Organizer regeneration and patterning of stem cell lineages in planarians

Abstract

Planarians are freshwater flatworms capable of whole-body regeneration. Like development, regeneration requires the establishment of tissue patterns and the specification of appropriate cell types. However, regeneration has the additional challenges of performing these tasks in the absence of developmental cues, and in the presence of pre-existing, differentiated tissues. Planarian head regeneration involves the anterior pole, which is a cluster of cells in the tip of head required for proper head patterning. We used transplantation to show that the head tip region, containing the anterior pole, has organizing activity. We sought to establish how the anterior pole is placed during regeneration. Anterior pole progenitors are specified medially and accumulate to form a cluster at the DV median plane. Pole progenitors are specified at the pre-existing midline, and coalesce to the DV boundary in the blastema. These findings demonstrate that during its formation, the anterior pole integrates positional information from the pre-existing tissues. This process places the anterior pole in the appropriate location to organize patterning during head regeneration. Regeneration in planarians involves two important components. Neoblasts are pluripotent stem cells that are the source of all new cells during regeneration. Position control genes (PCGs) are developmental signaling genes that are constitutively expressed in adult planarian muscle, and are thought to provide the molecular instructions for regeneration. How neoblasts respond to PCGs to produce regionally appropriate cells types remains unknown. To better understand this process, we characterized the planarian epidermis. Bulk RNA-sequencing of mature epidermal cells and subsequent in situ validation identified numerous spatial patterns. To understand when these patterns arose during differentiation, we performed single-cell sequencing (SCS) of epidermal progenitors. Positional identities present in the mature epidermis were also present in progenitors, with dorsal-ventral identity present in spatially distant neoblasts. Epidermal neoblasts were able change their DV identity upon inhibition of Bmp signaling. This demonstrates that neoblasts can respond to changes in their signaling environment, linking positional information from muscle to the generation of regionally-appropriate cells types.