A Global Vista of the Epigenomic State of the Mouse Submandibular Gland

The parotid, submandibular, and sublingual glands represent a trio of oral secretory glands whose primary function is to produce saliva, facilitate digestion of food, provide protection against microbes, and maintain oral health. While recent studies have begun to shed light on the global gene expression patterns and profiles of salivary glands, particularly those of mice, relatively little is known about the location and identity of transcriptional control elements. Here we have established the epigenomic landscape of the mouse submandibular salivary gland (SMG) by performing chromatin immunoprecipitation sequencing experiments for 4 key histone marks. Our analysis of the comprehensive SMG data sets and comparisons with those from other adult organs have identified critical enhancers and super-enhancers of the mouse SMG. By further integrating these findings with complementary RNA-sequencing based gene expression data, we have unearthed a number of molecular regulators such as members of the Fox family of transcription factors that are enriched and likely to be functionally relevant for SMG biology. Overall, our studies provide a powerful atlas of cis-regulatory elements that can be leveraged for better understanding the transcriptional control mechanisms of the mouse SMG, discovery of novel genetic switches, and modulating tissue-specific gene expression in a targeted fashion.

Database model and specification of GermOnline Release 2.0, a cross-species community annotation knowledgebase on germ cell differentiation

GermOnline is a web-accessible relational database that enables life scientists to make a significant and sustained contribution to the annotation of genes relevant for the fields of mitosis, meiosis, germ line development and gametogenesis across species. This novel approach to genome annotation includes a platform for knowledge submission and curation as well as microarray data storage and visualization hosted by a global network of servers.

AVAILABILITY

The database is accessible at http://www.germonline.org/. For convenient world-wide access we have set up a network of servers in Europe (http://germonline.unibas.ch/; http://germonline.igh.cnrs.fr/), Japan (http://germonline.biochem.s.u-tokyo.ac.jp/) and USA (http://germonline.yeastgenome.org/).

SUPPLEMENTARY INFORMATION

Extended documentation of the database is available through the link 'About GermOnline' at the websites.

GermOnline, a cross-species community knowledgebase on germ cell differentiation

GermOnline provides information and microarray expression data for genes involved in mitosis and meiosis, gamete formation and germ line development across species. The database has been developed, and is being curated and updated, by life scientists in cooperation with bioinformaticists. Information is contributed through an online form using free text, images and the controlled vocabulary developed by the GeneOntology Consortium. Authors provide up to three references in support of their contribution. The database is governed by an international board of scientists to ensure a standardized data format and the highest quality of GermOnline's information content. Release 2.0 provides exclusive access to microarray expression data from Saccharomyces cerevisiae and Rattus norvegicus, as well as curated information on approximately 700 genes from various organisms. The locus report pages include links to external databases that contain relevant annotation, microarray expression and proteome data. Conversely, the Saccharomyces Genome Database (SGD), S.cerevisiae GeneDB and Swiss-Prot link to the budding yeast section of GermOnline from their respective locus pages. GermOnline, a fully operational prototype subject-oriented knowledgebase designed for community annotation and array data visualization, is accessible at http://www.germonline.org. The target audience includes researchers who work on mitotic cell division, meiosis, gametogenesis, germ line development, human reproductive health and comparative genomics.

Benzobicyclo[8.4.0(1,6)0(8,13]tetradecenones: ring CD-taxoid models by planar tether controlled cycloaddition

[structure: see text]. A general approach to functionalized hexahydroanthracene dione skeletons is described. The planar dicarbonyl triene 15 cyclized spontaneously upon oxidation of the precursor diol 14 to the tricyclicdiketone 16. The adducts 16 and 2 were further transformed to the corresponding epoxides and oxetanes as a model for the D ring of taxoids.

CHF: a novel factor binding to cyclin A CHR corepressor element

Cell cycle modulation of cyclin A expression is due to the periodic relief of a transcriptional repression mediated by a bipartite negative DNA regulatory region. The 5' element (Cell Cycle Responsive Element: CCRE; cell Cycle Dependent Element: CDE) is clearly occupied in a cyclic manner in vivo, whereas the 3' element, whose sequence is shared by B-myb, cdc25C and cdc2 genes (cell Cycle gene Homology Region: CHR), is involved in more subtle interactions. Mutation of either element results in complete deregulation of cyclin A promoter activity. Whereas some reports claim that E2F/DP can bind to the CCRE/CDE, the nature of the protein(s) interacting with the CHR is unknown. In the present work we have characterized an activity present in quiescent cells and absent in cells blocked in S phase, which binds specifically to cyclin A CHR, but not to B-myb, or to cdc25C, or to cdc2 CHRs. A 90 kD protein, named CHF (cyclin A CHR binding factor), has been identified through preparative electrophoresis and UV crosslinking experiments. In order to address in more functional terms the binding of CHF to cyclin A CHR, we developed in vitro and in vivo oligonucleotide competition assays. Both in vitro transcription and in vivo microinjection experiments demonstrate that a functional difference exists between the composite CCRE/CDE-CHR repressor regions of cell cycle regulated genes such as cyclin A and cdc25C.

