Proline racemases: insights into Trypanosoma cruzi peptides containing D-proline

Proline racemases: insights into Trypanosoma cruzi peptides containing D-proline

Nicolas Coatnoan; Armand Berneman; Nathalie Chamond; Paola Minoprio⁺

Institut Pasteur, Laboratoire d'Immunobiologie des Infections à Trypanosoma, Département d'Immunologie, 25 Rue du Dr. Roux, 75724 Paris, Cedex 15, France

ABSTRACT

Trypanosoma cruzi proline racemases (TcPRAC) are homodimeric enzymes that interconvert the L and D-enantiomers of proline. At least two paralogous copies of proline racemase (PR) genes are present per parasite haploid genome and they are differentially expressed during T. cruzi development. Non-infective epimastigote forms that overexpress PR genes differentiate more readily into metacyclic infective forms that are more invasive to host cells, indicating that PR participates in mechanisms of virulence acquisition. Using a combination of biochemical and enzymatic methods, we show here that, in addition to free D-amino acids, non-infective epimastigote and infective metacyclic parasite extracts possess peptides composed notably of D-proline. The relative contribution of TcPRAC to D-proline availability and its further assembly into peptides was estimated through the use of wild-type parasites and parasites over-expressing TcPRAC genes. Our data suggest that D-proline-bearing peptides, similarly to the mucopeptide layer of bacterial cell walls, may be of benefit to T. cruzi by providing resistance against host proteolytic mechanisms.

Key words: proline racemase - TcPRAC - D-proline

The role of D-amino acids (D-AA) in physiopathological processes has occupied the attention of several groups for many years. Although believed to be "unnatural," D-AA residues are present in both prokaryotes and eukaryotes, in various organelles and tissues (Fisher 2007). Recent studies have shown that free D-AAs play relevant roles in various phisiological processes in higher organisms, including ontogeny, differentiation, neuronal and hormonal secretion, and even aging (Collingridge 1987, Fujii 2005, D'Aniello 2007). Interestingly, specific levels of D-AAs found in the human brain, cerebrospinal fluid and serum have been implicated with physiopathological processes, including schizophrenia, epilepsy, Alzheimer's brain degeneration and renal diseases (Nagata et al. 1987, Fisher et al. 1998, Fuchs et al. 2005).

Amino acid racemases (AAR), enzymes that reversibly catalyse the interconversion of free L-AAs and D-AAs, were until recently found only in prokaryotes and were primarily associated with the synthesis of bacterial cell layer components that operate as natural defences against proteolytic mechanisms set in motion by hosts (Walsh 1989, Caceres et al. 1997, Thompson et al. 1998). Accordingly, bacterial cell wall assembly requires complex biosynthetic processes that utilise both L-AAs and D-AAs to provide an envelope composed of peptidoglycans, teichoic acid, polysaccharides and proteins (Marraffini et al. 2006). For instance, D-alanine is required for mucopeptide synthesis and growth in most bacteria and it has been implicated with the acquisition of resistance to host-induced humoral and cellular defences against Listeria monocytogenes and Staphylococcus spp. (MacArthur & Archibald 1984, Thompson et al. 1998). The genetic modification of AAR and transferase genes has produced attenuated bacterial mutants that are unable to grow in standard in vitro and in vivo conditions and that exhibit high susceptibility to host defensins, protegrines, anti-bacterial peptides and cytotoxic T cell responses (Caceres et al. 1997, Peschel et al. 1999). More complex peptides bearing D-AAs were isolated from invertebrates and vertebrates, such as frog dermophin, crustacean and cephalopod neurohormones and cyanobacteria hepatotoxins (Montecucchi et al. 1981, Richter et al. 1987, Mignogna et al. 1998).

Proline racemase, a dimeric enzyme, was firstly isolated from the Clostridium sticklandii protobacterium by Cardinale and Abeles, in 1968. A eukaryotic proline racemase was first isolated and characterised from the human pathogenic parasite Trypanosoma cruzi (TcPRAC) (Reina-San-Martin et al. 2000), and more recently the enzyme has been found in other pathogens (Goytia et al. 2007, Chamond et al. 2009). The identification of intra-cellular and secreted isoforms of TcPRAC brought up the question of its role in parasite biology. Several hypotheses have been raised, such as that the enzyme may participate in the mechanisms leading to parasite differentiation by regulating intracellular proline metabolic pathways through the conversion of free L-proline (L-pro) internalised from the vector digestive tract (Chamond et al. 2003, 2005). It has also been hypothesised that TcPRAC contributes to the parasite's evasion strategies by making D-proline (D-pro) available for post-translational addition into peptide chains (Reina-San-Martin et al. 2000). This hypothesis is appealing since the incorporation of D-AA residues into peptide sequences would make them less immunogenic than those exclusively composed of L-analogues (Sela & Zisman 1997). In this regard, it has been reported that the addition of a single D-AA in the N-terminus of a protein is sufficient to confer general resistance to host proteolytic enzymes (Janeway & Humphrey 1970). Therefore, the incorporation of D-pro into expressed T. cruzi proteins could benefit the parasite by contributing to the initial lack of specificity of host B-cell responses and by improving parasite resistance to host cellular and extracellular proteases.

