Long-Range Signaling Activation and Local Inhibition Separate the Mesoderm and Endoderm Lineages

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FGF signalling as a mediator of lineage transitions—Evidence from embryonic stem cell differentiation

The fibroblast growth factor (FGF) signalling pathway is one of the most ubiquitous in biology. It has diverse roles in development, differentiation and cancer. Embryonic stem (ES) cells are in vitro cell lines capable of differentiating into all the lineages of the conceptus. As such they have the capacity to differentiate into derivatives of all three germ layers and to some extent the extra‐embryonic lineages as well. Given the prominent role of FGF signalling in early embryonic development, we explore the role of this pathway in early ES cell differentiation towards the major lineages of the embryo. As early embryonic differentiation is intricately choreographed at the level of morphogenetic movement, adherent ES cell culture affords a unique opportunity to study the basic steps in early lineage specification in the absence of ever shifting complex in vivo microenvironments. Thus recent experiments in ES cell differentiation are able to pinpoint specific FGF dependent lineage transitions that are difficult to resolve in vivo. Here we review the role of FGF signalling in early development alongside the ES cell data and suggest that FGF dependent signalling via phospho‐Erk activation maybe a major mediator of transitions in lineage specification. J. Cell. Biochem. 110: 10–20, 2010. © 2010 Wiley‐Liss, Inc.     

Absence of Nodal signaling promotes precocious neural differentiation in the mouse embryo

After implantation, mouse embryos deficient for the activity of the transforming growth factor-beta member Nodal fail to form both the mesoderm and the definitive endoderm. They also fail to specify the anterior visceral endoderm, a specialized signaling center which has been shown to be required for the establishment of anterior identity in the epiblast. Our study reveals that Nodal-/- epiblast cells nevertheless express prematurely and ectopically molecular markers specific of anterior fate. Our analysis shows that neural specification occurs and regional identities characteristic of the forebrain are established precociously in the Nodal-/- mutant with a sequential progression equivalent to that of wild-type embryo. When explanted and cultured in vitro, Nodal-/- epiblast cells readily differentiate into neurons. Genes normally transcribed in organizer-derived tissues, such as Gsc and Foxa2, are also expressed in Nodal-/- epiblast. The analysis of Nodal-/-;Gsc-/- compound mutant embryos shows that Gsc activity plays no critical role in the acquisition of forebrain characters by Nodal-deficient cells. This study suggests that the initial steps of neural specification and forebrain development may take place well before gastrulation in the mouse and highlights a possible role for Nodal, at pregastrula stages, in the inhibition of anterior and neural fate determination.     

The role of FGF signaling in guiding coordinate movement of cell groups

Cell migration influences cell-cell interactions to drive cell differentiation and organogenesis. To support proper development, cell migration must be regulated both temporally and spatially. Mesoderm cell migration in the Drosophila embryo serves as an excellent model system to study how cell migration is controlled and influences organogenesis. First, mesoderm spreading transforms the embryo into a multilayered form during gastrulation and, subsequently, cells originating from the caudal visceral mesoderm (CVM) migrate along the entire length of the gut. Here we review our studies, which have focused on the role of fibroblast growth factor (FGF) signaling, and compare and contrast these two different cell migration processes: mesoderm spreading and CVM migration. In both cases, FGF acts as a chemoattractant to guide cells’ directional movement but is likely not the only signal that serves this role. Furthermore, FGF likely modulates cell adhesion properties since FGF mutant phenotypes share similarities with those of cell adhesion molecules. Our working hypothesis is that levels of FGF signaling differentially influence cells’ response to result in either directional movement or changes in adhesive properties.     

KIF3B promotes a PI3K signaling gradient causing changes in a Shh protein gradient and suppressing polydactyly in mice

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AtDEK1 is essential for specification of embryonic epidermal cell fate

The specification of epidermal (L1) identity occurs early during plant embryogenesis. Here we show that, in Arabidopsis, AtDEK1 encodes a key component of the embryonic L1 cell-layer specification pathway. Loss of AtDEK1 function leads to early embryo lethality characterized by a severe loss of cell organization in the embryo proper and abnormal cell divisions within the suspensor. Markers for L1 identity, ACR4 and ATML1, are not expressed in homozygous mutant embryos. In order to clarify the function of AtDEK1 further, an RNAi knockdown approach was used. This allowed embryos to partially complete embryogenesis before losing AtDEK1 activity. Resulting seedlings showed a specific loss of epidermal cell identity within large portions of the cotyledons. In addition, meristem structure and function was systematically either reduced or entirely lost. AtDEK1 expression is not restricted to the L1 epidermal cell layer at any stage in development. This is consistent with AtDEK1 playing an upstream role in the continuous generation or interpretation of positional information required for epidermal specification. Our results not only identify a specific role for AtDEK1 during embryogenesis, but underline the potential key importance of L1 specification at the globular stage for subsequent progression through embryogenesis.     

