GTM-decon: guided-topic modeling of single-cell transcriptomes enables sub-cell-type and disease-subtype deconvolution of bulk transcriptomes.

Cell-type composition is an important indicator of health. We present Guided Topic Model for deconvolution (GTM-decon) to automatically infer cell-type-specific gene topic distributions from single-cell RNA-seq data for deconvolving bulk transcriptomes. GTM-decon performs competitively on deconvolving simulated and real bulk data compared with the state-of-the-art methods. Moreover, as demonstrated in deconvolving disease transcriptomes, GTM-decon can infer multiple cell-type-specific gene topic distributions per cell type, which captures sub-cell-type variations. GTM-decon can also use phenotype labels from single-cell or bulk data to infer phenotype-specific gene distributions. In a nested-guided design, GTM-decon identified cell-type-specific differentially expressed genes from bulk breast cancer transcriptomes.

Comparative transcriptomic analyses and single-cell RNA sequencing of the freshwater planarian Schmidtea mediterranea identify major cell types and pathway conservation.

BACKGROUND: In the Lophotrochozoa/Spiralia superphylum, few organisms have as high a capacity for rapid testing of gene function and single-cell transcriptomics as the freshwater planaria. The species Schmidtea mediterranea in particular has become a powerful model to use in studying adult stem cell biology and mechanisms of regeneration. Despite this, systematic attempts to define gene complements and their annotations are lacking, restricting comparative analyses that detail the conservation of biochemical pathways and identify lineage-specific innovations. RESULTS: In this study we compare several transcriptomes and define a robust set of 35,232 transcripts. From this, we perform systematic functional annotations and undertake a genome-scale metabolic reconstruction for S. mediterranea. Cross-species comparisons of gene content identify conserved, lineage-specific, and expanded gene families, which may contribute to the regenerative properties of planarians. In particular, we find that the TRAF gene family has been greatly expanded in planarians. We further provide a single-cell RNA sequencing analysis of 2000 cells, revealing both known and novel cell types defined by unique signatures of gene expression. Among these are a novel mesenchymal cell population as well as a cell type involved in eye regeneration. Integration of our metabolic reconstruction further reveals the extent to which given cell types have adapted energy and nucleotide biosynthetic pathways to support their specialized roles. CONCLUSIONS: In general, S. mediterranea displays a high level of gene and pathway conservation compared with other model systems, rendering it a viable model to study the roles of these pathways in stem cell biology and regeneration.

Genomics of apicomplexan parasites.

The increasing prevalence of infections involving intracellular apicomplexan parasites such as Plasmodium, Toxoplasma, and Cryptosporidium (the causative agents of malaria, toxoplasmosis, and cryptosporidiosis, respectively) represent a significant global healthcare burden. Despite their significance, few treatments are available; a situation that is likely to deteriorate with the emergence of new resistant strains of parasites. To lay the foundation for programs of drug discovery and vaccine development, genome sequences for many of these organisms have been generated, together with large-scale expression and proteomic datasets. Comparative analyses of these datasets are beginning to identify the molecular innovations supporting both conserved processes mediating fundamental roles in parasite survival and persistence, as well as lineage-specific adaptations associated with divergent life-cycle strategies. The challenge is how best to exploit these data to derive insights into parasite virulence and identify those genes representing the most amenable targets. In this review, we outline genomic datasets currently available for apicomplexans and discuss biological insights that have emerged as a consequence of their analysis. Of particular interest are systems-based resources, focusing on areas of metabolism and host invasion that are opening up opportunities for discovering new therapeutic targets.

The role of palmitoylation for protein recruitment to the inner membrane complex of the malaria parasite.

To survive and persist within its human host, the malaria parasite Plasmodium falciparum utilizes a battery of lineage-specific innovations to invade and multiply in human erythrocytes. With central roles in invasion and cytokinesis, the inner membrane complex, a Golgi-derived double membrane structure underlying the plasma membrane of the parasite, represents a unique and unifying structure characteristic to all organisms belonging to a large phylogenetic group called Alveolata. More than 30 structurally and phylogenetically distinct proteins are embedded in the IMC, where a portion of these proteins displays N-terminal acylation motifs. Although N-terminal myristoylation is catalyzed co-translationally within the cytoplasm of the parasite, palmitoylation takes place at membranes and is mediated by palmitoyl acyltransferases (PATs). Here, we identify a PAT (PfDHHC1) that is exclusively localized to the IMC. Systematic phylogenetic analysis of the alveolate PAT family reveals PfDHHC1 to be a member of a highly conserved, apicomplexan-specific clade of PATs. We show that during schizogony this enzyme has an identical distribution like two dual-acylated, IMC-localized proteins (PfISP1 and PfISP3). We used these proteins to probe into specific sequence requirements for IMC-specific membrane recruitment and their interaction with differentially localized PATs of the parasite.

