A transcriptionally [correction of transcriptively] active complex of APP with Fe65 and histone acetyltransferase Tip60

Amyloid-beta precursor protein (APP), a widely expressed cell-surface protein, is cleaved in the transmembrane region by gamma-secretase. gamma-Cleavage of APP produces the extracellular amyloid beta-peptide of Alzheimer's disease and releases an intracellular tail fragment of unknown physiological function. We now demonstrate that the cytoplasmic tail of APP forms a multimeric complex with the nuclear adaptor protein Fe65 and the histone acetyltransferase Tip60. This complex potently stimulates transcription via heterologous Gal4- or LexA-DNA binding domains, suggesting that release of the cytoplasmic tail of APP by gamma-cleavage may function in gene expression.

Defective lymphoid development in mice lacking expression of the common cytokine receptor gamma chain

The common gamma chain (gamma c) of the IL-2, IL-4, IL-7, IL-9, and IL-15 receptors is defective in humans with XSCID. Mice lacking gamma c expression had hypoplastic thymuses; the thymocytes responded to gamma c-independent mitogens, but not gamma c-dependent stimuli. Splenic T cells were diminished at 3 weeks of age, but CD4+ T cells markedly increased by 4 weeks. B cells were greatly diminished in contrast with the situation in XSCID. NK cells, gamma delta intestinal intraepithelial lymphocytes, dendritic epidermal T cells, peripheral lymph nodes, and gut-associated lymphoid tissue were absent. These findings underscore the importance of gamma c in lymphoid development. Moreover, differences in humans and mice lacking gamma c expression indicate species-specific differences in the roles of gamma c-dependent cytokines or in the existence of redundant pathways. These mice provide an important model for studying the pathophysiology provide an important model for studying the pathophysiology of and gene therapy for human XSCID.

Interleukin-2 receptor gamma chain: a functional component of the interleukin-7 receptor

The interleukin-2 receptor gamma chain (IL-2R gamma) is a necessary component of functional IL-2 receptors. IL-2R gamma mutations result in X-linked severe combined immunodeficiency (XSCID) in humans, a disease characterized by the presence of few or no T cells. In contrast, SCID patients with IL-2 deficiency and IL-2-deficient mice have normal numbers of T cells, suggesting that IL-2R gamma is part of more than one cytokine receptor. By using chemical cross-linking, IL-2R gamma was shown to be physically associated with the IL-7 receptor. The presence of IL-2R gamma augmented both IL-7 binding affinity and the efficiency of internalization of IL-7. These findings may help explain the defects of XSCID. Given its role in more than one cytokine receptor system, the common gamma chain (gamma c) is proposed as the designation for IL-2R gamma.

Requirement of CHROMOMETHYLASE3 for maintenance of CpXpG methylation

Epigenetic silenced alleles of the Arabidopsis SUPERMAN locus (the clark kent alleles) are associated with dense hypermethylation at noncanonical cytosines (CpXpG and asymmetric sites, where X = A, T, C, or G). A genetic screen for suppressors of a hypermethylated clark kent mutant identified nine loss-of-function alleles of CHROMOMETHYLASE3 (CMT3), a novel cytosine methyltransferase homolog. These cmt3 mutants display a wild-type morphology but exhibit decreased CpXpG methylation of the SUP gene and of other sequences throughout the genome. They also show reactivated expression of endogenous retrotransposon sequences. These results show that a non-CpG DNA methyltransferase is responsible for maintaining epigenetic gene silencing.

Cytochrome c release and apoptosis induced by mitochondrial targeting of nuclear orphan receptor TR3

TR3, an immediate-early response gene and an orphan member of the steroid-thyroid hormone-retinoid receptor superfamily of transcription factors, regulates apoptosis through an unknown mechanism. In response to apoptotic stimuli, TR3 translocates from the nucleus to mitochondria to induce cytochrome c release and apoptosis. Mitochondrial targeting of TR3, but not its DNA binding and transactivation, is essential for its proapoptotic effect. Our results reveal a mechanism by which a nuclear transcription factor translocates to mitochondria to initiate apoptosis.

