Hot off the presses! Jun 16 Dev Cell

The Jun 16 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:

• Tubulin Posttranslational Modifications: A Pushmi-Pullyu at Work?
  - Dev Cell 16(6):773-774 (2009)
  Numerous posttranslational modifications alter surface-exposed residues of tubulin within stable microtubules. The significance of one modification, glycylation, characteristic of ciliary and flagellar microtubules, has been particularly elusive. Two groups now identify the glycylation enzymes and determine the developmental consequences of their depletion. Glycylation enzymes and those responsible for another modification, glutamylation, work in opposition to one another in modifying microtubules.
• Bric-a-Brac at the Golgi
  - Dev Cell 16(6):775-776 (2009)
  Until now, Rho proteins were known as GTPases involved in cell polarity and morphogenesis. In a recent issue of Cell, Espinosa and coworkers show that RhoBTB3, a member of this family, is an ATPase involved in endosome-to-Golgi transport.
• Genetic DISC-section of Regeneration in Drosophila
  - Dev Cell 16(6):777-778 (2009)
  Although regeneration has long fascinated biologists, it remains a challenging field of study with much yet to learn at the molecular level. In this issue of Developmental Cell, Smith-Bolton et al. introduce a genetic ablation system in Drosophila melanogaster with the potential for large-scale identification of new regulators of regeneration.
• Head Muscles: Aliens Who Came in from the Cold?
  - Dev Cell 16(6):779-780 (2009)
  Two complementary studies by Harel et al. and Sambasivan et al. published in this issue of Developmental Cell show the overwhelming diversity in the developmental programs and embryonic origins of distinct muscle groups and their associated muscle stem cells.
• You Don't Need a PHD to Grow a Tumor
  - Dev Cell 16(6):781-782 (2009)
  Prolyl hydroxylases (PHDs) perceive intracellular oxygen tension and signal hypoxia-inducible factors (HIFs) to induce hypoxia-adaptive processes such as angiogenesis. A paper from Chan and colleagues in the June issue of Cancer Cell demonstrates that PHD2 inactivation in tumor cells accelerates tumor growth via HIF-independent, NF-κB-mediated changes in tumor angiogenesis.
• Asymmetric Cell Divisions: A View from Plant Development
  - Dev Cell 16(6):783-796 (2009)
  All complex multicellular organisms must solve the problem of generating diverse and appropriately patterned cell types. Asymmetric division, in which a single mother cell gives rise to daughters with distinct identities, is instrumental in the generation of cellular diversity and higher-level patterns. In animal systems, there exists considerable evidence for conserved mechanisms of polarization and asymmetric division. Here, we consider asymmetric cell divisions in plants, highlighting the unique aspects of plant cell biology and organismal development that constrain the process, but also emphasizing conceptual and mechanistic similarities with animal asymmetric divisions.
• Regenerative Growth in Drosophila Imaginal Discs Is Regulated by Wingless and Myc
  - Dev Cell 16(6):797-809 (2009)
  The study of regeneration would be aided greatly by systems that support large-scale genetic screens. Here we describe a nonsurgical method for inducing tissue damage and regeneration in Drosophila larvae by inducing apoptosis in the wing imaginal disc in a spatially and temporally regulated manner. Tissue damage results in localized regenerative proliferation characterized by altered expression of patterning genes and growth regulators as well as a temporary loss of markers of cell fate commitment. Wingless and Myc are induced by tissue damage and are important for regenerative growth. Furthermore, ectopic Myc enhances regeneration when other growth drivers tested do not. As the animal matures, the ability to regenerate is lost and cannot be restored by activation of Wingless or Myc. This system is conducive to forward genetic screens, enabling an unbiased search for genes that regulate both the extent of and the capacity for regeneration.
• Distinct Regulatory Cascades Govern Extraocular and Pharyngeal Arch Muscle Progenitor Cell Fates
  - Dev Cell 16(6):810-821 (2009)
  Genetic regulatory networks governing skeletal myogenesis in the body are well understood, yet their hierarchical relationships in the head remain unresolved. We show that either Myf5 or Mrf4 is necessary for initiating extraocular myogenesis. Whereas Mrf4 is dispensable for pharyngeal muscle progenitor fate, Tbx1 and Myf5 act synergistically for governing myogenesis in this location. As in the body, Myod acts epistatically to the initiating cascades in the head. Thus, complementary pathways, governed by Pax3 for body, and Tbx1 for pharyngeal muscles, but absent for extraocular muscles, activate the core myogenic network. These diverse muscle progenitors maintain their respective embryonic regulatory signatures in the adult. However, these signatures are not sufficient to ensure the specific muscle phenotypes, since the expected differentiated phenotype is not manifested when satellite cells are engrafted heterotopically. These findings identify novel genetic networks ! that may provide insights into myopathies which often affect only subsets of muscles.
