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Iterative, multimodal, and scalable single-cell profiling for discovery and characterization of signaling regulators.

Cell signaling plays a critical role in regulating cellular state, yet uncovering regulators of signaling pathways and understanding their molecular consequences remains challenging. Here, we present an iterative experimental and computational framework to identify and characterize regulators of signaling proteins, using the mTOR marker phosphorylated RPS6 (pRPS6) as a case study. We present a customized workflow that uses the 10x Flex assay to jointly profile intracellular protein levels, transcriptomes, and CRISPR perturbations in single cells. We use this to generate a "glossary" dataset of paired protein-RNA measurements across targeted perturbations, which we leverage to train a predictive model of pRPS6 levels based solely on transcriptomic data. Applying this model to a genome-wide Perturb-seq dataset enables in silico screening for pRPS6 and nominates novel regulators of mTOR signaling. Experimental validation confirms these predictions and reveals mechanistic diversity among hits, including changes in signaling output driven by anabolic activity, cellular proliferation and multiple stress pathways. Our work demonstrates how integrated experimental and computational approaches provide a scalable framework for multimodal phenotyping and discovery.

Journal Article↗

Identification of glycosylphosphatidylinositol-anchored proteins in Arabidopsis. A proteomic and genomic analysis.

In a recent bioinformatic analysis, we predicted the presence of multiple families of cell surface glycosylphosphatidylinositol (GPI)-anchored proteins (GAPs) in Arabidopsis (G.H.H. Borner, D.J. Sherrier, T.J. Stevens, I.T. Arkin, P. Dupree [2002] Plant Physiol 129: 486-499). A number of publications have since demonstrated the importance of predicted GAPs in diverse physiological processes including root development, cell wall integrity, and adhesion. However, direct experimental evidence for their GPI anchoring is mostly lacking. Here, we present the first, to our knowledge, large-scale proteomic identification of plant GAPs. Triton X-114 phase partitioning and sensitivity to phosphatidylinositol-specific phospholipase C were used to prepare GAP-rich fractions from Arabidopsis callus cells. Two-dimensional fluorescence difference gel electrophoresis and one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated the existence of a large number of phospholipase C-sensitive Arabidopsis proteins. Using liquid chromatography-tandem mass spectrometry, 30 GAPs were identified, including six beta-1,3 glucanases, five phytocyanins, four fasciclin-like arabinogalactan proteins, four receptor-like proteins, two Hedgehog-interacting-like proteins, two putative glycerophosphodiesterases, a lipid transfer-like protein, a COBRA-like protein, SKU5, and SKS1. These results validate our previous bioinformatic analysis of the Arabidopsis protein database. Using the confirmed GAPs from the proteomic analysis to train the search algorithm, as well as improved genomic annotation, an updated in silico screen yielded 64 new candidates, raising the total to 248 predicted GAPs in Arabidopsis.

Arabidopsis↗

Molecular characterization of maize acetylcholinesterase: a novel enzyme family in the plant kingdom.

Acetylcholinesterase (AChE) has been increasingly recognized in plants by indirect evidence of its activity. Here, we report purification and cloning of AChE from maize (Zea mays), thus providing to our knowledge the first direct evidence of the AChE molecule in plants. AChE was identified as a mixture of disulfide- and noncovalently linked 88-kD homodimers consisting of 42- to 44-kD polypeptides. The AChE hydrolyzed acetylthiocholine and propyonylthiocholine, but not S-butyrylthiocholine, and the AChE-specific inhibitor neostigmine bromide competitively inhibited its activity, implying that maize AChE functions in a similar manner as the animal enzyme. However, kinetic analyses indicated that maize AChE showed a lower affinity to substrates and inhibitors than animal AChE. The full-length cDNA of maize AChE gene is 1,471 nucleotides, which encode a protein having 394 residues, including a signal peptide. The deduced amino acid sequence exhibited no apparent similarity with that of the animal enzyme, although the catalytic triad was the same as in the animal AChE. In silico screening indicated that maize AChE homologs are widely distributed in plants but not in animals. These findings lead us to propose that the AChE family, as found here, comprises a novel family of the enzymes that is specifically distributed in the plant kingdom.

