Identification of an unexpected link between the Shh pathway and a G2/M regulator, the phosphatase CDC25B

Sonic hedgehog (Shh) signaling controls numerous aspects of vertebrate development, including proliferation of precursors in different organs. Identification of molecules that link the Shh pathway to cell cycle machinery is therefore of major importance for an understanding of the mechanisms underlying Shh-dependent proliferation. Here, we show that an actor in the control of entry into mitosis, the phosphatase CDC25B, is transcriptionally upregulated by the Shh/Gli pathway. Unlike other G2/M regulators, CDC25B is highly expressed in domains of Shh activity, including the ventral neural tube and the posterior limb bud. Loss- and gain-of-function experiments reveal that Shh contributes to CDC25B transcriptional activation in the neural tube both of chick and mouse embryos. Moreover, CDC25B transcripts are absent from the posterior limb bud of Shh-/- mice, while anterior grafts of Shh-expressing cells in the chicken limb bud induce ectopic CDC25B expression. Arresting the cell cycle does not reduce the level of CDC25B expression in the neural tube strongly suggesting that the upregulation of CDC25B is not an indirect consequence of the Shh-dependent proliferation. These data reveal an unexpected developmental link between the Shh pathway and a participant in G2/M control.

Globalisation reaches gene regulation: the case for vertebrate limb development

Analysis of key regulators of vertebrate limb development has revealed that the cis-regulatory regions controlling their expression are often located several hundred kilobases upstream of the transcription units. These far up- or down-stream cis-regulatory regions tend to reside within rather large, functionally and structurally unrelated genes. Molecular analysis is beginning to reveal the complexity of these large genomic landscapes, which control the co-expression of clusters of diverse genes by this novel type of long-range and globally acting cis-regulatory region. An increasing number of spontaneous mutations in vertebrates, including humans, are being discovered inactivating or altering such global control regions. Thereby, the functions of a seemingly distant but essential gene are disrupted rather than the closest.

Differential regulation of EP receptor isoforms during chondrogenesis and chondrocyte maturation

Regulation of chondrogenesis and chondrocyte maturation by prostaglandins has been a topic of interest during recent years. Particular focus on this area derives from the realization that inhibition of prostaglandin synthesis with non-steroidal anti-inflammatory drugs could impact these cartilage-related processes which are important in skeletal development and are recapitulated during bone healing either post-trauma or post-surgery. In addition to reviewing the relevant literature focused on prostaglandin synthesis and signaling through the G-protein coupled EP receptors, we present novel findings that establish the expression profile of EP receptors in chondroprogenitors and chondrocytes. Further, we begin to examine the signaling that may be involved with the transduction of PGE2 effects in these cells. Our findings suggest that EP2 and EP4 receptor activation of cAMP metabolism may represent a central axis of events that facilitate the impact of PGE2 on the processes of mesenchymal stem cell commitment to chondrogenesis and ultimate chondrocyte maturation.

Cadmium-induced postaxial forelimb ectrodactyly: association with altered sonic hedgehog signaling

Administration of CdSO(4) to C57BL/6 mice at day 9.5 of gestation induces a high incidence of postaxial forelimb ectrodactyly in the offspring. We propose that Cd(2+) exposure impairs the process of anterior/posterior formation in the limb bud, a process that is directed by Sonic hedgehog (Shh) signaling. We show that exposure of the mouse embryo to Cd(2+) disrupts Shh signaling as measured by polarizing activity of mouse limb bud ZPA grafted to a host chick wing, and activity of a Gli:luciferase reporter exposed to limb bud lysates. Yet the expression of Shh and its translation are not affected by Cd(2+) exposure. We propose that teratogen exposure affects the processing of Shh in the cells in which it is made.

