Latest Articles Include:
- Bombing My Mind
- Dev Cell 21(2):e1 (2011)
A paper from Itoh et al. has stuck with me since its publication date 8 years ago for many reasons, but primarily because it stretched my mind and pleased my eye. The major conclusion of the paper—that Mind bomb-mediated ubiquitination of Delta promotes its endocytosis, thereby activating Notch signaling in an adjacent cell—was an important advance in the fields of ubiquitin-mediated regulation, Delta-Notch activated signaling, and neuronal development. For these reasons the title and abstract caught my attention. However, this manuscript was a challenging read for a life-long yeast cell biologist. I had heard talks over the years from my developmental biology colleagues, but I wasn't used to looking carefully at zebrafish embryos. I had also never rigorously thought through the experiments that would be needed to test whether a signal was acting cell autonomously in vivo. The paper therefore took time, but it was well worth it: the data are spectacular, both in clarity and aesthetics, and the conclusions are significant. A paper with a protein named Mind bomb, magenta and green fluorescent micrographs, and a description of a new ! ubiquitin ligase has everything it takes to make my day. This PaperPick relates to "Mind Bomb Is a Ubiquitin Ligase that Is Essential for Efficient Activation of Notch Signaling by Delta" by M. Itoh, C.-H. Kim, G. Palardy, T. Oda, Y.-J. Jiang, D. Maust, S.-Y. Yeo, K. Lorick, G.J. Wright, L. Ariza-McNaughton, A.M. Weissman, J. Lewis, S.C. Chandrasekharappa, and A.B. Chitnis, published in January 2003. Video Abstract (23686 K) The first author of the original Developmental Cell paper, Dr. Motoyuki Itoh, introduces the mind bomb mutation, the work he did on it in Dr. Ajay Chitnis's laboratory, and some unanswered questions surrounding Notch ligand ubiquitination that persist to this day.
- Evolutionary Divergence in the Hedgehog Pathway
- Dev Cell 21(2):e2 (2011)
I like this paper because the findings that it reports took me completely by surprise—and serve as a constant reminder of the fallibility of my own scientific logic! A few years earlier, Rune Toftgård had invited me to give a talk at the Karolinska Institute about our analysis of the Hedgehog (HH) signaling pathway in Drosophila. During the course of my visit, Rune told me that he and his colleagues were planning to make a mouse knockout mutation of the Suppressor of fused (SUFU) gene, an exercise that I opined would be of only marginal value, given the dispensable nature of the orthologous gene in Drosophila. Indeed, my skepticism seemed well placed when we subsequently found that morpholino-mediated knockdown of SUFU has a rather subtle effect on HH signaling in zebrafish. But clearly undeterred by my advice, Toftgård and colleagues proceeded to generate a SUFU null mutation, the phenotype of which demonstrates its pivotal role in mammalian HH signaling! This PaperPick refers to "Genetic Elimination of Suppressor of Fused Reveals an Essential Repressor Function in the Mammalian Hedgehog Signaling Pathway," by J. Svärd, K. Heby-Henricson, M. Persson-Lek, B. Rozell, M. Lauth, A. Bergström, J. Ericson, R. Toftgård, and S. Teglund, published in February 2006. Video Abstract (17064 K) Drs. Toftgård, Teglund, and Heby-Henricson offer a personal account of the work published in this paper and highlight areas of current research on the mechanisms of Hedgehog signal transduction.
