Denervation causes fiber atrophy and myosin heavy chain co-expression in senescent skeletal muscle

Mitochondrial involvement and impact in aging skeletal muscle

Atrophy is a defining feature of aging skeletal muscle that contributes to progressive weakness and an increased risk of mobility impairment, falls, and physical frailty in very advanced age. Amongst the most frequently implicated mechanisms of aging muscle atrophy is mitochondrial dysfunction. Recent studies employing methods that are well-suited to interrogating intrinsic mitochondrial function find that mitochondrial respiration and reactive oxygen species emission changes are inconsistent between aging rat muscles undergoing atrophy and appear normal in human skeletal muscle from septuagenarian physically active subjects. On the other hand, a sensitization to permeability transition seems to be a general property of atrophying muscle with aging and this effect is even seen in atrophying muscle from physically active septuagenarian subjects. In addition to this intrinsic alteration in mitochondrial function, factors extrinsic to the mitochondria may also modulate mitochondrial function in aging muscle. In particular, recent evidence implicates oxidative stress in the aging milieu as a factor that depresses respiratory function in vivo (an effect that is not present ex vivo). Furthermore, in very advanced age, not only does muscle atrophy become more severe and clinically relevant in terms of its impact, but also there is evidence that this is driven by an accumulation of severely atrophied denervated myofibers. As denervation can itself modulate mitochondrial function and recruit mitochondrial-mediated atrophy pathways, future investigations need to address the degree to which skeletal muscle mitochondrial alterations in very advanced age are a consequence of denervation, rather than a primary organelle defect, to refine our understanding of the relevance of mitochondria as a therapeutic target at this more advanced age.

The Neuromuscular Junction: Aging at the Crossroad between Nerves and Muscle

Aging is associated with a progressive loss of muscle mass and strength and a decline in neurophysiological functions. Age-related neuromuscular junction (NMJ) plays a key role in musculoskeletal impairment that occurs with aging. However, whether changes in the NMJ precede or follow the decline of muscle mass and strength remains unresolved. Many factors such as mitochondrial dysfunction, oxidative stress, inflammation, changes in the innervation of muscle fibers, and mechanical properties of the motor units probably perform an important role in NMJ degeneration and muscle mass and strength decline in late life. This review addresses the primary events that might lead to NMJ dysfunction with aging, including studies on biomarkers, signaling pathways, and animal models. Interventions such as caloric restriction and exercise may positively affect the NMJ through this mechanism and attenuate the age-related progressive impairment in motor function.

Anaesthetic tricaine acts preferentially on neural voltage-gated sodium channels and fails to block directly evoked muscle contraction

Movements in animals arise through concerted action of neurons and skeletal muscle. General anaesthetics prevent movement and cause loss of consciousness by blocking neural function. Anaesthetics of the amino amide-class are thought to act by blockade of voltage-gated sodium channels. In fish, the commonly used anaesthetic tricaine methanesulphonate, also known as 3-aminobenzoic acid ethyl ester, metacaine or MS-222, causes loss of consciousness. However, its role in blocking action potentials in distinct excitable cells is unclear, raising the possibility that tricaine could act as a neuromuscular blocking agent directly causing paralysis. Here we use evoked electrical stimulation to show that tricaine efficiently blocks neural action potentials, but does not prevent directly evoked muscle contraction. Nifedipine-sensitive L-type Cav channels affecting movement are also primarily neural, suggesting that muscle Nav channels are relatively insensitive to tricaine. These findings show that tricaine used at standard concentrations in zebrafish larvae does not paralyse muscle, thereby diminishing concern that a direct action on muscle could mask a lack of general anaesthesia.

Habitual exercise plus dietary supplementation with milk fat globule membrane improves muscle function deficits via neuromuscular development in senescence-accelerated mice

We examined the effects of habitual exercise plus nutritional intervention through consumption of milk fat globule membrane (MFGM), a milk component, on aging-related deficits in muscle mass and function in senescence-accelerated P1 mice. Combining wheel-running and MFGM (MFGMEx) intake significantly attenuated age-related declines in quadriceps muscle mass (control: 318 ± 6 mg; MFGMEx: 356 ± 9 mg; P < 0.05) and in contractile force (1.4-fold and 1.5-fold higher in the soleus and extensor digitorum longus muscles, respectively). Microarray analysis of genes in the quadriceps muscle revealed that MFGMEx stimulated neuromuscular development; this was supported by significantly increased docking protein-7 (Dok-7) and myogenin mRNA expression. Treatment of differentiating myoblasts with MFGM-derived phospholipid or sphingolipid fractions plus mechanical stretching also significantly increased Dok-7 mRNA expression. These findings suggest that habitual exercise plus dietary MFGM improves muscle function deficits through neuromuscular development, and that phospholipid and sphingolipid in MFGM contribute to its physiological actions.

