Osteopenia in genetically diabetic DB/DB mice and effects of 1alpha-hydroxyvitamin D3 on the osteopenia. Basic Research Group

Interoceptive regulation of skeletal tissue homeostasis and repair

Recent studies have determined that the nervous system can sense and respond to signals from skeletal tissue, a process known as skeletal interoception, which is crucial for maintaining bone homeostasis. The hypothalamus, located in the central nervous system (CNS), plays a key role in processing interoceptive signals and regulating bone homeostasis through the autonomic nervous system, neuropeptide release, and neuroendocrine mechanisms. These mechanisms control the differentiation of mesenchymal stem cells into osteoblasts (OBs), the activation of osteoclasts (OCs), and the functional activities of bone cells. Sensory nerves extensively innervate skeletal tissues, facilitating the transmission of interoceptive signals to the CNS. This review provides a comprehensive overview of current research on the generation and coordination of skeletal interoceptive signals by the CNS to maintain bone homeostasis and their potential role in pathological conditions. The findings expand our understanding of intersystem communication in bone biology and may have implications for developing novel therapeutic strategies for bone diseases.

Endospanin Is a Candidate for Regulating Leptin Sensitivity

The hypothesis advanced is that endospanin, a highly conserved vesicle traffic protein in vertebrates, regulates leptin sensitivity in bone signaling. The effects of leptin on bones are well-studied but without consensus on whether the increases in leptin signaling stimulate bone gain or loss. The bone response may depend on leptin sensitivity, and endospanin is an established modulator of leptin sensitivity. An argument is advanced to develop zebrafish models for specific leptin signaling pathways. Zebrafish have well-developed molecular tools (e.g., CRISPR) and the advantage of non-destructive sampling of bones in the form of scales. Using these tools, experiments are described to substantiate the role of endospanin in zebrafish bone dynamics.

Systemic Parathyroid Hormone Enhances Fracture Healing in Multiple Murine Models of Type 2 Diabetes Mellitus

Type 2 diabetes mellitus (T2DM) is a multisystemic disease that afflicts more than 415 million people globally-the incidence and prevalence of T2DM continues to rise. It is well-known that T2DM has detrimental effects on bone quality that increase skeletal fragility, which predisposes subjects to an increased risk of fracture and fracture healing that results in non- or malunion. Diabetics have been found to have perturbations in metabolism, hormone production, and calcium homeostasis-particularly PTH expression-that contribute to the increased risk of fracture and decreased fracture healing. Given the perturbations in PTH expression and the establishment of hPTH (1-34) for use in age-related osteoporosis, it was determined logical to attempt to ameliorate the bone phenotype found in T2DM using hPTH (1-34). Therefore, the present study had two aims: (i) to establish a suitable murine model of the skeletal fragility present in T2DM because no current consensus model exists; and (ii) to determine the effects of hPTH (1-34) on bone fractures in T2DM. The results of the present study suggest that the polygenic mouse of T2DM, TALLYHO/JngJ, most accurately recapitulates the diabetic osteoporotic phenotype seen in humans and that the intermittent systemic administration of hPTH (1-34) increases fracture healing in T2DM murine models by increasing the proliferation of mesenchymal stem cells. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Effects of Leptin on the Skeleton

Leptin originates in adipocytes, including those in bone marrow, and circulates in concentrations 20 to 90 times higher than those in the cerebrospinal fluid. It has direct anabolic effects on osteoblasts and chondrocytes, but it also influences bone indirectly, via the hypothalamus and sympathetic nervous system, via changes in body weight, and via effects on the production of other hormones (e.g., pituitary). Leptin's role in bone physiology is determined by the balance of these conflicting effects. Reflecting this inconsistency, the leptin-deficient mouse has reduced length and bone mineral content of long bones but increased vertebral trabecular bone. A consistent bone phenotype in human leptin deficiency has not been established. Systemic leptin administration in animals and humans usually exerts a positive effect on bone mass, and leptin administration into the cerebral ventricles usually normalizes the bone phenotype in leptin-deficient mice. Reflecting the role of the sympathetic nervous system in mediating the central catabolic effects of leptin on the skeleton, β-adrenergic agonists and antagonists have major effects on bone in mice, but this is not consistently seen in humans. The balance of the central and peripheral effects of leptin on bone remains an area of substantial controversy and might vary between species and according to other factors such as body weight, baseline circulating leptin levels, and the presence of specific pathologies. In humans, leptin is likely to contribute to the positive relationship observed between adiposity and bone density, which allows the skeleton to respond appropriately to changes in soft tissue mass.

