Bisphosphonates improve trabecular bone mass and normalize cortical thickness in ovariectomized, osteoblast connexin43 deficient mice

S-Allylmercapto-N-Acetylcysteine (ASSNAC) Attenuates Osteoporosis in Ovariectomized (OVX) Mice

Osteoporosis is a bone-debilitating disease, demonstrating a higher prevalence in post-menopausal women due to estrogen deprivation. One of the main mechanisms underlying menopause-related bone loss is oxidative stress. S-allylmercapto-N-acetylcysteine (ASSNAC) is a nuclear factor erythroid 2-related factor 2 (Nrf2) activator and cysteine supplier, previously shown to have anti-oxidation protective effects in cultured cells and animal models. Here, we studied the therapeutic potential of ASSNAC with and without Alendronate in ovariectomized (OVX) female mice. The experimental outcome included (i) femur and L3 lumbar vertebra morphometry via Micro-Computed Tomography (μCT); (ii) bone remodeling (formation vs. resorption); and (iii) oxidative stress markers in bone marrow (BM) cells. Four weeks after OVX, there was a significant bone loss that remained evident after 8 weeks, as demonstrated via µCT in the femur (cortical and trabecular bone compartments) and vertebra (trabecular bone). ASSNAC at a dose of 50 mg/Kg/day prevented bone loss after the four-week treatment but had no significant effect after 8 weeks, while ASSNAC at a dose of 20 mg/Kg/day significantly protected against bone loss after 8 weeks of treatment. Alendronate prevented ovariectomy-induced bone loss, and combining it with ASSNAC further augmented this effect. OVX mice demonstrated high serum levels of both C-terminal cross-linked telopeptides of type I collagen (CTX) (bone resorption) and procollagen I N-terminal propeptide (P1NP) (bone formation) after 2 weeks, and these returned to control levels after 8 weeks. Alendronate, ASSNAC and their combination decreased CTX and increased P1NP. Alendronate induced oxidative stress as reflected by decreased glutathione and increased malondialdehyde (MDA) levels, and combining it with ASSNAC partially attenuated these changes. These results portray the therapeutic potential of ASSNAC for the management of post-menopausal osteoporosis. Furthermore, ASSNAC ameliorates the Alendronate-associated oxidative stress, suggesting its potential to prevent Alendronate side effects as well as improve its bone-protective effect.

Connexin 43 hemichannels and prostaglandin E2 release in anabolic function of the skeletal tissue to mechanical stimulation

Bone adapts to changes in the physical environment by modulating remodeling through bone resorption and formation to maintain optimal bone mass. As the most abundant connexin subtype in bone tissue, connexin 43 (Cx43)-forming hemichannels are highly responsive to mechanical stimulation by permitting the exchange of small molecules (<1.2 kDa) between bone cells and the extracellular environment. Upon mechanical stimulation, Cx43 hemichannels facilitate the release of prostaglandins E2 (PGE2), a vital bone anabolic factor from osteocytes. Although most bone cells are involved in mechanosensing, osteocytes are the principal mechanosensitive cells, and PGE2 biosynthesis is greatly enhanced by mechanical stimulation. Mechanical stimulation-induced PGE2 released from osteocytic Cx43 hemichannels acts as autocrine effects that promote β-catenin nuclear accumulation, Cx43 expression, gap junction function, and protects osteocytes against glucocorticoid-induced osteoporosis in cultured osteocytes. In vivo, Cx43 hemichannels with PGE2 release promote bone formation and anabolism in response to mechanical loading. This review summarizes current in vitro and in vivo understanding of Cx43 hemichannels and extracellular PGE2 release, and their roles in bone function and mechanical responses. Cx43 hemichannels could be a significant potential new therapeutic target for treating bone loss and osteoporosis.

Preclinical models of medication-related osteonecrosis of the jaw (MRONJ)

Medication-related osteonecrosis of the jaw (MRONJ) is a potentially severe adverse event affecting patients with cancer and patients with osteoporosis who have been treated with powerful antiresorptives (pARs) or angiogenesis inhibitors (AgIs). pARs, including nitrogen-containing bisphosphonates (N-BPs; e.g., zoledronic acid, alendronate) and anti-RANKL antibodies (e.g., denosumab), are used to manage bone metastases in patients with cancer or to prevent fragility fractures in patients with osteoporosis. Though significant advances have been made in understanding MRONJ, its pathophysiology is still not fully elucidated. Multiple species have been used in preclinical MRONJ research, including the rat, mouse, rice rat, rabbit, dog, sheep, and pig. Animal research has contributed immensely to advancing the MRONJ field, particularly, but not limited to, in developing models and investigating risk factors that were first observed in humans. MRONJ models have been developed using clinically relevant doses of systemic risk factors, like N-BPs, anti-RANKL antibodies, or AgIs. Specific local oral risk factors first noted in humans, including tooth extraction and inflammatory dental disease (e.g., periodontitis, periapical infection, etc.), were then added. Research in rodents, particularly the rat, and, to some extent, the mouse, across multiple laboratories, has contributed to establishing multiple relevant and complementary preclinical models. Models in larger species produced accurate clinical and histopathologic outcomes suggesting a potential role for confirming specific crucial findings from rodent research. We view the current state of animal models for MRONJ as good. The rodent models are now reliable enough to produce large numbers of MRONJ cases that could be applied in experiments testing treatment modalities. The course of MRONJ, including stage 0 MRONJ, is characterized well enough that basic studies of the molecular or enzyme-level findings in different MRONJ stages are possible. This review provides a current overview of the existing models of MRONJ, their more significant features and findings, and important instances of their application in preclinical research.

