ADP-ribosyl transferase activity in Trypanosoma brucei

The DNA damage response is developmentally regulated in the African trypanosome

Genomes are affected by a wide range of damage, which has resulted in the evolution of a number of widely conserved DNA repair pathways. Most of these repair reactions have been described in the African trypanosome Trypanosoma brucei, which is a genetically tractable eukaryotic microbe and important human and animal parasite, but little work has considered how the DNA damage response operates throughout the T. brucei life cycle. Using quantitative PCR we have assessed damage induction and repair in both the nuclear and mitochondrial genomes of the parasite. We show differing kinetics of repair for three forms of DNA damage, and dramatic differences in repair between replicative life cycle forms found in the testse fly midgut and the mammal. We find that mammal-infective T. brucei cells repair oxidative and crosslink-induced DNA damage more efficiently than tsetse-infective cells and, moreover, very distinct patterns of induction and repair of DNA alkylating damage in the two life cycle forms. We also reveal robust repair of DNA lesions in the highly unusual T. brucei mitochondrial genome (the kinetoplast). By examining mutants we show that nuclear alkylation damage is repaired by the concerted action of two repair pathways, and that Rad51 acts in kinetoplast repair. Finally, we correlate repair with cell cycle arrest and cell growth, revealing that induced DNA damage has strikingly differing effects on the two life cycle stages, with distinct timing of alkylation-induced cell cycle arrest and higher levels of damage induced death in mammal-infective cells. Our data reveal that T. brucei regulates the DNA damage response during its life cycle, a capacity that may be shared by many microbial pathogens that exist in variant environments during growth and transmission.

A chromosomal SIR2 homologue with both histone NAD-dependent ADP-ribosyltransferase and deacetylase activities is involved in DNA repair in Trypanosoma brucei

SIR2-like proteins have been implicated in a wide range of cellular events including chromosome silencing, chromosome segregation, DNA recombination and the determination of life span. We report here the molecular and functional characterization of a SIR2-related protein from the protozoan parasite Trypanosoma brucei, which we termed TbSIR2RP1. This protein is a chromosome-associated NAD-dependent enzyme which, in contrast to other known proteins of this family, catalyses both ADP-ribosylation and deacetylation of histones, particulary H2A and H2B. Under- or overexpression of TbSIR2RP1 decreased or increased, respectively, cellular resistance to DNA damage. Treatment of trypanosomal nuclei with a DNA alkylating agent resulted in a significant increase in the level of histone ADP-ribosylation and a concomitant increase in chromatin sensitivity to micrococcal nuclease. Both of these responses correlated with the level of TbSIR2RP1 expression. We propose that histone modification by TbSIR2RP1 is involved in DNA repair.

Histone synthesis in Trypanosoma cruzi

Trypanosoma cruzi is an ancient, parasitic eukaryote which does not undergo chromatin condensation during cell division. This behavior may be explained if one considers the strong amino acid sequence divergence of Trypanosoma histones compared to higher eukaryotes. In the latter organisms histone synthesis is coupled to DNA replication. Considering the nonconserved amino acid sequence of T. cruzi histones, as well as the absence of chromatin condensation in this organism, we have studied histone synthesis in relation to DNA replication in this parasite. We have found that core histones and a fraction of histone H1 are synthesized concomitantly to DNA replication. However, another fraction of histone H1 is constitutively synthesized.

In vitro poly-(ADP-ribosyl)ation of chromatin proteins in the rat tapeworm, Hymenolepis diminuta

1. (ADP-ribose)-transferase activity in crude chromatin of H. diminuta was demonstrated. 2. Chromatin proteins were ADP-ribosylated in vitro and selectively extracted. 60, 12 and 18% of the (ADP-ribose)n of chromatin proteins was associated with total histones, histone H1 and histone H2B, respectively. 3. The extent of oligo-(ADP-ribose) compared to total (ADP-ribose)n in the chromatin fraction, in the histone fraction, the histone H1 fraction and the histone H2B fraction was 45, 60, 26 and 49%, with an average chain length of 2.8, 2.1, 1.8 and 2.6, respectively. 4. Analysis of (ADP-ribosyl)n-ated proteins by acetic acid/urea polyacrylamide gel electrophoresis demonstrated that histone H1, histone H2B and a 35 kDa non-histone protein were major (ADP-ribose)n acceptors.

