Hot off the presses! Feb 15 dev cell

The Feb 15 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:

• Shedding Light on Mysterious Microtubules
  - dev cell 20(2):e1 (2011)
  We have known for decades that vertebrate kinetochores can nucleate microtubules. However, the role of such microtubules was unclear. Kitamura et al. investigated this issue by following a marked budding yeast centromere after its conditional reactivation to examine microtubule capture at unattached kinetochores. They found not only that yeast kinetochores can nucleate microtubules, but also that such microtubules facilitate attachment by decreasing the time required for spindle-pole-associated microtubules to make contact with unattached kinetochores. Historically, the fact that microtubules nucleated at kinetochores are opposite in polarity to those in mature spindle fibers was used to argue that they were not physiologically relevant for spindle assembly. Kitamura et al. have now shown that they, in fact, do play a role, and then rapidly depolymerize after spindle fiber attachments form. In sum, this paper provides a function for a previously mysterious microtubule ! population and outlines a surprising and dynamic mechanism through which kinetochore-originated microtubules assist spindle pole microtubules to efficiently "locate" unattached kinetochores. It also shows how this assistance mechanism is shut off to dim the "locator beacon" after correct attachments have been made. Understanding Kinetochore-Nucleated Microtubules (15231 K) Tomoyuki Tanaka, with the help of Etsushi Kitamura and Shinya Komoto, explains his group's key findings regarding the roles of kinetochore-generated microtubules in mitosis.
• What Developmental Biologists Can Learn from Plant Pathogens
  - dev cell 20(2):e2 (2011)
  Several good friends of mine study disease resistance in plants, and I have loosely followed the enormous advances made in this area. Still, for a long time, I could not get very excited about plant-pathogen interactions. This paper from the Carrington group finally made me realize that even a dyed-in-the-wool developmental biologist like myself must pay closer attention to the field. I was at first surprised to see a pathogen paper published in Developmental Cell, even though I remembered that plant viruses sometimes induced rather bizarre developmental abnormalities. In this paper, the Carrington group revealed that such defects were because of collateral damage caused by a viral protein that blocks an antiviral gene silencing pathway. Since this pathway involves small RNAs, the viral protein inadvertently also interferes with microRNAs important for development. Viral suppressor proteins have since become important tools for dissecting the role of small RNAs in plan! t development and physiology. Linking a Viral Protein to Plant MicroRNA Function and Development (32739 K) Jim Carrington discusses how his group came to examine the connection between plant RNA viruses and endogenous microRNA processing and function and where subsequent work has taken the field.
• The Only Constant Is Change
  - dev cell 20(2):141 (2011)
  
• Mitochondrial Fusion: Bax to the Fussure
  - dev cell 20(2):142-143 (2011)
  Mitochondria can be organized into highly interconnected networks through continuous cycles of fission and fusion. A recent study by Hoppins et al. (2011) published in Molecular Cell now suggests that Bax, more commonly known for its role in apoptosis, can promote mitochondrial fusion in a Mitofusin 2-dependent manner.
• A CRY in the Night
  - dev cell 20(2):144-145 (2011)
  CRY1 is essential for normal circadian clock function, but its transcriptional regulation by the clock has not been considered an important feature for its function. However, reporting in Cell, Ukai-Tadenuma et al. (2011) now show that rhythmic Cry1 expression in the early night is critical for clock function.
• A New Piece to the Unsolved Planar Cell Polarity Puzzle
  - dev cell 20(2):146-147 (2011)
  Wnt-Frizzled/Planar cell polarity (PCP) signaling is a conserved mechanism establishing cellular orientation across animal species. Many aspects of PCP-signaling regulation remain, however, poorly understood. A new paper establishes a potential link from Wnt5a to asymmetric PCP-factor localization via their phosphorylation.
• The Many Faces of RNAi
  - dev cell 20(2):148-161 (2011)
  Small non-coding RNAs, through association with Argonaute protein family members, have a variety of functions during the development of an organism. Although there is increased mechanistic understanding of the RNA interference (RNAi) pathways surrounding these small RNAs, how their effects are modulated by subcellular compartmentalization and cross-pathway functional interactions is only beginning to be explored. This review examines the current understanding of these aspects of RNAi pathways and the biological functions of these pathways.
• Wnt Signaling Gradients Establish Planar Cell Polarity by Inducing Vangl2 Phosphorylation through Ror2
  - dev cell 20(2):163-176 (2011)
  It is fundamentally important that signaling gradients provide positional information to govern morphogenesis of multicellular organisms. Morphogen gradients can generate different cell types in specific spatial order at distinct threshold concentrations. However, it is largely unknown whether and how signaling gradients also control cell polarities by acting as global cues. Here, we show that Wnt signaling gradient provides directional information to a field of cells. Vangl2, a core component in planar cell polarity, forms Wnt-induced receptor complex with Ror2 to sense Wnt dosages. Wnts dose-dependently induce Vangl2 phosphorylation of serine/threonine residues and Vangl2 activities depend on its levels of phosphorylation. In the limb bud, Wnt5a signaling gradient controls limb elongation by establishing PCP in chondrocytes along the proximal-distal axis through regulating Vangl2 phosphorylation. Our studies have provided new insight to Robinow syndrome, Brachydactyl! y Type B1, and spinal bifida which are caused by mutations in human ROR2, WNT5A, or VANGL.
