Potent ClpP agonists with anticancer properties bind with improved structural complementarity and alter the mitochondrial N-terminome.

The mitochondrial ClpP protease is responsible for mitochondrial protein quality control through specific degradation of proteins involved in several metabolic processes. ClpP overexpression is also required in many cancer cells to eliminate reactive oxygen species (ROS)-damaged proteins and to sustain oncogenesis. Targeting ClpP to dysregulate its function using small-molecule agonists is a recent strategy in cancer therapy. Here, we synthesized imipridone-derived compounds and related chemicals, which we characterized using biochemical, biophysical, and cellular studies. Using X-ray crystallography, we found that these compounds have enhanced binding affinities due to their greater shape and charge complementarity with the surface hydrophobic pockets of ClpP. N-terminome profiling of cancer cells upon treatment with one of these compounds revealed the global proteomic changes that arise and identified the structural motifs preferred for protein cleavage by compound-activated ClpP. Together, our studies provide the structural and molecular basis by which dysregulated ClpP affects cancer cell viability and proliferation.

Characterization of TR-107, a novel chemical activator of the human mitochondrial protease ClpP.

We recently described the identification of a new class of small-molecule activators of the mitochondrial protease ClpP. These compounds synthesized by Madera Therapeutics showed increased potency of cancer growth inhibition over the related compound ONC201. In this study, we describe chemical optimization and characterization of the next generation of highly potent and selective small-molecule ClpP activators (TR compounds) and demonstrate their efficacy against breast cancer models in vitro and in vivo. We selected one compound (TR-107) with excellent potency, specificity, and drug-like properties for further evaluation. TR-107 showed ClpP-dependent growth inhibition in the low nanomolar range that was equipotent to paclitaxel in triple-negative breast cancer (TNBC) cell models. TR-107 also reduced specific mitochondrial proteins, including OXPHOS and TCA cycle components, in a time-, dose-, and ClpP-dependent manner. Seahorse XF analysis and glucose deprivation experiments confirmed the inactivation of OXPHOS and increased dependence on glycolysis following TR-107 exposure. The pharmacokinetic properties of TR-107 were compared with other known ClpP activators including ONC201 and ONC212. TR-107 displayed excellent exposure and serum t1/2 after oral administration. Using human TNBC MDA-MB-231 xenografts, the antitumor response to TR-107 was investigated. Oral administration of TR-107 resulted in a reduction in tumor volume and extension of survival in the treated compared with vehicle control mice. ClpP activation in vivo was validated by immunoblotting for TFAM and other mitochondrial proteins. In summary, we describe the identification of highly potent new ClpP agonists with improved efficacy against TNBC, through targeted inactivation of OXPHOS and disruption of mitochondrial metabolism.

Functional cooperativity between the trigger factor chaperone and the ClpXP proteolytic complex.

A functional association is uncovered between the ribosome-associated trigger factor (TF) chaperone and the ClpXP degradation complex. Bioinformatic analyses demonstrate conservation of the close proximity of tig, the gene coding for TF, and genes coding for ClpXP, suggesting a functional interaction. The effect of TF on ClpXP-dependent degradation varies based on the nature of substrate. While degradation of some substrates are slowed down or are unaffected by TF, surprisingly, TF increases the degradation rate of a third class of substrates. These include λ phage replication protein λO, master regulator of stationary phase RpoS, and SsrA-tagged proteins. Globally, TF acts to enhance the degradation of about 2% of newly synthesized proteins. TF is found to interact through multiple sites with ClpX in a highly dynamic fashion to promote protein degradation. This chaperone-protease cooperation constitutes a unique and likely ancestral aspect of cellular protein homeostasis in which TF acts as an adaptor for ClpXP.

Development of Antibiotics That Dysregulate the Neisserial ClpP Protease.

