 |
|
|
|
|
|
|
|
      |
|
 |
|
| |||||||||||
| |||||||||||
| |||||||||||
| | | | ||||||||
|
In brief: June 2007Endocytosis A selective activity-dependent requirement for dynamin-1 in synaptic vesicle endocytosis. Dynamin-1 is a neuron-specific GTPase that is thought to mediate membrane fission during synaptic-vesicle endocytosis. De Camilli and colleagues now shed light on the functions of dynamin-1 in vivo by generating dynamin-1-knockout mice. Dynamin-1-/- mice died soon after birth. Compared with wild-type mice, rates of neuronal endocytosis were similar under resting conditions but were severely impaired during high levels of stimulation. Therefore, neurons have limited dynamin-1-independent mechanisms for vesicle retrieval. Cell division Alp7/TACC is a crucial target in Ran-GTPase-dependent spindle formation in fission yeast. Pim1, which forms part of the Ran-GTPase-dependent nuclear-transport machinery, regulates spindle formation in fission yeast. But the target of Ran for spindle formation is unknown. The authors showed that the microtubule-associated protein Alp7 is transported in the nucleus by the Ran system, and that it localizes close to the spindle pole body. alp7 mutants have mitotic defects that resemble those of pim1 mutants. Forced localization of Alp7 in the nucleus of pim1 mutants suppressed the mitotic defects and restored normal spindle formation. Epigenetics DNA methylation affects nuclear organization, histone modifications, and linker histone binding but not chromatin compaction. DNA methylation has been implicated in chromatin condensation and nuclear organization. Gilbert et al. used mouse embryonic stem cells that completely lacked DNA methylation to investigate the impact of this epigenetic mark. Cells that lacked DNA methylation showed increased clustering of chromocentres but, surprisingly, the compaction of chromatin was unaffected. The primary structure differed from wild-type cells at two levels; the authors observed increased levels of histone acetylation and decreased levels of histone H3 Lys9 dimethylation, as well as altered binding of linker histones. Evolution Evolution and diversification of lamprey antigen receptors: evidence for involvement of an AID-APOBEC family cytosine deaminase.
Jawless vertebrates, including lampreys and hagfish, do not have immunoglobulins but instead have variable lymphocyte receptors (VLRs). VLRs are somatically rearranged receptors composed of diverse leucine-rich repeats but the mechanism of their assembly is not known. Here, Rogozin and colleagues assembled a draft of the sea lamprey genome. Comparison of genomic sequences with mature VLR sequences revealed that mature VLRs are assembled from multiple genomic cassettes in a gene-conversion-like process. The authors also identified genes encoding two members of the AID-APOBEC cytosine deaminase family and found that these putative deaminases might be involved in VLR diversification and could therefore represent a primordial mechanism of the AID-induced DNA strand breaks that are required for immunoglobulin gene conversion. T-cell development RANK signals from CD4+3- inducer cells regulate development of Aire-expressing epithelial cells in the thymic medulla.
Central tolerance occurs in the thymus when developing thymocytes interact with specialized medullary thymic epithelial cells (mTECs) which express autoimmune regulator (AIRE), a transcription factor that controls the expression of tissue-restricted antigens by mTECs. To date, the molecular mechanisms controlling mTEC development have been undefined. Now, Rossi and colleagues show that CD4+CD3- lymphoid-tissue inducer cells — which have previously been associated with the development and function of secondary lymphoid tissue — are present in the thymus and express the RANK (receptor activator of nuclear factor- Autoimmunity TH17 cells contribute to uveitis and scleritis and are expanded by IL-2 and inhibited by IL-27/STAT1.
This study identifies an important role for interleukin-17 (IL-17)-producing T helper cells (TH17 cells) in human uveitis and scleritis. The authors found that the blood of patients with uveitis or scleritis contained more TH17 cells than the blood of healthy individuals. This expansion was driven by IL-2 but inhibited by interferon- Immune regulation Type I interferons protect neonates from acute inflammation through interleukin 10-producing B cells.
