Hot off the presses! Nov 15 Dev Cell

The Nov 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:

• Rab25 Mediates Integrin Recycling for Tumor Cell Migration in 3D
  - Dev Cell 21(5):e1 (2011)
  Prior to the publication of this report, the importance of endocytotic and exocytotic recycling of integrins was just beginning to be recognized. However, the findings in this report provided significant new insights into the mechanisms mediating integrin recycling and the critical importance of spatially regulated recycling in controlling specific aspects of cell migration and invasion. The Rab family GTPase RAB25, which has been implicated in tumor progression, was found to bind directly to β1 integrin and, through this interaction, specifically deliver α5β1-containing vesicles to the tips of pseudopodial extensions. This RAB25-integrin interaction was found to be critical for directional invasion of tumor cells in 3D extracellular matrix environments. I liked this report not only for the elegance of the experiments and fundamental insights into matrix receptor recycling, but also because it provided mechanistic insights into how aberrant vesicle trafficking and r! eceptor recycling could contribute to tumorigenesis. This PaperPick refers to "Rab25 associates with α5β1 integrin to promote invasive migration in 3D microenvironments," by P.T. Caswell, H.J. Spence, M. Parsons, D.P. White, K. Clark, K.W. Cheng, G.B. Mills, M.J. Humphries, A.J. Messent, K.I. Anderson, M.W. McCaffrey, B.W. Ozanne, and J.C. Norman, published in October 2007. Video Abstract
• Old Friends with a New Job
  - Dev Cell 21(5):e2 (2011)
  The paper by Kobayashi et al. triggered fond memories and curious questions, as one might expect when meeting a couple of old, almost forgotten friends. As a post doc in Larry Zipursky's laboratory, I worked in a team trying to understand how Boss/Sevenless signaling directs the presence of one and only one R7 cell in each ommatidium of the fly eye. Almost 20 years later, an odd sense of ownership contributed to my first knee-jerk reaction to the title of the paper claiming a role for Boss and Sevenless in early male gonads: Those mutants are fertile! How, then, could either Boss or Sevenless be important for the niche in which male germline stem cells reside? But, thinking back, neither boss nor sevenless mutants are blind, and yet they told us a lot about cell fate decisions in the fly visual system. Actually reading the paper, I was drawn into a fascinating story of how, early in the male embryonic gonads, Boss/Sevenless signaling helps in determining the number of ! somatic niche cells. This decision, much less flexible than one might expect, carries forward into the adult male, where niche size appears to determine the number of germline stem cells. This PaperPick refers to "Boss/Sev Signaling from Germline to Soma Restricts Germline-Stem-Cell-Niche Formation in the Anterior Region of Drosophila Male Gonads," by Y. Kitadate, S. Shigenobu, K. Arita, and S. Kobayashi, published in July 2007. Video Abstract
• Meiotic Pairing as a Polo Match
  - Dev Cell 21(5):805-806 (2011)
  In C. elegans, meiotic chromosome pairing is initiated by association of chromosomal sites known as pairing centers (PCs) with the nuclear periphery. The Dernburg and Zetka laboratories have shown that recruitment of Polo kinases to PCs at the nuclear envelope is essential to promote PC complex aggregation, pairing, and synapsis.
• Speed Reading for Genes: Bookmarks Set the Pace
  - Dev Cell 21(5):807-808 (2011)
  During mitosis, most transcription ceases. Mitotic gene bookmarking marks genes for reactivation to ensure reestablishment of transcription states and cell-cycle progression. In a recent issue of Nature Cell Biology, investigate how gene bookmarking leads to accelerated kinetics of transcriptional reactivation after mitosis.
• Autophagic Factors Cut to the Bone
  - Dev Cell 21(5):808-810 (2011)
  Autophagy is an intracellular membrane-trafficking pathway for the delivery of proteins and organelles to lysosomes for degradation and recycling. now describe an essential role for autophagic proteins in the trafficking and fusion of lysosomes at the site of bone resorption: the osteoclast ruffled border.
