Differentiation-dependent susceptibility of human muscle cells to Zika virus infection

Muscle cells are potential targets of many arboviruses, such as Ross River, Dengue, Sindbis, and chikungunya viruses, that may be involved in the physiopathological course of the infection. During the recent outbreak of Zika virus (ZIKV), myalgia was one of the most frequently reported symptoms. We investigated the susceptibility of human muscle cells to ZIKV infection. Using an in vitro model of human primary myoblasts that can be differentiated into myotubes, we found that myoblasts can be productively infected by ZIKV. In contrast, myotubes were shown to be resistant to ZIKV infection, suggesting a differentiation-dependent susceptibility. Infection was accompanied by a caspase-independent cytopathic effect, associated with paraptosis-like cytoplasmic vacuolization. Proteomic profiling was performed 24h and 48h post-infection in cells infected with two different isolates. Proteome changes indicate that ZIKV infection induces an upregulation of proteins involved in the activation of the Interferon type I pathway, and a downregulation of protein synthesis. This work constitutes the first observation of primary human muscle cells susceptibility to ZIKV infection, and differentiation-dependent restriction of infection from myoblasts to myotubes. Since myoblasts constitute the reservoir of stem cells involved in reparation/regeneration in muscle tissue, the infection of muscle cells and the viral-induced alterations observed here could have consequences in ZIKV infection pathogenesis.

Muscle cells are potential targets of many arboviruses, such as Ross River, Dengue, Sindbis, and chikungunya viruses, and may be involved in the disease manifestation. During the recent outbreak of Zika virus (ZIKV), myalgia was one of the most frequently reported symptoms. We investigated the susceptibility of human muscle cells to ZIKV infection. Using an in vitro model of human muscle stem cells (myoblasts) that can be differentiated into differentiated muscle cells (myotubes), we found that myoblasts can be infected by ZIKV. In contrast, myotubes were shown to be resistant to ZIKV infection. Infection induced the death of infected cells. Protein levels 24h and 48h post-infection indicate that ZIKV infection induces an upregulation of proteins involved in the activation of the Interferon type I pathway, and a downregulation of protein synthesis. This work constitutes the first observation of primary human muscle cells susceptibility to ZIKV infection, muscle stem cells being susceptible while differentiated muscle cells are resistant. Since myoblasts constitute the reservoir of stem cells involved in reparation/regeneration in muscle tissue, the infection of muscle cells and the viral-induced alterations observed here could have consequences during ZIKV infection.

Dermal and muscle fibroblasts and skeletal myofibers survive chikungunya virus infection and harbor persistent RNA

Chikungunya virus (CHIKV) is an arthritogenic alphavirus that acutely causes fever as well as severe joint and muscle pain. Chronic musculoskeletal pain persists in a substantial fraction of patients for months to years after the initial infection, yet we still have a poor understanding of what promotes chronic disease. While replicating virus has not been detected in joint-associated tissues of patients with persistent arthritis nor in various animal models at convalescent time points, viral RNA is detected months after acute infection. To identify the cells that might contribute to pathogenesis during this chronic phase, we developed a recombinant CHIKV that expresses Cre recombinase (CHIKV-3'-Cre). CHIKV-3'-Cre replicated in myoblasts and fibroblasts, and it induced arthritis during the acute phase in mice. Importantly, it also induced chronic disease, including persistent viral RNA and chronic myositis and synovitis similar to wild-type virus. CHIKV-3'-Cre infection of tdTomato reporter mice resulted in a population of tdTomato+ cells that persisted for at least 112 days. Immunofluorescence and flow cytometric profiling revealed that these tdTomato+ cells predominantly were myofibers and dermal and muscle fibroblasts. Treatment with an antibody against Mxra8, a recently defined host receptor for CHIKV, reduced the number of tdTomato+ cells in the chronic phase and diminished the levels of chronic viral RNA, implicating these tdTomato+ cells as the reservoir of chronic viral RNA. Finally, isolation and flow cytometry-based sorting of the tdTomato+ fibroblasts from the skin and ankle and analysis for viral RNA revealed that the tdTomato+ cells harbor most of the persistent CHIKV RNA at chronic time points. Therefore, this CHIKV-3'-Cre and tdTomato reporter mouse system identifies the cells that survive CHIKV infection in vivo and are enriched for persistent CHIKV RNA. This model represents a useful tool for studying CHIKV pathogenesis in the acute and chronic stages of disease.