The effects of the methylating agent 1,2-bis(methylsulfonyl)-1-methylhydrazine on morphology, DNA content and mitochondrial function of Trypanosoma brucei subspecies

Repeated exposure of trypanosomes in vitro or in vivo to low concentrations of the methylating agent 1,2-bis(methylsulfonyl)-1-methylhydrazine induces a series of moderately synchronous morphological and biochemical changes. Cell division halts and the long-slender bloodstream forms transform to short-stumpy forms via larger intermediate-stage cells which contain approximately double the normal G2 content of DNA. In common with naturally occurring short-stumpy trypanosomes, drug-induced short-stumpy forms do not infect rodents and when transferred to Cunningham's medium, transform to and replicate as procyclics. Furthermore, these short-stumpy forms exhibit alpha-ketoglutarate supported motility and oxygen consumption, acquire the ability to reduce nitroblue tetrazolium (NADH diaphorase positivity) and appear to be in the G1 or G0 stage of the cell cycle based upon DNA content.

Identification of ADPR-transferase activity in the rat tapeworm, Hymenolepis diminuta

1. The nuclear fraction of the rat tapeworm Hymenolepis diminuta (Cestoda) contains the enzyme adenosine diphosphoribosyl transferase (ADPR-transferase). 2. The enzyme catalyzes the postsynthetic modification of some nuclear proteins by the covalent attachment of the (ADP-ribose) moiety of NAD to such proteins. 3. The reaction is dependent on DNA which contains strand-breaks, and chain lengths equivalent to (ADP-ribose) is estimated. 4. The formation of polynucleotide products was competitively inhibited by 3-acetamidobezamide, with a Km of 125 microM. 5. The catalytic properties of ADPR-transferase in Hymenolepis diminuta are similar to those in T. brucei.

Leishmania mexicana amazonensis: ADP-ribosyltransferase antagonists specifically inhibit amastigote to promastigote differentiation

Leishmania mexicana amazonensis amastigotes were induced to differentiate by incubation at 27 C. Morphological transformation was studied both in untreated cultures and in cultures where DNA synthesis, and consequently the final stage in the production of promastigotes, was inhibited by hydroxyurea. 3-Methoxybenzamide and other antagonists of ADP-ribosyltransferase (ADPRT) specifically inhibited differentiation at a very early stage in both experimental systems. Cell proliferation (in the absence of hydroxyurea) was not inhibited by ADPRT antagonists--indeed greater multiplication of undifferentiated parasites was observed in the presence of these compounds. This indicated that the parasites were being diverted from differentiation to proliferation. Preincubation of the amastigotes with the ADPRT antagonists was required to produce this effect, providing further evidence that ADP-ribosylation of proteins is required for the initiation of differentiation in Leishmania.

Trypanosoma cruzi: differentiation to metacyclic trypomastigotes in the presence of ADP-ribosyltransferase inhibitors

The participation of ADP-ribosyltransferase in Trypanosoma cruzi differentiation to the metacyclic stage was evaluated by analyzing morphogenesis blockage by specific enzyme inhibitors: benzamide, 3-aminobenzamide, theophylline, and nicotinamide. In vitro assays showed a statistically significant reduction in the number of metacyclic forms only when any one of the four inhibitors was added during the period of interaction between epimastigote and Triatoma infestans intestinal homogenate or when present throughout the subsequent culture period in Grace's medium. When nicotinamide or benzamide was present during both interaction and culture period, morphogenesis was virtually abolished (less than or equal to 2%). In the in vivo assays, mice inoculated with parasites obtained from the insect vectors fed with trypomastigote-infected blood containing one of the four enzyme inhibitors developed lower parasitemias and showed longer survival in every case, compared with the respective controls. These findings suggest ADP-ribosyltransferase participation in T. cruzi differentiation both in vitro and in vivo.

