影响骨髓间质干细胞向成骨细胞分化的调控因素

长期的骨骼废用引起的骨质减少主要归因于骨形成的减少,成骨细胞由具有多向分化潜能的间充质细胞经骨原细胞、前成骨细胞分化而来,骨髓间质干细胞是骨髓来源的具有多向分化潜能的干细胞,本文综述了骨髓间质干细胞向成骨细胞分化的调控因素,有助于增加对骨丢失的理解,并进行预防和治疗,为航天员和骨骼废用病人创造更健康的生活。     

Playing with bone and fat

The relationship between bone and fat formation within the bone marrow microenvironment is complex and remains an area of active investigation. Classical in vitro and in vivo studies strongly support an inverse relationship between the commitment of bone marrow-derived mesenchymal stem cells or stromal cells to the adipocyte and osteoblast lineage pathways. In this review, we focus on the recent literature exploring the mechanisms underlying these differentiation events and discuss their implications relevant to osteoporosis and regenerative medicine.     

骨髓来源的间充质干细胞分化能力的鉴定

本文研究了人骨髓来源的间充质干细胞(MSCs)的成骨及成脂分化的潜能.通过加入诱导成骨的诱导剂,人的MSCs出现成骨分化的机箱,通过碱性磷酸酶活性测定,茜素红染色及主要调控基因BMP2和Runx2的表达,确定了MSCs具有成骨分化的潜能.对于成脂分化,通过油红O染色,及主要标志基因PPARγ的表达确定其具有成脂分化的潜能.所以,从骨髓分离的到的MSCs纯度达到标准,并且具有成骨成脂分化的多向潜能,是一种理想的实验模型细胞.     

Cannabinoid Receptor Type 1 Protects against Age- Related Osteoporosis by Regulating Osteoblast and Adipocyte Differentiation in Marrow Stromal Cells

Age-related osteoporosis is characterized by reduced bone formation and accumulation of fat in the bone marrow compartment. Here, we report that the type 1 cannabinoid receptor (CB1) regulates this process. Mice with CB1 deficiency (CB1^−/−) had increased peak bone mass due to reduced bone resorption, but developed age-related osteoporosis with reduced bone formation and accumulation of adipocytes in the bone marrow space. Marrow stromal cells from CB1^−/− mice had an enhanced capacity for adipocyte differentiation, a reduced capacity for osteoblast differentiation, and increased expression of phosphorylated CREB (pCREB) and PPARγ. Pharmacological blockade of CB1 receptors stimulated adipocyte differentiation, inhibited osteoblast differentiation, and increased cAMP and pCREB in osteoblast and adipocyte precursors. The CB1 receptor is therefore unique in that it regulates peak bone mass through an effect on osteoclast activity, but protects against age-related bone loss by regulating adipocyte and osteoblast differentiation of bone marrow stromal cells.     

The cytoskeletal regulatory scaffold protein GIT2 modulates mesenchymal stem cell differentiation and osteoblastogenesis

G protein-coupled receptor kinase interacting protein 2 (GIT2) is a signaling scaffold protein involved in the regulation of cytoskeletal structure, membrane trafficking, and G protein-coupled receptor internalization. Since dynamic cytoskeletal reorganization plays key roles both in osteoblast differentiation and in the maintenance of osteoclast polarity during bone resorption, we hypothesized that skeletal physiology would be altered in GIT2(-/-) mice. We found that adult GIT2(-/-) mice have decreased bone mineral density and bone volume in both the trabecular and cortical compartments. This osteopenia was associated with decreased numbers of mature osteoblasts, diminished osteoblastic activity, and increased marrow adiposity, suggesting a defect in osteoblast maturation. In vitro, mesenchymal stem cells derived from GIT2(-/-) mice exhibited impaired differentiation into osteoblasts and increased adipocyte differentiation, consistent with a role for GIT2 in mesenchymal stem cell fate determination. Despite elevated osteoclast inducing cytokines and osteoclast numbers, GIT2(-/-) mice also exhibit impaired bone resorption, consistent with a further role for GIT2 in regulating osteoclast function. Collectively, these findings underscore the importance of the cytoskeleton in both osteoblast and osteoclast function and demonstrate that GIT2 plays essential roles in skeletal metabolism, affecting both bone formation and bone resorption in vivo.     

EMF acts on rat bone marrow mesenchymal stem cells to promote differentiation to osteoblasts and to inhibit differentiation to adipocytes

The use of electromagnetic fields (EMFs) to treat nonunion fractures developed from observations in the mid‐1900s. Whether EMF directly regulates the bone marrow mesenchymal stem cells (MSCs), differentiating into osteoblasts or adipocytes, remains unknown. In the present study, we investigated the roles of sinusoidal EMF of 15 Hz, 1 mT in differentiation along these separate lineages using rat bone marrow MSCs. Our results showed that EMF promoted osteogenic differentiation of the stem cells and concurrently inhibited adipocyte formation. EMF increased alkaline phosphatase (ALP) activity and mineralized nodule formation, and stimulated osteoblast‐specific mRNA expression of RUNX2, ALP, BMP2, DLX5, and BSP. In contrast, EMF decreased adipogenesis and inhibited adipocyte‐specific mRNA expression of adipsin, AP‐2, and PPARγ2, and also inhibited protein expression of PPARγ2. These observations suggest that commitment of MSCs into osteogenic or adipogenic lineages is influenced by EMF. Bioelectromagnetics 31:277–285, 2010. © 2009 Wiley‐Liss, Inc.     

