Positional Information in Adult Bilaterian Tissues

Abstract

How the coordination of cellular and molecular processes guides the formation of complex biological structures and patterns is a multifaceted and longstanding question. In animal development, a single cell is able to become an organism with intricate morphology and organ systems, all arranged in a predictable, repeatable formation. In regeneration, missing tissue is reformed in a manner that recapitulates that which was lost, including shape, orientation, and internal structure. These challenging biological processes require positional information, which influences spatial fate and migration decisions to generate proper tissue formation. In this work, we investigated the mechanisms of positional information across the animal kingdom through the study of different organisms in Bilateria.

In planarians, positional information is regionally expressed by muscle cells in the form of genes encoding signaling molecules known as positional control genes (PCGs). Positional information in the planarian body is particularly influenced by putative organizer pole cells located at the extreme ends of the anterior-posterior axis. We identified nr4A as a gene required for the localization of pole cells to the extreme ends of the anterior-posterior (AP) axis. nr4A RNAi caused PCG expression to shift away from the extremes of the AP axis and the iterative duplication of anterior and posterior anatomy. This work reveals a phenotype in which positional information and the cells that influence positional information are no longer properly coordinated, leading to a state of positional disequilibrium.

Additionally, we sought to understand how the direction of positional information is established in the planarian body. Planarian regeneration involves a mechanism to distinguish anterior-facing wounds that require head regeneration from posterior-facing wounds that require tail regeneration. We found that RNAi of activin-2 interfered with this polarity resolution decision during regeneration, resulting in posterior-facing heads after amputation. We show that activin-2 influences asymmetric gene expression at amputations that is required for proper polarity resolution. This work reveals a role for Activin signaling in establishing proper positional information polarity during planarian regeneration.

We then sought to investigate how positional information functioned in adult tissues more generally in Bilateria. Acoels, a sister clade to the rest of Bilateria, also display a muscle-expressed PCG system that underlies their regenerative ability. This suggests that adult positional information could be evolutionarily ancient in Bilateria and might exist in many extant clades. We characterized regional gene expression in the regenerative axolotl limb and the non-regenerative mouse limb. This analysis revealed that, while developmental signaling factors rarely recapitulate developmental expression patterns in the limb, several developmental transcription factors are indeed expressed regionally as they are in development in limbs of both organisms. Other genes that regulate ECM, transcription, and signaling also display shared regional expression in limbs of both organisms. In both axolotl and mouse, connective tissue plays a primary role in this regional expression. This work provides a comprehensive characterization of regional gene expression in the vertebrate limb and is consistent with a model wherein connective tissue-driven adult positional information is an evolutionarily ancient and well-distributed characteristic of Bilateria.