Signal and propeptide processing of human tissue plasminogen activator: activity of a pro-tPA derivative

Identification, purification, and pharmacological activity analysis of Desmodus rotundus salivary plasminogen activator alpha1 (DSPAα1) expressed in transgenic rabbit mammary glands

Desmodus rotundus plasminogen activator alpha 1(DSPAα1) is a thrombolytic protein with advantages, such as a long half-life, high accuracy and specificity for thrombolysis, wide therapeutic window, and no neurotoxicity. To date, DSPAα1 has only been expressed in the Chinese hamster ovary, insect cells, transgenic tobacco plants, and Pichia pastoris. To the best of our knowledge, we are the first to report the expression of DSPAα1 in transgenic rabbit mammary glands, extract the product, and analyze its pharmacology activity. An efficient mammary gland-specific expression vector pCL25/DSPAα1 was transferred to prokaryotic zygotes in rabbits by microinjection to generate six DSPAα1 transgenic rabbits. The recombinant DSPAα1 (rDSPAα1) expression in transgenic rabbit milk was 1.19 ± 0.26 mg/mL. The rDSPAα1 purification protocol included pretreatment, ammonium sulfate precipitation, benzamidine affinity chromatography, cation exchange chromatography, and Cibacron blue affinity chromatography; approximately 98% purity was achieved using gel electrophoresis. According to sequencing results, the primary structure of rDSPAα1 was consistent with the theoretical design sequence, and its molecular weight was consistent with that of the natural protein. N-terminal sequencing results indicated rDSPAα1 to be a mature protein, as the goat signal peptide sequence of the expression vector was no longer detected. The fibrinolytic activity of rDSPAα1 was estimated to be 773,333 IU/mg. Fibrin-agarose plate assay and in vitro rat blood clot degradation assay showed that rDSPAα1 had strong thrombolytic activity. In conclusion, we report recombinant DSPAα1 with high thrombolytic activity expressed in transgenic rabbit mammary glands.

Impacts of tissue-type plasminogen activator (tPA) on neuronal survival

Tissue-type plasminogen activator (tPA) a serine protease is constituted of five functional domains through which it interacts with different substrates, binding proteins, and receptors. In the last years, great interest has been given to the clinical relevance of targeting tPA in different diseases of the central nervous system, in particular stroke. Among its reported functions in the central nervous system, tPA displays both neurotrophic and neurotoxic effects. How can the protease mediate such opposite functions remain unclear but several hypotheses have been proposed. These include an influence of the degree of maturity and/or the type of neurons, of the level of tPA, of its origin (endogenous or exogenous) or of its form (single chain tPA versus two chain tPA). In this review, we will provide a synthetic snapshot of our current knowledge regarding the natural history of tPA and discuss how it sustains its pleiotropic functions with focus on excitotoxic/ischemic neuronal death and neuronal survival.

Cathepsin D stimulates the activities of secreted plasminogen activators in the breast cancer acidic environment

Two proteases cathepsin D (cath D) and urokinase plasminogen activator (uPA) are tissue markers associated with an increased risk of metastasis in breast cancer. We investigated whether cath D, the major aspartyl protease overexpressed by breast cancer cells can trigger a proteolytic cascade via activation of plasminogens at the extracellular pH measured in hypoxic tumors. The effects of the aspartyl protease inhibitor pepstatin on the plasminogen activator (PA) system were analysed by conditioning media of human MDA-MB231 breast cancer cells at pH 6.6 and pH 7.4. Zymography analysis of culture media showed that pepstatin inhibited the secreted activity of tissue-type plasminogen activator (tPA) but not that of uPA. tPA was identified on the basis of the molecular weight, the immunoreactivity with relevant antibodies and the resistance to amiloride, a specific uPA inhibitor. The secreted tPA activity measured by a chromogenic assay in the presence of amiloride was also inhibited by pepstatin at pH 6.6. Surprisingly, pepstatin did not affect secreted tPA protein concentration but markedly increased the amount of the secreted plasminogen activator inhibitor-1 (PAI-1). We conclude that cath D overexpressed by these cells, stimulates at pH 6.6, but not at neutral pH, the extracellular PA proteolytic activity indirectly via PAI-1 proteolysis. This suggests that cath D at acidic pH close to the hypoxic regions of solid tumors, contributes to trigger a proteolytic cascade facilitating cancer cell invasion and metastasis.

