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Embryogenesis is guided by a limited set of signaling pathways that are reused at different times and places throughout development. How a context dependent signaling response is generated has been a central question of developmental biology, which can now be addressed with in vitro model systems. Our previous work in human embryonic stem cells (hESCs) established that pre-exposure of cells to WNT/{beta}-catenin signaling is sufficient to switch the output of ACTIVIN/SMAD2 signaling from pluripotency maintenance to mesendoderm (ME) differentiation. A body of previous literature has established the role of both pathways in ME differentiation. However, our work demonstrated that the two signals do not need to be present simultaneously and that hESCs have a means to record WNT signals. Here we demonstrate that hESCs have accessible chromatin at SMAD2 binding sites near pluripotency and ME-associated target genes and that WNT priming does not alter SMAD2 binding. Rather our results indicate that stable transcriptional output at ME genes results from WNT-dependent production of an additional SMAD2 co-factor, EOMES. We show that expression of EOMES can replace WNT signaling in ME differentiation, providing a mechanistic basis for WNT-priming and memory in early development.

Huntington's disease (HD) is a fatal neurodegenerative disease caused by expansion of CAG repeats in the Huntingtin gene (HTT). Neither its pathogenic mechanisms nor the normal functions of HTT are well understood. To model HD in humans, we engineered a genetic allelic series of isogenic human embryonic stem cell (hESC) lines with graded increases in CAG repeat length. Neural differentiation of these lines unveiled a novel developmental HD phenotype: the appearance of giant multinucleated telencephalic neurons at an abundance directly proportional to CAG repeat length, generated by a chromosomal instability and failed cytokinesis over multiple rounds of DNA replication. We conclude that disrupted neurogenesis during development is an important, unrecognized aspect of HD pathogenesis. To address the function of normal HTT protein we generated HTT+/− and HTT−/− lines. Surprisingly, the same phenotype emerged in HTT−/− but not HTT+/− lines. We conclude that HD is a developmental disorder characterized by chromosomal instability that impairs neurogenesis, and that HD represents a genetic dominant-negative loss of function, contrary to the prevalent gain-of-toxic-function hypothesis. The consequences of developmental alterations should be considered as a new target for HD therapies.

The recent FASEB Summer Research Conference entitled ‘The TGFβ Superfamily: Signaling in Development and Disease’ was held in August, 2011 in the spectacular setting of Il Ciocco, Lucca, amidst the olive trees in Tuscany, Italy. The organizers assembled an amazing forum, which included 53 speakers and 67 poster presentations from laboratories around the world, to showcase recent advances made in our understanding of the transforming growth factor-β (TGFβ) signaling pathway.

The TGFβ signaling pathway is a critical regulator of developmental processes and disease. The activity of TGFβ ligands is modulated by various families of soluble inhibitors that interfere with the interactions between ligands and receptors. In an unbiased, genome-wide RNAi screen to identify genes involved in ligand-dependent signaling, we unexpectedly identified the BMP/Activin/Nodal inhibitor Coco as an enhancer of TGFβ1 signaling. Coco synergizes with TGFβ1 in both cell culture and Xenopus explants. Molecularly, Coco binds to TGFβ1 and enhances TGFβ1 binding to its receptor Alk5. Thus, Coco acts both as an inhibitor and an enhancer of signaling depending on the ligand it binds. This finding raises the need for a global reconsideration of the molecular mechanisms regulating TGFβ signaling.

Patterning of the pre-gastrula embryo and subsequent neural induction post-gastrulation are very complex and intricate processes of which little,until recently, has been understood. The earliest decision in neural development, the choice between epidermal or neural fates, is regulated by bone morphogenetic protein (BMP) signaling within the ectoderm. Inhibition of BMP signaling is sufficient for neural induction. Many secreted BMP inhibitors are expressed exclusively within the organizer of the Xenopusgastrula embryo and therefore are predicted to act as bona fide endogenous neural inducers. Other cell-autonomous inhibitors of the BMP pathway are more widely expressed, such as the inhibitory Smads, Smad6 and Smad7. In this report we describe the biological and biochemical characterization of 51-B6, a novel member of Cerberus/Dan family of secreted BMP inhibitors, which we identified in a screen for Smad7-induced genes. This gene is expressed maternally in an animal to vegetal gradient, and its expression levels decline rapidly following gastrulation. In contrast to known BMP inhibitors, 51-B6 is broadly expressed in the ectoderm until the end of gastrulation. The timing,pattern of expression, and activities of this gene makes it unique when compared to other BMP/TGFβ/Wnt secreted inhibitors which are expressed only zygotically and maintained post-gastrulation. We propose that a function of 51-B6 is to block BMP and TGFβ signals in the ectoderm in order to regulate cell fate specification and competence prior to the onset of neural induction. In addition, we demonstrate that 51-B6 can act as a neural inducer and induce ectopic head-like structures in neurula staged embryos. Because of this embryological activity, we have renamed this clone Coco, after the Spanish word meaning head.