Quantitative Map of β-Lactone-Induced Virulence Regulation

Targeting caseinolytic protease P and its AAA1 chaperone for tuberculosis treatment

Caseinolytic protease P with its AAA1 chaperone, known as Mycobacterium tuberculosis (Mtb)ClpP1P2 proteolytic machinery, maintains protein homeostasis in Mtb cells and is essential for bacterial survival. It is regarded as an important biological target with the potential to address the increasingly serious issue of multidrug-resistant (MDR) TB. Over the past 10 years, many MtbClpP1P2-targeted modulators have been identified and characterized, some of which have shown potent anti-TB activity. In this review, we describe current understanding of the substrates, structure and function of MtbClpP1P2, classify the modulators of this important protein machine into several categories based on their binding subunits or pockets, and discuss their binding details; Such information provides insights for use in candidate drug research and development of TB treatments by targeting MtbClpP1P2 proteolytic machinery.

β-Lactams from the Ocean

The title of this essay is as much a question as it is a statement. The discovery of the β-lactam antibiotics-including penicillins, cephalosporins, and carbapenems-as largely (if not exclusively) secondary metabolites of terrestrial fungi and bacteria, transformed modern medicine. The antibiotic β-lactams inactivate essential enzymes of bacterial cell-wall biosynthesis. Moreover, the ability of the β-lactams to function as enzyme inhibitors is of such great medical value, that inhibitors of the enzymes which degrade hydrolytically the β-lactams, the β-lactamases, have equal value. Given this privileged status for the β-lactam ring, it is therefore a disappointment that the exemplification of this ring in marine secondary metabolites is sparse. It may be that biologically active marine β-lactams are there, and simply have yet to be encountered. In this report, we posit a second explanation: that the value of the β-lactam to secure an ecological advantage in the marine environment might be compromised by its close structural similarity to the β-lactones of quorum sensing. The steric and reactivity similarities between the β-lactams and the β-lactones represent an outside-of-the-box opportunity for correlating new structures and new enzyme targets for the discovery of compelling biological activities.

Cellular functions of the ClpP protease impacting bacterial virulence

Proteostasis mechanisms significantly contribute to the sculpting of the proteomes of all living organisms. ClpXP is a central AAA+ chaperone-protease complex present in both prokaryotes and eukaryotes that facilitates the unfolding and subsequent degradation of target substrates. ClpX is a hexameric unfoldase ATPase, while ClpP is a tetradecameric serine protease. Substrates of ClpXP belong to many cellular pathways such as DNA damage response, metabolism, and transcriptional regulation. Crucially, disruption of this proteolytic complex in microbes has been shown to impact the virulence and infectivity of various human pathogenic bacteria. Loss of ClpXP impacts stress responses, biofilm formation, and virulence effector protein production, leading to decreased pathogenicity in cell and animal infection models. Here, we provide an overview of the multiple critical functions of ClpXP and its substrates that modulate bacterial virulence with examples from several important human pathogens.

Tailored phenyl esters inhibit ClpXP and attenuate Staphylococcus aureus α-hemolysin secretion

Novel strategies against multidrug‐resistant bacteria are urgently needed in order to overcome the current silent pandemic. Manipulation of toxin production in pathogenic species serves as a promising approach to attenuate virulence and prevent infections. In many bacteria such as Staphylococcus aureus or Listeria monocyotgenes, serine protease ClpXP is a key contributor to virulence and thus represents a prime target for antimicrobial drug discovery. The limited stability of previous electrophilic warheads has prevented a sustained effect of virulence attenuation in bacterial culture. Here, we systematically tailor the stability and inhibitory potency of phenyl ester ClpXP inhibitors by steric shielding of the ester bond and fine‐tuning the phenol leaving group. Out of 17 derivatives, two (MAS‐19 and MAS‐30) inhibited S. aureus ClpP peptidase and ClpXP protease activities by >60 % at 1 μM. Furthermore, the novel inhibitors did not exhibit pronounced cytotoxicity against human and bacterial cells. Unlike the first generation phenylester AV170, these molecules attenuated S. aureus virulence markedly and displayed increased stability in aqueous buffer compared to the previous benchmark AV170.

