Malic Enzyme Couples Mitochondria with Aerobic Glycolysis in Osteoblasts

Bidirectional modulation of glycolysis using a multifunctional nanocomposite hydrogel promotes bone fracture healing in type 2 diabetes mellitus

Fracture healing in patients with type 2 diabetes mellitus (T2D) is markedly impaired, characterized by a prolonged inflammation phase and defective osteoblast differentiation at the fracture site. In this study, we identified aberrant cellular glycolysis at T2D fracture sites, with bone marrow mesenchymal stem cells (BMSCs) exhibiting suppressed glycolysis and macrophages displaying enhanced glycolysis, mediated by the dysregulation of hypoxia-inducible factor-1α (HIF-1α). To rectify these metabolic imbalances, we developed a multifunctional nanocomposite PN@MHV hydrogel. Myricitrin, a flavonoid glycoside, forms the MHV hydrogel by cross-linking with HA-PBA and PVA via hydrogen bonds, and upregulates glycolysis through HIF-1α, thus promoting osteoblast differentiation under high glucose environment. To further regulate the inflammatory microenvironment, we incorporated nanoparticles loaded with PX-478, a HIF-1α specific inhibitor, into the hydrogel, with folic acid covalently modified to target proinflammatory M1 macrophages. This PN@MHV hydrogel bidirectionally regulated glycolysis via HIF-1α, enhancing osteoblast differentiation while attenuating macrophage-mediated inflammation. Comprehensive in vitro and in vivo experiments in a T2D fracture mouse model confirmed the hydrogel's ability to improve the inflammatory microenvironment and accelerate bone healing. Our findings underscore the therapeutic potential of targeting cellular glycolysis as a promising approach for enhancing fracture healing in diabetic patients.

Organelle-oriented nanomedicines in tumor therapy: Targeting, escaping, or collaborating?

Precise tumor therapy is essential for improving treatment specificity, enhancing efficacy, and minimizing side effects. Targeting organelles is a key strategy for achieving this goal and is a frontier research area attracting a considerable amount of attention. The concept of organelle targeting has a significant effect on the structural design of the nanodrugs employed. Most notably, the intricate interactions among different organelles in a tumor cell essentially create a unified system. Unfortunately, this aspect might have been somewhat overlooked when existing organelle-targeting nanodrugs were designed. In this review, we underscore the synergistic relationship among the various organelles and advocate for a holistic view of organelle-targeting design. Through the integration of biology and material science, recent advancements in organelle targeting, escaping, and collaborating are consolidated to offer fresh perspectives for the development of antitumor nanomedicines.

The role of RAB12 in inhibiting osteogenic differentiation and driving metabolic dysregulation in osteoporosis

AIMS

The osteogenic differentiation of mesenchymal stem cells (MSCs) is crucial in osteoporosis, and the metabolic level of the bone microenvironment directly affects metabolic dysregulation in postmenopausal women. RAB12 is a member of the small GTPase Rab family proteins, known to play an important role in autophagy. However, the role of RAB12 in the osteogenic differentiation of osteoporotic hMSCs remains unclear.

MATERIALS AND METHOD

Immunohistochemical staining was used to validate the high expression of RAB12 in aged osteoporotic mouse models and ovariectomized (OVX) mouse models. Co-immunoprecipitation (Co-IP) and LC-MS/MS were employed to explore downstream proteins that may interact with RAB12. Adenovirus containing RAB12 siRNA sequences was injected into the tail vein of OVX osteoporotic mice to analyze the impact of the RAB12/PCBP1/GLUT1 axis on MSC osteogenic differentiation.

KEY FINDINGS

We found that RAB12 expression is upregulated in elderly osteoporotic patients and in osteoporotic mouse models. RAB12 negatively regulates the osteogenic differentiation of hMSCs both in vivo and in vitro. RAB12 interacts with the PCBP1 protein, affecting its autophagic degradation when its expression levels change. RAB12 regulates the transcriptional level of GLUT1 by influencing the autophagic degradation of PCBP1, thereby affecting MSC's regulation of glucose uptake, which in turn impacts MSC osteogenic differentiation and metabolic changes.

SIGNIFICANCE

RAB12 negatively regulates osteogenic differentiation through the PCBP1/GLUT1 axis, affecting glucose metabolism levels in the bone microenvironment. RAB12 may serve as a potential target for the treatment of osteoporosis and postmenopausal metabolic dysregulation.

Bone and muscle crosstalk in ageing and disease

Interorgan communication between bone and skeletal muscle is central to human health. A dysregulation of bone-muscle crosstalk is implicated in several age-related diseases. Ageing-associated changes in endocrine, inflammatory, nutritional and biomechanical stimuli can influence the differentiation capacity, function and survival of mesenchymal stem cells and bone-forming and muscle-forming cells. Consequently, the secretome phenotype of bone and muscle cells is altered, leading to impaired crosstalk and, ultimately, catabolism of both tissues. Adipose tissue acts as a third player in the bone-muscle interaction by secreting factors that affect bone and muscle cells. Physical exercise remains the key biological stimulus for bone-muscle crosstalk, either directly via the release of cytokines from bone, muscle or adipocytes, or indirectly through extracellular vesicles. Overall, bone-muscle crosstalk is considered an inherent process necessary to maintain the structure and function of both tissues across the life cycle. This Review summarizes the latest biomedical advances in bone-muscle crosstalk as it pertains to human ageing and disease. We also outline future research priorities to accommodate the understanding of this rapidly emerging field.

Uncovering mechanisms of thiazolidinediones on osteogenesis and adipogenesis using spatial fluxomics

OBJECTIVE

Insulin-sensitizing drugs, despite their broad use against type 2 diabetes, can adversely affect bone health, and the mechanisms underlying these side effects remain largely unclear. Here, we investigated the different metabolic effects of a series of thiazolidinediones, including rosiglitazone, pioglitazone, and the second-generation compound MSDC-0602K, on human mesenchymal stem cells (MSCs).

METHODS

We developed 13C subcellular metabolomic tracer analysis measuring separate mitochondrial and cytosolic metabolite pools, lipidomic network-based isotopologue models, and bioorthogonal click chemistry, to demonstrate that MSDC-0602K differentially affected bone marrow-derived MSCs (BM-MSCs) and adipose tissue-derived MSCs (AT-MSCs). In BM-MSCs, MSDC-0602K promoted osteoblastic differentiation and suppressed adipogenesis. This effect was clearly distinct from that of the earlier drugs and that on AT-MSCs.

RESULTS

Fluxomic data reveal unexpected differences between this drug's effect on MSCs and provide mechanistic insight into the pharmacologic inhibition of mitochondrial pyruvate carrier 1 (MPC). Our study demonstrates that MSDC-0602K retains the capacity to inhibit MPC, akin to rosiglitazone but unlike pioglitazone, enabling the utilization of alternative metabolic pathways. Notably, MSDC-0602K exhibits a limited lipogenic potential compared to both rosiglitazone and pioglitazone, each of which employs a distinct lipogenic strategy.

CONCLUSIONS

These findings indicate that the new-generation drugs do not compromise bone structure, offering a safer alternative for treating insulin resistance. Moreover, these results highlight the ability of cell compartment-specific metabolite labeling by click reactions and tracer metabolomics analysis of complex lipids to discover molecular mechanisms within the intersection of carbohydrate and lipid metabolism.

Unraveling the dynamics of osteoblast differentiation in MC3T3-E1 cells: Transcriptomic insights into matrix mineralization and cell proliferation

Unraveling the intricacies of osteoblast differentiation is crucial for advancing our comprehension of bone biology. This study investigated the complicated molecular events orchestrating osteoblast differentiation in MC3T3-E1 cells, a well-established in vitro culture model. Employing longitudinal RNA-sequencing analysis, we explored transcriptomic changes at the pivotal time points of 0, 1, 4, 7, 10, 14, and 21 days and categorized osteogenic differentiation into proliferation, matrix maturation, and mineralization stages. Notably, we observed a simultaneous increase in matrix mineralization and cell proliferation during the mineralization stage, accompanied by a positive correlation between proliferation-associated genes and those enriched in ossification. Additionally, we identified the presence of proliferating cells over the mineralizing matrix layers. These results could serve as a model for understanding the principles by which bone lining cells are formed on the calcified bone matrix and the mechanism by which new osteoblasts are recruited during the bone remodeling process.

