Time-course of mast cell accumulation in rat bone marrow after ovariectomy

Antipsychotic-induced bone loss: the role of dopamine, serotonin and adrenergic receptor signalling

Antipsychotics are commonly used in treating psychiatric disorders. These medications primarily target dopamine the serotonin receptors, they have some affinity to adrenergic, histamine, glutamate and muscarinic receptors. There is clinical evidence that antipsychotic use decreases BMD and increases fracture risk, with dopamine, serotonin and adrenergic receptor-signalling becoming an increasing area of focus where the presence of these receptors in osteoclasts and osteoblasts have been demonstrated. Osteoclasts and osteoblasts are the most important cells in the bone remodelling and the bone regeneration process where the activity of these cells determine the bone resorption and formation process in order to maintain healthy bone. However, an imbalance in osteoclast and osteoblast activity can lead to decreased BMD and increased fracture risk, which is also believed to be exacerbated by antipsychotics use. Therefore, the aim of this review is to provide an overview of the mechanisms of action of first, second and third generation antipsychotics and the expression profiles of dopamine, serotonin and adrenergic receptors during osteoclastogenesis and osteoblastogenesis.

Role of Mast-Cell-Derived RANKL in Ovariectomy-Induced Bone Loss in Mice

Mast cells may contribute to osteoporosis development, because patients with age-related or post-menopausal osteoporosis exhibit more mast cells in the bone marrow, and mastocytosis patients frequently suffer from osteopenia. We previously showed that mast cells crucially regulated osteoclastogenesis and bone loss in ovariectomized, estrogen-depleted mice in a preclinical model for post-menopausal osteoporosis and found that granular mast cell mediators were responsible for these estrogen-dependent effects. However, the role of the key regulator of osteoclastogenesis, namely, receptor activator of NFκB ligand (RANKL), which is secreted by mast cells, in osteoporosis development has, to date, not been defined. Here, we investigated whether mast-cell-derived RANKL participates in ovariectomy (OVX)-induced bone loss by using female mice with a conditional Rankl deletion. We found that this deletion in mast cells did not influence physiological bone turnover and failed to protect against OVX-induced bone resorption in vivo, although we demonstrated that RANKL secretion was significantly reduced in estrogen-treated mast cell cultures. Furthermore, Rankl deletion in mast cells did not influence the immune phenotype in non-ovariectomized or ovariectomized mice. Therefore, other osteoclastogenic factors released by mast cells might be responsible for the onset of OVX-induced bone loss.

Human mast cells induce osteoclastogenesis through cell surface RANKL

OBJECTIVES

We employed the co-culture of CD34⁺ stem cell-derived human mast cells (HMC) and human monocyte-derived osteoclast precursors to evaluate if mast cells contribute to the pathogenesis of osteoporosis through regulation of osteoclast proliferation and activation.

METHODS

Mature HMC and osteoclast precursors were cultured from monocytes isolated from human buffy coat. The osteoclast precursors were incubated with HMC or receptor activator of nuclear factor kappa-B ligand (RANKL) for a week prior to determination of osteoclast maturation through characterization by their morphology and tartrate resistant acid phosphatase (TRAP) expression. The bone absorption activity was determined by pit formation on osteo-assay plate.

RESULTS

Mature osteoclasts were identified following co-culture of osteoclast precursors with HMC for one week in the absence of RANKL and they were capable of bone resorption. These actions of HMC on osteoclasts were not affected by mast cell activators such anti-IgE or substance P but could be reversed by osteoprotegerin (OPG) in the co-culture system suggesting the involvement of RANKL. The expression of RANKL on the cell surface of HMC was confirmed by flow cytometry and the density was not affected by activation of HMC.

CONCLUSION

Our study provided direct evidence confirming the initiation of osteoclast proliferation and activation by mast cells through cell surface RANKL suggesting that mast cells may contribute to bone destruction in pathological conditions such as osteoporosis.

