Neuronal basic helix-loop-helix proteins (NEX and BETA2/Neuro D) regulate terminal granule cell differentiation in the hippocampus

The transcription factors neuronal helix-loop-helix protein (NEX)/mammalian atonal homolog 2 (Math-2), BETA2/neuronal determination factor (NeuroD), a nd NeuroD-related factor (NDRF)/NeuroD2 comprise a family of Drosophila ato nal-related basic helix-loop-helix (bHLH) proteins with highly overlapping expression in the developing forebrain. The ability of BETA2/NeuroD and NDR F to convert ectodermal cells into neurons after mRNA injection into Xenopu s oocytes suggested a role in specifying neuronal cell fate. However, neuro nal bHLH genes are largely transcribed in CNS neurons, which are fully comm itted. Here we analyze a defect in mice lacking BETA2/NeuroD, and in NEX*BE TA2/NeuroD double mutants, demonstrating that bHLH proteins are required in vivo for terminal neuronal differentiation. Most strikingly, presumptive g ranule cells of the dentate gyrus are generated but fail to mature, lack no rmal sodium currents, and show little dendritic arborization. Long-term hip pocampal slice cultures demonstrate secondary alterations of entorhinal and commissural/associational projections. The primary developmental arrest ap pears to be restricted to granule cells in which an autoregulatory system i nvolving all three neuronal bHLH genes has failed.