Isolation and characterisation of three moss-derived beta-tubulin promoters suitable for recombinant expression

Multifactorial analysis of terminator performance on heterologous gene expression in Physcomitrella

KEY MESSAGE

Characterization of Physcomitrella 3'UTRs across different promoters yields endogenous single and double terminators for usage in molecular pharming. The production of recombinant proteins for health applications accounts for a large share of the biopharmaceutical market. While many drugs are produced in microbial and mammalian systems, plants gain more attention as expression hosts to produce eukaryotic proteins. In particular, the good manufacturing practice (GMP)-compliant moss Physcomitrella (Physcomitrium patens) has outstanding features, such as excellent genetic amenability, reproducible bioreactor cultivation, and humanized protein glycosylation patterns. In this study, we selected and characterized novel terminators for their effects on heterologous gene expression. The Physcomitrella genome contains 53,346 unique 3'UTRs (untranslated regions) of which 7964 transcripts contain at least one intron. Over 91% of 3'UTRs exhibit more than one polyadenylation site, indicating the prevalence of alternative polyadenylation in Physcomitrella. Out of all 3'UTRs, 14 terminator candidates were selected and characterized via transient Dual-Luciferase assays, yielding a collection of endogenous terminators performing equally high as established heterologous terminators CaMV35S, AtHSP90, and NOS. High performing candidates were selected for testing as double terminators which impact reporter levels, dependent on terminator identity and positioning. Testing of 3'UTRs among the different promoters NOS, CaMV35S, and PpActin5 showed an increase of more than 1000-fold between promoters PpActin5 and NOS, whereas terminators increased reporter levels by less than tenfold, demonstrating the stronger effect promoters play as compared to terminators. Among selected terminator attributes, the number of polyadenylation sites as well as polyadenylation signals were found to influence terminator performance the most. Our results improve the biotechnology platform Physcomitrella and further our understanding of how terminators influence gene expression in plants in general.

Process Engineering of Biopharmaceutical Production in Moss Bioreactors via Model-Based Description and Evaluation of Phytohormone Impact

The moss Physcomitrella is an interesting production host for recombinant biopharmaceuticals. Here we produced MFHR1, a synthetic complement regulator which has been proposed for the treatment of diseases associated to the complement system as part of human innate immunity. We studied the impact of different operation modes for the production process in 5 L stirred-tank photobioreactors. The total amount of recombinant protein was doubled by using fed-batch or batch compared to semi-continuous operation, although the maximum specific productivity (mg MFHR1/g FW) increased just by 35%. We proposed an unstructured kinetic model which fits accurately with the experimental data in batch and semi-continuous operation under autotrophic conditions with 2% CO₂ enrichment. The model is able to predict recombinant protein production, nitrate uptake and biomass growth, which is useful for process control and optimization. We investigated strategies to further increase MFHR1 production. While mixotrophic and heterotrophic conditions decreased the MFHR1-specific productivity compared to autotrophic conditions, addition of the phytohormone auxin (NAA, 10 µM) to the medium enhanced it by 470% in shaken flasks and up to 230% and 260%, in batch and fed-batch bioreactors, respectively. Supporting this finding, the auxin-synthesis inhibitor L-kynurenine (100 µM) decreased MFHR1 production significantly by 110% and 580% at day 7 and 18, respectively. Expression analysis revealed that the MFHR1 transgene, driven by the Physcomitrella actin5 (PpAct5) promoter, was upregulated 16 h after NAA addition and remained enhanced over the whole process, whereas the auxin-responsive gene PpIAA1A was upregulated within the first 2 hours, indicating that the effect of auxin on PpAct5 promoter-driven expression is indirect. Furthermore, the day of NAA supplementation was crucial, leading to an up to 8-fold increase of MFHR1-specific productivity (0.82 mg MFHR1/g fresh weight, 150 mg accumulated over 7 days) compared to the productivity reported previously. Our findings are likely to be applicable to other plant-based expression systems to increase biopharmaceutical production and yields.

