Temporally controlled targeted somatic mutagenesis in embryonic surface ectoderm and fetal epidermal keratinocytes unveils two distinct developmental functions of BRG1 in limb morphogenesis and skin barrier formation

Abstract

Animal SWI2/SNF2 protein complexes containing either the brahma (BRM) or brahma-related gene 1 (BRG1) ATPase are involved in nucleosome remodelling and may control the accessibility of sequence-specific transcription factors to DNA. In vitro studies have indicated that BRM and BRG1 could regulate the expression of distinct sets of genes. However, as mice lacking BRM are viable and fertile, BRG1 might efficiently compensate for BRM loss. By contrast, as Brg1-null fibroblasts are viable but Brg1-null embryos die during the peri-implantation stage, BRG1 might exert cell-specific functions. To further investigate the in vivo role of BRG1, we selectively ablated Brg1 in keratinocytes of the forming mouse epidermis. We show that BRG1 is selectively required for epithelial-mesenchymal interactions in limb patterning, and during keratinocyte terminal differentiation, in which BRM can partially substitute for BRG1. By contrast, neither BRM nor BRG1 are essential for the proliferation and early differentiation of keratinocytes, which may require other ATP-dependent nucleosome-remodelling complexes. Finally, we demonstrate that cell-specific targeted somatic mutations can be created at various times during the development of mouse embryos cell-specifically expressing the tamoxifen-activatable Cre-ERT2 recombinase.

The SWI/SNF complex and cancer

“…Our data indicate that the loss of both BRG1 alleles induces apoptosis in non-transformed lung epithelial cells. BRG1 is required for the establishment and maintenance of epithelial polarity (Indra et al, 2005), an observation that is consistent with both the pro-apoptotic effects of BRG1 knockout in the lung as well as with the early embryonic lethality of BRG1 knockout in mice. On the other hand, knockout of both BRG1 alleles in urethane-induced tumors promotes tumorigenesis (Glaros et al, 2008).…”

“…RNAimediated knockdown of BAF60 in embryonic stem cells inhibits cardiovascular morphogenesis (Lickert et al, 2004). A dominant-negative BRG1 allele blocks myeloid differentiation of 32Dcl3 cells (Vradii et al, 2006), whereas loss of BRG1 in keratinocytes results in defects in limb patterning (Indra et al, 2005). Studies in embryonic stem cells have revealed that lack of BAF250 can compromise cell pluripotency and severely inhibit self-renewal, whereas functional BAF250 contributes to the proper expression of numerous genes involved in embryonic stem cell self-renewal, including Sox2, Utf1 and Oct4 (Yan et al, 2008) (Gao et al, 2008).…”

Epigenetic Regulation of Epidermal Differentiation

“…Loss of expression of Brg1 and Brm in the epidermis does not considerably affect proliferation, but does lead to aberrant expression of differentiation genes and results in defective formation of the skin barrier (Table 1) (Indra et al 2005). This is consistent with the known role of SWI/SNF in mediating exit from cell cycle in combination with Rb and HDAC (Zhang et al 2000).…”

Brg1 is required for stem cell maintenance in the murine intestinal epithelium in a tissue-specific manner

“…In order to achieve conditional inactivation of Brg1 in the small intestinal epithelium, we crossed mice expressing Cre-ER T2 recombinase under control of the Villin 1 promoter (Vil1) (Tg(Vil-cre/ESR1)23Syr, further abbreviated as VillinCreER T2 ) [20] to animals carrying loxP-flanked alleles of Brg1 [21,22]. The Cre-ER T2 encodes a fusion protein between the Cre recombinase and mutated estrogen receptor and is activated by exposure to tamoxifen.…”