The complex role of Rcor2: Regulates mesenchymal stromal cell differentiation in vitro but is dispensable in vivo.

Recent research has revealed several important pathways of epigenetic regulation leading to transcriptional changes in bone cells. Rest Corepressor 2 (Rcor2) is a coregulator of Lysine-specific histone demethylase 1 (Lsd1), a demethylase linked to osteoblast activity, hematopoietic stem cell differentiation and malignancy of different neoplasms. However, the role of Rcor2 in osteoblast differentiation has not yet been examined in detail. We have previously shown that Rcor2 is highly expressed in mesenchymal stromal cells (MSC) and particularly in the osteoblastic lineage. The role of Rcor2 in osteoblastic differentiation in vitro was further characterized and we demonstrate here that lentiviral silencing of Rcor2 in MC3T3-E1 cells led to a decrease in osteoblast differentiation. This was indicated by decreased alkaline phosphatase and von Kossa stainings as well as by decreased expression of several osteoblast-related marker genes. RNA-sequencing of the Rcor2-downregulated MC3T3-E1 cells showed decreased repression of Rcor2 target genes, as well as significant upregulation of majority of the differentially expressed genes. While the heterozygous, global loss of Rcor2 in vivo did not lead to a detectable bone phenotype, conditional deletion of Rcor2 in limb-bud mesenchymal cells led to a moderate decrease in cortical bone volume. These findings were not accentuated by challenging bone formation by ovariectomy or tibial fracture. Furthermore, a global deletion of Rcor2 led to decreased white adipose tissue in vivo and decreased the capacity of primary cells to differentiate into adipocytes in vitro. The conditional deletion of Rcor2 led to decreased adiposity in fracture callus. Taken together, these results suggest that epigenetic regulation of mesenchymal stromal cell differentiation is mediated by Rcor2, which could thus play an important role in defining the MSC fate.