bluuebunny/crossref_metadata_2025 huggingface dataset:
Sea urchin goosecoid function links fate specification along the animal-vegetal and oral-aboral embryonic axes
Authors: Angerer Lynne M. , Oleksyn David W. , Levine Amy M. , Li Xiaotao , Klein William H. , Angerer Robert C.
Published: April 2021

We have identified a single homolog of goosecoid, SpGsc, that regulates cell fates along both the animal-vegetal and oral-aboral axes of sea urchin embryos. SpGsc mRNA is expressed briefly in presumptive mesenchyme cells of the ∼200-cell blastula and, beginning at about the same time, accumulates in the presumptive oral ectoderm through pluteus stage. Loss-of-function assays with morpholine-substituted antisense oligonucleotides show that SpGsc is required for endoderm and pigment cell differentiation and for gastrulation. These experiments and gain-of-function tests by mRNA injection show that SpGsc is a repressor that antagonizes aboral ectoderm fate specification and promotes oral ectoderm differentiation. We show that SpGsc competes for binding to specific cis elements with SpOtx, a ubiquitous transcription activator that promotes aboral ectoderm differentiation. Moreover, SpGsc represses transcription in vivo from an artificial promoter driven by SpOtx. As SpOtx appears long before SpGsc transcription is activated, we propose that SpGsc diverts ectoderm towards oral fate by repressing SpOtx target genes. Based on the SpGsc-SpOtx example and other available data, we propose that ectoderm is first specified as aboral by broadly expressed activators, including SpOtx, and that the oral region is subsequently respecified by the action of negative regulators, including SpGsc. Accumulation of SpGsc in oral ectoderm depends on cell-cell interactions initiated by nuclear β-catenin function, which is known to be required for specification of vegetal tissues, because transcripts are undetectable in dissociated or in cadherin mRNA-injected embryos. This is the first identified molecular mechanism underlying the known dependence of oral-aboral ectoderm polarity on intercellular signaling.
TheArabidopsisnectary is an ABC-independent floral structure
Authors: Baum Stuart F. , Eshed Yuval , Bowman John L.
Published: April 2021

In contrast to the conservation of floral organ order in angiosperm flowers, nectary glands can be found in various floral and extrafloral positions. Since in Arabidopsis, the nectary develops only at the base of stamens, its specification was assayed with regard to the floral homeotic ABC selector genes. We show that the nectary can form independently of any floral organ identity gene but is restricted to the ‘third whorl’ domain in the flower. This domain is, in part, specified redundantly by LEAFY and UNUSUAL FLORAL ORGANS. Even though nectary glands arise from cells previously expressing the B class genes, their proper development requires the down-regulation of B class gene activity. While CRABS CLAW is essential for nectary gland formation, its ectopic expression is not sufficient to induce ectopic nectary formation. We show that in Arabidopsis multiple factors act to restrict the nectary to the flower, and surprisingly, some of these factors are LEAFY and UNUSUAL FLORAL ORGANS.
jing: a downstream target of slbo required for developmental control of border cell migration
Authors: Liu Yuru , Montell Denise J.
Published: April 2021

Epithelial to mesenchymal transitions and cell migration are important features of embryonic development and tumor metastasis. We are employing a systematic genetic approach to study the border cells in the Drosophila ovary, as a simple model for these cellular behaviors. Previously we found that expression of the basic-region/leucine zipper transcription factor, C/EBP, is required for the border cells to initiate their migration. Here we report the identification of a second nuclear factor, named JING (which means ‘still’), that is required for initiation of border cell migration. The jing locus was identified in a screen for mutations that cause border cell migration defects in mosaic clones. The jing mutant phenotype resembles that of slbo mutations, which disrupt the Drosophila C/EBP gene, but is distinct from other classes of border cell migration mutants. Expression of a jing-lacZ reporter in border cells requires C/EBP. Moreover, expression of jing from a heat-inducible promoter rescues the border cell migration defects of hypomorphic slbo mutants. The JING protein is most closely related to a mouse protein, AEBP2, which was identified on the basis of its ability to bind a small regulatory sequence within the adipocyte AP2 gene to which mammalian C/EBP also binds. We propose that the need to coordinate cell differentiation with nutritional status may be the link between mammalian adipocytes and Drosophila border cells that led to the conservation of C/EBP and AEBP2.
Reciprocal developmental changes in the roles of Bcl-w and Bcl-xL in regulating sensory neuron survival
Authors: Middleton Gayle , Wyatt Sean , Ninkina Natalia , Davies Alun M.
Published: April 2021

We have compared the roles of two anti-apoptotic members of the Bcl2 family, Bcl-w and Bcl-xL, in regulating the survival of sensory neurons during development. We used microinjection to introduce expression plasmids containing Bcl-w and Bcl-xL cDNAs in the sense and antisense orientations into the nuclei of BDNF-dependent nodose neurons and NGF-dependent trigeminal neurons at stages during and after the period of naturally occurring neuronal death. Whilst overexpression of either protein promoted neuronal survival in the absence of neurotrophins and microinjection of antisense constructs reduced neuronal survival in the presence of neurotrophins, the magnitude of these effects changed with age. Whereas Bcl-w overexpression became more effective in promoting neuronal survival with age, Bcl-xL overexpression became less effective, and whereas antisense Bcl-w became much more effective in killing neurotrophin-supplemented neurons with age, antisense Bcl-xL became much less effective in killing these neurons. There was a marked increased in Bcl-w mRNA and Bcl-w immunoreactive neurons and a decrease in Bcl-xL mRNA and Bcl-xL immunoreactive neurons in the trigeminal and nodose ganglia over this period of development. Our results demonstrate that both Bcl-w and Bcl-xL play an important anti-apoptotic role in regulating the survival of NGF- and BDNF-dependent neurons, and that reciprocal changes occur in the relative importance of these proteins with age. Whereas Bcl-xL plays a more important role during the period of naturally occurring neuronal death, Bcl-w plays a more important role at later stages.
Sonic hedgehog contributes to oligodendrocyte specification in the mammalian forebrain
Authors: Nery Susana , Wichterle Hynek , Fishell Gord
Published: April 2021

