GRK3 mediates desensitization of CRF1 receptors: a potential mechanism regulating stress adaptation

CRHR1 endocytosis: Spatiotemporal regulation of receptor signaling

Corticotropin releasing hormone (CRH) is crucial for basal and stress-initiated reactions in the hypothalamic-pituitary-adrenal axis (HPA) and extrahypothalamic brain circuits, where it acts as a neuromodulator to organize behavioral and humoral responses to stress. We review and describe cellular components and molecular mechanisms involved in CRH system signaling through G protein-coupled receptors (GPCRs) CRHR1 and CRHR2, under the current view of GPCR signaling from the plasma membrane but also from intracellular compartments, which establish the bases of signal resolution in space and time. Focus is placed on latest studies of CRHR1 signaling in physiologically significant contexts of the neurohormone function that disclosed new mechanistic features of cAMP production and ERK1/2 activation. We also introduce in a brief overview the pathophysiological function of the CRH system, underlining the need for a complete characterization of CRHRs signaling to design new and specific therapies for stress-related disorders.

Transcriptional Regulation, Signaling Pathways, and Subcellular Localization of Corticotropin-Releasing Factor Receptors in the Central Nervous System

Corticotropin-releasing factor (CRF) receptors CRF-R1 and CRF-R2 are differentially distributed in body tissues, and although they respond differentially to stimuli due to their association with different signaling pathways, both receptors have a fundamental role in the response and adaptation to stressful stimuli. Here, we summarize the reported data on different forms of CRF-R1 and CRF-R2 regulation as well as on their subcellular localization. Although the presence of R1 has been described at pre- and postsynaptic sites, R2 is mainly associated with postsynaptic densities. Different studies have provided valuable information on how these receptors regulate responses at a central level, elucidating different and sometimes synergistic roles in response to stress, but despite their high sequence identity, both receptors have been described to be differentially regulated both by their ligands and by transcriptional factors. To date, and from the point of view of their promoter sequences, it has not yet been reported how the different consensus sites identified in silico could be modulating the transcriptional regulation and expression of the receptors under different conditions, which strongly limits the full understanding of their differential functions, providing a wide field to increase and expand the study of the regulation and role of CRF receptors in the CRF system. SIGNIFICANCE STATEMENT: A large number of physiological functions related to the organization of the stress response in different body tissues are associated with the corticotropin-releasing factor system. This system also plays a relevant role in depression and anxiety disorders, as well as being a direct connection between stress and addiction. A better understanding of how the receptors of this system are regulated would help to expand the understanding of how these receptors respond differently to both drugs and stressful stimuli.

Disulfide Dimerization of Neuronal Calcium Sensor-1: Implications for Zinc and Redox Signaling

Neuronal calcium sensor-1 (NCS-1) is a four-EF-hand ubiquitous signaling protein modulating neuronal function and survival, which participates in neurodegeneration and carcinogenesis. NCS-1 recognizes specific sites on cellular membranes and regulates numerous targets, including G-protein coupled receptors and their kinases (GRKs). Here, with the use of cellular models and various biophysical and computational techniques, we demonstrate that NCS-1 is a redox-sensitive protein, which responds to oxidizing conditions by the formation of disulfide dimer (dNCS-1), involving its single, highly conservative cysteine C38. The dimer content is unaffected by the elevation of intracellular calcium levels but increases to 10–30% at high free zinc concentrations (characteristic of oxidative stress), which is accompanied by accumulation of the protein in punctual clusters in the perinuclear area. The formation of dNCS-1 represents a specific Zn²⁺-promoted process, requiring proper folding of the protein and occurring at redox potential values approaching apoptotic levels. The dimer binds Ca²⁺ only in one EF-hand per monomer, thereby representing a unique state, with decreased α-helicity and thermal stability, increased surface hydrophobicity, and markedly improved inhibitory activity against GRK1 due to 20-fold higher affinity towards the enzyme. Furthermore, dNCS-1 can coordinate zinc and, according to molecular modeling, has an asymmetrical structure and increased conformational flexibility of the subunits, which may underlie their enhanced target-binding properties. In HEK293 cells, dNCS-1 can be reduced by the thioredoxin system, otherwise accumulating as protein aggregates, which are degraded by the proteasome. Interestingly, NCS-1 silencing diminishes the susceptibility of Y79 cancer cells to oxidative stress-induced apoptosis, suggesting that NCS-1 may mediate redox-regulated pathways governing cell death/survival in response to oxidative conditions.

The GRKs Reactome: Role in Cell Biology and Pathology

G protein-coupled receptor kinases (GRKs) are protein kinases that function in concert with arrestins in the regulation of a diverse class of G protein-coupled receptors (GPCRs) signaling. Although GRKs and arrestins are key participants in the regulation of GPCR cascades, the complex regulatory mechanisms of GRK expression, its alternation, and their function are not thoroughly understood. Several studies together with the work from our lab in recent years have revealed the critical role of these kinases in various physiological and pathophysiological processes, including cardiovascular biology, inflammation and immunity, neurodegeneration, thrombosis, and hemostasis. A comprehensive understanding of the mechanisms underlying functional interactions with multiple receptor proteins and how these interactions take part in the development of various pathobiological processes may give rise to novel diagnostic and therapeutic strategies. In this review, we summarize the current research linking the role of GRKs to various aspects of cell biology, pathology, and therapeutics, with a particular focus on thrombosis and hemostasis.

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.

The Role of G Protein-coupled Receptor Kinases in Cancer

G protein-coupled receptors (GPCRs) are the largest family of plasma membrane receptors. Emerging evidence demonstrates that signaling through GPCRs affects numerous aspects of cancer biology such as vascular remolding, invasion, and migration. Therefore, development of GPCR-targeted drugs could provide a new therapeutic strategy to treating a variety of cancers. G protein-coupled receptor kinases (GRKs) modulate GPCR signaling by interacting with the ligand-activated GPCR and phosphorylating its intracellular domain. This phosphorylation initiates receptor desensitization and internalization, which inhibits downstream signaling pathways related to cancer progression. GRKs can also regulate non-GPCR substrates, resulting in the modulation of a different set of pathophysiological pathways. In this review, we will discuss the role of GRKs in modulating cell signaling and cancer progression, as well as the therapeutic potential of targeting GRKs.

The Psychoactive Agent Crocin Can Regulate Hypothalamic-Pituitary-Adrenal Axis Activity

Post-traumatic stress disorder (PTSD) occurs following life-threatening events. The activity of the hypothalamic-pituitary-adrenal (HPA) axis, which serves as the first line of defense against stress, is dysfunctional in this disorder. The current study aimed to investigate the role of Crocin in normalizing HPA function in an animal model of PTSD induced by electric foot shock. Rats were treated with Crocin 5 min prior to stress induction. The stimulus was re-introduced after 21 days, and we measured individual behaviors such as sniffing, rearing, grooming, and freezing. Enzyme-linked immunosorbent assays were performed to measure plasma levels of Corticosterone. On day 28, after rats were weighed and sacrificed, the adrenal and thymus glands were removed and subjected to real-time polymerase chain reaction to quantify the gene expression of corticotrophin-releasing hormone (CRH), glucocorticoid receptor (GluR), and arginine vasopressin (AVP). Our results demonstrate that rats re-exposed to a stressor developed characteristic symptoms of PTSD, but these were attenuated by Crocin. Treated rats showed significant changes in CRH expression in the hypothalamus, GluR expression in the pituitary, plasma Corticosterone levels, and freezing behavior. Together, these findings suggest that Crocin can regulate HPA axis activity in PTSD. It may serve an appropriate treatment for subjects who experience a traumatic event.

