Protein dynamics in iron-starved Mycobacterium tuberculosis revealed by turnover and abundance measurement using hybrid-linear ion trap-Fourier transform mass spectrometry

Mycobacterial extracellular vesicles and host pathogen interactions

Mycobacteria, like other bacteria, archaea and eukaryotic cells, naturally release extracellular vesicles (EVs) to interact with their environment. EVs produced by pathogenic bacteria are involved in many activities including cell-cell communication, immunomodulation, virulence and cell survival. Although EVs released by thick cell wall microorganisms like mycobacteria were recognized only recently, studies of Mycobacterium tuberculosis EVs already point to their important roles in host pathogen interactions, opening exciting new areas of investigation. This minireview will summarize the current understanding of mycobacterial EV biology and roles in pathogenesis and will discuss their potential therapeutic applications.

Mitochondrial Function, Biology, and Role in Disease: A Scientific Statement From the American Heart Association

Cardiovascular disease is a major leading cause of morbidity and mortality in the United States and elsewhere. Alterations in mitochondrial function are increasingly being recognized as a contributing factor in myocardial infarction and in patients presenting with cardiomyopathy. Recent understanding of the complex interaction of the mitochondria in regulating metabolism and cell death can provide novel insight and therapeutic targets. The purpose of this statement is to better define the potential role of mitochondria in the genesis of cardiovascular disease such as ischemia and heart failure. To accomplish this, we will define the key mitochondrial processes that play a role in cardiovascular disease that are potential targets for novel therapeutic interventions. This is an exciting time in mitochondrial research. The past decade has provided novel insight into the role of mitochondria function and their importance in complex diseases. This statement will define the key roles that mitochondria play in cardiovascular physiology and disease and provide insight into how mitochondrial defects can contribute to cardiovascular disease; it will also discuss potential biomarkers of mitochondrial disease and suggest potential novel therapeutic approaches.

Mechanisms of Phenotypic Rifampicin Tolerance in Mycobacterium tuberculosis Beijing Genotype Strain B0/W148 Revealed by Proteomics

The "successful" Russian clone B0/W148 of Mycobacterium tuberculosis Beijing is well-known for its capacity to develop antibiotic resistance. During treatment, resistant mutants can occur that have inheritable resistance to specific antibiotics. Next to mutations, M. tuberculosis has several mechanisms that increase their tolerance to a variety of antibiotics. Insights in the phenotypic mechanisms that contribute to drug tolerance will increase our understanding of how antibiotic resistance develops in M. tuberculosis. In this study, we examined the (phospho)proteome dynamics in M. tuberculosis Beijing strain B0/W148 when exposed to a high dose of rifampicin; one of the most potent first-line antibiotics. A total of 2,534 proteins and 191 phosphorylation sites were identified, and revealed the differential regulation of DosR regulon proteins, which are necessary for the development of a dormant phenotype that is less susceptible to antibiotics. By examining independent phenotypic markers of dormancy, we show that persisters of in vitro rifampicin exposure entered a metabolically hypoactive state, which yields rifampicin and other antibiotics largely ineffective. These new insights in the role of protein regulation and post-translational modifications during the initial phase of rifampicin treatment reveal a shortcoming in the antituberculosis regimen that is administered to 8-9 million individuals annually.

Mycobacterium tuberculosis in the Proteomics Era

The emerging field of proteomics has contributed greatly to improving our understanding of the human pathogen Mycobacterium tuberculosis over the last two decades. In this chapter we provide a comprehensive overview of mycobacterial proteome research and highlight key findings. First, studies employing a combination of two-dimensional gel electrophoresis and mass spectrometry (MS) provided insights into the proteomic composition, initially of the whole bacillus and subsequently of subfractions, such as the cell wall, cytosol, and secreted proteins. Comparison of results obtained under various culture conditions, i.e., acidic pH, nutrient starvation, and low oxygen tension, aiming to mimic facets of the intracellular lifestyle of M. tuberculosis, provided initial clues to proteins relevant for intracellular survival and manipulation of the host cell. Further attempts were aimed at identifying the biological functions of the hypothetical M. tuberculosis proteins, which still make up a quarter of the gene products of M. tuberculosis, and at characterizing posttranslational modifications. Recent technological advances in MS have given rise to new methods such as selected reaction monitoring (SRM) and data-independent acquisition (DIA). These targeted, cutting-edge techniques combined with a public database of specific MS assays covering the entire proteome of M. tuberculosis allow the simple and reliable detection of any mycobacterial protein. Most recent studies attempt not only to identify but also to quantify absolute amounts of single proteins in the complex background of host cells without prior sample fractionation or enrichment. Finally, we will discuss the potential of proteomics to advance vaccinology, drug discovery, and biomarker identification to improve intervention and prevention measures for tuberculosis.

