Signal Transduction by the JNK Group of MAP Kinases

Cells respond to changes in the physical and chemical properties of the environment. These changes include alterations in the amount of nutrients, growth factors, cytokines, and adhesion to the cell matrix. In addition, cells respond to physical stimulation mediated by osmolarity, heat, pH, redox, radiation, and mechanical stress. These physical and chemical cues control many aspects of cell function including migration, proliferation, differentiation, and death. The decision making process that cells employ to mount an appropriate response to a specific stimulus is critical for normal life. Many signal transduction pathways cooperate and participate in this process. Recent studies have established that mitogen-activated protein kinases (MAPK) play an important regulatory role. Genetic studies have identified five MAPK pathways in the budding yeast Saccharomyces cerevisiae (91Schaeffer H.J Weber M.J Mitogen-activated protein kinases specific messages from ubiquitous messengers.Mol. Cell. Biol. 1999; 19: 2435-2444Crossref PubMed Google Scholar). These MAPK are essential for mating (Fus3p), osmoregulation (Hog1p), sporulation (Smk1p), cell wall biosynthesis (Smk1p), and filamentation (Kss1p), and these enzymes form a group with related structures and biochemical properties. Each MAPK is activated by dual phosphorylation of a tripeptide motif (Thr-Xaa-Tyr) located in the activation loop (T-loop). This phosphorylation is mediated by a MAPK kinase (MAPKK) that is activated by phosphorylation by a MAPKK kinase (MAPKKK). These MAPK are therefore activated by a kinase signaling cascade. MAPK signaling pathways have also been identified in higher organisms. In mammals, three major groups of MAPK have been identified (91Schaeffer H.J Weber M.J Mitogen-activated protein kinases specific messages from ubiquitous messengers.Mol. Cell. Biol. 1999; 19: 2435-2444Crossref PubMed Google Scholar). Each of these groups of MAPK is activated by a protein kinase cascade (Figure 1). The ERK and p38 groups of MAPK are related to enzymes found in budding yeast and contain the dual phosphorylation motifs Thr-Glu-Tyr and Thr-Gly-Tyr, respectively. The c-Jun NH2-terminal kinases (JNK), also known as stress-activated MAP kinases (SAPK), represent a third group of MAPK that has been identified in mammals. JNK contains the dual phosphorylation motif Thr-Pro-Tyr. Biochemical studies led to the identification and purification of JNK as a “p54 microtubule-associated protein kinase” that was activated by cycloheximide (59Kyriakis J.M Avruch J pp54 microtubule-associated protein 2 kinase. A novel serine/threonine protein kinase regulated by phosphorylation and stimulated by poly-L-lysine.J. Biol. Chem. 1990; 265: 17355-17363Abstract Full Text PDF PubMed Google Scholar). JNK was found to bind the NH2-terminal activation domain of c-Jun (2Adler V Polotskaya A Wagner F Kraft A.S Affinity-purified c-Jun amino-terminal protein kinase requires serine/threonine phosphorylation for activity.J. Biol. Chem. 1992; 267: 17001-17005Abstract Full Text PDF PubMed Google Scholar, 41Hibi M Lin A Smeal T Minden A Karin M Identification of an oncoprotein- and UV-responsive protein kinase that binds and potentiates the c-Jun activation domain.Genes Dev. 1993; 7: 2135-2148Crossref PubMed Google Scholar) and to phosphorylate c-Jun on Ser-63 and Ser-73 (84Pulverer B.J Kyriakis J.M Avruch J Nikolakaki E Woodgett J.R Phosphorylation of c-jun mediated by MAP kinases.Nature. 