Macrophage function in the elderly and impact on injury repair and cancer

Older age is associated with deteriorating health, including escalating risk of diseases such as cancer, and a diminished ability to repair following injury. This rise in age-related diseases/co-morbidities is associated with changes to immune function, including in myeloid cells, and is related to immunosenescence. Immunosenescence reflects age-related changes associated with immune dysfunction and is accompanied by low-grade chronic inflammation or inflammageing. This is characterised by increased levels of circulating pro-inflammatory cytokines such as tumor necrosis factor (TNF), interleukin (IL)-1β and IL-6. However, in healthy ageing, there is a concomitant age-related escalation in anti-inflammatory cytokines such as transforming growth factor-β1 (TGF-β1) and IL-10, which may overcompensate to regulate the pro-inflammatory state. Key inflammatory cells, macrophages, play a role in cancer development and injury repair in young hosts, and we propose that their role in ageing in these scenarios may be more profound. Imbalanced pro- and anti-inflammatory factors during ageing may also have a significant influence on macrophage function and further impact the severity of age-related diseases in which macrophages are known to play a key role. In this brief review we summarise studies describing changes to inflammatory function of macrophages (from various tissues and across sexes) during healthy ageing. We also describe age-related diseases/co-morbidities where macrophages are known to play a key role, focussed on injury repair processes and cancer, plus comment briefly on strategies to correct for these age-related changes.

A Potential Alternative Strategy for Myoblast Transfer Therapy: The use of Sliced Muscle Grafts

Excellent long-term survival (up to 1 yr) of donor skeletal muscle cells was demonstrated using a mouse Y-chromosome specific probe, following the transplantation of grafts of whole muscles from male “normal” C57B1/10Sn mice into dystrophic muscles of female host mice. After the transplantation of equivalent sliced muscle grafts there was extensive movement of the male donor cells and fusion with host myofibres. This contrasts with the extremely poor survival of isolated myoblasts after injection into the same mouse model for Duchenne muscular dystrophy. The use of sliced muscle grafts may therefore represent a potential alternative approach to myoblast transfer therapy.

Effects of loaded voluntary wheel exercise on performance and muscle hypertrophy in young and old male C57Bl/6J mice

This study compared the capacity of young and old male C57Bl/6J mice to exercise with increasing resistance over 10 weeks, and its impact on muscle mass. Young mice (aged 15-25 weeks) were subjected to low (LR) and high (HR) resistance exercise, whereas only LR was used for old mice (107-117 weeks). Weekly patterns of voluntary wheel activity, food consumption and body weights were measured. Running patterns changed over time and with age, with two peaks of activity detected for young, but only one for old mice: speed and distance run was also less for old mice. The mass for six limb muscles was measured at the end of the experiment. The most pronounced increase in mass in response to exercise was for the soleus in young and old mice, and also quadriceps and gastrocnemius in young mice. Soleus and quadriceps muscles were analyzed histologically for myofiber number and size. A striking feature was the many small myofibers in response to exercise in young (but not old) soleus, whereas these were not present after exercise in young or old quadriceps. Overall, there was a striking difference in response to exercise between muscles and this was influenced by age.

Impact of fasting on the rhythmic expression of myogenic and metabolic factors in skeletal muscle of adult mice

Circadian rhythms and metabolism are tightly integrated, and rhythmic expression of metabolic factors is common in homeostatic processes. We measured the temporal changes in the expression of myogenic regulatory factors and expression and activity level of molecules involved in protein metabolism in skeletal muscles and livers in mice and examined the impact of fasting. Tissues were collected over 24 h (at zeitgeber times ZT1, ZT5, ZT9, ZT13, ZT17, ZT21, and ZT1 the following day) from adult male C57Bl/6J mice that had been either freely fed or fasted for 24 h. In skeletal muscle, there was a robust rise in the mRNA expression of the myogenic regulatory factors MyoD and myogenin during dark hours which was strongly suppressed by fasting. Circadian pattern was observed for mRNA of MuRF1, Akt1, and ribosomal protein S6 in muscles in fed and fasted mice and for Fbxo32 in fed mice. Activity (phosphorylation) levels of Akt(Ser473) displayed temporal regulation in fasted (but not fed) mice and were high at ZT9. Fasting caused significant reductions in phosphorylation for both Akt and S6 in muscles, indicative of inactivation. Hepatic phosphorylated Akt(Ser473) and S6(Ser235/236) proteins did not exhibit daily rhythms. Fasting significantly reduced hepatic Akt(473) phosphorylation compared with fed levels, although (unlike in muscle) it did not affect S6(Ser235/236) phosphorylation. This in vivo circadian study addresses for the first time the signaling activities of key molecules related to protein turnover and their possible cross-regulation of expression of genes related to protein degradation.

Blockade of TNF in vivo using cV1q antibody reduces contractile dysfunction of skeletal muscle in response to eccentric exercise in dystrophic mdx and normal mice

This study evaluated the contribution of the pro-inflammatory cytokine, tumour necrosis factor (TNF) to the severity of exercise-induced muscle damage and subsequent myofibre necrosis in mdx mice. Adult mdx and non-dystrophic C57 mice were treated with the mouse-specific TNF antibody cV1q before undergoing a damaging eccentric contraction protocol performed in vivo on a custom built mouse dynamometer. Muscle damage was quantified by (i) contractile dysfunction (initial torque deficit) immediately after the protocol, (ii) subsequent myofibre necrosis 48 h later. Blockade of TNF using cV1q significantly reduced contractile dysfunction in mdx and C57 mice compared with mice injected with the negative control antibody (cVaM) and un-treated mice. Furthermore, cV1q treatment significantly reduced myofibre necrosis in mdx mice. This in vivo evidence that cV1q reduces the TNF-mediated adverse response to exercise-induced muscle damage supports the use of targeted anti-TNF treatments to reduce the severity of the functional deficit and dystropathology in DMD.

Duchenne muscular dystrophy: focus on pharmaceutical and nutritional interventions

Duchenne muscular dystrophy is a lethal X-linked muscle disease resulting from a defect in the muscle membrane protein dystrophin. The absence of dystrophin leads to muscle membrane fragility, muscle death (necrosis) and eventual replacement of skeletal muscle by fat and fibrous connective tissue. Extensive muscle wasting and respiratory failure results in premature death often by the early 20s. This short review evaluates drug and nutritional interventions designed to reduce the severity of muscular dystrophy, while awaiting the outcome of research into therapies to correct the fundamental gene defect. Combinations of dietary supplementation with amino-acids such as creatine, specific anti-inflammatory drugs and perhaps drugs that target ion channels might have immediate realistic clinical benefits although rigorous research is required to determine optimal combinations of such interventions.

