Enhancers with cooperative Notch binding sites are more resistant to regulation by the Hairless co-repressor

Notch signaling controls many developmental processes by regulating gene expression. Notch-dependent enhancers recruit activation complexes consisting of the Notch intracellular domain, the Cbf/Su(H)/Lag1 (CSL) transcription factor (TF), and the Mastermind co-factor via two types of DNA sites: monomeric CSL sites and cooperative dimer sites called Su(H) paired sites (SPS). Intriguingly, the CSL TF can also bind co-repressors to negatively regulate transcription via these same sites. Here, we tested how synthetic enhancers with monomeric CSL sites versus dimeric SPSs bind Drosophila Su(H) complexes in vitro and mediate transcriptional outcomes in vivo. Our findings reveal that while the Su(H)/Hairless co-repressor complex similarly binds SPS and CSL sites in an additive manner, the Notch activation complex binds SPSs, but not CSL sites, in a cooperative manner. Moreover, transgenic reporters with SPSs mediate stronger, more consistent transcription and are more resistant to increased Hairless co-repressor expression compared to reporters with the same number of CSL sites. These findings support a model in which SPS containing enhancers preferentially recruit cooperative Notch activation complexes over Hairless repression complexes to ensure consistent target gene activation.

Cell signaling provides a basic means of communication during development. Many signaling pathways, including the Notch pathway, convert extracellular signals into changes in gene expression via transcription factors that bind specific DNA sequences. Importantly, the Notch pathway transcription factor can either form activating complexes upon Notch activation to stimulate gene expression or repression complexes with co-repressors to inhibit gene expression. Prior studies showed that the Notch activation complex binds DNA as either an independent complex on monomer binding sites or as two cooperative complexes (dimer) on paired binding sites. In this study, we used synthetic biology to examine how these two types of DNA sites impact the binding of Notch activation versus repression complexes and the output of Notch target gene expression. Our studies reveal that unlike the Notch activation complex, the repression complex does not cooperatively bind dimer sites. Moreover, our findings support the model that the enhanced stability of the Notch activation complex on dimer sites makes target genes with dimer sites less sensitive to the repression complex than target genes with only monomer sites. Thus, our studies reveal how target genes with different binding sites differ in sensitivity to the ratio of Notch activation to repression complexes.

Abstract 918: Pre-B-cell Leukemia Homeobox Interacting Protein 1 is a Novel Regulator of Growth Signaling in the Heart

The cellular mechanisms that allow for cardiomyocyte growth in different directions may yield important information about the development of hypertrophic and dilated cardiomyopathies, diseases which include the widening and lengthening growth of cardiomyocytes, respectively. We have previously found that genetic manipulations of the mitogen-activated protein kinase kinase 1 (MEK1) and extracellular signal-related kinase 1/2 (ERK1/2) signaling pathway cause cardiac hypertrophy and cardiomyocyte widening when the pathway is activated or dilated cardiomyopathy and cardiomyocyte lengthening when it is inhibited. The downstream effectors of the directional growth response controlled by these kinases have yet to be studied. Therefore, an unbiased phosphoproteomic screen was conducted on cardiomyocytes subjected to activation or inhibition of MEK1-ERK1/2 signaling. After validation of the screening hits with western and Phos-tag blots, Pre-B-cell leukemia homeobox interacting protein 1 (PBXIP1) emerged as an interesting target. Although it is known as a microtubule binding protein and regulator of proliferation in cancer cells, its function has never before been studied in the heart. Inhibition of MEK1-ERK1/2 increases phosphorylation of PBXIP1. However, activation of the pathway increases PBXIP1 protein expression, and as such we sought to determine the effects of PBXIP1 overexpression in the heart via adeno-associated virus 9 (AAV9) treatment. At four months of age, mice overexpressing PBXIP1 have significantly larger hearts than controls without a change in cardiac contractility. Intriguingly, histological analysis showed that cardiomyocytes highly infected with PBXIP1 AAV9 are significantly smaller in size compared to uninfected cells. Current studies are manipulating the PBXIP1 phosphorylation sites identified in the phosphoproteomic screen to determine the exact roles they may play in the cardiac growth response.