GLUL and rheumatoid arthritis: Genes associated with cartilage homeostasis were also upregulated in infiltrating leukocytes isolated from IL-17A/F–treated mice: PDK4, which encodes pyruvate dehydrogenase kinase 4, is required for chondrocyte proliferation and is suppressed in RA joints [113], while GLUL, which encodes glutamine synthetase, has been shown to be downregulated in cartilage from patients with severe osteoarthritis [114].