Ischemic preconditioning induces selective translocation of protein kinase C isoforms epsilon and eta in the heart of conscious rabbits without subcellular redistribution of total protein kinase C activity

Considerable controversy surrounds the role of protein kinase C (PKC) in ischemic preconditioning (PC). Previous studies have used pharmacological agents and/or measured total myocardial PKC activity; however, no information is available regarding the effects of PC on individual isoforms in vivo. We performed a comprehensive evaluation (using Western immunoblotting) of the expression and subcellular distribution of all 11 currently known PKC isoforms in the heart of conscious rabbits subjected to four different ischemic PC protocols known to induce early and/or late PC (one, three, or six cycles of 4-minute coronary occlusion [4'O]/4-minute reperfusion [4'R]; four cycles of 5-minute occlusion [5'O]/10-minute reperfusion [10'R]). Ten PKC isoforms (alpha, beta1/beta2, gamma, delta, epsilon, zeta, eta, iota, lambda, and mu) were found to be expressed in the rabbit heart. Quantitative immunoblotting demonstrated that as a subgroup, conventional PKCs (cPKCs) are more abundant than novel PKCs (nPKCs) (1445 versus 313 pg PKC/microg tissue protein, respectively) and that PKC alpha is the predominant isoform among the cPKCs (alpha, beta1, beta2, and gamma), representing 51% of this subgroup, and PKC epsilon is the most abundant among the nPKCs (delta, epsilon, zeta, and eta), accounting for 62% of this subgroup. None of the ischemic PC protocols examined caused appreciable changes in total PKC activity, in the subcellular distribution of total PKC activity, or in the subcellular distribution of PKC isoforms alpha, beta1/beta2, gamma, delta, zeta, iota, lambda, and mu. In contrast, all PC protocols caused significant translocation of PKC epsilon and PKC eta isoforms from the cytosolic to the particulate fraction. The particulate fraction of PKC epsilon increased in a dose-dependent fashion with the number of occlusion/reperfusion cycles performed, from 35+/-2% in the control group to 43+/-2% after one 4'O/5-minute reperfusion (5'R) cycle (P<.05), 52+/-2% after three cycles (P<.05 versus one cycle), and 66+/-3% after six cycles (P<.05 versus three cycles). The particulate fraction of PKC epsilon also increased, after four 5'O/10'R cycles, to 50+/-3% (P<.05 versus control). In contrast to PKC epsilon, the translocation of PKC eta was independent of the number of occlusion/reperfusion cycles performed. The particulate fraction of PKC eta increased from 67+/-3% in the control group to 84+/-2% after one 4'O/5'R cycle (P<.05), 84+/-2% after three 4'O/4'R cycles (P<.05), 86+/-3% after six 4'O/4'R cycles (P<.05), and 83+/-2% after four 5'O/10'R cycles (P<.05). When expressed as a percentage of control values, the increases in the particulate fraction of isoform epsilon were greater than those of isoform eta. The effects of 4'O without reperfusion were similar to those of one cycle of 4'O/5'R, indicating that 5'R did not attenuate isoform translocation. This is the first study to demonstrate PKC translocation after ischemic PC in vivo. The results indicate that in the conscious rabbit, ischemic PC causes selective translocation of the epsilon and eta isoforms without demonstrable changes in total myocardial PKC activity, implying that measurements of total PKC activity are not sufficiently sensitive to detect the involvement of PKC in PC. The results are consistent with the concept that the epsilon and eta isozymes play an important role in the genesis of ischemic PC in the conscious rabbit.

B cells inhibit induction of T cell-dependent tumor immunity

Cytotoxic T lymphocyte (CTL) mediated tumor immunity against major histocompatibility antigen (MHC) class I-positive but class II-negative tumors often requires help from CD4+ T cells. These CD4 cells are activated by MHC class II-positive cells that present tumor derived antigens. Considering that different antigen presenting cells, such as B cells, macrophages and dendritic cells compete for antigen and influence the outcome of an immune response, we analyzed tumor immunity in B cell-deficient mice. These mice appear normal with regard to T cell immunity and tolerance to some pure foreign antigens. We show here that the low immunogenicity of tumors is caused by B cells whose presence in the priming phase results in disabled CD4+ T cell help for CTL mediated tumor immunity. Instead, in the presence of B cells, a non-protective humoral immune response is induced. Our results may explain the enigmatic observation that tumor-reactive antibodies occur frequently in cancer patients.

