Current Drug Targets (v.13, #11)

Targeting Quorum Sensing and Competence Stimulation for Antimicrobial Chemotherapy by Nicholas E. Shepherd, Rosemary S. Harrison, David P. Fairlie (1348-1359).
Bacterial resistance to antibiotics is now a serious problem, with traditional classes of antibiotics having gradually become ineffective. New drugs are therefore needed to target and inhibit novel pathways that affect the growth of bacteria. An important feature in the survival of bacteria is that they coordinate their efforts together as a colony via secreted auto-inducing molecules. Competence stimulating peptides (CSPs) are among the quorum sensing pheromones involved in this coordination. These peptides activate a two-component system in gram-negative bacteria, binding to and activating a histidine kinase receptor called ComD, which phosphorylates a response regulator called ComE, leading to gene expression and induction of competence. Competent bacteria are able to take up exogenous DNA and incorporate it into their own genome. By this mechanism bacteria are able to acquire and share genes encoding antibiotic resistance. Despite having been studied for over 30 years, this pathway has only recently begun to be explored as a novel approach to modulating bacterial growth. Antagonists of ComD might block the signaling cascade that leads to competence, while overstimulation of ComD might also reduce bacterial growth. One possible approach to inhibiting ComD is to examine peptide sequences of CSPs that activate ComD and attempt to constrain them to bioactive conformations, likely to have higher affinity due to pre-organization for recognition by the receptor. Thus, small molecules that mimic an alpha helical epitope of CSPs, the putative ComD binding domain, have been shown here to inhibit growth of bacteria such as S. pneumoniae. Such alpha helix mimetics may be valuable clues to antibacterial chemotherapeutic agents that utilize a new mechanism to control bacterial growth.

Adaptors in Toll-Like Receptor Signaling and their Potential as Therapeutic Targets by Thomas Ve, Nicholas J. Gay, Ashley Mansell, Bostjan Kobe, Stuart Kellie (1360-1374).
To initiate the innate immune response, Toll-like receptors (TLRs) associate with cytoplasmic adaptor proteins through TIR (Toll/interleukin-1 receptor) domain interactions. The four principal signaling adaptor proteins include MyD88, MAL, TRIF and TRAM, and the fifth protein SARM, involved in negative regulation of TLR pathways, is usually considered a part of the TIR domain-containing adaptor protein group. Other TIR domain-containing proteins have also been shown to regulate these signaling pathways, including ST2 and SIGIRR, as well as several bacterial and viral TIR domain-containing proteins that modulate these pathways as virulence factors. TLR pathways and the adaptor proteins are associated with a number of diseases, including infection, sepsis, inflammatory, allergic and autoimmune diseases and cancer. We review our current understanding of the structure and function of adaptor proteins and their regulatory proteins, their association with disease and their potential as therapeutic targets in human disease.

Resolving Biofilm Infections: Current Therapy and Drug Discovery Strategies by Max V. Ranall, Mark S. Butler, Mark A. Blaskovich, Matthew A. Cooper (1375-1385).
Biofilms formed by pathogenic bacteria present a serious threat to human health as the efficacy of standard antibiotic therapeutic regimens is compromised by reduced microbial susceptibility within the biofilm environment. The discovery of improved therapies for biofilm elimination requires an understanding of biofilm formation and dispersal, and the development of assays to specifically analyze these dynamic processes. This review will discuss biofilm screening strategies suitable for drug discovery efforts, especially chemical and biological approaches that specifically target biofilm destruction.

Uropathogenic Escherichia coli Mediated Urinary Tract Infection by Makrina Totsika, Danilo Gomes Moriel, Adi Idris, Benjamin A. Rogers, Daniel J. Wurpel, Minh-Duy Phan, David L. Paterson, Mark A. Schembri (1386-1399).
Urinary tract infection (UTI) is among the most common infectious diseases of humans and is the most common nosocomial infection in the developed world. They cause significant morbidity and mortality, with approximately 150 million cases globally per year. It is estimated that 40-50% of women and 5% of men will develop a UTI in their lifetime, and UTI accounts for more than 1 million hospitalizations and $1.6 billion in medical expenses each year in the USA. Uropathogenic E. coli (UPEC) is the primary cause of UTI. This review presents an overview of the primary virulence factors of UPEC, the major host responses to infection of the urinary tract, the emergence of specific multidrug resistant clones of UPEC, antibiotic treatment options for UPEC-mediated UTI and the current state of vaccine strategies as well as other novel anti-adhesive and prophylactic approaches to prevent UTI. New and emerging themes in UPEC research are also discussed in the context of future outlooks.