Endocrinology and the brain: corticotropin-releasing hormone signaling

Corticotropin-releasing hormone (CRH) is a key player of basal and stress-activated responses in the hypothalamic-pituitary-adrenal axis (HPA) and in extrahypothalamic circuits, where it functions as a neuromodulator to orchestrate humoral and behavioral adaptive responses to stress. This review describes molecular components and cellular mechanisms involved in CRH signaling downstream of its G protein-coupled receptors (GPCRs) CRHR1 and CRHR2 and summarizes recent findings that challenge the classical view of GPCR signaling and impact on our understanding of CRHRs function. Special emphasis is placed on recent studies of CRH signaling that revealed new mechanistic aspects of cAMP generation and ERK1/2 activation in physiologically relevant contexts of the neurohormone action. In addition, we present an overview of the pathophysiological role of the CRH system, which highlights the need for a precise definition of CRHRs signaling at molecular level to identify novel targets for pharmacological intervention in neuroendocrine tissues and specific brain areas involved in CRH-related disorders.

New mechanisms and perspectives in nicotine withdrawal

Diseases associated with tobacco use constitute a major health problem worldwide. Upon cessation of tobacco use, an unpleasant withdrawal syndrome occurs in dependent individuals. Avoidance of the negative state produced by nicotine withdrawal represents a motivational component that promotes continued tobacco use and relapse after smoking cessation. With the modest success rate of currently available smoking cessation therapies, understanding mechanisms involved in the nicotine withdrawal syndrome are crucial for developing successful treatments. Animal models provide a useful tool for examining neuroadaptative mechanisms and factors influencing nicotine withdrawal, including sex, age, and genetic factors. Such research has also identified an important role for nicotinic receptor subtypes in different aspects of the nicotine withdrawal syndrome (e.g., physical vs. affective signs). In addition to nicotinic receptors, the opioid and endocannabinoid systems, various signal transduction pathways, neurotransmitters, and neuropeptides have been implicated in the nicotine withdrawal syndrome. Animal studies have informed human studies of genetic variants and potential targets for smoking cessation therapies. Overall, the available literature indicates that the nicotine withdrawal syndrome is complex, and involves a range of neurobiological mechanisms. As research in nicotine withdrawal progresses, new pharmacological options for smokers attempting to quit can be identified, and treatments with fewer side effects that are better tailored to the unique characteristics of patients may become available. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Using high resolution imaging to determine trafficking of corticotropin-releasing factor receptors in noradrenergic neurons of the rat locus coeruleus

Trafficking of G protein-coupled receptors (GPCRs) is a critical determinant of cellular sensitivity of neurons. To understand how endogenous or exogenous ligands impact cell surface expression of GPCRs, it is essential to employ approaches that achieve superior anatomical resolution at the synaptic level. In situations in which light and fluorescence microscopy techniques may provide only limited resolution, electron microscopy provides enhanced subcellular precision. Dual labeling immunohistochemistry employing visually distinct immunoperoxidase and immunogold markers has been an effective approach for elucidating complex receptor profiles at the synapse and to definitively establish the localization of individual receptors and neuromodulators to common cellular profiles. The immuno-electron microscopy approach offers the potential for determining membrane versus intracellular protein localization, as well as the association with various identifiable cellular organelles. Corticotropin-releasing factor (CRF) is an important regulator of endocrine, autonomic, immunological, behavioral and cognitive limbs of the stress response. Dysfunction of this neuropeptide system has been associated with several psychiatric disorders. This review summarizes findings from neuroanatomical studies, with superior spatial resolution, that indicate that the distribution of CRF receptors is a highly dynamic process that, in addition to being sexually dimorphic, involves complex regulation of receptor trafficking within extrasynaptic sites that have significant consequences for adaptations to stress, particularly within the locus coeruleus (LC), the major brain norepinephrine-containing nucleus.

Desensitization of human CRF2(a) receptor signaling governed by agonist potency and βarrestin2 recruitment

The primary goal was to determine agonist-specific regulation of CRF2(a) receptor function. Exposure of human retinoblastoma Y79 cells to selective (UCN2, UCN3 or stresscopins) and non-selective (UCN1 or sauvagine) agonists prominently desensitized CRF2(a) receptors in a rapid, concentration-dependent manner. A considerably slower rate and smaller magnitude of desensitization developed in response to the weak agonist CRF. CRF1 receptor desensitization stimulated by CRF, cortagine or stressin1-A had no effect on CRF2(a) receptor cyclic AMP signaling. Conversely, desensitization of CRF2(a) receptors by UCN2 or UCN3 did not cross-desensitize Gs-coupled CRF1 receptor signaling. In transfected HEK293 cells, activation of CRF2(a) receptors by UCN2, UCN3 or CRF resulted in receptor phosphorylation and internalization proportional to agonist potency. Neither protein kinase A nor casein kinases mediated CRF2(a) receptor phosphorylation or desensitization. Exposure of HEK293 or U2OS cells to UCN2 or UCN3 (100nM) produced strong βarrestin2 translocation and colocalization with membrane CRF2(a) receptors while CRF (1μM) generated only weak βarrestin2 recruitment. βarrestin2 did not internalize with the receptor, however, indicating that transient CRF2(a) receptor-arrestin complexes dissociate at or near the cell membrane. Since deletion of the βarrestin2 gene upregulated Gs-coupled CRF2(a) receptor signaling in MEF cells, a βarrestin2 mechanism restrains Gs-coupled CRF2(a) receptor signaling activated by urocortins. We further conclude that the rate and extent of homologous CRF2(a) receptor desensitization are governed by agonist-specific mechanisms affecting GRK phosphorylation, βarrestin2 recruitment, and internalization thereby producing unique signal transduction profiles that differentially affect the stress response.