Osteoporosis-decreased extracellular matrix stiffness impairs connexin 43-mediated gap junction intercellular communication in osteocytes

Osteocytes are the main sensitive and responsive cells for mechanical stimuli in bone. The connexin family enables them to communicate with each other via forming functional gap junctions. However, how osteoporosis-impaired extracellular mechanical property modulates gap junction intercellular communication in osteocytes remains elusive. In this study, we established an ovariectomy (OVX)-induced osteoporosis mouse model in vivo and a polydimethylsiloxane (PDMS)-based cell culture substrate model in vitro to explore the influence of extracellular matrix (ECM) stiffness on cell-to-cell communication in osteocytes. Firstly, we established an OVX-induced osteoporosis mouse model by characterizing the changes in radiography, morphology and histochemistry of femurs. Our results showed that osteoporosis decreased the bone matrix stiffness together with the changes including the loss of osteocytes and the decrease of protein markers. Meanwhile, the dendritic process interconnection and channel-forming protein, Cx43, were reduced in osteoporosis mice. Next we mimicked ECM stiffness changes in vitro by using PDMS substrates at ratios 1:5 for normal stiffness and 1:45 for osteoporosis stiffness. Our results showed that the decreased ECM stiffness reduced the number of dendritic processes in a single cell and gap junctions between adjacent osteocytes. We further detected the decreased expression of Cx43, in the substrate with decreased stiffness. Finally, we found that gap junction-based intercellular communication was reduced in living osteocytes in the substrate with decreased stiffness. This study demonstrates the correlation between ECM mechanical property and cell-to-cell communication in osteocytes and might pave the way for further exploration of osteoporosis in terms of biomechanics.

ATRAID regulates the action of nitrogen-containing bisphosphonates on bone

Nitrogen-containing bisphosphonates (N-BPs), such as alendronate, are the most widely prescribed medications for diseases involving bone, with nearly 200 million prescriptions written annually. Recently, widespread use of N-BPs has been challenged due to the risk of rare but traumatic side effects such as atypical femoral fracture (AFF) and osteonecrosis of the jaw (ONJ). N-BPs bind to and inhibit farnesyl diphosphate synthase, resulting in defects in protein prenylation. Yet, it remains poorly understood what other cellular factors might allow N-BPs to exert their pharmacological effects. Here, we performed genome-wide studies in cells and patients to identify the poorly characterized gene, ATRAID Loss of ATRAID function results in selective resistance to N-BP-mediated loss of cell viability and the prevention of alendronate-mediated inhibition of prenylation. ATRAID is required for alendronate inhibition of osteoclast function, and ATRAID-deficient mice have impaired therapeutic responses to alendronate in both postmenopausal and senile (old age) osteoporosis models. Last, we performed exome sequencing on patients taking N-BPs that suffered ONJ or an AFF. ATRAID is one of three genes that contain rare nonsynonymous coding variants in patients with ONJ or an AFF that is also differentially expressed in poor outcome groups of patients treated with N-BPs. We functionally validated this patient variation in ATRAID as conferring cellular hypersensitivity to N-BPs. Our work adds key insight into the mechanistic action of N-BPs and the processes that might underlie differential responsiveness to N-BPs in people.

Improving the Hip Fracture Risk Prediction Through 2D Finite Element Models From DXA Images: Validation Against 3D Models

Osteoporotic fracture incidence represents a major social and economic concern in the modern society, where the progressive graying of the population involves an highly increased fracture occurrence. Although the gold standard to diagnose osteoporosis is represented by the T-score measurement, estimated from the Bone Mineral Density (BMD) using Dual-energy X-ray Absorptiometry (DXA), the identification of the subjects at high risk of fracture still remains an issue. From this perspective, the purpose of this work is to investigate the role that DXA-based two-dimensional patient-specific finite element (FE) models of the proximal femur, in combination with T-score, could play in enhancing the risk of fracture estimation. With this aim, 2D FE models were built from DXA images of the 28 post-menopausal female subjects involved. A sideways fall condition was reproduced and a Risk of Fracture (RF^) was computed on the basis of principal strains criteria. The identified RF^ was then compared to that derived from the CT-based models developed in a previous study. The 2D and 3D RF^ turned out to be significantly correlated (Spearman's ρ = 0.66, p < 0.001), highlighting the same patients as those at higher risk. Moreover, the 2D RF^ resulted significantly correlated with the T-score (Spearman's ρ = −0.69, p < 0.001), and managed to better differentiate osteopenic patients, drawing the attention to some of them. The Hip Structural Analysis (HSA) variables explaining the majority of the variance of the 2D and 3D fracture risk were the same as well, i.e., neck-shaft angle and narrow neck buckling ratio. In conclusion, DXA-based FE models, developable from currently available clinical data, appear promising in supporting and integrating the present diagnostic procedure.

