The Vacuolar Morphogenesis Protein Vam6-Like Protein Vlp1 Is Required for Pathogenicity of Cryptococcus neoformans

UPS: Opportunities and challenges for gastric cancer treatment

Gastric cancer remains the fourth most frequently diagnosed malignancy and the fifth leading cause of cancer-related mortality worldwide owning to the lack of efficient drugs and targets for therapy. Accumulating evidence indicates that UPS, which consists of E1, E2, and E3 enzymes and proteasome, plays an important role in the GC tumorigenesis. The imbalance of UPS impairs the protein homeostasis network during development of GC. Therefore, modulating these enzymes and proteasome may be a promising strategy for GC target therapy. Besides, PROTAC, a strategy using UPS to degrade the target protein, is an emerging tool for drug development. Thus far, more and more PROTAC drugs enter clinical trials for cancer therapy. Here, we will analyze the abnormal expression enzymes in UPS and summarize the E3 enzymes which can be developed in PROTAC so that it can contribute to the development of UPS modulator and PROTAC technology for GC therapy.

The ubiquitin-proteasome system in melanin metabolism

BACKGROUND

The ubiquitin-proteasome system (UPS) is a highly conserved way of regulating intracellular protein balance. UPS mediates proteolysis and disruption of variation or misfolding, while finely regulating proteins involved in differentiation and other biological processes.

AIMS

The aim of this review is to systematically introduce UPS as a key regulator of melanin metabolism.

METHODS

Systematic search and retrospective review were performed on the published data.

RESULTS

Melanocyte-inducing transcription factor (MITF) is a substrate of the ubiquitin ligase VCHL1 and acts as a transcription factor to regulate the expression of key enzymes in melanin synthesis such as tyrosinase (TYR). The rate-limiting enzyme TYR is modified by the ubiquitin ligase Hrd1 during melanosynthesis. Melanin itself is also regulated by multiple ubiquitin ligases including Fbp1 and Vhl. By regulating the ubiquitination modification to target each link of melanin synthesis, it plays an important role in correcting the disorder of melanin metabolism. A number of chemical agents have been proven to inhibit the activity of ubiquitin ligase.

CONCLUSIONS

Drugs targeting E3 ligase and deubiquitinating enzymes have great potential in the treatment of melanin metabolism disorders.

Role of F-box Protein Cdc4 in Fungal Virulence and Sexual Reproduction of Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic yeast-like pathogen that mainly infects immunocompromised individuals and causes fatal meningitis. Sexual reproduction can promote the exchange of genetic material between different strains of C. neoformans, which is one of the reasons leading to the emergence of highly pathogenic and drug-resistant strains of C. neoformans. Although much research has been done on the regulation mechanism of Cryptococcus sexual reproduction, there are few studies on the sexual reproduction regulation of Cryptococcus by the ubiquitin-proteasome system. This study identified an F-box protein, Cdc4, which contains a putative F-box domain and eight WD40 domains. The expression pattern analysis showed that the CDC4 gene was expressed in various developmental stages of C. neoformans, and the Cdc4 protein was localized in the nucleus of cryptococcal cells. In vitro stress responses assays showed that the CDC4 overexpression strains are sensitive to SDS and MMS but not Congo red, implying that Cdc4 may regulate the cell membrane integrity and repair of DNA damage of C. neoformans. Fungal virulence assay showed that although the cdc4Δ mutant grows normally and can produce typical virulence factors such as capsule and melanin, the cdc4Δ mutant completely loses its pathogenicity in a mouse systemic-infection model. Fungal mating assays showed that Cdc4 is also essential for fungal sexual reproduction in C. neoformans. Although normal mating hyphae were observed during mating, the basidiospores' production was blocked in bilateral mating between cdc4Δ mutants. Fungal nuclei development assay showed that the nuclei failed to undergo meiosis after fusion inside the basidia during the bilateral mating of cdc4Δ mutants, indicating that Cdc4 is critical to regulating meiosis during cryptococcal mating. In summary, our study revealed that the F-box protein Cdc4 is critical for fungal virulence and sexual reproduction in C. neoformans.

