Red and White Blood Cell Counts Are Associated With Bone Marrow Adipose Tissue, Bone Mineral Density, and Bone Microarchitecture in Premenopausal Women

Effects of bone metabolism on hematopoiesis: A Mendelian randomization study

Objectives

Osteoblast is known to regulate hematopoiesis according to preclinical studies but the causal relationship in human remains uncertain. We aimed to evaluate causal relationships of bone mineral density (BMD) with blood cell traits using genetic data.

Methods

Summary statistics from the largest available genome-wide association study were retrieved for total body BMD (TBBMD), lumbar spine BMD (LSBMD), femoral neck BMD (FNBMD) and 29 blood cell traits including red blood cell, white blood cell and platelet-related traits. Using two-sample Mendelian randomization (MR) approach, inverse-variance weighted method was adopted as main univariable MR analysis. Multivariable MR (MVMR) analysis was conducted to evaluate whether the casual effect is independent of confounders.

Results

BMD was positively associated with reticulocyte-related traits, including high light scatter reticulocyte count and percentage, immature reticulocyte fraction, reticulocyte count and percentage, with causal effect estimate (beta) ranging from 0.023 to 0.064. Conversely, inverse association of BMD with hematocrit, hemoglobin, and red blood cell count was observed, with beta ranging from -0.038 to -0.019. The association remained significant in MVMR analysis after adjustment for confounders. For white blood cells, BMD was inversely associated with neutrophil count (beta: 0.029 to -0.019) and white blood cell count (beta: 0.024 to -0.02). Results across TBBMD, LSBMD, and FNBMD were consistent.

Conclusions

This study suggested bone metabolism had a causal effect on hematopoietic system in humans. Its causal effect on red blood cell traits was independent of confounders. Further studies on how improving bone health can reduce risk of hematological disorders are warranted.

Fatty infiltration in the musculoskeletal system: pathological mechanisms and clinical implications

Fatty infiltration denotes the anomalous accrual of adipocytes in non-adipose tissue, thereby generating toxic substances with the capacity to impede the ordinary physiological functions of various organs. With aging, the musculoskeletal system undergoes pronounced degenerative alterations, prompting heightened scrutiny regarding the contributory role of fatty infiltration in its pathophysiology. Several studies have demonstrated that fatty infiltration affects the normal metabolism of the musculoskeletal system, leading to substantial tissue damage. Nevertheless, a definitive and universally accepted generalization concerning the comprehensive effects of fatty infiltration on the musculoskeletal system remains elusive. As a result, this review summarizes the characteristics of different types of adipose tissue, the pathological mechanisms associated with fatty infiltration in bone, muscle, and the entirety of the musculoskeletal system, examines relevant clinical diseases, and explores potential therapeutic modalities. This review is intended to give researchers a better understanding of fatty infiltration and to contribute new ideas to the prevention and treatment of clinical musculoskeletal diseases.

Spatiotemporal expression and control of haemoglobin in space

It is now widely recognised that the environment in space activates a diverse set of genes involved in regulating fundamental cellular pathways. This includes the activation of genes associated with blood homoeostasis and erythropoiesis, with a particular emphasis on those involved in globin chain production. Haemoglobin biology provides an intriguing model for studying space omics, as it has been extensively explored at multiple -omic levels, spanning DNA, RNA, and protein analyses, in both experimental and clinical contexts. In this study, we examined the developmental expression of haemoglobin over time and space using a unique suite of multi-omic datasets available on NASA GeneLab, from the NASA Twins Study, the JAXA CFE study, and the Inspiration4 mission. Our findings reveal significant variations in globin gene expression corresponding to the distinct spatiotemporal characteristics of the collected samples. This study sheds light on the dynamic nature of globin gene regulation in response to the space environment and provides valuable insights into the broader implications of space omics research.

Deficiency of glucocorticoid receptor in bone marrow adipocytes has mild effects on bone and hematopoiesis but does not influence expansion of marrow adiposity with caloric restriction

Introduction

Unlike white adipose tissue depots, bone marrow adipose tissue (BMAT) expands during caloric restriction (CR). Although mechanisms for BMAT expansion remain unclear, prior research suggested an intermediary role for increased circulating glucocorticoids.