Reciprocal control of osteoblast/chondroblast and osteoblast/adipocyte differentiation of multipotential clonal human marrow stromal F/STRO-1(+) cells

The regulation of human bone marrow stromal precursor cell differentiation toward the chondrocyte, osteoblast or adipocyte lineages is not known. In this study, we assessed the lineage-specific differentiation and conversion of immortalized clonal F/STRO-1(+) A human fetal bone marrow stromal cells under the control of dexamethasone (Dex), indomethacin/insulin (Indo/Ins) and linoleic acid (LA). Under basal conditions, F/STRO-1(+) A cells expressed markers mRNAs or proteins of the osteoblast lineage [CBFA1, osteocalcin (OC), alkaline phosphatase (ALP), type 1 collagen], of the chondrocyte lineage (aggrecan, types 2, 9 and 10 collagen), and of the adipocyte lineage (PPARgamma2, C/EBPalpha, aP2, G3PDH, lipoprotein lipase, leptin). Treatment with Dex increased CBFA1, OC and ALP mRNA and protein levels. Exposure to LA enhanced expression of adipocytic genes and cytoplasmic triglycerides accumulation, and suppressed the Dex-induced stimulation of osteoblast marker genes. Indo/Ins stimulated the synthesis of aggrecan and type 2 collagen and increased types 9 and 10 collagen mRNA levels, and suppressed both basal and Dex-promoted expression of osteoblast markers. Conversely, stimulation of osteoblastogenesis by Dex suppressed both basal and Indo/Ins-stimulated chondrocyte genes. Thus, the clonal human fetal bone marrow stromal F/STRO-1(+) A cell line is a lineage-unrestricted common progenitor that expresses tripotential adipocyte, osteoblast or chondrocyte characteristics. Our data also show that differentiation towards one pathway in response to Dex, Indo/Ins and LA restricts expression of other lineage-specific genes, and provide evidence for a controlled reciprocal regulation of osteoblast/chondroblast and osteoblast/adipocyte differentiation of clonal F/STRO-1(+) human bone marrow stromal cells.     

Wnt 信号通路与骨髓间充质干细胞成骨之间的关系

Wnt信号通路是由Wnts诱发的一系列相互作用的分子组成。Wnt信号对骨髓间充质干细胞的影响在所有研究中均证实有明显作用,其可调节干细胞增殖、分化及凋亡。研究表明,抑制Wnt信号通路转导可使成骨细胞分化进程受阻,从而抑制骨形成;若诱导Wnt家族成员表达则可使成骨细胞特异性基因表达增加,促进骨形成。本文就Wnt信号通路的作用过程及其与骨髓间充质干细胞成骨诱导的关系做一综述。     

Osteoblastic cells: differentiation and trans-differentiation

The osteoblast is the bone forming cell and is derived from mesenchymal stem cells (MSC) present among the bone marrow stroma. MSC are capable of multi-lineage differentiation into mesoderm-type cells such as osteoblasts and adipocytes. Understanding the mechanisms underlying osteoblast differentiation from MSC is a central topic in bone biology that can provide insight into mechanisms of bone maintenance and also novel pharmacological targets to increase osteoblast differentiation and consequently bone formation.     

Mesenchymal stem cell aging: Mechanisms and influences on skeletal and non-skeletal tissues

The aging population and the incidence of aging-related diseases such as osteoporosis are on the rise. Aging at the tissue and organ levels usually involves tissue stem cells. Human and animal model studies indicate that aging affects two aspects of mesenchymal stem cell (MSC): a decrease in the bone marrow MSC pool and biased differentiation into adipocyte at the cost of osteoblast, which underlie the etiology of osteoporosis. Aging of MSC cells is also detrimental to some non-skeletal tissues, in particular the hematopoietic system, where MSCs serve as a niche component. In addition, aging compromises the therapeutic potentials of MSC cells, including cells isolated from aged individuals or cells cultured for many passages. Here we discuss the recent progress on our understanding of MSC aging, with a focus on the effects of MSC aging on bone remodeling and hematopoiesis and the mechanisms of MSC aging.     

