Movatterモバイル変換

[0]ホーム
URL:

Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
Thehttps:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

NIH NLM Logo
Log inShow account info
Access keysNCBI HomepageMyNCBI HomepageMain ContentMain Navigation
pubmed logo
Advanced Clipboard
User Guide

Full text links

MDPI full text link MDPI Free PMC article
Full text links

Actions

Share

Review
.2020 Aug 10;21(16):5728.
doi: 10.3390/ijms21165728.

Structural Complexity and Plasticity of Signaling Regulation at the Melanocortin-4 Receptor

Affiliations
Review

Structural Complexity and Plasticity of Signaling Regulation at the Melanocortin-4 Receptor

Gunnar Kleinau et al. Int J Mol Sci..

Abstract

The melanocortin-4 receptor (MC4R) is a class A G protein-coupled receptor (GPCR), essential for regulation of appetite and metabolism. Pathogenic inactivatingMC4R mutations are the most frequent cause of monogenic obesity, a growing medical and socioeconomic problem worldwide. The MC4R mediates either ligand-independent or ligand-dependent signaling. Agonists such as α-melanocyte-stimulating hormone (α-MSH) induce anorexigenic effects, in contrast to the endogenous inverse agonist agouti-related peptide (AgRP), which causes orexigenic effects by suppressing high basal signaling activity. Agonist action triggers the binding of different subtypes of G proteins and arrestins, leading to concomitant induction of diverse intracellular signaling cascades. An increasing number of experimental studies have unraveled molecular properties and mechanisms of MC4R signal transduction related to physiological and pathophysiological aspects. In addition, the MC4R crystal structure was recently determined at 2.75 Å resolution in an inactive state bound with a peptide antagonist. Underpinned by structural homology models of MC4R complexes simulating a presumably active-state conformation compared to the structure of the inactive state, we here briefly summarize the current understanding and key players involved in the MC4R switching process between different activity states. Finally, these perspectives highlight the complexity and plasticity in MC4R signaling regulation and identify gaps in our current knowledge.

Keywords: G protein-coupled receptor; melanocortin receptors; melanocortin-4 receptor; signal transduction.