Mast Cell Chymase Has a Negative Impact on Human Osteoblasts

Mast cells have been linked to osteoporosis and bone fractures, and in a previous study we found that mice lacking a major mast cell protease, chymase, develop increased diaphyseal bone mass. These findings introduce the possibility that mast cell chymase can regulate bone formation, but the underlying mechanism(s) has not previously been investigated. Here we hypothesized that chymase might exert such effects through a direct negative impact on osteoblasts, i.e., the main bone-building cells. Indeed, we show that chymase has a distinct impact on human primary osteoblasts. Firstly, chymase was shown to have pronounced effects on the morphological features of osteoblasts, including extensive cell contraction and actin reorganization. Chymase also caused a profound reduction in the output of collagen from the osteoblasts, and was shown to degrade osteoblast-secreted fibronectin and to activate pro-matrix metallopeptidase-2 released by the osteoblasts. Further, chymase was shown to have a preferential impact on the gene expression, protein output and phosphorylation status of TGFβ-associated signaling molecules. A transcriptomic analysis was conducted and revealed a significant effect of chymase on several genes of importance for bone metabolism, including a reduction in the expression of osteoprotegerin, which was confirmed at the protein level. Finally, we show that chymase interacts with human osteoblasts and is taken up by the cells. Altogether, the present findings provide a functional link between mast cell chymase and osteoblast function, and can form the basis for a further evaluation of chymase as a potential target for intervention in metabolic bone diseases.

Does anti-IgE therapy prevent chronic allergic asthma-related bone deterioration in asthmatic mice?

Current evidence on the association between allergic diseases and bone metabolism indicates asthma may be a potential risk factor for bone health. Using anti-IgE has been proven effective in allergic asthma treatment with a good safety profile; however, its effects on bone health are unknown. Thus, we aimed to investigate whether: (i) chronic allergic asthma (CAA) causes any meaningful changes in bone, and if any, (ii) anti-IgE therapy prevents any CAA-induced adverse alteration. A murine model was used to study CAA. Thirty-two BALB/c male-mice were assigned into four groups (eight-mice/group): Control, CAA (treated with saline), CAA + 100 µg of anti-IgE (CAA + 100AIgE), and CAA + 200 µg of anti-IgE (CAA + 200AIgE) groups. After immunization, saline or anti-IgE was performed intraperitoneally for 8-weeks (in five-sessions at 15-days interval). Three-point bending test was used for the mechanical analysis. Bone calcium (Ca²⁺) and phosphorus (P^3-) as well as Ca/P ratio were evaluated using inductively-coupled plasma-mass-spectrometer (ICP-MS). Compared to control, reductions observed in yield and ultimate moments, rigidity, energy-to-failure, yield and ultimate stresses, elastic modulus, toughness, and post-yield toughness parameters of the CAA group were found significant (P < 0.05). Similar declines were also detected regarding bone Ca²⁺, P^3- and Ca/P ratio (P < 0.05). Compared to control, we observed that 200 µg administration of anti-IgE in CAA + 200AIgE group hindered CAA-related impairments in mineral and mechanical characteristics of bone, while 100 µg in CAA + 100AIgE failed to do so. Our results showed CAA may cause bone loss, leading to a decrease in bone strength, and anti-IgE administration may dose-dependently inhibit these impairments in bone.

Mast Cells Trigger Disturbed Bone Healing in Osteoporotic Mice

Mast cells are important tissue-resident sensor and effector immune cells but also play a major role in osteoporosis development. Mast cells are increased in numbers in the bone marrow of postmenopausal osteoporotic patients, and mast cell-deficient mice are protected from ovariectomy (OVX)-induced bone loss. In this study, we showed that mast cell-deficient Mcpt5-Cre R-DTA mice were protected from OVX-induced disturbed fracture healing, indicating a critical role for mast cells in the pathomechanisms of impaired bone repair under estrogen-deficient conditions. We revealed that mast cells trigger the fracture-induced inflammatory response by releasing inflammatory mediators, including interleukin-6, midkine (Mdk), and C-X-C motif chemokine ligand 10 (CXCL10), and promote neutrophil infiltration into the fracture site in OVX mice. Furthermore, mast cells were responsible for reduced osteoblast and increased osteoclast activities in OVX mice callus, as well as increased receptor activator of NF-κB ligand serum levels in OVX mice. Additional in vitro studies with human cells showed that mast cells stimulate osteoclastogenesis by releasing the osteoclastogenic mediators Mdk and CXCL10 in an estrogen-dependent manner, which was mediated via the estrogen receptor alpha on mast cells. In conclusion, mast cells negatively affect the healing of bone fractures under estrogen-deficient conditions. Hence, targeting mast cells might provide a therapeutic strategy to improve disturbed bone repair in postmenopausal osteoporosis. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Immunoporosis: Role of Innate Immune Cells in Osteoporosis

