Central regulation of bone mass

Causal effects of osteoporosis on structural changes in specific brain regions: a Mendelian randomization study

Observational studies have reported that osteoporosis is associated with cortical changes in the brain. However, the inherent limitations of observational studies pose challenges in eliminating confounding factors and establishing causal relationships. And previous observational studies have not reported changes in specific brain regions. By employing Mendelian randomization, we have been able to infer a causal relationship between osteoporosis and a reduction in the surficial area (SA) of the brain cortical. This effect is partially mediated by vascular calcification. We found that osteoporosis significantly decreased the SA of global brain cortical (β = -1587.62 mm2, 95%CI: -2645.94 mm2 to -529.32 mm2, P = 0.003) as well as the paracentral gyrus without global weighted (β = - 19.42 mm2, 95%CI: -28.90 mm2 to -9.95 mm2, P = 5.85 × 10-5). Furthermore, we estimated that 42.25% and 47.21% of the aforementioned effects are mediated through vascular calcification, respectively. Osteoporosis leads to a reduction in the SA of the brain cortical, suggesting the presence of the bone-brain axis. Vascular calcification plays a role in mediating this process to a certain extent. These findings establish a theoretical foundation for further investigations into the intricate interplay between bone, blood vessels, and the brain.

Crosstalk Between the Neuroendocrine System and Bone Homeostasis

The homeostasis of bone microenvironment is the foundation of bone health and comprises 2 concerted events: bone formation by osteoblasts and bone resorption by osteoclasts. In the early 21st century, leptin, an adipocytes-derived hormone, was found to affect bone homeostasis through hypothalamic relay and the sympathetic nervous system, involving neurotransmitters like serotonin and norepinephrine. This discovery has provided a new perspective regarding the synergistic effects of endocrine and nervous systems on skeletal homeostasis. Since then, more studies have been conducted, gradually uncovering the complex neuroendocrine regulation underlying bone homeostasis. Intriguingly, bone is also considered as an endocrine organ that can produce regulatory factors that in turn exert effects on neuroendocrine activities. After decades of exploration into bone regulation mechanisms, separate bioactive factors have been extensively investigated, whereas few studies have systematically shown a global view of bone homeostasis regulation. Therefore, we summarized the previously studied regulatory patterns from the nervous system and endocrine system to bone. This review will provide readers with a panoramic view of the intimate relationship between the neuroendocrine system and bone, compensating for the current understanding of the regulation patterns of bone homeostasis, and probably developing new therapeutic strategies for its related disorders.

Causal associations of brain structure with bone mineral density: a large-scale genetic correlation study

In this study, we aimed to investigate the causal associations of brain structure with bone mineral density (BMD). Based on the genome-wide association study (GWAS) summary statistics of 1 325 brain imaging-derived phenotypes (BIDPs) of brain structure from the UK Biobank and GWAS summary datasets of 5 BMD locations, including the total body, femoral neck, lumbar spine, forearm, and heel from the GEFOS Consortium, linkage disequilibrium score regression (LDSC) was conducted to determine the genetic correlations, and Mendelian randomization (MR) was then performed to explore the causal relationship between the BIDPs and BMD. Several sensitivity analyses were performed to verify the strength and stability of the present MR outcomes. To increase confidence in our findings, we also performed confirmatory MR between BIDPs and osteoporosis. LDSC revealed that 1.93% of BIDPs, with a false discovery rate (FDR) < 0.01, were genetically correlated with BMD. Additionally, we observed that 1.31% of BIDPs exhibited a significant causal relationship with BMD (FDR < 0.01) through MR. Both the LDSC and MR results demonstrated that the BIDPs "Volume of normalized brain," "Volume of gray matter in Left Inferior Frontal Gyrus, pars opercularis," "Volume of Estimated Total Intra Cranial" and "Volume-ratio of brain segmentation/estimated total intracranial" had strong associations with BMD. Interestingly, our results showed that more left BIDPs were causally associated with BMD, especially within and around the left frontal region. In conclusion, a part of the brain structure causally influences BMD, which may provide important perspectives for the prevention of osteoporosis and offer valuable insights for further research on the brain-bone axis.

