Hot off the presses! May 01 Nat Cell Biol

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Latest Articles Include:

• Genetic privacy and piracy
  - Nat Cell Biol 11(5):509 (2009)
  
• Focus on host subversion
  - Nat Cell Biol 11(5):509 (2009)
  
• Virus entry by macropinocytosis
  - Nat Cell Biol 11(5):510-520 (2009)
  As obligatory intracellular parasites, viruses rely on host-cell functions for most aspects of their replication cycle. This is born out during entry, when most viruses that infect vertebrate and insect cells exploit the endocytic activities of the host cell to move into the cytoplasm. Viruses belonging to vaccinia, adeno, picorna and other virus families have been reported to take advantage of macropinocytosis, an endocytic mechanism normally involved in fluid uptake. The virus particles first activate signalling pathways that trigger actin-mediated membrane ruffling and blebbing. Usually, this is followed by the formation of large vacuoles (macropinosomes) at the plasma membrane, internalization of virus particles and penetration by the viruses or their capsids into the cytosol through the limiting membrane of the macropinosomes. We review the molecular machinery involved in macropinocytosis and describe what is known about its role in virus entry.
• Targeting of immune signalling networks by bacterial pathogens
  - Nat Cell Biol 11(5):521-526 (2009)
  Host defence against microbial pathogens requires appropriate coordination of multiple signalling pathways. These pathways are triggered by innate immune recognition of conserved microbial molecules, and initiate an inflammatory cascade that involves recruitment of leukocytes to the site of infection, activation of antimicrobial effector mechanisms and induction of an adaptive immune response that promotes clearance of infection and long-term immune memory. Microbial pathogens possess specialized proteins termed virulence factors, which interfere with host defence at several levels. Many virulence factors from diverse pathogens have been identified in recent years and their functions linked to disruption of essential processes of immune defence, from signalling to phagocytosis. Although the diversity of pathogens and virulence factors is immense, common themes have emerged with regard to how microbial pathogens interfere with immune responses. Here we discuss recent ad! vances in our understanding of how virulence factors target innate and adaptive immune responses, focusing on bacterial pathogens. We also propose that pathogens responsible for causing acute infection tend to target central components (hubs) of cellular signalling pathways, causing global disruption of the host response. By contrast, pathogens that cause chronic or persistent infections tend to target more peripheral signalling network components (nodes) to promote pathogen persistence.
• Viral avoidance and exploitation of the ubiquitin system
  - Nat Cell Biol 11(5):527-534 (2009)
  The versatility of ubiquitin in regulating protein function and cell behaviour through post-translational protein modification makes it a particularly attractive target for viruses. Here we review how viruses manipulate the ubiquitin system to favour their propagation by redirecting cellular ubiquitin enzymes or encoding their own ubiquitin components to enable replication, egress and immune evasion. These studies not only reveal the many cellular processes requiring ubiquitin but also illustrate how viruses usurp their host cells.
• From cell differentiation to the cell cycle: how failing in biochemistry led to success in morphology
  - Nat Cell Biol 11(5):535 (2009)
  
• Breaking a temporal barrier: signalling crosstalk regulates the initiation of border cell migration
  - Nat Cell Biol 11(5):536-538 (2009)
  
• Smurf1 zaps the talin head
  - Nat Cell Biol 11(5):538-540 (2009)
  
• SCAI blocks MAL-evolent effects on cancer cell invasion
  - Nat Cell Biol 11(5):540-542 (2009)
  
• Research highlights
  - Nat Cell Biol 11(5):543 (2009)
  