Initial docking of ER-derived vesicles requires Uso1p and Ypt1p but is independent of SNARE proteins

ER-to-Golgi transport in yeast may be reproduced in vitro with washed membranes, purified proteins (COPII, Uso1p and LMA1) and energy. COPII coated vesicles that have budded from the ER are freely diffusible but then dock to Golgi membranes upon the addition of Uso1p. LMA1 and Sec18p are required for vesicle fusion after Uso1p function. Here, we report that the docking reaction is sensitive to excess levels of Sec19p (GDI), a treatment that removes the GTPase, Ypt1p. Once docked, however, vesicle fusion is no longer sensitive to GDI. In vitro binding experiments demonstrate that the amount of Uso1p associated with membranes is reduced when incubated with GDI and correlates with the level of membrane-bound Ypt1p, suggesting that this GTPase regulates Uso1p binding to membranes. To determine the influence of SNARE proteins on the vesicle docking step, thermosensitive mutations in Sed5p, Bet1p, Bos1p and Sly1p that prevent ER-to-Golgi transport in vitro at restrictive temperatures were employed. These mutations do not interfere with Uso1p-mediated docking, but block membrane fusion. We propose that an initial vesicle docking event of ER-derived vesicles, termed tethering, depends on Uso1p and Ypt1p but is independent of SNARE proteins.

Activation and association of Stat3 with Src in v-Src-transformed cell lines

STAT proteins are a group of latent cytoplasmic transcription factors which function as signal transducers and activators of transcription. Stat1 and -2 were originally identified to function in interferon signaling, and Stat1 was also found to be activated by epidermal growth factor (EGF) and other cytokines. New members of the STAT gene family are identified. Among them, Stat3 has 52.5% amino acid sequence homology with Stat1 and is activated by platelet-derived growth factor (PDGF), colony-stimulating factor 1 (CSF-1), EGF, interleukin-6, and other cytokines. Treatment of cells with EGF activates Stat1 and Stat3, which become phosphorylated on tyrosine residues to form homo - or heterodimers and translocate into the nucleus, binding to the sis-inducible element (SIE) in the c-fos promoter. Somatic cell genetic analyses demonstrated that Jaks, a family of nontransmembrane protein tyrosine kinases, are required for the activation of Stat1 and Stat2 in interferon-treated cells. However, little is known about the activation of Stat3 by growth factors. Here we report that in all v-Src-transformed cell lines examined, Stat3 is constitutively activated to bind to DNA and the phosphorylation of tyrosine on Stat3 is enhanced by the induction of v-Src expression. We also report that Src is shown to be associated with Stat3 in vivo, as well as in vitro, and phosphorylates Stat3 in vitro. Stat3 is also activated by CSF-1, possibly through CSF-1 receptor-c Src association in NIH 3T3 cells overexpressing CSF-1 receptors. Together, the data suggest that Src is involved in activation of Stat3 in growth factor signal transduction.

Effects of DL-alpha-lipoic acid on peripheral nerve conduction, blood flow, energy metabolism, and oxidative stress in experimental diabetic neuropathy

Experimental diabetic peripheral neuropathy (DPN) is marked by impaired nerve conduction velocity (NCV), reduced nerve blood flow (NBF), and a variety of metabolic abnormalities in peripheral nerve that have been variously ascribed to hyperglycemia, abnormal fatty acid metabolism, ischemic hypoxia, and/or oxidative stress. Some investigators propose that NCV slowing in experimental DPN can be explained entirely on the basis of nerve energy depletion secondary to reduced NBF. This article reports highly selective effects of administration of the antioxidant DL-alpha-lipoic acid (LA) to streptozotocin-injected diabetic rats. LA improved digital sensory but not sciatic-tibial motor NCV, corrected endoneurial nutritive but not composite NBF, increased the mitochondrial oxidative state without correcting nerve energy depletion, and enhanced the accumulation of polyol pathway intermediates without worsening myo-inositol or taurine depletion. These studies implicate oxidative stress as an important pathophysiological factor in experimental DPN. They reveal complex interrelationships among nerve perfusion, energy metabolism, osmolyte content, conduction velocity, and oxidative stress that may reflect the heterogeneous and compartmentalized composition of peripheral nerve.

