Journal of Cell Biology

Molecular architecture of the kinetochore-microtubule attachment site is conserved between point and regional centromeres.

Publication Date: 2008 May 12 PMID: 18474626
Authors: Joglekar, A. P. - Bouck, D. - Finley, K. - Liu, X. - Wan, Y. - Berman, J. - He, X. - Salmon, E. D. - Bloom, K. S.
Journal: J Cell Biol

Point and regional centromeres specify a unique site on each chromosome for kinetochore assembly. The point centromere in budding yeast is a unique 150-bp DNA sequence, which supports a kinetochore with only one microtubule attachment. In contrast, regional centromeres are complex in architecture, can be up to 5 Mb in length, and typically support many kinetochore-microtubule attachments. We used quantitative fluorescence microscopy to count the number of core structural kinetochore protein complexes at the regional centromeres in fission yeast and Candida albicans. We find that the number of CENP-A nucleosomes at these centromeres reflects the number of kinetochore-microtubule attachments instead of their length. The numbers of kinetochore protein complexes per microtubule attachment are nearly identical to the numbers in a budding yeast kinetochore. These findings reveal that kinetochores with multiple microtubule attachments are mainly built by repeating a conserved structural subunit that is equivalent to a single microtubule attachment site.

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Dynamic localization of yeast Fus2p to an expanding ring at the cell fusion junction during mating.

Publication Date: 2008 May 12 PMID: 18474625
Authors: Paterson, J. M. - Ydenberg, C. A. - Rose, M. D.
Journal: J Cell Biol

Fus2p is a pheromone-induced protein associated with the amphiphysin homologue Rvs161p, which is required for cell fusion during mating in Saccharomyces cerevisiae. We constructed a functional Fus2p-green fluorescent protein (GFP), which exhibits highly dynamic localization patterns in pheromone-responding cells (shmoos): diffuse nuclear, mobile cytoplasmic dots and stable cortical patches concentrated at the shmoo tip. In mitotic cells, Fus2p-GFP is nuclear but becomes cytoplasmic as cells form shmoos, dependent on the Fus3p protein kinase and high levels of pheromone signaling. The rapid cytoplasmic movement of Fus2p-GFP dots requires Rvs161p and polymerized actin and is aberrant in mutants with compromised actin organization, which suggests that the Fus2p dots are transported along actin cables, possibly in association with vesicles. Maintenance of Fus2p-GFP patches at the shmoo tip cortex is jointly dependent on actin and a membrane protein, Fus1p, which suggests that Fus1p is an anchor for Fus2p. In zygotes, Fus2p-GFP forms a dilating ring at the cell junction, returning to the nucleus at the completion of cell fusion.

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Plakophilin 2: a critical scaffold for PKC{alpha} that regulates intercellular junction assembly.

Publication Date: 2008 May 12 PMID: 18474624
Authors: Bass-Zubek, A. E. - Hobbs, R. P. - Amargo, E. V. - Garcia, N. J. - Hsieh, S. N. - Chen, X. - Wahl, J. K. 3rd - Denning, M. F. - Green, K. J.
Journal: J Cell Biol

Plakophilins (PKPs) are armadillo family members related to the classical cadherin-associated protein p120(ctn). PKPs localize to the cytoplasmic plaque of intercellular junctions and participate in linking the intermediate filament (IF)-binding protein desmoplakin (DP) to desmosomal cadherins. In response to cell-cell contact, PKP2 associates with DP in plaque precursors that form in the cytoplasm and translocate to nascent desmosomes. Here, we provide evidence that PKP2 governs DP assembly dynamics by scaffolding a DP-PKP2-protein kinase Calpha (PKCalpha) complex, which is disrupted by PKP2 knockdown. The behavior of a phosphorylation-deficient DP mutant that associates more tightly with IF is mimicked by PKP2 and PKCalpha knockdown and PKC pharmacological inhibition, all of which impair junction assembly. PKP2 knockdown is accompanied by increased phosphorylation of PKC substrates, raising the possibility that global alterations in PKC signaling may contribute to pathogenesis of congenital defects caused by PKP2 deficiency.

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The class III PI(3)K Vps34 promotes autophagy and endocytosis but not TOR signaling in Drosophila.

Publication Date: 2008 May 12 PMID: 18474623
Authors: Juhasz, G. - Hill, J. H. - Yan, Y. - Sass, M. - Baehrecke, E. H. - Backer, J. M. - Neufeld, T. P.
Journal: J Cell Biol

Degradation of cytoplasmic components by autophagy requires the class III phosphatidylinositol 3 (PI(3))-kinase Vps34, but the mechanisms by which this kinase and its lipid product PI(3) phosphate (PI(3)P) promote autophagy are unclear. In mammalian cells, Vps34, with the proautophagic tumor suppressors Beclin1/Atg6, Bif-1, and UVRAG, forms a multiprotein complex that initiates autophagosome formation. Distinct Vps34 complexes also regulate endocytic processes that are critical for late-stage autophagosome-lysosome fusion. In contrast, Vps34 may also transduce activating nutrient signals to mammalian target of rapamycin (TOR), a negative regulator of autophagy. To determine potential in vivo functions of Vps34, we generated mutations in the single Drosophila melanogaster Vps34 orthologue, causing cell-autonomous disruption of autophagosome/autolysosome formation in larval fat body cells. Endocytosis is also disrupted in Vps34(-/-) animals, but we demonstrate that this does not account for their autophagy defect. Unexpectedly, TOR signaling is unaffected in Vps34 mutants, indicating that Vps34 does not act upstream of TOR in this system. Instead, we show that TOR/Atg1 signaling regulates the starvation-induced recruitment of PI(3)P to nascent autophagosomes. Our results suggest that Vps34 is regulated by TOR-dependent nutrient signals directly at sites of autophagosome formation.

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The tight junction protein complex undergoes rapid and continuous molecular remodeling at steady state.

