Role of B-Raf and MEK-1 in the central Nervous System

The Raf/MEK/ERK cascade is a highly conserved signal transduction module whose activation results in a number of different outcomes, depending on the cell type or the stimulus used. The B-Raf and MEK-1 kinases are highly expressed in neurons. Studies in cultured cells have implicated the Raf/MEK/ERK pathway in neuritogenesis and proliferation and have revealed a prosurvival role of B-Raf in primary neurons. Additionally, studies in vivo have implicated the ERK module in synaptic plasticity, learning and memory. Ablation of either B-Raf or Mek-1 results in embryonic lethality. To gain information about the function of these molecules in specific tissues and in the adult animal, we are using conditional knock-out mouse strains established in our lab (MEK-1) or obtained as a result of collaboration (B-Raf). In the course of a previous project, we have started to elucidate the role of B-Raf in the central nervous system and have found that B-Raf is required for the activation of the ERK module during postnatal life. Ablation of B-Raf in neuronal precursors causes severe hypomyelination and dwarfism, indicating that B-Raf plays non-redundant role in oligodendrocytes and in the neuronal control of somatic growth. The present project has two aims: to characterize the mechanisms responsible for the pleiotropic phenotypes caused by B-Raf ablation in neuronal precursors and to investigate the impact, if any, of MEK-1 ablation on neuronal development and thereby to gain insight in the proposed effector:target relationship between these two molecules. To achieve these goals, we will combine phenotype analysis in the whole organism (in vivo) as well as in cells (ex vivo) with biochemical experiments in ablated cells to elucidate the molecular basis of the phenotype. The results will help defining the function(s) of B-Raf and MEK-1 in this specific system, as well as their relevant biological targets. The information obtained will be instrumental in assessing the potential of these molecules as therapeutic targets and in directing the design and use of pharmacological kinase inhibitors.

Cells manage to survive, proliferate, and differentiate in their environment by interpreting the signals they receive from it and translating them into the right output. If signaling goes awry, even only in part of the cells, the whole organism is at risk. Signaling pathways are often organized in cascades in which one enzyme activates the next, leading to the amplification of the signal as it proceeds. One such signaling cascade is the Raf/MEK/ERK cascade, activated by stimuli that bring about cell proliferation, survival, or differentiation. Of the enzymes in this cascade, B-Raf and MEK-1 are highly expressed in neuronal cells. We showed that B-Raf is the main MEK/ERK activator in the central nervous system. Our studies have further revealed an essential function of Raf in axon growth, and, more specifically, of B-Raf in the differentiation of a specific class of neurons. We have also discovered that B- Raf is necessary for the differentiation of oligodendrocytes. In the central nervous system, these glial cells are responsible for the myelination of axons. This function is essential for life, and disruption of B-Raf in neuronal cells leads to an aggressive degenerative disease and to the death of the animal. Surprisingly, MEK-1 as a B-Raf effector has a redundant role and its ablation does not compromise myelination. The study highlights the importance of B-Raf as the main player among the three Raf isoforms in the development of the central nervous system and myelinogenesis.