Armeniaspirol analogues with more potent Gram-positive antibiotic activity show enhanced inhibition of the ATP-dependent proteases ClpXP and ClpYQ

Antibiotics with fundamentally new mechanisms of action such as the armeniaspirols, which target the ATP-dependent proteases ClpXP and ClpYQ, must be developed to combat antimicrobial resistance. While the mechanism of action of armeniaspirol against Gram-positive bacteria is understood, little is known about the structure-activity relationship for its antibiotic activity. Based on the preliminary data showing that modifications of armeniaspirol's N-methyl group increased antibiotic potency, we probed the structure-activity relationship of N-alkyl armeniaspirol derivatives. A series of focused derivatives were synthesized and evaluated for antibiotic activity against clinically relevant pathogens including methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus. Replacement of the N-methyl with N-hexyl, various N-benzyl, and N-phenethyl substituents led to substantial increases in antibiotic activity and potency for inhibition of both ClpYQ and ClpXP. Docking studies identified binding models for ClpXP and ClpYQ that were consistent with the inhibition data. This work confirms the role of ClpXP and ClpYQ in the mechanism of action of armeniaspirol and provides important lead compounds for further antibiotic development.

Recent advances in Clp protease modulation to address virulence, resistance and persistence of MRSA infection

The Clp protease is an AAA⁺ protease that executes abnormally folded or malfunctioning proteins, and has an important role in producing virulence factors, forming biofilms or persisters and developing methicillin-resistant Staphylococcus aureus (MRSA). Recent studies showed that Clp protease controls virulence via agr signaling and degrades antitoxins of the toxin-antitoxin system to modulate the formation of persisters and biofilms. In this review, we focus on recent developments concerning the virulence and persistence regulatory pathways and resistance-related mechanism of Clp protease in S. aureus, with an overview of the Clp modulators developed to treat MRSA infection.

Profiling of Haemophilus influenzae strain R2866 with carbohydrate-based covalent probes

We demonstrate the application of four covalent probes based on anomerically pure d-galactosamine and d-glucosamine scaffolds for the profiling of Haemophilus influenzae strain R2866. The probes have been used successfully for the labelling of target proteins not only in cell lysates, but also in intact cells. Differences in the labelling patterns between lysates and intact cells indicate that the probes can penetrate into the periplasm, but not the cytoplasm of H. influenzae. Analysis of selected target proteins by LC-MS/MS suggests predominant labelling of nucleotide-binding proteins, including several known antibacterial drug targets. Our protocols will aid the identification of molecular determinants of bacterial pathogenicity in Haemophilus influenzae and other bacterial pathogens.

Human ClpP protease, a promising therapy target for diseases of mitochondrial dysfunction

Human caseinolytic protease P (HsClpP), an ATP-dependent unfolding peptidase protein in the mitochondrial matrix, controls protein quality, regulates mitochondrial metabolism, and maintains the integrity and enzyme activity of the mitochondrial respiratory chain (RC). Studies show that abnormalities in HsClpP lead to mitochondrial dysfunction and various human diseases. In this review, we provide a comprehensive overview of the structure and biological function of HsClpP, and the involvement of its dysexpression or mutation in mitochondria for a panel of important human diseases. We also summarize the structural types and binding modes of known HsClpP modulators. Finally, we discuss the challenges and future directions of HsClpP targeting as promising approach for the treatment of human diseases of mitochondrial origin.

