Hot off the presses! May 17 dev cell

The May 17 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:

• At the Right Time and at the Right Place: Control of Cytokinesis in Mammals
  - dev cell 20(5):e1 (2011)
  Ensuring that cytokinesis occurs at the right time and in the right place is essential to maintain euploidy and cell-cycle progression. Cytokinesis is triggered by the RhoA GEF Ect2, which localizes to the spindle midzone during anaphase and activates RhoA at the overlying cell cortex to stimulate contractile ring formation. Using a chemical inhibitor of the Polo kinase Plk1, Petronczki et al. (2007) showed that Plk1 is essential for cytokinesis, because it specifically promotes Ect2 localization and interaction with its midzone receptor HsCyk-4. I liked this paper for three reasons. As a biologist, I was impressed because the paper explains an important part of biology: how a cell-cycle kinase determines when and where cytokinesis occurs. As a geneticist, I appreciate a "tight allele." The paper is a beautiful example of the power of chemical genetics to specifically and rapidly inhibit Polo at a specific cell-cycle stage. Finally, as a yeast cell-cycle researcher, it is always nice to see the dramatic conservation between yeast and mammals. In budding yeast, even though the bud neck and not the spindle midzone determines the site of cytokinesis, Polo kinase phosphorylates Rho GEFs to initiate contractile ring formation and cytokinesis. This PaperPick refers to "Polo-like Kinase 1 Triggers the Initiation of Cytokinesis in Human Cells by Promoting Recruitment of the RhoGEF Ect2 to the Central Spindle," by M. Petronczki, M. Glotzer, N. Kraut, and J.-M. Peters, published in May 2007. Video Abstract (20715 K) Here, Jan-Michael Peters discusses the work as a collaboration between Mark Petronczki (in his group), Michael Glotzer of the University of Chicago, and Norbert Kraut of Boehringer Ingelheim, who provided the key reagent: BI 2536, a chemical inhibitor of Plk1. The authors would like to note that their work on the role of Plk1 in cytokinesis was inspired by early observations made in the laboratory of David Glover and that the research groups of Prasad Jallepalli (Sloan Kettering) and Aaron Straight (Stanford) have also identified Plk1 as a key regulator of cleavage furrow formation in mammalian cells.
• What Makes an Osteoclast Special?
  - dev cell 20(5):e2 (2011)
  A paper published in Developmental Cell that I have continuously enjoyed over the years is "Induction and Activation of the Transcription Factor NFATc1 Integrate RANKL Signaling in Terminal Differentiation of Osteoclasts" by Takayanagi et al. I like this paper for many reasons. The first one is that it asks without preconceived idea a simple question: why the osteoclast differentiation factor RANKL, and not IL-1, triggers osteoclast differentiation, because both cytokines affect seemingly the same signaling cascade and transcription factors NF-KB and c-FOS. The approach was simple: a microarray of bone marrow stromal cells treated or not with RANKL or IL-1 to induce osteoclast differentiation. This analysis identified NFATc1, which the authors showed is the target of both c-FOS and NF-KB in osteoclasts. The authors checked all the boxes that need to be checked for a transcription factor determining cell differentiation, and more. Indeed, they identified calcium sig! naling as an activator of NFATc1, as well as genes whose expression is regulated by both c-FOS and NFATc1. This paper is beautifully written, the question posed is important, the experiments are of superb quality, and the advance for the field is highly significant. I always thought it was one of the better papers published in Developmental Cell pertaining to skeleton development. This PaperPick refers to "Induction and Activation of the Transcription Factor NFATc1 (NFAT2) Integrate RANKL Signaling in Terminal Differentiation of Osteoclasts," by H. Takayanagi, S. Kim, T. Koga, H. Nishina, M. Isshiki, H. Yoshida, A. Saiura, M. Isobe, T. Yokochi, J.-i. Inoue, E.F. Wagner, T.W. Mak, T. Kodama, and T. Taniguchi, published in December 2002. Video Abstract (112822 K) The first author of the original paper, Hiroshi Takayanagi, discusses how he decided at the time to investigate the transcriptional effectors of osteoclast RANKL signaling, within Tadatsugu Taniguchi's group.