Zinc finger protein 467 is a novel regulator of osteoblast and adipocyte commitment

Osteoblasts and adipocytes are derived from common mesenchymal progenitor cells. The bone loss of osteoporosis is associated with altered progenitor differentiation from an osteoblastic to an adipocytic lineage. cDNA microarrays and quantitative real-time PCR (Q-PCR) were carried out in a differentiating mouse stromal osteoblastic cell line, Kusa 4b10, to identify gene targets of factors that stimulate osteoblast differentiation including parathyroid hormone (PTH) and gp130-binding cytokines, oncostatin M (OSM) and cardiotrophin-1 (CT-1). Zinc finger protein 467 (Zfp467) was rapidly down-regulated by PTH, OSM, and CT-1. Retroviral overexpression and RNA interference for Zfp467 in mouse stromal cells showed that this factor stimulated adipocyte formation and inhibited osteoblast commitment compared with controls. Regulation of adipocyte markers, including peroxisome proliferator-activated receptor (PPAR) γ, C/EBPα, adiponectin, and resistin, and late osteoblast/osteocyte markers (osteocalcin and sclerostin) by Zfp467 was confirmed by Q-PCR. Intra-tibial injection of calvarial cells transduced with retroviral Zfp467 doubled the number of marrow adipocytes in C57Bl/6 mice compared with vector control-transduced cells, providing in vivo confirmation of a pro-adipogenic role of Zfp467. Furthermore, Zfp467 transactivated a PPAR-response element reporter construct and recruited a histone deacetylase complex. Thus Zfp467 is a novel co-factor that promotes adipocyte differentiation and suppresses osteoblast differentiation. This has relevance to therapeutic interventions in osteoporosis, including PTH-based therapies currently available, and may be of relevance for the use of adipose-derived stem cells for tissue engineering.     

The GDF11-FTO-PPARγ axis controls the shift of osteoporotic MSC fate to adipocyte and inhibits bone formation during osteoporosis

During osteoporosis, the shift of bone mesenchymal stem cell (BMSC) lineage commitment to adipocyte leads to the imbalance between bone mass and fat, which increases the risk of fracture. The mechanism underlying this process is not fully understood. Fat mass and obesity-associated protein (FTO) is an RNA demethylase that demethylates various methylated nucleic acids and participates in various physiological and pathological processes. Here we identified FTO as a regulator for BMSC fate determination during osteoporosis. FTO was up-regulated in bone marrow during aging or osteoporosis in human and mice in a GDF11(growth differentiation factor 11)-C/EBPα-dependent mechanism. The expression of FTO was also up-regulated during adipocyte differentiation of BMSCs whereas its expression was down-regulated during osteoblast differentiation. Gain-of-function and loss-of-function experiments showed that FTO favored the BMSCs to differentiate to adipocytes rather than osteoblasts. Further mechanism study demonstrated that FTO bound and demethylated the mRNA of the Peroxisome proliferator-activated receptor gamma (Pparg), leading to the increase in the expression of Pparg mRNA. Reversely, Pparg knockdown blocked the function of GDF11-FTO during osteoblast differentiation of BMSCs. Furthermore, conditionally genetic knockout of Fto in osteoblasts inhibited the development of osteopenia in mice. Collectively, our findings demonstrated that GDF11-FTO-Pparg axis promoted the shift of osteoporotic BMSC fate to adipocyte and inhibited bone formation during osteoporosis.     

Strontium ranelate rebalances bone marrow adipogenesis and osteoblastogenesis in senescent osteopenic mice through NFATc/Maf and Wnt signaling

With aging, bone marrow mesenchymal stromal cell (MSC) osteoblast differentiation decreases whereas MSC differentiation into adipocytes increases, resulting in increased adipogenesis and bone loss. Here, we investigated whether activation of cell signaling by strontium ranelate (SrRan) can reverse the excessive adipogenic differentiation associated with aging. In murine MSC cultures, SrRan increased Runx2 expression and matrix mineralization and decreased PPARγ2 expression and adipogenesis. This effect was associated with increased expression of the Wnt noncanonical representative Wnt5a and adipogenic modulator Maf and was abrogated by Wnt- and nuclear factor of activated T-cells (NFAT)c antagonists, implying a role for Wnt and NFATc/Maf signaling in the switch in osteoblastogenesis to adipogenesis induced by SrRan. To confirm this finding, we investigated the effect of SrRan in SAMP6 senescent mice, which exhibit decreased osteoblastogenesis, increased adipogenesis, and osteopenia. SrRan administration at a clinically relevant dose level increased bone mineral density, bone volume, trabecular thickness and number, as shown by densitometric, microscanning, and histomorphometric analyses in long bones and vertebrae. This attenuation of bone loss was related to increased osteoblast surface and bone formation rate and decreased bone marrow adipocyte volume and size. The restoration of osteoblast and adipocyte balance induced by SrRan was linked to increased Wnt5a and Maf expression in the bone marrow. The results indicate that SrRan acts on lineage allocation of MSCs by antagonizing the age-related switch in osteoblast to adipocyte differentiation via mechanisms involving NFATc/Maf and Wnt signaling, resulting in increased bone formation and attenuation of bone loss in senescent osteopenic mice.