bluuebunny/crossref_metadata_2025 huggingface dataset:
wingless signal and Zeste-white 3 kinase trigger opposing changes in the intracellular distribution of Armadillo
Authors: Peifer Mark , Sweeton Dari , Casey Michael , Wieschaus Eric
Published: April 2021

wingless/wnt-1 signaling directs cell fate during development. Genetic analysis in Drosophila identified genes that may encode components of the wingless signal transduction system. Drosophila Armadillo, homolog of vertebrate β-catenin, is required for wingless signaling. Unlike armadillo RNA, Armadillo protein accumulates non-uniformly in different cells of each embryonic segment. We found that cells alter their intracellular distribution of Armadillo in response to Wingless signal, accumulating increased levels of cytoplasmic Armadillo relative to those of membrane-associated protein. Levels of cytoplasmic Armadillo are also regulated by Zeste-White 3 kinase. Analysis of double mutants demonstrates that Armadillo’s role in wingless signaling is direct, and that Armadillo functions down-stream of both wingless and zeste-white 3. We present a model for the role of Armadillo stripes in transduction of wingless signal.
The establishment of bilateral asymmetry in sea urchin embryos
Authors: McCain Elizabeth R. , McClay David R.
Published: April 2021

Although much is known about the specification and determination of the two primary axes (animal/vegetal and dorsoventral or oral/aboral) in a number of embryos, little is understood about bilaterality. In the sea urchin, left/right asymmetry is crucial to normal development as the echinus or adult rudiment is positioned on the left side of the larva. We examined the establishment of bilateral asymmetry in Lytechnis variegatus embryos by determining the relationship of the first cleavage planes to the left/right axis. Embryos were bisected at different times to determine when the bilateral axis is committed. These lineage tracing and cell separation experiments demonstrated that the first cleavage plane divides the embryo into left and right halves, although this is conditional until after late blastula stage. The relationship between the specification of the dorsoventral axis and the bilateral axis was examined experimentally. In other species when the dorsal and ventral halves of early echinoderm embryos (preblastula) are separated, the dorsal half often reverses (180°) its dorsoventral axis. We asked whether those larvae with an inverted dorsoventral axis would shift the position of the echinus rudiment from the original left side to the new left side. If so, it would demonstrate that the larval asymmetry is dependent upon specification of the dorsoventral axis. Using a combination of lineage tracing and cell separation techniques, we show that the left/right asymmetry is specified with respect to the dorsoventral axis.
Early transcription inCaenorhabditis elegansembryos
Authors: Edgar Lois G. , Wolf Nurit , Wood William B.
Published: April 2021

We have analysed early transcription in devitellinized, cultured embryos of the nematode Caenorhabditis elegans by two methods: measurement of [32P]UTP uptake into TCA-precipitable material and autoradiographic detection of [3H]UTP labelling both in the presence and absence of α-amanitin. RNA synthesis was first detected at the 8-to 12-cell stage, and α-amanitin sensitivity also appeared at this time, during the cleavages establishing the major founder cell lineages. The requirements for maternally supplied versus embryonically produced gene products in early embryogenesis were examined in the same culture system by observing the effects of α-amanitin on cell division and the early stereotyped lineage patterns. In the presence of high levels of α-amanitin added at varying times from two cells onward, cell division continued until approximately the 100-cell stage and then stopped during a single round of cell division. The characteristic unequal early cleavages, orientation of cleavage planes and lineage-specific timing of early divisions were unaffected by α-amanitin in embryos up to 87 cells. These results indicate that embryonic transcription starts well before gastrulation in C. elegans embryos, but that although embryonic transcripts may have important early functions, maternal products can support at least the mechanics of the first 6 to 7 cell cycles.
Pax-3 expression in segmental mesoderm marks early stages in myogenic cell specification
Authors: Williams Brian A. , Ordahl Charles P.
Published: April 2021

Specification of the myogenic lineage begins prior to gastrulation and culminates in the emergence of determined myogenic precursor cells from the somites. The myoD family (MDF) of transcriptional activators controls late step(s) in myogenic specification that are closely followed by terminal muscle differentiation. Genes expressed in myogenic specification at stages earlier than MDFs are unknown. The Pax-3 gene is expressed in all the cells of the caudal segmental plate, the early mesoderm compartment that contains the precursors of skeletal muscle. As somites form from the segmental plate and mature, Pax-3 expression is progressively modulated. Beginning at the time of segmentation, Pax-3 becomes repressed in the ventral half of the somite, leaving Pax-3 expression only in the dermomyotome. Subsequently, differential modulation of Pax-3 expression levels delineates the medial and lateral halves of the dermomyotome, which contain precursors of axial (back) muscle and limb muscle, respectively. Pax-3 expression is then repressed as dermomyotome-derived cells activate MDFs. Quail-chick chimera and ablation experiments confirmed that the migratory precursors of limb muscle continue to express Pax-3 during migration. Since limb muscle precursors do not activate MDFs until 2 days after they leave the somite, Pax-3 represents the first molecular marker for this migratory cell population. A null mutation of the mouse Pax-3 gene, Splotch, produces major disruptions in early limb muscle development (Franz, T., Kothary, R., Surani, M. A. H., Halata, Z. and Grim, M. (1993) Anat. Embryol. 187, 153-160; Goulding, M., Lumsden, A. and Paquette, A. (1994)Development 120, 957-971). We conclude, therefore, that Pax-3 gene expression in the paraxial mesoderm marks earlier stages in myogenic specification than MDFs and plays a crucial role in the specification and/or migration of limb myogenic precursors.
Expression of inhibin subunits and follistatin during postimplantation mouse development: decidual expression of activin and expression of follistatin in primitive streak, somites and hindbrain
Authors: Albano R. M. , Arkell R. , Beddington R. S. P. , Smith J. C.
Published: April 2021

