Long-Term Survival of Transplanted Stem Cells in Immunocompetent Mice with Muscular Dystrophy
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The American Journal of Pathology, Vol. 173, No. 3, September 2008 Copyright © American Society for Investigative Pathology DOI: 10.2353/ajpath.2008.080259 Stem Cells, Tissue Engineering and Hematopoietic Elements Long-Term Survival of Transplanted Stem Cells in Immunocompetent Mice with Muscular Dystrophy Gregory Q. Wallace,* Karen A. Lapidos,*† blasts. Myoblasts can be cultured and expanded and Jordan S. Kenik,* and Elizabeth M. McNally*‡ have been tested for their ability to treat degenerative diseases of muscle.3–16 Under specific culture condi- From the Department of Medicine,* Section of Cardiology, and tions, myoblasts withdraw from the cell cycle and un- the Departments of Molecular Genetics and Cell Biology† and dergo terminal differentiation. In vivo, during myoblast Human Genetics,‡ The University of Chicago, Chicago, Illinois transplantation, a similar cell cycle withdrawal is thought to occur followed by terminal differentiation and the fu- sion of myoblasts to existing myofibers.17 In this setting, myoblast transfer has been hampered by limited engraft- Satellite cells refer to resident stem cells in muscle that ment thought to relate to poor migratory ability and lack of are activated in response to damage or disease for the long-term survival from rejection and ongoing muscle regeneration and repair of muscle fibers. The use of degeneration. An additional complication is that myo- stem cell transplantation to treat muscular diseases has blasts do not appear to replace the satellite cell niche. been limited by impaired donor cell survival attributed Duchenne muscular dystrophy (DMD) and a subset of to rejection and an unavailable stem cell niche. We iso- the limb girdle muscular dystrophies are caused by muta- lated a population of adult muscle mononuclear cells tions in genes encoding components of the dystrophin gly- (AMMCs) from normal, strain-matched muscle and coprotein complex.18,19 This complex consists of dystro- ␦ transplanted these cells into -sarcoglycan-null dystro- phin, dystroglycan, sarcoglycans, dystrobrevins, and phic mice. Distinct from other transplant studies, the syntrophins.20 The transmembrane dystrophin glycopro- recipient mice were immunocompetent with an intact tein complex links laminin-␣2 in the basal lamina to the endogenous satellite cell pool. We found that AMMCs actin cytoskeleton. Disruption of this protein chain leaves were 35 times more efficient at restoring sarcoglycan muscle membranes fragile and with permeability defects. compared with cultured myoblasts. Unlike cultured Clinically, the loss of dystrophin or sarcoglycans leads to myoblasts, AMMC-derived muscle fibers expressed sar- progressive muscle weakness that initially targets muscle coglycan protein throughout their entire length, consis- groups proximal to the trunk. In time, most musculature is tent with enhanced migratory ability. We examined the affected by muscle wasting. Pathologically, the muscle is re- capacity of single injections of AMMCs to provide long- placed ultimately by connective tissue and adipose infiltration term benefit for muscular dystrophy and found persis- indicating an overall failure of the satellite cell and regenerative tent regeneration after 6 months, consistent with aug- system of muscle. During aging in normal muscle, there is also mentation of the endogenous stem cell pool. Interestingly, a reduction in regenerative capacity.21–23 Mouse models of AMMCs were more effectively engrafted into aged dys- DMD and limb girdle muscular dystrophy lacking dystrophin trophic mice for the regeneration of large clusters of (mdx) and ␦-sarcoglycan (Sgcd-null), respectively, exhibit sarcoglycan-positive muscle fibers, which were pro- identical pathology to what is seen in human patients that tected from damage, suggesting that the stem cell begins as focal necrosis within muscle and eventual fibrofatty niche in older muscle remains permissive. (Am J infiltration.24–26 Although the underlying genetic defects and Pathol 2008, 173:792–802; DOI: 10.2353/ajpath.2008.080259) disease progression are understood in muscular dystrophy, therapy is not effective. Thus, cell-based approaches remain an attractive strategy for treating these disorders. Skeletal muscle is a dynamic tissue that regenerates after damage from exercise or disease. Muscle regeneration is mediated by satellite cells, which are defined by their Supported by the Muscular Dystrophy Association and the National Insti- position between the basal lamina and the sarcolemmal tutes of Health (grants HL61322, T32HL007381, and F32AR054700). 