Huntington's disease (HD) is an autosomal neurodegenerative disorder, caused by expansion of a glutamine repeat in the Huntingtin protein. Pathogenesis in HD includes the cytoplasmic cleavage of Huntingtin and release of an amino-terminal fragment capable of nuclear localization, where expanded-Huntingtin (Exp-Htt) might lead to aberrant transcriptional regulation, neuronal dysfunction and degeneration. Recent evidence, from hippocampal cell lines, also implicates altered interaction of Exp-Htt with components of the c-Jun N-terminal kinase (JNK) cascade. However, there is yet no proven implication of the JNK/c-Jun module in degeneration of striatal neurons, the more vulnerable cell population, in HD. In the present study, we used primary striatal neurons in culture to analyze c-Jun activation by Exp-Htt. Green fluorescent protein (GFP)-tagged exon 1 of human Huntingtin either in its normal (25Q, normal-Htt) or expanded (103Q, Exp-Htt) version was transiently transfected in these cells. We first set out, in our conditions, the time course of striatal degeneration produced by Exp-Htt, and found it occurred rapidly. At 48 h post-transfection, 60% of striatal neurons expressing Exp-Htt had apoptotic characteristics including DNA fragmentation and neuritic retraction. Most of these neurons also showed nuclear aggregates of GFP-Exp Htt. Kinetics of c-Jun activation were tested in transfected cells using immunocytochemical detection of phospho-c-Jun. We found a significant activation and induction of c-Jun in Exp-Htt but not normal-Htt-transfected neurons. Of interest, these events occurred prior to nuclear translocation of Exp-Htt. Finally, overexpression of a dominant negative version of c-Jun, as well as pharmacological inhibition of JNK strongly protected against DNA fragmentation and neuritic retraction induced by Exp-Htt. Thus our data suggest that c-Jun activation and induction, is an early event in the pathogenesis of HD, occurring prior to formation of nuclear aggregates of Exp-Htt.