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

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.

Role of CRF and the hypothalamic-pituitary-adrenal axis in stroke: revisiting temporal considerations and targeting a new generation of therapeutics

Ischaemic neurovascular stroke represents a leading cause of death in the developed world. Preclinical and human epidemiological evidence implicates the corticotropin-releasing factor (CRF) family of neuropeptides as mediators of acute neurovascular injury pathology. Preclinical investigations of the role of CRF, CRF receptors and CRF-dependent activation of the hypothalamic-pituitary-adrenal (HPA) axis have pointed toward a tissue-specific and temporal relationship between activation of these pathways and physiological outcomes. Based on the literature, the major phases of ischaemic stroke aetiology may be separated into an acute phase in which CRF and anti-inflammatory stress signalling are beneficial and a chronic phase in which these contribute to neural degeneration, toxicity and apoptotic signalling. Significant gaps in knowledge remain regarding the pathway, temporality and systemic impact of CRF signalling and stress biology in neurovascular injury progression. Heterogeneity among experimental designs poses a challenge to defining the apparent reciprocal relationship between neurological injury and stress metabolism. Despite these challenges, it is our opinion that the elucidated temporality may be best matched with an antibody against CRF with a half-life of days to weeks as opposed to minutes to hours as with small-molecule CRF receptor antagonists. This state-of-the-art review will take a multipronged approach to explore the expected potential benefit of a CRF antibody by modulating CRF and corticotropin-releasing factor receptor 1 signalling, glucocorticoids and autonomic nervous system activity. Additionally, this review compares the modulation of CRF and HPA axis activity in neuropsychiatric diseases and their counterpart outcomes post-stroke and assess lessons learned from antibody therapies in neurodegenerative diseases.

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.

Corticotropin-releasing factor receptor signaling and modulation: implications for stress response and resilience

Introduction In addition to their role in regulation of the hypothalamic-pituitary-adrenal-axis, corticotropin-releasing factor (CRF) and its related peptides, the urocortins, are important mediators of physiological and pathophysiological processes of the central nervous, cardiovascular, gastrointestinal, immune, endocrine, reproductive, and skin systems. Altered regulation of CRF-mediated adaptive responses to various stressful stimuli disrupts healthy function and might confer vulnerability to several disorders, including depression and anxiety. Methodology This narrative review was conducted through search and analysis of studies retrieved from online databases using a snowball method. Results This review covers aspects beginning with the discovery of CRF, CRF binding protein and their actions via interaction with CRF receptors type 1 and type 2. These are surface plasma membrane receptors, activation of which is associated with conformational changes and interaction with a variety of G-proteins and signaling pathways. We also reviewed the pharmacology and mechanisms of the receptor signaling modulatory activity of these receptors. Conclusion This review compiles and presents knowledge regarding the CRFergic system, including CRF related peptides, CRF binding protein, and CRF receptors, as well as some evidence that is potentially indicative of the biological roles of these entities in several physiological and pathophysiological processes.

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.

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.

β-arrestin signalling and bias in hormone-responsive GPCRs

G protein-coupled receptors (GPCRs) play crucial roles in the ability of target organs to respond to hormonal cues. GPCRs' activation mechanisms have long been considered as a two-state process connecting the agonist-bound receptor to heterotrimeric G proteins. This view is now challenged as mounting evidence point to GPCRs being connected to large arrays of transduction mechanisms involving heterotrimeric G proteins as well as other players. Amongst the G protein-independent transduction mechanisms, those elicited by β-arrestins upon their recruitment to the active receptors are by far the best characterized and apply to most GPCRs. These concepts, in conjunction with remarkable advances made in the field of GPCR structural biology and biophysics, have supported the notion of ligand-selective signalling also known as pharmacological bias. Interestingly, recent reports have opened intriguing prospects to the way β-arrestins control GPCR-mediated signalling in space and time within the cells. In the present paper, we review the existing evidence linking endocrine-related GPCRs to β-arrestin recruitement, signalling, pathophysiological implications and selective activation by biased ligands and/or receptor modifications. Emerging concepts surrounding β-arrestin-mediated transduction are discussed in the light of the peculiarities of endocrine systems.

Novel roles for β-arrestins in the regulation of pharmacological sequestration to predict agonist-induced desensitization of dopamine D3 receptors

BACKGROUND AND PURPOSE

In addition to typical GPCR kinase (GRK)-/β-arrestin-dependent internalization, dopamine D3 receptor employed an additional GRK-independent sequestration pathway. In this study, we investigated the molecular mechanism of this novel sequestration pathway.