Connexin 43 hemichannels protect bone loss during estrogen deficiency

Estrogen deficiency in postmenopausal women is a major cause of bone loss, resulting in osteopenia, osteoporosis, and a high risk for bone fracture. Connexin 43 (Cx43) hemichannels (HCs) in osteocytes play an important role in osteocyte viability, bone formation, and remodeling. We showed here that estrogen deficiency reduced Cx43 expression and HC function. To determine if functional HCs protect osteocytes and bone loss during estrogen deficiency, we adopted an ovariectomy model in wild-type (WT) and two transgenic Cx43 mice: R76W (dominant-negative mutant inhibiting only gap junction channels) and Cx43 Δ130–136 (dominant-negative mutant compromising both gap junction channels and HCs). The bone mineral density (BMD), bone structure, and histomorphometric changes of cortical and trabecular bones after ovariectomy were investigated. Our results showed that the Δ130–136 transgenic cohort had greatly decreased vertebral trabecular bone mass compared to WT and R76W mice, associated with a significant increase in the number of apoptotic osteocyte and empty lacunae. Moreover, osteoclast surfaces in trabecular and cortical bones were increased after ovariectomy in the R76W and WT mice, respectively, but not in ∆130–136 mice. These data demonstrate that impairment of Cx43 HCs in osteocytes accelerates vertebral trabecular bone loss and increase in osteocyte apoptosis, and further suggest that Cx43 HCs in osteocytes protect trabecular bone against catabolic effects due to estrogen deficiency.

Channels that form between cells and their extracellular environment help protect bone tissue from the damage wrought by low estrogen levels, a major cause of bone loss in post-menopausal women. Jean Jiang from the UT Health San Antonio and colleagues showed that depleting the estrogen hormone in mouse bone cells reduced levels of connexin 43 and impaired the protein’s ability to forms pores known as ‘hemichannels’. The researchers surgically removed the ovaries of various mouse strains to induce estrogen deficiencies. They found that transgenic mice without working hemichannels had reduced bone mass compared to normal mice or mice that could make hemichannels but lacked the ability for those channels to come together to form complete passageways. The findings highlight the importance of connexin 43 hemichannels in protecting bone tissue against osteoporosis.

Connexin43 regulates osteoprotegerin expression via ERK1/2 -dependent recruitment of Sp1

In bone, connexin43 expression in cells of the osteoblast lineage plays an important role in restraining osteoclastogenesis and bone resorption. While there is a consensus around the notion that the anti-osteoclastogenic factor, osteoprotegerin, is a driver of this effect, how connexin43 regulates osteoprotegerin gene expression is unclear. Here, we show that loss of connexin43 decreased osteoprotegerin gene expression and reduced ERK1/2 activation. Conversely, overexpression of connexin43 increased osteoprotegerin expression and enhanced ERK1/2 activation. This increase in phospho-ERK1/2 is required for connexin43 to induce transcription from the osteoprotegerin proximal promoter. Connexin43 increased promoter activity via a specific 200 base pair region of the osteoprotegerin promoter located at -1486 to -1286 with respect to the transcriptional start site, a region which includes four Sp1 binding elements. Further, activation of this promoter region required an intact functional connexin43, as hypomorphic or dominant negative connexin43 mutant constructs, including one with increased hemichannel activity, were unable to stimulate osteoprotegerin expression as strongly as wild type connexin43. Using chromatin immunoprecipitations, we show that connexin43 expression enhanced the recruitment of Sp1, but not Runx2, to the osteoprotegerin proximal promoter. In total, these data show that connexin43-dependent gap junctional communication among osteoblast cells permits efficient ERK1/2 activation. ERK1/2 signaling promotes the recruitment of the potent transcriptional activator, Sp1, to the osteoprotegerin proximal promoter, resulting in robust transcription of anti-osteoclastogenic factor, osteoprotegerin.

The effects of bisphosphonate on pain-related behavior and immunohistochemical analyses in hindlimb-unloaded mice

OBJECTIVE

The aim of this study was to evaluate skeletal pain associated with immobility-induced osteoporosis and to examine the inhibitory effect of bisphosphonate (BP) administration on pain in hindlimb-unloaded (HU) mice.

METHODS

The mechanism of osteoporotic pain in HU mice was evaluated through an examination of pain-related behavior, as well as immunohistochemical findings. In addition, the effects of alendronate (ALN), a potent osteoclast inhibitor, on these parameters were assessed.

RESULTS

HU mice with tail suspension developed bone loss and mechanical hyperalgesia in the hindlimbs. The HU mice showed an increase in the number of calcitonin gene-related peptide (CGRP)-immunoreactive neurons and in transient receptor potential channel vanilloid subfamily member 1 (TRPV1)-immunoreactive neurons in the dorsal root ganglions (DRGs) innervating the hindlimbs. Furthermore, administration of ALN prevented HU-induced bone loss, mechanical hyperalgesia, and upregulation of CGRP and TRPV1 expressions in DRG neurons of immobility-induced osteoporotic animal models.

CONCLUSIONS

HU mice appear to be a useful model for immobility-induced osteoporotic pain and hindlimb-unloading-induced bone loss, as well as upregulation of CGRP and TRPV1 expressions in DRG neurons, and BP treatment prevented bone loss and mechanical hyperalgesia. The inhibitory effect of BP on osteoclast function might contribute to improving osteoporosis-related pain.

