The promastigote surface antigen gene family of the Leishmania parasite: differential evolution by positive selection and recombination
© Devault and Bañuls; licensee BioMed Central Ltd. 2008
Received: 21 April 2008
Accepted: 24 October 2008
Published: 24 October 2008
PSA (promastigote surface antigen) is one of the major classes of membrane proteins present at the surface of the parasitic protozoan Leishmania. While it harbours leucine rich repeats, which are suggestive of its involvement in parasite-to-host physical interactions, its exact role is largely unknown. Furthermore, the extent of diversity of this gene family, both in copy number and sequence has not been established.
From the newly available complete genome sequences of L. major, L. infantum and L. braziliensis, we have established the complete list of PSA genes, based on the conservation of specific domain architecture. The latter includes an array of leucine rich repeats of unique signature flanked by conserved cysteine-rich domains. All PSA genes code either for secreted or membrane-anchored surface proteins. Besides the few previously identified PSA genes, which are shown here to be part of a relatively large subclass of PSA genes located on chromosome 12, this study identifies seven other PSA subtypes. The latter, whose genes lie on chromosomes 5, 9, 21 and 31 in all three species, form single gene (two genes in one instance) subfamilies, which phylogenetically cluster as highly related orthologs. On the other hand, genes found on chromosome 12 generally show high diversification, as reflected in greater sequence divergence between species, and in an extended set of divergent paralogs. Moreover, we show that the latter genes are submitted to strong positive selection. We also provide evidence that evolution of these genes is driven by intra- and intergenic recombination, thereby modulating the number of LRRs in protein and generating chimeric genes.
PSA is a Leishmania family of membrane-bound or secreted proteins, whose main signature consists in a specific LRR sequence. All PSA genes found in the genomes of three sequenced Leishmania species unambiguously distribute into eight subfamilies of orthologs. Seven of these are evolving relatively slowly and could correspond to basic functions related to parasite/host interactions. On the opposite, the other PSA gene class, which include all so far experimentally studied PSA genes, could be involved in more specialised adaptative functions.
Leishmania is a parasitic protozoan of the Trypanosomatidae family and is the pathogenic agent of leishmaniases in humans [1, 2]. These diseases are endemic in 88 countries on five continents, and cover a wide range of symptoms, from asymptomatic form and benign localised cutaneous lesions to mucocutaneous lesions and visceral outcome. While localised cutaneous lesions can be self-healing, the outcome of the other forms of the disease is more disabling, and the visceral forms are generally fatal if untreated by chemotherapy. The nature of these symptoms appears statistically associated with the parasite species. L. major and L. tropica mainly produce benign cutaneous forms, L. braziliensis, mucocutaneous forms and L. infantum and L.donovani generally cause visceral disorders. Leishmania undergoes two major developmental stages in its life cycle: one stage in an invertebrate host, the phlebotomine sand fly and one stage in a vertebrate host (wide range of hosts such as humans, canids, rodents, etc.). In the former, it proliferates in the sandfly gut as a flagellated form, promastigote, and later differentiates into the infectious metacyclic promastigote form while in vertebrate hosts it proliferates intracellularly as an a-flagellated form, amastigotes, after infection of macrophages. The parasite transits from one host to the other when the sandfly feeds by sucking the blood of the vertebrate host.
To understand host-parasite interactions at the molecular level, many studies have focused on the potential role of macromolecules present at the surface of the parasite. These include glycoinositolphospholipids (GIPL), lipophosphoglycan (LPG), as well as the membrane proteins proteophosphoglycan (PPG), MSP/GP63 endopeptidases, PSA (PSA-2/GP46) and amastins. Most of these membrane proteins are glycosylated and anchored via a covalent glycosylphosphatidylinositol (GPI) moiety . Some isoforms of PPG and PSA are also exported outside the parasite as soluble proteins [4, 5]. LPG plays an important role in binding to the phlebotomine midgut. Furthermore, in conjunction with the MSP proteins, it is involved in the first steps of the vertebrate host infection. They promote resistance to complement, binding to and internalisation in macrophages through the CR3 ligand and fibronectin receptors, and finally inhibition of the oxidative burst [6, 7]. GIPLs and the cysteine proteases CPB are involved in later stages of infection like protection against the nitric oxide production and modulation of the immune response .
