A comparative study of small RNAs in Toxoplasma gondii of distinct genotypes
© Wang et al.; licensee BioMed Central Ltd. 2012
Received: 26 July 2012
Accepted: 31 August 2012
Published: 3 September 2012
Toxoplasma gondii is an intracellular parasite with a significant impact on human health. Inside the mammalian and avian hosts, the parasite can undergo rapid development or remain inactive in the cysts. The mechanism that regulates parasite proliferation has not been fully understood. Small noncoding RNAs (sncRNA) such as microRNAs (miRNAs) are endogenous regulatory factors that can modulate cell differentiation and development. It is anticipated that hundreds of miRNAs regulate the expression of thousands of genes in a single organism. SncRNAs have been identified in T. gondii, however the profiles of sncRNAs expression and their potential regulatory function in parasites of distinct genotypes has largely been unknown.
The transcription profiles of miRNAs in the two genetically distinct strains, RH and ME49, of T. gondii were investigated and compared by a high-through-put RNA sequencing technique and systematic bioinformatics analysis. The expression of some of the miRNAs was confirmed by Northern blot analysis.
1,083,320 unique sequences were obtained. Of which, 17 conserved miRNAs related to 2 metazoan miRNA families and 339 novel miRNAs were identified. A total of 175 miRNAs showed strain-specific expression, of which 155 miRNAs were up-regulated in RH strain and 20 miRNAs were up-regulated in ME49 strain. Strain-specific expression of miRNAs in T. gondii could be due to activation of specific genes at different genomic loci or due to arm-switching of the same pre-miRNA duplex.
Evidence for the differential expression of miRNAs in the two genetically distinct strains of T. gondii has been identified and defined. MiRNAs of T. gondii are more species-specific as compared to other organisms, which can be developed as diagnostic biomarkers for toxoplasmosis. The data also provide a framework for future studies on RNAi-dependent regulatory mechanisms in the zoonotic parasite.
Toxoplasma gondii is an obligatory intracellular parasite that infects a wide range of hosts,including humans, animals and birds. It is considered to be one of the most widely distributed protozoan parasite with a sero-prevalence in humans of up to 30% worldwide . T. gondii is the etiological agent of toxoplasmosis which can be either life-threatening or long-term chronic infection.
The life cycle of Toxoplasma gondii is unusual in that the parasite is capable of indefinite proliferation in the hosts with either a sexual or an asexual cycle. The sexual cycle occurs only in the hosts of a feline species. The asexual cycle can occur in virtually any warm-blooded animals, which act as the intermediate hosts, ranging from chicken to baleen whales and humans. In the parasite’s life cycle, there are three fundamental morphotypes, named tachyzoites, bradyzoites and sporozoites. The development of T. gondii in the intermediate host involves an initial phase with a rapid proliferation of the tachyzoites, followed by the formation of tissue cysts containing slowly dividing or resting bradyzoites.
While the global population structure of T. gondii awaits further elucidation , three clonal lineages (Type I, II, and III) of T. gondii which comprise the majority of strains in both North America and Europe [3–5]. Recently, a fourth clonal lineage, designated haplogroup 12, has been identified based on isolates that are common in wild animals in the United States . The virulence of T. gondii is normally defined based on the LD50 in mice. Type I strain has been regarded as a more virulent strain in mice with an LD50 <10 [4, 7]. In contrast, type II (e.g., ME49) and type III strains are less lethal in mice (LD50 > 100) and the infections are usually less severe or asymptomatic . The clonal lineages also differ in a number of phenotypes such as growth rate, efficiency in migration and transmigration in tissues [7, 9].
Several studies have been carried out with aims to determine differential gene expression at various lifecycle stages of T. gondii[10–13]. Likewise, efforts were taken to reveal the factors that may modulate the virulence of T. gondii. ROP5, ROP16 and ROP18, which are rhoptry-derived factors, and their expression was found to correlate with parasite virulence [14–18]. Identification of strain-specific regulatory elements responsible for the distinct genotype will not only facilitate our understanding of parasite biology, but may also elucidate the association between genetically defined subpopulations (strains) and disease severity.
