ANGPTL4 binds to the leptin receptor to regulate ectopic bone formation.

Leptin protein was thought to be unique to leptin receptor (LepR), but the phenotypes of mice with mutation in LepR [db/db (diabetes)] and leptin [ob/ob (obese)] are not identical, and the cause remains unclear. Here, we show that db/db, but not ob/ob, mice had defect in tenotomy-induced heterotopic ossification (HO), implicating alternative ligand(s) for LepR might be involved. Ligand screening revealed that ANGPTL4 (angiopoietin-like protein 4), a stress and fasting-induced factor, was elicited from brown adipose tissue after tenotomy, bound to LepR on PRRX1+ mesenchymal cells at the HO site, thus promotes chondrogenesis and HO development. Disruption of LepR in PRRX1+ cells, or lineage ablation of LepR+ cells, or deletion of ANGPTL4 impeded chondrogenesis and HO in mice. Together, these findings identify ANGPTL4 as a ligand for LepR to regulate the formation of acquired HO.

Rheb1 is required for limb growth through regulating chondrogenesis in growth plate.

Ras homology enriched in the brain (Rheb) is well established as a critical regulator of cell proliferation and differentiation in response to growth factors and nutrients. However, the role of Rheb1 in limb development remains unknown. Here, we found that Rheb1 was dynamically expressed during the proliferation and differentiation of chondrocytes in the growth plate. Given that Prrx1+ limb-bud-like mesenchymal cells are the source of limb chondrocytes and are essential for endochondral ossification, we conditionally deleted Rheb1 using Prrx1-Cre and found a limb dwarfism in Prrx1-Cre; Rheb1fl/fl mice. Normalized to growth plate height, the conditional knockout (cKO) mice exhibited a significant decrease in column count of proliferative zones which was increased in hypertrophic zones resulting in decreased growth plate size, indicating abnormal endochondral ossification. Interestingly, although Rheb1 deletion profoundly inhibited the transcription factor Sox9 in limb cartilage; levels of runx2 and collagen type 2 were both increased. These novel findings highlight the essential role of Rheb1 in limb growth and indicate a complex regulation of Rheb1 in chondrocyte proliferation and differentiation.

Osteocytes regulate neutrophil development through IL-19: a potent cytokine for neutropenia treatment.

Osteocytes are the most abundant (90% to 95%) cells in bone and have emerged as an important regulator of hematopoiesis, but their role in neutrophil development and the underlying mechanisms remain unclear. Interleukin 19 (IL-19) produced predominantly by osteocytes stimulated granulopoiesis and neutrophil formation, which stimulated IL-19 receptor (IL-20Rß)/Stat3 signaling in neutrophil progenitors to promote their expansion and neutrophil formation. Mice with constitutive activation of mechanistic target of rapamycin complex (mTORC1) signaling in osteocytes (Dmp1-Cre) exhibited a dramatic increase in IL-19 production and promyelocyte/myelocytic expansion, whereas mTORC1 inactivation in osteocytes reduced IL-19 production and neutrophil numbers in mice. We showed that IL-19 administration stimulated neutrophil development, whereas neutralizing endogenous IL-19 or depletion of its receptor inhibited the process. Importantly, low-dose IL-19 reversed chemotherapy, irradiation, or chloramphenicol-induced neutropenia in mice more efficiently than granulocyte colony-stimulating factor. This evidence indicated that IL-19 was an essential regulator of neutrophil development and a potent cytokine for neutropenia treatment.

Interleukin 9 prevents immune thrombocytopenia in mice via JAK/STAT5 signaling.