EXPERIMENTAL APPROACH

Radioligand binding, flow cytometry and cell surface biotinylation assay were used to characterize trafficking properties of D2 and D3 receptors. Serine/threonine and N-linked glycosylation mutants of the D3 receptor were utilized to locate receptor regions involved in pharmacological sequestration and desensitization. Various point mutants of the D2 and D3 receptors, whose sequestration and desensitization properties were altered, were combined with knockdown cells of GRKs or β-arrestins to functionally correlate pharmacological sequestration and desensitization.

KEY RESULTS

The D3 receptor, but not the D2 receptor, showed characteristic trafficking behaviour in which receptors were shifted towards the more hydrophobic domains within the plasma membrane without translocation into other intracellular compartments. Among various amino acid residues tested, S145/S146, C147 and N12/19 were involved in pharmacological sequestration and receptor desensitization. Both pharmacological sequestration and desensitization of D3 receptor required β-arrestins, and functional relationship was observed between two processes when it was tested for D3 receptor variants and agonists.

CONCLUSIONS AND IMPLICATIONS

Pharmacological sequestration of D3 receptor accompanies movement of cell surface receptors into a more hydrophobic fraction within the plasma membrane and renders D3 receptor inaccessible to hydrophilic ligands. Pharmacological sequestration is correlated with desensitization of the D3 receptor in a Gβγ- and β-arrestin-dependent manner. This study provides new insights into molecular mechanism governing GPCR trafficking and desensitization.

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.

Arrestin-dependent but G-protein coupled receptor kinase-independent uncoupling of D2-dopamine receptors

We reconstituted D2 like dopamine receptor (D2R) and the delta opioid receptor (DOR) coupling to G-protein gated inwardly rectifying potassium channels (K(ir)3) and directly compared the effects of co-expression of G-protein coupled receptor kinase (GRK) and arrestin on agonist-dependent desensitization of the receptor response. We found, as described previously, that co-expression of a GRK and an arrestin synergistically increased the rate of agonist-dependent desensitization of DOR. In contrast, only arrestin expression was required to produce desensitization of D2R responses. Furthermore, arrestin-dependent GRK-independent desensitization of D2R-K(ir)3 coupling could be transferred to DOR by substituting the third cytoplasmic loop of DOR with that of D2R. The arrestin-dependent GRK-independent desensitization of D2R desensitization was inhibited by staurosporine treatment, and blocked by alanine substitution of putative protein kinase C phosphorylation sites in the third cytoplasmic loop of D2R. Finally, the D2R construct in which putative protein kinase C phosphorylation sites were mutated did not undergo significant agonist-dependent desensitization even after GRK co-expression, suggesting that GRK phosphorylation of D2R does not play an important role in uncoupling of the receptor.

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

The corticotropin-releasing factor (CRF) receptor 1 (CRFR1) is a target for the treatment of psychiatric diseases such as depression, schizophrenia, anxiety disorder, and bipolar disorder. The carboxyl-terminal tail of the CRFR1 terminates in a PDZ-binding motif that provides a potential site for the interaction of PSD-95/Discs Large/Zona Occludens 1 (PDZ) domain-containing proteins. In this study, we found that CRFR1 interacts with synapse-associated protein 97 (SAP97; also known as DLG1) by co-immunoprecipitation in human embryonic 293 (HEK 293) cells and cortical brain lysates and that this interaction is dependent upon an intact PDZ-binding motif at the end of the CRFR1 carboxyl-terminal tail. Similarly, we demonstrated that SAP97 is recruited to the plasma membrane in HEK 293 cells expressing CRFR1 and that mutation of the CRFR1 PDZ-binding motif results in the redistribution of SAP97 into the cytoplasm. Overexpression of SAP97 antagonized agonist-stimulated CRFR1 internalization, whereas single hairpin (shRNA) knockdown of endogenous SAP97 in HEK 293 cells resulted in increased agonist-stimulated CRFR1 endocytosis. CRFR1 was internalized as a complex with SAP97 resulting in the redistribution of SAP97 to endocytic vesicles. Overexpression or shRNA knockdown of SAP97 did not significantly affect CRFR1-mediated cAMP formation, but SAP97 knockdown did significantly attenuate CRFR1-stimulated ERK1/2 phosphorylation in a PDZ interaction-independent manner. Taken together, our studies show that SAP97 interactions with CRFR1 attenuate CRFR1 endocytosis and that SAP97 is involved in coupling G protein-coupled receptors to the activation of the ERK1/2 signaling pathway.