A novel pulse-chase SILAC strategy measures changes in protein decay and synthesis rates induced by perturbation of proteostasis with an Hsp90 inhibitor

Standard proteomics methods allow the relative quantitation of levels of thousands of proteins in two or more samples. While such methods are invaluable for defining the variations in protein concentrations which follow the perturbation of a biological system, they do not offer information on the mechanisms underlying such changes. Expanding on previous work [1], we developed a pulse-chase (pc) variant of SILAC (stable isotope labeling by amino acids in cell culture). pcSILAC can quantitate in one experiment and for two conditions the relative levels of proteins newly synthesized in a given time as well as the relative levels of remaining preexisting proteins. We validated the method studying the drug-mediated inhibition of the Hsp90 molecular chaperone, which is known to lead to increased synthesis of stress response proteins as well as the increased decay of Hsp90 "clients". We showed that pcSILAC can give information on changes in global cellular proteostasis induced by treatment with the inhibitor, which are normally not captured by standard relative quantitation techniques. Furthermore, we have developed a mathematical model and computational framework that uses pcSILAC data to determine degradation constants kd and synthesis rates Vs for proteins in both control and drug-treated cells. The results show that Hsp90 inhibition induced a generalized slowdown of protein synthesis and an increase in protein decay. Treatment with the inhibitor also resulted in widespread protein-specific changes in relative synthesis rates, together with variations in protein decay rates. The latter were more restricted to individual proteins or protein families than the variations in synthesis. Our results establish pcSILAC as a viable workflow for the mechanistic dissection of changes in the proteome which follow perturbations. Data are available via ProteomeXchange with identifier PXD000538.

Proteome turnover in bacteria: current status for Corynebacterium glutamicum and related bacteria

With the advent of high-resolution mass spectrometry together with sophisticated data analysis and interpretation algorithms, determination of protein synthesis and degradation rates (i.e. protein turnover) on a proteome-wide scale by employing stable isotope-labelled amino acids has become feasible. These dynamic data provide a deeper understanding of protein homeostasis and stress response mechanisms in microorganisms than well-established ‘steady state’ proteomics approaches. In this article, we summarize the technological challenges and solutions both on the biochemistry/mass spectrometry and bioinformatics level for turnover proteomics with a focus on chromatographic techniques. Although the number of available case studies for Corynebacterium glutamicum and related actinobacteria is still very limited, our review illustrates the potential of protein turnover studies for an improved understanding of questions in the area of biotechnology and biomedicine. Here, new insights from investigations of growth phase transition and different stress dynamics including iron, acid and heat stress for pathogenic but also for industrial actinobacteria are presented. Finally, we will comment on the advantages of integrated software solutions for biologists and briefly discuss the remaining technical challenges and upcoming possibilities for protein turnover analysis.

An Internal Standard-Assisted Synthesis and Degradation Proteomic Approach Reveals the Potential Linkage between VPS4B Depletion and Activation of Fatty Acid β-Oxidation in Breast Cancer Cells

