Glutamate heteroreceptor complexes in the brain

The existence of mGluR, NMDAR, AMPAR and putative KAR heteroreceptor complexes in synaptic and extrasynaptic regions of brain glutamate synapses represents a major integrative mechanism. Our aim in the current article is to analyze if the formation of the different types glutamate hetereceptor complexes involves the contribution of triplet amino acid homologies (protriplets) in a postulated receptor interface based on the triplet puzzle theory. Seven main sets (lists) of receptor pairs in databases were used containing various sets (lists) of human receptor heteromers and nonheteromers obtained from the available scientific publications including the publically available GPCR-hetnet database. Brain mGluR1-mGluR5 and mGluR2-mGluR4 isoreceptor complexes were demonstrated with a predominant extrasynaptic localization at a post- and prejunctional localization. The existence of putative mGluR4-mGluR7 heteroreceptor complexes in the basal ganglia is proposed. Metabotropic glutamate receptor subtypes also participated in the formation of a large number of heteroreceptor complexes like mGluR1-A1R, mGluR5-A2AR, mGluR5-D2R and D2R-A2AR-mGluR5, located in relation to glutamate synapses, especially in the basal ganglia. A putative mGluR1-GABAB1/2 heterocomplex may also exist. NMDAR heteroreceptor complexes were also demonstrated as a fundamental integrative mechanism in the glutamate synapse and its extrasynaptic membranes. It represented fundamental work on inter alia NMDAR-mGluR5, NMDAR-D1R and NMDAR-D2R heteroreceptor complexes involving both antagonistic and facilitatory allosteric receptor-receptor interactions. As to AMPA receptors, a heterocomplex was found for the interaction between IFNgR1 and the AMPAR mediated via the subunit GluA1 which may be of relevance for neuroinflammation. AMPAR-D2R heteroreceptor complexes were also demonstrated. Besides glutamate heteroreceptor complexes and their allosteric receptor-receptor interactions, a significant mechanism for the functional crosstalk can also be phosphorylation and/or reorganization of adapter proteins with dynamic binding to the two receptors modulating the allosteric receptor mechanism.

Existence of Brain 5-HT1A-5-HT2A Isoreceptor Complexes with Antagonistic Allosteric Receptor-Receptor Interactions Regulating 5-HT1A Receptor Recognition

Studies on serotonin-selective reuptake inhibitors have established that disturbances in the ascending 5-HT neuron systems and their 5-HT receptor subtypes and collateral networks to the forebrain contribute to the etiology of major depression and are targets for treatment. The therapeutic action of serotonin-selective reuptake inhibitors is of proven effectiveness, but the mechanisms underlying their effect are still unclear. There are many 5-HT subtypes involved; some need to be blocked (e.g., 5-HT2A, 5-HT3, and 5-HT7), whereas others need to be activated (e.g., postjunctional 5-HT1A and 5-HT4). These state-of-the-art developments are in line with the hypothesis that the development of major depression can involve an imbalance of the activity between different types of 5-HT isoreceptors. In the current study, using in situ proximity ligation assay (PLA), we report evidence for the existence of brain 5-HT1A-5-HT2A isoreceptor complexes validated in cellular models with bioluminescence resonance energy transfer (BRET²) assay. A high density of PLA-positive clusters visualizing 5-HT1A-5-HT2A isoreceptor complexes was demonstrated in the pyramidal cell layer of the CA1-CA3 regions of the dorsal hippocampus. A marked reduction in the density of PLA-positive clusters was observed in the CA1 and CA2 regions 24 h after a forced swim test session, indicating the dynamics of this 5-HT isoreceptor complex. Using a bioinformatic approach, previous work indicates that receptors forming heterodimers demonstrate triplet amino acid homologies. The receptor interface of the 5-HT1A-5-HT2A isoreceptor dimer was shown to contain the LLG and QNA protriplets in the transmembrane and intracellular domain, respectively. The 5-HT2A agonist TCB2 markedly reduced the affinity of the 5-HT1A agonist ipsapirone for the 5-HT1A agonist binding sites in the frontal lobe using the 5-HT1A radioligand binding assay. This action was blocked by the 5-HT2A antagonist ketanserin. It is proposed that the demonstrated 5-HT1A-5-HT2A isoreceptor complexes may play a role in depression through integration of 5-HT recognition, signaling and trafficking in the plasma membrane in two major 5-HT receptor subtypes known to be involved in depression. Antagonistic allosteric receptor-receptor interactions appear to be involved in this integrative process.

