Infectious Disorders - Drug Targets (Formerly Current Drug Targets - Infectious Disorders) - Volume 8, Issue 1, 2008

Most of the actions of estrogens are mediated by two isoforms of the estrogen receptor (ER) encoded by different genes, ERαand ERβ, which were cloned in 1986 and 1987, respectively. Both receptors function as ligand-dependent transcription factors to regulate gene expression according to the classical estrogen signalling pathway. However, estrogen signalling seems to be more complicated than the genomic pathway because several biological estrogen actions are too fast to be compatible with a transcriptional mechanism. Evidence has been accumulating indicating that not all of the physiological actions of estrogen can be explained according to a direct effect on gene transcription, and the involvement of signalling pathways related to cytoplasmatic proteins, growth factors and/or membrane-initiated responses has been reported. This mode of action has been termed non-classical, non-genomic or rapid effects of estradiol and it has been related to an increase in NO release, calcium homeostasis, cAMP accumulation or IGF pathway activation. However, the molecular mechanisms responsible are poorly understood. In this context, the existence of plasma membrane receptors has been proposed to explain the rapid actions of estrogen. What determines ERα targeting to the plasma membrane? The main question is, what is the role of these isoforms of ERα in/on the plasma membrane? It has been proposed that post-translational modifications of some ER proteins must occur to ensure targeting to the membrane, including palmitoylation, which could be related to facilitation of caveolin-1 binding, but more studies are needed to answer this question. Although the evidence suggests the existence of a close relationship between estrogen and sensitivity to the action of insulin, relatively few studies has attempted to solve the mystery exists about the molecular basis of this relationship in insulindependent tissues. Resolve these unknowns could have a major impact on long-term therapy, and that resistance to the action of insulin is the underlying cause of many diseases, for example, the aging female as Type 2 diabetes, cardiovascular disease circulatory, neurodegenerative diseases or some types of cancer. Recent data have revealed a surprising role for estradiol in regulating energy metabolism and opened new insights about the regulation of the intracellular insulin signalling and insulin secretion by ERα and ERβ. This new field of research promises radically to change our knowledge about the mechanism of actions of the estrogens and to contribute to understand better the therapeutic possibilities of the estrogens receptor in order to improve some aspects related to some menopause-relative diseases as insulin resistance, ictus, cancer and so on. The goal of these reviews is to highlight the important role that the fast or non-genomic estrogen actions has at different levels. In this way, Dr. Ripoll et al. show in their review that insulin release is controlled by estradiol due to the existence of ERα and ERβ in plasma membrane of β-cells, because 17α-estradiol regulates KATP channel activity and glucose-induced [Ca2+]i oscillations, eliciting changes in the activation of Ca2+-dependent transcription factors. Because an excess of white adipose tissue results in obesity and estrogens promote, maintain, and control the typical distribution of body fat and adipose tissue metabolism through still unknown mechanisms, Dr. Pallottini et al. show in their review the role of estrogens in adipose tissue differentiation and in the protection against the onset of obesity, and in this way they explain the underlying molecular mechanisms mediated by estrogen receptor isoform ERα and ERβ. Dr. Alonso and Dr. Gonzalez provide some evidences that suggest the existence of a narrow interrelation between the nongenomic action of estrogens and insulin sensitivity. The resolution of the unknown questions about the molecular mechanism of this interrelation would be able to have a great long-term therapeutic repercussion in several associated pathologies to the female aging, because insulin resistance could be the underlying cause of some of them. Finally, Dr. Gonzalez et al. analyzed present and future ways of the possible molecular mechanism involves in the neuroprotective effect of estrogens on brain. The relationship between insulin resistance states and neurodegenerative diseases associated with aging in females, and the cross-talk between estradiol and proteins includes in the IRS-1/PI3-k/Akt and IGF-1- IR signalling pathways in brain, will lead to a more complete understanding of the precise mechanism underlying estradiolmediated neuroprotection.

