bluuebunny/crossref_metadata_2025 huggingface dataset:
Differential requirement for STAT by gain-of-function and wild-type receptor tyrosine kinase Torso inDrosophila
Authors: Li Willis X. , Agaisse Herve , Mathey-Prevot Bernard , Perrimon Norbert
Published: April 2021

Malignant transformation frequently involves aberrant signaling from receptor tyrosine kinases (RTKs). These receptors commonly activate Ras/Raf/MEK/MAPK signaling but when overactivated can also induce the JAK/STAT pathway, originally identified as the signaling cascade downstream of cytokine receptors. Inappropriate activation of STAT has been found in many human cancers. However, the contribution of the JAK/STAT pathway in RTK signaling remains unclear. We have investigated the requirement of the JAK/STAT pathway for signaling by wild-type and mutant forms of the RTK Torso (Tor) using a genetic approach in Drosophila. Our results indicate that the JAK/STAT pathway plays little or no role in signaling by wild-type Tor. In contrast, we find that STAT, encoded by marelle (mrl; DStat92E), is essential for the gain-of-function mutant Tor (TorGOF) to activate ectopic gene expression. Our findings indicate that the Ras/Raf/MEK/MAPK signaling pathway is sufficient to mediate the normal functions of wild-type RTK, whereas the effects of gain-of-function mutant RTK additionally require STAT activation.
SUPERMAN attenuates positiveINNER NO OUTERautoregulation to maintain polar development ofArabidopsisovule outer integuments
Authors: Meister Robert J. , Kotow Louren M. , Gasser Charles S.
Published: April 2021

The outer integument of Arabidopsis ovules exhibits marked polarity in its development, growing extensively from the abaxial side, but only to a very limited extent from the adaxial side of the ovule. Mutations in two genes affect this asymmetric growth. In strong inner no outer (ino) mutants outer integument growth is eliminated, whereas in superman (sup) mutants integument growth on the adaxial side is nearly equal to wild-type growth on the abaxial side. Through complementation and reporter gene analysis, a region of INO 5′-flanking sequences was identified that contains sufficient information for appropriate expression of INO. Using this INO promoter (P-INO) we show that INO acts as a positive regulator of transcription from P-INO, but is not sufficient for de novo initiation of transcription in other plant parts. Protein fusions demonstrate nuclear localization of INO, consistent with a proposed role as a transcription factor for this member of the YABBY protein family. Through its ability to inhibit expression of the endogenous INO gene and transgenes driven by P-INO, SUP is shown to be a negative regulator of INO transcription. Substitution of another YABBY protein coding region (CRABS CLAW) for INO overcomes this negative regulation, indicating that SUP suppresses INO transcription through attenuation of the INO positive autoregulatory loop.
COP9 signalosome subunits 4 and 5 regulate multiple pleiotropic pathways inDrosophila melanogaster
Authors: Oron Efrat , Mannervik Mattias , Rencus Sigal , Harari-Steinberg Orit , Neuman-Silberberg Shira , Segal Daniel , Chamovitz Daniel A.
Published: April 2021

The COP9 signalosome (CSN) is an essential eight-subunit repressor of light-regulated development in Arabidopsis. This complex has also been identified in animals, though its developmental role remains obscure. CSN subunits have been implicated in various cellular processes, suggesting a possible role for the CSN as an integrator of multiple signaling pathways. In order to elucidate the function of the CSN in animals, a Drosophila model system has previously been established. Gel-filtration analysis with antibodies against CSN subunits 4, 5 and 7 revealed that these proteins act as a complex in Drosophila that is similar in size to the plant and mammalian complexes. Null mutations in either one of two subunits, CSN4 or CSN5, are larval lethal. Successful embryogenesis appears to be a consequence of maternal contribution of the complex. Biochemical analysis indicates that the different subunits are found in both CSN-dependent and CSN-independent forms, and that these forms are differentially affected by the mutations. Phenotypic characterization of these two mutants indicates that they show both shared and unique phenotypes, which suggest specific roles for each subunit. Both mutants have defective oocyte and embryo patterning, and defects in response to DNA damage, while csn5 mutants develop melanotic tumors and csn4 mutants have phenotypes reminiscent of defects in ecdysone signaling.
Specification of an anterior neuroectoderm patterning by Frizzled8a-mediated Wnt8b signalling during late gastrulation in zebrafish
Authors: Kim Seok-Hyung , Shin Jimann , Park Hae-Chul , Yeo Sang-Yeob , Hong Sung-Kook , Han Sangtae , Rhee Myungchull , Kim Cheol-Hee , Chitnis Ajay B. , Huh Tae-Lin
Published: April 2021

Wnts have been shown to provide a posteriorizing signal that has to be repressed in the anterior neuroectoderm for normal anteroposterior (AP) patterning. We have previously identified a zebrafish frizzled8a (fz8a) gene expressed in the presumptive anterior neuroectoderm as well as prechordal plate at the late gastrula stage. We have investigated the role of Fz8a-mediated Wnt8b signalling in anterior brain patterning in zebrafish. We show that in zebrafish embryos: (1) Wnt signalling has at least two different stage-specific posteriorizing activities in the anterior neuroectoderm, one before mid-gastrulation and the other at late gastrulation; (2) Fz8a plays an important role in mediating anterior brain patterning; (3) Wnt8b and Fz8a can functionally interact to transmit posteriorizing signals that determine the fate of the posterior diencephalon and midbrain in late gastrula embryos; and (4) Wnt8b can suppress fz8a expression in the anterior neuroectoderm and potentially affect the level and/or range of Wnt signalling. In conclusion, we suggest that a gradient of Fz8a-mediated Wnt8b signalling may play crucial role in patterning the posterior diencephalon and midbrain regions in the late gastrula.
MAPK-upstream protein kinase (MUK) regulates the radial migration of immature neurons in telencephalon of mouse embryo
Authors: Hirai Syu-ichi , Kawaguchi Atsumi , Hirasawa Ryutaro , Baba Masaya , Ohnishi Tetsuo , Ohno Shigeo
Published: April 2021