Publication Date: 2008 May 12 PMID: 18474622
Authors: Shen, L. - Weber, C. R. - Turner, J. R.
Journal: J Cell Biol

The tight junction defines epithelial organization. Structurally, the tight junction is comprised of transmembrane and membrane-associated proteins that are thought to assemble into stable complexes to determine function. In this study, we measure tight junction protein dynamics in live confluent Madin-Darby canine kidney monolayers using fluorescence recovery after photobleaching and related methods. Mathematical modeling shows that the majority of claudin-1 (76 +/- 5%) is stably localized at the tight junction. In contrast, the majority of occludin (71 +/- 3%) diffuses rapidly within the tight junction with a diffusion constant of 0.011 mum(2)s(-1). Zonula occludens-1 molecules are also highly dynamic in this region, but, rather than diffusing within the plane of the membrane, 69 +/- 5% exchange between membrane and intracellular pools in an energy-dependent manner. These data demonstrate that the tight junction undergoes constant remodeling and suggest that this dynamic behavior may contribute to tight junction assembly and regulation.

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Tektin 2 is required for central spindle microtubule organization and the completion of cytokinesis.

Publication Date: 2008 May 12 PMID: 18474621
Authors: Durcan, T. M. - Halpin, E. S. - Rao, T. - Collins, N. S. - Tribble, E. K. - Hornick, J. E. - Hinchcliffe, E. H.
Journal: J Cell Biol

During anaphase, the nonkinetochore microtubules in the spindle midzone become compacted into the central spindle, a structure which is required to both initiate and complete cytokinesis. We show that Tektin 2 (Tek2) associates with the spindle poles throughout mitosis, organizes the spindle midzone microtubules during anaphase, and assembles into the midbody matrix surrounding the compacted midzone microtubules during cytokinesis. Tek2 small interfering RNA (siRNA) disrupts central spindle organization and proper localization of MKLP1, PRC1, and Aurora B to the midzone and prevents the formation of a midbody matrix. Video microscopy revealed that loss of Tek2 results in binucleate cell formation by aberrant fusion of daughter cells after cytokinesis. Although a myosin II inhibitor, blebbistatin, prevents actin-myosin contractility, the microtubules of the central spindle are compacted. Strikingly, Tek2 siRNA abolishes this actin-myosin-independent midzone microtubule compaction. Thus, Tek2-dependant organization of the central spindle during anaphase is essential for proper midbody formation and the segregation of daughter cells after cytokinesis.

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Kank regulates RhoA-dependent formation of actin stress fibers and cell migration via 14-3-3 in PI3K-Akt signaling.

Publication Date: 2008 May 5 PMID: 18458160
Authors: Kakinuma, N. - Roy, B. C. - Zhu, Y. - Wang, Y. - Kiyama, R.
Journal: J Cell Biol

Phosphoinositide-3 kinase (PI3K)/Akt signaling is activated by growth factors such as insulin and epidermal growth factor (EGF) and regulates several functions such as cell cycling, apoptosis, cell growth, and cell migration. Here, we find that Kank is an Akt substrate located downstream of PI3K and a 14-3-3-binding protein. The interaction between Kank and 14-3-3 is regulated by insulin and EGF and is mediated through phosphorylation of Kank by Akt. In NIH3T3 cells expressing Kank, the amount of actin stress fibers is reduced, and the coexpression of 14-3-3 disrupted this effect. Kank also inhibits insulin-induced cell migration via 14-3-3 binding. Furthermore, Kank inhibits insulin and active Akt-dependent activation of RhoA through binding to 14-3-3. Based on these findings, we hypothesize that Kank negatively regulates the formation of actin stress fibers and cell migration through the inhibition of RhoA activity, which is controlled by binding of Kank to 14-3-3 in PI3K-Akt signaling.

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The formin mDia2 stabilizes microtubules independently of its actin nucleation activity.

Publication Date: 2008 May 5 PMID: 18458159
Authors: Bartolini, F. - Moseley, J. B. - Schmoranzer, J. - Cassimeris, L. - Goode, B. L. - Gundersen, G. G.
Journal: J Cell Biol

A critical microtubule (MT) polarization event in cell migration is the Rho/mDia-dependent stabilization of a subset of MTs oriented toward the direction of migration. Although mDia nucleates actin filaments, it is unclear whether this or a separate activity of mDia underlies MT stabilization. We generated two actin mutants (K853A and I704A) in a constitutively active version of mDia2 containing formin homology domains 1 and 2 (FH1FH2) and found that they still induced stable MTs and bound to the MT TIP proteins EB1 and APC, which have also been implicated in MT stabilization. A dimerization-impaired mutant of mDia2 (W630A) also generated stable MTs in cells. We examined whether FH1FH2mDia2 had direct activity on MTs in vitro and found that it bound directly to MTs, stabilized MTs against cold- and dilution-induced disassembly, and reduced the rates of growth and shortening during MT assembly and disassembly, respectively. These results indicate that mDia2 has a novel MT stabilization activity that is separate from its actin nucleation activity.

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Presynaptic Type III Neuregulin1-ErbB signaling targets {alpha}7 nicotinic acetylcholine receptors to axons.

Publication Date: 2008 May 5 PMID: 18458158
Authors: Hancock, M. L. - Canetta, S. E. - Role, L. W. - Talmage, D. A.
Journal: J Cell Biol

Type III Neuregulin1 (Nrg1) isoforms are membrane-tethered proteins capable of participating in bidirectional juxtacrine signaling. Neuronal nicotinic acetylcholine receptors (nAChRs), which can modulate the release of a rich array of neurotransmitters, are differentially targeted to presynaptic sites. We demonstrate that Type III Nrg1 back signaling regulates the surface expression of alpha7 nAChRs along axons of sensory neurons. Stimulation of Type III Nrg1 back signaling induces an increase in axonal surface alpha7 nAChRs, which results from a redistribution of preexisting intracellular pools of alpha7 rather than from increased protein synthesis. We also demonstrate that Type III Nrg1 back signaling activates a phosphatidylinositol 3-kinase signaling pathway and that activation of this pathway is required for the insertion of preexisting alpha7 nAChRs into the axonal plasma membrane. These findings, in conjunction with prior results establishing that Type III Nrg1 back signaling controls gene transcription, demonstrate that Type III Nrg1 back signaling can regulate both short-and long-term changes in neuronal function.

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Cdc14B depletion leads to centriole amplification, and its overexpression prevents unscheduled centriole duplication.

