Characterizing extracellular vesicles of human fungal pathogens

Abstract

Since their discovery in 2007, there has been growing awareness of the importance of fungal extracellular vesicles (EVs) for fungal physiology, host-pathogen interactions and virulence. Fungal EVs are nanostructures comprising bilayered membranes and molecules of various types that participate in several pathophysiological processes in fungal biology, including secretion, cellular communication, immunopathogenesis and drug resistance. However, many questions remain regarding the classification of EVs, their cellular origin, passage across the cell wall, experimental models for functional and compositional analyses, production in vitro and in vivo and biomarkers for EVs. Here, we discuss gaps in the literature of fungal EVs and identify key questions for the field. We present the history of fungal EV discovery, discuss five major unanswered questions in fungal EV biology and provide future perspectives for fungal EV research. We primarily focus our discussion on human fungal pathogens, but also extend it to include knowledge of other fungi, such as plant pathogens. With this Perspective we hope to stimulate new approaches and expand studies to understand the biology of fungal EVs.

Fig. 1 |. Composition of fungal EVs.

Fungal EVs can carry a wide variety of biomolecules, including proteins, polysaccharides, RNA, pigments and small molecules. The membrane structure of fungal EVs (inset) is still poorly understood, although microscopic evidence suggests a bilayered lipid membrane adorned with proteins and glycans. The presence of integral membrane proteins, as well as outer polysaccharides and mannoproteins decorating the vesicular surface, has been described in fungal EVs. Although other membrane components remain poorly characterized, it has been suggested that fungal EVs can be coated with lectins and pathogen-associated molecular patterns^–.

Fig. 2 |. Potential mechanisms of EV biogenesis in fungi.

Exosome biogenesis starts with plasma membrane invagination (i) and the subsequent endocytosis of external substances (represented as blue, irregular particles). This process leads to the formation of endosomes, followed by degradation of the ingested molecules. Endosomes can generate intraluminal vesicles (red spheres) by inward folding of their membranes, forming MVBs. MVBs can fuse with the plasma membrane and release intraluminal vesicles extracellularly. Alternatively, microvesicle-like EVs can form by membrane budding (ii). In fungi, these vesicles are deposited in the periplasmic space between the plasma membrane and cell wall, where they are directed to the outer space by as yet unknown mechanisms.