Guiding regeneration Many adult organisms can regenerate neural circuits after injury. However, it is not clear which guidance mechanisms operate to promote axon path finding in the adult. Scimone et al. addressed this question by investigating regeneration of the planarian visual system (see the Perspective by Roberts-Galbraith). Distinct muscle cell populations were found in close association with photoreceptor axons that, together with a neuron class, facilitated visual system assembly after diverse injuries or eye transplantations. These cells exhibited features similar to embryonic guidepost cells and were specified independently of eyes in precise locations by the action of adult positional information cues. Absence of these guidepost-like cells was associated with defective neuronal wiring in regeneration. Science, this issue p. eaba3203; see also p. 1428 Adult regeneration requires guidepost-like cells for precise rewiring of the eyes in the flatworm Schmidtea mediterranea. INTRODUCTION Multiple strategies exist to promote precise wiring of developing neuronal circuits. One strategy involves guidepost cells, which exist transiently in embryos. Guidepost cells can act as intermediate guidance targets for axons or by providing a scaffold that facilitates axonal targeting. Most guidance mechanisms become dispensable once the circuit is assembled. Loss of guidance mechanisms creates a potential limitation on regeneration of neuronal patterns—yet some animals are capable of functional regeneration of their nervous system. RATIONALE Assuming some adult animals have the ability to regenerate functional neuronal circuits, they must possess mechanism(s) for de novo repair of neuronal patterns. In this study, we aimed to characterize such mechanisms by studying regeneration of the planarian visual system after diverse injuries. RESULTS We identified a rare subset of muscle cells (notum+; frizzled 5/8-4+) concentrated at two precise anatomical locations and in tight association with photoreceptor axons. The first group of these cells was found near the eye, where visual axons project and fasciculate to form a bundle. The second group of these cells was found near choice points, where sorting of contralateral and ipsilateral axons occurs. Both groups of muscle cells were formed during regeneration of the visual system and were always tightly associated with axonal projections, consistent with a possible role in attraction to facilitate visual system assembly. In addition, we found that a notum+ set of neurons, located at the adult anterior brain commissure, regenerated before axonal midline crossing and was associated with optic chiasm regeneration. We reasoned that if the photoreceptor axon–associated notum+; frizzled 5/8-4+ muscle cells have a guidepost-like function, their formation should be independent of eye cells. Eyes transplanted to ectopic anatomical locations did not result in the formation of notum+; frizzled 5/8-4+ muscle cells. Furthermore, animals that were unable to generate eyes [ovo RNA interference (ovo RNAi) animals] were still capable of specifying these muscle cells at the right locations. In addition, we predicted that if these muscle cells were indeed guidepost-like cells, visual axon trajectories should be associated with them after eye transplantation into eyeless heads. In all instances, axons from transplanted eyes projected toward notum+; frizzled 5/8- 4+ muscle cells and often adjusted their trajectories after encountering them. We found that an array of signaling cues, which provide positional information essential for planarian patterning, was required for dictating the precise location of these guidepost-like cells. This provides a visual system–extrinsic mechanism for placing guidepost-like cells in the adult. Finally, with single-cell RNA sequencing and fluorescent in situ hybridization screening, we identified molecules and transcription factors expressed in these cells. RNAi studies reduced or eliminated muscle or neuronal guidepost-like cell subsets and resulted in aberrant patterns of visual axonal trajectories. CONCLUSION Adult molecular and cellular strategies for regenerating neuronal pattern in the absence of embryo-specific contexts must exist to overcome damage or loss after injury. Understanding these mechanisms might provide important insights for regenerative medicine. Here, we found adult guidepost-like cell populations, extrinsic to the visual system and placed by adult positional information, that promote normal visual system regeneration in planarians. Adult guidepost-like cells facilitate visual system regeneration in planarians. Muscle and neuron guidepost-like cells are present at key locations near the planarian visual system and are formed independently of photoreceptor axons. Regenerating and transplanted eyes target projections to guidepost-like cells. Positional information cues provide an eye-extrinsic mechanism to place guidepost-like cells in the adult. Loss of guidepost-like cells is associated with visual system disruption. wnt-5, slit, and ndk are involved in positional control of guidepost-like cell placement. fz5/8-4, frizzled 5/8-4. Neuronal circuits damaged or lost after injury can be regenerated in some adult organisms, but the mechanisms enabling this process are largely unknown. We used the planarian Schmidtea mediterranea to study visual system regeneration after injury. We identify a rare population of muscle cells tightly associated with photoreceptor axons at stereotyped positions in both uninjured and regenerating animals. Together with a neuronal population, these cells promote de novo assembly of the visual system in diverse injury and eye transplantation contexts. These muscle guidepost-like cells are specified independently of eyes, and their position is defined by an extrinsic array of positional information cues. These findings provide a mechanism, involving adult formation of guidepost-like cells typically observed in embryos, for axon pattern restoration in regeneration.

