Mesenchymal stem cells derived from human induced pluripotent stem cells improve the engraftment of myogenic cells by secreting urokinase-type plasminogen activator receptor (uPAR)

Background

Duchenne muscular dystrophy (DMD) is a severe X-linked recessive disease caused by mutations in the dystrophin gene. Transplantation of myogenic stem cells holds great promise for treating muscular dystrophies. However, poor engraftment of myogenic stem cells limits the therapeutic effects of cell therapy. Mesenchymal stem cells (MSCs) have been reported to secrete soluble factors necessary for skeletal muscle growth and regeneration.

Methods

We induced MSC-like cells (iMSCs) from induced pluripotent stem cells (iPSCs) and examined the effects of iMSCs on the proliferation and differentiation of human myogenic cells and on the engraftment of human myogenic cells in the tibialis anterior (TA) muscle of NSG-mdx^4Cv mice, an immunodeficient dystrophin-deficient DMD model. We also examined the cytokines secreted by iMSCs and tested their effects on the engraftment of human myogenic cells.

Results

iMSCs promoted the proliferation and differentiation of human myogenic cells to the same extent as bone marrow-derived (BM)-MSCs in coculture experiments. In cell transplantation experiments, iMSCs significantly improved the engraftment of human myogenic cells injected into the TA muscle of NSG-mdx^4Cv mice. Cytokine array analysis revealed that iMSCs produced insulin-like growth factor-binding protein 2 (IGFBP2), urokinase-type plasminogen activator receptor (uPAR), and brain-derived neurotrophic factor (BDNF) at higher levels than did BM-MSCs. We further found that uPAR stimulates the migration of human myogenic cells in vitro and promotes their engraftment into the TA muscles of immunodeficient NOD/Scid mice.

Conclusions

Our results indicate that iMSCs are a new tool to improve the engraftment of myogenic progenitors in dystrophic muscle.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02594-1.

Pharmacological activation of SERCA ameliorates dystrophic phenotypes in dystrophin-deficient mdx mice

Duchenne muscular dystrophy (DMD) is an X-linked genetic disorder characterized by progressive muscular weakness because of the loss of dystrophin. Extracellular Ca2+ flows into the cytoplasm through membrane tears in dystrophin-deficient myofibers, which leads to muscle contracture and necrosis. Sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) takes up cytosolic Ca2+ into the sarcoplasmic reticulum, but its activity is decreased in dystrophic muscle. Here, we show that an allosteric SERCA activator, CDN1163, ameliorates dystrophic phenotypes in dystrophin-deficient mdx mice. The administration of CDN1163 prevented exercise-induced muscular damage and restored mitochondrial function. In addition, treatment with CDN1163 for 7 weeks enhanced muscular strength and reduced muscular degeneration and fibrosis in mdx mice. Our findings provide preclinical proof-of-concept evidence that pharmacological activation of SERCA could be a promising therapeutic strategy for DMD. Moreover, CDN1163 improved muscular strength surprisingly in wild-type mice, which may pave the new way for the treatment of muscular dysfunction.

Perlecan Facilitates Neuronal Nitric Oxide Synthase Delocalization in Denervation-Induced Muscle Atrophy

Perlecan is an extracellular matrix molecule anchored to the sarcolemma by a dystrophin–glycoprotein complex. Perlecan-deficient mice are tolerant to muscle atrophy, suggesting that perlecan negatively regulates mechanical stress-dependent skeletal muscle mass. Delocalization of neuronal nitric oxide synthase (nNOS) from the sarcolemma to the cytosol triggers protein degradation, thereby initiating skeletal muscle atrophy. We hypothesized that perlecan regulates nNOS delocalization and activates protein degradation during this process. To determine the role of perlecan in nNOS-mediated mechanotransduction, we used sciatic nerve transection as a denervation model of gastrocnemius muscles. Gastrocnemius muscle atrophy was significantly lower in perinatal lethality-rescued perlecan-knockout (Hspg2^−/−-Tg) mice than controls (WT-Tg) on days 4 and 14 following surgery. Immunofluorescence microscopy showed that cell membrane nNOS expression was reduced by denervation in WT-Tg mice, with marginal effects in Hspg2^−/−-Tg mice. Moreover, levels of atrophy-related proteins—i.e., FoxO1a, FoxO3a, atrogin-1, and Lys48-polyubiquitinated proteins—increased in the denervated muscles of WT-Tg mice but not in Hspg2^−/−-Tg mice. These findings suggest that during denervation, perlecan promotes nNOS delocalization from the membrane and stimulates protein degradation and muscle atrophy by activating FoxO signaling and the ubiquitin–proteasome system.

iNOS is not responsible for RyR1 S-nitrosylation in mdx mice with truncated dystrophin

Background

Previous research indicated that nitric oxide synthase (NOS) is the key molecule for S-nitrosylation of ryanodine receptor 1 (RyR1) in DMD model mice (mdx mice) and that both neuronal NOS (nNOS) and inducible NOS (iNOS) might contribute to the reaction because nNOS is mislocalized in the cytoplasm and iNOS expression is higher in mdx mice. We investigated the effect of iNOS on RyR1 S-nitrosylation in mdx mice and whether transgenic expression of truncated dystrophin reduced iNOS expression in mdx mice or not.

Methods

Three- to 4-month-old C57BL/6 J, mdx, and transgenic mdx mice expressing exon 45–55-deleted human dystrophin (Tg/mdx mice) were used. We also generated two double mutant mice, mdx iNOS KO and Tg/mdx iNOS KO to reveal the iNOS contribution to RyR1 S-nitrosylation. nNOS and iNOS expression levels in skeletal muscle of these mice were assessed by immunohistochemistry (IHC), qRT-PCR, and Western blotting. Total NOS activity was measured by a citrulline assay. A biotin-switch method was used for detection of RyR1 S-nitrosylation. Statistical differences were assessed by one-way ANOVA with Tukey-Kramer post-hoc analysis.

Results

mdx and mdx iNOS KO mice showed the same level of RyR1 S-nitrosylation. Total NOS activity was not changed in mdx iNOS KO mice compared with mdx mice. iNOS expression was undetectable in Tg/mdx mice expressing exon 45–55-deleted human dystrophin, but the level of RyR1 S-nitrosylation was the same in mdx and Tg/mdx mice.

Conclusion

Similar levels of RyR1 S-nitrosylation and total NOS activity in mdx and mdx iNOS KO demonstrated that the proportion of iNOS in total NOS activity was low, even in mdx mice. Exon 45–55-deleted dystrophin reduced the expression level of iNOS, but it did not correct the RyR1 S-nitrosylation. These results indicate that iNOS was not involved in RyR1 S-nitrosylation in mdx and Tg/mdx mice muscles.

