The balance of concentration between Prokineticin 2β and Prokineticin 2 modulates the food intake by STAT3 signaling

Physical TRPV1-PKR2 interaction modulates PKR2 localization, activation and β-arrestin-2 recruitment

Transient Receptor Potential Vanilloid 1 (TRPV1) plays a central role in nociception and inflammation-induced hyperalgesia. Prokineticin receptors (PKR1 and PKR2) are G protein-coupled receptors (GPCRs) that are expressed both centrally and peripherally and induce various signalling pathways through the binding of their endogenous ligands, the prokineticins. The activation of PKRs is modulated by interaction with accessory proteins such as snapin and β-arrestin-2. The prokineticin system is involved in the mechanisms of pain perception by triggering a strong hyperalgesia that lowers the thresholds for thermal and mechanical stimuli. PKR1 and PKR2 are both expressed in dorsal root ganglia (DRG) neurons where they colocalise with TRPV1. In this work, we have identified the regions that mediate the physical interaction between TRPV1 and PKR2. We demonstrated that in the presence of the accessory protein snapin, the strength of binding between TRPV1 and PKR2 is increased. TRPV1 proves to be a physiological regulator of PKR2, modulating its activation, localisation and β-arrestin binding as well as PK2-induced mechanical allodynia. The results obtained show for the first time that TRPV1 regulates PKR2 signalling, suggesting a bidirectional interaction between the two systems.

Biochemical characterization of Prokineticin 2 binding to Prokineticin receptor 1 in zebrafish

Prokineticin 2 (PK2) binds to prokineticin receptor 1 and prokineticin receptor 2 (PKR1 and PKR2, respectively), two G protein-coupled receptors (GPCRs) that can mediate multiple signalling pathways by promoting the elevation of intracellular calcium and cAMP levels, phosphorylation of Akt and activation of ERK and STAT3. This work aims to evidence the conservation of protein sequence and the mechanism of PK2 binding to PKR1 to use the zebrafish model for the identification of new drugs as targets of prokineticin receptors. To this end, we first demonstrated that the zebrafish genes pk2 and pkr1 are phylogenetically related to orthologous mammalian genes by constructing evolutionary trees and performing syntenic analyses. Subsequently, by comparing the amino acid sequences, we showed that the interaction sites with PK2 are conserved in the zPKR1. Using GST pull-down and cross-linking experiments, we demonstrated the crucial role of the N-terminal region of zPKR1 for binding to the PK2. Finally, by expressing zPKR1 in CHO cells, we demonstrated the ability of zPKR1 to induce the activation of ERK and STAT3.

Genetic Polymorphisms of Prokineticins and Prokineticin Receptors Associated with Human Disease

Prokineticins (PKs) are low molecular weight proteins that exert their effects by binding to two seven-transmembrane G-protein-coupled receptors (prokineticin receptors, PKRs). The prokineticin system is an important player in the development of various diseases. Several polymorphisms that are associated with infertility, neuroendocrine disorders, Hirschsprung's syndrome (HSCR), idiopathic central precocious puberty (CPP) and congenital disorders such as Kallmann syndrome (KS) have been described for both the PKs and PKR genes. The aim of this study is to summarize and describe the impact of PK/PKR polymorphisms on the pathogenesis and outcome of the above diseases, highlighting the PK system as a therapeutic target and diagnostic biomarker in pathological conditions.

Intracerebroventricular prokineticin 2 infusion may play a role on the hypothalamus-pituitary-thyroid axis and energy metabolism

AIM

The hypothesis of this study is to determine the effects of intracerebroventricular (icv) prokineticin 2 infusion on food consumption and body weight and to elucidate whether it has effects on energy expenditure via the hypothalamus-pituitary-thyroid (HPT) axis in adipose tissue.

