphorylation. In addition, we found that in breast cancer cell lines with EGFR and c-Src co-localized to lipid rafts, phospho-inositide 3 kinase (PI3K) was also associated with lipid rafts. Together, the data herein suggest that lipid rafts provide a platform for the interaction of EGFR, c-Src, and PI3K, leading to activation of cellular survival signaling in breast cancer cells.

Dopamine receptors in human adipocytes: expression and functions

Introduction

Dopamine (DA) binds to five receptors (DAR), classified by their ability to increase (D1R-like) or decrease (D2R-like) cAMP. In humans, most DA circulates as dopamine sulfate (DA-S), which can be de-conjugated to bioactive DA by arylsulfatase A (ARSA). The objective was to examine expression of DAR and ARSA in human adipose tissue and determine whether DA regulates prolactin (PRL) and adipokine expression and release.

Methods

DAR were analyzed by RT-PCR and Western blotting in explants, primary adipocytes and two human adipocyte cell lines, LS14 and SW872. ARSA expression and activity were determined by qPCR and enzymatic assay. PRL expression and release were determined by luciferase reporter and Nb2 bioassay. Analysis of cAMP, cGMP, leptin, adiponectin and interleukin 6 (IL-6) was done by ELISA. Activation of MAPK and PI3 kinase/Akt was determined by Western blotting.

Results

DAR are variably expressed at the mRNA and protein levels in adipose tissue and adipocytes during adipogenesis. ARSA activity in adipocyte increases after differentiation. DA at nM concentrations suppresses cAMP, stimulates cGMP, and activates MAPK in adipocytes. Acting via D2R-like receptors, DA and DA-S inhibit PRL gene expression and release. Acting via D1R/D5R receptors, DA suppresses leptin and stimulates adiponectin and IL-6 release.

Conclusions

This is the first report that human adipocytes express functional DAR and ARSA, suggesting a regulatory role for peripheral DA in adipose functions. We speculate that the propensity of some DAR-activating antipsychotics to increase weight and alter metabolic homeostasis is due, in part, to their direct action on adipose tissue.

Mechanisms of antidepressant action: an integrated dopaminergic perspective

The molecular mechanisms that cause and maintain the major depressive disorder (MDD) are currently unknown. Consistently, antidepressant treatments are characterized by insufficient success rates. This causes high social costs and severe personal sufferings. In the present review we analyze some of the paradigms that are used to explain MDD, particularly from the perspective of the dopaminergic (DA) system. DA has been more classically associated with psychosis and substance abuse disorders, even though a role of DA in MDD has been proposed as well and some antidepressants with DA profile exist. In the present work, we review some of the molecular mechanisms that underpin MDD from the perspective of the dopaminergic system, in the hope of unifying some of the major theories of MDD - the monoaminergic, inflammatory, epigenetics, neurotrophin and anti-apoptotic theories. Several shared components of these theories are highlighted, partially accounted by the functions of the DA system (see supplementary video).

Signaling pathways leading to phosphorylation of Akt and GSK-3β by activation of cloned human and rat cerebral D₂and D₃ receptors

Although dopamine (DA) regulates the serine/threonine kinase Akt and its downstream substrate glycogen synthase kinase-3β (GSK-3β), the direct influence of dopaminergic receptors remains poorly characterized. Short-term incubation of Chinese hamster ovary (CHO)-expressed human (h)D(₂L) and hD₃) receptors with DA (maximal effect, 5-10 min) phosphorylated Akt (Thr308 and Ser473) and GSK-3β (Ser9), actions blocked by the selective D₂ and D₃ antagonists, 3-[4-(4-chlorophenyl)-4-hydroxypiperidin-l-yl]methyl-1H-indole (L741,626) and (3aR,9bS)-N[4-(8-cyano-1,3a,4,9b-tetrahydro-3H-benzopyrano[3,4-c]pyrrole-2-yl)-butyl] (4-phenyl)benzamide (S33084), respectively. Similar findings were acquired with the specific D₂/D₃ receptor agonist quinelorane, which also enhanced (10 min after administration) levels of p-Akt and p-GSK-3β in rat nucleus accumbens, an action blocked by the D₂/D₃ receptor antagonist raclopride. Akt and GSK-3β phosphorylation mediated via CHO-expressed hD(₂L) and hD₃ receptors was prevented by pertussis toxin and by inhibitors of insulin-like growth factor-1 receptors as well as phosphatidylinositol 3-kinase and Src. Likewise, chelation of intracellular Ca²+ and interference with an "atypical" phorbol ester-insensitive protein kinase C (PKC) abolished recruitment of Akt and GSK-3β. Inactivation of PKCμ blocked Akt and GSK-3β phosphorylation at hD(₂L) receptors. However, blockade of conventional PKC isoforms attenuated the actions of DA at hD₃ receptors only. Furthermore, phospholipase C (PLC), calmodulin, and Akt inhibitors abolished DA-induced GSK-3β phosphorylation by hD₃ receptors, whereas phosphorylation by hD(₂L) receptors partially involved calmodulin, Akt, and extracellular signal-regulated kinase (ERK) 1/2. In conclusion, at both hD(₂L) and hD₃ receptors, DA elicited a G(i/o)- and Ca²+/calmodulin-dependent phosphorylation of Akt and GSK-3β via transactivation of insulin-like growth factor 1 receptor. However, significant differences were seen regarding the involvement of PLC, calmodulin, and ERK1/2.

