Hot off the presses! Jun 14 dev cell

The Jun 14 issue of the dev cell is now up on Pubget (About dev cell): if you're at a subscribing institution, just click the link in the latest link at the home page. (Note you'll only be able to get all the PDFs in the issue if your institution subscribes to Pubget.)

Latest Articles Include:

• When Genetic Instability Is a Good Thing
  - dev cell 20(6):e1 (2011)
  Many years of cell-cycle research have been devoted to understanding the mechanisms that replicate and repair the genome so that cells can proliferate without accumulating the genetic damage that drives the initiation and progression of cancer. A classic example of genetic instability in cancer cells is the amplification of segments of the genome in a normally diploid cell type that increases the copy number and thus the expression of oncogenes. However, those who study cell-cycle control in a developmental context know that these types of events are often a normal part of development. This paper by the Orr-Weaver laboratory furthered our understanding of developmentally regulated gene amplification that occurs in Drosophila ovaries. The authors used competitive genome hybridization to a cDNA microarray to identify new follicle cell amplicons required for egg production. This work set the stage for using modern genomic techniques to comprehensively identify gene amplif! ication events that drive the synthesis of large amounts of gene product needed to produce certain structures during development. This PaperPick refers to "Gene amplification as a developmental strategy: isolation of two developmental amplicons in Drosophila," by Julie M. Claycomb, Matt Benasutti, Giovanni Bosco, Douglas D. Fenger, and Terry L. Orr-Weaver, published in January 2004. Video Abstract (28557 K) Dr. Orr-Weaver discusses her group's work on regulatory programs that amplify (or suppress) overreplication of cell-type-appropriate genes in the Drosophila embryo.
• Establishing Robust Left-Right Asymmetry in the Vertebrate Embryo
  - dev cell 20(6):e2 (2011)
  One of the most surprising things to emerge in the study of left-right asymmetry is that the mechanism by which symmetry is first broken (which was thought of as the most fundamental problem in the field) is actually not conserved across different classes of vertebrates. However, what is conserved is the downstream use of Nodal as a key left-sided inductive signal and of Lefty as a negative feedback inhibitor. This paper, from Hiroshi Hamada and coworkers, provided insight into why the Nodal-Lefty system is so special and evolutionarily maintained as the key determinant of left-right patterning, from snails to echinoderms to birds to mammals. The regulatory and biophysical properties of Nodal and Lefty provide a reaction-diffusion-like mechanism that can amplify a relatively small bias in differential gene expression between the left and right sides of the embryo. Thus, the Nodal-Lefty system enhances the expression activity on the side where it is slightly stronger to! begin with while concomitantly repressing the activity on the side where it is initially weaker. This mechanism elucidated by Hamada and colleagues can transform weak asymmetry (derived from any of a number of mechanisms, such as the directional rotation of cilia in mammals) into robust asymmetry. This PaperPick refers to "Generation of robust left-right asymmetry in the mouse embryo requires a self-enhancement and lateral-inhibition system," by Tetsuya Nakamura, Naoki Mine, Etsushi Nakaguchi, Atsushi Mochizuki, Masamichi Yamamoto, Kenta Yashiro, Chikara Meno, and Hiroshi Hamada, published in October 2006. Video Abstract (68252 K) Tetsuya Nakamura discusses the role of a Nodal-Lefty-based reaction-diffusion system in generating left-right asymmetry in the mouse embryo.
• Moving Right Along: How PP1 Helps Clear the Checkpoint
  - dev cell 20(6):733-734 (2011)
  Spindle checkpoint silencing is crucial for cell-cycle progression, but mechanisms underlying this process remain mysterious. Two papers, one in this issue of Developmental Cell (Meadows et al., 2011) and one in Current Biology (Rosenberg et al., 2011), begin to show how phosphatase PP1-gamma connects chromosome-microtubule attachment with anaphase entry.
• Balancing Parental Contributions in Plant Embryonic Gene Activation
  - dev cell 20(6):735-736 (2011)
  Little is known about chromatin remodeling events immediately after fertilization. A recent report by Autran et al. (2011) in Cell now shows that chromatin regulatory pathways that silence transposable elements are responsible for global delayed activation of gene expression in the early Arabidopsis embryo.
• Viperin Turns Coat in Cytomegalovirus Infection
  - dev cell 20(6):737-738 (2011)
  Cytomegalovirus infection is associated with cytoskeletal alterations and cell swelling (cytomegaly), which have been attributed to the viral mitochondria-localized inhibitor of apoptosis (vMIA) protein. In a recent issue of Science, Seo et al. (2011) showed that the antiviral host protein viperin is co-opted to function with vMIA for facilitating infection.
