Ureadepsipeptides as ClpP Activators

Sophisticated natural products as antibiotics

In this Review, we explore natural product antibiotics that do more than simply inhibit an active site of an essential enzyme. We review these compounds to provide inspiration for the design of much-needed new antibacterial agents, and examine the complex mechanisms that have evolved to effectively target bacteria, including covalent binders, inhibitors of resistance, compounds that utilize self-promoted entry, those that evade resistance, prodrugs, target corrupters, inhibitors of 'undruggable' targets, compounds that form supramolecular complexes, and selective membrane-acting agents. These are exemplified by β-lactams that bind covalently to inhibit transpeptidases and β-lactamases, siderophore chimeras that hijack import mechanisms to smuggle antibiotics into the cell, compounds that are activated by bacterial enzymes to produce reactive molecules, and antibiotics such as aminoglycosides that corrupt, rather than merely inhibit, their targets. Some of these mechanisms are highly sophisticated, such as the preformed β-strands of darobactins that target the undruggable β-barrel chaperone BamA, or teixobactin, which binds to a precursor of peptidoglycan and then forms a supramolecular structure that damages the membrane, impeding the emergence of resistance. Many of the compounds exhibit more than one notable feature, such as resistance evasion and target corruption. Understanding the surprising complexity of the best antimicrobial compounds provides a roadmap for developing novel compounds to address the antimicrobial resistance crisis by mining for new natural products and inspiring us to design similarly sophisticated antibiotics.

Borg extrachromosomal elements of methane-oxidizing archaea have conserved and expressed genetic repertoires

Borgs are huge extrachromosomal elements (ECE) of anaerobic methane-consuming "Candidatus Methanoperedens" archaea. Here, we used nanopore sequencing to validate published complete genomes curated from short reads and to reconstruct new genomes. 13 complete and four near-complete linear genomes share 40 genes that define a largely syntenous genome backbone. We use these conserved genes to identify new Borgs from peatland soil and to delineate Borg phylogeny, revealing two major clades. Remarkably, Borg genes encoding nanowire-like electron-transferring cytochromes and cell surface proteins are more highly expressed than those of host Methanoperedens, indicating that Borgs augment the Methanoperedens activity in situ. We reconstructed the first complete 4.00 Mbp genome for a Methanoperedens that is inferred to be a Borg host and predicted its methylation motifs, which differ from pervasive TC and CC methylation motifs of the Borgs. Thus, methylation may enable Methanoperedens to distinguish their genomes from those of Borgs. Very high Borg to Methanoperedens ratios and structural predictions suggest that Borgs may be capable of encapsulation. The findings clearly define Borgs as a distinct class of ECE with shared genomic signatures, establish their diversification from a common ancestor with genetic inheritance, and raise the possibility of periodic existence outside of host cells.

Knockout Mouse Studies Show That Mitochondrial CLPP Peptidase and CLPX Unfoldase Act in Matrix Condensates near IMM, as Fast Stress Response in Protein Assemblies for Transcript Processing, Translation, and Heme Production

LONP1 is the principal AAA+ unfoldase and bulk protease in the mitochondrial matrix, so its deletion causes embryonic lethality. The AAA+ unfoldase CLPX and the peptidase CLPP also act in the matrix, especially during stress periods, but their substrates are poorly defined. Mammalian CLPP deletion triggers infertility, deafness, growth retardation, and cGAS-STING-activated cytosolic innate immunity. CLPX mutations impair heme biosynthesis and heavy metal homeostasis. CLPP and CLPX are conserved from bacteria to humans, despite their secondary role in proteolysis. Based on recent proteomic-metabolomic evidence from knockout mice and patient cells, we propose that CLPP acts on phase-separated ribonucleoprotein granules and CLPX on multi-enzyme condensates as first-aid systems near the inner mitochondrial membrane. Trimming within assemblies, CLPP rescues stalled processes in mitoribosomes, mitochondrial RNA granules and nucleoids, and the D-foci-mediated degradation of toxic double-stranded mtRNA/mtDNA. Unfolding multi-enzyme condensates, CLPX maximizes PLP-dependent delta-transamination and rescues malformed nascent peptides. Overall, their actions occur in granules with multivalent or hydrophobic interactions, separated from the aqueous phase. Thus, the role of CLPXP in the matrix is compartment-selective, as other mitochondrial peptidases: MPPs at precursor import pores, m-AAA and i-AAA at either IMM face, PARL within the IMM, and OMA1/HTRA2 in the intermembrane space.

