The serially segmented (metameric) structures of vertebrates are based on somites that are periodically formed during embryogenesis. A 'clock and wavefront' model has been proposed to explain the underlying mechanism of somite formation, in which the periodicity is generated by oscillation of Notch components (the clock) in the posterior pre-somitic mesoderm (PSM). This temporal periodicity is then translated into the segmental units in the 'wavefront'. The wavefront is thought to exist in the anterior PSM and progress backwards at a constant rate; however, there has been no direct evidence as to whether the levels of Notch activity really oscillate and how such oscillation is translated into a segmental pattern in the anterior PSM. Here, we have visualized endogenous levels of Notch1 activity in mice, showing that it oscillates in the posterior PSM but is arrested in the anterior PSM. Somite boundaries formed at the interface between Notch1-activated and -repressed domains. Genetic and biochemical studies indicate that this interface is generated by suppression of Notch activity by mesoderm posterior 2 (Mesp2) through induction of the lunatic fringe gene (Lfng). We propose that the oscillation of Notch activity is arrested and translated in the wavefront by Mesp2.

The Notch-signalling pathway is important in establishing metameric pattern during somitogenesis. In mice, the lack of either of two molecules involved in the Notch-signalling pathway, Mesp2 or presenilin-1 (Ps1), results in contrasting phenotypes: caudalized versus rostralized vertebra. Here we adopt a genetic approach to analyse the molecular mechanism underlying the establishment of rostro-caudal polarity in somites. By focusing on the fact that expression of a Notch ligand, Dll1, is important for prefiguring somite identity, we found that Mesp2 initiates establishment of rostro-caudal polarity by controlling two Notch-signalling pathways. Initially, Mesp2 activates a Ps1-independent Notch-signalling cascade to suppress Dll1 expression and specify the rostral half of the somite. Ps1-mediated Notch-signalling is required to induce Dll1 expression in the caudal half of the somite. Therefore, Mesp2- and Ps1-dependent activation of Notch-signalling pathways might differentially regulate Dll1 expression, resulting in the establishment of the rostro-caudal polarity of somites.

Mesp2 is a transcription factor that plays fundamental roles in somitogenesis, and its expression is strictly restricted to the anterior presomitic mesoderm just before segment border formation. The transcriptional on-off cycle is linked to the segmentation clock. In our current study, we show that a T-box transcription factor, Tbx6, is essential for Mesp2 expression. Tbx6 directly binds to the Mesp2 gene upstream region and mediates Notch signaling, and subsequent Mesp2 transcription, in the anterior presomitic mesoderm. Our data therefore reveal that a mechanism, via Tbx6-dependent Notch signaling, acts on the transcriptional regulation of Mesp2. This finding uncovers an additional component of the interacting network of various signaling pathways that are involved in somitogenesis.enhancer ͉ transgenic mouse ͉ RBPJ ͉ luciferase assay

Apo (Apolipoprotein)B secretion, as well as albumin secretion, increased in 3D HepG2 and HepaRG spheroids. Liver metabolic gene expression was up-regulated in 3D HepaRG spheroids. These results suggest that hanging drop 3D cultures can improve hepatocellular responses as a functional liver.

SummaryGut epithelial organoids are routinely used to investigate intestinal biology; however, current culture methods are not amenable to genetic manipulation, and it is difficult to generate sufficient numbers for high-throughput studies. Here, we present an improved culture system of human induced pluripotent stem cell (iPSC)-derived intestinal organoids involving four methodological advances. (1) We adopted a lentiviral vector to readily establish and optimize conditioned medium for human intestinal organoid culture. (2) We obtained intestinal organoids from human iPSCs more efficiently by supplementing WNT3A and fibroblast growth factor 2 to induce differentiation into definitive endoderm. (3) Using 2D culture, followed by re-establishment of organoids, we achieved an efficient transduction of exogenous genes in organoids. (4) We investigated suspension organoid culture without scaffolds for easier harvesting and assays. These techniques enable us to develop, maintain, and expand intestinal organoids readily and quickly at low cost, facilitating high-throughput screening of pathogenic factors and candidate treatments for gastrointestinal diseases.

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During vertebrate embryogenesis, the formation of reiterated structures along the body axis is dependent upon the generation of the somite by segmentation of the presomitic mesoderm (PSM). Notch signaling plays a crucial role in both the generation and regulation of the molecular clock that provides the spatial information for PSM cells to form somites. In a screen for novel genes involved in somitogenesis, we identified a gene encoding a Wnt antagonist, Nkd1, which is transcribed in an oscillatory manner, and may represent a new member of the molecular clock constituents. The transcription of nkd1 is extremely downregulated in the PSM of vestigial tail (vt/vt), a hypomorphic mutant of Wnt3a, whereas nkd1 oscillations have a similar phase to lunatic fringe (L-fng) transcription and they are arrested in Hes7 (a negative regulator of Notch signaling) deficient embryos. These results suggest that the transcription of nkd1 requires Wnt3a, and that its oscillation patterns depend upon the function of Hes7. Wnt signaling has been postulated to be upstream of Notch signaling but we demonstrate in this study that a Wnt-signal-related gene may also be regulated by Notch signaling. Collectively, our data suggest that the reciprocal interaction of Notch and Wnt signals, and of their respective negative feedback loops, function to organize the segmentation clock required for somitogenesis.

In the course of an effort to identify the regulators for peroxisome proliferator-activated receptor g (PPARg)-dependent perilipin gene expression, we found that tribbles homolog 3 (TRB3), containing a single kinase domain without enzymatic activity, downregulates PPARg transcriptional activities by protein-protein interaction. We examined the role that TRB3 plays in adipocyte differentiation in 3T3-L1 cells. TRB3 gene and protein expression was increased during adipocyte differentiation concomitantly with an increase in the mRNA levels of CCAAT/enhancer binding protein homologous protein. The physical interaction between TRB3 and PPARg was also verified in 3T3-L1 adipocytes. Forced TRB3 expression in 3T3-L1 cells decreased the mRNA levels of PPARg-target genes and intracellular triglyceride levels, whereas knockdown of TRB3 expression by RNA interference increased them. TRB3 also inhibits PPARg-dependent adipocyte differentiation in lentivirus-mediated PPARg-expressing 3T3-L1 cells. These results provide evidence that TRB3 acts as a potent negative regulator of PPARg, a master regulator of adipocyte differentiation, and tightly controls adipogenesis.-Takahashi, Y., N. Ohoka, H. Hayashi, and R. Sato. TRB3 suppresses adipocyte differentiation by negatively regulating PPARg transcriptional activity. J. Lipid Res. 2008. 49: 880-892.