• Protein kinase D1 regulates cofilin-mediated F-actin reorganization and cell motility through slingshot
  - Nat Cell Biol 11(5):545-556 (2009)
  Dynamic actin remodelling processes at the leading edge of migrating tumour cells are concerted events controlled by a fine-tuned temporal and spatial interplay of kinases and phosphatases. Actin severing is regulated by actin depolymerizing factor (ADF)/cofilin, which regulates stimulus-induced lamellipodia protrusion and directed cell motility. Cofilin is activated by dephosphorylation through phosphatases of the slingshot (SSH) family. SSH activity is strongly increased by its binding to filamentous actin (F-actin); however, other upstream regulators remain unknown. Here we show that in response to RhoA activation, protein kinase D1 (PKD1) phosphorylates the SSH enzyme SSH1L at a serine residue located in its actin-binding motif. This generates a 14-3-3-binding motif and blocks the localization of SSH1L to F-actin-rich structures in the lamellipodium by sequestering it in the cytoplasm. Consequently, expression of constitutively active PKD1 in invasive tumour cells ! enhanced the phosphorylation of cofilin and effectively blocked the formation of free actin-filament barbed ends and directed cell migration.
• SCAI acts as a suppressor of cancer cell invasion through the transcriptional control of beta1-integrin
  - Nat Cell Biol 11(5):557-568 (2009)
  Gene expression reprogramming governs cellular processes such as proliferation, differentiation and cell migration through the complex and tightly regulated control of transcriptional cofactors that exist in multiprotein complexes. Here we describe SCAI (suppressor of cancer cell invasion), a novel and highly conserved protein that regulates invasive cell migration through three-dimensional matrices. SCAI acts on the RhoA–Dia1 signal transduction pathway and localizes in the nucleus, where it binds and inhibits the myocardin-related transcription factor MAL by forming a ternary complex with serum response factor (SRF). Genome-wide expression analysis surprisingly reveals that one of the strongest upregulated genes after suppression of SCAI is beta1-integrin. Decreased levels of SCAI are tightly correlated with increased invasive cell migration, and SCAI is downregulated in several human tumours. Functional analysis of the beta1-integrin gene strongly argues that SCAI! is a novel transcriptional cofactor that controls gene expression downstream of Dia1 to dictate changes in cell invasive behaviour.
• Border-cell migration requires integration of spatial and temporal signals by the BTB protein Abrupt
  - Nat Cell Biol 11(5):569-579 (2009)
  During development, elaborate patterns of cell differentiation and movement must occur in the correct locations and at the proper times. Developmental timing has been studied less than spatial pattern formation, and the mechanisms integrating the two are poorly understood. Border-cell migration in the Drosophila ovary occurs specifically at stage 9. Timing of the migration is regulated by the steroid hormone ecdysone, whereas spatial patterning of the migratory population requires localized activity of the JAK–STAT pathway. Ecdysone signalling is patterned spatially as well as temporally, although the mechanisms are not well understood. In stage 9 egg chambers, ecdysone signalling is highest in anterior follicle cells including the border cells. We identify the gene abrupt as a repressor of ecdysone signalling and border-cell migration. Abrupt protein is normally lost from border-cell nuclei during stage 9, in response to JAK–STAT activity. This contributes to the ! spatial pattern of the ecdysone response. Abrupt attenuates ecdysone signalling by means of a direct interaction with the basic helix–loop–helix (bHLH) domain of the P160 ecdysone receptor coactivator Taiman (Tai). Taken together, these findings provide a molecular mechanism by which spatial and temporal cues are integrated.
• KRAB-type zinc-finger protein Apak specifically regulates p53-dependent apoptosis
  - Nat Cell Biol 11(5):580-591 (2009)
