Latest Articles Include:
- Mentoring matters
- Nature Cell Biology 12(2):101 (2010)
Mentoring enhances the experience of mentees; however, mentors and their institutions will also reap the rewards of a mentorship culture.
- Tipping the scale: muscle versus fat
- Nature Cell Biology 12(2):102-104 (2010)
Adipocytes and scar tissue form during skeletal muscle degeneration. Two new studies reveal that adipocytes and fibroblasts in skeletal muscle derive from a population of bipotent progenitors that reside within muscle, but are not derived from the muscle lineage. These progenitor cells also have a surprising role in stimulating the restoration of muscle mass during regeneration.
- Mitochondria get a Parkin' ticket
- Nature Cell Biology 12(2):104-106 (2010)
Recent studies have revealed a prominent role of mitochondrial dysfunction in the development of one of the most common neurodegenerative disorders, Parkinson´s disease. The ubiquitin ligase Parkin and the protein kinase PINK1, whose mutations are associated with Parkinson´s disease, function in a pathway that links ubiquitylation with selective autophagy of damaged mitochondria.
- Cohesin and Cdk1: an anaphase barricade
- Nature Cell Biology 12(2):106-108 (2010)
Separation of sister chromatids at anaphase in metazoan cells requires only the cleavage of the kleisin subunit of centromeric cohesin, but efficient poleward movement of separated sisters requires the associated loss in Cdk1 activity. Activation of the anaphase-promoting complex/cyclosome ensures these events are coordinated.
- Research highlights
- Nature Cell Biology 12(2):109 (2010)
The RanGTP gradient surrounding chromosomes drives mitotic spindle assembly by allowing the local release of several factors from the importin–α/β heterodimer, including microtubule regulators. Eric Karsenti and colleagues (J. Cell Biol. 187, 813–829; 2009
- DNA zip codes control an ancient mechanism for gene targeting to the nuclear periphery
- Nature Cell Biology 12(2):111-118 (2010)
Many genes in Saccharomyces cerevisiae are recruited to the nuclear periphery after transcriptional activation. We have identified two gene recruitment sequences (GRS I and II) from the promoter of the INO1 gene that target the gene to the nuclear periphery. These GRSs function as DNA zip codes and are sufficient to target a nucleoplasmic locus to the nuclear periphery. Targeting requires components of the nuclear pore complex (NPC) and a GRS is sufficient to confer a physical interaction with the NPC. GRS I elements are enriched in promoters of genes that interact with the NPC, and genes that are induced by protein folding stress. Full transcriptional activation of INO1 and another GRS-containing gene requires GRS-mediated targeting of the promoter to the nuclear periphery. Finally, GRS I also functions as a DNA zip code in Schizosaccharomyces pombe, suggesting that this mechanism of targeting to the nuclear periphery has been conserved over approximately one billion ! years of evolution.
- PINK1/Parkin-mediated mitophagy is dependent on VDAC1 and p62/SQSTM1
Geisler S Holmström KM Skujat D Fiesel FC Rothfuss OC Kahle PJ Springer W - Nature Cell Biology 12(2):119-131 (2010)
Parkinson's disease is the most common neurodegenerative movement disorder. Mutations in PINK1 and PARKIN are the most frequent causes of recessive Parkinson's disease. However, their molecular contribution to pathogenesis remains unclear. Here, we reveal important mechanistic steps of a PINK1/Parkin-directed pathway linking mitochondrial damage, ubiquitylation and autophagy in non-neuronal and neuronal cells. PINK1 kinase activity and its mitochondrial localization sequence are prerequisites to induce translocation of the E3 ligase Parkin to depolarized mitochondria. Subsequently, Parkin mediates the formation of two distinct poly-ubiquitin chains, linked through Lys 63 and Lys 27. In addition, the autophagic adaptor p62/SQSTM1 is recruited to mitochondrial clusters and is essential for the clearance of mitochondria. Strikingly, we identified VDAC1 (voltage-dependent anion channel 1) as a target for Parkin-mediated Lys 27 poly-ubiquitylation and mitophagy. Moreover, p! athogenic Parkin mutations interfere with distinct steps of mitochondrial translocation, ubiquitylation and/or final clearance through mitophagy. Thus, our data provide functional links between PINK1, Parkin and the selective autophagy of mitochondria, which is implicated in the pathogenesis of Parkinson's disease.