Methods

In this study, we utilized a recently described mouse model (BMAd-Cre) to exclusively target bone marrow adipocytes (BMAds) for elimination of the glucocorticoid receptor (GR) (i.e. Nr3c1) whilst maintaining GR expression in other adipose depots.

Results

Mice lacking GR in BMAds (BMAd-Nr3c1 -/-) and control mice (BMAd-Nr3c1 +/+) were fed ad libitum or placed on a 30% CR diet for six weeks. On a normal chow diet, tibiae of female BMAd-Nr3c1-/- mice had slightly elevated proximal trabecular metaphyseal bone volume fraction and thickness. Both control and BMAd-Nr3c1-/- mice had increased circulating glucocorticoids and elevated numbers of BMAds in the proximal tibia following CR. However, no significant differences in trabecular and cortical bone were observed, and quantification with osmium tetroxide and μCT revealed no difference in BMAT accumulation between control or BMAd-Nr3c1 -/- mice. Differences in BMAd size were not observed between BMAd-Nr3c1-/- and control mice. Interestingly, BMAd-Nr3c1-/- mice had decreased circulating white blood cell counts 4 h into the light cycle.

Discussion

In conclusion, our data suggest that eliminating GR from BMAd has minor effects on bone and hematopoiesis, and does not impair BMAT accumulation during CR.

Dietary restriction plus exercise change gene expression of Cxcl12 abundant reticular cells in female mice

INTRODUCTION

Low energy availability due to excessive exercise lowers bone mass and impairs various physiological functions, including immunity and hematopoiesis. We focused on Cxcl12 abundant reticular (CAR) cells, which are bone marrow mesenchymal stem cells and are essential for the maintenance of hematopoietic and immune cells in bone marrow. We examine the functional changes in CAR cells resulting from dietary restriction combined with exercise.

MATERIALS AND METHODS

Five-week-old wild-type female mice were divided into an ad libitum group (CON), a 60% dietary restriction group (DR), an ad libitum with exercise group (CON + ex), and a 60% dietary restriction with exercise group (DR + ex). Blood parameters, bone structure parameters, and bone marrow fat volume were evaluated after 5 weeks. In addition, bone marrow CAR cells were isolated by cell sorting and analyzed for gene expression by RT-qPCR.

RESULTS

Bone mineral density (BMD) was significantly decreased in DR and DR + ex compared to CON and CON + ex. Especially, cortical bone mass and thickness were significantly decreased in DR and DR + ex groups, whereas trabecular bone mass was significantly increased. Bone marrow fat volume was significantly increased in DR and DR + ex groups compared to CON and CON + ex. The number of leukocytes in the blood was significantly decreased in the DR + ex group compared to the other three groups. RT-qPCR showed a significant decrease in gene expression of both Foxc1 and Runx2 in CAR cells of the DR + ex group compared to CON.

CONCLUSION

Dietary restriction combined with exercise promotes CAR cell differentiation into bone marrow adipocyte and suppresses osteoblast differentiation.

Deficits in volumetric bone mineral density, bone microarchitecture, and estimated bone strength in women with atypical anorexia nervosa compared to healthy controls

OBJECTIVE

Anorexia nervosa is associated with low bone mineral density (BMD) and deficits in bone microarchitecture and strength. Low BMD is common in atypical anorexia nervosa, in which criteria for anorexia nervosa are met except for low weight. We investigated whether women with atypical anorexia nervosa have deficits in bone microarchitecture and estimated strength at the peripheral skeleton.

METHOD

Measures of BMD and microarchitecture were obtained in 28 women with atypical anorexia nervosa and 27 controls, aged 21-46 years.

RESULTS

Mean tibial volumetric BMD, cortical thickness, and failure load were lower, and radial trabecular number and separation impaired, in atypical anorexia nervosa versus controls (p < .05). Adjusting for weight, deficits in tibial cortical bone variables persisted (p < .05). Women with atypical anorexia nervosa and amenorrhea had lower volumetric BMD and deficits in microarchitecture and failure load versus those with eumenorrhea and controls. Those with a history of overweight/obesity or fracture had deficits in bone microarchitecture versus controls. Tibial deficits were particularly marked. Less lean mass and longer disease duration were associated with deficits in high-resolution peripheral quantitative computed tomography (HR-pQCT) variables in atypical anorexia nervosa.

