Role of SAP97 protein in the regulation of corticotropin-releasing factor receptor 1 endocytosis and extracellular signal-regulated kinase 1/2 signaling

Dysregulated Signaling at Postsynaptic Density: A Systematic Review and Translational Appraisal for the Pathophysiology, Clinics, and Antipsychotics' Treatment of Schizophrenia

Emerging evidence from genomics, post-mortem, and preclinical studies point to a potential dysregulation of molecular signaling at postsynaptic density (PSD) in schizophrenia pathophysiology. The PSD that identifies the archetypal asymmetric synapse is a structure of approximately 300 nm in diameter, localized behind the neuronal membrane in the glutamatergic synapse, and constituted by more than 1000 proteins, including receptors, adaptors, kinases, and scaffold proteins. Furthermore, using FASS (fluorescence-activated synaptosome sorting) techniques, glutamatergic synaptosomes were isolated at around 70 nm, where the receptors anchored to the PSD proteins can diffuse laterally along the PSD and were stabilized by scaffold proteins in nanodomains of 50-80 nm at a distance of 20-40 nm creating "nanocolumns" within the synaptic button. In this context, PSD was envisioned as a multimodal hub integrating multiple signaling-related intracellular functions. Dysfunctions of glutamate signaling have been postulated in schizophrenia, starting from the glutamate receptor's interaction with scaffolding proteins involved in the N-methyl-D-aspartate receptor (NMDAR). Despite the emerging role of PSD proteins in behavioral disorders, there is currently no systematic review that integrates preclinical and clinical findings addressing dysregulated PSD signaling and translational implications for antipsychotic treatment in the aberrant postsynaptic function context. Here we reviewed a critical appraisal of the role of dysregulated PSD proteins signaling in the pathophysiology of schizophrenia, discussing how antipsychotics may affect PSD structures and synaptic plasticity in brain regions relevant to psychosis.

Basolateral amygdala corticotropin-releasing factor receptor type 1 regulates context-cocaine memory strength during reconsolidation in a sex-dependent manner

The basolateral amygdala (BLA) is a critical brain region for cocaine-memory reconsolidation. Corticotropin-releasing factor receptor type 1 (CRFR1) is densely expressed in the BLA, and CRFR1 stimulation can activate intra-cellular signaling cascades that mediate memory reconsolidation. Hence, we tested the hypothesis that BLA CRFR1 stimulation is necessary and sufficient for cocaine-memory reconsolidation. Using an instrumental model of drug relapse, male and female Sprague-Dawley rats received cocaine self-administration training in a distinct environmental context over 10 days followed by extinction training in a different context over 7 days. Next, rats were re-exposed to the cocaine-paired context for 15 min to initiate cocaine-memory retrieval and destabilization. Immediately or 6 h after this session, the rats received bilateral vehicle, antalarmin (CRFR1 antagonist; 500 ng/hemisphere), or corticotropin-releasing factor (CRF; 0.2, 30 or 500 ng/hemisphere) infusions into the BLA. Resulting changes in drug context-induced cocaine seeking (index of context-cocaine memory strength) were assessed three days later. Female rats self-administered more cocaine infusions and exhibited more extinction responding than males. Intra-BLA antalarmin treatment immediately after memory retrieval (i.e., when cocaine memories were labile), but not 6 h later (i.e., after memory reconsolidation), attenuated drug context-induced cocaine seeking at test independent of sex, relative to vehicle. Conversely, intra-BLA CRF treatment increased this behavior selectively in females, in a U-shaped dose-dependent fashion. In control experiments, a high (behaviorally ineffective) dose of CRF treatment did not reduce BLA CRFR1 cell-surface expression in females. Thus, BLA CRFR1 signaling is necessary and sufficient, in a sex-dependent manner, for regulating cocaine-memory strength.

Regulation of G protein-coupled receptor signaling by plasma membrane organization and endocytosis

The trafficking of G protein coupled-receptors (GPCRs) is one of the most exciting areas in cell biology because of recent advances demonstrating that GPCR signaling is spatially encoded. GPCRs, acting in a diverse array of physiological systems, can have differential signaling consequences depending on their subcellular localization. At the plasma membrane, GPCR organization could fine-tune the initial stages of receptor signaling by determining the magnitude of signaling and the type of effectors to which receptors can couple. This organization is mediated by the lipid composition of the plasma membrane, receptor-receptor interactions, and receptor interactions with intracellular scaffolding proteins. GPCR organization is subsequently changed by ligand binding and the regulated endocytosis of these receptors. Activated GPCRs can modulate the dynamics of their own endocytosis through changing clathrin-coated pit dynamics, and through the scaffolding adaptor protein β-arrestin. This endocytic regulation has signaling consequences, predominantly through modulation of the MAPK cascade. This review explores what is known about receptor sorting at the plasma membrane, protein partners that control receptor endocytosis, and the ways in which receptor sorting at the plasma membrane regulates downstream trafficking and signaling.