Rskalpha-actin/hIGF-1 transgenic mice with increased IGF-I in skeletal muscle and blood: impact on regeneration, denervation and muscular dystrophy

Human IGF-I was over-expressed in skeletal muscles of C57/BL6xCBA mice under the control of the rat skeletal alpha-actin gene promoter. RT-PCR verified expression of the transgene in skeletal muscle but not in the liver of 1- and 21-day old heterozygote transgenic mice. The concentration of endogenous mouse IGF-I, measured by an immunoassay which does not detect human IGF-I, was not significantly different between transgenic mice and wild-type littermates (9.5 +/- 0.8 and 13.3 +/- 1.9 ng/g in muscle; 158.3 +/- 18.6 and 132.9 +/- 33.1 ng/ml in plasma, respectively). In contrast, quantitation with antibodies to human IGF-I showed an increase in IGF-I of about 100 ng/ml in plasma and 150 ng/g in muscle of transgenic mice at 6 months of age. Transgenic males, compared to their age matched wild-type littermates, had a significantly higher body weight (38.6 +/- 0.53 g vs. 35.8 +/- 0.64 g at 6 months of age; P < 0.001), dry fat-free carcass mass (5.51 +/- 0.085 vs. 5.08 +/- 0.092 g; P < 0.001) and myofibrillar protein mass (1.62 +/- 0.045 vs. 1.49 +/- 0.048 g; P < 0.05), although the fractional content of fat in the carcass was lower (167 +/- 7.0 vs. 197 +/- 7.7 g/kg wet weight) in transgenic animals. There was no evidence of muscle hypertrophy and no change in the proportion of slow type I myofibres in the limb muscles of Rskalpha-actin/hIGF-I transgenic mice at 3 or 6 months of age. Phenotypic changes in Rskalpha-actin/hIGF-I mice are likely to be due to systemic as well as autocrine/paracrine effects of overproduction of IGF-I due to expression of the human IGF-I transgene. The effect of muscle specific over-expression of Rskalpha-actin/hIGF-I transgene was tested on: (i) muscle regeneration in auto-transplanted whole muscle grafts; (ii) myofibre atrophy following sciatic nerve transection; and (iii) sarolemmal damage and myofibre necrosis in dystrophic mdx muscle. No beneficial effect of muscle specific over-expression of Rskalpha-actin/hIGF-I transgene was seen in these three experimental models.

Strength at the extracellular matrix-muscle interface

Mechanical force is generated within skeletal muscle cells by contraction of specialized myofibrillar proteins. This paper explores how the contractile force generated at the sarcomeres within an individual muscle fiber is transferred through the connective tissue to move the bones. The initial key point for transfer of the contractile force is the muscle cell membrane (sarcolemma) where force is transferred laterally to the basement membrane (specialized extracellular matrix rich in laminins) to be integrated within the connective tissue (rich in collagens) before transmission to the tendons. Connections between (1) key molecules outside the myofiber in the basement membrane to (2) molecules within the sarcolemma of the myofiber and (3) the internal cytoplasmic structures of the cytoskeleton and sarcomeres are evaluated. Disturbances to many components of this complex interactive system adversely affect skeletal muscle strength and integrity, and can result in severe muscle diseases. The mechanical aspects of these crucial linkages are discussed, with particular reference to defects in laminin-alpha2 and integrin-alpha7. Novel interventions to potentially increase muscle strength and reduce myofiber damage are mentioned, and these are also highly relevant to muscle diseases and aging muscle.

A novel role for non-muscle gamma-actin in skeletal muscle sarcomere assembly

Existing models describing sarcomere assembly have arisen primarily from studies using cardiac muscle. In contrast to cardiac muscle, skeletal muscle differentiation is characterised by dramatic changes in protein expression, from non-muscle to muscle-specific isoforms before organisation of the sarcomeres. Consequently, little is understood of the potential influence of non-muscle cytoskeletal proteins on skeletal sarcomere assembly. To address this issue, transfectant (gamma33-B1) and control mouse C2 myoblasts were differentiated to form myotubes, and various stages of skeletal sarcomere assembly were studied. Organisation of non-muscle gamma-actin and co-localisation with sarcomeric alpha-actinin, an early marker of sarcomere assembly and a major component of Z lines, was noted. gamma-Actin was also identified in young myotubes with developing sarcomeric myofibrils in regenerating adult mouse muscle. Localisation of gamma-actin in a different area of the myotube to the muscle-specific sarcomeric alpha-actin also indicated a distinct role for gamma-actin. The effects of aberrant gamma-actin expression in other myoblast lines, further suggested a sequestering role for gamma-actin. These observations make the novel suggestion that non-muscle gamma-actin plays a role in skeletal sarcomere assembly both in vitro and in vivo. Consequently, a modified model is proposed which describes the role of gamma-actin in skeletal sarcomere assembly.

Evans Blue Dye as an in vivo marker of myofibre damage: optimising parameters for detecting initial myofibre membrane permeability

Evans Blue Dye (EBD) is widely used to study cellular membrane permeability and has recently been utilised in mdx mice to identify permeable skeletal myofibres that have become damaged as a result of muscular dystrophy. EBD has the potential to be a useful vital stain of myofibre permeability in other models of skeletal muscle injury and membrane-associated fragility. The parameters for its use for such purposes were optimised in the present study, of particular interest is the use of EBD to identify the onset of muscle damage. This study compared intravenous vs. intraperitoneal injection; tissue fixation; volume of EBD; time of availability in tissue; and persistence after injection in mdx mice (with endogenous muscle damage) and control mice. Satisfactory labelling of permeable myofibres was seen in frozen sections viewed with fluorescence microscopy when intraperitoneal injection of a 1% EBD solution injected at 1% volume relative to body mass was administered between 16 and 24 h prior to tissue sampling. EBD labelling was then assessed in three mouse models of experimental injury and repair-cut injury, whole muscle grafts, and exercise-induced muscle damage. These experiments demonstrated that (i) following a cut injury across myofibres, EBD penetrated up to 150 microm from the injury site over a 20-h period; (ii) EBD was present throughout myofibres of avascular whole muscle graft by one day after transplantation; and (iii) damaged myofibres were detected within 20 min after controlled lengthening-contraction exercise. This simple and inexpensive technique has sensitivity for the detection of increased myofibre permeability and/or sublethal damage that has advantages over other traditional histological techniques at the light microscopy level.

Leukaemia inhibitory factor increases myoblast replication and survival and affects extracellular matrix production: combined in vivo and in vitro studies in post-natal skeletal muscle

Leukaemia inhibitory factor (LIF) has been reported to specifically enhance myoblast proliferation in vitro and increase the number and size of myotubes in regenerating skeletal muscle in vivo. The present study specifically tests the effect of LIF on myoblast replication in vivo. Administration of exogenous LIF by slow release alginate gels in vivo sustained the level of myoblast proliferation at 2 days in regenerating crush-injured muscle. Since the extracellular matrix (ECM) plays an important role in regulating the effects of many growth factors, the hypothesis was tested, both in vivo and in vitro, that some of the beneficial effects of LIF are mediated by modulation of the ECM. The effects of LIF in vivo on the amount and localisation of the ECM molecules, fibronectin, tenascin-C, collagen type IV and laminin were assessed by immunohistochemistry on regenerating skeletal muscle but no influence of LIF on ECM composition was observed. In tissue culture, LIF increased BALB/c myoblast proliferation at day 3 on culture dishes coated with Matrigel and also increased the viability in vitro of BALB/c myoblasts grown under suboptimal conditions. Quantitation of the ECM produced by cultures (enzyme-linked immunosorbent assay) showed that LIF affected the amount of fibronectin, tenascin-C, collagen type IV and laminin produced by fusing myoblasts. No significant affect of LIF was seen on myotube formation either in vitro or in vivo. These combined in vitro and in vivo studies show an effect of LIF on ECM production in vitro, on myoblast survival and on in vivo myoblast replication.

