Video: the Fungus That Could Replace Plastic
Heterogeneity of the mycelium
Filamentous fungi form a network of hyphae called a colony. Aspergillus niger forms cm-scale macro-colonies on a solid surface and (sub)-mm-scale micro-colonies in liquid cultures. We have shown that distinct populations of micro-colonies can be distinguished in liquid cultures that are heterogeneous with respect to growth rate and expression of genes encoding secreted proteins. Gene expression is also heterogeneous between zones of micro- and macro-colonies. For instance, 10% of the genes is expressed in only one out of five concentric zones of a macro-colony. The underlying regulation is to a large extent related to differentiation processes. Gene expression is even heterogeneous within a zone of a colony. At the periphery of micro- and macro-colonies both highly and lowly active hyphae can be distinguished. The lowly active hyphae produce sufficient energy to grow at the same rate as the highly active hyphae. The latter group needs the extra energy to secrete proteins. Heterogeneity within a zone of a colony is remarkable considering the fact that all hyphae within the zone experience identical environmental conditions. Current research aims to identify mechanisms that result in and maintain the different levels of heterogeneity within a mycelium.
Secretion of structural proteins and immuno-modulating compounds
Many fungi produce a variety of Class I and/or Class II hydrophobins. These proteins fulfill a broad spectrum of functions, e.g. in formation of aerial reproductive structures and in attachment of pathogenic fungi to the surface of a host. Class I hydrophobins function by self-assembling into amyloid fibrils, which form two-dimensional amphipathic membranes at the hyphal surface. In contrast, Class II hydrophobins form membranes that readily dissociate. We have shown that Class I and Class II hydrophobins interact with each other. This indicates that fungal surfaces can be covered with a patchwork of assembled hydrophobins. The biophysical properties of hydrophobins differ and therefore the fungus can adapt its surface by varying the hydrophobin composition. We have also found that self-assembly of Class I hydrophobins into a functional amyloid layer is promoted by cell wall polysaccharides. This finding may impact research on amyloid proteins that cause human disease.
Aspergillus fumigatus (Af) is the most common causative agent of invasive pulmonary infections (IPA) in immunocompromised patients. Neutropenic patients with an Af infection are short in neutrophils, which play a crucial role in the defense against fungal pathogens. Af has to pass the lung-epithelial barrier to reach the underlying blood capillaries in order to cause systemic infections. So far, it is not known how passage of epithelium is accomplished and this is subject of our research. We have recently found evidence that Aspergillus secretes molecules that interact with immune receptors and our investigations are aimed to identify the nature of these components, their regulation, as well as their possible roles in modulating the immune response of the mucosal immune system in the lung lining fluids and effective passage of this barrier.
Formation of reproductive structures
We have identified a variety of regulatory proteins that are involved in mushroom formation and in the formation of asexual reproductive structures. For instance, VeA of Aspergillus niger regulates the formation of the conidiophore. In its absence the size of the spore head is reduced in size decreasing the numbers of spore chains that can start to develop. VeA is thus an essential protein in the competition of Aspergillus niger in the natural environment. In the case of the mushroom forming fungus Schizophyllum commune, we have identified regulators that respond to light. Moreover, we identified a regulator that inhibits vegetative growth and stimulates mushroom formation, while other regulatory proteins repress mushroom formation. Vegetative growth is supported by secreted enzymes that degrade the substrate. At this moment we reconstruct the regulatory network of mushroom formation and enzyme secretion and use this information to understand growth and development of commercial mushrooms.