Immune thrombocytopenia (ITP) is an autoimmune disorder characterized by autoimmune-mediated platelet destruction and impaired platelet production, which can lead to an increased risk of bleeding. The clinical management of ITP currently remains a challenge for hematologists. We explored the role of interleukin-9 (IL-9) in the treatment of CD41-induced ITP, and investigated its underlying mechanisms in a CD41-induced ITP mouse model. IL-9 treatment increased the numbers of mature megakaryocytes (CD41+CD42d+) and CD41+Sca-1+ cells in the bone marrow in these model mice, while IL-9 receptor (IL-9R) small interfering RNA (siRNA) inhibited the process. Moreover, phosphorylated signal transducer and activator of transcription 5 (STAT5), as a downstream molecule of IL-9R, was increased after IL-9 treatment. We next investigated the source of IL-9 in bone marrow, osteoblasts produced the highest level of IL-9. These results confirmed that IL-9 could prevent CD41-induced ITP in BALB/c mice by regulating osteoblasts and activating IL-9R/STAT5 signaling in megakaryocytes, thus providing further evidence for IL-9 as a promising therapeutic agent for the treatment of ITP.

Chondrocyte mTORC1 activation stimulates miR-483-5p via HDAC4 in osteoarthritis progression.

The hyperactivation of the mechanistic target of rapamycin complex 1 (mTORC1) in chondrocytes has been shown to accelerate the severity of destabilization of the medial meniscus-induced and age-related osteoarthritis (OA) phenotypes with aberrant chondrocyte hypertrophy and angiogenesis. Meanwhile, we previously reported that miR-483-5p is essential for the initiation and development of OA by stimulating chondrocyte hypertrophy and angiogenesis. The connection between mTORC1 and miR-483-5p activation in OA progression, however, remains unclear. In this study, we elucidated their relationship and identified the underlying mechanisms. The expression of miR-483-5p in the articular cartilage of cartilage-specific TSC1 knockout mice was assessed compared with control mice using the Agilent Mouse miRNA (8*60K) V19.0 array and real-time polymerase chain reaction (RT-PCR). The functional effects of the stimulation of miR-483-5p via histone deacetylase 4 (HDAC4) by mTORC1 in OA development, subsequently modulating its downstream targets matrilin 3 and tissue inhibitor of metalloproteinase 2, were examined by immunostaining, western blotting, and real-time PCR. This study revealed that miR-483-5p was responsible for mTORC1 activation-stimulated OA. Mechanistically, mTORC1 controls the HDAC4-dependent expression of miR-483-5p to stimulate chondrocyte hypertrophy, extracellular matrix degradation, and subchondral bone angiogenesis, and it consequently initiates and accelerates the development of OA. Our findings revealed a novel mTORC1-HDAC4-miR-483-5p pathway that is critical for OA development.

Osteoblasts support megakaryopoiesis through production of interleukin-9.

Severe thrombocytopenia is a significant challenge in patients undergoing myelosuppressive chemotherapy for malignancies. Understanding the biology of platelet-producing megakaryocytes development in the bone marrow microenvironment may facilitate the development of novel therapies to stimulate platelet production and prevent thrombocytopenia. We report here that osteoblasts supported megakaryopoiesis by secreting interleukin-9 (IL-9), which stimulated IL-9 receptor (IL-9R)/Stat3 signaling in promoting megakaryopoiesis. IL-9 production in osteoblasts was negatively regulated by the mechanistic target of rapamycin complex 1 (mTORC1) signaling in a NF-κB-dependent manner. Constitutive activation of mTORC1 inhibited IL-9 production in osteoblasts and suppressed megakaryocytic cells expansion, whereas mTORC1 inactivation increased IL-9 production and enhanced megakaryocyte and platelet numbers in mice. In mouse models, we showed that IL-9 administration stimulated megakaryopoiesis, whereas neutralizing endogenous IL-9 or IL-9R depletion inhibited the process. Importantly, we found that low doses of IL-9 efficiently prevented chemotherapy-induced thrombocytopenia (CIT) and accelerated platelet recovery after CIT. These data indicate that IL-9 is an essential regulator of megakaryopoiesis and a promising therapeutic agent for treatment of thrombocytopenia such as CIT.

