Comparative analysis of the human angiotensin II type 1a receptor heterologously produced in insect cells and mammalian cells

Expression and Characterization of Relaxin Family Peptide Receptor 1 Variants

G-protein coupled receptors (GPCR) transduce extracellular stimuli into the cell interior and are thus centrally involved in almost all physiological-neuronal processes. This essential function and association with many diseases or pathological conditions explain why GPCRs are one of the priority targets in medical and pharmacological research, including structure determination. Despite enormous experimental efforts over the last decade, both the expression and purification of these membrane proteins remain elusive. This is attributable to specificities of each GPCR subtype and the finding of necessary experimental in vitro conditions, such as expression in heterologous cell systems or with accessory proteins. One of these specific GPCRs is the leucine-rich repeat domain (LRRD) containing GPCR 7 (LGR7), also termed relaxin family peptide receptor 1 (RXFP1). This receptor is characterized by a large extracellular region of around 400 amino acids constituted by several domains, a rare feature among rhodopsin-like (class A) GPCRs. In the present study, we describe the expression and purification of RXFP1, including the design of various constructs suitable for functional/biophysical studies and structure determination. Based on available sequence information, homology models, and modern biochemical and genetic tools, several receptor variations with different purification tags and fusion proteins were prepared and expressed in Sf9 cells (small-scale), followed by an analytic fluorescence-detection size-exclusion chromatography (F-SEC) to evaluate the constructs. The most promising candidates were expressed and purified on a large-scale, accompanied by ligand binding studies using surface plasmon resonance spectroscopy (SPR) and by determination of signaling capacities. The results may support extended studies on RXFP1 receptor constructs serving as targets for small molecule ligand screening or structural elucidation by protein X-ray crystallography or cryo-electron microscopy.

Nanoparticles in Medicine: A Focus on Vascular Oxidative Stress

Nanotechnology has had a significant impact on medicine in recent years, its application being referred to as nanomedicine. Nanoparticles have certain properties with biomedical applications; however, in some situations, they have demonstrated cell toxicity, which has caused concern surrounding their clinical use. In this review, we focus on two aspects: first, we summarize the types of nanoparticles according to their chemical composition and the general characteristics of their use in medicine, and second, we review the applications of nanoparticles in vascular alteration, especially in endothelial dysfunction related to oxidative stress. This condition can lead to a reduction in nitric oxide (NO) bioavailability, consequently affecting vascular tone regulation and endothelial dysfunction, which is the first phase in the development of cardiovascular diseases. Therefore, nanoparticles with antioxidant properties may improve vascular dysfunction associated with hypertension, diabetes mellitus, or atherosclerosis.

Bombyx neuropeptide G protein-coupled receptor A7 is the third cognate receptor for short neuropeptide F from silkworm

The short neuropeptide F (sNPF) neuropeptides, closely related to vertebrate neuropeptide Y (NPY), have been suggested to exert pleiotropic effects on many physiological processes in insects. In the silkworm (Bombyx mori) two orphan G protein-coupled receptors, Bombyx neuropeptide G protein-coupled receptor (BNGR) A10 and A11, have been identified as cognate receptors for sNPFs, but other sNPF receptors and their signaling mechanisms in B. mori remain unknown. Here, we cloned the full-length cDNA of the orphan receptor BNGR-A7 from the brain of B. mori larvae and identified it as a receptor for Bombyx sNPFs. Further characterization of signaling and internalization indicated that BNGR-A7, -A10, and -A11 are activated by direct interaction with synthetic Bombyx sNPF-1 and -3 peptides. This activation inhibited forskolin or adipokinetic hormone-induced adenylyl cyclase activity and intracellular Ca²⁺ mobilization via a G_i/o-dependent pathway. Upon activation by sNPFs, BNGR-A7, -A10, and -A11 evoked ERK1/2 phosphorylation and underwent internalization. On the basis of these findings, we designated the receptors BNGR-A7, -A10, and -A11 as Bommo-sNPFR-1, -2, and -3, respectively. Moreover, the results obtained with quantitative RT-PCR analysis revealed that the three Bombyx sNPF receptor subtypes exhibit differential spatial and temporal expression patterns, suggesting possible roles of sNPF signaling in the regulation of a wide range of biological processes. Our findings provide the first in-depth information on sNPF signaling for further elucidation of the roles of the Bombyx sNPF/sNPFR system in the regulation of physiological activities.