1991; 353: 670-674Crossref PubMed Scopus (752) Google Scholar). Subsequently, JNK was molecularly cloned (22Derijard B Hibi M Wu I.H Barrett T Su B Deng T Karin M Davis R.J JNK1 a protein kinase stimulated by UV light and Ha-Ras that binds and phosphorylates the c-Jun activation domain.Cell. 1994; 76: 1025-1037Abstract Full Text PDF PubMed Scopus (2492) Google Scholar, 60Kyriakis J.M Banerjee P Nikolakaki E Dai T Rubie E.A Ahmad M.F Avruch J Woodgett J.R The stress-activated protein kinase subfamily of c-Jun kinases.Nature. 1994; 369: 156-160Crossref PubMed Scopus (2046) Google Scholar). JNK is activated by treatment of cells with cytokines (e.g., TNF and IL-1) and by exposure of cells to many forms of environmental stress (e.g., osmotic stress, redox stress, and radiation) (reviewed by 44Ip Y.T Davis R.J Signal transduction by the c-Jun N-terminal kinase (JNK)—from inflammation to development.Curr. Opin. Cell Biol. 1998; 10: 205-219Crossref PubMed Scopus (1138) Google Scholar). The purpose of this review is to summarize recent advances that have been made toward understanding the JNK signaling pathway. It is now known that JNK is required for embryonic morphogenesis and that this signaling pathway contributes to the regulation of cell proliferation and apoptosis. JNK also contributes to the function of some differentiated cells. Thus, the JNK signal transduction pathway is implicated in multiple physiological processes. Phosphorylation of c-Jun on the sites that are phosphorylated by JNK (Ser-63 and Ser-73) causes increased transcription activity (84Pulverer B.J Kyriakis J.M Avruch J Nikolakaki E Woodgett J.R Phosphorylation of c-jun mediated by MAP kinases.Nature. 1991; 353: 670-674Crossref PubMed Scopus (752) Google Scholar, 97Smeal T Binetruy B Mercola D.A Birrer M Karin M Oncogenic and transcriptional cooperation with Ha-Ras requires phosphorylation of c-Jun on serines 63 and 73.Nature. 1991; 354: 494-496Crossref PubMed Scopus (485) Google Scholar). Interestingly, JNK also phosphorylates other AP-1 proteins, including JunB, JunD, and ATF2 (44Ip Y.T Davis R.J Signal transduction by the c-Jun N-terminal kinase (JNK)—from inflammation to development.Curr. Opin. Cell Biol. 1998; 10: 205-219Crossref PubMed Scopus (1138) Google Scholar). In each case, the sites of phosphorylation correspond to Ser/Thr-Pro motifs located in the activation domain of the transcription factor. Substrate recognition by JNK requires a docking site to tether the kinase to the substrate (41Hibi M Lin A Smeal T Minden A Karin M Identification of an oncoprotein- and UV-responsive protein kinase that binds and potentiates the c-Jun activation domain.Genes Dev. 1993; 7: 2135-2148Crossref PubMed Google Scholar). The mechanism that accounts for JNK-dependent regulation of AP-1 transcription activity is unclear, but a role for the coactivator CBP/p300 has been proposed (5Arias J Alberts A.S Brindle P Claret F.X Smeal T Karin M Feramisco J Montminy M Activation of cAMP and mitogen responsive genes relies on a common nuclear factor.Nature. 1994; 370: 226-229Crossref PubMed Scopus (589) Google Scholar). Additional JNK-dependent processes may also contribute to the regulation of AP-1 activity. Thus, JNK may regulate the intrinsic histone acetylase activity of ATF2 (51Kawasaki H Schiltz L Chiu R Itakura K Taira K Nakatani Y Yokoyama K.K ATF-2 has intrinsic histone acetyltransferase activity which is modulated by phosphorylation.Nature. 2000; 405: 195-200Crossref PubMed Scopus (162) Google Scholar) and may regulate the ubiquitin-mediated degradation of AP-1 proteins (31Fuchs S.Y Fried V.A Ronai Z Stress-activated kinases regulate protein stability.Oncogene. 