Erk5 in Bone Marrow Mesenchymal Stem Cells Regulates Bone Homeostasis by Preventing Osteogenesis in Adulthood.

Extracellular signal-regulated kinase 5 (Erk5) belongs to the mitogen-activated protein kinase (MAPK) family. Previously, we demonstrated that Erk5 directly phosphorylates Smad-specific E3 ubiquitin protein ligase 2 (Smurf2) at Thr249 (Smurf2Thr249) to activate its E3 ubiquitin ligase activity. Although we have clarified the importance of Erk5 in embryonic mesenchymal stem cells (MSCs) on skeletogenesis, its role in adult bone marrow (BM)-MSCs on bone homeostasis remains unknown. Leptin receptor-positive (LepR+) BM-MSCs represent a major source of bone in adult bone marrow and are critical regulators of postnatal bone homeostasis. Here, we identified Erk5 in BM-MSCs as an important regulator of bone homeostasis in adulthood. Bone marrow tissue was progressively osteosclerotic in mice lacking Erk5 in LepR+ BM-MSCs with age, accompanied by increased bone formation and normal bone resorption in vivo. Erk5 deficiency increased the osteogenic differentiation of BM-MSCs along with a higher expression of Runx2 and Osterix, essential transcription factors for osteogenic differentiation, without affecting their stemness in vitro. Erk5 deficiency decreased Smurf2Thr249 phosphorylation and subsequently increased Smad1/5/8-dependent signaling in BM-MSCs. The genetic introduction of the Smurf2T249E mutant (a phosphomimetic mutant) suppressed the osteosclerotic phenotype in Erk5-deficient mice. These findings suggest that the Erk5-Smurf2Thr249 axis in BM-MSCs plays a critical role in the maintenance of proper bone homeostasis by preventing excessive osteogenesis in adult bone marrow.

Conditional inactivation of the L-type amino acid transporter LAT1 in chondrocytes models idiopathic scoliosis in mice.

Scoliosis, usually diagnosed in childhood and early adolescence, is an abnormal lateral curvature of the spine. L-type amino acid transporter 1 (LAT1), encoded by solute carrier transporter 7a5 (Slc7a5), plays a crucial role in amino acid sensing and signaling in specific cell types. We previously demonstrated the pivotal role of LAT1 on bone homeostasis in mice, and the expression of LAT1/SLC7A5 in vertebral cartilage of pediatric scoliosis patients; however, its role in chondrocytes on spinal homeostasis and implications regarding the underlying mechanisms during the onset and progression of scoliosis, remain unknown. Here, we identified LAT1 in mouse chondrocytes as an important regulator of postnatal spinal homeostasis. Conditional inactivation of LAT1 in chondrocytes resulted in a postnatal-onset severe thoracic scoliosis at the early adolescent stage with normal embryonic spinal development. Histological analyses revealed that Slc7a5 deletion in chondrocytes led to general disorganization of chondrocytes in the vertebral growth plate, along with an increase in apoptosis and a decrease in cell proliferation. Furthermore, loss of Slc7a5 in chondrocytes activated the general amino acid control (GAAC) pathway but inactivated the mechanistic target of rapamycin complex 1 (mTORC1) pathway in the vertebrae. The spinal deformity in Slc7a5-deficient mice was corrected by genetic inactivation of the GAAC pathway, but not by genetic activation of the mTORC1 pathway. These findings suggest that the LAT1-GAAC pathway in chondrocytes plays a critical role in the maintenance of proper spinal homeostasis by modulating cell proliferation and survivability.

The MAPK Erk5 is necessary for proper skeletogenesis involving a Smurf-Smad-Sox9 molecular axis.

Erk5 belongs to the mitogen-activated protein kinase (MAPK) family. Following its phosphorylation by Mek5, Erk5 modulates several signaling pathways in a number of cell types. In this study, we demonstrated that Erk5 inactivation in mesenchymal cells causes abnormalities in skeletal development by inducing Sox9, an important transcription factor of skeletogenesis. We further demonstrate that Erk5 directly phosphorylates and activates Smurf2 (a ubiquitin E3 ligase) at Thr249, which promotes the proteasomal degradation of Smad proteins and phosphorylates Smad1 at Ser206 in the linker region known to trigger its proteasomal degradation by Smurf1. Smads transcriptionally activated the expression of Sox9 in mesenchymal cells. Accordingly, removal of one Sox9 allele in mesenchymal cells from Erk5-deficient mice rescued some abnormalities of skeletogenesis. These findings highlight the importance of the Mek5-Erk5-Smurf-Smad-Sox9 axis in mammalian skeletogenesis.

