Yoshitomo Saita

Cytoplasmic superoxide causes bone fragility owing to low‐turnover osteoporosis and impaired collagen cross‐linking

The aging process correlates with the accumulation of cellular and tissue damage caused by oxidative stress. Although previous studies have suggested that oxidative stress plays a pathologic role in the development of bone fragility, little direct evidence has been found. In order to investigate the pathologic significance of oxidative stress in bones, we analyzed the bone tissue of mice deficient in cytoplasmic copper/zinc superoxide dismutase (CuZn‐SOD, encoded by the Sod1 gene; Sod1−/−). In this study, we showed for the first time that in vivo cytoplasmic superoxide caused a distinct weakness in bone stiffness and decreased BMD, aging‐like changes in collagen cross‐linking, and transcriptional alterations in the genes associated with osteogenesis. We also showed that the surface areas of osteoblasts and osteoclasts were decreased significantly in the lumbar vertebrae of Sod1−/− mice, indicating the occurrence of low‐turnover osteopenia. In vitro experiments demonstrated that intracellular oxidative stress induced cell death and reduced the proliferation in primary osteoblasts but not in osteoclasts, indicating that impaired osteoblast viability caused the decrease in osteoblast number and suppressed RANKL/M‐CSF osteoclastogenic signaling in bone. Furthermore, treatment with an antioxidant, vitamin C, effectively improved bone fragility and osteoblastic survival. These results imply that intracellular redox imbalance caused by SOD1 deficiency plays a pivotal role in the development and progression of bone fragility both in vivo and in vitro. We herein present a valuable model for investigating the effects of oxidative stress on bone fragility in order to develop suitable therapeutic interventions.

Osteoblastic bone formation is induced by using nanogel-crosslinking hydrogel as novel scaffold for bone growth factor

Bone regeneration for the defects in revision surgery of joint replacement is an increasingly important issue. To repair bone defects, bone cell activation by growth factors using synthetic resorbable scaffold is a useful and safe option. We examine the efficiency of nanogel‐crosslinking hydrogel as a novel synthetic scaffold for BMP to stimulate osteoblasts and to induce bone formation. Cholesterol‐bearing pullulan nanogel‐crosslinking hydrogel (CHPA/Hydrogel) was used to deliver BMP. The CHPA hydrogel pellets were implanted in vivo. Single implantation of CHPA/hydrogel containing low amounts of BMP induced osteoblastic activation and new bone formation in vivo. Furthermore, nanogel in a disc shape established recruitment of osteoblastic cells that vigorously formed bone to heal the calvarial defects, which did not heal spontaneously without it. In conclusion, CHPA/hydrogel serves as an efficient and versatile scaffold for the stimulation of osteoblasts to form bone and to repair defects via delivery of BMP. J. Cell. Physiol. 220: 1–7, 2009.

Sympathetic control of bone mass regulated by osteopontin

The sympathetic nervous system suppresses bone mass by mechanisms that remain incompletely elucidated. Using cell-based and murine genetics approaches, we show that this activity of the sympathetic nervous system requires osteopontin (OPN), a cytokine and one of the major members of the noncollagenous extracellular matrix proteins of bone. In this work, we found that the stimulation of the sympathetic tone by isoproterenol increased the level of OPN expression in the plasma and bone and that mice lacking OPN (OPN-KO) suppressed the isoproterenol-induced bone loss by preventing reduced osteoblastic and enhanced osteoclastic activities. In addition, we found that OPN is necessary for changes in the expression of genes related to bone resorption and bone formation that are induced by activation of the sympathetic tone. At the cellular level, we showed that intracellular OPN modulated the capacity of the β2-adrenergic receptor to generate cAMP with a corresponding modulation of cAMP-response element binding (CREB) phosphorylation and associated transcriptional events inside the cell. Our results indicate that OPN plays a critical role in sympathetic tone regulation of bone mass and that this OPN regulation is taking place through modulation of the β2-adrenergic receptor/cAMP signaling system.

