RBPjκ-dependent Notch signaling regulates mesenchymal progenitor cell proliferation and differentiation during skeletal development

The Notch pathway has recently been implicated in mesenchymal progenitor cell (MPC) differentiation from bone marrow-derived progenitors. However, whether Notch regulates MPC differentiation in an RBPjκ-dependent manner, specifies a particular MPC cell fate, regulates MPC proliferation and differentiation during early skeletal development or controls specific Notch target genes to regulate these processes remains unclear. To determine the exact role and mode of action for the Notch pathway in MPCs during skeletal development, we analyzed tissue-specific loss-of-function (Prx1Cre; Rbpjk ^f/f), gain-of-function (Prx1Cre; Rosa-NICD^f/+) and RBPjκ-independent Notch gain-of-function (Prx1Cre; Rosa-NICD ^f/+; Rbpjk^f/f) mice for defects in MPC proliferation and differentiation. These data demonstrate for the first time that the RBPjκ-dependent Notch signaling pathway is a crucial regulator of MPC proliferation and differentiation during skeletal development. Our study also implicates the Notch pathway as a general suppressor of MPC differentiation that does not bias lineage allocation. Finally, Hes1 was identified as an RBPjκ-dependent Notch target gene important for MPC maintenance and the suppression of in vitro chondrogenesis.