Transmembrane domain IV of the Gallus gallus VT2 vasotocin receptor is essential for forming a heterodimer with the corticotrophin releasing hormone receptor

Heat Stress Responses in Birds: A Review of the Neural Components

Heat stress is one of the major environmental conditions causing significant losses in the poultry industry and having negative impacts on the world's food economy. Heat exposure causes several physiological impairments in birds, including oxidative stress, weight loss, immunosuppression, and dysregulated metabolism. Collectively, these lead not only to decreased production in the meat industry, but also decreases in the number of eggs laid by 20%, and overall loss due to mortality during housing and transit. Mitigation techniques have been discussed in depth, and include changes in air flow and dietary composition, improved building insulation, use of air cooling in livestock buildings (fogging systems, evaporation panels), and genetic alterations. Most commonly observed during heat exposure are reduced food intake and an increase in the stress response. However, very little has been explored regarding heat exposure, food intake and stress, and how the neural circuitry responsible for sensing temperatures mediate these responses. That thermoregulation, food intake, and the stress response are primarily mediated by the hypothalamus make it reasonable to assume that it is the central hub at which these systems interact and coordinately regulate downstream changes in metabolism. Thus, this review discusses the neural circuitry in birds associated with thermoregulation, food intake, and stress response at the level of the hypothalamus, with a focus on how these systems might interact in the presence of heat exposure.

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.

Receptor⁻Receptor Interactions in Multiple 5-HT1A Heteroreceptor Complexes in Raphe-Hippocampal 5-HT Transmission and Their Relevance for Depression and Its Treatment

Due to the binding to a number of proteins to the receptor protomers in receptor heteromers in the brain, the term "heteroreceptor complexes" was introduced. A number of serotonin 5-HT1A heteroreceptor complexes were recently found to be linked to the ascending 5-HT pathways known to have a significant role in depression. The 5-HT1A⁻FGFR1 heteroreceptor complexes were involved in synergistically enhancing neuroplasticity in the hippocampus and in the dorsal raphe 5-HT nerve cells. The 5-HT1A protomer significantly increased FGFR1 protomer signaling in wild-type rats. Disturbances in the 5-HT1A⁻FGFR1 heteroreceptor complexes in the raphe-hippocampal 5-HT system were found in a genetic rat model of depression (Flinders sensitive line (FSL) rats). Deficits in FSL rats were observed in the ability of combined FGFR1 and 5-HT1A agonist cotreatment to produce antidepressant-like effects. It may in part reflect a failure of FGFR1 treatment to uncouple the 5-HT1A postjunctional receptors and autoreceptors from the hippocampal and dorsal raphe GIRK channels, respectively. This may result in maintained inhibition of hippocampal pyramidal nerve cell and dorsal raphe 5-HT nerve cell firing. Also, 5-HT1A⁻5-HT2A isoreceptor complexes were recently demonstrated to exist in the hippocampus and limbic cortex. They may play a role in depression through an ability of 5-HT2A protomer signaling to inhibit the 5-HT1A protomer recognition and signaling. Finally, galanin (1⁻15) was reported to enhance the antidepressant effects of fluoxetine through the putative formation of GalR1⁻GalR2⁻5-HT1A heteroreceptor complexes. Taken together, these novel 5-HT1A receptor complexes offer new targets for treatment of depression.

Neuroanatomical distribution and function of the vasopressin V1B receptor in the rat brain deciphered using specific fluorescent ligands

It is now accepted that vasopressin, through V_1A/V_1B receptors, centrally regulates cognitive functions such as memory, affiliation, stress, fear and depression. However, the respective roles of these receptor isoforms and their contribution to stress-related pathologies remain uncertain. The development of new therapeutic treatments requires a precise knowledge of the distribution of these receptors within the brain, which has been so far hampered by the lack of selective V_1B markers. In the present study, we have determined the pharmacological properties of three new potent rat V_1B fluorescent ligands and demonstrated that they constitute valuable tools for simultaneous visualization and activation of native V_1B receptors in living rat brain tissue. Thus, d[Leu⁴,Lys-Alexa 647)⁸]VP (analogue 3), the compound with the best affinity-selectivity/fluorescence ratio for the V_1B receptor emerged as the most promising. The rat brain regions most concerned by stress such as hippocampus, olfactory bulbs, cortex and amygdala display the highest V_1B fluorescent labelling with analogue 3. In the hippocampus CA₂, V_1B receptors are located on glutamatergic, not GABAergic neurones, and are absent from astrocytes. Using AVP-EGFP rats, we demonstrate the presence of V_1B autoreceptors on AVP-secreting neurones not only in the hypothalamus, but also sparsely in the hippocampus. Finally, using both electrophysiology and visualization of ERK phosphorylation, we show analogue 3-induced activation of the V_1B receptor in situ. This will help to analyse expression and functionality of V_1B receptors in the brain and contribute to further explore the AVPergic circuitry in normal and pathological conditions.

