Gli1 labels progenitors during chondrogenesis in postnatal mice

Skeletal growth promoted by endochondral ossification is tightly coordinated by self-renewal and differentiation of chondrogenic progenitors. Emerging evidence has shown that multiple skeletal stem cells (SSCs) participate in cartilage formation. However, as yet, no study has reported the existence of common long-lasting chondrogenic progenitors in various types of cartilage. Here, we identified Gli1+ chondrogenic progenitors (Gli1+ CPs), which were distinct from SSCs, were responsible for the lifelong generation of chondrocytes in the growth plate, vertebrae, ribs, and other cartilage. The absence of Gli1+ CPs led to cartilage defects and dwarfishness phenotype in mice. Furthermore, we found that the BMP signal played an important role in self-renewal and maintenance of Gli1+ CPs. The deletion of Bmpr1a caused the exhaustion of Gli1+ CPs, consequently disrupting columnar cartilage. Collectively, our data demonstrate that Gli1+ CPs are common long-term chondrogenic progenitors in multiple types of cartilage and are essential to maintain cartilage homeostasis. Overall design: Primary chondrocytes were stained with anti-CD45, anti-CD31 and anti-Ter119 primary antibody as described previously. Cell sorting was performed on a FACS Aria III cell sorter (BD Biosciences) to collect CD45-CD31-Ter119-tdTomato+ and CD45-CD31-Ter119-tdTomato- populations. The total RNA sample from each population was extracted separately using TRIzol and the RNeasy mini kit (Qiagen, Germany).