Following autophagy step by step

Autophagy is an evolutionarily conserved lysosomal degradation route for soluble components of the cytosol and organelles. There is great interest in identifying compounds that modulate autophagy because they may have applications in the treatment of major diseases including cancer and neurodegenerative disease. Hundeshagen and colleagues describe this month in BMC Biology a screening assay based on flow cytometry that makes it possible to track distinct steps in the autophagic process and thereby identify novel modulators of autophagy.     

"Molecular switches" on mGluR allosteric ligands that modulate modes of pharmacology

G-protein-coupled receptors (GPCRs) represent the largest class of drug targets, accounting for more than 40% of marketed drugs; however, discovery efforts for many GPCRs have failed to provide viable drug candidates. Historically, drug discovery efforts have focused on developing ligands that act at the orthosteric site of the endogenous agonist. Recently, efforts have focused on functional assay paradigms and the discovery of ligands that act at allosteric sites to modulate receptor function in either a positive, negative, or neutral manner. Allosteric modulators have numerous advantages over orthosteric ligands, including high subtype selectivity; the ability to mimic physiological conditions; the lack of densensitization, downregulation, and internalization; and reduced side effects. Despite these virtues, challenging issues have now arisen for allosteric modulators of metabotropic glutamate receptors (mGluRs): shallow SAR, ligand-directed trafficking, and the identification of subtle "molecular switches" that modulate the modes of pharmacology. Here, we will discuss the impact of modest structural changes to multiple mGluR allosteric ligands scaffolds that unexpectedly modulate pharmacology and raise concerns over metabolism and the pharmacology of metabolites.     

T-type Ca2+ channels in non-vascular smooth muscles

T-type Ca2+ current has been recorded in smooth muscle myocytes, and associated interstitial cells, isolated from the gastro-intestinal tract, urinary bladder, urethra, prostate gland, myometrium, vas deferens, lymphatic vessels and airways smooth muscle. By contrast, current through such channels has not been recorded from other tissues, such as the ureter. Whilst the properties of this Ca2+ current are similar in most of these cells, with respect to their voltage-dependence, ion selectivity and response to channel modulators, some differences have been recorded, most notably in the gastro-intestinal tract, and may demand a reappraisal of how a T-type Ca2+ current is characterised. The functions of such a current in different tissues remains uncertain. In most of smooth muscles discussed in this review, it is hypothesised that it underlies rhythmic or spontaneous electrical activity, especially in concert with other current-carrying systems, such as Ca2+-activated outward currents. Of equal interest is that the T-type Ca2+ channel may be a target for agents that modulate tissue function, especially in pathological conditions, or are the site of secondary effects of agents used in clinical medicine. For example, T-type Ca2+ channel modulators have been proposed to reduce overactive muscular activity in the gastro-intestinal or urinary tract, or function as tocolytic agents: and the action of volatile anaesthetics on them in airways smooth muscle requires consideration in their overall action.     

γ-Secretase inhibitors and modulators for Alzheimer's disease

γ-Secretase is a membrane embedded aspartyl protease complex with presenilin as the catalytic component. Along with β-secretase, this enzyme produces the amyloid β-protein of Alzheimer's disease (AD) from the amyloid β-protein precursor. Because of its key role in the pathogenesis of AD, γ-secretase has been a prime target for drug discovery, and many inhibitors of this protease have been developed. The therapeutic potential of these inhibitors is virtually negated by the fact that γ-secretase is an essential part of the Notch signaling pathway, rendering the compounds unacceptably toxic upon chronic exposure. However, these compounds have served as useful chemical tools for biological investigations. In contrast, γ-secretase modulators continue to be of keen interest as possible AD therapeutics. These modulators either shift amyloid β-protein production to shorter, less pathogenic peptides or inhibit the proteolysis of amyloid β-protein precursor selectively compared to that of Notch. The various chemical types of inhibitors and modulators will be discussed, along with their use as probes for basic biology and their potential as AD therapeutics.     

Targeting the homologous recombination pathway by small molecule modulators

During the last decade, the use of small molecule (MW <500 Da) compounds that modulate (inhibit or activate) important proteins of different biological pathways became widespread. Recently, the homologous recombination (HR) pathway emerged as a target for such modulators. Development of small molecule modulators pursues two distinct but not mutually exclusive purposes: to create a research tool to study the activities or functions of proteins of interest and to produce drugs targeting specific pathologies. Here, we review the progress of small molecule development in the area of HR.     

