Leptin functions peripherally to regulate differentiation of mesenchymal progenitor cells

Leptin impairs the therapeutic efficacy of adipose-derived mesenchymal stem cells by inducing apoptosis through NLRP3 inflammasomes activation

Mesenchymal stem cells (MSC) have been widely applied for regenerative medicine and the treatment of immune-disorders due to their multilineage differentiation and potent immunomodulatory properties. The therapeutic application of MSC post transplantation are influenced by various endogenous modulators. Leptin, a hormone primarily derived from adipose tissue, exerts a variety of physiological functions, in addition to the metabolic effects. In this study, we examined the effects of leptin on the viability of adipose-derived mesenchymal stem cells (ADSC) and its underlying molecular mechanisms with a particular focus on NLRP3 inflammasomes, which serve as signaling platform of the innate immune system. Leptin significantly decreased the viability of ADSC and induced apoptosis. Mechanistically, NLRP3 inflammasomes signaling critically contributes to leptin-induced apoptosis of ADSC by upregulating p53 and Puma. In addition, NLRP3 inflammasomes activation by leptin is mediated via ER stress induction and ROS accumulation. Finally, suppression of ADSC therapeutic efficacy by leptin and the critical role of NLRP3 inflammasomes in this phenomenon were confirmed in DSS-induced colitis model. Pre-conditioning with leptin before transplantation impaired the therapeutic efficacy and immunomodulatory function of ADSC, which were restored by treatment with a pharmacological inhibitor of NLRP3 inflammasomes. Taken together, the results suggest that leptin induces apoptotic cell death in ADSC and impairs the therapeutic effectiveness of ADSC by activating NLRP3 inflammasomes.

LepR-Expressing Cells in Bone and Periodontium

OBJECTIVE

LepR-expressing cells (LepR+ cells), a critical subpopulation of mesenchymal stem cells, have gained increasing attention in the last decade. LepR+ cells have been found to play a crucial role in maintaining bone and periodontal homeostasis. This review summarizes current research advances focusing on the role of LepR+ cells and their underlying regulatory molecular mechanisms in bones and periodontium, aiming to provide a better understanding of the therapeutic potential of this cell lineage.

METHODS

A literature review was conducted based on publications in PubMed over the past 20 years, summarizing the research progress on LepR+ cells in bone and periodontal tissues.

RESULTS

Current evidence revealed that LepR+ cells possess the ability of self-renewal and multilineage differentiation and are essential for bone turnover and periodontal tissue remodeling. In addition, LepR+ cells participate in the processes of bone fracture healing and alveolar socket healing. Moreover, under pathological conditions such as osteoporosis, bone marrow fibrosis, and periodontitis, LepR+ cells exhibit enhanced adipogenic or fibrogenic differentiation abilities.

CONCLUSION

Therapeutic approaches targeting the cell fate of LepR+ cells hold the potential to provide novel insights into bone/periodontal repair and regeneration therapy.

Overexpression of the receptor for resolvin E1 (ERV1) prevents early alveolar bone loss in leptin receptor deficiency-induced diabetes

BACKGROUND

This study was designed to test the hypothesis that the leptin receptor (LepR) regulates changes in periodontal tissues and that the overexpression of the receptor for resolvin E1 (ERV1) prevents age- and diabetes-associated alveolar bone loss.

METHODS

LepR-deficient transgenic (TG) mice were cross-bred with those overexpressing ERV1 (TG) to generate double-TG mice. In total, 95 mice were divided into four experimental groups: wild type (WT), TG, LepR deficient (db/db), and double transgenic (db/db TG). The groups were followed from 4 weeks up to 16 weeks of age. The natural progression of periodontal disease without any additional method of periodontitis induction was assessed by macroscopic and histomorphometric analyses. Osteoclastic activity was measured by tartrate-resistant acid phosphatase (TRAP) staining.

RESULTS

At 4 weeks, ERV1 overexpression prevented weight gain. From Week 8 onward, there was a significant increase in the weight of db/db mice with or without ERV1 overexpression compared to the WT mice, accompanied by an increase in glucose levels. By 8 weeks of age, the percentage of bone loss in the LepR deficiency groups was significantly greater compared to WT mice. ERV1 overexpression in the db/db TG mice prevented early alveolar bone loss; however, it did not impact the development of diabetic bone loss in aging mice after the onset of weight gain and diabetes.

CONCLUSIONS

The findings suggest that the overexpression of ERV1 prevents LepR-associated alveolar bone loss during the early phases of periodontal disease by delaying weight gain, diabetes onset, and associated inflammation; however, LepR deficiency increases susceptibility to naturally occurring inflammatory alveolar bone loss as the animal ages, associated with excess weight gain, onset of diabetes, and excess inflammation.

Adiposity and Mineral Balance in Chronic Kidney Disease

PURPOSE OF REVIEW

Bone homeostasis is balanced between formation and resorption activities and remain in relative equilibrium. Under disease states this process is disrupted, favoring more resorption over formation, leading to significant bone loss and fracture incidence. This aspect is a hallmark for patients with chronic kidney disease mineral and bone disorder (CKD-MBD) affecting a significant portion of the population, both in the United States and worldwide. Further study into the underlying effects of the uremic microenvironment within bone during CKD-MBD are critical as fracture incidence in this patient population not only leads to increased morbidity, but also increased mortality. Lack of bone homeostasis also leads to mineral imbalance contributing to cardiovascular calcifications. One area understudied is the possible involvement of bone marrow adipose tissue (BMAT) during the progression of CKD-MBD.

RECENT FINDINGS

BMAT accumulation is found during aging and in several disease states, some of which overlap as CKD etiologies. Importantly, research has found presence of BMAT inversely correlates with bone density and volume. Understanding the underlying molecular mechanisms for BMAT formation and accumulation during CKD-MBD may offer a potential therapeutic avenue to improve bone homeostasis and ultimately mineral metabolism.

Emerging roles of nerve-bone axis in modulating skeletal system

Over the past decades, emerging evidence in the literature has demonstrated that the innervation of bone is a crucial modulator for skeletal physiology and pathophysiology. The nerve-bone axis sparked extensive preclinical and clinical investigations aimed at elucidating the contribution of nerve-bone crosstalks to skeleton metabolism, homeostasis, and injury repair through the perspective of skeletal neurobiology. To date, peripheral nerves have been widely reported to mediate bone growth and development and fracture healing via the secretion of neurotransmitters, neuropeptides, axon guidance factors, and neurotrophins. Relevant studies have further identified several critical neural pathways that stimulate profound alterations in bone cell biology, revealing a complex interplay between the skeleton and nerve systems. In addition, inspired by nerve-bone crosstalk, novel drug delivery systems and bioactive materials have been developed to emulate and facilitate the process of natural bone repair through neuromodulation, eventually boosting osteogenesis for ideal skeletal tissue regeneration. Overall, this work aims to review the novel research findings that contribute to deepening the current understanding of the nerve-bone axis, bringing forth some schemas that can be translated into the clinical scenario to highlight the critical roles of neuromodulation in the skeletal system.

Function and Regulation of Bone Marrow Adipose Tissue in Health and Disease: State of the Field and Clinical Considerations

Bone marrow adipose tissue (BMAT) is a metabolically and clinically relevant fat depot that exists within bone. Two subtypes of BMAT, regulated and constitutive, reside in hematopoietic-rich red marrow and fatty yellow marrow, respectively, and exhibit distinct characteristics compared to peripheral fat such as white and brown adipose tissues. Bone marrow adipocytes (BMAds) are evolutionally preserved in most vertebrates, start development after birth and expand throughout life, and originate from unique progenitor populations that control bone formation and hematopoiesis. Mature BMAds also interact closely with other cellular components of the bone marrow niche, serving as a nearby energy reservoir to support the skeletal system, a signaling hub that contributes to both local and systemic homeostasis, and a final fuel reserve for survival during starvation. Though BMAT and bone are often inversely correlated, more BMAT does not always mean less bone, and the prevention of BMAT expansion as a strategy to prevent bone loss remains questionable. BMAT adipogenesis and lipid metabolism are regulated by the nervous systems and a variety of circulating hormones. This contributes to the plasticity of BMAT, including BMAT expansion in common physiological or pathological conditions, and BMAT catabolism under certain extreme circumstances, which are often associated with malnutrition and/or systemic inflammation. Altogether, this article provides a comprehensive overview of the local and systemic functions of BMAT and discusses the regulation and plasticity of this unique adipose tissue depot in health and disease. © 2024 American Physiological Society. Compr Physiol 14:5521-5579, 2024.

