Glutamate induces phosphorylation of Elk-1 and CREB, along with c-fos activation, via an extracellular signal-regulated kinase-dependent pathway in brain slices

In cell culture systems, the TCF Elk-1 represents a convergence point for e xtracellular signal-related kinase (ERK) and c-Jun N-terminal kinase/stress -activated protein kinase (JNK/SAPK) subclasses of mitogen-activated protei n kinase (MAPK) cascades. Its phosphorylation strongly potentiates its abil ity to activate transcription of the c-fos promoter through a ternary compl ex assembled on the c-fos serum response element. In rat brain postmitotic neurons, Elk-1 is strongly expressed (V. Sgambato, P. Vanhoutte, C. Pages, M. Regard, R. A. Hipskind, M. J. Besson, and J. Caboche, J. Neurosci. 18:21 4-226, 1998). However, its physiological role in these postmitotic neurons remains to be established. To investigate biochemically the signaling pathw ays targeting Elk-1 and c-fos in mature neurons, we used a semi-in vivo sys tem composed of brain slices stimulated with the excitatory neurotransmitte r glutamate. Glutamate treatment leads to a robust, progressive activation of the ERK and JNK/SAPK MAPK cascades. This corresponds kinetically to a si gnificant increase in Ser(383)-phosphorylated Elk-1 and the appearance of c -fos mRNA. Glutamate also causes increased levels of Ser(133)-phosphorylate d cyclic AMP-responsive element-binding protein (CREB) but only transiently relative to Elk-1 and c-fos. ERK and Elk-1 phosphorylation are blocked by the MARK kinase inhibitor PD98059, indicating the primary role of the ERK c ascade in mediating glutamate signaling to Elk-1 in the rat striatum in viv o. Glutamate-mediated CREB phosphorylation is also inhibited by PD98059 tre atment. Interestingly, KN62, which interferes with calcium-calmodulin kinas e (CaM-K) activity, leads to a reduction of glutamate-induced ERK activatio n and of CREB phosphorylation. These data indicate that ERK functions as a common component in two signaling pathways (ERK/Elk-1 and ERK/?/CREB) conve rging on the c-fos promoter in postmitotic neuronal cells and that CaM-Ks a ct as positive regulators of these pathways.