YAP (yes-associated protein) is a transcriptional factor that is negatively regulated by Hippo pathway, a conserved pathway for the development and size control of multiple organs. The exact function of YAP in bone homeostasis remains controversial. Here we provide evidence for YAP’s function in promoting osteogenesis, suppressing adipogenesis, and thus maintaining bone homeostasis. YAP is selectively expressed in osteoblast (OB)-lineage cells. Conditionally knocking out Yap in the OB lineage in mice reduces cell proliferation and OB differentiation and increases adipocyte formation, resulting in a trabecular bone loss. Mechanistically, YAP interacts with β-catenin and is necessary for maintenance of nuclear β-catenin level and Wnt/β-catenin signaling. Expression of β-catenin in YAP-deficient BMSCs (bone marrow stromal cells) diminishes the osteogenesis deficit. These results thus identify YAP-β-catenin as an important pathway for osteogenesis during adult bone remodeling and uncover a mechanism underlying YAP regulation of bone homeostasis.

Bone mass is maintained by balanced activity of osteoblasts and osteoclasts. Lrp4 (low-density lipoprotein receptor related protein 4) is a member of the LDL receptor family, whose mutations have been identified in patients with high-bone-mass disorders, such as sclerosteosis and van Buchem diseases. However, it remains unknown whether and how Lrp4 regulates bone-mass homeostasis in vivo. Here we provide evidence that Lrp4-null mutation or specific mutation in osteoblast-lineage cells increased cortical and trabecular bone mass, which was associated with elevated bone formation and impaired bone resorption. This phenotype was not observed in osteoclast-selective Lrp4 knockout mice. Mechanistic studies indicate that loss of Lrp4 function in osteoblast-lineage cells increased serum levels of sclerostin, a key factor for bone-mass homeostasis that interacts with Lrp4, but abolished the inhibition of Wnt/β-catenin signaling and osteoblastic differentiation by sclerostin. Concomitantly, sclerostin induction of RANKL (receptor activator of nuclear kappa B ligand) was impaired, leading to a lower ratio of RANKL over OPG (osteoprotegerin) (a key factor for osteoclastogenesis). Taken together, these results support the view for Lrp4 as a receptor of sclerostin to inhibit Wnt/β-catenin signaling and bone formation and identify Lrp4 as a critical player in bonemass homeostasis.one remodeling is a dynamic process essential for maintenance of skeletal integrity and bone homeostasis (1). Bone mass is tightly regulated by bone-forming osteoblasts (OBs) and bone-resorbing osteoclasts (OCs). OBs are differentiated from bone marrow stromal cells (BMSCs) or mesenchymal progenitor cells, whereas OCs are derived from hematopoietic bone marrow macrophages or myeloid monocytes (BMMs). The balance of bone formation and resorption is critical for maintenance of healthy bone mass. The imbalance of bone formation and resorption could result in high-bone-mass disorders such as sclerosteosis and van Buchem disease or bone loss such as osteoporosis.The canonical Wnt/β-catenin signaling is critical to regulate bone-mass homeostasis (1, 2). Binding of Wnt ligands to a dualreceptor complex of frizzled and Lrp5/6 leads to accumulation of cytoplasmic β-catenin and translocation of β-catenin into the nucleus to regulate gene expression. This pathway is required for commitment of mesenchymal stem cells to the OB lineage, OB precursor cell proliferation and differentiation, and OC genesis and activation (1-3). Clinically, Lrp5 mutations are associated with the osteoporosis-pseudoglioma syndrome, a low-bone-mass disorder (4), as well as with high-bone-mass disorders (5, 6).Lrp4 is a member of LDL family protein, containing a large extracellular region with multiple LDLa, EGF-like, and β-propeller repeats, a transmembrane domain, and a short C-terminal region. Lrp4 is a receptor of agrin (7,8), critical for neuromuscular junction formation. Mice lacking Lrp4 (null allele) die at birth because of inability to breathe (9). Lrp4 is also highly related to...

Reduced bone mineral density and hip fracture are frequently observed in patients with Alzheimer's disease (AD). However, mechanisms underlying their association remain poorly understood. Amyloid precursor protein (APP) is a transmembrane protein that is ubiquitously expressed in bone marrow stromal cells (BMSCs), osteoblasts (OBs), macrophages (BMMs), and osteoclasts (OCs). Mutations in the APP gene identified in early-onset AD patients are believed to cause AD. But little is known about APP's role in bone remodeling. Here, we present evidence for Swedish mutant APP (APPswe) in suppression of OB differentiation and function in culture and in mouse. APP expression in BMSCs increases during aging. Ubiquitous expression of APPswe in young adult Tg2576 transgenic mice (under the control of a prion promoter) recaptured skeletal "aging-like" deficits, including decreased OB genesis and bone formation, increased adipogenesis and bone marrow fat, and enhanced OC genesis and bone resorption. Remarkably, selective expression of APPswe in mature OB-lineage cells in TgAPPswe-Ocn mice (under the control of osteocalcin [Ocn] promoter-driven Cre) also decreased OB genesis and increased OC formation, resulting in a trabecular bone loss. These results thus suggest a cell-autonomous role for APPswe in suppressing OB formation and function, but a nonautonomous effect on OC genesis. Notably, increased adipogenesis and elevated bone marrow fat were detected in young adult Tg2576 mice, but not in TgAPPswe-Ocn mice, implying that APPswe in BMSCs and/or multicell types in bone marrow promotes bone marrow adipogenesis. Intriguingly, the skeletal aging-like deficits in young adult Tg2576 mice were prevented by treatment with N-acetyl-L-cysteine (NAC), an antioxidant, suggesting that reactive oxygen species (ROS) may underlie APPswe-induced osteoporotic deficits. Taken together, these results demonstrate a role for APPswe in suppressing OB differentiation and bone formation, implicate APPswe as a detrimental factor for AD-associated osteoporotic deficit, and reveal a potential clinical value of NAC in the treatment of osteoporotic deficits.