Uptake of horseradish peroxidase in denervated skeletal muscle occurs primarily at the endplate region

Novel clearance of muscle proteins by muscle cells

Blood levels of cardiac troponins (cTn) and myoglobin are analysed when myocardial infarction (MI) is suspected. Here we describe a novel clearance mechanism for muscle proteins by muscle cells. The complete plasma clearance profile of cTn and myoglobin was followed in rats after intravenous or intermuscular injections and analysed by PET and fluorescence microscopy of muscle biopsies and muscle cells. Compared with intravenous injections, only 5 % of cTnT, 0.6 % of cTnI and 8 % of myoglobin were recovered in the circulation following intramuscular injection. In contrast, 47 % of the renal filtration marker FITC-sinistrin and 81 % of cTn fragments from MI-patients were recovered after intramuscular injection. In addition, PET and biopsy analysis revealed that cTn was taken up by the quadriceps muscle and both cTn and myoglobin were endocytosed by cultured muscle cells. This local clearance mechanism could possibly be the dominant clearance mechanism for cTn, myoglobin and other muscle damage biomarkers released by muscle cells.

Skeletal muscle expression of clathrin and mannose 6-phosphate receptor in experimental chloroquine-induced myopathy

Previous studies suggest that the muscle fiber lysosome system plays a central role in the increased formation of autophagosomes and autolysosomes that occurs in the context of chloroquine-induced myopathy. The goal of this study was to characterize the contribution of receptor-mediated intracellular transport, particularly the endosomal pathway, to the abnormal accumulation of vacuoles in experimental chloroquine myopathy. Expression of the mannose 6-phosphate receptor (M6PR) and clathrin were analyzed in innervated and denervated rat soleus muscles after treatment with either saline or chloroquine. Accumulation of vacuoles was observed only in chloroquine-treated denervated muscles. Further, clathrin immunostaining and M6PR messenger ribonucleic acid (mRNA) were significantly increased in denervated soleus muscle from saline- and chloroquine-treated rats compared to contralateral, innervated muscles. However, there was no difference in clathrin levels when comparing saline- and chloroquine-treated denervated muscles. These data suggest that chloroquine activates the transport of newly synthesized lysosomal enzymes from the secretory pathway via the trans-Golgi network of the Golgi apparatus (an endosomal pathway) as well as autophagosome formation (an autophagic process) in skeletal muscles. Vacuoles may subsequently accumulate secondary to abnormal formation or turnover of autolysosomes at or after fusion of autophagosomes with early endosomes.

Membrane traffic in skeletal muscle

Skeletal muscle tissue is made up of highly organized multinuclear cells. The internal organization of the muscle cell is dictated by the necessary regular arrangement of repeated units within the protein myofibrils that mediate muscle contraction. Skeletal muscle cells have the usual membrane traffic pathways for partitioning newly synthesized proteins, internalizing cell surface receptors for hormones and nutrients, and mediating membrane repair. However, in muscle, these pathways must be further specialized to deal with targeting to and organizing muscle-specific membrane structures, satisfying the unique metabolic requirements of muscle and meeting the high demand for membrane repair in a tissue that is constantly under mechanical stress. Specialized membrane traffic pathways in muscle also play a role in the formation of muscle through fusion of myoblast membranes and the development of internal muscle-specific membrane structures during myogenesis and regeneration. It has recently become apparent that muscle-specific isoforms of proteins that are known to mediate ubiquitous membrane traffic pathways, as well as novel muscle-specific proteins, are involved in tissue-specific aspects of muscle membrane traffic. Here we describe the specialized membrane structures of skeletal muscle, how these are developed, maintained and repaired by specialized and generic membrane traffic pathways, and how defects in these pathways result in muscle disease.

