An in vivo requirement for the mediator subunit med14 in the maintenance of stem cell populations

XIST loss impairs mammary stem cell differentiation and increases tumorigenicity through Mediator hyperactivation

X inactivation (XCI) is triggered by upregulation of XIST, which coats the chromosome in cis, promoting formation of a heterochromatic domain (Xi). XIST role beyond initiation of XCI is only beginning to be elucidated. Here, we demonstrate that XIST loss impairs differentiation of human mammary stem cells (MaSCs) and promotes emergence of highly tumorigenic and metastatic carcinomas. On the Xi, XIST deficiency triggers epigenetic changes and reactivation of genes overlapping Polycomb domains, including Mediator subunit MED14. MED14 overdosage results in increased Mediator levels and hyperactivation of the MaSC enhancer landscape and transcriptional program, making differentiation less favorable. We further demonstrate that loss of XIST and Xi transcriptional instability is common among human breast tumors of poor prognosis. We conclude that XIST is a gatekeeper of human mammary epithelium homeostasis, thus unveiling a paradigm in the control of somatic cell identity with potential consequences for our understanding of gender-specific malignancies.

The Pol II preinitiation complex (PIC) influences Mediator binding but not promoter-enhancer looping

Knowledge of how Mediator and TFIID cross-talk contributes to promoter-enhancer (P-E) communication is important for elucidating the mechanism of enhancer function. We conducted an shRNA knockdown screen in murine embryonic stem cells to identify the functional overlap between Mediator and TFIID subunits on gene expression. Auxin-inducible degrons were constructed for TAF12 and MED4, the subunits eliciting the greatest overlap. Degradation of TAF12 led to a dramatic genome-wide decrease in gene expression accompanied by destruction of TFIID, loss of Pol II preinitiation complex (PIC) at promoters, and significantly decreased Mediator binding to promoters and enhancers. Interestingly, loss of the PIC elicited only a mild effect on P-E looping by promoter capture Hi-C (PCHi-C). Degradation of MED4 had a minor effect on Mediator integrity but led to a consistent twofold loss in gene expression, decreased binding of Pol II to Mediator, and decreased recruitment of Pol II to the promoters, but had no effect on the other PIC components. PCHi-C revealed no consistent effect of MED4 degradation on P-E looping. Collectively, our data show that TAF12 and MED4 contribute mechanistically in different ways to P-E communication but neither factor appears to directly control P-E looping, thereby dissociating P-E communication from physical looping.

Cyclin A2/cyclin-dependent kinase 1-dependent phosphorylation of Top2a is required for S phase entry during retinal development in zebrafish

Cyclin-dependent kinase 1 (CDK1) plays an essential role in cell cycle regulation. However, as mouse Cdk1 embryos die early, the role of CDK1 in regulating the cell cycle and embryo development remains unclear. Here, we showed that zebrafish cdk1^-/- embryos exhibit severe microphthalmia accompanied by multiple defects in S phase entry, M phase progression, and cell differentiation but not in interkinetic nuclear migration. We identified Top2a as a potential downstream target and cyclin A2 and cyclin B1 as partners of Cdk1 in cell cycle regulation via an in silico analysis. While depletion of either cyclin A2 or Top2a led to the decreased S phase entry in zebrafish retinal cells, the depletion of cyclin B1 led to M phase arrest. Moreover, phosphorylation of Top2a at serine 1213 (S1213) was nearly abolished in both cdk1 and ccna2 mutants, but not in ccnb1 mutants. Furthermore, overexpression of TOP2A^S1213D, the phosphomimetic form of human TOP2A, rescued S phase entry and alleviated the microphthalmia defects in both cdk1^-/- and ccna2^-/- embryos. Taken together, our data suggest that Cdk1 interacts with cyclin A2 to regulate S phase entry partially through Top2a phosphorylation and interacts with cyclin B1 to regulate M phase progression.

The Sec domain protein Scfd1 facilitates trafficking of ECM components during chondrogenesis

Chondrogenesis in the developing skeleton requires transformation of chondrocytes from a simple mesenchymal condensation to cells with a highly enriched extracellular matrix (ECM). This transition is in part accomplished by alterations in the chondrocyte protein transport machinery to cope with both the increased amount and large size of ECM components. In a zebrafish mutagenesis screen to identify genes essential for cartilage development, we uncovered a mutant that disrupts the gene encoding Sec1 family domain containing 1 (scfd1). Homozygous scfd1 mutant embryos exhibit a profound craniofacial abnormality caused by a failure of chondrogenesis. Loss of scfd1 was found to hinder ER to Golgi transport of ECM proteins and is accompanied with activation of the unfolded protein response in chondrocytes. We further demonstrate a conserved role for Scfd1 in differentiation of mammalian chondrocytes, in which loss of either SCFD1 or STX18, a SLY1 interacting t-SNARE, severely impair transport of type II collagen. These results show that the existence of a specific export pathway, mediated by a complex containing SCFD1 and STX18 that plays an essential role in secretion of large ECM proteins during chondrogenesis.

Deletion of Pr130 Interrupts Cardiac Development in Zebrafish

Protein phosphatase 2 regulatory subunit B, alpha (PPP2R3A), a regulatory subunit of protein phosphatase 2A (PP2A), is a major serine/threonine phosphatase that regulates crucial function in development and growth. Previous research has implied that PPP2R3A was involved in heart failure, and PR130, the largest transcription of PPP2R3A, functioning in the calcium release of sarcoplasmic reticulum (SR), plays an important role in the excitation-contraction (EC) coupling. To obtain a better understanding of PR130 functions in myocardium and cardiac development, two pr130-deletion zebrafish lines were generated using clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated proteins (Cas) system. Pr130-knockout zebrafish exhibited cardiac looping defects and decreased cardiac function (decreased fractional area and fractional shortening). Hematoxylin and eosin (H&E) staining demonstrated reduced cardiomyocytes. Subsequent transmission electron microscopy revealed that the bright and dark bands were narrowed and blurred, the Z- and M-lines were fogged, and the gaps between longitudinal myocardial fibers were increased. Additionally, increased apoptosis was observed in cardiomyocyte in pr130-knockout zebrafish compared to wild-type (WT). Taken together, our results suggest that pr130 is required for normal myocardium formation and efficient cardiac contractile function.

Med14 cooperates with brg1 in the differentiation of skeletogenic neural crest

Background

An intricate gene regulatory network drives neural crest migration and differentiation. How epigenetic regulators contribute to this process is just starting to be understood.

Results

We found that mutation of med14 or brg1 in zebrafish embryos resulted in a cluster of neural crest cell-related defects. In med14 or brg1 mutants, neural crest cells that form the jaw skeleton were specified normally and migrated to target sites. However, defects in their subsequent terminal differentiation were evident. Transplantation experiments demonstrated that med14 and brg1 are required directly in neural crest cells. Analysis of med14; brg1 double mutant embryos suggested the existence of a strong genetic interaction between members of the Mediator and BAF complexes.

Conclusions

These results suggest a critical role for Mediator and BAF complex function in neural crest development, and may also clarify the nature of defects in some craniofacial abnormalities.