Strähle U, Fischer N, Blader P
IGBMC, CNRS/INSERM/ULP, Illkirch, France.
Mech Dev. 1997 Mar;62(2):147-60. doi: 10.1016/s0925-4773(97)00657-6.
Proteins of the Netrin family have been implicated in axon guidance in both C. elegans and vertebrates. Here, we report the cloning and expression analysis of a zebrafish netrin homologue (net1). net1 is expressed in the floor plate and the anterior ventral neural tube. Its expression is ectopically induced by misexpression of sonic hedgehog (shh) and a dominant negative mutant of the regulatory subunit of protein kinase A (dnReg). Ectopic activation of net1, however, is restricted to distinct regions in the brain. Upon overexpression of shh or dnReg in cyclops mutants, which have strongly impaired net1 expression in the ventral neural tube, rescue of net1 expression was observed in the brain but not in the spinal cord. Ectopic expression of dnReg and Shh protein can be detected at high levels throughout injected embryos from pre-gastrula stages onwards suggesting that the competence of the neural plate to respond to Shh signalling activity differs regionally. Similar to net1, axial, the zebrafish homologue of mammalian HNF3beta, which is also expressed along the ventral neural tube, is ectopically induced in the brain of embryos injected with dnReg mRNA. Neurons differentiate normally within domains of ectopic net1 and axial expression. Thus, dorsal neuronal differentiation appears to be unaffected despite co-expression of a gene program specific for the ventral neural tube. This also suggests that these ectopically expressing regions have not differentiated into floor plate.
Netrin家族的蛋白质在秀丽隐杆线虫和脊椎动物的轴突导向中都发挥了作用。在此，我们报告斑马鱼netrin同源物（net1）的克隆及表达分析。net1在底板和前腹侧神经管中表达。其表达可由音猬因子（shh）和蛋白激酶A调节亚基的显性负性突变体（dnReg）的错误表达异位诱导。然而，net1的异位激活仅限于大脑中的特定区域。在独眼畸形突变体中过表达shh或dnReg时，其腹侧神经管中的net1表达严重受损，在大脑中可观察到net1表达的挽救，但在脊髓中未观察到。从原肠胚前期开始，在整个注射的胚胎中都能检测到高水平的dnReg和Shh蛋白的异位表达，这表明神经板对Shh信号活性的反应能力存在区域差异。与net1类似，哺乳动物HNF3β的斑马鱼同源物axial也沿腹侧神经管表达，在注射dnReg mRNA的胚胎大脑中被异位诱导。神经元在异位net1和axial表达区域内正常分化。因此，尽管共表达了腹侧神经管特有的基因程序，但背侧神经元分化似乎未受影响。这也表明这些异位表达区域并未分化为底板。
