bluuebunny/crossref_metadata_2025 huggingface dataset:
Expression of Sonic hedgehog and its putative role as a precursor cell mitogen in the developing mouse retina
Authors: Jensen Abbie M. , Wallace Valerie A.
Published: April 2021

We show that Sonic hedgehog and patched are expressed in adjacent domains in the developing mouse retina. Treatment of cultures of perinatal mouse retinal cells with the amino-terminal fragment of Sonic hedgehog protein results in an increase in the proportion of cells that incorporate bromodeoxuridine, in total cell numbers, and in rod photoreceptors, amacrine cells and Müller glial cells, suggesting that Sonic hedgehog promotes the proliferation of retinal precursor cells. These finding suggest that hedgehog and patched are part of a conserved signalling pathway in retinal development in mammals and insects.
Clathrin heavy chain is required for spore cell but not stalk cell differentiation in Dictyostelium discoideum
Authors: Niswonger Maria L. , Halloran Theresa J. O’
Published: April 2021

Previous studies of a clathrin-minus Dictyostelium cell line revealed important roles for clathrin heavy chain (clathrin) in endocytosis, secretion of lysosomal hydrolases and osmoregulation. In this paper, we examine the contribution of clathrin-mediated membrane traffic to development in Dictyostelium discoideum. Clathrin-minus cells were delayed in early development. When exposed to starvation conditions, clathrin-minus cells streamed and aggregated more slowly than wild-type cells. Although clathrin-minus cells displayed only 40% the level of extracellular cyclic AMP binding normally found in wild-type cells, they responded chemotactically to extracellular cyclic AMP. Clathrin-minus cells down-regulated cyclic AMP receptors, but only to half the extent of wild-type cells.

We found that the extent of development of clathrin-minus cells was variable and influenced by environmental conditions. Although the mutant cells always progressed beyond the tipped mound stage, the final structure varied from a finger-like projection to a short, irregular fruiting body. Microscopic examination of these terminal structures revealed the presence of intact stalks but a complete absence of spores. Clathrin-minus cells expressed prestalk (ecmA and ecmB) and prespore (psA and cotB) genes normally, but were blocked in expression of the sporulation gene spiA. Using clathrin-minus cells that had been transformed with various promoter-lacZ reporter constructs, we saw only partial sorting of clathrin-minus prestalk and prespore cells. Even when mixed with wild-type cells, clathrin-minus cells failed to sort correctly and never constructed functional spores.

These results suggest three roles for clathrin during Dictyostelium development. First, clathrin increases the efficiency of early development. Second, clathrin enables proper and efficient patterning of prestalk and prespore cells during culmination. Third, clathrin is essential for differentiation of mature spore cells.
Drosophila cubitus interruptus forms a negative feedback loop with patched and regulates expression of Hedgehog target genes
Authors: Hepker Jennifer , Wang Qun-Tian , Motzny Cynthia K. , Holmgren Robert , Orenic Teresa Vales
Published: April 2021

The Drosophila segment polarity gene cubitus interruptus (ci) encodes a zinc finger protein that is required for the proper patterning of segments and imaginal discs. Epistasis analysis indicates that ci functions in the Hedgehog (Hh) signal transduction pathway and is required to maintain wingless expression in the embryo. In this paper, the role of the Ci protein in the Hh signaling pathway is examined in more detail. Our results show that ectopic expression of ci in imaginal discs and the embryo activates the expression of Hh target genes. One of these target genes, patched, forms a negative feedback loop with ci that is regulated by Hh signal transduction. Activation is also achieved using the Ci zinc finger domain fused to a heterologous transactivation domain. Conversely, repression of Hh target genes occurs in animals expressing the Ci zinc finger domain fused to a repression domain. To examine Ci function in more detail, regions of the Ci protein that are responsible for its ability to transactivate and its subcellular distribution have been identified.
Reggie-1 and reggie-2, two cell surface proteins expressed by retinal ganglion cells during axon regeneration
Authors: Schulte Thomas , Paschke Katja A. , Laessing Ute , Lottspeich Friedrich , Stuermer Claudia A. O.
Published: April 2021

Fish – in contrast to mammals – regenerate retinal ganglion cell axons when the optic nerve is severed. Optic nerve injury leads to reexpression of proteins, which typically are first expressed in newly differentiated retinal ganglion cells and axons. Here we identified two new proteins of fish retinal ganglion cells, reggie-1 and reggie-2, with monoclonal antibody M802 and molecular cloning techniques. In normal fish, M802 stained the few retinal axons derived from newborn ganglion cells which in fish are added lifelong to the retinal margin. After optic nerve injury, however, M802 labeled all retinal ganglion cells and retinal axons throughout their path into tectum. Consistent with M802 staining, reggie-1 and reggie-2 mRNAs were present in lesioned retinal ganglion cells, as demonstrated by in situ hybridization, but were not detectable in their normal mature counterparts. In western blots with membrane proteins of the adult goldfish brain, M802 recognizes a 48×103Mr protein band. At the amino acid level, 48×103Mr reggie-1 and reggie-2 are 44% identical, lack transmem-brane and membrane anchor domains, but appear membrane associated by ionic interactions. Reggie-1 and reggie-2 are homologous to 35×103Mr ESA (human epidermal surface antigen) but are here identified as neuronal surface proteins, present on newly differentiated ganglion cells at the retinal margin and which are reex-pressed in mature ganglion cells upon injury and during axonal regeneration.
Cellular interpretation of multiple TGF-β signals: intracellular antagonism between activin/BVg1 and BMP-2/4 signaling mediated by Smads
Authors: Candia Albert F. , Watabe Tetsuro , Hawley Stephanie H. B. , Onichtchouk Darya , Zhang Ying , Derynck Rik , Niehrs Christof , Cho Ken W. Y.
Published: April 2021