Increased vulnerability of the brain norepinephrine system of females to corticotropin-releasing factor overexpression

Stress-related psychiatric disorders are more prevalent in women than men. As hypersecretion of the stress neuromediator, corticotropin-releasing factor (CRF) has been implicated in these disorders, sex differences in CRF sensitivity could underlie this disparity. Hyperarousal is a core symptom that is shared by stress-related disorders and this has been attributed to CRF regulation of the locus ceruleus (LC)-norepinephrine arousal system. We recently identified sex differences in CRF(1) receptor (CRF(1)) signaling and trafficking that render LC neurons of female rats more sensitive to CRF and potentially less able to adapt to excess CRF compared with male rats. The present study used a genetic model of CRF overexpression to test the hypothesis that females would be more vulnerable to LC dysregulation by conditions of excess CRF. In both male and female CRF overexpressing (CRF-OE) mice, the LC was more densely innervated by CRF compared with wild-type controls. Despite the equally dense CRF innervation of the LC in male and female CRF-OE mice, LC discharge rates recorded in slices in vitro were selectively elevated in female CRF-OE mice. Immunoelectron microscopy revealed that this sex difference resulted from differential CRF(1) trafficking. In male CRF-OE mice, CRF(1) immunolabeling was prominent in the cytoplasm of LC neurons, indicative of internalization, a process that would protect cells from excessive CRF. However, in female CRF-OE mice, CRF(1) labeling was more prominent on the plasma membrane, suggesting that the compensatory response of internalization was compromised. Together, the findings suggest that the LC-norepinephrine system of females will be particularly affected by conditions resulting in elevated CRF because of differences in receptor trafficking. As excessive LC activation has been implicated in the arousal components of stress-related psychiatric disorders, this may be a cellular mechanism that contributes to the increased incidence of these disorders in females.

Insights into mechanisms of corticotropin-releasing hormone receptor signal transduction

During evolution, mammals have developed remarkably similar molecular mechanisms to respond to external challenges and maintain survival. Critical regulators of these mechanisms are the family of 'stress'-peptides that consists of the corticotropin-releasing hormone (CRH) and urocortins (Ucns). These neuropeptides 'fine-tune' integration of an intricate series of physiological responses involving the autonomic, endocrine, immune, cardiovascular and reproductive systems, which induce a spectrum of behavioural and homeostatic changes. CRH and Ucns exert their actions by activating two types of CRH receptors (CRH-R), CRH-R1 and CRH-R2, which belong to the class-B1 family of GPCRs. The CRH-Rs exhibit signalling promiscuity facilitated by their ability to couple to multiple G-proteins and regulate diverse intracellular networks that involve intracellular effectors such as cAMP and an array of PKs in an agonist and tissue-specific manner, a property that allows them to exert unique roles in the integration of homeostatic mechanisms. We only now begin to unravel the plethora of CRH-R biological actions and the transcriptional and post-translational mechanisms such as alternative mRNA splicing or phosphorylation-mediated desensitization developed to tightly control CRH-Rs biological activity and regulate their physiological actions. This review summarizes the current understanding of CRH-R signalling complexity and regulatory mechanisms that underpin cellular responses to CRH and Ucns.

Endothelin-converting enzyme-1 actions determine differential trafficking and signaling of corticotropin-releasing factor receptor 1 at high agonist concentrations