Sex Differences in the Rat Hippocampal Opioid System After Oxycodone Conditioned Place Preference

Although opioid addiction has risen dramatically, the role of gender in addiction has been difficult to elucidate. We previously found sex-dependent differences in the hippocampal opioid system of Sprague-Dawley rats that may promote associative learning relevant to drug abuse. The present studies show that although female and male rats acquired conditioned place preference (CPP) to the mu-opioid receptor (MOR) agonist oxycodone (3 mg/kg, I.P.), hippocampal opioid circuits were differentially altered. In CA3, Leu-Enkephalin-containing mossy fibers had elevated levels in oxycodone CPP (Oxy) males comparable to those in females and sprouted in Oxy-females, suggesting different mechanisms for enhancing opioid sensitivity. Electron microscopy revealed that in Oxy-males delta opioid receptors (DORs) redistributed to mossy fiber-CA3 synapses in a manner resembling females that we previously showed is important for opioid-mediated long-term potentiation. Moreover, in Oxy-females DORs redistributed to CA3 pyramidal cell spines, suggesting the potential for enhanced plasticity processes. In Saline-injected (Sal) females, dentate hilar parvalbumin-containing basket interneuron dendrites had fewer MORs, however plasmalemmal and total MORs increased in Oxy-females. In dentate hilar GABAergic dendrites that contain neuropeptide Y, Sal-females compared to Sal-males had higher plasmalemmal DORs, and near-plasmalemmal DORs increased in Oxy-females. This redistribution of MORs and DORs within hilar interneurons in Oxy-females would potentially enhance disinhibition of granule cells via two different circuits. Together, these results indicate that oxycodone CPP induces sex-dependent redistributions of opioid receptors in hippocampal circuits in a manner facilitating opioid-associative learning processes and may help explain the increased susceptibility of females to opioid addiction acquisition and relapse.

The Corticotropin-Releasing Factor Family: Physiology of the Stress Response

The physiological stress response is responsible for the maintenance of homeostasis in the presence of real or perceived challenges. In this function, the brain activates adaptive responses that involve numerous neural circuits and effector molecules to adapt to the current and future demands. A maladaptive stress response has been linked to the etiology of a variety of disorders, such as anxiety and mood disorders, eating disorders, and the metabolic syndrome. The neuropeptide corticotropin-releasing factor (CRF) and its relatives, the urocortins 1–3, in concert with their receptors (CRFR1, CRFR2), have emerged as central components of the physiological stress response. This central peptidergic system impinges on a broad spectrum of physiological processes that are the basis for successful adaptation and concomitantly integrate autonomic, neuroendocrine, and behavioral stress responses. This review focuses on the physiology of CRF-related peptides and their cognate receptors with the aim of providing a comprehensive up-to-date overview of the field. We describe the major molecular features covering aspects of gene expression and regulation, structural properties, and molecular interactions, as well as mechanisms of signal transduction and their surveillance. In addition, we discuss the large body of published experimental studies focusing on state-of-the-art genetic approaches with high temporal and spatial precision, which collectively aimed to dissect the contribution of CRF-related ligands and receptors to different levels of the stress response. We discuss the controversies in the field and unravel knowledge gaps that might pave the way for future research directions and open up novel opportunities for therapeutic intervention.

Sex Differences in the Subcellular Distribution of Corticotropin-Releasing Factor Receptor 1 in the Rat Hippocampus following Chronic Immobilization Stress

