Lysosomes in skeletal muscle following denervation. Time course of horseradish peroxidase uptake and increase of lysosomal enzymes

Sarcopenia in critically ill patients

Sarcopenia occurring as a primary consequence of aging and secondary due to certain medical problems including chronic disease, malnutrition and inactivity is a progressive generalized loss of skeletal muscle mass, strength and function. The prevalence of sarcopenia increases with aging (approximately 5-13 % in the sixth and seventh decades). However, data showing the prevalence and clinical outcomes of sarcopenia in intensive care units (ICUs) are limited. A similar condition to sarcopenia in the ICU, called ICU-acquired weakness (ICU-AW), has been reported more frequently. Here, we aim to examine the importance of sarcopenia, especially ICU-AW, in ICU patients via related articles in Medline.

Rate constants of acetylcholine receptor internalization and degradation in mouse muscles

The rate constants for internalization and subsequent extrusion of acetylcholine receptors (AChRs) during degradation in adult innervated and denervated mouse diaphragm muscles were determined using proteinase K (PK) digestion. This procedure separated (125)I-alpha-bungarotoxin (Bgt)-labeled AChRs into PK-sensitive and PK-resistant compartments. The time course of the residual radioactivity in these two compartments suggested that they represented surface membrane and internalized compartments, respectively. The data were compatible with a mathematical model based on the assumption that during degradation of AChRs a surface compartment, A, fed an internal compartment, B, with an internalization rate constant (k(i)), and that B is drained from the cell with an extrusion rate constant (k(o)). Using the mathematical model, we were able to determine that k(i) and k(o) were, respectively, 0.068 (t(1/2) approximately 10.2 days) and 0.69-0.55 (t(1/2) approximately 1.0- 1.25 days) for innervated muscle and were, respectively, 0.69 (t(1/2) approximately 1.0 day) and 6.93 (t(1/2) approximately 0.1 day) for denervated muscle. Thus, the rate for internalization was about 8-10 times slower than that for extrusion from the cell for both the slowly degrading innervated (Rs) AChRs and for the rapidly degrading denervated (Rr) AChRs. This inequality between k(i) and k(o) therefore allows the combined quantity of A(t) + B(t), usually measured in AChR degradation studies, to approximate a single exponential.

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.

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.

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.

Dextrans as markers for endocytosis in innervated and denervated skeletal muscle

Fluorescence-labeled dextrans were evaluated as markers for endocytosis in skeletal muscle. Fluorescein isothiocyanate (FITC)-labeled dextrans (average molecular weight 3900 to 71200) showed a higher uptake in denervated than in innervated muscle both in vitro and in vivo. The in vitro uptake of FITC-dextran (35.600) increased linearly with time at 37 degrees C, and was almost completely inhibited by low temperature (4 degrees C). The uptake was not a pure bulk uptake, because a saturable component was evident from the concentration dependence and from competition experiments with unlabeled dextran. The uptake of FITC-labeled or rhodamine B isothiocyanate (RITC)-labeled dextrans in denervated muscle occurred mainly in small segments of the fibers centered around the denervated endplate region. However, not all denervated fibers showed such segments. Periodic acid Schiff's base staining for carbohydrates stained dextrans in denervated muscle fibers. Some staining, probably of lysosomes, was also observed in denervated muscle not exposed to dextran.

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.

Proteolytic enzyme activities during regeneration of the rat gastrocnemius muscle

The enzymatic activity of creatine phosphokinase and the lysosomal enzymes cathepsin D and acid phosphatase was followed during skeletal muscle regeneration after partial excision to the gastrocnemius muscle in the rat. For each time interval (1, 2, 5, 14 and 45 days) following injury, the activity of the regenerated muscle was compared with the activity in the contralateral sham operated muscle. The specific activity of creatine phosphokinase of the regenerated muscle showed a significant decrease (25%) during the first 2 days post injury and thereafter was comparable to that of the uninjured control muscle. The activity of cathepsin D was 2.3-4-fold significantly higher in the regenerated muscle than in the control intact muscle from day 1 until day 14 post-injury. At 45 days after partial excision, the activity of this enzyme was comparable to a normal muscle. However, the activity of another lysosomal enzyme (acid phosphatase) did not show any distinct changes from the level of this enzyme in the uninjured muscle during the course of muscle regeneration. It is suggested that elevation of lysosomal enzymes in skeletal muscle may not be confined to conditions of muscle wasting and degradation but also to differentiation and development processes such as during muscle regeneration following injury.

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.

