Identification of the HSP70-II gene in Leishmania braziliensis HSP70 locus: genomic organization and UTRs characterization
© Ramírez et al; licensee BioMed Central Ltd. 2011
Received: 12 July 2011
Accepted: 26 August 2011
Published: 26 August 2011
The heat stress suffered by Leishmania sp during its digenetic life-cycle is a key trigger for its stage differentiation. In Leishmania subgenera two classes of HSP70 genes differing in their 3' UTR were described. Although the presence of HSP70-I genes was previously suggested in Leishmania (Viannia) braziliensis, HSP70-II genes had been reluctant to be uncovered.
Here, we report the existence of two types of HSP70 genes in L. braziliensis and the genomic organization of the HSP70 locus. RT-PCR experiments were used to map the untranslated regions (UTR) of both types of genes. The 3' UTR-II has a low sequence identity (55-57%) when compared with this region in other Leishmania species. In contrast, the 5' UTR, common to both types of genes, and the 3' UTR-I were found to be highly conserved among all Leishmania species (77-81%). Southern blot assays suggested that L. braziliensis HSP70 gene cluster may contain around 6 tandemly-repeated HSP70-I genes followed by one HSP70-II gene, located at chromosome 28. Northern blot analysis indicated that levels of both types of mRNAs are not affected by heat shock.
This study has led to establishing the composition and structure of the HSP70 locus of L. braziliensis, complementing the information available in the GeneDB genome database for this species. L. braziliensis HSP70 gene regulation does not seem to operate by mRNA stabilization as occurs in other Leishmania species.
The Leishmania genus involves about 20 species that infect humans, causing different clinical manifestations ranging from self-healing cutaneous lesions (CL), mucosal lesions (MCL) to fatal visceral infections (VL) . More than 350 million people are considered at risk of contracting leishmaniases, and some 2 million new cases occur yearly . In Latin America, CL and MCL are neglected public health problems endemic in 22 countries. Many species of the subgenus Viannia cause the majority of CL cases but MCL is principally caused by Leishmania (Viannia) braziliensis. Canine leishmaniases, caused either by Leishmania infantum or by L. braziliensis, is also widespread in South America, being among the most important canine vector-borne diseases occurring in this region .
During its digenetic life cycle, the Leishmania parasite needs to adapt from the environmental (vector) temperature to the mammalian-host temperature (37°C). As a result, the heat shock response is induced and the heat shock proteins (HSPs) are expected to play important roles in the adaptation process, influencing the developmental change from promastigotes in sandflies to amastigotes in mammalian hosts [5–10]. Among HSPs, HSP70 is the most highly conserved in both sequence and function. Proteins of the HSP70 family are central components of many fundamental cellular processes, including the folding and assembly of newly synthesized proteins, refolding of misfolded and aggregated proteins, membrane translocation of organellar and secretory proteins, proteolytic degradation of unstable proteins, and control of regulatory protein activity [11–14].
Two classes of HSP70 genes, HSP70-I and HSP70-II, sharing the 5' untranslated region (UTR) and the coding region but differing in their 3' UTR, have been described in several Leishmania species like L. infantum, Leishmania major, Leishmania tropica, Leishmania mexicana and Leishmania amazonensis[15, 16]. In general, these genes are arranged in a single genomic cluster that contains five or six HSP70-I copies, followed by one HSP70-II copy. In L. infantum, it has been demonstrated that whereas HSP70-I mRNAs accumulate in response to heat shock treatment, and are translated at both 26 and 37°C, HSP70-II mRNAs do not show a temperature-dependent accumulation, but show preferential translation at heat shock temperatures .
Given that Leishmania genes are transcribed into polycistronic RNA precursors that need to be further processed into individual mRNAs by trans- splicing and polyadenylation, post-transcriptional regulation represents the main level of controlling gene expression in these parasites . Currently, multiple efforts are being dedicated to identify cis- and trans- elements involved in the modulation of mRNA processing, mRNA stabilization/destabilization, mRNA half-life, or translation efficiency. Although regulatory sequences have been identified in both 5' and 3' UTRs, most of them have been located in the 3' UTRs [18–23]. For instance, preferential translation of HSP83 in Leishmania requires a thermosensitive polypyrimidine-rich element (PPT) in the 3' UTR .