Publication Date: 2008 May 5 PMID: 18458157
Authors: Wu, J. - Cho, H. P. - Rhee, D. B. - Johnson, D. K. - Dunlap, J. - Liu, Y. - Wang, Y.
Journal: J Cell Biol

Centrosome duplication is tightly controlled in coordination with DNA replication. The molecular mechanism of centrosome duplication remains unclear. Previous studies found that a fraction of human proline-directed phosphatase Cdc14B associates with centrosomes. However, Cdc14B's involvement in centrosome cycle control has never been explored. Here, we show that depletion of Cdc14B by RNA interference leads to centriole amplification in both HeLa and normal human fibroblast BJ and MRC-5 cells. Induction of Cdc14B expression through a regulatable promoter significantly attenuates centriole amplification in prolonged S phase-arrested cells and proteasome inhibitor Z-L(3)VS-treated cells. This inhibitory function requires centriole-associated Cdc14B catalytic activity. Together, these results suggest a potential function for Cdc14B phosphatase in maintaining the fidelity of centrosome duplication cycle.

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Pitx2 is functionally important in the early stages of vascular smooth muscle cell differentiation.

Publication Date: 2008 May 5 PMID: 18458156
Authors: Shang, Y. - Yoshida, T. - Amendt, B. A. - Martin, J. F. - Owens, G. K.
Journal: J Cell Biol

Mechanisms that control vascular smooth muscle cell (SMC) differentiation are poorly understood. We identify Pitx2 as a previously unknown homeodomain transcription factor that is rapidly induced in an in vitro model of SMC differentiation from multipotent stem cells. Pitx2 induces expression of multiple SMC differentiation marker genes by binding to a TAATC(C/T) cis-element, by interacting with serum response factor, and by increasing histone acetylation levels within the promoters of SMC differentiation marker genes. Suppression of Pitx2 reduces expression of SMC differentiation marker genes in the early stages of SMC differentiation in vitro, whereas Prx1, another homeodomain protein, regulates SMC differentiation marker genes in fully differentiated SMCs. Pitx2, but not Prx1, knockout mouse embryos exhibit impaired induction of SMC differentiation markers in the dorsal aorta and branchial arch arteries. Our results demonstrate that Pitx2 functions to regulate the early stages of SMC differentiation.

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Kindlin-2 (Mig-2): a co-activator of beta3 integrins.

Publication Date: 2008 May 5 PMID: 18458155
Authors: Ma, Y. Q. - Qin, J. - Wu, C. - Plow, E. F.
Journal: J Cell Biol

Integrin activation is essential for dynamically linking the extracellular environment and cytoskeletal/signaling networks. Activation is controlled by integrins' short cytoplasmic tails (CTs). It is widely accepted that the head domain of talin (talin-H) can mediate integrin activation by binding to two sites in integrin beta's CT; in integrin beta(3) this is an NPLY(747) motif and the membrane-proximal region. Here, we show that the C-terminal region of integrin beta(3) CT, composed of a conserved TS(752)T region and NITY(759) motif, supports integrin activation by binding to a cytosolic binding partner, kindlin-2, a widely distributed PTB domain protein. Co-transfection of kindlin-2 with talin-H results in a synergistic enhancement of integrin alpha(IIb)beta(3) activation. Furthermore, siRNA knockdown of endogenous kindlin-2 impairs talin-induced alpha(IIb)beta(3) activation in transfected CHO cells and blunts alpha(v)beta(3)-mediated adhesion and migration of endothelial cells. Our results thus identify kindlin-2 as a novel regulator of integrin activation; it functions as a coactivator.

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Lloyd Trotman: of mice and men, cancer, and PTEN. Interviewed by Caitlin Sedwick.

Publication Date: 2008 May 5 PMID: 18458154
Authors: Trotman, L.
Journal: J Cell Biol

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You wrote it; you own it!

Publication Date: 2008 May 5 PMID: 18450642
Authors: Hill, E. - Rossner, M.
Journal: J Cell Biol

Authors of papers published in Rockefeller University Press journals (The Journal of Cell Biology, The Journal of Experimental Medicine, or The Journal of General Physiology) now retain copyright to their published work. This permits authors to reuse their own work in any way, as long as they attribute it to the original publication. Third parties may use our published materials under a Creative Commons license, six months after publication.

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All quiet on the neuronal front: NMDA receptor inhibition by prion protein.

Publication Date: 2008 May 5 PMID: 18443224
Authors: Steele, A. D.
Journal: J Cell Biol

The normal function of the prion protein (PrP)-the causative agent of mad cow or prion disease-has long remained out of reach. Deciphering PrP's function may help to unravel the complex chain of events triggered by PrP misfolding during prion disease. In this issue of the JCB, an exciting paper (Khosravani, H., Y. Zhang, S. Tsutsui, S. Hameed, C. Altier, J. Hamid, L. Chen, M. Villemaire, Z. Ali, F.R. Jirik, and G.W. Zamponi. 2008. J. Cell Biol. 181:551-565) connects diverse observations regarding PrP into a coherent framework whereby PrP dampens the activity of an N-methyl-d-aspartate (NMDA) receptor (NMDAR) subtype and reduces excitotoxic lesions. The findings of this study suggest that understanding the normal function of proteins associated with neurodegenerative disease may elucidate the molecular pathogenesis.

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CENP-E combines a slow, processive motor and a flexible coiled coil to produce an essential motile kinetochore tether.

Publication Date: 2008 May 5 PMID: 18443223
Authors: Kim, Y. - Heuser, J. E. - Waterman, C. M. - Cleveland, D. W.
Journal: J Cell Biol

The mitotic kinesin centromere protein E (CENP-E) is an essential kinetochore component that directly contributes to the capture and stabilization of spindle microtubules by kinetochores. Although reduction in CENP-E leads to high rates of whole chromosome missegregation, neither its properties as a microtubule-dependent motor nor how it contributes to the dynamic linkage between kinetochores and microtubules is known. Using single-molecule assays, we demonstrate that CENP-E is a very slow, highly processive motor that maintains microtubule attachment for long periods. Direct visualization of full-length Xenopus laevis CENP-E reveals a highly flexible 230-nm coiled coil separating its kinetochore-binding and motor domains. We also show that full-length CENP-E is a slow plus end-directed motor whose activity is essential for metaphase chromosome alignment. We propose that the highly processive microtubule-dependent motor activity of CENP-E serves to power chromosome congression and provides a flexible, motile tether linking kinetochores to dynamic spindle microtubules.