• A Sticky Wicket: Opposing Functions of p120-Catenin in Development and Cancer
  - dev cell 20(5):e3 (2011)
  It has been known for some time that cell-cell adhesion mediated by E-cadherin and catenins is important for development and cancer in epithelial tissues. Although β-catenin is upregulated in many cancers, p120-catenin is downregulated in most human cancers. Before this study, the molecular mechanism underlying β-catenin function in tumorigenesis was well on its way to being worked out, but little was known about p120. This paper piqued my interest because it showed that p120 was a major regulator of E-cadherin stability in the salivary gland. Its loss caused a major decrease in E-cadherin with severe defects in cell-cell adhesion and tissue morphology resembling intraepithelial hyperplasia. I liked this paper because it made me think about how looking at development gives important insights into cancer and about how two related molecules that bind to E-cadherin could behave in opposite ways to get the initial phases of tumorigenesis started. This PaperPick refers to "Blocked Acinar Development, E-Cadherin Reduction, and Intraepithelial Neoplasia upon Ablation of p120-Catenin in the Mouse Salivary Gland," by M.A. Davis and A.B. Reynolds, published in January 2006. Video Abstract (54216 K) Albert Reynolds discusses how his group came to examine the role of p120-catenin in the mouse salivary gland and the implications of their findings.
• Parsing p53 Transactivation
  - dev cell 20(5):573-574 (2011)
  Two functions of p53 are undisputed. Biologically, p53 is a potent tumor suppressor, whereas biochemically, it is a robust transcriptional activator of numerous target genes. Are these biological and biochemical functions of p53 related? The surprising answer, recently reported by Brady et al. (2011) in Cell, is minimally.
• There Is More to Life than Death: A Moonlighting Function of a Bcl-2 Member
  - dev cell 20(5):575-576 (2011)
  Members of the Bcl-2 family proteins are best known for their roles in apoptosis regulation. In this issue of Developmental Cell, Popgeorgiev et al. (2011) have uncovered a new, nonapoptotic role for a Bcl-2 homolog during early embryogenesis in zebrafish.
• Fine-Tuning Endothelial Notch: SIRT-ainly an Unexpected Mechanism
  - dev cell 20(5):577-578 (2011)
  Guarani et al. (2011), reporting in Nature, identify a mechanism for fine-tuning endothelial Notch signaling by SIRT1-mediated deacetylation. This regulation is critical for vascular sprouting and raises intriguing questions about therapeutic targeting of SIRT1 in angiogenesis and the potential mechanisms that link vascular growth and energy homeostasis.
• Coordinating Migratory Neuron Polarization by Numb-ing Communication
  - dev cell 20(5):578-580 (2011)
  An interplay between intrinsic polarity and extracellular cues guides neuronal migration during cerebellar development. In this issue of Developmental Cell, Zhou et al. (2011) demonstrate that Numb is the focal point in mediating the chemotactic response of migrating cerebellar granule cells to BDNF through its regulation of cell polarity.
• The Curious Case of the Soluble Protein
  - dev cell 20(5):581-582 (2011)
  How neurons tackle the challenge of soluble protein delivery to the distal axon has long puzzled neuroscientists. Reporting in Neuron, Scott et al. (2011) show that this axonal transport occurs through motor-dependent formation of dynamic heterogeneous protein complexes that pause upon complex disassembly and regain motility upon reassembly.
• The WTX Tumor Suppressor Regulates Mesenchymal Progenitor Cell Fate Specification
  - dev cell 20(5):583-596 (2011)
  WTX is an X-linked tumor suppressor targeted by somatic mutations in Wilms tumor, a pediatric kidney cancer, and by germline inactivation in osteopathia striata with cranial sclerosis, a bone overgrowth syndrome. Here, we show that Wtx deletion in mice causes neonatal lethality, somatic overgrowth, and malformation of multiple mesenchyme-derived tissues, including bone, fat, kidney, heart, and spleen. Inactivation of Wtx at different developmental stages and in primary mesenchymal progenitor cells (MPCs) reveals that bone mass increase and adipose tissue deficiency are due to altered lineage fate decisions coupled with delayed terminal differentiation. Specification defects in MPCs result from aberrant β-catenin activation, whereas alternative pathways contribute to the subsequently delayed differentiation of lineage-restricted cells. Thus, Wtx is a regulator of MPC commitment and differentiation with stage-specific functions in inhibiting canonical Wnt signaling. Fur! thermore, the constellation of anomalies in Wtx null mice suggests that this tumor suppressor broadly regulates MPCs in multiple tissues.