Members of the activin family are believed to act as mesoderm-inducing factors during early amphibian development. Little is known, however, about mesoderm formation in the mammalian embryo, and as one approach to investigating this we have studied activin and follistatin expression during early mouse development. Activins are homo- or heterodimers of the βA or βB subunits of inhibin, itself a heterodimer consisting of one of the β subunits together with an α subunit. Follistatin is a single-chain polypeptide which inhibits activin function.

Expression of the inhibin α chain could not be detected in embryonic or extraembryonic tissues at any of the stages studied (5.5 to 8.5 days) and expression of the βA and βB subunits could only be observed in the deciduum in cells surrounding the embryo. Expression of follistatin could also be detected in the deciduum, but in a pattern complementary to that of the β subunits. Embryonic expression of follistatin first occurred in the primitive streak, and at later stages transcripts were detectable in the somites and in rhombomeres 2, 4 and 6 of the hindbrain. These results are consistent with a role for activin in mesoderm formation in the mouse embryo, and suggest functions for follistatin in addition to its role as an inhibitor of activin.
Antagonistic activities of Suppressor of Hairless and Hairless control alternative cell fates in the Drosophila adult epidermis
Authors: Schweisguth François , Posakony James W.
Published: April 2021

Successive alternative cell fate choices in the imaginal disc epithelium lead to the differentiation of a relatively invariant pattern of multicellular adult sensory organs in Drosophila. We show here that the activity of Suppressor of Hairless is required for both the sensory organ precursor (SOP) versus epidermal cell fate decision, and for the trichogen (shaft) versus tormogen (socket) cell fate choice. Complete loss of Suppressor of Hairless function causes most proneural cluster cells to accumulate high levels of the achaete and Delta proteins and to adopt the SOP fate. Late or partial reduction in Suppressor of Hairless activity leads to the apparent transformation of the tormogen (socket) cell into a second trichogen (shaft) cell, producing a ‘double shaft’ phenotype. We find that overexpression of Suppres-sor of Hairless has the opposite phenotypic effects. SOP determination is prevented by an early excess of Suppres-sor of Hairless activity, while at a later stage, the trichogen (shaft) cell is transformed into a second tormogen (socket) cell, resulting in ‘double socket’ bristles. We conclude that, for two different cell fate decisions in adult sensory organ development, decreasing or increasing the level of Sup-pressor of Hairless function confers mutant phenotypes that closely resemble those associated with gain and loss of Hairless activity, respectively. These results, along with the intermediate SOP phenotype observed in Suppressor of Hairless; Hairless double mutant imaginal discs, suggest that the two genes act antagonistically to commit imaginal disc cells stably to alternative fates.
The ventral nervous system defective gene controls proneural gene expression at two distinct steps during neuroblast formation in Drosophila
Authors: Skeath James B. , Panganiban Grace F. , Carroll Sean B.
Published: April 2021

Within the Drosophila embryo, the formation of many neuroblasts depends on the functions of the proneural genes of the achaete-scute complex (AS-C): achaete (ac), scute (sc) and lethal of scute (l’sc), and the gene ventral nervous system defective (vnd). Here, we show that vnd controls neuroblast formation, in part, through its regulation of the proneural genes of the AS-C. vnd is absolutely required to activate ac, sc and l’sc gene expression in proneural clusters in specific domains along the medial column of the earliest arising neuroblasts. Using ac-lacZ reporter constructs, we determined that vnd controls proneural gene expression at two distinct steps during neuroblast formation through separable regulatory regions. First, vnd is required to activate proneural cluster formation within the medial column of every other neuroblast row through regulatory elements located 3′ to ac; second, through a 5′ regulatory region, vnd functions to increase or maintain proneural gene expression in the cell within the proneural cluster that normally becomes the neuroblast. By following neuroblast segregation in vnd mutant embryos, we show that the neuroectoderm forms normally and that the defects in neuroblast formation are specific to particular proneural clusters.
The survival of NGF-dependent but not BDNF-dependent cranial sensory neurons is promoted by several different neurotrophins early in their development
Authors: Buj-Bello Anna , Pinon Luzia G. P. , Davies Alun M.
Published: April 2021

