Osteoclasts secrete the chemotactic cytokine mim-1

Biologic Antiresorptive: Denosumab

Background

Osteoporosis is an age-related common bone disorder characterized by low bone mineral density and increased fragility fracture risk. Various Antiresorptive medications are being used to target osteoclast mediated bone resorption to prevent bone loss and reduce fracture risk.

About Denosumab

Denosumab is a novel biological antiresorptive drug that belongs to the class of monoclonal antibodies. It binds to and inhibits the cytokine receptor activator of nuclear factor kappa-B ligand (RANKL), which is requisite for osteoclast differentiation, function and survival.

Effectiveness

Denosumab has been shown to be a potent and effective therapy for osteoporosis, with clinical trial data demonstrating significant improvement in bone mineral density (BMD) and reductions in fracture risk at various skeletal sites for more than 10 years of treatment.

Safety Profile

Denosumab has a favourable benefit/risk profile, with low rates of complications such as infection, atypical femoral fracture and osteonecrosis of the jawbone.

Challenges

However, denosumab treatment requires continuous administration, as discontinuation leads to rapid bone mineral loss and increased risk of multiple vertebral fractures due to rebound of bone turnover. Therefore, modification to another anti-osteoporosis drug therapy after denosumab discontinuation is required to maintain bone health.

Conclusion

Denosumab is a promising biological antiresorptive therapy for osteoporosis that offers high efficacy and safety, but also poses challenges for long-term management.

The molecular structure and role of LECT2 or CHM-II in arthritis, cancer, and other diseases

Leukocyte cell-derived chemotaxin-2 (LECT2 or LECT-2), also called chondromodulin II (ChM-II or CHM2) plays a versatile role in various tissues. It was first identified as a chemotactic factor to promote the migration of neutrophils. It was also reported as a hepatokine to regulate glucose metabolism, obesity, and nonalcoholic fatty liver disease. As a secreted factor, LECT2 binds to several cell surface receptors CD209a, Tie1, and Met to regulate inflammatory reaction, fibrogenesis, vascular invasion, and tumor metastasis in various cell types. As an intracellular molecule, it is associated with LECT2-mediated amyloidosis, in which LECT2 misfolding results in insoluble fibrils in multiple tissues such as the kidney, liver, and lung. Recently, LECT2 was found to be associated with the development of rheumatoid arthritis and osteoarthritis, involving the dysregulation of osteoclasts, mesenchymal stem cells, osteoblasts, chondrocytes, and endothelial cells in the bone microenvironment. LECT2 is implicated in the development of cancers, such as hepatocellular carcinoma via MET-mediated PTP1B/Raf1/ERK signaling pathways and is proposed as a biomarker. The mechanisms by which LECT2 regulates diverse pathogenic conditions in various tissues remain to be fully elucidated. Further research to understand the role of LECT2 in a tissue tropism-dependent manner would facilitate the development of LECT2 as a biomarker for diagnosis and therapeutic target.

Gut microbiota and bone metabolism

Osteoporosis is the most common metabolic skeletal disease. It is characterized by the deterioration of the skeletal microarchitecture and bone loss, leading to ostealgia, and even bone fractures. Accumulating evidence has indicated that there is an inextricable relationship between the gut microbiota (GM) and bone homeostasis involving host-microbiota crosstalk. Any perturbation of the GM can play an initiating and reinforcing role in disrupting the bone remodeling balance during the development of osteoporosis. Although the GM is known to influence bone metabolism, the mechanisms associated with these effects remain unclear. Herein, we review the current knowledge of how the GM affects bone metabolism in health and disease, summarize the correlation between pathogen-associated molecular patterns of GM structural components and bone metabolism, and discuss the potential mechanisms underlying how GM metabolites regulate bone turnover. Deciphering the complicated relationship between the GM and bone health will provide new insights into the prevention and treatment of osteoporosis.

