The interaction of an epidermal growth factor/transforming growth factor alpha tail chimera with the human epidermal growth factor receptor reveals unexpected complexities

Binding of single-mutant epidermal growth factor (EGF) ligands alters the stability of the EGF receptor dimer and promotes growth signaling

The epidermal growth factor receptor (EGFR) is a membrane-anchored tyrosine kinase that is able to selectively respond to multiple extracellular stimuli. Previous studies have indicated that the modularity of this system may be caused by ligand-induced differences in the stability of the receptor dimer. However, this hypothesis has not been explored using single-mutant ligands thus far. Herein, we developed a new approach to identify residues responsible for functional divergence by selecting residues in the epidermal growth factor (EGF) ligand that are conserved among orthologs yet divergent between paralogs. Then, we mutated these residues and assessed the mutants' effects on the receptor using a combination of molecular dynamics (MD) and biochemical techniques. Although the EGF mutants had binding affinities for the EGFR comparable with the WT ligand, the EGF mutants showed differential patterns of receptor phosphorylation and cell growth in multiple cell lines. The MD simulations of the EGF mutants indicated that mutations had long-range effects on the receptor dimer interface. This study shows for the first time that a single mutation in the EGF is sufficient to alter the activation of the EGFR signaling pathway at the cellular level. These results also support that biased ligand-receptor signaling in the tyrosine kinase receptor system can lead to differential downstream outcomes and demonstrate a promising new method to study ligand-receptor interactions.

Superagonism at G protein-coupled receptors and beyond

Ligands targeting GPCRs can be categorized according to their intrinsic efficacy to trigger a specific, receptor-mediated response. A ligand endowed with the same level of efficacy as the endogenous agonist can be classified as a full agonist, whereas a compound that displays greater efficacy, that is, higher receptor signalling output than the endogenous agonist, can be called a superagonist. Subsequent to GPCR activation, an intracellular signalling cascade is set in motion, which may generate substantial amplification of the signal. This may obscure superagonism in pharmacological assays and, therefore, the definition of superagonism necessitates a combination of operational approaches, reduction of spare receptors or estimation of receptor activation close to the receptor level to quantify relative agonist efficacies in a particular system. The first part of this review will compare GPCR superagonism with superagonism in the field of immunology, where this term is well established. In the second part, known GPCR superagonists will be reviewed. Then, the experimental and analytical challenges in the deconvolution of GPCR superagonism will be addressed. Finally, the potential benefit of superagonism is discussed. The molecular mechanisms behind GPCR superagonism are not completely understood. However, crystallography shows that agonist binding alone is not sufficient for a fully active receptor state and that binding of the G protein is at least equally important. Accordingly, the emerging number of reported superagonists implies that ligand-induced receptor conformations more active than the ones stabilized by the endogenous agonist are indeed feasible. Superagonists may have therapeutic potential when receptor function is impaired or to induce negative feedback mechanisms. Linked Articles This article is part of a themed section on Molecular Pharmacology of G Protein-Coupled Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.20/issuetoc.

Structure-based discovery of fiber-binding compounds that reduce the cytotoxicity of amyloid beta

Amyloid protein aggregates are associated with dozens of devastating diseases including Alzheimer’s, Parkinson’s, ALS, and diabetes type 2. While structure-based discovery of compounds has been effective in combating numerous infectious and metabolic diseases, ignorance of amyloid structure has hindered similar approaches to amyloid disease. Here we show that knowledge of the atomic structure of one of the adhesive, steric-zipper segments of the amyloid-beta (Aβ) protein of Alzheimer’s disease, when coupled with computational methods, identifies eight diverse but mainly flat compounds and three compound derivatives that reduce Aβ cytotoxicity against mammalian cells by up to 90%. Although these compounds bind to Aβ fibers, they do not reduce fiber formation of Aβ. Structure-activity relationship studies of the fiber-binding compounds and their derivatives suggest that compound binding increases fiber stability and decreases fiber toxicity, perhaps by shifting the equilibrium of Aβ from oligomers to fibers.

DOI:http://dx.doi.org/10.7554/eLife.00857.001

Alzheimer’s disease is the most common form of dementia, estimated to affect roughly five million people in the United States, and its incidence is steadily increasing as the population ages. A pathological hallmark of Alzheimer’s disease is the presence in the brain of aggregates of two proteins: tangles of a protein called tau; and fibers and smaller units (oligomers) of a peptide called amyloid beta.

Many attempts have been made to screen libraries of natural and synthetic compounds to identify substances that might prevent the aggregation and toxicity of amyloid. Such studies revealed that polyphenols found in green tea and in the spice turmeric can inhibit the formation of amyloid fibrils. Moreover, a number of dyes reduce the toxic effects of amyloid on cells, although significant side effects prevent these from being used as drugs.

Structure-based drug design, in which the structure of a target protein is used to help identify compounds that will interact with it, has been used to generate therapeutic agents for a number of diseases. Here, Jiang et al. report the first application of this technique in the hunt for compounds that inhibit the cytotoxicity of amyloid beta. Using the known atomic structure of the protein in complex with a dye, Jiang et al. performed a computational screen of 18,000 compounds in search of those that are likely to bind effectively.

The compounds that showed the strongest predicted binding were then tested for their ability to interfere with the aggregation of amyloid beta and to protect cells grown in culture from its toxic effects. Compounds that reduced toxicity did not reduce the abundance of protein aggregates, but they appear to increase the stability of fibrils. This is consistent with other evidence suggesting that small, soluble forms (oligomers) of amyloid beta that break free from the fibrils may be the toxic agent in Alzheimer’s disease, rather than the fibrils themselves.

In addition to uncovering compounds with therapeutic potential in Alzheimer’s disease, this work presents a new approach for identifying proteins that bind to amyloid fibrils. Given that amyloid accumulation is a feature of many other diseases, including Parkinson’s disease, Huntington’s disease and type 2 diabetes, the approach could have broad therapeutic applications.

