Haptoglobin Acts as a TLR4 Ligand to Suppress Osteoclastogenesis via the TLR4-IFN-ß Axis.

Haptoglobin (Hp), a type of acute-phase protein, is known to have a systemic anti-inflammatory function and to modulate inflammation by directly affecting immune cells, such as T cells, dendritic cells, and macrophages. However, the effects of Hp on osteoclast differentiation are not well studied, even though osteoclast precursor cells belong to a macrophage-monocyte lineage. In this study, we found that the bone volume was reduced, and the number of osteoclasts was increased in Hp-deficient mice compared with wild-type mice. Moreover, our in vitro studies showed that Hp inhibits osteoclastogenesis by reducing the protein level of c-Fos at the early phase of osteoclast differentiation. We revealed that Hp-induced suppression of c-Fos was mediated by increased IFN-ß levels. Furthermore, Hp stimulated IFN-ß via a TLR4-dependent mechanism. These results demonstrate that Hp plays a protective role against excessive osteoclastogenesis via the Hp-TLR4-IFN-ß axis.

Recessive Mutations in ACPT, Encoding Testicular Acid Phosphatase, Cause Hypoplastic Amelogenesis Imperfecta.

Amelogenesis imperfecta (AI) is a heterogeneous group of genetic disorders affecting tooth enamel. The affected enamel can be hypoplastic and/or hypomineralized. In this study, we identified ACPT (testicular acid phosphatase) biallelic mutations causing non-syndromic, generalized hypoplastic autosomal-recessive amelogenesis imperfecta (AI) in individuals from six apparently unrelated Turkish families. Families 1, 4, and 5 were affected by the homozygous ACPT mutation c.713C>T (p.Ser238Leu), family 2 by the homozygous ACPT mutation c.331C>T (p.Arg111Cys), family 3 by the homozygous ACPT mutation c.226C>T (p.Arg76Cys), and family 6 by the compound heterozygous ACPT mutations c.382G>C (p.Ala128Pro) and 397G>A (p.Glu133Lys). Analysis of the ACPT crystal structure suggests that these mutations damaged the activity of ACPT by altering the sizes and charges of key amino acid side chains, limiting accessibility of the catalytic core, and interfering with homodimerization. Immunohistochemical analysis confirmed localization of ACPT in secretory-stage ameloblasts. The study results provide evidence for the crucial function of ACPT during amelogenesis.

Mitofusin 2, a mitochondria-ER tethering protein, facilitates osteoclastogenesis by regulating the calcium-calcineurin-NFATc1 axis.

Mitofusin 2 (Mfn2) is a mitochondrial outer membrane protein that participates in tethering mitochondria to the ER. Mitochondria-ER tethering has been demonstrated to play important roles in many cellular activities by regulating homeostasis of metabolites and calcium. Intracellular calcium signaling is crucial for the differentiation of osteoclasts, the bone-resorbing cells. In this study, we investigated whether Mfn2 plays a role in osteoclastogenesis by receptor activator of nuclear factor kappa B (RANKL) in primary cells. We found that RANKL increased Mfn2 expression during osteoclast formation from mouse bone marrow-derived macrophages (BMMs). When Mfn2 expression was suppressed in BMMs by using a siRNA-mediated gene knock-down system, osteoclast differentiation and activity of mature osteoclasts were reduced. Mfn2 knock-down also decreased the RANKL-mediated induction of NFATc1, the key transcription factor for osteoclast gene expression, without affecting c-Fos level. This effect on NFATc1 was associated with decreased calcium oscillation and calcineurin activity in Mfn2-deficient osteoclasts. Taken together, our results indicate that Mfn2 positively contributes to RANKL-induced osteoclast differentiation by regulating the calcium-calcieurin-NFATc1 axis, raising the importance of a previously under-recognized role of mitochondria in osteoclastogenesis.

