Kelly E. Dooling

The axolotl is a popular model organism in regenerative biology owing to its ability to regenerate amputated limbs and internal organs. The role of injury-derived signals in initiating the regenerative response is still not well understood, but the potential involvement of the stress response is of interest, as injury and stress are temporally linked. The dominant glucocorticoid response to stress varies among species, with corticosterone generally considered dominant in most amphibians, whereas cortisol predominates in others. Here we characterize the adrenal stress response in the axolotl and describe methods to measure axolotl stress hormones to facilitate their inclusion in future research involving axolotl development and regeneration. We describe an intricate and unexpected axolotl stress response that involves cortisol and corticosterone, each being dominant under different conditions. Corticosterone is preferably activated by the classical hypothalamus–pituitary–interrenal axis pathway, with both arginine vasotocin and adrenocorticotropic hormone promoting its synthesis and release. Under manual stress and direct stimuli with acetylcholine, cortisol is more prominent, suggesting an alternative mechanism involving sympathetic nerve signaling. In response to an amputation injury, both cortisol and corticosterone are increased, with corticosterone being dominant, suggesting an injury-specific response. Finally, when administering glucocorticoids directly and measuring classical physiological effects of glucocorticoid signaling, cortisol is more potent. We propose a hypothesis that axolotls rely on cortisol as their dominant glucocorticoid, functioning in part as an extension of the catecholamine system. By contrast, corticosterone is mainly regulated classically via the hypothalamus–pituitary–interrenal axis.

Salamanders are capable of regenerating whole limbs throughout life, a feat that is unmatched within tetrapods. Limb regeneration is dependent upon the formation of a blastema, which contains undifferentiated cells capable of giving rise to most cells of the regenerated limb. Innervation is required for regeneration, along with many signaling pathways, including FGF, BMP and Wnt, but the role of VEGF signaling during salamander limb regeneration is not well understood. Here we show that VEGF signaling is essential for limb regeneration and that blastema cells and limb fibroblasts display impaired proliferation in the absence of VEGF signaling. By performing analogous experiments in planaria, which lack vasculature, we show a potential evolutionarily conserved role for VEGF in the expansion of blastema tissues that is separable from angiogenesis. Moreover, loss of VEGF signaling reduces induction of EMT-like processes, suggesting VEGF signaling functions upstream of the expression of EMT transcription factors, including Snai2 . These findings highlight potential roles for VEGF signaling during regeneration which may extend beyond typical findings related to angiogenesis.

Previous studies have reported that amputation invokes body-wide responses in regenerative organisms, but most have not examined the implications of these changes beyond the region of tissue regrowth. Specifically, long-range epidermal responses to amputation are largely uncharacterized, with research on amputation-induced epidermal responses in regenerative organisms traditionally being restricted to the wound site. Here, we investigate the effect of amputation on long-range epidermal permeability in two evolutionarily distant, regenerative organisms: axolotls and planarians. We find that amputation triggers a long-range increase in epidermal permeability in axolotls, accompanied by a long-range epidermal downregulation in MAPK signaling. Additionally, we provide functional evidence that pharmacologically inhibiting MAPK signaling in regenerating planarians increases long-range epidermal permeability. These findings advance our knowledge of body-wide changes due to amputation in regenerative organisms and warrant further study on whether epidermal permeability dysregulation in the context of amputation may lead to pathology in both regenerative and non-regenerative organisms. GRAPHICAL

SUMMARY: Many species regenerate lost body parts following amputation. Most limb regeneration research has focused on the immediate injury site. Meanwhile, body-wide injury responses remain largely unexplored but may be critical for regeneration. Here, we discovered a role for the sympathetic nervous system in stimulating a body-wide stem cell activation response to amputation that drives enhanced limb regeneration in axolotls. This response is mediated by adrenergic signaling, which coordinates distant cellular activation responses via the α _2A -adrenergic receptor, and local regeneration responses via β-adrenergic receptors. Both α _2A – and β-adrenergic signaling act upstream of mTOR signaling. Notably, systemically-activated axolotls regenerate limbs faster than naïve animals, suggesting a potential selective advantage in environments where injury from cannibalism or predation is common. This work challenges the predominant view that cellular responses underlying regeneration are confined to the injury site and argues instead for body-wide cellular priming as a foundational step that enables localized tissue regrowth.