The endosomal/lysosomal system, in particular the endosomal sorting complexes required for transport (ESCRTs), plays an essential role in regulating the trafficking and destination of endocytosed receptors and their associated signaling molecules. Recently, we have shown that dysfunction and down-regulation of vacuolar protein sorting 4B (VPS4B), an ESCRT-III associated protein, under hypoxic conditions can lead to the abnormal accumulation of epidermal growth factor receptor (EGFR) and aberrant EGFR signaling in breast cancer. However, the pathophysiological consequences of VPS4B dysfunction remain largely elusive. In this study, we used an internal standard-assisted synthesis and degradation mass spectrometry (iSDMS) method, which permits the direct measurement of protein synthesis, degradation and protein dynamic expression, to address the effects of VPS4B dysfunction in altering EGF-mediated protein expression. Our initial results indicate that VPS4B down-regulation decreases the expression of many proteins involved in glycolytic pathways, while increased the expression of proteins with roles in mitochondrial fatty acid β-oxidation were up-regulated in VPS4B-depleted cells. This observation is also consistent with our previous finding that hypoxia can induce VPS4B down-regulated, suggesting that the adoption of fatty acid β-oxidation could potentially serve as an alternative energy source and survival mechanism for breast cancer cells in response to hypoxia-mediated VPS4B dysfunction.

Protein turnover quantification in a multilabeling approach: from data calculation to evaluation

Liquid chromatography coupled to tandem mass spectrometry in combination with stable-isotope labeling is an established and widely spread method to measure gene expression on the protein level. However, it is often not considered that two opposing processes are responsible for the amount of a protein in a cell--the synthesis as well as the degradation. With this work, we provide an integrative, high-throughput method--from the experimental setup to the bioinformatics analysis--to measure synthesis and degradation rates of an organism's proteome. Applicability of the approach is demonstrated with an investigation of heat shock response, a well-understood regulatory mechanism in bacteria, on the biotechnologically relevant Corynebacterium glutamicum. Utilizing a multilabeling approach using both heavy stable nitrogen as well as carbon isotopes cells are metabolically labeled in a pulse-chase experiment to trace the labels' incorporation in newly synthesized proteins and its loss during protein degradation. Our work aims not only at the calculation of protein turnover rates but also at their statistical evaluation, including variance and hierarchical cluster analysis using the rich internet application QuPE.

Metalloregulation of Gram-positive pathogen physiology

Owing to the unique redox potential of transition metals, many of these elements serve important roles as cofactors in numerous enzymes. However, the reactive nature of metal becomes an intracellular threat when these ions are present in excess. Therefore, all organisms require mechanisms for sensing small fluctuations in metal levels to maintain a controlled balance of uptake, efflux, and sequestration. The ability to sense metal ion concentration is especially important for the survival of pathogenic bacteria because host organisms can both restrict access to essential metals from invading pathogens and utilize the innate toxicity of certain metals for bacterial killing. Host-induced metal ion fluctuations must be rapidly sensed by pathogenic bacteria so that they can activate metal transport systems, alter their physiology to accommodate differences in metal concentrations, and regulate the expression of virulence factors.

Latent tuberculosis: what the host "sees"?

Mycobacterium tuberculosis (MTB), the causative agent of tuberculosis (TB), is the most successful pathogen of mankind and remains a major threat to global health as the leading cause of death due to a bacterial pathogen. Yet 90-95% of those who are infected with MTB remain otherwise healthy. These people are classified as "latently infected," but remain a reservoir from which active TB cases will continue to develop ("reactivation tuberculosis"). Latent infection is defined by the absence of clinical symptoms of TB in addition to a delayed hypersensitivity reaction to the purified protein derivative of MTB used in tuberculin skin test or a T-cell response to MTB-specific antigens. In the absence of reliable control measures for tuberculosis, understanding latent MTB infection and subsequent reactivation is a research priority. This review aims to summarize the recent findings in human and non-human primate models of tuberculosis that have led to new concepts of latent tuberculosis.

Label-free proteomics and systems biology analysis of mycobacterial phagosomes in dendritic cells and macrophages

Proteomics has been applied to study intracellular bacteria and phagocytic vacuoles in different host cell lines, especially macrophages (Mφs). For mycobacterial phagosomes, few studies have identified over several hundred proteins for systems assessment of the phagosome maturation and antigen presentation pathways. More importantly, there has been a scarcity in publication on proteomic characterization of mycobacterial phagosomes in dendritic cells (DCs). In this work, we report a global proteomic analysis of Mφ and DC phagosomes infected with a virulent, an attenuated, and a vaccine strain of mycobacteria. We used label-free quantitative proteomics and bioinformatics tools to decipher the regulation of phagosome maturation and antigen presentation pathways in Mφs and DCs. We found that the phagosomal antigen presentation pathways are repressed more in DCs than in Mφs. The results suggest that virulent mycobacteria might co-opt the host immune system to stimulate granuloma formation for persistence while minimizing the antimicrobial immune response to enhance mycobacterial survival. The studies on phagosomal proteomes have also shown promise in discovering new antigen presentation mechanisms that a professional antigen presentation cell might use to overcome the mycobacterial blockade of conventional antigen presentation pathways.

