Secretion of Mature IFN–α2 and Accumulation of Uncleaved Precursor by Bacillus subtilis Transformed with a Hybrid α–Amylase Signal Sequence–IFN–α2 Gene

The hollow porous sphere cell carrier for the dynamic 3D cell culture

Large-scale mammalian cell culture is essential in stem cell-based therapy, the production of the vaccine, and the manufacturing of therapeutic protein drugs. Due to the adherent growth characteristic of most mammalian cell types, the combination of cell carrier and bioreactor is a common choice in large-scale mammalian cell culture. Cell carriers are usually developed by polymer crosslinking, lithography, and emulsion drops; however, all these methods are difficult to control the uniformed porous structure and porous interior design. Therefore, unable to optimize the dynamic culture condition for cell proliferation, matrix production, and cell differentiation. Here we use fused deposition modelling (FDM) 3D printing technology to fabricate hollow porous spheres (HPS), based on which a novel dynamic 3D culture system has been established. In the meantime, computational fluid dynamics (CFD) simulations were conducted to study liquid flow behaviour in HPS. A dynamic cell seeding was developed and refined using the 3D culture system, which increased 32% (roughly) seeding efficiency compared to the traditional static cell seeding method. The cell proliferation analysis demonstrated that HPSs could speed up cell growth in dynamic cell culture. The HPS with a honeycomb-like structure showed the highest inner pore velocity (CFD analysis) and achieved the fastest cell proliferation and the highest cell viability. Overall, our study, for the first time, developed a 3D printed HPS cell culture device with a uniformed porous structure, which can effectively facilitate cell adhesion and proliferation in the dynamic cultural environment, thereby could be considered an ideal carrier candidate for the manufacturing of cells and cell-based products. Furthermore, this study provides a novel 3D dynamic culture system that can be further applied in cell culture and research in the future.

Extracellular production of human cystatin S and cystatin SA by Bacillus subtilis

We herein describe the development of a Bacillus subtilis system that can be used to produce large quantities of recombinant (r-) human salivary cystatins, a cysteine protease inhibitor of family 2 in the cystatin superfamily. The B. subtilis that lacked the alkaline protease E gene (DeltaaprE type mutant strain) was prepared by homologous recombination. The cDNA fragments coding for mature cystatins (S and SA) were ligated in frame to the DNA segment for the signal peptide of endoglucanase in the pHSP-US plasmid vector that was then use to transform the DeltaaprE type mutant strain of B. subtilis. The transformants carrying the expression vectors were cultivated in 5-L jar fermenters for 3 days at 30 degrees C. Both r-cystatin S and r-cystatin SA were successfully expressed and secreted into the culture broth, and were purified using a fast performance liquid chromatography system. The first use of DeltaaprE type mutant strain of B. subtilis made it possible to obtain a high yield of secreted protein, which makes this system an improvement over expression in Escherichia coli. We conclude that this system has high utility for expression of commercial quantities of secreted proteins.

The development of gene expression systems for filamentous fungi

Filamentous fungi have been used for decades in the commercial production of enzymes, antibiotics, and specialty chemicals. Traditionally, improving the yields of these products has involved either mutagenesis and screening or modification of fermentation conditions. Generally, selective breeding of strains has not been successful, because most of the commercially important fungal species lack a sexual cycle. For a few species, strain improvements have been made possible by employing the parasexual cycle for genetic crosses (30). The recent development of DNA-mediated transformation systems for several industrially important fungal species has spawned a flurry of research activity directed toward the development of gene expression systems for these microorganisms. This technology is now a viable means for novel and more directed approaches to improving existing fungal strains which produce enzymes or antibiotics. In addition, fungal expression systems are now being tested for the production of heterologous gene products such as mammalian pharmaceutical proteins. The goal of this review is to present a summary of the gene expression systems which have recently been developed for some filamentous fungi of commercial importance. To insure that the most recent developments are presented we have included data from not only scientific papers, but also from personal communications, abstracts, symposia, and our own laboratory.

