Foxp1 Controls Cell Fate Commitment And Senescence Of Mesenchymal Stem Cells During Skeletal Aging

Osteoporosis is one of the most prevalent skeletal diseases in aged population.Osteoporosis patients are featured by disrupted bone homeostasis,greater bone resorption than bone formation,osteodynia,decreased bone volume and bone mineral density,as well as increased risk of fracture.Aging highly contributes to the development of osteoporosis.In China,about 20%of over 50-year-old people suffer from osteoporosis;the prevalence of osteoporosis is swiftly increased as people get older from 40 to 60 years old.In osteoporosis patients,bone formation is reduced,whereas bone marrow adiposity is increased.These alterations in bone properties are closely related to the aging of bone marrow mesenchymal stem cells.Bone marrow mesenchymal stem cells（BMSCs）give rise to cells forming the skeleton,including osteoblasts and adipocytes.With aging,differential potential of BMSCs favors adipogenesis rather than osteogenesis,their self-renewal ability reduces gradually,and the cells undergo senescence.However,the molecular mechanisms underlie BMSCs aging have not been clarified clearly.In this study,we focused on the role of transcriptional factor Foxp1in the differentiation and aging process of BMSCs.Here we showed that Foxp1 expression level declined with age in an inverse manner with that of the senescence marker p16^Ink4a.We knocked out Foxp1 in BMSCs and osteo-chondroprogenitor cells using Prx1-Cre,we found that the conditional knock out mice exhibited senile osteoporosis phenotypes such as reduced bone mass and increased bone marrow adipocytes in young age,and the phenotypes became more severe as the mice got older.The BMSCs of Prx1-Cre;Foxp1^fl/fl mice presented premature senile characteristics including decreased proliferation rate,impaired self-renewal and preferring adipogenesis rather than osteogenesis.Meanwhile,we deleted Foxp1 conditionally in Nestin⁺BMSCs utilizing Nestin-Cre.The Nestin-Cre;Foxp1^fl/fl mice showed impaired bone formation and increased bone marrow adiposity.The adipogenic potential of BMSCs from Nestin-Cre;Foxp1^fl/fl mice was enhanced and the osteogenic potential was diminished,which were similar to those of Prx1-Cre;Foxp1^fl/fl mice.Forced expression of Foxp1 could inhibit adipogenesis and promote osteogenesis in primary bone marrow mesenchymal stem cells and mesenchymal cell line C3H10T1/2.At the molecular level,Foxp1 could also interact physically with C/EBPβ/δand directly bind to the promoter of PPARγto attenuate the transcriptional activation of PPARγ,thus Foxp1 prevented BMSCs from differentiating to adipocytes;Foxp1 may inhibit Notch signaling pathway activity through interacting with RBPjk to suppress its transactivation ability,consequently,Foxp1 stimulated osteogenesis of BMSCs.Moreover,Foxp1 regulated the expression of p16^Ink4a directly as a transcriptional repressor.Loss of p16^Ink4a partly rescued the osteoporosis phenotype in Prx1-Cre;Foxp1^fl/fl mice,indicating that Foxp1 inhibit BMSCs senescence,at least in part,by downregulating the expression of p16^Ink4a.Over-expression of FOXP1 in human BMSCs from aged donors improved both their proliferation rate and osteogenic potential.In conclusion,Foxp1 orchestrates cell fate commitment and senescence of BMSCs in an age-dependent and dosage-dependent manner.Foxp1 expression level in BMSCs declines with age.Foxp1 governs BMSCs fate determination through PPARγand Notch signaling.Foxp1controls cell senescence by regulating p16^Ink4a transcription.These findings not only present new insights into the regulation of BMSCs cell fate commitment and aging,but also provide a new therapeutic target of osteoporosis.