This study addresses the role of Sonic hedgehog (Shh) in promoting the generation of oligodendrocytes in the mouse telencephalon. We show that in the forebrain, expression of the early oligodendrocyte markers Olig2, plp/dm20 and PDGFRα corresponds to regions of Shh expression? To directly test if Shh can induce the development of oligodendrocytes within the telencephalon, we use retroviral vectors to ectopically express Shh within the mouse embryonic telencephalon. We find that infections with Shh-expressing retrovirus at embryonic day 9.5, result in ectopic Olig2 and PDGFRα expression by mid-embryogenesis. By postnatal day 21, cells expressing ectopic Shh overwhelmingly adopt an oligodendrocyte identity? To determine if the loss of telencephalic Shh correspondingly results in the loss of oligodendrocyte production, we studied Nkx2.1 mutant mice in which telencephalic expression of Shh is selectively lost. In accordance with Shh playing a role in oligodendrogenesis, within the medial ganglionic eminence ofNkx2.1 mutants, the early expression of PDGFRα is absent and the level of Olig2 expression is diminished in this region. In addition, in these same mutants, expression of both Shh and plp/dm20 is lost in the hypothalamus. Notably, in the prospective amygdala region where Shh expression persists in the Nkx2.1 mutant, the presence of plp/dm20 is unperturbed. Further supporting the idea that Shh is required for the in vivo establishment of early oligodendrocyte populations, expression of PDGFRα can be partially rescued by virally mediated expression of Shh in the Nkx2.1 mutant telencephalon. Interestingly, despite the apparent requirement for Shh for oligodendrocyte specification in vivo, all regions of either wild-type or Nkx2.1 mutant telencephalon are competent to produce oligodendrocytes in vitro. Furthermore, analysis of CNS tissue from Shh null animals definitively shows that, in vitro, Shh is not required for the generation of oligodendrocytes. We propose that oligodendrocyte specification is negatively regulated in vivo and thatShh generates oligodendrocytes by overcoming this inhibition. Furthermore, it appears that a Shh-independent pathway for generating oligodendrocytes exists.
The cell adhesion molecule Echinoid defines a new pathway that antagonizes the Drosophila EGF receptor signaling pathway
Authors: Bai Jia-Min , Chiu Wei-Hsin , Wang Jiunn-Chin , Tzeng Ting-Hue , Perrimon Norbert , Hsu Jui-Chou
Published: April 2021

Photoreceptor and cone cells in the Drosophila eye are recruited following activation of the epidermal growth factor receptor (EGFR) pathway. We have identified echinoid (ed) as a novel putative cell adhesion molecule that negatively regulates EGFR signaling. The ed mutant phenotype is associated with extra photoreceptor and cone cells. Conversely, ectopic expression of ed in the eye leads to a reduction in the number of photoreceptor cells. ed expression is independent of EGFR signaling and ED is localized to the plasma membrane of every cells throughout the eye disc. We present evidence that ed acts nonautonomously to generate extra R7 cells by a mechanism that is sina-independent but upstream of Tramtrack (TTK88). Together, our results support a model whereby ED defines an independent pathway that antagonizes EGFR signaling by regulating the activity, but not the level, of the TTK88 transcriptional repressor.
Merlin, the Drosophila homologue of neurofibromatosis-2, is specifically required in posterior follicle cells for axis formation in the oocyte
Authors: MacDougall Nina , Lad Yatish , Wilkie Gavin S. , Francis-Lang Helen , Sullivan William , Davis Ilan
Published: April 2021

In Drosophila, the formation of the embryonic axes is initiated by Gurken, a transforming growth factor α signal from the oocyte to the posterior follicle cells, and an unknown polarising signal back to the oocyte. We report that Drosophila Merlin is specifically required only within the posterior follicle cells to initiate axis formation. Merlin mutants show defects in nuclear migration and mRNA localisation in the oocyte. Merlin is not required to specify posterior follicle cell identity in response to the Gurken signal from the oocyte, but is required for the unknown polarising signal back to the oocyte. Merlin is also required non-autonomously, only in follicle cells that have received the Gurken signal, to maintain cell polarity and limit proliferation, but is not required in embryos and larvae. These results are consistent with the fact that human Merlin is encoded by the gene for the tumour suppressor neurofibromatosis-2 and is a member of the Ezrin-Radixin-Moesin family of proteins that link actin to transmembrane proteins. We propose that Merlin acts in response to the Gurken signal by apically targeting the signal that initiates axis specification in the oocyte.
A bipotential precursor population for pancreas and liver within the embryonic endoderm
Authors: Deutsch Gail , Jung Joonil , Zheng Minghua , Lóra José , Zaret Kenneth S.
Published: April 2021