The Influence of Obesity, Ovariectomy, and Greenshell Mussel Supplementation on Bone Mineral Density in Rats

Obesity is considered to impair long-term health by disturbing multiple physiological functions. However, it remains a controversial issue as to whether obesity has beneficial or detrimental effects on bone health in postmenopausal women. The aims of this study were to investigate the relationships between obesity and bone mineral density (BMD) under conditions of ovarian hormone deficiency in an animal model and to evaluate the potential health benefits of Greenshell mussel (GSM) on bone health. A total of 144 adult female Sprague-Dawley rats were fed from age 12 weeks on one of four diets (normal [ND]; ND + GSM; high fat/high sugar [HF/HS]; HF/HS + GSM; n = 36 per diet). At age 20 weeks, after a dual-energy X-ray absorptiometry (DXA) scan, 12 of the rats on each diet underwent ovariectomy (OVX) and the remaining rats were left intact. Twelve of the intact rats in each diet group were culled at age 26 weeks (short-term cohort). The remaining rats were culled at age 48 weeks (long-term cohort). Rats were DXA scanned before cull, then various fat pads were dissected. The results revealed that HF/HS rats and OVX rats dramatically increased body weight and fat deposition in correlation with leptin. In the long-term cohort, vertebral spine BMD rapidly declined after OVX. At termination, the OVX rats had decreased plasma bone turnover markers of CTX-1 and TRAP when compared with sham rats. Significantly higher BMD was found in OVX rats fed the HF/HS diet compared with ND, but this difference was not recapitulated in intact rats. BMD of right femur was significantly increased 5% to 10% by GSM in the short-term cohort. The data demonstrated that obesity can be beneficial by increasing BMD in OVX rats, and this may extrapolate to postmenopausal women as adipocyte-produced estrogen may slightly compensate for the reduction in ovarian hormones. Finally, the data showed that GSM may be beneficial to bone health by increasing BMD accrual. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

The Dramatic Role of IFN Family in Aberrant Inflammatory Osteolysis

Skeletal system has been considered a highly dynamic system, in which bone-forming osteoblasts and bone-resorbing osteoclasts go through a continuous remodeling cycle to maintain homeostasis of bone matrix. It has been well acknowledged that interferons (IFNs), acting as a subgroup of cytokines, not only have crucial effects on regulating immunology but also could modulate the dynamic balance of bone matrix. In the light of different isoforms, IFNs have been divided into three major categories in terms of amino acid sequences, recognition of specific receptors and biological activities. Currently, type I IFNs consist of a multi-gene family with several subtypes, of which IFN-α exerts pro-osteoblastogenic effects to activate osteoblast differentiation and inhibits osteoclast fusion to maintain bone matrix integrity. Meanwhile, IFN-β suppresses osteoblast-mediated bone remodeling as well as exhibits inhibitory effects on osteoclast differentiation to attenuate bone resorption. Type II IFN constitutes the only type, IFN-γ, which exerts regulatory effects on osteoclastic bone resorption and osteoblastic bone formation by biphasic ways. Interestingly, type III IFNs are regarded as new members of IFN family composed of four members, including IFN-λ1 (IL-29), IFN-λ2 (IL-28A), IFN-λ3 (IL-28B) and IFN-λ4, which have been certified to participate in bone destruction. However, the direct regulatory mechanisms underlying how type III IFNs modulate the metabolic balance of bone matrix, remains poorly elucidated. In this review, we have summarized functions of IFN family during physiological and pathological conditions and described the mechanisms by which IFNs maintain bone matrix homeostasis via affecting the osteoclast-osteoblast crosstalk. In addition, the potential therapeutic effects of IFNs on inflammatory bone destruction diseases such as rheumatoid arthritis (RA), osteoarthritis (OA) and infectious bone diseases are also well displayed, which are based on the predominant role of IFNs in modulating the dynamic equilibrium of bone matrix.

