From spores to fungal pellets: A new high-throughput image analysis highlights the structural development of Aspergillus niger.

Many filamentous fungi are exploited as cell factories in biotechnology. Cultivated under industrially relevant submerged conditions, filamentous fungi can adopt different macromorphologies ranging from dispersed mycelia over loose clumps to pellets. Central to the development of a pellet morphology is the agglomeration of spores after inoculation followed by spore germination and outgrowth into a pellet population, which is usually very heterogeneous. As the dynamics underlying population heterogeneity is not yet fully understood, we present here a new high-throughput image analysis pipeline based on stereomicroscopy to comprehensively assess the developmental program starting from germination up to pellet formation. To demonstrate the potential of this pipeline, we used data from 44 sampling times harvested during a 48 h submerged batch cultivation of the fungal cell factory Aspergillus niger. The analysis of up to 1700 spore agglomerates and 1500 pellets per sampling time allowed the precise tracking of the morphological development of the overall culture. The data gained were used to calculate size distributions and area fractions of spores, spore agglomerates, spore agglomerates within pellets, pellets, and dispersed mycelia. This approach eventually enables the quantification of culture heterogeneities and pellet breakage.

Identification of SclB, a Zn(II)2Cys6 transcription factor involved in sclerotium formation in Aspergillus niger.

Certain Aspergillus species such as Aspergillus flavus and A. parasiticus are well known for the formation of sclerotia. These developmental structures are thought to act as survival structures during adverse environmental conditions but are also a prerequisite for sexual reproduction. We previously described an A. niger mutant (scl-2) which formed sclerotium-like structures, suggesting a possible first stage of sexual development in this species. Several lines of evidence presented in this study support the previous conclusion that the sclerotium-like structures of scl-2 are indeed sclerotia. These included the observations that: (i) safranin staining of the sclerotia-like structures produced by the scl-2 mutant showed the typical cellular structure of a sclerotium; (ii) metabolite analysis revealed specific production of indoloterpenes, which have previously been connected to sclerotium formation; (iii) formation of the sclerotium-like structures is dependent on a functional NADPH complex, as shown for other fungi forming sclerotia. The mutation in scl-2 responsible for sclerotium formation was identified using parasexual crossing and bulk segregant analysis followed by high throughput sequencing and subsequent complementation analysis. The scl-2 strain contains a mutation that introduces a stop codon in the putative DNA binding domain of a previously uncharacterized Zn(II)2Cys6 type transcription factor (An08g07710). Targeted deletion of this transcription factor (sclB) confirmed its role as a repressor of sclerotial formation and in the promotion of asexual reproduction in A. niger. Finally, a genome-wide transcriptomic comparison of RNA extracted from sclerotia versus mycelia revealed major differences in gene expression. Induction of genes related to indoloterpene synthesis was confirmed and also let to the identification of a gene cluster essential for the production of aurasperones during sclerotium formation. Expression analysis of genes encoding other secondary metabolites, cell wall related genes, transcription factors, and genes related to reproductive processes identified many interesting candidate genes to further understand the regulation and biosynthesis of sclerotia in A. niger. The newly identified SclB transcription factor acts as a repressor of sclerotium formation and manipulation of sclB may represent a first prerequisite step towards engineering A. niger strains capable of sexual reproduction. This will provide exciting opportunities for further strain improvement in relation to protein or metabolite production in A. niger.

Engineering cofactor metabolism for improved protein and glucoamylase production in Aspergillus niger.