Osteoporosis or porous bone disorder is the result of an imbalance in an otherwise highly balanced physiological process known as 'bone remodeling'. The immune system is intricately involved in bone physiology as well as pathologies. Inflammatory diseases are often correlated with osteoporosis. Inflammatory mediators such as reactive oxygen species (ROS), and pro-inflammatory cytokines and chemokines directly or indirectly act on the bone cells and play a role in the pathogenesis of osteoporosis. Recently, Srivastava et al. (Srivastava RK, Dar HY, Mishra PK. Immunoporosis: Immunology of Osteoporosis-Role of T Cells. Frontiers in immunology. 2018;9:657) have coined the term "immunoporosis" to emphasize the role of immune cells in the pathology of osteoporosis. Accumulated pieces of evidence suggest both innate and adaptive immune cells contribute to osteoporosis. However, innate cells are the major effectors of inflammation. They sense various triggers to inflammation such as pathogen-associated molecular patterns (PAMPs), damage-associated molecular patterns (DAMPs), cellular stress, etc., thus producing pro-inflammatory mediators that play a critical role in the pathogenesis of osteoporosis. In this review, we have discussed the role of the innate immune cells in great detail and divided these cells into different sections in a systemic manner. In the beginning, we talked about cells of the myeloid lineage, including macrophages, monocytes, and dendritic cells. This group of cells explicitly influences the skeletal system by the action of production of pro-inflammatory cytokines and can transdifferentiate into osteoclast. Other cells of the myeloid lineage, such as neutrophils, eosinophils, and mast cells, largely impact osteoporosis via the production of pro-inflammatory cytokines. Further, we talked about the cells of the lymphoid lineage, including natural killer cells and innate lymphoid cells, which share innate-like properties and play a role in osteoporosis. In addition to various innate immune cells, we also discussed the impact of classical pro-inflammatory cytokines on osteoporosis. We also highlighted the studies regarding the impact of physiological and metabolic changes in the body, which results in chronic inflammatory conditions such as ageing, ultimately triggering osteoporosis.

Interaction between bone and immune cells: Implications for postmenopausal osteoporosis

Postmenopausal osteoporosis is a systemic disease characterized by the loss of bone mass and increased bone fracture risk largely resulting from significantly reduced levels of the hormone estrogen after menopause. Besides the direct negative effects of estrogen-deficiency on bone, indirect effects of altered immune status in postmenopausal women might contribute to ongoing bone destruction, as postmenopausal women often display a chronic low-grade inflammatory phenotype with altered cytokine expression and immune cell profile. In this context, it was previously shown that various immune cells interact with osteoblasts and osteoclasts either via direct cell-cell contact, or more likely via paracrine mechanisms. For example, specific subtypes of T lymphocytes express TNFα, which was shown to increase osteoblast apoptosis and to indirectly stimulate osteoclastogenesis via B cell-produced receptor-activator of NF-κB ligand (RANKL), thereby triggering bone loss during postmenopausal osteoporosis. T_h17 cells release interleukin-17 (IL-17), which directs mesenchymal stem cell differentiation towards the osteogenic lineage, but also indirectly increases osteoclast differentiation. B lymphocytes are a major regulator of osteoclast formation via granulocyte colony-stimulating factor secretion and the RANKL/osteoprotegerin system under estrogen-deficient conditions. Macrophages might act differently on bone cells dependent on their polarization profile and their secreted paracrine factors, which might have implications for the development of postmenopausal osteoporosis, because macrophage polarization is altered during disease progression. Likewise, neutrophils play an important role during bone homeostasis, but their over-activation under estrogen-deficient conditions contributes to osteoblast apoptosis via the release of reactive oxygen species and increased osteoclastogenesis via RANKL signaling. Furthermore, mast cells might be involved in the development of postmenopausal osteoporosis, because they store high levels of osteoclastic mediators, including IL-6 and RANKL, in their granules and their numbers are greatly increased in osteoporotic bone. Additionally, bone fracture healing is altered under estrogen-deficient conditions with the increased presence of pro-inflammatory cytokines, including IL-6 and Midkine, which might contribute to healing disturbances. Consequently, in addition to the direct negative influence of estrogen-deficiency on bone, immune cell alterations contribute to the pathogenesis of postmenopausal osteoporosis.