Prostaglandin EP2 receptor downstream of Notch signaling inhibits differentiation of human skeletal muscle progenitors in differentiation conditions

Understanding the signaling pathways that regulate proliferation and differentiation of muscle progenitors is essential for successful cell transplantation for treatment of Duchenne muscular dystrophy. Here, we report that a γ-secretase inhibitor, DAPT (N-[N-(3,5-difluorophenacetyl-L-alanyl)]-S-phenylglycine tertial butyl ester), which inhibits the release of NICD (Notch intercellular domain), promotes the fusion of human muscle progenitors in vitro and improves their engraftment in the tibialis anterior muscle of immune-deficient mice. Gene expression analysis revealed that DAPT severely down-regulates PTGER2, which encodes prostaglandin (PG) E2 receptor 2 (EP2), in human muscle progenitors in the differentiation condition. Functional analysis suggested that Notch signaling inhibits differentiation and promotes self-renewal of human muscle progenitors via PGE2/EP2 signaling in a cAMP/PKA-independent manner.

Tbx1 regulates inherited metabolic and myogenic abilities of progenitor cells derived from slow- and fast-type muscle

Skeletal muscle is divided into slow- and fast-type muscles, which possess distinct contractile and metabolic properties. Myogenic progenitors associated with each muscle fiber type are known to intrinsically commit to specific muscle fiber lineage during embryonic development. However, it is still unclear whether the functionality of postnatal adult myogenic cells is attributable to the muscle fiber in which they reside, and whether the characteristics of myogenic cells derived from slow- and fast-type fibers can be distinguished at the genetic level. In this study, we isolated adult satellite cells from slow- and fast-type muscle individually and observed that satellite cells from each type of muscle generated myotubes expressing myosin heavy chain isoforms similar to their original muscle, and showed different metabolic features. Notably, we discovered that slow muscle-derived cells had low potential to differentiate but high potential to self-renew compared with fast muscle-derived cells. Additionally, cell transplantation experiments of slow muscle-derived cells into fast-type muscle revealed that slow muscle-derived cells could better contribute to myofiber formation and satellite cell constitution than fast muscle-derived cells, suggesting that the recipient muscle fiber type may not affect the predetermined abilities of myogenic cells. Gene expression analyses identified T-box transcriptional factor Tbx1 as a highly expressed gene in fast muscle-derived myoblasts. Gain- and loss-of-function experiments revealed that Tbx1 modulated muscle fiber types and oxidative metabolism in myotubes, and that Tbx1 stimulated myoblast differentiation, but did not regulate myogenic cell self-renewal. Our data suggest that metabolic and myogenic properties of myogenic progenitor cells vary depending on the type of muscle from which they originate, and that Tbx1 expression partially explains the functional differences of myogenic cells derived from fast-type and slow-type muscles.

Premyogenic progenitors derived from human pluripotent stem cells expand in floating culture and differentiate into transplantable myogenic progenitors

Human induced pluripotent stem cells (hiPSCs) are a potential source for cell therapy of Duchenne muscular dystrophy. To reliably obtain skeletal muscle progenitors from hiPSCs, we treated hiPS cells with a Wnt activator, CHIR-99021 and a BMP receptor inhibitor, LDN-193189, and then induced skeletal muscle cells using a previously reported sphere-based culture. This protocol greatly improved sphere formation efficiency and stably induced the differentiation of myogenic cells from hiPS cells generated from both healthy donors and a patient with congenital myasthenic syndrome. hiPSC-derived myogenic progenitors were enriched in the CD57(−) CD108(−) CD271(+) ERBB3(+) cell fraction, and their differentiation was greatly promoted by TGF-β inhibitors. TGF-β inhibitors down-regulated the NFIX transcription factor, and NFIX short hairpin RNA (shRNA) improved the differentiation of iPS cell-derived myogenic progenitors. These results suggest that NFIX inhibited differentiation of myogenic progenitors. hiPSC-derived myogenic cells differentiated into myofibers in muscles of NSG-mdx^4Cv mice after direct transplantation. Our results indicate that our new muscle induction protocol is useful for cell therapy of muscular dystrophies.

Skeletal muscle generated from induced pluripotent stem cells - induction and application

Human induced pluripotent stem cells (hiPS cells or hiPSCs) can be derived from cells of patients with severe muscle disease. If skeletal muscle induced from patient-iPSCs shows disease-specific phenotypes, it can be useful for studying the disease pathogenesis and for drug development. On the other hand, human iPSCs from healthy donors or hereditary muscle disease-iPSCs whose genomes are edited to express normal protein are expected to be a cell source for cell therapy. Several protocols for the derivation of skeletal muscle from human iPSCs have been reported to allow the development of efficient treatments for devastating muscle diseases. In 2017, the focus of research is shifting to another stage: (1) the establishment of mature myofibers that are suitable for study of the pathogenesis of muscle disease; (2) setting up a high-throughput drug screening system; and (3) the preparation of highly regenerative, non-oncogenic cells in large quantities for cell transplantation, etc.

Calcitonin Receptor Signaling Inhibits Muscle Stem Cells from Escaping the Quiescent State and the Niche

Calcitonin receptor (Calcr) is expressed in adult muscle stem cells (muscle satellite cells [MuSCs]). To elucidate the role of Calcr, we conditionally depleted Calcr from adult MuSCs and found that impaired regeneration after muscle injury correlated with the decreased number of MuSCs in Calcr-conditional knockout (cKO) mice. Calcr signaling maintained MuSC dormancy via the cAMP-PKA pathway but had no impact on myogenic differentiation of MuSCs in an undifferentiated state. The abnormal quiescent state in Calcr-cKO mice resulted in a reduction of the MuSC pool by apoptosis. Furthermore, MuSCs were found outside their niche in Calcr-cKO mice, demonstrating cell relocation. This emergence from the sublaminar niche was prevented by the Calcr-cAMP-PKA and Calcr-cAMP-Epac pathways downstream of Calcr. Altogether, the findings demonstrated that Calcr exerts its effect specifically by keeping MuSCs in a quiescent state and in their location, maintaining the MuSC pool.

Mesoangioblast delivery of miniagrin ameliorates murine model of merosin-deficient congenital muscular dystrophy type 1A

Background

Merosin-deficient congenital muscular dystrophy type-1A (MDC1A) is characterized by progressive muscular dystrophy and dysmyelinating neuropathy caused by mutations of the α2 chain of laminin-211, the predominant laminin isoform of muscles and nerves. MDC1A has no available treatment so far, although preclinical studies showed amelioration of the disease by the overexpression of miniagrin (MAG). MAG reconnects orphan laminin-211 receptors to other laminin isoforms available in the extracellular matrix of MDC1A mice.

Methods

Mesoangioblasts (MABs) are vessel-associated progenitors that can form the skeletal muscle and have been shown to restore defective protein levels and motor skills in animal models of muscular dystrophies. As gene therapy in humans still presents challenging technical issues and limitations, we engineered MABs to overexpress MAG to treat MDC1A mouse models, thus combining cell to gene therapy.