• Phosphatidylinositol Synthase and Diacylglycerol Platforms Bust a Move
  - Dev Cell 21(5):810-812 (2011)
  challenge the dogma that phosphatidylinositol synthesis is restricted to the endoplasmic reticulum (ER) by showing that a mobile membrane compartment transports phosphatidylinositol synthase from the ER to numerous cellular compartments, including the plasma membrane. These findings significantly impact our view of phosphoinositide signaling in the cell.
• A Highly Dynamic ER-Derived Phosphatidylinositol-Synthesizing Organelle Supplies Phosphoinositides to Cellular Membranes
  - Dev Cell 21(5):813-824 (2011)
  Polyphosphoinositides are lipid signaling molecules generated from phosphatidylinositol (PtdIns) with critical roles in vesicular trafficking and signaling. It is poorly understood where PtdIns is located within cells and how it moves around between membranes. Here we identify a hitherto-unrecognized highly mobile membrane compartment as the site of PtdIns synthesis and a likely source of PtdIns of all membranes. We show that the PtdIns-synthesizing enzyme PIS associates with a rapidly moving compartment of ER origin that makes ample contacts with other membranes. In contrast, CDP-diacylglycerol synthases that provide PIS with its substrate reside in the tubular ER. Expression of a PtdIns-specific bacterial PLC generates diacylglycerol also in rapidly moving cytoplasmic objects. We propose a model in which PtdIns is synthesized in a highly mobile lipid distribution platform and is delivered to other membranes during multiple contacts by yet-to-be-defined lipid transfer! mechanisms.
• Dual Role of BKI1 and 14-3-3Â s in Brassinosteroid Signaling to Link Receptor with Transcription Factors
  - Dev Cell 21(5):825-834 (2011)
  The plasma membrane-localized plant steroid hormone receptor, BRASSINOSTEROID INSENSITIVE 1 (BRI1), is quiescent in the absence of steroids, largely due to a negative regulator, BRI1 KINASE INHIBITOR 1 (BKI1). Here, we report that the steroid-induced, plasma membrane-dissociated and phosphorylated BKI1 also plays positive roles in BR signaling by interacting with a subset of 14-3-3 proteins. The cytosolic fraction of BKI1 carboxyl terminal region enhances BR signaling. Mutations of two serine residues in this region lead to reduced phosphorylation by the BRI1 kinase and constitutive plasma membrane localization. The 14-3-3 proteins can interact with the phosphorylated BKI1 through a motif that contains the two phosphorylation sites to release inhibition of BRI1 by BKI1. Meanwhile, the cytosolic BKI1 antagonizes the 14-3-3 s and enhances accumulation of BRI1 EMS SUPPRESSOR 1 (BES1)/BRASSINAZOLE RESISTANT 1 (BZR1) in the nucleus to regulate BR-responses.
• The SCF-Fbxo40 Complex Induces IRS1Â Ubiquitination in Skeletal Muscle, Limiting IGF1 Signaling
  - Dev Cell 21(5):835-847 (2011)
  Insulin-like growth factor 1 (IGF1) induces skeletal muscle hypertrophy by activating the IGF1R/IRS1/PI3K/Akt pathway. However the effect of IGF1 in differentiated muscle is limited by IRS1 ubiquitination and proteasome-mediated breakdown. In skeletal muscle, IGF1R activation sensitizes IRS1 to degradation, and a screen for the responsible E3 ligase identified Fbxo40 as mediating this rapid turnover of IRS1, since IRS1 loss can be rescued by knockdown of Fbxo40. In biochemical assays, an SCF E3 ligase complex containing Fbxo40 directly ubiquitinates IRS1, and this activity is enhanced by increased tyrosine phosphorylation of IRS1. Fbxo40 is muscle specific in expression and is upregulated during differentiation. Knockdown of Fbxo40 induces dramatic hypertrophy of myofibers. Mice null for Fbxo40 have increased levels of IRS1 and demonstrate enhanced body and muscle size during the growth phase associated with elevated IGF1 levels. These findings establish an important m! eans of restraining IGF1's effects on skeletal muscle.