The pancreas emerges independently from dorsal and ventral domains of embryonic gut endoderm. Gene inactivation experiments in mice have identified factors required for dorsal pancreas development, but factors that initiate the ventral pancreas have remained elusive. In this study, we investigated the hypothesis that the emergence of the ventral pancreas is related to the emergence of the liver. We find that the liver and ventral pancreas are specified at the same time and in the same general domain of cells. Using embryo tissue explantation experiments, we find that the default fate of the ventral foregut endoderm is to activate the pancreas gene program. FGF signalling from the cardiac mesoderm diverts this endoderm to express genes for liver instead of those for pancreas. No evidence was found to indicate that the cell type choice for pancreas or liver involves a selection for growth or viability. Cardiac mesoderm or FGF induces the local expression of sonic hedgehog, which in turn is inhibitory to pancreas but not to liver. The bipotential precursor cell population for pancreas and liver in embryonic development and its fate selection by FGF has features that appear to be recapitulated in the adult pancreas and are reflected in the evolution of these organs.
Expression of activated Ras during Dictyostelium development alters cell localization and changes cell fate
Authors: Jaffer Zahara M. , Khosla Meenal , Spiegelman George B. , Weeks Gerald
Published: April 2021

There is now a body of evidence to indicate that Ras proteins play important roles in development. Dictyostelium expresses several ras genes and each appears to perform a distinct function. Previous data had indicated that the overexpression of an activated form of the major developmentally regulated gene, rasD, caused a major aberration in morphogenesis and cell type determination. We now show that the developmental expression of an activated rasG gene under the control of the rasD promoter causes a similar defect. Our results indicate that the expression of activated rasG in prespore cells results in their transdifferentiation into prestalk cells, whereas activated rasG expression in prestalk causes gross mislocalization of the prestalk cell populations.
Rescuing desmoplakin function in extra-embryonic ectoderm reveals the importance of this protein in embryonic heart, neuroepithelium, skin and vasculature
Authors: Gallicano G. Ian , Bauer Christoph , Fuchs Elaine
Published: April 2021

Desmosomes mediate intercellular adhesion through desmosomal cadherins, which interface with plakoglobin (PG) and desmoplakin (DP) to associate with the intermediate filament (IF) cytoskeleton. Desmosomes first assemble in the E3.5 mouse trophectoderm, concomitant with establishment of epithelial polarity and appearance of a blastocoel cavity. Increasing in size and number, desmosomes continue their prominence in extra-embryonic tissues, but as development proceeds, they also become abundant in a number of embryonic tissues, including heart muscle, epidermis and neuroepithelium. Previously, we explored the functional importance of desmosomes by ablating the Dsp gene. Homozygous Dsp mutant embryos progressed through implantation, but did not survive beyond E6.5, owing to a loss or instability of desmosomes and tissue integrity. We have now rescued the extra-embryonic tissues by aggregation of tetraploid (wild-type) and diploid (Dsp mutant) morulae. These animals survive several days longer, but die shortly after gastrulation, with major defects in the heart muscle, neuroepithelium and skin epithelium, all of which possess desmosomes, as well as the microvasculature, which does not. Interestingly, although wild-type endothelial cells of capillaries do not form desmosomes, they possess unusual intercellular junctions composed of DP, PG and VE-cadherin. The severity in phenotype and the breadth of defects in the Dsp mutant embryo is greater than PG mutant embryos, substantiating redundancy between PG and other armadillo proteins (e.g. β-catenin). The timing of lethality is similar to that of the VE-cadherin null embryo, suggesting that a participating cause of death may be a defect in vasculature, not reported for PG null embryos.
Specification of left-right asymmetry in the embryonic gut of Drosophila
Authors: Ligoxygakis Petros , Strigini Maura , Averof Michalis
Published: April 2021

Most animals exhibit stable left-right asymmetries in their body. Although significant progress has been made in elucidating the mechanisms that set up these asymmetries in vertebrates, nothing is known about them in Drosophila. This is usually attributed to the fact that no reversals of stable left-right asymmetries have been observed in Drosophila, although relevant surveys have been carried out. We have focused on the asymmetry of the proventriculus in the embryonic gut of Drosophila, an aspect of left-right asymmetry that is extremely stable in wild-type flies. We show that this asymmetry can be reversed by mutations in the dicephalic and wunen genes, which also cause reversals in the antero-posterior axis of the embryo relative to its mother. This is the first observation to suggest that left-right asymmetries in Drosophila can be reversed by genetic/developmental manipulations. It also suggests that maternal signals may initiate the specification of some left-right asymmetries in the embryo.
PAR-1 is required for the maintenance of oocyte fate in Drosophila
Authors: Huynh Jean-René , Shulman Joshua M. , Benton Richard , Johnston Daniel St
Published: April 2021

