High yielding recombinant Staphylokinase in bacterial expression system—cloning, expression, purification and activity studies

Split intein-mediated backbone cyclization enhances the stability and activity of staphylokinase, a potent fibrin-selective plasminogen activator

Staphylokinase (Sak), a small 15 kDa globular protein that is secreted by certain strains of Staphylococcus aureus, shows a potent fibrin-selective thrombolytic activity. Earlier work has shown that Sak could potentially become a low-cost alternative to currently used thrombolytic agents, such as tissue plasminogen activator (tPA). In attempts to improve its potential for clinical applications, numerous modifications of Sak have already been investigated. Here, we have characterized a novel Sak modification, cyclized Sak (cyc-Sak), which was prepared through split-intein mediated protein backbone cyclization. We have characterized the structure, stability and the activity of cyc-Sak using biophysical techniques, limited proteolysis studies and plasminogen (PG)-activation assays. Our results show that cyc-Sak possesses an identical structure, enhanced stability, resistance to proteolysis by exoproteases and improved PG-activation properties compared to its linear counterpart. It can be over-expressed with high yield in the cytoplasm of Escherichia coli and is easily purified in a two-step process. The intein-mediated cyclization occurs spontaneously in vivo during protein expression and does not necessitate further modification steps after purification of the protein. Furthermore, covalent Sak cyclization could be readily combined with other Sak modifications previously proposed, to generate an effective thrombolytic agent with lower immunogenicity and improved stability and activity.

Designing a multifunctional staphylokinase variant (SAK-2RGD-TTI) with appropriate thrombolytic activity in vitro

OBJECTIVE

Thrombin, platelets, and plasmin are three key factors involved in hemostasis and thrombolysis. Thrombolytic therapy with clinically approved drugs is often followed by recurrent thrombosis caused by thrombin-induced platelet aggregation from the clot debris. In order to minimize these problems, new constructs were designed for the expression of recombinant staphylokinase (rSAK) and also a fusion protein composed of staphylokinase, 20 amino acids containing 2 RGD followed by tsetse thrombin Inhibitor (SAK-2RGD-TTI) in Pichia pastoris.

RESULT

Modeling the tertiary structure of SAK-2RGD-TTI showed that the linker containing RGD and TTI did not interfere with proper folding of SAK. In laboratory testing, the purified SAK-2RGD-TTI (420 μg/mL) dissolved an average of 45% of the blood clot. The activity of the SAK-2RGD-TTI was also confirmed in various tests including human plasminogen activation assay, fibrin clot lysis assay, well diffusion method, activated partial thromboplastin time and platelet rich clot lysis assay.

CONCLUSION

Our findings suggest that SAK-2RGD-TTI has improved therapeutic properties preventing reocclussion. It further confirms that it is practicable to assemble and produce a hybrid multifunctional protein that targets hemostatic process at various stages.

Bacterial staphylokinase as a promising third-generation drug in the treatment for vascular occlusion

Vascular occlusion is one of the major causes of mortality and morbidity. Blood vessel blockage can lead to thrombotic complications such as myocardial infarction, stroke, deep venous thrombosis, peripheral occlusive disease, and pulmonary embolism. Thrombolytic therapy currently aims to rectify this through the administration of recombinant tissue plasminogen activator. Research is underway to design an ideal thrombolytic drug with the lowest risk. Despite the potent clot lysis achievable using approved thrombolytic drugs such as alteplase, reteplase, streptokinase, tenecteplase, and some other fibrinolytic agents, there are some drawbacks, such as high production cost, systemic bleeding, intracranial hemorrhage, vessel re-occlusion by platelet-rich and retracted secondary clots, and non-fibrin specificity. In comparison, bacterial staphylokinase, is a new, small-size plasminogen activator, unlike bacterial streptokinase, it hinders the systemic degradation of fibrinogen and reduces the risk of severe hemorrhage. A fibrin-bound plasmin-staphylokinase complex shows high resistance to a₂-antiplasmin-related inhibition. Staphylokinase has the potential to be considered as a promising thrombolytic agent with properties of cost-effective production and the least side effects.