Understanding the Role of GPCR Heteroreceptor Complexes in Modulating the Brain Networks in Health and Disease

The introduction of allosteric receptor-receptor interactions in G protein-coupled receptor (GPCR) heteroreceptor complexes of the central nervous system (CNS) gave a new dimension to brain integration and neuropsychopharmacology. The molecular basis of learning and memory was proposed to be based on the reorganization of the homo- and heteroreceptor complexes in the postjunctional membrane of synapses. Long-term memory may be created by the transformation of parts of the heteroreceptor complexes into unique transcription factors which can lead to the formation of specific adapter proteins. The observation of the GPCR heterodimer network (GPCR-HetNet) indicated that the allosteric receptor-receptor interactions dramatically increase GPCR diversity and biased recognition and signaling leading to enhanced specificity in signaling. Dysfunction of the GPCR heteroreceptor complexes can lead to brain disease. The findings of serotonin (5-HT) hetero and isoreceptor complexes in the brain over the last decade give new targets for drug development in major depression. Neuromodulation of neuronal networks in depression via 5-HT, galanin peptides and zinc involve a number of GPCR heteroreceptor complexes in the raphe-hippocampal system: GalR1-5-HT1A, GalR1-5-HT1A-GPR39, GalR1-GalR2, and putative GalR1-GalR2-5-HT1A heteroreceptor complexes. The 5-HT1A receptor protomer remains a receptor enhancing antidepressant actions through its participation in hetero- and homoreceptor complexes listed above in balance with each other. In depression, neuromodulation of neuronal networks in the raphe-hippocampal system and the cortical regions via 5-HT and fibroblast growth factor 2 involves either FGFR1-5-HT1A heteroreceptor complexes or the 5-HT isoreceptor complexes such as 5-HT1A-5-HT7 and 5-HT1A-5-HT2A. Neuromodulation of neuronal networks in cocaine use disorder via dopamine (DA) and adenosine signals involve A2AR-D2R and A2AR-D2R-Sigma1R heteroreceptor complexes in the dorsal and ventral striatum. The excitatory modulation by A2AR agonists of the ventral striato-pallidal GABA anti-reward system via targeting the A2AR-D2R and A2AR-D2R-Sigma1R heteroreceptor complex holds high promise as a new way to treat cocaine use disorders. Neuromodulation of neuronal networks in schizophrenia via DA, adenosine, glutamate, 5-HT and neurotensin peptides and oxytocin, involving A2AR-D2R, D2R-NMDAR, A2AR-D2R-mGluR5, D2R-5-HT2A and D2R-oxytocinR heteroreceptor complexes opens up a new world of D2R protomer targets in the listed heterocomplexes for treatment of positive, negative and cognitive symptoms of schizophrenia.

IL1R2, CCR2, and CXCR4 May Form Heteroreceptor Complexes with NMDAR and D2R: Relevance for Schizophrenia

The mild neuroinflammation hypothesis of schizophrenia was introduced by Bechter in 2001. It has been hypothesized that a hypofunction of glutamatergic signaling via N-methyl-D-aspartate receptors (NMDARs) and hyperactivation of dopamine D2 receptors play a role in schizophrenia. The triplet puzzle theory states that sets of triplet amino acid homologies guide two different receptors toward each other and contributes to the formation of a receptor heteromer. It is, therefore, proposed that putative NMDAR-C-C chemokine receptor type 2 (CCR2), NMDAR-C-X-C chemokine receptor type 4 (CXCR4), and NMDAR- interleukin 1 receptor type II (IL1R2) heteromers can be formed in the neuronal networks in mild neuroinflammation due to demonstration of Gly-Leu-Leu (GLL), Val-Ser-Thr (VST), and/or Ser-Val-Ser (SVS) amino acid homologies between these receptor protomers. This molecular process may underlie the ability to produce symptoms of schizophrenia in mild neuroinflammation. In this state, volume transmission (VT) is increased involving increased extracellular vesicle-mediated VT from microglia and astroglia. These vesicles may contain CCR2, CXCR4, and/or IL1R2 as well as their ligands and upon internalization by endocytic pathways into neurons can form heteroreceptor complexes with NMDAR in the plasma membrane with pathological allosteric receptor-receptor interactions involving increased internalization and reduced NMDAR signaling. The triplet puzzle theory also suggests the formation of putative D2R-CCR2, D2R-CXCR4, and D2R-IL1R2 heteromers in mild neuroinflammation in view of their demonstrated sets of Leu-Tyr-Ser (LYS), Leu-Pro-Phe (LPF), and/or Ser-Leu-Ala (SLA) triplet homologies. These D2R heteroreceptor complexes may also contribute to schizophrenia-like symptoms in mild neuroinflammation by enhancing D2R protomer function.