From Marrow to Bone and Fat: Exploring the Multifaceted Roles of Leptin Receptor Positive Bone Marrow Mesenchymal Stromal Cells

The bone marrow (BM) stromal cell microenvironment contains non-hematopoietic stromal cells called mesenchymal stromal cells (MSCs). MSCs are plastic adherent, form CFU-Fs, and give rise to osteogenic, adipogenic, chondrogenic progenitors, and most importantly provide HSC niche factor chemokine C-X-C motif ligand 12 (CXCL12) and stem cell factor (SCF). Different authors have defined different markers for mouse MSC identification like PDGFR+Sca-1+ subsets, Nestin+, or LepR+ cells. Of these, the LepR+ cells are the major source of SCF and CXCL12 in the BM microenvironment and play a major role in HSC maintenance and hematopoiesis. LepR+ cells give rise to most of the bones and BM adipocytes, further regulating the microenvironment. In adult BM, LepR+ cells are quiescent but after fracture or irradiation, they proliferate and differentiate into mesenchymal lineage osteogenic, adipogenic and/or chondrogenic cells. They also play a crucial role in the steady-state hematopoiesis process, as well as hematopoietic regeneration and the homing of hematopoietic stem cells (HSCs) after myeloablative injury and/or HSC transplantation. They line the sinusoidal cavities, maintain the trabeculae formation, and provide the space for HSC homing and retention. However, the LepR+ cell subset is heterogeneous; some subsets have higher adipogenic potential, while others express osteollineage-biased genes. Different transcription factors like Early B cell factor 3 (EBF3) or RunX2 help maintain this balance between the self-renewing and committed states, whether osteogenic or adipogenic. The study of LepR+ MSCs holds immense promise for advancing our understanding of HSC biology, tissue regeneration, metabolic disorders, and immune responses. In this review, we will discuss the origin of the BM resident LepR+ cells, different subtypes, and the role of LepR+ cells in maintaining hematopoiesis, osteogenesis, and BM adipogenesis following their multifaceted impact.

LRP1-deficient leptin receptor-positive cells in periodontal ligament tissue reduce alveolar bone mass by inhibiting bone formation

OBJECTIVE

Leptin receptor-positive (LepR+) periodontal ligament (PDL) cells play a crucial role in osteogenesis during tooth socket healing and orthodontic tooth movement; however, the factors regulating osteoblast differentiation remain unclear. This study aimed to demonstrate the function of low-density lipoprotein receptor-related protein 1 (LRP1) in alveolar bone formation by examining conditional knockout (cKO) mice lacking LRP1 in LepR+ cells.

DESIGN

Bone mass and formation were examined via bone morphometric analysis. Bone formation and resorption activities were determined via histochemical staining. Additionally, PDL cells collected from molars were induced to differentiate into osteoblasts with the addition of BMP2 and to mineralize with the addition of osteogenic medium. Osteoblast differentiation of PDL cells was examined by measuring the expression of osteoblast markers.

RESULTS

Bone morphometry analysis revealed decreased mineral apposition rate and alveolar bone mass in cKO mice. Additionally, cKO mice showed a decreased number of osterix-positive cells in the PDL. cKO mice had a large number of osteoclasts around the alveolar bone near the root apex and mesial surface of the tooth. In the PDL cells from cKO mice, inhibition of mineralized matrix formation and decreased expression of alkaline phosphatase, osterix, bone sialoprotein, and osteocalcin were observed even when BMP2 was added to the medium. BMP2, BMP4, and osteoprotegerin expression also decreased, but RANKL expression increased dominantly.

CONCLUSION

LRP1 in LepR+ cells promotes bone formation by stimulating osteoblast differentiation. Our findings can contribute to clinical research on bone diseases and help elucidate bone metabolism in the periodontal tissue.

ANGPTL4 binds to the leptin receptor to regulate ectopic bone formation

Leptin protein was thought to be unique to leptin receptor (LepR), but the phenotypes of mice with mutation in LepR [db/db (diabetes)] and leptin [ob/ob (obese)] are not identical, and the cause remains unclear. Here, we show that db/db, but not ob/ob, mice had defect in tenotomy-induced heterotopic ossification (HO), implicating alternative ligand(s) for LepR might be involved. Ligand screening revealed that ANGPTL4 (angiopoietin-like protein 4), a stress and fasting-induced factor, was elicited from brown adipose tissue after tenotomy, bound to LepR on PRRX1+ mesenchymal cells at the HO site, thus promotes chondrogenesis and HO development. Disruption of LepR in PRRX1+ cells, or lineage ablation of LepR+ cells, or deletion of ANGPTL4 impeded chondrogenesis and HO in mice. Together, these findings identify ANGPTL4 as a ligand for LepR to regulate the formation of acquired HO.

Designer Fat Cells: Adipogenic Differentiation of CRISPR-Cas9 Genome-Engineered Induced Pluripotent Stem Cells

Adipose tissue is an active endocrine organ that can signal bidirectionally to many tissues and organ systems in the body. With obesity, adipose tissue is a source of low-level inflammation that contributes to various co-morbidities and damage to downstream effector tissues. The ability to synthesize genetically engineered adipose tissue could have critical applications in studying adipokine signaling and the use of adipose tissue for novel therapeutic strategies. This study aimed to develop a method for non-viral adipogenic differentiation of genome-edited murine induced pluripotent stem cells (iPSCs) and to test the ability of such cells to engraft in mice in vivo . Designer adipocytes were created from iPSCs, which can be readily genetically engineered using CRISPR-Cas9 to knock out or insert individual genes of interest. As a model system for adipocyte-based drug delivery, an existing iPSC cell line that transcribes interleukin 1 receptor antagonist under the endogenous macrophage chemoattractant protein-1 promoter was tested for adipogenic capabilities under these same differentiation conditions. To understand the role of various adipocyte subtypes and their impact on health and disease, an efficient method was devised for inducing browning and whitening of IPSC-derived adipocytes in culture. Finally, to study the downstream effects of designer adipocytes in vivo , we transplanted the designer adipocytes into fat-free lipodystrophic mice as a model system for studying adipose signaling in different models of disease or repair. This novel translational tissue engineering and regenerative medicine platform provides an innovative approach to studying the role of adipose interorgan communication in various conditions.

DESIGNER FAT CELLS: ADIPOGENIC DIFFERENTIATION OF CRISPR-CAS9 GENOME-ENGINEERED INDUCED PLURIPOTENT STEM CELLS

Adipose tissue is an active endocrine organ that can signal bidirectionally to many tissues and organ systems in the body. With obesity, adipose tissue can serve as a source of low-level inflammation that contributes to various co-morbidities and damage to downstream effector tissues. The ability to synthesize genetically engineered adipose tissue could have critical applications in studying adipokine signaling and the use of adipose tissue for novel therapeutic strategies. This study aimed to develop a method for non-viral adipogenic differentiation of genome-edited murine induced pluripotent stem cells (iPSCs) and to test the ability of such cells to engraft in mice in vivo. Designer adipocytes were created from iPSCs, which can be readily genetically engineered using CRISPR-Cas9 to knock out or insert individual genes of interest. As a model system for adipocyte-based drug delivery, an existing iPSC cell line that transcribes interleukin 1 receptor antagonist under the endogenous macrophage chemoattractant protein-1 promoter was tested for adipogenic capabilities under these same differentiation conditions. To understand the role of various adipocyte subtypes and their impact on health and disease, an efficient method was devised for inducing browning and whitening of Ipsc-derived adipocytes in culture. Finally, to study the downstream effects of designer adipocytes in vivo, we transplanted the designer adipocytes into fat-free lipodystrophic mice as a model system for studying adipose signaling in different models of disease or repair. This novel translational tissue engineering and regenerative medicine platform provides an innovative approach to studying the role of adipose interorgan communication in various conditions.

