Insights Into The Trans-Splicing Based Expression Of Heat Shock Protein 90 In Giardia Lamblia
Heat shock proteins (Hsps) are a class of molecular chaperones which were first discovered as proteins up-regulated in response to heat stress in Drosophila. Later, it was found that these set of proteins get up-regulated as a general stress response associated with destabilization of native protein structures. Over a period of time, intricate involvement of Hsps in various biological processes has been well established. Heat shock protein 90 (Hsp90) is one of the important representative of this class of proteins. Hsp90 is an essential molecular chaperone which is evolutionarily conserved. It has a selective set of proteins to chaperone called as clients, which majorly include transcription factors and protein kinases. Through its interaction with its clients it modulates cell cycle, signal transduction, differentiation, development and evolution. Previous studies from Candida, Leishmania and Plasmodium have implicated Hsp90 to be involved in stage transition and growth. It is also critically involved in regulating growth of other protozoans such as Dictyostelium, Entamoeba and Trypanosoma. Thus, selective inhibition of Hsp90 has been explored as an intervention strategy against important human diseases such as cancer, malaria and other protozoan diseases. In Plasmodium falciparum, Hsp90 plays a critical role in stage transition. The parasite inside the human RBC develops from ring to trophozoite to schizont stage and inhibition of Hsp90 using specific pharmacological inhibitor arrests the growth of parasite at ring stage. In Dictyostelium, it has been observed that Hsp90 function is required for development. Inhibition of Hsp90 causes mound arrest and stops the cells from entering to its next developmental stage, fruiting bodies. In parallel, Hsp90 in Candida has been shown to be involved in morphogenesis. In nature Candida exists as a single cell yeast form and upon entry into the human host these yeast forms undergo morphogenesis to form virulent filamentous fungi. Inhibition of Hsp90 mimics temperature mediated morphogenesis. All together, these studies suggest that Hsp90 functions in a context dependent manner and each biological system explored has given new insights into the Hsp90 biology. Giardia lamblia, a protozoan parasite of humans and animals, is an important cause of diarrheal disease causing significant morbidity and also mortality in tropical countries. In the present study we focus on the biology of Hsp90 from Giardia lamblia. Giardia has a biphasic life cycle with infective cyst stage and pathogenic trophozoite stage. These cysts are present in the environment and enter mammalian host through oral route. They undergo a process called as excystation in the intestine giving rise to trophozoites. The trophozoites so formed colonize the upper part of the small intestine which causes the symptoms of giardiasis. Some of the trophozoites escape from the nutrition rich milieu of the upper part of small intestine to the lower part. In this region, trophozoites undergo a process called as encystation, wherein each trophozoite forms a cyst which escapes through faeces back into the environment. As seen in the life cycle of Giardia there are two major biological transitions, excystation and encystation; and till date no definitive player or pathway is known to regulate these processes. With the knowledge of Hsp90 playing an important role in similar biological transitions in other organisms we were encouraged to study role of Hsp90 in Giardia lamblia. Trans-splicing based generation of a full length Hsp90 in Giardia lamblia To understand the role of Hsp90, we first carried out sequence alignment of Hsp90 predicted ORFs in Giardia genome with yeast Hsp90. On alignment we observed that Hsp90 in Giardia is discontinuous and is annotated to be encoded by two different ORFs. Hsp90 in most organisms is coded by a single ORF with none to many cis-spliced introns. In a relatively intron poor organism G. lamblia, cytosolic Hsp90 is coded by two different ORFs separated by 777 kb in the genome. On multiple sequence alignment, we noticed that these two ORFs correspond to two independent regions of the Hsp90 protein. The ORFs are designated as hspN and hspC, containing the N-terminal and the C-terminal region of the protein respectively. We began our study by sequencing whole genome of Giardia lamblia clinical strain. Our genome sequencing confirmed the split nature of hsp90 and showed high ‘synteny’ between the other sequenced isolates. Using PCR based approach we have ruled out the possibility of having a full length gene in the genome. In contradiction to the genome result, we have observed a higher molecular weight protein in the lysate on proteomic analysis which was further confirmed by western blotting. The protein was observed to have a molecular weight of 80 kDa which could be a resultant of combination of two ORFs, suggesting the presence of a full length mRNA for Hsp90. PCR amplification using primers against both the fragments resulted in amplification of 2.1 kb product from the RNA pool of Giardia. Sequencing of this product showed that hspN and hspC were stitched together to form a mature messenger for full length Hsp90. In total our results suggest a post transcriptional process, trans-splicing, to be involved in the construction of Hsp90. The transition marked by this fusion coincides with the canonical GU¬AG splice site transitions as observed in other eukaryotes. Interestingly, a 26 nt near-complementary region was observed inside and upstream of hspN and hspC ORFs respectively. Put together these results suggest that the 26 nt complementary region acts as the positioning element to bring these two precursors in spatial proximity. With efficient spliceosomal activity these two precursor forms are trans-spliced to generate a full length cytosolic Hsp90 in Giardia. There are only four genes which have cis-spliced introns in the Giardia genome and the core components of the spliceosomal machinery are also present. The presence of canonical splice site in both the transcripts suggests that these transcripts are fused together by the spliceosomal machinery by the phenomenon of trans-splicing. The formation of full length Hsp90 RNA by its fragmented gene is the first example of trans-splicing in Giardia. To understand, are there any other genes which are also similarly trans-spliced we have carried out shotgun