Metabolic Reprogramming in Stromal and Immune Cells in Rheumatoid Arthritis and Osteoarthritis: Therapeutic Possibilities

Metabolic reprogramming of stromal cells, including fibroblast-like synoviocytes (FLS) and chondrocytes, as well as osteoclasts (OCs), are involved in the inflammatory and degenerative processes underlying rheumatoid arthritis (RA) and osteoarthritis (OA). In RA, FLS exhibit mTOR activation, enhanced glycolysis and reduced oxidative phosphorylation, fuelling inflammation, angiogenesis, and cartilage degradation. In OA, chondrocytes undergo metabolic rewiring, characterised by mTOR and NF-κB activation, mitochondrial dysfunction, and increased glycolysis, which promotes matrix metalloproteinase production, extracellular matrix (ECM) degradation, and angiogenesis. Macrophage-derived immunometabolites, including succinate and itaconate further modulate stromal cell function, acting as signalling molecules that modulate inflammatory and catabolic processes. Succinate promotes inflammation whilst itaconate is anti-inflammatory, suppressing inflammatory joint disease in models. Itaconate deficiency also correlates inversely with disease severity in RA in humans. Emerging evidence highlights the potential of targeting metabolic processes as promising therapeutic strategies for connective tissue disorders.

Interruption of mitochondrial symbiosis is associated with the development of osteoporosis

Mitochondria maintain bacterial traits because of their endosymbiotic origins, yet the host cell recognizes them as non-threatening since the organelles are compartmentalized. Nevertheless, the controlled release of mitochondrial components into the cytoplasm can initiate cell death, activate innate immunity, and provoke inflammation. This selective interruption of endosymbiosis as early as 2 billion years ago allowed mitochondria to become intracellular signaling hubs. Recent studies have found that the interruption of mitochondrial symbiosis may be closely related to the occurrence of various diseases, especially osteoporosis (OP). OP is a systemic bone disease characterized by reduced bone mass, impaired bone microstructure, elevated bone fragility, and susceptibility to fracture. The interruption of intra-mitochondrial symbiosis affects the energy metabolism of bone cells, leads to the imbalance of bone formation and bone absorption, and promotes the occurrence of osteoporosis. In this paper, we reviewed the mechanism of mitochondrial intersymbiosis interruption in OP, discussed the relationship between mitochondrial intersymbiosis interruption and bone marrow mesenchymal stem cells, osteoblasts and osteoclasts, as well as the inheritance and adaptation in the evolutionary process, and prospected the future research direction to provide new ideas for clinical treatment.

The inhibition of PINK1/Drp1-mediated mitophagy by hyperglycemia leads to impaired osteoblastogenesis in diabetes

Impaired bone quality and increased fracture risk are cardinal features of the skeleton in diabetes mellitus. Hyperglycemia-induced oxidative stress is proposed as a potential underlying mechanism, but the precise pathogenic mechanism remains incompletely understood. In this investigation, osteoblasts under high glucose exhibited heightened levels of reactive oxygen species, impaired mitochondrial membrane potential, and profound inhibition of late-stage osteoblast differentiation. Further analyses uncovered that high glucose resulted in the downregulation of the PINK1/Drp1 pathway in osteoblasts, consequently leading to impaired mitophagy. Conversely, the upregulation of PINK1/Drp1 pathway activated mitophagy, which restored the differentiation capacity of osteoblasts. Notably, in an STZ-induced diabetic mouse model, BMP9 upregulated the expression of PINK1/Drp1 in the bone tissue, leading to an improvement in bone quality and bone mineral density. These findings suggest that the PINK1/Drp1 pathway might be a potential therapeutic target to enhance osteogenic differentiation and treat diabetic osteoporosis.

Tgfβ signaling stimulates glycolysis to promote the genesis of synovial joint interzone in developing mouse embryonic limbs

The initial interzone cells for synovial joints originate from chondrocytes, but such critical transition is minimally understood. With single-cell RNA sequencing (scRNA-seq) of murine embryonic knee joint primordia, we discovered that heightened expression of glycolysis genes characterized developing interzone cells when compared to flanking chondrocytes. Conditional deletion of the glucose transporters Glut1 and/or Glut3, in either the incipient pre-skeletal mesenchyme with Prx1Cre or in chondrocytes with Col2Cre, disrupted interzone formation dose-dependently. In contrast, deletion of Glut1/3 in established interzone cells with Gdf5Cre did not have similar severe disruption of joint development. scRNA-seq revealed that Glut1/3 deletion by Prx1Cre impeded Tgfβ signaling in the developing interzone cells. Direct elimination of Tgfβ signaling with Prx1Cre partially phenocopied the deletion of Glut1/3 in impairing interzone formation. Tgfβ stimulated glycolysis in chondrocytes via activation of mTOR and Hif1α in vitro. The data support that the essential conversion of chondrocytes to interzone cells requires a transient elevation of glycolysis partly dependent on Tgfβ signaling.

Metabolomic characterization of MC3T3-E1pre-osteoblast differentiation induced by ipriflavone-loaded mesoporous nanospheres

This study reports on the metabolic changes accompanying the differentiation of MC3T3-E1 osteoprogenitor cells induced by mesoporous bioactive glass nanospheres (nMBG) loaded with ipriflavone (nMBG-IP). Ipriflavone (IP) is a synthetic isoflavone known for inhibiting bone resorption, maintaining bone density, and preventing osteoporosis. Delivering IP intracellularly is a promising strategy to modulate bone remodeling at significantly lower doses compared to free drug administration. Our results demonstrate that nMBG are efficiently internalized by pre-osteoblasts and, when loaded with IP, induce their differentiation. This differentiation process is accompanied by pronounced metabolic alterations, as monitored by NMR analysis of medium supernatants and cell extracts (exo- and endo-metabolomics, respectively). The main effects include an early-stage intensification of glycolysis and changes in several metabolic pathways, such as nucleobase metabolism, osmoregulatory and antioxidant pathways, and lipid metabolism. Notably, the metabolic impacts of nMBG-IP and free IP were very similar, whereas nMBG alone induced only mild changes in the intracellular metabolic profile without affecting the cells' consumption/secretion patterns or lipid composition. This finding indicates that the observed effects are primarily related to IP-induced differentiation and that nMBG nanospheres serve as convenient carriers with both efficient internalization and minimal metabolic impact. Furthermore, the observed link between pre-osteoblast differentiation and metabolism underscores the potential of utilizing metabolites and metabolic reprogramming as strategies to modulate the osteogenic process, for instance, in the context of osteoporosis and other bone diseases.

Energy metabolism in osteoprogenitors and osteoblasts: Role of the pentose phosphate pathway

Bioenergetic preferences of osteolineage cells, including osteoprogenitors and osteoblasts (OBs), are a matter of intense debate. Early studies pointed to OB reliance on glucose and aerobic glycolysis while more recent works indicated the importance of glutamine as a mitochondrial fuel. Aiming to clarify this issue, we performed metabolic tracing of 13C-labeled glucose and glutamine in human osteolineage cells: bone marrow stromal (a.k.a. mesenchymal stem) cells and bone marrow stromal cell-derived OBs. Glucose tracing showed noncanonical direction of glucose metabolism with high labeling of early glycolytic steps and the pentose phosphate pathway (PPP) but very low labeling of late glycolytic steps and the Krebs cycle. Labeling of Krebs cycle and late steps of glycolysis was primarily from glutamine. These data suggest that in osteolineage cells, glucose is metabolized primarily via the PPP while glutamine is metabolized in the mitochondria, also feeding into the late steps of glycolysis likely via the malate-aspartate shuttle. This metabolic setup did not change after induction of differentiation. To evaluate the importance of this setup for osteolineage cells, we used the inhibitors of either PPP or malate-aspartate shuttle and observed a significant reduction in both cell growth and ability to differentiate. In sum, we observed a distinct metabolic wiring in osteolineage cells with high flux of glucose through the PPP and glutamine flux fueling both mitochondria and late steps of glycolysis. This wiring likely reflects their unique capacity to rapidly proliferate and produce extracellular matrix, e.g., after bone fracture.