- Histone deacetylase and Cullin3–RENKCTD11 ubiquitin ligase interplay regulates Hedgehog signalling through Gli acetylation
- Nature Cell Biology 12(2):132-142 (2010)
Hedgehog signalling is crucial for development and is deregulated in several tumours, including medulloblastoma. Regulation of the transcriptional activity of Gli (glioma-associated oncogene) proteins, effectors of the Hedgehog pathway, is poorly understood. We show here that Gli1 and Gli2 are acetylated proteins and that their HDAC-mediated deacetylation promotes transcriptional activation and sustains a positive autoregulatory loop through Hedgehog-induced upregulation of HDAC1. This mechanism is turned off by HDAC1 degradation through an E3 ubiquitin ligase complex formed by Cullin3 and REN, a Gli antagonist lost in human medulloblastoma. Whereas high HDAC1 and low REN expression in neural progenitors and medulloblastomas correlates with active Hedgehog signalling, loss of HDAC activity suppresses Hedgehog-dependent growth of neural progenitors and tumour cells. Consistent with this, abrogation of Gli1 acetylation enhances cellular proliferation and transformation. ! These data identify an integrated HDAC- and ubiquitin-mediated circuitry, where acetylation of Gli proteins functions as an unexpected key transcriptional checkpoint of Hedgehog signalling.
- Mesenchymal progenitors distinct from satellite cells contribute to ectopic fat cell formation in skeletal muscle
- Nature Cell Biology 12(2):143-152 (2010)
Ectopic fat deposition in skeletal muscle is closely associated with several disorders, however, the origin of these adipocytes is not clear, nor is the mechanism of their formation. Satellite cells function as adult muscle stem cells but are proposed to possess multipotency. Here, we prospectively identify PDGFRα+ mesenchymal progenitors as being distinct from satellite cells and located in the muscle interstitium. We show that, of the muscle-derived cell populations, only PDGFRα+ cells show efficient adipogenic differentiation both in vitro and in vivo. Reciprocal transplantations between regenerating and degenerating muscles, and co-culture experiments revealed that adipogenesis of PDGFRα+ cells is strongly inhibited by the presence of satellite cell-derived myofibres. These results suggest that PDGFRα+ mesenchymal progenitors are the major contributor to ectopic fat cell formation in skeletal muscle, and emphasize that interaction between muscle cells and PDGFR! α+ mesenchymal progenitors, not the fate decision of satellite cells, has a considerable impact on muscle homeostasis.
- Muscle injury activates resident fibro/adipogenic progenitors that facilitate myogenesis
- Nature Cell Biology 12(2):153-163 (2010)
Efficient tissue regeneration is dependent on the coordinated responses of multiple cell types. Here, we describe a new subpopulation of fibro/adipogenic progenitors (FAPs) resident in muscle tissue but arising from a distinct developmental lineage. Transplantation of purified FAPs results in the generation of ectopic white fat when delivered subcutaneously or intramuscularly in a model of fatty infiltration, but not in healthy muscle, suggesting that the environment controls their engraftment. These cells are quiescent in intact muscle but proliferate efficiently in response to damage. FAPs do not generate myofibres, but enhance the rate of differentiation of primary myogenic progenitors in co-cultivation experiments. In summary, FAPs expand upon damage to provide a transient source of pro-differentiation signals for proliferating myogenic progenitors.