The radial migration of differentiating neurons provides an essential step in the generation of laminated neocortex, although its molecular mechanism is not fully understood. We show that the protein levels of a JNK activator kinase, MUK/DLK/ZPK, and JNK activity increase potently and temporally in newly generated neurons in developing mouse telencephalon during radial migration. The ectopic expression of MUK/DLK/ZPK in neural precursor cells in utero impairs radial migration, whereas it allows these cells to leave the ventricular zone and differentiate into neural cells. The MUK/DLK/ZPK protein is associated with dotted structures that are frequently located along microtubules and with Golgi apparatus in cultured embryonic cortical cells. In COS-1 cells, MUK/DLK/ZPK overexpression impairs the radial organization of microtubules without massive depolymerization. These results suggest that MUK/DLK/ZPK and JNK regulate radial cell migration via microtubule-based events.
Heart tube patterning inDrosophilarequires integration of axial and segmental information provided by theBithorax Complexgenes andhedgehogsignaling
Authors: Ponzielli Romina , Astier Martine , Chartier Aymeric , Gallet Armel , Thérond Pascal , Sémériva Michel
Published: April 2021

The Drosophila larval cardiac tube is composed of 104 cardiomyocytes that exhibit genetic and functional diversity. The tube is divided into the aorta and the heart proper that encompass the anterior and posterior parts of the tube, respectively. Differentiation into aorta and heart cardiomyocytes takes place during embryogenesis. We have observed living embryos to correlate morphological changes occurring during the late phases of cardiogenesis with the acquisition of organ function, including functional inlets, or ostiae.

Cardiac cells diversity originates in response to two types of spatial information such that cells differentiate according to their position, both within a segment and along the anteroposterior axis. Axial patterning is controlled by homeotic genes of the Bithorax Complex (BXC) which are regionally expressed within the cardiac tube in non-overlapping domains. Ultrabithorax (Ubx) is expressed in the aorta whereas abdominal A (abd-A) is expressed in the heart, with the exception of the four most posterior cardiac cells which express Abdominal B (Abd-B). Ubx and abd-A functions are required to confer an aorta or a heart identity on cardiomyocytes, respectively. The anterior limit of the expression domain of Ubx, abd-A and Abd-B is independent of the function of the other genes. In contrast, abd-A represses Ubx expression in the heart and ectopic overexpression of abd-A transforms aorta cells into heart cardiomyocytes. Taken together, these results support the idea that BXC homeotic genes in the cardiac tube conform to the posterior prevalence rule.

The cardiac tube is also segmentally patterned and each metamere contains six pairs of cardioblasts that are genetically diverse. We show that the transcription of seven up (svp), which is expressed in the two most posterior pairs of cardioblasts in each segment, is dependent on hedgehog (hh) signaling from the dorsal ectoderm. In combination with the axial information furnished by abd-A, the segmental hh-dependent information leads to the differentiation of the six pairs of svp-expressing cells into functional ostiae.

Movies available on-line
AnFgf8mouse mutant phenocopies human 22q11 deletion syndrome
Authors: Frank Deborah U. , Fotheringham Lori K. , Brewer Judson A. , Muglia Louis J. , Tristani-Firouzi Martin , Capecchi Mario R. , Moon Anne M.
Published: April 2021

Deletion of chromosome 22q11, the most common microdeletion detected in humans, is associated with a life-threatening array of birth defects. Although 90% of affected individuals share the same three megabase deletion, their phenotype is highly variable and includes craniofacial and cardiovascular anomalies, hypoplasia or aplasia of the thymus with associated deficiency of T cells, hypocalcemia with hypoplasia or aplasia of the parathyroids, and a variety of central nervous system abnormalities. Because ablation of neural crest in chicks produces many features of the deletion 22q11 syndrome, it has been proposed that haploinsufficiency in this region impacts neural crest function during cardiac and pharyngeal arch development. Few factors required for migration, survival, proliferation and subsequent differentiation of pharyngeal arch neural crest and mesoderm-derived mesenchyme into their respective cardiovascular, musculoskeletal, and glandular derivatives have been identified. However, the importance of epithelial-mesenchymal interactions and pharyngeal endoderm function is becoming increasingly clear.

Fibroblast growth factor 8 is a signaling molecule expressed in the ectoderm and endoderm of the developing pharyngeal arches and known to play an important role in survival and patterning of first arch tissues. We demonstrate a dosage-sensitive requirement for FGF8 during development of pharyngeal arch, pharyngeal pouch and neural crest-derived tissues. We show that FGF8 deficient embryos have lethal malformations of the cardiac outflow tract, great vessels and heart due, at least in part, to failure to form the fourth pharyngeal arch arteries, altered expression of Fgf10 in the pharyngeal mesenchyme, and abnormal apoptosis in pharyngeal and cardiac neural crest.

