A novel “reverse screening” to identify refolding additives for activin-A

Activin E upregulates uncoupling protein 1 and fibroblast growth factor 21 in brown adipocytes

Activin E activates brown and beige adipocytes and has been controversially implicated as a factor that induces obesity and fatty liver. Here, we sought to address this controversial issue by producing recombinant human activin E to evaluate its effects on HB2 brown adipocytes in vitro. Activin E increased uncoupling protein 1 (Ucp1) and fibroblast growth factor 21 (Fgf21) mRNA expression in the adipocytes. This upregulation was suppressed by SB431542, an inhibitor of activin receptor-like kinase (Alk) TGF-β type I receptors. SB431542 also inhibited the activin E-induced phosphorylation of Smad2/3. A promoter assay using a CAGA-Luc reporter and Alk expression vectors revealed that activin E activated the TGF-β/activin pathway via Alk7. The upregulation of Ucp1 and Fgf21 mRNA might be mediated through Alk7 and Smad2/3 phosphorylation. Activin E is a potential stimulator of energy expenditure by activating brown adipocytes and highlights its potential as a therapeutic target for treating obesity.

TGFBR3L is an inhibin B co-receptor that regulates female fertility

Follicle-stimulating hormone (FSH), a key regulator of ovarian function, is often used in infertility treatment. Gonadal inhibins suppress FSH synthesis by pituitary gonadotrope cells. The TGFβ type III receptor, betaglycan, is required for inhibin A suppression of FSH. The inhibin B co-receptor was previously unknown. Here, we report that the gonadotrope-restricted transmembrane protein, TGFBR3L, is the elusive inhibin B co-receptor. TGFBR3L binds inhibin B but not other TGFβ family ligands. TGFBR3L knockdown or overexpression abrogates or confers inhibin B activity in cells. Female Tgfbr3l knockout mice exhibit increased FSH levels, ovarian follicle development, and litter sizes. In contrast, female mice lacking both TGFBR3L and betaglycan are infertile. TGFBR3L’s function and cell-specific expression make it an attractive new target for the regulation of FSH and fertility.

Production, Isolation, and Structural Analysis of Ligands and Receptors of the TGF-β Superfamily

The ability to understand the molecular mechanisms by which secreted signaling proteins of the TGF-β superfamily assemble their cell surface receptors into complexes to initiate downstream signaling is dependent upon the ability to determine atomic-resolution structures of the signaling proteins, the ectodomains of the receptors, and the complexes that they form. The structures determined to date have revealed major differences in the overall architecture of the signaling complexes formed by the TGF-βs and BMPs, which has provided insights as to how they have evolved to fulfill their distinct functions. Such studies, have however, only been applied to a few members of the TGF-β superfamily, which is largely due to the difficulty of obtaining milligram-scale quantities of highly homogenous preparations of the disulfide-rich signaling proteins and receptor ectodomains of the superfamily. Here we describe methods used to produce signaling proteins and receptor ectodomains of the TGF-β superfamily using bacterial and mammalian expression systems and procedures to purify them to homogeneity.

Approaches for the generation of active papain-like cysteine proteases from inclusion bodies of Escherichia coli

Papain-like cysteine proteases are widely expressed, fulfill specific functions in extracellular matrix turnover, antigen presentation and processing events, and may represent viable drug targets for major diseases. In depth and rigorous studies of the potential for these proteins to be targets for drug development require sufficient amounts of protease protein that can be used for both experimental and therapeutic purposes. Escherichia coli was widely used to express papain-like cysteine proteases, but most of those proteases are produced in insoluble inclusion bodies that need solubilizing, refolding, purifying and activating. Refolding is the most critical step in the process of generating active cysteine proteases and the current approaches to refolding include dialysis, dilution and chromatography. Purification is mainly achieved by various column chromatography. Finally, the attained refolded proteases are examined regarding their protease structures and activities.

MB109 as bioactive human bone morphogenetic protein-9 refolded and purified from E. coli inclusion bodies

Background

The development of chemical refolding of transforming growth factor-beta (TGF-β) superfamily ligands has been instrumental to produce the recombinant proteins for biochemical studies and exploring the potential of protein therapeutics. The osteogenic human bone morphogenetic protein-2 (hBMP-2) and its Drosophila DPP homolog were the early successful cases of refolding into functional form. Despite the similarity in their three dimensional structure and amino acid sequences, several other TGF-β superfamily ligands could not be refolded readily by the same methods.

