Histological Effects of the Combined Administration of Eldecalcitol and a Parathyroid Hormone in the Metaphyseal Trabeculae of Ovariectomized Rats

Regional difference in the distribution of alkaline phosphatase, PHOSPHO1, and calcein labeling in the femoral metaphyseal trabeculae in parathyroid hormone-administered mice

OBJECTIVES

This study aimed to elucidate whether the administration of parathyroid hormone (PTH) results in remodeling- or modeling-based bone formation in different regions of the murine femora, and whether the PTH-driven bone formation would facilitate osteoblastic differentiation into osteocytes.

METHODS

Six-week-old male C57BL/6J mice were employed to examine the distribution of alkaline phosphatase (ALP), PHOSPHO1, podoplanin, and calcein labeling in two distinct long bone regions: the metaphyseal trabeculae close to the chondro-osseous junction (COJ) and those distant from the COJ in three mouse groups, a control group receiving a vehicle (sham group) and groups receiving hPTH (1-34) twice a day (PTH BID group) or four times a day (PTH QID group) for two weeks.

RESULTS

The sham group showed PHOSPHO1-reactive mature osteoblasts localized primarily at the COJ, whereas the PTH BID/QID groups exhibited extended lines of PHOSPHO1-reactive osteoblasts even in regions distant from the COJ. The PTH QID group displayed fragmented calcein labeling in trabeculae close to the COJ, whereas continuous labeling was observed in trabeculae distant from the COJ. Osteoblasts tended to express podoplanin and PHOSPHO1 independently in the close and distant regions of the sham group, while osteoblasts in the PTH-administered groups showed immunoreactivity of podoplanin and PHOSPHO1 together in the close and distant regions.

CONCLUSIONS

Administration of PTH may accelerate remodeling-based bone formation in regions close to the COJ while predominantly inducing modeling-based bone formation in distant regions. PTH appeared to simultaneously facilitate osteoblastic bone mineralization and differentiation into osteocytes in both remodeling- and modeling-based bone formation.

Immunohistochemical and Morphometric Assessment on the Biological Function and Vascular Endothelial Cells in the Initial Process of Cortical Porosity in Mice With PTH Administration

To clarify the cellular mechanism of cortical porosity induced by intermittent parathyroid hormone (PTH) administration, we examined the femoral cortical bone of mice that received 40 µg/kg/day (four times a day) human PTH (hPTH) (1-34). The PTH-driven cortical porosity initiated from the metaphyseal region and chronologically expanded toward the diaphysis. Alkaline phosphatase (ALP)-positive osteoblasts in the control mice covered the cortical surface, and endomucin-positive blood vessels were distant from these osteoblasts. In PTH-administered mice, endomucin-reactive blood vessels with TRAP-positive penetrated the ALP-positive osteoblast layer, invading the cortical bone. Statistically, the distance between endomucin-positive blood vessels and the cortical bone surface abated after PTH administration. Transmission electron microscopic observation demonstrated that vascular endothelial cells often pass through the flattened osteoblast layer and accompanied osteoclasts in the deep region of the cortical bone. The cell layers covering mature osteoblasts thickened with PTH administration and exhibited ALP, α-smooth muscle actin (αSMA), vascular cell adhesion molecule-1 (VCAM1), and receptor activator of NF-κB ligand (RANKL). Within these cell layers, osteoclasts were found near endomucin-reactive blood vessels. In PTH-administered femora, osteocytes secreted Dkk1, a Wnt inhibitor that affects angiogenesis, and blood vessels exhibited plasmalemma vesicle-associated protein, an angiogenic molecule. In summary, endomucin-positive blood vessels, when accompanied by osteoclasts in the ALP/αSMA/VCAM1/RANKL-reactive osteoblastic cell layers, invade the cortical bone, potentially due to the action of osteocyte-derived molecules such as DKK1.

Eldecalcitol Induces Minimodeling-Based Bone Formation and Inhibits Sclerostin Synthesis Preferentially in the Epiphyses Rather than the Metaphyses of the Long Bones in Rats

This study aimed to examine minimodeling-based bone formation between the epiphyses and metaphyses of the long bones of eldecalcitol (ELD)-administered ovariectomized rats. Sixteen-week-old female rats were divided into four groups: sham-operated rats receiving vehicle (Sham group), ovariectomized (OVX) rats receiving vehicle (Vehicle group), or ELDs (30 or 90 ng/kg BW, respectively; ELD30 and ELD90 groups). ELD administration increased bone volume and trabecular thickness, reducing the number of osteoclasts in both the epiphyses and metaphyses of OVX rats. The Sham and Vehicle groups exhibited mainly remodeling-based bone formation in both regions. The epiphyses of the ELD groups showed a significantly higher frequency of minimodeling-based bone formation than remodeling-based bone formation. In contrast, the metaphyses exhibited significantly more minimodeling-based bone formation in the ELD90 group compared with the ELD30 group. However, there was no significant difference between minimodeling-based bone formation and remodeling-based bone formation in the ELD90 group. While the minimodeling-induced new bone contained few sclerostin-immunoreactive osteocytes, the underlying pre-existing bone harbored many. The percentage of sclerostin-positive osteocytes was significantly reduced in the minimodeling-induced bone in the epiphyses but not in the metaphyses of the ELD groups. Thus, it seems likely that ELD could induce minimodeling-based bone formation in the epiphyses rather than in the metaphyses, and that ELD-driven minimodeling may be associated with the inhibition of sclerostin synthesis.

