Viral myocarditis in combination with genetic cardiomyopathy as a cause of sudden death. An autopsy series.

Sudden cardiac death (SCD) is a major public health issue worldwide. In the young (< 40 years of age), genetic cardiomyopathies and viral myocarditis, sometimes in combination, are the most frequent, but underestimated, causes of SCD. Molecular autopsy is essential for prevention. Several studies have shown an association between genetic cardiomyopathies and viral myocarditis, which is probably underestimated due to insufficient post-mortem investigations. We report on four autopsy cases illustrating the pathogenesis of these combined pathologies. In two cases, a genetic hypertrophic cardiomyopathy was diagnosed in combination with Herpes Virus Type 6 (HHV6) and/or Parvovirus-B19 (PVB19) in the heart. In the third case, autopsy revealed a dilated cardiomyopathy and virological analyses revealed acute myocarditis caused by three viruses: PVB19, HHV6 and Epstein-Barr virus. Genetic analyses revealed a mutation in the gene coding for desmin. The fourth case illustrated a channelopathy and a PVB19/HHV6 coinfection. Our four cases illustrate the highly probable deleterious role of cardiotropic viruses in the occurrence of SCD in subjects with genetic cardiomyopathies. We discuss the pathogenetic link between viral myocarditis and genetic cardiomyopathy. Molecular autopsy is essential in prevention of these SCD, and a close collaboration between cardiologists, pathologists, microbiologists and geneticians is mandatory.

Generation of human induced pluripotent stem cell lines from five patients with Myofibrillar myopathy carrying different heterozygous mutations in the DES gene.

Myofibrillar myopathy (MFM) is a rare genetic disorder characterized by muscular dystrophy that is often associated with cardiac disease. This disease is caused by mutations in several genes, among them DES (encoding desmin) is the most frequently affected. Peripheral blood mononuclear cells from 5 different MFM patients with different DES mutations were reprogrammed into induced pluripotent stem cells (IPSC) using non-integrative vectors. For each patient, one IPSC clone was selected and demonstrated pluripotency hallmarks without genomic abnormalities. SNP profiles were identical to the cells of origin and all the clones have the capacity to differentiate into all three germ layers.

Critical contribution of mitochondria in the development of cardiomyopathy linked to desmin mutation.

BACKGROUND: Beyond the observed alterations in cellular structure and mitochondria, the mechanisms linking rare genetic mutations to the development of heart failure in patients affected by desmin mutations remain unclear due in part, to the lack of relevant human cardiomyocyte models. METHODS: To shed light on the role of mitochondria in these mechanisms, we investigated cardiomyocytes derived from human induced pluripotent stem cells carrying the heterozygous DESE439K mutation that were either isolated from a patient or generated by gene editing. To increase physiological relevance, cardiomyocytes were either cultured on an anisotropic micropatterned surface to obtain elongated and aligned cardiomyocytes, or as a cardiac spheroid to create a micro-tissue. Moreover, when applicable, results from cardiomyocytes were confirmed with heart biopsies of suddenly died patient of the same family harboring DESE439K mutation, and post-mortem heart samples from five control healthy donors. RESULTS: The heterozygous DESE439K mutation leads to dramatic changes in the overall cytoarchitecture of cardiomyocytes, including cell size and morphology. Most importantly, mutant cardiomyocytes display altered mitochondrial architecture, mitochondrial respiratory capacity and metabolic activity reminiscent of defects observed in patient's heart tissue. Finally, to challenge the pathological mechanism, we transferred normal mitochondria inside the mutant cardiomyocytes and demonstrated that this treatment was able to restore mitochondrial and contractile functions of cardiomyocytes. CONCLUSIONS: This work highlights the deleterious effects of DESE439K mutation, demonstrates the crucial role of mitochondrial abnormalities in the pathophysiology of desmin-related cardiomyopathy, and opens up new potential therapeutic perspectives for this disease.

Generation of induced pluripotent stem cell line (TMOi001-A-11) carrying a homozygous deletion in the synemin gene using CRISPR/Cas9.

A number of genetic variants in the SYNM gene encoding for the intermediate filament synemin have been reported in patients with cardiomyopathies, skeletal myopathies, cancer and certain neurodegenerative disorders. To better understand its role, we generated a human induced pluripotent stem cell line with a homozygous deletion in the SYNM gene by CRISPR/Cas9 genome editing. The synemin-knockout human induced pluripotent stem cells exhibit typical morphology of pluripotent cells, expression of pluripotency markers, normal karyotype and differentiation capacity in the three germ layers. This line will allow us to investigate the role of synemin in cardiomyopathy upon differentiation into beating cardiomyocytes.