Productive infection of human skeletal muscle cells by pandemic and seasonal influenza A(H1N1) viruses

Besides the classical respiratory and systemic symptoms, unusual complications of influenza A infection in humans involve the skeletal muscles. Numerous cases of acute myopathy and/or rhabdomyolysis have been reported, particularly following the outbreak of pandemic influenza A(H1N1) in 2009. The pathogenesis of these influenza-associated myopathies (IAM) remains unkown, although the direct infection of muscle cells is suspected. Here, we studied the susceptibility of cultured human primary muscle cells to a 2009 pandemic and a 2008 seasonal influenza A(H1N1) isolate. Using cells from different donors, we found that differentiated muscle cells (i. e. myotubes) were highly susceptible to infection by both influenza A(H1N1) isolates, whereas undifferentiated cells (i. e. myoblasts) were partially resistant. The receptors for influenza viruses, α2-6 and α2-3 linked sialic acids, were detected on the surface of myotubes and myoblasts. Time line of viral nucleoprotein (NP) expression and nuclear export showed that the first steps of the viral replication cycle could take place in muscle cells. Infected myotubes and myoblasts exhibited budding virions and nuclear inclusions as observed by transmission electron microscopy and correlative light and electron microscopy. Myotubes, but not myoblasts, yielded infectious virus progeny that could further infect naive muscle cells after proteolytic treatment. Infection led to a cytopathic effect with the lysis of muscle cells, as characterized by the release of lactate dehydrogenase. The secretion of proinflammatory cytokines by muscle cells was not affected following infection. Our results are compatible with the hypothesis of a direct muscle infection causing rhabdomyolysis in IAM patients.

Characterization of apoptosis induced by grouper iridovirus in two newly established cell lines from barramundi, Lates calcarifer (Bloch)

Two new cell lines have been established from the muscle and swim bladder tissues of barramundi, Lates calcarifer, and designated as BM (barramundi muscle) and BSB (barramundi swimbladder), respectively. The cells multiplied well at 28 degrees C in Leibovitz's L-15 medium supplemented with 10% foetal bovine serum, and have been continuously subcultured more than 100 times to date. Morphologically, BM cells were mostly fibroblastic, whereas BSB were mostly epithelial. Both cell lines were susceptible to grouper iridovirus (GIV) and displayed characteristics of apoptosis after viral infection. The induction of apoptosis was further assayed in GIV-infected BM and BSB cells by various methods. The inhibition of cell growth by GIV was demonstrated by MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay. Morphological observations revealed typical apoptotic features in the infected cells, including cell shrinkage and rounding, chromosome condensation and formation of apoptotic body-like vesicles. Chromosome fragmentation was detected by DNA laddering and TUNEL assays. Finally, the appearance of phosphotidylserine on the outer leaflet of apoptotic cell membranes was confirmed by annexin V staining. This is the first report of apoptosis induced by GIV in fish cells.

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.

Rhabdomyolysis and acute renal failure associated with influenza virus type B infection

A patient with rhabdomyolysis-induced acute renal failure due to influenza B virus infection is presented. Influenza B infection caused rhabdomyolysis with efflux of myoglobin from myocytes, causing acute renal failure. In conclusion, influenza virus type B can cause severe rhabdomyolysis leading to acute renal failure.