Role and regulation of EGFR in actin remodeling in sperm capacitation and the acrosome reaction

To bind and fertilize the egg, the spermatozoon should undergo few biochemical and motility changes in the female reproductive tract collectively called capacitation. The capacitated spermatozoon binds to the egg zona pellucida, and then undergoes the acrosome reaction (AR), which allows its penetration into the egg. The mechanisms regulating sperm capacitation and the AR are not completely understood. In the present review, we summarize some data regarding the role and regulation of the epidermal growth factor receptor (EGFR) in these processes. In the capacitation process, the EGFR is partially activated by protein kinase A (PKA), resulting in phospholipase D (PLD) activation and actin polymerization. Protein kinase C alpha (PKCα), which is already activated at the beginning of the capacitation, also participates in PLD activation. Further activation of the EGFR at the end of the capacitation enhances intracellular Ca(2+) concentration leading to F-actin breakdown and allows the AR to take place. Under in vivo conditions, the EGFR can be directly activated by its known ligand epidermal growth factor (EGF), and indirectly by activating PKA or by transactivation mediated by G protein-coupled receptors (GPCRs) activation or by ouabain. Under physiological conditions, sperm PKA is activated mainly by bicarbonate, which activates the soluble adenylyl cyclase to produce cyclic adenosine monophosphate (cAMP), the activator of PKA. The GPCR activators angiotensin II or lysophosphatidic acid, as well as ouabain and EGF are physiological components present in the female reproductive tract.

Antipsychotic potential of quinazoline ErbB1 inhibitors in a schizophrenia model established with neonatal hippocampal lesioning

Hyper-signaling of the epidermal growth factor receptor family (ErbB) is implicated in the pathophysiology of schizophrenia. Various quinazoline inhibitors targeting ErbB1 or ErbB2 - 4 have been developed as anti-cancer agents and might be useful for antipsychotic treatment. In the present study, we used an animal model of schizophrenia established by neonatal hippocampal lesioning and evaluated the neurobehavioral consequences of ErbB1-inhibitor treatment. Subchronic administration of the ErbB1 inhibitor ZD1839 to the cerebroventricle of rats receiving neonatal hippocampal lesioning ameliorated deficits in prepulse inhibition as well as those in the latent inhibition of tone-dependent fear learning. There were no apparent adverse effects on basal learning scores or locomotor activity, however. The administration of other ErbB1 inhibitors, PD153035 and OSI-774, similarly attenuated the prepulse inhibition impairment of this animal model. In parallel, there were decreases in ErbB1 phosphorylation in animals treated with ErbB1 inhibitors. These results indicate an antipsychotic potential of quinazoline ErbB1 inhibitors. ErbB receptor tyrosine kinases may be novel therapeutic targets for schizophrenia or its related psychotic symptoms.