Osteoimmunology: The Conceptual Framework Unifying the Immune and Skeletal Systems

The immune and skeletal systems share a variety of molecules, including cytokines, chemokines, hormones, receptors, and transcription factors. Bone cells interact with immune cells under physiological and pathological conditions. Osteoimmunology was created as a new interdisciplinary field in large part to highlight the shared molecules and reciprocal interactions between the two systems in both heath and disease. Receptor activator of NF-κB ligand (RANKL) plays an essential role not only in the development of immune organs and bones, but also in autoimmune diseases affecting bone, thus effectively comprising the molecule that links the two systems. Here we review the function, gene regulation, and signal transduction of osteoimmune molecules, including RANKL, in the context of osteoclastogenesis as well as multiple other regulatory functions. Osteoimmunology has become indispensable for understanding the pathogenesis of a number of diseases such as rheumatoid arthritis (RA). We review the various osteoimmune pathologies, including the bone destruction in RA, in which pathogenic helper T cell subsets [such as IL-17-expressing helper T (Th17) cells] induce bone erosion through aberrant RANKL expression. We also focus on cellular interactions and the identification of the communication factors in the bone marrow, discussing the contribution of bone cells to the maintenance and regulation of hematopoietic stem and progenitors cells. Thus the time has come for a basic reappraisal of the framework for understanding both the immune and bone systems. The concept of a unified osteoimmune system will be absolutely indispensable for basic and translational approaches to diseases related to bone and/or the immune system.

BMP-2 and titanium particles synergistically activate osteoclast formation

A previous study showed that BMP-2 (bone morphogenetic protein-2) and wear debris can separately support osteoclast formation induced by the receptor activator of NF-κB ligand (RANKL). However, the effect of BMP-2 on wear debris-induced osteoclast formation is unclear. In this study, we show that neither titanium particles nor BMP-2 can induce osteoclast formation in RAW 264.7 mouse leukemic monocyte macrophage cells but that BMP-2 synergizes with titanium particles to enhance osteoclast formation in the presence of RANKL, and that at a low concentration, BMP-2 has an optimal effect to stimulate the size and number of multinuclear osteoclasts, expression of osteoclast genes, and resorption area. Our data also clarify that the effects caused by the increase in BMP-2 on phosphorylated SMAD levels such as c-Fos expression increased throughout the early stages of osteoclastogenesis. BMP-2 and titanium particles stimulate the expression of p-JNK, p-P38, p-IkB, and P50 compared with the titanium group. These data suggested that BMP-2 may be a crucial factor in titanium particle-mediated osteoclast formation.

Adenovirus-mediated expression of bone morphogenetic protein-2 activates titanium particle-induced osteoclastogenesis and this effect occurs in spite of the suppression of TNF-α expression by siRNA

The phagocytosis of wear particles by macrophages results in the secretion of pro-inflammatory cytokines, such as tumor necrosis factor-α (TNF-α), which play a major role in promoting osteoclast recruitment. The inhibition of TNF-α expression decreases osteoclastogenesis. In a previous study, we demonstrated that bone morphogenetic protein-2 (BMP-2) can activate wear debris-induced osteoclast recruitment in the presence of receptor activator of nuclear factor (NF)-κB ligand (RANKL); however, whether these effects are associated with pro-inflammatory cytokines remains unclear. In this study, we constructed an adenoviral vector carrying TNF-small interfering RNA (siRNA) (Ad-TNF-siRNA), as well as a vector carrying both the BMP-2 gene and TNF-α-siRNA (Ad-BMP-2-TNF-siRNA). The two adenoviral vectors significantly suppressed the expression of TNF-α; however, only treatment with Ad-TNF-siRNA significantly inhibited osteoclastogenesis. We demonstrate that the overexpression of BMP-2, despite the suppression of TNF-α expression by Ad-BMP-2-TNF-siRNA, increases the size and number of titanium (Ti) particle-induced multinuclear osteoclasts, the expression of osteoclast genes, as well as the resorption area. There were no differences observed between Ti particle-induced and Ad-BMP-2-TNF-siRNA-induced osteoclast formation. Moreover, Ad-BMP-2-TNF-siRNA directly acted upon osteoclast precursors by increasing the level of c-Fos, regulating other signaling pathways, such as p38 phosphorylated c-Jun N-terminal kinase (p-JNK) and phosphorylated IκB (p‑IκB). Taken together, these data demonstrate that treatment with Ad-BMP-2-TNF-siRNA increases wear debris-induced osteoclast formation by activating c-Fos and that these effects are not associated with pro-inflammatory cytokines.

