Progress in developing methylotrophic yeasts as expression systems

Yeast-based vaccines: New perspective in vaccine development and application

In presently licensed vaccines, killed or attenuated organisms act as a source of immunogens except for peptide-based vaccines. These conventional vaccines required a mass culture of associated or related organisms and long incubation periods. Special requirements during storage and transportation further adds to the cost of vaccine preparations. Availability of complete genome sequence, well-established genetic, inherent natural adjuvant and non-pathogenic nature of yeast species viz. Saccharomyces cerevisiae, Pichia pastoris makes them an ideal model system for the development of vaccines both for public health and for on-farm consumption. In this review, we compile the work in this emerging field during last two decades with major emphases on S. cerevisiae and P. pastoris which are routinely used worldwide for expression of heterologous proteins with therapeutic value against infectious diseases along with possible use in cancer therapy. We also pointed towards the developments in use of whole recombinant yeast, yeast surface display and virus-like particles as a novel strategy in the fight against infectious diseases and cancer along with other aspects including suitability of yeast in vaccines preparations, yeast cell wall component as an immune stimulator or modulator and present status of yeast-based vaccines in clinical trials.

Quinone-dependent alcohol dehydrogenases and FAD-dependent alcohol oxidases

This review considers quinone-dependent alcohol dehydrogenases and FAD-dependent alcohol oxidases, enzymes that are present in numerous methylotrophic eu- and prokaryotes and significantly differ in their primary and quaternary structure. The cofactors of the enzymes are bound to the protein polypeptide chain through ionic and hydrophobic interactions. Microorganisms containing these enzymes are described. Methods for purification of the enzymes, their physicochemical properties, and spatial structures are considered. The supposed mechanism of action and practical application of these enzymes as well as their producers are discussed.

Asn and asn: critical residues for in vitro biological activity of reteplase

Reteplase (rPA) is a thrombolytic agent used for the treatment of acute myocardial infarction. We studied the expression of rPA and its selected asparagine mutants after integration into the Pichia genome. Though methanol induction of the native and the rPA mutants showed similar expression levels (~200–250 mg/L), the mutants displayed significant loss of protease activity. Strikingly, the clot lysis activities of these mutants were considerably different. While mutation of Asn¹² (N12P) of the Kringle 2 domain showed delayed clot lysis activity (t_1/2 = 38 min) compared to the native rPA (t_1/2 = 33 min), a faster rate of clot lysis (t_1/2 = 27 min) was observed when the Asn²⁷⁸ (N278S) of the serine protease domain was mutated. Interestingly, the slowest clot lysis activity (t_1/2 = 49 min) demonstrated by the double mutant (N12P, N278S) suggests the dominant role of Asn¹² in regulating the fibrinolytic activity of rPA. The results presented in this paper indicate that the fibrinolytic and the proteolytic activities of rPA are independent of each other.

Choice of cellular protein expression system

Recombinant protein expression has become a standard laboratory tool, and a wide variety of systems and techniques are now in use. Because there are so many systems to choose from, the investigator has to be careful to use the combination that will give the best results for the protein being studied. This overview unit discusses expression and production choices, including post-translational modifications (e.g., glycosylation, acylation, sulfation, and removal of N-terminal methionine), in vivo and in vitro folding, and influence of downstream elements on expression.

Cloning and expression of the XPR2 gene from Yarrowia lipolytica in Pichia pastoris

Yarrowia lipolytica is a dimorphic yeast able to secrete different types of proteases depending on the pH of the environment. At neutral pH, the production of an extracellular alkaline protease (AEP) is induced. This protease could be useful in the leather, detergent, or food industries. The XPR2 gene, coding for AEP, was extracted from the pINA154 vector and cloned into the pHIL-D2 vector to obtain a new protease-producing recombinant Pichia pastoris strain. The gene was efficiently integrated in the P. pastoris genome and expressed from the AOX1 promoter actively induced by methanol. Finally, the protease was successfully secreted by P. pastoris GS115.

Recombinant expression systems in the pharmaceutical industry

In terms of downstream processing efficiency, secretory expression systems offer potential advantages for the production of recombinant proteins, compared with inclusion body forming cytosolic systems. However, for high-volume therapeutics like insulin, the product yields of the majority of the potentially available secretory systems is not yet fully competitive. Current strategies to improve productivity and secretion efficiency comprise: (1) enhancement of gene expression rates, (2) optimization of secretion signal sequences, (3) coexpression of chaperones and foldases, (4) creation of protease deficient mutants to avoid premature product degradation and (5) subsequent breeding and mutagenesis. For the production of non-glycosylated proteins and proteins, which are natively glycosylated but are also pharmacologically active without glycosylation, prokaryotes, which usually lack metabolic pathways for glycosylation, are theoretically the most suitable organisms and offer two alternatives: either Escherichia coli strains are conditioned to be efficient secreters or efficient native secreters like Bacillus species are accordingly developed. To fully exploit the secretory capacity of fungal species, a deeper understanding of their posttranslational modification physiology will be necessary to steer the degree and pattern of glycosylation, which influences both folding and secretion efficiency. Insect and mammalian cells display posttranslational modification patterns very similar or identical to humans, but in view of the entailed expenditures, their employment can only be justified if their modification machinery is required to ensure a desired pharmacological activity.

