Aspergillus nidulans biofilm formation modifies cellular architecture and enables light-activated autophagy

Abstract

After growing on surfaces, including those of medical and industrial importance, fungal biofilms self-generate internal microenvironments. We previously reported that gaseous microenvironments around founder Aspergillus nidulans cells change during biofilm formation causing microtubules (MTs) to disassemble under control of the hypoxic transcription factor SrbA. Here we investigate if biofilm formation might also promote changes to structures involved in exocytosis and endocytosis. During biofilm formation the ER remained intact but ER exit sites and the Golgi apparatus were modified as were endocytic actin patches. The biofilm driven changes required the SrbA hypoxic transcription factor and could be triggered by nitric oxide, further implicating gaseous regulation of biofilm cellular architecture. By tracking GFP-Atg8 dynamics, biofilm founder cells were also observed to undergo autophagy. Most notably, biofilm cells that had undergone autophagy were triggered into further autophagy by spinning disc confocal light. Our findings indicate that fungal biofilm formation modifies the secretory and endocytic apparatus and show biofilm cells can also undergo autophagy that is reactivated by light. The findings provide new insights into the changes occurring in fungal biofilm cell biology that potentially impact their unique characteristics, including antifungal drug resistance. [Media: see text]

Colony growth and biofilm formation of Aspergillus niger under simulated microgravity

“…We propose that young hyphae of A. niger mainly work as nutrient scavengers, having a highly active and compact cytoplasm, whereas old hyphae are important in setting the biofilm's three-dimensional structure, where only the cell wall is needed and thus the cytoplasm appears empty. Mature fungal colonies have been reported to be composed of both metabolic active, metabolic inactive as well as dead hyphae, the latter of which develop through autophagy (Boswell and Hopkins, 2008;Krijgsheld et al, 2013, Emri et al, 2018Kaur and Punekar, 2019;Khalid et al, 2019;Lingo et al, 2021). Our study supports these observations and further highlights that young hyphae at the edge of the colony are not significantly covered by an extracellular matrix.…”

“…This data was confirmed by transmission electron microscopy (TEM), for which we used another fixation method (high-pressure freezing instead of chemical fixation) in order to exclude the possibility that chemical fixation of SEM samples could have unintentionally altered hyphal morphologies (Figures 3C-F, 4). Indeed, autophagy in Aspergillus is known to be related with aging, is integral to nutrient and organelles recycling and is thought to facilitate growth of aerial hyphae and conidiophores (Kikuma et al, 2007;Nitsche et al, 2013;Lingo et al, 2021).…”

Recent Advances in Understanding the Human Fungal Pathogen Hypoxia Response in Disease Progression

“…As A. fumigatus biofilms mature, oxygen gradients are established within the biofilm, perhaps explaining the dependency on known hypoxia transcriptional regulators for biofilm maintenance and maturation (74). While the mechanisms remain to be fully defined, studies in the model filamentous fungus Aspergillus nidulans revealed that biofilm maturation involves the depolarization or disassembly of microtubules at the base of the biofilm and changes in endoplasmic reticulum exit sites, Golgi apparatus, and sites of endocytosis that are dependent on SrbA, thereby establishing dormancy (81,103). It is unknown whether A. fumigatus biofilm maturation involves microtubule depolarization and structural changes to subcellular organelles within the hypoxic regions of the biofilm.…”

Characterization of Aspergillus nidulans Biofilm Formation and Structure and Their Inhibition by Pea Defensin Psd2

“…A. nidulans is phylogenetically related to other species of economic importance, including A. niger and A. oryzae and clinically relevant species such as A. fumigatus (Galagan et al, 2005). Previous studies have described the A. nidulans adhesion in hydrophobic surfaces and their biofilm development, both of which are critical for the virulence of Aspergillus species (Shukla et al, 2017;He et al, 2018;Kadry et al, 2018;Lingo et al, 2021). Kadry et al (2018) used scanning electron microscopy (SEM) and found that A. nidulans strains (ΔgmtA and ΔgmtB) did not produce guanosine diphosphate mannose transporter (GMT), a transporter required for mannosylation of galactomannan and mannoproteins, exhibited smaller colony size, reduced sporulations, increased hyphal width and decreased based cell length.…”