Muscle breakdown and lysosomal activation (biochemistry)

Systematic Review and Meta-Analyses of Aminopeptidases as Prognostic Biomarkers in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the progressive loss of motor neurons in the spinal cord, brain stem, and cerebral cortex. Biomarkers for ALS are essential for disease detection and to provide information on potential therapeutic targets. Aminopeptidases catalyze the cleavage of amino acids from the amino terminus of protein or substrates such as neuropeptides. Since certain aminopeptidases are known to increase the risk of neurodegeneration, such mechanisms may reveal new targets to determine their association with ALS risk and their interest as a diagnostic biomarker. The authors performed a systematic review and meta-analyses of genome-wide association studies (GWASs) to identify reported aminopeptidases genetic loci associated with the risk of ALS. PubMed, Scopus, CINAHL, ISI Web of Science, ProQuest, LILACS, and Cochrane databases were searched to retrieve eligible studies in English or Spanish, published up to 27 January 2023. A total of 16 studies were included in this systematic review, where a series of aminopeptidases could be related to ALS and could be promising biomarkers (DPP1, DPP2, DPP4, LeuAP, pGluAP, and PSA/NPEPPS). The literature reported the association of single-nucleotide polymorphisms (SNPs: rs10260404 and rs17174381) with the risk of ALS. The genetic variation rs10260404 in the DPP6 gene was identified to be highly associated with ALS susceptibility, but meta-analyses of genotypes in five studies in a matched cohort of different ancestry (1873 cases and 1861 control subjects) showed no ALS risk association. Meta-analyses of eight studies for minor allele frequency (MAF) also found no ALS association for the "C" allele. The systematic review identified aminopeptidases as possible biomarkers. However, the meta-analyses for rs1060404 of DPP6 do not show a risk associated with ALS.

Impaired autophagy in sporadic inclusion-body myositis and in endoplasmic reticulum stress-provoked cultured human muscle fibers

The hallmark pathologies of sporadic inclusion-body myositis (s-IBM) muscle fibers are autophagic vacuoles and accumulation of ubiquitin-positive multiprotein aggregates that contain amyloid-beta or phosphorylated tau in a beta-pleated sheet amyloid configuration. Endoplasmic reticulum stress (ERS) and 26S proteasome inhibition, also associated with s-IBM, putatively aggrandize the accumulation of misfolded proteins. However, autophagosomal-lysosomal pathway formation and function, indicated by autophagosome maturation, have not been previously analyzed in this system. Here we studied the autophagosomal-lysosomal pathway using 14 s-IBM and 30 disease control and normal control muscle biopsy samples and our cultured human muscle fibers in a microenvironment modified to resemble aspects of s-IBM pathology. We report for the first time that in s-IBM, lysosomal enzyme activities of cathepsin D and B were decreased 60% (P < 0.01) and 40% (P < 0.05), respectively. We also detected two indicators of increased autophagosome maturation, the presence of LC3-II and decreased mammalian target of rapamycin-mediated phosphorylation of p70S6 kinase. Moreover, in cultured human muscle fibers, ERS induction significantly decreased activities of cathepsins D and B, increased levels of LC3-II, decreased phosphorylation of p70S6 kinase, and decreased expression of VMA21, a chaperone for assembly of lysosomal V-ATPase. We conclude that in s-IBM muscle, decreased lysosomal proteolytic activity might enhance accumulation of misfolded proteins, despite increased maturation of autophagosomes, and that ERS is a possible cause of s-IBM-impaired lysosomal function. Thus, unblocking protein degradation in s-IBM muscle fibers may be a desirable therapeutic strategy.

Dipeptidyl peptidase II (DPPII), a review

An increasing number of biological processes appear to be regulated by Pro-specific N-terminal processing. The proline-specific dipeptidyl peptidases (DPPs) like DPPIV, fibroblast activation protein alpha (FAP), DPPII, DPP8 and DPP9, because of their preference for cleavage after X-Pro in vitro, are likely to be involved in many of these processes. These DPPs are emerging as an important protease family with roles in the regulation of signaling by peptide hormones. Dipeptidyl peptidase II (DPPII, E.C. 3.4.14.2) is an intracellular protease that localizes to the vesicular system. It releases, preferably at acidic pH, N-terminal dipeptides from oligopeptides with Pro or Ala in the penultimate position. Despite the fact that the physiological role of DPPII still has not been elucidated, several suggestions were made on possible functions of the enzyme depending on its localization in different cells, body fluids and organs. DPPII was a.o. suggested to be involved in the processes of cell differentiation and in the protection from cell death, and to have a role in the degradation of collagen fragments, myofibrillar proteins and short neuropeptides. Moreover, changes in the level and distribution of the enzyme provided clues indicating additional roles in disease-related processes. Here we review the DPPII literature, aiming to bring more clarity in the disperse data on this subject and give a state of the art on DPPII research.