• Structural and Functional Studies of LRP6 Ectodomain Reveal a Platform for Wnt Signaling
  - Dev Cell 21(5):848-861 (2011)
  LDL-receptor-related protein 6 (LRP6), alongside Frizzled receptors, transduces Wnt signaling across the plasma membrane. The LRP6 ectodomain comprises four tandem β-propeller–EGF-like domain (PE) pairs that harbor binding sites for Wnt morphogens and their antagonists including Dickkopf 1 (Dkk1). To understand how these multiple interactions are integrated, we combined crystallographic analysis of the third and fourth PE pairs with electron microscopy (EM) to determine the complete ectodomain structure. An extensive inter-pair interface, conserved for the first-to-second and third-to-fourth PE interactions, contributes to a compact platform-like architecture, which is disrupted by mutations implicated in developmental diseases. EM reconstruction of the LRP6 platform bound to chaperone Mesd exemplifies a binding mode spanning PE pairs. Cellular and binding assays identify overlapping Wnt3a- and Dkk1-binding surfaces on the third PE pair, consistent with steric com! petition, but also suggest a model in which the platform structure supports an interplay of ligands through multiple interaction sites.
• Structural Basis of Wnt Signaling Inhibition by Dickkopf Binding to LRP5/6
  - Dev Cell 21(5):862-873 (2011)
  LDL receptor-related proteins 5 and 6 (LRP5/6) are coreceptors for Wnt growth factors, and also bind Dkk proteins, secreted inhibitors of Wnt signaling. The LRP5/6 ectodomain contains four β-propeller/EGF-like domain repeats. The first two repeats, LRP6(1-2), bind to several Wnt variants, whereas LRP6(3-4) binds other Wnts. We present the crystal structure of the Dkk1 C-terminal domain bound to LRP6(3-4), and show that the Dkk1 N-terminal domain binds to LRP6(1-2), demonstrating that a single Dkk1 molecule can bind to both portions of the LRP6 ectodomain and thereby inhibit different Wnts. Small-angle X-ray scattering analysis of LRP6(1-4) bound to a noninhibitory antibody fragment or to full-length Dkk1 shows that in both cases the ectodomain adopts a curved conformation that places the first three repeats at a similar height relative to the membrane. Thus, Wnts bound to either portion of the LRP6 ectodomain likely bear a similar spatial relationship to Frizzled co! receptors.
• Binary Regulation of Hippo Pathway by Merlin/NF2, Kibra, Lgl, and Melted Specifies and Maintains Postmitotic Neuronal Fate
  - Dev Cell 21(5):874-887 (2011)
  Patterning the Drosophila retina for color vision relies on postmitotic specification of photoreceptor subtypes. R8 photoreceptors express one of two light-sensing Rhodopsins, Rh5 or Rh6. This fate decision involves a bistable feedback loop between Melted, a PH-domain protein, and Warts, a kinase in the Hippo growth pathway. Here, we show that a subset of the Hippo pathway—Merlin, Kibra, and Lethal(2)giant larvae (Lgl), but not Expanded or Fat—is required for Warts expression and activity in R8 to specify Rh6 fate. Melted represses warts transcription to disrupt Hippo pathway activity and specify Rh5 fate. Therefore, R8 Hippo signaling exhibits ON-or-OFF regulation, promoting mutually exclusive fates. Furthermore, Merlin and Lgl are continuously required to maintain R8 neuronal subtypes. These results reveal roles for Merlin, Kibra, and Lgl in neuronal specification and maintenance and show that the Hippo pathway is reimplemented for sensory neuron fate by combinin! g canonical and noncanonical regulatory steps.
• Tao-1 Phosphorylates Hippo/MST Kinases to Regulate the Hippo-Salvador-Warts Tumor Suppressor Pathway
  - Dev Cell 21(5):888-895 (2011)
  Recent studies have shown that the Hippo-Salvador-Warts (HSW) pathway restrains tissue growth by phosphorylating and inactivating the oncoprotein Yorkie. How growth-suppressive signals are transduced upstream of Hippo remains unclear. We show that the Sterile 20 family kinase, Tao-1, directly phosphorylates T195 in the Hippo activation loop and that, like other HSW pathway genes, Tao-1 functions to restrict cell proliferation in developing imaginal epithelia. This relationship appears to be evolutionarily conserved, because mammalian Tao-1 similarly affects MST kinases. In S2 cells, Tao-1 mediates the effects of the upstream HSW components Merlin and Expanded, consistent with the idea that Tao-1 functions in tissues to regulate Hippo phosphorylation. These results demonstrate that one family of Ste20 kinases can activate another and identify Tao-1 as a component of the regulatory network controlling HSW pathway signaling, and therefore tissue growth, during development.