- The Nup107-160 complex and γ-TuRC regulate microtubule polymerization at kinetochores
Mishra RK Chakraborty P Arnaoutov A Fontoura BM Dasso M - Nature Cell Biology 12(2):164-169 (2010)
The metazoan nuclear pore complex (NPC) disassembles during mitosis, and many of its constituents distribute onto spindles and kinetochores, including the Nup107-160 sub-complex1, 2. We have found that Nup107-160 interacts with the γ-tubulin ring complex (γ-TuRC), an essential and conserved microtubule nucleator3, 4, and recruits γ-TuRC to unattached kinetochores. The unattached kinetochores nucleate microtubules in a manner that is regulated by Ran GTPase5; such microtubules contribute to the formation of kinetochore fibres (k-fibres)6, microtubule bundles connecting kinetochores to spindle poles. Our data indicate that Nup107-160 and γ-TuRC act cooperatively to promote spindle assembly through microtubule nucleation at kinetochores: HeLa cells lacking Nup107-160 or γ-TuRC were profoundly deficient in kinetochore-associated microtubule nucleation. Moreover, co-precipitated Nup107-160– γ-TuRC complexes nucleated microtubule formation in assays using purified tu! bulin. Although Ran did not regulate microtubule nucleation by γ-TuRC alone, Nup107-160–γ-TuRC complexes required Ran–GTP for microtubule nucleation. Collectively, our observations show that Nup107-160 promotes spindle assembly through Ran–GTP-regulated nucleation of microtubules by γ-TuRC at kinetochores, and reveal a relationship between nucleoporins and the microtubule cytoskeleton.
- Planar polarization of node cells determines the rotational axis of node cilia
- Nature Cell Biology 12(2):170-176 (2010)
Rotational movement of the node cilia generates a leftward fluid flow in the mouse embryo1 because the cilia are posteriorly tilted2, 3. However, it is not known how anterior-posterior information is translated into the posterior tilt of the node cilia. Here, we show that the basal body of node cilia is initially positioned centrally but then gradually shifts toward the posterior side of the node cells. Positioning of the basal body and unidirectional flow were found to be impaired in compound mutant mice lacking Dvl genes. Whereas the basal body was normally positioned in the node cells of Wnt3a−/− embryos, inhibition of Rac1, a component of the noncanonical Wnt signalling pathway, impaired the polarized localization of the basal body in wild-type embryos. Dvl2 and Dvl3 proteins were found to be localized to the apical side of the node cells, and their location was polarized to the posterior side of the cells before the posterior positioning of the basal body. The! se results suggest that posterior positioning of the basal body, which provides the posterior tilt to node cilia, is determined by planar polarization mediated by noncanonical Wnt signalling.
- 53BP1-dependent robust localized KAP-1 phosphorylation is essential for heterochromatic DNA double-strand break repair
- Nature Cell Biology 12(2):177-184 (2010)
DNA double-strand breaks (DSBs) trigger ATM (ataxia telangiectasia mutated) signalling and elicit genomic rearrangements and chromosomal fragmentation if misrepaired or unrepaired. Although most DSB repair is ATM-independent, ~15% of ionizing radiation (IR)-induced breaks persist in the absence of ATM-signalling1. 53BP1 (p53-binding protein 1) facilitates ATM-dependent DSB repair but is largely dispensable for ATM activation or checkpoint arrest. ATM promotes DSB repair within heterochromatin by phosphorylating KAP-1 (KRAB-associated protein 1, also known as TIF1β, TRIM28 or KRIP-1; ref. 2). Here, we show that the ATM signalling mediator proteins MDC1, RNF8, RNF168 and 53BP1 are also required for heterochromatic DSB repair. Although KAP-1 phosphorylation is critical for 53BP1-mediated repair, overall phosphorylated KAP-1 (pKAP-1) levels are only modestly affected by 53BP1 loss. pKAP-1 is transiently pan-nuclear but also forms foci overlapping with γH2AX in heterochro! matin. Cells that do not form 53BP1 foci, including human RIDDLE (radiosensitivity, immunodeficiency, dysmorphic features and learning difficulties) syndrome cells, fail to form pKAP-1 foci. 53BP1 amplifies Mre11–NBS1 accumulation at late-repairing DSBs, concentrating active ATM and leading to robust, localized pKAP-1. We propose that ionizing-radiation induced foci (IRIF) spatially concentrate ATM activity to promote localized alterations in regions of chromatin otherwise inhibitory to repair.
