Dystrophin delivery in dystrophin-deficient DMDmdx skeletal muscle by isogenic muscle-derived stem cell transplantation

Duchenne's muscular dystrophy (DMD) is a lethal muscle disease caused by a lack of dystrophin expression at the sarcolemma of muscle fibers. We investigated retroviral vector delivery of dystrophin in dystrophin-deficient DMD(mdx) (hereafter referred to as mdx) mice via an ex vivo approach using mdx muscle-derived stem cells (MDSCs). We generated a retrovirus carrying a functional human mini-dystrophin (RetroDys3999) and used it to stably transduce mdx MDSCs obtained by the preplate technique (MD3999). These MD3999 cells expressed dystrophin and continued to express stem cell markers, including CD34 and Sca-1. MD3999 cells injected into mdx mouse skeletal muscle were able to deliver dystrophin. Though a relatively low number of dystrophin-positive myofibers was generated within the gastrocnemius muscle, these fibers persisted for up to 24 weeks postinjection. The injection of cells from additional MDSC/Dys3999 clones into mdx skeletal muscle resulted in varying numbers of dystrophin-positive myofibers, suggesting a differential regenerating capacity among the clones. At 2 and 4 weeks postinjection, the infiltration of CD4- and CD8-positive lymphocytes and a variety of cytokines was detected within the injected site. These data suggest that the transplantation of retrovirally transduced mdx MDSCs can enable persistent dystrophin restoration in mdx skeletal muscle; however, the differential regenerating capacity observed among the MDSC/Dys3999 clones and the postinjection immune response are potential challenges facing this technology.     

Transplanted hematopoietic stem cells demonstrate impaired sarcoglycan expression after engraftment into cardiac and skeletal muscle

Pluripotent bone marrow-derived side population (BM-SP) stem cells have been shown to repopulate the hematopoietic system and to contribute to skeletal and cardiac muscle regeneration after transplantation. We tested BM-SP cells for their ability to regenerate heart and skeletal muscle using a model of cardiomyopathy and muscular dystrophy that lacks delta-sarcoglycan. The absence of delta-sarcoglycan produces microinfarcts in heart and skeletal muscle that should recruit regenerative stem cells. Additionally, sarcoglycan expression after transplantation should mark successful stem cell maturation into cardiac and skeletal muscle lineages. BM-SP cells from normal male mice were transplanted into female delta-sarcoglycan-null mice. We detected engraftment of donor-derived stem cells into skeletal muscle, with the majority of donor-derived cells incorporated within myofibers. In the heart, donor-derived nuclei were detected inside cardiomyocytes. Skeletal muscle myofibers containing donor-derived nuclei generally failed to express sarcoglycan, with only 2 sarcoglycan-positive fibers detected in the quadriceps muscle from all 14 mice analyzed. Moreover, all cardiomyocytes with donor-derived nuclei were sarcoglycan-negative. The absence of sarcoglycan expression in cardiomyocytes and skeletal myofibers after transplantation indicates impaired differentiation and/or maturation of bone marrow-derived stem cells. The inability of BM-SP cells to express this protein severely limits their utility for cardiac and skeletal muscle regeneration.     

骨髓间充质干细胞移植治疗mdx鼠：3个月疗效

背景：干细胞移植治疗肌营养不良症是目前的研究热点,相对造血干细胞移植,间充质干细胞移植风险较小。目的：观察骨髓间充质干细胞移植治疗Duchenne型肌营养不良鼠（mdx鼠）的疗效。方法：4周龄mdx鼠16只,随机分为治疗组与对照组,每组8只,经静脉移植及肌肉局部注射C57BL/6小鼠的骨髓间充质干细胞或等量生理盐水。结果与结论：移植3个月后,治疗组较对照组血清肌酸激酶水平下降,骨骼肌肌膜部分有dystrophin蛋白表达,而对照组检测不到dystrophin蛋白表达。但是两组的运动功能无明显改善。结果初步表明骨髓间充质干细胞移植对mdx鼠有一定的治疗作用,可能使肌细胞膜破坏减少,延缓病情发展。     