Enzymatic activity of α-L-fucosidase and L-fucokinase across vertebrate animal species

The oligosaccharide portion of glycoproteins is known to modulate protein structure, function, and turnover. Our laboratory is interested in the metabolism of L-fucose, a normal constituent of eukaryotic glycoproteins. L-fucose is unique in that it is the only levorotatory sugar utilized in mammalian systems. There is considerable interest in understanding the controls which determine the level of L-fucose attached to proteins, in order to generate stable and active glycoforms of protein for the treatment of disease. As part of a program to determine the controls on protein L-fucosylation, we have systematically determined the tissue distribution of the enzymes L-fucokinase and α-L-fucosidase in species across the vertebrate animal kingdom. In general, the level of α-L-fucosidase is higher than L-fucokinase level. The tissue with highest enzyme activity cannot be generalized, regardless of which enzyme is of interest. Furthermore, there is not a correlation between synthetic and catabolic enzyme activity within a tissue. L-fucokinase can be detected in all tissues examined. Interestingly, we have also detected ß-D-fucosidase activity, present in extraordinary levels in the liver and small intestine of snake. Whether this is due to a specific enzyme or whether it represents a broad specificity of the α-L-fucosidase is currently being investigated.     

Additive mixture effects of estrogenic chemicals in human cell-based assays can be influenced by inclusion of chemicals with differing effect profiles

A growing body of experimental evidence indicates that the in vitro effects of mixtures of estrogenic chemicals can be well predicted from the estrogenicity of their components by the concentration addition (CA) concept. However, some studies have observed small deviations from CA. Factors affecting the presence or observation of deviations could include: the type of chemical tested; number of mixture components; mixture design; and assay choice. We designed mixture experiments that address these factors, using mixtures with high numbers of components, chemicals from diverse chemical groups, assays with different in vitro endpoints and different mixture designs and ratios. Firstly, the effects of mixtures composed of up to 17 estrogenic chemicals were examined using estrogenicity assays with reporter-gene (ERLUX) and cell proliferation (ESCREEN) endpoints. Two mixture designs were used: 1) a 'balanced' design with components present in proportion to a common effect concentration (e.g. an EC(10)) and 2) a 'non-balanced' design with components in proportion to potential human tissue concentrations. Secondly, the individual and simultaneous ability of 16 potential modulator chemicals (each with minimal estrogenicity) to influence the assay outcome produced by a reference mixture of estrogenic chemicals was examined. Test chemicals included plasticizers, phthalates, metals, PCBs, phytoestrogens, PAHs, heterocyclic amines, antioxidants, UV filters, musks, PBDEs and parabens. In all the scenarios tested, the CA concept provided a good prediction of mixture effects. Modulation studies revealed that chemicals possessing minimal estrogenicity themselves could reduce (negatively modulate) the effect of a mixture of estrogenic chemicals. Whether the type of modulation we observed occurs in practice most likely depends on the chemical concentrations involved, and better information is required on likely human tissue concentrations of estrogens and of potential modulators. Successful prediction of the effects of diverse chemical combinations might be more likely if chemical profiling included consideration of effect modulation.     

Adipocytes under assault: Environmental disruption of adipose physiology

The burgeoning obesity epidemic has placed enormous strains on individual and societal health mandating a careful search for pathogenic factors, including the contributions made by endocrine disrupting chemicals (EDCs). In addition to evidence that some exogenous chemicals have the capacity to modulate classical hormonal signaling axes, there is mounting evidence that several EDCs can also disrupt metabolic pathways and alter energy homeostasis. Adipose tissue appears to be a particularly important target of these metabolic disruptions. A diverse array of compounds has been shown to alter adipocyte differentiation, and several EDCs have been shown to modulate adipocyte physiology, including adipocytic insulin action and adipokine secretion. This rapidly emerging evidence demonstrating that environmental contaminants alter adipocyte function emphasizes the potential role that disruption of adipose physiology by EDCs may play in the global epidemic of metabolic disease. Further work is required to better characterize the molecular targets responsible for mediating the effects of EDCs on adipose tissue. Improved understanding of the precise signaling pathways altered by exposure to environmental contaminants will enhance our understanding of which chemicals pose a threat to metabolic health and how those compounds synergize with lifestyle factors to promote obesity and its associated complications. This knowledge may also improve our capacity to predict which synthetic compounds may alter energy homeostasis before they are released into the environment while also providing critical evidentiary support for efforts to restrict the production and use of chemicals that pose the greatest threat to human metabolic health. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.     

