Broadening the spectrum of fluorescent protein tools for use in the encapsulated human fungal pathogen Cryptococcus neoformans

A Chimeric ORF Fusion Phenotypic Reporter for Cryptococcus neoformans

The plethora of genome sequences produced in the postgenomic age has not resolved many of our most pressing biological questions. Correlating gene expression with an interrogatable and easily observable characteristic such as the surrogate phenotype conferred by a reporter gene is a valuable approach to gaining insight into gene function. Many reporters including lacZ, amdS, and the fluorescent proteins mRuby3 and mNeonGreen have been used across all manners of organisms. Described here is an investigation into the creation of a robust, synthetic, fusion reporter system for Cryptococcus neoformans that combines some of the most useful fluorophores available in this system with the versatility of the counter-selectable nature of amdS. The reporters generated include multiple composition and orientation variants, all of which were investigated for differences in expression. Evaluation of known promoters from the TEF1 and GAL7 genes was undertaken, elucidating novel expression tendencies of these biologically relevant C. neoformans regulators of transcription. Smaller than lacZ but providing multiple useful surrogate phenotypes for interrogation, the fusion ORF serves as a superior whole-cell assay compared to traditional systems. Ultimately, the work described here bolsters the array of relevant genetic tools that may be employed in furthering manipulation and understanding of the WHO fungal priority group pathogen C. neoformans.

Survival in macrophages induces enhanced virulence in Cryptococcus

Cryptococcus is a ubiquitous environmental fungus and frequent colonizer of human lungs. Colonization can lead to diverse outcomes, from clearance to long-term colonization to life-threatening meningoencephalitis. Regardless of the outcome, the process starts with an encounter with phagocytes. Using the zebrafish model of this infection, we have noted that cryptococcal cells first spend time inside macrophages before they become capable of pathogenic replication and dissemination. What "licensing" process takes place during this initial encounter, and how are licensed cryptococcal cells different? To address this, we isolated cryptococcal cells after phagocytosis by cultured macrophages and found these macrophage-experienced cells to be markedly more virulent in both zebrafish and mouse models. Despite producing a thick polysaccharide capsule, they were still subject to phagocytosis by macrophages in the zebrafish. Analysis of antigenic cell wall components in these licensed cells demonstrated that components of mannose and chitin are more available for staining than they are in culture-grown cells or cells with capsule production induced in vitro. Cryptococcus is capable of exiting or transferring between macrophages in vitro, raising the likelihood that this fungus alternates between intracellular and extracellular life during growth in the lungs. Our results raise the possibility that intracellular life has its advantages over time, and phagocytosis-induced alteration in mannose and chitin exposure is one way that makes subsequent rounds of phagocytosis more beneficial to the fungus.IMPORTANCECryptococcosis begins in the lungs and can ultimately travel through the bloodstream to cause devastating infection in the central nervous system. In the zebrafish model, small amounts of cryptococcus inoculated into the bloodstream are initially phagocytosed and become far more capable of dissemination after they exit macrophages. Similarly, survival in the mouse lung produces cryptococcal cell types with enhanced dissemination. In this study, we have evaluated how phagocytosis changes the properties of Cryptococcus during pathogenesis. Macrophage-experienced cells (MECs) become "licensed" for enhanced virulence. They out-disseminate culture-grown cells in the fish and out-compete non-MECs in the mouse lung. Analysis of their cell surface demonstrates that MECs have increased availability of cell wall components mannose and chitin substances involved in provoking phagocytosis. These findings suggest how Cryptococcus might tune its cell surface to induce but survive repeated phagocytosis during early pathogenesis in the lung.

Cryptococcus neoformans adapts to the host environment through TOR-mediated remodeling of phospholipid asymmetry

Cryptococcus spp. are environmental fungi that first must adapt to the host environment before they can cause life-threatening meningitis in immunocompromised patients. Host CO2 concentrations are 100-fold higher than the external environment and strains unable to grow at host CO2 concentrations are not pathogenic. Using a genetic screening and transcriptional profiling approach, we report that the TOR pathway is critical for C. neoformans adaptation to host CO2 partly through Ypk1-dependent remodeling of phosphatidylserine asymmetry at the plasma membrane. We also describe a C. neoformans ABC/PDR transporter (PDR9) that is highly expressed in CO2-sensitive environmental strains, suppresses CO2-induced phosphatidylserine/phospholipid remodeling, and increases susceptibility to host concentrations of CO2. Interestingly, regulation of plasma membrane lipid asymmetry by the TOR-Ypk1 axis is distinct in C. neoformans compared to S. cerevisiae. Finally, host CO2 concentrations suppress the C. neoformans pathways that respond to host temperature (Mpk1) and pH (Rim101), indicating that host adaptation requires a stringent balance among distinct stress responses.