Results

Here, we report a comprehensive study on the variables of a rapid-dilution refolding method, including the concentrations of protein, salt, detergent and redox agents, pH, refolding duration and the presence of aggregation suppressors and host-cell contaminants, in order to identify the optimal condition to refold human BMP-9 (hBMP-9). To produce a recombinant form of hBMP-9 in E. coli cells, a synthetic codon-optimized gene was designed to encode the mature domain of hBMP-9 (Ser320 – Arg429) directly behind the first methionine, which we herein referred to as MB109. An effective purification scheme was also developed to purify the refolded MB109 to homogeneity with a final yield of 7.8 mg from 100 mg of chromatography-purified inclusion bodies as a starting material. The chemically refolded MB109 binds to ALK1, ActRIIb and BMPRII receptors with relatively high affinity as compared to other Type I and Type II receptors based on surface plasmon resonance analysis. Smad1-dependent luciferase assay in C2C12 cells shows that the MB109 has an EC₅₀ of 0.61 ng/mL (25 pM), which is nearly the same as hBMP-9.

Conclusion

MB109 is prone to be refolded as non-functional dimer and higher order multimers in most of the conditions tested, but bioactive MB109 dimer can be refolded with high efficiency in a narrow window, which is strongly dependent on the pH, refolding duration, the presence of aggregation suppressors and the concentrations of protein, salt and detegent. These results add to the current understanding of producing recombinant TGF-β superfamily ligands in the microbial E. coli system. An application of the technique to produce a large number of synthetic TGF-β chimeras for activity screen is also discussed.

Large scale modification of biomolecules using immobilized sortase A from Staphylococcus aureus

Recently, sortase A (SrtA) from Staphyloccus aureus moved into the focus of bioscience because of its ability to incorporate site specific modifications into proteins. The enzyme was mostly used to modify target proteins in an analytical scale, to study biomolecules in their cellular context. In this study, we show the applicability of SrtA mediated ligation for site specific modification of proteins in a large scale. Therefore, the reaction was first optimized using peptides and subsequently new reaction conditions were applied for the large scale biotinylation of interleukin-8. Furthermore, we established C-terminal immobilization of the SrtA on a PEG based resin and could demonstrate maintaining enzymatic activity. Immobilized SrtA significantly facilitates previous ligation protocols as the enzyme can be easily recycled. Also, the removal of excess reaction solution and the whole washing process is significantly accelerated, as centrifugation or filtration techniques can be applied instead of time-consuming chromatography steps.

Development and characterization of an automated high throughput screening method for optimization of protein refolding processes

Optimization of protein refolding parameters by automated, miniaturized, and parallelized high throughput screening is a powerful approach to meet the demand for fast process development with low material consumption. In this study, we validated methods applicable on a standard liquid handling robot for screening of refolding process parameters by dilution of denatured lysozyme in refolding buffer systems. Different approaches for the estimation of protein solubility and folding were validated concerning resolution and compatibility with the robotic system and with the complex buffer and protein structure composition. We established an indirect method to assess soluble lysozyme concentration independent of matrix effects and protein structure varieties by automated separation of aggregated protein, resolubilization, and measurement of absorption at 280 nm. Using this nonspecific solubility assays the correlation between favorable parameters for high active and soluble lysozyme yields were evaluated. An overlap of good refolding buffer compositions was found provided that the redox environment was controlled with redox reagents. In addition, the need to control unfolding conditions like time, temperature, lysozyme, and dithiothreitol concentration was pointed out as different feedstocks resulted in different refolding yields.

Single pH buffer refolding screen for protein from inclusion bodies

We previously reported the set up of an automated test for screening the refolding of recombinant proteins expressed as inclusion bodies in Escherichia coli[1]. The screen used 96 refolding buffers and was validated with 24 proteins, 70% of which remained soluble in at least one buffer. In the present paper, we have analyzed in more detail these experimental data to see if the refolding process can be driven by general rules. Notably, we found that proteins with an acidic isoelectric point (pI) refolded in buffers the average pH of which was alkaline and conversely. In addition, the number of refolding buffers wherein a protein remained soluble increased with the difference between its pI and the average pH of the buffers in which it refolded. A trend analysis of the other variables (ionic strength, detergents, etc.) was also performed. On the basis of this analysis, we devised and validated a new refolding screen made of a single buffer for acidic proteins and a single buffer for alkaline proteins.