Transactivation of PDGFRbeta by dopamine D4 receptor does not require PDGFRbeta dimerization

Growth factor-induced receptor dimerization and cross-phosphorylation are hallmarks of signal transduction via receptor tyrosine kinases (RTKs). G protein-coupled receptors (GPCRs) can activate RTKs through a process known as transactivation. The prototypical model of RTK transactivation involves ligand-mediated RTK dimerization and cross-phosphorylation. Here, we show that the platelet-derived growth factor receptor β (PDGFRβ) transactivation by the dopamine receptor D4 (DRD4) is not dependent on ligands for PDGFRβ. Furthermore, when PDGFRβ dimerization is inhibited and receptor phosphorylation is suppressed to near basal levels, the receptor maintains its ability to be transactivated and is still effective in signaling to ERK1/2. Hence, the DRD4-PDGFRβ-ERK1/2 pathway can occur independently of a PDGF-like ligand, PDGFRβ cross-phosphorylation and dimerization, which is distinct from other known forms of transactivation of RTKs by GPCRs.

From the cell to the clinic: a comparative review of the partial D₂/D₃receptor agonist and α2-adrenoceptor antagonist, piribedil, in the treatment of Parkinson's disease

Though L-3,4-dihydroxyphenylalanine (L-DOPA) is universally employed for alleviation of motor dysfunction in Parkinson's disease (PD), it is poorly-effective against co-morbid symptoms like cognitive impairment and depression. Further, it elicits dyskinesia, its pharmacokinetics are highly variable, and efficacy wanes upon long-term administration. Accordingly, "dopaminergic agonists" are increasingly employed both as adjuncts to L-DOPA and as monotherapy. While all recognize dopamine D(2) receptors, they display contrasting patterns of interaction with other classes of monoaminergic receptor. For example, pramipexole and ropinirole are high efficacy agonists at D(2) and D(3) receptors, while pergolide recognizes D(1), D(2) and D(3) receptors and a broad suite of serotonergic receptors. Interestingly, several antiparkinson drugs display modest efficacy at D(2) receptors. Of these, piribedil displays the unique cellular signature of: 1), signal-specific partial agonist actions at dopamine D(2)and D(3) receptors; 2), antagonist properties at α(2)-adrenoceptors and 3), minimal interaction with serotonergic receptors. Dopamine-deprived striatal D(2) receptors are supersensitive in PD, so partial agonism is sufficient for relief of motor dysfunction while limiting undesirable effects due to "over-dosage" of "normosensitive" D(2) receptors elsewhere. Further, α(2)-adrenoceptor antagonism reinforces adrenergic, dopaminergic and cholinergic transmission to favourably influence motor function, cognition, mood and the integrity of dopaminergic neurones. In reviewing the above issues, the present paper focuses on the distinctive cellular, preclinical and therapeutic profile of piribedil, comparisons to pramipexole, ropinirole and pergolide, and the core triad of symptoms that characterises PD-motor dysfunction, depressed mood and cognitive impairment. The article concludes by highlighting perspectives for clarifying the mechanisms of action of piribedil and other antiparkinson agents, and for optimizing their clinical exploitation.

Roles of the Akt/GSK-3 and Wnt signaling pathways in schizophrenia and antipsychotic drug action

Dopamine D(2) receptor antagonism is a unifying property of all antipsychotic drugs in clinical use. Remarkably, the effector molecules through which these medications exert their actions remain poorly characterized. Increasing attention is being focused on Akt/glycogen synthase kinase-3 (GSK-3) and wingless (Wnt) signaling pathways, which have been associated with schizophrenia in a number of genetic and postmortem studies. Antipsychotic medications may treat symptoms of psychosis, at least in part, through modulation of levels and activity of Akt, GSK-3, and Wnt-related intracellular signaling. The authors review evidence that Akt/GSK-3 and Wnt-related pathways are involved in the pathogenesis of schizophrenia as well as details of intracellular events related to these molecules mediated by both typical and atypical antipsychotic medications. Further study of Akt/GSK-3 and Wnt signaling may ultimately lead to alternative therapeutics of schizophrenia-related disorders.

Evaluation of bromocriptine in the treatment of acute severe peripartum cardiomyopathy: a proof-of-concept pilot study

BACKGROUND

Peripartum cardiomyopathy (PPCM) is a potentially life-threatening heart disease that occurs in previously healthy women. We identified prolactin, mainly its 16-kDa angiostatic and proapoptotic form, as a key factor in PPCM pathophysiology. Previous reports suggest that bromocriptine may have beneficial effects in women with acute onset of PPCM.