The absence of MyoD in regenerating skeletal muscle affects the expression pattern of basement membrane, interstitial matrix and integrin molecules that is consistent with delayed myotube formation

MyoD is a member of a skeletal muscle specific family of transcription factors which directs the events of myogenesis during development and regeneration. Muscle cells that lack MyoD show delayed fusion in vivo and in vitro and defects have been observed in vitro in the attachment of MyoD(-/-) myoblasts to complex substrates such as Matrigel. Since interactions with the extracellular matrix (ECM) are important during myoblast fusion (i. e. myotube formation), it was hypothesised that expression of ECM molecules or their receptors may be altered in MyoD(-/-) muscle. The production of basement membrane molecules such as collagen type IV and several laminins, the interstitial molecules fibronectin and tenascin-C, and the cell surface molecules integrin alpha5 and alpha6 were quantitated in vitro using ELISA on cultured cells from MyoD(-/-) and wild type mice. Differences were observed in the production of fibronectin, tenascin-C, collagen type IV, laminin-1 and integrin alpha5 between control and MyoD(-/-) myotubes in vitro. This corresponded with delayed fusion of myoblasts in MyoD(-/-) cultures. On the basis of these findings with respect to matrix expression in vitro, fluorescent immunohistochemistry was carried out on adult whole muscle autografts to examine whether the expression of these molecules, as well as integrin alpha7, was altered in the complex in vivo environment. Some minor differences in expression patterns were observed in MyoD(-/-) as compared to normal BALB/c autografts. The overall expression of matrix components was consistent with the delayed onset of myotube formation. These results suggest that the delay in myotube formation in MyoD(-/-) muscle is not a direct result of altered expression of the matrix molecules collagen type IV, laminins, fibronectin, tenascin-C, and integrins alpha5, alpha6 or alpha7.

The role of tumor necrosis factor-alpha (TNF-alpha) in skeletal muscle regeneration. Studies in TNF-alpha(-/-) and TNF-alpha(-/-)/LT-alpha(-/-) mice

The role of tumor necrosis factor-alpha (TNF-alpha), an important mediator of the inflammatory response after injury, was investigated in regenerating skeletal muscle. The pattern of expression of TNF-alpha during muscle regeneration was examined by immunohistochemistry in tissue sections of crush-injured or transplanted muscle autografts and in primary cultures of adult skeletal muscle. TNF-alpha was highly expressed in injured myofibers, inflammatory cells, endothelial cells, fibroblasts, and mast cells. Myoblasts and myotubes also expressed TNF-alpha in primary muscle cultures and tissue sections. The essential role of TNF-alpha and its homologue lymphotoxin-alpha (LT-alpha) during muscle regeneration was assessed by basic histology in TNF-alpha(-/-) and TNF-alpha(-/-)/LT-alpha(-/-) mice. No difference was apparent in the onset or pattern of muscle regeneration (i.e., inflammatory response, activation and fusion of myoblasts) between the two strains of null mice or between nulls and normal control mice. However, both strains of null mice appeared more prone to bystander damage of host muscle and regeneration distant from the site of injury/transplantation. Although expression of TNF-alpha may play an important role in muscle regeneration, the studies in the null mice show that redundancy within the cytokine system (or some other response) can effectively compensate for the absence of TNF-alpha in vivo.

Absence of MyoD increases donor myoblast migration into host muscle

Donor myoblast migration is a major limiting factor in the success of myoblast transfer therapy, a potential treatment for Duchenne muscular dystrophy. A possible strategy to promote the migration of donor myoblasts into host muscle is to enhance their proliferation and delay their fusion, two properties that are major characteristics of myoblasts in regenerating skeletal muscle in MyoD null (-/-) mice. Here we investigate whether the migration of MyoD (-/-) donor myoblasts into host muscle is enhanced in vivo. Sliced muscle grafts from male MyoD (-/-) or normal control (Balb/c) mice were transplanted into the muscles of female normal (Balb/c) host mice. Muscles were sampled at 1, 3, and 12 weeks after grafting, and the fate of male donor myoblasts within female host muscles determined by in situ hybridization with the mouse Y-chromosome-specific Y-1 probe. MyoD (-/-) donor myoblasts migrated into host muscle continuously over 1, 3, and 12 weeks after grafting, in contrast with Balb/c donor myoblasts, whose overall numbers and migratory distances did not increase significantly after 1 week. These results strongly support a role for elevated donor myoblast proliferation and/or their delayed fusion in enhancing migration into host muscle in vivo, and endorse the use of either genetically engineered donor myoblasts, or the administration of exogenous myoblast mitogens to improve donor myoblast migration in myoblast transfer therapy.

Complement and myoblast transfer therapy: donor myoblast survival is enhanced following depletion of host complement C3 using cobra venom factor, but not in the absence of C5

Myoblast transfer therapy (MTT) is a potential cell therapy for myopathies such as Duchenne Muscular Dystrophy and involves the injection of cultured muscle precursor cells ('myoblasts') isolated from normal donor skeletal muscles into dystrophic host muscle. The failure of donor myoblast survival following MTT is widely accepted as being due to the immune response of the host. The role of complement as one possible mechanism for the initial, very rapid death of myoblasts following MTT was investigated. Donor male myoblasts were injected into the tibialis anterior (TA) muscles of female host mice that were: (i) untreated; (ii) depleted of C3 complement (24 h prior to MTT) using cobra venom factor (CVF); and/or (iii) deficient in C5 complement. Quantification of surviving male donor myoblast DNA was performed using the Y-chromosome specific (Y1) probe on slot blots for samples taken at 0 h, 1 h, 24 h, 1 week and 3 weeks after MTT. Peripheral depletion of C3 was confirmed using double immunodiffusion, and local depletion of C3 in host TA muscles was confirmed by immunostaining of muscle samples. Cobra venom factor treatment significantly increased the initial survival of donor myoblasts, but there was a marked decline in myoblast numbers after 1 h and little long-term benefit by 3 weeks. Strain specific variation in the immediate survival of donor male myoblasts following MTT in untreated C57BL/10Sn, DBA-1 and DBA-2 (C5-deficient) female hosts was observed. Cobra venom factor depletion of C3 increased initial donor male myoblast survival (approximately twofold at 0 h) in C57BL/10Sn and DBA-1 host mice and approximately threefold in DBA-2 hosts at 0 h and 1 h after MTT. The rapid and extensive number (approximately 90%) of donor male myoblasts in untreated DBA-2 mice (that lack C5) indicates that activation of the membrane attack complex (MAC) plays no role in this massive initial cell death. The observation that myoblast survival was increased in all mice treated with CVF suggests that CVF may indirectly enhance donor myoblast survival by a mechanism possibly involving activated C3 fragments.

