Evolutionarily divergent mTOR remodels the translatome to drive rapid wound closure and regeneration

Abstract

An outstanding mystery in biology is why some species, such as the axolotl, can scarlessly heal and regenerate tissues while most mammals cannot. Here, we demonstrate that rapid activation of protein synthesis is a unique, and previously uncharacterized, feature of the injury response critical for limb regeneration in the axolotl (A. mexicanum). By applying polysome sequencing, we identify hundreds of transcripts, including antioxidants and ribosome components, which do not change in their overall mRNA abundance but are selectively activated at the level of translation from pre-existing mRNAs in response to injury. In contrast, we show that protein synthesis is not activated in response to digit amputation in the non-regenerative mouse. We further identify the mTORC1 pathway as a key upstream signal that mediates this regenerative translation response in the axolotl. Inhibition of this pathway is sufficient to suppress translation and axolotl regeneration. Surprisingly, although mTOR is highly evolutionarily conserved, we discover unappreciated expansions in mTOR protein sequence among urodele amphibians. By engineering an axolotl mTOR in human cells, we demonstrate that this change creates a hypersensitive kinase that may allow axolotls to maintain this pathway in a highly labile state primed for rapid activation. This may underlie metabolic differences and nutrient sensing between regenerative and non-regenerative species that are key to regeneration. Together, these findings highlight the unanticipated impact of the translatome on orchestrating the early steps of wound healing in highly regenerative species and provide a missing link in our understanding of vertebrate regenerative potential.

Molecular profile, source and lineage restriction of stem cells in an annelid regeneration model

“…The prevalent expression of ribosome-and cellular growth related genes in PSC-like cells during regeneration warranted an investigation of the role of TOR signaling in the activation or dedifferentiation or activity of stem cells in our model. As discussed above, TOR signaling has previously been implied in regeneration in many species, with a recent article demonstrating its crucial, regulatory role upstream of axolotl limb regeneration 49 . While the exact role this pathway plays in this process is not well understood, its activity seems to generally be required for regenerative stem cell proliferation.…”

“…The protein kinase Target of Rapamycin (TOR) has been implied in wound response and blastemal signaling in planarians, zebrafish and axolotl [49][50][51][52][53] , reviewed in 54 . Moreover, increased TOR signaling has also been observed in hypertranscriptomic cells 38 , as we observe them in the Platynereis regeneration process.…”

Insulin-like growth factor receptor / mTOR signaling elevates global translation to accelerate zebrafish fin regenerative outgrowth

“…Nonetheless, IGF/mTOR’s establishment and outgrowth roles could share a common mechanism in broadly elevated protein translation. Intriguingly, axolotl limb resection also triggers a rapid increase in mTOR-dependent protein activation essential to initiate regeneration (Zhulyn et al, 2021). This effect is most pronounced in epidermal cells, consistent with the early role of the wound epidermis in triggering a regenerative response.…”

Salamanders aid research into mechanisms of non‐canonical organ regeneration for clinical applications

“…It is well known that immunity is thought to be a key factor in the great regenerative capacity of salamanders [17, 18]; (2) The liver has a robust synthetic capacity and is responsible for synthesizing 90% of all soluble proteins [19]. Recent studies have shown that rapid protein synthesis is essential for limb regeneration in salamanders [20]; (3) The liver stores the energy substance hepatic glycogen. It is well known that organ regeneration is a process that is highly dependent on energy expenditure and requires rapid metabolism of energy in the early stages of regeneration [21–23].…”