- Transposon Silencing of Small RNAs
- Dev Cell 21(2):e3 (2011)
Transposons and fragments of transposable elements make up approximately half of the human genome; mobilization of these elements can destabilize the genome and lead to disease-associated mutations. In 2003, miRNAs and siRNAs were known to silence target mRNAs, but small RNAs had not been directly linked to transposon control. In August of that year, Aravin et al. reported the developmental profile of Drosophila small RNAs by using conventional sequencing technology. These pioneering studies identified a novel class of "repeat associated" siRNAs and hypothesized that they control transposon activity and chromatin structure. It is now clear that these "rasiRNAs" bind to Piwi clade Argonaute proteins and that Piwi-interacting RNAs (piRNAs) have a conserved function in genome maintenance and germline development. This PaperPick refers to "The Small RNA Profile during Drosophila melanogaster Development" by A.A. Aravin, M. Lagos-Quintana, A. Yalcin, M. Zavolan, D. Marks, B. Snyder, T. Gaasterland, J. Meyer, and T. Tuschl, published in August 2003. Video Abstract (63477 K) Dr. Thomas Tuschl discusses the context for the work on small RNA profiling during Drosophila development and elaborates on our current understanding of some of the classes of small RNAs originally described by Aravin et al.
- Regulation of Trunk Myogenesis by the Neural Crest: A New Facet of Neural Crest-Somite Interactions
- Dev Cell 21(2):187-188 (2011)
It is well established that the somitic mesoderm regulates early stages of neural crest development and further segmentation of crest-derived peripheral ganglia. The possibility that neural crest progenitors feed back on the somites was, however, not explored. Two recent studies provide evidence that the neural crest regulates somite-derived myogenesis by distinct mechanisms.
- Semaphorin Signals on the Road of Endothelial Tip Cells
- Dev Cell 21(2):189-190 (2011)
Blood vessels sprout toward avascular tissue in response to attractive proangiogenic factors. However, restricting signals are also required to coordinate the behavior of endothelial cells assembling the vascular network. Semaphorin cues are at the crossroad of this traffic, and they direct the behavior of endothelial tip cells leading the way.
- Balancing Act during Development: Lessons from a SUMO-less SF-1
- Dev Cell 21(2):191-192 (2011)
When a transcription factor is modified by small ubiquitin-like modifier (SUMO), this usually represses its transcriptional activity. In this issue of Developmental Cell, Lee et al. (2011) use a knockin mouse model to show that SUMO-less SF-1 binds and activates inappropriate targets, causing changes in cell fates and endocrine abnormalities.
- Pericytes: Developmental, Physiological, and Pathological Perspectives, Problems, and Promises
- Dev Cell 21(2):193-215 (2011)
Pericytes, the mural cells of blood microvessels, have recently come into focus as regulators of vascular morphogenesis and function during development, cardiovascular homeostasis, and disease. Pericytes are implicated in the development of diabetic retinopathy and tissue fibrosis, and they are potential stromal targets for cancer therapy. Some pericytes are probably mesenchymal stem or progenitor cells, which give rise to adipocytes, cartilage, bone, and muscle. However, there is still confusion about the identity, ontogeny, and progeny of pericytes. Here, we review the history of these investigations, indicate emerging concepts, and point out problems and promise in the field of pericyte biology.
- The Myosin Passenger Protein Smy1 Controls Actin Cable Structure and Dynamics by Acting as a Formin Damper
- Dev Cell 21(2):217-230 (2011)
Formins are a conserved family of proteins with robust effects in promoting actin nucleation and elongation. However, the mechanisms restraining formin activities in cells to generate actin networks with particular dynamics and architectures are not well understood. In S. cerevisiae, formins assemble actin cables, which serve as tracks for myosin-dependent intracellular transport. Here, we show that the kinesin-like myosin passenger-protein Smy1 interacts with the FH2 domain of the formin Bnr1 to decrease rates of actin filament elongation, which is distinct from the formin displacement activity of Bud14. In vivo analysis of smy1Δ mutants demonstrates that this "damper" mechanism is critical for maintaining proper actin cable architecture, dynamics, and function. We directly observe Smy1-3GFP being transported by myosin V and transiently pausing at the neck in a manner dependent on Bnr1. These observations suggest that Smy1 is part of a negative feedback mechanism! that detects cable length and prevents overgrowth.
