Limitations of nlsβ-galactosidase as a marker for studying myogenic lineage or the efficacy of myoblast transfer

Abstract

Background Nuclear localizing β‐galactosidase (nlsβ‐gal) is used as a marker for studying myoblast cell lineage and for evaluating myoblast survival after myoblast transfer, a procedure with potential use for gene complementation for muscular dystrophy. Usefulness of this construct depends on the establishment of the extent to which nlsβ‐gal or its mRNA may be translocated from the nucleus that encodes it to other noncoding myonuclei in hybrid myofibers and the ease with which the encoding and noncoding myonuclei can be distinguished. Previous in vitro studies (Ralston and Hall 1989. Science, 244:1066–1068) have suggested limited translocation of the fusion protein. We reexamined the extent to which nlsβ‐gal is translocated in hybrid myofibers, both in vitro and in vivo, and evaluated the extent to which one can rely on histochemistry to distinguish encoding from noncoding nuclei in these myofibers. Methods Myotubes formed in cocultures of a myoblast line (MM14 cells), stably transfected with a construct consisting of a nlsβ‐gal under the control of the myosin light chain 3F promoter and 3′ enhancer (3FlacZ10 cells), and [3H]‐thymidine‐labeled parental MM14 cells (plated at ratios of 1:6 or 1:20, respectively) were reacted with X‐gal. After autoradiography, the distance over which nlsβ‐gal was translocated in hybrid myotubes was determined. In vivo translocation of nlsβ‐gal was evaluated by injecting [3H]‐thymidine‐labeled 3FlacZ10 myoblasts into the regenerating extensor digitorum longus muscle of immunosuppressed normal and mdx (dystrophin deficient) mice. Sections stained with X‐gal and subjected to autoradiography permitted determination of the extent of nlsβ‐gal translocation in hybrid myofibers. Results In vitro: All nuclei in > 92% of hybrid myotubes showed evidence of nlsβ‐gal after exposure to X‐gal, suggesting extensive translocation. Within hybrid myotubes, MM14‐derived myonuclei ∼350 μm from a 3FlacZ10‐derived myonucleus showed evidence of nlsβ‐gal. In vivo: Similar translocation of nlsβ‐gal was observed in vivo. One week after myoblast transfer, donor‐derived myonuclei were distinguishable from host‐derived myonuclei containing nlsβ‐gal by the greater accumulation of reaction product in donor myonuclei after X‐gal staining. However, 2 weeks after injection, host myonuclei often contained a significant amount of nlsβ‐gal, and accumulation of reaction product could not be used as the criterion for identification of donor myonuclei. Conclusions Translocation of nlsβ‐gal (or its mRNA) is significantly greater than previously reported (Ralston and Hall 1989), resulting in large numbers of nlsβ‐gal positive noncoding myonuclei in hybrid myofibers. One week after myoblast transfer, distinguishing between nlsβ‐gal encoding and noncoding myonuclei in hybrid myofibers after X‐gal stainbers. One week after myoblast transfer, distinguishing between nlsβ‐gal encoding and non‐coding myonuclei in hybrid myofibers after X‐gal staining of sectioned muscle is feasible; however, by 2 weeks, nlsβ‐gal increases ...

Inducible Cre transgenic mouse strain for skeletal muscle-specific gene targeting

“…On the basis of the results of this analysis, we determined that there are approximately 0.91 myonuclei per fiber, a value comparable to the 0.88 myonuclei per fiber we recently reported [28]. Although these results demonstrate that the HSA-MCM strain can achieve a high level of recombination efficiency, they should be interpreted with caution as there is evidence suggesting that the β-galactosidase protein can translocate to “unlabeled” nuclei [29]. …”

Extraocular muscle satellite cells are high performance myo-engines retaining efficient regenerative capacity in dystrophin deficiency

“…7B). The detection of the nuclear reporter nLacZ in myonuclei upon donor cell engraftment is not necessarily an exact reflection of the number of donor-derived nuclei as the reporter protein, translated in the myofiber cytoplasm, may also enter host myonuclei within neighboring domains of the same myofiber (Kirillova et al, 2007; Yang et al., 1997). However, the degree of MLC3F-nLacZ nuclei in donor muscle is certainly related to the degree of engraftment and provides an efficient qualitative measure.…”

Transplanted primary neonatal myoblasts can give rise to functional satellite cells as identified using the Myf5nlacZl+ mouse

“…This observation indicates that donor-derived myoblasts had contributed to myofibre repair. Because of translocation of ␤-galactosidase message between myonuclei in a syncytium 18 the extent of muscle repair cannot be assessed, although at least one myonucleus per myofibre must be of donor origin.…”