Balancing signals derived from the TGFb family is crucial for regulating cell proliferation and differentiation, and in establishing the embryonic axis during development. TGFb/BMP signaling leads to the activation and nuclear translocation of Smad proteins, which activate transcription of speci®c target genes by recruiting P/CAF and p300. The two members of the ZEB family of zinc ®nger factors (ZEB-1/dEF1 and ZEB-2/SIP1) regulate TGFb/BMP signaling in opposite ways: ZEB-1/dEF1 synergizes with Smadmediated transcriptional activation, while ZEB-2/SIP1 represses it. Here we report that these antagonistic effects by the ZEB proteins arise from the differential recruitment of transcriptional coactivators (p300 and P/CAF) and corepressors (CtBP) to the Smads. Thus, while ZEB-1/dEF1 binds to p300 and promotes the formation of a p300±Smad transcriptional complex, ZEB-2/SIP1 acts as a repressor by recruiting CtBP. This model of regulation by ZEB proteins also functions in vivo, where they have opposing effects on the regulation of TGFb family-dependent genes during Xenopus development.

Precise control of cell proliferation and differentiation is critical for organogenesis. Geminin (Gem) has been proposed to link cell cycle exit and differentiation as a prodifferentiation factor and plays a role in neural cell fate acquisition. Here, we identified the SWI/SNF chromatin-remodeling protein Brg1 as an interacting partner of Gem. Brg1 has been implicated in cell cycle withdrawal and cellular differentiation. Surprisingly, we discovered that Gem antagonizes Brg1 activity during neurogenesis to maintain the undifferentiated cell state. Down-regulation of Gem expression normally precedes neuronal differentiation, and gain-and loss-of-function experiments in Xenopus embryos and mouse P19 cells demonstrated that Gem was essential to prevent premature neurogenesis. Misexpression of Gem also suppressed ectopic neurogenesis driven by Ngn and NeuroD. Gem's activity to block differentiation depended upon its ability to bind Brg1 and could be mediated by Gem's inhibition of proneural basic helix-loop-helix (bHLH)-Brg1 interactions required for bHLH target gene activation. Our data demonstrate a novel mechanism of Gem activity, through regulation of SWI/SNF chromatin-remodeling proteins, and indicate that Gem is an essential regulator of neurogenesis that can control the timing of neural progenitor differentiation and maintain the undifferentiated cell state.[Keywords: Brg1; Geminin; neurogenesis; proneural bHLH; SWI/SNF chromatin-remodeling complex; Xenopus] Supplemental material is available at http://www.genesdev.org.

Proneural basic helix-loop-helix proteins are key regulators of neurogenesis but their 'proneural' function is not well understood, partly because primary targets have not been systematically defined. Here, we identified direct transcriptional targets of the bHLH proteins Neurogenin and NeuroD and found that primary roles of these transcription factors are to induce regulators of transcription, signal transduction, and cytoskeletal rearrangement for neuronal differentiation and migration. We determined targets induced in both Xenopus and mouse, which represent evolutionarily conserved core mediators of Neurogenin and NeuroD activities. We defined consensus sequences for Neurogenin and NeuroD binding and identified responsive enhancers in seven shared target genes. These enhancers commonly contained clustered, conserved consensus-binding sites and drove neural-restricted transgene expression in Xenopus embryos. We then used this enhancer signature in a genome-wide computational approach to predict additional Neurogenin/ NeuroD target genes involved in neurogenesis. Taken together, these data demonstrate that Neurogenin and NeuroD preferentially recognize neurogenesis-related targets through an enhancer signature of clustered consensus-binding sites and regulate neurogenesis by activating a core set of transcription factors, which build a robust network controlling neurogenesis.

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Snail family transcriptional repressors regulate epithelial mesenchymal transitions during physiological and pathological processes. A conserved SNAG repression domain present in all vertebrate Snail proteins is necessary for repressor complex assembly. Here, we identify the Ajuba family of LIM proteins as functional corepressors of the Snail family via an interaction with the SNAG domain. Ajuba LIM proteins interact with Snail in the nucleus on endogenous E-cadherin promoters and contribute to Snail-dependent repression of E-cadherin. Using Xenopus neural crest as a model of in vivo Snail- or Slug-induced EMT, we demonstrate that Ajuba LIM proteins contribute to neural crest development as Snail/Slug corepressors and are required for in vivo Snail/Slug function. Because Ajuba LIM proteins are also components of adherens junctions and contribute to their assembly or stability, their functional interaction with Snail proteins in the nucleus suggests that Ajuba LIM proteins are important regulators of epithelia dynamics communicating surface events with nuclear responses.