Advances in Metabolic Engineering of Pichia pastoris Strains as Powerful Cell Factories

Pichia pastoris is the most widely used microorganism for the production of secreted industrial proteins and therapeutic proteins. Recently, this yeast has been repurposed as a cell factory for the production of chemicals and natural products. In this review, the general physiological properties of P. pastoris are summarized and the readily available genetic tools and elements are described, including strains, expression vectors, promoters, gene editing technology mediated by clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9, and adaptive laboratory evolution. Moreover, the recent achievements in P. pastoris-based biosynthesis of proteins, natural products, and other compounds are highlighted. The existing issues and possible solutions are also discussed for the construction of efficient P. pastoris cell factories.

Genome-scale metabolic model analysis of Pichia pastoris for enhancing the production of S-adenosyl-L-methionine

Komagataella phaffii, formerly Pichia pastoris (P. pastoris), is a promising methylotrophic yeast used in industry to produce recombinant protein and valuable metabolites. In this study, a genome-scale metabolic model (GEMs) was reconstructed and used to assess P. pastoris' metabolic capabilities for the production of S-adenosyl-L-methionine (AdoMet or SAM or SAMe) from individual carbon sources along with the addition of L-methionine. In a model-driven P. pastoris strain, the well-established genome-scale metabolic model iAUKM can be implemented to predict high valuable metabolite production. The model, iAUKM, was created by merging the previously published iMT1026 model and the draught model generated using Raven toolbox from the KEGG database which covered 2309 enzymatic reactions associated with 1033 metabolic genes and 1750 metabolites. The highly curated model was successful in capturing P. pastoris growth on various carbon sources, as well as AdoMet production under various growth conditions. Many overexpression gene targets for increasing AdoMet accumulation in the cell have been predicted for various carbon sources. Inorganic phosphatase (IPP) was one of the predicted overexpression targets as revealed from simulations using iAUKM. When IPP gene was integrated into P. pastoris, we found that AdoMet accumulation increased by 16% and 14% using glucose and glycerol as carbon sources, respectively. Our in silico results shed light on the factors limiting AdoMet production, as well as key pathways for rationalized engineering to increase AdoMet yield.

Metabolic engineering of Komagataella phaffii for synergetic utilization of glucose and glycerol

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Strains and Molecular Tools for Recombinant Protein Production in Pichia pastoris

Within the last two decades, the methylotrophic yeast Pichia pastoris (Komagataella phaffii) has become an important alternative to E. coli or mammalian cell lines for the production of recombinant proteins. Easy handling, strong promoters, and high cell density cultivations as well as the capability of posttranslational modifications are some of the major benefits of this yeast. The high secretion capacity and low level of endogenously secreted proteins further promoted the rapid development of a versatile Pichia pastoris toolbox. This chapter reviews common and new "Pichia tools" and their specific features. Special focus is given to expression strains, such as different methanol utilization, protease-deficient or glycoengineered strains, combined with application highlights. Different promoters and signal sequences are also discussed.

Transcriptome Analysis Unveils the Effects of Proline on Gene Expression in the Yeast Komagataella phaffii

Komagataella phaffii yeast is one of the most important biocompounds producing microorganisms in modern biotechnology. Optimization of media recipes and cultivation strategies is key to successful synthesis of recombinant proteins. The complex effects of proline on gene expression in the yeast K. phaffii was analyzed on the transcriptome level in this work. Our analysis revealed drastic changes in gene expression when K. phaffii was grown in proline-containing media in comparison to ammonium sulphate-containing media. Around 18.9% of all protein-encoding genes were differentially expressed in the experimental conditions. Proline is catabolized by K. phaffii even in the presence of other nitrogen, carbon and energy sources. This results in the repression of genes involved in the utilization of other element sources, namely methanol. We also found that the repression of AOX1 gene promoter with proline can be partially reversed by the deletion of the KpPUT4.2 gene.

What makes Komagataella phaffii non-conventional?