Evolving antimicrobial resistance has motivated the search for novel targets and alternative therapies. Caseinolytic protease (ClpP) has emerged as an enticing new target since its function is conserved and essential for bacterial fitness, and because its inhibition or dysregulation leads to bacterial cell death. ClpP protease function controls global protein homeostasis and is, therefore, crucial for the maintenance of the bacterial proteome during growth and infection. Previously, acyldepsipeptides (ADEPs) were discovered to dysregulate ClpP, leading to bactericidal activity against both actively growing and dormant Gram-positive pathogens. Unfortunately, these compounds had very low efficacy against Gram-negative bacteria. Hence, we sought to develop non-ADEP ClpP-targeting compounds with activity against Gram-negative species and called these activators of self-compartmentalizing proteases (ACPs). These ACPs bind and dysregulate ClpP in a manner similar to ADEPs, effectively digesting bacteria from the inside out. Here, we performed further ACP derivatization and testing to improve the efficacy and breadth of coverage of selected ACPs against Gram-negative bacteria. We observed that a diverse collection of Neisseria meningitidis and Neisseria gonorrhoeae clinical isolates were exquisitely sensitive to these ACP analogues. Furthermore, based on the ACP-ClpP cocrystal structure solved here, we demonstrate that ACPs could be designed to be species specific. This validates the feasibility of drug-based targeting of ClpP in Gram-negative bacteria.

ClpP protease activation results from the reorganization of the electrostatic interaction networks at the entrance pores.

Bacterial ClpP is a highly conserved, cylindrical, self-compartmentalizing serine protease required for maintaining cellular proteostasis. Small molecule acyldepsipeptides (ADEPs) and activators of self-compartmentalized proteases 1 (ACP1s) cause dysregulation and activation of ClpP, leading to bacterial cell death, highlighting their potential use as novel antibiotics. Structural changes in Neisseria meningitidis and Escherichia coli ClpP upon binding to novel ACP1 and ADEP analogs were probed by X-ray crystallography, methyl-TROSY NMR, and small angle X-ray scattering. ACP1 and ADEP induce distinct conformational changes in the ClpP structure. However, reorganization of electrostatic interaction networks at the ClpP entrance pores is necessary and sufficient for activation. Further activation is achieved by formation of ordered N-terminal axial loops and reduction in the structural heterogeneity of the ClpP cylinder. Activating mutations recapitulate the structural effects of small molecule activator binding. Our data, together with previous findings, provide a structural basis for a unified mechanism of compound-based ClpP activation.

Acyldepsipeptide Analogs Dysregulate Human Mitochondrial ClpP Protease Activity and Cause Apoptotic Cell Death.

Acyldepsipeptides (ADEPs) are potential antibiotics that dysregulate the activity of the highly conserved tetradecameric bacterial ClpP protease, leading to bacterial cell death. Here, we identified ADEP analogs that are potent dysregulators of the human mitochondrial ClpP (HsClpP). These ADEPs interact tightly with HsClpP, causing the protease to non-specifically degrade model substrates. Dysregulation of HsClpP activity by ADEP was found to induce cytotoxic effects via activation of the intrinsic, caspase-dependent apoptosis. ADEP-HsClpP co-crystal structure was solved for one of the analogs revealing a highly complementary binding interface formed by two HsClpP neighboring subunits but, unexpectedly, with HsClpP in the compact conformation. Given that HsClpP is highly expressed in multiple cancers and has important roles in cell metastasis, our findings suggest a therapeutic potential for ADEPs in cancer treatment.

Development and Characterization of Potent Cyclic Acyldepsipeptide Analogues with Increased Antimicrobial Activity.