During early embryogenesis of Xenopus, dorsoventral polarity of the mesoderm is established by dorsalizing and ventralizing agents, which are presumably mediated by the activity of an activin/BVg1-like protein and Bone Morphogenetic Proteins (BMP), respectively. Interestingly, these two TGF-β subfamilies are found in overlapping regions during mesoderm patterning. This raises the question of how the presumptive mesodermal cells recognize the multiple TGF-β signals and differentially interpret this information to assign a particular cell fate. In this study, we have exploited the well characterized model of Xenopus mesoderm induction to determine the intracellular interactions between BMP-2/4 and activin/BVg1 signaling cascades. Using a constitutively active BMP-2/4 receptor that transduces BMP-2/4 signals in a ligand-independent fashion, we demonstrate that signals provided by activin/BVg1 and BMP modulate each other’s activity and that this crosstalk occurs through intracellular mechanisms. In assays using BMP-2/4 and activin/BVg1-specific reporters, we determined that the specificity of BMP-2/4 and activin/BVg1 signaling is mediated by Smad1 and Smad2, respectively. These Smads should be considered as the mediators of the intracellular antagonism between BMP-2/4 and activin/BVg1 signaling possibly through sequestration of a limited pool of Smad4. Consistent with such a mechanism, Smad4 interacts functionally with both Smad1 and −2 to potentiate their signaling activities, and a dominant negative variant of Smad4 can inhibit both activin/BVg1 and BMP-2/4 mediated signaling Finally, we demonstrate that an activin/BVg1-dependent transcriptional complex contains both Smad2 and Smad4 and thereby provides a physical basis for the functional involvement of both Smads in TGF-β-dependent transcriptional regulation. Thus, Smad4 plays a central role in synergistically activating activin/BVg1 and BMP-dependent transcription and functions as an intracellular sensor for TGFβ-related signals.
Hoxd-12 differentially affects preaxial and postaxial chondrogenic branches in the limb and regulates Sonic hedgehog in a positive feedback loop
Authors: Knezevic Vladimir , Santo Ron De , Schughart Klaus , Huffstadt Ulrike , Chiang Chin , Mahon Kathleen A. , Mackem Susan
Published: April 2021

Several 5′ members of the Hoxd cluster are expressed in nested posterior-distal domains of the limb bud suggesting a role in regulating anteroposterior pattern of skeletal elements. While loss-of-function mutants have demonstrated a regulatory role for these genes in the developing limb, extensive functional overlaps between various different Hox genes has hampered elucidation of the roles played by individual members. In particular, the function of Hoxd-12 in the limb remains obscure. Using a gain-offunction approach, we find that Hoxd-12 misexpression in transgenic mice produces apparent transformations of anterior digits to posterior morphology and digit duplications, while associated tibial hemimelia and other changes indicate that formation/growth of certain skeletal elements is selectively inhibited. If the digital arch represents an anterior bending of the main limb axis, then the results are all reconcilable with a model in which Hoxd-12 promotes formation of postaxial chondrogenic condensations branching from this main axis (including the anteriormost digit) and selectively antagonizes formation of ‘true’ preaxial condensations that branch from this main axis (such as the tibia). Hoxd-12 misexpression can also induce ectopic Sonic hedgehog (Shh) expression, resulting in mirror-image polydactyly in the limb. Misexpression of Hoxd-12 in other lateral plate derivatives (sternum, pelvis) likewise phenocopies several luxoid/luxate class mouse mutants that all share ectopic Shh signalling. This suggests that feedback activation of Shh expression may be a major function of Hoxd-12. Hoxd-12 can bind to and transactivate the Shh promoter in vitro. Furthermore, expression of either exogenous Hoxd-11 or Hoxd-12 in cultured limb bud cells, together with FGF, induces expression of the endogenous Shh gene. Together these results suggest that certain 5′ Hoxd genes directly amplify the posterior Shh polarizing signal in a reinforcing positive feedback loop during limb bud outgrowth.
Pdd1p associates with germline-restricted chromatin and a second novel anlagen-enriched protein in developmentally programmed DNA elimination structures
Authors: Smothers James F. , Mizzen Craig A. , Tubbert Michelle M. , Cook Richard G. , Allis C. David
Published: April 2021