The important industrial protein production host Komagataella phaffii (syn Pichia pastoris) is classified as a non-conventional yeast. But what exactly makes K. phaffii non-conventional? In this review, we set out to address the main differences to the 'conventional' yeast Saccharomyces cerevisiae, but also pinpoint differences to other non-conventional yeasts used in biotechnology. Apart from its methylotrophic lifestyle, K. phaffii is a Crabtree-negative yeast species. But even within the methylotrophs, K. phaffii possesses distinct regulatory features such as glycerol-repression of the methanol-utilization pathway or the lack of nitrate assimilation. Rewiring of the transcriptional networks regulating carbon (and nitrogen) source utilization clearly contributes to our understanding of genetic events occurring during evolution of yeast species. The mechanisms of mating-type switching and the triggers of morphogenic phenotypes represent further examples for how K. phaffii is distinguished from the model yeast S. cerevisiae. With respect to heterologous protein production, K. phaffii features high secretory capacity but secretes only low amounts of endogenous proteins. Different to S. cerevisiae, the Golgi apparatus of K. phaffii is stacked like in mammals. While it is tempting to speculate that Golgi architecture is correlated to the high secretion levels or the different N-glycan structures observed in K. phaffii, there is recent evidence against this. We conclude that K. phaffii is a yeast with unique features that has a lot of potential to explore both fundamental research questions and industrial applications.

Global Transcriptome Profile of the Oleaginous Yeast Saitozyma podzolica DSM 27192 Cultivated in Glucose and Xylose

Unlike conventional yeasts, several oleaginous yeasts, including Saitozyma podzolica DSM 27192, possess the innate ability to grow and produce biochemicals from plant-derived lignocellulosic components such as hexose and pentose sugars. To elucidate the genetic basis of S. podzolica growth and lipid production on glucose and xylose, we performed comparative temporal transcriptome analysis using RNA-seq method. Approximately 3.4 and 22.2% of the 10,670 expressed genes were differentially (FDR < 0.05, and log2FC > 1.5) expressed under batch and fed batch modes, respectively. Our analysis revealed that a higher number of sugar transporter genes were significantly overrepresented in xylose relative to glucose-grown cultures. Given the low homology between proteins encoded by most of these genes and those of the well-characterised transporters, it is plausible to conclude that S. podzolica possesses a cache of putatively novel sugar transporters. The analysis also suggests that S. podzolica potentially channels carbon flux from xylose via both the non-oxidative pentose phosphate and potentially via the first steps of the Weimberg pathways to yield xylonic acid. However, only the ATP citrate lyase (ACL) gene showed significant upregulation among the essential oleaginous pathway genes under nitrogen limitation in xylose compared to glucose cultivation. Combined, these findings pave the way toward the design of strategies or the engineering of efficient biomass hydrolysate utilization in S. podzolica for the production of various biochemicals.

High efficiency CRISPR/Cas9 genome editing system with an eliminable episomal sgRNA plasmid in Pichia pastoris

Pichia pastoris is a methylotrophic yeast in which host heterologous expression of proteins has been developed owing to the strong inducible alcohol oxidase promoter (PAOX1). However, it is difficult to manipulate the genome in P. pastoris. Based on previous attempts to apply the CRISPR/Cas9 system in P. pastoris, a CRISPR/Cas9 system with episomal sgRNA plasmid was developed and 100 % genome editing efficiency, high multicopy gene editing and stable multigene editing were obtained without a sharp decline caused by multi-sgRNA. And 28/34 (∼82 %) sgRNAs tested were effective. The CGG may have a slightly higher and more stable cleavage efficiency than the other three NGG motifs, and a low GC content may be preferable for higher cleavage efficiency. This provides researchers with a stable genome editing tool that shows a high editing efficiency, shortening the experimentation period. Furthermore, we introduced dCas9 into P. pastoris and achieved target gene interference, expanding the CRISPR/Cas9 toolbox in P. pastoris.

Glycerol transporter 1 (Gt1) and zinc-regulated transporter 1 (Zrt1) function in different modes for zinc homeostasis in Komagataella phaffii (Pichia pastoris)

OBJECTIVES

To identify the zinc transport function of the membrane proteins Gt1 and Zrt1 in Komagataella phaffii (Pichia pastoris) and study their regulatory mode.