Corticotropin-releasing factor receptors (CRFR1) contribute to stress-induced adaptations in hippocampal structure and function that can affect learning and memory processes. Our prior studies showed that female rats with elevated estrogens compared to males have more plasmalemmal CRFR1 in CA1 pyramidal cells, suggesting a greater sensitivity to stress. Here, we examined the distribution of hippocampal CRFR1 following chronic immobilization stress (CIS) in female and male rats using immuno-electron microscopy. Without stress, total CRFR1 dendritic levels were higher in females in CA1 and in males in the hilus; moreover, plasmalemmal CRFR1 was elevated in pyramidal cell dendrites in CA1 in females and in CA3 in males. Following CIS, near-plasmalemmal CRFR1 increased in CA1 pyramidal cell dendrites in males but not to levels of control or CIS females. In CA3 and the hilus, CIS decreased cytoplasmic and total CRFR1 in dendrites in males only. These results suggest that in naive rats, CRF could induce a greater activation of CA1 pyramidal cells in females than males. Moreover, after CIS, which leads to even greater sex differences in CRFR1 by trafficking it to different subcellular compartments, CRF could enhance activation of CA1 pyramidal cells in males but to a lesser extent than either unstressed or CIS females. Additionally, CA3 pyramidal cells and inhibitory interneurons in males have heightened sensitivity to CRF, regardless of stress state. These sex differences in CRFR1 distribution and trafficking in the hippocampus may contribute to reported sex differences in hippocampus-dependent learning processes in baseline conditions and following chronic stress.

MAGI proteins can differentially regulate the signaling pathways of 5-HT2AR by enhancing receptor trafficking and PLC recruitment

MAGI proteins are Membrane-Associated Guanylate Kinase Inverted proteins that belong to the MAGUK family. They are scaffolding proteins that were shown to mediate the trafficking and signaling of various G protein-coupled receptors (GPCRs). They contain PDZ domains in their structure and many GPCRs interact with these proteins via the PDZ motifs on the carboxyl terminal end of a receptor. In a PDZ overlay assay performed with the carboxyl terminal tail of 5-HT_2AR, we were able to detect all three members of the MAGI subfamily, MAGI-1, MAGI-2 and MAGI-3 as interacting PDZ proteins. The PDZ motif of 5-HT_2AR consists of three amino acids; serine (S), cysteine (C) and valine (V). In this study, we characterize these 5-HT_2AR interactions with MAGI proteins. We first confirm the interaction using co-immunopricipitation and illustrate that the interaction is PDZ motif-dependent in human embryonic kidney (HEK 293) cells. We then assess the effects of overexpression and knockdown of the MAGI proteins on the internalization, trafficking and signaling of 5-HT_2AR. We find that knockdown of either MAGI-1 or MAGI-3 using siRNA results in a significant reduction in the internalization of 5-HT_2AR. As for signaling, we report here that MAGI proteins can modulate the signaling via the two transduction pathways that 5-HT_2AR can activate. We illustrate a significant effect of modulating MAGI proteins expression on 5-HT-stimulated IP formation. We demonstrate an enhancement in 5-HT_2AR-stimulated IP formation upon MAGI proteins overexpression. In addition, we show that knockdown of MAGI proteins with siRNA leads to a significant reduction in 5-HT_2AR-stimulated IP formation. Furthermore, we illustrate a significant increase in 5-HT-stimulated ERK1/2 phosphorylation upon MAGI proteins knockdown. Interestingly, this effect on ERK1/2 activation is PDZ motif-independent. We also suggest two possible mechanisms of regulation for the effect of MAGI proteins on 5-HT_2AR function. One mechanism involves the regulation of cell surface expression since we show that both MAGI-2 and MAGI-3 can enhance receptor trafficking to the plasma membrane when overexpressed in HEK 293 cells. The other mechanism points to regulation of second messengers in the signaling pathways. Specifically, we show that overexpression of any of the three MAGI proteins can enhance the recruitment of PLCβ3 to 5-HT_2AR. In addition, we report a negative effect for knocking down MAGI-3 on β-arrestin recruitment to the receptor and this effect is PDZ motif-independent. Taken together, our findings document distinct roles for the three MAGI proteins in regulating 5-HT_2AR trafficking and signaling and emphasize the importance of studying PDZ proteins and their interactions with GPCRs to regulate their function.

Interference of HTLV-1 Tax Protein with Cell Polarity Regulators: Defining the Subcellular Localization of the Tax-DLG1 Interaction

Human T cell leukemia virus (HTLV)-1 Tax (Tax) protein is very important in viral replication and cell transformation. Tax localizes in the nucleus and cytoplasm in association with organelles. Some activities of Tax depend on interactions with PDZ (PSD-95/Discs Large/Z0-1) domain-containing proteins such as Discs large protein 1 (DLG1) which is involved in cell polarity and proliferation. The DLG1 interaction results in a cytoplasmic co-localization pattern resembling vesicular aggregates, the nature of which is still unknown. To further explore the role of PDZ proteins in HTLV-1 cell transformation, we deeply investigated the Tax-DLG1 association. By fluorescence resonance energy transfer (FRET), we detected, for the first time, the direct binding of Tax to DLG1 within the cell. We showed that the interaction specifically affects the cellular distribution of not only DLG1, but also Tax. After studying different cell structures, we demonstrated that the aggregates distribute into the Golgi apparatus in spatial association with the microtubule-organizing center (MTOC). This study contributes to understand the biological significance of Tax-PDZ interactions.