Mosses: Versatile plants for biotechnological applications

Mosses have long been recognized as powerful experimental tools for the elucidation of complex processes in plant biology. Recent increases in the availability of sequenced genomes and mutant collections, the establishment of novel technologies for targeted mutagenesis, and the development of viable protocols for large-scale production in bioreactors are now transforming mosses into one of the most versatile tools for biotechnological applications. In the present review, we highlight the astonishing biotechnological potential of mosses and how these plants are being exploited for industrial, pharmaceutical, and environmental applications. We focus on the biological features that support their use as model organisms for basic and applied research, and how these are being leveraged to explore the biotechnological potential in an increasing number of species. Finally, we also provide an overview of the available moss cultivation protocols from an industrial perspective, offering insights into batch operations that are not yet well established or do not even exist in the literature. Our goal is to bolster the use of mosses as factories for the biosynthesis of molecules of interest and to show how these species can be harnessed for the generation of novel and commercially useful bioproducts.

Physcomitrella patens, a versatile synthetic biology chassis

During three decades the moss Physcomitrella patens has been developed to a superb green cell factory with the first commercial products on the market. In the past three decades the moss P. patens has been developed from an obscure bryophyte to a model organism in basic biology, biotechnology, and synthetic biology. Some of the key features of this system include a wide range of Omics technologies, precise genome-engineering via homologous recombination with yeast-like efficiency, a certified good-manufacturing-practice production in bioreactors, successful upscaling to 500 L wave reactors, excellent homogeneity of protein products, superb product stability from batch-to-batch, and a reliable procedure for cryopreservation of cell lines in a master cell bank. About a dozen human proteins are being produced in P. patens as potential biopharmaceuticals, some of them are not only similar to their animal-produced counterparts, but are real biobetters with superior performance. A moss-made pharmaceutical successfully passed phase 1 clinical trials, a fragrant moss, and a cosmetic moss-product is already on the market, highlighting the economic potential of this synthetic biology chassis. Here, we focus on the features of mosses as versatile cell factories for synthetic biology and their impact on metabolic engineering.

Combination of the Endogenous lhcsr1 Promoter and Codon Usage Optimization Boosts Protein Expression in the Moss Physcomitrella patens

The moss Physcomitrella patens is used both as an evo-devo model and biotechnological production system for metabolites and pharmaceuticals. Strong in vivo expression of genes of interest is important for production of recombinant proteins, e.g., selectable markers, fluorescent proteins, or enzymes. In this regard, the choice of the promoter sequence as well as codon usage optimization are two important inside factors to consider in order to obtain optimum protein accumulation level. To reliably quantify fluorescence, we transfected protoplasts with promoter:GFP fusion constructs and measured fluorescence intensity of living protoplasts in a plate reader system. We used the red fluorescent protein mCherry under 2x 35S promoter control as second reporter to normalize for different transfection efficiencies. We derived a novel endogenous promoter and compared deletion variants with exogenous promoters. We used different codon-adapted green fluorescent protein (GFP) genes to evaluate the influence of promoter choice and codon optimization on protein accumulation in P. patens, and show that the promoter of the gene of P. patens chlorophyll a/b binding protein lhcsr1 drives expression of GFP in protoplasts significantly (more than twofold) better than the commonly used 2x 35S promoter or the rice actin1 promoter. We identified a shortened 677 bp version of the lhcsr1 promoter that retains full activity in protoplasts. The codon optimized GFP yields significantly (more than twofold) stronger fluorescence signals and thus demonstrates that adjusting codon usage in P. patens can increase expression strength. In combination, new promotor and codon optimized GFP conveyed sixfold increased fluorescence signal.

