Molecular and cellular responses of the pathogenic fungus Lomentospora prolificans to the antifungal drug voriconazole

Abstract

The filamentous fungus Lomentospora (Scedosporium) prolificans is an emerging opportunistic pathogen associated with fatal infections in patients with disturbed immune function. Unfortunately, conventional therapies are hardly of any use against this fungus due to its intrinsic resistance. Therefore, we performed an integrated study of the L. prolificans responses to the first option to treat these mycoses, namely voriconazole, with the aim of unveiling mechanisms involved in the resistance to this compound. To do that, we used a wide range of techniques, including fluorescence and electron microscopy to study morphological alterations, ion chromatography to measure changes in cell-wall carbohydrate composition, and proteomics-based techniques to identify the proteins differentially expressed under the presence of the drug. Significantly, we showed drastic changes occurring in cell shape after voriconazole exposure, L. prolificans hyphae being shorter and wider than under control conditions. Interestingly, we proved that the architecture and carbohydrate composition of the cell wall had been modified in the presence of the drug. Specifically, L. prolificans constructed a more complex organelle with a higher presence of glucans and mannans. In addition to this, we identified several differentially expressed proteins, including Srp1 and heat shock protein 70 (Hsp70), as the most overexpressed under voriconazole-induced stress conditions. The mechanisms described in this study, which may be directly related to L. prolificans antifungal resistance or tolerance, could be used as targets to improve existing therapies or to develop new ones in order to successfully eliminate these mycoses.

Fig 1. Morphological changes on Lomentospora prolificans cells caused by Voriconazole (VRC) exposure.

Germination assays (A) were performed to analyze the effect of VRC on fungal cells. After 9 h of incubation cells were stained with calcofluor white and microscopically analysed (B) to determine their length (C), width (D), occupied area (E), and emitted fluorescence (F). Scale bar = 5 μm. Results are shown as mean ± SEM, n = 4. **p<0.01, ***p<0.0001 compared to non-treated cells. a.u., arbitrary units.

Fig 2. Ultrastructural analysis of voriconazole-induced changes on Lomentospora prolificans cells.

Transmission electron microscopy images (A) and measurements of cell wall thickness (B) of non-treated and 2 μg/ml voriconazole-treated fungal cells. Black lines highlight the thickness of the outer fibrillar layer. Results are shown as mean ± SEM, n ≥ 20 cells. ***p<0.0001 compared to non-treated cells.

Fig 3. Biochemical characterization of the carbohydrate composition of Lomentospora prolificans cell wall.

Carbohydrate compositional analysis of whole cell wall (A) and cell wall surface (B) upon exposure to 2 μg/ml voriconazole. Results are shown as mean ± SEM, n = 3. *p<0.05 compared to non-treated cells. Percentage of monosaccharide content in the whole cell wall (C) and surface (D).

Fig 4. Effect of cell membrane- and wall-disturbing agents on Lomentospora prolificans in the presence of voriconazole.

Decimal dilutions of conidial suspensions were spotted onto potato dextrose agar plates containing SDS (100 μg/ml), calcofluor white (500 μg/ml; CFW) or congo red (750 μg/ml; CR), and combined with 0, 2 or 4 μg/ml of voriconazole (VRC).

Fig 5. Effect of voriconazole on proteomic profiles of the Lomentospora prolificans cell surface subproteome.

Fungal cells were grown in absence (A) or presence (B) of 2 μg/ml voriconazole, and their surfaceomes resolved by two-dimensional electrophoresis. Arrows point to the most differentially expressed protein spots that were identified by LC-MS/MS (See Table 1).