1998; 17 (c): 1483-1490Crossref PubMed Google Scholar). A critical role for JNK appears to be the regulation of AP-1 transcription activity. This conclusion is supported by genetic analysis of Jun and JNK in Drosophila and by the analysis of AP-1 transcription activity in murine cells with targeted disruptions of genes that encode components of the JNK pathway (reviewed by 44Ip Y.T Davis R.J Signal transduction by the c-Jun N-terminal kinase (JNK)—from inflammation to development.Curr. Opin. Cell Biol. 1998; 10: 205-219Crossref PubMed Scopus (1138) Google Scholar). JNK appears to be essential for AP-1 activation caused by stress and some cytokines, but is not required for AP-1 activation in response to other stimuli (121Yang D Tournier C Wysk M Lu H.T Xu J Davis R.J Flavell R.A Targeted disruption of the MKK4 gene causes embryonic death, inhibition of c-Jun NH2-terminal kinase activation, and defects in AP-1 transcriptional activity.Proc. Natl. Acad. Sci. USA. 1997; 94 (a): 3004-3009Crossref PubMed Scopus (225) Google Scholar). The precise role of AP-1 in the response to JNK activation is likely to be modified by the activity of other transcription factors that interact with AP-1 on the promoters of target genes. The JNK protein kinases are encoded by three genes (Table 1). The Jnk1 and Jnk2 genes are expressed ubiquitously. In contrast, the Jnk3 gene has a more limited pattern of expression and is largely restricted to brain, heart, and testis. These genes are alternatively spliced to create ten JNK isoforms (37Gupta S Barrett T Whitmarsh A.J Cavanagh J Sluss H.K Derijard B Davis R.J Selective interaction of JNK protein kinase isoforms with transcription factors.EMBO J. 1996; 15: 2760-2770Crossref PubMed Scopus (0) Google Scholar). Transcripts derived from all three genes encode proteins with and without a COOH-terminal extension to create both 46 kDa nd 55 kDa isoforms. The functional significance of these splice variants is unclear. A second form of alternative splicing is restricted to the Jnk1 and Jnk2 genes and involves the selection of one of two alternative exons that encodes part of the kinase domain. This alternative splicing influences the substrate specificity of the JNK isoforms by altering the ability of JNK to interact with docking sites on substrates (37Gupta S Barrett T Whitmarsh A.J Cavanagh J Sluss H.K Derijard B Davis R.J Selective interaction of JNK protein kinase isoforms with transcription factors.EMBO J. 1996; 15: 2760-2770Crossref PubMed Scopus (0) Google Scholar). These docking sites are present in MAPKK, MAPK phosphatases, and MAPK substrates and appear to mediate interactions with a common surface on JNK (104Tanoue T Adachi M Moriguchi T Nishida E A conserved docking motif in MAP kinases common to substrates, activators and regulators.Nat. Cell Biol. 2000; 2: 110-116Crossref PubMed Scopus (491) Google Scholar). Initial studies indicated that the docking and substrate specificities of JNK1 and JNK2 were different. For example, c-Jun was preferentially bound and phosphorylated by JNK1, while ATF2 was preferentially bound and phosphorylated by JNK2 (48Kallunki T Su B Tsigelny I Sluss H.K Derijard B Moore G Davis R Karin M JNK2 contains a specificity-determining region responsible for efficient c-Jun binding and phosphorylation.Genes Dev. 1994; 8: 2996-3007Crossref PubMed Google Scholar, 95Sluss H.K Barrett T Derijard B Davis R.J Signal transduction by tumor necrosis factor mediated by JNK protein kinases.Mol. Cell. Biol. 1994; 14: 8376-8384Crossref PubMed Google Scholar). However, it is now clear that these differences reflect the particular spliced isoforms that were examined. Different tissues express distinct repertoires of spliced JNK isoforms and the particular spliced isoform that preferentially targets a specific substrate can be encoded by either the Jnk1 or the Jnk2 genes (37Gupta S Barrett T Whitmarsh A.J Cavanagh J Sluss H.K Derijard B Davis R.J Selective interaction of JNK protein kinase isoforms with transcription factors.EMBO