Project leader : J. Lafont
Participant : F. Mallein-Gerin, M. Pasdeloup
The articular chondrocytes are normaly under chronic hypoxia.
During development, cartilage is a transient tissue precluding the skeletal bone. Remplacement of primary cartilage by bone is possible after vascularisation which allows osteoblast precursors invasion. However, some specific location where cartilage still lies (such as articular cartilage) will stay avascular. This cartilage is at the interface between several tissues and plays a mechanical protection of long bone. Its homeostasis is regulated by simple diffusion of soluble factors released by surrounding tissues (subchondral bone, synovial fluid). Thus resident cells of adult cartilage survive in a low oxygen environment (hypoxic gradient) compared to atmosphere.
Observation of columnar chondrocytes from human explant of articular cartilage (optical microscopy).
Hypoxia regulates expression of chondrocyte markers.
During cartilage formation, chondrocytes are in a hypoxic environment, which induces expression of VEGF, a survival factor for the cells. Beyond this effect, hypoxia stimulates the expression of matrix genes specific to cartilage (coding the two main proteins of cartilage, aggrecan and type II collagen). We demonstrate using human articular chondrocytes (primary culture), hypoxia increases the expression of chondrocyte markers, among which the transcription factor SOX9. This transcription factor is part of a transcriptionnal machinery that controls the expression of COL2A1 and Aggrecan genes. Under hypoxic conditions, HIF-2alpha (Hypoxia-Inducible Factor) stimulates the expression of SOX9 and modulates a set of genes associated with the chondrocyte phenotype.
The chondrocytes are thus adapted to hypoxic life, and their normal physiology depends on these conditions. The effects of hypoxia on the biology of chondrocytes is of high interest in cartilage engineering. We study the underlying mechanisms of such a hypoxic regulation (signaling, epigenetics) in order to identify new targets and to propose molecular tools, allowing a better control of the function of chondrocyte.
Caracterisation of chondrocytes from human articular cartilage (RNA-Seq). Neovascularisation appears in the OA cartilage whereas the absence of vessels is specific of the healthy cartilage.
Selected publications :
1. Lafont JE, Poujade FA, Pasdeloup M, Neyret P, Mallein-Gerin F. (2016). Hypoxia potentiates the BMP-2 driven COL2A1 stimulation in human articular chondrocytes via p38 MAPK. Osteoarthritis & Cartilage. 2016 May ;24(5):856-67. doi : 10.1016/j.joca.2015.11.017.
2. Thoms BL, Dudek KA, Lafont JE and Murphy CL (2013). Hypoxia promotes production and inhibits destruction of human articular cartilage. Arthritis and Rheum. 2013 May 65(5)1302-12. doi : 10.1002/art.37867.
3. Dudek KA§, Lafont JE§, Martinez-Sanchez A, Murphy CL. (2010). Type II collagen expression is regulated by tissue-specific miR-675 in human articular chondrocytes. The Journal of Biol. Chem. 285 (32) : 24381-7. doi : 10.1074/jbc.M110.111328.
4. Murphy CL, BL Thoms, RJ Vaghjiani and JE Lafont (2009). HIF-mediated articular chondrocyte function: prospects for cartilage repair. Arth. Res. and Ther. 11(1):213
5. Lafont JE, Talma S, Hopfgarten C, Murphy CL (2008). Hypoxia promotes the differentiated human articular chondrocyte phenotype through SOX9-dependent and -independent pathways. The Journal of Biol. Chem. 283 (8) 4778-4786.
Pr. S. Lustig and E. Servien, Hôpital de la Croix-rousse, Lyon
Dr. R. Debret, équipe fonctionnalité et dynamique du tissu cutané (UMR 5305-LBTI)