RESULTS

Two membrane proteins that might have zinc transport function were found in K. phaffii. GT1 was known to encode a glycerol transporter belonging to the Major Facilitator Superfamily. ZRT1 was predicted to resemble the zinc transporter gene in Saccharomyces cerevisiae. Consistent with the prediction, protein plasma-membrane localizations were confirmed by ultracentrifugation and confocal microscopy. Their zinc binding abilities were identified by ITC in vitro, and the impaired zinc uptake activity caused by their deficiencies was confirmed by zinc fluorescence quantification in vivo. Furthermore, zinc excess could turn the two channels off, while zinc deficiency induced their expressions. Gt1 could only function to maintain zinc homeostasis in glycerol, while the block of Gt1 function might lead to Zrt1 upregulation in glucose.

CONCLUSIONS

The zinc transport capabilities of Gt1 and Zrt1 were identified in vivo and in vitro. Their regulatory mode to maintain zinc homeostasis in K. phaffii is a new inspiration.

Transcriptional regulatory proteins in central carbon metabolism of Pichia pastoris and Saccharomyces cerevisiae

System-wide interactions in living cells and discovery of the diverse roles of transcriptional regulatory proteins that are mediator proteins with catalytic domains and regulatory subunits and transcription factors in the cellular pathways have become crucial for understanding the cellular response to environmental conditions. This review provides information for future metabolic engineering strategies through analyses on the highly interconnected regulatory networks in Saccharomyces cerevisiae and Pichia pastoris and identifying their components. We discuss the current knowledge on the carbon catabolite repression (CCR) mechanism, interconnecting regulatory system of the central metabolic pathways that regulate cell metabolism based on nutrient availability in the industrial yeasts. The regulatory proteins and their functions in the CCR signalling pathways in both yeasts are presented and discussed. We highlight the importance of metabolic signalling networks by signifying ways on how effective engineering strategies can be designed for generating novel regulatory circuits, furthermore to activate pathways that reconfigure the network architecture. We summarize the evidence that engineering of multilayer regulation is needed for directed evolution of the cellular network by putting the transcriptional control into a new perspective for the regulation of central carbon metabolism of the industrial yeasts; furthermore, we suggest research directions that may help to enhance production of recombinant products in the widely used, creatively engineered, but relatively less studied P. pastoris through de novo metabolic engineering strategies based on the discovery of components of signalling pathways in CCR metabolism. KEY POINTS: • Transcriptional regulation and control is the key phenomenon in the cellular processes. • Designing de novo metabolic engineering strategies depends on the discovery of signalling pathways in CCR metabolism. • Crosstalk between pathways occurs through essential parts of transcriptional machinery connected to specific catalytic domains. • In S. cerevisiae, a major part of CCR metabolism is controlled through Snf1 kinase, Glc7 phosphatase, and Srb10 kinase. • In P. pastoris, signalling pathways in CCR metabolism have not yet been clearly known yet. • Cellular regulations on the transcription of promoters are controlled with carbon sources.

Characterization and application of a putative transcription factor (SUT2) in Pichia pastoris

Pichia pastoris is able to metabolize methanol via a specific MUT (methanol utilization) pathway. Based on the powerful AOX1 (Alcohol Oxidase 1) promoter, the P. pastoris expression system has become one of the most widely used eukaryotic expression systems. The molecular mechanisms of methanol metabolic regulation remain unclearly understood, so it is important to identify and develop new transcriptional regulators. Our previous studies suggested that the expression of SUT2 could be induced by methanol but is repressed by glycerol, which indicates that SUT2 may be involved in methanol metabolism through an unknown mechanism. SUT2 encodes a putative transcription factor-like protein harboring a Gal4-like Zn₂Cys₆ DNA-binding domain in Pichia pastoris, and its homolog in Saccharomyces cerevisiae regulates sterol uptake and synthesis. This study shows that the overexpression of SUT2 promoted the expression of AOX1 and increases ergosterol content in cells. Furthermore, via truncation of the putative SUT2 promoter at diverse loci, the - 973 base pair (bp) to - 547 bp region to the ATG was shown to be the core element of the inducible promoter P_SUT2, which strongly responds to the methanol signal. The transcriptional start site of SUT2, "A" at the 22nd bp upstream of ATG, was determined with 5'-rapid amplification of cDNA ends. A forward-loop cassette was constructed with MXR1 (Methanol Expression Regulator 1, a positive transcription factor of P_AOX1) promoted by P_SUT2, enabling moderate elevation in the expression level of Mxr1 and high activity of P_AOX1 without damaging cellular robustness further boosting the production of heterologous proteins. The P_AOX1-driven expression of enhanced green fluorescent protein in this novel system was improved by 18%, representing a promising method for extrinsic protein production. SUT2 may play roles in methanol metabolism by participating in sterol biosynthesis. P_SUT2 was characterized as a novel inducible promoter in P. pastoris and a P_SUT2-driven MXR1 forward-loop cassette was constructed to enhance the P_AOX1 activity, laying a foundation for further development and application of P. pastoris expression system.