Results

MABs synthesize and secrete only negligible amount of laminin-211 either in vitro or in vivo. MABs engineered to deliver MAG and injected in muscles of MDC1A mice showed amelioration of muscle histology, increased expression of laminin receptors in muscle, and attenuated deterioration of motor performances. MABs did not enter the peripheral nerves, thus did not affect the associated peripheral neuropathy.

Conclusions

Our study demonstrates the potential efficacy of combining cell with gene therapy to treat MDC1A.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-015-0055-5) contains supplementary material, which is available to authorized users.

Low intensity training of mdx mice reduces carbonylation and increases expression levels of proteins involved in energy metabolism and muscle contraction

High intensity training induces muscle damage in dystrophin-deficient mdx mice, an animal model for Duchenne muscular dystrophy. However, low intensity training (LIT) rescues the mdx phenotype and even reduces the level of protein carbonylation, a marker of oxidative damage. Until now, beneficial effects of LIT were mainly assessed at the physiological level. We investigated the effects of LIT at the molecular level on 8-week-old wild-type and mdx muscle using 2D Western blot and protein-protein interaction analysis. We found that the fast isoforms of troponin T and myosin binding protein C as well as glycogen phosphorylase were overcarbonylated and downregulated in mdx muscle. Some of the mitochondrial enzymes of the citric acid cycle were overcarbonylated, whereas some proteins of the respiratory chain were downregulated. Of functional importance, ATP synthase was only partially assembled, as revealed by Blue Native PAGE analysis. LIT decreased the carbonylation level and increased the expression of fast isoforms of troponin T and of myosin binding protein C, and glycogen phosphorylase. In addition, it increased the expression of aconitate hydratase and NADH dehydrogenase, and fully restored the ATP synthase complex. Our study demonstrates that the benefits of LIT are associated with lowered oxidative damage as revealed by carbonylation and higher expression of proteins involved in energy metabolism and muscle contraction. Potentially, these results will help to design therapies for DMD based on exercise mimicking drugs.

Doublecortin marks a new population of transiently amplifying muscle progenitor cells and is required for myofiber maturation during skeletal muscle regeneration

Muscle satellite cells are indispensable for muscle regeneration, but the functional diversity of their daughter cells is unknown. Here, we show that many Pax7+MyoD− cells locate both beneath and outside the basal lamina during myofiber maturation. A large majority of these Pax7+MyoD− cells are not self-renewed satellite cells, but have different potentials for both proliferation and differentiation from Pax7+MyoD+ myoblasts (classical daughter cells), and are specifically marked by expression of the doublecortin (Dcx) gene. Transplantation and lineage-tracing experiments demonstrated that Dcx-expressing cells originate from quiescent satellite cells and that the microenvironment induces Dcx in myoblasts. Expression of Dcx seems to be necessary for myofiber maturation because Dcx-deficient mice exhibited impaired myofiber maturation resulting from a decrease in the number of myonuclei. Furthermore, in vitro and in vivo studies suggest that one function of Dcx in myogenic cells is acceleration of cell motility. These results indicate that Dcx is a new marker for the Pax7+MyoD− subpopulation, which contributes to myofiber maturation during muscle regeneration.

Capsaicin mimics mechanical load-induced intracellular signaling events: involvement of TRPV1-mediated calcium signaling in induction of skeletal muscle hypertrophy

Mechanical load-induced intracellular signaling events are important for subsequent skeletal muscle hypertrophy. We previously showed that load-induced activation of the cation channel TRPV1 caused an increase in intracellular calcium concentrations ([Ca²⁺]_i) and that this activated mammalian target of rapamycin (mTOR) and promoted muscle hypertrophy. However, the link between mechanical load-induced intracellular signaling events, and the TRPV1-mediated increases in [Ca²⁺]_i are not fully understood. Here we show that administration of the TRPV1 agonist, capsaicin, induces phosphorylation of mTOR, p70S6K, S6, Erk1/2 and p38 MAPK, but not Akt, AMPK or GSK3β. Furthermore, the TRPV1-induced phosphorylation patterns resembled those induced by mechanical load. Our results continue to highlight the importance of TRPV1-mediated calcium signaling in load-induced intracellular signaling pathways.

Impaired viability of muscle precursor cells in muscular dystrophy with glycosylation defects and amelioration of its severe phenotype by limited gene expression

A group of muscular dystrophies, dystroglycanopathy is caused by abnormalities in post-translational modifications of dystroglycan (DG). To understand better the pathophysiological roles of DG modification and to establish effective clinical treatment for dystroglycanopathy, we here generated two distinct conditional knock-out (cKO) mice for fukutin, the first dystroglycanopathy gene identified for Fukuyama congenital muscular dystrophy. The first dystroglycanopathy model-myofiber-selective fukutin-cKO [muscle creatine kinase (MCK)-fukutin-cKO] mice-showed mild muscular dystrophy. Forced exercise experiments in presymptomatic MCK-fukutin-cKO mice revealed that myofiber membrane fragility triggered disease manifestation. The second dystroglycanopathy model-muscle precursor cell (MPC)-selective cKO (Myf5-fukutin-cKO) mice-exhibited more severe phenotypes of muscular dystrophy. Using an isolated MPC culture system, we demonstrated, for the first time, that defects in the fukutin-dependent modification of DG lead to impairment of MPC proliferation, differentiation and muscle regeneration. These results suggest that impaired MPC viability contributes to the pathology of dystroglycanopathy. Since our data suggested that frequent cycles of myofiber degeneration/regeneration accelerate substantial and/or functional loss of MPC, we expected that protection from disease-triggering myofiber degeneration provides therapeutic effects even in mouse models with MPC defects; therefore, we restored fukutin expression in myofibers. Adeno-associated virus (AAV)-mediated rescue of fukutin expression that was limited in myofibers successfully ameliorated the severe pathology even after disease progression. In addition, compared with other gene therapy studies, considerably low AAV titers were associated with therapeutic effects. Together, our findings indicated that fukutin-deficient dystroglycanopathy is a regeneration-defective disorder, and gene therapy is a feasible treatment for the wide range of dystroglycanopathy even after disease progression.