Inhibition of mammalian cell proliferation by genetically selected peptide aptamers that functionally antagonize E2F activity

The p16-cyclin D-pRB-E2F pathway is frequently deregulated in human tumors. This critical regulatory pathway controls the G1/S transition of the mammalian cell cycle by positive and negative regulation of E2F-responsive genes required for DNA replication. To assess the value of the transcription factors E2Fs as targets for antiproliferative strategies, we have initiated a program aiming to develop inhibitors targeting specifically these proteins in vitro and in vivo. The cellular activity of E2F is the result of the heterodimeric association of two families of proteins, E2Fs and DPs, which then bind DNA. Here, we use a two hybrid approach to isolate from combinatorial libraries peptide aptamers that specifically interact with E2Fs DNA binding and dimerization domains. One of these is a potent inhibitor of E2F binding activity in vitro and in mammalian fibroblasts, blocks cells in G1, and the free variable region from this aptamer has the same effect. Our experiments argue that the variable region of this aptamer is structured, and that it functions by binding E2F with a motif that resembles a DP heterodimerization region, and blocking E2F's association with DP. These results show that cell proliferation can be inhibited using genetically-selected synthetic peptides that specifically target protein-protein interaction motifs within cell cycle regulators. These results also emphasize the critical role of the E2F pathway for cell proliferation and might allow the design of novel antiproliferative agents targeting the cyclin/CDK-pRB-E2F pathway.

cdk1- and cdk2-mediated phosphorylation of MyoD Ser200 in growing C2 myoblasts: role in modulating MyoD half-life and myogenic activity

We have examined the role of protein phosphorylation in the modulation of the key muscle-specific transcription factor MyoD. We show that MyoD is highly phosphorylated in growing myoblasts and undergoes substantial dephosphorylation during differentiation. MyoD can be efficiently phosphorylated in vitro by either purified cdk1-cyclin B or cdk1 and cdk2 immunoprecipitated from proliferative myoblasts. Comparative two-dimensional tryptic phosphopeptide mapping combined with site-directed mutagenesis revealed that cdk1 and cdk2 phosphorylate MyoD on serine 200 in proliferative myoblasts. In addition, when the seven proline-directed sites in MyoD were individually mutated, only substitution of serine 200 to a nonphosphorylatable alanine (MyoD-Ala200) abolished the slower-migrating hyperphosphorylated form of MyoD, seen either in vitro after phosphorylation by cdk1-cyclin B or in vivo following overexpression in 10T1/2 cells. The MyoD-Ala200 mutant displayed activity threefold higher than that of wild-type MyoD in transactivation of an E-box-dependent reporter gene and promoted markedly enhanced myogenic conversion and fusion of 10T1/2 fibroblasts into muscle cells. In addition, the half-life of MyoD-Ala200 protein was longer than that of wild-type MyoD, substantiating a role of Ser200 phosphorylation in regulating MyoD turnover in proliferative myoblasts. Taken together, our data show that direct phosphorylation of MyoD Ser200 by cdk1 and cdk2 plays an integral role in compromising MyoD activity during myoblast proliferation.

[Role of the multifunctional Trio protein in the control of the Rac1 and RhoA gtpase signaling pathways]

The small GTPases Cdc42, Rac and RhoA have important regulatory roles in mediating cytoskeletal rearrangements, MAP kinase cascades and induction of G1 cell cycle progression. The activity of the GTPases is regulated by guanine nucleotide exchange factors (GEFs) which accelerate their GDP/GTP exchange rate, and thereby activate them. All the GEFs for the Rho-GTPases family share two conserved domains: the DH domain (for Dbl-homology domain) responsible for the enzymatic activity, and the PH domain, probably responsible for the proper localization of the molecule. Trio is a multifunctional protein that is comprised of two functional Rho-GEFs domains and a serine/threonine kinase domain. We have shown in vitro and in vivo that the first GEF domain (GEFD1) activates Rac1, while the second GEF domain (GEFD2) acts on RhoA. Moreover, the co-expression of both domains induces simultaneously the activation of both GTPases. To our knowledge, this is the first example of a member of the Rho-GEF family, that contains two functional exchange factor domains, with restricted and different specificity. We are currently investigating how these GEF domains are activated, by addressing the role of the PH domains in GTPases activation by Trio. We have shown that: 1) the PH1 of Trio is necessary for Rac activation by the GEFD1; 2) the PH1 of Trio targets the molecule to the cytoskeleton; 3) the GEFD1 domain of Trio binds, in a two-hybrid screen, the actin binding protein filamin. These data suggest that the PH1 targets Trio to the cytoskeleton close to Rac and its effectors, probably via interaction with the actin-binding protein filamin, consistent with a role of Trio in actin cytoskeleton remodeling.