The role of transmitter diffusion and flow versus extracellular vesicles in volume transmission in the brain neural-glial networks

Two major types of intercellular communication are found in the central nervous system (CNS), namely wiring transmission (point-to-point communication, the prototype being synaptic transmission with axons and terminals) and volume transmission (VT; communication in the extracellular fluid and in the cerebrospinal fluid (CSF)) involving large numbers of cells in the CNS. Volume and synaptic transmission become integrated inter alia through the ability of their chemical signals to activate different types of receptor protomers in heteroreceptor complexes located synaptically or extrasynaptically in the plasma membrane. The demonstration of extracellular dopamine (DA) and serotonin (5-HT) fluorescence around the DA and 5-HT nerve cell bodies with the Falck-Hillarp formaldehyde fluorescence method after treatment with amphetamine and chlorimipramine, respectively, gave the first indications of the existence of VT in the brain, at least at the soma level. There exist different forms of VT. Early studies on VT only involved spread including diffusion and flow of soluble biological signals, especially transmitters and modulators, a communication called extrasynaptic (short distance) and long distance (paraaxonal and paravascular and CSF pathways) VT. Also, the extracellular vesicle type of VT was demonstrated. The exosomes (endosome-derived vesicles) appear to be the major vesicular carriers for VT but the larger microvesicles also participate. Both mainly originate at the soma-dendritic level. They can transfer lipids and proteins, including receptors, Rab GTPases, tetraspanins, cholesterol, sphingolipids and ceramide. Within them there are also subsets of mRNAs and non-coding regulatory microRNAs. At the soma-dendritic membrane, sets of dynamic postsynaptic heteroreceptor complexes (built up of different types of physically interacting receptors and proteins) involving inter alia G protein-coupled receptors including autoreceptors, ion channel receptors and receptor tyrosine kinases are hypothesized to be the molecular basis for learning and memory. At nerve terminals, the presynaptic heteroreceptor complexes are postulated to undergo plastic changes to maintain the pattern of multiple transmitter release reflecting the firing pattern to be learned by the heteroreceptor complexes in the postsynaptic membrane.

FGFR1-5-HT1A Heteroreceptor Complexes: Implications for Understanding and Treating Major Depression

The serotonin and neurotrophic factor hypotheses of depression are well known. The discovery of brain fibroblast growth factor receptor 1 (FGFR1)-5 hydroxytryptamine receptor 1A (5-HT1A) heteroreceptor complexes, and their enhancement of neuroplasticity, offers an integration of these hypotheses at the molecular level. They were first described in the hippocampus and later in midbrain 5-HT neurons, where these heterocomplexes are enriched in 5-HT1A autoreceptors. Combined FGF2 and 5-HT1A agonist treatment increased the formation of these heterocomplexes and the facilitatory allosteric receptor-receptor interactions within them led to the enhancement of FGFR1 signaling and was associated with the development of antidepressant effects. We discuss these findings with regard to a theory of motifs critically involved in these interactions and suggest that these complexes represent novel targets for antidepressants.

Evidence for the existence of FGFR1-5-HT1A heteroreceptor complexes in the midbrain raphe 5-HT system

The ascending midbrain 5-HT neurons known to contain 5-HT1A autoreceptors may be dysregulated in depression due to a reduced trophic support. With in situ proximity ligation assay (PLA) and supported by co-location of the FGFR1 and 5-HT1A immunoreactivities in midbrain raphe 5-HT cells, evidence for the existence of FGFR1-5-HT1A heteroreceptor complexes were obtained in the dorsal and median raphe nuclei of the Sprague-Dawley rat. Their existence in the rat medullary raphe RN33B cell cultures was also established. After combined FGF-2 and 8-OH-DPAT treatment, a marked and significant increase in PLA positive clusters was found in the RN33B cells. Similar results were reached upon coactivation by agonists in HEK293T cells using the Fluorescent Resonance Energy Transfer (FRET) technique resulting in increased FRETmax and reduced FRET50 values. The heteroreceptor complex formation was dependent on TMV of the 5-HT1A receptor since it was blocked by incubation with TMV but not with TMII. Taken together, the 5-HT1A autoreceptors by being recruited into a FGFR1-5-HT1A heteroreceptor complex in the midbrain raphe 5-HT nerve cells may develop a novel function, namely a trophic role in many midbrain 5-HT neuron systems originating from the dorsal and medianus raphe nuclei.