Osteoporotic hip fracture prediction: is T-score based criterion enough? A Hip Structural Analysis based model

At present, the current gold-standard for osteoporosis diagnosis is based on bone mineral density measurement, which, however, has been demonstrated to poorly estimate fracture risk. Further parameters in the hands of the clinicians are represented by the Hip Structural Analysis (HSA) variables, which include geometric information of the proximal femur cross-section. The purpose of this study was to investigate the suitability of HSA parameters as additional hip fracture risk predictors. With this aim, twenty-eight three-dimensional patient-specific models of the proximal femur were built from CT images and a sideways fall condition was reproduced by finite element analyses. A tensile or compressive predominance based on minimum and maximum principal strains was determined at each volume element and a Risk Factor (RF) was calculated. The power of HSA variables combinations to predict the maximum superficial RF values was assessed by multivariate linear regression analysis. The optimal regression model, identified through the Akaike information criterion, only comprises two variables, the buckling ratio and the neck-shaft angle. In order to validate the study, the model was tested on two additional patients who suffered a hip fracture after a fall. The results classified the patients in the high risk level, confirming the prediction power of the adopted model.

CMHX008, a PPARγ partial agonist, enhances insulin sensitivity with minor influences on bone loss

Traditional thiazolidinediones (TZDs), such as rosiglitazone, are peroxisome proliferator-activated receptor γ (PPARγ) potent agonists that can be used to treat type 2 diabetes but carry unwanted effects, including increased risk for fracture. The present work aimed to compare the insulin-sensitizing efficacies and bone-loss side effects of CMHX008, a novel TZDs-like PPARγ partial agonist, with those of rosiglitazone. A TR-FRET PPARγ competitive binding assay was used to compare the binding affinity between CMHX008 and rosiglitazone. Mice were administered vehicle, CMHX008 or rosiglitazone for 16 weeks. Mesenchymal stem cells (MSCs) were used to examine differences in differentiation into osteoblasts after compounds treatment. TR-FRET showed lower affinity to PPARγ by CMHX008 compared with rosiglitazone. Mice treated with CMHX008 showed insulin sensitization similar to that of mice treated with rosiglitazone, which was related to the significant inhibition of PPARγ Ser273 phosphorylation and improved insulin sensitivity by facilitating the phosphorylation of insulin receptor and Akt in adipose tissues. Micro-CT and histomorphometric analyses demonstrated that the degree of trabecular bone loss after treatment with CMHX008 was weaker than that observed with rosiglitazone, as evidenced by consistent changes in BV/TV, Tb.N, Tb.Th, Tb.Sp, and the mineral apposition rate. MSCs treated with CMHX008 showed higher ALP activity and mRNA levels of bone formation markers than did cells treated with rosiglitazone in the osteoblast differentiation test. Thus, CMHX008 showed insulin-sensitizing effects similar to those of rosiglitazone with a lower risk of bone loss, suggesting that PPARγ sparing eliminates the skeletal side effects of TZDs while maintaining their insulin-sensitizing properties.

Strontium and bisphosphonate coated iron foam scaffolds for osteoporotic fracture defect healing

The purpose of this work was to investigate new bone formation in macroporous iron foams coated with strontium (FeSr) or bisphosphonate (FeBiP) compared to plain iron foam (Fe) and empty defect in a critical size metaphyseal bone defect model in ovariectomized rats. 60 female rats were subjected to bilateral ovariectomy and multi-deficient diet for 3 months. A 4 mm wedge shaped metaphyseal osteotomy was created, fixed with a mini-plate and subsequently filled with Fe, FeSr, FeBiP or left empty. After 6 weeks, μCt analysis revealed a statistically significant increased bone formation at the implant interface in FeSr compared to FeBiP (p = 0.035) and Fe (p = 0.002), respectively. Increased mineralized tissue was also seen within the pores in FeSr (p = 0.023) compared to Fe. Histomorphometry revealed significantly increased bone formation at the implant interface in FeSr (p < 0.001) and FeBiP (p = 0.006) compared to plain Fe with increased osteoblast and decreased osteoclast activity in combination with increased BMP2 and decreased RANKL/OPG in immunohistochemistry. ToF-SIMS analysis showed overlapping Ca signals with Fe for both FeSr and FeBiP thereby indicating tissue in-growth into the scaffolds. In conclusion, iron foam with strontium or bisphosphonate coating are of further interest in metaphyseal fracture defects in osteopenic bone.

Heterozygous deletion of both sclerostin (Sost) and connexin43 (Gja1) genes in mice is not sufficient to impair cortical bone modeling

Connexin43 (Cx43) is the main gap junction protein expressed in bone forming cells, where it modulates peak bone mass acquisition and cortical modeling. Genetic ablation of the Cx43 gene (Gja1) results in cortical expansion with accentuated periosteal bone formation associated with decreased expression of the Wnt inhibitor sclerostin. To determine whether sclerostin (Sost) down-regulation might contribute to periosteal expansion in Gja1 deficient bones, we took a gene interaction approach and crossed mice harboring germline null alleles for Gja1 or Sost to generate single Gja1^+/–and Sost^+/–and double Gja1^+/–;Sost^+/–heterozygous mice. In vivo μCT analysis of cortical bone at age 1 and 3 months confirmed increased thickness in Sost^–/–mice, but revealed no cortical abnormalities in single Gja1^+/–or Sost^+/–mice. Double heterozygous Gja1^+/–Sost^+/–also showed no differences in mineral density, cortical thickness, width or geometry relative to wild type control mice. Likewise, 3-point bending measurement of bone strength revealed no significant differences between double Gja1^+/–;Sost^+/–or single heterozygous and wild type mice. Although these data do not exclude a contribution of reduced sclerostin in the cortical expansion seen in Gja1 deficient bones, they are not consistent with a strong genetic interaction between Sost and Gja1 dictating cortical modeling.

