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Biomedical subjects

Freddy Radtke

Publications and source records attributed to Freddy Radtke.

At least 19 recordsLinked to original sources
PubMed2007

Notch1 and Notch2 receptors influence progressive hair graying in a dose-dependent manner.

The Notch signaling pathway is involved in diverse biological processes such as cell fate decisions or stem cell maintenance. In this study, we assessed the role of this pathway for melanocyte development and hair pigmentation using RBP-Jkappa, Notch1, and Notch2 conditional knockout mice. Disruption of the Notch pathway by inactivating RBP-Jkappa in the melanocyte lineage using Tyr::Cre mice led to a severe coat color dilution. Similarly, hair graying was observed when Notch1 and/or Notch2 receptors were ablated in melanocytes. This phenotype was proportional to the number of floxed Notch alleles, with the most pronounced effect seen in Tyr::Cre/degrees; Notch1(flox/flox); Notch2(flox/flox) mice. Deletion of Notch1 and/or Notch2 in melanoblasts did not induce a congenital defect. The number of Dct-expressing cells at embryonic stages was not affected, but melanocytes located within the hair matrix progressively disappeared during the first regeneration of the hair follicle. In contrast, non-follicular melanocytes and pigmentation in the dermis and in the choroid were not affected. We suggest that both Notch1 and Notch2 receptors contribute to the maintenance of melanoblasts and melanocyte stem cells, and are essential for proper hair pigmentation.

Alleles↗
PubMed2006

Critical role of endothelial Notch1 signaling in postnatal angiogenesis.

Notch receptors are important mediators of cell fate during embryogenesis, but their role in adult physiology, particularly in postnatal angiogenesis, remains unknown. Of the Notch receptors, only Notch1 and Notch4 are expressed in vascular endothelial cells. Here we show that blood flow recovery and postnatal neovascularization in response to hindlimb ischemia in haploinsufficient global or endothelial-specific Notch1(+/-) mice, but not Notch4(-/-) mice, were impaired compared with wild-type mice. The expression of vascular endothelial growth factor (VEGF) in response to ischemia was comparable between wild-type and Notch mutant mice, suggesting that Notch1 is downstream of VEGF signaling. Treatment of endothelial cells with VEGF increases presenilin proteolytic processing, gamma-secretase activity, Notch1 cleavage, and Hes-1 (hairy enhancer of split homolog-1) expression, all of which were blocked by treating endothelial cells with inhibitors of phosphatidylinositol 3-kinase/protein kinase Akt or infecting endothelial cells with a dominant-negative Akt mutant. Indeed, inhibition of gamma-secretase activity leads to decreased angiogenesis and inhibits VEGF-induced endothelial cell proliferation, migration, and survival. Overexpression of the active Notch1 intercellular domain rescued the inhibitory effects of gamma-secretase inhibitors on VEGF-induced angiogenesis. These findings indicate that the phosphatidylinositol 3-kinase/Akt pathway mediates gamma-secretase and Notch1 activation by VEGF and that Notch1 is critical for VEGF-induced postnatal angiogenesis. These results suggest that Notch1 may be a novel therapeutic target for improving angiogenic response and blood flow recovery in ischemic limbs.

Amyloid Precursor Protein Secretases↗
PubMed2006

Induction of cardiogenesis in embryonic stem cells via downregulation of Notch1 signaling.

Embryonic stem cells represent an attractive source of cardiomyocytes for cell-replacement therapies. However, before embryonic stem cells can be successfully used for the treatment of cardiac diseases, the precise molecular mechanisms that underlie their cardiogenic differentiation must be identified. A network of intrinsic and extrinsic factors regulates embryonic stem cell self-renewal and differentiation into a variety of different cell lineages. Here, we show that Notch signaling takes place in some but not all embryonic stem cells and that the Notch pathway is shut down during the course of differentiation concomitantly with downregulation of Notch receptor and ligand expression. Moreover, gain- and loss-of-function experiments for Notch signaling components show that this pathway is a crucial regulator of cardiomyocyte differentiation within ES cells. Differentiation of ES cells into cardiomyocytes is favored by inactivation of the Notch1 receptor, whereas endogenous Notch signaling promotes differentiation of ES cells into the neuronal lineage. We conclude that Notch signaling influences the cell fate decision between mesodermal and the neuroectodermal cell fates during embryonic stem cell differentiation. These findings should help to optimize the production of specific cell types via modulation of the Notch pathways and, in particular, to improve the production of embryonic stem cell-derived cardiomyocytes.