Early gene expression profiles of anabolic and catabolic molecules in murine bone after a single PTH injection

The current study examined the gene expression profiles of anabolic and catabolic molecules after a single parathyroid hormone (PTH) injection in mice. No significant changes were observed in alkaline phosphatase area/tissue volume, tartrate-resistant acid phosphatase-positive osteoclasts, or static bone histomorphometry parameters. However, a sudden and significant decrease in Runx2 expression occurred at 1.5 h post-injection followed by immediate elevation, while sclerostin level was initially downregulated but gradually recovered. Meanwhile, Rankl expression initially increased and then returned to baseline. The prolonged elevation of anabolic molecules and transient increase in catabolic molecules may contribute to the anabolic effect of PTH treatment.

Differential osteoblastic activity in primary metaphyseal trabecular and secondary trabeculae of c-fos deficient mice

OBJECTIVES

It has been highlighted that osteoblastic activities in remodeling-based bone formation are coupled with osteoclastic bone resorption while those in modeling-based bone formation are independent of osteoclasts. This study aimed to verify whether modeling-based bone formation can occur in the absence of osteoclasts.

METHODS

We performed histochemical analyses on the bone of eight-week-old male wild-type and c-fos-/- mice. Histochemical analyses were conducted on primary trabeculae near the chondro-osseous junction (COJ), sites of modeling-based bone formation, and secondary trabeculae, sites of remodeling-based bone formation, in the femora and tibiae of mice.

RESULTS

Alkaline phosphatase (ALP) immunoreactivity, a marker of osteoblastic lineages, was observed in the metaphyseal trabeculae of wild-type mice, while ALP was scattered throughout the femora of c-fos-/- mice. PHOSPHO1, an enzyme involved in matrix vesicle-mediated mineralization, was predominantly detected in primary trabeculae and also within short lines of osteoblasts in secondary trabeculae of wild-type mice. In contrast, femora of c-fos-/- mice showed several patches of PHOSPHO1 positivity in the primary trabeculae, but there were hardly any patches of PHOSPHO1 in secondary trabeculae. Calcein labeling was consistently observed in primary trabeculae close to the COJ in both wild-type and c-fos-/- mice; however, calcein labeling in the secondary trabeculae was only detected in wild-type mice. Transmission electron microscopic examination demonstrated abundant rough endoplasmic reticulum in the osteoblasts in secondary trabeculae of wild-type mice, but not in those of c-fos-/- mice.

CONCLUSIONS

Osteoblastic activities at the sites of modeling-based bone formation may be maintained in the absence of osteoclasts.

Recent advance of small-molecule drugs for clinical treatment of osteoporosis: A review

Osteoporosis is a metabolic bone disorder typified by a reduction in bone mass and structural degradation of bone tissue, leading to heightened fragility and vulnerability to fractures. The incidence of osteoporosis increases with age, making it a significant public health challenge. The pathogenesis of osteoporosis involves an imbalance between osteoblast-mediated bone formation and resorption. The current treatment options for osteoporosis include bisphosphonates, hormone replacement therapy (HRT), selective estrogen receptor modulators (SERMs), and denosumab. The recent advances in small-molecule drugs for the clinical treatment of osteoporosis offer promising options for improving bone health and reducing fracture risk. This review aims to provide an overview of the clinical applications and synthetic routes of representative small-molecule drugs for the treatment of osteoporosis. A comprehensive understanding of the synthetic methods of drug molecules for osteoporosis may inspire the development of new, more effective, and practical synthetic techniques for treating this condition.

Immunolocalization of Enzymes/Membrane Transporters Related to Bone Mineralization in the Metaphyses of the Long Bones of Parathyroid-Hormone-Administered Mice

The present study aimed to demonstrate the immunolocalization and/or gene expressions of the enzymes and membrane transporters involved in bone mineralization after the intermittent administration of parathyroid hormone (PTH). The study especially focused on TNALP, ENPP1, and PHOSPHO1, which are involved in matrix vesicle-mediated mineralization, as well as PHEX and the SIBLING family, which regulate mineralization deep inside bone. Six-week-old male mice were subcutaneously injected with 20 μg/kg/day of human PTH (1-34) two times per day (n = 6) or four times per day (n = 6) for two weeks. Additionally, control mice (n = 6) received a vehicle. Consistently with an increase in the volume of the femoral trabeculae, the mineral appositional rate increased after PTH administration. The areas positive for PHOSPHO1, TNALP, and ENPP1 in the femoral metaphyses expanded, and the gene expressions assessed by real-time PCR were elevated in PTH-administered specimens when compared with the findings in control specimens. The immunoreactivity and/or gene expressions of PHEX and the SIBLING family (MEPE, osteopontin, and DMP1) significantly increased after PTH administration. For example, MEPE immunoreactivity was evident in some osteocytes in PTH-administered specimens but was hardly observed in control specimens. In contrast, mRNA encoding cathepsin B was significantly reduced. Therefore, the bone matrix deep inside might be further mineralized by PHEX/SIBLING family after PTH administration. In summary, it is likely that PTH accelerates mineralization to maintain a balance with elevated matrix synthesis, presumably by mediating TNALP/ENPP1 cooperation and stimulating PHEX/SIBLING family expression.