DISCUSSION

Women with atypical anorexia nervosa have lower volumetric BMD and deficits in bone microarchitecture and strength at the peripheral skeleton versus controls, independent of weight, and particularly at the tibia. Women with atypical anorexia nervosa and amenorrhea, less lean mass, longer disease duration, history of overweight/obesity, or fracture history may be at higher risk. This is salient as deficits in HR-pQCT variables are associated with increased fracture risk.

PUBLIC SIGNIFICANCE

Atypical anorexia nervosa is a psychiatric disorder in which psychological criteria for anorexia nervosa are met despite weight being in the normal range. We demonstrate that despite weight in the normal range, women with atypical anorexia nervosa have impaired bone density, structure, and strength compared to healthy controls. Whether this translates to an increased risk of incident fracture in this population requires further investigation.

Bone marrow adipocyte: Origin, biology and relationship with hematological malignancy

Bone marrow adipose tissue (BMAT) has been histologically recognized for decades. In this study, we performed a bibliometric analysis to quantitatively analyze the clusters of keywords of BMAT and hematopoiesis to better understand BMAT and hematopoiesis. Starting with conclusive keywords, our results demonstrated that BMAds is distinct from extramedullary adipose tissues and maintains a routine but dynamic accumulation throughout an individual's life. Various pathophysiological factors take part in dysregulation of the adipose-osteogenic balance throughout life. Bone marrow adipocytes (BMAds) are also contradictorily involved in normal hematopoiesis, and positively participate in the occurrence and progression of hematologic malignancies, exerting a chemoprotective role in tumor treatment. Mechanically, metabolic reprogramming and abnormal secretory profile of BMAds and tumor cells play a critical role in the chemotherapy resistance. Overall, we hope that this work will provide new ideas for relevant future research on BMAds.

Bone marrow adiposity modulation after long duration spaceflight in astronauts

Space travel requires metabolic adaptations from multiple systems. While vital to bone and blood production, human bone marrow adipose (BMA) tissue modulation in space is unknown. Here we show significant downregulation of the lumbar vertebrae BMA in 14 astronauts, 41 days after landing from six months' missions on the International Space Station. Spectral analyses indicated depletion of marrow adipose reserves. We then demonstrate enhanced erythropoiesis temporally related to low BMA. Next, we demonstrated systemic and then, local lumbar vertebrae bone anabolism temporally related to low BMA. These support the hypothesis that BMA is a preferential local energy source supplying the hypermetabolic bone marrow postflight, leading to its downregulation. A late postflight upregulation abolished the lower BMA of female astronauts and BMA modulation amplitude was higher in younger astronauts. The study design in the extreme environment of space can limit these conclusions. BMA modulation in astronauts can help explain observations on Earth.

Rehmannia Glutinosa Polysaccharide Regulates Bone Marrow Microenvironment via HIF-1α/NF-κB Signaling Pathway in Aplastic Anemia Mice

Aplastic anemia (AA), a rare disorder, is associated with bone marrow microenvironment (BMM). Presently, AA treatment is of great difficulty. This study aimed to explore the mechanism of action of Rehmannia glutinosa polysaccharide (RGP) in AA. Busulfan was used to induce AA in BALB/c mice; blood cell count and Ray's Giemsa staining were used to assess the severity of hematopoietic failure; HE was performed to assess the pathological state of the marrow cavity; ELISA was performed to assess IL-4, IL-10, IL-6, IL-12, IL-1β, TNF-α, MCP-1, VEGF, and EPO; and WB was performed to evaluate the effects of RGP on the HIF-1α/NF-κB signaling. Significant downregulation of hemocyte levels in the blood and nucleated cells in the bone marrow was reversed by RGP and Cyclosporine A (CA). Compared with the AA group, dilating blood sinusoids, inflammation, hematopoiesis, decreased bone marrow cells and megakaryocytes were alleviated by RGP and CA, and the HIF-1α/NF-κB signaling was inhibited too. Notably, RGP was more effective when used in combination with CA. In this study, we established a relationship between BMM and the HIF-1α/NF-κB signaling pathway and found that RGP regulates BMM by suppressing the activation of the HIF-1α/NF-κB signaling. Thus, RGP exerts a pharmacological effect on AA.