In recent years, the discovery of numerous small RNAs has increased our knowledge in post-transcriptional gene regulation in cell development and other biological processes. Small RNAs, such as microRNA (miRNA), small interfering RNA (siRNA), and Piwi-associated RNA (piRNA), are regulatory elements that can modulate gene expression at the post-transcriptional level. MiRNAs, a class of ≈ 22 nucleotide small-RNA sequences that participate in the post-transcriptional regulation of gene expression , have been known to play critical roles in diverse biological processes, including development, viral defense, metabolism, and apoptosis [20–23]. The primary transcripts (pri-miRNAs) of miRNA genes are generated by either RNA polymerase II  or RNA polymerase III . A single pri-miRNA may contain from one to six miRNA precursors. They are processed by RNase complexes (Drosha and DGCR8) into ≈ 70 nucleotide fragments with a stem-loop structure called pre-miRNA. The pre-miRNAs are then exported to the cytoplasm by Exportin-5  and the hairpin is cleaved by the RNase III enzyme (Dicer) to miRNA duplexes . A single miRNA can silence a number of genes while one gene can be targeted by several miRNAs [28, 29]. Previous studies have found that T. gondii has a complete machinery for small RNA generation and small RNA-mediated gene regulation . There are mainly two classes of small regulatory RNAs derived from the T. gondii genome, namely miRNA and rdsRNA. And it was found that rdsRNAs were consistently more abundant in the highly virulent Toxoplasma isotype-I (RH) than in other two strains (RPU and CTG). Further, it was recently reported that the Argonaute of T. gondii (TgAgo) is methylated and its activity is Mg2+-dependent . However the differential expression of miRNA in parasites of distinct genotypes has not been well investigated.
In this study, we systematically studied the expression of miRNAs in type I (RH) and type II strain (ME49) T. gondii by high-through-put RNA sequencing technology and bioinformatic analysis. A number of T. gondii specific miRNAs was identified. Meanwhile, differentially expressed miRNAs between the two strains of T. gondii were detected. The results demonstrated profound differences between the two strains of T. gondii in miRNA expression.
Tachyzoites of T. gondii RH strain and ME49 strain were propagated in mice. The tachyzoites were purified by density-gradient centrifugation on Percoll . The study of using laboratory animals was reviewed and approved by the Ethical Committee of Jilin University.
Total RNA of T. gondii (both RH and ME49 strain) was prepared using Trizol reagent (Invitrogen, SF, USA) according to the manufacturer’s protocol. The integrity of total RNA was examined by standard agarose gel electrophoresis, and RNA purity was reflected by the 260/280 nm absorbance ratio and the concentration was determined using a Nanodrop 1000 machine (Thermo Scientific CA, USA). The purified total RNA was stored at-80°C until use.
Construction of small RNA libraries and sequencing
For small RNA library construction and deep sequencing, RNA samples from RH and ME49 strains of T. gondii were prepared as follows: for each strain, equal quantities (10 μg) of RNA isolated from tachyzoites were pooled. Small RNA molecules in the range of 18–30 nt RNA was purified after polyacrylamide gel electrophoresis (PAGE) and ligated with proprietary adapters to the 5′ and 3′ termini. The samples were used as templates for cDNA synthesis. The cDNA was amplified to produce sequencing libraries which were subjected to Solexa’s sequencing-by-synthesis method. Sequencing was carried out at the Beijing Genomics Institute (BGI, China). Two separated runs with two batches of RNAs were carried out in the sequencing. Only sequences with high quality were included in the analysis.
Mapping sequence reads to the reference genome
Individual sequence reads with the base quality scores were produced by Solexa. Clean reads were obtained after removing of the low quality reads, such as adaptor null reads, insert null reads, 5′ adaptor contaminants, and reads with polyA tail and ambiguous bases. Adapter sequences were trimmed from both ends of clean reads before analysis. All identical sequences were counted and eliminated from the initial data set. The resulting set of unique sequences with associated read counts was referred to as sequence tags. The unique reads were mapped onto the T. gondii genome (http://www.toxodb.org) using the program SOAP (http://soap.genomics.org.cn) .