Endocytosis in skeletal muscle fibers

Defining the organization of endocytic pathway in multinucleated skeletal myofibers is crucial to understand the routing of membrane proteins, such as receptors and glucose transporters, through this system. Here we analyzed the organization of the endocytic trafficking pathways in isolated rat myofibers. We found that sarcolemmal-coated pits and transferrin receptors were concentrated in the I band areas. Fluid phase markers were taken up into vesicles in the same areas along the whole length of the fibers and were then delivered into structures around and between the nuclei. These markers also accumulated beneath the neuromuscular and myotendinous junctions. The recycling compartment, labeled with transferrin, appeared as perinuclear and interfibrillar dots that partially colocalized with the GLUT4 compartment. Low-density lipoprotein, a marker of the lysosome-directed pathway, was transported into sparsely distributed perinuclear and interfibrillar dots that contacted microtubules. A majority of these dots did not colocalize with internalized transferrin, indicating that the recycling and the lysosome-directed pathways were distinct. In conclusion, the I band areas were active in endocytosis along the whole length of the multinucleated myofibers. The sorting endosomes distributed in a cross-striated fashion while the recycling and late endosomal compartments showed perinuclear and interfibrillar localizations and followed the course of microtubules.

Secretion of plasminogen activator and lysosomal enzymes from mouse skeletal muscle: effect of denervation

Levels of hydrolytic enzymes increase in skeletal muscle after denervation and their activities in the extracellular matrix appear to be important for interaction between muscle and nerve. Using enzymatic assays for beta-glucuronidase, beta-galactosidase, and plasminogen activator, we show that secretion of these enzymes from mouse skeletal muscle increases after denervation and that drugs interfering with the secretory pathway or the reuptake of enzymes modulate this release. Thus, brefeldin A inhibited secretion of plasminogen activator activity and mannan increased secreted amounts of beta-glucuronidase, but not of beta-galactosidase, in denervated muscle. In innervated muscle, brefeldin A decreased secreted activity of plasminogen activator, but mannan had no effect on secretion of either beta-glucuronidase or beta-galactosidase. Furthermore, secretion of plasminogen activator was temperature dependent. These observations, together with previous studies, suggest that secretion of hydrolytic enzymes from adult skeletal muscle may be of physiological significance in nerve-muscle communication.

Expression and distribution of sodium channels in short- and long-term denervated rodent skeletal muscles

1. Loose-patch voltage-clamp recordings were made from rat and mouse skeletal muscle fibres denervated for up to 6 weeks. Innervated muscles possessed a Na+ current density of 107 +/- 3.3 mA cm-2 in endplate membrane, and 6.3 +/- 0.6 mA cm-2 in extrajunctional membrane. This high concentration of Na+ channels at the endplate was gradually reduced following denervation. After 6 weeks of denervation, the endplate Na+ channel concentration was reduced by 40-50%, and the density of Na+ channels in extrajunctional membrane was increased by about 30%. 2. The tetrodotoxin (TTX)-resistant form of the Na+ channel appeared after 3 days of denervation and comprised approximately 43% of the endplate Na+ channels 5-6 days after denervation. Subsequently, TTX-resistant Na+ channels were reduced in density to approximately 25% of the postjunctional Na+ channels and remained at this level up to 6 weeks after denervation. 3. RNase protection analysis showed that mRNA encoding the TTX-resistant Na+ channel was virtually absent in innervated muscle, rose > 50-fold after 3 days of denervation, then decreased by 95% 6 weeks after denervation. The density of TTX-resistant Na+ channels correlated qualitatively with changes in mRNA levels. 4. These results suggest that the density of Na+ channels at neuromuscular junctions is maintained by two mechanisms, one influenced by the nerve terminal and the other independent of innervation.

Endocytotic activity of mouse skeletal muscle fibres after long-term denervation

The endocytotic activity of skeletal muscle fibres and its relation to the denervated endplate region has been studied using horseradish peroxidase (HRP) as marker for endocytosis. In muscles denervated for a short time period (10-20 days) HRP-uptake occurred in small segments of the muscle fibres near the centre of the muscle (endplate region). After long-term denervation (6-12 months) similar segments with high endocytotic activity were seen preferentially in more peripheral parts of the muscle fibres. Ultrastructural characteristics of segments with high endocytotic activity from long-term denervated muscle fibres include a proliferating transverse tubular system, HRP-containing bodies of different sizes with some very large vacuoles extending over several sarcomeres. These characteristics are similar to those described previously for HRP-uptake in the endplate region of short-term denervated muscle (Tågerud et al., J. Neurol. Sci., 75 (1986) 141) except that no recognizable endplate structures were observed in the present study. The results are discussed in relation to the fate of the denervated endplate and the receptive capacity for synapse formation in long-term denervated muscle.