PubMed Disclaimer

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Scheme of MC4R interrelations. The figure reflects functional-structural aspects related to MC4R signal transduction. Middle: Potential interaction of an MC4R dimer with a heterotrimeric G protein (surface representation). (A) represents the binding of AgRP, (B) the binding of α-MSH, and (C) the potential binding of sodium. (D) indicates an association with other proteins such as Kir channels and (E) arrestin binding with the MC4R. (F) The recently described crystal structure of the MC4R in the inactive state is bound to a peptide antagonist (SHU9119).
Figure 2
Figure 2
Sequence alignment of the MC4R and homologous MCRs in comparison to bovine rhodopsin (bOPSD), the β2-adrenergic receptor (ADRB2), and the sphingosine 1-phosphate receptor 1 (S1PR1). This comparison visualizes predicted structural dimensions of each MC4R part, also assigned according to the recently solved MC4R inactive-state structure (PDB ID: 6W25) [21], such as the helices or loops (IL, intracellular loops; ELs, extracellular loops; TMH, transmembrane helices). Highly conserved positions, according to the unifying Ballesteros and Weinstein numbering scheme for class A GPCRs [83], are indicated by respective numbers. This alignment was visualized using the BioEdit software [84]. Specific background colors indicating conservation (Blossum62 matrix) and reflecting biophysical properties of the amino acid side chains as follows: black—proline, blue—positively charged, cyan/green—aromatic and hydrophobic, green—hydrophobic, red—negatively charged, gray—hydrophilic, dark-red—cysteine, and magenta—histidine.
Figure 3
Figure 3
Presumed MC4R active-state models. (A) Independent of various details, this receptor shows structural features that are common for GPCRs such as seven transmembrane helices (H1–7) and corresponding connection loops (extracellular loops, EL1-3, and intracellular loops, IL1-3), the additional membrane surface helix H8 as well as the N-terminus (Nt) and C-terminus (Ct). As recently proposed [39], α-MSH may bind into a crevice between EL1, EL2 and EL3 and transitions to the transmembrane helices. A disulfide bridge (yellow sticks) in the EL3 has also been proposed for the MC2R [85]. (B) The second MC4R model variant is intended to show how parts of the N-terminus (magenta, sticks) might interact as intramolecular ligands, which should lead to a permanent basal signaling activity as previously proposed [65,79]. Details of this interaction [79] are shown in (B1) (labeled side chains, green for MC4R, magenta for the N-terminus). In addition, a second previously proposed disulfide bridge in the extracellular region should be located between the N-terminus (C40) and the EL3 (C279), whereby alanine mutations of these two cysteines have no impact on MC4R functionalities [86]. In the solved inactive-state crystal structure of MC4R [21], no disulfide bridge between C40 and C279 can be observed, only between C271 and C277 in EL3. (C) It would also be hypothetically conceivable that in an MC4R homodimer (see oligomerization section below), the N-termini between the two individual monomers transinteract from one to the other, respectively, in contrast to a monomeric receptor constellation (cis activation (B1)).
Figure 4
Figure 4
The crystal structure of the MC4R in an inactive state bound with the antagonist SHU9119 [21]. (A) The structure shows a number of important details of the MC4R, such as an unkinked helix 5. Instead of a highly conserved proline in class A GPCRs, a methionine is located at the corresponding position 5.50 (deep green). The NP7.50xxY motif in helix 7 (the numbers 5.50 and 7.50 are used according to the unifying numbering scheme for amino acids in the transmembrane helices of class A GPCRs proposed by Ballesteros and Weinstein [83]), which is highly conserved in class A GPCR, is a less conserved DPxxY motif in the MCR subtypes. However, in the crystallized MC4R structure, D7.49 has been mutated to an asparagine (NP7.50xxY) motif. The peptidic antagonist SHU9119 (cyan) binds between the EL2 and several helix-loop transitions. (B) SHU9119 binding is characterized by hydrogen bonds (lack dotted lines) to hydrophilic amino acid residues (red sticks) and backbone atoms in TMH 2–4 and the EL2. Further participating residues in TMH6 and TMH7 are hydrophobic and aromatic (green sticks). Experimental studies combined with electron densities (resolution of the crystal structure is 2.75 Å) between D122, D126, and E100 indicating calcium binding in this region, which is also associated with ligand binding. (C) The empty ligand binding pocket (inner surface, gray) reflects the dimension of this crevice, which is large in the MCRs compared to other GPCRs due to the very short EL2.
Figure 5
Figure 5
The putative sodium binding site with two aspartates in the MC4R. Sequence specificity of MCRs is a negatively charged aspartate in the DPxxY motif of transmembrane helix (H) seven, whereas in most class A GPCRs, an asparagine (NPxxY) is usually present. The MC4R aspartate in TMH7 could affect the ion binding properties of the MC4R, including the H-bond network with water molecules or the exact spatial location of the cation. In addition to the two aspartates located in TMH2 and TMH7, further amino acids, such as those from helix 3 (e.g., S136), are involved in the formation of the sodium binding site.
Figure 6
Figure 6
Potential extracellular cation binding sites at the MC4R. In addition to the above-described sodium binding sites between TMH2, -3, and -7 (Figure 5) and the detected calcium binding site close to the extracellular part (Figure 4) [21], several additional extracellular amino acids (negatively charged as Asp and Glu, or His and Cys as zinc binding motifs) should contribute to copper, calcium, or zinc ion binding as indicated in the model.
Figure 7
Figure 7
Binding site comparison between the MC4R bound with SHU9119 and the MC1R, and α-MSH in complex with the MC4R. (A) The MC4R (light-magenta backbone) binds SHU9119 into a cavity between the extracellular loops (ELs) and the transmembrane helices (H). Visualized are amino acids covering the ligand binding site (lines). Sticks (labeled) represent residues in the binding site that vary compared to the MC1R. These four different amino acids are also shown for the MC1R (blue) at a superimposed MC1R model based on the MC4R crystal structure. Specifically, the difference in corresponding MC4R S188 and MC1R Y183 is likely highly important for SHU9119 binding, because this ligand interacts with the side chain of S188 that should differ in the MC1R with a tyrosine at this position. In contrast to an antagonistic effect of SHU9119 on the MC4R, this ligand activates the MC1R. (B) Because all peptidic MCR ligands share a core motif essential for recognition by the receptor (see also C), few essential interactions observed for SHU9119 in the complex crystal structure with the MC4R may also be involved in MSH binding, e.g., (α-MSH/receptor) H6/T101 and R8/D126. Using these interactions as constraints for α-MSH docking into the MC4R, W9, in particular, may be differentially located in the receptor compared to W7 of SHU9119. This is essentially caused by the large and bulky aromatic D-Nal (D-Naphtylalanine) in SHU9119 instead of the F7 in α-MSH, which leads to differences in the orientation of the tryptophan. (C) This sequence comparison highlights the similarities and distinct features of MC4R ligands. Of note, the chemical difference between SHU9119 (antagonist) and melanotan II (agonist) is located at a single position (red, D-Nal vs. D-Phe), leading to the opposite ligand effect (Nle: norleucine).
Figure 8
Figure 8
Putative MC4R dimer or oligomer constellations with bound G protein.A: The MC4R can represent dimers at the IL2–TMH4 interface; however, other interfaces, such as the TMH1–helix 8 interface, cannot be excluded for oligomers. In the supposed arrangement of an interface between IL2 and TMH4, for steric reasons, only one G protein molecule can likely bind (B). It remains unknown how MC4R oligomerization affects β-arrestin binding (C) or how receptor protomers in oligomers differ from each other in their ligand binding kinetics through mutual influence, or how they react to simultaneously available arrestin and G protein molecules.
See this image and copyright information in PMC

