Project description:Repertoires of transcriptional enhancers orchestrate gene expression during embryonic development, thereby shaping the forms and functions of organs. Within these repertoires individual enhancers display spatially distinct or overlapping activities that collectively build up the expression domain of cognate genes. However, the temporal specificity of these enhancers - how their activities change over developmental time to dynamically influence gene expression - remains uncharacterized. Here, we observed that temporally restricted enhancer repertoires are embedded at numerous loci associated with mouse limb development. To monitor how such enhancer repertoires govern gene transcription in vivo across extensive developmental periods, we introduce the regulatory trajectory framework. This paradigm conceptually involves transcriptional initiation, marking the beginning of gene expression, followed by its maintenance over time, and ultimately decommissioning, leading to gene repression. To track and sort cells undergoing these distinct phases, we devised a transgenic recorder approach at the Shox2 model locus. Through this method, we discovered that cells maintaining Shox2 transcription in early and late limb development relies on distinct, temporally restricted enhancer repertoires. We demonstrate that eliminating early- or late-acting enhancers only transiently affects Shox2 expression indicating that these enhancer repertoires function independently. Additionally, we found that changes in the 3D topology of the locus associate with enhancer activities and that a rapid loss of enhancer-promoter contacts occurs during decommissioning. Finally, we show that the decommissioning of the Shox2 locus can be actively driven by Hoxd13, a gene which expression is known to antagonize Shox2. Overall, our work uncovers the dependency of developmental genes on enhancers with temporally restricted activities to generate complex expression patterns over time and shed light on the dynamics of enhancer-promoter interactions.

Project description:ChIP-Sequencing on Shox2-HA E12.5 and E13.5 Limb and Palate, as well as Pbx on E12.5 limb . Abstract: Vertebrate appendage patterning is programmed by Hox-TALE factors-bound regulatory elements. However, it remains enigmatic which cell lineages are commissioned by Hox-TALE factors to generate regional specific pattern and whether other Hox-TALE co-factors exist. In this study, we investigated the transcriptional mechanisms controlled by the Shox2 transcriptional regulator in limb patterning. Harnessing an osteogenic lineage-specific Shox2 inactivation approach we show that despite widespread Shox2 expression in multiple cell lineages, lack of the stylopod observed upon Shox2 deficiency is a specific result of Shox2 loss of function in the osteogenic lineage. ChIP-Seq revealed robust interaction of Shox2 with cis-regulatory enhancers clustering around skeletogenic genes that are also bound by Hox-TALE factors, supporting a lineage autonomous function of Shox2 in osteogenic lineage fate determination and skeleton patterning. Pbx ChIP-Seq further allowed the genome-wide identification of cis-regulatory modules exhibiting co-occupancy of Pbx, Meis, and Shox2 transcriptional regulators. Integrative analysis of ChIP-Seq and RNA-Seq data and transgenic enhancer assays indicate that Shox2 patterns the stylopod as a repressor via interaction with enhancers active in the proximal limb mesenchyme and antagonizes the repressive function of TALE factors in osteogenesis. Shox2/TALE For ChIP-Seq, the list of libraries below, including controls, were generated [listed in the format of (antibody)-target-tissue-stage]: (Î±-HA)-Shox2-Limb-E12.5, (Î±-HA)-Shox2-Limb-E13.5, (Î±-HA)-Shox2-Palate-E12.5, (Î±-HA)-Shox2-Limb/Palate-E12.5, (Î±-Pbx)-Pbx-Limb-E12.5, Input (control), (Î±-HA)-Mixed Limb/Palate from Shox2+/+ mice-E12.5 (control). *The attached signal tracks(*.bigwig) were generated by âbdgcmp (MACS2) to filter out background signal(by filtering against the signal track obtained from (Î±-HA)-Mixed Limb/Palate from Shox2+/+ mice-E12.5 (control)) and subsequently convert to bigwig for analysis and visualization.

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Temporal constraints on enhancer usage shape the regulation of limb gene transcription

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