Overexpression and functional characterisation of the human melanocortin 4 receptor in Sf9 cells

Mutations in MC4R facilitate the angiogenic activity in patients with orbital venous malformation

IMPACT STATEMENT

The detailed molecular mechanism of orbital venous malformation (OVM) is still not clear. Using whole exome sequencing, 4 types of melanocortin 4 receptor (MC4R) mutation were detected in 7 of 27 patients with OVM, and all types of MC4R mutations resulted in the upregulation of MC4R expression. In vitro study indicated that MC4R has impacts on the proliferation, cell cycle, migration, and tube formation of the endothelial cells. Moreover, MC4R mutations altered the downstream signaling, including cAMP concentration and the expression levels of several PI3K/AKT/mTOR downstream genes, including p21, cyclin B1, ITGA10, and ITGA11. MC4R mutations may lead to the pathogenesis of OVM through modulating the downstream signaling to alter the angiogenic activity of endothelial cells.

Sf9 cells: a versatile model system to investigate the pharmacological properties of G protein-coupled receptors

The Sf9 cell/baculovirus expression system is widely used for high-level protein expression, often with the purpose of purification. However, proteins may also be functionally expressed in the defined Sf9 cell environment. According to the literature, the pharmacology of G-protein-coupled receptors (GPCRs) functionally reconstituted in Sf9 cells is similar to the receptor properties in mammalian cells. Sf9 cells express both recombinant GPCRs and G-proteins at much higher levels than mammalian cells. Sf9 cells can be grown in suspension culture, providing an inexpensive way of obtaining large protein amounts. Co-infection with various baculoviruses allows free combination of GPCRs with different G-proteins. The absence of constitutively active receptors in Sf9 cells provides an excellent signal-to background ratio in functional assays, allowing the detection of agonist-independent receptor activity and of small ligand-induced signals including partial agonistic and inverse agonistic effects. Insect cell Gα(i)-like proteins mostly do not couple productively to mammalian GPCRs. Thus, unlike in mammalian cells, Sf9 cells do not require pertussis toxin treatment to obtain a Gα(i)-free environment. Co-expression of GPCRs with Gα(i1), Gα(i2), Gα(i3) or Gα(o) in Sf9 cells allows the generation of a selectivity profile for these Gα(i/o)-isoforms. Additionally, GPCR-G-protein combinations can be compared with defined 1:1 stoichiometry by expressing GPCR-Gα fusion proteins. Sf9 cells can also be employed for ligand screening in medicinal chemistry programs, using radioligand binding assays or functional assays, like the steady-state GTPase- or [(35)S]GTPγS binding assay. This review shows that Sf9 cells are a versatile model system to investigate the pharmacological properties of GPCRs.

Abundant expression and purification of biologically active mitochondrial citrate carrier in baculovirus-infected insect cells

Heterologous expression of recombinant proteins is an essential technology for protein characterization. A major obstacle to investigating the biochemical properties of membrane proteins is the difficulty in obtaining sufficient amounts of functional protein. Here we report the successful expression of the tricarboxylate (or citrate) carrier (CIC) of eel (Anguilla anguilla) from Spodoptera frugiperda (Sf9) cells using the baculovirus expression system. The recombinant CIC was purified by affinity chromatography on Ni(2+)-NTA agarose; the yield of the purified active protein was 0.4-0.5 mg/l of culture. The transport characteristics of the recombinant CIC and the effects of inhibitors on transport are similar to those determined for eel liver mitochondrial CIC. Because the CIC is one member of an extensive family of mitochondrial transport proteins, it is likely that the procedure used in this study to express and purify this carrier can be successfully applied to other mitochondrial transport proteins, thus providing sufficient protein for functional characterization.

Increased expression of the integral membrane protein ErbB2 in Chinese hamster ovary cells expressing the anti-apoptotic gene Bcl-xL

Receptor tyrosine kinases (RTKs) are the second largest family of membrane receptors and play a key role in the regulation of vital cellular processes, such as control of cell growth, differentiation, metabolism, and migration. The production of whole-length RTKs in large quantities for biophysical or structural characterization, however, is a challenge. In this study, a cell engineering strategy using the anti-apoptotic Bcl-2 family protein, Bcl-x(L), was tested as a potential method for increasing stable expression levels of a recombinant RTK membrane protein in Chinese hamster ovary (CHO) cells. Wild-type and CHO cells stably overexpressing heterologous Bcl-x(L) were transformed with the gene for a model RTK membrane protein, ErbB2, on a plasmid also containing the Zeocin resistance gene. While CHO cells exhibited a gradual decrease in expression with passaging, CHO-bcl-x(L) cells offered an increased and sustained level of ErbB2 expression following continuous passaging over more than 33 days in culture. The increased ErbB2 expression in CHO-bcl-x(L) cells was evident both in stable transfected pools and in clonal isolates, and demonstrated both in Western blot analysis and flow cytometry. Furthermore, the sustained high-level protein expression in CHO-bcl-x(L) cells does not alter the correct membrane localization of the ErbB2 protein. Our results demonstrate that cellular engineering, specifically anti-apoptosis engineering, can provide increased and stable ErbB2 membrane protein expression in mammalian cells. This approach may also be useful for other membrane proteins in which large quantities are needed for biophysical and structural studies.

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.

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

Angiotensin II type 1a receptor (AT1aR) is a member of GPCR superfamily and it plays crucial roles in the regulation of blood pressure, hormone secretion and renal functions. Here, we report functional overexpression and characterization of the human AT1aR in insect cells using the baculovirus system and in mammalian cells using the Semliki Forest virus system. The recombinant receptor was expressed at a level of 29-32 pmol/mg and it binds to angiontensin II with high affinity (Kd=0.98-1.1 nM). Angiotensin II stimulated accumulation of inositol phosphate and phosphorylation of MAP kinase was also observed, which indicated that the recombinant AT1aR could couple to endogenous Galphaq protein. Confocal laser scanning microscopy revealed that the recombinant receptor was predominantly localized in the plasma membrane and agonist induced internalization of the recombinant AT1aR was also observed. The recombinant AT1aR was expressed in glycosylated form and in vivo inhibition of glycosylation suppressed its surface expression.

Co-operative regulation of ligand binding to melanocortin receptor subtypes: Evidence for interacting binding sites

This study evaluates the binding the melanocyte stimulating hormone peptide analogue [125I]NDP-MSH to melanocortin receptors MC1, MC3, MC4 and MC5 in insect cell membranes produced by baculovirus expression systems. The presence of Ca2+ was found to be mandatory to achieve specific [125I]NDP-MSH binding to the melanocortin receptors. Although association kinetics of [125I]NDP-MSH followed the regularities of simple bimolecular reactions, the dissociation of [125I]NDP-MSH from the melanocortin receptors was heterogeneous. Eleven linear and cyclic MSH peptides studied displaced the [125I]NDP-MSH binding to the studied melanocortin receptors, with the shapes of their competition curves varying from biphasic or shallow to super-steep (Hill coefficients ranging from 0.4 to 1.5). Notably the same peptide often gave highly different patterns on different melanocortin receptor subtypes; e.g. the MC4 receptor selective antagonist HS131 gave a Hill coefficient of 1.5 on the MC1 receptor but 0.5-0.7 on the MC(3-5) receptors. Adding a mask of one of the peptides to block its high affinity binding did not prevent other competing peptides to yield biphasic competition curves. The data indicate that the binding of MSH peptides to melanocortin receptors are governed by a complex dynamic homotropic co-operative regulations.