Muscle wasting induced by HTLV-1 tax-1 protein: an in vitro and in vivo study

Besides tropical spastic paraparesis/human T-cell leukemia virus type-1 (HTLV-1)-associated myelopathy, the human retrovirus HTLV-1 causes inflammatory disorders such as myositis. Although the pathogenesis of HTLV-1-associated myositis is primarily unknown, a direct effect of cytokines or viral proteins in myocytotoxicity is suspected. We have developed an in vitro cell culture model to study the interactions between primary human muscle cells and HTLV-1 chronically infected cells. When HTLV-1-infected cell lines were added to differentiated muscle cultures, cytopathic changes such as fiber shrinking were observed as early as 1 day after contact. This was accompanied by alterations in desmin and vimentin organization, occurring in the absence of muscle cell infection but with Tax-1 present in myotubes. Cytopathic changes were also observed when infected culture supernatants were added to the muscle cells. Fiber atrophy and cytoskeletal disorganization were confirmed in muscle biopsies from two HTLV-1-infected patients with myositis. Transduction of cultured muscle cells with a lentiviral vector containing the HTLV-1 Tax gene reproduced such effects in vitro. The present data indicate that the myocytotoxicity that is observed in HTLV-1-associated myopathies can be due to a direct effect of the Tax-1 protein expressed in infected inflammatory cells, in the absence of muscle cell infection.

Adenovirus Vectors Based on Human Adenovirus Type 19a Have High Potential for Human Muscle-Directed Gene Therapy

Until recently, adenovirus-based gene therapy has been almost exclusively based on human adenovirus serotype 5 (Ad5). The aim of this study was to systematically compare the efficiency of transduction of primary muscle cells from various species by two adenoviral vectors from subgroups C and D. Transduction of a panel of myoblasts demonstrated a striking specificity of an Ad19a-based replication-defective E1-deleted vector (Ad19aEGFP) for human cells, whereas the Ad5-based vector had high affinity for nonhuman primate myoblasts. Transgene expression correlated well with cell-associated vector genomes. Up to 6.59% of the initially applied Ad19aEGFP vector particles were taken up by human myoblasts, as compared with 0.1% of the corresponding Ad5 vector. Remarkably, Ad19aEGFP but not Ad5EGFP efficiently transduced differentiated human myotubes, an in vitro model for skeletal muscle transduction. Uptake of Ad19aEGFP vector particles in human myotubes was 12-fold more efficient than that of Ad5EGFP. Moreover, both vectors demonstrated an early block at the level of vector uptake in mouse myoblasts and rat L6 cells. Investigation of the underlying mechanism for binding and uptake of the two vectors by human myoblasts showed high susceptibility for Ad19a to neuraminidase and wheat germ agglutinin (WGA) lectin, whereas Ad5-mediated transduction was dependent on binding to the coxsackie-adenovirus receptor (CAR) and sensitive to soluble RGD peptide and heparin. Our study offers insights into species-dependent factors that determine Ad tropism and, moreover, provides a basis for application of the novel Ad19a-based vector for gene transfer into human skeletal muscle.

Recombinant adeno-associated virus vectors for gene therapy

Recombinant adeno-associated virus (rAAV) vectors are based on a non-pathogenic human parvovirus (AAV) that is unique in its ability to persist in human cells without causing any pathologic effects. Studies of the potential barriers to rAAV-mediated transduction of relatively resistant cells has led to an understanding of the mechanisms of cell attachment and entry, cytoplasmic translocation, nuclear entry, conversion to active double-stranded DNA, activation of transcription and establishment of persistent molecular forms. Each of these areas is individually discussed, as are recent applications in vivo in preclinical models and clinical trials.

[Virus-like inclusions in the myocytes of the skeletal muscle in lateral amyotrophic sclerosis]

Microscopic examination of musculus gastrocnemius biopsies was made in four cases of sporadic lateral amyotrophic sclerosis (LAS). The validity of the clinical diagnosis was confirmed by detected neurotrophic atrophy of the muscular fibers typical for LAS. Electron microscopic study revealed virus-like inclusions 200-450 nm in size in sarcoplasm of myocytes of all the patients. The inclusions consist of lined cells of hexagonal shape at the distance of 37-41 nm from each other. The inclusions resemble enteroviruses but are not identical to them both by size and structure of their elements. There were also specific ultrastructural changes of myocytes corresponding to viral infection. The above virus-like inclusions should be considered as specific structures formed as a result of metabolic shifts caused by productive action on the cell of infective or unknown factor.