Selection of novel TB vaccine candidates and their evaluation as DNA vaccines against aerosol challenge

Putative TB vaccine candidates were selected from lists of genes induced in response to in vivo-like stimuli, such as low oxygen and carbon starvation or growth in macrophages, and tested as plasmid DNA vaccines for their ability to protect against Mycobacterium tuberculosis challenge in a guinea pig aerosol infection model. This vaccination method was chosen as it induces the Th1 cell-mediated immune response required against intracellular pathogens such as M. tuberculosis. Protection was assessed in the guinea pig model in terms of mycobacteria present in the lungs at 30 days post-challenge. Protection achieved by the novel candidates was compared to BCG (positive control) and saline (negative control). Four vaccines encoding for proteins such as PE and PPE proteins, a zinc metalloprotease and an acyltransferase, gave a level of protection that was statistically better than saline in the lungs. These findings have enabled us to focus on a sub-set of vaccine candidates for further evaluation using additional vaccination strategies.

Production of Desmodus rotundus salivary plasminogen activator alpha1 (DSPAalpha1) in tobacco is hampered by proteolysis

The high fibrin specificity of Desmodus rotundus salivary plasminogen activator alpha1 (DSPAalpha1 or desmoteplase (INN)) makes it a promising candidate for the treatment of acute ischemic stroke. In the current study we explored the use of transgenic tobacco plants and BY-2 suspension cells as alternative production platforms for this drug. Four different N-terminal signal peptides, from plants and animals, were used to translocate the recombinant DSPAalpha1 protein to the endomembrane system. Intact recombinant DSPAalpha1 was produced in transgenic plants and BY-2 cells, although a certain degree of degradation was observed in immunoblotted extracts. The choice of signal peptide had no major influence on the degradation pattern or recombinant protein accumulation, which reached a maximum level of 38 microg/g leaf material. N-terminal sequencing of purified, His6-tagged DSPAalpha1 revealed only minor changes in the position of signal peptide cleavage compared to the same protein expressed in Chinese hamster ovary cells. However, correctly processed recombinant DSPAalpha1 was also detected. The enzymatic activity of the recombinant protein was confirmed using an in vitro assay with unpurified and purified samples, demonstrating that plants are suitable for the production of functional DSPAalpha1. In contrast to whole plant cell extracts, no recombinant DSPAalpha1 was detected in the culture supernatant of transgenic BY-2 cells. Further analysis showed that recombinant DSPAalpha1 is subject to proteolysis and that endogenous secreted BY-2 proteases are responsible for DSPAalpha1 degradation in the culture medium. The addition of a highly concentrated protease inhibitor mixture or 5 mM EDTA reduced DSPAalpha1 proteolysis, improving the accumulation of intact product in the culture medium. Strategies to improve the plant cell suspension system for the production of secreted recombinant proteins are discussed.

Retroviral vector-mediated expression of hirudin by human vascular endothelial cells: implications for the design of retroviral vectors expressing biologically active proteins

We constructed a hirudin cDNA cassette, HV-1.1, that encodes mature hirudin variant-1 fused to the signal peptide of human tissue-type plasminogen activator (t-PA). The cassette was subcloned into retroviral vectors and used to transduce human vascular endothelial cells in vitro. Hirudin antigen and activity were measured by ELISA and thrombin inhibition assays, respectively. Transduced cells secreted up to 35 +/- 2 ng/10(6) cells/24 h of biologically active hirudin; expression was stable for at least 7 weeks. Recombinant hirudin, expressed from the HV-1.1 cassette, had a specific activity of 7.1 +/- 0.2 antithrombin units per microgram (ATU/microgram), compared with specific activities of approximately 12 ATU/microgram for both native leech hirudin and recombinant hirudin produced in yeast. Protein sequencing and mass spectroscopic analysis revealed the presence of an extra N-terminal serine residue, indicating aberrant cleavage of the t-PA signal peptide and likely accounting for the diminished activity. We therefore constructed a second cDNA cassette, HV-1.2, in which hirudin secretion was directed by the signal peptide of human growth hormone. Hirudin expressed from the HV-1.2 cassette had a specific activity of 13.5 +/- 0.2 ATU/microgram. Protein sequencing and mass spectroscopic analysis demonstrated proper cleavage of the growth hormone signal peptide. Thus, we achieved high level retrovirus-mediated secretion of biologically active hirudin from endothelial cells in vitro. Use of these vectors may permit sustained local antagonism of thrombin activity in vivo.