Imatinib attenuates severe mouse dystrophy and inhibits proliferation and fibrosis-marker expression in muscle mesenchymal progenitors

Imatinib mesylate inhibits signaling of tyrosine kinase receptors, including PDGFRα, and has been used for human cancer therapy. Recent studies have indicated that imatinib is also effective in treatment of some chronic diseases with fibrosis. Fibrosis is the feature of Duchenne muscular dystrophy. It has been reported that imatinib attenuates fibrosis in mdx mice. Recently we revealed that PDGFRα is specifically expressed in muscle mesenchymal progenitors, which are the origin of muscle fibrosis. Here, we show that imatinib ameliorates the muscular pathology of DBA/2-mdx, a more severe mouse muscular dystrophy. In addition, imatinib inhibits both the proliferation and fibrosis marker expression induced by PDGF-AA in muscle mesenchymal progenitors in vitro. Importantly, the effective dose of imatinib on muscle mesenchymal progenitors did not inhibit myoblast proliferation. These results suggest that imatinib targets mesenchymal progenitors, and that a therapeutic strategy targeting mesenchymal progenitors could be a potential treatment for muscular dystrophies.

Activation of calcium signaling through Trpv1 by nNOS and peroxynitrite as a key trigger of skeletal muscle hypertrophy

Skeletal muscle atrophy occurs in aging and pathological conditions, including cancer, diabetes and AIDS. Treatment of atrophy is based on either preventing protein-degradation pathways, which are activated during atrophy, or activating protein-synthesis pathways, which induce muscle hypertrophy. Here we show that neuronal nitric oxide synthase (nNOS) regulates load-induced hypertrophy by activating transient receptor potential cation channel, subfamily V, member 1 (TRPV1). The overload-induced hypertrophy was prevented in nNOS-null mice. nNOS was transiently activated within 3 min after overload. This activation promoted formation of peroxynitrite, a reaction product of nitric oxide with superoxide, which was derived from NADPH oxidase 4 (Nox4). Nitric oxide and peroxynitrite then activated Trpv1, resulting in an increase of intracellular Ca(2+) concentration ([Ca(2+)](i)) that subsequently triggered activation of mammalian target of rapamycin (mTOR). Notably, administration of the TRPV1 agonist capsaicin induced hypertrophy without overload and alleviated unloading- or denervation-induced atrophy. These findings identify nitric oxide, peroxynitrite and [Ca(2+)](i) as the crucial mediators that convert a mechanical load into an intracellular signaling pathway and lead us to suggest that TRPV1 could be a new therapeutic target for treating muscle atrophy.

Laminin regulates postnatal oligodendrocyte production by promoting oligodendrocyte progenitor survival in the subventricular zone

The laminin family of extracellular matrix proteins are expressed broadly during embryonic brain development, but are enriched at ventricular and pial surfaces where laminins mediate radial glial attachment during corticogenesis. In the adult brain, however, laminin distribution is restricted, yet is found within the vascular basal lamina and associated fractones of the ventricular zone (VZ)-subventricular zone (SVZ) stem cell niche, where laminins regulate adult neural progenitor cell proliferation. It remains unknown, however, if laminins regulate the wave of oligodendrogenesis that occurs in the neonatal/early postnatal VZ-SVZ. Here we report that Lama2, the gene that encodes the laminin α2-subunit, regulates postnatal oligodendrogenesis. At birth, Lama2-/- mice had significantly higher levels of dying oligodendrocyte progenitor cells (OPCs) in the OPC germinal zone of the dorsal SVZ. This translated into fewer OPCs, both in the dorsal SVZ well as in an adjacent developing white matter tract, the corpus callosum. In addition, intermediate progenitor cells that give rise to OPCs in the Lama2-/- VZ-SVZ were mislocalized and proliferated nearer to the ventricle surface. Later, delays in oligodendrocyte maturation (with accompanying OPC accumulation), were observed in the Lama2-/- corpus callosum, leading to dysmyelination by postnatal day 21. Together these data suggest that prosurvival laminin interactions in the developing postnatal VZ-SVZ germinal zone regulate the ability, or timing, of oligodendrocyte production to occur appropriately.

Slow-dividing satellite cells retain long-term self-renewal ability in adult muscle

Satellite cells are muscle stem cells that have important roles in postnatal muscle growth and adult muscle regeneration. Although fast- and slow-dividing populations in activated satellite cells have been observed, the functional differences between them remain unclear. Here we elucidated the relationship between proliferation behaviour and satellite cell function. To assess the frequency of cell division, satellite cells isolated from mouse EDL muscle were labelled with the fluorescent dye PKH26, stimulated to proliferate and then sorted by FACS. The vast majority of activated satellite cells were PKH26low fast-dividing cells, whereas PKH26high slow-dividing cells were observed as a minority population. The fast-dividing cells generated a higher number of differentiated and self-renewed cells compared with the slow-dividing cells. However, cells derived from the slow-dividing population formed secondary myogenic colonies when passaged, whereas those from the fast-dividing population rapidly underwent myogenic differentiation without producing self-renewing cells after a few rounds of cell division. Furthermore, slow-dividing cells transplanted into injured muscle extensively contributed to muscle regeneration in vivo. Id1, a HLH protein, was expressed by all activated satellite cells, but the expression level varied within the slow-dividing cell population. We show that the slow-dividing cells retaining long-term self-renewal ability are restricted to an undifferentiated population that express high levels of Id1 protein (PKH26highId1high population). Finally, genome-wide gene expression analysis described the molecular characteristics of the PKH26highId1high population. Taken together, our results indicate that undifferentiated slow-dividing satellite cells retain stemness for generating progeny capable of long-term self-renewal, and so might be essential for muscle homeostasis throughout life.

Hesr1 and Hesr3 are essential to generate undifferentiated quiescent satellite cells and to maintain satellite cell numbers

Satellite cells, which are skeletal muscle stem cells, divide to provide new myonuclei to growing muscle fibers during postnatal development, and then are maintained in an undifferentiated quiescent state in adult skeletal muscle. This state is considered to be essential for the maintenance of satellite cells, but their molecular regulation is unknown. We show that Hesr1 (Hey1) and Hesr3 (Heyl) (which are known Notch target genes) are expressed simultaneously in skeletal muscle only in satellite cells. In Hesr1 and Hesr3 single-knockout mice, no obvious abnormalities of satellite cells or muscle regenerative potentials are observed. However, the generation of undifferentiated quiescent satellite cells is impaired during postnatal development in Hesr1/3 double-knockout mice. As a result, myogenic (MyoD and myogenin) and proliferative (Ki67) proteins are expressed in adult satellite cells. Consistent with the in vivo results, Hesr1/3-null myoblasts generate very few Pax7(+) MyoD(-) undifferentiated cells in vitro. Furthermore, the satellite cell number gradually decreases in Hesr1/3 double-knockout mice even after it has stabilized in control mice, and an age-dependent regeneration defect is observed. In vivo results suggest that premature differentiation, but not cell death, is the reason for the reduced number of satellite cells in Hesr1/3 double-knockout mice. These results indicate that Hesr1 and Hesr3 are essential for the generation of adult satellite cells and for the maintenance of skeletal muscle homeostasis.

