Dissecting components of key transcriptional networks is essential for understanding complex developmental processes and phenotypes. Genetic studies have highlighted the role of members of the Mef2 family of transcription factors as essential regulators in myogenesis from flies to man. To understand how these transcription factors control diverse processes in muscle development, we have combined chromatin immunoprecipitation analysis with gene expression profiling to obtain a temporal map of Mef2 activity during Drosophila embryonic development. This global approach revealed three temporal patterns of Mef2 enhancer binding, providing a glimpse of dynamic enhancer use within the context of a developing embryo. Our results provide mechanistic insight into the regulation of Mef2's activity at the level of DNA binding and suggest cooperativity with the bHLH protein Twist. The number and diversity of new direct target genes indicates a much broader role for Mef2, at all stages of myogenesis, than previously anticipated.

SummarySustained spermatogenesis in adult males and fertility recovery following germ cell depletion are dependent on undifferentiated spermatogonia. We previously demonstrated a key role for the transcription factor SALL4 in spermatogonial differentiation. However, whether SALL4 has broader roles within spermatogonia remains unclear despite its ability to co-regulate genes with PLZF, a transcription factor required for undifferentiated cell maintenance. Through development of inducible knockout models, we show that short-term integrity of differentiating but not undifferentiated populations requires SALL4. However, SALL4 loss was associated with long-term functional decline of undifferentiated spermatogonia and disrupted stem cell-driven regeneration. Mechanistically, SALL4 associated with the NuRD co-repressor and repressed expression of the tumor suppressor genes Foxl1 and Dusp4. Aberrant Foxl1 activation inhibited undifferentiated cell growth and survival, while DUSP4 suppressed self-renewal pathways. We therefore uncover an essential role for SALL4 in maintenance of undifferentiated spermatogonial activity and identify regulatory pathways critical for germline stem cell function.

Whole genome duplication in teleost fish reveals that a few changes in non-regulatory genomic sequences are a source for generating new enhancers.

Our objectives were to compare the cellular and molecular effects of aggregating bovine embryonic vs. somatic cell nuclear transfer (ECNT vs. SCNT) embryos and to determine whether aggregation can improve cattle cloning efficiency. We reconstructed cloned embryos from: 1) morula-derived blastomeres, 2) six adult male ear skin fibroblast lines, 3) one fetal female lung fibroblast line (BFF), and 4) two transgenic clonal strains derived from BFF. Embryos were cultured either singularly (1X) or as aggregates of three (3X). In vitro-fertilized (IVF) 1X and 3X embryos served as controls. After aggregation, the in vitro development of ECNT but not that of SCNT or IVF embryos was strongly compromised. The inner cell mass (ICM), total cell (TC) numbers, and ICM:TC ratios significantly increased for all the aggregates. The relative concentration of the key embryonic transcript POU5F1 (or OCT4) did not correlate with these increases, remaining unchanged in the ECNT and IVF aggregates and decreasing significantly in the SCNT aggregates. Overall, the IVF and 3X ECNT but not the 1X ECNT embryos had significantly higher relative POU5F1 levels than the SCNT embryos. High POU5F1 levels correlated with high in vivo survival, while no such correlation was noted for the ICM:TC ratios. Development to weaning was more than doubled in the ECNT aggregates (10/51 or 20% vs. 7/85 or 8% for 3X vs. 1X, respectively; P < 0.05). In contrast, the SCNT and IVF controls showed no improvement in survival. These data reveal striking biological differences between embryonic and somatic clones in response to aggregation.