In vivo effects of protease inhibitors on chickens with hereditary muscular dystrophy

Increased resistance of the collagen in avian dystrophic muscle to collagenolytic attack: evidence for increased crosslinking

Fibrous collagen from normal (line 412) and dystrophic (line 413) chicken pectoral muscles was examined for susceptibility to bacterial collagenase by a pH-stat titration method to assess the kinetics of peptide bond hydrolysis. These experiments showed that there was a profound difference in the kinetics of hydrolysis using fibrous collagen purified from normal and dystrophic muscle. Although normal collagen was readily hydrolyzed by the collagenase, dystrophic collagen was not hydrolyzed in the presence of 200 microM Ca and was hydrolyzed slowly in the presence of 5 mM Ca. At this high Ca concentration, the Km for collagenase was six times higher in dystrophic collagen than in normal, whereas the maximum reaction rate (Vmax) remained the same. After CNBr digestion, peptides from both sources were readily hydrolyzed by collagenase in the presence of 200 microM Ca, and there was no noticeable difference in the reaction kinetics. The above results were not modified whether the chicken was sacrificed on day 4 or day 98 post-hatching, and the insusceptibility to collagenase preceded the clinical manifestations of dystrophy. The structure of both fibrous and CNBr-treated collagen from normal and dystrophic muscle was compared. One and two-dimensional gel electrophoreses showed identical peptide maps. Gel filtration of CNBr peptides showed in the excluded volume the presence of twice as much high-molecular-weight (presumably crosslinked) peptides from dystrophic collagen compared with normal. We conclude that the intermolecular crosslinks are more extensively formed in dystrophic collagen and that this increased crosslinking results in the increased resistance of the dystrophic collagen to collagenase.

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.

Oxidative stress and muscular dystrophy

Oxidative stress may be the fundamental basis of many of the structural, functional and biochemical changes characteristic of the inherited muscular dystrophies in animals and humans. The presence of by-products of oxidative damage, and the compensatory increases in cellular antioxidants, both indicate oxidative stress may be occurring in dystrophic muscle. Changes in the proportions and metabolism of cellular lipids, abnormal functions of cellular membranes, altered activity of membrane-bound enzymes such as the SR Ca2+-ATPase, disturbances in cellular protein turnover and energy production and a variety of other changes all indicate that these inherited muscular dystrophies appear more like the results of oxidative stress to muscle than any other type of underlying muscle disturbance. Particular details of these altered characteristics of dystrophic muscle, in combination with current knowledge on the processes of oxidative damage to cells, may provide some insight into the underlying biochemical defect responsible for the disease, as well as direct research towards the ultimate goal of an effective treatment.

Abnormal response to calmodulin in vitro of dystrophic chicken muscle membrane Ca2+-ATPase activity

A skeletal muscle membrane fraction enriched in sarcoplasmic reticulum (SR) contained Ca2+-ATPase activity which was stimulated in vitro in normal chickens (line 412) by 6 nM purified bovine calmodulin (33% increase over control, P less than 0.001). In contrast, striated muscle from chickens (line 413) affected with an inherited form of muscular dystrophy, but otherwise genetically similar to line 412, contained SR-enriched Ca2+-ATPase activity which was resistant to stimulation in vitro by calmodulin. Basal levels of Ca2+-ATPase activity (no added calmodulin) were comparable in muscles of unaffected and affected animals, and the Ca2+ optima of the enzymes in normal and dystrophic muscle were identical. Purified SR vesicles, obtained by calcium phosphate loading and sucrose density gradient centrifugation, showed the same resistance of dystrophic Ca2+-ATPase to exogenous calmodulin as the SR-enriched muscle membrane fraction. Dystrophic muscle had increased Ca2+ content compared to that of normal animals (P less than 0.04) and has been previously shown to contain increased levels of immuno- and bioactive calmodulin and of calmodulin mRNA. The calmodulin resistance of the Ca2+-ATPase in dystrophic muscle reflects a defect in regulation of cell Ca2+ metabolism associated with elevated cellular Ca2+ and calmodulin concentrations.