BACKGROUND: Nicotinamide adenine dinucleotide phosphate (NADPH) is an important cofactor ensuring intracellular redox balance, anabolism and cell growth in all living systems. Our recent multi-omics analyses of glucoamylase (GlaA) biosynthesis in the filamentous fungal cell factory Aspergillus niger indicated that low availability of NADPH might be a limiting factor for GlaA overproduction. RESULTS: We thus employed the Design-Build-Test-Learn cycle for metabolic engineering to identify and prioritize effective cofactor engineering strategies for GlaA overproduction. Based on available metabolomics and 13C metabolic flux analysis data, we individually overexpressed seven predicted genes encoding NADPH generation enzymes under the control of the Tet-on gene switch in two A. niger recipient strains, one carrying a single and one carrying seven glaA gene copies, respectively, to test their individual effects on GlaA and total protein overproduction. Both strains were selected to understand if a strong pull towards glaA biosynthesis (seven gene copies) mandates a higher NADPH supply compared to the native condition (one gene copy). Detailed analysis of all 14 strains cultivated in shake flask cultures uncovered that overexpression of the gsdA gene (glucose 6-phosphate dehydrogenase), gndA gene (6-phosphogluconate dehydrogenase) and maeA gene (NADP-dependent malic enzyme) supported GlaA production on a subtle (10%) but significant level in the background strain carrying seven glaA gene copies. We thus performed maltose-limited chemostat cultures combining metabolome analysis for these three isolates to characterize metabolic-level fluctuations caused by cofactor engineering. In these cultures, overexpression of either the gndA or maeA gene increased the intracellular NADPH pool by 45% and 66%, and the yield of GlaA by 65% and 30%, respectively. In contrast, overexpression of the gsdA gene had a negative effect on both total protein and glucoamylase production. CONCLUSIONS: This data suggests for the first time that increased NADPH availability can indeed underpin protein and especially GlaA production in strains where a strong pull towards GlaA biosynthesis exists. This data also indicates that the highest impact on GlaA production can be engineered on a genetic level by increasing the flux through the pentose phosphate pathway (gndA gene) followed by engineering the flux through the reverse TCA cycle (maeA gene). We thus propose that NADPH cofactor engineering is indeed a valid strategy for metabolic engineering of A. niger to improve GlaA production, a strategy which is certainly also applicable to the rational design of other microbial cell factories.

Comparative genomics of the aconidial Aspergillus niger strain LDM3 predicts genes associated with its high protein secretion capacity.

Aspergillus niger is widely used as a cell factory for homologous and heterologous protein production. As previous studies reported that reduced sporulation favors protein secretion in A. niger, in this study, we conducted a comparative genomic analysis of the non-sporulating industrially exploited A. niger strain LDM3 in China and the reference protein secretion strain CBS 513.88 to predict the key genes that might define the genetic basis of LDM3's high protein-producing potential in silico. After sequencing using a hybrid approach combining Illumina and PacBio sequencing platforms, a high-quality genome sequence of LDM3 was obtained which harbors 11,209 open reading frames (ORFs). LDM3 exhibits large chromosomal rearrangements in comparison to CBS 513.88. An alignment of the two genome sequences revealed that the majority of the 457 ORFs uniquely present in LDM3 possessed predicted functions in redox pathways, protein transport, and protein modification processes. In addition, bioinformatic analyses revealed the presence of 656 ORFs in LDM3 with non-synonymous mutations encoding for proteins related to protein translation, protein modification, protein secretion, metabolism, and energy production. We studied the impact of two of these on protein production in the established lab strain N402. Both tupA and prpA genes were selected because available literature suggested their involvement in asexual sporulation of A. niger. Our co-expression network analysis supportively predicted the role of tupA in protein secretion and the role of prpA in energy generation, respectively. By knockout experiments, we showed that the ΔtupA mutant displayed reduced sporulation (35%) accompanied by higher total protein secretion (65%) compared to its parental strain. Such an effect was, however, not observed in the ΔprpA mutant.

Transcriptomic insights into the roles of the transcription factors Clr1, Clr2 and Clr4 in lignocellulose degradation of the thermophilic fungal platform Thermothelomyces thermophilus.

