| |||
|
|
|||
|
      |
|
 |
|
| |||||||||||
| |||||||||||
| |||||||||||
| | | | ||||||||
|
In brief: January 2007Cell division Kinetochore microtubule dynamics and attachment stability are regulated by Hec1.
The conserved KMN network constitutes the core microtubule-binding site of the kinetochore.
Binding of microtubules (MTs) to the kinetochore is essential for chromosome segregation. But which components of the kinetochore complex directly bind MTs was previously unknown. Two groups now provide insights. Cheeseman et al. recently identified the KNL-1/Mis12/Ndc80 (KMN) network as being essential for kinetochore–MT interactions. By analysing the KMN network by in vitro reconstitution with purified components, they now show that the Ndc80 complex and KNL-1 have distinct, but synergistic, MT-binding activities. The MT-binding activity of the Ndc80 complex is reduced following phosphorylation of Ndc80 by Aurora B kinase, and this activity might contribute to Aurora B-dependent elimination of incorrect kinetochore–MT attachments. DeLuca et al. also show that the Ndc80 complex is essential for kinetochore–MT interactions. Specifically, the N terminus of Ndc80 (HEC1 in humans) regulates the attachment stability of the MT plus-ends to kinetochores, and plus-end polymerization and depolymerization. The N terminus of HEC1 is phosphorylated by Aurora B in vitro, and cells that express nonphosphorylatable HEC1 mutants show an increased number of erroneous MT attachments, hyperstretched centromeres (due to abnormal MT turnover) and chromosome mis-segregation. Systems biology Variability and memory of protein levels in human cells.
The variability in protein levels among cells has been well studied in prokaryotes. Sigal et al. now report fluctuations in the levels of 20 fluorescently tagged endogenous proteins in human cells. Individual protein levels varied considerably among cells, and although mixing between high and low levels occurred for all proteins, in many cases the variation persisted for several generations. In addition, the levels of proteins that function in the same biological pathway were better correlated than those in different pathways. Telomeres The DNA damage machinery and homologous recombination pathway act consecutively to protect human telomeres.
Telomeres protect chromosomes from being recognized as damaged DNA and from triggering DNA-damage checkpoints. So why do telomeres depend on the checkpoint proteins ATM and ATR? The authors show that DNA-repair factors, such as ATM and ATR, function in two independent steps at telomeres to ensure efficient telomere replication and subsequent processing of the telomere ends. Telomere ends need to be recognized as damaged DNA to complete end replication and to form correctly functioning protective caps. T cells Gfi-1 plays an important role in IL-2-mediated Th2 cell expansion.
The transcriptional repressor growth-factor independent 1 (GFI1) has been implicated in promoting clonal expansion of T helper 2 (TH2) cells in response to interleukin-2 (IL-2). This has now been confirmed by the generation of conditional knockout mice that lack Gfi1 in CD4+ T cells. CD4+ T cells from conditional Gfi1-/- mice developed normally but, when placed in TH2-cell-polarizing conditions in vitro, the clonal expansion of TH2 cells was impaired. This defect also occurred in vivo, as fewer TH2 cells developed in conditional Gfi1-/- mice than in control mice after infection with Schistosoma mansoni. Consistent with a role for GFI1 in IL-2-mediated proliferation, the activation of STAT5 (signal transducer and activator of transcription 5) by IL-2 was reduced in Gfi1-/- T cells. However, overexpression of STAT5 in Gfi1-/- T cells did not rescue the proliferation defect, indicating that GFI1 functions downstream of, or in parallel with, STAT5 signaling to achieve optimal TH2-cell expansion. Immunotherapy An essential role for Akt1 in dendritic cell function and tumor immunotherapy.
Current dendritic cell (DC)-based vaccines for tumour therapy often fall short because the transfused DCs tend to be short-lived and only transiently active. So, the authors sought ways to improve DC survival and maturation, so that better T-cell responses could be induced. When DCs were deprived of GM-CSF (granulocyte/macrophage colony-stimulating factor), the amount of AKT protein declined before DC death. Exposure of the DCs to lipopolysaccharide and CD40-specific antibody restored AKT expression and prolonged DC survival. The role for AKT in supporting DC survival and maturation was confirmed by expressing a constitutively active, lipid-raft-targeted form of AKT (MF- Autoimmunity Responses against islet antigens in NOD mice are prevented by tolerance to proinsulin but not IGRP.