Regulation of gene expression of vasotocin and corticotropin-releasing hormone receptors in the avian anterior pituitary by corticosterone

The effect of chronic stress (CS) on gene expression of the chicken arginine vasotocin (AVT) and corticotropin-releasing hormone (CRH) receptors [VT2R, VT4R, CRH-R1, and CRH-R2] was examined by measuring receptor mRNA levels in the anterior pituitary gland of the chicken after chronic immobilization stress compared to acute stress (AS). Radioimmunoassay results showed that blood circulating corticosterone (CORT) levels in the CS group were significantly decreased compared to that of birds in the AS group (P<0.05). The VT2R and CRH-R2 mRNA in CS birds were significantly decreased to that of controls. The VT4R mRNA was significantly decreased compared to controls in AC birds and was further decreased in the CS group compared to controls (P<0.05). The CRH-R1 mRNA was significantly decreased in the AS birds compared to controls. However, there was no significant difference of CRH-R1 mRNA between acute stress and chronic stress birds. Using primary anterior pituitary cell cultures, the effect of exogenous CORT on VT/CRH receptor gene expression was examined. Receptor mRNA levels were measured after treatment of CORT followed by AVT/CRH administration. The CORT pretreatment resulted in a dose-dependent decrease of proopiomelanocortin heteronuclear RNA, a molecular marker of a stress-induced anterior pituitary. Without CORT pretreatment of anterior pituitary cell cultures, the VT2R, VT4R and CRH-R1mRNA levels were significantly increased within 15 min and then decreased at 1 h and 6 h by AVT/CRH administration (P<0.05). Pretreatment of CORT in anterior pituitary cells induced a dose-dependent increase of VT2R, VT4R and CRH-R2 mRNA levels, and a significant decrease of CRH-R1 mRNA levels at only the high dose (10 ng/ml) of CORT (P<0.05).Taken together, results suggest a modulatory role of CORT on the regulation of VT/CRH receptor gene expression in the avian anterior pituitary gland dependent upon CORT levels.

A possible mechanism contributing to the synergistic action of vasotocin (VT) and corticotropin-releasing hormone (CRH) receptors on corticosterone release in birds

Arginine vasotocin (AVT) and corticotropin-releasing hormone (CRH) are two neuronal regulators in the hypothalamic-pituitary-adrenal (HPA) axis that modulate biological responses to stress in avian species. When AVT and CRH are administered together in vitro or in vivo, levels of adrenocorticotropic hormone (ACTH) or plasma corticosterone (CORT) are released, respectively, in a synergistic manner. The underlying mechanism of this greater than additive stress response was investigated by expressing the vasotocin receptor type 2 (VT2R) and CRH receptor type 1 (CRH-R1), both G-protein coupled receptors, in HeLa cells. Fluorescence resonance energy transfer (FRET) analysis provided the evidence for heterodimerization of the VT2R/CRH-R1 in the presence of their respective ligands, AVT and CRH. The VT2R and CRH-R1 were tagged at the C-terminal ends with either cyan fluorescent protein (CFP) or yellow fluorescent protein (YFP), and a VT2R chimera was constructed by replacing the fourth transmembrane region (TM4) of the VT2R with TM-IV of the β2-adrenergic receptor (β2AR). When VT2R/β2AR chimera and CRH-R1 were expressed in HeLa cells, heterodimerization was partly disrupted. Taken together, these data indicate that TM-IV of the VT2R may provide an important interface for effective receptor dimerization, suggesting that direct molecular interaction between VT2R and CRH-R1 receptors plays a role in mediating an enhanced interaction between these two receptors. Their interaction at the anterior pituitary level may potentiate the endocrine output of the avian HPA system.

