ChIP-seq and In Vivo Transcriptome Analyses of the Aspergillus fumigatus SREBP SrbA Reveals a New Regulator of the Fungal Hypoxia Response and Virulence

Abstract

The Aspergillus fumigatus sterol regulatory element binding protein (SREBP) SrbA belongs to the basic Helix-Loop-Helix (bHLH) family of transcription factors and is crucial for antifungal drug resistance and virulence. The latter phenotype is especially striking, as loss of SrbA results in complete loss of virulence in murine models of invasive pulmonary aspergillosis (IPA). How fungal SREBPs mediate fungal virulence is unknown, though it has been suggested that lack of growth in hypoxic conditions accounts for the attenuated virulence. To further understand the role of SrbA in fungal infection site pathobiology, chromatin immunoprecipitation followed by massively parallel DNA sequencing (ChIP-seq) was used to identify genes under direct SrbA transcriptional regulation in hypoxia. These results confirmed the direct regulation of ergosterol biosynthesis and iron uptake by SrbA in hypoxia and revealed new roles for SrbA in nitrate assimilation and heme biosynthesis. Moreover, functional characterization of an SrbA target gene with sequence similarity to SrbA identified a new transcriptional regulator of the fungal hypoxia response and virulence, SrbB. SrbB co-regulates genes involved in heme biosynthesis and demethylation of C4-sterols with SrbA in hypoxic conditions. However, SrbB also has regulatory functions independent of SrbA including regulation of carbohydrate metabolism. Loss of SrbB markedly attenuates A. fumigatus virulence, and loss of both SREBPs further reduces in vivo fungal growth. These data suggest that both A. fumigatus SREBPs are critical for hypoxia adaptation and virulence and reveal new insights into SREBPs' complex role in infection site adaptation and fungal virulence.

Fungal sensing of host environment

“…C. albicans senses H 2 O 2 and via Hog1 SAPK pathway induces expression of detoxifying enzymes at the surface of the cell wall, and concomitantly, via activation of Rad35, induces hyphal formation (Dantas Ada et al, 2015). In A. fumigatus, sterol regulatory element-binding proteins were found to be relevant for adaptation to hypoxia and virulence in the host (Chung et al, 2014). Interestingly, these transcription factors directly link the ergosterol biosynthesis pathway with carbohydrate metabolism.…”

Aspergillosis and stem cell transplantation: An overview of experimental pathogenesis studies

“…80 A. fumigatus SrbA and SrbB, members of the highly conserved sterol regulatory element binding protein (SREBP) family of transcription factors, have been reported to play critical roles in fungal adaptation to hypoxia. 81,82 In vitro, SrbA is required for normal hyphal growth and cell wall morphology under hypoxic conditions, and increases ergosterol biosynthesis, thereby contributing to azole antifungal resistance. 81 Interestingly, SrbA also regulates iron acquisition in response to hypoxia and low iron conditions by increasing siderophore biosynthesis.…”

“…83 SrbB mediates similar functions to SrbA but also regulates heme biosynthesis, as well as carbohydrate and sterol metabolism under hypoxic conditions. 82 Mutants deficient in SrbA display significantly reduced growth and are avirulent in both neutropenic and non-neutropenic mouse models, while SrbB-deficient mutants are markedly reduced in virulence in a non-neutropenic mouse model. [81][82][83] The virulence of SrbB-deficient mutants in neutropenic mouse models has not been reported.…”

“…82 Mutants deficient in SrbA display significantly reduced growth and are avirulent in both neutropenic and non-neutropenic mouse models, while SrbB-deficient mutants are markedly reduced in virulence in a non-neutropenic mouse model. [81][82][83] The virulence of SrbB-deficient mutants in neutropenic mouse models has not been reported. Notably, mutants deficient in both SrbA and SrbB are more impaired in growth and virulence than mutants deficient in either protein.…”

“…Notably, mutants deficient in both SrbA and SrbB are more impaired in growth and virulence than mutants deficient in either protein. 82 This observation suggests that SrbA and SrbB regulate both shared and distinct genes that are crucial to fungal metabolism and virulence. Another protein in the SREBP family, SrbC, remains to be characterized.…”