DOI:http://dx.doi.org/10.7554/eLife.00857.002

Repetitive dissociation from crocidolite asbestos acts as persistent signal for epidermal growth factor receptor

Mesothelioma is an incurable form of cancer located most commonly in the pleural lining of the lungs and is associated almost exclusively with the inhalation of asbestos. The binding of asbestos to epidermal growth factor receptor (EGFR), a transmembrane signal protein, has been proposed as a trigger for downstream signaling of kinases and expression of genes involved in cell proliferation and inhibition of apoptosis. Here, we investigate the molecular binding of EGFR to crocidolite (blue asbestos; Na2(Fe(2+),Mg)3Fe2(3+)Si8O22(OH)2) in buffer solution. Atomic force microscopy measurements revealed an attractive force of interaction (i.e., bond) as EGFR was pulled from contact with long fibers of crocidolite. The rupture force of this bond increased with loading rate. According to the Bell model, the off-rate of bond dissociation (k(off)) for EGFR was 22 s(-1). Similar experiments with riebeckite crystals, the nonasbestiform variety of crocidolite, yielded a k(off) of 8 s(-1). These k(off) values on crocidolite and riebeckite are very rapid compared to published values for natural agonists of EGFR like transforming growth factor and epidermal growth factor. This suggests binding of EGFR to the surfaces of these minerals could elicit a response that is more potent than biological hormone or cytokine ligands. Signal transduction may cease for endogenous ligands due to endocytosis and subsequent degradation, and even riebeckite particles can be cleared from the lungs due to their short, equant habit. However, the fibrous habit of crocidolite leads to lifelong persistence in the lungs where aberrant, repetitious binding with EGFR may continually trigger the activation switch leading to chronic expression of genes involved in oncogenesis.

Experimental inhibition of peptide fibrillogenesis by synthetic peptides, carbohydrates and drugs

Peptide fibrillogenesis generally begins by the transformation of normally soluble proteins into elongated aggregates which are called as amyloid. These fibrils mainly consist of ß-sheets. They share certain common characteristics such as a cross-ß x-ray diffraction pattern, association with other common proteins and typical staining by the dye Congo Red. The individual form of the deposit consists of a disease-specific peptide/protein. The disease-specific protein serves as the basis for the classification of the amyloids. The association of fibril-forming peptides/proteins with diseases makes them primary disease-targets. Understanding the molecular interactions involved in the fibril formation becomes the foremost requirement to characterize the target. Interference with these interactions of ß-sheets in vitro prevents and sometimes reverses the fibril assembly. A small molecule capable of interfering with the formation of fibril could have therapeutic applications in these diseases. This anti-aggregation approach appears to be a viable treatment option. A search for such a molecule is pursued actively world over. All types of compounds and approaches to slow down or prevent the aggregation process have been described in the literature. These efforts are reviewed in this chapter.

Surface-active properties of nitrogen heterocyclic and dialkylamino derivates of hexadecylphosphocholine and cetyltrimethylammonium bromide

The physico-chemical properties of dialkylamino and nitrogen heterocyclic analogues of hexadecylphosphocholine (HPC) and cetyltrimethylammonium bromide (CTAB) were investigated. The surface properties, such as the critical micelle concentration (cmc), the surface tension value at the cmc (gamma(cmc)), and the surface area at the surface saturation per head group (A(cmc)) were determined by means of surface tension measurements. Micelle size was determined using the dynamic light scattering method. The influence of dialkylamino groups and heterocyclic ring size on surface-active properties was investigated. Surface activity and micellar size of prepared analogues of HPC and CTAB were mutually compared.

Synthesis and antimicrobial activity of a series of optically active quaternary ammonium salts derived from phenylalanine

We synthesized nine quaternary ammonium compounds (QUATs) starting from phenylalanine, N-alkyl-N,N-dimethyl-(1-hydroxy-3-phenylpropyl)-2-ammonium bromides, which were prepared as optically pure substances. Five compounds were prepared as S-enantiomers and four compounds as R-enantiomers. These compounds were evaluated by their activities against bacteria and fungi. Three microbial strains were used in the study: the gram-negative bacteria Escherichia coli, the gram-positive bacteria Staphylococcus aureus and the fungi Candida albicans. The activities were expressed as minimum bactericidal or fungicidal concentrations (MBC). The most active compounds were (2S)-N-tetradecyl-N,N-dimethyl-(1-hydroxy-3-phenylpropyl)-2-ammonium bromide and (2R)-N-tetradecyl-N,N-dimethyl-(1-hydroxy-3-phenylpropyl)-2-ammonium bromide, with MBC values exceeding those of commercial benzalkoniumbromide (BAB) used as standard. The relationships between structure and biological activity of the tested QUATs were quantified by the bilinear model (QSAR) and are discussed.

Potential roles of Arnt2 in zebrafish larval development

The aryl hydrocarbon receptor nuclear translocator (ARNT) is a basic helix-loop-helix-PAS heterodimeric transcription factor that dimerizes with other basic helix-loop-helix-PAS proteins to mediate biological responses. The function of ARNT2 is poorly understood. Here we provide an initial characterization of the zebrafish arnt2 null (arnt2(-/-)) mutant to identify functions of Arnt2 during development. Arnt2(-/-) mutant zebrafish develop normally until 120 hours postfertilization (hpf ) when morphological changes and functional deficits occur. The C-start escape response initiated by either touch or startle stimuli is absent in the mutants. Brain ventricle size is markedly increased at 120 hpf. Heart ventricles are enlarged, with decreased ventricle wall thickness. A cardiac arrhythmia, characterized by missing beats, is also observed in the mutants. This is associated with bradycardia in arnt2(-/-) larvae. Dilated liver sinusoids merge abnormally to form an extensive, labyrinth-like network of vascular channels. External appearance of arnt2(-/-) larvae at 120 hpf is indistinguishable from wild type except that the swim bladder is not inflated. The arnt2(-/-) mutants are not debilitated when phenotypic effects are first detected at 120 hpf that culminate in mortality, 4 days later around 216 hpf. Gross morphological assessment of the development of forebrain, midbrain, and hindbrain regions, neuromasts and Mauthner neurons, inner ear semicircular canals and otoliths, primary motor neurons, trigeminal ganglia, and trunk skeletal muscles, before or when the arnt2(-/-) phenotype was observed, failed to demonstrate a difference from wild type. The only effect in arnt2(-/-) larvae that occurred before 120 hpf was a decrease in expression of sim1, an Arnt2 dimerization partner, in the hypothalamus and ventral thalamus at 72 hpf. Further research is needed to determine if the primary functions of Arnt2 occur during the larval stage, when the phenotype is observed, or earlier in development.