Methodological considerations when studying the skeletal response to glucose intolerance using the diet-induced obesity model

The prevalence of obesity and type 2 diabetes mellitus (T2DM) continues to rise, and as a result, research aimed at understanding the molecular basis for the co-morbidities has become an area of much scientific interest. Among the more recently recognized chronic complications of T2DM is the increased risk of fracture, especially hip fracture, that has been reported independent of bone mineral density (BMD). A widely used animal model to study how the development and progression of impaired glucose tolerance affect the skeleton has been the diet-induce obesity (DIO) model. As the name implies, this model employs the use of a version of high-fat diets to induce obesity and the subsequent metabolic perturbations that occur with T2DM. Although the model offers a number of advantages, the literature reveals some inconsistent results. Upon further review, discrepancies in the choice of the experimental high-fat diets and the control diets have become a point of major concern. The variability between diets and study design has made it difficult to compare data and results across studies. Therefore, this review aims to provide guidelines that should be employed when designing studies using DIO models of T2DM.

The brain in bone and fuel metabolism

Obesity and osteoporosis have become major public health challenges worldwide. The brain is well established as a pivotal regulator of energy homeostasis, appetite and fuel metabolism. However, there is now clear evidence for regulation between the brain and bone. Similarly, evidence also indicates that the involvement of the brain in bone and adipose regulation is both related and interdependent. The hypothalamus, with its semi-permeable blood brain barrier, is one of the most powerful regulatory regions within the body, integrating and relaying signals not only from peripheral tissues but also from within the brain itself. Two main neuronal populations within the arcuate nucleus of the hypothalamus regulate energy homeostasis: The orexigenic, appetite-stimulating neurons that co-express neuropeptide Y and agouti-related peptide and the anorexigenic, appetite-suppressing neurons that co-express proopiomelanocortin and cocaine- and amphetamine related transcript. From within the arcuate, these four neuropeptides encompass some of the most powerful control of energy homeostasis in the entire body. Moreover, they also regulate skeletal homeostasis, identifying a co-ordination network linking the processes of bone and energy homeostasis. Excitingly, the number of central neuropeptides and neural factors known to regulate bone and energy homeostasis continues to grow, with cannabinoid receptors and semaphorins also involved in bone homeostasis. These neuronal pathways represent a growing area of research that is identifying novel regulatory axes between the brain and the bone, and links with other homeostatic networks; thereby revealing a far more complex, and interdependent bone biology than previously envisioned. This review examines the current understanding of the central regulation of bone and energy metabolism.

Neuronal control of bone and muscle

The functional interplay between bone and muscle that enables locomotion is a fundamental aspect of daily life. However, other interactions between bone and muscle continue to attract attention as our understanding of the breath and importance of this inter-relationship continues to expand. Of particular interest is the regulatory connection between bone and muscle, which adds a new insight to the coordination of the bone/muscle unit. We have appreciated the importance of neuronal signaling to the control of bone turnover and muscle contraction, but recent data indicate that neuronal inputs control a far wider range of bone and muscle physiology than previously appreciated. This review outlines the role of the sympathetic nervous system and neuronal/neuropeptide inputs upon the regulation of bone and muscle tissue, and the potential for co-regulatory actions, particularly involving the sympathetic nervous system. This article is part of a Special Issue entitled "Muscle Bone Interactions".

22-Oxacalcitriol attenuates bone loss in nonobese type 2 diabetes

Active vitamin D is a major therapeutic agent for bone disease. Although some studies have reported that vitamin D ameliorates bone disease related to diabetes, the mechanism remains unclear. Our study investigated the effect of the vitamin D receptor activator 22-oxacalcitriol (OCT) on bone disease in a rat model of diabetes. OCT was administered at a dose of 0.2μg/kg three times per week for 10weeks. We performed blood and urine analyses, single energy X-ray absorptiometry, micro-computed tomography, bone histomorphometry, and oxidative stress assessment in rats at 30weeks of age. OCT did not affect hemoglobin A1c or serum calcium levels. Bone mineral density (BMD), bone volume in the cortical and trabecular bones, and bone turnover were decreased in rats with diabetes. OCT treatment increased BMD and bone formation and tended to increase bone volume in the trabecular bone, but did not change bone volume in the cortical bone or bone resorption. The urinary oxidative stress marker 8-hydroxydeoxyguanosine (8-OHdG) excretion and the number of 8-OHdG-positive cells in bone were increased in rats with diabetes, and OCT treatment suppressed these increases. Our data suggest that OCT attenuated bone loss in a rat model of diabetes. This attenuation may be partially mediated by improved bone formation resulting from the antioxidative effect of OCT.

