Novel application of flow cytometry: determination of muscle fiber types and protein levels in whole murine skeletal muscles and heart

CAP2 deficiency delays myofibril actin cytoskeleton differentiation and disturbs skeletal muscle architecture and function

Actin filaments (F-actin) are key components of sarcomeres, the basic contractile units of skeletal muscle myofibrils. A crucial step during myofibril differentiation is the sequential exchange of α-actin isoforms from smooth muscle (α-SMA) and cardiac (α-CAA) to skeletal muscle α-actin (α-SKA) that, in mice, occurs during early postnatal life. This "α-actin switch" requires the coordinated activity of actin regulators because it is vital that sarcomere structure and function are maintained during differentiation. The molecular machinery that controls the α-actin switch, however, remains enigmatic. Cyclase-associated proteins (CAP) are a family of actin regulators with largely unknown physiological functions. We here report a function for CAP2 in regulating the α-actin exchange during myofibril differentiation. This α-actin switch was delayed in systemic CAP2 mutant mice, and myofibrils remained in an undifferentiated stage at the onset of the often excessive voluntary movements in postnatal mice. The delay in the α-actin switch coincided with the onset of motor function deficits and histopathological changes including a high frequency of type IIB ring fibers. Our data suggest that subtle disturbances of postnatal F-actin remodeling are sufficient for predisposing muscle fibers to form ring fibers. Cofilin2, a putative CAP2 interaction partner, has been recently implicated in myofibril actin cytoskeleton differentiation, and the myopathies in cofilin2 and CAP2 mutant mice showed striking similarities. We therefore propose a model in which CAP2 and cofilin2 cooperate in actin regulation during myofibril differentiation.

Postnatal training of 129/Sv mice confirms the long-term influence of early exercising on the motor properties of mice

A previous study showed that motor experiences during critical periods of development durably affect the motor properties of adult C57BL/6J mice. However, dependence on early environmental features may vary with the genetic profile. To evaluate the contribution of the genetic background on external influences to motricity, we performed the same experiment in a 129/Sv mouse strain that show a strongly different motor profile. Mice were subjected to endurance training (enriched environment or forced treadmill), hypergravity (chronic centrifugation), or simulated microgravity (hindlimb unloading) between postnatal days 10 and 30. They were then returned to standard housing until testing at the age of nine months. The endurance-trained mice showed a fast-slow shift in the deep zone of the tibialis. In addition, mice reared in the enriched environment showed a modified gait and body posture, and improved performance on the rotarod, whereas forced treadmill training did not affect motor output. Hypergravity induced a fast-slow shift in the superficial zone of the tibialis, with no consequence on motor output. Hindlimb unloading provoked an increased percentage of immature hybrid fibres in the tibialis and a shift in the soleus muscle. When compared with similarly reared C57BL/6J mice, 129/Sv mice showed qualitative differences attributable to the lower efficiency of early training due to their lower basal motor activity level. Nevertheless, the results are essentially consistent in both strains, and support the hypothesis that early motor experience influences the muscle phenotype and motor output.

Effects of exercise and dietary epigallocatechin gallate and β-alanine on skeletal muscle in aged mice

Aging leads to sarcopenia and loss of physical function. We examined whether voluntary wheel running, when combined with dietary supplementation with (-)-epigallocatechin-3-gallate (EGCG) and β-alanine (β-ALA), could improve muscle function and alter gene expression in the gastrocnemius of aged mice. Seventeen-month-old BALB/cByJ mice were given access to a running wheel or remained sedentary for 41 days while receiving either AIN-93M (standard feed) or AIN-93M containing 1.5 mg·kg(-1) EGCG and 3.43 mg·kg(-1) β-ALA. Mice underwent tests over 11 days from day 29 to day 39 of the study period, including muscle function testing (grip strength, treadmill exhaustive fatigue, rotarod). Following a rest day, mice were euthanized and gastrocnemii were collected for analysis of gene expression by quantitative PCR. Voluntary wheel running (VWR) improved rotarod and treadmill exhaustive fatigue performance and maintained grip strength in aged mice, while dietary intervention had no effect. VWR increased gastrocnemius expression of several genes, including those encoding interleukin-6 (Il6, p = 0.001), superoxide dismutase 1 (Sod1, p = 0.046), peroxisome proliferator-activated receptor gamma coactivator 1-α (Ppargc1a, p = 0.013), forkhead box protein O3 (Foxo3, p = 0.005), and brain-derived neurotrophic factor (Bdnf, p = 0.008), while reducing gastrocnemius levels of the lipid peroxidation marker 4-hydroxynonenal (p = 0.019). Dietary intervention alone increased gastrocnemius expression of Ppargc1a (p = 0.033) and genes encoding NAD-dependent protein deacetylase sirtuin-1 (Sirt1, p = 0.039), insulin-like growth factor I (Igf1, p = 0.003), and macrophage marker CD11b (Itgam, p = 0.016). Exercise and a diet containing β-ALA and EGCG differentially regulated gene expression in the gastrocnemius of aged mice, while VWR but not dietary intervention improved muscle function. We found no synergistic effects between dietary intervention and VWR.

