CHO cell engineering to prevent polypeptide aggregation and improve therapeutic protein secretion.

The ability to efficiently produce recombinant proteins in a secreted form is highly desirable and cultured mammalian cells such as CHO cells have become the preferred host as they secrete proteins with human-like post-translational modifications. However, attempts to express high levels of particular proteins in CHO cells may consistently result in low yields, even for non-engineered proteins such as immunoglobulins. In this study, we identified the responsible faulty step at the stage of translational arrest, translocation and early processing for such a "difficult-to-express" immunoglobulin, resulting in improper cleavage of the light chain and its precipitation in an insoluble cellular fraction unable to contribute to immunoglobulin assembly. We further show that proper processing and secretion were restored by over-expressing human signal receptor protein SRP14 and other components of the secretion pathway. This allowed the expression of the difficult-to-express protein to high yields, and it also increased the production of an easy-to-express protein. Our results demonstrate that components of the secretory and processing pathways can be limiting, and that engineering of the secretory pathway may be used to improve the secretion efficiency of therapeutic proteins from CHO cells.

Metabolic labeling and protein linearization technology allow the study of proteins secreted by cultured cells in serum-containing media.

Supernatants from cell cultures (also called conditioned media, CMs) are commonly analyzed to study the pool of secreted proteins (secretome). To reduce the exogenous protein background, serum-free media are often used to obtain CMs. Serum deprivation, however, can severely affect cell viability and phenotype, including protein secretion. We present a strategy to analyze the proteins secreted by cells in fetal bovine serum-containing CMs, which combines the advantage of metabolic labeling and protein concentration linearization techniques. Incubation of CMs with a hexapeptide ligand library was used to reduce the dynamic range of the samples and led to the identification of 3 times more proteins than in untreated CM samples. Labeling with a deuterated amino acid was used to distinguish between cellular proteins and homologous bovine proteins contained in the medium. Application of the strategy to two breast cancer cell lines led to the identification of proteins secreted in different amounts and which could correlate with their varying degree of aggressiveness. Selected reaction monitoring (SRM)-based quantitation of three proteins of interest in the crude samples yielded data in good agreement with the results from concentration-equalized samples.

Long-term efficacy of ciliary muscle gene transfer of three sFlt-1 variants in a rat model of laser-induced choroidal neovascularization.

Inhibition of vascular endothelial growth factor (VEGF) has become the standard of care for patients presenting with wet age-related macular degeneration. However, monthly intravitreal injections are required for optimal efficacy. We have previously shown that electroporation enabled ciliary muscle gene transfer results in sustained protein secretion into the vitreous for up to 9 months. Here, we evaluated the long-term efficacy of ciliary muscle gene transfer of three soluble VEGF receptor-1 (sFlt-1) variants in a rat model of laser-induced choroidal neovascularization (CNV). All three sFlt-1 variants significantly diminished vascular leakage and neovascularization as measured by fluorescein angiography (FA) and flatmount choroid at 3 weeks. FA and infracyanine angiography demonstrated that inhibition of CNV was maintained for up to 6 months after gene transfer of the two shortest sFlt-1 variants. Throughout, clinical efficacy was correlated with sustained VEGF neutralization in the ocular media. Interestingly, treatment with sFlt-1 induced a 50% downregulation of VEGF messenger RNA levels in the retinal pigment epithelium and the choroid. We demonstrate for the first time that non-viral gene transfer can achieve a long-term reduction of VEGF levels and efficacy in the treatment of CNV.Gene Therapy advance online publication, 27 June 2013; doi:10.1038/gt.2013.36.

Development of a coculture model of encapsulated cells.

