Coordinate control of axon defasciculation and myelination by laminin-2 and -8

Schwann cells form basal laminae (BLs) containing laminin-2 (Ln-2; heterotrimer α2β1γ1) and Ln-8 (α4β1γ1). Loss of Ln-2 in humans and mice carrying α2-chain mutations prevents developing Schwann cells from fully defasciculating axons, resulting in partial amyelination. The principal pathogenic mechanism is thought to derive from structural defects in Schwann cell BLs, which Ln-2 scaffolds. However, we found loss of Ln-8 caused partial amyelination in mice without affecting BL structure or Ln-2 levels. Combined Ln-2/Ln-8 deficiency caused nearly complete amyelination, revealing Ln-2 and -8 together have a dominant role in defasciculation, and that Ln-8 promotes myelination without BLs. Transgenic Ln-10 (α5β1γ1) expression also promoted myelination without BL formation. Rather than BL structure, we found Ln-2 and -8 were specifically required for the increased perinatal Schwann cell proliferation that attends myelination. Purified Ln-2 and -8 directly enhanced in vitro Schwann cell proliferation in collaboration with autocrine factors, suggesting Lns control the onset of myelination by modulating responses to mitogens in vivo.

AAV vector-mediated microdystrophin expression in a relatively small percentage of mdx myofibers improved the mdx phenotype

Duchenne muscular dystrophy (DMD) is a lethal disorder of skeletal muscle caused by mutations in the dystrophin gene. Adeno-associated virus (AAV) vector-mediated gene therapy is a promising approach to the disease. Although a rod-truncated microdystrophin gene has been proven to ameliorate dystrophic phenotypes, the level of microdystrophin expression required for effective gene therapy by an AAV vector has not been determined yet. Here, we constructed a recombinant AAV type 2 vector, AAV2-MCKDeltaCS1, expressing microdystrophin (DeltaCS1) under the control of a muscle-specific MCK promoter and injected it into TA muscles of 10-day-old and 5-week-old mdx mice. AAV2-MCKDeltaCS1-mediated gene transfer into 5-week-old mdx muscle resulted in extensive and long-term expression of microdystrophin and significantly improved force generation. Interestingly, 10-day-old injected muscle expressed microdystrophin in a limited number of myofibers but showed hypertrophy of microdystrophin-positive muscle fibers and considerable recovery of contractile force. Thus, we concluded that AAV2-MCKDeltaCS1 could be a powerful tool for gene therapy of DMD.

Mac-1(low) early myeloid cells in the bone marrow-derived SP fraction migrate into injured skeletal muscle and participate in muscle regeneration

Recent studies have shown that bone marrow (BM) cells, including the BM side population (BM-SP) cells that enrich hematopoietic stem cells (HSCs), are incorporated into skeletal muscle during regeneration, but it is not clear how and what kinds of BM cells contribute to muscle fiber regeneration. We found that a large number of SP cells migrated from BM to muscles following injury in BM-transplanted mice. These BM-derived SP cells in regenerating muscles expressed different surface markers from those of HSCs and could not reconstitute the mouse blood system. BM-derived SP/Mac-1(low) cells increased in number in regenerating muscles following injury. Importantly, our co-culture studies with activated satellite cells revealed that this fraction carried significant potential for myogenic differentiation. By contrast, mature inflammatory (Mac-1(high)) cells showed negligible myogenic activities. Further, these BM-derived SP/Mac-1(low) cells gave rise to mononucleate myocytes, indicating that their myogenesis was not caused by stochastic fusion with host myogenic cells, although they required cell-to-cell contact with myogenic cells for muscle differentiation. Taken together, our data suggest that neither HSCs nor mature inflammatory cells, but Mac-1(low) early myeloid cells in the BM-derived SP fraction, play an important role in regenerating skeletal muscles.

Purification and cell-surface marker characterization of quiescent satellite cells from murine skeletal muscle by a novel monoclonal antibody

A novel monoclonal antibody, SM/C-2.6, specific for mouse muscle satellite cells was established. SM/C-2.6 detects mononucleated cells beneath the basal lamina of skeletal muscle, and the cells co-express M-cadherin. Single fiber analyses revealed that M-cadherin+ mononucleated cells attaching to muscle fibers are stained with SM/C-2.6. SM/C-2.6+ cells, which were freshly purified by FACS from mouse skeletal muscle, became MyoD+ in vitro in proliferating medium, and the cells differentiated into desmin+ and nuclear-MyoD+ myofibers in vitro when placed under differentiation conditions. When the sorted cells were injected into mdx mouse muscles, donor cells differentiated into muscle fibers. Flow cytometric analyses of SM/C-2.6+ cells showed that the quiescent satellite cells were c-kit-, Sca-1-, CD34+, and CD45-. More, SM/C-2.6+ cells were barely included in the side population but in the main population of cells in Hoechst dye efflux assay. These results suggest that SM/C-2.6 identifies and enriches quiescent satellite cells from adult mouse muscle, and that the antibody will be useful as a powerful tool for the characterization of cellular and molecular mechanisms of satellite cell activation and proliferation.

