Studies of OC-STAMP in Osteoclast Fusion: A New Knockout Mouse Model, Rescue of Cell Fusion, and Transmembrane Topology

DC-STAMP: A Key Regulator in Osteoclast Differentiation

Osteoimmunology research is a new emerging research field that investigates the links between the bone and immune responses. Results from osteoimmunology studies suggest that bone is not only an essential component of the musculoskeletal system, but is also actively involved in immune regulation. Many important factors involved in immune regulation also participate in bone homeostasis. Bone homeostasis is achieved by a coordinated action between bone-synthesizing osteoblasts and bone-degrading osteoclasts. An imbalanced action between osteoblasts and osteoclasts often results in pathological bone diseases: osteoporosis is caused by an excessive osteoclast activity, whereas osteopetrosis results from an increased osteoblast activity. This review focuses on dendritic cell-specific transmembrane protein (DC-STAMP), an important protein currently considered as a master regulator of osteoclastogenesis. Of clinical relevance, the frequency of circulating DC-STAMP+ cells is elevated during the pathogenesis of psoriatic diseases. Intriguingly, recent results suggest that DC-STAMP also plays an imperative role in bone homeostasis by regulating the differentiation of both osteoclasts and osteoblasts. This article summarizes our current knowledge on DC-STAMP by focusing on its interacting proteins, its regulation on osteoclastogenesis-related genes, its possible involvement in immunoreceptor tyrosine-based inhibitory motif (ITIM)-mediated signaling cascade, and its potential of developing therapeutics for clinical applications. J. Cell. Physiol. 231: 2402-2407, 2016. © 2016 Wiley Periodicals, Inc.

The Dark Side of Cell Fusion

Cell fusion is a physiological cellular process essential for fertilization, viral entry, muscle differentiation and placental development, among others. In this review, we will highlight the different cancer cell-cell fusions and the advantages obtained by these fusions. We will specially focus on the acquisition of metastatic features by cancer cells after fusion with bone marrow-derived cells. The mechanism by which cancer cells fuse with other cells has been poorly studied thus far, but the presence in several cancer cells of syncytin, a trophoblastic fusogen, leads us to a cancer cell fusion mechanism similar to the one used by the trophoblasts. The mechanism by which cancer cells perform the cell fusion could be an interesting target for cancer therapy.

Advanced glycation end products biphasically modulate bone resorption in osteoclast-like cells

Advanced glycation end products (AGEs) disturb bone remodeling during aging, and this process is accelerated in diabetes. However, their role in modulation of osteoclast-induced bone resorption is controversial, with some studies indicating that AGEs enhance bone resorption and others showing the opposite effect. We determined whether AGEs present at different stages of osteoclast differentiation affect bone resorption differently. Based on increased levels of tartrate-resistant acid phosphatase (TRAP) and cathepsin K (CTSK), we identified day 4 of induction as the dividing time of cell fusion stage and mature stage in RAW264.7 cell-derived osteoclast-like cells (OCLs). AGE-modified BSA (50-400 μg/ml) or control BSA (100 μg/ml) was then added at the beginning of each stage. Results showed that the presence of AGEs at the cell fusion stage reduced pit numbers, resorption area, and CTSK expression. Moreover, expression of receptor activator of nuclear factor-κB (RANK) as well as the number of TRAP-positive cells, nuclei per OCL, actin rings, and podosomes also decreased. However, the presence of AGEs at the mature stage enlarged the resorption area markedly and increased pit numbers slightly. Intriguingly, only the number of nuclei per OCL and podosomes increased. These data indicate that AGEs biphasically modulate bone resorption activity of OCLs in a differentiation stage-dependent manner. AGEs at the cell fusion stage reduce bone resorption dramatically, mainly via suppression of RANK expression in osteoclast precursors, whereas AGEs at the mature stage enhance bone resorption slightly, most likely by increasing the number of podosomes in mature OCLs.