The zinc finger transcriptional repressor Blimp1/Prdm1 is dispensable for early axis formation but is required for specification of primordial germ cells in the mouse

Blimp1, a zinc-finger containing DNA-binding transcriptional repressor,functions as a master regulator of B cell terminal differentiation. Considerable evidence suggests that Blimp1 is required for the establishment of anteroposterior axis formation and the formation of head structures during early vertebrate development. In mouse embryos, Blimp1 is strongly expressed in axial mesendoderm, the tissue known to provide anterior patterning signals during gastrulation. Here, we describe for the first time the defects caused by loss of Blimp1 function in the mouse. Blimp1 deficient embryos die at mid-gestation, but surprisingly early axis formation, anterior patterning and neural crest formation proceed normally. Rather, loss of Blimp1 expression disrupts morphogenesis of the caudal branchial arches and leads to a failure to correctly elaborate the labyrinthine layer of the placenta. Blimp1mutant embryos also show widespread blood leakage and tissue apoptosis, and,strikingly, Blimp1 homozygous mutants entirely lack PGCs. At the time of PGC allocation around 7.25 days post coitum, Blimp1 heterozygous embryos exhibit decreased numbers of PCGs. Thus Blimp1 probably acts to turn off the default pathway that allows epiblast cells to adopt a somatic cell fate, and shifts the transcriptional program so that they become exclusively allocated into the germ cell lineage.

The contribution of chicken embryology to the understanding of vertebrate limb development

The chicken is an excellent model organism for studying vertebrate limb development, mainly because of the ease of manipulating the developing limb in vivo. Classical chicken embryology has provided fate maps and elucidated the cell-cell interactions that specify limb pattern. The first defined chemical that can mimic one of these interactions was discovered by experiments on developing chick limbs and, over the last 15 years or so, the role of an increasing number of developmentally important genes has been uncovered. The principles that underlie limb development in chickens are applicable to other vertebrates and there are growing links with clinical genetics. The sequence of the chicken genome, together with other recently assembled chicken genomic resources, will present new opportunities for exploiting the ease of manipulating the limb.

Bone development: interaction of molecular components and biophysical forces

A deeper understanding of skeletal development comes from knowledge of the molecular components, cell and tissue structure, and biophysical and vascular mechanisms underlying physiologic function. Endochondral and intramembranous bone formation mechanisms are active during long bone synthesis. Most endochondral growth in length occurs at the physes by the coordinated actions of cell proliferation, matrix synthesis, and chondrocyte hypertrophy. Several intrinsic molecules act to modulate physeal structure and function whereas extrinsic molecules, such as hormones, provide systemic regulation of growth. Biophysical forces develop intrinsically within the growing bone, serving to enlarge it in three dimensions, while extrinsic forces resist and channel expansion into normal recognizable and functional forms. Newer imaging modalities, such as magnetic resonance imaging, are highly effective in assessing epiphyseal vascularity and differentiating fibrous, cartilaginous, mineralized and osseous tissues in prenatal and postnatal developing bone. This study illustrates the interplay between the molecular and biophysical aspects of normal bone growth, shows ways in which altered development leads to abnormal structures, and provides examples where biologic and biophysical interventions can affect bone development in favorable ways.

Zebrafish Polycomb group gene ph2α is required for epiboly and tailbud formation acting downstream of FGF signaling

We analyzed Polycomb group gene ph2alpha functionally in zebrafish embryos by a gene knock-down procedure using morpholino antisense oligos. Inhibition of ph2alpha message translation resulted in abnormal epibolic movements as well as a thick tailbud or incomplete covering of the yolk plug. At the 24hpf stage, morphants had short trunks and tails, phenotypes similar to those with disturbances in FGF signaling. Accordingly, we looked at the effects of ph2alpha expression upstream and downstream of the FGF pathway. Treatment with SU5402, an inhibitor of Fgfrs, or injection of dominant-negative Fgfr1 DNA markedly reduced ph2alpha expression in the tailbud. In addition, cells expressing mRNAs for no tail, spadetail, myoD, and papc, which are involved in FGF-related development of posterior mesoderm, were distributed abnormally. Collectively, the data argue that ph2alpha is required for epiboly and tailbud formation, acting downstream of the FGF signaling pathway.

Expression of interleukin-17B in mouse embryonic limb buds and regulation by BMP-7 and bFGF

Interleukin-17B (IL-17B) is a member of interleukin-17 family that displays a variety of proinflammatory and immune modulatory activities. In this study, we found that IL-17B mRNA was maximally expressed in the limb buds of 14.5 days post coitus (dpc) mouse embryo and declined to low level at 19.5 dpc. By immunohistochemical staining, the strongest IL-17B signals were observed in the cells of the bone collar in the primary ossification center. The chondrocytes in the resting and proliferative zones were stained moderately, while little staining was seen in the hypertrophic zone. Furthermore, in both C3H10T1/2 and MC3T3-E1 cells, the IL-17B mRNA was up-regulated by recombinant human bone morphogenetic protein-7, but down-regulated by basic fibroblast growth factor via the extracellular signal-regulated kinase pathway. This study provides the first evidence that IL-17B is expressed in the mouse embryonic limb buds and may play a role in chondrogenesis and osteogenesis.