Recent work has shown that the survival of the nerve growth factor (NGF)-dependent trigeminal ganglion neurons of the mouse embryo is promoted by brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) during the early stages of target field innervation (Buchman and Davies, (1993)Development, 118, 989-1001). The present study was undertaken to ascertain if responsiveness to multiple neurotrophins is a universal feature of the early stages of neuronal development or is restricted to only certain kinds of neurons. To address this issue, we took advantage of the accessibility, from an early developmental stage, of several populations of cranial sensory neurons in the chicken embryo that depend for survival on just one or two known neurotrophins during the phase of naturally occurring cell death. During the mid-embryonic period (E10 to E12) when the number of sensory neurons is declining due to naturally occurring neuronal death, the neurons of the jugular ganglion and the dorsomedial part of the trigeminal ganglion (DMTG) were supported by NGF, the neurons of the ventrolateral part of the trigeminal ganglion (VLTG) were supported by BDNF and the nodose ganglion contained a major subset of neurons supported by BDNF and a minor subset supported by NT-3. Earlier in development (E6), the survival of DMTG and jugular neurons was additionally promoted by BDNF and NT-3. In contrast, E6 VLTG neurons did not exhibit a survival response to either NGF or NT-3, and E6 nodose neurons did not exhibit a survival response to NGF. The loss of the early survival response of DMTG and jugular neurons to BDNF and NT-3 was due to a marked shift in the doseresponse to these neurotrophins (by more than four orders of magnitude between E6 and E8). These findings indicate that the responsiveness of certain populations of neurons to neurotrophins becomes restricted during development and raise the possibility that neuron-target interactions involving multiple neurotrophins regulate neuronal survival in vivo.
Gap gene properties of the pair-rule gene runt during Drosophila segmentation
Authors: Tsai Chihcheng , Gergen J. Peter
Published: April 2021

The Drosophila Runt protein is a member of a new family of transcriptional regulators that have important roles in processes extending from pattern formation in insect embryos to leukemogenesis in humans. We used ectopic expression to investigate runt’s function in the pathway of Drosophila segmentation. Transient over-expression of runt under the control of a Drosophila heat-shock promoter caused stripe-specific defects in the expression patterns of the pair-rule genes hairy and even-skippedbut had a more uniform effect on the secondary pair-rule gene fushi tarazu. Surprisingly, the expression of the gap segmentation genes, which are upstream of runt in the segmentation hierarchy was also altered in hs/runt embryos. A subset of these effects were interpreted as due to an antagonistic effect of runt on transcriptional activation by the maternal morphogen bicoid. In support of this, expression of synthetic reporter gene constructs containing oligomerized binding sites for the Bicoid protein was reduced in hs/runt embryos. Finally, genetic experiments demonstrated that regulation of gap gene expression by runt is a normal component of the regulatory program that generates the segmented body pattern of the Drosophila embryo.
Developmental potential of trunk neural crest cells in the mouse
Authors: Serbedzija George N. , Bronner-Fraser Marianne , Fraser Scott E.
Published: April 2021

The availability of naturally occurring and engineered mutations in mice which affect the neural crest makes the mouse embryo an important experimental system for studying neural crest cell differentiation. Here, we determine the normal developmental potential of neural crest cells by performing in situ cell lineage analysis in the mouse by microinjecting lysinated rhodamine dextran (LRD) into individual dorsal neural tube cells in the trunk. Labeled progeny derived from single cells were found in the neural tube, dorsal root ganglia, sympathoadrenal derivatives, presumptive Schwann cells and/or pigment cells. Most embryos contained labeled cells both in the neural tube and at least one neural crest derivative, and numerous clones contributed to multiple neural crest derivatives. The time of injection influenced the derivatives populated by the labeled cells. Injections at early stages of migration yielded labeled progeny in both dorsal and ventral neural crest derivatives, whereas those performed at later stages had labeled cells only in more dorsal neural crest derivatives, such as dorsal root ganglion and presumptive pigment cells. The results suggest that in the mouse embryo: (1) there is a common precursor for neural crest and neural tube cells; (2) some neural crest cells are multipotent; and (3) the timing of emigration influences the range of possible neural crest derivatives.
The fates of the blastomeres of the 16-cell zebrafish embryo
Authors: Strehlow David , Heinrich Gerhard , Gilbert Walter
Published: April 2021

We present a fate map for the 16-cell-stage blastomeres of the zebrafish embryo Brachydanio rerio. We injected high molecular weight fluorescent dextran into cleavage-stage cells to observe the contributions of the descendants of the first 16 cells to the adult. The patterns derived from these early cells are similar, but not identical among different embryos. Furthermore, two-color injections showed that sister blastomeres at the 16-cell stage regularly contribute to different sets of adult structures. A few of the scored tissues could not be mapped in a manner consistent with the predicted axes. Other tissues were mapped to several of the 16 blastomeres. Many of the tissues map with high probability to a few 16-cell-stage blastomeres. Thus, based on 112 injections in 56 different embryos, we have constructed a fate map by assigning probabilities for the contribution of each blastomere to each of 31 tissues in the 26-hour embryo.
The spatial and temporal dynamics of Sax1(CHox3) homeobox gene expression in the chick’s spinal cord
Authors: Spann Perah , Ginsburg Malka , Rangini Zehava , Fainsod Abraham , Eyal-Giladi Hefzibah , Gruenbaum Yosef
Published: April 2021

