Local Insulin-like Growth Factor I expression is essential for Purkinje neuron survival at birth
Résumé
IGF1, an anabolic and neuroprotective factor, promotes neuronal survival by blocking apoptosis. It is released into the bloodstream by the liver, or synthesized locally by muscles and neural cells, acting in an autocrine or paracrine fashion. Intriguingly, genetic studies conducted in invertebrate and murine models also suggest that an excess of IGF1 signaling may trigger neurodegeneration. This emphasizes the importance of gaining a better understanding of the mechanisms controlling IGF1 regulation and gene transcription. In the cerebellum, Igf1 expression is activated just before birth in a subset of Purkinje cells (PCs). Mice carrying a null mutation for HLH transcription factor EBF2 feature PC apoptosis at birth. We show that Igf1 is sharply downregulated in Ebf2 null PCs starting prior to the onset of PC death. In vitro, EBF2 binds a conserved distal Igf1 promoter region. The pro-survival PI3K signaling pathway is strongly inhibited in mutant cerebella. Finally, organotypic cerebellar cultures treated with an IGF1 competitive inhibitor exhibit a significant increase in Purkinje cell death and, consistently, Ebf2 null organotypic cultures respond to IGF1 treatment by sharply reducing Purkinje cell apoptosis. Our findings reveal that IGF1 is required for PC survival in the neonatal cerebellum, and identify a new mechanism controlling its local expression in the CNS.
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