Engineered Fluorescent Strains of Cryptococcus neoformans: a Versatile Toolbox for Studies of Host-Pathogen Interactions and Fungal Biology, Including the Viable but Nonculturable State

Cryptococcus neoformans is an opportunistic fungal pathogen known for its remarkable ability to infect and subvert phagocytes. This ability provides survival and persistence within the host and relies on phenotypic plasticity. The viable but nonculturable (VBNC) phenotype was recently described in C. neoformans, whose study is promising in understanding the pathophysiology of cryptococcosis. The use of fluorescent strains is improving host interaction research, but it is still underexploited. Here, we fused histone H3 or the poly(A) binding protein (Pab) to enhanced green fluorescent protein (eGFP) or mCherry, obtaining a set of C. neoformans transformants with different colors, patterns of fluorescence, and selective markers (hygromycin B resistance [Hygr] or neomycin resistance [Neor]). We validated their similarity to the parental strain in the stress response, the expression of virulence-related phenotypes, mating, virulence in Galleria mellonella, and survival within murine macrophages. PAB-GFP, the brightest transformant, was successfully applied for the analysis of phagocytosis by flow cytometry and fluorescence microscopy. Moreover, we demonstrated that an engineered fluorescent strain of C. neoformans was able to generate VBNC cells. GFP-tagged Pab1, a key regulator of the stress response, evidenced nuclear retention of Pab1 and the assembly of cytoplasmic stress granules, unveiling posttranscriptional mechanisms associated with dormant C. neoformans cells. Our results support that the PAB-GFP strain is a useful tool for research on C. neoformans. IMPORTANCE Cryptococcus neoformans is a human-pathogenic yeast that can undergo a dormant state and is responsible for over 180,000 deaths annually worldwide. We engineered a set of fluorescent transformants to aid in research on C. neoformans. A mutant with GFP-tagged Pab1 improved fluorescence-based techniques used in host interaction studies. Moreover, this mutant induced a viable but nonculturable phenotype and uncovered posttranscriptional mechanisms associated with dormant C. neoformans. The experimental use of fluorescent mutants may shed light on C. neoformans-host interactions and fungal biology, including dormant phenotypes.

Use of red, far-red, and near-infrared light in imaging of yeasts and filamentous fungi

Abstract

While phototoxicity can be a useful therapeutic modality not only for eliminating malignant cells but also in treating fungal infections, mycologists aiming to observe morphological changes or molecular events in fungi, especially when long observation periods or high light fluxes are warranted, encounter problems owed to altered regulatory pathways or even cell death caused by various photosensing mechanisms. Consequently, the ever expanding repertoire of visible fluorescent protein toolboxes and high-resolution microscopy methods designed to investigate fungi in vitro and in vivo need to comply with an additional requirement: to decrease the unwanted side effects of illumination. In addition to optimizing exposure, an obvious solution is red-shifted illumination, which, however, does not come without compromises. This review summarizes the interactions of fungi with light and the various molecular biology and technology approaches developed for exploring their functions on the molecular, cellular, and in vivo microscopic levels, and outlines the progress towards reducing phototoxicity through applying far-red and near-infrared light.

Key points

• Fungal biological processes alter upon illumination, also under the microscope

• Red shifted fluorescent protein toolboxes decrease interference by illumination

• Innovations like two-photon, lightsheet, and near IR microscopy reduce phototoxicity

Identification and characterisation of sPEPs in Cryptococcus neoformans

Short open reading frame (sORF)-encoded peptides (sPEPs) have been found across a wide range of genomic locations in a variety of species. To date, their identification, validation, and characterisation in the human fungal pathogen Cryptococcus neoformans has been limited due to a lack of standardised protocols. We have developed an enrichment process that enables sPEP detection within a protein sample from this polysaccharide-encapsulated yeast, and implemented proteogenomics to provide insights into the validity of predicted and hypothetical sORFs annotated in the C. neoformans genome. Novel sORFs were discovered within the 5' and 3' UTRs of known transcripts as well as in "non-coding" RNAs. One novel candidate, dubbed NPB1, that resided in an RNA annotated as "non-coding", was chosen for characterisation. Through the creation of both specific point mutations and a full deletion allele, the function of the new sPEP, Npb1, was shown to resemble that of the bacterial trans-translation protein SmpB.

Beyond protein tagging: Rewiring the genetic code of fluorescent proteins - A review

Fluorescent proteins (FP) are an integral part of modern biology due to its diverse biochemical and photophysical properties. The boundaries of FP have been extended through conventional mutagenesis and directed evolution approaches. Engineering of FP based on the standard genetic code consisting of 20 amino acids with limited functional groups restrict its diversification. Degeneracy of genetic code has helped in covering this substantial gap through genetic code engineering, wherein introduction of unnatural amino acid (UAA) analogues resulted in a collection of FP with varying properties. This review features the work carried till date in the area of FP incorporated with UAAs and explores strategies employed for incorporation, impact of UAAs in chromophore and surrounding residues and changes in inherent properties of FP. The long-standing association of FP as a tool for high throughput screening of orthogonal aaRS/tRNA pairs used in site specific incorporation of UAAs is expounded. Insertion of UAAs in FP has enabled their use in contemporary fields such as biophotovoltaics, bioremediation, biosensors, biomaterials and imaging of acidic vesicles. Thus, expansion of genetic code of FP is envisaged to rejig the existing spectra of colors and future research initiative in this direction is expected to glow brighter and brighter.