Tyrosine kinase Fyn promotes osteoarthritis by activating the ß-catenin pathway.

OBJECTIVES: To investigate the role of tyrosine kinase Fyn in the development of osteoarthritis (OA) and the underlying mechanisms, and to define whether targeting Fyn could prevent OA in mice. METHODS: Cartilage samples from normal and aged mice were analysed with proteome-wide screening. Fyn expression was examined with immunofluorescence in human and age-dependent or experimental mouse OA cartilage samples. Experimental OA in Fyn-knockout mice was induced by destabilisation of the medial meniscus. Primary cultured mouse chondrocytes were treated with proinflammatory cytokine interleukin-1ß. The inhibitor of Src kinase family, AZD0530 (saracatinib), and inhibitor of Fyn, PP1, were used to treat experimental OA in mice. RESULTS: Fyn expression was markedly upregulated in human OA cartilage and in cartilage from aged mice and those with post-traumatic OA. Fyn accumulates in articular chondrocytes and interacts directly with and phosphorylates ß-catenin at Tyr142, which stabilises ß-catenin and promotes its nuclear translocation. The deletion of Fyn effectively delayed the development of post-traumatic and age-dependent OA in mice. Fyn inhibitors AZD0530 and PP1 significantly attenuated OA progression by blocking the ß-catenin pathway and reducing the levels of extracellular matrix catabolic enzymes in the articular cartilage. CONCLUSIONS: Fyn accumulates and activates ß-catenin signalling in chondrocytes, accelerating the degradation of the articular cartilage and OA development. Targeting Fyn is a novel and potentially therapeutic approach to the treatment of OA.

TSC1 deletion in fibroblasts alleviates lipopolysaccharide-induced acute kidney injury.

Mechanistic target of rapamycin complex 1 (mTORC1) signaling is active in inflammation, but its involvement in septic acute kidney injury (AKI) has not been shown. mTORC1 activation (p-S6) in renal fibroblasts was increased in a mouse AKI model induced by 1.5 mg/kg lipopolysaccharide (LPS). Deletion of tuberous sclerosis complex 1 (TSC1), an mTORC1 negative regulator, in fibroblasts (Fibro-TSC1-/-) inhibited the elevation of serum creatinine and blood urea nitrogen in AKI compared with that in TSC1fl/fl control mice. Endothelin-1 (EDN1) and phospho-Jun-amino-terminal kinase (p-JNK) were up-regulated in Fibro-TSC1-/- renal fibroblasts after LPS challenge. Rapamycin, an mTORC1 inhibitor, and bosentan, an EDN1 antagonist, eliminated the difference in renal function between TSC1fl/fl and Fibro-TSC1-/- mice after LPS injection. Rapamycin restored LPS-induced up-regulation of EDN1, endothelin converting enzyme-1 (ECE1), and p-JNK in TSC1-knockdown mouse embryonic fibroblasts (MEFs). SP600125, a Jun-amino-terminal kinase (JNK) inhibitor, attenuated LPS-induced enhancement of EDN1 and ECE1 in TSC1-knockdown MEFs without a change in phospho-S6 ribosomal protein (p-S6) level. The results indicate that mTORC1-JNK-dependent up-regulation of ECE1 elevated EDN1 in TSC1-knockout renal fibroblasts and contributed to improvement of renal function in Fibro-TSC1-/- mice with LPS-induced AKI. Renal fibroblast mTORC1 plays an important role in septic AKI.

Mecp2 fine-tunes quiescence exit by targeting nuclear receptors.

