Notch pathway targets proangiogenic regulator Sox17 to restrict angiogenesis.

RATIONALE: The Notch pathway stabilizes sprouting angiogenesis by favoring stalk cells over tip cells at the vascular front. Because tip and stalk cells have different properties in morphology and function, their transcriptional regulation remains to be distinguished. Transcription factor Sox17 is specifically expressed in endothelial cells, but its expression and role at the vascular front remain largely unknown. OBJECTIVE: To specify the role of Sox17 and its relationship with the Notch pathway in sprouting angiogenesis. METHODS AND RESULTS: Endothelial-specific Sox17 deletion reduces sprouting angiogenesis in mouse embryonic and postnatal vascular development, whereas Sox17 overexpression increases it. Sox17 promotes endothelial migration by destabilizing endothelial junctions and rearranging cytoskeletal structure and upregulates expression of several genes preferentially expressed in tip cells. Interestingly, Sox17 expression is suppressed in stalk cells in which Notch signaling is relatively high. Notch activation by overexpressing Notch intracellular domain reduces Sox17 expression both in primary endothelial cells and in retinal angiogenesis, whereas Notch inhibition by delta-like ligand 4 (Dll4) blockade increases it. The Notch pathway regulates Sox17 expression mainly at the post-transcriptional level. Furthermore, endothelial Sox17 ablation rescues vascular network from excessive tip cell formation and hyperbranching under Notch inhibition in developmental and tumor angiogenesis. CONCLUSIONS: Our findings demonstrate that the Notch pathway restricts sprouting angiogenesis by reducing the expression of proangiogenic regulator Sox17.

Cardiovascular and hematopoietic defects associated with Notch1 activation in embryonic Tie2-expressing populations.

Notch signaling is critical for the development and maintenance of the cardiovasculature, with loss-of-function studies defining roles of Notch1 in the endothelial/hematopoietic lineages. No in vivo studies have addressed complementary gain-of-function strategies within these tissues to define consequences of Notch activation. We developed a transgenic model of Cre recombinase-mediated activation of a constitutively active mouse Notch1 allele (N1ICD(+)) and studied transgene activation in Tie2-expressing lineages. The in vivo phenotype was compared to effects of Notch1 activation on endothelial tubulogenesis, paracrine regulation of smooth muscle cell proliferation, and hematopoiesis. N1ICD(+) embryos showed midgestation lethality with defects in angiogenic remodeling of embryonic and yolk sac vasculature, cardiac development, smooth muscle cell investment of vessels, and hematopoietic differentiation. Angiogenic defects corresponded with impaired endothelial tubulogenesis in vitro following Notch1 activation and paracrine inhibition of smooth muscle cells when grown with Notch1-activated endothelial cells. Flow cytometric analysis of hematopoietic and endothelial precursor populations demonstrated a significant loss of CD71(+)/Ter119(+) populations with an active N1ICD(+) allele and a corresponding increase in c-Kit(+)/CD71 and Flk1(+) populations, suggesting a developmental block during the transition between c-Kit- and Ter119-expressing erythroblasts. Cardiovascular lineages are sensitive to an imbalance in Notch signaling, with aberrant activation reflecting a vascular phenotype comparable to a loss-of-function Notch1 mutation.

Expression analysis of the novel gene collagen triple helix repeat containing-1 (Cthrc1).

We recently identified collagen triple helix repeat containing-1 (Cthrc1) as a novel gene induced in adventitial fibroblasts after arterial injury. Cthrc1 is a 30 kDa secreted protein that has the ability to inhibit collagen matrix synthesis. Cthrc1 is also glycosylated and retains a signal sequence consistent with the presence of Cthrc1 in the extracellular space. In injured arteries and skin wounds, we have found Cthrc1 expression to be associated with myofibroblasts and sites of collagen matrix deposition. Furthermore, we demonstrated that Cthrc1 inhibits collagen matrix deposition in vitro. Using in situ hybridization and immunohistochemistry, we characterized the expression domains of Cthrc1 during murine embryonic development and in postnatal tissues. In mouse embryos, Cthrc1 was expressed in the visceral endoderm, notochord, neural tube, developing kidney, and heart. Abundant expression of Cthrc1 was observed in the developing skeleton, i.e., in cartilage primordia, in growth plate cartilage with exclusion of the hypertrophic zone, in the bone matrix and periostium. Bones from adults showed expression of Cthrc1 only in the bone matrix and periostium while the articular cartilage lacked expression. Cthrc1 is typically expressed at epithelial-mesenchymal interfaces that include the epidermis and dermis, basal corneal epithelium, airway epithelium, esophagus epithelium, choroid plexus epithelium, and meninges. In the adult kidney, collecting ducts and distal tubuli expressed Cthrc1. Collectively, the sites of Cthrc1 expression overlap considerably with those reported for TGF-beta family members and interstitial collagens. The present study provides useful information towards the understanding of potential Cthrc1 functions.