Efficient introduction of plasmid DNA into Trypanosoma brucei and transcription of a transfected chimeric gene

Electroporation induces efficient transient transfection of Trypanosoma brucei, and the introduced DNA can be transcribed into RNA. When we delivered a high-voltage electric pulse to cells mixed with radiolabeled pBR322, approximately equal to 15% of the plasmid DNA was taken up by the parasites. When transfecting DNA contained a segment of T. brucei ribosomal DNA that included the 5' end of the rRNA gene, the introduced plasmid directed expression of RNA; this RNA expression was shown both by dot blots and by S1 nuclease protection assays carried out under conditions specific for probe hybridization to RNA. In the absence of the ribosomal region, analogous transcription did not occur. We optimized this trypanosomal expression system with regard to electric shock strength, concentration of input DNA, and incubation time after electric shock. This technique enabling specific trypanosome DNA expression in vivo should facilitate the molecular analysis of T. brucei gene expression.

Detection of adenosine diphosphate-ribosyl transferase activity in the filarial worm, Onchocerca volvulus

The existence of the nuclear enzyme ADP-ribosyl transferase in the filarial worm Onchocerca volvulus was demonstrated. The enzyme activity was observed in the nuclear preparation from the parasitic organism. Poly(ADP-ribose) was identified as the reaction product by the isolation of phosphoribosyl-AMP and 5'AMP as the major products of snake-venom phosphodiesterase digestion. The temperature and pH optima for the enzyme were 25 degrees C and 8.5, respectively. The apparent Km value exhibited by the substrate NAD+, is 750 microM and the activity of the enzyme is inhibited by four chemical classes of inhibitors, nicotinamides, methylxanthines, thymidine and aromatic amides.

Transient formation of DNA strand breaks during the induced differentiation of a human promyelocytic leukaemic cell line, HL-60

During the induced differentiation of the human promyelocytic leukaemic cell line, HL-60, along the myelocytic lineage, DNA strand-breaks are formed. These breaks which are formed in the face of a proficient DNA repair mechanism, are only transiently maintained and subsequently become religated. The ligation of these breaks requires the activity of the nuclear adenosine diphosphoribosyl transferase (ADPRT). Inhibition of nuclear ADPRT, an enzyme totally dependent on the presence of DNA strand-breaks for its activity and required for efficient DNA repair in eukaryotic cells, blocks the religation of these breaks but not their formation. The inhibition of DNA strand ligation in the differentiating HL-60 cells results in loss of viability and cell death.

Effect of 3-aminobenzamide on antigenic variation of Trypanosoma brucei

African trypanosomes, like Trypanosoma brucei, depend on antigenic variation to evade the immune response of the vertebrate host. An antigenic switch corresponds to the activation of a variable surface glycoprotein (VSG) gene from a large silent repertoire. Most switches require the duplicative transposition of a VSG gene, which involves strand breaks in DNA and subsequent repair. The nuclear enzyme adenosine-diphosphoribosyl transferase (ADPRT), which is dependent on the presence of DNA strand breaks for its activity, might be involved in this process because it has a regulatory role in DNA repair in all eukaryotic cells studied so far. In previous work, the presence of ADPRT activity was demonstrated in T. brucei. Moreover, it was also shown in isolated trypanosomes the ADPRT activity, which is stimulated by the induction of DNA strand breaks, could be blocked by the competitive inhibitor 3-aminobenzamide. Here we report experiments using rats which were infected with small numbers of T. brucei expressing VSG gene 118. After two days, the rats were coupled to a continuous intraperitoneal infusion system administrating 3-aminobenzamide in 0.9% NaCl (81.4 mM) at a rate of 0.65 ml/hr/rat for a period of up to five days. Control rats received only a 0.9% NaCl infusion. At days 1, 3 and 5, 250 microliters blood was obtained from a tail artery. Plasma 3-aminobenzamide was determined using a new high performance liquid chromatography method, developed for these experiments. In most rats the plasma concentrations were maintained between 0.8 and 1.2 mM. The rate of antigenic switching was determined by quantitating the fraction of trypanosomes that had lost their VSG 118 coat, using antibody against VSG 118 and a limiting dilution in mice. The average switching rate found was 2.0 X 10(-6) in controls and 1.3 X 10(-7) in drug-treated rats (15-fold reduction). This suggests that ADPRT is required for completing most antigenic switching events. We discuss the possibility that drug-resistant switching only involves non-duplicative VSG gene activation.