Secretion of human interleukin-2 fused with green fluorescent protein in recombinant Pichia pastoris

A novel system to map protein interactions reveals evolutionarily conserved immune evasion pathways on transmissible cancers

Around 40% of humans and Tasmanian devils (Sarcophilus harrisii) develop cancer in their lifetime, compared to less than 10% for most species. In addition, devils are affected by two of the three known transmissible cancers in mammals. Immune checkpoint immunotherapy has transformed human medicine, but a lack of species-specific reagents has limited checkpoint immunology in most species. We developed a cut-and-paste reagent development system and used the fluorescent fusion protein system to show that immune checkpoint interactions are conserved across 160,000,000 years of evolution, CD200 is highly expressed on transmissible tumor cells, and coexpression of CD200R1 can block CD200 surface expression. The system's versatility across species was demonstrated by fusing a fluorescent reporter to a camelid-derived nanobody that binds human programmed death ligand 1. The evolutionarily conserved pathways suggest that naturally occurring cancers in devils and other species can be used to advance our understanding of cancer and immunological tolerance.

Downregulation by organic nitrogen of AOX1 promoter used for controlled expression of foreign genes in the yeast Pichia pastoris

Pichia pastoris is a well-established cell factory for recombinant protein synthesis. Various optimization strategies of processes based on AOX1 promoter have been investigated, including methanol co-feeding with glycerol or sorbitol during the induction stage. Compared with carbon sources, comparatively little research has been devoted to the effects of nitrogen sources. Several reports have described the benefits of adding casamino acids (CA) to the recombinant protein production medium, however, without considering its effects at the gene expression level. Using enhanced green fluorescent protein as a reporter protein, monitored using flow cytometry, CA was shown to downregulate AOX1 promoter induction. Despite higher growth rates, cultures containing CA exhibited slower transition to the induced state, whereas metabolite analysis revealed that methanol consumption was reduced in the presence of CA compared with its absence. The repressive effect of CA was further confirmed by analysing the synthesis of extracellular recombinant Candida antarctica lipase under control of the AOX1 promoter. These findings highlight nitrogen source selection as an important consideration for AOX1-based protein production.

Casamino acids facilitate the secretion of recombinant dengue virus serotype-3 envelope domain III in Pichia pastoris

Background

Dengue is a viral disease spread to humans by mosquitoes. Notably, there are four serotypes of dengue viruses (DENV) that places ~40 % of the global population at risk of infection. However, lack of a suitable drug or a preventive vaccine exacerbates the matter further. Envelope domain-III (EDIII) antigen of dengue virus (DENV) has garnered much attention as a promising vaccine candidate for dengue, in addition to its use as a diagnostic intermediate. Hence developing a method for efficient production of high quality recombinant EDIII is important for research and industrial purpose.

Results

In this work, a Pichia pastoris system was optimized for the secretory over-expression of DENV serotype-3 EDIII under the control of methanol inducible AOX1 promoter. Temperature alone had a significant impact upon the amount of secretory EDIII, with 2.5-fold increase upon reducing the induction temperature from 30 to 20 °C. However surprisingly, supplementation of culture media with Casamino acids (CA), further augmented secretory EDIII titer, with a concomitant drop of intracellular EDIII levels at both temperatures. Though, reduction in intracellular retention of EDIII was more prominent at 20 °C than 30 °C. This suggests that CA supplementation facilitates overexpressing P. pastoris cells to secrete more EDIII by reducing the proportion retained intracellularly. Moreover, a bell-shaped correlation was observed between CA concentration and secretory EDIII titer. The maximum EDIII expression level of 187 mg/L was achieved under shake flask conditions with induction at 20 °C in the presence of 1 % CA. The overall increase in EDIII titer was ~9-fold compared to un-optimized conditions. Notably, mouse immune-sera, generated using this purified EDIII antigen, efficiently neutralized the DENV.

Conclusions

The strategy described herein could enable fulfilling the mounting demand for recombinant EDIII as well as lay direction to future studies on secretory expression of recombinant proteins in P. pastoris with CA as a media supplement.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-016-0243-3) contains supplementary material, which is available to authorized users.

