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Latest Articles Include:

• Freedom of scientific expression
  - Nat Cell Biol 11(7):785 (2009)
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• Building confidence: the transition from student to professor
  - Nat Cell Biol 11(7):786 (2009)
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• Chromosome congression: on the bi-orient express
  - Nat Cell Biol 11(7):787-789 (2009)
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• Cavin fever: regulating caveolae
  - Nat Cell Biol 11(7):789-791 (2009)
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• Delta traffic takes a sh-Arp turn
  - Nat Cell Biol 11(7):791-793 (2009)
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• A reader for centromeric chromatin
  - Nat Cell Biol 11(7):793-795 (2009)
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• Research highlights
  - Nat Cell Biol 11(7):796 (2009)
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• Arabidopsis cortical microtubules position cellulose synthase delivery to the plasma membrane and interact with cellulose synthase trafficking compartments
  - Nat Cell Biol 11(7):797-806 (2009)
  Plant cell morphogenesis relies on the organization and function of two polymer arrays separated by the plasma membrane: the cortical microtubule cytoskeleton and cellulose microfibrils in the cell wall. Studies using in vivo markers confirmed that one function of the cortical microtubule array is to drive organization of cellulose microfibrils by guiding the trajectories of active cellulose synthase (CESA) complexes in the plasma membrane, thus orienting nascent microfibrils. Here we provide evidence that cortical microtubules also position the delivery of CESA complexes to the plasma membrane and interact with small CESA-containing compartments by a mechanism that permits motility driven by microtubule depolymerization. The association of CESA compartments with cortical microtubules was greatly enhanced during osmotic stress and other treatments that limit cellulose synthesis. On recovery from osmotic stress, delivery of CESA complexes to the plasma membrane was obse! rved in association with microtubule-tethered compartments. These results reveal multiple functions for the microtubule cortical array in organizing CESA in the cell cortex.
• SDPR induces membrane curvature and functions in the formation of caveolae
  - Nat Cell Biol 11(7):807-814 (2009)
  Caveolae are plasma membrane invaginations with a characteristic flask-shaped morphology. They function in diverse cellular processes, including endocytosis. The mechanism by which caveolae are generated is not fully understood, but both caveolin proteins and PTRF (polymerase I and transcript release factor, also known as cavin) are important. Here we show that loss of SDPR (serum deprivation protein response) causes loss of caveolae. SDPR binds directly to PTRF and recruits PTRF to caveolar membranes. Overexpression of SDPR, unlike PTRF, induces deformation of caveolae and extensive tubulation of the plasma membrane. The B-subunit of Shiga toxin (STB) also induces membrane tubulation and these membrane tubes also originate from caveolae. STB colocalizes extensively with both SDPR and caveolin 1. Loss of caveolae reduces the propensity of STB to induce membrane tubulation. We conclude that SDPR is a membrane-curvature-inducing component of caveolae, and that STB-induce! d membrane tubulation is facilitated by caveolae.
• The Arp2/3 complex and WASp are required for apical trafficking of Delta into microvilli during cell fate specification of sensory organ precursors
  - Nat Cell Biol 11(7):815-824 (2009)
  Cell fate decisions mediated by the Notch signalling pathway require direct cell–cell contact between adjacent cells. In Drosophila melanogaster, an external sensory organ (ESO) develops from a single sensory organ precursor (SOP) and its fate specification is governed by differential Notch activation. Here we show that mutations in actin-related protein-3 (Arp3) compromise Notch signalling, leading to a fate transformation of the ESO. Our data reveal that during ESO fate specification, most endocytosed vesicles containing the ligand Delta traffic to a prominent apical actin-rich structure (ARS) formed in the SOP daughter cells. Using immunohistochemistry and transmission electron microscopy (TEM) analyses, we show that the ARS contains numerous microvilli on the apical surface of SOP progeny. In Arp2/3 and WASp mutants, the surface area of the ARS is substantially reduced and there are significantly fewer microvilli. More importantly, trafficking of Delta-positive v! esicles from the basal area to the apical portion of the ARS is severely compromised. Our data indicate that WASp-dependent Arp2/3 actin polymerization is crucial for apical presentation of Delta, providing a mechanistic link between actin polymerization and Notch signalling.