Absence of desmin slightly prolongs myoblast proliferation and delays fusion in vivo in regenerating grafts of skeletal muscle

The expression of desmin, a muscle-specific intermediate filament protein, is upregulated during skeletal myogenesis, but its role in the myogenic process is unclear. Postnatal skeletal muscle regeneration occurs to completion in desmin null (-/-) mice, however, only late time points (i.e., days 7 and 21) in the myogenic process have been examined. This study observes the early events in skeletal muscle regeneration (i.e., from 3 days) in desmin (-/-) mice. Whole muscle autografts were performed in desmin (-/-) and control normal (Balb/c) mice. Muscle samples were taken on days 3, 5, 6, 7, 8, 9 and 11 after transplantation, and regeneration was assessed by graft morphology, patterns of cell proliferation and quantitation of myotube numbers. At day 5 myotube formation was delayed in the desmin (-/-) grafts compared to the normal controls. Immunohistochemical analysis of proliferating cell nuclear antigen demonstrated a very high proportion of proliferating cells in the periphery of desmin (-/-) whole muscle grafts at day 5 compared to the controls, where mitosis in this area was negligible. This strongly indicates t hat myoblast proliferation is prolonged during postnatal myogenesis in the absence of desmin. By day 6 there was no marked morphological difference between desmin (-/-) and normal control whole muscle grafts, although the zonal pattern of myoblast replication was slightly delayed in the desmin (-/-) mice until day 8. These results indicate a slightly extended phase of myoblast proliferation with delayed fusion in vivo in mature regenerating desmin (-/-) skeletal muscle.

Problems and solutions in myoblast transfer therapy

Duchenne muscular dystrophy is a severe X-linked neuromuscular disease that affects approximately 1/3500 live male births in every human population, and is caused by a mutation in the gene that encodes the muscle protein dystrophin. The characterization and cloning of the dystrophin gene in 1987 was a major breakthrough and it was considered that simple replacement of the dystrophin gene would ameliorate the severe and progressive skeletal muscle wasting characteristic of Duchenne muscular dystrophy. After 20 years, attempts at replacing the dystrophin gene either experimentally or clinically have met with little success, but there have been many significant advances in understanding the factors that limit the delivery of a normal dystrophin gene into dystrophic host muscle. This review addresses the host immune response and donor myoblast changes underlying some of the major problems associated with myoblast-mediated dystrophin replacement, presents potential solutions, and outlines other novel therapeutic approaches.

Myotube formation is delayed but not prevented in MyoD-deficient skeletal muscle: studies in regenerating whole muscle grafts of adult mice

We compared the time course of myogenic events in vivo in regenerating whole muscle grafts in MyoD(-/-) and control BALB/c adult mice using immunohistochemistry and electron microscopy. Immunohistochemistry with antibodies to desmin and myosin revealed a striking delay by about 3 days in the formation of myotubes in MyoD(-/-) autografts compared with BALB/c mice. However, myotube formation was not prevented, and autografts in both strains appeared similar by 8 days. Electron microscopy confirmed myotube formation in 8- but not 5-day MyoD(-/-) grafts. This pattern was not influenced by cross-transplantation experiments between strains examined at 5 days. Antibodies to proliferating cell nuclear antigen demonstrated an elevated level of replication by MyoD(-/-) myoblasts in autografts, and replication was sustained for about 3 days compared with controls. These data indicate that the delay in the onset of differentiation and hence fusion is related to extended proliferation of the MyoD(-/-) myoblasts. Overall, although muscle regeneration was delayed it was not impaired in MyoD(-/-) mice in this model.

Why do cultured transplanted myoblasts die in vivo? DNA quantification shows enhanced survival of donor male myoblasts in host mice depleted of CD4+ and CD8+ cells or Nk1.1+ cells

Overcoming the massive and rapid death of injected donor myoblasts is the primary hurdle for successful myoblast transfer therapy (MTT), designed as a treatment for the lethal childhood myopathy Duchenne muscular dystrophy. The injection of male myoblasts into female host mice and quantification of surviving male DNA using the Y-chromosome-specific (Y1) probe allows the speed and extent of death of donor myoblasts to be determined. Cultured normal C57BL/10Sn male donor myoblasts were injected into untreated normal C57BL/10Sn and dystrophic mdx female host mice and analyzed by slot blots using a 32P-labeled Y1 probe. The amount of male DNA from donor myoblasts showed a remarkable decrease within minutes and by 1 h represented only about 10-18% of the 2.5 x 10(5) cells originally injected (designated 100%). This declined further over 1 week to approximately 1-4%. The host environment (normal or dystrophic) as well as the extent of passaging in tissue culture (early "P3" or late "P15-20" passage) made no difference to this result. Modulation of the host response by CD4+/CD8+ -depleting antibodies administered prior to injection of the cultured myoblasts dramatically enhanced donor myoblast survival in dystrophic mdx hosts (15-fold relative to untreated hosts after 1 week). NK1.1 depletion also dramatically enhanced donor myoblast survival in dystrophic mdx hosts (21-fold after 1 week) compared to untreated hosts. These results provide a strategic approach to enhance donor myoblast survival in clinical trials of MTT.

Exposure to tissue culture conditions can adversely affect myoblast behavior in vivo in whole muscle grafts: implications for myoblast transfer therapy

The effects of tissue culture conditions on the viability of myoblasts in whole muscles transplanted in vivo were investigated. Whole male (SJL/J) donor muscles were exposed to various tissue culture reagents and proteolytic enzymes, and allografted into female (SJL/J) host mice. Desmin immunohistochemistry was used to assess the numbers of myogenic cells (as an index of myoblast viability and the extent of regeneration) in tissue sections of whole-muscle grafts sampled on days 7 and 14. DNA quantitation with a Y-chromosome-specific probe was used to determine the total Y-1 sequence DNA (as an index of myoblast survival and proliferation) in whole-muscle grafts sampled on days 1, 3, and 7. In grafts exposed to serum-free medium, there was a delay in myoblast fusion at 7 days that was recovered by 14 days, but exposure to serum (10% or 20%) had a prolonged adverse effect on myotube formation at 14 days. DNA quantitation demonstrated that either serum-free culture medium or 10% serum enhanced the number of male cells within whole-muscle grafts at 7 days. Proteolytic digestion (even for 5 min) of whole muscles prior to grafting was extremely detrimental to myoblast survival and viability at 7 and 14 days. The unexpected finding of adverse effects of tissue culture conditions on the regeneration of whole-muscle grafts in vivo appears to parallel the major problem of the rapid death of isolated cultured donor myoblasts after injection in myoblast transfer therapy. The use of whole-muscle grafts provides an alternative and sensitive model to analyze the crucial effects of various tissue culture components on the subsequent survival and proliferation of myogenic cells in vivo.