Newborns and infants are highly vulnerable to infection, even though their immune systems are fairly well developed. As shown here, this might be because a CD5+ B-cell subset, which is prevalent in neonates but rare in adults, limits the production of pro-inflammatory cytokines by dendritic cells (DCs) in response to microorganisms by producing copious amounts of the regulatory cytokine interleukin-10 (IL-10). In addition, IL-10 production by CD5+ B cells protected neonatal mice from overexuberant responses following an acute inflammatory challenge that was otherwise lethal in adult mice and in neonatal mice that lack CD5+ B cells or IL-10. Interestingly, rather than promoting inflammation, DC-derived type I interferons stimulated the secretion of IL-10 by CD5+ B cells, thereby acting to reinforce the regulatory loop in neonatal mice. MicroRNA The tumor suppressor microRNA let-7 represses the HMGA2 oncogene The high mobility group A2 (HMGA2) protein is overexpressed in various tumours. Lee and Dutta showed that HMGA2 mRNA is destabilized by the microRNA let-7, reducing proliferation in lung cancer cells. The tumour suppressor let-7 targets the 3' untranslated region of HMGA2 mRNA, proposed to be a site of oncogenic translocation that could result in HMGA2 overexpression. Therapy Analysis of the apoptotic and therapeutic activities of histone deacetylase inhibitors by using a mouse model of B cell lymphoma Histone deacetlyase inhibitors (HDACIs), such as vorinostat, are undergoing clinical validation as anticancer agents for use in patients with haematological malignancies and solid tumours. However, it is not clear how these inhibitors elicit an anti-tumour effect. Using the Eµ-Myc mouse model of B-cell lymphoma, Ricky Johnstone, Paul Marks and colleagues showed that vorinostat selectively induced lymphoma cell death by activating apoptosis, which was independent of p53 and death receptor pathways. However, sensitivity to vorinostat in vivo was dependent on the expression of the BH3-only pro-apoptotic proteins BID and BIM. These insights could provide a rationale for stratifying patients that are likely to respond to HDACIs. Leukemia Modeling the initiation and progression of human acute leukemia in mice John Dick and colleagues have developed a mouse model that recapitulates human mixed-lineage leukaemia (MLL). They showed that the disease is sustained by multipotent leukaemia-initiating cells characterized by rearrangements in the immunoglobulin heavy chain (IgH) genes. This mouse model provides a powerful new tool with which to dissect the mechanisms that underlie the development and progression of leukaemia. Plant genetics Multiple signals from damaged chloroplasts converge on a common pathway to regulate nuclear gene expression. Most of the genes that control the development and function of plastids and mitochondria are in the nucleus. So, as well as nuclear control of organelle gene expression, there are signals that allow nuclear gene expression to respond appropriately to physiological changes in organelles. The authors show that the three known retrogade signalling pathways between damaged Arabidopsis thaliana chloroplasts and the nucleus actually feed into the same protein in the chloroplast, GENOMES UNCOUPLED 1, which then transmits an integrated signal to the nucleus. Gene regulation Control of alternative RNA splicing and gene expression by eukaryotic riboswitches. Riboswitches, which are common in prokaryotes, are regions in mRNAs that are structurally altered when bound by a metabolite, thereby modifying gene expression. The authors now show how a riboswitch functions in a eukaryote. In Neurospora crassa, thiamine controls the expression of the genes involved in its own metabolism, such as NMT1. Thiamine binds to a riboswitch in the mRNA of NMT1 and alters its conformation, changing the availability of splice sites, which reduces the expression of the functional protein. Development The hedgehog-binding proteins GAS1 and CDO cooperate to positively regulate SHH signaling during mouse development. Gas1 extends the range of hedgehog action by facilitating its signaling. Hedgehog functions as a morphogen in several developmental contexts, but it is not fully known how the concentration gradient is refined and then interpreted into differential cellular activity. The membrane