The Fgf8 mutants described herein display the complete array of cardiovascular, glandular and craniofacial phenotypes seen in human deletion 22q11 syndromes. This represents the first single gene disruption outside the typically deleted region of human chromosome 22 to fully recapitulate the deletion 22q11 phenotype. FGF8 may operate directly in molecular pathways affected by deletions in 22q11 or function in parallel pathways required for normal development of pharyngeal arch and neural crest-derived tissues. In either case, Fgf8 may function as a modifier of the 22q11 deletion and contribute to the phenotypic variability of this syndrome.
Fgf8is required for pharyngeal arch and cardiovascular development in the mouse
Authors: Abu-Issa Radwan , Smyth Graham , Smoak Ida , Yamamura Ken-ichi , Meyers Erik N.
Published: April 2021

We present here an analysis of cardiovascular and pharyngeal arch development in mouse embryos hypomorphic for Fgf8. Previously, we have described the generation of Fgf8 compound heterozygous (Fgf8neo/–) embryos. Although early analysis demonstrated that some of these embryos have abnormal left-right (LR) axis specification and cardiac looping reversals, the number and type of cardiac defects present at term suggested an additional role for Fgf8 in cardiovascular development. Most Fgf8neo/– mutant embryos survive to term with abnormal cardiovascular patterning, including outflow tract, arch artery and intracardiac defects. In addition, these mutants have hypoplastic pharyngeal arches, small or absent thymus and abnormal craniofacial development. Neural crest cells (NCCs) populate the pharyngeal arches and contribute to many structures of the face, neck and cardiovascular system, suggesting that Fgf8 may be required for NCC development. Fgf8 is expressed within the developing pharyngeal arch ectoderm and endoderm during NCC migration through the arches. Analysis of NCC development in Fgf8neo/– mutant embryos demonstrates that NCCs are specified and migrate, but undergo cell death in areas both adjacent and distal to where Fgf8 is normally expressed. This study defines the cardiovascular defects present in Fgf8 mutants and supports a role for Fgf8 in development of all the pharyngeal arches and in NCC survival.
The activation and maintenance ofPax2expression at the mid-hindbrain boundary is controlled by separate enhancers
Authors: Pfeffer Peter L. , Payer Bernhard , Reim Gerlinde , di Magliano Marina Pasca , Busslinger Meinrad
Published: April 2021

Pax2 is the earliest known gene to be expressed throughout the mid-hindbrain region in late gastrula embryos of the mouse and is essential for the formation of an organizing center at the midbrain-hindbrain boundary (MHB), which controls midbrain and cerebellum development. We have used transgenic analysis to identify three MHB-specific enhancers in the upstream region of the mouse Pax2 gene. A 120 bp enhancer (at –3.7 kb) in cooperation with the endogenous promoter was sufficient to induce transgene expression in the anterior neural plate of late gastrula embryos, while it was already inactivated again at the MHB during somitogenesis. The activity of this early enhancer was severely reduced by mutation of three homeodomain-binding sites, two of which are part of a recognition sequence for POU homeodomain proteins. Oct3/4 (Pou5f1), the mouse ortholog of zebrafish Pou2, efficiently bound to this sequence, suggesting its involvement in the regulation of the early Pax2 enhancer. Starting at the four-somite stage, Pax2 is expressed at the MHB under the control of two enhancers located at –4.1 kb and –2.8 kb. The distal late enhancer contains a 102 bp sequence that is not only highly conserved between the mouse and pufferfish Pax2 genes, but also contributes to the enhancer activity of both genes in transgenic mice. The proximal 410 bp enhancer, which overlaps with a kidney-specific regulatory element, contains a functional Pax2/5/8-binding site and thus maintains Pax2 expression at the MHB under auto- and cross-regulatory control by Pax2/5/8 proteins. Importantly, the early and proximal late enhancers are not only sufficient but also necessary for expression at the MHB in the genomic context of the Pax2 locus, as their specific deletion interfered with correct temporal expression of a large Pax2 BAC transgene. Hence, separate enhancers under the control of distinct transcription factors activate and maintain Pax2 expression at the MHB.
Pitx2is required at multiple stages of pituitary organogenesis: pituitary primordium formation and cell specification
Authors: Suh Hoonkyo , Gage Philip J. , Drouin Jacques , Camper Sally A.
Published: April 2021

Analysis of an allelic series in mice revealed that the Pitx2 homeobox gene is required at multiple stages of pituitary development. It is necessary for initiating expansion of Rathke’s pouch and maintaining expression of the fetal-specific transcription factors Hesx1 and Prop1. At later stages Pitx2 is necessary for specification and expansion of the gonadotropes and Pit1 lineage within the ventral and caudomedial anterior pituitary. Mechanistically, this is due to the dependence of several critical lineage-specific transcription factors, Pit1, Gata2, Egr1 and Sf1, on a threshold level of PITX2. The related Pitx1 gene has a role in hormone gene transcription, and it is important late in ontogeny for the final expansion of the differentiated cell types. Pitx1 and Pitx2 have overlapping functions in the expansion of Rathke’s pouch, revealing the sensitivity of pituitary organogenesis to the dosage of the PITX family. The model developed for PITX gene function in pituitary development provides a better understanding of the etiology of Rieger syndrome and may extend to other PITX-sensitive developmental processes.
Pax6is required to regulate the cell cycle and the rate of progression from symmetrical to asymmetrical division in mammalian cortical progenitors
Authors: Estivill-Torrus Guillermo , Pearson Helen , van Heyningen Veronica , Price David J. , Rashbass Penny
Published: April 2021

