Khattak S , Schuez M , Richter T , Knapp D , Haigo SL , Sandoval-Guzmán T , Hradlikova K , Duemmler A , Kerney R , Tanaka EM .

	Figure 1. Transgenic Axolotls with Cell-type-Specific Transgene Expression of EGFP(A) Live axolotl larva expressing membrane-tethered EGFP in neurons under the control of mouse βIII-tubulin promoter.(B) Limb of live transgenic axolotl expressing EGFP in Schwann cells. The transgenic animal is expressing EGFP under the control of mouse CNP promoter and marking all myelin-positive cells in the body.(C) Xenopus Keratin 12 promoter driving EGFP in epidermis of live axolotl. Here, a limb image is shown.(D) Fluorescence image of limb on a live animal showing EGFP expression in muscle driven by the Xenopus Cardiac Actin (Car Act) promoter.(E) Limb of live transgenic axolotl where Xenopus Col2α1 promoter is driving EGFP in cartilage.Scale bars, 1 mm (B, D, and E) and 2 mm (A and C). See also Figure S1.
	Figure 2. In Vivo Tracking of Nerves and Muscle during Normal and Intercalary Limb Regeneration(A–C) Live tracking of growing nerves in a βIIItub:GAP43-EGFP transgenic animal. Nerves as seen in a 6 day limb blastema (A), early digit-forming (Notch) stage (B), and late digit (Palette)-regeneration stage (C) of the same animal. The dashed white lines in (A) and (C) represent the amputation plane.(D) Scheme of hand blastema transplantation to follow EGFP muscle versus other cell types during intercalary regeneration. A double-transgenic (CAGGs:Cherrynuc;CarAct:EGFP) hand blastema was transplanted onto an upper-arm stump of a nontransgenic white mutant host and allowed to regenerate.(E) Regenerated limb from the double-transgenic hand blastema graft. The white dashed line shows the amputation plane of the host stump (to where the hand blastema was transplanted). Muscle fibers (green) are present in upper-arm regions during intercalary regeneration, whereas the cherry-positive cells largely populate the hand region of the regenerate.Scale bars, 100 μm (A), 200 μm (B), 300 μm (C), and 1 mm (E).
	Figure 3. Neural Stem Cell Expression via AxSox2:Cre-ERT2-nucGFP and Marking of Neural Stem Cell Descendents via CRE-Mediated Recombination(A) Scheme of double transgenic used to label SOX2+ cells in the brain. LoxP reporter (CAGGs:loxP-EGFP-loxP-Cherry) was crossed with a transgenic axolotl where the Cre-ERT2 gene is under the control of the axolotl Sox2 promoter.(B) Bright-field image of head of Sox2: Cre-ERT2 transgenic animal line (F1s).(C) EGFP fluorescence image of the head shown in (B) showing brain-specific expression.(D–G) Immunostaining of head cross-section of Sox2: Cre-ERT2 driver animal. nucGFP expression is seen in (D), SOX2 immunofluorescence is in (E), nuclei are stained for DAPI in (F), and (G) shows the merged image. (G′) Higher-resolution image of inset in (G) showing colocalization of EGFP signal with immunostaining for SOX2 in the ventricular zone of the brain section.(H) Bright-field image of head of double-transgenic animal, AxSox2:Cre-ERT2-nucGFP; CAGGs:loxP-EGFP-loxP-Cherry.(I) Cherry fluorescence image of the head shown in (H). Clear Cherry fluorescence is seen in brain without tamoxifen administration.(J–M) Cross-section of head of AxSox2:Cre-ERT2-nucGFP; CAGGs:loxP-EGFP-loxP-Cherry showing expression in Cherry (J), SOX2 immunofluorescence (K)-positive cells in the brain. (L) shows DAPI+ nuclei, whereas (M) is the merged image. A broader fluorescence is seen compared to the nucGFP signal in the AxSox2:Cre-ERT2-nucGFP driver, indicating that Sox2+ cells have contributed to new neurogenesis in the brain. (M′) Higher-resolution inset of (M) brain section showing Cherry-positive cells in SOX2+ cells as well as their putative descendants.Scale bars, 2 mm (C and I), 200 μm (G and M), and 30 μm (G′ and M′).
