larly inhaled by humans, are protected by surface hydrophobins from recognition by the
human immune system (Aimanianda et al.
2009) and expelled or efficiently eliminated in
immunocompetent individuals by neutrophilinduced programmed cell death (Shlezinger
et al. 2017). By contrast, immunosuppressed
individuals are at risk of developing invasive
aspergillosis, which initially starts as a lung
infection that can eventually disseminate
through the blood stream and cause severe
and potentially lethal cerebral infections. Investigations focusing on the role of the central
UPR regulatory proteins IreA (Ire1) and HacA
(Hac1) revealed important contributions of the
UPR to fungal virulence. In infection models
using immunosuppressed (triamcinolone acetonide) mice, deletion of hacA results in reduced
virulence, and ireA deletion mutants were
found to be completely avirulent (Feng et al.
2011; Richie et al. 2009). These observations
were connected to reduced ER and cell wall
stress resistance and decreased thermotolerance of both mutant strains. Differences
between ireA and hacA mutants with respect
to the ER stress response were investigated by
genome-wide expression analysis, revealing 391
and 1157 differentially expressed genes in hacA
and ireA deletion mutants (in comparison to
the WT), respectively, of which 243 genes are
shared between both mutant strains (Feng et al.
2011; Richie et al. 2011b). In higher eukaryotes,
ER stress results in the global reduction of
translation initiation by phosphorylation of
eIF2a by PERK (PKR-like endoplasmic reticulum kinase) (Ron and Walter 2007). A similar
mechanism contributing to phosphorylation of
eIF2a under amino acid imbalance is mediated
by Gcn2/CpcC and important for virulence in
A. fumigatus (Krappmann et al. 2004). ER stress
has not been reported to reduce translation
initiation frequency in fungi. Conversely, analysis of the A. fumigatus translatome by polysome profiling under ER stress-inducing
conditions and growth at increased temperatures revealed a predominantly increased ribosome association of mRNAs. Responses to the
ER stress-inducing drugs DTT and tunicamycin
are rapid and distinct with limited overlap
between both treatments (DTT, total 717
mRNAs; TM total, 876 mRNAs; overlap, 233
mRNAs). The response to a temperature shift
from 25
C to 37
C is associated with the
increased translation of 372 mRNAs, overlapping with DTT and TM treatment in case of 44
mRNAs (Krishnan et al. 2014).
Degradation of terminally misfolded proteins is realized by the ER-associated degradation (ERAD) pathway, promoting the export of
substrate proteins into the cytoplasm for ubiquitylation and subsequent degradation by the
26S proteasome. Deletion of derA, encoding a
predicted Derlin ortholog, an ER membrane
protein involved in retro-translocation during
ERAD, or hrdA (HMG-coA Reductase Degradation), encoding the ER membrane-localized E3
ubiquitin ligase, did not affect ER stress resistance or virulence (Krishnan et al. 2013; Richie
et al. 2011b). The parallel deletion of both genes
leads to reduced resistance against cell wall and
ER stress but does not affect virulence (Grahl
et al. 2012; Krishnan et al. 2013). derA deletion
mutants showed increased UPR activity, and
the double deletion mutant of derA and hacA
is severely affected in hyphal growth, antifungal
drug resistance, and protease secretion and displayed strongly reduced virulence in triamcinolone acetonide immunosuppressed mice
(Krishnan et al. 2013, Richie et al. 2011b). Similarly, UPR activity is increased upon conditional repression of the Saccharomyces
cerevisiae stt3 ortholog, encoding the catalytic
subunit of the OST (Oligosaccharyl Transferase) complex, mediating N-glycosylation in
the ER (Li et al. 2011). Activity of the UPR
during host colonization is apparently
connected to growth at mammalian body temperature, nutrient deprivation and hypoxia
(Krishnan and Askew 2014). The adaptation to
hypoxic conditions is mediated by the SREBP
(sterol regulatory element-binding protein)
transcription factor SreA (Chung et al. 2014),
which is activated by signal peptide peptidasemediated cleavage of its ER transmembrane
domain and required for full virulence of A.
fumigatus (Bat-Ochir et al. 2016). The clinical
relevance of the UPR is further corroborated
via its role in antifungal drug resistance. hacA
mutants showed increased sensitivity towards
caspofungin, itraconazole, fluconazole, and
56
R. Harting and K. Heimel
human immune system (Aimanianda et al.