N-cadherin Regulation of Bone Growth and Homeostasis Is Osteolineage Stage-Specific

N-cadherin inhibits osteogenic cell differentiation and canonical Wnt/β-catenin signaling in vitro. However, in vivo both conditional Cdh2 ablation and overexpression in osteoblasts lead to low bone mass. We tested the hypothesis that N-cadherin has different effects on osteolineage cells depending upon their differentiation stage. Embryonic conditional osteolineage Cdh2 deletion in mice results in defective growth, low bone mass, and reduced osteoprogenitor number. These abnormalities are prevented by delaying Cdh2 ablation until 1 month of age, thus targeting only committed and mature osteoblasts, suggesting they are the consequence of N-cadherin deficiency in osteoprogenitors. Indeed, diaphyseal trabecularization actually increases when Cdh2 is ablated postnatally. The sclerostin-insensitive Lrp5^A214V mutant, associated with high bone mass, does not rescue the growth defect, but it overrides the low bone mass of embryonically Cdh2-deleted mice, suggesting N-cadherin interacts with Wnt signaling to control bone mass. Finally, bone accrual and β-catenin accumulation after administration of an anti-Dkk1 antibody are enhanced in N-cadherin-deficient mice. Thus, although lack of N-cadherin in embryonic and perinatal age is detrimental to bone growth and bone accrual, in adult mice loss of N-cadherin in osteolineage cells favors bone formation. Hence, N-cadherin inhibition may widen the therapeutic window of osteoanabolic agents. © 2017 American Society for Bone and Mineral Research.

Connexin43 and the Intercellular Signaling Network Regulating Skeletal Remodeling

PURPOSE OF THE REVIEW

This review highlights recent developments into how intercellular communication through connexin43 facilitates bone modeling and remodeling.

RECENT FINDINGS

Connexin43 is required for both skeletal development and maintenance, particularly in cortical bone, where it carries out multiple functions, including preventing osteoclastogenesis, restraining osteoprogenitor proliferation, promoting osteoblast differentiation, coordinating organized collagen matrix deposition, and maintaining osteocyte survival. Emerging data shows that connexin43 regulates both the exchange of small molecules among osteoblast lineage cells and the docking of signaling proteins to the gap junction, affecting the efficiency of signal transduction. Understanding how and what connexin43 communicates to coordinate tissue remodeling has therapeutic implications in bone. Altering the information shared by intercellular communication and/or targeting the recruitment of signaling machinery to the gap junction could be used to impact the skeletal homeostatic set point, either driving osteogenesis or inhibiting resorption.

Defective signaling, osteoblastogenesis and bone remodeling in a mouse model of connexin 43 C-terminal truncation

In skeletal tissue, loss or mutation of the gap junction protein connexin 43 (Cx43, also known as GJA1) in cells of the osteoblast lineage leads to a profound cortical bone phenotype and defective tissue remodeling. There is mounting evidence in bone cells that the C-terminus (CT) of Cx43 is a docking platform for signaling effectors and is required for efficient downstream signaling. Here, we examined this function, using a mouse model of Cx43 CT-truncation (Gja1 K258Stop). Relative to Gja1+/- controls, male Gja1-/K258Stop mice have a cortical bone phenotype that is remarkably similar to those reported for deletion of the entire Cx43 gene in osteoblasts. Furthermore, we show that the Cx43 CT binds several signaling proteins that are required for optimal osteoblast function, including PKCδ, ERK1 and ERK2 (ERK1/2, also known as MAPK3 and MAPK1, respectively) and β-catenin. Deletion of the Cx43 CT domain affects these signaling cascades, impacting osteoblast proliferation, differentiation, and collagen processing and organization. These data imply that, at least in bone, Cx43 gap junctions not only exchange signals, but also recruit the appropriate effector molecules to the Cx43 CT in order to efficiently activate signaling cascades that affect cell function and bone acquisition.

Role of connexins and pannexins during ontogeny, regeneration, and pathologies of bone

Electron micrographs revealed the presence of gap junctions in osteoblastic cells over 40 years ago. These intercellular channels formed from connexins are present in bone forming osteoblasts, bone resorbing osteoclasts, and osteocytes (mature osteoblasts embedded in the mineralized bone matrix). More recently, genetic and pharmacologic studies revealed the role of connexins, and in particular Cx43, in the differentiation and function of all bone types. Furthermore, mutations in the gene encoding Cx43 were found to be causally linked to oculodentodigital dysplasia, a condition that results in an abnormal skeleton. Pannexins, molecules with similar structure and single-membrane channel forming potential as connexins when organized as hemichannels, are also expressed in osteoblastic cells. The function of pannexins in bone and cartilage is beginning to be uncovered, but more research is needed to determine the role of pannexins in bone development, adult bone mass and skeletal homeostasis. We describe here the current knowledge on the role of connexins and pannexins on skeletal health and disease.