RhoA GTPase and serum response factor control selectively the expression of MyoD without affecting Myf5 in mouse myoblasts

MyoD and Myf5 belong to the family of basic helix-loop-helix transcription factors that are key operators in skeletal muscle differentiation. MyoD and Myf5 genes are selectively activated during development in a time and region-specific manner and in response to different stimuli. However, molecules that specifically regulate the expression of these two genes and the pathways involved remain to be determined. We have recently shown that the serum response factor (SRF), a transcription factor involved in activation of both mitogenic response and muscle differentiation, is required for MyoD gene expression. We have investigated here whether SRF is also involved in the control of Myf5 gene expression, and the potential role of upstream regulators of SRF activity, the Rho family G-proteins including Rho, Rac, and CDC42, in the regulation of MyoD and Myf5. We show that inactivation of SRF does not alter Myf5 gene expression, whereas it causes a rapid extinction of MyoD gene expression. Furthermore, we show that RhoA, but not Rac or CDC42, is also required for the expression of MyoD. Indeed, blocking the activity of G-proteins using the general inhibitor lovastatin, or more specific antagonists of Rho proteins such as C3-transferase or dominant negative RhoA protein, resulted in a dramatic decrease of MyoD protein levels and promoter activity without any effects on Myf5 expression. We further show that RhoA-dependent transcriptional activation required functional SRF in C2 muscle cells. These data illustrate that MyoD and Myf5 are regulated by different upstream activation pathways in which MyoD expression is specifically modulated by a RhoA/SRF signaling cascade. In addition, our results establish the first link between RhoA protein activity and the expression of a key muscle regulator.

The two guanine nucleotide exchange factor domains of Trio link the Rac1 and the RhoA pathways in vivo

Trio contains two functional guanine nucleotide exchange factors (GEF) domains for the Rho-like GTPases and a serine/threonine kinase domain. In vitro, GEF domain 1(GEFD1) is specifically active on Rac1, while GEF domain 2 (GEFD2) targets RhoA. To determine whether Trio could activate Rac1 and RhoA in vivo, we measured the effect of Trio on Mitogen Activated Protein Kinase (MAPK) pathways and cytoskeletal rearrangements events mediated by the two GTPases. We show that: (i) the GEFD1 domain of Trio triggers the MAPK pathway leading to Jun kinase (JNK) activation and the production of membrane ruffles; (ii) co-expression of the TrioGEFD1 domain with a dominant-negative form of Rac blocked JNK induction, whereas a dominant-negative form of Cdc42 did not; (iii) a deletion mutant of TrioGEFD1 lacking a region important for exchange activity could not stimulate JNK activity; (iv) in contrast, the TrioGEFD2 domain does not stimulate JNK activity and induces the formation of stress fibers, as does activated RhoA; (v) furthermore, co-expression of both GEF domains induces simultaneously the formation of ruffles and stress fibers. Trio, therefore represents a unique member of the Rho-GEFs family possessing two functional domains of distinct specificities, that allow it to link Rho and Rac signaling pathway in vivo.

Esterase-triggered fluorescence of fluorogenic oligonucleotides

In the prooligonucleotide approach, a step of activation by cellular esterases is necessary for the removal of internucleoside phosphate masking groups and subsequent intracellular delivery of active antisense oligonucleotides. The efficacy of this approach implies that prooligonucleotides, once they are taken up by cells, are demasked by esterases during their course to their nucleic acid targets. In this regard, a method for labeling oligomers with esterase-activable fluorogenic tag was designed. The two phenolic functions of carboxyfluorescein were protected by pivaloyl groups, yielding a nonfluorescent lactone which was further activated as a N-hydroxysuccinimide ester. Two nuclease-resistant phosphorothioate 18-mer and methylphosphonate 19-mer oligodeoxynucleosides were attached to this biprotected fluorescein derivative via an amino linker at the 5'-end of the oligomers. The two conjugates were assayed for their carboxyesterase substrate ability in different biological media. In the presence of purified esterases or when incubated in serum or cell extracts, both oligonucleotide conjugates became fluorescent. In addition, the phosphorothioate oligoconjugate was microinjected into the cytoplasm of human fibroblasts, and a fast cytoplasmic release of fluorescence was observed with a rapid translocation of the fluorescent oligomer into the nucleus.

