Mice lacking the ERK1 isoform of MAP kinase are unimpaired in emotional learning.

The cellular processes underlying learning and the formation of memory involve the regulation of synaptic strength as well as the establishment of new synaptic connections. The mechanisms by which neuronal activity is translated into changes in synaptic organization have received considerable attention. The recognition that intracellular signaling pathways involving protein kinases are essential intermediates in the induction of long-term changes in synaptic strength represents a significant advance in our understanding of the molecular mechanisms of learning and memory. The MAP kinases have been shown to be critically involved in the formation of long-term memory (Kornhauser and Greenberg 1997). Originally discovered as regulators of cell division and differentiation, mitogen-activated protein kinases (MAPKs) are abundantly expressed in neurons in the mature central nervous system. Recently, their role in these nondividing, terminally differentiated neurons has come under investigation. A number of previous studies have demonstrated a major role for MAPK in mammalian associative learning. Our laboratory has previously shown activation of extracellular signal-regulated kinase (ERK) isoforms of MAPK in the rat hippocampus following a cue and contextual fear-conditioning paradigm. In addition, intraperitoneal injection of an inhibitor of MAPK kinase (MEK), the upstream activator of MAPK, blocked fear conditioning in these animals (Atkins et al. 1998). Preventing MAPK activation with a MEK inhibitor also impaired fear learning in the mouse (Selcher et al. 1999). In a further refinement, Schafe et al. (1999) demonstrated selective blockade of long-term, but not short-term, fear conditioning following intraventricular infusion of a MEK inhibitor. Together with reports utilizing other associative learning paradigms including conditioned taste aversion (Berman et al. 1998) and step-down inhibitory avoidance (Walz et al. 1999), these fear-conditioning studies solidly implicate MAPK in the formation of associative memories. Regulation of the MAPK family of enzymes involves a core cascade of three protein kinases. Signal transduction through this pathway begins with the activation of ras and the physical interaction of this small G protein with the serine/threonine kinases Raf-1 or B-Raf, resulting in their activation. The Raf kinases then phosphorylate and activate MEK (or MAPKK). MEK1 and MEK2 are dedicated dual-specificity kinases that activate both ERK1 (p44 MAPK) and ERK2 (p42 MAPK) by phosphorylating these enzymes on threonine and tyrosine residues (e.g., Thr 183 and Tyr 185 for ERK2), provoking their enzymatic activation. On activation, the ERKs are translocated to the nucleus, where they phosphorylate transcription factors, leading to changes in gene expression (English et al. 1999). Although a role for the ERKs in a number of behavioral paradigms has been established, it is unclear which of the two ERK isoforms contributes to learning. The inhibitors of the MAPK cascade used in previous learning studies (PD098059, U0126, and SL327) are quite selective in their inhibition of MEK but, unfortunately, do not differentiate between the two ERK isoforms. Previous studies using knockout and transgenic mice have elucidated the specific role of individual isoforms in other protein kinase families known to be critical for synaptic plasticity and learning (Silva et al. 1992; Abeliovich et al. 1993; Abel et al. 1997; Weeber et al. 2000). Therefore, to investigate the specific function of the ERK isoforms in learning and memory, we employed mice in which one ERK isoform was genetically inactivated. Specifically, we characterized mice lacking the ERK1 isoform of MAPK in an attempt to determine the behavioral contribution of this protein kinase.