Perspective: The current state of Cre driver mouse lines in skeletal research: Challenges and opportunities

The Cre/Lox system has revolutionized the ability of biomedical researchers to ask very specific questions about the function of individual genes in specific cell types at specific times during development and/or disease progression in a variety of animal models. This is true in the skeletal biology field, and numerous Cre driver lines have been created to foster conditional gene manipulation in specific subpopulations of bone cells. However, as our ability to scrutinize these models increases, an increasing number of issues have been identified with most driver lines. All existing skeletal Cre mouse models exhibit problems in one or more of the following three areas: (1) cell type specificity-avoiding Cre expression in unintended cell types; (2) Cre inducibility-improving the dynamic range for Cre in inducible models (negligible Cre activity before induction and high Cre activity after induction); and (3) Cre toxicity-reducing the unwanted biological effects of Cre (beyond loxP recombination) on cellular processes and tissue health. These issues are hampering progress in understanding the biology of skeletal disease and aging, and consequently, identification of reliable therapeutic opportunities. Skeletal Cre models have not advanced technologically in decades despite the availability of improved tools, including multi-promoter-driven expression of permissive or fragmented recombinases, new dimerization systems, and alternative forms of recombinases and DNA sequence targets. We review the current state of skeletal Cre driver lines, and highlight some of the successes, failures, and opportunities to improve fidelity in the skeleton, based on successes pioneered in other areas of biomedical science.

Leptin accelerates BMSC transformation into vertebral epiphyseal plate chondrocytes by activating SENP1-mediated deSUMOylation of SIRT3

Bone marrow mesenchymal stem cells (BMSCs) are capable of multidirectional differentiation, and engrafted BMSCs can be used to replace damaged chondrocytes for treatment of intervertebral disc disease. However, chondroblast differentiation of implanted BMSCs is inhibited by the anoxic environment of the articular cavity. Here, we found that leptin enhanced the transformation of BMSCs into chondrocytes under hypoxic conditions. BMSCs isolated from mice were cultured in medium supplemented with leptin under hypoxia. The expression of MFN1/2 and OPA1 were increased only in BMSCs cultured in an anoxic environment. In addition, in hypoxic environments cell energy metabolism relies on glycolysis regulated by leptin, rather than by mitochondrial oxidation. The expression of the de-SUMOylation protease SENP1 was elevated, leading to SIRT3-mediated activation of PGC-1α; these processes were regulated by CREB phosphorylation, and promoted mitochondrial fusion and cell differentiation. The chondrogenic activity of BMSCs isolated from SIRT3-knockout mice was lower than that of BMSCs isolated from wildtype mice. Implantation of SIRT3-knockout murine-derived BMSCs did not significantly improve the articular cartilage layer of the disc. In conclusion, the hypoxic microenvironment promoted BMSC differentiation into chondrocytes, whereas osteoblast differentiation was inhibited. SENP1 activated SIRT3 through the deSUMOylation of mitochondria and eliminated the antagonistic effect of SIRT3 acetylation on phosphorylation. When phosphorylation activity of CREB was increased, phosphorylated CREB is then transferred to the nucleus, affecting PGC-1α. This promotes mitochondrial fusion and differentiation of BMSCs. Leptin not only maintains chondrogenic differentiation homeostasis of BMSCs, but also provides energy for differentiation of BMSCs under hypoxic conditions through glycolysis.

Leptin receptor in osteocytes promotes cortical bone consolidation in female mice

Bone strength is partially determined during cortical bone consolidation, a process comprising coalescence of peripheral trabecular bone and its progressive mineralisation. Mice with genetic deletion of suppressor of cytokine signalling 3 (Socs3), an inhibitor of STAT3 signalling, exhibit delayed cortical bone consolidation, indicated by high cortical porosity, low mineral content, and low bone strength. Since leptin receptor (LepR) is expressed in the osteoblast lineage and is suppressed by SOCS3, we evaluated whether LepR deletion in osteocytes would rectify the Dmp1cre.Socs3fl/fl bone defect. First, we tested LepR deletion in osteocytes by generating Dmp1cre.LepRfl/fl mice and detected no significant bone phenotype. We then generated Dmp1cre.Socs3fl/fl.LepRfl/fl mice and compared them to Dmp1cre.Socs3fl/fl controls. Between 6 and 12 weeks of age, both Dmp1cre.Socs3fl/fl.LepRfl/fl and control (Dmp1cre.Socs3fl/fl) mice showed an increasing proportion of more heavily mineralised bone, indicating some cortical consolidation with time. However, at 12 weeks of age, rather than resolving the phenotype, delayed consolidation was extended in female Dmp1cre.Socs3fl/fl.LepRfl/fl mice. This was indicated in both metaphysis and diaphysis by greater proportions of low-density bone, lower proportions of high-density bone, and greater cortical porosity than Dmp1cre.Socs3fl/fl controls. There was also no change in the proportion of osteocytes staining positive for phospho-STAT3, suggesting the effect of LepR deletion in Dmp1cre.Socs3fl/fl mice is STAT3-independent. This identifies a new role for leptin signalling in bone which opposes our original hypothesis. Although LepR in osteocytes has no irreplaceable physiological role in normal bone maturation, when STAT3 is hyperactive, LepR in Dmp1Cre-expressing cells supports cortical consolidation.

Adipokines and coronary artery calcification in incident dialysis participants

PURPOSE

Adipokines have been associated with increased risk of cardiovascular disease. Our aim was to determine if adipokine levels are associated with coronary artery calcification (CAC) as well as all-cause mortality in incident dialysis patients.

METHODS

In patients new to dialysis, we prospectively investigated the association of adiponectin, leptin and resistin with coronary artery calcification measured by ECG-gated computer tomography. Participants were recruited a median of two months after starting dialysis.

RESULTS

The mean age was 50.0 (12.6) years and 31.1% were women. About 42% percent had BMI > 30. Higher adiponectin levels were inversely associated with CAC progression as change in Agatston score [-155.1 (-267.9, -42.2), p = 0.008] or change in CAC volumes between scans [-2.8 (-4.9, -0.6), p = 0.01]. Higher leptin levels were associated with CAC progression [110.4 (34.3-186.6), p = 0.005]. Decreased leptin [HR 0.5 (0.3-0.9), p = 0.05] was associated with all-cause mortality in adjusted models. There was no significant association between all-cause mortality and adiponectin [1.4 (0.6-3.4), p = 0.4] or resistin [HR 1.7 (0.5-5.0), p = 0.4].

CONCLUSION

High adiponectin protects against CAC progression, but is not associated with increased all-cause mortality. Higher leptin, as well as higher leptin to adiponectin ratio, is associated with CAC progression. Lower leptin levels were associated with all-cause mortality. The association of adipokines and cardiovascular disease in individuals on dialysis is complex and requires further study.

An integrative bioinformatics approach to decipher adipocyte-induced transdifferentiation of osteoblast

In human, bone loss is associated with increased marrow adipose tissue and recent data suggest that medullary adipocytes could play a role in osteoporosis by acting on neighboring bone-forming osteoblasts. Supporting this hypothesis, we previously showed, in a coculture model based on human bone marrow stromal cells, that factors secreted by adipocytes induced the conversion of osteoblasts towards an adipocyte-like phenotype. In this work, we employed an original integrative bioinformatics approach connecting proteomic and transcriptomic data from adipocytes and osteoblasts, respectively, to investigate the mechanisms underlying their crosstalk. Our analysis identified a total of 271 predicted physical interactions between adipocyte-secreted proteins and osteoblast membrane protein coding genes and proposed three pathways for their potential contribution to osteoblast transdifferentiation, the PI3K-AKT, the JAK2-STAT3 and the SMAD pathways. Our findings demonstrated the effectiveness of our integrative omics strategy to decipher cell-cell communication events.

Synergistic stimulation of osteoblast differentiation of rat mesenchymal stem cells by leptin and 25(OH)D3 is mediated by inhibition of chaperone-mediated autophagy

Background

Vitamin D is important for the mineralization of bones by stimulating osteoblast differentiation of bone marrow mesenchymal stem cells (BMMSCs). BMMSCs are a target of vitamin D action, and the metabolism of 25(OH)D₃ to biologically active 1α,25(OH)₂D₃ in BMMSCs promotes osteoblastogenesis in an autocrine/paracrine manner. Our previous study with human BMMSCs showed that megalin is required for the 25(OH)D₃-DBP complex to enter cells and for 25(OH)D₃ to stimulate osteoblast differentiation in BMMSCs. Furthermore, we reported that leptin up-regulates megalin in those cells. Leptin is a known inhibitor of PI3K/AKT-dependent chaperone-mediated autophagy (CMA). In this study, we tested the hypothesis that leptin acts synergistically with 25(OH)D₃ to promote osteoblastogenesis in rat BMMSCs by a mechanism that entails inhibition of PI3K/AKT-dependent CMA.

Methods

BMMSCs were isolated from rat bone marrow (4-week-old male SD rats); qRT-PCR and western immunoblots or immunofluorescence were used to evaluate the expression of megalin, ALP, COL1A1, RUNX2, OSX, OSP, and CMA in rBMMSCs. The osteoblast differentiation was evaluated by ALP activity, ALP staining, and calcium deposition. The viability of rBMMSCs was assessed with the CCK-8 kit. Biosynthesis of 1α,25(OH)₂D₃ was measured by a Rat 1α,25(OH)₂D₃ ELISA Kit.