The PAR-1 kinase is required for the posterior localisation of the germline determinants in C. elegans and Drosophila, and localises to the posterior of the zygote and the oocyte in each case. We show that Drosophila PAR-1 is also required much earlier in oogenesis for the selection of one cell in a germline cyst to become the oocyte. Although the initial steps in oocyte determination are delayed, three markers for oocyte identity, the synaptonemal complex, the centrosomes and Orb protein, still become restricted to one cell in mutant clones. However, the centrosomes and Orb protein fail to translocate from the anterior to the posterior cortex of the presumptive oocyte in region 3 of the germarium, and the cell exits meiosis and becomes a nurse cell. Furthermore, markers for the minus ends of the microtubules also fail to move from the anterior to the posterior of the oocyte in mutant clones. Thus, PAR-1 is required for the maintenance of oocyte identity, and plays a role in microtubule-dependent localisation within the oocyte at two stages of oogenesis. Finally, we show that PAR-1 localises on the fusome, and provides a link between the asymmetry of the fusome and the selection of the oocyte.
Distinct sites of origin of oligodendrocytes and somatic motoneurons in the chick spinal cord: oligodendrocytes arise from Nkx2.2-expressing progenitors by a Shh-dependent mechanism
Authors: Soula Cathy , Danesin Cathy , Kan Paulette , Grob Magali , Poncet Christian , Cochard Philippe
Published: April 2021

In the vertebrate spinal cord, oligodendrocytes arise from the ventral part of the neuroepithelium, a region also known to generate somatic motoneurons. The emergence of oligodendrocytes, like that of motoneurons, depends on an inductive signal mediated by Sonic hedgehog. We have defined the precise timing of oligodendrocyte progenitor specification in the cervico-brachial spinal cord of the chick embryo. We show that ventral neuroepithelial explants, isolated at various development stages, are unable to generate oligodendrocytes in culture until E5 but become able to do so in an autonomous way from E5.5. This indicates that the induction of oligodendrocyte precursors is a late event that occurs between E5 and E5.5, precisely at the time when the ventral neuroepithelium stops producing somatic motoneurons. Analysis of the spatial restriction of oligodendrocyte progenitors, evidenced by their expression of O4 or PDGFRμ, indicate that they always lie within the most ventral Nkx2.2-expressing domain of the neuroepithelium, and not in the adjacent domain characterized by Pax6 expression from which somatic motoneurons emerge. We then confirm that Shh is necessary between E5 and E5.5 to specify oligodendrocyte precursors but is no longer required beyond this stage to maintain ongoing oligodendrocyte production. Furthermore, Shh is sufficient to induce oligodendrocyte formation from ventral neuroepithelial explants dissected at E5. Newly induced oligodendrocytes expressed Nkx2.2 but not Pax6, correlating with the in vivo observation. Altogether, our results show that, in the chick spinal cord, oligodendrocytes originate from Nkx2.2-expressing progenitors.
Notch signaling represses the glial fate in fly PNS
Authors: Van De Bor Véronique , Giangrande Angela
Published: April 2021

By using gain-of-function mutations it has been proposed that vertebrate Notch promotes the glial fate. We show in vivo that glial cells are produced at the expense of neurons in the peripheral nervous system of flies lacking Notch and that constitutively activated Notch produces the opposite phenotype. Notch acts as a genetic switch between neuronal and glial fates by negatively regulating glial cell deficient/glial cells missing, the gene required in the glial precursor to induce gliogenesis. Moreover, Notch represses neurogenesis or gliogenesis, depending on the sensory organ type. Numb, which is asymmetrically localized in the multipotent cell that produces the glial precursor, induces glial cells at the expense of neurons. Thus, a cell- autonomous mechanism inhibits Notch signaling.
Steroid regulation of autophagic programmed cell death during development
Authors: Lee Cheng-Yu , Baehrecke Eric H.
Published: April 2021

Apoptosis and autophagy are morphologically distinct forms of programmed cell death. While autophagy occurs during the development of diverse organisms and has been implicated in tumorigenesis, little is known about the molecular mechanisms that regulate this type of cell death. Here we show that steroid-activated programmed cell death of Drosophila salivary glands occurs by autophagy. Expression of p35 prevents DNA fragmentation and partially inhibits changes in the cytosol and plasma membranes of dying salivary glands, suggesting that caspases are involved in autophagy. The steroid-regulated BR-C, E74A and E93 genes are required for salivary gland cell death. BR-C and E74A mutant salivary glands exhibit vacuole and plasma membrane breakdown, but E93 mutant salivary glands fail to exhibit these changes, indicating that E93 regulates early autophagic events. Expression of E93 in embryos is sufficient to induce cell death with many characteristics of apoptosis, but requires the H99 genetic interval that contains the rpr, hid and grim proapoptotic genes to induce nuclear changes diagnostic of apoptosis. In contrast, E93 expression is sufficient to induce the removal of cells by phagocytes in the absence of the H99 genes. These studies indicate that apoptosis and autophagy utilize some common regulatory mechanisms.
TheC. eleganshomolog of the murine cystic kidney disease geneTg737functions in a ciliogenic pathway and is disrupted inosm-5mutant worms
Authors: Haycraft Courtney J. , Swoboda Peter , Taulman Patrick D. , Thomas James H. , Yoder Bradley K.
Published: April 2021

Cilia and flagella are important organelles involved in diverse functions such as fluid and cell movement, sensory perception and embryonic patterning. They are devoid of protein synthesis, thus their formation and maintenance requires the movement of protein complexes from the cytoplasm into the cilium and flagellum axoneme by intraflagellar transport (IFT), a conserved process common to all ciliated or flagellated eukaryotic cells. We report that mutations in the Caenorhabditis elegans gene Y41g9a.1 are responsible for the ciliary defects in osm-5 mutant worms. This was confirmed by transgenic rescue of osm-5(p813) mutants using the wild-type Y41g9a.1 gene. osm-5 encodes a tetratricopeptide repeat (TPR)-containing protein that is the homolog of murine polaris (Tg737), a protein associated with cystic kidney disease and left-right axis patterning defects in the mouse. osm-5 is expressed in ciliated sensory neurons in C. elegans and its expression is regulated by DAF-19, an RFX-type transcription factor that governs the expression of other genes involved in cilia formation in the worm. Similar to murine polaris, the OSM-5 protein was found to concentrate at the cilium base and within the cilium axoneme as shown by an OSM-5::GFP translational fusion and immunofluorescence. Furthermore, time-lapse imaging of OSM-5::GFP fusion protein shows fluorescent particle migration within the cilia. Overall, the data support a crucial role for osm-5 in a conserved ciliogenic pathway, most likely as a component of the IFT process.

