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Heart failure (HF) represents one of the leading causes of morbidity and mortality in developed nations today. Although this disease process represents a final common endpoint for several entities, including hypertension, coronary artery disease, and cardiomyopathy, a predominant characteristic of end-stage HF is an altered beta-adrenergic receptor signaling cascade. In the heart, beta-adrenergic receptors (beta ARs), members of the superfamily of G-protein-coupled receptors (GPCRs), modulate cardiac function by controlling chronotropic, inotropic, and lusitropic responses to catecholamines of the sympathetic nervous system. In HF, beta ARs are desensitized and downregulated in a maladaptive response to chronic stimulation. This process is largely mediated by G-protein-coupled receptor kinases (GRKs), which phosphorylate GPCRs leading to functional uncoupling. The most abundant cardiac GRK, known as GRK2 or beta AR kinase 1 (beta ARK1), is increased in human HF, and has been implicated in the pathogenesis of dysfunctional cardiac beta AR signaling. The association of beta ARs and GRKs with impaired cardiac function has been extensively studied using transgenic mouse models, which have demonstrated that beta ARK1 plays a vital role in the regulation of myocardial beta AR signaling. These findings have caused beta ARs and GRKs to be regarded as potential therapeutic targets, and gene therapy strategies have been used to manipulate the beta AR signaling pathway in myocardium, leading to improved function in the compromised heart. Ultimately, these genetic modifications of the heart may represent new potential therapies for human HF.
J Mol Cell Cardiol 2004 Jul
PMID:Genetic manipulation of myocardial beta-adrenergic receptor activation and desensitization. 1524 31

Antibodies have proved invaluable in the study of G-protein-coupled receptors (GPCRs). The utility of these immunoglobulin probes for investigation of protein structures and functions arises from their selectivity as well as their versatility. Antibodies can be used to analyze GPCR size, abundance, distribution, turnover, modification, interaction with other proteins, and functional properties. In this chapter, techniques for the generation and characterization of receptor-selective antibodies are described. Two protocols are given for the generation of antibodies: (1) development of polyclonal antibodies (PAbs) against synthetic peptides corresponding to a specific site within a GPCR and (2) selection of synthetic single-chain fragment variable (scFv) monoclonal antibodies (MAbs) from libraries expressed on the surface of bacteriophage. Immunoblot and enzyme-linked immunosorbent assays for characterization of the selectivity and affinity of such antibodies are described. Finally, methods are given for improvement of the titer and specificity of PAbs.
Methods Mol Biol 2004
PMID:Generation, use, and validation of receptor-selective antibodies. 1525 Apr 85

G-protein-coupled receptor mRNAs are expressed at low levels and therefore present a challenge for the study of their sites and levels of expression. In situ hybridization (ISH) and Northern blotting are powerful methods for the localization of mRNAs and the study of regulation of mRNA expression. ISH combines the power of precise cellular localization with the ability to perform semiquantitative analysis of the mRNA level, whereas Northern blotting has the ability to identify genetic splice variants, or to study multiple RNA molecules sequentially in the same tissue samples. These protocols give step-by-step instructions for the performance of these techniques, and the analysis of the data that can be obtained using them.
Methods Mol Biol 2004
PMID:Identification of G-protein-coupled receptor mRNA expression by Northern blotting and in situ hybridization. 1525 Apr 88

It is now clear that nearly all G-protein-coupled receptors (GPCRs) are phosphorylated and palmitolyated. The process of receptor phosphorylation has been extensively studied because it offers a regulatory mechanism that is both rapid and dynamic. However, it has recently become clear that palmitoyaltion of GPCRs at C-terminal cysteine residues may also offer dynamic receptor modification. A growing number of GPCRs have been demonstrated to undergo rapid agonist-mediated changes in their palmitoylation status with functional implications to receptor signaling. This chapter aims to outline the methods we have used to investigate agonist-mediated changes in GPCR phosphorylation and palmitoylation.
Methods Mol Biol 2004
PMID:G-protein-coupled receptor phosphorylation and palmitoylation. 1525 Apr 98

Reversible phosphorylation is important for G-protein-coupled receptor (GPCR) signaling, desensitization, and endocytosis, yet the precise location and role of in vivo phosphorylation sites is unknown for most receptors. This chapter describes a powerful analytical method for the direct identification of GPCR phosphorylation sites by two-dimensional (2D) phosphopeptide mapping. The GPCR of interest is isolated from 32P-labeled cells by immunoprecipitation and transferred to nitrocellulose membranes. In situ cleavage by trypsin releases phosphopeptides that are separated by a combination of high-voltage electrophoresis and chromatography. Phosphoamino acid analysis and Edman sequencing of isolated phosphopeptides reveals information that can lead to the direct identification of GPCR phosphorylation sites. Furthermore, the 2D phosphopeptide mapping technique allows the analysis of temporal and positional changes in the GPCR phosphorylation pattern under different physiological conditions.
Methods Mol Biol 2004
PMID:Identification of G-protein-coupled receptor phosphorylation sites by 2D phosphopeptide mapping. 1525 Apr 99

