Notch and Hedgehog in the thymus/parathyroid common primordium: Crosstalk in organ formation.

The avian thymus and parathyroids (T/PT) common primordium derives from the endoderm of the third and fourth pharyngeal pouches (3/4PP). The molecular mechanisms that govern T/PT development are not fully understood. Here we study the effects of Notch and Hedgehog (Hh) signalling modulation during common primordium development using in vitro, in vivo and in ovo approaches. The impairment of Notch activity reduced Foxn1/thymus-fated and Gcm2/Pth/parathyroid-fated domains in the 3/4PP and further compromised the development of the parathyroid glands. When Hh signalling was abolished, we observed a reduction in the Gata3/Gcm2- and Lfng-expression domains at the median/anterior and median/posterior territories of the pouches, respectively. In contrast, the Foxn1 expression-domain at the dorsal tip of the pouches expanded ventrally into the Lfng-expression domain. This study offers novel evidence on the role of Notch signalling in T/PT common primordium development, in an Hh-dependent manner.

Portrayal of the Notch system in embryonic stem cell-derived embryoid bodies.

We portrayed the Notch system in embryonic stem cell (ESC)-derived embryoid bodies (EBs) differentiating under the standard protocols used to assess yolk sac (YS) hematopoiesis in vitro. Notch receptors and Notch ligands were detected in virtually all cells throughout EB development. Notch 1 and Notch 2, but not Notch 4, were visualized in the nucleus of EB cells, and all these receptors were also observed as patent cytoplasmic foci. Notch ligands (Delta-like 1 and 4, Jagged 1 and 2) were immunodetected mostly as cytoplasmic foci. Widespread Notch 1 activation was evident at days 2-4 of EB differentiation, the time window of hemangioblast generation in this in vitro system. EBs experienced major spatial remodeling beyond culture day 4, the time point coincident with the transition between primitive and multilineage waves of YS hematopoiesis in vitro. At day 6, where definitive YS hematopoiesis is established in EBs, these exhibit an immature densely packed cellular region (DCR) surrounded by a territory of mesodermal-like cells and an outer layer of endodermal cells. Immunolabeling of Notch receptors and ligands was usually higher in the DCR. Our results show that Notch system components are continuously and abundantly expressed in the multicellular environments arising in differentiating EBs. In such an active Notch system, receptors and ligands do not accumulate extensively at the cell surface but instead localize at cytoplasmic foci, an observation that fits current knowledge on endocytic modulation of Notch signaling. Our data thus suggest that Notch may function as a territorial modulator during early development, where it may eventually influence YS hematopoiesis.

From the outside, from within: Biological and therapeutic relevance of signal transduction in T-cell acute lymphoblastic leukemia.

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematological cancer that arises from clonal expansion of transformed T-cell precursors. In this review we summarize the current knowledge on the external stimuli and cell-intrinsic lesions that drive aberrant activation of pivotal, pro-tumoral intracellular signaling pathways in T-cell precursors, driving transformation, leukemia expansion, spread or resistance to therapy. In addition to their pathophysiological relevance, receptors and kinases involved in signal transduction are often attractive candidates for targeted drug development. As such, we discuss also the potential of T-ALL signaling players as targets for therapeutic intervention.

The notch ligand delta-like 4 regulates multiple stages of early hemato-vascular development.

BACKGROUND: In mouse embryos, homozygous or heterozygous deletions of the gene encoding the Notch ligand Dll4 result in early embryonic death due to major defects in endothelial remodeling in the yolk sac and embryo. Considering the close developmental relationship between endothelial and hematopoietic cell lineages, which share a common mesoderm-derived precursor, the hemangioblast, and many key regulatory molecules, we investigated whether Dll4 is also involved in the regulation of early embryonic hematopoiesis. METHODOLOGY/PRINCIPAL FINDINGS: Using Embryoid Bodies (EBs) derived from embryonic stem cells harboring hetero- or homozygous Dll4 deletions, we observed that EBs from both genotypes exhibit an abnormal endothelial remodeling in the vascular sprouts that arise late during EB differentiation, indicating that this in vitro system recapitulates the angiogenic phenotype of Dll4 mutant embryos. However, analysis of EB development at early time points revealed that the absence of Dll4 delays the emergence of mesoderm and severely reduces the number of blast-colony forming cells (BL-CFCs), the in vitro counterpart of the hemangioblast, and of endothelial cells. Analysis of colony forming units (CFU) in EBs and yolk sacs from Dll4(+/-) and Dll4(-/-) embryos, showed that primitive erythropoiesis is specifically affected by Dll4 insufficiency. In Dll4 mutant EBs, smooth muscle cells (SMCs) were seemingly unaffected and cardiomyocyte differentiation was increased, indicating that SMC specification is Dll4-independent while a normal dose of this Notch ligand is essential for the quantitative regulation of cardiomyogenesis. CONCLUSIONS/SIGNIFICANCE: This study highlights a previously unnoticed role for Dll4 in the quantitative regulation of early hemato-vascular precursors, further indicating that it is also involved on the timely emergence of mesoderm in early embryogenesis.

The spatial organization of centromeric heterochromatin during normal human lymphopoiesis: evidence for ontogenically determined spatial patterns.

It is believed that pericentromeric heterochromatin may play a major role in the epigenetic regulation of gene expression. We have previously shown that centromeres in human peripheral blood cells aggregate into distinct "myeloid" and "lymphoid" spatial patterns, suggesting that the three-dimensional organization of centromeric heterochromatin in interphase may be ontogenically determined during hematopoietic differentiation. To investigate this possibility, the spatial patterns of association of different centromeres were analyzed in hematopoietic progenitors and compared with those in early-B and early-T cells, mature B and T lymphocytes, and, additionally, mature granulocytes and monocytes. We show that those patterns change during lymphoid differentiation, with major spatial arrangements taking place at different stages during T and B cell differentiation. Heritable patterns of centromere association are observed, which can occur either at the level of the common lymphoid progenitor, or in early-T or early-B committed cells. A correlation of the observed patterns of centromere association with the gene content of the respective chromosomes further suggests that the variation in the composition of these heterochromatic structures may contribute to the dynamic relocation of genes in different nuclear compartments during cell differentiation, which might have functional implications for cell-stage-specific gene expression.