2009) and expelled or efficiently eliminated in
immunocompetent individuals by neutrophilinduced programmed cell death (Shlezinger
et al. 2017). By contrast, immunosuppressed
individuals are at risk of developing invasive
aspergillosis, which initially starts as a lung
infection that can eventually disseminate
through the blood stream and cause severe
and potentially lethal cerebral infections. Investigations focusing on the role of the central
UPR regulatory proteins IreA (Ire1) and HacA
(Hac1) revealed important contributions of the
UPR to fungal virulence. In infection models
using immunosuppressed (triamcinolone acetonide) mice, deletion of hacA results in reduced
virulence, and ireA deletion mutants were
found to be completely avirulent (Feng et al.
2011; Richie et al. 2009). These observations
were connected to reduced ER and cell wall
stress resistance and decreased thermotolerance of both mutant strains. Differences
between ireA and hacA mutants with respect
to the ER stress response were investigated by
genome-wide expression analysis, revealing 391
and 1157 differentially expressed genes in hacA
and ireA deletion mutants (in comparison to
the WT), respectively, of which 243 genes are
shared between both mutant strains (Feng et al.
2011; Richie et al. 2011b). In higher eukaryotes,
ER stress results in the global reduction of
translation initiation by phosphorylation of
eIF2a by PERK (PKR-like endoplasmic reticulum kinase) (Ron and Walter 2007). A similar
mechanism contributing to phosphorylation of
eIF2a under amino acid imbalance is mediated
by Gcn2/CpcC and important for virulence in
A. fumigatus (Krappmann et al. 2004). ER stress
has not been reported to reduce translation
initiation frequency in fungi. Conversely, analysis of the A. fumigatus translatome by polysome profiling under ER stress-inducing
conditions and growth at increased temperatures revealed a predominantly increased ribosome association of mRNAs. Responses to the
ER stress-inducing drugs DTT and tunicamycin
are rapid and distinct with limited overlap
between both treatments (DTT, total 717
mRNAs; TM total, 876 mRNAs; overlap, 233
mRNAs). The response to a temperature shift
from 25
C to 37
C is associated with the
increased translation of 372 mRNAs, overlapping with DTT and TM treatment in case of 44
mRNAs (Krishnan et al. 2014).
Degradation of terminally misfolded proteins is realized by the ER-associated degradation (ERAD) pathway, promoting the export of
substrate proteins into the cytoplasm for ubiquitylation and subsequent degradation by the
26S proteasome. Deletion of derA, encoding a
predicted Derlin ortholog, an ER membrane
protein involved in retro-translocation during
ERAD, or hrdA (HMG-coA Reductase Degradation), encoding the ER membrane-localized E3
ubiquitin ligase, did not affect ER stress resistance or virulence (Krishnan et al. 2013; Richie
et al. 2011b). The parallel deletion of both genes
leads to reduced resistance against cell wall and
ER stress but does not affect virulence (Grahl
et al. 2012; Krishnan et al. 2013). derA deletion
mutants showed increased UPR activity, and
the double deletion mutant of derA and hacA
is severely affected in hyphal growth, antifungal
drug resistance, and protease secretion and displayed strongly reduced virulence in triamcinolone acetonide immunosuppressed mice
(Krishnan et al. 2013, Richie et al. 2011b). Similarly, UPR activity is increased upon conditional repression of the Saccharomyces
cerevisiae stt3 ortholog, encoding the catalytic
subunit of the OST (Oligosaccharyl Transferase) complex, mediating N-glycosylation in
the ER (Li et al. 2011). Activity of the UPR
during host colonization is apparently
connected to growth at mammalian body temperature, nutrient deprivation and hypoxia
(Krishnan and Askew 2014). The adaptation to
hypoxic conditions is mediated by the SREBP
(sterol regulatory element-binding protein)
transcription factor SreA (Chung et al. 2014),
which is activated by signal peptide peptidasemediated cleavage of its ER transmembrane
domain and required for full virulence of A.
fumigatus (Bat-Ochir et al. 2016). The clinical
relevance of the UPR is further corroborated
via its role in antifungal drug resistance. hacA
mutants showed increased sensitivity towards
caspofungin, itraconazole, fluconazole, and
56
R. Harting and K. Heimel