Foxk1 promotes bone formation through inducing aerobic glycolysis

Transcription factor Foxk1 can regulate cell proliferation, differentiation, metabolism, and promote skeletal muscle regeneration and cardiogenesis. However, the roles of Foxk1 in bone formation is unknown. Here, we found that Foxk1 expression decreased in the bone tissue of aged mice and osteoporosis patients. Knockdown of Foxk1 in primary murine calvarial osteoblasts suppressed osteoblast differentiation and proliferation. Conditional knockout of Foxk1 in preosteoblasts and mature osteoblasts in mice exhibited decreased bone mass and mechanical strength due to reduced bone formation. Mechanistically, we identified Foxk1 targeted the promoter region of many genes of glycolytic enzyme by CUT&Tag analysis. Lacking of Foxk1 in primary murine calvarial osteoblasts resulted in reducing aerobic glycolysis. Inhibition of glycolysis by 2DG hindered osteoblast differentiation and proliferation induced by Foxk1 overexpression. Finally, specific overexpression of Foxk1 in preosteoblasts, driven by a preosteoblast specific osterix promoter, increased bone mass and bone mechanical strength of aged mice, which could be suppressed by inhibiting glycolysis. In summary, these findings reveal that Foxk1 plays a vital role in the osteoblast metabolism regulation and bone formation stimulation, offering a promising approach for preventing age-related bone loss.

Mitochondria in skeletal system-related diseases

Skeletal system-related diseases, such as osteoporosis, arthritis, osteosarcoma and sarcopenia, are becoming major public health concerns. These diseases are characterized by insidious progression, which seriously threatens patients' health and quality of life. Early diagnosis and prevention in high-risk populations can effectively prevent the deterioration of these patients. Mitochondria are essential organelles for maintaining the physiological activity of the skeletal system. Mitochondrial functions include contributing to the energy supply, modulating the Ca2+ concentration, maintaining redox balance and resisting the inflammatory response. They participate in the regulation of cellular behaviors and the responses of osteoblasts, osteoclasts, chondrocytes and myocytes to external stimuli. In this review, we describe the pathogenesis of skeletal system diseases, focusing on mitochondrial function. In addition to osteosarcoma, a characteristic of which is active mitochondrial metabolism, mitochondrial damage occurs during the development of other diseases. Impairment of mitochondria leads to an imbalance in osteogenesis and osteoclastogenesis in osteoporosis, cartilage degeneration and inflammatory infiltration in arthritis, and muscle atrophy and excitationcontraction coupling blockade in sarcopenia. Overactive mitochondrial metabolism promotes the proliferation and migration of osteosarcoma cells. The copy number of mitochondrial DNA and mitochondria-derived peptides can be potential biomarkers for the diagnosis of these disorders. High-risk factor detection combined with mitochondrial component detection contributes to the early detection of these diseases. Targeted mitochondrial intervention is an effective method for treating these patients. We analyzed skeletal system-related diseases from the perspective of mitochondria and provided new insights for their diagnosis, prevention and treatment by demonstrating the relationship between mitochondria and the skeletal system.

Association between triglycerides and lumbar bone mineral density in Chinese patients with osteoporotic fractures: a retrospective cross-sectional study

The association between lumbar bone mineral density (BMD) and triglyceride (TG) levels has been extensively studied; however, the results remain controversial. Therefore, this research aimed to elucidate the association of TG levels with lumbar BMD in patients with osteoporotic fractures (OPFs) who have undergone surgery. This cross-sectional study analyzed 3,558 OPF patients (aged 50 years and above) who were admitted to the First People's Hospital of Kunshan and assessed their TG levels, lumbar BMD, and other variables. The outcome variable of this research was bone density, whereas the baseline glycerol trihydrate levels were considered as the exposure variable. An analysis adjustment was conducted for various covariates, including age, gender, body mass index (BMI), and other baseline laboratory and clinical results. Furthermore, the potential non-linear relationships were assessed via the smooth curve fitting, and threshold effect analyses. The mean age of 3,558 included OPF patients was 68.87 ± 10.55 years. In the fully adjusted multivariate regression analysis, a positive correlation was found between TG levels and lumbar BMD (β = 0.015, 95% CI: 0.001-0.028, p = 0.033). Furthermore, the threshold effect analysis revealed a curvilinear relationship between TG levels and lumbar BMD, with a turning point at 1.26 mmol/L. Moreover, on both sides of the turning point, different patterns were observed. On the left side, TG levels were positively correlated with lumbar BMD. However, despite higher TG levels, the differences in lumbar BMD on the right side of the turning point, were not statistically significant, indicating a lack of significant association (p = 0.712). In summary, this research indicated that in OPF patients, higher TG levels were significantly positively associated with lumbar BMD. Furthermore, there was a threshold value of 1.26 mmol/L, indicating that TG levels in OPF patients with concomitant hypertriglyceridemia should be maintained within the normal range, and reducing TG levels below 1.26 mmol/L requires continuous monitoring. This approach effectively controls TG levels without adversely impacting lumbar BMD.

Wnt7b overexpression in osteoblasts stimulates bone formation and reduces obesity in mice on a high-fat diet

Previous studies have shown that Wnt7b potently stimulates bone formation by promoting osteoblast differentiation and activity. As high-fat feeding leads to obesity and systemic metabolic dysregulation, here we investigate the potential benefit of Wnt7b overexpression in osteoblasts on both bone and whole-body metabolism in mice fed with a high-fat diet (HFD). Wnt7b overexpression elicited massive overgrowth of trabecular and cortical bone but seemed to ameliorate body fat accumulation in mice with prolonged HFD feeding. In addition, Wnt7b overexpression modestly improved glucose tolerance in male mice on HFD. Collectively, the results indicate that targeted overexpression of Wnt7b in osteoblasts not only stimulates bone formation but also improves certain aspects of global metabolism in overnourished mice.

Insulin resistance, bone health, and fracture risk

Insulin resistance, defined as an impaired biological response to insulin stimulation in target tissues, arises most frequently in the presence of central obesity. Although obesity is generally associated with increased bone mass, recent data challenge this view and, if complicated by T2DM, obese patients are at high risk for fragility fractures. IR may play a key role in this increased fracture risk through effects on bone quality rather than bone quantity. Further understanding of the mechanisms and approaches to prevent osteoporotic fractures in IR-related diseases is needed.

CLINICAL RELEVANCE

The dramatic increase in obesity and metabolic syndrome (MetS) over the last half-century has led to a worldwide epidemic of type 2 diabetes mellitus (T2DM) as well as in the incidence of insulin resistance (IR). IR is defined as an impaired biological response to insulin stimulation in target tissues and is primarily related to the liver, muscle, and adipose tissue. The most frequent underlying cause is central obesity, and it is known that excess abdominal adipose tissue secretes increased amounts of free fatty acids, which directly affects insulin signalling, reduces glucose uptake in muscle, and triggers excessive triglyceride synthesis and gluconeogenesis in the liver. When pancreatic β cells are unable to secrete the higher levels of insulin needed, T2DM, the main complication of IR, occurs.

OBSERVATIONS

Although obesity is generally associated with increased bone mass, recent data challenge this view and highlight the multifaceted nature of the obesity-bone relationship. Patients with T2DM are at significant risk for well-known complications of diabetes, including retinopathy, nephropathy, macrovascular disease, and neuropathy, but it is clear that they are also at high risk for fragility fractures. Moreover, recent data provide strong evidence that IR may key role in the increased fracture risk observed in both obesity and T2DM.

CONCLUSIONS

In this concise review article, the role of IR in increased risk of osteoporotic fractures in MetS, obesity, and T2DM is discussed and summarised, including consideration of the need for fracture risk assessment as a 'preventive measure', especially in patients with T2DM and chronic MetS with abdominal obesity. Personalised and targeted diagnostic and therapeutic approaches to prevent osteoporotic fractures in IR-related diseases are needed and could make significant contributions to health outcomes.

Itaconate is a metabolic regulator of bone formation in homeostasis and arthritis

OBJECTIVES

Bone remodelling is a highly dynamic process dependent on the precise coordination of osteoblasts and haematopoietic-cell derived osteoclasts. Changes in core metabolic pathways during osteoclastogenesis, however, are largely unexplored and it is unknown whether and how these processes are involved in bone homeostasis.

METHODS

We metabolically and transcriptionally profiled cells during osteoclast and osteoblast generation. Individual gene expression was characterised by quantitative PCR and western blot. Osteoblast function was assessed by Alizarin red staining. immunoresponsive gene 1 (Irg1)-deficient mice were used in various inflammatory or non-inflammatory models of bone loss. Tissue gene expression was analysed by RNA in situ hybridisation.

RESULTS

We show that during differentiation preosteoclasts rearrange their tricarboxylic acid cycle, a process crucially depending on both glucose and glutamine. This rearrangement is characterised by the induction of Irg1 and production of itaconate, which accumulates intracellularly and extracellularly. While the IRG1-itaconate axis is dispensable for osteoclast generation in vitro and in vivo, we demonstrate that itaconate stimulates osteoblasts by accelerating osteogenic differentiation in both human and murine cells. This enhanced osteogenic differentiation is accompanied by reduced proliferation and altered metabolism. Additionally, supplementation of itaconate increases bone formation by boosting osteoblast activity in mice. Conversely, Irg1-deficient mice exhibit decreased bone mass and have reduced osteoproliferative lesions in experimental arthritis.

