Aspergillus fumigatusDoes Not Require Fatty Acid Metabolism via Isocitrate Lyase for Development of Invasive Aspergillosis

Abstract

Aspergillus fumigatus is the most prevalent airborne filamentous fungus causing invasive aspergillosis in immunocompromised individuals. Only a limited number of determinants directly associated with virulence are known, and the metabolic requirements of the fungus to grow inside a host have not yet been investigated. Previous studies on pathogenic microorganisms, i.e., the bacterium Mycobacterium tuberculosis and the yeast Candida albicans, have revealed an essential role for isocitrate lyase in pathogenicity. In this study, we generated an isocitrate lyase deletion strain to test whether this strain shows attenuation in virulence. Results have revealed that isocitrate lyase from A. fumigatus is not required for the development of invasive aspergillosis. In a murine model of invasive aspergillosis, the wild-type strain, an isocitrate lyase deletion strain, and a complemented mutant strain were similarly effective in killing mice. Moreover, thin sections demonstrated invasive growth of all strains. Additionally, thin sections of lung tissue from patients with invasive aspergillosis stained with anti-isocitrate lyase antibodies remained negative. From these results, we cannot exclude the use of lipids or fatty acids as a carbon source for A. fumigatus during invasive growth. Nevertheless, test results do imply that the glyoxylate cycle from A. fumigatus is not required for the anaplerotic synthesis of oxaloacetate under infectious conditions. Therefore, an antifungal drug inhibiting fungal isocitrate lyases, postulated to act against Candida infections, is assumed to be ineffective against A. fumigatus.

Methylcitrate synthase from Aspergillus fumigatus is essential for manifestation of invasive aspergillosis

“…Mutants of C. albicans with a defective isocitrate lyase also displayed a dramatic reduction in virulence (Lorenz and Fink, 2001), which confirms the important role of the glyoxylate cycle in virulence of several pathogenic microorganisms. In contrast, investigations on A. fumigatus have shown that isocitrate lyase mutants retained the same virulence as the wild-type (Schöbel et al, 2007). Therefore, it is unlikely that A. fumigatus strongly depends on lipids as carbon sources during invasive growth.…”

“…Our study, therefore, demonstrates that utilization of carbon sources available in the human host can differ considerably for various pathogens. The main causative organism for development of invasive aspergillosis, A. fumigatus, is not dependent on the glyoxylate cycle (Schöbel et al, 2007) but, as shown here, requires the methylcitrate cycle and thus most likely the degradation of proteins for manifestation of the infection. Incomplete amino-acid degradation leads to the accumulation of toxic propionyl-CoA that causes self-poisoning of the fungal cell.…”

“…In case of the wild-type and the complemented strain, therefore, the activity cannot be attributed solely to a single enzyme. Nevertheless, the increased citrate synthase activity on olive oil, when compared with glucose, is most likely due to the glyoxylate cycle, which is active on the former but not on the latter carbon source (Ebel et al, 2006;Schöbel et al, 2007).…”

“…On day 3 two mice, and on days 4 and 5 one mouse, from each group infected with one of the respective strains were sacrificed for detailed investigation of the lungs. For each lung, the inflammatory index and that of fungal invasion was determined from the histological sections as described (Schöbel et al, 2007). The use of solely A.…”

Gene Expression Profiling in the Human Pathogenic Dermatophyte Trichophyton rubrum during Growth on Proteins

“…In addition, the glyoxylate cycle is absent in mammals and therefore represents a potential drug target. Whereas this pathway appears to be dispensable for A. fumigatus-induced invasive aspergillosis (26,28), it was found to be virulence associated in infections by other microbial pathogens such as Mycobacterium tuberculosis and the yeast Candida albicans (17,21). In these pathogens, enzymes of the glyoxylate pathway were shown to contribute to the persistence of the microbes in phagocytic immune cells, a function which remains elusive for dermatophytes.…”