• Spindle Checkpoint Silencing Requires Association of PP1 to Both Spc7 and Kinesin-8 Motors
  - dev cell 20(6):739-750 (2011)
  The spindle checkpoint is the prime cell-cycle control mechanism that ensures sister chromatids are bioriented before anaphase takes place. Aurora B kinase, the catalytic subunit of the chromosome passenger complex, both destabilizes kinetochore attachments that do not generate tension and simultaneously maintains the spindle checkpoint signal. However, it is unclear how the checkpoint is silenced following chromosome biorientation. We demonstrate that association of type 1 phosphatase (PP1Dis2) with both the N terminus of Spc7 and the nonmotor domains of the Klp5-Klp6 (kinesin-8) complex is necessary to counteract Aurora B kinase to efficiently silence the spindle checkpoint. The role of Klp5 and Klp6 in checkpoint silencing is specific to this class of kinesin and independent of their motor activities. These data demonstrate that at least two distinct pools of PP1, one kinetochore associated and the other motor associated, are needed to silence the spindle checkpoint.
• GGA3 Functions as a Switch to Promote Met Receptor Recycling, Essential for Sustained ERK and Cell Migration
  - dev cell 20(6):751-763 (2011)
  Cells are dependent on correct sorting of activated receptor tyrosine kinases (RTKs) for the outcome of growth factor signaling. Upon activation, RTKs are coupled through the endocytic machinery for degradation or recycled to the cell surface. However, the molecular mechanisms governing RTK recycling are poorly understood. Here, we show that Golgi-localized gamma ear-containing Arf-binding protein 3 (GGA3) interacts selectively with the Met/hepatocyte growth factor RTK when stimulated, to sort it for recycling in association with "gyrating" clathrin. GGA3 loss abrogates Met recycling from a Rab4 endosomal subdomain, resulting in pronounced trafficking of Met toward degradation. Decreased Met recycling attenuates ERK activation and cell migration. Met recycling, sustained ERK activation, and migration require interaction of GGA3 with Arf6 and an unexpected association with the Crk adaptor. The data show that GGA3 defines an active recycling pathway and support a bro! ader role for GGA3-mediated cargo selection in targeting receptors destined for recycling.
• Sonic Hedgehog Shedding Results in Functional Activation of the Solubilized Protein
  - dev cell 20(6):764-774 (2011)
  All Hedgehog (Hh) proteins are released from producing cells despite being synthesized as N- and C-terminally lipidated, membrane-tethered molecules. Thus, a cellular mechanism is needed for Hh solubilization. We previously suggested that a disintegrin and metalloprotease (ADAM)-mediated shedding of Sonic hedgehog (ShhNp) from its lipidated N and C termini results in protein solubilization. This finding, however, seemed at odds with the established role of N-terminal palmitoylation for ShhNp signaling activity. We now resolve this paradox by showing that N-palmitoylation of ShhNp N-terminal peptides is required for their proteolytic removal during solubilization. These peptides otherwise block ShhNp zinc coordination sites required for ShhNp binding to its receptor Patched (Ptc), explaining the essential yet indirect role of N-palmitoylation for ShhNp function. We suggest a functional model in which membrane-tethered multimeric ShhNp is at least partially autoinhibited! in trans but is processed into fully active, soluble multimers upon palmitoylation-dependent cleavage of inhibitory N-terminal peptides.
• Overlapping Roles and Collective Requirement for the Coreceptors GAS1, CDO, and BOC in SHH Pathway Function
  - dev cell 20(6):775-787 (2011)
  Secreted Hedgehog (HH) ligands signal through the canonical receptor Patched (PTCH1). However, recent studies implicate three additional HH-binding, cell-surface proteins, GAS1, CDO, and BOC, as putative coreceptors for HH ligands. A central question is to what degree these coreceptors function similarly and what their collective requirement in HH signal transduction is. Here we provide evidence that GAS1, CDO, and BOC play overlapping and essential roles during HH-mediated ventral neural patterning of the mammalian neural tube. Specifically, we demonstrate two important roles for these molecules: an early role in cell fate specification of multiple neural progenitors and a later role in motor neuron progenitor maintenance. Most strikingly, genetic loss-of-function experiments indicate an obligatory requirement for GAS1, CDO, and BOC in HH pathway activity in multiple tissues.
