SLAT negatively regulates RANKL-induced osteoclast differentiation

Joint Association Analysis Identified 18 New Loci for Bone Mineral Density

Bone mineral density (BMD) at various skeletal sites have shared genetic determinants. In the present study, aiming to identify shared loci associated with BMD, we conducted a joint association study of a genomewide association study (GWAS) and a meta-analysis of BMD at different skeletal sites: (i) a single GWAS of heel BMD in 142,487 individuals from the UK Biobank, and (ii) a meta-analysis of 30 GWASs of total body (TB) BMD in 66,628 individuals from the Genetic Factors for Osteoporosis (GEFOS) Consortium. The genetic correlation coefficient of the two traits was estimated to be 0.57. We performed joint association analysis with a recently developed statistical method multi-trait analysis of GWAS (MTAG) to account for trait heterogeneity and sample overlap. The joint association analysis combining samples of up to 209,115 individuals identified 18 novel loci associated with BMD at the genomewide significance level (α = 5.0 × 10^-8 ), explaining an additional 0.43% and 0.60% of heel-BMD and TB-BMD heritability, respectively. The vast majority of the identified lead SNPs or their proxies exerted local expression quantitative trait loci (cis-eQTL) activity. Credible risk variants, defined as those SNPs located within 500 kilobases (kb) of the lead SNP and with p values within two orders of magnitude of the lead SNP, were enriched in transcription factor binding sites (p = 3.58 × 10^-4 ) and coding regions (p = 5.71 × 10^-4 ). Fifty-six candidate genes were prioritized at these novel loci using multiple sources of information, including several genes being previously reported to play a role in bone biology but not reported in previous GWASs (PPARG, FBN2, DEF6, TNFRSF19, and NFE2L1). One newly identified gene, SCMH1, was shown to upregulate the expression of several bone biomarkers, including alkaline phosphatase (ALP), collagen type 1 (COL-I), osteocalcin (OCN), osteopontin (OPN), and runt-related transcription factor 2 (RUNX2), in mouse osteoblastic MC3T3-E1 cells, highlighting its regulatory role in bone formation. Our results may provide useful candidate genes for future functional investigations. © 2019 American Society for Bone and Mineral Research.

Association of Genetic Variants of RANK, RANKL, and OPG with Ankylosing Spondylitis Clinical Features in Taiwanese

Ankylosing spondylitis (AS) is a chronic inflammatory disease that leads to spinal ankylosis. The receptor activator of the nuclear factor-kappa (RANK), RANK ligand, and osteoprotegerin (OPG) (RANK/RANKL/OPG) pathway plays critical roles in bone metabolism and the immune system. The current study was aimed at investigating whether six single-nucleotide polymorphisms (SNPs) within the RANK, RANKL, and OPG genes essential for bone homeostasis are associated with AS. Genotype distributions, allele and haplotype frequencies, were compared between 1120 AS patients and 1435 healthy controls and among AS patients with stratification by syndesmophyte formation, onset age, and HLA-B27 positivity. We found that RANKL SNPs were associated with AS syndesmophyte formation. Notably, the RANKL SNP haplotype rs7984870C/rs9533155G/rs9525641C was negatively associated with AS susceptibility and appeared to protect against syndesmophyte formation in AS. Functionally, RANKL promoter SNPs (rs9525641 C/T and rs9533155 G/C) affected DNA-protein complex formation and promoter activity in promoter reporter analyses. The OPG SNP haplotype rs2073618G/rs3102735T was significantly associated with HLA-B27 negativity in AS patients. Furthermore, AS patients with syndesmophyte formation had significantly lower levels of soluble RANKL levels than those without syndesmophyte formation. Our data suggested a role for RANKL in AS susceptibility and severity.

MiR-30a attenuates osteoclastogenesis via targeting DC-STAMP-c-Fos-NFATc1 signaling

Osteoclast is a kind of unique cells which is responsible for bone matrix absorption. It was widely reported that microRNAs (miRNAs) could regulate several physiological processes, including osteoclastogenesis. In our study, microarray analysis showed that miR-30a was down-regulated during osteoclastogenesis after RANKL (receptor activator of nuclear factor κB ligand) stimulation. Osteoclasts and actin-ring formation as well as bone resorption were inhibited when miR-30a was overexpressed in osteoclast precursor cells. Meantime, when miR-30a was inhibited in osteoclast precursor cells, osteoclasts and actin-ring formation as well as bone resorption were promoted. Furthermore, we discovered that miR-30a overexpression inhibited the protein levels of DC-STAMP, c-Fos and NFATc1. However, when DC-STAMP was inhibited by using a DC-STAMP siRNA, we could not detect the inhibition effect of osteoclastogenesis and bone resorption induced by miR-30a. In conclusion, miR-30a attenuated osteoclastogenesis via suppression of DC-STAMP-c-Fos-NFATc1 signaling pathway. On these grounds, this study may reveal a new promising target for the treatment of osteoporosis and other osteopenic disorders.