CONCLUSION

In summary, we identify itaconate, generated as a result of the metabolic rewiring during osteoclast differentiation, as a previously unrecognised regulator of osteoblasts.

Metabolic reprogramming in skeletal cell differentiation

The human skeleton is a multifunctional organ made up of multiple cell types working in concert to maintain bone and mineral homeostasis and to perform critical mechanical and endocrine functions. From the beginning steps of chondrogenesis that prefigures most of the skeleton, to the rapid bone accrual during skeletal growth, followed by bone remodeling of the mature skeleton, cell differentiation is integral to skeletal health. While growth factors and nuclear proteins that influence skeletal cell differentiation have been extensively studied, the role of cellular metabolism is just beginning to be uncovered. Besides energy production, metabolic pathways have been shown to exert epigenetic regulation via key metabolites to influence cell fate in both cancerous and normal tissues. In this review, we will assess the role of growth factors and transcription factors in reprogramming cellular metabolism to meet the energetic and biosynthetic needs of chondrocytes, osteoblasts, or osteoclasts. We will also summarize the emerging evidence linking metabolic changes to epigenetic modifications during skeletal cell differentiation.

The multifaceted roles of mitochondria in osteoblasts: from energy production to mitochondrial-derived vesicle secretion

Mitochondria in osteoblasts have been demonstrated to play multiple crucial functions in bone formation from intracellular adenosine triphosphate production to extracellular secretion of mitochondrial components. The present review explores the current knowledge about mitochondrial biology in osteoblasts, including mitochondrial biogenesis, bioenergetics, oxidative stress generation, and dynamic changes in morphology. Special attention is given to recent findings, including mitochondrial donut formation in osteoblasts, which actively generates mitochondrial-derived vesicles (MDVs), followed by extracellular secretion of small mitochondria and MDVs. We also discuss the therapeutic effects of targeting osteoblast mitochondria, highlighting their potential applications in improving bone health.

METTL14-mediated methylation of SLC25A3 mitigates mitochondrial damage in osteoblasts, leading to the improvement of osteoporosis

PURPOSE

Osteoporosis is linked to impaired function of osteoblasts, and decreased expression of METTL14 may result in abnormal differentiation of these bone-forming cells. However, the specific impact of METTL14 on osteoblast differentiation and its underlying mechanisms are not yet fully understood.

METHODS AND RESULTS

This study discovered a positive correlation between METTL14 expression and bone formation in specimens from osteoporosis patients and ovariectomized (OVX) mice. Additionally, METTL14 targeting of SLC25A3 contributed to the restoration of mitochondrial ROS levels and mitochondrial membrane potential in osteoblasts and promoted osteoblast differentiation. Moreover, in vivo experiments showed that METTL14 enhanced bone formation, and therapeutic introduction of METTL14 countered the decrease in bone formation in OVX mice.

CONCLUSIONS

Overall, these findings emphasize the crucial role of the METTL14/SLC25A3 signaling axis in osteoblast activity, suggesting that this axis could be a potential target for improving osteoporosis.

HIF1 activation safeguards cortical bone formation against impaired oxidative phosphorylation

Energy metabolism, through pathways such as oxidative phosphorylation (OxPhos) and glycolysis, plays a pivotal role in cellular differentiation and function. Our study investigates the impact of OxPhos disruption in cortical bone development by deleting mitochondrial transcription factor A (TFAM). TFAM controls OxPhos by regulating the transcription of mitochondrial genes. The cortical bone, constituting the long bones' rigid shell, is sheathed by the periosteum, a connective tissue layer populated with skeletal progenitors that spawn osteoblasts, the bone-forming cells. TFAM-deficient mice presented with thinner cortical bone, spontaneous midshaft fractures, and compromised periosteal cell bioenergetics, characterized by reduced ATP levels. Additionally, they exhibited an enlarged periosteal progenitor cell pool with impaired osteoblast differentiation. Increasing hypoxia-inducible factor 1a (HIF1) activity within periosteal cells substantially mitigated the detrimental effects induced by TFAM deletion. HIF1 is known to promote glycolysis in all cell types. Our findings underscore the indispensability of OxPhos for the proper accrual of cortical bone mass and indicate a compensatory mechanism between OxPhos and glycolysis in periosteal cells. The study opens new avenues for understanding the relationship between energy metabolism and skeletal health and suggests that modulating bioenergetic pathways may provide a therapeutic avenue for conditions characterized by bone fragility.

Metabolic regulation of skeletal cell fate and function

Bone development and bone remodelling during adult life are highly anabolic processes requiring an adequate supply of oxygen and nutrients. Bone-forming osteoblasts and bone-resorbing osteoclasts interact closely to preserve bone mass and architecture and are often located close to blood vessels. Chondrocytes within the developing growth plate ensure that bone lengthening occurs before puberty, but these cells function in an avascular environment. With ageing, numerous bone marrow adipocytes appear, often with negative effects on bone properties. Many studies have now indicated that skeletal cells have specific metabolic profiles that correspond to the nutritional microenvironment and their stage-specific functions. These metabolic networks provide not only skeletal cells with sufficient energy, but also biosynthetic intermediates that are necessary for proliferation and extracellular matrix synthesis. Moreover, these metabolic pathways control redox homeostasis to avoid oxidative stress and safeguard cell survival. Finally, several intracellular metabolites regulate the activity of epigenetic enzymes and thus control the fate and function of skeletal cells. The metabolic profile of skeletal cells therefore not only reflects their cellular state, but can also drive cellular activity. Insight into skeletal cell metabolism will thus not only advance our understanding of skeletal development and homeostasis, but also of skeletal disorders, such as osteoarthritis, diabetic bone disease and bone malignancies.

Energy homeostasis in the bone

The bone serves as an energy reservoir and actively engages in whole-body energy metabolism. Numerous studies have determined fuel requirements and bioenergetic properties of bone under physiological conditions as well as the dysregulation of energy metabolism associated with bone metabolic diseases. Here, we review the main sources of energy in bone cells and their regulation, as well as the endocrine role of the bone in systemic energy homeostasis. Moreover, we discuss metabolic changes that occur as a result of osteoporosis. Exploration in this area will contribute to an enhanced comprehension of bone energy metabolism, presenting novel possibilities to address metabolic diseases.

Extracellular matrix stiffness as an energy metabolism regulator drives osteogenic differentiation in mesenchymal stem cells

The biophysical factors of biomaterials such as their stiffness regulate stem cell differentiation. Energy metabolism has been revealed an essential role in stem cell lineage commitment. However, whether and how extracellular matrix (ECM) stiffness regulates energy metabolism to determine stem cell differentiation is less known. Here, the study reveals that stiff ECM promotes glycolysis, oxidative phosphorylation, and enhances antioxidant defense system during osteogenic differentiation in MSCs. Stiff ECM increases mitochondrial fusion by enhancing mitofusin 1 and 2 expression and inhibiting the dynamin-related protein 1 activity, which contributes to osteogenesis. Yes-associated protein (YAP) impacts glycolysis, glutamine metabolism, mitochondrial dynamics, and mitochondrial biosynthesis to regulate stiffness-mediated osteogenic differentiation. Furthermore, glycolysis in turn regulates YAP activity through the cytoskeletal tension-mediated deformation of nuclei. Overall, our findings suggest that YAP is an important mechanotransducer to integrate ECM mechanical cues and energy metabolic signaling to affect the fate of MSCs. This offers valuable guidance to improve the scaffold design for bone tissue engineering constructs.

Lipid Peroxidation-Related Redox Signaling in Osteosarcoma

Oxidative stress and lipid peroxidation play important roles in numerous physiological and pathological processes, while the bioactive products of lipid peroxidation, lipid hydroperoxides and reactive aldehydes, act as important mediators of redox signaling in normal and malignant cells. Many types of cancer, including osteosarcoma, express altered redox signaling pathways. Such redox signaling pathways protect cancer cells from the cytotoxic effects of oxidative stress, thus supporting malignant transformation, and eventually from cytotoxic anticancer therapies associated with oxidative stress. In this review, we aim to explore the status of lipid peroxidation in osteosarcoma and highlight the involvement of lipid peroxidation products in redox signaling pathways, including the involvement of lipid peroxidation in osteosarcoma therapies.