Quality control in eukaryotic membrane protein overproduction

The overexpression of authentically folded eukaryotic membrane proteins in milligramme quantities is a fundamental prerequisite for structural studies. One of the most commonly used expression systems for the production of mammalian membrane proteins is the baculovirus expression system in insect cells. However, a detailed analysis by radioligand binding and comparative Western blotting of G protein-coupled receptors and a transporter produced in insect cells showed that a considerable proportion of the expressed protein was misfolded and incapable of ligand binding. In contrast, production of the same membrane proteins in stable inducible mammalian cell lines suggested that the majority was folded correctly. It was noted that detergent solubilisation of the misfolded membrane proteins using either digitonin or dodecylmaltoside was considerablyless efficient than using sodium dodecyl sulfate or foscholine-12, whilst these detergents were equally efficient at solubilising correctly folded membrane proteins. This provides a simple and rapid test to suggest whether heterologously expressed mammalian membrane proteins are indeed correctly folded, without requiring radioligand binding assays. This will greatly facilitate the high-throughput production of fully functional membrane proteins for structural studies.

From Recombinant Expression to Crystals: A Step-by-Step Guide to GPCR Crystallography

G protein-coupled receptors (GPCRs) are the primary targets of drugs prescribed for many human pathophysiological conditions such as hypertension, allergies, schizophrenia, asthma, and various types of cancer. High-resolution structure determination of GPCRs has been a key focus area in GPCR biology to understand the basic mechanism of their activation and signaling and to materialize the long-standing dream of structure-based drug design on these versatile receptors. There has been tremendous effort at this front in the past two decades and it has culminated into crystal structures of 27 different receptors so far. The recent progress in crystallization and structure determination of GPCRs has been driven by innovation and cutting-edge developments at every step involved in the process of crystallization. Here, we present a step-by-step description of various steps involved in GPCR crystallization starting from recombinant expression to obtaining diffracting crystals. We also discuss the next frontiers in GPCR biology that are likely to be a primary focus for crystallography efforts in the next decade or so.

Membrane preparation and solubilization

Membrane proteins play an essential role in several biological processes like ion transport, signal transduction, and electron transfer to name a few. For structural and functional studies of integral membrane proteins, it is critically important to isolate proteins from the membrane using biological detergents. Detergents disrupt the native lipid components of the native membrane and encase the membrane protein in an unnatural environment in aqueous solution. However, a particular membrane protein is best solubilized in a specific detergent; therefore, screening for the optimal detergent is essential. Apart from keeping the membrane protein monodispered in solution, the detergent has to be compatible with downstream processes to isolate and characterize a membrane protein. Over the past several years, a number of membrane proteins have been successfully isolated for structural and functional studies that allowed an outline of general strategies for isolating a novel membrane protein of interest.

The atypical N-glycosylation motif, Asn-Cys-Cys, in human GPR109A is required for normal cell surface expression and intracellular signaling

Asparagine-linked glycosylation (N-glycosylation) is necessary for the proper folding of secreted and membrane proteins, including GPCRs. Thus, many GPCRs possess the N-glycosylation motif Asn-X-Ser/Thr at their N-termini and/or extracellular loops. We found that human GPR109A (hGPR109A) has an N-glycosylation site at Asn(17) in the N-terminal atypical motif, Asn(17)-Cys(18)-Cys(19). Why does hGPR109A require the atypical motif, rather than the typical sequence? Here we show that Asn(17)-Cys(18)-Cys(19) sequence of hGPR109A possesses 2 biologic roles. First, Asn(17)-X-Cys(19) contributed to hGPR109A N-glycosylation by acting as an atypical motif. This modification is required for the normal surface expression of hGPR109A, as evidenced by the reduced surface expression of the nonglycosylated mutants, hGPR109A/N17A, and the finding that hGPR109A/C19S and hGPR109A/C19T, which are N-glycosylated at Asn(17), exhibited expression similar to the wild-type receptor. Second, the X-Cys(18)-Cys(19) dicysteine is indispensable for hGPR109A function. Substitution of Cys(18) or Cys(19) residue to Ala impaired Gi-mediated signaling via hGPR109A. We propose the disulfide bond formations of these residues with other Cys existed in the extracellular loops for the proper folding. Together, these results suggest that the atypical motif Asn(17)-Cys(18)-Cys(19) is crucial for the normal surface trafficking and function of hGPR109A.

