Trypanosoma cruzi: use of herpes simplex virus-thymidine kinase as a negative selectable marker

Alpha-Herpesvirus Thymidine Kinase Genes Mediate Viral Virulence and Are Potential Therapeutic Targets

Alpha-herpesvirus thymidine kinase (TK) genes are virulence-related genes and are nonessential for viral replication; they are often preferred target genes for the construction of gene-deleted attenuated vaccines and genetically engineered vectors for inserting and expressing foreign genes. The enzymes encoded by TK genes are key kinases in the nucleoside salvage pathway and have significant substrate diversity, especially the herpes simplex virus 1 (HSV-1) TK enzyme, which phosphorylates four nucleosides and various nucleoside analogues. Hence, the HSV-1 TK gene is exploited for the treatment of viral infections, as a suicide gene in antitumor therapy, and even for the regulation of stem cell transplantation and treatment of parasitic infection. This review introduces the effects of α-herpesvirus TK genes on viral virulence and infection in the host and classifies and summarizes the current main application domains and potential uses of these genes. In particular, mechanisms of action, clinical limitations, and antiviral and antitumor therapy development strategies are discussed.

Conditional removal of selectable markers in Trypanosoma cruzi using a site-specific recombination tool: proof of concept

The scarcity of molecular tools for genetic manipulation is a critical obstacle for functional genomics studies on Trypanosoma cruzi. The current study adapted an inducible site-specific recombination system based on Dimerizable CRE recombinase (DiCRE). Two vectors for stable transfection were created, a first one to express inactive portions of DiCRE recombinase, and a second plasmid containing the loxP sites to test DiCRE activity. After integrating both constructs into the T. cruzi genome, it was shown that DiCRE recombinase can be efficiently used to manipulate its genome by allowing the removal of selectable markers thus generating homogeneous populations. The DiCRE recombinase success allows conditional knockout and the removal of selectable markers without prior parasite modification, which also facilitate the transferring of DiCRE recombinase to different T. cruzi strains.

Functional genomics approaches in parasitic helminths

As research on parasitic helminths is moving into the post-genomic era, an enormous effort is directed towards deciphering gene function and to achieve gene annotation. The sequences that are available in public databases undoubtedly hold information that can be utilized for new interventions and control but the exploitation of these resources has until recently remained difficult. Only now, with the emergence of methods to genetically manipulate and transform parasitic worms will it be possible to gain a comprehensive understanding of the molecular mechanisms involved in nutrition, metabolism, developmental switches/maturation and interaction with the host immune system. This review focuses on functional genomics approaches in parasitic helminths that are currently used, to highlight potential applications of these technologies in the areas of cell biology, systems biology and immunobiology of parasitic helminths.

Transgenesis and neuronal ablation in parasitic nematodes: revolutionary new tools to dissect host-parasite interactions

Ease of experimental gene transfer into viral and prokaryotic pathogens has made transgenesis a powerful tool for investigating the interactions of these pathogens with the host immune system. Recent advances have made this approach feasible for more complex protozoan parasites. By contrast, the lack of a system for heritable transgenesis in parasitic nematodes has hampered progress toward understanding the development of nematode-specific cellular responses. Recently, however, significant strides towards such a system have been made in several parasitic nematodes, and the possible applications of these in immunological research should now be contemplated. In addition, methods for targeted cell ablation have been successfully adapted from Caenorhabditis elegans methodology and applied to studies of neurobiology and behaviour in Strongyloides stercoralis. Together, these new technical developments offer exciting new tools to interrogate multiple aspects of the host-parasite interaction following nematode infection.

HygR and PurR plasmid vectors for episomal transfection of Trypanosoma cruzi

This work describes the development and functional testing of two episomes for stable transfection of Trypanosoma cruzi. pHygD contained the 5'- and 3'- flanking regions of the gene encoding the cathepsin B-like protease of T. cruzi as functional trans-splicing and polyadenylation signals for the hygR ORF. Evidence is presented to support extrachromosomal maintenance and organization as tandem repeats in transfected parasites. pPac was derived from pHygD by replacement of the entire hygR ORF with a purR coding region. The ability to modify pHygD and the availability of the complete DNA sequence make these plasmids useful tools for the genetic manipulation of T. cruzi.

New selectable markers and single crossover integration for the highly versatile Plasmodium knowlesi transfection system

Plasmodium knowlesi provides a highly versatile transfection system for malaria, since it enables rapid genetic modification of the parasite both in vivo as well as in vitro. However, it is not possible to perform multiple genetic manipulations within one parasite line because of a lack of selectable markers. In an effort to develop additional selectable markers for this parasite, positive and negative selectable markers that have recently been successfully used in Plasmodium falciparum were tested. It was shown that the positive selectable markers human dihydrofolate reductase (hdhfr), blasticidin S deaminase (bsd) and neomycin phosphotransferase II (neo) all conferred drug resistance to P. knowlesi when introduced as episomes. The plasmid containing the hdhfr selectable marker was not only successfully introduced as circular form, but also as linear fragment, demonstrating for the first time single crossover integration in P. knowlesi. Thymidine kinase was tested for its potential as negative selectable marker and it was shown that recombinant P. knowlesi parasites expressing thymidine kinase from episomes were highly sensitive to ganciclovir compared to wild-type P. knowlesi. The availability of new positive selectable markers and a strong candidate for a negative selectable marker for P. knowlesi, in combination with the opportunity to perform targeted single crossover integration in P. knowlesi, significantly increases the flexibility of this transfection system, making it one of the most versatile systems available for Plasmodium.