Mitochondrial enzyme FAHD1 reduces ROS in osteosarcoma

This study investigated the impact of overexpressing the mitochondrial enzyme Fumarylacetoacetate hydrolase domain-containing protein 1 (FAHD1) in human osteosarcoma epithelial cells (U2OS) in vitro. While the downregulation or knockdown of FAHD1 has been extensively researched in various cell types, this study aimed to pioneer the exploration of how increased catalytic activity of human FAHD1 isoform 1 (hFAHD1.1) affects human cell metabolism. Our hypothesis posited that elevation in FAHD1 activity would lead to depletion of mitochondrial oxaloacetate levels. This depletion could potentially result in a decrease in the flux of the tricarboxylic acid (TCA) cycle, thereby accompanied by reduced ROS production. In addition to hFAHD1.1 overexpression, stable U2OS cell lines were established overexpressing a catalytically enhanced variant (T192S) and a loss-of-function variant (K123A) of hFAHD1. It is noteworthy that homologs of the T192S variant are present in animals exhibiting increased resistance to oxidative stress and cancer. Our findings demonstrate that heightened activity of the mitochondrial enzyme FAHD1 decreases cellular ROS levels in U2OS cells. However, these results also prompt a series of intriguing questions regarding the potential role of FAHD1 in mitochondrial metabolism and cellular development.

Tricarboxylic Acid Cycle Regulation of Metabolic Program, Redox System, and Epigenetic Remodeling for Bone Health and Disease

Imbalanced osteogenic cell-mediated bone gain and osteoclastic remodeling accelerates the development of osteoporosis, which is the leading risk factor of disability in the elderly. Harmonizing the metabolic actions of bone-making cells and bone resorbing cells to the mineralized matrix network is required to maintain bone mass homeostasis. The tricarboxylic acid (TCA) cycle in mitochondria is a crucial process for cellular energy production and redox homeostasis. The canonical actions of TCA cycle enzymes and intermediates are indispensable in oxidative phosphorylation and adenosine triphosphate (ATP) biosynthesis for osteogenic differentiation and osteoclast formation. Knockout mouse models identify these enzymes' roles in bone mass and microarchitecture. In the noncanonical processes, the metabolites as a co-factor or a substrate involve epigenetic modification, including histone acetyltransferases, DNA demethylases, RNA m6A demethylases, and histone demethylases, which affect genomic stability or chromatin accessibility for cell metabolism and bone formation and resorption. The genetic manipulation of these epigenetic regulators or TCA cycle intermediate supplementation compromises age, estrogen deficiency, or inflammation-induced bone mass loss and microstructure deterioration. This review sheds light on the metabolic functions of the TCA cycle in terms of bone integrity and highlights the crosstalk of the TCA cycle and redox and epigenetic pathways in skeletal tissue metabolism and the intermediates as treatment options for delaying osteoporosis.

Excess glucocorticoids inhibit murine bone turnover via modulating the immunometabolism of the skeletal microenvironment

Elevated bone resorption and diminished bone formation have been recognized as the primary features of glucocorticoid-associated skeletal disorders. However, the direct effects of excess glucocorticoids on bone turnover remain unclear. Here, we explored the outcomes of exogenous glucocorticoid treatment on bone loss and delayed fracture healing in mice and found that reduced bone turnover was a dominant feature, resulting in a net loss of bone mass. The primary effect of glucocorticoids on osteogenic differentiation was not inhibitory; instead, they cooperated with macrophages to facilitate osteogenesis. Impaired local nutrient status - notably, obstructed fatty acid transportation - was a key factor contributing to glucocorticoid-induced impairment of bone turnover in vivo. Furthermore, fatty acid oxidation in macrophages fueled the ability of glucocorticoid-liganded receptors to enter the nucleus and then promoted the expression of BMP2, a key cytokine that facilitates osteogenesis. Metabolic reprogramming by localized fatty acid delivery partly rescued glucocorticoid-induced pathology by restoring a healthier immune-metabolic milieu. These data provide insights into the multifactorial metabolic mechanisms by which glucocorticoids generate skeletal disorders, thus suggesting possible therapeutic avenues.

TMEM119 (c.G143A, p.S48L) Mutation Is Involved in Primary Failure of Eruption by Attenuating Glycolysis-Mediated Osteogenesis

Primary failure of eruption (PFE) is a rare oral disease with an incidence rate of 0.06%. It is characterized by abnormal eruption mechanisms that disrupt tooth eruption. The underlying pathogenic genetic variant and mechanism of PFE remain largely unknown. The purpose of this study was to explore the role of a novel transmembrane protein 119 (TMEM119) mutation in two PFE patients in a Chinese family. Information collection was performed on the family with a diagnosis of PFE, and blood samples from patients and healthy family members were extracted. Whole-exome sequencing was performed. Bioinformatics analysis revealed that a heterozygous variant in the TMEM119 gene (c.G143A, p.S48L) was a disease-associated mutation in this family. Recombinant pcDNA3.1 plasmid-containing wild-type and mutant TMEM119 expression cassettes were successfully constructed and transfected into MC3T3-E1 cells, respectively. The results of in vitro analysis suggested that the subcellular distribution of the TMEM119 protein was transferred from the cell cytoplasm to the nucleus, and the ability of cells to proliferate and migrate as well as glycolytic and mineralized capacities were reduced after mutation. Furthermore, rescue assays showed that activating transcription factor 4 (ATF4) overexpression rescued the attenuated glycolysis and mineralization ability of cells. Results of in vivo analysis demonstrated that TMEM119 was mainly expressed in the alveolar bone around the mouse molar germs, and the expression level increased with tooth eruption, demonstrated using immunohistochemistry and immunofluorescence. Collectively, the novel TMEM119 mutation is potentially pathogenic in the PFE family by affecting the glucose metabolism and mineralized function of osteoblasts, including interaction with ATF4. Our findings broaden the gene mutation spectrum of PFE and further elucidate the pathogenic mechanism of PFE.

POZ/BTB and AT hook containing zinc finger 1 (PATZ1) suppresses differentiation and regulates metabolism in human embryonic stem cells

Human embryonic stem cells (hESCs) can proliferate infinitely (self-renewal) and give rise to almost all types of somatic cells (pluripotency). Hence, understanding the molecular mechanism of pluripotency regulation is important for applications of hESCs in regenerative medicine. Here we report that PATZ1 is a key factor that regulates pluripotency and metabolism in hESCs. We found that depletion of PATZ1 is associated with rapid downregulation of master pluripotency genes and prominent deceleration of cell growth. We also revealed that PATZ1 regulates hESC pluripotency though binding the regulatory regions of OCT4 and NANOG. In addition, we demonstrated PATZ1 is a key node in the OCT4/NANOG transcriptional network. We further revealed that PATZ1 is essential for cell growth in hESCs. Importantly, we discovered that depletion of PATZ1 drives hESCs to exploit glycolysis which energetically compensates for the mitochondrial dysfunction. Overall, our study establishes the fundamental role of PATZ1 in regulating pluripotency in hESCs. Moreover, PATZ1 is essential for maintaining a steady metabolic homeostasis to refine the stemness of hESCs.

Mitochondrial Dysfunction in Periodontitis and Associated Systemic Diseases: Implications for Pathomechanisms and Therapeutic Strategies

Periodontitis is a chronic infectious disorder damaging periodontal tissues, including the gingiva, periodontal ligament, cementum, and alveolar bone. It arises from the complex interplay between pathogenic oral bacteria and host immune response. Contrary to the previous view of "energy factories", mitochondria have recently been recognized as semi-autonomous organelles that fine-tune cell survival, death, metabolism, and other functions. Under physiological conditions, periodontal tissue cells participate in dynamic processes, including differentiation, mineralization, and regeneration. These fundamental activities depend on properly functioning mitochondria, which play a crucial role through bioenergetics, dynamics, mitophagy, and quality control. However, during the initiation and progression of periodontitis, mitochondrial quality control is compromised due to a range of challenges, such as bacterial-host interactions, inflammation, and oxidative stress. Currently, mounting evidence suggests that mitochondria dysfunction serves as a common pathological mechanism linking periodontitis with systemic conditions like type II diabetes, obesity, and cardiovascular diseases. Therefore, targeting mitochondria to intervene in periodontitis and multiple associated systemic diseases holds great therapeutic potential. This review provides advanced insights into the interplay between mitochondria, periodontitis, and associated systemic diseases. Moreover, we emphasize the significance of diverse therapeutic modulators and signaling pathways that regulate mitochondrial function in periodontal and systemic cells.