Acetylcholinesterase↗

Mycobacterium tuberculosis RmlC epimerase (Rv3465): a promising drug-target structure in the rhamnose pathway.

The Mycobacterium tuberculosis rmlC gene encodes dTDP-4-keto-6-deoxyglucose epimerase, the third enzyme in the M. tuberculosis dTDP-L-rhamnose pathway which is essential for mycobacterial cell-wall synthesis. Because it is structurally unique, highly substrate-specific and does not require a cofactor, RmlC is considered to be the most promising drug target in the pathway, and the M. tuberculosis rmlC gene was selected in the initial round of TB Structural Genomics Consortium targets for structure determination. The 1.7 A native structure determined by the consortium facilities is reported and implications for in silico screening of ligands for structure-guided drug design are discussed.

Amino Acid Sequence↗

Structure of pyrR (Rv1379) from Mycobacterium tuberculosis: a persistence gene and protein drug target.

The Mycobacterium tuberculosis pyrR gene (Rv1379) encodes a protein that regulates the expression of pyrimidine-nucleotide biosynthesis (pyr) genes in a UMP-dependent manner. Because pyrimidine biosynthesis is an essential step in the progression of TB, the gene product pyrR is an attractive antitubercular drug target. The 1.9 A native structure of Mtb pyrR determined by the TB Structural Genomics Consortium facilities in trigonal space group P3(1)21 is reported, with unit-cell parameters a = 66.64, c = 154.72 A at 120 K and two molecules in the asymmetric unit. The three-dimensional structure and residual uracil phosphoribosyltransferase activity point to a common PRTase ancestor for pyrR. However, while PRPP- and UMP-binding sites have been retained in Mtb pyrR, a distinct dimer interaction among subunits creates a deep positively charged cleft capable of binding pyr mRNA. In silico screening of pyrimidine-nucleoside analogs has revealed a number of potential lead compounds that, if bound to Mtb pyrR, could facilitate transcriptional attenuation, particularly cyclopentenyl nucleosides.

Amino Acid Sequence↗

Genomic Characterization of Aeromonas dhakensis Isolated From a Fatal Dolphin Case.

Aeromonas dhakensis has emerged as a significant pathogen affecting both aquatic animals and humans; however, genomic data for isolates from marine mammals remain scarce. In this study, we characterised the genome of A. dhakensis strain KDL-001, isolated from a fatal dolphin case, using whole-genome sequencing and comparative genomics. Taxonomic analyses, including MLST and average nucleotide identity (ANI), confirmed the isolate as A. dhakensis. Core-genome phylogeny further revealed that KDL-001 is closely related to strains derived from fish and aquatic environments. Notably, in silico screening of virulence-associated genes showed that the virulence-associated gene profile of the dolphin isolate was broadly comparable to those of other A. dhakensis strains, with no isolate-specific virulence-associated genes being identified within the limits of this analysis. These findings demonstrate that the dolphin-derived isolate is genomically comparable to previously described A. dhakensis strains and possesses conserved virulence-associated genes commonly found within the species.

Animals↗

In silico prediction of drug binding to CYP2D6: identification of a new metabolite of metoclopramide.

Patients with cancer often take many different classes of drugs to treat the effects of their malignancy and the side effects of treatment, as well as their comorbidities. The potential for drug-drug interactions that may affect the efficacy of anticancer treatment is high, and a major source of such interactions is competition for the drug-metabolizing enzymes, cytochromes P450 (P450s). We have examined a series of 20 drugs commonly prescribed to cancer patients to look for potential interactions via CYP2D6. We used a homology model of CYP2D6, together with molecular docking techniques, to perform an in silico screen for binding to CYP2D6. Experimental IC50 values were determined for these compounds and compared with the model predictions to reveal a correlation with a regression coefficient of r2= 0.61. Importantly, the docked conformation of the commonly prescribed antiemetic metoclopramide predicted a new site of metabolism that was further investigated through in vitro analysis with recombinant CYP2D6. An aromatic N-hydroxy metabolite of metoclopramide, consistent with predictions from our modeling studies, was identified by high-performance liquid chromatography/mass spectrometry. This metabolite was found to represent a major product of metabolism in human liver microsomes, and CYP2D6 was identified as the main P450 isoform responsible for catalyzing its formation. In view of the prevalence of interindividual variation in the CYP2D6 genotype and phenotype, we suggest that those experiencing adverse reactions with metoclopramide, e.g., extrapyramidal syndrome, are likely to have a particular CYP2D6 genotype/phenotype. This warrants further investigation.