PSA proteins have been detected in both promastigote and amastigote stages [9, 10]. However, the levels drastically increase from exponentially growing to stationary phase parasites, which suggests that PSA are strongly overexpressed in metacyclic promastigotes. This overexpression seems linked with the virulence status of the parasites since serial passaged in vitro cultured promastigotes with low virulence do not show this burst of PSA expression in stationary phase . This expression profile is consistent with the only known role for PSA: resistance to complement lysis . But other roles are possible, either in the late parasite-phlebotomine or metacyclic/amastigote-macrophage interactions. The most suggestive functional determinant in the PSA2 primary structure is the presence of leucine rich repeats (LRR). LRRs are primarily known to be involved in protein-protein and protein-glycolipids interactions. Interestingly, many known LRR containing proteins are involved in host-pathogen interactions. Yersinia invasin, Listeria internalins and Streptococci LrrG protein are surface proteins that play key roles in binding and internalisation of these bacteria in their mammalian hosts [12, 13]. Likewise, the mammalian Toll-like receptor and Nod families , and the plant NBS-LRR resistance proteins  are primary actors of the innate immunological system that recognize and bind proteins, glycolipids and nucleic acids of pathogens.
As for the other membrane proteins above, PSAs are encoded by multicopy genes. Up to now, only one or two PSA genes have been described in L. amazonensis , L. major  and L. infantum , and before the complete sequencing of L. major genome, the number of copies in a single species was largely unknown. Here we have taken advantage of the newly available complete genome sequences of the three species L. major, L. infantum and L. braziliensis [18, 19] to draw the precise structure of the PSA gene family. After analysis of the gene phylogeny, which might serve in the identification of distinct PSA functions, we have investigated for the presence of recombination and positive selection as driving forces acting on the evolution of this multigene family.
The PSAgene family
The main signature of PSA proteins is their specific 24–25 amino-acids LRR motif, of consensus sequence G(T/S)LPxxWxx (M/I)xxLxxLxLxxxx(x)(V/L/I)(S/T) (Fig. 1). The most striking feature of this motif relative to other LRRs is the presence of quasi invariant glycine, proline and tryptophane residues at positions 1, 4 and 7, respectively. This motif is not present in the predicted proteins from the fully sequenced genomes of two other trypanosomatidae, T. brucei and T. cruzi, and no clear homologs of PSA could be identified in these organisms. The only PSA domain for which homology was found in the other trypanosomatidae is the Thr/Ser-rich domain. The T(n)KP2 and T(n)EAPT repeats found in most of these domains are present in T. cruzi mucins. However, while it makes up the bulk of the mucin protein architecture, this domain is not always present in PSA proteins (see Fig. 2), and mucins do not contain any LRR repeat. Therefore, PSA proteins and mucins are likely to play different roles. Actually, the LRRs mostly similar to those of Leishmania PSA proteins belong to the plant NBS-LRR resistance proteins.
PSA genes are dispersed on chromosomes (Chr) 5, 9, 12, 21 and 31 for all three Leishmania species. Chromosomes 9 and 21 contain a single gene, while Chr 5 contains 2 genes, and Chr 31, 3 genes (4 in L. major). PSA genes localised on different chromosomes (and also those of the different Chr31 loci) are sufficiently divergent to define PSA gene classes (see below). Inside each chromosome, all PSA genes are clustered in tandem, except those of Chr 5 and gene LbrM31_V2.3700, which are well separated. The PSA subfamily present on Chr 12 is the most complex (except in L. braziliensis where there is only one gene). In L. major, the 24 Chr 12 PSA genes belong to a single cluster of tandem repeats, with many of these genes separated by a single non-related intervening gene. This pattern is also observed for the 7 Chr12 PSA genes of L. infantum. All PSA genes arranged in clusters have the same orientation relative to transcription. The only PSA genes studied so far in vivo belong to the Chr12 array. Although the PSA genes we have identified on the other chromosomes show significant sequence divergence with the Chr12 genes (see below), the conservation of the precise domain organisation between all these genes justifies their being included in the same unique PSA family.