Importance of the Chiral Centers of Jasmonic Acid in the Responses of Plants (Activities and Antagonism between Natural and Synthetic Analogs)

The importance of the two chiral centers at C-3 and C-7 in the molecular structure of jasmonic acid in plant responses was investigated. We separated methyl jasmonate (MeJA) into (3R)- and (3S)-isomers with a fixed stereochemistry at C-3, but epimerization at C-7 is possible. The four isomers of the nonepimerizable analog 7-methyl MeJA were synthesized. These six esters and their corresponding acids were tested in three bioassays: (a) senescence in sunflower (Helianthus annuus) cotyledons; (b) proteinase inhibitor II gene expression in transgenic tobacco (Nicotiana tabacum) with [beta]-glucuronidase as a biochemical reporter; and (c) seed germination in Brassica napus and wheat (Triticum aestivum). The esters and acids had similar activities in the three assays, with the ester being more effective than its acid. The (3R)-stereochemistry was critical for jasmonate activity. Although activity was reduced after substituting the C-7 proton with a methyl group, the analogs with (3R,7R)- or (3R,7S)-stereochemistry were active in some of the assays. Although the four isomers of 7-methyl MeJA were inactive or only weakly active in the senescence assay, they could overcome the senescence-promoting effect of (3R)-MeJA. The strongest antagonistic effect was observed with the (3R,7S)-isomer.

Characterization and physiological importance of a novel cell cycle regulated protein kinase in Xenopus laevis oocytes that phosphorylates cyclin B2

We have partially purified a specific cyclin B2 kinase (cyk) from prophase oocytes of Xenopus laevis after an ATP-gamma-S activation step. Phosphopeptide analysis identified Ser53 as the major in vitro phosphorylation site for cyk in cyclin B2. Using a synthetic peptide derived from cyclin B2 encompassing Ser53 (cyktide) as a substrate, cyk was shown to be activated during progesterone-induced maturation, with a peak of activity between 40 and 50% maturation. A sustained high cyk activity was observed in oscillating egg extracts. Microinjection of cyk-phosphorylated cyclin B2 into prophase oocytes accelerated progesterone-induced maturation by about 2 h, indicating that cyclin B2 is a relevant substrate for cyk and that the function of cyk is situated upstream of cdc2-cyclin B activation. Microinjection of cyk-phosphorylated cyktide or a combination of cyk and cyclin B1 into G2 fibroblasts induced significant changes in cell morphology, reminiscent of a premature prophase-like phenotype. Similarly, addition of cyk-phosphorylated cyktide in cyclin B1-dependent interphase extracts resulted in histone H1 kinase activation.

Growth and differentiation of C2 myogenic cells are dependent on serum response factor

In order to study to what extent and at which stage serum response factor (SRF) is indispensable for myogenesis, we stably transfected C2 myogenic cells with, successively, a glucocorticoid receptor expression vector and a construct allowing for the expression of an SRF antisense RNA under the direction of the mouse mammary tumor virus long terminal repeat. In the clones obtained, SRF synthesis is reversibly down-regulated by induction of SRF antisense RNA expression by dexamethasone, whose effect is antagonized by the anti-hormone RU486. Two kinds of proliferation and differentiation patterns have been obtained in the resulting clones. Some clones with a high level of constitutive SRF antisense RNA expression are unable to differentiate into myotubes; their growth can be blocked by further induction of SRF antisense RNA expression by dexamethasone. Other clones are able to differentiate and are able to synthesize SRF, MyoD, myogenin, and myosin heavy chain at confluency. When SRF antisense RNA expression is induced in proliferating myoblasts by dexamethasone treatment, cell growth is blocked and cyclin A concentration drops. When SRF antisense RNA synthesis is induced in arrested confluent myoblasts cultured in a differentiation medium, cell fusion is blocked and synthesis of not only SRF but also MyoD, myogenin, and myosin heavy chain is inhibited. Our results show, therefore, that SRF synthesis is indispensable for both myoblast proliferation and myogenic differentiation.