In vivo cell tracking of mouse embryonic myoblasts and fast fibers during development

Fast and slow TnI are co-expressed in E11.5 embryos, and fast TnI is present from the very beginning of myogenesis. A novel green fluorescent protein (GFP) reporter mouse lines (FastTnI/GFP lines) that carry the primary and secondary enhancer elements of the mouse fast troponin I (fast TnI), in which reporter expression correlates precisely with distribution of the endogenous fTnI protein was generated. Using the FastTnI/GFP mouse model, we characterized the early myogenic events in mice, analyzing the migration of GFP+ myoblasts, and the formation of primary and secondary myotubes in transgenic embryos. Interestingly, we found that the two contractile fast and slow isoforms of TnI are expressed during the migration of myoblasts from the somites to the limbs and body wall, suggesting that both participate in these events. Since no sarcomeres are present in myoblasts, we speculate that the function of fast TnI in early myogenesis is, like Myosin and Tropomyosin, to participate in cell movement during the initial myogenic stages. genesis

Coronary adventitial cells are linked to perivascular cardiac fibrosis via TGFβ1 signaling in the mdx mouse model of Duchenne muscular dystrophy

In Duchenne muscular dystrophy (DMD), progressive accumulation of cardiac fibrosis promotes heart failure. While the cellular origins of fibrosis in DMD hearts remain enigmatic, fibrotic tissue conspicuously forms near the coronary adventitia. Therefore, we sought to characterize the role of coronary adventitial cells in the formation of perivascular fibrosis. Utilizing the mdx model of DMD, we have identified a population of Sca1+, PDGFRα+, CD31-, and CD45- coronary adventitial cells responsible for perivascular fibrosis. Histopathology of dystrophic hearts revealed that Sca1+ cells extend from the adventitia and occupy regions of perivascular fibrosis. The number of Sca1+ adventitial cells increased two-fold in fibrotic mdx hearts vs. age matched wild-type hearts. Moreover, relative to Sca1-, PDGFRα+, CD31-, and CD45- cells and endothelial cells, Sca1+ adventitial cells FACS-sorted from mdx hearts expressed the highest level of Collagen1α1 and 3α1, Connective tissue growth factor, and Tgfβr1 transcripts. Surprisingly, mdx endothelial cells expressed the greatest level of the Tgfβ1 ligand. Utilizing Collagen1α1-GFP reporter mice, we confirmed that the majority of Sca1+ adventitial cells expressed type I collagen, an abundant component of cardiac fibrosis, in both wt (71%±4.1) and mdx (77%±3.5) hearts. In contrast, GFP+ interstitial fibroblasts were PDGFRα+ but negative for Sca1. Treatment of cultured Collagen1α1-GFP+ adventitial cells with TGFβ1 resulted in increased collagen synthesis, whereas pharmacological inhibition of TGFβR1 signaling reduced the fibrotic response. Therefore, perivascular cardiac fibrosis by coronary adventitial cells may be mediated by TGFβ1 signaling. Our results implicate coronary endothelial cells in mediating cardiac fibrosis via transmural TGFβ signaling, and suggest that the coronary adventitia is a promising target for developing novel anti-fibrotic therapies.

Actin nemaline myopathy mouse reproduces disease, suggests other actin disease phenotypes and provides cautionary note on muscle transgene expression

Mutations in the skeletal muscle α-actin gene (ACTA1) cause congenital myopathies including nemaline myopathy, actin aggregate myopathy and rod-core disease. The majority of patients with ACTA1 mutations have severe hypotonia and do not survive beyond the age of one. A transgenic mouse model was generated expressing an autosomal dominant mutant (D286G) of ACTA1 (identified in a severe nemaline myopathy patient) fused with EGFP. Nemaline bodies were observed in multiple skeletal muscles, with serial sections showing these correlated to aggregates of the mutant skeletal muscle α-actin-EGFP. Isolated extensor digitorum longus and soleus muscles were significantly weaker than wild-type (WT) muscle at 4 weeks of age, coinciding with the peak in structural lesions. These 4 week-old mice were ~30% less active on voluntary running wheels than WT mice. The α-actin-EGFP protein clearly demonstrated that the transgene was expressed equally in all myosin heavy chain (MHC) fibre types during the early postnatal period, but subsequently became largely confined to MHCIIB fibres. Ringbinden fibres, internal nuclei and myofibrillar myopathy pathologies, not typical features in nemaline myopathy or patients with ACTA1 mutations, were frequently observed. Ringbinden were found in fast fibre predominant muscles of adult mice and were exclusively MHCIIB-positive fibres. Thus, this mouse model presents a reliable model for the investigation of the pathobiology of nemaline body formation and muscle weakness and for evaluation of potential therapeutic interventions. The occurrence of core-like regions, internal nuclei and ringbinden will allow analysis of the mechanisms underlying these lesions. The occurrence of ringbinden and features of myofibrillar myopathy in this mouse model of ACTA1 disease suggests that patients with these pathologies and no genetic explanation should be screened for ACTA1 mutations.