Laminin alpha4 and integrin alpha6 are upregulated in regenerating dy/dy skeletal muscle: comparative expression of laminin and integrin isoforms in muscles regenerating after crush injury

The expression of laminin isoforms and laminin-binding integrin receptors known to occur in muscle was investigated during myogenic regeneration after crush injury. Comparisons were made between dystrophic 129ReJ dy/dy mice, which have reduced laminin alpha2 expression, and their normal littermates. The overall histological pattern of regeneration after crush injury was similar in dy/dy and control muscle, but proceeded faster in dy/dy mice. In vitro studies revealed a greater yield of mononuclear cells extracted from dy/dy muscle and a reduced proportion of desmin-positive cells upon in vitro cultivation, reflecting the presence of inflammatory cells and "preactivated" myoblasts due to ongoing regenerative processes within the endogenous dystrophic lesions. Laminin alpha1 was not detectable in skeletal muscle. Laminin alpha2 was present in basement membranes of mature myofibers and newly formed myotubes in control and dy/dy muscles, albeit weaker in dy/dy. Laminin alpha2-negative myogenic cells were detected in dy/dy and control muscle, suggesting the involvement of other laminin alpha chains in early myogenic differentiation, such as laminin alpha4 and alpha5 which were both transiently expressed in basement membranes of newly formed myotubes of dy/dy and control mice. Integrin beta1 was expressed on endothelial cells, muscle fibers, and peripheral nerves in uninjured muscle and broadened after crush injury to the interstitium where it occurred on myogenic and nonmyogenic cells. Integrin alpha3 was not expressed in uninjured or regenerating muscle, while integrin alpha6 was expressed mainly on endothelial cells and peripheral nerves in uninjured muscle. Upon crush injury integrin alpha6 increased in the interstitium mainly on nonmyogenic cells, including infiltrating leukocytes, endothelial cells, and fibroblasts. In dy/dy muscle, integrin alpha6 occurred on some newly formed myotubes. Integrin alpha7 was expressed on muscle fibers at the myotendinous junction and showed weak and irregular expression on muscle fibers. After crush injury, integrin alpha7 expression extended to the newly formed myotubes and some myoblasts. However, many myoblasts and newly formed myotubes were integrin alpha7 negative. No marked difference was observed in integrin alpha7 expression between dy/dy and control muscle, either uninjured or after crush injury. Only laminin alpha4 and integrin alpha6 expression patterns were notably different between dy/dy and control muscle. Expression of both molecules was more extensive in dy/dy muscle, especially in the interstitium of regenerating areas and on newly formed myotubes. In view of the faster myogenic regeneration observed in dy/dy mice, the data suggest that laminin alpha4 and integrin alpha6 support myogenic regeneration. However, whether these accelerated myogenic effects are a direct consequence of the reduced laminin alpha2 expression in dy/dy mice, or an accentuation of the ongoing regenerative events in focal lesions in the muscle, requires further investigation.

Immunobiology and the future of myoblast transfer therapy

Myoblast transfer therapy (MTT) is a cell-mediated gene transfer method aimed at the restoration of normal dystrophin expression in Duchenne muscular dystrophy (DMD). Initial clinical MTT trials were conducted amid much controversy, as they were based on very few animal studies. Unfortunately, the trials were of little therapeutic benefit. As a result, there has been a renaissance of interest in experimental studies in animal models. In MTT, myoblasts are obtained by muscle biopsy from normal, i.e., dystrophin-positive, donors, expanded in culture, and injected directly into the muscles of dystrophic recipients. The major requirement for successful MTT is the survival of injected donor myoblasts in the host environment. However, a vast majority of donor cells fail to survive for more than 1 h after injection, and very few last beyond the first week. This review on the immunological aspects of MTT focuses in particular on the roles of specific components of the host immune response, the effects of tissue culture on donor cells, and strategies under development to circumvent the problem of donor myoblast death after injection in vivo.

Enhanced migration and fusion of donor myoblasts in dystrophic and normal host muscle

Sliced male C57Bl/10Sn (H2-b) donor muscles were grafted into the female histocompatible muscles of untreated, FK506-treated, and T-cell depleted (with or without thymic tolerization) dystrophic (mdx; H2-b) and normal (C57Bl/10Sn; H2-b) hosts, and also into histoincompatible normal (Balb/c; H2-d) hosts. The fate of male donor nuclei was monitored on tissue sections by in situ hybridization with a Y-chromosome specific probe. The results demonstrate that the dystrophic environment is more conducive than normal muscle to donor myoblast migration, with the distance moved being threefold greater at 12 weeks in dystrophic hosts. T-cell depletion was significantly more effective than FK506 treatment at enhancing donor myoblast emigration in both histocompatible and histoincompatible hosts at 3 weeks. Furthermore, the effects of T-cell depletion were sustained in histoincompatible hosts at 12 weeks. These data endorse the use of host T-cell depletion as a promising long-term strategy to improve myoblast transfer therapy (MTT) in the clinical situation.

Muscle regeneration: molecular aspects and therapeutic implications

With respect to diverse clinical applications for muscle regeneration, this paper discusses the latest markers for identifying skeletal muscle precursor cells in regenerating muscle, the implications of alternative sources of myogenic precursor cells and putative stem cells, and the current status of administration of exogenous factors to enhance muscle repair.

The timing between skeletal muscle myoblast replication and fusion into myotubes, and the stability of regenerated dystrophic myofibres: an autoradiographic study in mdx mice

In mdx mice, a model for Duchenne muscular dystrophy, the timing between the replication of myoblasts and their incorporation into myotubes was determined autoradiographically. Thirty-eight mdx mice aged 23 d were injected with tritiated thymidine to label myoblasts replicating early in the dystrophic process. At intervals from 8 h to 30 d after injection the tibialis anterior muscles were removed, processed for autoradiography and analysed for labelled central myonuclei (derived from the progeny of myoblasts which had been labelled at 23 d). At 8 h after injection there were no labelled central myonuclei, showing that the labelled myoblasts had not fused within this time. At 1 d, 2 % of central myonuclei were labelled, at 2 d, up to 32% were labelled, at 3 d approximately 60% were labelled, and at 4 d the labelling peaked at 74%. In the 27 mice sampled from 5-30 d after injection, the levels of central myonuclear labelling varied enormously: from 1-63%. However, there was a consistent decrease in the numbers of labelled central myonuclei with time. This may have been due to dilution of the relative numbers of labelled myonuclei due to other, nonlabelled, myoblasts replicating after the availability of tritiated thymidine, and fusing. It was also possible that labelled myofibres underwent subsequent necrosis and were eliminated from the muscle. The proposal that a regenerated myofibre can undergo a subsequent cycle of necrosis and regeneration was supported by evidence of some necrotic myofibres with labelled and unlabelled central nuclei. These results have implications for understanding the cellular biology and pathology of dystrophic muscle, particularly in relation to myoblast transfer therapy as a potential treatment of Duchenne muscular dystrophy.