In the proliferative zone of the developing cerebral cortex, multipotential progenitors predominate early in development and divide to increase the progenitor pool. As corticogenesis progresses, proportionately fewer progenitors are produced and, instead, cell divisions yield higher numbers of postmitotic neurones or glial cells. As the switch from the generation of progenitors to that of differentiated cells occurs, the orientation of cell division alters from predominantly symmetrical to predominantly asymmetrical. It has been hypothesised that symmetrical divisions expand the progenitor pool, whereas asymmetrical divisions generate postmitotic cells, although this remains to be proved. The molecular mechanisms regulating these processes are poorly understood.

The transcription factor Pax6 is highly expressed in the cortical proliferative zone and there are morphological defects in the Pax6Sey/Sey (Pax6 null) cortex, but little is known about the principal cellular functions of Pax6 in this region. We have analysed the cell-cycle kinetics, the progenitor cleavage orientation and the onset of expression of differentiation markers in Pax6Sey/Sey cortical cells in vivo and in vitro. We showed that, early in corticogenesis at embryonic day (E) 12.5, the absence of Pax6 accelerated cortical development in vivo, shortening the cell cycle and the time taken for the onset of expression of neural-specific markers. This also occurred in dissociated culture of isolated cortical cells, indicating that the changes were intrinsic to the cortical cells. From E12.5 to E15.5, proportions of asymmetrical divisions increased more rapidly in mutant than in wild-type embryos. By E15.5, interkinetic nuclear migration during the cell cycle was disrupted and the length of the cell cycle was significantly longer than normal in the Pax6Sey/Sey cortex, with a lengthening of S phase.

Together, these results show that Pax6 is required in developing cortical progenitors to control the cell-cycle duration, the rate of progression from symmetrical to asymmetrical division and the onset of expression of neural-specific markers.
Erythropoietin receptor signalling is required for normal brain development
Authors: Yu Xiaobing , Shacka John J. , Eells Jeffrey B. , Suarez-Quian Carlos , Przygodzki Ronald M. , Beleslin-Cokic Bojana , Lin Chyuan-Sheng , Nikodem Vera M. , Hempstead Barbara , Flanders Kathleen C. , Costantini Frank , Noguchi Constance Tom
Published: April 2021

Erythropoietin, known for its role in erythroid differentiation, has been shown to be neuroprotective during brain ischaemia in adult animal models. Although high levels of erythropoietin receptor are produced in embryonic brain, the role of erythropoietin during brain development is uncertain. We now provide evidence that erythropoietin acts to stimulate neural progenitor cells and to prevent apoptosis in the embryonic brain. Mice lacking the erythropoietin receptor exhibit severe anaemia and defective cardiac development, and die at embryonic day 13.5 (E13.5). By E12.5, in addition to apoptosis in foetal liver, endocardium and myocardium, the erythropoietin receptor null mouse shows extensive apoptosis in foetal brain. Lack of erythropoietin receptor affects brain development as early as E10.5, resulting in a reduction in the number of neural progenitor cells and increased apoptosis. Corresponding in vitro cultures of cortical cells from Epor–/– mice also exhibited decreases in neuron generation compared with normal controls and increased sensitivity to low oxygen tension with no surviving neurons in Epor–/– cortical cultures after 24 hour exposure to hypoxia. The viability of primary Epor+/+ rodent embryonic cortical neurons was further increased by erythropoietin stimulation. Exposure of these cultures to hypoxia induced erythropoietin expression and a tenfold increase in erythropoietin receptor expression, increased cell survival and decreased apoptosis. Cultures of neuronal progenitor cells also exhibited a proliferative response to erythropoietin stimulation. These data demonstrate that the neuroprotective activity of erythropoietin is observed as early as E10.5 in the developing brain, and that induction of erythropoietin and its receptor by hypoxia may contribute to selective cell survival in the brain.
Tail gut endoderm and gut/genitourinary/tail development: a new tissue-specific role forHoxa13
Authors: de Santa Barbara Pascal , Roberts Drucilla J.
Published: April 2021

Hoxa13 is expressed early in the caudal mesoderm and endoderm of the developing hindgut. The tissue-specific roles of Hoxa13 function have not been described. Hand-foot-genital syndrome, a rare dominantly inherited human malformation syndrome characterized by distal extremity and genitourinary anomalies, is caused by mutations in the HOXA13 gene. We show evidence that one specific HOXA13 mutation likely acts as a dominant negative in vivo. When chick HFGa13 is overexpressed in the chick caudal endoderm early in development, caudal structural malformations occur. The phenotype is specific to HFGa13 expression in the posterior endoderm, and includes taillessness and severe gut/genitourinary (GGU) malformations. Finally, we show that chick HFGa13 negatively regulates expression of Hoxd13 and antagonizes functions of both endogenous Hoxa13 and Hoxd13 proteins. We suggest a fundamental role for epithelial specific expression of Hoxa13 in the epithelial-mesenchymal interaction necessary for tail growth and posterior GGU patterning.
scuteexpression inCalliphora vicinareveals an ancestral pattern of longitudinal stripes on the thorax of higher Diptera
Authors: Pistillo Daniela , Skaer Nick , Simpson Pat
Published: April 2021