Antiemetics↗

The antimalarial potential of 4-quinolinecarbinolamines may be limited due to neurotoxicity and cross-resistance in mefloquine-resistant Plasmodium falciparum strains.

The clinical potential of mefloquine has been compromised by reports of adverse neurological effects. A series of 4-quinolinecarbinolamines were compared in terms of neurotoxicity and antimalarial activity in an attempt to identify replacement drugs. Neurotoxicity (MTT [thiazolyl blue reduction] assay) was assessed by exposure of cultured embryonic rat neurons to graded concentrations of the drugs for 20 min. The 50% inhibitory concentration (IC(50)) of mefloquine was 25 microM, while those of the analogs were 19 to 200 microM. The relative (to mefloquine) therapeutic indices of the analogs were determined after using the tritiated hypoxanthine assay for assessment of the antimalarial activity of the analogs against mefloquine-sensitive (W2) and -resistant (D6 and TM91C235) Plasmodium falciparum strains. Five analogs, WR157801, WR073892, WR007930, WR007333, and WR226253, were less neurotoxic than mefloquine and exhibited higher relative therapeutic indices (RTIs) against TM91C235 (2.9 to 12.2). Conventional quinoline antimalarials were generally less neurotoxic (IC(50)s of 400, 600, and 900 for amodiaquine, chloroquine, and quinine) or had higher RTIs (e.g., 30 for halofantrine against TM91C235). The neurotoxicity data for the 4-quinolinecarbinolamines were used to develop a three-dimensional (3D), function-based pharmacophore. The crucial molecular features correlated with neurotoxicity were a hydrogen bond acceptor (lipid) function, an aliphatic hydrophobic function, and a ring aromatic function specifically distributed in the 3D surface of the molecule. Mapping of the 3D structures of a series of structurally diverse quinolines to the pharmacophore allowed accurate qualitative predictions of neurotoxicity (or not) to be made. Extension of this in silico screening approach may aid in the identification of less-neurotoxic quinoline analogs.

Animals↗

Crystal structure of a putative methyltransferase from Mycobacterium tuberculosis: misannotation of a genome clarified by protein structural analysis.

Bioinformatic analyses of whole genome sequences highlight the problem of identifying the biochemical and cellular functions of many gene products that are at present uncharacterized. The open reading frame Rv3853 from Mycobacterium tuberculosis has been annotated as menG and assumed to encode an S-adenosylmethionine (SAM)-dependent methyltransferase that catalyzes the final step in menaquinone biosynthesis. The Rv3853 gene product has been expressed, refolded, purified, and crystallized in the context of a structural genomics program. Its crystal structure has been determined by isomorphous replacement and refined at 1.9 A resolution to an R factor of 19.0% and R(free) of 22.0%. The structure strongly suggests that this protein is not a SAM-dependent methyltransferase and that the gene has been misannotated in this and other genomes that contain homologs. The protein forms a tightly associated, disk-like trimer. The monomer fold is unlike that of any known SAM-dependent methyltransferase, most closely resembling the phosphohistidine domains of several phosphotransfer systems. Attempts to bind cofactor and substrate molecules have been unsuccessful, but two adventitiously bound small-molecule ligands, modeled as tartrate and glyoxalate, are present on each monomer. These may point to biologically relevant binding sites but do not suggest a function. In silico screening indicates a range of ligands that could occupy these and other sites. The nature of these ligands, coupled with the location of binding sites on the trimer, suggests that proteins of the Rv3853 family, which are distributed throughout microbial and plant species, may be part of a larger assembly binding to nucleic acids or proteins.

Amino Acid Sequence↗

ywfE in Bacillus subtilis codes for a novel enzyme, L-amino acid ligase.