Daily oral supplementation of Hochu-Ekki-To prevents osteoclastic activation and bone loss in ovariectomized mice.

Bone remodeling is sophisticatedly regulated by two different cell types: bone-resorbing osteoclasts and bone-forming osteoblasts. Hochu-Ekki-To, a Japanese traditional herbal medicine, is commonly used for the treatment of chronic diseases or frailty after an illness; however, its effects on metabolic bone diseases such as osteoporosis are not well known. We herein report that daily oral Hochu-Ekki-To administration significantly inhibits osteoclast activation as well as the reduction in bone volume in ovariectomized mice. Our results suggest that supplementation with Hochu-Ekki-To might be beneficial for the prophylaxis and treatment of metabolic bone diseases associated with abnormal osteoclast activation.

mTOR regulates skeletogenesis through canonical and noncanonical pathways.

The mechanistic/mammalian target of rapamycin (mTOR) regulates various cellular processes, in part through incorporation into distinct protein complexes. The mTOR complex 1 (mTORC1) contains the Raptor subunit, while mTORC2 specifically contains the Rictor subunit. Mouse genetic studies, including ours, have revealed a critical role for mTOR in skeletogenesis through its expression in undifferentiated mesenchymal cells. In addition, we have recently revealed that mTORC1 expression in chondrocytes is crucial for skeletogenesis. Recent work indicates that mTOR regulates cellular functions, depending on the context, through both complex-dependent (canonical pathway) and complex-independent roles (noncanonical pathway). Here, we determined that mTOR regulates skeletal development through the noncanonical pathway, as well as the canonical pathway, in a cell-type and context-specific manner. Inactivation of Mtor in undifferentiated mesenchymal cells or chondrocytes led to either severe hypoplasia in appendicular skeletons or a severe and generalized chondrodysplasia, respectively. Moreover, Rictor deletion in undifferentiated mesenchymal cells or chondrocytes led to mineralization defects in some skeletal components. Finally, we revealed that simultaneous deletion of Raptor and Rictor in undifferentiated mesenchymal cells recapitulated the appendicular skeletal phenotypes of Mtor deficiency, whereas chondrocyte-specific Raptor and Rictor double-mutants exhibited milder hypoplasia of appendicular and axial skeletons than those seen upon Mtor deletion. These findings indicate that mTOR regulates skeletal development mainly through the canonical pathway in undifferentiated mesenchymal cells, but at least in part through the noncanonical pathway in chondrocytes.

mTORC1 Overactivation Leads to Abnormalities in Skeletal Development.

The mechanistic/mammalian target of rapamycin complex-1 (mTORC1) integrates multiple signaling pathways and regulates various cellular processes. Tuberous sclerosis complex 1 (Tsc1) and complex 2 (Tsc2) are critical negative regulators of mTORC1. Mouse genetic studies, including ours, have revealed that inactivation of mTORC1 in undifferentiated mesenchymal cells and chondrocytes leads to severe skeletal abnormalities, indicating a pivotal role for mTORC1 in skeletogenesis. Here, we show that hyperactivation of mTORC1 influences skeletal development through its expression in undifferentiated mesenchymal cells at the embryonic stage. Inactivation of Tsc1 in undifferentiated mesenchymal cells by paired-related homeobox 1 (Prx1)-Cre-mediated recombination led to skeletal abnormalities in appendicular skeletons. In contrast, Tsc1 deletion in chondrocytes using collagen type II α1 (Col2a1)-Cre or in osteoprogenitors using Osterix (Osx)-Cre did not result in skeletal defects in either appendicular or axial skeletons. These findings indicate that Tsc complex-mediated chronic overactivation of mTORC1 influences skeletal development at the embryonic stage through its expression in undifferentiated mesenchymal cells but not in chondrocytes or osteoprogenitors.

Genetic analysis of Runx2 function during intramembranous ossification.

