Elk-1 a transcription factor with multiple facets in the brain

Abstract

The ternary complex factor (TCF) Elk-1 is a transcription factor that regulates immediate early gene (IEG) expression via the serum response element (SRE) DNA consensus site. Elk-1 is associated with a dimer of serum response factor (SRF) at the SRE site, and its phosphorylation occurs at specific residues in response to mitogen-activated protein kinases (MAPKs), including c-Jun-N terminal kinase (JNK), p38/MAPK, and extracellular-signal regulated kinase (ERK). This phosphorylation event is critical for triggering SRE-dependent transcription. Although MAPKs are fundamental actors for the instatement and maintenance of memory, and much investigation of their downstream signaling partners have been conducted, no data yet clearly implicate Elk-1 in these processes. This is partly due to the complexity of Elk-1 sub-cellular localization, and hence functions, within neurons. Elk-1 is present in its resting state in the cytoplasm, where it colocalizes with mitochondrial proteins or microtubules. In this particular sub-cellular compartment, overexpression of Elk-1 is toxic for neuronal cells. When phosphorylated by the MAPK/ERK, Elk-1 translocates to the nucleus where it is implicated in regulating chromatin remodeling, SRE-dependent transcription, and neuronal differentiation. Another post-translational modification is the conjugation to SUMO (Small Ubiquitin-like MOdifier), which relocalizes Elk-1 in the cytoplasm. Thus, Elk-1 plays a dual role in neuronal functions: pro-apoptotic within the cytoplasm, and pro-differentiation within the nucleus. To address the role of Elk-1 in the brain, one must be aware of its multiple facets, and design molecular tools that will shut down Elk-1 expression, trafficking, or activation, in specific neuronal compartments. We summarize in this review the known molecular functions of Elk-1, its regulation in neuronal cells, and present evidence of its possible implication in model systems of synaptic plasticity, learning, but also in neurodegenerative diseases.

Keywords: A; ERK signaling; brain plasticity; chromatin remodeling; gene regulation; long-term neuronal adaptation; memory formation.

Figure 1

Diagram illustrating functional domains and major post-translational modifications of the Elk-1 protein. The ETS domain (or A) domain lies within the n-terminus end of the protein and is responsible for Elk-1 binding to DNA. The B domain is involved in the binding of Elk-1 to a dimer of its cofactor, the SRF. The R domain is crucial for the repression of Elk-1 transcriptional activity. It contains the lysine residues that are susceptible to be SUMOylated, a post-translational event that reinforce the repression exerted by the R domain. The C (or transactivation) domain contains the amino acids that are phosphorylated by MAP kinases. Among these residues, phosphorylation of Serine 383 and Serine 389 is a crucial event to activate Elk-1-mediated transcription. The two domains depicted in green are involved in the binding of Elk-1 to activated MAP kinases. The D (or DEJL) domain is responsible for the binding of Elk-1 to activated MAP kinases of the ERK, JNK, and p38 subtypes. The DEF (or FXFP) domain is more specific since it is only required for the binding of Elk-1 to activated ERK. The NES and NLS motifs are involved in nuclear export and import of Elk-1, respectively.

Figure 2

Synthetic view of the impact of Elk-1 post-translational modifications on its cytoplasm-to-nucleus trafficking and functions in neurons (see main text for details).