Introduction: Thermothelomyces thermophilus, formerly known as Myceliophthora thermophila, is used in industry to produce lignocellulolytic enzymes and heterologous proteins. However, the transcriptional network driving the expression of these proteins remains elusive. As a first step to systematically uncover this network, we investigated growth, protein secretion, and transcriptomic fingerprints of strains deficient in the cellulolytic transcriptional regulators Clr1, Clr2, and Clr4, respectively. Methods: The genes encoding Clr1, Clr2, and Clr4 were individually deleted using split marker or the CRISPR/Cas12a technology and the resulting strains as well as the parental strain were cultivated in bioreactors under chemostat conditions using glucose as the carbon source. During steady state conditions, cellulose was added instead of glucose to study the genetic and cellular responses in all four strains to the shift in carbon source availability. Results: Notably, the clr1 and clr2 deletion strains were unable to continue to grow on cellulose, demonstrating a key role of both regulators in cellulose catabolism. Their transcriptomic fingerprints uncovered not only a lack of cellulase gene expression but also reduced expression of genes predicted to encode hemicellulases, pectinases, and esterases. In contrast, the growth of the clr4 deletion strain was very similar compared to the parental strain. However, a much stronger expression of cellulases, hemicellulases, pectinases, and esterases was observed. Discussion: The data gained in this study suggest that both transcriptional regulators Clr1 and Clr2 activate the expression of genes predicted to encode cellulases as well as hemicellulases, pectinases, and esterases. They further suggest that Clr1 controls the basal expression of cellulases and initiates the main lignocellulolytic response to cellulose via induction of clr2 expression. In contrast, Clr4 seems to act as a repressor of the lignocellulolytic response presumably via controlling clr2 expression. Comparative transcriptomics in all four strains revealed potentially new regulators in carbohydrate catabolism and lignocellulolytic enzyme expression that define a candidate gene list for future analyses.

Colony growth and biofilm formation of Aspergillus niger under simulated microgravity.

The biotechnology- and medicine-relevant fungus Aspergillus niger is a common colonizer of indoor habitats such as the International Space Station (ISS). Being able to colonize and biodegrade a wide range of surfaces, A. niger can ultimately impact human health and habitat safety. Surface contamination relies on two key-features of the fungal colony: the fungal spores, and the vegetative mycelium, also known as biofilm. Aboard the ISS, microorganisms and astronauts are shielded from extreme temperatures and radiation, but are inevitably affected by spaceflight microgravity. Knowing how microgravity affects A. niger colony growth, in particular regarding the vegetative mycelium (biofilm) and spore production, will help prevent and control fungal contaminations in indoor habitats on Earth and in space. Because fungal colonies grown on agar can be considered analogs for surface contamination, we investigated A. niger colony growth on agar in normal gravity (Ground) and simulated microgravity (SMG) conditions by fast-clinorotation. Three strains were included: a wild-type strain, a pigmentation mutant (ΔfwnA), and a hyperbranching mutant (ΔracA). Our study presents never before seen scanning electron microscopy (SEM) images of A. niger colonies that reveal a complex ultrastructure and biofilm architecture, and provide insights into fungal colony development, both on ground and in simulated microgravity. Results show that simulated microgravity affects colony growth in a strain-dependent manner, leading to thicker biofilms (vegetative mycelium) and increased spore production. We suggest that the Rho GTPase RacA might play a role in A. niger's adaptation to simulated microgravity, as deletion of ΔracA leads to changes in biofilm thickness, spore production and total biomass. We also propose that FwnA-mediated melanin production plays a role in A. niger's microgravity response, as ΔfwnA mutant colonies grown under SMG conditions showed increased colony area and spore production. Taken together, our study shows that simulated microgravity does not inhibit A. niger growth, but rather indicates a potential increase in surface-colonization. Further studies addressing fungal growth and surface contaminations in spaceflight should be conducted, not only to reduce the risk of negatively impacting human health and spacecraft material safety, but also to positively utilize fungal-based biotechnology to acquire needed resources in situ.

Aspergillus niger Spores Are Highly Resistant to Space Radiation.