• The Sterile 20-like Kinase Tao-1 Controls Tissue Growth by Regulating the Salvador-Warts-Hippo Pathway
  - Dev Cell 21(5):896-906 (2011)
  The Salvador-Warts-Hippo (SWH) pathway is a complex signaling network that controls both developmental and regenerative tissue growth. Using a genetic screen in Drosophila melanogaster, we identified the sterile 20-like kinase, Tao-1, as an SWH pathway member. Tao-1 controls various biological phenomena, including microtubule dynamics, animal behavior, and brain development. Here we describe a role for Tao-1 as a regulator of epithelial tissue growth that modulates activity of the core SWH pathway kinase cassette. Tao-1 functions together with Hippo to activate Warts-mediated repression of Yorkie. Tao-1's ability to control SWH pathway activity is evolutionarily conserved because human TAO1 can suppress activity of the Yorkie ortholog, YAP. Human TAO1 controls SWH pathway activity by phosphorylating, and activating, the Hippo ortholog, MST2. Given that SWH pathway activity is subverted in many human cancers, our findings identify human TAO kinases as potential tumor su! ppressor genes.
• Anterior Visceral Endoderm Directs Ventral Morphogenesis and Placement of Head and Heart via BMP2 Expression
  - Dev Cell 21(5):907-919 (2011)
  In amniotes, ventral folding morphogenesis achieves gut internalization, linear heart tube formation, ventral body wall closure, and encasement of the fetus in extraembryonic membranes. Impairment of ventral morphogenesis results in human birth defects involving body wall, gut, and heart malformations and in mouse misplacement of head and heart. Absence of knowledge about genetic pathways and cell populations directing ventral folding in mammals has precluded systematic study of cellular mechanisms driving this vital morphogenetic process. We report tissue-specific mouse mutant analyses identifying the bone morphogenetic protein (BMP) pathway as a key regulator of ventral morphogenesis. BMP2 expressed in anterior visceral endoderm (AVE) signals to epiblast derivatives during gastrulation to orchestrate initial stages of ventral morphogenesis, including foregut development and positioning of head and heart. These findings identify unanticipated functions for the AVE in ! organizing the gastrulating embryo and indicate that visceral endoderm-expressed BMP2 coordinates morphogenetic cell behaviors in multiple epiblast lineages. Video Abstract View Within Article
• Kinetochore-Dependent Microtubule Rescue Ensures Their Efficient and Sustained Interactions in Early Mitosis
  - Dev Cell 21(5):920-933 (2011)
  How kinetochores regulate microtubule dynamics to ensure proper kinetochore-microtubule interactions is unknown. Here, we studied this during early mitosis in Saccharomyces cerevisiae. When a microtubule shrinks and its plus end reaches a kinetochore bound to its lateral surface, the microtubule end attempts to tether the kinetochore. This process often fails and, responding to this failure, microtubule rescue (conversion from shrinkage to growth) occurs, preventing kinetochore detachment from the microtubule end. This rescue is promoted by Stu2 transfer (ortholog of vertebrate XMAP215/ch-TOG) from the kinetochore to the microtubule end. Meanwhile, microtubule rescue distal to the kinetochore is also promoted by Stu2, which is transported by a kinesin-8 motor Kip3 along the microtubule from the kinetochore. Microtubule extension following rescue facilitates interaction with other widely scattered kinetochores, diminishing long delays in collecting the complete set of k! inetochores by microtubules. Thus, kinetochore-dependent microtubule rescue ensures efficient and sustained kinetochore-microtubule interactions in early mitosis.