• Vangl2 Promotes Wnt/Planar Cell Polarity-like Signaling by Antagonizing Dvl1-Mediated Feedback Inhibition in Growth Cone Guidance
  - dev cell 20(2):177-191 (2011)
  Although a growing body of evidence supports that Wnt-Frizzled signaling controls axon guidance from vertebrates to worms, whether and how this is mediated by planar cell polarity (PCP) signaling remain elusive. We show here that the core PCP components are required for Wnt5a-stimulated outgrowth and anterior-posterior guidance of commissural axons. Dishevelled1 can inhibit PCP signaling by increasing hyperphosphorylation of Frizzled3 and preventing its internalization. Vangl2 antagonizes that by reducing Frizzled3 phosphorylation and promotes its internalization. In commissural axon growth cones, Vangl2 is predominantly localized on the plasma membrane and is highly enriched on the tips of the filopodia as well as in patches of membrane where new filopodia emerge. Taken together, we propose that the antagonistic functions of Vangl2 and Dvl1 (over Frizzled3 hyperphosphorylation and endocytosis) allow sharpening of PCP signaling locally on the tips of the filopodia to s! ense directional cues, Wnts, eventually causing turning of growth cones.
• ERK5 Regulates Muscle Cell Fusion through Klf Transcription Factors
  - dev cell 20(2):192-205 (2011)
  In skeletal muscle differentiation, muscle-specific genes are regulated by two groups of transcription factors, the MyoD and MEF2 families, which work together to drive the differentiation process. Here, we show that ERK5 regulates muscle cell fusion through Klf transcription factors. The inhibition of ERK5 activity suppresses muscle cell fusion with minimal effects on the expression of MyoD, MEF2, and their target genes. Promoter analysis coupled to microarray assay reveals that Klf-binding motifs are highly enriched in the promoter regions of ERK5-dependent upregulated genes. Remarkably, Klf2 and Klf4 expression are also upregulated during differentiation in an ERK5-dependent manner, and knockdown of Klf2 or Klf4 specifically suppresses muscle cell fusion. Moreover, we show that Sp1 transcription factor links ERK5 to Klf2/4, and that nephronectin, a Klf transcriptional target, is involved in muscle cell fusion. Therefore, an ERK5/Sp1/Klf module plays a key role in th! e fusion process during skeletal muscle differentiation.
• Synaptojanin 1-Mediated PI(4,5)P2 Hydrolysis Is Modulated by Membrane Curvature and Facilitates Membrane Fission
  - dev cell 20(2):206-218 (2011)
  Phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] plays a fundamental role in clathrin-mediated endocytosis. However, precisely how PI(4,5)P2 metabolism is spatially and temporally regulated during membrane internalization and the functional consequences of endocytosis-coupled PI(4,5)P2 dephosphorylation remain to be explored. Using cell-free assays with liposomes of varying diameters, we show that the major synaptic phosphoinositide phosphatase, synaptojanin 1 (Synj1), acts with membrane curvature generators/sensors, such as the BAR protein endophilin, to preferentially remove PI(4,5)P2 from curved membranes as opposed to relatively flat ones. Moreover, in vivo recruitment of Synj1's inositol 5-phosphatase domain to endophilin-induced membrane tubules results in fragmentation and condensation of these structures largely in a dynamin-dependent fashion. Our study raises the possibility that geometry-based mechanisms may contribute to spatially restricting PI(4,5)P2 elim! ination during membrane internalization and suggests that the PI(4,5)P2-to-PI4P conversion achieved by Synj1 at sites of high curvature may cooperate with dynamin to achieve membrane fission.
• IKK Regulates Cell Elongation through Recycling Endosome Shuttling
  - dev cell 20(2):219-232 (2011)
  IKK-related kinases are key regulators of innate immunity and oncogenesis. While their effects on transcription are well characterized, their cytoplasmic functions remain poorly understood. Drosophila IKK-related kinase, IKK, regulates cytoskeletal organization and cell elongation. Here, we demonstrate that IKK is activated locally at the tip of growing mechanosensory bristles and regulates the rapid shuttling of recycling endosomes, independent of its roles in F-actin organization and caspase signaling. IKK regulates the localization of recycling endosome regulators Rab11 and Dynein and phosphorylates their adaptor molecule, Nuclear fallout (Nuf). Nuf's negative regulation by IKK suggests that local activation of IKK inhibits Dynein on incoming recycling endosomes, converting them for outward transport. Mammalian IKK-related kinases also regulate the recycling endosomes' distribution by phosphorylating the Nuf homolog Rab11-FIP3. Our results establish an evolutionaril! y conserved function of IKK-related kinases in regulating recycling endosome dynamics and point to a key role of endosome dynamics in cell morphogenesis.