Prokaryotic Substrate-Binding Proteins as Targets for Antimicrobial Therapies by Rafael M. Counago, Christopher A. McDevitt, Miranda P. Ween, Bostjan Kobe (1400-1410).
The rapid emergence of multidrug-resistant bacteria over the last two decades has catalyzed a shift away from traditional antibiotic development strategies and encouraged the search for unconventional drug targets. Prokaryotic substrate- binding proteins (SBPs), together with their cognate ATP-binding cassette (ABC) transporters, facilitate the unidirectional, transbilayer movement of specific extracytosolic cargoes against a concentration gradient, powered by ATP hydrolysis. In Gram-negative bacteria, SBPs are found in the periplasmic space, whereas in Gram-positive organisms these proteins are anchored to the outer cell wall by a lipid moiety. SBPs are vital components of the substrate-translocation machinery, as they determine cargo specificity and are involved in coupling the cargo uptake process with ABC transporter- mediated ATP hydrolysis. In this review, we focus on “Cluster A-1” divalent metal-binding proteins from within the SBP family. Acquisition of transition row metal ions is essential for bacterial colonization and virulence and Cluster A-1 SBPs play an integral role in this process. Cluster A-1 SBPs lack homologs in humans, bypass the need to deliver compounds into the bacterial cell, and are therefore potential drug targets against Gram-positive bacteria. Here we discuss the role SBPs play in the prokaryotic substrate-translocation machinery with emphasis in the substrate-binding mechanism of Cluster A-1 SBPs, the role of these proteins in virulence and their potential use as drug targets.

RNA Interference for Viral Infections by Stephen J. Blake, Fawzi F. Bokhari, Nigel A.J. McMillan (1411-1420).
The treatment of viral infections has relied on pre-emptive vaccination or use of a limited range of anti-viral drugs. However, the majority of viruses have no available drugs and treatment is merely supportive. RNA interference (RNAi) offers the ability to directly and rapidly treat virus infections via the targeting of viral genes. Indeed, clinical trials have already been undertaken with promising results. Here we review the current state of the RNAi field for the treatment of viral infections such as HIV, human papillomavirus and HCV. We also review novel strategies including the concept of targeting self-genes to limit viral infection and activating the immune system for improved outcomes. Finally we examine innovative approaches being pursued at the Australian Infectious Diseases Research Centre including the use of highthroughput siRNA screens to identify new antiviral targets.

Wzy-Dependent Bacterial Capsules as Potential Drug Targets by Daniel J. Ericsson, Alistair Standish, Bostjan Kobe, Renato Morona (1421-1431).
The bacterial capsule is a recognized virulence factor in pathogenic bacteria. It likely works as an antiphagocytic barrier by minimizing complement deposition on the bacterial surface. With the continual rise of bacterial pathogens resistant to multiple antibiotics, there is an increasing need for novel drugs. In the Wzy-dependent pathway, the biosynthesis of capsular polysaccharide (CPS) is regulated by a phosphoregulatory system, whose main components consist of bacterial-tyrosine kinases (BY-kinases) and their cognate phosphatases. The ability to regulate capsule biosynthesis has been shown to be vital for pathogenicity, because different stages of infection require a shift in capsule thickness, making the phosphoregulatory proteins suitable as drug targets. Here, we review the role of regulatory proteins focusing on Streptococcus pneumoniae, Staphylococcus aureus, and Escherichia coli and discuss their suitability as targets in structure-based drug design.

Recent Advances in the Development of Thioredoxin Reductase Inhibitors as Anticancer Agents by Yang Liu, Yijing Li, Shenghui Yu, Guisen Zhao (1432-1444).
Redox homeostasis is crucial for the cellular viability and normal function which balance is maintained by two major cellular antioxidant systems, including glutathione system and thioredoxin system. Thioredoxin system, including thioredoxin (Trx), thioredoxin reductase (TrxR) and NADPH, exhibits a wide range of functions such as regulation of redox state and cell apoptosis. Particularly, Trx functions as a protein disulfide reductase which is essential for the function of Trx system. However, the bioactivity of Trx is closely dependent on its reducing form. According to the information, TrxR is the only cellular enzyme to catalyze the NADPH-dependent reduction of Trx. Besides the reduction of some protein disulfide like Trx, TrxR still has a broad substrate specificity to reduce some small molecules like 5, 5 '-dithiobis-2- nitrobenzoic acid (DTNB). The reduction of Trx or its own direct action towards its various substrates endows TrxR with a wide range of cellular functions. Recent studies have elucidated that TrxR was upregulated in many malignant tumors and inhibition of TrxR could prevent the tumor initiation and progression, suggesting TrxR to be a promising target for cancer therapy and the high nucleophilic and accessible selenocysteine (Sec) active site might be the prime target for drug design. Various kinds of TrxR inhibitors have been developed as anticancer agents for years. In this review, TrxR inhibitors are divided into three classes, including metal-containing inhibitors, naturally occurring products and their derivatives and other newly emerged inhibitors. The last five years reports about TrxR inhibitors of each class will be introduced and their novel inhibiting mechanisms will be discussed.