Anisotropic dense collagen hydrogels with two ranges of porosity to mimic the skeletal muscle extracellular matrix.

Despite the crucial role of the extracellular matrix (ECM) in the organotypic organization and function of skeletal muscles, most 3D models do not mimic its specific characteristics, namely its biochemical composition, stiffness, anisotropy, and porosity. Here, a novel 3D in vitro model of muscle ECM was developed reproducing these four crucial characteristics of the native ECM. An anisotropic hydrogel mimicking the muscle fascia was obtained thanks to unidirectional 3D printing of dense collagen with aligned collagen fibrils. The space between the different layers was tuned to generate an intrinsic network of pores (100 µm) suitable for nutrient and oxygen diffusion. By modulating the gelling conditions, the mechanical properties of the construct reached those measured in the physiological muscle ECM. This artificial matrix was thus evaluated for myoblast differentiation. The addition of large channels (600 µm) by molding permitted to create a second range of porosity suitable for cell colonization without altering the physical properties of the hydrogel. Skeletal myoblasts embedded in Matrigel®, seeded within the channels, organized in 3D, and differentiated into multinucleated myotubes. These results show that porous and anisotropic dense collagen hydrogels are promising biomaterials to model skeletal muscle ECM.

Generation of an Adequate Perfusion Network within Dense Collagen Hydrogels Using Thermoplastic Polymers as Sacrificial Matrix to Promote Cell Viability.

Dense collagen hydrogels are promising biomaterials for several tissue-engineering applications. They exhibit high mechanical properties, similar to physiological extracellular matrices, and do not shrink under cellular activity. However, they suffer from several drawbacks, such as weak nutrient and O2 diffusion, impacting cell survival. Here, we report a novel strategy to create a perfusion system within dense and thick collagen hydrogels to promote cell viability. The 3D printing of a thermoplastic filament (high-impact polystyrene, HIPS) with a three-wave shape is used to produce an appropriate sacrificial matrix. The HIPS thermoplastic polymer allows for good shape fidelity of the filament and does not collapse under the mechanical load of the collagen solution. After the collagen gels around the filament and dissolves, a channel is generated, allowing for adequate and rapid hydrogel perfusion. The dissolution process does not alter the collagen hydrogel's physical or chemical properties, and the perfusion is associated with an increased fibroblast survival. Here, we report the novel utilization of thermoplastics to generate a perfusion network within biomimetic collagen hydrogels.

Desmin Modulates Muscle Cell Adhesion and Migration.

Cellular adhesion and migration are key functions that are disrupted in numerous diseases. We report that desmin, a type-III muscle-specific intermediate filament, is a novel cell adhesion regulator. Expression of p.R406W mutant desmin, identified in patients with desmin-related myopathy, modified focal adhesion area and expression of adhesion-signaling genes in myogenic C2C12 cells. Satellite cells extracted from desmin-knock-out (DesKO) and desmin-knock-in-p.R405W (DesKI-R405W) mice were less adhesive and migrated faster than those from wild-type mice. Moreover, we observed mislocalized and aggregated vinculin, a key component of cell adhesion, in DesKO and DesKI-R405W muscles. Vinculin expression was also increased in desmin-related myopathy patient muscles. Together, our results establish a novel role for desmin in cell-matrix adhesion, an essential process for strength transmission, satellite cell migration and muscle regeneration. Our study links the patho-physiological mechanisms of desminopathies to adhesion/migration defects, and may lead to new cellular targets for novel therapeutic approaches.

3D models of dilated cardiomyopathy: Shaping the chemical, physical and topographical properties of biomaterials to mimic the cardiac extracellular matrix.

The pathophysiology of dilated cardiomyopathy (DCM), one major cause of heart failure, is characterized by the dilation of the heart but remains poorly understood because of the lack of adequate in vitro models. Current 2D models do not allow for the 3D organotypic organization of cardiomyocytes and do not reproduce the ECM perturbations. In this review, the different strategies to mimic the chemical, physical and topographical properties of the cardiac tissue affected by DCM are presented. The advantages and drawbacks of techniques generating anisotropy required for the cardiomyocytes alignment are discussed. In addition, the different methods creating macroporosity and favoring organotypic organization are compared. Besides, the advances in the induced pluripotent stem cells technology to generate cardiac cells from healthy or DCM patients will be described. Thanks to the biomaterial design, some features of the DCM extracellular matrix such as stiffness, porosity, topography or chemical changes can impact the cardiomyocytes function in vitro and increase their maturation. By mimicking the affected heart, both at the cellular and at the tissue level, 3D models will enable a better understanding of the pathology and favor the discovery of novel therapies.