Does Hip Bone Density Differ between Paretic and Non-Paretic Sides in Hemiplegic Stroke Patients? and Its Relationship with Physical Impairment

Background

Bone loss after stroke escalates the risk of fractures, mainly in the hip, leading to further disability in individuals with stroke. We aimed to investigate the skeletal effect of bone mineral density (BMD) based on the duration of onset of stroke, compare the BMD of the paretic and non-paretic sides, and elucidate the relationship between BMD and disability variables.

Methods

The 31 male hemiplegic stroke patients between 20 and 70 years of age with cerebral infarction or hemorrhage were considered in this study. Subacute and chronic cases included 13 and 18 patients with lag time from the onset of 1 to 6 months and beyond 6 months, respectively. BMD in the lumbar, paretic, and non-paretic hip as well as the disability variables were analyzed retrospectively.

Results

The subacute group showed a significant reduction in the femoral neck BMD on the paretic side compared to that on the non-paretic side based on T-scores (P=0.013). Bone loss was significantly correlated with lower limb muscle strength and overall physical impairment (P<0.05). The chronic group demonstrated significant reduction in femur neck and total femur BMD on the paretic side compared to that on the non-paretic side based on T-scores (P=0.002 and P<0.001, respectively). T-scores of BMD in the chronic phase were not significantly associated with the clinical parameters.

Conclusions

Early screening of bilateral hip BMD in the early stages after stroke, monitoring, and timely implementation of prevention strategies are important to minimize subsequent bone loss and prevent possible complications in patients who experience stroke.

P2X7 receptor acts as an efficient drug target in regulating bone metabolism system

Skeletal system is a highly dynamic system going through continuous resorption and reconstruction to maintain homeostasis, which is influenced by numerous factors. Once the balance is disrupted, various kinds of bone diseases may occur such as osteoporosis. It has been well known that ATP (adenosine triphosphate), an important signaling molecule, is important in maintaining the dynamic balance of bone matrix. ATP mainly functions through P2X receptors, a kind of ATP receptors expressed by various kinds of bone cells to regulate the whole network of skeleton system. Among P2X receptors, P2X7 plays a crucial role in bone since P2X7 is widely expressed by bone cells and the mutation of P2X7 receptor is associated with kinds of bone diseases. It's acknowledged that P2X7 acts as a potential therapeutic target for clinical treatment of bone-related diseases but further investigations are needed for the practical application. However, since P2X7 has a complicated effect in many aspects, the exact role of P2X7 in skeleton system is ambiguous. This review discusses the function of P2X7 in bone and other cells and their general effect on skeleton system, especially focusing on the possible clinical application for bone diseases.

Stroke Prevents Exercise-induced Gains in Bone Microstructure But Not Composition in Mice

Ischemic stroke induces rapid loss in bone mineral density up to 13 times greater than during normal aging, leading to markedly increased risk of fracture. Little is known about skeletal changes following stroke beyond density loss. In this study we use a mild-moderate middle cerebral artery occlusion model to determine the effects of ischemic stroke without bedrest on bone microstructure, dynamic bone formation, and tissue composition. Twenty-seven 12-week-old male C57Bl/6J mice received either a stroke or sham surgery and then either received daily treadmill exercise or remained sedentary for four weeks. All mice were ambulatory immediately following stroke, and limb coordination during treadmill exercise was unaffected by stroke, indicating similar mechanical loading across limbs for surgery groups. Stroke did not directly detriment microstructure, but exercise only stimulated adaptation in sham group, not stroke group, with increased bone volume fraction and trabecular thickness in the sham distal femoral metaphysis. Stroke differentially decreased cortical area in the affected limb relative to the unaffected limb of the distal femoral metaphysis, and endosteal bone formation rate in the affected tibial diaphysis. Although exercise failed to improve bone microstructure following stroke, exercise increased mineral-to-matrix content in stroke but not sham. Together, these results show that stroke inhibits exercise-induced changes to femoral microstructure but not tibial composition, even without changes to gait. Similarly, affected-unaffected limb differences in cortical bone structure and bone formation rate in ambulatory mice show that stroke affects bone health even without bedrest.

