Receptor for Advanced Glycation End Products-Mediated Signaling Impairs the Maintenance of Bone Marrow Mesenchymal Stromal Cells in Diabetic Model Mice

Effect of type 2 diabetes mellitus microenvironment on osteogenic capacity of bone marrow mesenchymal stem cells

Type 2 diabetes mellitus (T2DM) often leads to delayed bone regeneration such as slow healing of fractures and bone defects. The number, status and osteogenic differentiation capacity of bone marrow mesenchymal stem cells (BMSCs) are extremely important in bone healing and bone regeneration. The T2DM microenvironment can have irreversible negative effects on BMSCs. In this paper, we review the molecular expression and altered proliferation, migration, and osteogenic differentiation capacity of BMSCs in the microenvironment of T2DM, it provides a new perspective to restore the normal function of T2DM-BMSCs, so as to save the damaged bone regeneration capacity.

AMPK, a hub for the microenvironmental regulation of bone homeostasis and diseases

AMP-activated protein kinase (AMPK), a crucial regulatory kinase, monitors energy levels, conserving ATP and boosting synthesis in low-nutrition, low-energy states. Its sensitivity links microenvironmental changes to cellular responses. As the primary support structure and endocrine organ, the maintenance, and repair of bones are closely associated with the microenvironment. While a series of studies have explored the effects of specific microenvironments on bone, there is lack of angles to comprehensively evaluate the interactions between microenvironment and bone cells, especially for bone marrow mesenchymal stem cells (BMMSCs) which mediate the differentiation of osteogenic lineage. It is noteworthy that accumulating evidence has indicated that AMPK may serve as a hub between BMMSCs and microenvironment factors, thus providing a new perspective for us to understand the biology and pathophysiology of stem cells and bone. In this review, we emphasize AMPK's pivotal role in bone microenvironment modulation via ATP, inflammation, reactive oxygen species (ROS), calcium, and glucose, particularly in BMMSCs. We further explore the use of AMPK-activating drugs in the context of osteoarthritis and osteoporosis. Moreover, building upon the foundation of AMPK, we elucidate a viewpoint that facilitates a comprehensive understanding of the dynamic relationship between the microenvironment and bone homeostasis, offering valuable insights for prospective investigations into stem cell biology and the treatment of bone diseases.

Associations of marrow fat fraction with MR imaging based trabecular bone microarchitecture in first-time diagnosed type 1 diabetes mellitus

Purpose

To determine whether there are alterations in marrow fat content in individuals first-time diagnosed with type 1 diabetes mellitus (T1DM) and to explore the associations between marrow fat fraction and MRI-based findings in trabecular bone microarchitecture.

Method

A case-control study was conducted, involving adults with first-time diagnosed T1DM (n=35) and age- and sex-matched healthy adults (n=46). Dual-energy X-ray absorptiometry and 3 Tesla-MRI of the proximal tibia were performed to assess trabecular microarchitecture and vertebral marrow fat fraction. Multiple linear regression analysis was used to test the associations of marrow fat fraction with trabecular microarchitecture and bone density while adjusting for potential confounding factors.

Results

In individuals first-time diagnosed with T1DM, the marrow fat fraction was significantly higher (p < 0.001) compared to healthy controls. T1DM patients also exhibited higher trabecular separation [median (IQR): 2.19 (1.70, 2.68) vs 1.81 (1.62, 2.10), p < 0.001], lower trabecular volume [0.45 (0.30, 0.56) vs 0.53 (0.38, 0.60), p = 0.013], and lower trabecular number [0.37 (0.26, 0.44) vs 0.41 (0.32, 0.47), p = 0.020] compared to controls. However, bone density was similar between the two groups (p = 0.815). In individuals with T1DM, there was an inverse association between marrow fat fraction and trabecular volume (r = -0.69, p < 0.001) as well as trabecular number (r = -0.55, p < 0.001), and a positive association with trabecular separation (r = 0.75, p < 0.001). Marrow fat fraction was independently associated with total trabecular volume (standardized β = -0.21), trabecular number (β = -0.12), and trabecular separation (β = 0.57) of the proximal tibia after adjusting for various factors including age, gender, body mass index, physical activity, smoking status, alcohol consumption, blood glucose, plasma glycated hemoglobin, lipid profile, and bone turnover biomarkers.

Conclusions

Individuals first-time diagnosed with T1DM experience expansion of marrow adiposity, and elevated marrow fat content is associated with MRI-based trabecular microstructure.

High glucose microenvironment and human mesenchymal stem cell behavior

High glucose (HG) culture conditions in vitro and persistent exposure to hyperglycemia in diabetes patients are detrimental to stem cells, analogous to any other cell type in our body. It interferes with diverse signaling pathways, i.e. mammalian target of rapamycin (mTOR)-phosphoinositide 3-kinase (PI3K)-Akt signaling, to impact physiological cellular functions, leading to low cell survival and higher cell apoptosis rates. While elucidating the underlying mechanism responsible for the apoptosis of adipose tissue-derived mesenchymal stem cells (MSCs), a recent study has shown that HG culture conditions dysregulate mTOR-PI3K-Akt signaling in addition to mitochondrial malfunctioning due to defective mitochondrial membrane potential (MtMP) that lowers ATP production. This organelle-level dysfunction energy-starves the cells and increases oxidative stress and ultrastructural abnormalities. Disruption of the mitochondrial electron transport chain produces an altered mitochondrial NAD+/NADH redox state as evidenced by a low NAD+/NADH ratio that primarily contributes to the reduced cell survival in HG. Some previous studies have also reported altered mitochondrial membrane polarity (causing hyperpolarization) and reduced mitochondrial cell mass, leading to perturbed mitochondrial homeostasis. The hostile microenvironment created by HG exposure creates structural and functional changes in the mitochondria, altering their bioenergetics and reducing their capacity to produce ATP. These are significant data, as MSCs are extensively studied for tissue regeneration and restoring their normal functioning in cell-based therapy. Therefore, MSCs from hyperglycemic donors should be cautiously used in clinical settings for cell-based therapy due to concerns of their poor survival rates and increased rates of post engraftment proliferation. As hyperglycemia alters the bioenergetics of donor MSCs, rectifying the loss of MtMP may be an excellent target for future research to restore the normal functioning of MSCs in hyperglycemic patients.