protein, growth-arrest specific 1 (GAS1) was previously thought to be a negative regulator of hedgehog signalling. However, these two papers demonstrate in several developmental contexts that GAS1 positively regulates hedgehog. They also show that hedgehog signalling then negatively influences Gas1 expression in a negative-feedback loop. This is in contrast to the previously identified feedback loop in which the hedgehog receptor PTC1 transduces the hedgehog signal to upregulate its own expression, and the excess PTC1 soaks up the hedgehog signal. The authors propose a model in which these two different feedback mechanisms act in tandem to refine the hedgehog signal for proper cell-fate specification. Ion channels TRPM8 is required for cold sensation in mice. Attenuated cold sensitivity in TRPM8 null mice. The transient receptor potential melastatin 8 cation channel (TRPM8) is thought to be involved in thermosensation. Two studies used TRPM8-deficient mice to examine the role of TRPM8 in cold sensation in vivo. TRPM8-deficient mice had greatly reduced sensitivity to innocuous cooling and to noxious cooling chemicals. However, they did not display reduced sensitivity to noxious cold, suggesting that other cold receptors are also involved in the sensation of painful cold. Addiction A molecular basis of analgesic tolerance to cannabinoids. Cannabinoids have analgesic properties, but repeated use leads to analgesic tolerance, possibly caused by downregulation of the type 1 cannabinoid receptor (CB1). Prolonged exposure to a CB1-agonist downregulated CB1 in vitro and in vivo, and this effect required expression of G-protein-associated sorting protein 1 (GASP1). Moreover, repeated agonist administration produced analgesic tolerance in mice, which was reduced in animals expressing a dominant-negative form of GASP1. This indicates that GASP1 is crucial in regulating CB1 trafficking, providing a potential target for strategies to reduce the development of analgesic tolerance to cannabinoids. Neurodegenerative disorders Ccr2 deficiency impairs microglial accumulation and accelerates progression of Alzheimer-like disease. Microglia accumulate in plaques in the brains of patients with Alzhemer's disease, but whether their presence is beneficial or harmful is unclear. CC-chemokine receptor 2 (CCR2) is expressed by microglia and may facilitate their recruitment to the brain. The authors showed that knocking out Ccr2 in a transgenic mouse model of Alzhemer's disease reduced microglial accumulation, increased the accumulation of amyloid- Learning and memory Involvement of β-site APP cleaving enzyme 1 (BACE1) in amyloid precursor protein-mediated enhancement of memory and activity-dependent synaptic plasticity.
Psychiatric disorders Behavioral phenotypes of Disc1 missense mutations in mice. The authors showed that mice with one missense mutation in the gene disrupted in schizophrenia 1 (DISC1) had a depression-like phenotype, whereas a different DISC1 mutation altered prepulse and latent inhibition, thought to model information-processing deficits in schizophrenia. The depression- and schizophrenia-like phenotypes responded to antidepressant and antipsychotic treatment, respectively. Both mutations reduced DISC1 binding to phosphodiesterase-4B (PDEB4), suggesting that altered DISC1–PDE4B binding underlies the behavioural phenotypes. This study confirms a possible involvement of DISC1 in several psychiatric disorders, which might therefore have common underlying mechanisms. Angiogenesis Inhibition of angiogenesis by the antifungal drug itraconazole.
Angiogenesis — the formation of new blood vessels — is implicated in diseases such as cancer, diabetic neuropathy and rheumatoid arthritis. By screening of a library of approved drugs, Chong and colleagues identified the antifungal drug itraconazole as an angiogenesis inhibitor with in vivo activity. In addition, the authors showed that human lanosterol 14 Learning and memory eIF2 Repeated synaptic activation results in new gene-expression patterns that lead to long-term memory (LTM). Costa-Mattioli and colleagues showed that mice with reduced phosphorylation of the translation initiation factor eIF2 Neurological disease Effects of pan- and subtype-selective N-methyl-D-aspartate receptor antagonists on cortical spreading depression in the rat: therapeutic potential for migraine.
