bluuebunny/crossref_metadata_2025 huggingface dataset:
The allocation of epiblast cells to the embryonic heart and other mesodermal lineages: the role of ingression and tissue movement during gastrulation
Authors: Tam Patrick P. L. , Parameswaran Maala , Kinder Simon J. , Weinberger Ron P.
Published: April 2021

The cardiogenic potency of cells in the epiblast of the early primitive-streak stage (early PS) embryo was tested by heterotopic transplantation. The results of this study show that cells in the anterior and posterior epiblast of the early PS-stage embryos have similar cardiogenic potency, and that they differentiated to heart cells after they were transplanted directly to the heart field of the late PS embryo. That the epiblast cells can acquire a cardiac fate without any prior act of ingression through the primitive streak or movement within the mesoderm suggests that neither morphogenetic event is critical for the specification of the cardiogenic fate. The mesodermal cells that have recently ingressed through the primitive streak can express a broad cell fate that is characteristic of the pre-ingressed cells in the host when they were returned to the epiblast. However, mesoderm cells that have ingressed through the primitive streak did not contribute to the lateral plate mesoderm after transplantation back to the epiblast, implying that some restriction of lineage potency may have occurred during ingression. Early PS stage epiblast cells that were transplanted to the epiblast of the mid PS host embryos colonised the embryonic mesoderm but not the extraembryonic mesoderm. This departure from the normal cell fate indicates that the allocation of epiblast cells to the mesodermal lineages is dependent on the timing of their recruitment to the primitive streak and the morphogenetic options that are available to the ingressing cells at that instance.
Retinoid signaling is required for the establishment of a ZPA and for the expression of Hoxb-8, a mediator of ZPA formation
Authors: Lu Hui-Chen , Revelli Jean-Pierre , Goering Lisa , Thaller Christina , Eichele Gregor
Published: April 2021

We show that retinoid receptor antagonists applied to the presumptive wing region block the formation of a zone of polarizing activity (ZPA). This suggests a direct relationship between retinoid signaling and the establishment of the ZPA. We provide evidence that the Hox gene, Hoxb-8, is a direct target of retinoid signaling since exogenously applied RA rapidly induces this gene in the absence of protein synthesis and, moreover, retinoid receptor antagonists down-regulate Hoxb-8 expression. In addition, we find that, in the lateral plate mesoderm, the domains of Hoxb-8 expression and of polarizing activity are coextensive. Taken together, these findings support the hypothesis that retinoids are required for the establishment of a ZPA, and that retinoids act, at least in part, through Hoxb-8, a gene associated with ZPA formation (Charité et al., 1994).
Requirement for engrailed and invected genes reveals novel regulatory interactions between engrailed/invected, patched, gooseberry and wingless during Drosophila neurogenesis
Authors: Bhat Krishna Moorthi , Schedl Paul
Published: April 2021

During neurogenesis, the transmembrane protein Patched (Ptc) promotes a wingless (wg)-mediated specification of a neuronal precursor cell, NB4-2, by repressing gooseberry (gsb). In this study, novel interactions of these genes with engrailed (en) and invected (inv) during neurogenesis have been uncovered. While in row 4 cells Ptc represses gsb and wg, in row 5 cells en/inv relieve Ptc repression of gsb by a non-autonomous mechanism that does not involve hedgehog (hh). This differential regulation of gsb leads to the specification of NB5-3 and NB4-2 identities to two distinct neuroblasts. The uncoupling of the ptc-gsb regulatory circuit also enables gsb to promote Wg expression in row 5 cells. Our results suggest that the en/inv⟶ptc⟶gsb⟶wg pathway uncovered here and the hh⟶wg are distinct pathways that function to maintain wild-type level of Wg. Our results also indicate that Hh is not the only ligand for Ptc and similarly Ptc is not the only receptor for Hh.
Sequence and embryonic expression of the amphioxus engrailed gene (AmphiEn): the metameric pattern of transcription resembles that of its segment-polarity homolog in Drosophila
Authors: Holland Linda Z. , Kene Mamata , Williams Nic A. , Holland Nicholas D.
Published: April 2021

Vertebrate segmentation has been proposed as an evolutionary inheritance either from some metameric protostome or from a more closely related deuterostome. To address this question, we studied the developmental expression of AmphiEn, the engrailed gene of amphioxus, the closest living invertebrate relative of the vertebrates. In neurula embryos of amphioxus, AmphiEn is expressed along the anteroposterior axis as metameric stripes, each located in the posterior part of a nascent or newly formed segment. This pattern resembles the expression stripes of the segment-polarity gene engrailed, which has a key role in establishing and maintaining the metameres in embryos of Drosophila and other metameric protostomes. Later, amphioxus embryos express AmphiEn in non-metameric patterns — transiently in the embryonic ectoderm and dorsal nerve cord. Nerve cord expression occurs in a few cells approximately midway along the rostrocaudal axis and also in a conspicuous group of anterior cells in the cerebral vesicle at a level previously identified as corresponding to the vertebrate diencephalon. Compared to vertebrate engrailed expression at the midbrain/hindbrain boundary, AmphiEn expression in the cerebral vesicle is relatively late. Thus, it is uncertain whether the cerebral vesicle expression marks the rostral end of the amphioxus hindbrain; if it does, then amphioxus may have little or no homolog of the vertebrate midbrain. The segmental expression of AmphiEn in forming somites suggests that the functions of engrailed homologs in establishing and main- taining a metameric body plan may have arisen only once during animal evolution. If so, the protostomes and deuterostomes probably shared a common segmented ancestor.
5-HT2B receptor-mediated serotonin morphogenetic functions in mouse cranial neural crest and myocardiac cells
Authors: Choi Doo-Sup , Ward Simon J. , Messaddeq Nadia , Launay Jean-Marie , Maroteaux Luc
Published: April 2021