Skeletal muscle lipid content and oxidative activity in relation to muscle fiber type in aging and metabolic syndrome

One of the most noticeable effects of aging is the reduction in skeletal muscle mass and strength (sarcopenia). The metabolic syndrome (MS) is also prevalent in old subjects, but its relevance to skeletal muscle characteristics has poorly been investigated. Immunohistochemical studies were performed with muscle biopsies from young (22 years) and old (73 years) men with and without MS to reveal age-dependent and MS-associated modifications of fiber-type characteristics. Atrophy of type II fibers and altered fiber shape characterized muscle aging in lean healthy men. In contrast, increased cross-sectional area of the most abundant type I and type IIA fibers, and reduced cytochrome c oxidase content in all fiber types, characterized MS. Aging and particularly MS were associated with accumulation of intramyocellular lipid droplets. Although lipids mostly accumulated in type I fibers, matrix-assisted laser desorption/ionization-mass spectrometry imaging of intramyocellular lipids did not distinguish fiber types, but clearly separated young, old, and MS subjects. In conclusion, our study suggests that MS in the elderly persons is associated with alterations in skeletal muscle at a fiber-type specific level. Overall, these fiber type-specific modifications may be important both for the age-related loss of muscle mass and strength and for the increased prevalence of MS in elderly subjects.

Quantitative quadruple-label immunofluorescence of mitochondrial and cytoplasmic proteins in single neurons from human midbrain tissue

•

We developed an assay to quantify respiratory chain deficiencies in single neurons.

•

Quadruple-label immunofluorescence was combined with quantitative image analysis.

•

The single-cell assay was applied to tyrosine hydroxylase-positive midbrain neurons.

•

The expression of complexes I and IV was determined relative to mitochondrial mass.

•

The assay proved specific in patients with known respiratory chain deficiencies.

Background

Respiratory chain (RC) deficiencies are found in primary mtDNA diseases. Focal RC defects are also associated with ageing and neurodegenerative disorders, e.g. in substantia nigra (SN) neurons from Parkinson's disease patients. In mitochondrial disease and ageing, mtDNA mutational loads vary considerably between neurons necessitating single cell-based assessment of RC deficiencies. Evaluating the full extent of RC deficiency within SN neurons is challenging because their size precludes investigations in serial sections. We developed an assay to measure RC abnormalities in individual SN neurons using quadruple immunofluorescence.

New method

Using antibodies against subunits of complex I (CI) and IV, porin and tyrosine hydroxylase together with IgG subtype-specific fluorescent labelled secondary antibodies, we quantified the expression of CI and CIV compared to mitochondrial mass in dopaminergic neurons. CI:porin and CIV:porin ratios were determined relative to a standard control.

Results

Quantification of expression of complex subunits in midbrain sections from patients with mtDNA disease and known RC deficiencies consistently showed reduced CI:porin and/or CIV:porin ratios.

Comparison with existing method(s)

The standard histochemical method to investigate mitochondrial dysfunction, the cytochrome c oxidase/succinate dehydrogenase assay, measures CIV and CII activities. To also study CI in a patient, immunohistology in additional sections, i.e. in different neurons, is required. Our method allows correlation of the expression of CI, CIV and mitochondrial mass at a single cell level.

Conclusion

Quantitative quadruple-label immunofluorescence is a reliable tool to measure RC deficiencies in individual neurons that will enable new insights in the molecular mechanisms underlying inherited and acquired mitochondrial dysfunction.

