Retinoid X receptor-mediated transdifferentiation cascade in budding tunicates

Area of Science:

• Developmental Biology
• Molecular Biology
• Chordate Evolution

Background:

• Retinoic acid (RA) is crucial for development and regeneration in many animals.
• Tunicates, as chordates, offer insights into conserved developmental pathways.
• The specific role of RA signaling in tunicate bud regeneration remains incompletely understood.

Purpose of the Study:

• To elucidate the gene regulatory network governing retinoic acid-induced transdifferentiation in the budding tunicate, Polyandrocarpa misakiensis.
• To investigate the differential roles of Retinoic Acid Receptor (RAR) and Retinoid X Receptor (RXR) in RA signaling.
• To understand the downstream targets and pathways activated by RA in tunicate regeneration.

Main Methods:

• In vivo and in vitro experiments using Polyandrocarpa misakiensis buds and cell lines.
• Application of different retinoic acid isomers (all-trans RA, 13-cis RA, 9-cis RA).
• Gene expression analysis using mRNA induction, small interfering RNA (siRNA) knockdown, and pharmacological inhibitors (ERK inhibitor).

Main Results:

• RA treatment induced RAR and RXR expression, with distinct ligand preferences compared to vertebrates.
• RXR mRNA, but not RAR, could induce transdifferentiation-related genes (RXR, ERK, MYC) and β-catenin (β-CTN) in an RA-dependent or independent manner.
• RXR, ERK, and β-CTN activities were significantly attenuated by RXR siRNA; ERK inhibition blocked dedifferentiation, and β-CTN siRNA suppressed morphogenesis.

Conclusions:

• RA's primary role in P. misakiensis transdifferentiation is positive feedback regulation of RXR, rather than direct RAR activation.
• This mechanism highlights an ancient mode of RA signaling potentially conserved in chordates.
• RXR, ERK, and β-catenin form a critical pathway for RA-mediated regeneration and body axis formation in tunicates.