Understanding how novel antifungal compounds work in target cells is useful not only in facilitating the discovery of new drugs but also new tools that can be used for further exploration of the targeted biological pathways and their regulation. Various genomic and genetic technologies have been developed in the model yeast Saccharomyces cerevisiae, and have been successfully used to identify drug target pathways. This review discusses the methods developed for some of these technologies, and how they have been used to evaluate the cellular pathways affected by a variety of therapeutic drugs and inhibitors. The advantages and disadvantages of each method are considered, and new advances are highlighted where applicable. The investigation of the mechanism of action of new antifungal compounds will undoubtedly lead to the development of new antifungal therapies targeting new fungal pathways that are more specific and less toxic than currently available antifungal drugs.

Synthesis of farnesyl pyrophosphate (FPP), a key intermediate of the isoprenoid biosynthesis pathway, is catalyzed by FPP synthase (FPPS). Antiprotozoal properties of bisphosphonates, which target FPPS, have generated interest in FPPS as a potential antiprotozoal drug target. The genes encoding FPPS from parasitic protozoa were assessed to analyze structural and functional features of the enzyme. Comparisons of the FPPS from the parasitic protozoa and search for conserved motifs revealed that FPPS from both apicomplexan and trypanosomatid parasites show characteristic conserved regions for example first aspartate rich motif (FARM) contained within II conserved domain and the second aspartate rich motif (SARM) contained within VI conserved domain. Phylogenetic analysis of FPPS generated a tree with three distinct clusters. Overall topology of the phylogenic tree constructed with small subunit ribosomal RNA sequences was almost similar to that constructed with FPPS sequences. Comparative homology modeling and structural comparisons of FPPS from the parasitic protozoa provided significant insights into common and distinct characteristics of the enzyme. The critical interacting residues of the isopentenyl pyrophosphate binding site are conserved across the enzymes from the family except for malarial FPPS where the C-terminal residues from the BXB motif of helix J were missing. Variations noticed in aromatic residue pairs at the fourth and fifth position upstream of the FARM, which play important role in determination of chain length of the polyprenyl products, may produce functional differences among protozoan FPPSs. The structural comparison of protozoan FPPS may be useful in designing common or selective FPPS inhibitors as potential broad spectrum or selective antiprotozoal agents.

Technology surrounding genomics, or the study of an organism's genome and its gene use, has advanced rapidly resulting in an abundance of readily available genomic data. Although genomics is extremely valuable, proteins are ultimately responsible for controlling most aspects of cellular function. The field of proteomics, or the study of the full array of proteins produced by an organism, has become the premier arena for the identification and characterization of proteins. Yet the task of characterizing a proteomic profile is more complex, in part because many unique proteins can be produced by the same gene product and because proteins have more diverse chemical structures making sequencing and identification more difficult. Proteomic profiles of a particular organism, tissue or cell are influenced by a variety of environmental stimuli, including those brought on by infectious disease. The intent of this review is to highlight applications of proteomics used in the study of pathogenesis, etiology and pathology of infectious disorders. While many infectious agents have been the target of proteomic studies, this review will focus on those infectious diseases which rank among the highest in worldwide mortalities, such as HIV/AIDS, tuberculosis, malaria, measles, and hepatitis.