Slow-type C-protein in dystrophic chicken fast pectoralis muscle

C-protein isoform expression in hereditary dystrophic chicken skeletal muscle was compared with that in normal chicken muscle during postnatal development by immunocytochemical and immunoblot methods. In the pectoralis muscle (PM) of both normal and dystrophic chicken, slow- and fast-type C-proteins were coexpressed in the vast majority of myofibers at neonatal age, but the slow C-protein disappeared, leaving continued expression of only the fast-type C-protein as muscle development progressed up to 2 weeks posthatch. In the dystrophic chicken PM, however, myofibers containing slow-type C-protein reappeared about 1 month posthatch and increased in number with the progression of muscular dystrophy. We conclude that C-protein isoform expression in dystrophic myofibers resembles that in neonatal myofibers and that the expression of slow-type C-protein can be seen as a marker for chicken muscular dystrophy.

Abnormal expression of the calmodulin gene in muscle from the dystrophic chicken

Compared to that of genetically-related normal chickens, pectoralis muscle from the dystrophic chicken contained increased calmodulin measured by radioimmunoassay. Determined by the dot blot procedure, expression of the calmodulin gene was enhanced in muscle from affected animals. The bioactivity of the gene product was normal. Together with previous studies reporting increased cell Ca2+ content in dystrophic muscle, the current findings of increased sarcoplasmic calmodulin suggest the latter is a cellular response to defective Ca2+ transport at the level of cell efflux or intracellular organelle (sarcoplasmic reticulum) uptake.

Effects of percutaneous electrical stimulation on functional ability, plasma creatine kinase, and pectoralis musculature of normal and genetically dystrophic chickens

The breast musculature of genetically dystrophic Line 413 and genetically related normal Line 412 chickens were treated in three separate trials with high-frequency electrical stimulation (ES). Beginning on days 7 or 14 ex ovo, each bird received three ES treatments per week. Each stimulation cycle repeated five times per day consisted of 15 s "on" followed by 50 s "off". In the third trial only, the birds were additionally treated beginning day 3 ex ovo with either leucine (100 mg/kg) or the proteinase inhibitor Ep475 (10 mg/kg). ES significantly delayed the onset of righting disability in the dystrophic chickens. However, this improvement was temporary and could be masked by single treatments of either leucine or Ep475. Plasma creatine kinase activities were increased generally in both the stimulated normal and dystrophic birds. In two trials ES increased the relative muscle mass, and in one trial increased protein. ES had little effect on normal muscle mass or protein. However, ES treatment together with either leucine or Ep475 appeared to improve both normal and dystrophic muscle mass and protein. Furthermore ES decreased dystrophic muscle calcium but not acetylcholinesterase activity. On the other hand, ES had no effect on the total normal muscle calcium but increased normal acetylcholinesterase values. In both normal and dystrophic muscle samples, ES treatment in combination with leucine appeared to increase the mean muscle fiber diameters and number of myonuclei, and in the case of the dystrophic muscle, appeared to decrease the relative proportion of vacuolated, degenerating, and intensely oxidative histochemical fibers. In general, stimulation (especially in combination with leucine) appears to alter in varying degrees the phenotypic expression of the muscle disease exhibited in the dystrophic chicken.

Stiffness and contractile properties of avian normal and dystrophic muscle bundles as measured by sinusoidal length perturbations

