Extracellular matrix and osmolarity influence the development and homeostasis of skeletal tissues through Rho GTPase-mediated alteration of the actin cytoskeleton. This study investigated whether the actin-branching Arp2/3 complex, a downstream effector of the Rho GTPases Cdc42 and Rac1, plays a critical role in maintaining the health of matrix-rich and osmotically loaded intervertebral discs and cartilage. Mice with constitutive intervertebral disc and cartilage-specific deletion of the critical Arp2/3 subunit Arpc2 (Col2-Cre; Arpc2f/f) developed chondrodysplasia and spinal defects. Since these mice did not survive to adulthood, we generated mice with inducible Arpc2 deletion in disc and cartilage (Acan-CreERT2; Arpc2f/f). Inactivation of Arp2/3 at skeletal maturity resulted in growth plate closure, loss of proteoglycan content in articular cartilage, and degenerative changes in the intervertebral disc at 1 year of age. Chondrocytes with Arpc2 deletion showed compromised cell spreading on both collagen and fibronectin. Pharmacological inhibition of Cdc42 and Arp2/3 prevented the osmoadaptive transcription factor TonEBP/NFAT5 from recruiting co-factors in response to a hyperosmolarity challenge. Together, these findings suggest that Arp2/3 plays a critical role in cartilaginous tissues through the regulation of cell-extracellular matrix interactions and modulation of TonEBP-mediated osmoadaptation.
Steven Tessier, Alexandra C Doolittle, Kimheak Sao, Jeremy D. Rotty, James E. Bear, Veronica Ulici, Richard F. Loeser, Irving M. Shapiro, Brian O. Diekman, Makarand V. Risbud
Background: Inflammation is implicated in many aging-related disorders. In animal models, menopause leads to increased gut permeability and inflammation. Our primary objective was to determine if gut permeability increases during the menopause transition (MT) in women. Our exploratory objectives were to examine whether greater gut permeability is associated with more inflammation and lower bone mineral density (BMD).Methods: We included 65 women from the Study of Women’s Health Across the Nation. Key measures were markers of gut permeability (gut barrier dysfunction [fatty acid binding protein 2 [FABP2]) and immune activation secondary to gut microbial translocation (lipopolysaccharide binding protein [LBP], soluble CD14 [sCD14]); inflammation (high-sensitivity CRP); and lumbar spine (LS) or total hip (TH) BMD. Results: In our primary analysis, FABP2, LBP, and sCD14 increased by 22.8% (P = 0.001), 3.7% (P = 0.05), and 8.9% (P = 0.0002), respectively from pre- to postmenopause. In exploratory, repeated measures, mixed-effects linear regression (adjusted for age at the premenopausal visit, body mass index, race/ethnicity, and study site), greater gut permeability was associated with greater inflammation, and lower LS and TH BMD. Conclusions: Gut permeability increases during the MT. Greater gut permeability is associated with more inflammation and lower BMD. Future studies should examine the longitudinal associations of gut permeability, inflammation, and BMD.Funding: NIH, Department of Health and Human Services, through the National Institute on Aging, National Institute of Nursing Research, and NIH Office of Research on Women’s Health (U01NR004061, U01AG012505, U01AG012535, U01AG012531, U01AG012539, U01AG012546, U01AG012553, U01AG012554, U01AG012495).
Albert Shieh, Marta Epeldegui, Arun S Karlamangla, Gail A. Greendale
Dietary salt consumption leads to cutaneous Na+ storage and is associated with various disorders, including osteopenia. Here, we explore the impact of Na+ and the osmoprotective transcription factor nuclear factor of activated T cell 5 (NFAT5) on bone density and osteoclastogenesis. Compared with treatment of mice with high-salt diet, low-salt diet (LSD) increased bone density, decreased osteoclast numbers, and elevated Na+ content and Nfat5 levels in the BM. This response to LSD was dependent on NFAT5 expressed in myeloid cells. Simulating in vivo findings, we exposed osteoclast precursors and osteoblasts to elevated Na+ content (high-salt conditions; HS¢), resulting in increased NFAT5 binding to the promotor region of RANKL decoy receptor osteoprotegerin (OPG). These data not only demonstrate that NFAT5 in myeloid cells determines the Na+ content in BM, but that NFAT5 is able to govern the expression of the osteoprotective gene OPG. This provides insights into mechanisms of Na+-induced cessation of osteoclastogenesis and offers potentially new targets for treating salt-induced osteopenia.