• Peroxiredoxin Stabilization of DE-Cadherin Promotes Primordial Germ Cell Adhesion
  - dev cell 20(2):233-243 (2011)
  Regulated adhesion between cells and their environment is critical for normal cell migration. We have identified mutations in a gene encoding the Drosophila hydrogen peroxide (H2O2)-degrading enzyme Jafrac1, which lead to germ cell adhesion defects. During gastrulation, primordial germ cells (PGCs) associate tightly with the invaginating midgut primordium as it enters the embryo; however, in embryos from jafrac1 mutant mothers this association is disrupted, leaving some PGCs trailing on the outside of the embryo. We observed similar phenotypes in embryos from DE-cadherin/shotgun (shg) mutant mothers and were able to rescue the jafrac1 phenotype by increasing DE-cadherin levels. This and our biochemical evidence strongly suggest that Jafrac1-mediated reduction of H2O2 is required to maintain DE-cadherin protein levels in the early embryo. Our results present in vivo evidence of a peroxiredoxin regulating DE-cadherin-mediated adhesion.
• Trophectoderm Lineage Determination in Cattle
  - dev cell 20(2):244-255 (2011)
  The trophectoderm (TE) and inner cell mass (ICM) are committed and marked by reciprocal expression of Cdx2 and Oct4 in mouse late blastocysts. We find that the TE is not committed at equivalent stages in cattle, and that bovine Cdx2 is required later, for TE maintenance, but does not repress Oct4 expression. A mouse Oct4 (mOct4) reporter, repressed in mouse TE, remained active in the cattle TE; bovine Oct4 constructs were not repressed in the mouse TE. mOct4 has acquired Tcfap2 binding sites mediating Cdx2-independent repression—cattle, humans, and rabbits do not contain these sites and maintain high Oct4 levels in the TE. Our data suggest that the regulatory circuitry determining ICM/TE identity has been rewired in mice, to allow rapid TE differentiation and early blastocyst implantation. These findings thus emphasize ways in which mice may not be representative of the earliest stages of mammalian development and stem cell biology.
• Translocation of the Cytoplasmic Domain of ADAM13 to the Nucleus Is Essential for Calpain8-a Expression and Cranial Neural Crest Cell Migration
  - dev cell 20(2):256-263 (2011)
  ADAMs are transmembrane metalloproteases that control cell behavior by cleaving both cell adhesion and signaling molecules. The cytoplasmic domain of ADAMs can regulate the proteolytic activity by controlling the subcellular localization and/or the activation of the protease domain. Here, we show that the cytoplasmic domain of ADAM13 is cleaved and translocates into the nucleus. Preventing this translocation renders the protein incapable of promoting cranial neural crest (CNC) cell migration in vivo, without affecting its proteolytic activity. In addition, the cytoplasmic domain of ADAM13 regulates the expression of multiple genes in CNC, including the protease Calpain8-a. Restoring the expression of Calpain8-a is sufficient to rescue CNC migration in the absence of the ADAM13 cytoplasmic domain. This study shows that the cytoplasmic domain of ADAM metalloproteases can perform essential functions in the nucleus of cells and may contribute substantially to the overall f! unction of the protein.
• Transcriptional Activation of Arabidopsis Axis Patterning Genes WOX8/9 Links Zygote Polarity to Embryo Development
  - dev cell 20(2):264-270 (2011)
  In most flowering plants, the apical-basal body axis is established by an asymmetric division of the polarized zygote. In Arabidopsis, early embryo patterning is regulated by WOX homeobox genes, which are coexpressed in the zygote but become restricted to apical (WOX2) and basal (WOX8/9) cells. How the asymmetry of zygote division is regulated and connected to the daughter cell fates is largely unknown. Here, we show that expression of WOX8 is independent of the axis patterning signal auxin, but, together with the redundant gene WOX9, is activated in the zygote, its basal daughter cell, and the hypophysis by the zinc-finger transcription factor WRKY2. In wrky2 mutants, egg cells polarize normally but zygotes fail to reestablish polar organelle positioning from a transient symmetric state, resulting in equal cell division and distorted embryo development. Both defects are rescued by overexpressing WOX8, indicating that WRKY2-dependent WOX8 transcription links zygote pol! arization with embryo patterning.
• Mechanistic Basis of Otolith Formation during Teleost Inner Ear Development
  - dev cell 20(2):271-278 (2011)
  Otoliths, which are connected to stereociliary bundles in the inner ear, serve as inertial sensors for balance. In teleostei, otolith development is critically dependent on flow forces generated by beating cilia; however, the mechanism by which flow controls otolith formation remains unclear. Here, we have developed a noninvasive flow probe using optical tweezers and a viscous flow model in order to demonstrate how the observed hydrodynamics influence otolith assembly. We show that rotational flow stirs and suppresses precursor agglomeration in the core of the cilia-driven vortex. The velocity field correlates with the shape of the otolith and we provide evidence that hydrodynamics is actively involved in controlling otolith morphogenesis. An implication of this hydrodynamic effect is that otolith self-assembly is mediated by the balance between Brownian motion and cilia-driven flow. More generally, this flow feature highlights an alternative biological strategy for co! ntrolling particle localization in solution.