Current Drug Targets - Volume 12, Issue 7, 2011

In the early nineties of the last century the possibility that human pathogens can be investigated in simple hosts was either not seriously taken into consideration or even denied by most researchers. Pioneering work in Frederick Ausubel's lab paved the way for the use of surrogate hosts in the study of host - pathogen interactions [1]. In retrospect it is not surprising that the investigation of host - pathogen interactions is possible in simple, non-natural hosts since underlying fundamental cellular processes are conserved from yeast to man. Likewise, many of the involved genes and their functions are conserved. The identification and analysis of crucial host or pathogen factors in non-natural hosts lead to a better understanding of the complex cross-talk between host and pathogen. Identified proteins may also constitute potential new antimicrobial drug targets and must then, 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. Acanthamoebae are free-living amoebae and among the most prevalent protozoa found in the environment. They feed on bacteria by phagocytosis, however, some bacteria are able to survive and multiply within the amoebae. The intracellular growth of bacteria has been associated with enhanced environmental survival of the bacteria, increased virulence and decreased sensitivity to antibiotic substances (Sandstrom et al., this issue). The potential of the professional phagocyte Dictyostelium discoideum as an alternative model to higher organisms for host-pathogen interaction studies is discussed in the second review. Because hostpathogen interactions necessarily involve two organisms, it is desirable to be able to genetically manipulate both the pathogen and its host. Particularly suited are those hosts, like Dictyostelium, whose genome sequence is known and annotated and for which excellent genetic and cell biological tools are available in order to dissect the complex crosstalk between host and pathogen (Bozzaro and Eichinger, this issue). The use of the model plant Arabidopsis thaliana accelerated our understanding of different 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 plant pathogens, results obtained with the plant system might be applicable to the animal system (Schlaich, this issue). The manuscript “C. elegans: an all in one model for antimicrobial drug discovery” aims at presenting the potential of the invertebrate Caenorhabditis elegans as an alternative model to mammalian systems for host-pathogen interaction studies. The advantages and limitations of this nematode for in vivo antimicrobial drug screenings and recent developments as well as perspectives concerning high-throughput approaches are discussed (Squiban and Kurz, this issue). Drosophila melanogaster whose basal immune response is well understood is a widely used model organism to decipher host-pathogen interactions. The review by Limmer et al. focuses mainly on infections with two categories of pathogens, the well-studied Gram-negative bacterium Pseudomonas aeruginosa and infections by fungi of medical interest. These examples provide an overview over the current knowledge on Drosophila-pathogen interactions and illustrate the approaches that can be used to study these interactions (Limmer et al., this issue). In vivo imaging in combination with advanced tools for genomic and large scale mutant analysis is one of the strengths of the zebrafish. The organism offers excellent possibilities as a high-throughput drug screening model for immune-related diseases, including inflammatory and infectious diseases and cancer. The review by Meijer and Spaink discusses the current knowledge on receptors and downstream signaling components that are involved in the zebrafish embryo'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 considered synonymous 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 mechanisms of disease pathogenesis and the development of treatment strategies. The review by Herrero et al. describes the application of mouse animal models of alphaviral diseases to better understand the mechanisms that contribute to disease and to define the role 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, from Acanthamoeba to Mouse, to dissect host - pathogen interactions and to unravel novel drug targets.

Acanthamoebae are free-living amoebae distributed worldwide. They are among the most prevalent protozoa found in the environment, and have been isolated from a wide variety of public water supplies, swimming pools, bottled water, 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 the host, resulting in new properties of the bacteria. The intracellular growth of bacteria has been associated with enhanced environmental survival of the bacteria, increased virulence and increased resistance against antibiotic substances. The advantage of utilising free-living amoebae is that research can be carried out on non-mammalian cells as a model based on natural reality to study bacterial virulence and pathogenicity. Amoebae are easy to handle experimentally compared with mammalian cells and allow studies on host factors for host-parasite interactions. Bacteria are easily manipulated genetically, which creates the possibility of research on mutants to study bacteria-host interactions. Thus utilising this non-mammalian model can result in better understanding of interactions between prokaryotic and eukaryotic cells and assist 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 a number of advantages and has steadily increased in recent years. Infection-specific genes can often only be studied in a very limited way in man and even in the mouse model their analysis is usually expensive, time consuming and technically challenging or sometimes even impossible. In contrast, their functional analysis in D. discoideum and other simple model organisms is often easier, faster and cheaper. Because host-pathogen interactions necessarily involve two organisms, it is desirable 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 are available in order to dissect the complex crosstalk between host and pathogen. The review focusses on host-pathogen interactions of D. discoideum with Legionella pneumophila, mycobacteria, and Salmonella typhimurium which replicate intracellularly.

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

One approach to identify new drugs with antimicrobial activities is to screen large libraries of molecules directly for their capacity to block the growth of bacterial or fungal monocultures. A more relevant way to assess both a product's efficacy and its potential cytotoxicity is undoubtedly to use an in vivo infection system. Testing banks containing thousands of natural or chemically synthesized molecules with rodents is generally neither desirable nor feasible. Therefore, invertebrate model organisms could represent a valuable alternative. In this review, we present the worm 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 hostpathogen interactions. Infection of C. elegans induces a number of defense mechanisms, some of which are similar to those seen in mammalian innate immunity. Further, it has been demonstrated that several microbial virulence mechanisms required for full pathogenicity in mammals are also necessary for infection in nematodes. Based on these facts, a number of innovative antimicrobial drug screens have been carried out successfully and the development of new tools to monitor the interaction between worm and microbes in vivo opens promising perspectives.