Histochemical assessment of osteoclast-like giant cells in Rankl-/- mice

OBJECTIVES

We examined mice with gene deletion of Receptor activator of nuclear factor-κB (Rank) ligand (Rankl) to histologically clarify whether they contained progenitor cells committed to osteoclastic differentiation up to the stage requiring RANK/RANKL signaling.

METHODS

The tibiae and femora of ten-week-old male wild-type, c-fos^-/-, and Rankl^-/- mice were used for immunohistochemistry and transmission electron microscopy (TEM).

RESULTS

In Rankl^-/- mice, we observed osteoclast-like giant cells, albeit in low numbers, with single or two nuclei, engulfing the mineralized extracellular matrix. TEM revealed that these giant cells contained large numbers of mitochondria, vesicles/vacuoles, and clear zone-like structures but no ruffled borders. They often engulfed fragmented bony/cartilaginous components of the extracellular matrix that had been degraded. Additionally, osteoclast-like giant cells exhibited immunoreactivity for vacuolar H⁺-ATPase, galectin-3, and siglec-15 but not for tartrate-resistant acid phosphatase, cathepsin K, or MMP-9, all of which are classical hallmarks of osteoclasts. Furthermore, osteoclast-like giant cells were ephrinB2-positive as they were near EphB4-positive osteoblasts that are also positive for alkaline phosphatase and Runx2 in Rankl^-/- mice. Unlike Rankl^-/- mice, c-fos^-/- mice lacking osteoclast progenitors and mature osteoclasts had no ephrinB2-positive osteoclast-like cells or alkaline phosphatase-positive/Runx2-reactive osteoblasts. This suggests that similar to authentic osteoclasts, osteoclast-like giant cells might have the potential to activate osteoblasts in Rankl^-/- mice.

CONCLUSIONS

It seems plausible that osteoclast-like giant cells may have acquired some osteoclastic traits and the ability to resorb mineralized matrices even when the absence of RANK/RANKL signaling halted the osteoclastic differentiation cascade.

Histochemical assessment on osteoclasts in long bones of toll-like receptor 2 (TLR2) deficient mice

OBJECTIVE

Toll-like receptor 2 (TLR2), recognizes a wide variety of pathogen-associated molecular patterns such as lipopolysaccharides, peptidoglycans, and lipopeptides, and is generally believed to be present in monocytes, macrophages, dendritic cells, and vascular endothelial cells. However, no histological examination of osteoclasts, which differentiate from precursors common to macrophages/monocytes, has been performed in a non-infected state of TLR2 deficiency. The objective of this study was to examine the histological properties and function of osteoclasts in the long bones of 8-week-old male TLR2 deficient (TLR2^-/-) mice to gain insight into TLR2 function in biological circumstances without microbial infection.

METHODS

Eight-week-old male wild-type and TLR2^-/- mice were fixed with paraformaldehyde solution, and their tibiae and femora were used for micro-CT analysis, immunohistochemistry, transmission electron microscopy, and real-time PCR analysis.

RESULTS

TLR2^-/- tibiae and femora exhibited increased bone volume of metaphyseal trabeculae and elevated numbers of TRAP-positive osteoclasts. However, the number of multinucleated TRAP-positive osteoclasts was reduced, whereas mononuclear TRAP-positive cells increased, despite the high expression levels of Dc-Stamp and Oc-Stamp. Although TRAP-positive multinucleated and mononuclear osteoclasts showed the immunoreactivity and elevated expression of RANK and siglec-15, they revealed weak cathepsin K-positivity and less incorporation of the mineralized bone matrix, and often missing ruffled borders. It seemed likely that, despite the increased numbers, TLR2^-/- osteoclasts reduced cell fusion and bone resorption activity.

CONCLUSION

It seems likely that even without bacterial infection, TLR2 might participate in cell fusion and subsequent bone resorption of osteoclasts.

Evocalcet Rescues Secondary Hyperparathyroidism-driven Cortical Porosity in CKD Male Rats

To elucidate the effect of evocalcet, a new oral calcimimetic to bone of secondary hyperparathyroidism (SHPT) with chronic kidney disease (CKD), the rats were 5/6 nephrectomized and fed on a high-phosphate diet. The treated rats were then divided into vehicle groups and evocalcet administered groups. The rats in the vehicle groups exhibited increased levels of serum PTH and inorganic phosphate (Pi) levels, high bone turnover, and severe cortical porosity, mimicking SHPT (CKD-SHPT rats). The cortical bone of the CKD-SHPT rats showed broad demineralization around the osteocytes, suppression of Phex/small integrin-binding ligand N-linked glycoprotein-mediated mineralization in the periphery of the osteocytic lacunae, and increased levels of osteocytic cell death, all of which were considered as the first steps of cortical porosity. In contrast, evocalcet ameliorated the increased serum PTH levels, the enlarged osteocytic lacunae, and the cortical porosity of the CKD-SHPT rats. Osteocytes of CKD-SHPT rats strongly expressed PTH receptor and Pit1/Pit2, which sense extracellular Pi, indicating that PTH and Pi affected these osteocytes. Cell death of cultured osteocytes increased in a Pi concentration-dependent manner, and PTH administration rapidly elevated Pit1 expression and enhanced osteocytic death, indicating the possibility that the highly concentrated serum PTH and Pi cause severe perilacunar osteolysis and osteocytic cell death. It is likely therefore that evocalcet not only decreases serum PTH but also reduces the exacerbation combined with PTH and Pi to the demineralization of osteocytic lacunae and osteocytic cell death, thereby protecting cortical porosity in CKD-SHPT rats.