MATERIAL AND METHODS

A total of 40 rats were used in the study and 4 groups were established: Control, Sham, Prokineticin 1.5 and Prokineticin 4.5 (n=10). Except for the Control group, rats were treated intracerebroventricularly via osmotic minipumps, the Sham group was infused with aCSF (vehicle), and the Prokineticin 1.5 and Prokineticin 4.5 groups were infused with 1.5 nMol and 4.5 nMol prokineticin 2, respectively. Food and water consumption and body weight were monitored during 7-day infusion in all groups. At the end of the infusion, the rats were decapitated and serum TSH, fT4 and fT3 levels were determined by ELISA. In addition, PGC-1α and UCP1 gene expression levels in white adipose tissue (WAT) and brown adipose tissue (BAT), TRH from rat hypothalamic tissue were determined by real-time PCR.

RESULTS

Icv prokineticin 2 (4.5 nMol) infusion had no effect on water consumption but reduced daily food consumption and body weight (p<0.05). Icv prokineticin 2 (4.5 nMol) infusion significantly increased serum TSH, fT4 and fT3 levels when compared to Control and Sham groups (p<0.05). Also, icv prokineticin 2 (4.5 nMol) infusion increased the expression of TRH in the hypothalamus tissue and expression of PGC-1α UCP1 in the WAT and BAT (p<0.05).

CONCLUSION

Icv prokineticin 2 (4.5 nMol) infusion may suppress food consumption via its receptors in the hypothalamus and reduce body weight by stimulating energy expenditure and thermogenesis in adipose tissue through the HPT axis.

MRAP2a Binds and Modulates Activity and Localisation of Prokineticin Receptor 1 in Zebrafish

The prokineticin system plays a role in hypothalamic neurons in the control of energy homeostasis. Prokineticin receptors (PKR1 and PKR2), like other G-protein-coupled receptors (GPCRs) are involved in the regulation of energy intake and expenditure and are modulated by the accessory membrane protein 2 of the melanocortin receptor (MRAP2). The aim of this work is to characterise the interaction and regulation of the non-melanocortin receptor PKR1 by MRAP2a in zebrafish (zMRAP2a) in order to use zebrafish as a model for the development of drugs targeting accessory proteins that can alter the localisation and activity of GPCRs. To this end, we first showed that zebrafish PKR1 (zPKR1) is able to interact with both zMRAP2a and human MRAP2 (hMRAP2). This interaction occurs between the N-terminal region of zPKR1 and the C-terminal domain of zMRAP2a, which shows high sequence identity with hMRAP2 and a similar propensity for dimer formation. Moreover, we demonstrated that in Chinese hamster ovary (CHO) cells, zMRAP2a or hMRAP2 are able to modulate zPKR1 activation induced by zebrafish PK2 (zPK2) resulting in an impaired ERK and STAT3 activation.

Mapping the interaction site for β-arrestin-2 in the prokineticin 2 receptor

G protein-coupled receptors (GPCRs) are a family of cell membrane receptors that couple and activate heterotrimeric G proteins and their associated intracellular signalling processes after ligand binding. Although the carboxyl terminal of the receptors is essential for this action, it can also serve as a docking site for regulatory proteins such as the β-arrestins. Prokineticin receptors (PKR1 and PKR2) are a new class of GPCRs that are able to activate different classes of G proteins and form complexes with β-arrestins after activation by the endogenous agonists PK2. The aim of this work was to define the molecular determinants within PKR2 that are required for β-arrestin-2 binding and to investigate the role of β-arrestin-2 in the signalling pathways induced by PKR2 activation. Our data show that PKR2 binds constitutively to β-arrestin-2 and that this process occurs through the core region of the receptor without being affected by the carboxy-terminal region. Indeed, a PKR2 mutant lacking the carboxy-terminal amino acids retains the ability to bind constitutively to β-arrestin-2, whereas a mutant lacking the third intracellular loop does not. Overall, our data suggest that the C-terminus of PKR2 is critical for the stability of the β-arrestin-2-receptor complex in the presence of PK2 ligand. This leads to the β-arrestin-2 conformational change required to initiate intracellular signalling that ultimately leads to ERK phosphorylation and activation.