Age-associated changes in the response of skeletal muscle cells to exercise and regeneration

This paper looks at the effects of aging on the response of skeletal muscle to exercise from the perspective of the behavior of muscle precursor cells (widely termed satellite cells or myoblasts) and regeneration. The paper starts by outlining the ways in which skeletal muscle can respond to damage resulting from exercise or other trauma. The age-related changes within skeletal muscle tissue and the host environment that may affect the proliferation and fusion of myoblasts in response to injury in old animals are explored. Finally, in vivo and in vitro data concerning the wide range of signaling molecules that stimulate satellite cells and other aspects of regeneration are discussed with respect to aging. Emphasis is placed on the important role of the host environment, inflammatory cells, growth factors and their receptors (particularly for FGF-2), and the extracellular matrix.

The host environment determines strain-specific differences in the timing of skeletal muscle regeneration: cross-transplantation studies between SJL/J and BALB/c mice

The difference in the timing of the regeneration process of skeletal muscle between SJL/J and BALB/c mice was investigated using grafts of whole skeletal muscle (both autografts and allografts). Histological, autoradiographic and immunohistochemical techniques were used in the investigation. Infiltration of leucocytes into autografts, numbers of desmin-positive myogenic cells and myotube formation were all more advanced in the SJL/J compared with BALB/c mice. Furthermore, autoradiographic evidence showed that myoblasts in the SJL/J autografts were synthesising DNA 12 h earlier than myoblasts in BALB/c autografts. In allografts, where SJL/J host mice received BALB/c grafts, and vice versa, leucocyte infiltration and myotube formation occurred earlier in the BALB/c muscles grafted into SJL/J hosts, than in the reverse situation with BALB/c hosts. The results show that, at least for whole muscle grafts, it is the host environment which determines the speed and outcome of the regenerative process.

In vitro assessment of the biological activity of basic fibroblast growth factor released from various polymers and biomatrices

The kinetics of controlled release of basic fibroblast growth factor (bFGF) from polymers (sutures, polycarbonate, Hydron, and Elvax), biopolymers (alginate), and biomatrices (lens capsules), and conditions for storage of bFGF (temperature, plastic type, heparin) were evaluated in vitro. Tissue culture proliferation bioassays with 3T3 fibroblasts, showed that only lens capsules with bFGF had a sustained release of bFGF for up to three weeks. The other materials released all of the 'bound' bFGF with two hours or produced an inflammatory response in vivo. Therefore, the lens tissue had the most potential for controlled long-term delivery of bFGF in vivo. These studies emphasise the importance of in vitro analysis of release kinetics of growth factors from a range of materials as a basis for potential in vivo applications.

Macrophages and dendritic cells in normal and regenerating murine skeletal muscle

Mononuclear phagocytes and MHC class II+ dendritic cells (DC) were identified in frozen sections of skeletal muscle using a panel of pan-specific antimacrophage (MOMA-2, SER-4, Mac-1, F4/80), anti-major histocompatibility complex (MHC) class II (M5/114) and anti-DC (NLDC-145, N418, M342) monoclonal antibodies. Uninjured and regenerating skeletal muscle were investigated in SJL/J and BALB/c mice, strains with known differences in muscle regenerative capacity. Resident tissue macrophages and MHC class II+ DC were present within uninjured mouse muscle. A subpopulation of DC were positive for the pan-DC markers, N418 and M342, and negative for the lymphoid DC marker NLDC-145. Following crush injury, the macrophage population increased by day 2, became marked by day 3, and had decreased by day 6. In contrast, the number of MHC class II+ cells around the injury site increased steadily after injury and remained high at day 6. The numbers of macrophages and DC detected by immunohistochemical staining were consistently higher in SJL/J than BALB/c muscles. This study confirms that macrophages are a significant component of normal murine skeletal muscle and that these cells increase dramatically after injury. Furthermore the data also reveal for the first time that DC are present in normal skeletal muscle and that MHC class II+ cells, including DC, increase after injury. The presence of DC in muscle has important implications for the understanding of the immunobiology of muscle and immune-mediated processes such as the host versus graft responses following muscle transplants and autoimmune diseases affecting this tissue.

A potential alternative strategy for myoblast transfer therapy: the use of sliced muscle grafts

Excellent long-term survival (up to 1 yr) of donor skeletal muscle cells was demonstrated using a mouse Y-chromosome specific probe, following the transplantation of grafts of whole muscles from male "normal" C57B1/10Sn mice into dystrophic muscles of female host mice. After the transplantation of equivalent sliced muscle grafts there was extensive movement of the male donor cells and fusion with host myofibres. This contrasts with the extremely poor survival of isolated myoblasts after injection into the same mouse model for Duchenne muscular dystrophy. The use of sliced muscle grafts may therefore represent a potential alternative approach to myoblast transfer therapy.

The exogenous administration of basic fibroblast growth factor to regenerating skeletal muscle in mice does not enhance the process of regeneration

The effects, in vivo, of the exogenous administration of bFGF on myogenesis of regenerating skeletal muscle was assessed either morphometrically or autoradiographically in three separate models of muscle injury in mice: crush-injured, denervated, and dystrophic (mdx) muscles. The bFGF was administered at various doses and different time schedules, sometimes in combination with heparin, into injured tibialis anterior muscles of mice. Delivery of the bFGF was either by direct intramuscular injection or by the sustained release from 888polymers (Hydron or Elvax) implanted into the muscles. The bioactivity of bFGF was confirmed in vitro by measuring its ability to stimulate the proliferation of BALB/c-3T3 fibroblasts and muscle precursor cell lines. The ability of bFGF to stimulate angiogenesis in vivo was confirmed by the implantation of controlled-release polymers containing bFGF into the normally avascular cornea of rats. No measurable effect of bFGF was seen in any of the models of skeletal muscle injury under these experimental conditions, indicating that the availability of biologically active bFGF is not a limiting factor in the regeneration of skeletal muscle following injury.

