JNK1 is not essential for TNF-mediated joint disease

Abstract

Tumour necrosis factor (TNF) signalling molecules are considered as promising therapeutic targets of antirheumatic therapy. Among them, mitogen-activated protein kinases are thought to be of central importance. Herein, we investigate the role in vivo of TNF-alpha signalling through c-Jun N-terminal kinase (JNK)1 in destructive arthritis. Human TNF transgenic (hTNFtg) mice, which develop inflammatory arthritis, were intercrossed with JNK1-deficient (JNK1-/-) mice. Animals (n = 35) of all four genotypes (wild-type, JNK1-/-, hTNFtg, JNK1-/-hTNFtg) were assessed for clinical and histological signs of arthritis. Clinical features of arthritis (swelling and decreased grip strength) developed equally in hTNFtg and JNK1-/-hTNFtg mice. Histological analyses revealed no differences in the quantity of synovial inflammation and bone erosions or in the cellular composition of the synovial infiltrate. Bone destruction and osteoclast formation were observed to a similar degree in hTNFtg and JNK1-/-hTNFtg animals. Moreover, cartilage damage, as indicated by proteoglycan loss in the articular cartilage, was comparable in the two strains. Intact phosphorylation of JNK and c-Jun as well as expression of JNK2 in the synovial tissue of JNK1-/-hTNFtg mice suggests that signalling through JNK2 may compensate for the deficiency in JNK1. Thus, JNK1 activation does not seem to be essential for TNF-mediated arthritis.

Figure 1

Clinical course of arthritis in human tumour necrosis factor transgenic (hTNFtg) and JNK1^-/-hTNFtg mice. Joint swelling (a) and grip strength (b) were assessed. No statistically significant differences between the two groups were detected. Vertical bars indicate standard deviation. Control animals (wild-type and JNK1^-/-) showed no signs of arthritis (not shown). JNK, c-Jun N-terminal kinase.

Figure 2

Histological assessment of synovial inflammation and joint destruction. No signs of arthritis are seen in the tarsal joints of wild-type (wt) and JNK1 (c-Jun N-terminal kinase 1) knockout (JNK1^-/-) mice, whereas severe inflammation (I) and numerous erosions (E) are observed in human TNF transgenic (hTNFtg) and intercrossed (JNK1^-/-hTNFtg) mice. H-&-E-stained sections; magnification 50×.

Figure 3

Quantitative analysis of synovial inflammation and joint destruction. There were no differences in the extent of inflammatory tissue between human tumour necrosis factor transgenic (hTNFtg) and intercrossed (JNK1^-/-hTNFtg) mice. Mean areas of inflammation (a) and erosions (b) were comparable in these two strains. Vertical bars indicate standard deviation. JNK, c-Jun N-terminal kinase.

Figure 4

Synovial osteoclasts in human tumour necrosis factor transgenic (hTNFtg) and JNK1^-/-hTNFtg mice. As demonstrated by staining with tartrate-resistant acid phosphatase (a), there are abundant osteoclasts (OC) at the site of erosions in hTNFtg and intercrossed (JNK1^-/-hTNFtg) mice, which all developed arthritis (magnification 100×). The number of osteoclasts was similar in the two animal groups (b). Vertical bars indicate standard deviation. JNK, c-Jun N-terminal kinase.

Figure 5

Evaluation of proteoglycan loss in human tumour necrosis factor transgenic (hTNFtg) and JNK1^-/-hTNFtg mice. As shown by toluidine blue staining (a), arthritis leads to loss of proteoglycan (arrows) in the cartilage of hTNFtg and intercrossed JNK1^-/-hTNFtg mice (magnification 100×). The area of early cartilage damage (b) was similar in the two groups with arthritis, whereas the joints of wild-type (wt) and JNK1 knockout (JNK1^-/-) animals were unaffected. Vertical bars indicate standard deviation. JNK, c-Jun N-terminal kinase.

Figure 6

Analysis of JNK activation in human tumour necrosis factor transgenic (hTNFtg) and JNK1^-/-/hTNFtg mice. Synovial inflammatory tissue of the mice were stained with antibodies against the phosphorylated form of JNK (a), total JNK2 (b), and the phosphorylated forms of c-Jun (c). Positive cells are stained in brown (arrows; magnification 400×). Results of quantification (percentages of positive cells) are given in separate bar charts. Vertical bars indicate standard deviation. JNK, c-Jun N-terminal kinase; p-c-Jun, phosphorylated c-Jun; p-JNK, phosphorylated JNK. 1 cm = (a) 40 μm, (b, c) 30 μm.