Quiescence (G0) maintenance and exit are crucial for tissue homeostasis and regeneration in mammals. Here, we show that methyl-CpG binding protein 2 (Mecp2) expression is cell cycle-dependent and negatively regulates quiescence exit in cultured cells and in an injury-induced liver regeneration mouse model. Specifically, acute reduction of Mecp2 is required for efficient quiescence exit as deletion of Mecp2 accelerates, while overexpression of Mecp2 delays quiescence exit, and forced expression of Mecp2 after Mecp2 conditional knockout rescues cell cycle reentry. The E3 ligase Nedd4 mediates the ubiquitination and degradation of Mecp2, and thus facilitates quiescence exit. A genome-wide study uncovered the dual role of Mecp2 in preventing quiescence exit by transcriptionally activating metabolic genes while repressing proliferation-associated genes. Particularly disruption of two nuclear receptors, Rara or Nr1h3, accelerates quiescence exit, mimicking the Mecp2 depletion phenotype. Our studies unravel a previously unrecognized role for Mecp2 as an essential regulator of quiescence exit and tissue regeneration.

Whitening of brown adipose tissue inhibits osteogenic differentiation via secretion of S100A8/A9.

The mechanism by which brown adipose tissue (BAT) regulates bone metabolism is unclear. Here, we reveal that BAT secretes S100A8/A9, a previously unidentified BAT adipokine (batokine), to impair bone formation. Brown adipocytes-specific knockout of Rheb (RhebBAD KO), the upstream activator of mTOR, causes BAT malfunction to inhibit osteogenesis. Rheb depletion induces NF-κB dependent S100A8/A9 secretion from brown adipocytes, but not from macrophages. In wild-type mice, age-related Rheb downregulation in BAT is associated with enhanced S100A8/A9 secretion. Either batokines from RhebBAD KO mice, or recombinant S100A8/A9, inhibits osteoblast differentiation of mesenchymal stem cells in vitro by targeting toll-like receptor 4 on their surfaces. Conversely, S100A8/A9 neutralization not only rescues the osteogenesis repressed in the RhebBAD KO mice, but also alleviates age-related osteoporosis in wild-type mice. Collectively, our data revealed an unexpected BAT-bone crosstalk driven by Rheb-S100A8/A9, uncovering S100A8/A9 as a promising target for the treatment, and potentially, prevention of osteoporosis.

MYL3 protects chondrocytes from senescence by inhibiting clathrin-mediated endocytosis and activating of Notch signaling.

As the unique cell type in articular cartilage, chondrocyte senescence is a crucial cellular event contributing to osteoarthritis development. Here we show that clathrin-mediated endocytosis and activation of Notch signaling promotes chondrocyte senescence and osteoarthritis development, which is negatively regulated by myosin light chain 3. Myosin light chain 3 (MYL3) protein levels decline sharply in senescent chondrocytes of cartilages from model mice and osteoarthritis (OA) patients. Conditional deletion of Myl3 in chondrocytes significantly promoted, whereas intra-articular injection of adeno-associated virus overexpressing MYL3 delayed, OA progression in male mice. MYL3 deficiency led to enhanced clathrin-mediated endocytosis by promoting the interaction between myosin VI and clathrin, further inducing the internalization of Notch and resulting in activation of Notch signaling in chondrocytes. Pharmacologic blockade of clathrin-mediated endocytosis-Notch signaling prevented MYL3 loss-induced chondrocyte senescence and alleviated OA progression in male mice. Our results establish a previously unknown mechanism essential for cellular senescence and provide a potential therapeutic direction for OA.

Down-regulated GAS6 impairs synovial macrophage efferocytosis and promotes obesity-associated osteoarthritis.