Both tension and stiffness as a function of muscle length were measured under relaxing conditions on isolated small bundles of chemically skinned myofibers from normal and dystrophic chicken pectoral muscles. It was shown that the dystrophic muscle was stiffer than normal muscle and developed more tension for the same amount of stretch. A fraction of stiffness was not removed by extraction with either 0.6 M KI or with 5 M guanidine HCl mixed with 1% mercaptoethanol. The stiffness of dystrophic muscle was also unaffected by treatment with bacterial collagenase under conditions that destroyed the stiffness of tendon. Nyquist plots of normal and dystrophic muscles during calcium-activated isometric contraction were very similar and were characteristic of fast-twitch muscle, as evidenced by three clear exponential processes. The normal appearance of the Nyquist plot of dystrophic muscle demonstrates that cross-bridge function is not altered, and the characteristic slowing of contraction and relaxation is not a consequence of a fast-to-slow transformation of muscle types. The increased stiffness of dystrophic muscle may be a very fundamental change in the biomechanics of dystrophy. We postulate that the stiffness is mediated by an altered form of collagen, which is collagenase-resistant by virtue of excessive crosslinking.

Characterization of lymphocyte interferons with different species specificities from normal and genetically dystrophic chickens

Lymphocytes from thymus and spleen of normal (Line 412) and genetically dystrophic (Line 413) chickens produce two types of interferons (IFNs) with different host cell specificities. The first type, referred to as ChIFN-alpha, demonstrates antiviral activity on primary normal chicken embryo (CE) cells. This activity is stable at 60 degrees C for 1 h and, in this respect, ChIFN-alpha is similar to the standard ChIFN-beta. In contrast, the second type, referred to as ChIFN-alpha 1, demonstrates antiviral activity in human and simian cells but not in primary CE cells. This activity is labile at 60 degrees C for 1 h. The amount of these two types of IFNs produced in lymphocytes from the spleen of dystrophic chickens was fourfold greater than that produced from normal chickens under similar experimental conditions. In contrast to the lymphocytes from thymus and spleen, the lymphocytes from the bursa of both the normal and dystrophic chickens produced only one type of IFN, namely ChIFN-alpha 1. The development of antiviral state in human cells by ChIFN-alpha 1 requires host RNA synthesis. Although ChIFN-alpha 1 has antiviral properties similar to HuIFN-alpha in human cells, the two IFNs are not antigenically related.

31P-NMR studies of normal and dystrophic chicken muscle

Phosphorus-31 nuclear magnetic resonance (31P-NMR) analysis was performed on normal (line 412) and dystrophic (line 413) superficial pectoralis muscles excised from chickens between 15 and 116 days after hatching. An apparent alteration in dystrophic muscle energy metabolism was abolished by pretreatment with curare and was attributed to muscle hyperexcitability. Time-dependent 31P-NMR studies demonstrated no apparent difference in the overall tissue adenosine triphosphatase (ATPase) activity of dystrophic as compared to normal muscle. After 55 days of age, a resonance signal attributed to serine ethanolamine phosphodiester (SEPDE) was observed only in dystrophic muscle. Adding the paramagnetic cation Mn2+ to the buffer surrounding the muscle resulted in an approximate 80% decrement in the dystrophic SEPDE signal without apparent alteration of the other phosphatic signals in either the normal or dystrophic muscle. This would argue against any generalized membrane defect in dystrophic chicken muscle and suggest that SEPDE is in a compartment accessible to Mn2+.

In vivo effects of three calcium blockers on chickens with inherited muscular dystrophy

Genetically homozygous Line 413 dystrophic chickens were given in separate trials daily i.p. injections of aqueous solutions of the calcium blocker drugs, diltiazem, verapamil, or nifedipine. At a dosage of 20 mg/kg/day, drug therapy in each case significantly prolonged the functional ability of the dystrophic chickens as quantitated regularly by a standardized test for righting ability. Enhanced functional ability, however, was not generally accompanied by a decrease in the usually high plasma creatine kinase activity. In addition, there was no change in the pectoralis muscle mass or protein with any of the drug treatments. Moreover, no significant reduction in the abnormally high total muscle calcium was found with calcium blocker treatment. Also, there was no marked change in the histopathology of muscle from the drug-treated dystrophic chickens. We concluded that drugs with calcium entry blocker activity offer only limited benefit in retarding dystrophic symptoms expressed in the chicken (viz., short-term enhancement in righting ability).