Molecular insights into the dynamic modulation of bacterial ClpP function and oligomerization by peptidomimetic boronate compounds

Bacterial caseinolytic protease P subunit (ClpP) is important and vital for cell survival and infectivity. Recent publications describe and discuss the complex structure-function relationship of ClpP and its processive activity mediated by 14 catalytic sites. Even so, there are several aspects yet to be further elucidated, such as the paradoxical allosteric modulation of ClpP by peptidomimetic boronates. These compounds bind to all catalytic sites, and in specific conditions, they stimulate a dysregulated degradation of peptides and globular proteins, instead of inhibiting the enzymatic activity, as expected for serine proteases in general. Aiming to explore and explain this paradoxical effect, we solved and refined the crystal structure of native ClpP from Staphylococcus epidermidis (Se), an opportunistic pathogen involved in nosocomial infections, as well as ClpP in complex with ixazomib at 1.90 Å and 2.33 Å resolution, respectively. The interpretation of the crystal structures, in combination with complementary biochemical and biophysical data, shed light on how ixazomib affects the ClpP conformational state and activity. Moreover, SEC-SAXS and DLS measurements show, for the first time, that a peptidomimetic boronate compound also induces the assembly of the tetradecameric structure from isolated homomeric heptameric rings of a gram-positive organism.

Structure of Staphylococcus aureus ClpP Bound to the Covalent Active-Site Inhibitor Cystargolide A

The caseinolytic protease is a highly conserved serine protease, crucial to prokaryotic and eukaryotic protein homeostasis, and a promising antibacterial and anticancer drug target. Herein, we describe the potent cystargolides as the first natural β-lactone inhibitors of the proteolytic core ClpP. Based on the discovery of two clpP genes next to the cystargolide biosynthetic gene cluster in Kitasatospora cystarginea, we explored ClpP as a potential cystargolide target. We show the inhibition of Staphylococcus aureus ClpP by cystargolide A and B by different biochemical methods in vitro. Synthesis of semisynthetic derivatives and probes with improved cell penetration allowed us to confirm ClpP as a specific target in S. aureus cells and to demonstrate the anti-virulence activity of this natural product class. Crystal structures show cystargolide A covalently bound to all 14 active sites of ClpP from S. aureus, Aquifex aeolicus, and Photorhabdus laumondii, and reveal the molecular mechanism of ClpP inhibition by β-lactones, the predominant class of ClpP inhibitors.

ClpP Peptidase as a Plausible Target for the Discovery of Novel Antibiotics

Antimicrobial resistance (AMR) to currently available antibiotics/drugs is a global threat. It is desirable to develop new drugs that work through a novel target(s) to avoid drug resistance. This review discusses the potential of the caseinolytic protease P (ClpP) peptidase complex as a novel target for finding novel antibiotics, emphasising the ClpP's structure and function. ClpP contributes to the survival of bacteria via its ability to destroy misfolded or aggregated proteins. In consequence, its inhibition may lead to microbial death. Drugs inhibiting ClpP activity are currently being tested, but no drug against this target has been approved yet. It was demonstrated that Nblocked dipeptides are essential for activating ClpP's proteolytic activity. Hence, compounds mimicking these dipeptides could act as inhibitors of the formation of an active ClpP complex. Drugs, including Bortezomib, Cisplatin, Cefmetazole, and Ixazomib, inhibit ClpP activation. However, they were not approved as drugs against the target because of their high toxicity, likely due to the presence of strong electrophiles in their warheads. The modifications of these warheads could be a good strategy to reduce the toxicity of these molecules. For instance, a boronate warhead was replaced by a chloromethyl ketone, and this new molecule was shown to exhibit selectivity for prokaryotic ClpP. A better understanding of the structure and function of the ClpP complex would benefit the search for compounds mimicking N-blocked dipeptides that would inhibit ClpP complex activity and cause bacterial death.

Targeting bacterial degradation machinery as an antibacterial strategy

The exploitation of a cell's natural degradation machinery for therapeutic purposes is an exciting research area in its infancy with respect to bacteria. Here, we review current strategies targeting the ClpCP system, which is a proteolytic degradation complex essential in the biology of many bacterial species of scientific interest. Strategies include using natural product antibiotics or acyldepsipeptides to initiate the up- or down-regulation of ClpCP activity. We also examine exciting recent forays into BacPROTACs to trigger the degradation of specific proteins of interest through the hijacking of the ClpCP machinery. These strategies represent an important emerging avenue for combatting antimicrobial resistance.