Moss-made pharmaceuticals: from bench to bedside

Over the past two decades, the moss Physcomitrella patens has been developed from scratch to a model species in basic research and in biotechnology. A fully sequenced genome, outstanding possibilities for precise genome-engineering via homologous recombination (knockout moss), a certified GMP production in moss bioreactors, successful upscaling to 500 L wave reactors, excellent homogeneity of protein glycosylation, remarkable batch-to-batch stability and a safe cryopreservation for master cell banking are some of the key features of the moss system. Several human proteins are being produced in this system as potential biopharmaceuticals. Among the products are tumour-directed monoclonal antibodies with enhanced antibody-dependent cytotoxicity (ADCC), vascular endothelial growth factor (VEGF), complement factor H (FH), keratinocyte growth factor (FGF7/KGF), epidermal growth factor (EGF), hepatocyte growth factor (HGF), asialo-erythropoietin (asialo-EPO, AEPO), alpha-galactosidase (aGal) and beta-glucocerebrosidase (GBA). Further, an Env-derived multi-epitope HIV protein as a candidate vaccine was produced, and first steps for a metabolic engineering of P. patens have been made. Some of the recombinant biopharmaceuticals from moss bioreactors are not only similar to those produced in mammalian systems such as CHO cells, but are of superior quality (biobetters). The first moss-made pharmaceutical, aGal to treat Morbus Fabry, is in clinical trials.

The potential of Physcomitrella patens as a platform for the production of plant-based vaccines

The moss Physcomitrella patens has a number of advantages for the production of biopharmaceuticals, including: i) availability of standardized conditions for cultivation in bioreactors; ii) not being part of the food chain; iii) high biosafety; iv) availability of highly efficient transformation methods; v) a haploid, fully sequenced genome providing genetic stability and uniform expression; vi) efficient gene targeting at the nuclear level allows for the generation of mutants with specific post-translational modifications (e.g., glycosylation patterns); and vii) oral formulations are a viable approach as no toxic effects are attributed to ingestion of this moss. In the light of this panorama, this opinion paper analyzes the possibilities of using P. patens for the production of oral vaccines and presents some specific cases where its use may represent significant progress in the field of plant-based vaccine development. The advantages represented by putative adjuvant effects of endogenous secondary metabolites and producing specific glycosylation patterns are highlighted.

Functional expression and characterisation of membrane transport proteins

Membrane transporters set the framework organising the complexity of plant metabolism in cells, tissues and organisms. Their substrate specificity and controlled activity in different cells is a crucial part for plant metabolism to run pathways in concert. Transport proteins catalyse the uptake and exchange of ions, substrates, intermediates, products and cofactors across membranes. Given the large number of metabolites, a wide spectrum of transporters is required. The vast majority of in silico annotated membrane transporters in plant genomes, however, has not yet been functionally characterised. Hence, to understand the metabolic network as a whole, it is important to understand how transporters connect and control the metabolic pathways of plant cells. Heterologous expression and in vitro activity studies of recombinant transport proteins have highly improved their functional analysis in the last two decades. This review provides a comprehensive overview of the recent advances in membrane protein expression and functional characterisation using various host systems and transport assays.

Glycoprotein production in moss bioreactors

Complex multimeric recombinant proteins such as therapeutic antibodies require a eukaryotic expression system. Transgenic plants may serve as promising alternatives to the currently favored mammalian cell lines or hybridomas. In contrast to prokaryotic systems, posttranslational modifications of plant and human proteins resemble each other largely, among those, protein N-glycosylation of the complex type. However, a few plant-specific sugar residues may cause immune reactions in humans, representing an obstacle for the broad use of plant-based systems as biopharmaceutical production hosts. The moss Physcomitrella patens represents a flexible tissue-culture system for the contained production and secretion of recombinant biopharmaceuticals in photobioreactors. The recent synthesis of therapeutic proteins as a scFv antibody fragment or the large and heavily modified complement regulator factor H demonstrate the versatility of this expression system. A uniquely efficient gene targeting mechanism can be employed to precisely engineer the glycosylation machinery for recombinant products. In this way, P. patens lines with non-immunogenic optimized glycan structures were created. Therapeutic antibodies produced in these strains exhibited antibody-dependent cellular cytotoxicity superior to the same molecules synthesized in mammalian cell lines.