The ATP-dependent carboxylate-amine/thiol ligase superfamily is known to contain enzymes catalyzing the formation of various types of peptide, such as d-alanyl-d-alanine, polyglutamate, and gamma-peptide, but, curiously, no enzyme synthesizing alpha-dipeptides of l-amino acids is known. We attempted to find such an enzyme. By in silico screening based on the consensus sequence of the superfamily followed by an in vitro assay with purified enzyme to avoid the degradation of the peptide(s) synthesized, ywfE of Bacillus subtilis was found to code for the activity forming l-alanyl-l-glutamine from l-alanine and l-glutamine with hydrolysis of ATP to ADP. No AMP was formed, supporting the idea that the enzyme belongs to the superfamily. Surprisingly, the enzyme accepted a wide variety of l-amino acids. Among 231 combinations of l-amino acids tested, reaction products were obtained for 111 combinations and 44 kinds of alpha-dipeptides were confirmed by high-performance liquid chromatography analyses, while no tripeptide or longer peptide was detected and the d-amino acids were inert. From these results, we propose that ywfE encodes a new member of the superfamily, l-amino acid ligase.

Adenosine Triphosphate↗

Vibrio cholerae strain typing and phylogeny study based on simple sequence repeats.

Vibrio cholerae is the etiological agent of cholera. Its natural reservoir is the aquatic environment. To date, practical typing of V. cholerae is mainly serological and requires about 200 antisera. Simple sequence repeats (SSR), also termed VNTR (for variable number of tandem repeats), provide a source of high genomic polymorphism used in bacterial typing. Here we describe an SSR-based typing method that combines the variation in highly mutable SSR loci, with that of shorter, relatively more stable mononucleotide repeat (MNR) loci, for accurate and rapid typing of V. cholerae. In silico screening of the V. cholerae genome revealed thousands of perfect SSR tracts with an average frequency of one SSR every 152 bp. A panel of 32 V. cholerae strains, representing both clinical and environmental isolates, was tested for polymorphism in SSR loci. Two strategies were applied to identify SSR variation: polymorphism of SSR tracts longer than 12 bp (L-SSR) assessed by capillary fragment-size analysis and MNR polymorphism assessed by sequencing. The nine L-SSR loci tested were all polymorphic, displaying 2 to 13 alleles per locus. Sequence analysis of eight MNR-containing loci (MNR-multilocus sequence typing [MLST]) provided information on both variations in the MNR tract itself, and single nucleotide polymorphism (SNP) in their flanking sequences. Phylogenetic analysis of the combined SSR data showed a clear discrimination between the clinical strains belonging to O1 and O139 serogroups, and the environmental isolates. Furthermore, discrimination between 27 strains of the 32 strains was achieved. SSR-based typing methods combining L-SSR and MNR-MLST were found to be efficient for V. cholerae typing.

Bacterial Typing Techniques↗

Selective targeting of indel-inferred differences in spatial structures of homologous proteins.

The eukaryotic pathogen Leishmania donovani possesses a housekeeping protein Elongation-Factor-1alpha (EF-1alpha) which has been found to be unexpectedly involved in the pathogen's virulence. Because it is associated with virulence and essential for cell survival, this protein is an attractive choice for drug targeting; however, its sequence is highly similar (> 80% sequence identity) to that of its human homolog, rendering it a risky choice for a drug target. The chief difference between these two proteins has been found to be a 12 amino acid sequence present in human EF-1alpha but absent from leishmania EF-1alpha. Furthermore, it has been shown that this 12 amino acid insert in the human sequence corresponds to a hairpin loop on the surface of the protein. In this study, we searched for those spatial features in leishmania EF-1alpha that are impacted or obscured by the extra hairpin loop in the human counterpart. We have also conducted a large-scale in silico screening for small molecules that could plausibly bind to these protein features. While experimental evidence is required to verify our results, our findings thus far appear to support this approach as a new strategy for the development of antagonists against pathogenic targets having close human homologs.

Amino Acid Sequence↗

Novel subcellular locations and functions for secretory pathway Ca2+/Mn2+-ATPases.

