Prx1 and 3.2kb Col1a1 promoters target distinct bone cell populations in transgenic mice

Abstract

Bones consist of a number of cell types including osteoblasts and their precursor cells at various stages of differentiation. To analyze cellular organization within the bone, we generated Col1a1CreER-DsRed transgenic mice that express, in osteoblasts, CreER and DsRed under the control of a mouse 3.2 kb Col1a1 promoter. We further crossed Col1a1CreER-DsRed mice with Prx1CreER-GFP mice that express CreER and GFP in osteochondro progenitor cells under the control of a 2.4 kb Prx1 promoter. Since the 3.2 kb Col1a1 promoter becomes active in osteoblasts at early stages of differentiation, and Prx1CreER-GFP-expressing periosteal cells show endogenous Col1a1 expression, we expected to find a cell population in which both the 2.4 kb Prx1 promoter and the 3.2 kb Col1a1 promoter are active. However, our histological and flow cytometric analyses demonstrated that these transgenes are expressed in distinct cell populations. In the periosteum of long bones, Col1a1CreER-DsRed is expressed in the innermost layer directly lining the bone surface, while Prx1CreER-GFP-expressing cells are localized immediately outside of the Col1a1CreER-DsRed-expressing osteoblasts. In the calvaria, Prx1CreER-GFP-expressing cells are also localized in the cranial suture mesenchyme. Our experiments further showed that Col1a1CreER-DsRed-expressing cells lack chondrogenic potential, while the Prx1CreER-GFP-expressing cells show both chondrogenic and osteogenic potential. Our results indicate that Col1a1CreER-DsRed-expressing cells are committed osteoblasts, while Prx1CreER-GFP-expressing cells are osteochondro progenitor cells. The Prx1CreER-GFP and Col1a1CreER-DsRed transgenes will offer novel approaches for analyzing lineage commitment and early stages of osteoblast differentiation under physiologic and pathologic conditions.

Ubiquitin‐specific protease USP 34 controls osteogenic differentiation and bone formation by regulating BMP 2 signaling

“…Next, we investigated the in vivo function of USP34 in BMP2induced bone formation by performing calvarial injection experiments. Prx1-Cre recombines in progenitors not only from long bone, but also in those from calvaria (Elefteriou & Yang, 2011;Xiong et al, 2011;Ouyang et al, 2014). Subcutaneous injection of BMP2 onto the calvaria induces the local recruitment and differentiation of cranial mesenchymal cells into osteoblasts, resulting in the de novo formation of cancellous bone (Mundy et al, 1999;Addison et al, 2014).…”

ERK1 and ERK2 Regulate Chondrocyte Terminal Differentiation During Endochondral Bone Formation

“…These observations suggest that the expansion of the zone of hypertrophic chondrocytes in cKO osx mice is caused by ERK1/2 inactivation in hypertrophic chondrocytes. We further inactivated ERK1/2 in osteoblasts by using the Col1a1CreER-DsRed transgene that does not show Cre recombinase activity in chondrocytes (9) . We found no significant difference in the width of the zone of hypertrophic chondrocytes in Col1a1CreER-DsRed; ERK1 −/− ; ERK2 flox/flox mice compared to controls at E18.5, even though ERK2 was efficiently deleted from osteoblasts in these animals (Fig.…”

Bone repair and stem cells

“…Therefore, periosteal stem cells are likely to retain the capability to regenerate a growth plate after fracture, at least in early life. Interestingly, Prrx1 expression becomes confined to the periosteum in the cambium layer during a postnatal period, immediately outside the osteoblasts lining the bone surface [26]. A lineage-tracing study using Prrx1-creERt reveals that these periosteal cells give rise to some of the chondrocytes and osteoblasts in the fracture callus [27•], suggesting the possibility that Prrx1 -expressing cells in the periosteum include stem cells retaining characteristics similar to those of limb bud mesenchymal cells during early morphogenesis.…”