Calcium misregulation and the pathogenesis of muscular dystrophy

Although the exact nature of the relationship between calcium and the pathogenesis of Duchenne muscular dystrophy (DMD) is not fully understood, this is an important issue which has been addressed in several recent reviews (Alderton and Steinhardt, 2000a, Gailly, 2002, Allen et al., 2005). A key question when trying to understand the cellular basis of DMD is how the absence or low level of expression of dystrophin, a cytoskeletal protein, results in the slow but progressive necrosis of muscle fibres. Although loss of cytoskeletal and sarcolemmal integrity which results from the absence of dystrophin clearly plays a key role in the pathogenesis associated with DMD, a number of lines of evidence also establish a role for misregulation of calcium ions in the DMD pathology, particularly in the cytoplasmic space just under the sarcolemma. A number of calcium-permeable channels have been identified which can exhibit greater activity in dystrophic muscle cells, and exIsting evidence suggests that these may represent different variants of the same channel type (perhaps the transient receptor potential channel, TRPC). In addition, a prominent role for calcium-activated proteases in the DMD pathology has been established, as well as modulation of other intracellular regulatory proteins and signaling pathways. Whether dystrophin and its associated proteins have a direct role in the regulation of calcium ions, calcium channels or intracellular calcium stores, or indirectly alters calcium regulation through enhancement of membrane tearing, remains unclear. Here we focus on areas of consensus or divergence amongst the existing literature, and propose areas where future research would be especially valuable.

Function and genetics of dystrophin and dystrophin-related proteins in muscle

The X-linked muscle-wasting disease Duchenne muscular dystrophy is caused by mutations in the gene encoding dystrophin. There is currently no effective treatment for the disease; however, the complex molecular pathology of this disorder is now being unravelled. Dystrophin is located at the muscle sarcolemma in a membrane-spanning protein complex that connects the cytoskeleton to the basal lamina. Mutations in many components of the dystrophin protein complex cause other forms of autosomally inherited muscular dystrophy, indicating the importance of this complex in normal muscle function. Although the precise function of dystrophin is unknown, the lack of protein causes membrane destabilization and the activation of multiple pathophysiological processes, many of which converge on alterations in intracellular calcium handling. Dystrophin is also the prototype of a family of dystrophin-related proteins, many of which are found in muscle. This family includes utrophin and alpha-dystrobrevin, which are involved in the maintenance of the neuromuscular junction architecture and in muscle homeostasis. New insights into the pathophysiology of dystrophic muscle, the identification of compensating proteins, and the discovery of new binding partners are paving the way for novel therapeutic strategies to treat this fatal muscle disease. This review discusses the role of the dystrophin complex and protein family in muscle and describes the physiological processes that are affected in Duchenne muscular dystrophy.

Lysosomal enzyme activities in skeletal muscle of patients with neuromuscular diseases

Activities of nine lysosomal enzymes and pH-dependent isozyme patterns of alpha-mannosidase were examined in the skeletal muscle of patients with neuromuscular diseases, and the ratios of these enzyme activities in leukocytes to those in myocytes (L/M ratio) were determined. The activities of enzymes with a high L/M ratio were markedly increased in the muscles of patients with Duchenne muscular dystrophy (DMD), myotonic dystrophy (MyD), or polymyositis (PM). In contrast, those which showed a low L/M ratio were increased in the muscles of amyotrophic lateral sclerosis (ALS) and disuse muscle atrophy (DUA). The isozyme pattern of alpha-mannosidase in DMD muscle resembled that in leukocytes, while those in ALS and DUA muscle resembled that in normal muscle. These results may suggest that the increased activity of lysosomal enzymes in the muscles of patients with DMD, MyD, or PM is due primarily to infiltrating leukocytes, while that in patients with ALS or DUA is due to intramyofiber lysosomes.

