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Biomedical subjects

Dirk Bohmann

Publications and source records attributed to Dirk Bohmann.

At least 19 recordsLinked to original sources
PubMed2006

Chameau HAT and DRpd3 HDAC function as antagonistic cofactors of JNK/AP-1-dependent transcription during Drosophila metamorphosis.

Gene regulation by AP-1 transcription factors in response to Jun N-terminal kinase (JNK) signaling controls essential cellular processes during development and in pathological situations. Here, we report genetic and molecular evidence that the histone acetyltransferase (HAT) Chameau and the histone deacetylase DRpd3 act as antagonistic cofactors of DJun and DFos to modulate JNK-dependent transcription during thorax metamorphosis and JNK-induced apoptosis in Drosophila. We demonstrate in cultured cells that DFos phosphorylation mediated by JNK signaling plays a central role in coordinating the dynamics of Chameau and DRpd3 recruitment and function at AP-1-responsive promoters. Activating the pathway stimulates the HAT function of Chameau, promoting histone H4 acetylation and target gene transcription. Conversely, in response to JNK signaling inactivation, DRpd3 is recruited and suppresses histone acetylation and transcription. This study establishes a direct link among JNK signaling, DFos phosphorylation, chromatin modification, and AP-1-dependent transcription and its importance in a developing organism.

Acetylation↗
PubMed2006

JNK signaling coordinates integrin and actin functions during Drosophila embryogenesis.

Epithelial movements are key morphogenetic events in animal development. They are driven by multiple mechanisms, including signal-dependent changes in cytoskeletal organization and in cell adhesion. Such processes must be controlled precisely and coordinated to accurately sculpt the three-dimensional form of the developing organism. By observing the Drosophila epidermis during embryonic development using confocal time-lapse microscopy, we have investigated how signaling through the Jun-N-terminal kinase (JNK) pathway governs the tissue sheet movements that result in dorsal closure (DC). We find that JNK controls the polymerization of actin into a cable at the epidermal leading edge as previously suggested, as well as the joining (zipping) of the contralateral epithelial cell sheets. Here, we show that zipping is mediated by regulation of the integrins myospheroid and scab. Our data demonstrate that JNK signaling regulates a set of target genes that cooperate to facilitate epithelial movement and closure.

Actins↗
PubMed2005

Non-cell-autonomous induction of tissue overgrowth by JNK/Ras cooperation in a Drosophila tumor model.

The role of c-Jun N-terminal kinase (JNK) signaling in cancer is enigmatic, and both tumor-promoting and tumor-suppressing functions have been ascribed to JNK pathway components. We have used the Drosophila eye to investigate the function of the JNK pathway in three different tumor models of increasing malignancy. Benign lesions caused by loss of the neoplastic tumor suppressor gene scribble can efficiently be eliminated by JNK-induced apoptosis. In such a scenario, the eye reverts to a wild-type phenotype, indicating that the JNK pathway prevents tumor formation. The situation changes in the case of aggressive tissue overgrowth, which can be induced by oncogenic activation of the Ras/Raf pathway in the eye, or in malignant invasive tumors resulting when Raf activation is combined with loss of scribble. The growth of these more aggressive tumor types is significantly, yet incompletely, suppressed by JNK-mediated apoptosis. Remarkably, oncogenic Raf and JNK cooperate in these tumors, to induce massive hyperplasia in adjacent wild-type tissue. Thus, depending on the genetic context, JNK signaling can eradicate tumors by removing premalignant cells, or promote aberrant overgrowth in tissues surrounding primary lesions.

Animals↗
PubMed2005

Synaptic and genomic responses to JNK and AP-1 signaling in Drosophila neurons.