Connexins in the skeleton

Shaping of the skeleton (modeling) and its maintenance throughout life (remodeling) require coordinated activity among bone forming (osteoblasts) and resorbing cells (osteoclasts) and osteocytes (bone embedded cells). The gap junction protein connexin43 (Cx43) has emerged as a key modulator of skeletal growth and homeostasis. The skeletal developmental abnormalities present in oculodentodigital and craniometaphyseal dysplasias, both linked to Cx43 gene (GJA1) mutations, demonstrate that the skeleton is a major site of Cx43 action. Via direct action on osteolineage cells, including altering production of pro-osteoclastogenic factors, Cx43 contributes to peak bone mass acquisition, cortical modeling of long bones, and maintenance of bone quality. Cx43 also contributes in diverse ways to bone responsiveness to hormonal and mechanical signals. Skeletal biology research has revealed the complexity of Cx43 function; in addition to forming gap junctions and "hemichannels", Cx43 provides a scaffold for signaling molecules. Hence, Cx43 actively participates in generation and modulation of cellular signals driving skeletal development and homeostasis. Pharmacological interference with Cx43 may in the future help remedy deterioration of bone quality occurring with aging, disuse and hormonal imbalances.

St. John's Wort (Hypericum perforatum) stimulates human osteoblastic MG-63 cell proliferation and attenuates trabecular bone loss induced by ovariectomy

BACKGROUND/OBJECFTIVES

The effect of St. John's Wort extract (SJW) on MG-63 cell proliferation and trabecular bone loss induced by ovariectomy was examined.

MATERIALS/METHODS

Proliferation, expression of estrogen receptor (ER) α and ER β, and gene expressions of osteoprotegerin (OPG), osteocalcin (OC) and alkaline phosphatase (ALP) were examined in MG-63 cells treated with or without SJW. Ovariectomized rats were treated with SJW at the dose of 100 or 200 mg/kg/day, β-estradiol-3-benzoate (E2), or vehicle only (OVX-C), and sham operated rats were treated with vehicle only (Sham-C). Serum ALP and C-telopeptide (CTX), and femoral trabecular bone loss were examined.

RESULTS

SJW increased MG-63 cell proliferation and expression of ER α and ER β, and positive effect was shown on gene expressions of ALP, OC and OPG. SJW also showed estrogen like effect on bone associated with slowing down in trabecular bone loss. Histopathology by H&E showed rats treated with SJW displayed denser structure in metaphyseal region of distal femur compared with rats in OVX-C. SJW was shown to reduce serum CTX in OVX rats.

CONCLUSION

The present study provides new insight in preventing estrogen deficiency induced bone loss of SJW and possibility for its application in bone health supplement.

Mandibular inferior cortical bone thickness on panoramic radiographs in patients using bisphosphonates

OBJECTIVE

The goal of this study was to detect dimensional changes in the mandibular cortical bone associated with bisphosphonate (BP) use and to correlate measurements of the cortical bone with the cumulative dose of BPs.

STUDY DESIGN

Mandibular inferior cortical bone thickness (MICBT) was measured under the mental foramen on panoramic radiographs of patients with and without bisphosphonate-related osteonecrosis of the jaws (BRONJ) taking BPs and controls.

RESULTS

Patients with BRONJ had the highest mean MICBT (6.81 ± 1.35 mm), compared with patients without BRONJ taking BPs (5.44 ± 1.09 mm) and controls (4.79 ± 0.85 mm) (P < .01). Mean MICBT of patients with BRONJ was significantly higher than that of patients without BRONJ taking BPs. There was a correlation between MICBT and cumulative dose of zolendronate.

CONCLUSIONS

Measurement of MICBT on panoramic radiographs is a potentially useful tool for the detection of dimensional changes associated with BP therapy.

Deletion of connexin43 in osteoblasts/osteocytes leads to impaired muscle formation in mice

It is well-established that muscle forces are necessary for bone development as well as proper bone modeling and remodeling. Recent work has also suggested that bone acts as an endocrine organ that can influence the development of other organs. Connexin43 (Cx43), a gap junction protein that transduces mechanical signals, is an important determinant of cortical bone modeling. Using an osteoblast/osteocyte-specific ablation of the Cx43 gene (Gja1) driven by the 2.3-kb Col1 α1 promoter (cKO) in the mouse, in this study we confirmed reduced cortical bone thickness and density with expanded bone marrow cavity in the cKO humerus. Surprisingly, Gja1 deletion in bone cells also affected skeletal muscle development, resulting in lower fast muscle weight, grip strength, and maximum absolute and specific tetanic forces (60% to 80%, 85%, and 50%, respectively, of WT mice). The normally fast twitch extensor digitorum longus (EDL) muscle exhibited increased slow twitch fibers in cKO mice. These muscle defects were accompanied by a 40% to 60% reduction in mRNA abundance for genes encoding osteocalcin in the humerus, relative to WT mice. Accordingly, both carboxylated and undercarboxylated isoforms of osteocalcin were reduced by over 30% in the circulation of cKO mice. Moreover, the active, undercarboxylated isoform of osteocalcin (glu-OC) promoted myotube formation in C2C12 myoblast cultures, and glu-OC injections to cKO mice rescued EDL muscle cross-sectional area and grip strength in vivo. These findings demonstrate that Cx43 in osteoblasts/osteocytes indirectly modulates skeletal muscle growth and function, potentially via an endocrine effect of glu-OC.