Similar articles

See all similar articles

Cited by

See all "Cited by" articles

References

    1. De Mendoza A., Sebe-Pedros A., Ruiz-Trillo I. The evolution of the GPCR signaling system in eukaryotes: Modularity, conservation, and the transition to metazoan multicellularity. Genome Biol. Evol. 2014;6:606–619. doi: 10.1093/gbe/evu038. - DOI - PMC - PubMed
    1. Schoneberg T., Hofreiter M., Schulz A., Rompler H. Learning from the past: Evolution of GPCR functions. Trends Pharmacol. Sci. 2007;28:117–121. doi: 10.1016/j.tips.2007.01.001. - DOI - PubMed
    1. Limbird L.E. The receptor concept: A continuing evolution. Mol. Interv. 2004;4:326–336. doi: 10.1124/mi.4.6.6. - DOI - PubMed
    1. Alexander S.P., Christopoulos A., Davenport A.P., Kelly E., Mathie A., Peters J.A., Veale E.L., Armstrong J.F., Faccenda E., Harding S.D., et al. The Concise Guide to Pharmacology 2019/20: G protein-coupled receptors. Br. J. Pharmacol. 2019;176:S21–S141. doi: 10.1111/bph.14748. - DOI - PMC - PubMed
    1. Kobilka B.K., Deupi X. Conformational complexity of G-protein-coupled receptors. Trends Pharmacol. Sci. 2007;28:397–406. doi: 10.1016/j.tips.2007.06.003. - DOI - PubMed

Publication types

MeSH terms

Substances

Related information

Grants and funding

LinkOut - more resources

Full text links
MDPI full text link MDPI Free PMC article
Cite
Send To

NCBI Literature Resources

MeSHPMCBookshelfDisclaimer

The PubMed wordmark and PubMed logo are registered trademarks of the U.S. Department of Health and Human Services (HHS). Unauthorized use of these marks is strictly prohibited.

[8]ページ先頭
©2009-2025 Movatter.jp