BACKGROUND: The transcription factor AP-1 positively controls synaptic plasticity at the Drosophila neuromuscular junction. Although in motor neurons, JNK has been shown to activate AP-1, a positive regulator of growth and strength at the larval NMJ, the consequences of JNK activation are poorly studied. In addition, the downstream transcriptional targets of JNK and AP-1 signaling in the Drosophila nervous system have yet to be identified. Here, we further investigated the role of JNK signaling at this model synapse employing an activated form of JNK-kinase; and using Serial Analysis of Gene Expression and oligonucleotide microarrays, searched for candidate early targets of JNK or AP-1 dependent transcription in neurons. RESULTS: Temporally-controlled JNK induction in postembryonic motor neurons triggers synaptic growth at the NMJ indicating a role in developmental plasticity rather than synaptogenesis. An unexpected observation that JNK activation also causes a reduction in transmitter release is inconsistent with JNK functioning solely through AP-1 and suggests an additional, yet-unidentified pathway for JNK signaling in motor neurons. SAGE profiling of mRNA expression helps define the neural transcriptome in Drosophila. Though many putative AP-1 and JNK target genes arose from the genomic screens, few were confirmed in subsequent validation experiments. One potentially important neuronal AP-1 target discovered, CG6044, was previously implicated in olfactory associative memory. In addition, 5 mRNAs regulated by RU486, a steroid used to trigger conditional gene expression were identified. CONCLUSION: This study demonstrates a novel role for JNK signaling at the larval neuromuscular junction and provides a quantitative profile of gene transcription in Drosophila neurons. While identifying potential JNK/AP-1 targets it reveals the limitations of genome-wide analyses using complex tissues like the whole brain.

Animals↗
PubMed2005

JNK extends life span and limits growth by antagonizing cellular and organism-wide responses to insulin signaling.

Aging of a eukaryotic organism is affected by its nutrition state and by its ability to prevent or repair oxidative damage. Consequently, signal transduction systems that control metabolism and oxidative stress responses influence life span. When nutrients are abundant, the insulin/IGF signaling (IIS) pathway promotes growth and energy storage but shortens life span. The transcription factor Foxo, which is inhibited by IIS, extends life span in conditions of low IIS activity. Life span can also be increased by activating the stress-responsive Jun-N-terminal kinase (JNK) pathway. Here we show that JNK requires Foxo to extend life span in Drosophila. JNK antagonizes IIS, causing nuclear localization of Foxo and inducing its targets, including growth control and stress defense genes. JNK and Foxo also restrict IIS activity systemically by repressing IIS ligand expression in neuroendocrine cells. The convergence of JNK signaling and IIS on Foxo provides a model to explain the effects of stress and nutrition on longevity.

Active Transport, Cell Nucleus↗
PubMed2005

DREF is required for efficient growth and cell cycle progression in Drosophila imaginal discs.

Based on overexpression studies and target gene analyses, the transcription factor DNA replication-related element factor (DREF) has been proposed to regulate growth and replication in Drosophila melanogaster. Here we present loss-of-function experiments to analyze the contribution of DREF to these processes. RNA interference-mediated extinction of DREF function in vivo demonstrates a requirement for the protein for normal progression through the cell cycle and consequently for growth of imaginal discs and the derived adult organs. We show that DREF regulates the expression of genes that are required for the transition of imaginal disc cells through S phase. In conditions of suppressed apoptosis, DREF activation can cause overgrowth of developing organs. These data establish DREF as a global regulator of transcriptional programs that mediate cell proliferation and organ growth during animal development.

Animals↗
PubMed2004

Toward the computable transcriptome.

Applying a combination of innovative approaches to understanding neuronal gene regulation in C. elegans, an article in the latest Developmental Cell (Wenick and Hobert, 2004) gives hope that reading the genome's transcriptional regulatory code may one day be possible.

Animals↗
PubMed2004

DNA damage induces downregulation of histone gene expression through the G1 checkpoint pathway.

Activation of the G(1) checkpoint following DNA damage leads to inhibition of cyclin E-Cdk2 and subsequent G(1) arrest in higher eucaryotes. Little, however, is known about the molecular events downstream of cyclin E-Cdk2 inhibition. Here we show that, in addition to the inhibition of DNA synthesis, ionizing radiation induces downregulation of histone mRNA levels in mammalian cells. This downregulation occurs at the level of transcription and requires functional p53 and p21(CIP1/WAF1) proteins. We demonstrate that DNA damage induced by ionizing radiation results in the suppression of phosphorylation of NPAT, an in vivo substrate of cyclin E-Cdk2 kinase and an essential regulator of histone gene transcription, and its dissociation from histone gene clusters in a p53/p21-dependent manner. Inhibition of Cdk2 activity by specific inhibitors in the absence of DNA damage similarly disperses NPAT from histone gene clusters and represses histone gene expression. Our results thus suggest that inhibition of Cdk2 activity following DNA damage results in the downregulation of histone gene transcription through dissociation of NPAT from histone gene clusters.

