Yihong Wan

PPAR-γ regulates osteoclastogenesis in mice

Osteoclasts are bone-resorbing cells derived from hematopoietic precursors of the monocyte-macrophage lineage. Regulation of osteoclast function is central to the understanding of bone diseases such as osteoporosis, rheumatoid arthritis and osteopetrosis. Although peroxisome proliferator–activated receptor-γ (PPAR-γ) has been shown to inhibit osteoblast differentiation, its role, if any, in osteoclasts is unknown. This is a clinically crucial question because PPAR-γ agonists, “such as thiazolidinediones—” a class of insulin-sensitizing drugs, have been reported to cause a higher rate of fractures in human patients. Here we have uncovered a pro-osteoclastogenic effect of PPAR-γ by using a Tie2Cre/flox mouse model in which PPAR-γ is deleted in osteoclasts but not in osteoblasts. These mice develop osteopetrosis characterized by increased bone mass, reduced medullary cavity space and extramedullary hematopoiesis in the spleen. These defects are the result of impaired osteoclast differentiation and compromised receptor activator of nuclear factor-κB ligand signaling and can be rescued by bone marrow transplantation. Moreover, ligand activation of PPAR-γ by rosiglitazone exacerbates osteoclast differentiation in a receptor-dependent manner. Our examination of the underlying mechanisms suggested that PPAR-γ functions as a direct regulator of c-fos expression, an essential mediator of osteoclastogenesis. Therefore, PPAR-γ and its ligands have a previously unrecognized role in promoting osteoclast differentiation and bone resorption.

Fibroblast growth factor 21 promotes bone loss by potentiating the effects of peroxisome proliferator-activated receptor γ

The endocrine hormone fibroblast growth factor 21 (FGF21) is a powerful modulator of glucose and lipid metabolism and a promising drug for type 2 diabetes. Here we identify FGF21 as a potent regulator of skeletal homeostasis. Both genetic and pharmacologic FGF21 gain of function lead to a striking decrease in bone mass. In contrast, FGF21 loss of function leads to a reciprocal high-bone-mass phenotype. Mechanistically, FGF21 inhibits osteoblastogenesis and stimulates adipogenesis from bone marrow mesenchymal stem cells by potentiating the activity of peroxisome proliferator-activated receptor γ (PPAR-γ). Consequently, FGF21 deletion prevents the deleterious bone loss side effect of the PPAR-γ agonist rosiglitazone. Therefore, FGF21 is a critical rheostat for bone turnover and a key integrator of bone and energy metabolism. These results reveal that skeletal fragility may be an undesirable consequence of chronic FGF21 administration.

The starvation hormone, fibroblast growth factor-21, extends lifespan in mice

Fibroblast growth factor-21 (FGF21) is a hormone secreted by the liver during fasting that elicits diverse aspects of the adaptive starvation response. Among its effects, FGF21 induces hepatic fatty acid oxidation and ketogenesis, increases insulin sensitivity, blocks somatic growth and causes bone loss. Here we show that transgenic overexpression of FGF21 markedly extends lifespan in mice without reducing food intake or affecting markers of NAD+ metabolism or AMP kinase and mTOR signaling. Transcriptomic analysis suggests that FGF21 acts primarily by blunting the growth hormone/insulin-like growth factor-1 signaling pathway in liver. These findings raise the possibility that FGF21 can be used to extend lifespan in other species. DOI: http://dx.doi.org/10.7554/eLife.00065.001

miR-34a Blocks Osteoporosis and Bone Metastasis by Inhibiting Osteoclastogenesis and Tgif2

