Cell lineage determination in the mouse

During the peri-implantation development of the mouse embryo from the blastocyst through gastrulation, Pou5f1 (OCT-4) down-regulation is closely linked to the initial step of lineage allocation to extraembryonic and embryonic somatic tissues. Subsequently, differentiation of the lineage precursors is subject to inductive tissue interactions and intercellular signalling that regulate cell proliferation and the acquisition of lineage-specific morphological and molecular characteristics. A notable variation of this process of lineage specification is the persistence of Pou5f1 activity throughout the differentiation of the primordial germ cells, which may underpin their ability to produce pluripotent progeny either as stem cells (embryonic germ cells) in vitro or as gametes in vivo. Nevertheless, intercellular signalling still plays a critical role in the specification of the primordial germ cells. The findings that primordial germ cells can be induced from any epiblast cells and that they share common progenitors with other somatic cells provide compelling evidence for the absence of a pre-determined germ line in the mouse embryo.     

Acceleration of X-chromosome gene order evolution in the cattle lineage

The gene order on the X chromosome of eutherians is generally highly conserved, although an increase in the rate of rearrangement has been reported in the rodent lineage. Conservation of the X chromosome is thought to be caused by selection related to maintenance of dosage compensation. However, we herein reveal that the cattle (Btau4.0) lineage has experienced a strong increase in the rate of X-chromosome rearrangement, much stronger than that previously reported for rodents. We also show that this increase is not matched by a similar increase on the autosomes and cannot be explained by assembly errors. Furthermore, we compared the difference in two cattle genome assemblies: Btau4.0 and Btau6.0 (Bos taurus UMD3.1). The results showed a discrepancy between Btau4.0 and Btau6.0 cattle assembly version data, and we believe that Btau6.0 cattle assembly version data are not more reliable than Btau4.0. [BMB Reports 2013; 46(6): 310-315]     

A dominant clonal lineage of Streptococcus uberis in cattle in Germany

Bovine mastitis causes enormous economic losses in the dairy industry with Streptococcus uberis as one of the most common bacterial pathogens causing clinical and subclinical variations. In most cases mastitis can be cured by intramammary administration of antimicrobial agents. However, the severity of the clinical manifestations can vary greatly from mild to severe symtoms. In this study, a comparative genomic analysis of 24 S. uberis isolates from three dairy farms in Germany, affected by different courses of infection was conducted. While there were sporadic mild infections in farm A and B, a large number of infections were observed within a very short period of time in farm C. The comparison of virulence genes, antimicrobial resistance genes and prophage regions revealed no features that might be responsible for this severe course. However, almost all isolates from farm C showed the same, novel MLST profile (ST1373), thus a clonal outbreak cannot be excluded, whereby the actual reason for the particular virulence remains unknown. This study demonstrates the importance of extensive metagenomic studies, including the host genomes and the environment, to gain further evidence on the pathogenicity of S. uberis.

     

Mesodermal cell lineage determination and proto-oncogene expression

Previously, we have established a rat embryo fibroblastic cell line that is characterized as immortal, nontumorigenic and retains a diploid karyotype. We report here that this cell line will spontaneously differentiate along mesodermal cell lineages to form myotubes and adipocytes. We have isolated and characterized lineage-determined (nondifferentiated) preadipocytes and myoblasts. Using these cell lines we have investigated the expression of proto-oncogenes concomitant with lineage-specific determination. No major changes in proto-oncogene RNA levels were observed that correlated with lineage determination. These results would suggest that none of the 15 proto-oncogenes used in these experiments are involved in mesodermal lineage determination; but this does not rule out the possibility that other proto-oncogenes not tested or currently unknown may be involved. However, these differentiating subclones and nondifferentiating fibroblasts that exhibit a normal karyotype will be an excellent model-system to investigate the molecular basis of cell lineage determination.     