Candidate gene sequencing reveals mutations causing hypoplastic amelogenesis imperfecta.

OBJECTIVE: Amelogenesis imperfecta (AI) is a rare hereditary disorder affecting the quality and quantity of the tooth enamel. The purpose of this study was to identify the genetic etiology of hypoplastic AI families based on the candidate gene approach. MATERIALS AND METHODS: We recruited three Turkish families with hypoplastic AI and performed a candidate gene screening based on the characteristic clinical feature to find the pathogenic genetic etiology. RESULTS: The candidate gene sequencing of the LAMB3 gene for family 1 revealed a heterozygous nonsense mutation in the last exon [c.3431C > A, p.(Ser1144*)]. FAM20A gene sequencing for families 2 and 3 identified a homozygous deletion [c.34_35delCT, p.(Leu12Alafs*67)] and a homozygous deletion-insertion (c.1109 + 3_1109 + 7delinsTGGTC) mutation, respectively. CONCLUSION: The candidate gene approach can be successfully used to identify the genetic etiology of the AI in some cases with characteristic clinical features. CLINICAL RELEVANCE: Identification of the genetic etiology of the AI will help both the family members and dentist understand the nature of the disorder. Characteristic clinical feature can suggest possible genetic causes.

Caveolin-1 regulates osteoclastogenesis and bone metabolism in a sex-dependent manner.

Lipid raft microdomains have important roles in various cellular responses. Caveolae are a specialized type of lipid rafts that are stabilized by oligomers of caveolin proteins. Here, we show that caveolin-1 (Cav-1) plays a crucial role in the regulation of osteoclastogenesis. We found that caveolin-1 was dramatically up-regulated by receptor activator of nuclear factor κB ligand (RANKL), the osteoclast differentiation factor. Knockdown of Cav-1 reduced osteoclastogenesis and induction of NFATc1, the master transcription factor for osteoclastogenesis, by RANKL. Consistent with the in vitro results, injection of caveolin-1 siRNA onto mice calvariae showed reduction in RANKL-induced bone resorption and osteoclast formation. Moreover, Cav-1(-/-) female mice had higher bone volume and lower osteoclast number compared with wild type mice. However, Cav-1(-/-) male mice had both osteoclast and osteoblast numbers higher than wild type mice with no difference in bone volume. The sex dependence in the effect of Cav-1 deficiency was partly attributed to decreased receptor activator of nuclear factor κB and increased cFms expression in osteoclast precursors of female and male mice, respectively. Taken together, these data demonstrate that Cav-1 has a complicated but critical role for osteoclastogenesis.

Tetraspanin 7 regulates sealing zone formation and the bone-resorbing activity of osteoclasts.

Tetraspanin family proteins regulate morphology, motility, fusion, and signaling in various cell types. We investigated the role of the tetraspanin 7 (Tspan7) isoform in the differentiation and function of osteoclasts. Tspan7 was up-regulated during osteoclastogenesis. When Tspan7 expression was reduced in primary precursor cells by siRNA-mediated gene knock-down, the generation of multinuclear osteoclasts was not affected. However, a striking cytoskeletal abnormality was observed: the formation of the podosome belt structure was inhibited and the microtubular network were disrupted by Tspan7 knock-down. Decreases in acetylated microtubules and levels of phosphorylated Src and Pyk2 in Tspan7 knock-down cells supported the involvement of Tspan7 in cytoskeletal rearrangement signaling in osteoclasts. This cytoskeletal defect interfered with sealing zone formation and subsequently the bone-resorbing activity of mature osteoclasts on dentin surfaces. Our results suggest that Tspan7 plays an important role in cytoskeletal organization required for the bone-resorbing function of osteoclasts by regulating signaling to Src, Pyk2, and microtubules.

A single recurrent mutation in the 5'-UTR of IFITM5 causes osteogenesis imperfecta type V.

