Cell-Autonomous and Non-cell-autonomous Function of Hox Genes Specify Segmental Neuroblast Identity in the Gnathal Region of the Embryonic CNS in <i>Drosophila</i>

During central nervous system (CNS) development neural stem cells (Neuroblasts, NBs) have to acquire an identity appropriate to their location. In thoracic and abdominal segments of Drosophila, the expression pattern of Bithorax-Complex Hox genes is known to specify the segmental identity of NBs prior to their delamination from the neuroectoderm. Compared to the thoracic, ground state segmental units in the head region are derived to different degrees, and the precise mechanism of segmental specification of NBs in this region is still unclear. We identified and characterized a set of serially homologous NB-lineages in the gnathal segments and used one of them (NB6-4 lineage) as a model to investigate the mechanism conferring segment-specific identities to gnathal NBs. We show that NB6-4 is primarily determined by the cell-autonomous function of the Hox gene Deformed (Dfd). Interestingly, however, it also requires a non-cell-autonomous function of labial and Antennapedia that are expressed in adjacent anterior or posterior compartments. We identify the secreted molecule Amalgam (Ama) as a downstream target of the Antennapedia-Complex Hox genes labial, Dfd, Sex combs reduced and Antennapedia. In conjunction with its receptor Neurotactin (Nrt) and the effector kinase Abelson tyrosine kinase (Abl), Ama is necessary in parallel to the cell-autonomous Dfd pathway for the correct specification of the maxillary identity of NB6-4. Both pathways repress CyclinE (CycE) and loss of function of either of these pathways leads to a partial transformation (40%), whereas simultaneous mutation of both pathways leads to a complete transformation (100%) of NB6-4 segmental identity. Finally, we provide genetic evidences, that the Ama-Nrt-Abl-pathway regulates CycE expression by altering the function of the Hippo effector Yorkie in embryonic NBs. The disclosure of a non-cell-autonomous influence of Hox genes on neural stem cells provides new insight into the process of segmental patterning in the developing CNS.在中枢神经系统（central nervous system, CNS）发育过程中，成神经细胞（Neuroblasts, NBs，一类神经干细胞）需要获得与其所处位置匹配的细胞身份。在果蝇的胸腹部体节中，双胸复合体（Bithorax-Complex）Hox基因的表达模式已被证实可在成神经细胞从神经外胚层脱离前，指定其体节身份。相较于胸部体节，头部区域的基础体节单位存在不同程度的特化，而成神经细胞在该区域的体节特化精确机制仍未明晰。我们在颌部体节（gnathal segments）中鉴定并表征了一系列序列同源的成神经细胞谱系，并选取其中的NB6-4谱系作为模型，探究赋予颌部成神经细胞体节特异性身份的分子机制。研究表明，NB6-4的细胞身份主要由Hox基因变形（Deformed, Dfd）的细胞自主性功能决定。值得注意的是，其同时依赖于在相邻前后区室中表达的头足蛋白（labial）与触角足（Antennapedia）的非细胞自主性功能。我们鉴定出分泌分子Amalgam (Ama)是触角足复合体（Antennapedia-Complex）Hox基因头足蛋白、变形、性梳减少（Sex combs reduced）以及触角足的下游调控靶标。结合其受体神经粘连蛋白（Neurotactin, Nrt）与效应激酶阿伯逊酪氨酸激酶（Abelson tyrosine kinase, Abl），Ama可与细胞自主性的Dfd通路协同作用，共同精准指定NB6-4的颚体节身份。两条通路均可抑制细胞周期蛋白E（CyclinE, CycE）的表达：任一通路功能缺失会导致40%的部分体节身份转化，而两条通路同时突变则会使NB6-4的体节身份发生100%的完全转化。最后，我们通过遗传学实验证实，Ama-Nrt-Abl通路可通过调控胚胎成神经细胞中河马通路（Hippo）效应因子Yorkie的功能，进而调节CyclinE的表达。本次研究揭示了Hox基因通过非细胞自主性途径影响神经干细胞的机制，为发育中中枢神经系统的体节模式形成过程提供了全新的研究视角。