Histochemical assessment on osteocytic osteolysis in lactating mice fed with a calcium-insufficient diet

OBJECTIVES

This study aimed to examine if feeding lactating mice a calcium-insufficient diet while simultaneously administering alendronate (ALN) could potentially induce osteocytic osteolysis.

METHODS

Lactating mice were fed calcium (Ca)-insufficient diets with or without ALN administration, and then their femurs were examined for TRAP and ALP, and observed by Kossa staining and transmission electron microscopy (TEM). Mice that had been fed a Ca-insufficient diet were then fed a ⁴⁴Ca-containinig diet, and their tibial sections were examined by isotope microscopy.

RESULTS

Mice fed a Ca-insufficient diet had a reduced number of TRAP-positive osteoclasts after ALN administration. ALN-treated, lactating mice fed a Ca-insufficient diet had enlarged lacunae in their cortical bones, and TEM imaging demonstrated expanded regions between osteocytes and lacunar walls. In ALN-treated lactating mice fed a Ca-insufficient diet, huge areas of demineralized bone matrix occurred, centered around blood vessels in the cortical bone. Isotope microscopy showed ⁴⁴Ca in the vicinity of the osteocytic lacunae, and in the broad, previously demineralized region around the blood vessels in the cortical bone of lactating mice fed a ⁴⁴Ca-sufficient diet.

CONCLUSIONS

Bone demineralization likely takes place in the periphery of the osteocytic lacunae and in the broad regions around the blood vessels of lactating mice when they are exposed to severely reduced serum Ca through a Ca-insufficient diet coupled with ALN administration.

Deletion of Tfam in Prx1-Cre expressing limb mesenchyme results in spontaneous bone fractures

INTRODUCTION

Osteoblasts require substantial amounts of energy to synthesize the bone matrix and coordinate skeleton mineralization. This study analyzed the effects of mitochondrial dysfunction on bone formation, nano-organization of collagen and apatite, and the resultant mechanical function in mouse limbs.

MATERIALS AND METHODS

Limb mesenchyme-specific Tfam knockout (Tfam^f/f;Prx1-Cre: Tfam-cKO) mice were analyzed morphologically and histologically, and gene expressions in the limb bones were assessed by in situ hybridization, qPCR, and RNA sequencing (RNA-seq). Moreover, we analyzed the mitochondrial function of osteoblasts in Tfam-cKO mice using mitochondrial membrane potential assay and transmission electron microscopy (TEM). We investigated the pathogenesis of spontaneous bone fractures using immunohistochemical analysis, TEM, birefringence analyzer, microbeam X-ray diffractometer and nanoindentation.

RESULTS

Forelimbs in Tfam-cKO mice were significantly shortened from birth, and spontaneous fractures occurred after birth, resulting in severe limb deformities. Histological and RNA-seq analyses showed that bone hypoplasia with a decrease in matrix mineralization was apparent, and the expression of type I collagen and osteocalcin was decreased in osteoblasts of Tfam-cKO mice, although Runx2 expression was unchanged. Decreased type I collagen deposition and mineralization in the matrix of limb bones in Tfam-cKO mice were associated with marked mitochondrial dysfunction. Tfam-cKO mice bone showed a significantly lower Young's modulus and hardness due to poor apatite orientation which is resulted from decreased osteocalcin expression.

CONCLUSION

Mice with limb mesenchyme-specific Tfam deletions exhibited spontaneous limb bone fractures, resulting in severe limb deformities. Bone fragility was caused by poor apatite orientation owing to impaired osteoblast differentiation and maturation.

Immunolocalization of endomucin-reactive blood vessels and α-smooth muscle actin-positive cells in murine nasal conchae

OBJECTIVES

Recently, the biological functions of endomucin-positive blood vessels and closely associated αSMA-positive cells in long bones have been highlighted. The surrounding tissues of the flat bones, such as nasal bones covered with mucosa and lamina propria, are different from those of the long bones, indicating the different distributions of endomucin-positive blood vessels and αSMA-reactive cells in nasal bones. This study demonstrates the immunolocalization of endomucin-reactive blood vessels and αSMA-positive cells in the nasal conchae of 3- and 7-week-old mice.

METHODS

The nasal conchae of 3-week-old and 7-week-old male C57BL/6J mice were used for immunoreaction of endomucin, CD34, PDGFbb, TRAP, and c-kit.