Sax1 (previously CHox3) is a chicken homeobox gene belonging to the same homeobox gene family as the Drosophila NK1 and the honeybee HHO genes. Sax1 transcripts are present from stage 2 H&H until at least 5 days of embryonic development. However, specific localization of Sax1 transcripts could not be detected by in situ hybridization prior to stage 8−, when Sax1 transcripts are specifically localized in the neural plate, posterior to the hindbrain. From stages 8− to 15 H&H, Sax1 continues to be expressed only in the spinal part of the neural plate. The anterior border of Sax1 expression was found to be always in the transverse plane separating the youngest somite from the yet unsegmented mesodermal plate and to regress with similar dynamics to that of the segregation of the somites from the mesodermal plate. The posterior border of Sax1 expression coincides with the posterior end of the neural plate. In order to study a possible regulation of Sax1 expression by its neighboring tissues, several embryonic manipulation experiments were performed. These manipulations included: removal of somites, mesodermal plate or notochord and transplantation of a young ectopic notochord in the vicinity of the neural plate or transplantation of neural plate sections into the extraembryonic area. The results of these experiments revealed that the induction of the neural plate by the mesoderm has already occurred in full primitive streak embryos, after which Sax1 is autonomously regulated within the spinal part of the neural plate.
An axial domain of HOM/Hox gene expression is formed by morphogenetic alignment of independently specified cell lineages in the leech Helobdella
Authors: Nardelli-Haefliger Denise , Bruce Ashley E. E. , Shankland Marty
Published: April 2021

The homeobox gene Lox2, a member of the HOM/Hox gene class, is expressed in a restricted domain along the anteroposterior (A-P) body axis of the leech Helobdella. The segmental tissues of the leech embryo arise from the parallel merger of five distinct and bilaterally paired cell lineages generated by embryonic stem cells or teloblasts. Injection of cell lineage tracers coupled with anti-LOX2 immunochemistry reveals that all five teloblast lineages generate central nervous system neurons that express the LOX2 protein, and that each lineage expresses LOX2 within a similar domain of body segments. Some lineally identified neurons display anti-LOX2 immunoreactivity over the entire expression domain, but the OM7 neuron has a distinctively high level of LOX2 expression, which is restricted to the seventh midbody ganglion.

To ascertain the role of positional information in the axial patterning of LOX2 expression, we performed focal cell ablations that displaced one or another of the teloblast lineages out of segmental register with the other axial tissues. Such displacements brought about a corresponding shift in the LOX2 expression of the perturbed lineage, and had little or no effect on the LOX2 expression of the other, unperturbed lineages. This result indicates that the axial domain of LOX2 expression is not specified by positional cues acting coordinately across the various teloblast lineages, nor would it seem that the expression domain is imprinted from one lineage to the others. Rather, the different teloblast lineages acquire their axial patterns independently, and secondarily bring these patterns into alignment along the A-P axis through a process of morphogenetic assembly.
A distal tyrosinase upstream element stimulates gene expression in neural-crest-derived melanocytes of transgenic mice: position-independent and mosaic expression
Authors: Porter Susan D. , Meyer Cathy J.
Published: April 2021

We have assessed the importance of a melanocyte-specific DNase I hypersensitive site and matrix attachment region situated 15 kb upstream of the mouse tyrosinase gene by analysis in transgenic mice. Transgenes containing all, part, or none of this region linked to the tyrosinase promoter and human tyrosinase cDNA were introduced into genetically albino mice, and pigmentation and transgene message levels were analyzed in the resulting transgenic lines. The effect of the upstream region was to enhance significantly gene expression in melanocytes, and to provide position-independent expression of the transgene. Two exceptions to complete position indepen-dence were seen; these lines displayed a mosaic expression pattern in which the transgene was expressed fully in some melanocyte clones but less so in others, resulting in trans-verse stripes of colours ranging from near white to dark grey. Unexpectedly, pigmentation in the eye of all trans-genic lines containing the upstream region was non-uniform, in that the neural-crest-derived melanocytes of the choroid and anterior iris contained significantly more pigment than those derived from the optic cup (retinal pigment epithelium and posterior iris). Transgenes con-taining a small part or none of the upstream region were expressed poorly and in a position-dependent manner; of those lines that were visibly pigmented, expression was equal in the neural crest and optic-cup-derived cells of the eye. Mice with transgenes containing DNA sequences encompassing the hypersensitive site but lacking most of the matrix attachment region were, on average, poorer expressors than those containing the entire upstream region; the highest expressing line of this series, however, had a pigmentation level similar to the lines with the full upstream region. Thus, full transcriptional enhancement activity may lie within the segment containing the hyper-sensitive site, but position-independent expression may require the flanking matrix attachment region containing sequences.
The Caenorhabditis elegans NK-2 class homeoprotein CEH-22 is involved in combinatorial activation of gene expression in pharyngeal muscle
Authors: Okkema Peter G. , Fire Andrew
Published: April 2021

The pharyngeal muscles of Caenorhabditis elegans are single sarcomere muscles used for feeding. Like vertebrate cardiac and smooth muscles, C. elegans pharyngeal muscle does not express any of the known members of the MyoD family of myogenic factors. To identify mechanisms regulating gene expression in this tissue, we have characterized a pharyngeal muscle-specific enhancer from myo-2, a myosin heavy chain gene expressed exclusively in pharyngeal muscle. Assaying enhancer function in transgenic animals, we identified three subelements, designated A, B and C, that contribute to myo-2 enhancer activity. These subelements are individually inactive; however, any combination of two or more subelements forms a functional enhancer. The B and C subelements have distinct cell type specificities. A duplication of B activates transcription in a subset of pharyngeal muscles (m3, m4, m5 and m7). A duplication of C activates transcription in all pharyngeal cells, muscle and non-muscle. Thus, the activity of the myo-2 enhancer is regulated by a combination of pharyngeal muscle-type-specific and organ-specific signals. Screening a cDNA expression library, we identified a gene encoding an NK-2 class homeodomain protein, CEH-22, that specifically binds a site necessary for activity of the B subelement. CEH-22 protein is first expressed prior to myogenic differentiation and is present in the same subset of pharyngeal muscles in which B is active. Expression continues throughout embryonic and larval development. This expression pattern suggests CEH-22 plays a key role in pharyngeal muscle-specific activity of the myo-2 enhancer.
Wnt-1-dependent regulation of local E-cadherin and αN-catenin expression in the embryonic mouse brain
Authors: Shimamura Kenji , Hirano Shinji , McMahon Andrew P. , Takeichi Masatoshi
Published: April 2021