Mechanistic study of the Aldo-keto reductase family 1 member A1 in regulating mesenchymal stem cell fate decision toward adipogenesis and osteogenesis

AIMS

Akr1A1 is a glycolytic enzyme catalyzing the reduction of aldehyde to alcohol. This study aims to delineate the role of Akr1A1 in regulating the adipo-osteogenic lineage differentiation of mesenchymal stem cells (MSCs).

MAIN METHODS

MSCs derived from human bone marrow and Wharton Jelly together with gain- and loss-of-function analysis as well as supplementation with the S-Nitrosoglutathione reductase (GSNOR) inhibitor N6022 were used to study the function of Akr1A1 in controlling MSC lineage differentiation into osteoblasts and adipocytes.

KEY FINDINGS

Akr1A1 expression, PKM2 activity, and lactate production were found to be decreased in osteoblast-committed MSCs, but PGC-1α increased to induce mitochondrial oxidative phosphorylation. Increased Akr1A1 inhibited the SIRT1-dependent pathway for decreasing the expressions of PGC-1α and TAZ but increasing PPAR γ in adipocyte-committed MSCs, hence promoting glycolysis in adipogenesis. In contrast, Akr1A1 expression, PKM2 activity and lactate production were all increased in adipocyte-differentiated cells with decreased PGC-1α for switching energy utilization to glycolytic metabolism. Reduced Akr1A1 expression in osteoblast-committed cells relieves its inhibition of SIRT1-mediated activation of PGC-1α and TAZ for facilitating osteogenesis and mitochondrial metabolism.

SIGNIFICANCE

Several metabolism-involved regulators including Akr1A1, SIRT1, PPARγ, PGC-1α and TAZ were differentially expressed in osteoblast- and adipocyte-committed MSCs. More importantly, Akr1A1 was identified as a new key regulator for controlling the MSC lineage commitment in favor of adipogenesis but detrimental to osteogenesis. Such information should be useful to develop perspective new therapeutic agents to reverse the adipo-osteogenic differentiation of BMSCs, in a way to increase in osteogenesis but decrease in adipogenesis.

Effects of PTH on osteoblast bioenergetics in response to glucose

Parathyroid hormone acts through its receptor, PTHR1, expressed on osteoblasts, to control bone remodeling. Metabolic flexibility for energy generation has been demonstrated in several cell types dependent on substrate availability. Recent studies have identified a critical role for PTH in regulating glucose, fatty acid and amino acid metabolism thus stimulating both glycolysis and oxidative phosphorylation. Therefore, we postulated that PTH stimulates increased energetic output by osteoblasts either by increasing glycolysis or oxidative phosphorylation depending on substrate availability. To test this hypothesis, undifferentiated and differentiated MC3T3E1C4 calvarial pre-osteoblasts were treated with PTH to study osteoblast bioenergetics in the presence of exogenous glucose. Significant increases in glycolysis with acute ∼1 h PTH treatment with minimal effects on oxidative phosphorylation in undifferentiated MC3T3E1C4 in the presence of exogenous glucose were observed. In differentiated cells, the increased glycolysis observed with acute PTH was completely blocked by pretreatment with a Glut1 inhibitor (BAY-876) resulting in a compensatory increase in oxidative phosphorylation. We then tested the effect of PTH on the function of complexes I and II of the mitochondrial electron transport chain in the absence of glycolysis. Utilizing a novel cell plasma membrane permeability mitochondrial (PMP) assay, in combination with complex I and II specific substrates, slight but significant increases in basal and maximal oxygen consumption rates with 24 h PTH treatment in undifferentiated MC3T3E1C4 cells were noted. Taken together, our data demonstrate for the first time that PTH stimulates both increases in glycolysis and the function of the electron transport chain, particularly complexes I and II, during high energy demands in osteoblasts.

Malate-Based Biodegradable Scaffolds Activate Cellular Energetic Metabolism for Accelerated Wound Healing

The latest advancements in cellular bioenergetics have revealed the potential of transferring chemical energy to biological energy for therapeutic applications. Despite efforts, a three-dimensional (3D) scaffold that can induce long-term bioenergetic effects and facilitate tissue regeneration remains a big challenge. Herein, the cellular energetic metabolism promotion ability of l-malate, an important intermediate of the tricarboxylic acid (TCA) cycle, was proved, and a series of bioenergetic porous scaffolds were fabricated by synthesizing poly(diol l-malate) (PDoM) prepolymers via a facial one-pot polycondensation of l-malic acid and aliphatic diols, followed by scaffold fabrication and thermal-cross-linking. The degradation products of the developed PDoM scaffolds can regulate the metabolic microenvironment by entering mitochondria and participating in the TCA cycle to elevate intracellular adenosine triphosphate (ATP) levels, thus promoting the cellular biosynthesis, including the production of collagen type I (Col1a1), fibronectin 1 (Fn1), and actin alpha 2 (Acta2/α-Sma). The porous PDoM scaffold was demonstrated to support the growth of the cocultured mesenchymal stem cells (MSCs) and promote their secretion of bioactive molecules [such as vascular endothelial growth factor (VEGF), transforming growth factor-β1 (TGF-β1), and basic fibroblast growth factor (bFGF)], and this stem cells-laden scaffold architecture was proved to accelerate wound healing in a critical full-thickness skin defect model on rats.

Skeletal muscle-derived extracellular vesicles transport glycolytic enzymes to mediate muscle-to-bone crosstalk

Identification of cues originating from skeletal muscle that govern bone formation is essential for understanding the crosstalk between muscle and bone and for developing therapies for degenerative bone diseases. Here, we identified that skeletal muscle secreted multiple extracellular vesicles (Mu-EVs). These Mu-EVs traveled through the bloodstream to reach bone, where they were phagocytized by bone marrow mesenchymal stem/stromal cells (BMSCs). Mu-EVs promoted osteogenic differentiation of BMSCs and protected against disuse osteoporosis in mice. The quantity and bioactivity of Mu-EVs were tightly correlated with the function of skeletal muscle. Proteomic analysis revealed numerous proteins in Mu-EVs, some potentially regulating bone metabolism, especially glycolysis. Subsequent investigations indicated that Mu-EVs promoted the glycolysis of BMSCs by delivering lactate dehydrogenase A into these cells. In summary, these findings reveal that Mu-EVs play a vital role in BMSC metabolism regulation and bone formation stimulation, offering a promising approach for treating disuse osteoporosis.

Glycometabolic reprogramming in cementoblasts: A vital target for enhancing cell mineralization

Cementum, a constituent part of periodontal tissues, has important adaptive and reparative functions. It serves to attach the tooth to alveolar bone and acts as a barrier delimit epithelial growth and bacteria evasion. A dynamic and highly responsive cementum is essential for maintaining occlusal relationships and the integrity of the root surface. It is a thin layer of mineralized tissue mainly produced by cementoblasts. Cementoblasts are osteoblast-like cells essential for the restoration of periodontal tissues. In recent years, glucose metabolism has been found to be critical in bone remodeling and osteoblast differentiation. However, the glucose metabolism of cementoblasts remains incompletely understood. First, immunohistochemistry staining and in vivo tracing with 18 F-fluorodeoxyglucose (18 F-FDG) revealed significantly higher glucose metabolism in cementum formation. To test the bioenergetic pathways of cementoblast differentiation, we compared the bioenergetic profiles of mineralized and unmineralized cementoblasts. As a result, we observed a significant increase in the consumption of glucose and production of lactate, coupled with the higher expression of glycolysis-related genes. However, the expression of oxidative phosphorylation-related genes was downregulated. The verified results were consistent with the RNA sequencing results. Likewise, targeted energy metabolomics shows that the levels of glycolytic metabolites were significantly higher in the mineralized cementoblasts. Seahorse assays identified an increase in glycolytic flux and reduced oxygen consumption during cementoblast mineralization. Apart from that, we also found that lactate dehydrogenase A (LDHA), a key glycolysis enzyme, positively regulates the mineralization of cementoblasts. In summary, cementoblasts mainly utilized glycolysis rather than oxidative phosphorylation during the mineralization process.