Degeneration of neuromuscular junction in age and dystrophy

Functional denervation is a hallmark of aging sarcopenia as well as of muscular dystrophy. It is thought to be a major factor reducing skeletal muscle mass, particularly in the case of sarcopenia. Neuromuscular junctions (NMJs) serve as the interface between the nervous and skeletal muscular systems, and thus they may receive pathophysiological input of both pre- and post-synaptic origin. Consequently, NMJs are good indicators of motor health on a systemic level. Indeed, upon sarcopenia and dystrophy, NMJs morphologically deteriorate and exhibit altered characteristics of primary signaling molecules, such as nicotinic acetylcholine receptor and agrin. Since a remarkable reversibility of these changes can be observed by exercise, there is significant interest in understanding the molecular mechanisms underlying synaptic deterioration upon aging and dystrophy and how synapses are reset by the aforementioned treatments. Here, we review the literature that describes the phenomena observed at the NMJ in sarcopenic and dystrophic muscle as well as to how these alterations can be reversed and to what extent. In a second part, the current information about molecular machineries underlying these processes is reported.

Long-term high-level exercise promotes muscle reinnervation with age

The histologic features of aging muscle suggest that denervation contributes to atrophy, that immobility accelerates the process, and that routine exercise may protect against loss of motor units and muscle tissue. Here, we compared muscle biopsies from sedentary and physically active seniors and found that seniors with a long history of high-level recreational activity up to the time of muscle biopsy had 1) lower loss of muscle strength versus young men (32% loss in physically active vs 51% loss in sedentary seniors); 2) fewer small angulated (denervated) myofibers; 3) a higher percentage of fiber-type groups (reinnervated muscle fibers) that were almost exclusive of the slow type; and 4) sparse normal-size muscle fibers coexpressing fast and slow myosin heavy chains, which is not compatible with exercise-driven muscle-type transformation. The biopsies from the old physically active seniors varied from sparse fiber-type groupings to almost fully transformed muscle, suggesting that coexpressing fibers appear to fill gaps. Altogether, the data show that long-term physical activity promotes reinnervation of muscle fibers and suggest that decades of high-level exercise allow the body to adapt to age-related denervation by saving otherwise lost muscle fibers through selective recruitment to slow motor units. These effects on size and structure of myofibers may delay functional decline in late aging.

CIR-Myo News: Proceedings of the 2014 Spring Padua Muscle Days: Terme Euganee and Padova (Italy), April 3-5, 2014

Not available.

Respiratory chain deficiency in aged spinal motor neurons

Sarcopenia, muscle wasting, and strength decline with age, is an important cause of loss of mobility in the elderly individuals. The underlying mechanisms are uncertain but likely to involve defects of motor nerve, neuromuscular junction, and muscle. Loss of motor neurons with age and subsequent denervation of skeletal muscle has been recognized as one of the contributing factors. This study investigated aspects of mitochondrial biology in spinal motor neurons from elderly subjects. We found that protein components of complex I of mitochondrial respiratory chain were reduced or absent in a proportion of aged motor neurons–a phenomenon not observed in fetal tissue. Further investigation showed that complex I-deficient cells had reduced mitochondrial DNA content and smaller soma size. We propose that mitochondrial dysfunction in these motor neurons could lead to the cell loss and ultimately denervation of muscle fibers.

Aging related changes in determinants of muscle force generating capacity: a comparison of muscle aging in men and male rodents

Human aging is associated with a progressive decline in skeletal muscle mass and force generating capacity, however the exact mechanisms underlying these changes are not fully understood. Rodents models have often been used to enhance our understanding of mechanisms of age-related changes in human skeletal muscle. However, to what extent age-related alterations in determinants of muscle force generating capacity observed in rodents resemble those in humans has not been considered thoroughly. This review compares the effect of aging on muscle force generating determinants (muscle mass, fiber size, fiber number, fiber type distribution and muscle specific tension), in men and male rodents at similar relative age. It appears that muscle aging in male F344*BN rat resembles that in men most; 32-35-month-old rats exhibit similar signs of muscle weakness to those of 70-80-yr-old men, and the decline in 36-38-month-old rats is similar to that in men aged over 80 yrs. For male C57BL/6 mice, age-related decline in muscle force generating capacity seems to occur only at higher relative age than in men. We conclude that the effects on determinants of muscle force differ between species as well as within species, but qualitatively show the same pattern as that observed in men.

