RGS Protein Physiology and Pathophysiology

1. Auflage 2015
ISBN: 978-0-12-802954-1
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, Band Volume 133, 226 Seiten

Reihe: Progress in Molecular Biology and Translational Science

RGS Protein Physiology and Pathophysiology describes the current, state-of-the-art research occurring in the laboratories of leaders in the RGS protein field that utilize genetic mouse models to interrogate the function of RGS proteins in vivo. Each chapter describes the elucidated role of a specific RGS protein or family of RGS proteins in normal physiology and/or disease with particular emphasis on how these discoveries inform healthcare and drug discovery. The work is a timely reference as drugs targeting G protein coupled receptors represent 40% of currently marketed therapeutics. - Brings together information on the current state of the RGS protein field - Contains comprehensive descriptions of the known pathophysiological and physiological functions of RGS proteins, the first such undertaking - Gives particular emphasis to the ways these discoveries inform healthcare and drug discovery

Autoren/Hrsg.

Leseproben

Chapter One Introduction
G Protein-coupled Receptors and RGS Proteins
Adele Stewart2; Rory A. Fisher1    Department of Pharmacology, University of Iowa Carver College of Medicine, Iowa City, Iowa, USA
1 Corresponding author: email address: [e-mail]
2 Present address: Department of Pharmacology, Vanderbilt University, Nashville, Tennessee, USA Abstract
Here, we provide an overview of the role of regulator of G protein-signaling (RGS) proteins in signaling by G protein-coupled receptors (GPCRs), the latter of which represent the largest class of cell surface receptors in humans responsible for transducing diverse extracellular signals into the intracellular environment. Given that GPCRs regulate virtually every known physiological process, it is unsurprising that their dysregulation plays a causative role in many human diseases and they are targets of 40–50% of currently marketed pharmaceuticals. Activated GPCRs function as GTPase exchange factors for Ga subunits of heterotrimeric G proteins, promoting the formation of Ga-GTP and dissociated Gß? subunits that regulate diverse effectors including enzymes, ion channels, and protein kinases. Termination of signaling is mediated by the intrinsic GTPase activity of Ga subunits leading to reformation of the inactive Gaß? heterotrimer. RGS proteins determine the magnitude and duration of cellular responses initiated by many GPCRs by functioning as GTPase-accelerating proteins (GAPs) for specific Ga subunits. Twenty canonical mammalian RGS proteins, divided into four subfamilies, act as functional GAPs while almost 20 additional proteins contain nonfunctional RGS homology domains that often mediate interaction with GPCRs or Ga subunits. RGS protein biochemistry has been well elucidated in vitro, but the physiological functions of each RGS family member remain largely unexplored. This book summarizes recent advances employing modified model organisms that reveal RGS protein functions in vivo, providing evidence that RGS protein modulation of G protein signaling and GPCRs can be as important as initiation of signaling by GPCRs. Keywords GPCR G proteins RGS proteins Signal transduction 1 GPCR Physiology, Pathophysiology, and Pharmacology
G protein-coupled receptors (GPCRs) represent the largest class of cell surface receptors and are responsible for transducing extracellular signals in the form of peptides, neurotransmitters, hormones, odorants, light, ions, nucleotides, or amino acids into the intracellular environment. It is now believed that the GPCR superfamily contains over 1000 genes in humans, comprising ~ 2% of all gene-encoding DNA.1,2 Given the diversity of GCPR stimuli and the abundance of GPCR-encoding genes in the human genome, it is not surprising that GPCR dysregulation plays a causative role in many human maladies including cardiovascular diseases, neuropsychiatric disorders, metabolic syndromes, carcinogenesis, and viral infections.3–6 In fact, it is estimated that 40–50% of currently marketed pharmaceuticals target GPCRs, arguably the most remunerative drug class with worldwide sales totaling $47 billion in 2003.3 Though new GPCR-targeted drugs are in the pharmaceutical industry pipeline,7 a number of challenges have emerged in the development of novel therapeutics aimed at disrupting or enhancing signaling through GPCRs. In particular, for many years, a lack of high-resolution crystal structures made in silico bioinformatic drug screening challenging. The recently solved structure of the ß2-adrenergic receptor in complex with Gas8 (amongst others) will likely facilitate such efforts in the coming years. Additional hurdles in GPCR drug development include agonist-induced receptor desensitization and tolerance; activation or inhibition of multiple GPCR effector cascades; a lack of selectivity between ligand-specific receptor subtypes; and the possibility of off-target effects due to receptor expression in multiple cells, tissues or organs in the body.7 Though receptor targeting is ideal due to the lack of need for intracellular drug trafficking, it is now believed that GPCR effectors and regulators may also be viable drug targets and might represent a means to improve therapeutic efficacy and specificity. 2 GPCR Signal Transduction: Heterotrimeric G Proteins