CDC2-CDC28 Kinases↗
PubMed2004

Deregulated repression of c-Jun provides a potential link to its role in tumorigenesis.

The transcription factor c-Jun cooperates with oncogenic alleles of ras in malignant transformation. Constitutively active Ras causes, via activation of mitogen activated protein kinases, phosphorylation of c-Jun which is essential for subsequent target gene activation and tumorigenesis. Studying the mechanisms controlling c-Jun activity we found that its transcription activation function is actively repressed by a presumably multimeric repressor complex that includes histone deacetylase 3 as a critical subunit. Suppression of c-Jun is relieved by MAP kinase-mediated phosphorylation and/or titration of inhibitor components. The viral tumorigenic counterpart of c-Jun, v-Jun, escapes this inhibition, suggesting deregulated transcriptional activity of c-Jun as a relevant cause for carcinogenesis.

Animals↗
PubMed2003

JNK phosphorylation relieves HDAC3-dependent suppression of the transcriptional activity of c-Jun.

The AP-1 transcription factor c-Jun is a prototypical nuclear effector of the JNK signal transduction pathway. The integrity of JNK phosphorylation sites at serines 63/73 and at threonines 91/93 in c-Jun is essential for signal-dependent target gene activation. We show that c-Jun phosphorylation mediates dissociation of an inhibitory complex, which is associated with histone deacetylase 3 (HDAC3). The subsequent events that ultimately cause increased mRNA synthesis are independent of c-Jun phosphorylation and its interaction with JNK. These findings provide an 'activation by de-repression' model as an explanation for the stimulatory function of JNK on c-Jun.

3T3 Cells↗
PubMed2003

JNK signaling confers tolerance to oxidative stress and extends lifespan in Drosophila.

Changes in the genetic makeup of an organism can extend lifespan significantly if they promote tolerance to environmental insults and thus prevent the general deterioration of cellular function that is associated with aging. Here, we introduce the Jun N-terminal kinase (JNK) signaling pathway as a genetic determinant of aging in Drosophila melanogaster. Based on expression profiling experiments, we demonstrate that JNK functions at the center of a signal transduction network that coordinates the induction of protective genes in response to oxidative challenge. JNK signaling activity thus alleviates the toxic effects of reactive oxygen species (ROS). In addition, we show that flies with mutations that augment JNK signaling accumulate less oxidative damage and live dramatically longer than wild-type flies. Our work thus identifies the evolutionarily conserved JNK signaling pathway as a major genetic factor in the control of longevity.

Aging↗
PubMed2002

Menin uncouples Elk-1, JunD and c-Jun phosphorylation from MAP kinase activation.

Menin, a nuclear protein encoded by the tumor suppressor gene MEN1, interacts with the AP-1 transcription factor JunD and inhibits its transcriptional activity. In addition, overexpression of Menin counteracts Ras-induced tumorigenesis. We show that Menin inhibits ERK-dependent phosphorylation and activation of both JunD and the Ets-domain transcription factor Elk-1. We also show that Menin represses the inducible activity of the c-fos promoter. Furthermore, Menin expression inhibits Jun N-terminal kinase (JNK)-mediated phosphorylation of both JunD and c-Jun. Kinase assays show that Menin overexpression does not interfere with activation of either ERK2 or JNK1, suggesting that Menin acts at a level downstream of MAPK activation. An N-terminal deletion mutant of Menin that cannot inhibit JunD phosphorylation by JNK, can still repress JunD phosphorylation by ERK2, suggesting that Menin interferes with ERK and JNK pathways through two distinct inhibitory mechanisms. Taken together, our data suggest that Menin uncouples ERK and JNK activation from phosphorylation of their nuclear targets Elk-1, JunD and c-Jun, hence inhibiting accumulation of active Fos/Jun heterodimers. This study provides new molecular insights into the tumor suppressor function of Menin and suggests a mechanism by which Menin may interfere with Ras-dependent cell transformation and oncogenesis.