Programmed DNA rearrangements, including DNA diminution, characterize the differentiation of somatic from germline nuclei in several developmental systems. Pdd1p (Programmed DNA degradation protein 1), a development-restricted polypeptide, has been implicated in heterochromatin assembly and DNA degradation during ciliate macronuclear development. Here, cross-linking and co-immunoprecipitation were used to verify that Pdd1passociated chromatin is enriched in germline-restricted DNA. Pdd1p-associated proteins include general core histones and a second anlagen-enriched polypeptide (Pdd2p, formerly known as p43). Immunoblotting analyses demonstrate that, like Pdd1p, Pdd2p is developmentally regulated and present in conjugating cells during the time of germline DNA rearrangements and degradation. Pdd2p is post-translationally modified by phosphorylation at a time in development corresponding to dephosphorylation of Pdd1p and the formation of heterochromatic DNA elimination structures. Following gene cloning, the derived amino acid sequence of the PDD2 gene predicts a novel polypeptide containing multiple putative phosphorylation sites. In situ analyses, using both light and electron microscopy, demonstrate that Pdd1p and Pdd2p co-localize in DNA elimination structures within developing macronuclei. However, unlike Pdd1p, which also localizes to apoptotic macronuclei, Pdd2p appears to be restricted to a higher degree to germline DNA elimination structures. Taken together, the data presented here demonstrate a physical link between Pdd1p and germline-restricted chromatin and establish Pdd2p as the second member of a small group of developmentally restricted polypeptides implicated in programmed DNA elimination.
Differential H4 acetylation of paternal and maternal chromatin precedes DNA replication and differential transcriptional activity in pronuclei of 1-cell mouse embryos
Authors: Adenot Pierre G. , Mercier Yvan , Renard Jean-Paul , Thompson Eric M.
Published: April 2021

In the mouse embryo, transcriptional activation begins during S/G2 phase of the first cell cycle when paternal and maternal chromatin are still in separate nuclear entities within the same cytoplasm. At this time, the male pronucleus exhibits greater transcriptional activity than the female pronucleus. Since acetylation of histones in the nucleosome octamer exerts a regulatory influence on gene expression, we investigated changes in histone acetylation during the remodeling of paternal and maternal chromatin from sperm entry through to minor genome activation and mitosis. We found (1) neither mature sperm nor metaphase II maternal chromatin stained for hyperacetylated histone H4; (2) immediately following fertilization, hyperacetylated H4 was associated with paternal but not maternal chromatin while, in parthenogenetically activated oocytes, maternal chromatin became hyperacetylated; (3) in zygotes, differential levels and patterns of hyperacetylated H4 between male and female pronuclei persisted throughout most of G1 with histone deacetylases and acetyltransferases already active at this time; (4) when transcriptional differences are observed in S/G2, male and female pronuclei have equivalent levels of H4 hyperacetylation and DNA replication was not required to attain this equivalence and (5) in contrast to the lack of H4 hyperacetylation on gametic chromatin, chromosomes at the first mitosis showed distinct banding patterns of H4 hyperacetylation. These results suggest that sperm chromatin initially out-competes maternal chromatin for the pool of hyperacetylated H4 in the oocyte, that hyperacetylated H4 participates in the process of histone-protamine exchange in the zygote, and that differences in H4 acetylation in male and female pronuclei during G1 are translated across DNA replication to transcriptional differences in S/G2. Prior to fertilization, neither paternal nor maternal chromatin show memory of H4 hyperacetylation patterns but, by the end of the first cell cycle, before major zygotic genome activation at the 2-cell stage, chromosomes already show hyperacetylated H4 banding patterns.
Two copies of a subelement from the Vg1 RNA localization sequence are sufficient to direct vegetal localization in Xenopus oocytes
Authors: Gautreau Denise , Cote Colette A. , Mowry Kimberly L.
Published: April 2021

Localization of mRNA has emerged as a fundamental mechanism for generating polarity during development. In vertebrates, one example of this phenomenon is Vg1 RNA, which is localized to the vegetal cortex of Xenopus oocytes. Vegetal localization of Vg1 RNA is directed by a 340-nt sequence element contained within its 3′ untranslated region. To investigate how such cis-acting elements function in the localization process, we have undertaken a detailed analysis of the precise sequence requirements for vegetal localization within the 340-nt localization element. We present evidence for considerable redundancy within the localization element and demonstrate that critical sequences lie at the ends of the element. Importantly, we show that a subelement from the 5′ end of the Vg1 localization element is, when duplicated, sufficient to direct vegetal localization. We suggest that the Vg1 localization element is composed of smaller, redundant sequence motifs and identify one such 6-nt motif as essential for localization. These results allow insight into what constitutes an RNA localization signal and how RNA sequence elements may act in the localization process.
Novel regulatory interactions revealed by studies of murine limb pattern in Wnt-7a and En-1 mutants
Authors: Cygan Jennifer A. , Johnson Randy L. , McMahon Andrew P.
Published: April 2021