Enhancement of neovascularization in regenerating skeletal muscle by the sustained release of erucamide from a polymer matrix

The angiogenic agent erucamide (cis-13-docosenamide), incorporated into a polymeric biomaterial (Elvax 40P, a copolymer of ethylene and vinyl acetate), was used to determine whether angiogenesis can be increased in the regenerating skeletal muscle, and whether the enhanced revascularization improves the new muscle formation. The angiogenic nature of this lipid was confirmed in a rat cornea-micropocket assay, prior to insertion of small strips of the polymer containing either 3 micrograms, 300 micrograms erucamide or only polymer as a control into the mid-region of crush-injured tibialis anterior (TA) muscles of forty-five adult male BALB/c mice. All TA muscles were sampled ten days after injury and analyzed morphometrically. Statistical analyses of the mean blood vessel area density in lesions from twelve perfused TA muscles (three from each of the erucamide-treated or control group), revealed a dose-dependent angiogenic effect of erucamide: a dosage of 3 micrograms increased mean blood vessel area density to 5.1% compared to 2.0% in controls, due to numerous large caliber, thin-walled vessels, whereas the mean vessel area density in both the 30-micrograms (3.5%) and 300-micrograms (1.5%) doses were similar to controls. However, at all three doses tested, erucamide did not significantly alter the degree of new muscle formation, connective tissue deposition, or removal of necrotic debris.

Extracellular matrix, growth factors, genetics: their influence on cell proliferation and myotube formation in primary cultures of adult mouse skeletal muscle

Cell proliferation and myotube formation in response to growth factors on various extracellular matrices (ECM) were investigated in primary skeletal muscle cultures from adult SJL/J and BALB/c mice. There was no difference between the rates of proliferation from primary cultures of SJL/J and Balb/c mice measured at 48 h in response to a range of concentrations of PDGF-AA, -AB, -BB, TGF beta 1, or LIF (added at 24 h). SJL/J primary cultures were more responsive to bFGF (which was the most potent mitogen) than were BALB/c cultures. Comparison of dose response curves to bFGF and TGF beta 1 grown on gelatin or Matrigel showed that the nature of the ECM did not have a significant affect. More myotubes formed at 4 days in SJL/J than in parallel BALB/c cultures on gelatin or Matrigel (P < 0.05). On gelatin more myotubes with 4 or more nuclei were formed in cultures from SJL/J than BALB/c muscles (P < 0.05); however, on Matrigel these myotubes occurred with similar frequency. Myotube formation examined in BALB/c muscle cultures grown on collagen i.v., entactin-free laminin, and fibronectin showed that none of these ECM components alone supported large myotube formation (4 or more nuclei) as well as did Matrigel, although fibronectin was as effective as Matrigel with respect to the total number of myotubes formed. Parallel experiments carried out using the myogenic H-2Kb(27) cell line showed similar effects with the exception of laminin which enhanced large myotube formation and desmin expression in the H-2Kb(27) but not in the primary muscle cultures. The greater sensitivity in mitogenic response to bFGF and the more extensive myotube formation seen in SJL/J compared with BALB/c cultures in vitro reflects the superior capacity for muscle regeneration of SJL/J mice in vivo.

The genotype of bone marrow-derived inflammatory cells does not account for differences in skeletal muscle regeneration between SJL/J and BALB/c mice

This study determined whether the genotype of bone marrow-derived inflammatory cells contributes to the more pronounced leukocytic exudation and extensive new muscle formation seen in SJL/J compared with BALB/c mice after a crush-injury (Mitchell et al. 1992). Female SJL/J mice were whole-body irradiated and reconstituted with male bone marrow from the BALB/c strain, and irradiated BALB/c females reconstituted with male SJL/J bone marrow. The mice were allowed to recover for 3 weeks and the tibialis anterior muscle (in a leg which had been protected from irradiation) was injured by crushing. At 3 and 10 days after injury the extent of necrotic debris, mononuclear leukocytic infiltration and new muscle formation was assessed in the muscles. The SJL/J mice reconstituted with BALB/c bone marrow showed extensive mononuclear leukocytic infiltration and clearance of necrotic debris when compared with BALB/c mice reconstituted with SJL/J bone marrow, and these strain-specific differences mirrored those seen with control bone marrow reconstituted hosts and non-irradiated hosts. The results show that the genotype of the bone marrow-derived macrophages is not responsible for the superior regeneration of crush-injured skeletal muscle in SJL/J mice, and it appears that factors intrinsic to the muscle tissue may be of central importance.

Retarded myogenic cell replication in regenerating skeletal muscles of old mice: an autoradiographic study in young and old BALBc and SJL/J mice

The patterns of skeletal muscle precursor cell replication after crush injury were compared by the use of autoradiographic techniques, in young (4-week-old) and old (39-week-old) BALBc and SJL/J mice. Similar comparisons were made between cut and crush lesions in old BALBc muscle. Muscle precursor cell replication commenced at 18-24 h after injury in both young and old muscles from both strains of mice. In young BALBc muscle the peak of myogenic activity at 60 h was 36 h earlier than in old mice. SJL/J muscle responded more rapidly than did BALBc: in young SJL/J the peak myogenic activity was at 46 h (14 h earlier than in young BALBc muscle), and in old SJL/J muscle the peak activity at 72 h was 24 h earlier than in old BALBc muscle. In all mice (both young and old) myogenic cell replication was substantially reduced by 120 h after injury. A comparison of the timing of muscle precursor cell replication in cut and crush lesions in old BALBc mice revealed a more rapid response in the cut lesion; this difference between the lesions is comparable with data from identical lesions in 6-8-week-old BALBc mice (McGeachie and Grounds 1987). However, the peak of myogenic replication in the older mice in the present study was some 26-36 h later than in the younger 6-8-week-old mice. These experiments show that, whilst muscle precursor cell replication commences at approximately the same time (about 24 h) after injury in young and old mice, the peak level of activity is delayed by some 24-36 h in old mice. In addition, the SJL/J mouse strain responds more rapidly and prolifically to muscle injury than does the BALBc strain.

The time course of basic fibroblast growth factor expression in crush-injured skeletal muscles of SJL/J and BALB/c mice

Staining for basic fibroblast growth factor (bFGF), a potent mitogen, was examined in muscle recovering from a crush injury and compared between two mouse strains with distinctly different capacities for muscle regeneration to determine if bFGF staining and the steps and outcome of repair were related. Immunofluorescence studies on intact and crushed tibialis anterior muscle were carried out at 0, 1, 3, 6, and 12 h postcrush in SJL/J mice, and at 1, 2, 3, 5, 7, and 11 days after injury in both SJL/J and BALB/c mice (n = 2-4). Disrupted fibers showed increased sarcoplasmic staining for bFGF as little as 3-6 h after injury prior to infiltration with intensely fluorescent mononuclear cells (at 12-24 h). Fiber bFGF was maintained in SJL/J muscles for 2 days, but was lower in most damaged fibers of BALB/c muscles at the same time. Surviving stumps of crushed fibers, once sealed, exhibited sarcoplasmic extensions, some of which stained intensely for bFGF. These processes appeared to connect adjacent fiber stumps, and many were noted in association with aligned mononuclear cells (presumptive myoblasts) at the site of new myotube formation. In representative sections there were more bFGF-positive mononuclear cells present in SJL/J than BALB/c muscles. Intense bFGF localization marked newly regenerating myotubes in both SJL/J and BALB/c muscles, and such myotubes were more frequent and larger in SJL/J muscles. More bFGF was present in regenerating muscles of SJL/J compared with BALB/c mice (with respect to damaged myofibers, mononuclear cells, and myotubes) and this correlates with the superior new muscle formation seen in SJL/J mice. These studies support the idea of a positive relation between bFGF in damaged fibers, the bFGF-positive mononuclear cells, and the speed and success of muscle regeneration.