Structurally, GPCRs are characterized by seven membrane-spanning alpha helices with an extracellular N-terminal tail, often, but not exclusively, involved in ligand binding, and intracellular loops and a C-terminus involved in guanine-nucleotide regulatory protein (G protein) coupling and receptor regulation. Ligand binding is believed to induce a conformational change in the receptor that promotes G protein association.9 Activated receptors function as guanine nucleotide exchange factors (GEFs) for the a subunit of the heterotrimeric G protein complex. Ga will then transition from its inactive guanosine diphosphate (GDP)-bound form to the active guanosine triphosphate (GTP)-bound monomer, dissociating from the Gß? dimer (Fig. 1). There are four families of Ga subunits in mammals (Gas, Gai, Gaq, and Ga12/13), which differ in their specific effector coupling, downstream signaling, and net cellular response. GPCR coupling to Ga subunits is highly selective allowing for ligand-specific modulation of downstream signaling in cells. Ga subunits contain two characterized functional domains: a GTP-binding cassette homologous to that found in Ras-like small GTPases and a helical insertion. GCPRs trigger a conformational change in the three flexible “switch” regions of the GTP-binding domain. The helical insertion, conversely, is unique to heterotrimeric G proteins and functions to sequester the guanine nucleotide in the GTP-binding domain. Nucleotide dissociation requires displacement of this structure, a process facilitated by active GPCRs.10,11 Both GTP-bound Ga and Gß? activate effector molecules, which include enzymes, ion channels, and protein kinases.3 Deactivation of G-protein signaling occurs by the intrinsic hydrolysis of GTP to GDP by the Ga subunit, which occurs at a rate that varies among the G-protein subfamilies.12 Figure 1 Canonical regulation of GPCR signaling by RGS proteins. Agonist binding to GPCRs induces a conformation change that facilitates the exchange of GDP for GTP on the a subunit of the heterotrimeric complex. Both GTP-bound Ga in the active form and the released Gß? dimer can then go on to stimulate a number of downstream effectors. RGS proteins are GAPs for Ga, which function to terminate signaling through GPCRs by accelerating the intrinsic GTPase activity of Ga and promoting reassociation of the heterotrimeric complex with the receptor at the cell membrane. Five genes encode Gß subunits and twelve genes encode the varying G? isoforms resulting in an impressive diversity of possible dimeric Gß? complexes.13 Gß and G? subunits form obligate heterodimers in vivo as Gß requires G? for proper protein folding.14 G? proteins have a simple structure containing two a-helices joined by a linker loop, which form a coiled-coil interaction with the N-terminal a-helix of Gß.15 The remainder of the Gß subunit consists of a ß-propeller motif composed of tryptophan-aspartic acid (WD) repeats forming arrangements of antiparallel ß sheets. Crystal structures of effector-bound Gß? complexes have revealed that this ß-propeller structure is intimately involved in effector coupling.16,17 Unsurprisingly, this effector-binding site largely overlaps with the region responsible for interaction between Gß? dimers and the switch II region of Ga, which explains the lack of Gß? signaling when sequestered in the heterotrimeric G protein complex.12 It is known that some Gß and G? subunits preferentially interact18–20 leading to the supposition that there may be some selectivity in Gß? dimer receptor/G protein coupling and effector activation. Indeed, studies in individual Gß and G? knockout models have revealed unique phenotypic consequences for loss of specific subunits implying that these proteins are not as interchangeable as was originally believed.21 3 G Protein Regulation
Regulation of GPCRs is complex with multiple layers of interconnected signaling pathways activated upon receptor simulation that feedback to impact receptor function. The best characterized GPCR regulatory mechanisms are mediated by G protein-coupled receptor kinases (GRKs), arrestins, and regulator of G protein-signaling (RGS) proteins. The Gß? dimer facilitates membrane targeting of GRKs resulting in GRK-mediated GPCR phosphorylation. This modification recruits ß-arrestins, which sterically hinder further G-protein coupling to the receptor.22 Though their role in GPCR desensitization has been well characterized, it is now appreciated that arrestins are multifunctional scaffolds...