Improving the production of E. coli β-lactamase in Bacillus subtilis: the effect of glucose, pH and temperature on the production level

Bacillus subtilis has been considered a promising host for the production of foreign proteins. However, proteases released by the host organism can often cause rapid breakdown of secreted heterologous proteins. Here we report that the addition of 6% glucose and 100 mM potassium phosphate to the growth medium significantly reduces the degradation of E. coli TEM beta-lactamase secreted from B. subtilis, when applying an expression system based on B. amyloliquefaciens alpha-amylase. The yield of beta-lactamase was increased 10-20-fold when compared to the yield in Luria medium. The promoter of B. amyloliquefaciens alpha-amylase gene is repressed by glucose. However, here we show that the repression does not take place in a multicopy plasmid, thus enabling our approach to efficiently reduce the protease action by catabolite repression. We have also studied the role of pH and temperature on the beta-lactamase production in laboratory scale bioreactors. Low temperature and low pH are both favorable for a high level beta-lactamase production by the high copy plasmid construction.

Cloning and characterization of a gene encoding extracellular metalloprotease from Streptomyces lividans

The prt gene, encoding a protease (Prt) from Streptomyces lividans TK24, was cloned and sequenced. An S. lividans host with plasmid-borne prt secreted 200 micrograms/ml of a 22-kDa Prt into the culture medium. Prt is classified as a metalloprotease since its activity is significantly inhibited by 1,10-phenanthroline or EDTA. The region upstream from prt codes for an incomplete open reading frame (ORF) oriented opposite to prt. This ORF has a strong similarity to a gene family (lysR) whose members regulate the transcription of structural genes required for either biosynthesis or degradation.

Optimization of the signal-sequence cleavage site for secretion from Bacillus subtilis of a 34-amino acid fragment of human parathyroid hormone

We have effected the secretion from Bacillus subtilis of a 34-amino acid (aa) fragment of human parathyroid hormone (PTH,1-34), using a Bacillus amyloliquefaciens neutral protease signal sequence. The secretion efficiency depended on the aa sequence near the signal-sequence cleavage site. We constructed a series of gene fusions encoding different pairs of aa between the signal sequence and PTH,1-34. There was a correlation between those polypeptides which were efficiently secreted and the potential for a beta-turn in the region just beyond the signal-sequence cleavage site. Based on this correlation, we constructed a gene fusion which specified Gly rather than Ala at the C terminus of the signal sequence, thus creating a beta-turn potential at the end of the signal sequence. The change provided a slight increase in secretion efficiency.

Secretory synthesis of human interleukin-2 by Streptomyces lividans

To study the ability of Streptomyces lividans to produce heterologous proteins by secretion, we directly fused DNA encoding the leader peptide of the alpha-amylase inhibitor, tendamistat, produced by Streptomyces tendae, with DNA encoding the mature part of interleukin-2 (IL-2). Such cloned fusion constructs are translated in S. lividans, in spite of the quite different codon usage. The active Il-2 is secreted into the culture broth, though the amounts are much less than that of the alpha-amylase inhibitor. The presence of IL-2 in the supernatants could be demonstrated both by an activity assay and by immunoblotting. In addition to the secreted form, three different species of Il-2 antibody immunoreactive proteins, with different Mrs, are either present in the cells or attached to the cells. This indicates that inefficient processing and translocation of the precursor is a major reason for the low activities found in the supernatant.