Notch1-induced brain tumor models the sonic hedgehog subgroup of human medulloblastoma.

While activation of the Notch pathway is observed in many human cancers, it is unknown whether elevated Notch1 expression is sufficient to initiate tumorigenesis in most tissues. To test the oncogenic potential of Notch1 in solid tumors, we expressed an activated form of Notch1 (N1ICD) in the developing mouse brain. N1ICD;hGFAP-cre mice were viable but developed severe ataxia and seizures, and died by weaning age. Analysis of transgenic embryo brains revealed that N1ICD expression induced p53-dependent apoptosis. When apoptosis was blocked by genetic deletion of p53, 30% to 40% of N1ICD;GFAP-cre;p53(+/-) and N1ICD;GFAP-cre;p53(-/-) mice developed spontaneous medulloblastomas. Interestingly, N1ICD-induced medulloblastomas most closely resembled the sonic hedgehog subgroup of human medulloblastoma at the molecular level. Surprisingly, N1ICD-induced tumors do not maintain high levels of the Notch pathway gene expression, except for Notch2, showing that initiating oncogenic events may not be decipherable by analyzing growing tumors in some cases. In summary, this study shows that Notch1 has an oncogenic potential in the brain when combined with other oncogenic hits, such as p53 loss, and provides a novel mouse model of medulloblastoma. Cancer Res; 73(17); 5381-90. ©2013 AACR.

Lmx1a encodes a rostral set of mesodiencephalic dopaminergic neurons marked by the Wnt/B-catenin signaling activator R-spondin 2.

Recent developments in molecular programming of mesodiencephalic dopaminergic (mdDA) neurons have led to the identification of many transcription factors playing a role in mdDA specification. LIM homeodomain transcription factor Lmx1a is essential for chick mdDA development, and for the efficient differentiation of ES-cells towards a dopaminergic phenotype. In this study, we aimed towards a more detailed understanding of the subtle phenotype in Lmx1a-deficient (dreher) mice, by means of gene expression profiling. Transcriptome analysis was performed, to elucidate the exact molecular programming underlying the neuronal deficits after loss of Lmx1a. Subsequent expression analysis on brain sections, confirmed that Nurr1 is regulated by Lmx1a, and additional downstream targets were identified, like Pou4f1, Pbx1, Pitx2, C130021l20Rik, Calb2 and Rspo2. In line with a specific, rostral-lateral (prosomer 2/3) loss of expression of most of these genes during development, Nurr1 and C130021l20Rik were affected in the SNc of the mature mdDA system. Interestingly, this deficit was marked by the complete loss of the Wnt/b-catenin signaling activator Rspo2 in this domain. Subsequent analysis of Rspo2-/- embryos revealed affected mdDA neurons, partially phenocopying the Lmx1a mutant. To conclude, our study revealed that Lmx1a is essential for a rostral-lateral subset of the mdDA neuronal field, where it might serve a critical function in modulating proliferation and differentiation of mdDA progenitors through the regulation of the Wnt activator Rspo2.

Mutations in R-spondin 4 (RSPO4) underlie inherited anonychia.

Recently, we reported that mutations in the R-spondin 4 (RSPO4) gene underlie inherited anonychia/hyponychia. Here, we studied five consanguineous Pakistani families with recessive inheritance of a combination of anonychia and hyponychia. Homozygous mutations were identified in the RSPO4 gene in all five families. Three families had a splice site mutation at the exon 2-intron 2 boundary. One family had a 26 bp deletion encompassing the start codon, and the final family had a missense mutation changing the initiating methionine to isoleucine. We demonstrated by in situ hybridization that Rspo4 is exclusively expressed in the mesenchyme underlying the digit tip epithelium in the mouse at embryonic day 14.5 (e14.5). These findings expand our understanding of the role of RSPO4 in nail development and disease.