Expression of Dog Microdystrophin in Mouse and Dog Muscles by Gene Therapy

Duchenne muscular dystrophy (DMD) is characterized by the absence of dystrophin. Several previous studies demonstrated the feasibility of delivering microdystrophin complementary DNA (cDNA) into mouse and normal nonhuman primate muscles by ex vivo gene therapy. However, these animal models do not reproduce completely the human DMD phenotype, while the dystrophic dog model does. To progress toward the use of the best animal model of DMD, a dog microdystrophin was transduced into human and dystrophic dog muscle precursor cells (MPCs) with a lentivirus before their transplantation into mouse muscles. One month following MPC transplantation, myofibers expressing the dog microdystrophin were observed. We also used another approach to introduce this transgene into myofibers, i.e., the electrotransfer of a plasmid coding for the dog microdystrophin. The plasmid was injected into mouse and dog muscles, and brief electric pulses were applied in the region of injection. Two weeks later, the transgene was detected in both animals. Therefore, ex vivo gene therapy and electrotransfer are two possible methods to introduce a truncated version of dystrophin into myofibers of animal models and eventually into myofibers of DMD patients.     

Matching host muscle and donor myoblasts for myosin heavy chain improves myoblast transfer therapy

Intensive efforts have been made to develop an effective therapy for Duchenne muscular dystrophy (DMD). Although myoblast transplantation has been found capable of transiently delivering dystrophin and improving the strength of the injected dystrophic muscle, this approach has been hindered by the immune rejection problems as well as the poor survival and limited spread of the injected cells. In the present study, we have investigated whether the careful selection of donor myoblasts and host muscle for the myosin heavy chain expression (MyHCs) plays a role in the success of myoblast transfer. Highly purified normal myoblasts derived from the m. soleus and m. gastrocnemius white of normal mice were transplanted into the m. soleus (containing 70% of type I fibers) and gastrocnemius white (100% of type II fibers) of dystrophin deficient mdx mice. At several time-points after injection (10, 20 and 30 days), the number of dystrophin-positive fibers was monitored and compared among the different groups. A significantly higher number and better persistence of dystrophin-positive myofibers were observed when the injected muscle and donor myoblasts expressed a similar MyHC in comparison with myoblast transfer between host muscle and donor myoblasts that were not matched for MyHC. These results suggest that careful matching between the injected myoblasts and injected muscle for the MyHC expression can improve the efficiency of myoblast-mediated gene transfer to skeletal muscle. Gene Therapy (2000) 7, 428-437.     

Local and distant transfection of mdx muscle fibers with dystrophin and LacZ genes delivered in vivo by synthetic microspheres.

Patterns of dystrophin and beta-galactosidase expression were examined in mdx mice after i.m. injections of synthetic microspheres (MF-2) loaded with full-length (pHSADy) or mini-dystrophin gene (pSG5dys) cDNA plasmid constructs or with LacZ marker gene (pCMV-LacZ). A single injection of 25 microg pHSADy into quadriceps femoris muscle resulted in 6.8% of dystrophin positive myofibers (DPM) in a given muscle; 8.4% of DPM in glutaeus muscle and 4.3% of DPM in quadriceps femoris muscle of contralateral limb on day 21 after exposure compared with only 0.6% DPM in intact (non-injected) mdx mice. A high proportion of DPM (17.6% and 10.8%, respectively) was registered in both injected and contralateral muscles after mini- gene cDNA administration. MF-2/dystrophin cDNA particles were detected by FISH analysis in about 60-70% of myofiber nuclei in muscles of injected and contralateral limbs 7 days after application. The presence of human dystrophin cDNA and its products in all skeletal muscles and in different internal organs was proven by PCR and RT-PCR analysis. Patches of beta-galactosidase expression were abundant in injected muscle, and frequent in the contralateral and other skeletal muscles as well as in diaphragm, heart and lungs. High levels of dystrophin cDNA expression, and an efficient distant transfection effect with preferential intranuclei inclusion of MF-2 vehicle, are very encouraging for the development of a new constructive strategy in gene therapy trials of DMD.     

Genetic correction of dystrophin deficiency and skeletal muscle remodeling in adult MDX mouse via transplantation of retroviral producer cells.