Bioreactor systems for in vitro production of foreign proteins using plant cell cultures

Plant cells have been demonstrated to be an attractive heterologous expression host (using whole plants and in vitro plant cell cultures) for foreign protein production in the past 20years. In recent years in vitro liquid cultures of plant cells in a fully contained bioreactor have become promising alternatives to traditional microbial fermentation and mammalian cell cultures as a foreign protein expression platform, due to the unique features of plant cells as a production host including product safety, cost-effective biomanufacturing, and the capacity for complex protein post-translational modifications. Heterologous proteins such as therapeutics, antibodies, vaccines and enzymes for pharmaceutical and industrial applications have been successfully expressed in plant cell culture-based bioreactor systems including suspended dedifferentiated plant cells, moss, and hairy roots, etc. In this article, the current status and emerging trends of plant cell culture for in vitro production of foreign proteins will be discussed with emphasis on the technological progress that has been made in plant cell culture bioreactor systems.

Functional cross-kingdom conservation of mammalian and moss (Physcomitrella patens) transcription, translation and secretion machineries

Plants and mammals are separated by a huge evolutionary distance. Consequently, biotechnology and genetics have traditionally been divided into 'green' and 'red'. Here, we provide comprehensive evidence that key components of the mammalian transcription, translation and secretion machineries are functional in the model plant Physcomitrella patens. Cross-kingdom compatibility of different expression modalities originally designed for mammalian cells, such as native and synthetic promoters and polyadenylation sites, viral and cellular internal ribosome entry sites, secretion signal peptides and secreted product proteins, and synthetic transactivators and transrepressors, was established. This mammalian expression portfolio enabled constitutive, conditional and autoregulated expression of different product genes in a multicistronic expression format, optionally adjusted by various trigger molecules, such as butyrolactones, macrolide antibiotics and ethanol. Capitalizing on a cross-kingdom-compatible expression platform, we pioneered a prototype biopharmaceutical manufacturing scenario using microencapsulated transgenic P. patens protoplasts cultivated in a Wave Bioreactor. Vascular endothelial growth factor 121 (VEGF(121)) titres matched those typically achieved by standard protonema populations grown in stirred-tank bioreactors. The full compatibility of mammalian expression systems in P. patens further promotes the use of moss as a cost-effective alternative for the manufacture of complex biopharmaceuticals, and as a valuable host system to advance synthetic biology in plants.

Moss bioreactors producing improved biopharmaceuticals

Plants may serve as superior production systems for complex recombinant pharmaceuticals. Current strategies for improving plant-based systems include the development of large-scale production facilities as well as the optimisation of protein modifications. While post-translational modifications of plant proteins generally resemble those of mammalian proteins, certain plant-specific protein-linked sugars are immunogenic in humans, a fact that restricts the use of plants in biopharmaceutical production so far. The moss Physcomitrella patens was developed as a contained tissue culture system for recombinant protein production in photo-bioreactors. By targeted gene replacements, moss strains were created with non-immunogenic humanised glycan patterns. These were proven to be superior to currently used mammalian cell lines in producing antibodies with enhanced effectiveness.

In vivo glyco-engineered antibody with improved lytic potential produced by an innovative non-mammalian expression system

Recent studies have demonstrated that the reduction of the core fucosylation on N-glycans of human IgGs is responsible for a clearly enhanced antibody-dependent cellular cytotoxicity (ADCC). This finding might give access to improved active therapeutic antibodies. Here, the expression of the tumor antigen-specific antibody IGN311 was performed in a glyco-optimized strain of the moss Physcomitrella patens. Removal of plant specific N-glycan structures in this plant expression host was achieved by targeted knockout of corresponding genes and included quantitative elimination of core fucosylation. Antibodies transiently expressed and secreted by such genetically modified moss protoplasts assembled correctly, showed an unaltered antigen-binding affinity and, in extensive tests, revealed an up to 40-fold enhanced ADCC. Thus, the glyco-engineered moss-based transient expression platform combines a rapid technology with the subsequent analysis of glycooptimized therapeutics with regard to advanced properties.

