Isolation and purification of a biologically active human platelet-derived growth factor BB expressed in Escherichia coli

Protein expression and purification of bioactive growth factors for use in cell culture and cellular agriculture

Mitogenic growth factors are major cost drivers in serum-free media, contributing up to 95% of the total cost. Here, we present a streamlined workflow detailing cloning, expression testing, protein purification, and bioactivity screening that allows for low-cost production of bioactive growth factors including basic fibroblast growth factor and transforming growth factor β1. This generalized procedure can be used for multiple families of growth factors with minor modification, and the outputs are bioactive and suitable for cell culture applications. For complete details on the use and execution of this protocol, please refer to Venkatesan, et al.1.

Recombinant production of growth factors for application in cell culture

Culturing eukaryotic cells has widespread applications in research and industry, including the emerging field of cell-cultured meat production colloquially referred to as “cellular agriculture”. These applications are often restricted by the high cost of growth medium necessary for cell growth. Mitogenic protein growth factors (GFs) are essential components of growth medium and account for upwards of 90% of the total costs. Here, we present a set of expression constructs and a simplified protocol for recombinant production of functionally active GFs, including FGF2, IGF1, PDGF-BB, and TGF-β1 in Escherichia coli. Using this E. coli expression system, we produced soluble GF orthologs from species including bovine, chicken, and salmon. Bioactivity analysis revealed orthologs with improved performance compared to commercially available alternatives. We estimated that the production cost of GFs using our methodology will significantly reduce the cost of cell culture medium, facilitating low-cost protocols tailored for cultured meat production and tissue engineering.

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Developed methodology for low-cost production of soluble, bioactive GFs

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Purified GFs were active on NIH-3T3 and bovine satellite cells

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Some GF orthologs outperformed commercially sourced GFs

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Production of GFs using these methods can foster significant cost savings

Bacterial expression, correct membrane targeting and functional folding of the HIV-1 membrane protein Vpu using a periplasmic signal peptide

Viral protein U (Vpu) is a type-III integral membrane protein encoded by Human Immunodeficiency Virus-1 (HIV- 1). It is expressed in infected host cells and plays several roles in viral progeny escape from infected cells, including down-regulation of CD4 receptors. But key structure/function questions remain regarding the mechanisms by which the Vpu protein contributes to HIV-1 pathogenesis. Here we describe expression of Vpu in bacteria, its purification and characterization. We report the successful expression of PelB-Vpu in Escherichia coli using the leader peptide pectate lyase B (PelB) from Erwinia carotovora. The protein was detergent extractable and could be isolated in a very pure form. We demonstrate that the PelB signal peptide successfully targets Vpu to the cell membranes and inserts it as a type I membrane protein. PelB-Vpu was biophysically characterized by circular dichroism and dynamic light scattering experiments and was shown to be an excellent candidate for elucidating structural models.

Characterization of three putative Lon proteases of Thermus thermophilus HB27 and use of their defective mutants as hosts for production of heterologous proteins

In the genome of a thermophilic bacterium, Thermus thermophilus HB27, three genes, TTC0418, TTC0746 and TTC1975, were annotated as ATP-dependent protease La (Lon). Sequence comparisons indicated that TTC0418 and TTC0746 showed significant similarities to bacterial LonA-type proteases, such as Escherichia coli Lon protease, especially in regions corresponding to domains for ATP-binding and hydrolysis, and for proteolysis, but TTC1975 exhibited a similarity only at the C-terminal proteolytic domain. The enzymatic analyses, using purified recombinant proteins produced by E. coli, revealed that TTC0418 and TTC0746 exhibited peptidase and protease activities against two synthetic peptides and casein, respectively, in an ATP-dependent manner, and at the same time, both the enzymes had significant ATPase activities in the presence of substrates. On the other hand, TTC1975 possessed a protease activity against casein, but addition of ATP did not enhance this activity. Moreover, a T. thermophilus mutant deficient in both TTC0418 and TTC0746 showed a similar growth characteristic to an E. coli lon mutant, i.e., a growth defect lag after a nutritional downshift. These results indicate that TTC0418 and TTC0746 are actually members of bacterial LonA-type proteases with different substrate specificities, whereas TTC1975 should not be classified as a Lon protease. Finally, the effects of mutations deficient in these proteases were assessed on production of several heterologous gene products from Pyrococcus horikoshii and Geobacillus stearothermophilus. It was shown that TTC0746 mutation was more effective in improving production than the other two mutations, especially for production of P. horikoshii alpha-mannosidase and G. stearothermophilus alpha-amylase, indicating that the TTC0746 mutant of T. thermophilus HB27 may be useful for production of heterologous proteins from thermophiles and hyperthermophiles.