Runt-related transcription factor 2 (Runx2) is an essential transcriptional regulator of osteoblast differentiation and its haploinsufficiency leads to cleidocranial dysplasia because of a defect in osteoblast differentiation during bone formation through intramembranous ossification. The cellular origin and essential period for Runx2 function during osteoblast differentiation in intramembranous ossification remain poorly understood. Paired related homeobox 1 (Prx1) is expressed in craniofacial mesenchyme, and Runx2 deficiency in cells of the Prx1 lineage (in mice referred to here as Runx2prx1 (-/-)) resulted in defective intramembranous ossification. Runx2 was heterogeneously expressed in Prx1-GFP(+) cells located at the intrasutural mesenchyme in the calvaria of transgenic mice expressing GFP under the control of the Prx1 promoter. Double-positive cells for Prx1-GFP and stem cell antigen-1 (Sca1) (Prx1(+)Sca1(+) cells) in the calvaria expressed Runx2 at lower levels and were more homogeneous and primitive than Prx1(+)Sca1(-) cells. Osterix (Osx) is another transcriptional determinant of osteoblast lineages expressed by osteoblast precursors; Osx is highly expressed by Prx1(-)Runx2(+) cells at the osteogenic front and on the surface of mineralized bone in the calvaria. Runx2 deficiency in cells of the Osx lineage (in mice referred to here as Runx2osx (-/-)) resulted in severe defects in intramembranous ossification. These findings indicate that the essential period of Runx2 function in intramembranous ossification begins at the Prx1(+)Sca1(+) mesenchymal stem cell stage and ends at the Osx(+)Prx1(-)Sca1(-) osteoblast precursor stage.

Hypoxic Stress Upregulates the Expression of Slc38a1 in Brown Adipocytes via Hypoxia-Inducible Factor-1α.

The availability of amino acid in the brown adipose tissue (BAT) has been shown to be altered under various conditions; however, little is known about the possible expression and pivotal role of amino acid transporters in BAT under physiological and pathological conditions. The present study comprehensively investigated whether amino acid transporters are regulated by obesogenic conditions in BAT in vivo. Moreover, we investigated the mechanism underlying the regulation of the expression of amino acid transporters by various stressors in brown adipocytes in vitro. The expression of solute carrier family 38 member 1 (Slc38a1; gene encoding sodium-coupled neutral amino acid transporter 1) was preferentially upregulated in the BAT of both genetic and acquired obesity mice in vivo. Moreover, the expression of Slc38a1 was induced by hypoxic stress through hypoxia-inducible factor-1α, which is a master transcription factor of the adaptive response to hypoxic stress, in brown adipocytes in vitro. These results indicate that Slc38a1 is an obesity-associated gene in BAT and a hypoxia-responsive gene in brown adipocytes.

The transcriptional modulator Ifrd1 is a negative regulator of BMP-2-dependent osteoblastogenesis.

We previously demonstrated that the transcriptional coactivator/repressor interferon-related developmental regulator 1 (Ifrd1) was expressed in osteoblasts and participated in the regulation of bone homeostasis. However, it remains unclear how Ifrd1 expression itself is regulated in osteoblasts. In the present study, we investigated the upstream regulatory mechanisms of Ifrd1 in osteoblasts during osteoblastogenesis. Ifrd1 protein expression and runt-related transcription factor 2, the master regulator of osteoblastogenesis, were markedly upregulated by bone morphogenetic protein 2 (BMP-2) stimulation in primary osteoblasts. Moreover, BMP-2 stimulation significantly induced Ifrd1 mRNA expression and promoter activity in osteoblasts. LDN193189, an inhibitor of activin-like kinase 2/3, almost completely inhibited the BMP-2-induced increase in Ifrd1 protein expression. There were at least two putative Smad-binding elements in the 5'-flanking region, which was highly conserved between mouse and human Ifrd1 genes. Co-introduction of both Smad4 and Smad1 significantly increased Ifrd1 promoter activity in osteoblasts. In addition, BMP-2 induced the recruitment of Smad1 to the Ifrd1 promoter in osteoblasts. Moreover, BMP-2-dependent osteoblastogenesis was further enhanced in Ifrd1 knocked-down osteoblasts, as determined by the intensity of Alizarin red stain and marker gene expression. These results suggest that BMP-2 directly induces Ifrd1 expression at the transcriptional level in osteoblasts via the Smad pathway, and Ifrd1 negatively regulates BMP-2-dependent osteoblastogenesis.

ATF3 deficiency in chondrocytes alleviates osteoarthritis development.