Regulation and evaluation of five methanol-inducible promoters in the methylotrophic yeast Candida boidinii

We isolated the promoter regions of five methanol-inducible genes (P(AOD1), alcohol oxidase; P(DAS1), dihydroxyacetone synthase; P(FDH1), formate dehydrogenase; P(PMP20), Pmp20; and P(PMP47), Pmp47) from the Candida boidinii genome, and evaluated their strength and studied their regulation using the acid phosphatase gene of Saccharomyces cerevisiae (ScPHO5) as the reporter. Of the five promoters, P(DAS1) was the strongest methanol-inducible promoter whose strength was approximately 1.5 times higher than that of the commonly used P(AOD1) in methanol-induced cells. Although the expression of P(AOD1) and P(DAS1) was completely repressed by the presence of glucose, formate-induced expression of P(FDH1) was not repressed by glucose. Expression under P(PMP47), another methanol-inducible promoter, was highly induced by oleate. The induction kinetics of P(PMP47) and P(DAS1) revealed that methanol induces the expression of peroxisome membrane protein Pmp47, earlier than the expression of matrix enzyme dihydroxyacetone synthase (Das1p), and that this information is contained in the promoter region of the respective gene. This is the first report which evaluates several methanol-inducible promoters in parallel in the methylotrophic yeast.

Vectors for rapid selection of integrants with different plasmid copy numbers in the yeast Hansenula polymorpha DL1

Plasmids with different selectable markers were constructed and used to transform the Hansenula polymorpha strain DL1. It was shown that, depending on the host mutant strain, the use of these plasmids enables rapid selection of transformants with plasmids integrated in low (1-2), moderate (6-9) or high (up to 100) copy numbers. The vectors and mutant described are potentially useful for the construction of efficient producers of heterologous proteins in H. polymorpha.

Autonomous plasmid replication in Aspergillus nidulans: AMA1 and MATE elements

With few exceptions, in eukaryotic organisms the presence of a chromosomal replicator on a circular vector molecule is not sufficient to confer on it the ability to persist and replicate extrachromosomally. However, it is possible to isolate from genomes of some filamentous fungi DNA fragments which can provide extrachromosomal maintenance of plasmids. In Aspergillus nidulans, two functional classes of such sequences can be distinguished: effective plasmid replicators (e.g., AMA1) and transformation enhancers (e.g., ANS1 or MATEs), which apparently are able to initiate aberrant replication, leading to vector rearrangement and multimerization and eventually resulting in chromosomal integration. We discuss the similarity of these events to DNA amplification in other eukaryotes. A model is suggested which accounts for the formation of effective replicating plasmids as a result of sequence amplification. The model is based on the observation that in some organisms, including A. nidulans and Schizosaccharomyces pombe, duplication of an inefficient replicator enhances its efficiency dramatically. Some structural traits of transformation enhancers in A. nidulans imply a role for topoisomerases in amplification and replication of circular DNA molecules. We discuss practical applications of replicative vectors for gene cloning and expression studies.

Application of yeasts in gene expression studies: a comparison of Saccharomyces cerevisiae, Hansenula polymorpha and Kluyveromyces lactis -- a review

From the onset of gene technology yeasts have been among the most commonly used host cells for the production of heterologous proteins. At the beginning of this new development the attention in molecular biology and biotechnology focused on the use of the best characterized species, Saccharomyces cerevisiae, leading to an increasing number of production systems for recombinant compounds. In recent years alternative yeasts became accessible for the techniques of modern molecular genetics and, thereby, for potential applications in biotechnology. In this respect Kluyveromyces lactis, and the methylotrophs Hansenula polymorpha and Pichia pastoris have been proven to offer significant advantages over the traditional baker's yeast for the production of certain proteins. In the following article, the present status of the various yeast systems is discussed.

Inducible expression of a heterologous protein in Hansenula polymorpha using the alcohol oxidase 1 promoter of Pichia pastoris

Pichia pastoris (Pp) and Hansenula polymorpha (Hp) are methylotrophic yeasts commonly used for industrial purposes. Growth of either of these yeasts in the presence of methanol as the carbon source results in high-level induction of alcohol oxidase expression. The respective alcohol oxidase genes, AOX1 in Pp and MOX in Hp, have similar regulatory characteristics. Our studies show that the Pp AOX1 promoter (AOX1p) can be used for methanol-induced expression of a heterologous gene in Hp. Furthermore, the size of an AOX1p-heterologous gene-AOX1 terminator cassette transcript synthesized in Hp is indistinguishable from that synthesized in Pp suggesting that transcription both initiates and terminates at the same sites in both yeast species. Induction of AOX1p in Hp demonstrates that the methanol-inducible regulatory mechanism in Hp is able to recognize and activate the Pp promoter in spite of extensive sequence variations between AOX1p and MOXp.