Fibrosis and adipogenesis originate from a common mesenchymal progenitor in skeletal muscle

Accumulation of adipocytes and collagen type-I-producing cells (fibrosis) is observed in muscular dystrophies. The origin of these cells had been largely unknown, but recently we identified mesenchymal progenitors positive for platelet-derived growth factor receptor alpha (PDGFRα) as the origin of adipocytes in skeletal muscle. However, the origin of muscle fibrosis remains largely unknown. In this study, clonal analyses show that PDGFRα(+) cells also differentiate into collagen type-I-producing cells. In fact, PDGFRα(+) cells accumulated in fibrotic areas of the diaphragm in the mdx mouse, a model of Duchenne muscular dystrophy. Furthermore, mRNA of fibrosis markers was expressed exclusively in the PDGFRα(+) cell fraction in the mdx diaphragm. Importantly, TGF-β isoforms, known as potent profibrotic cytokines, induced expression of markers of fibrosis in PDGFRα(+) cells but not in myogenic cells. Transplantation studies revealed that fibrogenic PDGFRα(+) cells mainly derived from pre-existing PDGFRα(+) cells and that the contribution of PDGFRα(-) cells and circulating cells was limited. These results indicate that mesenchymal progenitors are the main origin of not only fat accumulation but also fibrosis in skeletal muscle.

Establishment of bipotent progenitor cell clone from rat skeletal muscle

The present study describes the isolation, cloning and characterization of adipogenic progenitor cells from rat skeletal muscle. Among the obtained 10 clones, the most highly adipogenic progenitor, 2G11 cells, were further characterized. In addition to their adipogenicity, 2G11 cells retain myogenic potential as revealed by formation of multinucleated myotubes when co-cultured with myoblasts. 2G11 cells were resistant to an inhibitory effect of basic fibroblast growth factor on adipogenesis, while adipogenesis of widely used preadipogenic cell line, 3T3-L1 cells, was suppressed almost completely by the same treatment. In vivo transplantation experiments revealed that 2G11 cells are able to possess both adipogenicity and myogenicity in vivo. These results indicate the presence of bipotent progenitor cells in rat skeletal muscle, and suggest that such cells may contribute to ectopic fat formation in skeletal muscle.

Reactive gliosis of astrocytes and Müller glial cells in retina of POMGnT1-deficient mice

Protein O-linked mannose beta1, 2-N-acetylglucosaminyltransferase 1 (POMGnT1) is an enzyme that catalyzes the transfer of N-acetylglucosamine to O-mannose of glycoproteins. Alpha-dystroglycan, a substrate of POMGnT1, is concentrated around the blood vessels, in the outer plexiform layer (OPL), and in the inner limiting membrane (ILM) of the retina. Mutations of the POMGnT1 gene in humans cause muscle-eye-brain (MEB) disease. Several ocular abnormalities including retinal dysplasia, ERG abnormalities, and retinal detachments have been reported in patients with MEB. We have analyzed the eyes of POMGnT1-deficient mice, generated by standard gene targeting technique, to study the retinal abnormalities. Clinical examination of adult mutant mice revealed a high incidence (81% by 12-months-of-age) of retinal detachments. Sheathing of the retinal vessels and the presence of ectopic fibrous tissues around the optic nerve head were also found. Histological examinations showed focal retinal detachment associated with GFAP immunopositivity. The ILM of the mutant mice was disrupted with ectopic cells near the disruptions. The expression of Dp71, a shorter isoform of dystrophin, was severely reduced in the ILM and around retinal blood vessels of POMGnT1-deficient mice. The expression of Dp427, Dp260, Dp140 were also reduced in the OPL of the mutant mice. Electroretinographic (ERG) analyses showed reduced a- and b-wave amplitudes. Examinations of flat mounts revealed abnormal vascular network associated with highly irregular astrocytic processes. In addition, ER-TR7-positive fibrous tissue was found closely associated with reactive astrocytes especially around the optic nerve head. Our results suggest that altered glycosylation of alpha-DG may be responsible for the reactive gliosis and reticular fibrosis in the retina, and the subsequent developments of retinal dysplasia, abnormal ERGs, and retinal detachment in the mutant mice.

Post-translational maturation of dystroglycan is necessary for pikachurin binding and ribbon synaptic localization

Pikachurin, the most recently identified ligand of dystroglycan, plays a crucial role in the formation of the photoreceptor ribbon synapse. It is known that glycosylation of dystroglycan is necessary for its ligand binding activity, and hypoglycosylation is associated with a group of muscular dystrophies that often involve eye abnormalities. Because little is known about the interaction between pikachurin and dystroglycan and its impact on molecular pathogenesis, here we characterize the interaction using deletion constructs and mouse models of muscular dystrophies with glycosylation defects (Large(myd) and POMGnT1-deficient mice). Pikachurin-dystroglycan binding is calcium-dependent and relatively less sensitive to inhibition by heparin and high NaCl concentration, as compared with other dystroglycan ligand proteins. Using deletion constructs of the laminin globular domains in the pikachurin C terminus, we show that a certain steric structure formed by the second and the third laminin globular domains is necessary for the pikachurin-dystroglycan interaction. Binding assays using dystroglycan deletion constructs and tissue samples from Large-deficient (Large(myd)) mice show that Large-dependent modification of dystroglycan is necessary for pikachurin binding. In addition, the ability of pikachurin to bind to dystroglycan prepared from POMGnT1-deficient mice is severely reduced, suggesting that modification of the GlcNAc-β1,2-branch on O-mannose is also necessary for the interaction. Immunofluorescence analysis reveals a disruption of pikachurin localization in the photoreceptor ribbon synapse of these model animals. Together, our data demonstrate that post-translational modification on O-mannose, which is mediated by Large and POMGnT1, is essential for pikachurin binding and proper localization, and suggest that their disruption underlies the molecular pathogenesis of eye abnormalities in a group of muscular dystrophies.

Gene therapy for muscle disease

The molecular mechanisms of Duchenne muscular dystrophy (DMD) have been extensively investigated since the discovery of the dystrophin gene in 1986. Nonetheless, there is currently no effective treatment for DMD. Recent reports, however, indicate that adenoassociated viral (AAV) vector-mediated transfer of a functional dystrophin cDNA into the affected muscle is a promising strategy. In addition, antisense-mediated exon skipping technology has been emerging as another promising approach to restore dystrophin expression in DMD muscle. Ongoing clinical trials show restoration of dystrophin in DMD patients without serious side effects. Here, we summarize the recent progress in gene therapy, with an emphasis on exon skipping for DMD.