Classical embryological experiments have demonstrated that dorsal-ventral patterning of the vertebrate limb is dependent upon ectodermal signals. One such factor is Wnt-7a, a member of the Wnt family of secreted proteins, which is expressed in the dorsal ectoderm. Loss of Wnt-7a results in the appearance of ventral characteristics in the dorsal half of the distal limb. Conversely, En-1, a homeo-domain transcription factor, is expressed exclusively in the ventral ectoderm, where it represses Wnt-7a. En-1 mutants have dorsal characteristics in the ventral half of the distal limb. Experiments in the chick suggest that the dorsalizing activity of Wnt-7a in the mesenchyme is mediated through the regulation of the LIM-homeodomain transcription factor Lmx-1. Here we have examined the relationship between Wnt-7a, En-1 and Lmx-1b, a mouse homolog of chick Lmx-1, in patterning the mammalian limb. We find that Wnt-7a is required for Lmx-1b expression in distal limb mesenchyme, and that Lmx-1b activation in the ventral mesenchyme of En-1 mutants requires Wnt-7a. Consistent with Lmx-1b playing a primary role in dorsal-ization of the limb, we find a direct correlation between regions of the anterior distal limb in which Lmx-1b is misregulated during limb development and the localization of dorsal-ventral patterning defects in Wnt-7a and En-1 mutant adults. Thus, ectopic Wnt-7a expression and Lmx-1b activation underlie the dorsalized En-1 phenotype, although our analysis also reveals a Wnt-7a-independent activity for En-1 in the repression of pigmentation in the ventral epidermis. Finally, we demonstrate that ectopic expression of Wnt-7a in the ventral limb ectoderm of En-1 mutants results in the formation of a second, ventral apical ectodermal ridge (AER) at the junction between Wnt-7a-expressing and nonexpressing ectoderm. Unlike the normal AER, ectopic AER formation is dependent upon Wnt-7a activity, indicating that distinct genetic mechanisms may be involved in primary and secondary AER formation.
Deficient development and maintenance of postsynaptic specializations in mutant mice lacking an ‘adult’ acetylcholine receptor subunit
Authors: Missias Andrea C. , Mudd Jacqueline , Cunningham Jeanette M. , Steinbach Joseph H. , Merlie John P. , Sanes Joshua R.
Published: April 2021

At many synapses, ‘fetal’ neurotransmitter receptor subunits are replaced by ‘adult’ subunits as development proceeds. To assess the significance of such transitions, we deleted the gene encoding the adult acetylcholine receptor (AChR) ε subunit, which replaces its fetal counterpart, the γ subunit, at the skeletal neuromuscular junction during early postnatal life. Several aspects of postnatal maturation, including synapse elimination, proceeded normally in the absence of the adult AChR, but structural development of the endplate was compromised. Later, inadequate compensation by the γ subunit led to severely reduced AChR density in mutant endplates relative to controls. This decreased density led to a profound reorganization of AChR-associated components of the postsynaptic membrane and cytoskeleton. Together, these results suggest novel roles for AChRs in assembly of the postsynaptic apparatus.
Analysis of Hoxd-13 and Hoxd-11 misexpression in chick limb buds reveals that Hox genes affect both bone condensation and growth
Authors: Goff Deborah J. , Tabin Clifford J.
Published: April 2021

Hox genes are important regulators of limb pattern in vertebrate development. Misexpression of Hox genes in chicks using retroviral vectors provides an opportunity to analyze gain-of-function phenotypes and to assess their modes of action. Here we report the misexpression phenotype for Hoxd-13 and compare it to the misexpression phenotype of Hoxd-11. Hoxd-13 misexpression in the hindlimb results in a shortening of the long bones, including the femur, the tibia, the fibula and the tarsometatarsals. Mutations in an alanine repeat region in the N-terminus of Hoxd-13 have recently been implicated in human synpolydactyly (Muragaki, Y., Mundlos, S., Upton, J. and Olsen, B. R. (1996) Science 272, 548-551). N-terminal truncations of Hoxd-13 which lack this repeat were constructed and were found to produce a similar, although slightly milder, mis-expression phenotype than the full-length Hoxd-13.

The stage of bone development regulated by Hox genes has not previously been examined. The changes in bone lengths caused by Hoxd-13 misexpression are late phenotypes that first become apparent during the growth phase of the bones. Analysis of tritiated thymidine uptake by the tibia and fibula demonstrates that Hox genes can pattern the limb skeleton by regulating the rates of cell division in the proliferative zone of growing cartilage. Hoxd-11, in contrast to Hoxd-13, acts both at the initial cartilage condensation phase in the foot and during the later growth phase in the lower leg.

Ectopic Hoxd-13 appears to act in a dominant negative manner in regions where it is not normally expressed. We propose a model in which all Hox genes are growth promoters, regulating the expression of the same target genes, with some Hox genes being more effective promoters of growth than other Hox genes. According to this model, the overall rate of growth in a given region is the result of the combined action of all of the Hox genes expressed in that region competing for the same target genes.
Mutations in lottchen cause cell fate transformations in both neuroblast and glioblast lineages in the Drosophila embryonic central nervous system
Authors: Buescher Marita , Chia William
Published: April 2021

The Drosophila embryonic central nervous system (CNS) develops from a stereotyped pattern of neuronal progenitor cells called neuroblasts (NB). Each NB has a unique identity that is defined by the time and position of its formation and a characteristic combination of genes it expresses. Each NB generates a specific lineage of neurons and/or glia. Here we describe the genetic and phenotypic analysis of lottchen (ltt), a novel gene whose loss of function causes a change in the identity of at least one NB as well as cell fate transformations within the lateral glioblast lineage. In wildtype embryos the parental NB of the motoneuron RP2 is NB4-2. ltt embryos are distinguished by an additional RP2-like neuron which appears later in development. We show that the two RP2 neurons are derived from two distinct GMC4-2a-like cells that do not share the same parental NB, indicating that a second NB has acquired the potential to produce a GMC and a neuron which is normally restricted to the NB4-2 lineage. Moreover, the ltt mutations lead to a loss of correctly specified longitudinal glia; this coincides with severely defective longitudinal connectives. Therefore, lottchen plays a role in specifying the identity of both neuroblast and glioblast lineages in the Drosophila embryonic CNS. We discuss the possibility that ltt may act to differentiate NB identity along the medial lateral axis.
Notochord alters the permissiveness of myotome for pathfinding by an identified motoneuron in embryonic zebrafish
Authors: Beattie Christine E. , Eisen Judith S.
Published: April 2021