Effects of estrogen deficiency followed by streptozotocin-induced diabetes on periodontal tissues of female rats

Although both estrogen deficiency and diabetes contribute to periodontal tissue deterioration, the combined effects of these conditions on periodontium is unknown. Thus, we analyzed the combined effects of ovariectomy followed by streptozotocin (STZ)-induced diabetes on periodontal tissues of rats. Twenty adult rats were ovariectomized (OVX) or SHAM-operated (SHAM). After 3 weeks, the rats received an intraperitoneal injection of STZ (60 mg/kg/body weight) to induce diabetes or vehicle (blank) solution. The groups were assigned as follows (n = 5): SHAM-vehicle (SHAM), OVX-vehicle (OVX), SHAM + STZ (SHAM-Di), and OVX + STZ (OVX-Di). Seven weeks post-diabetes induction, the rats were euthanized. Blood samples were collected for glucose measurements and maxillae were processed for paraffin embedding. Sections stained with hematoxylin/eosin, Masson's trichrome, and picrosirius-red were used for alveolar bone loss and collagen fiber analysis in the lamina propria. Immunohistochemistry was performed for runt-related transcription factor 2 (Runx2), matrix metalloproteinase 9 (MMP-9), and tryptase detection. Alveolar bone loss and fewer collagen fibers were observed in the OVX-Di group, collagen fibers with irregular organization, and MMP-9 immunoreactivity were more evident in diabetic groups, and MMP-9-positive osteoclasts on alveolar bone surface were noticed in all groups. The OVX-Di group showed lower Runx2 immunoreactivity (osteoblast formation marker), and more tryptase-positive cells (mast cell marker) in the alveolar bone marrow. Our results indicate that estrogen depletion, followed by STZ-induced diabetes, promotes periodontal tissue deterioration that is more evident than both interventions applied alone. Furthermore, our results points to a possible participation of bone-derived mast cells in this process.

The Role of Mast Cells in Bone Metabolism and Bone Disorders

Mast cells (MCs) are important sensor and effector cells of the immune system that are involved in many physiological and pathological conditions. Increasing evidence suggests that they also play an important role in bone metabolism and bone disorders. MCs are located in the bone marrow and secrete a wide spectrum of mediators, which can be rapidly released upon activation of mature MCs following their differentiation in mucosal or connective tissues. Many of these mediators can exert osteocatabolic effects by promoting osteoclast formation [e.g., histamine, tumor necrosis factor (TNF), interleukin-6 (IL-6)] and/or by inhibiting osteoblast activity (e.g., IL-1, TNF). By contrast, MCs could potentially act in an osteoprotective manner by stimulating osteoblasts (e.g., transforming growth factor-β) or reducing osteoclastogenesis (e.g., IL-12, interferon-γ). Experimental studies investigating MC functions in physiological bone turnover using MC-deficient mouse lines give contradictory results, reporting delayed or increased bone turnover or no influence depending on the mouse model used. By contrast, the involvement of MCs in various pathological conditions affecting bone is evident. MCs may contribute to the pathogenesis of primary and secondary osteoporosis as well as inflammatory disorders, including rheumatoid arthritis and osteoarthritis, because increased numbers of MCs were found in patients suffering from these diseases. The clinical observations could be largely confirmed in experimental studies using MC-deficient mouse models, which also provide mechanistic insights. MCs also regulate bone healing after fracture by influencing the inflammatory response toward the fracture, vascularization, bone formation, and callus remodeling by osteoclasts. This review summarizes the current view and understanding of the role of MCs on bone in both physiological and pathological conditions.

Presence of mast cells and the expression of metalloproteinase 9 in the gingiva of ovariectomized rats with periodontal disease

Background

The host's answer has an important role in periodontal disease, and the mast cells have a prime role. Such cells seem to be influenced by estrogen deficiency. The objective was to evaluate the mast cells and the expression of metalloproteinase(MMP)-9 in periodontal disease induced in ovariectomized rats.

Methods

For that purpose, 36 rats were used; 18 ovariectomized (OVX) and another 18 Sham-operated (SHAM). After 60 days the periodontal disease was induced by a ligature around the first lower right molars (group P). The opposite side was the control group (group C). The euthanasia occurred 3, 7 and 14 days after the placement of the ligature. The gingiva was removed and analyzed histochemically and immunohistochemically to quantify the mast cells and to analyze MMP 9 expression.

Results

By comparing the groups SHAM-P and C and groups OVX-C and P, it was noted that mast cells from group C were higher than P in all experimental periods. When comparing groups SHAM-C and OVX-C, significant factors were not found. When comparing groups SHAM-P and OVX-P, there was an inclination for mast cells reduction with time. The MMP-9 expression was related to the presence of periodontitis.