Development and characterization of islet-derived mesenchymal stem cells from clinical grade neonatal porcine cryopreserved islets

BACKGROUND

Porcine tissues display a great potential as donor tissues in xenotransplantation, including cell therapy. Cryopreserving clinical grade porcine tissue and using it as a source for establishing therapeutic cells should be advantageous for transportation and scheduled manufacturing of MSCs. Of note, we previously performed encapsulated porcine islet transplantation for the treatment of unstable type 1 diabetes mellitus in the clinical setting. It has been reported that co-transplantation of islets and Mesenchymal stem cells (MSCs) enhanced efficacy. We assume that co-transplantation of porcine islets and porcine islet-derived MSCs could improve the efficacy of clinical islet xenotransplantation.

METHODS

MSCs were established from fresh and cryopreserved non-clinical grade neonatal porcine islets and bone marrow (termed non-clinical grade npISLET-MSCs and npBM-MSCs, respectively), as well as from cryopreserved clinical grade neonatal porcine islets (termed clinical grade npISLET-MSCs). Subsequently, the cell proliferation rate and diameter, surface marker expression, adipogenesis, osteogenesis, and colony-forming efficiency of the MSCs were assessed.

RESULTS

Cell proliferation rate and diameter did not differ between clinical grade and non-clinical grade npISLET-MSCs. However, non-clinical grade npBM-MSCs were significantly shorter and smaller than both npISLET-MSCs (p < 0.05). MSC markers (CD29, CD44, and CD90) were strongly expressed in clinical grade npISLET-MSCs and non-clinical grade npISLET-MSCs and npBM-MSCs. The expression of MSC-negative markers CD31, CD34, and SLA-DR was low in all MSCs. Clinical grade npISLET-MSCs derived from adipose and osteoid tissues were positive for Oil Red and alkaline phosphatase staining. The results of colony-forming assay were not significantly different between clinical grade npISLET-MSCs and non-clinical grade npBM-MSCs.

CONCLUSION

The method described herein was successful in of developing clinical grade npISLET-MSCs from cryopreserved islets. Cryopreserved clinical grade porcine islets could be an excellent stable source of MSCs for cell therapy.

Dynamics of Ex Vivo Mesenchymal Stromal Cell Potency under Continuous Perfusion

Mesenchymal stromal cells (MSCs) are a candidate for cell immunotherapy due to potent immunomodulatory activity found in their secretome. Though studies on their secreted substances have been reported, the time dynamics of MSC potency remain unclear. Herein, we report on the dynamics of MSC secretome potency in an ex vivo hollow fiber bioreactor using a continuous perfusion cell culture system that fractionated MSC-secreted factors over time. Time-resolved fractions of MSC-conditioned media were evaluated for potency by incubation with activated immune cells. Three studies were designed to characterize MSC potency under: (1) basal conditions, (2) in situ activation, and (3) pre-licensing. Results indicate that the MSC secretome is most potent in suppressing lymphocyte proliferation during the first 24 h and is further stabilized when MSCs are prelicensed with a cocktail of pro-inflammatory cytokines, IFNγ, TNFα, and IL-1β. The evaluation of temporal cell potency using this integrated bioreactor system can be useful in informing strategies to maximize MSC potency, minimize side effects, and allow greater control for the duration of ex vivo administration approaches.

Role of Advanced Glycation End-Products and Oxidative Stress in Type-2-Diabetes-Induced Bone Fragility and Implications on Fracture Risk Stratification

Type 2 diabetes (T2D) and osteoporosis (OP) are major causes of morbidity and mortality that have arelevant health and economic burden. Recent epidemiological evidence suggests that both of these disorders are often associated with each other and that T2D patients have an increased risk of fracture, making bone an additional target of diabetes. As occurs for other diabetic complications, the increased accumulation of advanced glycation end-products (AGEs) and oxidative stress represent the major mechanisms explaining bone fragility in T2D. Both of these conditions directly and indirectly (through the promotion of microvascular complications) impair the structural ductility of bone and negatively affect bone turnover, leading to impaired bone quality, rather than decreased bone density. This makes diabetes-induced bone fragility remarkably different from other forms of OP and represents a major challenge for fracture risk stratification, since either the measurement of BMD or the use of common diagnostic algorithms for OP have a poor predictive value. We review and discuss the role of AGEs and oxidative stress on the pathophysiology of bone fragility in T2D, providing some indications on how to improve fracture risk prediction in T2D patients.

Protective Effects of Resveratrol Against Adenomyosis in a Mouse Model

Adenomyosis is a uterine condition in which endometrial glands and stroma are commonly pathologically observed in the myometrium. In this study, we sought to determine the effect of resveratrol on the progression of adenomyosis. Adenomyosis was induced in mice given tamoxifen neonatally. All mice were subjected to body weight measurement and hotplate testing every four weeks beginning four weeks after birth. All mice with adenomyosis were randomly separated into 3 groups at 16 weeks: untreated, low-dose resveratrol (25 mg/kg), and high-dose resveratrol (50 mg/kg). After 3 weeks of treatment, final hotplate test and body weight measurement were performed, and the uterine horn blood samples were collected. Adenomyosis in mice caused body weight loss and uterine weight gain, reduced hotplate latency, and progression of endometrial fibrosis. The underlying biological process could be coupled with the overexpression of many cells' proliferation and immune-regulation-related genes. Resveratrol treatment could slow the progression of adenomyosis by enhancing hotplate latency, lowering endometrial fibrosis, and restoring cell proliferation- and immune-regulation-associated gene expression levels in endometrium and plasma. However, resveratrol treatment also reduced the body weight and uterine weight. In conclusion, Resveratrol might be a potential compound for treating patients with adenomyosis.

