Highly active promoters and native secretion signals for protein production during extremely low growth rates in Aspergillus niger

Protein production dynamics and physiological adaptation of recombinant Komagataella phaffii at near-zero growth rates

BACKGROUND

Specific productivity (qP) in yeast correlates with growth, typically peaking at intermediate or maximum specific growth rates (μ). Understanding the factors limiting productivity at extremely low μ might reveal decoupling strategies, but knowledge of production dynamics and physiology in such conditions is scarce. Retentostats, a type of continuous cultivation, enable the well-controlled transition to near-zero µ through the combined retention of biomass and limited substrate supply. Recombinant Komagataella phaffii (syn Pichia pastoris) secreting a bivalent single domain antibody (VHH) was cultivated in aerobic, glucose-limited retentostats to investigate recombinant protein production dynamics and broaden our understanding of relevant physiological adaptations at near-zero growth conditions.

RESULTS

By the end of the retentostat cultivation, doubling times of approx. two months were reached, corresponding to µ = 0.00047 h-1. Despite these extremely slow growth rates, the proportion of viable cells remained high, and de novo synthesis and secretion of the VHH were observed. The average qP at the end of the retentostat was estimated at 0.019 mg g-1 h-1. Transcriptomics indicated that genes involved in protein biosynthesis were only moderately downregulated towards zero growth, while secretory pathway genes were mostly regulated in a manner seemingly detrimental to protein secretion. Adaptation to near-zero growth conditions of recombinant K. phaffii resulted in significant changes in the total protein, RNA, DNA and lipid content, and lipidomics revealed a complex adaptation pattern regarding the lipid class composition. The higher abundance of storage lipids as well as storage carbohydrates indicates that the cells are preparing for long-term survival.

CONCLUSIONS

In conclusion, retentostat cultivation proved to be a valuable tool to identify potential engineering targets to decouple growth and protein production and gain important insights into the physiological adaptation of K. phaffii to near-zero growth conditions.

The α-(1,3)-glucan synthase gene agsE impacts the secretome of Aspergillus niger

Aspergillus niger is widely used as a cell factory for the industrial production of enzymes. Previously, it was shown that deletion of α-1-3 glucan synthase genes results in smaller micro-colonies in liquid cultures of Aspergillus nidulans. Also, it has been shown that small wild-type Aspergillus niger micro-colonies secrete more protein than large mirco-colonies. We here assessed whether deletion of the agsC or agsE α-1-3 glucan synthase genes results in smaller A. niger micro-colonies and whether this is accompanied by a change in protein secretion. Biomass formation was not affected in the deletion strains but pH of the culture medium had changed from 5.2 in the case of the wild-type to 4.6 and 6.4 for ΔagsC and ΔagsE, respectively. The diameter of the ΔagsC micro-colonies was not affected in liquid cultures. In contrast, diameter of the ΔagsE micro-colonies was reduced from 3304 ± 338 µm to 1229 ± 113 µm. Moreover, the ΔagsE secretome was affected with 54 and 36 unique proteins with a predicted signal peptide in the culture medium of MA234.1 and the ΔagsE, respectively. Results show that these strains have complementary cellulase activity and thus may have complementary activity on plant biomass degradation. Together, α-1-3 glucan synthesis (in)directly impacts protein secretion in A. niger.

CRISPR/Cas9-Mediated Multiplexed Genome Editing in Aspergillus oryzae

Aspergillus oryzae has great potential and competitive advantages to be developed as an excellent expression system, owing to its powerful protein secretion ability, complex post-translational modification, and safety characteristics. However, the low efficiency of genetic modification and gene function analysis is an urgent problem to be solved in A. oryzae and other filamentous fungal systems. Therefore, establishing efficient genetic transformation and multiplexed genome editing tools is significant for developing A. oryzae expression systems, and revealing its intrinsic mechanisms. In this study, the high-efficiency transformation of A. oryzae was achieved by optimizing the preparation conditions of protoplasts, and the random editing efficiency of the CRISPR/Cas9 system in A. oryzae for single and double genes reached 37.6% and 19.8%, respectively. With the aid of the selection marker, such as color or resistance, the editing efficiency of single and double genes can reach 100%. Based on the developed CRISPR/Cas9 genome editing method, the heterologous lipase gene (TLL) achieves precise integration at different genetic loci in one step. The efficient and accurate acquisition of positive transformants indicated that the morphological gene yA could be used as a helpful selection marker for genome editing in A. oryzae. In conclusion, the developed system improves the efficiency of transformation and multiplexed genome editing for A. oryzae. It provides a practical method for developing the A. oryzae high-efficiency expression system for heterologous proteins.