Iron-sparing response of Mycobacterium avium subsp. paratuberculosis is strain dependent

Background

Two genotypically and microbiologically distinct strains of Mycobacterium avium subsp. paratuberculosis (MAP) exist - S and C MAP strains that primarily infect sheep and cattle, respectively. Concentration of iron in the cultivation medium has been suggested as one contributing factor for the observed microbiologic differences. We recently demonstrated that S strains have defective iron storage systems, leading us to propose that these strains might experience iron toxicity when excess iron is provided in the medium. To test this hypothesis, we carried out transcriptional and proteomic profiling of these MAP strains under iron-replete or -deplete conditions.

Results

We first complemented M. smegmatisΔideR with IdeR of C MAP or that derived from S MAP and compared their transcription profiles using M. smegmatis mc²155 microarrays. In the presence of iron, sIdeR repressed expression of bfrA and MAP2073c, a ferritin domain containing protein suggesting that transcriptional control of iron storage may be defective in S strain. We next performed transcriptional and proteomic profiling of the two strain types of MAP under iron-deplete and -replete conditions. Under iron-replete conditions, C strain upregulated iron storage (BfrA), virulence associated (Esx-5 and antigen85 complex), and ribosomal proteins. In striking contrast, S strain downregulated these proteins under iron-replete conditions. iTRAQ (isobaric tag for relative and absolute quantitation) based protein quantitation resulted in the identification of four unannotated proteins. Two of these were upregulated by a C MAP strain in response to iron supplementation. The iron-sparing response to iron limitation was unique to the C strain as evidenced by repression of non-essential iron utilization enzymes (aconitase and succinate dehydrogenase) and upregulation of proteins of essential function (iron transport, [Fe-S] cluster biogenesis and cell division).

Conclusions

Taken together, our study revealed that C and S strains of MAP utilize divergent metabolic pathways to accommodate in vitro iron stress. The knowledge of the metabolic pathways these divergent responses play a role in are important to 1) advance our ability to culture the two different strains of MAP efficiently, 2) aid in diagnosis and control of Johne's disease, and 3) advance our understanding of MAP virulence.

Advances in protein turnover analysis at the global level and biological insights

The concept of a dynamic state of body constituents, a precursor of the modern term of proteome dynamics, was conceived over a century ago. But, not until recently can we examine the dynamics of individual "constituents" for example, proteins at a truly global level. The path of advancement in our understanding of protein turnover at the global level is marked by the introduction of some key technological innovations. These methods include the isotopic tracer technique in the 1930s, the two-dimensional gel electrophoresis technique in the 1970s, the sector mass spectrometer that could analyze isotopomers of peptides in the early 1990s, the 2D gel/MALDI-TOF proteomics technology in the late 1990s, the booming liquid chromatography/mass spectrometry proteomics technology in this decade, and the recently emerging protein-tagging approaches that offer single-cell resolution for protein turnover measurements. The long-standing inquiry raised in the 1950s about the existence of a dynamic state in different organisms at different physiological conditions can now be answered with an individual "constituent" resolution on a truly global scale. Now it appears that protein degradation is not necessarily an end to the protein function. Rather, it can be the start of a new function because protein degradation clears the way for the action of other proteins. Protein turnover participates in a multi-layer complex regulatory network and shares equal importance with gene transcription and protein translation. The advances in technologies for protein turnover analysis and the improved understanding of the biological role of protein turnover will likely help to solve some long-standing biomedical problems such as the tuberculosis disease that at the present day still affects one-third of the world population.