Our knowledge on the organization and expression of HSP70 genes in Leishmania species of the subgenus Viannia is scanty. Although, the sequence for the L. braziliensis genome has recently been published , unfortunately, the genomic sequence found in the GeneDB database presents several gaps that hinder to elucidate the organization of the HSP70 locus in L. braziliensis. Moreover, in a preliminary work, it was documented by hybridization assays the existence of HSP70-I genes in L. braziliensis, but evidence on the presence of HSP70-II genes was not obtained . In this work, we have determined the 5' and 3' UTRs for the HSP70 L. braziliensis genes, demonstrating the existence of the HSP70-II gene in this Viannia species, and established the organization of the HSP70 locus. Also the expression of both types of HSP70 genes was assessed.
Promastigotes of L. braziliensis ( MHOM/BR/75/M2904) were cultured in vitro at 26°C in Schneiders`s insect medium (Sigma Aldrich, Inc., St. Louis, USA) supplemented with 20% heat-inactivated fetal calf serum (Eurobio, Inc., Les Ulis, France), and 0.1 μg/mL of 6-Biopterin (Sigma Aldrich, Inc., St. Louis, USA). For heat shock treatments, 10 mL-aliquots of L. braziliensis promastigote cultures at logarithmic phase (5-9 × 106 promastigotes mL-1) were transferred into 50 mL flasks, and incubated at 32°C, 35°C or 37°C for two hours. Afterwards, parasites were harvested for DNA or RNA extraction.
Southern and Northern blot analyses
Total DNA from L. braziliensis cells was isolated using the phenol-chloroform-isoamilic alcohol method . Two μg of DNA were digested with several restriction enzymes according to the manufacturer specifications (Promega, Inc., Madison, WI, USA), electrophoresed on 0.8% low electroendosmosis agarose gels (Conda Pronadisa, Inc., Madrid, Spain), and transferred to nylon membrane (Roche, Inc., Mannheim Germany) by standard methods . Total RNA from promastigotes was isolated using the Total Quick RNA Cells and Tissues (TALENT, Inc., Trieste, Italy). Four μg per line were separated on 1.5% (w/v) low electroendosmosis agarose/MOPS/formaldehyde gels and transferred to nylon membranes. L. braziliensis chromosomes were prepared from promastigotes, harvested during log phase, washed and embedded in 0.6% low melting agarose (GIBCO BRL, Inc., N.Y, USA) plugs, which were finally soaked in a lysis solution (0.5 M EDTA, pH 9; 1% SDS, 1 mg/mL proteinase K) at 50°C during 48 h.
After three washes with 0.2 M EDTA for 2 h each one, the blocks were loaded directly into the wells of 1% agarose NA gel (Amersham Bioscience, Inc., Uppsala, Sweden), sealed in place, separated by pulsed homogeneous electric field gel electrophoresis (PFGE) using a Pharmacia Biotech Gene Navigator apparatus at 100 V, 120° separation angle and switch times varying from 100 s/7 h; 200 s/10 h and 500 s/20 h, and transferred to nylon membranes. The following probes were used: 3' UTR-I (clone pTLb3HSP70-D, Sma I/Eco RI digested), 3' UTR-II (clone pTLb3H70-11B, Hin cII/Eco RI digested), and intercoding (IR-HSP70, clone pLbHSP70-IR-E, Pst I digested). They were labeled with digoxigenin by randomly primed synthesis using the DIG High Prime DNA Labeling kit (Roche, Inc., Mannheim, Germany). Hybridizations and immunological detection were performed using the Detection Starter kit II (Roche, Inc., Mannheim, Germany) according to manufacturer's instructions. Finally, membranes were exposed on Curix RP2 plus medical X-Ray film (AGFA, Mortsel, Belgium).