Activating transcription factor 3 (Atf3) has been implicated in the pathogenesis of various diseases, including cancer and inflammation, as well as in the regulation of cell proliferation and differentiation. However, the involvement of Atf3 in developmental skeletogenesis and joint disease has not been well studied to date. Here, we show that Atf3 is a critical mediator of osteoarthritis (OA) development through its expression in chondrocytes. ATF3 expression was markedly up-regulated in the OA cartilage of both mice and humans. Conditional deletion of Atf3 in chondrocytes did not result in skeletal abnormalities or affect the chondrogenesis, but alleviated the development of OA generated by surgically inducing knee joint instability in mice. Inflammatory cytokines significantly up-regulated Atf3 expression through the nuclear factor-kB (NF-kB) pathway, while cytokine-induced interleukin-6 (Il6) expression was repressed, in ATF3-deleted murine and human chondrocytes. Mechanistically, Atf3 deficiency decreased cytokine-induced Il6 transcription in chondrocytes through repressing NF-kB signalling by the attenuation of the phosphorylation status of IkB and p65. These findings suggest that Atf3 is implicated in the pathogenesis of OA through modulation of inflammatory cytokine expression in chondrocytes, and the feed-forward loop of inflammatory cytokines/NF-kB/Atf3 in chondrocytes may be a novel therapeutic target for the treatment for OA. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Amelioration of the Development of Osteoarthritis by Daily Intake of ß-Cryptoxanthin.

ß-Cryptoxanthin, which is primarily obtained from citrus fruits such as Satsuma mandarins, is a major carotenoid routinely found in human serum. Recently, we demonstrated that daily oral intake of ß-cryptoxanthin prevented ovariectomy-induced bone loss and ameliorated neuropathic pain in mice. Although ß-cryptoxanthin exerts preventive effects on various lifestyle-related diseases, there have been no studies on the effect of ß-cryptoxanthin on the development of osteoarthritis, the most common degenerative joint disease, which frequently leads to loss of ability and stiffness in the elderly. Here we showed that daily oral administration of ß-cryptoxanthin significantly prevented the development of osteoarthritis developed by surgically inducing knee joint instability in mice in vivo. Furthermore, in vitro experiments revealed that ß-cryptoxanthin markedly inhibited the expression of inflammatory cytokines and enzymes critical for the degradation of the extracellular matrix in primary chondrocytes. Our results suggest that oral supplementation of ß-cryptoxanthin would be beneficial for the maintenance of joint health and as prophylaxis against osteoarthritis.

Daily oral intake of ß-cryptoxanthin ameliorates neuropathic pain.

ß-cryptoxanthin, a xanthophyll carotenoid, exerts preventive effects on various lifestyle-related diseases. Here, we found that daily oral administration of ß-cryptoxanthin significantly ameliorated the development of tactile allodynia following spinal nerve injury but was ineffective in mechanical allodynia in an inflammatory pain model in mice. Our results suggest that ß-cryptoxanthin supplementation would be beneficial for the prophylaxis of neuropathic pain.

Daily intake of ß-cryptoxanthin prevents bone loss by preferential disturbance of osteoclastic activation in ovariectomized mice.

Although ß-cryptoxanthin, a xanthophyll carotenoid, has been shown to exert an anabolic effect on bone calcification, little attention has been paid thus far to the precise mechanism of bone remodeling. Daily oral administration of ß-cryptoxanthin significantly inhibited osteoclastic activation as well as reduction of bone volume in ovariectomized mice. In vitro studies revealed that ß-cryptoxanthin inhibited differentiation and maturation of osteoclasts by repression of the nuclear factor-κB-dependent transcriptional pathway. Our results suggest that supplementation with ß-cryptoxanthin would be beneficial for prophylaxis and for therapy of metabolic bone diseases associated with abnormal osteoclast activation.

Repression of adipogenesis through promotion of Wnt/ß-catenin signaling by TIS7 up-regulated in adipocytes under hypoxia.

Although tetradecanoyl phorbol acetate induced sequence-7 (TIS7) has been identified as a co-activator/repressor of gene transcription in different eukaryotic cells, little attention has been paid to the functionality of TIS7 in adipocytes. Here, we evaluated the possible role of TIS7 in mechanisms underlying the regulation of adipogenesis. TIS7 expression was preferentially up-regulated in white adipose tissues (WAT) of obesity model mice as well as in pre-adipocytic 3T3-L1 cells cultured under hypoxic conditions. TIS7 promoter activity was selectively enhanced by activating transcription factor-6 (ATF6) among different transcription factors tested, while induction of TIS7 by hypoxic stress was markedly prevented by knockdown of ATF6 by shRNA in 3T3-L1 cells. Overexpression of TIS7 markedly inhibited Oil Red O staining and expression of particular adipogenic genes in 3T3-L1 cells. TIS7 synergistically promoted gene transactivation mediated by Wingless-type mouse mammary tumor virus integration site family (Wnt)/ß-catenin, while blockade of the Wnt/ß-catenin pathway by a dominant negative form of T-cell factor-4 (DN-TCF4) markedly prevented the inhibition of adipogenesis in 3T3-L1 cells with TIS7 overexpression. TIS7 predominantly interacted with ß-catenin in the nucleus of WAT in the genetically obese ob/ob mice as well as in 3T3-L1 cells cultured under hypoxic conditions. Both knockdown of TIS7 by shRNA and introduction of DN-TCF4 similarly reversed the hypoxia-induced inhibition of adipogenic gene expression in 3T3-L1 cells. These findings suggest that TIS7 could play a pivotal role in adipogenesis through interacting with ß-catenin to promote the canonical Wnt signaling in pre-adipocytes under hypoxic stress such as obesity.