Results

The combination of leptin and 25(OH)D₃ treatment significantly enhanced osteoblast differentiation as shown by ALP activity, ALP staining, and calcium deposition, the expression of osteogenic genes ALP, COL1A1, RUNX2, OSX, and OSP by qRT-PCR and western immunoblots in rBMMSCs. Leptin enhanced the expression of megalin and synthesis of 1α,25(OH)₂D₃ in rBMMSCs. Our data showed that leptin inhibited CMA activity of rBMMSCs by activating PI3K/AKT signal pathway; the ability of leptin to enhance 25(OH)D₃ promoted osteoblast differentiation of rBMMSCs was weakened by the PI3K/AKT signal pathway inhibitor.

Conclusions

Our data reveal the mechanism by which leptin and 25(OH)D₃ promote osteoblast differentiation in rBMMSCs. Leptin promoted the expression of megalin by inhibiting CMA, increased the utilization of 25(OH)D₃ by rBMMSCs, and enhanced the ability of 25(OH)D₃ to induce osteoblast differentiation of rBMMSCs. PI3K/AKT is at least partially involved in the regulation of CMA. These data indicate the importance of megalin in BMMSCs for vitamin D’s role in skeletal health.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02623-z.

Molecular mechanism of anti-adipogenic effect of vitexin in differentiating hMSCs

In this study, we evaluated a detailed molecular mechanism of anti-adipogenic activity of vitexin, apigenin flavone glucoside, present in germinated fenugreek seeds, in differentiating human mesenchymal stem cells (hMSCs). The lipid content of differentiated adipocytes was estimated by ORO staining. Effect on mitotic clonal expansion was checked by cell cycle analysis. Expression of early and terminal adipocyte differentiation markers, anti- and pro-adipogenic transcription factors and signalling intermediates regulating them was evaluated at RNA and protein level. We found vitexin to be non-cytotoxic up to 20 μM at which intracellular lipid accumulation was significantly decreased. Cell cycle analysis suggested that vitexin does not affect mitotic clonal expansion. Expression of early and late differentiation markers, such as CEBPα, CEBPβ, PPARγ, FABP4, perilipin, adiponectin and Glut4 was significantly reduced in the presence of vitexin. Expression of KLF4 and KLF15, positive regulators of PPARγ, was decreased, whereas that of negative regulators, namely KLF2, GATA2, miR20a, miR27a, miR27b, miR128, miR130a, miR130b, miR182 and miR548 increased with vitexin treatment. This effect was mediated by the activation of the AMP-activated protein kinase (AMPK) pathway via the activation of LepR and additionally by inhibiting ROS. Thus, our results showed that vitexin regulates the expression of PPARγ and inhibits adipogenesis of hMSCs at an early stage of differentiation.

Role of myeloid cell leptin signaling in the regulation of glucose metabolism

Although innate immunity is linked to metabolic health, the effect of leptin signaling in cells from the innate immune system on glucose homeostasis has not been thoroughly investigated. We generated two mouse models using Cre-lox methodology to determine the effect of myeloid cell-specific leptin receptor (Lepr) reconstitution and Lepr knockdown on in vivo glucose metabolism. Male mice with myeloid cell-specific Lepr reconstitution (Lyz2Cre⁺Lepr^loxTB/loxTB) had better glycemic control as they aged compared to male mice with whole-body transcriptional blockade of Lepr (Lyz2Cre⁻Lepr^loxTB/loxTB). In contrast, Lyz2Cre⁺Lepr^loxTB/loxTB females only had a trend for diminished hyperglycemia after a prolonged fast. During glucose tolerance tests, Lyz2Cre⁺Lepr^loxTB/loxTB males had a mildly improved plasma glucose profile compared to Cre⁻ controls while Lyz2Cre⁺Lepr^loxTB/loxTB females had a similar glucose excursion to their Cre⁻ controls. Myeloid cell-specific Lepr knockdown (Lyz2Cre⁺Lepr^flox/flox) did not significantly alter body weight, blood glucose, insulin sensitivity, or glucose tolerance in males or females. Expression of the cytokine interleukin 10 (anti-inflammatory) tended to be higher in adipose tissue of male Lyz2Cre⁺Lepr^loxTB/loxTB mice (p = 0.0774) while interleukin 6 (pro-inflammatory) was lower in male Lyz2Cre⁺Lepr^flox/flox mice (p < 0.05) vs. their respective controls. In conclusion, reconstitution of Lepr in cells of myeloid lineage has beneficial effects on glucose metabolism in male mice.

Leptin Chronic Effect on Differentiation, Ion Currents and Protein Expression in N1E-115 Neuroblastoma Cells

&lt;b&gt;Background and Objective:&lt;/b&gt; Arcuate nucleus (ARC), a component of appetite-regulatory factors, contains populations of both orexigenic and anorexigenic neurons and one of the fundamental components of its system is leptin. Studies have evidenced the critical neurotrophic role in the development of ARC. To determine such effects on neuron development, N1E-115 neuroblastoma cells were used as an ARC model. &lt;b&gt;Materials and Methods:&lt;/b&gt; N1E-115 neuroblastoma cells were treated with leptin [10 nM] for 24, 48 and 72 hrs. Dimethyl sulfoxide (DMSO) 1.5% was used as a known drug that promotes neurite expression. Cells percentage (%) that developed neurites was evaluated by bright field microscopy. Patch-clamp electrophysiology was used to analyze membrane ion currents, RT-PCR for quantifying changes in mRNA expression of anorexic peptides, proopiomelanocortin (POMC) and cocaine and amphetamine-related transcript (CART), in addition to principal Na&lt;sub&gt;v&lt;/sub&gt;, Ca&lt;sub&gt;v&lt;/sub&gt; ion channel subunits. &lt;b&gt;Results:&lt;/b&gt; N1E-115 cells treated with leptin show neurite expression after 24 hrs of treatment, similar effects were obtained with DMSO. Leptin (time-dependent) increases the inward current in comparison with the control value at 72 hrs. Outward currents were not affected by leptin. Leptin and DMSO increased Na&lt;sup&gt;+&lt;/sup&gt; and Ca&lt;sup&gt;2+&lt;/sup&gt; current without changes in the kinetic properties. Lastly, leptin promotes an increase in mRNA level expression of transcripts to POMC, CART, Na&lt;sub&gt;v&lt;/sub&gt;1.2 and Ca&lt;sub&gt;v&lt;/sub&gt;1.3. &lt;b&gt;Conclusion:&lt;/b&gt; Leptin chronic treatment promotes neurite expression, Up-regulation of Na&lt;sup&gt;+&lt;/sup&gt; and Ca&lt;sup&gt;2+&lt;/sup&gt; ion channels determining neuronal excitability, besides increasing the mRNA level expression of anorexic peptides POMC and CART in neuroblastoma N1E-115.

A bone-specific adipogenesis pathway in fat-free mice defines key origins and adaptations of bone marrow adipocytes with age and disease

Bone marrow adipocytes accumulate with age and in diverse disease states. However, their origins and adaptations in these conditions remain unclear, impairing our understanding of their context-specific endocrine functions and relationship with surrounding tissues. In this study, by analyzing bone and adipose tissues in the lipodystrophic ‘fat-free’ mouse, we define a novel, secondary adipogenesis pathway that relies on the recruitment of adiponectin-negative stromal progenitors. This pathway is unique to the bone marrow and is activated with age and in states of metabolic stress in the fat-free mouse model, resulting in the expansion of bone marrow adipocytes specialized for lipid storage with compromised lipid mobilization and cytokine expression within regions traditionally devoted to hematopoiesis. This finding further distinguishes bone marrow from peripheral adipocytes and contributes to our understanding of bone marrow adipocyte origins, adaptations, and relationships with surrounding tissues with age and disease.

