Specific positioning of the casein gene cluster in active nuclear domains in luminal mammary epithelial cells

Abstract

The nuclear organization of mammary epithelial cells has been shown to be sensitive to the three-dimensional microenvironment in several models of cultured cells. However, the relationships between the expression and position of genes have not often been explored in animal tissues. We therefore studied the localization of milk protein genes in the nuclei of luminal mammary epithelial cells during lactation as well as in two non-expressing cells, i.e., hepatocytes and the less differentiated embryonic fibroblasts. We compared the position of a cluster of co-regulated genes, encoding caseins (CSN), with that of the whey acidic protein (WAP) gene which is surrounded by genes displaying different expression profiles. We show that the position of the CSN cluster relative to various nuclear compartments is correlated with its activity. In luminal cells, the CSN cluster loops out from its chromosome territory and is positioned in the most euchromatic regions, and frequently associated with elongating RNA polymerase II-rich zones. In hepatocytes and embryonic fibroblasts, the cluster is found preferentially closer to the nuclear periphery. Interestingly, we had previously observed a very peripheral position of the CSN locus in the nuclei of HC11 mammary epithelial cells weakly expressing milk protein genes. We thus show that cultured cell lines are not fully representative of the nuclear organization of genes in a complex and highly organized tissue such as the mammary gland and propose that the spatial positioning of the locus is important to ensuring the optimum control of CSN gene activity observed in the mammary tissue.

DNA Methylation and Transcription in a Distal Region Upstream from the Bovine AlphaS1 Casein Gene after Once or Twice Daily Milking

“…At the other extreme, one copy of demethylated DNA is present in every cell and both copies are demethylated in 20% of cells. This latter hypothesis, under which all cells are expected to express one copy of the CSN gene, fits better with our histological findings (Figure 3) and in situ hybridization results, which revealed an accumulation of CSN transcripts in a large proportion of luminal epithelial cells during lactation [47].…”

Chromatin loops, gene positioning, and gene expression

“…Here localization was stated to be predictive for the developmental state of the T cell (Merkenschlager et al, 2004). Localization of inactive genes to the nuclear periphery was also found for the human CFTR locus (Zink et al, 2004; Ballester et al, 2008) and the casein cluster in mammary glands (Kress et al, 2011). …”

Cow management modulates gene expression in the mammary gland, a possible epigenetics role

“…At the end of lactation, the CpG sites are highly methylated again until the next lactation. Kress et al (2011) reported another regulation mechanism. These authors showed an association between the localization of milk protein genes in the nuclei of luminal mammary epithelial cells and their expression.…”