The fracture sites of atypical femoral fractures are associated with the weight-bearing lower limb alignment

PURPOSE Atypical femoral fractures (AFFs) are stress-related fractures that are speculated to associate with long-term treatment with bisphosphonates for osteoporosis. A history of AFF is a high risk factor for the development of a subsequent AFF in the same location of the contralateral femur, suggesting that a patients individual anatomical factor(s) are related to the fracture site of AFFs. In this study, we investigated the radiographs of fourteen AFFs (four bilateral fractures among ten patients) treated at six hospitals associated with our university between 2005 and 2010. The fracture site and standing femorotibial angle (FTA), which reflects the mechanical axis of the lower limb, were measured on weight-bearing lower limb radiographs. The fracture site and FTA of patients with typical femoral fractures (TFF) were compared to those of patients with AFFs. The correlations were examined using Spearmans rank correlation coefficients. The fracture locations in the femora were almost the same in the patients with bilateral AFFs. There was a positive correlation between the fracture site and the standing FTA in the patients with AFFs (r=0.82, 95% confidence interval; 0.49 to 0.94), indicating that the larger the standing FTA (varus alignment), the more distal the site of the fracture in the femur. The FTA of the patients with atypical diaphyseal femoral fracture were significantly larger compared to that of those with not only atypical subtrochanteric fractures but also TFFs. In conclusion, the fracture sites of AFFs are associated with the standing lower limb alignment, while those of TFFs are not.

Anabolic action of parathyroid hormone regulated by the β2-adrenergic receptor

Parathyroid hormone (PTH), the major calcium-regulating hormone, and norepinephrine (NE), the principal neurotransmitter of sympathetic nerves, regulate bone remodeling by activating distinct cell-surface G protein-coupled receptors in osteoblasts: the parathyroid hormone type 1 receptor (PTHR) and the β2-adrenergic receptor (β2AR), respectively. These receptors activate a common cAMP/PKA signal transduction pathway mediated through the stimulatory heterotrimeric G protein. Activation of β2AR via the sympathetic nervous system decreases bone formation and increases bone resorption. Conversely, daily injection of PTH (1–34), a regimen known as intermittent (i)PTH treatment, increases bone mass through the stimulation of trabecular and cortical bone formation and decreases fracture incidences in severe cases of osteoporosis. Here, we show that iPTH has no osteoanabolic activity in mice lacking the β2AR. β2AR deficiency suppressed both iPTH-induced increase in bone formation and resorption. We showed that the lack of β2AR blocks expression of iPTH-target genes involved in bone formation and resorption that are regulated by the cAMP/PKA pathway. These data implicate an unexpected functional interaction between PTHR and β2AR, two G protein-coupled receptors from distinct families, which control bone formation and PTH anabolism.

Nanogel‐based delivery system enhances PGE2 effects on bone formation

Recovery of bone loss is one of the active research issues in bone medicine due to the need for efficient measures for bone gain. We examined here a novel drug delivery system using a nanogel of cholesterol‐bearing pullulan (CHP) in combination with prostaglandin E2 (PGE2). PGE2 or PGE2/CHP, vehicle (saline containing 0.06% ethanol and 0.02% Tween 80) or CHP were injected on to the calvariae of mice once every day for 5 days per week for 4 weeks. Low dosage of PGE2 (0.6 µg) alone or CHP alone did not induce new bone formation in this system. In contrast, PGE2 (0.6 µg)/CHP induced new bone formation. Bone formation activities of PGE2 was enhanced by CHP nanogels only at the site of injection (calvaria) but not in the distant sites of the skeleton, showing that PGE2/CHP could avoid systemic effects. In spite of the fact that previously reported animal models of bone formation by PGE2 were associated with loss of body weight, bone formation based on PGE2/CHP did not associate with loss of body weight. Furthermore, only a single application of PGE2 in combination with nanogel cross‐linking hydrogel sphere (PGE2/CHP‐PEO) induced new bone formation. Thus, nanogel‐based delivery system is an efficient delivery system of bone anabolic agent, PGE2. J. Cell. Biochem. 101:1063–1070, 2007.

Mechanical overloading causes mitochondrial superoxide and SOD2 imbalance in chondrocytes resulting in cartilage degeneration

Mechanical stress and aging are major risk factors of cartilage degeneration. Human studies have previously reported that oxidative damage increased, while SOD2 protein was reciprocally downregulated in osteoarthritic degenerated cartilage. However, it remains unclear whether mitochondrial superoxide imbalance in chondrocytes causes cartilage degeneration. We herein demonstrate that mechanical loading promoted mitochondrial superoxide generation and selective Sod2 downregulation in chondrocytes in vivo and that mitochondrial superoxide inducer also downregulated Sod2 expression in chondrocytes in vitro. A genetically manipulated model revealed that Sod2 deficiency in chondrocytes also resulted in mitochondrial superoxide overproduction and dysfunction, thus leading to cartilage degeneration. Intra-articular injection of a permeable antioxidant effectively suppressed the mechanical loading-induced mitochondrial superoxide generation and cartilage degeneration in mice. Our findings demonstrate that mitochondrial superoxide plays a pivotal role in the development and progression of osteoarthritis, and the mitochondrial superoxide balance may therefore be a promising target for the treatment of cartilage degeneration.