Whitlockite-Enabled Hydrogel for Craniofacial Bone Regeneration

Growth-factor-free bone regeneration remains a challenge in craniofacial engineering. Here, we engineered an osteogenic niche composed of a commercially modified alginate hydrogel and whitlockite microparticles (WHMPs), which impart tunable physicochemical properties that can direct osteogenesis of human gingival mesenchymal stem cells (GMSCs). Our in vitro studies demonstrate that WHMPs induce osteogenesis of GMSCs more effectively than previously demonstrated hydroxyapatite microparticles (HApMPs). Alginate-WHMP hydrogels showed higher elasticity without any adverse effects on the viability of the encapsulated GMSCs. Moreover, the alginate-WHMP hydrogels upregulate the mitogen-activated protein kinase (MAPK) pathway, which in turn orchestrates several osteogenic markers, such as RUNX2 and OCN, in the encapsulated GMSCs. Concurrent coculture studies with human osteoclasts demonstrate that GMSCs encapsulated in alginate-WHMP hydrogels downregulate osteoclastic activity, potentially due to release of Mg2+ ions from the WHMPs along with secretion of osteoprotegerin from the GMSCs. In vivo studies demonstrated that the GMSCs encapsulated in our osteogenic niche were able to promote bone repair in calvarial defects in murine models. Altogether, our results confirmed the development of a promising treatment modality for craniofacial bone regeneration based on an injectable growth-factor-free hydrogel delivery system.

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.

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.

Cortical bone maturation in mice requires SOCS3 suppression of gp130/STAT3 signalling in osteocytes

Bone strength is determined by its dense cortical shell, generated by unknown mechanisms. Here we use the Dmp1Cre:Socs3f/f mouse, with delayed cortical bone consolidation, to characterise cortical maturation and identify control signals. We show that cortical maturation requires a reduction in cortical porosity, and a transition from low to high density bone, which continues even after cortical shape is established. Both processes were delayed in Dmp1Cre:Socs3f/f mice. SOCS3 (suppressor of cytokine signalling 3) inhibits signalling by leptin, G-CSF, and IL-6 family cytokines (gp130). In Dmp1Cre:Socs3f/f bone, STAT3 phosphorylation was prolonged in response to gp130-signalling cytokines, but not G-CSF or leptin. Deletion of gp130 in Dmp1Cre:Socs3f/f mice suppressed STAT3 phosphorylation in osteocytes and osteoclastic resorption within cortical bone, leading to rescue of the corticalisation defect, and restoration of compromised bone strength. We conclude that cortical bone development includes both pore closure and accumulation of high density bone, and that these processes require suppression of gp130-STAT3 signalling in osteocytes.

Signature quality attributes of CD146+ mesenchymal stem/stromal cells correlate with high therapeutic and secretory potency

CD146⁺ bone marrow-derived mesenchymal stem/stromal cells (BM-MSCs) play key roles in the perivascular niche, skeletogenesis, and hematopoietic support; however, comprehensive evaluation of therapeutic potency has yet to be determined. In this study, in vitro inflammatory priming to crude human BM-MSCs (n = 8) captured a baseline of signature responses, including enriched CD146⁺ with coexpression of CD107a^High , CXCR4^High , and LepR^High , transcriptional profile, enhanced secretory capacity, and robust immunomodulatory secretome and function, including immunopotency assays (IPAs) with stimulated immune cells. These signatures were significantly more pronounced in CD146⁺ (POS)-sorted subpopulation than in the CD146^- (NEG). Mechanistically, POS BM-MSCs showed a markedly higher secretory capacity with significantly greater immunomodulatory and anti-inflammatory protein production upon inflammatory priming compared with the NEG BM-MSCs. Moreover, IPAs with stimulated peripheral blood mononuclear cells and T lymphocytes demonstrated robust immunosuppression mediated by POS BM-MSC while inducing significant frequencies of regulatory T cells. in vivo evidence showed that POS BM-MSC treatment promoted pronounced M1-to-M2 macrophage polarization, ameliorating inflammation/fibrosis of knee synovium and fat pad, unlike treatment with NEG BM-MSCs. These data correlate the expression of CD146 with innately higher immunomodulatory and secretory capacity, and thus therapeutic potency. This high-content, reproducible evidence suggests that the CD146⁺ (POS) MSC subpopulation are the mediators of the beneficial effects achieved using crude BM-MSCs, leading to translational implications for improving cell therapy and manufacturing.

Leptin acts on mesenchymal stem cells to promote chemoresistance in osteosarcoma cells

Leptin signaling influences osteoblastogenesis and modulates the fate of mesenchymal stem cells (MSCs) during bone and cartilage regeneration. Although MSCs abound in the osteosarcoma (OS) microenvironment, and leptin exhibits pro-tumorigenic properties, leptin's influence on OS progression and chemoresistant signaling in MSCs remains unclear. Using cell viability and apoptosis assays, we showed that medium conditioned by leptin-treated human MSCs promotes cisplatin resistance in cultured human OS cells. Moreover, GFP-LC3 expression and chloroquine treatment experiments showed that this effect is mediated by stimulation of autophagy in OS cells. TGF-β expression in MSCs was upregulated by leptin and suppressed by leptin receptor knockdown. Silencing TGF-β in MSCs also abolished OS cell chemoresistance induced by leptin-conditioned medium. Cisplatin resistance was also induced when leptin-conditioned MSCs were co-injected with MG-63 OS cells to generate subcutaneous xenografts in nude mice. Finally, we observed a significant correlation between autophagy-associated gene expression in OS clinical samples and patient prognosis. We conclude that leptin upregulates TGF-β in MSCs, which promotes autophagy-mediated chemoresistance in OS cells.

Increase in Leptin and PPAR-γ Gene Expression in Lipedema Adipocytes Differentiated in vitro from Adipose-Derived Stem Cells

Lipedema is a painful loose connective tissue disorder characterized by a bilaterally symmetrical fat deposition in the lower extremities. The goal of this study was to characterize the adipose-derived stem cells (ASCs) of healthy and lipedema patients by the expression of stemness markers and the adipogenic and osteogenic differentiation potential. Forty patients, 20 healthy and 20 with lipedema, participated in this study. The stromal vascular fraction (SVF) was obtained from subcutaneous thigh (SVF-T) and abdomen (SVF-A) fat and plated for ASCs characterization. The data show a similar expression of mesenchymal markers, a significant increase in colonies (p < 0.05) and no change in the proliferation rate in ASCs isolated from the SVF-T or SVF-A of lipedema patients compared with healthy patients. The leptin gene expression was significantly increased in lipedema adipocytes differentiated from ASCs-T (p = 0.04) and the PPAR-γ expression was significantly increased in lipedema adipocytes differentiated from ASCs-A (p = 0.03) compared to the corresponding cells from healthy patients. No significant changes in the expression of genes associated with inflammation were detected in lipedema ASCs or differentiated adipocytes. These results suggest that lipedema ASCs isolated from SVF-T and SVF-A have a higher adipogenic differentiation potential compared to healthy ASCs.

The relationship between bone marrow adipose tissue and bone metabolism in postmenopausal osteoporosis

Postmenopausal osteoporosis (PMOP) is a prevalent skeletal disorder associated with menopause-related estrogen withdrawal. PMOP is characterized by low bone mass, deterioration of the skeletal microarchitecture, and subsequent increased susceptibility to fragility fractures, thus contributing to disability and mortality. Accumulating evidence indicates that abnormal expansion of marrow adipose tissue (MAT) plays a crucial role in the onset and progression of PMOP, in part because both bone marrow adipocytes and osteoblasts share a common ancestor lineage. The cohabitation of MAT adipocytes, mesenchymal stromal cells, hematopoietic cells, osteoblasts and osteoclasts in the bone marrow creates a microenvironment that permits adipocytes to act directly on other cell types in the marrow. Furthermore, MAT, which is recognized as an endocrine organ, regulates bone remodeling through the secretion of adipokines and cytokines. Although an enhanced MAT volume is linked to low bone mass and fractures in PMOP, the detailed interactions between MAT and bone metabolism remain largely unknown. In this review, we examine the possible mechanisms of MAT expansion under estrogen withdrawal and further summarize emerging findings regarding the pathological roles of MAT in bone remodeling. We also discuss the current therapies targeting MAT in osteoporosis. A comprehensive understanding of the relationship between MAT expansion and bone metabolism in estrogen deficiency conditions will provide new insights into potential therapeutic targets for PMOP.

Leptin in Tumor Microenvironment

Leptin is a hormone that plays a major role as mediator of long-term regulation of energy balance, suppressing food intake, and stimulating weight loss. More recently, important physiological roles other than controlling appetite and energy expenditure have been suggested for leptin, including neuroendocrine, reparative, reproductive, and immune functions. These emerging peripheral roles let hypothesize that leptin can modulate also cancer progression. Indeed, many studies have demonstrated that elevated chronic serum concentrations of leptin, frequently seen in obese subjects, represent a stimulatory signal for tumor growth. Current knowledge indicates that also different non-tumoral cells resident in tumor microenvironment may respond to leptin creating a favorable soil for cancer cells. In addition, leptin is produced also within the tumor microenvironment creating the possibility for paracrine and autocrine action. In this review, we describe the main mechanisms that regulate peripheral leptin availability and how leptin can shape tumor microenvironment.

