Spleen serves as a reservoir of osteoclast precursors through vitamin D-induced IL-34 expression in osteopetrotic op/op mice.
Osteoclasts are generated from monocyte/macrophage-lineage precursors in response to colony-stimulating factor 1 (CSF-1) and receptor activator of nuclear factor-κB ligand (RANKL). CSF-1-mutated CSF-1(op/op) mice as well as RANKL(-/-) mice exhibit osteopetrosis (OP) caused by osteoclast deficiency. We previously identified RANKL receptor (RANK)/CSF-1 receptor (CSF-1R) double-positive cells as osteoclast precursors (OCPs), which existed in bone in RANKL(-/-) mice. Here we show that OCPs do not exist in bone but in spleen in CSF-1(op/op) mice, and spleen acts as their reservoir. IL-34, a newly discovered CSF-1R ligand, was highly expressed in vascular endothelial cells in spleen in CSF-1(op/op) mice. Vascular endothelial cells in bone also expressed IL-34, but its expression level was much lower than in spleen, suggesting a role of IL-34 in the splenic generation of OCPs. Splenectomy (SPX) blocked CSF-1-induced osteoclastogenesis in CSF-1(op/op) mice. Osteoclasts appeared in aged CSF-1(op/op) mice with up-regulation of IL-34 expression in spleen and bone. Splenectomy blocked the age-associated appearance of osteoclasts. The injection of 2-methylene-19-nor-(20S)-1α,25(OH)(2)D(3) (2MD), a potent analog of 1α,25-dihidroxyvitamin D(3), into CSF-1(op/op) mice induced both hypercalcemia and osteoclastogenesis. Administration of 2MD enhanced IL-34 expression not only in spleen but also in bone through a vitamin D receptor-mediated mechanism. Either splenectomy or siRNA-mediated knockdown of IL-34 suppressed 2MD-induced osteoclastogenesis. These results suggest that IL-34 plays a pivotal role in maintaining the splenic reservoir of OCPs, which are transferred to bone in response to diverse stimuli, in CSF-1(op/op) mice. The present study also suggests that the IL-34 gene in vascular endothelial cells is a unique target of vitamin D.
Division of Hard Tissue Research, Institute for Oral Science, School of Dentistry, Matsumoto Dental University, Nagano 399-0781, Japan.
Yuko
Y
Nakamichi
Toshihide
T
Mizoguchi
Atsushi
A
Arai
Yasuhiro
Y
Kobayashi
Masahiro
M
Sato
Josef M
JM
Penninger
Hisataka
H
Yasuda
Shigeaki
S
Kato
Hector F
HF
DeLuca
Tatsuo
T
Suda
Nobuyuki
N
Udagawa
Naoyuki
N
Takahashi
D000818:Animals; D007150:Immunohistochemistry; D007378:Interleukins / Q000494:pharmacology*; D051379:Mice; D008822:Mice, Transgenic; D010010:Osteoclasts / Q000473:pathology*; D010022:Osteopetrosis / Q000378:metabolism / Q000473:pathology*; D013154:Spleen / Q000473:pathology*; D014807:Vitamin D / Q000494:pharmacology*
D007378:Interleukins; C552364:interleukin-34, mouse; D014807:Vitamin D