Competitive and silent antagonism of recombinant 5-HT1B receptors by amiloride

Functional Characterization of Sodium Channel Inhibitors at the Delta-Opioid Receptor

Existing pharmacotherapies acting on the opioid receptor system have been extensively used to treat chronic pain and addictive disorders. Nevertheless, the adverse side effects associated with opioid therapy underscore the need for concerted measures to develop safer analgesics. A promising avenue of research stems from the characterization of a sodium-dependent allosteric regulation site housed within the delta-opioid receptor and several other G protein-coupled receptors (GPCRs), thereby revealing the presence of a cluster of sodium and water molecules lodged in a cavity thought to be present only in the inactive conformation of the receptor. Studies into the structure-function relationship of said pocket demonstrated its critical involvement in the functional control of GPCR signaling. While the sodium pocket has been proposed to be present in the majority of class A GPCRs, the shape of this allosteric cavity appears to have significant structural variation among crystallographically solved GPCRs, making this site optimal for the design of new allosteric modulators that will be selective for opioid receptors. The size of the sodium pocket supports the accommodation of small molecules, and it has been speculated that promiscuous amiloride and 5'-substituted amiloride-related derivatives could target this cavity within many GPCRs, including opioid receptors. Using pharmacological approaches, we have described the selectivities of 5'-substituted amiloride-related derivatives, as well as the hitherto undescribed activity of the NHE1 inhibitor zoniporide toward class A GPCRs. Our investigations into the structural features of the delta-opioid receptor and its ensuing signaling activities suggest a bitopic mode of overlapping interactions involving the orthosteric site and the juxtaposed Na⁺ pocket, but only at the active or partially active opioid receptor.

Harnessing Ion-Binding Sites for GPCR Pharmacology

Endogenous ions play important roles in the function and pharmacology of G-protein coupled receptors (GPCRs). Historically the evidence for ionic modulation of GPCR function dates to 1973 with studies of opioid receptors, where it was demonstrated that physiologic concentrations of sodium allosterically attenuated agonist binding. This Na+-selective effect was distinct from effects of other monovalent and divalent cations, with the latter usually counteracting sodium's negative allosteric modulation of binding. Since then, numerous studies documenting the effects of mono- and divalent ions on GPCR function have been published. While ions can act selectively and nonselectively at many sites in different receptors, the discovery of the conserved sodium ion site in class A GPCR structures in 2012 revealed the unique nature of Na+ site, which has emerged as a near-universal site for allosteric modulation of class A GPCR structure and function. In this review, we synthesize and highlight recent advances in the functional, biophysical, and structural characterization of ions bound to GPCRs. Taken together, these findings provide a molecular understanding of the unique roles of Na+ and other ions as GPCR allosteric modulators. We will also discuss how this knowledge can be applied to the redesign of receptors and ligand probes for desired functional and pharmacological profiles. SIGNIFICANCE STATEMENT: The function and pharmacology of GPCRs strongly depend on the presence of mono and divalent ions in experimental assays and in living organisms. Recent insights into the molecular mechanism of this ion-dependent allosterism from structural, biophysical, biochemical, and computational studies provide quantitative understandings of the pharmacological effects of drugs in vitro and in vivo and open new avenues for the rational design of chemical probes and drug candidates with improved properties.

Allosteric modulation of G protein-coupled receptors by amiloride and its derivatives. Perspectives for drug discovery?

The function of G protein‐coupled receptors (GPCRs) can be modulated by compounds that bind to other sites than the endogenous orthosteric binding site, so‐called allosteric sites. Structure elucidation of a number of GPCRs has revealed the presence of a sodium ion bound in a conserved allosteric site. The small molecule amiloride and analogs thereof have been proposed to bind in this same sodium ion site. Hence, this review seeks to summarize and reflect on the current knowledge of allosteric effects by amiloride and its analogs on GPCRs. Amiloride is known to modulate adenosine, adrenergic, dopamine, chemokine, muscarinic, serotonin, gonadotropin‐releasing hormone, GABA_B, and taste receptors. Amiloride analogs with lipophilic substituents tend to be more potent modulators than amiloride itself. Adenosine, α‐adrenergic and dopamine receptors are most strongly modulated by amiloride analogs. In addition, for a few GPCRs, more than one binding site for amiloride has been postulated. Interestingly, the nature of the allosteric effect of amiloride and derivatives varies considerably between GPCRs, with both negative and positive allosteric modulation occurring. Since the sodium ion binding site is strongly conserved among class A GPCRs it is to be expected that amiloride also binds to class A GPCRs not evaluated yet. Investigating this typical amiloride‐GPCR interaction further may yield general insight in the allosteric mechanisms of GPCR ligand binding and function, and possibly provide new opportunities for drug discovery.