Loss of heterozygosity (LOH) at human chromosome 18q, which includes the gene Deleted in Colorectal Cancer (DCC), has been linked to colorectal and many other human cancers. DCC encodes the receptor for the axon guidance molecule Netrin (Net) and functions during neural development in a variety of organisms. However, since its discovery in the 1990s, the status of DCC as a tumor suppressor has been debated, primarily due to a lack of support for this hypothesis in animal models. A recent study from our laboratory capitalized on the genetic tractability of Drosophila melanogaster to demonstrate that this gene functions as an invasive tumor suppressor, thereby providing the first direct link between DCC loss and metastatic phenotypes in an animal model for cancer. Two subsequent studies from other laboratories have demonstrated that DCC suppresses tumor progression and metastasis in murine colorectal and mammary tumor models. Combined, these findings have prompted the rebirth of DCC as a tumor suppressor and highlighted the need for continued analysis of DCC function in animal models for human cancer.

Co-Morbidity and Self Medication in Schizophrenia: Involvement of Endogenous Morphine Signaling Mechanisms by Richard M. Kream, Hana Kuzelova, Milena Kralickova, Radek Ptacek, George B. Stefano (1454-1457).
For over 30 years, empirical studies have demonstrated expression of chemically authentic morphine by diverse animal tissues and organs systems. De novo biosynthesis of endogenous morphine by animal cells displays striking similarities to the multi-enzyme mediated biosynthetic pathway previously characterized in great biochemical and molecular detail in opium poppy (Papaver somniferum). The committed enzyme step within this pathway involves an asymmetric Pictet-Spengler condensation of dopamine (DA) and 3,4 dihydroxyphenylacetaldehyde (DOPAL), the oxidation product of L- 3,4-dihydroxyphenylalanine (L-DOPA), to form the essential intermediate precursor tetrahydropapaveroline (THP). We have hypothesized that endogenous morphine is synthesized within peripheral sites via conversion of THP in a regulated biosynthetic pathway, or conversely, THP may be directly transported into the CNS and converted to endogenous morphine within a similar biosynthetic pathway. The fundamental chemical relationship of the prototype catecholamine DA and its immediate precursor L-DOPA to endogenous morphine expression indicates a novel reciprocally interactive mechanism that links catecholamine and “morphinergic” pathways in the activation and inhibition of key physiological responses, including higher order neural integration. Dysregulation of interactive DAergic and “morphinergic” signaling pathways within CNS foci may contribute to the etiological factors driving co-morbid behavioral syndromes in major psychiatric disorders. Our short review is designed to provide insights on comorbidity and self-medication in schizophrenia from a novel perspective involving endogenous morphine signaling mechanisms.

Recent Approaches to Novel Antibacterials Designed After LPS Structure and Biochemistry by Luca Gabrielli, Alice Capitoli, Davide Bini, Francesca Taraballi, Cristina Lupo, Laura Russo, Laura Cipolla (1458-1471).
Lipopolysaccharides (LPSs), which constitute the lipid portion of the outer leaflet of Gram-negative bacteria, are essential for growth, and are responsible for a variety of biological effects associated with Gram-negative sepsis. LPSs are amphiphilic molecules comprising three regions: lipid A, the core region, and a polysaccharide portion; the lipid A was proven to represent the toxic principle of endotoxic active lipopolysaccharides. In addition, it is known that the minimal conserved structure of LPS is the lipophylic oligoasaccharidic structure containing Kdo residues linked to the-LipA moiety. Thus, the design and development of novel antibacterial drugs can focus on different aspects, related to the biosynthesis and chemical features of LPS: 1) Inhibitors of lipid A biosynthesis 2) Inhibitors of Kdo biosynthesis. Both Kdo and Lipid A are needed for the construction of the minimum structural element Kdo2-LipidA, needed for bacterial survival. Any inhibitors acting on the biogenetic pathway of this molecule can act as antibacterial. 3) Antagonists of the interaction between endotoxins and the host receptors: LPS is recognised by the CD14 and the Toll-like receptor (TLR)- 4/MD2 complex, where Lipid A is the crucial moiety in the interaction. Any drug acting as an antagonist of this process can have antisepsis potential. Considerable efforts have been made in this direction to identify natural or synthetic molecules able to interfere with the interaction between LPS and inflammatory cells. This review will highlight recent efforts in the design and biological activity of enzyme inhibitors and antagonist acting on the 3 key aspects outlined above.