Ontogeny of inhibitory neurons in the auditory hindbrain

在发育中的组织中指定独特的细胞命运对任何多细胞生物体都是基本的。哺乳动物的后脑含有许多不同的神经元群，这些神经元群共同构成了耳蜗核复合体(Cnc)、上橄榄复合体(SOC)和外侧丘系(LL)。这些细胞中有相当一部分是抑制性神经元，它们以中间神经元的形式出现，甚至形成单独的核。这些神经元在频谱和时序分析、声源位置和听流分离中起着关键作用。对小鼠的遗传学分析表明，中枢神经系统的抑制性神经元是Ptf1a+的直系细胞，而SOC的许多抑制性神经元是EN1+的直系细胞。然而，分子伴侣和下游因素仍有待研究。此外，对于听觉后脑中的一些抑制性神经元群体，其个体发育尚不清楚。为了缩小这些差距，我们将表征Lbx1+和Lhx1+谱系对听觉后脑的贡献。这两种转录因子都与其他脑区的Ptf1a+谱系有关。初步数据显示，如果不是全部，也有许多抑制神经元是Lbx1+。此外，在SOC中还观察到Lbx1+线形细胞，与此结构中缺少Ptf1a+线形细胞形成对比。在这个项目中，我们将首先定义Lbx1+谱系对听觉后脑的确切贡献。其次，为了确定这种转录因子的功能，将分析Lbx1-/-小鼠在细胞命运、细胞丢失和异常轴突寻路中的开关。此外，通过比较已知的Lbx1下游作用因子在野生型和Lbx1-/-小鼠中的表达模式，将识别听觉后脑中依赖Lbx1的遗传程序。第三，我们将确定Lhx1+直系细胞对听觉后脑的贡献。最后，我们将使用获得的数据来解决听觉后脑的进化，这是该领域的另一个悬而未决的问题。目前的观点认为，哺乳动物的听觉后脑核团代表着进化的新颖性，而它们在鸟类中功能相同的核团反映了趋同进化产生的同型结构。这一过程可以通过招募不同或相似的遗传程序来进行。后者将支持进化过程中的发展制约的概念。为了解决这个问题，我们将描述Ptf1a/Lbx1相关的基因调控网络在鸡中也是如此，因为Ptf1a+直系细胞也对鸟类抑制性听神经元起作用。总而言之，这种系统的方法将产生关于跨四足动物的抑制性听神经元个体发育的全面知识。因此，它还将为神经系统的进化发展过程提供新的见解。

The specification of unique cell fates within the developing tissue is fundamental for any multicellular organism. The mammalian hindbrain harbors many different neuronal populations, which together constitute the cochlear nucleus complex (CNC), the superior olivary complex (SOC), and the lateral lemniscus (LL). A substantial number of these cells are inhibitory neurons that occur as interneurons or form even separate nuclei. These neurons play pivotal roles in the analysis of spectrum and timing, location of sound sources, and auditory stream segregation. Genetic analyses in mice revealed that inhibitory neurons of the CNC are Ptf1a+ lineal cells and many inhibitory neurons in the SOC are En1+ lineal cells. However, molecular partners and downstream factors remain to be investigated. Furthermore, for some inhibitory neuronal populations in the auditory hindbrain, their ontogeny is still unknown. To close these gaps, we will characterize the contribution of the Lbx1+ and Lhx1+ lineages to the auditory hindbrain. Both transcription factors have been associated with the Ptf1a+ lineage in other brain regions. Preliminary data indicate that many if not all inhibitory neurons in the CNC are Lbx1+. Further Lbx1+ lineal cells were observed in the SOC, contrasting the absence of Ptf1a+ lineal cells in this structure. Within this project, we will first define the precise contribution of the Lbx1+ lineage to the auditory hindbrain. Second, to define the function of this transcription factor, Lbx1-/- mice will be analyzed with respect to switches in cell fate, cell loss, and abnormal axonal pathfinding. Furthermore, the Lbx1 dependent genetic program in the auditory hindbrain will be identified by comparing the expression pattern of known Lbx1 downstream acting factors in wildtype and Lbx1-/- mice. Third, we will determine the contribution of Lhx1+ lineal cells to the auditory hindbrain. Finally, we will use the obtained data to address the evolution of the auditory hindbrain, another outstanding question in the field. Current view holds that mammalian auditory hindbrain nuclei represent evolutionary novelties and that their functionally equivalent nuclei in birds reflect homoplasious structures, generated by convergent evolution. This process can occur by recruitment of distinct or similar genetic programs. The latter would support the concept of developmental constraints within evolutionary processes. To address this issue, we will characterize the Ptf1a/Lbx1 associated gene regulatory network also in chicken, as Ptf1a+ lineal cells contribute to avian inhibitory auditory neurons as well. Altogether, this systematic approach will generate comprehensive knowledge concerning the ontogeny of inhibitory auditory neurons across tetrapods. Thereby it will also provide new insight into evolutionary developmental processes in the nervous system.