Animals↗
PubMed2006

Multiple functions of Notch signaling in self-renewing organs and cancer.

In recent years a substantial body of evidence has accumulated to support the notion that signaling pathways known to be important during embryonic development play important roles in regulating self-renewing tissues. Moreover, the same pathways are often deregulated during tumorigenesis due to mutations of key elements of these pathways. The Notch signaling cascade meets all of the above-mentioned criteria. We discuss here the pleiotropic roles of the Notch signaling pathway in three different self-renewing organs (intestine, hematopoietic system and skin) and how its deregulation is involved in tumorigenesis.

Animals↗
PubMed2006

From gut homeostasis to cancer.

The mammalian intestine has one of the highest turnover rates in the body. The intestinal epithelium is completely renewed in less than a week. It is divided into spatially distinct compartments in the form of finger-like projections and invaginations that are dedicated to specific functions. Intestinal cells are constantly produced from a stem cell reservoir that gives rise to proliferating transient amplifying cells, which subsequently differentiate and migrate to the correct compartment before dying after having fulfilled their physiological function. In recent years, a substantial body of evidence has accumulated to support the concept that signaling pathways known to be crucial for embryonic development of multiple organisms play a critical role in tightly regulating and controlling the self-renewing process of the intestine. Moreover, the same pathways appear to be deregulated in several hereditary and sporadic colorectal cancer syndromes due to activating and/or inactivating mutations of key components of such pathways. In this review we discuss recent findings demonstrating that differentiation and homeostasis of the intestine are controlled by developmental pathways such as Wnt, Notch, TGF-beta and Hedgehog, and illustrate how their deregulation contributes to intestinal neoplasia.

Animals↗
PubMed2005

Notch signaling is required for normal prostatic epithelial cell proliferation and differentiation.

Notch pathway is crucial for stem/progenitor cell maintenance, growth and differentiation in a variety of tissues. Using a transgenic cell ablation approach, we found in our previous study that cells expressing Notch1 are crucial for prostate early development and re-growth. Here, we further define the role of Notch signaling in regulating prostatic epithelial cell growth and differentiation using biochemical and genetic approaches in ex vivo or in vivo systems. Treatment of developing prostate grown in culture with inhibitors of gamma-secretase/presenilin, which is required for Notch cleavage and activation, caused a robust increase in proliferation of epithelial cells co-expressing cytokeratin 8 and 14, lack of luminal/basal layer segregation and dramatically reduced branching morphogenesis. Using conditional Notch1 gene deletion mouse models, we found that inactivation of Notch1 signaling resulted in profound prostatic alterations, including increased tufting, bridging and enhanced epithelial proliferation. Cells within these lesions co-expressed both luminal and basal cell markers, a feature of prostatic epithelial cells in predifferentiation developmental stages. Microarray analysis revealed that the gene expression in a number of genetic networks was altered following Notch1 gene deletion in prostate. Furthermore, expression of Notch1 and its effector Hey-1 gene in human prostate adenocarcinomas were found significantly down-regulated compared to normal control tissues. Taken together, these data suggest that Notch signaling is critical for normal cell proliferation and differentiation in the prostate, and deregulation of this pathway may facilitate prostatic tumorigenesis.

Amyloid Precursor Protein Secretases↗
PubMed2005

Jagged1 signals in the postnatal subventricular zone are required for neural stem cell self-renewal.