Anti-infective therapy using species-specific activators of Staphylococcus aureus ClpP

The emergence of methicillin-resistant Staphylococcus aureus isolates highlights the urgent need to develop more antibiotics. ClpP is a highly conserved protease regulated by ATPases in bacteria and in mitochondria. Aberrant activation of  bacterial ClpP is an alternative method of discovering antibiotics, while it remains difficult to develop selective  Staphylococcus aureus ClpP activators that can avoid disturbing Homo sapiens ClpP functions. Here, we use a structure-based design to identify (R)- and (S)-ZG197 as highly selective Staphylococcus aureus ClpP activators. The key structural elements in Homo sapiens ClpP, particularly W146 and its joint action with the C-terminal motif, significantly contribute to the discrimination of the activators. Our selective activators display wide antibiotic properties towards an array of multidrug-resistant staphylococcal strains in vitro, and demonstrate promising antibiotic efficacy in zebrafish and murine skin infection models. Our findings indicate that the species-specific activators of Staphylococcus aureus ClpP are exciting therapeutic agents to treat staphylococcal infections.

Noise in a Metabolic Pathway Leads to Persister Formation in Mycobacterium tuberculosis

Tuberculosis is difficult to treat due to dormant cells formed in response to immune stress and stochastically formed persisters, both of which are tolerant of antibiotics. Bactericidal antibiotics kill by corrupting their energy-dependent targets. We reasoned that stochastic variation, or noise, in the expression of an energy-generating component will produce rare persister cells. In sorted M. tuberculosis cells grown on acetate, there is considerable cell-to-cell variation in the level of mRNA coding for AckA, the acetate kinase. Quenching the noise by overexpressing ackA sharply decreases persisters, showing that it acts as the main persister gene under these conditions. This demonstrates that a low energy mechanism is responsible for the formation of M. tuberculosis persisters. Entrance into a low-energy state driven by noise in expression of energy-producing enzymes is likely a general mechanism by which bacteria produce persisters. IMPORTANCE M. tuberculosis infection requires the administration of multiple antibiotics for a prolonged period of time. Treatment difficulty is generally attributed to M. tuberculosis entrance into a nonreplicative, antibiotic-tolerant state. M. tuberculosis enters this nonreplicative state in response to immune stress. However, a small population of cells enter a nonreplicative, multidrug-tolerant state under normal growth conditions, absent any stress. These cells are termed persisters. The mechanisms by which persisters enter a nonreplicative state are largely unknown. Here, we show that, as with other bacteria, M. tuberculosis persisters are low-energy cells formed stochastically during normal growth. Additionally, we identify the natural variation in the expression of energy producing genes as a source of the stochastic entrance of M. tuberculosis into the low-energy persister state. These findings have important implications for understanding the heterogeneous nature of M. tuberculosis infection and will aid in designing better treatment regimens against this important human pathogen.

Engineered peptide PLG0206 overcomes limitations of a challenging antimicrobial drug class

The absence of novel antibiotics for drug-resistant and biofilm-associated infections is a global public health crisis. Antimicrobial peptides explored to address this need have encountered significant development challenges associated with size, toxicity, safety profile, and pharmacokinetics. We designed PLG0206, an engineered antimicrobial peptide, to address these limitations. PLG0206 has broad-spectrum activity against >1,200 multidrug-resistant (MDR) ESKAPEE clinical isolates, is rapidly bactericidal, and displays potent anti-biofilm activity against diverse MDR pathogens. PLG0206 displays activity in diverse animal infection models following both systemic (urinary tract infection) and local (prosthetic joint infection) administration. These findings support continuing clinical development of PLG0206 and validate use of rational design for peptide therapeutics to overcome limitations associated with difficult-to-drug pharmaceutical targets.

