The c-Jun NH2-terminal protein kinases (JNK) are a family of MAP kinases that are preferentially activated by cell stress-inducing signals, such as trophic-withdrawal, heat shock and UV. JNKs have been implicated in several physiological functions including regulation of apoptosis, inflammatory responses, cell proliferation, differentiation and tissue morphogenesis. For example, JNK may play a role in the induction of apoptosis in neurons during development. However, the mechanism for JNK-mediated neuronal apoptosis is undefined. Using arsenite as a model for toxicant-induced neuronal apoptosis during development, we have begun to elucidate the molecular and cellular mechanisms underlying JNK-induced neuronal cell death. Sodium arsenite is an environmental toxicant that causes developmental defects in the CNS. Our data indicate that JNK isoforms may be differentially regulated during neuronal apoptosis. There are three JNK genes; JNK1, JNK2, and JNK3, cortical neurons express kinase activities for all JNKs. We showed that JNK contributes to arsenite-induced apoptosis in cultured neurons. Furthermore, JNK3 but not JNK1/2 is activated by arsenite. Since JNK3 is the only neural specific JNK isoform, it may provide a neurospecific target for blocking neuronal apoptosis. We propose to test the hypothesis that JNK3 but not JNK1 or 2, may be important for stress-induced neuronal apoptosis both in vivo and in vitro. We propose to address this issue using primary cultured cortical neurons, hippocampal slice cultures and high precision stereotactic delivery of apoptotic agents to the hippocampus and cortex. Moreover, we propose to use JNK3-/- mice as well as transgenic mice over-expressing JBD specifically in neurons in the hippocampus and cortex to elucidate the function of JNK in neuronal apoptosis in vivo. We also propose a novel mechanism for JNK induction of apoptosis in which JNK activation induces post-translational modification that leads to caspase activation, and transcriptional regulation of cell death genes.