Neural stem cells (NSCs) in the postnatal mammalian brain self-renew and are a source of neurons and glia. To date, little is known about the molecular and cellular mechanisms regulating the maintenance and differentiation of these multipotent progenitors. We show that Jagged1 is required by mitotic cells in the subventricular zone (SVZ) and stimulates self-renewal of multipotent epidermal growth factor-dependent NSCs. Jagged1-expressing cells line the adult SVZ and are juxtaposed to Notch1-expressing cells, some of which are putative NSCs. In vitro, endogenous Jagged1 acts through Notch1 to promote NSC maintenance and multipotency. In vivo, reducing Jagged1/Notch1 signaling decreases the number of proliferating cells in the SVZ. In addition, soluble Jagged1 promotes self-renewal and neurogenic potential of multipotent neural progenitors in vitro. Our findings suggest a central role for Jagged1 in the NSC niche in the SVZ for maintaining a population of NSCs in the postnatal brain.

Animals↗
PubMed2005

Notch1 is essential for postnatal hair follicle development and homeostasis.

Notch genes encode evolutionarily conserved large, single transmembrane receptors, which regulate many cell fate decisions and differentiation processes during fetal and postnatal life. Multiple Notch receptors and ligands are expressed in both developing and adult epidermis and hair follicles. Proliferation and differentiation of these two ectodermal-derived structures have been proposed to be controlled in part by the Notch pathway. Whether Notch signaling is involved in postnatal hair homeostasis is currently unknown. Here, we investigate and compare the role of the Notch1 receptor during embryonic hair follicle development and postnatal hair homeostasis using Cre-loxP based tissue specific and inducible loss-of-function approaches. During embryonic development, tissue-specific ablation of Notch1 does not perturb formation and patterning of hair follicle placodes. However, Notch1 deficient hair follicles invaginate prematurely into the dermis. Embryonic as well as postnatal inactivation of Notch1 shortly after birth or in adult mice results in almost complete hair loss followed by cyst formation. The first hair cycle of Notch1 deficient mice is characterized by shortened anagen and a premature entry into catagen. These data show that Notch1 is essential for late stages of hair follicle development during embryogenesis as well as for post-natal hair follicle development and hair homeostasis.

Animals↗
PubMed2005

Notch/gamma-secretase inhibition turns proliferative cells in intestinal crypts and adenomas into goblet cells.

The self-renewing epithelium of the small intestine is ordered into stem/progenitor crypt compartments and differentiated villus compartments. Recent evidence indicates that the Wnt cascade is the dominant force in controlling cell fate along the crypt-villus axis. Here we show a rapid, massive conversion of proliferative crypt cells into post-mitotic goblet cells after conditional removal of the common Notch pathway transcription factor CSL/RBP-J. We obtained a similar phenotype by blocking the Notch cascade with a gamma-secretase inhibitor. The inhibitor also induced goblet cell differentiation in adenomas in mice carrying a mutation of the Apc tumour suppressor gene. Thus, maintenance of undifferentiated, proliferative cells in crypts and adenomas requires the concerted activation of the Notch and Wnt cascades. Our data indicate that gamma-secretase inhibitors, developed for Alzheimer's disease, might be of therapeutic benefit in colorectal neoplastic disease.

Adenoma↗
PubMed2005

Essential role of endothelial Notch1 in angiogenesis.

