Abstract:
Polyamines are ubiquitous components of all living cells and their depletion usually causes growth arrest or cytostasis, a strategy employed for treatment of West-African trypanosomiasis. In the malaria parasite, Plasmodium falciparum, polyamine biosynthesis is regulated by the uniquely bifunctional protein, Sadenosylmethionine decarboxylase/ornithine decarboxylase (PfAdoMetDC/ODC). The unique nature of this protein could provide a selective mechanism for antimalarial treatment. To validate polyamine depletion and specifically PfAdoMetDC/ODC, as drug target for antimalarial therapeutic intervention, polyamine biosynthesis was completely restrained via the inhibition of both catalytic sites of PfAdoMetDC/ODC with DFMO and MDL73811. The physiological effects during the resulting cytostasis were studied with a comprehensive functional genomics approach. The study was preceded by various assays to determine the treatment dosage that would result in complete cytostasis, without non-specific chemical cytotoxicity. The results obtained revealed that the cytostatic mechanism with growth arrest of the treated parasites and normal progression of the untreated controls require special consideration for basic comparisons of response in terms of the assay methodology used and data analysis. This is particularly important when studying a multistage organism such as P. falciparum, which constantly develops and change during the intraerythrocytic developmental cycle, such that growth arrest compared to normal progression would result in significant differences merely due to stage. This critical principle was kept in mind throughout the investigation and was applied to the relative quantification of RNA, proteins and metabolites via a relative time zero approach as opposed to the standard parallel time point comparison. Three independent functional genomics investigations, namely transcriptomics, proteomics and metabolomics were conducted, in which highly synchronised 3D7 parasite cultures were treated during the schizont stage and parasites were sampled during a time course at three time points (just before and during cytostasis). Transcriptome analysis revealed the occurrence of a generalised transcriptional arrest just prior to the growth arrest. To our knowledge this is the first time that transcriptional arrest as the preceding mechanism of cytostasis due to polyamine depletion, was demonstrated. However, despite the transcriptional arrest, the abundance of 538 transcripts was differentially affected and included three perturbation-specific compensatory transcriptional responses: the increased abundance of the transcripts for lysine decarboxylase and ornithine aminotransferase (OAT) and the decreased abundance of that for S-adenosylmethionine synthetase (AdoMet synthetase). Pearson correlations indicated more subtle effects of the perturbation on the proteome and even more so on the metabolome where homeostasis was generally maintained, except downstream to the enzymatic blockade at PfAdoMetDC/ODC. The perturbation-specific compensatory roles of OAT in the regulation of ornithine and AdoMet synthetase in the regulation of AdoMet were confirmed on both the protein and metabolite levels, confirming their biological relevance. The results provide evidence that P. falciparum respond to alleviate the detrimental effects of polyamine depletion via the regulation of its transcriptome and subsequently the proteome and metabolome, which supports a role for transcriptional control in the regulation of polyamine and methionine metabolism within the parasite. The study concludes that polyamines are essential molecules for parasite survival and that PfAdoMetDC/ODC is a valid target for antimalarial drug development.