MicroRNA-139-5p mediates BMSCs impairment in diabetes by targeting HOXA9/c-Fos

The properties and functions of BMSCs were altered by the diabetic microenvironment, and its mechanism was not very clear. In recent years, the regulation of the function of BMSCs by microRNA has become a research hotspot, meanwhile, HOX genes also have been focused on and involved in multiple functions of stem cells. In this study, we investigated the role of miR-139-5p in diabetes-induced BMSC impairment. Since HOXA9 may be a target gene of miR-139-5p, we speculated that miR-139-5p/HOXA9 might be involved in regulating the biological characteristics and the function of BMSCs in diabetes. We demonstrated that the miR-139-5p expression was increased in BMSCs derived from STZ-induced diabetic rats. MiR-139-5p mimics were able to inhibit cell proliferation, and migration and promoted senescence and apoptosis in vitro. MiR-139-5p induced the down-regulated expression of HOXA9 and c-Fos in BMSCs derived from normal rats. Moreover, miR-139-5p inhibitors reversed the tendency in diabetic-derived BMSCs. Further, gain-and-loss function experiments indicated that miR-139-5p regulated the functions of BMSCs by targeting HOXA9 and c-Fos. In vivo wound model experiments showed that the downregulation of miR-139-5p further promoted the epithelialization and angiogenesis of diabetic BMSC-mediated skin. In conclusion, induction of miR-139-5p upregulation mediated the impairment of BMSCs through the HOXA9/c-Fos pathway in diabetic rats. Therefore, miR-139-5p/HOXA9 might be an important therapeutic target in treating diabetic BMSCs and diabetic complications in the future.

Methylglyoxal - an advanced glycation end products (AGEs) precursor - Inhibits differentiation of human MSC-derived osteoblasts in vitro independently of receptor for AGEs (RAGE)

A major precursor of advanced glycation end-products (AGEs) - methylglyoxal (MG) - is a reactive carbonyl metabolite that originates from glycolytic pathways. MG formation and accumulation has been implicated in the pathogenesis of diabetes and age-related chronic musculoskeletal disorders. Human bone marrow-derived stromal cells (BMSCs) are multipotent cells that have the potential to differentiate into cells of mesenchymal origin including osteoblasts, but the role of MG on their differentiation is unclear. We therefore evaluated the effect of MG on proliferation and differentiation of BMSC-derived osteoblasts. Cells were treated with different concentrations of MG (600, 800 and 1000 μM). Cell viability was assessed using a Cell Counting Kit-8 assay. Alkaline phosphatase (ALP) activity and calcium deposition assays were performed to evaluate osteoblast differentiation and mineralization. Gene expression was measured using qRT-PCR, whereas AGE specific receptor (RAGE) and collagen 1 were examined by immunocytochemistry and Western blotting. RAGE knockdown was performed by transducing RAGE specific short hairpin RNAs (shRNAs) using lentivirus. During osteogenic differentiation, MG treatment resulted in reduction of cell viability (27.7 %), ALP activity (45.5 %) and mineralization (82.3 %) compared to untreated cells. MG significantly decreased expression of genes involved in osteogenic differentiation - RUNX2 (2.8 fold), ALPL (3.2 fold), MG detoxification through glyoxalase - GLO1 (3 fold) and collagen metabolism - COL1A1 (4.9 fold), COL1A2 (6.8 fold), LOX (5.4 fold) and PLOD1 (1.7 fold). MG significantly reduced expression of collagen 1 (53.3 %) and RAGE (43.1 %) at protein levels. Co-treatment with a MG scavenger - aminoguanidine - prevented all negative effects of MG. RAGE-specific knockdown during MG treatment did not reverse the effects on cell viability, osteogenic differentiation or collagen metabolism. In conclusion, MG treatment can negatively influence the collagen metabolism and differentiation of BMSCs-derived osteoblasts through a RAGE independent mechanism.

Advanced Glycation End Products, Bone Health, and Diabetes Mellitus

Advanced glycation end products (AGEs), the compounds resulting from the non-enzymatic glycosylation between reducing sugars and proteins, are derived from food or produced de novo. Over time, more and more endogenous and exogenous AGEs accumulate in various organs such as the liver, kidneys, muscle, and bone, threatening human health. Among these organs, bone is most widely reported. AGEs accumulating in bone reduce bone strength by participating in bone structure formation and breaking bone homeostasis by binding their receptors to alter the proliferation, differentiation, and apoptosis of cells involved in bone remodeling. In this review, we summarize the research about the effects of AGEs on bone health and highlight their associations with bone health in diabetes patients to provide some clues toward the discovery of new treatment and prevention strategies for bone-related diseases caused by AGEs.

Crosstalk Between Senescent Bone Cells and the Bone Tissue Microenvironment Influences Bone Fragility During Chronological Age and in Diabetes

Bone is a complex organ serving roles in skeletal support and movement, and is a source of blood cells including adaptive and innate immune cells. Structural and functional integrity is maintained through a balance between bone synthesis and bone degradation, dependent in part on mechanical loading but also on signaling and influences of the tissue microenvironment. Bone structure and the extracellular bone milieu change with age, predisposing to osteoporosis and increased fracture risk, and this is exacerbated in patients with diabetes. Such changes can include loss of bone mineral density, deterioration in micro-architecture, as well as decreased bone flexibility, through alteration of proteinaceous bone support structures, and accumulation of senescent cells. Senescence is a state of proliferation arrest accompanied by marked morphological and metabolic changes. It is driven by cellular stress and serves an important acute tumor suppressive mechanism when followed by immune-mediated senescent cell clearance. However, aging and pathological conditions including diabetes are associated with accumulation of senescent cells that generate a pro-inflammatory and tissue-destructive secretome (the SASP). The SASP impinges on the tissue microenvironment with detrimental local and systemic consequences; senescent cells are thought to contribute to the multimorbidity associated with advanced chronological age. Here, we assess factors that promote bone fragility, in the context both of chronological aging and accelerated aging in progeroid syndromes and in diabetes, including senescence-dependent alterations in the bone tissue microenvironment, and glycation changes to the tissue microenvironment that stimulate RAGE signaling, a process that is accelerated in diabetic patients. Finally, we discuss therapeutic interventions targeting RAGE signaling and cell senescence that show promise in improving bone health in older people and those living with diabetes.