During zebrafish development, identified motoneurons innervate cell-specific regions of each trunk myotome. One motoneuron, CaP, extends an axon along the medial surface of the ventral myotome. To learn how this pathway is established during development, the CaP axon was used as an assay to ask whether other regions of the myotome were permissive for normal CaP pathfinding. Native myotomes were replaced with donor myotomes in normal or reversed dorsoventral orientations and CaP pathfinding was assayed. Ventral myotomes were permissive for CaP axons, even when they were taken from older embryos, suggesting that the CaP pathway remained present on ventral myotome throughout development. Dorsal myotomes from young embryos were also permissive for CaP axons, however, older dorsal myotomes were non-permissive, showing that permissiveness of dorsal myotome for normal CaP pathfinding diminished over time. This process appears to depend on signals from the embryo, since dorsal myotomes matured in vitro remained permissive for CaP axons. Genetic mosaics between wild-type and floating head mutant embryos revealed notochord involvement in dorsal myotome change of permissiveness. Dorsal and ventral myotomes from both younger and older floating head mutant embryos were permissive for CaP axons. These data suggest that initially both dorsal and ventral myotomes are permissive for CaP axons but as development proceeds, there is a notochord-dependent decrease in permissiveness of dorsal myotome for CaP axonal outgrowth. This change participates in restricting the CaP pathway to the ventral myotome and thus to neuromuscular specificity.
Nucleocytoplasmic localisation of extradenticle protein is spatially regulated throughout development in Drosophila
Authors: Aspland Simon E. , White Robert A. H.
Published: April 2021

The extradenticle protein is a homeodomain transcription factor which has an important role regulating the DNA-binding specificity of homeotic selector proteins. We have made a monoclonal antibody against extradenticle and have studied the expression of the protein in the embryo and in imaginal discs. We find that extradenticle is initially uniformly distributed as expected but strikingly is excluded from nuclei until gastrulation. During the extended germ band stage the protein remains predominantly cytoplasmic and does not accumulate in nuclei until germ band retraction. Nuclear accumulation occurs in a highly spatially regulated pattern. In the imaginal discs the nuclear accumulation of extradenticle is also spatially regulated and, in the wing and leg discs, distal regions exhibit cytoplasmic extradenticle whereas proximally the protein is nuclear. We suggest that this regulation of the sub-cellular localisation of extradenticle is important for the interactions between extradenticle and the homeotic selector proteins and that extradenticle is not simply a ubiquitously available cofactor.
Cell-cell association directed mitotic spindle orientation in the early development of the marine shrimp Sicyonia ingentis
Authors: Wang Steven W. , Griffin Fred J. , Clark Wallis H.
Published: April 2021

During early cleavages of Sicyonia ingentis embryos, mitotic spindle orientations differ between blastomeres and change in a predictable manner with each successive mitosis. From 2nd through 7th cleavages, spindles orient at a 90° angle with respect to the spindle of the parent blastomere. Thus, spindle orientation is parallel to the cleavage plane that formed the blastomere. To determine if specific spindle orientations were intrinsic properties of individual blastomeres, we altered blastomere associations and asked how mitotic spindle orientation was affected in successive cleavages using laser scanning confocal microscopy. Linear embryos were constructed by dissociating 4-cell embryos and recombining the blastomeres in a linear array. The ensuing cleavage (3rd embryonic cleavage) of these linear embryos was parallel to the long axis of the embryo, resulting in four parallel pairs of blastomeres which lay in a common plane that was parallel to the substratum. The 4th cleavage produced a linear embryo with the 16 blastomeres arranged in four parallel quartets. Then, in preparation for 5th cleavage, spindles oriented at a 45° angle (not parallel as in normal development) with respect to the previous cleavage plane. When 8-cell linear embryos were separated into linear half-embryos, subsequent spindle orientations were not like those observed for intact 8-cell linear embryos, but rather regressed to the orientation seen in 4-cell linear embryos. We suggest that the reorientation of mitotic spindles during early cleavage of S. ingentis is neither an intrinsic property nor age dependent, but rather is cell contact related. Further, these results in conjunction with observations of non-manipulated embryos suggest that spindle poles (centrosomes) avoid cytoplasmic regions adjacent to where there is cell-cell contact during early development.
Expression and function of decapentaplegic and thick veins during the differentiation of the veins in the Drosophila wing
Authors: de Celis Jose F.
Published: April 2021

The differentiation of the veins in the Drosophila wing involves the coordinate activities of several signal transduction pathways, including those mediated by the transmembrane receptors Torpedo and Notch. In this report, the role of the signalling molecule Decapentaplegic during vein differentiation has been analysed. It is shown that decapentaplegic is expressed in the pupal veins under the control of genes that establish vein territories in the imaginal disc. Decapentaplegic, acting through its receptor Thick veins, activates vein differentiation and restricts expression of both veinlet and the Notch-ligand Delta to the developing veins. Genetic combinations between mutations that increase or reduce Notch, veinlet and decapentaplegic activities suggest that the maintenance of the vein differentiation state during pupal development involves cross-regulatory interactions between these pathways.
SDC-3 coordinates the assembly of a dosage compensation complex on the nematode X chromosome
Authors: Davis Tamara L. , Meyer Barbara J.
Published: April 2021

