Focal adhesion protein vinculin inhibits Mef2c-driven sclerostin expression in osteocytes to promote bone formation in mice

  1. Yishu Wang
  2. Jianmei Huang
  3. Sixiong Lin
  4. Lei Qin
  5. Dingyu Hao
  6. Peijun Zhang
  7. Shaochuan Huo
  8. Xuenong Zou
  9. Di Chen
  10. Guozhi Xiao

  • Curated by eLife

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    eLife assessment

    This is an interesting and valuable study describing the importance of a focal adhesion protein vinculin in osteocytes in controlling bone formation by regulating the expression of sclerostin, which inhibits bone formation. The data are generally convincing and support the conclusions. Some additional investigation and discussions are required to further strengthen the conclusion and interpretation.

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Abstract

The focal adhesion (FA) is the structural basis of the cell-extracellular matrix crosstalk and plays important roles in control of organ formation and function. Here we show that expression of FA protein vinculin is dramatically reduced in osteocytes in patients with aging-related osteoporosis. Deleting vinculin using the mouse 10-kb Dmp1-Cre transgenic mice causes a severe osteopenia in both young and aged mice by impairing osteoblast function without affecting bone resorption. Vinculin loss impairs the anabolic response of skeleton to mechanical loading in mice. At the molecular level, vinculin knockdown increases, while vinculin overexpression decreases, sclerostin expression in osteocytes without impacting expression of Mef2c, a major transcriptional regulator of the Sost gene, which encodes sclerostin. Vinculin interacts with Mef2c and vinculin loss enhances Mef2c nuclear translocation and binding to the Sost enhancer ECR5 to promote sclerostin production in osteocytes. Deleting Sost expression reverses the osteopenic phenotypes caused by vinculin loss in mice. We demonstrate that estrogen is a major regulator of vinculin expression in osteocytes and that more importantly vinculin loss mediates estrogen deficiency induction of bone loss in mice. Thus, we demonstrate a novel mechanism through which vinculin inhibits the Mef2c-driven sclerostin expression in osteocytes to promote bone formation.

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  1. eLife

    Author Response

    Reviewer #1 (Public Review):

    The study by Yang et al. reports a new mechanistic role of vinculin in inhibiting the Mef2c nuclear translation and sclerostin expression in osteocytes and promoting bone formation. The authors showed the reduction of vinculin in aged bone human bone samples. A 10kb DMP-1-Cre mouse model was generated that deleted vinculin in osteocytes. They found that vinculin deletion caused bone loss and decreased bone formation associated with increased sclerostin expression. This increase does not affect the protein level of transcription factor Met2c but interestingly enhances nuclear translocation. Vinculin is interested in Mef2c and appears to retain Mef2c in the cytosol. As expected, as a component of the mechanosensory focal adhesion complex, bone formation via tibial loading was decreased in vinculin deletion. Intriguingly, the bone loss associated with estrogen deficiency through ovariectomy was attenuated. Overall, the study unveiled an important role concerning a key player of focal adhesion and the study was well designed and executed. The paper would be strengthened by including a more thorough discussion including variables such as male vs. female, and cortical vs. trabecular bone as the vinculin deletion appeared to primarily affect trabecular bone while mechanical loading exerts anabolic effects on both bone types. The effect of estrogen deficiency effect is interesting and is worth some discussion.

    Strengths:

    The paper shows a novel mechanism that vinculin retains Mef2c in the cytosol via protein interaction to prevent it from migrating to the nucleus and increases transcription of sclerostin, an inhibitory factor for Wnt/β-catenin signaling, a critical pathway for osteoblast activity and bone formation. They employed various in vivo and in vitro models as well as human tissue samples including generating conditional knockout of vinculin in osteocytes in vivo and vinculin gene knockdown in MLO-Y4 cells. They also used physiological/pathological relevant models, tibial loading, and ovariectomy to study the role of vinculin under mechanical loading and estrogen deficiency. The adopted standard techniques to study bone properties include microCT, bone formation, bone histomorphometry, histochemistry as well as biochemical assays such as immunoprecipitation, ChIP assays, etc.

    The study is comprehensive and thorough and the noticeable uniqueness is that after observing the phenotypes from in vivo data, they further explored the underlying mechanisms using cell models. The experiments in general are well-designed and presented with adequate repeats and statistical analysis. The paper is also logically written and the figures were clearly labeled.

    We highly thank the reviewer for his/her positive comments and helpful suggestions.

    Minor weaknesses:

    More discussion is necessary concerning the potential difference in responses between male and female. Most of the studies were conducted in male mice except ovariectomy mice.

    During the revision, we have added new results from µCT analysis. Our new results showed that vinculin loss significantly reduced bone mass in 6-month-old female mice (Figure 2-figure supplement 1. a-d).