- Cohesin cleavage and Cdk inhibition trigger formation of daughter nuclei
- Nature Cell Biology 12(2):185-192 (2010)
The metaphase–anaphase transition is orchestrated through proteolysis of numerous proteins by a ubiquitin protein ligase called the anaphase-promoting complex or cyclosome (APC/C)1. A crucial aspect of this process is sister chromatid separation, which is thought to be mediated by separase, a thiol protease activated by the APC/C. Separase cleaves cohesin, a ring-shaped complex that entraps sister DNAs2, 3. It is a matter of debate whether cohesin-independent forces also contribute to sister chromatid cohesion4, 5, 6. Using 4D live-cell imaging of Drosophila melanogaster syncytial embryos blocked in metaphase (via APC/C inhibition), we show that artificial cohesin cleavage7 is sufficient to trigger chromosome disjunction. This is nevertheless insufficient for correct chromosome segregation. Kinetochore–microtubule attachments are rapidly destabilized by the loss of tension caused by cohesin cleavage in the presence of high Cdk1 (cyclin-dependent kinase 1) activity,! as occurs when the APC/C cannot destroy mitotic cyclins. Metaphase chromosomes undergo a bona fide anaphase when cohesin cleavage is combined with Cdk1 inhibition. We conclude that only two key events, opening of cohesin rings and downregulation of Cdk1, are sufficient to drive proper segregation of chromosomes in anaphase.
- Regulation of NF-κB inhibitor IκBα and viral replication by a KSHV microRNA
- Nature Cell Biology 12(2):193-199 (2010)
Kaposi's sarcoma-associated herpesvirus (KSHV) is causally linked to several acquired immune deficiency syndrome-related malignancies, including Kaposi's sarcoma, primary effusion lymphoma (PEL) and a subset of multicentric Castleman's disease1. Control of viral lytic replication is essential for KSHV latency, evasion of the host immune system and induction of tumours1. Here, we show that deletion of a 14 microRNA (miRNA) cluster from the KSHV genome significantly enhances viral lytic replication as a result of reduced NF-κB activity. The miRNA cluster regulates the NF-κB pathway by reducing expression of IκBα protein, an inhibitor of NF-κB complexes. Computational and miRNA seed mutagenesis analyses were used to identify KSHV miR-K1, which directly regulates the IκBα protein level by targeting the 3′UTR of its transcript. Expression of miR-K1 is sufficient to rescue NF-κB activity and inhibit viral lytic replication, whereas inhibition of miR-K1 in KSHV-infe! cted PEL cells has the opposite effect. Thus, KSHV encodes an miRNA to control viral replication by activating the NF-κB pathway. These results demonstrate an important role for KSHV miRNAs in regulating viral latency and lytic replication by manipulating the host survival pathway.
- Phosphorylation of STIM1 underlies suppression of store-operated calcium entry during mitosis
- Nature Cell Biology 12(2):199 (2010)
Introduction ; published online 1 November 2009; corrected after print, 12 January 2010 In both the HTML and PDF versions of this letter, statements regarding the suppression of SOCE during mitosis have been altered as follows: on page 1456 (abstract, line 2), the following text has replaced the previous text, "Store-operated Ca2+ entry (SOCE) and Ca2+ release-activated Ca2+ currents (Icrac) are strongly suppressed during cell division, the only known physiological situation in which Ca2+ store depletion is uncoupled from the activation of Ca2+ influx." On page 1465 (right column, second paragraph, line 5), the following text has replaced the previous text, "The only known physiological situation in which SOCE seems to be strongly negatively regulated is during cell division16–19, 37." An additional citation has been added to the reference list: 37. Machaca K. & Huan S.Store-operated calcium entry inactivates at the germinal vesicle breakdown stage of Xenopus meiosis. J. Biol. Chem. 275, 38710–38715 (2000). In the Methods, references 38, 39 and 40 have been changed from 37, 38 and 39, respectively.