Red fluorescent Xenopus laevis: a new tool for grafting analysis
© Waldner et al; licensee BioMed Central Ltd. 2009
Received: 23 December 2008
Accepted: 23 June 2009
Published: 23 June 2009
Fluorescent proteins such as the green fluorescent protein (GFP) have widely been used in transgenic animals as reporter genes. Their use in transgenic Xenopus tadpoles is especially of interest, because large numbers of living animals can easily be screened. To track more than one event in the same animal, fluorescent markers that clearly differ in their emission spectrum are needed.
We established the transgenic Xenopus laevis strain tom3 that expresses ubiquitously red fluorescence from the tdTomato gene through all larval stages and in the adult animal. This new tool was applied to track transplanted blastemas obtained after tail amputation. The blastema can regenerate ectopic tails marked by red fluorescence in the host animal. Surprisingly, we also found contribution of the host animal to form the regenerate.
We have established a useful new tool to label grafts in Xenopus transplantation experiments.
The green fluorescent protein (GFP) has successfully been used as a marker gene in the past years. It was also applied to transgenic Xenopus either to label grafts (e. g.  or cell lineages by using specific promoters (e. g. [2–4]). The GFP derivatives cyan fluorescent protein (CFP) and yellow fluorescent protein (YFP) expanded the color range and were also successfully used in transgenic Xenopus [5–7]. Although both markers can be used independently by applying appropriate filter sets, there is some overlap of the emission spectra. Therefore, to track more than one event in the same animal reliably, fluorescent markers that clearly differ in their emission spectrum are needed. The red fluorescent protein DsRed has been isolated from Discosoma sp. to be used as a new tracer with higher wavelength emission, but the long maturation time and poor solubility of the tetrameric DsRed protein  prevented its widespread use in transgenic animals. We also observed toxic effects in Xenopus of DsRed under the control of the ubiquitously active CMV promoter (MR, unpublished data). Nevertheless, some transgenic animals were made using DsRed in zebrafish  or its faster maturating derivatives in Drosophila , mice  and rats . Progress was achieved a few years ago, when variants of monomeric molecules derived from DsRed were generated giving rise to different color shades of red . All derivatives are characterized by a shorter maturation time and an improved solubility as compared to the wild type DsRed protein, with tdTomato showing the highest brightness and photo stability. To generate a new tool to label grafts we established in the present study a ubiquitous red fluorescent transgenic Xenopus laevis strain using the CMV promoter driven tdTomato sequence.
We applied this newly established tool to track cell fate during Xenopus tail regeneration. After tail amputation of the tadpole, a mass of proliferative cells is formed underneath the wound epidermis within 24 hours. This mass of cells is commonly called the 'blastema', and is able to regenerate to an imperfect copy of the amputated tail within a few days (reviewed in [14, 15]). Although this regenerated tail has less well organized myotomes, the tadpoles are able to swim. Recent studies suggest that V-ATPase dependent proton flux  and apoptosis  are essential for blastema formation. However, nothing is known whether the blastema generates ectopic tails upon transplantation or whether the interaction with the surrounding amputated tissue is needed. We therefore performed transplantation experiments using the red fluorescent Xenopus strain and followed the fate of the graft as well as of the surrounding host tissue.