Bone-resorbing osteoclasts significantly contribute to osteoporosis and bone metastases of cancer. MicroRNAs play important roles in physiology and disease, and present tremendous therapeutic potential. Nonetheless, how microRNAs regulate skeletal biology is underexplored. Here we identify miR-34a as a novel and critical suppressor of osteoclastogenesis, bone resorption and the bone metastatic niche. miR-34a is downregulated during osteoclast differentiation. Osteoclastic miR-34a-overexpressing transgenic mice exhibit lower bone resorption and higher bone mass. Conversely, miR-34a knockout and heterozygous mice exhibit elevated bone resorption and reduced bone mass. Consequently, ovariectomy-induced osteoporosis, as well as bone metastasis of breast and skin cancers, are diminished in osteoclastic miR-34a transgenic mice, and can be effectively attenuated by miR-34a nanoparticle treatment. Mechanistically, we identify transforming growth factor-β-induced factor 2 (Tgif2) as an essential direct miR-34a target that is pro-osteoclastogenic. Tgif2 deletion reduces bone resorption and abolishes miR-34a regulation. Together, using mouse genetic, pharmacological and disease models, we reveal miR-34a as a key osteoclast suppressor and a potential therapeutic strategy to confer skeletal protection and ameliorate bone metastasis of cancers.

PGC1β Mediates PPARγ Activation of Osteoclastogenesis and Rosiglitazone-Induced Bone Loss

Long-term usage of rosiglitazone, a synthetic PPARgamma agonist, increases fracture rates among diabetic patients. PPARgamma suppresses osteoblastogenesis while activating osteoclastogenesis, suggesting that rosiglitazone decreases bone formation while sustaining or increasing bone resorption. Using mouse models with genetically altered PPARgamma, PGC1beta, or ERRalpha, here we show that PGC1beta is required for the resorption-enhancing effects of rosiglitazone. PPARgamma activation indirectly induces PGC1beta expression by downregulating beta-catenin and derepressing c-jun. PGC1beta, in turn, functions as a PPARgamma coactivator to stimulate osteoclast differentiation. Complementarily, PPARgamma also induces ERRalpha expression, which coordinates with PGC1beta to enhance mitochondrial biogenesis and osteoclast function. ERRalpha knockout mice exhibit osteoclast defects, revealing ERRalpha as an important regulator of osteoclastogenesis. Strikingly, PGC1beta deletion in osteoclasts confers complete resistance to rosiglitazone-induced bone loss. These findings identify PGC1beta as an essential mediator for the PPARgamma stimulation of osteoclastogenesis by targeting both PPARgamma itself and ERRalpha, thus activating two distinct transcriptional programs.

PPARγ in bone homeostasis.

The nuclear receptor peroxisome proliferator-activated receptor (PPAR)γ is a crucial cellular and metabolic switch that regulates many physiologic and disease processes. Emerging evidence reveals that PPARγ is also a key modulator of skeletal remodeling. Long-term use of rosiglitazone, a synthetic PPARγ agonist and a drug to treat insulin resistance, increases fracture rates among patients with diabetes. Recent studies have revealed that PPARγ activation not only suppresses osteoblastogenesis, but also activates osteoclastogenesis, thereby decreasing bone formation while sustaining or increasing bone resorption. The pro-osteoclastogenic effect of rosiglitazone is mediated by a transcriptional network comprised of PPARγ, PPAR-gamma coactivator 1β and estrogen-related receptor α, which promotes both osteoclast differentiation and mitochondrial activation. Therefore, PPARγ plays dual roles in bone homeostasis by regulating both mesenchymal and hematopoietic lineages.

Neuronal and Nonneuronal Cholinergic Structures in the Mouse Gastrointestinal Tract and Spleen