Reduced bone formation markers, and altered trabecular and cortical bone mineral densities of non-paretic femurs observed in rats with ischemic stroke: A randomized controlled pilot study

Background

Immobility and neural damage likely contribute to accelerated bone loss after stroke, and subsequent heightened fracture risk in humans.

Objective

To investigate the skeletal effect of middle cerebral artery occlusion (MCAo) stroke in rats and examine its utility as a model of human post-stroke bone loss.

Methods

Twenty 15-week old spontaneously hypertensive male rats were randomized to MCAo or sham surgery controls. Primary outcome: group differences in trabecular bone volume fraction (BV/TV) measured by Micro-CT (10.5 micron istropic voxel size) at the ultra-distal femur of stroke affected left legs at day 28. Neurological impairments (stroke behavior and foot-faults) and physical activity (cage monitoring) were assessed at baseline, and days 1 and 27. Serum bone turnover markers (formation: N-terminal propeptide of type 1 procollagen, PINP; resorption: C-terminal telopeptide of type 1 collagen, CTX) were assessed at baseline, and days 7 and 27.

Results

No effect of stroke was observed on BV/TV or physical activity, but PINP decreased by -24.5% (IQR -34.1, -10.5, p = 0.046) at day 27. In controls, cortical bone volume (5.2%, IQR 3.2, 6.9) and total volume (6.4%, IQR 1.2, 7.6) were higher in right legs compared to left legs, but these side-to-side differences were not evident in stroke animals.

Conclusion

MCAo may negatively affect bone formation. Further investigation of limb use and physical activity patterns after MCAo is required to determine the utility of this current model as a representation of human post-stroke bone loss.

The y6 receptor suppresses bone resorption and stimulates bone formation in mice via a suprachiasmatic nucleus relay

The neuropeptide Y system is known to play an important role in the regulation of bone homeostasis and while the functions of its major receptors, Y1R and Y2R, in this process have become clearer, the contributions of other Y-receptors, like the y6 receptor (y6R), are unknown. Y6R expression is restricted to the suprachiasmatic nucleus (SCN) of the hypothalamus, an area known to regulate circadian rhythms, and the testis. Here we show that lack of y6R signalling, results in significant reduction in bone mass, but no changes in bone length. Male and female y6R knockout (KO) mice display reduced cortical and cancellous bone volume in axial and appendicular bones. Mechanistically, the reduction in cancellous bone is the result of an uncoupling of bone remodelling, leading to an increase in osteoclast surface and number, and a reduction in osteoblast number, osteoid surface, mineralizing surface and bone formation rate. y6R KO mice displayed increased numbers of osteoclast precursors and produced greater numbers of osteoclasts in RANKL-treated cultures. They also produced fewer CFU-ALP osteoblast precursors in the marrow and showed reduced mineralization in primary osteoblastic cultures, as well as reduced expression for the osteoblast lineage marker, alkaline phosphatase, in bone isolates. The almost exclusive location of y6Rs in the hypothalamus suggests a critical role of central neuronal pathways controlling this uncoupling of bone remodelling which is in line with known actions or other Y-receptors in the brain. In conclusion, y6R signalling is required for maintenance of bone mass, with loss of y6R uncoupling bone remodelling and resulting in a negative bone balance. This study expands the scope of hypothalamic regulation of bone, highlighting the importance for neural/endocrine coordination and their marked effect upon skeletal homeostasis.

Forum on bone and skeletal muscle interactions: summary of the proceedings of an ASBMR workshop

Annual costs are enormous for musculoskeletal diseases such as osteoporosis and sarcopenia and for bone and muscle injuries, costing billions annually in health care. Although it is clear that muscle and bone development, growth, and function are connected, and that muscle loads bone, little is known regarding cellular and molecular interactions between these two tissues. A conference supported by the National Institutes of Health (NIH) and the American Society for Bone and Mineral Research (ASBMR) was held in July 2012 to address the enormous burden of musculoskeletal disease. National and international experts in either bone or muscle presented their findings and their novel hypotheses regarding muscle-bone interactions to stimulate the exchange of ideas between these two fields. The immediate goal of the conference was to identify critical research themes that would lead to collaborative research interactions and grant applications focusing on interactions between muscle and bone. The ultimate goal of the meeting was to generate a better understanding of how these two tissues integrate and crosstalk in both health and disease to stimulate new therapeutic strategies to enhance and maintain musculoskeletal health.