METHODS AND RESULTS

A prospective, single-center, randomized, open-label, proof-of-concept pilot study of women with newly diagnosed PPCM receiving standard care (PPCM-Std; n=10) versus standard care plus bromocriptine for 8 weeks (PPCM-Br, n=10) was conducted. Because mothers receiving bromocriptine could not breast-feed, the 6-month outcome of their children (n=21) was studied as a secondary end point. Blinded clinical, hemodynamic, and echocardiographic assessments were performed at baseline and 6 months after diagnosis. Cardiac magnetic resonance imaging was performed 4 to 6 weeks after diagnosis in PPCM-Br patients. There were no significant differences in baseline characteristics, including serum 16-kDa prolactin levels and cathepsin D activity, between the 2 study groups. PPCM-Br patients displayed greater recovery of left ventricular ejection fraction (27% to 58%; P=0.012) compared with PPCM-Std patients (27% to 36%) at 6 months. One patient in the PPCM-Br group died compared with 4 patients in the PPCM-Std group. Significantly fewer PPCM-Br patients (n=1, 10%) experienced the composite end point of poor outcome defined as death, New York Heart Association functional class III/IV, or left ventricular ejection fraction <35% at 6 months compared with the PPCM-Std patients (n=8, 80%; P=0.006). Cardiac magnetic resonance imaging revealed no intracavitary thrombi. Infants of mothers in both groups showed normal growth and survival.

CONCLUSIONS

In this trial, the addition of bromocriptine to standard heart failure therapy appeared to improve left ventricular ejection fraction and a composite clinical outcome in women with acute severe PPCM, although the number of patients studied was small and the results cannot be considered definitive. Larger-scale multicenter and blinded studies are in progress to test this strategy more robustly.

Enhanced hypothalamic leptin signaling in mice lacking dopamine D2 receptors

The dopamine D(2) receptor (D2R) plays a critical role in diverse neurophysiological functions. D2R knock-out mice (D2R(-/-)) show reduced food intake and body weight while displaying an increased basal energy expenditure level, compared with their wild type littermates. Thus, these mice show a lean phenotype. D2R(-/-) mice displayed increased leptin sensitivity, and leptin injection induced increased phosphorylation of the hypothalamic signal transducer and activator of transcription 3 (STAT3) in D2R(-/-) mice relative to wild type littermates. Using double immunofluorescence histochemistry, we have demonstrated that D2Rs are present in leptin-sensitive STAT3-positive cells in the arcuate nucleus of the hypothalamus and that leptin injection induces STAT3 phosphorylation in hypothalamic neurons expressing D2Rs. Stimulation of D2R by the D2R agonist quinpirole suppressed the leptin-induced STAT3 phosphorylation and nuclear trans-localization of phospho-STAT3 in the hypothalamus of wild type mice. However, this regulation was not detected in the D2R(-/-) mice. Treatment of D2R agonist and antagonist could modulate the leptin-induced food intake and body weight changes in wild type mice but not in D2R(-/-) mice. Together, our findings suggest that the interaction between the dopaminergic system and leptin signaling in hypothalamus is important in control of energy homeostasis.

IL-1beta induces MMP-9 expression via a Ca2+-dependent CaMKII/JNK/c-JUN cascade in rat brain astrocytes

Interleukin (IL)-1beta has been shown to induce matrix metalloproteinase (MMP)-9 expression through mitogen-activated protein kinases, including JNK, in rat brain astrocyte-1 (RBA-1) cells. However, little is known about whether JNK activated by Ca(2+)-dependent CaMKII is associated with MMP-9 expression induced by IL-1beta. Here, we report that the Ca(2+)/CaMKII/JNK/c-Jun participates in the MMP-9 expression induced by IL-1beta. Zymographic, Western blotting, and RT-PCR analyses showed that IL-1beta-induced expression of MMP-9 mRNA and protein was attenuated by Ca(2+) chelator (BAPTA), and the inhibitors of ER Ca(2+)-ATPase (thapsigargin), CaMKII (KN-62), and JNK1/2 (SP600125). IL-1beta also stimulated phosphorylation of CaMKII and JNK1/2, and increase in intracellular Ca(2+) ([Ca(2+)](i)), which were inhibited by pretreatment with BAPTA, thapsigargin (TG), KN-62, or SP600125. Furthermore, the upregulation of MMP-9 protein was blocked by transfection with c-Jun or CaMKII short hairpin RNA (shRNA). We further confirmed that IL-1beta stimulated c-Jun associated with AP-1-binding sites within MMP-9 promoter (-87 to -80 bp and -511 to -497 bp) by immunoprecipitation and chromatin immunoprecipitation (ChIP)-PCR assays. The activation and recruitment of c-Jun to MMP-9 promoter were inhibited by pretreatment with BAPTA, TG, KN-62, or SP600125. Moreover, IL-1beta-induced MMP-9 gene transcription by AP-1 was confirmed by transfection with a MMP-9 promoter-luciferase reporter plasmid with a distal AP-1-binding site (-511 to -497 bp) adjacent to an Ets-binding site-mutation (mt-AP1/Ets-MMP-9). These results demonstrated that in RBA-1 cells, JNK/c-Jun activation was mediated through a Ca(2+)-dependent CaMKII pathway that promoted transcription factor c-Jun/AP-1 recruitment and eventually led to increase in MMP-9 expression by IL-1beta.