Comparison of expression optimization of new derivative of staphylokinase (SAK-2RGD-TTI) with the rSAK

Staphylokinase (SAK) is a promising thrombolytic agent for the treatment of patients suffering from blood-clotting disorders. To increase the potency of SAK and to minimize vessel reocclusion, a new construct bearing SAK motif fused to tsetse thrombin inhibitor (TTI) via a 20-amino acid linker with 2 RGD (2 × arginine-glycine-aspartic acid inhibiting platelet aggregation via attachment to integrin receptors of platelet) was codon optimized and expressed comparatively in Pichia pastoris GS115 as a Mut⁺ strain and KM71H as a Mut^s strain. Fusion protein was optimized in terms of best expression condition and fibrinolytic activity and compared with the rSAK. Expression level of the designed construct reached up to 175 mg/L of the culture medium after 72-hr stimulation with 2.5% methanol and remained steady for 3-4 days. The highest expression was obtained at the range of 2-3% methanol. The SAK-2RGD-TT (relative activity >82%) was more active at 25-37 °C than rSAK (relative activity of 93%). Further, it showed relative activity >80% at pH ranges of 7-9. Western blot analysis showed two bands of nearly 27 and 24 kDa at ratio of 5 to 3, respectively. The specific fibrinolytic activity of the SAK-2RGD-TTI was measured as 8,269 U/mg, and 19,616 U/mg for the nonpurified and purified proteins, respectively. Deglycosylation by using tunicamycin in culture medium resulted in higher fibrinolytic activity of SAK-2RGD-TTI (2.2 fold). Consequently, compared to the rSAK, at the same equimolar proportion, addition of RGD and TTI fragments could increase fibrinolytic activity. Also, P. pastoris can be considered as an efficient host for overexpression of the soluble SAK-2RGD-TTI with high activity without requiring a complicated purification procedure.

High-level expression of a biologically active staphylokinase in Pichia pastoris

Staphylokinase (SAK) as the third generation thrombolytic molecule is a promising agent for the treatment of thrombosis. SAK variant of SAKфC was expressed in Pichia pastoris strains KM71H and GS115. The codon adaptation index of SAK was improved from 0.75 to 0.89. The expression of recombinant SAK (rSAK) reached to its maximum (310 mg/L of the culture medium) after 48-hr stimulation with 3% methanol and remained steady until day 5. The maximum activity of the enzyme was at pH 8.6 and 37°C. It was highly active at temperatures 20-37°C and pH ranges of 6.8-9 (relative residual activity more than 80%). It was determined that rSAK was 73.8% of the total proteins secreted by P. pastoris KM71H into the culture media. The specific activities of rSAK were measured as 9,002 and 21,042 U/mg for the nonpurified and purified proteins, respectively. The quantity of the purified protein (>99% purity) was 720 µg/mL with a purification factor of 2.34. Western blot analysis showed two bands of nearly 22 and 18.6 kDa. It was concluded that P. pastoris is a proper host for expression of biologically active and endotoxin-free rSAK due to its high expression and low protein impurity in culture supernatant.

Temperature-sensitive liposome-mediated delivery of thrombolytic agents

BACKGROUND

Clinical efficacy of thrombolytic drugs is limited by lack of specific delivery and requires large therapeutic doses which increase toxicity. Encapsulating these drugs in temperature-sensitive liposomes and applying hyperthermia to deliver thrombolytic agents locally to thrombus might theoretically favourably alter the therapeutic window. The objectives of this study were to formulate liposomes encapsulating thrombolytics and assess thrombolytic activity following hyperthermia.

METHODS

Three liposome formulations were investigated: temperature-sensitive liposome (TSL, DPPC:DSPE-PEG2000 (mol% 95:5)), low temperature-sensitive liposome (LTSL, DPPC:MSPC:DSPE-PEG2000 (mol% 85.3:9.7:5)), and traditional temperature-sensitive liposome (TTSL, DPPC:HSPC:Chol:DSPE-PEG2000 (mol% 55:25:15:5)). To characterise temperature-dependent release of high molecular weight cargo from each formulation, fluorescein-conjugated dextrans (70 kDa) were loaded and release was quantified via spectrophotometry. Staphylokinase (SAK), urokinase, and tissue-type plasminogen activator were also loaded individually into each liposome formulation. Leakage at 37 °C and release at 38-44 °C were quantified via chromogenic enzymatic activity assay. Clot lysis was evaluated by measuring mass of blood clots before and after thrombolytic liposome treatment.