Ro 32-0432 attenuates mecamylamine-precipitated nicotine withdrawal syndrome in mice

G protein-coupled receptor kinase 5 is noted to mediate a number of signal transduction cascades involved in the causation of nicotine withdrawal syndrome. Therefore, the present study investigated the effect of Ro 32-0432, a G protein-coupled receptor kinase 5 inhibitor, on propagation of nicotine dependence and resultant withdrawal signs in subchronic nicotine mouse model. Our experimental protocol consisted of administration of nicotine, (2.5 mg/kg, subcutaneously), four times daily for 7 days. In order to precipitate nicotine withdrawal, mice were given one injection of mecamylamine (3 mg/kg, intraperitoneally) 1 h after the last nicotine injection on the test day (day 8). Behavioral observations were made for a period of 30 min immediately after mecamylamine treatment. Withdrawal syndrome was quantitated in terms of a composite withdrawal severity score, jumping frequency, nicotine-induced hyperalgesia by tail flick method, and withdrawal syndrome-related anxiety was assessed by elevated plus maze test results. Ro 32-0432 dose dependently attenuated mecamylamine-induced nicotine withdrawal syndrome in mice. It is concluded that Ro 32-0432 attenuates the propagation of nicotine dependence and reduce withdrawal signs possibly by G protein-coupled receptor kinase 5 activation-linked mechanisms.

Genes, Environment, and Time: The Vietnam Era Twin Study of Aging (VETSA)

The Vietnam Era Twin Study of Aging (VETSA) is a large-scale investigation of cognitive aging from middle to later age. The intended sample of 1440 twin subjects is recruited from the Vietnam Era Twin Registry (VETR), a registry of middle-aged male-male twin pairs who both served in the military during the Vietnam conflict (1965–1975). VETSA employs a multitrait multimethod approach to cognitive assessment to focus on the genetic and environmental contributions to cognitive processes over time, as well as the relative contributions to cognitive aging from health, social, personality, and other contextual factors. The cognitive domains of episodic memory, working memory, abstract reasoning, and inhibitory executive functioning are assessed through neuropsychological testing. In addition, VETSA obtains the participant's score on the Armed Forces Qualification Test, taken at the time of induction into the military around age 20 years, and readministers the test. Two other projects — VETSA Cortisol and VETSA Magnetic Resonance Imaging — are also in progress using subsamples of the VETSA twins. Prior waves of data collection by VETSA investigators using the VETR have provided historical data on physical and mental health, while future waves of VETSA data collection are planned every 5 years. These methods will provide data on multiple pheno-types in the same individuals with regard to genetic and environmental contributions to cognitive functioning over time, personality and interpersonal risk and protective factors, stress and cortisol regulation, and structural brain correlates of aging processes.

Molecular and cell signaling targets for PTSD pathophysiology and pharmacotherapy

The reasons for differences in vulnerability or resilience to the development of posttraumatic stress disorder (PTSD) are unclear. Here we review key genetic diatheses and molecular targets especially signaling pathways that mediate responses to trauma and severe stress and their potential contribution to the etiology of PTSD. Sensitization of glucocorticoid receptor (GR) signaling and dysregulation of GR modulators FKBP5, STAT5B, Bcl-2, and Bax have been implicated in PTSD pathophysiology. Furthermore, Akt, NFκB, MKP-1, and p11, which are G protein-coupled receptor (GPCR) pathway molecules, can promote or prevent sustained high anxiety- and depressive-like behavior following severe stress. Agonist-induced activation of the corticotropin releasing factor CRF(1) receptor is crucial for survival in the context of serious danger or trauma, but persistent CRF(1) receptor hypersignaling when a threatening or traumatic situation is no longer present is maladaptive. CRF(1) receptor single nucleotide polymorphisms (SNPs) can confer susceptibility or resilience to childhood trauma while a SNP for the PAC1 receptor, another class B1 GPCR, has been linked genetically to PTSD. GRK3 phosphorylation of the CRF(1) receptor protein and subsequent binding of βarrestin2 rapidly terminate Gs-coupled CRF(1) receptor signaling by homologous desensitization. A deficient GRK-βarrestin2 mechanism would result in excessive CRF(1) receptor signaling thereby contributing to PTSD and co-morbid posttraumatic depression. Clinical trials are needed to assess if small molecule CRF(1) receptor antagonists are effective prophylactic agents when administered immediately after trauma. βarrestin2-biased agonists for CRF receptors and possibly other GPCRs implicated in PTSD, however, may prove to be novel pharmacotherapy with greater selectivity and therapeutic efficacy. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Differential effects of inescapable stress on locus coeruleus GRK3, alpha2-adrenoceptor and CRF1 receptor levels in learned helpless and non-helpless rats: a potential link to stress resilience

Exposure of rats to unpredictable, inescapable stress results in two distinct behaviors during subsequent escape testing. One behavior, suggestive of lack of stress resilience, is prolonged escape latency compared to non-stressed rats and is labeled learned helplessness (LH). The other behavior suggestive of stress resilience is normal escape latency and is labeled non-helpless (NH). This study examines the effects of unpredictable, inescapable tail-shock stress (TSS) on alpha(2)-adrenoceptor (α(2A)-AR) and corticotropin-releasing factor 1 receptor (CRF(1)-R) regulation as well as protein levels of G protein-coupled receptor kinase 3 (GRK3), GRK2, tyrosine hydroxylase (TH) plus carbonylated protein levels in locus coeruleus (LC), amygdala (AMG), cortex (COR) and striatum (STR). In NH rats, α(2A)-AR and CRF(1)-R were significantly down-regulated in LC after TSS. No changes in these receptor levels were observed in the LC of LH rats. GRK3, which phosphorylates receptors and thereby contributes to α(2A)-AR and CRF(1)-R down-regulation, was reduced in the LC of LH but not NH rats. GRK2 levels were unchanged. In AMG, GRK3 but not GRK2 levels were reduced in LH but not NH rats, and receptor regulation was impaired in LH rats. In STR, no changes in GRK3 or GRK2 levels were observed. Finally, protein carbonylation, an index of oxidative stress, was increased in the LC and AMG of LH but not NH rats. We suggest that reduced stress resilience after TSS may be related to oxidative stress, depletion of GRK3 and impaired regulation of α(2A)-AR and CRF(1)-R in LC.

Corticotropin-releasing factor receptors induce calcium mobilization through cross-talk with Gq-coupled receptors

The cross-talk between corticotropin-releasing factor (CRF) and muscarinic receptors was investigated by measuring evoked transient increases in cytosolic calcium concentration. HEK293 cells stably expressing human CRF type 1 (hCRF(1)) and type 2(a) (hCRF(2(a))) receptors were stimulated with the muscarinic receptor agonist carbachol and shortly after by a CRF agonist. Unexpectedly, this second response was enhanced when compared to stimulating naive cells either with carbachol or CRF agonist only. Priming with 100 microM carbachol increased the maximal CRF agonist response and shifted its concentration-response curve to the left to attain almost the same potency as for stimulating the production of the natural second messenger cyclic AMP. Yet, priming did not affect CRF agonist-stimulated cyclic AMP production itself. Carbachol priming was not restricted to recombinant CRF receptors only since endogenously expressed beta(2)-adrenoceptors also started to produce a robust calcium signal. Without priming no such signal was observed. Similar findings were made in the human retinoblastoma cell line Y79 for endogenously expressed CRF(1) receptors and the type 1 pituitary adenylate cyclase-activating polypeptide receptors but not for the CRF(2(a)) receptors. This differentiation between CRF(1) and CRF(2) receptors was further supported by use of selective agonists and antagonists. The results suggest that stimulating a Gq-coupled receptor shortly before stimulating a Gs-coupled receptor may result in a parallel signaling event on top of the classical cyclic AMP pathway.