Changes in turbidity during microtubule assembly in brain extracts from normal chickens and chickens with muscular dystrophy

When extracts of brain from normal and dystrophic chickens were incubated at 30 degrees C under conditions that favored microtubule assembly, the increase in turbidity of the extract from the dystrophic animal was approximately 50% less than the increase in the turbidity of the extract from the normal animal. In developmental studies using age-matched normal and dystrophic chickens this difference in turbidity was observed only with chickens 65 days ex-ovo or older. The extent of microtubule assembly, as determined by a sedimentation methodology, was a linear function of the total protein concentration in the extract and was equal in extracts from normal and dystrophic chickens. In contrast, the total increase in turbidity was much larger than could be accounted for by sedimenting microtubules and varied in a non-linear manner with the total protein concentration. There was no increase in turbidity when brain extracts that were prepared by centrifugation at 100,000 g X 40 min were incubated at 30 degrees C. The condensation reaction of the tubulin in these high speed extracts with microtubules was equal in the extracts from normal and dystrophic animals. These studies indicate that a developmentally regulated difference in brain extracts from normal and dystrophic chickens can be demonstrated by turbidimetric methods but this difference is unrelated to the formation of microtubules in these extracts. These studies support the concept that muscular dystrophy is also accompanied by a change in the central nervous system.

Drugs in muscular dystrophy of the chicken: corticosterone-21-acetate

In a previous series of 22-day evaluations of 31 compounds, only corticosterone-21-acetate (C-21-A) increased righting ability of genetically dystrophic chickens to a greater extent than the standard of comparison, methysergide maleate. In the present study, C-21-A was subjected to longer-term trials of up to 48 days, and additional signs of the myopathy were examined. The highest doses of C-21-A increased righting ability for the duration of the trials, decreased the typically elevated plasma levels of creatine kinase (CK) activity by more than 80%, and improved morphology of the dystrophic pectoralis major muscle at the light microscopic level. The major adverse effect of C-21-A, reduction of body weight, was consistently observed at the relatively high doses needed to increase righting ability. That alone, however, could not account for increased righting ability, and plasma CK activity was decreased even at doses that did not reduce body weight. The results show that C-21-A is the most effective compound yet tested in this system and, perhaps more significantly, provides the first evidence that it is possible to identify compounds that improve muscle morphology in a hereditary myopathy using a short-term, step-wise system.

Limited benefit to genetically dystrophic chickens from a synthetic proteinase inhibitor: Ep475

Chickens with inherited muscular dystrophy (Line 413) were treated in two separate trials with daily intraperitoneal injections of 10% DMSO-water solutions containing the proteinase inhibitors, Ep475 and E64. Drug therapy in each case significantly prolonged the functional ability of the treated chickens. Diluent control chickens around day 35 ex ovo characteristically reached a maximum ability to right from the supine position in a standardized functional test for muscle weakness. Subsequently, the control chickens were found to decline progressively in their ability to right. Treatment with the proteinase inhibitors had no effect on the typically elevated levels of plasma creatine kinase activity. In a histological analysis of the affected pectoralis major muscle, drug treatment had no effect on the relative distribution of degenerating, and vacuolated fibers, inflammatory cells, and abnormal fiber diameters. An exception was seen in decreased necrotic fibers of chickens treated with high doses of Ep475. Moreover, both inhibitors had positive effects on two biochemical abnormalities common to the dystrophic pectoralis muscle: increase in noncollagen protein, and reduction in total calcium.

Tissue and subcellular distribution of enzymes inactivating leupeptin

Leupeptin, a proteinase inhibitor obtained from culture filtrates of Streptomyces sp., has been proposed as a valuable agent for the restriction of proteolysis in vivo. We have previously shown that mammalian tissues possess an enzymic system that is capable of inactivating leupeptin (Beynon RJ, Brown CP & Butler PE, 1981, Acta Biol. Med. Germ. 40, 1539-1546). This paper demonstrates the ubiquitous distribution of the inactivating system throughout mammalian tissues and provides evidence for the location of the enzyme(s) in the soluble fraction of the cell.