Something old, something new: challenges and developments in Aspergillus niger biotechnology

The filamentous ascomycete fungus Aspergillus niger is a prolific secretor of organic acids, proteins, enzymes and secondary metabolites. Throughout the last century, biotechnologists have developed A. niger into a multipurpose cell factory with a product portfolio worth billions of dollars each year. Recent technological advances, from genome editing to other molecular and omics tools, promise to revolutionize our understanding of A. niger biology, ultimately to increase efficiency of existing industrial applications or even to make entirely new products. However, various challenges to this biotechnological vision, many several decades old, still limit applications of this fungus. These include an inability to tightly control A. niger growth for optimal productivity, and a lack of high-throughput cultivation conditions for mutant screening. In this mini-review, we summarize the current state-of-the-art for A. niger biotechnology with special focus on organic acids (citric acid, malic acid, gluconic acid and itaconic acid), secreted proteins and secondary metabolites, and discuss how new technological developments can be applied to comprehensively address a variety of old and persistent challenges.

Developing Aspergillus niger as a cell factory for food enzyme production

Aspergillus niger has become one of the most important hosts for food enzyme production due to its unique food safety characteristics and excellent protein secretion systems. A series of food enzymes such as glucoamylase have been commercially produced by A. niger strains, making this species a suitable platform for the engineered of strains with improved enzyme production. However, difficulties in genetic manipulations and shortage of expression strategies limit the progress in this regard. Moreover, several mycotoxins have recently been detected in some A. niger strains, which raises the necessity for a regulatory approval process for food enzyme production. With robust strains, processing engineering strategies are also needed for producing the enzymes on a large scale, which is also challenging for A. niger, since its culture is aerobic, and non-Newtonian fluid properties are developed during submerged culture, making mixing and aeration very energy-intensive. In this article, the progress and challenges of developing A. niger for the production of food enzymes are reviewed, including its genetic manipulations, strategies for more efficient production of food enzymes, and elimination of mycotoxins for product safety.

Heterologous expression and characterization of Penicillium citrinum nuclease P1 in Aspergillus niger and its application in the production of nucleotides

Nuclease P1 gene (nuc P1) which was cloned from Penicillium citrinum and expressed in A. niger Bdel4 with the low-background extracellular protein. The expression strategy of multi-copy nuc P1 in the A. niger with the linker of 2A peptide was applied to improve the enzyme activity of nuclease P1, the highest activity up to 77.6 U/mL. After Ni-chelate purification, the specific enzyme activity, the optimum temperature and pH were 32.4 U/mg, 65 °C and 5.3 respectively. The recombination nuclease P1 was activated by addition of Mg²⁺, Zn²⁺ and Cu²⁺, and inhibited by addition of Ca²⁺, Fe²⁺, Mn²⁺, Ni²⁺, Co²⁺, Mg²⁺, K⁺ and EDTA. Furthermore, the enzyme hydrolyses yeast RNA efficiently into 5'- nucleotides. Through enzymolysis, the highest concentration of nucleotides achieved 15.12 mg/mL, and 75U nuclease P1 is suitable amount should be added to the enzymolysis system.