The filamentous fungus Aspergillus niger is one of the main contaminants of the International Space Station (ISS). It forms highly pigmented, airborne spores that have thick cell walls and low metabolic activity, enabling them to withstand harsh conditions and colonize spacecraft surfaces. Whether A. niger spores are resistant to space radiation, and to what extent, is not yet known. In this study, spore suspensions of a wild-type and three mutant strains (with defects in pigmentation, DNA repair, and polar growth control) were exposed to X-rays, cosmic radiation (helium- and iron-ions) and UV-C (254 nm). To assess the level of resistance and survival limits of fungal spores in a long-term interplanetary mission scenario, we tested radiation doses up to 1000 Gy and 4000 J/m2. For comparison, a 360-day round-trip to Mars yields a dose of 0.66 ± 0.12 Gy. Overall, wild-type spores of A. niger were able to withstand high doses of X-ray (LD90 = 360 Gy) and cosmic radiation (helium-ion LD90 = 500 Gy; and iron-ion LD90 = 100 Gy). Drying the spores before irradiation made them more susceptible toward X-ray radiation. Notably, A. niger spores are highly resistant to UV-C radiation (LD90 = 1038 J/m2), which is significantly higher than that of other radiation-resistant microorganisms (e.g., Deinococcus radiodurans). In all strains, UV-C treated spores (1000 J/m2) were shown to have decreased biofilm formation (81% reduction in wild-type spores). This study suggests that A. niger spores might not be easily inactivated by exposure to space radiation alone and that current planetary protection guidelines should be revisited, considering the high resistance of fungal spores.

A Penicillium rubens platform strain for secondary metabolite production.

We present a Penicillium rubens strain with an industrial background in which the four highly expressed biosynthetic gene clusters (BGC) required to produce penicillin, roquefortine, chrysogine and fungisporin were removed. This resulted in a minimal secondary metabolite background. Amino acid pools under steady-state growth conditions showed reduced levels of methionine and increased intracellular aromatic amino acids. Expression profiling of remaining BGC core genes and untargeted mass spectrometry did not identify products from uncharacterized BGCs. This platform strain was repurposed for expression of the recently identified polyketide calbistrin gene cluster and achieved high yields of decumbenone A, B and C. The penicillin BGC could be restored through in vivo assembly with eight DNA segments with short overlaps. Our study paves the way for fast combinatorial assembly and expression of biosynthetic pathways in a fungal strain with low endogenous secondary metabolite burden.

Practical guidance for the implementation of the CRISPR genome editing tool in filamentous fungi.

BACKGROUND: Within the last years, numerous reports described successful application of the CRISPR nucleases Cas9 and Cpf1 for genome editing in filamentous fungi. However, still a lot of efforts are invested to develop and improve protocols for the fungus and genes of interest with respect to applicability, scalability and targeting efficiencies. These efforts are often hampered by the fact that-although many different protocols are available-none have systematically analysed and compared different CRISPR nucleases and different application procedures thereof for the efficiency of single- and multiplex-targeting approaches in the same fungus. RESULTS: We present here data for successful genome editing in the cell factory Thermothelomyces thermophilus, formerly known as Myceliophthora thermophila, using the three different nucleases SpCas9, FnCpf1, AsCpf1 guided to four different gene targets of our interest. These included a polyketide synthase (pks4.2), an alkaline protease (alp1), a SNARE protein (snc1) and a potential transcription factor (ptf1). For all four genes, guide RNAs were developed which enabled successful single-targeting and multiplex-targeting. CRISPR nucleases were either delivered to T. thermophilus on plasmids or preassembled with in vitro transcribed gRNA to form ribonucleoproteins (RNPs). We also evaluated the efficiency of single oligonucleotides for site-directed mutagenesis. Finally, we were able to scale down the transformation protocol to microtiter plate format which generated high numbers of positive transformants and will thus pave the way for future high-throughput investigations. CONCLUSION: We provide here the first comprehensive analysis and evaluation of different CRISPR approaches for a filamentous fungus. All approaches followed enabled successful genome editing in T. thermophilus; however, with different success rates. In addition, we show that the success rate depends on the respective nuclease and on the targeted gene locus. We finally present a practical guidance for experimental considerations aiming to guide the reader for successful implementation of CRISPR technology for other fungi.

Correction to: Practical guidance for the implementation of the CRISPR genome editing tool in filamentous fungi.

[This corrects the article DOI: 10.1186/s40694-019-0079-4.].

Aspergillus niger is a superior expression host for the production of bioactive fungal cyclodepsipeptides.