BACKGROUND: Notch signaling influences binary cell fate decisions in a variety of tissues. The Notch1 receptor is widely expressed during embryogenesis and is essential for embryonic development. Loss of global Notch1 function results in early embryonic lethality, but the cell type responsible for this defect is not known. Here, we identify the endothelium as the primary target tissue affected by Notch1 signaling. METHODS AND RESULTS: We generated an endothelium-specific deletion of Notch1 using Tie2Cre and conditional Notch1(flox/flox) mice. Mutant embryos lacking endothelial Notch1 died at approximately embryonic day 10.5 with profound vascular defects in placenta, yolk sac, and embryo proper, whereas heterozygous deletion had no effect. In yolk sacs of mutant embryos, endothelial cells formed a primary vascular plexus indicative of intact vasculogenesis but failed to induce the secondary vascular remodeling required to form a mature network of well-organized large and small blood vessels, which demonstrates a defect in angiogenesis. These vascular defects were also evident in the placenta, where blood vessels failed to invade the placental labyrinth, and in the embryo proper, where defective blood vessel maturation led to pericardial and intersomitic hemorrhage. Enhanced activation of caspase-3 was detected in endothelial and neural cells of mutant mice, which resulted in enhanced apoptotic degeneration of somites and the neural tube. CONCLUSIONS: These findings recapitulate the vascular phenotype of global Notch1-/- mutants and indicate an essential cell-autonomous role of Notch1 signaling in the endothelium during vascular development. These results may have important clinical implications with regard to Notch1 signaling in adult angiogenesis.

Animals↗
PubMed2005

Self-renewal and cancer of the gut: two sides of a coin.

The intestinal epithelium follows the paradigms of stem cell biology established for other self-renewing tissues. With a unique topology, it constitutes a two-dimensional structure folded into valleys and hills: the proliferative crypts and the differentiated villi. Its unprecedented self-renewal rate appears reflected in a high susceptibility to malignant transformation. The molecular mechanisms that control homeostatic self-renewal and those that underlie colorectal cancer are remarkably symmetrical. Here, we discuss the biology of the intestinal epithelium, emphasizing the roles played by Wnt, bone morphogenic protein, and Notch signaling cascades in epithelial self-renewal and cancer.

Adenomatous Polyposis Coli↗
PubMed2005

Notch signaling in hematopoiesis and lymphopoiesis: lessons from Drosophila.

The evolutionarily conserved Notch signaling pathway regulates a broad spectrum of cell fate decisions and differentiation processes during fetal and postnatal life. It is involved in embryonic organogenesis as well as in the maintenance of homeostasis of self-renewing systems. In this article, we review the role of Notch signaling in the hematopoietic system with particular emphasis on lymphocyte development and highlight the similarities in Notch function between Drosophila and mammalian differentiation processes. Recent studies indicating that aberrant NOTCH signaling is frequently linked to the induction of T leukemia in humans will also be discussed.

Animals↗
PubMed2005

Notch1 signals through Jagged2 to regulate apoptosis in the apical ectodermal ridge of the developing limb bud.

The Notch family of receptors is involved in a wide variety of developmental processes, including cell fate specification, cell proliferation, and cell survival decisions during cell differentiation and tissue morphogenesis. Notch1 and Notch ligands are expressed in the developing limbs, and Notch signalling has been implicated in the formation of a variety of tissues that comprise the limb, such as the skeleton, musculature, and vasculature. Notch signalling has also been implicated in regulating overall limb size. We have used a conditional allele of Notch1 in combination with two different Cre transgenic lines to delete Notch1 function either in the limb mesenchyme or in the apical ectodermal ridge (AER) and limb ectoderm. We demonstrate that Notch signalling, involving Notch1 and Jagged2, is required to regulate the number of Fgf8-expressing cells that comprise the AER and that regulation of the levels of fibroblast growth factor signalling is important for the freeing of the digits during normal limb formation. Regulation of the extent of the AER is achieved by Notch signalling positively regulating apoptosis in the AER. We also demonstrate that Notch1 is not required for proper formation of all the derivatives of the limb mesenchyme.

Animals↗
PubMed2005

Inducible inactivation of Notch1 causes nodular regenerative hyperplasia in mice.