Absence of Desmin Results in Impaired Adaptive Response to Mechanical Overloading of Skeletal Muscle.

Background: Desmin is a muscle-specific protein belonging to the intermediate filament family. Desmin mutations are linked to skeletal muscle defects, including inherited myopathies with severe clinical manifestations. The aim of this study was to examine the role of desmin in skeletal muscle remodeling and performance gain induced by muscle mechanical overloading which mimics resistance training. Methods: Plantaris muscles were overloaded by surgical ablation of gastrocnemius and soleus muscles. The functional response of plantaris muscle to mechanical overloading in desmin-deficient mice (DesKO, n = 32) was compared to that of control mice (n = 36) after 7-days or 1-month overloading. To elucidate the molecular mechanisms implicated in the observed partial adaptive response of DesKO muscle, we examined the expression levels of genes involved in muscle growth, myogenesis, inflammation and oxidative energetic metabolism. Moreover, ultrastructure and the proteolysis pathway were explored. Results: Contrary to control, absolute maximal force did not increase in DesKO muscle following 1-month mechanical overloading. Fatigue resistance was also less increased in DesKO as compared to control muscle. Despite impaired functional adaptive response of DesKO mice to mechanical overloading, muscle weight and the number of oxidative MHC2a-positive fibers per cross-section similarly increased in both genotypes after 1-month overloading. However, mechanical overloading-elicited remodeling failed to activate a normal myogenic program after 7-days overloading, resulting in proportionally reduced activation and differentiation of muscle stem cells. Ultrastructural analysis of the plantaris muscle after 1-month overloading revealed muscle fiber damage in DesKO, as indicated by the loss of sarcomere integrity and mitochondrial abnormalities. Moreover, the observed accumulation of autophagosomes and lysosomes in DesKO muscle fibers could indicate a blockage of autophagy. To address this issue, two main proteolysis pathways, the ubiquitin-proteasome system and autophagy, were explored in DesKO and control muscle. Our results suggested an alteration of proteolysis pathways in DesKO muscle in response to mechanical overloading. Conclusion: Taken together, our results show that mechanical overloading increases the negative impact of the lack of desmin on myofibril organization and mitochondria. Furthermore, our results suggest that under these conditions, the repairing activity of autophagy is disturbed. Consequently, force generation is not improved despite muscle growth, suggesting that desmin is required for a complete response to resistance training in skeletal muscle.

Impaired Binding to Junctophilin-2 and Nanostructural Alteration in CPVT Mutation.

[Figure: see text].

Aldehyde dehydrogenases contribute to skeletal muscle homeostasis in healthy, aging, and Duchenne muscular dystrophy patients.