Asymmetric flaccid paralysis: a neuromuscular presentation of West Nile virus infection

The neuromuscular aspects of West Nile virus (WNV) infection have not been characterized in detail. We have studied a group of six patients with proven WNV infection. All cases presented with acute, severe, asymmetric, or monolimb weakness, with minimal or no sensory disturbance after a mild flu-like prodrome. Four cases also had facial weakness. Three of our cases had no encephalitic signs or symptoms despite cerebrospinal fluid pleocytosis. Electrophysiological studies showed severe denervation in paralyzed limb muscles, suggesting either motor neuron or multiple ventral nerve root damage. This localization is supported further by the finding of abnormal signal intensity confined to the anterior horns on a lumbar spine magnetic resonance imaging. Muscle biopsies from three patients showed scattered necrotic fibers, implicating mild direct or indirect muscle damage from the WNV infection. In summary, we describe a group of patients with acute segmental flaccid paralysis with minimal or no encephalitic or sensory signs. We have localized the abnormality to either the spinal motor neurons or their ventral nerve roots. It will be important for physicians to consider WNV infection in patients with acute asymmetric paralysis with or without encephalitic symptoms.

Targeting functional subtypes of spinal motoneurons and skeletal muscle fibers in vivo by intramuscular injection of adenoviral and adeno-associated viral vectors

We report that functional subtypes of spinal motoneurons and skeletal muscle fibers can be selectively transduced using replication-defective adenoviral (ADV) or adeno-associated (AAV) viral vectors. After intramuscular injection in adult rodents, ADV vectors transduced both fast-twitch and slow-twitch skeletal muscle fibers. Intramuscular injection of ADV vectors also caused transduction of spinal motoneurons and dorsal root ganglion cells. However, only neurons innervating the injected muscle were transduced, as shown by co-injection of a retrograde axonal tracer. In adult male rats it is therefore possible to transduce fast or slow spinal motoneurons and muscle fibers selectively since in these animals, the extensor digitorum longus and soleus muscles contain almost exclusively fast or slow motor units, respectively. In rats, AAV vectors transduced muscle fibers in the predominantly fast extensor digitorum longus but not in the predominantly slow soleus muscle. We did not observe any transduction of spinal motoneurons following intramuscular injection of AAV vectors. These results show that physiologically and clinically important subpopulations of cells in the neuromuscular system can be selectively transduced by viral vectors.

Post-mitotic, differentiated myotubes efficiently produce retroviral vector from hybrid adeno-retrovirus templates

We have examined the ability of proliferating myoblasts and post-mitotic, differentiated myotubes to produce retroviral vector using hybrid adeno-retroviral vectors as templates. We show that production of retroviral vector from myoblasts peaks 48 h after adenoviral infection at 4.8 x 10(4) cfu/ml and is scarcely detectable by 96 h. Both fully and partially differentiated myotubes were able to generate a sustained increase in the levels of retroviral vector compared with myoblasts peaking 48 h at 1.4 x 10(5) cfu/ml and 1.8 x 10(5) cfu/ml, respectively. Addition of the cell cycle inhibitor aphidicolin (5 microg/ml) had no effect on the production of retroviral vector from fully differentiated myotubes, but resulted in an 80% increase in vector production from partially differentiated myotubes. Thus indicating that retroviral vector production is more efficient in post-mitotic myotubes and is independent of muscle cell cycle progression.

Characterization of heterokaryons between skeletal myoblasts and somatic cells formed by fusion with HVJ (Sendai virus); effects on myogenic differentiation

In skeletal myogenic differentiation, myoblasts fuse with myogenic cells spontaneously, but do not fuse with non-myogenic cells either in vivo or in vitro, suggesting that the fusion of myoblasts with non-myogenic cells is unsuitable for differentiation. To understand the inevitability of the fusion among myoblasts, we prepared heterokaryons in crosses between quail myoblasts transformed with a temperature-sensitive mutant of Rous sarcoma virus (QM-RSV cells) and rodent non-myogenic cells, such as tumor cells, fibroblasts, or neurogenic cells by HVJ (Sendai virus) and examined how myogenic differentiation was influenced in the prepared heterokaryons, focusing on myogenin expression and myofibril formation as markers of differentiation. When presumptive QM-RSV cells were fused with non-myogenic cells by HVJ and induced to differentiate, both myogenin expression and myofibril formation were suppressed. When myotubes of QM-RSV cells that had already expressed myogenin and formed myofibrils were fused with non-myogenic cells, both myogenin and myofibrils disappeared. Especially, fibrous structures of myofibrils were significantly lost and dots or aggregations of F-actin were formed within 24 hr after formation of heterokaryons. However, the fusion of presumptive or differentiated QM-RSV cells with rodent myoblasts did not disturb myogenin expression or myofibril formation. These results suggest that mutual fusion of myoblasts is indispensable for normal myogenic differentiation irrespective of the species, and that some factors inhibiting myogenic differentiation exist in the cytoplasm of non-myogenic cells, but not in myoblasts.