Phylogeny of the PSAgene family
An alignment of all PSA protein sequences from all three Leishmania species was generated. For phylogenic analysis, only the domains present in all PSA proteins were included in the alignment. The Thr/Ser and Cys-rich/transmembrane C-terminal domains were thus discarded, as well as the central LRR domain (except for the last LRR motif of each sequence, which was retained). LRR units probably evolve not only by nucleotide substitution but also by birth and loss of repeats following recombination events. Therefore, alignments of rather distant LRR domains (with low similarity between repeats and varying number of repeats) may not reflect properly their evolution. This truncated alignment was transformed into its DNA coding version (Additional file 1), which was used to generate a phylogenetic tree by PhyML (Fig. 2). This tree shows the existence of eight PSA subfamilies which can be defined by their chromosome localisation: Chr5 (A and B), 9, 12, 21 and three subfamilies on Chr31 (A, B, C). Subfamilies Chr5A and B are the most distantly related and pair apart from all other clusters. Among the latter, only the Chr21/Chr31A subfamilies can be phylogenetically linked with high confidence bootstrap values. Each of the three Leishmania species has at least one gene in each of these eight subfamilies. The L. infantum gene orthologs always group with those of L. major, which is expected from the known species tree. The relatively short branch length separating orthologs in comparison to those separating the inparalogs (ancestors of the seven gene subfamilies) might suggest that speciation of the three Leishmania is a late event in the evolution history of the PSA genes. In keeping with this interpretation, a tree with similar relative branch lengths was inferred when using the rate of synonymous substitutions (dS), a more neutral distance metric (not shown).
Chr5A, B, 9, 21, 31A, B, C subfamilies of PSAgenes
While the Chr12 subfamily is most diversified, having very different numbers (1 to 24) of rather distantly related paralogs in the three species, the PSA genes lying on the other chromosomes form very simple clusters. For each of the latter subfamilies, each species contains a single gene (except L. major, which has two highly similar paralogs for the Chr31A cluster) and the orthologs or pseudo-orthologs are quite similar (median = 75% amino acid identity for all three species and 93% for L. infantum/L. major orthologs). This degree of homology between orthologs, which was calculated on the same domains used above for phylogenetic tree determination, was also observed over the entire LRR domain. Indeed, confident alignments covering this domain can be obtained inside each of the Chr5A and B, 9, 21, 31A, B, and C clusters since the number of LRRs differ at most by one inside each subfamily and the similarity between orthologs is (median = 80% amino acid identity for all three species and 96% for L. infantum/L. major orthologs) (see Additional files 2, 3, 4, 5, 6, 7, 8 for sequence alignments). On the other hand, similarity between genes of different subfamilies is rather low over this domain (median = 32% amino acid identity, and much difference in number of repeats). This may suggest that the LRR domains characteristic of each cluster represent specific binding properties, which could confer specialized roles to their respective PSA proteins. Another link between these subfamilies and functional specialisation is given by the nature of the C-terminal domain. Indeed, all members of the same subfamily share the same membrane anchorage/secretion determinant. In particular, all genes of the phylogenetically linked Chr21 and 31A subgroups are predicted to code for secreted proteins (Fig. 2).
Chr12 subfamily of PSAgenes
Recombination in PSAgenes
More profound rearrangements can be created by intergenic recombinations (both at coding and non-coding sites), which lead to whole gene duplication and deletions. One such rearrangement can be traced in the L. major Chr12 PSA gene cluster, where PSA genes are organised in tandem, tough often separated by one of two unrelated genes of type A or B. The distribution of these intervening genes together with the phylogenic signature of each PSA genes suggest that the two pairs of very similar paralogs LmjF12.0810/LmjF12.0940 and LmjF12.0830/LmjF12.0960 were created by a same segmental duplication involving a bloc of five genes (see Additional file 10).