Cytochrome P450 CYP1A1: wider roles in cancer progression and prevention

CYP1A1 is one of the main cytochrome P450 enzymes, examined extensively for its capacity to activate compounds with carcinogenic properties. Continuous exposure to inhalation chemicals and environmental carcinogens is thought to increase the level of CYP1A1 expression in extrahepatic tissues, through the aryl hydrocarbon receptor (AhR). Although the latter has long been recognized as a ligand-induced transcription factor, which is responsible for the xenobiotic activating pathway of several phase I and phase II metabolizing enzymes, recent evidence suggests that the AhR is involved in various cell signaling pathways critical to cell cycle regulation and normal homeostasis. Disregulation of these pathways is implicated in tumor progression. In addition, it is becoming increasingly evident that CYP1A1 plays an important role in the detoxication of environmental carcinogens, as well as in the metabolic activation of dietary compounds with cancer preventative activity. Ultimately the contribution of CYP1A1 to cancer progression or prevention may depend on the balance of procarcinogen activation/detoxication and dietary natural product extrahepatic metabolism.

The development of quantum dot calibration beads and quantitative multicolor bioassays in flow cytometry and microscopy

The use of fluorescence calibration beads has been the hallmark of quantitative flow cytometry. It has enabled the direct comparison of interlaboratory data as well as quality control in clinical flow cytometry. In this article, we describe a simple method for producing color-generalizable calibration beads based on streptavidin functionalized quantum dots. Based on their broad absorption spectra and relatively narrow emission, which is tunable on the basis of dot size, quantum dot calibration beads can be made for any fluorophore that matches their emission color. In an earlier publication, we characterized the spectroscopic properties of commercial streptavidin functionalized dots (Invitrogen). Here we describe the molecular assembly of these dots on biotinylated beads. The law of mass action is used to readily define the site densities of the dots on the beads. The applicability of these beads is tested against the industry standard, namely commercial fluorescein calibration beads. The utility of the calibration beads is also extended to the characterization surface densities of dot-labeled epidermal growth factor ligands as well as quantitative indicators of the binding of dot-labeled virus particles to cells.

Review: hypoxia-inducible factor-1 (HIF-1): a novel transcription factor in immune reactions

Hypoxia-inducible factor-1 (HIF-1) is a dimeric transcriptional complex that has been recognized primarily for its role in the maintenance of oxygen and energy homoeostasis. The HIF-1alpha subunit is O(2) labile and is degraded by the proteasome following prolyl-hydroxylation and ubiquitination in normoxic cells. The present review summarizes evidence that HIF-1 is also involved in immune reactions. Immunomodulatory peptides, including interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-alpha), stimulate HIF-1 dependent gene expression even in normoxic cells. Both the hypoxic and the cytokine-induced activation of HIF-1 involve the phosphatidylinositol- 3-kinase (PI3K) and the mitogen-activated protein kinase (MAPK) signaling pathways. In addition, heat shock proteins (HSP) and other cofactors interact with HIF-1 subunits. HIF-1 increases the transcription of several genes for proteins that promote blood flow and inflammation, including vascular endothelial growth factor (VEGF), heme oxygenase-1, endothelial and inducible nitric oxide synthase (NOS) and cyclooxygenase-2 (COX-2). The pharmacologic activation of the HIF-1 complex can be desirable in ischemic and inflammatory disorders. In contrast, HIF-1 blockade may be beneficial to prevent tumor angiogenesis and tumor growth.

Mechanism-Based Fluorescent Reporter for Protein Kinase A Detection

A novel mechanism-based fluorescent reporter was designed for the detection of protein kinase A (PKA), which is known to mediate a variety of cellular responses in most eukaryotic cells. The probe consists of a specific binding peptide sequence, LRRRRFAFC, conjugated with 2'-thioethyl-5-(or -6)-carboxyfluoresceinamide (FAMS; 2) and 5-(or 6-)carboxytetramethylrhodamine (TAMRA) at the cysteine and leucine residues, respectively. In the absence of PKA, the two fluorophores associate by hydrophobic interactions, forming an intramolecular ground-state dimer; this results in fluorescein quenching (>93 %). Upon PKA addition, the reporter reacts with the sulfhydryl functionality at Cys199 through a disulfide-exchange mechanism. FAMS is subsequently released, resulting in significant fluorescence amplification. The remaining peptide sequence, which acts as an inhibitor, is attached covalently to the enzyme. Our results suggest that this type of sensors could have far-reaching applications in the molecular sensing of enzymes.

Temperature dependence of the nucleation constant rate in β amyloid fibrillogenesis

Beta-amyloid peptide (A beta), in fibrillar form, is the primary constituent of senile plaques, a defining feature of Alzheimer's disease (AD). In solution assays, fibrils form with a lag time, interpreted as a nucleation/condensation-dependent process. The kinetics of fibrillogenesis is controlled by two key parameters: nucleation and elongation rate constants. We report here the study of the temperature dependence of the nucleation rate constant on an A beta monomer concentration of 18.4 microM at pH 7.4 and at temperatures ranging from 302 to 318 K. We found that the nucleation constant varied as in the Arrhenius law, giving an activation energy of 311.2 kJ mol(-1). The corresponding values of enthalpy of activation (deltaH*), entropy of activation (deltaS*) and Gibbs energy of activation (deltaG*) were evaluated by Eyring's equation of absolute reaction rate. A Gibbs energy of activation of approximately 110 kJ mol(-1) was obtained.

