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The tail of the frog tadpole, comprising spinal cord, muscle, and notochord, regenerates following partial amputation. We show that, in Xenopus, this occurs throughout development, except for a "refractory period" between stages 45 and 47, when tails heal over without regeneration. Regeneration can be enabled during this refractory period by activation of either the BMP or Notch signaling pathways. Conversely, regeneration can be prevented during the later, regenerative, stages by inhibition of either pathway. BMP signaling will cause regeneration of all tissues, whereas Notch signaling activates regeneration of spinal cord and notochord, but not muscle. An activated form of Msx1 can promote regeneration in the same way as BMP signaling. Epistasis experiments suggest that BMP signaling is upstream of Notch signaling but exerts an independent effect on muscle regeneration. The results demonstrate that regenerative capability can be enabled by genetic modifications that reactivate specific components of the developmental program.
Figure 2. Analysis of Developmental Gene Expression in Nonregenerating and Regenerating Tail TissuesIn situ hybridization of genes involved in tail development.(A, E, I, M, Q, and U) Stage 30 embryos.(B, F, J, N, R, and V) Nonregenerating tail stumps, 50% amputated at stage 47 and fixed 3 days afterward.(C, G, K, O, S, and W) Stage 49 tail stumps, 50% amputated and fixed immediately.(D, H, L, P, T, and X) Regenerating tail stumps, 50% amputated at stage 49 and fixed 3 days later. Amputation level is shown by the black line.(A–D) Xbra is expressed in the developing tail bud (yellow arrow) and reexpressed in the tip of regenerating tails (white arrow).(E–H) X-delta-1 is expressed in the posterior wall of the tail bud (yellow arrow) and fin and is reexpressed in the fin and bud region (white arrow) of regenerating tails.(I–L) lfng is expressed in the dorsal tail bud (yellow arrow) and is expressed weakly at the cut surface of the spinal cord in nonregenerating tail stumps and more strongly in dorsal components of the regenerating tail (white arrows).(M–P) X-Notch-1 is expressed in somites, fin margin, and throughout the developing tail bud (yellow arrow) and is reexpressed in the blastema (white arrow), presomitic mesoderm, and fin margin.(Q–T) Msx2 is expressed in the tail bud leading edge (yellow arrow) and reexpressed throughout the blastema (white arrow) and fin margin.(U–X) Msx1 is expressed in the dorsal tail bud (yellow arrow) and reexpressed dorsally in the regenerating blastema (white arrow). All are lateral views, oriented with anterior to the left.
Figure 1. Axial Regenerative Ability in Xenopus laevis Tadpoles Is Lost at Stage 45/47 and Regained at Stage 48 and Later. White arrowheads show the level of tail amputation where appropriate. (A-C) Tadpoles were subjected to removal of 50% of the postanal tail at different stages of development and allowed to recover for 7 days. Stage 42 (A), stage 47 (B), and stage 48 (C).(D) Tadpoles subjected to 50% tail removal at stage 46 do not regenerate a tail, but develop normally, and the remaining stump is resorbed normally during metamorphosis. Left to right: stage 58; stage 63; stage 66.(E and F) Tadpole tails amputated at stage 46 (E) will regenerate if they are cut again after stage 48 (F). (G-P) Tail stumps partially cleared in glycerol and viewed under Nomarski optics. (G-K) Regeneration process in stage 49 tadpoles. Wound epidermis is indicated by black arrows. (G) Stage 49 tail fixed immediately after amputation. (H) After 24 hr, the wound epithelium has formed and blastemal cells are appearing. (I) Within 48 hr, blastemal cells have migrated into the center of the lesion and are proliferating ('cone 'stage). (J) After 3 days, the proximal blastemal cells are beginning to redifferentiate into the constituent tissues of the tail. (K) By 4-5 days, new notochord, spinal cord, fin, and presomitic mesoderm have formed and melanocytes (black pigment cells) are migrating into the new tail. (L-P) In contrast, tails cut at stage 47 do not regenerate or form a blastema, and the cut surface becomes covered with a skin-like epithelium (white arrows). (L) By 24 hr postamputation, a skin-like epithelium has formed over the cut surface of the tail stump (white arrows). The tissue behind this epithelium does not form a blastema. (M-P) The wound area does not alter over time, and there is no regeneration of tail tissues. (M-P) After 48 hr (M), 3 days (N), 4 days (O), and 7 days (P) postamputation. All tail stumps are lateral views oriented anterior to the left, and dorsal uppermost. b, regeneration bud; f, fin; n, notochord; s, spinal cord.
Figure 3. Properties of the Type of Constructs Used in This Study
(A and B) Embryos transgenic for a myc-tagged protein under the control of the HSP70 promoter were either heat-shocked at stage 14 (B) or not (A), and the expression was visualized by immunostaining (dark blue).