Emergent dynamics of thymocyte development and lineage determination

Experiments have generated a plethora of data about the genes, molecules, and cells involved in thymocyte development. Here, we use a computer-driven simulation that uses data about thymocyte development to generate an integrated dynamic representation—a novel technology we have termed reactive animation (RA). RA reveals emergent properties in complex dynamic biological systems. We apply RA to thymocyte development by reproducing and extending the effects of known gene knockouts: CXCR4 and CCR9. RA simulation revealed a previously unidentified role of thymocyte competition for major histocompatability complex presentation. We now report that such competition is required for normal anatomical compartmentalization, can influence the rate of thymocyte velocities within chemokine gradients, and can account for the disproportion between single-positive CD4 and CD8 lineages developing from double-positive precursors.     

Cell lineage and determination of cell fate in ascidian embryos

A detailed cell lineage of ascidian embryos has been available since the turn of the century. This cell lineage was deduced from the segregation of pigmented egg cytoplasmic regions into particular blastomeres during embryogenesis. The invariant nature of the cell lineage, the segregation of specific egg cytoplasmic regions into particular blastomeres, and the autonomous development of most embryonic cells suggests that cell fate is determined primarily by cytoplasmic determinants. Modern studies have provided strong evidence for the existence of cytoplasmic determinants, especially in the primary muscle cells, yet the molecular identity, localization, and mode of action of these factors are still a mystery. Recent revisions of the classic cell lineage and demonstrations of the lack of developmental autonomy in certain embryonic cells suggest that induction may also be an important mechanism for the determination of cell fate in ascidians. There is strong evidence for the induction of neural tissue and indirect evidence for inductive interactions in the development of the secondary muscle cells. In contrast to the long-accepted dogma, specification of cell fate in ascidians appears to be established by a combination of cytoplasmic determinants and inductive cell interactions.     

Trophectoderm cell subpopulations in the periimplantation mouse blastocyst

Trophectoderm of the preimplantation mouse blastocyst is composed of two cell subpopulations relative to their proximity to the inner cell mass. The polar trophectoderm overlying the inner cell mass proliferates to form the ectoplacental cone, and the mural trophectoderm endoreplicates and gives rise to giant cells. We examined specific differences in the two trophectoderm cell populations using a lectin (Dolichos biflorus) to detect cell surface characteristics and a simple sugar (D-Gal) to detect differences in incorporation. During the first day of delayed implantation, the mural trophectoderm presented twice as many lectin binding sites as did the polar trophectoderm. The mural trophectoderm of both nondelaying and delayed implantation blastocysts showed a greater rate of incorporation of the tritiated sugar by presenting more reduced silver grains in radioautograms. These results indicate that the mural trophectoderm and polar trophectoderm are two distinct cell types in the periimplantation blastocyst.     

Neuronal potential and lineage determination by neural stem cells

How do neural stem cells ensure that they give rise to the right number and type of neurons at the right time? Over the past year several regulatory mechanisms have been identified, including promotion of neurogenesis by proneural bHLH genes, instruction of gliogenesis by Notch, and cell-intrinsic changes in the neurogenic capacity of stem cells in culture and in vivo.     

L-myc and N-myc influence lineage determination in the central nervous system.

The N-myc and the L-myc proto-oncogenes are expressed during embryonal development mainly in the developing brain. Studies of their expression in single neuroepithelial cells revealed that neural precursors not yet committed to the glial or the neuronal lineage expressed both genes, but after lineage commitment they expressed either N-myc or L-myc. Moreover, enforced expression of L-myc in the neural precursor cell line 2.3D caused neuronal differentiation, while the expression of N-myc promoted glial differentiation. These results indicate that L-myc and N-myc play critical roles in lineage determination for the central nervous system.     