During embryogenesis, serotonin has been reported to be involved in craniofacial and cardiovascular morphogenesis. The detailed molecular mechanisms underlying these functions, however remain unknown. From mouse and human species, we have recently reported the cloning of 5-HT2B receptors which share signal transduction pathways with other 5-HT2 receptor subtypes (5-HT2A and 5-HT2C). In addition to phospholipase C stimulation, it appears that these three subtypes of receptor transduce a common serotonin-induced mitogenic activity, which could be important for cell differentiation and proliferation. We have first investigated the expression of 5-HT2 receptor mRNAs in the mouse embryo. Interestingly, a peak of 5-HT2B receptor mRNA expression was detected 8-9 days postcoitum, whereas there was only low level 5-HT2A and no 5-HT2C receptor mRNA expression at this stage. Expression of this receptor was confirmed by binding assays using a 5-HT2-specific ligand which revealed a peak of binding to membrane preparations from 9 days post-coitum embryos. In addition, whole mount in situ hybridisation and immunohistochemistry on similar stage embryos detected 5-HT2B expression in neural crest cells, heart myocardium and somites. The requirement for functional 5-HT2B receptors between 8 and 9 days postcoitum is supported by culture of embryos exposed to 5-HT2-specific ligands; 5-HT2B high-affinity antagonist such as ritanserin, induced morphological defects in the cephalic region, heart and neural tube. These antagonistic treatments interfere with cranial neural crest cell migration, induce their apoptosis, and are responsible for abnormal sarcomeric organisation of the subepicardial layer and for the absence of the trabecular cell layer in the ventricular myocardium. This report indicates for the first time that 5-HT2B receptors are actively mediating the action of serotonin on embryonic morphogenesis, probably by pre-venting the differentiation of cranial neural crest cells and myocardial precursor cells.
Ethylene provides positional information on cortical cell division but is not involved in Nod factor-induced root hair tip growth in Rhizobium-legume interaction
Authors: Heidstra Renze , Yang Wei Cai , Yalcin Yesim , Peck Scott , Emons AnneMie , Kammen Ab van , Bisseling Ton
Published: April 2021

Nod factors secreted by Rhizobium leguminosarum bv. viciae induce root hair deformation, involving a reinitiation of tip growth, and the formation of nodule primordia in Vicia sativa (vetch). Ethylene is a potent inhibitor of cortical cell division, an effect that can be counteracted by applying silver ions (Ag+) or aminoethoxy-vinylglycine (AVG). In contrast to the inhibitory effect on cortical cell division, ethylene promotes the formation of root hairs (which involves tip growth) in the root epidermis of Arabidopsis. We investigate the possible paradox concerning the action of ethylene, putatively promoting Nod factor induced tip growth whilst, at the same time, inhibiting cortical cell division.

We show, by using the ethylene inhibitors AVG and Ag+, that ethylene has no role in the reinitiation of root hair tip growth induced by Nod factors (root hair deformation) in vetch. However, root hair formation is controlled, at least in part, by ethylene. Furthermore, we show that ACC oxidase, which catalizes the last step in ethylene biosynthesis, is expressed in the cell layers opposite the phloem in that part of the root where nodule primordia are induced upon inoculation with Rhizobium. Therefore, we test whether endogenously produced ethylene provides positional information controlling the site where nodule primordia are formed by determining the position of nodules formed on pea roots grown in the presence of AVG or Ag+.
In situ dissection of the Fab-7 region of the bithorax complex into a chromatin domain boundary and a Polycomb-response element
Authors: Mihaly Jozsef , Hogga Ilham , Gausz Janos , Gyurkovics Henrik , Karch Frangois
Published: April 2021

Parasegmental (PS)-specific expression of the homeotic genes of the bithorax-complex (BX-C) appears to depend upon the subdivision of the complex into a series of functionally independent cis-regulatory domains. Fab-7 is a regulatory element that lies between iab-6 and iab-7 (the PS11- and PS12-specific cis-regulatory domains, respectively). Deletion of Fab-7 causes ectopic expression of iab- 7 in PS11 (where normally only iab-6 is active). Two models have been proposed to account for the dominant Fab-7 phenotype. The first considers that Fab-7 functions as a boundary element that insulates iab-6 and iab-7. The second model envisages that Fab-7 contains a silencer element that keeps iab-7 repressed in parasegments anterior to PS12. Using a P-element inserted in the middle of the Fab-7 region (the blt transposon), we have generated an extensive collection of new Fab-7 mutations that allow us to subdivide Fab-7 into a boundary element and a Polycomb-respond element (PRE). The boundary lies within 1 kb of DNA on the proximal side of the blt trans- poson (towards iab-6). Deletions removing this element alone cause a complex gain- and loss-of-function phenotype in PS11; in some groups of cells, both iab-6 and iab-7 are active, while in others both iab-6 and iab-7 are inactive. Thus, deletion of the boundary allows activating as well as repressing activities to travel between iab-6 and iab-7. We also provide evidences that the boundary region contains an enhancer blocker element. The Polycomb- response element lies within 0.5 kb of DNA immediately distal to the boundary (towards iab-7). Deletions removing the PRE alone do not typically cause any visible phenotype as homozygotes. Interestingly, weak ectopic activation of iab-7 is observed in hemizygous PRE deletions, suggesting that the mechanisms that keep iab-7 repressed in the absence of this element may depend upon chromosome pairing. These results help to reconcile the previously contradictory models on Fab-7 function and to shed light on how a chromatin domain boundary and a nearby PRE concur in the setting up of the appropriate PS-specific expression of the Abd-B gene of the BX-C.
Atm deficiency results in severe meiotic disruption as early as leptonema of prophase I
Authors: Barlow C. , Liyanage M. , Moens P.B. , Tarsounas M. , Nagashima K. , Brown K. , Rottinghaus S. , Jackson S.P. , Tagle D. , Ried T. , Wynshaw-Boris A.
Published: April 2021