Neural regulation of muscle acetylcholine receptor epsilon- and alpha-subunit gene promoters in transgenic mice

The effects of denervation were investigated in mice with transgenes containing promoter elements from the muscle acetylcholine receptor epsilon- and alpha-subunit genes. The promoter sequences were coupled to a nuclear localization signal-beta-galactosidase fusion gene (nlacZ) as a reporter. While many postsynaptic specializations form in the embryo, expression of the epsilon subunit is induced during the first two postnatal weeks. When muscles were denervated at birth, before the onset of epsilon expression, epsilon nlacZ still appeared at the former synaptic sites on schedule. This result suggests that the nerve leaves a localized "trace" in the muscle that can continue to regulate transcription. An additional finding was that epsilon nlacZ expression was much stronger in denervated than in intact muscles. This suggests that the epsilon promoter is similar to the other subunits in containing elements that are activated on cessation of neural activity. However, even after denervation, epsilon nlacZ expression was always confined to the synaptic region whereas alpha nlacZ expression increased in nuclei along the entire length of the fiber. This suggests that while the epsilon gene is similar in its activity dependence to other subunit genes, it is unique in that local nerve-derived signals are essential for its expression. Consequently, inactivity enhances epsilon expression only in synaptic nuclei where such signals are present, but enhances expression throughout the muscle fiber. Truncations and an internal deletion of the epsilon promoter indicate that cis-elements essential for the response to synaptic signals are contained within 280 bp of the transcription start site. In contrast to these results in young animals, denervation in older animals leads to an unexpected reduction in nlacZ activity. However, mRNA measurements indicated that transgene expression was increased in these animals. This discordance between nlacZ mRNA and enzyme activity, demonstrates a previously unknown limitation of nlacZ as a reporter gene in transgenic animals.

Protein secretion from mouse skeletal muscle: coupling of increased exocytotic and endocytotic activities in denervated muscle

The secretion of proteins labelled by incorporation of radioactive amino acids was studied in innervated and 10 to 13-day-denervated mouse skeletal muscle. The secretion of 3H-leucine-labelled proteins, expressed per mg muscle wet weight, increased after denervation, and the kinetics of the secretory process was also altered in denervated muscle. Separation of secreted 35S-methionine-labelled proteins by sodium dodecyl sulphate polyacrylamide gel electrophoresis followed by autoradiography revealed some denervation-induced alterations in the pattern of secreted proteins. The secretion from both innervated and denervated muscle was highly temperature sensitive and was reversibly inhibited by brefeldin A, a drug that blocks forward membrane transport from the endoplasmic reticulum/Golgi apparatus. This drug was also found to inhibit the uptake of fluorescein isothiocyanate-labelled dextran in denervated muscle but had no effect on the endocytotic activity of innervated muscle. This lends support to the hypothesis that the increased endocytotic activity in denervated muscle is coupled to a high secretory activity.

Effect of denervation on overdevelopment of chloroquine-induced autophagic vacuoles in skeletal muscles

We studied how denervation affects the overdevelopment of autophagic vacuoles in muscles of chloroquine-treated rats. The number of autophagic vacuoles increased significantly in the chloroquine-treated soleus muscles after denervation as compared to similarly treated contralateral, innervated muscles. No vacuoles were present in the denervated and innervated muscles of saline-treated rats. After denervation, the autophagic vacuoles in chloroquine-treated muscle contained numerous glycogen particles and various heterogeneous materials. A biochemical study showed no significant difference in the activities of lysosomal proteases and hydrolases in the chloroquine- and saline-treated muscles after denervation, although these activities were markedly increased in comparison to the same activities in the contralateral, innervated muscles. Chloroquine treatment by itself did not, but denervation with or without chloroquine treatment did enhance the biochemical activities of lysosomal enzymes in the animals. We speculate that denervation induces the marked accumulation of autophagic vacuoles in chloroquine-induced myopathy.