Therapeutic Potential of Targeting Prokineticin Receptors in Diseases

The prokineticins (PKs) were discovered approximately 20 years ago as small peptides inducing gut contractility. Today, they are established as angiogenic, anorectic, and proinflammatory cytokines, chemokines, hormones, and neuropeptides involved in variety of physiologic and pathophysiological pathways. Their altered expression or mutations implicated in several diseases make them a potential biomarker. Their G-protein coupled receptors, PKR1 and PKR2, have divergent roles that can be therapeutic target for treatment of cardiovascular, metabolic, and neural diseases as well as pain and cancer. This article reviews and summarizes our current knowledge of PK family functions from development of heart and brain to regulation of homeostasis in health and diseases. Finally, the review summarizes the established roles of the endogenous peptides, synthetic peptides and the selective ligands of PKR1 and PKR2, and nonpeptide orthostatic and allosteric modulator of the receptors in preclinical disease models. The present review emphasizes the ambiguous aspects and gaps in our knowledge of functions of PKR ligands and elucidates future perspectives for PK research. SIGNIFICANCE STATEMENT: This review provides an in-depth view of the prokineticin family and PK receptors that can be active without their endogenous ligand and exhibits "constitutive" activity in diseases. Their non- peptide ligands display promising effects in several preclinical disease models. PKs can be the diagnostic biomarker of several diseases. A thorough understanding of the role of prokineticin family and their receptor types in health and diseases is critical to develop novel therapeutic strategies with safety concerns.

Special Issue "GPCR: Roles in Cell Development and Disease"

We are pleased to present the following Special Issue of the International Journal of Molecular Sciences (IJMS), entitled "GPCR: Roles in Cell Development and Disease" [...].

Non-Peptide Agonists and Antagonists of the Prokineticin Receptors

The prokineticin family comprises a group of secreted peptides that can be classified as chemokines based on their structural features and chemotactic and immunomodulatory functions. Prokineticins (PKs) bind with high affinity to two G protein-coupled receptors (GPCRs). Prokineticin receptor 1 (PKR1) and prokineticin receptor 2 (PKR2) are involved in a variety of physiological functions such as angiogenesis and neurogenesis, hematopoiesis, the control of hypothalamic hormone secretion, the regulation of circadian rhythm and the modulation of complex behaviors such as feeding and drinking. Dysregulation of the system leads to an inflammatory process that is the substrate for many pathological conditions such as cancer, pain, neuroinflammation and neurodegenerative diseases such as Alzheimer's and Parkinson's disease. The use of PKR's antagonists reduces PK2/PKRs upregulation triggered by various inflammatory processes, suggesting that a pharmacological blockade of PKRs may be a successful strategy to treat inflammatory/neuroinflammatory diseases, at least in rodents. Under certain circumstances, the PK system exhibits protective/neuroprotective effects, so PKR agonists have also been developed to modulate the prokineticin system.

Arginine 125 Is an Essential Residue for the Function of MRAP2

MRAP2 is a small simple transmembrane protein arranged in a double antiparallel topology on the plasma membrane. It is expressed in the paraventricular nucleus of the hypothalamus, where it interacts with various G protein-coupled receptors, such as the prokineticin receptors, and regulates energy expenditure and appetite. The aim of this work was to analyze the functional role of the specific arginine residue at position 125 of MRAP2, which affects protein conformation, dimer formation, and PKR2 binding. Results obtained with the MRAP2 mutants R125H and R125C, which are found in human patients with extreme obesity, and mouse MRAP2, in which arginine 125 is normally replaced by histidine, were compared with those obtained with human MRAP2. Understanding the mechanism by which MRAP2 regulates G protein-coupled receptors helps in elucidating the metabolic pathways involved in metabolic dysfunction and in developing new drugs as specific targets of the MRAP2-PKR2 complex.

Identification of Regions Involved in the Physical Interaction between Melanocortin Receptor Accessory Protein 2 and Prokineticin Receptor 2

Melanocortin Receptor Accessory Protein 2 (MRAP2) modulates the trafficking and signal transduction of several G-protein-coupled receptors (GPCRs) involved in the control of energy homeostasis, such as Prokineticin receptors (PKRs). They bind the endogenous ligand prokineticin 2 (PK2), a novel adipokine that has an anorexic effect and modulates thermoregulation and energy homeostasis. In the present work, we used biochemical techniques to analyze the mechanism of interaction of MRAP2 with PKR2 and we identified the specific amino acid regions involved in the complex formation. Our results indicate that MRAP2 likely binds to the N-terminal region of PKR2, preventing glycosylation and consequently the correct receptor localization. We also identified a C-terminal region of MRAP2 that is critical for the interaction with PKR2. Consequently, we analyzed the role of the prokineticin transduction system in the regulation of MRAP2 expression in tissues involved in the control of food intake: at the central level, in hypothalamic explants, and at the peripheral level, in adipocytes. We demonstrated the modulation of MRAP2 expression by the prokineticin transduction system.