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Rac1 accumulates in the nucleus during the G2 phase of the cell cycle and promotes cell division.

Publication Date: 2008 May 5 PMID: 18443222
Authors: Michaelson, D. - Abidi, W. - Guardavaccaro, D. - Zhou, M. - Ahearn, I. - Pagano, M. - Philips, M. R.
Journal: J Cell Biol

Rac1 regulates a wide variety of cellular processes. The polybasic region of the Rac1 C terminus functions both as a plasma membrane-targeting motif and a nuclear localization sequence (NLS). We show that a triproline N-terminal to the polybasic region contributes to the NLS, which is cryptic in the sense that it is strongly inhibited by geranylgeranylation of the adjacent cysteine. Subcellular fractionation demonstrated endogenous Rac1 in the nucleus and Triton X-114 partition revealed that this pool is prenylated. Cell cycle-blocking agents, synchronization of cells stably expressing low levels of GFP-Rac1, and time-lapse microscopy of asynchronous cells revealed Rac1 accumulation in the nucleus in late G2 and exclusion in early G1. Although constitutively active Rac1 restricted to the cytoplasm inhibited cell division, activated Rac1 expressed constitutively in the nucleus increased the mitotic rate. These results show that Rac1 cycles in and out of the nucleus during the cell cycle and thereby plays a role in promoting cell division.

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FIP200, a ULK-interacting protein, is required for autophagosome formation in mammalian cells.

Publication Date: 2008 May 5 PMID: 18443221
Authors: Hara, T. - Takamura, A. - Kishi, C. - Iemura, S. - Natsume, T. - Guan, J. L. - Mizushima, N.
Journal: J Cell Biol

Autophagy is a membrane-mediated intracellular degradation system. The serine/threonine kinase Atg1 plays an essential role in autophagosome formation. However, the role of the mammalian Atg1 homologues UNC-51-like kinase (ULK) 1 and 2 are not yet well understood. We found that murine ULK1 and 2 localized to autophagic isolation membrane under starvation conditions. Kinase-dead alleles of ULK1 and 2 exerted a dominant-negative effect on autophagosome formation, suggesting that ULK kinase activity is important for autophagy. We next screened for ULK binding proteins and identified the focal adhesion kinase family interacting protein of 200 kD (FIP200), which regulates diverse cellular functions such as cell size, proliferation, and migration. We found that FIP200 was redistributed from the cytoplasm to the isolation membrane under starvation conditions. In FIP200-deficient cells, autophagy induction by various treatments was abolished, and both stability and phosphorylation of ULK1 were impaired. These results suggest that FIP200 is a novel mammalian autophagy factor that functions together with ULKs.

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Augmin: a protein complex required for centrosome-independent microtubule generation within the spindle.

Publication Date: 2008 May 5 PMID: 18443220
Authors: Goshima, G. - Mayer, M. - Zhang, N. - Stuurman, N. - Vale, R. D.
Journal: J Cell Biol

Since the discovery of gamma-tubulin, attention has focused on its involvement as a microtubule nucleator at the centrosome. However, mislocalization of gamma-tubulin away from the centrosome does not inhibit mitotic spindle formation in Drosophila melanogaster, suggesting that a critical function for gamma-tubulin might reside elsewhere. A previous RNA interference (RNAi) screen identified five genes (Dgt2-6) required for localizing gamma-tubulin to spindle microtubules. We show that the Dgt proteins interact, forming a stable complex. We find that spindle microtubule generation is substantially reduced after knockdown of each Dgt protein by RNAi. Thus, the Dgt complex that we name "augmin" functions to increase microtubule number. Reduced spindle microtubule generation after augmin RNAi, particularly in the absence of functional centrosomes, has dramatic consequences on mitotic spindle formation and function, leading to reduced kinetochore fiber formation, chromosome misalignment, and spindle bipolarity defects. We also identify a functional human homologue of Dgt6. Our results suggest that an important mitotic function for gamma-tubulin may lie within the spindle, where augmin and gamma-tubulin function cooperatively to amplify the number of microtubules.

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Prion protein attenuates excitotoxicity by inhibiting NMDA receptors.

Publication Date: 2008 May 5 PMID: 18443219
Authors: Khosravani, H. - Zhang, Y. - Tsutsui, S. - Hameed, S. - Altier, C. - Hamid, J. - Chen, L. - Villemaire, M. - Ali, Z. - Jirik, F. R. - Zamponi, G. W.
Journal: J Cell Biol

It is well established that misfolded forms of cellular prion protein (PrP [PrP(C)]) are crucial in the genesis and progression of transmissible spongiform encephalitis, whereas the function of native PrP(C) remains incompletely understood. To determine the physiological role of PrP(C), we examine the neurophysiological properties of hippocampal neurons isolated from PrP-null mice. We show that PrP-null mouse neurons exhibit enhanced and drastically prolonged N-methyl-d-aspartate (NMDA)-evoked currents as a result of a functional upregulation of NMDA receptors (NMDARs) containing NR2D subunits. These effects are phenocopied by RNA interference and are rescued upon the overexpression of exogenous PrP(C). The enhanced NMDAR activity results in an increase in neuronal excitability as well as enhanced glutamate excitotoxicity both in vitro and in vivo. Thus, native PrP(C) mediates an important neuroprotective role by virtue of its ability to inhibit NR2D subunits.

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Telomere dysfunction and cell survival: roles for distinct TIN2-containing complexes.