Osteoclasts: New Insights

Osteoclasts, the bone-resorbing cells, play a pivotal role in skeletal development and adult bone remodeling. They also participate in the pathogenesis of various bone disorders. Osteoclasts differentiate from cells of the monocyte/macrophage lineage upon stimulation of two essential factors, the monocyte/macrophage colony stimulating factor (M-CSF) and receptor activation of NF-κB ligand (RANKL). M-CSF binds to its receptor c-Fms to activate distinct signaling pathways to stimulate the proliferation and survival of osteoclast precursors and the mature cell. RANKL, however, is the primary osteoclast differentiation factor, and promotes osteoclast differentiation mainly through controlling gene expression by activating its receptor, RANK. Osteoclast function depends on polarization of the cell, induced by integrin αvβ3, to form the resorptive machinery characterized by the attachment to the bone matrix and the formation of the bone-apposed ruffled border. Recent studies have provided new insights into the mechanism of osteoclast differentiation and bone resorption. In particular, c-Fms and RANK signaling have been shown to regulate bone resorption by cross-talking with those activated by integrin αvβ3. This review discusses new advances in the understanding of the mechanisms of osteoclast differentiation and function.

Bone cell communication factors and Semaphorins

Bone tissue is continuously renewed throughout adult life by a process called 'remodeling', which involves a dynamic interplay among bone cells including osteoclasts, osteoblasts and osteocytes. For example, a tight coupling between bone resorption and formation is essential for the homeostasis of the skeletal system. Studies on the coupling mechanism in physiological and pathological settings have revealed that osteoclasts or osteoclastic bone resorption promote bone formation through the production of diverse coupling factors. The classical coupling factors are the molecules that promote bone formation after resorption, but there may be distinct mechanisms at work in various phases of bone remodeling. A recent study revealed that the Semaphorin 4D expressed by osteoclasts inhibits bone formation, which represents a mechanism by which coupling is dissociated. Furthermore, it has been demonstrated that osteoblastic expression of Semaphorin 3A exerts an osteoprotective effect by both suppressing bone resorption and increasing bone formation. Thus, recent advances have made it increasingly clear that bone remodeling is regulated by not only classical coupling factors, but also molecules that mediate cell-cell communication among bone cells. We propose that such factors be called bone cell communication factors, which control the delicate balance of the interaction of bone cells so as to maintain bone homeostasis.

Expression and characterization of c-Myb in prenatal odontogenesis

The transcription factor c-Myb is involved in the control of cell proliferation, survival and differentiation. As these processes accompany the morphogenesis of developing teeth, this work investigates the possible role of c-Myb during odontogenesis. Analysis of the expression of c-Myb in the monophyodont mouse was followed by similar analysis in a diphyodont species, the pig, which has a dentition more closely resembling that of the human. The distribution of c-Myb was correlated with the pattern of proliferation and apoptosis and the tooth phenotype of c-Myb mutant mice was also assessed. In the mouse, c-Myb expression was detected throughout prenatal development of the first molar tooth. Negative temporospatial correlation was found between c-Myb expression and apoptosis, while c-Myb expression positively correlated with proliferation. c-Myb-positive cells, however, were more abundant than the proliferating cell nuclear antigen positive cells, suggesting other roles of c-Myb in odontogenesis. In the minipig, in contrast to the mouse, there was an asymmetrical arrangement of c-Myb positive cells, with a higher presence on the labial side of the tooth germ and dental lamina. A cluster of negative cells was situated in the mesenchyme close to the tooth bud. At later stages, the number of positive cells decreased and these cells were situated in the upper part of the dental papilla in the areas of future cusp formation. The expression of c-Myb in both species was strong in the odontoblasts and ameloblasts at the stage of dentin and enamel production suggesting a possible novel role of c-Myb during tooth mineralization.