The role of lipid peroxidation in McArdle's disease: applications for treatment of other myopathies

Experimental therapies for McArdle's disease have been directed toward increasing substrate availability to exercising muscle. Such therapies to date have proven largely unsuccessful. These include administration of isoproterenol to increase blood flow, glucagon treatment to elevate serum glucose and increased dietary fat intake. Each of these therapies also results in greater levels of unesterified fatty acids in blood. More recently, a high protein diet is suggested to provide increased amounts of amino acids which would be available as fuel sources. We hypothesize that the absence of myophosphorylase in McArdle's disease creates an imbalance between the enzymes of the redox systems that control the generation, propagation and inactivation of free radicals. This occurs because muscle cells are forced to rely more heavily on fatty acid oxidation. The resulting free radical damage to cellular components disrupts metabolic control and increases the permeability of membranes. Elevated levels of Ca2+ in the sarcoplasm activate proteases, phospholipases and other catabolic enzymes initiating muscle fatigue and cramping. Lipid peroxidation is a consequence of normal muscle activity and may occur unchecked in individuals with McArdle's disease. Continued muscle activity in the absence of a favorable nutritional environment may promote the progression of the disease by increasing susceptibility to oxidative stress.

The pentose phosphate pathway in skeletal muscle under patho-physiological conditions. A combined histochemical and biochemical study

Over the last 30 years, research into the neuromuscular apparatus, has expanded greatly. Multidisciplinary investigations have rapidly advanced our understanding both of diseases and of the basic neuromuscular mechanisms. The mode of pathological reaction of the neuromuscular apparatus is now quite well understood. The most notable aspect of the reaction of the injured neuromuscular apparatus is the remarkably stereotyped character of the resulting pathological changes as demonstrated by a wide variety of harmful causes, producing surprisingly similar effects. The findings of our combined histochemical and biochemical investigations presented in this monograph, are in complete harmony with the stereotyped character of the pathological changes. For example, it is particularly striking that many affected muscle fibres of patients with muscular dystrophies, congenital myopathies, inflammatory myopathies, metabolic myopathies, endocrine myopathies, or with diseases of the lower motor neuron, display an enhanced activity of both oxidative enzymes of the pentose phosphate pathway. Likewise, we found that experimental animals with disordered skeletal muscles, provoked by different types of agents or treatments, reveal the same marked rise in activity of GPDH and PGDH in the muscle fibres, with a positive correlation between the activity of both enzymes. Other findings of our investigations point to a positive correlation between the activity of GPDH and PGDH on the one hand and that of the non-oxidative enzymes of the pentose phosphate pathway, the enzymes TA, TK, RPI and RPE on the other hand. The rise in activity of PGDH and, in particular, of GPDH is regulated by two different mechanisms. The first represents a rapid control mechanism based on the stimulation of both oxidative enzymes of the pentose phosphate pathway by NADP+ and on their inhibition by NADPH. The other mechanism represents a long-term effect directed at the synthesis of the enzymes. It is this type of mechanism which is responsible for the rise in activity of GPDH and PGDH we observed. The findings obtained with the applied enzyme histochemical techniques clearly demonstrated that the rise in activity of both enzymes is not homogeneously distributed in the disordered skeletal muscles of man and experimental animals. For that reason, in order to obtain reliable quantitative information about enzyme activities in the muscle fibres themselves, the application of biochemical assays on a micro-scale was indispensable. The biochemical assay of enzyme activities was performed on histologically and histochemically selected dissected muscle specimens.(ABSTRACT TRUNCATED AT 400 WORDS)

Rapid neural regulation of muscle urokinase-like plasminogen activator as defined by nerve crush

Muscle plasminogen activators (PAs), such as urokinase-like PA and, to a lesser extent, tissue PA, increase dramatically after denervation induced by axotomy. The PA/plasmin system has also been implicated in degradation of specific components of the muscle fiber basement membrane after local activation of plasminogen. These results suggest that neural regulation of muscle extracellular matrix metabolism accompanies or precedes regeneration after injury and is mediated by activation of PAs. In the present study, we have used nerve crush to explore the neural regulation of muscle PA activities directly after subtotal axon interruption and during the process of reinnervation. Muscle contraction after nerve stimulation and estimation of choline acetyltransferase activity were used to monitor reinnervation. Within 24 hr of nerve crush, muscle urokinase (but not tissue PA) activity rose in soluble and membrane-bound muscle fractions, as shown by an amidolytic assay and a fibrin zymography. Membrane-bound activity was 5-fold higher than cytosol activity, but there was no shift between cellular compartments during the time course of denervation. Coincident with the return of choline acetyltransferase activity and muscle contractility, muscle urokinase returned almost to baseline levels. These results show tight regulation of muscle urokinase levels by some neural influence.