	Figure 4. Tight Temporal Control of Cre/loxP-Mediated Gene Expression Using the ERT2-cre-ERT2 System(A) Schematic diagram of the Cre driver. The CAGGs promoter is driving the ERT2-cre-ERT2-T2A-nuc-EGFP cassette.(B) Limb of CAGGs:ERT2-Cre-ERT2-T2A-EGFP-nuc transgenic animal in green and red channel.(C) Schema of mating between CAGGs:loxP-EGFP-loxP-Cherry and CAGGs:ERT2-Cre-ERT2-T2A-EGFP-nuc animals.(D and F) Limb and tail of a double-transgenic (CAGGs:loxP-EGFP-loxP-Cherry; CAGGs:ERT2-Cre-ERT2-T2A-EGFP-nuc) animal showing EGFP and Cherry expression levels before tamoxifen induction.(E and G) Robust Cherry expression is observed in limb and tail of double-transgenic (CAGGs:loxP-EGFP-loxP-Cherry; CAGGs:ERT2-Cre-ERT2-T2A-EGFP-nuc) animals after administration of 4-OHT.Scale bars, 1 mm (B and D–F) and 500 μm (G). See also Figures S2, S3, S4, and S5.
	Figure 5. Temporal Induction of Gene Expression in Co2A1+ Cells Using Col2A1:ERT2-Cre-ERT2-T2A-EGFP-nuc Transgenic Axolotls(A) Scheme of mating between loxP reporter animal (CAGGs:loxP-EGFP-STOP-loxP-Cherry) and the driver animal (Col2A1:ERT2-Cre-ERT2-T2A-EGFP-nuc).(B and D) Double-transgenic animal shows no Cherry fluorescence in head (B) and tail (D) before tamoxifen induction. High-exposure times were used for imaging so that overall tissue architecture could be seen.(C and E) Double-transgenic head (C) and tail (E) images after tamoxifen induction. Clear Cherry expression is observed only in the skeletal elements of the head and vertebral column in the tail. Half-exposure time from control (C and E) was used.(F–I) Cross-section of head showing colocalization of Cherry (F) with Collagen type II antibody staining (G). DAPI delineates nuclei in blue. (I) represents merged image.(J) High-resolution image of inset marked in (I) showing Cherry-positive cells associated with Col2A1 staining in head cartilage.Scale bars, 2 mm (C, E, and I) and 100 μm (J).
	Figure 6. Inducible Overexpression of the Cell-Cycle Inhibitor p16INK4 Represses Spinal Cord Regeneration(A) To overexpress human p16INK4A, a transgenic animal was made where the p16INK4A-T2A-Cherry gene was cloned behind a floxed GFP cassette (CAGGs:loxP-EGFP-STOP-loxP-p16INK4AT2A-Cherry). This animal was crossed with a transgenic animal where the 4-OHT-inducible Cre was driven by the ubiquitous CAGGs promoter (CAGGs:ERT2-Cre-ERT2-T2A-EGFP-nuc). The progenies of this mating were screened and injected intraperitoneally with 4-OHT and examined live for induction of Cherry expression 5 days later.(B and C) Phenotype of single-transgenic control and double-transgenic experimental animals ectopically expressing p16INK4A, following 4-OHT intraperitoneal injection and tail amputation. No Cherry induction is seen in single-transgenic animals (B), whereas the double-transgenic animals induced Cherry expression (C). Images were taken 4 days after tail amputation. White dotted lines indicate the amputation plane; white arrows demarcate the extent of ependymal tube outgrowth.(D) Quantitation of the regenerate spinal cord outgrowth in p16INK4A-expressing transgenic animals and nonexpressing controls. The length of the ependymal tube is significantly reduced in the p16INK4A-expressing animals (paired t-test and the Mann-Whitney U test, p <0.001).Scale bars, 1 mm (B and C).

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