E-cadherin is transiently expressed in local regions of the embryonic mouse brain, which include several patchy areas on the mesencephalon and diencephalon and their roof plate and part of cerebellar rudiments. In the present study, we compared this E-cadherin expression with that of Wnt-1, which occurs in specific zones in the embryonic brain, and found certain spatiotemporal relations between them: Wnt-1 expression tended to run parallel or overlap with peripheries of the E-cadherin-positive areas. For example, in the dorsal midline, Wnt-1 was expressed at the middle of the roof plate, while E-cadherin was absent in the middle zone but detected in two arrays of marginal roof plate cells. Furthermore, αN-catenin, a cadherin-associated protein, was found to occur at the roof plate of the mesencephalon and diencephalon, coinciding with Wnt-1 expression. The expression of these molecules was then studied in two alleles of the Wnt-1 mutation, Wnt-1sw and Wnt-1neo. In mice homozygous for these mutant genes, E-cadherin expression in the roof plate was up-regulated; the middle E-cadherin-negative zone disappeared. Moreover, E-cadherin expression in the roof plate began earlier in the mutant mice than in wild-type mice. On the contrary, αN-catenin expression in the dorsal midline was suppressed in these mutants. These changes in cadherin and catenin expression occurred at the level of mRNA expression. These results suggest that the Wnt-1 signal is, either directly or indirectly, involved in the regulation of expression of E-cadherin and αN-catenin in restricted regions of the embryonic brain. This mechanism may contribute to the patterning of the expression of these adhesion-related proteins in the embryonic brain.
Exposure to retinoic acid before or after the onset of somitogenesis reveals separate effects on rhombomeric segmentation and 3′ HoxB gene expression domains
Authors: Wood Heather , Pall Gurman , Morriss-Kay Gillian
Published: April 2021

We have compared the relationship between the patterns of altered morphogenesis and of altered gene expression in mouse embryos exposed to excess retinoic acid (RA) (a) just before and (b) just after the onset of somitic segmentation (day 7.75 to day 8.25). Exposure to RA prior to the onset of somitic segmentation results in suppression of rhombomeric (but not somitic) segmentation, and conversion of the genetic identity of the whole preotic hindbrain to that of rhombomere 4. In contrast, exposure to RA at early somite stages results in near-normal rhombomeric segmentation; rhombomeric gene expression domains indicate that only rhombomere 2 has changed its genetic identity to that of rhombomere 4, the other preotic segments showing normal expression patterns for HoxB genes and Krox-20. The results indicate that RA has separable effects (1) on the genes mediating the process of rhombomeric segmentation per se, such as Krox-20, and (2) on the genes that influence the nature of the structures that subsequently develop from the individual rhombomeres, such as the Hox genes.
Two maternal genes, apx-1 and pie-1, are required to distinguish the fates of equivalent blastomeres in the early Caenorhabditis elegans embryo
Authors: Mango Susan E. , Thorpe Christopher J. , Martin Paula R. , Chamberlain Stephen H. , Bowerman Bruce
Published: April 2021

In a 4-cell Caenorhabditis elegans embryo, two sister blas- tomeres called ABa and ABp are born with equivalent developmental potential, but eventually produce distinct patterns of cell fate. The different fates of ABa and ABp are specified at least in part by inductive interactions with neighboring blastomeres. Previous studies indicate that, at the 4-cell stage, a signal from the posterior-most blas- tomere, P2, is required for ABp to produce at least one of its unique cell types. This P2/ABp interaction depends on glp-1, a putative receptor for intercellular interactions.

To investigate this early induction further, we isolated mutants in which ABp developed abnormally. We describe the effects of recessive mutations in apx-1, a maternal gene that appears to be required for P2 to signal ABp. In embryos from mothers homozygous for mutations in apx- 1 (apx-1 embryos), ABp fails to produce its characteristic cell types. Instead, ABp from apx-1 embryos develops more like its sister ABa: it produces ABa-like pharyngeal cells and it recapitulates ABa-like cell lineages. Because mutations in apx-1 affect the development of only the ABp blastomere, we suggest that the wild-type gene encodes a component of the P2/ABp signalling pathway. To explain the observation that ABp in apx-1 embryos adopts an ABa- like fate, we propose a model in which the P2 signal is required to break the initial equivalence of ABa and ABp. We performed two independent tests of this model. First, we examined ABp development in pie-1 mutant embryos, in which P2 adopts the identity of another blastomere. We find that, in pie-1 embryos, ABp fails to produce its char- acteristic cell types and instead adopts a fate similar to that of ABa. We conclude that the changed identity of P2 in pie- 1 embryos prevents the P2/ABp interaction. As a second test, we examined ABp development in wild-type embryos after physically removing P2. These operated embryos produce extra pharyngeal cells, consistent with our proposal that a signal from P2 breaks the initially equiva- lent developmental state of ABa and ABp. We discuss the possibility that apx-1 acts as a ligand in this glp-1- dependent signalling pathway.
marbles mutants: uncoupling cell determination and nuclear migration in the developing Drosophila eye
Authors: Fischer-Vize Janice A. , Mosley Kathleen L.
Published: April 2021