Neuron-specific expression of CuZnSOD prevents the loss of muscle mass and function that occurs in homozygous CuZnSOD-knockout mice

Deletion of copper-zinc superoxide dismutase (CuZnSOD) in Sod1(-/-) mice leads to accelerated loss of muscle mass and force during aging, but the losses do not occur with muscle-specific deletion of CuZnSOD. To determine the role of motor neurons in the muscle decline, we generated transgenic Sod1(-/-) mice in which CuZnSOD was expressed under control of the synapsin 1 promoter (SynTgSod1(-/-) mice). SynTgSod1(-/-) mice expressed CuZnSOD in brain, spinal cord, and peripheral nerve, but not in other tissues. Sciatic nerve CuZnSOD content in SynTgSod1(-/-) mice was ~20% that of control mice, but no reduction in muscle mass or isometric force was observed in SynTgSod1(-/-) mice compared with control animals, whereas muscles of age-matched Sod1(-/-) mice displayed 30-40% reductions in mass and force. In addition, increased oxidative damage and adaptations in stress responses observed in muscles of Sod1(-/-) mice were absent in SynTgSod1(-/-) mice, and degeneration of neuromuscular junction (NMJ) structure and function occurred in Sod1(-/-) mice but not in SynTgSod1(-/-) mice. Our data demonstrate that specific CuZnSOD expression in neurons is sufficient to preserve NMJ and skeletal muscle structure and function in Sod1(-/-) mice and suggest that redox homeostasis in motor neurons plays a key role in initiating sarcopenia during aging.

Increased sensitivity to mitochondrial permeability transition and myonuclear translocation of endonuclease G in atrophied muscle of physically active older humans

Mitochondrial dysfunction is implicated in skeletal muscle atrophy and dysfunction with aging, with strong support for an increased mitochondrial-mediated apoptosis in sedentary rodent models. Whether this applies to aged human muscle is unknown, nor is it clear whether these changes are caused by sedentary behavior. Thus, we examined mitochondrial function [respiration, reactive oxygen species (ROS) emission, and calcium retention capacity (CRC)] in permeabilized myofibers obtained from vastus lateralis muscle biopsies of healthy physically active young (23.7±2.7 yr; mean±SD) and older (71.2±4.9 yr) men. Although mitochondrial ROS and maximal respiratory capacity were unaffected, the acceptor control ratio was reduced by 18% with aging, suggesting mild uncoupling of oxidative phosphorylation. CRC was reduced by 50% with aging, indicating sensitization of the mitochondrial permeability transition pore (mPTP) to apoptosis. Consistent with the mPTP sensitization, older muscles showed a 3-fold greater fraction of endonuclease G (a mitochondrial proapoptotic factor)-positive myonuclei. Aged muscles also had lower mitophagic potential, based on a 43% reduction in Parkin to the voltage-dependent anion channel (VDAC) protein ratio. Collectively, these results show that mitochondrial-mediated apoptotic signaling is increased in older human muscle and suggest that accumulation of dysfunctional mitochondria with exaggerated apoptotic sensitivity is due to impaired mitophagy.

Genistein exposure during the early postnatal period favors the development of obesity in female, but not male rats

Genistein (Gen), the primary isoflavone in soy, has been shown to adversely affect various endocrine-mediated endpoints in rodents and humans. Soy formula intake by human infants has been associated with early age at menarche and decreased female-typical behavior in girls. Adipose deposition and expansion are also hormonally regulated and Gen has been shown to alter these processes. However, little is known about the impact of early-life soy intake on metabolic homeostasis in adulthood. The current study examined the impact of early-life Gen exposure on adulthood body composition (by magnetic resonance imaging) and the molecular signals mediating adipose expansion. From postnatal day (PND) 1 to 22, rat pups were daily orally dosed with 50mg/kg Gen to mimic blood Gen levels in human infants fed soy formula. Female but not male Gen-exposed rats had increased fat/lean mass ratio, fat mass, adipocyte size and number, and decreased muscle fiber perimeter. PND22 Gen-exposed females, but not males, had increased expression of adipogenic factors, including CCAAT/enhancer binding protein alpha (Cebpα), CCAAT/enhancer binding protein beta (Cebpβ), and peroxisome proliferator-activated receptor gamma (Pparγ). Furthermore, Wingless-related MMTV integration site 10b (Wnt10b), a critical regulator of adipogenic cell fate determination, was hypermethylated and had decreased expression in adipose of PND22 Gen-exposed females. These data suggest that developmental Gen exposure in rats has gender-specific effects on adiposity that closely parallel the effects of a postweaning high-fat diet and underscore the importance of considering timing of exposure and gender when establishing safety recommendations for early-life dietary Gen intake.