  Only a few p53 regulators have been shown to participate in the selective control of p53-mediated cell cycle arrest or apoptosis. How p53-mediated apoptosis is negatively regulated remains largely unclear. Here we report that Apak (ATM and p53-associated KZNF protein), a Krüppel-associated box (KRAB)-type zinc-finger protein, binds directly to p53 in unstressed cells, specifically downregulates pro-apoptotic genes, and suppresses p53-mediated apoptosis by recruiting KRAB-box-associated protein (KAP)-1 and histone deacetylase 1 (HDAC1) to attenuate the acetylation of p53. Apak inhibits p53 activity by interacting with ATM, a previously identified p53 activator. In response to stress, Apak is phosphorylated by ATM and dissociates from p53, resulting in activation of p53 and induction of apoptosis. These findings revealed Apak to be a negative regulator of p53-mediated apoptosis and showed the dual role of ATM in p53 regulation.
• RAD18 transmits DNA damage signalling to elicit homologous recombination repair
  - Nat Cell Biol 11(5):592-603 (2009)
  To maintain genome stability, cells respond to DNA damage by activating signalling pathways that govern cell-cycle checkpoints and initiate DNA repair. Cell-cycle checkpoint controls should connect with DNA repair processes, however, exactly how such coordination occurs in vivo is largely unknown. Here we describe a new role for the E3 ligase RAD18 as the integral component in translating the damage response signal to orchestrate homologous recombination repair (HRR). We show that RAD18 promotes homologous recombination in a manner strictly dependent on its ability to be recruited to sites of DNA breaks and that this recruitment relies on a well-defined DNA damage signalling pathway mediated by another E3 ligase, RNF8. We further demonstrate that RAD18 functions as an adaptor to facilitate homologous recombination through direct interaction with the recombinase RAD51C. Together, our data uncovers RAD18 as a key factor that orchestrates HRR through surveillance of the D! NA damage signal.
• Persistent transcription-blocking DNA lesions trigger somatic growth attenuation associated with longevity
  - Nat Cell Biol 11(5):604-615 (2009)
  The accumulation of stochastic DNA damage throughout an organism's lifespan is thought to contribute to ageing. Conversely, ageing seems to be phenotypically reproducible and regulated through genetic pathways such as the insulin-like growth factor-1 (IGF-1) and growth hormone (GH) receptors, which are central mediators of the somatic growth axis. Here we report that persistent DNA damage in primary cells from mice elicits changes in global gene expression similar to those occurring in various organs of naturally aged animals. We show that, as in ageing animals, the expression of IGF-1 receptor and GH receptor is attenuated, resulting in cellular resistance to IGF-1. This cell-autonomous attenuation is specifically induced by persistent lesions leading to stalling of RNA polymerase II in proliferating, quiescent and terminally differentiated cells; it is exacerbated and prolonged in cells from progeroid mice and confers resistance to oxidative stress. Our findings sugg! est that the accumulation of DNA damage in transcribed genes in most if not all tissues contributes to the ageing-associated shift from growth to somatic maintenance that triggers stress resistance and is thought to promote longevity.
• Telomere recombination requires the MUS81 endonuclease
  - Nat Cell Biol 11(5):616-623 (2009)
  Telomerase-negative cancer cells maintain their telomeres through the alternative lengthening of telomeres (ALT) pathway1, 2, 3. Although a growing body of evidence demonstrates that the ALT mechanism is a post-replicative telomere recombination process, molecular details of this pathway are largely unknown. Here we demonstrate that MUS81, a DNA structure specific recombination endonuclease, has a key role in the maintenance of telomeres in human ALT cells. We find that MUS81 specifically localizes to ALT-associated promyelocytic leukaemia (PML) nuclear bodies (APBs) and associates with telomeric DNA in ALT cells, which is enriched during the G2 phase of the cell cycle. Depletion of MUS81 results in the reduction of ALT-specific telomere recombination and leads to proliferation arrest of ALT cells. In addition, the endonuclease activity of MUS81 is required for recombination-based ALT cell survival, and the interaction of MUS81 with the telomeric repeat-binding factor ! TRF2 regulates this enzymatic activity, thereby maintaining telomere recombination. Thus, our results suggest that MUS81 is involved in the maintenance of ALT cell survival at least in part by homologous recombination of telomeres.