In Drosophila the stereotyped arrangement of sensory bristles on the notum is determined by the tightly regulated control of transcription of the achaete-scute (ac-sc) genes which are expressed in small proneural clusters of cells at the sites of each future bristle. Expression relies on a series of discrete cis-regulatory elements present in the ac-sc gene complex that are the target of the transcriptional activators pannier (pnr) and the genes of the iroquois complex. Stereotyped bristle patterns are common among species of acalyptrate Schizophora such as Drosophila, and are thought to have derived from an ancestral pattern of four longitudinal rows extending the length of the scutum, through secondary loss of bristles. To investigate evolutionary changes in bristle patterns and ac-sc regulation by pnr, we have isolated homologues of these genes from Calliphora vicina, a species of calyptrate Schizophora separated from Drosophila by at least 100 million years. Calliphora vicina displays a pattern of four rows of bristles on the scutum resembling the postulated ancestral one. We find that sc in Calliphora is expressed in two longitudinal stripes on the medial scutum that prefigure the development of the rows of acrostichal and dorsocentral bristles. This result suggests that a stripe-like expression pattern of sc may be an ancestral feature and may have preceded the evolution of proneural clusters. The implications for the evolution of the cis-regulatory elements responsible for sc expression in the proneural clusters of Drosophila, and function of Pnr are discussed.
Cell migration in the postembryonic development of the fish lateral line
Authors: Sapède Dora , Gompel Nicolas , Dambly-Chaudière Christine , Ghysen Alain
Published: April 2021

We examine at the cellular level the postembryonic development of the posterior lateral line in the zebrafish. We show that the first wave of secondary neuromasts is laid down by a migrating primordium, primII. This primordium originates from a cephalic region much like the primordium that formed the primary line during embryogenesis. PrimII contributes to both the lateral and the dorsal branches of the posterior lateral line. Once they are deposited by the primordium, the differentiating neuromasts induce the specialisation of overlying epidermal cells into a pore-forming annulus, and the entire structure begins to migrate ventrally across the epithelium. Thus the final two-dimensional pattern depends on the combination of two orthogonal processes: anteroposterior waves of neuromast formation and dorsoventral migration of individual neuromasts. Finally, we examine how general these migratory processes can be by describing two fish species with very different adult patterns, Astyanax fasciatus (Mexican blind cavefish) and Oryzias latipes (medaka). We show that their primary patterns are nearly identical to that observed in zebrafish embryos, and that their postembryonic growth relies on the same combination of migratory processes that we documented in the case of the zebrafish.
Onset of the segmentation clock in the chick embryo: evidence for oscillations in the somite precursors in the primitive streak
Authors: Jouve Caroline , Iimura Tadahiro , Pourquie Olivier
Published: April 2021

Vertebrate somitogenesis is associated with a molecular oscillator, the segmentation clock, which is defined by the periodic expression of genes related to the Notch pathway such as hairy1 and hairy2 or lunatic fringe (referred to as the cyclic genes) in the presomitic mesoderm (PSM). Whereas earlier studies describing the periodic expression of these genes have essentially focussed on later stages of somitogenesis, we have analysed the onset of the dynamic expression of these genes during chick gastrulation until formation of the first somite. We observed that the onset of the dynamic expression of the cyclic genes in chick correlated with ingression of the paraxial mesoderm territory from the epiblast into the primitive streak. Production of the paraxial mesoderm from the primitive streak is a continuous process starting with head mesoderm formation, while the streak is still extending rostrally, followed by somitic mesoderm production when the streak begins its regression. We show that head mesoderm formation is associated with only two pulses of cyclic gene expression. Because such pulses are associated with segment production at the body level, it suggests the existence of, at most, two segments in the head mesoderm. This is in marked contrast to classical models of head segmentation that propose the existence of more than five segments. Furthermore, oscillations of the cyclic genes are seen in the rostral primitive streak, which contains stem cells from which the entire paraxial mesoderm originates. This indicates that the number of oscillations experienced by somitic cells is correlated with their position along the AP axis.
MAX1andMAX2control shoot lateral branching inArabidopsis
Authors: Stirnberg Petra , van de Sande Karin , Leyser H. M. Ottoline
Published: April 2021

Plant shoots elaborate their adult form by selective control over the growth of both their primary shoot apical meristem and their axillary shoot meristems. We describe recessive mutations at two loci in Arabidopsis, MAX1 and MAX2, that affect the selective repression of axillary shoots. All the first order (but not higher order) axillary shoots initiated by mutant plants remain active, resulting in bushier shoots than those of wild type. In vegetative plants where axillary shoots develop in a basal to apical sequence, the mutations do not clearly alter node distance, from the shoot apex, at which axillary shoot meristems initiate but shorten the distance at which the first axillary leaf primordium is produced by the axillary shoot meristem. A small number of mutant axillary shoot meristems is enlarged and, later in development, a low proportion of mutant lateral shoots is fasciated. Together, this suggests that MAX1 and MAX2 do not control the timing of axillary meristem initiation but repress primordia formation by the axillary meristem. In addition to shoot branching, mutations at both loci affect leaf shape. The mutations at MAX2 cause increased hypocotyl and petiole elongation in light-grown seedlings. Positional cloning identifies MAX2 as a member of the F-box leucine-rich repeat family of proteins. MAX2 is identical to ORE9, a proposed regulator of leaf senescence (Woo, H. R., Chung, K. M., Park, J.-H., Oh, S. A., Ahn, T., Hong, S. H., Jang, S. K. and Nam, H. G. (2001) Plant Cell13, 1779-1790). Our results suggest that selective repression of axillary shoots involves ubiquitin-mediated degradation of as yet unidentified proteins that activate axillary growth.
Sexually dimorphic development of mouse primordial germ cells: switching from oogenesis to spermatogenesis
Authors: Adams Ian R. , McLaren Anne
Published: April 2021