Duchenne muscular dystrophy (DMD) is an X-linked, lethal disease caused by mutations of the dystrophin gene. No effective therapy is available, but dystrophin gene transfer to skeletal muscle has been proposed as a treatment for DMD. We have developed a strategy for efficient in vivo gene transfer of dystrophin cDNA into regenerating skeletal muscle. Retroviral producer cells, which release a vector carrying the therapeutically active dystrophin minigene, were mitotically inactivated and transplanted in adult nude/mdx mice. Transplantation of 3 x 10(6) producer cells in a single site of the tibialis anterior muscle resulted in the transduction of between 5.5 and 18% total muscle fibers. The same procedure proved also feasible in immunocompetent mdx mice under short-term pharmacological immunosuppression. Minidystrophin expression was stable for up to 6 mo and led to alpha-sarcoglycan reexpression. Muscle stem cells could be transduced in vivo using this procedure. Transduced dystrophic skeletal muscle showed evidence of active remodeling reminiscent of the genetic normalization process which takes place in female DMD carriers. Overall, these results demonstrate that retroviral-mediated dystrophin gene transfer via transplantation of producer cells is a valid approach towards the long-term goal of gene therapy of DMD.     

Autologous transplantation of muscle precursor cells modified with a lentivirus for muscular dystrophy: human cells and primate models.

Duchenne muscular dystrophy (DMD) is characterized by the absence of dystrophin. We tested the ability of lentiviral vectors to deliver a transgene into myogenic cells before their transplantation. Enhanced green fluorescent protein (eGFP) transgene was efficiently transferred into cells and eGFP-positive fibers were generated following transplantation. An eGFP-micro-dystrophin transgene under the control of a cytomegalovirus promoter was then transferred with the same viral vector but caused some toxicity to the mono-nucleated cells. We then used instead a muscle creatine kinase promoter. Dystrophin expression was observed in the muscle fibers after the transplantation of such genetically modified cells into mdx and severe combined immunodeficient mice. Micro-dystrophin expression was also observed in monkey muscles a month after allogenic or autologous transplantation of genetically modified myoblasts. Therapeutic exon skipping was induced by infecting myoblasts of a DMD patient, deleted for dystrophin exons 49 and 50, with a lentivirus expressing a U7 small nuclear RNA containing antisense sequences against exon 51. The modification led to correct exon skipping and to the expression of a quasi-dystrophin in vitro and in vivo. These results demonstrate the feasibility of lentiviral-based ex vivo gene therapy for DMD.     

Dystrophin expression in host muscle following transplantation of muscle precursor cells modified with the phiC31 integrase

Duchenne muscular dystrophy (DMD) is the most severe muscular dystrophy. It is caused by the absence of dystrophin in muscle fibers. The autologous transplantation of genetically corrected muscle precursor cells (MPCs) is a possible cure for DMD. A non-viral method of genetic modification was tested in this study. The co-transfection (nucleofection) of a phiC31 integrase and a transgene expressing plasmid in MPCs led to an increased stable expression in vitro. The stable expression of a small transgene (eGFP) in muscle fibers was initially demonstrated following the transplantation of the genetically modified cells. The stable expression of a truncated version of dystrophin as well as the full-length dystrophin fused with eGFP was then demonstrated in MPCs obtained from an mdx mice. The transplantation of these cells led not only to the expression of these fusion proteins in muscle fibers but also to the reconstitution of the dystrophin complex. Human MPCs were also genetically modified with a plasmid coding for the full-length human dystrophin gene fused with eGFP and transplanted in severe combined immuno deficient mice leading to the expression of eGFP dystrophin in muscle fibers. This work indicates that cell transplantation after correction of MPCs with phiC31 integrase is a possible approach to treat DMD.     