Regulation of nicotinic acetylcholine receptor turnover by MuRF1 connects muscle activity to endo/lysosomal and atrophy pathways

Muscle atrophy is a process of muscle wasting induced under a series of catabolic stress conditions, such as denervation, disuse, cancer cachexia, heart and renal failure, AIDS, and aging. Neuromuscular junctions (NMJs), the synapses between motor neurons and muscle fibers undergo major changes in atrophying muscles, ranging from mild morphological alterations to complete disintegration. In this study, we hypothesized that remodeling of NMJs and muscle atrophy could be linked together. To test this, we examined if a major atrophy-promoting E3 ubiquitin ligase, MuRF1, is involved in the maintenance of NMJs. Immunofluorescence revealed that MuRF1 is highly enriched close to the NMJ. Affinity precipitation and in vivo imaging showed that MuRF1 interacts in endocytic structures with both, acetylcholine receptor, the primary postsynaptic protein of the NMJ, as well as with Bif-1, an autophagy- and endocytosis-regulating factor. In vivo imaging, radio labeling, and weighing approaches demonstrated that metabolic destabilization of acetylcholine receptors and muscle atrophy induced by denervation were significantly rescued in MuRF1-KO animals. Notably, interaction with Bif-1, and the rescue of AChR lifetime and muscle atrophy were specific to MuRF1 but not MuRF2. Our data demonstrate an involvement of MuRF1 in membrane protein-turnover, including the degradation of AChRs at the NMJ under atrophying conditions where MuRF1 also interacts and associates with Bif-1.

Role of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) in denervation-induced atrophy in aged muscle: facts and hypotheses

Aging-related loss of muscle mass, a biological process named sarcopenia, contributes to mobility impairment, falls, and physical frailty, resulting in an impaired quality of life in older people. In view of the aging of our society, understanding the underlying mechanisms of sarcopenia is a major health-care imperative. Evidence obtained from human and rodent studies demonstrates that skeletal muscle denervation/reinnervation cycles occur with aging, and that progressive failure of myofiber reinnervation is a major cause of the accelerating phase of sarcopenia in advanced age. However, the mechanisms responsible for the loss of myofiber innervation with aging remain unknown. The two major strategies that counteract sarcopenia, that is, caloric restriction and endurance training, are well known to protect neuromuscular junction (NMJ) integrity, albeit through undefined mechanisms. Interestingly, both of these interventions better preserve PGC-1α expression with aging, a transcriptional coactivator which has recently been shown to regulate key proteins involved in maintaining NMJ integrity. We therefore propose that the aging-related decline in PGC-1α may be a central mechanism promoting instability of the NMJ and consequently, aging-related alterations of myofiber innervation in sarcopenia. Similarly, the promotion of PGC-1α expression by both caloric restriction and exercise training may be fundamental to their protective benefits for aging muscle by better preserving NMJ integrity.

Sequence of age-associated changes to the mouse neuromuscular junction and the protective effects of voluntary exercise

Loss of connections between motor neurons and skeletal muscle fibers contribute to motor impairment in old age, but the sequence of age-associated changes that precede loss of the neuromuscular synapse remains uncertain. Here we determine changes in the size of neuromuscular synapses within the tibialis anterior muscle across the life span of C57BL/6J mice. Immunofluorescence, confocal microscopy and morphometry were used to measure the area occupied by nerve terminal synaptophysin staining and postsynaptic acetylcholine receptors at motor endplates of 2, 14, 19, 22, 25 and 28month old mice. The key findings were: 1) At middle age (14-months) endplate acetylcholine receptors occupied 238±11 µm² and nerve terminal synaptophysin 168±14 µm² (mean ± SEM). 2) Between 14-months and 19-months (onset of old age) the area occupied by postsynaptic acetylcholine receptors declined 30%. At many endplates the large acetylcholine receptor plaque became fragmented into multiple smaller acetylcholine receptor clusters. 3) Between 19- and 25-months, the fraction of endplate acetylcholine receptors covered by synaptophysin fell 21%. By 28-months, half of the endplates imaged retained ≤50 µm² area of synaptophysin staining. 4) Within aged muscles, the degree to which an endplate remained covered by synaptophysin did not depend upon the total area of acetylcholine receptors, nor upon the number of discrete receptor clusters. 5) Voluntary wheel-running exercise, beginning late in middle-age, prevented much of the age-associated loss of nerve terminal synaptophysin. In summary, a decline in the area of endplate acetylcholine receptor clusters at the onset of old age was followed by loss of nerve terminal synaptophysin from the endplate. Voluntary running exercise, begun late in middle age, substantially inhibited the loss of nerve terminal from aging motor endplates.