Vertebrate limb development: from Harrison's limb disk transplantations to targeted disruption of Hox genes

Various animal organs have long been used to investigate the cellular and molecular nature of embryonic growth and morphogenesis. Among those organs, the tetrapod limb has been preferentially used as a model system for elucidating general patterning mechanisms. At the appropriate time during the embryonic period, the limb territories are first determined at the right positions along the cephalocaudal axis of the animal body, and soon the limb buds grow out from the flanks as mesenchymal cell masses covered by simple ectoderm. The position, number, and identity of the limbs depend on the expression of specific Hox genes. Limb morphogenesis occurs along three axes, which become gradually fixed: first the anteroposterior axis, then the dorsoventral, and finally the proximodistal axis, along which the bulk of limb growth occurs. Growth of the limb in amniotes depends on the formation of the apical ectodermal ridge, which, by secreting many members of the fibroblast growth factors family, attracts lateral plate and somitic mesodermal cells, keeps these cells in the progress zone proliferating, and prevents their differentiation until an appropriate time period. Mutual interactions between mesoderm and ectoderm are important in the growth process, and signaling regions have been identified, such as the zone of polarizing activity, the dorsal limb ectoderm, and the apical ectodermal ridge. Several molecules have been found to play leading roles in various biological processes relevant to morphogenesis. Besides its intrinsic merit as a model for unraveling the mechanisms of development, the limb deserves considerable clinical interest because defects of limb development are the most common single category of congenital abnormalities.

Heparin-binding EGF-like growth factor shows transient left–right asymmetrical expression in mouse myotome pairs

Heparin-binding EGF-like growth factor (HB-EGF) is a potent mitogen and chemoattractant for diverse cell types including, keratinocytes, fibroblasts and vascular smooth muscle cells. In adult mice, skeletal muscle and endothelial cells prominently express HB-EGF, although analysis of embryonic expression has been limited to studies of heart and kidney development. Here we survey HB-EGF mRNA expression in E7.5-E15 mouse embryos and show that HB-EGF is expressed in branchial arches, limb buds and, transiently, in mature somites between E9.25 and E11. This somitic expression is restricted to the myotomal compartment. Intriguingly, within myotome pairs, the expression of HB-EGF is stronger on the left side of the body, whilst cognate receptors, ErbB1 and ErbB4, are symmetrically expressed in left and right somite pairs. In iv/iv mutant embryos, with inverted left-right body axis, the expression of HB-EGF was also inverted, now being stronger in myotomes on the right side of the body. Thus, the expression of HB-EGF in myotome pairs is regulated by global cues that define the left-right body axis.

Microbubble-enhanced sonoporation: efficient gene transduction technique for chick embryos

The gene transduction technique is a useful method to study gene functions that underlie vertebrate embryogenesis. In this study, a new gene transduction technique is reported using microbubble-enhanced sonoporation (hereafter referred to as sonoporation) to achieve ectopic and transient gene expression for several embryonic organs including embryonic chick limb bud mesenchymes. The technique proposed in this study has the advantages of 1) relatively simple gene transduction procedures, and 2) efficient exogenous gene transduction and expression with lower damages to embryos. Green fluorescent protein (GFP) or LacZ was misexpressed in limb bud mesenchymes by sonoporation, with the introduced expression transiently detected in the injected sites. Most of the transduced chick embryos survived without showing significant embryonic abnormalities or cell death after sonoporation. To demonstrate its efficacy for assessing the effect of transient gene transduction, the Shh (sonic hedgehog) was transduced into the developing chick limb bud. The transduced limb bud displayed limb malformations including partial digit duplication. Advantages and possible future applications in relation to this method are discussed.