RESULTS

While we identified abundant endomucin-reactive blood vessels in the lamina propria neighboring the bone, not all were positive for endomucin. More CD34-reactive cells and small blood vessels were observed in the nasal conchae of 3-week-old mice than in those of 7-week-old mice. Some αSMA-positive cells in the nasal conchae surrounded the blood vessels, indicating vascular smooth muscle cells, while other αSMA-immunopositive fibroblastic cells were detected throughout the lamina propria. αSMA-positive cells did not co-localize with C-kit-immunoreactivity, thereby indicating that the αSMA-positive cells may be myofibroblasts rather than undifferentiated mesenchymal cells.

CONCLUSIONS

Unlike long bones, nasal conchae contain endomucin-positive as well as endomucin-negative blood vessels and exhibit numerous αSMA-positive fibroblastic cells throughout the lamina propria neighboring the bone. Apparently, the distribution patterns of endomucin-positive blood vessels and αSMA-positive cells in nasal conchae are different from those in long bones.

Tmem174, a regulator of phosphate transporter prevents hyperphosphatemia

Renal type II sodium-dependent inorganic phosphate (Pi) transporters NaPi2a and NaPi2c cooperate with other organs to strictly regulate the plasma Pi concentration. A high Pi load induces expression and secretion of the phosphaturic hormones parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) that enhance urinary Pi excretion and prevent the onset of hyperphosphatemia. How FGF23 secretion from bone is increased by a high Pi load and the setpoint of the plasma Pi concentration, however, are unclear. Here, we investigated the role of Transmembrane protein 174 (Tmem174) and observed evidence for gene co-expression networks in NaPi2a and NaPi2c function. Tmem174 is localized in the renal proximal tubules and interacts with NaPi2a, but not NaPi2c. In Tmem174-knockout (KO) mice, the serum FGF23 concentration was markedly increased but increased Pi excretion and hypophosphatemia were not observed. In addition, Tmem174-KO mice exhibit reduced NaPi2a responsiveness to FGF23 and PTH administration. Furthermore, a dietary Pi load causes marked hyperphosphatemia and abnormal NaPi2a regulation in Tmem174-KO mice. Thus, Tmem174 is thought to be associated with FGF23 induction in bones and the regulation of NaPi2a to prevent an increase in the plasma Pi concentration due to a high Pi load and kidney injury.

Characterization, pharmacokinetics, and pharmacodynamics of anti-Siglec-15 antibody and its potency for treating osteoporosis and as follow-up treatment after parathyroid hormone use

Recent studies have established the idea that Siglec-15 is involved in osteoclast differentiation and/or function, and it is anticipated that therapies suppressing Siglec-15 function can be used to treat bone diseases such as osteoporosis. We have produced rat monoclonal anti-Siglec-15 antibody (32A1) and successively generated humanized monoclonal anti-Siglec-15 antibody (DS-1501a) from 32A1. Studies on the biological properties of DS-1501a showed its specific binding affinity to Siglec-15 and strong activity to inhibit osteoclastogenesis. 32A1 inhibited multinucleation of osteoclasts and bone resorption (pit formation) in cultured mouse bone marrow cells. 32A1 also inhibited pit formation in cultured human osteoclast precursor cells. Maximum serum concentration and serum exposure of DS-1501a in rats were increased in a dose-dependent manner after single subcutaneous or intravenous administration. Furthermore, single administration of DS-1501a significantly suppressed bone resorption markers with minimal effects on bone formation markers and suppressed the decrease in bone mineral density (BMD) of the lumbar vertebrae in ovariectomized (OVX) rats. In histological analysis, the osteoclasts distant from the chondro-osseous junction of the tibia tended to be flattened, shrunken, and functionally impaired in 32A1-treated rats, while alkaline phosphatase-positive osteoblasts were observed throughout the metaphyseal trabeculae. In addition, we compared the efficacy of 32A1 with that of alendronate (ALN) as follow-up medicine after treatment with parathyroid hormone (PTH) using mature established osteoporosis rats. The beneficial effect of PTH on bone turnover disappeared 8 weeks after discontinuing the treatment. The administration of 32A1 once every 4 weeks for 8 weeks suppressed bone resorption and bone formation when the treatment was switched from PTH to 32A1, leading to the maintenance of BMD and bone strength. Unlike with ALN, the onset of suppression of bone resorption with 32A1 was rapid, while the suppression of bone formation was mild. The improvement of bone mass, beneficial bone turnover balance, and suppression of osteoclast differentiation/multinucleation achieved by 32A1 were supported by histomorphometry. Notably, the effects of 32A1 on bone strength, not only structural (extrinsic) but also material (intrinsic) properties, were significantly greater than those of ALN. Since the effect of 32A1 on BMD was moderate, its effect on bone strength could not be fully explained by the increase in BMD. The beneficial balance of bone turnover caused by 32A1 might, at least in part, be responsible for the improvement in bone quality. This is the first report describing the effects of anti-Siglec-15 antibody in OVX rats; the findings suggest that this antibody could be an excellent candidate for treating osteoporosis, especially in continuation therapy after PTH treatment, due to its rapid action and unprecedented beneficial effects on bone quality.