Genetic background affects properties of satellite cells and mdx phenotypes

Duchenne muscular dystrophy (DMD) is the most common lethal genetic disorder of children. The mdx (C57BL/10 background, C57BL/10-mdx) mouse is a widely used model of DMD, but the histopathological hallmarks of DMD, such as the smaller number of myofibers, accumulation of fat and fibrosis, and insufficient regeneration of myofibers, are not observed in adult C57BL/10-mdx except for in the diaphragm. In this study, we showed that DBA/2 mice exhibited decreased muscle weight, as well as lower myofiber numbers after repeated degeneration-regeneration cycles. Furthermore, the self-renewal efficiency of satellite cells of DBA/2 is lower than that of C57BL/6. Therefore, we produced a DBA/2-mdx strain by crossing DBA/2 and C57BL/10-mdx. The hind limb muscles of DBA/2-mdx mice exhibited lower muscle weight, fewer myofibers, and increased fat and fibrosis, in comparison with C57BL/10-mdx. Moreover, remarkable muscle weakness was observed in DBA/2-mdx. These results indicate that the DBA/2-mdx mouse is a more suitable model for DMD studies, and the efficient satellite cell self-renewal ability of C57BL/10-mdx might explain the difference in pathologies between humans and mice.

Generation of transplantable, functional satellite-like cells from mouse embryonic stem cells

Satellite cells are myogenic stem cells responsible for the postnatal regeneration of skeletal muscle. Here we report the successful in vitro induction of Pax7-positive satellite-like cells from mouse embryonic stem (mES) cells. Embryoid bodies were generated from mES cells and cultured on Matrigel-coated dishes with Dulbecco's modified Eagle medium containing fetal bovine serum and horse serum. Pax7-positive satellite-like cells were enriched by fluorescence-activated cell sorting using a novel anti-satellite cell antibody, SM/C-2.6. SM/C-2.6-positive cells efficiently differentiate into skeletal muscle fibers both in vitro and in vivo. Furthermore, the cells demonstrate satellite cell characteristics such as extensive self-renewal capacity in subsequent muscle injury model, long-term engraftment up to 24 wk, and the ability to be secondarily transplanted with remarkably high engraftment efficiency compared to myoblast transplantation. This is the first report of transplantable, functional satellite-like cells derived from mES cells and will provide a foundation for new therapies for degenerative muscle disorders.

Reduced proliferative activity of primary POMGnT1-null myoblasts in vitro

Protein O-linked mannose beta1,2-N-acetylglucosaminyltransferase 1 (POMGnT1) is an enzyme that transfers N-acetylglucosamine to O-mannose of glycoproteins. Mutations of the POMGnT1 gene cause muscle-eye-brain (MEB) disease. To obtain a better understanding of the pathogenesis of MEB disease, we mutated the POMGnT1 gene in mice using a targeting technique. The mutant muscle showed aberrant glycosylation of alpha-DG, and alpha-DG from mutant muscle failed to bind laminin in a binding assay. POMGnT1(-/-) muscle showed minimal pathological changes with very low-serum creatine kinase levels, and had normally formed muscle basal lamina, but showed reduced muscle mass, reduced numbers of muscle fibers, and impaired muscle regeneration. Importantly, POMGnT1(-/-) satellite cells proliferated slowly, but efficiently differentiated into multinuclear myotubes in vitro. Transfer of a retrovirus vector-mediated POMGnT1 gene into POMGnT1(-/-) myoblasts completely restored the glycosylation of alpha-DG, but proliferation of the cells was not improved. Our results suggest that proper glycosylation of alpha-DG is important for maintenance of the proliferative activity of satellite cells in vivo.

Residual laminin-binding activity and enhanced dystroglycan glycosylation by LARGE in novel model mice to dystroglycanopathy

Hypoglycosylation and reduced laminin-binding activity of α-dystroglycan are common characteristics of dystroglycanopathy, which is a group of congenital and limb-girdle muscular dystrophies. Fukuyama-type congenital muscular dystrophy (FCMD), caused by a mutation in the fukutin gene, is a severe form of dystroglycanopathy. A retrotransposal insertion in fukutin is seen in almost all cases of FCMD. To better understand the molecular pathogenesis of dystroglycanopathies and to explore therapeutic strategies, we generated knock-in mice carrying the retrotransposal insertion in the mouse fukutin ortholog. Knock-in mice exhibited hypoglycosylated α-dystroglycan; however, no signs of muscular dystrophy were observed. More sensitive methods detected minor levels of intact α-dystroglycan, and solid-phase assays determined laminin binding levels to be ∼50% of normal. In contrast, intact α-dystroglycan is undetectable in the dystrophic Large^myd mouse, and laminin-binding activity is markedly reduced. These data indicate that a small amount of intact α-dystroglycan is sufficient to maintain muscle cell integrity in knock-in mice, suggesting that the treatment of dystroglycanopathies might not require the full recovery of glycosylation. To examine whether glycosylation defects can be restored in vivo, we performed mouse gene transfer experiments. Transfer of fukutin into knock-in mice restored glycosylation of α-dystroglycan. In addition, transfer of LARGE produced laminin-binding forms of α-dystroglycan in both knock-in mice and the POMGnT1 mutant mouse, which is another model of dystroglycanopathy. Overall, these data suggest that even partial restoration of α-dystroglycan glycosylation and laminin-binding activity by replacing or augmenting glycosylation-related genes might effectively deter dystroglycanopathy progression and thus provide therapeutic benefits.

Downstream utrophin enhancer is required for expression of utrophin in skeletal muscle

BACKGROUND

Duchenne muscular dystrophy is caused by the absence of the muscle cytoskeletal protein dystrophin. Utrophin is an autosomal homologue of dystrophin, and overexpression of utrophin is expected to compensate for the dystrophin deficit. We previously reported that the 5.4-kb 5'-flanking region of the utrophin gene containing the A-utrophin core promoter did not drive transgene expression in heart and skeletal muscle. To clarify the regulatory mechanism of utrophin expression, we generated a nuclear localization signal-tagged LacZ transgenic (Tg) mouse, in which the LacZ gene was driven by the 129-bp downstream utrophin enhancer (DUE) and the 5.4-kb 5'-flanking region of the utrophin promoter.

METHODS

Two Tg lines were established. The levels of transgene mRNA expression in several tissues were examined by reverse transcriptase-polymerase chain reaction (RT-PCR) and quantitative RT-PCR. Cryosections of several tissues were stained with haematoxylin and eosin and X-gal.

RESULTS

The transgene expression patterns were consistent with endogenous utrophin in several tissues including heart and skeletal muscle. Transgene expression was also up-regulated more in regenerating muscle than in nonregenerating muscle. Moreover, utrophin expression was augmented in the skeletal muscle of DUE Tg/dystrophin-deficient mdx mice through cross-breeding experiments. We finally established cultures of primary myogenic cells from this Tg mouse and found that utrophin up-regulation during muscle differentiation depends on the DUE motif.

CONCLUSIONS

Our results showed that DUE is indispensable for utrophin expression in skeletal muscle and heart, and primary myogenic cells from this Tg mice provide a high through-put screening system for drugs that up-regulate utrophin expression.

