A forkhead Transcription Factor Is Wound-Induced at the Planarian Midline and Required for Anterior Pole Regeneration

Planarian regeneration requires positional information to specify the identity of tissues to be replaced as well as pluripotent neoblasts capable of differentiating into new cell types. We found that wounding elicits rapid expression of a gene encoding a Forkhead-family transcription factor, FoxD. Wound-induced FoxD expression is specific to the ventral midline, is regulated by Hedgehog signaling, and is neoblast-independent. FoxD is subsequently expressed within a medial subpopulation of neoblasts at wounds involving head regeneration. Ultimately, FoxD is co-expressed with multiple anterior markers at the anterior pole. Inhibition of FoxD with RNA interference (RNAi) results in the failure to specify neoblasts expressing anterior markers (notum and prep) and in anterior pole formation defects. FoxD(RNAi) animals fail to regenerate a new midline and to properly pattern the anterior blastema, consistent with a role for the anterior pole in organizing pattern of the regenerating head. Our results suggest that wound signaling activates a forkhead transcription factor at the midline and, if the head is absent, FoxD promotes specification of neoblasts at the prior midline for anterior pole regeneration.

涡虫（Planarian）的再生过程需同时具备两类关键条件：一是用于确定待替换组织身份的位置信息，二是可分化为各类新生细胞类型的成体多能干细胞（neoblasts）。本研究发现，创伤刺激可快速诱导一个编码叉头框家族转录因子FoxD的基因的表达。创伤诱导的FoxD表达具有腹侧中线特异性，其表达受刺猬信号通路（Hedgehog signaling）调控，且不依赖于成体多能干细胞。在涉及头部再生的创伤部位，FoxD随后会在成体多能干细胞的内侧亚群中表达。最终，FoxD会与多种前体标记基因在前极（anterior pole）处共表达。通过RNA干扰（RNAi）抑制FoxD的表达后，无法特化出表达前体标记基因notum与prep的成体多能干细胞，同时会出现前极形成缺陷。经FoxD(RNAi)处理的涡虫无法再生出新的中线结构，也无法正确构建前部再生芽基，这与前极在调控再生头部的模式形成中发挥的功能相一致。本研究结果表明，创伤信号会在中线处激活一个叉头框转录因子；若缺失头部，FoxD会促进原有中线处的成体多能干细胞定向特化，以完成前极的再生。