Rankl-induced osteoclastogenesis leads to loss of mineralization in a medaka osteoporosis model

Abstract

SUMMARYOsteoclasts are macrophage-related bone resorbing cells of hematopoietic origin. Factors that regulate osteoclastogenesis are of great interest for investigating the pathology and treatment of bone diseases such as osteoporosis. In mammals, receptor activator of NF-B ligand (Rankl) is a regulator of osteoclast formation and activation: its misexpression causes osteoclast stimulation and osteoporotic bone loss. Here, we report an osteoporotic phenotype that is induced by overexpression of Rankl in the medaka model. We generated transgenic medaka lines that express GFP under control of the cathepsin K promoter in osteoclasts starting at 12 days post-fertilization (dpf), or Rankl together with CFP under control of a bi-directional heat-shock promoter. Using long-term confocal time-lapse imaging of double and triple transgenic larvae, we monitored in vivo formation and activation of osteoclasts, as well as their interaction with osteoblasts. Upon Rankl induction, GFP-positive osteoclasts are first observed in the intervertebral regions and then quickly migrate to the surface of mineralized neural and haemal arches, as well as to the centra of the vertebral bodies. These osteoclasts are TRAP (tartrate-resistant acid phosphatase) and cathepsin K positive, mononuclear and highly mobile with dynamically extending protrusions. They are exclusively found in tight contact with mineralized matrix. Rankl-induced osteoclast formation resulted in severe degradation of the mineralized matrix in vertebral bodies and arches. In conclusion, our in vivo imaging approach confirms a conserved role of Rankl in osteoclastogenesis in teleost fish and provides new insight into the cellular interactions during bone resorption in an animal model that is useful for genetic and chemical screening.

miR-218 is involved in the negative regulation of osteoclastogenesis and bone resorption by partial suppression of p38MAPK-c-Fos-NFATc1 signaling: Potential role for osteopenic diseases

“…In this study, the down-regulation of miR-218 was confirmed during the process of RANKL-induced osteoclastogenesis, in both osteoclast precursors of BMMs and RAW 264.7. Accumulating evidences have proven that RANKL can induce osteoclast formation by various signaling, which will facilitate the development of osteoporosis [22,23]. In our studies, we found that miR-218 overexpression significantly abrogated RANKL-induced cell-cell fusion of TRAP-positive mononuclear pre-osteoclasts to become multinuclear mature osteoclasts, concomitant with the down-regulation of Trap and Cathepsin K levels, both the master regulators of osteoclastogenesis.…”

A vertebrate specific and essential role for sp7/osterix in osteogenesis revealed by gene knock-out in the teleost medaka

“…Microdissection of cartilage was performed according to DeLaurier et al (2010). Imaging was performed according to To et al (2012).…”

The enigmas of bone without osteocytes

“…58,64,65 Given that signaling proteins like sclerostin and RANKL allow osteocytes to act as significant regulators of osteoblast and osteoclast activity and the interplay between them, controlling the rate and extent of bone formation and resorption, it is appealing to ask what control mechanisms exist in bone without osteocytes, and which mechanisms may be conserved between fishes and other vertebrates. Although these questions are largely unaddressed in fishes (but see To et al 66 ), it is clear that, in general, the regulatory measures attributed to osteocytes are not simply absent in acellular bone. Moreover, acellular bone does not appear as mammalian bone with the osteocytes simply removed or dead, a manipulation that alters SCL and RANKL levels and can have drastic effects on the balance between bone gain and loss.…”

“…18 Clearly, although osteocytes are lacking, bone production and removal do not run rampant in acellular fish bone, although RANKL has recently been shown to exist and have a conserved role in the skeleton of an acellular teleost fish. 66 This suggests alternative regulation factors and/or control pathways, perhaps via osteoblast control of RANKL, although parathyroid hormone (known to promote osteoblast expression of RANKL in mammals) is present in quite different form in fishes, and fishes lack parathyroid glands. 60,67 Determination of the precise triggering and activation machineries involved, the sequence of gene expression in regulation and coordination, and the cells that take part in sensing the need for fish bone modeling can be made by a combination of histological, biochemical and molecular genetics methods.…”