The Fate Status of Stem Cells in Diabetes and its Role in the Occurrence of Diabetic Complications

Diabetes mellitus (DM) is becoming a growing risk factor for public health worldwide. It is a very common disease and is widely known for its susceptibility to multiple complications which do great harm to the life and health of patients, some even lead to death. To date, there are many mechanisms for the complications of diabetes, including the generation of reactive oxygen species (ROS) and the abnormal changes of gas transmitters, which ultimately lead to injuries of cells, tissues and organs. Normally, even if injured, the body can quickly repair and maintain its homeostasis. This is closely associated with the repair and regeneration ability of stem cells. However, many studies have demonstrated that stem cells happen to be damaged under DM, which may be a nonnegligible factor in the occurrence and progression of diabetic complications. Therefore, this review summarizes how diabetes causes the corresponding complications by affecting stem cells from two aspects: stem cells dysfunctions and stem cells quantity alteration. In addition, since mesenchymal stem cells (MSCs), especially bone marrow mesenchymal stem cells (BMMSCs), have the advantages of strong differentiation ability, large quantity and wide application, we mainly focus on the impact of diabetes on them. The review also puts forward the basis of using exogenous stem cells to treat diabetic complications. It is hoped that through this review, researchers can have a clearer understanding of the roles of stem cells in diabetic complications, thus promoting the process of using stem cells to treat diabetic complications.

Comparison of concentrated fresh mononuclear cells and cultured mesenchymal stem cells from bone marrow for bone regeneration

Autologous bone marrow mononuclear cell (BMMNC) transplantation has been widely studied in recent years. The fresh cell cocktail in BMMNCs, without going through the in vitro culture process, helps to establish a stable microenvironment for osteogenesis, and each cell type may play a unique role in bone regeneration. Our study compared the efficacy of concentrated fresh BMMNCs and cultured bone marrow‐derived mesenchymal stem cells (BMSCs) in Beagle dogs for the first time. Fifteen‐millimeter segmental bone defects were created in the animals' tibia bones. In BMMNCs group, the defects were repaired with concentrated fresh BMMNCs combined with β‐TCP (n = 5); in cultured BMSC group, with in vitro cultured and osteo‐induced BMSCs combined with β‐TCP (n = 5); in scaffold‐only group, with a β‐TCP graft alone (n = 5); and in blank group, nothing was grafted (n = 3). The healing process was monitored by X‐rays and single photon emission computed tomography. The animals were sacrificed 12 months after surgery and their tibias were harvested and analyzed by microcomputed tomography and hard tissue histology. Moreover, the microstructure, chemical components, and microbiomechanical properties of the regenerated bone tissue were explored by multiphoton microscopy, Raman spectroscopy and nanoindentation. The results showed that BMMNCs group promoted much more bone regeneration than cultured BMSC group. The grafts in BMMNCs group were better mineralized, and they had collagen arrangement and microbiomechanical properties similar to the contralateral native tibia bone. These results indicate that concentrated fresh bone marrow mononuclear cells may be superior to in vitro expanded stem cells in segmental bone defect repair.

Bone marrow fat: friend or foe in people with diabetes mellitus?

Global trends in the prevalence of overweight and obesity put the adipocyte in the focus of huge medical interest. This review highlights a new topic in adipose tissue biology, namely the emerging pathogenic role of fat accumulation in bone marrow (BM). Specifically, we summarize current knowledge about the origin and function of BM adipose tissue (BMAT), provide evidence for the association of excess BMAT with diabetes and related cardiovascular complications, and discuss potential therapeutic approaches to correct BMAT dysfunction. There is still a significant uncertainty about the origins and function of BMAT, although several subpopulations of stromal cells have been suggested to have an adipogenic propensity. BM adipocytes are higly plastic and have a distinctive capacity to secrete adipokines that exert local and endocrine functions. BM adiposity is abundant in elderly people and has therefore been interpreted as a component of the whole-body ageing process. BM senescence and BMAT accumulation has been also reported in patients and animal models with Type 2 diabetes, being more pronounced in those with ischaemic complications. Understanding the mechanisms responsible for excess and altered function of BMAT could lead to new treatments able to preserve whole-body homeostasis.

Downregulated brain and muscle aryl hydrocarbon receptor nuclear translocator-like protein-1 inhibits osteogenesis of BMSCs through p53 in type 2 diabetes mellitus

The bone marrow mesenchymal stem cells (BMSCs)-mediated abnormal bone metabolism can delay and impair the bone remodeling process in type 2 diabetes mellitus (T2DM). Our previous study demonstrated that the downregulation of brain and muscle aryl hydrocarbon receptor nuclear translocator-like protein 1 (BMAL1), a circadian clock protein, inhibited the Wnt/β-catenin pathway via enhanced GSK-3β in diabetic BMSCs. In this article, we confirmed that the downregulated BMAL1 in T2DM played an inhibitory role in osteogenic differentiation of BMSCs. Upregulation of BMAL1 in the diabetic BMSCs significantly recovered the expression pattern of osteogenic marker genes and alkaline phosphatase (Alp) activity. We also observed an activation of the p53 signaling pathways, exhibited by increased p53 and p21 in diabetic BMSCs. Downregulation of p53 resulting from overexpression of BMAL1 was detected, and when we applied p53 gene silencing (shRNA) and the p53 inhibitor, pifithrin-α (PFT-α), the impaired osteogenic differentiation ability of diabetic BMSCs was greatly restored. However, there was no change in the level of expression of BMAL1. Taken together, our results first revealed that BMAL1 regulated osteogenesis of BMSCs through p53 in T2DM, providing a novel direction for further exploration of the mechanism underlying osteoporosis in diabetes.

[Effects of advanced glycation end products on osteoclasts at different stages of differentiation]

OBJECTIVE

To explore the effect of advanced glycation end products (AGEs) on osteoclasts at different stages of differentiation.