The Promoter Toolbox for Recombinant Gene Expression in Trichoderma reesei

The ascomycete Trichoderma reesei is one of the main fungal producers of cellulases and xylanases based on its high production capacity. Its enzymes are applied in food, feed, and textile industry or in lignocellulose hydrolysis in biofuel and biorefinery industry. Over the last years, the demand to expand the molecular toolbox for T. reesei to facilitate genetic engineering and improve the production of heterologous proteins grew. An important instrument to modify the expression of key genes are promoters to initiate and control their transcription. To date, the most commonly used promoter for T. reesei is the strong inducible promoter of the main cellobiohydrolase cel7a. Beside this one, there is a number of alternative inducible promoters derived from other cellulase- and xylanase encoding genes and a few constitutive promoters. With the advances in genomics and transcriptomics the identification of new constitutive and tunable promoters with different expression strength was simplified. In this review, we will discuss new developments in the field of promoters and compare their advantages and disadvantages. Synthetic expression systems constitute a new option to control gene expression and build up complex gene circuits. Therefore, we will address common structural features of promoters and describe options for promoter engineering and synthetic design of promoters. The availability of well-characterized gene expression control tools is essential for the analysis of gene function, detection of bottlenecks in gene networks and yield increase for biotechnology applications.

Antifungal Peptides of the AFP Family Revisited: Are These Cannibal Toxins?

The emergence and spread of pathogenic fungi resistant to currently used antifungal drugs represents a serious challenge for medicine and agriculture. The use of smart antimicrobials, so-called “dirty drugs” which affect multiple cellular targets, is one strategy to prevent resistance. Of special interest is the exploitation of the AFP family of antimicrobial peptides, which include its founding member AFP from Aspergillus giganteus. This latter is a highly potent inhibitor of chitin synthesis and affects plasma membrane integrity in many human and plant pathogenic fungi. A transcriptomic meta-analysis of the afp-encoding genes in A. giganteus and A. niger predicts a role for these proteins during asexual sporulation, autophagy, and nutrient recycling, suggesting that AFPs are molecules important for the survival of A. niger and A. giganteus under nutrient limitation. In this review, we discuss parallels which exist between AFPs and bacterial cannibal toxins and provide arguments that the primary function of AFPs could be to kill genetically identical siblings. We hope that this review inspires computational and experimental biologists studying alternative explanations for the nature and function of antimicrobial peptides beyond the general assumption that they are mere defense molecules to fight competitors.

Effect of CBM1 and linker region on enzymatic properties of a novel thermostable dimeric GH10 xylanase (Xyn10A) from filamentous fungus Aspergillus fumigatus Z5

Xylanase with a high thermostability will satisfy the needs of raising the temperature of hydrolysis to improve the rheology of the broth in industry of biomass conversion. In this study, a xylanase gene (xyn10A), predicted to encode a hydrolase domain of GH10, a linker region and a CBM1 domain, was cloned from a superior lignocellulose degrading strain Aspergillus fumigatus Z5 and successfully expressed in Pichia pastoris X33. Xyn10A has a specific xylanase activity of 34.4 U mg⁻¹, and is optimally active at 90 °C and pH 6.0. Xyn10A shows quite stable at pHs ranging from 3.0 to 11.0, and keeps over 40% of xylanase activity after incubation at 70 °C for 1 h. Removal of CBM1 domain has a slight negative effect on its thermostability, but the further cleavage of linker region significantly decreased its stability at high temperature. The transfer of CBM1 and linker region to another GH10 xylanase can help to increase the thermostability. In addition, hydrolase domains between the two Xyn10A proteins naturally formed a dimer structure, which became more thermostable after removing the CBM1 or/and linker region. This thermostable Xyn10A is a suitable candidate for the highly efficient fungal enzyme cocktails for biomass conversion.

Reconstructing Dynamic Promoter Activity Profiles from Reporter Gene Data

Accurate characterization of promoter activity is important when designing expression systems for systems biology and metabolic engineering applications. Promoters that respond to changes in the environment enable the dynamic control of gene expression without the necessity of inducer compounds, for example. However, the dynamic nature of these processes poses challenges for estimating promoter activity. Most experimental approaches utilize reporter gene expression to estimate promoter activity. Typically the reporter gene encodes a fluorescent protein that is used to infer a constant promoter activity despite the fact that the observed output may be dynamic and is a number of steps away from the transcription process. In fact, some promoters that are often thought of as constitutive can show changes in activity when growth conditions change. For these reasons, we have developed a system of ordinary differential equations for estimating dynamic promoter activity for promoters that change their activity in response to the environment that is robust to noise and changes in growth rate. Our approach, inference of dynamic promoter activity (PromAct), improves on existing methods by more accurately inferring known promoter activity profiles. This method is also capable of estimating the correct scale of promoter activity and can be applied to quantitative data sets to estimate quantitative rates.