Global Inventory of ClpP- and ClpX-Regulated Proteins in Staphylococcus aureus

Staphylococcus aureus represents an opportunistic pathogen, which utilizes elaborate quorum sensing mechanisms to precisely control the expression and secretion of virulence factors. Previous studies indicated a role of the ClpXP proteolytic system in controlling pathogenesis. While detailed transcriptome data for S. aureus ClpP and ClpX knockout mutants is available, corresponding studies on the proteome and secretome level are largely lacking. To globally decipher the functional roles of ClpP and ClpX, we utilized S. aureus genomic deletion mutants of the corresponding genes for in-depth proteomic liquid chromatography-mass spectrometry (LC-MS)/MS analysis. These studies were complemented by an inactive ClpP active-site mutant strain to monitor changes solely depending on the activity and not the presence of the protein. A comparison of these strains with the wildtype revealed, e.g., downregulation of virulence, purine/pyrimidine biosynthesis, iron uptake, and stress response. Correspondingly, the integration of metabolomics data showed a reduction in the subset of purine and pyrimidine metabolite levels. Interestingly, a comparison between the ClpP knockout and ClpP S98A active-site mutant strains revealed characteristic differences. These results are not only of fundamental importance to understand the cellular role of ClpXP but also have implications for the development of novel virulence inhibitor classes.

Chemical tools for selective activity profiling of bacterial penicillin-binding proteins

Penicillin-binding proteins (PBPs) are membrane-associated proteins involved in the biosynthesis of peptidoglycan (PG), the main component of bacterial cell walls. These proteins were discovered and named for their affinity to bind the β-lactam antibiotic penicillin. The importance of the PBPs has long been appreciated; however, specific roles of individual family members in each bacterial strain, as well as their protein-protein interactions, are yet to be understood. The apparent functional redundancy of the 4-18 PBPs that most eubacteria possess makes determination of their individual roles difficult. Existing techniques to study PBPs are not ideal because they do not directly visualize protein activity and can suffer from artifacts and perturbations of native PBP function. Therefore, development of new methods for studying the roles of individual PBPs in cell wall synthesis is required. We recently generated a library of fluorescent chemical probes containing a β-lactone scaffold that specifically targets the PBPs, enabling the visualization of their catalytic activity. Herein, we describe a general protocol to label and detect the activity of individual PBPs in Streptococcus pneumoniae using our fluorescent β-lactone probes.

Sample Preparation by Easy Extraction and Digestion (SPEED) - A Universal, Rapid, and Detergent-free Protocol for Proteomics Based on Acid Extraction

The main challenge of bottom-up proteomic sample preparation is to extract proteomes in a manner that enables efficient protein digestion for subsequent mass spectrometric analysis. Today's sample preparation strategies are commonly conceptualized around the removal of detergents, which are essential for extraction but strongly interfere with digestion and LC-MS. These multi-step preparations contribute to a lack of reproducibility as they are prone to losses, biases and contaminations, while being time-consuming and labor-intensive. We report a detergent-free method, named Sample Preparation by Easy Extraction and Digestion (SPEED), which consists of three mandatory steps, acidification, neutralization and digestion. SPEED is a universal method for peptide generation from various sources and is easily applicable even for lysis-resistant sample types as pure trifluoroacetic acid (TFA) is used for highly efficient protein extraction by complete sample dissolution. The protocol is highly reproducible, virtually loss-less, enables very rapid sample processing and is superior to the detergent/chaotropic agent-based methods FASP, ISD-Urea and SP3 for quantitative proteomics. SPEED holds the potential to dramatically simplify and standardize sample preparation while improving the depth of proteome coverage especially for challenging samples.