The discovery that the human Jagged1 gene (JAG1) is the Alagille syndrome disease gene indicated that Notch signaling has an important role in bile duct homeostasis. The functional study of this signaling pathway has been difficult because mice with targeted mutations in Jagged1, Notch1, or Notch2 have an embryonic lethal phenotype. We have previously generated mice with inducible Notch1 disruption using an interferon-inducible Cre-recombinase transgene in combination with the loxP flanked Notch1 gene. We used this conditional Notch1 knockout mouse model to investigate the role of Notch1 signaling in liver cell proliferation and differentiation. Deletion of Notch1 did not result in bile duct paucity, but, surprisingly, resulted in a continuous proliferation of hepatocytes. In conclusion, within weeks after Notch1 inactivation, the mice developed nodular regenerative hyperplasia without vascular changes in the liver.

Animals↗
PubMed2005

Paradigms of notch signaling in mammals.

Notch proteins regulate a broad spectrum of cell fate decisions and differentiation processes during fetal and postnatal life. These proteins are involved in organogenesis during embryonic development as well as in the maintenance of homeostasis of self-renewing systems. The paradigms of Notch function, such as stem and progenitor cell maintenance, lineage specification mediated by binary cell fate decisions, and induction of terminal differentiation, were initially established in invertebrates and subsequently confirmed in mammals. Moreover, aberrant Notch signaling is linked to tumorigenesis. In this review, we discuss the origin of postulated Notch functions, give examples from different mammalian organ systems, and try to relate them to the hematopoietic system.

Animals↗
PubMed2004

Jagged1-dependent Notch signaling is dispensable for hematopoietic stem cell self-renewal and differentiation.

Jagged1-mediated Notch signaling has been suggested to be critically involved in hematopoietic stem cell (HSC) self-renewal. Unexpectedly, we report here that inducible Cre-loxP-mediated inactivation of the Jagged1 gene in bone marrow progenitors and/or bone marrow (BM) stromal cells does not impair HSC self-renewal or differentiation in all blood lineages. Mice with simultaneous inactivation of Jagged1 and Notch1 in the BM compartment survived normally following a 5FU-based in vivo challenge. In addition, Notch1-deficient HSCs were able to reconstitute mice with inactivated Jagged1 in the BM stroma even under competitive conditions. In contrast to earlier reports, these data exclude an essential role for Jagged1-mediated Notch signaling during hematopoiesis.

Animals↗
PubMed2004

Fibroblast growth factor receptor signaling promotes radial glial identity and interacts with Notch1 signaling in telencephalic progenitors.

The Notch and fibroblast growth factor (FGF) pathways both regulate cell fate specification during mammalian neural development. We have shown previously that Notch1 activation in the murine forebrain promotes radial glial identity. This result, together with recent evidence that radial glia can be progenitors, suggested that Notch1 signaling might promote progenitor and radial glial character simultaneously. Consistent with this idea, we found that in addition to promoting radial glial character in vivo, activated Notch1 (ActN1) increased the frequency of embryonic day 14.5 (E14.5) ganglionic eminence (GE) progenitors that grew into neurospheres in FGF2. Constitutive activation of C-promoter binding factor (CBF1), a Notch pathway effector, also increased neurosphere frequency in FGF2, suggesting that the effect of Notch1 on FGF responsiveness is mediated by CBF1. The observation that ActN1 promoted FGF responsiveness in telencephalic progenitors prompted us to examine the effect of FGF pathway activation in vivo. We focused on FGFR2 because it is expressed in radial glia in the GEs where ActN1 increases FGF2 neurosphere frequency, but not in the septum where it does not. Like ActN1, activated FGFR2 (ActFGFR2) promoted radial glial character in vivo. However, unlike ActN1, ActFGFR2 did not enhance neurosphere frequency at E14.5. Additional analysis demonstrated that, unexpectedly, neither ActFGFR2 nor ActFGFR1 could replace the need for ligand in promoting neurosphere proliferation. This study suggests that telencephalic progenitors with radial glial morphology are maintained by interactions between the Notch and FGF pathways, and that the mechanisms by which FGF signaling promotes radial glial character in vivo and progenitor proliferation in vitro can be uncoupled.

Animals↗