Acyldepsipeptide Analogues: A Future Generation Antibiotics for Tuberculosis Treatment

Acyldepsipeptides (ADEPs) are a new class of emerging antimicrobial peptides (AMPs), which are currently explored for treatment of pathogenic infections, including tuberculosis (TB). These cyclic hydrophobic peptides have a unique bacterial target to the conventional anti-TB drugs, and present a therapeutic window to overcome Mycobacterium Tuberculosis (M. tb) drug resistance. ADEPs exerts their antibacterial activity on M. tb strains through activation of the protein homeostatic regulatory protease, the caseinolytic protease (ClpP1P2). ClpP1P2 is normally regulated and activated by the ClpP-ATPases to degrade misfolded and toxic peptides and/or short proteins. ADEPs bind and dysregulate all the homeostatic capabilities of ClpP1P2 while inducing non-selective proteolysis. The uncontrolled proteolysis leads to M. tb cell death within the host. ADEPs analogues that have been tested possess cytotoxicity and poor pharmacokinetic and pharmacodynamic properties. However, these can be improved by drug design techniques. Moreover, the use of nanomaterial in conjunction with ADEPs would yield effective synergistic effect. This new mode of action has potential to combat and eradicate the extensive multi-drug resistance (MDR) problem that is currently faced by the public health pertaining bacterial infections, especially TB.

Genomic characterization of Staphylococcus aureus isolates causing osteoarticular infections in otherwise healthy children

Background

Pediatric osteoarticular infections are commonly caused by Staphylococcus aureus. The contribution of S. aureus genomic variability to pathogenesis of these infections is poorly described.

Methods

We prospectively enrolled 47 children over 3 1/2 years from whom S. aureus was isolated on culture—12 uninfected with skin colonization, 16 with skin abscesses, 19 with osteoarticular infections (four with septic arthritis, three with acute osteomyelitis, six with acute osteomyelitis and septic arthritis and six with chronic osteomyelitis). Isolates underwent whole genome sequencing, with assessment for 254 virulence genes and any mutations as well as creation of a phylogenetic tree. Finally, isolates were compared for their ability to form static biofilms and compared to the genetic analysis.

Results

No sequence types predominated amongst osteoarticular infections. Only genes involved in evasion of host immune defenses were more frequently carried by isolates from osteoarticular infections than from skin colonization (p = .02). Virulence gene mutations were only noted in 14 genes (three regulating biofilm formation) when comparing isolates from subjects with osteoarticular infections and those with skin colonization. Biofilm results demonstrated large heterogeneity in the isolates’ capacity to form static biofilms, with healthy control isolates producing more robust biofilm formation.

Conclusions

S. aureus causing osteoarticular infections are genetically heterogeneous, and more frequently harbor genes involved in immune evasion than less invasive isolates. However, virulence gene carriage overall is similar with infrequent mutations, suggesting that pathogenesis of S. aureus osteoarticular infections may be primarily regulated at transcriptional and/or translational levels.

Tamarixetin Attenuated the Virulence of Staphylococcus aureus by Directly Targeting Caseinolytic Protease P

Staphylococcus aureus, especially drug-resistant S. aureus infections, is a worldwide healthcare challenge. There is a growing focus on antivirulence therapy against S. aureus. Caseinolytic protease p (ClpP) is a protein hydrolase essential for pathogenicity in S. aureus. A flavonoid compound, tamarixetin, which was screened in this work, was specifically able to inhibit the hydrolytic activity of ClpP on the fluorescent substrate Suc-LY-AMC with an IC₅₀ of 49.73 μM, without affecting the growth of methicillin-resistant S. aureus strain USA300 and was without obvious cytotoxicity. Further assays found that tamarixetin inhibited the transcription of hla, agr, RNAIII, pvl, PSM-α, and spa genes as well as suppressed the protein expression levels of Hla and PVL. Moreover, tamarixetin was observed to dramatically inhibit the hemolytic activity of hla in S. aureus. Consistent with that of S. aureus USA300-ΔclpP, tamarixetin was shown to increase urease expression. The thermal shift and cellular thermal shift assays showed that tamarixetin markedly changed the thermal stability of ClpP. The dissociation constant (K_D) value of tamarixetin with ClpP was 2.52 × 10^-6 M measured by surface plasmon resonance. The molecular docking and ClpP point mutation results also demonstrated that tamarixetin had a strong interaction with ClpP. In vivo study showed that tamarixetin was effective in protecting mice from S. aureus pneumonia by increasing survival, reducing lung tissue load, and slowing down the infiltration of inflammatory factors. In addition, tamarixetin was able to enhance the antibacterial activity of cefotaxime in combination. In conclusion, tamarixetin was promising as a ClpP inhibitor for S. aureus infections.