The core enhancer is essential for proper timing of MyoD activation in limb buds and branchial arches

Transgenic analyses have defined two transcriptional enhancers that regulate MyoD expression in mammals, the core enhancer and distal regulatory region; these enhancers exhibit complementary activities and together are sufficient to recapitulate MyoD expression in developing and mature skeletal muscle. The core enhancer is activated in presumptive muscle cells and determined myoblasts, suggesting an important role in initiating MyoD expression. Here, targeted mutagenesis in the mouse is used to identify necessary and redundant core enhancer functions. The core enhancer is essential for the timely initiation of MyoD expression in limb buds and branchial arches, as enhancer deletion delayed MyoD activation by 1 to 2 days in these muscle lineages. Functionally, this delay in MyoD transcription delayed the onset of muscle differentiation, as assayed by expression of the gene encoding for the early differentiation marker, Myogenin. In addition to these lineage-specific defects, a generalized, modest reduction in MyoD expression was observed in all muscle lineages and at all embryonic stages examined. Interestingly, however, a specific defect was not observed in the nascent myocytes at the medial and lateral aspects of the myotome, suggesting the existence of at least one other enhancer with this specificity. The core enhancer was also dispensable for Myf-5- and Pax-3-dependent regulation of MyoD transcription. These data demonstrate a differential requirement for core enhancer activity in muscle lineages derived from migratory precursors and suggest redundancy in cis regulatory mechanisms controlling myotomal MyoD expression.

Targeted disruption of the mouse rho-associated kinase 2 gene results in intrauterine growth retardation and fetal death

Rho-associated kinase (ROCK), including the ROCK-I and ROCK-II isoforms, is a protein kinase involved in signaling from Rho to actin cytoskeleton. However, in vivo functions of each ROCK isoform remain largely unknown. We generated mice deficient in ROCK-II by gene targeting. ROCK-II(-/-) embryos were found at the expected Mendelian frequency until 13.5 days postcoitum, but approximately 90% died thereafter in utero. ROCK-II(-/-) mice of both genders that survived were born runts, subsequently developed without gross abnormality, and were fertile. Whole-mount staining for a knocked-in lacZ reporter gene revealed that ROCK-II was highly expressed in the labyrinth layer of the placenta. Disruption of architecture and extensive thrombus formation were found in the labyrinth layer of ROCK-II(-/-) mice. While no obvious alteration in actin filament structures was found in the labyrinth layer of ROCK-II(-/-) placenta and stress fibers were formed in cultured ROCK-II(-/-) trophoblasts, elevated expression of plasminogen activator inhibitor 1 was found in ROCK-II(-/-) placenta. These results suggest that ROCK-II is essential in inhibiting blood coagulation and maintaining blood flow in the endothelium-free labyrinth layer and that loss of ROCK-II leads to thrombus formation, placental dysfunction, intrauterine growth retardation, and fetal death.

Retinoic acid inhibits chondrogenesis of mesenchymal cells by sustaining expression of N-cadherin and its associated proteins

Retinoic acid (RA) is a well-known regulator of chondrocyte phenotype. RA inhibits chondrogenic differentiation of mesenchymal cells and also causes loss of differentiated chondrocyte phenotype. The present study investigated the mechanisms underlying RA regulation of chondrogenesis. RA treatment in chondrifying mesenchymal cells did not affect precartilage condensation, but blocked progression from precartilage condensation to cartilage nodule formation. This inhibitory effect of RA was independent of protein kinase C and extracellular signal-regulated protein kinase, which are positive and negative regulators of cartilage nodule formation, respectively. The progression from precartilage condensation to cartilage nodule requires downregulation of N-cadherin expression. However, RA treatment caused sustained expression of N-cadherin and its associated proteins including alpha- and beta-catenin suggesting that modulation of expression of these molecules is associated with RA-induced inhibition of chondrogenesis. This hypothesis was supported by the observation that disruption of the actin cytoskeleton by cytochalasin D (CD) blocks RA-induced sustained expression of cell adhesion molecules and overcomes RA-induced inhibition of chondrogenesis. Taken together, our results suggest RA inhibits chondrogenesis by stabilizing cell-to-cell interactions at the post-precartilage condensation stage.

The progress zone model for specifying positional information

The Progress Zone Model proposes that positional information in a growing system can be specified by the time the cells spend in a zone where growth occurs. In the vertebrate limb, the Progress Zone is specified by the Apical Ectodermal Ridge. The best evidence for the model is that killing cells in the zone at an early stage leads to loss of proximal structure as cells remain much longer in the zone as it becomes repopulated.