Assigning significance in label-free quantitative proteomics to include single-peptide-hit proteins with low replicates

Selecting differentially regulated proteins with an assignment of statistical significance remains difficult for proteins with a single-peptide hit or a small fold-change when sample replicates are limited. This study presents a label-free quantitative proteomics scheme that was used to select differentially regulated proteins with single-peptide hits and with <2-fold change at a 5% false discovery rate. The scheme incorporated a labeled internal control into two unlabeled samples to facilitate error modeling when there were no replicates for the unlabeled samples. The results showed that, while both a power law global error model with a signal-to-noise ratio statistic (PLGEM-STN) and a constant fold-change threshold could be used, neither of them alone was stringent enough to select differentially regulated proteins at a 5% false discovery rate. Thus, the rule of minimum number of permuted significant pairings (MPSP) was introduced to reduce false discovery rates in combination with PLGEM-STN or a fold-change threshold. MPSP played a critical role in extending the selection of differentially regulated proteins to those with single-peptide hits or with lower fold-changes. Although the approaches were demonstrated for limited sample replicates, they should also be applicable to the situation where more sample replicates are available.

Systems biology approaches to understanding mycobacterial survival mechanisms

The advent of high-throughput platforms for the interrogation of biological systems at the cellular and molecular level have allowed living cells to be observed and understood at a hitherto unprecedented level of detail and have enabled the construction of comprehensive, predictive in silico models. Here, we review the application of such high-throughput, systems-biological techniques to mycobacteria-specifically to the pernicious human pathogen Mycobacterium tuberculosis (MTb) and its ability to survive in human hosts. We discuss the development and application of transcriptomic, proteomic, regulomic, and metabolomic techniques for MTb as well as the development and application of genome-scale in silico models. Thus far, systems-biological approaches have largely focused on in vitro models of MTb growth; reliably extending these approaches to in vivo conditions relevant to infection is a significant challenge for the future that holds the ultimate promise of novel chemotherapeutic interventions.

Acid stress response of a mycobacterial proteome: insight from a gene ontology analysis

Acidity in vesicles of macrophages is a general signal that bacteria respond to during infection. Mycobacteria are particularly capable of resisting the acidification in macrophages that engulf the bacteria. In this work, we used label-free quantitative proteomics to study the Mycobacterium smegmatis proteome under acid stress so as to gain an insight into the acidic adaptation in mycobacteria. We quantified 1032 proteins. With a 3-fold change threshold, 20 and 52 proteins were found regulated at false discovery rates of 5% and 14% respectively. We performed a systems analysis based on gene ontology for the global proteome expression profile. We found that the most significant changes induced by the acid stress include a downregulation of transmembrane transporter activity and an upregulation of enzymes involved in fatty acid metabolism. The results suggest that reduced transmembrane transport and increased fatty acid metabolism probably contribute to or associate with acid tolerance in mycobacteria.

A systems biology approach to study the phagosomal proteome modulated by mycobacterial infections

Systems biology and proteomics have recently contributed significantly to the insight into the biogenesis and immunity-related functions of the phagosome. To gain insight into the modulation of the phagosomal proteome by the wild-type Mycobacterium tuberculosis H37Rv reference strain, an attenuated mutant of the H37Rv strain, and the BCG Pasteur vaccine strain, we employed the nano-liquid chromatography/LTQ-FTMS based proteomics approach and a systems biology approach to analyze the bacillus-containing phagosomes purified from the bone-marrow-derived BMA3.A3 macrophages infected with the three different mycobacterial strains. We identified 322 proteins at a false-discovery rate of 2%. These proteins were quantified with a label-free proteomics method. All but one of these proteins is mouse proteins. The gene ontology analysis of these mouse proteins suggests that lysosomal proteins represented <3% of the detected proteins, supporting the observation that these mycobacterial strains inhibit or limit the phagosome maturation process. The results also indicate that the endoplasmic reticulum (ER) proteins do not constitute a major part of the phagosome proteome, supporting the phagosome maturation model of the role of ER in phagosome biogenesis. This phagosome maturation model is in contrast to the phagocytosis model which predicts that half of the phagosome membrane is derived from ER. This pilot study demonstrates that a combination of proteomics, multivariate analysis, and systems biology promises to bring forward new insights into the mycobacterial pathogenesis and the interconnected phagosome biology.