Synthesis and Structure–Activity Relationship of Thioacetamide-Triazoles against Escherichia coli

Infections due to Gram-negative bacteria are increasingly dangerous due to the spread of multi-drug resistant strains, emphasizing the urgent need for new antibiotics with alternative modes of action. We have previously identified a novel class of antibacterial agents, thioacetamide-triazoles, using an antifolate targeted screen and determined their mode of action which is dependent on activation by cysteine synthase A. Herein, we report a detailed examination of the anti-E. coli structure–activity relationship of the thioacetamide-triazoles. Analogs of the initial hit compounds were synthesized to study the contribution of the aryl, thioacetamide, and triazole sections. A clear structure–activity relationship was observed generating compounds with excellent inhibition values. Substitutions to the aryl ring were generally best tolerated, including the introduction of thiazole and pyridine heteroaryl systems. Substitutions to the central thioacetamide linker section were more nuanced; the introduction of a methyl branch to the thioacetamide linker substantially decreased antibacterial activity, but the isomeric propionamide and N-benzamide systems retained activity. Changes to the triazole portion of the molecule dramatically decreased the antibacterial activity, further indicating that 1,2,3-triazole is critical for potency. From these studies, we have identified new lead compounds with desirable in-vitro ADME properties and in-vivo pharmacokinetic properties.

The Role of Integration Host Factor in Escherichia coli Persister Formation

Persisters represent a small subpopulation of cells that are tolerant of killing by antibiotics and are implicated in the recalcitrance of chronic infections to antibiotic therapy. One general theme has emerged regarding persisters formed by different bacterial species, namely, a state of relative dormancy characterized by diminished activity of antibiotic targets. Within this framework, a number of studies have linked persister formation to stochastic decreases in energy-generating components, leading to low ATP and target activity. In this study, we screen knockouts in the main global regulators of Escherichia coli for their effect on persisters. A knockout in integration host factor (IHF) had elevated ATP and a diminished level of persisters. This was accompanied by an overexpression of isocitrate dehydrogenase (Icd) and a downregulation of isocitrate lyase (AceA), two genes located at the bifurcation between the tricarboxylic acid (TCA) cycle and the glyoxylate bypass. Using a translational ihfA-mVenus fusion, we sort out rare bright cells, and this subpopulation is enriched in persisters. Our results suggest that noise in the expression of ihf produces rare cells with low Icd/high AceA, diverting substrates into the glyoxylate bypass, which decreases ATP, leading to antibiotic-tolerant persisters. We further examine noise in a simple model, the lac operon, and show that a knockout of the lacI repressor increases expression of the operon and decreases persister formation. Our results suggest that noise quenching by overexpression serves as a general approach to determine the nature of persister genes in a variety of bacterial species and conditions. IMPORTANCE Persisters are phenotypic variants that survive exposure to antibiotics through temporary dormancy. Mutants with increased levels of persisters have been identified in clinical isolates, and evidence suggests these cells contribute to chronic infections and antibiotic treatment failure. Understanding the underlying mechanism of persister formation and tolerance is important for developing therapeutic approaches to treat chronic infections. In this study, we examine a global regulator, IHF, that plays a role in persister formation. We find that noise in expression of IHF contributes to persister formation, likely by regulating the switch between the TCA cycle that efficiently produces energy and the glyoxylate bypass. We extend this study to a simple model lac operon and show that when grown on lactose as the sole carbon source, noise in its expression influences ATP levels and determines persister formation. This noise is quenched by overexpression of the lac operon, providing a simple approach to test the involvement of a gene in persister formation.

LipE guided discovery of isopropylphenyl pyridazines as pantothenate kinase modulators

Pantothenate kinase (PANK) is the critical regulator of intracellular levels of coenzyme A and has emerged as an attractive target for treating neurological and metabolic disorders. This report describes the optimization, synthesis, and full structure–activity relationships of a new chemical series of pantothenate competitive PANK inhibitors. Potent drug-like molecules were obtained by optimizing a high throughput screening hit, using lipophilic ligand efficiency (LipE) derived from human PANK3 IC₅₀ values to guide ligand development. X-ray crystal structures of PANK3 with index inhibitors from the optimization were determined to rationalize the emerging structure activity relationships. The analysis revealed a key bidentate hydrogen bonding interaction between pyridazine and R306′ as a major contributor to the LipE gain observed in the optimization. A tractable series of PANK3 modulators with nanomolar potency, excellent LipE values, desirable physicochemical properties, and a well-defined structural binding mode was produced from this study.

