Current Protein and Peptide Science - Volume 15, Issue 7, 2014

This special issue is based on a mini-symposium in the area of neurosciences with the title "Understanding the role of heteroreceptor complexes in the central nervous system" held at the Nobel Forum, Karolinska Institutet on December 17th, 2012, organized by Kjell Fuxe, Dasiel O. Borroto-Escuela and Luigi F. Agnati. It consists of seven mini-reviews in the field receptor heteromers. The early work on negative cooperativity and neuropeptide-monoamine receptor-receptor interactions in the central nervous system gave the first indications of the existence of homomers and heteromers of G-protein coupled receptors (GPCR), respectively, and the GPCR field began to expand from monomers into dimers and receptor mosaics (higher-order dimers). It was underlined that the existence of receptor heteromers with allosteric receptor-receptor interactions increases the diversity and bias of GPCR recognition and signalling. The molecular phenomenon of allosteric receptor-receptor interactions is proposed to give a better understanding of brain function through molecular integration of signals. An alteration in specific receptor-receptor interactions is in fact considered to play a role in pathogenic mechanisms leading to several diseases, inter alia Parkinson’s disease, hypertension, schizophrenia, addiction and depression. It is a new principle in molecular medicine. Therefore, pharmacological targeting of receptor-receptor interactions in heteromers will become an important area for developing more selective drugs with reduced side-effects including heterobivalent compounds and optimal types of combined treatments. In other words , it will lead to novel strategies for treatment, and finally novel drugs for treatment of disease. The first mini-review by Dr. Tena-Campus and colleagues introduces the field of GPCR oligomerization as emerging signalling units with new opportunities for drug design and discusses the technologies involved for detection of receptor heteromers. Then the issue moves into examples of receptor-receptor interactions in the DA and neuropeptide field. Dr. Van Craenenbroeck and colleagues presents an article on the role of dimerization in the biogenesis of DA D4 receptors and thus in their maturation. Dr. Zaida Diaz-Cabiale and colleagues describe the existence of galanin receptor-neuropeptide Y receptor interactions in the brain including galanin receptor-neuropeptide Y Y1 interactions in the brain stem. Indications are obtained that the receptor target for galanin fragment 1-15 is instead a GalR1-GalR2 heteromer. Then the special issue enters into the role of receptor-receptor interactions in putative striatal GPCR heteromers in Parkinson’s disease and schizophrenia. Dr. Beggiato and colleagues discuss the role of antagonistic adenosine A2A-D2 receptorreceptor interactions in the striato-pallidal GABA neurons and their relevance for treatment of Parkinson’s disease. They give the rationale for the introduction of A2A receptor antagonists in clinical trials in this disease based on these antagonistic receptor- receptor interactions which become even more strongly developed in animal models of Parkinson’s disease. Dr. Luca Ferraro and colleagues instead discuss in detail the antagonistic Neurotensin NTS1-dopamine D2 receptor-receptor interactions in putative receptor heteromers in the dorsal and ventral striatum. Their involvement in striato-pallidal GABA and mesocorticolimbic DA communication is discussed with focus on their relevance for Parkinson’s disease, schizophrenia and their treatments. Dr. Di Liberto and colleagues deal with the role of receptor-receptor interactions in brain trophism and plasticity with focus on interactions between G protein-coupled receptor-Receptor Tyrosine Kinase, specially the cholinergic and fibroblast growth factor receptor 1 (FGFR1). mAChR–FGFR1 interactions are indicated leading to transactivation of FGFR1 with potential relevance for cognition. Luigi Agnati and colleagues in the last paper of this special issue suggest a unified perspective for integrative brain actions through “ neurosemeiotics” and “ free energy minimization”. Especially the Bio-semeiotics concept of “adaptor” may involve the receptor-receptor interactions in heteroreceptor complexes. Through such “adaptors” a code may be produced that give meaning to the sensory stimuli reaching the cortical regions of the brain . We hope the readers will find the articles in this special issue of interest and may give some inspiration to enter this exciting field of receptor research in the CNS which opens up a novel understanding of the molecular events that may lead to neurological and mental diseases and offer novel strategies for their treatments. The editors are grateful to the authors for their fine contributions to this special issue.

G-protein-coupled receptors (GPCRs) are a widespread family of transmembrane receptors with different physiologically relevant functions. Alterations in the structure and function of these receptors at different levels (ligand binding, signaling and trafficking) may result in a number of pathological conditions which represent a major health problem. Mutations in these receptors are also linked to different inherited diseases for which there is no cure to date. Rationale design, based on receptor structural knowledge, is needed for the discovery of novel drugs with higher selectivity and less side effects. In fact, about 50% of the drugs currently under development target this kind of receptors. Oligomerization among GPCRs has been clearly established from experimental, particularly in vitro, studies. Moreover, homo and heterodimerization provide new unexpected clues for explaining the molecular mechanisms underlying some diseases in which GPCRs signaling might be affected. In this review we will analyze GPCRs structure and function for a better understanding of the dimerization process and the experimental approaches currently used to detect such interactions. Furthermore, how drugs targeting heteromers can represent new opportunities to tackle novel and safer treatments of some pathologies will be described. Recent results, in this regard, will be reported as encouraging examples in the field. Finally, the newest technologies available for developing drugs targeting heteromers will also be reviewed highlighting the importance of bivalent ligands that emerge as very powerful molecules interacting with heteromers.

