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The thyroid hormone receptor TR␣1 regulates intestinal development and homeostasis by controlling epithelial proliferation in the crypts. This involves positive control of the Wnt/␤-catenin pathway. To further investigate the effect of thyroid hormone-TR␣1 signaling on the intestinal epithelium proliferating compartment, we performed a comparative transcription profile analysis on laser microdissected crypt cells recovered from wild type animals with normal or perturbed hormonal status, as well as from TR knock-out mice. Statistical analysis and an in silico approach allowed us to identify 179 differentially regulated genes and to group them into organized functional networks. We focused on the "cell cycle/cell proliferation" network and, in particular, on the Frizzled-related protein sFRP2, whose expression was greatly increased in response to thyroid hormones. In vitro and in vivo analyses showed that the expression of sFRP2 is directly regulated by TR␣1 and that it activates ␤-catenin signaling via Frizzled receptors. Indeed, sFRP2 stabilizes ␤-catenin, activates its target genes, and enhances cell proliferation. In conclusion, these new data, in conjunction with our previous results, indicate a complex interplay between TR␣1 and components of the Wnt/␤-catenin pathway. Moreover, we describe in this study a novel mechanism of action of sFRP2, responsible for the activation of ␤-catenin signaling.

Thyroid hormones and the control of cell proliferation or cell differentiation: paradox or duality?. Molecular and Cellular Endocrinology, Elsevier, 2009, 313 (1-2) This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. SummaryAmphibian metamorphosis perfectly illustrates a key paradox: thyroid hormones control diverse cellular processes depending on the tissue context. This point is also reinforced by a recent accumulation of evidence. For example, thyroid hormones and their nuclear receptor TRs have been described to function in different systems in synergy and/or in antagonism with other signaling pathways. This interaction helps explain their pleiotropic roles. This review summarizes the most important advances in this field, focusing in particular on the key action of thyroid hormones in controlling the balance between the processes of cell proliferation and cell differentiation in a few organs, with special attention paid to the intestine. We highlight similarities between the cellular and molecular events occurring during postnatal intestinal maturation at metamorphosis in amphibians, and comparable events observed at weaning in mice. 1-Introduction1.1 Thyroid hormone production Thyroid hormones (THs), L-thyroxine or T4 and 3,5,3'-L-triiodothyronine or T3, are essential for the development of several organs, including the central nervous system, skeleton, heart, intestine, skeletal muscle and sensory organs. Moreover, they also have important regulatory effects on oxygen consumption and metabolic rate (Oppenheimer et al., 1987). The follicular cells of the thyroid gland synthesize and secrete both hormones, but T4 is the most abundant. This process is under the control of circulating THs levels through the hypothalamuspituitary-thyroid feedback regulatory loop (rev. in Fredric and Wondisford, 2004). The intracellular concentration of T3 is dependent upon the uptake of T3 and T4, and their subsequent metabolism (Bianco and Kim, 2006). In fact, both hormones are actively transported across the cell membrane by specific transporter proteins, of which monocarboxylate transporter-8 is the best characterized (Dumitrescu et al., 2004;Friesema et al., 2004; rev. in Refetoff andDumitrescu, 2007 andVisser et al., 2007). Three iodothyronine deiodinase selenoenzymes (D1, D2 and D3) regulate TH activation and catabolism (Bianco and Kim, 2006). D1 and D2 catalyze the 5'-deiodination of T4 to its active metabolite T3. T3 is generated from the activity of D1, which is the main source of circulating T3. In contrast, D2 is the isoenzyme primarily responsible for local production of T3 in target cells. T3 derived from ...

The intestinal epithelium is very peculiar for its continuous cell renewal, fuelled by multipotent stem cells localized within the crypts of Lieberkühn. Several lines of evidence have established the evolutionary conserved RNA-binding protein Musashi1 as a marker of adult stem cells, including those of the intestinal epithelium, and revealed its roles in stem cell self-renewal and cell fate determination. Previous studies from our laboratories have shown that Musashi1 controls stem cell-like features in medulloblastoma, glioblastoma and breast cancer cells, and has pro-proliferative and pro-tumorigenic properties in intestinal epithelial progenitor cells in vitro. In order to undertake a detailed study of Musashi1’s function in the intestinal epithelium in vivo, we have generated a mouse model, referred to as v-Msi, overexpressing Musashi1 specifically in the entire intestinal epithelium. Compared with wild type litters, v-Msi1 mice exhibited increased intestinal crypt size accompanied by enhanced proliferation. Comparative transcriptomics by RNA-seq revealed Musashi1’s association with gut stem cell signature, cell cycle, DNA replication and drug metabolism. Finally, we identified and validated three novel mRNA targets that are stabilized by Musashi1, Ccnd1 (Cyclin D1), Cdk6 and Sox4. In conclusion, the targeted expression of Musashi1 in the intestinal epithelium in vivo increases the cell proliferation rate and strongly suggests its action on stem cells activity. This is due to the modulation of a complex network of gene functions and pathways including drug metabolism, cell cycle and DNA synthesis and repair.

The generation of induced pluripotent stem (iPS) cells holds great promise in regenerative medicine. The use of the transcription factors Oct4, Sox2, Klf4 and c-Myc for reprogramming is extensively documented, but comparatively little is known about soluble molecules promoting reprogramming. Here we identify the secreted cue Netrin-1 and its receptor DCC, described for their respective survival/death functions in normal and oncogenic contexts, as reprogramming modulators. In various somatic cells, we found that reprogramming is accompanied by a transient transcriptional repression of Netrin-1 mediated by an Mbd3/Mta1/Chd4-containing NuRD complex. Mechanistically, Netrin-1 imbalance induces apoptosis mediated by the receptor DCC in a p53-independent manner. Correction of the Netrin-1/DCC equilibrium constrains apoptosis and improves reprogramming efficiency. Our work also sheds light on Netrin-1's function in protecting embryonic stem cells from apoptosis mediated by its receptor UNC5b, and shows that the treatment with recombinant Netrin-1 improves the generation of mouse and human iPS cells.

UBXN-2, a substrate adaptor of the AAA ATPase CDC-48/p97, is required to coordinate centrosome maturation timing with mitosis.

Thyroid hormones control various aspects of gut development and homeostasis. The best-known example is in gastrointestinal tract remodeling during amphibian metamorphosis. It is well documented that these hormones act via the TR nuclear receptors, which are hormone-modulated transcription factors. Several studies have shown that thyroid hormones regulate the expression of several genes in the Notch signaling pathway, indicating a possible means by which they participate in the control of gut physiology. However, the mechanisms and biological significance of this control have remained unexplored. Using multiple in vivo and in vitro approaches, we show that thyroid hormones positively regulate Notch activity through the TRα1 receptor. From a molecular point of view, TRα1 indirectly controls Notch1, Dll1, Dll4 and Hes1 expression but acts as a direct transcriptional regulator of the Jag1 gene by binding to a responsive element in the Jag1 promoter. Our findings show that the TRα1 nuclear receptor plays a key role in intestinal crypt progenitor/stem cell biology by controlling the Notch pathway and hence the balance between cell proliferation and cell differentiation.