Bone marrow ablation demonstrates that excess endogenous parathyroid hormone plays distinct roles in trabecular and cortical bone

Foxf2 represses bone formation via Wnt2b/β-catenin signaling

Differentiation of mesenchymal stem cells (MSCs) into osteoblasts is a critical process for proper skeletal development and acquisition/maintenance of bone mass. However, since this regulatory mechanism has not yet been fully elucidated, the treatment of severe osteoporosis and fractures is a challenge. Here, through a comprehensive analysis of gene expression during the differentiation of MSCs into osteoblasts, we show that the forkhead transcription factor Foxf2 is a crucial regulator of this process. Foxf2 expression transiently increased during MSC osteoblastic differentiation. Overexpression of Foxf2 in MSCs inhibited osteoblastic differentiation, and conversely, knockdown of Foxf2 expression promoted this process. Osteoprogenitor-specific Foxf2 knockout mice developed a high bone mass phenotype due to increased bone formation. RNA-seq analysis and molecular experiments revealed that Foxf2 regulation of bone formation is mediated by Wnt2b. Knockdown of Foxf2 in mouse femurs enhanced bone regeneration in vivo. FOXF2 expression was correlated with hip bone mineral density in postmenopausal women with low bone mass. Finally, inhibition of FOXF2 promoted osteoblastic differentiation of human MSCs. This study uncovers a critical role of Foxf2 in the differentiation of MSCs into osteoblasts and provides insight into the pathogenesis associated with bone-related diseases such as osteoporosis and nonunion after fracture.

Marrow aspiration in aged mice: intramedullary osteogenesis, reduced mechano-adaptation, increased marrow fat

Introduction: With age, the number of adipocytes and osteoclasts increases, the number of osteoblasts decreases, and mechano-adaptation is impaired.Objectives: Using marrow aspiration, which has a known osteogenic effect in young mice, we sought to recruit osteoblast progenitors to mediate the mechano-adaptive response to in vivo tibial loading.Methods: First, we assessed bone formation and marrow adiposity in the tibiae of old mice (>20 months) sacrificed 1, 2, and 4 weeks after unilateral marrow aspiration. Then, we examined the effects of marrow aspiration on mechano-adaptation in aged mice using tibial loading.Results: Two weeks after aspiration, aspirated tibiae had more bone than contralateral tibiae due to the formation of bone in the medullary canal. Two weeks and four weeks after marrow aspiration, the volume of marrow adipose tissue was higher in the aspirated tibiae, compared to contralateral tibiae. Histomorphometry indicated that aspiration increased non-periosteal (endosteal, intracortical, intramedullary) bone formation, compared to the contralateral tibia.  Mice with marrow aspiration had reduced periosteal bone formation in the contralateral tibia, compared to mice that had loading alone. Loading-induced periosteal bone formation was higher in mice that had loading alone, compared to mice that had aspiration + loading, indicating that aspiration further reduced the mechano-adaptive response.Conclusion: These data demonstrate that, in old mice, bone forms in the medullary canal following aspiration. Adiposity is increased following marrow aspiration, and periosteal mechano-adaptation is reduced.

Rictor ablation in BMSCs inhibits bone metastasis of TM40D cells by attenuating osteolytic destruction and CAF formation

The mTOR complex 2 (mTORC2) is recognized as a promising target for breast cancer treatment. As mTORC2-specific inhibitors do not yet exist, studies into the role of mTORC2 in cancer are performed by deleting Rictor or by RNAi-mediated Rictor silencing. The purpose of this study was to explore the effects of Rictor ablation in bone mesenchymal stromal cells (BMSCs) on bone metastasis of breast cancer. First, female mice with the genotype of Prx1-Cre;Rictor^f/f (hereafter RiCKO) or Rictor^f/f (as control) were injected intratibially with cells of the breast cancer cell line (TM40D) at 4 months of age. Three weeks later, osteolytic bone destruction was detected in metastatic legs by X-ray and micro-CT. We found that Rictor ablation in BMSCs inhibited TM40D-induced osteolytic bone destruction and resulted in greater bone volume maintenance in vivo. Lower CTX-I serum level, a decreased number of TRAP+ osteoclasts and lower Cathepsin-K expression observed at the tumor-bone interface indicated that osteoclastogenesis was inhibited in RiCKO mice. Additionally, co-culture experiments confirmed that Rictor deletion in BMSCs diminished osteoclast differentiation partly via down regulation of RANKL expression. Furthermore, Rictor deficiency was found to reduce the transition of BMSCs to CAFs coupled with decreased secretion of cytokines (IL-6, RANKL, TGFβ), which resulted in lower chemotaxis and less proliferation in TM40D cells. These results suggest that Rictor ablation in BMSCs plays dual roles in breast cancer bone metastasis: (1) repression of osteolytic bone destruction; (2) inhibition of tumor growth.