Publication Date: 2008 May 5 PMID: 18443218
Authors: Kim, S. H. - Davalos, A. R. - Heo, S. J. - Rodier, F. - Zou, Y. - Beausejour, C. - Kaminker, P. - Yannone, S. M. - Campisi, J.
Journal: J Cell Biol

Telomeres are maintained by three DNA-binding proteins (telomeric repeat binding factor 1 [TRF1], TRF2, and protector of telomeres 1 [POT1]) and several associated factors. One factor, TRF1-interacting protein 2 (TIN2), binds TRF1 and TRF2 directly and POT1 indirectly. Along with two other proteins, TPP1 and hRap1, these form a soluble complex that may be the core telomere maintenance complex. It is not clear whether subcomplexes also exist in vivo. We provide evidence for two TIN2 subcomplexes with distinct functions in human cells. We isolated these two TIN2 subcomplexes from nuclear lysates of unperturbed cells and cells expressing TIN2 mutants TIN2-13 and TIN2-15C, which cannot bind TRF2 or TRF1, respectively. In cells with wild-type p53 function, TIN2-15C was more potent than TIN2-13 in causing telomere uncapping and eventual growth arrest. In cells lacking p53 function, TIN2-15C was more potent than TIN2-13 in causing telomere dysfunction and cell death. Our findings suggest that distinct TIN2 complexes exist and that TIN2-15C-sensitive subcomplexes are particularly important for cell survival in the absence of functional p53.

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Polo-like kinase is required for Golgi and bilobe biogenesis in Trypanosoma brucei.

Publication Date: 2008 May 5 PMID: 18443217
Authors: de Graffenried, C. L. - Ho, H. H. - Warren, G.
Journal: J Cell Biol

A bilobed structure marked by TbCentrin2 regulates Golgi duplication in the protozoan parasite Trypanosoma brucei. This structure must itself duplicate during the cell cycle for Golgi inheritance to proceed normally. We show here that duplication of the bilobed structure is dependent on the single polo-like kinase (PLK) homologue in T. brucei (TbPLK). Depletion of TbPLK leads to malformed bilobed structures, which is consistent with an inhibition of duplication and an increase in the number of dispersed Golgi structures with associated endoplasmic reticulum exit sites. These data suggest that the bilobe may act as a scaffold for the controlled assembly of the duplicating Golgi.

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{alpha}4{beta}1 integrin and erythropoietin mediate temporally distinct steps in erythropoiesis: integrins in red cell development.

Publication Date: 2008 Apr 21 PMID: 18426982
Authors: Eshghi, S. - Vogelezang, M. G. - Hynes, R. O. - Griffith, L. G. - Lodish, H. F.
Journal: J Cell Biol

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A central function for perlecan in skeletal muscle and cardiovascular development.

Publication Date: 2008 Apr 21 PMID: 18426981
Authors: Zoeller, J. J. - McQuillan, A. - Whitelock, J. - Ho, S. Y. - Iozzo, R. V.
Journal: J Cell Biol

Perlecan's developmental functions are difficult to dissect in placental animals because perlecan disruption is embryonic lethal. In contrast to mammals, cardiovascular function is not essential for early zebrafish development because the embryos obtain adequate oxygen by diffusion. In this study, we use targeted protein depletion coupled with protein-based rescue experiments to investigate the involvement of perlecan and its C-terminal domain V/endorepellin in zebrafish development. The perlecan morphants show a severe myopathy characterized by abnormal actin filament orientation and disorganized sarcomeres, suggesting an involvement of perlecan in myopathies. In the perlecan morphants, primary intersegmental vessel sprouts, which develop through angiogenesis, fail to extend and show reduced protrusive activity. Live videomicroscopy confirms the abnormal swimming pattern caused by the myopathy and anomalous head and trunk vessel circulation. The phenotype is partially rescued by microinjection of human perlecan or endorepellin. These findings indicate that perlecan is essential for the integrity of somitic muscle and developmental angiogenesis and that endorepellin mediates most of these biological activities.

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ErbB2 directly activates the exchange factor Dock7 to promote Schwann cell migration.

Publication Date: 2008 Apr 21 PMID: 18426980
Authors: Yamauchi, J. - Miyamoto, Y. - Chan, J. R. - Tanoue, A.
Journal: J Cell Biol

The cellular events that precede myelination in the peripheral nervous system require rapid and dynamic morphological changes in the Schwann cell. These events are thought to be mainly controlled by axonal signals. But how signals on the axons are coordinately organized and transduced to promote proliferation, migration, radial sorting, and myelination is unknown. We describe that the axonal signal neuregulin-1 (NRG1) controls Schwann cell migration via activation of the atypical Dock180-related guanine nucleotide exchange factor (GEF) Dock7 and subsequent activation of the Rho guanine triphosphatases (GTPases) Rac1 and Cdc42 and the downstream c-Jun N-terminal kinase. We show that the NRG1 receptor ErbB2 directly binds and activates Dock7 by phosphorylating Tyr-1118. Dock7 knockdown, or expression of Dock7 harboring the Tyr-1118-to-Phe mutation in Schwann cells, attenuates the effects of NRG1. Thus, Dock7 functions as an intracellular substrate for ErbB2 to promote Schwann cell migration. This provides an unanticipated mechanism through which ligand-dependent tyrosine phosphorylation can trigger the activation of Rho GTPase-GEFs of the Dock180 family.

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Remodeling of cellular cytoskeleton by the acid sphingomyelinase/ceramide pathway.

Publication Date: 2008 Apr 21 PMID: 18426979
Authors: Zeidan, Y. H. - Jenkins, R. W. - Hannun, Y. A.
Journal: J Cell Biol

The chemotherapeutic agent cisplatin is widely used in treatment of solid tumors. In breast cancer cells, cisplatin produces early and marked changes in cell morphology and the actin cytoskeleton. These changes manifest as loss of lamellipodia/filopodia and appearance of membrane ruffles. Furthermore, cisplatin induces dephosphorylation of the actin-binding protein ezrin, and its relocation from membrane protrusions to the cytosol. Because cisplatin activates acid sphingomyelinase (ASMase), we investigate here the role of the ASMase/ceramide (Cer) pathway in mediating these morphological changes. We find that cisplatin induces a transient elevation in ASMase activity and its redistribution to the plasma membrane. This translocation is blocked upon overexpression of a dominant-negative (DN) ASMase(S508A) mutant and by a DN PKCdelta. Importantly; knockdown of ASMase protects MCF-7 cells from cisplatin-induced cytoskeletal changes including ezrin dephosphorylation. Reciprocally, exogenous delivery of D-e-C16-Cer, but not dihydro-C16-Cer, recapitulates the morphotropic effects of cisplatin. Collectively, these results highlight a novel tumor suppressor property for Cer and a function for ASMase in cisplatin-induced cytoskeletal remodeling.