X chromosome expression in C. elegans is controlled by a chromosome-wide regulatory process called dosage compensation that specifically reduces by half the level of transcripts made from each hermaphrodite X chromosome. This process equalizes X expression between the sexes (XX hermaphrodites and XO males), despite their two-fold difference in X chromosome dose, and thereby prevents sex- specific lethality. Dosage compensation is achieved by a protein complex that associates with X in a sex-specific fashion to modulate gene expression. SDC-3, a protein that coordinately controls both sex determination and dosage compensation, activates dosage compensation by directing the dosage compensation protein complex to the hermaphrodite X chromosomes. We show that SDC-3 coordinates this assembly through its own sex-specific association with X. SDC-3 in turn requires other members of the dosage compensation gene hierarchy for its stability and its X localization. In addition, SDC-3 requires its own zinc finger motifs and an amino-terminal region for its X association. Our experiments suggest the possible involvement of zinc finger motifs in X chromosome recognition and the amino- terminal region in interactions with other dosage compensation proteins.
SHAGGY and DISHEVELLED exert opposite effects on wingless and decapentaplegic expression and on positional identity in imaginal discs
Authors: Heslip Tim R. , Theisen Heidi , Walker Heli , Marsh J. Lawrence
Published: April 2021

The finding that Wingless (WG) and Decapentaplegic (DPP) suppress each others transcription provides a mechanism for creating developmental territories in fields of cells. Here, we address the mechanism of that antagonism. The dishevelled (dsh) and shaggy (sgg) genes encode intracellular proteins generally thought of as downstream of WG signaling. We have investigated the effects of changing either DSH or SGG activity on both cell fate and wg and dpp expression. At the level of cell fate in discs, DSH antagonizes SGG activity. At the level of gene expression, SGG positively regulates dpp expression and negatively regulates wg expression while DSH activity suppresses dpp expression and promotes wg expression. Sharp borders of gene expression correlating precisely with clone boundaries suggest that the effects of DSH and SGG on transcription of wg and dpp are not mediated by secreted factors but rather act through intracellular effectors. The interactions described here suggest a model for the antagonism between WG and DPP that is mediated via SGG. The model incorporates autoactivation and lateral inhibition, which are properties required for the production of stable patterns. The regulatory interactions described exhibit extensive ability to organize new pattern in response to manipulation or injury.
Subsets of retinal progenitors display temporally regulated and distinct biases in the fates of their progeny
Authors: Alexiades Macrene R. , Cepko Constance L.
Published: April 2021

Cell fate determination in the developing vertebrate retina is characterized by the sequential generation of seven classes of cells by multipotent progenitor cells. Despite this order of genesis, more than one cell type is generated at any time; for example, in the rat, several cell types are born during the prenatal period, while others are born postnatally. In order to examine whether there are classes of progenitor cells with distinct developmental properties contributing to this developmental progression, we examined antigen expression in progenitor cells during rat retinal development. Two markers of amacrine and horizontal cells, the VC1.1 epitope and syntaxin, were found to be expressed on a subset of progenitors in a temporally regulated manner that closely paralleled the birthdays of these cell types. In order to investigate which cell types were produced by the progenitors expressing these markers, fluorescent latex microspheres covalently coupled to VC1.1 antibodies were used to indelibly label VC1.1+ progenitor cells and their progeny. Early in retinal development, VC1.1+ progenitors generated a high percentage of amacrine and horizontal cells, but no cone photoreceptors. During this same period, a comparable number of cone photoreceptors were generated by VC1.1- progenitors. In the late embryonic and early postnatal period, VC1.1+ progenitors continued to generate predominantly amacrine cells, but also gave rise to an increasing number of rod photoreceptors. These findings demonstrate that expression of these two markers by progenitors is highly correlated with a bias towards the production of amacrine and horizontal cells. The fact that subsets of progenitors with temporally regulated and distinct biases are intermingled within the retinal neuroepithelium provides a basis for understanding how different cell types are generated both simultaneously and in a particular order by multipotent progenitors during retinal development.
Segmental and neuronal architecture of the hindbrain of Krox-20 mouse mutants
Authors: Schneider-Maunoury Sylvie , Seitanidou Tania , Charnay Patrick , Lumsden Andrew
Published: April 2021