Dopamine D2 heteroreceptor complexes and their receptor-receptor interactions in ventral striatum: novel targets for antipsychotic drugs

This review is focused on the D2 heteroreceptor complexes within the ventral striatum with their receptor-receptor interactions and relevance for the treatment of schizophrenia. A "guide-and-clasp" manner for receptor-receptor interactions is proposed where "adhesive guides" may be amino acid triplet homologies, which were determined for different kinds of D2 heteroreceptor complexes. The first putative D2 heteroreceptor complex to be discovered in relation to schizophrenia was the A2A-D2 heteroreceptor complex where antagonistic A2A-D2 receptor-receptor interactions were demonstrated after A2A agonist treatment in the ventral striatum. The A2A agonist CGS 21680 with atypical antipsychotic properties may at least in part act by increasing β-arrestin2 signaling over the D2 protomer in the A2A-D2 heteroreceptor complex in the ventral striatum. The antagonistic NTS1-D2 interactions in the NTS1-D2 heteroreceptor complex in the ventral striatum are proposed as one molecular mechanism for the potential antipsychotic effects of NT. Indications were obtained that the psychotic actions of the 5-HT2AR hallucinogens LSD and DOI can involve enhancement of D2R protomer signaling via a biased agonist action at the 5-HT2A protomer in the D2-5-HT2A heteroreceptor complex in the ventral striatum. Facilitatory allosteric D2likeR-OTR interactions in heteroreceptor complexes in nucleus accumbens may have a role in social and emotional behaviors. By blocking the D2 protomers of these heteroreceptor complexes, antipsychotics can fail to reduce the negative symptoms of schizophrenia. The discovery of different types of D2 heteroreceptor complexes gives an increased understanding of molecular mechanisms involved in causing schizophrenia and new strategies for its treatment and understanding the side effects of antipsychotics.

Moonlighting proteins and protein-protein interactions as neurotherapeutic targets in the G protein-coupled receptor field

There is serious interest in understanding the dynamics of the receptor-receptor and receptor-protein interactions in space and time and their integration in GPCR heteroreceptor complexes of the CNS. Moonlighting proteins are special multifunctional proteins because they perform multiple autonomous, often unrelated, functions without partitioning into different protein domains. Moonlighting through receptor oligomerization can be operationally defined as an allosteric receptor-receptor interaction, which leads to novel functions of at least one receptor protomer. GPCR-mediated signaling is a more complicated process than previously described as every GPCR and GPCR heteroreceptor complex requires a set of G protein interacting proteins, which interacts with the receptor in an orchestrated spatio-temporal fashion. GPCR heteroreceptor complexes with allosteric receptor-receptor interactions operating through the receptor interface have become major integrative centers at the molecular level and their receptor protomers act as moonlighting proteins. The GPCR heteroreceptor complexes in the CNS have become exciting new targets for neurotherapeutics in Parkinson's disease, schizophrenia, drug addiction, and anxiety and depression opening a new field in neuropsychopharmacology.

G protein-coupled receptor heterodimerization in the brain

G protein-coupled receptors (GPCRs) play critical roles in cellular processes and signaling and have been shown to form heteromers with diverge biochemical and/or pharmacological activities that are different from those of the corresponding monomers or homomers. However, despite extensive experimental results supporting the formation of GPCR heteromers in heterologous systems, the existence of such receptor heterocomplexes in the brain remains largely unknown, mostly because of the lack of appropriate methodology. Herein, we describe the in situ proximity ligation assay procedure underlining its high selectivity and sensitivity to image GPCR heteromers with confocal microscopy in brain sections. We describe here how the assay is performed and discuss advantages and disadvantages of this method compared with other available techniques.

Integrin triplets of marine sponges in the murine and human MHCI-CD8 interface and in the interface of human neural receptor heteromers and subunits

Based on our theory, main triplets of amino acid residues have been discovered in cell-adhesion receptors (integrins) of marine sponges, which participate as homologies in the interface between two major immune molecules, MHC class I (MHCI) and CD8αβ. They appear as homologies also in several human neural receptor heteromers and subunits. The obtained results probably mean that neural and immune receptors also utilize these structural integrin triplets to form heteromers and ion channels, which are required for a tuned and integrated intracellular and intercellular communication and a communication between cells and the extracellular matrix with an origin in sponges, the oldest multicellular animals.