V1b and CRHR1 receptor heterodimerization mediates synergistic biological actions of vasopressin and CRH

Vasopressin (AVP) and CRH synergistically regulate adrenocorticotropin and insulin release at the level of the pituitary and pancreas, respectively. Here, we first extended these AVP and CRH coregulation processes to the adrenal medulla. We demonstrate that costimulation of chromaffin cells by AVP and CRH simultaneously induces a catecholamine secretion exceeding the one induced by each hormone alone, thus demonstrating a net potentiation. To further elucidate the molecular mechanisms underlying this synergism, we coexpressed human V1b and CRH receptor (CRHR)1 receptor in HEK293 cells. In this heterologous system, AVP also potentiated CRH-stimulated cAMP accumulation in a dose-dependent and saturable manner. This effect was only partially mimicked by phorbol ester or inhibited by a phospholipase C inhibitor respectively. This finding suggests the existence of an new molecular mechanism, independent from second messenger cross talk. Similarly, CRH potentiated the AVP-induced inositol phosphates production. Using bioluminescence resonance energy transfer, coimmunoprecipitation, and receptor rescue experiments, we demonstrate that V1b and CRHR1 receptors assemble as heterodimers. Moreover, new pharmacological properties emerged upon receptors cotransfection. Taken together, these data strongly suggest that direct molecular interactions between V1b and CRHR1 receptors play an important role in mediating the synergistic interactions between these two receptors.

Heteromerization of G protein-coupled receptors: relevance to neurological disorders and neurotherapeutics

Because G protein-coupled receptors (GPCRs) are numerous, widely expressed and involved in major physiological responses, they represent a relevant therapeutic target for drug discovery, particularly regarding pharmacological treatments of neurological disorders. Among the biological phenomena regulating receptor function, GPCR heteromerization is an important emerging area of interest and investigation. There is increasing evidence showing that heteromerization contributes to the pharmacological heterogeneity of GPCRs by modulating receptor ontogeny, activation and recycling. Although in many cases the physiological relevance of receptor heteromerization has not been fully established, the unique pharmacological and functional properties of heteromers are likely to lead to new strategies in clinical medicine. This review describes the main GPCR heteromers and their implications for major neurological disorders such as Parkinson's disease, schizophrenia and addiction. A better understanding of molecular mechanisms underlying drug interactions related to the targeting of receptor heteromers could provide more specific and efficient therapeutic agents for the treatment of brain diseases.

Making structural sense of dimerization interfaces of delta opioid receptor homodimers

Opioid receptors, like other members of the G protein-coupled receptor (GPCR) family, have been shown to associate to form dimers and/or oligomers at the plasma membrane. Whether this association is stable or transient is not known. Recent compelling evidence suggests that at least some GPCRs rapidly associate and dissociate. We have recently calculated binding affinities from free energy estimates to predict transient association between mouse delta opioid receptor (DOR) protomers at a symmetric interface involving the fourth transmembrane (TM4) helix (herein termed “4” dimer). Here we present disulfide cross-linking experiments with DOR constructs with cysteines substituted at the extracellular ends of TM4 or TM5 that confirm the formation of DOR complexes involving these helices. Our results are consistent with the involvement of TM4 and/or TM5 at the DOR homodimer interface, but possibly with differing association propensities. Coarse-grained (CG) well-tempered metadynamics simulations of two different dimeric arrangements of DOR involving TM4 alone or with TM5 (herein termed “4/5” dimer) in an explicit lipid−water environment confirmed the presence of two structurally and energetically similar configurations of the 4 dimer, as previously assessed by umbrella sampling calculations, and revealed a single energetic minimum of the 4/5 dimer. Additional CG umbrella sampling simulations of the 4/5 dimer indicated that the strength of association between DOR protomers varies depending on the protein region at the interface, with the 4 dimer being more stable than the 4/5 dimer.

G protein-coupled receptor oligomerization for what?