BACKGROUND: Aldehyde dehydrogenases (ALDHs) are key players in cell survival, protection, and differentiation via the metabolism and detoxification of aldehydes. ALDH activity is also a marker of stem cells. The skeletal muscle contains populations of ALDH-positive cells amenable to use in cell therapy, whose distribution, persistence in aging, and modifications in myopathic context have not been investigated yet. METHODS: The Aldefluor® (ALDEF) reagent was used to assess the ALDH activity of muscle cell populations, whose phenotypic characterizations were deepened by flow cytometry. The nature of ALDH isoenzymes expressed by the muscle cell populations was identified in complementary ways by flow cytometry, immunohistology, and real-time PCR ex vivo and in vitro. These populations were compared in healthy, aging, or Duchenne muscular dystrophy (DMD) patients, healthy non-human primates, and Golden Retriever dogs (healthy vs. muscular dystrophic model, Golden retriever muscular dystrophy [GRMD]). RESULTS: ALDEF+ cells persisted through muscle aging in humans and were equally represented in several anatomical localizations in healthy non-human primates. ALDEF+ cells were increased in dystrophic individuals in humans (nine patients with DMD vs. five controls: 14.9 ± 1.63% vs. 3.6 ± 0.39%, P = 0.0002) and dogs (three GRMD dogs vs. three controls: 10.9 ± 2.54% vs. 3.7 ± 0.45%, P = 0.049). In DMD patients, such increase was due to the adipogenic ALDEF+ /CD34+ populations (11.74 ± 1.5 vs. 2.8 ± 0.4, P = 0.0003), while in GRMD dogs, it was due to the myogenic ALDEF+ /CD34- cells (3.6 ± 0.6% vs. 1.03 ± 0.23%, P = 0.0165). Phenotypic characterization associated the ALDEF+ /CD34- cells with CD9, CD36, CD49a, CD49c, CD49f, CD106, CD146, and CD184, some being associated with myogenic capacities. Cytological and histological analyses distinguished several ALDH isoenzymes (ALDH1A1, 1A2, 1A3, 1B1, 1L1, 2, 3A1, 3A2, 3B1, 3B2, 4A1, 7A1, 8A1, and 9A1) expressed by different cell populations in the skeletal muscle tissue belonging to multinucleated fibres, or myogenic, endothelial, interstitial, and neural lineages, designing them as potential new markers of cell type or of metabolic activity. Important modifications were noted in isoenzyme expression between healthy and DMD muscle tissues. The level of gene expression of some isoenzymes (ALDH1A1, 1A3, 1B1, 2, 3A2, 7A1, 8A1, and 9A1) suggested their specific involvement in muscle stability or regeneration in situ or in vitro. CONCLUSIONS: This study unveils the importance of the ALDH family of isoenzymes in the skeletal muscle physiology and homeostasis, suggesting their roles in tissue remodelling in the context of muscular dystrophies.

Polyacrylamide Hydrogels with Rigidity-Independent Surface Chemistry Show Limited Long-Term Maintenance of Pluripotency of Human Induced Pluripotent Stem Cells on Soft Substrates.

In general, cells are cultured and adapted to the in vitro rigidities of plastic or glass ranging between 1 and 10 GPa, which is very far from physiological values that are mostly in the kilopascal range. Stem cells however show a high sensitivity to the rigidity of their culture environment, which impacts their differentiation program. Here, we address the impact of rigidity on the long-term maintenance of pluripotency in human induced pluripotent stem cells (hiPSCs) to determine whether soft substrates could provide a new standard for hiPSC expansion and maintenance. To do this, we set up a fabrication process of polyacrylamide-based culture supports with a rigidity-decoupled surface chemistry. Soft elastic substrates with uniform and reproducible physicochemical properties were designed. The maintenance of pluripotency of two hiPSCs lines on substrates with stiffnesses ranging from 3 to 25 kPa was studied with an identical chemical coating consisting of a truncated recombinant vitronectin with defined surface density. Based on the analysis of cellular adhesion, survival, growth kinetics, three-dimensional distribution, and gene and protein expressions, we demonstrate that below 25 kPa hiPSCs do not maintain pluripotency on long-term culture, while pluripotency and self-renewal capacities are maintained above 25 kPa. In contrast to previous studies, no drift toward a specific germ line lineage was revealed. On soft substrates, cell colonies started to grow in three-dimensional (3D), suggesting that softness allows cells to limit contact with the synthetic matrix and to build their own microenvironment. These observations drastically limit the benefit of using standardized soft substrates to expand hiPSCs, at least with the current culture conditions. The development of a robust technology for the design of soft substrates nevertheless opens up perspectives to fine-tune physicochemical properties of the culture environment in addition to or in replacement of soluble growth factors to finely direct cell fate.

Nanofibrous clinical-grade collagen scaffolds seeded with human cardiomyocytes induces cardiac remodeling in dilated cardiomyopathy.

Limited data are available on the effects of stem cells in non-ischemic dilated cardiomyopathy (DCM). Since the diffuse nature of the disease calls for a broad distribution of cells, this study investigated the scaffold-based delivery of human induced pluripotent stem cell-derived cardiomyocytes (hiPS-CM) in a mouse model of DCM. Nanofibrous scaffolds were produced using a clinical grade atelocollagen which was electrospun and cross-linked under different conditions. As assessed by scanning electron microscopy and shearwave elastography, the optimum crosslinking conditions for hiPS-CM colonization proved to be a 10% concentration of citric acid crosslinking agent and 150 min of post-electrospinning baking. Acellular collagen scaffolds were first implanted in both healthy mice and those with induced DCM by a cardiac-specific invalidation of serum response factor (SRF). Seven and fourteen days after implantation, the safety of the scaffold was demonstrated by echocardiography and histological assessments. The subsequent step of implantation of the scaffolds seeded with hiPS-CM in DCM induced mice, using cell-free scaffolds as controls, revealed that after fourteen days heart function decreased in controls while it remained stable in the treated mice. This pattern was associated with an increased number of endothelial cells, in line with the greater vascularity of the scaffold. Moreover, a lesser degree of fibrosis consistent with the upregulation of several genes involved in extracellular matrix remodeling was observed. These results support the interest of the proposed hiPS-CM seeded electrospun scaffold for the stabilization of the DCM outcome with potential for its clinical use in the future.

