Bone cell communication factors and Semaphorins

Abstract

-6396/12 www.nature.com/bonekey IntroductionThe bony skeleton enables various crucial processes, such as locomotive activity, the storage of calcium and the harboring of hematopoietic stem cells. Bone is a dynamic organ that is continuously being broken down by osteoclasts and subsequently rebuilt with new bone by osteoblasts throughout the course of one ' s adult life. These activities occur in response to various hormones, cytokines, chemokines and biomechanical external stimuli. 1,2 This process, called bone remodeling, is a prerequisite for the normal bone homeostasis that maintains both bone quality and strength. 2 An imbalance of bone resorption and formation is often central to metabolic bone diseases in humans and animals. For example, excessive resorption by osteoclasts results in pathological bone destruction, as seen in osteoporosis, autoimmune arthritis, Paget ' s disease, periodontitis and bone tumors. 1,3,4 Therefore, bone remodeling must be regulated both temporally and spatially so that the aged or damaged bone is replaced by an essentially equivalent amount of new bone. 1,5,6 The bone remodeling is carried out within the temporary anatomic structures, so-called basic multicellular unit (BMU), which consists of a group of osteoclasts in front forming the cutting cone and a group of osteoblasts behind forming the closing cone, by associating with blood supply and the peripheral innervation. 7,8 The bone remodeling is a cycle consisting of three phases: ' initiation ' of bone resorption by osteoclasts, the ' transition ' from resorption to new bone formation (also well known as a ' reversal ' period) and the ' bone formation ' ( Figure 1 ). 6,9 The entire process is achieved by the coordinated actions of osteoclasts, osteoblasts and other osteoblast lineage cells, including bone-lining cells and osteocytes. The most obvious example is the osteoclastogenesis regulatory system by receptor activator of nuclear-B ligand (RANKL) and its decoy receptor osteoprotegerin (OPG), which are produced by osteoblast lineage cells. However, key questions remain REVIEW Bone cell communication factors and SemaphorinsTakako Negishi-Koga 1 , 2 , 3 and Hiroshi Takayanagi 1 , 2 , 3 , 4 Bone tissue is continuously renewed throughout adult life by a process called ' remodeling ' , which involves a dynamic interplay among bone cells including osteoclasts, osteoblasts and osteocytes. For example, a tight coupling between bone resorption and formation is essential for the homeostasis of the skeletal system. Studies on the coupling mechanism in physiological and pathological settings have revealed that osteoclasts or osteoclastic bone resorption promote bone formation through the production of diverse coupling factors. The classical coupling factors are the molecules that promote bone formation after resorption, but there may be distinct mechanisms at work in various phases of bone remodeling. A recent study revealed that the Semaphorin 4D expressed by osteoclasts inhibits bone formation, which represents a mechanism by which...

LIS1 Regulates Osteoclastogenesis through Modulation of M-SCF and RANKL Signaling Pathways and CDC42

“…These pathways converge to induce and activate nuclear factor of activated T cells 1 (NFATc1), a master transcription factor of osteoclast differentiation 4, 5. Systemic hormones and other cytokines/growth factors in the bone marrow microenvironment regulate osteoclast number or activities through controlling the expression of M-CSF and RANKL in other cell types of bone marrow or modulating the downstream signaling pathways of these two cytokines in osteoclast lineage cells 6.…”

Ubiquitin E3 Ligase LNX2 is Critical for Osteoclastogenesis In Vitro by Regulating M-CSF/RANKL Signaling and Notch2

“…RANKL, on the other hand, induces the expression of osteoclast-specific genes and promotes the survival of osteoclasts by activating NF-κB, JNK, and intracellular calcium/NFATc1 pathways [7, 8]. Other soluble and membrane-bound cytokines and factors in bone marrow microenvironment exert their influence on osteoclast differentiation and function through modulating M-CSF, RANKL, and/or their downstream signaling pathways [9]. …”

Osteoclast-secreted SLIT3 coordinates bone resorption and formation

“…This consists of 3 consecutive phases: the initiation of bone resorption by osteoclasts; the transition from catabolism to anabolism; and then bone formation by osteoblasts, followed by termination (1). Each of these phases is finely controlled by humoral factors or molecules mediating the communication among bone cells to maintain skeletal integrity (2). As an imbalance of bone remodeling leads to metabolic bone disorders such as osteoporosis (3), a better understanding of the molecular mechanisms regulating its various phases is crucial to the development of better approaches to the prevention and treatment of metabolic bone diseases.…”