Dopamine receptors are G protein-coupled receptors critically involved in locomotion, reward, and cognitive processes. Export of dopamine receptors to the plasma membrane is thought to follow the default secretory pathway, whereby proteins travel from the endoplasmatic reticulum (ER), through the Golgi apparatus, to arrive at the cell surface. Several observations indicate that trafficking from the ER to the plasma membrane is tightly regulated, and that correct folding in the ER acts as a bottle neck to the maturation of the dopamine D4 receptors. The dopamine D4 receptor is an interesting receptor since it has a polymorphic region in its third intracellular loop, resulting in receptor isoforms of varying length and amino acid composition. Correct folding is enhanced by: (1) interaction with specific proteins, such as ER resident chaperones, (2) interaction with pharmacological chaperones, for example, ligands that are membrane permeable and can bind to the receptor in the ER, and (3) receptor dimerization; the assembly of multisubunit proteins into a quaternary structure is started in the ER before cell surface delivery, which helps in correct folding and subsequent expression. These interactions help the process of GPCR folding, but more importantly they ensure that only properly folded proteins proceed from the ER to the trans-Golgi network. In this review we will mainly focus on the role of receptor dimerization in dopamine D4 receptor maturation.

The presence of Galanin and Neuropeptide Y and/or their receptors in several areas of the brain involved in memory, mood, cardiovascular control and food intake indicates that Galanin, and Neuropeptide Y could equilibrate the physiological actions of each other. There is evidence for the existence of interactions between Galanin Receptor and Neuropeptide Y Receptor in the nucleus of the solitarii tract (NTS), hypothalamus and dorsal raphe nucleus probably taking place with the formation of heteromers between Galanin Receptor and Neuropeptide Y Y1 Receptor. The galanin fragment (Gal 1-15) preferring receptors may instead be formed by the GalR1-GalR2 heteromer which in the NTS may interact with Neuropeptide Y Y2 receptors. These receptor heteromers may be one key molecular mechanism for Galanin and its N-terminal fragment (Galanin 1-15) to modulate the function of different types of glia–neuronal networks in the CNS, especially the emotional, metabolic and cardiovascular networks.

Striatal dopamine adenosine A2A and D2 receptors interact to modulate some aspects of motor and motivational function. The demonstration of A2A/D2 receptor heteromerization in living cells constituted a progress for understanding the neurobiology of dopamine D2 and adenosine A2A receptors. In fact, the existence of putative striatalA2A/D2 receptor heteromers has been suggested to be important for striatal function under both normal and pathological conditions, such as Parkinson’s disease. Consequently, the antagonistic A2A-D2 receptor interactions in a putative striatal receptor heteromer on striato-pallidal GABA neuron led to the introduction of A2A receptor antagonists as possible anti- Parkinsonian drugs. The present mini-review briefly summarizes the main findings supporting the presence of antagonistic A2A-D2 receptor interactions in putative receptor heteromers in the basal ganglia. Special emphasis is given to in vivo microdialysis findings demonstrating the functional role putative A2A/D2 heteromers on striato-pallidal GABA neurons play in the modulation of this pathway, in which A2A receptors inhibit D2 receptor signaling. The possible relevance of compounds targeting the putative striatal A2A/D2 heteromer in the Parkinson’s disease pharmacological treatment is also discussed.

The tridecapeptide neurotensin (NT) acts as neurotransmitter in the central nervous system and in the periphery. NT and NT receptors are largely localized in dopamine (DA)-enriched regions of the mammalian brain. Accordingly, numerous studies indicate the presence of close functional interactions between DA neurons and the peptide. Among others mechanisms, it has been suggested that NT could modulate nigrostriatal, mesolimbic and meso-cortical DA transmission through an antagonistic receptor-receptor interaction between the NT receptor subtype 1 (NTS1) and the dopamine D2 receptor (D2R). In particular, it was originally demonstrated that the peptide reduces the D2R agonist affinity in striatal sections and in striatal membrane preparations. These effects could be a consequence of the direct allosteric NTS1/D2 receptor interactions leading to a decrease in the DA agonist affinity at the D2 receptor. Several neurochemical, biochemical and co-immunoprecipitation data have successively reinforced the indication of the presence of direct NTS1-D2 receptor interactions in the mammalian brain. The present mini-review attempts to provide a summary of current knowledge, mainly emerging from our microdialysis studies, supporting the presence of a NTS1/D2 receptor heteromer in the brain. The pre and post-synaptic mechanisms underlying the involvement of this heteromer in the striatopallidal GABA and mesocorticolimbic DA neurotransmission are discussed especially for their relevance in Parkinson’s disease and schizophrenia, respectively.