Nuclear factor-κB is a common upstream signal for growth differentiation factor-5 expression in brown adipocytes exposed to pro-inflammatory cytokines and palmitate.

We have previously demonstrated that genetic and acquired obesity similarly led to drastic upregulation in brown adipose tissue (BAT), rather than white adipose tissue, of expression of both mRNA and corresponding protein for the bone morphogenic protein/growth differentiation factor (GDF) member GDF5 capable of promoting brown adipogenesis. In this study, we evaluated expression profiles of GDF5 in cultured murine brown pre-adipocytes exposed to pro-inflammatory cytokines and free fatty acids (FFAs), which are all shown to play a role in the pathogenesis of obesity. Both interleukin-1ß (IL-1ß) and tumor necrosis factor-α (TNF-α) were effective in up-regulating GDF5 expression in a concentration-dependent manner, while similar upregulation was seen in cells exposed to the saturated FFA palmitate, but not to the unsaturated FFA oleate. In silico analysis revealed existence of the putative nuclear factor-κB (NF-κB) binding site in the 5'-flanking region of mouse GDF5, whereas introduction of NF-κB subunits drastically facilitated both promoter activity and expression of GDF5 in brown pre-adipocytes. Chromatin immunoprecipitation analysis confirmed significant facilitation of the recruitment of NF-κB to the GDF5 promoter in lysed extracts of BAT from leptin-deficient ob/ob obese mice. Upregulation o GDF5 expression was invariably inhibited by an NF-κB inhibitor in cultured brown pre-adipocytes exposed to IL-1ß, TNF-α and palmitate. These results suggest that obesity leads to upregulation of GDF5 expression responsible for the promotion of brown adipogenesis through a mechanism relevant to activation of the NF-κB pathway in response to particular pro-inflammatory cytokines and/or saturated FFAs in BAT.

PI3K/Akt is involved in brown adipogenesis mediated by growth differentiation factor-5 in association with activation of the Smad pathway.

We have previously demonstrated promotion by growth differentiation factor-5 (GDF5) of brown adipogenesis for systemic energy expenditure through a mechanism relevant to activating the bone morphological protein (BMP) receptor/mothers against decapentaplegic homolog (Smad)/peroxisome proliferator-activated receptor gamma co-activator 1α (PGC-1α) pathway. Here, we show the involvement of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway in brown adipogenesis mediated by GDF5. Overexpression of GDF5 in cells expressing adipocyte protein-2 markedly accelerated the phosphorylation of Smad1/5/8 and Akt in white and brown adipose tissues. In brown adipose tissue from heterozygous GDF5(Rgsc451) mutant mice expressing a dominant-negative (DN) GDF5 under obesogenic conditions, the basal phosphorylation of Smad1/5/8 and Akt was significantly attenuated. Exposure to GDF5 not only promoted the phosphorylation of both Smad1/5/8 and Akt in cultured brown pre-adipocytes, but also up-regulated Pgc1a and uncoupling protein-1 expression in a manner sensitive to the PI3K/Akt inhibitor Ly294002 as well as retroviral infection with DN-Akt. GDF5 drastically promoted BMP-responsive luciferase reporter activity in a Ly294002-sensitive fashion. Both Ly294002 and DN-Akt markedly inhibited phosphorylation of Smad5 in the nuclei of brown pre-adipocytes. These results suggest that PI3K/Akt signals play a role in the GDF5-mediated brown adipogenesis through a mechanism related to activation of the Smad pathway.

Prevention of bone loss after ovariectomy in mice with preferential overexpression of the transcription factor paired box-5 in osteoblasts.

