Pattern formation and essential responses for regeneration in planarians

Abstract

The fundamental requirements for regeneration are poorly understood. Planarians can robustly regenerate all tissues after injury, involving stem cells, patterning cues and a set of cellular and molecular responses collectively called the "missing tissue" or "regenerative" response. The missing tissue response has long been considered a fundamental requirement of planarian regeneration. follistatin, which encodes an extracellular Activin inhibitor, is required for the missing tissue response after head amputation, and for subsequent regeneration. We found that follistatin is required for the missing tissue response regardless of the wound context, but only causes regeneration failure after head amputation. This head regeneration failure involves follistatin-mediated regulation of Wnt signaling at wounds, and is not a consequence of a diminished missing tissue response.

We found that all tested contexts of regeneration, including head regeneration, could occur with a defective missing tissue response, however, at a slower pace. Our findings suggest that in the absence of major cellular and molecular programs induced by large injuries, regulation of wound-induced Wnt signaling to enable regenerative re-patterning along with continuous tissue turnover can mediate successful regeneration in essentially any wound context. Wnt signaling regulates primary body axis formation across the Metazoa, with high Wnt signaling specifying posterior identity. Whether a common Wnt-driven transcriptional program accomplishes this broad role is poorly understood. We identified genes acutely affected after Wnt signaling inhibition in the posterior of two regenerative species, the planarian Schmidtea mediterranea and the acoel Hofstenia miamia, which are separated by >550 million years of evolution.

Wnt signaling was found to maintain positional information in muscle and regional gene expression in multiple differentiated cell types. sp5, Hox genes, and Wnt pathway components are down-regulated rapidly after [beta]-catenin RNAi in both species. brachyury, a vertebrate Wnt target, also displays Wnt-dependent expression in Hofstenia. Planarian sp5 inhibits Wnt-dependent expression of trunk genes in the tail, promoting separate tail-trunk body domains. We propose that common regulation of a small gene set - Hox, sp5, and brachyury - might underlie the widespread utilization of Wnt signaling in primary axis patterning across the Bilateria.