MEF2C regulates osteoclastogenesis and pathologic bone resorption via c-FOS

Osteoporosis is a metabolic bone disease with dysregulated coupling between bone resorption and bone formation,which results in decreased bone mineral density.The MEF2C locus,which encodes the transcription factor MADS box transcription enhancer factor 2,polypeptide C(MEF2C),is strongly associated with adult osteoporosis and osteoporotic fractures.Although the role of MEF2C in bone and cartilage formation by osteoblasts,osteocytes,and chondrocytes has been studied,the role of MEF2C in osteoclasts,which mediate bone resorption,remains unclear.In this study,we identified MEF2C as a positive regulator of human and mouse osteoclast differentiation.While decreased MEF2C expression resulted in diminished osteoclastogenesis,ectopic expression of MEF2C enhanced osteoclast generation.Using transcriptomic and bioinformatic approaches,we found that MEF2C promotes the RANKL-mediated induction of the transcription factors c-FOS and NFATc1,which play a key role in osteoclastogenesis.Mechanistically,MEF2C binds to FOS regulatory regions to induce c-FOS expression,leading to the activation of NFATC1 and downstream osteoclastogenesis.Inducible deletion of Mef2c in mice resulted in increased bone mass under physiological conditions and protected mice from bone erosion by diminishing osteoclast formation in K/BxN serum induced arthritis,a murine model of inflammatory arthritis.Our findings reveal direct regulation of osteoclasts by MEF2C,thus adding osteoclasts as a cell type in which altered MEF2C expression or function can contribute to pathological bone remodeling.

Augmenting MNK1/2 activation by c-FMS proteolysis promotes osteoclastogenesis and arthritic bone erosion

Osteoclasts are bone-resorbing cells that play an essential role in homeostatic bone remodeling and pathological bone erosion.Macrophage colony stimulating factor(M-CSF)is abundant in rheumatoid arthritis(RA).However,the role of M-CSF in arthritic bone erosion is not completely understood.Here,we show that M-CSF can promote osteoclastogenesis by triggering the proteolysis of c-FMS,a receptor for M-CSF,leading to the generation of FMS intracellular domain(FICD)fragments.Increased levels of FICD fragments positively regulated osteoclastogenesis but had no effect on inflammatory responses.Moreover,myeloid cell-specific FICD expression in mice resulted in significantly increased osteoclast-mediated bone resorption in an inflammatory arthritis model.The FICD formed a complex with DAP5,and the FICD/DAP5 axis promoted osteoclast differentiation by activating the MNK1/2/EIF4E pathway and enhancing NFATcl protein expression.Moreover,targeting the MNK1/2 pathway diminished arthritic bone erosion.These results identified a novel role of c-FMS proteolysis in osteoclastogenesis and the pathogenesis of arthritic bone erosion.

SREBP2 restricts osteoclast differentiation and activity by regulating IRF7 and limits inflammatory bone erosion

Osteoclasts are multinucleated bone-resorbing cells,and their formation is tightly regulated to prevent excessive bone loss.However,the mechanisms by which osteoclast formation is restricted remain incompletely determined.Here,we found that sterol regulatory element binding protein 2(SREBP2)functions as a negative regulator of osteoclast formation and inflammatory bone loss.Cholesterols and SREBP2,a key transcription factor for cholesterol biosynthesis,increased in the late phase of osteoclastogenesis.

Osteoclasts are not a source of SLIT3

The axon guidance cue SLIT3 was identified as an osteoanabolic agent in two recent reports. However, these reports conflict in their nomination of osteoblasts versus osteoclasts as the key producers of skeletal SLIT3 and additionally offer conflicting data on the effects of SLIT3 on osteoclastogenesis. Here, aiming to address this discrepancy, we found no observable SLIT3 expression during human or mouse osteoclastogenesis and the only modest SLIT3-mediated effects on osteoclast differentiation. Conditional deletion of SLIT3 in cathepsin K(CTSK)-positive cells, including osteoclasts, had no effect on the number of osteoclast progenitors, in vitro osteoclast differentiation, overall bone mass, or bone resorption/formation parameters. Similar results were observed with the deletion of SLIT3 in Lys M-positive cells, including osteoclast lineage cells. Consistent with this finding, bone marrow chimeras made from Slit3-/-donors that lacked SLIT3 expression at all stages of osteoclast development displayed normal bone mass relative to controls. Taken in context, multiple lines of evidence were unable to identify the physiologic function of osteoclast-derived SLIT3,indicating that osteoblasts are the major source of skeletal SLIT3.