Morphogenesis of a multicellular structure requires not only that cells are specified to express particular gene products, but also that cells move to adopt characteristic shapes and positions. Little is known about how these two aspects of morphogenesis are coordinated. The developing Drosophila compound eye is a monolayer, in which cells are suspended between apical and basal membranes and assemble sequentially into hundreds of unit eyes, or facets, guided by a series of cell interactions. As cells are determined to join the facet, their nuclei and cell bodies rise apically and then settle into position in the cell group. The final nuclear positions determine the shape of the individual cells. We have identified a Drosophila gene called marbles which is required for the apical nuclear migrations that accompany cell determination during eye development. In marbles mutant eyes, the sequence of cell specification that leads to the formation of facets occurs almost normally despite the failure of nuclear migration in many cells. The marbles mutant phenotype reveals that during Drosophila eye development cell determination does not require nuclear migration.
sidecar pollen, an Arabidopsis thaliana male gametophytic mutant with aberrant cell divisions during pollen development
Authors: Chen Yun-Chia Sophia , McCormick Sheila
Published: April 2021

During pollen development each product of meiosis undergoes a stereotypical pattern of cell divisions to give rise to a three-celled gametophyte, the pollen grain. First an asymmetric mitosis generates a larger vegetative cell and a smaller generative cell, then the generative cell undergoes a second mitosis to give rise to two sperm cells. It is unknown how this pattern of cell divisions is controlled. We have identified an Arabidopsis gene, SIDECAR POLLEN, which is required for the normal cell division pattern during pollen development. In the genetic background of the NoO ecotype, sidecar pollen heterozygotes have about 45% wild-type pollen, 48% aborted pollen and 7% pollen with an extra cell. Homozygous sidecar pollen plants have about 20% wild-type pollen, 53% aborted pollen and 27% extra-celled pollen. Similar ratios of sidecar pollen phenotypes are seen in the Columbia ecotype but sidecar pollen is a gametophytic lethal in the Landsberg erecta ecotype. Thus this allele of sidecar pollen shows differential gametophytic penetrance and variable expressivity in different genetic backgrounds. The extra cell has the cell identity of a vegetative cell and is produced prior to any asymmetric microspore mitosis. Pollen tetrad analysis directly demonstrates that SIDECAR POLLEN is indeed expressed in male gametophytes. To our knowledge, scp is the first male gametophytic mutation to be described in Arabidopsis.
A Xenopus nodal-related gene that acts in synergy with noggin to induce complete secondary axis and notochord formation
Authors: Lustig K. D. , Kroll K. , Sun E. , Ramos R. , Elmendorf H. , Kirschner M. W.
Published: April 2021

Using a paracrine assay to screen for signaling proteins that could respecify ectodermal tissue, we isolated a Xenopus gene related to the mouse gene nodal, a member of the TGFβ superfamily. The gene is expressed in three regions in the early Xenopus embryo: first in the gastrula organizer, then in two stripes of cells flanking the posterior notochord in late neurulae, and finally in lateral plate mesoderm restricted to the left side of tailbud-stage embryos. Ectopic expression of the gene induces muscle formation in ectodermal explants and partial secondary axes in whole embryos. Together with noggin, another secreted protein also present in the organizer, it induces notochord formation in ectodermal explants and complete secondary axes in whole embryos. These results suggest that the nodal-related gene may act together with noggin to induce axial pattern during gastrulation and also may play a role in left-right asymmetry generation in the post-gastrula embryo.
Nerve dependency of regeneration: the role of Distal-less and FGF signaling in amphibian limb regeneration
Authors: Mullen Lina M. , Bryant Susan V. , Torok Maureen A. , Blumberg Bruce , Gardiner David M.
Published: April 2021

Dlx-3, a homolog of Drosophila Dll, has been isolated from an axolotl blastema cDNA library, and its expression in developing and regenerating limbs characterized. The normal expression pattern, and the changes that occur during experimental treatments, indicate a correlation between Dlx-3 expression and the establishment of the outgrowth-permitting epidermis. Dlx-3 is expressed at high levels in a distal-to-proximal gradient in the epidermis of developing limb buds, and is upregulated in the apical ectodermal cap (AEC) during limb regeneration. Expression is maximal at the late bud stage of regeneration, coincident with the transition from the early phase of nerve dependency to the later phase of nerve independence. Dlx-3 expression in the epidermis is rapidly downregulated by denervation during the nerve-dependent phase and is unaffected by denervation during the nerve-independent phase. We investigated this relationship between nerves and Dlx-3 expression by implanting FGF-2 beads into regenerates that had been denervated at a nerve-dependent stage. Dlx-3 expression was maintained by FGF-2 after denervation, and regeneration progressed to completion. In addition, we detected FGF-2 protein in the AEC and in nerves, and observed that the level of expression in both tissues decreases dramatically in response to denervation. We conclude that both limb development and regeneration require a permissive epidermis, characterized by Dlx-3 and FGF expression, both of which are maintained by FGF through an autocrine loop. The transformation of the limb epidermis into a functional AEC that produces and responds to FGF autocatalytically, is presumed to be induced by FGF. Since nerves appear to be a source of this priming FGF, it is possible that a member of the FGF family of growth factors is the elusive neurotrophic factor of limb regeneration.
RtoA links initial cell type choice to the cell cycle in Dictyostelium
Authors: Wood Salli A. , Ammann Robin R. , Brock Debra A. , Li Lily , Spann Timothy , Gomer Richard H.
Published: April 2021

