Molecular Mechanisms of Conidial Germination in Aspergillus spp

Abstract

Aspergilli produce conidia for reproduction or to survive hostile conditions, and they are highly effective in the distribution of conidia through the environment. In immunocompromised individuals, inhaled conidia can germinate inside the respiratory tract, which may result in invasive pulmonary aspergillosis. The management of invasive aspergillosis has become more complex, with new risk groups being identified and the emergence of antifungal resistance. Patient survival is threatened by these developments, stressing the need for alternative therapeutic strategies. As germination is crucial for infection, prevention of this process might be a feasible approach. A broader understanding of conidial germination is important to identify novel antigermination targets. In this review, we describe conidial resistance against various stresses, transition from dormant conidia to hyphal growth, the underlying molecular mechanisms involved in germination of the most common Aspergillus species, and promising antigermination targets. Germination of Aspergillus is characterized by three morphotypes: dormancy, isotropic growth, and polarized growth. Intra- and extracellular proteins play an important role in the protection against unfavorable environmental conditions. Isotropically expanding conidia remodel the cell wall, and biosynthetic machineries are needed for cellular growth. These biosynthetic machineries are also important during polarized growth, together with tip formation and the cell cycle machinery. Genes involved in isotropic and polarized growth could be effective antigermination targets. Transcriptomic and proteomic studies on specific Aspergillus morphotypes will improve our understanding of the germination process and allow discovery of novel antigermination targets and biomarkers for early diagnosis and therapy.

Keywords: Aspergillus; conidia; dormant; germination; isotropic growth; polarized growth.

FIG 1

Schematic representation of the conidial cell.

FIG 2

Transition between different morphotypes during germination. Important biological processes for each morphotype are mentioned in boxes.

FIG 3

Regulatory relationship between the central regulators, velvet proteins, and MybA. BrlA, AbaA, and WetA are defined as the central regulators for conidiation. abaA, wetA, velC, and vosA have putative BrlA response elements [BREs; 5′-(C/A)(G/A)AGG(G/A)-3′] in their promoter regions. brlA, wetA, velC, vosA, velB, veA, and abaA itself have putative AbaA response elements (AREs; 5′-CATTCY-3′, where Y is a pyrimidine) in their promoter regions. vosA, velB, and wetA have putative WetA response elements (WREs; 5′-CCGYTTGCGGC-3′) in their promoter regions. WetA also plays a role in cell wall integrity, spore viability, and stress tolerance. AbaA positively regulates expression of wetA, vosA, and velB. These genes potentially have MybA binding sites in their promoters. Additionally, cspA expression was totally switched of in a ΔmybA mutant. MybA, so far, has been described only for A. fumigatus. CspA is involved in spore viability and cell wall permeability. The VelB-VosA heterodimer is involved in spore maturation, cell wall integrity, trehalose biogenesis, spore viability, and thermal, oxidative, and UV stress resistance. The VelB-VosA and VelB-VeA heterodimers negatively control germination.

FIG 4

The HOG-MAPK pathway. The HOG-MAPK pathway is responsible for conidia stress tolerance. SskA is the final receptor of the TcsB-YpdA-SskA two-component signaling system and is the key activator of SskB (MAPKKK). Pbs2 (MAPKK) is activated by SskB. MpkC (MAPK) and SakA (MAPK) are the main effectors of the HOG pathway, activated by Pbs2. SakA and MpkC are involved in activating the CWI pathway. Transcription factor AtfA and the hypothetical osmotic stress regulator OsrA are activated by SakA. DprA and DprB are involved in the protection against oxidative, osmotic, and pH stress. DprC is involved in the protection against freeze stress.

FIG 5

Regulatory pathways involved in conidial germination. The pathways shown are based on the knowledge from A. nidulans and A. fumigatus studies. G proteins are essential for signal transduction. Gα binds GDP and forms a complex with Gβ and Gγ that is associated with the GPCR. Exchange of GDP for GTP on Gα leads to dissociation from Gβ and Gγ and the GPCR. Gα-ATP and Gβγ dimer both regulate downstream effector proteins for various biological processes. AcyA is activated by Gα and produces cAMP, which binds to PKAR to release the regulatory subunit from the catalytic subunit. Active PkaC can now phosphorylate downstream targets. GEFs activate Ras by exchanging Ras-bound GDP for GTP. GAPs stimulate hydrolysis of Ras-bound GTP to GDP, thereby deactivating Ras. Calcineurin is a heterodimer composed of a regulatory subunit (CnaB) and a catalytic subunit (CnaA); the heterodimer is activated by Ca²⁺ and CaM. A downstream effector of calcineurin is the transcription factor CrzA. Hsp90 interacts with calcineurin, thereby orchestrating cell wall integrity and conidiation.

FIG 6

(Isotropic growth) Swelling of the conidium is the initial stage of the isotropic growth phase. Swelling is due to an increase in osmotic pressure as well as the modification of the conidial cell wall by glycosylhydrolases. The rodlet layer is degraded by aspartic proteases. (Polarized growth) The cell wall is modified, and biosynthesis of new polysaccharides leads to different polysaccharides on the surface of the cell wall, such as α-glucans, galactomannan, and galactosaminogalactan (GAG). (Synthesis of new cell wall polysaccharides) β-(1,3)-Glucan, β-(1,3)-β-(1,4)-glucan, chitin, α-(1,3)-glucan, galactomannan, and GAG are the main components of the cell wall. β-(1,3)-Glucan is synthesized by glucan synthase Fks1 and modified by Bgt1 to -3, Gel1 to -7, and glucanases BglA to -C. Chitin is synthesized by chitin synthases (ChsA to -D, -F, and -G and CsmA and -B) and modified by transglycosidases Crh1 to -5 and chitinase ChiA. α-(1,3)-Glucan is synthesized by Ags1 to -3 synthases.