Abstract
[Introduction] AKAP13 (Protein Kinase A Anchoring Protein 13) is a guanine nucleotides exchange factor (GEF) that activates RhoA as well as a scaffold protein integrating multiple signaling cascades. Previous study has shown that Akap13 hetero knockout mice develop osteoporotic changes and result in decreased osteogenesis and we believe AKAP13 is involved in the early stages of osteogenesis. On the other hand, the involvement of Wnt signaling in bone metabolism has been extensively studied, however the detailed mechanism has not been elucidated. Recent study showed that focal adhesion kinase (FAK)-integrin contributes to bone formation via Wnt signaling pathway. Thus, it is suggested that Akap13 may influence osteogenesis via Wnt signaling through FAK mediated integrin pathway that is linked to Rho GTPase. In this study, we investigated the involvement of AKAP13 in bone formation and the relationship with Wnt signaling. [Methods] uCT analyses of femurs of Akap13 heterozygous-conventional knock-out mice were performed. We transfected the Akap13 or control siRNA into osteoblastic cells and examined the Wnt-related genes expression and osteogenic markers by real time RT-PCR. Osteogenic marker expression was examined after application of RhoA expression vector with or without Akap13 siRNA to osteoblastic cells. [Results] Mice haploinsufficient for Akap13 (Akap13+/-) displayed reduced bone mineral density resembling the changes observed in osteoporotic bone. Colony forming unit-fibroblast numbers were diminished in Akap13+/- mice. Osteoblast numbers in periosteum and extracellular matrix production in Akap13+/- mice was reduced compared to control littermates. Runx2 and Alp mRNAs were both reduced in femora of Akap13+/- mice. Akap13 knockdown showed a significant decrease of Lef1 mRNA expression in MC3T3-E1 cells as well as a reduction of Runx2 and ALP mRNA expression in immortalized bone marrow stem cells (BMSCs). RhoA overexpression in BMSCs showed Alp mRNA increment in RhoA induction dose dependent manner. Reduction of Alp mRNA by AKAP13 knockdown was amplified in higher RhoA overexpression. [Discussion] The Akap13 knockdown showed decreased expression of Wnt-related genes in osteoblastic cells, suggesting the possibility that AKAP13 plays an important role in regulating osteogenesis through Wnt signaling. AKAP13 may be involved in or contribute directly to Wnt signaling through interaction with RhoA by mechanical stress or stimulation of extracellular matrix.<!--EndFragment-->
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