Chemical Modulation of Human Mitochondrial ClpP: Potential Application in Cancer Therapeutics

The human ClpP proteolytic complex (HsClpP) is a serine protease located in the mitochondrial matrix and participates in the maintenance of the mitochondrial proteome among other cellular functions. HsClpP typically forms a multimeric complex with the AAA+ protein unfoldase HsClpX. Notably, compared to that of normal, healthy cells, the expression of HsClpP in many types of solid and nonsolid cancers is found to be upregulated. While the exact role of HsClpP in tumorigenesis is not clear, certain types of cancers are highly dependent on the protease for cell proliferation and metastasis. In light of these observations, recent research has focused on the discovery and characterization of small organic molecules that can target and modulate HsClpP activity. These include compounds that inhibit HsClpP's proteolytic activity via covalent modification of its catalytic Ser residue as well as those that activate and dysregulate HsClpP by displacing HsClpX to negate its regulatory role. Importantly, several of these compounds have been shown to induce HsClpP-dependent apoptotic cell death in a variety of cancerous cells. This review provides an overview of these research efforts and highlights the various types of small molecule modulators of HsClpP activity with respect to their potential use as cancer therapeutics.

Biosynthesis and chemical diversity of β-lactone natural products

Covering: up to 2018 β-Lactones are strained rings that are useful organic synthons and pharmaceutical warheads. Over 30 core scaffolds of β-lactone natural products have been described to date, many with potent bioactivity against bacteria, fungi, or human cancer cell lines. β-Lactone natural products are chemically diverse and have high clinical potential, but production of derivatized drug leads has been largely restricted to chemical synthesis partly due to gaps in biochemical knowledge about β-lactone biosynthesis. Here we review recent discoveries in enzymatic β-lactone ring closure via ATP-dependent synthetases, intramolecular cyclization from seven-membered rings, and thioesterase-mediated cyclization during release from nonribosomal peptide synthetase assembly lines. We also comprehensively cover the diversity and taxonomy of source organisms for β-lactone natural products including their isolation from bacteria, fungi, plants, insects, and marine sponges. This work identifies computational and experimental bottlenecks and highlights future directions for genome-based discovery of biosynthetic gene clusters that may produce novel compounds with β-lactone rings.

Recent Advances in Targeting Human Mitochondrial AAA+ Proteases to Develop Novel Cancer Therapeutics

The mitochondrion is a vital organelle that performs diverse cellular functions. In this regard, the cell has evolved various mechanisms dedicated to the maintenance of the mitochondrial proteome. Among them, AAA+ ATPase-associated proteases (AAA+ proteases) such as the Lon protease (LonP1), ClpXP complex, and the membrane-bound i-AAA, m-AAA and paraplegin facilitate the clearance of misfolded mitochondrial proteins to prevent the accumulation of cytotoxic protein aggregates. Furthermore, these proteases have additional regulatory functions in multiple biological processes that include amino acid metabolism, mitochondria DNA transcription, metabolite and cofactor biosynthesis, maturation and turnover of specific respiratory and metabolic proteins, and modulation of apoptosis, among others. In cancer cells, the increase in intracellular ROS levels promotes tumorigenic phenotypes and increases the frequency of protein oxidation and misfolding, which is compensated by the increased expression of specific AAA+ proteases as part of the adaptation mechanism. The targeting of AAA+ proteases has led to the discovery and development of novel anti-cancer compounds. Here, we provide an overview of the molecular characteristics and functions of the major mitochondrial AAA+ proteases and summarize recent research efforts in the development of compounds that target these proteases.

Thinking Outside the Box-Novel Antibacterials To Tackle the Resistance Crisis

The public view on antibiotics as reliable medicines changed when reports about "resistant superbugs" appeared in the news. While reasons for this resistance development are easily spotted, solutions for re-establishing effective antibiotics are still in their infancy. This Review encompasses several aspects of the antibiotic development pipeline from very early strategies to mature drugs. An interdisciplinary overview is given of methods suitable for mining novel antibiotics and strategies discussed to unravel their modes of action. Select examples of antibiotics recently identified by using these platforms not only illustrate the efficiency of these measures, but also highlight promising clinical candidates with therapeutic potential. Furthermore, the concept of molecules that disarm pathogens by addressing gatekeepers of virulence will be covered. The Review concludes with an evaluation of antibacterials currently in clinical development. Overall, this Review aims to connect select innovative antimicrobial approaches to stimulate interdisciplinary partnerships between chemists from academia and industry.