1,2 membrane. Satellite cells are maintained throughout Accepted for publication May 23, 2008. the life of the organism and are thought to asymmetrically Address reprint requests to E.M. McNally, M.D., Ph.D., The University of divide to simultaneously replenish the satellite cell pool Chicago, 5841 S. Maryland Ave., MC6088, Chicago, IL 60637. E-mail: and produce myogenic precursor cells known as myo- [email protected]. 792 Long-Term Regeneration in Muscular Dystrophy 793 AJP September 2008, Vol. 173, No. 3 Cell-based therapies for muscular dystrophies have the nine, the cytoplasmic and transmembrane domains.24,35 combined benefits of boosting regenerative capacity and The Sgcd allele was backcrossed heterozygously 10 gen- simultaneously delivering normal copies of genes to dystro- erations with C57BL6/J mice and then intercrossed to gen- phic muscle. For long-term efficacy, adding a self-renewing erate recipient animals. Donors were 6- to 10-week-old, source of stem cells to the endogenous stem cell pool of sex-mismatched, C57BL/6J littermates. In experiments that muscle is advantageous. Early attempts at cell-based ther- used GFP transgenic mice, the donors were 6- to 10-week- apy for muscular dystrophy relied on transfer of immortal- old, sex-mismatched C57BL/6-Tg(ACTB-EGFP)1Osb/J mice ized myoblast cultures to mdx mice.27 These studies dem- (The Jackson Laboratory, Bar Harbor, ME) with an en- onstrated that myoblast transfer could restore dystrophin to hanced GFP transgene under the control of a chicken a small percentage of recipient myofibers and inspired clin- -actin promoter and cytomegalovirus enhancer. Ani- ical trials in human DMD patients.10,12–14 Subsequent stud- mals were housed, treated, and handled according to ies have identified the limitations of myoblast cell lines and guidelines of the University of Chicago Institutional Ani- cultured myoblasts. Cultured myoblasts are hampered by mal Care and Use Committee, the Animal Welfare Act, their inability to migrate throughout myofibers, limiting their and the National Institutes of Health Guide for the Care muscle contribution to 60 to 900 m from the injection and Use of Laboratory Animals. site.28–30 Another shortcoming is that myoblasts quickly die after injection,15,31,32 at least partially because of host im- mune responses.16,33 Isolation of AMMCs Recent studies suggested that culture conditions pro- Mononuclear cells were isolated from hindlimb muscles of mote partial cell differentiation, thus diminishing stem cell 6- to 10-week-old syngeneic wild-type donors. Muscles capabilities and limiting effectiveness of cultured cells to were finely minced with razor blades. Collagenase/dispase 34 contribute to multiple rounds of muscle regeneration. solution (2.4 U/ml dispase II, 2.5 mmol/L CaCl , 1% colla- 34 2 Montarras and colleagues showed that freshly isolated genase D; Roche Diagnostics, Indianapolis, IN) was added cells regenerated muscle three times more efficiently at 2 ml/g tissue followed by a 45-minute incubation at 37°C. than cells exposed to culture conditions. Consequently, Every 15 minutes during digestion, tissue slurries were trit- cultured myoblasts have the advantage of ex vivo expan- urated 15 times through a 5-ml pipette. The digestion was sion but through this process differentiate sufficiently so stopped by adding F-10 Ham’s media supplemented with that they do not efficiently replace satellite cells. 20% fetal bovine serum and 1% penicillin/streptomycin. To expand on these findings, we transplanted a pop- Digested muscle was passed through filters with pore sizes ulation of freshly isolated, adult muscle mononuclear of 100 m, 70 m, and 40 m before centrifugation at cells (AMMCs) into immunocompetent Sgcd-null mice. 200 ϫ g for 5 minutes. Pellets were resuspended in sup- Compared to mdx mice, Sgcd-null mice have increased plemented F-10 Ham’s media and Ficoll-Paque Plus (Am- cardiac and skeletal muscle necrosis and do not have ersham Biosciences, Piscataway, NJ) was layered beneath revertant fibers that may complicate interpreting the re- the cell suspensions followed by centrifugation at 800 ϫ g 24,25 generative potential of donor cells. We found that for 15 minutes with the brake off. AMMCs were collected at ϫ AMMCs regenerated muscle 35 more effectively than the media/Ficoll interface and pellet cells were collected cultured myoblasts. Transplantation of AMMCs resulted after removing the supernatant. After washing, cells were in robust expression of sarcoglycan throughout the resuspended in phosphate-buffered saline (PBS)/0.5% bo- length of the transplanted muscle, even when competing vine serum albumin at 107 cells/ml and kept on ice until against endogenous satellite cells. Subsequent injections transplanted. of AMMCs yielded additional donor-derived fibers in im- munocompetent recipients, suggesting that AMMCs con- tain a population of immunoprivileged cells with myogenic Primary Myoblast Cultures potential.