Cardiotrophin Like Cytokine Factor 1 (CLCF1) alleviates bone loss in osteoporosis mouse models by suppressing osteoclast differentiation through activating interferon signaling and repressing the nuclear factor-κB signaling pathway

A growing body of evidence suggests that immune factors that regulate osteoclast differentiation and bone resorption might be promising therapeutic agents for the treatment of osteoporosis. The expression of CLCF1, an immune cell-derived molecule, has been reported to be reduced in patients with postmenopausal osteoporosis. This suggests that it may be involved in bone remodeling. Thus, we explored the functional role of CLCF1 in osteoclastogenesis and bone loss associated with osteoporosis. Surprisingly, the administration of recombinant CLCF1 repressed excessive bone loss in ovariectomized mice and prevented RANKL-induced bone loss in calvarial mouse model. Likewise, the addition of recombinant CLCF1 to RANKL-stimulated monocytes resulted in a significant suppression in the number of differentiated osteoclasts with small resorption areas being observed on dentine slices in vitro. At the same dosage, CLCF1 did not exhibit any detectable negative effects on the differentiation of osteoblasts. Mechanistically, the inhibition of osteoclast differentiation by the CLCF1 treatment appears to be related to the activation of interferon signaling (IFN) and the suppression of the NF-κB signaling pathway. Interestingly, the expression of the main components of IFN-signaling namely, STAT1 and IRF1, was detected in macrophages as early as 1 h after stimulation with CLCF1. Consistent with these results, the blockade of STAT1 in macrophages abolished the inhibitory effect of CLCF1 on osteoclast differentiation in vitro. These collective findings point to a novel immunoregulatory function of CLCF1 in bone remodeling and highlight it as a potentially useful therapeutic agent for the treatment of osteoporosis.

Histochemical characteristics on minimodeling-based bone formation induced by anabolic drugs for osteoporotic treatment

Modeling, the changes of bone size and shape, often takes place at the developmental stages, whereas bone remodeling-replacing old bone with new bone-predominantly occurs in adults. Unlike bone remodeling, bone formation induced by modeling i.e., minimodeling (microscopic modeling in cancellous bone) is independent of osteoclastic bone resorption. Although recently-developed drugs for osteoporotic treatment could induce minimodeling-based bone formation in addition to remodeling-based bone formation, few reports have demonstrated the histological aspects of minimodeling-based bone formation. After administration of eldecalcitol or romosozumab, unlike teriparatide treatment, mature osteoblasts formed new bone by minimodeling, without developing thick preosteoblastic layers. The histological characteristics of minimodeling-based bone formation is quite different from remodeling, as it is not related to osteoclastic bone resorption, resulting in convex-shaped new bone and smooth cement lines called arrest lines. In this review, we will show histological properties of minimodeling-based bone formation by osteoporotic drugs.

Altered immunolocalization of FGF23 in murine femora metastasized with human breast carcinoma MDA-MB-231 cells

INTRODUCTION

After the onset of bone metastasis, tumor cells appear to modify surrounding microenvironments for their benefit, and particularly, the levels of circulating fibroblast growth factor (FGF) 23 in patients with tumors have been highlighted.

MATERIALS AND METHODS

We have attempted to verify if human breast carcinoma MDA-MB-231 cells metastasized in the long bone of nu/nu mice would synthesize FGF23. Serum concentrations of calcium, phosphate (Pi) and FGF23 were measured in control nu/nu mice, bone-metastasized mice, and mice with mammary gland injected with MDA-MB-231 cells mimicking primary mammary tumors.

RESULTS AND CONCLUSIONS

MDA-MB-231 cells revealed intense FGF23 reactivity in metastasized lesions, whereas MDA-MB-231 cells cultured in vitro or when injected into the mammary glands (without bone metastasis) showed weak FGF23 immunoreactivity. Although the bone-metastasized MDA-MB-231 cells abundantly synthesized FGF23, osteocytes adjacent to the FGF23-immunopositive tumors, unlike intact osteocytes, showed no FGF23. Despite significantly elevated serum FGF23 levels in bone-metastasized mice, there was no significant decrease in the serum Pi concentration when compared with the intact mice and mice with a mass of MDA-MB-231 cells in mammary glands. The metastasized femora showed increased expression and FGFR1 immunoreactivity in fibroblastic stromal cells, whereas femora of control mice showed no obvious FGFR1 immunoreactivity. Taken together, it seems likely that MDA-MB-231 cells synthesize FGF23 when metastasized to a bone, and thus affect FGFR1-positive stromal cells in the metastasized tumor nest in a paracrine manner.

Comparative immunolocalization of tissue nonspecific alkaline phosphatase and ectonucleotide pyrophosphatase/phosphodiesterase 1 in murine bone

OBJECTIVE

This study aimed to demonstrate the immunolocalization and gene expression of tissue nonspecific alkaline phosphatase (TNALP) and ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) in osteoblasts, preosteoblasts, and osteocytes of murine bone to provide clues for a better understanding of the supply of phosphate ions (Pi) during bone mineralization.

METHODS

Six-week-old male C57BL/6J mice (n = 6) were fixed with a paraformaldehyde solution, and the right femora were extracted for immunodetection of TNALP and ENPP1, while the left tibiae were used for reverse transcription polymerase chain reaction to evaluate Tnalp and Enpp1 gene expression.