Laminin alpha1 chain reduces muscular dystrophy in laminin alpha2 chain deficient mice

Laminin (LN) alpha2 chain deficiency in humans and mice leads to severe forms of congenital muscular dystrophy (CMD). Here, we investigated whether LNalpha1 chain in mice can compensate for the absence of LNalpha2 chain and prevent the development of muscular dystrophy. We generated mice expressing a LNalpha1 chain transgene in skeletal muscle of LNalpha2 chain deficient mice. LNalpha1 is not normally expressed in muscle, but the transgenically produced LNalpha1 chain was incorporated into muscle basement membranes, and normalized the compensatory changes of expression of certain other laminin chains (alpha4, beta2). In 4-month-old mice, LNalpha1 chain could fully prevent the development of muscular dystrophy in several muscles, and partially in others. The LNalpha1 chain transgene not only reversed the appearance of histopathological features of the disease to a remarkable degree, but also greatly improved health and longevity of the mice. Correction of LNalpha2 chain deficiency by LNalpha1 chain may serve as a paradigm for gene therapy of CMD in patients.

Laminin α2 Is Essential for Odontoblast Differentiation Regulating Dentin Sialoprotein Expression

Laminin alpha2 is subunit of laminin-2 (alpha2beta1gamma1), which is a major component of the muscle basement membrane. Although the laminin alpha2 chain is expressed in the early stage of dental mesenchyme development and localized in the tooth germ basement membrane, its expression pattern in the late stage of tooth germ development and molecular roles are not clearly understood. We analyzed the role of laminin alpha2 in tooth development by using targeted mice with a disrupted lama2 gene. Laminin alpha2 is expressed in dental mesenchymal cells, especially in odontoblasts and during the maturation stage of ameloblasts, but not in the pre-secretory or secretory stages of ameloblasts. Lama2 mutant mice have thin dentin and a widely opened dentinal tube, as compared with wild-type and heterozygote mice, which is similar to the phenotype of dentinogenesis imperfecta. During dentin formation, the expression of dentin sialoprotein, a marker of odontoblast differentiation, was found to be decreased in odontoblasts from mutant mice. Furthermore, in primary cultures of dental mesenchymal cells, dentin matrix protein, and dentin sialophosphoprotein, mRNA expression was increased in laminin-2 coated dishes but not in those coated with other matrices, fibronectin, or type I collagen. Our results suggest that laminin alpha2 is essential for odontoblast differentiation and regulates the expression of dentin matrix proteins.

Expression profiling of cytokines and related genes in regenerating skeletal muscle after cardiotoxin injection: a role for osteopontin

To examine the roles of cytokines in muscle regeneration, we injected cardiotoxin into mouse tibialis anterior muscle and examined the expression profiles of cytokines and related genes in the regeneration process. Expression of 40, 64, and 7 genes among 522 genes spotted on a cytokine expression array were increased more than fivefold at 48 hours, 96 hours, and 7 days after toxin injection, respectively, when compared with those of the control muscle. Especially the levels of mRNA for chemokines and chemokine receptors, many of which are potent regulators of macrophages, were highly elevated 48 hours after injury. The expression of osteopontin (OPN), a versatile regulator of inflammation and tissue repair, was up-regulated more than 118-fold in regenerating muscle at 48 hours after injury. Northern blotting confirmed that the expression of OPN was highest at 48 hours after cardiotoxin injection and declined sharply thereafter. Immunohistochemistry showed that OPN was detected both in the cytoplasm of macrophages and in necrotic muscle infiltrated with macrophages. Our studies suggest OPN may serve as an adhesion molecule that promotes macrophage binding to necrotic fibers and may be an important mediator in the early phase of muscle regeneration.