N-cadherin restrains PTH activation of Lrp6/β-catenin signaling and osteoanabolic action

Interaction between parathyroid hormone/parathyroid hormone-related peptide receptor 1 (PTHR1) and low-density lipoprotein receptor-related protein 6 (Lrp6) is important for parathyroid hormone (PTH) signaling and anabolic action. Because N-cadherin has been shown to negatively regulate canonical Wnt/β-catenin signaling, we asked whether N-cadherin alters PTH signaling and stimulation of bone formation. Ablation of the N-cadherin gene (Cdh2) in primary osteogenic lineage cells resulted in increased Lrp6/PTHR1 interaction in response to PTH1-34 , associated with enhanced PTH-induced PKA signaling and PKA-dependent β-catenin C-terminus phosphorylation, which promotes β-catenin transcriptional activity. β-catenin C-terminus phosphorylation was abolished by Lrp6 knockdown. Accordingly, PTH1-34 stimulation of Tcf/Lef target genes, Lef1 and Axin2, was also significantly enhanced in Cdh2-deficient cells. This enhanced responsiveness to PTH extends to the osteo-anabolic effect of PTH, as mice with a conditional Cdh2 deletion in Osx+ cells treated with intermittent doses of PTH1-34 exhibited significantly larger gains in trabecular bone mass relative to control mice, the result of accentuated osteoblast activity. Therefore, N-cadherin modulates Lrp6/PTHR1 interaction, restraining the intensity of PTH-induced β-catenin signaling, and ultimately influencing bone formation in response to intermittent PTH administration.

Bisphosphonates and connexin 43: a critical review of evidence

Bisphosphonates (BPs) are drugs commonly used in the treatment of various disease arising or affecting bone tissue. There is a standard use in bone neoplasia and metastasis, hormonal and developmental disorders as well as for compensation of adverse effects in several medical therapies. Many in-vivo and in-vitro studies have assessed the efficacy of this drug and its function in cellular scale. In this concern, BPs are described to inhibit the resorptive function of osteoclasts and to prevent apoptosis of osteoblasts and osteocytes. They can preserve the osteocytic network, reduce fracture rate, and increase the bone mineral content, which is therapeutically used. Connexin 43 (Cx43) is a crucial molecule for basal regulation of bone homeostasis, development, and differentiation. It is described for signal transduction in many physiological and pathological stimuli and recently to be involved in BP action.

Do bisphosphonates affect bone healing? A meta-analysis of randomized controlled trials

Background

Whether bisphosphonates affect indirect bone healing is still unclear.

Method

We carried out a comprehensive search strategy. Only randomized controlled trials were included. Two reviewers independently assessed methodological qualities and extracted outcome data. Analysis was performed with RevMan 5.2.

Results

Eight eligible randomized controlled trials with 2,508 patients were included. Meta-analysis results showed that no statistically significant differences were founded in indirect bone healing in short time (within 3 months) (relative risk (RR) 1.40, relative the control group; 95% CI 0.36 to 5.49) and in long-term (more than 12 months) postoperation (RR 1.0; 95% CI 0.98 to 1.02) between bisphosphonates infusion groups and control groups. There were no statistically significant differences of indirect bone healing between early and delay bisphosphonates administration groups. Bisphosphonates infusion after lumbar infusion surgery could promote bone healing and shorten fusion time in 6 months postoperation (RR 1.35; 95% CI 1.11 to 1.66).

Conclusion

There was no clinically detectable delay to fracture healing via external callus formation following bisphosphonates treatment. Considering the benefit aspects of bisphosphonates for osteoporosis treatment, we recommend bisphosphonates infusion after fracture fixation surgery and lumbar fusion surgery.

Connexin 43 hemichannels and intracellular signaling in bone cells

Cell function and survival are controlled by intracellular signals, and modulated by surrounding cells and the extracellular environment. Connexin channels participate in these processes by mediating cell-to-cell communication. In bone cells, gap junction channels were detected in the early 1970s, and are present among bone resorbing osteoclasts, bone forming osteoblasts, and osteocytes - mature osteoblasts embedded in the mineralized matrix. These channels are composed mainly by Cx43, although the expression of other connexins (45, 46, and 37) has also been reported. It is now believed that undocked Cx43 hemichannels (connexons) formed in unopposed cell membranes facing the extracellular environment participate in the interaction of bone cells with the extracellular environment, and in their communication with neighboring cells. Thus, we and others demonstrated the presence of active hemichannels in osteoblastic and osteocytic cells. These hemichannels open in response to pharmacological and mechanical stimulation. In particular, preservation of the viability of osteoblasts and osteocytes by the anti-osteoporotic drugs bisphosphonates depends on Cx43 expression in vitro and in vivo, and is mediated by undocked hemichannels. Cx43 hemichannels are also required for the release of prostaglandins and ATP by osteocytes, and for cell survival induced by mechanical stimulation in vitro. Moreover, they are required for the anti-apoptotic effect of parathyroid hormone in osteoblastic cells. This review summarizes the current knowledge on the presence and function of undocked connexons, and the role of hemichannel regulation for the maintenance of bone cell viability and, potentially, bone health.

Molecular mechanisms of osteoblast/osteocyte regulation by connexin43

Osteoblasts, osteocytes, and osteoprogenitor cells are interconnected into a functional network by gap junctions formed primarily by connexin43 (Cx43). Over the past two decades, it has become clear that Cx43 is important for the function of osteoblasts and osteocytes. This connexin contributes to the acquisition of peak bone mass and is a major modulator of cortical modeling. We review key data from human and mouse genetics on the skeletal consequences of ablation or mutation of the Cx43 gene (Gja1) and the molecular mechanisms by which Cx43 regulates the differentiation, function, and survival of osteogenic lineage cells. We also discuss putative second messengers that are communicated by Cx43 gap junctions, the role of hemichannels, and the function of Cx43 as a scaffold for signaling molecules. Current knowledge demonstrates that Cx43 is more than a passive channel; rather, it actively participates in the generation and modulation of cellular signals that drive skeletal development and homeostasis.