• Boc and Gas1 Each Form Distinct Shh Receptor Complexes with Ptch1 and Are Required for Shh-Mediated Cell Proliferation
  - dev cell 20(6):788-801 (2011)
  Hedgehog (Hh) proteins regulate important developmental processes, including cell proliferation and differentiation. Although Patched acts as the main Hh receptor in Drosophila, Hh signaling absolutely requires the additional Hh-binding proteins Ihog and Boi. Here we show that, unexpectedly, cerebellar granule neuron progenitors (CGNPs) lacking Boc and Cdon, the vertebrate orthologs of Ihog and Boi, still proliferate in response to Hh. This is because in their absence, Gas1, an Hh-binding protein not present in Drosophila, mediates Hh signaling. Consistently, only CGNPs lacking all three molecules—Boc, Cdon, and Gas1—have a complete loss of Hh-dependent proliferation. In a complementary manner, we find that a mutated Hh ligand that binds Patched1 but not Boc, Cdon, or Gas1 cannot activate Hh signaling. Together, this demonstrates an absolute requirement for Boc, Cdon, and Gas1 in Hh signaling and reveals a distinct requirement for ligand-binding components that dis! tinguishes the vertebrate and invertebrate Hh receptor systems.
• Hedgehog Activates Fused through Phosphorylation to Elicit a Full Spectrum of Pathway Responses
  - dev cell 20(6):802-814 (2011)
  In flies and mammals, extracellular Hedgehog (Hh) molecules alter cell fates and proliferation by regulating the levels and activities of Ci/Gli family transcription factors. How Hh-induced activation of transmembrane Smoothened (Smo) proteins reverses Ci/Gli inhibition by Suppressor of Fused (SuFu) and kinesin family protein (Cos2/Kif7) binding partners is a major unanswered question. Here we show that the Fused (Fu) protein kinase is activated by Smo and Cos2 via Fu- and CK1-dependent phosphorylation. Activated Fu can recapitulate a full Hh response, stabilizing full-length Ci via Cos2 phosphorylation and activating full-length Ci by antagonizing Su(fu) and by other mechanisms. We propose that Smo/Cos2 interactions stimulate Fu autoactivation by concentrating Fu at the membrane. Autoactivation primes Fu for additional CK1-dependent phosphorylation, which further enhances kinase activity. In this model, Smo acts like many transmembrane receptors associated with cytopl! asmic kinases, such that pathway activation is mediated by kinase oligomerization and trans-phosphorylation.
• PDGFRβ Signaling Regulates Mural Cell Plasticity and Inhibits Fat Development
  - dev cell 20(6):815-826 (2011)
  Mural cells (pericytes and vascular smooth muscle cells) provide trophic and structural support to blood vessels. Vascular smooth muscle cells alternate between a synthetic/proliferative state and a differentiated/contractile state, but the dynamic states of pericytes are poorly understood. To explore the cues that regulate mural cell differentiation and homeostasis, we have generated conditional knockin mice with activating mutations at the PDGFRβ locus. We show that increased PDGFRβ signaling drives cell proliferation and downregulates differentiation genes in aortic vascular smooth muscle. Increased PDGFRβ signaling also induces a battery of immune response genes in pericytes and mesenchymal cells and inhibits differentiation of white adipocytes. Mural cells are emerging as multipotent progenitors of pathophysiological importance, and we identify PDGFRβ signaling as an important in vivo regulator of their progenitor potential.
• SLIT/ROBO1 Signaling Suppresses Mammary Branching Morphogenesis by Limiting Basal Cell Number
  - dev cell 20(6):827-840 (2011)
  In the field of breast biology, there is a growing appreciation for the "gatekeeping function" of basal cells during development and disease processes yet mechanisms regulating the generation of these cells are poorly understood. Here, we report that the proliferation of basal cells is controlled by SLIT/ROBO1 signaling and that production of these cells regulates outgrowth of mammary branches. We identify the negative regulator TGF-β1 upstream of Robo1 and show that it induces Robo1 expression specifically in the basal layer, functioning together with SLIT2 to restrict branch formation. Loss of SLIT/ROBO1 signaling in this layer alone results in precocious branching due to a surplus of basal cells. SLIT2 limits basal cell proliferation by inhibiting canonical WNT signaling, increasing the cytoplasmic and membrane pools of β-catenin at the expense of its nuclear pool. Together, our studies provide mechanistic insight into how specification of basal cell number in! fluences branching morphogenesis.