Extended tropism of an adenoviral vector does not circumvent the maturation-dependent transducibility of mouse skeletal muscle

BACKGROUND

Efficient adenoviral gene delivery to mature skeletal muscle has been hindered by different factors. The low levels of adenoviral attachment receptor (CAR) that have been reported in this tissue may be a limiting factor. Therefore, adenoviral transduction of mature muscle may be improved by extending the tropism of the adenoviral vectors to attachment receptors that are highly expressed in mature myofibers. In this study, we have investigated whether an extended tropism adenoviral vector which additionally attaches to the broadly expressed heparan-containing receptors (AdPK) can bypass the maturation-dependent adenoviral transducibility of mouse skeletal muscle.

METHODS

The adenoviral vector AdPK carrying the LacZ gene was evaluated as a gene delivery vehicle in mouse skeletal muscle at different maturities in vitro and in vivo. The viral transduction efficiencies were determined by histochemical and ONPG analysis of the beta-galactosidase activity level.

RESULTS

Higher transduction efficiencies were detected in immature muscle from normal mice, and in mature muscle from merosin-deficient dy/dy mice (carrying myofibers with an impaired extracellular matrix) and dystrophin-deficient mdx mice (showing a high level of myoblast activity) when compared to mature muscle from normal mice.

CONCLUSION

Despite the enhanced attachment characteristics, the extended tropism adenoviral vector is, similarly to the wild-type adenoviral vector in previous studies, still hindered by both a protective extracellular matrix and the diminished myoblast-mediation in mature muscle.

Activation of the JNK pathway is important for cardiomyocyte death in response to simulated ischemia

Multiple signaling pathways, including the c-Jun N-terminal kinase (JNK) pathway, are activated in myocardial ischemia and reperfusion (MI/R) and correlate with cell death. However, the role of the JNK pathway in MI/R-induced cell death is poorly understood. In a rabbit model, we found that ischemia followed by reperfusion resulted in JNK activation which could be detected in cytosol as well as in mitochondria. To address the functional role of the JNK activation, we examined the consequences of blockade of JNK activation in isolated cardiomyocytes under conditions of simulated ischemia. The JNK activity was stimulated approximately sixfold by simulated ischemia and reperfusion (simulated MI). When a dominant negative mutant of JNK kinase-2 (dnJNKK2), an upstream regulator of JNK, and JNK-interacting protein-1 (JIP-1) were expressed in myocytes by recombinant adenovirus, the activation of JNK by simulated MI was reduced 53%. Furthermore, the TNFalpha-activated JNK activity in H9c2 cells was completely abolished by dnJNKK2 and JIP-1. In correlation, when dnJNKK2 and JIP-1 were expressed in cardiomyocytes, both constructs significantly reduced cell death after simulated MI compared to vector controls. We conclude that activation of the JNK cascade is important for cardiomyocyte death in response to simulated ischemia.

Cationic lipids and polymers are able to enhance adenoviral infection of cultured mouse myotubes

Adenovirus-mediated gene transfer has been used to promote efficient expression of various reporter and therapeutic transgenes such as minidystrophin in skeletal muscle tissue. However, down-regulation of the adenovirus internalisation receptors, alpha(v)/beta3 and alpha(v)beta5, in adult myofibres and in mature cultured myotubes makes them less susceptible to infection than neonatal muscle or cultured myoblasts. It has been reported elsewhere that adenoviral transduction of cells that are normally refractory to infection can be enhanced by complexing virus particles with cationic lipids or cationic polymers. In this study we describe increased levels of adenovirus-mediated transduction of cultured C2C12 myotubes, when the vector is complexed with either of the cationic lipids Lipofectamine or 1,3-dioleoyloxy-2-(6-carboxyspermyl)propylamide (DOSPER) or the cationic polymer polyethylenimine. The presence of polycations allowed a smaller dose of adenovirus vector to be used to attain the same level of infection seen with adenovirus alone, which has important relevance to future in vivo studies. Electron microscopic analysis of adenovirus/polycation complexes showed large aggregates as opposed to single adenovirus particles in the absence of polycations. Finally, by complexing adenovirus particles with polycations, partial protection against the neutralising effect of adenovirus antiserum was observed.

