Glutamate Induces Phosphorylation of Elk-1 and CREB, Along with c -fos Activation, via an Extracellular Signal-Regulated Kinase-Dependent Pathway in Brain Slices

Abstract

In cell culture systems, the TCF Elk-1 represents a convergence point for extracellular signal-related kinase (ERK) and c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) subclasses of mitogen-activated protein kinase (MAPK) cascades. Its phosphorylation strongly potentiates its ability to activate transcription of the c-fos promoter through a ternary complex assembled on the c-fos serum response element. In rat brain postmitotic neurons, Elk-1 is strongly expressed (V. Sgambato, P. Vanhoutte, C. Pagès, M. Rogard, R. A. Hipskind, M. J. Besson, and J. Caboche, J. Neurosci. 18:214–226, 1998). However, its physiological role in these postmitotic neurons remains to be established. To investigate biochemically the signaling pathways targeting Elk-1 and c-fos in mature neurons, we used a semi-in vivo system composed of brain slices stimulated with the excitatory neurotransmitter glutamate. Glutamate treatment leads to a robust, progressive activation of the ERK and JNK/SAPK MAPK cascades. This corresponds kinetically to a significant increase in Ser³⁸³-phosphorylated Elk-1 and the appearance of c-fos mRNA. Glutamate also causes increased levels of Ser¹³³-phosphorylated 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 MAPK kinase inhibitor PD98059, indicating the primary role of the ERK cascade in mediating glutamate signaling to Elk-1 in the rat striatum in vivo. Glutamate-mediated CREB phosphorylation is also inhibited by PD98059 treatment. Interestingly, KN62, which interferes with calcium-calmodulin kinase (CaM-K) activity, leads to a reduction of glutamate-induced ERK activation and of CREB phosphorylation. These data indicate that ERK functions as a common component in two signaling pathways (ERK/Elk-1 and ERK/?/CREB) converging on the c-fospromoter in postmitotic neuronal cells and that CaM-Ks act as positive regulators of these pathways.

ACKNOWLEDGMENTS

We thank Parke-Davis for the generous gift of PD98059. We also thank N. Kayadjanian, E. Valjent, and M. Leonhard for helpful technical assistance.

This work was supported by the University Pierre and Marie Curie, the CNRS, the Fondation pour la Recherche Medicale (R.A.H.), Institut Lilly, and Biomed Program (BMHY-CT-97-2215).