The Bones of Children With Obesity

Excess adiposity in childhood may affect bone development, ultimately leading to bone frailty. Previous reports showing an increased rate of extremity fractures in children with obesity support this fear. On the other hand, there is also evidence suggesting that bone mineral content is higher in obese children than in normal weight peers. Both adipocytes and osteoblasts derive from multipotent mesenchymal stem cells (MSCs) and obesity drives the differentiation of MSCs toward adipocytes at the expense of osteoblast differentiation. Furthermore, adipocytes in bone marrow microenvironment release a number of pro-inflammatory and immunomodulatory molecules that up-regulate formation and activation of osteoclasts, thus favoring bone frailty. On the other hand, body adiposity represents a mechanical load, which is beneficial for bone accrual. In this frame, bone quality, and structure result from the balance of inflammatory and mechanical stimuli. Diet, physical activity and the hormonal milieu at puberty play a pivotal role on this balance. In this review, we will address the question whether the bone of obese children and adolescents is unhealthy in comparison with normal-weight peers and discuss mechanisms underlying the differences in bone quality and structure. We anticipate that many biases and confounders affect the clinical studies conducted so far and preclude us from achieving robust conclusions. Sample-size, lack of adequate controls, heterogeneity of study designs are the major drawbacks of the existing reports. Due to the increased body size of children with obesity, dual energy absorptiometry might overestimate bone mineral density in these individuals. Magnetic resonance imaging, peripheral quantitative CT (pQCT) scanning and high-resolution pQCT are promising techniques for the accurate estimate of bone mineral content in obese children. Moreover, no longitudinal study on the risk of incident osteoporosis in early adulthood of children and adolescents with obesity is available. Finally, we will address emerging dietary issues (i.e., the likely benefits for the bone health of polyunsaturated fatty acids and polyphenols) since an healthy diet (i.e., the Mediterranean diet) with balanced intake of certain nutrients associated with physical activity remain the cornerstones for achieving an adequate bone accrual in young individuals regardless of their adiposity degree.

ΔFosB Requires Galanin, but not Leptin, to Increase Bone Mass via the Hypothalamus, but both are needed to increase Energy expenditure

Energy metabolism and bone homeostasis share several regulatory pathways. The AP1 transcription factor ΔFosB and leptin both regulate energy metabolism and bone, yet whether their pathways intersect is not known. Transgenic mice overexpressing ΔFosB under the control of the Enolase 2 (ENO2) promoter exhibit high bone mass, high energy expenditure, low fat mass, and low circulating leptin levels. Because leptin is a regulator of bone and ΔFosB acts on leptin-responsive ventral hypothalamic (VHT) neurons to induce bone anabolism, we hypothesized that regulation of leptin may contribute to the central actions of ΔFosB in the VHT. To address this question, we used adeno-associated virus (AAV) expression of ΔFosB in the VHT of leptin-deficient ob/ob mice and genetic crossing of ENO2-ΔFosB with ob/ob mice. In both models, leptin deficiency prevented ΔFosB-triggered reduction in body weight, increase in energy expenditure, increase in glucose utilization, and reduction in pancreatic islet size. In contrast, leptin deficiency failed to prevent ΔFosB-triggered increase in bone mass. Unlike leptin deficiency, galanin deficiency blocked both the metabolic and the bone ΔFosB-induced effects. Overall, our data demonstrate that, while the catabolic energy metabolism effects of ΔFosB require intact leptin and galanin signaling, the bone mass-accruing effects of ΔFosB require galanin but are independent of leptin. © 2019 American Society for Bone and Mineral Research.

Obesity and calcification of the ligaments of the spine: a comprehensive CT analysis of the entire spine in a random trauma population

BACKGROUND CONTEXT

Obesity, which is currently surging to epidemic levels within the United States, has been linked to hyperostotic conditions like diffuse idiopathic skeletal hyperostosis (DISH) and ossification of the posterior longitudinal ligament (OPLL). Excess adipose tissue and insulin-resistance may cause a systemic increase in serum levels of proinflammatory cytokines and these signals can affect bone metabolism. Spinal ligaments and discs may have receptors for these signaling molecules. Anecdotal observations at this institution suggested that there is a clinically important subset of younger patients with obesity and multilevel stenosis in the presence of unusual calcification of the spinal ligaments that is distinct from DISH.

PURPOSE

To determine if there is an association between truncal obesity and calcifications of the spine in nonelderly adults.

STUDY DESIGN/SETTING

This is a retrospective analysis of 214 sequential trauma patients between the ages of 29 and 50. Patients' age, sex, truncal obesity, history of hypertension, and diabetes were assessed for association with ligamentous calcification of the spine.

PATIENT SAMPLE

Sequential trauma patients were chosen from our institution's trauma database between 2006 and 2007.

METHODS

Full spine computed tomography (CT) imaging was examined for bone formation in the region of the anterior longitudinal ligament (ALL) and annulus, posterior longitudinal ligament (PLL) and annulus, and the ligamentum flavum (LF). Visceral and subcutaneous abdominal fat were also evaluated. The authors report no study funding sources or conflicts of interest.

OUTCOME MEASURES

Calcification of the ALL, PLL, and LF were assigned a score at each level and then combined for a total calcification score (TCS) for the entire spine. Obesity was estimated using a truncal body mass index (TBMI) by using a previously validated CT derived truncal total adiposity volume (TAV).

RESULTS

ALL calcification was associated with age, male gender, hypertension, and increased adiposity. PLL calcification was significantly associated with age and hypertension. LF calcification was only associated with increased obesity.

CONCLUSIONS

In our analysis of nonelderly patients, LF calcification was independently associated with truncal obesity. This implies obesity plays a greater role in calcification than could be accounted for by simply age-related degeneration or gender.

Shared Autonomic Pathways Connect Bone Marrow and Peripheral Adipose Tissues Across the Central Neuraxis

Bone marrow adipose tissue (BMAT) is increased in both obesity and anorexia. This is unique relative to white adipose tissue (WAT), which is generally more attuned to metabolic demand. It suggests that there may be regulatory pathways that are common to both BMAT and WAT and also those that are specific to BMAT alone. The central nervous system (CNS) is a key mediator of adipose tissue function through sympathetic adrenergic neurons. Thus, we hypothesized that central autonomic pathways may be involved in BMAT regulation. To test this, we first quantified the innervation of BMAT by tyrosine hydroxylase (TH) positive nerves within the metaphysis and diaphysis of the tibia of B6 and C3H mice. We found that many of the TH+ axons were concentrated around central blood vessels in the bone marrow. However, there were also areas of free nerve endings which terminated in regions of BMAT adipocytes. Overall, the proportion of nerve-associated BMAT adipocytes increased from proximal to distal along the length of the tibia (from ~3-5 to ~14-24%), regardless of mouse strain. To identify the central pathways involved in BMAT innervation and compare to peripheral WAT, we then performed retrograde viral tract tracing with an attenuated pseudorabies virus (PRV) to infect efferent nerves from the tibial metaphysis (inclusive of BMAT) and inguinal WAT (iWAT) of C3H mice. PRV positive neurons were identified consistently from both injection sites in the intermediolateral horn of the spinal cord, reticular formation, rostroventral medulla, solitary tract, periaqueductal gray, locus coeruleus, subcoeruleus, Barrington's nucleus, and hypothalamus. We also observed dual-PRV infected neurons within the majority of these regions. Similar tracings were observed in pons, midbrain, and hypothalamic regions from B6 femur and tibia, demonstrating that these results persist across mouse strains and between skeletal sites. Altogether, this is the first quantitative report of BMAT autonomic innervation and reveals common central neuroanatomic pathways, including putative "command" neurons, involved in coordinating multiple aspects of sympathetic output and facilitation of parallel processing between bone marrow/BMAT and peripheral adipose tissue.

MECHANISMS IN ENDOCRINOLOGY: Bone marrow adiposity and bone, a bad romance?

