Current Drug Targets (v.12, #7)

In the early nineties of the last century the possibility that human pathogens can be investigated in simple hosts was eithernot seriously taken into consideration or even denied by most researchers. Pioneering work in Frederick Ausubel’s lab pavedthe way for the use of surrogate hosts in the study of host - pathogen interactions [1]. In retrospect it is not surprising that theinvestigation of host - pathogen interactions is possible in simple, non-natural hosts since underlying fundamental cellularprocesses are conserved from yeast to man. Likewise, many of the involved genes and their functions are conserved. Theidentification and analysis of crucial host or pathogen factors in non-natural hosts lead to a better understanding of the complexcross-talk between host and pathogen. Identified proteins may also constitute potential new antimicrobial drug targets and mustthen, preferably in the natural host, be further validated.In this special issue the use of eight model organisms to study host - pathogen interactions are described. Acanthamoebaeare free-living amoebae and among the most prevalent protozoa found in the environment. They feed on bacteria byphagocytosis, however, some bacteria are able to survive and multiply within the amoebae. The intracellular growth of bacteriahas been associated with enhanced environmental survival of the bacteria, increased virulence and decreased sensitivity toantibiotic substances (Sandstrom et al., this issue). The potential of the professional phagocyte Dictyostelium discoideum as analternative model to higher organisms for host-pathogen interaction studies is discussed in the second review. Because hostpathogeninteractions necessarily involve two organisms, it is desirable to be able to genetically manipulate both the pathogenand its host. Particularly suited are those hosts, like Dictyostelium, whose genome sequence is known and annotated and forwhich excellent genetic and cell biological tools are available in order to dissect the complex crosstalk between host andpathogen (Bozzaro and Eichinger, this issue). The use of the model plant Arabidopsis thaliana accelerated our understanding ofdifferent plant-parasite interactions. Meanwhile hundreds of genes are known that are involved in different defence reactions,offering many new targets for drug development. Since some pathogenic strategies are conserved between animal and plantpathogens, results obtained with the plant system might be applicable to the animal system (Schlaich, this issue). Themanuscript -C. elegans: an all in one model for antimicrobial drug discovery- aims at presenting the potential of theinvertebrate Caenorhabditis elegans as an alternative model to mammalian systems for host-pathogen interaction studies. Theadvantages and limitations of this nematode for in vivo antimicrobial drug screenings and recent developments as well asperspectives concerning high-throughput approaches are discussed (Squiban and Kurz, this issue). Drosophila melanogasterwhose basal immune response is well understood is a widely used model organism to decipher host-pathogen interactions. Thereview by Limmer et al. focuses mainly on infections with two categories of pathogens, the well-studied Gram-negativebacterium Pseudomonas aeruginosa and infections by fungi of medical interest. These examples provide an overview over thecurrent knowledge on Drosophila-pathogen interactions and illustrate the approaches that can be used to study theseinteractions (Limmer et al., this issue). In vivo imaging in combination with advanced tools for genomic and large scale mutantanalysis is one of the strengths of the zebrafish. The organism offers excellent possibilities as a high-throughput drug screeningmodel for immune-related diseases, including inflammatory and infectious diseases and cancer. The review by Meijer andSpaink discusses the current knowledge on receptors and downstream signaling components that are involved in the zebrafishembryo’s innate immune response and summarizes recent insights gained from the use of bacterial infection models,particularly Mycobacterium marinum (Meijer and Spaink, this issue). The guinea pig model of disease has been consideredsynonymous with the experimental laboratory animal since the nineteenth century. In the review by Anthony Hickey the use of the guinea pig as a laboratory animal, aspects of immunology, viral pathogens and host - pathogen models are discussed(Hickey, this issue). Mouse animal models, which mimic human disease, are invaluable tools for understanding the mechanismsof disease pathogenesis and the development of treatment strategies. The review by Herrero et al. describes the application ofmouse animal models of alphaviral diseases to better understand the mechanisms that contribute to disease and to define therole that the immune response may have on disease pathogenesis with the view of providing the foundation for new treatments(Herrero et al., this issue).In summary, the manuscripts of this special issue highlight the potential of the different model organisms, fromAcanthamoeba to Mouse, to dissect host - pathogen interactions and to unravel novel drug targets.