CRF receptor 1 (CRF(1)), a key neuroendocrine mediator of the stress response, has two known agonists corticotropin-releasing factor (CRF) and urocortin 1 (Ucn1). Here we report that endothelin-converting enzyme-1 (ECE-1) differentially degrades CRF and Ucn1; ECE-1 cleaves Ucn1, but not CRF, at critical residue Arginine-34/35', which is essential for ligand-receptor binding. At near K(D) agonist concentration (30 nm), both Ucn1- and CRF-mediated Ca(2+) mobilization are ECE-1 dependent. Interestingly, at high agonist concentration (100 nm), Ucn1-mediated Ca(2+) mobilization remains ECE-1 dependent, whereas CRF-mediated mobilization becomes independent of ECE-1 activity. At high agonist concentration, ECE-1 inhibition disrupted Ucn1-, but not CRF-induced CRF(1) recycling and resensitization, but did not prolong the association of CRF(1) with β-arrestins. RNA interference-mediated knockdown of Rab suggests that both Ucn1- and CRF-induced CRF(1) resensitization is dependent on activity of Rab11, but not of Rab4. CRF(1) behaves like a class A G protein-coupled receptor with respect to transient β-arrestins interaction. We propose that differential degradation by ECE-1 is a novel mechanism by which CRF(1) receptor is protected from overactivation by physiologically relevant high concentrations of higher affinity ligand to mediate distinct resensitization and downstream signaling.

Prolonged and site-specific over-expression of corticotropin-releasing factor reveals differential roles for extended amygdala nuclei in emotional regulation

Corticotropin-releasing factor (CRF) has a key role in the central stress response, and altered levels of this neuropeptide are linked to stress-related psychopathologies such as anxiety and depression. These disorders are associated with the inability to properly regulate stress response, specifically following exposure to prolonged stressful stimuli. Therefore, the current study assessed the effects of prolonged and site-specific over-expression of CRF, which mimics the state of chronic production, in extended amygdala nuclei that are known to be involved in mediating anxiety-like states. We first constructed and generated lentiviruses that overexpress (OE) CRF in a robust and stable manner, and then generated two male mouse models continuously over-expressing CRF, either at the central nucleus of the amygdala (CeA), or at the dorsolateral subdivision of the bed nucleus of the stria terminalis (BNSTdl). After 4 months, behavioral assessments were conducted for anxiety and depressive indices on these mice. Surprisingly, prolonged CRF OE at the CeA attenuated stress-induced anxiety-like behaviors, whereas prolonged CRF OE in the BNSTdl increased depressive-like behaviors, without affecting anxiety levels. These results show possible differential roles for CRF expressed by distinct loci of the extended amygdala, in mediating stress-induced emotional behaviors.

Temporal profiling of orexin receptor-arrestin-ubiquitin complexes reveals differences between receptor subtypes

Orexin G protein-coupled receptors (OxRs) and their cognate agonists have been implicated in a number of disorders since their recent discovery, ranging from narcolepsy to formation of addictive behavior. Bioluminescence resonance energy transfer assays of agonist-occupied OxRs provided evidence for a strong dose-dependent interaction with both trafficking proteins β-arrestin 1 and 2 that required unusually high agonist concentrations compared with inositol phosphate signaling. This appears to be reflected in functional differences in potency with respect to orexin A (OxA) and OxR2-dependent ERK1/2 phosphorylation after 90 min compared with 2 min, potentially consistent with β-arrestin-mediated versus G protein-mediated signaling, respectively. Furthermore, extended bioluminescence resonance energy transfer kinetic data monitoring OxA-dependent receptor-β-arrestin and β-arrestin-ubiquitin proximity suggested subtype-specific differences in receptor trafficking, with OxR2 activation resulting in more sustained receptor-β-arrestin-ubiquitin complex formation than elicited by OxR1 activation. Enzyme-linked immunosorbent assay (ELISA) data also revealed that OxR1 underwent significantly more rapid recycling compared with OxR2. Finally, we have observed sustained OxA-dependent ERK1/2 phosphorylation in the presence of OxR2 compared with OxR1. Although both OxR subtypes could be classified as class B receptors for β-arrestin usage based on the initial strength of interaction with both β-arrestins, our temporal profiling revealed tangible differences between OxR subtypes. Consequently, OxR1 appears to fit uneasily into the commonly used β-arrestin classification scheme. More importantly, it is hoped that this improved profiling capability, enabling the subtleties of protein complex formation, stability, and duration to be assessed in live cells, will help unlock the therapeutic potential of targeting these receptors.