In Dictyostelium, initial cell type choice is correlated with the cell-cycle phase of the cell at the time of starvation. We have isolated a mutant, ratioA (rtoA), with a defect in this mechanism that results in an abnormally high percentage of prestalk cells. The rtoA gene has been cloned and sequenced and codes for a novel protein. The cell cycle is normal in rtoA. In the wild type, prestalk cells differentiate from those cells in S or early G2 phase at starvation and prespore cells from cells in late G2 or M phase at starvation. In rtoA mutants, both prestalk and prespore cells originate randomly from cells in any phase of the cell cycle at starvation.
Cadherin-mediated cell adhesion and cell motility in Drosophila trachea regulated by the transcription factor Escargot
Authors: Tanaka-Matakatsu Miho , Uemura Tadashi , Oda Hiroki , Takeichi Masatoshi , Hayashi Shigeo
Published: April 2021

Coordination of cell motility and adhesion is essential for concerted movement of tissues during animal morphogenesis. The Drosophila tracheal network is formed by branching, migration and fusion of tubular ectodermal epithelia. Tracheal tip cells, located at the end of each branch that is going to fuse, extend filopodia to search for targets and later change their cell shape to a seamless ring to allow passage of lumen. The cell adhesion molecule DE-cadherin accumulates at the site of contact to form a ring that marks the site of lumen entry and is essential for the fusion. DE-cadherin expression in tip cells of a subset of branches is dependent on escargot, a zinc finger gene expressed in all tip cells. Such escargot mutant tip cells failed to adhere to each other and continued to search for alternative targets by extending long filopodia. We present evidence indicating escargot positively regulates transcription of the DE-cadherin gene, shotgun. Overexpression of DE-cadherin rescued the defect in one of the fusion points in escargot mutants, demonstrating an essential role of DE-cadherin in target recognition and identifying escargot as a key regulator of cell adhesion and motility in tracheal morphogenesis.
Targeted disruption of the Hoxb-2 locus in mice interferes with expression of Hoxb-1 and Hoxb-4
Authors: Barrow Jeffery R. , Capecchi Mario R.
Published: April 2021

Mice with a disruption in the hoxb-2 locus were generated by gene targeting. 75% of the hoxb-2 mutant homozygotes died within 24 hours of birth. While a majority of these mice had severe sternal defects that compromised their ability to breathe, some had relatively normal sternum morphology, suggesting that one or more additional factor(s) contributed to neonatal lethality. At 3-3.5 weeks of age, half of the remaining hoxb-2 homozygotes became weak and subsequently died. All of the mutants that survived to 3 weeks of age showed marked facial paralysis similar to, but more severe than, that reported for hoxb-1 mutant homozygotes (Goddard, J. M., Rossel, M., Manley, N. R. and Capecchi, M. R. (1996) Development 122, 3217-3228). As for the hoxb-1 mutations, the facial paralysis observed in mice homozygous for the hoxb-2 mutation results from a failure to form the somatic motor component of the VIIth (facial) nerve which controls the muscles of facial expression. Features of this phenotype closely resemble the clinical signs associated with Bell’s Palsy and Moebius Syndrome in humans. The sternal defects seen in hoxb-2 mutant mice are similar to those previously reported for hoxb-4 mutant mice (Ramirez-Solis, R., Zheng, H., Whiting, J., Krumlauf, R. and Bradley. A. (1993) Cell 73, 279-294). The above results suggest that the hoxb-2 mutant phenotype may result in part from effects of the hoxb-2 mutation on the expression of both hoxb-1 and hoxb-4. Consistent with this proposal, we found that the hoxb-2 mutation disrupts the expression of hoxb-1 in cis. In addition, the hoxb-2 mutation changes the expression of hoxb-4 and the hoxb-4 mutation, in turn, alters the pattern of hoxb-2 expression. Hoxb-2 and hoxb-4 appear to function together to mediate proper closure of the ventral thoracic body wall. Failure in this closure results in severe defects of the sternum.
Hikaru genki protein is secreted into synaptic clefts from an early stage of synapse formation in Drosophila
Authors: Hoshino Mikio , Suzuki Emiko , Nabeshima Yo-ichi , Hama Chihiro
Published: April 2021