Endocytosis and autophagy in dying neurons: an ultrastructural study in chick embryos

In an effort to understand naturally occurring neuronal death in the developing isthmo-optic nucleus, we have accentuated one of its most probably causes, failure to receive adequate trophic maintenance from the axonal terminal zone in the retina, and have studied the dying neurons ultrastructurally. Retrograde trophic maintenance was blocked by means of intraocularly injected colchicine, which caused all the isthmo-optic neurons to die by just one of the two or more kinds of cell death that they undergo during normal development. The present paper deals with the very prominent cytoplasmic aspects of this kind of cell death, notably the uptake of exogeneous horseradish peroxidase and autophagy. There were also nuclear changes, which are dealt with mainly in the accompanying paper (Clarke and Hornung, J. Comp. Neurol. 283:438-449,'89). Numerous cytoplasmic vacuoles occurred in both soma and dendrites, and they were of three main kinds, of which the smallest (less than 0.5 microns diameter) had unstructured contents, whereas the larger two (1-2 microns and 2-7 microns) were secondary lysosomes (mostly residual bodies). Intravascularly injected horseradish peroxidase labeled all three kinds of vacuole but not the free cytoplasm, indicating that the uptake was by endocytosis rather than by leakage through holes in the membrane, as is confirmed by our failure to detect any such holes. We suspect that the smallest vacuoles are the primary endosomes, that these subsequently fuse with vacuoles of the intermediate kind, and that the largest vacuoles are formed by the fusion of these latter. The purpose of the endocytosis may be to channel the plasma membrane piecemeal into the lysosomes for destruction.

Cytotoxic action of the lysosomotropic agent L-leucine methyl ester on mammalian skeletal muscle. The role of the sarcoplasmic reticulum in producing myofilament damage

Incubation in vitro of mouse or rat diaphragms or mouse soleus muscle with the lysosomotropic agent leucine methyl ester (10 mM) produced a slowly-developing swelling of the sarcoplasmic reticulum (SR), showing that this organelle is able to take up amino acid methyl esters and conforming with previous suggestions that the SR may serve some of the functions of the lysosomal system in muscle cells. Cellular damage followed, sometimes associated with the shrinkage of the SR, but experiments with inhibitors of lysosomal cathepsins suggest that acid hydrolases were not implicated in this damage. It is suggested that the system producing myofilament damage is located on the SR and that it may be directly activated by membrane perturbation.

Regulation of lysosomal proteolysis in burn injury

The regulation of the lysosomal pathway by burn injury was investigated using a local burn model in which one hindlimb of a rat is scaled at 85 degrees C for 3.6 s. By two days postinjury the rate of net protein breakdown in the incubated soleus muscle from the burned leg is doubled. The activity of cathepsin D increases 40% to 50% and that of cathepsins B and L increase 80% to 100%. The activity of a lysosomal nonprotease activity, N-acetylglucosaminidase, is not significantly increased. The latency of lysosomal enzymes (an estimate of cellular autophagy) did not change at any time postburn. Inhibitors of autophagy (ie, leucine and 3-methyladenine) inhibited net protein breakdown and increased latency to a similar extent in muscles from control and burned legs. Thus, there is no evidence that a change in cellular autophagy is responsible for the increased proteolysis seen in intact muscle. However, burn-induced changes in alternative routes of protein sequestration cannot be excluded. Burn did not increase either receptor-mediated or fluid phase endocytosis by incubated soleus muscle. Burn injury also did not reduce the inhibitory action of cytoplasmic inhibitors of cathepsins B and L in skeletal muscle. However, burn injury markedly stimulated the synthesis of glycoproteins in the microsomal fraction without affecting overall protein synthesis. This increase in synthesis preceded the rise in lysosomal protease activity. These results support the possibility that induction of lysosomal protease synthesis may underlie burn-induced increases in muscle proteolysis.

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.

Cytochemical localization of acid phosphatase in striated muscle

Normal skeletal and cardiac striated muscle from adult rats was incubated for the cytochemical detection of acid phosphatase activity with cerium as the capture metal. Results from these experiments show that normal striated muscle has a greater number of acid phosphatase-positive structures, which are presumed to be lysosomes, than has been indicated by several previous cytochemical studies.

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.

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

The spatial distribution of horseradish peroxidase (HRP) uptake has been studied by light- and electron microscopy in the denervated hemidiaphragm of the mouse. Segments with high HRP uptake were observed in a band centrally located in the denervated muscle. This distribution is similar to the well-known innervation pattern of the diaphragm. Ultrastructural studies demonstrated a high incidence of postsynaptic folds in close proximity of fibre areas with high intracellular content of HRP. 8-12 days after denervation a large number of fibres showed segments of high HRP uptake. 2-4 days after denervation very few such segments were observed. Biochemical studies also demonstrated an increase in HRP uptake after denervation occurring primarily in the endplate region. The activities of the lysosomal enzymes N-acetyl-beta-D-glucosaminidase, acid phosphatase and cathepsin D all increased after denervation, most prominently in the endplate region. It is suggested that the observed segmental uptake of HRP and lysosomal activation reflects a process for rapid membrane turnover in denervated muscle.