Type 1 diabetes is characterized by immune responses to several autoantigens expressed by pancreatic Natural killer cells Cross-talk with myeloid accessory cells regulates human natural killer cell interferon- The early production of interferon- T-cell migration Physiological and aberrant regulation of memory T cell trafficking by the co-stimulatory molecule CD28.
It is well known that CD28 is a co-stimulatory molecule for antigen-induced T-cell activation. But does CD28 signalling have any other physiological functions? Here the authors showed that activation of CD28 on T cells induced integrin clustering and increased integrin-mediated migration in vitro. Engagement of CD28 in vivo led to the migration of memory T cells to non-lymphoid sites, which was independent of T-cell-receptor-derived signals and expression of specific homing molecules. This is in contrast to cytotoxic T-lymphocyte antigen 4 (CTLA4)-mediated signals, which prevent tissue infiltration by memory T cells. This new physiological function of CD28 has important implications for targeting CD28 in human disease therapy. Differentiation GATA-3 maintains the differentiation of the luminal cell fate in the mammary gland. During puberty, the mammary gland differentiates from multipotent progenitor cells into myoepithelial cells or luminal epithelial cells, the cell type from which breast cancer develops. Zena Werb and colleagues now show that the transcription factor GATA3 is essential to this process. Using an RNA-based genome-wide microarray screen they show that Gata3 is the most highly expressed transcription factor in mouse differentiating and mature mammary epithelium. Furthermore, a conditional Gata3 knockout mouse was used to show that the transcription factor is essential for the pubertal development and maintenance of the mammary gland. Gata3 loss in adult mice resulted in the proliferation of undifferentiated cells and the detachment of existing epithelial cells from the basement membrane. Reduced expression of GATA3 in breast cancers is associated with a poor prognosis. This work suggests that GATA3 loss might have a causal role in breast cancer progression and metastasis. Tumorigenesis Deletion of DDB1 in mouse brain and lens leads to p53-dependent elimination of proliferating cells Damaged DNA binding protein 1 (DDB1) is a component of the Cullin4A ubiquitin ligase and is involved in DNA repair, cell-cycle regulation and DNA replication. Using mouse models, Stephen Goff and colleagues showed that Ddb1 is required for embryogenesis, and the conditional inactivation of Ddb1 results in the apoptosis of proliferating cells owing to the accumulation of cell-cycle regulators and genomic instability. Conditional loss of Trp53 in Ddb1-null tissues partially suppressed apoptosis, but the rescued cells showed overproliferation and histological defects that might represent pre-neoplastic lesions. Metastasis The Spemann organizer gene, Goosecoid, promotes tumour metastasis Are regulators of gastrulation reactivated in tumour cells to confer invasive and metastatic properties? Robert Weinberg and colleagues show that GSC, which encodes goosecoid, a homeobox transcription factor involved in cell migration, was overexpressed in breast tumour samples and could be induced by transforming growth factor- RNA world A late-acting quality control process for mature eukaryotic rRNAs.
The existence of quality-control pathways for mRNAs is well established; this study now shows that ribosomal RNAs (rRNAs) are also subject to such monitoring mechanisms. The authors showed that rRNAs that contain point mutations in sites that are essential for ribosome function are significantly downregulated in Saccharomyces cerevisiae. The mechanism that is involved results in decreased stability of rRNAs that have already been assembled into ribosomes or ribosomal subunits. This provides a means by which eukaryotic cells might ensure the structural and functional integrity of the protein-translating machinery. Technology P[acman]: a BAC transgenic platform for targeted insertion of large DNA fragments in Drosophila melanogaster.
The integration of large DNA fragments into the Drosophila melanogaster genome is technically challenging, but this study describes a method that overcomes this by combining three technologies. Recombineering allows the retrieval of DNA fragments that are larger than 100 kb and their subsequent site-directed mutagenesis in conditionally amplifiable BACs. Insertion of these large BAC-carried transgenes can then be driven by the bacteriophage Cancer genomics Novel patterns of genome rearrangement and their association with survival in breast cancer.