Movies available on-line:

http://www.biologists.com/Development/movies/dev3342.html
Hedgehog signaling patterns the tracheal branches
Authors: Glazer Lillian , Shilo Ben-Zion
Published: April 2021

The elaborate branching pattern of the Drosophila tracheal system originates from ten tracheal placodes on both sides of the embryo, each consisting of about 80 cells. Simultaneous cell migration from each tracheal pit in six different directions gives rise to the stereotyped branching pattern. Each branch contains a fixed number of cells. Previous work has shown that in the dorsoventral axis, localized activation of the Dpp, Wnt and EGF receptor (DER) pathways, subdivides the tracheal pit into distinct domains. We present the role of the Hedgehog (Hh) signaling system in patterning the tracheal branches. Hh is expressed in segmental stripes abutting the anterior border of the tracheal placodes. Induction of patched expression, which results from activation by Hh, demonstrates that cells in the anterior half of the tracheal pit are activated. In hh-mutant embryos migration of all tracheal branches is absent or stalled. These defects arise from a direct effect of Hh on tracheal cells, rather than by indirect effects on patterning of the ectoderm. Tracheal cell migration could be rescued by expressing Hh only in the tracheal cells, without rescuing the ectodermal defects. Signaling by several pathways, including the Hh pathway, thus serves to subdivide the uniform population of tracheal cells into distinct cell types that will subsequently be recruited into the different branches.
Depletion of definitive gut endoderm inSox17-null mutant mice
Authors: Kanai-Azuma Masami , Kanai Yoshiakira , Gad Jacqueline M. , Tajima Youichi , Taya Choji , Kurohmaru Masamichi , Sanai Yutaka , Yonekawa Hiromichi , Yazaki Kazumori , Tam Patrick P. L. , Hayashi Yoshihiro
Published: April 2021

In the mouse, the definitive endoderm is derived from the epiblast during gastrulation, and, at the early organogenesis stage, forms the primitive gut tube, which gives rise to the digestive tract, liver, pancreas and associated visceral organs. The transcription factors, Sox17 (a Sry-related HMG box factor) and its upstream factors, Mixer (homeobox factor) and Casanova (a novel Sox factor), have been shown to function as endoderm determinants in Xenopus and zebrafish, respectively. However, whether the mammalian orthologues of these genes are also involved with endoderm formation is not known. We show that Sox17–/– mutant embryos are deficient of gut endoderm. The earliest recognisable defect is the reduced occupancy by the definitive endoderm in the posterior and lateral region of the prospective mid- and hindgut of the headfold-stage embryo. The prospective foregut develops properly until the late neural plate stage. Thereafter, elevated levels of apoptosis lead to a reduction in the population of the definitive endoderm in the foregut. In addition, the mid- and hindgut tissues fail to expand. These are accompanied by the replacement of the definitive endoderm in the lateral region of the entire length of the embryonic gut by cells that resemble the visceral endoderm. In the chimeras, although Sox17-null ES cells can contribute unrestrictedly to ectodermal and mesodermal tissues, few of them could colonise the foregut endoderm and they are completely excluded from the mid- and hindgut endoderm. Our findings indicate an important role of Sox17 in endoderm development in the mouse, highlighting the idea that the molecular mechanism for endoderm formation is likely to be conserved among vertebrates.
Co-ordinating retinal histogenesis: early cell cycle exit enhances early cell fate determination in theXenopusretina
Authors: Ohnuma Shin-ichi , Hopper Susannah , Wang Kevin C. , Philpott Anna , Harris William A.
Published: April 2021

The laminar arrays of distinct cell types in the vertebrate retina are built by a histogenic process in which cell fate is correlated with birth order. To explore this co-ordination mechanistically, we altered the relative timing of cell cycle exit in the developing Xenopus retina and asked whether this affected the activity of neural determinants. We found that Xath5, a bHLH proneural gene that promotes retinal ganglion cell (RGC) fate, (Kanekar, S., Perron, M., Dorsky, R., Harris, W. A., Jan, L. Y., Jan, Y. N. and Vetter, M. L. (1997) Neuron19, 981-994), does not cause these cells to be born prematurely. To drive cells out of the cell cycle early, therefore, we misexpressed the cyclin kinase inhibitor, p27Xic1. We found that early cell cycle exit potentiates the ability of Xath5 to promote RGC fate. Conversely, the cell cycle activator, cyclin E1, which inhibits cell cycle exit, biases Xath5-expressing cells toward later neuronal fates. We found that Notch activation in this system caused cells to exit the cell cycle prematuely, and when it is misexpressed with Xath5, it also potentiates the induction of RGCs. The potentiation is counteracted by co-expression of cyclin E1. These results suggest a model of histogenesis in which the activity of factors that promote early cell cycle exit enhances the activity of factors that promote early cellular fates.
The zinc-finger transcription factor Klf4 is required for terminal differentiation of goblet cells in the colon
Authors: Katz Jonathan P. , Perreault Nathalie , Goldstein Bree G. , Lee Catherine S. , Labosky Patricia A. , Yang Vincent W. , Kaestner Klaus H.
Published: April 2021