Acanthamoeba-Bacteria: A Model to Study Host Interaction with Human Pathogens by Gunnar Sandstrom, Amir Saeed, Hadi Abd (936-941).
Acanthamoebae are free-living amoebae distributed worldwide. They are among the most prevalent protozoafound in the environment, and have been isolated from a wide variety of public water supplies, swimming pools, bottledwater, ventilation ducts, soil, air, surgical instruments, contact lenses, dental treatment units and hospitals.Acanthamoebae feed on bacteria by phagocytosis, but some bacteria are able to survive and sometimes multiply in thehost, resulting in new properties of the bacteria. The intracellular growth of bacteria has been associated with enhancedenvironmental survival of the bacteria, increased virulence and increased resistance against antibiotic substances. Theadvantage of utilising free-living amoebae is that research can be carried out on non-mammalian cells as a model based onnatural reality to study bacterial virulence and pathogenicity. Amoebae are easy to handle experimentally compared withmammalian cells and allow studies on host factors for host-parasite interactions. Bacteria are easily manipulatedgenetically, which creates the possibility of research on mutants to study bacteria-host interactions. Thus utilising thisnon-mammalian model can result in better understanding of interactions between prokaryotic and eukaryotic cells andassist in the development of new therapeutic agents to recognise and treat infections.

The use of simple hosts such as Dictyostelium discoideum in the study of host pathogen interactions offers anumber of advantages and has steadily increased in recent years. Infection-specific genes can often only be studied in avery limited way in man and even in the mouse model their analysis is usually expensive, time consuming and technicallychallenging or sometimes even impossible. In contrast, their functional analysis in D. discoideum and other simple modelorganisms is often easier, faster and cheaper. Because host-pathogen interactions necessarily involve two organisms, it isdesirable to be able to genetically manipulate both the pathogen and its host. Particularly suited are those hosts, like D.discoideum, whose genome sequence is known and annotated and for which excellent genetic and cell biological tools areavailable in order to dissect the complex crosstalk between host and pathogen. The review focusses on host-pathogeninteractions of D. discoideum with Legionella pneumophila, mycobacteria, and Salmonella typhimurium which replicateintracellularly.

In the last twenty years, the use of Arabidopsis as a model plant sped up discoveries at the molecular levels indifferent plant-parasite interactions. Nowadays, we know of probably hundreds of genes that are involved in the one or theother defence reaction, offering hundreds of targets for drug development. Even more interesting, identifying crucialregulatory components might allow to influence the various defence pathways as needed. Moreover, since somepathogenic strategies are conserved between animal and plant pathogens, results obtained with one system might beapplicable to the other.

C. elegans: An All in One Model for Antimicrobial Drug Discovery by B. Squiban, C. Leopold Kurz (967-977).
One approach to identify new drugs with antimicrobial activities is to screen large libraries of moleculesdirectly for their capacity to block the growth of bacterial or fungal monocultures. A more relevant way to assess both aproduct’s efficacy and its potential cytotoxicity is undoubtedly to use an in vivo infection system. Testing bankscontaining thousands of natural or chemically synthesized molecules with rodents is generally neither desirable norfeasible. Therefore, invertebrate model organisms could represent a valuable alternative. In this review, we present theworm C. elegans as a suitable host model for the evaluation and characterization of drug effects in a pathogenesis context.This simple organism has been of great value in many fields of biology and is currently intensely used in studies of hostpathogeninteractions. Infection of C. elegans induces a number of defense mechanisms, some of which are similar tothose seen in mammalian innate immunity. Further, it has been demonstrated that several microbial virulence mechanismsrequired for full pathogenicity in mammals are also necessary for infection in nematodes. Based on these facts, a numberof innovative antimicrobial drug screens have been carried out successfully and the development of new tools to monitorthe interaction between worm and microbes in vivo opens promising perspectives.