Restriction site free cloning (RSFC) plasmid family for seamless, sequence independent cloning in Pichia pastoris

BACKGROUND

Tagging proteins is a standard method facilitating protein detection, purification or targeting. When tagging a certain protein of interest, it is challenging to predict which tag will give optimal results and will not interfere with protein folding, activity or yields. Ideally, multiple tags and positions are tested which however complicates molecular cloning and expression vector generation. In conventional cloning, tags are either added on PCR primers (requiring a distinct primer and PCR product per tag) or provided on the vector (typically leaving a restriction site scar).

RESULTS

Here we report a vector family of 40 plasmids allowing simple, seamless fusions of a single PCR product with various N- and C-terminal tags, signal sequences and promoters. The restriction site free cloning (RSFC) strategy presented in this paper relies on seamless cloning using type IIS restriction endonucleases. After cutting out a stuffer (placeholder) fragment from the vectors, a single PCR product can be directly inserted in frame into all 40 plasmids using blunt end or TA ligations, requiring only verification of the orientation. We have established a RSFC vector family for the commonly used protein expression host Pichia pastoris and demonstrated the system with the secretory expression of horseradish peroxidase (HRP). HRP fusions to four tags (Myc, FLAG, His, Strep) and two fusion proteins (GFP and MBP) showed a 31-fold difference in volumetric activities. C-terminal tagging caused in some cases almost a complete loss of function, whereas N-terminal tags showed moderate differences.

CONCLUSIONS

The RSFC vectors provide an unprecedented toolbox for expression optimization in P. pastoris. The results obtained with HRP underline the importance of comparing different tags to maximize activities of fusion proteins. In a similar fashion the RSFC strategy can be applied in other expression hosts to screen for optimal promoters, signal sequences or to facilitate the evaluation of (iso-) enzyme families.

Recombinant protein production in the eukaryotic protozoan parasite Leishmania tarentolae: a review

Leishmania tarentolae is a trypanosomatid protozoan parasite of the gecko, and has been established as a new eukaryotic expression system for the production of recombinant proteins. It seems that a protozoan parasite is a curious choice as the expression host; however, Trypanosomatidae are rich in glycoproteins with a pattern of glycosylation closely related to those in mammals and higher vertebrates. Thus, one of the main advantages of a L. tarentolae expression system is the mammalian-type posttranslational modification of target proteins. Although there are few examples of recombinant protein expression using this system, it can be an attractive alternative to using mammalian cells. This chapter presents an overview of the newly developed protein expression system based on L. tarentolae.

Plasma phospholipid transfer protein fused with green fluorescent protein is secreted by HepG2 cells and displays phosphatidylcholine transfer activity

Phospholipid transfer protein (PLTP) is a serum glycoprotein with a central role in high-density lipoprotein metabolism. We created a fusion protein in which enhanced green fluorescent protein (EGFP) was fused to the carboxyl-terminus of PLTP. Stably transfected HepG2 cells, which overexpress this fusion protein, were generated. PLTP-EGFP was translocated into the ER and fluoresced within the biosynthetic pathway, showing a marked concentration in the Golgi complex. The transfected cells secreted into the growth medium phospholipid transfer activity 7-fold higher than that of the mock-transfected controls. The medium of the PLTP-EGFP - expressing cells displayed EGFP fluorescence, demonstrating that both the PLTP and the EGFP moieties had attained a biologically active conformation. However, the specific activity of PLTP-EGFP in the medium was markedly reduced as compared with that of endogenous PLTP. This suggests that the EGFP attached to the carboxyl-terminal tail of PLTP interferes with the interaction of PLTP with its substrates or with the lipid transfer process itself. Fluorescently tagged PLTP is a useful tool for elucidating the intracellular functions of PLTP and the interaction of exogenously added PLTP with cells, and will provide a means of monitoring the distribution of exogenously added PLTP between serum lipoprotein subspecies.