The problem of antibiotic resistance has prompted the search for new antibiotics with novel mechanisms of action. Analogues of the A54556 cyclic acyldepsipeptides (ADEPs) represent an attractive class of antimicrobial agents that act through dysregulation of caseinolytic protease (ClpP). Previous studies have shown that ADEPs are active against Gram-positive bacteria (e.g., MRSA, VRE, PRSP (penicillin-resistant Streptococcus pneumoniae)); however, there are currently few studies examining Gram-negative bacteria. In this study, the synthesis and biological evaluation of 14 novel ADEPs against a variety of pathogenic Gram-negative and Gram-positive organisms is outlined. Optimization of the macrocyclic core residues and N-acyl side chain culminated in the development of 26, which shows potent activity against the Gram-negative species Neisseria meningitidis and Neisseria gonorrheae and improved activity against the Gram-positive organisms Staphylococcus aureus and Enterococcus faecalis in comparison with known analogues. In addition, the co-crystal structure of an ADEP-ClpP complex derived from N. meningitidis was solved.

Inhibition of the Mitochondrial Protease ClpP as a Therapeutic Strategy for Human Acute Myeloid Leukemia.

From an shRNA screen, we identified ClpP as a member of the mitochondrial proteome whose knockdown reduced the viability of K562 leukemic cells. Expression of this mitochondrial protease that has structural similarity to the cytoplasmic proteosome is increased in leukemic cells from approximately half of all patients with AML. Genetic or chemical inhibition of ClpP killed cells from both human AML cell lines and primary samples in which the cells showed elevated ClpP expression but did not affect their normal counterparts. Importantly, Clpp knockout mice were viable with normal hematopoiesis. Mechanistically, we found that ClpP interacts with mitochondrial respiratory chain proteins and metabolic enzymes, and knockdown of ClpP in leukemic cells inhibited oxidative phosphorylation and mitochondrial metabolism.

Novel function discovery with GeneMANIA: a new integrated resource for gene function prediction in Escherichia coli.

MOTIVATION: The model bacterium Escherichia coli is among the best studied prokaryotes, yet nearly half of its proteins are still of unknown biological function. This is despite a wealth of available large-scale physical and genetic interaction data. To address this, we extended the GeneMANIA function prediction web application developed for model eukaryotes to support E.coli. RESULTS: We integrated 48 distinct E.coli functional interaction datasets and used the GeneMANIA algorithm to produce thousands of novel functional predictions and prioritize genes for further functional assays. Our analysis achieved cross-validation performance comparable to that reported for eukaryotic model organisms, and revealed new functions for previously uncharacterized genes in specific bioprocesses, including components required for cell adhesion, iron-sulphur complex assembly and ribosome biogenesis. The GeneMANIA approach for network-based function prediction provides an innovative new tool for probing mechanisms underlying bacterial bioprocesses. CONTACT: gary.bader@utoronto.ca; mohan.babu@uregina.ca SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.

Total synthesis and antibacterial testing of the A54556 cyclic acyldepsipeptides isolated from Streptomyces hawaiiensis.

The first total synthesis of all six known A54556 acyldepsipeptide (ADEP) antibiotics from Streptomyces hawaiiensis is reported. This family of compounds has a unique mechanism of antibacterial action, acting as activators of caseinolytic protease (ClpP). Assembly of the 16-membered depsipeptide core was accomplished via a pentafluorophenyl ester-based macrolactamization strategy. Late stage amine deprotection was carried out under neutral conditions by employing a mild hydrogenolysis strategy, which avoids the formation of undesired ring-opened depsipeptide side products encountered during deprotection of acid-labile protecting groups. The free amines were found to be significantly more reactive toward late stage amide bond formation as compared to the corresponding ammonium salts, giving final products in excellent yields. A thorough NMR spectroscopic analysis of these compounds was carried out to formally assign the structures and to aid with the spectroscopic assignment of ADEP analogues. The identity of two of the structures was confirmed by comparison with biologically produced samples from S. hawaiiensis. An X-ray crystallographic analysis of an ADEP analogue reveals a conformation similar to that found in cocrystal structures of ADEPs with ClpP protease. The degree of antibacterial activity of the different compounds was evaluated in vitro using MIC assays employing both Gram-positive and Gram-negative strains and a fluorescence-based biochemical assay.

Activators of cylindrical proteases as antimicrobials: identification and development of small molecule activators of ClpP protease.