proteomic analysis of the total cell lysate obtained from Giardia trophozoites. Using Hsp90 as template, in our proteomic datasets, we have designed an algorithm for identification of additional trans-spliced gene products at the protein level. We have identified a total of 476 proteins of which hypothetical proteins constitute the major class followed by metabolic enzymes. We have compared the theoretical molecular weights for the identified proteins with the experimentally determined mass. Any discrepancy in the molecular mass was further analyzed and we assigned a gene to be potentially trans-spliced based on three criteria: if they were encoded by two or more different ORFs (loci), absence of a single full length counterpart and presence of splice sites with branch point and positional elements. Using this algorithm we were able to identify dynein as a potential candidate of trans-splicing reaction which was confirmed by the nucleotide sequence analysis of the predicted ORFs. Interestingly, dynein gene fragments were observed to be scattered on different chromosomes with minor splice sites unlike hsp90 genes. In vivo Expression of Hsp90 sub-fragments, HspN and HspC In the mature Hsp90 mRNA formed upon trans-splicing, 33 additional codons are present right between hspN and hspC sequences and they were acquired from the upstream region of hspC ORF. The 33 codons encode for an important region of Hsp90 which harbours the conserved catalytic “Arg” residue; suggesting that the full length Giardia Hsp90 (GlHsp90) formed could be an active ATPase. To confirm the same we have carried out in vitro characterization of trans-spliced Hsp90. Towards this, we have cloned, expressed and purified His tag-GlHsp90. As a first step, highly purified protein was used to assess its efficiency in binding to it cognate ligand, ATP, and the known inhibitors. Our binding studies show that GlHsp90 binds to ATP with a dissociation constant of 628 M and to its inhibitors, GA and 17AAG with 1.5 μM and 17.5 μM respectively. The bound ATP will be subsequently cleaved by Hsp90 which is an essential step in the chaperone cycle. As determined in our ATPase assay we observed that GlHsp90 hydrolyzes bound ATP with the catalytic efficiency of 4.4 × 10-5μM-1.min-1which confirms that Hsp90 generated upon trans-splicing is an active ATPase. The uniqueness of the hsp90 gene arrangement in Giardia posed a new question. Do these gene fragments also get translated? Our results suggest that HspN and HspC are poly¬adenylated. In order to determine the levels of these transcripts we performed qRT-PCR using primers specific to HspN, HspC and GlHsp90. We have observed that, in comparison with HspN transcript level, HspC and GlHsp90 transcripts are 15 and 75 folds higher respectively. To check for the presence of translation products of these transcripts, we have re-analyzed our proteomic datasets wherein we could identify peptides corresponding to HspN and HspC in their respective molecular weight region, 45 to 35 kDa. To confirm the proteomic data, western blot analysis was performed for trophozoite lysate on both 1D and 2D gels using anti-HspN antibody. Two specific bands (1D) / spots (2D) corresponding to the full length Hsp90 and HspN were identified. Gel filtration analysis revealed that HspN co¬eluted with full length Hsp90 thereby suggesting that both the proteins are in a same complex. With the background that HspN and HspC are present at the protein level, we asked if these fragments in combination can hydrolyse ATP. We reconstituted recombinant HspN and HspC in equimolar amounts and scored for the hydrolysis of ATP. However, no Pi release was observed. To determine whether HspN and HspC could modulate Hsp90 function, ATPase activity was monitored in the presence of HspN or HspC, in vitro. It was observed that ATPase activity was inhibited by both the fragments thus suggesting that HspN and HspC negatively regulate Hsp90 ATPase activity. Role of Hsp90 in Giardia encystation Giardia has a biphasic life cycle with proliferative trophozoites and latent cyst stage. In Giardia, in vitro encystation was established nearly two decades back by modulating the medium conditions. However, the mechanism and triggers underlying this transition are not well characterized. To understand whether Hsp90 has any role in this transition, in vitro conversion of trophozoites to cysts was achieved. The cysts obtained showed all the characteristic features of mature Giardia cyst with cyst wall protein 1 (CWP1) on the cyst wall and four nuclei as determined by immunofluorescence analysis. Further, the levels of Hsp90 in trophozoites were compared with mature cysts at both transcript and protein levels and it was found that cysts show more than 50% reduction in the level of Hsp90 in comparison with normal trophozoites. In accordance, exogenous inhibition of Hsp90 using 17AAG promoted the formation of cysts in vitro by 60 folds in a dose dependent manner; however, the window period of Hsp90 function compromise plays an important role in this process. Higher numbers of cysts were obtained from the cells treated with inhibitors during pre-encystation condition but inhibition of Hsp90 during encystation did not affect the formation of cysts, suggesting that Hsp90 down-regulation plays an important role during commitment towards encystation. To further show that cyst formation is a specific response to Hsp90 inhibition we have carried out encystation in the presence of metranidazole and from heat shocked cells; however, in both the conditions we did not observe any significant change in cyst formation, thus confirming that Hsp90 plays an important role during encystation in Giardia lamblia. Summary In Conclusion, Our study throws light on a unique aspect of Hsp90 biology in Giardia Lamblia, wherein the formation of the full length protein is dependent on a unique trans splicing reaction of its gene components representing different domains. We have also shown that HsP90 fragments, HspN and HspC, are also expressed in Trophozoites. Our in vitro data suggests that these fragments possibly regulate the function of Hsp90. Furthermore, the full length of Hsp90 plays an important role in stage transition in Giardia wherein inhibition of Hsp90 induces encystations. The study has opened many new avenues for research. Understanding the exact role of HspN and HspC in vivo will provide better appreciation for the evolution of such a complex biogenesis of an essential protein.
- Biochemistry (BC)