In this chapter we describe methods for detecting the ubiquitination state of G-protein-coupled receptors (GPCRs). This involves coexpression of a GPCR with an epitope-tagged ubiquitin construct in a heterologous expression system. Modification by ubiquitin of the GPCR resulting from agonist activation is detected by immunoprecipation and subsequent immunoblotting for the epitope-tagged ubiquitin. We use here the chemokine receptor CXCR4 as the model receptor; however, this could be easily modified to detect the ubiquitination state of any GPCR.
Methods Mol Biol 2004
PMID:Ubiquitination of G-protein-coupled receptors. 1525 May

Complex networks of protein-protein interactions are key determinants of cellular function, including those regulated by G-protein-coupled receptors (GPCRs). Formation of either stable or transitory complexes are involved in regulating all aspects of receptor function, from ligand binding through to signal transduction, desensitization, resensitization and downregulation. Today, 50% of all recently launched drugs are targeted against GPCRs. This particular class of proteins is extremely useful as a drug target because the receptors are partly located outside the cell, simplifying bioavailability and delivery of drugs directed against them. However, being located within the cell membrane causes difficulties for the study of GPCR function and bioluminescence resonance energy transfer (BRET), a naturally occurring phenomenon, represents a newly emerging, powerful tool with which to investigate and monitor dynamic interactions involving this receptor class. BRET is a noninvasive, highly sensitive technique, performed as a simple homogeneous assay. involving the proximity-dependent transfer of energy from an energy donor to acceptor resulting in the emission of light. This technology has several advantages over alternative approaches as the detection occurs within live cells, in real time, and is not restricted to a particular cellular compartment. The use of such biophysical techniques as BRET, will not only increase our understanding of the nature of GPCR regulation and the protein complexes involved, but could also potentially lead to the development of novel therapeutics that modulate these interactions.
Methods Mol Biol 2004
PMID:Study of G-protein-coupled receptor-protein interactions by bioluminescence resonance energy transfer. 1525 May 2

The versatility, sensitivity, and feasibility of fluorescence methods are very attractive to study protein-protein interaction at low levels of protein expression. However, one of the most severe limits in protein chemistry has been the difficulty of introducing site-specific fluorescent labels. The development of genetically encoded fluorescent probes, that is, green fluorescent protein (GFP) and its variants therefore opened up a broad field of novel applications. To characterize protein-protein interactions and determine detailed spatio-temporal dynamics of partners that are molecularly well characterized, fluorescence energy transfer methods are excellent nondestructive tools in living cells. Cellular responses to external factors are extensively based on direct molecular interaction and especially G-protein-coupled receptors (GPCRs) have been shown to interact with an unexpected level of complexity. Classical models of signal transduction describe GPCRs as monomeric proteins, while recent studies using fluorescence resonance energy transfer (FRET) and other methods show that GPCRs can also function as homo- or heterodimers. Theoretical background information on FRET technology and its diverse applications are summarized here. A detailed description of a spectroscopic method for FRET studies in the field of GPCR interaction is presented to facilitate and propagate studies to increase our understanding of protein-protein interactions involving GPCRs.
Methods Mol Biol 2004
PMID:Fluorescence resonance energy transfer to study receptor dimerization in living cells. 1525 May 3

Many protein interactions with G-protein-coupled receptors (GPCRs) appear to influence receptor signaling and functional regulation. There is great interest therefore in methods for the identification of novel or unanticipated GPCR binding proteins. A proven method for identifying such protein interactions is the yeast two-hybrid screen, which involves screening the protein products of a cDNA library with a selected domain derived from a GPCR. Once it is established that a candidate protein produces a specific positive interaction within the yeast two-hybrid system, it is important to demonstrate further that this interaction is likely to occur in vivo. Coimmunoprecipitation, in which proteins of interest are copurified with the receptor under study, is a good way to address this important issue. Together, the yeast two-hybrid screen and coimmunoprecipitation are a useful way to identify and sort through candidate GPCR-interacting proteins prior to analysis in physiological studies.
Methods Mol Biol 2004
PMID:Identification of protein interactions by yeast two-hybrid screening and coimmunoprecipitation. 1525 May 4

Accumulating examples have demonstrated that knock-out and knock-in mice of G-protein-coupled receptors (GPCRs) are useful in elucidating physiological functions of the receptor in vivo. GPCR knock-out and knock-in are achieved by either (1) manipulation of the endogenous locus of the receptor gene or (2) transgenic expression of the modified receptor. Historically speaking, the first generation knock-outs made the best use of homologous recombination in embryonic stem (ES) cells and their totipotency to introduce the desired mutation into the endogenous receptor locus. In the second-generation knock-outs using the Cre/loxP system, the disruption of the receptor gene is cell-type specific or region-specific but is irreversible in principle. In contrast, transgenic expression in the receptor knock-out mice of the wild-type receptor protein under a tissue- and stage-specific promoter (conditional "rescue" of the receptor knock-out) can be easily applied to create "reversible" or "inducible" knock-out of the receptor. This is called the third generation knock-out. In the following sections, we introduce examples of the materials and methods based on our in vivo analyses of the metabotropic glutamate receptor-subtype 1 (mGluR1).
Methods Mol Biol 2004
PMID:Receptor knock-out and knock-in strategies. 1525 May 6


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