Expression and activity of serum response factor is required for expression of the muscle-determining factor MyoD in both dividing and differentiating mouse C2C12 myoblasts

To understand the mechanism by which the serum response factor (SRF) is involved in the process of skeletal muscle differentiation, we have assessed the effect of inhibiting SRF activity or synthesis on the expression of the muscle-determining factor MyoD. Inhibition of SRF activity in mouse myogenic C2C12 cells through microinjection of either the SRE oligonucleotide (which acts by displacing SRF proteins from the endogenous SRE sequences), purified SRF-DB (a 30-kDa portion of SRF containing the DNA-binding domain of SRF, which acts as a dominant negative mutant in vivo), or purified anti-SRF antibodies rapidly prevents the expression of MyoD. Moreover, the rapid shutdown of MyoD expression after in vivo inhibition of SRF activity is observed not only in proliferating myoblasts but also in myoblasts cultured under differentiating conditions. Additionally, by using a cellular system expressing a glucocorticoid-inducible antisense-SRF (from aa 74 to 244) we have shown that blocking SRF expression by dexamethasone induction of antisense SRF results in the lack of MyoD expression as probed by both immunofluorescence and Northern blot analysis. Taken together these data demonstrate that SRF expression and activity are required for the expression of the muscle-determining factor MyoD.

Solid-phase synthesis and cellular localization of a C- and/or N-terminal labelled peptide

We report the solid-phase synthesis by the Fmoc strategy of a peptide containing a cysteamide group at its C-terminus. This peptide was subjected to further modifications including the linkage of fluorophores, namely lucifer yellow and coumarin respectively, at the C- and/or N-terminals. After incubation with living cultured cells these two probes were localized and it is concluded that the post-synthesis modifications can strongly modify the localization of the peptide.

Regulation of transcription factor localization: fine-tuning of gene expression

Import of 'nuclear' proteins into the nucleus, in particular, transcription factors, is not a constitutive process; instead it appears to be modulated in response to external stimuli, cell-cycle progression and developmental cues. Examples of such regulation involve direct phosphorylation of the transported protein, masking of the nuclear localization signal(s), cytoplasmic retention by binding to an anchoring protein, modulation of the import machinery itself and possible interplay between these different mechanisms. As such, nucleo-cytoplasmic traffic constitutes an important regulatory checkpoint in the control of gene expression.

TGF-beta 1 and cAMP attenuate cyclin A gene transcription via a cAMP responsive element through independent pathways

Transforming growth factor beta (TGF-beta) is a potent inhibitor of the proliferation of many cell lines. The expression of Cyclin A is down-regulated by TGF-beta 1 in Chinese hamster lung fibroblasts and most of this effect is mediated at the transcriptional level through a cAMP-responsive element (CRE), but does not require a functional cAMP-dependent protein kinase. However, activation of the cAMP pathway in these cells gives rise to a strong inhibition of proliferation, paralleled by a down-regulation of Cyclin A promoter activity. This effect requires the integrity of the CRE, suggesting a role for CRE-binding proteins in late G1/S controls.

Delayed cyclin A and B1 degradation in non-transformed mammalian cells

Cyclins A and B are known to exhibit significant differences in their function, cellular distribution and timing of degradation at mitosis. On the basis of observations in marine invertebrates and Xenopus, it was proposed that cyclin destruction triggers cdc2 kinase inactivation and anaphase onset. However, this model has recently been questioned, both in Xenopus and in budding yeast. In this report, we present evidence for delayed degradation of both cyclins A and B1 in non-transformed mammalian cells. Indeed, by means of indirect immunofluorescence and confocal microscopy, we show that cyclins A and B1 are present up to anaphase in REF52, Hs68, human primary fibroblasts and NRK epithelial cells. In marked contrast, cyclin A is shown to be degraded within metaphase and cyclin B just at the transition to anaphase in HeLa and two transformed cell lines, derivatives of normal NRK and REF52. These results further support the notion that cyclin destruction might be not correlated with anaphase onset in normal cells and highlight a significant difference in the fate of mitotic cyclins between transformed and non-transformed cells.

Abscisic Acid Structure-Activity Relationships in Barley Aleurone Layers and Protoplasts (Biological Activity of Optically Active, Oxygenated Abscisic Acid Analogs)

Optically active forms of abscisic acid (ABA) and their oxygenated metabolites were tested for their biological activity by examining the effects of the compounds on the reversal of gibberellic acid-induced [alpha]-amylase activity in barley (Hordeum vulgare cv Himalaya) aleurone layers and the induction of gene expression in barley aleurone protoplasts transformed with a chimeric construct containing the promoter region of an albumin storage protein gene. Promotion of the albumin storage protein gene response had a more strict stereochemical requirement for elicitation of an ABA response than inhibition of [alpha]-amylase gene expression. The naturally occurring stereoisomer of ABA and its metabolites were more effective at eliciting an ABA-like response. ABA showed the highest activity, followed by 7[prime]-hydroxyABA, with phaseic acid being the least active. Racemic 8[prime]-hydroxy-2[prime],3[prime]-dihydroABA, an analog of 8[prime]-hydroxyABA, was inactive, whereas racemic 2[prime],3[prime]-dihydroABA was as effective as ABA. The differences in response of the same tissue to the ABA enantiomers lead us to conclude that there exists more than one type of ABA receptor and/or multiple signal transduction pathways in barley aleurone tissue.