Epidermal growth factor family members: endogenous mediators of the ovulatory response

Previous studies showed that epidermal growth factor (EGF) and TGFalpha mimic the action of LH on the resumption of oocyte maturation. We tested whether EGF-like agents, such as amphiregulin (AR), epiregulin (ER), and betacellulin (BTC), also mediate the LH stimulation of the ovulatory response in the rat. LH induced transient follicular expression of AR, ER, and BTC mRNA, reaching a maximum after 3-h incubation. Furthermore, the addition of ER, AR, and BTC to the culture medium could mimic some of LH actions. AR and ER fully simulated LH-induced resumption of meiosis in vitro, whereas BTC was less effective. To study the putative involvement of EGF-like factors in mediation of LH signal, the effect of the EGF receptor kinase inhibitor AG1478 was tested. When added with LH, AG1478, but not its inactive analog AG43, reduced EGF receptor phosphorylation and oocyte maturation compared with follicles treated with LH only. In addition to the inhibition of resumption of meiosis, AG1478 administration into the bursa (3 microg/bursa) resulted in 51% (P < 0.0005) inhibition of ovulation in the treated ovaries, compared with the untreated contralateral ones, as well as to the vehicle-treated ovaries (P < 0.02). LH, as well as ER, induced the expression of genes associated with the ovulatory response like rat hyaluronan synthase-2, cyclooxygenase-2, and TNFalpha-stimulated gene 6 mRNA, whereas AG1478 inhibited this effect of LH. Release of EGF-like factors from the membrane is dependent on activated metalloproteases. Indeed, Galardin, a broad-spectrum metalloprotease inhibitor, but not a specific matrix metalloprotease 2 and 9 inhibitor, suppressed meiotic maturation induced by LH. Conversely, meiotic maturation induced by ER was not affected by Galardin, thus, supporting the notion that LH releases follicular membrane-bound EGF-like agents. In summary, EGF-like factors such as ER, AR, and BTC seem to mediate, at least partially, the LH stimulation of oocyte maturation, ovulatory enzyme expression, and ovulation.

Proline, a multifaceted signalling molecule in plant responses to abiotic stress: understanding the physiological mechanisms

Abiotic stresses have a detrimental impact on plant growth and productivity and are a major threat to sustainable crop production in rapidly changing environments. Proline, an important amino acid, plays an important role in maintaining the metabolism and growth of plants under abiotic stress conditions. Many insights indicate a positive relationship between proline accumulation and tolerance of plants to various abiotic stresses. Because of its metal chelator properties, it acts as a molecular chaperone, an antioxidative defence molecule that scavenges reactive oxygen species (ROS), as well as having signalling behaviour to activate specific gene functions that are crucial for plant recovery from stresses. It also acts as an osmoprotectant, a potential source to acquire nitrogen as well as carbon, and plays a significant role in the flowering and development of plants. Overproduction of proline in plant cells contributes to maintaining cellular homeostasis, water uptake, osmotic adjustment and redox balance to restore the cell structures and mitigate oxidative damage. Many reports reveal that transgenic plants, particularly those overexpressing genes tailored for proline accumulation, exhibit better adaptation to abiotic stresses. Therefore, this review aims to provide a comprehensive update on proline biosynthesis and accumulation in plants and its putative regulatory roles in mediating plant defence against abiotic stresses. Additionally, the current and future directions in research concerning manipulation of proline to induce gene functions that appear promising in genetics and genomics approaches to improve plant adaptive responses under changing climate conditions are also highlighted.

Metformin regulates the incretin receptor axis via a pathway dependent on peroxisome proliferator-activated receptor-α in mice

AIMS/HYPOTHESIS

Metformin is widely used for the treatment of type 2 diabetes. Although it reduces hepatic glucose production, clinical studies show that metformin may reduce plasma dipeptidyl peptidase-4 activity and increase circulating levels of glucagon-like peptide 1 (GLP-1). We examined whether metformin exerts glucoregulatory actions via modulation of the incretin axis.

