The melanocortin-2 receptor accessory protein (MRAP) family interacts with and regulates the signaling of diverse G protein-coupled receptors (GPCRs). MRAP2 modifies the signaling of three distinct GPCRs, melanocortin-4 receptor (MC4R), MC3R, and ghrelin receptor (GHSR), all essential for appetite regulation. The nature of MRAP2/GPCR complexes and whether there are shared mechanisms for complex assembly, critical structural regions, or consistent effects on receptor signaling remains unknown. Here, we show that all three GPCRs preferentially interact with MRAP2 as 1:1 complexes and MRAP2 binding disrupts GPCR homodimerization. MRAP2 interacts with shared receptor transmembrane regions to promote GPCR signaling and impairs β-arrestin-2 recruitment to prolong signaling and delay internalization. Deletion of the MRAP2 cytoplasmic region impairs GPCR signaling by modulating constitutive activity. Human MRAP2 variants associated with overweight/obesity modify the constitutive activity of all three GPCRs. Thus, MRAP2 regulates GPCR function using shared molecular mechanisms, and we provide further evidence for the importance of GHSR constitutive activity.
Copyright © 2026 The Author(s). Published by Elsevier Inc. All rights reserved.

Excitatory peptide (EP) and CCHamide (CCHa) are protostome neuropeptides originally identified in lophotrochozoans (including annelids and mollusks) and arthropods, respectively, and are homologous to the deuterostome endothelin (ET) and gastrin-releasing peptide (GRP)/neuromedin-B (NMB) systems. These peptides are brain-gut peptides: in vertebrates, GRP/NMB function as satiety peptides, whereas arthropod CCHa displays species-specific actions, either inhibiting or promoting feeding. However, the mechanisms by which these peptides modulate feeding circuits remain unknown. Here, we investigated the EP/CCHa signaling pathway in Aplysia, a mollusk with a well-defined feeding circuit. We identified a single precursor encoding Aplysia EP/CCHa (apEP/CCHa). Mass spectrometry demonstrated that an apEP/CCHa peptide is present in the central ganglia. In situ hybridization and immunohistochemistry revealed apEP/CCHa-positive neurons in the CNS, immunopositive cells in the gut, and immunopositive fibers in the gut-innervating esophageal nerve. To identify potential targets, we cloned two novel apEP/CCHa receptors. Phylogenetically, one receptor clusters with lophotrochozoan EP/CCHa receptors, whereas the other unexpectedly clusters with arthropod receptors, suggesting independent lineages for the two receptors. Single-cell RNA sequencing showed that both receptors are expressed in the key feeding central pattern generator (CPG) interneurons B20 and B34. Functionally, apEP/CCHa inhibited food intake in vivo and converted ingestive motor programs to egestive ones in vitro. At the circuit level, apEP/CCHa modulated excitability of B20 and B34, and two additional interneurons (B40, B64). In summary, we demonstrate that apEP/CCHa is a brain-gut peptide that functions as a satiation signal, and identify specific feeding CPG elements through which apEP/CCHa regulates motivational state transitions.
Copyright © 2026 The Authors. Published by Elsevier Inc. All rights reserved.

The heterodimeric GABAB receptor, composed of GB1 and GB2 subunits, is a metabotropic G protein-coupled receptor (GPCR) activated by the neurotransmitter GABA. GABA binds to the extracellular domain of GB1 to activate G proteins through GB2. Here we show that GABAB receptors can be activated by mechanical forces, such as traction force and shear stress, in a GABA-independent manner. This GABA-independent mechano-activation of GABAB receptor is mediated by a direct interaction between integrins and the extracellular domain of GB1, indicating that GABAB receptor and integrin form a mechano-transduction complex. Mechanistically, shear stress promotes the binding of integrin to GB1 and induces an allosteric re-arrangement of GABAB receptor transmembrane domains towards an active conformation, culminating in receptor activation. Furthermore, we demonstrate that shear stress-induced GABAB receptor activation plays a crucial role in astrocyte remodeling. These findings reveal a role of GABAB receptor in mechano-transduction, uncovering a ligand-independent activation mechanism for GPCRs.
© 2025. The Author(s).

Neuropeptides play a crucial role in modulating behavior, with certain signaling systems, such as those for Neuropeptide Y and Cholecystokinin (CCK), being clearly conserved across protostomes and deuterostomes. However, evolutionary relationships for other signaling systems are less evident. Buccalin (Buc) and allatostatin A (AstA) were independently discovered in the mollusk Aplysia californica and the arthropod Diploptera punctate and are now recognized as homologous. These peptides share a conserved C-terminal Y/F-X-F-G-L/I-amide motif. Bioinformatic evidence further suggests homology between this signaling system and the deuterostome galanin (GAL) and kisspeptin (KISS) signaling systems. Nonetheless, uncertainties persist, particularly due to the limited characterization of Buc/AstA receptors in lophotrochozoans, the superphylum that encompasses mollusks and annelids. To date, only one AstA receptor has been experimentally characterized in the annelid Platynereis dumerilii, while no receptors have been characterized in Aplysia. In this study, we combined bioinformatics, molecular biology, and a cell-based assay to identify an Aplysia buccalin receptor, designated as apBuc/AstA-R. The 19 mature buccalin peptides encoded by the buccalin precursor activated apBuc/AstA-R in a dose-dependent manner, with EC50 values ranging from 23 to 320 nM. Importantly, the potencies of buccalin A and B corresponded to their previously characterized physiological effects. To assess cross-activity, we tested whether Drosophila AstAs and human KISS/GAL can activate apBuc/AstA-R. While receptor activation was observed with Drosophila AstAs and human KISS, human GAL was not active. Phylogenetic and chromosomal gene analyses reinforced the idea that the protostome Buc/AstA signaling system is orthologous to the deuterostome GAL systems and paralogous with the KISS system. Additionally, bioinformatic data suggest the presence of a KISSR-like receptor in Aplysia, though it was not activated by either human KISS/GAL or by Aplysia and Drosophila Buc/AstAs. Overall, our study provides the first comprehensive characterization of the Buc/AstA signaling system in Aplysia and provides further evidence for its close evolutionary relationship with the deuterostome GAL and KISS signaling systems.
© 2025 The Authors. Published by American Chemical Society.

The melanocortin-4 receptor is a G protein-coupled receptor and a key regulator of appetite and metabolism. It can interact with the melanocortin-receptor accessory protein 2, a single transmembrane helix protein known to interact with several different G protein-coupled receptors. However, the consequences of this interaction are not completely understood. Here we report that co-expression of melanocortin-receptor accessory protein 2 has multiple effects on the melanocortin-4 receptor: it enhances G protein-mediated signaling and simultaneously impairs β-arrestin2 recruitment and, consequently, internalization. In addition, co-expression of melanocortin-receptor accessory protein 2 leads to an increased number of monomers of melanocortin-4 receptor by disrupting receptor oligomers. A structural homology model of the active state melanocortin-4 receptor - melanocortin-receptor accessory protein 2 - Gαs complex suggests interaction sites that are relevant for receptor activation. Our data indicate that melanocortin-receptor accessory protein 2 is an accessory protein that interacts with and influences melanocortin-4 receptor structure, biasing its signaling towards G protein-mediated effects.
© 2025. The Author(s).