Cell-cell nanotubes: Tunneling through several types of synapses

Nanotube can be generally seen as a nanoscale cylindrical structure. Membrane (or tunneling) nanotube (TNT) is a cytoplasmic tunnel between two cells. Such direct cell-cell channel is used for a physical transport of biochemical cargo, whereas nanotubular networks between cells may be a novel principle of communicative and integrative biology. Recently, TNTs and their networks were discovered in plant cells and then they were reported also in animal cells. Just the reverse, a notion of plant synapse has been also proposed only recently, long after the corresponding notion of neuronal synapse in animals. However, both TNTs and synapses seem to be closely related and evolutionary conserved structures through different types of cells. Accordingly, this mini-review aims to demonstrate that TNTs may represent one of the deep functional similarities between neuronal, immune, viral and plant synapses.

Integrin triplets of marine sponges in human D2 receptor heteromers

The evidence for the existence of receptor heteromers opens up a new field for a better understanding of neural transmission. Based on our theory, we have discovered main triplets of amino acid residues in cell-adhesion receptors of marine sponges, which appear also as homologies in several dopamine D2 receptor heteromers of human brain. The obtained results probably mean a general molecular mechanism for receptor-receptor interactions in heteromers originated from the lowest animals (marine sponges).

Extrasynaptic neurotransmission in the modulation of brain function. Focus on the striatal neuronal-glial networks

Extrasynaptic neurotransmission is an important short distance form of volume transmission (VT) and describes the extracellular diffusion of transmitters and modulators after synaptic spillover or extrasynaptic release in the local circuit regions binding to and activating mainly extrasynaptic neuronal and glial receptors in the neuroglial networks of the brain. Receptor-receptor interactions in G protein-coupled receptor (GPCR) heteromers play a major role, on dendritic spines and nerve terminals including glutamate synapses, in the integrative processes of the extrasynaptic signaling. Heteromeric complexes between GPCR and ion-channel receptors play a special role in the integration of the synaptic and extrasynaptic signals. Changes in extracellular concentrations of the classical synaptic neurotransmitters glutamate and GABA found with microdialysis is likely an expression of the activity of the neuron-astrocyte unit of the brain and can be used as an index of VT-mediated actions of these two neurotransmitters in the brain. Thus, the activity of neurons may be functionally linked to the activity of astrocytes, which may release glutamate and GABA to the extracellular space where extrasynaptic glutamate and GABA receptors do exist. Wiring transmission (WT) and VT are fundamental properties of all neurons of the CNS but the balance between WT and VT varies from one nerve cell population to the other. The focus is on the striatal cellular networks, and the WT and VT and their integration via receptor heteromers are described in the GABA projection neurons, the glutamate, dopamine, 5-hydroxytryptamine (5-HT) and histamine striatal afferents, the cholinergic interneurons, and different types of GABA interneurons. In addition, the role in these networks of VT signaling of the energy-dependent modulator adenosine and of endocannabinoids mainly formed in the striatal projection neurons will be underlined to understand the communication in the striatal cellular networks.

On the origin of the triplet puzzle of homologies in receptor heteromers: immunoglobulin triplets in different types of receptors

Based on our theory, we have discovered main triplets of amino acid residues in the GABAB1 receptor and several other neural receptors which seem to come from immunoglobulin chains and appear also as homologies in receptor heteromers. The obtained results strengthen our hypothesis that these triplets may "guide-and-clasp" receptor-receptor interactions playing a role, e.g., in neuroinflammation disorders.

Moonlighting characteristics of G protein-coupled receptors: focus on receptor heteromers and relevance for neurodegeneration

It is proposed that the moonlighting concept can be applied to G protein coupled receptors (GPCRs) as, obviously, they can carry out different types of functions. The same motifs in, for example, the third intracellular loop, can moonlight by switching between receptor-receptor interactions and interactions with signaling proteins such as G proteins or calmodulin. A "guide-and-clasp" manner of receptor-receptor interactions has been proposed where the "adhesive guides" may be the triplet homologies. As an example, the triplets AAR (or RAA) and AAE (or EAA) homologies in A(2A) R-D2 R heteromers may guide-and-clasp binding not only of the two protomers but also of calmodulin and G(i) . A beautiful moonlighting phenomenon in the A(2A) R-D2 R heteromer is that the positively charged D2 R N-terminal third intracellular loop epitope (VLRRRRKRVN) may switch between bindings to the negatively charged A(2A) R epitope (SAQEpSQGNT), localized in the medium segment of the C terminus of the A2A receptor to several negative epitopes of calmodulin. Furthermore, overlapping motifs may favor moonlighting to G(i/o) via inter alia electrostatic interaction between triplets AAR(in D2 R third intracellular loop) and AAE (G(i/alpha1) ) (and/or their symmetric variants) contributing to guide-and-clasp D2 R-G(i) interactions Thus, moonlighting in GPCR heteromers can take place via allosteric receptor-receptor interactions and is also described in D1 R-D2 R, D2 R-5-HT2 R,and A1 R-P2Y1 heteromers. Allosteric receptor-receptor interactions in GPCR-receptor tyrosine kinases (RTKs) heteromers and postulated ion channel receptor-RTK heteromers-like, for example, AMPA-NMDA-TrkB heteromers may lead to moonlighting of the participating GPCR and RTK protomers altering, for example, the pattern of the five major signaling pathways of the RTKs favoring MAPK and/or mTOR signaling with high relevance for neurodegenerative processes and depression induced atrophy of neurons. Moonlighting may also develop in the intracellular loops and C-terminal of the GPCRs as a result of dynamic allosteric interactions between different types of G proteins and other receptor interacting proteins in these domains of the receptor.