中文翻译：

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肌肉和神经元路标样细胞促进涡虫视觉系统再生

引导再生 许多成体生物体在受伤后可以再生神经回路。然而，尚不清楚哪些引导机制可以促进成人的轴突路径发现。Scimone 等人。通过研究涡虫视觉系统的再生解决了这个问题（参见 Roberts-Galbraith 的观点）。发现不同的肌肉细胞群与光感受器轴突密切相关，这些轴突与神经元类一起促进了各种损伤或眼移植后的视觉系统组装。这些细胞表现出类似于胚胎路标细胞的特征，并且通过成人位置信息线索的作用在精确位置独立于眼睛指定。这些路标样细胞的缺失与再生中的神经元接线缺陷有关。科学，这个问题 p。eaba3203; 另见 p. 1428 成年再生需要路标样细胞来精确重新布线扁虫 Schmidtea mediterranea 的眼睛。简介 存在多种策略来促进发育中的神经元回路的精确布线。一种策略涉及路标细胞，它们在胚胎中短暂存在。路标细胞可以作为轴突的中间指导目标，或通过提供促进轴突靶向的支架。一旦电路组装好，大多数引导机制就变得可有可无。引导机制的丧失对神经元模式的再生产生了潜在的限制——然而一些动物能够对其神经系统进行功能性再生。基本原理假设一些成年动物有能力再生功能性神经回路，它们必须具有从头修复神经元模式的机制。在这项研究中，我们旨在通过研究不同损伤后涡虫视觉系统的再生来描述这种机制。结果 我们发现了一个罕见的肌肉细胞亚群（notum+；frizzled 5/8-4+），集中在两个精确的解剖位置，并与光感受器轴突紧密相关。第一组这些细胞是在眼睛附近发现的，视觉轴突在这里突出并束缚形成一束。第二组这些细胞是在选择点附近发现的，在选择点附近会发生对侧和同侧轴突的分类。两组肌肉细胞都是在视觉系统再生过程中形成的，并且总是与轴突投射紧密相关，这与促进视觉系统组装的吸引力的可能作用一致。此外，我们发现位于成人前脑连合处的一组 notum+ 神经元在轴突中线交叉之前再生，并与视交叉再生相关。我们推断，如果光感受器轴突相关的 notum+；卷曲的 5/8-4+ 肌肉细胞具有类似路标的功能，它们的形成应该独立于眼细胞。移植到异位解剖位置的眼睛不会导致 notum+ 的形成；卷曲的 5/8-4+ 肌肉细胞。此外，无法产生眼睛的动物 [卵 RNA 干扰 (卵 RNAi) 动物] 仍然能够在正确的位置指定这些肌肉细胞。此外，我们预测，如果这些肌肉细胞确实是路标样细胞，那么在将眼睛移植到无眼头后，视觉轴突轨迹应该与它们相关联。在所有情况下，移植眼的轴突都投射到 notum+；卷曲 5/8- 4+ 肌肉细胞，并经常在遇到它们后调整它们的轨迹。我们发现需要一系列信号线索来指示这些路标样细胞的精确位置，这些信号线索提供涡虫图案化所必需的位置信息。这提供了一种视觉系统——用于在成人体内放置路标样细胞的外在机制。最后，通过单细胞 RNA 测序和荧光原位杂交筛选，我们鉴定了在这些细胞中表达的分子和转录因子。RNAi 研究减少或消除了肌肉或神经元路标样细胞亚群，并导致视觉轴突轨迹的异常模式。结论 必须存在在没有胚胎特异性环境的情况下再生神经元模式的成人分子和细胞策略，以克服受伤后的损伤或损失。了解这些机制可能为再生医学提供重要的见解。在这里，我们发现成年路标样细胞群，外在于视觉系统并由成年位置信息放置，促进真涡虫的正常视觉系统再生。成年路标样细胞促进涡虫的视觉系统再生。肌肉和神经元路标样细胞存在于涡虫视觉系统附近的关键位置，并且独立于光感受器轴突形成。再生和移植的眼睛将投射到路标样细胞上。位置信息提示提供了一种眼睛外在机制，可以在成人体内放置路标样细胞。路标样细胞的丢失与视觉系统中断有关。wnt-5、slit 和 ndk 参与路标样细胞放置的位置控制。fz5/8-4，卷曲的 5/8-4。受伤后受损或丢失的神经元回路可以在一些成年生物体中再生，但实现这一过程的机制在很大程度上是未知的。我们使用涡虫 Schmidtea mediterranea 来研究受伤后的视觉系统再生。我们在未受伤和再生动物的刻板位置上发现了一个罕见的与感光器轴突紧密相关的肌肉细胞群。与神经元群一起，这些细胞在不同的损伤和眼移植环境中促进视觉系统的从头组装。这些肌肉路标样细胞的指定独立于眼睛，它们的位置由一组外部位置信息线索定义。这些发现提供了一种机制，涉及通常在胚胎中观察到的路标样细胞的成体形成，用于再生中的轴突模式恢复。