Chapter 5 - Escorts Take the Lead: Molecular Chaperones as Therapeutic Targets

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The functional and physiological diversity of transmembrane receptors results from factors that influence the pharmacology, signaling, and trafficking of these receptors. Receptor mutations and other modifications may lead to misfolding, intracellular retention, and ineffective signaling of transmembrane receptors. The importance of such mutations is highlighted by the fact that various diseases have been linked to mutations that lead to ineffective signaling of these receptors, resulting from the retention of receptors in intracellular compartments. Studies focused on understanding the regulation of trafficking and cell surface expression of newly synthesized receptors have highlighted molecular chaperones as key regulators of receptor maturation and sorting. In this chapter, we discuss the functions of molecular chaperones in the regulation of seven-transmembrane-containing G-protein-coupled receptor function and trafficking and explore ways in which chaperones can serve as novel therapeutic targets.

Introduction

A large family of over 800 genes encodes receptor proteins that are characterized by a signature seven-transmembrane structure. Members of this family include sensory receptors for taste, odorants, and light as well as receptors for many hormones, neurotransmitters, chemokines, and ions. These seven-transmembrane receptors (also known as heptahelical receptors) are commonly referred to as G-protein-coupled receptors (GPCRs) because they mediate their effects through the activation of a variety of heterotrimeric (α, β, γ-subunits) guanine nucleotide-binding G proteins. These GPCRs regulate many physiological processes, and the mechanism by which GPCRs translate extracellular signals into cellular changes has been an area of active research for many years.

Initial theories of GPCR signaling involved agonist binding leading to the activation of the receptor, resulting in dissociation of the G protein into an α subunit and a βγ subunit. Both these subunits have been shown to activate or inhibit various downstream effector molecules. However, further advances in the field of GPCR signaling have demonstrated that the mechanisms by which cell surface receptors orchestrate cellular changes are more complex. The recognition of the importance of GPCR oligomerization, the discovery of regulators of G protein signaling (RGS) proteins, and the identification of accessory/chaperone molecules are just some of the factors that have contributed to the expansion of the role and function of GPCRs.

Not only do the GPCRs regulate a plethora of physiological processes but drugs that target these receptors account for most of the medicines sold worldwide. These drugs target these seven-transmembrane receptors directly or target other proteins that are crucial for signaling through these receptors. This chapter focuses on the functions of molecular chaperones in the regulation of transmembrane receptor function and trafficking and explores ways in which chaperones can serve as novel therapeutic targets.

Section snippets

Molecular Chaperones and Accessory Proteins

The molecular chaperone concept was first proposed by John Ellis in 1987; he proposed that the term molecular chaperone be used to describe “a class of cellular proteins whose function is to ensure that the folding of certain other polypeptide chains and their assembly into oligomeric structures occur correctly.”1 There is a commonly held misconception that molecular chaperones are solely involved in ensuring proper protein folding. While many chaperones are involved in stabilizing unfolded

GPCR Maturation and Postendoplasmic Reticulum Trafficking

GPCRs are synthesized by ribosomes attached at the cytosolic face of the endoplasmic reticulum (ER). During biosynthesis, these proteins are targeted by their hydrophobic signal sequences to the translocation complex which facilitates cotranslational entry into the ER lumen. Insertion of transmembrane domains into the membrane is driven by the translocation complex and orientation signals contained in the protein's polypeptidic chain. This membrane insertion is assisted by molecular chaperones

Regulated Translocation to Intracellular Compartments and/or the Plasma Membrane

In the most simplified model of GPCR trafficking, GPCRs are expressed on the cell surface following biosynthetic sorting and are then endocytosed in response to activation by agonists. Increasingly, there is evidence that for some GPCRs, this might not always be the case. This is demonstrated by studies performed studying the trafficking of the protease-activated receptor (PAR) family. Irreversible activation of the PAR1 and PAR2 thrombin receptors by cleavage results in internalization and

GPCR Oligomerization

GPCRs physically associate with other cellular proteins, including a large variety of soluble intracellular proteins such as β-arrestins, membrane proteins such as receptor-activity-modifying proteins (RAMPs) as well as other GPCRs. The recognition of GPCR homo- and heteromerization has generated numerous possibilities for expanding the roles and functions of GPCRs. Dimerization of GPCRs has been implicated in modulating a number of functional properties of the receptors, including ligand

Specific Molecular Chaperones for GPCRs

It has become evident that, in addition to using general chaperones for the folding and sorting of newly synthesized proteins, some proteins may require more specialized assistance and utilize specific chaperones. Some of these specialized chaperones or enzymes directly participate in the folding of their cognate proteins. In the case of GPCRs, a few specialized chaperones have been reported to be involved in GPCR folding. The functions of these chaperones are summarized in Table I. NinaA (n

The Melanocortin-2 Receptor and MRAP

The melanocortin family of receptors is a group of five structurally related GPCRs that play diverse physiological roles in mammals.79 All the melanocortin receptors cause an increase in cAMP levels when stimulated by agonists. However, the receptors differ in their affinity for physiological agonist. Melanocortin-2 is unique because it is the only melanocortin receptor that is selectively regulated by adrenocorticotropin hormone. Melanocortin-2 is also unique in the way that it requires an

Opioid Receptors and RTP4

As discussed earlier, the three subtypes of opioid receptors, δ, κ, and μ have the ability to form both homomers and heteromers.86 In addition, there are varied pharmacological profiles for different homomer and heteromer receptor pairs. The clinical importance of heteromerization of opioid receptors is highlighted by the fact that despite μ-opioid receptors mediating, most of the pain-relieving effects of morphine, antagonism of δ-opioid receptors leads to a reduction in the tolerance that

Pharmacological Chaperones

As previously discussed, some receptor proteins do not achieve proper cell surface trafficking when expressed in heterologous systems. This is usually due to the absence of proper endogenous chaperones which lead to the intracellular retention of these receptors by the cell's quality-control system. One method to overcome this intracellular retention of receptor proteins is through the use of small molecules that act as chaperones and aid in the proper sorting and trafficking of receptors. Such

Conclusions

With recent advances in techniques to study the pharmacology, signaling, and trafficking of transmembrane receptors we are left with a better understanding of how this tightly regulated network of receptors work. We now have a great appreciation of the role of processes such as receptor oligomerization, receptor desensitization and interaction with accessory proteins and cofactors. Advances in the field of molecular chaperone biology have provided insights into what can happen when receptors

Acknowledgments

The authors thank Drs. Raphael Rozenfeld and Ivone Gomes for critically reading the manuscript. These studies were supported by National Institutes of Health Grants DA008863 and DA019521 (to L.A.D.).

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