Infertility is a common feature of the human disorder ataxia-telangiectasia and Atm-deficient mice are completely infertile. To gain further insight into the role of ATM in meiosis, we examined meiotic cells in Atm-deficient mice during development. Spermatocyte degeneration begins between postnatal days 8 and 16.5, soon after entry into prophase I of meiosis, while oocytes degenerate late in embryogenesis prior to dictyate arrest. Using electron microscopy and immunolocalization of meiotic proteins in mutant adult spermatocytes, we found that male and female gametogenesis is severely disrupted in Atm-deficient mice as early as leptonema of prophase I, resulting in apoptotic degeneration. A small number of mutant cells progress into later stages of meiosis, but no cells proceed beyond prophase I. ATR, a protein related to ATM, DMC1, a RAD51 family member, and RAD51 are mislocalized to chromatin and have reduced localization to developing synaptonemal complexes in spermatocytes from Atm-deficient mice, suggesting dysregulation of the orderly progression of meiotic events. ATM protein is normally present at high levels primarily in ova cytoplasm of developing ovarian follicles, and in the nucleus of spermatogonia and to a lesser extent in spermatoctyes, but without localization to the synaptonemal complex. We propose a model in which ATM acts to monitor meiosis by participation in the regulation or surveillance of meiotic progression, similar to its role as a monitor of mitotic cell cycle progression.
Eph receptors and ephrins: effectors of morphogenesis
Authors: Holder Nigel , Klein Rüdiger
Published: April 2021

Eph receptor tyrosine kinases and their ligands, the ephrins, appear to lie functionally at the interface between pattern formation and morphogenesis. We review the role of Eph and ephrin signalling in the formation of segmented structures, in the control of axon guidance and cell migration and in the development of the vasculature. We address the question of how the specificity of response is achieved and discuss the specificity of ephrin-Eph interactions and the significance of structural domains in Eph receptors.
Prospero distinguishes sibling cell fate without asymmetric localization in the Drosophila adult external sense organ lineage
Authors: Manning Laurina , Doe Chris Q.
Published: April 2021

The adult external sense organ precursor (SOP) lineage is a model system for studying asymmetric cell division. Adult SOPs divide asymmetrically to produce IIa and IIb daughter cells; IIa generates the external socket (tormogen) and hair (trichogen) cells, while IIb generates the internal neuron and sheath (thecogen) cells. Here we investigate the expression and function of prospero in the adult SOP lineage. Although Prospero is asymmetrically localized in embryonic SOP lineage, this is not observed in the adult SOP lineage: Prospero is first detected in the IIb nucleus and, during IIb division, it is cytoplasmic and inherited by both neuron and sheath cells. Subsequently, Prospero is downregulated in the neuron but maintained in the sheath cell. Loss of prospero function leads to ‘double bristle’ sense organs (reflecting a IIb- to-IIa transformation) or ‘single bristle’ sense organs with abnormal neuronal differentiation (reflecting defective IIb development). Conversely, ectopic prospero expression results in duplicate neurons and sheath cells and a complete absence of hair/socket cells (reflecting a IIa- to-IIb transformation). We conclude that (1) despite the absence of asymmetric protein localization, prospero expression is restricted to the IIb cell but not its IIa sibling, (2) prospero promotes IIb cell fate and inhibits IIa cell fate, and (3) prospero is required for proper axon and dendrite morphology of the neuron derived from the IIb cell. Thus, prospero plays a fundamental role in establishing binary IIa/IIb sibling cell fates without being asymmetrically localized during SOP division. Finally, in contrast to previous studies, we find that the IIb cell divides prior to the IIa cell in the SOP lineage.
Actomyosin contraction of the posterior hemisphere is required for inversion of the Volvox embryo
Authors: Nishii Ichiro , Ogihara Satoshi
Published: April 2021

During inversion of a Volvox embryo, a series of cell shape changes causes the multicellular sheet to bend outward, and propagation of the bend from the anterior to the posterior pole eventually results in an inside-out spherical sheet of cells. We use fluorescent and electron microscopy to study the behavior of the cytoskeleton in cells undergoing shape changes. Microtubules are aligned parallel to the cell’s long axis and become elongated in the bend. Myosin and actin filaments are arrayed perinuclearly before inversion. In inversion, actin and myosin are located in a subnuclear position throughout the uninverted region but this localization is gradually lost towards the bend. Actomyosin inhibitors cause enlargement of the embryo. The bend propagation is inhibited halfway and, as a consequence, the posterior hemisphere remains uninverted. The arrested posterior hemisphere will resume and complete inversion even in the presence of an actomyosin inhibitor if the anterior hemisphere is removed microsurgically. We conclude that the principal role of actomyosin in inversion is to cause a compaction of the posterior hemisphere; unless the equatorial diameter of the embryo is reduced in this manner, it is too large to pass through the opening defined by the already-inverted anterior hemisphere.
The smad5 mutation somitabun blocks Bmp2b signaling during early dorsoventral patterning of the zebrafish embryo
Authors: Hild Marc , Dick Alexander , Rauch Gerd-Jörg , Meier Andrea , Bouwmeester Tewis , Haffter Pascal , Hammerschmidt Matthias
Published: April 2021