Infectious disorders have always received special attention due to their global importance in human health. Recently this area has been in special focus due to increased global biological threats, bioterrorism and prominent media coverage given to some recent infectious disease epidemics [1, 2]. Global climate changes have also led to an increased risk of infectious diseases [3]. Reemergence of the infectious diseases and continuous emergence of drug resistance strains of the pathogens underscores the need for identification of new agents; indeed, the building and continuous augmentation of an “armamentarium of multiple drugs” is necessary to cope up with the problem of further development of resistance [4]. The conventional approaches to drug discovery, particularly the technologies of in vitro individual target-based and pathogen culture-based screening, may not be sufficient to sustain this level of innovation and drug discovery/development. Additionally, morbidity and mortality attributable to tropical diseases, particularly the parasitic infections including malaria, leishmaniasis and trypanosomiasis (new and old world) are staggering. Together they are a tremendous burden in morbidity, mortality, and economic hardship. More than half of the world's population is currently at risk of infection with one or the other tropical disease pathogens. In addition, it has been estimated that almost one third of world's population has been exposed to tuberculosis. Therefore TB and tropical infections collectively are a major global health predicament. Despite of this, tropical parasitic diseases and TB have been largely ignored in relation to advances in modern drug discovery. Infectious Disorders Drug Targets (IDDT) shall continue with the mission to provide a high impact platform for discussions on new discoveries, recent developments and critical evaluation of the knowledge on novel drug & vaccine targets of infectious disease pathogens as well as recent technological advancements in this important area related to global health. The technological advancements and accumulation of wealth of information on infectious disease pathogens have resulted into an augmented interest and exponential rise in the knowledge regarding distinct biochemical, molecular and functional characteristics of potential target enzyme as well as metabolic pathways of the pathogens [5-9]. A few selected examples of these advancements are sequencing of pathogen genomes; whole pathogen genome expression analysis, target and pathways analysis through DNA/protein microarrays & global expression profiling of the pathogen genomes; high throughput structural and functional genomics; molecular targets-based high throughput screening approaches; generation of large compound libraries directed to special molecular-targets; fragmented screening of privileged chemical libraries; live cell bio-imaging technologies; renewed interests and application of high impact technologies in natural products as important source of novel pharmacophores. These developments have resulted into a paradigm shift in approach of new drug discovery against the infectious disorders. The pathogen genome data, functional genomics and system biology approaches have helped in construction/reconstruction of almost complete metabolic maps of the pathogens [10]. This information has been useful in identification of metabolic pathways, enzymes, receptors, cellular function, which are unique to the pathogens [11]. Such knowledge can be directly applied to molecular-targets based drug-design. The process of new drug discovery is a mammoth task which requires gigantic investments, and in light of the low level of profitability associated with the treatments for tropical parasitic diseases, discovery efforts have not been sufficiently systematic, rigorous and comprehensive. Although the incidences of parasitic infections are mostly centered in tropical regions, the impacts, especially economic, of the disease are global. Emergence of few Private-Public Partnerships (PPPs), for example Medicines for Malaria Venture (MMV); Drugs for Neglected Diseases Initiatives (DNDi); Global Funds to fight AIDS, Tuberculosis and Malaria; and interests of governmental and non-profit agencies has greatly stimulated the research for discovery and development of new drugs against the neglected infectious diseases [12]. Continuing with the legacy of predecessors future missions of IDDT would be targeted to present critical analysis of common target enzymes/metabolic pathways of the infectious disease pathogens, compilation of up-to-date information on molecular approaches for controlling emerging infectious diseases, SARS and other related viral pathogens, skin infectious diseases, Prion diseases, neglected eukaryotic infectious pathogens e.g., parasitic helminthes, Apicomplexan parasites and trypanosomatids. An special IDDT issue dedicated to in silico approaches in study of pathogen targets, including the comparative structural analysis of potential common target enzymes shall be applied to new drug discovery research.

Numerous experimental and clinical data show that the physiological actions of insulin and sexual steroids interact in target tissues for these hormones. In the other hand, sexual steroids has effects on peripheral tissues, and since the skeletal muscle is the main responsible for peripheral glucose uptake, it would be possible that the sexual steroids induce directly in the muscle a decrease of the sensibility of this tissue to insulin action. Some of the biological actions of the estrogens are too fast like to be compatible with this classical mechanism of action, and this mechanism has been called not classical, non-genomic or rapid actions of the estrogens. Moreover, some experiments have shown that low concentrations of estradiol, induce an increase in the rate of IRS-1 phosphorylation, promotes the association between IRS-1 and the subunit of PI3-k, p85α, causes a decrease in the rate of IRS-1 serine phosphorylation and increases the rate of Akt phosphorylation. Therefore, the evidences suggest the existence of a narrow interrelation between the estrogens and insulin sensitivity, but relatively few studies have tried to resolve the molecular base of this relation in insulindependent tissues. The resolution of these unknown questions would be able to have a great long-term therapeutic repercussion. In this sense, we should not forget that insulin resistance is the underlying cause of several associated pathologies to the female aging, as Type 2 diabetes, cardio-circulatory pathology or neurodegenerative disease.