Role of CRF receptor signaling in stress vulnerability, anxiety, and depression

Markers of hyperactive central corticotropin releasing factor (CRF) systems and CRF-related single nucleotide polymorphisms (SNPs) have been identified in patients with anxiety and depressive disorders. Designing more effective antagonists may now be guided by data showing that small molecules bind to transmembrane domains. Specifically, CRF(1) receptor antagonists have been developed as novel anxiolytic and antidepressant treatments. Because CRF(1) receptors become rapidly desensitized by G protein-coupled receptor kinase (GRK) and beta-arrestin mechanisms in the presence of high agonist concentrations, neuronal hypersecretion of synaptic CRF alone may be insufficient to account for excessive central CRF neurotransmission in stress-induced affective pathophysiology. In addition to desensitizing receptor function, GRK phosphorylation and beta-arrestin binding can shift a G protein-coupled receptor (GPCR) to signal selectively via the extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK-MAPK) or Akt pathways independent of G proteins. Also, Epac-dependent CRF(1) receptor signaling via the ERK-MAPK pathway has been found to potentiate brain-derived neurotrophic factor (BDNF)-stimulated TrkB signaling. Thus, genetic or acquired abnormalities in GRK and beta-arrestin function may be involved in the pathophysiology of stress-induced anxiety and depression.

Decreased GRK3 but not GRK2 expression in frontal cortex from bipolar disorder patients

Overactivation of G-protein-mediated functions and altered G-protein regulation have been reported in bipolar disorder (BD) brain. Further, drugs effective in treating BD are reported to up-regulate expression of G-protein receptor kinase (GRK) 3 in rat frontal cortex. We therefore hypothesized that some G-protein subunits and GRK levels would be reduced in the brain of BD patients. We determined protein and mRNA levels of G-protein beta and gamma subunits, GRK2, and GRK3 in post-mortem frontal cortex from 10 BD patients and 10 age-matched controls by using immunoblots and real-time RT-PCR. There were statistically significant decreases in protein and mRNA levels of G-protein subunits beta and gamma and of GRK3 in BD brain but not a significant difference in the GRK2 level. Decreased expression of G-protein subunits and of GRK3 may alter neurotransmission, leading to disturbed cognition and behaviour in BD.

Identification of brain target neurons using a fluorescent conjugate of corticotropin-releasing factor

Corticotropin-releasing factor (CRF) is a peptide well known for its role in coordinating various neuroendocrine, autonomic, and behavioral components of the vertebrate stress response, including rapid enhancement of locomotor activity. Although CRF's locomotor enhancing properties are well documented, the neuronal mechanisms and specific target neurons that underlie the peptide's effect on locomotor behavior remain poorly understood. In the present study, we describe the synthesis and functional characteristics of a CRF rhodamine analogue TAMRA-X conjugate mixture (CRF-TAMRA 1), to be used for tracking this peptide's internalization into target neurons in the brainstem of an amphibian, the roughskin newt (Taricha granulosa). CRF-TAMRA 1 conjugate administration into the lateral cerebral ventricle resulted in a rapid, endosomal-like internalization of fluorescence into brainstem medullary neurons. In addition, central CRF-TAMRA 1 administration produced neurobehavioral effects comparable to the native peptide, effects that were blocked by pre-treatment with the CRF receptor antagonist, alpha-helical CRF. Taken together, our results show the efficacy of CRF-TAMRA 1 as a novel tool for tracking CRF internalization into targets neurons in vivo and ultimately, aiding in elucidating the neuronal mechanisms and circuitry underlying CRF's influence on behavioral and physiological responses to stress.

Role and action in the pituitary corticotroph of corticotropin-releasing factor (CRF) in the hypothalamus

Corticotropin-releasing factor (CRF), produced in the hypothalamic paraventricular nucleus (PVN) in response to stress, stimulates the synthesis and secretion of adrenocorticotropin (ACTH) via CRF receptor type 1 (CRF(1) receptor) in the anterior pituitary (AP) of mammals. CRF is critical for the circadian rhythmicity of the hypothalamic-pituitary-adrenal axis and the augmented release of ACTH from the pituitary in response to the stress. A higher molecular weight form of immunoreactive beta-endorphin, putative proopiomelanocortin (POMC), is increased in CRF-knockout mice (CRF KO), suggesting the important role of CRF in the processing of POMC. In fact, CRF is able to modulate the processing of POMC through changes in prohormone convertase (PC)-1 expression levels. Multiple forms of ACTH-related peptides containing unprocessed ones are present in some cases of ACTH-producing tumors, presumably without action of PC-1 under the control of CRF. Following CRF-activated stimulation of the receptor signaling, CRF(1) receptor is down-regulated and desensitized. In fact, CRF facilitates the degradation of CRF(1) receptor mRNA via the protein kinase A pathway. Prolonged agonist activation of CRF(1) receptor leads to a loss of responsiveness, or desensitization of the receptor. G protein-coupled receptor kinase 2 is involved in desensitization of CRF(1) receptor by CRF in the corticotroph.

Leukocyte analysis from WHIM syndrome patients reveals a pivotal role for GRK3 in CXCR4 signaling

Leukocytes from individuals with warts, hypogammaglobulinemia, infections, and myelokathexis (WHIM) syndrome, a rare immunodeficiency, and bearing a wild-type CXCR4 ORF (WHIM(WT)) display impaired CXCR4 internalization and desensitization upon exposure to CXCL12. The resulting enhanced CXCR4-dependent responses, including chemotaxis, probably impair leukocyte trafficking and account for the immunohematologic clinical manifestations of WHIM syndrome. We provided here evidence that GPCR kinase-3 (GRK3) specifically regulates CXCL12-promoted internalization and desensitization of CXCR4. GRK3-silenced control cells displayed altered CXCR4 attenuation and enhanced chemotaxis, as did WHIM(WT) cells. These findings identified GRK3 as a negative regulator of CXCL12-induced chemotaxis and as a candidate responsible for CXCR4 dysfunction in WHIM(WT) leukocytes. Consistent with this, we showed that GRK3 overexpression in both leukocytes and skin fibroblasts from 2 unrelated WHIM(WT) patients restored CXCL12-induced internalization and desensitization of CXCR4 and normalized chemotaxis. Moreover, we found in cells derived from one patient a profound and selective decrease in GRK3 products that probably resulted from defective mRNA synthesis. Taken together, these results have revealed a pivotal role for GRK3 in regulating CXCR4 attenuation and have provided a mechanistic link between the GRK3 pathway and the CXCR4-related WHIM(WT) disorder.