Two nuclear localization signals present in the basic-helix 1 domains of MyoD promote its active nuclear translocation and can function independently

MyoD, a member of the family of helix-loop-helix myogenic factors that plays a crucial role in skeletal muscle differentiation, is a nuclear phosphoprotein. Using microinjection of purified MyoD protein into rat fibroblasts, we show that the nuclear import of MyoD is a rapid and active process, being ATP and temperature dependent. Two nuclear localization signals (NLSs), one present in the basic region and the other in the helix 1 domain of MyoD protein, are demonstrated to be functional in promoting the active nuclear transport of MyoD. Synthetic peptides spanning these two NLSs and biochemically coupled to IgGs can promote the nuclear import of microinjected IgG conjugates in muscle and nonmuscle cells. Deletion analysis reveals that each sequence can function independently within the MyoD protein since concomittant deletion of both sequences is required to alter the nuclear import of this myogenic factor. In addition, the complete cytoplasmic retention of a beta-galactosidase-MyoD fusion mutant protein, double deleted at these two NLSs, argues against the existence of another functional NLS motif in MyoD.

Nuclear localization of the testis determining gene product SRY

We have studied the expression of the human SRY protein (termed p27SRY) in two different cell lines by using specific antibodies. Confocal microscopy enabled us to localize p27SRY precisely in the nucleus in a discrete punctuate pattern. Furthermore, through microinjection experiments, we have demonstrated that the localization of the p27SRY protein into the nucleus was an event involving the NH2-terminal part of the high mobility group (HMG) domain. With the help of several synthetic peptides and various p27SRY mutants, we have characterized a bipartite basic motif in this part of the protein corresponding to a nuclear localization signal. This nuclear localization signal appears to be highly conserved in SRY box- and HMB box-containing proteins, suggesting common properties of nuclear targeting within the HMG box protein family.

Implications for cAMP-dependent protein kinase in the maintenance of the interphase state

The cAMP dependent protein kinase (A-kinase) is one of the first and best studied kinases in mammalian cells. There is extensive evidence that A-kinase activity acts antagonistically toward mitotic entry both in oocyte and somatic cells. Firstly, A-kinase seems to directly compromise the activation process of the cdc2 cyclin B mitotic kinase. Secondly, as shown by specific in vivo inhibition of A-kinase using microinjection of a stable form of its inhibitor peptide PKI, A-kinase modulates several key interphase cellular processes including cytoskeletal dynamics, transcription, chromatin structure and nuclear localization. We discuss the potential mechanisms involved in the down regulation of A-kinase activity at the interphase/mitosis transition.

The serum response factor nuclear localization signal: general implications for cyclic AMP-dependent protein kinase activity in control of nuclear translocation

We have identified a basic sequence in the N-terminal region of the 67-kDa serum response factor (p67SRF or SRF) responsible for its nuclear localization. A peptide containing this nuclear localization signal (NLS) translocates rabbit immunoglobulin G (IgG) into the nucleus as efficiently as a peptide encoding the simian virus 40 NLS. This effect is abolished by substituting any two of the four basic residues in this NLS. Overexpression of a modified form of SRF in which these basic residues have been mutated confirms the absolute requirement for this sequence, and not the other basic amino acid sequences adjacent to it, in the nuclear localization of SRF. Since this NLS is in close proximity to potential phosphorylation sites for the cAMP-dependent protein kinase (A-kinase), we further investigated if A-kinase plays a role in the nuclear location of SRF. The nuclear transport of SRF proteins requires basal A-kinase activity, since inhibition of A-kinase by using either the specific inhibitory peptide PKIm or type II regulatory subunits (RII) completely prevents the nuclear localization of plasmid-expressed tagged SRF or an SRF-NLS-IgG conjugate. Direct phosphorylation of SRF by A-kinase can be discounted in this effect, since mutation of the putative phosphorylation sites in either the NLS peptide or the encoded full-length SRF protein had no effect on nuclear transport of the mutants. Finally, in support of an implication of A-kinase-dependent phosphorylation in a more general mechanism affecting nuclear import, we show that the nuclear transport of a simian virus 40-NLS-conjugated IgG or purified cyclin A protein is also blocked by inhibition of A-kinase, even though neither contains any potential sites for phosphorylation by A-kinase or can be phosphorylated by A-kinase in vitro.