Cloning UTR sequences and in silico analyses
First-strand cDNA synthesis was carried out from L. braziliensis total RNA using an oligo-dT primer and a cDNA synthesis kit (LKB Pharmacia, New Jersey, USA). Amplification of the UTRs was performed from poly-T primed-cDNA using specific primers: LbSL (5'CGCTATATAAGTATCAGTTTC3') and Lb181 (5'TGCAACCCGATCATGACCAAG 3') for the 5' UTR, and Lb1824 (5'GATCATGACCAAGATGTACCAGAG 3') and Poly T Eco RI (5'CGGAATTCTTTTTTTTTTTTTTTTTTT 3') for the 3' UTR-I (see Additional file 1). Briefly, 20 μL reactions containing 2 μL of cDNA, 1X reaction buffer (10 mM Tris-HCl pH 9.0, 50 mM KCl, 0.1% Triton X-100), 1.5 mM MgCl2, 0.7 mM of dNTP mix, 0.2 μM of each primer, 2 M betain, and 0.06 units per μL of expand high fidelity enzyme (Roche, Branford, USA) were prepared. For 3' UTR-II amplification, 1 mM MgCl2 and 0.5 μM of each primer were used. An MJ Research PTC-100 DNA thermocycler was used for the reaction with the following amplification profile: 95°C/5 min (initial denaturation), 35 cycles at 92°C/0.5 min, 58°C/0.5 min and 72°C/1 min, with a final incubation at 72°C for 10 min. All the amplified fragments were resolved in agarose gels and visualized under UV exposure after ethidium bromide staining. RT-PCR products were excised from gels, purified using GFX Gel Band Purification kit (Amersham Biosciences, GE Healthcare, Chalfont St. Giles, Buckinghamshire, England) and cloned in the pGEM®-T Easy plasmid (Promega, Madison, WI, USA); pCR®II (Invitrogen, California, USA) or pCR2.1 (Invitrogen, California, USA) plasmids. The following clones were obtained: pLbHSP70-5B containing the 5' UTR, pTLb3HSP70-D for the 3' UTR-I, pLb3H70-11B for the 3' UTR-II, and pLbHSP70-IR-E for the intercoding region. The sequences were determined using the Big Dye Terminators v3.1 kit (Applied Biosystem, California, USA) by automatic sequencing at the Servicio de Genómica (Parque Científico de Madrid, Universidad Autónoma de Madrid). In order to deduce the HSP70 mRNA UTRs from other Leishmania species LALIGN (http://www.ch.embnet.org/software/LALIGN_form.html) analyzes were performed. A ClustalW analysis (http://www.ebi.ac.uk/Tools/clustalw2/index.html) for multiple alignments of sequences, were carried out to determine the similarity among them.
Results and discussion
The L. braziliensis HSP70 locus contains two types of HSP70 genes
Gene structure and expression of the HSP70 protein have been well characterized in L. infantum[7, 15, 16, 27] and other trypanosomatids of medical importance as L. major, Trypanosoma brucei[29, 30], and Trypanosoma cruzi[31–36]; nonetheless, little is known about their genes in L. braziliensis. A previous study showed the presence of the HSP70-I genes in this parasite; however, it was not determined their copy number. These authors also reported that using the 3' UTR-II region from L. infantum as probe, it was not possible to detect the presence of HSP70-II genes in L. braziliensis.
After determining the extent of the UTRs, it was possible to define the intergenic region (IR) within the L. braziliensis HSP70 locus. The IR, not included in the mature mRNA, is defined as the sequence beginning downstream of the polyadenylation site and ending immediately upstream of the spliced leader acceptor site of a downstream gene. Thus, the IR between LbrM28.2990 and LbrM28.2980 genes was found to be 237 nucleotides in length; this region was almost identical (99.6%) to that cloned for this work (pLbHSP70-IR-E clone, GenBank accession number JF449366), suggesting a high degree of conservation of this region along the HSP70 cluster.
Comparison of L. braziliensis HSP70 UTRs with their homologues in other Leishmania species
The characterization of UTRs for L. braziliensis HSP70 genes (Figures 2, 3 and 4) has evidenced the existence of a remarkable conservation of the HSP70 locus along the genus Leishmania, extending previous studies  to a species of the Viannia subgenus. The 5' UTR cloned in this work was found to be highly conserved with the equivalent regions of the LbrM28_v2.2990 and LbrM28_v2.2980 genes (98% and 99.5%, respectively). The comparison with the HSP70 5' UTR of other Leishmania species showed also a remarkable sequence identity (77-81%). Noticeably, this region was well conserved among all Leishmania species analyzed except for two exclusive sequences of L. braziliensis (Figure 2).
Comparison of the 3' UTR-I from L. braziliensis with those from other Leishmania species revealed identities between 71-73%. Furthermore, stretches of identical nucleotides are present in the 3' UTR-I in all the species analyzed (Figure 3 and 4). Again, it was found that L. braziliensis sequence is the most divergent among the analyzed sequences. Thus, the L. braziliensis 3' UTR-I lacks several regions common to the other Leishmania species; in particular, there are two important sequence gaps, of 60 and 77 nts, located at the beginning and the middle of the region, respectively (Figure 3 and 4).
