Development of the Layer 5 Pyramidal Neuron Subgroup Expressing Er81

在发育中的大脑皮层，锥体神经元在生发区产生，并迁移到发育中的皮质板层，最早产生的占据最深的层，最后产生更浅的层。在生发区内产生的锥体神经元的类型受基因表达的不同调控控制，其中一些已经被识别出来。我们还不知道特定基因的表达如何导致轴突生长、靶点选择或生理特性的特定模式。ER81在发育早期的脑室区域的分裂细胞中表达，然后在皮质板区有丝分裂后的L5神经元中表达。在Pax6突变体和Ngn2突变体中，Er81的表达都被破坏，这意味着Er81受这两个基因的调控。这表明Er81在神经元分化和皮质分层/迁移中发挥了早期作用，类似于它在嗅球发育中的作用。在后期，大多数表达L5Er81的神经元也表达Ctip2，但不表达Otx1。Ctip2控制皮质脊髓轴突突起的形成，而Otx1控制皮层/桥脑投射突起的形成。虽然一些研究表明Er81是Ctip2的下游，但我们的初步微阵列和qPCR数据表明Er81控制Ctip2，这一点在Er81 KO中看到的Ctip2上调中得到了证实。Ctip2的缺失导致CST轴突的丢失，而表达减少则导致皮质锥体神经元的错误指定，提示Ctip2和Er81控制轴突指定和L5投射的侧支撤回的过程。在皮质外，ER81特异性表达于L5神经元的靶核，如上丘和下橄榄，为研究皮质连接的特异性提供了一条令人兴奋的途径。ER81在脊髓回路的形成中也发挥了类似的作用，在脊髓中，Er81由特定的运动神经元和感觉神经元表达，在突变体中，这些相互连接的形成存在缺陷。Er81和Ctip2均受Fezf2调控。在Fezf2突变体中，表达L5Ctip2的细胞存在错误规范，相反，表达Tbr1和Satb2的细胞迁移到L6，并通过前连合/胼胝体发送轴突。Sat b2表达于L5腓骨头投射神经元，并抑制Ctip2的表达。在Er81KO小鼠中，我们发现表达Ctip2的神经元数量增加，并有膝盖体投射，这表明Er81通过抑制Satb2来控制Ctip2。可以看出，ER81在建立大脑皮层连通性方面发挥了核心作用。在过去的几年里，我们对皮层神经元规范的分子控制的理解有所提高，我们已经产生了一些非常令人兴奋的初步数据，支持了拟议的实验。虽然这个复杂的基因网络调节着特定锥体细胞的发育，但我们对这些网络的变化如何允许特定的区域连接--这是大脑连接的一个基本过程--知之甚少。随着大脑发育的临床相关研究确定了与导致大脑功能改变的发育障碍有关的新基因，了解正常转录网络的细节是至关重要的。虽然现在暗示特定基因如何影响皮质连接的知识可以使未来的治疗成为可能，但如果我们要在大脑修复中使用基于细胞的治疗，这将是必不可少的。目前的研究几乎完全集中在从固有的和实验诱导的干细胞中产生可识别的神经细胞类别，还没有对这些细胞如何在连接方面进行控制的理解。

In the developing cerebral cortex, pyramidal neurons are generated in the germinal zones and migrate into the developing cortical plate, with the earliest generated occupying the deepest layers, and the last generated the more superficial layers. Within the germinal zones the type of pyramidal neuron generated is controlled by differential regulation of gene expression, some of which have been identified. We do not yet know how expression of particular genes leads to particular patterns of axon outgrowth, target selection, or physiological properties. Er81 is expressed at early stages of cortical development in the ventricular zone in dividing cells and then later in post-mitotic L5 neurons in the cortical plate. In both Pax6 mutants and Ngn2 mutants the expression of Er81 is disrupted, implying Er81 is regulated by both of these genes. This suggests an early role for Er81 in neuronal differentiation and cortical lamination/migration, similar to its role in olfactory bulb development. At later stages, most L5 Er81 expressing neurons also express Ctip2 but not Otx1. Ctip2 controls the formation of corticospinal axon outgrowth, whilst Otx1 controls outgrowth of collicular/pontine projections. Whilst some studies have shown ER81 to be down-stream of Ctip2, our preliminary microarray and qPCR data show that Er81 controls Ctip2, which is confirmed by the up-regulation of Ctip2 seen in the Er81 KO. Loss of Ctip2 results in a loss of CST axons, whilst reduced expression results in mis-specification of cortical pyramidal neurons, suggesting that Ctip2 and Er81 control the processes of axonal specification and collateral withdrawal of L5 projections. Outside the cortex, Er81 is expressed specifically in target nuclei of L5 neurons, such as the superior colliculus and inferior olive, offering an exciting avenue for investigation of specification of cortical connectivity. A similar role has been shown for Er81 in formation of circuitry in the spinal cord where ER81 is expressed by specific motor and sensory neurons and there are defects in the formation of these interconnections in mutants. Er81 and Ctip2 are both regulated by Fezf2. In Fezf2 mutants there is a mis-specification of the L5 Ctip2 expressing cells, which instead express Tbr1 and Satb2, migrate to L6 and send axons through the anterior commissure/corpus callosum. Satb2 is expressed in L5 callosal projection neurons and represses the expression of Ctip2. In the Er81 KO mouse we showed an increase in the number of Ctip2-expressing neurons with callosal projections suggesting that Er81 controls Ctip2 by suppressing Satb2. Er81 can be seen to play a central role in the establishment of cortical connectivity. In the last few years our understanding of the molecular control of cortical neuronal specification has improved and we have produced some very exciting preliminary data underpinning the proposed experiments. Whilst this complex network of genes regulates the development of particular pyramidal cells, we have very little idea about how changes in these networks allow for specific regional connectivity which is a fundamental process of brain wiring. As clinically related studies of brain development identify new gene associations with developmental disorders that result in altered brain function, it is essential to understand the details of the normal transcriptional networks. Whist it is perhaps premature to suggest that knowledge of how specific genes influence cortical wiring could enable future therapies, this will be essential if we are ever to use cell based therapies in brain repair. Current research has focused almost exclusively on the generation of identifiable neuronal cell classes from both innate and experimentally induced stem cells, no understanding of how such cells might be controlled in terms of their connectivity has yet been made.