- Genome-Wide RNAi Screens Identify Genes Required for Ricin and PE Intoxications
- Dev Cell 21(2):231-244 (2011)
Protein toxins such as Ricin and Pseudomonas exotoxin (PE) pose major public health challenges. Both toxins depend on host cell machinery for internalization, retrograde trafficking from endosomes to the ER, and translocation to cytosol. Although both toxins follow a similar intracellular route, it is unknown how much they rely on the same genes. Here we conducted two genome-wide RNAi screens identifying genes required for intoxication and demonstrating that requirements are strikingly different between PE and Ricin, with only 13% overlap. Yet factors required by both toxins are present from the endosomes to the ER, and, at the morphological level, the toxins colocalize in multiple structures. Interestingly, Ricin, but not PE, depends on Golgi complex integrity and colocalizes significantly with a medial Golgi marker. Our data are consistent with two intertwined pathways converging and diverging at multiple points and reveal the complexity of retrograde membrane traffi! cking in mammalian cells.
- Shaping Cells and Organs in Drosophila by Opposing Roles of Fat Body-Secreted Collagen IV and Perlecan
- Dev Cell 21(2):245-256 (2011)
Basement membranes (BMs) are resilient polymer structures that surround organs in all animals. Tissues, however, undergo extensive morphological changes during development. It is not known whether the assembly of BM components plays an active morphogenetic role. To study in vivo the biogenesis and assembly of Collagen IV, the main constituent of BMs, we used a GFP-based RNAi method (iGFPi) designed to knock down any GFP-trapped protein in Drosophila. We found with this method that Collagen IV is synthesized by the fat body, secreted to the hemolymph (insect blood), and continuously incorporated into the BMs of the larva. We also show that incorporation of Collagen IV determines organ shape, first by mechanically constricting cells and second through recruitment of Perlecan, which counters constriction by Collagen IV. Our results uncover incorporation of Collagen IV and Perlecan into BMs as a major determinant of organ shape and animal form.
- Cell Surface Heparan Sulfate Chains Regulate Local Reception of FGF Signaling in the Mouse Embryo
- Dev Cell 21(2):257-272 (2011)
Heparan sulfate (HS) proteoglycans modulate the activity of multiple growth factors on the cell surface and extracellular matrix. However, it remains unclear how the HS chains control the movement and reception of growth factors into targeted receiving cells during mammalian morphogenetic processes. Here, we found that HS-deficient Ext2 null mutant mouse embryos fail to respond to fibroblast growth factor (FGF) signaling. Marker expression analyses revealed that cell surface-tethered HS chains are crucial for local retention of FGF4 and FGF8 ligands in the extraembryonic ectoderm. Fine chimeric studies with single-cell resolution and expression studies with specific inhibitors for HS movement demonstrated that proteolytic cleavage of HS chains can spread FGF signaling to adjacent cells within a short distance. Together, the results show that spatiotemporal expression of cell surface-tethered HS chains regulate the local reception of FGF-signaling activity during mammal! ian embryogenesis.
- Neural Crest Cell Lineage Restricts Skeletal Muscle Progenitor Cell Differentiation through Neuregulin1-ErbB3 Signaling
- Dev Cell 21(2):273-287 (2011)
Coordinating the balance between progenitor self-renewal and myogenic differentiation is required for a regulated expansion of the developing muscles. Previous observation that neural crest cells (NCCs) migrate throughout the somite regions, where trunk skeletal muscles first emerge, suggests a potential role for these cells in influencing early muscle formation. However, specific signaling interactions between NCCs and skeletal muscle cells remain unknown. Here we show that mice with specific NCC and peripheral nervous system defects display impaired survival of skeletal muscle and show skeletal muscle progenitor cell (MPC) depletion due to precocious commitment to differentiation. We show that reduced NCC-derived Neuregulin1 (Nrg1) in the somite region perturbs ErbB3 signaling in uncommitted MPCs. Using a combination of explant culture experiments and genetic ablation in the mouse, we demonstrate that Nrg1 signals provided by the NCC lineage play a critical role in s! ustainable myogenesis, by restraining MPCs from precocious differentiation.