Recent advances in understanding the role of proteostasis

Maintenance of a functional proteome is achieved through the mechanism of proteostasis that involves precise coordination between molecular machineries assisting a protein from its conception to demise. Although each organelle within a cell has its own set of proteostasis machinery, inter-organellar communication and cell non-autonomous signaling bring forth the multidimensional nature of the proteostasis network. Exposure to extrinsic and intrinsic stressors can challenge the proteostasis network, leading to the accumulation of aberrant proteins or a decline in the proteostasis components, as seen during aging and in several diseases. Here, we summarize recent advances in understanding the role of proteostasis and its regulation in aging and disease, including monogenetic and infectious diseases. We highlight some of the emerging as well as unresolved questions in proteostasis that need to be addressed to overcome pathologies associated with damaged proteins and to promote healthy aging.

At the Crossroads of Bioenergetics and Antibiotic Discovery

Dr. Vladimir Skulachev was my mentor, and his pioneering work in the field of bioenergetics inspired the discoveries described in this review, written in the form of a personal account of events. Examining basic mechanisms of chemiosmotic coupling unexpectedly led us to transenvelope multidrug resistance pumps (MDR pumps) that severely limit development of novel antibiotics. One of the major advances of Skulachev and his group was the discovery of the mitochondrial membrane potential with the use of permeant cations such as TPP⁺, which served as electric probes. We describe our finding of their natural counterparts in plants, where they act as antimicrobials. The most challenging problems in antimicrobial drug discovery are antibiotic tolerance of chronic infections caused by dormant persister cells; antibiotic resistance, responsible for the current antimicrobial resistance crisis (AMR); and finding novel compounds acting against Gram-negative bacteria, protected by their powerful multidrug resistance pumps. Our study of persisters shows that these are rare cells formed by stochastic fluctuation in expression of Krebs cycle enzymes, leading to a drop in ATP, target shutdown, and antibiotic tolerance. Searching for compounds that can corrupt targets in the absence of ATP, we identified acyldepsipeptide (ADEP) that activates the ClpP protease, forcing cells to self-digest. Growing previously uncultured bacteria led us to teixobactin, a novel cell wall acting antibiotic. Teixobactin avoids efflux by targeting lipid II and lipid III, precursors of peptidoglycan and wall teichoic acid, located on the surface. The targets are immutable, and teixobactin is the first antibiotic with no detectable resistance. Our search for compounds acting against Gram-negative bacteria led to the discovery of darobactins, which also hit a surface target, the essential chaperone BamA.

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.

Reprogramming of the Caseinolytic Protease by ADEP Antibiotics: Molecular Mechanism, Cellular Consequences, Therapeutic Potential

Rising antibiotic resistance urgently calls for the discovery and evaluation of novel antibiotic classes and unique antibiotic targets. The caseinolytic protease Clp emerged as an unprecedented target for antibiotic therapy 15 years ago when it was observed that natural product-derived acyldepsipeptide antibiotics (ADEP) dysregulated its proteolytic core ClpP towards destructive proteolysis in bacterial cells. A substantial database has accumulated since on the interaction of ADEP with ClpP, which is comprehensively compiled in this review. On the molecular level, we describe the conformational control that ADEP exerts over ClpP, the nature of the protein substrates degraded, and the emerging structure-activity-relationship of the ADEP compound class. On the physiological level, we review the multi-faceted antibacterial mechanism, species-dependent killing modes, the activity against carcinogenic cells, and the therapeutic potential of the compound class.

Recalcitrant Staphylococcus aureus Infections: Obstacles and Solutions

Antibiotic treatment failure of Staphylococcus aureus infections is very common. In addition to genetically encoded mechanisms of antibiotic resistance, numerous additional factors limit the efficacy of antibiotics in vivo Identifying and removing the barriers to antibiotic efficacy are of major importance, as even if new antibiotics become available, they will likely face the same barriers to efficacy as their predecessors. One major obstacle to antibiotic efficacy is the proficiency of S. aureus to enter a physiological state that is incompatible with antibiotic killing. Multiple pathways leading to antibiotic tolerance and the formation of tolerant subpopulations called persister cells have been described for S. aureus Additionally, S. aureus is a versatile pathogen that can infect numerous tissues and invade a variety of cell types, of which some are poorly penetrable to antibiotics. It is therefore unlikely that there will be a single solution to the problem of recalcitrant S. aureus infection. Instead, specific approaches may be required for targeting tolerant cells within different niches, be it through direct targeting of persister cells, sensitization of persisters to conventional antibiotics, improved penetration of antibiotics to particular niches, or any combination thereof. Here, we examine two well-described reservoirs of antibiotic-tolerant S. aureus, the biofilm and the macrophage, the barriers these environments present to antibiotic efficacy, and potential solutions to the problem.