Endogenous modulators of glucocorticoid receptor function also regulate purified protein kinase C

Modulator-1 and -2, proposed to be novel ether-linked aminophosphoglycerides, were originally identified as regulators of glucocorticoid receptor function (Bodine, P. V., and Litwack, G. (1990) J. Biol. Chem. 265, 9544-9554). We now demonstrate that these modulators are also potent new stimulators of protein kinase C activity in vitro. These endogenous biomolecules regulate purified protein kinase C activity in a biphasic and dose-dependent pattern, as determined by histone phosphorylation. Modulators, at concentrations within their apparent cellular range, stimulate protein kinase C-catalyzed histone phosphorylation 2-4-fold when added separately, or 10-12-fold when added together. This enhancement of kinase activity apparently is specific for protein kinase C, since neither protein kinase M, nor cAMP-dependent protein kinase A are stimulated by the modulators. The stimulation of purified protein kinase C occurs only when the enzyme has been initially activated by calcium, phosphatidylserine, and diacylglycerol, indicating that the modulators do not simply substitute for one of the enzyme cofactors. In addition, the modulators appear to interact directly with protein kinase C, perhaps with the regulatory domain of the enzyme, since these biomolecules inhibit the binding of phorbol ester to purified protein kinase C. Finally, time-course studies of protein kinase C-catalyzed histone phosphorylation indicate that the velocity of the enzyme reaction is increased by the modulators. Taken together, these results suggest that the modulators are a new class of regulators of protein kinase C.     

Novel indolylmaleimide acts as GSK-3β inhibitor in human neural progenitor cells

The Wnt pathway is involved in cellular processes linked to either proliferation or differentiation. Therefore small molecules offer an attractive opportunity to modulate this pathway, whereas the key enzyme GSK-3β is of special interest. In this study, non-symmetrically substituted indolylmaleimides have been synthesized and their ability to function as GSK-3β inhibitors has been investigated in a human neural progenitor cell line. Among the newly synthesized compounds, the substance IM-12 showed a significant activity in several biological tests which was comparable or even outplayed the effects of the known GSK-3β inhibitor SB-216763. Furthermore the treatment of human neural progenitor cells with IM-12 resulted in an increase of neuronal cells. Therefore we conclude that indolylmaleimides act via the canonical Wnt signalling pathway by inhibition of the key enzyme GSK-3β.     

Discovery of pan autophagy inhibitors through a high-throughput screen highlights macroautophagy as an evolutionarily conserved process across 3 eukaryotic kingdoms

Due to the involvement of macroautophagy/autophagy in different pathophysiological conditions such as infections, neurodegeneration and cancer, identification of novel small molecules that modulate the process is of current research and clinical interest. In this work, we developed a luciferase-based sensitive and robust kinetic high-throughput screen (HTS) of small molecules that modulate autophagic degradation of peroxisomes in the budding yeast Saccharomyces cerevisiae. Being a pathway-specific rather than a target-driven assay, we identified small molecule modulators that acted at key steps of autophagic flux. Two of the inhibitors, Bay11 and ZPCK, obtained from the screen were further characterized using secondary assays in yeast. Bay11 inhibited autophagy at a step before fusion with the vacuole whereas ZPCK inhibited the cargo degradation inside the vacuole. Furthermore, we demonstrated that these molecules altered the process of autophagy in mammalian cells as well. Strikingly, these molecules also modulated autophagic flux in a novel model plant, Aponogeton madagascariensis. Thus, using small molecule modulators identified by using a newly developed HTS autophagy assay, our results support that macroautophagy is a conserved process across fungal, animal and plant kingdoms.     

Seven sirtuins for seven deadly diseases ofaging

Sirtuins are a class of NAD⁺-dependent deacetylases having beneficial health effects. This extensive review describes the numerous intracellular actions of the seven mammalian sirtuins, their protein targets, intracellular localization, the pathways they modulate, and their role in common diseases of aging. Selective pharmacological targeting of sirtuins is of current interest in helping to alleviate global disease burden. Since all sirtuins are activated by NAD⁺, strategies that boost NAD⁺ in cells are of interest. While most is known about SIRT1, the functions of the six other sirtuins are now emerging. Best known is the involvement of sirtuins in helping cells adapt energy output to match energy requirements. SIRT1 and some of the other sirtuins enhance fat metabolism and modulate mitochondrial respiration to optimize energy harvesting. The AMP kinase/SIRT1–PGC-1α–PPAR axis and mitochondrial sirtuins appear pivotal to maintaining mitochondrial function. Downregulation with aging explains much of the pathophysiology that accumulates with aging. Posttranslational modifications of sirtuins and their substrates affect specificity. Although SIRT1 activation seems not to affect life span, activation of some of the other sirtuins might. Since sirtuins are crucial to pathways that counter the decline in health that accompanies aging, pharmacological agents that boost sirtuin activity have clinical potential in treatment of diabetes, cardiovascular disease, dementia, osteoporosis, arthritis, and other conditions. In cancer, however, SIRT1 inhibitors could have therapeutic value. Nutraceuticals such as resveratrol have a multiplicity of actions besides sirtuin activation. Their net health benefit and relative safety may have originated from the ability of animals to survive environmental changes by utilizing these stress resistance chemicals in the diet during evolution. Each sirtuin forms a key hub to the intracellular pathways affected.     