Establishment of a red fluorescent Xenopus laevisstrain
Ectopic tail regeneration due to blastema transplantation
In this study we established the red fluorescent transgenic Xenopus strain tom3 expressing the CMV promoter driven tdTomato sequence. This is in contrast to the use of DsRed as a transgene that is not compatible with normal development and thus precludes the establishment of a transgenic strain (MR, unpublished data). We could recently confirm the value of tdTomato as stable transgene to mark a Xenopus strain containing a functional Cre recombinase . Adult animals of the tom3 strain show ubiquitous and homogenous expression of the red fluorescent protein indicating that the CMV promoter can be used to drive transgene expression also at later stages of development. Therefore, the CMV promoter may be equivalent to the murine ROSA26 promoter that has been proposed for widespread and persistent transgene expression in Xenopus laevis . This transgenic strain can thus be used to label grafts isolated at any time point in development. Animals of the tom3 strain showing ubiquitous and strong expression of tdTomato developed absolutely normal suggesting that tdTomato has a high biocompatibility as proposed for a recently engineered noncytotoxic DsRed variant, which has the drawback of a reduced brightness . When we transplanted labelled blastemas obtained after tail amputation we found that cells marked by red fluorescence can form an ectopic regenerate, but do not migrate into the host tadpole. We even observed ectopic regeneration in the head pointing to a strong degree of self organization of the blastema. As an orientation of the blastema used for transplantation was not possible, we cannot exclude that transplants not properly orientated failed to outgrow. The regenerate contains the typical tail structures including muscle, spinal cord and notochord. This implies that the blastema formed 24 hours after amputation contains precursor cells to all these tissues and that their differentiation can proceed autonomously of the amputated stump. However, we cannot exclude that the transplanted blastema contained part of the terminal bulb of the spinal cord as well as of the bullet-shaped notochord cells that partially extend into the regeneration bud . The fact that melanophores were present in all regenerates is consistent with a previous study  showing that these cells are derived from neural crest precursors and further strengthens the concept of cell recruitment by the blastema. In all cases the ectopic tail lacks the broad tail fin present in the in situ regenerate (data not shown). Possibly the supportive structure of the stump is needed for the formation of the tail fin. Furthermore, the ectopic tail is less well organized lacking proper segmentation and typically with a twisted appearance compared to the normal regenerate. This is reminiscent to the twisted tail regenerates observed upon spinal cord ablation in Xenopus . We assume that this deficiency in proper morphology but also the inability for movement of the ectopic tail are possibly due to the lack of a connection to the spinal cord of the host animal.
After tail amputation in Xenopus a wound epidermis is formed by migrating cells within the first 24 hours. It is proposed that this wound epidermis signals to the underlying cells to form the blastema (reviewed in [14, 15]). Our experiments using labelled host cells expand this model, because our data reveal that the transplanted blastema also recruits cells of the host to form the regenerate. The tissue identity of these cells as well as the underlying mechanism have to be determined in future studies.
In summary, the newly established red fluorescent tom3 strain offers a powerful tool to further investigate the process of regeneration in Xenopus. In combination with the existing green and yellow [1, 7] fluorescent strains, three different cell types can now be labelled in transplantation experiments.
The tdTomato sequence was isolated from pRSET-BtdTomato  as a BamHI/EcoRI fragment and cloned into pBluescriptIISK+. From the obtained plasmid the BamHI/XhoI fragment was isolated and subcloned into pCSGFP2  replacing the GFP2 sequence to generate pCSCMV:tdTomato.
The original protocol using restriction enzyme mediated integration  was modified by using frozen sperm nuclei and omitting the egg extract to get more normal developing larvae . Individual founders were marked as described . The pCSCMV:tdTomato construct was digested by SalI and the fragment lacking the bacterial DNA was purified for transgenesis. Fluorescence microscopy was done with a Leica MZ/FLIII or a Zeiss Axioplan stereomicroscope with the appropriate filters as described . Xenopus stages are as defined .
About 1 cm of the tail of stage 50 tadpoles was amputated. The tadpoles were kept in fresh water for 24 hours. The newly formed blastema was cut off and introduced under the skin of the host tadpole by using sharp forceps.
The tissue was fixed and stained with the monoclonal antibodies Xen1 (3B1), 12/101, or anti-Collagen type II (Clone II-4CII, MP Biomedicals) essentially as described .
The expression plasmid containing the tdTomato gene was kindly provided by Roger Y. Tsien. The Xen1 (3B1) antibody developed by Ariel Ruiz i Altaba and the 12/101 antibody developed by Jeremy P. Brockes were obtained from the Developmental Studies Hybridoma Bank developed under the auspices of the NICHD and maintained by The University of Iowa, Department of Biological Sciences, Iowa City, IA 52242. This work was funded by the Deutsche Forschungsgemeinschaft (RY 5/8-2).
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