Bone marrow adipocytes (BMA-) constitute an original and heterogeneous fat depot whose development appears interlinked with bone status throughout life. The gradual replacement of the haematopoietic tissue by BMA arises in a well-ordered way during childhood and adolescence concomitantly to bone growth and continues at a slower rate throughout the adult life. Importantly, BM adiposity quantity is found well associated with bone mineral density (BMD) loss at different skeletal sites in primary osteoporosis such as in ageing or menopause but also in secondary osteoporosis consecutive to anorexia nervosa. Since BMA and osteoblasts originate from a common mesenchymal stem cell, adipogenesis is considered as a competitive process that disrupts osteoblastogenesis. Besides, most factors secreted by bone and bone marrow cells (ligands and antagonists of the WNT/β-catenin pathway, BMP and others) reciprocally regulate the two processes. Hormones such as oestrogens, glucocorticoids, parathyroid and growth hormones that control bone remodelling also modulate the differentiation and the activity of BMA. Actually, BMA could also contribute to bone loss through the release of paracrine factors altering osteoblast and/or osteoclast formation and function. Based on clinical and fundamental studies, this review aims at presenting and discussing these current arguments that support but also challenge the involvement of BMA in the bone mass integrity.

Novel leptin receptor signaling mutants identify location and sex-dependent modulation of bone density, adiposity, and growth

Leptin, a hormone primarily produced by adipocytes, contributes to the regulation of bone health by modulating bone density, growth and adiposity. Upon leptin binding, multiple sites of the long form of the leptin receptor (LepRb) are phosphorylated to trigger activation of downstream signaling pathways. To address the role of LepRb-signaling pathways in bone health, we compared the effects of three LepRb mutations on bone density, adiposity, and growth in male and female mice. The ∆65 mutation, which lacks the known tyrosine phosphorylation sites, caused obesity and the most dramatic bone phenotype marked by excessive bone adiposity, osteoporosis, and decreased growth, consistent with the phenotype of db/db and ob/ob mice that fully lack leptin receptor signaling. Mutation of LepRb Tyr ₁₁₃₈ , which results in an inability to recruit and phosphorylate signal transducer and activator of transcription 3, also caused obesity, but bone loss and adiposity were more dominant in male mice and no growth defect was observed. In contrast, mutation of LepRb Tyr ₉₈₅ , which blocks SHP2/SOCS3 recruitment to LepRb and contributes to leptin hypersensitivity, promoted increased femur bone density only in male mice, while marrow adiposity and bone growth were not affected. Additional analyses of vertebral trabecular bone volume indicate that only the Tyr ₁₁₃₈ mutant mice exhibit bone loss in vertebrae. Together, our findings suggest that the phosphorylation status of specific sites of the LepRb contribute to the sex- and location-dependent bone responses to leptin. Unraveling the mechanisms by which leptin responses are sex- and location-dependent can contribute to the development of uniquely targeted osteoporosis therapies.

Paracrine and endocrine actions of bone-the functions of secretory proteins from osteoblasts, osteocytes, and osteoclasts

The skeleton is a dynamic organ that is constantly remodeled. Proteins secreted from bone cells, namely osteoblasts, osteocytes, and osteoclasts exert regulation on osteoblastogenesis, osteclastogenesis, and angiogenesis in a paracrine manner. Osteoblasts secrete a range of different molecules including RANKL/OPG, M-CSF, SEMA3A, WNT5A, and WNT16 that regulate osteoclastogenesis. Osteoblasts also produce VEGFA that stimulates osteoblastogenesis and angiogenesis. Osteocytes produce sclerostin (SOST) that inhibits osteoblast differentiation and promotes osteoclast differentiation. Osteoclasts secrete factors including BMP6, CTHRC1, EFNB2, S1P, WNT10B, SEMA4D, and CT-1 that act on osteoblasts and osteocytes, and thereby influenceaA osteogenesis. Osteoclast precursors produce the angiogenic factor PDGF-BB to promote the formation of Type H vessels, which then stimulate osteoblastogenesis. Besides, the evidences over the past decades show that at least three hormones or "osteokines" from bone cells have endocrine functions. FGF23 is produced by osteoblasts and osteocytes and can regulate phosphate metabolism. Osteocalcin (OCN) secreted by osteoblasts regulates systemic glucose and energy metabolism, reproduction, and cognition. Lipocalin-2 (LCN2) is secreted by osteoblasts and can influence energy metabolism by suppressing appetite in the brain. We review the recent progresses in the paracrine and endocrine functions of the secretory proteins of osteoblasts, osteocytes, and osteoclasts, revealing connections of the skeleton with other tissues and providing added insights into the pathogenesis of degenerative diseases affecting multiple organs and the drug discovery process.

Microenvironmental factors that regulate mesenchymal stem cells: lessons learned from the study of heterotopic ossification

Bone marrow contains a non-hematopoietic, clonogenic, multipotent population of stromal cells that are later called mesenchymal stem cells (MSC). Similar cells that share many common features with MSC are also found in other organs, which are thought to contribute both to normal tissue regeneration and to pathological processes such as heterotopic ossification (HO), the formation of ectopic bone in soft tissue. Understanding the microenvironmental factors that regulate MSC in vivo is essential both for understanding the biology of the stem cells and for effective translational applications of MSC. Unfortunately, this important aspect has been largely underappreciated. This review tries to raise the attention and highlight this critical issue by updating the relevant literature along with discussions of the key issues in the area.

Connecting Bone and Fat: The Potential Role for Sclerostin

Sclerostin (SOST), a protein secreted from mature osteocytes in response to mechanical unloading and other stimuli, inhibits the osteogenic Wnt/β-catenin pathway in mesenchymal stem cells (MSCs) impeding their ability to differentiate into mineralizing osteoblasts.

PURPOSE

This review summarizes the crosstalk between adipose tissue and bone. It also reviews the origin, regulation, and role of SOST in osteogenesis and brings attention to an emerging role of this protein in the regulation of adipogenesis.

RECENT FINDINGS

Bone-derived molecules that drive MSC adipogenesis have not previously been identified, but recent findings suggest that SOST signaling may induce adipogenesis. In vivo SOST acts locally to induce changes in bone and, in vitro, increases adipogenesis in 3T3-L1 preadipocytes.

SUMMARY

SOST is able to induce adipogenesis in certain preadipocytes, however bone-specific studies are needed to determine the effect of local SOST concentrations in healthy and disease models on bone marrow adipose tissue.

Whole-Body Vibration Mimics the Metabolic Effects of Exercise in Male Leptin Receptor-Deficient Mice

Whole-body vibration (WBV) has gained attention as a potential exercise mimetic, but direct comparisons with the metabolic effects of exercise are scarce. To determine whether WBV recapitulates the metabolic and osteogenic effects of physical activity, we exposed male wild-type (WT) and leptin receptor-deficient (db/db) mice to daily treadmill exercise (TE) or WBV for 3 months. Body weights were analyzed and compared with WT and db/db mice that remained sedentary. Glucose and insulin tolerance testing revealed comparable attenuation of hyperglycemia and insulin resistance in db/db mice following TE or WBV. Both interventions reduced body weight in db/db mice and normalized muscle fiber diameter. TE or WBV also attenuated adipocyte hypertrophy in visceral adipose tissue and reduced hepatic lipid content in db/db mice. Although the effects of leptin receptor deficiency on cortical bone structure were not eliminated by either intervention, exercise and WBV increased circulating levels of osteocalcin in db/db mice. In the context of increased serum osteocalcin, the modest effects of TE and WBV on bone geometry, mineralization, and biomechanics may reflect subtle increases in osteoblast activity in multiple areas of the skeleton. Taken together, these observations indicate that WBV recapitulates the effects of exercise on metabolism in type 2 diabetes.

Exenatide Improves Bone Quality in a Murine Model of Genetically Inherited Type 2 Diabetes Mellitus

Type 2 diabetes mellitus (T2DM) is associated with skeletal complications, including an increased risk of fractures. Reduced blood supply and bone strength may contribute to this skeletal fragility. We hypothesized that long-term administration of Exenatide, a glucagon-like peptide-1 receptor agonist, would improve bone architecture and strength of T2DM mice by increasing blood flow to bone, thereby stimulating bone formation. In this study, we used a model of obesity and severe T2DM, the leptin receptor-deficient db/db mouse to assess alterations in bone quality and hindlimb blood flow and to examine the beneficial effects of 4 weeks administration of Exenatide. As expected, diabetic mice showed marked alterations in bone structure, remodeling and strength, and basal vascular tone compared with lean mice. Exenatide treatment improved trabecular bone mass and architecture by increasing bone formation rate, but only in diabetic mice. Although there was no effect on hindlimb perfusion at the end of this treatment, Exenatide administration acutely increased tibial blood flow. While Exenatide treatment did not restore the impaired bone strength, intrinsic properties of the matrix, such as collagen maturity, were improved. The effects of Exenatide on in vitro bone formation were further investigated in primary osteoblasts cultured under high-glucose conditions, showing that Exenatide reversed the impairment in bone formation induced by glucose. In conclusion, Exenatide improves trabecular bone mass by increasing bone formation and could protect against the development of skeletal complications associated with T2DM.