In the whole animal, metabolic regulations are set by reciprocal interactions between various organs, via the blood circulation. At present, analyses of such interactions require numerous and uneasily controlled in vivo experiments. In a search for an alternative to in vivo experiments, our work aims at developing a coculture system in which different cell types are isolated in polymer capsules and grown in a common environment. The signals exchanged between cells from various origins are, thus, reproducing the in vivo intertissular communications. With this perspective, we evaluated a new encapsulation system as an artificial housing for liver cells on the one hand and adipocytes on the other hand. Murine hepatocytes were encapsulated with specially designed multicomponent capsules formed by polyelectrolyte complexation between sodium alginate, cellulose sulphate and poly(methylene-coguanidine) hydrochloride, of which the permeability has been characterized. We demonstrated the absence of cytotoxicity and the excellent biocompatibility of these capsules towards primary culture of murine hepatocytes. Encapsulated hepatocytes retain their specific functions--transaminase activity, urea synthesis, and protein secretion--during the first four days of culture in minimum medium. Mature adipocytes, isolated from mouse epidydimal fat, were embedded in alginate beads. Measurement of protein secretion shows an identical profile between free and embedded adipocytes. We finally assessed the properties of encapsulated hepatocytes, cryopreserved over a periods of up to four months. The perspective of using encapsulated cells in coculture are discussed, since this system may represent a promising tool for fundamental research, such as analyses of drug metabolism, intercellular regulations, and metabolic pathways, as well as for the establishment of a tissue bank for storage and supply of murine hepatocytes.

Involvement of nuclear factor-kappaB in macrophage migration inhibitory factor gene transcription up-regulation induced by interleukin- 1 beta in ectopic endometrial cells.

OBJECTIVE: To investigate the involvement of the nuclear factor (NF)-kappaB in the interleukin (IL)-1 beta-mediated macrophage migration inhibitory factor (MIF) gene activation. DESIGN: Prospective study. SETTING: Human reproduction research laboratory. PATIENT(S): Nine women with endometriotic lesions. INTERVENTION(S): Endometriotic lesions were obtained during laparoscopic surgery. MAIN OUTCOME MEASURE(S): The MIF protein secretion was analyzed by ELISA, MIF mRNA expression by quantitative real-time polymerase chain reaction (PCR), NF-kappaB translocation into the nucleus by electrophoresis mobility shift assay, I kappaB phosphorylation and degradation by Western blot, and human MIF promoter activity by transient cell transfection. RESULT(S): This study showed a significant dose-dependent increase of MIF protein secretion and mRNA expression, the NF-kappaB translocation into the nucleus, I kappaB phosphorylation, I kappaB degradation, and human MIF promoter activity in endometriotic stromal cells in response to IL-1 beta. Curcumin (NF-kappaB inhibitor) significantly inhibited all these IL-1 beta-mediated effects. Analysis of the activity of deletion constructs of the human MIF promoter and a computer search localized two putative regulatory elements corresponding to NF-kappaB binding sites at positions -2538/-2528 bp and -1389/-1380 bp. CONCLUSION(S): This study suggests the involvement of the nuclear transcription factor NF-kappaB in MIF gene activation in ectopic endometrial cells in response to IL-1 beta and identifies a possible pathway of endometriosis-associated inflammation and ectopic cell growth.

Securin and separase modulate membrane traffic by affecting endosomal acidification.

Securin and separase play a key role in sister chromatid separation during anaphase. However, a growing body of evidence suggests that in addition to regulating chromosome segregation, securin and separase display functions implicated in membrane traffic in Caenorhabditis elegans and Drosophila. Here we show that in mammalian cells both securin and separase associate with membranes and that depletion of either protein causes robust swelling of the trans-Golgi network (TGN) along with the appearance of large endocytic vesicles in the perinuclear region. These changes are accompanied by diminished constitutive protein secretion as well as impaired receptor recycling and degradation. Unexpectedly, cells depleted of securin or separase display defective acidification of early endosomes and increased membrane recruitment of vacuolar (V-) ATPase complexes, mimicking the effect of the specific V-ATPase inhibitor Bafilomycin A1. Taken together, our findings identify a new functional role of securin and separase in the modulation of membrane traffic and protein secretion that implicates regulation of V-ATPase assembly and function.

Epidermal growth factor (EGF) stimulation of cultured brain cells. II. Increased production of extracellular soluble proteins.