Effects of pulsed electromagnetic fields on the mRNA expression of CAII and RANK in ovariectomized rats

The present study was designed to determine the effects of pulsed electromagnetic fields (PEMFs) on the mRNA expression of the carbonic anhydrase II (CAII) and receptor activator of NF-κB (RANK) in ovariectomized rats. A total of 48 SD rats were randomly divided into four groups [Sham, OVX, PEMFs, and E(2) (premarin)], 12 rats in each group. Rats in the Sham group received sham ovariectomy, while rats in OVX, PEMFs, and E(2) groups received ovariectomy. Twelve weeks following the surgery, rats (whole body) in the PEMFs group were exposed to PEMFs for 30 days with 3.8 mT, 8 Hz, and 40 min per day; rats in the E(2) group were administered premarin (0.0625 mg/kg/d; intragastric administration 1-2 ml/100 g). Rats in the Sham and OVX groups housed in the same conditions. At the end of intervention, the level of serum estradiol of rats was measured. The gene expression of CAII and RANK in the left ilium of rats was determined with real-time fluorescent-nested quantitative polymerase chain reaction. Compared with the Sham group, the level of serum estradiol in the ovariectomized group was significantly decreased (P < 0.05); compared with the OVX group, CAIImRNA expression was significantly decreased in the PEMFs group and E group (P < 0.05, 0.01, respectively). Compared with the E group, RANKmRNA expression was significantly higher in the PEMFs group (P < 0.05); although RANKmRNA expression decreased in PEMFs group, no statistically significant difference was found between PEMF group and OVX group (P = 0.82). These data suggest that PEMFs could regulate the expression of CAIImRNA in ovariectomized rats.

Disorders of bone remodeling

The skeleton provides mechanical support for stature and locomotion, protects vital organs, and controls mineral homeostasis. A healthy skeleton must be maintained by constant bone modeling to carry out these crucial functions throughout life. Bone remodeling involves the removal of old or damaged bone by osteoclasts (bone resorption) and the subsequent replacement of new bone formed by osteoblasts (bone formation). Normal bone remodeling requires a tight coupling of bone resorption to bone formation to guarantee no alteration in bone mass or quality after each remodeling cycle. However, this important physiological process can be derailed by a variety of factors, including menopause-associated hormonal changes, age-related factors, changes in physical activity, drugs, and secondary diseases, which lead to the development of various bone disorders in both women and men. We review the major diseases of bone remodeling, emphasizing our current understanding of the underlying pathophysiological mechanisms.

Effects of pulsed electromagnetic fields on the mRNA expression of RANK and CAII in ovariectomized rat osteoclast-like cell

This study was designed to determine the effects of pulsed electromagnetic fields (PEMF) on the mRNA expression of the receptor activator of NF-kappa-B (RANK) and carbonic anhydrase II (CA II) in ovariectomized rat osteoclast-like cells. Marrow cells were harvested from femora and tibiae of rats, from which the ovaries had been totally excised, and cultured in 6-well chamber slides. After 1 day of incubation, the marrow cells were exposed to PEMF for 3 days with 3.8 mT, 8 Hz, and 40 min per day. Osteoclast-like cells were confirmed by both tartrate resistant acid phosphatase (TRAP) stain and bone resorption assay. The expression of RANK and CA II mRNA was determined with real-time fluorescent-nested quantitative polymerase chain reaction. Compared with the sham group, the level of serum estradiol in the ovariectomized group was significantly decreased ( p < 0.05). The numbers of multinucleated, TRAP-positive osteoclast-like cells and resorption pits formed were observed. In invitro study, the expression of RANK and CA II were measured in sham, ovariectomized without PEMF, and ovariectomized with PEMF treatment. Compared with the ovariectomized (PEMF) experimental group and sham group, CA II mRNA expression was significantly increased in the ovariectomized control group ( p < 0.05, 0.01, respectively). Compared with the sham group, RANK mRNA expression was significantly increased in the ovariectomized control group ( p < 0.05). These data suggest that PEMF could regulate the expression of RANK and CA II mRNA in the marrow culture system.