The authors screened diploid breast tumours with a representational oligonucleotide microarray analysis (ROMA), which can detect genomic amplifications and deletions at high resolution. This technique uses a representation that is much less complex than the whole human genome, but still allows the identification of several patterns of variation in genomic copy number. Among these, the 'firestorm' signature, which is characterized by multiple amplifications that are limited to single chromosome arms, was predictive of aggressive disease and poor survival. Molecular neuroscience Phospholipase C An itch causes an irresistible urge to scratch, but the underlying mechanisms are unclear. A new study shows that activation of the histamine 1 receptor on C-fibre sensory neurons causes an increase in intracellular calcium levels, which is mediated by the enzyme phospholipase C Calcium Signaling Regional interaction of endoplasmic reticulum Ca2+ signals between soma and dendrites through rapid luminal Ca2+ diffusion.
Although the role of the endoplasmic reticulum (ER) in Ca2+ signalling events is well established, the structural and functional organization of the neuronal ER is poorly understood. Choi et al. provide strong evidence that the ER in dendrites and the soma is interconnected. Using fluorescent Ca2+ markers, they measured changes in Ca2+ concentration in distinct regions of isolated dopamine neurons after local or whole cell stimulation. The results point towards the ER as a continuous, signal-integrating organelle in which Ca2+ can move rapidly between distinct parts of the neuron. Repair The Nogo–Nogo receptor pathway limits a spectrum of adult CNS axonal growth.
Several factors expressed in the CNS inhibit axonal outgrowth; these include Nogo-A, which is mainly expressed by glial cells, and the axonal Nogo receptor NgR1. The authors assessed the regenerative capability of two strains of transgenic mice that failed to show an enhanced regenerative phenotype after spinal hemisection in previous studies: nogo-aatg/atg and ngr1-/-. This study used another lesion paradigm, unilateral pyramidotomy, and showed regeneration in both strains verified by pronounced collateral sprouting of intact corticospinal tract fibres across the midline into the denervated spinal cord. Identification of the molecular basis for this difference might reveal clues to efficient therapies after spinal cord injury. Stem cells Developmental origin of a bipotential myocardial and smooth muscle cell precursor in the mammalian heart.
Multipotent embryonic Isl1+ progenitor cells lead to cardiac, smooth muscle, and endothelial cell diversification.
Cardiac disorders represent one field in which there is currently great interest in the potential of therapeutic strategies based on stem cells, but the identification of the optimal cell types for such strategies is a major challenge. The endothelial, cardiac and smooth muscle cells needed for cardiogenesis have been thought to arise from distinct embryonic precursors. However, these two papers in Cell report multipotent cardiovascular progenitor cells that are capable of developing into two and three of these cell types, respectively, which might be valuable in therapeutic cardiovascular tissue regeneration in disorders such as heart failure. Parkinson's Disease Activation of tyrosine kinase receptor signaling pathway by rasagiline facilitates neurorescue and restoration of nigrostriatal dopamine neurons in MPTP-induced parkinsonism.
Developing agents that have the potential to modify or reverse the pathogenesis of Parkinson's disease, rather than just target the symptoms, is a major challenge. Sagi and colleagues investigated the effects of rasagiline — an approved monoamine oxidase (MAO)-B inhibitor that is known to have neuroprotective activities unrelated to its ability to inhibit MAO-B — in the standard MPTP mouse model of Parkinson's disease. They show that rasagiline has neurorescue/neurotrophic activity, and provide several lines of evidence that a mechanism involving induction of neurotrophic factors and activation of the Ras–PI3K–Akt survival pathway is important in this activity. | |
| ||||||||
| |||||||||||
 | |||||||||||
| |||||||||||
| |||||||||||
| |||||||||||
HOME | SIGNALING UPDATE | MOLECULE PAGES | DATA CENTER | ABOUT US | |||||||||||
| |||||||||||
AKT). Transfer of MF-
-cells, including proinsulin and IGRP (islet-specific glucose-6-phosphatase catalytic-subunit-related protein). But do immune responses to the individual proteins develop independently or does a response spread from one protein to another? To study this, the authors assessed the specificity of T cells arising in non-obese diabetic (NOD) mice that were made tolerant to proinsulin by overexpressing proinsulin-2 in antigen-presenting cells. These mice failed to mount T-cell responses to both proinsulin and IGRP, and this protected the mice from the development of insulitis. By contrast, mice overexpressing IGRP failed to mount responses to IGRP but still generated responses to proinsulin and developed insulitis, providing direct evidence that the immune response spreads from proinsulin to IGRP.
responses to malaria
C31 integrase, which guarantees integration at specific sites in the genome.