ClpP is a cylindrical serine protease whose ability to degrade proteins is regulated by the unfoldase ATP-dependent chaperones. ClpP on its own can only degrade small peptides. Here, we used ClpP as a target in a high-throughput screen for compounds, which activate the protease and allow it to degrade larger proteins, hence, abolishing the specificity arising from the ATP-dependent chaperones. Our screen resulted in five distinct compounds, which we designate as Activators of Self-Compartmentalizing Proteases 1 to 5 (ACP1 to 5). The compounds are found to stabilize the ClpP double-ring structure. The ACP1 chemical structure was considered to have drug-like characteristics and was further optimized to give analogs with bactericidal activity. Hence, the ACPs represent classes of compounds that can activate ClpP and that can be developed as potential novel antibiotics.

Dual interaction of factor H with C3d and glycosaminoglycans in host-nonhost discrimination by complement.

The alternative pathway of complement is important in innate immunity, attacking not only microbes but all unprotected biological surfaces through powerful amplification. It is unresolved how host and nonhost surfaces are distinguished at the molecular level, but key components are domains 19-20 of the complement regulator factor H (FH), which interact with host (i.e., nonactivator surface glycosaminoglycans or sialic acids) and the C3d part of C3b. Our structure of the FH19-20:C3d complex at 2.3-Å resolution shows that FH19-20 has two distinct binding sites, FH19 and FH20, for C3b. We show simultaneous binding of FH19 to C3b and FH20 to nonactivator surface glycosaminoglycans, and we show that both of these interactions are necessary for full binding of FH to C3b on nonactivator surfaces (i.e., for target discrimination). We also show that C3d could replace glycosaminoglycan binding to FH20, thus providing a feedback control for preventing excess C3b deposition and complement amplification. This explains the molecular basis of atypical hemolytic uremic syndrome, where mutations on the binding interfaces between FH19-20 and C3d or between FH20 and glycosaminoglycans lead to complement attack against host surfaces.

Structural and theoretical studies indicate that the cylindrical protease ClpP samples extended and compact conformations.

The highly conserved ClpP protease consists of two heptameric rings that interact by the interdigitation of an alpha-helix beta strand handle domain motif to form a tetradecameric cylinder. We previously proposed that protease dynamics results in the temporary unstructuring of interacting pairs of handle domains, opening transient equatorial side pores that allow for peptide egress. Here, we report the structure of an Escherichia coli ClpP mutant in which each opposing pair of protomers is linked by a disulfide bond. This structure resembles the compact structures of Streptococcus pneumoniae, Mycobacterium tuberculosis, and Plasmodium falciparum ClpPs, rather than the active, extended structures that have previously been determined for E. coli ClpPs. The structural data, along with normal mode analysis, support a model whereby the ClpP cylinder switches dynamically between an active extended state required for substrate degradation and an inactive compact state allowing peptide product release.

Tamoxifen enhances the Hsp90 molecular chaperone ATPase activity.

BACKGROUND: Hsp90 is an essential molecular chaperone that is also a novel anti-cancer drug target. There is growing interest in developing new drugs that modulate Hsp90 activity. METHODOLOGY/PRINCIPAL FINDINGS: Using a virtual screening approach, 4-hydroxytamoxifen, the active metabolite of the anti-estrogen drug tamoxifen, was identified as a putative Hsp90 ligand. Surprisingly, while all drugs targeting Hsp90 inhibit the chaperone ATPase activity, it was found experimentally that 4-hydroxytamoxifen and tamoxifen enhance rather than inhibit Hsp90 ATPase. CONCLUSIONS/SIGNIFICANCE: Hence, tamoxifen and its metabolite are the first members of a new pharmacological class of Hsp90 activators.

Mutational analyses reveal that the staphylococcal immune evasion molecule Sbi and complement receptor 2 (CR2) share overlapping contact residues on C3d: implications for the controversy regarding the CR2/C3d cocrystal structure.