- Notch Initiates the Endothelial-to-Mesenchymal Transition in the Atrioventricular Canal through Autocrine Activation of Soluble Guanylyl Cyclase
- Dev Cell 21(2):288-300 (2011)
The heart is the most common site of congenital defects, and valvuloseptal defects are the most common of the cardiac anomalies seen in the newborn. The process of endothelial-to-mesenchymal transition (EndMT) in the cardiac cushions is a required step during early valve development, and Notch signaling is required for this process. Here we show that Notch activation induces the transcription of both subunits of the soluble guanylyl cyclase (sGC) heterodimer, GUCY1A3 and GUCY1B3, which form the nitric oxide receptor. In parallel, Notch also promotes nitric oxide (NO) production by inducing Activin A, thereby activating a PI3-kinase/Akt pathway to phosphorylate eNOS. We thus show that the activation of sGC by NO through a Notch-dependent autocrine loop is necessary to drive early EndMT in the developing atrioventricular canal (AVC).
- Semaphorin-PlexinD1 Signaling Limits Angiogenic Potential via the VEGF Decoy Receptor sFlt1
- Dev Cell 21(2):301-314 (2011)
Sprouting angiogenesis expands the embryonic vasculature enabling survival and homeostasis. Yet how the angiogenic capacity to form sprouts is allocated among endothelial cells (ECs) to guarantee the reproducible anatomy of stereotypical vascular beds remains unclear. Here we show that Sema-PlxnD1 signaling, previously implicated in sprout guidance, represses angiogenic potential to ensure the proper abundance and stereotypical distribution of the trunk's segmental arteries (SeAs). We find that Sema-PlxnD1 signaling exerts this effect by antagonizing the proangiogenic activity of vascular endothelial growth factor (VEGF). Specifically, Sema-PlxnD1 signaling ensures the proper endothelial abundance of soluble flt1 (sflt1), an alternatively spliced form of the VEGF receptor Flt1 encoding a potent secreted decoy. Hence, Sema-PlxnD1 signaling regulates distinct but related aspects of angiogenesis: the spatial allocation of angiogenic capacity within a primary vessel and sp! rout guidance.
- Eliminating SF-1 (NR5A1) Sumoylation In Vivo Results in Ectopic Hedgehog Signaling and Disruption of Endocrine Development
- Dev Cell 21(2):315-327 (2011)
Sumoylation is generally considered a repressive mark for many transcription factors. However, the in vivo importance of sumoylation for any given substrate remains unclear and is questionable because the extent of sumoylation appears exceedingly low for most substrates. Here, we permanently eliminated SF-1/NR5A1 sumoylation in mice (Sf-1K119R, K194R, or 2KR) and found that Sf-12KR/2KR mice failed to phenocopy a simple gain of SF-1 function or show elevated levels of well-established SF-1 target genes. Instead, mutant mice exhibited marked endocrine abnormalities and changes in cell fate that reflected an inappropriate activation of hedgehog signaling and other potential SUMO-sensitive targets. Furthermore, unsumoylatable SF-1 mutants activated Shh and exhibited preferential recruitment to Shh genomic elements in cells. We conclude that the sumoylation cycle greatly expands the functional capacity of transcription factors such as SF-1 and is leveraged during developmen! t to achieve cell-type-specific gene expression in multicellular organisms.
- Repo-Man Coordinates Chromosomal Reorganization with Nuclear Envelope Reassembly during Mitotic Exit
- Dev Cell 21(2):328-342 (2011)
Repo-Man targets protein phosphatase 1 γ (PP1γ) to chromatin at anaphase onset and regulates chromosome structure during mitotic exit. Here, we show that a Repo-Man:PP1 complex forms in anaphase following dephosphorylation of Repo-Man. Upon activation, the complex localizes to chromosomes and causes the dephosphorylation of histone H3 (Thr3, Ser10, and Ser28). In anaphase, Repo-Man has both catalytic and structural functions that are mediated by two separate domains. A C-terminal domain localizes Repo-Man to bulk chromatin in early anaphase. There, it targets PP1 for the dephosphorylation of histone H3 and possibly other chromosomal substrates. An N-terminal domain localizes Repo-Man to the chromosome periphery later in anaphase. There, it is responsible for the recruitment of nuclear components such as Importin β and Nup153 in a PP1-independent manner. These observations identify Repo-Man as a key factor that coordinates chromatin remodeling and early events of nuc! lear envelope reformation during mitotic exit.