RANKL-responsive epigenetic mechanism reprograms macrophages into bone-resorbing osteoclasts

Monocyte/macrophage lineage cells are highly plastic and can differentiate into various cells under different environmental stimuli. Bone-resorbing osteoclasts are derived from the monocyte/macrophage lineage in response to receptor activator of NF-κB ligand (RANKL). However, the epigenetic signature contributing to the fate commitment of monocyte/macrophage lineage differentiation into human osteoclasts is largely unknown. In this study, we identified RANKL-responsive human osteoclast-specific superenhancers (SEs) and SE-associated enhancer RNAs (SE-eRNAs) by integrating data obtained from ChIP-seq, ATAC-seq, nuclear RNA-seq and PRO-seq analyses. RANKL induced the formation of 200 SEs, which are large clusters of enhancers, while suppressing 148 SEs in macrophages. RANKL-responsive SEs were strongly correlated with genes in the osteoclastogenic program and were selectively increased in human osteoclasts but marginally presented in osteoblasts, CD4+ T cells, and CD34+ cells. In addition to the major transcription factors identified in osteoclasts, we found that BATF binding motifs were highly enriched in RANKL-responsive SEs. The depletion of BATF1/3 inhibited RANKL-induced osteoclast differentiation. Furthermore, we found increased chromatin accessibility in SE regions, where RNA polymerase II was significantly recruited to induce the extragenic transcription of SE-eRNAs, in human osteoclasts. Knocking down SE-eRNAs in the vicinity of the NFATc1 gene diminished the expression of NFATc1, a major regulator of osteoclasts, and osteoclast differentiation. Inhibiting BET proteins suppressed the formation of some RANKL-responsive SEs and NFATc1-associated SEs, and the expression of SE-eRNA:NFATc1. Moreover, SE-eRNA:NFATc1 was highly expressed in the synovial macrophages of rheumatoid arthritis patients exhibiting high-osteoclastogenic potential. Our genome-wide analysis revealed RANKL-inducible SEs and SE-eRNAs as osteoclast-specific signatures, which may contribute to the development of osteoclast-specific therapeutic interventions.

RBP-J is required for M2 macrophage polarization in response to chitin and mediates expression of a subset of M2 genes

Development of alternatively activated （M2） macrophage phenotypes is a complex process that is coordinately regulated by a plethora of pathways and factors. Here, we report that RBP-J, a DNA-binding protein that integrates signals from multiple pathways including the Notch pathway, is critically involved in polarization of M2 macrophages. Mice deficient in RBP-J in the myeloid compartment exhibited impaired M2 phenotypes in vivo in a chitin-induced model of M2 polarization. Consistent with the in vivo findings, M2 polarization was partially compromised in vitro in Rbpj-deficient macrophages as demonstrated by reduced expression of a subset of M2 effector molecules including arginase 1. Functionally, myeloid Rbpj deficiency impaired M2 effector functions including recruitment of eosinophils and suppression of T cell proliferation. Collectively, we have identified RBP- Jas an essential regulator of differentiation and function of alternatively activated macrophages.

Engagement of TLR and Dectin-1/Syk Signaling Is Required for Activation of Notch Targets in Dendritic Cells

Pattern-recognition receptors,such as toll-like receptors(TLRs),detect a wide range of microbial products and initiate innate immune responses leading to the production of inflammatory mediators.In addition,TLR signaling also activates expression of Notch target genes that play crucial roles in suppression of TLR-triggered inflammatory responses.However,whether TLR signaling pathways engaged by other classes of pattern-recognition receptors induce expression of Notch target genes remains unclear.Here we demonstrate that zymosan,a stimulus for TLR2 and dectin-1,strongly induces expression of multiple Notch target genes in both human and murine dendritic cells.Mechanistically,induction of Notch targets by zymosan is both TLR2-and Syk-dependent through activation of mitogen-activated protein kinases and the transcription factor c-Fos.Hence,our data reveals a novel mechanism that efficient induction of Notch target genes requires engagement of TLR and dectin-1/Syk signaling pathways.

Three paralogous clusters of the miR-17-92 family of microRNAs restrain IL-12-mediated immune defense

MicroRNAs(miRNAs)have been widely implicated in immune regulation,but evidence for the coordinated function of paralogous miRNA clusters remains scarce.Here,by using genetically modified mice with individual or combined cluster deficiencies,we found that three paralogous clusters of the miR-17-92 family of miRNAs collectively suppressed IL-12 production in macrophages.Accordingly,miR-17-92 family miRNAs deficiencies resulted in heightened production of IL-12 and thus enhanced the host defense against intracellular pathogen Listeria monocytogenes in vivo.Mechanistically,different members of the miR-17-92 family of miRNAs acted on a common target,PTEN,to inhibit IL-12 expression by modulating the PI3K-Akt-GSK3 pathway.In addition,the expression of miR-17-92 family miRNAs was collectively inhibited by the transcription factor RBP-J,and RBP-J-associated macrophage functional defects were genetically rescued by deleting three clusters of miR-17-92 family miRNAs on a RBP-J null background.Thus,our results illustrated key roles of three clusters of miR-17-92 family miRNAs in cooperatively controlling IL-12-mediated immune responses and identified miR-17-92 family miRNAs as functional targets of RBP-J in macrophages.