OUP accepted manuscript

Background

Fungal infections are common life-threatening diseases amongst immunodeficient individuals. Invasive fungal disease is commonly treated with an azole antifungal agent, resulting in selection pressure and the emergence of drug resistance. Antifungal resistance is associated with higher mortality rates and treatment failure, making the current clinical management of fungal disease very challenging. Clinical isolates from a variety of fungi have been shown to contain mutations in the MSH2 gene, encoding a component of the DNA mismatch repair pathway. Mutation of MSH2 results in an elevated mutation rate that can increase the opportunity for selectively advantageous mutations to occur, accelerating the development of antifungal resistance.

Objectives

To characterize the molecular mechanisms causing the microevolutionary emergence of antifungal resistance in msh2 mismatch repair mutants of Cryptococcus neoformans.

Methods

The mechanisms resulting in the emergence of antifungal resistance were investigated using WGS, characterization of deletion mutants and measuring ploidy changes.

Results

The genomes of resistant strains did not possess mutations in ERG11 or other genes of the ergosterol biosynthesis pathway. Antifungal resistance was due to small contributions from mutations in many genes. MSH2 does not directly affect ploidy changes.

Conclusions

This study provides evidence that resistance to fluconazole can evolve independently of ERG11 mutations. A common microevolutionary route to the emergence of antifungal resistance involves the accumulation of mutations that alter stress signalling, cellular efflux, membrane trafficking, epigenetic modification and aneuploidy. This complex pattern of microevolution highlights the significant challenges posed both to diagnosis and treatment of drug-resistant fungal pathogens.

Vam6/Vps39/TRAP1-domain proteins influence vacuolar morphology, iron acquisition and virulence in Cryptococcus neoformans

The pathogenic fungus Cryptococcus neoformans must overcome iron limitation to cause disease in mammalian hosts. Previously, we reported a screen for insertion mutants with poor growth on haem as the sole iron source. In this study, we characterised one such mutant and found that the defective gene encoded a Vam6/Vps39/TRAP1 domain-containing protein required for robust growth on haem, an important iron source in host tissue. We designated this protein Vps3 based on reciprocal best matches with the corresponding protein in Saccharomyces cerevisiae. C. neoformans encodes a second Vam6/Vps39/TRAP1 domain-containing protein designated Vam6/Vlp1, and we found that this protein is also required for robust growth on haem as well as on inorganic iron sources. This protein is predicted to be a component of the homotypic fusion and vacuole protein sorting complex involved in endocytosis. Further characterisation of the vam6Δ and vps3Δ mutants revealed perturbed trafficking of iron acquisition functions (e.g., the high affinity iron permease Cft1) and impaired processing of the transcription factor Rim101, a regulator of haem and iron acquisition. The vps3Δ and vam6Δ mutants also had pleiotropic phenotypes including loss of virulence in a mouse model of cryptococcosis, reduced virulence factor elaboration and increased susceptibility to stress, indicating pleiotropic roles for Vps3 and Vam6 beyond haem use in C. neoformans. TAKE AWAYS: Two Vam6/Vps39/TRAP1-domain proteins, Vps3 and Vam6, support the growth of Cryptococcus neoformans on haem. Loss of Vps3 and Vam6 influences the trafficking and expression of iron uptake proteins. Loss of Vps3 or Vam6 eliminates the ability of C. neoformans to cause disease in a mouse model of cryptococcosis.

More Than Just Cleaning: Ubiquitin-Mediated Proteolysis in Fungal Pathogenesis

Ubiquitin-proteasome mediated protein turnover is an important regulatory mechanism of cellular function in eukaryotes. Extensive studies have linked the ubiquitin-proteasome system (UPS) to human diseases, and an array of proteasome inhibitors have been successfully developed for cancer therapy. Although still an emerging field, research on UPS regulation of fungal development and virulence has been rapidly advancing and has generated considerable excitement in its potential as a target for novel drugs. In this review, we summarize UPS composition and regulatory function in pathogenic fungi, especially in stress responses, host adaption, and fungal pathogenesis. Emphasis will be given to UPS regulation of pathogenic factors that are important for fungal pathogenesis. We also discuss future potential therapeutic strategies for fungal infections based on targeting UPS pathways.