Pichia pastoris, a methylotrophic yeast, is an established system for the production of heterologous proteins, particularly biopharmaceuticals and industrial enzymes. To maximise and optimise the production of recombinant products, recent molecular research has focused on numerous issues including the design of expression vectors, optimisation of gene copy number, co-expression of secretory proteins such as chaperones, engineering of glycosylation and secretory pathways, etc. However, the physiological effects of different cultivation strategies are often difficult to separate from the molecular effects of the gene construct (e.g., cellular stress through over-expression or incorrect post-translational processing). Hence, overall system optimisation is difficult, even though it is urgently required in order to describe and understand the behaviour of new molecular constructs. This review focuses on particular aspects of recombinant protein production related to variations in biomass growth and their implications for strain design and screening, as well as on the concept of rational comparisons between cultivation systems for the development of specific production processes in bioreactors. The relationship between specific formation rates of secreted recombinant proteins, qp, and specific growth rates, μ, has been analysed in a conceptual attempt to compare different systems, particularly those based on AOX1/methanol and GAP/glucose, and this has now evolved into a pivotal concept for bioprocess engineering of P. pastoris.

Matching both the construction of a recombinant strain and the process design with the characteristics of the target protein has the potential to significantly enhance bioprocess performance, robustness, and reproducibility. The factors affecting the physiological state of recombinant Pichia pastoris Mut ؉ (methanol utilization-positive) strains and their cell membranes were quantified at the individual cell level using a combination of staining with fluorescent dyes and flow cytometric enumeration. Cell vitalities were found to range from 5 to 95% under various process conditions in high-cell-density fed-batch cultures, with strains producing either porcine trypsinogen or horseradish peroxidase extracellularly. Impaired cell vitality was observed to be the combined effect of production of recombinant protein, low pH, and high cell density. Vitality improved when any one of these stress factors was excluded. At a pH value of 4, which is commonly applied to counter proteolysis, recombinant strains exhibited severe physiological stress, whereas strains without heterologous genes were not affected. Physiologically compromised cells were also found to be increasingly sensitive to methanol when it accumulated in the culture broth. The magnitude of the response varied when different reporters were combined with either the native AOX1 promoter or its d6* variant, which differ in both strength and regulation. Finally, the quantitative assessment of the physiology of individual cells enables the implementation of innovative concepts in bioprocess development. Such concepts are in contrast to the frequently used paradigm, which always assumes a uniform cell population, because differentiation between the individual cells is not possible with methods commonly used.Changes to the product quantity and quality as well as the robustness of bioprocesses can be triggered by a number of factors which affect the physiological state of the microorganisms. The expression of a foreign gene, the processing of the recombinant protein, and the exposure of the cell to metabolites, inductors, substrates, unfavorable environmental conditions, high cell density (HCD), and/or "aging" can all result in markedly different physiological responses (17).The interpretation of bioprocess data often fails to reflect the actual state of individual cells within a population. It is based typically on performance characterization using concentration measurements and the simplifying assumption of uniform ("averaged") performance of each cell. This practice (39) provides an example of the failure to distinguish between a homogenous population of equally compromised microorganisms and a heterogeneous population of cells each performing differently. An understanding of the state of the individual cells is critical in achieving high efficiency in recombinant protein production. Only by knowing the number of (vital) cells that express the target molecule at the highest possible rate can the number of such cells within the population be maximized and the propo...

Natural tools for recombinant protein production show technological limitations. Available natural promoters for gene expression in Pichia pastoris are either constitutive, weak or require the use of undesirable substances or procedures for induction. Here we show the application of deletion variants based on the well known methanol inducible AOX1 promoter and small synthetic promoters, where cis-acting elements were fused to core promoter fragments. They enable differently regulated target protein expression and at the same time to replace methanol induction by a glucose or glycerol feeding strategy. Trypsinogen, the precursor of the serine protease trypsin, was expressed using these different promoters. Depending on the applied promoter the production window (i.e. the time of increasing product concentration) changed significantly. In fedbatch processes trypsinogen yields before induction with methanol were up to 10 times higher if variants of the AOX1 promoter were applied. In addition, the starting point of autoproteolytic product degradation can be predetermined by the promoter choice.

The key to optimizing productivity during industrial fermentations is the ability to rapidly monitor and interpret the physiological state of single microbial cells in a population and to recognize and characterize different sub-populations. Here, a flow cytometry-based method for the reproducible detection of changes in membrane function and/or structure of recombinant E. coli JM101 (pSPZ3) expressing xylene monooxygenase (XMO), was developed. XMO expression led to compromised but not permeabilized cell membranes. This was deduced from the fact that recombinant cells only stained with ethidium bromide (EB) and not with propidium iodide (PI). During the glucose-limited fedbatch cultivation, an increase from 25% to 95% of EB-stained cells was observed, occurring between 2 and 5 h after induction. Control experiments confirmed that this increase was due to the recombinant protein production and not caused by any possible effects of varying substrate availability, high cell density, plasmid replication or the presence of the inducing agent. We hypothesize that the integration of the recombinant protein into the cell membrane physically disrupted the functionality of the efflux pumps, thus resulting in EB-staining of the recombinant cells. This method enabled us to detect changes in the physiological state of single cells 2-4 h before other indications of partial cell damage, such as unbalanced growth, acetate accumulation and an increased CO(2) production rate, were observed. This method therefore shows promise with respect to the further development of an early-warning system to prevent sudden productivity decreases in processes with recombinant E. coli expressing heterologous membrane proteins.