Conclusions

It was concluded that periodontitis led to mast cells reduction and MMP-9 increase. The ovariectomy itself did not alter the MMP-9 expression and did not influence the presence of mast cells in rat papilla, however, when associated to inflammation led to a reduction of mast cells.

Increased Bone Mass in Female Mice Lacking Mast Cell Chymase

Here we addressed the potential impact of chymase, a mast-cell restricted protease, on mouse bone phenotype. We show that female mice lacking the chymase Mcpt4 acquired a persistent expansion of diaphyseal bone in comparison with wild type controls, reaching a 15% larger diaphyseal cross sectional area at 12 months of age. Mcpt4-/- mice also showed increased levels of a bone anabolic serum marker and higher periosteal bone formation rate. However, they were not protected from experimental osteoporosis, suggesting that chymase regulates normal bone homeostasis rather than the course of osteoporosis. Further, the absence of Mcpt4 resulted in age-dependent upregulation of numerous genes important for bone formation but no effects on osteoclast activity. In spite of the latter, Mcpt4-/- bones had increased cortical porosity and reduced endocortical mineralization. Mast cells were found periosteally and, notably, bone-proximal mast cells in Mcpt4-/- mice were degranulated to a larger extent than in wild type mice. Hence, chymase regulates degranulation of bone mast cells, which could affect the release of mast cell-derived factors influencing bone remodelling. Together, these findings reveal a functional impact of mast cell chymase on bone. Further studies exploring the possibility of using chymase inhibitors as a strategy to increase bone volume may be warranted.

Acute estrogen surge enhances inflammatory nociception without altering spinal Fos expression

Chronic pain is a major neurological disorder that can manifest differently between genders or sexes. The complex actions of sex hormones may underlie these differences; previous studies have suggested that elevated estrogen levels can enhance pain perception. The purpose of this study was to investigate the hypothesis that acute, activational effects of estradiol (E2) increase persistent inflammatory nociception, and anatomically where this modulation occurs. Spinal expression of Fos is widely used as a marker of nociceptive activation. This study used formalin-evoked nociception in ovariectomized (OVX) adult female rats and measured late-phase hindlimb flinching and Fos expression in the spinal cord, and their modification by acute estrogen supplementation similar to a proestrus surge. Six days after ovariectomy, female rats were injected subcutaneously (s.c.) with 10μg/kg E2 or vehicle. Twenty-four hours later, 50μL of 1.25% or 100μL of 5% formalin was injected into the right hindpaw; hindlimb flinches were counted, and spinal cords removed 2h after formalin injection. The numbers of Fos-expressing neurons in sections of the lumbar spinal cord were analyzed using immunohistochemistry. Formalin-induced inflammation produced a dose-dependent increase in late-phase hindlimb flinching, and E2 pretreatment increased flinching following 5%, but not 1.25% formalin injection. Despite the modification of behavior by E2, the number of spinal Fos-positive neurons was not altered by E2 pretreatment. These findings demonstrate that an acute proestrus-like surge in serum estrogen can produce a stimulus-intensity-dependent increase in inflammation-evoked nociceptive behavior. However, the lack of effect on spinal Fos expression suggests that this enhancement of nociceptive signaling by estrogen is independent of changes in peripheral activation of, expression of the immediate early gene Fos by, or signal throughput of spinal nociceptive neurons.

Suppression of mast cell activity contributes to the osteoprotective effect of an herbal formula containing Herba Epimedii, Fructus Ligustri Lucidi and Fructus Psoraleae

Objectives

Mast cells are believed to contribute to the pathogenesis of osteoporosis as their number is increased in osteoporotic bones. Herba Epimedii, Fructus Ligustri Lucidi and Fructus Psoraleae are three Chinese herbs traditionally for tonifying the ‘kidney system’ and a herbal formula (ELP) containing the respective herbs at the weight ratio of 5 : 4 : 1 was shown to prevent osteoporosis. This study evaluated if suppression of mast cell accumulation and activity contribute to the anti-osteoporotic action of ELP.

Methods

The herbs were boiled under reflux to produce the aqueous extract that was further concentrated under reduced pressure and lyophilized. An in-vivo rat osteoporosis model using hind limb unloading was employed for studying the accumulation of mast cells. The human mast cell line, LAD2, was employed to evaluate the mast cell modulating action of ELP.