The use of LP533401 as a therapeutic option for renal osteodystrophy affects, renal calcium handling, vitamin D metabolism, and bone health in uremic rats

BACKGROUND

Klotho is a key regulator of phosphate and Ca²⁺-transport in the kidney. Recently, we showed that treatment with LP533401 improved bone health in rats with chronic kidney disease (CKD) via the normalization of serum phosphate resulting from the reduced renal expression of phosphate cotransporters, including Klotho.

METHODS

We evaluated the effect of LP533401 therapy on Klotho-expression-dependent Ca²⁺-transporters, renal calcium handling, and the potential consequences for the bone of uremic rats.

RESULTS

Treatment with LP533401 and its vehicle resulted in the inhibition of transient receptor potential vanilloid receptor subtypes 5 and 6 (TRPV5, TRPV6) and calbindin (CaBP-28k, CaBP-9k) expression. The compensatory acceleration in renal expression of Na+/Ca²⁺-exchanger, 25-hydroxyvitamin d-1α-hydroxylase (CYP27B1), the intensification of vitamin D metabolism, and disruption of sophisticated balance between 1,25-dihydroxyvitamin D-serotonin was observed, especially in rats treated with LP533401. The imbalance between 1,25-dihydroxyvitamin D-serotonin levels led to intensified bone remodeling and improvement in bone geometry, mineral status, and strength in animals treated with LP533401.

CONCLUSION

The modulation of circulating serotonin and its relation to other regulators of calcium handling can play an important role in calcium homeostasis and bone integrity in CKD rats treated with LP533401.

LP533401 restores bone health in 5/6 nephrectomized rats by a decrease of gut-derived serotonin and regulation of serum phosphate through the inhibition of phosphate co-transporters expression in the kidneys

LP533401 is an orally bioavailable small molecule that inhibits tryptophan hydroxylase-1, an enzyme responsible for the synthesis of gut-derived serotonin (GDS). Recently, we showed that increased GDS in rats with chronic kidney disease (CKD) affected bone strength and metabolism. We tested the hypothesis that treatment with LP533401 could reverse CKD-induced bone loss in uremia. Sixteen weeks after 5/6 nephrectomy, rats were randomized into untreated (CKD), treated with vehicle (VEH) and LP533401 at a dose of 30 or 100 mg/kg daily for 8 weeks. Treatment with LP533401 decreased serotonin turnover and restored bone mineral status, microarchitecture, and strength in CKD rats to the values observed in the controls. In parallel with the reduction of serotonin, serum phosphate levels also decreased, particularly in the LP533401, 100 mg/kg group. The mechanism underlying this phenomenon resulted from decreased expression of the renal VDR/FGF1R/Klotho/Npt2a/Npt2c axis, leading to elevated phosphate excretion in the kidneys. The elevated urinary phosphate excretion resulted in improved bone mineral status and strength in LP533401-treated rats. Unexpectedly, the standard VEH used in this model was able to reduce renal VDR/FGF1R/Klotho/Npt2a expression, leading to a compensatory increase in Npt2c mRNA levels, secondary disturbances in phosphate-regulated hormones and partial improvement in the mineral status of the trabecular bone. The decrease of serotonin synthesis together with the simultaneous reduction of renal Npt2a and Npt2c expression in rats treated with LP533401, 100 mg/kg led to an increase in 1,25(OH)₂D₃ levels; this mechanism seems to be particularly beneficial in relation to the mineral status of cortical bone.