Expression, binding, and signaling properties of CRF2(a) receptors endogenously expressed in human retinoblastoma Y79 cells: passage-dependent regulation of functional receptors

Endogenous expression of the corticotropin-releasing factor type 2a receptor [CRF2(a)] but not CRF2(b) and CRF2(c) was observed in higher passage cultures of human Y79 retinoblastoma cells. Functional studies further demonstrated an increase in CRF2(a) mRNA and protein levels with higher passage numbers (> 20 passages). Although the CRF1 receptor was expressed at higher levels than the CRF2(a) receptor, both receptors were easily distinguishable from one another by selective receptor ligands. CRF(1)-preferring or non-selective agonists such as CRF, urocortin 1 (UCN1), and sauvagine stimulated cAMP production in Y79 to maximal responses of approximately 100 pmoles/10(5) cells, whereas the exclusive CRF2 receptor-selective agonists UCN2 and 3 stimulated cAMP production to maximal responses of approximately 25-30 pmoles/10(5) cells. UCN2 and 3-mediated cAMP stimulation was potently blocked by the approximately 300-fold selective CRF2 antagonist antisauvagine (IC50 = 6.5 +/- 1.6 nmol/L), whereas the CRF(1)-selective antagonist NBI27914 only blocked cAMP responses at concentrations > 10 microL. When the CRF(1)-preferring agonist ovine CRF was used to activate cAMP signaling, NBI27914 (IC50 = 38.4 +/- 3.6 nmol/L) was a more potent inhibitor than antisauvagine (IC50 = 2.04 +/- 0.2 microL). Finally, UCN2 and 3 treatment potently and rapidly desensitized the CRF2 receptor responses in Y79 cells. These data demonstrate that Y79 cells express functional CRF1 and CRF2a receptors and that the CRF2(a) receptor protein is up-regulated during prolonged culture.

Carboxyl-terminal and intracellular loop sites for CRF1 receptor phosphorylation and beta-arrestin-2 recruitment: a mechanism regulating stress and anxiety responses

The primary goal was to test the hypothesis that agonist-induced corticotropin-releasing factor type 1 (CRF(1)) receptor phosphorylation is required for beta-arrestins to translocate from cytosol to the cell membrane. We also sought to determine the relative importance to beta-arrestin recruitment of motifs in the CRF(1) receptor carboxyl terminus and third intracellular loop. beta-Arrestin-2 translocated significantly more rapidly than beta-arrestin-1 to agonist-activated membrane CRF(1) receptors in multiple cell lines. Although CRF(1) receptors internalized with agonist treatment, neither arrestin isoform trafficked with the receptor inside the cell, indicating that CRF(1) receptor-arrestin complexes dissociate at or near the cell membrane. Both arrestin and clathrin-dependent mechanisms were involved in CRF(1) receptor internalization. To investigate molecular determinants mediating the robust beta-arrestin-2-CRF(1) receptor interaction, mutagenesis was performed to remove potential G protein-coupled receptor kinase phosphorylation sites. Truncating the CRF(1) receptor carboxyl terminus at serine-386 greatly reduced agonist-dependent phosphorylation but only partially impaired beta-arrestin-2 recruitment. Removal of a serine/threonine cluster in the third intracellular loop also significantly reduced CRF(1) receptor phosphorylation but did not alter beta-arrestin-2 recruitment. Phosphorylation was abolished in a CRF(1) receptor possessing both mutations. Surprisingly, this mutant still recruited beta-arrestin-2. These mutations did not alter membrane expression or cAMP signaling of CRF(1) receptors. Our data reveal the involvement of at least the following two distinct receptor regions in beta-arrestin-2 recruitment: 1) a carboxyl-terminal motif in which serine/threonine residues must be phosphorylated and 2) an intracellular loop motif configured by agonist-induced changes in CRF(1) receptor conformation. Deficient beta-arrestin-2-CRF(1) receptor interactions could contribute to the pathophysiology of affective disorders by inducing excessive CRF(1) receptor signaling.

Activation of the CRF(1) receptor causes ERK1/2 mediated increase in GRK3 expression in CATH.a cells

G-protein coupled receptor kinase 3 (GRK3) mediates desensitization of alpha(2)-adrenergic (alpha(2)-AR) and CRF(1) receptors. CRF(1) receptors, alpha(2)-AR and GRK3, are localized to the primary source of noradrenergic inputs to higher brain centers critical in both the response to stress and the development of depression, namely, locus coeruleus (LC). This study utilizing CATH.a cells (derived from the LC), demonstrates for the first time, that the stress hormone, CRF selectively up-regulates GRK3 expression via an ERK1/2-mediated mechanism accompanied by the activation of Sp-1 and Ap-2 transcription factors. This observation has important implications for the regulation of stress signaling in the brain.

Chronic treatment with mood stabilizers increases membrane GRK3 in rat frontal cortex

BACKGROUND

G-protein receptor kinases (GRKs) are a family of serine/threonine kinases involved in the homologous desensitization of agonist activated G-protein coupled receptors (GPCRs). G-protein coupled receptor supersensitivity, possibly as a result of decreased GRK, has been suggested in affective disorders.

METHODS

We used immunobloting to determine if chronic, therapeutically relevant doses of lithium (Li+), carbamazepine (CBZ), and valproate (VPA), would increase GRK2/3 protein levels in rat frontal cortex.

RESULTS

Chronic Li+ (24%) and CBZ (44%) significantly increased GRK3 in the membrane but not cytosol fractions. Chronic VPA had no effect on GRK3. G-protein receptor kinase 2 protein levels were unchanged by all treatments. The GRK3 membrane to cytosol ratio was increased significantly in Li+ and CBZ treated rats.

CONCLUSIONS

These results show that chronically administered Li+ and CBZ, but not VPA, increase the translocation of GRK3 from cytosol to membrane, possibly correcting supersensitivity of GPCRs in bipolar disorder.

Corticotropin releasing factor (CRF) receptor signaling in the central nervous system: new molecular targets

Corticotropin-releasing factor (CRF) and the related urocortin peptides mediate behavioral, cognitive, autonomic, neuroendocrine and immunologic responses to aversive stimuli by activating CRF(1) or CRF(2) receptors in the central nervous system and anterior pituitary. Markers of hyperactive central CRF systems, including CRF hypersecretion and abnormal hypothalamic-pituitary-adrenal axis functioning, have been identified in subpopulations of patients with anxiety, stress and depressive disorders. Because CRF receptors are rapidly desensitized in the presence of high agonist concentrations, CRF hypersecretion alone may be insufficient to account for the enhanced CRF neurotransmission observed in these patients. Concomitant dysregulation of mechanisms stringently controlling magnitude and duration of CRF receptor signaling also may contribute to this phenomenon. While it is well established that the CRF(1) receptor mediates many anxiety- and depression-like behaviors as well as HPA axis stress responses, CRF(2) receptor functions are not well understood at present. One hypothesis holds that CRF(1) receptor activation initiates fear and anxiety-like responses, while CRF(2) receptor activation re-establishes homeostasis by counteracting the aversive effects of CRF(1) receptor signaling. An alternative hypothesis posits that CRF(1) and CRF(2) receptors contribute to opposite defensive modes, with CRF(1) receptors mediating active defensive responses triggered by escapable stressors, and CRF(2) receptors mediating anxiety- and depression-like responses induced by inescapable, uncontrollable stressors. CRF(1) receptor antagonists are being developed as novel treatments for affective and stress disorders. If it is confirmed that the CRF(2) receptor contributes importantly to anxiety and depression, the development of small molecule CRF(2) receptor antagonists would be therapeutically useful.