Obesity has always been considered a significant risk factor in osteoarthritis (OA) progression, but the underlying mechanism of obesity-related inflammation in OA synovitis remains unclear. The present study found that synovial macrophages infiltrated and polarized in the obesity microenvironment and identified the essential role of M1 macrophages in impaired macrophage efferocytosis using pathology analysis of obesity-associated OA. The present study revealed that obese OA patients and Apoe-/- mice showed a more pronounced synovitis and enhanced macrophage infiltration in synovial tissue, accompanied by dominant M1 macrophage polarization. Obese OA mice had a more severe cartilage destruction and increased levels of synovial apoptotic cells (ACs) than OA mice in the control group. Enhanced M1-polarized macrophages in obese synovium decreased growth arrest-specific 6 (GAS6) secretion, resulting in impaired macrophage efferocytosis in synovial ACs. Intracellular contents released by accumulated ACs further triggered an immune response and lead to a release of inflammatory factors, such as TNF-α, IL-1ß, and IL-6, which induce chondrocyte homeostasis dysfunction in obese OA patients. Intra-articular injection of GAS6 restored the phagocytic capacity of macrophages, reduced the accumulation of local ACs, and decreased the levels of TUNEL and Caspase-3 positive cells, preserving cartilage thickness and preventing the progression of obesity-associated OA. Therefore, targeting macrophage-associated efferocytosis or intra-articular injection of GAS6 is a potential therapeutic strategy for obesity-associated OA.

Exosome Release Delays Senescence by Disposing of Obsolete Biomolecules.

Accumulation of obsolete biomolecules can accelerate cell senescence and organism aging. The two efficient intracellular systems, namely the ubiquitin-proteasome system and the autophagy-lysosome system, play important roles in dealing with cellular wastes. However, how multicellular organisms orchestrate the processing of obsolete molecules and delay aging remains unclear. Herein, it is shown that prevention of exosome release by GW4869 or Rab27a-/- accelerated senescence in various cells and mice, while stimulating exosome release by nutrient restriction delays aging. Interestingly, exosomes isolate from serum-deprived cells or diet-restricted human plasma, enriched with garbage biomolecules, including misfolded proteins, oxidized lipids, and proteins. These cellular wastes can be englobed by macrophages, eventually, for disintegration in vivo. Inhibition of nutrient-sensing mTORC1 signaling increases exosome release and delays senescence, while constitutive activation of mTORC1 reduces exosome secretion and exacerbates senescence in vitro and in mice. Notably, inhibition of exosome release attenuates nutrient restriction- or rapamycin-delayed senescence, supporting a key role for exosome secretion in this process. This study reveals a potential mechanism by which stimulated exosome release delays aging in multicellular organisms, by orchestrating the harmful biomolecules disposal via exosomes and macrophages.

Loss of RanGAP1 drives chromosome instability and rapid tumorigenesis of osteosarcoma.

Chromothripsis is a catastrophic event of chromosomal instability that involves intensive fragmentation and rearrangements within localized chromosomal regions. However, its cause remains unclear. Here, we show that reduction and inactivation of Ran GTPase-activating protein 1 (RanGAP1) commonly occur in human osteosarcoma, which is associated with a high rate of chromothripsis. In rapidly expanding mouse osteoprogenitors, RanGAP1 deficiency causes chromothripsis in chr1q, instant inactivation of Rb1 and degradation of p53, consequent failure in DNA damage repair, and ultrafast osteosarcoma tumorigenesis. During mitosis, RanGAP1 anchors to the kinetochore, where it recruits PP1-γ to counteract the activity of the spindle-assembly checkpoint (SAC) and prevents TOP2A degradation, thus safeguarding chromatid decatenation. Loss of RanGAP1 causes SAC hyperactivation and chromatid decatenation failure. These findings demonstrate that RanGAP1 maintains mitotic chromosome integrity and that RanGAP1 loss drives tumorigenesis through its direct effects on SAC and decatenation and secondary effects on DNA damage surveillance.

Targeting of mTORC2 prevents cell migration and promotes apoptosis in breast cancer.