Signaling by members of the TGFβ superfamily is thought to be transduced by Smad proteins. Here, we describe a zebrafish mutant in smad5, designated somitabun (sbn). The dominant maternal and zygotic effect of the sbntc24 mutation is caused by a change in a single amino acid in the L3 loop of Smad5 protein which transforms Smad5 into an antimorphic version, inhibiting wild-type Smad5 and related Smad proteins. sbn mutant embryos are strongly dorsalized, similarly to mutants in Bmp2b, its putative upstream signal. Double mutant analyses and RNA injection experiments show that sbn and bmp2b interact and that sbn acts downstream of Bmp2b signaling to mediate Bmp2b autoregulation during early dorsoventral (D-V) pattern formation. Comparison of early marker gene expression patterns, chimera analyses and rescue experiments involving temporally controlled misexpression of bmp or smad in mutant embryos reveal three phases of D-V patterning: an early sbn- and bmp2b-independent phase when a coarse initial D-V pattern is set up, an intermediate sbn- and bmp2b-dependent phase during which the putative morphogenetic Bmp2/4 gradient is established, and a later sbn-independent phase during gastrulation when the Bmp2/4 gradient is interpreted and cell fates are specified.
Expression of Trk receptors in the developing mouse trigeminal ganglion: in vivo evidence for NT-3 activation of TrkA and TrkB in addition to TrkC
Authors: Huang Eric J. , Wilkinson George A. , Fariñas Isabel , Backus Carey , Zang Keling , and Sharyl L. Wong , Reichardt Louis F.
Published: April 2021

Animals lacking neurotrophin-3 (NT-3) are born with deficits in almost all sensory ganglia. Among these, the trigeminal ganglion is missing 70% of the normal number of neurons, a deficit which develops during the major period of neurogenesis between embryonic stages (E) 10.5 and E13.5. In order to identify the mechanisms for this deficit, we used antisera specific for TrkA, TrkB, and TrkC to characterize and compare the expression patterns of each Trk receptor in trigeminal ganglia of wild type and NT-3 mutants between E10.5 and E15.5. Strikingly, TrkA, TrkB, and TrkC proteins appear to be exclusively associated with neurons, not precursors. While some neurons show limited co-expression of Trk receptors at E11.5, by E13.5 each neuron expresses only one Trk receptor. Neuronal birth dating and cell counts show that in wild-type animals all TrkB- and TrkC-expressing neurons are generated before E11.5, while the majority of TrkA-expressing neurons are generated between E11.5 and E13.5. In mice lacking NT-3, the initial formation of the ganglion, as assessed at E10.5, is similar to that in wild-type animals. At E11.5, however, the number of TrkC-expressing neurons is dramatically reduced and the number of TrkC-immunopositive apoptotic profiles is markedly elevated. By E13.5, TrkC-expressing neurons are virtually eliminated. At E11.5, compared to wild type, the number of TrkB-expressing neurons is also reduced and the number of TrkB immunoreactive apoptotic profiles is increased. TrkA neurons are also reduced in the NT-3 mutants, but the major deficit develops between E12.5 and E13.5 when elevated numbers of TrkA-immunoreactive apoptotic profiles are detected. Normal numbers of TrkA- and TrkB-expressing neurons are seen in a TrkC-deficient mutant. Therefore, our data provide evidence that NT-3 supports the survival of TrkA-, TrkB- and TrkC-expressing neurons in the trigeminal ganglion by activating directly each of these receptors in vivo.
The Caenorhabditis elegans gene ncc-1 encodes a cdc2-related kinase required for M phase in meiotic and mitotic cell divisions, but not for S phase
Authors: Boxem Mike , Srinivasanxs Dayalan G. , van den Heuvel Sander
Published: April 2021

We have identified six protein kinases that belong to the family of cdc2-related kinases in Caenorhabditis elegans. Results from RNA interference experiments indicate that at least one of these kinases is required for cell-cycle progression during meiosis and mitosis. This kinase, encoded by the ncc-1 gene, is closely related to human Cdk1/Cdc2, Cdk2 and Cdk3 and yeast CDC28/cdc2+. We addressed whether ncc-1 acts to promote passage through a single transition or multiple transitions in the cell cycle, analogous to Cdks in vertebrates or yeasts, respectively. We isolated five recessive ncc-1 mutations in a genetic screen for mutants that resemble larval arrested ncc-1(RNAi) animals. Our results indicate that maternal ncc-1 product is sufficient for embryogenesis, and that zygotic expression is required for cell divisions during larval development. Cells that form the postembryonic lineages in wild-type animals do not enter mitosis in ncc-1 mutants, as indicated by lack of chromosome condensation and nuclear envelope breakdown. However, progression through G1 and S phase appears unaffected, as revealed by expression of ribonucleotide reductase, incorporation of BrdU and DNA quantitation. Our results indicate that C. elegans uses multiple Cdks to regulate cell-cycle transitions and that ncc-1 is the C. elegans ortholog of Cdk1/Cdc2 in other metazoans, required for M phase in meiotic as well as mitotic cell cycles.
egl-27 generates anteroposterior patterns of cell fusion in C. elegans by regulating Hox gene expression and Hox protein function
Authors: Ch’ng QueeLim , Kenyon Cynthia
Published: April 2021