Klf4 (formerly GKLF) is a zinc-finger transcription factor expressed in the epithelia of the skin, lungs, gastrointestinal tract and several other organs. In vitro studies have suggested that Klf4 plays an important role in cell proliferation and/or differentiation. Mice homozygous for a null mutation in Klf4 die within 15 hours of birth and show selective perturbation of late-stage differentiation structures in the epidermis, but the function of Klf4 in the gastrointestinal tract has not been investigated. To address this issue, we have generated Klf4–/– mice by homologous recombination in embryonic stem cells. In this study, we provide the first in vivo evidence that Klf4 is a goblet cell-specific differentiation factor in the colon. Klf4–/– mice exhibit normal cell proliferation and cell death rates in the colon on postnatal day 1. However, Klf4–/– mice demonstrate a 90% decrease in the number of goblet cells in the colon, show abnormal expression of the goblet cell-specific marker Muc2 by in situ hybridization, have abnormal staining of the colonic epithelium with Alcian Blue for acidic mucins, and lack normal goblet cell morphology by ultrastructural analysis. All other epithelial cell types are present in the colon of Klf4–/– mice. In summary, Klf4 plays a crucial role in colonic epithelial cell differentiation in vivo.
An anterior function for theDrosophilaposterior determinant Pumilio
Authors: Gamberi Chiara , Peterson David S. , He Luming , Gottlieb Ellen
Published: April 2021

Bicoid is a key determinant of anterior Drosophila development. We demonstrate that the prototypical Puf protein Pumilio temporally regulates bicoid (bcd) mRNA translation via evolutionarily conserved Nanos response elements (NRE) in its 3′UTR. Disruption of Pumilio-bcd mRNA interaction by either Pumilio or bcd NRE mutations caused delayed bcd mRNA deadenylation and stabilization, resulting in protracted Bicoid protein expression during embryogenesis. Phenotypically, embryos from transgenic mothers that harbor bcd NRE mutations exhibited dominant anterior patterning defects and we discovered similar head defects in embryos from pum– mothers. Hence, Pumilio is required for normal anterior development. Since bcd mRNA resides outside the posterior gradient of the canonical partner of Pumilio, Nanos, our data suggest that Pumilio can recruit different partners to specifically regulate distinct mRNAs.
Transformation of shoots into roots inArabidopsisembryos mutant at theTOPLESSlocus
Authors: Long Jeff A. , Woody Scott , Poethig Scott , Meyerowitz Elliot M. , Barton M. Kathryn
Published: April 2021

We describe a novel phenotype in Arabidopsis embryos homozygous for the temperature-sensitive topless-1 mutation. This mutation causes the transformation of the shoot pole into a root. Developing topless embryos fail to express markers for the shoot apical meristem (SHOOT MERISTEMLESS and UNUSUAL FLORAL ORGANS) and the hypocotyl (KNAT1). By contrast, the pattern of expression of root markers is either duplicated (LENNY, J1092) or expanded (SCARECROW). Shifts of developing topless embryos between permissive and restrictive temperatures show that apical fates (cotyledons plus shoot apical meristem) can be transformed to basal fates (root) as late as transition stage. As the apical pole of transition stage embryos shows both morphological and molecular characteristics of shoot development, this demonstrates that the topless 1 mutation is capable of causing structures specified as shoot to be respecified as root. Finally, our experiments fail to show a clear link between auxin signal transduction and topless-1 mutant activity: the development of the apical root in topless mutant individuals is not dependent on the activity of the predicted auxin response factor MONOPTEROS nor is the expression of DR5, a proposed ‘auxin maximum reporter’, expanded in the apical domain of topless embryos.
Commissureless is required both in commissural neurones and midline cells for axon guidance across the midline
Authors: Georgiou Marios , Tear Guy
Published: April 2021

In the absence of Commissureless (Comm) function, axons are unable to extend across the central nervous system midline. Comm downregulates levels of Roundabout (Robo), a receptor for the midline repellent Slit, in order to allow axons to cross the midline. comm transcript is expressed at high levels in the midline glia and Comm protein accumulates on axons at the midline. This has led to the hypothesis that Comm moves from the midline glia to the axons, where it can reduce Robo levels. We have found that expression of Comm in the midline cells is unable to rescue the comm phenotype and that tagged versions of Comm are not transferred to axons. A re-examination of Comm protein expression and the use of targeted RNA interference reveal that correct midline crossing requires that Comm is expressed in the commissural axons and midline glia. We suggest that accumulation of Comm protein at the midline spatially limits Comm activity and prevents it from being active on the contralateral side of the central nervous system.
Mechanisms of Gurken-dependentpiperegulation and the robustness of dorsoventral patterning inDrosophila
Authors: Peri Francesca , Technau Martin , Roth Siegfried
Published: April 2021