Dissecting the genetic effect of the CRH system on anxiety and stress-related behaviour

Corticotropin-releasing hormone (CRH) plays a central role in the adaptation of the body to stress. CRH integrates the endocrine, autonomic and behavioural responses to stress acting as a secretagogue within the line of the hypothalamic pituitary adrenocortical (HPA) system and as a neurotransmitter modulating synaptic transmission in the central nervous system. Accumulating evidence suggests that the neuroendocrine and behavioural symptoms observed in patients suffering from major depression are at least in part linked to a hyperactivity of the CRH system. Genetic modifications of the CRH system by conventional and conditional gene targeting strategies in the mouse allowed us to study the endogenous mechanisms underlying HPA system regulation and CRH-related neuronal circuitries involved in pathways mediating anxiety and stress-related behaviour.

Internalization of the human CRF receptor 1 is independent of classical phosphorylation sites and of beta-arrestin 1 recruitment

The corticotropin releasing factor receptor 1 (CRFR1) belongs to the superfamily of G-protein coupled receptors. Though CRF is involved in the aetiology of several stress-related disorders, including depression and anxiety, details of CRFR1 regulation such as internalization remain uncharacterized. In the present study, agonist-induced internalization of CRFR1 in HEK293 cells was visualized by confocal microscopy and quantified using the radioligand 125I-labelled sauvagine. Recruitment of beta-arrestin 1 in response to receptor activation was demonstrated by confocal microscopy. The extent of 125I-labelled sauvagine stimulated internalization was significantly impaired by sucrose, indicating the involvement of clathrin-coated pits. No effect on the extent of internalization was observed in the presence of the second messenger dependent kinase inhibitors H-89 and staurosporine, indicating that cAMP-dependent protein kinase and protein kinase C are not prerequisites for CRFR1 internalization. Surprisingly, deletion of all putative phosphorylation sites in the C-terminal tail, as well as a cluster of putative phosphorylation sites in the third intracellular loop, did not affect receptor internalization. However, these mutations almost abolished the recruitment of beta-arrestin 1 following receptor activation. In conclusion, we demonstrate that CRFR1 internalization is independent of phosphorylation sites in the C-terminal tail and third intracellular loop, and the degree of beta-arrestin 1 recruitment.

Simultaneous alpha2B- and beta2-adrenoceptor activation sensitizes the alpha2B-adrenoceptor for agonist-induced down-regulation

We recently reported that alpha(2A)-adrenoceptor (AR) desensitization and down-regulation occurs after 24-h treatment with epinephrine (EPI) (0.3 microM) in BE(2)-C cells that express both alpha(2)- and beta(2)-ARs. The same concentration of norepinephrine (NE) has no effect. The effect of EPI is prevented by beta(2)-AR blockade and is associated with an increase in G protein-coupled receptor kinase 3 (GRK3) expression. Because differences in agonist-induced down-regulation of the alpha(2A)-versus alpha(2B)-ARs have been reported, the present study examines the effects of simultaneous activation of alpha(2B)- and beta(2)-ARs on alpha(2B)-AR number and signaling. We studied NG108 cells that naturally express alpha(2B)-ARs, and BN17 cells, NG108 cells transfected to express the human beta(2)-AR. In NG108 cells, alpha(2B)-AR desensitization and down-regulation require treatment with 20 microM EPI or NE; GRK expression was not changed. In BN17 cells expressing beta(2)-ARs, the threshold EPI concentration for alpha(2B)-AR desensitization and down-regulation was reduced to 0.3 microM; 10 microM NE was required for the same effect. Furthermore, 24-h EPI or NE treatments that produced desensitization also resulted in a selective 2-fold up-regulation of GRK3; GRK2 was unchanged. The beta-AR antagonist alprenolol (1 microM) and GRK3 antisense (but not sense) DNA blocked 0.3 microM EPI- and 10 microM NE-induced desensitization and down-regulation of the alpha(2B)-AR as well as GRK3 up-regulation. In conclusion, simultaneous activation of alpha(2B)- and beta(2)-ARs results in a 67-fold decrease in the threshold concentration of EPI required for alpha(2B)-AR down-regulation. This lower threshold for down-regulation is associated with alpha(2B)- and beta(2)-AR dependent up-regulation of GRK3 expression.

Regulation of corticotropin-releasing hormone receptor type 1alpha signaling: structural determinants for G protein-coupled receptor kinase-mediated phosphorylation and agonist-mediated desensitization

Attenuation of CRH receptor type 1 (CRH-R1) signaling activity might involve desensitization and uncoupling of CRH-R1 from intracellular effectors. We investigated the desensitization of native CRH-R in human myometrial cells from pregnant women and recombinant CRH-R1alpha stably overexpressed in human embryonic kidney (HEK) 293 cells. In both cell types, CRH-R1-mediated adenylyl cyclase activation was susceptible to homologous desensitization induced by pretreatment with high concentrations of CRH. Time course studies showed half-maximal desensitization occurring after approximately 40 min of pretreatment and full recovery of CRH-R1alpha functional response within 2 h of removal of CRH pretreatment. In HEK 293 cells, desensitization of CRH-R1alpha was associated with receptor phosphorylation and subsequent endocytosis. To analyze the mechanism leading to CRH-R1alpha desensitization, we overexpressed a truncated beta-arrestin (319-418) and performed coimmunoprecipitation and G protein-coupled receptor kinase (GRK) translocation studies. We found that GRK3 and GRK6 are the main isoforms that interact with CRH-R1alpha, and that recruitment of GRK3 requires Gbetagamma-subunits as well as beta-arrestin. Site-directed mutagenesis of Ser and Thr residues in the CRH-R1alpha C terminus, identified Thr399 as important for GRK-induced receptor phosphorylation and desensitization.We conclude that homologous desensitization of CRH-R1alpha involves the coordinated action of multiple GRK isoforms, Gbeta gamma dimers and beta-arrestin. Based on our identification of key amino acid(s) for GRK-dependent phosphorylation, we demonstrate the importance of the CRH-R1alpha carboxyl tail for regulation of receptor activity.