Mechanical Overloading Increases Maximal Force and Reduces Fragility in Hind Limb Skeletal Muscle from Mdx Mouse.

There is fear that mechanical overloading (OVL; ie, high-force contractions) accelerates Duchenne muscular dystrophy. Herein, we determined whether short-term OVL combined with wheel running, short-term OVL combined with irradiation, and long-term OVL are detrimental for hind limb mdx mouse muscle, a murine model of Duchene muscular dystrophy exhibiting milder dystrophic features. OVL was induced by the surgical ablation of the synergic muscles of the plantaris muscle, a fast muscle susceptible to contraction-induced muscle damage in mdx mice. We found that short-term OVL combined with wheel and long-term OVL did not worsen the deficit in specific maximal force (ie, absolute maximal force normalized to muscle size) and histological markers of muscle damage (percentage of regenerating fibers and fibrosis) in mdx mice. Moreover, long-term OVL did not increase the alteration in calcium homeostasis and did not deplete muscle cell progenitors expressing Pax 7 in mdx mice. Irradiation before short-term OVL, which is believed to inhibit muscle regeneration, was not more detrimental to mdx than control mice. Interestingly, short-term OVL combined with wheel and long-term OVL markedly improved the susceptibility to contraction-induced damage, increased absolute maximal force, induced hypertrophy, and promoted a slower, more oxidative phenotype. Together, these findings indicate that OVL is beneficial to mdx muscle, and muscle regeneration does not mask the potentially detrimental effect of OVL.

Effect of voluntary physical activity initiated at age 7 months on skeletal hindlimb and cardiac muscle function in mdx mice of both genders.

INTRODUCTION: The effects of voluntary activity initiated in adult mdx (C57BL/10ScSc-DMD(mdx) /J) mice on skeletal and cardiac muscle function have not been studied extensively. METHODS: We studied the effects of 3 months of voluntary wheel running initiated at age 7 months on hindlimb muscle weakness, increased susceptibility to muscle contraction-induced injury, and left ventricular function in mdx mice. RESULTS: We found that voluntary wheel running did not worsen the deficit in force-generating capacity and the force drop after lengthening contractions in either mdx mouse gender. It increased the absolute maximal force of skeletal muscle in female mdx mice. Moreover, it did not affect left ventricular function, structural heart dimensions, cardiac gene expression of inflammation, fibrosis, or remodeling markers. CONCLUSION: These results indicate that voluntary activity initiated at age 7 months had no detrimental effects on skeletal or cardiac muscles in either mdx mouse gender.

Investigating the role of GXXXG motifs in helical folding and self-association of plasticins, Gly/Leu-rich antimicrobial peptides.

Plasticins (PTC) are dermaseptin-related antimicrobial peptides characterized by a large number of leucine and glycine residues arranged in GXXXG motifs that are often described to promote helix association within biological membranes. We report the structure and interaction properties of two plasticins, PTC-B1 from Phyllomedusa bicolor and a cationic analog of PTC-DA1 from Pachymedusa dacnicolor, which exhibit membrane-lytic activities on a broad range of microorganisms. Despite a high number of glycine, CD and NMR spectroscopy show that the two plasticins adopt mainly alpha-helical conformations in a wide variety of environments such as trifluoroethanol, detergent micelles and lipid vesicles. In DPC and SDS, plasticins adopt well-defined helices that lie parallel to the micelle surface, all glycine residues being located on the solvent-exposed face. Spectroscopic data and cross-linking experiments indicate that the GXXXG repeats in these amphipathic helices do not provide a strong oligomerization interface, suggesting a different role from GXXXG motifs found in transmembrane helices.

Long-term functional benefits of human embryonic stem cell-derived cardiac progenitors embedded into a fibrin scaffold.