BACKGROUND: Fungal cyclodepsipeptides (CDPs) are non-ribosomally synthesized peptides produced by a variety of filamentous fungi and are of interest to the pharmaceutical industry due to their anticancer, antimicrobial and anthelmintic bioactivities. However, both chemical synthesis and isolation of CDPs from their natural producers are limited due to high costs and comparatively low yields. These challenges might be overcome by heterologous expression of the respective CDP-synthesizing genes in a suitable fungal host. The well-established industrial fungus Aspergillus niger was recently genetically reprogrammed to overproduce the cyclodepsipeptide enniatin B in g/L scale, suggesting that it can generally serve as a high production strain for natural products such as CDPs. In this study, we thus aimed to determine whether other CDPs such as beauvericin and bassianolide can be produced with high titres in A. niger, and whether the generated expression strains can be used to synthesize new-to-nature CDP derivatives. RESULTS: The beauvericin and bassianolide synthetases were expressed under control of the tuneable Tet-on promoter, and titres of about 350-600 mg/L for bassianolide and beauvericin were achieved when using optimized feeding conditions, respectively. These are the highest concentrations ever reported for both compounds, whether isolated from natural or heterologous expression systems. We also show that the newly established Tet-on based expression strains can be used to produce new-to-nature beauvericin derivatives by precursor directed biosynthesis, including the compounds 12-hydroxyvalerate-beauvericin and bromo-beauvericin. By feeding deuterated variants of one of the necessary precursors (d-hydroxyisovalerate), we were able to purify deuterated analogues of beauvericin and bassianolide from the respective A. niger expression strains. These deuterated compounds could potentially be used as internal standards in stable isotope dilution analyses to evaluate and quantify fungal spoilage of food and feed products. CONCLUSION: In this study, we show that the product portfolio of A. niger can be expanded from enniatin to other CDPs such as beauvericin and bassianolide, as well as derivatives thereof. This illustrates the capability of A. niger to produce a range of different peptide natural products in titres high enough to become industrially relevant.

Correction to: Aspergillus niger is a superior expression host for the production of bioactive fungal cyclodepsipeptides.

[This corrects the article DOI: 10.1186/s40694-018-0048-3.].

Human immune cells' behavior and survival under bioenergetically restricted conditions in an in vitro fracture hematoma model.

The initial inflammatory phase of bone fracture healing represents a critical step for the outcome of the healing process. However, both the mechanisms initiating this inflammatory phase and the function of immune cells present at the fracture site are poorly understood. In order to study the early events within a fracture hematoma, we established an in vitro fracture hematoma model: we cultured hematomas forming during an osteotomy (artificial bone fracture) of the femur during total hip arthroplasty (THA) in vitro under bioenergetically controlled conditions. This model allowed us to monitor immune cell populations, cell survival and cytokine expression during the early phase following a fracture. Moreover, this model enabled us to change the bioenergetical conditions in order to mimic the in vivo situation, which is assumed to be characterized by hypoxia and restricted amounts of nutrients. Using this model, we found that immune cells adapt to hypoxia via the expression of angiogenic factors, chemoattractants and pro-inflammatory molecules. In addition, combined restriction of oxygen and nutrient supply enhanced the selective survival of lymphocytes in comparison with that of myeloid derived cells (i.e., neutrophils). Of note, non-restricted bioenergetical conditions did not show any similar effects regarding cytokine expression and/or different survival rates of immune cell subsets. In conclusion, we found that the bioenergetical conditions are among the crucial factors inducing the initial inflammatory phase of fracture healing and are thus a critical step for influencing survival and function of immune cells in the early fracture hematoma.

The Aspergillus niger RmsA protein: A node in a genetic network?

Many cells and organisms go through polarized growth phases during their life. Cell polarization is achieved by local accumulation of signaling molecules which guide the cytoskeleton and vesicular trafficking to specific parts of the cell and thus ensure polarity establishment and maintenance. Polarization of signaling molecules is also fundamental for the lifestyle of filamentous fungi such as Aspergillus niger and essential for their morphogenesis, development and survival under environmental stress conditions. Considerable advances in our understanding on the protagonists and processes mediating polarized growth in filamentous fungi have been made over the past years. However, how the interplay of different signaling pathways is coordinated has yet to be determined. We found that the A. niger RmsA protein is central for the polarization of actin at the hyphal tip but also of vital importance for the metabolism, viability and stress resistance of A. niger. This suggests that RmsA could occupy an important position in the global network of pathways that balance growth, morphogenesis and survival of A. niger.