CYR61/CCN1 Regulates Sclerostin Levels and Bone Maintenance

Abstract

CYR61/CCN1 is a matricellular protein that resides in the extracellular matrix, but serves regulatory rather than structural roles. CYR61/CCN1 is found in mineralized tissues and has been shown to influence bone healing in vivo and osteogenic differentiation in vitro. In this study we generated Cyr61 bone-specific knockout mice to examine the physiological role of CYR61/CCN1 in bone development and maintenance in vivo. Extensive analysis of Cyr61 conditional knockout mice showed a significant decrease in both trabecular and cortical bone mass as compared to WT littermates. Our data suggest that CYR61/CCN1 exerts its effects on mature osteoblast/osteocyte function to modulate bone mass. Specifically, changes were observed in osteocyte/osteoblast expression of RankL, VegfA, and Sost. The increase in RankL expression was correlated with a significant increase in osteoclast number; decreased VegfA expression was correlated with a significant decrease in bone vasculature; increased Sost expression was associated with decreased Wnt signaling, as revealed by decreased Axin2 expression and increased adiposity in the bone marrow. Although the decreased number of vascular elements in bone likely contributes to the low bone mass phenotype in Cyr61 conditional knockout mice, this cannot explain the observed increase in osteoclasts and the decrease in Wnt signaling. We conducted in vitro assays using UMR-106 osteosarcoma cells to explore the role CYR61/CCN1 plays in modulating Sost mRNA and protein expression in osteocytes and osteoblasts. Overexpression of CYR61/CCN1 can suppress Sost expression in both control and Cyr61 knockout cells, and blocking Sost with siRNA can rescue Wnt responsiveness in Cyr61 knockout cells in vitro. Overall, our data suggest that CYR61/CCN1 modulates mature osteoblast and osteocyte function to regulate bone mass through angiogenic effects as well as by modulating Wnt signaling, at least in part through the Wnt antagonist Sost. © 2018 American Society for Bone and Mineral Research.

YAP and TAZ Mediate Osteocyte Perilacunar/Canalicular Remodeling

“…(67) Similarly, deletion of CYR61 at various stages of the osteoblast lineage (Osterix-Cre, Collagen1 (2.3 kb)-Cre, and Osteocalcin-Cre) resulted in low bone mass phenotypes of similar severity, suggesting mature osteoblasts/osteocytes are the primary source of CYR61 in bone. (19) Deletion of CYR61 from Osteocalcin-expressing mature osteoblasts also increased osteoclast numbers in vivo, (19) consistent with the effects of osteocyte-conditional YAP/TAZ deletion here. CYR61 inhibits osteoclastogenesis in vitro through a RANKL-independent mechanism, (24) while CTGF has been observed to promote osteoclast-precursor fusion through interaction with dendritic cellspecific transmembrane protein (DC-STAMP).…”

“…(22) CTGF also enhances osteoblastogenesis in vitro, in part by inhibiting Notch signaling and inducing HES-1 transcription and NFAT transactivation. (19) Deletion of CYR61 from Osteocalcin-expressing mature osteoblasts also increased osteoclast numbers in vivo, (19) consistent with the effects of osteocyte-conditional YAP/TAZ deletion here. (67) Similarly, deletion of CYR61 at various stages of the osteoblast lineage (Osterix-Cre, Collagen1 (2.3 kb)-Cre, and Osteocalcin-Cre) resulted in low bone mass phenotypes of similar severity, suggesting mature osteoblasts/osteocytes are the primary source of CYR61 in bone.…”

“…(18,19) CYR61 and CTGF have been implicated in activation of osteoblastogenesis (20)(21)(22)(23) and inhibition of osteoclastogenesis. (18,19) CYR61 and CTGF have been implicated in activation of osteoblastogenesis (20)(21)(22)(23) and inhibition of osteoclastogenesis.…”

“…Osteoblast/osteoclast coordinated bone remodeling is regulated in part through the CCN family of matricellular growth factors, cysteine-rich angiogenic inducer-61 (CYR61, CCN1), and connective tissue growth factor (CTGF, CCN2). (18,19) CYR61 and CTGF have been implicated in activation of osteoblastogenesis (20)(21)(22)(23) and inhibition of osteoclastogenesis. (24) Both CYR61 and CTGF are downstream gene targets of the paralogous transcriptional regulators Yes associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ).…”

Osteocytes remodel bone by TGF-β-induced YAP/TAZ signaling

“…Indirect bone remodeling is regulated in part through the CCN family of matricellular growth factors, cysteine-rich angiogenic inducer-61 (Cyr61) and connective tissue growth factor (Ctgf) (17,18). Cyr61 and Ctgf have been implicated in activation of osteoblastogenesis (19)(20)(21)(22) and inhibition of osteoclastogenesis (23).…”