Mechanical load regulates bone growth via periosteal Osteocrin

Background Several Cre reporter strains of mice have been described, in which a lacZ gene is turned on in cells expressing Cre recombinase, as well as their daughter cells, following Cre-mediated excision of a loxP-flanked transcriptional "stop" sequence. These mice are useful for cell lineage tracing experiments as well as for monitoring the expression of Cre transgenes. The green fluorescent protein (GFP) and variants such as EYFP and ECFP offer an advantage over lacZ as a reporter, in that they can be easily visualized without recourse to the vital substrates required to visualize β-gal in living tissue. Results In view of the general utility of targeting the ubiquitously expressed ROSA26 locus, we constructed a generic ROSA26 targeting vector. We then generated two reporter lines of mice by inserting EYFP or ECFP cDNAs into the ROSA26 locus, preceded by a loxP-flanked stop sequence. These strains were tested by crossing them with transgenic strains expressing Cre in a ubiquitous (β-actin-Cre) or a cell-specific (Isl1-Cre and En1-Cre) pattern. The resulting EYFP or ECFP expression patterns indicated that the reporter strains function as faithful monitors of Cre activity. Conclusions In contrast to existing lacZ reporter lines, where lacZ expression cannot easily be detected in living tissue, the EYFP and ECFP reporter strains are useful for monitoring the expression of Cre and tracing the lineage of these cells and their descendants in cultured embryos or organs. The non-overlapping emission spectra of EYFP and ECFP make them ideal for double labeling studies in living tissues.

Estrogen prevents osteoporotic bone loss by attenuating bone resorption; however, the molecular basis for this is unknown. Here, we report a critical role for the osteoclastic estrogen receptor alpha (ERalpha) in mediating estrogen-dependent bone maintenance in female mice. We selectively ablated ERalpha in differentiated osteoclasts (ERalpha(DeltaOc/DeltaOc)) and found that ERalpha(DeltaOc/DeltaOc) females, but not males, exhibited trabecular bone loss, similar to the osteoporotic bone phenotype in postmenopausal women. Further, we show that estrogen induced apoptosis and upregulation of Fas ligand (FasL) expression in osteoclasts of the trabecular bones of WT but not ERalpha(DeltaOc/DeltaOc) mice. The expression of ERalpha was also required for the induction of apoptosis by tamoxifen and estrogen in cultured osteoclasts. Our results support a model in which estrogen regulates the life span of mature osteoclasts via the induction of the Fas/FasL system, thereby providing an explanation for the osteoprotective function of estrogen as well as SERMs.

Bone is comprised of separate inner endosteal and outer periosteal compartments, each with distinct contributions to bone physiology and each maintaining separate pools of cells due to physical separation by the bone cortex. While the skeletal stem cell giving rise to endosteal osteoblasts has been extensively studied, the identification of a periosteal stem cell has been elusive 1–5 . Here, we identify a periosteal stem cell (PSC) present in the long bones and calvarium of mice that displays clonal multipotency, self-renewal and sits at the apex of a differentiation hierarchy. Single cell and bulk transcriptional profiling show that PSCs display distinct transcriptional signatures in comparison with both other skeletal stem cells and mature mesenchymal cells. While other skeletal stem cells form bone via an initial cartilage template using the endochondral pathway 4 , PSCs form bone via a direct intramembranous route, providing a cellular basis for the divergence between intramembranous versus endochondral developmental pathways. However there is plasticity in this division, as PSCs acquire endochondral bone formation capacity in response to injury. Genetic blockade of the ability of PSCs to give rise to bone-forming osteoblasts results in selective impairments in cortical bone architecture and defects in fracture healing. A cell analogous to PSCs is present in the human periosteum, raising the possibility that PSCs are attractive targets for drug and cellular therapy for skeletal disorders. Moreover, the identification of PSCs provides evidence that bone contains multiple pools of stem cells, each with distinct physiologic functions.