Novel Biological Functions of the NsdC Transcription Factor in Aspergillus fumigatus

Abstract

The fungal zinc finger transcription factor NsdC is named after, and is best known for, its essential role in sexual reproduction (never in sexual development). In previous studies with Aspergillus nidulans, it was also shown to have roles in promotion of vegetative growth and suppression of asexual conidiation. In this study, the function of the nsdC homologue in the opportunistic human pathogen A. fumigatus was investigated. NsdC was again found to be essential for sexual development, with deletion of the nsdC gene in both MAT1-1 and MAT1-2 mating partners of a cross leading to complete loss of fertility. However, a functional copy of nsdC in one mating partner was sufficient to allow sexual reproduction. Deletion of nsdC also led to decreased vegetative growth and allowed conidiation in liquid cultures, again consistent with previous findings. However, NsdC in A. fumigatus was shown to have additional biological functions including response to calcium stress, correct organization of cell wall structure, and response to the cell wall stressors. Furthermore, virulence and host immune recognition were affected. Gene expression studies involving chromatin immunoprecipitation (ChIP) of RNA polymerase II (PolII) coupled to next-generation sequencing (Seq) revealed that deletion of nsdC resulted in changes in expression of over 620 genes under basal growth conditions. This demonstrated that this transcription factor mediates the activity of a wide variety of signaling and metabolic pathways and indicates that despite the naming of the gene, the promotion of sexual reproduction is just one among multiple roles of NsdC. IMPORTANCE Aspergillus fumigatus is an opportunistic human fungal pathogen and the main causal agent of invasive aspergillosis, a life-threatening infection especially in immunocompromised patients. A. fumigatus can undergo both asexual and sexual reproductive cycles, and the regulation of both cycles involves several genes and pathways. Here, we have characterized one of these genetic determinants, the NsdC transcription factor, which was initially identified in a screen of transcription factor null mutants showing sensitivity when exposed to high concentrations of calcium. In addition to its known essential roles in sexual reproduction and control of growth rate and asexual reproduction, we have shown in the present study that A. fumigatus NsdC transcription factor has additional previously unrecognized biological functions including calcium tolerance, cell wall stress response, and correct cell wall organization and functions in virulence and host immune recognition. Our results indicate that NsdC can play novel additional biological functions not directly related to its role played during sexual and asexual processes.

The Heterotrimeric Transcription Factor CCAAT-Binding Complex and Ca 2+ -CrzA Signaling Reversely Regulate the Transition between Fungal Hyphal Growth and Asexual Reproduction

“…During this process, the initiation of asexual reproduction is repressed through the suppression of the developmental inductive transcription factor gene brlA , a conserved essential conidiation gene in fungi, and its upstream developmental activation pathway ( 27 ). Previous studies have verified that in the model filamentous fungus A. nidulans or A. fumigatus , dysfunction of NsdC, NsdD, MpkB, VosA, SfgA, VeA, or VelB results in the activation of conidiation after a period of vegetative growth under liquid culture conditions ( 26 – 28 , 30 , 34 ), suggesting that negative regulators exist for suppressing submerged conidiation to maintain constant vegetative growth with increasing mycelial pellets.…”

“…However, based on these data alone, we could not exclude other possible mechanisms whereby submerged conidiation induced by the CBC mutants is due to derepression of other negative regulators of conidiation. In A. nidulans or A. fumigatus , it has been reported that the vosA , veA , velB , sfgA , nsdC , nsdD , and mpkB mutants also displayed asexual conidiation phenotypes similar to those of the CBC mutants in liquid medium ( 26 – 28 , 30 , 34 ). We then compared the expression differences of the abovementioned genes in the hapB mutant and its parental wild-type strain.…”

“…In addition to its involvement in this central regulatory pathway, later studies identified that brlA expression could be affected by several upstream developmental activators, fluG , flbA , flbB , flbC , flbD , and flbE , and mutations in these genes result in “fluffy” colonies with downregulated brlA ( 8 , 15 , 16 , 21 – 25 ). Moreover, through combinatorial genetic studies, SfgA, VosA, VeA, VelB, VelC, NsdC, and NsdD, which repress the expression of brlA as negative regulators of conidiation, have been found ( 26 – 30 ). Notably, current knowledge of conidiation-related regulators in Aspergillus was mainly obtained from studies under solid-culture conditions.…”

“…Thus, the transition from vegetative growth to conidiation in Aspergillus species is a crucial process contributing to the long-term survival, propagation, and fitness of fungi in response to various environmental conditions ( 32 , 33 ). To date, several reports have demonstrated that the loss of each of the velvet regulators VosA, VeA, and VelB; SfgA (the FluG suppressor); and NsdC and NsdD (the sexual reproduction activators) causes abnormal submerged hyperconidiation ( 26 – 28 , 30 , 34 ), suggesting that these regulators may exert negative influence on asexual conidiation in liquid-submerged culture. However, whether or how these genes work together with a known central regulatory pathway and its upstream developmental activators is still poorly understood.…”

Natural Variation and the Role of Zn2Cys6 Transcription Factors SdrA, WarA and WarB in Sorbic Acid Resistance of Aspergillus niger

“…One transcription factor is a homologue of the general stress response regulator AtfA [40]. Three transcription factors which, upon deletion, reduced the resistance towards sorbic acid are homologues of genes regulating siderophores and iron uptake, HapX, NsdC and AcuK [42][43][44]. AcuK is known to be essential for growth on gluconeogenic carbon sources, and its reduced resistance could possibly be caused by a metabolic imbalance and reduced catabolism of sorbic acid.…”