We used biochemical and enzymatic approaches to verify the presence of D-AAs, and more precisely that of D-pro, in extracts of epimastigote (EPI) and metacyclic (META) forms of T. cruzi. Soluble (SF) and non soluble (NSF) parasite fractions were prepared, subjected to successive digestions with endoproteases and amino and carboxy- peptidases, and further fractionated by preparative thin-layer chromatography. All positive layer sections that were deduced from ninhydrin-sensitive borders of the plate were scrapped off, resuspended in water and tested for the presence of D-AAs. D-amino acid oxidase (D-AAOx-) positive fractions were further separated by thin-layer chromatography using chiral plates to separate free AA stereoisomers and small peptides. When compared to simultaneously run L and D-AA standards, the ninhydrin-stained plates containing the extracts from the EPI and META forms (Fig. 1A, B) illustrate the resolution of the method and the chiral separation of the extracts. First, with regard to the spots on the NSF chromatograms, the major differences observed between the EPI and META forms were related to the migration path fingerprint. It can be observed that after the drastic physical and proteolytic treatment of the samples, the resulting insoluble material contains proteins that were not degraded (stained in red) as well as non-protein polymers that do not react with ninhydrin generating a white background path. These macromolecules that interfere with the migration of the samples may explain why some components are carried farther up the plates than others. In addition, varying polarities of the molecules composing the EPI and META NSFs may also explain the varying strength of adsorption to the absorbent and the observed migration profiles. The data also show that most of the EPI and META extract proteins were digested and their ensuing contents are thus found in SF fractions. However, although qualitative, the technique highlights increasing amounts of material in the chromatograms from META SFs as compared to those from EPI SFs. Duplicates of the plates were then stained with isatin (1H-indole-2,3-dione), a proline-specific dye, and this clearly revealed the presence of both D-pro and L-pro in the SFs (Fig. 1C, D). Interestingly, spots corresponding to D and L-pro were more intense in META SFs than in EPI SFs.

In an attempt to quantitate the presence of D-AA bound to peptide chains, all the fractions were assayed for D-AA content throughout all stages of fractionation. Since EPI and META forms differ in size, nucleic acid and protein contents, data were adjusted to the water volume corresponding to 10⁸ parasites, as described by Rohloff, in 2003 (Table). To assess the impact of proline uptake on the availability of substrate for the intracellular isoforms of TcPRAC, the experiments used EPIs conventionally grown in LIT medium and METAs that had issued from EPIs differentiating in TAU medium supplemented with 10 mM (1X) or 30 mM (3X) L-pro. The Table depicts the total D-AA concentration obtained from the EPI and META forms. The total concentration of putative D-AAs bound to polypeptides or to small peptides was deduced from the total amount of D-AAs obtained before and after the enzymatic processing of the samples. The data from samples analysed before any protease treatment revealed that extracts from META parasites possess three times more free D-AAs than EPIs. Moreover, a two-fold greater D-AA concentration was observed in the samples from META extracts (0.31 mM) as compared to EPI extracts (0.15 mM) after biochemical fractionation, indicating the presence of D-AAs bound to peptide chains. Interestingly, while extracts from META parasites generated in 3X L-pro-containing medium showed modestly elevated free D-AA content (4.10 mM) relative to 1X-derived METAs (3.96 mM), they yielded five times more bound D-AAs (0.83 mM) than EPIs. Although the DAAOx test cannot discriminate different D-AAs, intracellular increases in D-pro resulting from racemisation of L-pro by TcPRAC may well account for these differences. Nevertheless, the results obtained suggest that the concentration of free D-AAs and in particular that of D-AAs bound to small peptides or to larger polypeptides, increases during metacyclogenesis.