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A cell-based reglucosylation assay demonstrates the role of GT1 in the quality control of a maturing glycoprotein.

Publication Date: 2008 Apr 21 PMID: 18426978
Authors: Pearse, B. R. - Gabriel, L. - Wang, N. - Hebert, D. N.
Journal: J Cell Biol

The endoplasmic reticulum (ER) protein GT1 (UDP-glucose: glycoprotein glucosyltransferase) is the central enzyme that modifies N-linked carbohydrates based upon the properties of the polypeptide backbone of the maturing substrate. GT1 adds glucose residues to nonglucosylated proteins that fail the quality control test, supporting ER retention through persistent binding to the lectin chaperones calnexin and calreticulin. How GT1 functions in its native environment on a maturing substrate is poorly understood. We analyzed the reglucosylation of a maturing model glycoprotein, influenza hemagglutinin (HA), in the intact mammalian ER. GT1 reglucosylated N-linked glycans in the slow-folding stem domain of HA once the nascent chain was released from the ribosome. Maturation mutants that disrupted the oxidation or oligomerization of HA also supported region-specific reglucosylation by GT1. Therefore, GT1 acts as an ER quality control sensor by posttranslationally reglucosylating glycans on slow-folding or nonnative domains to recruit chaperones specifically to critical aberrant regions.

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eIF4GI links nutrient sensing by mTOR to cell proliferation and inhibition of autophagy.

Publication Date: 2008 Apr 21 PMID: 18426977
Authors: Ramirez-Valle, F. - Braunstein, S. - Zavadil, J. - Formenti, S. C. - Schneider, R. J.
Journal: J Cell Biol

Translation initiation factors have complex functions in cells that are not yet understood. We show that depletion of initiation factor eIF4GI only modestly reduces overall protein synthesis in cells, but phenocopies nutrient starvation or inhibition of protein kinase mTOR, a key nutrient sensor. eIF4GI depletion impairs cell proliferation, bioenergetics, and mitochondrial activity, thereby promoting autophagy. Translation of mRNAs involved in cell growth, proliferation, and bioenergetics were selectively inhibited by reduction of eIF4GI, as was the mRNA encoding Skp2 that inhibits p27, whereas catabolic pathway factors were increased. Depletion or overexpression of other eIF4G family members did not recapitulate these results. The majority of mRNAs that were translationally impaired with eIF4GI depletion were excluded from polyribosomes due to the presence of multiple upstream open reading frames and low mRNA abundance. These results suggest that the high levels of eIF4GI observed in many breast cancers might act to specifically increase proliferation, prevent autophagy, and release tumor cells from control by nutrient sensing.

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Genome-wide analysis of signaling networks regulating fatty acid-induced gene expression and organelle biogenesis.

Publication Date: 2008 Apr 21 PMID: 18426976
Authors: Saleem, R. A. - Knoblach, B. - Mast, F. D. - Smith, J. J. - Boyle, J. - Dobson, C. M. - Long-O'Donnell, R. - Rachubinski, R. A. - Aitchison, J. D.
Journal: J Cell Biol

Reversible phosphorylation is the most common posttranslational modification used in the regulation of cellular processes. This study of phosphatases and kinases required for peroxisome biogenesis is the first genome-wide analysis of phosphorylation events controlling organelle biogenesis. We evaluate signaling molecule deletion strains of the yeast Saccharomyces cerevisiae for presence of a green fluorescent protein chimera of peroxisomal thiolase, formation of peroxisomes, and peroxisome functionality. We find that distinct signaling networks involving glucose-mediated gene repression, derepression, oleate-mediated induction, and peroxisome formation promote stages of the biogenesis pathway. Additionally, separate classes of signaling proteins are responsible for the regulation of peroxisome number and size. These signaling networks specify the requirements of early and late events of peroxisome biogenesis. Among the numerous signaling proteins involved, Pho85p is exceptional, with functional involvements in both gene expression and peroxisome formation. Our study represents the first global study of signaling networks regulating the biogenesis of an organelle.

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The highly conserved nuclear lamin Ig-fold binds to PCNA: its role in DNA replication.

Publication Date: 2008 Apr 21 PMID: 18426975
Authors: Shumaker, D. K. - Solimando, L. - Sengupta, K. - Shimi, T. - Adam, S. A. - Grunwald, A. - Strelkov, S. V. - Aebi, U. - Cardoso, M. C. - Goldman, R. D.
Journal: J Cell Biol

This study provides insights into the role of nuclear lamins in DNA replication. Our data demonstrate that the Ig-fold motif located in the lamin C terminus binds directly to proliferating cell nuclear antigen (PCNA), the processivity factor necessary for the chain elongation phase of DNA replication. We find that the introduction of a mutation in the Ig-fold, which alters its structure and causes human muscular dystrophy, inhibits PCNA binding. Studies of nuclear assembly and DNA replication show that lamins, PCNA, and chromatin are closely associated in situ. Exposure of replicating nuclei to an excess of the lamin domain containing the Ig-fold inhibits DNA replication in a concentration-dependent fashion. This inhibitory effect is significantly diminished in nuclei exposed to the same domain bearing the Ig-fold mutation. Using the crystal structures of the lamin Ig-fold and PCNA, molecular docking simulations suggest probable interaction sites. These findings also provide insights into the mechanisms underlying the numerous disease-causing mutations located within the lamin Ig-fold.

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Aurora B kinase and protein phosphatase 1 have opposing roles in modulating kinetochore assembly.

Publication Date: 2008 Apr 21 PMID: 18426974
Authors: Emanuele, M. J. - Lan, W. - Jwa, M. - Miller, S. A. - Chan, C. S. - Stukenberg, P. T.
Journal: J Cell Biol

The outer kinetochore binds microtubules to control chromosome movement. Outer kinetochore assembly is restricted to mitosis, whereas the inner kinetochore remains tethered to centromeres throughout the cell cycle. The cues that regulate this transient assembly are unknown. We find that inhibition of Aurora B kinase significantly reduces outer kinetochore assembly in Xenopus laevis and human tissue culture cells, frog egg extracts, and budding yeast. In X. leavis M phase extracts, preassembled kinetochores disassemble after inhibiting Aurora B activity with either drugs or antibodies. Kinetochore disassembly, induced by Aurora B inhibition, is rescued by restraining protein phosphatase 1 (PP1) activity. PP1 is necessary for kinetochores to disassemble at the exit from M phase, and purified enzyme is sufficient to cause disassembly on isolated mitotic nuclei. These data demonstrate that Aurora B activity is required for kinetochore maintenance and that PP1 is necessary and sufficient to disassemble kinetochores. We suggest that Aurora B and PP1 coordinate cell cycle-dependent changes in kinetochore assembly though phosphorylation of kinetochore substrates.