Secretory pathway Ca2+/Mn2+-ATPases (SPCAs) are important for maintenance of cellular Ca2+ and Mn2+ homeostasis, and, to date, all SPCAs have been found to localize to the Golgi apparatus. The single Drosophila SPCA gene (SPoCk) was identified by an in silico screen for novel Ca2+-ATPases. It encoded three SPoCk isoforms with novel, distinct subcellular specificities in the endoplasmic reticulum (ER) and peroxisomes in addition to the Golgi. Furthermore, expression of the peroxisome-associated SPoCk isoform was sexually dimorphic. Overexpression of organelle-specific SPoCk isoforms impacted on cytosolic Ca2+ handling in both cultured Drosophila cells and a transporting epithelium, the Drosophila Malpighian (renal) tubule. Specifically, the ER isoform impacted on inositol-trisphosphate-mediated Ca2+ signaling and the Golgi isoform impacted on diuresis, whereas the peroxisome isoform colocalized with Ca2+ "spherites" and impacted on calcium storage and transport. Interfering RNA directed against the common exons of the three SPoCk isoforms resulted in aberrant Ca2+ signaling and abolished neuropeptide-stimulated diuresis by the tubule. SPoCk thus contributed to both of the contrasting requirements for Ca2+ in transporting epithelia: to transport or store Ca2+ in bulk without compromising its use as a signal.

Animals↗

Prohibitin is a novel target gene of vitamin D involved in its antiproliferative action in breast cancer cells.

Previously, we showed that N-methyl-N-nitrosourea-transformed MCF12F breast epithelial cells exhibited differential expression of several genes, including up-regulation of prohibitin and elevated sensitivity to a relatively noncalcemic vitamin D analogue, 1alpha-hydroxyvitamin D5 [1alpha(OH)D5]. In this report, we evaluated the functional significance of prohibitin in relation to the cellular response to vitamin D. The in silico screening for putative transcription factor binding sites identified two vitamin D receptor (VDR)/retinoid X receptor binding sites in the 1-kb promoter region of prohibitin. Prohibitin up-regulation by 1alpha(OH)D5 treatment at both transcriptional and translational levels was confirmed by real-time reverse transcription-PCR and Western blot analysis in breast cancer cells, identifying prohibitin as a vitamin D target gene. Confocal microscopic analysis showed that prohibitin was localized in the nuclei of MCF-7 cells and a portion of prohibitin was colocalized with VDR, but direct physical interaction between VDR and prohibitin in cell lysates was not detectable. In MCF-7 cells expressing tetracycline-inducible prohibitin (Tet-On model), the overexpression of prohibitin inhibited cell proliferation and enhanced vitamin D-induced antiproliferative activity. Knockdown of prohibitin was accompanied by increased number of cells incorporating bromodeoxyuridine in the whole population and increased cell distribution in the S phase of cell cycle. In addition, prohibitin level had no significant effect on the vitamin D-induced transactivation of CYP24, a VDR target gene. This is the first report to suggest that prohibitin serves as a novel vitamin D target gene, which is involved in the antiproliferative action of vitamin D without affecting CYP24 transactivation in breast cancer cells.

Base Sequence↗

Experimental study of human expressed sequences newly identified in silico as tumor specific.

In our previous studies, we used global computational differential display of ESTs that belong to UNIGENE clusters and identified human sequences differentially expressed in human tumors, as well as a considerable amount of transcripts represented only in tumor-derived cDNA libraries. Most of the tumor-specific EST clusters are derived from the plurality of the tumor types originated in tissues of both ectodermal and mesodermal origin. We found that many of such tumor-specific ESTs do not contain long open reading frames and cannot be classified as protein-encoding genes. To experimentally assess patterns of expression of these EST clusters, we studied four of them in PCR experiments on Clontech MTC panels. The experimental data confirm the results obtained by in silico screening, i.e. tumor specificity of their expression. We suggest that a significant increase in the expression of non-coding RNA is a fundamental feature of cancer cells, and that such transcripts could serve as markers for the diagnosis or monitoring of human malignancies.

DNA, Neoplasm↗

A glucose kinase from Mycobacterium smegmatis.