Overexpression of membrane proteins in mammalian cells for structural studies

The number of structures of integral membrane proteins from higher eukaryotes is steadily increasing due to a number of innovative protein engineering and crystallization strategies devised over the last few years. However, it is sobering to reflect that these structures represent only a tiny proportion of the total number of membrane proteins encoded by a mammalian genome. In addition, the structures determined to date are of the most tractable membrane proteins, i.e., those that are expressed functionally and to high levels in yeast or in insect cells using the baculovirus expression system. However, some membrane proteins that are expressed inefficiently in these systems can be produced at sufficiently high levels in mammalian cells to allow structure determination. Mammalian expression systems are an under-used resource in structural biology and represent an effective way to produce fully functional membrane proteins for structural studies. This review will discuss examples of vertebrate membrane protein overexpression in mammalian cells using a variety of viral, constitutive or inducible expression systems.

The significance of G protein-coupled receptor crystallography for drug discovery

Crucial as molecular sensors for many vital physiological processes, seven-transmembrane domain G protein-coupled receptors (GPCRs) comprise the largest family of proteins targeted by drug discovery. Together with structures of the prototypical GPCR rhodopsin, solved structures of other liganded GPCRs promise to provide insights into the structural basis of the superfamily's biochemical functions and assist in the development of new therapeutic modalities and drugs. One of the greatest technical and theoretical challenges to elucidating and exploiting structure-function relationships in these systems is the emerging concept of GPCR conformational flexibility and its cause-effect relationship for receptor-receptor and receptor-effector interactions. Such conformational changes can be subtle and triggered by relatively small binding energy effects, leading to full or partial efficacy in the activation or inactivation of the receptor system at large. Pharmacological dogma generally dictates that these changes manifest themselves through kinetic modulation of the receptor's G protein partners. Atomic resolution information derived from increasingly available receptor structures provides an entrée to the understanding of these events and practically applying it to drug design. Supported by structure-activity relationship information arising from empirical screening, a unified structural model of GPCR activation/inactivation promises to both accelerate drug discovery in this field and improve our fundamental understanding of structure-based drug design in general. This review discusses fundamental problems that persist in drug design and GPCR structural determination.

Role of shear stress and stretch in vascular mechanobiology

Blood vessels are under constant mechanical loading from blood pressure and flow which cause internal stresses (endothelial shear stress and circumferential wall stress, respectively). The mechanical forces not only cause morphological changes of endothelium and blood vessel wall, but also trigger biochemical and biological events. There is considerable evidence that physiologic stresses and strains (stretch) exert vasoprotective roles via nitric oxide and provide a homeostatic oxidative balance. A perturbation of tissue stresses and strains can disturb biochemical homeostasis and lead to vascular remodelling and possible dysfunction (e.g. altered vasorelaxation, tone, stiffness, etc.). These distinct biological endpoints are caused by some common biochemical pathways. The focus of this brief review is to point out some possible commonalities in the molecular pathways in response to endothelial shear stress and circumferential wall stretch.

Role of asparagine-linked glycosylation in cell surface expression and function of the human adrenocorticotropin receptor (melanocortin 2 receptor) in 293/FRT cells

