Site-1 Protease Is Essential to Growth Plate Maintenance and Is a Critical Regulator of Chondrocyte Hypertrophic Differentiation in Postnatal Mice

Abstract

Site-1 protease (S1P) is a proprotein convertase with essential functions in lipid homeostasis and unfolded protein response pathways. We previously studied a mouse model of cartilage-specific knock-out of S1P in chondroprogenitor cells. These mice exhibited a defective cartilage matrix devoid of type II collagen protein (Col II) and displayed chondrodysplasia with no endochondral bone formation even though the molecular program for endochondral bone development appeared intact. To gain insights into S1P function, we generated and studied a mouse model in which S1P is ablated in postnatal chondrocytes. Postnatal ablation of S1P results in chondrodysplasia. However, unlike early embryonic ablations, the growth plates of these mice exhibit a lack of Ihh, PTHrP-R, and Col10 expression indicating a loss of chondrocyte hypertrophic differentiation and thus disruption of the molecular program required for endochondral bone development. S1P ablation results in rapid growth plate disruption due to intracellular Col II entrapment concomitant with loss of chondrocyte hypertrophy suggesting that these two processes are related. Entrapment of Col II in the chondrocytes of the prospective secondary ossification center precludes its development. Trabecular bone formation is dramatically diminished in the primary spongiosa and is eventually lost. The primary growth plate is eradicated by apoptosis but is gradually replaced by a fully functional new growth plate from progenitor stem cells capable of supporting new bone growth. Our study thus demonstrates that S1P has fundamental roles in the preservation of postnatal growth plate through chondrocyte differentiation and Col II deposition and functions to couple growth plate maturation to trabecular bone development in growing mice.Site-1 protease (S1P 3 ; also known as the membrane-bound transcription factor protease, site-1) is a proprotein convertase that plays a vital role in maintaining lipid homeostasis and unfolded protein response through regulated intramembrane proteolysis pathways. During lipid homeostasis, S1P is involved in the proteolytic maturation of the latent endoplasmic reticulum membrane-bound sterol regulatory element-binding protein transcription factors that direct the synthesis of proteins involved in cholesterol and fatty acid synthesis and uptake (1, 2). During unfolded protein response, S1P is involved in the maturation of the endoplasmic reticulum membrane-bound transcription factors ATF6 (3), old astrocyte specifically induced substance (4), and cAMP-responsive element-binding protein H (5).Several recent studies have also implicated a role for S1P in skeletal development, although the exact molecular nature of this requirement is not known. Mammalian skeletal development takes place largely through the process of endochondral ossification in which mesenchymal cell condensates differentiate to form chondrocytes. Chondrocytes secrete the type II collagen (Col II)-rich cartilaginous anlagen to form a template for the development of the skeletal structura...

Endoplasmic Reticulum (ER) Stress Inducible Factor Cysteine-Rich with EGF-Like Domains 2 (Creld2) Is an Important Mediator of BMP9-Regulated Osteogenic Differentiation of Mesenchymal Stem Cells

“…The C/EBP family member CHOP is a multifunctional ER-induced transcription factor and also involved in regulating bone formation [38]–[40]. It has been reported that the Site-1 protease (S1P) is necessary for a specialized ER stress response required for endochondral ossification and growth plate development [41], [42]. Furthermore, we have recently demonstrated that MAPK signaling pathway is involved in regulating BMP9-induced osteogenic differentiation [43], [44].…”

Effects of aging on articular cartilage homeostasis

“…Site-1 protease regulates components of UPR pathways. Cartilage specific deletions results in abnormal growth plate development due to intracellular accumulation of type II collagen and chondrocyte apoptosis[131]. …”

Site-1 protease regulates skeletal stem cell population and osteogenic differentiation in mice

“…8J), just above the hypertrophic zone of the growth plate. In previous studies, apoptosis was effected by abnormal proCol IIB entrapment that induced UPR and delayed SOC formation (Patra et al, 2011). However, no abnormal Col II entrapment was observed at P5 (Fig.…”