• CPAP is a cell-cycle regulated protein that controls centriole length
  - Nat Cell Biol 11(7):825-831 (2009)
  Centriole duplication involves the growing of a procentriole (progeny centriole) next to the proximal end of each pre-existing centriole (parental centriole). The molecular mechanisms that regulate procentriole elongation remain obscure. We show here that expression of the centriolar protein CPAP (centrosomal P4.1-associated protein) is carefully regulated during the cell cycle, with the protein being degraded in late mitosis. Depletion of CPAP inhibited centrosome duplication, whereas excess CPAP induced the formation of elongated procentriole-like structures (PLSs), which contain stable microtubules and several centriolar proteins. Ultrastructural analysis revealed that these structures are similar to procentrioles with elongated microtubules. Overexpression of a CPAP mutant (CPAP-377EE) that does not bind to tubulin dimers significantly inhibited the formation of CPAP-induced PLSs. Together, these results suggest that CPAP is a new regulator of centriole length and ! its intrinsic tubulin-dimer binding activity is required for procentriole elongation.
• Chromosome congression in the absence of kinetochore fibres
  - Nat Cell Biol 11(7):832-838 (2009)
  Proper chromosome congression (the process of aligning chromosomes on the spindle) contributes to accurate and faithful chromosome segregation. It is widely accepted that congression requires kinetochore fibres (K-fibres), microtubule bundles that extend from the kinetochores to spindle poles1, 2. Here, we demonstrate that chromosomes in human cells co-depleted of HSET (human kinesin-14)3, 4 and hNuf2 (human Ndc80/Hec1-complex component)5 can congress to the metaphase plate in the absence of K-fibres. However, the chromosomes are not stably maintained at the metaphase plate under these conditions. Chromosome congression in HSET + hNuf2 co-depleted cells required the plus-end directed motor CENP-E (centromere protein E; kinesin-7 family member)6, which has been implicated in the gliding of mono-oriented kinetochores alongside adjacent K-fibres7. Thus, proper end-on attachment of kinetochores to microtubules is not necessary for chromosome congression. Instead, our data ! support the idea that congression allows unattached chromosomes to move to the middle of the spindle where they have a higher probability of establishing connections with both spindle poles. These bi-oriented connections are also used to maintain stable chromosome alignment at the spindle equator.
• Lateral microtubule bundles promote chromosome alignment during acentrosomal oocyte meiosis
  - Nat Cell Biol 11(7):839-844 (2009)
  Although centrosomes serve to organize microtubules in most cell types, oocyte spindles form and mediate meiotic chromosome segregation in their absence. Here, we used high-resolution imaging of both bipolar and experimentally generated monopolar spindles in Caenorhabditis elegans to reveal a surprising organization of microtubules and chromosomes within acentrosomal structures. We found that homologous chromosome pairs (bivalents) are surrounded by microtubule bundles running along their sides, whereas microtubule density is extremely low at chromosome ends despite a high concentration of kinetochore proteins at those regions. Furthermore, we found that the chromokinesin KLP-19 (kinesin-like protein 19) is targeted to a ring around the centre of each bivalent and provides a polar ejection force that is required for congression. Together, these observations create a new picture of chromosome–microtubule association in acentrosomal spindles and reveal a mechanism by w! hich metaphase alignment can be achieved using this organization. Specifically, we propose that ensheathment by lateral microtubule bundles places spatial constraints on the chromosomes, thereby promoting biorientation, and that localized motors mediate movement along these bundles, thereby promoting alignment.
