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

  1. Fangfang Song
  2. Won Dong Lee
  3. Tyler Marmo
  4. Xing Ji
  5. Chao Song
  6. Xueyang Liao
  7. Rebecca Seeley
  8. Lutian Yao
  9. Haoran Liu
  10. Fanxin Long

  • Curated by eLife

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    eLife assessment

    In this study, the authors made important progress in understanding bone metabolic defects of T2D. They have established a valuable model that could mimic some aspects of T2D in mice. Particularly, the study provided important evidence showing bone turnover and metabolism were in defects, and changes in glycolysis would rescue bone defects in T2D. Overall, the authors provide compelling evidence from dynamic histomorphometry, C13 isotype labeling in vivo, scRNA-seq, and metabolic assays to demonstrate that the defective glucose metabolism causes osteopenia associated with T2D.

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Abstract

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 13 C-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.

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  1. Version published to 10.7554/elife.85714 on eLife
  2. eLife

    Author Response

    Reviewer #2 (Public Review):

    Despite high bone mineral density, increased fracture risk has been associated with T2D in humans. In this study, the authors established a model that could mimic some aspects of T2D in mice and then study bone turnover and metabolism in detail.

    Strengths

    This is an exciting study, the methods are detailed and well done, and the results are presented coherently and support the conclusions.

    Previous work from Dr. Long's group over this last decade has established a requirement for glycolysis in osteoblast differentiation. They showed the requirement for glycolysis not only for the anabolic action of PTH but also as an effector downstream of Wnt signaling. Using the T2D mouse model they have generated, they test if manipulating glycolysis and oxidative phosphorylation can rescue some of the detrimental effects on bone in this model. They use several novel approaches, they use glucose-labeling studies that are relatively underutilized, and it provides some insights into defective TCA cycle. They also utilize BMSCs that have been sorted for performing single-cell sequencing studies to identify specific populations modified with T2D. Unfortunately, the results are modest and need some clarification on what these populations add to the story.

    We appreciate the positive comments. Although T2D had only modest effect on the relative pool size of each cell population, the changes in metabolic pathways (glycolysis and oxphos) in several clusters were notable and provided support to the central notion that T2D altered cellular metabolism in osteoblast-lineage and other bone marrow cells.

    The authors use two approaches: a drug (Metformin) and a number of mouse genetic models to over-express genes involved in the glycolytic pathway using Dox inducible models. The results with overexpressing HIF1 and PFKFB3 show a potential rescue of bone defects with T2D, and Glut1 overexpression does not rescue T2D-induced bone loss.

    Concerns

    The authors have generated several overexpression models to manipulate the glycolytic pathway to recuse T2D-induced bone loss. The use of DOX in drinking water has been shown to affect mitochondrial metabolism. Did the authors control for these effects? Since both the groups of mice got the DOX in drinking water, there is internal control.

    The experiments were controlled for any potential effects of DOX per se as all animals were subjected to the same DOX regimen.

    Only one of the rescue experiments had control with the Chow diet. There are some studies that have shown a high-fat diet to be protective of bone loss in TID models.

    We have now added the chow diet control for the Hif1a rescue experiment as well (Fig. 7).

    The use of metformin to correct metabolic dysfunction and, thereby, bone mass is an exciting result. Did the authors test to see if they had in any way rescued this phenotype because of reducing ROS levels? The decrease in OxsPhos seen with the seahorse experiments suggests there could be mitochondrial dysfunction often associated with ROS generation.

    I appreciate the reviewer’s insight here. We have not examined ROS levels but agree that changes in ROS levels could potentially contribute to the bone phenotype in diabetes.

    All of the experiments used male mice (because STZ use and ease of T2D establishment in males). It would be better if this were made clear in the title.

    The title has been revised to specify male mice.

    Is the T2D model presented really represent what is observed in humans? Some experiments to test the other factors implicated in T2D and whether those are modulated in the rescue experiments might help address this.

    Our T2D model exhibited all typical features of T2D patients, those including obesity, glucose intolerance and insulin resistance. We have shown that metformin modestly improved glucose tolerance and insulin sensitivity in the T2D mice (Fig. 6C, E). We have not examined whether those global metabolic features were modulated in the genetic rescue experiments which targeted only osteoblasts.

  3. eLife

    eLife assessment

    In this study, the authors made important progress in understanding bone metabolic defects of T2D. They have established a valuable model that could mimic some aspects of T2D in mice. Particularly, the study provided important evidence showing bone turnover and metabolism were in defects, and changes in glycolysis would rescue bone defects in T2D. Overall, the authors provide compelling evidence from dynamic histomorphometry, C13 isotype labeling in vivo, scRNA-seq, and metabolic assays to demonstrate that the defective glucose metabolism causes osteopenia associated with T2D.