METHODS

Raw264.7 cells cultured in vitro were induced for osteoclastogenesis using RANKL, and the stages of differentiation of the osteoclasts were determined with TRAP staining. The cells were then randomly divided into control group, early-stage AGEs intervention group and late-stage AGEs intervention group. The viability of the cells after AGEs treatment was assessed using CCK-8 method. The cells were examined after the induction for osteoclastogenesis using TRAP staining, and the expression levels of RANK, NFATC-1, TRAF-6, TRAP and CTSK mRNAs were tested with RT-PCR; the expressions of CTSK and RANK proteins were detected using Western boltting.

RESULTS

We defined the initial 3 days of induction as the early stage of differentiation and the time beyond 3 days as the late stage of differentiation of Raw264.7 cells. Intervention with AGEs at 100 mg/L produced no significant effects on the viability of the cells, but AGEs suppressed the cell proliferation at a concentration exceeding 100 mg/L. The number of osteolasts in the early- and late-stage intervention groups was greater than that in the control group, but the cell count differed significantly only between the early-stage intervention group and control group (P < 0.05). The gene expressions of RANK, NFATC-1, TRAF-6, TRAP and CTSK all increased after the application of AGEs in both the early and late stages of differentiation, but the changes were significant only in the early-stage intervention group (P < 0.05). The changes in CTSK and RANK protein expressions were consistent with their mRNA expressions.

CONCLUSIONS

AGEs can affect the differentiation of osteoclasts differently when applied at different stages, and intervention with AGEs at the early stage produces stronger effect to promote osteoclast differentiation than its application at a late stage.

Insulin Signaling in Bone Marrow Adipocytes

PURPOSE OF REVIEW

The goal of this review is to discuss the role of insulin signaling in bone marrow adipocyte formation, metabolic function, and its contribution to cellular senescence in relation to metabolic bone diseases.

RECENT FINDINGS

Insulin signaling is an evolutionally conserved signaling pathway that plays a critical role in the regulation of metabolism and longevity. Bone is an insulin-responsive organ that plays a role in whole body energy metabolism. Metabolic disturbances associated with obesity and type 2 diabetes increase a risk of fragility fractures along with increased bone marrow adiposity. In obesity, there is impaired insulin signaling in peripheral tissues leading to insulin resistance. However, insulin signaling is maintained in bone marrow microenvironment leading to hypermetabolic state of bone marrow stromal (skeletal) stem cells associated with accelerated senescence and accumulation of bone marrow adipocytes in obesity. This review summarizes current findings on insulin signaling in bone marrow adipocytes and bone marrow stromal (skeletal) stem cells and its importance for bone and fat metabolism. Moreover, it points out to the existence of differences between bone marrow and peripheral fat metabolism which may be relevant for developing therapeutic strategies for treatment of metabolic bone diseases.

Advanced Glycation End Products (AGEs), Receptor for AGEs, Diabetes, and Bone: Review of the Literature

Diabetes compromises bone cell metabolism and function, resulting in increased risk of fragility fracture. Advanced glycation end products (AGEs) interact with the receptor for AGEs (RAGE) and can make a meaningful contribution to bone cell metabolism and/or alter function. Searches in PubMed using the key words "advanced glycation end-product," "RAGE," "sRAGE," "bone," and "diabetes" were made to explain some of the clinical outcomes of diabetes in bone metabolism through the AGE-RAGE signaling pathway. All published clinical studies were included in tables. The AGE-RAGE signaling pathway participates in diabetic complications, including diabetic osteopathy. Some clinical results in diabetic patients, such as reduced bone density, suppressed bone turnover markers, and bone quality impairment, could be potentially due to AGE-RAGE signaling consequences. However, the AGE-RAGE signaling pathway has some helpful roles in the bone, including an increase in osteogenic function. Soluble RAGE (sRAGE), as a ligand decoy, may increase in either conditions of RAGE production or destruction, and then it cannot always reflect the AGE-RAGE signaling. Recombinant sRAGE can block the AGE-RAGE signaling pathway but is associated with some limitations, such as accessibility to AGEs, an increase in other RAGE ligands, and a long half-life (24 hours), which is associated with losing the beneficial effect of AGE/RAGE. As a result, sRAGE is not a helpful marker to assess activity of the RAGE signaling pathway. The recombinant sRAGE cannot be translated into clinical practice due to its limitations.

The impact of forced swimming on expression of RANKL and OPG in a type 2 diabetes mellitus rat model

OBJECTIVE

The importance of swimming in bone metabolism during type 2 diabetes (T2DM) is not well known. Receptor activator of nuclear factor-kB ligand (RANKL)/ osteoprotegerin (OPG) system as a critical pathway in bone remodeling may play a role in pathogenesis of T2DM. Hence, we tested this pathway and the possible beneficial effects of swim training on T2DM.

MATERIALS AND METHODS

Forty male rats were assigned to groups (n = 10): control(C), diabetic (D), exercised control (E), and diabetic exercised (DE). One week after the induction of diabetes, animals were subjected to swim. At the end of training, fasting blood sugar, insulin, bone and serum OPG and RANKL levels were measured.

RESULTS

Diabetes significantly increased OPG and decreased RANKL mRNAs and proteins in bone and serum and swim training could reverse these changes to control.

CONCLUSION

Swim training could partially compensate T2DM associated changes of bone and serum OPG/RANKL in rats.

Cell Based Therapy for Type 1 Diabetes: Should We Take Hyperglycemia Into Account?