We have recently shown that the transcription factor Paired box-5 (Pax5) promotes bone formation in vivo and osteoblastogenesis in vitro. Here, we demonstrated the involvement of Pax5 in bone remodeling after ovariectomy (OVX). A significant increase was seen in vertebrae bone volume in transgenic mice preferentially overexpressing Pax5 in osteoblasts by using the mouse α1(I)Collagen promoter, whereas OVX significantly reduced vertebrae bone volume in wild-type (WT) mice without significantly affecting that in Pax5 transgenic mice. Preferential osteoblastic Pax5 overexpression invariably led to significant increases in osteoblastic and osteoclastic parameters in mice with sham operation. However, OVX significantly increased osteoclastic parameters in WT mice, without additionally increasing osteoblastic and osteoclastic parameters in Pax5 transgenic mice. These results suggest that osteoblastic Pax5 would play a role in OVX-induced bone loss through a mechanism relevant to the promotion of both osteoblastic bone formation and osteoclastic bone resorption in vivo.

l-Type amino acid transporter 1 in hypothalamic neurons in mice maintains energy and bone homeostasis.

Hypothalamic neurons regulate body homeostasis by sensing and integrating changes in the levels of key hormones and primary nutrients (amino acids, glucose, and lipids). However, the molecular mechanisms that enable hypothalamic neurons to detect primary nutrients remain elusive. Here, we identified l-type amino acid transporter 1 (LAT1) in hypothalamic leptin receptor-expressing (LepR-expressing) neurons as being important for systemic energy and bone homeostasis. We observed LAT1-dependent amino acid uptake in the hypothalamus, which was compromised in a mouse model of obesity and diabetes. Mice lacking LAT1 (encoded by solute carrier transporter 7a5, Slc7a5) in LepR-expressing neurons exhibited obesity-related phenotypes and higher bone mass. Slc7a5 deficiency caused sympathetic dysfunction and leptin insensitivity in LepR-expressing neurons before obesity onset. Importantly, restoring Slc7a5 expression selectively in LepR-expressing ventromedial hypothalamus neurons rescued energy and bone homeostasis in mice deficient for Slc7a5 in LepR-expressing cells. Mechanistic target of rapamycin complex-1 (mTORC1) was found to be a crucial mediator of LAT1-dependent regulation of energy and bone homeostasis. These results suggest that the LAT1/mTORC1 axis in LepR-expressing neurons controls energy and bone homeostasis by fine-tuning sympathetic outflow, thus providing in vivo evidence of the implications of amino acid sensing by hypothalamic neurons in body homeostasis.

Amelioration by the natural polyamine spermine of cartilage and bone destruction in rats with collagen-induced arthritis.

We investigated pharmacological properties of naturally occurring polyamines on cartilage and bone destruction seen in joints of rats with collagen-induced arthritis (CIA). Daily supplementation of spermine (SPM), but not spermidine, significantly inhibited increases in the hind paw volume and arthritis score in CIA rats, in addition to the increased mRNA expression of receptor activator of nuclear factor-κB ligand in both cartilage and synovial tissues. Histological analysis clearly revealed a drastic prevention by SPM of the cartilage and bone destruction in synovial joints of CIA rats. Particular natural polyamines would be beneficial for the prophylaxis of synovial joint destruction in rheumatoid arthritis.

The role of CDK8 in mesenchymal stem cells in controlling osteoclastogenesis and bone homeostasis.

Bone marrow mesenchymal stem cells (MSCs) are critical regulators of postnatal bone homeostasis. Osteoporosis is characterized by bone volume and strength deterioration, partly due to MSC dysfunction. Cyclin-dependent kinase 8 (CDK8) belongs to the transcription-related CDK family. Here, CDK8 in MSCs was identified as important for bone homeostasis. CDK8 level was increased in aged MSCs along with the association with aging-related signals. Mouse genetic studies revealed that CDK8 in MSCs plays a crucial role in bone resorption and homeostasis. Mechanistically, CDK8 in MSCs extrinsically controls osteoclastogenesis through the signal transducer and transcription 1 (STAT1)-receptor activator of the nuclear factor κ Β ligand (RANKL) axis. Moreover, aged MSCs have high osteoclastogenesis-supporting activity, partly through a CDK8-dependent manner. Finally, pharmacological inhibition of CDK8 effectively repressed MSC-dependent osteoclastogenesis and prevented ovariectomy-induced osteoclastic activation and bone loss. These findings highlight that the CDK8-STAT1-RANKL axis in MSCs could play a crucial role in bone resorption and homeostasis.