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In brief: July 2007Stem cells DNA repair is limiting for haematopoietic stem cells during ageing. Deficiencies in DNA damage repair limit the function of haematopoietic stem cells with age. A principal mechanism of ageing is thought to be the inability to maintain tissue homeostasis owing to the accumulation of DNA damage in stem cells. Having discovered a mouse strain with a hypomorphic mutation in LIG4, a non-homologous end-joining (NHEJ) DNA-repair protein, Nijnik and colleagues found that inefficient NHEJ resulted in progressive loss of bone-marrow-derived stem-cell populations, as manifested through loss of erythropoiesis, a feature of normal ageing. Rossi et al. examined the consequences of deficiencies in nucleotide-excision repair, telomere maintenance and NHEJ in haematopoietic stem cells. Although deficiencies in these pathways did not affect the sizes of the stem-cell reserve populations, they did cause stem cells to lose their capacity to self-renew and proliferate following stress. These studies suggest that inherited and genetic factors that increase the levels of DNA damage might be key determinants of stem-cell capacity and, therefore, ageing. Cell cycle Maximal chromosome compaction occurs by axial shortening in anaphase and depends on Aurora kinase. During cell division, chromosome compaction begins in prometaphase and must be complete before genetic material is segregated. Now, for the first time, Ellenberg and colleagues have monitored chromosome compaction in real time using time-lapse confocal microscopy. Contrary to expectations that chromosomes are most compact in metaphase, compaction continues after sister-chromatid segregation and reaches its maximum in late anaphase. The authors propose that this additional compaction minimizes segregation errors by removing chromosome arms from the cytokinetic plane, thereby preventing the cleavage furrow from damaging them. Aurora B and dynamic microtubules are required for compaction during anaphase. Immunological synapses Opposing effects of PKC The formation of an immunological synapse is essential for T-cell priming but how it is stabilized in naive T cells is not known. The immunological synapse is a radially symmetrical structure composed of supramolecular activation clusters (SMACs). Sims et al. show that protein kinase C Signalling Proinflammatory stimuli induce IKK The inhibition of signalling pathways is important to maintain balance during an immune response, but the events that restrict inflammatory gene transcription in the nucleus are poorly understood. Protein inhibitor of activated STAT1 (PIAS1) is known to block the binding of certain pro-inflammatory transcription factors to their target genes. In this study, Liu et al. show that phosphorylation of PIAS1 at Ser90 by IKK Mast cells Mast cells promote atherosclerosis by releasing proinflammatory cytokines. Mast cells are known to accumulate in human atherosclerotic lesions, although their functional significance here has been unclear. Here, Sun et al. establish that mast cells directly participate in a mouse model of atherogenesis by releasing the pro-inflammatory cytokines interleukin-6 (IL-6) and interferon- Inflammation TRAIL limits excessive host immune responses in bacterial meningitis. In this report, Hoffmann et al. provide the first evidence that tumour-necrosis-factor-related apoptosis-inducing ligand (TRAIL) can act as an anti-inflammatory cytokine in meningitis. TRAIL has important regulatory functions in the host immune response. Examining the anti-inflammatory effects of TRAIL in mice with experimental meningitis, the authors found that TRAIL-deficient mice showed prolonged acute inflammation and increased apoptosis of leukocytes in the hippocampus. These effects were reversed by the administration of recombinant TRAIL or by reconstitution of haematopoiesis with wild-type bone-marrow cells; administering recombinant TRAIL into the subarachnoid space of wild-type mice with meningitis also reduced inflammation and apoptosis. Interestingly, patients with bacterial meningitis showed increased intrathecal synthesis of TRAIL. So, TRAIL may have therapeutic potential as an anti-inflammatory agent in invasive infections. Tumour suppressors Wilms tumour suppressor WTX negatively regulates WNT/
Angiogenesis Endothelial Akt signalling is rate-limiting for rapamycin inhibition of mouse mammary tumour progression. It has been propopsed that the mammalian target of rapamycin (mTOR) inhibitor rapamycin inhibits Akt in endothelial cells and therefore has anti-angiogenic activity. Laura Benjamin and colleagues show that, in contrast to tumour cells, endothelial cells have a bimodal Akt response to rapamycin. That is, in endothelial cells, low doses of rapamycin (1 ng ml-1) increased phosphorylated Akt (pAkt) levels, but higher doses (4–50 ng ml-1) dose-dependently decreased pAkt. However, in tumour cells, all doses of rapamycin (up to 50 ng ml-1) increased pAkt. Rapamycin inhibited tumorigenesis in a mouse model of breast cancer, and the expression of constitutively activated Akt in the endothelium of these mice reduced the anti-tumour efficacy rapamycin. As in cultured cells, tumour cells from the mice had an opposite Akt response from tumour endothelial cells after rapamycin treatment. These data indicate that the anti-tumour activity of rapamycin may be due to the inhibition