Osteocytes, the most abundant and long-lived cells in bone, are the master regulators of bone remodeling. In addition to their functions in endocrine regulation and calcium and phosphate metabolism, osteocytes are the major responsive cells in force adaptation due to mechanical stimulation. Mechanically induced bone formation and adaptation, disuse-induced bone loss and skeletal fragility are mediated by osteocytes, which sense local mechanical cues and respond to these cues in both direct and indirect ways. The mechanotransduction process in osteocytes is a complex but exquisite regulatory process between cells and their environment, between neighboring cells, and between different functional mechanosensors in individual cells. Over the past two decades, great efforts have focused on finding various mechanosensors in osteocytes that transmit extracellular mechanical signals into osteocytes and regulate responsive gene expression. The osteocyte cytoskeleton, dendritic processes, Integrin-based focal adhesions, connexin-based intercellular junctions, primary cilium, ion channels, and extracellular matrix are the major mechanosensors in osteocytes reported so far with evidence from both in vitro and in vitro studies. This review aims to give a systematic introduction to osteocyte mechanobiology, provide details of osteocyte mechanosensors, and discuss the roles of osteocyte mechanosensitive signaling pathways in the regulation of bone homeostasis.

Background To comprehensively analyze the global level and trends of prevalence, incidence and years lived with disability (YLDs) for low back pain (LBP) from 1990 to 2019 by age, sex and sociodemographic index (SDI). Methods Publicly available modelled data and methods were obtained from the Global Burden of Diseases (GBD) study 2019, and used to evaluate the global burden of LBP through a systematic analysis. Results Globally, the age-standardized prevalence, incidence and YLDs rate of LBP were slightly decreased from 1990 to 2019, but the number of the prevalent cases, incident cases and YLDs had substantially increased, and LBP remains the leading cause of YLDs in 2019 worldwide. The number of prevalent cases was increased with age and peaked at the age of 45–54 years for both sexes, and the global prevalence rate was higher in females than in males and increased with age, peaking at the 80–84 age group in both sexes in 2019. Overall, a positive association between the age-standardized YLD rate and SDI was observed over the past thirty years. At the national revel, the United States, Denmark and Switzerland had the three highest levels of age-standardized prevalence, while Zambia, Zimbabwe and Canada showed the highest increase in the age-standardized prevalence during 1990–2019. Conclusions LBP is a major public health issue globally, and its burden remains high. Increasing population awareness about its risk factors and preventive measures for LBP are needed to reduce the future burden of this condition. The translational potential of this article Due to the high prevalence and heavy burden of LBP globally, it is important to update its epidemiological data. This systematic analysis provides researchers and healthcare policy makers with up-to-date, comprehensive and comparable information on global LBP burden, which is of clinical translational significance.

The signals that control skeletogenesis are incompletely understood. Here we show that deleting Kindlin-2 in Prx1-expressing mesenchymal progenitors in mice causes neonatal lethality, chondrodysplasia, and loss of the skull vault. Kindlin-2 ablation reduces chondrocyte density by decreasing cell proliferation and increasing apoptosis, and disrupts column formation, thus impairing the formation of the primary ossification center and causing severe limb shortening. Remarkably, Kindlin-2 localizes to not only focal adhesions, but also to the nuclei of chondrocytes. Loss of Kindlin-2 reduces, while overexpression of Kindlin-2 increases, Sox9 expression. Furthermore, overexpression of Sox9 restores the defects in chondrogenic differentiation induced by Kindlin-2 deletion in vitro. Additionally, Kindlin-2 ablation inhibits TGF-β1-induced Smad2 phosphorylation and chondrocyte differentiation. Finally, deleting Kindlin-2 in chondrocytes directly impairs chondrocyte functions, resulting in progressive dwarfism and kyphosis in mice. These studies uncover a previously unrecognized function for Kindlin-2 and a mechanism for regulation of the chondrocyte differentiation program and chondrogenesis.