The early history of the polarizing region: from classical embryology to molecular biology

The polarizing region of the developing limb bud is one of the best known examples of a cell-cell signalling centre that mediates patterning in vertebrate embryos. This article traces some highlights in the history of the polarizing region from its discovery by John Saunders and early work that defined polarizing activity through a period in which modelling was pre-eminent, right up to the discovery of defined molecules with polarizing activity. There is a particular focus on the discovery that retinoic acid could mimic signalling of the polarizing activity and this finding is then set in the context of more recent work which implicates Shh and BMPs in mediating polarizing activity.

Bmp, Fgf and Wnt signalling in programmed cell death and chondrogenesis during vertebrate limb development: the role of Dickkopf-1

Dickkopf-1 (Dkk-1) is a potent head inducer in Xenopus. This effect can be attributed to its capability to specifically inhibit Wnt/beta-catenin signalling. Recent data point to a crucial role for Dkk-1 in the control of programmed cell death during vertebrate limb development. In this paper, we present a comparative expression analysis of Dkk-1, Bmp-4 and Sox-9 as well as data on the regulation of Dkk-1 by Wnt. Finally, we summarize the current knowledge of its potential function in the developing limb and present a model how the interplay of the Bmp, Fgf and Wnt signalling pathways might differentially regulate programmed cell death versus chondrogenic differentiation in limb mesodermal cells.

How serendipity shaped a life; an interview with John W. Saunders, Jr. by John F. Fallon

John W. Saunders Jr. is an outstanding contributor to the field of Developmental Biology. His analyses of the apical ectodermal ridge, discovery and study of the zone of polarizing activity, insights into cell death in development, and analytical studies of feather patterns are part of a legacy to developmental biology. The body of his published work remains central to the understanding of limb development and is a major reason for the premiere place that the developmental biology of limbs holds in our research and teaching today. Beyond these things known to nearly everyone, there is John's role as teacher that is equally impressive. His one-on-one style, in small groups or from the podium is engaging, encompassing, and above all else, enthusiastic about the study of the development of living things. His love of developmental biology comes through to students of all ages and is inspirational. And, of course, inimitable charm accompanies the substance of any interaction with John. He still teaches in the Embryology Course at MBL Woods Hole. Recent students say that hearing his lectures and his involvement in the laboratory are highlights of the course. His continued knowledge of science and delight in new advances is a model for students to follow and they recognize it. John Saunders is a scientist and educator par excellence. His contributions have stood the test of time. His personal interactions with colleagues and students have enriched their lives in innumerable ways, large and small. His is a lifetime of outstanding achievements. In this interview, he reflects on his six--going on seven--decades in science and his personal enjoyment of recent advances in Developmental Biology.

The mouse polydactylous mutation, luxate (lx), causes anterior shift of the anteroposterior border in the developing hindlimb bud

Pattern formation along the anterior-posterior axis of the vertebrate limb is established upon activation of Sonic Hedgehog (SHH) in the zone of polarizing activity (ZPA). Since many mouse mutants with preaxial polydactyly show ectopic expression of Shh at the anterior margin of the limb buds, it has been thought to be a primary defect caused by these mutations. We show here that the mouse mutation luxate (lx) exhibits dose-dependent reduction in the size of the Fgf8 expression domain in the ectoderm from the initial stage of limb development. This aberration was independent of Fgf10 expression in the limb mesenchyme. Shh was induced in the mesenchyme underlying the posterior end of the Fgf8 expression domain, indicating an anterior shift of Shh expression in lx hindlimb buds. Prior to the ectopic induction of Shh, the expression domains of genes downstream from Shh, namely dHAND, Gli1, Ptc and Gre, which are normally expressed in posterior mesenchyme of limb buds, expanded anteriorly on the lx hindlimb buds. Conversely, the expression domains of anterior mesenchymal markers such as Gli3and Alx4 decreased in size. Thus, ectopic Shh is not a primary defect of the lx mutation. Rather, our results indicate that the lx mutation affects the positioning of the anteroposterior border in developing hindlimb buds.

