FGF and other Ras/MAPK pathway activators counterbalance BMP action during neurogenesis, bone formation, and other aspects of vertebrate development and homeostasis. BMP receptors signal through C-terminal phosphorylation and nuclear translocation of the transcription factor Smad1, whereas MAPKs catalyze inhibitory phosphorylation in the Smad1 linker region. Here we show that linker phosphorylation restricts Smad1 activity by enabling Smad1 recognition by the HECT-domain ubiquitin ligase Smurf1. Besides causing Smad1 polyubiquitination, Smurf1 binding inhibits the interaction of Smad1 with the nuclear translocation factor Nup214. Consequently, MAPK-dependent Smurf1 binding leads Smad1 alternatively to degradation or cytoplasmic retention. Smad1 linker phosphorylation and Smurf1 act as interdependent inputs to control BMP signaling during mouse osteoblast differentiation and Xenopus neural development. Linker phosphorylation is triggered also by BMP, providing feedback control. The interplay between linker phosphorylation, Smurf-dependent ubiquitination, and nucleoporin exclusion enables regulation of BMP action by diverse signals and biological contexts.

The role of microRNAs in embryonic cell fate specification is largely unknown. In vertebrates, the miR-430/427/302 family shows a unique expression signature and is exclusively expressed during early embryogenesis. Here, we comparatively address the embryonic function of miR-302 in human embryonic stem cells (hESCs) and its ortholog miR-427 in Xenopus laevis. Interestingly, we found that this miRNA family displays species-specific target selection among ligands of the Nodal pathway, with a striking conservation of the inhibitors, Lefties, but differential targeting of the activators, Nodals. The Nodal pathway plays a crucial role in germ layer specification. Accordingly, by gain and loss of function experiments in hESCs, we show that miR-302 promotes the mesendodermal lineage at the expense of neuroectoderm formation. Similarly, depletion of miR-427 in Xenopus embryos hinders the organizer formation and leads to severe dorsal mesodermal patterning defects. These findings suggest a crucial role for the miR-430/427/302 family in vertebrate embryogenesis by controlling germ layer specification.

Hepatocyte growth factor induces proliferation, motility and differentiation of epithelial cells through the tyrosine kinase receptor encoded by the MET proto‐oncogene. The cytoplasmic portion of Met (referred to as cyto‐Met) is activated but only weakly transforming. In order to determine the effect of activated Met on hepatocytes, we have targeted truncated Met expression to the liver by incorporating the cDNA into a vector carrying the entire human a‐1‐antitrypsin transcriptional unit. Transgenic expression in the liver of truncated human Met, containing the regulatory and the catalytic cytoplasmic domains, renders hepatocytes constitutively resistant to apoptosis and reproducibly permits immortalization. The emerging stable cell lines are not transformed and maintain a highly differentiated phenotype judged by the retention of epithelial cell polarity and the expression of hepatocyte‐enriched transcription factors as well as hepatic products.

Cellular quiescence is a dormant but reversible cellular state in which cell-cycle entry and proliferation are prevented. Recent studies both in vivo and in vitro demonstrate that quiescence is actively maintained through synergistic interactions between intrinsic and extrinsic signals. Subtypes of adult mammalian stem cells can be maintained in this poised, quiescent state, and subsequently reactivated upon tissue injury to restore homeostasis. However, quiescence can become deregulated in pathological settings. In this review, we discuss the recent advances uncovering intracellular signaling pathways, transcriptional changes, and extracellular cues within the stem cell niche that control induction and exit from quiescence in tissue stem cells. We discuss the implications of quiescence as well as the pharmacological and genetic approaches that are being explored to either induce or prevent quiescence as a therapeutic strategy.

Understanding how distinct cell types arise from multipotent progenitor cells is a major quest in stem cell biology. The liver and pancreas share many aspects of their early development and possibly originate from a common progenitor. However, how liver and pancreas cells diverge from a common endoderm progenitor population and adopt specific fates remains elusive. Using RNA sequencing (RNA-seq), we defined the molecular identity of liver and pancreas progenitors that were isolated from the mouse embryo at two time points, spanning the period when the lineage decision is made. The integration of temporal and spatial gene expression profiles unveiled mutually exclusive signaling signatures in hepatic and pancreatic progenitors. Importantly, we identified the noncanonical Wnt pathway as a potential developmental regulator of this fate decision and capable of inducing the pancreas program in endoderm and liver cells. Our study offers an unprecedented view of gene expression programs in liver and pancreas progenitors and forms the basis for formulating lineage-reprogramming strategies to convert adult hepatic cells into pancreatic cells.

Epithelial-to-mesenchymal transition (EMT) is a coordinated process, occurring both during morphogenesis and tumor progression, that allows epithelial cells to dissociate from initial contacts and migrate to secondary sites. The transcriptional repressors of the Snail family induce EMT in different epithelial cell lines and their expression is strictly correlated with EMT during the development and progression of carcinomas. We have previously shown that EMT in hepatocytes correlates with the downregulation of hepatic differentiation key factors HNFs (hepatocyte nuclear factors), and in particular of HNF4alpha. Here, we demonstrate that Snail overexpression is sufficient (i) to induce EMT in hepatocytes with conversion of morphology, downregulation of several epithelial adhesion molecules, reduction of proliferation and induction of matrix metalloproteinase 2 expression and, (ii) most relevantly, to repress the transcription of the HNF4alpha gene through a direct binding to its promoter. These finding demonstrate that Snail is at the crossroads of the regulation of EMT in hepatocytes by a dual control of epithelial morphogenesis and differentiation.

Vital organs such as the pancreas and the brain lack the capacity for effective regeneration. To overcome this limitation, an emerging strategy consists of converting resident tissue-specific cells into the cell types that are lost due to disease by a process called in vivo lineage reprogramming. Here we discuss recent breakthroughs in regenerating pancreatic β-cells and neurons from various cell types, and highlight fundamental challenges that need to be overcome for the translation of in vivo lineage reprogramming into therapy.

Met murine hepatocyte (MMH) lines were established from livers of transgenic mice expressing constitutively active human Met. These lines harbor two cell types: epithelial cells resembling the parental populations and flattened cells with multiple projections and a dispersed growth habit that are designated palmate. Epithelial cells express the liver-enriched transcription factors HNF4 and HNF1α, and proteins associated with epithelial cell differentiation. Treatments that modulate their differentiation state, including acidic FGF, induce hepatic functions. Palmate cells show none of these properties. However, they can differentiate along the hepatic cell lineage, giving rise to: (a) epithelial cells that express hepatic transcription factors and are competent to express hepatic functions; (b) bile duct-like structures in three-dimensional Matrigel cultures. Derivation of epithelial from palmate cells is confirmed by characterization of the progeny of individually fished cells. Furthermore, karyotype analysis confirms the direction of the phenotypic transition: palmate cells are diploid and the epithelial cells are hypotetraploid. The clonal isolation of the palmate cell, an immortalized nontransformed bipotential cell that does not yet express the liver-enriched transcription factors and is a precursor of the epithelial-hepatocyte in MMH lines, provides a new tool for the study of mechanisms controlling liver development.