A procedure for preparing and transplanting skeletal muscle cells from α-actin-RFP transgenic donors into immune compromised homozygous rag2 mutant zebrafish has been demonstrated (Protocol Section 1, Figure 1A and Figure 2). Skeletal muscle tissue was prepared from α-actin-RFP transgenic donors and the resulting single cell suspension contained 84.3% viable cells as assessed by DAPI exclusion following Flow Cytometry analysis (Figure 2B). RFP-positive cells comprised 35.3% of this single cell suspension (Figure 2C). Transplantation of cells into the dorsal skeletal muscle of rag2 homozygous mutant recipient fish led to consistent and strong engraftment as assessed by differentiation of single cells into multinucleated fibers (1 x 106 cells injected per fish, Table 1, Figure 2D-I). Wild type recipient fish failed to engraft muscle fibers over the 30-day experiment (n = 13). By 10 days post transplantation, 9 out of 14 rag2 homozygous mutant zebrafish contained RFP-positive muscle fibers near the site of injection (64.3%, Figure 2E,F). Importantly, engrafted RFP-positive muscle persisted to 30 days post-transplantation (Figure 2G-I), with a subset of animals being followed for 115 days post-engraftment and exhibiting robust and persistent muscle engraftment (data not shown). These results are similar to those reported previously by our group23 using the same protocol (Table 1).
We have also presented a method for the generation, preparation and transplantation of ERMS tumor cells into the peritoneal cavity of rag2 homozygous mutant recipient fish (Protocol Section 2, Figure 1B and Figure 3). ERMS were generated in double transgenic myf5-GFP; mylpfa-mCherry fish that have been shown to allow the visualization of intra-tumoral heterogeneity and functional analysis of tumor cell subpopulations following transplantation11. However, further molecular characterization of each subpopulation is difficult because fish are small when they develop ERMS between 10 to 30 days of life and the number of tumor cells are limiting for downstream applications. One solution is to expand tumor cell numbers by engrafting ERMS into adult recipient zebrafish. To date, similar experiments have been completed using CG1-strain syngeneic fish and required in excess of 4 generations of backcrossing to develop syngeneic lines that were transgenic for myf5-GFP; mylpfa-mCherry. To circumvent these issues, we demonstrated the utility of immune compromised rag2 homozygous mutant recipient zebrafish to engraft primary ERMS from a AB-strain zebrafish. All primary ERMS engrafted into rag2 homozygous mutant animals, facilitating expansion of the tumor (Table 1). Similar results were recently reported where 24 of 27 rag2 homozygous mutant zebrafish engrafted ERMS, while 0 of 7 wild type siblings engrafted disease23. A representative example of an engrafted ERMS is shown at 30 days post-transplantation in Figure 3E. Engrafted ERMS share histological features of embryonal rhabdomyosarcoma, similar to that found in the primary tumor (Figure 3B and 3F). FACS analysis confirmed that ERMS contained functionally distinct tumor propagating cells and differentiated cells that express myf5-GFP and/or mylpfa-mCherry. Survival rates following the intraperitoneal injection procedure were in excess of 95%. Recipient zebrafish commonly succumb from tumor burden after the 30 days post-transplantation time point.

Figure 1. Protocol schematic for (A) normal and (B) malignant skeletal muscle cell transplantation into rag2 homozygous mutant zebrafish. Optional steps are marked with (*).

Figure 2. Skeletal muscle engraftment into rag2 homozygous mutant zebrafish. (A) α-actin-RFP transgenic donor zebrafish. (B) Cell viability of isolated muscle cell suspension as assessed by DAPI dye exclusion and flow cytometry. (C) Proportion of RFP-positive cells found within the muscle cell suspension from α-actin-RFP donor (red), compared to a wild type control (grey). (D-E) Merged bright field and fluorescent images of wild type animals (D) or rag2 homozygous mutant fish (E) at 30 days post-transplantation. (F) Engraftment rates over time. Red denotes number of engrafted animals while grey shows non-engrafted fish. Number of animals analyzed at each time point are indicated. (G-I) High magnification images of boxed region in panel E shown at 10 (G), 20 (H) and 30 (I) days post-transplantation, showing retention of differentiated muscle fibers over time (arrowheads). Scale bars equal 2 mm. Please click here to view a larger version of this figure.

Figure 3. Transplantation of myf5-GFP; mylpfa-mCherry ERMS into rag2 homozygous mutant zebrafish. (A-D) rag2-kRASG12D induced primary ERMS arising in AB-strain myf5-GFP; mylpfa-mCherry zebrafish at 30 days of life. (E-H) rag2 homozygous mutant zebrafish engrafted with ERMS and analyzed at 30 days post-transplantation. (A, E) Merged bright field and fluorescent images of primary and transplanted ERMS. Tumor area is outlined and arrowhead indicates injection site in E. (B, F) Hematoxylin- and eosin-stained paraffin sections of primary (B) and engrafted ERMS (F) showing areas of increased cellularity associated with cancer. (C, G) Cell viability as assessed by DAPI dye exclusion and flow cytometry. (D, H) Fluorescent tumor cell sub-populations, as assessed by flow cytometry. Scale bars equal 2 mm (A, E) and 50 μm (B, F). Please click here to view a larger version of this figure.

Table 1. Engraftment results for muscle and ERMS cell transplantation. (*) denotes previously reported data using the same techniques23. Data is reprinted with permission from Nature Methods. Please click here to view a larger version of this table.