High molecular weight kininogen, the extracellular inhibitor of thiol proteases, is deficient in hamsters with muscular dystrophy

High molecular weight kininogen has been shown to be the principal plasma inhibitor of cellular thiol proteases including cathepsins B, H and L and calpains 1 and 2. Since these same enzymes have been reported to be elevated in animals with muscular dystrophy, we studied plasmas from hamsters with muscular dystrophy and compared these to normal hamster plasma. The ability of plasma to inhibit purified platelet calpain was assayed and found to be 62% of normal. Since low molecular weight kininogen can also inhibit calpain, the coagulant activity of kininogen, an activity unique for high molecular weight kininogen, was determined in dystrophic hamster plasma and found to be 69% of normal in close agreement with the calpain inhibitory activity. The contribution of the other plasma calpain inhibitor alpha 2-macroglobulin appeared small since inactivation with methylamine did not alter the ability to inhibit calpain in either normal or dystrophic plasma. We conclude that there is a selective deficiency of plasma high molecular weight kininogen in dystrophic hamsters, an abnormality which could play a role in the pathogenesis of this disorder.

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.

Are lysosomal enzymes involved in rapid damage in vertebrate muscle cells? A study of the separate pathways leading to cellular damage

Experiments with lysosomotropic agents suggest that the sarcotubular system subserves some of the functions of the lysosomal apparatus in frog skeletal muscle. Dinitrophenol or A23187 trigger lysosome labilization and myofilament damage in mammalian cardiac muscle. Lysolecithin labilizes isolated liver lysosomes, but has no action following phospholipase A2 activation in vivo. Zinc ions or a pHi of 7.5 do not protect against myofilament damage. In fractions from mammalian cardiac muscle, calcium and calmodulin do not cause lysosomal labilization whereas cGMP does but only at high concentration (10(-4) M). It is concluded that lysosomal hydrolases play no significant part in rapid muscle damage. It is suggested that rises in [Ca]i activate two separate pathways causing (i) myofilament damage; (ii) sarcolemmal (and possibly lysosomal) membrane damage via phospholipase A2 and lipoxygenase activity. Dinitrophenol triggers both pathways independently and thus may cause lysosome labilization. The possibility that the sarcoplasmic reticulum is the site generating myofilament damage is discussed.

Free radicals: a potential pathogenic mechanism in inherited muscular dystrophy

Despite years of intensive work, the biochemical defect responsible for the pathogenesis of inherited muscular dystrophy has not been identified either in humans or animal models. This review examines evidence in support of the hypothesis that free radicals may be responsible for muscle degeneration in this disorder. A variety of cellular abnormalities noted in dystrophic muscles can be accounted for by free radical mediated damage. In addition, chemical by-products associated with free radical damage are found in dystrophic muscle tissue from humans and animals with this disease. Various enzymatic antioxidant systems can be enhanced as a normal cellular response to oxidative stress, and such changes are seen both in dystrophic muscle cells and certain other tissues of dystrophic animals. An increased level of free radical damage would follow from either: enhanced production of free radical species, or a deficient component of the cellular antioxidant system, such as vitamin E. The free radical hypothesis of muscular dystrophy can account for data supporting several alternative theories of the pathogenesis of this disease, as well as other observations which have not previously been explained.

Plasminogen activator in mammalian skeletal muscle: characteristics of effect of denervation on urokinase-like and tissue activator