The production of extracellular soluble proteins was studied in serum-free aggregating cell cultures of fetal rat telencephalon labeled on culture day 7 with a mixture of radioactive amino acid precursors. Cultures treated continuously with epidermal growth factor (EGF; 20 ng/ml) showed a generally increased protein secretion and a particularly enhanced production of a few distinct extracellular proteins. The time lag of this response after an initial dose of EGF (25 ng/ml) on day 7 was 48 h. The total macromolecular radioactivity that accumulated within 96 h of labeling in the media of EGF-treated cultures was 175% of untreated controls, whereas no difference was found in the proportions of intracellular amino acid incorporation. Cultures which received a single dose of EGF (25 ng/ml) on day 1 showed still a greatly increased protein secretion on day 7. Prevention of extracellular protein accumulation by reducing the initial cell number and increasing the rate of media changes did not affect the EGF-induced stimulation of the two glial enzymes, glutamine synthetase and 2',3'-cyclic nucleotide 3'-phosphohydrolase. The results suggest that both the increased production of extracellular proteins and the enhanced development of glial enzymatic activities reflect the stimulated phenotypic expression of EGF-sensitive brain cells.

Regulation of the expression of Fit insect toxin locus genes in the root-associated biocontrol pseudomonad CHA0

The root-colonizing Pseudomonas fluorescens strain CHA0 is a biocontrol agent of soil-borne plant diseases caused by fungal and oomycete pathogens. Remarkably, this plant-beneficial pseudomonad is also endowed with potent insecticidal activity that depends on the production of a large protein toxin termed Fit (for P. fluorescens insecticidal toxin). In our present work, the genomic locus encoding the P. fluorescens insect toxin is subjected to a detailed molecular analysis. The Fit toxin gene fitD is flanked upstream by the fitABC genes and downstream by the fitE gene that encode the ABC transporter, membrane fusion, and outer membrane efflux components of a type I protein secretion system predicted to function in toxin export. The fitF, fitG, and fitH genes located downstream of fitE code for regulatory proteins having domain structures typical of signal transduction histidine kinases, LysR-type transcriptional regulators, and response regulators, respectively. The role of these insect toxin locus-associated control elements is being investigated with mutants defective for the regulatory genes and with GFP-based reporter fusions to putative promoter regions upstream of the transporter genes fitA and fitE, the toxin gene fitD, and the regulatory genes fitF and fitH. Our preliminary findings suggest that the three regulators interact with known global regulators of biocontrol factor expression to control Fit toxin expression and secretion.

Macrophage migration inhibitory factor is involved in a positive feedback loop increasing aromatase expression in endometriosis.

Immune-endocrine interplay may play a major role in the pathogenesis of endometriosis. In the present study, we have investigated the interaction between macrophage migration inhibitory factor (MIF), a major pro-inflammatory and growth-promoting factor markedly expressed in active endometriotic lesions, and estradiol (E(2)) in ectopic endometrial cells. Our data showed a significant increase of MIF protein secretion and mRNA expression in endometriotic cells in response to E(2). MIF production was blocked by Fulvestrant, an estrogen receptor (ER) antagonist, and induced by ERα and ERβ selective agonists propyl-pyrazole-triol (PPT) and diarylpropionrile (DPN), respectively, thus demonstrating a specific receptor-mediated effect. Cell transfection with MIF promoter construct showed that E(2) significantly stimulates MIF promoter activity. Interestingly, our data further revealed that MIF reciprocally stimulates aromatase protein and mRNA expression via a posttranscriptional mRNA stabilization mechanism, that E(2) itself can upregulate aromatase expression, and that inhibition of endogenous MIF, using MIF specific siRNA, significantly inhibits E(2)-induced aromatase. Thus, the present study revealed the existence of a local positive feedback loop by which estrogen acts directly on ectopic endometrial cells to upregulate the expression of MIF, which, in turn, displays the capability of inducing the expression of aromatase, the key and rate-limiting enzyme for estrogen synthesis. Such interplay may have a considerable impact on the development of endometriosis.

