A novel miR17/protein tyrosine phosphatase-oc/EphA4 regulatory axis of osteoclast activity

Strategies of Macrophages to Maintain Bone Homeostasis and Promote Bone Repair: A Narrative Review

Bone homeostasis (a healthy bone mass) is regulated by maintaining a delicate balance between bone resorption and bone formation. The regulation of physiological bone remodeling by a complex system that involves multiple cells in the skeleton is closely related to bone homeostasis. Loss of bone mass or repair of bone is always accompanied by changes in bone homeostasis. However, due to the complexity of bone homeostasis, we are currently unable to identify all the mechanisms that affect bone homeostasis. To date, bone macrophages have been considered a third cellular component in addition to osteogenic spectrum cells and osteoclasts. As confirmed by co-culture models or in vivo experiments, polarized or unpolarized macrophages interact with multiple components within the bone to ensure bone homeostasis. Different macrophage phenotypes are prone to resorption and formation of bone differently. This review comprehensively summarizes the mechanisms by which macrophages regulate bone homeostasis and concludes that macrophages can control bone homeostasis from osteoclasts, mesenchymal cells, osteoblasts, osteocytes, and the blood/vasculature system. The elaboration of these mechanisms in this narrative review facilitates the development of macrophage-based strategies for the treatment of bone metabolic diseases and bone defects.

miRNAs in osteoclast biology

MicroRNAs (miRNAs) are a class of short RNA molecules that mediate the regulation of gene activity through interactions with target mRNAs and subsequent silencing of gene expression. It has become increasingly clear the miRNAs regulate many diverse aspects of bone biology, including bone formation and bone resorption processes. The role of miRNAs specifically in osteoclasts has been of recent investigation, due to clinical interest in discovering new paradigms to control excessive bone resorption, as is observed in multiple conditions including aging, estrogen deprivation, cancer metastases or glucocorticoid use. Therefore understanding the role that miRNAs play during osteoclastic differentiation is of critical importance. In this review, we highlight and discuss general aspects of miRNA function in osteoclasts, including exciting data demonstrating that miRNAs encapsulated in extracellular vesicles (EVs) either originating from osteoclasts, or signaling to osteoclast from divergent sites, have important roles in bone homeostasis.

Involvement of Noncoding RNAs in the Differentiation of Osteoclasts

As the most important bone-resorbing cells, osteoclasts play fundamental roles in bone remodeling and skeletal health. Much effort has been focused on identifying the regulators of osteoclast metabolism. Noncoding RNAs (ncRNAs) reportedly regulate osteoclast formation, differentiation, survival, and bone-resorbing activity to participate in bone physiology and pathology. The present review intends to provide a general framework for how ncRNAs and their targets regulate osteoclast differentiation and the important events of osteoclastogenesis they are involved in, including osteoclast precursor generation, early differentiation, mononuclear osteoclast fusion, and multinucleated osteoclast function and survival. This framework is beneficial for understanding bone biology and for identifying the potential biomarkers or therapeutic targets of bone diseases. The review also summarizes the results of in vivo experiments and classic experiment methods for osteoclast-related researches.

Non-coding RNAs in bone remodelling and bone metastasis: Mechanisms of action and translational relevance

Bone metastases are frequent complications in patients with advanced cancer, which can be fatal or may rapidly impede the quality of life of patients. Current treatments for patients with bone metastases are palliative. Therefore, a better understanding of the molecular mechanisms that precede the overt development of skeletal lesions could lead to better therapeutic interventions. In this review, we present evidence that non-coding RNAs (ncRNAs) such as long ncRNAs, microRNAs, and circular RNAs are emerging as master regulators of bone metastasis formation. We highlight potential opportunities for the therapeutic targeting of ncRNAs. Furthermore, we discuss the possibility that ncRNAs may be used as biomarkers in the context of bone metastases, which might provide insight for improving the response to current bone-targeting therapies. LINKED ARTICLES: This article is part of a themed issue on The molecular pharmacology of bone and cancer-related bone diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.9/issuetoc.

Phosphorylation of the phosphatase PTPROt at Tyr399 is a molecular switch that controls osteoclast activity and bone mass in vivo

Bone resorption by osteoclasts is essential for bone homeostasis. The kinase Src promotes osteoclast activity and is activated in osteoclasts by the receptor-type tyrosine phosphatase PTPROt. In other contexts, however, PTPROt can inhibit Src activity. Through in vivo and in vitro experiments, we show that PTPROt is bifunctional and can dephosphorylate Src both at its inhibitory residue Tyr⁵²⁷ and its activating residue Tyr⁴¹⁶ Whereas wild-type and PTPROt knockout mice exhibited similar bone masses, mice in which a putative C-terminal phosphorylation site, Tyr³⁹⁹, in endogenous PTPROt was replaced with phenylalanine had increased bone mass and reduced osteoclast activity. Osteoclasts from the knock-in mice also showed reduced Src activity. Experiments in cultured cells and in osteoclasts derived from both mouse strains demonstrated that the absence of phosphorylation at Tyr³⁹⁹ caused PTPROt to dephosphorylate Src at the activating site pTyr⁴¹⁶ In contrast, phosphorylation of PTPROt at Tyr³⁹⁹ enabled PTPROt to recruit Src through Grb2 and to dephosphorylate Src at the inhibitory site Tyr⁵²⁷, thus stimulating Src activity. We conclude that reversible phosphorylation of PTPROt at Tyr³⁹⁹ is a molecular switch that selects between its opposing activities toward Src and maintains a coherent signaling output, and that blocking this phosphorylation event can induce physiological effects in vivo. Because most receptor-type tyrosine phosphatases contain potential phosphorylation sites at their C termini, we propose that preventing phosphorylation at these sites or its consequences may offer an alternative to inhibiting their catalytic activity to achieve therapeutic benefit.