• Pairing Centers Recruit a Polo-like Kinase to Orchestrate Meiotic Chromosome Dynamics in C. elegans
  - Dev Cell 21(5):934-947 (2011)
  Faithful segregation of homologous chromosomes during meiosis requires pairing, synapsis, and crossing-over. In C. elegans, homolog pairing and synapsis depend on pairing centers (PCs), special regions near one end of each chromosome that interact with the nuclear envelope (NE) and cytoplasmic microtubules. Here, we report that PCs are required for nuclear reorganization at the onset of meiosis. We demonstrate that PCs recruit the Polo-like kinase PLK-2 to induce NE remodeling, chromosome pairing, and synapsis. Recruitment of PLK-2 is also required to mediate a cell cycle delay and selective apoptosis of nuclei containing unsynapsed chromosomes, establishing a molecular link between these two quality control mechanisms. This work reveals unexpected functions for the conserved family of Polo-like kinases, and advances our understanding of how meiotic processes are properly coordinated to ensure transmission of genetic information from parents to progeny.
• Polo Kinases Establish Links between Meiotic Chromosomes and Cytoskeletal Forces Essential for Homolog Pairing
  - Dev Cell 21(5):948-958 (2011)
  During meiosis, chromosomes must find and align with their homologous partners. SUN and KASH-domain protein pairs play a conserved role by establishing transient linkages between chromosome ends and cytoskeletal forces across the intact nuclear envelope (NE). In C. elegans, a pairing center (PC) on each chromosome mediates homolog pairing and linkage to the microtubule network. We report that the polo kinases PLK-1 and PLK-2 are targeted to the PC by ZIM/HIM-8-pairing proteins. Loss of plk-2 inhibits chromosome pairing and licenses synapsis between nonhomologous chromosomes, indicating that PLK-2 is required for PC-mediated interhomolog interactions. plk-2 is also required for meiosis-specific phosphorylation of SUN-1 and establishment of dynamic SUN/KASH (SUN-1/ZYG-12) modules that promote homolog pairing. Our results provide key insights into the regulation of homolog pairing and reveal that targeting of polo-like kinases to the NE by meiotic chromosomes establishes ! the conserved linkages to cytoskeletal forces needed for homology assessment.
• The E3Â Ubiquitin-Ligase HACE1 Catalyzes the Ubiquitylation of Active Rac1
  - Dev Cell 21(5):959-965 (2011)
  Rac1 small GTPase controls essential aspects of cell biology and is a direct target of numerous bacterial virulence factors. The CNF1 toxin of pathogenic Escherichia coli addresses Rac1 to ubiquitin-proteasome system (UPS). We report the essential role of the tumor suppressor HACE1, a HECT-domain containing E3 ubiquitin-ligase, in the targeting of Rac1 to UPS. HACE1 binds preferentially GTP-bound Rac1 and catalyzes its polyubiquitylation. HACE1 expression increases the ubiquitylation of Rac1, when the GTPase is activated by point mutations or by the GEF-domain of Dbl. RNAi-mediated depletion of HACE1 blocks the ubiquitylation of active Rac1 and increases GTP-bound Rac1 cellular levels. HACE1 antagonizes cell isotropic spreading, a hallmark of Rac1 activation, and is required for endothelial cell monolayer invasion by bacteria. Together, these data establish the role of the HACE1 E3 ubiquitin-ligase in controlling Rac1 ubiquitylation and activity.
• Autophagy Proteins Regulate the Secretory Component of Osteoclastic Bone Resorption
  - Dev Cell 21(5):966-974 (2011)
  Osteoclasts resorb bone via the ruffled border, whose complex folds are generated by secretory lysosome fusion with bone-apposed plasma membrane. Lysosomal fusion with the plasmalemma results in acidification of the resorptive microenvironment and release of CatK to digest the organic matrix of bone. The means by which secretory lysosomes are directed to fuse with the ruffled border are enigmatic. We show that proteins essential for autophagy, including Atg5, Atg7, Atg4B, and LC3, are important for generating the osteoclast ruffled border, the secretory function of osteoclasts, and bone resorption in vitro and in vivo. Further, Rab7, which is required for osteoclast function, localizes to the ruffled border in an Atg5-dependent manner. Thus, autophagy proteins participate in polarized secretion of lysosomal contents into the extracellular space by directing lysosomes to fuse with the plasma membrane. These findings are in keeping with a putative link between autophagy ! genes and human skeletal homeostasis.
• The Permeability Transition Pore Controls Cardiac Mitochondrial Maturation and Myocyte Differentiation
  - Dev Cell 21(5):975 (2011)