- The WD40 Repeat PtdIns(3)P-Binding Protein EPG-6 Regulates Progression of Omegasomes to Autophagosomes
- Dev Cell 21(2):343-357 (2011)
PtdIns(3)P plays critical roles in the autophagy pathway. However, little is known about how PtdIns(3)P effectors act with autophagy proteins in autophagosome formation. Here we identified an essential autophagy gene in C. elegans, epg-6, which encodes a WD40 repeat-containing protein with PtdIns(3)P-binding activity. EPG-6 directly interacts with ATG-2. epg-6 and atg-2 regulate progression of omegasomes to autophagosomes, and their loss of function causes accumulation of enlarged early autophagic structures. Another WD40 repeat PtdIns(3)P effector, ATG-18, plays a distinct role in autophagosome formation. We also established the hierarchical relationship of autophagy genes in degradation of protein aggregates and revealed that the UNC-51/Atg1 complex, EPG-8/Atg14, and binding of lipidated LGG-1 to protein aggregates are required for omegasome formation. Our study demonstrates that autophagic PtdIns(3)P effectors play distinct roles in autophagosome formation and also ! provides a framework for understanding the concerted action of autophagy genes in protein aggregate degradation.
- Selective Autophagy Regulates Insertional Mutagenesis by the Ty1 Retrotransposon in Saccharomyces cerevisiae
- Dev Cell 21(2):358-365 (2011)
Macroautophagy (autophagy) is a bulk degradation system for cytoplasmic components and is ubiquitously found in eukaryotic cells. Autophagy is induced under starvation conditions and plays a cytoprotective role by degrading unwanted cytoplasmic materials. The Ty1 transposon, a member of the Ty1/copia superfamily, is the most abundant retrotransposon in the yeast Saccharomyces cerevisiae and acts to introduce mutations in the host genome via Ty1 virus-like particles (VLPs) localized in the cytoplasm. Here we show that selective autophagy downregulates Ty1 transposition by eliminating Ty1 VLPs from the cytoplasm under nutrient-limited conditions. Ty1 VLPs are targeted to autophagosomes by an interaction with Atg19. We propose that selective autophagy safeguards genome integrity against excessive insertional mutagenesis caused during nutrient starvation by transposable elements in eukaryotic cells.
- Steep Differences in Wingless Signaling Trigger Myc-Independent Competitive Cell Interactions
- Dev Cell 21(2):366-374 (2011)
Wnt signaling is a key regulator of development that is often associated with cancer. Wingless, a Drosophila Wnt homolog, has been reported to be a survival factor in wing imaginal discs. However, we found that prospective wing cells survive in the absence of Wingless as long as they are not surrounded by Wingless-responding cells. Moreover, local autonomous overactivation of Wg signaling (as a result of a mutation in APC or axin) leads to the elimination of surrounding normal cells. Therefore, relative differences in Wingless signaling lead to competitive cell interactions. This process does not involve Myc, a well-established cell competition factor. It does, however, require Notum, a conserved secreted feedback inhibitor of Wnt signaling. We suggest that Notum could amplify local differences in Wingless signaling, thus serving as an early trigger of Wg signaling-dependent competition.
- Shaping BMP Morphogen Gradients through Enzyme-Substrate Interactions
- Dev Cell 21(2):375-383 (2011)
- Differential Regulation of Protrusion and Polarity by PI(3)K during Neutrophil Motility in Live Zebrafish
- Dev Cell 21(2):384 (2011)