Recombinant adeno-associated virus type 8-mediated extensive therapeutic gene delivery into skeletal muscle of alpha-sarcoglycan-deficient mice

Autosomal recessive limb-girdle muscular dystrophy type 2D (LGMD 2D) is caused by mutations in the alpha-sarcoglycan gene (alpha-SG). The absence of alpha-SG results in the loss of the SG complex at the sarcolemma and compromises the integrity of the sarcolemma. To establish a method for recombinant adeno-associated virus (rAAV)-mediated alpha-SG gene therapy into alpha-SG-deficient muscle, we constructed rAAV serotypes 2 and 8 expressing the human alpha-SG gene under the control of the ubiquitous cytomegalovirus promoter (rAAV2-alpha-SG and rAAV8-alpha-SG). We compared the transduction profiles and evaluated the therapeutic effects of a single intramuscular injection of rAAVs into alpha-SG-deficient (Sgca(-/-)) mice. Four weeks after rAAV2 injection into the tibialis anterior (TA) muscle of 10-day-old Sgca(-/-) mice, transduction of the alpha-SG gene was localized to a limited area of the TA muscle. On the other hand, rAAV8-mediated alpha-SG expression was widely distributed in the hind limb muscle, and persisted for 7 months without inducing cytotoxic and immunological reactions, with a reversal of the muscle pathology and improvement in the contractile force of the Sgca(-/-) muscle. This extensive rAAV8-mediated alpha-SG transduction in LGMD 2D model animals paves the way for future clinical application.

Muscle CD31(-) CD45(-) side population cells promote muscle regeneration by stimulating proliferation and migration of myoblasts

CD31(-) CD45(-) side population (SP) cells are a minor SP subfraction that have mesenchymal stem cell-like properties in uninjured skeletal muscle but that can expand on muscle injury. To clarify the role of these SP cells in muscle regeneration, we injected green fluorescent protein (GFP)-positive myoblasts with or without CD31(-) CD45(-) SP cells into the tibialis anterior muscles of immunodeficient NOD/scid mice or dystrophin-deficient mdx mice. More GFP-positive fibers were formed after co-transplantation than after transplantation of GFP-positive myoblasts alone in both mdx and NOD/scid muscles. Moreover, grafted myoblasts were more widely distributed after co-transplantation than after transplantation of myoblasts alone. Immunohistochemistry with anti-phosphorylated histone H3 antibody revealed that CD31(-) CD45(-) SP cells stimulated cell division of co-grafted myoblasts. Genome-wide gene expression analyses showed that these SP cells specifically express a variety of extracellular matrix proteins, membrane proteins, and cytokines. We also found that they express high levels of matrix metalloproteinase-2 mRNA and gelatinase activity. Furthermore, matrix metalloproteinase-2 derived from CD31(-) CD45(-) SP cells promoted migration of myoblasts in vivo. Our results suggest that CD31(-) CD45(-) SP cells support muscle regeneration by promoting proliferation and migration of myoblasts. Future studies to further define the molecular and cellular mechanisms of muscle regeneration will aid in the development of cell therapies for muscular dystrophy.

Suppression of macrophage functions impairs skeletal muscle regeneration with severe fibrosis

When damaged, skeletal muscle regenerates. In the early phases of regeneration, inflammatory cells such as neutrophils/granulocytes and macrophages infiltrate damaged muscle tissue. To reveal the roles of macrophages during skeletal muscle regeneration, we injected an antibody, AFS98 that blocks the binding of M-CSF to its receptor into normal mice that received muscle damages. Anti-M-CSF receptor administration suppressed macrophage but not neutrophil infiltration. Histological study indicated that suppression of macrophages function leads to the incomplete muscle regeneration. In addition FACS and immunohistochemical study showed that the acute lack of macrophages delayed proliferation and differentiation of muscle satellite cells in vivo. Furthermore, mice injected with the anti-M-CSF receptor antibody exhibited not only adipogenesis, but also significant collagen deposition, i.e., fibrosis and continuous high expression of connective tissue growth factor. Finally we indicate that these fibrosis markers were strongly enriched in CD90(+) cells that do not include myogenic cells. These results indicate that macrophages directly affect satellite cell proliferation and that a macrophage deficiency severely impairs skeletal muscle regeneration and causes fibrosis.

NO production results in suspension-induced muscle atrophy through dislocation of neuronal NOS

Forkhead box O (Foxo) transcription factors induce muscle atrophy by upregulating the muscle-specific E3 ubiquitin ligases MuRF-1 and atrogin-1/MAFbx, but other than Akt, the upstream regulators of Foxos during muscle atrophy are largely unknown. To examine the involvement of the dystrophin glycoprotein complex (DGC) in regulation of Foxo activities and muscle atrophy, we analyzed the expression of DGC members during tail suspension, a model of unloading-induced muscle atrophy. Among several DGC members, only neuronal NOS (nNOS) quickly dislocated from the sarcolemma to the cytoplasm during tail suspension. Electron paramagnetic resonance spectrometry revealed production of NO in atrophying muscle. nNOS-null mice showed much milder muscle atrophy after tail suspension than did wild-type mice. Importantly, nuclear accumulation of dephosphorylated Foxo3a was not evident in nNOS-null muscle, and neither MuRF-1 nor atrogin-1/MAFbx were upregulated during tail suspension. Furthermore, an nNOS-specific inhibitor, 7-nitroindazole, significantly prevented suspension-induced muscle atrophy. The NF-kappaB pathway was activated in both wild-type and nNOS-null muscle during tail suspension. We also show that nNOS was involved in the mechanism of denervation-induced atrophy. We conclude that nNOS/NO mediates muscle atrophy via regulation of Foxo transcription factors and is a new therapeutic target for disuse-induced muscle atrophy.

Autologous transplantation of SM/C-2.6(+) satellite cells transduced with micro-dystrophin CS1 cDNA by lentiviral vector into mdx mice

Duchenne muscular dystrophy (DMD) is a lethal muscle disorder caused by mutations in the dystrophin gene. Transplantation of autologous myogenic cells genetically corrected ex vivo is a possible treatment for this disorder. In order to test the regenerative efficiency of freshly isolated satellite cells, we purified quiescent satellite cells from limb muscles of 8-12-week-old green fluorescent protein-transgenic (GFP-Tg) mice using SM/C-2.6 (a recently developed monoclonal antibody) and flow cytometry. Freshly isolated satellite cells were shown to participate in muscle regeneration more efficiently than satellite cell-derived myoblasts passaged in vitro do, when transplanted into tibialis anterior (TA) muscles of 8-12-week-old cardiotoxin-injected C57BL/6 mice and 5-week-old dystrophin-deficient mdx mice, and analyzed at 4 weeks after injection. Importantly, expansion of freshly isolated satellite cells in vitro without passaging had no detrimental effects on their regenerative capacity. Therefore we directly isolated satellite cells from 5-week-old mdx mice using SM/C-2.6 antibody and cultured them with lentiviral vectors expressing micro-dystrophin CS1. The transduced cells were injected into TA muscles of 5-week-old mdx mice. At 4 weeks after transplantation, the grafted cells efficiently contributed to regeneration of mdx dystrophic muscles and expressed micro-dystrophin at the sarcolemma. These results suggest that there is potential for lentiviral vector-mediated ex vivo gene therapy for DMD.