Although the G protein-coupled receptor (GPCR) oligomerization has been questioned during the last decade, under some premises the existence of a supramolecular organization of these receptors begins now to be widely accepted by the scientific community. Indeed, GPCR oligomers may enhance the diversity and performance by which extracellular signals are transferred to the G proteins in the process of receptor transduction, although the mechanism that underlie this phenomenon remains still unexplained. Recently, a trans-conformational switching model has been proposed as a mechanism allowing direct inhibition of receptor activation. Thus, heterotropic receptor-receptor allosteric regulations are behind the GPCR oligomeric function. Accordingly, we revise here how GPCR oligomerization impinge in several important receptor functions like biosynthesis, plasma membrane diffusion or velocity, pharmacology and signaling. Overall, the rationale of receptor oligomerization might lie in the cellular need of sensing complex extracellular signals and to translate into a simple computational mode.

Activation of Type 1 CRH receptor isoforms induces serotonin release from human carcinoid BON-1N cells: an enterochromaffin cell model

CRH and 5-hydroxytryptamine (5-HT) are expressed in human colonic enterochromaffin (EC) cells, but their interactions at the cellular level remain largely unknown. The mechanistic and functional relationship between CRH and 5-HT systems in EC cells was investigated in a human carcinoid cloned BON cell line (BON-1N), widely used as an in vitro model of EC cell function. First, we identified multiple CRH(1) splice variants, including CRH(1a), CRH(1c), CRH(1f), and a novel form lacking exon 4, designated here as CRH(1i), in the BON-1N cells. The expression of CRH(1i) was also confirmed in human brain cortex, pituitary gland, and ileum. Immunocytochemistry and immunoblot analysis confirmed that BON-1N cells were CRH(1) and 5-HT positive. CRH, urocortin (Ucn)-1, and cortagine, a selective CRH(1) agonist, all increased intracellular cAMP, and this concentration-dependent response was inhibited by CRH(1)-selective antagonist NBI-35965. CRH and Ucn-1, but not Ucn-2, stimulated significant ERK1/2 phosphorylation. In transfected human embryonic kidney-293 cells, CRH(1i) isoforms produced a significant increase in pERK1/2 in response to CRH(1) agonists that was sensitive to NBI-35965. CRH and Ucn-1 stimulated 5-HT release that reached a maximal increase of 3.3- and 4-fold at 10(-8) m over the basal level, respectively. In addition, exposure to CRH for 24-h up-regulated tryptophan hydroxylase-1 mRNA levels in the BON-1N cells. These findings define the expression of EC cell-specific CRH(1) isoforms and activation of CRH(1)-dependent pathways leading to 5-HT release and synthesis; thus, providing functional evidence of a link exists between CRH and 5-HT systems, which have implications in stress-induced CRH(1) and 5-HT-mediated stimulation of lower intestinal function.

Lessons from free energy simulations of delta-opioid receptor homodimers involving the fourth transmembrane helix

Several G protein-coupled receptors (GPCRs), including opioid receptors deltaOR, muOR, and kappaOR, have been reported to form stable dimers or oligomers in lipid bilayers and cell membranes. This notion has been recently challenged by imaging data supporting a transient nature of GPCR association. Here we use umbrella sampling reconstructed free energies of deltaOR homodimers involving the fourth transmembrane helix to predict their association constant. The results of these simulations, combined with estimates of diffusion-limited association rates, suggest a short lifetime for deltaOR homodimers in the membrane, in agreement with recent trends.

Increasingly accurate dynamic molecular models of G-protein coupled receptor oligomers: Panacea or Pandora's box for novel drug discovery?

For years, conventional drug design at G-protein coupled receptors (GPCRs) has mainly focused on the inhibition of a single receptor at a usually well-defined ligand-binding site. The recent discovery of more and more physiologically relevant GPCR dimers/oligomers suggests that selectively targeting these complexes or designing small molecules that inhibit receptor-receptor interactions might provide new opportunities for novel drug discovery. To uncover the fundamental mechanisms and dynamics governing GPCR dimerization/oligomerization, it is crucial to understand the dynamic process of receptor-receptor association, and to identify regions that are suitable for selective drug binding. This minireview highlights current progress in the development of increasingly accurate dynamic molecular models of GPCR oligomers based on structural, biochemical, and biophysical information that has recently appeared in the literature. In view of this new information, there has never been a more exciting time for computational research into GPCRs than at present. Information-driven modern molecular models of GPCR complexes are expected to efficiently guide the rational design of GPCR oligomer-specific drugs, possibly allowing researchers to reach for the high-hanging fruits in GPCR drug discovery, i.e. more potent and selective drugs for efficient therapeutic interventions.