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The peroxisome: a production in four acts.

Publication Date: 2008 Apr 21 PMID: 18426973
Authors: Rout, M. P.
Journal: J Cell Biol

A cell regulates the number, size, and kind of each organelle it possesses in response to its particular role in an environment or tissue. Yet we still know little about how the molecular signaling networks within each cell perform such regulation. In this issue, Saleem et al. (Saleem, R.A., B. Knoblach, F.D. Mast, J.J. Smith, J. Boyle, C.M. Dobson, R. Long-O'Donnell, R.A. Rachubinski, and J.D. Aitchison. 2008. J. Cell Biol. 181:281-292) show for the first time how groups of kinases and phosphatases are organized to control when and how a cell assembles one kind of organelle, the peroxisome.

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Sir John Gurdon: godfather of cloning. Interviewed by Ruth Williams.

Publication Date: 2008 Apr 21 PMID: 18426972
Authors: Gurdon, S. J.
Journal: J Cell Biol

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Spinophilin and the immune synapse.

Publication Date: 2008 Apr 21 PMID: 18411314
Authors: Seed, B. - Xavier, R.
Journal: J Cell Biol

Extensive alterations in cellular organization are known to accompany the responses of sensitized T cells to target cells presenting an antigen of interest. Now, equally if not more dramatic changes are found to take place in cells presenting an antigen. With the help of a spinophilin-GFP fusion protein, Bloom et al. (Bloom, O., J.J. Unternaehrer, A. Jiang, J.-S. Shin, L. Delamarre, P. Allen, and I. Mellman. 2008. J. Cell Biol. 181:203-211) have captured a remarkable polarization of the cellular architecture of dendritic cells presenting an antigen to T cells.

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Decoding ARE-mediated decay: is microRNA part of the equation?

Publication Date: 2008 Apr 21 PMID: 18411313
Authors: von Roretz, C. - Gallouzi, I. E.
Journal: J Cell Biol

Messenger ribonucleic acids (mRNAs) containing adenine/uridine-rich elements (AREs) in their 3' untranslated region are particularly labile, allowing for the regulation of expression for growth factors, oncoproteins, and cytokines. The regulators, effectors, and location of ARE-mediated decay (AMD) have been investigated by many groups in recent years, and several links have been found between AMD and microRNA-mediated decay. We highlight these similarities, along with recent advances in the field of AMD, and also mention how there is still much left unknown surrounding this specialized mode of mRNA decay.

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Spinophilin participates in information transfer at immunological synapses.

Publication Date: 2008 Apr 21 PMID: 18411312
Authors: Bloom, O. - Unternaehrer, J. J. - Jiang, A. - Shin, J. S. - Delamarre, L. - Allen, P. - Mellman, I.
Journal: J Cell Biol

The adaptive immune response is initiated by the presentation of peptides bound to major histocompatibility complex molecules on dendritic cells (DCs) to antigen-specific T lymphocytes at a junction termed the immunological synapse. Although much attention has been paid to cytoplasmic events on the T cell side of the synapse, little is known concerning events on the DC side. We have sought signal transduction components of the neuronal synapse that were also expressed by DCs. One such protein is spinophilin, a scaffolding protein of neuronal dendritic spines that regulates synaptic transmission. In inactive, immature DCs, spinophilin is located throughout the cytoplasm but redistributes to the plasma membrane upon stimulus-induced maturation. In DCs interacting with T cells, spinophilin is polarized dynamically to contact sites in an antigen-dependent manner. It is also required for optimal T cell activation because DCs derived from mice lacking spinophilin exhibit defects in antigen presentation both in vitro and in vivo. Thus, spinophilin may play analogous roles in information transfer at both neuronal and immunological synapses.

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Cell polarization during monopolar cytokinesis.

Publication Date: 2008 Apr 21 PMID: 18411311
Authors: Hu, C. K. - Coughlin, M. - Field, C. M. - Mitchison, T. J.
Journal: J Cell Biol

During cytokinesis, a specialized set of proteins is recruited to the equatorial region between spindle poles by microtubules and actin filaments, enabling furrow assembly and ingression before cell division. We investigate the mechanisms underlying regional specialization of the cytoskeleton in HeLa cells undergoing drug-synchronized monopolar cytokinesis. After forced mitotic exit, the cytoskeleton of monopolar mitotic cells is initially radially symmetric but undergoes a symmetry-breaking reaction that simultaneously polarizes microtubules and the cell cortex, with a concentration of cortical furrow markers into a cap at one side of the cell. Polarization requires microtubules, F-actin, RhoA, Myosin II activity, and Aurora B kinase activity. Aurora B localizes to actin cables in a gap between the monopolar midzone and the furrow-like cortex, suggesting a communication between them. We propose that feedback loops between cortical furrow components and microtubules promote symmetry breaking during monopolar cytokinesis and regional specialization of the cytoskeleton during normal bipolar cytokinesis.

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Flightless-I regulates proinflammatory caspases by selectively modulating intracellular localization and caspase activity.

Publication Date: 2008 Apr 21 PMID: 18411310
Authors: Li, J. - Yin, H. L. - Yuan, J.
Journal: J Cell Biol

Caspase-1 and caspase-11 are proinflammatory caspases that regulate cytokine production and leukocyte migration during pathogen infection. In an attempt to identify new intracellular regulators of caspase-11, we found that Flightless-I, a member of the gelsolin superfamily of actin-remodeling proteins, interacts and regulates both caspase-11 and caspase-1. Flightless-I targets caspase-11 to the Triton X-100-insoluble cytoskeleton fraction and the cell leading edge. In addition, Flightless-I inhibits caspase-1 activation and caspase-1-mediated interleukine-1beta (IL-1beta) maturation. The physiological relevance of these findings is supported by the opposing effects of Flightless-I overexpression and knockdown on caspase-1 activity and IL-1beta maturation. Our results suggest that Flightless-I may be a bona fide caspase-1 inhibitor that acts through a mechanism similar to that of cytokine response modifier A, a potent caspase-1 inhibitor from the cowpox virus. Our study provides a new mechanism controlling the localization and activation of proinflammatory caspases.