Eph receptors and ephrin signaling pathways: a role in bone homeostasis

The maintenance of bone homeostasis is tightly controlled, and largely dependent upon cellular communication between osteoclasts and osteoblasts, and the coupling of bone resorption to bone formation. This tight coupling is essential for the correct function and maintenance of the skeletal system, repairing microscopic skeletal damage and replacing aged bone. A range of pathologic diseases, including osteoporosis and cancer-induced bone disease, disrupt this coupling and cause subsequent alterations in bone homeostasis. Eph receptors and their associated ligands, ephrins, play critical roles in a number of cellular processes including immune regulation, neuronal development and cancer metastasis. Eph receptors are also expressed by cells found within the bone marrow microenvironment, including osteoclasts and osteoblasts, and there is increasing evidence to implicate this family of receptors in the control of normal and pathological bone remodeling.

Proteome analysis of matrix vesicles isolated from femurs of chicken embryo

Matrix vesicles (MVs) are extracellular organelles that initiate mineral formation, accumulating inorganic phosphate (P(i)) and calcium leading to the formation of hydroxyapatite (HA) crystals, the main mineral component of bones. MVs are produced during bone formation, as well as during the endochondral calcification of cartilage. MVs are released into the extracellular matrix from osseous cells such as osteoblasts and hypertrophic chondrocytes. In this report, using 1-D SDS-PAGE, in-gel tryptic digestion and an LC-MS-MS/MS protein identification protocol, we characterized the proteome of MVs isolated from chicken embryo (Gallus gallus) bones and cartilage. We identified 126 gene products, including proteins related to the extracellular matrix and ion transport, as well as enzymes, cytoskeletal, and regulatory proteins. Among the proteins recognized for the first time in MVs were aquaporin 1, annexin A1 (AnxA1), AnxA11, glycoprotein HT7, G(i) protein alpha2, and scavenger receptor type B. The pathways for targeting the identified proteins into MVs and their particular functions in the biomineralization process are discussed. Obtaining a knowledge of the functions and roles of these proteins during embryonic mineralization is a prerequisite for the overall understanding of the initial mineral formation mechanisms.

Osteoclast-osteoblast communication

Cells in osteoclast and osteoblast lineages communicate with each other through cell-cell contact, diffusible paracrine factors and cell-bone matrix interaction. Osteoclast-osteoblast communication occurs in a basic multicellular unit (BMU) at the initiation, transition and termination phases of bone remodeling. At the initiation phase, hematopoietic precursors are recruited to the BMU. These precursors express cell surface receptors including c-Fms, RANK and costimulatory molecules, such as osteoclast-associated receptor (OSCAR), and differentiate into osteoclasts following cell-cell contact with osteoblasts, which express ligands. Subsequently, the transition from bone resorption to formation is mediated by osteoclast-derived 'coupling factors', which direct the differentiation and activation of osteoblasts in resorbed lacunae to refill it with new bone. Bidirectional signaling generated by interaction between ephrinB2 on osteoclasts and EphB4 on osteoblast precursors facilitates the transition. Such interaction is likely to occur between osteoclasts and lining cells in the bone remodeling compartment (BRC). At the termination phase, bone remodeling is completed by osteoblastic bone formation and mineralization of bone matrix. Here, we describe molecular communication between osteoclasts and osteoblasts at distinct phases of bone remodeling.

Dynamic change in expression of LECT2 during liver regeneration after partial hepatectomy in mice

Leukocyte-cell-derived chemotaxin 2 (LECT2) was first isolated from the culture fluid of phytohemagglutinin-activated human T-cell leukemia SKW-3 cells and was found to be expressed in the human, bovine and murine livers. To further analyze the role of LECT2 in the liver, we investigated the expression of mouse LECT2 (mLECT2) during liver regeneration after partial hepatectomy (PHx) using immunohistochemical and in situ hybridization techniques. Mouse LECT2 protein and mRNA were detected in most hepatocytes in normal mouse; however, at 30 min after PHx, they were not detected in liver tissue. At 2 h after PHx, expression of mLECT2 protein was seen in hepatocytes surrounding the central vein, although mRNA expression levels were still low. At 6 h after PHx, a marked number of hepatocytes expressing mLECT2 protein and mRNA were seen throughout the liver, and at 12 h after PHx, hepatocytes expressing mLECT2 protein and mRNA further increased in number. However, expression levels of mLECT2 protein and mRNA at 24 h after PHx were significantly lower when compared with levels after 12 h. These results indicate that LECT2 triggers the early events of regeneration with concomitant suppression of hepatocyte proliferation.