• Distinct Origins and Genetic Programs of Head Muscle Satellite Cells
  - Dev Cell 16(6):822-832 (2009)
  Adult skeletal muscle possesses a remarkable regenerative capacity, due to the presence of satellite cells, adult muscle stem cells. We used fate-mapping techniques in avian and mouse models to show that trunk (Pax3+) and cranial (MesP1+) skeletal muscle and satellite cells derive from separate genetic lineages. Similar lineage heterogeneity is seen within the head musculature and satellite cells, due to their shared, heterogenic embryonic origins. Lineage tracing experiments with Isl1Cre mice demonstrated the robust contribution of Isl1+ cells to distinct jaw muscle-derived satellite cells. Transplantation of myofiber-associated, Isl1-derived satellite cells into damaged limb muscle contributed to muscle regeneration. In vitro experiments demonstrated the cardiogenic nature of cranial- but not trunk-derived satellite cells. Finally, overexpression of Isl1 in the branchiomeric muscles of chick embryos inhibited skeletal muscle differentiation in vitro and in vivo, sugg! esting that this gene plays a role in the specification of cardiovascular and skeletal muscle stem cell progenitors.
• Methylation of the Sterol Nucleus by STRM-1 Regulates Dauer Larva Formation in Caenorhabditis elegans
  - Dev Cell 16(6):833-843 (2009)
  In response to pheromone(s), Caenorhabditis elegans interrupts its reproductive life cycle and enters diapause as a stress-resistant dauer larva. This decision is governed by a complex system of neuronal and hormonal regulation. All the signals converge onto the nuclear hormone receptor DAF-12. A sterol-derived hormone, dafachronic acid (DA), supports reproductive development by binding to DAF-12 and inhibiting its dauer-promoting activity. Here, we identify a methyltransferase, STRM-1, that modulates DA levels and thus dauer formation. By modifying the substrates that are used for the synthesis of DA, STRM-1 can reduce the amount of hormone produced. Loss of STRM-1 function leads to elevated levels of DA and inefficient dauer formation. Sterol methylation was not previously recognized as a mechanism for regulating hormone activity. Moreover, the C-4 sterol nucleus methylation catalyzed by STRM-1 is unique to nematodes and thus could be a target for therapeutic strateg! ies against parasitic nematode infections.
• Effects of Age on Meiosis in Budding Yeast
  - Dev Cell 16(6):844-855 (2009)
  In humans, the frequency with which meiotic chromosome mis-segregation occurs increases with age. Whether age-dependent meiotic defects occur in other organisms is unknown. Here, we examine the effects of replicative aging on meiosis in budding yeast. We find that aged mother cells show a decreased ability to initiate the meiotic program and fail to express the meiotic inducer IME1. The few aged mother cells that do enter meiosis complete this developmental program but exhibit defects in meiotic chromosome segregation and spore formation. Furthermore, we find that mutations that extend replicative life span also extend the sexual reproductive life span. Our results indicate that in budding yeast, the ability to initiate and complete the meiotic program as well as the fidelity of meiotic chromosome segregation decrease with cellular age and are controlled by the same pathways that govern aging of asexually reproducing yeast cells.
• Metabolic Control of Oocyte Apoptosis Mediated by 14-3-3ζ-Regulated Dephosphorylation of Caspase-2
  - Dev Cell 16(6):856-866 (2009)
  Xenopus oocyte death is partly controlled by the apoptotic initiator caspase-2 (C2). We reported previously that oocyte nutrient depletion activates C2 upstream of mitochondrial cytochrome c release. Conversely, nutrient-replete oocytes inhibit C2 via S135 phosphorylation catalyzed by calcium/calmodulin-dependent protein kinase II. We now show that C2 phosphorylated at S135 binds 14-3-3ζ, thus preventing C2 dephosphorylation. Moreover, we determined that S135 dephosphorylation is catalyzed by protein phosphatase-1 (PP1), which directly binds C2. Although C2 dephosphorylation is responsive to metabolism, neither PP1 activity nor binding is metabolically regulated. Rather, release of 14-3-3ζ from C2 is controlled by metabolism and allows for C2 dephosphorylation. Accordingly, a C2 mutant unable to bind 14-3-3ζ is highly susceptible to dephosphorylation. Although this mechanism was initially established in Xenopus, we now demonstrate similar control of murine C2 by pho! sphorylation and 14-3-3 binding in mouse eggs. These findings provide an unexpected evolutionary link between 14-3-3 and metabolism in oocyte death.