As Pichia pastoris (syn. Komagataella sp.) yeast can secrete pure recombinant proteins at high rates, it is a desirable production system. The function of a novel synthetic variant of the AOX1 promoter was characterised comprehensively using a strain secreting Candida antarctica lipase B (CALB) as a model. A new time-saving approach was introduced to determine, in only one experiment, the hitherto unknown relationship between specific product formation rate (q p) and specific growth rate (μ). Tight control of recombinant protein formation was possible in the absence of methanol, while using glycerol as a sole carbon/energy source. CALB was not synthesised during batch cultivation in excess glycerol (>10 g l−1) and at a growth rate close to μ max (0.15 h−1). Between 0.017 and 0.115 h−1 in glycerol-limited fedbatch cultures, basal levels of q p > 0.4 mg g−1 h−1 CALB were reached, independent of the μ at which the culture grew. At μ > 0.04 h−1, an elevated q p occurred temporarily during the first 20 h after changing to fedbatch mode and decreased thereafter to basal. In order to accelerate the determination of the q p(μ) relationship (kinetics of product formation), the entire μ range was covered in a single fedbatch experiment. By linearly increasing and decreasing glycerol addition rates, μ values were repeatedly shifted from 0.004 to 0.074 h−1 and vice versa. Changes in q p were related to changes in μ. A rough estimation of μ range suitable for production was possible in a single fedbatch, thus significantly reducing the experimental input over previous approaches comprising several experiments.

Highly active variants of minimal hammerhead ribozymes are generated by the replacement of substantial parts of stem-loop structures with pyrene building blocks.

To take full advantage of recombinant Pichia pastoris (Komagataella phaffii) as a production system for heterologous proteins, the complex protein secretory process should be understood and optimised by circumventing bottlenecks. Typically, little or no attention has been paid to the fate of newly synthesised protein inside the cell, or its passage through the secretory pathway, and only the secreted product is measured. However, the system’s productivity (i.e. specific production rate qp), includes productivity of secreted (qp,extra) plus intracellularly accumulated (qp,intra) protein. In bioreactor cultivations with P. pastoris producing penicillin G acylase, we studied the dynamics of product formation, i.e. both the specific product secretion (qp,extra) and product retention (qp,intra) as functions of time, as well as the kinetics, i.e. productivity in relation to specific growth rate (μ). Within the time course, we distinguished (I) an initial phase with constant productivities, where the majority of product accumulated inside the cells, and qp,extra, which depended on μ in a bell-shaped manner; (II) a transition phase, in which intracellular product accumulation reached a maximum and productivities (intracellular, extracellular, overall) were changing; (III) a new phase with constant productivities, where secretion prevailed over intracellular accumulation, qp,extra was linearly related to μ and was up to three times higher than in initial phase (I), while qp,intra decreased 4–6-fold. We show that stress caused by heterologous protein production induces cellular imbalance leading to a secretory bottleneck that ultimately reaches equilibrium. This understanding may help to develop cultivation strategies for improving protein secretion from P. pastoris.Key Points• A novel concept for industrial bioprocess development.• A Relationship between biomass growth and product formation in P. pastoris.• A Three (3) phases of protein production/secretion controlled by the AOX1-promoter.• A Proof of concept in production of industrially relevant penicillin G acylase.

Health and safety concerns, enhanced quality criteria, and environmental sustainability, have prompted investigations into production using recombinant yeasts as a feasible alternative for isolation of proteins from natural animal or plant sources, as well as for processes utilising either mammalian cell cultures or bacterial systems. An overview of recent research papers and review articles provides readers with a comprehensive insight into the field of next-generation yeast expression systems. Major breakthroughs in recombinant yeast technology linked to Pichia pastoris are (i) the public availability of tools to generate proteins with tailored and highly homogenous N-glycan structures, similar to the forms assembled in humans, (ii) the recent accomplishment of the annotation of its genome sequence, and finally, (iii) the presence of the first few (non-glycosylated) therapeutic proteins in Pichia on the market. The P. pastoris expression platform is now well developed, as proven by multiple products used in human and veterinary medicine and in industry (e.g., enzymes for chemical synthesis and for the modification/synthesis of pharmaceuticals, drug target proteins used for structural analysis or for high throughput screening, proteins for diagnostics, proteinous biomaterials, vaccines, and therapeutic proteins). Nevertheless, the complexity of protein analysis (monitoring) continues to restrict process development for recombinant products. Drawing on combined expertise in molecular biology and process technology, the Institute of Biotechnology (IBT) at the Zurich University of Applied Science (ZHAW) and its international partners have developed solutions which (i) fully eliminate (or partially reduce) the use of methanol, which is undesirable in high-cell-density and high-productivity processes, (ii) match both strain construction and process design with the target protein characteristics to the benefit of the cells' physiological shape, and (iii) allow multi-gene expressions to be balanced to achieve custom tailored and reproducible protein quality at the level of (engineered) posttranslational modifications. In addition to enabling superior product quality specifications to be achieved with reduced development time, these innovations have helped the industries involved to minimise financial risks and the risk of failure, as well as create an opportunity for (new) drugs with improved functionality at low cost.