Role of HuD and other RNA-binding proteins in neural development and plasticity.

Transcription factors have traditionally been viewed as the main determinants of gene expression. Yet, in recent years it has become apparent that RNA-binding proteins also play a critical role in determining the levels of expression of a large number of genes. Once mRNAs are transcribed, RNA-binding proteins can control all subsequent steps in their function, from alternative splicing and translation to mRNA transport and stability. In the nervous system, a large number of genes are regulated post-transcriptionally via the interaction of their mRNAs with specific RNA-binding proteins. This type of regulation is particularly important in the control of the temporal and spatial pattern of gene expression during neural development. This review will discuss the function of the embryonic lethal abnormal vision (ELAV)/Hu family of nervous system-specific RNA-binding proteins, with a special emphasis on HuD, a member of this family that controls GAP-43 mRNA stability and expression. In addition, we will present recent findings on other neural RNA-binding proteins: the ribonucleoprotein K homology (KH)-domain proteins, Fragile X mental retardation protein (FMRP), quakinguiable protein (QKI), and Nova-1. Together with the ELAV/Hu family, these proteins are essential for proper neural development and in some cases for plasticity in the mature brain. The biological significance of these proteins is evident not only by their evolutionary conservation but also by the magnitude of problems arising from autoimmune reactions against them or from mutations affecting their expression or function.