Expression analysis of HSP70 genes in L. braziliensis
Copy number and chromosomal location of HSP70 genes in L. braziliensis
Hybridization of the membrane with the 3' UTR-II probe showed a sole hybridization fragment (5.1-kb in Xho I+Sma I digested DNA, and 4.1-kb in Xho I+Bam HI digested DNA) (Figure 8B, lanes 3 and 4, respectively), in agreement with the existence of a unique HSP70-II gene, which was located at the 3' end of the locus (Figure 8D). According to the number of fragments observed in the lanes containing Bam HI partially digested DNA (Figure 8A, lanes 5 to 8), and taking into account the size of the hybridizing Xho I-fragments which should contain the complete locus (Figure 8A, lane 2), it was estimated the presence of around seven HSP70 genes in the L. braziliensis HSP70 locus, as is shown in Figure 8D which summarizes the genomic organization of the L. braziliensis HSP70 locus.
The presence of two hybridization fragments, larger than 20 kb, in the lane containing Xho I-digested DNA (Figure 8A, lane 2) has no direct explanation. Two hypotheses may be invoked to explain this unexpected finding. On the one hand, it can be postulated the existence of two identical HSP70 tandems that, furthermore, should be found in the same chromosome. Thus, PFGE separation and hybridization assays showed that HSP70 genes are located in a chromosome of approximately 1.5 Mb (Figure 8C), which would correspond to the chromosome 28, according to the location of the HSP70 locus in the GeneDB database. The other hypothesis is the existence of allelic polymorphisms either within the locus (affecting the number of HSP70 gene copies) or in its boundaries (affecting one of the Xho I restriction sites). Although the size and hybridization intensity of these Xho I restriction fragments supports the second alternative, and the resolution of the PFGE assay do not permit distinguish between the two size different sister chromosomes, further work is needed to clarify this finding.
The present work was intended to establish the genomic organization of HSP70 genes in L. braziliensis. Firstly, by RT-PCR and cloning, we identified two types of 3' UTR sequences, demonstrating that also in L. braziliensis two types of HSP70 genes exist, a feature shared with other Leishmania species. In addition, our analyses support the existence of at least seven HSP70 genes arranged in a head-to-tail manner. In summary, the HSP70 locus in L. braziliensis, like in Leishmania subgenus species, is composed of the two types of genes (HSP70-I and HSP70-II), the number and the relative position of these genes being very similar in the Leishmania genus. This finding is of especial value taking into account that L. (V.) braziliensis complex is considered as the earliest divergent species of the genus Leishmania. The strict conservation of the HSP70 gene array in all Leishmania species analyzed suggests that this type of arrangement must have an important functional role. Indeed, as recently reported, the HSP70-II gene in L. infantum is extremely relevant for virulence and intracellular survival of the parasite .
Additionally, we have experimentally mapped the 5' and 3' UTR of both types of HSP70 genes of L. braziliensis. After comparing with the genomic sequences, the position of processing signals, such as the trans-splicing and polyadenylation sites as well as the C-rich polypyrimidine tracts, were determined. The distance between these elements is in agreement with previously reported range of distances .
Former studies in L. braziliensis reported that after probing the mRNA blot with the HSP70 coding region, two hybridization RNA species very close in size were observed, corresponding the lower molecular weight species to that hybridizing with a 3' UTR-I probe . Our Northern blot analysis supported these findings and revealed that the larger RNA corresponds to the HSP70-II transcript, confirming the existence of the HSP70-II genes in this parasite. Although, the size difference between these transcripts could not be explained by the sole difference in size of both 3' UTRs, it is likely that the differences are due to different length of the poly(A) tail [42, 43]. Noticeably, it is considered that unstable mRNAs carry shorter poly (A) tails [44, 45]. Northern blot assays showed that the steady-state levels of both transcripts are not affected by the temperature of incubation. Consequently, in contrast to the species of the Leishmania subgenus, L. braziliensis HSP70-I transcripts are not stabilized upon heat shock.
heat shock protein
70 kDa heat shock protein
pulsed field gel electrophoresis
thermosensitive polypyrimidine-rich element
activated protein kinase c receptor (guanine nucleotide-binding protein beta subunit- like protein).
CJP's lab was supported by Colciencias (Colombia), Research project No. 1203-405-20233. JMR's lab was supported by grants from the Ministerio de Ciencia y Tecnología (BFU2009-08986) and the Fondo de Investigaciones Sanitarias (ISCIII-RETIC RD06/0021/0008-FEDER). CAR was supported by Colciencias, Programa Nacional de Doctorados, convocatoria 2008.
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