Daniel Wilinski

New insights into the blood-stage transcriptome of Plasmodium falciparum using RNA-Seq

Recent advances in high‐throughput sequencing present a new opportunity to deeply probe an organisms transcriptome. In this study, we used Illumina‐based massively parallel sequencing to gain new insight into the transcriptome (RNA‐Seq) of the human malaria parasite, Plasmodium falciparum. Using data collected at seven time points during the intraerythrocytic developmental cycle, we (i) detect novel gene transcripts; (ii) correct hundreds of gene models; (iii) propose alternative splicing events; and (iv) predict 5′ and 3′ untranslated regions. Approximately 70% of the unique sequencing reads map to previously annotated protein‐coding genes. The RNA‐Seq results greatly improve existing annotation of the P. falciparum genome with over 10% of gene models modified. Our data confirm 75% of predicted splice sites and identify 202 new splice sites, including 84 previously uncharacterized alternative splicing events. We also discovered 107 novel transcripts and expression of 38 pseudogenes, with many demonstrating differential expression across the developmental time series. Our RNA‐Seq results correlate well with DNA microarray analysis performed in parallel on the same samples, and provide improved resolution over the microarray‐based method. These data reveal new features of the P. falciparum transcriptional landscape and significantly advance our understanding of the parasites red blood cell‐stage transcriptome.

Host-parasite Interactions Revealed by Plasmodium falciparum Metabolomics

Intracellular pathogens have devised mechanisms to exploit their host cells to ensure their survival and replication. The malaria parasite Plasmodium falciparum relies on an exchange of metabolites with the host for proliferation. Here we describe a mass spectrometry-based metabolomic analysis of the parasite throughout its 48 hr intraerythrocytic developmental cycle. Our results reveal a general modulation of metabolite levels by the parasite, with numerous metabolites varying in phase with the developmental cycle. Others differed from uninfected cells irrespective of the developmental stage. Among these was extracellular arginine, which was specifically converted to ornithine by the parasite. To identify the biochemical basis for this effect, we disrupted the plasmodium arginase gene in the rodent malaria model P. berghei. These parasites were viable but did not convert arginine to ornithine. Our results suggest that systemic arginine depletion by the parasite may be a factor in human malarial hypoargininemia associated with cerebral malaria pathogenesis.

Co-inhibition of Plasmodium falciparum S-Adenosylmethionine Decarboxylase/Ornithine Decarboxylase Reveals Perturbation-specific Compensatory Mechanisms by Transcriptome, Proteome, and Metabolome Analyses

Polyamines are ubiquitous components of all living cells, and their depletion usually causes cytostasis, a strategy employed for treatment of West African trypanosomiasis. To evaluate polyamine depletion as an anti-malarial strategy, cytostasis caused by the co-inhibition of S-adenosylmethionine decarboxylase/ornithine decarboxylase in Plasmodium falciparum was studied with a comprehensive transcriptome, proteome, and metabolome investigation. Highly synchronized cultures were sampled just before and during cytostasis, and a novel zero time point definition was used to enable interpretation of results in lieu of the developmentally regulated control of gene expression in P. falciparum. Transcriptome analysis revealed the occurrence of a generalized transcriptional arrest just prior to the growth arrest due to polyamine depletion. However, the abundance of 538 transcripts was differentially affected and included three perturbation-specific compensatory transcriptional responses as follows: the increased abundance of the transcripts for lysine decarboxylase and ornithine aminotransferase and the decreased abundance of that for S-adenosylmethionine synthetase. Moreover, the latter two compensatory mechanisms were confirmed on both protein and metabolite levels confirming their biological relevance. In contrast with previous reports, the results provide evidence that P. falciparum responds to alleviate the detrimental effects of polyamine depletion via regulation of its transcriptome and subsequently the proteome and metabolome.