Increased endocytotic and lysosomal activities in denervated type I and type II muscle fibres

Previous work has shown that increased endocytotic and lysosomal activities occur in the endplate region of denervated skeletal muscle fibres. This, however, does not engage all fibres of a muscle at a given time after denervation. The present study was carried out in order to determine if both type I (slow) and type II (fast) muscle fibres can react to denervation by increased endocytotic and lysosomal activities. Uptake of horseradish peroxidase as a marker for endocytosis was studied in conjunction with acid phosphatase staining for lysosomal activity in type I and type II fibres of the denervated mouse hemidiaphragm. Fibre typing was performed using a monoclonal antibody against fast skeletal myosin and by adenosine triphosphatase staining. The results show that increased endocytosis and lysosomal activation occur in both type I and type II fibres after denervation.

Regulation of turnover and number of acetylcholine receptors at neuromuscular junctions

The number and metabolic stability of acetylcholine receptors (AChRs) at neuromuscular junctions of rat tibialis anterior (TA) and soleus (SOL) muscles were examined after denervation, paralysis by continuous application of tetrodotoxin to the nerve, or denervation and direct stimulation of the muscle through implanted electrodes. After 18 days of denervation AChR half-life declined from about 10 days to 2.3 days (TA) or 3.6 days (SOL) and after 18 days of nerve conduction block to 3.1 days (TA). In contrast, the total number of AChRs per endplate was unaffected by these treatments. Denervation for 33 days had no further effect on AChR half-life but reduced the total number of AChRs to about 54% (SOL) or 38% (TA) of normal. Direct stimulation of the 33-day denervated SOL from day 18 restored normal AChR stability and counteracted muscle atrophy but had no effect on the decline in AChR number. The results indicate that motoneurons control the stability of junctional AChRs through evoked muscle activity and the number of junctional AChRs through trophic factors.

High endocytotic and lysosomal activities in segments of rat myotubes differentiated in vitro

Endocytosis and the lysosome system have been studied in rat myotubes differentiated in vitro. Horseradish peroxidase was used as marker for endocytosis and was found to accumulate unevenly in the myotubes. Small segments of myotubes display very high endocytotic activity. Similar segments contained numerous lysosomes, as seen by the accumulation of neutral red or histochemical staining for acid phosphatase. The segments also contained accumulations of acetylcholine receptors as determined by binding of tetramethyl rhodamine-labelled alpha-bungarotoxin. Unstained segments in living cultures could be recognized by phase-contrast microscopy since they often appeared somewhat dilated and were not as well spread on the culture surface as the main parts of the myotubes. Ultrastructurally, the segments contained an intensely proliferating tubular system in communication with the extracellular space, which therefore probably represents the developing transverse tubular system. The segments also contained endocytosed marker within large phagosomes. Contractile filaments occurred in the segments but were frequently less well-organized than in other parts of the myotubes. The described characteristics of the segments in rat myotubes differentiated in vitro bear resemblance to some of the characteristics of the denervated endplate region of adult muscle.

Effects of botulinum toxin induced muscle paralysis on endocytosis and lysosomal enzyme activities in mouse skeletal muscle

The effects of botulinum toxin (type A) induced muscle paralysis on endocytosis and lysosomal enzyme activities in skeletal muscle were compared with the effects of surgical denervation. Muscle atrophy, measured as decrease in total muscle protein content, was as large or larger after botulinum toxin treatment as after denervation. Endocytic activity, measured as the in vitro uptake of horseradish peroxidase, and the specific activities of the lysosomal enzymes N-acetyl-beta-D-glucosaminidase and cathepsin D were all increased six days after denervation. Only the specific activity of cathepsin D was increased six days after botulinum toxin poisoning. The uptake of horseradish peroxidase and the specific activity of N-acetyl-beta-D-glucosaminidase were also increased eleven days after poisoning. Transverse sections of eleven days botulinum poisoned muscles from animals injected with horseradish peroxidase showed fibres with dense peroxidase staining similar to those seen in denervated muscle although they seemed to occur less frequently. The results show that increases in endocytic activity and lysosomal enzyme activities may occur in skeletal muscle without the presence of degenerating axons. The differences in effects of surgical denervation and botulinum toxin induced paralysis are discussed in terms of what is known about the mechanism of action of botulinum toxin and the possible functional roles of the two lysosomal enzymes studied.