• OsHAL3 mediates a new pathway in the light-regulated growth of rice
  - Nat Cell Biol 11(7):845-851 (2009)
  Plants show distinct morphologies in different light conditions through a process called photomorphogenesis. A predominant feature of photomorphogenesis is the reduced growth of seedlings under light conditions compared with darkness. For this adaptive event, the most well-known molecular mechanism involves photoreceptor-mediated inhibition of cell elongation1, 2, 3, 4. However, it is not known whether additional pathways exist. Here, we describe a newly discovered pathway of light-modulated plant growth mediated by the halotolerance protein HAL3, a flavin mononucleotide (FMN)-binding protein involved in cell division5, 6, 7, 8. We found that light, especially blue light, suppresses growth of rice seedlings by reducing the activity of Oryza sativa (Os) HAL3. Both in vitro and in vivo studies showed that OsHAL3 is structurally inactivated by light through photo-oxidation and by direct interaction with photons. In addition, the transcriptional expression of OsHAL3 is syn! ergistically regulated by different light conditions. Further investigation suggested that OsHAL3 promotes cell division by recruiting a ubiquitin system, rather than by its 4'-phosphopantothenoylcysteine (PPC) decarboxylase activity. Our results uncover a new mechanism for light-regulated plant growth, namely, light not only inhibits cell elongation but also suppresses cell division through HAL3 and E3 ubiquitin ligase. This study thus brings new insights into our understanding of plant photomorphogenesis.
• Increasing organismal healthspan by enhancing mitochondrial protein quality control
  - Nat Cell Biol 11(7):852-858 (2009)
  Degradation of damaged proteins by members of the protein quality control system is of fundamental importance in maintaining cellular homeostasis. In mitochondria, organelles which both generate and are targets of reactive oxygen species (ROS), a number of membrane bound and soluble proteases are essential components of this system. Here we describe the regulation of Podospora anserina LON (PaLON) levels, an AAA+ family serine protease localized in the matrix fraction of mitochondria. Constitutive overexpression of PaLon results in transgenic strains of the fungal ageing model P. anserina showing increased ATP-dependent serine protease activity. These strains display lower levels of carbonylated (aconitase) and carboxymethylated proteins, reduced secretion of hydrogen peroxide and a higher resistance against exogenous oxidative stress. Moreover, they are characterized by an extended lifespan without impairment of vital functions such as respiration, growth and fertilit! y. The reported genetic manipulation proved to be a successful intervention in organismal ageing and it led to an increase in the healthy lifespan, the healthspan, of P. anserina.
• Cell shape changes indicate a role for extrinsic tensile forces in Drosophila germ-band extension
  - Nat Cell Biol 11(7):859-864 (2009)
  Drosophila germ-band extension (GBE) is an example of the convergence and extension movements that elongate and narrow embryonic tissues. To understand the collective cell behaviours underlying tissue morphogenesis, we have continuously quantified cell intercalation and cell shape change during GBE. We show that the fast, early phase of GBE depends on cell shape change in addition to cell intercalation. In antero-posterior patterning mutants such as those for the gap gene Krüppel, defective polarized cell intercalation is compensated for by an increase in antero-posterior cell elongation, such that the initial rate of extension remains the same. Spatio-temporal patterns of cell behaviours indicate that an antero-posterior tensile force deforms the germ band, causing the cells to change shape passively. The rate of antero-posterior cell elongation is reduced in twist mutant embryos, which lack mesoderm. We propose that cell shape change contributing to germ-band extens! ion is a passive response to mechanical forces caused by the invaginating mesoderm.
• BRIT1/MCPH1 links chromatin remodelling to DNA damage response
  - Nat Cell Biol 11(7):865-872 (2009)
  To detect and repair damaged DNA, DNA-damage-response proteins need to overcome the barrier of condensed chromatin to gain access to DNA lesions1. ATP-dependent chromatin remodelling is one of the fundamental mechanisms used by cells to relax chromatin in DNA repair2, 3. However, the mechanism mediating their recruitment to DNA lesions remains largely unknown. BRIT1 (also known as MCPH1) is an early DNA-damage-response protein that is mutated in human primary microcephaly4, 5, 6, 7, 8. Here we report a previously unknown function of BRIT1 as a regulator of the ATP-dependent chromatin remodelling complex SWI–SNF in DNA repair. After damage to DNA, BRIT1 increases its interaction with SWI–SNF through ATM/ATR-dependent phosphorylation on the BAF170 subunit. This increase in binding affinity provides a means by which SWI–SNF can be specifically recruited to and maintained at DNA lesions. Loss of BRIT1 causes impaired chromatin relaxation as a result of decreased asso! ciation of SWI–SNF with chromatin. This explains the decreased recruitment of repair proteins to DNA lesions and the reduced efficiency of repair in BRIT1-deficient cells, resulting in impaired cell survival after DNA damage. Our findings therefore identify BRIT1 as a key molecule that links chromatin remodelling with response to DNA damage in the control of DNA repair, and its dysfunction contributes to human disease.