J. 1996; 15: 2760-2770Crossref PubMed Scopus (0) Google Scholar). The analysis of Jnk gene disruptions in mice confirms that there is extensive complementation between the Jnk genes and that there are also tissue-specific defects in signal transduction that may reflect the JNK isoform profile of individual tissues (Table 1). This complicates the analysis of Jnk knockout mice and indicates the need for studies of compound mutants that lack expression of all JNK isoforms (107Tournier C Hess P Yang D.D Xu J Turner T.K Nimnual A Bar-Sagi D Jones S.N Flavell R.A Davis R.J Requirement of JNK for stress-induced activation of the cytochrome c-mediated death pathway.Science. 2000; 288: 870-874Crossref PubMed Scopus (1204) Google Scholar).Table 1Components of the Mammalian JNK Signal Transduction PathwayAlternative NameJNK Pathway CharacterizationaReferences: 1Ichijo, H., et al. Science 275, 90–94 (1997); 2Wang, X.S., et al. J. Biol. Chem. 271, 31607–31611 (1996); 3Wang, X.S., et al. Biochem. Biophys. Res. Commun. 253, 33–37 (1998); 4Hirai, S., et al. Oncogene 12, 641–650 (1996); 5Fan, G., et al. J. Biol. Chem. 271, 24788–24793 (1996); 6Sakuma, H., et al. J. Biol. Chem. 272, 28622–28629 (1997); 7Minden, A., et al. Science 266, 1719–1723 (1994); 8Yan, M., et al. Nature 372, 798–800 (1994); 9Lange-Carter, C.A., et al. Science 260, 315–319 (1993); 10Yujiri, T., et al. J. Biol. Chem. 274, 12605–12610 (1999); 11Minamino, T., et al. Proc. Natl. Acad. Sci. USA 96, 15127–15132 (1999); 12Yujiri, T., et al. Proc. Natl. Acad. Sci. USA 97, 7272–7277 (2000); 13Yujiri, T., et al. Science 282, 1911–1914 (1998); 14Xia, Y., et al. Proc. Natl. Acad. Sci. USA 97, 5243–5248 (2000); 15Blank, J.L., et al. J. Biol. Chem. 271, 5361–5368 (1996); 16Yang, J., et al. Nat. Genet. 24, 309–313 (2000); 17Gerwins, P., et al. J. Biol. Chem. 272, 8288–8295 (1997); 18Takekawa, M., et al. EMBO J. 16, 4973–4982 (1997); 19Dorow, D.S., et al. Eur. J. Biochem. 213, 701–710 (1993); 20Hirai, S., et al. J. Biol. Chem. 272, 15167–15173 (1997); 21Rana, A., et al. J. Biol. Chem. 271, 19025–19028 (1996); 22Teramoto, H., et al. J. Biol. Chem. 271, 27225–27228 (1996); 23Tibbles, L.A., et al. EMBO J. 15, 7026–7035 (1996); 24Yamaguchi, K., et al. Science 270, 2008–2011 (1995); 25Salmeron, A., et al. EMBO J. 15, 817–826 (1996); 26Derijard, B., et al. Science 267, 682–685 (1995); 27Lin, A., et al. Science 268, 286–290 (1995); 28Swat, W., et al. Immunity 8, 625–634 (1998); 29Yang, D., et al. Proc. Natl. Acad. Sci. USA 94, 3004–3009 (1997); 30Nishina, H., et al. J. Exp. Med. 186, 941–953 (1997); 31Nishina, H., et al. Development 126, 505–516 (1999); 32Ganiatsas, S., et al. Proc. Natl. Acad. Sci. USA 95, 6881–6886 (1998); 33Tournier, C., et al. Proc. Natl. Acad. Sci. USA 94, 7337–7342 (1997); 34Moriguchi, T., et al. EMBO J. 16, 7045–7053 (1997); 35Yao, Z., et al. J. Biol. Chem. 272, 32378–32383 (1997); 36Wu, Z., et al. Mol. Cell Biol. 17, 7407–7416 (1997); 37Lu, X., et al. J. Biol. Chem. 272, 24751–24754 (1997); 38Lawler, S., et al. FEBS Lett. 414, 153–158 (1997); 39Dong, C., et al. Nature 405, 91–94 (2000); 40Derijard, B., et al. Cell 76, 1025–1037 (1994); 41Sanchez, I., et al. Nature 372, 794–798 (1994); 42Gupta, S., et al. EMBO J. 15, 2760–2770 (1996); 43Dong, C., et al. Science 282, 2092–2095 (1998); 44Tournier, C., et al. Science 288, 870–874 (2000); 45Kuan, C.Y., et al. Neuron 22, 667–676 (1999); 46Sabapathy, K., et al. Mech. Dev. 89, 115–124 (1999); 47Constant, S.L., et al. J. Immunol 165, 2671–2676 (2000); 48Kallunki, T., et al. Genes Dev 8, 2996–3007 (1994); 49Sluss, H.K., et al. Mol. Cell. Biol. 14, 8376–8384 (1994); 50Yang, D.D., et al. Immunity 9, 575–585 (1998); 51Sabapathy, K., et al. Curr. Biol. 9, 116–125 (1999); 52Chu, W.M., et