Fumarase activity in NAD-dependent malic enzyme, MaeA, from Escherichia coli

NAD-dependent malic enzymes catalyze NAD reduction to NADH while converting malate to pyruvate and CO2. In this study, NAD was reduced to NADH by MaeA, NAD-dependent malic enzyme from Escherichia coli, when fumarate was used as substrate. This suggested that MaeA catalyzed the conversion of fumarate to malate and then malate to pyruvate. The K0.5 value for fumarate was determined as 13 mM, different from previously characterized fumarases in Escherichia coli. Fumarate inhibited the malic enzyme activity of MaeA where NAD reduction to NADH was examined in the presence of malate as substrate. Human ME2, an NAD-dependent malic enzyme, also converted NAD to NADH in the presence of fumarate, suggesting that the duplex activity as fumarase and malic enzyme might be conserved in various NAD-dependent malic enzymes. MaeB, NADP-dependent malic enzyme from Escherichia coli, did not reduce NADP to NADPH in the presence of fumarate, suggesting the fumarase activities of MaeA and ME2 were specific.

Genetic activation of glycolysis in osteoblasts preserves bone mass in type I diabetes

Type I diabetes (T1D) impairs bone accrual in patients, but the mechanism is unclear. Here in a murine monogenic model for T1D, we demonstrate that diabetes suppresses bone formation resulting in a rapid loss of both cortical and trabecular bone. Single-cell RNA sequencing uncovers metabolic dysregulation in bone marrow osteogenic cells of diabetic mice. In vivo stable isotope tracing reveals impaired glycolysis in diabetic bone that is highly responsive to insulin stimulation. Remarkably, deletion of the insulin receptor reduces cortical but not trabecular bone. Increasing glucose uptake by overexpressing Glut1 in osteoblasts exacerbates bone defects in T1D mice. Conversely, activation of glycolysis by Pfkfb3 overexpression preserves both trabecular and cortical bone mass in the face of diabetes. The study identifies defective glucose metabolism in osteoblasts as a pathogenic mechanism for osteopenia in T1D, and furthermore implicates boosting osteoblast glycolysis as a potential bone anabolic therapy.

Glutathione limits RUNX2 oxidation and degradation to regulate bone formation

Reactive oxygen species (ROS) are natural products of mitochondrial oxidative metabolism and oxidative protein folding. ROS levels must be well controlled, since elevated ROS has been shown to have deleterious effects on osteoblasts. Moreover, excessive ROS is thought to underlie many of the skeletal phenotypes associated with aging and sex steroid deficiency in mice and humans. The mechanisms by which osteoblasts regulate ROS and how ROS inhibits osteoblasts are not well understood. Here, we demonstrate that de novo glutathione (GSH) biosynthesis is essential in neutralizing ROS and establish a proosteogenic reduction and oxidation reaction (REDOX) environment. Using a multifaceted approach, we demonstrate that reducing GSH biosynthesis led to acute degradation of RUNX2, impaired osteoblast differentiation, and reduced bone formation. Conversely, reducing ROS using catalase enhanced RUNX2 stability and promoted osteoblast differentiation and bone formation when GSH biosynthesis was limited. Highlighting the therapeutic implications of these findings, in utero antioxidant therapy stabilized RUNX2 and improved bone development in the Runx2+/- haplo-insufficient mouse model of human cleidocranial dysplasia. Thus, our data establish RUNX2 as a molecular sensor of the osteoblast REDOX environment and mechanistically clarify how ROS negatively impacts osteoblast differentiation and bone formation.

Sustained Release of BMSC-EVs from 3D Printing Gel/HA/nHAP Scaffolds for Promoting Bone Regeneration in Diabetic Rats

Extracellular vesicles (EVs) play an important role in intercellular communication, and the function of EVs mainly depends on the state of source cells. To determine the effect of diabetic microenvironment on EVs secreted by bone marrow mesenchymal stem cells (BMSCs), this work explores the effect of normal glucose (5.5 mm) cultured BMSCs derived EVs (NG-EVs) and high glucose (30 mm) cultured BMSCs derived EVs (HG-EVs) in regulating the migration, proliferation and osteoblastic differentiation of BMSCs in vitro. In order to improve the bioavailability of EVs, this work constructs a sustained release system of polydopamine (PDA) functionalized 3D printing gelatin/hyaluronic acid/nano-hydroxyapatite (Gel/HA/nHAP) scaffolds (S/PDA) and verifies its function in the calvarial defect model of diabetic rats. This work confirms that both NG-EVs and HG-EVs can promote proliferation and migration, inhibit apoptosis and promote osteogenic differentiation, but the function of HG-EVs is weaker than that of NG-EVs. Therefore, EVs secreted by autologous cells of diabetic patients are not suitable for self-repair. This work hopes that the 3D printing scaffold designed for sustained-release EVs will provide a new strategy for acellular tissue engineering bone repair in diabetic patients.

Cellular redox homeostasis maintained by malic enzyme 2 is essential for MYC-driven T cell lymphomagenesis

T cell lymphomas (TCLs) are a group of rare and heterogeneous tumors. Although proto-oncogene MYC has an important role in driving T cell lymphomagenesis, whether MYC carries out this function remains poorly understood. Here, we show that malic enzyme 2 (ME2), one of the NADPH-producing enzymes associated with glutamine metabolism, is essential for MYC-driven T cell lymphomagenesis. We establish a CD4-Cre; Myc flox/+transgenic mouse mode, and approximately 90% of these mice develop TCL. Interestingly, knockout of Me2 in Myc transgenic mice almost completely suppresses T cell lymphomagenesis. Mechanistically, by transcriptionally up-regulating ME2, MYC maintains redox homeostasis, thereby increasing its tumorigenicity. Reciprocally, ME2 promotes MYC translation by stimulating mTORC1 activity through adjusting glutamine metabolism. Treatment with rapamycin, an inhibitor of mTORC1, blocks the development of TCL both in vitro and in vivo. Therefore, our findings identify an important role for ME2 in MYC-driven T cell lymphomagenesis and reveal that MYC-ME2 circuit may be an effective target for TCL therapy.

Suppression of the human malic enzyme 2 modifies energy metabolism and inhibits cellular respiration

Human mitochondrial NAD(P)⁺-dependent malic enzyme (ME2) is well-known for its role in cell metabolism, which may be involved in cancer or epilepsy. We present potent ME2 inhibitors based on cyro-EM structures that target ME2 enzyme activity. Two structures of ME2-inhibitor complexes demonstrate that 5,5'-Methylenedisalicylic acid (MDSA) and embonic acid (EA) bind allosterically to ME2's fumarate-binding site. Mutagenesis studies demonstrate that Asn35 and the Gln64-Tyr562 network are required for both inhibitors' binding. ME2 overexpression increases pyruvate and NADH production while decreasing the cell's NAD⁺/NADH ratio; however, ME2 knockdown has the opposite effect. MDSA and EA inhibit pyruvate synthesis and thus increase the NAD⁺/NADH ratio, implying that these two inhibitors interfere with metabolic changes by inhibiting cellular ME2 activity. ME2 silence or inhibiting ME2 activity with MDSA or EA decreases cellular respiration and ATP synthesis. Our findings suggest that ME2 is crucial for mitochondrial pyruvate and energy metabolism, as well as cellular respiration, and that ME2 inhibitors could be useful in the treatment of cancer or other diseases that involve these processes.

Nutrient-Dependent Mitochondrial Fission Enhances Osteoblast Function

BACKGROUND

The bone synthesizing function of osteoblasts (OBs) is a highly demanding energy process that requires nutrients. However, how nutrient availability affects OBs behavior and bone mineralization remain to be fully understood.

METHODS

MC3T3-E1 cell line and primary OBs (OBs) cultures were treated with physiological levels of glucose (G; 5.5 mM) alone or with the addition of palmitic acid (G+PA) at different concentrations. Mitochondria morphology and activity were evaluated by fluorescence microscopy, qPCR, and oxygen consumption rate (OCR) measurement, and OBs function was assessed by mineralization assay.

RESULTS

The addition of non-lipotoxic levels of 25 μM PA to G increased mineralization in OBs. G+25 μM PA exposure reduced mitochondria size in OBs, which was associated with increased activation of dynamin-related protein 1, a mitochondrial fission protein, enhanced mitochondria OCR and ATP production, and increased expression of oxidative phosphorylation genes. Treatment with Mdivi-1, a putative inhibitor of mitochondrial fission, reduced osteogenesis and mitochondrial respiration in OBs.

CONCLUSIONS

Our results revealed that OBs function was enhanced in the presence of glucose and PA at 25 μM. This was associated with increased OBs mitochondrial respiration and dynamics. These results suggest a role for nutrient availability in bone physiology and pathophysiology.