A review of rodent models of type 2 diabetic skeletal fragility

Evidence indicating that adult type 2 diabetes (T2D) is associated with increased fracture risk continues to mount. Unlike osteoporosis, diabetic fractures are associated with obesity and normal to high bone mineral density, two factors that are typically associated with reduced fracture risk. Animal models will likely play a critical role in efforts to identify the underlying mechanisms of skeletal fragility in T2D and to develop preventative treatments. In this review we critically examine the ability of current rodent models of T2D to mimic the skeletal characteristics of human T2D. We report that although there are numerous rodent models of T2D, few have undergone thorough assessments of bone metabolism and strength. Further, we find that many of the available rodent models of T2D have limitations for studies of skeletal fragility in T2D because the onset of diabetes is often prior to skeletal maturation and bone mass is low, in contrast to what is seen in adult humans. There is an urgent need to characterize the skeletal phenotype of existing models of T2D, and to develop new models that more closely mimic the skeletal effects seen in adult-onset T2D in humans.

Neuropeptide Y is a critical modulator of leptin's regulation of cortical bone

Leptin signaling is required for normal bone homeostasis; however, loss of leptin results in differing effects on cortical and cancellous bone, as well as altered responses between the axial and appendicular regions. Local β-adrenergic actions are responsible for the greater cancellous bone volume in leptin-deficient (ob/ob) mice; however, the mechanism responsible for the opposing reduction in cortical bone in ob/ob mice is not known. Here we show that blocking the leptin-deficient increase in neuropeptide Y (NPY) expression reverses the cortical bone loss in ob/ob mice. Mice null for both NPY and leptin (NPY(-/-) ob/ob), display greater cortical bone mass in both long-bones and vertebra, with NPY(-/-) ob/ob mice exhibiting thicker and denser cortical bone, associated with greater endocortical and periosteal mineral apposition rate (MAR), compared to ob/ob animals. Importantly, these cortical changes occurred without significant increases in body weight, with NPY(-/-) ob/ob mice showing significantly reduced adiposity compared to ob/ob controls, most likely due to the reduced respiratory exchange ratio seen in these animals. Interestingly, cancellous bone volume was not different between NPY(-/-) ob/ob and ob/ob, suggesting that NPY is not influencing the adrenergic axis. Taken together, this work demonstrates the critical role of NPY signaling in the regulation of bone and energy homeostasis, and more importantly, suggests that reduced leptin levels or leptin resistance, which occurs in obesity, could potentially inhibit cortical bone formation via increased central NPY signaling.

Bariatric surgery, bone loss, obesity and possible mechanisms

Bariatric surgery remains the most effective treatment for severely obese patients. However, the potential long-term effects of bariatric surgical procedures on health, including bone health, are only partially understood. The goal of this review was to present data on the impact of bariatric surgery on bone metabolism and to analyse possible reasons for the loss of bone mass that frequently occurs after bariatric surgery. Such factors include nutritional deficiencies, rapid weight loss per se, effects of fat-derived adipokines and gut-derived appetite-regulatory hormones. However, the relative roles of these factors in skeletal regulation and the mechanisms by which they work are not yet fully defined. Our review was focussed on the complex relationship between body weight, fat mass and bone mass, as well as peripheral and central mediators potentially involved in the dual regulation of both energy and bone homeostasis. We also review the data on the inverse relationship between central obesity, bone marrow fat and osteoporosis. As the number of bariatric operations increases, it is imperative to recognize mechanisms responsible for bariatric surgery-induced bone loss, with careful monitoring of bone health including long-term fracture incidence in patients undergoing these procedures.