RESULTS

The LTSL formulation had optimal release characteristics with maximum release at 41.3 °C. Release of dextrans from LTSLs was observed to be 11.5 ± 1.5%, 79.7 ± 1.6%, and 93.6 ± 3.7% after 15 min in plasma at 37°, 39°, and 41.3 °C, respectively. The SAK LTSL had the highest release/leakage ratio and demonstrated greater clot lysis.

CONCLUSIONS

The SAK LTSL achieves significant clot lysis in vitro. When combined with local hyperthermia, the SAK LTSL potentially produces sufficient thrombolysis while minimising systemic side effects.

Construction of novel chimeric proteins through the truncation of SEC2 and Sak from Staphylococcus aureus

It is an usual clinical phenomenon that cancer patients are prone to thrombosis. Until now, there have been no efficient methods or appropriate drugs to prevent and cure tumor thrombus. Therefore, the construction of a bifunctional chimeric protein for the treatment of cancer, complicated with thrombosis, is of great significance. Utilizing the superantigenic activity of staphylococcal enterotoxin C2 (SEC2) and the thrombolytic activity of staphylokinase (Sak), Sak-linker-SEC2 and SEC2-linker-Sak were constructed which had good anti-tumor and thrombolytic activities at the same time. Due to the intrinsic emetic activity of SEC2 and high molecular weight (MW) of chimeric proteins (44 kDa), their clinical applications will be restricted. In this study, novel chimeric proteins including ΔSEC2-ΔSak and ΔSak-ΔSEC2 were constructed through the truncation of SEC2 and Sak without 9-Ala linker and His-tag. Compared with the former, both the truncated proteins preserved nearly the same anti-tumor and thrombolytic activities. In addition, their MWs were only 29 kDa and their immunoreactivities were slightly lower than that of Sak-linker-SEC2 and SEC2-linker-Sak, respectively. Therefore, the novel chimeric proteins possessed merits and characteristics, such as low MS, low immunogenicity, and difunctionality which the former had not. It will be of great interest if the above-mentioned proteins can be used to cure Trousseau syndrome in clinic.

Expression of recombinant staphylokinase in the methylotrophic yeast Hansenula polymorpha

Background

Currently, the two most commonly used fibrinolytic agents in thrombolytic therapy are recombinant tissue plasminogen activator (rt-PA) and streptokinase (SK). Whereas SK has the advantage of substantially lower costs when compared to other agents, it is less effective than either rt-PA or related variants, has significant allergenic potential, lacks fibrin selectivity and causes transient hypotensive effects in high dosing schedules. Therefore, development of an alternative fibrinolytic agent having superior efficacy to SK, approaching that of rt-PA, together with a similar or enhanced safety profile and advantageous cost-benefit ratio, would be of substantial importance. Pre-clinical data suggest that the novel fibrinolytic recombinant staphylokinase (rSAK), or related rSAK variants, could be candidates for such development. However, since an efficient expression system for rSAK is still lacking, it has not yet been fully developed or evaluated for clinical purposes. This study’s goal was development of an efficient fermentation process for the production of a modified, non-glycosylated, biologically active rSAK, namely rSAK-2, using the well-established single cell yeast Hansenula polymorpha expression system.

Results

The development of an efficient large scale (80 L) Hansenula polymorpha fermentation process of short duration for rSAK-2 production is described. It evolved from an initial 1mL HTP methodology by successive scale-up over almost 5 orders of magnitude and improvement steps, including the optimization of critical process parameters (e.g. temperature, pH, feeding strategy, medium composition, etc.). Potential glycosylation of rSAK-2 was successfully suppressed through amino acid substitution within its only N-acetyl glycosylation motif. Expression at high yields (≥ 1g rSAK-2/L cell culture broth) of biologically active rSAK-2 of expected molecular weight was achieved.

Conclusion

The optimized production process described for rSAK-2 in Hansenula polymorpha provides an excellent, economically superior, manufacturing platform for a promising therapeutic fibrinolytic agent.

Expression of a staphylokinase, a thrombolytic agent in Arabidopsis thaliana

A gene encoding staphylokinase from Staphylococcus aureus was cloned into the plant transformation binary vector pCAMBIA 1304. The transgene was introduced into the genome of A. thaliana via in planta Agrobacterium tumefaciens-mediated genetic transformation. The presence of the staphylokinase gene was confirmed by PCR in 60% of the investigated plants. The presence of the fusion protein (119 kDa) was confirmed by SDS-PAGE and Western blot analysis in protein extracts from putative transgenics. Furthermore, the amidolytic assay confirmed the activity of SAK in protein extracts in 23 out of 45 transgenic lines of A. thaliana plants.