• Talin phosphorylation by Cdk5 regulates Smurf1-mediated talin head ubiquitylation and cell migration
  - Nat Cell Biol 11(5):624-630 (2009)
  Cell migration is a dynamic process that requires temporal and spatial regulation of integrin activation and focal adhesion assembly/disassembly1. Talin, an actin and beta-integrin tail-binding protein, is essential for integrin activation and focal adhesion formation2, 3. Calpain-mediated cleavage of talin has a key role in focal adhesion turnover3; however, the talin head domain, one of the two cleavage products, stimulates integrin activation, localizes to focal adhesions and maintains cell edge protrusions2, 4, 5, suggesting that other steps, downstream of talin proteolysis, are required for focal adhesion disassembly. Here we show that talin head binds Smurf1, an E3 ubiquitin ligase involved in cell polarity and migration6, 7, more tightly than full-length talin does and that this interaction leads to talin head ubiquitylation and degradation. We found that talin head is a substrate for Cdk5, a cyclin-dependent protein kinase that is essential for cell migration, ! synaptic transmission and cancer metastasis8, 9, 10, 11. Cdk5 phosphorylated talin head at Ser 425, inhibiting its binding to Smurf1, thus preventing talin head ubiquitylation and degradation. Expression of the mutant talS425A, which resists Cdk5 phosphorylation thereby increasing its susceptibility to Smurf1-mediated ubiqitylation, resulted in extensive focal adhesion turnover and inhibited cell migration. Thus, talin head produced by calpain-induced cleavage of talin is degraded through Smurf1-mediated ubiquitylation; moreover, phosphorylation by Cdk5 regulates the binding of Smurf1 to talin head, controlling talin head turnover, adhesion stability and ultimately, cell migration.
• Production of offspring from a germline stem cell line derived from neonatal ovaries
  - Nat Cell Biol 11(5):631-636 (2009)
  The idea that females of most mammalian species have lost the capacity for oocyte production at birth1, 2, 3, 4, 5 has been challenged recently by the finding that juvenile and adult mouse ovaries possess mitotically active germ cells6. However, the existence of female germline stem cells (FGSCs) in postnatal mammalian ovaries still remains a controversial issue among reproductive biologists and stem cell researchers6, 7, 8, 9, 10. We have now established a neonatal mouse FGSC line, with normal karyotype and high telomerase activity, by immunomagnetic isolation and culture for more than 15 months. FGSCs from adult mice were isolated and cultured for more than 6 months. These FGSCs were infected with GFP virus and transplanted into ovaries of infertile mice. Transplanted cells underwent oogenesis and the mice produced offspring that had the GFP transgene. These findings contribute to basic research into oogenesis and stem cell self-renewal and open up new possibilities ! for use of FGSCs in biotechnology and medicine.
• Bone morphogenetic protein heterodimers assemble heteromeric type I receptor complexes to pattern the dorsoventral axis
  Little SC Mullins MC - Nat Cell Biol 11(5):637-643 (2009)
  Patterning the embryonic dorsoventral axis of both vertebrates and invertebrates requires signalling through bone morphogenetic proteins (BMPs)1. Although a well-studied process, the identity of the physiologically relevant BMP signalling complex in the Drosophila melanogaster embryo is controversial2, 3, is generally inferred from cell culture studies and has not been investigated in vertebrates. Here, we demonstrate that dorsoventral patterning in zebrafish, Danio rerio, requires two classes of non-redundant type I BMP receptors, Alk3/6 and Alk8 (activin-like kinases 3/6 and 8). We show, under physiological conditions in the embryo, that these two type I receptor classes form a complex in a manner that depends on Bmp2 and Bmp7. We found that both Bmp2–7 heterodimers, as well as Bmp2 and Bmp7 homodimers, form in the embryo. However, only recombinant ligand heterodimers can activate BMP signalling in the early embryo, whereas a combination of Bmp2 and Bmp7 homodimers! cannot. We propose that only heterodimers, signalling through two distinct classes of type I receptor, possess sufficient receptor affinity in an environment of extracellular antagonists to elicit the signalling response required for dorsoventral patterning.
