In vivo studies have shown T cells to be central to the mechanism by which estrogen deficiency induces bone loss, but the mechanism involved remains, in part, undefined. In vitro, T cells from ovariectomized mice produce increased amounts of tumor necrosis factor (TNF), which augments receptor activator of NF-B ligand (RANKL)-induced osteoclastogenesis. However, both the mechanism and the relevance of this phenomenon in vivo remain to be established. In this study, we found that ovariectomy increased the number of bone marrow T cell-producing TNF without altering production of TNF per T cell. Attesting to the essential contribution of TNF, ovariectomy induced rapid bone loss in wild type (wt) mice but failed to do so in TNF-deficient (TNF ؊͞؊ ) mice. Furthermore, ovariectomy induced bone loss, which was absent in T cell-deficient nude mice, was restored by adoptive transfer of wt T cells, but not by reconstitution with T cells from TNF ؊͞؊ mice. These findings demonstrate the key causal role of T cell-produced TNF in the bone loss after estrogen withdrawal. Finally, ovariectomy caused bone loss in wt mice and in mice lacking p75 TNF receptor but failed to do so in mice lacking the p55 TNF receptor. These findings demonstrate that enhanced T cell production of TNF resulting from increased bone marrow T cell number is a key mechanism by which estrogen deficiency induces bone loss in vivo. The data also demonstrate that the bone-wasting effect of TNF in vivo is mediated by the p55 TNF receptor.ovariectomy ͉ osteoporosis ͉ mouse ͉ pQCT I t is now recognized that one of the main mechanisms by which estrogen deficiency causes bone loss is by stimulating osteoclast formation (1), a process induced by the simultaneous stimulation of osteoclast precursors by macrophage colonystimulating factor (M-CSF) and a tumor necrosis factor (TNF)-related factor known as receptor activator of NF-B ligand (RANKL) (also known as OPGL, TRANCE, or ODF) (2-4).In physiologic, unstimulated conditions, the differentiation of osteoclast precursors into mature osteoclasts in the bone marrow depends on the production of M-CSF by monocytes and stromal cells and RANKL by stromal cells and osteoblasts (5). However, in stimulated conditions, additional bone marrow cells contribute to regulating osteoclast formation by producing soluble and membrane-bound pro-and antiosteoclastogenic cytokines. Among them are naïve and activated T cells, which modulate osteoclast formation trough increased production of RANKL (6-8), osteoprotegerin (9), and IFN-␥ (10).During inflammation and autoimmune arthritis, activated T cell production of RANKL promotes bone resorption and bone loss (6) whereas release of IFN-␥ limits T cell-induced bone wasting (10). Recent studies from our laboratory have disclosed that activated T cells play an essential causal role not only in inflammation-induced bone loss, but also in the bone wasting induced by estrogen deficiency (11). In fact, whereas ovariectomy (ovx) stimulated bone resorption and induced rapid bone loss in T cell-re...

T cell-produced cytokines play a pivotal role in the bone loss caused by inflammation, infection, and estrogen deficiency. IFN-γ is a major product of activated T helper cells that can function as a pro-or antiresorptive cytokine, but the reason why IFN-γ has variable effects in bone is unknown. Here we show that IFN-γ blunts osteoclast formation through direct targeting of osteoclast precursors but indirectly stimulates osteoclast formation and promotes bone resorption by stimulating antigen-dependent T cell activation and T cell secretion of the osteoclastogenic factors RANKL and TNF-α. Analysis of the in vivo effects of IFN-γ in 3 mouse models of bone loss -ovariectomy, LPS injection, and inflammation via silencing of TGF-β signaling in T cells -reveals that the net effect of IFN-γ in these conditions is that of stimulating bone resorption and bone loss. In summary, IFN-γ has both direct anti-osteoclastogenic and indirect pro-osteoclastogenic properties in vivo. Under conditions of estrogen deficiency, infection, and inflammation, the net balance of these 2 opposing forces is biased toward bone resorption. Inhibition of IFN-γ signaling may thus represent a novel strategy to simultaneously reduce inflammation and bone loss in common forms of osteoporosis.

Expansion of the pool of tumor necrosis factor (TNF)-␣-producing T cells is instrumentalovariectomy ͉ osteoporosis ͉ tumor necrosis factor ͉ sex steroids

While N6-methyladenosine (m6A), the most abundant internal modification in eukaryotic mRNA, is linked to cell differentiation and tissue development, the biological significance of m6A modification in mammalian glial development remains unknown. Here, we identify a novel m6A reader, Prrc2a (Proline rich coiled-coil 2 A), which controls oligodendrocyte specification and myelination. Nestin-Cre-mediated knockout of Prrc2a induces significant hypomyelination, decreased lifespan, as well as locomotive and cognitive defects in a mouse model. Further analyses reveal that Prrc2a is involved in oligodendrocyte progenitor cells (OPCs) proliferation and oligodendrocyte fate determination. Accordingly, oligodendroglial-lineage specific deletion of Prrc2a causes a similar phenotype of Nestin-Cre-mediated deletion. Combining transcriptome-wide RNA-seq, m6A-RIP-seq and Prrc2a RIP-seq analysis, we find that Olig2 is a critical downstream target gene of Prrc2a in oligodendrocyte development. Furthermore, Prrc2a stabilizes Olig2 mRNA through binding to a consensus GGACU motif in the Olig2 CDS (coding sequence) in an m6A-dependent manner. Interestingly, we also find that the m6A demethylase, Fto, erases the m6A modification of Olig2 mRNA and promotes its degradation. Together, our results indicate that Prrc2a plays an important role in oligodendrocyte specification through functioning as a novel m6A reader. These findings suggest a new avenue for the development of therapeutic strategies for hypomyelination-related neurological diseases.

PTH promotes bone catabolism by targeting bone marrow stromal cells (SCs) and their osteoblastic progeny. Here we show that a continuous infusion of PTH that mimics hyperparathyroidism fails to induce osteoclast formation, bone resorption and cortical bone loss in mice lacking T cells. T cells provide proliferative and survival cues to SCs and sensitize SCs to PTH through CD40 Ligand (CD40L), a surface molecule of activated T cells that induces CD40 signaling in SCs. As a result, deletion of T cells or T cell expressed CD40L blunts the bone catabolic activity of PTH by decreasing bone marrow SC number, RANKL/OPG production and osteoclastogenic activity. Therefore, T cells play an essential permissive role in hyperparathyroidism as they influence SC proliferation, lifespan and function through CD40L. T cell-SC cross-talk pathways may thus provide pharmacological targets for PTH induced bone disease.