Positive selection of PSAgenes
3' Intergenic region
Expression of some PSA genes of Chr12 has been shown to be stage specific. Strong increases in the levels of both mRNA and protein were observed during metacyclogenesis in vitro . This regulation was shown to depend on the 3' intergenic region (IR) of the PSA gene . The question arises, then, as to whether the different classes of PSA might be regulated the same way. We therefore wished to compare the phylogeny of the 3' (IR) to that obtained above for the coding sequence. Around 2 kb of the 3' IR of PSA genes from all three Leishmania species (with few exceptions for L. infantum and L. braziliensis due to gaps in their sequences) were analysed by local BLAST and aligned by Dialign2. We found that the 3' IRs are conserved between all orthologs (and pseudo-orthologs for he Chr12 clade), but show no significant homology between genes of the 8 different PSA clades (05, 09, 12, 21, 31A, B, C). While the similarities found for pairwise comparisons involving L. braziliensis sequences are variable and relatively low, those of the other comparisons are high. The L. infantum/L. major orthologs of clades 5, 9, 21, 31A, B, C are equally well conserved (ranging from 87 to 92% identity; see Additional file 11 for sequence alignments), as are the set of pseudo-orthologs and paralogs of clade 12 (median = 95% identity; see Additional file 12 for sequence alignment). Yet, the alignment of the 3' IR of these Chr12 PSA genes is quite mosaic, showing large blocks of deletions/insertions. Phylogenetic analysis of an alignment retaining 1.2 kb of domains conserved in at least 70% of the genes revealed that Chr12 genes cluster according to the species (not shown). This is reminiscent of the extremities of the coding regions (Fig. 2). Functional studies will be necessary to determine if this high degree of conservation is the result of gene conversion or rather, of purifying selection driven by imperatives of gene expression. In any case, if indeed the 3' IR of the different PSA clades play a role in the control of gene expression, either the pattern of expression is variable between the clades or the cues governing this control are subtler than the primary sequence per se.
Discussion and Conclusion
Thanks to the whole genome sequencing of three Leishmania species, we could draw the complete picture of the PSA gene family. Up to now, only a few genes belonging to the Chr12 subfamily had been characterised. Also, L. braziliensis was thought not to contain any gene for that subfamily. We have proposed a nomenclature for all PSA genes (see Fig. 2 and 3). We have kept the PSA letters, even tough it is now known that these genes are expressed both in promastigote and amastigote stages and code not only for membrane proteins at the surface of the parasite but also for excreted soluble proteins. Follow the chromosome number and other symbols that refer to the phylogenetic PSA gene clusters.
PSAs are basically LRR repeats containing proteins, which are either membrane-bound at the surface of Leishmania or soluble as excreted proteins. These features suggest that this protein family could be involved in host-parasite interactions. The diversity found in PSAs, which concerns mainly the LRR domain, would be consistent with this role. This diversity is not accounted only by the sequence divergence of the LRR repeats but also by the number of these repeats present in each protein, which varies from 2 to 25 and could influence both the shape and the space occupied by PSAs, and thereby their binding properties. Two other factors of diversity in PSA reside outside the LRR domain. One concerns the Thr/Ser-rich domain, whose length varies considerably and can be totally absent in some PSAs. Although there is some indication that PSAs could be glycosylated [24, 25], both the sites of glycosylation and the nature of the amino acid-glycan linkage are unknown. Most of these Thr/Ser-rich domains contain a signature present in T. cruzi mucins which is known to be the site of O-glycosylation. Finally, membrane attachment determinants (which result mostly in anchoring through GPI) define two destinations for PSA proteins: excretion or attachment to the parasite's surface. Although the majority are predicted to be membrane bound, PSAs predicted to be secreted are found both as strongly conserved proteins between species (PSA21 in all three species; LiPSA12A and LmPSA12A) and as species specific proteins (LiPSA12I and most LmPSA12C). This suggests that the presence of these C-terminal truncated forms of PSAs is neither fortuitous nor the products of pseudogenes, but that secretion plays a fundamental role in Leishmania.
