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Abstract：
Subplate (SP) neurons are among the earliest-born neurons in the cerebral cortex and heterogeneous in terms of gene expression. SP neurons consist mainly of projection neurons, which begin to extend their axons to specific target areas very early during development. However, the relationships between axon projection and gene expression patterns of the SP neurons, and their remnant layer 6b (L6b) neurons, are largely unknown. In this study, we analyzed the corticocortical projections of L6b/SP neurons in the mouse cortex and searched for a marker gene expressed in L6b/SP neurons that have ipsilateral inter-areal projections. Retrograde tracing experiments demonstrated that L6b/SP neurons in the primary somatosensory cortex (S1) projected to the primary motor cortex (M1) within the same cortical hemisphere at postnatal day (PD) 2 but did not show any callosal projection. This unilateral projection pattern persisted into adulthood. Our microarray analysis identified the gene encoding a β subunit of voltage-gated potassium channel (Kcnab1) as being expressed in L6b/SP. Double labeling with retrograde tracing and in situ hybridization demonstrated that Kcnab1 was expressed in the unilaterally-projecting neurons in L6b/SP. Embryonic expression was specifically detected in the SP as early as embryonic day (E) 14.5, shortly after the emergence of SP. Double immunostaining experiments revealed different degrees of co-expression of the protein product Kvβ1 with L6b/SP markers Ctgf (88%), Cplx3 (79%), and Nurr1 (58%), suggesting molecular subdivision of unilaterally-projecting L6b/SP neurons. In addition to expression in L6b/SP, scattered expression of Kcnab1 was observed during postnatal stages without layer specificity. Among splicing variants with three alternative first exons, the variant 1.1 explained all the cortical expression mentioned in this study. Together, our data suggest that L6b/SP neurons have corticocortical projections and Kcnab1 expression defines a subpopulation of L6b/SP neurons with a unilateral inter-areal projection.
摘要：
子板（SP）神经元是大脑皮层中最早出生的神经元之一，在基因表达方面具有异质性。SP神经元主要由投射神经元组成，在开发过程中很早就开始将轴突扩展到特定的目标区域。然而,轴突投影与SP神经元基因表达模式的关系,和它们的剩余层6b(L6b)神经元，基本上是未知的。在这项研究中,我们分析了小鼠皮质中L6b/SP神经元的皮质皮质投射,并搜索了在具有同侧区间投射的L6b/SP神经元中表达的标记基因.逆行追踪实验表明，在出生后第2天（PD），初级体感皮层（S1）中的L6b/SP神经元投射到同一皮质半球内的初级运动皮层（M1），但未显示任何call骨投射。这种单方面的投射模式一直持续到成年。我们的微阵列分析鉴定了编码电压门控钾通道（Kcnab1）β亚基的基因在L6b/SP中表达。逆行示踪和原位杂交的双重标记表明，Kcnab1在L6b/SP的单侧投射神经元中表达。在胚胎中特异性地检测到SP的表达早在胚胎日(E)14.5，在SP出现后不久。双重免疫染色实验显示蛋白产物Kvβ1与L6b/SP标记物Ctgf(88%)有不同程度的共表达,Cplx3（79%），和Nurr1（58%），提示单侧投射L6b/SP神经元的分子细分。除了在L6b/SP中表达外，在出生后阶段观察到Kcnab1的分散表达，没有层特异性。在具有三个选择性第一外显子的剪接变体中，变体1.1解释了本研究中提到的所有皮质表达。一起,我们的数据表明,L6b/SP神经元具有皮质皮质投射,Kcnab1表达定义了具有单侧区间投射的L6b/SP神经元亚群.