A novel autonomously replicating sequence (ARS) for multiple integration in the yeast Hansenula polymorpha DL-1

Several autonomously replicating sequences of Hansenula polymorpha DL-1 (HARSs) with the characteristics of tandem integration were cloned by an enrichment procedure and analyzed for their functional elements to elucidate the mechanism of multiple integration in tandem repeats. All plasmids harboring newly cloned HARSs showed a high frequency of transformation and were maintained episomally before stabilization. After stabilization, the transforming DNA was stably integrated into the chromosome. HARS36 was selected for its high efficiency of transformation and tendency for integration. Several tandemly repeated copies of the transforming plasmid containing HARS36 (pCE36) integrated into the vicinity of the chromosomal end. Bal 31 digestion of the total DNA from the integrants followed by Southern blotting generated progressive shortening of the hybridization signal, indicating the telomeric localization of the transforming plasmids on the chromosome. The minimum region of HARS36 required for its HARS activity was analyzed by deletion analyses. Three important regions, A, B, and C, for episomal replication and integration were detected. Analysis of the DNA sequences of regions A and B required for the episomal replication revealed that region A contained several AT-rich sequences that showed sequence homology with the ARS core consensus sequence of Saccharomyces cerevisiae. Region B contained two directly repeated sequences which were predicted to form a bent DNA structure. Deletion of the AT-rich core in region A resulted in a complete loss of ARS activity, and deletion of the repeated sequences in region B greatly reduced the stability of the transforming plasmid and resulted in retarded cell growth. Region C was required for the facilitated chromosomal integration of transforming plasmids.

High-level secretion of fungal glucoamylase using the Candida boidinii gene expression system

The methylotrophic yeast, Canadida boidinii, was investigated as an expression host for secretory enzyme production. The cDNA of Rhizopus oryzae glucoamylase was placed under the C. boidinii alcohol oxidase (AODl) promoter. A transformant integrated with a single-copy expression cassette to the chromosome produced glucoamylase into the medium to a high amount when the cells were grown on methanol or methanol plus glycerol as (a) carbon source(s). The transformant C. boidinii cells were grown up to ca. 95 g dry cell weight/liter medium, and the concentration of glucoamylase in the medium reached 3.4 g/liter. This showed that the signal sequence from Rhizopus glucoamylase functioned very efficiently in C. boidinii. Next, secreted glucoamylase from C. boidinii was purified and compared with the enzyme produced in S. cerevisiae. The enzyme produced in C. boidinii was found to have higher molecular weight than that produced in S. cerevisiae, which was due to the difference of the N-linked glycosylated sugar structure of the produced proteins.

Identification of multiple plasmids released from recombinant genomes of Hansenula polymorpha by transformation of Escherichia coli

Total DNAs isolated from two Hansenula polymorpha (Pichia angusta) strains having chromosomal single or tandem multiple integrations of a pUC18-derived expression plasmid produced Escherichia coli transformants which contained plasmids of different size and/or organization than that of the expression plasmid. Evidence that plasmid-like structures are formed in H. polymorpha and that their formation is stimulated by DNA damage is presented in this study.

Expression of Saccharomyces adenylate kinase gene in Candida boidinii under the regulation of its alcohol oxidase promoter

The methylotrophic yeast, Candida boidinii, was investigated as a new efficient host for heterologous gene expression. The Saccharomyces cerevisiae adenylate kinase gene (ADK1) was used as the first example for heterologous enzyme production in C. boidinii. C. boidinii cells were transformed with plasmids harboring the S. cerevisiae ADK1 gene under the alcohol oxidase (C. boidinii AOD1) promoter. The chromosome-integrant strains produced adenylate kinase protein corresponding to 22%-28% of the total soluble proteins in an enzymatically active form. When the three-copy integrative transformant was grown for 60 h on methanol-glycerol medium in a 1.5-l jar fermentor, adenylate kinase was produced intracellularly with a yield of up to 2 milligrams culture medium. As the expression of the S. cerevisiae ADK1 in C. boidinii was under similar regulation to that of the C. boidinii AOD1, the previously cloned 1.7-kb AOD1 promoter fragment was proved to harbor sufficient cis elements for AOD1 regulation and found to be an efficient promoter for heterologous gene expression.

The yeast Kluyveromyces lactis as an efficient host for heterologous gene expression

Several different yeast species have been developed into systems for efficient heterologous gene expression. In this paper we review foreign gene expression in the dairy yeast Kluyveromyces lactis. This yeast presents several advantageous properties in comparison to other yeast species. These include its impressive secretory capacities, its excellent fermentation characteristics on large scale, its food grade status and the availability of both episomal and integrative expression vectors. Moreover, in contrast to the methylotrophic yeasts that are frequently used for the expression of foreign genes, K. lactis does not require explosion-proof fermentation equipment. Here, we present an overview of the available tools for heterologous gene expression in K. lactis (available promoters, vector systems, etc). Also, the production of prochymosin, human serum albumin and pancreatic phospholipase by K. lactis is discussed in more detail, and used to rate the achievements of K. lactis with respect to other micro-organisms in which these proteins have been produced.