Bone marrow adipose tissue is associated with fracture history in anorexia nervosa

Although bone mineral density (BMD) is decreased and fracture risk increased in anorexia nervosa, BMD does not predict fracture history in this disorder. We assessed BMD, bone microarchitecture, and bone marrow adipose tissue (BMAT) in women with anorexia nervosa and found that only BMAT was associated with fracture history.

INTRODUCTION

Anorexia nervosa (AN) is a psychiatric disorder characterized by low body weight, low BMD, and increased risk of fracture. Although BMD is reduced and fracture risk elevated, BMD as assessed by DXA does not distinguish between individuals with versus those without prior history of fracture in AN. Despite having decreased peripheral adipose tissue stores, individuals with AN have enhanced bone marrow adipose tissue (BMAT), which is inversely associated with BMD. Whether increased BMAT is associated with fracture in AN is not known.

METHODS

We conducted a cross-sectional study in 62 premenopausal women, including 34 with AN and 28 normal-weight women of similar age. Fracture history was collected during patient interviews and BMD measured by DXA, BMAT by 1H-MRS, and parameters of bone microarchitecture by HR-pQCT.

RESULTS

Sixteen women (47.1%) with AN reported prior history of fracture compared to 11 normal-weight women (39.3%, p = 0.54). In the entire group and also the subset of women with AN, there were no significant differences in BMD or parameters of bone microarchitecture in women with prior fracture versus those without. In contrast, women with AN with prior fracture had greater BMAT at the spine and femur compared to those without (p = 0.01 for both).

CONCLUSION

In contrast to BMD and parameters of bone microarchitecture, BMAT is able to distinguish between women with AN with prior fracture compared to those without. Prospective studies will be necessary to understand BMAT's potential pathophysiologic role in the increased fracture risk in AN.

Unravelling genetic causality of haematopoiesis on bone metabolism in human

OBJECTIVE

Haematopoiesis was shown to regulate bone metabolism in in vivo studies. However, whether haematopoiesis has causal effects on bone health has never been investigated in humans. We aimed to evaluate the causal relationships of blood traits with bone mineral density (BMD) and fracture.

DESIGN AND METHODS

Using two-sample Mendelian randomization, causal relationship of 29 blood traits with estimated BMD (eBMD), total body BMD (TBBMD), lumbar spine BMD (LSBMD), femoral neck BMD (FNBMD) and fracture were evaluated by inverse-variance weighted (IVW) method and multiple sensitivity analyses. Relevant genetic data were obtained from the largest possible publicly available genome-wide association studies.

RESULTS

Eight genetically determined red blood cell traits showed positive causal effects on eBMD, with beta estimates ranging from 0.009 (mean corpuscular haemoglobin) to 0.057 (haemoglobin concentration), while three white blood cell traits, including lymphocyte count (beta: -0.020; 95% CI: -0.033 to -0.007), neutrophil count (beta: -0.020; 95% CI: -0.035 to -0.006) and white blood cell count (beta: -0.027; 95% CI: -0.039 to -0.014), were inversely associated with eBMD. Causal effects for six of these blood traits were validated on TBBMD, LSBMD, FNBMD and/or fracture. The association of reticulocyte count (beta: 0.040; 95% CI: 0.016 to 0.063), haemoglobin (beta: 0.058; 95% CI: 0.021 to 0.094) and mean corpuscular haemoglobin concentration (beta: 0.030; 95% CI: 0.007 to 0.054) with eBMD remained significant in multivariable IVW analyses adjusted for other blood traits.

CONCLUSION

This study provided evidence that haematopoietic system might regulate the skeletal system in humans and suggested the possible pathophysiology of bone diseases among people with haematological diseases.