    It is interesting that the cKO of vinculin in osteocytes primarily affects trabecular bones with limited effect on cortical bones. However, sclerostin is increased in cortical bones. The promotion of bone formation by mechanical loading appears to affect both cortical and trabecular bones. If focal adhesion is a key mechanosensory complex, how to reconcile the different responses in the cKO model?

    We thank the reviewer for raising this good point. In fact, we do not know why there was no marked cortical bone loss in cKO mice. During the revision, we performed three-point bending analysis to determine whether vinculin loss impaired in the mechanical properties of the long bone and found that the ultimate force and total energy absorption before fracture were decreased in the femur of 3-month-old male cKO compared to those in control mice (Figure 3-figure supplement 1. e, f). Furthermore, our new results from the calcein double labeling experiments showed that both MAR and BFR of femur cortical bones were slightly but significantly reduced in cKO mice relative to those in control mice (Figure 3-figure supplement 1. a-c).

    The OVX response is interesting and it is worthwhile to elaborate more regarding the potential underlying mechanism and what's the relationship between estrogen and mechanical loading and if the action of estrogen on vinculin shares any similar mechanisms with mechanical loading, etc.

    We feel that the relationship between estrogen and mechanical loading could be quite complex, which deserves further investigation in the future. Thank you for this good point.

    Reviewer #3 (Public Review):

    This study by Wang et al. investigates the role of the focal adhesion protein vinculin in osteocytes and its effect on bone mass. First, they showed decreased levels of vinculin in osteocytes in trabecular bone from aged individuals compared to young, suggesting a potential role for vinculin in regulating bone mass with aging. Next, they deleted vinculin in late osteoblasts and osteocytes in young and older mice and found decreased bone mineral density and trabecular bone mass. This was due to impaired bone formation, which the authors attributed to increased sclerostin levels. Further in vitro experiments showed that vinculin regulates sclerostin via the transcription factor Mefc2. Conditional knockout of vinculin in late osteoblasts and osteocytes had no effect on the bone of mice lacking Sost, further implicating an essential role for sclerostin in mediating the effects of vinculin in osteocytes. Interestingly, the vinculin conditional knockout mice had an impaired response to mechanical loading, suggesting an important role for vinculin in the osteocyte mechanoresponse. Finally, the authors showed that while ovariectomy increased osteoclast formation and bone resorption in control mice, it had no effect on the bone of the vinculin conditional knockout mice.

    Overall, the authors show convincing data for the important role of vinculin in osteocytes in regulating the anabolic effects of bone formation under physiological conditions. They also show that osteocyte vinculin may be a regulator of bone resorption under conditions mimicking postmenopausal osteoporosis. However, not all of the conclusions are fully supported by the data.

    Strengths:

    The use of both in vivo and in vitro approaches to determine the role of vinculin in osteocytes provides compelling evidence for its importance under basal conditions and in regulating the anabolic effects of mechanical loading. The in vitro assays nicely demonstrate a potential mechanism through Mef2c/ECR5.

    The creation of the vinculin and Sost double conditional knockout mouse model provides further convincing evidence for the causative role of sclerostin in the effects of vinculin knockout in osteocytes.

    The use of both young and older male mice links nicely with the human samples where vinculin expression appears to be reduced in osteocytes with aging. The authors need to be careful in describing 14-month-old mice as aged though, as these mice would not be typically thought of as old.

    Weaknesses:

    The methods section is lacking in basic details (e.g., there is no information on the CRISPR deletion of Vcl in the MLO-Y4 cells). While referencing their previous papers is fine, a brief description of the methods should be included in this paper.

    During the revision, we have added the method of CRISPR deletion of Vcl in the MLO-Y4 cells (page 22, line 16-20).

    While much of the data linking vinculin to sclerostin is convincing, it is surprising that the authors show decreased trabecular bone volume in the vinculin cKO mice, yet show increased sclerostin levels in the cortical bone. If increased sclerostin is responsible for impaired bone formation in the vinculin cKO mice, why is there no cortical bone phenotype?

    Please see our response to above Point 3.

    It would be important for the authors to also show the sclerostin immunostaining in the trabecular bone of these animals.

    During the revision, we utilized IHC to demonstrate that cKO mice also displayed increased level of the sclerostin protein in cortical bone compared to the control group (Figure 4-figure supplement 1. a, b).

    The authors do not provide any potential explanation for the effects of vinculin cKO in the ovariectomized mice. Under physiological conditions, osteocyte vinculin has no effect on osteoclast number or bone resorption. How is osteocyte vinculin affecting osteoclasts after ovariectomy? Are there differences in the osteocyte expression of Rankl or Opg in response to the loss of estrogen in the vinculin cKO and control mice?