The biochemical and histological analysis of subcutaneous calcitonin and intramedullary methylprednisolone on bone repair after bone marrow ablation: an experimental comparative study in rats

Background

Although, glucocorticoid (GC) and calcitonin-induced changes in bone repair have been studied previously, the exact effects of these on fracture healing remain controversial. Hence, the purpose of this experimental study is to determine biochemical and histological effects of locally administrated GC and systemically administrated calcitonin on the kinetics of healing response after bone marrow ablation in rats.

Methods

After having undergone marrow ablation, a steroid-treated group of rats (n = 24) received a single dose of intramedullary methylprednisolone (2 mg/kg), a calcitonin-treated group (n = 24) received intermittently administrated subcutaneous salmon calcitonin (16 IU/kg), and a control group (n = 24) received intramedullary saline (25 μl).

Results

Blood samples taken on days 1, 3, 7, 9, and 15 after ablation showed an increase in serum calcium, alkaline phosphatase (ALP), and phosphate levels in the Calcitonin and Control groups. Levels of calcium and ALP peaked on day 7 after ablation. However, an increase in phosphate levels indicated a biphasic reaction that peaked on the third and ninth day after ablation. Hypercalcemia was not observed in Steroid group because of the inhibition of osteoclastic bone resorption. In that group, the serum levels of ALP and phosphate were lower than baseline levels. The levels of urinary calcium excretion peaked 3 to 7 days after marrow ablation in the control group and 7 to 9 days after that procedure in the steroid group. Histologic evaluation showed that the rats in the control group demonstrated the expected healing period according to the histological grades and that a delay in healing occurred in the calcitonin group after day 9 because of the inhibition of osteoclastic bone resorption. All rats in the steroid group exhibited a decrease and delayed healing response.

Conclusion

Total serum calcium, phosphate, and ALP levels increased after bilateral tibial bone marrow ablation and urine calcium and hydroxyproline excretion also increased as a factor of bone resorption. Subcutaneously administrated salmon calcitonin did not affect biochemical changes after marrow ablation. Single-dose intramedullary methylprednisolone inhibited extra-tibial bone resorption induced by cytokines after bone marrow ablation.

Mechanisms of vasodilation to PTH 1-84, PTH 1-34, and PTHrP 1-34 in rat bone resistance arteries

Parathyroid hormone (PTH) augments bone metabolism and bone mass when given intermittently. Enhanced blood flow is requisite to support high tissue metabolism. The bone arteries are responsive to all three PTH analogs, which may serve to augment skeletal blood flow during intermittent PTH administration.

INTRODUCTION

PTH augments bone metabolism. Yet, mechanisms by which PTH regulates bone blood vessels are unknown. We deciphered (1) endothelium-dependent and endothelium-independent vasodilation to PTH 1-84, PTH 1-34, and PTHrP 1-34, (2) the signaling pathways (i.e., endothelial nitric oxide synthase [eNOS], cyclooxygenase [COX], protein kinase C [PKC], and protein kinase A [PKA]), and (3) receptor activation.

METHODS

Femoral principal nutrient arteries (PNAs) were given cumulative doses (10(-13)-10(-8) M) of PTH 1-84, PTH 1-34, and PTHrP 1-34 with and without signaling pathway blockade. Vasodilation was also determined following endothelial cell removal (i.e., denudation), PTH 1 receptor (PTH1R) inhibition and to sodium nitroprusside (SNP; a nitric oxide [NO] donor).

RESULTS

Vasodilation was lowest to PTH 1-34, and maximal dilation was highest to PTHrP 1-34. Inhibition of eNOS reduced vasodilation to PTH 1-84 (-80 %), PTH 1-34 (-66 %), and PTHrP 1-34 (-48 %), evidencing the contribution of NO. Vasodilation following denudation was eliminated (PTH 1-84 and PTHrP 1-34) and impaired (PTH 1-34, 17 % of maximum), highlighting the importance of endothelial cells for PTH signaling. Denuded and intact PNAs responded similarly to SNP. Both PKA and PKC inhibition diminished vasodilation in all three analogs to varying degrees. PTH1R blockade reduced vasodilation to 1, 12, and 12 % to PTH 1-84, PTH 1-34, and PTHrP 1-34, respectively.