Virulence on the Fly: Drosophila melanogaster as a Model Genetic Organism to Decipher Host-Pathogen Interactions by Stefanie Limmer, Jessica Quintin, Charles Hetru, Dominique Ferrandon (978-999).
To gain an in-depth grasp of infectious processes one has to know the specific interactions between thevirulence factors of the pathogen and the host defense mechanisms. A thorough understanding is crucial for identifyingpotential new drug targets and designing drugs against which the pathogens might not develop resistance easily. Modelorganisms are a useful tool for this endeavor, thanks to the power of their genetics. Drosophila melanogaster is widelyused to study host-pathogen interactions. Its basal immune response is well understood and is briefly reviewed here.Considerations relevant to choosing an adequate infection model are discussed. This review then focuses mainly oninfections with two categories of pathogens, the well-studied Gram-negative bacterium Pseudomonas aeruginosa andinfections by fungi of medical interest. These examples provide an overview over the current knowledge on Drosophilapathogeninteractions and illustrate the approaches that can be used to study those interactions. We also discuss theusefulness and limits of Drosophila infection models for studying specific host-pathogen interactions and high-throughputdrug screening.

Host-Pathogen Interactions Made Transparent with the Zebrafish Model by Annemarie H. Meijer, Herman P. Spaink (1000-1017).
The zebrafish holds much promise as a high-throughput drug screening model for immune-related diseases,including inflammatory and infectious diseases and cancer. This is due to the excellent possibilities for in vivo imaging incombination with advanced tools for genomic and large scale mutant analysis. The context of the embryo’s developingimmune system makes it possible to study the contribution of different immune cell types to disease progression.Furthermore, due to the temporal separation of innate immunity from adaptive responses, zebrafish embryos and larvaeare particularly useful for dissecting the innate host factors involved in pathology. Recent studies have underscored theremarkable similarity of the zebrafish and human immune systems, which is important for biomedical applications. Thisreview is focused on the use of zebrafish as a model for infectious diseases, with emphasis on bacterial pathogens.Following a brief overview of the zebrafish immune system and the tools and methods used to study host-pathogeninteractions in zebrafish, we discuss the current knowledge on receptors and downstream signaling components that areinvolved in the zebrafish embryo’s innate immune response. We summarize recent insights gained from the use ofbacterial infection models, particularly the Mycobacterium marinum model, that illustrate the potential of the zebrafishmodel for high-throughput antimicrobial drug screening.

The guinea pig model of disease has been considered synonymous with the experimental laboratory animalsince the nineteenth century. Recently we have reviewed the use of this species in models of bacterial infectious disease.The present review extends to viral diseases for which the guinea pig is less frequently considered the relevant animalmodel. The use of the guinea pig as a laboratory animal, aspects of immunology, viral pathogens and host-pathogenmodels are discussed. As a small and relatively inexpensive model for infection and immunity the guinea pig has asignificant future but there are substantial requirements for development of validated quantitative analytical methods forimmunological and disease biomarkers if it is to reach its potential.

Applications of Animal Models of Infectious Arthritis in Drug Discovery: A focus on Alphaviral Disease by Lara Herrero, Michelle Nelson, Jayaram Bettadapura, Michelle E. Gahan, Suresh Mahalingam (1024-1036).
Animal models, which mimic human disease, are invaluable tools for understanding the mechanisms of diseasepathogenesis and development of treatment strategies. In particular, animal models play important roles in the area ofinfectious arthritis. Alphaviruses, including Ross River virus (RRV), o'nyong-nyong virus, chikungunya virus (CHIKV),mayaro virus, Semliki Forest virus and sindbis virus, are globally distributed and cause transient illness characterized byfever, rash, myalgia, arthralgia and arthritis in humans. Severe forms of the disease result in chronic incapacitatingarthralgia and arthritis. The mechanisms of how these viruses cause musculoskeletal disease are ill defined. In recentyears, the use of a mouse model for RRV-induced disease has assisted in unraveling the pathobiology of infection and indiscovering novel drugs to ameliorate disease. RRV as an infection model has the potential to provide key insights intosuch disease processes, particularly as many viruses, other than alphaviruses, are known to cause infectious arthritides.The emergence and outbreak of CHIKV in many parts of the world has necessitated the need to develop animal models ofCHIKV disease. The development of non-human primate models of CHIKV disease has given insights into viral tropismand disease pathogenesis and facilitated the development of new treatment strategies. This review highlights theapplication of animal models of alphaviral diseases in the fundamental understanding of the mechanisms that contribute todisease and for defining the role that the immune response may have on disease pathogenesis, with the view of providingthe foundation for new treatments.