Designer TGFβ superfamily ligands with diversified functionality

Transforming Growth Factor – beta (TGFβ) superfamily ligands, including Activins, Growth and Differentiation Factors (GDFs), and Bone Morphogenetic Proteins (BMPs), are excellent targets for protein-based therapeutics because of their pervasiveness in numerous developmental and cellular processes. We developed a strategy termed RASCH (Random Assembly of Segmental Chimera and Heteromer), to engineer chemically-refoldable TGFβ superfamily ligands with unique signaling properties. One of these engineered ligands, AB208, created from Activin-βA and BMP-2 sequences, exhibits the refolding characteristics of BMP-2 while possessing Activin-like signaling attributes. Further, we find several additional ligands, AB204, AB211, and AB215, which initiate the intracellular Smad1-mediated signaling pathways more strongly than BMP-2 but show no sensitivity to the natural BMP antagonist Noggin unlike natural BMP-2. In another design, incorporation of a short N-terminal segment from BMP-2 was sufficient to enable chemical refolding of BMP-9, without which was never produced nor refolded. Our studies show that the RASCH strategy enables us to expand the functional repertoire of TGFβ superfamily ligands through development of novel chimeric TGFβ ligands with diverse biological and clinical values.

Acyl cystamine: small-molecular foldase mimics accelerating oxidative refolding of disulfide-containing proteins

Based on the structural characteristic of Protein disulfide isomerases and DsbA that have hydrophobic regions around the active sites, hydrophobic alkyl tails are linked to cystamine to create new small molecular foldase mimics, acyl cystamine. Both the oxidizing power and oxidation specificity of cystamine are enhanced by n-octanoyl or n-hexanoyl tail. N-octanoyl and n-hexanoyl cystamine are very effective to facilitate oxidative protein refolding at strong reducing environments. In the presence of 0.42 mM DTT, the activity recovery of lysozyme is over 90% by 90-min refolding with 0.1 mM n-octanoyl cystamine and 0.1 mM cystamine as oxidant, while almost no activity is recovered with 0.2 mM GSSG by 160-min refolding. For the refolding of 0.2 mg/mL lysozyme, with 0.6 mM n-hexanoyl cystamine and 1.12 mM residual DTT as redox agents, the activity recovery reaches as high as 93% after refolding for only 20 min. For ribonuclease A (RNase A) refolding, with 0.4 mM n-hexanoyl cystamine and 1.30 mM DTT, the recovery of activity reaches as high as 90% within 3 h. Thus, with n-octanoyl or n-hexanoyl cystamine as the oxidants, the necessity to remove excess DTT in the reduced and denatured protein solutions can be greatly alleviated. With a moderate hydrophobicity, n-hexanoyl cystamine is promising for application in oxidative protein refolding at an extensive concentration range. It is observed that in the oxidative refolding of 0.2 mg/mL lysozyme and RNase A, only about half of n-hexanoyl cystamine is needed when compared to cystamine to achieve the same kinetic effect.

Arginine improves protein elution in hydrophobic interaction chromatography

The effects of arginine on protein binding and elution in hydrophobic interaction chromatography (HIC) were examined using recombinant human interleukin-6 (IL-6) and activin-A. Binding of IL-6 in the presence of ammonium sulfate (AS) was tested using low- and high-substituted phenyl-sepharose. While inclusion of arginine during loading of IL-6 resulted in incomplete binding to the low-substituted phenyl-sepharose, binding was complete to the high-substituted phenyl-sepharose. Arginine facilitated elution of IL-6 from both columns. These results demonstrate that arginine weakens hydrophobic interactions between IL-6 and the phenyl-sepharose. More drastic results were obtained using activin-A, which showed undetectable recovery from phenyl-sepharose. Although no apparent elution of activin-A was observed from butyl-sepharose in aqueous buffer alone, the addition of arginine to the buffer resulted in partial elution recovery and, together with ethanol, resulted in greatly improved recovery of the protein. Two arginine derivatives, acetylarginine and agmatine, were also effective. These results show that arginine improves protein elution in HIC.

Correlation Between Thermal Aggregation and Stability of Lysozyme with Salts Described by Molar Surface Tension Increment: An Exceptional Propensity of Ammonium Salts as Aggregation Suppressor

Protein aggregation is a critical problem for biotechnology and pharmaceutical industries. Despite the fact that soluble proteins have been used for many applications, our understanding of the effect of the solution chemistry on protein aggregation still remains to be elucidated. This paper investigates the process of thermal aggregation of lysozyme in the presence of various types of salts. The simple law was found; the aggregation rate of lysozyme increased with increasing melting temperature of the protein (T (m)) governed by chemical characteristics of additional salts. Ammonium salts were, however, ruled out; the aggregation rates of lysozyme in the presence of the ammonium salts were smaller than the ones estimated from T (m). Comparing with sodium salts, ammonium salts increased the solubility of the hydrophobic amino acids, indicating that ammonium salts adsorb the hydrophobic region of proteins, which leads to the decrease in aggregation more effectively than sodium salts. The positive relation between aggregation rate and T (m) was described by another factor such as the surface tension of salt solutions. Fourier transform infrared spectral analysis showed that the thermal aggregates were likely to form beta-sheet in solutions that give high molar surface tension increment. These results suggest that protein aggregation is attributed to the surface free energy of the solution.