Transcriptional regulatory networks of methanol-independent protein expression in Pichia pastoris under the AOX1 promoter with trans-acting elements engineering

To explore the differences in the intracellular transcriptional mechanism in carbon-derepressed and wild-type Pichia pastoris strains fed with three different carbon sources. RNA in carbon-derepressed (Δmig1Δmig2Δnrg1-Mit1; Mut) and wild-type (WT) P. pastoris fed with three different carbon sources (dextrose, glycerol, and methanol) were sequenced. Differentially expressed genes (DEGs) associated with these carbon sources were obtained and clustered into modules using weighted gene co-expression network analysis (WGCNA). Signaling pathway enrichment analysis was performed using KEGG, and protein to protein interaction (PPI) network was also constructed. A total of 2536 DEGs were obtained from three intersections, and some of them were enriched in carbon sources and involved in carbon metabolism, secondary metabolisms, and amino acid biosynthesis. Two modules, MEgreenyellow (involved in protease, oxidative phosphorylation, endoplasmic reticulum protein processing, folate carbon pool, and glycerol phospholipid metabolism pathways) and MEmidnightblue (involved in protease, endocytosis, steroid biosynthesis, and hippo signaling pathways) were significantly correlated with the strain type. Eight hub genes and two sub-networks were obtained from PPI network. Sub-network A enriched in proteasomes pathway while sub-network B enriched in ribosome pathway. The genes involved in carbon metabolism, secondary metabolic, and amino acid biosynthesis pathways changed significantly under different carbon sources. The changes in proteasome and ribosome activities play roles in carbohydrate metabolism in the methanol-free PAOX1 start-up Mut strain.

High level extracellular production of recombinant human interferon alpha 2b in glycoengineered Pichia pastoris: culture medium optimization, high cell density cultivation and biological characterization

AIMS

The present study was aimed at design of experiments (DoE)- and artificial intelligence-based culture medium optimization for high level extracellular production of a novel recombinant human interferon alpha 2b (huIFNα2b) in glycoengineered Pichia pastoris and its characterization.

METHODS AND RESULTS

The artificial neural network-genetic algorithm model exhibited improved huIFNα2b production and better predictability compared to response surface methodology. The optimized medium exhibited a fivefold increase in huIFNα2b titre compared to the complex medium. A maximum titre of huIFNα2b (436 mg l^-1 ) was achieved using the optimized medium in the bioreactor. Real-time capacitance data from dielectric spectroscopy were utilized to model the growth kinetics with unstructured models. Biological characterization by antiproliferative assay proved that the purified recombinant huIFNα2b was biologically active, exhibiting growth inhibition on breast cancer cell line.

CONCLUSIONS

Culture medium optimization resulted in enhanced production of huIFNα2b in glycoengineered P. pastoris at both shake flask and bioreactor level. The purified huIFNα2b was found to be N-glycosylated and biologically active.

SIGNIFICANCE AND IMPACT OF THE STUDY

DoE-based medium optimization strategy significantly improved huIFNα2b production. The antiproliferative activity of huIFNα2b substantiates its potential scope for application in cancer therapy.