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.

Identification of gene pathways altered by deletion of the androgen receptor specifically in mineralizing osteoblasts and osteocytes in mice

Androgens play a key role in skeletal growth and maintenance in males and can mediate their actions, at least in part, via the androgen receptor (AR) in osteoblasts. To investigate the mechanisms by which androgens exert their effects via the AR in mineralizing osteoblasts and osteocytes, we identified gene targets/pathways regulated by the AR using targeted gene expression and microarray approaches on bone isolated from mice in which the AR is specifically deleted in mineralizing osteoblasts and osteocytes (mOBL-ARKOs). Gene ontology mining indicated a number of biological processes to be affected in the bones of mOBL-ARKOs including skeletal and muscular system development and carbohydrate metabolism. All genes identified to have altered expression in the bones of mOBL-ARKOs were confirmed by Q-PCR for their androgen responsiveness in an androgen deprivation and replacement mouse model. The osteoblast genes Col1a1 and Bglap and the osteoclast genes Ctsk and RANKL (Tnfs11) were upregulated in the bones of mOBL-ARKOs, consistent with the increased matrix synthesis, mineralization, and bone resorption observed previously in these mice. Of significant interest, we identified genes involved in carbohydrate metabolism (adiponectin and Dpp4) and in growth and development (GH, Tgfb (Tgfb2), Wnt4) as potential targets of androgen action via the AR in mineralizing osteoblasts.

The neuropeptide VIP regulates the expression of osteoclastogenic factors in osteoblasts

Osteoclast formation is controlled by stromal cells/osteoblasts expressing macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL), crucial for osteoclast progenitor cell proliferation, survival and differentiation, and osteoprotegerin (OPG) that inhibits the interaction between RANKL and its receptor RANK. Recent data have strongly indicated that the nervous system plays an important role in bone biology. In the present study, the effects of the neuropeptide vasoactive intestinal peptide (VIP), present in peptidergic skeletal nerve fibers, on the expression of RANKL, OPG, and M-CSF in osteoblasts and stromal cells have been investigated. VIP and pituitary adenylate cyclase-activating polypeptide 38 (PACAP-38), but not secretin, stimulated rankl mRNA expression in mouse calvarial osteoblasts. In contrast, VIP inhibited the mRNA expressions of opg and m-csf, effects shared by PACAP-38, but not by secretin. VIP did not affect rankl, opg, or m-csf mRNA expression in mouse bone marrow stromal cells (BMSCs). The effects by VIP on the mRNA expression of rankl, opg, and m-csf were all potentiated by the cyclic AMP phosphodiesterase inhibitor rolipram. In addition, VIP robustly enhanced the phosphorylation of ERK and the stimulatory effect by VIP on rankl mRNA was inhibited by the MEK1/2 inhibitor PD98059. These observations demonstrate that activation of VPAC(2) receptors in osteoblasts enhances the RANKL/OPG ratio by mechanisms mediated by cyclic AMP and ERK pathways suggesting an important role for VIP in bone remodeling.

Hypothalamic leptin gene therapy prevents weight gain without long-term detrimental effects on bone in growing and skeletally mature female rats