Laminin alpha2 deficiency and muscular dystrophy; genotype-phenotype correlation in mutant mice

Deficiency of laminin alpha2 is the cause of one of the most severe muscular dystrophies in humans and other species. It is not yet clear how particular mutations in the laminin alpha2 chain gene affect protein expression, and how abnormal levels or structure of the protein affect disease. Animal models may be valuable for such genotype-phenotype analysis and for determining mechanism of disease as well as function of laminin. Here, we have analyzed protein expression in three lines of mice with mutations in the laminin alpha2 chain gene and in two lines of transgenic mice overexpressing the human laminin alpha2 chain gene in skeletal muscle. The dy(3K)/dy(3K) experimental mutant mice are completely deficient in laminin alpha2; the dy/dy spontaneous mutant mice have small amounts of apparently normal laminin; and the dy(W)/dy(W) mice express even smaller amounts of a truncated laminin alpha2, lacking domain VI. Interestingly, all mutants lack laminin alpha2 in peripheral nerve. We have demonstrated previously, that overexpression of the human laminin alpha2 in skeletal muscle in dy(2J)/dy(2J) and dy(W)/dy(W) mice under the control of a striated muscle-specific creatine kinase promoter substantially prevented the muscular dystrophy in these mice. However, dy(W)/dy(W) mice, expressing the human laminin alpha2 under the control of the striated muscle-specific portion of the desmin promoter, still developed muscular dystrophy. This failure to rescue is apparently because of insufficient production of laminin alpha2. This study provides additional evidence that the amount of laminin alpha2 is most critical for the prevention of muscular dystrophy. These data may thus be of significance for attempts to treat congenital muscular dystrophy in human patients.

Adeno-associated virus vector-mediated gene transfer into dystrophin-deficient skeletal muscles evokes enhanced immune response against the transgene product

Duchenne muscular dystrophy (DMD) is an X-linked, lethal muscular disorder caused by a defect in the DMD gene. AAV vector-mediated micro-dystrophin cDNA transfer is an attractive approach to treatment of DMD. To establish effective gene transfer into skeletal muscle, we examined the transduction efficiency of an AAV vector in skeletal muscles of dystrophin-deficient mdx mice. When an AAV vector encoding the LacZ gene driven by a CMV promoter (AAV-CMVLacZ) was introduced, beta-galactosidase expression markedly decreased in mdx muscle 4 weeks after injection due to immune responses against the transgene product. We also injected AAV-CMVLacZ into skeletal muscles of mini-dystrophin-transgenic mdx mice (CVBA3'), which show ameliorated phenotypes without overt signs of muscle degeneration. AAV vector administration, however, evoked substantial immune responses in CVBA3' muscle. Importantly, AAV vector using muscle-specific MCK promoter also elicited responses in mdx muscle, but at a considerably later period. These results suggested that neo-antigens introduced by AAV vectors could evoke immune reactions in mdx muscle, since increased permeability allowed a leakage of neo-antigens from the dystrophin-deficient sarcolemma of muscle fibers. However, resident antigen-presenting cells, such as myoblasts, myotubes and regenerating immature myofibers, might also play a role in the immune response.

Alpha1-syntrophin-deficient skeletal muscle exhibits hypertrophy and aberrant formation of neuromuscular junctions during regeneration

Alpha1-syntrophin is a member of the family of dystrophin-associated proteins; it has been shown to recruit neuronal nitric oxide synthase and the water channel aquaporin-4 to the sarcolemma by its PSD-95/SAP-90, Discs-large, ZO-1 homologous domain. To examine the role of alpha1-syntrophin in muscle regeneration, we injected cardiotoxin into the tibialis anterior muscles of alpha1-syntrophin-null (alpha1syn-/-) mice. After the treatment, alpha1syn-/- muscles displayed remarkable hypertrophy and extensive fiber splitting compared with wild-type regenerating muscles, although the untreated muscles of the mutant mice showed no gross histological change. In the hypertrophied muscles of the mutant mice, the level of insulin-like growth factor-1 transcripts was highly elevated. Interestingly, in an early stage of the regeneration process, alpha1syn-/- mice showed remarkably deranged neuromuscular junctions (NMJs), accompanied by impaired ability to exercise. The contractile forces were reduced in alpha1syn-/- regenerating muscles. Our results suggest that the lack of alpha1-syntrophin might be responsible in part for the muscle hypertrophy, abnormal synapse formation at NMJs, and reduced force generation during regeneration of dystrophin-deficient muscle, all of which are typically observed in the early stages of Duchenne muscular dystrophy patients.