Agnes Schröder, Patrick Neubert, Jens Titze, Aline Bozec, Wolfgang Neuhofer, Peter Proff, Christian Kirschneck, Jonathan Jantsch
Observations in transgenic α-Klotho (Kl) mice (KlTg) defined the antiaging role of soluble Klotho (sKL130). A genetic translocation that elevates sKL levels in humans is paradoxically associated with increased circulating fibroblast growth factor 23 (FGF23) levels and the potential of both membrane KL (mKL135) and sKL130 to act as coreceptors for FGF23 activation of fibroblast growth factor receptors (FGFRs). Neither FGF23 expression nor the contributions of FGF23, mKL135, and sKL130 codependent and independent functions have been investigated in KlTg mice. In the current study, we examined the effects of Kl overexpression on FGF23 levels and functions in KlTg mice. We found that mKL135 but not sKL130 stimulated FGF23 expression in osteoblasts, leading to elevated Fgf23 bone expression and circulating levels in KlTg mice. Elevated FGF23 suppressed 1,25(OH)2D and parathyroid hormone levels but did not cause hypophosphatemic rickets in KlTg mice. KlTg mice developed low aldosterone–associated hypertension but not left ventricular hypertrophy. Mechanistically, we found that mKL135 and sKL130 are essential cofactors for FGF23-mediated ERK activation but that they inhibited FGF23 stimulation of PLC-γ and PI3K/AKT signaling. Thus, increased longevity in KlTg mice occurs in the presence of excess FGF23 that interacts with mKL and sKL to bias FGFR pathways.
Zhousheng Xiao, Gwendalyn King, Salvatore Mancarella, Undral Munkhsaikhan, Li Cao, Chun Cai, Leigh Darryl Quarles
Mammalian focal adhesion proteins Pinch1 and Pinch2 regulate integrin activation and cell–extracellular matrix adhesion and migration. Here, we show that deleting Pinch1 in osteocytes and mature osteoblasts using the 10-kb mouse Dmp1-Cre and Pinch2 globally (double KO; dKO) results in severe osteopenia throughout life, while ablating either gene does not cause bone loss, suggesting a functional redundancy of both factors in bone. Pinch deletion in osteocytes and mature osteoblasts generates signals that inhibit osteoblast and bone formation. Pinch-deficient osteocytes and conditioned media from dKO bone slice cultures contain abundant sclerostin protein and potently suppress osteoblast differentiation in primary BM stromal cells (BMSC) and calvarial cultures. Pinch deletion increases adiposity in the BM cavity. Primary dKO BMSC cultures display decreased osteoblastic but enhanced adipogenic, differentiation capacity. Pinch loss decreases expression of integrin β3, integrin-linked kinase (ILK), and α-parvin and increases that of active caspase-3 and -8 in osteocytes. Pinch loss increases osteocyte apoptosis in vitro and in bone. Pinch loss upregulates expression of both Rankl and Opg in the cortical bone and does not increase osteoclast formation and bone resorption. Finally, Pinch ablation exacerbates hindlimb unloading–induced bone loss and impairs active ulna loading–stimulated bone formation. Thus, we establish a critical role of Pinch in control of bone homeostasis.