Allosteric sodium in class A GPCR signaling

Despite their functional and structural diversity, G-protein-coupled receptors (GPCRs) share a common mechanism of signal transduction via conformational changes in the seven-transmembrane (7TM) helical domain. New major insights into this mechanism come from the recent crystallographic discoveries of a partially hydrated sodium ion that is specifically bound in the middle of the 7TM bundle of multiple class A GPCRs. This review discusses the remarkable structural conservation and distinct features of the Na(+) pocket in this most populous GPCR class, as well as the conformational collapse of the pocket upon receptor activation. New insights help to explain allosteric effects of sodium on GPCR agonist binding and activation, and sodium's role as a potential co-factor in class A GPCR function.

Investigations of the signaling cascade involved in diuretic hormone stimulation of Malpighian tubule fluid secretion in Rhodnius prolixus

In insects, the excretory system is comprised of the Malpighian tubules (MTs) and the hindgut, which collectively function to maintain ionic and osmotic balance of the haemolymph and rid the organism of toxic compounds or elements in excess. Secretion by the Malpighian tubules of insects is regulated by a variety of hormones including peptidergic factors as well as biogenic amines. In Rhodnius prolixus, two endogenous diuretic hormones have been identified; the biogenic amine serotonin (5-hydroxytryptamine, 5-HT) and the corticotropin releasing factor-related peptide, RhoprCRF. Both factors significantly increase secretion by MTs and are known to elevate intracellular levels of cAMP. Interestingly, applying sub-maximal doses of these two diuretic factors in combination on isolated MTs in vitro reveals synergistic effects as rates of fluid secretion are significantly higher than would be expected if rates of secretion from MTs treated with each factor alone were summed. This observed synergism suggests that different downstream targets may be activated by the two diuretic factors, but that some cellular elicitors may be shared since cAMP is elevated in response to either diuretic hormone. This study investigated the signaling cascade involved in the diuretic hormone regulation of Malpighian tubule fluid secretion. Bioassays were performed in physiological as well as modified salines (e.g. calcium-free) alone or in the presence of a variety of pharmacological compounds that interfere with prospective intracellular targets, such as the apical cation/H(+) exchanger. Intriguingly, only amiloride yielded differential effects on the two diuretics with 5HT-stimulated secretion being blocked, whereas in contrast, RhoprCRF-stimulated secretion was unaffected. In addition, experiments examining the role of extracellular and intracellular calcium on fluid secretion rate showed that both diuretics are dependent on intracellular calcium availability. Finally, fluid secretion stimulated by either diuretic hormone was also sensitive to inhibition of cAMP-dependent protein kinase A. Taken together, these results suggest that each diuretic hormone activates pathways dependent upon intracellular calcium and cAMP.

The role of a sodium ion binding site in the allosteric modulation of the A(2A) adenosine G protein-coupled receptor

The function of G protein-coupled receptors (GPCRs) can be modulated by a number of endogenous allosteric molecules. In this study, we used molecular dynamics, radioligand binding, and thermostability experiments to elucidate the role of the recently discovered sodium ion binding site in the allosteric modulation of the human A(2A) adenosine receptor, conserved among class A GPCRs. While the binding of antagonists and sodium ions to the receptor was noncompetitive in nature, the binding of agonists and sodium ions appears to require mutually exclusive conformational states of the receptor. Amiloride analogs can also bind to the sodium binding pocket, showing distinct patterns of agonist and antagonist modulation. These findings suggest that physiological concentrations of sodium ions affect functionally relevant conformational states of GPCRs and can help to design novel synthetic allosteric modulators or bitopic ligands exploiting the sodium ion binding pocket.