Ligand depletion negatively controls the mitogenic activity of epidermal growth factor

EGF activates the ErbB1 receptor, but there appears only a limited correlation between its receptor binding affinity and mitogenic activity. This is indicated by our present observation that in cells with high ErbB1 expression, including SUM102 breast tumor cells, low affinity EGF/Notch chimeras have similarly high mitogenic activity as EGF, in spite of the fact that EGF is superior in inducing receptor tyrosine phosphorylation and p42/p44 MAP-kinase activity. However, as a result of receptor-mediated internalisation high-affinity ligands such as EGF are depleted much more rapidly from the extracellular medium than low-affinity EGF/Notch chimeras. As a consequence, the mitogenic activity of EGF on ErbB1 overexpressing cells is limited by substantial degradation of internalised ligand in the period before cells enter S-phase, a phenomenon that is not observed for low affinity mutant ligands. The mitogenic activity of EGF on ErbB1 overexpressing cells does therefore not only depend on the applied concentration but also on the total amount of ligand added, and is strongly underestimated when tested in a limited assay volume. No such dependence on the incubation volume was observed for EGF activity on cells with low ErbB1 expression levels and on cells for which EGF is growth inhibitory.

Amyloid inhibitors and beta-sheet breakers

Compelling evidence indicates that a key pathological event in Alzheimer's disease is the misfolding and aggregation of normal soluble amyloid-beta peptide into beta-sheet-rich oligomeric structures which have a neurotoxic activity and ability to form insoluble amyloid deposits that accumulate in the brain. beta-sheet breakers constitute a new class of drugs that are designed to specifically bind amyloid-beta peptide blocking and/or reversing the misfolding process. In this article we review this approach and summarize the data supporting the view that beta-sheet breakers could be serious candidates to combat this devastating disease.

Multiple promoters drive tissue-specific expression of the human M2 muscarinic acetylcholine receptor gene

Despite the wealth of information on the functional and pharmacological properties of the M2 muscarinic receptor, we know relatively little of structure and regulation of the M2 receptor gene. Here, we describe the organisation of the human M2 gene and its promoters. Four exons are present in the 5' untranslated region of the human M2 mRNA distributed over 146 kb on chromosome 7 which produce eight different splice variants in the IMR-32 neuroblastoma cell line. The unexpectedly large size of this gene indicates that transcription initiates much further upstream of the coding region than earlier studies had indicated. We present evidence that there are three distinct human M2 promoters. Analysis of endogenous transcripts revealed that promoter 2 is preferentially used in neuroblastoma cells, whereas promoter 1 in cardiac cells. All promoters are highly conserved across human, mouse, rat and pig. They contain multiple start sites and none possess a TATA-box. In addition, we describe another M2 promoter that is specific for rat. We show that GATA-4 transcription factor binds to two sites within the regulatory regions of the M2 gene using reporter gene assays, electromobility shift assays and mutational analysis.

An autocatalytic reaction as a model for the kinetics of the aggregation of beta-amyloid

Alzheimer's disease is the commonest form of senile dementia, affecting almost 20 million people worldwide. This neurodegenerative disorder is characterized by amyloid deposition in senile plaques, composed primarily of fibrils of an aggregated peptide, beta-amyloid. Fibrillation of beta-amyloid is a nucleation-dependent polymerization process, which is controlled by two kinetics parameters: the nucleation rate and the elongation or growth rate. As the kinetics of fibrillation is strongly dependent on the presence of trace amounts of fibrils, we suggest that the aggregation of beta-amyloid is a model of autocatalytic reaction. A mathematical analysis, permitting quantitative monitoring of the kinetics of fibrillogenesis of beta-amyloid, nucleation, and elongation constants, is presented. The model was checked by applying it to the aggregation of the fragment 1-40 of the beta-amyloid. Understanding of these rate constants may facilitate the study of the effect of substances used for controlling fibril creation and growth. The disaggregating effect of dodecyl trimethylammonium bromide, a cationic surfactant, was easily quantified by means of the model.

Effects of estrogen on muscarinic acetylcholine receptors in the rat hippocampus

The aim of the present study was to investigate whether different estrogen manipulations have effects on the expression of muscarinic acetylcholine receptors (mAChRs) in the adult female rat hippocampus. Hippocampus was obtained from rats in proestrus (control), ovariectomized for 2, 10 and 15 days, ovariectomized for 15 days and treated with 17beta-estradiol for 7 days, and treated with 17beta-estradiol immediately after ovariectomy for 21 days. Rats' estrogen status was monitored by measuring estradiol plasma levels and uterus relative weight. [3H]quinuclidinyl benzilate ([3H]QNB) binding studies indicated that ovariectomy time-dependently increases the number of mAChRs in hippocampus when compared to those obtained from control rats. Estradiol treatments for 21 days avoid the effect of ovariectomy. However, the estradiol treatments for 7 days after 15 days of ovariectomy slightly change the number of mAChRs. In conclusion, these results showed that ovariectomy time-dependently increases mAChRs number in the rat hippocampus. In addition, these data suggest that treatment with estradiol initiated within a specific period of time after the loss of ovarian function may be effective at preventing specific effects of hormone deprivation on hippocampus.

The thyroid hormone receptor beta gene: structure and functions in the brain and sensory systems

Thyroid hormone profoundly influences the development of the vertebrate nervous system. The thyroid hormone receptor beta gene (Thrb) is a key mediator of many of these actions. The Thrb gene is complex, spanning up to 400 kb in mammals, and differentially expresses distinct receptor subtypes through independent tissue-specific promoters and alternative splicing. These receptors serve a range of functions in the brain as well as particularly sensitive functions in the auditory and visual sensory systems. The Thrb gene illustrates how versatility in neurodevelopmental control can be achieved at the receptor level.