Hox genes pattern the fates of the ventral ectodermal Pn.p cells that lie along the anteroposterior (A/P) body axis of C. elegans. In these cells, the Hox genes are expressed in sequential overlapping domains where they control the ability of each Pn.p cell to fuse with the surrounding syncytial epidermis. The activities of Hox proteins are sex-specific in this tissue, resulting in sex-specific patterns of cell fusion: in hermaphrodites, the mid-body cells remain unfused, whereas in males, alternating domains of syncytial and unfused cells develop. We have found that the gene egl-27, which encodes a C. elegans homologue of a chromatin regulatory factor, specifies these patterns by regulating both Hox gene expression and Hox protein function. In egl-27 mutants, the expression domains of Hox genes in these cells are shifted posteriorly, suggesting that egl-27 influences A/P positional information. In addition, egl-27 controls Hox protein function in the Pn.p cells in two ways: in hermaphrodites it inhibits MAB-5 activity, whereas in males it permits a combinatorial interaction between LIN-39 and MAB-5. Thus, by selectively modifying the activities of Hox proteins, egl-27 elaborates a simple Hox expression pattern into complex patterns of cell fates. Taken together, these results implicate egl-27 in the diversification of cell fates along the A/P axis and suggest that chromatin reorganization is necessary for controlling Hox gene expression and Hox protein function.
Loss of Nkx2.1 homeobox gene function results in a ventral to dorsal molecular respecification within the basal telencephalon: evidence for a transformation of the pallidum into the striatum
Authors: Sussel Lori , Marin Oscar , Kimura Shioko , Rubenstein John L. R.
Published: April 2021

The telencephalon is organized into distinct longitudinal domains: the cerebral cortex and the basal ganglia. The basal ganglia primarily consists of a dorsal region (striatum) and a ventral region (pallidum). Within the telencephalon, the anlage of the pallidum expresses the Nkx2.1 homeobox gene. A mouse deficient in Nkx2.1 function does not form pallidal structures, lacks basal forebrain TrkA-positive neurons (probable cholinergic neurons) and has reduced numbers of cortical cells expressing GABA, DLX2 and calbindin that migrate from the pallidum through the striatum and into the cortex. We present evidence that these phenotypes result from a ventral-to-dorsal transformation of the pallidal primordium into a striatal-like anlage.
Proximal to distal cell communication in the Drosophila leg provides a basis for an intercalary mechanism of limb patterning
Authors: Goto Satoshi , Hayashi Shigeo
Published: April 2021

Proximodistal patterning in the Drosophila leg is elaborated from the circular arrangement of the proximal domain expressing escargot and homothorax, and the distal domain expressing Distal-less that are allocated during embryogenesis. The distal domain differentiates multiply segmented distal appendages by activating additional genes such as dachshund. Secreted signaling molecules Wingless and Decapentaplegic, expressed along the anterior-posterior compartment boundary, are required for activation of Distal-less and dachshund and repression of homothorax in the distal domain. However, whether Wingless and Decapentaplegic are sufficient for the circular pattern of gene expression is not known. Here we show that a proximal gene escargot and its activator homothorax regulate proximodistal patterning in the distal domain. Clones of cells expressing escargot or homothorax placed in the distal domain induce intercalary expression of dachshund in surrounding cells and reorient planar cell polarity of those cells. Escargot and homothorax-expressing cells also sort out from other cells in the distal domain. We suggest that inductive cell communication between the proximodistal domains, which is maintained in part by a cell-sorting mechanism, is the cellular basis for an intercalary mechanism of the proximodistal axis patterning of the limb.
P0 and PMP22 mark a multipotent neural crest-derived cell type that displays community effects in response to TGF-β family factors
Authors: Hagedorn Lilian , Suter Ueli , Sommer Lukas
Published: April 2021

Protein zero (P0) and peripheral myelin protein 22 (PMP22) are most prominently expressed by myelinating Schwann cells as components of compact myelin of the peripheral nervous system (PNS), and mutants affecting P0 and PMP22 show severe defects in myelination. Recent expression studies suggest a role of P0 and PMP22 not only in myelination but also during embryonic development. Here we show that, in dorsal root ganglia (DRG) and differentiated neural crest cultures, P0 is expressed in the glial lineage whereas PMP22 is also detectable in neurons. In addition, however, P0 and PMP22 are both expressed in a multipotent cell type isolated from early DRG. Like neural crest stem cells (NCSCs), this P0/PMP22-positive cell gives rise to glia, neurons and smooth-muscle-like cells in response to instructive extracellular cues. In cultures of differentiating neural crest, a similar multipotent cell type can be identified in which expression of P0 and PMP22 precedes the appearance of neural differentiation markers. Intriguingly, this P0/PMP22-positive progenitor exhibits fate restrictions dependent on the cellular context in which it is exposed to environmental signals. While single P0/PMP22-positive progenitor cells can generate smooth muscle in response to factors of the TGF-β family, communities of P0/PMP22-positive cells interpret TGF-β factors differently and produce neurons or undergo increased cell death instead of generating smooth-muscle-like cells. Our data are consistent with a model in which cellular association of postmigratory multipotent progenitors might be involved in the suppression of a non-neural fate in forming peripheral ganglia.
Craniofacial, vestibular and bone defects in mice lacking the Distal-less-related gene Dlx5
Authors: Acampora Dario , Merlo Giorgio R. , Paleari Laura , Zerega Barbara , Postiglione Maria Pia , Mantero Stefano , Bober Eva , Barbieri Ottavia , Simeone Antonio , Levi Giovanni
Published: April 2021