Agonist-induced endocytosis and receptor phosphorylation mediate resensitization of dopamine D(2) receptors

The regulatory mechanisms and functional roles of agonist-induced internalization of G protein-coupled receptors (GPCRs) were analyzed using mutant dopamine D(2) receptors (D(2)Rs) in which all possible GPCR kinase (GRK) phosphorylation sites were mutated or the affinity for beta-arrestins was altered. Agonist-induced internalization of D(2)Rs involved a phosphorylation-dependent component, which was mediated by serine/threonine (S/T) residues in the second loop and T225 in the third loop, and a phosphorylation-independent component. GRK2-mediated enhancement of the internalization and inhibition of D(2)R signaling did not involve receptor phosphorylation, and only the former required the enzymatic activity of GRK2. The phosphorylation-deficient mutant (D(2)R-intracellular loop 2/3) recycled more slowly and showed more agonist-induced desensitization than did the wild-type D(2)R, suggesting that receptor phosphorylation mediates the recycling of the internalized receptors and enhances receptor resensitization. Blockade of the agonist-induced internalization of D(2)R-intracellular loop 2/3 provoked desensitization as in wild-type D(2)R, suggesting that certain cellular processes other than receptor dephosphorylation occurring within the endocytic vesicle are responsible for the resensitization of D(2)R. When dissociation between D(2)R and beta-arrestin was inhibited or when the expression of cellular beta-arrestins was decreased, agonist-induced desensitization of D(2)R did not occur, suggesting that dissociation from beta-arrestin is the main cellular process required for resensitization of D(2)R and is achieved through agonist-induced internalization. These results indicate that, in the regulation of some GPCRs, phosphorylation-independent association with beta-arrestin plays a major role in agonist-induced desensitization.

Brainstem reticulospinal neurons are targets for corticotropin-releasing factor-Induced locomotion in roughskin newts

Stress-induced release or central administration of corticotropin-releasing factor (CRF) enhances locomotion in a wide range of vertebrates, including the roughskin newt, Taricha granulosa. Although CRF's stimulatory actions on locomotor behavior are well established, the target neurons through which CRF exerts this effect remain unknown. To identify these target neurons, we utilized a fluorescent conjugate of CRF (CRF-TAMRA 1) to track this peptide's internalization into reticulospinal and other neurons in the medullary reticular formation (MRF), a region critically involved in regulating locomotion. Epifluorescent and confocal microscopy revealed that CRF-TAMRA 1 was internalized by diverse MRF neurons, including reticulospinal neurons retrogradely labeled with Cascade Blue dextran. In addition, we immunohistochemically identified a distinct subset of serotonin-containing neurons, located throughout the medullary raphé, that also internalized the fluorescent CRF-TAMRA 1 conjugate. Chronic single-unit recordings obtained from microwire electrodes in behaving newts revealed that intracerebroventricular (icv) administration of CRF-TAMRA 1 increased medullary neuronal firing and that appearance of this firing was associated with, and strongly predictive of, episodes of CRF-induced locomotion. Furthermore, icv administered CRF-TAMRA 1 produced behavioral and neurophysiological effects identical to equimolar doses of unlabeled CRF. Collectively, these findings provide the first evidence that CRF directly targets reticulospinal and serotonergic neurons in the MRF and indicate that CRF may enhance locomotion via direct effects on the hindbrain, including the reticulospinal system.

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.

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.

Stress-induced intracellular trafficking of corticotropin-releasing factor receptors in rat locus coeruleus neurons

Corticotropin-releasing factor (CRF) activates locus coeruleus (LC)-norepinephrine neurons during stress. Previous stress or CRF administration attenuates the magnitude of this response by decreasing postsynaptic sensitivity to CRF. Here we describe the fate of CRF receptors (CRFr) in LC neurons after stress. Rats were exposed to swim stress or handling and perfused 1 or 24 h later. Sections through the LC were processed for immunogold-silver labeling of CRFr. CRFr in LC dendrites was present on the plasma membrane and within the cytoplasm. In control rats, the ratio of cytoplasmic to total dendritic labeling was 0.55 +/- 0.01. Swim stress increased this ratio to 0.77 +/- 0.01 and 0.80 +/- 0.02 at 1 and 24 h after stress, respectively. Internalized CRFr was associated with different organelles at different times after stress. At 1 h after stress, CRFr was often associated with early endosomes in dendrites and perikarya. By 24 h, more CRFr was associated with multivesicular bodies, suggesting that some of the internalized receptor is targeted for degradation. In perikarya, more internalized CRFr was associated with Golgi apparatus 24 vs. 1 h after stress. This is suggestive of changes in CRFr synthesis. Alternatively, this may indicate communication between multivesicular bodies and Golgi apparatus in the process of recycling. Administration of the selective CRF(1) antagonist, antalarmin, before swim stress attenuated CRFr internalization. The present demonstration of stress-induced internalization of CRFr in LC neurons provides evidence that CRF is released in the LC during swim stress to activate this system and initiate cellular trafficking of the receptor that determines subsequent sensitivity of LC neurons to CRF.