SIGNIFICANCE STATEMENT

We conducted a novel Mendelian randomization study investigating the causal relationship of blood cells with bone mineral density. Red and white blood cell traits have positive and inverse causal relationship with bone mineral density, respectively, suggesting a potential link of haematopoietic system with the skeletal system in humans. Current findings suggest individuals with related haematological diseases, such as anaemia and leukocytosis, may have a lifelong increased risk of osteoporosis and/or fracture. Given that complete blood count is commonly performed in clinical setting, whether complete blood count can be used to predict fracture risk warrants further investigation.

Osteoblast Lineage Support of Hematopoiesis in Health and Disease

In mammals, hematopoiesis migrates to the bone marrow during embryogenesis coincident with the appearance of mineralized bone, where hematopoietic stem cells (HSCs) and their progeny are maintained by the surrounding microenvironment or niche, and sustain the entirety of the hematopoietic system. Genetic manipulation of niche factors and advances in cell lineage tracing techniques have implicated cells of both hematopoietic and nonhematopoietic origin as important regulators of hematopoiesis in health and disease. Among them, cells of the osteoblast lineage, from stromal skeletal stem cells to matrix-embedded osteocytes, are vital niche residents with varying capacities for hematopoietic support depending on stage of differentiation. Here, we review populations of osteoblasts at differing stages of differentiation and summarize the current understanding of the role of the osteoblast lineage in supporting hematopoiesis. © 2022 American Society for Bone and Mineral Research (ASBMR).

Bone mass in Saudi women aged 20-40 years: the association with obesity and vitamin D deficiency

This study describes that low bone density is prevalent in premenopausal Saudi women, especially women of normal weight and vitamin D deficiency. Although BMD is higher in obese young women, this may not be beneficial later in life in conjunction with persistent vitamin D deficiency.

INTRODUCTION

Not attaining peak bone mass is one crucial factor contributing to the risk of developing osteoporosis and suffering fractures in later life. The objectives of this study were to describe the normal range of bone mineral density (BMD) and bone mineral content (BMC) in premenopausal Saudi women in relation to obesity and vitamin D insufficiency.

METHODS

A cross-sectional study involving 312 healthy Saudi women aged 20-40. All women were clinically examined. BMD (g/cm2) and BMC (g) assessed at total body (TB), femoral neck (FN) and lumbar spine (LS) were performed using dual-energy X-ray absorptiometry (DXA). Obesity was defined as BMI ≥ 30 kg/m2 and vitamin D deficiency defined as 25(OH)D < 50 nmol/L.

RESULTS

Almost half of the studied women were obese, and the majority (86.2%) were deficient in vitamin D. Mean BMD in TB 1.060 ± 0.091, FN 0.918 ± 0.153 and LS 1.118 ± 0.123 g/cm2, while TB-BMC 2077 ± 272 g. When classified by BMI, the proportion with low bone density was 2-3 times higher among the normal weight compared to the obese women, p < 0.001. In the cohort overall, ~ 19% of these young premenopausal women had osteopenia or osteoporosis at the femoral neck, but 26% in normal weight, vitamin D deficient women.

CONCLUSION

This study shows low bone density in premenopausal Saudi women, particularly those with normal weight. While obesity appears to confer some protection against vitamin D deficiency at this age, this is assumed to change in later life.

Lipolysis of bone marrow adipocytes is required to fuel bone and the marrow niche during energy deficits

To investigate roles for bone marrow adipocyte (BMAd) lipolysis in bone homeostasis, we created a BMAd-specific Cre mouse model in which we knocked out adipose triglyceride lipase (ATGL, Pnpla2 gene). BMAd-Pnpla2-/- mice have impaired BMAd lipolysis, and increased size and number of BMAds at baseline. Although energy from BMAd lipid stores is largely dispensable when mice are fed ad libitum, BMAd lipolysis is necessary to maintain myelopoiesis and bone mass under caloric restriction. BMAd-specific Pnpla2 deficiency compounds the effects of caloric restriction on loss of trabecular bone in male mice, likely due to impaired osteoblast expression of collagen genes and reduced osteoid synthesis. RNA sequencing analysis of bone marrow adipose tissue reveals that caloric restriction induces dramatic elevations in extracellular matrix organization and skeletal development genes, and energy from BMAd is required for these adaptations. BMAd-derived energy supply is also required for bone regeneration upon injury, and maintenance of bone mass with cold exposure.