Functional Characterisation of ClpP Mutations Conferring Resistance to Acyldepsipeptide Antibiotics in Firmicutes

Acyldepsipeptide (ADEP) is an exploratory antibiotic with a novel mechanism of action. ClpP, the proteolytic core of the caseinolytic protease, is deregulated towards unrestrained proteolysis. Here, we report on the mechanism of ADEP resistance in Firmicutes. This bacterial phylum contains important pathogens that are relevant for potential ADEP therapy. For Staphylococcus aureus, Bacillus subtilis, enterococci and streptococci, spontaneous ADEP-resistant mutants were selected in vitro at a rate of 10-6 . All isolates carried mutations in clpP. All mutated S. aureus ClpP proteins characterised in this study were functionally impaired; this increased our understanding of the mode of operation of ClpP. For molecular insights, crystal structures of S. aureus ClpP bound to ADEP4 were determined. Well-resolved N-terminal domains in the apo structure allow the pore-gating mechanism to be followed. The compilation of mutations presented here indicates residues relevant for ClpP function and suggests that ADEP resistance will occur at a lower rate during the infection process.

Cell division protein FtsZ: from structure and mechanism to antibiotic target

Antimicrobial resistance to virtually all clinically applied antibiotic classes severely limits the available options to treat bacterial infections. Hence, there is an urgent need to develop and evaluate new antibiotics and targets with resistance-breaking properties. Bacterial cell division has emerged as a new antibiotic target pathway to counteract multidrug-resistant pathogens. New approaches in antibiotic discovery and bacterial cell biology helped to identify compounds that either directly interact with the major cell division protein FtsZ, thereby perturbing the function and dynamics of the cell division machinery, or affect the structural integrity of FtsZ by inducing its degradation. The impressive antimicrobial activities and resistance-breaking properties of certain compounds validate the inhibition of bacterial cell division as a promising strategy for antibiotic intervention.

Imipridone Anticancer Compounds Ectopically Activate the ClpP Protease and Represent a New Scaffold for Antibiotic Development

The imipridones ONC201 and ONC212 selectively kill cancer cells and have been ascribed multiple mechanisms-of-action. Genome-wide CRISPR knockout screens revealed that loss of the mitochondrial proteases CLPP and MIPEP confer strong resistance to both compounds...

Systematic genetic interaction profiles can reveal the mechanisms-of-action of bioactive compounds. The imipridone ONC201, which is currently in cancer clinical trials, has been ascribed a variety of different targets. To investigate the genetic dependencies of imipridone action, we screened a genome-wide clustered regularly interspaced short palindromic repeats (CRISPR) knockout library in the presence of either ONC201 or its more potent analog ONC212. Loss of the mitochondrial matrix protease CLPP or the mitochondrial intermediate peptidase MIPEP conferred strong resistance to both compounds. Biochemical and surrogate genetic assays showed that impridones directly activate CLPP and that MIPEP is necessary for proteolytic maturation of CLPP into a catalytically competent form. Quantitative proteomic analysis of cells treated with ONC212 revealed degradation of many mitochondrial as well as nonmitochondrial proteins. Prompted by the conservation of ClpP from bacteria to humans, we found that the imipridones also activate ClpP from Escherichia coli, Bacillus subtilis, and Staphylococcus aureus in biochemical and genetic assays. ONC212 and acyldepsipeptide-4 (ADEP4), a known activator of bacterial ClpP, caused similar proteome-wide degradation profiles in S. aureus. ONC212 suppressed the proliferation of a number of Gram-positive (S. aureus, B. subtilis, and Enterococcus faecium) and Gram-negative species (E. coli and Neisseria gonorrhoeae). Moreover, ONC212 enhanced the ability of rifampin to eradicate antibiotic-tolerant S. aureus persister cells. These results reveal the genetic dependencies of imipridone action in human cells and identify the imipridone scaffold as a new entry point for antibiotic development.