Systemic Bud Induction and Retinoic Acid Signaling Underlie Whole Body Regeneration in the Urochordate Botrylloides leachi

Abstract

Regeneration in adult chordates is confined to a few model cases and terminates in restoration of restricted tissues and organs. Here, we study the unique phenomenon of whole body regeneration (WBR) in the colonial urochordate Botrylloides leachi in which an entire adult zooid is restored from a miniscule blood vessel fragment. In contrast to all other documented cases, regeneration is induced systemically in blood vessels. Multiple buds appear simultaneously in newly established regeneration niches within vasculature fragments, stemming from composites of pluripotent blood cells and terminating in one functional zooid. We found that retinoic acid (RA) regulates diverse developmental aspects in WBR. The homologue of the RA receptor and a retinaldehyde dehydrogenase-related gene were expressed specifically in blood cells within regeneration niches and throughout bud development. The addition of RA inhibitors as well as RNA interference knockdown experiments resulted in WBR arrest and bud malformations. The administration of all-trans RA to blood vessel fragments resulted in doubly accelerated regeneration and multibud formation, leading to restored colonies with multiple zooids. The Botrylloides system differs from known regeneration model systems by several fundamental criteria, including epimorphosis without the formation of blastema and the induction of a “multifocal regeneration niche” system. This is also to our knowledge the first documented case of WBR from circulating blood cells that restores not only the soma, but also the germ line. This unique Botrylloides WBR process could serve as a new in vivo model system for regeneration, suggesting that RA signaling may have had ancestral roles in body restoration events.

From carrot to clinic: an overview of the retinoic acid signaling pathway

“…Moreover, the appendicularian tunicate Oikopleura dioica has lost its RA signaling complement almost in its entirety, with only RXR being present [50,51]. At least in ascidian tunicates, RA signaling appears to be active with roles in specification of ectoderm and endoderm [172,173], development and morphogenesis of anterior CNS [50,174] and adhesive organs (palps) [173,175], and in whole body regeneration [176].…”

Reconsidering regeneration in metazoans: an evo-devo approach

“…Positional information of blastema cells during limb regeneration in amphibians is affected by retinoids, mainly by influencing the expression of homeobox genes (Gardiner et al, 1995;Mercader et al, 2005). RA also has a role in the transdifferentiation of epithelial cells during gut regeneration, budding, and whole body regeneration in colonial ascidians (Hisata et al, 1998;Kamimura et al, 2000;Rinkevich et al, 2007). By directing intracellular signals, the Hippo pathway intersects directly with several master signaling cascades and acts as an integration point, particularly maintaining the correct organ size during development and homeostasis across many metazoans (Pan, 2007;Mauviel et al, 2011;Varelas and Wrana, 2012).…”

Vertebrate-like regeneration in the invertebrate chordate amphioxus

“…In contrast, among vertebrate models, studies have highlighted dedifferentiation of existing structures or the role of muscle satellite cells (3). Bridging the gap, ambulacrarian deuterostomes and urochordates show considerable regenerative capacity (1,(4)(5)(6) (Fig. 1A).…”