The Dlx5 gene encodes a Distal-less-related DNA-binding homeobox protein first expressed during early embryonic development in anterior regions of the mouse embryo. In later developmental stages, it appears in the branchial arches, the otic and olfactory placodes and their derivatives, in restricted brain regions, in all extending appendages and in all developing bones. We have created a null allele of the mouse Dlx5 gene by replacing exons I and II with the E. coli lacZ gene. Heterozygous mice appear normal. β-galactosidase activity in Dlx5+/− embryos and newborn animals reproduces the known pattern of expression of the gene. Homozygous mutants die shortly after birth with a swollen abdomen. They present a complex phenotype characterised by craniofacial abnormalities affecting derivatives of the first four branchial arches, severe malformations of the vestibular organ, a delayed ossification of the roof of the skull and abnormal osteogenesis. No obvious defect was observed in the patterning of limbs and other appendages. The defects observed in Dlx5−/− mutant animals suggest multiple and independent roles of this gene in the patterning of the branchial arches, in the morphogenesis of the vestibular organ and in osteoblast differentiation.
Dlx5 regulates regional development of the branchial arches and sensory capsules
Authors: Depew Michael J. , Liu Jen Kuei , Long Jason E. , Presley Robert , Meneses Juanito J. , Pedersen Roger A. , Rubenstein John L. R.
Published: April 2021

We report the generation and analysis of mice homozygous for a targeted deletion of the Dlx5 homeobox gene. Dlx5 mutant mice have multiple defects in craniofacial structures, including their ears, noses, mandibles and calvaria, and die shortly after birth. A subset (28%) exhibit exencephaly. Ectodermal expression of Dlx5 is required for the development of olfactory and otic placode-derived epithelia and surrounding capsules. The nasal capsules are hypoplastic (e.g. lacking turbinates) and, in most cases, the right side is more severely affected than the left. Dorsal otic vesicle derivatives (e.g. semicircular canals and endolymphatic duct) and the surrounding capsule, are more severely affected than ventral (cochlear) structures. Dlx5 is also required in mandibular arch ectomesenchyme, as the proximal mandibular arch skeleton is dysmorphic. Dlx5 may control craniofacial development in part through the regulation of the goosecoid homeobox gene. goosecoid expression is greatly reduced in Dlx5 mutants, and both goosecoid and Dlx5 mutants share a number of similar craniofacial malformations. Dlx5 may perform a general role in skeletal differentiation, as exemplified by hypomineralization within the calvaria. The distinct focal defects within the branchial arches of the Dlx1, Dlx2 and Dlx5 mutants, along with the nested expression of their RNAs, support a model in which these genes have both redundant and unique functions in the regulation of regional patterning of the craniofacial ectomesenchyme.
Zebrafish narrowminded suggests a genetic link between formation of neural crest and primary sensory neurons
Authors: Artinger Kristin Bruk , Chitnis Ajay B. , Mercola Mark , Driever Wolfgang
Published: April 2021

In the developing vertebrate nervous system, both neural crest and sensory neurons form at the boundary between non-neural ectoderm and the neural plate. From an in situ hybridization based expression analysis screen, we have identified a novel zebrafish mutation, narrowminded (nrd), which reduces the number of early neural crest cells and eliminates Rohon-Beard (RB) sensory neurons. Mosaic analysis has shown that the mutation acts cell autonomously suggesting that nrd is involved in either the reception or interpretation of signals at the lateral neural plate boundary. Characterization of the mutant phenotype indicates that nrd is required for a primary wave of neural crest cell formation during which progenitors generate both RB sensory neurons and neural crest cells. Moreover, the early deficit in neural crest cells in nrd homozygotes is compensated later in development. Thus, we propose that a later wave can compensate for the loss of early neural crest cells but, interestingly, not the RB sensory neurons. We discuss the implications of these findings for the possibility that RB sensory neurons and neural crest cells share a common evolutionary origin.
Successive patterns of clonal cell dispersion in relation to neuromeric subdivision in the mouse neuroepithelium
Authors: Mathis Luc , Sieur Johan , Voiculescu Octavian , Charnay Patrick , Nicolas Jean-François
Published: April 2021