Circulating red cells usually remain of host origin after bone marrow transplantation for severe combined immunodeficiency

Background: Patients with severe combined immunodeficiency (SCID) treated with allogeneic bone marrow transplantation often receive a milder conditioning regimen than patients who undergo transplantation for hematologic malignancy, and they regularly retain circulating white cells of host origin. The origin of circulating red cells following successful bone marrow transplantation to treat SCID is not known. Study Design and Methods: Review of the medical records identified all patients with SCID who underwent ABO-mismatched bone marrow transplantation at the University of California, San Francisco, between 1982 and 1994. The ABO and Rh phenotype at>6 months after transplantation was determined for all successful transplants by review of the medical record or the taking of a fresh blood sample for analysis. Patient-conditioning and donor bone marrow-preparative regimens were reviewed to assess their possible influence on the red cell phenotype after successful bone marrow transplantation. Results: Nine of 35 SCID patients who underwent successful transplantation received marrow from ABO-mismatched donors. Eight of the nine patients had only host red cells circulating at 6 to 84 months after transplantation, while one patient had only donor red cells circulating at 48 months after transplantation. None of the patients had circulating red cells of both host and donor origin. Conditioning regimens included cyclophosphamide and antithymocyte globulin for all nine patients; only three patients also received total body irradiation. Seven of the nine patients received related-donor, HLA- mismatched bone marrow, and two patients received HLA-identical bone marrow; eight patients received T-cell-depleted bone marrow. The one patient whose red cell phenotype converted to that of the donor received T-cell-depleted, haploidentical marrow, and the preparative regimen included chemotherapy and total body irradiation. Conclusion: SCID patients successfully treated with allogeneic bone marrow transplantation typically fail to show circulating red cells of donor phenotype; this finding is in contrast to the universal presence of circulating donor red cells following successful bone marrow transplantation to treat hematologic malignancies and other diseases. The milder conditioning regimens typically given to patients with SCID, along with T-cell depletion and HLA mismatching, may play a role in this different outcome. It is not known whether the inability to find circulating red cells of donor origin is due to a failure to engraft donor pluripotent stem cells or a failure of engrafted donor stem cells to differentiate along the erythroid lineage.     

Modulation of Starling forces and muscle fiber maturity permits adenovirus-mediated gene transfer to adult dystrophic (mdx) mice by the intravascular route

Duchenne muscular dystrophy (DMD) and other inherited myopathies lead to progressive destruction of most skeletal muscles in the body, including those responsible for maintaining respiration. DMD is a fatal disorder caused by defects in the dystrophin gene. Recombinant adenovirus vectors (AdV) are considered a promising means for therapeutic delivery of a functional dystrophin gene to DMD muscles. If AdV-mediated dystrophin gene replacement in DMD is to be successful, development of a systemic delivery method for targeting the large number of diseased muscles will be required. In this study we investigated two major factors preventing efficient AdV-mediated gene transfer to skeletal muscles of adult animals after intravascular AdV administration: (1) an inability of AdV particles to breach the endothelial barrier and enter into contact with myofibers, and (2) a relatively nonpermissive myofiber population for AdV infection due at least in part to insufficient levels of the coxsackie/adenovirus attachment receptor (CAR). On the basis of established principles governing the transendothelial flux of macromolecules, we further hypothesized that an alteration in Starling forces (increased hydrostatic and decreased osmotic pressures) within the intravascular compartment would facilitate AdV transendothelial flux via convective transport. In addition, experimental muscle regeneration was employed to increase the prevalence of immature myofibers in which CAR expression is upregulated. Here we report that by employing the above-described strategy, high-level heterologous reporter gene expression was achievable in hindlimb muscles of normal rats as well as dystrophic (mdx) mice (genetic homolog of DMD) after a single intraarterial injection of AdV. Microsphere studies confirmed enhanced transport into muscle of fluorescent tracer particles in the size range of AdV, and there was a high concordance between CAR upregulation and myofiber transduction after intraarterial AdV delivery. Furthermore, in mdx mice examined 10 days after intraarterial AdV delivery, the aforementioned procedures had no adverse effects on the force-generating capacity of targeted muscles. These findings have implications for eventual AdV-mediated gene therapy of generalized skeletal muscle diseases such as DMD using a systemic intraarterial delivery approach.     