of endothelial Akt signalling. Metastasis Increased tumour cell dissemination and cellular senescence in the absence of Gerhard Christofori and colleagues have used the Rip1Tag2 mouse model of pancreatic Tumour suppressors Identification of the JNK signalling pathway as a functional target of the tumour suppressor PTEN. Loss of PTEN (phosphatase and tensin homologue) in cancers leads to the activation of Akt. However, the activation of Akt alone does not fully explain all the biological consequences of PTEN loss. Using gene set enrichment analysis (GSEA) of isogenic pairs of human cancer cell lines with and without short hairpin RNA knockdown of PTEN, Charles Sawyers and colleagues have identified the JUN N-terminal kinase (JNK) pathway as a target of PTEN. JNK activity is higher in cells that lack PTEN, and genetic studies indicated that JNK and Akt are activated independently through phosphatidylinositol 3-kinase (PI3K). Furthermore, JNK inhibition was more effective in cells that lacked PTEN. In human prostate cancer tissue microarrays, phosphorylated Akt staining was strongly correlated with phosphorylated JNK. Therefore, JNK may be a crucial component of the PTEN pathway, and might be able to be exploited therapeutically. Nuclear receptors Abrogation of nuclear receptors Nr4a3 and Nr4a1 leads to development of acute myeloid leukemia. The orphan nuclear receptors NR4A1 (NUR77) and NR4A3 (NOR1) have been implicated in many processes, including apoptosis, proliferation, inflammation and differentiation in both haematopoietic and non-haematopoietic cells. Orla Conneely and colleagues show that knocking out both of these highly homologous genes in mice leads to the rapid development of acute myeloid leukaemia (AML) that is fatal by 30 days after birth. Decreased expression of NR4A1 and NR4A3 transcripts was common in leukaemic blasts from 46 patients with AML, regardless of the cytogenetic profile, indicating that therapeutic approaches involving these genes might be beneficial. Signalling Contact-dependent inhibition of EGFR signalling by Nf2/Merlin. The neurofibromatosis type 2 (NF2) protein (also known as merlin) is a tumour suppressor closely related to the ezrin/radixin/moesin (ERM) proteins. Nf2 deficiency abrogates contact-dependent inhibition of proliferation and formation of stable adherens junctions between cells. Andrea McClatchey and colleagues now show, using Nf2-/- mouse embryonic fibroblasts (MEFs), that merlin coordinates adherens junction stabilization by inhibiting epidermal growth factor receptor (EGFR) signalling in confluent cells. Merlin associates with EGFR through the PDZ-domain containing adaptor NHE-RF1, sequestering EGFR to a membrane compartment where it cannot be internalized or signal to downstream effectors. Furthermore, the proliferation of confluent Nf2-/- MEFs is inhibited by EGFR inhibitors, indicating a possible therapeutic strategy for Nf2 deficient tumours. Network biology MicroRNA-mediated feedback and feedforward loops are recurrent network motifs in mammals.
Gene regulatory networks in bacteria and yeast are made up of recurring motifs that resemble the components of electrical circuits. This paper shows that similar motifs exist in the microRNA-mediated circuits of mammals. Genome-wide expression data shows that the expression of mRNAs tends to be highly correlated, either positively or negatively, with that of the microRNAs that regulate them. Positive correlation might represent a feedback control mechanism, whereas negative correlation implies that the microRNAs provide an extra level of control to ensure robustness of gene expression. RNA interference RNA interference-mediated suppression and replacement of human rhodopsin in vivo. Mutational heterogeneity presents a challenge to the therapeutic correction of genetic disease. In mice carrying a dominant mutation in the rhodopsin gene, which is mutated in retinitis pigmentosa, the authors suppressed wild-type and mutant rhodopsin expression by RNAi. The simultaneous expression of a replacement rhodopsin gene that was refractory to RNAi owing to a modified codon composition resulted in the expression of functional rhodopsin. The same therapeutic molecules could be used to correct a range of rhodopsin mutations. Development Transcription factor modularity in a gene-centered C. elegans core neuronal protein–DNA interaction network. This study describes the mapping of a core network of transcription factors and their target genes in C. eleganssensory neurons, providing insights into how the architectures of such networks relate to their functions. The network consists of two distinct modules: one contains transcription factors that are involved in reproduction and target genes that are expressed in both neurons and other tissues, whereas the other is enriched for transcription factors with targets that are mainly expressed in neurons. Technology Genome-wide mapping of in vivo protein–DNA interactions. A new method, ChIPSeq, involving chromatin immunoprecipitation and ultra-high-throughput DNA sequencing allows high-resolution genome-wide mapping of protein–DNA interactions. ChIPSeq has several advantages over methods that couple ChIP to microarray analysis: for example, it can be performed on any sequenced genome and single-copy sites that might be under-represented in microarrays are accessible. Through ChIPSeq, the authors identified 1,946 binding sites for