Wenner P, O'Donovan M J, Matise M P
National Institutes of Health, National Institute of Neurological Disorders and Stroke, Lab of Neural Control, Bethesda, Maryland 20892-4455, USA.
J Neurophysiol. 2000 Nov;84(5):2651-7. doi: 10.1152/jn.2000.84.5.2651.
A number of homeodomain transcription factors have been implicated in controlling the differentiation of various types of neurons including spinal motoneurons. Some of these proteins are also expressed in spinal interneurons, but their function is unknown. Progress in understanding the role of transcription factors in interneuronal development has been slow because the synaptic connections of interneurons, which in part define their identity, are difficult to establish. Using whole cell recording in the isolated spinal cord of chick embryos, we assessed the synaptic connections of lumbosacral interneurons expressing the Engrailed-1 (En1) transcription factor. Specifically we established whether En1-expressing interneurons made direct connections with motoneurons and whether they constitute a single interneuron class. Cells were labeled with biocytin and subsequently processed for En1 immunoreactivity. Our findings indicate that the connections of En1-expressing cells with motoneurons and with sensory afferents were diverse, suggesting that the population was heterogeneous. In addition, the synaptic connections we tested were similar in interneurons that expressed the En1 protein and in many that did not. The majority of sampled En1 cells did, however, exhibit a direct synaptic connection to motoneurons that is likely to be GABAergic. Because our physiological methods underestimate the number of direct connections with motoneurons, it is possible that the great majority, perhaps all, En1-expressing cells make direct synaptic connections with motoneurons. Our results raise the possibility that En1 could be involved in interneuron-motoneuron connectivity but that its expression is not restricted to a distinct functional subclass of ventral interneuron. These findings constrain hypotheses about the role of En-1 in interneuron development and function.
许多同源结构域转录因子参与了包括脊髓运动神经元在内的各类神经元的分化控制。其中一些蛋白质也在脊髓中间神经元中表达，但其功能尚不清楚。由于中间神经元的突触连接（部分决定了它们的身份）难以建立，因此在理解转录因子在中间神经元发育中的作用方面进展缓慢。我们利用鸡胚离体脊髓的全细胞记录，评估了表达Engrailed-1（En1）转录因子的腰骶部中间神经元的突触连接。具体来说，我们确定了表达En1的中间神经元是否与运动神经元直接相连，以及它们是否构成单一的中间神经元类别。细胞用生物素标记，随后进行En1免疫反应性处理。我们的研究结果表明，表达En1的细胞与运动神经元和感觉传入纤维的连接是多样的，这表明该群体是异质的。此外，我们测试的突触连接在表达En1蛋白的中间神经元和许多不表达En1蛋白的中间神经元中是相似的。然而，大多数抽样的En1细胞确实表现出与运动神经元的直接突触连接，这种连接可能是γ-氨基丁酸能的。由于我们的生理学方法低估了与运动神经元的直接连接数量，很可能绝大多数（也许是所有）表达En1的细胞都与运动神经元建立了直接突触连接。我们的结果提出了一种可能性，即En1可能参与中间神经元与运动神经元的连接，但它的表达并不局限于腹侧中间神经元的一个独特功能亚类。这些发现限制了关于En-1在中间神经元发育和功能中作用的假设。