• PP1-mediated dephosphorylation of phosphoproteins at mitotic exit is controlled by inhibitor-1 and PP1 phosphorylation
  - Nat Cell Biol 11(5):644-651 (2009)
  Loss of cell division cycle 2 (Cdc2, also known as Cdk1) activity after cyclin B degradation is necessary, but not sufficient, for mitotic exit. Proteins phosphorylated by Cdc2 and downstream mitotic kinases must be dephosphorylated. We report here that protein phosphatase-1 (PP1) is the main catalyst of mitotic phosphoprotein dephosphorylation. Suppression of PP1 during early mitosis is maintained through dual inhibition by Cdc2 phosphorylation and the binding of inhibitor-1. Protein kinase A (PKA) phosphorylates inhibitor-1, mediating binding to PP1. As Cdc2 levels drop after cyclin B degradation, auto-dephosphorylation of PP1 at its Cdc2 phosphorylation site (Thr 320) allows partial PP1 activation. This promotes PP1-regulated dephosphorylation at the activating site of inhibitor-1 (Thr 35) followed by dissociation of the inhibitor-1–PP1 complex and then full PP1 activation to promote mitotic exit. Thus, Cdc2 both phosphorylates multiple mitotic substrates and inhi! bits their PP1-mediated dephosphorylation.
• Arginine methylation of Piwi proteins catalysed by dPRMT5 is required for Ago3 and Aub stability
  - Nat Cell Biol 11(5):652-658 (2009)
  Piwi family proteins are essential for germline development and bind piwi-interacting RNAs (piRNAs)1, 2, 3. The grandchildless gene aub of Drosophila melanogaster encodes the piRNA-binding protein Aubergine (Aub), which is essential for formation of primordial germ cells (PGCs)4. Here we report that Piwi family proteins of mouse, Xenopus laevis and Drosophila contain symmetrical dimethylarginines (sDMAs). We found that Piwi proteins are expressed in Xenopus oocytes and we identified numerous Xenopus piRNAs. We report that the Drosophila homologue of protein methyltransferase 5 (dPRMT5, csul/dart5), which is also the product of a grandchildless gene5, 6, is required for arginine methylation of Drosophila Piwi, Ago3 and Aub proteins in vivo. Loss of dPRMT5 activity led to a reduction in the levels of piRNAs, Ago3 and Aub proteins, and accumulation of retrotransposons in the Drosophila ovary. Our studies explain the relationship between aub and dPRMT5 (csul/dart5) genes b! y demonstrating that dPRMT5 is the enzyme that methylates Aub. Our findings underscore the significance of sDMA modification of Piwi proteins in the germline and suggest an interacting pathway of genes that are required for piRNA function and PGC specification.
• WIP1 phosphatase is a negative regulator of NF-kappaB signalling
  - Nat Cell Biol 11(5):659-666 (2009)
  Post-translational modifications of NF-kappaB through phosphorylations enhance its transactivation potential. Much is known about the kinases that phosphorylate NF-kappaB, but little is known about the phosphatases that dephosphorylate it. By using a genome-scale siRNA screen, we identified the WIP1 phosphatase as a negative regulator of NF-kappaB signalling. WIP1-mediated regulation of NF-kappaB occurs in both a p38-dependent and independent manner. Overexpression of WIP1 resulted in decreased NF-kappaB activation in a dose-dependent manner, whereas WIP1 knockdown resulted in increased NF-kappaB function. We show that WIP1 is a direct phosphatase of Ser 536 of the p65 subunit of NF-kappaB. Phosphorylation of Ser 536 is known to be essential for the transactivation function of p65, as it is required for recruitment of the transcriptional co-activator p300. WIP1-mediated regulation of p65 regulated binding of NF-kappaB to p300 and hence chromatin remodelling. Consistent! with our results, mice lacking WIP1 showed enhanced inflammation. These results provide the first genetic proof that a phosphatase directly regulates NF-kappaB signalling in vivo.