The restriction of Pipe, a potential glycosaminoglycan-modifying enzyme, to ventral follicle cells of the egg chamber is essential for dorsoventral axis formation in the Drosophila embryo. pipe repression depends on the TGFα-like ligand Gurken, which activates the Drosophila EGF receptor in dorsal follicle cells. An analysis of Raf mutant clones shows that EGF signalling is required cell-autonomously in all dorsal follicle cells along the anteroposterior axis of the egg chamber to repress pipe. However, the autoactivation of EGF signalling important for dorsal follicle cell patterning has no influence on pipe expression. Clonal analysis shows that also the mirror-fringe cassette suggested to establish a secondary signalling centre in the follicular epithelium is not involved in pipe regulation. These findings support the view that the pipe domain is directly delimited by a long-range Gurken gradient. Pipe induces ventral cell fates in the embryo via activation of the Spätzle/Toll pathway. However, large dorsal patches of ectopic pipe expression induced by Raf clones rarely affect embryonic patterning if they are separated from the endogenous pipe domain. This indicates that potent inhibitory processes prevent pipe dependent Toll activation at the dorsal side of the egg.
Epithelial cell cycling predicts p53 responsiveness to γ-irradiation during post-natal mammary gland development
Authors: Minter Lisa M. , Dickinson Ellen S. , Naber Stephen P. , Jerry D. Joseph
Published: April 2021

The tumor suppressor gene, TP53, plays a major role in surveillance and repair of radiation-induced DNA damage. In multiple cell types, including mammary epithelial cells, abrogation of p53 (encoded by Trp53) function is associated with increased tumorigenesis. We examined γ-irradiated BALB/c-Trp53+/+ and -Trp53–/– female mice at five stages of post-natal mammary gland development to determine whether radiation-induced p53 activity is developmentally regulated. Our results show that p53-mediated responses are attenuated in glands from irradiated virgin and lactating mice, as measured by induction of p21/WAF1 (encoded by Cdkn1a) and apoptosis, while irradiated early- and mid-pregnancy glands exhibit robust p53 activity. There is a strong correlation between p53-mediated apoptosis and the degree of cellular proliferation, independent of the level of differentiation. In vivo, proliferation is intimately influenced by steroid hormones. To determine whether steroid hormones directly modulate p53 activity, whole organ cultures of mammary glands were induced to proliferate using estrogen plus progesterone or epidermal growth factor plus transforming growth factor-α and p53 responses to γ-irradiation were measured. Regardless of mitogens used, proliferating mammary epithelial cells show comparable p53 responses to γ-irradiation, including expression of nuclear p53 and p21/WAF1 and increased levels of apoptosis, compared to non-proliferating irradiated control cultures. Our study suggests that differences in radiation-induced p53 activity during post-natal mammary gland development are influenced by the proliferative state of the gland, and may be mediated indirectly by the mitogenic actions of steroid hormones in vivo.
Zebrafish mutants identify an essential role for laminins in notochord formation
Authors: Parsons Michael J. , Pollard Steven M. , Saúde Leonor , Feldman Benjamin , Coutinho Pedro , Hirst Elizabeth M. A. , Stemple Derek L.
Published: April 2021

Basement membranes are thought to be essential for organ formation, providing the scaffold on which individual cells organize to form complex tissues. Laminins are integral components of basement membranes. To understand the development of a simple vertebrate organ, we have used positional cloning to characterize grumpy and sleepy, two zebrafish loci known to control notochord formation, and find that they encode laminin β1 and laminin γ1, respectively. Removal of either chain results in the dramatic loss of laminin 1 staining throughout the embryo and prevents formation of the basement membrane surrounding the notochord. Notochord cells fail to differentiate and many die by apoptosis. By transplantation, we demonstrate that, for both grumpy and sleepy, notochord differentiation can be rescued by exogenous sources of the missing laminin chain, although notochordal sources are also sufficient for rescue. These results demonstrate a clear in vivo requirement for laminin β1 and laminin γ1 in the formation of a specific vertebrate organ and show that laminin or the laminin-dependent basement membrane is essential for the differentiation of chordamesoderm to notochord.
A pathway of signals regulating effector and initiator caspases in the developingDrosophilaeye
Authors: Yu Sun-Yun , Yoo Soon Ji , Yang Lihui , Zapata Cynthia , Srinivasan Anu , Hay Bruce A. , Baker Nicholas E.
Published: April 2021

Regulated cell death and survival play important roles in neural development. Extracellular signals are presumed to regulate seven apparent caspases to determine the final structure of the nervous system. In the eye, the EGF receptor, Notch, and intact primary pigment and cone cells have been implicated in survival or death signals. An antibody raised against a peptide from human caspase 3 was used to investigate how extracellular signals controlled spatial patterning of cell death. The antibody crossreacted specifically with dying Drosophila cells and labelled the activated effector caspase Drice. It was found that the initiator caspase Dronc and the proapoptotic gene head involution defective were important for activation in vivo. Dronc may play roles in dying cells in addition to activating downstream effector caspases. Epistasis experiments ordered EGF receptor, Notch, and primary pigment and cone cells into a single pathway that affected caspase activity in pupal retina through hid and Inhibitor of Apoptosis Proteins. None of these extracellular signals appeared to act by initiating caspase activation independently of hid. Taken together, these findings indicate that in eye development spatial regulation of cell death and survival is integrated through a single intracellular pathway.
Gliogenesis inDrosophila: genome-wide analysis of downstream genes ofglial cells missingin the embryonic nervous system
Authors: Egger Boris , Leemans Ronny , Loop Thomas , Kammermeier Lars , Fan Yun , Radimerski Tanja , Strahm Martin C. , Certa Ulrich , Reichert Heinrich
Published: April 2021