We made use of the laacz procedure of single-cell labelling to visualize clones labelled before neuromere formation, in 12.5-day mouse embryos. This allowed us to deduce two successive phases of cell dispersion in the formation of the rhombencephalon: an initial anterior-posterior (AP) cell dispersion, followed by an asymmetrical dorsoventral (DV) cell distribution during which AP cell dispersion occurs in territories smaller than one rhombomere. We conclude that the general arrest of AP cell dispersion precedes the onset of morphological segmentation and is not imposed by the interface between adjacent rhombomeres. This demonstrates a major change in the mode of epithelial growth that precedes or accompanies the formation of neuromeres. We also deduced that the period of DV cell dispersion in the neuroepithelium is followed by a coherent growth phase. These results suggest a cell organization on a Cartesian grid, the coordinates of which correspond to the AP and DV axis of the neural tube. A similar sequence of AP cell dispersion followed by an arrest of AP cell dispersion, a preferential DV cell dispersion and then by a coherent neuroepithelial growth, is also observed in the spinal cord and mesencephalon. This demonstrates that a similar cascade of cell events occurs in these different domains of the CNS. In the prosencephalon, differences in spatial constraints may explain the variability in the orientation of cell clusters. Genetic and clonal patterning in the AP and DV dimensions follow the same spatial sequence. An interesting possibility is that these successive patterns of cell growth facilitate the acquisition of positional information.
Targeted ablation of secretin-producing cells in transgenic mice reveals a common differentiation pathway with multiple enteroendocrine cell lineages in the small intestine
Authors: Rindi Guido , Ratineau Christelle , Ronco Anne , Candusso Maria Elena , Tsai Ming-Jer , Leiter Andrew B.
Published: April 2021

The four cell types of gut epithelium, enteroendocrine cells, enterocytes, Paneth cells and goblet cells, arise from a common totipotent stem cell located in the mid portion of the intestinal gland. The secretin-producing (S) cell is one of at least ten cell types belonging to the diffuse neuroendocrine system of the gut. We have examined the developmental relationship between secretin cells and other enteroendocrine cell types by conditional ablation of secretin cells in transgenic mice expressing herpes simplex virus 1 thymidine kinase (HSVTK). Ganciclovir-treated mice showed markedly increased numbers of apoptotic cells at the crypt-villus junction. Unexpectedly, ganciclovir treatment induced nearly complete ablation of enteroendocrine cells expressing cholecystokinin and peptide YY/glucagon (L cells) as well as secretin cells, suggesting a close developmental relationship between these three cell types. In addition, ganciclovir reduced the number of enteroendocrine cells producing gastric inhibitory polypeptide, substance-P, somatostatin and serotonin. During recovery from ganciclovir treatment, the enteroendocrine cells repopulated the intestine in normal numbers, suggesting that a common early endocrine progenitor was spared. Expression of BETA2, a basic helix-loop-helix protein essential for differentiation of secretin and cholecystokinin cells was examined in the proximal small intestine. BETA2 expression was seen in all enteroendocrine cells and not seen in nonendocrine cells. These results suggest that most small intestinal endocrine cells are developmentally related and that a close developmental relationship exists between secretin-producing S cells and cholecystokinin-producing and L type enteroendocrine cells. In addition, our work shows the existence of a multipotent endocrine-committed cell type and locates this hybrid multipotent cell type to a region of the intestine populated by relatively immature cells.
Transplacental delivery of the Wnt antagonist Frzb1 inhibits development of caudal paraxial mesoderm and skeletal myogenesis in mouse embryos
Authors: Borello Ugo , Coletta Marcello , Tajbakhsh Shahragim , Leyns Luc , Robertis Eddy M. De , Buckingham Margaret , Cossu Giulio
Published: April 2021

Axial structures (neural tube/notochord) and surface ectoderm activate myogenesis in the mouse embryo; their action can be reproduced, at least in part, by several molecules such as Sonic hedgehog and Wnts. Recently, soluble Wnt antagonists have been identified. Among those examined only Frzb1 was found to be expressed in the presomitic mesoderm and newly formed somites and thus its possible role in regulating myogenesis was investigated in detail. When presomitic mesoderm or newly formed somites were cultured with axial structures and surface ectoderm on a feeder layer of C3H10T1/2 cells expressing Frzb1, myogenesis was abolished or severely reduced in presomitic mesoderm and the three most recently formed somites. In contrast, no effect was observed on more mature somites. Inhibition of myogenesis did not appear to be associated with increased cell death since the final number of cells in the explants grown in the presence of Frzb1 was only slightly reduced in comparison with controls. In order to examine the possible function of Frzb1 in vivo, we developed a method based on the overexpression of the soluble antagonist by transient transfection of WOP cells with a Frzb1 expression vector and injection of transfected cells into the placenta of pregnant females before the onset of maternofoetal circulation. Frzb1, secreted by WOP cells, accumulated in the embryo and caused a marked reduction in size of caudal structures. Myogenesis was strongly reduced and, in the most severe cases, abolished. This was not due to a generalized toxic effect since only several genes downstream of the Wnt signaling pathway such as En1, Noggin and Myf5 were downregulated; in contrast, Pax3 and Mox1 expression levels were not affected even in embryos exhibiting the most severe phenotypes. Taken together, these results suggest that Wnt signals may act by regulating both myogenic commitment and expansion of committed cells in the mouse mesoderm.
Specificity of TGFβ signaling is conferred by distinct type I receptors and their associated SMAD proteins in Caenorhabditis elegans
Authors: Krishna Srikant , Maduzia Lisa L. , Padgett Richard W.
Published: April 2021

