Schwab M H, Druffel-Augustin S, Gass P, Jung M, Klugmann M, Bartholomae A, Rossner M J, Nave K A
Zentrum für Molekulare Biologie (ZMBH), University of Heidelberg, D-69120 Heidelberg, Germany.
J Neurosci. 1998 Feb 15;18(4):1408-18. doi: 10.1523/JNEUROSCI.18-04-01408.1998.
Basic helix-loop-helix (bHLH) genes have emerged as important regulators of neuronal determination and differentiation in vertebrates. Three putative neuronal differentiation factors [NEX for neuronal helix-loop-helix protein-1 (mammalian atonal homolog-2), neuroD (beta-2), and NDRF for neuroD-related factor (neuroD2)] are highly homologous to each other in the bHLH region and comprise a new bHLH subfamily. To study the role of NEX, the first bHLH protein identified in this group, we have disrupted the NEX gene by homologous recombination. NEX-deficient mice have no obvious developmental defect, and CNS neurons appear fully differentiated. To investigate further whether the absence of NEX is compensated for by neuroD and NDRF, we compared the spatiotemporal expression of all three genes. We demonstrate, by in situ hybridization, that the transcription patterns of NEX, neuroD, and NDRF genes are highly overlapping in the developing CNS of normal rats between embryonic day 12 and adult stages but are not strictly identical. The most prominent transcription of each gene marks the dorsal neuroepithelium of the telencephalon in early development and is sustained in the adult neocortex, hippocampus, and cerebellum. In general, neuroD provides the earliest marker of neuronal differentiation in any given region compared with NDRF or NEX. Whereas a few CNS regions are specific for neuroD, no region was detected in which solely NEX or NDRF is expressed. This suggests that the function of the mutant NEX gene in neuronal differentiation is compensated for by neuroD and NDRF and that, in analogy with myogenic bHLH proteins, neuronal differentiation factors are at least in part equivalent in function.
碱性螺旋-环-螺旋（bHLH）基因已成为脊椎动物神经元决定和分化的重要调节因子。三种假定的神经元分化因子[神经元螺旋-环-螺旋蛋白-1的NEX（哺乳动物无调性同源物-2）、神经D（β-2）以及神经D相关因子的NDRF（神经D2）]在bHLH区域彼此高度同源，构成一个新的bHLH亚家族。为了研究该组中首个被鉴定的bHLH蛋白NEX的作用，我们通过同源重组破坏了NEX基因。NEX基因缺陷型小鼠没有明显的发育缺陷，中枢神经系统神经元似乎已完全分化。为了进一步研究神经D和NDRF是否能补偿NEX的缺失，我们比较了这三个基因的时空表达。我们通过原位杂交证明，在正常大鼠发育中的中枢神经系统中，从胚胎第12天到成年阶段，NEX、神经D和NDRF基因的转录模式高度重叠，但并不完全相同。每个基因最显著的转录在发育早期标记端脑的背侧神经上皮，并在成年新皮层、海马体和小脑中持续存在。一般来说，与NDRF或NEX相比，神经D在任何给定区域提供了最早的神经元分化标记。虽然少数中枢神经系统区域对神经D具有特异性，但未检测到仅表达NEX或NDRF的区域。这表明神经D和NDRF补偿了突变的NEX基因在神经元分化中的功能，并且与肌源性bHLH蛋白类似，神经元分化因子在功能上至少部分等效。