Plasmid electrotransfer of eye ciliary muscle: principles and therapeutic efficacy using hTNF-alpha soluble receptor in uveitis.

Due to its small size and particular isolating barriers, the eye is an ideal target for local therapy. Recombinant protein ocular delivery requires invasive and painful repeated injections. Alternatively, a transfected tissue might be used as a local producer of transgene-encoded therapeutic protein. We have developed a nondamaging electrically mediated plasmid delivery technique (electrotransfer) targeted to the ciliary muscle, which is used as a reservoir tissue for the long-lasting expression and secretion of therapeutic proteins. High and long-lasting reporter gene expression was observed, which was restricted to the ciliary muscle. Chimeric TNF-alpha soluble receptor (hTNFR-Is) electrotransfer led to elevated protein secretion in aqueous humor and to drastic inhibition of clinical and histological inflammation scores in rats with endotoxin-induced uveitis. No hTNFR-Is was detected in the serum, demonstrating the local delivery of proteins using this method. Plasmid electrotransfer to the ciliary muscle, as performed in this study, did not induce any ocular pathology or structural damage. Local and sustained therapeutic protein production through ciliary muscle electrotransfer is a promising alternative to repeated intraocular protein administration for a large number of inflammatory, degenerative, or angiogenic diseases.

SOCS-1 protein prevents Janus Kinase/STAT-dependent inhibition of beta cell insulin gene transcription and secretion in response to interferon-gamma.

In the pathogenesis of type I diabetes mellitus, activated leukocytes infiltrate pancreatic islets and induce beta cell dysfunction and destruction. Interferon (IFN)-gamma, tumor necrosis factor-alpha and interleukin (IL)-1 beta play important, although not completely defined, roles in these mechanisms. Here, using the highly differentiated beta Tc-Tet insulin-secreting cell line, we showed that IFN-gamma dose- and time-dependently suppressed insulin synthesis and glucose-stimulated secretion. As described previously IFN-gamma, in combination with IL-1 beta, also induces inducible NO synthase expression and apoptosis (Dupraz, P., Cottet, S., Hamburger, F., Dolci, W., Felley-Bosco, E., and Thorens, B. (2000) J. Biol. Chem. 275, 37672--37678). To assess the role of the Janus kinase/signal transducer and activator of transcription (STAT) pathway in IFN-gamma intracellular signaling, we stably overexpressed SOCS-1 (suppressor of cytokine signaling-1) in the beta cell line. We demonstrated that SOCS-1 suppressed cytokine-induced STAT-1 phosphorylation and increased cellular accumulation. This was accompanied by a suppression of the effect of IFN-gamma on: (i) reduction in insulin promoter-luciferase reporter gene transcription, (ii) decrease in insulin mRNA and peptide content, and (iii) suppression of glucose-stimulated insulin secretion. Furthermore, SOCS-1 also suppressed the cellular effects that require the combined presence of IL-1 beta and IFN-gamma: induction of nitric oxide production and apoptosis. Together our data demonstrate that IFN-gamma is responsible for the cytokine-induced defect in insulin gene expression and secretion and that this effect can be completely blocked by constitutive inhibition of the Janus kinase/STAT pathway.

Microsomal triglyceride transfer protein promotes the secretion of Xenopus laevis vitellogenin A1.

Vitellogenins (Vtg) are ancient lipid transport and storage proteins and members of the large lipid transfer protein (LLTP) gene family, which includes insect apolipophorin II/I, apolipoprotein B (apoB), and the microsomal triglyceride transfer protein (MTP). Lipidation of Vtg occurs at its site of synthesis in vertebrate liver, insect fat body, and nematode intestine; however, the mechanism of Vtg lipid acquisition is unknown. To explore whether Vtg biogenesis requires the apoB cofactor and LLTP family member, MTP, Vtg was expressed in COS cells with and without coexpression of the 97-kDa subunit of human MTP. Expression of Vtg alone gave rise to a approximately 220-kDa apoprotein, which was predominantly confined to an intracellular location. Coexpression of Vtg with human MTP enhanced Vtg secretion by 5-fold, without dramatically affecting its intracellular stability. A comparison of wild type and a triglyceride transfer-defective form of MTP revealed that both were capable of promoting Vtg secretion, whereas only wild type MTP could promote the secretion of apoB41 (amino-terminal 41% of apoB). These studies demonstrate that the biogenesis of Vtg is MTP-dependent and that MTP is the likely ancestral member of the LLTP gene family.

