As research in this field moves forward, however, trying to describe receptor heteromers is becoming a significant conceptual challenge. The literature presents a bewildering array of terms, and there is a need for standardization based on operationally clear definitions. Thus, we present a proposal for a consensus nomenclature, based on a classical definition of ‘receptor’ and designed to encompass not only G protein–coupled receptors (GPCRs) but also other known transmembrane receptors. Based on the proposed nomenclature, we also give recommendations for the identification of receptor heteromers in native tissues. Research on receptor heteromers is poised to revolutionize basic tenets of pharmacology and take rational drug development to a new level of specificity and efficacy. We envision that the adoption of the proposed nomenclature system and experimental criteria will advance communication (and thereby progress) in the field.

It is well known that receptor proteins often have quaternary structures; namely, they represent an assembly of two or more different polypeptide chains, called subunits⁶, that may or may not derive from the same gene. We propose that the term ‘heteromeric receptor’ be used to define a dimeric or oligomeric receptor for which the minimal functional unit is composed of two or more different subunits that are not functional on their own. This definition would apply to ligand-gated ion channels (ionotropic receptors) such as glutamate N-methyl-D-aspartate (NMDA) receptors (Fig. 1a) or most nicotinic acetylcholine receptors^8,9. The term would also be used for some GPCRs and some tyrosine kinase receptors, such as receptors for glial cell line–derived neurotrophic factor (GDNF) family ligands, in which subunits are responsible either for the association with the ligand or for the catalytic response¹⁰. Similarly, the γ-aminobutyric acid B (GABA_B) receptor, a GPCR, is composed of two seven-transmembrane (7TM) proteins, GABA_B1 and GABA_B2, that are involved in ligand recognition and cell signaling, respectively². According to the definition of receptor cited above, neither subunit of the GABA_B receptor is a receptor because neither protomer is fully functional on its own. Hence, the GABA_B receptor should be referred to as a heteromeric GPCR (Fig. 1b). Some taste receptors, for which genetic deletion of one of the subunits leads to suppression of the receptor function², would also be called heteromeric GPCRs. If the receptor subunits are identical, they would constitute a ‘homomeric receptor’. This is the case for some ionotropic receptors, such as the α₇ nicotinic acetylcholine receptor⁹, and also some tyrosine kinase receptors, such as those for neurotrophins, which require a ligand-induced dimerization or an alteration of a constitutive dimerization interface to become functional¹¹.

As defined thus far, neither the term ‘heteromeric receptor’ nor the term ‘receptor heteromer’ would apply to a hetero-oligomeric species that in this context would be a protein complex composed of a receptor (as defined above) plus another membrane protein that modifies the biochemical properties of the receptor, such as, for example, some GPCRs associated with receptor activity–modifying proteins (RAMPs) or with ‘orphan GPCRs’. According to the definition of receptor, true orphan GPCRs, whose activities are likely controlled by ligand-independent mechanisms, might be better referred to as ‘orphan 7TM proteins’ to distinguish them from those that are likely to be regulated by an as-yet unidentified ligand¹⁵. Obviously, an orphan 7TM protein would be reclassified as a GPCR if a ligand were identified, so the former term can only be applied provisionally. An example of this type of hetero-oligomer is the orphan 7TM protein GPR50 binding to the melatonin MT₁ receptor, thereby modifying its functional properties¹⁵ (Fig. 1e). Additionally, three different RAMPs (RAMP1, RAMP2 and RAMP3) that are single transmembrane proteins have been identified so far¹⁶. The calcitonin (CT) receptor has high affinity for CT, and its association with any of the RAMPs results in a different receptor with high affinity for the CT-family neuropeptide amylin (AMY)¹⁶ (Fig. 1f). We propose for those cases that do not fit the definitions of either heteromeric receptor or receptor heteromer that the name of the associated modifying protein be added to the name of the receptor (for example, MT₁-GPR50 receptor). When the associated modifying protein changes the ligands that are preferentially recognized by the complex, we propose to continue naming the complex based on the ligand that binds preferentially while specifying the identity of the proteins contributing to the complex (for example, the AMY₁ receptor, formed by CT receptor and RAMP1).

