Endocytosis and lysosomal enzyme activities in dystrophic muscle: the effect of denervation

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.

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.

Different mechanisms of increased proteolysis in atrophy induced by denervation or unweighting of rat soleus muscle

Mechanisms of accelerated proteolysis were compared in denervated and unweighted (by tail-cast suspension) soleus muscles. In vitro and in vivo proteolysis were more rapid and lysosomal latency was lower in denervated than in unweighted muscle. In vitro, lysosomotropic agents (eg, chloroquine, methylamine) did not lessen the increase in proteolysis caused by unweighting, but abolished the difference in proteolysis between denervated and unweighted muscle. Leucine methylester, an indicator of lysosome fragility, lowered latency more in denervated than in unweighted muscle. 3-Methyladenine, which inhibits phagosome formation, increased latency similarly in all muscles tested. Mersalyl, a thiol protease inhibitor, and 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate hydrochloride (TMB-8), which antagonizes sarcoplasmic reticulum release of Ca2+, reduced accelerated proteolysis caused by unweighting without diminishing the faster proteolysis due to denervation. Calcium ionophore (A23187) increased proteolysis more so in unweighted than control muscles whether or not Ca2+ was present. Different mechanisms of accelerated proteolysis were studied further by treating muscles in vivo for 24 hours with chloroquine or mersalyl. Chloroquine diminished atrophy of the denervated but not the unweighted muscle, whereas mersalyl prevented atrophy of the unweighted but not of the denervated muscle, both by inhibiting in vivo proteolysis. These results suggest that (1) atrophy of denervated, but not of unweighted, soleus muscle involves increased lysosomal proteolysis, possibly caused by greater permeability of the lysosome, and (2) cytosolic proteolysis is important in unweighting atrophy, involving some role of Ca2(+)-dependent proteolysis and/or thiol proteases.

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.

The effect of stretch removal on muscle weight and proteolytic enzyme activity in normal and dystrophic chicken muscles

Hypertrophy was induced in the patagialis (PAT) muscle of 6-week-old normal and dystrophic chicks by passive stretch for 1 week. Stretch was then removed and muscle weights and activities of the proteolytic enzymes cathepsin C, cathepsin D, and leucine aminopeptidase (LAPase) were measured after 3, 5, 7, 10, and 14 days. In both genotypes, weights of stretch-released muscles dropped progressively for 7 days relative to control muscles, after which they were not significantly different. At the time of stretch release, proteolytic enzyme activities were approximately twice as high in stretched normal muscles as in normal control muscles. In dystrophic chicks there was no difference in activities between stretched and control muscles. However, the activities of the enzymes in dystrophic muscles were already about 4 times higher than in normal control muscles. After stretch release, the enzyme activities in normal muscle progressively fell for 10 days, after which they were not different from normal control muscles. In dystrophic muscles the enzyme activities remained elevated and were not different from dystrophic control muscle activities at any time. We conclude that degradative enzyme activities in normal muscle closely parallel changes in muscle weight, whereas in dystrophic muscle proteolytic enzymes remain elevated and constant whether the muscle is gaining or losing weight.