Micro-dystrophin cDNA ameliorates dystrophic phenotypes when introduced into mdx mice as a transgene

The adeno-associated virus vector is a good tool for gene transfer into skeletal muscle, but the length of a gene that can be incorporated is limited. To develop a gene therapy for Duchenne muscular dystrophy, we generated a series of rod-truncated micro-dystrophin cDNAs: M3 (one rod repeat, 3.9 kb), AX11 (three rod repeats, 4.4 kb), and CS1 (four rod repeats, 4.9 kb). These micro-dystrophins, driven by a CAG promoter, were used to produce transgenic (Tg) mdx mice and all three micro-dystrophins were shown to localize at the sarcolemma together with the expression of dystrophin-associated proteins. Among them, CS1 greatly improved dystrophic phenotypes of mdx mice and contractile force of the diaphragm in particular was restored to the level of normal C57BL/10 mice. AX11 modestly ameliorated the dystrophic pathology, but, importantly, M3-Tg mdx mice still showed severe dystrophic phenotypes. These data suggest that the rod structure, and its length in particular, is crucial for the function of micro-dystrophin.

Muscle regeneration by reconstitution with bone marrow or fetal liver cells from green fluorescent protein-gene transgenic mice

The myogenic potential of bone marrow and fetal liver cells was examined using donor cells from green fluorescent protein (GFP)-gene transgenic mice transferred into chimeric mice. Lethally irradiated X-chromosome-linked muscular dystrophy (mdx) mice receiving bone marrow cells from the transgenic mice exhibited significant numbers of fluorescence+ and dystrophin+ muscle fibres. In order to compare the generating capacity of fetal liver cells with bone marrow cells in neonatal chimeras,these two cell types from the transgenic mice were injected into busulfantreated normal or mdx neonatal mice, and muscular generation in the chimeras was examined. Cardiotoxin-induced (or -uninduced, for mdx recipients) muscle regeneration in chimeras also produced fluorescence+ muscle fibres. The muscle reconstitution efficiency of the bone marrow cells was almost equal to that of fetal liver cells. However, the myogenic cell frequency was higher in fetal livers than in bone marrow. Among the neonatal chimeras of normal recipients, several fibres expressed the fluorescence in the cardiotoxin-untreated muscle. Moreover,fluorescence+ mononuclear cells were observed beneath the basal lamina of the cardiotoxin-untreated muscle of chimeras, a position where satellite cells are localizing. It was also found that mononuclear fluorescence+ and desmin+ cells were observed in the explantation cultures of untreated muscles of neonatal chimeras. The fluorescence+ muscle fibres were generated in the second recipient mice receiving muscle single cells from the cardiotoxin-untreated neonatal chimeras. The results suggest that both bone marrow and fetal liver cells may have the potential to differentiate into muscle satellite cells and participate in muscle regeneration after muscle damage as well as in physiological muscle generation.

Interleukin 6 induces overexpression of the sarcolemmal utrophin in neonatal mdx skeletal muscle

Duchenne muscular dystrophy (DMD) is an X-linked lethal disorder caused by a defect in the DMD gene, which encodes the cytoskeletal protein dystrophin. Utrophin is an autosomal homolog of the DMD gene product dystrophin, and augmented expression of endogenous utrophin is expected to provide an alternative therapeutic approach to DMD. We previously reported that an immune response against a beta-galactosidase-expressing adenovirus vector, AxCALacZ, resulted in an accumulation of endogenous utrophin on the extrasynaptic sarcolemma in dystrophin-deficient mdx mice. To determine which cytokine is involved in the regulation of utrophin expression, we directly injected several cytokines separately into neonatal mdx muscles and tested whether the expression of utrophin is increased on the sarcolemma. Importantly, among the cytokines tested, solely interleukin 6 (IL-6) successfully increased expression of utrophin. Moreover, the increase in utrophin mRNA was detected in recombinant IL-6-injected mdx muscles by quantitative real-time reverse transcriptase-polymerase chain reaction. Further, IL-6 expression was elevated in AxCALacZ-infected mdx muscle at an early stage, and anti-IL-6 receptor (IL-6R) antibody treatment blocked enhanced utrophin expression in AxCALacZ-infected mdx muscle. We should point out, however, that overexpression of utrophin due to recombinant IL-6 treatment lasted only 1 week. In addition, expression of utrophin was not evident in normal C57BL/10 neonatal muscles injected with IL-6. Taken together, these results suggest that IL-6 can induce overexpression of utrophin on the extrasynaptic sarcolemma but requires preexisting factors in neonatal mdx muscle to fully regulate utrophin expression.