• Epidermal growth factor-like domain 7 (EGFL7) modulates Notch signalling and affects neural stem cell renewal
  - Nat Cell Biol 11(7):873-880 (2009)
  Epidermal growth factor-like domain 7 (EGFL7) is a secreted factor implicated in cellular responses such as cell migration and blood vessel formation; however the molecular mechanisms underlying the effects of EGFL7 are largely unknown. Here we have identified transmembrane receptors of the Notch family as EGFL7-binding molecules. Secreted EGFL7 binds to a region in Notch involved in ligand-mediated receptor activation, thus acting as an antagonist of Notch signalling. Expression of EGFL7 in neural stem cells (NSCs) in vitro decreased Notch-specific signalling and consequently, reduced proliferation and self-renewal of NSCs. Such altered Notch signalling caused a shift in the differentiation pattern of cultured NSCs towards an excess of neurons and oligodendrocytes. We identified neurons as a source of EGFL7 in the brain, suggesting that brain-derived EGFL7 acts as an endogenous antagonist of Notch signalling that regulates proliferation and differentiation of subventr! icular zone-derived adult NSCs.
• TGF- activates Akt kinase through a microRNA-dependent amplifying circuit targeting PTEN
  - Nat Cell Biol 11(7):881-889 (2009)
  Akt kinase is activated by transforming growth factor-1 (TGF-) in diabetic kidneys, and has important roles in fibrosis, hypertrophy and cell survival in glomerular mesangial cells1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11. However, the mechanisms of Akt activation by TGF- are not fully understood. Here we show that TGF- activates Akt in glomerular mesangial cells by inducing the microRNAs (miRNAs) miR-216a and miR-217, both of which target PTEN (phosphatase and tensin homologue), an inhibitor of Akt activation. These miRNAs are located within the second intron of a non-coding RNA (RP23-298H6.1-001). The RP23 promoter was activated by TGF- and miR-192 through E-box-regulated mechanisms, as shown previously3. Akt activation by these miRs led to glomerular mesangial cell survival and hypertrophy, which were similar to the effects of activation by TGF-. These studies reveal a mechanism of Akt activation through PTEN downregulation by two miRs, which are regulated by upstream miR-1! 92 and TGF-. Due to the diversity of PTEN function12, 13, this miR-amplifying circuit may have key roles, not only in kidney disorders, but also in other diseases.
• The tyrosine kinase Stitcher activates Grainy head and epidermal wound healing in Drosophila
  - Nat Cell Biol 11(7):890-895 (2009)
  Epidermal injury initiates a cascade of inflammation, epithelial remodelling and integument repair at wound sites. The regeneration of the extracellular barrier and damaged tissue repair rely on the precise orchestration of epithelial responses triggered by the injury1, 2. Grainy head (Grh) transcription factors induce gene expression to crosslink the extracellular barrier in wounded flies and mice3, 4. However, the activation mechanisms and functions of Grh factors in re-epithelialization remain unknown. Here we identify stitcher (stit), a new Grh target in Drosophila melanogaster. stit encodes a Ret-family receptor tyrosine kinase required for efficient epidermal wound healing. Live imaging analysis reveals that Stit promotes actin cable assembly during wound re-epithelialization. Stit activation also induces extracellular signal-regulated kinase (ERK) phosphorylation along with the Grh-dependent expression of stit and barrier repair genes at the wound sites. The tra! nscriptional stimulation of stit on injury triggers a positive feedback loop increasing the magnitude of epithelial responses. Thus, Stit activation upon wounding coordinates cytoskeletal rearrangements and the level of Grh-mediated transcriptional wound responses.