the neuron-restrictive silencer factor (NSFR) and several previously unknown NSFR targets. Plasticity Neuregulin-1 enhances depolarization-induced GABA release The Neuregulin-1 receptor ErbB4 controls glutamatergic synapse maturation and plasticity Mutations in Neuregulin 1 (NRG1) and ERBB4 have been associated with schizophrenia. Two studies now show that NRG1 increases the release of the inhibitory neurotransmitter GABA in cortical slices in an ERBB4-dependent manner, and that NRG1–ERBB4 signalling contributes to hippocampal glutamatergic transmission, by stabilizing AMPA receptors and dendritic spines. These findings highlight a key role for NRG1 and ERBB4 in both excitatory and inhibitory circuits in the adult brain. Neurotransmission How synaptotagmin promotes membrane fusion SNARES and synaptotagmin-1 mediate calcium-triggered fusion of synaptic vesicles with the presynaptic plasma membrane. The authors show that calcium increases local membrane curvature by inducing the insertion of synaptotagmin's two cytoplasmic domains into the lipid bilayer; this brings the vesical and plasma membranes closer together and promotes SNARE-mediated fusion. Other proteins containing these domains also induce membrane curvature upon the addition of calcium, suggesting that this might be a general mechanism for membrane fusion. Behaviour A role for brain-specific homeobox factor Bsx in the control of hyperphagia and locomotory behaviour Maintaing a stable weight requires a balance between energy expenditure and food intake. Neuropeptide Y (NPY) and agouti-related peptide (AGRP) promote feeding behaviour and weight gain. The evolutionarily conserved transcription factor BSX has now been found to regulate the expression of these peptides in the hypothalamus. BSX-deficient mice exhibit reduced expression of NPY and AGRP, are about 50% less active and show reduced hyperphagia after fasting. Loss of BSX rescues the obese phenotype of mice that are deficient in the anorexigenic peptide leptin, suggesting that BSX might be a potential target for controlling obesity. Axon growth Genetic modulation of BDNF signalling affects the outcome of axonal competition in vivo Activity-dependent competition between axons influences branch formation and stability, but the underlying molecular mechanisms are unknown. The authors show that in the mouse olfactory system, competitive neuronal environments change the axonal arborisation pattern by pruning silent axon arbors. Such elimination is regulated by the total level or activity-dependent release of brain-derived neurotrophic factor (BDNF); this effect might be mediated by the neurotrophin receptor p75(NTR). Circadian rhythms microRNA modulation of circadian-clock period and entrainment The detailed mechanisms involved in timing the circadian clock in the suprachiasmatic nucleus (SCN) are not completely understood. This study revealed that the microRNA miR-219, whose rhythmic expression is itself driven by the SCN clock, modulates the length of the circadian period, whereas miR-132, which is light-induced, regulates circadian-clock resetting by light. In vitro, both microRNAs can affect cellular excitability and modulate CLOCK- and BMAL1-dependent expression of the PER1 clock gene, providing a novel, post-translational mechanism of circadian-clock timing. Cancer MET amplification leads to gefitinib resistance in lung cancer by activating ERBB3 signaling.
Although the kinase inhibitor gefitinib is effective in cancer that has epidermal growth factor receptor (EGFR)-activating mutations, these tumours invariably develop drug resistance. Engelman and colleagues describe a gefitinib-sensitive lung-cancer cell line that developed resistance to gefitinib as a result of focal amplification of the MET kinase proto-oncogene, which led to the ERBB3-dependent activation of phosphatidylinositol 3 kinase. Inhibition of MET signalling restored gefitinib sensitivity. Thus, clinical combination of MET-kinase and EGFR-kinase inhibitors could be therapeutically beneficial. Amyloid diseases Connecting TNF- Increased brain deposition of amyloid Genomics Announcing the ENCODE results.
The Encyclopedia of DNA Elements (ENCODE) project has been a massive collaborative effort to identify and analyze the functional elements of the human genome. The June issue of Genome Research contains 25 research reports describing the validation of results from the pilot ENCODE project, using both experimental and computational methods for characterizing functional elements. A summary of ENCODE's most recent exciting biological findings can be found in Nature. RNA interference Suppressing off-target effects.
Along with small interfering RNA, morpholino phosphorodiamidate antisense oligonucleotides (MOs) are effective agents for knocking down gene expression. Off-target effects, however, can be a major problem in the application of both technologies. Robu et al. now show that one consequence of MO knockdown is the upregulation of a p53-dependent cell death pathway. They propose the concurrent inhibition of p53 as a viable strategy to minimize off-target effects resulting from MO knockdown. | |
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and WASp on symmetry breaking and relocation of the immunological synapse.
-mediated phosphorylation of PIAS1 to restrict inflammation and immunity.
B (NF-
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-catenin signalling.