Putative epithelial-mesenchymal transitions during salamander limb regeneration: Current perspectives and future investigations

Abstract

Previous studies have implicated epithelial-mesenchymal transition (EMT) in salamander limb regeneration. In this review, we describe putative roles for EMT during each stage of limb regeneration in axolotls and other salamanders. We hypothesize that EMT and EMT-like gene expression programs may regulate three main cellular processes during limb regeneration: (1) keratinocyte migration during wound closure; (2) transient invasion of the stump by epithelial cells undergoing EMT; and (3) use of EMT-like programs by non-epithelial blastemal progenitor cells to escape the confines of their niches. Finally, we propose nontraditional roles for EMT during limb regeneration that warrant further investigation, including alternative EMT regulators, stem cell activation, and fibrosis induced by aberrant EMT.

Keywords: axolotl; blastema; epithelial–mesenchymal transition; limb regeneration; mesenchymal–epithelial transition.

FIGURE 1

Stages of salamander limb regeneration. Upon amputation, salamanders undergo a limb regeneration response through sequential, programmed phases: wound closure, blastema formation, and differentiation. This process results in a fully regenerated limb.

FIGURE 2

Putative roles of EMT and EMT-related processes at each stage of limb regeneration. (A) Traditional EMT of keratinocytes during wound closure. (B) Possible regulatory role for peripheral innervation. (C) Potential transient stump invasion from the epithelium, and mobilization of stump tissue-resident progenitor cells via EMT-like processes. (D) Reverse MET and MET-like processes, which may occur during late-stage regeneration. Abbreviations: EMT, epithelial–mesenchymal transition; MET, mesenchymal–epithelial transition.

FIGURE 3

Hypothetical roles of EMT in salamander limb regeneration. A–E Summary of current perspectives on EMT and EMT-related processes from initial injury (A) to full limb regeneration (B) in salamanders. (C) Epidermal cells undergoing traditional EMT and migrating across the wound site. The basement membrane is depicted in blue. (D) A muscle satellite cell initiating EMT-like programs during blastema creation. Arrowheads in the box on the right point to possible non-traditional regulators of EMT, which we suggest warrant further investigation. (E) Differentiation of a putative tenocyte progenitor cell, with remodeling of cell–cell junctions. (F–H) Topics on EMT during limb regeneration that warrant future experimentation. (F) Future investigation of the relationship between aberrant EMT and fibrosis, which may be mediated by Areg. The cartoon depicts a Masson’s trichrome stain, with blue representing collagen deposition, and dots depicting nuclei of connective and muscle tissue. (G) Future investigation of EMT-like responses in systemically activated stem cells, possibly driven by peripheral nerve signaling. Blue dots represent systemically activated stem cells. (H) Future investigation of the effects of manipulating EMT-TFs in the blastema on regeneration. Abbreviations: EMT, epidermal–mesenchymal transition; EMT-TF, epidermal–mesenchymal transition–transcript factors; IFG, insulin growth factor; MET, mesenchymal–epithelial transition; PDGF, platelet-derived growth factor; SHH, sonic hedgehog; TGF-beta, transforming growth factor-beta; WE, wound epidermis.