Protein turnover in mycobacterial proteomics

Understanding the biology of Mycobacterium tuberculosis is one of the primary challenges in current tuberculosis research. Investigation of mycobacterial biology using the systems biology approach has deciphered much information with regard to the bacilli and tuberculosis pathogenesis. The modulation of its environment and the ability to enter a dormant phase are the hallmarks of M. tuberculosis. Until now, proteome studies have been able to understand much about the role of various proteins, mostly in growing M. tuberculosis cells. It has been difficult to study dormant M. tuberculosis by conventional proteomic techniques with very few proteins being found to be differentially expressed. Discrepancy between proteome and transcriptome studies lead to the conclusion that a certain aspect of the mycobacterial proteome is not being explored. Analysis of protein turnover may be the answer to this dilemma. This review, while giving a gist of the proteome response of mycobacteria to various stresses, analyzes the data obtained from abundance studies versus data from protein turnover studies in M. tuberculosis. This review brings forth the point that protein turnover analysis is capable of discerning more subtle changes in protein synthesis, degradation, and secretion activities. Thus, turnover studies could be incorporated to provide a more in-depth view into the proteome, especially in dormant or persistent cells. Turnover analysis might prove helpful in drug discovery and a better understanding of the dynamic nature of the proteome of mycobacteria.

An assessment of false discovery rates and statistical significance in label-free quantitative proteomics with combined filters

Background

Many studies have provided algorithms or methods to assess a statistical significance in quantitative proteomics when multiple replicates for a protein sample and a LC/MS analysis are available. But, confidence is still lacking in using datasets for a biological interpretation without protein sample replicates. Although a fold-change is a conventional threshold that can be used when there are no sample replicates, it does not provide an assessment of statistical significance such as a false discovery rate (FDR) which is an important indicator of the reliability to identify differentially expressed proteins. In this work, we investigate whether differentially expressed proteins can be detected with a statistical significance from a pair of unlabeled protein samples without replicates and with only duplicate LC/MS injections per sample. A FDR is used to gauge the statistical significance of the differentially expressed proteins.

Results

We have experimented to operate on several parameters to control a FDR, including a fold-change, a statistical test, and a minimum number of permuted significant pairings. Although none of these parameters alone gives a satisfactory control of a FDR, we find that a combination of these parameters provides a very effective means to control a FDR without compromising the sensitivity. The results suggest that it is possible to perform a significance analysis without protein sample replicates. Only duplicate LC/MS injections per sample are needed. We illustrate that differentially expressed proteins can be detected with a FDR between 0 and 15% at a positive rate of 4–16%. The method is evaluated for its sensitivity and specificity by a ROC analysis, and is further validated with a [¹⁵N]-labeled internal-standard protein sample and additional unlabeled protein sample replicates.

Conclusion

We demonstrate that a statistical significance can be inferred without protein sample replicates in label-free quantitative proteomics. The approach described in this study would be useful in many exploratory experiments where a sample amount or instrument time is limited. Naturally, this method is also suitable for proteomics experiments where multiple sample replicates are available. It is simple, and is complementary to other more sophisticated algorithms that are not designed for dealing with a small number of sample replicates.

Principal Component Analysis of Proteome Dynamics in Iron-starved Mycobacterium Tuberculosis

The goal of this study is to use principal component analysis (PCA) for multivariate analysis of proteome dynamics based on both protein abundance and turnover information generated by high-resolution mass spectrometry. We previously reported assessing protein dynamics in iron-starved Mycobacterium tuberculosis, revealing interesting interconnection among the cellular processes involving protein synthesis, degradation, and secretion (Anal. Chem. 80, 6860-9). In this study, we use target-decoy database search approach to select peptides for quantitation at a false discovery rate of 4.2%. We further use PCA to reduce the data dimensions for simpler interpretation. The PCA results indicate that the protein turnover and relative abundance properties are approximately orthogonal in the data space defined by the first three principal components. We show the potential of the Hotelling's T2 (T2) value as a quantifiable index for comparing changes between protein functional categories. The T2 value represents the gross change of a protein in both abundance and turnover. Close examination of the antigen 85 complex demonstrates that T2 correctly predicts the coordinated changes of the antigen 85 complex proteins. The multi-dimensional protein dynamics data further reveal the secretion of the antigen 85 complex. Overall, this study demonstrates PCA as an effective means to facilitate interpretation of the multivariate proteome dynamics dataset which otherwise would remain a significant challenge using traditional methods.