MAGI Proteins Regulate the Trafficking and Signaling of Corticotropin-Releasing Factor Receptor 1 via a Compensatory Mechanism

Corticotropin-releasing factor (CRF) receptor1 (CRFR1) is associated with psychiatric illness and is a proposed target for the treatment of anxiety and depression. Similar to many G protein-coupled receptors (GPCRs), CRFR1 harbors a PDZ (PSD-95/Disc Large/Zona Occludens)-binding motif at the end of its carboxyl-terminal tail. The interactions of PDZ proteins with GPCRs are crucial for the regulation of receptor function. In the present study, we characterize the interaction of all members of the membrane-associated guanylate kinase with inverted orientation PDZ (MAGI) proteins with CRFR1. We show using co-immunoprecipitation that CRFR1 interacts with MAGI-1 and MAGI-3 in human embryonic kidney (HEK293) cells in a PDZ motif-dependent manner. We find that overexpression as well as knockdown of MAGI proteins result in a significant reduction in CRFR1 endocytosis. This effect is dependent on an intact PDZ binding motif for MAGI-2 and MAGI-3 but not MAGI-1. We show that the alteration in expression levels of MAGI-1, MAGI-2 or MAGI-3 can interfere with β-arrestin recruitment to CRFR1. This could explain the effects observed with receptor internalization. We also find that knockdown of endogenous MAGI-1, MAGI-2 or MAGI-3 in HEK293 cells can lead to an enhancement in ERK1/2 signaling but has no effect on cAMP formation. Interestingly, we observe a compensation effect between MAGI-1 and MAGI-3. Taken together, our data suggest that the MAGI proteins, MAGI-1, MAGI-2 and MAGI-3 can regulate β-arrestin-mediated internalization of CRFR1 as well as its signaling and that there is a compensatory mechanism involved in regulating the function of the MAGI subfamily.

Resequencing and Association Analysis of Six PSD-95-Related Genes as Possible Susceptibility Genes for Schizophrenia and Autism Spectrum Disorders

PSD-95 associated PSD proteins play a critical role in regulating the density and activity of glutamate receptors. Numerous previous studies have shown an association between the genes that encode these proteins and schizophrenia (SZ) and autism spectrum disorders (ASD), which share a substantial portion of genetic risks. We sequenced the protein-encoding regions of DLG1, DLG2, DLG4, DLGAP1, DLGAP2, and SynGAP in 562 cases (370 SZ and 192 ASD patients) on the Ion PGM platform. We detected 26 rare (minor allele frequency <1%), non-synonymous mutations, and conducted silico functional analysis and pedigree analysis when possible. Three variants, G344R in DLG1, G241S in DLG4, and R604C in DLGAP2, were selected for association analysis in an independent sample set of 1315 SZ patients, 382 ASD patients, and 1793 healthy controls. Neither DLG4-G241S nor DLGAP2-R604C was detected in any samples in case or control sets, whereas one additional SZ patient was found that carried DLG1-G344R. Our results suggest that rare missense mutations in the candidate PSD genes may increase susceptibility to SZ and/or ASD. These findings may strengthen the theory that rare, non-synonymous variants confer substantial genetic risks for these disorders.

