Cellular Membranes, a Versatile Adaptive Composite Material

Cellular membranes belong to the most vital yet least understood biomaterials of live matter. For instance, its biomechanical requirements substantially vary across species and subcellular sites, raising the question how membranes manage to adjust to such dramatic changes. Central to its adaptability at the cell surface is the interplay between the plasma membrane and the adjacent cell cortex, forming an adaptive composite material that dynamically adjusts its mechanical properties. Using a hypothetical composite material, we identify core challenges, and discuss how cellular membranes solved these tasks. We further muse how pathological changes in material properties affect membrane mechanics and cell function, before closing with open questions and future challenges arising when studying cellular membranes.

Cell surface targeting of the Melanocortin 5 Receptor (MC5R) requires serine-rich terminal motifs

The Melanocortin 5 Receptor (MC5R) is a cell surface receptor that belongs to the class of G-protein coupled receptors (GPCRs), which comprises an intracellular carboxylic domain, seven transmembrane helices and an extracellular amino terminal. Over the last few years, MC5R has been implicated in the regulation of lipid metabolism in exocrine glands, muscle and even in adipose tissue and its function is quite dependent on its correct cell membrane targeting. In this context, the purpose of this work was to study the role of MC5R N-terminus in the receptor trafficking from the endoplasmic reticulum (ER) through the Golgi complex to the plasma membrane. Analysis of N-terminal deleted forms of MC5R revealed that the first 21 amino acids contain the information responsible for the receptor cell surface expression and the removal of further amino acids interfere with the receptor synthesis. In this setting, several mutant forms of the receptor were created by site directed mutagenesis of the MC5R first 21 amino acids and their presence at the plasma membrane was assessed. We have found that two small motifs, constituted by residues Ser4/Ser5 and Ser17/Glu18, are clearly involved in the correct targeting of MC5R to the cell surface. Fluorescence microscopy analysis has revealed that MC5R constructs with mutations in those residues are mainly retained at the ER/Golgi complex. Furthermore, the homodimerization ability of the receptor is maintained in these mutant forms, suggesting that other mechanisms are involved in the regulation of the anterograde transport of MC5R by those N-terminal domains.

Pharmacological chaperones for the misfolded melanocortin-4 receptor associated with human obesity

The melanocortin-4 receptor (MC4R) plays a vital role in regulating energy homeostasis. Mutations in the MC4R cause early-onset severe obesity. The majority of loss of function MC4R mutants are retained intracellularly, many of which are not terminally misfolded and can be stabilized and targeted to the plasma membrane by different chaperones. Some of the mutants might be functional once coaxed to the cell surface. Molecular chaperones and chemical chaperones correct the misfolding of some mutant MC4Rs. However, their therapeutic application is very limited due to their non-specific mechanism of action and, for chemical chaperone, high dosage needed to be effective. Several pharmacological chaperones have been identified for the MC4R and Ipsen 5i and Ipsen 17 are the most potent and efficacious. Here we provide a comprehensive review on how different approaches have been applied to rescue misfolded MC4R mutants. This article is part of a Special Issue entitled: Melanocortin Receptors - edited by Ya-Xiong Tao.

How genetic errors in GPCRs affect their function: Possible therapeutic strategies

Activating and inactivating mutations in numerous human G protein-coupled receptors (GPCRs) are associated with a wide range of disease phenotypes. Here we use several class A GPCRs with a particularly large set of identified disease-associated mutations, many of which were biochemically characterized, along with known GPCR structures and current models of GPCR activation, to understand the molecular mechanisms yielding pathological phenotypes. Based on this mechanistic understanding we also propose different therapeutic approaches, both conventional, using small molecule ligands, and novel, involving gene therapy.

Chaperoning G protein-coupled receptors: from cell biology to therapeutics

G protein-coupled receptors (GPCRs) are membrane proteins that traverse the plasma membrane seven times (hence, are also called 7TM receptors). The polytopic structure of GPCRs makes the folding of GPCRs difficult and complex. Indeed, many wild-type GPCRs are not folded optimally, and defects in folding are the most common cause of genetic diseases due to GPCR mutations. Both general and receptor-specific molecular chaperones aid the folding of GPCRs. Chemical chaperones have been shown to be able to correct the misfolding in mutant GPCRs, proving to be important tools for studying the structure-function relationship of GPCRs. However, their potential therapeutic value is very limited. Pharmacological chaperones (pharmacoperones) are potentially important novel therapeutics for treating genetic diseases caused by mutations in GPCR genes that resulted in misfolded mutant proteins. Pharmacoperones also increase cell surface expression of wild-type GPCRs; therefore, they could be used to treat diseases that do not harbor mutations in GPCRs. Recent studies have shown that indeed pharmacoperones work in both experimental animals and patients. High-throughput assays have been developed to identify new pharmacoperones that could be used as therapeutics for a number of endocrine and other genetic diseases.