Bovine sperm acrosome reaction induced by G-protein-coupled receptor agonists is mediated by epidermal growth factor receptor transactivation

We have previously demonstrated the presence of active epidermal growth factor receptor (EGFR) and its involvement in sperm capacitation and the acrosome reaction; however, the mechanism of EGFR activation was not clear. We show here that the sperm EGFR can be transactivated by angiotensin II or by lysophosphatydic acid, two ligands which activate specific G-protein-coupled receptors (GPCR), or by directly activating protein kinase A using 8Br-cAMP. This transactivation occurs in noncapacitated sperm and is mediated by PKA, SRC and a metalloproteinase. We also show that the EGFR is activated in sperm incubated under in vitro capacitation conditions, without any added ligand, but not in bicarbonate-deficient medium or when PKA is blocked. Despite the fact that EGFR is activated in capacitated sperm, this state is not sufficient to induce the acrosome reaction. We conclude that the EGFR is stimulated during capacitation via PKA activation, while further activation of the EGFR in capacitated sperm is required in order to induce the acrosome reaction. The acrosome reaction can be induced by GPCR via the transactivation of the EGFR by a signaling pathway involving PKA, SRC and metalloproteinase and the EGFR down-stream effectors PI3K, PLC and PKC.

Adenosine A(1) receptor-mediated transactivation of the EGF receptor produces a neuroprotective effect on cortical neurons in vitro

AIM

To understand the mechanism of the transactivation of the epidermal growth factor receptor (EGFR) mediated by the adenosine A(1) receptor (A(1)R).

METHODS

Primary cultured rat cortical neurons subjected to oxygen-glucose deprivation (OGD) and HEK293/A(1)R cells were treated with the A(1)R-specific agonist N(6)-cyclopentyladenosine (CPA). Phospho-EGFR, Akt, and ERK1/2 were observed by Western blot. An interaction between EGFR and A(1)R was detected using immunoprecipitation and immunocytochemistry.

RESULTS

The A(1)R agonist CPA causes protein kinase B (Akt) activation and protects primary cortical neurons from oxygen-glucose deprivation (OGD) insult. A(1)R and EGFR co-localize in the membranes of neurons and form an immunocomplex. A(1)R stimulation induces significant EGFR phosphorylation via a PI3K and Src kinase signaling pathway; this stimulation provides a neuroprotective effect in cortical neurons. CPA leads to sustained phosphorylation of extracellularly regulated kinases 1 and 2 (ERK1/2) in cortical neurons, but only to transient phosphorylation in HEK 293/A(1)R cells. The response to the A(1)R agonist is mediated primarily through EGFR transactivation that is dependent on pertussis toxin (PTX)-sensitive G(i) protein and metalloproteases in HEK 293/A(1)R.

CONCLUSION

A(1)R-mediated EGFR transactivation confers a neuroprotective effect in primary cortical neurons. PI3 kinase and Src kinase play pivotal roles in this response.Acta Pharmacologica Sinica (2009) 30: 889-898; doi: 10.1038/aps.2009.80.