    During the revision, we used IHC staining to measure the expression of Rankl and Opg in the cortical bone of control and cKO mice treated with or without OVX surgery. The results showed that in cKO mice, the increase in Rankl induced by OVX was lower than that in the control group, while the increase in Opg induced by OVX was higher than that in the control group. The Rankl/Opg ratio was decreased in cKO mice induced by OVX (Figure 7-figure supplement 1. a-d).

    From their in vitro experiments, the authors deduce that loss of vinculin affects osteocyte attachment. However, their images would suggest that it is the formation of dendrites that is strongly inhibited in the cells lacking vinculin. It is surprising that no investigation of osteocyte dendrite number or connectivity was performed in the vinculin cKO mice. This is particularly important as a decrease in osteocyte dendrites and connectivity has been observed in the bones of aged mice (see Tiede-Lewis et al., Aging. 2017) and osteocyte dendrites are important for mechanosensation.

    Please see our response to above Point 4.

  2. eLife

    eLife assessment

    This is an interesting and valuable study describing the importance of a focal adhesion protein vinculin in osteocytes in controlling bone formation by regulating the expression of sclerostin, which inhibits bone formation. The data are generally convincing and support the conclusions. Some additional investigation and discussions are required to further strengthen the conclusion and interpretation.

  3. eLife

    Reviewer #1 (Public Review):

    The study by Yang et al. reports a new mechanistic role of vinculin in inhibiting the Mef2c nuclear translation and sclerostin expression in osteocytes and promoting bone formation. The authors showed the reduction of vinculin in aged bone human bone samples. A 10kb DMP-1-Cre mouse model was generated that deleted vinculin in osteocytes. They found that vinculin deletion caused bone loss and decreased bone formation associated with increased sclerostin expression. This increase does not affect the protein level of transcription factor Met2c but interestingly enhances nuclear translocation. Vinculin is interested in Mef2c and appears to retain Mef2c in the cytosol. As expected, as a component of the mechanosensory focal adhesion complex, bone formation via tibial loading was decreased in vinculin deletion. Intriguingly, the bone loss associated with estrogen deficiency through ovariectomy was attenuated. Overall, the study unveiled an important role concerning a key player of focal adhesion and the study was well designed and executed. The paper would be strengthened by including a more thorough discussion including variables such as male vs. female, and cortical vs. trabecular bone as the vinculin deletion appeared to primarily affect trabecular bone while mechanical loading exerts anabolic effects on both bone types. The effect of estrogen deficiency effect is interesting and is worth some discussion.

    Strengths:
    The paper shows a novel mechanism that vinculin retains Mef2c in the cytosol via protein interaction to prevent it from migrating to the nucleus and increases transcription of sclerostin, an inhibitory factor for Wnt/β-catenin signaling, a critical pathway for osteoblast activity and bone formation.
    They employed various in vivo and in vitro models as well as human tissue samples including generating conditional knockout of vinculin in osteocytes in vivo and vinculin gene knockdown in MLO-Y4 cells. They also used physiological/pathological relevant models, tibial loading, and ovariectomy to study the role of vinculin under mechanical loading and estrogen deficiency. The adopted standard techniques to study bone properties include microCT, bone formation, bone histomorphometry, histochemistry as well as biochemical assays such as immunoprecipitation, ChIP assays, etc.

    The study is comprehensive and thorough and the noticeable uniqueness is that after observing the phenotypes from in vivo data, they further explored the underlying mechanisms using cell models. The experiments in general are well-designed and presented with adequate repeats and statistical analysis. The paper is also logically written and the figures were clearly labeled.

    Minor weaknesses:
    More discussion is necessary concerning the potential difference in responses between male and female. Most of the studies were conducted in male mice except ovariectomy mice.
    It is interesting that the cKO of vinculin in osteocytes primarily affects trabecular bones with limited effect on cortical bones. However, sclerostin is increased in cortical bones. The promotion of bone formation by mechanical loading appears to affect both cortical and trabecular bones. If focal adhesion is a key mechanosensory complex, how to reconcile the different responses in the cKO model?
    The OVX response is interesting and it is worthwhile to elaborate more regarding the potential underlying mechanism and what's the relationship between estrogen and mechanical loading and if the action of estrogen on vinculin shares any similar mechanisms with mechanical loading, etc.