Key findings

Mast cell number in the tibiae of hind limb unloaded rats increased significantly during the course of osteoporosis. ELP treatment (10 g/kg/day) prevented both osteoporosis and mast cell accumulation in these rats. Furthermore, ELP significantly inhibited histamine and tumour necrosis factor-α release from LAD2 cells.

Conclusion

Mast cells contributed to hormone independent osteoporosis. The suppression of mast cell accumulation and activation may contribute to the anti-osteoporotic action of ELP.

Quantitative comparison of the inhibitory effects of GW5638 and tamoxifen on angiogenesis in the cornea pocket assay

GW5638 is a novel tissue-selective estrogen receptor (ER) modulator. Structurally, it is a derivative of tamoxifen that is known for its inhibitory effects on angiogenesis in an ER-independent manner. Therefore, it is possible that GW5638 has the same effects as tamoxifen on angiogenesis. To test this hypothesis, we used the rat cornea pocket assay and developed a new method that could precisely determine the total projected area of microvessels induced by basic fibroblast growth factor (bFGF) in the cornea. Animals in the study were treated with corn oil (control group), tamoxifen, or GW5638. After treatment, we observed that both GW5638 and tamoxifen could inhibit angiogenesis in the cornea (P<0.05) and that the inhibitory effects were not mediated by blocking functions of estrogen. Meanwhile, GW5638 had minimal effects on the body weight of animals whereas tamoxifen significantly reduced the body weight. Based on these observations, we concluded that GW5638 was as effective as tamoxifen in antiangiogenic treatment but less toxic than tamoxifen.

Histamine mediates osteoclastic resorption only during the acute phase of bone loss in ovariectomized rats

Short-term studies have shown that histamine is involved, via its H2 receptors (H2R), in the mediator network regulating trabecular bone loss in long bones of ovariectomized (OVX) rats. It is not known whether this effect of histamine persists over time or involves other skeletal sites. In this study, rats were maintained for 6 months postOVX and treated daily with saline or famotidine (10 mg kg(-1)), an H2R antagonist. At the end of the experimental period, femur trabecular bone mass was markedly decreased in OVX rats, whether or not they were treated with famotidine. In contrast, in the fourth lumbar vertebra, where bone loss starts later than in the femur, famotidine treatment attenuated the decline in trabecular bone volume, protected the trabecular architecture, maintained the thickness of the cortices and reduced the numbers of osteoclasts and tartrate-resistant acid phosphatase-positive preosteoclasts, whereas it had no influence on bone formation parameters. In vertebral bone marrow of OVX rats, the numbers of mast cells (MCs) and non-MC histamine-producing cells increased, while famotidine treatment significantly diminished both cell populations. These data show that H2R antagonism does not protect trabecular bone mass in the long term, and that short-term protection involves all bones. Histamine is involved during the early phase of strong osteoclastic resorption but not during the late phase of slower resorption, suggesting that different mediator networks control the two phases of destruction. Histamine would be part of the network mediating the early phase.

Serotonin regulates osteoclast differentiation through its transporter

5-HTT mediates antidepressant-sensitive clearance of 5-HT after its release into neural synapses. We found increased expression of 5-HTT in RANKL-induced osteoclast-like cells. Fluoxetine, an inhibitor of 5-HTT, reduced osteoclast differentiation but not activation. Reserpine, an inhibitor of 5-HT intracellular transport, potentiated differentiation. These results indicate a role for 5-HTT in osteoclast function and suggest that commonly used antidepressive agents may affect bone mass.

INTRODUCTION

Interactions between the serotonergic and skeletal systems are suggested by various clinical observations but are poorly understood.

MATERIALS AND METHODS

Using gene microarrays, we found that the serotonin transporter (5-HTT) was strongly expressed in RANKL-induced osteoclasts. Using RANKL stimulation of RAW264.7 cells and mouse bone marrow cells as a model system for osteoclast differentiation, we studied the possible role/s of the different components of the serotonin (5-HT) system on the differentiation process.