Cortical spreading depression (CSD) — a transient wave of neuronal depolarization — is associated with the underlying pathophysiology of migraine. Peeters and colleagues showed that memantine, a clinically used N-methyl-D-aspartate (NMDA) glutamate receptor (NMDAR) blocker, and two antagonists that are selective for NMDAR containing the NR2B subunit, decreased CSD events at therapeutically relevant doses. This suggests that NMDAR antagonists might be useful for the treatment of migraine and other SD disorders. Gene regulation A tool to upregulate gene expression.
Whereas RNAi has come into its own as a means to knock down gene expression, no good corresponding tool has been available to do the opposite: upregulate gene expression. Xiao et al. describe the development of a cell-permeable synthetic transcription factor mimic, which activates gene expression in living cells by binding to a specially designed promoter. They were able to achieve a fivefold upregulation of expression in HeLa cells. Genomics A gene expression resource for fission yeast.
With the goal of understanding the multiple levels of regulation of gene expression, Lackner et al. present genome-wide data sets identifying key gene expression intermediates in the fission yeast Schizosaccharomyces pombe. Using microarray analysis, they collected data under standardized conditions, revealing new insights about systems-level regulation from transcription to translation. Techniques & applications RNA visualization in live bacterial cells using fluorescent protein complementation Localization and visualization of RNA in live bacterial cells is now possible thanks to a new method that uses a protein complementation assay. An RNA-binding protein (eIF4A) was split into two domains, each of which was fused to inactive fragments of a marker green fluorescent protein (GFP). Simultaneous binding of the eIF4A fragments to an RNA aptamer on a target RNA brings the two inactive fragments of GFP together, resulting in reconstitution of fluorescence and detection of the RNA of interest. Experiments with three different RNAs in Escherichia coli validated the technique: fluorescence depended on target RNA expression and could be quantified according to the level of gene expression; background was much lower than with similar techniques using a direct GFP reporter; and the complex assembled on the target RNA did not seem to interfere with its function or localization. The technique revealed spatial organization of transcription in E. coli. Untranslated message accumulated at the cell poles, which could be a storage site for unused RNAs. Bacterial pathogenesis Borrelia burgdorferi intercepts host hormonal signals to regulate expression of outer surface protein A The agent of Lyme disease, Borrelia burgdorferi, is spread by biting Ixodes ticks and infects mammalian hosts to complete its replication cycle. The borrelial outer-surface protein A (OspA) is required for tick colonization. OspA is highly expressed by tick-borne bacteria but is downregulated in mammalian hosts. When ticks feed on infected mammals the OspA protein is upregulated and B. burgdorferi re-colonizes ticks. Scheckelhoff and colleagues showed that the mammalian stress hormones epinephrine and norepinephrine induce OspA expression in B. burgdorferi. Blocking the action of these hormones downregulated OspA in vitro and reduced the uptake of bacteria from infected mice, but only at the later stages of infection, not at the peak of bacterial dissemination. Further work will be needed to pinpoint how B. burgdorferi senses host hormones to aid completion of its infectious cycle. | |
| ||||||||
| |||||||||||
 | |||||||||||
| |||||||||||
| |||||||||||
| |||||||||||
HOME | SIGNALING UPDATE | MOLECULE PAGES | DATA CENTER | ABOUT US | |||||||||||
| |||||||||||
B) ligand. These cells control the development of mature AIRE+ mTECs from RANK+AIRE- mTEC progenitor cells. When RANK- thymic stromal cells were transplanted into immunodeficient hosts, this resulted in autoimmunity similar to the disease that occurs when AIRE is defective, confirming a key role for RANK as a regulator of central tolerance.
(IFN
protein and accelerated disease progression in these animals. This provides support for a neuroprotective role of microglia, which are likely to be involved in amyloid-
-demethylase (14DM, CYP51) is essential for endothelial cell proliferation, suggesting that lanosterol 14DM is a new target for angiogenesis inhibitors.