Bone marrow adipose tissue in metabolic health

Recent studies have highlighted the role of bone marrow adipose tissue (BMAT) as a regulator of skeletal homeostasis and energy metabolism. While long considered an inert filler, occupying empty spaces from bone loss and reduced hematopoiesis, BMAT is now considered a secretory and metabolic organ that responds to nutritional challenges and secretes cytokines, which indirectly impact energy and bone metabolism. The recent advances in our understanding of the function of BMAT have been enabled by novel noninvasive imaging techniques, which allow longitudinal assessment of BMAT in vivo following interventions. This review will focus on the latest advances in our understanding of BMAT and its role in metabolic health. Imaging techniques to quantify the content and composition of BMAT will be discussed.

Conditioned medium from human tonsil-derived mesenchymal stem cells inhibits glucocorticoid-induced adipocyte differentiation

Obesity, which has become a major global health problem, involves a constitutive increase in adipocyte differentiation signaling. Previous studies show that mesenchymal stem cells (MSCs) induce weight loss and glycemic control. However, the mechanisms by which MSCs regulate adipocyte differentiation are not yet known. In this study, we investigated the effects of conditioned medium obtained from human tonsil-derived MSCs (T-MSC CM) on adipocyte differentiation. We found that T-MSC CM attenuated adipocyte differentiation from early stages via inhibiting glucocorticoid signaling. T-MSC CM also increased the phosphorylation of p38 mitogen-activated protein kinase and glucocorticoid receptors and decreased the subsequent nucleus translocation of glucocorticoid receptors. Chronic treatment of mice with synthetic glucocorticoids induced visceral and bone marrow adipose tissue expansion, but these effects were not observed in mice injected with T-MSC CM. Furthermore, T-MSC CM injection protected against reductions in blood platelet counts induced by chronic glucocorticoid treatment, and enhanced megakaryocyte differentiation was also observed. Collectively, these results demonstrate that T-MSC CM exerts inhibitory effects on adipocyte differentiation by regulating glucocorticoid signal transduction. These findings suggest that the therapeutic application of T-MSC CM could reduce obesity by preventing adipose tissue expansion.

Report From the 6th International Meeting on Bone Marrow Adiposity (BMA2020)

The 6th International Meeting on Bone Marrow Adiposity (BMA) entitled "Marrow Adiposity: Bone, Aging, and Beyond" (BMA2020) was held virtually on September 9th and 10th, 2020. The mission of this meeting was to facilitate communication and collaboration among scientists from around the world who are interested in different aspects of bone marrow adiposity in health and disease. The BMA2020 meeting brought together 198 attendees from diverse research and clinical backgrounds spanning fields including bone biology, endocrinology, stem cell biology, metabolism, oncology, aging, and hematopoiesis. The congress featured an invited keynote address by Ormond MacDougald and ten invited speakers, in addition to 20 short talks, 35 posters, and several training and networking sessions. This report summarizes and highlights the scientific content of the meeting and the progress of the working groups of the BMA society (http://bma-society.org/).

Bone marrow adipose tissue: Role in bone remodeling and energy metabolism

Bone marrow adipose tissue (BMAT) has been considered for several decades as a silent bystander that fills empty space left in bone marrow following age-related decrease in hematopoiesis. However, recently new discoveries revealed BMAT as a secretory and metabolically active organ contributing to bone and whole-body energy metabolism. BMAT exhibits metabolic functions distinct from extramedullary adipose depots, relevant to its role in regulation of energy metabolism and its contribution to fracture risk observed in metabolic bone diseases. This review discusses novel insights of BMAT with particular emphasis on its contribution to the regulation of bone homeostasis. We also discuss the role of BMAT in regulation of fuel utilization and energy use that affect skeletal stem cell functions.