Fourier transform infrared spectroscopy used to evidence the prevention of beta-sheet formation of amyloid beta(1-40) peptide by a short amyloid fragment

Reflectance Fourier transform infrared (FT-IR) microspectroscopy was applied to study the prevention of beta-sheet formation of amyloid beta (Abeta)(1-40) peptide by co-incubation with a hexapeptide containing a KLVFF sequence (Abeta(15-20) fragment). Second-derivative spectral analysis was used to locate the position of the overlapping components of the amide I band of Abeta peptide and assigned them to different secondary components. The result indicates that each intact sample of Abeta(15-20) fragment or Abeta(1-40) peptide previously incubated in distilled water at 37 degrees C transformed their secondary structure from 1649 (1651) or 1653cm(-1) to 1624cm(-1), suggesting the transformation from alpha-helix and/or random coil structures to beta-sheet structure. By co-incubating both samples with different molar ratio in distilled water at 37 degrees C, the structural transformation was not found for Abeta(1-40) peptide after 24h-incubation. But the beta-sheet formation of Abeta(1-40) peptide after 48h-incubation was evidenced from the appearance of the IR peak at 1626cm(-1) by adding a little amount of Abeta(15-20) fragment. There was no beta-sheet formation of Abeta(1-40) peptide after addition with much amount of Abeta(15-20) fragment, however, suggesting the higher amount of Abeta(15-20) fragment used might inhibit the beta-sheet formation of Abeta(1-40) peptide. The more Abeta(15-20) fragment used made the more stable structure of Abeta(1-40) peptide and the less beta-sheet formation of Abeta(1-40) peptide. The study indicates that the reflectance FT-IR microspectroscopy can easily evidence the prevention of beta-sheet formation of Abeta(1-40) peptide by a short amyloid fragment.

Transcription factor REST dependent proteins are comparable between Down Syndrome and control brains: challenging a hypothesis

Impairment of the RE-1-silencing transcription factor (REST) and REST-dependent genes in Down Syndrome (DS) neuronal progenitor cells and neurospheres has been published recently. As dysregulation of this system has been shown at the RNA level and considering the long and unpredictable way from RNA to proteins, and as it is the proteins that do the function in brain, we decided to test this hypothesis at the protein level. Cortex of brains of patients with Down Syndrome at the early second trimester were used. REST-dependent structures as synapsin I, brain derived neurotrophic factor BDNF and neuronal growth-associated protein SCG10 were determined at the protein level using immunoblotting. Proteins were comparably expressed in fetal Down syndrome and control brains. Even when normalized versus housekeeping genes (glyceraldehyde-6-phosphate-dehydrogenease) and a marker for neuronal density (neuron-specific enolase) DS results were resembling controls. Therefore, we cannot confirm the REST-hypothesis by our studies in the 18/19th week of gestation at the protein level in brain and taking into account that the hypothesis was based upon studies in progenitor cells.

Structure and transcription of the human m3 muscarinic receptor gene

We have isolated and characterized the human m3 muscarinic receptor gene and its promoter. Using 5' rapid amplification of cDNA ends (RACE), internal polymerase chain reaction (PCR), and homology searching to identify EST clones, we determined that the cDNA encoding the m3 receptor comprises 4,559 bp in 8 exons, which are alternatively spliced to exclude exons 2, 4, 6, and/or 7; the receptor coding sequence occurs within exon 8. Analysis of P1 artificial chromosome (PAC) and bacterial artificial chromosome (BAC) clones and of PCR- amplified genomic DNA, and homology searching of human chromosome 1 sequence provided from the Sanger Centre (Hinxton, Cambridge, UK) revealed that the m3 muscarinic receptor gene spans at least 285 kb. A promoter fragment containing bp -1240 to +101 (relative to the most 5' transcription start site) exhibited considerable transcriptional activity during transient transfection in cultured subconfluent, serum-fed canine tracheal myocytes, and 5' deletion analysis of promoter function revealed the presence of positive transcriptional regulatory elements between bp -526 and -269. Sequence analysis disclosed three potential AP-2 binding sites in this region; five more AP-2 consensus binding motifs occur between bp -269 and +101. Cotransfection with a plasmid expressing human AP-2alpha substantially increased transcription from m3 receptor promoter constructs containing 526 or 269 bp of 5' flanking DNA. Furthermore, m3 receptor promoter activity was enhanced by long-term serum deprivation of canine tracheal myocytes, a treatment that is known to increase AP-2 transcription-promoting activity in these cells. Together, these data suggest that expression of the human m3 muscarinic receptor gene is regulated in part by AP-2 in airway smooth muscle.

Solution structure of the mEGF/TGFalpha44-50 chimeric growth factor

The solution structure of the growth factor chimera mEGF/TGFalpha44-50 has been determined using an extended version of the dyana procedure for calculating structures from NMR data. The backbone fold and preferred orientation of the domains of the chimera are similar to those found in previous studies of EGF structures, and several H-bonds used as input constraints in those studies were found independently in the chimera. This shows that the modified activity of the chimera does not result from a major structural change. However, the improved precision of the structure presented here allows the origin of some unusual chemical shifts found in all of these compounds to be explained, as well as the results obtained from some site-specific mutants. Further studies of the properties of this chimeric growth factor should help to elucidate the mechanism(s) of hetero- and homodimerization of the c-erbB receptors.

Characterization of receptors of insect cytokine, growth-blocking peptide, in human keratinocyte and insect Sf9 cells

Insect cytokine, growth-blocking peptide (GBP), enhances cell proliferation of human keratinocyte cells with a potency almost equivalent to that of human epidermal growth factor (EGF). GBP consists of 25 amino acid residues containing a core region that shows a striking similarity to the C-terminal beta-loop domain of EGF and disordered N and C termini. The present study demonstrates that, although GBP lacks the N-terminal half-portion of EGF molecule, at least five amino acids of the disordered N-terminal six-amino acid region are indispensable for affecting the cell growth activity of GBP. Upon stimulating mitogenesis in keratinocyte cells, GBP directly binds and activates their EGF receptors. GBP also effects proliferative activity on insect Sf9 cells through the binding and activation of the specific receptor, which consists of a heterodimeric complex: a binding subunit (60 kDa) and a tyrosine phosphorylation subunit (58 kDa). These results indicate that GBP enhances cell proliferation of human keratinocyte and insect Sf9 cells through the activation of EGF and GBP receptors, respectively.