Identification of a Novel Alternatively Spliced Form of Inflammatory Regulator SWAP-70-Like Adapter of T Cells

Activation of naive CD4⁺ T cells results in the development of several distinct subsets of effector Th cells, including Th2 cells that play a pivotal role in allergic inflammation and helminthic infections. SWAP-70-like adapter of T cells (SLAT), also known as Def6 or IBP, is a guanine nucleotide exchange factor for small GTPases, which regulates CD4⁺ T cell inflammatory responses by controlling Ca²⁺/NFAT signaling. In this study, we have identified a novel alternatively spliced isoform of SLAT, named SLAT2, which lacks the region encoded by exons 2-7 of the Def6 gene. SLAT2 was selectively expressed in differentiated Th2 cells after the second round of in vitro stimulation, but not in differentiated Th1, Th17, or regulatory T (Treg) cells. Functional assays revealed that SLAT2 shared with SLAT the ability to enhance T cell receptor- (TCR-) mediated activation of NFAT and production of IL-4 but was unable to enhance TCR-induced adhesion to ICAM-1. Ectopic expression of SLAT2 or SLAT in Jurkat T cells resulted in the expression of distinct forms of filopodia, namely, short versus long ones, respectively. These results demonstrate that modulating either SLAT2 or SLAT protein expression could play critical roles in cytokine production and actin reorganization during inflammatory immune responses.

Def6 Restrains Osteoclastogenesis and Inflammatory Bone Resorption

Inflammatory bone resorption mediated by osteoclasts is a major cause of morbidity and disability in many inflammatory disorders, including rheumatoid arthritis (RA). The mechanisms that regulate osteoclastogenesis and bone resorption in inflammatory settings are complex and have not been well elucidated. In this study, we identify the immunoregulator differentially expressed in FDCP 6 homolog (Def6) as a novel inhibitor of osteoclastogenesis in physiological and inflammatory conditions. Def6 deficiency in Def6^-/- mice enhanced the sensitivity of osteoclast precursors to the physiological osteoclastogenic inducer receptor activator for NF-κB ligand, and Def6^-/- osteoclasts formed actin rings. Furthermore, Def6 deficiency markedly increased TNF-α-induced osteoclastogenesis in vitro and in vivo and enhanced bone resorption in an inflammatory osteolysis mouse model. TNF-α serum levels correlated negatively with Def6 expression levels in osteoclast precursors obtained from RA patients, and the osteoclastogenic capacity of the osteoclast precursors was significantly inversely correlated with their Def6 expression levels, indicating that Def6 functions as an inhibitor of excessive osteoclast formation and bone destruction in RA. Mechanistically, Def6 suppressed osteoclastogenesis and the expression of key osteoclastogenic factors NFATc1, B lymphocyte-induced maturation protein-1, and c-Fos by regulating an endogenous IFN-β-mediated autocrine feedback loop. The Def6-dependent pathway may represent a novel therapeutic target to prevent pathological bone destruction.

Production and Functional Characterization of Murine Osteoclasts Differentiated from ER-Hoxb8-Immortalized Myeloid Progenitor Cells

In vitro differentiation into functional osteoclasts is routinely achieved by incubation of embryonic stem cells, induced pluripotent stem cells, or primary as well as cryopreserved spleen and bone marrow-derived cells with soluble receptor activator of nuclear factor kappa-B ligand and macrophage colony-stimulating factor. Additionally, osteoclasts can be derived from co-cultures with osteoblasts or by direct administration of soluble receptor activator of nuclear factor kappa-B ligand to RAW 264.7 macrophage lineage cells. However, despite their benefits for osteoclast-associated research, these different methods have several drawbacks with respect to differentiation yields, time and animal consumption, storage life of progenitor cells or the limited potential for genetic manipulation of osteoclast precursors. In the present study, we therefore established a novel protocol for the differentiation of osteoclasts from murine ER-Hoxb8-immortalized myeloid stem cells. We isolated and immortalized bone marrow cells from wild type and genetically manipulated mouse lines, optimized protocols for osteoclast differentiation and compared these cells to osteoclasts derived from conventional sources. In vitro generated ER-Hoxb8 osteoclasts displayed typical osteoclast characteristics such as multi-nucleation, tartrate-resistant acid phosphatase staining of supernatants and cells, F-actin ring formation and bone resorption activity. Furthermore, the osteoclast differentiation time course was traced on a gene expression level. Increased expression of osteoclast-specific genes and decreased expression of stem cell marker genes during differentiation of osteoclasts from ER-Hoxb8-immortalized myeloid progenitor cells were detected by gene array and confirmed by semi-quantitative and quantitative RT-PCR approaches. In summary, we established a novel method for the quantitative production of murine bona fide osteoclasts from ER-Hoxb8 stem cells generated from wild type or genetically manipulated mouse lines. These cells represent a standardized and theoretically unlimited source for osteoclast-associated research projects.

MicroRNA-26a regulates RANKL-induced osteoclast formation

Osteoclasts are unique cells responsible for the resorption of bone matrix. MicroRNAs (miRNAs) are involved in the regulation of a wide range of physiological processes. Here, we examined the role of miR-26a in RANKL-induced osteoclastogenesis. The expression of miR-26a was up-regulated by RANKL at the late stage of osteoclastogenesis. Ectopic expression of an miR-26a mimic in osteoclast precursor cells attenuated osteoclast formation, actin-ring formation, and bone resorption by suppressing the expression of connective tissue growth factor/CCN family 2 (CTGF/CCN2), which can promote osteoclast formation via up-regulation of dendritic cell-specific transmembrane protein (DC-STAMP). On the other hand, overexpression of miR-26a inhibitor enhanced RANKL-induced osteoclast formation and function as well as CTGF expression. In addition, the inhibitory effect of miR-26a on osteoclast formation and function was prevented by treatment with recombinant CTGF. Collectively, our results suggest that miR-26a modulates osteoclast formation and function through the regulation of CTGF.