Gene therapy for muscular dystrophies: current status and future prospects

Since the identification in 1987 of the gene for Duchenne muscular dystrophy (DMD), research on the molecular pathogenesis of muscular dystrophy has progressed extensively. In particular, discovery of the DMD gene product, dystrophin, led to the identification of dystrophin-associated proteins and, subsequently, the recognition of other types of muscular dystrophy caused by the defects in each of the sarcoglycan genes. On the other hand, effective therapy for DMD has not yet been established. Some of the viral vectors, such as adeno-associated virus vectors or lentiviral vector, have been proven to enable the long-term expression of the exogenous gene without overt host immune reactions. However, dystrophin cDNAs are too large (14kb) to be accommodated in these viral vectors. To solve this problem, we and other research groups succeeded in truncating full-length dystrophin cDNA to small dystrophin cDNA (4 to 5kb), the products of which protect dystrophin-deficient mdx muscle from contraction-induced membrane damage when introduced by viral vectors or as a transgene into mdx mice. The usefulness of these truncated dystrophin cDNAs should be confirmed using other animal models such as dystrophic dogs. To develop successful treatment of DMD, the authors believe that several different approaches should be used, such as cell transfer therapy, drug design to up-regulate utrophin, or a strategy to repair the mutation in vivo.

Association of neuronal nitric oxide synthase (nNOS) with alpha1-syntrophin at the sarcolemma

alpha1-syntrophin is a PDZ-containing dystrophin-associated protein, expressed predominantly in striated muscle and brain. alpha1-syntrophin null mice generated by gene targeting technique showed no overt muscular dystrophic phenotype. Though other dystrophin-associated proteins were localized at the sarcolemma, neuronal nitric oxide synthase (nNOS) was selectively lost from the membrane fraction but remained in the cytoplasm. Thus, the alpha1-syntrophin null mice are useful in the elucidation of the functional importance of nNOS targeting at the sarcolemma. In addition, the mice would facilitate identification of other signaling molecules, which are targeted to dystrophin complex via interaction with alpha1-syntrophin.

Schwann cell myelination occurred without basal lamina formation in laminin alpha2 chain-null mutant (dy3K/dy3K) mice

The laminin alpha2 chain is a major component of basal lamina in both skeletal muscle and the peripheral nervous system. Laminin alpha2 chain deficiency causes merosin-deficient congenital muscular dystrophy, which affects not only skeletal muscles, but also the peripheral and central nervous systems. It has been reported that the formation of basal lamina is required for myelination in the peripheral nervous system. In fact, the spinal root of dystrophic mice (dy/dy mice), whose laminin alpha2 chain expression is greatly reduced, shows lack of basal lamina and clusters of naked axons. To investigate the role of laminin alpha2 chain and basal lamina in vivo, we examined the peripheral nervous system of dy3K/dy3K mice, which are null mutants of laminin alpha2 chain. The results indicate the presence of myelination although Schwann cells lacked basal lamina in the spinal roots of dy3K/dy3K mice, suggesting that basal lamina is not an absolute requirement for myelination in vivo. Immunohistochemically, the expression of laminin alpha4 chain was increased and laminin alpha5 chain was preserved in the endoneurium of the spinal root. Laminin alpha4 and alpha5 chains may play the critical role in myelination instead of laminin alpha2 chain in dy3K/dy3K mice. In addition, the motor conduction velocity of the sciatic nerve was significantly reduced compared with that of wild-type littermate. This reduction in conduction velocity may be due to small axon diameter, thin myelin sheath and the patchy disruption of the basal lamina of the nodes of Ranvier in dy3K/dy3K mice.

Merosin and congenital muscular dystrophy

Merosin (also called as Laminin-2) is an isoform of laminin comprised of the alpha2, beta1 and gamma1 chains. In European populations, half of the patients with classical congenital muscular dystrophy have mutations of the LAMA2 gene (6q22-23) and present reduced or absence of laminin alpha2 chain. This form is generally referred to as merosin-deficient CMD. Merosin-deficient CMD is characterized by involvement of not only skeletal muscle but also central and peripheral nervous systems: Extensive brain white matter abnormalities are found by magnetic resonance imaging (MRI). However, most patients show no mental retardation. Recent case studies reported that some patients have several structural abnormalities such as abnormal cerebral cortical gyration, hypoplasia of cerebellum and pons, and dilation of ventricles. At present, functions of merosin related to muscle degeneration have not been fully elucidated. In addition, the mechanisms responsible for pathogenesis of diffuse brain white matter abnormalities remain to be determined. As mouse models for merosin-deficient CMD, three spontaneous mutants(dy, dy(2J), dy(PAS1)) and two mutants named dy(W) and dy(3K) by targeted gene disruption have been reported. These mice will help to elucidate the pathogenesis of merosin-deficient CMD and serve to develop therapy.