The role of bHLH transcription factor NEX in neuronal differentiation and experience-dependent plasticity

Abstract

NEX (neuronal helix-loop-helix protein) is an atonal-related basic helix-loop-helix (bHLH) transcription factor that belongs to the subfamily of neuronal differentiation factors, which also includes NeuroD and NDRF. NEX is expressed in postmitotic pyramidal neurons throughout CNS development and in the adult brain. In rodents, expression starts around embryonic day 11 in postmitotic pyramidal neurons of the cortical plate. In the adult mouse brain expression is maintained in hippocampus proper, amygdala, entorhinal cortex and subiculum. Recent in vitro studies have suggested that NEX promotes neurite outgrowth and regeneration and facilitates neuronal survival. Since inactivation of the Nex gene per se does not have an obvious impact on forebrain formation in the mouse, it is likely that the closely related proteins NeuroD and NDRF can compensate for the loss of NEX protein. However, sustained NEX expression in adult pyramidal neurons of several brain areas, which are implicated in memory formation and retrieval, suggests a function of NEX in neuronal plasticity and higher cognitive functions. The goal of this study was to investigate the function of NEX during murine telencephalon development and in experience-dependent plasticity of the adult brain. To study the role of NEX in the adult brain, a microarray analysis of the hippocampal CA3 region of NEX null mutants versus wildtype animals was performed. In order to test the effect of the NEX null mutation on experience-induced plasticity, an enriched environment paradigm was applied prior to transcriptome analysis. The comparison of gene expression profiles of null mutant and control animals revealed a differential regulation of metabolic pathways and several genes involved in learning and memory. In line with increased metabolic activity of hippocampal neurons, NEX null mutants exhibited hyperactivity in the open field, light-dark preference and plus maze test. Further behavioural studies demonstrated a learning delay in spatial learning tasks and impaired contextual memory formation in NEX null mutants. The spatiotemporal expression pattern of NEX during development and altered expression of cell cycle genes in null mutants suggested a role for NEX in cell cycle exit control and neuronal differentiation. To study the role of NEX during early cortical development a transgenic mouse line was generated, in which NEX expression can be induced by Cre-recombinase. NEX overexpression in neuronal progenitors, but not in postmitotic neurons, resulted in the reduction of cerebral hemisphere size and abnormal cortical layer formation. Massive apoptosis of ventricular zone progenitors and ectopic expression of neuronal differentiation markers indicated premature cell cycle exit and impaired migration. Additionally, a transgenic mouse line allowing doxycycline-regulated control of gene expression in postmitotic pyramidal neurons was created during this study. The NEX-htTA transgenic line is currently being used to study mechanisms of cortical lamination.