On the existence and function of galanin receptor heteromers in the central nervous system

Galanin receptor (GalR) subtypes 1-3 linked to central galanin neurons may form heteromers with each other and other types of G protein-coupled receptors in the central nervous system (CNS). These heteromers may be one molecular mechanism for galanin peptides and their N-terminal fragments (gal 1-15) to modulate the function of different types of glia-neuronal networks in the CNS, especially the emotional and the cardiovascular networks. GalR-5-HT1A heteromers likely exist with antagonistic GalR-5-HT1A receptor-receptor interactions in the ascending midbrain raphe 5-HT neuron systems and their target regions. They represent a novel target for antidepressant drugs. Evidence is given for the existence of GalR1-5-HT1A heteromers in cellular models with trans-inhibition of the protomer signaling. A GalR1-GalR2 heteromer is proposed to be a galanin N-terminal fragment preferring receptor (1-15) in the CNS. Furthermore, a GalR1-GalR2-5-HT1A heterotrimer is postulated to explain why only galanin (1-15) but not galanin (1-29) can antagonistically modulate the 5-HT1A receptors in the dorsal hippocampus rich in gal fragment binding sites. The results underline a putative role of different types of GalR-5-HT1A heteroreceptor complexes in depression. GalR antagonists may also have therapeutic actions in depression by blocking the antagonistic GalR-NPYY1 receptor interactions in putative GalR-NPYY1 receptor heteromers in the CNS resulting in increases in NPYY1 transmission and antidepressant effects. In contrast the galanin fragment receptor (a postulated GalR1-GalR2 heteromer) appears to be linked to the NPYY2 receptor enhancing the affinity of the NPYY2 binding sites in a putative GalR1-GalR2-NPYY2 heterotrimer. Finally, putative GalR-α2-adrenoreceptor heteromers with antagonistic receptor-receptor interactions may be a widespread mechanism in the CNS for integration of galanin and noradrenaline signals also of likely relevance for depression.

Fibroblast growth factor receptor 1- 5-hydroxytryptamine 1A heteroreceptor complexes and their enhancement of hippocampal plasticity

BACKGROUND

The hippocampus and its 5-hydroxytryptamine transmission plays an important role in depression related to its involvement in limbic circuit plasticity.

METHODS

The analysis was made with bioluminescence resonance energy transfer, co-immunoprecipitation, in situ proximity ligation assay, binding assay, in cell western and the forced swim test.

RESULTS

Using bioluminescence resonance energy transfer analysis, fibroblast growth factor receptor 1 (FGFR1)-5-hydroxytryptamine 1A (5-HT1A) receptor complexes have been demonstrated and their specificity and agonist modulation characterized. Their presence based on co-immunoprecipitation and proximity ligation assay has also been indicated in hippocampal cultures and rat dorsal hippocampal formation showing a neuronal location. In vitro assays on extracellular signal-regulated kinases 1 and 2 phosphorylation have shown synergistic increases in signaling on coactivation with fibroblast growth factor 2 (FGF2) and a 5-HT1A agonist, and dependent on the heteroreceptor interface. In vitro and in vivo studies also revealed a 5-HT1A agonist induced phosphorylation of FGFR1 and extracellular signal-regulated kinase 1/2 in rat hippocampus without changing FGF2 levels. Co-activation of the heteroreceptor also resulted in synergistic increases in extensions of PC12 cells and neurite densities and protrusions in primary hippocampal cultures dependent on the receptor interface. The combined acute and repeated intracerebroventricular treatment with FGF2 and 8-OH-DPAT was found to produce evidence of highly significant antidepressant actions in the forced swim test.