We recently characterized an interaction between the Staphylococcus aureus immune evasion molecule Staphylococcus aureus binder of Ig (Sbi) and complement C3, an interaction mediated primarily through the binding of C3d(g) to Sbi domain IV. Events related to these studies prompted us to investigate via mutagenesis the binding interface of C3d for Sbi domain IV (Sbi-IV), as well as to revisit the controversial issue of the complement receptor 2 (CR2) binding site of C3d. Specifically, we had shown that Sbi domains III and IV fragment binding to C3dg inhibited the latter's binding to CR2. Moreover, a published cocrystal structure of C3d bound to complement inhibitory C-terminal domain of extracellular fibrinogen-binding protein (Efb-C), a structural and functional homolog of Sbi-IV, showed Efb-C binding to a region on the concave face of C3d previously implicated in CR2 binding by our mutagenesis data but not confirmed in the CR2(short consensus repeat [SCR]1-2):C3d cocrystal structure. We have now analyzed by surface plasmon resonance the binding of a series of variant C3dg molecules to biosensor-bound Sbi-IV or CR2(SCR1-2). We found that mutations to the concave face acidic pocket of C3d significantly affected binding to both Sbi-IV and CR2, although there was divergence in which residues were most important in each case. By contrast, no binding defects were seen for mutations made to the sideface of C3d implicated from the cocrystal structure to be involved in binding CR2(SCR1-2). The results with Sbi-IV suggest a mode of binding highly similar to that visualized in the Efb-C:C3d complex. The results with CR2 confirm our earlier mapping studies and cast even further doubt on the physiologic relevance of the complex visualized in the C3d:CR2 cocrystal.

The FGL2-FcgammaRIIB pathway: a novel mechanism leading to immunosuppression.

Fibrinogen-like protein 2 (FGL2) is a multifunctional protein, which has been implicated in the pathogenesis of allograft and xenograft rejection. Previously, FGL2 was shown to inhibit maturation of BM-derived DC and T-cell proliferation. The mechanism of the immunosuppressive activity of FGL2 remains poorly elucidated. Here, we focus on identification of FGL2-specific receptor(s) and their ability to modulate APC activity and allograft survival. Using flow cytometry and surface plasmon resonance analysis, we show that FGL2 binds specifically to Fc gamma receptor (FcgammaR)IIB and FcgammaRIII receptors, which are expressed on the surface of APC, including B lymphocytes, macrophages and DC. Antibody to FcgammaRIIB and FcgammaRIII, or deficiency of these receptors, abrogated FGL2 binding. FGL2 inhibited the maturation of BMDC from FcgammaRIIB+/+ mice but not from FcgammaRIIB(-/-) mice and induced apoptosis in the FcgammaRIIB+ mouse B-cell line (A20) but not the A20IIA1.6 cell line that does not express FcgammaRIIB. Recombinant FGL2 infused into FcgammaRIIB+/+ (C57BL/6J, H-2b) mice but not FcgammaRIIB(-/-) mice inhibited rejection of fully mismatched BALB/cJ (H-2d) skin allografts. The identification of specific receptor binding has important implications for the pathogenesis of immune-mediated disease and suggests a potential for targeted FGL2 therapy.

Structure-function analysis of the C3 binding region of Staphylococcus aureus immune subversion protein Sbi.

Among the recently discovered Staphylococcus aureus immune evasion proteins, Sbi is unique in its ability to interact with components of both the adaptive and innate immune systems of the host. Sbi domains I and II (Sbi-I and Sbi-II) bind IgG. Sbi domain IV (residues 198-266) binds the central complement protein C3. When linked to Sbi-III, Sbi-IV induces a futile consumption of complement via alternative pathway activation, whereas isolated Sbi-IV specifically inhibits the alternative pathway without complement consumption. Here we have determined the three-dimensional structure of Sbi-IV by NMR spectroscopy, showing that Sbi-IV adopts a three-helix bundle fold similar to those of the S. aureus complement inhibitors Efb-C, Ehp, and SCIN. The (1)H-(15)N HSQC spectrum of Sbi-III indicates that this domain, essential for futile complement consumption, is natively unfolded, at least when isolated from the rest of Sbi. Sbi-IV and Sbi-III-IV both bind C3dg with 1:1 stoichiometry and submicromolar affinity. Despite low overall sequence identity, Sbi possesses the same residues as Efb at two positions essential for Efb-C binding to C3d. Mutation to alanine of either of these residues, Arg-231 and Asn-238, abolishes both Sbi-IV binding to C3dg and Sbi-IV alternative pathway inhibition. The almost complete conservation of Sbi-III and Sbi-IV amino acid sequences across more than 30 strains isolated from human and animal hosts indicates that the unique mechanism of Sbi in complement system subversion is a feature of infections of both humans and economically important animals.