In C. elegans, the TGFβ-like type II receptor daf-4 is required for two distinct signaling pathways. In association with the type I receptor daf-1, it functions in the dauer pathway. In addition, it is also required for body size determination and male tail patterning, roles which do not require daf-1. In an effort to determine how two different signals are transmitted through daf-4, we looked for other potential signaling partners for DAF-4. We have cloned and characterized a novel type I receptor and show that it is encoded by sma-6. Mutations in sma-6 generate the reduced body size (Sma) and abnormal mail tail (Mab) phenotypes identical to those observed in daf-4 and sma-2, sma-3, sma-4 mutants (C. elegans Smads), indicating that they function in a common signaling pathway. However, mutations in sma-6, sma-2, sma-3, or sma-4 do not produce constitutive dauers, which demonstrates that the unique biological functions of daf-4 are mediated by distinct type I receptors functioning in parallel pathways. We propose that the C. elegans model for TGFβ-like signaling, in which distinct type I receptors determine specificity, may be a general mechanism of achieving specificity in other organisms. These findings distinguish between the manner in which signaling specificity is achieved in TGFβ-like pathways and receptor tyrosine-kinase (RTK) pathways.
Mosaic analysis of the dominant mutant,Gnarley1-R, reveals distinct lateral and transverse signaling pathways during maize leaf development
Authors: Foster Toshi , Veit Bruce , Hake Sarah
Published: April 2021

Maize leaves are organized into two major domains along the proximal-distal axis: a broad flat blade at the distal end of the leaf, and a narrow, thickened sheath that encircles the stem. Between the blade and sheath are two wedge-shaped tissues called auricles, and the ligule, an epidermally derived fringe. Members of the Knotted1 (Kn1) family of mutations change the shape and position of both ligule and auricle, thus disturbing the overall pattern of the leaf. Here we present the results of a mosaic analysis of Gnarley1-R (Gn1-R), which like members of the Kn1 family, affects the ligule and auricle. Gn1-R is distinct, however, in altering the dimensions of cells that make up sheath tissue. To gain insight into the Gn1-R phenotype, we performed a mosaic analysis using X-ray induced chromosome breakage to generate wild-type (gn1+/−) sectors in otherwise Gn1-R leaves. These sectors allowed us to determine whether Gn1-R acts non-autonomously to influence adjacent cells. Most aspects of the Gn1-R phenotype, such as ligule position, inhibition of auricle development, and sheath thickness showed autonomy in the lateral dimension (leaf width). In contrast, all aspects of the Gn1-R phenotype were non-autonomous in the transverse dimension (leaf thickness), suggesting that signaling occurs between cell layers in the leaf. These results support a model for distinct signaling pathways along lateral versus transverse axes of a developing leaf.
Glycogen synthase kinase-3 (GSK-3) is regulated during Dictyostelium development via the serpentine receptor cAR3
Authors: Plyte Simon E. , O’Donovan Emma , Woodgett James R. , Harwood Adrian J.
Published: April 2021

Glycogen synthase kinase-3 (GSK-3) is required during metazoan development to mediate the effects of the extracellular signal wingless/Wnt-1 and hence is necessary for correct cell type specification. GSK-3 also regulates cell fate during Dictyostelium development, but in this case it appears to mediate the effects of extracellular cAMP. By direct measurement of GSK-3 kinase activity during Dictyostelium development, we find that there is a rise in activity at the initiation of multicellular development which can be induced by cAMP. The timing of the rise correlates with the requirement for the Dictyostelium homologue of GSK-3, GSKA, to specify cell fate. We show that loss of the cAMP receptor cAR3 almost completely abolishes the rise in kinase activity and causes a mis-specification of cell fate that is equivalent to that seen in a gskA− mutant. The phenotype of a cAR3− mutant however is less severe than loss of gskA and ultimately gives rise to an apparently wild-type fruiting body. These results indicate that in Dictyostelium extracellular cAMP acts via cAR3 to cause a rise in GSKA kinase activity which regulates cell type patterning during the initial stages of multicellularity.
Signaling through the stromal epidermal growth factor receptor is necessary for mammary ductal development
Authors: Wiesen Jane F. , Young Peter , Werb Zena , Cunha Gerald R.
Published: April 2021

Stromal-epithelial interactions are critical in determining patterns of growth, development and ductal morphogenesis in the mammary gland, and their perturbations are significant components of tumorigenesis. Growth factors such as epidermal growth factor (EGF) contribute to these reciprocal stromal-epithelial interactions. To determine the role of signaling through the EGF receptor (EGFR) in mammary ductal growth and branching, we used mice with a targeted null mutation in the Egfr. Because Egfr−/− mice die perinatally, transplantation methods were used to study these processes. When we transplanted neonatal mammary glands under the renal capsule of immuno-compromised female mice, we found that EGFR is essential for mammary ductal growth and branching morphogenesis, but not for mammary lobulo-alveolar development. Ductal growth and development was normal in transplants of mammary epithelium from Egfr−/− mice into wild-type (WT) gland-free fat pads and in tissue recombinants prepared with WT stroma, irrespective of the source of epithelium (StromaWT/Epi−/−, StromaWT/EpiWT). However, ductal growth and branching was impaired in tissue recombinants prepared with Egfr−/− stroma (Stroma−/−/EpiWT, Stroma−/−/Epi−/−). Thus, for ductal morphogenesis, signaling through the EGFR is required only in the stromal component, the mammary fat pad. These data indicate that the EGFR pathway plays a key role in the stromal-epithelial interactions required for mammary ductal growth and branching morphogenesis. In contrast, signaling through the EGFR is not essential for lobulo-alveolar development. Stimulation of lobulo-alveolar development in the mammary gland grafts by inclusion of a pituitary isograft under the renal capsule as a source of prolactin resulted in normal alveolar development in both Egfr−/− and wild-type transplants. Through the use of tissue recombinants and transplantation, we have gained new insights into the nature of stromal-epithelial interactions in the mammary gland, and how they regulate ductal growth and branching morphogenesis.