Identification of susceptibility loci contributing to a complex disease using conventional segregation, linkage, and association methods

We set out to apply conventional analytic methods to a GAW data set of nuclear families with an oligogenic disease that has a population prevalence of 0.023. We chose methods generally applied to disorders with at least one major gene. Our approaches included: 1) complex segregation analysis under two models of ascertainment, 2) linkage analysis assuming either a single-locus trait with possible genetic heterogeneity or a two-locus trait, and 3) allelic association studies using both a case/control approach and the haplotype relative risk (HRR) test. The association study was the only analysis of the three that provided evidence for genes playing a role in the etiology of this disorder.

Response of Cultured Maize Cells to (+)-Abscisic Acid, (-)-Abscisic Acid, and Their Metabolites

The metabolism and effects of (+)-S- and (-)-R-abscisic acid (ABA) and some metabolites were studied in maize (Zea mays L. cv Black Mexican Sweet) suspension-cultured cells. Time-course studies of metabolite formation were performed in both cells and medium via analytical high-performance liquid chromatography. Metabolites were isolated and identified using physical and chemical methods. At 10 [mu]M concentration and 28[deg] C, (+)-ABA was metabolized within 24 h, yielding natural (-)-phaseic acid [(-)-PA] as the major product. The unnatural enantiomer (-)-ABA was less than 50% metabolized within 24 h and gave primarily (-)-7[prime]-hydroxyABA [(-)-7[prime]-HOABA], together with (+)-PA and ABA glucose ester. The distribution of metabolites in cells and medium was different, reflecting different sites of metabolism and membrane permeabilities of conjugated and nonconjugated metabolites. The results imply that (+)-ABA was oxidized to (-)-PA inside the cell, whereas (-)-ABA was converted to (-)-7[prime]-HOABA at the cell surface. Growth of maize cells was inhibited by both (+)- and (-)-ABA, with only weak contributions from their metabolites. The concentration of (+)-ABA that caused a 50% inhibition of growth of maize cells was approximately 1 [mu]M, whereas that for its metabolite (-)-PA was approximately 50 [mu]M. (-)-ABA was less active than (+)-ABA, with 50% growth inhibition observed at about 10 [mu]M. (-)-7[prime]-HOABA was only weakly active, with 50% inhibition caused by approximately 500 [mu]M. Time-course studies of medium pH indicated that (+)-ABA caused a transient pH increase (+0.3 units) at 6 h after addition that was not observed in controls or in samples treated with (-)-PA. The effect of (-)-ABA on medium Ph was marginal. No racemization at C-1[prime] of (+)-ABA, (-)-ABA, or metabolites was observed during the studies.

Effects of Abscisic Acid Metabolites and Analogs on Freezing Tolerance and Gene Expression in Bromegrass (Bromus inermis Leyss) Cell Cultures

Optical isomers and racemic mixtures of abscisic acid (ABA) and the ABA metabolites abscisyl alcohol (ABA alc), abscisyl aldehyde (ABA ald), phaseic acid (PA), and 7[prime]hydroxyABA (7[prime]OHABA) were studied to determine their effects on freezing tolerance and gene expression in bromegrass (Bromus inermis Leyss) cell-suspension cultures. A dihydroABA analog (DHABA) series that cannot be converted to PA was also investigated. Racemic ABA, (+)-ABA, ([plus or minus])-DHABA, and (+)-DHABA were the most active in inducing freezing tolerance, (-)-ABA, ([plus or minus])-7[prime]OHBA, (-)-DHABA, ([plus or minus])-ABA ald, and ([plus or minus])-ABA alc had a moderate effect, and PA was inactive. If the relative cellular water content decreased below 82%, dehydrin gene expression increased. Except for (-)-ABA, increased expression of dehydrin genes and increased accumulation of responsive to ABA (RAB) proteins were linked to increased levels of frost tolerance. PA had no effect on the induction of RAB proteins; however, ([plus or minus])- and (+)-DHABA were both active, which suggests that PA is not involved in freezing tolerance. Both (+)-ABA and (-)-ABA induced dehydrin genes and the accumulation of RAB proteins to similar levels, but (-)-ABA was less effective than (+)-ABA at increasing freezing tolerance. The (-)-DHABA analog was inactive, implying that the ring double bond is necessary in the (-) isomers for activating an ABA response.