Hypothalamic leptin gene therapy normalizes the mosaic skeletal phenotype of leptin-deficient ob/ob mice. However, it is not clear whether increased hypothalamic leptin alters bone metabolism in animals already producing the hormone. The objective of this study was to evaluate the long duration effects of recombinant adeno-associated virus-rat leptin (rAAV-Lep) hypothalamic gene therapy on weight gain and bone metabolism in growing and skeletally mature leptin-replete female Sprague-Dawley rats. Rats were either unoperated or implanted with cannulas in the third ventricle of the hypothalamus and injected with either rAAV-Lep or rAAV-GFP (control vector encoding green fluorescent protein) and maintained on standard rat chow fed ad libitum for either 5 or 10 weeks (starting at 3 months of age) or 18 weeks (starting at 9 months of age). Tibias, femurs, or lumbar vertebrae were analyzed by micro-computed tomography and/or histomorphometry. In comparison with age-matched rAAV-GFP rats, rAAV-Lep rats maintained a lower body weight for the duration of studies. At 5 weeks after vector administration, rAAV-Lep rats had lower cancellous bone volume and bone marrow adiposity but higher osteoblast perimeter compared with nonoperated controls. However, these values did not differ between the two groups at 10 weeks after vector administration. Differences in cancellous bone volume and architecture were not detected between the rAAV-Lep and rAAV-GFP groups at either time point. Also, rAAV-Lep had no negative effects on bone in the 9-month-old skeletally mature rats at 18 weeks after vector administration. We hypothesize that the transient reductions in bone mass and bone marrow adiposity at 5 weeks after vector administration were due to hypothalamic surgery. We conclude that increased hypothalamic leptin, sufficient to prevent weight gain, has minimal specific effects (rAAV-Lep versus rAAV-GFP) on bone metabolism in normal female rats.

Elevated Fra-1 expression causes severe lipodystrophy

A shift from osteoblastogenesis to adipogenesis is one of the underlying mechanisms of decreased bone mass and increased fat during aging. We now uncover a new role for the transcription factor Fra-1 in suppressing adipogenesis. Indeed, Fra1 (Fosl1) transgenic (Fra1tg) mice, which developed progressive osteosclerosis as a result of accelerated osteoblast differentiation, also developed a severe general lipodystrophy. The residual fat of these mice appeared immature and expressed lower levels of adipogenic markers, including the fatty acid transporter Cd36 and the CCAAT/enhancer binding protein Cebpa. Consequently accumulation of triglycerides and free fatty acids were detected in the serum of fasting Fra1tg mice. Fra-1 acts cell autonomously because the adipogenic differentiation of Fra1 transgenic primary osteoblasts was drastically reduced, and overexpression of Fra-1 in an adipogenic cell line blocked their differentiation into adipocytes. Strikingly, Cebpa was downregulated in the Fra-1-overexpressing cells and Fra-1 could bind to the Cebpa promoter and directly suppress its activity. Thus, our data add to the known common systemic control of fat and bone mass, a new cell-autonomous level of control of cell fate decision by which the osteogenic transcription factor Fra-1 opposes adipocyte differentiation by inhibiting C/EBPα.

Increased PTHrP and decreased estrogens alter bone turnover but do not reproduce the full effects of lactation on the skeleton

During lactation, calcium is mobilized from the maternal skeleton to supply the breast for milk production. This results in rapid but fully reversible bone loss. Prior studies have suggested that PTHrP, secreted from the breast, and estrogen deficiency, due to suckling-induced central hypogonadism, combine to trigger bone resorption. To determine whether this combination was sufficient to explain bone loss during lactation, we raised PTHrP levels and decreased levels of estrogens in nulliparous mice. PTHrP was infused via osmotic minipumps and estrogens were decreased either by using leuprolide, a long-acting GnRH agonist, or by surgical ovariectomy (OVX). Bone mineral density declined by 23.2 ± 1.3% in the spine and 16.8 ± 1.9% in the femur over 10 d of lactation. This was accompanied by changes in trabecular architecture and an increase in both osteoblast and osteoclast numbers. OVX and PTHrP infusion both induced a modest decline in bone mineral density over 10 d, but leuprolide treatment did not. The combination of OVX and PTHrP was more effective than either treatment alone, but there was no interaction between PTHrP and leuprolide. None of the treatments reproduced the same degree of bone loss caused by lactation. However, both forms of estrogen deficiency led to an increase in osteoclasts, whereas infusion of PTHrP increased both osteoblasts and osteoclasts. Therefore, although the combination of PTHrP and estrogen deficiency contributes to bone loss, it is insufficient to reproduce the full response of the skeleton to lactation, suggesting that other factors also regulate bone metabolism during this period.