Distribution and possible function of an adrenomedullin-like peptide in the developing chick limb bud

Adrenomedullin (AM) is a multifunctional peptide that exhibits discrete domains of expression during mouse embryogenesis consistent with a role in regulating growth and differentiation during morphogenesis. Here we report that AM immunoreactivity is present at high levels throughout the apical ectodermal ridge (AER) of the chick limb bud as the AER is directing the outgrowth and patterning of underlying limb mesoderm. Immunostaining is particularly strong along the surfaces of the contiguous cells of the AER. AM immunoreactivity attenuates as the AER regresses and is absent from the distal apical ectoderm of stage 20 limbless mutant limb buds which fail to develop an AER. To explore the possible role of AM in AER activity, we examined the effect of exogenous AM and an AM inhibitor on the in vitro morphogenesis of limb mesoderm, cultured in the presence and absence of the AER. Although exogenous AM cannot substitute for the AER in promoting outgrowth of limb mesoderm in vitro, a specific AM antagonist, AM(22-52), impairs the outgrowth and proliferation of limb mesoderm cultured in the presence of the AER. This is consistent with the possibility that inhibition of endogenous AM activity in the AER impairs the ability of the AER to promote limb morphogenesis. Taken together, these studies suggest that an AM-like molecule may function in an autocrine fashion to regulate some aspect of AER activity.

Interplay between the molecular signals that control vertebrate limb development

Vertebrate limbs display three obvious axes of asymmetry. These three axes are referred to as proximal-distal (Pr-D; shoulder to digit tips), anterior-posterior (A-P; thumb to little finger), and dorsal-ventral (D-V; back of hand to palm). At a molecular level, it is now possible to define the signals that control patterning of each of the three axes of the developing limb. These signals do not work in isolation though but rather their activity must be integrated such that the various limb elements are coordinately formed with relation to these three axes. This review will provide an overview of the intricate medley amongst the molecular signals that serve to establish and coordinate patterning information along the three primary axes of the limb.

Cell adhesiveness and affinity for limb pattern formation

Stage-dependent cell sorting in vitro is an intriguing property that mesenchymal cells of a chick limb bud have. We previously proposed that N-cadherin, a cell adhesion molecule, is involved in the sorting process and is likely to be a component of the mechanism of proximal-distal patterning in the developing limb (Yajima et al., (1999) Dev. Dynam. 216:274-284). Here, we present more direct evidence that N-cadherin is one of the molecules responsible for regulation of stage-dependent cell sorting in vitro. Our results suggest that N-cadherin, which accumulates in the distal region of the chick limb bud as limb development proceeds, is related to the positional identity that gives rise to the different shapes and numbers of cartilaginous elements along the proximal-distal axis. In this article we also give insights into positional identity which is mediated by Hoxgenes and cell surface property during limb development.

Negative feedback regulation of FGF signaling levels by Pyst1/MKP3 in chick embryos

BACKGROUND

The importance of endogenous antagonists in intracellular signal transduction pathways is becoming increasingly recognized. There is evidence in cultured mammalian cells that Pyst1/MKP3, a dual specificity protein phosphatase, specifically binds to and inactivates ERK1/2 mitogen-activated protein kinases (MAPKs). High-level Pyst1/Mkp3 expression has recently been found at many sites of known FGF signaling in mouse embryos, but the significance of this association and its function are not known.

RESULTS

We have cloned chicken Pyst1/Mkp3 and show that high-level expression in neural plate correlates with active MAPK. We show that FGF signaling regulates Pyst1 expression in developing neural plate and limb bud by ablating and/or transplanting tissue sources of FGFs and by applying FGF protein or a specific FGFR inhibitor (SU5402). We further show by applying a specific MAP kinase kinase inhibitor (PD184352) that Pyst1 expression is regulated via the MAPK cascade. Overexpression of Pyst1 in chick embryos reduces levels of activated MAPK in neural plate and alters its morphology and retards limb bud outgrowth.

CONCLUSIONS

Pyst1 is an inducible antagonist of FGF signaling in embryos and acts in a negative feedback loop to regulate the activity of MAPK. Our results demonstrate both the importance of MAPK signaling in neural induction and limb bud outgrowth and the critical role played by dual specificity MAP kinase phosphatases in regulating developmental outcomes in vertebrates.