• Genome-wide Analysis of Simultaneous GATA1/2, RUNX1, FLI1, and SCL Binding in Megakaryocytes Identifies Hematopoietic Regulators
  - dev cell 20(5):597-609 (2011)
  Hematopoietic differentiation critically depends on combinations of transcriptional regulators controlling the development of individual lineages. Here, we report the genome-wide binding sites for the five key hematopoietic transcription factors—GATA1, GATA2, RUNX1, FLI1, and TAL1/SCL—in primary human megakaryocytes. Statistical analysis of the 17,263 regions bound by at least one factor demonstrated that simultaneous binding by all five factors was the most enriched pattern and often occurred near known hematopoietic regulators. Eight genes not previously appreciated to function in hematopoiesis that were bound by all five factors were shown to be essential for thrombocyte and/or erythroid development in zebrafish. Moreover, one of these genes encoding the PDZK1IP1 protein shared transcriptional enhancer elements with the blood stem cell regulator TAL1/SCL. Multifactor ChIP-Seq analysis in primary human cells coupled with a high-throughput in vivo perturbation scr! een therefore offers a powerful strategy to identify essential regulators of complex mammalian differentiation processes.
• Numb Links Extracellular Cues to Intracellular Polarity Machinery to Promote Chemotaxis
  - dev cell 20(5):610-622 (2011)
  Cell polarization is essential throughout development for proliferation, migration, and differentiation. However, it is not known how extracellular cues correctly orient cell polarity at distinct stages of development. Here, we show that the endocytic adaptor protein Numb, previously characterized for its role in cell proliferation, subsequently plays an important role in cell migration. In neural precursors stimulated with the chemotactic factor BDNF, Numb binds to activated TrkB, the BDNF receptor, and functions both as an endocytic regulator for TrkB and as a scaffold for aPKC (aPKC). Thus, Numb promotes BDNF-dependent aPKC activation. Interestingly, Numb is also a substrate of aPKC. When phosphorylated, Numb exhibits increased efficacy in binding TrkB and in promoting a chemotactic response to BDNF. Therefore, Numb functions in a feed-forward loop to promote chemotaxis of neural precursors, linking BDNF, an extracellular cue, to aPKC, a critical component of the in! trinsic polarity machinery.
• Competition between Blown Fuse and WASP for WIP Binding Regulates the Dynamics of WASP-Dependent Actin Polymerization In Vivo
  - dev cell 20(5):623-638 (2011)
  Dynamic rearrangements of the actin cytoskeleton play a key role in numerous cellular processes. In Drosophila, fusion between a muscle founder cell and a fusion competent myoblast (FCM) is mediated by an invasive, F-actin-enriched podosome-like structure (PLS). Here, we show that the dynamics of the PLS is controlled by Blown fuse (Blow), a cytoplasmic protein required for myoblast fusion but whose molecular function has been elusive. We demonstrate that Blow is an FCM-specific protein that colocalizes with WASP, WIP/Solitary, and the actin focus within the PLS. Biochemically, Blow modulates the stability of the WASP-WIP complex by competing with WASP for WIP binding, leading to a rapid exchange of WASP, WIP and G-actin within the PLS, which, in turn, actively invades the adjacent founder cell to promote fusion pore formation. These studies identify a regulatory protein that modulates the actin cytoskeletal dynamics by controlling the stability of the WASP-WIP complex.