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DDA3 recruits microtubule depolymerase Kif2a to spindle poles and controls spindle dynamics and mitotic chromosome movement.

Publication Date: 2008 Apr 21 PMID: 18411309
Authors: Jang, C. Y. - Wong, J. - Coppinger, J. A. - Seki, A. - Yates, J. R. 3rd - Fang, G.
Journal: J Cell Biol

Dynamic turnover of the spindle is a driving force for chromosome congression and segregation in mitosis. Through a functional genomic analysis, we identify DDA3 as a previously unknown regulator of spindle dynamics that is essential for mitotic progression. DDA3 depletion results in a high frequency of unaligned chromosomes, a substantial reduction in tension across sister kinetochores at metaphase, and a decrease in the velocity of chromosome segregation at anaphase. DDA3 associates with the mitotic spindle and controls microtubule (MT) dynamics. Mechanistically, DDA3 interacts with the MT depolymerase Kif2a in an MT-dependent manner and recruits Kif2a to the mitotic spindle and spindle poles. Depletion of DDA3 increases the steady-state levels of spindle MTs by reducing the turnover rate of the mitotic spindle and by increasing the rate of MT polymerization, which phenocopies the effects of partial knockdown of Kif2a. Thus, DDA3 represents a new class of MT-destabilizing protein that controls spindle dynamics and mitotic progression by regulating MT depolymerases.

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Constitutive phosphorylation of MDC1 physically links the MRE11-RAD50-NBS1 complex to damaged chromatin.

Publication Date: 2008 Apr 21 PMID: 18411308
Authors: Spycher, C. - Miller, E. S. - Townsend, K. - Pavic, L. - Morrice, N. A. - Janscak, P. - Stewart, G. S. - Stucki, M.
Journal: J Cell Biol

The MRE11-RAD50-Nijmegen breakage syndrome 1 (NBS1 [MRN]) complex accumulates at sites of DNA double-strand breaks (DSBs) in microscopically discernible nuclear foci. Focus formation by the MRN complex is dependent on MDC1, a large nuclear protein that directly interacts with phosphorylated H2AX. In this study, we identified a region in MDC1 that is essential for the focal accumulation of the MRN complex at sites of DNA damage. This region contains multiple conserved acidic sequence motifs that are constitutively phosphorylated in vivo. We show that these motifs are efficiently phosphorylated by caseine kinase 2 (CK2) in vitro and directly interact with the N-terminal forkhead-associated domain of NBS1 in a phosphorylation-dependent manner. Mutation of these conserved motifs in MDC1 or depletion of CK2 by small interfering RNA disrupts the interaction between MDC1 and NBS1 and abrogates accumulation of the MRN complex at sites of DNA DSBs in vivo. Thus, our data reveal the mechanism by which MDC1 physically couples the MRN complex to damaged chromatin.

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Phosphorylation of SDT repeats in the MDC1 N terminus triggers retention of NBS1 at the DNA damage-modified chromatin.

Publication Date: 2008 Apr 21 PMID: 18411307
Authors: Melander, F. - Bekker-Jensen, S. - Falck, J. - Bartek, J. - Mailand, N. - Lukas, J.
Journal: J Cell Biol

DNA double-strand breaks (DSBs) trigger accumulation of the MRE11-RAD50-Nijmegen breakage syndrome 1 (NBS1 [MRN]) complex, whose retention on the DSB-flanking chromatin facilitates survival. Chromatin retention of MRN requires the MDC1 adaptor protein, but the mechanism behind the MRN-MDC1 interaction is unknown. We show that the NBS1 subunit of MRN interacts with the MDC1 N terminus enriched in Ser-Asp-Thr (SDT) repeats. This interaction was constitutive and mediated by binding between the phosphorylated SDT repeats of MDC1 and the phosphate-binding forkhead-associated domain of NBS1. Phosphorylation of the SDT repeats by casein kinase 2 (CK2) was sufficient to trigger MDC1-NBS1 interaction in vitro, and MDC1 associated with CK2 activity in cells. Inhibition of CK2 reduced SDT phosphorylation in vivo, and disruption of the SDT-associated phosphoacceptor sites prevented the retention of NBS1 at DSBs. Together, these data suggest that phosphorylation of the SDT repeats in the MDC1 N terminus functions to recruit NBS1 and, thereby, increases the local concentration of MRN at the sites of chromosomal breakage.

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Leptin affects endocardial cushion formation by modulating EMT and migration via Akt signaling cascades.

Publication Date: 2008 Apr 21 PMID: 18411306
Authors: Nath, A. K. - Brown, R. M. - Michaud, M. - Sierra-Honigmann, M. R. - Snyder, M. - Madri, J. A.
Journal: J Cell Biol

Blood circulation is dependent on heart valves to direct blood flow through the heart and great vessels. Valve development relies on epithelial to mesenchymal transition (EMT), a central feature of embryonic development and metastatic cancer. Abnormal EMT and remodeling contribute to the etiology of several congenital heart defects. Leptin and its receptor were detected in the mouse embryonic heart. Using an ex vivo model of cardiac EMT, the inhibition of leptin results in a signal transducer and activator of transcription 3 and Snail/vascular endothelial cadherin-independent decrease in EMT and migration. Our data suggest that an Akt signaling pathway underlies the observed phenotype. Furthermore, loss of leptin phenocopied the functional inhibition of alphavbeta3 integrin receptor and resulted in decreased alphavbeta3 integrin and matrix metalloprotease 2, suggesting that the leptin signaling pathway is involved in adhesion and migration processes. This study adds leptin to the repertoire of factors that mediate EMT and, for the first time, demonstrates a role for the interleukin 6 family in embryonic EMT.

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