Analyses were made of the fibrinolytic, plasminogen-activating system in skeletal muscle to determine if a regulating influence of the nerve could be detected on these enzymes. Young male mice underwent right sciatic neurectomy. Extracts were prepared from denervated muscle at 2-17 d after axotomy and compared with controls. Using a cascade-style biochemical assay (Rånby, M., B. Norrman, and P. Wallén, 1982, Thromb. Res., 27:743-748) we found that low levels of plasminogen activator (PA) were present in adult, innervated mouse muscle, but that denervation resulted in a marked time-dependent increase in enzyme activity. Qualitative separation showed an eightfold increase in urokinase-like PA with moderate elevation of tissue PA activity after 10 d. Fibrin zymography (Granelli-Piperno, A., and E. Reich, 1978, J. Exp. Med., 148:223-234) revealed clear zones of lysis corresponding to molecular masses of 48 kD for urokinase-like PA and 75 kD for tissue PA, consistent with the molecular masses found for these enzymes in other tissues of the mouse (Danø, K., P. A. Andreasen, J. Grøndahl-Hansen, P. Kristensen, L. S. Nielsen, and L. Skriver, 1985, Adv. Cancer Res., 44:139-266). In other studies we have shown that PA-activated plasmin readily attacks critical adhesive basement membrane molecules. The present results indicate that enzymes involved in plasminogen activation, particularly urokinase-like PA, rapidly increase after axotomy, suggesting they may have a role early in muscle denervation. Similar alterations in PA activity might underlie the elimination of polyneuronal innervation during mammalian muscle development. Certain neuromuscular diseases may also involve activation of these enzymes, resulting in degradation of basement membrane zone components and, therefore, warrant further study.

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.

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.

Effects of stretch and denervation on protease activities of normal and dystrophic chicken muscle

Changes of protease activities that follow passive stretch, denervation, and denervation plus stretch were followed in the patagialis muscle of normal and dystrophic chicks between 6 and 7 weeks of age. The baseline activities of cathepsin C, cathepsin D, and leucine aminopeptidase in dystrophic muscle were 2 to 3.5 times higher than in normal muscle. Passive stretch and denervation induced increases in protease activities by 40 to 120% in normal muscle, whereas the same treatments did not significantly affect the activities of the enzymes in dystrophic muscle. We conclude that the level of protease activity in dystrophic chicken muscle at 6 weeks of age had already attained a maximum limit and could not be increased even by denervation. In spite of protease activities, which were not different from control dystrophic muscle, denervated dystrophic muscles lost muscle weight rapidly whether they were stretched or not. They weighed 60% less than the innervated control muscle after 7 days. Inherently high protease activities in dystrophic muscle do not vary at this age regardless of whether or not the muscle is gaining or losing weight.

Successful treatment of murine muscular dystrophy with the proteinase inhibitor leupeptin

Mice with genetic muscular dystrophy were treated with intraperitoneal injections of the proteinase inhibitor leupeptin, beginning before the onset of weakness. A significant number of the treated animals failed to develop histological evidence of dystrophy, compared with controls. Leupeptin treatment prevented (or delayed) the onset of muscular dystrophy in this experiment.

Myofibrillar protein degradation in the chicken. 3-Methylhistidine release in vivo and in vitro in normal and genetically muscular-dystrophic chickens

Myofibrillar protein degradation was measured in 4-week-old normal (line 412) and genetically muscular-dystrophic (line 413) New Hampshire chickens by monitoring the rates of 3-methylhistidine excretion in vivo and in vitro. A method of perfusing breast and wing muscles was developed and the rate of 3-methylhistidine release in vitro was measured between 30 and 90min of perfusion. During this perfusion period, 3-methylhistidine release from the muscle preparation was linear, indicating that changes in 3-methylhistidine concentration of the perfusate were the result of myofibrillar protein degradation. Furthermore, the viability of the perfused muscle was maintained during this interval. After 60min of perfusion, ATP, ADP and creatine phosphate concentrations in pectoral muscle were similar to muscle freeze-clamped in vivo. Rates of glucose uptake and lactate production were constant during the perfusion. In dystrophic-muscle preparations, the rate of 3-methylhistidine release in vitro (nmol/h per g of dried muscle) was elevated 2-fold when compared with that in normal muscle. From these data the fractional degradation rates of myofibrillar protein in normal and dystrophic pectoral muscle were calculated to be 12 and 24% respectively. Daily 3-methylhistidine excretion (nmol/day per g body wt.) in vivo was elevated 1.35-fold in dystrophic chickens. Additional studies revealed that the anti-dystrophic drugs diphenylhydantoin and methylsergide, which improve righting ability of dystrophic chickens, did not alter 3-methylhistidine release in vitro. This result implies that changes in myofibrillar protein turnover are not the primary lesion in avian muscular dystrophy. From tissue amino acid analysis, the myofibrillar 3-methylhistidine content per g dry weight of muscle was similar in normal and dystrophic pectoral muscle. More than 96% of the 3-methylhistidine present in pectoral muscle was associated with the myofibrillar fraction. Dystrophic myofibrillar protein contained significantly less 3-methylhistidine (nmol/g of myofibrillar protein) than protein from normal muscle. This observation supports the hypothesis that there may be a block in the biochemical maturation and development of dystrophic muscle after hatching. Free 3-methylhistidine (nmol/g wet wt.) was elevated in dystrophic muscle, whereas blood 3-methylhistidine concentrations were similar in both lines. In summary, the increased myofibrillar protein catabolism demonstrated in dystrophic pectoral muscle correlates with the increased lysosomal cathepsin activity in this tissue as reported by others.