During embryogenesis, primordial germ cells (PGCs) have the potential to enter either spermatogenesis or oogenesis. In a female genital ridge, or in a non-gonadal environment, PGCs develop as meiotic oocytes. However, male gonadal somatic cells inhibit PGCs from entering meiosis and direct them to a spermatogenic fate. We have examined the ability of PGCs from male and female embryos to respond to the masculinising environment of the male genital ridge, defining a temporal window during which PGCs retain a bipotential fate. To help understand how PGCs respond to the male gonadal environment, we have identified molecular differences between male PGCs that are committed to spermatogenesis and bipotential female PGCs. Our results suggest that one way in which PGCs respond to this masculinising environment is to synthesise prostaglandin D2. We show that this signalling molecule can partially masculinise female embryonic gonads in culture, probably by inducing female supporting cells to differentiate into Sertoli cells. In the developing testis, prostaglandin D2 may act as a paracrine factor to induce Sertoli cell differentiation. Thus part of the response of PGCs to the male gonadal environment is to generate a masculinising feedback loop to ensure male differentiation of the surrounding gonadal somatic cells.
her1and thenotchpathway function within the oscillator mechanism that regulates zebrafish somitogenesis
Authors: Holley Scott A. , Jülich Dörthe , Rauch Gerd-Jörg , Geisler Robert , Nüsslein-Volhard Christiane
Published: April 2021

Somite formation is thought to be regulated by an unknown oscillator mechanism that causes the cells of the presomitic mesoderm to activate and then repress the transcription of specific genes in a cyclical fashion. These oscillations create stripes/waves of gene expression that repeatedly pass through the presomitic mesoderm in a posterior-to-anterior direction. In both the mouse and the zebrafish, it has been shown that the notch pathway is required to create the stripes/waves of gene expression. However, it is not clear if the notch pathway comprises part of the oscillator mechanism or if the notch pathway simply coordinates the activity of the oscillator among neighboring cells. In the zebrafish, oscillations in the expression of a hairy-related transcription factor, her1 and the notch ligand deltaC precede somite formation. Our study focuses on how the oscillations in the expression of these two genes is affected in the mutants aei/deltaD and des/notch1, in ‘morpholino knockdowns’ of deltaC and her1 and in double ‘mutant’ combinations. This analysis indicates that these oscillations in gene expression are created by a genetic circuit comprised of the notch pathway and the notch target gene her1. We also show that a later function of the notch pathway can create a segmental pattern even in the absence of prior oscillations in her1 and deltaC expression.

Supplementary data available at http://www.eb.tuebingen.mpg.de/papers/holley_dev_2002.html
Short Stop provides an essential link between F-actin and microtubules during axon extension
Authors: Lee Seungbok , Kolodziej Peter A.
Published: April 2021

Coordination of F-actin and microtubule dynamics is important for cellular motility and morphogenesis, but little is known about underlying mechanisms. short stop (shot) encodes an evolutionarily conserved, neuronally expressed family of rod-like proteins required for sensory and motor axon extension in Drosophila melanogaster. We identify Shot isoforms that contain N-terminal F-actin and C-terminal microtubule-binding domains, and that crosslink F-actin and microtubules in cultured cells. The F-actin- and microtubule-binding domains of Shot are required in the same molecule for axon extension, though the length of the connecting rod domain can be dramatically reduced without affecting activity. Shot therefore functions as a cytoskeletal crosslinker in axon extension, rather than mediating independent interactions with F-actin and microtubules. A Ca2+-binding motif located adjacent to the microtubule-binding domain is also required for axon extension, suggesting that intracellular Ca2+ release may regulate Shot activity. These results suggest that Shot coordinates regulated interactions between F-actin and microtubules that are crucial for neuronal morphogenesis.
Exploring the myriapod body plan: expression patterns of the ten Hox genes in a centipede
Authors: Hughes Cynthia L. , Kaufman Thomas C.
Published: April 2021

The diversity of the arthropod body plan has long been a fascinating subject of study. A flurry of recent research has analyzed Hox gene expression in various arthropod groups, with hopes of gaining insight into the mechanisms that underlie their evolution. The Hox genes have been analyzed in insects, crustaceans and chelicerates. However, the expression patterns of the Hox genes have not yet been comprehensively analyzed in a myriapod. We present the expression patterns of the ten Hox genes in a centipede, Lithobius atkinsoni, and compare our results to those from studies in other arthropods. We have three major findings. First, we find that Hox gene expression is remarkably dynamic across the arthropods. The expression patterns of the Hox genes in the centipede are in many cases intermediate between those of the chelicerates and those of the insects and crustaceans, consistent with the proposed intermediate phylogenetic position of the Myriapoda. Second, we found two ‘extra’ Hox genes in the centipede compared with those in Drosophila. Based on its pattern of expression, Hox3 appears to have a typical Hox-like role in the centipede, suggesting that the novel functions of the Hox3 homologs zen and bicoid were adopted somewhere in the crustacean-insect clade. In the centipede, the expression of the gene fushi tarazu suggests that it has both a Hox-like role (as in the mite), as well as a role in segmentation (as in insects). This suggests that this dramatic change in function was achieved via a multifunctional intermediate, a condition maintained in the centipede. Last, we found that Hox expression correlates with tagmatic boundaries, consistent with the theory that changes in Hox genes had a major role in evolution of the arthropod body plan.
Sensory neurons of the Atonal lineage pioneer the formation of glomeruli within the adultDrosophilaolfactory lobe
Authors: Jhaveri Dhanisha , Rodrigues Veronica
Published: April 2021