Involvement of G protein-coupled receptor kinase 4 and 6 in rapid desensitization of dopamine D1 receptor in rat IEC-6 intestinal epithelial cells

Dopamine-induced inhibition of Na(+)-K(+)-ATPase has been suggested to play a role in the regulation of Na(+) absorption at the intestinal level, and these effects were mediated by dopamine D(1)-like receptors. The aim of this work was to evaluate the effect of the activation of the D(1)-like receptors on the activity of the Na(+)/H(+) exchanger (NHE) in the rat intestinal epithelial cell line IEC-6. The presence of D(1) receptors was confirmed by immunoblotting. The dopamine D(1)-like receptor agonist SKF-38393 produced a concentration-dependent inhibition of NHE activity and stimulation of adenylyl cyclase (AC), this being antagonized by the D(1) selective antagonist SKF-83566. Effects of SKF-38393 on NHE and AC activities were maximal at 5 min of exposure to the agonist and rapidly diminished with no effect at 25 min. Exposure of cells for 25 min to dibutyryl-cAMP (0.5 mM) or to the AC activator forskolin (3 microM) effectively inhibited NHE activity. Pretreatment of cells with heparin (1 microM), a nonselective G protein-coupled receptor kinase (GRK) inhibitor, prevented the loss of effects on NHE activity after 25 min exposure to SKF-38393. The presence of GRK4, GRK6A, and GRK6B was confirmed by immunoblotting. Overnight treatment with the anti-GRK4-6 antibody complexed with Lipofectin was also effective in preventing loss of the effects of SKF-38393 on NHE and AC activities. It is concluded that dopamine D(1) receptors in IEC-6 rapidly desensitize to D(1)-like agonist stimulation and GRK4 and 6 appear to be involved in agonist-mediated responsiveness and desensitization.

Corticotropin-releasing factor and urocortin differentially modulate rat Purkinje cell dendritic outgrowth and differentiation in vitro

The precise outgrowth and arborization of dendrites is crucial for their function as integrators of signals relayed from axons and, hence, the functioning of the brain. Proper dendritic differentiation is particularly resonant for Purkinje cells as the intrinsic activity of this cell-type is governed by functionally distinct regions of its dendritic tree. Activity-dependent mechanisms, driven by electrical signaling and trophic factors, account for the most active period of dendritogenesis. An as yet unexplored trophic modulator of Purkinje cell dendritic development is corticotropin-releasing factor (CRF) and family member, urocortin, both of which are localized in climbing fibers. Here, we use rat organotypic cerebellar slice cultures to investigate the roles of CRF and urocortin on Purkinje cell dendritic development. Intermittent exposure (12 h per day for 10 days in vitro) of CRF and urocortin induced significantly more dendritic outgrowth (45% and 70%, respectively) and elongation (25% and 15%, respectively) compared with untreated cells. Conversely, constant exposure to CRF and urocortin significantly inhibited dendritic outgrowth. The trophic effects of CRF and urocortin are mediated by the protein kinase A and mitogen-activating protein kinase pathways. The study shows unequivocally that CRF and urocortin are potent regulators of dendritic development. However, their stimulatory or inhibitory effects are dependent upon the degree of expression of these peptides. Furthermore, the effects of CRF and urocortin on neuronal differentiation and re-modeling may provide a cellular basis for pathologies such as major depression, which show perturbations in the expression of these stress peptides.

Corticotropin releasing hormone receptors: two decades later

Hypothalamic corticotropin releasing hormone (CRH) regulates pituitary ACTH secretion and mediates behavioral and autonomic responses to stress, through interaction with type 1 plasma membrane receptors (CRHR1) located in pituitary corticotrophs and the brain. Although the CHRI are essential for ACTH responses to stress, their number in the pituitary gland does not correlate with corticotroph responsiveness, suggesting that activation of a small number of receptors is sufficient for maximum ACTH production. CRH binding and hybridization studies in adrenalectomized, glucocorticoid-treated or stressed rats revealed divergent changes in CRH receptors and CRH1 mRNA in the pituitary, with a reduction in receptor binding but normal or elevated expression of CHR1 mRNA levels. Western blot analysis of CRHR1 protein in pituitary membranes from adrenalectomized rats showed unchanged receptor mRNA levels and increased CRHR1 protein, despite binding down-regulation, suggesting that decreased binding is due to homologous desensitization, rather than reduced receptor synthesis. In contrast, decreased CRH binding following glucocorticoid administration is associated with a reduction in CRHR1 protein, suggesting inhibition of CRH1 mRNA translation. The regulation of CRHR1 translation may involve binding of cytosolic proteins, and a minicistron in the 5'-UTR of the CRHR1 mRNA. It is likely that post-transcriptional regulatory mechanisms that permit rapid changes in CRH receptor activity are important for adaptation of corticotroph responsiveness to continuous changes in physiological demands.

Mediation of corticotropin releasing factor type 1 receptor phosphorylation and desensitization by protein kinase C: a possible role in stress adaptation

Protein kinase C (PKC)-mediated desensitization of the corticotropin releasing factor type 1 (CRF1) receptor was investigated in human retinoblastoma Y79 and transfected COS-7 cells. Because stimulation of Y79 cells with CRF resulted in large ( approximately 30-fold) increases in intracellular cAMP accumulation without changing inositol phosphate levels, the CRF1 receptor expressed in retinoblastoma cells couples to Gs, but not to Gq, and predominantly signals via the protein kinase A cascade. Direct activation of PKC by treatment with the phorbol ester phorbol 12-myristate 13-acetate (PMA) or 1,2-dioctanoyl-sn-glycerol (DOG) desensitized CRF1 receptors in Y79 cells, reducing the maximum for CRF- (but not forskolin)-stimulated cAMP accumulation by 56.3 +/- 1.2% and 40.4 +/- 2.1%, respectively (p < 0.001). Pretreating Y79 cells with the PKC inhibitor bisindolylmaleimide I (BIM) markedly inhibited PMA's desensitizing action on CRF-stimulated cAMP accumulation, but did not affect homologous CRF1 receptor desensitization. Retinoblastoma cells were found to express PKCalpha, betaI, betaII, delta, lambda, and RACK1. When alpha and beta isoforms of PKC were down-regulated 80 to 90% by a 48-h PMA exposure, PMA-induced CRF1 receptor desensitization was abolished. In transfected COS-7 cells the magnitude of CRF1 receptor phosphorylation after a 5-min exposure to PMA was 2.32 +/- 0.21-fold greater compared with the basal level. Pretreating COS-7 cells with BIM abolished PMA-induced CRF1 receptor phosphorylation. These studies demonstrate that protein kinase C (possibly alpha and beta isoforms) has an important role in the phosphorylation and heterologous desensitization of the CRF1 receptor.

Evidence that a single nucleotide polymorphism in the promoter of the G protein receptor kinase 3 gene is associated with bipolar disorder

In a genome-wide linkage survey, we have previously shown evidence suggesting that the chromosome 22q12 region contains a susceptibility locus for bipolar disorder (BPD). Two independent family sets yielded lod scores suggestive of linkage at markers in this region near the gene G protein receptor kinase 3 (GRK3). GRK3 is an excellent candidate risk gene for BPD since GRK3 is expressed widely in the brain, and since GRKs play key roles in the homologous desensitization of G protein-coupled receptor signaling. We have also previously shown GRK3 expression to be induced by amphetamine in an animal model of mania using microarray-based expression profiling. To identify possible functional mutations in GRK3, we sequenced the putative promoter region, all 21 exons, and intronic sequence flanking each exon, in 14-22 individuals with BPD. We found six sequence variants in the 5'-UTR/promoter region, but no coding or obvious splice variants. Transmission disequilibrium analyses of one set of 153 families indicated that two of the 5'-UTR/promoter variants are associated with BPD in families of northern European Caucasian ancestry. A supportive trend towards association to one of these two variants (P-5) was then subsequently obtained in an independent sample of 237 families. In the combined sample, the P-5 variant had an estimated allele frequency of 3% in bipolar subjects, and displayed a transmission to non-transmission ratio of 26 : 7.7 (chi(2)=9.6, one-sided P value=0.0019). Altogether, these data support the hypothesis that a dysregulation in GRK3 expression alters signaling desensitization, and thereby predisposes to the development of BPD.