Nicotine sensitization and analysis of brain-derived neurotrophic factor in adolescent beta-arrestin-2 knockout mice

Nicotine sensitization and levels of brain-derived neurotrophic factor (BDNF) were analyzed in adolescent beta-arrestin-2 knockout (betaA-2 KO) and wild type (WT) mice. The beta-arrestin-2 protein has been shown to be important in G-protein hydrolysis and receptor internalization. Four- to five-week-old adolescent betaA-2 KO and WT C57/Bl6 mice were administered either nicotine (0.5 mg/kg free base) or saline 10 min before being placed into a locomotor arena on each of 7 (Experiment 1) or 14 (Experiment 2) consecutive days. A nicotine challenge was given 7 days after sensitization was complete. In Experiment 1, betaA-2 KO mice administered nicotine or saline and WT mice administered nicotine demonstrated significant hypoactivity during early in testing, and neither WT nor betaA-2 KO mice administered nicotine demonstrated sensitization. On the nicotine challenge, WT mice administered nicotine demonstrated significantly higher activity levels compared to all groups, and this same group demonstrated significantly higher levels of accumbal BDNF compared to all groups. In Experiment 2, betaA-2 KO mice were again hypoactive compared to WT mice, whereas WT mice administered nicotine demonstrated significant hypoactivity during initial testing and significantly higher levels of activity compared to all other groups late in testing. On the nicotine challenge, WT mice that received nicotine demonstrated a significant increase in activity compared to all groups, and showed increased accumbal BDNF compared to all groups. These results show that the beta-arrestin-2 protein is important in induction and expression of nicotine sensitization as well as nicotine's effects on accumbal BDNF.

G protein-coupled receptor kinase 4 (GRK4) regulates the phosphorylation and function of the dopamine D3 receptor

During conditions of moderate sodium excess, the dopaminergic system regulates blood pressure and water and electrolyte balance by engendering natriuresis. Dopamine exerts its effects on dopamine receptors, including the dopamine D(3) receptor. G protein-coupled receptor kinase 4 (GRK4), whose gene locus (4p16.3) is linked to essential hypertension, desensitizes the D(1) receptor, another dopamine receptor. This study evaluated the role of GRK4 on D(3) receptor function in human proximal tubule cells. D(3) receptor co-segregated in lipid rafts and co-immunoprecipitated and co-localized in human proximal tubule cells and in proximal and distal tubules and glomeruli of kidneys of Wistar Kyoto rats. Bimolecular fluorescence complementation and confocal microscopy revealed that agonist activation of the receptor initiated the interaction between D(3) receptor and GRK4 at the cell membrane and promoted it intracellularly, presumably en route to endosomal trafficking. Of the four GRK4 splice variants, GRK4-gamma and GRK4-alpha mediated a 3- and 2-fold increase in the phosphorylation of agonist-activated D(3) receptor, respectively. Inhibition of GRK activity with heparin or knockdown of GRK4 expression via RNA interference completely abolished p44/42 phosphorylation and mitogenesis induced by D(3) receptor stimulation. These data demonstrate that GRK4, specifically the GRK4-gamma and GRK4-alpha isoforms, phosphorylates the D(3) receptor and is crucial for its signaling in human proximal tubule cells.

The presence and function of dopamine type 2 receptors in boar sperm: a possible role for dopamine in viability, capacitation, and modulation of sperm motility

Several studies have shown that dopamine and other catecholamines are present in oviduct luminal fluid. We recently reported that dopamine type 2 receptors (DRD2) are present in a wide range of mammalian sperm, suggesting a role for dopaminergic signaling in events such as fertilization, capacitation, and sperm motility. In the present study, we used Western blot analysis to show that boar sperm express DRD2 and that their activation with dopamine (100 nM) has a positive effect on cell viability that can be correlated with AKT/PKB phosphorylation. Bromocriptine (100 nM) and dopamine (100 nM and 10 muM) increased tyrosine phosphorylation during the capacitation period. Immunofluorescence analysis indicated that DRD2 localization is dynamic and depends on the capacitation stage, colocalizing with tyrosine phosphorylated proteins in the acrosome and midpiece region of capacitated boar sperm. This association was confirmed by coimmunoprecipitation analysis. We also showed that bromocriptine (100 nM) and low-concentration dopamine (100 nM and 10 muM) increased total and progressive motility of sperm. However, high concentrations of dopamine (1 mM) decreased tyrosine phosphorylation and motility in in vitro sperm capacitation assays. This can be explained by the presence of the dopamine transporters (DAT, official symbol SLC6A3) in sperm, as demonstrated by Western blot analysis and immunocytochemistry. Taken together, our results support the idea that dopamine may have a fundamental role during sperm capacitation and motility in situ in the female upper reproductive tract.