“Are melanocortin receptors constitutively active in vivo?”

Is the 5-hydroxytryptamine 7 Receptor Constitutively Active in the Vasculature? A Study in Veins/Vein

ABSTRACT

The 5-hydroxytryptamine 7 (5-HT 7 ) receptor is reported to have considerable constitutive activity when transfected into cells. Constitutive activity-receptor activity in the absence of known agonist-is important for understanding the contributions of a receptor to (patho)physiology. We test the hypothesis that the 5-HT 7 receptor possesses constitutive activity in a physiological situation. Isolated veins from male and female Sprague Dawley rats were used as models for measuring isometric force; the abdominal vena cava possesses a functional 5-HT 7 receptor that mediates relaxation, whereas the small mesenteric vein does not. Compounds reported to act as inverse agonists were investigated for their ability to cause contraction (moving a constitutively active relaxant receptor to an inactive state, removing relaxation). Compared with a vehicle control, clozapine, risperidone, ketanserin, and SB269970 caused no contraction in the isolated male abdominal vena cava. By contrast, methiothepin caused a concentration-dependent contraction of the male but not female abdominal vena cava, although with low potency (-log EC 50 [M] = 5.50 ± 0.45) and efficacy (∼12% of contraction to endothelin-1). Methiothepin-induced contraction was not reduced by the 5-HT 7 receptor antagonist (SB269970, 1 μM, not active in the vena cava). These same compounds showed little to no effect in the isolated mesenteric vein. We conclude that the 5-HT 7 receptor in the isolated veins of the Sprague Dawley rat does not possess constitutive activity. We raise the question of the physiological relevance of constitutive activity of this receptor important to such diverse physiological functions as sleep, circadian rhythm, temperature, and blood pressure regulation.

The constitutive activity of melanocortin-4 receptors in cAMP pathway is allosterically modulated by zinc and copper ions

Melanocortin-4 receptors (MC₄ R) are unique among G-protein-coupled receptors (GPCRs) as they have endogenous ligands that can exhibit inverse agonistic properties in the case of elevated basal activity. It is known that the constitutive activity of GPCRs strongly affects the ligand-dependent physiological responses, but little is known about these regulatory mechanisms. Since several metal ions have been shown to be important modulators of the signal transduction of GPCRs, we hypothesized that metal ions regulate the basal activity of MC₄ Rs. Implementation of a fluorescence anisotropy assay and novel redshifted fluorescent peptides enabled kinetic characterization of ligand binding to MC₄ R expressed on budded baculoviruses. We show that Ca²⁺ is required for high-affinity ligand binding, but Zn²⁺ and Cu²⁺ in the presence of Ca²⁺ behave as negative allosteric modulators of ligand binding to MC₄ R. FRET-based cAMP biosensor was used to measure the activation of MC₄ R stably expressed in CHO-K1 cells. At low micromolar concentrations, Zn²⁺ caused MC₄ R-dependent activation of the cAMP pathway, whereas Cu²⁺ reduced the activity of MC₄ R even below the basal level. These findings indicate that at physiologically relevant concentrations can Zn²⁺ and Cu²⁺ function as MC₄ R agonists or inverse agonists, respectively. This means that depending on the level of constitutive activity induced by Zn²⁺ ions, the pharmacological effect of orthosteric ligands of MC₄ R can be switched from a partial to an inverse agonist. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. More information about the Open Science badges can be found at https://cos.io/our-services/open-science-badges/.

Melanocortin-4 receptor-regulated energy homeostasis

The melanocortin system provides a conceptual blueprint for the central control of energetic state. Defined by four principal molecular components--two antagonistically acting ligands and two cognate receptors--this phylogenetically conserved system serves as a prototype for hierarchical energy balance regulation. Over the last decade the application of conditional genetic techniques has facilitated the neuroanatomical dissection of the melanocortinergic network and identified the specific neural substrates and circuits that underscore the regulation of feeding behavior, energy expenditure, glucose homeostasis and autonomic outflow. In this regard, the melanocortin-4 receptor is a critical coordinator of mammalian energy homeostasis and body weight. Drawing on recent advances in neuroscience and genetic technologies, we consider the structure and function of the melanocortin-4 receptor circuitry and its role in energy homeostasis.

Constitutive activity in melanocortin-4 receptor: biased signaling of inverse agonists

The melanocortin-4 receptor (MC4R) is a critical regulator of energy homeostasis, including both energy intake and energy expenditure. It mediates the actions of a number of hormones on energy balance. The endogenous ligands for MC4R include peptide agonists derived from processing of proopiomelanocortin and the antagonist Agouti-related peptide (AgRP). Wild-type MC4R has some basal (constitutive) activity. Naturally occurring and laboratory-generated mutations have been identified, which results in either increased or decreased basal activities. Impaired basal signaling has been suggested to be a cause of dysregulated energy homeostasis and early-onset obesity, although several constitutively active mutations have also been identified from obese patients. AgRP and several small-molecule antagonists have been shown to be inverse agonists in the Gs-cAMP pathway. However, in the extracellular signal-regulated kinase (ERK) 1/2 pathway, we showed that these inverse agonists are potent agonists, demonstrating convincingly that they are biased ligands. We also showed that some mutations that do not cause constitutive activation in the Gs-cAMP pathway cause constitutive activation in the ERK1/2 pathway, suggesting that they are biased receptors. The physiological and potential pathophysiological relevance of the biased constitutive signaling in MC4R and therapeutic potential remain to be investigated.