Secretory expression in Escherichia coli and Bacillus subtilis of human interferon alpha genes directed by staphylokinase signals

A DNA segment covering the signal sequence coding region, the ribosome binding site, and the promoter of the staphylokinase (sak) 42D gene (Behnke and Gerlach 1987) was cloned into pUC19 to form a portable expression-secretion unit (ESU). Fusion of human interferon alpha 1 (hIFN alpha 1) and hybrid hIFN alpha 1/2 genes to this sak ESU resulted in secretory expression of the two gene products in both Escherichia coli and Bacillus subtilis. While most of the IFN alpha was exported to the periplasmic space of E. coli, about 99% was secreted to the culture medium by recombinant B. subtilis strains. The total yield in E. coli was 1.2 x 10(5) IU/ml. This level of expression and export led to instability of the recombinant strains that was spontaneously relieved in vivo by inactivation of the sak ESU through insertion of an IS1 element. No such instability was observed with B. subtilis although expression and secretion levels reached even 3 x 10(6) IU/ml. Proteolytic degradation of IFN alpha by extracellular proteases was avoided by a combination of constitutive expression and secretion during the logarithmic growth phase and the use of exoprotease-reduced host strains. The IFN alpha 1 protein purified from B. subtilis culture supernatant was correctly processed, carried the expected 11 amino acid N-terminal elongation that resulted from DNA manipulations and proved to be homogenous in Western blotting experiments. The same recombinant plasmid that directed efficient secretion of hIFN alpha 1 in B. subtilis gave poor yields when introduced into Streptococcus sanguis.

Expression of the MuIFN alpha 7 gene in Bacillus subtilis using the levansucrase system

The mouse interferon alpha 7 gene, the signal sequence of which has been removed by oligonucleotide-directed mutagenesis, was introduced into a Bacillus subtilis secretion vector containing the promoter and the signal sequence of the B. subtilis levansucrase gene. Different B. subtilis strains were transformed with the fused levansucrase-interferon gene; their cell extracts and culture supernatants tested for antiviral activity and the IFN alpha 7 protein showed the presence of IFN alpha 7 only in the cell extracts. To promote IFN alpha 7 secretion, constructs were realized in order to restore the alpha helix conformation of the signal sequence of levansucrase and interferon protein junction. Our results suggest that factors other than the structure of the peptide around the cleavage site are involved in the secretion of IFN alpha 7 by B. subtilis.

Expression and secretion of human atrial natriuretic alpha-factor in Bacillus subtilis using the subtilisin signal peptide

Using the signal peptide of the Bacillus subtilis subtilisin gene (aprE) and a synthetic cDNA corresponding to the mature region of the human atrial natriuretic alpha-factor (hANF), we have constructed a secretion vector. B. subtilis cells, when transformed with this vector, secrete immunoreactive hANF peptides into the medium at about 500 micrograms/liter. The hANF is the first human gene product to be secreted from B. subtilis using this signal peptide. We have used promoters active during vegetative growth or sporulation and hosts deficient in several extracellular proteases but some proteolysis of the secretion products still occurs. In addition, both cell growth and sporulation are adversely affected by hANF production. Possible explanations for this observation are inefficient secretion of the atrial hormone or toxicity of the precursor or mature peptide.

Secretion and processing of staphylococcal nuclease by Bacillus subtilis

We have studied the secretion and processing of Staphylococcus aureus nuclease in Bacillus subtilis. We show that the initial species of nuclease found in the cell supernatants during short-term radioactive labeling (pulse-chase) had a molecular weight of approximately 18,800 and comigrated in a sodium dodecyl sulfate-polyacrylamide gel with staphylococcal nuclease B. This nuclease B form was processed to the mature nuclease A extracellularly by a phenylmethylsulfonyl fluoride-sensitive protease. The nuclease B-processing site is a consensus signal peptidase site, and the processing of nuclease B was coupled to secretion as judged by pulse-chase experiments. The nuclease A was shown by microsequencing of the N terminus to be 2 amino acid residues shorter than the nuclease A described for S. aureus Foggi. The nuclease B form was still the first species found in the culture supernatant after removal of the N-terminal 26 amino acids of the native 60-amino-acid signal peptide. However, removal of the N-terminal 72 amino acids abolishes secretion of any nuclease form and leads to the intracellular accumulation of nuclease.