[Bone and adipose tissue formation]

Leptin has been described to have a crucial role in bone homeostasis by systemic as well as local action. Systemically, leptin seems to inhibit bone formation controlled by a feedback loop including osteocalcin and insulin. Even though the action seems to be bone site specific, as well as gender- and time-dependent, the results showing the interaction of these three factors are in part still inconsistent. In this article the complex effects of leptin, insulin, and osteocalcin on bone and fat metabolism are summarized.

Marrow Adipose Tissue: Trimming the Fat

Marrow adipose tissue (MAT) is a unique fat depot, located in the skeleton, that has the potential to contribute to both local and systemic metabolic processes. In this review we highlight several recent conceptual developments pertaining to the origin and function of MAT adipocytes; consider the relationship of MAT to beige, brown, and white adipose depots; explore MAT expansion and turnover in humans and rodents; and discuss future directions for MAT research in the context of endocrine function and metabolic disease. MAT has the potential to exert both local and systemic effects on metabolic homeostasis, skeletal remodeling, hematopoiesis, and the development of bone metastases. The diversity of these functions highlights the breadth of the potential impact of MAT on health and disease.

Leptin Receptor Promotes Adipogenesis and Reduces Osteogenesis by Regulating Mesenchymal Stromal Cells in Adult Bone Marrow

Skeletal stem cells (SSCs) that are the major source of osteoblasts and adipocytes in adult bone marrow express leptin receptor (LepR). To test whether LepR regulates SSC function, we conditionally deleted Lepr from limb bone marrow stromal cells, but not from the axial skeleton or hypothalamic neurons, using Prx1-Cre. Prx1-Cre;Lepr(fl/fl) mice exhibited normal body mass and normal hematopoiesis. However, limb bones from Prx1-Cre;Lepr(fl/fl) mice exhibited increased osteogenesis, decreased adipogenesis, and accelerated fracture healing. Leptin increased adipogenesis and reduced osteogenesis by activating Jak2/Stat3 signaling in bone marrow stromal cells. A high-fat diet increased adipogenesis and reduced osteogenesis in limb bones from wild-type mice, but not from Prx1-Cre;Lepr(fl/fl) mice. This reflected local effects of LepR on osteogenesis and adipogenesis by bone marrow stromal cells and systemic effects on bone resorption. Leptin/LepR signaling regulates adipogenesis and osteogenesis by mesenchymal stromal cells in the bone marrow in response to diet and adiposity.

Expansion of Bone Marrow Adipose Tissue During Caloric Restriction Is Associated With Increased Circulating Glucocorticoids and Not With Hypoleptinemia

Bone marrow adipose tissue (MAT) accounts for up to 70% of bone marrow volume in healthy adults and increases further in clinical conditions of altered skeletal or metabolic function. Perhaps most strikingly, and in stark contrast to white adipose tissue, MAT has been found to increase during caloric restriction (CR) in humans and many other species. Hypoleptinemia may drive MAT expansion during CR but this has not been demonstrated conclusively. Indeed, MAT formation and function are poorly understood; hence, the physiological and pathological roles of MAT remain elusive. We recently revealed that MAT contributes to hyperadiponectinemia and systemic adaptations to CR. To further these observations, we have now performed CR studies in rabbits to determine whether CR affects adiponectin production by MAT. Moderate or extensive CR decreased bone mass, white adipose tissue mass, and circulating leptin but, surprisingly, did not cause hyperadiponectinemia or MAT expansion. Although this unexpected finding limited our subsequent MAT characterization, it demonstrates that during CR, bone loss can occur independently of MAT expansion; increased MAT may be required for hyperadiponectinemia; and hypoleptinemia is not sufficient for MAT expansion. We further investigated this relationship in mice. In females, CR increased MAT without decreasing circulating leptin, suggesting that hypoleptinemia is also not necessary for MAT expansion. Finally, circulating glucocorticoids increased during CR in mice but not rabbits, suggesting that glucocorticoids might drive MAT expansion during CR. These observations provide insights into the causes and consequences of CR-associated MAT expansion, knowledge with potential relevance to health and disease.

Changes in Skeletal Integrity and Marrow Adiposity during High-Fat Diet and after Weight Loss

The prevalence of obesity has continued to rise over the past three decades leading to significant increases in obesity-related medical care costs from metabolic and non-metabolic sequelae. It is now clear that expansion of body fat leads to an increase in inflammation with systemic effects on metabolism. In mouse models of diet-induced obesity, there is also an expansion of bone marrow adipocytes. However, the persistence of these changes after weight loss has not been well described. The objective of this study was to investigate the impact of high-fat diet (HFD) and subsequent weight loss on skeletal parameters in C57Bl6/J mice. Male mice were given a normal chow diet (ND) or 60% HFD at 6 weeks of age for 12, 16, or 20 weeks. A third group of mice was put on HFD for 12 weeks and then on ND for 8 weeks to mimic weight loss. After these dietary challenges, the tibia and femur were removed and analyzed by micro computed-tomography for bone morphology. Decalcification followed by osmium staining was used to assess bone marrow adiposity, and mechanical testing was performed to assess bone strength. After 12, 16, or 20 weeks of HFD, mice had significant weight gain relative to controls. Body mass returned to normal after weight loss. Marrow adipose tissue (MAT) volume in the tibia increased after 16 weeks of HFD and persisted in the 20-week HFD group. Weight loss prevented HFD-induced MAT expansion. Trabecular bone volume fraction, mineral content, and number were decreased after 12, 16, or 20 weeks of HFD, relative to ND controls, with only partial recovery after weight loss. Mechanical testing demonstrated decreased fracture resistance after 20 weeks of HFD. Loss of mechanical integrity did not recover after weight loss. Our study demonstrates that HFD causes long-term, persistent changes in bone quality, despite prevention of marrow adipose tissue accumulation, as demonstrated through changes in bone morphology and mechanical strength in a mouse model of diet-induced obesity and weight loss.

Hypothalamic Leptin Gene Therapy Reduces Bone Marrow Adiposity in ob/ob Mice Fed Regular and High-Fat Diets

Low bone mass is often associated with elevated bone marrow adiposity. Since osteoblasts and adipocytes are derived from the same mesenchymal stem cell (MSC) progenitor, adipocyte formation may increase at the expense of osteoblast formation. Leptin is an adipocyte-derived hormone known to regulate energy and bone metabolism. Leptin deficiency and high-fat diet-induced obesity are associated with increased marrow adipose tissue (MAT) and reduced bone formation. Short-duration studies suggest that leptin treatment reduces MAT and increases bone formation in leptin-deficient ob/ob mice fed a regular diet. Here, we determined the long-duration impact of increased hypothalamic leptin on marrow adipocytes and osteoblasts in ob/ob mice following recombinant adeno-associated virus (rAAV) gene therapy. Eight- to 10-week-old male ob/ob mice were randomized into four groups: (1) untreated, (2) rAAV-Lep, (3) rAAV-green fluorescent protein (rAAV-GFP), or (4) pair-fed to rAAV-Lep. For vector administration, mice were injected intracerebroventricularly with either rAAV-leptin gene therapy (rAAV-Lep) or rAAV-GFP (9 × 10(7) particles) and maintained for 30 weeks. In a second study, the impact of increased hypothalamic leptin levels on MAT was determined in mice fed high-fat diets; ob/ob mice were randomized into two groups and treated with either rAAV-Lep or rAAV-GFP. At 7 weeks post-vector administration, half the mice in each group were switched to a high-fat diet for 8 weeks. Wild-type (WT) controls included age-matched mice fed regular or high-fat diet. High-fat diet resulted in a threefold increase in MAT in WT mice, whereas MAT was increased by leptin deficiency up to 50-fold. Hypothalamic leptin gene therapy increased osteoblast perimeter and osteoclast perimeter with minor change in cancellous bone architecture. The gene therapy decreased MAT levels in ob/ob mice fed regular or high-fat diet to values similar to WT mice fed regular diet. These findings suggest that leptin plays an important role in regulating the differentiation of MSCs to adipocytes and osteoblasts, a process that may be dysregulated by high-fat diet. However, the results also illustrate that reducing MAT by increasing leptin levels does not necessarily result in increased bone mass.