PSD-95 regulates CRFR1 localization, trafficking and β-arrestin2 recruitment

Corticotropin-releasing factor (CRF) is a neuropeptide commonly associated with the hypothalamic-pituitary adrenal axis stress response. Upon release, CRF activates two G protein-coupled receptors (GPCRs): CRF receptor 1 (CRFR1) and CRF receptor 2 (CRFR2). Although both receptors contribute to mood regulation, CRFR1 antagonists have demonstrated anxiolytic and antidepressant-like properties that may be exploited in the generation of new pharmacological interventions for mental illnesses. Previous studies have demonstrated CRFR1 capable of heterologously sensitizing serotonin 2A receptor (5-HT2AR) signaling: another GPCR implicated in psychiatric disease. Interestingly, this phenomenon was dependent on Postsynaptic density 95 (PSD-95)/Disc Large/Zona Occludens (PDZ) interactions on the distal carboxyl termini of both receptors. In the current study, we demonstrate that endogenous PSD-95 can be co-immunoprecipitated with CRFR1 from cortical brain homogenate, and this interaction appears to be primarily via the PDZ-binding motif. Additionally, PSD-95 colocalizes with CRFR1 within the dendritic projections of cultured mouse neurons in a PDZ-binding motif-dependent manner. In HEK 293 cells, PSD-95 overexpression inhibited CRFR1 endocytosis, whereas PSD-95 shRNA knockdown enhanced CRFR1 endocytosis. Although PSD-95 does not appear to play a significant role in CRF-mediated cAMP or ERK1/2 signaling, PSD-95 was demonstrated to suppress β-arrestin2 recruitment: providing a potential mechanism for PSD-95's inhibition of endocytosis. In revisiting previously documented heterologous sensitization, PSD-95 shRNA knockdown did not prevent CRFR1-mediated enhancement of 5-HT2AR signaling. In conclusion, we have identified and characterized a novel functional relationship between CRFR1 and PSD-95 that may have implications in the design of new treatment strategies for mental illness.

Phosphodiesterase 4D acts downstream of Neuropilin to control Hedgehog signal transduction and the growth of medulloblastoma

Alterations in Hedgehog (Hh) signaling lead to birth defects and cancers including medulloblastoma, the most common pediatric brain tumor. Although inhibitors targeting the membrane protein Smoothened suppress Hh signaling, acquired drug resistance and tumor relapse call for additional therapeutic targets. Here we show that phosphodiesterase 4D (PDE4D) acts downstream of Neuropilins to control Hh transduction and medulloblastoma growth. PDE4D interacts directly with Neuropilins, positive regulators of Hh pathway. The Neuropilin ligand Semaphorin3 enhances this interaction, promoting PDE4D translocation to the plasma membrane and cAMP degradation. The consequent inhibition of protein kinase A (PKA) enhances Hh transduction. In the developing cerebellum, genetic removal of Neuropilins reduces Hh signaling activity and suppresses proliferation of granule neuron precursors. In mouse medulloblastoma allografts, PDE4D inhibitors suppress Hh transduction and inhibit tumor growth. Our findings reveal a new regulatory mechanism of Hh transduction, and highlight PDE4D as a promising target to treat Hh-related tumors.

DOI:http://dx.doi.org/10.7554/eLife.07068.001

A communication system in cells called the Hedgehog signaling pathway plays an essential role in the formation of tissues and organs in animal embryos. The activity of the pathway is carefully controlled during development and if Hedgehog signaling is disrupted it can lead to developmental defects and particular types of cancer. Some of these cancers can be treated with a drug called vismodegib, which targets a particular molecule in the Hedgehog signaling pathway. However, tumor cells can become resistant to this drug, so researchers are hoping to find new therapies that target other aspects of the signaling pathway.

Hedgehog signaling promotes the division of brain cells called granule neuron precursor cells (or GNP cells for short). If the signaling pathway is over-active it can trigger the GNP cells to divide more than they should. This can lead to medulloblastoma, which is the most common type of brain tumor that affects children. Proteins called Neuropilins—which bind to molecules known as Semaphorins—promote Hedgehog signaling and the formation of medulloblastoma, but it was not clear how this works.

Here Ge et al. studied the role of Neuropilin in cultured cells and in the cerebellum of mice. The experiments show that Semaphorin 3 promotes the accumulation of an enzyme called PDE4D at the cell membrane. PDE4D interacts with Neuropilin and blocks the activity of another enzyme that normally inhibits Hedgehog signaling. In mice that lack Neuropilin and Semophorin 3, the GNP cells are less able to divide, which leads to abnormal development of the cerebellum.

Further experiments show that drugs that target PDE4D inhibit both the Hedgehog pathway and the growth of tumors that are resistant to vismodegib treatment. Ge et al.'s findings uncover a new way in which Hedgehog signaling is regulated and highlight a potential new strategy for treating medulloblastoma and other similar tumors. Current PDE4D inhibitors are associated with severe side effects, so the next challenge is to develop new drugs that have fewer side effects.