Peripheral leptin regulates bone formation

Substantial evidence does not support the prevailing view that leptin, acting through a hypothalamic relay, decreases bone accrual by inhibiting bone formation. To clarify the mechanisms underlying regulation of bone architecture by leptin, we evaluated bone growth and turnover in wild-type (WT) mice, leptin receptor-deficient db/db mice, leptin-deficient ob/ob mice, and ob/ob mice treated with leptin. We also performed hypothalamic leptin gene therapy to determine the effect of elevated hypothalamic leptin levels on osteoblasts. Finally, to determine the effects of loss of peripheral leptin signaling on bone formation and energy metabolism, we used bone marrow (BM) from WT or db/db donor mice to reconstitute the hematopoietic and mesenchymal stem cell compartments in lethally irradiated WT recipient mice. Decreases in bone growth, osteoblast-lined bone perimeter and bone formation rate were observed in ob/ob mice and greatly increased in ob/ob mice following subcutaneous administration of leptin. Similarly, hypothalamic leptin gene therapy increased osteoblast-lined bone perimeter in ob/ob mice. In spite of normal osteoclast-lined bone perimeter, db/db mice exhibited a mild but generalized osteopetrotic-like (calcified cartilage encased by bone) skeletal phenotype and greatly reduced serum markers of bone turnover. Tracking studies and histology revealed quantitative replacement of BM cells following BM transplantation. WT mice engrafted with db/db BM did not differ in energy homeostasis from untreated WT mice or WT mice engrafted with WT BM. Bone formation in WT mice engrafted with WT BM did not differ from WT mice, whereas bone formation in WT mice engrafted with db/db cells did not differ from the low rates observed in untreated db/db mice. In summary, our results indicate that leptin, acting primarily through peripheral pathways, increases osteoblast number and activity.

The role of vitamin D in the metabolic homeostasis of diabetic bone

Most studies across a variety of geographic locations suggest that vitamin D insufficiency is more common in individuals with type 1 diabetes (T1D) compared to the general population. In type 2 diabetes (T2D), while obesity is commonplace and lower vitamin D levels are present in obese adolescents and adults, the association between vitamin D insufficiency and T2D is less clear. Studies suggest that the relationship between T2D and vitamin D may be concurrently influenced by ethnicity, geography, BMI and age. None-the-less, diabetic osteopathy is a significant co-morbidity of both forms of diabetes, and is characterized by micro-architectural changes that decrease bone quality leading to an increased risk for bone fracture in both disorders. The question remains, however, to what degree vitamin D homeostasis contributes to or exacerbates skeletal pathology in diabetes. Proposed mechanisms for vitamin D deficiency in diabetes include: 1) genetic predisposition (T1D); 2) increased BMI (T2D); 3) concurrent albuminuria (T1D or T2D); or 4) exaggerated renal excretion of vitamin D metabolites or vitamin D binding protein (T1D, T2D, animal models). The specific effects of vitamin D treatment on diabetic osteoporosis have been examined in rodents, and demonstrate skeletal improvements even in the face of untreated diabetes. However, human clinical trial data examining whether vitamin D status can be directly related to or is predictive of bone quality and fracture risk in those with diabetes is still needed. Herein, we provide a review of the literature linking vitamin D, diabetes and skeletal health.

Skeletal phenotype of the leptin receptor-deficient db/db mouse

Leptin, a major hormonal product of the adipocyte, regulates appetite and reproductive function through its hypothalamic receptors. The leptin receptor is present in osteoblasts and chondrocytes, and previously we have shown leptin to be an anabolic bone factor in vitro, stimulating osteoblast proliferation and inhibiting osteoclastogenesis. Leptin increases bone mass and reduces bone fragility when administered peripherally but also can indirectly reduce bone mass when administered into the central nervous system. However, data from animal models deficient in either leptin (ob/ob) or its receptor (db/db) remain contradictory. We compared the bone phenotype of leptin receptor-deficient (db/db) and wild-type mice using micro-computed tomographic (µCT) analysis of the proximal tibias and vertebrae. In the tibia, db/db mice had reduced percent trabecular bone volume (13.0 ± 1.62% in wild-type versus 6.01 ± 0.601% in db/db mice, p = .002) and cortical bone volume (411 ± 21.5 µm(3) versus 316 ± 3.53 µm(3), p = .0014), trabecular thickness (48.4 ± 001.07 µm versus 45.1 ± 0.929 µm, p = .041) and trabecular number (2.68 ± 0.319 mm(-1) versus 1.34 ± 0.148 mm(-1), p = .0034). In the fifth lumbar vertebral body, the trabecular thickness and cortical thickness were decreased in the db/db versus wild-type mice (0.053 ± 0.0011 mm versus 0.047 ± 0.0013 mm, p = .0002 and 0.062 ± 0.00054 mm versus 0.056 ± 0.0009 mm, p = .0001), respectively, whereas the trabecular and cortical percent bone volume and trabecular number did not reach significance. The total (endosteal and periosteal) cortical perimeter (12.2 ± 0.19 mm versus 13.2 ± 0.30 mm, p = .01) was increased. The serum osteocalcin levels were reduced in the db/db mice, suggesting that bone formation rates are decreased. The material properties of db/db femurs were determined by three-point bending and nanoindentation, showing decreased bone strength (13.3 ± 0.280 N versus 7.99 ± 0.984 N, p = .0074) and material stiffness (28.5 ± 0.280 GPa versus 25.8 ± 0.281 GPa, p < .0001). These results demonstrate that bone mass and strength are reduced in the absence of leptin signaling, indicating that leptin acts in vivo as an anabolic bone factor. This concurs with results of in vitro studies and of peripheral leptin administration in vivo and suggests that leptin's direct effects on bone cells are likely to override its actions via the central nervous system.