Current achievements in the production of complex biopharmaceuticals with moss bioreactors

Transgenic plants are promising alternatives for the low-cost and safe pathogen-free production of complex recombinant pharmaceutical proteins (molecular farming). Plants as higher eukaryotes perform posttranslational modifications similar to those of mammalian cells. However, plant-specific protein N-glycosylation was shown to be immunogenic, a fact that represents a drawback for many plant systems in biopharmaceutical production. The moss Physcomitrella patens offers unique properties as a contained system for protein production. It is grown in the predominant haploid gametophytic stage as tissue suspension cultures in photobioreactors. Efficient secretory signals and a transient transfection system allow the secretion of freshly synthesized proteins to the surrounding medium. The key advantage of Physcomitrella compared to other plant systems is the feasibility of targeted gene replacements. By this means, moss strains with non-immunogenic humanized glycan patterns were created. Here we present an overview of the relevant aspects for establishing moss as a production system for recombinant biopharmaceuticals.

High-level expression of secreted complex glycosylated recombinant human erythropoietin in the Physcomitrella Delta-fuc-t Delta-xyl-t mutant

The highly glycosylated peptide hormone erythropoietin (EPO) plays a key role in the regulation of erythrocyte maturation. Currently, marketed EPO is produced by recombinant technology in mammalian cell cultures. The complementary DNA (cDNA) for human EPO (hEPO) was transiently and stably expressed in the moss Physcomitrella patens wild-type and Delta-fuc-t Delta-xyl-t mutant, the latter containing N-glycans lacking the plant-specific, core-bound alpha1,3-fucose and beta1,2-xylose. New expression vectors were designed based on a Physcomitrella ubiquitin gene-derived promoter for the expression of hEPO cDNA. Transient expression in protoplasts was much stronger at 10 than at 20 degrees C. In Western blot analysis, the molecular size of moss-produced recombinant human EPO (rhEPO) was identified to be 30 kDa, and it accumulated in the medium of transiently transformed protoplasts to high levels around 0.5 microg/mL. Transgenic Physcomitrella Delta-fuc-t Delta-xyl-t mutant lines expressing EPO cDNA showed secretion of rhEPO through the cell wall to the culture medium. In 5- and 10-L photobioreactor cultures, secreted rhEPO accumulated to high levels above 250 microg/g dry weight of moss material after 6 days. Silver staining of rhEPO on sodium dodecylsulphate-polyacrylamide gel electrophoresis (SDS-PAGE) taken from the bioreactor culture demonstrated a high purity of the over-expressed secreted rhEPO, with a very low background of endogenous moss proteins. Peptide mapping of rhEPO produced by the Physcomitrella Delta-fuc-t Delta-xyl-t mutant indicated correct processing of the plant-derived signal peptide. All three N-glycosylation sites of rhEPO were occupied by complex-type N-glycans completely devoid of the plant-specific core sugar residues fucose and xylose.

The moss Physcomitrella patens

The moss Physcomitrella patens, like seed plants, shows alternation of generations, but its gametophyte, the haploid phase of the life cycle, is dominant, making it ideal for genetic studies. Crosses show direct segregations, so F2 or test crosses are unnecessary. Mutagenesis yields mutants, the phenotype of which is directly evident. Haploid tissue can be propagated vegetatively, allowing the maintenance of mutants blocked early in development. Protoplasts, isolated from filamentous gametophytic tissue, regenerate directly into filamentous tissue, providing an abundant supply of single haploid cells for transformation. Recombination occurs at a high frequency between genomic sequences in transforming DNA and the corresponding chromosomal sequences, allowing precise inactivation or modification of genes. RNAi technology allows the inactivation of the expression of gene families and the partial knockdown of essential genes. Over 100,000 ESTs have been sequenced and annotated, and sequencing of the genome should be completed by the end of 2005.