Acetylcholine, acting on both nicotinic receptors (nAChRs) and muscarinic receptors (mAChRs), plays a role in the regulation of synaptic plasticity, being involved in the regulation of cellular processes and cognitive functions, such as learning, memory and attention. Recently, G protein coupled receptors (GPCRs), including mAChRs, have been reported to transactivate tyrosine-kinase receptors (RTK), such as epidermal growth factor receptor (EGFR), and initiate their intracellular signaling. In this minireview we have first analysed the RTK transactivation mechanisms, involving cholinergic receptors, and thereafter the interplay between AChR and neurotrophic factor systems built up by FGF2 and fibroblast growth factor receptor 1 (FGFR1). Although mAChR and FGFR1 activate common signaling pathways, playing similar roles in the regulation of central nervous system (CNS) plasticity and trophism, this analysis revealed that at the present there are no data reporting an involvement of cholinergic receptors in the FGFR1 transactivation. However, here we reported preliminary results on FGFR1 transactivation by mAChRs, suggesting a possible interaction between mAChR and neurotrophic factor receptors, with potential relevance for cognitive functions.

Two far-reaching theoretical approaches, namely “Neuro-semeiotics” (NS) and “Free-energy Minimization” (FEM), have been recently proposed as frames within which to put forward heuristic hypotheses on integrative brain actions. In the present paper these two theoretical approaches are briefly discussed in the perspective of a recent model of brain architecture and information handling based on what we suggest calling Jacob’s tinkering principle, whereby “to create is to recombine!”. The NS and FEM theoretical approaches will be discussed from the perspective both of the Roamer-Type Volume Transmission (especially exosome-mediated) of intercellular communication and of the impact of receptor oligomers and Receptor-Receptor Interactions (RRIs) on signal recognition/decoding processes. In particular, the Bio-semeiotics concept of “adaptor” will be used to analyze RRIs as an important feature of NS. Furthermore, the concept of phenotypic plasticity of cells will be introduced in view of the demonstration of the possible transfer of receptors (i.e., adaptors) into a computational network via exosomes (see also Appendix). Thus, Jacob’s tinkering principle will be proposed as a theoretical basis for some learning processes both at the network level (Turing-like type of machine) and at the molecular level as a consequence of both the plastic changes in the adaptors caused by the allosteric interactions in the receptor oligomers and the intercellular transfer of receptors. Finally, on the basis of NS and FEM theories, a unified perspective for integrative brain actions will be proposed.

The secretion of extracellular membrane vesicles (EMVs) is a common phenomenon that occurs in archaea, bacteria, and mammalian cells. EMVs contain biologically active proteins, which have diverse roles in biological processes. The outer membrane vesicles (OMVs) of Gram-negative bacteria and membrane vesicles (MVs) of Gram-positive bacteria have been discovered in various species. The main issues related to bacterial EMVs are their virulence, biogenesis mechanisms, host cell interaction mechanisms, and their potential use as new vaccine candidates. Recently, proteomics has become an essential tool for the characterization of EMVs. Proteomics is useful for the identification, quantification, and protein-protein interaction analysis of EMV protein components. This review describes the current understanding of secretory EMVs based on proteomic methods and the characteristics of various bacterial secretory EMVs. Finally, evidence for their potential roles and future applications are discussed.

Aim of this review is to study the role of the TRPV2 channel, a member of the TRPV subfamily of TRP channels, in tumor progression. Physiologically, the triggering of TRPV2 by agonists/activators (e.g., growth factors, hormones and cannabinoids), by inducing TRPV2 translocation from the endosome to the plasmatic membrane, inhibit cell proliferation and induce necrosis and/or apoptosis. Thus, loss or alterations of TRPV2 proliferative and apoptotic signals, results in uncontrolled proliferation and augmented resistance to apoptotic stimuli. For example in prostate cancer cells, the TRPV2 activation following lysophospholipid or adrenomedullin stimulation enhances the invasiveness of cancer cells; furthermore, the increased malignancy of castration-resistant prostate cancer cells was associated with enhanced TRPV2 expression, mainly in metastatic prostate cancer cells. In addition, the TRPV2 cellular functions may also to be related to the presence of TRPV2 variants, able to interfere with the physiological functions of normal TRPV2 channels. In this regard, bladder cancer tumors show loss or reduction of a short TRPV2 variant during cancer progression, with increased malignancy and invasiveness. High expression of TRPV2 was also observed more frequently in esophageal squamous cell carcinoma patients with advanced pT stage, lymph node metastasis and advanced pathological stage.

This review paper aims at discussing the major recent achievements in the field of the heparin-binding proteins (HBPs), primarily chemokines and defensins, and their complexes with glycosaminoglycans examined by solution nuclear magnetic resonance spectroscopy. As opposed to the HBPs involved in coagulation (mainly antithrombin and thrombin), and growth factors (especially fibroblast growth factors 1 and 2), which were extensively analyzed in the past, inflammatory chemokines turned out to be the most thoroughly studied HBPs nowadays. Defensins, which structurally and functionally resemble the chemokines, are also under intensive current investigation.