By demonstrating that SHH signaling plateaus in large blastemas while digit patterning continues to scale, this work overturns assumed morphogen coordination and identifies discordant scaling as the fundamental—and pharmacologically rescuable—barrier to adult organ regeneration.
Our work shows that regeneration of an adult body part, such as the salamander limb, requires morphogen signaling ranges to expand and adapt to tissue size. Even in highly regenerative animals such the axolotl, the spatial domains of key signalling pathways do not scale in a perfectly coordinated manner, resulting in defects in the number of digits regenerated in very large animals.This finding is an eye-opener for future endeavours to regenerate adult-sized organs.
Axolotl limb regeneration across a 10-fold size range reveals that different signaling pathways scale independently, with SHH-mediated growth signaling exhibiting size-dependent feedback that saturates in large tissues while digit patterning maintains linear scaling, resulting in discordant scaling that causes regenerative defects in the largest animals that can be rescued by restoring SHH-dependent growth through either ligand supplementation or amputation at smaller tissue dimensions.
Orders of magnitude distinguish an organ’s size in adulthood from its size when patterning was established during embryonic development. The prospect of engineering adult organ regeneration raises the fundamental question of whether regenerative stem cell patterning should be elicited at embryonic or adult scale. Axolotls regenerate their limbs at all stages of post-embryonic life, encompassing an order of magnitude increase in animal size, but the mechanisms allowing this robust capacity remain unclear. Limb regeneration occurs by the formation of an embryonic-like progenitor zone, the blastema, whose dimensions increase with animal size, suggesting that at least some aspects of limb regeneration must scale. Here, by combining spatial transcriptomics, biophysical modelling, quantitative imaging, and functional perturbations, we found distinct scaling signatures among key signaling pathways, arguing against a body-size-dependent hormonal scaling mechanism. SHH signaling showed partial scaling that saturated in largest adult sizes, with a correspondingly early termination of blastema growth while Wnt9a-modulated digit patterning scaled at all animal sizes. As a result of this differential scaling, digit patterning wavelength was mismatched with respect to domain growth in the largest blastemas, preventing the addition of the last digit. In agreement with this model, regenerative failure can be rescued by timed supplementation of SHH. Our results show that coordinated scaling of morphogen signaling is a key requirement for adult organ regeneration.