Diabetes mellitus is characterized by long standing hyperglycemia leading to numerous life-threatening complications. For type 1 diabetes mellitus, resulting from selective destruction of insulin producing cells by exaggerated immune reaction, the only effective therapy remains exogenous insulin administration. Despite accurate compliance to treatment of certain patients, transient episodes of hyperglycemia cannot be completely eliminated by this symptomatic treatment. Novel immunotherapeutic approaches based on tolerogenic dendritic cells, T regulatory cells and mesenchymal stem cells (MSCs) have been tested in clinical trials, endeavoring to directly modulate the autoimmune destruction process in pancreas. However, hyperglycemia itself affects the immune system and the final efficacy of cell-based immunotherapies could be affected by the different glycemic control of enrolled patients. The present review explores the impact of hyperglycemia on immune cells while providing greater insight into the molecular mechanisms of high glucose action and subsequent metabolic reprogramming of different immune cells. Furthermore, over-production of mitochondrial reactive oxygen species, formation of advanced glycation end products as a consequence of hyperglycemia and their downstream signalization in immune cells are also discussed. Since hyperglycemia in patients with type 1 diabetes mellitus might have an impact on immune-interventional treatment, the maintenance of a tight glucose control seems to be beneficial in patients considered for cell-based therapy.

RAGE Signaling in Skeletal Biology

PURPOSE OF REVIEW

The receptor for advanced glycation end products (RAGE) and several of its ligands have been implicated in the onset and progression of pathologies associated with aging, chronic inflammation, and cellular stress. In particular, the role of RAGE and its ligands in bone tissue during both physiological and pathological conditions has been investigated. However, the extent to which RAGE signaling regulates bone homeostasis and disease onset remains unclear. Further, RAGE effects in the different bone cells and whether these effects are cell-type specific is unknown. The objective of the current review is to describe the literature over RAGE signaling in skeletal biology as well as discuss the clinical potential of RAGE as a diagnostic and/or therapeutic target in bone disease.

RECENT FINDINGS

The role of RAGE and its ligands during skeletal homeostasis, tissue repair, and disease onset/progression is beginning to be uncovered. For example, detrimental effects of the RAGE ligands, advanced glycation end products (AGEs), have been identified for osteoblast viability/activity, while others have observed that low level AGE exposure stimulates osteoblast autophagy, which subsequently promotes viability and function. Similar findings have been reported with HMGB1, another RAGE ligand, in which high levels of the ligand are associated with osteoblast/osteocyte apoptosis, whereas low level/short-term administration stimulates osteoblast differentiation/bone formation and promotes fracture healing. Additionally, elevated levels of several RAGE ligands (AGEs, HMGB1, S100 proteins) induce osteoblast/osteocyte apoptosis and stimulate cytokine production, which is associated with increased osteoclast differentiation/activity. Conversely, direct RAGE-ligand exposure in osteoclasts may have inhibitory effects. These observations support a conclusion that elevated bone resorption observed in conditions of high circulating ligands and RAGE expression are due to actions on osteoblasts/osteocytes rather than direct actions on osteoclasts, although additional work is required to substantiate the observations. Recent studies have demonstrated that RAGE and its ligands play an important physiological role in the regulation of skeletal development, homeostasis, and repair/regeneration. Conversely, elevated levels of RAGE and its ligands are clearly related with various diseases associated with increased bone loss and fragility. However, despite the recent advancements in the field, many questions regarding RAGE and its ligands in skeletal biology remain unanswered.

Mesenchymal stem cell dysfunction in diabetes

Diabetes mellitus (DM) is a chronic disease that results in a variety of systemic complications. Recently, stem cell-based therapies have been proposed as potential modalities to manage DM related complications. Mesenchymal stem cell (MSC) based therapies are often considered as an ideal stem cell-based treatment for DM management due to their immunosuppressive characteristics, anti-inflammatory properties and differentiation potential. While MSCs show tremendous promise, the underlying functional deficits of MSCs in DM patients is not well understood. Using the MEDLINE database to define these functional deficits, our search yielded 1826 articles of which 33 met our inclusion criteria. This allowed us to review the topic and illuminate four major molecular categories by which MSCs are compromised in both Type 1 DM and Type II DM models which include: (1) changes in angiogenesis/vasculogenesis, (2) altered pro-inflammatory cytokine secretion, (3) increased oxidative stress markers and (4) impaired cellular differentiation and decreased proliferation. Knowledge of the deficits in MSC function will allow us to more clearly assess the efficacy of potential biologic therapies for reversing these dysfunctions when treating the complications of diabetic disease.

Guiding the Differentiation Direction of Pancreatic Islet-Derived Stem Cells by Glycated Collagen

The microenvironment is an important factor of stem cells regulating their maintenance, survival, and differentiation. The glycation of proteins with reducing sugars through nonenzymatic reactions induces the collagen cross-linking, which causes tissue stiffening, which is enhanced during aging and diabetes. In this study, we aimed to analyze the effect of glycated collagen on the stem cell culture and differentiation. The collagen type 1 was modified by glycation with mannose, rhamnose, arabinose, and glucose. After the culture of mesenchymal stem cells on the coated surfaces with glycated collagen, the differences in cell adhesion, proliferation, and differentiation were compared. The results showed that the modifications did not induce apoptosis or cause cell death. However, the culture of cells on modified collagens improved the proliferation. It was found that the mannose-modified collagen stimulated the adipogenic differentiation of stem cells, and rhamnose-modified collagen supports the differentiation into both osteogenic and insulin-producing cells. The low concentration of monosaccharides during glycation process improved the characteristics of the matrix protein in favor of stem cell differentiation. Modification of the collagen by glycation might be used as a tool to improve natural polymers for material-induced stem cell differentiation in the future.

Mesenchymal Stem Cell-Based Immunomodulation: Properties and Clinical Application

Mesenchymal stem cells (MSCs) are multipotent stem cells characterized by self-renewal, production of clonal cell populations, and multilineage differentiation. They exist in nearly all tissues and play a significant role in tissue repair and regeneration. Additionally, MSCs possess wide immunoregulatory properties via interaction with immune cells in both innate and adaptive immune systems, leading to immunosuppression of various effector functions. Numerous bioactive molecules secreted by MSCs, particularly cytokines, growth factors, and chemokines, exert autocrine/paracrine effects that modulate the physiological processes of MSCs. These invaluable virtues of MSCs provide new insight into potential treatments for tissue damage and inflammation. In particular, their extensive immunosuppressive properties are being explored for promising therapeutic application in immune disorders. Recently, clinical trials for MSC-mediated therapies have rapidly developed for immune-related diseases following reports from preclinical studies declaring their therapeutic safety and efficacy. Though immunotherapy of MSCs remains controversial, these clinical trials pave the way for their widespread therapeutic application in immune-based diseases. In this review, we will summarize and update the latest research findings and clinical trials on MSC-based immunomodulation.