Most of breast cancers are resistant to mammalian target of rapamycin complex 1 (mTORC1) inhibitors rapamycin and rapalogs. Recent studies indicate mTORC2 is emerging as a promising cancer therapeutic target. In this study, we compared the inhibitory effects of targeting mTORC1 with mTORC2 on a variety of breast cancer cell lines and xenograft. We demonstrated that inhibition of mTORC1/2 by mTOR kinase inhibitors PP242 and OSI-027 effectively suppress phosphorylation of Akt (S473) and breast cancer cell proliferation. Targeting of mTORC2 either by kinase inhibitors or rictor knockdown, but not inhibition of mTORC1 either by rapamycin or raptor knockdown promotes serum starvation- or cisplatin-induced apoptosis. Furthermore, targeting of mTORC2 but not mTORC1 efficiently prevent breast cancer cell migration. Most importantly, in vivo administration of PP242 but not rapamycin as single agent effectively prevents breast tumor growth and induces apoptosis in xenograft. Our data suggest that agents that inhibit mTORC2 may have advantages over selective mTORC1 inhibitors in the treatment of breast cancers. Given that mTOR kinase inhibitors are in clinical trials, this study provides a strong rationale for testing the use of mTOR kinase inhibitors or combination of mTOR kinase inhibitors and cisplatin in the clinic.

mTORC1 is a target of nordihydroguaiaretic acid to prevent breast tumor growth in vitro and in vivo.

Nordihydroguaiaretic acid (NDGA) is a natural phenolic compound isolated from the creosote bush Larrea divaricata, which has anti-tumor activities both in vitro and in vivo. Its analogs are in clinical development for use in refractory solid tumors. But the mechanisms underlying the anti-cancer effect of NDGA are not fully understood. In this study, we identified mammalian target of rapamycin complex 1 (mTORC1) as a target of NDGA both in cultured breast cancer cells and in xenograft models. NDGA effectively inhibited basal level of mTORC1 but not mTORC2 activity in breast cancer cell lines. NDGA also suppressed mTORC1 downstream signaling such as expression of cyclin D1, hypoxia-inducible factor-α and VEGF, and prevented proliferation in breast cancer cells. Although NDGA stimulated AMP-activated protein kinase (AMPK)/tuberous sclerosis complex 2 (TSC2) signaling, which negatively regulates mTORC1, AMPK and TSC2 deletion could not diminish the inhibition of mTORC1 by NDGA. Subsequent studies revealed that NDGA may also direct target mTORC1 complex because NDGA suppressed amino acids- and insulin-stimulated mTORC1 and acted like rapamycin to disrupt mTOR-Raptor interaction. Most importantly, NDGA repressed breast tumor growth and targeted mTORC1 and its downstream signaling in xenograft models. Together our data provide a novel mechanism for NDGA activity which could help explain its anti-cancer activity. Disruption of mTOR-Raptor complex and activation of AMPK/TSC signaling may contribute to inhibitory effects of NDGA against mTORC1. Our data also raise the possibility that NDGA, as an mTORC1 inhibitor, may have a broad spectrum of action on breast cancers.

DNMT1 is a negative regulator of osteogenesis.

The role and underlying mechanisms of DNA methylation in osteogenesis/chondrogenesis remain poorly understood. We here reveal DNA methyltransferase 1 (DNMT1), which is responsible for copying DNA methylation onto the newly synthesized DNA strand after DNA replication, is overexpressed in sponge bone of people and mice with senile osteoporosis and required for suppression of osteoblast (OB) differentiation of mesenchymal stem cells (MSCs) and osteoprogenitors. Depletion of DNMT1 results in demethylation at the promoters of key osteogenic genes such as RORA and Fgfr2, and consequent upregulation of their transcription in vitro. Mechanistically, DNMT1 binds exactly to the promoters of these genes and are responsible for their 5-mc methylation. Conversely, simultaneous depletion of RORA or Fgfr2 blunts the effects of DNMT1 silencing on OB differentiation, suggesting RORA or Fgfr2 may be crucial for modulating osteogenic differentiation downstream of DNMT1. Collectively, these results reveal DNMT1 as a key repressor of OB differentiation and bone formation while providing us a new rationale for specific inhibition of DNMT1 as a potential therapeutic strategy to treat age-related bone loss.

