The right ventricle in tetralogy of Fallot: adaptation to sequential loading.

Right ventricular dysfunction is a major determinant of outcome in patients with complex congenital heart disease, as in tetralogy of Fallot. In these patients, right ventricular dysfunction emerges after initial pressure overload and hypoxemia, which is followed by chronic volume overload due to pulmonary regurgitation after corrective surgery. Myocardial adaptation and the transition to right ventricular failure remain poorly understood. Combining insights from clinical and experimental physiology and myocardial (tissue) data has identified a disease phenotype with important distinctions from other types of heart failure. This phenotype of the right ventricle in tetralogy of Fallot can be described as a syndrome of dysfunctional characteristics affecting both contraction and filling. These characteristics are the end result of several adaptation pathways of the cardiomyocytes, myocardial vasculature and extracellular matrix. As long as the long-term outcome of surgical correction of tetralogy of Fallot remains suboptimal, other treatment strategies need to be explored. Novel insights in failure of adaptation and the role of cardiomyocyte proliferation might provide targets for treatment of the (dysfunctional) right ventricle under stress.

Polycomb protein RYBP activates transcription factor Plagl1 during in vitro cardiac differentiation of mouse embryonic stem cells.

RING1 and YY1 binding protein (RYBP) is primarily known to function as a repressor being a core component of the non-canonical polycomb repressive complexes 1 (ncPRC1s). However, several ncPRC1-independent functions of RYBP have also been described. We previously reported that RYBP is essential for mouse embryonic development and that Rybp null mutant embryonic stem cells cannot form contractile cardiomyocytes (CMCs) in vitro. We also showed that PLAGL1, a cardiac transcription factor, which is often mutated in congenital heart diseases (CHDs), is not expressed in Rybp-null mutant CMCs. However, the underlying mechanism of how RYBP regulates Plagl1 expression was not revealed. Here, we demonstrate that RYBP cooperated with NKX2-5 to transcriptionally activate the P1 and P3 promoters of the Plagl1 gene and that this activation is ncPRC1-independent. We also show that two non-coding RNAs residing in the Plagl1 locus can also regulate the Plagl1 promoters. Finally, PLAGL1 was able to activate Tnnt2, a gene important for contractility of CMCs in transfected HEK293 cells. Our study shows that the activation of Plagl1 by RYBP is important for sarcomere development and contractility, and suggests that RYBP, via its regulatory functions, may contribute to the development of CHDs.

RYBP regulates Pax6 during in vitro neural differentiation of mouse embryonic stem cells.

We have previously reported that RING1 and YY1 binding protein (RYBP) is important for central nervous system development in mice and that Rybp null mutant (Rybp-/-) mouse embryonic stem (ES) cells form more progenitors and less terminally differentiated neural cells than the wild type cells in vitro. Accelerated progenitor formation coincided with a high level of Pax6 expression in the Rybp-/- neural cultures. Since Pax6 is a retinoic acid (RA) inducible gene, we have analyzed whether altered RA signaling contributes to the accelerated progenitor formation and impaired differentiation ability of the Rybp-/- cells. Results suggested that elevated Pax6 expression was driven by the increased activity of the RA signaling pathway in the Rybp-/- neural cultures. RYBP was able to repress Pax6 through its P1 promoter. The repression was further attenuated when RING1, a core member of ncPRC1s was also present. According to this, RYBP and PAX6 were rarely localized in the same wild type cells during in vitro neural differentiation. These results suggest polycomb dependent regulation of Pax6 by RYBP during in vitro neural differentiation. Our results thus provide novel insights on the dynamic regulation of Pax6 and RA signaling by RYBP during mouse neural development.

RYBP is important for cardiac progenitor cell development and sarcomere formation.

We have previously established that epigenetic regulator RING1 and YY1 binding protein (RYBP) is required for the contractility of embryonic stem (ES) cell derived cardiomyocytes (CMCs), suggesting its essential role in contractility. In order to investigate the underlying molecular events of this phenotype, we compared the transcriptomic profile of the wild type and Rybp null mutant ES cells and CMCs differentiated from these cell lines. We identified genes related to ion homeostasis, cell adhesion and sarcomeric organization affected in the Rybp null mutant CMCs, by using hierarchical gene clustering and Gene Ontology analysis. We have also demonstrated that the amount of RYBP is drastically reduced in the terminally differentiated wild type CMCs whilst it is broadly expressed in the early phase of differentiation when progenitors form. We also describe that RYBP is important for the proper expression of key cardiac transcription factors including Mesp1, Shh and Mef2c. These findings identify Rybp as a gene important for both early cardiac gene transcription and consequent sarcomere formation necessary for contractility. Since impairment of sarcomeric function and contractility plays a central role in reduced cardiac pump function leading to heart failures in human, current results might be relevant to the pathophysiology of cardiomyopathies.

Evolving Role of RING1 and YY1 Binding Protein in the Regulation of Germ-Cell-Specific Transcription.

Separation of germline cells from somatic lineages is one of the earliest decisions of embryogenesis. Genes expressed in germline cells include apoptotic and meiotic factors, which are not transcribed in the soma normally, but a number of testis-specific genes are active in numerous cancer types. During germ cell development, germ-cell-specific genes can be regulated by specific transcription factors, retinoic acid signaling and multimeric protein complexes. Non-canonical polycomb repressive complexes, like ncPRC1.6, play a critical role in the regulation of the activity of germ-cell-specific genes. RING1 and YY1 binding protein (RYBP) is one of the core members of the ncPRC1.6. Surprisingly, the role of Rybp in germ cell differentiation has not been defined yet. This review is focusing on the possible role of Rybp in this process. By analyzing whole-genome transcriptome alterations of the Rybp-/- embryonic stem (ES) cells and correlating this data with experimentally identified binding sites of ncPRC1.6 subunits and retinoic acid receptors in ES cells, we propose a model how germ-cell-specific transcription can be governed by an RYBP centered regulatory network, underlining the possible role of RYBP in germ cell differentiation and tumorigenesis.