MicroRNAs (miRNAs) are short non-coding RNAs that have been implicated in fine-tuning gene regulation, although the precise roles of many are still unknown. Pancreatic development is characterized by the complex sequential expression of a gamut of transcription factors. We have performed miRNA expression profiling at two key stages of mouse embryonic pancreas development, e14.5 and e18.5. miR-124a2 expression was strikingly increased at e18.5 compared with e14.5, suggesting a possible role in differentiated ␤-cells. Among the potential miR124a gene targets identified by biocomputation, Foxa2 is known to play a role in ␤-cell differentiation. To evaluate the impact of miR-124a2 on gene expression, we overexpressed or down-regulated miR-124a2 in MIN6 ␤-cells. As predicted, miR-124a2 regulated Foxa2 gene expression, and that of its downstream target, pancreatic duodenum homeobox-1 (Pdx-1). Foxa2 has been described as a master regulator of pancreatic development and also of genes involved in glucose metabolism and insulin secretion, including the ATP-sensitive K ؉ (K ATP ) channel subunits, Kir6.2 and Sur-1. Correspondingly, miR-124a2 overexpression decreased, and anti-miR-124a2 increased Kir6.2 and Sur-1 mRNA levels. Moreover, miR-124a2 modified basal and glucose-or KCl-stimulated intracellular free Ca 2؉ concentrations in single MIN6 and INS-1 (832/13) ␤-cells, without affecting the secretion of insulin or co-transfected human growth hormone, consistent with an altered sensitivity of the ␤-cell exocytotic machinery to Ca 2؉ . In conclusion, whereas the precise role of microRNA-124a2 in pancreatic development remains to be deciphered, we identify it as a regulator of a key transcriptional protein network in ␤-cells responsible for modulating intracellular signaling.Studies implicating small regulatory RNAs in the control of gene expression have demonstrated that transcriptional regulation is controlled not only by protein factors, but also by small endogenous RNA molecules of ϳ19 -23 nucleotides in length called microRNAs (miRNAs or miRs) 3 (1, 2). The first miRNAs discovered were lin-4 and let-7, which are crucial for regulating the developmental timing in the nematode, Caenorhabditis elegans (1, 3). Since this initial report, several hundred miRNAs have been identified in plants and animals that regulate diverse biological processes ranging from cell metabolism to cell differentiation and growth, apoptosis, and immune responses (4 -8). Moreover, it has been shown that miRNAs are characterized by differential spatial and temporal expression patterns supporting their role in such processes (3, 9). miRNAs serve as regulators of gene expression by binding to complementary sites on their target transcripts and, by an ill-defined mechanism, significantly induce the cleavage of mRNA or the repression of translation, depending on the partial or complete sequence homology, respectively (2, 10 -12). It has been estimated that miRNA genes represent ϳ1% of the genome of complex organisms. It appears that they share a certain...

A high frequency (about 60%) of ret rearrangements in papillary thyroid carcinomas of children exposed to radioactive fallout in Belarus after the Chernobyl accident, has been reported by three recent studies Ito et al., 1994;Klugbauer et al., 1995). These studies suggested that the radiation exposure may be a direct inducer of activating rearrangements in the ret gene. In order to con®rm the postulated link between irradiation and the role of the ret protooncogene in thyroid tumorigenesis, we analysed for the presence of ret activating rearrangements using RT ± PCR, XL ± PCR, Southern blot and direct sequencing techniques, 39 human thyroid tumors (19 papillary carcinomas and 20 follicular adenomas), from patients who had received external radiation for benign or malignant conditions. As controls, we studied 39 spontaneous' tumors (20 papillary carcinomas and 19 follicular adenomas). Our data concerning the radiationassociated tumors, showed that: (1) the overall frequency of ret rearrangements was 84% in papillary carcinomas (16/19) and 45% (9/20) in follicular adenomas; (2) in contrast with the results obtained in the Chernobyl tumors, the most frequently observed chimeric gene was RET/PTC1 instead of the RET/PTC3 and (3) all the tumors were negative for RET/PTC2. In the`spontaneous' tumors, only the papillary carcinomas presented a ret rearrangement (15% : 3/20): 1 RET/PTC1, 1 RET/ PTC3 and 1 uncharacterized. In conclusion, our results con®rm the crucial role played by the ret proto-oncogene activating rearrangements in the development of radiation-associated thyroid tumors appearing after therapeutic or accidental ionizing irradiation, and show, for the ®rst time, the presence of RET/PTC genes in follicular adenomas appeared after external irradiation.

The importance of mesenchymal-epithelial interactions in the proliferation of pancreatic progenitor cells is well established. Here, we provide evidence that the mesenchyme also controls the timing of ␤-cell differentiation. When rat embryonic pancreatic epithelium was cultured without mesenchyme, we found first rapid induction in epithelial progenitor cells of the transcription factor neurogenin3 (Ngn3), a master gene controlling endocrine cell-fate decisions in progenitor cells; then ␤-cell differentiation occurred. In the presence of mesenchyme, Ngn3 induction was delayed, and few ␤-cells developed. This effect of the mesenchyme on Ngn3 induction was mediated by cell-cell contacts and required a functional Notch pathway. We then showed that associating Ngn3-expressing epithelial cells with mesenchyme resulted in poor ␤-cell development via a mechanism mediated by soluble factors. Thus, in addition to its effect upstream of Ngn3, the mesenchyme regulated ␤-cell differentiation downstream of Ngn3. In conclusion, these data indicate that the mesenchyme controls the timing of ␤-cell differentiation both upstream and downstream of Ngn3. Diabetes 55:582-589, 2006