Hypoxia-dependent protein expression: erythropoietin

Normal cell homeostasis relies on the ordered flow of nutrients and substrates through metabolic pathways. Any perturbation of this flow eventually leads to dysfunction, impairment of defense mechanisms, loss of viability and death. High altitude and pathological hypoxia represent a serious and frequent cause for the loss of cell viability. Although organisms customarily respond by triggering adaptive or maladaptive mechanisms, all forms of life eventually succumb to hypoxia if it is severe enough, irrespectively of the primary cause. This paper reviews one of the mechanisms by which organisms respond to hypoxia: erythropoiesis. Although such response is not always beneficial, the discovery of the biochemical mechanisms underlying erythropoiesis has triggered an active field of research that is actually applying lessons learned in the mountains to a more clinical environment.

Hypoxia-inducible factor 1alpha and 1beta proteins share common signaling pathways in human prostate cancer cells

Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric transcription factor consisting alpha and beta subunits. It is critically involved in cancer cell hypoxia adaptation, glycolysis, and angiogenesis. HIF-1beta is associated with HIF-1 functions as a dimerization partner of HIF-1alpha, and is on the other hand associated with carcinogenesis via dioxin signaling. Regulation of HIF-1beta protein expression was investigated in human prostate cancer (PCA) cells. HIF-1beta protein was expressed constitutively under nonhypoxic conditions in all human PCA cells tested, and was up-regulated by hypoxia, CoCl2, EGF, serum, or PMA in moderate levels. Compared to that of HIF-1alpha, the constitutive, serum-, EGF-, and PMA-increased HIF-1beta protein expression were also inhibited by selective PI3K or FRAP/TOR inhibitors but in higher doses. Hypoxia partially reversed the dose dependent inhibition of HIF-1beta. These results suggest that HIF-1alpha and beta share common signaling pathways for nuclear protein accumulation.

Design of peptides undergoing self-catalytic alpha-to-beta transition and amyloidogenesis

Improved understanding of amyloidogenic peptides and proteins such as prion proteins and Alzheimer's beta peptides has attracted much attention to the elucidation of the molecular mechanisms of such amyloidogenesis. As a representative, in the prion protein, the conformational transitions from alpha-helix to beta-structure undergo along with the amyloidogenesis in a self-catalytic manner. Moreover, recent studies by the de novo design of peptides and proteins as well as the amyloidogenesis of peptides and proteins including pathogenic protein mutants have provided insight into the conformational changes essential to amyloidogenesis and correct folding.

The EGF domain: requirements for binding to receptors of the ErbB family

Epidermal growth factor (EGF) has been the prototype growth-stimulating peptide for many years. It has a characteristic structure with three disulfide bridges, which is essential for its activity. However, many other proteins, including both growth factors and proteins with unrelated functions, have similar EGF-like domains. This indicates that besides a characteristic conformation provided by the EGF-like domain, specific amino acids are required to provide specificity in protein functioning. Currently, more than 10 different growth factors with an EGF-like domain have been characterized which all exert their action by binding to the four members of the erbB family of receptors. In this review, studies are described on the structure-function relationship of these EGF-like growth factor molecules in an attempt to analyze the individual amino acids that determine their binding specificity to the individual members of the erbB family.

Early expression of angiogenesis factors in acute myocardial ischemia and infarction

BACKGROUND

When the myocardium is deprived of blood, a process of ischemia, infarction, and myocardial remodeling is initiated. Hypoxia-inducible factor 1 (HIF-1) is a transcriptional activator of vascular endothelial growth factor (VEGF) and is critical for initiating early cellular responses to hypoxia. We investigated the temporal and spatial patterns of expression of the alpha subunit of HIF-1 (HIF-1alpha) and VEGF in specimens of human heart tissue to elucidate the early molecular responses to myocardial hypoxia.

METHODS

Ventricular-biopsy specimens from 37 patients undergoing coronary bypass surgery were collected. The specimens were examined by microscopy for evidence of ischemia, evolving infarction, or a normal histologic appearance. The specimens were also analyzed with the reverse-transcriptase polymerase chain reaction for HIF-1alpha and VEGF messenger RNA (mRNA) expression and by immunohistochemical analysis for the location of the HIF-1alpha and VEGF proteins.

RESULTS

HIF-1alpha mRNA was detected in myocardial specimens with pathological evidence of acute ischemia (onset, <48 hours before surgery) or early infarction (onset, <24 hours before surgery). In contrast, VEGF transcripts were seen in specimens with evidence of acute ischemia or evolving infarction (onset, 24 to 120 hours before surgery). Patients with normal ventricles or evidence of infarction in the distant past had no detectable levels of either VEGF mRNA or HIF-1alpha mRNA. HIF-1alpha immunoreactivity was detected in the nuclei of myocytes and endothelial cells, whereas VEGF immunoreactivity was found in the cytoplasm of endothelial cells lining capillaries and arterioles.

CONCLUSIONS

An increase in the level of HIF-1alpha is an early response to myocardial ischemia or infarction. This response defines, at a molecular level, one of the first adaptations of human myocardium to a deprivation of blood. HIF-1alpha is a useful temporal marker of acutely jeopardized myocardium.

Angiogenesis: how a tumor adapts to hypoxia

The growth of new blood vessels from the preexisting vascular tree, also known as angiogenesis, occurs in situations such as wound and fracture healing, arthritis, cardiovascular and cerebral ischemia, and nearly every type of cancer known. Vascular endothelial growth factor (VEGF) has been shown to play a crucial role in these events. Hypoxia-dependent VEGF induction is mediated by hypoxia-inducible factor-1 (HIF-1). HIF-1 is a heterodimeric transcription factor tightly regulated by oxygen concentration. In this short review, we summarize recent data concerning the control of HIF-1 activity and notably the regulation of HIF-1alpha subunit by phosphorylation and the ubiquitin proteasomal degradation system. A complete knowledge of this mechanism could, by the design of new antiangiogenic strategies, have a strong impact in clinical oncology.