In order to gain further insights into the participation of TcPRAC isoforms in the biosynthesis of D-Pro, experiments were conducted to compare wild-type (WT) parasites to mutants overexpressing different TcPRAC genes (Chamond et al. 2005). Thus, the paralogous genes TcPRACA (A⁺) and TcPRACB (B⁺⁺) were overexpressed in non-infective EPI forms using appropriate vectors to obtain stable chromosomal integration of these genes in sense and antisense orientations. It is worth noting that non-infective forms of the parasite expressing full length antisense TcPRACB RNA [functional knockdown (KD)] are not viable, whereas functional TcPRACA-KD (A^-) EPIs survive only poorly even under low selection pressure for recombinant parasites. Consistent with our hypothesis, extracts from parasites overexpressing TcPRAC genes display more D-AA-containing peptides than WT or KD controls after physical and biochemical fractionation. Specifically, the META/EPI D-AA ratio obtained from parasites that developed in higher concentrations of proline (~5.5) was greater than the META/EPI DAA ratio from WT (~2). Furthermore, parasites overexpressing the intracytoplasmic isoform of proline racemase (B⁺⁺) that had differentiated in 1X or 3X proline conditions displayed higher META/EPI D-AA ratios than the respective WT controls. Interestingly, although they present a relative increase in META/EPI D-AA ratios, overexpressors of the secreted version of TcPRAC (A⁺) show lower levels of D-AAs than WT or B⁺⁺ parasites. Since TcPRACA transcripts appear to be more highly expressed at the end of metacyclogenesis (Fig. 3), it is tempting to consider that racemisation of intracellular proline during this stage of development instead depends on the cytoplasmic version of the enzyme (TcPRACB). Fig. 2B shows preliminary data from chiral thin layer chromatography (TLC) plates of extracts of two and three SF fractions obtained from METAs overexpressing TcPRACA (A⁺) and TcPRACB (B⁺⁺), respectively, compared to WT parasites. Given that the plates were developed with isatin, which specifically stains proline, these experiments demonstrate that in addition to free D-AAs, META extracts submitted to successive proteolytic treatments specifically contain elevated amounts of D- proline bound to small peptides or polypeptide chains. Corroborating the above hypothesis, although the methodology used is not quantitative, SF fractions of B⁺⁺ META mutants extracted from two zones of the TLC chromatogram exhibit considerable amounts of D-pro as compared to the amount observed in equivalent protein amounts of WT META SF extracts (Fig. 2).

The presence of D-AA in (poly) peptide chains has been credited to various strategies, including replacement or addition of stereoisomers in peptides recently synthesised by ribosomes, post-translational conversion of non-chiral D-residues by amino acid synthetases, and polymerisation of monomers in an assembly-line-like biosynthesis by non-ribosomal peptide synthetases (Xie & van der Donk 2004, Jilek et al. 2005). The pharmacokinetic advantages of synthetic peptides and their extended half-life and improved stability and resistance to proteases (Janeway & Humphrey 1970, Thompson et al. 1998) have generated considerable interest. Moreover, data are accumulating on the differential immunogenic properties of D-enantiomers and the poor ability of D-polymers to activate B lymphocyte responses (Sela & Zisman 1997). Some examples include the induction of immunological paralysis following the injection of multichain polypeptides built exclusively of D-residues and the discrimination and stereospecificity of antibodies raised against peptides whose sequences are composed of L or D-AAs. The origin of free D-AAs is less clear than that of protein bound D-AAs. However, the recent discovery of a functional proline racemase in T. cruzi and other important pathogens (Goytia et al. 2007, Chamond et al. 2009) has uncovered endogenous and intracellular means of D-pro production, raising the question of its specific functions. Moreover, the identification in parasite extracts of D-AAs, and more specifically of D-pro, may reflect cell adaptive responses and point to more general mechanisms for the regulation of protein and peptide expression by D-AA racemases. Since T. cruzi needs TcPRAC activity for metacyclogenesis, as suggested by our previous data (Chamond et al. 2005), the enzyme could be involved in parasite metabolic processes leading to the post-translational modification of recently synthesised proteins or peptides. It is also possible that the parasite uses racemisation to synthesise and/or express new peptides bearing D-pro to avoid or reduce host-induced proteolysis. The identities of the parasite proteins bearing D-pro should be investigated, as these may reveal attractive targets involved in parasite evasion.

ACKNOWLEDGEMENTS

To A. Cosson, for handling some cultures.

Received 10 March 2009

Accepted 8 June 2009

Financial support: Institut Pasteur Fonds dédiés n 17 (Sanofi-Aventis, Ministère de la Recherche)

NCo and AB contributed equally to this work.

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Publication Dates

• Publication in this collection
  21 July 2009
• Date of issue
  July 2009

History

• Received
  10 Mar 2009
• Accepted
  08 June 2009