Role of HTRA1 in bone formation and regeneration: In vitro and in vivo evaluation

Abstract

The role of mammalian high temperature requirement protease A1 (HTRA1) in somatic stem cell differentiation and mineralized matrix formation remains controversial, having been demonstrated to impart either anti- or pro-osteogenic effects, depending on the in vitro cell model used. The aim of this study was therefore to further evaluate the role of HTRA1 in regulating the differentiation potential and lineage commitment of murine mesenchymal stem cells in vitro, and to assess its influence on bone structure and regeneration in vivo. Our results demonstrated that short hairpin RNA-mediated ablation of Htra1 in the murine mesenchymal cell line C3H10T1/2 increased the expression of several osteogenic gene markers, and significantly enhanced matrix mineralization in response to BMP-2 stimulation. These effects were concomitant with decreases in the expression of chondrogenic gene markers, and increases in adipogenic gene expression and lipid accrual. Despite the profound effects of loss-of-function of HTRA1 on this in vitro osteochondral model, these were not reproduced in vivo, where bone microarchitecture and regeneration in 16-week-old Htra1-knockout mice remained unaltered as compared to wild-type controls. By comparison, analysis of femurs from 52-week-old mice revealed that bone structure was better preserved in Htra1-knockout mice than age-matched wild-type controls. These findings therefore provide additional insights into the role played by HTRA1 in regulating mesenchymal stem cell differentiation, and offer opportunities for improving our understanding of how this multifunctional protease may act to influence bone quality.

HtrA1 Is Specifically Up-Regulated in Active Keloid Lesions and Stimulates Keloid Development

“…These observations imply that HtrA1 plays a role in the pathogenesis of various diseases by modulating proteins in the ECM or cell surface. Although controversial, HtrA1 has been proposed as a key molecule in osteogenesis and chondrogenesis [ 14 , 17 , 18 ]. HtrA1 expression is induced during hypertrophic change in chondrocytes, with the up-regulation of the type X collagen marker in keloid lesions [ 9 , 18 ].…”

Murine osteoclasts secrete serine protease HtrA1 capable of degrading osteoprotegerin in the bone microenvironment

“…HtrA3 may complement the function of HtrA1 in humans. The deletion of HtrA1 in mice has been shown to influence the aged bone phenotype; bone volume and repair from fractures were not affected in 16-week-old HtrA1-deficient mice, but bone volume increased significantly in 52-week-old HtrA1-deficient mice 31 . HtrA1 deficiency may have suppressed osteoclastogenesis due to the reduced RANKL/OPG ratio in old mice.…”

“…10). In this regard, it appears that there is redundancy in proteolytic activity between HtrA3 and HtrA1 31 . We confirmed that HtrA3 is also able to degrade OPG (Supplementary Fig.…”

“…Bone matrix proteins, such as fibronectin and aggrecan, are substrates of HtrA1 35,36 . HtrA1 has been reported to play a role in the suppression of signals induced by bone morphogenetic proteins and transforming growth factor β via changes in extracellular matrix degradation 17,18,31 . HtrA1 was also found to be expressed at the ossification point in bone marrow, and that the expression was upregulated with the maturation of osteoblasts 32,41 .…”

Molecular structure and the role of high‐temperature requirement protein 1 in skeletal disorders and cancers

“…Some inconsistent results regarding the effects of HTRA1 on bone formation may be due to complex interactions between HTRA1 and TGF‐β family proteins in different cells and dosages of HTRA1 used in each experimental design. Further understanding the role of HTRA1 in bone formation and bone resorption might help to develop HTRA1 a therapeutic target in bone metabolic diseases such as osteoporosis …”