Dual roles for c-Jun N-terminal kinase in developmental and stress responses in cerebellar granule neurons

c-Jun N-terminal kinases (JNKs) typically respond strongly to stress, are i mplicated in brain development, and are believed to mediate neuronal apopto sis. Surprisingly, however, JNK does not respond characteristically to stre ss in cultured cerebellar granule (CBG) neurons, a widely exploited CNS mod el for studies of death and development, despite the regulation of its subs trate c-Jun. To understand this anomaly, we characterized JNK regulation in CBG neurons. We find that the specific activity of CBG JNK is elevated con siderably above that from neuron-like cell lines (SH-SY5Y, PC12); however, similar elevated activities are found in brain extracts. This activity does not result from cellular stress because the stress-activated protein kinas e p38 is not activated. We identify a minor stress-sensitive pool of JNK th at translocates with mitogen-activated protein kinase kinase-4 (MKK4) into the nucleus. However, the major pool of total activity is cytoplasmic, resi ding largely in the neurites, suggesting a non-nuclear role for JNK in neur ons. A third JNK pool is colocalized with MKK7 in the nucleus, and specific activities of both increase during neuritogenesis, nuclear JNK activity in creasing 10-fold, whereas c-Jun expression and activity decrease. A role fo r JNK during differentiation is supported by modulation of neuritic archite cture after expression of dominant inhibitory regulators of the JNK pathway . Channeling of JNK signaling away from c-Jun during differentiation is con sistent with the presence in the nucleus of the JNK/MKK7 scaffold protein J NK-interacting protein, which inhibits JNK-c-Jun interaction. We propose a model in which distinct pools of JNK serve different functions, providing a basis for understanding multifunctional JNK signaling in differentiating n eurons.