The vertebrate hindbrain is transiently segmented during its early development with the formation of reiterated bulges, the rhombomeres (r). The Krox-20 gene, which encodes a zinc finger transcription factor, has been shown previously to be implicated in the maintenance of r3 and r5 (Schneider-Maunoury, S., Topilko, P., Seitanidou, T., Levi, G., Cohen-Tannoudji, M., Pournin, S., Babinet, C. and Charnay, P. (1993) Cell 75, 1199-1214; Swiatek, P. J. and Gridley, T. (1993) Genes Dev. 7, 2071-2084. However, it was not clear from these analyses how extensive the deletion of r3 and r5 was and whether the overall segmentation and internal architecture of the hindbrain was affected. We have now reinvestigated these issues by analysis of rhombomere boundaries, using both morphological and molecular markers, and of the fate of specific motor neuron populations, using retrograde and anterograde carbocyanine dye tracing. We conclude that r3 and r5 and their derivatives are completely eliminated in Krox-20−/ − embryos while overall hindbrain segmentation is maintained. In addition, we show that the disappearance of these territories has important consequences for even-numbered rhombomeres as well, in particular on axonal navigation: (i) a population of r6 motoneurons, presumably normally fated to join the glossopharyngeal nerve, has its axons misrouted toward the facial exit point in r4; (ii) the trigeminal motor axons are also misrouted, presumably because of the proximity of the trigeminal and facial exit points. They fasciculate with facial axons outside the neural tube and enter the second branchial arch instead of the first arch. This navigational error could explain the disappearance, at around 17.5 dpc, of the trigeminal motor nucleus in Krox-20−/ − embryos by inadequate supply of essential, possibly arch-specific survival factors.
Hedgehog activity, independent of Decapentaplegic, participates in wing disc patterning
Authors: Luis Mullor José , Calleja Manuel , Capdevila Javier , Guerrero Isabel
Published: April 2021

In the Drosophila wing imaginal disc, the Hedgehog (Hh) signal molecule induces the expression of decapentaplegic (dpp) in a band of cells abutting the anteroposterior (A/P) compartment border. It has been proposed that Dpp organizes the patterning of the entire wing disc. We have tested this proposal by studying the response to distinct levels of ectopic expression of Hh and Dpp, using the sensory organ precursors (SOPs) of the wing and notum and the presumptive wing veins as positional markers. Here, we show that Dpp specifies the position of most SOPs in the notum and of some of them in the wing. Close to the A/P compartment border, however, SOPs are specified by Hh rather than by Dpp alone. We also show that late signaling by Hh, after setting up dpp expression, is responsible for the formation of vein 3 and the scutellar region, and also for the determination of the distance between veins 3 and 4. One of the genes that mediates the Hh signal is the zinc-finger protein Cubitus interruptus (Ci). These results indicate that Hh has a Dpp-independent morphogenetic effect in the region of the wing disc near the A/P border.
Determination of the unequal fate of cotyledons of a one-leaf plant, Monophyllaea
Authors: Tsukaya Hirokazu
Published: April 2021

One-leaf plants, belonging to the family Gesneriaceae, were described for the first time more than 150 years ago. One such unusual plant, Monophyllaea, has only one leaf at maturity. Only one of the two cotyledons grows continuously, without the formation of true leaves, and this feature, known as anisocotyledonous development, has been repeatedly mentioned in textbooks of plant morphology. However, the mechanism for the determination of the one-leaf phenotype remains to be ascertained.

In this study, meristematic regions were identified, by monitoring DNA synthesis, at the base of both cotyledons just after germination, while no such regions were found in the shoot apex. Surgical experiments with seedlings and analysis of the anisocotyledonous development revealed that the fate of the cotyledons is determined during their growth. Anisocotyledonous development seems to be the result of competition between the two cotyledons. The mechanism that governs the development of the shoot in the genus Monophyllaea is discussed in relation to apical dominance, which is the common mechanism that regulates shoot development in many plants.
Loss of cerebum function ventralizes the zebrafish embryo
Authors: Fisher Shannon , Amacher Sharon L. , Halpern Marnie E.
Published: April 2021

Recent studies implicate ventrally derived signals, in addition to dorsal ones emanating from the organizer, in patterning the vertebrate gastrula. We have identified five overlapping deficiencies that uncover the zebrafish cerebum locus and dramatically alter dorsal-ventral polarity at gastrulation. Consistent with the properties of experimentally ventralized amphibian embryos, cerebum mutants exhibit reduced neurectodermal gene expression domains and an increase in derivatives of ventral mesoderm. Structures derived from paraxial and lateral mesoderm also are reduced; however, dorsal axial mesodermal derivatives, such as the hatching gland and notochord, are largely spared. The pleiotropic action of cerebum deficiencies, and the differential response of affected tissues, suggest that the cerebum gene may normally function as an inhibitor of ven-tralizing signals, a function previously ascribed to Noggin and Chordin in Xenopus. Analysis of the cerebum phenotype provides genetic evidence for the existence of ventralizing signals in the zebrafish gastrula and for antagonists of those signals.
Lingual deficits in BDNF and NT3 mutant mice leading to gustatory and somatosensory disturbances, respectively
Authors: Nosrat Christopher A. , Blomlöf Johan , ElShamy Wael M. , Ernfors Patrik , Olson Lars
Published: April 2021

A combination of anatomical, histological and physiological data from wild-type and null-mutated mice have established crucial roles for BDNF and NT3 in gustatory and somatosensory innervation of the tongue, and indeed for proper development of the papillary surface of the tongue. BDNF is expressed in taste buds, NT3 in many surrounding epithelial structures. Absence of BDNF in mice leads to severely malformed taste bud-bearing papillae and severe reduction of taste buds, a loss of proper innervation of remaining taste buds and a loss of taste discrimination although not of the suckling reflex per se. In contrast, absence of NT3 leads to a massive loss of somatosensory innervation of lingual structures. These findings demonstrate distinct roles for BDNF and NT3 in the establishment of the complex innervation apparatus of the tongue with non-overlapping roles for the lingual gustatory and somatosensory systems. The distinction between different sensory modalities, being dependent on either BDNF or NT3 may also have clinical implications.
A sensory axon repellent secreted from ventral spinal cord explants is neutralized by antibodies raised against collapsin-1
Authors: Shepherd Iain T. , Luo Yuling , Lefcort Frances , Reichardt Louis F. , Raper Jonathan A.
Published: April 2021