DOI:http://dx.doi.org/10.7554/eLife.07068.002

Corticotropin-Releasing Hormone Receptor Type 1 (CRHR1) Clustering with MAGUKs Is Mediated via Its C-Terminal PDZ Binding Motif

The corticotropin-releasing hormone receptor type 1 (CRHR1) plays an important role in orchestrating neuroendocrine, behavioral, and autonomic responses to stress. To identify molecules capable of directly modulating CRHR1 signaling, we performed a yeast-two-hybrid screen using the C-terminal intracellular tail of the receptor as bait. We identified several members of the membrane-associated guanylate kinase (MAGUK) family: postsynaptic density protein 95 (PSD95), synapse-associated protein 97 (SAP97), SAP102 and membrane associated guanylate kinase, WW and PDZ domain containing 2 (MAGI2). CRHR1 is co-expressed with the identified MAGUKs and with the additionally investigated PSD93 in neurons of the adult mouse brain and in primary hippocampal neurons, supporting the probability of a physiological interaction in vivo. The C-terminal PDZ (PSD-95, discs large, zona occludens 1) binding motif of CRHR1 is essential for its physical interaction with MAGUKs, as revealed by the CRHR1-STAVA mutant, which harbors a functionally impaired PDZ binding motif. The imitation of a phosphorylation at Thr413 within the PDZ binding motif also disrupted the interaction with MAGUKs. In contrast, distinct PDZ domains within the identified MAGUKs are involved in the interactions. Expression of CRHR1 in primary neurons demonstrated its localization throughout the neuronal plasma membrane, including the excitatory post synapse, where the receptor co-localized with PSD95 and SAP97. The co-expression of CRHR1 and respective interacting MAGUKs in HEK293 cells resulted in a clustered subcellular co-localization which required an intact PDZ binding motif. In conclusion, our study characterized the PDZ binding motif-mediated interaction of CRHR1 with multiple MAGUKs, which directly affects receptor function.

Role of cystic fibrosis transmembrane conductance regulator-associated ligand (CAL) in regulating the trafficking and signaling of corticotropin-releasing factor receptor 1

Corticotropin releasing factor (CRF) receptor1 (CRFR1) is associated with psychiatric illness and is a proposed target for the treatment of anxiety and depression. Like many G protein-coupled receptors (GPCRs), CRFR1 harbors a PDZ (PSD95/Disc Large/Zona Occludens 1)-binding motif at the end of its carboxyl terminal tail. The interactions of PDZ proteins with GPCRs are crucial for the regulation of their receptor function. In the present study, we characterize the interaction of the cystic fibrosis transmembrane conductance regulator-associated ligand (CAL) with CRFR1. We show using co-immunoprecipitation that the two proteins interact in human embryonic kidney (HEK293) cells in a PDZ motif-dependent manner. We find that the interaction occurs at the Golgi apparatus and that overexpression of CAL retains a proportion of CRFR1 in the intracellular compartment and prevents trafficking to the cell surface. We also demonstrate a significant reduction in the levels of receptor at the plasma membrane upon CAL overexpression, as well as a reduction in internalization. We find that the overexpression of CAL in HEK293 cells resulted in a significant decrease in CRF-stimulated extracellular-regulated protein kinase 1/2 (ERK1/2) phosphorylation, but has no effect on cAMP signaling mediated by the receptor. This effect was dependent on an intact PDZ motif and knockdown of CAL expression using CAL siRNA results in a significant enhancement in ERK1/2 signaling. We show that CAL contributes to the regulation of CRFR1 glycosylation and utilize glycosylation-deficient CRFR1 mutants to further examine the role of glycosylation in the cell surface trafficking of CRFR1. We find that the mutation of Asn residues 90 and 98 results in a reduction in cell surface CRFR1 that is comparable to the effect of CAL overexpression and that these mutants are retained in the Golgi apparatus. Mutation of Asn residues 90 and 98 also results in a decrease in the efficacy for CRF-stimulated cAMP formation mediated by CRFR1. Taken together, our data suggest that CAL can regulate the anterograde trafficking, the internalization as well as the signaling of CRFR1 via modulating the post-translational modifications that the receptor undergoes at the Golgi apparatus.