Osteocyte TSC1 promotes sclerostin secretion to restrain osteogenesis in mice.

Osteocytes secrete the glycoprotein sclerostin to inhibit bone formation by osteoblasts, but how sclerostin production is regulated in osteocytes remains unclear. Here, we show that tuberous sclerosis complex 1 (TSC1) in osteocytes promotes sclerostin secretion through inhibition of mechanistic target of rapamycin complex 1 (mTORC1) and downregulation of Sirt1. We generated mice with DMP1-Cre-directed Tsc1 gene deletion ( Tsc1 CKO) to constitutively activate mTORC1 in osteocytes. Although osteocyte TSC1 disruption increased RANKL expression and osteoclast formation, it markedly reduced sclerostin production in bone, resulting in severe osteosclerosis with enhanced bone formation in mice. Knockdown of TSC1 activated mTORC1 and decreased sclerostin, while rapamycin inhibited mTORC1 and increased sclerostin mRNA and protein expression levels in MLO-Y4 osteocyte-like cells. Furthermore, mechanical loading activated mTORC1 and prevented sclerostin expression in osteocytes. Mechanistically, TSC1 promotes sclerostin production and prevents osteogenesis through inhibition of mTORC1 and downregulation of Sirt1, a repressor of the sclerostin gene Sost. Our findings reveal a role of TSC1/mTORC1 signalling in the regulation of osteocyte sclerostin secretion and bone formation in response to mechanical loading in vitro. Targeting TSC1 represents a potential strategy to increase osteogenesis and prevent bone loss-related diseases.

Exosome Release Is Regulated by mTORC1.

Exosomes are small membrane-bound vesicles released into extracellular spaces by many types of cells. These nanovesicles carry proteins, mRNA, and miRNA, and are involved in cell waste management and intercellular communication. In the present study, it is shown that exosome release, which leads to net loss of cellular membrane and protein content, is negatively regulated by mechanistic target of rapamycin complex 1 (mTORC1). It is found that in cells and animal models exosome release is inhibited by sustained activation of mTORC1, leading to intracellular accumulation of CD63-positive exosome precursors. Inhibition of mTORC1 by rapamycin or nutrient and growth factor deprivation stimulates exosome release, which occurs concomitantly with autophagy. The drug-stimulated release is blocked by siRNA-mediated downregulation of small GTPase Rab27A. Analysis of the cargo content in exosomes released from rapamycin-treated cells reveals that inhibition of mTORC1 does not significantly alter its majority protein and miRNA profiles. These observations demonstrate that exosome release, like autophagy, is negatively regulated by mTORC1 in response to changes in nutrient and growth factor conditions.

Bone and plasma citrate is reduced in osteoporosis.

High concentration of citrate exists in bone of humans and all osteo-vertebrates, and citrate incorporation imparts important biomechanical and other functional properties to bone. However, which cells are responsible for citrate production in bone remains unclear and whether the citrate component changes with bone loss during osteoporosis is also not known. Here, we show that the citrate content is markedly reduced in the bone of mice or rats with age-related, ovariectomy-induced or retinoic acid-induced bone loss. Plasmic citrate is also downregulated in osteoporotic animals. Importantly, the plasmic citrate level of aged osteoporotic males is significantly lower than that of young healthy males and positively correlates with human lumbar spine bone mineral density (BMD) and total hip BMD. Furthermore, citrate production increases with in vitro osteoblastic differentiation, accompanied by upregulation of proteins involved in citrate secretion, suggesting that osteoblasts are highly specialized cells that produce citrate in bone. Our findings establish a novel relationship between citrate content and bone loss-related diseases such as osteoporosis, suggesting a critical role of bone citrate in the maintenance of the citrate balance in the circulation. Serum citrate level may thus represent a novel marker for osteoporosis.