Lack of Schnurri-2 Expression Associates with Reduced Bone Remodeling and Osteopenia *

Regulation of bone remodeling determines the levels of bone mass and its imbalance causes major skeletal diseases such as osteoporosis. A zinc finger protein, Schnurri-2 (SHN-2), was recently demonstrated to regulate bone morphogenetic protein-dependent adipogenesis and lymphogenesis. However, the role of SHN-2 in bone is not known. Here, we investigated the effects of Shn-2 deficiency on bone metabolism and cell function in Shn-2-null mice. Lack of SHN-2 expression reduced bone remodeling by suppressing both osteoblastic bone formation and osteoclastic bone resorption activities in vivo. Shn-2 deficiency suppressed osterix and osteocalcin expression as well as in vitro mineralization. Conversely, Shn-2 overexpression enhanced osteocalcin promoter activity and bone morphogenetic protein-dependent osteoblastic differentiation. Shn-2 deficiency suppressed Nfatc1 and c-fos expression leading to reduction of tartrate-resistant acid phosphatase-positive cell development in vivo as well as in the cultures of bone marrow cells. These studies demonstrate that SHN-2 regulates the activities of critical transcription factors required for normal bone remodeling.

Correlation between synovitis detected on enhanced-magnetic resonance imaging and a histological analysis with a patient-oriented outcome measure for Japanese patients with end-stage knee osteoarthritis receiving joint replacement surgery

Osteoarthritis (OA) is a disease that primarily results in the degeneration and destruction of the articular cartilage. However, synovitis that occurs secondarily by this primary phenomenon is crucial for both the structural and symptomatic progression of the disease. The Japanese Knee Osteoarthritis Measure (JKOM) was created as an outcome measure for Japanese patients with knee OA. This study was conducted to determine whether synovitis in knee OA correlates with the current disability of patients with knee OA who required total knee arthroplasty (TKA). Thirty-four Japanese patients with end-stage knee OA who required TKA were included in this study. The visual analog scale (VAS, 0–100) for pain and the JKOM score, as well as the Western Ontario and McMaster Universities Arthritis Index (WOMAC), were examined before the operation. Synovial samples were taken at the time of the operation. A histological analysis and gadolinium-enhanced magnetic resonance imaging (Gd-MRI) were conducted to evaluate synovitis. Correlations between the synovitis score evaluated by histological analysis and Gd-MRI with either the pain VAS score or the JKOM score were examined using Spearman’s rank correlation coefficient. Neither the synovitis scores evaluated by the histological analysis nor those by a Gd-MRI correlated with the pain VAS score (n = 34, r = 0.25, p = 0.18 and r = 0.08, p = 0.75, respectively) and WOMAC (n = 14, r = 0.35, p = 0.22 and r = 0.45, p = 0.16, respectively) of the patients. However, they significantly correlated with the JKOM score of the patients (n = 34, r = 0.55, p = 0.001 and r = 0.71, p = 0.001, respectively). The severity of synovitis in OA was closely correlated with the current functional impairment and disability of the patients receiving TKA with end-stage knee OA.

Cytoplasmic reactive oxygen species and SOD1 regulate bone mass during mechanical unloading.

Oxidative stress contributes to the pathogenesis of age‐related diseases as well as bone fragility. Our previous study demonstrated that copper/zinc superoxide dismutase (Sod1)‐deficient mice exhibit the induction of intracellular reactive oxygen species (ROS) and bone fragility resulting from low‐turnover bone loss and impaired collagen cross‐linking (Nojiri et al. J Bone Miner Res. 2011;26:2682–94). Mechanical stress also plays an important role in the maintenance of homeostasis in bone tissue. However, the molecular links between oxidative and mechanical stresses in bone tissue have not been fully elucidated. We herein report that mechanical unloading significantly increased intracellular ROS production and the specific upregulation of Sod1 in bone tissue in a tail‐suspension experiment. We also reveal that Sod1 loss exacerbated bone loss via reduced osteoblastic abilities during mechanical unloading. Interestingly, we found that the administration of an antioxidant, vitamin C, significantly attenuated bone loss during unloading. These results indicate that mechanical unloading, in part, regulates bone mass via intracellular ROS generation and the Sod1 expression, suggesting that activating Sod1 may be a preventive strategy for ameliorating mechanical unloading–induced bone loss.