A Bright Future for Fluorescence Imaging of Fungi in Living Hosts

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

Investigating Candida glabrata Urinary Tract Infections (UTIs) in Mice Using Bioluminescence Imaging

Urinary tract infections (UTIs) are quite common and mainly caused by bacteria such as Escherichia coli. However, when patients have urinary catheters, fungal infections comprise up to 15% of these types of infections. Moreover, fungal UTIs have a high mortality, due to rapid spreading of the fungi to the kidneys. Most fungal UTIs are caused by Candida species, among which Candida albicans and Candida glabrata are the most common. C. glabrata is an opportunistic pathogenic yeast, phylogenetically quite close to Saccharomyces cerevisiae. Even though it is commonly isolated from the urinary tract and rapidly acquires resistance to antifungals, its pathogenesis has not been studied extensively in vivo. In vivo studies require high numbers of animals, which can be overcome by the use of non-invasive imaging tools. One such tool, bioluminescence imaging, has been used successfully to study different types of C. albicans infections. For C. glabrata, only biofilms on subcutaneously implanted catheters have been imaged using this tool. In this work, we investigated the progression of C. glabrata UTIs from the bladder to the kidneys and the spleen. Furthermore, we optimized expression of a red-shifted firefly luciferase in C. glabrata for in vivo use. We propose the first animal model using bioluminescence imaging to visualize C. glabrata in mouse tissues. Additionally, this UTI model can be used to monitor antifungal activity in vivo over time.

Application of Two-Photon Microscopy to Study Sclerotium cepivorum Berk Sclerotia Isolated from Naturally Infested Soil and Produced In Vitro

The danger of Sclerotium cepivorum lies in the strength of its survival structure: sclerotia. Sclerotia comprising hardened mycelium contains food reserves that allow it to remain dormant for long period, which makes the sclerotia-infested soil useless to grow any crop of the Allium species, including onion and garlic. This paper would be the first report on the application of two-photon fluorescence microscopy to the analysis of the structure of sclerotia from S. cepivorum. For this study and, in order to test the method, two different types of sclerotia were used: (1) sclerotia isolated from naturally infested soil and (2) sclerotia produced in vitro (from 20-day-old cultures). Both types of sclerotia were processed by cryopreservation and eight µm histological cuts were used to obtain an autofluorescence image. For both sclerotia, the fluorescence spectrum has three peak signals at their wall. Sclerotia from infested soil presented fluorescence peaks at 400-436, 436-475, and 515-575 nm, while signals from sclerotia produced in vitro presented fluorescence peaks at 400-442, 500-600, and 655-700 nm. Peaks at the violet electromagnetic region (400-436 and 400-442) are like that of the signals reported by the melanin. This study showed that two-photon microscopy is a novel and valuable tool for the study of sclerotia structure and their fluorescence signal, and the possibility of using it as a specific marker to direct detection in the field should be explored.

Establishment of the monomeric yellow-green fluorescent protein mNeonGreen for life cell imaging in mycelial fungi

The engineered monomeric version of the lancelet Branchiostoma lanceolatum fluorescent protein, mNeonGreen (mNG), has several positive characteristics, such as a very bright fluorescence, high photostability and fast maturation. These features make it a good candidate for the utilization as fluorescent tool for cell biology and biochemical applications in filamentous fungi. We report the generation of plasmids for the expression of the heterologous mNG gene under the control of an inducible and a constitutive promoter in the filamentous ascomycete Sordaria macrospora and display a stable expression of mNG in the cytoplasm. To demonstrate its usefulness for labeling of organelles, the peroxisomal targeting sequence serine-lysine-leucine (SKL) was fused to mNG. Expression of this tagged version led to protein import of mNG into peroxisomes and their bright fluorescence in life cell imaging.

Expansion Microscopy for Cell Biology Analysis in Fungi

Super-resolution microscopy has evolved as a powerful method for subdiffraction-resolution fluorescence imaging of cells and cellular organelles, but requires sophisticated and expensive installations. Expansion microscopy (ExM), which is based on the physical expansion of the cellular structure of interest, provides a cheap alternative to bypass the diffraction limit and enable super-resolution imaging on a conventional fluorescence microscope. While ExM has shown impressive results for the magnified visualization of proteins and RNAs in cells and tissues, it has not yet been applied in fungi, mainly due to their complex cell wall. Here we developed a method that enables reliable isotropic expansion of ascomycetes and basidiomycetes upon treatment with cell wall degrading enzymes. Confocal laser scanning microscopy (CLSM) and structured illumination microscopy (SIM) images of 4.5-fold expanded sporidia of Ustilago maydis expressing fluorescent fungal rhodopsins and hyphae of Fusarium oxysporum or Aspergillus fumigatus expressing either histone H1-mCherry together with Lifeact-sGFP or mRFP targeted to mitochondria, revealed details of subcellular structures with an estimated spatial resolution of around 30 nm. ExM is thus well suited for cell biology studies in fungi on conventional fluorescence microscopes.