Animals↗
PubMed2002

The DEXD/H-box RNA helicase RHII/Gu is a co-factor for c-Jun-activated transcription.

Tandem affinity purification (TAP) and mass spectrometric peptide sequencing showed that the DEAD-box RNA helicase RHII/Gu is a functional interaction partner of c-Jun in human cells. The N-terminal transcription activation region of, c-Jun interacts with a C-terminal domain of RHII/Gu. This interaction is stimulated by anisomycin treatment in a manner that is concurrent with, but independent of, c-Jun phosphorylation. A possible explanation for this effect is provided by the observation that RHII/Gu translocates from nucleolus to nucleoplasm upon anisomycin or UV treatment or when JNK signaling is activated by overexpression of a constitutively active form of MEKK1 kinase. Several experiments show that the RNA helicase activity of RHII/Gu supports c-Jun-mediated target gene activation: dominant-negative forms of RHII/Gu, as well as a neutralizing antibody against the enzyme, significantly interfered with c-Jun target gene activity but not with transcription in general. These findings clarify the mechanism of c-Jun-mediated transcriptional regulation, and provide evidence for an involvement of RHII/Gu in stress response and in RNA polymerase II-catalyzed transcription in mammalian cells.

Animals↗
PubMed2002

An essential function of AP-1 heterodimers in Drosophila development.

Fos and Jun proteins homo- or heterodimerize to form functional AP-1 transcription factors. Drosophila mutants lacking either Jun or Fos display indistinguishable dorsal open phenotypes, indicating an essential function of both Jun and Fos for embryonic dorsal closure. Here we present experiments to determine the basis for this dual requirement. By combining mutant alleles and transgenes expressing Fos and Jun variants with altered dimerization preferences, fly lines were generated in which only specifically defined dimer variants can form. Phenotypic analysis of these mutants reveals that homodimers of Fos or of Jun cannot replace the function of the heterodimeric complex. This defect is not explained by the lower stability of homodimers as compared to heterodimers, because 'pseudo-homodimers' which are as stable as native Jun-Fos heterodimers cannot substitute for their function. We conclude that Jun and Fos play complementary roles that are both required for signal transduction and gene activation during dorsal closure.

Animals↗
PubMed2002

Transrepression of AP-1 by nuclear receptors in Drosophila.

Mammalian cell culture studies have shown that several members of the nuclear receptor super family such as glucocorticoid receptor, retinoic acid receptor and thyroid hormone receptor can repress the activity of AP-1 proteins by a mechanism that does not require the nuclear receptor to bind to DNA directly, but that is otherwise poorly understood. Several aspects of nuclear receptor function are believed to rely on this inhibitory mechanism, which is referred to as transrepression. This study presents evidence that nuclear receptor-mediated transrepression of AP-1 occurs in Drosophila melanogaster. In two different developmental situations, embryonic dorsal closure and wing development, several nuclear receptors, including Seven up, Tailless, and Eagle antagonize AP-1. The inhibitory interactions with nuclear receptors are integrated with other modes of AP-1 regulation, such as mitogen-activated protein kinase signaling. A potential role of nuclear receptors in setting a threshold of AP-1 activity required for the manifestation of a cellular response is discussed.

Animals↗
PubMed2002

A genomic switch at the transition from cell proliferation to terminal differentiation in the Drosophila eye.

Organogenesis involves cell proliferation followed by complex determination and differentiation events that are intricately controlled in time and space. The instructions for these different steps are, to a large degree, implicit in the gene expression profiles of the cells that partake in organogenesis. Combining fluorescence-activated cell sorting and SAGE, we analyzed genomic expression patterns in the developing eye of Drosophila melanogaster. Genomic activity changes as cells pass from an uncommitted proliferating progenitor state through determination and differentiation steps toward a specialized cell fate. Analysis of the upstream sequences of genes specifically expressed during the proliferation phase of eye development implicates the transcription factor DREF and its inhibitor dMLF in the control of cell growth in this organ.

Animals↗