Suppression of piriform cortex activity in rat by corticotropin-releasing factor 1 and serotonin 2A/C receptors

The piriform cortex (PC) is richly innervated by corticotropin-releasing factor (CRF) and serotonin (5-HT) containing axons arising from central amygdala and Raphe nucleus. CRFR₁ and 5-HT_2A/2CRs have been shown to interact in manner where CRFR activation subsequently potentiates the activity of 5-HT_2A/2CRs. The purpose of this study was to determine how the activation of CRFR₁ and/or 5-HT₂Rs modulates PC activity at both the circuit and cellular level. Voltage sensitive dye imaging showed that CRF acting through CRFR₁ dampened activation of the Layer II of PC and interneurons of endopiriform nucleus. Application of the selective 5-HT_2A/CR agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) following CRFR₁ activation potentiated this effect. Blocking the interaction between CRFR₁ and 5-HT₂R with a Tat-CRFR₁-CT peptide abolished this potentiation. Application of forskolin did not mimic CRFR₁ activity but instead blocked it, while a protein kinase A antagonist had no effect. However, activation and antagonism of protein kinase C (PKC) either mimicked or blocked CRF modulation, respectively. DOI had no effect when applied alone indicating that the prior activation of CRFR₁ receptors was critical for DOI to show significant effects similar to CRF. Patch clamp recordings showed that both CRF and DOI reduced the synaptic responsiveness of Layer II pyramidal neurons. CRF had highly variable effects on interneurons within Layer III, both increasing and decreasing their excitability, but DOI had no effect on the excitability of this group of neurons. These data show that CRF and 5-HT, acting through both CRFR₁ and 5-HT_2A/CRs, reduce the activation of the PC. This modulation may be an important blunting mechanism of stressor behaviors mediated through the olfactory cortex.

Minireview: Role of intracellular scaffolding proteins in the regulation of endocrine G protein-coupled receptor signaling

The majority of hormones stimulates and mediates their signal transduction via G protein-coupled receptors (GPCRs). The signal is transmitted into the cell due to the association of the GPCRs with heterotrimeric G proteins, which in turn activates an extensive array of signaling pathways to regulate cell physiology. However, GPCRs also function as scaffolds for the recruitment of a variety of cytoplasmic protein-interacting proteins that bind to both the intracellular face and protein interaction motifs encoded by GPCRs. The structural scaffolding of these proteins allows GPCRs to recruit large functional complexes that serve to modulate both G protein-dependent and -independent cellular signaling pathways and modulate GPCR intracellular trafficking. This review focuses on GPCR interacting PSD95-disc large-zona occludens domain containing scaffolds in the regulation of endocrine receptor signaling as well as their potential role as therapeutic targets for the treatment of endocrinopathies.

PDZK1/NHERF3 differentially regulates corticotropin-releasing factor receptor 1 and serotonin 2A receptor signaling and endocytosis

The corticotropin-releasing factor receptor 1 (CRFR1) and serotonin 2A receptor (5-HT2AR) are linked to cellular mechanisms underlying stress anxiety and depression. Both receptors are members of the G protein-coupled receptor (GPCR) superfamily and encode class I PSD-95/DiscsLarge/Zona Occludens 1 (PDZ) binding motifs (-S/T-x-V/I/L) at the end of their carboxyl-terminal tails. We have identified PDZK1, also referred to as Na(+)/H(+) exchange regulatory cofactor 3 (NHERF3) as both a CRFR1- and 5-HT2AR-interacting protein. We have examined whether PDZK1 plays a role in regulating both CRFR1 and 5-HT2AR activity. We find that while PDZK1 interactions with CRFR1 are PDZ binding motif-dependent, PDZK1 associates with 5-HT2AR in a PDZ binding motif-independent manner and CRFR1 expression, but not 5-HT2AR expression, redistributes PDZK1 to the plasma membrane in PDZ binding motif-dependent manner. PDZK1, negatively regulates 5-HT2AR endocytosis and has no effect upon 5-HT2AR-mediated ERK1/2 phosphorylation. In contrast, PDZK1 overexpression does not affect CRFR1 endocytosis, but selectively increases CRFR1-stimulated ERK1/2 phosphorylation. Similar to what has been previously reported for PSD-95 and SAP97, PDZK1 positively influences 5-HT2AR-stimulated inositol phosphate formation, but does not contribute to the regulation of CRFR1-mediated cAMP signaling. Taken together, these results indicate that PDZK1 differentially regulates the signaling and trafficking of CRFR1 and 5-HT2AR via PDZ-dependent and -independent mechanisms, respectively.

