Transcriptional Regulation of the Hypoxia Response in Aspergillus fumigatus by a family of Sterol Regulatory Element Binding Proteins

Ref ID: 19532


D Chung1, BM Barker2, CC Carey3, SJ Blosser4, B Merriman4, K Morohashi5, A. Mazurie2,
TK Mitchell6, RA Cramer1*

Author address:

1Microbiology and Immunology, Geisel School of Medicine at Dartmouth, Hanover, USA
2Pathogen Genomics Division, Translational Genomics Institute and Northern Arizona University, Flagstaff,
3Bioinformatics Core Dept. Microbiology, Montana State

Full conference title:

6th Advances Against Aspergillosis 2014


Sterol Regulatory Element Binding Proteins (SREBPs) are basic helix-loop-helix (bHLH)
transcription factors conserved from fungi to human. In mammals, three SREBPs, SREBP-1a,
SREBP-1c and SREBP-2 have been characterized and they are involved in cholesterol biosynthesis
and lipid metabolism. Studies in fungi have shown that SREBPs are critical for growth in low oxygen
conditions (hypoxia) as well as regulation of sterol biosynthesis genes. Adaptation to hypoxia is
particularly important for pathogenic fungi to develop diseases given that sites of fungal infection in
host are hypoxic. Our lab has reported the importance of the A. fumigatus SREBP, SrbA in growth in
hypoxia, drug susceptibility, and virulence. Recently our lab identified additional SREBPs and we
aim to understand how these 3 SREBP transcription factors coordinate gene regulation in response
to hypoxia and develop a novel therapeutic approach to inhibit activation of this virulence associated
genetic network in vivo.
RNAseq and Chromatin Immunoprecipitation (ChIP)-sequencing were performed with wild type
and a srbA null mutant in normoxia or hypoxia. Based on the genome-wide analyses we identified
additional SREBPs, designated SrbB and SrbC. We generated a series of deletion mutants with all
possible SREBP gene combinations. To understand genetic relationships among the three SREBPs,
we performed quantitative real-time PCR and ChIP-quantitative PCR. To test virulence, we utilized
our steroid murine model of invasive pulmonary aspergillosis.
srbB is transcriptionally regulated by SrbA through SrbA direct binding on the srbB promoter. In
hypoxia, a srbB null mutant shows reduced growth and biomass production compared to wild type,
whereas a srbC null mutant shows enhanced hypoxia growth compared to wild-type. Notably, both
916;srbB and 916;srbC mutants show attenuated virulence compared to WT in the murine aspergillosis
model. Double or triple knock-out mutants show that deletion of srbA is dominant to that of srbB
or srbC in terms of hypoxia growth. Interestingly, restored srbB expression partially complements
hypoxia growth in 916;srbA. In early exposure to hypoxia (30 min) SrbB is a negative regulator of
srbA and srbC transcription by direct promoter binding, and SrbB and SrbC transcriptionally
regulate each other in a negative feedback loop. Moreover, mRNA levels of two major ergosterol
biosynthesis genes, erg11A and erg25A is almost completely lost in the 916;srbA916;srbB916;srbC triple
mutant in normoxia and hypoxia compared to other mutants and wild type.
Our data suggest that SrbA, SrbB, and SrbC are involved in hypoxia adaptation, virulence, and
antifugal drug responses in A. fumigatus. Therefore, understanding transcriptional regulation of the
hypoxia response by SREBPs will provide new critical information to augment existing antifungal
therapies and/or develop new therapeutic approaches.

Abstract Number: 59

Conference Year: 2014

Link to conference website:

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