al. Immunity 11, 721–731 (1999); 53Mohit, A.A., et al. Neuron 14, 67–78 (1995); 54Yang, D.D., et al. Nature 389, 865–870 (1997); 55Dickens, M., et al. Science 277, 693–696 (1997); 56Whitmarsh, A.J., et al. Science 281, 1671–1674 (1998); 57Bonny, C., et al. J. Biol. Chem. 273, 1843–1846 (1998); 58Negri, S., et al. Genomics 64, 324–330 (2000); 59Yasuda, J., et al. Mol. Cell. Biol. 19, 7245–7254 (1999); 60Ito, M., et al. Mol. Cell. Biol. 19, 7539–7548 (1999); 61Kelkar, N., et al. Mol. Cell. Biol. 20, 1030–1043 (2000).Gene DisruptionaReferences: 1Ichijo, H., et al. Science 275, 90–94 (1997); 2Wang, X.S., et al. J. Biol. Chem. 271, 31607–31611 (1996); 3Wang, X.S., et al. Biochem. Biophys. Res. Commun. 253, 33–37 (1998); 4Hirai, S., et al. Oncogene 12, 641–650 (1996); 5Fan, G., et al. J. Biol. Chem. 271, 24788–24793 (1996); 6Sakuma, H., et al. J. Biol. Chem. 272, 28622–28629 (1997); 7Minden, A., et al. Science 266, 1719–1723 (1994); 8Yan, M., et al. Nature 372, 798–800 (1994); 9Lange-Carter, C.A., et al. Science 260, 315–319 (1993); 10Yujiri, T., et al. J. Biol. Chem. 274, 12605–12610 (1999); 11Minamino, T., et al. Proc. Natl. Acad. Sci. USA 96, 15127–15132 (1999); 12Yujiri, T., et al. Proc. Natl. Acad. Sci. USA 97, 7272–7277 (2000); 13Yujiri, T., et al. Science 282, 1911–1914 (1998); 14Xia, Y., et al. Proc. Natl. Acad. Sci. USA 97, 5243–5248 (2000); 15Blank, J.L., et al. J. Biol. Chem. 271, 5361–5368 (1996); 16Yang, J., et al. Nat. Genet. 24, 309–313 (2000); 17Gerwins, P., et al. J. Biol. Chem. 272, 8288–8295 (1997); 18Takekawa, M., et al. EMBO J. 16, 4973–4982 (1997); 19Dorow, D.S., et al. Eur. J. Biochem. 213, 701–710 (1993); 20Hirai, S., et al. J. Biol. Chem. 272, 15167–15173 (1997); 21Rana, A., et al. J. Biol. Chem. 271, 19025–19028 (1996); 22Teramoto, H., et al. J. Biol. Chem. 271, 27225–27228 (1996); 23Tibbles, L.A., et al. EMBO J. 15, 7026–7035 (1996); 24Yamaguchi, K., et al. Science 270, 2008–2011 (1995); 25Salmeron, A., et al. EMBO J. 15, 817–826 (1996); 26Derijard, B., et al. Science 267, 682–685 (1995); 27Lin, A., et al. Science 268, 286–290 (1995); 28Swat, W., et al. Immunity 8, 625–634 (1998); 29Yang, D., et al. Proc. Natl. Acad. Sci. USA 94, 3004–3009 (1997); 30Nishina, H., et al. J. Exp. Med. 186, 941–953 (1997); 31Nishina, H., et al. Development 126, 505–516 (1999); 32Ganiatsas, S., et al. Proc. Natl. Acad. Sci. USA 95, 6881–6886 (1998); 33Tournier, C., et al. Proc. Natl. Acad. Sci. USA 94, 7337–7342 (1997); 34Moriguchi, T., et al. EMBO J. 16, 7045–7053 (1997); 35Yao, Z., et al. J. Biol. Chem. 272, 32378–32383 (1997); 36Wu, Z., et al. Mol. Cell Biol. 17, 7407–7416 (1997); 37Lu, X., et al. J. Biol. Chem. 272, 24751–24754 (1997); 38Lawler, S., et al. FEBS Lett. 414, 153–158 (1997); 39Dong, C., et al. Nature 405, 91–94 (2000); 40Derijard, B., et al. Cell 76, 1025–1037 (1994); 41Sanchez, I., et al. Nature 372, 794–798 (1994); 42Gupta, S., et al. EMBO J. 15, 2760–2770 (1996); 43Dong, C., et al. Science 282, 2092–2095 (1998); 44Tournier, C., et al. Science 288, 870–874 (2000); 45Kuan, C.Y., et al. Neuron 22, 667–676 (1999); 46Sabapathy, K., et al. Mech. Dev. 89, 115–124 (1999); 47Constant, S.L., et al. J. Immunol 165, 2671–2676 (2000); 48Kallunki, T., et al. Genes Dev 8, 2996–3007 (1994); 49Sluss, H.K., et al. Mol. Cell. Biol. 14, 8376–8384 (1994); 50Yang, D.D., et al. Immunity 9, 575–585 (1998); 51Sabapathy, K., et al. Curr. Biol. 9, 116–125 (1999); 52Chu, W.M., et al. Immunity 11, 721–731 (1999); 53Mohit, A.A., et al. Neuron 14, 67–78 (1995); 54Yang, D.D., et al. Nature 389, 865–870 (1997); 55Dickens, M., et al. Science 277, 693–696 (1997); 56Whitmarsh, A.J., et al. Science 281, 1671–1674 (1998); 57Bonny, C., et al. J. Biol. Chem. 273, 1843–1846 (1998); 58Negri, S., et