HETEROMERIZATION OF G-PROTEIN-COUPLED RECEPTORS: IMPLICATIONS FOR CENTRAL NERVOUS SYSTEM FUNCTION AND DYSFUNCTION
Co-Chairs:
Sergi Ferré, M.D., Ph.D.
National Institute on Drug Abuse
David Shurtleff, Ph.D.
National Institute on Drug Abuse
Overview
During the present decade, evidence has accumulated indicating
that heteromerization of neurotransmitter receptors confers
functional entities that possess different biochemical characteristics
with respect to the individual components of the heteromer. This
symposium will update the roles of dopamine, opioid, and
adenosine receptor heteromers in the central nervous system, and
their implications for neuropsychiatric disorders, including drug
addiction.
Basic Concepts in G-Protein-Coupled Receptors Homodimerization and Heterodimerization
Rafael Franco, Ph.D.
Until recently, heptahelical G-protein-coupled receptors (GPCRs) were considered to be expressed as monomers on the cell surface of neuronal and non-neuronal cells. It is now becoming evident that this view must be overtly changed, because these receptors can form homodimers, heterodimers, and higher order oligomers on the plasma membrane. We discuss some of the basics and some new concepts of receptor homo- and heteromerization.
Dimers-oligomers modify pharmacology, trafficking, and signaling of receptors. First of all, GPCR homodimers must be considered as the main molecules that are targeted by neurotransmitters or by drugs. Thus, binding data must be fitted to dimer-based models. In these models, it is considered that the conformational changes transmitted within the dimer molecule lead to cooperativity. Cooperativity must be taken into account in the binding of agonist and antagonist drugs, and also in the binding of the so-called allosteric modulators. Cooperativity results from the intramolecular crosstalk in the homodimer. As an intramolecular crosstalk in the heterodimer, the binding of one neurotransmitter to one receptor often affects the binding of the second neurotransmitter to the partner receptor. Coactivation of the two receptors in a heterodimer can completely change the signaling pathway triggered by the neurotransmitter, as well as the trafficking of the receptors. Heterodimer-specific drugs or dual drugs able to simultaneously activate the two receptors in the heterodimer emerge as novel and promising drugs for a variety of central nervous system therapeutic applications.
Heteromers of Dopamine Receptors
Susan R. George, M.D.
[Slides not available]
Signaling through each of the five dopamine receptors has been shown to occur primarily via the activation or attenuation of adenylyl cyclase activity through coupling to Gs- and Gi-family G proteins. There have been reports of a Gq-coupled D1-like dopamine receptor in the brain, but the definitive identification of this entity has proven elusive. We have shown that hetero-oligomerization of D1
and D2 receptors generates a novel signaling complex, distinct from D1 or D2 homo-oligomers.
We have identified a novel hetero-oligomeric dopamine-signaling complex in the brain, which consists of D1 and D2 receptors, that couples to Gq/11, generates robust intracellular calcium release when both receptors are coactivated, and increases phosphorylation of calmodulin-dependent protein kinase IIa. This is the first indication that dopamine can directly activate the fast-signaling calcium mechanism in the brain. Hetero-oligomerization has novel implications for signal transduction, such as increasing the repertoire of G-protein-coupled receptors signaling pathways and effector mechanisms available for endogenous ligands. The D1–D2 hetero-oligomer will have considerable significance for understanding the physiology of dopamine systems in the brain, and will also be a novel target for drug discovery once their physiological functions have been fully elucidated.
Modulation of Function by Opioid Receptor Dimerization
Lakshmi A. Devi, Ph.D.
[Slides not available]
Opioid receptors belong to the super family of G-protein-coupled receptors characterized by their seven transmembrane domains. The activation of these receptors by narcotic analgesics or by endogenous opioid peptides leads to the activation of inhibitory G-proteins followed by the activation of multiple signal transduction pathways. A number of investigations have suggested that opioid receptor types interact with each other. Previous studies using receptor-selective antagonists, antisense oligonucleotides, or animals lacking opioid receptors have suggested that these interactions modulate receptor activity. We examined opioid receptor interactions (homotypic and heterotypic) using biochemical, biophysical, and pharmacological techniques.
We show that µ and δ opioid receptors physically associate with each other to form heterodimers that exhibit altered agonist affinity, efficacy, and/or potency. Using receptor type-selective antibodies, we immunoisolated interacting complexes from heterologous cells as well as endogenous tissue. Finally, we show that chronic morphine treatment upregulates the levels of µ-δ heterodimers in vivo and leads to the formation of new signaling complexes, which in turn lead to a switch in signaling by µ-δ heterodimers. Taken together, these results suggest that µ-δ heterodimers can be used as unique targets for the development of novel drugs and therapies for the treatment of chronic pain and other pathologies.
Adenosine Receptor Heteromers and Their Integrative Role in Striatal Function
Sergi Ferré, M.D., Ph.D.
By analyzing the functional role of adenosine receptor heteromers, a series of new concepts was reviewed that should modify our classical views of neurotransmission in the central nervous system. Neurotransmitter receptors cannot be considered as single functional units anymore. Heteromerization of neurotransmitter receptors confers functional entities, which possess different biochemical characteristics with respect to the individual components of the heteromer. Some of these characteristics can be used as a “biochemical fingerprint” to identify neurotransmitter receptor heteromers in the central nervous system. This is exemplified by changes in binding characteristics that are dependent on co-activation of the receptor units of different adenosine receptor heteromers. Neurotransmitter receptor heteromers can act as “processors” of computations that modulate cell signaling, sometimes critically involved in the control of pre- and post-synaptic neurotransmission. For instance, the adenosine A1–A2A receptor heteromer acts as a concentration-dependent switch that controls striatal glutamatergic neurotransmission. Neurotransmitter receptor heteromers play a particularly important integrative role in the "local module" (the minimal portion of one or more neurons and/or one or more glial cells that operate as independent integrative units), where they act as processors mediating computations that convey information from diverse volume-transmitted signals. For instance, the adenosine A2A-dopamine D2 receptor heteromers work as integrators of two different neurotransmitters in the striatal spine module.
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