CONCLUSIONS

Vasodilation of femoral PNAs to the PTH analogs occurred via activation of the endothelial cell PTH1R for NO-mediated events. PTH 1-84 and PTHrP 1-34 primarily stimulated PKA signaling, and PTH 1-34 equally stimulated PKA and PKC signaling.

Bone marrow ablation demonstrates that estrogen plays an important role in osteogenesis and bone turnover via an antioxidative mechanism

To assess the effect of estrogen deficiency on osteogenesis and bone turnover in vivo, 8-week-old mice were sham-operated or bilaterally ovariectomized (OVX), and after 8 weeks, mechanical bone marrow ablation (BMX) was performed and newly formed bone tissue was analyzed from 6 days to 2 weeks after BMX. Our results demonstrated that OVX mice following BMX displayed 2 reversed phase changes, one phase observed at 6 and 8 days after BMX delayed osteogenesis accompanied by a delay in osteoclastogenesis, and the other phase observed at 12 and 14 days after BMX increased osteoblastic activity and osteoclastic activity. Furthermore, we asked whether impaired osteogenesis caused by estrogen deficiency was associated with increased oxidative stress, and oxidative stress parameters were examined in bone tissue from sham-operated and OVX mice and OVX mice were administrated with antioxidant N-acetyl-l-cysteine (NAC) or vehicle after BMX. Results demonstrated that estrogen deficiency induced oxidative stress in mouse bone tissue with reduced antioxidase levels and activity, whereas NAC administration almost rescued the abnormalities in osteogenesis and bone turnover caused by OVX. Results from this study indicate that estrogen deficiency resulted in primarily impaired osteogenesis and subsequently accelerated bone turnover by increasing oxidative stress and oxidative stress promises to be an effective target in the process of treatment of postmenopausal osteoporosis.

Endogenous parathyroid hormone-related protein compensates for the absence of parathyroid hormone in promoting bone accrual in vivo in a model of bone marrow ablation

To assess the effect of hypoparathyroidism on osteogenesis and bone turnover in vivo, bone marrow ablation (BMXs) were performed in tibias of 8-week-old wild-type and parathyroid hormone-null (PTH(-/-)) mice and newly formed bone tissue was analyzed from 5 days to 3 weeks after BMX. At 1 week after BMX, trabecular bone volume, osteoblast numbers, alkaline phosphatase-positive areas, type I collagen-positive areas, PTH receptor-positive areas, calcium sensing receptor-positive areas, and expression of bone formation-related genes were all decreased significantly in the diaphyseal regions of bones of PTH(-/-) mice compared to wild-type mice. In contrast, by 2 weeks after BMX, all parameters related to osteoblastic bone accrual were increased significantly in PTH(-/-) mice. At 5 days after BMX, active tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts had appeared in wild-type mice but were undetectable in PTH(-/-) mice, Both the ratio of mRNA levels of receptor activator of NF-κB ligand (RANKL)/osteoprotegerin (OPG) and TRAP-positive osteoclast surface were still reduced in PTH(-/-) mice at 1 week but were increased by 2 weeks after BMX. The expression levels of parathyroid hormone-related protein (PTHrP) at both mRNA and protein levels were upregulated significantly at 1 week and more dramatically at 2 weeks after BMX in PTH(-/-) mice. To determine whether the increased newly formed bones in PTH(-/-) mice at 2 weeks after BMX resulted from the compensatory action of PTHrP, PTH(-/-) PTHrP(+/-) mice were generated and newly formed bone tissue was compared in these mice with PTH(-/-) and wild-type mice at 2 weeks after BMX. All parameters related to osteoblastic bone formation and osteoclastic bone resorption were reduced significantly in PTH(-/-) PTHrP(+/-) mice compared to PTH(-/-) mice. These results demonstrate that PTH deficiency itself impairs osteogenesis, osteoclastogenesis, and osteoclastic bone resorption, whereas subsequent upregulation of PTHrP in osteogenic cells compensates by increasing bone accrual.