RESULTS

Osteoclast 5-HTT exhibited typical 5-HT uptake activity that was inhibitable by fluoxetine (Prozac). Fluoxetine reduced osteoclast differentiation but did not inhibit the activation of preformed osteoclasts, whereas the addition of 5-HT itself enhanced differentiation. Fluoxetine-treated osteoclast precursors had reduced NF-kappa B activation and elevated inhibitory protein kappa B alpha (I kappa B alpha) levels compared with untreated cells. 5-HT, on the other hand, resulted in activation of NF-kappa B. Reserpine inhibition of intracellular transport of 5-HT into cytoplasmic vesicles potentiated RANKL-induced osteoclast formation, suggesting the importance of intracellular 5-HT in regulating osteoclast differentiation. Reserpine also modestly enhanced the expression of the osteoclast marker TRACP in the absence of RANKL.

CONCLUSIONS

Taken together, these data suggest that the 5-HT system plays an important role in bone homeostasis through effects on osteoclast differentiation and implies that commonly used antidepressive agents may affect bone mass.

Histamine participates in the early phase of trabecular bone loss in ovariectomized rats

We have previously reported that cimetidine, a reference H2 receptor antagonist, attenuates the initial osteoclastic burst and subsequent trabecular bone loss induced by ovariectomy (ovx) in rats. This study was designed to determine whether these effects are specific to H2 antagonism. To this end, we compared the effects of two H2 receptor antagonists, cimetidine and famotidine. In addition, we analyzed the response of histamine-producing cells to these inhibitors. Seventy-two 90-day-old female Sprague-Dawley rats were ovariectomized or sham-operated, and received single daily intramuscular injections of cimetidine (125 mg/kg), famotidine (10 mg/kg), or vehicle. The animals were killed 14 days after surgery and their femurs were processed for histomorphometry. Trabecular bone volume was reduced by 30% in ovx rats and by 15% in cimetidine- and famotidine-treated rats. Architectural parameters were reduced by about 20% in ovx rats. Cimetidine and famotidine attenuated these consequences of ovx by about 50%. Trabecular connectivity was deteriorated by ovx, while cimetidine and famotidine attenuated this effect. Resorption parameters were increased by ovx, while cimetidine and famotidine prevented this increase. Kinetic bone formation parameters were increased by ovx, while cimetidine and famotidine had no influence. Neither cimetidine nor famotidine had any observable effect in sham-treated rats. Mast cell numbers increased by 250% in ovx rats and by only 40% in H2 antagonists-treated ovx rats. A resident histamine-positive, non-mast cell, population found in bone marrow was increased by 25% by ovx. Interestingly, cimetidine and famotidine reduced this population in both sham-operated and ovx rats, famotidine being more potent than cimetidine. These results show that H(2) receptor blockade partially prevents the consequences of castration on cancellous bone resorption in female rats, and strongly suggest that histamine participates in the mediator network regulating estrogen deficiency induced bone resorption. A large population of histamine-producing cells, which differ morphologically from mast cells and belong to an immature marrow population, may be a source of histamine in this model. The H(2) blockers targeted this population, and this effect appeared to explain the anti-resorptive action of the two drugs.

Periimplant Bone Healing under Experimental Hepatic Osteodystrophy Induced by a Choline-Deficient Diet: A Histomorphometric Study in Rats

BACKGROUND

Osseointegration and success of an implant involve the interaction of local and systemic factors such as bone metabolic diseases.

PURPOSE

The purpose of this study was to evaluate the effect of experimental hepatic osteodystrophy induced by a choline-deficient diet on periimplant bone healing.

MATERIALS AND METHODS

Laminar titanium implants were placed in the tibias of five groups of Wistar rats: those with a (1). controlled diet for 15 days; (2). choline-deficient diet for 15 days; (3). controlled diet for 30 days; (4). choline-deficient diet for 30 days; (5). choline-deficient diet for 15 days and a controlled diet for 15 days (refeeding). Body weight and food intake, hematocrit, and hemoglobinemia were evaluated. The animals were killed at 15 or 30 days post implantation. The liver, kidneys, and tibias were resected and fixed in 20% formalin solution. The tibias were radiographed and processed for histomorphometric evaluation of the periimplant bone area.

RESULTS

Histologic studies revealed steatosis in the liver but no alterations in the kidneys. Rats fed a choline-deficient diet showed periimplant bone healing with marked qualitative and quantitative alterations. The periimplant bone area was 28% and 75% lower in experimental animals than in controls at 15 and 30 days, respectively.

CONCLUSIONS

Liver alterations caused by a choline-deficient diet alter periimplant osteogenesis qualitatively and quantitatively.

