Lethal Phenotype-Based Database Screening Identifies Ceramide as a Negative Regulator of Primitive Streak Formation.

In early embryogenesis, the primitive streak (PrS) generates the mesendoderm and is essential for organogenesis. However, because the PrS is a minute and transient tissue, elucidating the mechanism of its formation has been challenging. We performed comprehensive screening of 2 knockout mouse databases based on the fact that failure of PrS formation is lethal. We identified 812 genes involved in various cellular functions and responses that might be linked to PrS formation, with the category of greatest abundance being "Metabolism." In this study, we focused on genes of sphingolipid metabolism and investigated their roles in PrS formation using an in vitro mouse ES cell differentiation system. We show here that elevated intracellular ceramide negatively regulates gene expression essential for PrS formation and instead induces neurogenesis. In addition, sphingosine-1-phosphate (a ceramide derivative) positively regulates neural maturation. Our results indicate that ceramide regulates both PrS formation and the induction of neural differentiation.

Disruption of the thymic microenvironment is associated with thymic involution of transitional cell cancer.

INTRODUCTION: During bladder tumorigenesis, thymopoiesis is usually downregulated. Considering that the thymus is the site of most T-cell development, this phenomenon may be related to thymic involution. However, the mechanisms involved in this phenomenon remain to be elucidated. MATERIALS AND METHODS: An MB 49 murine bladder tumor model was used to identify mechanisms that might underlie this process. RESULTS: The thymuses of tumor-bearing mice showed less cellularity than those of healthy mice. Involution was found to be associated with less proliferation and more apoptosis of thymic epithelial cells (TEC). Foxn1, KGF, and IL-7, three factors known to be involved in thymic development, were also downregulated in the thymuses of tumor bearers. When these mice were intravenously injected with KGF, the thymic microenvironment, thymopoiesis, and T-cell differentiation all returned to near normal status. CONCLUSIONS: The decreases in thymopoiesis and impaired T-cell differentiation may be attributable to changes in the thymic microenvironment. Improving the function of TEC, rather than T-cell progenitors, should be the focus of therapy.

Acetylcholine receptors regulate gene expression that is essential for primitive streak formation in murine embryoid bodies.

Muscarinic acetylcholine receptors (mAchRs) are critical components of the cholinergic system, which is the key regulator of both the central and peripheral nervous systems in mammals. Interestingly, several components of the cholinergic system, including mAchRs and choline acetyltransferase (ChAT), have recently been found to be expressed in mouse embryonic stem (ES) cells and human placenta. These results raise the intriguing possibility that mAchRs play physiological roles in the regulation of early embryogenesis. Early embryogenesis can be mimicked in vitro using an ES cell-based culture system in which the cells form a primitive streak-like structure and efficiently develop into mesodermal progenitors. Here we report that chemical inhibitors specifically targeting mAchRs suppressed the expression of genes essential for primitive streak formation, including Wnt3, and thereby blocked mesodermal progenitor differentiation. Interestingly, mAchR inhibitors also reduced the expression of Cyp26a1, an enzyme involved in the catabolism of retinoic acid (RA). RA is an important regulator of Wnt3 signaling. Our study presents evidence indicating that mAchRs influence RA signaling necessary for the induction of the primitive streak. To our knowledge, this is the first report showing that mAchRs have important functions not only in adult mammals but also during early mammalian embryogenesis.

The mevalonate pathway regulates primitive streak formation via protein farnesylation.

The primitive streak in peri-implantation embryos forms the mesoderm and endoderm and controls cell differentiation. The metabolic cues regulating primitive streak formation remain largely unknown. Here we utilised a mouse embryonic stem (ES) cell differentiation system and a library of well-characterised drugs to identify these metabolic factors. We found that statins, which inhibit the mevalonate metabolic pathway, suppressed primitive streak formation in vitro and in vivo. Using metabolomics and pharmacologic approaches we identified the downstream signalling pathway of mevalonate and revealed that primitive streak formation requires protein farnesylation but not cholesterol synthesis. A tagging-via-substrate approach revealed that nuclear lamin B1 and small G proteins were farnesylated in embryoid bodies and important for primitive streak gene expression. In conclusion, protein farnesylation driven by the mevalonate pathway is a metabolic cue essential for primitive streak formation.

A Modified Murine Embryonic Stem Cell Test for Evaluating the Teratogenic Effects of Drugs on Early Embryogenesis.

Mammalian fetal development is easily disrupted by exogenous agents, making it essential to test new drug candidates for embryotoxicity and teratogenicity. To standardize the testing of drugs that might be used to treat pregnant women, the U.S. Food and Drug Administration (FDA) formulated special grade categories, labeled A, B, C, D and X, that define the level of risk associated with the use of a specific drug during pregnancy. Drugs in categories (Cat.) D and X are those with embryotoxic and/or teratogenic effects on humans and animals. However, which stages of pregnancy are affected by these agents and their molecular mechanisms are unknown. We describe here an embryonic stem cell test (EST) that classifies FDA pregnancy Cat.D and Cat.X drugs into 4 classes based on their differing effects on primitive streak formation. We show that ~84% of Cat.D and Cat.X drugs target this period of embryogenesis. Our results demonstrate that our modified EST can identify how a drug affects early embryogenesis, when it acts, and its molecular mechanism. Our test may thus be a useful addition to the drug safety testing armamentarium.

Transcriptome analysis of murine thymic epithelial cells reveals age­associated changes in microRNA expression.

Age-related thymic involution is accompanied by a decrease in thymopoiesis and, thus, a deficiency in T cell-mediated immunity in the elderly. A number of events involved in thymic involution have been discovered; however, it remains unclear as to whether they are causes or consequences of thymic involution. These events include the degeneration of T cell progenitors, as well as the deterioration of the thymic stroma, which is a characteristic of thymic epithelial cell loss due to increased apoptosis and decreased cell proliferation. MicroRNAs (miRNAs) are believed to play important roles in the regulation of cell death and proliferation during the aging process. In the present study, we compared the transcriptional levels of various miRNAs in TECs from young and aged mice using microarray analysis. Quantitative PCR was performed to confirm the changes in the expression of miRNAs in the different age groups. Possible downstream targets and pathways of these miRNAs were predicted by performing bioinformatics analysis. To the best of our knowledge, this is the first study to systematically analyze the expression of miRNAs in mouse TECs and to demonstrate that miRNA expression is altered with thymic aging.