  4. eLife

    Reviewer #1 (Public Review):

    T2D in youth has been reported to reduce bone mass due to impaired bone anabolism, but the underlying mechanisms are not fully understood. The authors study the relationship between T2DM (Type 2 Diabetes Mellitus) and "skeletal fragility." Specifically, they look at glucose metabolism defects in osteoblasts during T2DM and their impacts on osteoblast activity. The results are novel as they elucidate the effects of low-dose STZ models of T2DM on osteoblast function and the function of osteoblasts from those mice in terms of glycolysis, glucose uptake, and function. Additionally, it covers recovery of glucose metabolic effects through overexpression of Hif1a or Pfkfb3 (targeted to osteoblasts) and metformin treatment. The role of Hif1a and Pfkfb3 in osteoblasts with regard to the rescue of T2DM bone effects is critical to the novelty of the paper and may benefit from being included and emphasized in the title and/or abstract. The study of osteoblasts and their glucose metabolism has been studied but not extensively at the mechanism level. The approach of using a mouse model is good for youth-onset T2D. It would be helpful if the author could include a bit more in the abstract about the critical role of Hif1a and Pfkfb3 in osteoblasts in recovery from T2DM treatment's bone effects in vivo.

  5. eLife

    Reviewer #2 (Public Review):

    Despite high bone mineral density, increased fracture risk has been associated with T2D in humans. In this study, the authors established a model that could mimic some aspects of T2D in mice and then study bone turnover and metabolism in detail.

    Strengths
    This is an exciting study, the methods are detailed and well done, and the results are presented coherently and support the conclusions.
    Previous work from Dr. Long's group over this last decade has established a requirement for glycolysis in osteoblast differentiation. They showed the requirement for glycolysis not only for the anabolic action of PTH but also as an effector downstream of Wnt signaling. Using the T2D mouse model they have generated, they test if manipulating glycolysis and oxidative phosphorylation can rescue some of the detrimental effects on bone in this model.
    They use several novel approaches, they use glucose-labeling studies that are relatively underutilized, and it provides some insights into defective TCA cycle. They also utilize BMSCs that have been sorted for performing single-cell sequencing studies to identify specific populations modified with T2D. Unfortunately, the results are modest and need some clarification on what these populations add to the story.
    The authors use two approaches: a drug (Metformin) and a number of mouse genetic models to over-express genes involved in the glycolytic pathway using Dox inducible models. The results with overexpressing HIF1 and PFKFB3 show a potential rescue of bone defects with T2D, and Glut1 overexpression does not rescue T2D-induced bone loss.

    Concerns
    The authors have generated several overexpression models to manipulate the glycolytic pathway to recuse T2D-induced bone loss. The use of DOX in drinking water has been shown to affect mitochondrial metabolism. Did the authors control for these effects? Since both the groups of mice got the DOX in drinking water, there is internal control.
    Only one of the rescue experiments had control with the Chow diet. There are some studies that have shown a high-fat diet to be protective of bone loss in TID models.
    The use of metformin to correct metabolic dysfunction and, thereby, bone mass is an exciting result. Did the authors test to see if they had in any way rescued this phenotype because of reducing ROS levels? The decrease in OxsPhos seen with the seahorse experiments suggests there could be mitochondrial dysfunction often associated with ROS generation.
    All of the experiments used male mice (because STZ use and ease of T2D establishment in males). It would be better if this were made clear in the title.
    Is the T2D model presented really represent what is observed in humans? Some experiments to test the other factors implicated in T2D and whether those are modulated in the rescue experiments might help address this.

  6. eLife

    Reviewer #3 (Public Review):

    The manuscript entitled "Osteoblast-intrinsic defect in glucose metabolism impairs bone formation in type II diabetic mice" by Song et al. showed that osteoblast activity was compromised due to impaired glucose metabolism using a youth-onset T2D mouse model. The investigators induced youth-onset T2D in 22-week-old C57BL/6J male mice by a high-fat diet (HFD) starting at 6 weeks of age and injection of low-dose streptozotocin three times at 12-week-old. Then they demonstrated that metformin promoted glycolysis and osteoblast differentiation in vitro and increased bone mass in the diabetic mice. It was also demonstrated that targeted overexpression of Hif1a or Pfkfb3, but not Glut1, in osteoblasts reduced bone loss in T2D mice. Overall, the investigators made a great effort to characterize the changes in metabolism in the bone of the B6/C57 mice by HFD and metformin with microCT, dynamic histomorphometry, C13 isotype labeling in vivo, scRNA-seq and metabolic assays with bone marrow mesenchymal cells in vitro.

  7. Version published to 10.1101/2023.01.16.524248v1 on bioRxiv