Histamine stimulates production of osteoclast differentiation factor/receptor activator of nuclear factor-kappaB ligand by osteoblasts

Histamine H(1),H(2), and H(3) receptors are expressed by osteoblastic MC3T3-E1 (E1) cells derived from mouse calvaria. Expression of the osteoclast differentiation factor (ODF)/receptor activator of nuclear factor-kappaB ligand (RANKL) transcript was induced in E1 cells and bone marrow stromal cells (ST2). Histamine markedly increased the steady-state level of ODF/RANKL mRNA in a dose-dependent manner. The effect of histamine on expression of ODF/RANKL mRNA by E1 cells was transient, with a peak at 6h. Western blot analysis revealed that histamine increased production of ODF/RANKL protein by E1 cells at 12h. In cocultures of E1 cells and mouse bone marrow cells, histamine stimulated osteoclastogenesis in the presence of 1,25-dihydroxyvitamin D(3) and this effect was blocked by preincubation with neutralizing antibody against ODF/RANKL. These results suggest that histamine regulates osteoclastogenesis, at least in part, through induction of ODF/RANKL expression by osteoblasts and bone marrow stromal cells.

Estrogen receptor-related receptor alpha impinges on the estrogen axis in bone: potential function in osteoporosis

The orphan nuclear estrogen receptor-related receptor alpha (ERRalpha) is expressed by osteoblastic cells and plays a functional role in osteoprogenitor proliferation and differentiation. To dissect further the role of ERRalpha in bone, we investigated the effects of estrogen (E2) on ERRalpha both in vitro and in vivo. Chronic treatment of fetal rat calvaria cells with E2-stimulated bone nodule formation and up-regulated ERRalpha mRNA expression at early (10 h and d 8) but not later times in culture, suggesting a link between ERRalpha and E2 during osteoprogenitor proliferation. ERRalpha mRNA levels were significantly lower in ovariectomized adult rat bones vs. those of sham-operated rats early (1 d and 1 wk) post surgery, but levels returned to control levels thereafter. ERRalpha is also expressed in osteoclasts (tartrate-resistant acid phosphatase + multinucleated cells) in vivo and in vitro (RAW 264.7 cells) and ovariectomization lowered the OPG/receptor activator of nuclear factor kappaB ligand expression ratio. Down-regulation of ERRalpha expression via antisense treatment of rat calvaria cells not only inhibited osteogenesis but also increased adipocyte colony formation and changed the OPG/receptor activator of nuclear factor kappaB ligand ratio. These data suggest that ERRalpha is regulated by estrogen in bone in which it may play a functional role at several levels (osteoblasts, adipocytes, and osteoclasts) in E2 deficiency diseases such as osteoporosis.

Short-term prevention of osteoclastic resorption and osteopenia in ovariectomized rats treated with the H(2) receptor antagonist cimetidine

Ovariectomy rapidly induces strong osteoclast differentiation, leading to a marked loss of cancellous bone in the rat appendicular skeleton. As we found that histamine inhibition prevented periosteal bone resorption in rats, we tested the hypothesis that cimetidine, an H(2) receptor antagonist, prevents the osteoclastic burst and subsequent trabecular bone loss in this setting. Forty female Sprague-Dawley rats were ovariectomized (ovx) or sham-operated. Rats from each group received daily intramuscular injections of cimetidine (125 mg/kg per day) or vehicle. The animals were killed 14 days after surgery, and their femora were processed for morphometry. Cimetidine had no effect on serum estradiol levels in the control and ovx rats. BV/TV was reduced by 36% in the ovx rats, and by 10% in the cimetidine treated rats (p < 0.01). Tb.N and Tb.Wi were significantly reduced by 30% in the ovx rats and by 15% ovx-treated ones. OcS/BS did not change in the treated ovx rats, but increased 3.7-fold in the untreated ovx ones (p < 0.001). The N.Oc/TBPm increased markedly in the ovx rats (2.6-fold, p < 0.0001 vs. controls), but only slightly in the cimetidine-treated animals (+18%, p < 0.05 vs. controls), with a significant difference between the cimetidine-treated and -untreated ovx animals (p < 0.001). Cimetidine had no effect on these parameters in sham-operated animals. These results show that histamine inhibition by an H(2) receptor antagonist partially prevents the consequences of castration on cancellous bone, possibly by an action on osteoclast differentiation. Interestingly, cimetidine had no effect on basal resorption along trabecular bone. Histamine inhibition by H(2) blockers warrants further investigation in this model of osteopenia.