Osteogenesis imperfecta (OI) is a heterogenous group of genetic disorders of bone fragility. OI type V is an autosomal-dominant disease characterized by calcification of the forearm interosseous membrane, radial head dislocation, a subphyseal metaphyseal radiodense line, and hyperplastic callus formation; the causative mutation involved in this disease has not been discovered yet. Using linkage analysis in a four-generation family and whole-exome sequencing, we identified a heterozygous mutation of c.-14C>T in the 5'-untranslated region of a gene encoding interferon-induced transmembrane protein 5 (IFITM5). It completely cosegregated with the disease in three families and occurred de novo in five simplex individuals. Transfection of wild-type and mutant IFITM5 constructs revealed that the mutation added five amino acids (Met-Ala-Leu-Glu-Pro) to the N terminus of IFITM5. Given that IFITM5 expression and protein localization is restricted to the skeletal tissue and IFITM5 involvement in bone formation, we conclude that this recurrent mutation would have a specific effect on IFITM5 function and thus cause OI type V.

NFI-C regulates osteoblast differentiation via control of osterix expression.

In bone marrow, bone marrow stromal cells (BMSCs) have the capacity to differentiate into osteoblasts and adipocytes. Age-related osteoporosis is associated with a reciprocal decrease of osteogenesis and an increase of adipogenesis in bone marrow. In this study, we demonstrate that disruption of nuclear factor I-C (NFI-C) impairs osteoblast differentiation and bone formation, and increases bone marrow adipocytes. Interestingly, NFI-C controls postnatal bone formation but does not influence prenatal bone development. We also found decreased NFI-C expression in osteogenic cells from human osteoporotic patients. Notably, transplantation of Nfic-overexpressing BMSCs stimulates osteoblast differentiation and new bone formation, but inhibits adipocyte differentiation by suppressing peroxisome proliferator-activated receptor gamma expression in Nfic(-/-) mice showing an age-related osteoporosis-like phenotype. Finally, NFI-C directly regulates Osterix expression but acts downstream of the bone morphogenetic protein-2-Runx2 pathway. These results suggest that NFI-C acts as a transcriptional switch in cell fate determination between osteoblast and adipocyte differentiation in BMSCs. Therefore, regulation of NFI-C expression in BMSCs could be a novel therapeutic approach for treating age-related osteoporosis.

5-Lipoxygenase mediates RANKL-induced osteoclast formation via the cysteinyl leukotriene receptor 1.

5-Lipoxygenase (5-LO) catalyzes the formation of two major groups of leukotrienes, leukotriene B4 and cysteinyl leukotrienes (CysLTs), and it has been implicated as a promising drug target to treat various inflammatory diseases. However, its role in osteoclastogenesis has not been investigated. In this study, we used mouse bone marrow-derived macrophages (BMMs) to show that 5-LO inhibitor suppresses RANKL-induced osteoclast formation. Inhibition of 5-LO was associated with impaired activation of multiple signaling events downstream of RANK, including ERK and p38 phosphorylation, and IκB degradation, followed by a decrease in NFATc1 expression. Ectopic overexpression of a constitutively active form of NFATc1 partly rescued the antiosteoclastogenic effect of 5-LO inhibitor. The knockdown of 5-LO in BMMs also resulted in a significant reduction in RANKL-induced osteoclast formation, accompanied by decreased expression of NFATc1. Similar effects were shown with CysLT receptor (CysLTR)1/2 antagonist and small RNA for CysLTR1 in BMMs, indicating the involvement of CysLT and CysLTR1 in 5-LO-mediated osteoclastogenesis. Finally, 5-LO inhibitor suppressed LPS-induced osteoclast formation and bone loss in the in vivo mouse experiments, suggesting a potential therapeutic strategy for treating diseases involving bone destruction. Taken together, the results of this study demonstrate that 5-LO is a key mediator of RANKL-induced osteoclast formation and possibly a novel therapeutic target for bone-resorption diseases.