Edited by Robb Krumlauf, Stowers Institute for Medical Research, Kansas City, MO, and approved July 24, 2018 (received for review March 2, 2018)
The evolutionary changes behind the loss in regenerative potential from salamanders to mammals remain largely elusive. Lizards, representing an intermediary species between the two, possess a limited ability to regenerate their tails. Here, we probe the mechanisms behind the differing regenerative patterns between lizards and salamanders, and we find that neural stem cells within the regenerated spinal cords are distinct cell populations that regulate divergent tail regeneration patterns. This finding sheds light on the factors that govern regenerative ability as well as the loss of this capability and brings us one step closer to eventually elucidating strategies to allow for mammalian regeneration.
While lizards and salamanders both exhibit the ability to regenerate amputated tails, the outcomes achieved by each are markedly different. Salamanders, such as Ambystoma mexicanum, regenerate nearly identical copies of original tails. Regenerated lizard tails, however, exhibit important morphological differences compared with originals. Some of these differences concern dorsoventral patterning of regenerated skeletal and spinal cord tissues; regenerated salamander tail tissues exhibit dorsoventral patterning, while regrown lizard tissues do not. Additionally, regenerated lizard tails lack characteristically roof plate-associated structures, such as dorsal root ganglia. We hypothesized that differences in neural stem cells (NSCs) found in the ependyma of regenerated spinal cords account for these divergent regenerative outcomes. Through a combination of immunofluorescent staining, RT-PCR, hedgehog regulation, and transcriptome analysis, we analyzed NSC-dependent tail regeneration. Both salamander and lizard Sox2+ NSCs form neurospheres in culture. While salamander neurospheres exhibit default roof plate identity, lizard neurospheres exhibit default floor plate. Hedgehog signaling regulates dorsalization/ventralization of salamander, but not lizard, NSCs. Examination of NSC differentiation potential in vitro showed that salamander NSCs are capable of neural differentiation into multiple lineages, whereas lizard NSCs are not, which was confirmed by in vivo spinal cord transplantations. Finally, salamander NSCs xenogeneically transplanted into regenerating lizard tail spinal cords were influenced by native lizard NSC hedgehog signals, which favored salamander NSC floor plate differentiation. These findings suggest that NSCs in regenerated lizard and salamander spinal cords are distinct cell populations, and these differences contribute to the vastly different outcomes observed in tail regeneration.
Author contributions: A.X.S., R.S.T., and T.P.L. designed research; A.X.S., R.L., M.L.H., and T.P.L. performed research; A.X.S., R.L., M.L.H., and T.P.L. analyzed data; and A.X.S. and T.P.L. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1803780115/-/DCSupplemental.
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