Aging increases the oxidation of dichlorohydrofluorescein in single isolated skeletal muscle fibers at rest, but not during contractions

An increase in the activity of reactive oxygen species (ROS) has been implicated in the mechanisms of loss of skeletal muscle that occurs during aging, but few studies have attempted to directly assess activities in intact muscle fibers. The current project used the nonspecific fluorescent probe for ROS and reactive nitrogen species, 5-(and-6)-chloromethyl-2',7'-dichlorodihydrofluorescein (CM-DCFH), in single, isolated, mature skeletal muscle fibers from adult and old mice in addition to biochemical measurements of key regulatory proteins for ROS in muscles of these animals. Data confirmed the changes in key regulatory processes for ROS (increased glutathione peroxidase 1 and catalase activities and reduced total glutathione content) previously reported in muscle from old mice and showed increased CM-DCFH oxidation in muscle fibers from old mice at rest and indicate that these changes are likely due to an increase in generation of oxidants rather than a lack of scavenging capacity. The increased CM-DCFH oxidation persisted even when cellular defenses against oxidants were increased by loading fibers from young and old mice with glutathione. During contractile activity, and in contrast to the increase observed in fibers from young mice, there was no further increase in CM-DCFH oxidation in muscle fibers from old mice. These data also suggest that the defect in short-term adaptations to contractions that occurs in old mice may be related to a diminished, or absent, increase in the muscle generation of ROS and/or reactive nitrogen species that normally accompanies contractile activity in young mice.

Reference values for vastus lateralis fiber size and type in healthy subjects over 40 years old: a systematic review and metaanalysis

Skeletal muscle atrophy is a major systemic impairment in chronic diseases. Yet its determinants have been hard to identify because a clear research definition has not been agreed upon. The reduction in muscle fiber cross-sectional area (CSA) is a widely acknowledged marker of muscle atrophy, but no reference values for the muscle fiber CSA at the age of the onset of chronic disease have ever been published. Thus, we aimed to systematically review the studies providing data on fiber CSA and fiber type proportion in the vastus lateralis of the quadriceps of healthy subjects (age >40 yr) and then to pool and analyze the data from the selected studies to determine reference values for fiber CSA. We followed the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) and identified 19 studies, including 423 subjects that matched the inclusion criteria. On the basis of fiber type and gender, the mean fiber CSA and the lower limits of normal (LLNs) were (%type I*60) + 1,743 μm(2) and (%type I*60) - 718 μm(2), respectively, for men; and (%type I*70) + 139 μm(2) and (%type I*70) - 1,485 μm(2), respectively, for women. There was no significant heterogeneity among subgroups of fiber type and gender. The pooled type I fiber proportion was 50.3% (LLN = 32.9%). In multivariate analysis, fiber CSA was significantly correlated with Vo2 peak (r = 190.92; P = 0.03), and type I fiber proportion was correlated with age (r = -0.024; P = 0.005), body mass index (r = 0.096; P = 0.005), and Vo2 peak (r = -0.053; P = 0.005). Our metaanalysis of a homogeneous set of studies is the first to provide valuable LLNs for fiber CSA according to fiber type and gender. This analysis will be improved by prospective assessment in well-characterized healthy subjects.

The effects of target skeletal muscle cells on dorsal root ganglion neuronal outgrowth and migration in vitro