RESULTS

TNALP was intensely localized on the basolateral cell membranes of mature osteoblasts and preosteoblastic cells. There was little immunoreactivity of TNALP on the secretory surface of the osteoblasts and no TNALP reactivity in the osteocytes. In contrast, ENPP1 was observed throughout the cytoplasm of mature osteoblasts and osteocytes embedded in bone but was not observed in preosteoblasts. Together, despite the fact that the osteoid is a site of matrix vesicle-mediated mineralization, ENPP1, which inhibits mineralization by providing pyrophosphates, was localized in close proximity of the osteoid, whereas TNALP, which facilitates mineralization by providing Pi, was relatively distant from the osteoid.

CONCLUSION

It seems likely that the differential localization of TNALP and ENPP1 around the osteoid observed at the microscopic level may provide preferential micro-circumstance for a balanced concentration of Pi and pyrophosphate for bone mineralization.

Vitamin K-Dependent γ-Glutamyl Carboxylase in Sertoli Cells Is Essential for Male Fertility in Mice

γ-Glutamyl carboxylase (GGCX) is a vitamin K (VK)-dependent enzyme that catalyzes the γ-carboxylation of glutamic acid residues in VK-dependent proteins. The anticoagulant warfarin is known to reduce GGCX activity by inhibiting the VK cycle and was recently shown to disrupt spermatogenesis. To explore GGCX function in the testis, here, we generated Sertoli cell-specific Ggcx conditional knockout (Ggcx scKO) mice and investigated their testicular phenotype. Ggcx scKO mice exhibited late-onset male infertility. They possessed morphologically abnormal seminiferous tubules containing multinucleated and apoptotic germ cells, and their sperm concentration and motility were substantially reduced. The localization of connexin 43 (Cx43), a gap junction protein abundantly expressed in Sertoli cells and required for spermatogenesis, was distorted in Ggcx scKO testes, and Cx43 overexpression in Sertoli cells rescued the infertility of Ggcx scKO mice. These results highlight GGCX activity within Sertoli cells, which promotes spermatogenesis by regulating the intercellular connection between Sertoli cells and germ cells.

Intermittent PTH Administration Increases Bone-Specific Blood Vessels and Surrounding Stromal Cells in Murine Long Bones

To verify whether PTH acts on bone-specific blood vessels and on cells surrounding these blood vessels, 6-week-old male mice were subjected to vehicle (control group) or hPTH [1-34] (20 µg/kg/day, PTH group) injections for 2 weeks. Femoral metaphyses were used for histochemical and immunohistochemical studies. In control metaphyses, endomucin-positive blood vessels were abundant, but αSMA-reactive blood vessels were scarce. In the PTH-administered mice, the lumen of endomucin-positive blood vessels was markedly enlarged. Moreover, many αSMA-positive cells were evident near the blood vessels, and seemed to derive from those vessels. These αSMA-positive cells neighboring the blood vessels showed features of mesenchymal stromal cells, such as immunopositivity for c-kit and tissue nonspecific alkaline phosphatase (TNALP). Thus, PTH administration increased the population of perivascular/stromal cells positive for αSMA and c-kit, which were likely committed to the osteoblastic lineage. To understand the cellular events that led to increased numbers and size of bone-specific blood vessels, we performed immunohistochemical studies for PTH/PTHrP receptor and VEGF. After PTH administration, PTH/PTHrP receptor, VEGF and its receptor flk-1 were consistently identified in both osteoblasts and blood vessels (endothelial cells and surrounding perivascular cells). Our findings suggest that exogenous PTH increases the number and size of bone-specific blood vessels while fostering perivascular/stromal cells positive for αSMA/TNALP/c-kit.

Immunocytochemical assessment of cell differentiation of podoplanin-positive osteoblasts into osteocytes in murine bone

In this study, we examined the immunolocalization of podoplanin/E11, CD44, actin filaments, and phosphorylated ezrin in the osteoblasts on the verge of differentiating into osteocytes in murine femora and tibiae. When observing under stimulated emission depletion microscopy, unlike podoplanin-negative osteoblasts, podoplanin-positive osteoblasts showed a rearranged assembly of actin filaments along the cell membranes which resembled that of embedded osteocytes. In the metaphysis, i.e., the bone remodeling site, CD44-bearing osteoclasts were either proximal to or in contact with podoplanin-positive osteoblasts, but the podoplanin-positive osteoblasts also localized CD44 on their own cell surface. These podoplanin-positive osteoblasts, which either possessed CD44 on their cell surface or were close to CD44-bearing osteoclasts, showed phosphorylated ezrin-positivity on the cell membranes. Therefore, the CD44/podoplanin interaction on the cell surface may be involved in the osteoblastic differentiation into osteocytes in the metaphyses, via the mediation of podoplanin-driven ezrin phosphorylation and the subsequent reorganized assembly of actin filaments. Consistently, the protein expression of phosphorylated ezrin was increased after CD44 administration in calvarial culture. Conversely, in modeling sites such as the cortical bones, podoplanin-positive osteoblasts were uniformly localized at certain intervals even without contact with CD44-positive bone marrow cells; furthermore, they also exhibited phosphorylated ezrin immunoreactivity along their cell membranes. Taken together, it seems likely that the CD44/podoplanin interaction is involved in osteoblastic differentiation into osteocytes in the bone remodeling area but not in modeling sites.