成体脂肪来源的Flk-1+间充质干细胞用于杜氏肌营养不良症治疗的可行性研究

为了探讨成体脂肪来源的间充质干细胞（adipose derived mesenchymal stemcell,AD-MSC）移植治疗杜氏肌营养不良症（Duchenne muscular dystrophy,DMD）的可行性,从成年GFP小鼠脂肪组织中分离得到间充质干细胞（mesen-chymal stem cell,MSC）,用流式细胞术分析其细胞表型和细胞周期;在体外分别以成肌和成内皮诱导体系诱导AD-MSC的定向分化,通过免疫荧光染色和RT-PCR进行鉴定;经尾静脉移植AD-MSC到CTX肌肉损伤模型小鼠和mdx小鼠（DMD动物模型）体内,通过RT-PCR和免疫荧光染色检测供体细胞的分布和分化情况,并进行统计分析。结果表明：从GFP小鼠脂肪组织中分离得到的Flk-1＋MSC可以在体外分化为肌肉细胞和血管内皮细胞;细胞移植后,可在损伤肌肉部位检测到GFP阳性的肌纤维、血管内皮细胞和肌肉干细胞;mdx小鼠移植后肌膜上抗肌萎缩蛋白（dystro-phin）的表达部分恢复,骨骼肌中心核肌纤维比例也大大降低。结论：AD-MSC是一种较为理想的供移植治疗DMD的干细胞,它不仅可以定向归巢到损伤的肌肉组织中,参与肌纤维重建,而且能改善组织供血,缓解DMD病征,同时部分供体细胞以干细胞形式存在,可以代替原来肌肉干细胞的功能,不断修复损伤的肌肉组织。     

Intramuscular Transplantation of Human Postnatal Myoblasts Generates Functional Donor-Derived Satellite Cells

Myogenic cell transplantation is an experimental approach for the treatment of myopathies. In this approach, transplanted cells need to fuse with pre-existing myofibers, form new myofibers, and generate new muscle precursor cells (MPCs). The last property was fully reported following myoblast transplantation in mice but remains poorly studied with human myoblasts. In this study, we provide evidence that the intramuscular transplantation of postnatal human myoblasts in immunodeficient mice generates donor-derived MPCs and specifically donor-derived satellite cells. In a first experiment, cells isolated from mouse muscles 1 month after the transplantation of human myoblasts proliferated in vitro as human myoblasts. These cells were retransplanted in mice and formed myofibers expressing human dystrophin. In a second experiment, we observed that inducing muscle regeneration 2 months following transplantation of human myoblasts led to myofiber regeneration by human-derived MPCs. In a third experiment, we detected by immunohistochemistry abundant human-derived satellite cells in mouse muscles 1 month after transplantation of postnatal human myoblasts. These human-derived satellite cells may correspond totally or partially to the human-derived MPCs evidenced in the first two experiments. Finally, we present evidence that donor-derived satellite cells may be produced in patients that received myoblast transplantation.     

Interplay of IKK/NF-kappaB signaling in macrophages and myofibers promotes muscle degeneration in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a lethal X-linked disorder associated with dystrophin deficiency that results in chronic inflammation and severe skeletal muscle degeneration. In DMD mouse models and patients, we find that IkappaB kinase/NF-kappaB (IKK/NF-kappaB) signaling is persistently elevated in immune cells and regenerative muscle fibers. Ablation of 1 allele of the p65 subunit of NF-kappaB was sufficient to improve pathology in mdx mice, a model of DMD. In addition, conditional deletion of IKKbeta in mdx mice elucidated that NF-kappaB functions in activated macrophages to promote inflammation and muscle necrosis and in skeletal muscle fibers to limit regeneration through the inhibition of muscle progenitor cells. Furthermore, specific pharmacological inhibition of IKK resulted in improved pathology and muscle function in mdx mice. Collectively, these results underscore the critical role of NF-kappaB in the progression of muscular dystrophy and suggest the IKK/NF-kappaB signaling pathway as a potential therapeutic target for DMD.     

