Dawn of a New RAMPage

Abstract

Receptor activity-modifying proteins (RAMPs) interact with G-protein-coupled receptors (GPCRs) to modify their functions, imparting significant implications upon their physiological and therapeutic potentials. Resurging interest in identifying RAMP-GPCR interactions has recently been fueled by coevolution studies and orthogonal technological screening platforms. These new studies reveal previously unrecognized RAMP-interacting GPCRs, many of which expand beyond Class B GPCRs. The consequences of these interactions on GPCR function and physiology lays the foundation for new molecular therapeutic targets, as evidenced by the recent success of erenumab. Here, we highlight recent papers that uncovered novel RAMP-GPCR interactions, human RAMP-GPCR disease-causing mutations, and RAMP-related human pathologies, paving the way for a new era of RAMP-targeted drug development.

Keywords: CGRP; G-protein-coupled receptor; adrenomedullin; coevolution; erenumab; receptor activity-modifying proteins (RAMPs).

Figure 1:. Role of RAMP1-CLR in disease

(A) Graphical representation of the CGRP receptor components CLR and RAMP1. (B) Schematic depicting release of CGRP from peripheral nerves and its potential targets including lymphatic endothelial cells, CNS nerves, and smooth muscle cells. (C) CGRP can also stimulate CNS nerves, and other cell types in the CNS, to incite migraines. Erenumab, the first FDA-approved monoclonal antibody against a GPCR, targets the RAMP1-CLR receptor to help treat migraine. (D) CGRP can bind to its receptor on lymphatic endothelial cells, which is linked to lacteal innervation and lipid uptake in the small intestine. (E) CGRP can also bind to receptors on smooth muscle cells, which can have physiological effects in the colon. Human studies have shown dysregulation of CGRP signaling in patients with diverticulitis disease (DD), highlighting the importance of this signaling axis in human disease. (F) CGRP is specifically upregulated in a distinct population of Type 2 Innate Lymphoid Cells found in the lungs in response to specific cytokine cues released by lung-infiltrated N. brasiliensis worms during helminth infection and constrains the magnitude of the innate immune response.

Figure 2.. Recently identified human mutations in the AM-RAMP2-CLR signaling axis.

(A) As of 2019, in humans, a single pathologic mutation was identified in CLR, specifically on extracellular loop 1 of the receptor, and 6 heterozygous pathologic RAMP2 variants were identified, with mutations on the cleaved signaling peptide of RAMP2, and both the extracellular and cytosolic portion of the membrane-localized protein, with no mutations identified in the transmembrane domain. These mutations are spatially displayed on a schematic of CLR bound to RAMP2 and adrenomedullin, with each colored circle corresponding to one of the six identified RAMP2 variants and the star corresponding to the identified CLR mutation. (B) In humans, homozygous deletion of Valine 205 in CLR results in developmental arrest of lymphangiogenesis associated with lethal fluid accumulation, known as non-immune hydrops fetalis, while heterozygous carriers display female subfertility. Lymphatic deletion of CLR in mice results in similar phenotypes as seen in humans. (C) 6 pathologic RAMP2 variants were identified in an exome sequencing study of patients with primary open angle glaucoma (POAG) and each of the 6 variants were linked to functional impairments in AM-RAMP2-CLR signaling and deleterious effects on retinal ganglion nerve health.

Figure 3:. Pathophysiological roles of RAMP3.

(A) Depiction of the receptor complex RAMP3-GPER/GPR30 and its ligand estradiol. (B) Murine studies looked at the link between RAMP3-GPER/GPR30 and heart disease by crossing RAMP3 knockout mice onto a heart disease-prone genetic background. This in vivo activation of GPER/GPR30 resulted in significant reduction in heart disease parameters that was both RAMP3 and sex dependent. (C) The RAMP3-ACKR3 receptor is a decoy-receptor for the ligand adrenomedullin (AM), in that it binds AM, but does not result in G-protein signaling. (D) Recently, RAMP3 was shown to alter the decoy activity of ACKR3 through a recycling mechanism, which promoted plasma membrane re-sensitization of ACKR3. This decoy activity was shown to be important during guided cell migration in murine retinal angiogenesis. (D) RAMP3 can also interact with CLR to form a receptor for adrenomedullin 2 (AM2). (E) This signaling axis was investigated in obese and non-obese human patients, where it was found that RAMP3 mRNA levels were increased in obese patients, which correlates with RAMP3 knockout mice phenotypes. These studies highlight the importance of continuing to study RAMP3 in human disease, particularly metabolic disorders.