Regeneration is an elegant and complex process informed by both local and long-range signals. Many current studies on regeneration are largely limited to investigations of local modulators within a canonical cohort of model organisms. Enhanced genetic tools increasingly enable precise temporal and spatial perturbations within these model regenerators, and these have primarily been applied to cells within the local injury site. Meanwhile, many aspects of broader spatial regulators of regeneration have not yet been examined with the same level of scrutiny. Recent studies have shed important insight into the significant effects of environmental cues and circulating factors on the regenerative process. These observations highlight that consideration of more systemic and possibly more broadly acting cues will also be critical to fully understand complex tissue regeneration. In this review, we explore the ways in which systemic cues and circulating factors affect the initiation of regeneration, the regenerative process, and its outcome. As this is a broad topic, we conceptually divide the factors based on their initial input as either external cues (for example, starvation and light/dark cycle) or internal cues (for example, hormones); however, all of these inputs ultimately lead to internal responses. We consider studies performed in a diverse set of organisms, including vertebrates and invertebrates. Through analysis of systemic mediators of regeneration, we argue that increased investigation of these “systemic factors” could reveal novel insights that may pave the way for a diverse set of therapeutic avenues.
Animals exhibit extreme diversity in regenerative ability. This likely reflects different, lineage-specific selective pressures in their evolutionary histories, but how specific molecular features of regenerative programs help solve species-specific challenges has not been examined in detail. Here we discover that, in the highly-regenerative axolotl salamander, a conserved, body-wide stem cell activation response triggered in response to limb removal primes undisturbed limbs for regeneration upon subsequent amputation. This response should be particularly useful to salamanders, which frequently lose limbs in response to cannibalism. We further demonstrate the body-wide response requires both peripheral nervous system input at these distant sites and mTOR signaling. We defined gene expression changes within the nerves and nearby tissues, harboring responsive stem cells, leading to identification of candidate genetic pathways influencing distant stem cell activation following amputation. Functional experimentation confirmed a requirement for adrenergic signaling in amputation-induced activation of distant stem cells. These findings reveal a direct link between systemic cellular activation responses to local tissue damage and overall regenerative ability. Similar systemic activation responses to tissue removal have been observed in animals with widely differing regenerative abilities (e.g., planaria to mice), suggesting that it is the responses downstream of these signals, likely sculpted by differing evolutionary pressures, that ultimately distinguish regenerators from non-regenerators.
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