CONCLUSIONS

The findings indicate that neurotrophic and antidepressant effects of 5-HT in brain may, in part, be mediated by activation of the 5-HT1A receptor protomer in the hippocampal FGFR1-5-HT1A receptor complex enhancing the FGFR1 signaling.

Agonist-induced formation of FGFR1 homodimers and signaling differ among members of the FGF family

Fibroblast growth factor receptor 1 (FGFR1) is known to be activated by homodimerization in the presence of both the FGF agonist ligand and heparan sulfate glycosaminoglycan. FGFR1 homodimers in turn trigger a variety of downstream signaling cascades via autophosphorylation of tyrosine residues in the cytoplasmic domain of FGFR1. By means of Bioluminescence Energy Resonance Transfer (BRET) as a sign of FGFR1 homodimerization, we evaluated in HEK293T cells the effects of all known FGF agonist ligands on homodimer formation. A significant correlation between BRET(2) signaling and ERK1/2 phosphorylation was observed, leading to a further characterization of the binding and signaling properties of the FGF subfamilies. FGF agonist ligand-FGFR1 binding interactions appear as the main mechanism for the control of FGFR1 homodimerization and MAPK signaling which demonstrated a high correlation. The bioinformatic analysis demonstrates the interface of the two pro-triplets SSS (Ser-Ser-Ser) and YGS (Tyr-Gly-Ser) located in the extracellular and intracellular domain of the FGFR1. These pro-triplets are postulated participate in the FGFR1 homodimerization interface interaction. The findings also reveal that FGF agonist ligands within the same subfamily of the FGF gene family produced similar increases in FGFR1 homodimer formation and MAPK signaling. Thus, the evolutionary relationship within this gene family appears to have a distinct functional relevance.

On the existence of a possible A2A-D2-β-Arrestin2 complex: A2A agonist modulation of D2 agonist-induced β-arrestin2 recruitment

Given that coactivation of adenosine A(2A) (A(2A)R) and dopamine D(2) (D(2)R) receptors results in the coaggregation, cointernalization, and codesensitization of the A(2A)R and D(2)R and the role of scaffolding protein β-arrestin2 in the desensitization, internalization, and signaling of G-protein-coupled receptors, in this study we explored the ability of the A(2A)R agonist CGS21680 in A(2A)R-D(2)R-coexpressing cells to modulate the D(2)R agonist-induced recruitment of β-arrestin2 to the D(2)R by means of proximity-based bioluminescence resonance energy transfer (BRET(2)) and co-trafficking analysis. We found evidence that CGS21680 can increase the maximal BRET(2) signal between β-arrestin2(RLuc) and D(2L)R(GFP2) upon D(2)R activation, by increasing the potency of the D(2)R agonist to exert this action. In addition, this change was associated with an increased formation of cytoplasmic clusters containing β-arrestin2(GFP2) and D(2L)R(YFP) as seen from the co-trafficking analysis. Furthermore, the A(2A)R agonist advanced the time for the increase in Akt phosphorylation obtained with the D(2)R agonist. Finally, using a novel bioinformatics approach to predict the protein-protein interface, we have also found that amino acid pro-triplets TNY, LLS, RAF, and VSR may be crucial for the -induced β-arrestin2 recruitment by A(2A)R-D(2)R heteromers. Taken together, the results indicate that the antagonistic A(2A)R-D(2)R allosteric receptor-receptor interaction in A(2A)R-D(2)R heteromers favors β-arrestin2 recruitment to the D(2L)R protomer with subsequent cointernalization associated with a reduced time onset of Akt phosphorylation followed by a rapid dephosphorylation. Thus, β-arrestin2 action becomes more rapid and short-lasting and, in this way, mimics G-protein-mediated signaling.

Dopamine D2 and 5-hydroxytryptamine 5-HT(₂A) receptors assemble into functionally interacting heteromers