Interaction of human complement with Sbi, a staphylococcal immunoglobulin-binding protein: indications of a novel mechanism of complement evasion by Staphylococcus aureus.

Staphylococcal immunoglobulin-binding protein, Sbi, is a 436-residue protein produced by many strains of Staphylococcus aureus. It was previously characterized as being cell surface-associated and having binding capacity for human IgG and beta(2)-glycoprotein I. Here we show using small angle x-ray scattering that the proposed extracellular region of Sbi (Sbi-E) is an elongated molecule consisting of four globular domains, two immunoglobulin-binding domains (I and II) and two novel domains (III and IV). We further show that together domains III and IV (Sbi-III-IV), as well as domain IV on its own (Sbi-IV), bind complement component C3 via contacts involving both the C3dg fragment and the C3a anaphylatoxin domain. Preincubation of human serum with either Sbi-E or Sbi-III-IV is inhibitory to all complement pathways, whereas domain IV specifically inhibits the alternative pathway. Monitoring C3 activation in serum incubated with Sbi fragments reveals that Sbi-E and Sbi-III-IV both activate the alternative pathway, leading to consumption of C3. By contrast, inhibition of this pathway by Sbi-IV does not involve C3 consumption. The observation that Sbi-E activates the alternative pathway is counterintuitive to intact Sbi being cell wall-associated, as recruiting complement to the surface of S. aureus would be deleterious to the bacterium. Upon re-examination of this issue, we found that Sbi was not associated with the cell wall fraction, but rather was found in the growth medium, consistent with it being an excreted protein. As such, our data suggest that Sbi helps mediate bacterial evasion of complement via a novel mechanism, namely futile fluid-phase consumption.

The complement regulator C4b-binding protein (C4BP) interacts with both the C4c and C4dg subfragments of the parent C4b ligand: evidence for synergy in C4BP subsite binding.

C4b-binding protein (C4BP) is a multimeric serum protein that is a potent regulator of the classical and lectin complement pathways. The binding site for C4b has been localized to complement control protein (CCP) domains 1-3 of the C4BP alpha-chain and, in particular, to a cluster of positively charged amino acids predicted to be at the interface between CCP 1 and CCP 2. To determine the regions of C4b contributing to C4BP binding, we have examined via surface plasmon resonance technology the binding of the C4c and C4dg subfragments of C4b to C4BP. At half-physiologic ionic strength, specific and saturable binding was observed for both C4c and C4dg. C4c exhibited much greater ionic strength sensitivity in its binding than did C4dg. Analysis of the effect on binding of the subfragments to various C4b-binding-defective C4BP mutants, together with cross-competition experiments, suggests that the subsites in C4BP for C4c and C4dg are adjacent, but distinct. Additionally, we observed synergy in subsite filling such that the presence of C4dg enhanced the extent of C4c binding over its basal level, and vice versa. The enhanced binding of C4c in the presence of C4dg was not due to an increase in affinity but rather reflected a 2-3-fold increase in the number of sites capable of binding C4c. This suggests the existence of a conformational equilibrium between high- and low-affinity states in the C4c binding subsite within each C4BP subunit, an equilibrium which is shifted in favor of the high-affinity state by the filling of the C4dg subsite.