Regulation of mammalian O2 homeostasis by hypoxia-inducible factor 1

Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric basic-helix-loop-helix-PAS transcription factor consisting of HIF-1 alpha and HIF-1 beta subunits. HIF-1 alpha expression and HIF-1 transcriptional activity increase exponentially as cellular O2 concentration is decreased. Several dozen target genes that are transactivated by HIF-1 have been identified, including those encoding erythropoietin, glucose transporters, glycolytic enzymes, and vascular endothelial growth factor. The products of these genes either increase O2 delivery or allow metabolic adaptation to reduced O2 availability. HIF-1 is required for cardiac and vascular development and embryonic survival. In fetal and postnatal life, HIF-1 is required for a variety of physiological responses to chronic hypoxia. HIF-1 expression is increased in tumor cells by multiple mechanisms and may mediate adaptation to hypoxia that is critical for tumor progression. HIF-1 thus appears to function as a master regulator of O2 homeostasis that plays essential roles in cellular and systemic physiology, development, and pathophysiology.

Phosphorylation of rat serine 105 or mouse threonine 217 in C/EBP beta is required for hepatocyte proliferation induced by TGF alpha

We report that TGF alpha induces activation of the p90 ribosomal S kinase (RSK), which results in the phosphorylation of rat C/EBP beta on Ser-105 and of mouse C/EBP beta on Thr-217 and concomitantly stimulates proliferation in differentiated hepatocytes. Moreover, C/EBP beta-/- mouse hepatocytes respond to TGF alpha when wild-type C/EBP beta is reexpressed, whereas they remain refractory to the growth effect of TGF alpha when expressing phosphoacceptor mutants rat C/EBP beta Ala-105 or mouse C/EBP beta Ala-217. In contrast, C/EBP beta-/- hepatocytes expressing the phosphorylation mimic mutants, rat C/EBP beta Asp-105 or mouse C/EBP beta Glu-217, exhibited marked proliferation in the absence of TGF alpha. Thus, a site-specific phosphorylation of the transcription factor C/EBP beta is critical for hepatocyte proliferation induced by TGF alpha and other stimuli that activate RSK.

PD 102807, a novel muscarinic M4 receptor antagonist, discriminates between striatal and cortical muscarinic receptors coupled to cyclic AMP

In membranes of Chinese hamster ovary cells expressing the cloned human M1-M4 muscarinic receptor subtypes, PD 102807, a novel M4 selective antagonist, was found to counteract the M4 receptor-induced stimulation of [35S]-GTPgammaS binding to membrane G proteins with a pK(B) of 7.40, a value which was 63-, 33- and 10-fold higher than those displayed at M1 (pK(B) = 5.60), M2 (pK(B) = 5.88) and M3 (pK(B) = 6.39) receptor subtypes, respectively. In rat striatal membranes, PD 102807 antagonized the muscarinic inhibition of dopamine (DA) D1 receptor-stimulated adenylyl cyclase with a pK(B) value of 7.36. In contrast, in membranes of rat frontal cortex, PD 102807 displayed lower potencies in antagonizing either the muscarinic facilitation of corticotropin releasing hormone (CRH)-stimulated adenylyl cyclase (pK(B) = 5.79) or inhibition of Ca2+/calmodulin (Ca2+/CaM)-stimulated enzyme activity (pK(B) = 5.95). In each response investigated, PD 102807 interacted with muscarinic receptors in a manner typical of a simple competitive antagonist. These data provide additional evidence that PD 102807 is a M4-receptor preferring antagonist and that this compound can discriminate the striatal muscarinic receptors inhibiting DA D1 receptor activity from the cortical receptors mediating the potentiation of CRH receptor signalling and the inhibition of Ca2+/CaM-stimulated adenylyl cyclase activity.

A novel transduction mechanism mediating dopamine-induced vascular relaxation: opening of BKCa channels by cyclic AMP-induced stimulation of the cyclic GMP-dependent protein kinase

Dopamine dilates the coronary, renal and other vascular beds; however, the signaling pathway underlying this effect is unclear. In this study the signal-transduction process mediating dopamine-induced relaxation of porcine coronary arteries was investigated in isolated vessels and single arterial myocytes. Dopamine-induced relaxation of arteries was mediated through the DA- receptor and involved K+ efflux, and subsequent patch-clamp studies demonstrated that either dopamine or fenoldopam, a selective DA-1 agonist, increased the opening probability of the large-conductance, calcium- and voltage-activated K+ (BKCa) channel in coronary myocytes. Moreover, blockade of this channel by iberiotoxin prevented dopamine-induced coronary relaxation. Dopamine stimulation of BKCa channels was completely prevented by a DA-1-receptor antagonist, but was unaffected by propranolol. Furthermore, inhibiting adenylyl cyclase activity prevented stimulation of BKCa channel activity, whereas chlorophenylthio (CPT)-cyclic adenosine monophosphate (AMP), a membrane-permeable analog of cyclic AMP, mimicked the effects of dopamine. Interestingly, inhibiting the cyclic AMP-dependent protein kinase (PKA) did not affect the response to dopamine, whereas dopamine-induced channel activity was completely blocked by inhibiting the activity of the cyclic guanosine monophosphate (GMP)-dependent protein kinase (PKG). These findings demonstrate that activation of DA-1 receptors causes stimulation of BKCa channel activity by a mechanism involving cyclic AMP-dependent stimulation of PKG, but not PKA, and further suggest that this cross-reactivity mediates dopamine-induced coronary vasodilation.

Genetic regulation of glutamate receptor ion channels

Transcriptional and translational regulation of glutamate receptor expression determines one of the key phenotypic features of neurons in the brain--the properties of their excitatory synaptic receptors. Up- and down-regulation of various glutamate receptor subunits occur throughout development, following ischemia, seizures, repetitive activation of afferents, or chronic administration of a variety of drugs. The promoters of the genes that encode the NR1, NR2B, NR2C, GluR1, GluR2, and KA2 subunits share several characteristics that include multiple transcriptional start sites within a CpG island, lack of TATA and CAAT boxes, and neuronal-selective expression. In most cases, the promoter regions include overlapping Sp1 and GSG motifs near the major initiation sites, and a silencer element, to guide expression in neurons. Manipulating the levels of glutamate receptors in vivo by generating transgenic and knockout mice has enhanced understanding of the role of specific glutamate receptor subunits in long-term potentiation and depression, learning, seizures, neural pattern formation, and survival. Neuron-specific glutamate receptor promoter fragments may be employed in the design of novel gene-targeting constructs to deliver future experimental transgene and therapeutic agents to selected neurons in the brain.