Neuropeptide Y and sex hormone interactions in humoral and neuronal regulation of bone and fat

The hypothalamus regulates the skeleton and adipose tissue via endocrine mechanisms. Changes in sex steroid levels in menopause and aging are central to the associated changes in bone mass and adiposity. Whereas many of these effects occur via direct actions on osteoblasts or adipocytes, sex hormones can also mediate effects on bone and adipose tissue via interaction with neuronal pathways. A key hypothalamic regulator of bone and adipose tissue is neuropeptide Y (NPY), which coordinately influences these tissues via effects on neuroendocrine and sympathetic nervous output. Better understanding of the interaction between NPY and sex steroids in regulating skeletal and energy homeostasis could lead to more effective treatments for osteoporosis and obesity.

Interactions between breast, bone, and brain regulate mineral and skeletal metabolism during lactation

Mammalian reproduction requires that nursing mothers transfer large amounts of calcium to their offspring through milk. As a result, lactation is associated with dramatic alterations in bone and mineral metabolism, including reversible bone loss. One theme that has emerged from recent studies examining these adaptations is that the lactating breast actively participates in regulating bone and mineral metabolism. This review will detail our current knowledge of interactions between the breast, skeleton, and hypothalamus during lactation and will consider implications that this reproductive physiology has for the pathophysiology of osteoporosis and breast cancer.

Cross-talk among bone cells

PURPOSE OF REVIEW

Bone cells such as osteoclasts, osteoblasts, and osteocytes 'talk' to each other throughout adult life to maintain bone integrity. This review highlights frontier areas of research on intercellular communication among bone cells.

RECENT FINDINGS

Bone cells communicate to regulate the balance between bone resorption and bone formation. Recent findings have suggested that bone remodeling compartment is critical for osteoclast-osteoblast communication during bone remodeling. New molecules and mechanisms for bone cell communication, including 'coupling' of bone formation to resorption, have been revealed. Osteoclastic regulation of pH within the bone remodeling compartment is a mechanism that has been posited to rapidly activate osteoblastic bone formation, whereas osteocytes in the bone matrix regulate osteoclasts and osteoblasts on the bone surface through the lacuno-canaliculi network.

SUMMARY

Differentiation, activation, and apoptosis of bone cells are often dependent on the status of other types of bone cells. Bone cells in different lineages achieve intercellular communication not only by ligand-receptor interactions but also by molecules and ions traveling in the extracellular space or across gap junctions, processes that depend profoundly on the four-dimensional (space and time) architecture of bone tissue.

Frequency of osteoporosis in 46 men with methamphetamine abuse hospitalized in a National Hospital

BACKGROUND

Methamphetamine, a derivative of amphetamine, has been well known to cause mental problems in humans; however, its physical effects are little known. Despite relevant information on the effect of methamphetamine abuse on bone quality being available, data regarding the frequency of osteoporosis in methamphetamine abusers are limited.

METHODS

We selected 46 hospitalized male methamphetamine abusers and 188 reference male controls in whom any conditions affecting bone metabolism were ruled out. Bone mineral density (BMD) in the lumbar spine was measured by dual energy X-ray absorptiometry (DXA). We compared the BMD between methamphetamine abusers and controls and evaluated the frequency of osteoporosis in both groups.

RESULTS

The mean BMD value was lower in methamphetamine abusers (mean+/-SD, 0.71+/-0.07 g/cm(2)) than in the controls (mean+/-SD, 0.98+/-0.14 g/cm(2)). The frequency of osteoporosis was 22% according to WHO diagnostic guidelines, and osteopenia at the lumbar spine was 76%. The correlation between the extent of methamphetamine abuse and BMD was very clear.

CONCLUSION

There was considerable loss of bone mineral in a high percentage of methamphetamine abusers. Our study is the first clinical study to determine the frequency of osteoporosis in male methamphetamine abusers.