Bacterial TLR2/6 Ligands Block Ciliogenesis, Derepress Hedgehog Signaling, and Expand the Neocortex

Microbial components have a range of direct effects on the fetal brain. However, little is known about the cellular targets and molecular mechanisms that mediate these effects. Neural progenitor cells (NPCs) control the size and architecture of the brain and understanding the mechanisms regulating NPCs is crucial to understanding brain developmental disorders. We identify ventricular radial glia (vRG), the primary NPC, as the target of bacterial cell wall (BCW) generated during the antibiotic treatment of maternal pneumonia. BCW enhanced proliferative potential of vRGs by shortening the cell cycle and increasing self-renewal. Expanded vRGs propagated to increase neuronal output in all cortical layers. Remarkably, Toll-like receptor 2 (TLR2), which recognizes BCW, localized at the base of primary cilia in vRGs and the BCW-TLR2 interaction suppressed ciliogenesis leading to derepression of Hedgehog (HH) signaling and expansion of vRGs. We also show that TLR6 is an essential partner of TLR2 in this process. Surprisingly, TLR6 alone was required to set the number of cortical neurons under healthy conditions. These findings suggest that an endogenous signal from TLRs suppresses cortical expansion during normal development of the neocortex and that BCW antagonizes that signal through the TLR2/cilia/HH signaling axis changing brain structure and function. Overall design: Comparative spatial transcriptomics of mouse brain sections obtained at E12 after either S. pneumoniae + ampicillin challenge or after ampicillin control.

微生物组分对胎儿大脑存在多种直接作用，但目前针对介导此类作用的细胞靶点与分子机制的认知仍较为匮乏。神经祖细胞（Neural progenitor cells, NPCs）可调控大脑的尺寸与结构，阐明其调控机制对于理解大脑发育障碍至关重要。本研究鉴定出，母体肺炎接受氨苄青霉素治疗过程中产生的细菌细胞壁（bacterial cell wall, BCW）的作用靶点为主要神经祖细胞——室管膜放射状胶质细胞（ventricular radial glia, vRG）。BCW可通过缩短细胞周期、增强自我更新能力，提升vRG的增殖潜能；扩增的vRG会进一步增殖，使所有皮层层的神经元产出量增加。值得注意的是，可识别BCW的Toll样受体2（Toll-like receptor 2, TLR2）定位于vRG的初级纤毛基部，BCW与TLR2的相互作用会抑制纤毛发生，进而解除Hedgehog（HH）信号通路的抑制并促进vRG扩增。本研究同时证实，Toll样受体6（Toll-like receptor 6, TLR6）是TLR2在该过程中的必需伴侣蛋白。令人意外的是，在健康状态下，仅TLR6即可调控皮层神经元的数量。上述研究结果表明，在新皮层正常发育过程中，Toll样受体介导的内源性信号会抑制皮层扩张；而BCW通过TLR2/纤毛/HH信号轴拮抗该信号，从而改变大脑的结构与功能。实验整体设计：分别对经肺炎链球菌（S. pneumoniae）+氨苄青霉素攻毒、或仅给予氨苄青霉素对照处理的小鼠，在胚胎12天（E12）采集脑组织切片，开展比较空间转录组学分析。