BACKGROUND: Cardiac-committed cells and biomimetic scaffolds independently improve the therapeutic efficacy of stem cells. In this study we tested the long-term effects of their combination. METHODS: Eighty immune-deficient rats underwent permanent coronary artery ligation. Five to 7 weeks later, those with an echocardiographically measured ejection fraction (EF) ≤55% were re-operated on and randomly allocated to receive a cell-free fibrin patch (n = 25), a fibrin patch loaded with 700,000 human embryonic stem cells (ESC) pre-treated to promote early cardiac differentiation (SSEA-1(+) progenitors [n = 30]), or to serve as sham-operated animals (n = 25). Left ventricular function was assessed by echocardiography at baseline and every month thereafter until 4 months. Hearts were then processed for assessment of fibrosis and angiogenesis and a 5-component heart failure score was constructed by integrating the absolute change in left ventricular end-systolic volume (LVESV) between 4 months and baseline, and the quantitative polymerase chain reaction (qPCR)-based expression of natriuretic peptides A and B, myosin heavy chain 7 and periostin. All data were recorded and analyzed in a blinded manner. RESULTS: The cell-treated group consistently yielded better functional outcomes than the sham-operated group (p = 0.002 for EF; p = 0.01 for LVESV). Angiogenesis in the border zone was also significantly greater in the cell-fibrin group (p = 0.006), which yielded the lowest heart failure score (p = 0.04 vs sham). Engrafted progenitors were only detected shortly after transplantation; no grafted cells were identified after 4 months. There was no teratoma identified. CONCLUSIONS: A fibrin scaffold loaded with ESC-derived cardiac progenitors resulted in sustained improvement in contractility and attenuation of remodeling without sustained donor cell engraftment. A paracrine effect, possibly on innate reparative responses, is a possible mechanism for this enduring effect.

Synemin acts as a regulator of signalling molecules during skeletal muscle hypertrophy.

Synemin, a type IV intermediate filament (IF) protein, forms a bridge between IFs and cellular membranes. As an A-kinase-anchoring protein, it also provides temporal and spatial targeting of protein kinase A (PKA). However, little is known about its functional roles in either process. To better understand its functions in muscle tissue, we generated synemin-deficient (Synm(-) (/-)) mice. Synm(-) (/-) mice displayed normal development and fertility but showed a mild degeneration and regeneration phenotype in myofibres and defects in sarcolemma membranes. Following mechanical overload, Synm(-) (/-) mice muscles showed a higher hypertrophic capacity with increased maximal force and fatigue resistance compared with control mice. At the molecular level, increased remodelling capacity was accompanied by decreased myostatin (also known as GDF8) and atrogin (also known as FBXO32) expression, and increased follistatin expression. Furthermore, the activity of muscle-mass control molecules (the PKA RIIα subunit, p70S6K and CREB1) was increased in mutant mice. Finally, analysis of muscle satellite cell behaviour suggested that the absence of synemin could affect the balance between self-renewal and differentiation of these cells. Taken together, our results show that synemin is necessary to maintain membrane integrity and regulates signalling molecules during muscle hypertrophy.

Myofiber androgen receptor promotes maximal mechanical overload-induced muscle hypertrophy and fiber type transition in male mice.

The first aim of this study was to examine the role of myofiber androgen receptor (AR) in male mice on muscle performance gain and remodeling-induced muscle mechanical overloading (OVL) that mimics resistance training. The response of OVL in mice in which AR is selectively ablated in myofibers (AR(skm-/y)) was compared with that of wild-type (WT) mice. In addition, we determined whether the synthetic anabolic androgen nandrolone administration affects the OVL response. We found that OVL increased absolute maximal force and fatigue resistance in both mouse genotypes (P < .05). However, the absolute maximal force increased more in AR(skm-/y) mice as compared with WT mice (+88% vs +63%) (P < .05). Muscle weight increased less in response to OVL in AR(skm-/y) mice (+54%) than in WT mice (+115%) (P < .05). The fiber number per cross-section similarly increased in both mouse genotypes after OVL (P < .05). In contrast to WT mice, the diameter of the fibers expressing myosin heavy chain (MHC)-2x decreased after OVL in AR(skm-/y) mice (P < .05). The MHC-2b to MHC-2a fiber type transition in response to OVL was reduced in AR(skm-/y) mice as compared with WT mice (P < .05). Finally, nandrolone administration during OVL did not further improve absolute maximal force and fatigue resistance and markedly alter muscle remodeling in both mouse genotypes. Together, our results indicate that myofiber AR is required for a complete response to OVL and that exogenous androgens do not increase muscle performance during intensive remodeling in male mice.