Trophectoderm mechanics direct epiblast shape upon embryo implantation

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Cellular determination in the Xenopus retina is independent of lineage and birth date

Xenopus embryos injected with tritiated thymidine throughout the stages of embryonic retinal neurogenesis showed that more than 95% of the embryonic retinal cells are born within a 25 hr period. While there are shallow central to peripheral, dorsal to ventral, and interlaminar gradients of neurogenesis in these eyes, throughout most of this 25 hr period, postmitotic cells are being added to all sectors and layers. Small clones of differentiated retinal neurons and glia derived from single neuroepithelial cells injected with HRP. These clones were elongated radially. They were also composed of many different combinations of cell types, suggesting a mechanism whereby determination is arbitrarily and independently assigned to postmitotic cells. Such a model, when tested statistically, fits our data very well. We present a scheme for cellular determination in the Xenopus retina in which a coherent group of clonally related cells stretch out radially as lamination begins. This brings different cells into different microenvironments. Local interactions in these microenvironments then lead the cells toward specific fates.     

Making or breaking the heart: from lineage determination to morphogenesis

The cues governing cardiac cell-fate decisions, cardiac differentiation, and three-dimensional morphogenesis are rapidly being elucidated. Several themes are emerging that are relevant for childhood and adult heart disease and the growing field of stem cell biology. This review will consider our current understanding of cardiac cell-fate determination and cardiogenesis--largely derived from developmental studies in model organisms and human genetic approaches--and examine future implications for diagnosis, prevention, and treatment of heart disease in the young and old.     

Notch1 signaling regulates chondrogenic lineage determination through Sox9 activation

Notch signaling is involved in several cell lineage determination processes during embryonic development. Recently, we have shown that Sox9 is most likely a primary target gene of Notch1 signaling in embryonic stem cells (ESCs). By using our in vitro differentiation protocol for chondrogenesis from ESCs through embryoid bodies (EBs) together with our tamoxifen-inducible system to activate Notch1, we analyzed the function of Notch signaling and its induction of Sox9 during EB differentiation towards the chondrogenic lineage. Temporary activation of Notch1 during early stages of EB, when lineage determination occurs, was accompanied by rapid and transient Sox9 upregulation and resulted in induction of chondrogenic differentiation during later stages of EB cultivation. Using siRNA targeting Sox9, we knocked down and adjusted this early Notch1-induced Sox9 expression peak to non-induced levels, which led to reversion of Notch1-induced chondrogenic differentiation. In contrast, continuous Notch1 activation during EB cultivation resulted in complete inhibition of chondrogenic differentiation. Furthermore, a reduction and delay of cardiac differentiation observed in EBs after early Notch1 activation was not reversed by siRNA-mediated Sox9 knockdown. Our data indicate that Notch1 signaling has an important role during early stages of chondrogenic lineage determination by regulation of Sox9 expression.     

c-Fos and bone loss: A proto-oncogene regulates osteoclast lineage determination

Development of gene transfer systems provides a key tool for understanding gene function. Exciting and often unexpected consequences from embryo manipulations are yielding insights into molecular mechanisms underlying development under normal and pathogenic states, and are providing animal models for diseases. Contributing to this progress is the elegant work on c-fos⁽¹⁾, where Wagner and coworkers identify this proto-oncogene as a primary factor which directs cell differentiation along the osteoclast/macrophage lineages, and thus regulates bone remodeling. Their studies support a link between skeletogenesis, marrow formation and hematopoiesis, and may help to delineate mechanisms underlying the oncogenic transformation of skeletal and hematopoietic cells.     

Protein kinase C in erythroid and megakaryocytic differentiation: possible role in lineage determination

Erythroid differentiation of normal human hematopoietic progenitor cells was drastically inhibited by phorbol ester, 12-myristate 13-acetate (PMA), an agent known to activate the class of serine-threonine kinases, protein kinase C (PKC). This inhibition was accompanied by augmented megakaryocytic differentiation as demonstrated by expression of megakaryocyte-specific mRNAs and proteins. These effects of PMA were reversed by two specific antagonists of PKC. Analysis of single colonies transferred from cultures not containing PMA to PMA-containing cultures indicated that, in this system, PMA exerts megakaryocytic differentiating activity directly on cells which may have already initiated a progression toward the erythroid pathway of differentiation. These results suggest that modulation of PKC activity plays a role in erythroid and megakaryocytic differentiation, and may constitute an important selective signal between these pathways during normal blood cell development.