METHODS

Metformin action was assessed in Glp1r(-/-), Gipr(-/-), Glp1r:Gipr(-/-), Pparα (also known as Ppara)(-/-) and hyperglycaemic obese wild-type mice with or without the GLP-1 receptor (GLP1R) antagonist exendin(9-39). Experimental endpoints included glucose tolerance, plasma insulin levels, gastric emptying and food intake. Incretin receptor expression was assessed in isolated islets from metformin-treated wild-type and Pparα(-/-) mice, and in INS-1 832/3 beta cells with or without peroxisome proliferator-activated receptor (PPAR)-α or AMP-activated protein kinase (AMPK) antagonists.

RESULTS

In wild-type mice, metformin acutely increased plasma levels of GLP-1, but not those of gastric inhibitory polypeptide or peptide YY; it also improved oral glucose tolerance and reduced gastric emptying. Metformin significantly improved oral glucose tolerance despite loss of incretin action in Glp1r(-/-), Gipr(-/-) and Glp1r(-/-) :Gipr(-/-) mice, and in wild-type mice fed a high-fat diet and treated with exendin(9-39). Levels of mRNA transcripts for Glp1r, Gipr and Pparα were significantly increased in islets from metformin-treated mice. Metformin directly increased Glp1r expression in INS-1 beta cells via a PPAR-α-dependent, AMPK-independent mechanism. Metformin failed to induce incretin receptor gene expression in islets from Pparα(-/-) mice.

CONCLUSIONS/INTERPRETATION

As metformin modulates multiple components of the incretin axis, and enhances expression of the Glp1r and related insulinotropic islet receptors through a mechanism requiring PPAR-α, metformin may be mechanistically well suited for combination with incretin-based therapies.

Ectopic hypermethylation of flower-specific genes in Arabidopsis

BACKGROUND

Arabidopsis mutations causing genome-wide hypomethylation are viable but display a number of specific developmental abnormalities, including some that resemble known floral homeotic mutations. We previously showed that one of the developmental abnormalities present in an antisense-METHYLTRANSFERASEI (METI) transgenic line resulted from ectopic hypermethylation of the SUPERMAN gene.

RESULTS

Here, we investigate the extent to which hypermethylation of SUPERMAN occurs in several hypomethylation mutants, and describe methylation effects at a second gene, AGAMOUS. SUPERMAN gene hypermethylation occurred at a high frequency in several mutants that cause overall decreases in genomic DNA methylation. The hypermethylation pattern was largely similar in the different mutant backgrounds. Genetic analysis suggests that hypermethylation most likely arose either during meiosis or somatically in small sectors of the plant. A second floral development gene, AGAMOUS, also became hypermethylated and silenced in an Arabidopsis antisense-METI line.

CONCLUSIONS

These results suggest that ectopic hypermethylation of specific genes in mutant backgrounds that show overall decreases in methylation may be a widespread phenomenon that could explain many of the developmental defects seen in Arabidopsis methylation mutants. This resembles a phenomenon seen in cancer cells, which can simultaneously show genome-wide hypomethylation and hypermethylation of specific genes. Comparison of the methylated sequences in SUPERMAN and AGAMOUS suggests that hypermethylation could involve DNA secondary structures formed by pyrimidine-rich sequences.

Intracellular disposition and metabolism of fluorescently-labeled unmodified and modified oligonucleotides microinjected into mammalian cells