During embryogenesis, different subclasses of sensory neurons extend central projections to specific locations in the spinal cord. Muscle and cutaneous afferents initially project to the same location in the dorsal cord. Later, specific muscle afferents leave other afferents behind and project into the ventral cord. Previous studies have shown that ventral spinal cord explants secrete a repellent for sensory neurites. We now find that antibodies to collapsin-1 neutralize this repellent activity. Additional data suggest that all afferents respond to collapsin-1 when they are first confined to the dorsal cord, but that ventrally projecting muscle afferents become collapsin-1 insensitive as they project into the ventral cord. Our results suggest that the transient dorsal expression of collapsin-1 prevents all efferents from entering the cord early and sustained ventral expression prevents dorsally terminating afferents from entering the ventral cord later.
The role of Pax-6 in eye and nasal development
Authors: Grindley Justin C. , Davidson Duncan R. , Hill Robert E.
Published: April 2021

Small eye (Sey) mice homozygous for mutations in the Pax-6 gene have no lenses and no nasal cavities. We have examined the ontogeny of eye and nasal defects in Sey/Sey embryos and have related the defects seen to the pattern of Pax-6 mRNA expression in the mouse during normal eye and nasal development.

There are two principal components of the early eye, the neural ectoderm of the optic vesicle, which forms the retina, and the overlying surface ectoderm, which forms the lens and cornea. By studying these interacting tissues in normal and Sey/Sey embryos, we have identified processes for which Pax-6 is important and can thus suggest possible roles for the Pax-6 gene.

Pax-6 is essential for the formation of lens placodes from surface ectoderm. In normal development, early Pax-6 mRNA expression in a broad domain of surface ectoderm is downregulated, but expression is specifically maintained in the developing lens placode. Moreover, other Pax-6expressing tissues are frequently those that have can transdifferentiate into lens. Thus, phenotype and expression together suggest a role for Pax-6 in lens determination.

At least some functions of Pax-6 can be separated from the influence of other tissues. Early Sey/Sey optic vesicles are abnormally broad and fail to constrict proximally.

These defects occur prior to the time of lens placode formation and probably reflect a requirement for Pax-6 in neural ectoderm. In surface ectoderm domains, where Pax6 expression is known to be independent of the presence of an optic vesicle, Pax-6 function is required for the maintenance of its own transcription.

The mutual dependency of lens and optic vesicle development can also be studied using the Small eye mutation. Using region-specific markers we find that, in the morphologically abnormal Sey/Sey optic vesicles, aspects of normal proximo-distal specification nevertheless persist, despite the complete absence of lens.

Like the lens, the nasal cavities develop from ectodermal placodes that normally express Pax-6 mRNA, fail to form in Sey/Sey mice and show Pax-6-dependent Pax-6 mRNA regulation. Analysis of patterns of programmed cell death and absence of nasal region expression from an Msx-1 transgene in Sey/Sey embryos suggest a requirement for Pax-6 in the transition from presumptive nasal ectoderm to placode, and that Msx-1, or genes regulating it, are possible targets for Pax-6.
Left-right asymmetry of a nodal-related gene is regulated by dorsoanterior midline structures during Xenopus development
Authors: Lohr Jamie L. , Danos Maria C. , Yost H. Joseph
Published: April 2021

Development of asymmetry along the left-right axis is a critical step in the formation of the vertebrate body plan. Disruptions of normal left-right patterning are associated with abnormalities of multiple organ systems, including significant congenital heart disease. The mouse nodal gene, and its homologues in chick and Xenopus, are among the first genes known to be asymmetrically expressed along the left-right axis before the development of organ asymmetry. Alterations in the expression pattern of mouse nodal and the chick homologue (cNR-1) have been associated with defects in the development of left-right asymmetry and cardiac looping (Levin, M., Johnson, R. L., Stern, C. D., Kuehn, M. and Tabin, C. (1995) Cell 82, 803-814; Collignon, J., Varlet, I. and Robertson, E. J. (1996) Nature 381, 155-158; Lowe, L. A., Supp, D. M., Sampath, K., Yokoyama, T., Wright, C. V. E., Potter, S. S., Overbeek, P. and Kuehn, M. R. (1996) Nature 381, 158-161). Here, we show that the normal expression patterns of the Xenopus nodal-related gene (Xnr-1) are variable in a large population of embryos and that Xnr-1 expression is altered by treatments that perturb normal left-right development. The incidence of abnormal Xnr-1 expression patterns correlates well with cardiac reversal rates in both control and experimentally treated Xenopus embryos. Furthermore, dorsal midline structures, including notochord and/or hypochord and neural floorplate, regulate Xnr-1 expression prior to the specification of cardiac left-right orientation by repression of Xnr-1 expression in the right lateral plate mesoderm during closure of the neural tube. The correlation of Xnr-1 expression and orientation of cardiac looping suggests that Xnr-1 is a component of the left-right signaling pathway required for the specification of cardiac orientation in Xenopus, and that dorsal midline structures normally act to repress the signaling pathway on the right side of the embryo.