Accumulating evidence demonstrates that acetylcholine can directly modulate immune function in peripheral tissues including the spleen and gastrointestinal tract. However, the anatomical relationships between the peripheral cholinergic system and immune cells located in these lymphoid tissues remain unclear due to inherent technical difficulties with currently available neuroanatomical methods. In this study, mice with specific expression of the tdTomato fluorescent protein in choline acetyltransferase (ChAT)‐expressing cells were used to label preganglionic and postganglionic cholinergic neurons and their projections to lymphoid tissues. Notably, our anatomical observations revealed an abundant innervation in the intestinal lamina propria of the entire gastrointestinal tract principally originating from cholinergic enteric neurons. The aforementioned innervation frequently approached macrophages, plasma cells, and lymphocytes located in the lamina propria and, to a lesser extent, lymphocytes in the interfollicular areas of Peyers patches. In addition to the above innervation, we observed labeled epithelial cells in the gallbladder and lower intestines, as well as Microfold cells and T‐cells within Peyers patches. In contrast, we found only a sparse innervation in the spleen consisting of neuronal fibers of spinal origin present around arterioles and in lymphocyte‐containing areas of the white pulp. Lastly, a small population of ChAT‐expressing lymphocytes was identified in the spleen including both T‐ and B‐cells. In summary, this study describes the variety of cholinergic neuronal and nonneuronal cells in a position to modulate gastrointestinal and splenic immunity in the mouse. J. Comp. Neurol. 521:3741–3767, 2013.

Maternal western diet causes inflammatory milk and TLR2/4-dependent neonatal toxicity

For all newborn mammals, mothers milk is the perfect nourishment, crucial for their postnatal development. Here we report that, unexpectedly, maternal western diet consumption in mice causes the production of toxic milk that contains excessive long chain and saturated fatty acids, which triggers ceramide accumulation and inflammation in the nursing neonates, manifested as alopecia. This neonatal toxicity requires Toll-like-receptors (TLR), but not gut microbiota, because TLR2/4 deletion or TLR4 inhibition confers resistance, whereas germ-free mice remain sensitive. These findings unravel maternal western diet-induced inflammatory milk secretion as a novel aspect of the metabolic syndrome at the maternal offspring interface.

New therapeutic targets for cancer bone metastasis

Bone metastases are dejected consequences of many types of tumors including breast, prostate, lung, kidney, and thyroid cancers. This complicated process begins with the successful tumor cell epithelial-mesenchymal transition, escape from the original site, and penetration into the circulation. The homing of tumor cells to the bone depends on both tumor-intrinsic traits and various molecules supplied by the bone metastatic niche. The colonization and growth of cancer cells in the osseous environment, which awaken their dormancy to form micro- and macro-metastasis, involve an intricate interaction between the circulating tumor cells and local bone cells including osteoclasts, osteoblasts, adipocytes, and macrophages. We discuss the most recent advances in the identification of new molecules and novel mechanisms during each step of bone metastasis that may serve as promising therapeutic targets.

Osteoclast Progenitors Reside in the Peroxisome Proliferator-Activated Receptor γ-Expressing Bone Marrow Cell Population

ABSTRACT Osteoclasts are bone-resorbing cells essential for skeletal development, homeostasis, and regeneration. They derive from hematopoietic progenitors in the monocyte/macrophage lineage and differentiate in response to RANKL. However, the precise nature of osteoclast progenitors is a longstanding and important question. Using inducible peroxisome proliferator-activated receptor γ (PPARγ)-tTA TRE-GFP (green fluorescent protein) reporter mice, we show that osteoclast progenitors reside specifically in the PPARγ-expressing hematopoietic bone marrow population and identify the quiescent PPARγ+ cells as osteoclast progenitors. Importantly, two PPARγ-tTA TRE-Cre-controlled genetic models provide compelling functional evidence. First, Notch activation in PPARγ+ cells causes high bone mass due to impaired osteoclast precursor proliferation. Second, selective ablation of PPARγ+ cells by diphtheria toxin also causes high bone mass due to decreased osteoclast numbers. Furthermore, PPARγ+ cells respond to both pathological and pharmacological resorption-enhancing stimuli. Mechanistically, PPARγ promotes osteoclast progenitors by activating GATA2 transcription. These findings not only identify the long-sought-after osteoclast progenitors but also establish unprecedented tools for their visualization, isolation, characterization, and genetic manipulation.