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.

Agonist-induced internalization of corticotropin-releasing factor receptors in noradrenergic neurons of the rat locus coeruleus

Corticotropin-releasing factor (CRF) acts within the locus coeruleus (LC), to modulate activity of the LC-norepinephrine (NE) system. Combining molecular and cellular approaches, we demonstrate CRF receptor (CRFr) mRNA expression in Sprague-Dawley rat LC and provide the first in vivo evidence for agonist-induced internalization of CRFr. CRFr mRNA was detected in LC micropunches by RT-PCR. In dual labelling immunofluorescence studies, tyrosine hydroxylase (TH) containing neurons exhibited CRFr labelling. At the ultrastructural level, immunogold-silver labelling for CRFr was localized to the plasma membrane of TH-immunoperoxidase labelled dendrites. CRF (100 ng) injection into the LC produced a robust neuronal activation that peaked 10-15 min after injection and was maintained for the duration of the recording. This was associated with CRFr internalization in LC neurons that was apparent at 5 and 30 min after injection. By 5 min after injection the ratio of cytoplasmic to total dendritic CRFr-labelling was 0.81 +/- 0.01 in rats injected with CRF and 0.59 +/- 0.02 in rats injected with artificial cerebrospinal fluid (ACSF; P < 0.0001). Enhanced internalization of CRFr was maintained at 30 min after CRF injection, with the ratio being 0.86 +/- 0.02 for CRF-injected cases and 0.57 +/- 0.03 for ACSF-injected cases (P < 0.0001). Internalized CRFr was associated with early endosomes, indicative of degradation or recycling. Agonist-induced CRFr internalization in LC neurons may underlie acute desensitization to CRF or stress. This process may be a pivotal target by which stressors or pharmacological agents regulate the sensitivity of the LC-NE system to CRF and subsequent stressors.

Differential responses of corticotropin-releasing hormone receptor type 1 variants to protein kinase C phosphorylation

Corticotropin-releasing hormone (CRH) regulates diverse biological functions in mammals, through activation of two types of specific G protein-coupled receptors that are expressed as multiple mRNA spliced variants. In most cells, the type 1alpha CRH receptor (CRH-R1alpha) preferentially activates the G(s)-adenylyl cyclase signaling cascade. CRH-R1alpha-mediated signaling activity is impaired by insertion of 29 amino acids in the first intracellular loop, a sequence modification that is characteristic of the human-specific CRH-R1beta variant. In various tissues, CRH signaling events are regulated by protein kinase C (PKC). The CRH receptors contain multiple putative PKC phosphorylation sites that represent potential targets. To investigate this, we expressed recombinant CRH-R1alpha or CRH-R1beta in human embryonic kidney 293 cells and analyzed signaling events after PKC activation. Agonist (oxytocin) or phorbol 12-myristate 13-acetate-induced activation of PKC led to phosphorylation of both CRH-R1 variants. However, CRH-R1alpha and CRH-R1beta exhibited different functional responses to PKC-induced phosphorylation, with only the CRH-R1beta susceptible to cAMP signaling desensitization. This was associated with a significant decrease of accessible CRH-R1beta receptors expressed on the cell surface. Both CRH-R1 variants were susceptible to homologous desensitization and internalization following treatment with CRH; however, PKC activation increased internalization of CRH-R1beta but not CRH-R1alpha in a beta-arrestin-independent manner. Our findings indicate that CRH-R1alpha and -R1beta exhibit differential responses to PKC-induced phosphorylation, and this might represent an important mechanism for functional regulation of CRH signaling in target cells.

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.