The first centers for processing of odor information by animals lie in the olfactory lobe. Sensory neurons from the periphery synapse with interneurons in anatomically recognizable units, termed glomeruli, seen in both insects and vertebrates. The mechanisms that underlie the formation of functional maps of the odor-world in the glomeruli within the olfactory lobe remains unclear. We address the basis of sensory targeting in the fruitfly Drosophila and show that one class of sensory neurons, those of the Atonal lineage, plays a crucial role in glomerular patterning. Atonal-dependent neurons pioneer the segregation of other classes of sensory neurons into distinct glomeruli. Furthermore, correct sensory innervation is necessary for the arborization of projection neurons into glomeruli and for the elaboration of processes of central glial cells into the lobe.
Inhibition of aggregation of the slime mould dictyostelium discoideum by a factor diffusing from escherichia coli
Authors: Shaffer B. M.
Published: April 2021

A factor diffuses from Escherichia colt that temporarily or permanently prevents separate pre-aggregation cells of Dictyostelium discoideum from forming aggregation centres or streams. Centres are inhibited at a greater distance from the bacteria than are streams. The inhibitor does not support the slime-mould’s growth, and it has different properties from previously discovered aggregation-centre inhibitors released by the amoebae themselves. The factor, as produced, does not inhibit Polysphondylium violaceum cells from forming centres or streams. Yet although E. coli itself attracts pre-aggregation P. violaceum cells, it does not mimic a P. violaceum aggregate in inducing them to form streams.
Moiré patterns in electron micrographs of a bacterial membrane
Authors: Glauert Audrey M.
Published: April 2021

A regular hexagonal array of morphological units is observed in negatively stained preparations of a membrane from the surface of the spherical bacterium Micrococcus radiodurans. Fragments of this membrane are frequently folded so that structural details from two or more layers are superimposed. The appearance of the resulting moiré patterns is illustrated and the need to be aware of the possibility of the formation of these patterns in similar biological specimens is stressed.
Observations on scale production in pyramimonas amylifera conrad
Authors: Manton Irene
Published: April 2021

Scale production has been studied after glutaraldehyde fixation with the primary object of tracing the source of flagellar scales which this species is known to possess (Manton, Oates & Parke, 1963). Large numbers of all types of flagellar scales, unmixed with body-type scales, are detectable at all times, stored in an orderly arrangement within a vesicular scale reservoir of characteristic morphology which is described. These scales are liberated through a narrow duct opening to the cell exterior near the flagellar bases. This duct is thought to be temporary and subject to reformation with minor change of site, since open ducts are more frequently encountered in darkness than by day ; some putative developmental stages are illustrated. Observations on other details of scale production include demonstration of the formation of all types of body scales within the Golgi cistemae, sometimes together with a few flagella-type scales. The larger body-type scales are uniformly oriented within the Golgi cistemae in relation to the position of the subtending endoplasmic reticulum. This orientation is lost before liberation to the cell surface, which occurs from moderate-sized detached vesicles in an unknown manner. The implications of these findings for an understanding of Golgi structure and of the basic problem of scale arrangement on the receiving sites are discussed in a preliminary way.
Distribution of pyrenoids among some brown algae
Authors: Evans L. V.
Published: April 2021

The distribution of pyrenoids among some orders of the brown algae has been investigated with the electron microscope and a report on their occurrence is given, with particular reference to the results obtained with the light microscope by Simon (1954). Some illustrated detail of the structure of the pyrenoids and of the chromatophores is included.

Pyrenoids were found to be present in the representatives examined of the Ectocarpales, Sphacelariales, Scytosiphonales and Dictyosiphonales. Excepting the Sphacelariales, this is in agreement with the results of Simon. Pyrenoids were found to be definitely absent in all members of the Dictyotales and Laminariales examined, and disagreement is expressed with Bouck (1965) who reported their presence in Chorda filum, a member of the latter order. The situation in the Fucales is the subject of another communication elsewhere.

The presence or absence of pyrenoids is regarded as an additional taxonomic character of possible phyletic use when more is known. The results are discussed from the standpoint of their possible value in assessing the relative position of the Phaeophyceae as a whole. Brown algal lamellations are composed of three, or occasionally four, parallel thylakoids which do not cohere and are not aggregated into stacks. This is thought to be more primitive than the condition in some of the other groups of the Chromophyta where there is adherence of thylakoids into stacks of two or three members as, for example, in the Haptophyceae, Xanthophyceae and Chrysophyceae. The Phaeophyceae are, however, regarded as much less primitive than the Rhodophyceae, where the widely spaced parallel thylakoids are arranged singly.
Fine structure of swarmers of Cladophora and Chaetomorpha: II. the chloroplast
Authors: Robinson D. G.
Published: April 2021