To gain an in-depth grasp of infectious processes one has to know the specific interactions between the virulence factors of the pathogen and the host defense mechanisms. A thorough understanding is crucial for identifying potential new drug targets and designing drugs against which the pathogens might not develop resistance easily. Model organisms are a useful tool for this endeavor, thanks to the power of their genetics. Drosophila melanogaster is widely used 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 on infections with two categories of pathogens, the well-studied Gram-negative bacterium Pseudomonas aeruginosa and infections by fungi of medical interest. These examples provide an overview over the current knowledge on Drosophilapathogen interactions and illustrate the approaches that can be used to study those interactions. We also discuss the usefulness and limits of Drosophila infection models for studying specific host-pathogen interactions and high-throughput drug screening.

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 in combination with advanced tools for genomic and large scale mutant analysis. The context of the embryo's developing immune 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 larvae are particularly useful for dissecting the innate host factors involved in pathology. Recent studies have underscored the remarkable similarity of the zebrafish and human immune systems, which is important for biomedical applications. This review 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-pathogen interactions in zebrafish, we discuss the current knowledge on receptors and downstream signaling components that are involved in the zebrafish embryo's innate immune response. We summarize recent insights gained from the use of bacterial infection models, particularly the Mycobacterium marinum model, that illustrate the potential of the zebrafish model for high-throughput antimicrobial drug screening.

The guinea pig model of disease has been considered synonymous with the experimental laboratory animal since 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 animal model. The use of the guinea pig as a laboratory animal, aspects of immunology, viral pathogens and host-pathogen models are discussed. As a small and relatively inexpensive model for infection and immunity the guinea pig has a significant future but there are substantial requirements for development of validated quantitative analytical methods for immunological and disease biomarkers if it is to reach its potential.

Animal models, which mimic human disease, are invaluable tools for understanding the mechanisms of disease pathogenesis and development of treatment strategies. In particular, animal models play important roles in the area of infectious 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 by fever, rash, myalgia, arthralgia and arthritis in humans. Severe forms of the disease result in chronic incapacitating arthralgia and arthritis. The mechanisms of how these viruses cause musculoskeletal disease are ill defined. In recent years, the use of a mouse model for RRV-induced disease has assisted in unraveling the pathobiology of infection and in discovering novel drugs to ameliorate disease. RRV as an infection model has the potential to provide key insights into such 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 of CHIKV disease. The development of non-human primate models of CHIKV disease has given insights into viral tropism and disease pathogenesis and facilitated the development of new treatment strategies. This review highlights the application of animal models of alphaviral diseases in the fundamental understanding of the mechanisms that contribute to disease and for defining the role that the immune response may have on disease pathogenesis, with the view of providing the foundation for new treatments.

Proteins of glutamatergic NMDA receptor signaling pathways have been studied as targets for intervention in a variety of neuropathological conditions, including neurodegenerations, epilepsy, neuropathic pain, drug addiction, and schizophrenia. High activity NMDA-blocking agents have been designed to treat some of these disorders; however, their effect is often compromised by undesirable side effects. Therefore, alternative ways of modulating NMDA receptor function need to be sought after. The opening of the NMDA receptor ion channel requires occupation of two distinct binding sites, the glutamate site and the glycine site. It has been shown that D-serine, rather than glycine, can trigger the physiological NMDA receptor function. D-serine is a product of the activity of a specific enzyme, serine racemase (SR), which was identified a decade ago. SR has therefore emerged as a new potential target for the NMDA-receptor-based diseases. There is evidence linking increased levels of D-Ser to amyotrophic lateral sclerosis and Alzheimer's disease and decreased concentrations of Dserine to 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 and inhibition of mammalian SRs. It summarizes approaches that have been applied to design SR inhibitors and lists the known active compunds. Based on biochemical and docking analyses, i) we delineate for the first time the ATP binding site of human SR, ii) we suggest possible mechanisms of action for the active compounds. In the end, we discuss the SR features that make the discovery of its inhibitors a challenging, yet very important, task of medicinal chemistry.

Phosphatidylinositol 3-kinases (PI3Ks) are key molecules in the signal transduction pathways initiated by the binding of extracellular signals to their cell surface receptors. The PI3K family of enzymes comprises eight catalytic isoforms 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 connection to 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 large number 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 these enzymes. Currently, a wide range of selective PI3K inhibitors have been tested in preclinical studies and some have entered clinical trials in oncology. However, due to the complexity of PI3K signaling pathways, developing an effective anti-cancer therapy may be difficult. The biggest challenge in curing cancer patients with various signaling pathway abnormalities is to target multiple components of different signal transduction pathways with mechanism-based combinatorial treatments. In this article we will give an overview of the complex role of PI3K isoforms in human diseases and discuss their potential as drug targets. In addition, we will describe the drugs currently used in clinical trials, as well as promising emerging candidates.

Cyclooxygenase-2 (Cox-2) is an inducible enzyme involved in the conversion of arachidonic acid to prostaglandin and other eicosanoids. Molecular pathology studies have revealed that Cox-2 is over-expressed in cancer and stroma cells during tumor progression, and anti-cancer chemo-radiotherapies induce expression of Cox-2 in cancer cells. Elevated tumor Cox-2 is associated with increased angiogenesis, tumor invasion and promotion of tumor cell resistance 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 on Cox-2 inhibitors as a beneficial target for cancer chemotherapy. The mode of action of Cox-2 and its inhibitors remains unclear. Further clinical application needs to be investigated for comprehending Cox-2 biological functions and establishing it as an effective target in cancer therapy.