Nuclear import of the myogenic factor MyoD requires cAMP-dependent protein kinase activity but not the direct phosphorylation of MyoD

MyoD is a nuclear phosphoprotein that belongs to the family of myogenic regulatory factors and acts in the transcriptional activation of muscle-specific genes. We have investigated the role of cAMP-dependent protein kinase (A-kinase) in modulating the nuclear locale of MyoD. Purified MyoD protein microinjected into the cytoplasm of rat embryo fibroblasts is rapidly translocated into the nucleus. Inhibition of A-kinase activity through injection of the specific inhibitory peptide PKI prevents this nuclear localisation. This inhibition of nuclear location is specifically reversed by injection of purified A-kinase catalytic subunit, showing the requirement for A-kinase in the nuclear import of MyoD. Site-directed mutagenesis of all the putative sites for A-kinase-dependent phosphorylation on MyoD, substituting serine or threonine residues for the non-phosphorylatable amino acid alanine, had no effect on nuclear import of mutated MyoD. These data exclude the possibility that the effect of A-kinase on the nuclear translocation of MyoD is mediated by direct phosphorylation of MyoD and imply that A-kinase operates through phosphorylation of components involved in the nuclear transport of MyoD.

Phosphorylated CREB binds specifically to the nuclear protein CBP

Cyclic AMP-regulated gene expression frequently involves a DNA element known as the cAMP-regulated enhancer (CRE). Many transcription factors bind to this element, including the protein CREB, which is activated as a result of phosphorylation by protein kinase A. This modification stimulates interaction with one or more of the general transcription factors or, alternatively, allows recruitment of a co-activator. Here we report that CREB phosphorylated by protein kinase A binds specifically to a nuclear protein of M(r) 265K which we term CBP (for CREB-binding protein). Fusion of a heterologous DNA-binding domain to the amino terminus of CBP enables the chimaeric protein to function as a protein kinase A-regulated transcriptional activator. We propose that CBP may participate in cAMP-regulated gene expression by interacting with the activated phosphorylated form of CREB.

Serum response factor p67SRF is expressed and required during myogenic differentiation of both mouse C2 and rat L6 muscle cell lines

The 67-kD serum response factor (p67SRF) is a ubiquitous nuclear transcription factor that acts by direct binding to a consensus DNA sequence, the serum response element (SRE), present in the promoter region of numerous genes. Although p67SRF was initially implicated in the activation of mitogen-stimulated genes, the identification of a sequence similar to SRE, the CArG box motif, competent to interact with SRE binding factors in many muscle-specific genes, has led to speculation that, in addition to its function in cell proliferation, p67SRF may play a role in muscle differentiation. Indirect immunofluorescence using affinity-purified antibodies specifically directed against p67SRF reveals that this factor is constitutively expressed and localized in the nucleus of two skeletal muscle cell lines: rat L6 and mouse C2 myogenic cells during myogenic differentiation. This result was further confirmed through immunoblotting and Northern blot analysis. Furthermore, specific inhibition of p67SRF in vivo through microinjection of purified p67SRF antibodies prevented the myoblast-myotube transition and the expression of muscle-specific genes such as the protein troponin T. We further showed that anti-p67SRF injection also inhibited the expression of the myogenic factor myogenin, implying an early requirement for p67SRF in muscle differentiation. These results demonstrate that p67SRF is involved in the process of skeletal muscle differentiation. The potential action of p67SRF via CArG sequences is discussed.

Generation of full-length cDNA recombinant vectors for the transient expression of human fibronectin in mammalian cell lines

In order to study the roles of the different alternatively spliced variants of human fibronectin (FN) as well as of its binding sites and structural domains in the processes of extracellular matrix assembly, cell adhesion, and cell migration, we constructed expression vectors coding for different human full-length FN polypeptides and deleted versions of these constructs. We expressed them transiently in mammalian cells by calcium phosphate transfection and microinjection techniques. While the deleted recombinants were expressed by both transfection and microinjection, the full-length recombinants could be only expressed by microinjection. Calcium phosphate transfection leads to the accumulation of recombinant FN in cytoplasmic vesicles of both matrix-forming and nonforming cells. On the contrary, microinjection leads to the accumulation of recombinant FN in cytoplasmic vesicles in cells that do not form a matrix, but to the rapid incorporation into the extracellular matrix of matrix-forming cells in addition to a cytoplasmic localization. Identical results were obtained when the FN signal and propeptides were replaced by those of E-cadherin.

Nuclear localization of c-Fos, but not v-Fos proteins, is controlled by extracellular signals

We report here that transport of the protein product of the c-fos proto-oncogene from the cytoplasm, where it is synthesized, into the nucleus, where it operates as part of the AP-1 transcription complex, is not spontaneous but depends on the continuous stimulation of cells by serum factors. A labile protein inhibitor of transport, the effect of which is reversed by cAMP, is responsible for retention of c-Fos protein within the cytoplasm of serum-starved fibroblasts. In contrast, v-Fos proteins transduced by the murine retroviruses FBJ and FBR, which remain nuclear in the absence of serum, evade the translocation control, which therefore appears to contribute to their tumorigenic potential.