• Regulated Offloading of Cytoplasmic Dynein from Microtubule Plus Ends to the Cortex
  - dev cell 20(5):639-651 (2011)
  Cytoplasmic dynein mediates spindle orientation from the cell cortex through interactions with astral microtubules, but neither the mechanism governing its cortical targeting nor the regulation thereof is well understood. Here we show that yeast dynein offloads from microtubule plus ends to the daughter cell cortex. Mutants with an engineered peptide inserted between the tail domain and the motor head retain wild-type motor activity but exhibit enhanced offloading and cortical targeting. Conversely, shortening the "neck" sequence between the tail and motor domains precludes offloading from the microtubule plus ends. Furthermore, chimeric mutants with mammalian dynein "neck" sequences rescue targeting and function. These findings provide direct support for an active microtubule-mediated delivery process that appears to be regulated by a conserved masking/unmasking mechanism.
• ADF/Cofilin Regulates Secretory Cargo Sorting at the TGN via the Ca2+ ATPase SPCA1
  - dev cell 20(5):652-662 (2011)
  Actin-severing proteins ADF/cofilin are required for the sorting of secretory cargo at the trans-Golgi network (TGN) in mammalian cells. How do these cytoplasmic proteins interact with the cargoes in the lumen of the TGN? Put simply, how are these two sets of proteins connected across the TGN membrane? Mass spectrometry of cofilin1 immunoprecipitated from HeLa cells revealed the presence of actin and the Ca2+ ATPase SPCA1. Moreover, cofilin1 was localized to the TGN and bound to SPCA1 via dynamic actin. SPCA1 knockdown, like ADF/cofilin1 knockdown, inhibited Ca2+ uptake into the TGN and caused missorting of secretory cargo. These defects were rescued by the overexpression of the TGN-localized SPCA1. We propose that ADF/cofilin-dependent severing of actin filaments exposes and promotes the activation of SPCA1, which pumps Ca2+ into the lumen of the TGN for the sorting of the class of secretory cargo that binds Ca2+.
• The Apoptotic Regulator Nrz Controls Cytoskeletal Dynamics via the Regulation of Ca2+ Trafficking in the Zebrafish Blastula
  - dev cell 20(5):663-676 (2011)
  Bcl-2 family members are key regulators of apoptosis. Their involvement in other cellular processes has been so far overlooked. We have studied the role of the Bcl-2 homolog Nrz in the developing zebrafish. Nrz was found to be localized to the yolk syncytial layer, a region containing numerous mitochondria and ER membranes. Nrz knockdown resulted in developmental arrest before gastrulation, due to free Ca2+ increase in the yolk cell, activating myosin light chain kinase, which led to premature contraction of actin-myosin cables in the margin and separation of the blastomeres from the yolk cell. In the yolk syncytial layer, Nrz appears to prevent the release of Ca2+ from the endoplasmic reticulum by directly interacting with the IP3R1 Ca2+ channel. Thus, the Bcl-2 family may participate in early development, not only by controlling apoptosis but also by acting on cytoskeletal dynamics and cell movements via Ca2+ fluxes inside the embryo.
• MPK-1 ERK Controls Membrane Organization in C. elegans Oogenesis via a Sex-Determination Module
  - dev cell 20(5):677-688 (2011)
  Tissues that generate specialized cell types in a production line must coordinate developmental mechanisms with physiological demand, although how this occurs is largely unknown. In the Caenorhabditis elegans hermaphrodite, the developmental sex-determination cascade specifies gamete sex in the distal germline, while physiological sperm signaling activates MPK-1/ERK in the proximal germline to control plasma membrane biogenesis and organization during oogenesis. We discovered repeated utilization of a self-contained negative regulatory module, consisting of NOS-3 translational repressor, FEM-CUL-2 (E3 ubiquitin ligase), and TRA-1 (Gli transcriptional repressor), which acts both in sex determination and in physiological demand control of oogenesis, coordinating these processes. In the distal germline, where MPK-1 is not activated, TRA-1 represses the male fate as NOS-3 functions in translational repression leading to inactivation of the FEM-CUL-2 ubiquitin ligase. In th! e proximal germline, sperm-dependent physiological MPK-1 activation results in phosphorylation-based inactivation of NOS-3, FEM-CUL-2-mediated degradation of TRA-1 and the promotion of membrane organization during oogenesis.