Large-scale propagation of recombinant adherent cells that secrete a stable form of human glandular kallikrein, hK2

Human glandular kallikrein 2 (hK2) is a trypsin-like serine protease that is expressed predominantly in the prostate epithelium and has 78% aa identity with prostate-specific antigen (PSA). hK2 has been recognized as a potential prostate cancer marker and has been demonstrated to be highly expressed in prostate cancer compared to benign prostatic tissue. Purification and characterization of hK2 have been impeded due to its lower expression in bodily fluids and tissues compared to PSA and its ability to autodegrade. Therefore, to study biochemical and biological characteristics of hK2, a stable and enzymatically inactive mutant form of hK2, hK2(A217V), was expressed in a hamster cell line, AV12-664 (AV12-hK2(A217V)). AV12-hK2(A217V) cells secreted prohK2(A217V) (phK2(A217V)) in the spent medium at approximately 2.5 microgram/ml. Since AV12-hK2(A217V) are adherent cells, it was necessary to develop an efficient system to propagate large numbers of cells to obtain significant quantities of phK2(A217V). In this paper, we compared ceramic core bioreactor and microcarrier beads as alternatives to static culture to propagate adherent cells. Considering production levels, ease of operation, cost effectiveness, and labor, microcarrier beads were found to be a better alternative. Our findings led to the development of a general protocol for large-scale propagation of adherent cells on microcarrier beads eliminating the need for propagating AV12-hK2(A217V) in culture flasks or bioreactors. Microcarrier beads coated with AV12-hK2(A217V) cells could be propagated in 1- or 3-liter spinner flasks and were passed from one spinner to the next in a manner analogous to static culture or could be frozen and later used as inoculum for subsequent spinners. Using this protocol, >40 liters of spent medium was harvested within 30 days, which in turn was used to purify phK2(A217V). phK2(A217V) purified from spent medium of cells grown either on microcarrier beads or in culture flasks were biochemically similar as indicated by HIC-HPLC profile followed by sequencing of relevant peaks.

Affinity binding of a vampire bat plasminogen activator to SEC resins

DSPAalpha1 is a recombinant form of the vampire bat plasminogen activator which we have produced in mammalian cell culture. During the development of a recovery process for DSPAalpha1 we observed an unexpected binding interaction between this protein and several types of gel filtration chromatography resins. Under typical operating conditions using neutral pH buffers, we found that DSPA flows through the sizing resin and is fractionated, as expected, according to its molecular size. However, DSPA applied under certain acidic conditions (<pH 3) binds tightly to the Sephacryl series of resins. The protein is not released until solvent conditions are changed, specifically the pH is raised above 3. From the results presented we conclude that this unexpected interaction with the gel filtration media is not simply an ion exchange nor a hydrophobic interaction, but rather a more complex, mixed mode "affinity" like binding. Several structural features of the DSPA protein which may be involved in this unique binding have been examined, including binding after inactivation of its active site, chemical deglycosylation, chemical denaturation, or limited proteolysis. The "affinity" interactions persist despite these treatments and lead us to conclude that there may be a unique peptide sequence(s) within the protein which is responsible for the binding interaction to Sephacryl resins.

In vivo expression of mutant preproendothelins: hierarchy of processing events but no strict requirement of Trp-Val at the processing site

Endothelin-1 (ET-1), a 21-residue vasoconstrictor peptide, originates in human cells from a 212-amino acid precursor (preproET-1). Big ET-1, an intermediate form of 38 amino acids, is generated by cleavage at basic-pair residues of proET-1, while a specific "ET-converting enzyme" was proposed to process the unusual Trp-Val site at positions 21 and 22 of big ET-1. We have previously shown that expression of synthetic RNA encoding human preproET-1 in Xenopus oocytes results in secretion of putative ET-1 and big ET-1. Here, to further dissect the processing pathway of preproET-1, we designed and expressed in oocytes a set of preproET-1 mutants. Four mutants affecting the Trp-Val site always originated putative ET-1(s) at levels comparable to the wild type, suggesting that there is only a conformational requirement for cleavage at this site. An Arg-->Ile mutation at the basic-pair site after the C terminus of big ET-1 fully inhibited the formation of both big ET-1 and ET-1, indicating that processing at this site is an early event and that big ET-1 is an obligate intermediate for the synthesis of ET-1 in vivo. Also, a truncated mutant bearing a stop codon after the C terminus of the big ET-1 sequence was totally stable and further processed into mature big ET-1 and ET-1, indicating that the second part of the precursor is not necessary for maturation.