The molecular mechanisms underlying the regulation of the biological activity of corticotropin-releasing hormone receptors: implications for physiology and pathophysiology

The CRH receptor (CRH-R) is a member of the secretin family of G protein-coupled receptors. Wide expression of CRH-Rs in the central nervous system and periphery ensures that their cognate agonists, the family of CRH-like peptides, are capable of exerting a wide spectrum of actions that underpin their critical role in integrating the stress response and coordinating the activity of fundamental physiological functions, such as the regulation of the cardiovascular system, energy balance, and homeostasis. Two types of mammal CRH-R exist, CRH-R1 and CRH-R2, each with unique splicing patterns and remarkably distinct pharmacological properties, but similar signaling properties, probably reflecting their distinct and sometimes contrasting biological functions. The regulation of CRH-R expression and activity is not fully elucidated, and we only now begin to fully understand the impact on mammalian pathophysiology. The focus of this review is the current and evolving understanding of the molecular mechanisms controlling CRH-R biological activity and functional flexibility. This shows notable tissue-specific characteristics, highlighted by their ability to couple to distinct G proteins and activate tissue-specific signaling cascades. The type of activating agonist, receptor, and target cell appears to play a major role in determining the overall signaling and biological responses in health and disease.

Differential regulation of corticotropin releasing factor 1alpha receptor endocytosis and trafficking by beta-arrestins and Rab GTPases

The corticotropin releasing factor (CRF) type 1alpha receptor, a member of the G protein-coupled receptor (GPCR) subfamily B, is involved in the aetiology of anxiety and depressive disorders. In the present study, we examined the internalization and trafficking of the CRF1alpha receptor in both human embryonic kidney (HEK)293 cells and primary cortical neurons. We found that CRF1alpha receptor activation leads to the selective recruitment of beta-arrestin2 in both HEK293 cells and neurons. We observed distinct distribution patterns of CRF1alpha receptor and beta-arrestin2 in HEK293 cells and cortical neurons. In HEK293 cells, beta-arrestin2-green fluorescent protein (GFP) co-localized with CRF1alpha receptor in vesicles at the plasma membrane but was dissociated from the receptor in endosomes. In contrast, in primary cortical neurons, beta-arrestin2 and CRF1alpha receptor were internalized in distinct endocytic vesicles. By bioluminescence resonance energy transfer, we demonstrated that beta-arrestin2 association with CRF1alpha receptor was increased in cells transfected with G protein-coupled receptor kinase (GRK)3 and GRK6 and decreased in cells transfected with GRK2 and GRK5. In both HEK293 cells and cortical neurons, internalized CRF1alpha receptor transited from Rab5-positive early endosomes to Rab4-positive recycling endosomes and was not targeted to lysosomes. However, CRF1alpha receptor resensitization was blocked by the overexpression of wild-type, but not dominant-negative, Rab5 and Rab4 GTPases. Taken together, our results suggest that beta-arrestin trafficking differs between HEK293 cells and neurons, and that CRF1alpha receptor resensitization is regulated in an atypical manner by Rab GTPases.

The glucagon-like peptide-2 receptor C terminus modulates beta-arrestin-2 association but is dispensable for ligand-induced desensitization, endocytosis, and G-protein-dependent effector activation

Classic models of receptor desensitization and internalization have been largely based on the behavior of Family A G-protein-coupled receptors (GPCRs). The glucagon-like peptide-2 receptor (GLP-2R) is a member of the Family B glucagon-secretin GPCR family, which exhibit significant sequence and structural differences from the Family A receptors in their intracellular and extracellular domains. To identify structural motifs that regulate GLP-2R signaling and cell surface receptor expression, we analyzed the functional properties of a series of mutant GLP-2Rs. The majority of the C-terminal receptor tail was dispensable for GLP-2-induced cAMP accumulation, ERK1/2 activation, and endocytosis in transfected cells. However, progressive truncation of the C terminus reduced cell surface receptor expression, altered agonist-induced GLP-2R trafficking, and abrogated protein kinase A-mediated heterologous receptor desensitization. Elimination of the distal 21 amino acids of the receptor was sufficient to promote constitutive receptor internalization and prevent agonist-induced recruitment of beta-arrestin-2. Site-directed mutagenesis identified specific amino acid residues within the distal GLP-2R C terminus that mediate the stable association with beta-arrestin-2. Surprisingly, although the truncated mutant receptors failed to interact with beta-arrestin-2, they underwent homologous desensitization and subsequent resensitization with kinetics similar to that observed with the wild-type GLP-2R. Our data suggest that, although the GLP-2R C terminus is not required for coupling to cellular machinery regulating signaling or desensitization, it may serve as a sorting signal for intracellular trafficking. Taken together with the previously demonstrated clathrin and dynamin-independent, lipid-raft-dependent pathways for internalization, our data suggest that GLP-2 receptor signaling has evolved unique structural and functional mechanisms for control of receptor trafficking, desensitization, and resensitization.