Identification and Characterization of a New Splicing Variant of Prokineticin 2

Prokineticin 2 (PROK2) is a secreted bioactive peptide that regulates a variety of biological responses via two GPCRs, the prokineticin receptors (PROKRs). The aim of this study was to characterize a new alternatively spliced product of the prok2 gene consisting of four exons. The 40-amino acid peptide, designated PROK2C, is encoded by exon 1 and exon 4, and its expression was detected in the hippocampus and spinal cord of mice. PROK2C was expressed in a heterologous system, Pichia pastoris, and its binding specificity to the amino-terminal regions of PROKR1 and PROKR2 was investigated by GST pull-down experiments. In addition, the introduction of the unnatural amino acid p-benzoyl-L-phenylalanine using amber codon suppression technology demonstrated the role of tryptophan at position 212 of PROKR2 for PROK2C binding by photoactivatable cross-linking. The functional significance of this new isoform was determined in vivo by nociceptive experiments, which showed that PROK2C elicits strong sensitization of peripheral nociceptors to painful stimuli. In order to analyze the induction of PROK2C signal transduction, STAT3 and ERK phosphorylation levels were determined in mammalian CHO cells expressing PROKR1 and PROKR2. Our data show by in vivo and in vitro experiments that PROK2C can bind and activate both prokineticin receptors.

Prokineticin-Receptor Network: Mechanisms of Regulation

Prokineticins are a new class of chemokine-like peptides that bind their G protein-coupled receptors, PKR1 and PKR2, and promote chemotaxis and the production of pro-inflammatory cytokines following tissue injury or infection. This review summarizes the major cellular and biochemical mechanisms of prokineticins pathway regulation that, like other chemokines, include: genetic polymorphisms; mRNA splice modulation; expression regulation at transcriptional and post-transcriptional levels; prokineticins interactions with cell-surface glycosaminoglycans; PKRs degradation, localization, post-translational modifications and oligomerization; alternative signaling responses; binding to pharmacological inhibitors. Understanding these mechanisms, which together exert substantial biochemical control and greatly enhance the complexity of the prokineticin-receptor network, leads to novel opportunities for therapeutic intervention. In this way, besides targeting prokineticins or their receptors directly, it could be possible to indirectly influence their activity by modulating their expression and localization or blocking the downstream signaling pathways.

Versatile Role of Prokineticins and Prokineticin Receptors in Neuroinflammation

Prokineticins are a new class of chemokine-like peptides involved in a wide range of biological and pathological activities. In particular, prokineticin 2 (PK2), prokineticin receptor 1 (PKR1) and prokineticin receptor 2 (PKR2) play a central role in modulating neuroinflammatory processes. PK2 and PKRs, which are physiologically expressed at very low levels, are strongly upregulated during inflammation and regulate neuronal-glial interaction. PKR2 is mainly overexpressed in neurons, whereas PKR1 and PK2 are mainly overexpressed in astrocytes. Once PK2 is released in inflamed tissue, it is involved in both innate and adaptive responses: it triggers macrophage recruitment, production of pro-inflammatory cytokines, and reduction of anti-inflammatory cytokines. Moreover, it modulates the function of T cells through the activation of PKR1 and directs them towards a pro-inflammatory Th1 phenotype. Since the prokineticin system appears to be upregulated following a series of pathological insults leading to neuroinflammation, we will focus here on the involvement of PK2 and PKRs in those pathologies that have a strong underlying inflammatory component, such as: inflammatory and neuropathic pain, Alzheimer's disease, Parkinson's disease, multiple sclerosis, stroke, obesity, diabetes, and gastrointestinal inflammation.