The development of neural circuits is regulated by a large number of factors that are localized at distinct neural sites. We report here the localization of one of these factors, hikaru genki (hig) protein, at synaptic clefts in the pupal and adult nervous systems of Drosophila. In hig mutants, unusually frequent bursting activity of the muscles and abnormal motor behavior during the adult stage suggest the misfunction of neuromuscular circuitry. Our immunohistochemical analyses revealed that hig protein, produced by neurons, is secreted from the presynaptic terminals into the spaces between the presynaptic and postsynaptic terminals. In addition, we have found that the localization of this protein in the synaptic spaces temporally correlates with its functional requirement during a critical period that occurs in the middle stage of pupal formation, a period when a number of dendrite and axon growth cones meet to form synapses. These findings indicate that hig protein functions in the formation of functional neural circuits from the early stages of synapse formation.
Two Pax-binding sites are required for early embryonic brain expression of an Engrailed-2 transgene
Authors: Song Dong-Li , Chalepakis Georges , Gruss Peter , Joyner Alexandra L.
Published: April 2021

The temporally and spatially restricted expression of the mouse Engrailed (En) genes is essential for development of the midbrain and cerebellum. The regulation of En-2 expression was studied using in vitro protein-DNA binding assays and in vivo expression analysis in transgenic mice to gain insight into the genetic events that lead to regionalization of the developing brain. A minimum En-2 1.0 kb enhancer fragment was defined and found to contain multiple positive and negative regulatory elements that function in concert to establish the early embryonic midhindbrain expression. Furthermore, the mid-hindbrain regulatory sequences were shown to be structurally and functionally conserved in humans.

The mouse paired-box-containing genes Pax-2, Pax-5 and Pax-8 show overlapping expression with the En genes in the developing brain. Significantly, two DNA-binding sites for Pax-2, Pax-5 and Pax-8 proteins were identified in the 1.0 kb En-2 regulatory sequences, and mutation of the binding sites disrupted initiation and maintenance of expression in transgenic mice. These results present strong molecular evidence that the Pax genes are direct upstream regulators of En-2 in the genetic cascade controlling midhindbrain development. These mouse studies, taken together with others in Drosophila and zebrafish on the role of Pax genes in controlling expression of En family members, indicate that a Pax-En genetic pathway has been conserved during evolution.
Mouse preimplantation blastocysts adhere to cells expressing the transmembrane form of heparin-binding EGF-like growth factor
Authors: Raab Gerhard , Kover Karen , Paria Bibhash C. , Dey Sudhansu K. , Ezzell Robert M. , Klagsbrun Michael
Published: April 2021

Previous studies have shown that heparin-binding epidermal growth factor (EGF)-like growth factor (HB-EGF) mRNA is synthesized in the mouse uterine luminal epithelium, temporally, just prior to implantation, and spatially, only at the site of blastocyst apposition (Das, S. K., Wang, X. N., Paria, B. C., Damm, D., Abraham, J. A., Klagsbrun, M., Andrews, G. K. and Dey, S. K. (1994) Development 120, 1071–1083). HB-EGF is synthesized as a transmembrane protein (HB-EGFTM) that can be processed to release the soluble growth factor. An antibody that cross-reacts only with the transmembrane form detected HB-EGFTM in uterine luminal epithelium in a spatial manner similar to that of HB-EGF mRNA. HB-EGFTM is a juxtacrine growth factor that mediates cell-cell contact. To ascertain if HB-EGFTM could be an adhesion factor for blastocysts, a mouse cell line synthesizing human HB-EGFTM was co-cultured with mouse blastocysts. Cells synthesizing HB-EGFTM adhered to day-4 mouse blasto-cysts more extensively than parental cells or cells synthesizing a constituitively secreted form of HB-EGF. Adhesion of cells synthesizing HB-EGFTM to blastocysts was inhibited by excess recombinant HB-EGF but less so by TGF-α. Adhesion was also inhibited by the synthetic peptide P21 corresponding to the HB-EGF heparin binding domain, and by incubating the blastocysts with heparinase. In addition, adhesion to delayed implanting dormant blastocysts, which lack EGF receptor (EGFR), was diminished relative to normal blastocysts. These results suggested that adhesion between blastocysts and cells synthesizing HB-EGFTM was mediated via interactions with both blastocyst EGFR and heparan sulfate proteoglycan (HSPG). It was concluded that HB-EGFTM, which is synthesized exclusively in the luminal epithelium at the site of blastocyst apposition, and which is a juxtacrine adhesion factor for blastocysts, could be one of the mediators of blastocyst adhesion to the uterus in the process of implantation.
The C. elegans gene vab-8 guides posteriorly directed axon outgrowth and cell migration
Authors: Wightman Bruce , Clark Scott G. , Taskar Anna M. , Forrester Wayne C. , Maricq Andres V. , Bargmann Cornelia I. , Garriga Gian
Published: April 2021

The assembly of the nervous system in the nematode C. elegans requires the directed migrations of cells and growth cones along the anteroposterior and dorsoventral body axis. We show here that the gene vab-8 is essential for most posteriorly directed migrations of cells and growth cones. Mutations in vab-8 disrupt fourteen of seventeen posteriorly directed migrations, but only two of seventeen anteriorly directed and dorsoventral migrations. For two types of neurons that extend axons both anteriorly and posteriorly, vab-8 mutations disrupt only the growth of the posteriorly directed axon. vab-8 encodes two genetic activities that function in the guidance of different migrations. Our results suggest that most posteriorly directed cell and growth cone migrations are guided by a common mechanism involving the vab-8 gene.