The intracellular distribution and metabolism of microinjected fluorescently-labeled oligonucleotides (ODNs) have been evaluated using confocal fluorescence microscopy. Fluorescent phosphodiester ODNs, microinjected into the cytoplasm of mammalian cells, rapidly accumulate within the nucleus; the fluorescence disappears with a half-life of 15-20 minutes. Microinjected fluorescent phosphorothioate ODNs remain in the nucleus for more than 24 hours. The persistence of fluorescence depends on the length of the ODN. Modification of the 3' end of phosphodiester ODNs does not significantly slow the rapid disappearance of fluorescence, although certain 3' modifications localize ODNs into the cytoplasm. Using specially designed ODNs, endonuclease activity is demonstrated to exist in the cytoplasm and nucleus. Modification of the 2' position of the ribose rings of a fluorescent phosphodiester oligodeoxynucleotide with O-methyl or O-allyl does not alter its intracellular distribution; however, the 2'-O-allyl modification stabilizes the persistence of fluorescence more than 60-fold compared to the 2'-deoxy control. Thus, the experiments indicate that somatic cells contain nucleolytic activities which degrade microinjected ODNs; however, chemical modification can dramatically circumvent this process.

alpha-Phenyl-tert-butyl-nitrone reduces cortical infarct and edema in rats subjected to focal ischemia

The neuroprotective effects of the spin-trapping agent alpha-phenyl-N-tert-butyl nitrone (PBN) were evaluated in rats subjected to focal cerebral ischemia produced by permanent middle cerebral artery (MCA) and ipsilateral common carotid artery (CCA) occlusion. PBN was given i.p. at 100 mg/kg at initial times of administration of 0.5 h prior to ischemia (group 2), 0.5 (group 3), 5 (group 4) and 12 h (group 5) after ischemia. Additional doses of PBN (100 mg/kg) were administered as follows: Group 2, at 24 h; Group 3, at 5, 17, 29 and 41 h; Group 4, at 17, 29 and 41 h; Group 5, at 24 and 36 h. Animals were sacrificed 48 h after MCA occlusion and infarct volumes were calculated from triphenyetetrazolium stained 1.5 mm slices of the forebrain. PBN significantly attenuated cortical infarct volume and cerebral edema in all of the treated rats compared with those in ischemic control (group 1) rats, with no significant differences between the different PBN treated groups. The percentage of infarct volume in ischemic control rats was 22.7 +/- 1.0, while those in PBN-treated groups were: 9.6 +/- 2.0, P < 0.01 (group 2); 12.2 +/- 2.2, P < 0.01 (group 3); 11.1 +/- 2.9, P < 0.01 (group 4) and 14.4 +/- 2.5, P < 0.01 (group 5). Furthermore, neurological behavior tests showed that PBN decreased the neurological deficit scores in rats initially treated either prior to or for up to 12 h after ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Serine phosphorylation and negative regulation of Stat3 by JNK

STATs are activated by various cytokines and growth factors via tyrosine phosphorylation, which leads to sequential dimer formation, nuclear translocation, binding to specific DNA sequences, and regulation of gene expression. Recently, serine phosphorylation of Stat3 on Ser-727 by ERK has been identified in response to epidermal growth factor (EGF). Here, we report that Ser-727 phosphorylation of Stat3 can also be induced by JNK and activated either by stress or by its upstream kinase and that various stress treatments induce serine phosphorylation of Stat3 in the absence of tyrosine phosphorylation. Inhibitors of ERK and p38 did not inhibit UV-induced Stat3 serine phosphorylation, suggesting that neither of them is involved. We further demonstrate that JNK1, activated by its upstream kinase MKK7, negatively regulated the tyrosine phosphorylation and DNA binding and transcriptional activities of Stat3 stimulated by EGF. Correspondingly, pretreatment of cells with UV reduced the EGF-stimulated tyrosine phosphorylation and phosphotyrosine-dependent activities of Stat3. The inhibitory effect was not observed for Stat1. Our results suggest that Stat3 is a target of JNK that may regulate Stat3 activity via both Ser-727 phosphorylation-dependent and -independent mechanisms.

Protein kinase C delta associates with and phosphorylates Stat3 in an interleukin-6-dependent manner