Osteoclast-derived activity in the coupling of bone formation to resorption

The cells of bone and the immune system communicate by means of soluble and membrane-bound cytokines and growth factors. Through local signalling mechanisms, cells of the osteoblast lineage control the formation and activity of osteoclasts and, therefore, the resorption of bone. Both T and B lymphocytes produce activators and inhibitors of osteoclast formation. A local 'coupling factor' linking bone resorption to subsequent formation in the bone multicellular unit (BMU) has long been proposed as the key regulator of the bone remodelling process, but never identified. There is evidence in support of the view that the coupling mechanism is dependent on growth factors released from the bone matrix during resorption, or is generated from maturing osteoblasts. We argue that osteoclasts contribute in important ways to the transiently activated osteoclast, and stimulate osteoblast lineage cells to begin replacing the resorbed bone in each BMU.

Molecular cloning of leukocyte cell-derived chemotaxin 2 in rainbow trout

In humans, leukocyte cell-derived chemotaxin 2 (LECT2) is a 16kDa chemotactic protein that consists of 133 amino acids and three intramolecular disulphide bonds. Although it was originally demonstrated to have a chemotactic function in vitro, recent data sustain a further multifunctional role of LECT2 that extends from cell growth, differentiation, damage/repair process and carcinogenesis to autoimmune diseases. The in vivo function of LECT2 protein still remains obscure. In order to study the phylogeny of LECT2, a full-length cDNA clone of LECT2 gene, 720 bp in size, was isolated in rainbow trout (Oncorhynchus mykiss). Its deduced amino acid sequence of 156 residues, presents 40, 45 and 61% overall identity to human, mouse and carp LECT2 proteins, respectively. In contrast to mammalian LECT2 protein, trout LECT2 protein reveals two potential N-glycosylation sites. Phylogenetic analysis shows that trout LECT2 is clustered with the known homologous proteins. Trout LECT2 mRNA is predominately expressed in liver and spleen, showing lower expression in kidney, intestine, heart and brain.

Differential effects of calmodulin and protein kinase C antagonists on bone resorption and acid transport activity

Tamoxifen inhibits bone resorption by disrupting calmodulin-dependent processes. Since tamoxifen inhibits protein kinase C in other cells, we compared the effects of tamoxifen and the PKC inhibitor, bis indolylmaleimide II (bIM), on bone resorption and acid transport activity in isolated membrane vesicles. Bis indolylmaleimide inhibited bone resorption 50% with an IC50 approximately 3 microM, as well as acid transport activity in a concentration -dependent manner with an IC50 of approximately 0.4 IM. The IC50 of bIM for inhibiting acid transport activity was similar to that of calmodulin antagonists. The potassium ionophore, valinomycin, failed to restore bIM or tamoxifen-dependent inhibition of acid transport, suggesting that bIM and tamoxifen both inhibit H(+)-ATPase activity. Half maximal inhibitory concentrations of tamoxifen and bIM were not additive in acid transport assays, suggesting different sites of action. Furthermore, exogenous calmodulin blocked tamoxifen, but not bIM, -dependent inhibition of acid transport. We also compared the effects of tamoxifen and bIM on phosphorylation of proteins in isolated membrane fractions as determined by 32P incorporation and autoradiography. Tamoxifen had no effect on protein phosphorylation in contrast to bIM, which inhibited phosphorylation of eight proteins with different apparent kinetics. The data suggest that, while tamoxifen and bIM both affect H(+)-ATPase activity, the mechanisms of action are different.