PSA genes distribute into eight subfamilies. In view of the found orthologies, the most parsimonious scenario would be that the last common ancestor (LCA) of L. major, L. infantum and L. braziliensis had nine genes: one in each of chromosomes 9, 12 and 21, two on chromosome 5 and three on chromosome 31. Among the PSA subfamilies, the Chr12 subfamily is the most diversified one, at least in L. major and L. infantum. This is brought about by nucleotide substitutions as well as unequal gene recombination, which modulates both the number of LRRs in PSA proteins and the number of genes in the family, and generates chimeric genes. The evolution of the Chr12 subfamily of PSA genes is probably best explained by a birth-and-loss process, by which the gene family is either expanded or contracted, while PSA genes are individually submitted to diversifying selection. We have detected clear evidence of positive selection for the Chr12 genes in both L. major and L. infantum, while the existence of orthologs, divergent paralogs, as well as probable pseudogenes (LmjF12.1005, LinJ12_v4.0662) all point to a birth-and-loss model. We note, however, that the extremities of the coding regions and the 3' IR of the Chr12 PSA genes are all species specific (no orthologies can be inferred) and remarkably similar (some stretches are even devoid of synonymous substitutions), suggesting that some gene homogenisation might have taken place by gene conversion. In contrast to the Chr12 subfamily, the seven other subfamilies seem to evolve in a more conservative way. Not only all genes of Chr5, 9, 21 and 31 from the three Leishmania species cluster as more similar orthologs but evidence of recombination is scarce. One finds only one pair of paralogs (the L. major Chr31A subfamily), and the number of LRR repeats is the same for each group of orthologs (except one difference in Chr21 subfamily). Furthermore, diversifying selection apparently does not act on these genes, as evidenced by dN/dS values well under those obtained for the Chr12 genes. We would therefore postulate the latter subfamilies to be involved in basic slowly evolving host-parasite interactions, whereas the Chr12 subfamily could be linked to more dynamic adaptative responses like optimisation of cellular infection (internalisation and maintenance in the macrophage) and escape from the immune system. The latter could also be related to the Leishmania species identities like the nature of the sand fly species infected and the clinical symptoms. Still, diversification of Chr12 genes apparently does not offer any advantage for L. braziliensis. At this stage, it is hard to find out what kind of adaptative role these genes could play in L. major and L. infantum, which would be dispensable for L. braziliensis.
Diversifying selection has been shown to drive evolution of genes involved in host-pathogen interactions. These include virulence genes, as found in Streptococci  and Listeria monocytogenes , as well as resistance genes, like the numerous plant NBS-LRR genes , the primate Trim5α  genes involved in restriction of retroviruses replication and many gene classes involved in the Drosophila innate immune system . Interestingly, the Listeria virulence gene inlA and the NBS-LRR genes possess LRR domains, which were shown to evolve under positive selection. In the present work, we found that in the Chr12 PSA gene subfamily of two Leishmania species, LRR domains evolve under strong positive selection. Most notably, the several residues submitted to diversifying selection are predicted to reside at the protein's surface. The importance of this evolving LRR protein interface in the parasite's life cycle awaits the determination of the precise PSA biological function and the identification of eventual host PSA interactors.
