Polycomb Regulates Mesoderm Cell Fate-Specification in Embryonic Stem Cells through Activation and Repression Mechanisms

Abstract

Polycomb complexes (PRC1 and PRC2) are essential regulators of epigenetic gene silencing in embryonic and adult stem cells. Emerging evidence suggests that the core subunit composition regulates distinct biological processes, yet little is known about the mechanistic underpinnings of how differently composed Polycomb complexes instruct and maintain cell fate. Here we find that Mel18, also known as Pcgf2 and one of six Pcgf paralogs, uniquely regulates PRC1 to specify mesoderm cell fate in embryonic stem cells. Mechanistically, Mel18 functions as a classical Polycomb protein during early cardiac mesoderm differentiation by repressing pluripotency, lineage specification, late cardiac differentiation, and negative regulators of the BMP pathway. However, Mel18 also positively regulates expression of key mesoderm transcription factors, revealing an unexpected function of Mel18 in gene activation during cardiac differentiation. Taken together, our findings reveal that Mel18 is required to specify PRC1 function in both a context- and stage-specific manner.

Zrf1 controls mesoderm lineage genes and cardiomyocyte differentiation

“…Recently, Mel18, a subunit of PRC1 was shown to impact on mesoderm differentiation by directly controlling the expression of transcription factors essential for early cardiac mesoderm cell specification. 48 Still, many open questions remain as to how PRCs regulate transcription during cardiomyogenesis.…”

“…Based on our data and a previously published report showing the importance of PRC1 in cardiac mesoderm differentiation 48 we decided to focus on the role of Zrf1 in cardiac differentiation. First we analyzed specific signaling genes important for cardiogenic mesoderm induction, which represents the first step of cardiomyogenesis (Fig.…”

From Flies to Mice: The Emerging Role of Non-Canonical PRC1 Members in Mammalian Development

“…Blocking of Pcgf2 up-regulates the expression of the WNT/TCF target Jagged1, a Notch ligand, and consequently activates the Notch pathway [304]. PCGF2 represses pluripotency genes, lineage specification genes, late cardiac differentiation genes, and negative regulators of the Bmp pathway but positively regulates expression of key mesoderm transcription factors (Tbx5, Tbx20, Nkx2.5, Mixl1 and Myl7) during cardiac differentiation due to direct activation of several mesoderm specific genes (Gata4, Hand1, Lhx1 and Six2) [305]. Pcgf2/Mel18 depletion by two specific shRNAs suggested that the protein is not necessary to the self-renewal of ES cells and required for the stability of PHC1 and CBX7 proteins [305].…”

“…PCGF2 represses pluripotency genes, lineage specification genes, late cardiac differentiation genes, and negative regulators of the Bmp pathway but positively regulates expression of key mesoderm transcription factors (Tbx5, Tbx20, Nkx2.5, Mixl1 and Myl7) during cardiac differentiation due to direct activation of several mesoderm specific genes (Gata4, Hand1, Lhx1 and Six2) [305]. Pcgf2/Mel18 depletion by two specific shRNAs suggested that the protein is not necessary to the self-renewal of ES cells and required for the stability of PHC1 and CBX7 proteins [305]. Genome wide ChIP coupled massive parallel sequencing analysis of PCGF2/MEL18 binding sites in during cardiac differentiation of ES cells revealed that PCGF2 has two antagonistic functions.…”

“…However, at the same time, PCGF2/MEL18 is a positive regulator of key mesodermal transcription factors. The protein is required to specify PRC1 function in a developmental stage and context specific manner [305].…”