Human c-Jun N-terminal kinase expression and activation in the nervous system

Differential expression and localization of c-Jun N-terminal kinases (JNKs) in the human brain may reflect transduction of a variety of extracellular stimuli to selective cellular responses. Of the three JNKs, JNK1 and 2 are widely distributed in tissues and JNK3 is predominantly restricted to brain where it is expressed in neurons. Although there is considerable molecular conservation among all three JNKs, we distinguished expression of each by in situ hybridization, immunoblot analysis with a panel of antibodies, and stress-activation using c-Jun as substrate. In the human central nervous system (CNS), there are at least 10 isoforms: JNK3alpha1 and JNK1alpha1 were the major JNK isoforms expressed; JNK2 was not detected. On immunoblots of brain homogenates, antibody selectivity identified JNK3alpha1 as a 45-kDa protein, JNK1alpha1, a slightly lower band at 44 kDa, and a 50-kDa band of unknown specificity. Recombinant human JNK3alpha1, transfected either into CHO, COS-1, or Neuro2A (N2A) cells, was strongly expressed as a 45-kDa protein in each. Transfected JNK3alpha1, and endogenous JNK1, each immunoprecipitated from N2A cells, phosphorylated recombinant forms of human c-Jun. Kinase activity of each JNK was modestly stimulated in N2A cells by anisomycin but not by ceramide, UV irradiation, or heat shock. Endogenous JNK activation, especially at a low level, may reflect a chronic and cumulative stress process that contributes to hyperphosphorylation of cytoskeletal proteins such as those found in Alzheimer's disease (AD), and ultimately, induction of apoptosis.