Clonally variant gene families in Plasmodium falciparum share a common activation factor

The genome of the malaria parasite Plasmodium falciparum contains several multicopy gene families, including var, rifin, stevor and Pfmc‐2TM. These gene families undergo expression switching and appear to play a role in antigenic variation. It has recently been shown that forcing parasites to express high copy numbers of transcriptionally active, episomal var promoters led to gradual downregulation and eventual silencing of the entire var gene family, suggesting that a limiting titratable factor plays a role in var gene activation. Through similar experiments using rifin, stevor or Pfmc‐2TM episomal promoters we show that promoter titration can be used as a general method to downregulate multicopy gene families in P. falciparum. Additionally, we show that promoter titration with var, rifin, stevor or Pfmc‐2TM episomal promoters results in downregulation of expression not only of the family to which the episomal promoter belongs, but also members of the other gene families, suggesting that the var‐specific titratable factor previously described is shared by all four families. Further, transcriptionally active promoters from different families colocalize within the same subnuclear expression site, indicating that the role that nuclear architecture plays in var gene regulation also likely applies to the other multicopy gene families of P. falciparum.

Divergence of Pumilio/fem-3 mRNA Binding Factor (PUF) Protein Specificity through Variations in an RNA-binding Pocket

Background: PUF protein RNA recognition is critical for target gene regulation. Results: A chemically conserved binding pocket in a subset of PUF proteins recognizes cytosine at different positions upstream of the core PUF recognition sequence. Conclusion: A specialized cytosine-binding pocket introduces qualitative and quantitative differences in RNA recognition by PUF proteins. Significance: Simple adaptations can diversify PUF protein RNA recognition. mRNA control networks depend on recognition of specific RNA sequences. Pumilio-fem-3 mRNA binding factor (PUF) RNA-binding proteins achieve that specificity through variations on a conserved scaffold. Saccharomyces cerevisiae Puf3p achieves specificity through an additional binding pocket for a cytosine base upstream of the core RNA recognition site. Here we demonstrate that this chemically simple adaptation is prevalent and contributes to the diversity of RNA specificities among PUF proteins. Bioinformatics analysis shows that mRNAs associated with Caenorhabditis elegans fem-3 mRNA binding factor (FBF)-2 in vivo contain an upstream cytosine required for biological regulation. Crystal structures of FBF-2 and C. elegans PUF-6 reveal binding pockets structurally similar to that of Puf3p, whereas sequence alignments predict a pocket in PUF-11. For Puf3p, FBF-2, PUF-6, and PUF-11, the upstream pockets and a cytosine are required for maximal binding to RNA, but the quantitative impact on binding affinity varies. Furthermore, the position of the upstream cytosine relative to the core PUF recognition site can differ, which in the case of FBF-2 originally masked the identification of this consensus sequence feature. Importantly, other PUF proteins lack the pocket and so do not discriminate upstream bases. A structure-based alignment reveals that these proteins lack key residues that would contact the cytosine, and in some instances, they also present amino acid side chains that interfere with binding. Loss of the pocket requires only substitution of one serine, as appears to have occurred during the evolution of certain fungal species.

RNA regulatory networks diversified through curvature of the PUF protein scaffold.

Proteins bind and control mRNAs, directing their localization, translation and stability. Members of the PUF family of RNA-binding proteins control multiple mRNAs in a single cell, and play key roles in development, stem cell maintenance and memory formation. Here we identified the mRNA targets of a S. cerevisiae PUF protein, Puf5p, by ultraviolet-crosslinking-affinity purification and high-throughput sequencing (HITS-CLIP). The binding sites recognized by Puf5p are diverse, with variable spacer lengths between two specific sequences. Each length of site correlates with a distinct biological function. Crystal structures of Puf5p–RNA complexes reveal that the protein scaffold presents an exceptionally flat and extended interaction surface relative to other PUF proteins. In complexes with RNAs of different lengths, the protein is unchanged. A single PUF protein repeat is sufficient to induce broadening of specificity. Changes in protein architecture, such as alterations in curvature, may lead to evolution of mRNA regulatory networks.

Recurrent rewiring and emergence of RNA regulatory networks.