Controlled expression of recombinant proteins in Physcomitrella patens by a conditional heat-shock promoter: a tool for plant research and biotechnology

The ability to express tightly controlled amounts of endogenous and recombinant proteins in plant cells is an essential tool for research and biotechnology. Here, the inducibility of the soybean heat-shock Gmhsp17.3B promoter was addressed in the moss Physcomitrella patens, using beta-glucuronidase (GUS) and an F-actin marker (GFP-talin) as reporter proteins. In stably transformed moss lines, Gmhsp17.3B-driven GUS expression was extremely low at 25 degrees C. In contrast, a short non-damaging heat-treatment at 38 degrees C rapidly induced reporter expression over three orders of magnitude, enabling GUS accumulation and the labelling of F-actin cytoskeleton in all cell types and tissues. Induction levels were tightly proportional to the temperature and duration of the heat treatment, allowing fine-tuning of protein expression. Repeated heating/cooling cycles led to the massive GUS accumulation, up to 2.3% of the total soluble proteins. The anti-inflammatory drug acetyl salicylic acid (ASA) and the membrane-fluidiser benzyl alcohol (BA) also induced GUS expression at 25 degrees C, allowing the production of recombinant proteins without heat-treatment. The Gmhsp17.3B promoter thus provides a reliable versatile conditional promoter for the controlled expression of recombinant proteins in the moss P. patens.

A fast and flexible PEG-mediated transient expression system in plants for high level expression of secreted recombinant proteins

Plant expression systems offer a valuable alternative to traditional systems for the production of recombinant biopharmaceuticals. A highly efficient polyethyleneglycol (PEG)-mediated transient expression system for secreted recombinant proteins in plants has been developed. The human vascular endothelial growth factor 121 (rhVEGF) has been successfully expressed and efficiently secreted into the culture medium by transiently transformed moss protoplasts. In order to obtain secretion efficiency data, different expressed signal peptides were analysed and time course studies were performed with expression constructs containing different promoters. The transformation procedure was optimised for high level expression (up to 10 microg/ml) and successfully performed even with a transgenic glyco-engineered strain lacking plant-specific immunogenic sugar residues in N-glycans. The amount of rhVEGF was produced in such quantity that it allowed for the analysis of biological activity, silver-staining and Western blotting, revealing the correct formation and processing of the homodimer. This fast and flexible transient expression system enables feasibility studies and construct optimisation to be concluded within a few days, thus avoiding the time consuming step of having to generate stably transformed lines.

Use of Physcomitrella patens actin 5' regions for high transgene expression: importance of 5' introns

We have isolated four actin (Act) genes from Physcomitrella patens and used their corresponding 5' regions for recombinant expression of the human vascular endothelial growth factor (rhVEGF121) in transiently transformed Physcomitrella protoplasts and in stable transformed lines. In the transient system, we found up to 11-fold activity of the corresponding 5' regions as compared with that of the plant constitutive 35S promoter. Moreover, the use of an optimised expression vector in which the human VEGF signal peptide was exchanged with a plant signal peptide resulted in an additional 7-fold increase in secreted rhVEGF. We found that the 5' introns of PpAct1, PpAct5 and PpAct7 are essential for high expression. The enhancing mechanisms of the introns, however, seem to be different since in the case of PpAct1, the expression level is stimulated only in the presence of the endogenous promoter, whereas the 5' introns of PpAct5 and PpAct7 stimulate expression also in combination with the 35S promoter. Beyond this, the isolated 5' regions are shown to be useful for high expression levels in transgenic moss lines with values of secreted rhVEGF up to 96 microg g(-1) dry weight.