Yishu Wang, Qinnan Yan, Yiran Zhao, Xin Liu, Simin Lin, Peijun Zhang, Liting Ma, Yumei Lai, Xiaochun Bai, Chuanju Liu, Chuanyue Wu, Jian Q. Feng, Di Chen, Huiling Cao, Guozhi Xiao
Osteolytic bone lesions and hypercalcemia are common, serious complications in adult T cell leukemia/lymphoma (ATL), an aggressive T cell malignancy associated with human T cell leukemia virus type 1 (HTLV-1) infection. The HTLV-1 viral oncogene HBZ has been implicated in ATL tumorigenesis and bone loss. In this study, we evaluated the role of HBZ on ATL-associated bone destruction using HTLV-1 infection and disease progression mouse models. Humanized mice infected with HTLV-1 developed lymphoproliferative disease and continuous, progressive osteolytic bone lesions. HTLV-1 lacking HBZ displayed only modest delays to lymphoproliferative disease but significantly decreased disease-associated bone loss compared with HTLV-1–infected mice. Gene expression array of acute ATL patient samples demonstrated increased expression of RANKL, a critical regulator of osteoclasts. We found that HBZ regulated RANKL in a c-Fos–dependent manner. Treatment of HTLV-1–infected humanized mice with denosumab, a monoclonal antibody against human RANKL, alleviated bone loss. Using patient-derived xenografts from primary human ATL cells to induce lymphoproliferative disease, we also observed profound tumor-induced bone destruction and increased c-Fos and RANKL gene expression. Together, these data show the critical role of HBZ in driving ATL-associated bone loss through RANKL and identify denosumab as a potential treatment to prevent bone complications in ATL patients.
Jingyu Xiang, Daniel A. Rauch, Devra D. Huey, Amanda R. Panfil, Xiaogang Cheng, Alison K. Esser, Xinming Su, John C. Harding, Yalin Xu, Gregory C. Fox, Francesca Fontana, Takayuki Kobayashi, Junyi Su, Hemalatha Sundaramoorthi, Wing Hing Wong, Yizhen Jia, Thomas J. Rosol, Deborah J. Veis, Patrick L. Green, Stefan Niewiesk, Lee Ratner, Katherine N. Weilbaecher
Recently we demonstrated that ablation of the DNA methyltransferase enzyme, Dnmt3b, resulted in catabolism and progression of osteoarthritis (OA) in murine articular cartilage through a mechanism involving increased mitochondrial respiration. In this study, we identify 4-aminobutyrate aminotransferase (Abat) as a downstream target of Dnmt3b. Abat is an enzyme that metabolizes γ-aminobutyric acid to succinate, a key intermediate in the tricarboxylic acid cycle. We show that Dnmt3b binds to the Abat promoter, increases methylation of a conserved CpG sequence just upstream of the transcriptional start site, and inhibits Abat expression. Dnmt3b deletion in articular chondrocytes results in reduced methylation of the CpG sequence in the Abat promoter, which subsequently increases expression of Abat. Increased Abat expression in chondrocytes leads to enhanced mitochondrial respiration and elevated expression of catabolic genes. Overexpression of Abat in murine knee joints via lentiviral injection results in accelerated cartilage degradation following surgical induction of OA. In contrast, lentiviral-based knockdown of Abat attenuates the expression of IL-1β–induced catabolic genes in primary murine articular chondrocytes in vitro and also protects against murine articular cartilage degradation in vivo. Strikingly, treatment with the FDA-approved small-molecule Abat inhibitor, vigabatrin, significantly prevents the development of injury-induced OA in mice. In summary, these studies establish Abat as an important new target for therapies to prevent OA.
Jie Shen, Cuicui Wang, Jun Ying, Taotao Xu, Audrey McAlinden, Regis J. O’Keefe
Age is a well-established risk factor for impaired bone fracture healing. Here, we identify a role for apolipoprotein E (ApoE) in age-associated impairment of bone fracture healing and osteoblast differentiation, and we investigate the mechanism by which ApoE alters these processes. We identified that, in both humans and mice, circulating ApoE levels increase with age. We assessed bone healing in WT and ApoE–/– mice after performing tibial fracture surgery: bone deposition was higher within fracture calluses from ApoE–/– mice. In vitro recombinant ApoE (rApoE) treatment of differentiating osteoblasts decreased cellular differentiation and matrix mineralization. Moreover, this rApoE treatment decreased osteoblast glycolytic activity while increasing lipid uptake and fatty acid oxidation. Using parabiosis models, we determined that circulating ApoE plays a strong inhibitory role in bone repair. Using an adeno-associated virus–based siRNA system, we decreased circulating ApoE levels in 24-month-old mice and demonstrated that, as a result, fracture calluses from these aged mice displayed enhanced bone deposition and mechanical strength. Our results demonstrate that circulating ApoE as an aging factor inhibits bone fracture healing by altering osteoblast metabolism, thereby identifying ApoE as a new therapeutic target for improving bone repair in the elderly.