Bioinformatic analysis of T. gondii small RNAs
Sequences were, at the first step, searched against Metazoa mature miRNA of Sanger miRBase allowing two mismatches to identify homologs of known Metazoa miRNAs using the program Patscan . Sequence tags of more than 5 reads that matched perfectly or near-perfectly (no more than 2 mismatches and mismatch not positioned in seed region) to metazoan mature miRNAs were assumed to be conserved miRNA candidates. Secondly, the remaining sequences were screened against the non-coding RNA database  and the T. gondii genome database, and some non-coding RNAs such as rRNA, tRNA and snoRNAs were identified and filtered out for further analysis. Thirdly, We searched for the inverted repeats (step loops or hairpin structure) among the remained sequences which did not match to the miRNA database by using the software Einverted of Emboss  with the parameters following parameters: threshold = 30, match score = 3, mismatch score = 3, gap penalty = 6, and maximum repeat length = 240 as described . Each inverted repeat was extended 10 nt on each side, and the secondary structure of the inverted repeat was predicted by RNAfold . Unique reads in the inverted repeats were evaluated by MirCheck  using modified parameters that are more suitable for organisms such as worms and protozoa. Finally, miRNA precursors that passed MirCheck were inspected manually in order to remove the false prediction (Additional file 1: Figure S1). Further, miRNAs with the same sequences derived from pre-miRNAs (the sequences may not be completely the same) located at different genomic loci are indicated with an additional dash-number suffix. Sequences of T. gondii-specific miRNAs have been deposited in the miRNA database (miRBase).
MiRNAs with statistically significant differences in relative abundance (as reflected by TPM values) between the two libraries (corresponding to the two distinct strains of the parasites) were analyzed with IDEG6 and edger . We used the general Chi-square method as it is most frequently used in such analysis [40, 41]. MiRNA with a P value ≤ 0.01 were deemed to be significantly different between the samples of the two distinct strains of parasites.
Northern blot analysis of miRNA expression
Twenty micrograms of total RNA was loaded in each well of a 12.5% polyacrylamide gel containing 42% urea and run in 0.5× TBE buffer. Following the electrophoresis, RNA was transferred by capillary transferring to a Hybond-N + nylon membranes (GE Healthsystems, Uppsala, Sweden). After UV cross-linking, the membranes were baked for 1 h at 80°C. Probes complementary to small RNA sequences were end-labeled with DIG at 5′ Termini (TaKaRa, Dalian, China). Pre-hybridization of the membrane was performed by overnight incubation at 53°C followed by hybridization overnight at the same temperature in Northernmax Hybridization buffer (Ambion, CA, USA). After hybridization, the membranes were washed four times for 30 min in 2×SSC, 0.05% SDS and twice for 15 min in 0.1×SSC, 0.1% SDS at RT. Chemiluminescent signal was detected using a DIG Detection Kit (Roche, Germany) following the manufacturer’s instructions. The oligonucleotide probes used for hybridization are as follows:
Tgo-novel-40 probe: TTCCTG*ATCCTA*TTTAG*CAG*GT
Tgo-novel-12-1 probe: CCA*CTTCAG*TCTTCA*AAG*TTCT
Tgo-novel-41 probe: AGGG*CGG*ACTG*GCTG*CTTCCAGCA
Tgo-novel-1-1-3p probe: GACAG*TGCTCG*GGTCCG*CAACA*CGCC
Tgo-novel-14-2 probe: ACA*CTT*CCCTT*CTCG*CCG
(LNA (locked nucleic acid) substitutions are indicated by a “*”).
An overview of the small RNA sequencing results
Identification of miRNAs in T. gondii
Characterization of the expression of T. gondii miRNAs
Based on their genomic locations, T. gondii miRNAs were categorized into three types (Additional file 6: Table S4 and Additional file 7: Table S5) named intronic, intergenic and UTR-derived miRNAs. The numbers of miRNAs derived from the 3 genomic regions were 33, 305 and 18 respectively. No miRNA genes were found to be located in exons. Thus miRNA genes were predominantly intergenic in T. gondii. This observation was in agreement with a previous study on Schistosoma japonicum which suggested that most miRNA genes have their own control elements (or promoters) in the genome . Further, it was found that a miRNA can be generated from several pre-miRNAs encoded by genes located in different genomic loci. Thus miRNAs with the same sequence but derived from different pre-miRNAs were named with an additional dash-number suffix (Additional file 6: Table S4 and Additional file 7: Table S5).
Comparison of the novel miRNAs expressed in ME49 and RH revealed strain-specific expression patterns in the two distinct strains of T. gondii. For instance, the number of reads of tgo-novel-40 in ME49 was 4000 times more than that in RH. Similarly, the expression level of tgo-novel-1-1-5p, tgo-novel-12-1, tgo-novel-41 and tgo-novel-15-1 was significantly higher in ME49 strain. On the contrary, tgo-novel-14-2, was highly expressed in RH, the reads number was about 10 times as much as that in ME49 (Additional file 7: Table 5 and Additional file 8: Table S6).