Osteocytic Osteolysis in PTH-treated Wild-type and Rankl-/- Mice Examined by Transmission Electron Microscopy, Atomic Force Microscopy, and Isotope Microscopy

To demonstrate the ultrastructure of osteocytic osteolysis and clarify whether osteocytic osteolysis occurs independently of osteoclastic activities, we examined osteocytes and their lacunae in the femora and tibiae of 11-week-old male wild-type and Rankl^-/- mice after injection of human parathyroid hormone (PTH) [1-34] (80 µg/kg/dose). Serum calcium concentration rose temporarily 1 hr after PTH administration in wild-type and Rankl^-/- mice, when renal arteries and veins were ligated. After 6 hr, enlargement of osteocytic lacunae was evident in the cortical bones of wild-type and Rankl^-/- mice, but not so in their metaphyses. Von Kossa staining and transmission electron microscopy showed broadly demineralized bone matrix peripheral to enlarged osteocytic lacunae, which contained fragmented collagen fibrils and islets of mineralized matrices. Nano-indentation by atomic force microscopy revealed the reduced elastic modulus of the PTH-treated osteocytic perilacunar matrix, despite the microscopic verification of mineralized matrix in that region. In addition, ⁴⁴Ca deposition was detected by isotope microscopy and calcein labeling in the eroded osteocytic lacunae of wild-type and Rankl^-/- mice. Taken together, our findings suggest that osteocytes can erode the bone matrix around them and deposit minerals on their lacunar walls independently of osteoclastic activity, at least in the murine cortical bone. (J Histochem Cytochem 68: -XXX, 2020).

Histochemical examination on principal collagen fibers in periodontal ligaments of ascorbic acid-deficient ODS-od/od rats

In this study, we aimed to clarify the role of ascorbic acid in collagen synthesis in periodontal ligaments using osteogenic disorder Shionogi (ODS)/ShiJcl-od/od rats lacking L-gulonolactone oxidase. These rats cannot synthesize ascorbic acid in vivo. Eight-week-old ODS/ShiJcl-od/od male rats were administered ascorbic acid solution at a concentration of 200 mg/dL (control group, n = 6) or ascorbic acid solution at concentration of 0.3 mg/dL (insufficient group, n = 12). Six rats of the insufficient group were then given with ascorbic acid solution at concentration of 200 mg/dL for additional 3 weeks (rescued group, n = 6), and then, their mandibles were histochemically examined. Consequently, the insufficient group specimens were seen to possess fewer collagen fibers, and silver impregnation revealed numerous fine, reticular fiber-like fibrils branching off from collagen in the periodontal ligaments. In control group, faint immunoreactivities for matrix metalloproteinase (MMP)2 and cathepsin H were seen in the periphery of blood vessels and throughout the ligament, respectively. In contrast, in the insufficient group, intense MMP2-immunoreactivity was observed to be associated with collagen fibrils in the periodontal ligaments, and cathepsin H-immunopositivity was seen in ligamentous cells. The rescued group showed abundant collagen fibers filling the periodontal ligament space. Under transmission electron microscopy, ligamentous fibroblasts incorporated collagen fibrils into tubular endosomes/lysosomes while simultaneously synthesizing collagen fibril bundles. Thus, ascorbic acid insufficiency affected the immunolocalization of cathepsin H and MMP2; however, ligamentous fibroblasts appear to possess the potential to synthesize collagen fibers when supplied with ascorbic acid.

Sequential Treatment with Eldecalcitol After PTH Improves Bone Mechanical Properties of Lumbar Spine and Femur in Aged Ovariectomized Rats

Parathyroid hormone (PTH) analogs have a powerful anabolic effect on bone and are used in the treatment of patients with severe osteoporosis. However, there are limitations to how long they can be safely administered. Withdrawal of PTH results in the cancelation of its effects, necessitating subsequent treatment to maintain the bone quantity and quality. This study assessed the effects of Eldecalcitol (ELD), an active vitamin D3 derivative, after PTH in estrogen-deficient osteoporotic rats. Six-month-old female rats were ovariectomized, and PTH administration was started 7 weeks later. After 4 weeks of PTH treatment, the animals were divided into three groups and either continued to receive PTH (PTH-PTH), or were switched to ELD (PTH-ELD) or vehicle (PTH-Veh) for an additional 4 weeks. In the femur, increased BMD by 4 weeks treatment of PTH was significantly reduced in PTH-Veh but not in PTH-PTH and PTH-ELD. The same tendency was observed in the lumbar vertebrae. MicroCT imaging and histomorphometry analysis revealed that the favorable bone structure changes by PTH administration were also maintained in the femurs and tibias of the PTH-PTH and PTH-ELD groups. Increased bone strength by 4-week treatment of PTH in lumber also maintained in PTH-ELD. Furthermore, minimodeling was observed in the PTH-ELD group. These results demonstrate that treatment with ELD sequentially following PTH prevented the bone quantity and strength reduction that accompanies PTH withdrawal in estrogen-deficient rats.