The role of proteases in experimental glucocorticoid myopathy

Mature, male, New Zealand white rabbits were treated with the synthetic glucocorticoid betamethasone (0.3 mg/kg body weight/day) for 2 weeks. The glucocorticoid treatment caused a 30% decrease in muscle weight of the type 2 psoas muscle, but had no apparent effect on the type 1 soleus muscle. Cathepsin D activity was elevated twofold in the psoas muscle of treated rabbits, a finding suggesting an active role for lysosomes in mediating muscle breakdown in glucocorticoid-induced myopathy of the rabbit. There was no detectable alkaline serine protease activity in the muscles from either treated or control rabbits. Alkaline protease is localized in mast cells in some species, particularly the rat. Toluidine blue staining for mast cells was absent in rabbit muscles, a finding indicating that this species does not contain these cells. This protease, previously implicated in glucocorticoid myopathy, apparently plays no role in rabbit myopathy. There was no detectable elevation of the Ca-activated protease in muscles from glucocorticoid-treated animals. This finding suggests that if this protease plays a role in muscle degradation, its activity is controlled in vivo by special conditions (such as elevated CA levels, inhibitors, and compartmentalizations).

Protein synthesis and degradation in skeletal muscle of normal and dystrophic hamsters

Dystrophic hamsters (BIO 53.58) had lower body weights and gastrocnemius muscle weights than normal hamsters (BIO RB). Dystrophic muscle contained less protein than normal muscle. The proportion of collagenous to noncollagenous protein remained unchanged. Loss of protein in the dystrophic muscle was the result of an increase in the rate of protein degradation. This was accompanied by higher activities of two lysosomal proteases, cathepsins B and D. The net effect of the increase in protein degradation was blunted by an increase in the rate of synthesis of total protein and myosin. The faster rate of synthesis in dystrophic muscle was partially due to an increase in the concentration of cellular RNA. Rates of peptide-chain initiation and protein synthesis decreased in muscles of normal hamsters perfused in the absence of insulin. In the presence of insulin, these processes were maintained at higher rates. However, the rate of protein synthesis in dystrophic muscle appeared less insulin-dependent than normal muscle. Protein degradation was inhibited by insulin in both types of muscle.

2,4-dinitrophenol, lysosomal breakdown and rapid myofilament degradation in vertebrate skeletal muscle

1. The possibility that rapid Ca2+-uptake by skeletal muscle mitochondria may cause local reductions in pHi (by H+/Ca2+ exchange) and so promote lysosomal breakdown has been explored using amphibian and mammalian preparations. Recent studies suggested that such a sequence of events is possible in cardiac muscle. 2. However, extensive muscle damage can still be initiated in skeletal muscle when the mitochondria are uncoupled so that Ca2+-uptake is prevented. 3. DNP alone induces extensive myofilament degradation which is similar to that produced by A23187 and caffeine and described previously. 4. It is suggested that (a) the known action of DNP in promoting lysosomal labilization in living cells is produced by mitochondrial uncoupling and the release of stored Ca2+, (b) raised [Ca2+]i promotes lysosomal breakdown in skeletal muscle, so that the hydrolases released effect myofilament dissolution rapidly. 5. DNP also rapidly causes septation and division of the mitochondria in mammalian skeletal muscle.

Catalase, superoxide dismutase, glutathione reductase and thiobarbituric acid-reactive products in normal and dystrophic human muscle

The level of thiobarbituric acid-reactive products and the specific activities of catalase and glutathione reductase were significantly higher in muscles from patients with major forms of muscular dystrophies over those of control subjects. Superoxide dismutase activity was not altered in dystrophic muscles. These findings indicate the occurrence in human dystrophic muscles of an increased level of lipid peroxidation and the possible activation of certain enzymes that could conceivably inhibit lipid peroxidation in vivo.