Development of the Layer 5 Pyramidal Neuron Subgroup Expressing Er81

点击翻译按钮获取中文摘要

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.