Osteoblastic erythropoietin is not required for bone mass accrual.

Erythropoietin (EPO), primarily produced by interstitial fibroblasts in the kidney during adulthood, and its receptor are well-known for their crucial role in regulating erythropoiesis. Recent research has unveiled an additional function of circulating EPO in the control of bone mass accrual and homeostasis through its receptor, which is expressed in both osteoblasts and osteoclasts. Notably, cells of the osteoblast lineage can produce and secrete functional EPO upon activation of the hypoxia signaling pathway. However, the physiological relevance of osteoblastic EPO remains to be fully elucidated. This study aimed to investigate the potential role of osteoblastic EPO in regulating bone mass accrual and erythropoiesis in young adult mice. To accomplish this, we employed a mutant mouse model lacking EPO specifically in mesenchymal progenitors and their descendants. Our findings indicate that in vivo loss of EPO in the osteoblast lineage does not significantly affect either bone mass accrual or erythropoiesis in young adult mice. Further investigations are necessary to comprehensively understand the potential contribution of EPO produced and secreted by osteoblast cells during aging, repair, and under pathological conditions.

HIF1 activation safeguards cortical bone formation against impaired oxidative phosphorylation.

Energy metabolism, through pathways such as oxidative phosphorylation (OxPhos) and glycolysis, plays a pivotal role in cellular differentiation and function. Our study investigates the impact of OxPhos disruption in cortical bone development by deleting Mitochondrial Transcription Factor A (TFAM). TFAM controls OxPhos by regulating the transcription of mitochondrial genes. The cortical bone, constituting the long bones' rigid shell, is sheathed by the periosteum, a connective tissue layer populated with skeletal progenitors that spawn osteoblasts, the bone-forming cells. TFAM-deficient mice presented with thinner cortical bone, spontaneous midshaft fractures, and compromised periosteal cell bioenergetics, characterized by reduced ATP levels. Additionally, they exhibited an enlarged periosteal progenitor cell pool with impaired osteoblast differentiation. Increasing Hypoxia-Inducible Factor 1a (HIF1) activity within periosteal cells significantly mitigated the detrimental effects induced by TFAM deletion. HIF1 is known to promote glycolysis in all cell types. Our findings underscore the indispensability of OxPhos for the proper accrual of cortical bone mass and indicate a compensatory mechanism between OxPhos and glycolysis in periosteal cells. The study opens new avenues for understanding the relationship between energy metabolism and skeletal health and suggests that modulating bioenergetic pathways may provide a therapeutic avenue for conditions characterized by bone fragility.