al. Genomics 64, 324–330 (2000); 59Yasuda, J., et al. Mol. Cell. Biol. 19, 7245–7254 (1999); 60Ito, M., et al. Mol. Cell. Biol. 19, 7539–7548 (1999); 61Kelkar, N., et al. Mol. Cell. Biol. 20, 1030–1043 (2000).MAPKKKASK1MAPKKK51, 2ASK2MAPKKK63DLKMUK, ZPK4, 5LZK6MEKK17–910–14MEKK21510MEKK31516MEKK4MTK117, 18MLK119MLK2MST20MLK3SPRK, PTK121–23TAK124Tpl-2Cot25MAPKKMKK4SEK1, SERK1, SKK1, JNKK18, 26, 2728–32MKK7SEK2, SKK4, JNKK233–3839MAPKJNK1SAPKγ, SAPK1c40–4239, 43–47JNK2SAPKα, SAPK1a41, 42, 48, 4939, 44–46, 50–52JNK3SAPKβ, SAPK1b, p49F1241, 42, 5354ScaffoldsJIP1IB155–57JIP2IB258, 59JIP3JSAP60, 61a References: 1Ichijo, H., et al. Science 275, 90–94 (1997); 2Wang, X.S., et al. J. Biol. Chem. 271, 31607–31611 (1996); 3Wang, X.S., et al. Biochem. Biophys. Res. Commun. 253, 33–37 (1998); 4Hirai, S., et al. Oncogene 12, 641–650 (1996); 5Fan, G., et al. J. Biol. Chem. 271, 24788–24793 (1996); 6Sakuma, H., et al. J. Biol. Chem. 272, 28622–28629 (1997); 7Minden, A., et al. Science 266, 1719–1723 (1994); 8Yan, M., et al. Nature 372, 798–800 (1994); 9Lange-Carter, C.A., et al. Science 260, 315–319 (1993); 10Yujiri, T., et al. J. Biol. Chem. 274, 12605–12610 (1999); 11Minamino, T., et al. Proc. Natl. Acad. Sci. USA 96, 15127–15132 (1999); 12Yujiri, T., et al. Proc. Natl. Acad. Sci. USA 97, 7272–7277 (2000); 13Yujiri, T., et al. Science 282, 1911–1914 (1998); 14Xia, Y., et al. Proc. Natl. Acad. Sci. USA 97, 5243–5248 (2000); 15Blank, J.L., et al. J. Biol. Chem. 271, 5361–5368 (1996); 16Yang, J., et al. Nat. Genet. 24, 309–313 (2000); 17Gerwins, P., et al. J. Biol. Chem. 272, 8288–8295 (1997); 18Takekawa, M., et al. EMBO J. 16, 4973–4982 (1997); 19Dorow, D.S., et al. Eur. J. Biochem. 213, 701–710 (1993); 20Hirai, S., et al. J. Biol. Chem. 272, 15167–15173 (1997); 21Rana, A., et al. J. Biol. Chem. 271, 19025–19028 (1996); 22Teramoto, H., et al. J. Biol. Chem. 271, 27225–27228 (1996); 23Tibbles, L.A., et al. EMBO J. 15, 7026–7035 (1996); 24Yamaguchi, K., et al. Science 270, 2008–2011 (1995); 25Salmeron, A., et al. EMBO J. 15, 817–826 (1996); 26Derijard, B., et al. Science 267, 682–685 (1995); 27Lin, A., et al. Science 268, 286–290 (1995); 28Swat, W., et al. Immunity 8, 625–634 (1998); 29Yang, D., et al. Proc. Natl. Acad. Sci. USA 94, 3004–3009 (1997); 30Nishina, H., et al. J. Exp. Med. 186, 941–953 (1997); 31Nishina, H., et al. Development 126, 505–516 (1999); 32Ganiatsas, S., et al. Proc. Natl. Acad. Sci. USA 95, 6881–6886 (1998); 33Tournier, C., et al. Proc. Natl. Acad. Sci. USA 94, 7337–7342 (1997); 34Moriguchi, T., et al. EMBO J. 16, 7045–7053 (1997); 35Yao, Z., et al. J. Biol. Chem. 272, 32378–32383 (1997); 36Wu, Z., et al. Mol. Cell Biol. 17, 7407–7416 (1997); 37Lu, X., et al. J. Biol. Chem. 272, 24751–24754 (1997); 38Lawler, S., et al. FEBS Lett. 414, 153–158 (1997); 39Dong, C., et al. Nature 405, 91–94 (2000); 40Derijard, B., et al. Cell 76, 1025–1037 (1994); 41Sanchez, I., et al. Nature 372, 794–798 (1994); 42Gupta, S., et al. EMBO J. 15, 2760–2770 (1996); 43Dong, C., et al. Science 282, 2092–2095 (1998); 44Tournier, C., et al. Science 288, 870–874 (2000); 45Kuan, C.Y., et al. Neuron 22, 667–676 (1999); 46Sabapathy, K., et al. Mech. Dev. 89, 115–124 (1999); 47Constant, S.L., et al. J. Immunol 165, 2671–2676 (2000); 48Kallunki, T., et al. Genes Dev 8, 2996–3007 (1994); 49Sluss, H.K., et al. Mol. Cell. Biol. 14, 8376–8384 (1994); 50Yang, D.D., et al. Immunity 9, 575–585 (1998); 51Sabapathy, K., et al. Curr. Biol. 9, 116–125 (1999); 52Chu, W.M., et al. Immunity 11, 721–731 (1999); 53Mohit, A.A., et al. Neuron 14, 67–78 (1995); 54Yang, D.D., et al. Nature 389, 865–870 (1997); 55Dickens, M., et al. Science 277, 693–696 (1997); 56Whitmarsh, A.J., et al. Science 281, 1671–1674 (1998); 57Bonny, C., et