Blocking the Myostatin Signal With a Dominant Negative Receptor Improves the Success of Human Myoblast Transplantation in Dystrophic Mice

Duchenne muscular dystrophy (DMD) is a recessive disease caused by a dystrophin gene mutation. Myoblast transplantation permits to introduce the dystrophin gene in dystrophic muscle fibers. However, the success of this approach is reduced by the short duration of the regeneration following the transplantation, which reduces the number of hybrid fibers. Myostatin (MSTN) is a negative regulator of skeletal muscle development and responsible for limiting regeneration. It binds with high affinity to the activin type IIB receptor (ActRIIB). Our aim was to verify whether the success of the myoblast transplantation is enhanced by blocking the MSTN signal with expression of a dominant negative mutant of ActRIIB (dnActRIIB). In vitro, blocking MSTN activity with a lentivirus carrying dnActRIIB increased proliferation and fusion of human myoblasts because MSTN regulates the expression of several myogenic regulatory factors. In vivo, myoblasts infected with the dnActRIIB lentivirus were transplanted in immunodeficient dystrophic mice. Dystrophin immunostaining of tibialis anterior (TA) cross-sections of these mice 1 month post-transplantation revealed more human dystrophin-positive myofibers following the transplantation of dnActRIIB myoblasts than of control myoblasts. Thus, blocking the MSTN signal with dnActRIIB improved the success of myoblast transplantation by increasing the myoblast proliferation and fusion and changed the expression of myogenic regulatory factors.     

异基因脐血干细胞移植后STR基因座位的表达分析

本研究分析杜氏型肌营养不良（DMD）病人异基因脐带血干细胞移植后STR基因座位的表达。利用PCR—SSO方法检测患者、供者HLA-A、B、DR位点，并利用STR—PCR方法检测术后各个器官的STR-基因位点的表达情况。结果表明：患者与供者在HLA中、低分辨情况下A、B、DR位点全相合，患者的胸骨骨髓显示为供者独立植入，脾、左上肺、前臂、肌舌、左肝、胃、右颞叶、膈肌、右支气管、左心室、右肾均为嵌合状态。结论：造血干细胞移植术后，供者的基因可在实体器官表达，形成嵌合状态.     

Flow cytometric characterization of myogenic cell populations obtained via the preplate technique: potential for rapid isolation of muscle-derived stem cells.

Myoblast transplantation has been investigated as a therapy for muscle-related diseases and as a gene delivery vehicle for therapeutic recombinant proteins. Clinical successes involving muscle cell transplantation have been limited, in part because of poor donor cell survival, and the heterogeneous nature of myogenic donor cells has largely been ignored. We have previously reported an isolation technique, preplating, that results in purified myogenic cells that are capable of significantly higher rates of donor cell survival leading to enhanced gene transfer to skeletal muscle. Characterization of these purified cells revealed that they display markers common to stem cells and are capable of multilineage differentiation. This study was performed to phenotypically characterize, by flow cytometry, muscle-derived cell populations obtained by the preplate technique for the purpose of eventually developing a method to quickly identify and isolate viable muscle cells best suited for transplantation. Muscle cell cultures were analyzed for expression of the surface proteins Sca-1, c-Kit, and CD34. We found that the preplate technique purifies distinct myogenic cell subpopulations expressing CD34 alone (Sca-1 negative) and Sca-1 alone (CD34 negative), but that this expression is subject to change with time in culture. Isolation and transplantation of phenotypically pure Sca-1-positive myogenic cells, obtained by magnetic cell sorting, demonstrates the ability to quickly select viable myogenic cells capable of regenerating skeletal muscle and restoring dystrophin expression within dystrophic host skeletal muscle. Flow cytometric described phenotypes will aid in the rapid isolation of specific donor cell populations for muscle cell transplants and muscle cell-mediated gene therapies, thereby enhancing their future success.     

Poly(ethylene imine)-poly(ethylene glycol) copolymers facilitate efficient delivery of antisense oligonucleotides to nuclei of mature muscle cells of mdx mice