• TTLL3 Is a Tubulin Glycine Ligase that Regulates the Assembly of Cilia
  - Dev Cell 16(6):867-876 (2009)
  In most ciliated cell types, tubulin is modified by glycylation, a posttranslational modification of unknown function. We show that the TTLL3 proteins act as tubulin glycine ligases with chain-initiating activity. In Tetrahymena, deletion of TTLL3 shortened axonemes and increased their resistance to paclitaxel-mediated microtubule stabilization. In zebrafish, depletion of TTLL3 led to either shortening or loss of cilia in several organs, including the Kupffer's vesicle and olfactory placode. We also show that, in vivo, glutamic acid and glycine ligases oppose each other, likely by competing for shared modification sites on tubulin. We propose that tubulin glycylation regulates the assembly and dynamics of axonemal microtubules and acts either directly or indirectly by inhibiting tubulin glutamylation.
• Apical Secretion in Epithelial Tubes of the Drosophila Embryo Is Directed by the Formin-Family Protein Diaphanous
  - Dev Cell 16(6):877-888 (2009)
  Apical localization of filamentous actin (F-actin) is a common feature of epithelial tubes in multicellular organisms. However, its origins and function are not known. We demonstrate that the Diaphanous (Dia)/Formin actin-nucleating factor is required for generation of apical F-actin in diverse types of epithelial tubes in the Drosophila embryo. Dia itself is apically localized both at the RNA and protein levels, and apical localization of its activators, including Rho1 and two guanine exchange factor proteins (Rho-GEFs), contributes to its activity. In the absence of apical actin polymerization, apical-basal polarity and microtubule organization of tubular epithelial cells remain intact; however, secretion through the apical surface to the lumen of tubular organs is blocked. Apical secretion also requires the Myosin V (MyoV) motor, implying that secretory vesicles are targeted to the apical membrane by MyoV-based transport, along polarized actin filaments nucleated by! Dia. This mechanism allows efficient utilization of the entire apical membrane for secretion.
• Dynamin Participates in the Maintenance of Anterior Polarity in the Caenorhabditis elegans Embryo
  - Dev Cell 16(6):889-900 (2009)
  Cell polarity is crucial for the generation of cell diversity. Recent evidence suggests that the actin cytoskeleton plays a key role in establishment of embryonic polarity, yet the mechanisms that maintain polarity cues in particular membrane domains during development remain unclear. Dynamin, a large GTPase, functions in both endocytosis and actin dynamics. Here, the Caenorhabditis elegans dynamin ortholog, DYN-1, maintains anterior polarity cues. DYN-1-GFP foci are enriched in the anterior cortex in a manner dependent on the anterior polarity proteins, PAR-6 and PKC-3. Membrane internalization and actin comet formation are enriched in the anterior, and are dependent on DYN-1. PAR-6-labeled puncta are also internalized from cortical accumulations of DYN-1-GFP. Our results demonstrate a mechanism for the spatial and temporal regulation of endocytosis in the anterior of the embryo, contributing to the precise localization and maintenance of polarity factors within a dyn! amic plasma membrane.
• PP2A Antagonizes Phosphorylation of Bazooka by PAR-1 to Control Apical-Basal Polarity in Dividing Embryonic Neuroblasts
  - Dev Cell 16(6):901-908 (2009)
  Bazooka/Par-3 (Baz) is a key regulator of cell polarity in epithelial cells and neuroblasts (NBs). Phosphorylation of Baz by PAR-1 and aPKC is required for its function in epithelia, but little is known about the dephosphorylation mechanisms that antagonize the activities of these kinases or about the relevance of Baz phosphorylation for NB polarity. We found that protein phosphatase 2A (PP2A) binds to Baz via its structural A subunit. By using phospho-specific antibodies, we show that PP2A dephosphorylates Baz at the conserved serine residue 1085 and thereby antagonizes the kinase activity of PAR-1. Loss of PP2A function leads to complete reversal of polarity in NBs, giving rise to an "upside-down" polarity phenotype. Overexpression of PAR-1 or Baz, or mutation of 14-3-3 proteins that bind phosphorylated Baz, causes essentially the same phenotype, indicating that the balance of PAR-1 and PP2A effects on Baz phosphorylation determines NB polarity.
• Hedgehog and Bmp Polarize Hematopoietic Stem Cell Emergence in the Zebrafish Dorsal Aorta
  - Dev Cell 16(6):909-916 (2009)
  Hematopoietic stem cells (HSCs) are first detected in the floor of the embryonic dorsal aorta (DA), and we investigate the signals that induce the HSC program there. We show that while continued Hedgehog (Hh) signaling from the overlying midline structures maintains the arterial program characteristic of the DA roof, a ventral Bmp4 signal induces the blood stem cell program in the DA floor. This patterning of the DA by Hh and Bmp is the mirror image of that in the neural tube, with Hh favoring dorsal rather than ventral cell types, and Bmp favoring ventral rather than dorsal. With the majority of current data supporting a model whereby HSCs derive from arterial endothelium, our data identify the signal driving this conversion. These findings are important for the study of the production of HSCs from embryonic stem cells and establish a paradigm for the development of adult stem cells.