al. J. Biol. Chem. 273, 1843–1846 (1998); 58Negri, S., et al. Genomics 64, 324–330 (2000); 59Yasuda, J., et al. Mol. Cell. Biol. 19, 7245–7254 (1999); 60Ito, M., et al. Mol. Cell. Biol. 19, 7539–7548 (1999); 61Kelkar, N., et al. Mol. Cell. Biol. 20, 1030–1043 (2000). Open table in a new tab Mice deficient of JNK1 or JNK2 appear to be morphologically normal. However, these mice are immunodeficient due to severe defects in T cell function (20Constant S.L Dong C Yang D.D Wysk M Davis R.J Flavell R.A JNK1 is required for T cell-mediated immunity against Leischmania major infection.J. Immunol. 2000; 165: 2671-2676PubMed Google Scholar). No evidence for a defect in T cell activation (proliferation and IL-2 secretion) was obtained. Instead, the deficiency was identified as a requirement of JNK for the appropriate differentiation of CD4 T helper cells into effector cells (23Dong C Yang D.D Wysk M Whitmarsh A.J Davis R.J Flavell R.A Defective T cell differentiation in the absence of Jnk1.Science. 1998; 282: 2092-2095Crossref PubMed Scopus (461) Google Scholar, 24Dong C Yang D.D Tournier C Whitmarsh A.J Xu J Davis R.J Flavell R.A JNK is required for effector T-cell function but not for T-cell activation.Nature. 2000; 405: 91-94Crossref PubMed Scopus (235) Google Scholar, 123Yang D.D Conze D Whitmarsh A.J Barrett T Davis R.J Rincon M Flavell R.A Differentiation of CD4+ T cells to Th1 cells requires MAP kinase JNK2.Immunity. 1998; 9: 575-585Abstract Full Text Full Text PDF PubMed Scopus (346) Google Scholar). The mechanism was reported to be mediated, in part, by alterations in the function of the NFAT1 transcription factor (23Dong C Yang D.D Wysk M Whitmarsh A.J Davis R.J Flavell R.A Defective T cell differentiation in the absence of Jnk1.Science. 1998; 282: 2092-2095Crossref PubMed Scopus (461) Google Scholar, 19Chow C.W Dong C Flavell R.A Davis R.J c-Jun NH2-terminal kinase inhibits targeting of the protein phosphatase calcineurin to NFATc1.Mol. Cell. Biol. 2000; 20: 5227-5234Crossref PubMed Scopus (96) Google Scholar) and by defects in IFNγ secretion (123Yang D.D Conze D Whitmarsh A.J Barrett T Davis R.J Rincon M Flavell R.A Differentiation of CD4+ T cells to Th1 cells requires MAP kinase JNK2.Immunity. 1998; 9: 575-585Abstract Full Text Full Text PDF PubMed Scopus (346) Google Scholar). In contrast, one group has reported that JNK2 may be required for T cell activation at low levels of stimulation (88Sabapathy K Hu Y Kallunki T Schreiber M David J.P Jochum W Wagner E.F Karin M JNK2 is required for efficient T-cell activation and apoptosis but not for normal lymphocyte development.Curr. Biol. 1999; 9 (a): 116-125Abstract Full Text Full Text PDF PubMed Scopus (239) Google Scholar) and that the mechanism is mediated, in part, by JNK-dependent regulation of IL-2 mRNA stability (17Chen C.Y Gherzi R Andersen J.S Gaietta G Jurchott K Royer H.D Mann M Karin M Nucleolin and YB-1 are required for JNK-mediated interleukin-2 mRNA stabilization during T-cell activation.Genes Dev. 2000; 14: 1236-1248PubMed Google Scholar). Nevertheless, there is agreement amongst all of these reports that at moderate and high levels of immune challenge, JNK is not required for T cell activation. However, JNK is required for effector T cell function (24Dong C Yang D.D Tournier C Whitmarsh A.J Xu J Davis R.J Flavell R.A JNK is required for effector T-cell function but not for T-cell activation.Nature. 