Expression and Function of Transmembrane-4 Superfamily (Tetraspanin) Proteins in Osteoclasts: Reciprocal Roles of Tspan-5 and NET-6 during Osteoclastogenesis

Background: Osteoclasts are bone-resorbing multinuclear polykaryons essential for bone remodeling, formed through cell fusion of mononuclear macrophage/monocyte lineage precursor cells upon stimulation by the RANK/RANKL system. Recent studies have revealed that a family of tetraspanin proteins, such as CD9, is critically involved in the cell fusion/polykaryon formation of these cell types. Until now, however, there is limited knowledge about the types of tetraspanins expressed in osteoclasts and their precursors. Methods: The expression of different tetraspanin proteins in a monocyte/macrophage-lineage osteoclast precursor cell line, RAW264.7, was cyclopedically investigated using RT-PCR with specific primers and quantitative real-time RT-PCR. The function of two kinds of tetraspanins, Tspan-5 and NET-6, whose expression pattern was altered by RANKL stimulation, was examined by transfecting gene-specific short-interfering RNAs into these cell types. Results: Of the 17 tetraspanins in mammalian hematopoietic cells, RAW264.7 cells express mRNA for 12 different kinds of tetraspanins, namely, CD9, CD37, CD53, CD63, CD81, CD82, CD151, NAG-2, NET-6, SAS, Tspan-3, and Tspan-5. Interestingly, during their maturation into osteoclasts upon RANKL stimulation, the transcript for Tspan-5 is up-regulated, whereas that for NET-6 is down-regulated. Targeted inhibition of Tspan-5 by using gene-specific RNA interference suppressed RANKL-induced cell fusion during osteoclastogenesis, whereas inhibition of NET-6 augmented the osteoclastogenesis itself. These results suggest that Tspan-5 and NET-6 have a reciprocal function during osteoclastogenesis, i.e., positive and negative regulation by Tspan-5 and NET-6, respectively. RANKL regulates osteoclastogenesis by altering the balances of these tetraspanin proteins. Conclusions: These data indicate that a diversity of tetraspanins is expressed in osteoclast precursors, and that cell fusion during osteoclastogenesis is regulated by cooperation of distinct tetraspanin family proteins such as Tspan-5 and NET-6. This study indicates that functional alterations of tetraspanin family proteins may have therapeutic potential in diseases where osteoclasts play a major role, such as rheumatoid arthritis and osteoporosis

Effect of correction of hindfoot valgus deformity on ankle joint pain relief in rheumatoid arthritis cases: A report of two cases

We often see painful ankle joint destruction with painful hindfoot valgus deformity in rheumatoid arthritis. Our policy in such cases has been to first correct the hindfoot deformity in the subtalar joint with fusion, but then ankle joint pain has been observed. Two women with rheumatoid arthritis underwent correction and fusion surgery for hindfoot valgus deformity. They had been using wheelchairs because of severe pain in the ankle joint and hindfoot despite extensive medical treatment. After surgery, both patients complained of no pain in the hindfoot. Furthermore, dramatic pain reduction in the ankle joint was also observed especially in a case without ankle joint instability. Consequently, the patients could walk without any support. Correction of valgus hindfoot deformity contributes to centralizing the weight-bearing line in the ankle joint, leading to ankle joint pain relief. It appears possible to preserve the ankle joint without additional ankle surgery even in rheumatoid arthritis cases, if ankle is stable