Osteoblast-intrinsic defect in glucose metabolism impairs bone formation in type II diabetic male mice

Skeletal fragility is associated with type 2 diabetes mellitus (T2D), but the underlying mechanism is not well understood. Here, in a mouse model for youth-onset T2D, we show that both trabecular and cortical bone mass is reduced due to diminished osteoblast activity. Stable isotope tracing in vivo with 13C-glucose demonstrates that both glycolysis and glucose fueling of the TCA cycle are impaired in diabetic bones. Similarly, Seahorse assays show suppression of both glycolysis and oxidative phosphorylation by diabetes in bone marrow mesenchymal cells as a whole, whereas single-cell RNA sequencing reveals distinct modes of metabolic dysregulation among the subpopulations. Metformin not only promotes glycolysis and osteoblast differentiation in vitro, but also improves bone mass in diabetic mice. Finally, osteoblast-specific overexpression of either Hif1a, a general inducer of glycolysis, or Pfkfb3 which stimulates a specific step in glycolysis, averts bone loss in T2D mice. The study identifies osteoblast-intrinsic defects in glucose metabolism as an underlying cause of diabetic osteopenia, which may be targeted therapeutically.

Lysosomal acid lipase regulates bioenergetic process during the cytodifferentiation of human periodontal ligament cells

Lipid metabolism is one of energy metabolic pathways that produce adenosine triphosphate (ATP). In this pathway, lysosomal acid lipase (LAL) encoded by Lipase A (LIPA), plays an important role in catalyzing lipids to fatty acids (FAs), which drive oxidative phosphorylation (OXPHOS) and generate ATP. Previously, we found that a LIPA single nucleotide polymorphism rs143793106, which decreases the LAL activity, suppressed the cytodifferentiation of human periodontal ligament (HPDL) cells. However, the mechanisms underlying that suppression are still not fully clarified. Thus, we aimed to investigate the mechanisms regulating the cytodifferentiation of HPDL cells by LAL in terms of energy metabolism. We performed the osteogenic induction of HPDL cells with or without Lalistat-2, a LAL inhibitor. To visualize lipid droplet (LD) utilization, we performed confocal microscopy on HPDL cells. We also performed real-time PCR to analyze the gene expression of calcification-related and metabolism-related genes. Furthermore, we measured the ATP production rate from two major energy production pathways, OXPHOS and glycolysis, and OXPHOS-related parameters of HPDL cells during their cytodifferentiation. We found that LDs were utilized during the cytodifferentiation of HPDL cells. Alkaline phosphatase (ALPL), collagen type 1 alpha 1 chain (COL1A1), ATP synthase F1 subunit alpha (ATP5F1A), and carnitine palmitoyltransferase 1A (CPT1A) mRNA expressions were upregulated, whereas lactate dehydrogenase A (LDHA) mRNA expression was downregulated. Additionally, total ATP production rate was significantly increased. In contrast, in the presence of Lalistat-2, LD utilization was inhibited and ALPL, COL1A1, and ATP5F1A mRNA expression was downregulated. Additionally, ATP production rate and spare respiratory capacity of the OXPHOS pathway were decreased in HPDL cells during their cytodifferentiation. Collectively, the defect of LAL in HPDL cells decreased LD utilization and OXPHOS capacity, resulting in reduced energy to sustain the adequate ATP production required for the cytodifferentiation of HPDL cells. Thus, LAL is important for periodontal tissue homeostasis as a regulator of bioenergetic process of HPDL cells.

Role of the Mitochondrial Permeability Transition in Bone Metabolism and Aging

The mitochondrial permeability transition pore (MPTP) and its positive regulator, cyclophilin D (CypD), play important pathophysiological roles in aging. In bone tissue, higher CypD expression and pore activity are found in aging; however, a causal relationship between CypD/MPTP and bone degeneration needs to be established. We previously reported that CypD expression and MPTP activity are downregulated during osteoblast (OB) differentiation and that manipulations in CypD expression affect OB differentiation and function. Using a newly developed OB-specific CypD/MPTP gain-of-function (GOF) mouse model, we here present evidence that overexpression of a constitutively active K166Q mutant of CypD (caCypD) impairs OB energy metabolism and function, and bone morphological and biomechanical parameters. Specifically, in a spatial-dependent and sex-dependent manner, OB-specific CypD GOF led to a decrease in oxidative phosphorylation (OxPhos) levels, higher oxidative stress, and general metabolic adaptations coincident with the decreased bone organic matrix content in long bones. Interestingly, accelerated bone degeneration was present in vertebral bones regardless of sex. Overall, our work confirms CypD/MPTP overactivation as an important pathophysiological mechanism leading to bone degeneration and fragility in aging. © 2023 American Society for Bone and Mineral Research (ASBMR).

Intracellular glucose starvation inhibits osteogenic differentiation in human periodontal ligament cells

BACKGROUND

Periodontal ligament cells (PDLCs), as mesenchymal cells in the oral cavity, are closely linked to periodontal tissue regeneration. However, the effect of local glucose deficiency on periodontal tissue regeneration, such as immediately post-surgery, remains unknown.

OBJECTIVE

In the present study, we investigated the effect of a low-glucose environment on the proliferation and osteogenic differentiation of PDLCs.

MATERIALS AND METHODS

We used media with five glucose concentrations (100, 75, 50, 25, and 0 mg/dL) and focused on the effects of a low-glucose environment on the proliferation, osteogenic differentiation, and autophagy of PDLCs. Additionally, we focused on changes in lactate production in a low-glucose environment and investigated the involvement of lactate with AZD3965, a monocarboxylate transporter-1 (MCT-1) inhibitor.

RESULTS

The low-glucose environment inhibited PDLCs proliferation, migration, and osteogenic differentiation, and induced the expression of the autophagy-related factors LC3 and p62. Lactate and ATP production were decreased under low-glucose conditions. The addition of AZD3965 (MCT-1 inhibitor) in normal glucose conditions caused a similar trend as in low-glucose conditions on PDLCs.

CONCLUSION

Our results suggest lactate production through glucose metabolism in the osteogenic differentiation of PDLCs. A low-glucose environment decreased lactate production, inhibiting cell proliferation, migration, and osteogenic differentiation and inducing autophagy in PDLCs.

The role of mitochondria in the peri-implant microenvironment

NEW FINDINGS

What is the topic of this review? In this review, we consider the key role of mitochondria in the peri-implant milieu, including the regulation of mitochondrial reactive oxygen species and mitochondrial metabolism in angiogenesis, the polarization of macrophage immune responses, and bone formation and bone resorption during osseointegration. What advances does it highlight? Mitochondria contribute to the behaviours of peri-implant cell lines based on metabolic and reactive oxygen species signalling modulations, which will contribute to the research field and the development of new treatment strategies for improving implant success.

ABSTRACT

Osseointegration is a dynamic biological process in the local microenvironment adjacent to a bone implant, which is crucial for implant performance and success of the implant surgery. Recently, the role of mitochondria in the peri-implant microenvironment during osseointegration has gained much attention. Mitochondrial regulation has been verified to be essential for cellular events in osseointegration and as a therapeutic target for peri-implant diseases in the peri-implant microenvironment. In this review, we summarize our current knowledge of the key role of mitochondria in the peri-implant milieu, including the regulation of mitochondrial reactive oxygen species and mitochondrial metabolism in angiogenesis, the polarization of macrophage immune responses, and bone formation and resorption during osseointegration, which will contribute to the research field and the development of new treatment strategies to improve implant success. In addition, we indicate limitations in our current understanding of the regulation of mitochondria in osseointegration and suggest topics for further study.

Mitochondrial Transfer Regulates Cell Fate Through Metabolic Remodeling in Osteoporosis

Mitochondria are the powerhouse of eukaryotic cells, which regulate cell metabolism and differentiation. Recently, mitochondrial transfer between cells has been shown to direct recipient cell fate. However, it is unclear whether mitochondria can translocate to stem cells and whether this transfer alters stem cell fate. Here, mesenchymal stem cell (MSC) regulation is examined by macrophages in the bone marrow environment. It is found that macrophages promote osteogenic differentiation of MSCs by delivering mitochondria to MSCs. However, under osteoporotic conditions, macrophages with altered phenotypes, and metabolic statuses release oxidatively damaged mitochondria. Increased mitochondrial transfer of M1-like macrophages to MSCs triggers a reactive oxygen species burst, which leads to metabolic remodeling. It is showed that abnormal metabolism in MSCs is caused by the abnormal succinate accumulation, which is a key factor in abnormal osteogenic differentiation. These results reveal that mitochondrial transfer from macrophages to MSCs allows metabolic crosstalk to regulate bone homeostasis. This mechanism identifies a potential target for the treatment of osteoporosis.