Stat3 is activated by phosphorylation on Tyr-705, which leads to dimer formation, nuclear translocation, and regulation of gene expression. Serine phosphorylation of Stat3 by mitogen-activated protein kinase has also been observed in cells responding to epidermal growth factor and shown to affect its tyrosine phosphorylation and transcriptional activity. Serine phosphorylation of Stat3 is also induced by interleukin-6 (IL-6) stimulation, which is shown to be independent of mitogen-activated protein kinase and sensitive to the Ser/Thr kinase inhibitor H7. In this study, we investigated whether protein kinase C (PKC) is the kinase that is induced and responsible for Stat3 serine phosphorylation by IL-6 stimulation and which isoform of PKCs is likely to be involved. Here, we report that Stat3 was specifically associated with PKC delta in vivo in an IL-6-dependent manner in several cell types. Furthermore, Stat3 was phosphorylated by PKC delta in vivo on Ser-727, which could be inhibited either by a specific PKC delta inhibitor or by a dominant-negative mutant of PKC delta. Finally, we showed that the phosphorylation of Stat3 by PKC delta led to a negative regulation of Stat3 DNA binding and transcriptional activity. These results indicate that PKC delta is likely to be the kinase that phosphorylates Stat3 in response to IL-6 stimulation and suggest a possible regulatory role of PKC delta on Stat3 function.

Conserved plant genes with similarity to mammalian de novo DNA methyltransferases

DNA methylation plays a critical role in controlling states of gene activity in most eukaryotic organisms, and it is essential for proper growth and development. Patterns of methylation are established by de novo methyltransferases and maintained by maintenance methyltransferase activities. The Dnmt3 family of de novo DNA methyltransferases has recently been characterized in animals. Here we describe DNA methyltransferase genes from both Arabidopsis and maize that show a high level of sequence similarity to Dnmt3, suggesting that they encode plant de novo methyltransferases. Relative to all known eukaryotic methyltransferases, these plant proteins contain a novel arrangement of the motifs required for DNA methyltransferase catalytic activity. The N termini of these methyltransferases contain a series of ubiquitin-associated (UBA) domains. UBA domains are found in several ubiquitin pathway proteins and in DNA repair enzymes such as Rad23, and they may be involved in ubiquitin binding. The presence of UBA domains provides a possible link between DNA methylation and ubiquitin/proteasome pathways.

Repression of Stat3 activity by activation of mitogen-activated protein kinase (MAPK)

STAT proteins are activated by phosphorylation at specific tyrosine residue at the carboxy-terminus which is required for dimer-formation, nuclear translocation, DNA binding and transcriptional activity in cells treated with cytokines and growth factors. Recent studies have indicated that STATs are also phosphorylated by MAPK, or extracellular signal-regulated kinase (ERK) on serine. We investigated the role of ERK on the regulation of STAT activity. Here, we report that ERK2 activated by its upstream kinase, MEK1, represses Stat3 transcriptional activity induced by Src or Jak-2. To unravel the mechanism of repression, we further showed that Stat3 DNA binding activity and its tyrosine phosphorylation are also inhibited under the same conditions. ERK2 phosphorylates Stat3 on three serine-containing peptides and decreases its tyrosine phosphorylation induced by EGF treatment. We also detected an association of ERK2 and Stat3 in vivo which is modulated positively by activation of ERK2, but negatively by Jak2. We propose that MAP kinase cascade may negatively regulate Stat3 activities by decreasing its tyrosine phosphorylation and also possibly by association.

Silicon surfaces as electron acceptors: dative bonding of amines with Si(001) and Si(111) surfaces

The bonding of the trimethylamine (TMA) and dimethylamine (DMA) with crystalline silicon surfaces has been investigated using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, and density-functional computational methods. XPS spectra show that TMA forms stable dative-bonded adducts on both Si(001) and Si(111) surfaces that are characterized by very high N(1s) binding energies of 402.2 eV on Si(001) and 402.4 eV on Si(111). The highly ionic nature of these adducts is further evidenced by comparison with other charge-transfer complexes and through computational chemistry studies. The ability to form these highly ionic charge-transfer complexes between TMA and silicon surfaces stems from the ability to delocalize the donated electron density between different types of chemically distinct atoms within the surface unit cells. Corresponding studies of DMA on Si(001) show only dissociative adsorption via cleavage of the N-H bond. These results show that the unique geometric structures present on silicon surfaces permit silicon atoms to act as excellent electron acceptors.