The dorsal neural tube: A dynamic setting for cell fate decisions

The dorsal neural tube first generates neural crest cells that exit the neural primordium following an epithelial‐to‐mesenchymal conversion to become sympathetic ganglia, Schwann cells, dorsal root sensory ganglia, and melanocytes of the skin. Following the end of crest emigration, the dorsal midline of the neural tube becomes the roof plate, a signaling center for the organization of dorsal neuronal cell types. Recent lineage analysis performed before the onset of crest delamination revealed that the dorsal tube is a highly dynamic region sequentially traversed by fate‐restricted crest progenitors. Furthermore, prospective roof plate cells were shown to originate ventral to presumptive crest and to progressively relocate dorsalward to occupy their definitive midline position following crest delamination. These data raise important questions regarding the mechanisms of cell emigration in relation to fate acquisition, and suggest the possibility that spatial and/or temporal information in the dorsal neural tube determines initial segregation of neural crest cells into their derivatives. In addition, they emphasize the need to address what controls the end of neural crest production and consequent roof plate formation, a fundamental issue for understanding the separation between central and peripheral lineages during development of the nervous system. © 2010 Wiley Periodicals, Inc. Develop Neurobiol 70: 796–812, 2010.     

Activin promotes differentiation of cultured mouse trophoblast stem cells towards a labyrinth cell fate

Prolonged maintenance of trophoblast stem (TS) cells requires fibroblast growth factor (FGF) 4 and embryonic fibroblast feeder cells or feeder cell-conditioned medium. Previous studies have shown that TGF-β and Activin are sufficient to replace embryonic fibroblast-conditioned medium. Nodal, a member of the TGF-β superfamily, is also known to be important in vivo for the maintenance of TS cells in the developing placenta. Our current studies indicate that TS cells do not express the Nodal co-receptor, Cripto, and do not respond directly to active Nodal in culture. Conversely, Activin subunits and their receptors are expressed in the placenta and TS cell cultures, with Activin predominantly expressed by trophoblast giant cells (TGCs). Differentiation of TS cells in the presence of TGC-conditioned medium or exogenous Activin results in a reduction in the expression of TGC markers. In line with TGC-produced Activin representing the active component in TGC-conditioned medium, this differentiation-inhibiting effect can be reversed by the addition of follistatin. Additional experiments in which TS cells were differentiated in the presence or absence of exogenous Activin or TGF-β show that Activin but not TGF-β results in the maintenance of expression of TS cell markers, prolongs the expression of syncytiotrophoblast markers, and significantly delays the expression of spongiotrophoblast and TGC markers. These results suggest that Activin rather than TGF-β (or Nodal) acts directly on TS cells influencing both TS cell maintenance and cell fate, depending on whether the cells are also exposed to FGF4.     

CD4 co-receptor dependent signaling promotes competency for re-stimulation induced cell death of effector T cells

The elimination of activated T cells by FAS-mediated signaling is an important immunoregulatory mechanism used to maintain homeostasis and prevent tissue damage. T cell receptor-dependent signals are required to confer sensitivity to FAS-mediated re-stimulation-induced cell death (RICD), however, the nature of these signals is not well understood. In this report, we show, using T cells from CD4-deficient mice reconstituted with a tail-less CD4 transgene, that CD4-dependent signaling events are a critical part of the competency signal required for RICD. This is in part due to defects in FAS receptor signaling complex formation as shown by decreased recruitment of FAS and caspase 8 into lipid rafts following antigen re-stimulation in the absence of CD4-dependent signals. In addition, in the absence of CD4-dependent signals, effector T cells have a selective defect in IL-2 secretion following peptide re-stimulation, while provision of exogenous IL-2 during re-stimulation partially restores susceptibility to RICD. Importantly, IL-2 production and proliferation after primary peptide stimulation is comparable between wild type and CD4-deficient T cells indicating that the requirement for CD4-dependent signaling events for IL-2 production is developmentally regulated and is particularly critical in previously activated effector T cells. In total, our results indicate that CD4 co-receptor dependent signaling events specifically regulate effector T cell survival and function. Further, these data suggest that CD4-dependent signaling events may protect against the decreased IL-2 production and resistance to cell death seen during chronic immune stimulation.     

Repetitive cellular patterns in the secondary phloem of conifer and dicot trees,and a hypothesis for their development

Abstract

The radial fusiform cell files of the secondary phloem of conifers and dicots are composed of different cell types?–?fibres, parenchyma and sieve cells (in conifers), or sieve elements plus companion cells (in dicots). These cell types are arranged in characteristic, species-specific sequences along the radii of the files. The sequences are replicated in adjacent files and this leads to tangential bands of similar cell type. Moreover, the sequences are developed repetitively so that a sequence found in one year's growth increment of phloem is repeated in the next increment. In some species, many repetitions of the same sequence occur within one annual increment. A general hypothesis has been developed to account for the radial sequences of cell types. It is proposed that there is a gradient of a phloem-promoting morphogen, a series of morphogen thresholds for the determination of each phloem cell type, and a particular spatio-temporal pattern of periclinal cell division in the phloem domain of the vascular cambium that generates a corresponding pattern of cell displacement through the morphogen gradient in the immediately post-mitotic zone of cell determination. The feasibility of the hypothesis was supported by means of simulation which, using a constant set of initial conditions, could reproduce very nearly all the radial sequences of cell types found in the secondary phloem of a range of species of conifers and woody dicots. The tangential banding of the various cell types suggests that cell production and cell determination are events which occur synchronously across the radial files. The repeating blocks of cell types may constitute functional modules of phloem tissue, and the constituent cells probably have particular patterns of symplasmic connections and mechano-structural properties.     