Freeze-etched chloroplast thylakoids in swarmers of Chaetomorpha reveal the in situ arrangment of these thylakoids and also a completely new fracture face. This face, which is completely smooth, does not bear any resemblance to any freeze-etched fracture face previously recorded. The relative merits of thylakoid models are discussed in relation to this new fracture face.
Germinal vesicle breakdown in the mouse oocyte
Authors: Calarco Patricia G. , Donahue R. P. , Szollosi D.
Published: April 2021

Germinal vesicle breakdown in mouse oocytes in vivo and in vitro has been examined by electron microscopy. In vitro oocytes were studied immediately after release from follicles and at various times (0·5–11 h) in culture. Approximately 30 min after oocyte release, chromatin condensation begins along the convoluted nuclear envelope (NE). Chromatin granules are common in all condensing chromosomes. Heterochromatin, visible from early condensation until chromosomes are of uniform density, often is observed near the kinetochores. The nucleolus breaks down after peripheral incorporation of separate nucleolus-associated bodies composed of 25-nm diameter fibrils. These bodies are later found free in the cytoplasm. As chromosome condensation progresses, the NE becomes highly convoluted, then discontinuous, finally forming NE doublets. Spindle formation begins with the appearance near the NE of small medium-dense areas from which microtubules emanate. No centrioles are present. Dark granules and mitochondria move centrally in the oocyte and surround the spindle. Peripheral cortical granules and large aggregations of multivesicular bodies are present at all stages. The Golgi apparatus is not well developed. Very little rough endoplasmic reticulum is present, although free ribosomal clusters are common. There are no significant ultrastructural differences between eggs maturing in vivo and in vitro.
Physiology and ultrastructure of phasic and tonic skeletal muscle fibres in the locust, Schistocerca gregaria
Authors: Cochrane D. G. , Elder H. Y. , Usherwood P. N. R.
Published: April 2021

Insect muscle fibres can be classed as either phasic or tonic according to their response to potassium depolarization. The phasic fibres contract only transiently during prolonged potassium depolarization, whereas the tonic fibres give a sustained contracture.

The extensor tibiae muscle in the metathoracic leg of the locust contains both tonic (T/et) and phasic (P/et) fibres; the electrical, mechanical and ultrastructural properties of these fibres have been compared with those of phasic fibres from the retractor unguis muscle (P/ru) in the same leg. A broad correlation has been established between the mechanical response and the amount of sarcoplasmic reticulum (SR) in the fibres.

At maximal body length the rise time to peak twitch tension for the T/et fibres was found to be 790 ± 60 ms, for the P/et fibres 59 ± 2· 5 ms and for the P/ru fibres, 30 ± 1·1 ms. The half-decay times for the isometric twitch contractions were 2950 ± 88 ms for the T/et fibres, 119 ± 4·2 ms for the P/et, and 35 ± 2 ·3 ms for the P/ru. The P/et and P/ru gave brief isometric contractures during potassium depolarization; under the same treatment the T/et fibres remained contracted throughout the treatment period.

The major structural differences between the 3 types lies in the SR. Expressed as percentages of total fibre volume, the SR represents in the T/et 1·1 %, in the P/et 6·8 %, and in the P/ru 19 %. The surface area of the SR, in terms of μm2/μm3 of fibre volume is 1·0 ± 0·1 in the T/et, 2·9 ±0·2 in the P/et and 11·9 ± 1·0 in the P/ru. Microtubules, often associated with elements of the SR, are sparsely distributed amongst the contractile elements in the T/et fibres. All 3 muscle types have a well developed T-system which forms dyadic associations with the SR. Larger-diameter Z-invaginations which conduct tracheoles into the muscles also give rise to ‘longitudinal T-tubules’, particularly in the T/et fibres. Dyads arise by association of cisternae of the SR: (i) with T-tubules sensu strictu, (ii) with Z-invaginations and T-tubule-like extensions from them, and (iii) directly with the plasma membrane at the surface of the fibre.
Regulation of thymidylate synthetase activity in cultured mammalian cells
Authors: Conrad Abigail H. , Ruddle F. H.
Published: April 2021

Changes in thymidylate synthetase specific activity in Don Chinese hamster cells grown in vitro have been examined during the culture cycle and after exposure of lag- and log-phase cultures to drugs which inhibit DNA, RNA, and protein synthesis. During the culture cycle enzyme activity was low during lag phase, rose 6- to 8-fold before log phase, fluctuated between 5-5 and 9 nmol dTMP/h/107 cells during log phase, and declined to base level during stationary phase. Puromycin prevented all increases in enzyme specific activity and caused a decrease in enzyme activity when applied to log-phase cultures. Actinomycin D prevented the initial rise in enzyme activity if applied during early lag phase but caused a pronounced increase in enzyme activity above control levels when applied during log phase.

High thymidine concentration (1 min) stopped cell division in log-phase cultures but did not alter the log-phase plateau level of thymidylate synthetase activity. Fluorodeoxyuridine stopped cell division and depressed enzyme activity to varying degrees depending upon its concentration, but at concentrations less than 10− M enzyme activity eventually returned to normal log-phase levels and cell division resumed if puromycin was not present. Methotrexate stopped cell division and caused a 3- to 4-fold increase in enzyme activity above control levels if puromycin was not present. This increase occurred in the presence of actinomycin D but was retarded by addition of thymidine when actinomycin D was not present.

These experiments suggest that the regulation of thymidylate synthetase activity in log-phase cells is complex and may involve thymidine triphosphate.