Carbon metabolism and regulation is poorly understood in mycobacteria, a genus that includes some major pathogenic species like Mycobacterium tuberculosis and Mycobacterium leprae. Here, we report the identification of a glucose kinase from Mycobacterium smegmatis. This enzyme serves in glucose metabolism and global carbon catabolite repression in the related actinomycete Streptomyces coelicolor. The gene, msmeg1356 (glkA), was found by means of in silico screening. It was shown that it occurs in the same genetic context in all so far sequenced mycobacterial species, where it is located in a putative tricistronic operon together with a glycosyl hydrolase and a putative malonyl-CoA transacylase. Heterologous expression of glkA in an Escherichia coli glucose kinase mutant led to the restoration of glucose growth, which provided in vivo evidence for glucose kinase function. GlkA(Msm) was subsequently overproduced in order to study its enzymatic features. We found that it can form a dimer and that it efficiently phosphorylates glucose at the expense of ATP. The affinity constant for glucose was with 9 mM about eight times higher and the velocity was about tenfold slower when compared to the parallel measured glucose kinase of S. coelicolor. Both enzymes showed similar substrate specificity, which consists in an ATP-dependent phosphorylation of glucose and no, or very inefficient, phosphorylation of the glucose analogues 2-deoxyglucose and methyl alpha-glucoside. Hence, our data provide a basis for studying the role of mycobacterial glucose kinase in vivo to unravel possible catalytic and regulatory functions.

Adenosine Triphosphate↗

The proapoptotic factor Nix is coexpressed with Bcl-xL during terminal erythroid differentiation.

Transcriptional profiles of cultured primary human erythroid cells were examined to identify those genes involved in the control of erythroid growth during the terminal phase of maturation. Our in silico screening strategy indicated that a hypoxia-inducible proapoptotic member of the Bcl-2 gene family called Nix is expressed during erythropoiesis. We next performed Northern blot analyses and determined that the 1.4-kb Nix transcript is expressed at lower levels in erythroleukemia cells than reticulocytes. Polymerase chain reaction (PCR)-based transcriptional patterning confirmed the increased expression of Nix during human erythropoiesis with a pattern similar to that of Bcl-xL and glycophorin A and opposite that of Bcl-2. Western blot analyses revealed Nix protein levels that were lower than expected due to increased proteosomal degradation. The expression of Nix and Bcl-xL proteins decreased relative to glyceraldehyde-3-phosphate dehydrogenase (GAPDH) control on the removal of erythropoietin (EPO) from the culture medium. Immunocytochemical analyses demonstrated a similar perinuclear mitochondrial expression pattern for both proteins in hemoglobinized precursors. On the basis of these data, we propose that the proapoptotic factor Nix is a highly regulated effector of growth during terminal erythroid maturation.

Apoptosis↗

VaxiJen: a server for prediction of protective antigens, tumour antigens and subunit vaccines.

BACKGROUND: Vaccine development in the post-genomic era often begins with the in silico screening of genome information, with the most probable protective antigens being predicted rather than requiring causative microorganisms to be grown. Despite the obvious advantages of this approach--such as speed and cost efficiency--its success remains dependent on the accuracy of antigen prediction. Most approaches use sequence alignment to identify antigens. This is problematic for several reasons. Some proteins lack obvious sequence similarity, although they may share similar structures and biological properties. The antigenicity of a sequence may be encoded in a subtle and recondite manner not amendable to direct identification by sequence alignment. The discovery of truly novel antigens will be frustrated by their lack of similarity to antigens of known provenance. To overcome the limitations of alignment-dependent methods, we propose a new alignment-free approach for antigen prediction, which is based on auto cross covariance (ACC) transformation of protein sequences into uniform vectors of principal amino acid properties. RESULTS: Bacterial, viral and tumour protein datasets were used to derive models for prediction of whole protein antigenicity. Every set consisted of 100 known antigens and 100 non-antigens. The derived models were tested by internal leave-one-out cross-validation and external validation using test sets. An additional five training sets for each class of antigens were used to test the stability of the discrimination between antigens and non-antigens. The models performed well in both validations showing prediction accuracy of 70% to 89%. The models were implemented in a server, which we call VaxiJen. CONCLUSION: VaxiJen is the first server for alignment-independent prediction of protective antigens. It was developed to allow antigen classification solely based on the physicochemical properties of proteins without recourse to sequence alignment. The server can be used on its own or in combination with alignment-based prediction methods. It is freely-available online at the URL: http://www.jenner.ac.uk/VaxiJen.

Algorithms↗