Targets of neuronal innervations play a vital role in regulating the survival and differentiation of innervating neurotrophin-responsive neurons. During development, neurons extend axons to their targets, and then their survival become dependent on the trophic substances secreted by their target cells. Sensory endings were present on myoblasts, myotubes, and myofibers in all intrafusal bundles regardless of age. The interdependence of sensory neurons and skeletal muscle (SKM) cells during both embryonic development and the maintenance of the mature functional state has not been fully understood. In the present study, neuromuscular cocultures of organotypic dorsal root ganglion (DRG) explants and dissociate SKM cells were established. Using this culture system, the morphological relationship between DRG neurons and SKM cells, neurites growth and neuronal migration were investigated. The migrating neurons were determined by fluorescent labeling of microtubule-associated protein-2 (MAP-2) and neurofilament 200 (NF-200) or growth-associated protein 43 (GAP-43). The expression of NF-200 and GAP-43 and their mRNAs was evaluated by Western blot assay and real time-PCR analysis. The results reveal that DRG explants showed more dense neurites outgrowth in neuromuscular cocultures as compared with that in the culture of DRG explants alone. The number of total migrating neurons (the MAP-2-expressing neurons) and the percentage NF-200-immunoreactive (IR) and GAP-43-IR neurons increased significantly in the presence of SKM cells. The levels of NF-200 and GAP-43 and their mRNAs increased significantly in neuromuscular cocultures as compared with that in the culture of DRG explants alone. These results suggested that target SKM cells play an important role in regulating neuronal protein synthesis, promoting neuritis outgrowth and neuronal migration of DRG explants in vitro. These results not only provide new clues for a better understanding of the association of SKM cells with DRG sensory neurons during development, they may also have implications for axonal regeneration after nerve injury.

Automated methods for the analysis of skeletal muscle fiber size and metabolic type

It is of interest to quantify the size, shape, and metabolic subtype of skeletal muscle fibers in many areas of biomedical research. To do so, skeletal muscle samples are sectioned transversely to the length of the muscle and labeled for extracellular or membrane proteins to delineate the fiber boundaries and additionally for biomarkers related to function or metabolism. The samples are digitally photographed and the fibers "outlined" for quantification of fiber cross-sectional area (CSA) using pointing devices interfaced to a computer, which is tedious, prone to error, and can be nonobjective. Here, we review methods for characterizing skeletal muscle fibers and describe new automated techniques, which rapidly quantify CSA and biomarkers. We discuss the applications of these methods to the characterization of mitochondrial dysfunctions, which underlie a variety of human afflictions, and we present a novel approach, utilizing images from the online Human Protein Atlas to predict relationships between fiber-specific protein expression, function, and metabolism.

Severe atrophy of slow myofibers in aging muscle is concealed by myosin heavy chain co-expression

Although slow myofibers are considered less susceptible to atrophy with aging, slow fiber atrophy may have been underestimated previously. First, the marked atrophy of the aging rat soleus (Sol) muscle cannot be explained by the atrophy of only the fast fibers, due to their low abundance. Second, the increase in small fibers co-expressing both fast and slow myosin heavy chains (MHC) in the aging rat Sol is proportional to a decline in pure MHC slow fibers (Snow et al., 2005), suggesting that these MHC co-expressing fibers represent formerly pure slow fibers. Thus, we examined the size and proportion of MHC slow, MHC fast, and MHC fast-slow co-expressing fibers in the Sol and mixed region of the gastrocnemius (Gas) muscle in young adult (YA) and senescent (SEN) rats. Our results suggest that formerly pure MHC slow fibers are the source of MHC co-expressing fibers with aging in both muscle regions. Accounting for the atrophy of these fibers in calculating MHC slow fiber atrophy with aging revealed that MHC slow fibers atrophy on average by 40% in the Sol and by 38% in the mixed Gas, values which are similar to the 60% and 31% atrophy of pure MHC fast fibers in the Sol and mixed Gas, respectively. Probing for the atrophy-dependent ubiquitin ligase, MAFbx (atrogin 1), it was suggested that former slow fibers acquire atrophy potential via the up-regulation of MAFbx coincident with the co-expression of fast MHC. These results redefine the impact of aging on slow fiber atrophy, and emphasize the necessity of addressing the atrophy of fast and slow fibers in seeking treatments for aging muscle atrophy.

Effect of nutritional interventions and resistance exercise on aging muscle mass and strength

Sarcopenia, defined as the age-related loss of muscle mass, has a negative effect on strength, functional independence and overall quality of life. Sarcopenia is a multifactorial phenomenon characterized by changes in muscle morphology, protein and hormonal kinetics, oxidative stress, inflammation, physical activity and nutrition. It is well known that resistance exercise increases aging muscle mass and strength and these physiological adaptations from exercise may be further enhanced with certain nutritional interventions. Research indicates that essential amino acids and milk-based proteins, creatine monohydrate, essential fatty acids, and vitamin D may all have beneficial effects on aging muscle biology.