Proteins of glutamatergic NMDA receptor signaling pathways have been studied as targets for intervention in avariety of neuropathological conditions, including neurodegenerations, epilepsy, neuropathic pain, drug addiction, andschizophrenia. High activity NMDA-blocking agents have been designed to treat some of these disorders; however, theireffect is often compromised by undesirable side effects. Therefore, alternative ways of modulating NMDA receptorfunction need to be sought after.The opening of the NMDA receptor ion channel requires occupation of two distinct binding sites, the glutamate site andthe glycine site. It has been shown that D-serine, rather than glycine, can trigger the physiological NMDA receptorfunction. D-serine is a product of the activity of a specific enzyme, serine racemase (SR), which was identified a decadeago. SR has therefore emerged as a new potential target for the NMDA-receptor-based diseases. There is evidence linkingincreased levels of D-Ser to amyotrophic lateral sclerosis and Alzheimer’s disease and decreased concentrations of Dserineto schizophrenia.SR is a pyridoxal-5’-phosphate dependent enzyme found in the cytosol of glial and neuronal cells. It is activated by ATP,divalent cations like Mg2+ or Ca2+, and reducing agents. This paper reviews the present literature on the activity andinhibition of mammalian SRs. It summarizes approaches that have been applied to design SR inhibitors and lists theknown active compounds. Based on biochemical and docking analyses, i) we delineate for the first time the ATP bindingsite of human SR, and ii) we suggest possible mechanisms of action for the active compounds. In the end, we discuss theSR features that make the discovery of its inhibitors a challenging, yet very important, task of medicinal chemistry.

Phosphatidylinositol 3-Kinase Isoforms as Novel Drug Targets by Karolina Blajecka, Anna Borgstrom, Alexandre Arcaro (1056-1081).
Phosphatidylinositol 3-kinases (PI3Ks) are key molecules in the signal transduction pathways initiated by thebinding of extracellular signals to their cell surface receptors. The PI3K family of enzymes comprises eight catalyticisoforms subdivided into three classes and control a variety of cellular processes including proliferation, growth,apoptosis, migration and metabolism. Deregulation of the PI3K pathway has been extensively investigated in connectionto cancer, but is also involved in other commonly occurring diseases such as chronic inflammation, autoimmunity,allergy, atherosclerosis, cardiovascular and metabolic diseases. The fact that the PI3K pathway is deregulated in a largenumber of human diseases, and its importance for different cellular responses, makes it an attractive drug target.Pharmacological PI3K inhibitors have played a very important role in studying cellular responses involving theseenzymes. Currently, a wide range of selective PI3K inhibitors have been tested in preclinical studies and some haveentered clinical trials in oncology. However, due to the complexity of PI3K signaling pathways, developing an effectiveanti-cancer therapy may be difficult. The biggest challenge in curing cancer patients with various signaling pathwayabnormalities is to target multiple components of different signal transduction pathways with mechanism-basedcombinatorial treatments. In this article we will give an overview of the complex role of PI3K isoforms in human diseasesand discuss their potential as drug targets. In addition, we will describe the drugs currently used in clinical trials, as wellas promising emerging candidates.

Biology of Cox-2: An Application in Cancer Therapeutics by Zakir Khan, Noor Khan, Ram P. Tiwari, Nand K. Sah, GBKS Prasad, Prakash S. Bisen (1082-1093).
Cyclooxygenase-2 (Cox-2) is an inducible enzyme involved in the conversion of arachidonic acid toprostaglandin and other eicosanoids. Molecular pathology studies have revealed that Cox-2 is over-expressed in cancerand stroma cells during tumor progression, and anti-cancer chemo-radiotherapies induce expression of Cox-2 in cancercells. Elevated tumor Cox-2 is associated with increased angiogenesis, tumor invasion and promotion of tumor cellresistance to apoptosis. Several experimental and clinical studies have established potent anti-cancer activity of NSAID(Non-steroidal anti-inflammatory drugs) and other Cox-2 inhibitors such as celecoxib. Much attention is being focused onCox-2 inhibitors as a beneficial target for cancer chemotherapy. The mode of action of Cox-2 and its inhibitors remainsunclear. Further clinical application needs to be investigated for comprehending Cox-2 biological functions andestablishing it as an effective target in cancer therapy.