Identification of megakaryocytes as a target of advanced glycation end products in diabetic complications in bone marrow

AIMS

To define the possible effect of diabetic conditions on megakaryocytes, the long-know precursors of platelets and lately characterized modulator of hematopoietic stem quiescence-activation transition.

METHODS

Megakaryoblastic MEG-01 cell culture and TPO/SCF/IL-3-induced differentiation of human umbilical blood mononuclear cells toward megakaryocytes were used to test effects of glycated bovine serum albumin (BSA-AGEs). The ob/ob mice and streptozotocin-treated mice were used as models of hyperglycemia. MTT was used to measure cell proliferation, FACS for surface marker and cell cycle, and RT-qPCR for the expression of interested genes. Megakaryocytes at different stages in marrow smear were checked under microscope.

RESULTS

When added in MEG-01 cultures at 200 μg/ml, BSA-AGEs increased proliferation of cells and enhanced mRNA expression of RAGE, VEGFα and PF4 in the cells. None of cell cycle distribution, PMA-induced platelet-like particles production, expression of GATA1/NF-E2/PU-1/IL-6/OPG/PDGF in MEG-01 cells nor TPO/SCF/IL-3 induced umbilical cord blood cells differentiation into megakaryocyte was affected by BSA-AGEs. In the ob/ob diabetic mice, MKs percentages in marrow cells and platelets in peripheral blood were significantly increased compared with control mice. In streptozotocin-induced diabetic mice, however, MKs percentage in marrow cells was decreased though peripheral platelet counts were not altered. Gene expression assay showed that the change in MKs in these two diabetic conditions might be explained by the alteration of GATA1 and NF-E2 expression, respectively.

CONCLUSIONS

Diabetic condition in animals might exert its influence on hematopoiesis via megakaryocytes-the newly identified modulator of hematopoietic stem cells in bone marrow.

Accumulation of carboxymethyl-lysine (CML) in human cortical bone

Advanced glycation end-products (AGEs) are a category of post translational modification associated with the degradation of the structural properties of multiple different types of tissues. Typically, AGEs are the result of a series of post-translational modification reactions between sugars and proteins through a process known as non-enzymatic glycation (NEG). Increases in the rate of NEG of bone tissue are associated with type 2 diabetes and skeletal fragility. Current methods of assessing NEG and its impact on bone fracture risk involve measurement of pentosidine or total fluorescent AGEs (fAGEs). However, pentosidine represents only a small fraction of possible fAGEs present in bone, and neither pentosidine nor total fAGE measurement accounts for non-fluorescent AGEs, which are known to form in significant amounts in skin and other collagenous tissues. Carboxymethyl-lysine (CML) is a non-fluorescent AGE that is often measured and has been shown to accumulate in tissues such as skin, heart, arteries, and intervertebral disks, but is currently not assessed in bone. Here we show the localization of CML to collagen I using mass spectrometry for the first time in human bone. We then present a new method using demineralization followed by heating and trypsin digestion to measure CML content in human bone and demonstrate that CML in bone is 40-100 times greater than pentosidine (the current most commonly used marker of AGEs in bone). We then establish the viability of CML as a measurable AGE in bone by showing that levels of CML, obtained from bone using this technique, increase with age (p<0.05) and are correlated with previously reported measures of bone toughness. Thus, CML is a viable non-fluorescent AGE target to assess AGE accumulation and fragility in bone. The method developed here to extract and measure CML from human bone could facilitate the development of a new diagnostic assay to evaluate fracture risk and potentially lead to new therapeutic approaches to address bone fragility.

The differentiation of mesenchymal stem cells to vascular cells regulated by the HMGB1/RAGE axis: its application in cell therapy for transplant arteriosclerosis

BACKGROUND

Mesenchymal stem cell (MSC) transplantation shows promise for treating transplant arteriosclerosis, at least partly via promoting endothelial regeneration. However, the efficacy and safety are still under investigation especially regarding recent findings that neointimal smooth muscle cells are derived from MSC-like cells. The high mobility group box 1 (HMGB1)/receptor for advanced glycation end-product (RAGE) axis is involved in regulating proliferation, migration, and differentiation of MSCs, and therefore it can be presumably applied to improve the outcome of cell therapy. The aim of the current study was to investigate this hypothesis.

METHODS

Rat MSCs were treated with HMGB1 or modified with HMGB1 vectors to activate the HMGB1/RAGE axis. RAGE was targeted and inhibited by specific short hairpin RNA vectors. We assessed the capacity for cell proliferation, migration, and differentiation after vector transfection in vitro and in a rat model of transplant arteriosclerosis. The expression of CD31 and α-smooth muscle actin (αSMA) was determined to evaluate the differentiation of MSCs to endothelial cells and smooth muscle cells.

RESULTS

Exogenous HMGB1 treatment and transfection with HMGB1 vectors promoted MSC migration and vascular endothelial growth factor (VEGF)-induced differentiation to CD31+ cells while inhibiting their proliferation and platelet-derived growth factor (PDGF)-induced differentiation to αSMA+ cells. Such an effect was blocked by RAGE knockdown. HMGB1-modified cells preferably migrated to graft neointima and differentiated to CD31+ cells along with significant relief of transplant arteriosclerosis and inhibition of HMGB1 and RAGE expression in graft vessels. RAGE knockdown inhibited cell migration to graft vessels.

CONCLUSIONS

HMGB1 stimulated MSCs to migrate and differentiate to endothelial cells via RAGE signaling, which we translated to successful application in cell therapy for transplant arteriosclerosis.