A small molecule agonist THIQ as a novel pharmacoperone for intracellularly retained melanocortin-4 receptor mutants

Although mutations in the melanocortin-4 receptor (MC4R) gene cause severe early-onset obesity, we still do not have effective approaches to correct the defects of these mutations. Several antagonists have been identified as pharmacoperones of the MC4R whereas no agonist of the MC4R has been reported. In the present study, we investigated the effect of a small molecule agonist of the MC4R, THIQ, on the cell surface expression and signaling of ten intracellularly retained MC4R mutants using different cell lines. We showed that THIQ increased the cell surface expression of three mutants (N62S, C84R, and C271Y) and two of them (N62S and C84R) had increased signaling in HEK293 cells. Interestingly, THIQ increased the signaling of two other mutants (P78L and P260Q) without increasing their cell surface expression in HEK293 cells. In neuronal cells, THIQ exhibited a more potent effect, correcting the cell surface expression and signaling of seven mutants (N62S, I69R, P78L, C84R, W174C, P260Q, and C271Y). Other mutants were not rescued by THIQ. We also showed that THIQ did not rescue MC4R mutants defective in ligand binding or signaling or one intracellularly retained mutant of the melanocortin-3 receptor. In summary, we demonstrated that a small molecule agonist acted as a pharmacoperone of the MC4R rescuing the cell surface expression and signaling of some intracellularly retained MC4R mutants.

Ipsen 5i is a Novel Potent Pharmacoperone for Intracellularly Retained Melanocortin-4 Receptor Mutants

Inactivating mutations of the melanocortin-4 receptor (MC4R) cause early-onset severe obesity in humans. Comprehensive functional studies show that most of the inactivating mutants of the MC4R are retained intracellularly. In the present study, we investigated whether a small molecule inverse agonist of the MC4R, Ipsen 5i, could act as a pharmacoperone and correct the cell surface expression and function of intracellularly retained mutant MC4Rs using multiple cell lines, including HEK293 and two neuronal cell lines. We showed that Ipsen 5i rescued the cell surface expression of all 11 intracellularly retained mutant MC4Rs studied herein in at least one cell line. Ipsen 5i functionally rescued seven mutants in all cell lines used. One mutant (Y157S) was functionally rescued in HEK293 cells but not in the two neuronal cell lines. Ipsen 5i increased cell surface expression of three mutants (S58C, G98R, and F261S) but did not affect signaling. Ipsen 5i had no effect on mutant MC4Rs with other defects (Δ88-92, D90N, I102S) or no defect (N274S). It also did not affect trafficking of a misrouted MC3R mutant (I335S). Cell impermeable peptide ligands of the MC4R or cell permeable small molecule ligand of δ opioid receptor could not rescue misrouted mutant MC4R. In summary, we demonstrated that Ipsen 5i was a novel potent pharmacoperone of the MC4R, correcting trafficking and signaling of a significant portion (73%) of intracellularly retained mutants. Additional studies are needed to demonstrate its in vivo efficacy.

Exposure of MC4R to agonist in the endoplasmic reticulum stabilizes an active conformation of the receptor that does not desensitize

Melanocortin-4 receptor (MC4R) is a G protein-coupled receptor expressed in neurons of the hypothalamus where it regulates food intake. MC4R responds to an agonist, α-melanocyte-stimulating hormone (α-MSH) and to an antagonist/inverse agonist, agouti-related peptide (AgRP), which are released by upstream neurons. Binding to α-MSH leads to stimulation of receptor activity and suppression of food intake, whereas AgRP has opposite effects. MC4R cycles constantly between the plasma membrane and endosomes and undergoes agonist-mediated desensitization by being routed to lysosomes. MC4R desensitization and increased AgRP expression are thought to decrease the effectiveness of MC4R agonists as an antiobesity treatment. In this study, α-MSH, instead of being delivered extracellularly, is targeted to the endoplasmic reticulum (ER) of neuronal cells and cultured hypothalamic neurons. We find that the ER-targeted agonist associates with MC4R at this location, is transported to the cell surface, induces constant cAMP and AMP kinase signaling at maximal amplitude, abolishes desensitization of the receptor, and promotes both cell-surface expression and constant signaling by an obesity-linked MC4R variant, I316S, that otherwise is retained in the ER. Formation of the MC4R/agonist complex in the ER stabilizes the receptor in an active conformation that at the cell surface is insensitive to antagonism by AgRP and at the endosomes is refractory to routing to the lysosomes. The data indicate that targeting agonists to the ER can stabilize an active conformation of a G protein-coupled receptor that does not become desensitized, suggesting a target for therapy.

Structural determinants regulating cell surface targeting of melanocortin receptors

Melanocortin receptors (MCRs) belong to the G-protein-coupled receptor family of transmembrane proteins. They recognize specific ligands named melanocortins that are mainly produced in the pituitary and hypothalamus. Newly synthesized MCRs at the endoplasmic reticulum are subjected to quality control mechanisms that screen for the correct structure, folding or processing, essential for their proper cell surface expression. Some motifs, located at the N- or C-terminus or even on transmembrane and in loop regions, have been implicated in these biological processes. This article reviews these specific domains and the role of accessory proteins and post-translation modifications in MCRs' targeting to cell surface. Additionally, promising approaches involving pharmacological stabilization of misfolded and misrouted mutant MCRs, which improve their forward transport, are reported. Understanding the MCRs' structural determinants fundamental for their proper cell surface integration is essential for correcting abnormalities found in some diseases.