In conclusion, we have identified eight PSA subfamilies. The phylogenetic distance separating them, which is paralleled by the divergence of their LRR domains, appears large enough to reflect functional specificities. The strong conservation of the orthologs of the subfamilies 5A, 5B, 9, 21, 31A, B and C suggest that they play essential roles. Generation of knock outs and gene expression analysis of all individual PSA genes will be needed to explore the importance of these genes in the different stages of the life cycle of Leishmania, and eventually in adaptation of the parasites to their specific insect or mammalian host. In comparison to the seven subfamilies above, the PSA12 subfamily is evolving much more rapidly, (at least in L. major and L. infantum) as revealed by the high number of paralogs and the presence of strong diversifying selection. Sequencing of these genes in several strains of L. major and L. infantum is expected to show a high incidence of polymorphism.
Alignment of PSA amino acid and nucleotide sequences and identification of membrane anchorage determinants
PSA protein and DNA sequences were extracted from the L. major, L. infantum and L. braziliensis database http://www.genedb.org/ after BLAST analysis. The latest versions of the genome sequences were used, i.e. 5.2 for L. major, 2.0 for L. braziliensis and 3.0 (and 4.0 for chromosome 12) for L. infantum. Protein sequences were aligned with ClustalX 2.0  followed by manual correction. These alignments were transposed to the corresponding genomic DNA alignments by protal2dna http://mobyle.pasteur.fr/cgi-bin/MobylePortal/portal.py?form=tranalign. The presence of signal peptide and transmembrane domains was reported as found in annotations of the Leishmania genomes http://www.genedb.org/. The prediction of GPI-anchorage was performed by GPI-SOM http://gpi.unibe.ch/. Genomic sequences flanking the PSA ORFs in 3' were aligned with Dialign 2.2.1 .
Phylogenetic trees of Fig. 2 and 3 were obtained from nucleotide sequence alignments by maximum likelihood (PhyML v2.4.4) . The selection of the nucleotide substitution model (and eventually of amino acids substitution) was determined by ModelGenerator http://distributed.cs.nuim.ie/multiphyl.php. The HKY+I+G (Fig. 2) and TN93 +G+I (Fig. 3) nucleotide models were used, with the base frequency, Ts/Tv ratio, and gamma distribution parameter estimated by PhyML. These trees were compared to those obtained by PhyML from the corresponding amino acid sequences (WAG+G model of amino acids substitution) or with a distance method implemented in MEGA v3.1  using both DNA (TN93) or amino acids (JTT model) alignments. Occasionally (see text in Results section), a codon model (dS, number of synonymous substitutions per synonymous sites; Nei-Gojobori method, JC model) as implemented in MEGA was used to calculate distance matrix. Trees were annotated and visualised by Treedyn (v194.3) .
One level of positive selection analysis involved calculation of dN/dS ratios between pairs of sequences (MEGA, Nei-Gojobori method, JC model). ω = dN/dS < 1 corresponds to purifying (negative) selection, ω = 1 to neutral evolution (absence of selection), and ω > 1 indicating adaptive evolution (positive selection). The maximum likelihood based method implemented in the codeml program of the PAML package (v3.15)  was used to detect positive selection as well as identifying positively selected sites, when the number of related sequences was sufficiently high. To detect positive selection, we performed likelihood ratio test (LRT) analysis between three pairs of models to determine whether particular models provided a significantly better fit to the data: M1a ("Nearly-Neutral" model) vs. M2a ("PositiveSelection" model), M7 ("beta" model) vs. M8 ("beta&ω" model) and M8a vs. M8. Only models M2a and M8 allow for site categories evolving under positive selection (see Yang et al. 2005  for further details on the models). Positive selection is inferred when the (2Δλ) statistic is greater than critical values 1) of the Chi square distribution for a degree of freedom of 2 (M1a/M2a. and M7/M8 comparisons) or 2) of a 50:50 mixture of point mass 0 and Chi square (critical values are 2.71 at 5% and 5.41 at 1%; M8a/M8 comparison). Only predicted positively selected sites with posterior BEB (Bayes empirical Bayes) probabilities greater than 0.95 or 0.90 were retained.
We thank F.Chevenet for advice in running the Treedyn software, A. Kajava for structure modelling of PSA LRRs and E. Desmarais for critical reading of the manuscript.
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