Flow cytometry analysis: a quantitative method for collagen VI deficiency screening

Mutations in COL6A1, COL6A2 and COL6A3 genes result in collagen VI myopathies: Ullrich congenital muscular dystrophy (UCMD), Bethlem myopathy (BM) and intermediate phenotypes. At present, none of the existing diagnostic techniques for evaluating collagen VI expression is quantitative, and the detection of subtle changes in collagen VI expression remains challenging. We investigated flow cytometry analysis as a means of quantitatively measuring collagen VI in primary fibroblasts and compared this method with the standard method of fibroblast collagen VI immunohistochemical analysis. Eight UCMD and five BM molecularly confirmed patients were studied and compared to five controls. Flow cytometry analysis consistently detected a reduction of collagen VI of at least 60% in all UCMD cases. In BM cases the levels of collagen VI were variable but on average 20% less than controls. Flow cytometry analysis provides an alternative method for screening for collagen VI deficiency at the protein level in a quantitative, time and cost-effective manner.

Investigation of changes in skeletal muscle alpha-actin expression in normal and pathological human and mouse hearts

We have developed a quantitative antibody-based assay to measure the content of skeletal muscle alpha-actin relative to cardiac alpha-actin. We found 21 +/- 2% skeletal muscle alpha-actin content in normal heart muscle of adult man and mouse. In end stage failing heart 53 +/- 5% of striated actin was skeletal muscle alpha-actin and in samples of inter-ventricular septum from patients with hypertrophic obstructive cardiomyopathy (HOCM) skeletal muscle alpha-actin was 72 +/- 2% of sarcomeric actin. Thin filaments containing actin isolated from normal and HOCM heart muscle were functionally indistinguishable when studied by quantitative in vitro motility assay. We also found elevated skeletal muscle alpha-actin (60 +/- 7%) in a mouse model of dilated cardiomyopathy.

Reduced plasma membrane expression of dysferlin mutants is attributed to accelerated endocytosis via a syntaxin-4-associated pathway

Ferlins are an ancient family of C2 domain-containing proteins, with emerging roles in vesicular trafficking and human disease. Dysferlin mutations cause inherited muscular dystrophy, and dysferlin also shows abnormal plasma membrane expression in other forms of muscular dystrophy. We establish dysferlin as a short-lived (protein half-life approximately 4-6 h) and transitory transmembrane protein (plasma membrane half-life approximately 3 h), with a propensity for rapid endocytosis when mutated, and an association with a syntaxin-4 endocytic route. Dysferlin plasma membrane expression and endocytic rate is regulated by the C2B-FerI-C2C motif, with a critical role identified for C2C. Disruption of C2C dramatically reduces plasma membrane dysferlin (by 2.5-fold), due largely to accelerated endocytosis (by 2.5-fold). These properties of reduced efficiency of plasma membrane expression due to accelerated endocytosis are also a feature of patient missense mutant L344P (within FerI, adjacent to C2C). Importantly, dysferlin mutants that demonstrate accelerated endocytosis also display increased protein lability via endosomal proteolysis, implicating endosomal-mediated proteolytic degradation as a novel basis for dysferlin-deficiency in patients with single missense mutations. Vesicular labeling studies establish that dysferlin mutants rapidly transit from EEA1-positive early endosomes through to dextran-positive lysosomes, co-labeled by syntaxin-4 at multiple stages of endosomal transit. In summary, our studies define a transient biology for dysferlin, relevant to emerging patient therapeutics targeting dysferlin replacement. We introduce accelerated endosomal-directed degradation as a basis for lability of dysferlin missense mutants in dysferlinopathy, and show that dysferlin and syntaxin-4 similarly transit a common endosomal pathway in skeletal muscle cells.

Rescue of skeletal muscle alpha-actin-null mice by cardiac (fetal) alpha-actin

Skeletal muscle alpha-actin (ACTA1) is the major actin in postnatal skeletal muscle. Mutations of ACTA1 cause mostly fatal congenital myopathies. Cardiac alpha-actin (ACTC) is the major striated actin in adult heart and fetal skeletal muscle. It is unknown why ACTC and ACTA1 expression switch during development. We investigated whether ACTC can replace ACTA1 in postnatal skeletal muscle. Two ACTC transgenic mouse lines were crossed with Acta1 knockout mice (which all die by 9 d after birth). Offspring resulting from the cross with the high expressing line survive to old age, and their skeletal muscles show no gross pathological features. The mice are not impaired on grip strength, rotarod, or locomotor activity. These findings indicate that ACTC is sufficiently similar to ACTA1 to produce adequate function in postnatal skeletal muscle. This raises the prospect that ACTC reactivation might provide a therapy for ACTA1 diseases. In addition, the mouse model will allow analysis of the precise functional differences between ACTA1 and ACTC.