G protein-coupled receptor 30 (GPR30) forms a plasma membrane complex with membrane-associated guanylate kinases (MAGUKs) and protein kinase A-anchoring protein 5 (AKAP5) that constitutively inhibits cAMP production

GPR30, or G protein-coupled estrogen receptor, is a G protein-coupled receptor reported to bind 17β-estradiol (E2), couple to the G proteins Gs and Gi/o, and mediate non-genomic estrogenic responses. However, controversies exist regarding the receptor pharmacological profile, effector coupling, and subcellular localization. We addressed the role of the type I PDZ motif at the receptor C terminus in receptor trafficking and coupling to cAMP production in HEK293 cells and CHO cells ectopically expressing the receptor and in Madin-Darby canine kidney cells expressing the native receptor. GPR30 was localized both intracellularly and in the plasma membrane and subject to limited basal endocytosis. E2 and G-1, reported GPR30 agonists, neither stimulated nor inhibited cAMP production through GPR30, nor did they influence receptor localization. Instead, GPR30 constitutively inhibited cAMP production stimulated by a heterologous agonist independently of Gi/o. Moreover, siRNA knockdown of native GPR30 increased cAMP production. Deletion of the receptor PDZ motif interfered with inhibition of cAMP production and increased basal receptor endocytosis. GPR30 interacted with membrane-associated guanylate kinases, including SAP97 and PSD-95, and protein kinase A-anchoring protein (AKAP) 5 in the plasma membrane in a PDZ-dependent manner. Knockdown of AKAP5 or St-Ht31 treatment, to disrupt AKAP interaction with the PKA RIIβ regulatory subunit, decreased inhibition of cAMP production, and St-Ht31 increased basal receptor endocytosis. Therefore, GPR30 forms a plasma membrane complex with a membrane-associated guanylate kinase and AKAP5, which constitutively attenuates cAMP production in response to heterologous agonists independently of Gi/o and retains receptors in the plasma membrane.

G protein-coupled receptors: what a difference a 'partner' makes

G protein-coupled receptors (GPCRs) are important cell signaling mediators, involved in essential physiological processes. GPCRs respond to a wide variety of ligands from light to large macromolecules, including hormones and small peptides. Unfortunately, mutations and dysregulation of GPCRs that induce a loss of function or alter expression can lead to disorders that are sometimes lethal. Therefore, the expression, trafficking, signaling and desensitization of GPCRs must be tightly regulated by different cellular systems to prevent disease. Although there is substantial knowledge regarding the mechanisms that regulate the desensitization and down-regulation of GPCRs, less is known about the mechanisms that regulate the trafficking and cell-surface expression of newly synthesized GPCRs. More recently, there is accumulating evidence that suggests certain GPCRs are able to interact with specific proteins that can completely change their fate and function. These interactions add on another level of regulation and flexibility between different tissue/cell-types. Here, we review some of the main interacting proteins of GPCRs. A greater understanding of the mechanisms regulating their interactions may lead to the discovery of new drug targets for therapy.

Sorting of β1-adrenergic receptors is mediated by pathways that are either dependent on or independent of type I PDZ, protein kinase A (PKA), and SAP97

The β1-adrenergic receptor (β1-AR) is a target for treatment of major cardiovascular diseases, such as heart failure and hypertension. Recycling of agonist-internalized β1-AR is dependent on type I PSD-95/DLG/ZO1 (PDZ) in the C-tail of the β1-AR and on protein kinase A (PKA) activity (Gardner, L. A., Naren, A. P., and Bahouth, S. W. (2007) J. Biol. Chem. 282, 5085-5099). We explored the effects of point mutations in the PDZ and in the activity of PKA on recycling of the β1-AR and its binding to the PDZ-binding protein SAP97. These studies indicated that β1-AR recycling was inhibited by PKA inhibitors and by mutations in the PDZ that interfered with SAP97 binding. The trafficking effects of short sequences differing in PDZ and SAP97 binding were examined using chimeric mutant β1-AR. β1-AR chimera containing the type I PDZ of the β2-adrenergic receptor that does not bind to SAP97 failed to recycle except when serine 312 was mutated to aspartic acid. β1-AR chimera with type I PDZ sequences from the C-tails of aquaporin-2 or GluR1 recycled in a SAP97- and PKA-dependent manner. Non-PDZ β1-AR chimera derived from μ-opioid, dopamine 1, or GluR2 receptors promoted rapid recycling of chimeric β1-AR in a SAP97- and PKA-independent manner. Moreover, the nature of the residue at position -3 in the PDZ regulated whether the β1-AR was internalized alone or in complex with SAP97. These results indicate that divergent pathways were involved in trafficking the β1-AR and provide a roadmap for its trafficking via type I PDZs versus non-PDZs.