Endoplasmic reticulum stress and fungal pathogenesis

The gateway to the secretory pathway is the endoplasmic reticulum (ER), an organelle that is responsible for the accurate folding, post-translational modification and final assembly of up to a third of the cellular proteome. When secretion levels are high, errors in protein biogenesis can lead to the accumulation of abnormally folded proteins, which threaten ER homeostasis. The unfolded protein response (UPR) is an adaptive signaling pathway that counters a buildup in misfolded and unfolded proteins by increasing the expression of genes that support ER protein folding capacity. Fungi, like other eukaryotic cells that are specialized for secretion, rely upon the UPR to buffer ER stress caused by fluctuations in secretory demand. However, emerging evidence is also implicating the UPR as a central regulator of fungal pathogenesis. In this review, we discuss how diverse fungal pathogens have adapted ER stress response pathways to support the expression of virulence-related traits that are necessary in the host environment.     

Chitin and ergosterol combined to measure total and living fungal biomass in ectomycorrhizas

We have studied the chitin and ergosterol contents of ectomycorrhizal roots in three sets of experiments to evaluate them as indicators of fungal biomass. The first set of experiments showed that ageing had a marked effect on ergosterol concentrations. The ergosterol content of 7-month-old, brown, shrunken Pinus sylvestris L.– Paxillus involutus (Fr.) Fr. mycorrhizas was found to be only 10% of that found in white, turgid, 1- or 4-month-old specimens. This supports the hypothesis that the compound is a good indicator of living fungal biomass. Ageing had a lesser effect on chitin concentrations since the chitin levels found in 7-month-old mycorrhizas were still 60% of the levels found in 1- and 4-month-old specimens.
Consequently, the chitin:ergosterol ratio increased from about 14 to 19 in 1- and 4-month-old mycorrhizas respectively to about 110 in 7-month-old mycorrhizas. In the second set of experiments, we found that variation in plant growth had no effect on the chitin:ergosterol ratio in whole root systems of either Alnus incana (L.) Moench or Pinus sylvestris mycorrhizal with Paxillus involutus . In the third set of experiments, we found a constant relationship between the two marker concentrations in 10-month-old root systems of Pinus sylvestris , regardless of fungal species involved, using Paxillus involutus , Piloderma croceum Erikss. & Hjorts and Suillus variegatus (Fr.) O. Kuntze as test organisms. Taken together, the results of this study suggest that both chitin and ergosterol give reliable, but different, relative measures of fungal biomass in mycorrhizal roots. Furthermore, we demonstrate that, in combination, the two chemical markers can be used to estimate both total and living fungal biomass (derived from the chitin:ergosterol ratio).     

Chitin synthesis in zygotes of Ascaris suum

In Ascaris suum chitin is formed in the zygote immediately after oocyte fertilization, and its synthesis is completed in the eggs from the distal half of the uterus. Incorporation of radiocarbon [14C] glucose into chitin of the eggshell was 40-fold higher than incorporation of [14C] glucosamine. The same rank order also holds for the incorporation of label from these isotopes into the glycogen of the ovaries. A large part of the radiolabel was incorporated first into oocyte glycogen and only after fertilization was it incorporated into eggshell chitin. Actinomycin D inhibited chitin synthesis in the eggs from the distal half of the uterus and it significantly reduced incorporation of radiocarbon from glucose into chitin.     

Chitin synthesis inhibitors: old molecules and new developments

Abstract Chitin is the most abundant natural aminopolysaccharide and serves as a structural component of extracellular matrices. It is found in fungal septa, spores, and cell walls, and in arthropod cuticles and peritrophic matrices, squid pens, mollusk shells, nematode egg shells, and some protozoan cyst walls. As prokaryotes, plants and vertebrates including humans do not produce chitin, its synthesis is considered as an attractive target site for fungicides, insecticides, and acaricides. Although no chitin synthesis inhibitor has been developed into a therapeutic drug to treat fungal infections in humans, a larger number of compounds have been successfully launched worldwide to combat arthropod pests in agriculture and forestry. This review summarizes the latest advances on the mode of action of chitin synthesis inhibitors with a special focus on those molecules that act on a postcatalytic step of chitin synthesis.     

A Candida-based view of fungal sex and pathogenesis

Analysis of the genomes of multiple Candida species reveals shared traits associated with pathogenesis, but also striking differences in sexual reproduction.     

Chitin deacetylase activity of the rust Uromyces viciae-fabae is controlled by fungal morphogenesis

Abstract The broad bean rust fungus Uromyces viciae-fabae exhibits chitin only on surfaces of those infection structures which in nature are formed on the plant cuticle, but not on those differentiated in the intercellular space of the host leaf. Chitin deacetylase, an enzyme which converts chitin to chitosan, has been studied during in vitro differentiation of rust infection structures. Radiometrie and gel electrophoretic analyses of crude extracts and extracellular washing fluids have shown that chitin deacetylase activity massively increases when the fungus starts to penetrate through the stomata, and that formation of the enzyme is strictly differentiation-specifically controlled. The extracellular portion of chitin deacetylase activity was about 53% in 24-h-old differentiated infection structures. Five isoforms with apparent molecular masses of 48.1, 30.7, 25.2, 15.2 and 12.7 kDa were detectable after substrate SDS-PAGE. The enzyme is temperature-sensitive and has a pH optimum of 5.5-6.0.     

Chitin synthesis inhibitors prevent cyst formation by Entamoeba trophozoites

Cysts of Entamoeba invadens obtained under axenic culture conditions have been reported to be similar to cysts of the human intestinal parasite E histolytica both in morphology and chitin presence in their wails. Mature E. invadens cyst forms, isolated from cultures following discontinuous Percoll gradient sedimentation were resistant (>80%) to detergent treatment. Addition of chitin synthesis inhibitors such as Polyoxin D and Nikkomycin (50 μg/ml) to cultures in encystation media markedly inhibited (>85%) the formation of detergent resistant cysts and prevented the incorporation of radiolabeled chitin precursor N-acetyl[³H]glucosamine. These findings suggest that chitin synthesis inhibitors may serve as drugs which specifically block the life cycle of the Entamoeba parasite.     

Photoactivated perylenequinone toxins in fungal pathogenesis of plants

Several genera of plant pathogenic fungi produce photoactivated perylenequinone toxins involved in pathogenesis of their hosts. These toxins are photosensitizers, absorbing light energy and generating reactive oxygen species that damage the membranes of the host cells. Studies with toxin-deficient mutants and on the involvement of light in symptom development have documented the importance of these toxins in successful pathogenesis of plants. This review focuses on the well studied perylenequinone toxin, cercosporin, produced by species in the genus Cercospora. Significant progress has been made recently on the biosynthetic pathway of cercosporin, with the characterization of genes encoding a polyketide synthase and a major facilitator superfamily transporter, representing the first and last steps of the biosynthetic pathway, as well as important regulatory genes. In addition, the resistance of Cercospora fungi to cercosporin and to the singlet oxygen that it generates has led to the use of these fungi as models for understanding cellular resistance to photosensitizers and singlet oxygen. These studies have shown that resistance is complex, and have documented a role for transporters, transient reductive detoxification, and quenchers in cercosporin resistance.     

Fungome: Annotating proteins implicated in fungal pathogenesis

Sequencing genomes of different pathogenic fungi produced plethora of genetic information. This "omics" data might be of great interest to probe strain diversity, identify virulence factors and complementary genes in other fungal species, and importantly in predicting the role of proteins specific to pathogenesis in humans. We propose a component called "fungome" for those fungal proteins implicated in pathogenesis which, we believe, will allow researchers to improve the annotation of fungal proteins.