Extent of structural asymmetry in homodimeric proteins: prevalence and relevance.

Most homodimeric proteins have symmetric structure. Although symmetry is known to confer structural and functional advantage, asymmetric organization is also observed. Using a non-redundant dataset of 223 high-resolution crystal structures of biologically relevant homodimers, we address questions on the prevalence and significance of asymmetry. We used two measures to quantify global and interface asymmetry, and assess the correlation of several molecular and structural parameters with asymmetry. We have identified rare cases (11/223) of biologically relevant homodimers with pronounced global asymmetry. Asymmetry serves as a means to bring about 2:1 binding between the homodimer and another molecule; it also enables cellular signalling arising from asymmetric macromolecular ligands such as DNA. Analysis of these cases reveals two possible mechanisms by which possible infinite array formation is prevented. In case of homodimers associating via non-topologically equivalent surfaces in their tertiary structures, ligand-dependent mechanisms are used. For stable dimers binding via large surfaces, ligand-dependent structural change regulates polymerisation/depolymerisation; for unstable dimers binding via smaller surfaces that are not evolutionarily well conserved, dimerisation occurs only in the presence of the ligand. In case of homodimers associating via interaction surfaces with parts of the surfaces topologically equivalent in the tertiary structures, steric hindrance serves as the preventive mechanism of infinite array. We also find that homodimers exhibiting grossly symmetric organization rarely exhibit either perfect local symmetry or high local asymmetry. Binding of small ligands at the interface does not cause any significant variation in interface asymmetry. However, identification of biologically relevant interface asymmetry in grossly symmetric homodimers is confounded by the presence of similar small magnitude changes caused due to artefacts of crystallisation. Our study provides new insights regarding accommodation of asymmetry in homodimers.

Accommodation of profound sequence differences at the interfaces of eubacterial RNA polymerase multi-protein assembly.

Evolutionarily divergent proteins have been shown to change their interacting partners. RNA polymerase assembly is one of the rare cases which retain its component proteins in the course of evolution. This ubiquitous molecular assembly, involved in transcription, consists of four core subunits (alpha, beta, betaprime, and omega), which assemble to form the core enzyme. Remarkably, the orientation of the four subunits in the complex is conserved from prokaryotes to eukaryotes although their sequence similarity is low. We have studied how the sequence divergence of the core subunits of RNA polymerase is accommodated in the formation of the multi-molecular assembly, with special reference to eubacterial species. Analysis of domain composition and order of the core subunits in >85 eubacterial species indicates complete conservation. However, sequence analysis indicates that interface residues of alpha and omega subunits are more divergent than those of beta, betaprime, and sigma70 subunits. Although beta and betaprime are generally well-conserved, residues involved in interaction with divergent subunits are not conserved. Insertions/deletions are also observed near interacting regions even in case of the most conserved subunits, beta and betaprime. Homology modelling of three divergent RNA polymerase complexes, from Helicobacter pylori, Mycoplasma pulmonis and Onion yellows phytoplasma, indicates that insertions/deletions can be accommodated near the interface as they generally occur at the periphery. Evaluation of the modeled interfaces indicates that they are physico-chemically similar to that of the template interfaces in Thermus thermophilus, indicating that nature has evolved to retain the obligate complex in spite of substantial substitutions and insertions/deletions.

Evolutionary divergence of Plasmodium falciparum: sequences, protein-protein interactions, pathways and processes.

In this article we review the organism-wide biological data available for Plasmodium falciparum (P. falciparum), a malarial parasite, in relation to the data available for other organisms. We provide comparisons at different levels such as amino acid sequences of proteins encoded in the genomes, protein-protein interaction features, metabolic and signaling pathways and processes. Our comparative analyses highlights that P. falciparum is highly diverged compared to most other eukaryotes at all these levels. Despite the extensive variation some of the physical associations between proteins, such as RNA polymerase complex and CDK-cyclin complex are expected to be conserved given their fundamental importance and ubiquitous nature. We also discuss examples of protein-protein interactions across human and P. falciparum potentially happening during pathogenesis.