Non-enzymatic glycation of bone collagen modifies osteoclastic activity and differentiation

Type I collagen, the major organic component of bone matrix, undergoes a series of post-translational modifications that occur with aging, such as the non-enzymatic glycation. This spontaneous reaction leads to the formation of advanced glycation end products (AGEs), which accumulate in bone tissue and affect its structural and mechanical properties. We have investigated the role of matrix AGEs on bone resorption mediated by mature osteoclasts and the effects of exogenous AGEs on osteoclastogenesis. Using in vitro resorption assays performed on control- and AGE-modified bone and ivory slices, we showed that the resorption process was markedly inhibited when mature osteoclasts were seeded on slices containing matrix pentosidine, a well characterized AGE. More specifically, the total area resorbed per slice, and the area degraded per resorption lacuna created by osteoclasts, were significantly decreased in AGE-containing slices. This inhibition of bone resorption was confirmed by a marked reduction of the release of type I collagen fragments generated by the collagenolytic enzymes secreted by osteoclasts in the culture medium of AGE-modified mineralized matrices. This effect is likely to result from decreased solubility of collagen molecules in the presence of AGEs, as documented by the reduction of pepsin-mediated digestion of AGE-containing collagen. We found that AGE-modified BSA totally inhibited osteoclastogenesis in vitro, most likely by impairing the commitment of osteoclast progenitors into pre-osteoclastic cells. Although the mechanisms remain unknown, AGEs might interfere with osteoclastic differentiation and activity through their interaction with specific cell-surface receptors, because we showed that both osteoclast progenitors and mature osteoclasts expressed different AGEs receptors, including receptor for AGEs (RAGEs). These results suggest that AGEs decreased osteoclast-induced bone resorption, by altering not only the structural integrity of bone matrix proteins but also the osteoclastic differentiation process. We suggest that AGEs may play a role in the alterations of bone remodeling associated with aging and diabetes.

Streptozotocin-induced diabetes influences the activity of ecto-nucleoside triphosphate diphosphohydrolase 1 of rat osseous plate membranes

We report the kinetic characterization of an ecto-nucleosidetriphosphate diphosphohydrolase 1 from rat osseous plate membranes in streptozotocin-induced diabetic rats, which arises during ectopic mineralization twenty days after a subcutaneous implantation of demineralized bone matrix, Insulin deficiency decreased the ecto-nucleoside triphosphate diphosphohydrolase activity from 1293.1 +/- 39.8 (control rats) to 556.0 +/- 8.2 nmol Pi/(min mg). Two families of ATP hydrolyzing sites showed cooperative effects with specific activities of 256.2 +/- 7.7 nmol Pi/(min mg) and 299.8 +/- 8.9 nmol Pi/(min mg), and studies on the stimulation of the enzyme by magnesium and calcium ions showed that the decrease in enzyme activity results from changes in the affinity of the enzyme for these ions. To our knowledge this is the first study associating the effects of type I diabetes with an ecto-nucleoside triphosphate diphosphohydrolase activity from rat osseous plate membranes.

Isoflavone consumption does not increase the bone mass in osteopenic obese female zucker rats

Some controversy exists in the literature concerning the effects of leptin on bone metabolism. Thus we have compared femoral bone density and biochemical markers of bone metabolism in male and female fatty (leptin-resistant) Zucker rats and their lean homozygous controls at 3 and 6 months of age. At 3 months, no differences concerning total, diaphyseal (cortical bone), and distal metaphyseal (trabecular bone) femoral bone densities, plasma osteocalcin concentrations, and urinary deoxypyridinoline excretion were observed between fatty and lean rats. On the opposite, at 6 months of age, in both males and females, total, diaphyseal, and distal metaphyseal femoral bone densities and plasma osteocalcin concentrations were lower in Zucker than in lean rats. Soybean isoflavone consumption (40 microg/g body weight/day for 90 days, a dose which prevents osteopenia following ovariectomy both in lean Zucker homozygous controls and in Wistar rats) by obese female Zucker rats had no significant effect upon their bone mass.