Direct identification could be achieved by taking advantage of selective probes (for example, specific antibodies or labeled selective ligands) that could discriminate between the receptor heteromer and other configurations of the individual components. However, so far, specific antibodies have only been reported for cannabinoid CB₁ receptor homomers²¹, and no specific receptor heteromer ligand has yet been found. The compound 6′-guanidinonaltrindole (6′-GNTI) has been shown to be a selective agonist for opioid δ-κ receptor heteromers, but (albeit with lower potency) it also acts as a δ receptor antagonist²². As a result, we must rely on indirect approaches for the identification of a receptor heteromer in native tissues by discovering characteristic biochemical signatures and elucidating the receptor domains or epitopes that determine the receptor heteromer’s quaternary structure. For example, a biochemical characteristic could be first identified in an artificial cell system, which can then be used as a ‘biochemical fingerprint’ to demonstrate its presence in the native tissue. Importantly, detection of this fingerprint must be contingent upon true heteromerization and not the mere co-expression of the receptors.

A strong suggestion that a biochemical fingerprint is specific for a receptor heteromer can be obtained by showing that it is abolished or altered when the heteromerization is disrupted, or alternatively when the quaternary structure of the heteromer is significantly modified without disrupting heteromerization. This could be shown with biophysical techniques (for instance, a significant decrease in the RET signal). This strategy requires identification of the domains or epitopes (of at least one of the receptors) that form the interaction surface in the heteromer; this may allow the construction of appropriate mutant or chimeric receptors, or the design of peptides that can selectively occupy and disrupt the receptor heteromer interface. A better delineation of this interface may also allow for a more productive approach using transgenic animals. It might be possible, for example, to generate a knock-in animal expressing a mutated receptor that fails to heteromerize with the other units of the receptor heteromer in transfected cells. In this paradigm, a differential ability to co-immunoprecipitate the two receptors from wild-type but not from the knock-in animals would support the existence of the receptor heteromer in native tissue, as long as the distribution of the mutated receptor does not change relative to that of the wild-type receptor, and as long as the expression of the partner receptor also remains unaltered.

Allosteric interactions between receptor units have been considered a common biochemical characteristic of a number of receptor heteromers^1,3,23. These interactions were initially called “intramembrane receptor-receptor interactions” because they were first observed in crude membrane preparations of brain tissue¹. In the typical intramembrane receptor-receptor interaction, stimulation of one receptor leads to changes in the binding characteristics of an adjacent receptor, such as decreased or increased affinity for an agonist. Using extensively washed membrane preparations, this constitutes a strong indication that the ligand triggers an intermolecular change from which a new biochemical property, characteristic of the receptor heteromer, has now emerged. In many cases, the same kind of interaction has been shown in both cotransfected cells and native tissues; this could be interpreted as an indication of the existence of receptor heteromers in vivo (see refs. ^24,25 for recent examples). However, the major challenge is to demonstrate that the direct physical interaction of the two receptors is necessary for the modification of their signaling. Thus, for a true allosteric interaction in the receptor heteromer, the biochemical signature should be characteristic of the receptor heteromer and not of, for instance, downstream cross-talk effects at the level of G proteins or other signaling effectors, as it has been recently shown for a receptor heteromer consisting of serotonin 5-HT_2A and glutamate metabotropic mGlu₂ receptors²⁵. Nevertheless, an allosteric interaction in the receptor heteromer can in principle be identified by its particularly fast kinetics. For example, in the adrenergic α_2A-opioid μ receptor heteromer, allosteric effects took less than 500 ms, which is the time required for G protein activation by a receptor²⁶. This makes indirect (G protein–mediated) effects very unlikely.

Different approaches are being explored for the selective targeting of receptor heteromers. A current strategy is to screen for compounds that selectively target one of the receptors that constitute the receptor heteromer²². Another approach is to develop bivalent ligands that can interact simultaneously and specifically with both receptors in a receptor heteromer. In a recent study, an opioid μ receptor agonist–opioid δ receptor antagonist bivalent compound was developed by linking two moieties with a spacer²⁹. Recent studies suggest that opioid δ-μ receptor heteromers modulate opioid μ receptor–mediated tolerance and dependence and that opioid δ-μ receptor bivalent ligands of precise spacer length exhibit a higher potency than morphine and the potential to achieve analgesia without tolerance and dependence²⁹. However, such compounds, due to their large size, do not exhibit optimal drug-like properties³⁰, which could be overcome by using a combination of small molecules that selectively target each unit in the receptor heteromer.