More Than Just a Bandage: Closing the Gap Between Injury and Appendage Regeneration

Abstract

The remarkable regenerative capabilities of amphibians have captured the attention of biologists for centuries. The frogs Xenopus laevis and Xenopus tropicalis undergo temporally restricted regenerative healing of appendage amputations and spinal cord truncations, injuries that are both devastating and relatively common in human patients. Rapidly expanding technological innovations have led to a resurgence of interest in defining the factors that enable regenerative healing, and in coupling these factors to human therapeutic interventions. It is well-established that early embryonic signaling pathways are critical for growth and patterning of new tissue during regeneration. A growing body of research now indicates that early physiological injury responses are also required to initiate a regenerative program, and that these differ in regenerative and non-regenerative contexts. Here we review recent insights into the biophysical, biochemical, and epigenetic processes that underlie regenerative healing in amphibians, focusing particularly on tail and limb regeneration in Xenopus. We also discuss the more elusive potential mechanisms that link wounding to tissue growth and patterning.

Keywords: Xenopus; epigenetic; innate immune; limb bud; proliferation; reactive oxygen species; regeneration; tail.

FIGURE 1

Cellular processes activated by injury in Xenopus tail regeneration. A regenerative stage 41 tadpole is shown, prior to the onset of independent feeding and the refractory period. Responses to injury that are critical for regeneration include (A) formation of reactive oxygen species such as H₂O₂ through the action of NOX complexes (purple) and p22-phox/cyba (light purple); (B) bioelectrical signaling mediated by ion channel activation; (C) recruitment of innate immune cell types such as macrophages; (D) epigenetic modifications that affect chromatin accessibility and transcription, and (E) activation of proliferation of blastemal cells and tissue-specific progenitors.

FIGURE 2

Integrative model of regeneration. Experimental evidence reviewed here suggests the five modalities of early wound response and regeneration discussed in this paper are interconnected. Experimentally determined, indirect connections are denoted by dashed lines. (A) ROS has been shown to be one of the most early activated signaling modalities and is upstream of ion channel activity, proliferation, epigenetic modification, and transcription factor activation. (B) Membrane depolarization and ion channel activity have been shown to act upstream of proliferation and innate immune cell recruitment. (C) Innate immune cells are shown to act upstream of proliferation and perhaps release cytokines necessary for successful regeneration. (D) Epigenetic modifications have been profiled to look at repressive and active marks in regeneration. Presence of these marks along with the epigenetic enzymes that remodel chromatin have been shown to be important for regeneration. (E) Cell proliferation appears to be downstream of early wound responses and perhaps part of a transition from wound repair to regeneration.

FIGURE 3

Comparison of physiological phenomena during healing between regenerative and non-regenerative organisms. (A) Color coded boxes correspond to either ROS (purple), changes in membrane potential (yellow), innate immune cell recruitment (orange), epigenetic reprogramming (blue), or tissue-specific stem cell proliferation and differentiation (green). (B-D) Summarization of physiological phenomena either known to be associated with regenerative healing, known to be associated with scarring, or not yet studied (unknown) in regenerative and non-regenerative contexts or species. “Pro-regen” refers to epimorphic regeneration as described in this review. “Pro-scarring” refers to injuries that undergo wound healing and scarring. (B) Regeneration or scarring of the epithelium in tadpoles (left), late-metamorphic froglets with minimal regeneration (middle), non-regenerative mammals (right). (C) Regeneration or scarring of the spinal cord. (D) Regeneration or scarring of the amphibian limb and mammalian digit tip. ¹In adult frogs and mammals, the epidermal layer regenerates, however, the dermis is replaced by fibrous tissue.