Cloning, soluble expression and purification of high yield recombinant hGMCSF in Escherichia coli

Expression of human granulocyte macrophage colony stimulating factor (hGMCSF), a cytokine of therapeutic importance, as a thioredoxin (TRX) fusion has been investigated in Escherichia coli BL21 (DE3) codon plus cells. The expression of this protein was low when cloned under the T7 promoter without any fusion tags. High yield of GMCSF was achieved (∼88 mg/L of fermentation broth) in the shake flask when the gene was fused to the E. coli TRX gene. The protein was purified using a single step Ni(2+)-NTA affinity chromatography and the column bound fusion tag was removed by on-column cleavage with enterokinase. The recombinant hGMCSF was expressed as a soluble and biologically active protein in E. coli, and upon purification, the final yield was ∼44 mg/L in shake flask with a specific activity of 2.3 × 10(8) U/mg. The results of Western blot and RP-HPLC analyses, along with biological activity using the TF-1 cell line, established the identity of the purified hGMCSF. In this paper, we report the highest yield of hGMCSF expressed in E. coli. The bioreactor study shows that the yield of hGMCSF could be easily scalable with a yield of ∼400 mg/L, opening up new opportunities for large scale production hGMCSF in E. coli.

A novel prokaryotic vector for identification and selection of recombinants: direct use of the vector for expression studies in E. coli

Background

The selection of bacterial recombinants that harbour a desired insert, has been a key factor in molecular cloning and a series of screening procedures need to be performed for selection of clones carrying the genes of interest. The conventional cloning techniques are reported to have problems such as screening high number of colonies, generation of false positives, setting up of control ligation mix with vector alone etc.

Results

We describe the development of a novel dual cloning/expression vector, which enables to screen the recombinants directly and expression of the gene of interest. The vector contains Green fluorescence protein (GFP) as the reporter gene and is constructed in such a way that the E. coli cells upon transformation with this vector does not show any fluorescence, but readily fluoresce upon insertion of a foreign gene of interest. The same construct could be easily used for screening of the clones and expression studies by mere switching to specific hosts.

Conclusions

This is the first vector reported that takes the property of colour or fluorescence to be achieved only upon cloning while all the other vectors available commercially show loss of colour or loss of fluorescence upon cloning. As the fluorescence of GFP depends on the solubility of the protein, the intensity of the fluorescence would also indicate the extent of solubility of the expressed target protein.

Yeast extract mediated autoinduction of lacUV5 promoter: an insight

We report a simple and cost-effective autoinducible media component responsible for the autoinduction of proteins in Escherichia coli under lacUV5 promoter system. Yeast extract (YE) at high concentration was found to stimulate the expression of T7 RNA polymerase in BL21(DE3) cells while such an effect was not seen in BL21A1 cells. A systematic study on the effect of varying concentrations of YE indicated several folds higher expression of genes viz., human granulocyte colony stimulating factor (rhGCSF), human interferon alpha 2b (rhIFN-alpha2b) and Staphylokinase (rSAK) in BL21(DE3) cells in the absence of any specific inducer like IPTG or additional lactose. Additional investigations on the inducible component of the YE revealed the presence of significant amount of endogenous lactose as the contributory factor for the observed autoinduction phenomenon. This paper highlights the easy scalability of the use of the present media component for large-scale production in biotechnology industry.

High-level expression of non-glycosylated and active staphylokinase from Pichia pastoris

Staphylokinase (SAK) is a promising thrombolytic agent for treating blood-clotting disorders. Recombinant SAK (rSAK) was produced after integration of the gene into Pichia pastoris genome. The recombinant Pichia carrying multiple insertions of the SAK gene yielded high-level (approximately 1 g/l) of extracellular glycosylated rSAK (approximately 18 kDa) with negligible plasminogen activation activity. Addition of tunicamycin during the induction phase resulted in expression of non-glycosylated and highly active rSAK (approximately 15 kDa) from the same clone. Two simple steps of ion-exchange chromatography produced an homogenous rSAK of >95% purity which suitable for future structural and functional studies.