Basic research and clinical applications of bisphosphonates in bone disease: what have we learned over the last 40 years?

It is now 40 years since bisphosphonates (BPs) were first used in the clinic. So, it is timely to provide a brief review of what we have learned about these agents in bone disease. BPs are bone-specific and have been classified into two major groups on the basis of their distinct molecular modes of action: amino-BPs and non-amino-BPs. The amino-BPs are more potent and they inhibit farnesyl pyrophosphate synthase (FPPS), a key enzyme of the mavalonate/cholesterol biosynthetic pathway, while the non-amino-BPs inhibit osteoclast activity, by incorporation into non-hydrolyzable analogs of ATP. Both amino-BPs and non-amino-BPs can protect osteoblasts and osteocytes against apoptosis. The BPs are widely used in the clinic to treat various diseases characterized by excessive bone resorption, including osteoporosis, myeloma, bone metastasis, Legg-Perthes disease, malignant hyperparathyroidism, and other conditions featuring bone fragility. This review provides insights into some of the adverse effects of BPs, such as gastric irritation, osteonecrosis of the jaw, atypical femoral fractures, esophageal cancer, atrial fibrillation, and ocular inflammation. In conclusion, this review covers the biochemical and molecular mechanisms of action of BPs in bone, particularly the discovery that BPs have direct anti-apoptotic effects on osteoblasts and osteocytes, and the current situation of BP use in the clinic.

Connexin43 modulates post-natal cortical bone modeling and mechano-responsiveness

Recent advances have established connexin43 (Cx43) as a key regulator of osteoblast function and of bone response to mechanical stimuli. Work by independent laboratories has consistently demonstrated postnatal development of larger than normal cross-section of long bones after conditional ablation of the Cx43 gene, Gja1, selectively in osteoblasts and/or osteocytes. This phenotype is caused by excessive endocortical bone resorption associated with periosteal expansion and cortical thinning. Review of published data suggests that the earlier in the osteogenic lineage is Gja1 deleted, the more severe is the cortical phenotype, implying functional roles of Cx43 at different stages of the osteoblast differentiation program. Such cortical modeling abnormalities resemble the changes occurring in the cortex upon disuse or aging. Indeed, Cx43 deficiency desensitizes endocortical osteoclasts from activation induced by removal of mechanical load, thus preventing medullary area expansion. The action of Cx43 on cancellous bone is controversial. Furthermore, the absence of Cx43 in osteoblasts and osteocytes results in activation of periosteal bone formation at lower strains than in wild-type bones, suggesting that Cx43 deficiency increased cortical sensitivity to mechanical load. Thus, Cx43 modulates cortical bone modeling in homeostatic conditions and in response to mechanical load by restraining both endocortical bone resorption and periosteal bone formation. Cx43 may represent a novel pharmacologic target for improving cortical bone strength through modulation of mechano-responsiveness.

Extracellular norepinephrine clearance by the norepinephrine transporter is required for skeletal homeostasis

Changes in bone remodeling induced by pharmacological and genetic manipulation of β-adrenergic receptor (βAR) signaling in osteoblasts support a role of sympathetic nerves in the regulation of bone remodeling. However, the contribution of endogenous sympathetic outflow and nerve-derived norepinephrine (NE) to bone remodeling under pathophysiological conditions remains unclear. We show here that differentiated osteoblasts, like neurons, express the norepinephrine transporter (NET), exhibit specific NE uptake activity via NET and can catabolize, but not generate, NE. Pharmacological blockade of NE transport by reboxetine induced bone loss in WT mice. Similarly, lack of NE reuptake in norepinephrine transporter (Net)-deficient mice led to reduced bone formation and increased bone resorption, resulting in suboptimal peak bone mass and mechanical properties associated with low sympathetic outflow and high plasma NE levels. Last, daily sympathetic activation induced by mild chronic stress was unable to induce bone loss, unless NET activity was blocked. These findings indicate that the control of endogenous NE release and reuptake by presynaptic neurons and osteoblasts is an important component of the complex homeostatic machinery by which the sympathetic nervous system controls bone remodeling. These findings also suggest that drugs antagonizing NET activity, used for the treatment of hyperactivity disorders, may have deleterious effects on bone accrual.

Beyond gap junctions: Connexin43 and bone cell signaling

Connexin43 (Cx43) is the most abundant gap junction protein expressed in bone cells and plays a central role in cell-to-cell communication in the skeleton. Findings of the last decade uncovered functions of Cx43 hemichannels expressed on unopposed plasma cell membranes as mediators of the communication between bone cells and their extracellular milieu. Additionally, through its cytoplasmic C-terminus domain, Cx43 serves as a scaffolding protein that associates with structural and signaling molecules leading to regulation of intracellular signaling, independent of channel activity. This perspective discusses the evidence demonstrating that via these diverse mechanisms Cx43 is a key component of the intracellular machinery responsible for signal transduction in bone in response to pharmacologic, hormonal and mechanical stimuli. This advance in the knowledge of the role of connexins increases our understanding of the pathophysiological mechanisms that regulate bone cell function and provides new opportunities to treat bone diseases.