• β-Catenin-Dependent FGF Signaling Sustains Cell Survival in the Anterior Embryonic Head by Countering Smad4
  - dev cell 20(5):689-699 (2011)
  Growing evidence suggests that FGFs secreted from embryonic signaling centers are key mediators of cell survival. However, the mechanisms regulating FGF-dependent cell survival remain obscure. At the rostral end of the embryo, for example, ablation of FGF signaling leads to the rapid death of the precursor cells that form the anterior head, including the telencephalon. Here, we outline a core genetic circuit that regulates survival in the embryonic mouse head: WNT signaling through β-catenin directly maintains FGF expression and requires FGF function in vivo to oppose proapoptotic TGF-β signaling through SMAD4. Moreover, these antagonistic pathways converge on the transcriptional regulation of apoptosis, and genes such as Cdkn1a, suggesting a mechanism for how signaling centers in the embryonic head regulate cell survival.
• A Screen for Conditional Growth Suppressor Genes Identifies the Drosophila Homolog of HD-PTP as a Regulator of the Oncoprotein Yorkie
  - dev cell 20(5):700-712 (2011)
  Mammalian cancers depend on "multiple hits," some of which promote growth and some of which block apoptosis. We screened for mutations that require a synergistic block in apoptosis to promote tissue overgrowth and identified myopic (mop), the Drosophila homolog of the candidate tumor-suppressor and endosomal regulator His-domain protein tyrosine phosphatase (HD-PTP). We find that Myopic regulates the Salvador/Warts/Hippo (SWH) tumor suppressor pathway: Myopic PPxY motifs bind conserved residues in the WW domains of the transcriptional coactivator Yorkie, and Myopic colocalizes with Yorkie at endosomes. Myopic controls Yorkie endosomal association and protein levels, ultimately influencing expression of some Yorkie target genes. However, the antiapoptotic gene diap1 is not affected, which may explain the conditional nature of the myopic growth phenotype. These data establish Myopic as a Yorkie regulator and implicate Myopic-dependent association of Yorkie with endos! omal compartments as a regulatory step in nuclear outputs of the SWH pathway.
• Bone Regenerates via Dedifferentiation of Osteoblasts in the Zebrafish Fin
  - dev cell 20(5):713-724 (2011)
  While mammals have a limited capacity to repair bone defects, zebrafish can completely regenerate amputated bony structures of their fins. Fin regeneration is dependent on formation of a blastema, a progenitor cell pool accumulating at the amputation plane. It is unclear which cells the blastema is derived from, whether it forms by dedifferentiation of mature cells, and whether blastema cells are multipotent. We show that mature osteoblasts dedifferentiate and form part of the blastema. Osteoblasts downregulate expression of intermediate and late bone differentiation markers and induce genes expressed by bone progenitors. Dedifferentiated osteoblasts proliferate in a FGF-dependent manner and migrate to form part of the blastema. Genetic fate mapping shows that osteoblasts only give rise to osteoblasts in the regenerate, indicating that dedifferentiation is not associated with the attainment of multipotency. Thus, bone can regenerate from mature osteoblasts via dediffer! entiation, a finding with potential implications for human bone repair.
• Fate Restriction in the Growing and Regenerating Zebrafish Fin
  - dev cell 20(5):725-732 (2011)
  We use transposon-based clonal analysis to identify the lineage classes that make the adult zebrafish caudal fin. We identify nine distinct lineage classes, including epidermis, melanocyte/xanthophore, iridophore, intraray glia, lateral line, osteoblast, dermal fibroblast, vascular endothelium, and resident blood. These lineage classes argue for distinct progenitors, or organ founding stem cells (FSCs), for each lineage, which retain fate restriction throughout growth of the fin. Thus, distinct FSCs exist for the four neuroectoderm lineages, and dermal fibroblasts are not progenitors for fin ray osteoblasts; however, artery and vein cells derive from a shared lineage in the fin. Transdifferentiation of cells or lineages in the regeneration blastema is often postulated. However, our studies of single progenitors or FSCs reveal no transfating or transdifferentiation between these lineages in the regenerating fin. This result shows that, the same as in growth, lineages re! tain fate restriction when passed through the regeneration blastema.