Pitfalls of enzyme-based molecular anticancer dietary manipulations: food for thought

Dietary approaches to cancer chemoprevention increasingly have focused on single nutrients or phytochemicals to stimulate one or another enzymatic metabolizing system. These procedures, which aim to boost carcinogen detoxification or inhibit carcinogen bioactivation, fail to take into account the multiple and paradoxical biological outcomes of enzyme modulators that make their effects unpredictable. Here, we critically examine the scientific and medical evidence for the idea that the physiological roles of specific enzymes may be manipulated by regular, long-term administration of isolated nutrients and other chemicals derived from food plants. Instead, we argue that consumption of healthful diets is most likely to reduce mutagenesis and cancer risk, and that research efforts and dietary recommendations should be redirected away from single nutrients to emphasize the improvement of dietary patterns as a principal strategy for public health policy.     

A solid-phase method for synthesis of dimeric and trimeric ligands: Identification of potent bivalent ligands of 14-3-3σ

Multivalent protein-protein interactions including bivalent and trivalent interactions play a critical role in mediating a wide range of biological processes. Hence, there is a significant interest in developing molecules that can modulate those signaling pathways mediated by multivalent interactions. For example, multimeric molecules capable of binding to a receptor protein through a multivalent interaction could serve as modulators of such interactions. However, it is challenging to efficiently generate such multimeric ligands. Here, we have developed a facile solid-phase method that allows for the rapid generation of (homo- and hetero-) dimeric and trimeric protein ligands. The feasibility of this strategy was demonstrated by efficiently synthesizing fluorescently-labeled dimeric peptide ligands, which led to dramatically increased binding affinities (~400-fold improvement) relative to a monomeric 14-3-3σ protein ligand.     

A systematic protocol for the characterization of Hsp90 modulators

Several Hsp90 modulators have been identified including the N-terminal ligand geldanamycin (GDA), the C-terminal ligand novobiocin (NB), and the co-chaperone disruptor celastrol. Other Hsp90 modulators elicit a mechanism of action that remains unknown. For example, the natural product gedunin and the synthetic anti-spermatogenic agent H2-gamendazole, recently identified Hsp90 modulators, manifest biological activity through undefined mechanisms. Herein, we report a series of biochemical techniques used to classify such modulators into identifiable categories. Such studies provided evidence that gedunin and H2-gamendazole both modulate Hsp90 via a mechanism similar to celastrol, and unlike NB or GDA.     

Natural intracellular peptides can modulate the interactions of mouse brain proteins and thimet oligopeptidase with 14-3-3ε and calmodulin

Protein interactions are crucial for most cellular process. Thus, rationally designed peptides that act as competitive assembly inhibitors of protein interactions by mimicking specific, determined structural elements have been extensively used in clinical and basic research. Recently, mammalian cells have been shown to contain a large number of intracellular peptides of unknown function. Here, we investigate the role of several of these natural intracellular peptides as putative modulators of protein interactions that are related to Ca(2+) -calmodulin (CaM) and 14-3-3ε, which are proteins that are related to the spatial organization of signal transduction within cells. At concentrations of 1-50 μM, most of the peptides that are investigated in this study modulate the interactions of CaM and 14-3-3ε with proteins from the mouse brain cytoplasm or recombinant thimet oligopeptidase (EP24.15) in vitro, as measured by surface plasmon resonance. One of these peptides (VFDVELL; VFD-7) increases the cytosolic Ca(2+) concentration in a dose-dependent manner but only if introduced into HEK293 cells, which suggests a wide biological function of this peptide. Therefore, it is exciting to suggest that natural intracellular peptides are novel modulators of protein interactions and have biological functions within cells.