Genetic manipulation of kinetoplastida

During the 1980s, many kinetoplastid genes were cloned and their function inferred from homology with genes from other organisms, location of the corresponding proteins or expression in heterologous systems. Up until 1990, before the availability of DNA transfection methodology, we could not analyze the function of kinetoplastid genes within the organisms themselves. Since then, it has become possible to create and complement mutants, to overexpress foreign proteins in the parasites, to knock out genes and even to switch off essential functions. However, these methods are not equally applicable in all parasites. Here, Christine Clayton highlights the differences and similarities between the most commonly used model organisms, and assesses the relative advantages of different approaches and parasites for different types of investigation.

Virulence in Trypanosoma cruzi infection correlates with the expression of a distinct family of sialidase superfamily genes

The overall success of Trypanosoma cruzi depends on its ability to invade the host and establish a long-term infection. Little is known of the genetic factors responsible for observed differences in virulence from strain to strain in T. cruzi. A virulent T. cruzi line was derived from an attenuated parental line by two passages through mice. To identify virulence genes a subtraction library was constructed and screened for cDNA expressed exclusively in the virulent line. One cDNA hybridized to 3.5 and 4.5 Kb RNA present in virulent trypomastigotes but absent in attenuated trypomastigotes. Sequence analysis showed the cDNA to encode an 85 kDa protein with homology to members of the sialidase/trans-sialidase superfamily and has been designated vp85.1. The highest amino acid sequence similarity was to a previously described T. cruzi sialidase-homologue pseudogene [Takle, G.B., O'Conner, J., Young, A.J. and Cross, G.A.M. (1992) Mol. Biochem. Parasitol. 56, 117-128]. The vp85.1 amino acid sequence has higher homology to members of the 160 kDa flagellar-associated antigen family, FL-160, than to other 85 kDa expressed sialidase superfamily members. Southern blot analysis of virulent and attenuated lines demonstrated a complex hybridization pattern consistent with a multiple gene copy family that was identical in both lines. Antibody directed against recombinant vp85.1 peptide recognized proteins between 95 and 115 kDa in total virulent parasite lysates which were absent in attenuated lysates. Peptide N-glycosidase F treatment reduced the high molecular weight bands to 85 kDa, indicating vp85 is an N-linked glycoprotein. Immunofluorescence with anti-vp85.1 demonstrated surface localization of vp85.1 on virulent, but not attenuated, trypomastigotes. We postulate this new subfamily of trans-sialidases may play a role in virulence.

Expression of heterologous proteins in stable insect cell culture

Stable transformed insect cell lines have been used for producing many highly processed heterologous proteins. Current research has focused on development of new expression and selection systems, and enhancement of vector stability. Defining the variation of modification and processing capabilities between cell lines will further enhance complex protein production from insect cells.

A cell cycle model for the tachyzoite of Toxoplasma gondii using the Herpes simplex virus thymidine kinase

Toxoplasma gondii (RH strain) tachyzoites were transfected with a plasmid containing a fusion of the chloramphenicol acetyl transferase and the Herpes simplex virus-2 thymidine kinase coding regions and transgenic parasites obtained by chloramphenicol selection. CTK11, a single high expressing clone was isolated based on immunofluorescence and contained approximately five integrated copies of the fusion sequence. Lysates prepared from this clone displayed thymidine kinase activity of 2.9 pmol min(-1) microg(-1) protein, whereas thymidine kinase activity was not detected in lysates from the parental RH strain. Growth of CTK11 tachyzoites was fully inhibited in 5 microM ganciclovir and thymidine and in 2.5 microM 5-bromo-2'-deoxyuridine. While the inhibitory effects of ganciclovir were lethal, low concentrations of thymidine (10 microM) were largely reversible. Asynchronously growing CTK11 tachyzoites were found to contain major G1 (1 N) and S phase (1 N+) distributions as determined by relative propidium iodide fluorescence and with reference to the haploid (1 N) DNA content of a T. gondii sporozoite population. CTK11 tachyzoites blocked 4 h in 10 microM thymidine exhibited mean fluorescence consistent with a 1 N complement of DNA indicating growth was arrested in G1. Following the removal of excess thymidine, parasites immediately entered S phase, thus confirming the late G1 block. Parasites with a 2 N complement of DNA (G2 + M) first appear at 2 h post-release, while 1 N (G1) parasites re-appear at 3 h suggesting the length of S phase is < or = 2 h and that of G2 + M is < or = 1 h. Within 7 h, parasites had transited G2 + M and much of G1 and re-entered S of the subsequent cell cycle--a time consistent with the doubling of these parasites in culture. Thus, the CTK11 tachyzoite cell cycle is similar to those of higher eukaryotic cells and is characterized by major G1 and S phases and a relatively short G2 + M.