Mesenchymal stromal/stem cells as potential therapy in diabetic retinopathy

Diabetic retinopathy (DR) is a multifactorial microvascular disease induced by hyperglycemia and subsequent metabolic abnormalities. The resulting cell stress causes a sequela of events that ultimately can lead to severe vision impairment and blindness. The early stages are characterized by activation of glia and loss of pericytes, endothelial cells (EC) and neuronal cells. The integrity of the retinal microvasculature becomes affected, and, as a possible late response, macular edema may develop as a common reason for vision loss in patients with non-proliferative DR. Moreover, the local ischemia can trigger vasoproliferation leading to vision-threating proliferative DR (PDR) in humans. Available treatment options include control of metabolic and hemodynamic factors. Timely intervention of advanced DR stages with laser photocoagulation, intraocular anti-vascular endothelial growth factor (VEGF) or glucocorticoid drugs can reduce vision loss. As the pathology involves cell loss of both the vascular and neuroglial compartments, cell replacement strategies by stem and progenitor cells have gained considerable interest in the past years. Compared to other disease entities, so far little is known about the efficacy and potential mode of action of cell therapy in treatment of DR. In preclinical models of DR different cell types have been applied ranging from embryonic or induced pluripotent stem cells, hematopoietic stem cells, and endothelial progenitor cells to mesenchymal stromal cells (MSC). The latter cell population can combine various modes of action (MoA), thus they are among the most intensely tested cell types in cell therapy. The aim of this review is to discuss the rationale for using MSC as potential cell therapy to treat DR. Accordingly, we will revise identified MoA of MSCs and speculate how these may support the repair of the damaged retina.

Advanced glycation end products dietary restriction effects on bacterial gut microbiota in peritoneal dialysis patients; a randomized open label controlled trial

The modern Western diet is rich in advanced glycation end products (AGEs). We have previously shown an association between dietary AGEs and markers of inflammation and oxidative stress in a population of end stage renal disease (ESRD) patients undergoing peritoneal dialysis (PD). In the current pilot study we explored the effects of dietary AGEs on the gut bacterial microbiota composition in similar patients. AGEs play an important role in the development and progression of cardiovascular (CVD) disease. Plasma concentrations of different bacterial products have been shown to predict the risk of incident major adverse CVD events independently of traditional CVD risk factors, and experimental animal models indicates a possible role AGEs might have on the gut microbiota population. In this pilot randomized open label controlled trial, twenty PD patients habitually consuming a high AGE diet were recruited and randomized into either continuing the same diet (HAGE, n = 10) or a one-month dietary AGE restriction (LAGE, n = 10). Blood and stool samples were collected at baseline and after intervention. Variable regions V3-V4 of 16s rDNA were sequenced and taxa was identified on the phyla, genus, and species levels. Dietary AGE restriction resulted in a significant decrease in serum N^ε-(carboxymethyl) lysine (CML) and methylglyoxal-derivatives (MG). At baseline, our total cohort exhibited a lower relative abundance of Bacteroides and Alistipes genus and a higher abundance of Prevotella genus when compared to the published data of healthy population. Dietary AGE restriction altered the bacterial gut microbiota with a significant reduction in Prevotella copri and Bifidobacterium animalis relative abundance and increased Alistipes indistinctus, Clostridium citroniae, Clostridium hathewayi, and Ruminococcus gauvreauii relative abundance. We show in this pilot study significant microbiota differences in peritoneal dialysis patients’ population, as well as the effects of dietary AGEs on gut microbiota, which might play a role in the increased cardiovascular events in this population and warrants further studies.

Diabetes, bone and glucose-lowering agents: basic biology

Skeletal fragility often accompanies diabetes and does not appear to correlate with low bone mass or trauma severity in individuals with diabetes. Instead (and in contrast to those with osteoporotic bone disease), bone remodelling and bone turnover are compromised in both type 1 and type 2 diabetes, contributing to defective bone material quality. This review is one of a pair discussing the relationship between diabetes, bone and glucose-lowering agents; an accompanying review is provided in this issue of Diabetologia by Ann Schwartz (DOI: 10.1007/s00125-017-4283-6 ). This review presents basic science evidence that, alongside other organs, bone is affected in diabetes via impairments in glucose metabolism, toxic effects of glucose oxidative derivatives (advance glycation end-products [AGEs]), and via impairments in bone microvascular function and muscle endocrine function. The cellular and molecular basis for the effects of diabetes on bone are discussed, as is the impact of diabetes on the stem cell niche and fracture healing. Furthermore, the safety of clinically approved glucose-lowering therapies and the possibility of developing a single therapy that would be beneficial for both insulin sensitisation and diabetes bone syndrome are outlined.

Brain and muscle aryl hydrocarbon receptor nuclear translocator-like protein-1 cooperates with glycogen synthase kinase-3β to regulate osteogenesis of bone-marrow mesenchymal stem cells in type 2 diabetes

Type 2 diabetes mellitus (T2DM) is associated with inhibited osteogenesis of bone marrow mesenchymal stem cells (BMSCs). Brain and muscle ARNT-like protein 1 (BMAL1) has been linked to the T2DM-related bone remodeling, however, the specific mechanism is still unclear. Herein, we aimed to determine the role of BMAL1 in T2DM-induced suppression of BMSCs osteogenesis. Inhibited osteogenesis and BMAL1 expression were showed in diabetic BMSCs. And while β-catenin and T cell factor (TCF) expression were decreased, the glycogen synthase kinase-3β (GSK-3β) and nemo-like kinase (NLK) expression were increased in diabetic BMSCs. Moreover, over-expression of BMAL1 led to recovered osteogenesis ability and activation of Wnt/β-catenin pathway, which was partially due to inhibition of GSK-3β caused by over-expression of BMAL1. Taken together, our findings provide new insights into the role of BMAL1 in T2DM-induced suppression of BMSCs osteogenesis. Over-expressed BMAL1 could recover BMSCs osteogenesis in T2DM partially by decreasing GSK-3β expression to activate Wnt/β-catenin pathway. BMAL1 may have a potential use in repairing diabetic bone metabolic disorders.