Intrinsic differences in MyoD and myogenin expression between primary cultures of SJL/J and BALB/C skeletal muscle

The time course of expression of the skeletal muscle-specific regulatory genes MyoD and myogenin was studied in primary cultures of skeletal muscle from adult SJL/J and BALB/c mice. In situ detection of expression with MyoD and myogenin riboprobes and myogenin antibody showed that the onset of expression of these genes occurred earlier in cells from SJL/J mice. Probe-positive cells and myotubes were also more frequent in cultures from SJL/J mice than in BALB/c. The onset of expression of MyoD and myogenin was delayed in cultured cells relative to the time course seen following injury in vivo. Myogenin protein was demonstrated in replicating cells and all myogenin-positive cells expressed desmin. The observed strain-specific difference infers a greater intrinsic myogenicity of cells in SJL/J muscle in vitro and reflects the superior capacity for new muscle formation previously reported in SJL/J mice in vivo.

Age-related changes in replication of myogenic cells in mdx mice: quantitative autoradiographic studies

Cell replication in muscle was measured by tritiated thymidine (3H-TdR) incorporation and autoradiography, in mdx mice from 2-44 weeks of age. Pre-mitotic labelling (within 1 h of 3H-TdR injection) was determined in 16 mice aged from 15 to 300 days. In 30 further mdx mice, one leg was irradiated 1 h after 3H-TdR injection to block DNA synthesis. Post-mitotic labelling was measured in both legs 10-15 days later. Between 20 and 60 days of age a very high proportion (up to 2%) of muscle (satellite cell) nuclei were replicating pre-mitotically; from 80-300 days cell replication was detectable but at much lower levels. Centrally placed nuclei within muscle fibres appeared at 24 days, increased rapidly to 50% by 50-100 days, declining thereafter to 25% at 300 days. In post-mitotic samples, labelled myotubes and labelled peripheral muscle nuclei (satellite cell nuclei and myonuclei) appeared at 28 days and were present in the mdx muscles through to 310 days, indicating continued cell replication and muscle regeneration. Myogenic cell replication was both retarded and inhibited by irradiation. These data demonstrate that muscle cell replication in mdx mice commences at about 3 weeks of age, is maximal at 4-8 weeks, but continues at lower levels until at least 44 weeks.

The migration and intermixing of donor and host glia on nitrocellulose polymers implanted into cortical lesion cavities in adult mice and rats

The fate of neonatal glia (mostly glial fibrillary acidic protein-positive astrocytes), cultured on nitrocellulose papers and implanted into cortical lesion cavities, was examined in adult mice and rats. In mice, a Y-chromosome-specific probe and in situ hybridization techniques were used to identify male cells. Male-female grafts allowed visualization of donor glia and their behaviour after transplantation; female-male grafts allowed an analysis of how host cells responded to the presence of the implants. There was substantial intermixing of cells, with many donor glia migrating away from the implants and host cells migrating onto both sides of the nitrocellulose paper. In rats, donor glia were labelled with fluorescein-conjugated latex microspheres prior to transplantation on nitrocellulose polymers. The rat data were broadly consistent with those obtained from the mouse; moreover, immunohistochemical studies in rats suggested that the majority of host cells migrating onto the previously cell-coated papers were astrocytes. In a number of studies, glia-coated polymers have been used in an attempt to promote the regrowth of axons across lesion sites in the brain and spinal cord. The present work suggests that both transplanted and host glia may influence the regenerative growth seen in such implants.

The role of macrophages in skeletal muscle regeneration with particular reference to chemotaxis

The results from this investigation suggest that chemotactic factor(s) from damaged myofibers attract polymorphonuclear leukocytes (PML) and macrophages to the site of injury, while exudate macrophages but not PML induce a strong positive chemotactic response in myogenic cells. The AB and BB isoforms of platelet-derived growth factor (PDGF), transforming growth factor-beta (TGF-beta), basic fibroblast growth factor (bFGF), and leukemia inhibitory factor (LIF) (all of these are secreted by macrophages) were also shown to be chemoattractants for muscle precursor cells (MPC). The AA isoform of PDGF did not appear to have any such chemotactic effect on MPC. Macrophages were also shown, under tissue culture conditions, to stimulate the proliferation of MPC. This mitogenic effect was similarly demonstrated for the BB isoform of PDGF, bFGF, and TGF-beta but not for the AA or AB isoforms of PDGF nor for LIF. The results indicate that macrophages play a pivotal role in the orchestration of muscle fiber reconstitution.

Fusion of myogenic cells to the newly sealed region of damaged myofibres in skeletal muscle regeneration

In regenerating skeletal muscle, sarcoplasmic extensions containing variable numbers of nuclei, widely referred to as 'buds' or 'stumps', are formed at the ends of damaged myofibres. In this paper we investigated whether the nuclei seen in the buds results from fusion of myogenic cells or from migration of myonuclei to the sealed ends of damaged myofibres in murine muscle regenerating after crush injury. The fusion of mononuclear and multinucleate myogenic cells to the buds was demonstrated by transmission electron microscopy. In order to elucidate the frequency and kinetics of cytoplasmic continuity between myotubes and sealed myofibres, we labelled the damaged myofibres with carbocyanine dye DiI (which inserts into the lipid bilayer and travels down continuous membranes) and the samples were then examined by confocal scanning microscopy. This technique showed that there was little fusion between myotubes and myofibres during the first 6 days after crush injury, but significant fusion had occurred by the tenth day especially at the newly sealed region of the damaged myofibre. A scheme for the repair of damaged skeletal muscle is presented.

Specific cloning of DNA fragments unique to the dog Y chromosome

A novel technique that enabled the specific cloning of a DNA fragment unique to the dog Y chromosome is described. The method involves competitive hybridization of DNA prepared from male dog lymphocytes with biotin-labeled DNA prepared from female dog lymphocytes. The biotinylated female-female and male-female hybrid DNA fragments were removed by capture with streptavidin-coated paramagnetic particles. Full-length double-stranded DNA was generated from the remaining fragments by using the Klenow fragment of DNA polymerase I, followed by direct cloning using a low-background ligation technique. Analysis of putative recombinant clones derived by this method has led to the identification of a fragment that hybridizes specifically to male dog DNA. The clones were selected initially on the basis of a differential signal obtained when hybridized to dilutions of male and female dog DNA immobilized on neural nylon membrane. To evaluate its suitability as a probe for trans-sexually grafted cells in transplantation studies, the fragment was labeled with digoxigenin and hybridized in situ to male and female dog tissue sections. The clone designated number 6.2 hybridized strongly to male dog nuclei. The cloning strategy employed could be extended to other studies in which competitive reassociation can be used to identify unique DNA sequences.