• Dynamic Coordination of Innate Immune Signaling and Insulin Signaling Regulates Systemic Responses to Localized DNA Damage
  - dev cell 20(6):841-854 (2011)
  Metazoans adapt to changing environmental conditions and to harmful challenges by attenuating growth and metabolic activities systemically. Recent studies in mice and flies indicate that endocrine signaling interactions between insulin/IGF signaling (IIS) and innate immune signaling pathways are critical for this adaptation, yet the temporal and spatial hierarchy of these signaling events remains elusive. Here, we identify and characterize a program of signaling interactions that regulates the systemic response of the Drosophila larva to localized DNA damage. We provide evidence that epidermal DNA damage induces an innate immune response that is kept in check by systemic repression of IIS activity. IIS repression induces NFκB/Relish signaling in the fat body, which is required for recovery of IIS activity in a second phase of the systemic response to DNA damage. This systemic response to localized DNA damage thus coordinates growth and metabolic activities across tiss! ues, ensuring growth homeostasis and survival of the animal.
• Inositol Trisphosphate-Induced Ca2+ Signaling Modulates Auxin Transport and PIN Polarity
  - dev cell 20(6):855-866 (2011)
  The phytohormone auxin is an important determinant of plant development. Directional auxin flow within tissues depends on polar localization of PIN auxin transporters. To explore regulation of PIN-mediated auxin transport, we screened for suppressors of PIN1 overexpression (supo) and identified an inositol polyphosphate 1-phosphatase mutant (supo1), with elevated inositol trisphosphate (InsP3) and cytosolic Ca2+ levels. Pharmacological and genetic increases in InsP3 or Ca2+ levels also suppressed the PIN1 gain-of-function phenotypes and caused defects in basal PIN localization, auxin transport and auxin-mediated development. In contrast, the reductions in InsP3 levels and Ca2+ signaling antagonized the effects of the supo1 mutation and disrupted preferentially apical PIN localization. InsP3 and Ca2+ are evolutionarily conserved second messengers involved in various cellular functions, particularly stress responses. Our findings implicate them as modifiers of cell polar! ity and polar auxin transport, and highlight a potential integration point through which Ca2+ signaling-related stimuli could influence auxin-mediated development.
• Perinuclear Cohibin Complexes Maintain Replicative Life Span via Roles at Distinct Silent Chromatin Domains
  - dev cell 20(6):867-879 (2011)
  Heterochromatin, or silent chromatin, preferentially resides at the nuclear envelope. Telomeres and rDNA repeats are the two major perinuclear silent chromatin domains of Saccharomyces cerevisiae. The Cohibin protein complex maintains rDNA repeat stability in part through silent chromatin assembly and perinuclear rDNA anchoring. We report here a role for Cohibin at telomeres and show that functions of the complex at chromosome ends and rDNA maintain replicative life span. Cohibin binds LEM/SUN domain-containing nuclear envelope proteins and telomere-associated factors. Disruption of Cohibin or the envelope proteins abrogates telomere localization and silent chromatin assembly within subtelomeres. Loss of Cohibin limits Sir2 histone deacetylase localization to chromosome ends, disrupts subtelomeric DNA stability, and shortens life span even when rDNA repeats are stabilized. Restoring telomeric Sir2 concentration abolishes chromatin and life span defects linked to the lo! ss of telomeric Cohibin. Our work uncovers roles for Cohibin complexes and reveals relationships between nuclear compartmentalization, chromosome stability, and aging.
• Gene Regulation by MAPK Substrate Competition
  - dev cell 20(6):880-887 (2011)
  Developing tissues are patterned by coordinated activities of signaling systems, which can be integrated by a regulatory region of a gene that binds multiple transcription factors or by a transcription factor that is modified by multiple enzymes. Based on a combination of genetic and imaging experiments in the early Drosophila embryo, we describe a signal integration mechanism that cannot be reduced to a single gene regulatory element or a single transcription factor. This mechanism relies on an enzymatic network formed by mitogen-activated protein kinase (MAPK) and its substrates. Specifically, anteriorly localized MAPK substrates, such as Bicoid, antagonize MAPK-dependent downregulation of Capicua, a repressor that is involved in gene regulation along the dorsoventral axis of the embryo. MAPK substrate competition provides a basis for ternary interaction of the anterior, dorsoventral, and terminal patterning systems. A mathematical model of this interaction can expla! in gene expression patterns with both anteroposterior and dorsoventral polarities.