Molecular signature of quiescent satellite cells in adult skeletal muscle

Skeletal muscle satellite cells play key roles in postnatal muscle growth and regeneration. To study molecular regulation of satellite cells, we directly prepared satellite cells from 8- to 12-week-old C57BL/6 mice and performed genome-wide gene expression analysis. Compared with activated/cycling satellite cells, 507 genes were highly upregulated in quiescent satellite cells. These included negative regulators of cell cycle and myogenic inhibitors. Gene set enrichment analysis revealed that quiescent satellite cells preferentially express the genes involved in cell-cell adhesion, regulation of cell growth, formation of extracellular matrix, copper and iron homeostasis, and lipid transportation. Furthermore, reverse transcription-polymerase chain reaction on differentially expressed genes confirmed that calcitonin receptor (CTR) was exclusively expressed in dormant satellite cells but not in activated satellite cells. In addition, CTR mRNA is hardly detected in nonmyogenic cells. Therefore, we next examined the expression of CTR in vivo. CTR was specifically expressed on quiescent satellite cells, but the expression was not found on activated/proliferating satellite cells during muscle regeneration. CTR-positive cells reappeared at the rim of regenerating myofibers in later stages of muscle regeneration. Calcitonin stimulation delayed the activation of quiescent satellite cells. Our data provide roles of CTR in quiescent satellite cells and a solid scaffold to further dissect molecular regulation of satellite cells. Disclosure of potential conflicts of interest is found at the end of this article.

Injection of a recombinant AAV serotype 2 into canine skeletal muscles evokes strong immune responses against transgene products

Using murine models, we have previously demonstrated that recombinant adeno-associated virus (rAAV)-mediated microdystrophin gene transfer is a promising approach to treatment of Duchenne muscular dystrophy (DMD). To examine further therapeutic effects and the safety issue of rAAV-mediated microdystrophin gene transfer using larger animal models, such as dystrophic dog models, we first investigated transduction efficiency of rAAV in wild-type canine muscle cells, and found that rAAV2 encoding beta-galactosidase effectively transduces canine primary myotubes in vitro. Subsequent rAAV2 transfer into skeletal muscles of normal dogs, however, resulted in low and transient expression of beta-galactosidase together with intense cellular infiltrations in vivo, where cellular and humoral immune responses were remarkably activated. In contrast, rAAV2 expressing no transgene elicited no cellular infiltrations. Co-administration of immunosuppressants, cyclosporine and mycophenolate mofetil could partially improve rAAV2 transduction. Collectively, these results suggest that immune responses against the transgene product caused cellular infiltration and eliminated transduced myofibers in dogs. Furthermore, in vitro interferon-gamma release assay showed that canine splenocytes respond to immunogens or mitogens more susceptibly than murine ones. Our results emphasize the importance to scrutinize the immune responses to AAV vectors in larger animal models before applying rAAV-mediated gene therapy to DMD patients.

Gene therapy for Duchenne muscular dystrophy

Gene therapy has great potential to treat Duchenne muscular dystrophy. Among many proposed strategies to deliver a therapeutic gene to muscle, recombinant adeno-associated virus-mediated gene transfer is the most promising. The recent isolation of new adeno-associated virus serotypes from human and nonhuman primates provides the opportunity to develop vectors that can achieve the long-term expression of a therapeutic gene in muscles of the entire body without detrimental effects. To translate the results from small animal models to clinical trials in humans, further work using larger animal models, such as dystrophic dogs or nonhuman primates, is required. This review also discusses recent progress in other gene transfer-related therapeutic approaches, including targeted exon skipping and gene correction.

Functional heterogeneity of side population cells in skeletal muscle

Skeletal muscle regeneration has been exclusively attributed to myogenic precursors, satellite cells. A stem cell-rich fraction referred to as side population (SP) cells also resides in skeletal muscle, but its roles in muscle regeneration remain unclear. We found that muscle SP cells could be subdivided into three sub-fractions using CD31 and CD45 markers. The majority of SP cells in normal non-regenerating muscle expressed CD31 and had endothelial characteristics. However, CD31(-)CD45(-) SP cells, which are a minor subpopulation in normal muscle, actively proliferated upon muscle injury and expressed not only several regulatory genes for muscle regeneration but also some mesenchymal lineage markers. CD31(-)CD45(-) SP cells showed the greatest myogenic potential among three SP sub-fractions, but indeed revealed mesenchymal potentials in vitro. These SP cells preferentially differentiated into myofibers after intramuscular transplantation in vivo. Our results revealed the heterogeneity of muscle SP cells and suggest that CD31(-)CD45(-) SP cells participate in muscle regeneration.

The utrophin promoter A drives high expression of the transgenic LacZ gene in liver, testis, colon, submandibular gland, and small intestine

BACKGROUND

Duchenne muscular dystrophy (DMD) is caused by the absence of the muscle cytoskeletal protein dystrophin. Utrophin is an autosomal homologue of dystrophin, and overexpression of the protein is expected to compensate for the defect of dystrophin. The utrophin gene has two promoters, A and B, and promoter A of the utrophin gene is a possible target of pharmacological interventions for DMD because A-utrophin is up-regulated in dystrophin-deficient mdx skeletal and cardiac muscles. To investigate the utrophin promoter A activity in vivo, we generated nuclear localization signal-tagged LacZ transgenic mice, where the LacZ gene was driven by the 5-kb flanking region of the A-utrophin gene.

METHODS

Four transgenic lines were established by mating four independent founders with C57BL/6J mice. The levels of mRNA for beta-galactosidase in several tissues were examined by RT-PCR. Cryosections from several tissues were stained with hematoxylin and eosin (H&E) and with 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-Gal).

RESULTS

The 5-kb upstream region of the A-utrophin gene showed high transcriptional activity in liver, testis, colon, submandibular gland, and small intestine, consistent with the endogenous expression of utrophin protein. Surprisingly, the levels of both beta-gal protein and mRNA for the transgene in cardiac and skeletal muscles were extremely low, even in nuclei near the neuromuscular junctions. These results indicate that the regulation of the utrophin gene in striated muscle is different from that in non-muscle tissues.

CONCLUSIONS

Our results clearly showed that the utrophin A promoter is not sufficient to drive expression in muscle, but other regulatory elements are required.