Control elements of muscarinic receptor gene expression

Studies describing the structures of the M1, M2 and M4 muscarinic acetylcholine receptors (mAChR) genes and the genetic elements that control their expression are reviewed. In particular, we focus on the role of the neuron-restrictive silencer element/restriction element-1 (NRSE/RE-1) in the regulation of the M4 mAChR gene. The NRSE/RE-1 was first identified as a genetic control element that prevents the expression of the SCG-10 and type II sodium channel (NaII) genes in non-neuronal cells in culture. The NRSE/RE-1 inhibits gene expression by binding the repressor/silencer protein NRSF/REST, which is present in many non-neuronal cell lines and tissues. Our studies show that although the expression of the M4 mAChR gene is inhibited by NRSF/REST, this inhibition is not always complete. Rather, the efficiency of silencing by NRSF/REST is different in different cells. A plausible explanation for this differential silencing is that the NRSF/RE-1 interacts with distinct sets of promoter binding proteins in different types of cells. We hypothesize that modulation of NRSF/REST silencing activity by these proteins contributes to the cell-specific pattern of expression of the M4 mAChR in neuronal and non-neuronal cells. Recent studies that suggest a more complex role for the NRSE/RE-1 in regulating gene expression are also discussed.

Repression and activation of muscarinic receptor genes

The specific cellular response to muscarinic receptor activation is dependent upon appropriate expression of each of the five muscarinic receptor genes by individual cells. Here we summarise recent work describing some of the genomic regulatory elements and transcriptional mechanisms that control expression of the M1 and M4 genes.

Hypoxia-mediated regulation of gene expression in mammalian cells

The molecular mechanism underlying oxygen sensing in mammalian cells has been extensively investigated in the areas of glucose transport, glycolysis, erythropoiesis, angiogenesis and catecholamine metabolism. Expression of functionally operative representative proteins in these specific areas, such as the glucose transporter 1, glycolytic enzymes, erythropoietin, vascular endothelial growth factor and tyrosine hydroxylase are all induced by hypoxia. Recent studies demonstrated that both transcriptional activation and post-transcriptional mechanisms are important to the hypoxia-mediated regulation of gene expression. In this article, the cis-acting elements and trans-acting factors involved in the transcriptional activation of gene expression will be reviewed. In addition, the mechanisms of post-transcriptional mRNA stabilization will also be addressed. We will discuss whether these two processes of regulation of hypoxia-responsive genes are mechanistically linked and co-operative in nature.

The VHL tumour-suppressor gene paradigm

The VHL tumour-suppressor gene (TSG) has a critical 'gatekeeper' role in regulating growth and differentiation of human kidney cells, and inactivation of the VHL gene is the most frequent genetic event in human kidney cancer. There are many similarities between the genetics of the VHL and retinoblastoma TSGs, but the VHL tumourigenesis model is more complex. Here, we examine the current knowledge of the genetics and functional aspects of the VHL TSG, and emphasize how the VHL gene provides a paradigm that illustrates many aspects of TSG biology.

Identification of the minimal requirements for binding to the human epidermal growth factor (EGF) receptor using chimeras of human EGF and an EGF repeat of Drosophila Notch

Many proteins contain so-called epidermal growth factor (EGF)-like domains that share the characteristic spacing of cysteines and glycines with members of the EGF family. They are, however, functionally unrelated, despite the fact that the three-dimensional structure of these EGF-like domains, also, is often very similar to that of the EGF receptor agonists. In the present study, we linked an EGF-like repeat from the Drosophila Notch protein to the N- and C-terminal linear tail sequences of human EGF (hEGF), and we showed that this chimera (E1N6E) is unable to bind or activate the hEGF receptor. This recombinant protein was then used as a basic construct for identifying the minimal requirements for high affinity EGF receptor binding and activation. We selectively reintroduced a limited number of important hEGF-derived residues, and by using this unique approach, we were able to make hEGF/Notch chimeras that, compared with wild type hEGF, showed nearly 100% binding affinity and mitogenic activity on HER-14 cells expressing the hEGF receptor.

Differential endocytic routing of homo- and hetero-dimeric ErbB tyrosine kinases confers signaling superiority to receptor heterodimers

Both homo- and hetero-dimers of ErbB receptor tyrosine kinases mediate signaling by a large group of epidermal growth factor (EGF)-like ligands. However, some ligands are more potent than others, although they bind to the same direct receptor. In addition, signaling by receptor heterodimers is superior to homodimers. We addressed the mechanism underlying these two features of signal tuning by using three ligands: EGF; transforming growth factor alpha (TGFalpha); and their chimera, denoted E4T, which act on cells singly expressing ErbB-1 as a weak, a strong, and a very strong agonist, respectively. Co-expression of ErbB-2, a developmentally important co-receptor whose expression is frequently elevated in human cancers, specifically potentiated EGF signaling to the level achieved by TGFalpha, an effect that was partially mimicked by ErbB-3. Analysis of the mechanism underlying this trans-potentiation implied that EGF-driven homodimers of ErbB-1 are destined for intracellular degradation, whereas the corresponding heterodimers with ErbB-2 or with ErbB-3, dissociate in the early endosome. As a consequence, in the presence of either co-receptor, ErbB-1 is recycled to the cell surface and its signaling is enhanced. This latter route is followed by TGFalpha-driven homodimers of ErbB-1, and also by E4T-bound receptors, whose signaling is further enhanced by repeated cycles of binding and dissociation from the receptors. We conclude that alternative endocytic routes of homo- and hetero-dimeric receptor complexes may contribute to tuning and diversification of signal transduction. In addition, the ability of ErbB-2 to shunt ligand-activated receptors to recycling may explain, in part, its oncogenic potential.