Adipose tissue has recently been described as one of the major endocrine gland that plays a role in energy homeostasis, lipid metabolism, immune response, and reproduction. An excess of white adipose tissue, caused by a complex interaction between genetic, hormonal, behavioral, and environmental factors, results in obesity: a heterogeneous disorder that predisposes humans to a variety of diseases. Among several hormones, estrogens promote, maintain, and control the typical distribution of body fat and adipose tissue metabolism through still unknown mechanisms. These steroids are known to regulate fat mass, adipose deposition and differentiation, and adipocyte metabolism. Moreover, estrogen deficiency results in increases in adipose tissue, preferentially in visceral fat, which would link obesity to the susceptibility of related disorders. In this review the role of estrogens in adipose tissue differentiation and in the protection against the onset of obesity will be discussed with particular attention being drawn to the underlying molecular mechanisms mediated by estrogen receptor isoforms ERα and ERβ.

Rapid estrogen actions are triggered after estrogens are bound to a variety of proteins in organelles other than the nucleus. Those include classic estrogen receptors ERα and ERβ, novel membrane proteins that behave as estrogen receptors such as GPR30, ion channels, and other ligand receptors. In pancreatic α and β-cells, estrogens binding to a non-classical membrane estrogen receptors at physiological concentrations regulate ion channels and [Ca2+]i signals, provoking important physiological responses. In β-cells, 17β- estradiol regulates KATP channel activity and glucose-induced [Ca2+]i oscillations, eliciting changes in insulin release and the activation of Ca2+-dependent transcription factors. In α-cells, 17β-estradiol abolishes low glucose-induced [Ca2+]i oscillations.

The incidence of neurodegenerative diseases is higher in postmenopausal women that young women. In this sense, Alzheimer's and Parkinson's diseases, ischemic brain injury and memory or cognitive dysfunction increase dramatically when the ovarian function declines. On the other hand, insulin resistance represents an independent factor in the etiology of age-associated coronary and cerebrovascular disease. Therefore, depression, neurodegenerative diseases such as Alzheimer's and Parkinson's diseases and memory or cognitive dysfunction should be considered, in some cases, a result of metabolic syndrome, and that postmenopausal women are more vulnerable that young women to these diseases. Several studies have suggested that the molecular mechanism by which estradiol exerts its neuroprotective effects involves activation of the PI3-k signalling pathway, which is activated by insulin and IGF-1. Therefore, it seems possible that ERα can interact with these signalling pathways, mainly with PI3-k and IRS-1, to promote neuroprotective effects in the brain. In particular, IGF-I seems to be particularly important in the process of neuroprotection; it can reverse age-related effects and attenuate the age-related decrease in cerebral glucose utilization. Moreover, gonadal hormones have been found to regulate IGF-I receptor. Therefore, it seems clear that the interaction of both systems plays a role in the prevention of neuronal age-related effects. These findings suggest that by interacting with some components of the IGF-I signalling pathway, ERα affects the actions of IGF-I in the brain and suggest future avenues of research. The relationship between insulin resistance states associated with aging in females, and the cross-talk between estradiol and proteins includes in the IRS-1/PI3-k/Akt and IGF-1-IR signalling pathways, will lead to a more complete understanding of the precise mechanism underlying estradiol-mediated neuroprotection. Numerous clinical studies have demonstrated that the incidence of neurodegenerative diseases in higher in postmenopausal women that young women. In this sense, Alzheimer's and Parkinson's disease, ischemic brain injury and memory or cognitive dysfunction increase dramatically when the ovarian function declines. Moreover, estrogen replacement therapy seems to be a good element in order to decrease the risk and/or severity of neurodegenerative conditions, and it would be able to improve some aspects related to memory and learning process.