Osteoblast-specific targeting of soluble colony-stimulating factor-1 increases cortical bone thickness in mice

The soluble and membrane-bound forms of CSF-1 are synthesized by osteoblasts and stromal cells in the bone microenvironment. Transgenic mice, generated to selectively express sCSF-1 in bone, showed increased cortical thickness in the femoral diaphysis caused by new bone formation along the endosteal surface. The ability of sCSF-1 to enhance bone cell activity in vivo is potentially relevant for increasing cortical bone in a variety of disorders.

INTRODUCTION

The soluble form of colony-stimulating factor-1 (sCSF-1) and the membrane-bound form of CSF-1 (mCSF-1) have been shown to support osteoclastogenesis in vitro; however, the effect of each peptide on bone remodeling in vivo is unclear. To determine the effect of sCSF-1, selectively expressed in bone, the skeletal phenotype of transgenic mice harboring the human sCSF-1 cDNA under the control of the osteocalcin promoter was assessed.

METHODS

At 5 and 14 weeks, mice were analyzed for CSF-1 protein levels, weighed, and X-rayed, and femurs were removed for peripheral quantitative computed tomography, histology, and histomorphometry.

RESULTS

High levels of human sCSF-1 were detected in bone extracts and, to a lesser extent, in plasma. Adult transgenic mice showed normal body weight and increased circulating monocytic cells. At 5 weeks, the femoral diaphysis was similar in CSF-1T and wt/wt littermates. However, by 14 weeks, the femoral diaphysis in CSF-1T mice showed increased cortical thickness and bone mineral density. In contrast to the diaphysis, the femoral metaphysis of CSF-1T mice showed normal cancellous bone comparable with wt/wt littermates at each time point. Histological sections demonstrated increased woven bone along the endosteal surface of the diaphysis and intracortical remodeling. Fluorochrome-labeling analysis confirmed endocortical bone formation in CSF-1T, with a 3.1-fold increase in the percentage of double-labeled surfaces and a 3.6-fold increase in the bone formation rate compared with wt/wt mice. Although remodeling resulted in a slightly porous cortex, sCSF-1 preferentially stimulated endocortical bone formation, leading to increased cortical thickness.

CONCLUSIONS

These findings indicate that sCSF-1 is a key determinant of bone cell activity in the corticoendosteal envelope.

Gingipain RgpB is excreted as a proenzyme in the vimA-defective mutant Porphyromonas gingivalis FLL92

We have previously shown that the unique vimA (virulence-modulating) gene could modulate proteolytic activity in Porphyromomas gingivalis. Although a reduction in cysteine protease activity was observed in the vimA-defective mutant, P. gingivalis FLL92, compared to that of the wild-type strain, no changes were seen in the expression of the gingipain genes. This result might suggest posttranscriptional regulation of protease expression. To determine whether there was a defect in the translation, transport, or maturation of the gingipains, P. gingivalis FLL92 was further characterized. In contrast to the wild-type strain, a 90% reduction was seen in both Rgp and Kgp protease activities in strain FLL92 during the exponential growth phase. These activities, however, increased to approximately 60% of that of the wild-type strain during stationary phase. Throughout all the growth phases, Rgp and Kgp activities were mostly soluble, in contrast to those of the wild-type strain. Western blot analyses identified unique Rgp- and Kgp-immunoreactive bands in extracellular protein fractions from FLL92 grown to late exponential phase. Also, the RgpB proenzyme was identified in this fraction by mass spectrometry. In addition, in vitro protease activity could be induced by a urea denaturation-renaturation cycle in this fraction. These results indicate that protease activity in P. gingivalis may be growth phase regulated, possibly by multiple mechanisms. Furthermore, the gingipain RgpB is excreted in an inactive form in the vimA mutant. In addition, these results provide the first evidence of posttranslational regulation of protease activity in P. gingivalis and may suggest an important role for the vimA gene in protease activation in this organism.

