critical for virulence in human pathogenic
fungi such as A. fumigatus, C. neoformans,
and Paracoccidioides spp. (Bien and Espenshade 2010; Chun et al. 2007; Chung et al.
2014; Grahl et al. 2012; Lima Pde et al. 2015).
In N. crassa, the SREBP pathway is highly
induced under lignocellulolytic conditions and
negatively regulates expression of oxygenconsuming enzymes and protein secretion
(Reilly et al. 2015; Qin et al. 2017). SREBP mediates suppression of the central ER stress
response genes bip-1 and pdi-1 in a hac-1 and
ire-1 independent manner. Importantly, the
inability for efficient cellulase secretion in hac1 deletion strains is compensated by the concurrent deletion of the gene encoding the
SREBP homolog SAH-2 (Short Aerial Hyphae
2). In mammalian cells, the interplay between
UPR and the SREBP-mediated hypoxia
response is of high medical relevance. The
Hac1 homolog XBP1 is required for tumor
growth under hypoxic conditions and promotes tumor progression in triple-negative
breast cancer cell lines by forming a transcriptional complex with HIF1 (Hypoxia-Induced
Factor 1) that regulates expression of HIF1 target genes (Chen et al. 2014).
V. The UPR in Biotechnology of
Filamentous Fungi
The application of the UPR in white biotechnology is of long-standing interest as UPR
activity provides strongly increased capacities
for protein production and secretion (Heimel
2015). Comprehensive studies on the UPR in
biotechnologically relevant filamentous fungi
such as A. niger, Trichoderma reesei, and,
more recently, Neurospora crassa substantially
contributed to our current understanding of
the UPR. Various approaches for harnessing
the positive effects of the UPR have been
described, aiming to increase yields of heterologous and endogenous secreted proteins.
Genomic approaches detailing on the cellular
and transcriptional responses to ER stress
within the process of biotechnological protein
production revealed distinct requirements for
optimizing the production of different proteins
of interest. So far, generally applicable strategies that fully exploit the potential of the UPR
for high-level protein production and secretion
remain to be established. In the following, UPRrelated approaches for engineering the secretory capacity in important filamentous model
fungi will be described.
A. Aspergillus niger
Aspergillus niger is used for more than
100 years in white biotechnology for the production of metabolites, proteins, and enzymes
including citric acid, gluconic acid, glucoamylase, and heterologous proteins (Cairns et al.
2018). Several genes encoding central components of the ER folding machinery, such as the
ER chaperone BipA, the foldases PdiA and
CypB (Cyclophilin B), and the lectin chaperone
calnexin ClxA, were identified as being induced
under various stress conditions, including
secretion stress (Derkx and Madrid 2001;
Jeenes et al. 1997; Ngiam et al. 2000; van Gemeren et al. 1997). Transcriptional activation of
these genes is mediated by the Hac1 homolog
HacA (Mulder et al. 2004), recognizing the consensus
sequence
CAN(G/A)NTGT/GCCT
(Mulder et al. 2006). Genome-wide expression
studies identified a core set of 40 genes presumably required for high-level expression of A.
niger glucoamylase (Kwon et al. 2012). In contrast to most yeasts and dimorphic fungi, deletion of hacA or expression of the intronless
hacA mRNA (leading to constitutive UPR activity) reduces vegetative growth and induces formation of aberrant hyphae, complicating the
functional characterization and application of
HacA in biotechnological protein production
(Heimel 2015; Mulder and Nikolaev 2009;
Mulder et al. 2006; Mulder et al. 2004). Consequently, attempts for increasing the production
of glucoamylase, manganese peroxidase, thaumatin, preprochymosin, or tissue plasminogen
activator focused on the overexpression of
genes encoding ER chaperones and foldases,
including bipA, pdiA, and clxA, or the deletion
66
R. Harting and K. Heimel
fungi such as A. fumigatus, C. neoformans,
and Paracoccidioides spp. (Bien and Espenshade 2010; Chun et al. 2007; Chung et al.
2014; Grahl et al. 2012; Lima Pde et al. 2015).
In N. crassa, the SREBP pathway is highly
induced under lignocellulolytic conditions and
negatively regulates expression of oxygenconsuming enzymes and protein secretion
(Reilly et al. 2015; Qin et al. 2017). SREBP mediates suppression of the central ER stress
response genes bip-1 and pdi-1 in a hac-1 and
ire-1 independent manner. Importantly, the
inability for efficient cellulase secretion in hac1 deletion strains is compensated by the concurrent deletion of the gene encoding the
SREBP homolog SAH-2 (Short Aerial Hyphae
2). In mammalian cells, the interplay between
UPR and the SREBP-mediated hypoxia
response is of high medical relevance. The
Hac1 homolog XBP1 is required for tumor
growth under hypoxic conditions and promotes tumor progression in triple-negative
breast cancer cell lines by forming a transcriptional complex with HIF1 (Hypoxia-Induced
Factor 1) that regulates expression of HIF1 target genes (Chen et al. 2014).
V. The UPR in Biotechnology of
Filamentous Fungi
The application of the UPR in white biotechnology is of long-standing interest as UPR
activity provides strongly increased capacities
for protein production and secretion (Heimel
2015). Comprehensive studies on the UPR in
biotechnologically relevant filamentous fungi
such as A. niger, Trichoderma reesei, and,
more recently, Neurospora crassa substantially
contributed to our current understanding of
the UPR. Various approaches for harnessing
the positive effects of the UPR have been
described, aiming to increase yields of heterologous and endogenous secreted proteins.
Genomic approaches detailing on the cellular
and transcriptional responses to ER stress
within the process of biotechnological protein
production revealed distinct requirements for
optimizing the production of different proteins
of interest. So far, generally applicable strategies that fully exploit the potential of the UPR
for high-level protein production and secretion
remain to be established. In the following, UPRrelated approaches for engineering the secretory capacity in important filamentous model
fungi will be described.
A. Aspergillus niger
Aspergillus niger is used for more than
100 years in white biotechnology for the production of metabolites, proteins, and enzymes
including citric acid, gluconic acid, glucoamylase, and heterologous proteins (Cairns et al.
2018). Several genes encoding central components of the ER folding machinery, such as the
ER chaperone BipA, the foldases PdiA and
CypB (Cyclophilin B), and the lectin chaperone
calnexin ClxA, were identified as being induced
under various stress conditions, including
secretion stress (Derkx and Madrid 2001;
Jeenes et al. 1997; Ngiam et al. 2000; van Gemeren et al. 1997). Transcriptional activation of
these genes is mediated by the Hac1 homolog
HacA (Mulder et al. 2004), recognizing the consensus
sequence
CAN(G/A)NTGT/GCCT
(Mulder et al. 2006). Genome-wide expression
studies identified a core set of 40 genes presumably required for high-level expression of A.
niger glucoamylase (Kwon et al. 2012). In contrast to most yeasts and dimorphic fungi, deletion of hacA or expression of the intronless
hacA mRNA (leading to constitutive UPR activity) reduces vegetative growth and induces formation of aberrant hyphae, complicating the
functional characterization and application of
HacA in biotechnological protein production
(Heimel 2015; Mulder and Nikolaev 2009;
Mulder et al. 2006; Mulder et al. 2004). Consequently, attempts for increasing the production
of glucoamylase, manganese peroxidase, thaumatin, preprochymosin, or tissue plasminogen
activator focused on the overexpression of
genes encoding ER chaperones and foldases,
including bipA, pdiA, and clxA, or the deletion
66
R. Harting and K. Heimel