2000; 405: 91-94Crossref PubMed Scopus (235) Google Scholar). An important question concerns why T cells are sensitive to defects in JNK1 or JNK2 expression. This may result from the pattern of JNK expression in murine T cells (111Weiss L Whitmarsh A.J Yang D.D Rincon M Davis R.J Flavell R.A Regulation of c-Jun NH(2)-terminal kinase (Jnk) gene expression during T cell activation.J. Exp. Med. 2000; 191: 139-146Crossref PubMed Scopus (71) Google Scholar). Immature T cells (thymocytes) express high levels of JNK1 and JNK2. However, JNK expression is down-regulated in peripheral T cells. JNK is therefore expressed at low levels in naîve T cells, but the expression of JNK is up-regulated following immune challenge. The low level of JNK expression in naîve T cells may account for the sensitivity of these cells to targeted disruption of the Jnk genes. Insight into the function of JNK protein kinases has recently been achieved through the determination of the atomic structure of JNK3 (118Xie X Gu Y Fox T Coll J.T Fleming M.A Markland W Caron P.R Wilson K.P Su M.S Crystal structure of JNK3 a kinase implicated in neuronal apoptosis.Structure. 1998; 6: 983-991Abstract Full Text Full Text PDF PubMed Google Scholar). Figure 2 illustrates the structure of the inactive complex of JNK3 with an ATP analog. The overall fold is typical of protein kinases and is similar to other MAPK and consists of two domains with an active site cleft. One significant difference between JNK3 and other MAPK is that the ATP binding site is well-ordered in the inactive structure. The low activity appears to result from misalignment of active site residues and the location of the T-loop, which blocks access of substrates to the active site. MAPKK activate JNK by phosphorylation of the T-loop on Thr and Tyr (Figure 2). Inactivation of JNK is mediated by a group of phosphatases, including Ser phosphatases, Tyr phosphatases, and dual specificity phosphatases (53Keyse S.M Protein phosphatases and the regulation of mitogen-activated protein kinase signalling.Curr. Opin. Cell Biol. 2000; 12: 186-192Crossref PubMed Scopus (571) Google Scholar). The mechanism of JNK activation by dual phosphorylation is unclear, but it is likely that this phosphorylation may alter the structure of the T-loop and cause realignment of the NH2- and COOH-terminal domains to create a functional active site. A crystal structure of activated JNK3 will be required to identify the structural changes that occur during activation. The JNK protein kinases are activated by phosphorylation on Thr and Tyr by MKK4 (also known as SEK1) and MKK7. These protein kinases are expressed as a group of alternatively spliced isoforms. Three MKK4 protein kinases with distinct NH2-terminal regions have been identified and six MKK7 protein kinase isoforms with different NH2 termini and COOH termini have been described. The mechanism that creates the different MKK4 isoforms has not been defined. In contrast, detailed studies of the Mkk7 gene demonstrate that the isoforms result from both alternative splicing and the utilization of different promoters (106Tournier C Whitmarsh A.J Cavanagh J Barrett T Davis R.J The MKK7 gene encodes a group of c-Jun NH2-terminal kinase kinases.Mol. Cell. Biol. 1999; 19: 1569-1581Crossref PubMed Google Scholar). These different forms of MKK4 and MKK7 are biochemically distinct (basal activity and inducibility) and are differentially activated by upstream MAPKKK (106Tournier C Whitmarsh A.J Cavanagh J Barrett T Davis R.J The MKK7 gene encodes a group of c-Jun NH2-terminal kinase kinases.Mol. Cell. Biol. 1999; 19: 1569-1581Crossref PubMed Google Scholar). The MKK7 protein kinase is primarily activated by cytokines (e.g., TNF and IL-1) and MKK4 is primarily activated by environmental stress. Comparison of the biochemical properties of MKK4 and MKK7 demonstrates that while both protein kinases can activate JNK by dual phosphorylation on Thr and Tyr, there are significant differences in substrate specificity. First, MKK4, but not MKK7, can also activate p38 MAPK. W