(C and D) Tadpoles transgenic for GFP under the control of the HSP70 promoter were heat-shocked (D) or not (C), and expression was viewed by fluorescence.
(E and F) Embryos heat-shocked at stage 14 develop normally in the absence of the HSP70-Noggin-γCrys-GFP transgene, whereas their transgenic siblings develop multiple defects consistent with BMP inhibition.
Note the GFP expression in the lenses of the eyes in transgenics.
(G) Transgene constructs used in this study.
Figure 4. Activation of BMP or Notch Signaling Pathways during the Refractory Stages Promotes Regeneration of Tail TissuesNontransgenic .
(A) and transgenic tadpole tails (C-E) shown 7 days after removal of 50% of the tail at stage 47. All tadpoles received a heat shock 3-4 hr before amputation and subsequent daily heat shocks.
(A) Wild-type (WT) tadpoles as controls for transgenics. No regeneration of tail tissues from the stump has occurred.
(B) Tadpoles transgenic for the HSP70-Alk3-γCrys-GFP construct can regenerate their tails completely.
(C) Tadpoles transgenic for the HSP70-NICD-γCrys-GFP construct partially regenerate their tails, reforming the spinal cord and notochord.
(D) Tadpoles transgenic for the HSP70-eveMsx1-γCrys-GFP construct can regenerate their tails completely.
(E) Replacing eveMsx1 with the nonfunctional, N-terminal-deleted δNMsx1 in the transgene cassette abolishes regeneration ability. White arrows show the level of amputation. Tails are shown in lateral view, anterior to the left and dorsal uppermost.
(F�H) Analysis of Msx1 expression (blue staining) in refractory stage tadpoles 2 days after amputation and heat shock.
(F) Msx1 is expressed in HSP70-Alk3-γCrys-GFP transgenics.
(G) Msx1 is not expressed in HSP70-NICD-γCrys-GFP transgenics.
(H) Msx1 is not expressed in nontransgenic sibling controls.
(I-N) Analysis of tissue composition of 7 day regenerates during the normal refractory period. Immunohistochemical staining (black/
Figure 5. Inhibition of the BMP or Notch Signaling Pathways Prevents Tail RegenerationNontransgenic (A and D) and transgenic F0 tadpoles (B and C) shown 7 days after removal of the posterior 50% of the tail at stage 50/52. (A)�(C) are viewed with both fluorescent and low incident light such that the GFP in the lens can be detected, indicating the presence of the transgene. The tadpoles received a heat shock 3�4 hr before amputation and subsequent daily heat shocks.(A) A wild-type (WT) tadpole can regenerate a complete tail in 7 days. White arrowheads indicate the level of amputation.(B) Transgenic tadpole carrying the HSP70-Noggin-γCrys-GFP construct. Tail regeneration was completely blocked.(C) Transgenic tadpole carrying the HSP70-tBr-γCrys-GFP construct. Tail regeneration was completely blocked.(D) Stage 49 WT tadpole cultured in 10 μM MG132 immediately after removal of the posterior half of the tail, for 7 days. Tail regeneration was completely blocked.(E�H) Regeneration phenotype is linked to inheritance of the transgene in F1 animals derived from crossing a male individual carrying the HSP70-Noggin-γCrys-GFP to a wild-type female. Following heat shocks and amputation, transgenic animals failed to regenerate their tails (E) and Msx1 (blue staining) was not expressed in the stump (F). WT siblings regenerated normally (G), and expressed Msx1 in the blastema (H). Black arrowheads in (G) and a black line in (H) mark the level of amputation.
Figure 6. Epistasis Experiments Suggest that BMP Acts Upstream of Notch in Tail Regeneration (A-D) Effect of a Notch inhibitor on BMP pathway-driven refractory stage regeneration. Tadpoles were amputated at stage 47 and given daily heat shocks for 1 week. (A) Wild-type (WT) tadpoles do not regenerate at this stage.(B) Sibling tadpoles expressing the HSP70-Alk3-γCrys-GFP regenerate all the tail tissues.(C and D) Neither WT (C) nor transgenic tadpoles (D) will regenerate any tailtissue following treatment with 10 μM Notch proteolysis inhibitor MG132.(E and F) Effect of combined inhibition of BMP signaling and activation of the Notch pathway. The tadpoles express both HSP70-tBr-γCrys-GFP and HSP70-NICD-γCrys-RFP. Following amputation after stage 50, when WT animals regenerate tails normally, these double transgenics regenerate only notochord and spinal cord tissue. No new muscle is formed. White arrowheads mark the site of amputation in (B, E, and F).