The serine protease hepsin mediates urinary secretion and polymerisation of Zona Pellucida domain protein uromodulin.

Uromodulin is the most abundant protein in the urine. It is exclusively produced by renal epithelial cells and it plays key roles in kidney function and disease. Uromodulin mainly exerts its function as an extracellular matrix whose assembly depends on a conserved, specific proteolytic cleavage leading to conformational activation of a Zona Pellucida (ZP) polymerisation domain. Through a comprehensive approach, including extensive characterisation of uromodulin processing in cellular models and in specific knock-out mice, we demonstrate that the membrane-bound serine protease hepsin is the enzyme responsible for the physiological cleavage of uromodulin. Our findings define a key aspect of uromodulin biology and identify the first in vivo substrate of hepsin. The identification of hepsin as the first protease involved in the release of a ZP domain protein is likely relevant for other members of this protein family, including several extracellular proteins, as egg coat proteins and inner ear tectorins.

Insulin secretion is regulated by the glucose-dependent production of islet beta cell macrophage migration inhibitory factor.

Macrophage migration inhibitory factor (MIF), originally identified as a cytokine secreted by T lymphocytes, was found recently to be both a pituitary hormone and a mediator released by immune cells in response to glucocorticoid stimulation. We report here that the insulin-secreting beta cell of the islets of Langerhans expresses MIF and that its production is regulated by glucose in a time- and concentration-dependent manner. MIF and insulin colocalize by immunocytochemistry within the secretory granules of the pancreatic islet beta cells, and once released, MIF appears to regulate insulin release in an autocrine fashion. In perifusion studies performed with isolated rat islets, immunoneutralization of MIF reduced the first and second phase of the glucose-induced insulin secretion response by 39% and 31%, respectively. Conversely, exogenously added recombinant MIF was found to potentiate insulin release. Constitutive expression of MIF antisense RNA in the insulin-secreting INS-1 cell line inhibited MIF protein synthesis and decreased significantly glucose-induced insulin release. MIF is therefore a glucose-dependent, islet cell product that regulates insulin secretion in a positive manner and may play an important role in carbohydrate metabolism.

Blocking VLDL secretion causes hepatic steatosis but does not affect peripheral lipid stores or insulin sensitivity in mice

The liver secretes triglyceride-rich VLDLs, and the triglycerides in these particles are taken up by peripheral tissues, mainly heart, skeletal muscle, and adipose tissue. Blocking hepatic VLDL secretion interferes with the delivery of liver-derived triglycerides to peripheral tissues and results in an accumulation of triglycerides in the liver. However, it is unclear how interfering with hepatic triglyceride secretion affects adiposity, muscle triglyceride stores, and insulin sensitivity. To explore these issues, we examined mice that cannot secrete VLDL [due to the absence of microsomal triglyceride transfer protein (Mttp) in the liver]. These mice exhibit markedly reduced levels of apolipoprotein B-100 in the plasma, along with reduced levels of triglycerides in the plasma. Despite the low plasma triglyceride levels, triglyceride levels in skeletal muscle were unaffected. Adiposity and adipose tissue triglyceride synthesis rates were also normal, and body weight curves were unaffected. Even though the blockade of VLDL secretion caused hepatic steatosis accompanied by increased ceramides and diacylglycerols in the liver, the mice exhibited normal glucose tolerance and were sensitive to insulin at the whole-body level, as judged by hyperinsulinemic euglycemic clamp studies. Normal hepatic glucose production and insulin signaling were also maintained in the fatty liver induced by Mttp deletion. Thus, blocking VLDL secretion causes hepatic steatosis without insulin resistance, and there is little effect on muscle triglyceride stores or adiposity