  4. eLife

    Reviewer #2 (Public Review):

    In this interesting study, Wang et al. demonstrated a critical role of the key focal adhesion protein vinculin in the control of bone mass in mice. Specifically, the authors deleted vinculin expression by using the mouse 10-kb Dmp1-Cre transgenic mice that were reported to primarily target osteocytes and mature osteoblasts. The authors found that vinculin loss in these cells caused severe osteopenia in mice due to impairment of osteoblast and bone formation with minimal impact on osteoclast formation and bone resorption. Interestingly, the vinculin loss also reduced the mechanical loading induction of bone formation in mice. Mechanically, the authors found that vinculin knockdown increased, while vinculin overexpression decreased, sclerostin expression in osteocytes without affecting that of Mef2c, a major transcriptional regulator of the Sost gene, which encodes sclerostin. Mechanistically, the authors found that vinculin protein bound to Mef2c and vinculin loss increased Mef2c nuclear translocation and binding to the Sost enhancer ECR5. Deleting Sost expression largely reversed the osteopenic phenotypes caused by vinculin deletion. Finally, the authors demonstrated that estrogen promoted vinculin expression in osteocytes and that vinculin loss abolished the estrogen deficiency induction of bone loss in mice. In this study, with a tremendous amount of convincing in vitro and in vivo data, the authors have established a critical role of vinculin in bone and defined a novel mechanism that regulates bone mass. The findings from this study are important and interesting.

  5. eLife

    Reviewer #3 (Public Review):

    This study by Wang et al. investigates the role of the focal adhesion protein vinculin in osteocytes and its effect on bone mass. First, they showed decreased levels of vinculin in osteocytes in trabecular bone from aged individuals compared to young, suggesting a potential role for vinculin in regulating bone mass with aging. Next, they deleted vinculin in late osteoblasts and osteocytes in young and older mice and found decreased bone mineral density and trabecular bone mass. This was due to impaired bone formation, which the authors attributed to increased sclerostin levels. Further in vitro experiments showed that vinculin regulates sclerostin via the transcription factor Mefc2. Conditional knockout of vinculin in late osteoblasts and osteocytes had no effect on the bone of mice lacking Sost, further implicating an essential role for sclerostin in mediating the effects of vinculin in osteocytes. Interestingly, the vinculin conditional knockout mice had an impaired response to mechanical loading, suggesting an important role for vinculin in the osteocyte mechanoresponse. Finally, the authors showed that while ovariectomy increased osteoclast formation and bone resorption in control mice, it had no effect on the bone of the vinculin conditional knockout mice.

    Overall, the authors show convincing data for the important role of vinculin in osteocytes in regulating the anabolic effects of bone formation under physiological conditions. They also show that osteocyte vinculin may be a regulator of bone resorption under conditions mimicking postmenopausal osteoporosis. However, not all of the conclusions are fully supported by the data.

    Strengths:

    The use of both in vivo and in vitro approaches to determine the role of vinculin in osteocytes provides compelling evidence for its importance under basal conditions and in regulating the anabolic effects of mechanical loading. The in vitro assays nicely demonstrate a potential mechanism through Mef2c/ECR5.

    The creation of the vinculin and Sost double conditional knockout mouse model provides further convincing evidence for the causative role of sclerostin in the effects of vinculin knockout in osteocytes.

    The use of both young and older male mice links nicely with the human samples where vinculin expression appears to be reduced in osteocytes with aging. The authors need to be careful in describing 14-month-old mice as aged though, as these mice would not be typically thought of as old.

    Weaknesses:

    The methods section is lacking in basic details (e.g., there is no information on the CRISPR deletion of Vcl in the MLO-Y4 cells). While referencing their previous papers is fine, a brief description of the methods should be included in this paper.

    While much of the data linking vinculin to sclerostin is convincing, it is surprising that the authors show decreased trabecular bone volume in the vinculin cKO mice, yet show increased sclerostin levels in the cortical bone. If increased sclerostin is responsible for impaired bone formation in the vinculin cKO mice, why is there no cortical bone phenotype? It would be important for the authors to also show the sclerostin immunostaining in the trabecular bone of these animals.

    The authors do not provide any potential explanation for the effects of vinculin cKO in the ovariectomized mice. Under physiological conditions, osteocyte vinculin has no effect on osteoclast number or bone resorption. How is osteocyte vinculin affecting osteoclasts after ovariectomy? Are there differences in the osteocyte expression of Rankl or Opg in response to the loss of estrogen in the vinculin cKO and control mice?

    From their in vitro experiments, the authors deduce that loss of vinculin affects osteocyte attachment. However, their images would suggest that it is the formation of dendrites that is strongly inhibited in the cells lacking vinculin. It is surprising that no investigation of osteocyte dendrite number or connectivity was performed in the vinculin cKO mice. This is particularly important as a decrease in osteocyte dendrites and connectivity has been observed in the bones of aged mice (see Tiede-Lewis et al., Aging. 2017) and osteocyte dendrites are important for mechanosensation.

  6. Version published to 10.1101/2022.11.09.515817v1 on bioRxiv