Significance Regulatory networks change during evolution. A protein that controls many genes in one species may control a different set of genes in another. We examined how mRNA networks evolve, focusing on the PUF (Pumilio and FBF) family of RNA-binding proteins. These govern stability and translation of hundreds of mRNAs and enable coordinate regulation of discrete biological outcomes. To understand how RNA networks evolve, we used knowledge of the RNA specificity of each PUF protein to predict its mRNA targets and directly identified mRNAs bound to each protein in divergent fungi via biochemical methods. We find networks controlled by one protein switch during evolution to be controlled by another and that proteins with different specificities can share, gain, or lose batteries of mRNAs. Alterations in regulatory networks contribute to evolutionary change. Transcriptional networks are reconfigured by changes in the binding specificity of transcription factors and their cognate sites. The evolution of RNA–protein regulatory networks is far less understood. The PUF (Pumilio and FBF) family of RNA regulatory proteins controls the translation, stability, and movements of hundreds of mRNAs in a single species. We probe the evolution of PUF–RNA networks by direct identification of the mRNAs bound to PUF proteins in budding and filamentous fungi and by computational analyses of orthologous RNAs from 62 fungal species. Our findings reveal that PUF proteins gain and lose mRNAs with related and emergent biological functions during evolution. We demonstrate at least two independent rewiring events for PUF3 orthologs, independent but convergent evolution of PUF4/5 binding specificity and the rewiring of the PUF4/5 regulons in different fungal lineages. These findings demonstrate plasticity in RNA regulatory networks and suggest ways in which their rewiring occurs.

Clonally variant gene families in P. falciparum share a common activation factor

The genome of the malaria parasite Plasmodium falciparum contains several multicopy gene families, including var, rifin, stevor and Pfmc‐2TM. These gene families undergo expression switching and appear to play a role in antigenic variation. It has recently been shown that forcing parasites to express high copy numbers of transcriptionally active, episomal var promoters led to gradual downregulation and eventual silencing of the entire var gene family, suggesting that a limiting titratable factor plays a role in var gene activation. Through similar experiments using rifin, stevor or Pfmc‐2TM episomal promoters we show that promoter titration can be used as a general method to downregulate multicopy gene families in P. falciparum. Additionally, we show that promoter titration with var, rifin, stevor or Pfmc‐2TM episomal promoters results in downregulation of expression not only of the family to which the episomal promoter belongs, but also members of the other gene families, suggesting that the var‐specific titratable factor previously described is shared by all four families. Further, transcriptionally active promoters from different families colocalize within the same subnuclear expression site, indicating that the role that nuclear architecture plays in var gene regulation also likely applies to the other multicopy gene families of P. falciparum.

Reply to Hogan: Direct evidence of RNA–protein interactions and rewiring

We agree with G. J. Hogan (1) that evolution of PUF–RNA interactions is an important problem, but disagree with his characterization of our report (2). First, we cite Hogan et al. (3) and Jiang et al. (4) seven and four times, respectively; credit them with proposing Puf3-regulon rewiring; describe Hogan’s work (3) as “elegant”; and credit it with proposing Puf4/5 duplication and divergence. We also explicitly state that our biochemical data support their earlier analyses. Second, Hogan’s (1) assertion that “the majority of results presented by the authors have been previously published” is untrue. Using crosslinking and immunopurification, we directly identify mRNAs bound in Neurospora crassa by its Puf3 (Nc\_Puf3) and Puf4/5 (Nc\_Puf4/5) proteins for the first … [↵][1]1To whom correspondence should be addressed. Email: wickens{at}biochem.wisc.edu. [1]: #xref-corresp-1-1

Clonally variant gene families inPlasmodium falciparumshare a common activation factor

The genome of the malaria parasite Plasmodium falciparum contains several multicopy gene families, including var, rifin, stevor and Pfmc‐2TM. These gene families undergo expression switching and appear to play a role in antigenic variation. It has recently been shown that forcing parasites to express high copy numbers of transcriptionally active, episomal var promoters led to gradual downregulation and eventual silencing of the entire var gene family, suggesting that a limiting titratable factor plays a role in var gene activation. Through similar experiments using rifin, stevor or Pfmc‐2TM episomal promoters we show that promoter titration can be used as a general method to downregulate multicopy gene families in P. falciparum. Additionally, we show that promoter titration with var, rifin, stevor or Pfmc‐2TM episomal promoters results in downregulation of expression not only of the family to which the episomal promoter belongs, but also members of the other gene families, suggesting that the var‐specific titratable factor previously described is shared by all four families. Further, transcriptionally active promoters from different families colocalize within the same subnuclear expression site, indicating that the role that nuclear architecture plays in var gene regulation also likely applies to the other multicopy gene families of P. falciparum.