An ultrastructural study of the segmental uptake of horseradish peroxidase in the endplate region of denervated skeletal muscle fibres

The segmental uptake of horseradish peroxidase (HRP) in the endplate region of denervated skeletal muscle fibres has been studied ultrastructurally using a method for selecting single muscle fibres with high segmental peroxidase staining from denervated mouse tibialis anterior muscle. Segments containing large peroxidase positive phagosomes could already be seen 10-15 min after i.v. injection of HRP. Such segments were still present 24 h after HRP injection. The localization of phagosomes, deep in the fibres rather than immediately under the sarcolemma, suggests that the uptake occurs from t-tubuli. Vivid proliferation of t-tubuli, consisting of vesiculation, enlargement and encircling of cytoplasmic components, was also observed. The HRP accumulates in phagosomes of varying size and shape. Similar membrane-limited bodies without or with very weak peroxidase staining were also observed. The peroxidase-positive phagosomes participate in autophagic processes as suggested by their content of undegraded cellular material. Golgi profiles, which occurred deep in the muscle fibres, and enlarged components of the sarcoplasmic reticulum were frequently encountered in the segments. Myofibrillar degeneration occurs in the segments and progresses with time after denervation. The described segments may be related to the increased membrane turnover in denervated muscle fibres and/or they may be related to processes aimed at establishing new synaptic contacts.

Biochemical and ultrastructural effects of chloroquine on horseradish peroxidase uptake and lysosomal enzyme activities in innervated and denervated mouse skeletal muscle

The effects of chloroquine treatment on horseradish peroxidase (HRP) uptake and lysosomal enzyme activities in innervated and denervated mouse skeletal muscle have been studied using biochemical, histochemical and ultrastructural techniques. Chloroquine treatment caused a large (59-101%) increase in the activity of cathepsin D in both innervated and denervated muscle. The activity of N-acetyl-beta-D-glucosaminidase also increased slightly in denervated muscle. No effect was observed on acid phosphatase activity. The in vivo uptake of HRP in innervated and denervated muscle was unaffected by chloroquine treatment. The results show that the activities of certain lysosomal enzymes may increase in skeletal muscle without an increase in endocytic activity. This is discussed in comparison to what is seen in denervated and dystrophic muscle. Histochemical and ultrastructural studies showed the HRP uptake to occur segmentally in denervated muscle fibres from untreated as well as chloroquine-treated animals. Ultrastructurally the peroxidase-positive phagosomes occurring in these segments were found to contain increased levels of undegraded material after chloroquine treatment suggesting that these phagosomes are of a lysosomal nature and also participate in autophagic processes.

Receptor-mediated uptake of horseradish peroxidase in innervated and denervated skeletal muscle

The in vitro uptake of [3H]inulin and horseradish peroxidase (HRP) has been studied in innervated and 6 days denervated extensor digitorum longus muscle of the mouse. Both markers were taken up at a higher rate in denervated muscle. The increase in uptake after denervation was, however, larger for HRP than for [3H]inulin. After 2 h incubation at 37 degrees C, pH 7.3, in the presence of equimolar concentrations of HRP and [3H]inulin (approx. 2.1 microM), the uptake of HRP was approx. 8 times as great as the uptake of [3H]inulin in the same innervated muscles. In denervated muscle the HRP uptake was approx. 19 times as great as the [3H]inulin uptake in the same muscles. Various possible explanations of these differences in uptake have been considered and tested experimentally. [3H]Inulin uptake in skeletal muscle has previously been shown to obey bulk kinetics. The present investigation shows the HRP uptake to obey saturation kinetics. The HRP uptake shows dependency on divalent cations and is reduced if incubation is carried out at pH 6.4. The uptake of HRP, when used at a low, non-saturating concentration (10 micrograms/ml approx. 0.25 microM), is inhibited greater than or equal to 60% by yeast mannan (0.1 mg/ml), ribonuclease B (0.1 mg/ml, approx. 7.4 microM), mannose (30 mM), monodansylcadaverine (1 mM), chloroquine (100 microM), trifluoperazine (25 microM) or maleic acid (2 mM). It is concluded that HRP is taken up in innervated and denervated skeletal muscle by a process of receptor-mediated endocytosis and that this uptake is under neurotrophic control.