Simple, rapid, high-purity preparation of recombinant human platelet-derived growth factor-BB

Recombinant human platelet-derived growth factor-BB (rhPDGF-BB) is used to treat full-thickness diabetic ulcers and is being investigated for use in other chronic ulcers, non-healing wounds, and periodontal defects. A simple, novel method for expression and purification of rhPDGF-BB from Escherichia coli is now described. This method produces the dimeric protein in high yield (10-12 mg/g wet cell mass) and with a purity >95%. rhPDGF-BB was exclusively found in inclusion bodies (IBs) representing approx. 30% of the total cell proteins. The IBs were extracted and the monomer purified by RP-HPLC. The purified rhPDGF-B monomer was then refolded using Tris buffer and subsequently dimerized to produce biologically active rhPDGF-BB. This product was composed of two polypeptide chains, each approx. 12 kDa. The final product exhibited specific activity in a fibroblast proliferation assay indistinguishable from that of the WHO reference standard.

A new generation of vectors with increased induction ratios by overimposing a second regulatory level by attenuation

A major drawback of regulated gene expression from vectors bearing strong promoters is the associated high basal expression level. Simple regulatory systems have an intrinsic limitation in the range of induction, and attempts to mutate promoters to reduce basal expression usually result in concomitant reduction of induced levels. We have explored the possibility of reducing basal levels of gene expression while keeping induced levels intact by incorporating an additional regulatory circuit controlling a different step of the expression process. We have integrated the nasFEDCBA transcriptional attenuation system of Klebsiella oxytoca into a cascade expression circuit based on different regulatory elements of Pseudomonas putida, and also into a system based on the tac promoter, to expand their regulatory capacity. Basal expression from the promoters of these circuits was reduced by more than 10-fold by the nasF attenuator sequence while keeping the induced levels intact in the presence of the antiterminator protein, thus increasing the induction ratio by up to 1700-fold. In addition, using different combinations of regulatory elements and inducing conditions, we were able to obtain a broad range of expression levels. These vectors and the concept of their design will be very useful in regulating overproduction of heterologous proteins both at laboratory and industrial scales.

Human plasma fibronectin potentiates the mitogenic activity of platelet-derived growth factor and complements its wound healing effects

Integrin-mediated cell adhesion and growth factor stimuli are both required for optimal control of cell proliferation. In the context of skin injury, cell-derived fibronectin and platelet-derived growth factor play important roles in the stimulation of cell proliferation and migration, activities that are crucial to the healing process. To assess the ability of exogenously supplied plasma-derived fibronectin to stimulate wound repair and to study its ability to cooperate with platelet-derived growth factor-BB during healing, we devised a novel topical delivery formulation that allows the controlled release of both molecules to a wound. Using this topical formulation and the rabbit ear model of dermal wound healing, we show that plasma fibronectin is a potent stimulator of the wound healing process. We also show that administration of fibronectin and platelet-derived growth factor-BB in combination has additive wound healing effects. Finally, we report novel findings on the ability of soluble plasma fibronectin to potentiate the mitogenic effects of platelet-derived growth factor-BB in vitro. These findings not only show that optimal concentrations of exogenous fibronectin administered using an effective delivery system stimulate wound healing; they also suggest that PDGF-BB should be administered with fibronectin to achieve optimal therapeutic stimulation of wound healing.

Renaturation of heterodimeric platelet-derived growth factor from inclusion bodies of recombinant Escherichia coli using size-exclusion chromatography

A procedure for renaturation of heterodimeric platelet-derived growth factor (PDGF-AB) from inclusion bodies of recombinant Escherichia coli using size-exclusion chromatography is described. Either prepurified or crude PDGF-AB inclusion bodies solubilized with guanidinium hydrochloride were subjected to buffer exchange from denaturing to renaturing conditions during chromatography. Renaturation of PDGF-AB involves folding of the solubilized and unfolded molecules into dimerization competent monomers during size-exclusion chromatography and subsequent dimerization of folded monomers into the biologically active heterodimeric growth factor. Optimized conditions result in an overall yield of 75% active PDGF-AB with respect to size-exclusion chromatography and subsequent dimerization. The described approach allows renaturation at high protein concentrations and circumvents aggregation which is observed when refolding is carried out by dilution.