Molecular mechanisms underlying osteoclast formation and activation

Osteoporosis is one of the leading causes of morbidity in the elderly and is characterized by a progressive loss of total bone mass and bone density. Bone loss in osteoporosis is due to the persistent excess of osteoclastic bone resorption over osteoblastic bone formation. Receptor activator of NFkappaB ligand (RANKL) critically regulates both osteoclast differentiation and activation. TRAFs appear to be central coupling molecules in the signal transduction pathways that regulate osteoclastogenesis, cathepsin K is the major mediator of osteoclastic bone resorption, and sex steroids and aging also affect osteoclastogenesis and osteoclast activity. However, bone homeostasis depends upon the intimate coupling of bone formation and bone resorption, wherein both osteoclasts and osteoblasts exert vital stimulatory and inhibitory effects upon each other via molecules such as RANKL, TGFbeta, PDGF, BMP2, and Mim-1. This review will highlight some of the major features of the complex circuit of cytokines, growth factors, and hormones that underlies the formation and function of osteoclasts and the dynamic equilibrium that marks the interaction between osteoclasts and osteoblasts.

Expression, oxidative refolding, and characterization of six-histidine-tagged recombinant human LECT2, a 16-kDa chemotactic protein with three disulfide bonds

Human LECT2 is a 16-kDa chemotactic protein that consists of 133 amino acids and three intramolecular disulfide bonds. Here, we present the oxidative refolding of (His)(6)-LECT2, an N-terminally (His)(6)-tagged recombinant protein of human LECT2. (His)(6)-LECT2 was overproduced in Escherichia coli in the form of insoluble aggregates, solubilized with 8 M urea in the presence of 10 mM DTT, and purified and refolded on Ni-NTA agarose by lowering the urea concentration before the elution. This process, however, gave a mixture of oligomers of (His)(6)-LECT2 as well as the monomer, whose composition was as low as 36%. The oligomers formed as a result of incorrect intermolecular disulfide bonds. After the refolding on Ni-NTA agarose (step 1), the disulfide bonds were shuffled using a glutathione redox buffer (step 2) and the remaining thiols were completely oxidized (step 3) to improve the yield of correctly folded, monomeric (His)(6)-LECT2. The monomer composition was significantly improved to 81% by the three-step refolding method and the monomer thus obtained was shown to have the same conformation as the authentic LECT2 produced in CHO cells by CD and NMR spectroscopies. The yield of (His)(6)-LECT2 was 1.0 mg/L E. coli culture and was 16 times as high as that in our previous report, in which (His)(6)-LECT2 was purified from the soluble fractions of E. coli cell lysates.

Myb induced myeloid protein 1 (Mim-1) is an acetyltransferase

We have screened protein extracts from chicken blood cells for acetyltransferases. An in gel acetyltransferase assay revealed that a 32 kDa protein, which is more prevalent in whole blood when compared with erythrocyte cells, possessed an auto-acetylation activity. This protein was purified by a series of chromatographic steps, sequenced by Edman degradation and subsequently identified as Myb induced myeloid protein (Mim-1). Mim-1 has similarities to the conserved acetyltransferase motifs found in the GNAT superfamily of proteins and also contains three minimal GK acetylation motifs. These data identify Mim-1 as an acetyltransferase.

Transcription of chromosomal rRNA genes by both RNA polymerase I and II in yeast uaf30 mutants lacking the 30 kDa subunit of transcription factor UAF

UAF, a yeast RNA polymerase I transcription factor, contains Rrn5p, Rrn9p, Rrn10p, histones H3 and H4, and uncharacterized protein p30. Mutants defective in RRN5, RRN9 or RRN10 are unable to transcribe rDNA by polymerase I and grow extremely slowly, but give rise to variants able to grow by transcribing chromosomal rDNA by polymerase II. Thus, UAF functions as both an activator of polymerase I and a silencer of polymerase II for rDNA transcription. We have now identified the gene for subunit p30. This gene, UAF30, is not essential for growth, but its deletion decreases the cellular growth rate. Remarkably, the deletion mutants use both polymerase I and II for rDNA transcription, indicating that the silencer function of UAF is impaired, even though rDNA transcription by polymerase I is still occurring. A UAF complex isolated from the uaf30 deletion mutant was found to retain the in vitro polymerase I activator function to a large extent. Thus, Uaf30p plays only a minor role in its activator function. Possible reasons for slow growth caused by uaf30 mutations are discussed.