A sphingolipid synthesis‐related protein OrmA in Aspergillus fumigatus is responsible for azole susceptibility and virulence

Abstract

Previous studies identified that the budding yeast Saccharomyces cerevisiae have two sphingolipid synthesis‐related proteins, Orm1p and Orm2p, that negatively regulate the activities of SPT, which is a key rate‐limiting enzyme in sphingolipid synthesis. However, little is known about whether sphingolipids in the cell membrane, which are closely related to ergosterols, could affect the efficacy of azole drugs, which target to the ergosterol biosynthesis. In this study, through genome‐wide homologue search analysis, we found that the Aspergillus fumigatus genome only contains one Orm homologue, referred to as OrmA for which the protein expression could be induced by azole antifungals in a dose‐dependent manner. Deletion of ormA caused hypersensitivity to azoles, and adding the sphingolipid synthesis inhibitor myriocin rescued the azole susceptibility induced by lack of ormA. In contrast, overexpression of OrmA resulted in azole resistance, indicating that OrmA is a positive azole‐response regulator. Further mechanism analysis verified that OrmA is related to drug susceptibility by affecting endoplasmic reticulum stress responses in an unfolded protein response pathway‐HacA‐dependent manner. Lack of ormA led to an abnormal profile of sphingolipid ceramide components accompanied by hypersensitivity to low temperatures. Furthermore, deletion of OrmA significantly reduced virulence in an immunosuppressed mouse model. The findings in this study collectively suggest that the sphingolipid metabolism pathway in A. fumigatus plays a critical role in azole susceptibility and fungal virulence.

Sphingolipids: Regulators of azole drug resistance and fungal pathogenicity

“…Although Orm proteins are, overall, highly conserved in animals, plants, and fungi, the Orm proteins in fungi have specific characteristics. Notably, there are seven conserved amino acid residues-RRRRSSS-near the N-terminal region of fungal Orm proteins (Zhai et al, 2019); the three serine residues can be phosphorylated, leading to inactivation of OrmA. These data indicate that these specific sites of OrmA may serve as targets for developing novel antifungal therapies, and, hence, inhibitors that act on these specific sites might improve the efficacy of azole drugs without any toxicity to host cells.…”

“…Many studies highlight that sphingolipids are signaling molecules that play an important role in several cellular processes (Chen et al ., 2013; Montefusco et al ., 2013; Huang et al ., 2014; Rego et al ., 2014; Teixeira et al ., 2016; Villasmil et al ., 2016; Mohammad et al ., 2018; Liu et al ., 2019). Interestingly, recent studies have confirmed that sphingolipids could contribute to azole resistance (Gao et al ., 2018; Garnaud et al ., 2018; Zhai et al ., 2019). Therefore, in this section, we will focus on the role of sphingolipids as signaling molecules in azole drug resistance.…”

“…However, the molecular mechanism underlying resistance to azoles arising from the UPR pathway is still unclear, and the downstream response factors responsible for azole resistance are still unknown. Recently, however, a research has discovered that HacA regulates sphingolipid biosynthesis to respond to azole drug stress in A. fumigatus (Zhai et al ., 2019), and found that the UPR pathway is required for normal expression of the sphingolipid biosynthesis‐related protein OrmA (Figure 2b); thus, OrmA is a downstream response factor of the UPR pathway (Zhai et al ., 2019). Cells in which ormA is deleted fail to effectively respond to azole stress, while cells overexpressing ormA respond better to azole stress than the wild type; thus, deletion of ormA causes hypersensitivity to azoles, and overexpression of ormA results in azole resistance (Zhai et al ., 2019).…”

“…Recently, however, a research has discovered that HacA regulates sphingolipid biosynthesis to respond to azole drug stress in A. fumigatus (Zhai et al ., 2019), and found that the UPR pathway is required for normal expression of the sphingolipid biosynthesis‐related protein OrmA (Figure 2b); thus, OrmA is a downstream response factor of the UPR pathway (Zhai et al ., 2019). Cells in which ormA is deleted fail to effectively respond to azole stress, while cells overexpressing ormA respond better to azole stress than the wild type; thus, deletion of ormA causes hypersensitivity to azoles, and overexpression of ormA results in azole resistance (Zhai et al ., 2019). Additionally, the deletion of ormA results in abnormal cell membrane lipid components, which may be another possible reason for the increased susceptibility to azoles.…”

“…In recent years, the role of sphingolipids in pathogenic fungi, in terms of azole resistance and fungal pathogenicity, has emerged as an interesting field. Some sphingolipids, such as long‐chain bases (LCBs), ceramides, and glucosylceramide (GlcCer), have been shown to be involved in the regulation of azole resistance and virulence in pathogenic fungi (Vandenbosch et al ., 2012; Del Poeta et al ., 2014; Gao et al ., 2018; Munshi et al ., 2018; Zhai et al ., 2019). This regulation is important because it opens avenues for new, sphingolipid‐targeted therapeutic approaches.…”