In view of the co-distribution of dopamine D(₂L)R and 5-hydroxytryptamine 5-HT(₂A) receptors (D(₂L)R and 5-HT(₂A)R, respectively) within inter alia regions of the dorsal and ventral striatum and their role as a target of antipsychotic drugs; in this study we assessed the potential existence of D(₂L)R-5-HT(₂A)R heteromers in living cells and the functional consequences of this interaction. Thus, by means of a proximity-based bioluminescence resonance energy transfer (BRET) approach we demonstrated that the D(₂L)R and the 5-HT(₂A)R form stable and specific heteromers when expressed in HEK293T mammalian cells. Furthermore, when the D(₂L)R-5-HT(₂A)R heteromeric signaling was analyzed we found that the 5-HT(₂A)R-mediated phospholipase C (PLC) activation was synergistically enhanced by the concomitant activation of the D(₂L)R as shown in a NFAT-luciferase reporter gene assay and a specific and significant rise of the intracellular calcium levels were observed when both receptors were simultaneously activated. Conversely, when the D(2L)R-mediated adenylyl cyclase (AC) inhibition was assayed we showed that costimulation of D(₂L)R and 5-HT(₂A)R within the heteromer led to inhibition of the D(₂L)R functioning, thus suggesting the existence of a 5-HT(₂A)R-mediated D(₂L)R trans-inhibition phenomenon. Finally, a bioinformatics study reveals that the triplet amino acid homologies LLT (Leu-Leu-Thr) and AIS (Ala-Ile-Ser) in TM1 and TM3, respectively of the D₂R-5-HT(₂A)R may be involved in the receptor interface. Overall, the presence of the D(₂L)R-5-HT(₂A)R heteromer in discrete brain regions is postulated based on the existence of D(₂L)R-5-HT(₂A) receptor-receptor interactions in living cells and their codistribution inter alia in striatal regions. Possible novel therapeutic strategies for treatment of schizophrenia should be explored by targeting this heteromer.

Why chemokines are cytokines while their receptors are not cytokine ones?

Chemotactic cytokines, or chemokines, are a large family of small proteins, which are distinguished from other cytokines in that they are the only members of the cytokine family that act on G-protein coupled receptor superfamily. This minireview tries to answer the title question by structure/function analysis of chemokines, cytokines, and their receptors. We also consider secretion of chemokines/cytokines in health and disease as well as expression of their receptors both in immune system and brain. Our analysis suggests that cytokine and chemokine receptors may share similar architecture with Toll-like receptors. Such similarity hints a similar way of their functioning as molecular switches controlled by protein-protein interactions. Hence, we pay attention to the related receptor-receptor associations and evolutionary conserved leucine-rich motifs.

Molecular networks of brain and immunity

Exciting complexity of natural phenomena can be based on rather simple biophysical principles. For example, the genetic code is based on a double-helix of DNA formed by planar geometry of weak hydrogen bounds. On the examples of cytokine networks, immune synapse, psychoneuroimmunology and systems biology, this review paper attempts to show how molecular networks both in brain and immunity can be studied using common principles of protein-protein interactions.

A simple mathematical model of cooperativity in receptor mosaics based on the “symmetry rule”

The phenomenon of receptor-receptor interactions was hypothesized about 20 years ago. It has been demonstrated by now that receptor-receptor interactions between G-protein coupled receptors (GPCRs) occur at plasma membrane level and result in the reciprocal modulation of their binding characteristics (i.e., cooperativity). One of the most important feature of this phenomenon is the concept of cluster of receptors, or receptor mosaic (RM). However, no proper mathematical approach has still been available to characterize RMs as far as their receptor composition, receptor topography and order of receptor activation inside the RM. This paper tries to fill the gap. A simple mathematical approach to the cooperativity in RMs formed by dimers of identical receptors and/or by iso-receptors is proposed. To this aim the so-called "symmetry rule" has been considered. This approach allows to describe by means of a simple energy function the effects of receptor composition (number of dimers), spatial organisation (respective location of the dimers) and order of activation (order according to which the single receptors are ligated) on the integrative cooperativity (index) of the RMs.

Receptor–Receptor Interactions, Receptor Mosaics, and Basic Principles of Molecular Network Organization: Possible Implications for Drug Development

The phenomenon of receptor-receptor interactions was hypothesized by Agnati and Fuxe in the 1980s, and several indirect proofs were provided in the following years by means of in vitro binding experiments and in vivo experiments in physiological and pathological animal models. This paper aims to outline some of the most important features and consequences of this phenomenon in the frame of the structural and functional aspects of molecular networks. In particular, the concepts of receptor mosaic (RM), and of horizontal and vertical molecular networks (HMNs, VMNs, respectively) are illustrated. To discuss some aspects of the functional organization of molecular networks, not only new data on protein-protein interactions but also the biochemical mechanism of cooperativity will be used. On this basis, some theoretical deductions can be drawn that allow a tentative classification of the RMs and the proposal of the extension of the concept of branching point introduced for enzymes to the possible switching role of some RMs in directing signals to various VMNs. Finally, the cooperativity phenomenon and the so-called symmetry rule will be used to introduce a proper mathematical approach that characterizes RMs as to their receptor composition, receptor topography, and order of receptor activation inside the RM. These new data on G protein-coupled receptors and molecular network organization indicate possible new approaches for drug development.