Wnt signaling in hematopoiesis: Crucial factors for self‐renewal,proliferation, and cell fate decisions

A large number of studies from many different laboratories have implicated the Wnt signaling pathway in regulation of hematopoiesis. However, different inducible gain‐ and loss‐of‐function approaches yielded controversial and some times contradictory results. In this prospect we will review the current ideas on Wnt signaling in hematopoiesis and early lymphopoiesis. Reviewing this large body of knowledge let us to hypothesize that different levels of activation of the pathway, dosages of Wnt signaling required and the interference by other signals in the context of Wnt activation collectively explain these controversies. Besides differences in dosage, differences in biological function of Wnt proteins in various blood cell types also is a major factor to take into account. Our own work has shown that while in the thymus Wnt signaling provides cytokine‐like, proliferative stimuli to developing thymocytes, canonical Wnt signaling in HSC regulates cell fate decisions, in particular self‐renewal versus differentiation. J. Cell. Biochem. 109: 844–849, 2010. © 2010 Wiley‐Liss, Inc.     

Establishment of a transitory dorsal-biased window of localized Ca signaling in the superficial epithelium following the mid-blastula transition in zebrafish embryos

Using complementary luminescent- and fluorescent-based Ca²⁺ imaging techniques, we have re-examined the Ca²⁺ dynamics that occur during the Blastula Period (BP) of zebrafish development. We confirm that aperiodic, localized Ca²⁺ transients are generated predominately in the superficial epithelial cells (SECs). At the start of the BP, these Ca²⁺ transients are distributed homogeneously throughout the entire superficial epithelium. Following the mid-blastula transition (MBT), however, their distribution becomes asymmetrical, where a significantly greater number are generated in the presumptive dorsal quadrant of the superficial epithelium. This asymmetry in Ca²⁺ signaling lasts for around 60 min, after which the total number of transients generated from the entire superficial epithelium falls to less than one per minute until the end of the BP. We have thus called this asymmetry the “dorsal-biased Ca²⁺ signaling window”. The application of pharmacological agents indicates that the post-MBT SEC Ca²⁺ transients are generated via the phosphatidylinositol (PI) signaling pathway. This suggests that the previously reported ventralizing function attributed to the homogeneously distributed PI pathway-generated SEC Ca²⁺ transients is most likely to occur earlier in development, prior to the MBT.     

Morphogenetic apoptosis: a mechanism for correcting discontinuities in morphogen gradients

Smooth gradients of the morphogens Hh, Dpp, and Wg are required for proper development of Drosophila imaginal discs. Here, it is reported that, when a discontinuity is generated between two adjacent cells in the reception of either the Dpp or Wg signal, then cells on either side of the discontinuity boundary undergo apoptosis by activating the c-Jun N-terminal Kinase (JNK) pathway. Furthermore, in the medial region of the wing imaginal disc, the JNK pathway is also activated if cells do not receive the proper levels of Dpp and Hh signals. These observations suggest that cells within a developing field have the ability to access their spatial positions by comparing the level of morphogen signal they receive with that of their neighbors. This phenomenon is likely related to the process of cell competition, and we suggest that it is an evolutionarily important mechanism that helps prevent abnormal tissue specification and growth during development.     

Targeting cell signaling pathways for drug discovery: an old lock needs a new key

In this age of targeted therapy, the failure of most current drug-discovery efforts to yield safe, effective, and inexpensive drugs has generated widespread concern. Successful drug development has been stymied by a general focus on target selection rather than clinical safety and efficacy. The very process of validating the targets themselves is inefficient and in many cases leads to drugs having poor efficacy and undesirable side effects. Indeed, some rationally designed drugs (e.g., inhibitors of receptor tyrosine kinases, tumor necrosis factor (TNF), cyclooxygenase-2 (COX-2), vascular endothelial growth factor (VEGF), bcr-abl, and proteasomes) are ineffective against cancers and other inflammatory conditions and produce serious side effects. Since any given cancer carries mutations in an estimated 300 genes, this raises an important question about how effective these targeted therapies can ever be against cancer. Thus, it has become necessary to rethink drug development strategies. This review analyzes the shortcomings of rationally designed target-specific drugs against cancer cell signaling pathways and evaluates the available options for future drug development.