Neuroendocrine pharmacology of stress

Exposure to hostile conditions initiates responses organized to enhance the probability of survival. These coordinated responses, known as stress responses, are composed of alterations in behavior, autonomic function and the secretion of multiple hormones. The activation of the renin-angiotensin system and the hypothalamic-pituitary-adrenocortical axis plays a pivotal role in the stress response. Neuroendocrine components activated by stressors include the increased secretion of epinephrine and norepinephrine from the sympathetic nervous system and adrenal medulla, the release of corticotropin-releasing factor (CRF) and vasopressin from parvicellular neurons into the portal circulation, and seconds later, the secretion of pituitary adrenocorticotropin (ACTH), leading to secretion of glucocorticoids by the adrenal gland. Corticotropin-releasing factor coordinates the endocrine, autonomic, behavioral and immune responses to stress and also acts as a neurotransmitter or neuromodulator in the amygdala, dorsal raphe nucleus, hippocampus and locus coeruleus, to integrate brain multi-system responses to stress. This review discussed the role of classical mediators of the stress response, such as corticotropin-releasing factor, vasopressin, serotonin (5-hydroxytryptamine or 5-HT) and catecholamines. Also discussed are the roles of other neuropeptides/neuromodulators involved in the stress response that have previously received little attention, such as substance P, vasoactive intestinal polypeptide, neuropeptide Y and cholecystokinin. Anxiolytic drugs of the benzodiazepine class and other drugs that affect catecholamine, GABA(A), histamine and serotonin receptors have been used to attenuate the neuroendocrine response to stressors. The neuroendocrine information for these drugs is still incomplete; however, they are a new class of potential antidepressant and anxiolytic drugs that offer new therapeutic approaches to treating anxiety disorders. The studies described in this review suggest that multiple brain mechanisms are responsible for the regulation of each hormone and that not all hormones are regulated by the same neural circuits. In particular, the renin-angiotensin system seems to be regulated by different brain mechanisms than the hypothalamic-pituitary-adrenal system. This could be an important survival mechanism to ensure that dysfunction of one neurotransmitter system will not endanger the appropriate secretion of hormones during exposure to adverse conditions. The measurement of several hormones to examine the mechanisms underlying the stress response and the effects of drugs and lesions on these responses can provide insight into the nature and location of brain circuits and neurotransmitter receptors involved in anxiety and stress.

Induction of G protein-coupled receptor kinases 2 and 3 contributes to the cross-talk between mu and ORL1 receptors following prolonged agonist exposure

The molecular mechanism(s) underlying cross-tolerance between mu and opioid receptor-like 1 (ORL1) receptor agonists were investigated using two human neuroblastoma cell lines endogenously expressing these receptors and G protein-coupled receptor kinases (GRKs). Prolonged (24 h) activation of the mu receptor desensitized both mu and ORL1 receptor-mediated inhibition of forskolin-stimulated cAMP accumulation and upregulated GRK2 levels in SH-SY5Y and BE(2)-C cells. Prolonged ORL1 activation increased GRK2 levels and desensitized both receptors in SH-SY5Y cells. Upregulation of GRK2 correlated with increases in levels of transcription factors Sp1 or AP-2. PD98059, an upstream inhibitor of extracellular signal-regulated kinases 1 and 2 (ERK1/2), reversed all these events. Pretreatment with orphanin FQ/nociceptin (OFQ/N) also upregulated GRK3 levels in both cell lines, and desensitized both receptors in BE(2)-C cells. Protein kinase C (PKC), but not ERK1/2, inhibition blocked OFQ/N-mediated GRK3 induction and mu and ORL1 receptor desensitization in BE(2)-C cells. Antisense DNA treatment confirmed the involvement of GRK2/3 in mu and ORL1 desensitization. Here, we demonstrate for the first time a role for ERK1/2-mediated GRK2 induction in the development of tolerance to mu agonists, as well as cross-tolerance to OFQ/N. We also demonstrate that chronic OFQ/N-mediated desensitization of ORL1 and mu receptors occurs via cell-specific pathways, involving ERK1/2-dependent GRK2, or PKC-dependent and ERK1/2-independent GRK3 induction.

The CRF peptide family and their receptors: yet more partners discovered

Abnormal signaling at corticotropin-releasing factor CRF1 and CRF2 receptors might contribute to the pathophysiology of stress-related disorders such as anxiety, depression and eating disorders, in addition to cardiac and inflammatory disorders. Recently, molecular characterization of CRF1 and CRF2 receptors and the cloning of novel ligands--urocortin, stresscopin-related peptide/urocortin II, and stresscopin/urocortin III--have revealed a far-reaching physiological importance for the family of CRF peptides. Although the physiological roles of the CRF2 receptor remain to be defined, the preclinical and clinical development of specific small-molecule antagonists of the CRF1 receptor opens new avenues for the treatment of psychiatric and neurological disorders.

Persistent corticotropin-releasing factor(1) receptor desensitization and downregulation in the human neuroblastoma cell line IMR-32

Brain corticotropin-releasing factor (CRF) systems integrate various responses to stress. Pathological responses to stress may result from errors in CRF receptor regulation in response to changes in synaptic CRF levels. To establish an in vitro model to study brain CRF receptors, we characterized the CRF-induced modulation of CRF(1) receptors in the human neuroblastoma cell line, IMR-32. Treatment with CRF decreased CRF(1) receptor binding and desensitized CRF-induced increases in cAMP. The decrease in binding had an EC(50) of approximately 10 nM, was maximal by 30 min, and was blocked by the CRF receptor antagonist [D-Phe(12), Nle(21,38), C(alpha)-MeLeu(37)]CRF(12-41). The desensitization was homologous as vasoactive intestinal polypeptide-induced increases in cAMP were unchanged, and elevation of cAMP did not alter CRF(1) receptor binding. Treatment with CRF for up to 24 h did not alter CRF(1) receptor mRNA levels, suggesting that a posttranscriptional mechanism maintains the decrease in receptor binding. Interestingly, recovery of CRF receptor binding and CRF-stimulated cAMP production was only partial following exposure to 100 nM CRF. In contrast, receptor binding recovered to control levels following exposure to 10 nM CRF. These data suggest that exposure to high doses of CRF result in permanent changes characterized by only partial recovery. Identifying the mechanisms underlying this partial recovery may provide insights into mechanisms underlying the acute and chronic effects of stress on CRF receptor regulation.