Lubricin Contributes to Homeostasis of Articular Cartilage by Modulating Differentiation of Superficial Zone Cells

Abstract

Lubricin encoded by the proteoglycan 4 (Prg4) gene is produced from superficial zone (SFZ) cells of articular cartilage and synoviocytes, which is indispensable for lubrication of joint surfaces. Loss‐of‐function of human and mouse Prg4 results in early‐onset arthropathy accompanied by lost SFZ cells and hyperplastic synovium. Here, we focused on increases in the thickness of articular cartilage in Prg4‐knockout joints and analyzed the underlying mechanisms. In the late stage of articular cartilage development, the articular cartilage was thickened at 2 to 4 weeks and the SFZ disappeared at 8 weeks in Prg4‐knockout mice. Similar changes were observed in cultured Prg4‐knockout femoral heads. Cell tracking showed that Prg4‐knockout SFZ cells at 1 week of age expanded to deep layers after 1 week. In in vitro experiments, overexpression of Prg4 lacking a mucin‐like domain suppressed differentiation of ATDC5 cells markedly, whereas pellets of Prg4‐knockout SFZ cells showed enhanced differentiation. RNA sequencing identified matrix metalloproteinase 9 (Mmp9) as the top upregulated gene by Prg4 knockout. Mmp9 expressed in the SFZ was further induced in Prg4‐knockout mice. The increased expression of Mmp9 by Prg4 knockout was canceled by IκB kinase (IKK) inhibitor treatment. Phosphorylation of Smad2 was also enhanced in Prg4‐knockout cell pellets, which was canceled by the IKK inhibitor. Expression of Mmp9 and phosphorylated Smad2 during articular cartilage development was enhanced in Prg4‐knockout joints. Lubricin contributes to homeostasis of articular cartilage by suppressing differentiation of SFZ cells, and the nuclear factor‐kappa B‐Mmp9‐TGF‐β pathway is probably responsible for the downstream action of lubricin. © 2020 American Society for Bone and Mineral Research (ASBMR).

The superficial zone of articular cartilage

“…As described above, Prg4 knockout mice display increased articular cartilage thickness [ 7 ]; however, the underlying molecular mechanism was unknown. We confirmed that the SFZ disappeared at eight weeks in the Prg4 knockout mice, and their cartilage was significantly thicker than that of wild-type (WT) mice [ 15 ]. In addition to the enhanced cartilage degeneration caused by the SFZ disappearance, ectopic endochondral ossification was observed with aging in the knockout mice [ 15 ].…”

“…We confirmed that the SFZ disappeared at eight weeks in the Prg4 knockout mice, and their cartilage was significantly thicker than that of wild-type (WT) mice [ 15 ]. In addition to the enhanced cartilage degeneration caused by the SFZ disappearance, ectopic endochondral ossification was observed with aging in the knockout mice [ 15 ]. Cell tracking indicated that Prg4 homo-knockout SFZ cells abnormally expand to the DZ layers, compared with Prg4 hetero-knockout cells [ 15 ].…”

“…Recently, we reported other roles of PRG4, such as modulating the differentiation of SFZ cells [ 15 ]. As described above, Prg4 knockout mice display increased articular cartilage thickness [ 7 ]; however, the underlying molecular mechanism was unknown.…”

Divergence in chondrogenic potential between in vitro and in vivo of adipose- and synovial-stem cells from mouse and human

“…BMP signaling is crucial for skeletogenesis, particularly for endochondral development. On the contrary, TGFβ signaling is crucial for joint development [48,49] and homeostasis of articular cartilage [42,50,51]. In fact, TGFβ1 is constantly supplied to the synovial fluid from cartilage and synovium [52,53].…”

Mechanical Cues: Bidirectional Reciprocity in the Extracellular Matrix Drives Mechano-Signalling in Articular Cartilage

“…It contributes to the highly lubricated cartilage surface [8] and is critical for SZ integrity since mice lacking Prg4 exhibit a loss of cartilage structure, change in stiffness and frictional properties [9] and develop cartilage wear as they age [10,11]. Mechanistically this may occur via the nuclear factor-κB-matrix metalloproteinase 9-transforming growth factor beta (NFκB-MMP9-TGFβ) axis since lubricin is increased by TGFβ and lubricin suppresses SZ MMP9 expression preventing chondrocyte differentiation [7,12]. Recently, β-catenin signalling has also been linked to lubricin's role in maintaining cartilage surface integrity since deletion of β-catenin in Prg4-expressing SZ cells leads to failure of the SZ and enhanced progression of osteoarthritis (OA) [13].…”