Rong Huang, Xiaohua Zong, Puviindran Nadesan, Janet L. Huebner, Virginia B. Kraus, James P. White, Phillip J. White, Gurpreet S. Baht
Bone provides supportive microenvironments for hematopoietic stem cells (HSCs) and mesenchymal stem cells (MSCs) and is a frequent site of metastasis. While incidences of bone metastases increase with age, the properties of the bone marrow microenvironment that regulate dormancy and reactivation of disseminated tumor cells (DTCs) remain poorly understood. Here, we elucidate the age-associated changes in the bone secretome that trigger proliferation of HSCs, MSCs, and DTCs in the aging bone marrow microenvironment. Remarkably, a bone-specific mechanism involving expansion of pericytes and induction of quiescence-promoting secretome rendered this proliferative microenvironment resistant to radiation and chemotherapy. This bone-specific expansion of pericytes was triggered by an increase in PDGF signaling via remodeling of specialized type H blood vessels in response to therapy. The decline in bone marrow pericytes upon aging provides an explanation for loss of quiescence and expansion of cancer cells in the aged bone marrow microenvironment. Manipulation of blood flow — specifically, reduced blood flow — inhibited pericyte expansion, regulated endothelial PDGF-B expression, and rendered bone metastatic cancer cells susceptible to radiation and chemotherapy. Thus, our study provides a framework to recognize bone marrow vascular niches in age-associated increases in metastasis and to target angiocrine signals in therapeutic strategies to manage bone metastasis.
Amit Singh, Vimal Veeriah, Pengjun Xi, Rossella Labella, Junyu Chen, Sara G. Romeo, Saravana K. Ramasamy, Anjali P. Kusumbe
We induced chronic kidney disease (CKD) with adenine in WT mice, mice with osteocyte-specific deletion of Cyp27b1, encoding the 25-hydroxyvitamin D 1(OH)ase [Oct-1(OH)ase–/–], and mice with global deletion of Cyp27b1 [global-1α(OH)ase–/–]; we then compared extraskeletal calcification. After adenine treatment, mice displayed increased blood urea nitrogen, decreased serum 1,25(OH)2D, and severe hyperparathyroidism. Skeletal expression of Cyp27b1 and of sclerostin and serum sclerostin all increased in WT mice but not in Oct-1α(OH)ase–/– mice or global-1α(OH)ase–/– mice. In contrast, skeletal expression of BMP2 and serum BMP2 rose in the Oct-1α(OH)ase–/– mice and in the global-1α(OH)ase–/– mice. Extraskeletal calcification occurred in muscle and blood vessels of mice with CKD and was highest in Oct-1α(OH)ase–/–mice. In vitro, recombinant sclerostin (100 ng/mL) significantly suppressed BMP2-induced osteoblastic transdifferentiation of vascular smooth muscle A7r5 cells and diminished BMP2-induced mineralization. Our study provides evidence that local osteocytic production of 1,25(OH)2D stimulates sclerostin and inhibits BMP2 production in murine CKD, thus mitigating osteoblastic transdifferentiation and mineralization of soft tissues. Increased osteocytic 1,25(OH)2D production, triggered by renal malfunction, may represent a “primary defensive response” to protect the organism from ectopic calcification by increasing sclerostin and suppressing BMP2 production.
Loan Nguyen-Yamamoto, Ken-Ichiro Tanaka, Rene St–Arnaud, David Goltzman
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