Activating transcription factor 4 (ATF4) is a critical transcription factor for osteoblast (OBL) function and bone formation; however, a direct role in osteoclasts (OCLs) has not been established. Here, we targeted expression of ATF4 to the OCL lineage using the Trap promoter or through deletion of Atf4 in mice. OCL differentiation was drastically decreased in Atf4 -/-bone marrow monocyte (BMM) cultures and bones. Coculture of Atf4 -/-BMMs with WT OBLs or a high concentration of RANKL failed to restore the OCL differentiation defect. Conversely, Trap-Atf4-tg mice displayed severe osteopenia with dramatically increased osteoclastogenesis and bone resorption. We further showed that ATF4 was an upstream activator of the critical transcription factor Nfatc1 and was critical for RANKL activation of multiple MAPK pathways in OCL progenitors. Furthermore, ATF4 was crucial for M-CSF induction of RANK expression on BMMs, and lack of ATF4 caused a shift in OCL precursors to macrophages. Finally, ATF4 was largely modulated by M-CSF signaling and the PI3K/AKT pathways in BMMs. These results demonstrate that ATF4 plays a direct role in regulating OCL differentiation and suggest that it may be a therapeutic target for treating bone diseases associated with increased OCL activity.

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Parathyroid hormone (PTH) is a potent anabolic agent for the treatment of osteoporosis. However, its mechanism of action in osteoblast and bone is not well understood. In this study, we show that the anabolic actions of PTH in bone are severely impaired in both growing and adult ovariectomized mice lacking bone-related activating transcription factor 4 (ATF4). Our study demonstrates that ATF4 deficiency suppresses PTH-stimulated osteoblast proliferation and survival and abolishes PTH-induced osteoblast differentiation, which, together, compromise the anabolic response. We further demonstrate that the PTH-dependent increase in osteoblast differentiation is correlated with ATF4-dependent up-regulation of Osterix. This regulation involves interactions of ATF4 with a specific enhancer sequence in the Osterix promoter. Furthermore, actions of PTH on Osterix require this same element and are associated with increased binding of ATF4 to chromatin. Taken together these experiments establish a fundamental role for ATF4 in the anabolic actions of PTH on the skeleton.

SUMMARYT cells are strongly regulated by oxidizing environments and amino acid restriction. How T cells reprogram metabolism to adapt to these extracellular stress situations is not well understood. Here, we show that oxidizing environments and amino acid starvation induce ATF4 in CD4+ T cells. We also demonstrate that Atf4-deficient CD4+ T cells have defects in redox homeostasis, proliferation, differentiation, and cytokine production. We further reveal that ATF4 regulates a coordinated gene network that drives amino acid intake, mTORC1 activation, protein translation, and an anabolic program for de novo synthesis of amino acids and glutathione. ATF4 also promotes catabolic glycolysis and glutaminolysis and oxidative phosphorylation and thereby provides precursors and energy for anabolic pathways. ATF4-deficient mice mount reduced Th1 but elevated Th17 immune responses and develop more severe experimental allergic encephalomyelitis (EAE). Our study demonstrates that ATF4 is critical for CD4+ T cell-mediated immune responses through driving metabolic adaptation.

Resveratrol, a naturally occurring stilbene, induced apoptosis in human breast cancer MCF-7 cells. The mechanism of this effect was dependent on mitogen-activated protein kinase (MAPK, ERK1/2) activation and was associated with serine phosphorylation and acetylation of p53. Treatment of MCF-7 cells with resveratrol in the presence of 17β-oestradiol (E2) further enhanced MAPK activation, but E2 blocked resveratrol-induced apoptosis, as measured by nucleosome ELISA and DNA fragmentation assays. E2 inhibited resveratrol-stimulated phosphorylation of serines 15, 20 and 392 of p53 and acetylation of p53 in a concentration- and time-dependent manner. These effects of E2 on resveratrol action were blocked by ICI 182,780 (ICI), an inhibitor of the nuclear oestrogen receptor-α (ER). ICI 182,780 did not block the actions of resveratrol, alone. Electrophoretic mobility studies of p53 binding to DNA and of p21 expression indicated that E2 inhibited resveratrol-induced, p53-directed transcriptional activity. These results suggest that E2 inhibits p53-dependent apoptosis in MCF-7 cells by interfering with post-translational modifications of p53 which are essential for p53-dependent DNA binding and consequent stimulation of apoptotic pathways. These studies provide insight into the complex pathways by which apoptosis is induced by resveratrol in E2-depleted and -repleted environments.