• Centromere assembly requires the direct recognition of CENP-A nucleosomes by CENP-N
  - Nat Cell Biol 11(7):896-902 (2009)
  Centromeres are specialized chromosomal domains that direct kinetochore assembly during mitosis. CENP-A (centromere protein A), a histone H3-variant present exclusively in centromeric nucleosomes, is thought to function as an epigenetic mark that specifies centromere identity. Here we identify the essential centromere protein CENP-N as the first protein to selectively bind CENP-A nucleosomes but not H3 nucleosomes. CENP-N bound CENP-A nucleosomes in a DNA sequence-independent manner, but did not bind soluble CENP-A–H4 tetramers. Mutations in CENP-N that reduced its affinity for CENP-A nucleosomes caused defects in CENP-N localization and had dominant effects on the recruitment of CENP-H, CENP-I and CENP-K to centromeres. Depletion of CENP-N using siRNA (short interfering RNA) led to similar centromere assembly defects and resulted in reduced assembly of nascent CENP-A into centromeric chromatin. These data suggest that CENP-N interprets the information encoded within! CENP-A nucleosomes and recruits other proteins to centromeric chromatin that are required for centromere function and propagation.
• Essential role for eIF4GI overexpression in the pathogenesis of inflammatory breast cancer
  - Nat Cell Biol 11(7):903-908 (2009)
  Inflammatory breast cancer (IBC) is the most lethal form of primary breast cancer1. IBC lethality derives from generation of tumour emboli, which are non-adherent cell clusters that rapidly spread by a form of continuous invasion known as passive metastasis2, 3, 4, 5. In most cancers, expression of E-cadherin, an epithelial marker, is indicative of low metastatic potential6, 7. In IBC, E-cadherin is overexpressed8 and supports formation of tumour emboli by promoting tumour cell interactions rather than adherence to stroma2, 3, 9. E-cadherin, a surface component of adherens junctions, is anchored by interaction with p120 catenin (p120). We show that the unique pathogenic properties of IBC result in part from overexpression of the translation initiation factor eIF4GI in most IBCs. eIF4GI reprograms the protein synthetic machinery for increased translation of mRNAs with internal ribosome entry sites (IRESs) that promote IBC tumour cell survival and formation of tumour emb! oli. Overexpression of eIF4GI promotes formation of IBC tumour emboli by enhancing translation of IRES-containing p120 mRNAs. These findings provide a new understanding of translational control in the development of advanced breast cancer.
• Transmission and spreading of tauopathy in transgenic mouse brain
  - Nat Cell Biol 11(7):909-913 (2009)
  Hyperphosphorylated tau makes up the filamentous intracellular inclusions of several neurodegenerative diseases, including Alzheimer's disease1. In the disease process, neuronal tau inclusions first appear in the transentorhinal cortex from where they seem to spread to the hippocampal formation and neocortex2. Cognitive impairment becomes manifest when inclusions reach the hippocampus, with abundant neocortical tau inclusions and extracellular -amyloid deposits being the defining pathological hallmarks of Alzheimer's disease. An abundance of tau inclusions, in the absence of -amyloid deposits, defines Pick's disease, progressive supranuclear palsy, corticobasal degeneration and other diseases1. Tau mutations cause familial forms of frontotemporal dementia, establishing that tau protein dysfunction is sufficient to cause neurodegeneration and dementia3, 4, 5. Thus, transgenic mice expressing mutant (for example, P301S) human tau in nerve cells show the essential feature! s of tauopathies, including neurodegeneration and abundant filaments made of hyperphosphorylated tau protein6, 8. By contrast, mouse lines expressing single isoforms of wild-type human tau do not produce tau filaments or show neurodegeneration7, 8. Here we have used tau-expressing lines to investigate whether experimental tauopathy can be transmitted. We show that injection of brain extract from mutant P301S tau-expressing mice into the brain of transgenic wild-type tau-expressing animals induces assembly of wild-type human tau into filaments and spreading of pathology from the site of injection to neighbouring brain regions.
• p53 controls cancer cell invasion by inducing the MDM2-mediated degradation of Slug
  - Nat Cell Biol 11(7):914 (2009)
  Introduction Received 29 October 2008; accepted 19 February 2009; published online 17 May 2009; corrected after print 19 May 2009 In the version of this article initially published, the Myc–Ub label in Figure 3c was incorret. Arrowheads were missing in Fig. 5d and 5e. A plus (+) was misplaced from the curve in Fig. 7c. The correct versions of these figures are shown below. These errors have been corrected in the HTML and PDF versions of the article.