GFP tagging sheds light on protein translocation: implications for key methods in cell biology

Testing the CRISPR-Cas9 and glmS ribozyme systems in Leishmania tarentolae

Leishmania parasites include important pathogens and model organisms and are even used for the production of recombinant proteins. However, functional genomics and the characterization of essential genes are often limited in Leishmania because of low-throughput technologies for gene disruption or tagging and the absence of components for RNA interference. Here, we tested the T7 RNA polymerase-dependent CRISPR-Cas9 system by Beneke et al. and the glmS ribozyme-based knock-down system in the model parasite Leishmania tarentolae. We successfully deleted two reference genes encoding the flagellar motility factor Pf16 and the salvage-pathway enzyme adenine phosphoribosyltransferase, resulting in immotile and drug-resistant parasites, respectively. In contrast, we were unable to disrupt the gene encoding the mitochondrial flavoprotein Erv. Cultivation of L. tarentolae in standard BHI medium resulted in a constitutive down-regulation of an episomal mCherry-glmS reporter by 40 to 60%. For inducible knock-downs, we evaluated the growth of L. tarentolae in alternative media and identified supplemented MEM, IMDM and McCoy's 5A medium as candidates. Cultivation in supplemented MEM allowed an inducible, glucosamine concentration-dependent down-regulation of the episomal mCherry-glmS reporter by more than 70%. However, chromosomal glmS-tagging of the genes encoding Pf16, adenine phosphoribosyltransferase or Erv did not reveal a knock-down phenotype. Our data demonstrate the suitability of the CRISPR-Cas9 system for the disruption and tagging of genes in L. tarentolae as well as the limitations of the glmS system, which was restricted to moderate efficiencies for episomal knock-downs and caused no detectable phenotype for chromosomal knock-downs.

Redesign of Genetically Encoded Biosensors for Monitoring Mitochondrial Redox Status in a Broad Range of Model Eukaryotes

The development of genetically encoded redox biosensors has paved the way toward chemically specific, quantitative, dynamic, and compartment-specific redox measurements in cells and organisms. In particular, redox-sensitive green fluorescent proteins (roGFPs) have attracted major interest as tools to monitor biological redox changes in real time and in vivo. Most recently, the engineering of a redox relay that combines glutaredoxin (Grx) with roGFP2 as a translational fusion (Grx1-roGFP2) led to a biosensor for the glutathione redox potential ( EGSH). The expression of this probe in mitochondria is of particular interest as mitochondria are the major source of oxidants, and their redox status is closely connected to cell fate decisions. While Grx1-roGFP2 can be expressed in mammalian mitochondria, it fails to enter mitochondria in various nonmammalian model organisms. Here we report that inversion of domain order from Grx1-roGFP2 to roGFP2-Grx1 yields a biosensor with perfect mitochondrial targeting while fully maintaining its biosensor capabilities. The redesigned probe thus allows extending in vivo observations of mitochondrial redox homeostasis to important nonmammalian model organisms, particularly plants and insects.

No need for labels: the autofluorescence of Leishmania tarentolae mitochondria and the necessity of negative controls

Background

Fluorescence microscopy is a powerful tool to study the morphology and function of subcellular compartments or to determine the localization of proteins. The method is also regularly used for the analysis of parasitic protists including kinetoplastida.

Results

Here, we report a significant autofluorescence of Leishmania tarentolae mitochondria. The autofluorescence, presumably caused by flavoproteins, was detectable using a variety of cell fixation protocols and had a maximum emission at approximately 538 nm. Stable signals were obtained with xenon lamps as a light source and filter sets that are commonly used for the detection of green fluorescent protein.

Conclusions

On the one hand, we present a methodological approach to examine mitochondrial morphology or to study the colocalization of mitochondrial proteins without additional staining or labeling procedures. On the other hand, under certain experimental conditions, mitochondrial autofluorescence can result in false positive signals, demonstrating the necessity to analyze unlabeled cells as negative controls.