Genome-wide gene expression profiling analysis of Leishmania major and Leishmania infantum developmental stages reveals substantial differences between the two species

Posted by Hamed Shateri Najafabadi

The title of this post is in fact the title of a recent paper published by Annie Rochette and her colleagues in BMC Genomics (2008, 9:255). This work, which has been done in Barbara Papadopoulou‘s lab at Laval University, reveals unexpected differences between developmental regulation of genes at mRNA level between the two closely related trypanosomatids Leishmania major and Leishmania infantum. I asked Annie to write a summary of her paper in her own point of view. I hope you agree with me that the author’s point of view should be well reflected in the abstract of the paper, so was the case for this article. Here is the abstract as Annie sent to me:

“Leishmania parasites cause a diverse spectrum of diseases in humans ranging from spontaneously healing skin lesions (e.g., L. major) to life-threatening visceral diseases (e.g., L. infantum). The high conservation in gene content and genome organization between Leishmania major and Leishmania infantum contrasts their distinct pathophysiologies, suggesting that highly regulated hierarchical and temporal changes in gene expression may be involved. We used a multispecies DNA oligonucleotide microarray to compare whole-genome expression patterns of promastigote (sandfly vector) and amastigote (mammalian macrophages) developmental stages between L. major and L. infantum. Seven percent of the total L. infantum genome and 9.3% of the L. major genome were differentially expressed at the RNA level throughout development. The main variations were found in genes involved in metabolism, cellular organization and biogenesis, transport and genes encoding unknown function. Remarkably, this comparative global interspecies analysis demonstrated that only 10-12% of the differentially expressed genes were common to L. major and L. infantum. Differentially expressed genes are randomly distributed across chromosomes further supporting a posttranscriptional control, which is likely to involve a variety of 3’UTR elements. This study highlighted substantial differences in gene expression patterns between L. major and L. infantum. These important species-specific differences in stage-regulated gene expression may contribute to the disease tropism that distinguishes L. major from L. infantum.”

Thanks to Annie and her colleagues for this beautiful paper.

I would also like to highlight another paper by Nagalakshmi and colleagues which was published in Science about a month ago. In this work, the transcriptome of yeast is analyzed, but not using microarrays. They used massive high-throughput Illumina sequencing to sequence the whole transcriptome of yeast. This approach, in addition to providing precise estimates for the extent at which each part of the genome is transcribed, gives a plethora of other information that is extremely difficult to gain by routine microarray analysis. First of all, it does not need any a priori assumption regarding the regions that are being transcribed, similar to tiling arrays with the difference that the resolution is several folds higher than any affordable tiling array. It also provides information regarding post-transcriptional modifications of RNAs, such as splicing, alternative splicing, poly-adenylation, etc (see Hani’s blog). Trypanosomatids have surprised us several times, by showing us that a mature RNA can look nothing like its precursor due to the high extent of editing and trans-splicing. They have shown us that it is possible to transcribe almost half of a complete chromosome in just one huge RNA, or that a chromosome can be extensively transcribed from both strands. I am sure these surprises will be nothing once we have the data from sequencing the whole transcriptome of a trypanosomatid species; two will be better!


Altruism in Leishmania: apoptotic parasites are required for infectivity of metacyclic promastigotes

Posted by Kasra Hassani

Suppression of the innate immune response and inhibition of activation of phagocytes that would otherwise kill the parasites has long been established as mechanisms of immune evasion and persistence among Leishmania parasites.

In their paper, van Zandbergen et al. have indicated presence of a high ratio (more than 40%) of apoptotic cells in the metacyclic/stationary phage parasites. They have characterized these cells by occurrence of phosphatidyl serine (PS) in the outer leaflet of plasma membrane as well as PS-binding protein Anexin A5(AnxA5). The majority of AnxA5+ cells have been shown to be apoptotic and different in morphology to infective parasites and they have shown that depletion of these apoptotic cells from the infective population substantially abrogates infectivity.

Apoptotic cells induce production of TGF-beta and IL-10 which are anti-inflammatory cytokines; these cytokines are produced as well by neutrophils when they phagocyte apoptotic Leishmania. Apoptotic parasites also hamper secretion of TNF-alpha, all of which results in inactivation of neutrophils and later macrophages and their inability to kill the phagocytosed parasites.

This is an interesting example of altruism among single-cell populations; the authors have suggested that apoptosis is probably triggered in late log phase and stationary phase promastigotes in the sandfly midgut due to nutrient depletion prior to their entry into the mammalian host.

Studying the secretome of Leishmania donovani

Posted by Kasra Hassani

In this paper, Silverman et al. have pointed to two interesting subjects: first, what proteins are generally secreted from Leishmania, and second, how are these proteins secreted. In an extensive proteomic analysis, they have pointed out 151 proteins that they believe are being actively secreted out of stationary promastigotes of Leishmania donovani. These proteins belong to a wide variety of groups, such as proteases, antioxidants, nucleases etc. and each might play roles in survival of the parasite within its hosts and modulation of the immune response. Identification of these proteins opens up many opportunities for further studies that promote understanding their function and possible therapeutic targets in continuing studies.

Another interesting finding of Silverman et al. was that among these secreted proteins only 2 contain a classical amino-terminal secretion signal, which means that Leishmania largely might benefit from non-classical secretion pathways such as exosomes. Exosomes have been studied previously in human B cells and dendritic cells and it is actually interesting to point out that there is striking correspondence between the proteome content of these exosomes and Leishmania’s secretome (except for the proteins for which Leishmania does not have an ortholog). The authors have proposed the release of exosomes from the surface of the cell and especially from the flagellar pocket to be an important pathway of protein secretion by Leishmania and they have observed vesicular budding from the parasite surface by STM.