Glyco-engineering of moss lacking plant-specific sugar residues

The commercial production of complex pharmaceutical proteins from human origin in plants is currently limited through differences in protein N-glycosylation pattern between plants and humans. On the one hand, plant-specific alpha(1,3)-fucose and beta(1,2)-xylose residues were shown to bear strong immunogenic potential. On the other hand, terminal beta(1,4)-galactose, a sugar common on N-glycans of pharmaceutically relevant proteins, e.g., antibodies, is missing in plant N-glycan structures. For safe and flexible production of pharmaceutical proteins, the humanisation of plant protein N-glycosylation is essential. Here, we present an approach that combines avoidance of plant-specific and introduction of human glycan structures. Transgenic strains of the moss Physcomitrella patens were created in which the alpha(1,3)-fucosyltransferase and beta(1,2)-xylosyltransferase genes were knocked out by targeted insertion of the human beta(1,4)-galactosyltransferase coding sequence in both of the plant genes (knockin). The transgenics lacked alpha(1,3)-fucose and beta(1,2)-xylose residues, whereas beta(1,4)-galactose residues appeared on protein N-glycans. Despite these significant biochemical changes, the plants did not differ from wild type with regard to overall morphology under standard cultivation conditions. Furthermore, the glyco-engineered plants secreted a transiently expressed recombinant human protein, the vascular endothelial growth factor, in the same concentration as unmodified moss, indicating that the performed changes in glycosylation did not impair the secretory pathway of the moss. The combined knockout/knockin approach presented here, leads to a new generation of engineered moss and towards the safe and flexible production of correctly processed pharmaceutical proteins with humanised N-glycosylation profiles.

Enhanced recovery of a secreted recombinant human growth factor using stabilizing additives and by co-expression of human serum albumin in the moss Physcomitrella patens

The production of pharmaceutical proteins in plants provides a valuable alternative to other traditional eukaryotic expression systems from economic and safety perspectives. The moss Physcomitrella patens allows the expression and secretion of complex target proteins into a simple aqueous maintenance medium, which facilitates downstream processing by rendering it less complex. To address the question of whether the addition of protein-stabilizing substances enhances the recovery of a target protein secreted into the culture medium, several additives at different concentrations were tested in a small-scale screening system. Although polyvinylpyrrolidone (PVP) and human serum albumin (HSA) showed a significant impact on protein levels, supplementation of the medium with these substances was accompanied by certain limitations in upstream processes, such as foam formation (HSA), and in downstream processes, such as reduced binding efficiency on chromatography columns (PVP), respectively. In order to reap the benefit of the enhancing effect and to avoid the given negative aspects, we developed a new strategy based on the recombinant expression of HSA in plants that are already capable of expressing a target protein. First, we analysed the expression and secretion of recombinant HSA in transiently and stably transformed wild-type (WT) plants. HSA was then co-expressed in Physcomitrella plants transgenic for human vascular endothelial growth factor (VEGF). Even with high expression levels of recombinant human VEGF (rhVEGF), the co-expression of recombinant HSA (rHSA) resulted in 48%-102% higher recovery of the target protein without concomitant negative effects on the upstream process. This strategy enables the enhanced recovery of target protein and does not require the addition of foreign components directly to the culture medium.

Molecular tools to study Physcomitrella patens

The moss Physcomitrella patens has become a suitable model plant system for the analysis of diverse aspects of modern plant biology. The research strategies have been influenced by the implementation of state-of-the-art cell culture and molecular biology techniques. The forthcoming completion of the Physcomitrella genome sequencing project will generate many open questions, the examination of which will rely on a diverse set of molecular tools. Within this article, we intend to introduce the essential cell culture and molecular biology techniques which have been adopted in recent years to make Physcomitrella amenable to a wide range of genetic analyses. Many research groups have made valuable contributions to improve the methodology for the study of Physcomitrella. We would like to apologise to all colleagues whose important contributions could not be cited within this manuscript.