In Drosophila, the glial cells missing (gcm) gene encodes a transcription factor that controls the determination of glial versus neuronal fate. In gcm mutants, presumptive glial cells are transformed into neurons and, conversely, when gcm is ectopically misexpressed, presumptive neurons become glia. Although gcm is thought to initiate glial cell development through its action on downstream genes that execute the glial differentiation program, little is known about the identity of these genes. To identify gcm downstream genes in a comprehensive manner, we used genome-wide oligonucleotide arrays to analyze differential gene expression in wild-type embryos versus embryos in which gcm is misexpressed throughout the neuroectoderm. Transcripts were analyzed at two defined temporal windows during embryogenesis. During the first period of initial gcm action on determination of glial cell precursors, over 400 genes were differentially regulated. Among these are numerous genes that encode other transcription factors, which underscores the master regulatory role of gcm in gliogenesis. During a second later period, when glial cells had already differentiated, over 1200 genes were differentially regulated. Most of these genes, including many genes for chromatin remodeling factors and cell cycle regulators, were not differentially expressed at the early stage, indicating that the genetic control of glial fate determination is largely different from that involved in maintenance of differentiated cells. At both stages, glial-specific genes were upregulated and neuron-specific genes were downregulated, supporting a model whereby gcm promotes glial development by activating glial genes, while simultaneously repressing neuronal genes. In addition, at both stages, numerous genes that were not previously known to be involved in glial development were differentially regulated and, thus, identified as potential new downstream targets of gcm. For a subset of the differentially regulated genes, tissue-specific in vivo expression data were obtained that confirmed the transcript profiling results. This first genome-wide analysis of gene expression events downstream of a key developmental transcription factor presents a novel level of insight into the repertoire of genes that initiate and maintain cell fate choices in CNS development.
A common translational control mechanism functions in axial patterning and neuroendocrine signaling inDrosophila
Authors: Clark Ira E. , Dobi Krista C. , Duchow Heather K. , Vlasak Anna N. , Gavis Elizabeth R.
Published: April 2021

Translational repression of maternal nanos (nos) mRNA by a cis-acting Translational Control Element (TCE) in the nos 3′UTR is critical for anterior-posterior patterning of the Drosophila embryo. We show, through ectopic expression experiments, that the nos TCE is capable of repressing gene expression at later stages of development in neuronal cells that regulate the molting cycle. Our results predict additional targets of TCE-mediated repression within the nervous system. They also suggest that mechanisms that regulate maternal mRNAs, like TCE-mediated repression, may function more widely during development to spatially or temporally control gene expression.
PTEN is essential for cell migration but not for fate determination and tumourigenesis in the cerebellum
Authors: Marino Silvia , Krimpenfort Paul , Leung Carly , van der Korput Hetty A. G. M. , Trapman Jan , Camenisch Isabelle , Berns Anton , Brandner Sebastian
Published: April 2021

PTEN is a tumour suppressor gene involved in cell cycle control, apoptosis and mediation of adhesion and migration signalling. Germline mutations of PTEN in humans are associated with familial tumour syndromes, among them Cowden disease. Glioblastomas, highly malignant glial tumours of the central nervous system frequently show loss of PTEN. Recent reports have outlined some aspects of PTEN function in central nervous system development. Using a conditional gene disruption approach, we inactivated Pten in mice early during embryogenesis locally in a region specific fashion and later during postnatal development in a cell-specific manner, to study the role of PTEN in differentiation, migration and neoplastic transformation. We show that PTEN is required for the realisation of normal cerebellar architecture, for regulation of cell and organ size, and for proper neuronal and glial migration. However, PTEN is not required for cell differentiation and lack of PTEN is not sufficient to induce neoplastic transformation of neuronal or glial cells
Disruption of the gene encoding the cell adhesion molecule DdCAD-1 leads to aberrant cell sorting and cell-type proportioning duringDictyosteliumdevelopment
Authors: Wong Estella , Yang Chunzhong , Wang Jun , Fuller Danny , Loomis William F. , Siu Chi-Hung
Published: April 2021

The cadA gene in Dictyostelium encodes the Ca2+-dependent cell adhesion molecule DdCAD-1, which is expressed soon after the initiation of development. To investigate the biological role of DdCAD-1, the cadA gene was disrupted by homologous recombination. The cadA-null cells showed a 50% reduction in EDTA-sensitive cell adhesion. The remaining EDTA-sensitive adhesion sites were resistant to dissociation by anti-DdCAD-1 antibody, suggesting that they were distinct adhesion sites. Cells that lacked DdCAD-1 were able to complete development and form fruiting bodies. However, they displayed abnormal slug morphology and culmination was delayed by ∼6 hours. The yield of spores was reduced by ∼50%. The proportion of prestalk cells in cadA– slugs showed a 2.5-fold increase over the parental strain. When cadA– cells were transfected with pcotB::GFP to label prespore cells, aberrant cell-sorting patterns in slugs became apparent. When mutant prestalk cells were mixed with wild-type prespore cells, mutant prestalk cells were unable to return to the anterior position of chimeric slugs, suggesting defects in the sorting mechanism. The wild-type phenotype was restored when cadA– cells were transfected with a cadA-expression vector. These results indicate that, in addition to cell-cell adhesion, DdCAD-1 plays a role in cell type proportioning and pattern formation.