Validation of miRNAs expression by Northern blot
MiRNAs are recognized as critical regulators in gene expression at the post-transcriptional level. Previous studies have found that T. gondii possesses a complete RNA silencing pathway which suggests that small noncoding RNAs may play a critical role in the parasite development and its parasitization in the hosts . In this study, the profiles of small RNA populations of the two distinct strains of T. gondii were investigated.
The distribution of small RNAs in the two small RNA libraries generated after deep sequencing was compared. A predominant number of small ncRNAs was strain-specific (Figure 1 and Figure 2) and the strain-specific small ncRNA including miRNAs were more in RH strain than in ME49 strain (Figure 1). The difference in miRNA numbers found in the two libraries could be due to the less presentation of ME49 genomic sequences in the databases which may affect small RNA identification. Further, there was a clear difference in the tendency of the length distribution in small RNAs between the two strains. In RH, the 26 nt RNAs represented the predominant species (Figure 3, Additional file 5: Table S3), while in ME49, about 18.92% of the total small RNAs were 21 nt in size, which was the most abundant class. This might be due to the genetic differences between the two parasite strains, which also suggested that the two parasite strains are biologically different. Previous study reported that the structural features of pre-miRNA hairpins might influence the efficiency of Dicer binding and specificity of precursor cleavage, which leads to the length diversity of miRNAs [46, 47]. The reasons of strain-specific length distribution of the small ncRNAs between the two strains of T. gondii remain vague, further studies are needed to dissect the mechanism in sncRNA processing that may be associated with strain-specific gene regulation.
In total, we identified 17 conserved miRNA and 339 species-specific miRNAs in the two strains of T. gondii, of which 7 miRNAs sequences were reported by Braun et al. in 2010 . Interestingly, about 5% of T. gondii miRNAs were categorized as conserved and accounted for less than 1% of the read counts, whereas more than 99% of the remaining miRNAs were recognized as species-specific. This observation supports the earlier finding that T. gondii possesses a RNA-associated gene regulation machinery which is phylogenetically diverged from mammals but more similar to plants . Further, let-7 and lin-4, the two most conserved miRNAs in metazoan, were not found in T. gondii indicting that the fine-tuning mechanism of miRNAs in T. gondii was distinct from other species.
During the process of the biosynthesis of miRNAs, miRNA and miRNA* (or miR*) were generated by enzymatic cleavage of the 70–80 nt precursor hairpin. The functional strand (miRNA) of the small RNA duplex is preferentially loaded into the RISC as the guide strand, while the other strand, the passenger strand (miRNA*), is degraded [44, 45]. However, recent studies suggested that mature miRNAs can be generated from both strands of the pre-miRNA duplex [48–50]. In C. elegans and related nematodes, it has been reported that the diversity of miRNAs was, at least partially, due to the arm-switching and hairpin shifting . We found that tgo-novel-1 changed the miRNA strand with arm switching of the same hairpin between the two strains of T. gondii. In RH strain, the dominant miRNA of tgo-novel-1 was derived from the 3′ arm, while in ME49 it seemed that mature miRNA was only derived from the 5′ arm. Due to the fact that the sequences of miRNAs derived from the two arms of the same hairpin were complementary, they likely regulate different target sequences. Further dissection of the function of the miRNAs derived from the two arms of the same hairpin might lead to deep understanding of the parasite biology.
In summary, 17 conserved miRNAs related to 2 metazoan miRNA families and 339 novel miRNAs were identified in the two genetically different strains of T. gondii. The majority of miRNAs were species-specific, which supports the finding that T. gondii is an evolutionarily diverged organism from other protozoana. The difference in expression abundance of certain miRNAs as well as the arm-switching in pre-miRNA processing leading to different miRNA species in the two parasite strains suggested that there was a fine-tuning mechanism of miRNA biogenesis in distinct strains of T. gondii. Understanding the genetic factors that regulate T. gondii gene expression could contribute to the development of specific tools to control the transmission of the parasite.
This study was supported by the National Natural Science Foundation of China (NSFC 30625029), and the national science & technology specific projects (2008ZX-10004-011).
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