Expression system for heterologous protein expression in the filamentous fungus Aspergillus unguis

Heterologous protein expression in filamentous fungi is advantageous, especially in the context of large scale production of high volume low value recombinant proteins. However, such systems are rare and not available in public domain. A novel filamentous fungus - Aspergillus unguis NII 08123 was used as host for developing a protein expression system. An expression cassette was assembled using A. nidulans glyceraldehyde 3 phosphate dehydrogenase promoter (Pgapd), tryptophan synthase transcription terminator (TtrpC) and hygromycin resistance gene (hph) as selection marker. The enhanced green fluorescent protein (GFP) gene from Aequorea victoria was used as the model test protein for the evaluation of the expression system. The genetic transformation of this novel fungus was optimized through electroporation. Use of heterologous signal peptides resulted in high levels of secreted expression. The fungal host-expression system combination was tested successfully for the expression of the recombinant therapeutic protein-human interferon beta (HuIFNβ).

Promoters from the itaconate cluster of Ustilago maydis are induced by nitrogen depletion

Background

Ustilago maydis is known for its natural potential to produce a broad range of valuable chemicals, such as itaconate, from both industrial carbon waste streams and renewable biomass. Production of itaconate, and many other secondary metabolites, is induced by nitrogen limitation in U. maydis. The clustered genes responsible for itaconate production have recently been identified, enabling the development of new expression tools that are compatible with biotechnological processes.

Results

Here we report on the investigation of two of the native promoters, P_tad1 and P_mtt1, from the itaconate cluster of U. maydis MB215. For both promoters the specific activation upon nitrogen limitation, which is known to be the trigger for itaconate production in Ustilago, could be demonstrated by gfp expression. The promoters cover a broad range of expression levels, especially when combined with the possibility to create single- and multicopy construct integration events. In addition, these reporter constructs enable a functional characterization of gene induction patterns associated with itaconate production.

Conclusions

The promoters are well suited to induce gene expression in response to nitrogen limitation, coupled to the itaconate production phase, which contributes towards the further improvement of organic acid production with Ustilago.

A Transcriptome Meta-Analysis Proposes Novel Biological Roles for the Antifungal Protein AnAFP in Aspergillus niger

Understanding the genetic, molecular and evolutionary basis of cysteine-stabilized antifungal proteins (AFPs) from fungi is important for understanding whether their function is mainly defensive or associated with fungal growth and development. In the current study, a transcriptome meta-analysis of the Aspergillus niger γ-core protein AnAFP was performed to explore co-expressed genes and pathways, based on independent expression profiling microarrays covering 155 distinct cultivation conditions. This analysis uncovered that anafp displays a highly coordinated temporal and spatial transcriptional profile which is concomitant with key nutritional and developmental processes. Its expression profile coincides with early starvation response and parallels with genes involved in nutrient mobilization and autophagy. Using fluorescence- and luciferase reporter strains we demonstrated that the anafp promoter is active in highly vacuolated compartments and foraging hyphal cells during carbon starvation with CreA and FlbA, but not BrlA, as most likely regulators of anafp. A co-expression network analysis supported by luciferase-based reporter assays uncovered that anafp expression is embedded in several cellular processes including allorecognition, osmotic and oxidative stress survival, development, secondary metabolism and autophagy, and predicted StuA and VelC as additional regulators. The transcriptomic resources available for A. niger provide unparalleled resources to investigate the function of proteins. Our work illustrates how transcriptomic meta-analyses can lead to hypotheses regarding protein function and predict a role for AnAFP during slow growth, allorecognition, asexual development and nutrient recycling of A. niger and propose that it interacts with the autophagic machinery to enable these processes.