Muscle maturation: implications for gene therapy

Skeletal muscle is a promising target tissue for gene therapy, for both muscle and non-muscle disorders. A variety of methods have been studied to transfer genes into skeletal muscle, including retroviral, adenoviral and herpes simplex viral vectors. However, various factors impede muscle-based viral gene therapy. Here, we discuss why some viral vectors cannot efficiently transduce mature muscle fibers, and describe some new approaches to overcome this barrier.

Viral gene delivery to skeletal muscle: insights on maturation-dependent loss of fiber infectivity for adenovirus and herpes simplex type 1 viral vectors

The mechanisms causing age-dependent loss of muscle fiber infectivity observed in vivo for both adenoviral (Ad) and herpes simplex virus type 1 (HSV-1) gene delivery vectors remain poorly understood. Here we investigate the possible bases for this phenomenon using the novel application of enzymatically isolated, viable, single muscle fibers. We show that maturation-dependent loss of fiber infectivity is recapitulated in single fibers, and, thus, is not solely due to host immune response. Using localized irradiation of muscle in vivo, we show data suggesting that Ad infectivity of differentiated myofibers depends, at least in part, on myoblasts to mediate fiber transduction. On the other hand, infection of single fibers by HSV-1 is not affected by irradiation. Using confocal microscopy, we show that the basal lamina of myogenic cells efficiently infected by HSV-1 is structurally less organized than that of fibers resistant to infection by HSV-1. As well, we show that single myofibers isolated from adult, basal lamina-defective mice (merosin-deficient, dy/dy) are at least 10-fold more susceptible to infection by HSV-1 than are myofibers isolated from control mice. Together, these observations support the hypothesis that the basal lamina acts as a physical barrier to HSV-1 infection of mature muscle.

Rabies viral antigen in human tongues and salivary glands

Lingual and major salivary tissue samples from three cases of rabies were stained with the immunoperoxidase (ABC) technique. All tissue blocks had been embedded in paraffin 4-10 years before. The first antibody used was monoclonal antirabies nucleocapsin (N) mouse antibody (HAM). Four out of five pieces of tongue from two cases showed a large amount of granular staining indicating rabies antigen (RVAg) inside serous glandular cells, terminal nerves, muscle cells and covering epithelial cells including taste cells. In the tissue probes from the third case only minimal granular staining was found, probably due to complete absence of the serous gland. In contrast to the tongue, only a little weakly reacting material was found in 4 out of 9 probes of salivary gland, either in acini or in nerve fibres. The amount of RVAg is evidently much greater in the human tongue than in major salivary glands, whereas major salivary glands from infected dogs, foxes and skunks reportedly contain much RVAg. As the human tongue's serous gland appears to be a preferred location for RVAg, it may be a source of oral infection.

Production of defective virus by terminally differentiated myotubes infected with Rous sarcoma virus

The generally accepted concept that the replication of Rous sarcoma virus (RSV) is dependent on host cell DNA synthesis was reexamined. As the host we used terminally differentiated myotubes (MT), in which no cellular DNA synthesis is observed. As an extension of our previous study which indicated that RSV-infected MT produce various virus components, we examined viral particles produced by infected MT. Electron microscopy showed presence of viral particles released from infected MT. Immunoprecipitation analysis revealed that these particles contained an equal amount of the gag but a decreased amount of the env proteins as compared with the particles from infected chicken embryo fibroblasts (CEF). Consequently, viral particles from infected MT had an infectivity only 6% of that of particles from infected CEF cells. In a parallel experiment, we microinjected molecularly cloned RSV DNA into MT. In contrast to the infection mediated by viral particles, both MT and CEF cells produced the same amount of infectious particles when microinjected with viral DNA. We conclude that RSV replicates in the complete absence of host DNA synthesis, though infectivity of the progeny virus depends on the initial condition of the infection.