Pcgf6, a Polycomb Group Protein, Regulates Mesodermal Lineage Differentiation in Murine ESCs and Functions in iPS Reprogramming

Abstract

Polycomb group (PcG) proteins comprise evolutionary conserved factors with essential functions for embryonic development and adult stem cells. PcG proteins constitute two main multiprotein polycomb repressive complexes (PRC1 and PRC2) that operate in a hierarchical manner to silence gene transcription. Functionally distinct PRC1 complexes are defined by Polycomb group RING finger protein (Pcgf) paralogs. So far, six Pcgf paralogs (Pcgf1-6) have been identified as defining components of different PCR1-type complexes. Paralog-specific functions are not well understood. Here, we show that Pcgf6 is the only Pcgf paralog with high expression in undifferentiated embryonic stem cells (ESCs). Upon differentiation Pcgf6 expression declines. Following Pcgf6 kockdown (KD) in ESCs, the expression of pluripotency genes decreased, while mesodermal-and spermatogenesis-specific genes were derepressed. Concomitantly with the elevated expression of mesodermal lineage markers, Pcgf6 KD ESCs showed increased hemangioblastic and hematopoietic activities upon differentiation suggesting a function of Pcgf6 in repressing mesodermal-specific lineage genes. Consistant with a role in pluripotency, Pcgf6 replaced Sox2 in the generation of germline-competent induced pluripotent stem (iPS) cells. Furthermore, Pcgf6 KD in mouse embryonic fibroblasts reduced the formation of ESC-like colonies in OSKMdriven reprogramming. Together, these analyses indicate that Pcgf6 is nonredundantly involved in maintaining the pluripotent nature of ESCs and it functions in iPS reprogramming. STEM CELLS 2014;32:3112-3125

Polycomb Group Protein Pcgf6 Acts as a Master Regulator to Maintain Embryonic Stem Cell Identity

“…Pcgf6 has been shown to be required to maintain ESC self-renewal by an RNAi screen33 and to be the major Posterior sex combs (Psc) component in Cbx7-bound PRC1 complexes in ESCs34. In a recent study, Pcgf6 was identified as a key regulator of mouse ESC pluripotency and self renewal, iPS reprogramming and mesodermal differentiation35. Knockdown of Pcgf6 was shown to decrease expression of core ESC transcription factors Oct4, Sox2 and Nanog, reduce cellular reprogramming and increase mesodermal and testes-specific gene expression35.…”

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

“…The genomic binding profile of PCGF6 is similar to TRXG activators and coincides to active chromatin marks. Knock down of Pcgf6 reduced the expression of core pluripotency regulators Oct4, Sox2 and Nanog [338,340]. Conversely Pcgf6 overexpression prevented downregulation of these factors, impaired differentiation and enhanced reprograming [340].…”

“…NcPRC1.6 type complexes are the most abundant ncPRC1s found in mouse ES cells and have a key role in maintaining pluripotency. During neuronal and hematopoietic differentiation, their amount declines [206,338].…”

“…Consistent with a role in pluripotency PCGF6 could replace SOX2 in the generation of germline competent induced pluripotent stem cells (iPS). Zdzieblo et al found that PCGF6 represses genes involved in spermatogenesis and mesodermal differentiation [338]. PCGF6 is expressed in meiotic and post-meiotic male germ cells in the testis.…”

Functional Crosstalk Between Lysine Methyltransferases on Histone Substrates: The Case of G9A/GLP and Polycomb Repressive Complex 2

“…Indeed, while PRC2 has been shown to facilitate the acquisition of induced pluripotency in different somatic cells (44,101,114,176), G9a/GLP and H3K9 methylation have been instead suggested as a barrier of cellular reprogramming (25,142). Probably, the requirement of PRC2 for an efficient generation of iPSCs relies on its capacity to maintain the silencing of somatic cells transcriptional programs while being involved in repression of pluripotency genes; H3K9 methylation needs instead to be overcome for the reacquisition of pluripotency and an efficient cellular reprogramming (25).…”