Analysis of microtubules and F-actin structures in hyphae and conidia development of the opportunistic human pathogenic black yeast Aureobasidium pullulans

Growth and division mode plasticity is dependent on cell density in marine-derived black yeasts

The diversity and ecological contribution of the fungus kingdom in the marine environment remain understudied. A recent survey in the Atlantic (Woods Hole, MA, USA) brought to light the diversity and unique biological features of marine fungi. The study revealed that black yeast species undergo an unconventional cell division cycle, which has not been documented in conventional model yeast species such as Saccharomyces cerevisiae (budding yeast) and Schizosaccharomyces pombe (fission yeast). The prevalence of this unusual property is unknown. Here, I collected and identified 65 marine fungi species across 40 genera from the surface ocean water, sediment, and the surface of macroalgae (seaweeds) in the Pacific (Sugashima, Toba, Japan). The Sugashima collection largely did not overlap with the Woods Hole collection and included several unidentifiable species, further illustrating the diversity of marine fungi. Three black yeast species were isolated, two of which were commonly found in Woods Hole (Aureobasidium pullulans and Hortaea werneckii). Surprisingly, their cell division mode was dependent on cell density, and the previously reported unconventional division mode was reproduced only at a certain cell density. For all three black yeast species, cells underwent filamentous growth with septations at low cell density and immediately formed buds at high cell density. At intermediate cell density, two black yeasts (H. werneckii and an unidentifiable species) showed rod cells undergoing septation at the cell equator. In contrast, all eight budding yeast species showed a consistent division pattern regardless of cell density. This study suggests the plastic nature of the growth/division mode of marine-derived black yeast.

Microtubules and Actin Cytoskeleton of Cryptococcus neoformans as Targets for Anticancer Agents to Potentiate a Novel Approach for New Antifungals

BACKGROUND

We investigated the targeting of microtubules (MT) and F-actin cytoskeleton (AC) of the human pathogenic yeast Cryptococcus neoformans with agents for cancer therapy, in order to examine whether this yeast cytoskeleton could become a new antifungal target for the inhibition of cell division.

METHODS

Cells treated with 10 cytoskeleton inhibitors in yeast extract peptone dextrose medium were investigated by phase-contrast and fluorescence microscopy, and growth inhibition was estimated by cell counts using a Bürker chamber and measuring absorbance for 6 days.

RESULTS

Docetaxel, paclitaxel, vinblastine sulfate salt, cytochalasin D and chlorpropham [isopropyl N-(3-chlorophenyl) carbamate] did not inhibit proliferation. The MT inhibitors methyl benzimidazole-2-ylcarbamate (BCM), nocodazole, thiabendazole (TBZ) and vincristine (VINC) disrupted MT and inhibited mitoses, but anucleated buds emerged on cells that increased in size, vacuolated and seemed to die after 2 days. The response of the cells to the presence of the actin inhibitor latrunculin A (LA) included the disappearance of actin patches, actin cables and actin rings; this arrested budding and cell division. However, in 3-4 days, resistant budding cells appeared in all 5 inhibitors. Disruption of the MT and AC and inhibition of cell division and budding persisted only when the MT and AC inhibitors were combined, i.e. VINC + LA, BCM + LA or TBZ + LA.

CONCLUSION

The MT and AC of C. neoformans are new antifungal targets for the persistent inhibition of cell division by combined F-actin and MT inhibitors.

Actin ring formation around the cell nucleus of long-neck yeast

The unique long-neck yeast Fellomyces fuzhouensis has F-actin cables and cortical patches. Here, we describe a new F-actin structure present in fungi, a perinuclear F-actin collar ring around the cell nucleus. This F-actin structure can be visualized by fluorescent microscopic imaging of rhodamine-phalloidin-stained F-actin in cells treated with the mitotic drug isopropyl N-(3-chlorophenyl) carbamate or the microtubule inhibitor thiabendazol or when cells were grown in cut dried radish medium or yeast extract pepton dextrose (YEPD) medium. In contrast, these structures were absent in cells treated with Latrunculin A. The hypothetical functions of the F-actin ring are discussed.

Cytoskeletal structures, ultrastructural characteristics and the capsule of the basidiomycetous yeast Cryptococcus laurentii

The cytoskeleton, capsule and cell ultrastructure were studied during the cell cycle of Cryptococcus laurentii. In an encapsulated strain, cytoplasmic microtubules and a mitotic spindle were detected. Mitosis was preceded by migration of the nucleus into the bud. F-actin failed to be visualised by rhodamine-phalloidin (RhPh) in encapsulated cells and therefore an acapsular strain was used. The following actin structures were found: actin dots, actin cables and cytokinetic ring. Ultrastructural studies showed the presence of a nucleus in the bud before mitosis. A collar-shaped structure was seen at the base of bud emergence. A lamellar cell wall and a rough outer surface of the cells were detected. Cytoskeletal structures found in C. laurentii are similar to those in Cryptococcus neoformans, which is a serious human pathogen.

The cytoskeleton in the unique cell reproduction by conidiogenesis of the long-neck yeast Fellomyces (Sterigmatomyces) fuzhouensis

The morphology of conidiogenesis and associated changes in microtubules, actin distribution and ultrastructure were studied in the basidiomycetous yeast Fellomyces fuzhouensis by phase-contrast, fluorescence, and electron microscopy. The interphase cell showed a central nucleus with randomly distributed bundles of microtubules and actin, and actin patches in the cortex. The conidiogenous mother cell developed a slender projection, or stalk, that contained cytoplasmic microtubules and actin cables stretched parallel to the longitudinal axis and actin patches accumulated in the tip. The conidium was produced on this stalk. It contained dispersed cytoplasmic microtubules, actin cables, and patches concentrated in the cortex. Before mitosis, the nucleus migrated through the stalk into the conidium and cytoplasmic microtubules were replaced by a spindle. Mitosis started in the conidium, and one daughter nucleus then returned to the mother via an eccentrically elongated spindle. The cytoplasmic microtubules reappeared after mitosis. A strong fluorescence indicating accumulated actin appeared at the base of the conidium, where the cytoplasm cleaved eccentrically. Actin patches then moved from the stalk together with the retracting cytoplasm to the mother and conidium. No septum was detected in the long neck by electron microscopy, only a small amount of fine "wall material" between the conidium and mother cell. Both cells developed a new wall layer, separating them from the empty neck. The mature conidium disconnected from the empty neck at the end-break, which remained on the mother as a tubular outgrowth. Asexual reproduction by conidiogenesis in the long-neck yeast F. fuzhouensis has unique features distinguishing it from known asexual forms of reproduction in the budding and fission yeasts. Fellomyces fuzhouensis develops a unique long and narrow neck during conidiogenesis, through which the nucleus must migrate into the conidium for eccentric mitosis. This is followed by eccentric cytokinesis. We found neither an actin cytokinetic ring nor a septum in the long neck, from which cytoplasm retracted back to mother cell after cytokinesis. Both the conidium and mother were separated from the empty neck by the development of a new lateral wall (initiated as a wall plug). The cytoskeleton is clearly involved in all these processes.

Aureobasidium pullulans morphology: two adapted polysaccharide stains

Morphological stages of Aureobasidium pullulans were investigated utilizing different media ingredients and were visualized by bright-field microscopy. A polysaccharide stain was developed to stain chlamydospores, cell walls, hyphae, and conidia, since current staining techniques do not reveal subcellular details to identify fungi, especially those that exhibit polysaccharide secretions.