pH-dependent conformational change of gastric mucin leads to sol-gel transition

We present dynamic light scattering (DLS) and hydrophobic dye-binding data in an effort to elucidate a molecular mechanism for the ability of gastric mucin to form a gel at low pH, which is crucial to the barrier function of gastric mucus. DLS measurements of dilute mucin solutions were not indicative of intermolecular association, yet there was a steady fall in the measured diffusion coefficient with decreasing pH, suggesting an apparent increase in size. Taken together with the observed rise in depolarized scattering ratio with decreasing pH, these results suggest that gastric mucin undergoes a conformational change from a random coil at pH >/= 4 to an anisotropic, extended conformation at pH < 4. The increased binding of mucin to hydrophobic fluorescent with decreasing pH indicates that the change to an extended conformation is accompanied by exposure of hydrophobic binding sites. In concentrated mucin solutions, the structure factor S(q, t) derived from DLS measurements changed from a stretched exponential decay at pH 7 to a power-law decay at pH 2, which is characteristic of a sol-gel transition. We propose that the conformational change facilitates cross-links among mucin macromolecules through hydrophobic interactions at low pH, which in turn leads to a sol-gel transition when the mucin solution is sufficiently concentrated.

Longitudinal versus cross-sectional evaluations of leukocyte telomere length dynamics: age-dependent telomere shortening is the rule

BACKGROUND

Leukocyte telomere length (LTL) is considered a biomarker of human aging and based on cross-sectional studies it shortens with age. However, longitudinal studies reported that many adults display LTL lengthening.

METHODS

Using Southern blots, we compared cross-sectional rates of age-related LTL change across a ∼20 year age range with those based on longitudinal evaluations in three surveys (S1, S2, and S3) with three time intervals: S1-S2 (5.8 years), S2-S3 (6.6 years), and S1-S3 (12.4 years). Hierarchical linear modeling was used to explore LTL dynamics using LTL data from S1, S2, and S3.

RESULTS

Cross-sectionally, mean LTL shortenings were 24.6, 25.4, and 23.6 bp/y at S1, S2, and S3, respectively. Longitudinally, more variation was observed in the rate of LTL change during the shorter than longer follow-up periods. Furthermore, using simple differences in LTL, 14.4% and 10.7% of individuals displayed LTL lengthening during S1-S2 and S2-S3, respectively, but only 1.5% during S1-S3 (p < 0.001). The estimated mean rate of LTL shortening based on averaging empirical Bayes' estimates of LTL from a parsimonious hierarchical linear modeling model was 31 bp/y with a range from 23 to 47 bp/y with none of the participants showing LTL lengthening over the average 12.4 years of follow-up.

CONCLUSIONS

As aging displays a unidirectional progression, it is unlikely that LTL elongates with age. LTL elongation in longitudinal studies primarily reflects measurement errors of LTL in relation to the duration of follow-up periods.

Smad1 interacts with homeobox DNA-binding proteins in bone morphogenetic protein signaling

Bone morphogenetic proteins (BMP) transduce their signals into the cell through a family of mediator proteins known as Smads. Upon phosphorylation by the BMP receptors, Smad1 interacts with Smad4 and translocates into the nucleus where the complex recruits DNA-binding protein(s) to activate specific gene transcription. However, the DNA-binding protein(s) involved in BMP signaling has not been identified. Using a yeast two-hybrid approach, we found that Smad1 interacts with Hoxc-8, a homeodomain transcription factor. The interaction between Smad1 and Hoxc-8 was confirmed by a "pull-down" assay and a co-immunoprecipitation experiment in COS-1 cells. Interestingly, purified Smad1 inhibited Hoxc-8 binding to the osteopontin Hoxc-8 site in a concentration-dependent manner. Transient transfection studies showed that native osteopontin promoter activity was elevated upon BMP stimulation. Consistent with the gel shift assay, overexpression of Hoxc-8 abolished the BMP stimulation. When a wild type or mutant Hoxc-8 binding element was linked to an SV40 promoter-driven reporter gene, the wild type but not the mutant Hoxc-8 binding site responded to BMP stimulation. Again, overexpression of Hoxc-8 suppressed the BMP-induced activity of the wild type reporter construct. Our findings suggest that Smad1 interaction with Hoxc-8 dislodges Hoxc-8 from its DNA binding element, resulting in the induction of gene expression.