Preclinical models for investigating how bone marrow adipocytes influence bone and hematopoietic cellularity

Laboratory mice are a crucial preclinical model system for investigating bone marrow adipocyte (BMAd)-bone and BMAd-hematopoiesis interactions. In this review, we evaluate the suitability of mice to model common human diseases related to osteopenia or hematopoietic disorders, point out consistencies and discrepancies among different studies, and provide insights into model selection. Species, age, sex, skeletal site, and treatment protocol should all be considered when designing future studies.

Bone Marrow Adipocytes-Role in Physiology and Various Nutritional Conditions in Human and Animal Models

In recent years, adipose tissue has attracted a lot of attention. It is not only an energy reservoir but also plays important immune, paracrine and endocrine roles. BMAT (bone marrow adipose tissue) is a heterogeneous tissue, found mostly in the medullary canal of the long bones (tibia, femur and humerus), in the vertebrae and iliac crest. Adipogenesis in bone marrow cavities is a consequence of ageing or may accompany pathologies like diabetes mellitus type 1 (T1DM), T2DM, anorexia nervosa, oestrogen and growth hormone deficiencies or impaired haematopoiesis and osteoporosis. This paper focuses on studies concerning BMAT and its physiology in dietary interventions, like obesity in humans and high fat diet in rodent studies; and opposite: anorexia nervosa and calorie restriction in animal models.

Exercise Increases Bone in SEIPIN Deficient Lipodystrophy, Despite Low Marrow Adiposity

Exercise, typically beneficial for skeletal health, has not yet been studied in lipodystrophy, a condition characterized by paucity of white adipose tissue, with eventual diabetes, and steatosis. We applied a mouse model of global deficiency of Bscl2 (SEIPIN), required for lipid droplet formation. Male twelve-week-old B6 knockouts (KO) and wild type (WT) littermates were assigned six-weeks of voluntary, running exercise (E) versus non-exercise (N=5-8). KO weighed 14% less than WT (p=0.01) and exhibited an absence of epididymal adipose tissue; KO liver Plin1 via qPCR was 9-fold that of WT (p=0.04), consistent with steatosis. Bone marrow adipose tissue (BMAT), unlike white adipose, was measurable, although 40.5% lower in KO vs WT (p=0.0003) via 9.4T MRI/advanced image analysis. SEIPIN ablation's most notable effect marrow adiposity was in the proximal femoral diaphysis (-56% KO vs WT, p=0.005), with relative preservation in KO-distal-femur. Bone via μCT was preserved in SEIPIN KO, though some quality parameters were attenuated. Running distance, speed, and time were comparable in KO and WT. Exercise reduced weight (-24% WT-E vs WT p<0.001) but not in KO. Notably, exercise increased trabecular BV/TV in both (+31%, KO-E vs KO, p=0.004; +14%, WT-E vs WT, p=0.006). The presence and distribution of BMAT in SEIPIN KO, though lower than WT, is unexpected and points to a uniqueness of this depot. That trabecular bone increases were achievable in both KO and WT, despite a difference in BMAT quantity/distribution, points to potential metabolic flexibility during exercise-induced skeletal anabolism.

Obesity and Bone Health: A Complex Link

So far, the connections between obesity and skeleton have been extensively explored, but the results are inconsistent. Obesity is thought to affect bone health through a variety of mechanisms, including body weight, fat volume, bone formation/resorption, proinflammatory cytokines together with bone marrow microenvironment. In this review, we will mainly describe the effects of adipokines secreted by white adipose tissue on bone cells, as well as the interaction between brown adipose tissue, bone marrow adipose tissue, and bone metabolism. Meanwhile, this review also reviews the evidence for the effects of adipose tissue and its distribution on bone mass and bone-related diseases, along with the correlation between different populations with obesity and bone health. And we describe changes in bone metabolism in patients with anorexia nervosa or type 2 diabetes. In summary, all of these findings show that the response of skeleton to obesity is complex and depends on diversified factors, such as mechanical loading, obesity type, the location of adipose tissue, gender, age, bone sites, and secreted cytokines, and that these factors may exert a primary function in bone health.