Asparagine-linked glycosylation (N-glycosylation) of G protein-coupled receptors may be necessary for functions ranging from agonist binding, folding, maturation, stability, and internalization. Human melanocortin 2 receptor (MC2R) possesses putative N-glycosylation sites in its N-terminal extracellular domain; however, to date, the role of MC2R N-glycosylation has yet to be investigated. The objective of the present study is to examine whether N-glycosylation is essential or not for cell surface expression and cAMP production in native and MC2R accessory protein (MRAP alpha, -beta, or -dCT)-expressing cells using 293/FRT transfected with Myc-MC2R. Western blot analyses performed with or without endoglycosidase H, peptide:N-glycosidase F or tunicamycin treatments and site-directed mutagenesis revealed that MC2R was glycosylated in the N-terminal domain at its two putative N-glycosylation sites (Asn(12)-Asn(13)-Thr(14) and Asn(17)-Asn(18)-Ser(19)). In the absence of human MRAP coexpression, N-glycosylation of at least one of the two sites was necessary for MC2R cell surface expression. However, when MRAP was present, cell surface expression of MC2R mutants was either rescued entirely with the N17-18Q (QQNN) and N12-13Q (NNQQ) mutants or partially with the unglycosylated N12-13, 17-18Q (QQQQ) mutant. Functional and expression analyses revealed a discrepancy between wild-type (WT) and QQQQ cell surface receptor levels and maximal cAMP production with a 4-fold increase in EC(50) values. Taken together, these results indicate that the absence of MC2R N-glycosylation abrogates to a large extent MC2R cell surface expression in the absence of MRAPs, whereas when MC2R is N-glycosylated, it can be expressed at the plasma membrane without MRAP assistance.

Employing Rhodobacter sphaeroides to functionally express and purify human G protein-coupled receptors

G protein-coupled receptors (GPCRs) represent the largest class of cell surface receptors and play crucial roles in many cellular and physiological processes. Functional production of recombinant GPCRs is one of the main bottlenecks to obtaining structural information. Here, we report the use of a novel bacterial expression system based on the photosynthetic bacteriumRhodobacter sphaeroidesfor the production of human recombinant GPCRs. The advantage of employingR. sphaeroidesas a host lies in the fact that it provides much more membrane surface per cell compared to other typical expression hosts. The system was tailored to overexpress recombinant receptors under the control of the moderately strong and highly regulated superoperonic photosynthetic promoterpufQ. We tested this system for the expression of some class A GPCRs, namely, the human adenosine A2a receptor (A2aR), the human angiotensin AT1a receptor (AT1aR) and the human bradykinin B2 receptor (B2R). Several different constructs were examined and functional production of the recombinant receptors was achieved. The best-expressed receptor, AT1aR, was solubilized and affinity-purified. To the best of our knowledge, this is the first report of successful use of a bacterial host –R. sphaeroides– to produce functional recombinant GPCRs under the control of a photosynthetic gene promoter.

Heterologous expression and comparative characterization of the human neuromedin U subtype II receptor using the methylotrophic yeast Pichia pastoris and mammalian cells

Neuromedin U (a neuropeptide) plays regulatory roles in feeding, anxiety, smooth muscle contraction, blood flow and pain. The physiological actions of NmU are mediated via two recently identified G protein-coupled receptors namely the neuromedin U type 1 receptor (NmU(1)R) and the neuromedin U type 2 receptor (NmU(2)R). Despite their crucial roles in cell physiology, structural information on these receptors is limited, mainly due to their low expression levels in native tissues. Here, we report the overexpression of the human NmU(2)R in the methylotrophic yeast Pichia pastoris and baby hamster kidney (BHK) cells using the Semliki Forest virus (SFV) system. The recombinant receptor was expressed as a fusion protein with three different affinity tags namely, the Flag tag, the histidine 10 tag and the biotinylation domain of Propionobacterium shermanii. Expression level of the recombinant receptor was 6-9pmol/mg under optimized conditions, which is significantly higher than the expression level in the native tissues. The recombinant receptor binds to its endogenous ligand neuromedin U with high affinity (Kd=0.8-1.0nM) and the binding constant for the recombinant receptor is similar to that of the wild type NmU(2)R. Enzymatic deglycosylation suggested that the recombinant NmU(2)R was glycosylated in P. pastoris, but not in BHK cells. Confocal laser scanning microscopy and immunogold labelling experiment revealed that the recombinant receptor was predominantly localized in the intracellular membranes. To our knowledge, this is the first report of heterologous overexpression of an affinity tagged recombinant NmU(2)R and it should facilitate further characterization of this receptor.