Antisense oligonucleotides (AO) can facilitate dystrophin expression via targeted exon skipping in cultured cells of Duchenne muscular dystrophy (DMD) patients and in the mouse model of DMD (mdx mice). However, the lack of effective means to deliver AO to myonuclei remains the foremost limitation to their usefulness in DMD gene therapy. In this study we show that copolymers of cationic poly(ethylene imine) (PEI) and poly(ethylene glycol) (PEG) facilitated efficient cellular uptake and nuclear delivery of AO in mature skeletal muscle fibers isolated from mdx mice. Confocal analysis of dual fluorescently tagged PEG-PEI-AO polyplexes, 24 hr after transfection, showed that the copolymer and AO were colocalized within punctate membrane- associated structures. Importantly, AO was efficiently translocated into myonuclei, whereas the copolymer was mostly excluded. The morphology of all transfected myofibers was perfectly maintained with no indication of damage or cytotoxicity. Quantitative fluorescence analysis showed that transfection with PEG-PEI-AO resulted in a 6-fold higher uptake of AO into myonuclei compared with transfections of AO alone. Interestingly, transfections with rhodamine-labeled PEG-PEI copolymers yielded an approximately 2- fold higher uptake of AO into myonuclei compared with transfections of unlabeled copolymers. Attempts to further increase AO delivery by addition of insulin-transferrin-selenium (ITS) to the medium showed no further improvement in AO delivery. Dose-response analysis indicated saturation of endocytotic uptake of the polyplex. Overall, we conclude that PEG-PEI copolymers represent high-capacity, nontoxic carriers for efficient delivery of AO to nuclei of mature myofibers.     

Regeneration capacity of skeletal muscle

The organotypic stem cell of skeletal muscle has previously been known as satellite cell. They allow muscle fiber growth during ontogenesis, enable fiber hypertrophy and are responsible for the very efficient repair of muscle fibers. This efficient apparatus is to some degree counterbalanced by an enormous use of the satellite cell pool: fiber atrophy probably is accompanied by loss of myonuclei such that every reversal of atrophy is bound to use new myonuclei i.e. satellite cells. How often in life does this occur? Hard to say. Moreover, the potent repair capacity is challenged by an unexpected vulnerability of skeletal muscle fibers: Passive stretching of contracted muscles may cause multiple "microdamage," disruption of contractile elements or tiny areas of true necrosis (focal necrosis). How often does this happen? Well, for many of us at least once per year when we go up and down mountains during vacation time, followed by sour muscles. Others may decide to change his/her (locomotor) behaviour by severe onset of jogging; it may happen that they suffer kidney failure on Monday due to muscle microdamage and the transfer of myoproteins into the serum over weekend. Also 20 minutes of stepping up and down something like a chair will do: There is a remarkable increase in kreatin kinase and other muscle derived proteins which lasts for days and is bound to reflect some muscle damage. How about sportsmen and worker who repeatedly use their muscles in such a way? We don't have answers yet to most of these questions, but considerable amount of information has been collected over the last years both in animal and--less--in human. What is common in all cases of growth and repair is the proliferation of the satellite cells and their consequent incorporation and fusion with the parent fiber. This way focal damage is repaired often without visible reminders. We would run out of satellite cells were they not stem cells: After division one daughter remains a satellite cell while the other is free to divide. Divide how often? Important for the human cells since the cell ages and proliferates slower and slower till it stops to divide at all, at least in culture. The same is true for the new satellite cell. This we know from recent experiments in which human biopsies derived myogenic cells were grown in vitro and in vivo (by implanting them into skeletal muscles of immunoincompetent mice): Growth correlates negatively with age of the donor. Between age 2 and some 70 years, about two divisions are performed by each satellite cell in human vastus lateralis and biceps brachii muscle in 10 years in the average. Most important for the older among us: at age 76 there are still some 13 divisions left before complete exhaustion. However, there are diseases like Duchenne Muscular Dystrophy (DMD) in which muscle fibers lack a structural protein with the effect of enhanced vulnerability to mechanical stress. There the enhanced use of the satellite cell pool makes the remaining growth capacity in an 8-years-old child as low as otherwise found at age 80. Some time ago, implantation of genetically intact myoblasts obtained from healthy relatives has been proposed as a treatment of DMD. Every logic would have predicted that some local implantation of whatever numbers of cells was bound to fail rescue the complete masculature or at least the muscles for breathing. The human as guinea pig? Now, even years later, we still collect the basic information on growth of human myoblasts and start thinking of ways for systemic application and quantitatively relevant incorporation of the myogenic stem cell or other--possibly pluripotent--stem cells derived from bone marrow.