Immunocytochemical studies of cathepsin D in human skeletal muscle

The distribution of cathepsin D, an acidic endopeptidase, was localized by immunocytochemistry in human skeletal muscle obtained from 34 persons with a variety of neuromuscular disorders. Normal human skeletal muscle contained small amounts of cathepsin D, all of which was found close to the sarcolemmal membrane. Immunoreactive cathepsin D was present in the cytoplasm of many infiltrating phagocytic cells and was increased in skeletal muscle fibers from patients with muscular dystrophies, inflammatory myopathies, rhabdomyolysis, acid maltase deficiency, and neurogenic atrophy. In cases of Duchenne type muscular dystrophy, the increase in cathepsin D was especially prominent in small regenerating fibers, in which it was visualized at the ultrastructural level in lysosome-like organelles and extralysosomal locations. The function of cathepsin D in skeletal muscle is unclear, but the present findings suggest a possible role in muscle regeneration and repair. Such a role would necessitate careful selection of drugs which interfere with proteolytic activity if they are to be used as therapeutic agents in treating neuromuscular diseases.

Paradoxical effect of leupeptin in vivo on cathepsin B activity

Leupeptin is a potent inhibitor of cathepsin B in vitro and is presumed to act in a similar manner in vivo. It is currently being used in several laboratories to examine the role of lysosomal proteinases such as cathepsin B in mouse models of muscular dystrophy. This report clearly demonstrates that leupeptin in adequate concentrations in vivo, is a potent stimulator of cathepsin B activity in striated muscle, heart, liver and kidney of the mouse. This paradoxical effect indicates that care is required in the interpretation of the results of the use of leupeptin as a cathepsin B inhibitor in vivo and that its use as an antiprotease for therapeutic purposes may be limited. Studies on CBZ-Phe-Ala-CHN2 demonstrated that this agent, when administered in vivo, inhibited Cathepsin B in the tissues assayed.

Parenteral branched-chain amino acid treatment and avian dystrophy

Genetically homozygous line 413 dystrophic chickens were given twice-daily intraperitoneal injections of solutions containing branched-chain amino acids (BCCA-leucine, valine, isoleucine) either alone or in combination; and their alpha-ketoacid analogs (alpha-ketoisocaproic and alpha-ketoisovaleric acids). Another trial consisted of an amino acid mixture containing BCAA. Amino acid supplementation in each case significantly prolonged righting ability measured regularly by a standardized flip-test procedure. Enhanced functional ability was not generally accompanied by a decrease in plasma creatine kinase activity. However, a measurable increase in the affected pectoralis major muscle mass and protein content (female chickens in particular) was found with BCAA therapy. Moreover, the increase in muscle bulk was attended in some cases by a reduction in the relative number of degenerating fibers quantitated microscopically. Contrariwise, the amino acid mixture caused a reduction in pectoralis muscle mass. It is concluded that parenteral BCAA therapy offers limited benefit in retarding dystrophic symptoms in the chickens.

Fragmentation analysis of normal and dystrophic avian muscle

A difference between normal and dystrophic avian muscle was demonstrated by comparing the patterns of fragmentation of muscle during homogenization. Fragmentation was monitored by morphological methods and by viscometry. This method of fragmentation analysis depends on the principle that the viscosity of a suspension is an exponential function of the partial volume fraction occupied by the suspended particles; the more a tissue is fragmented into smaller pieces, the greater the viscosity of the resulting homogenate. Increasing the duration of homogenization of either fresh muscle in buffer or glycerinated muscle in relaxing solution gradually increased the viscosity of the homogenate of normal muscle, whereas the viscosity of the homogenate of dystrophic muscle remained approximately constant. This difference in viscosity indicated that dystrophic muscle was sheared into larger pieces which were resistant to further fragmentation. Electron microscopic examination showed that the homogenate of dystrophic muscle contained rows of sarcomeres, whereas normal muscle was sheared into amorphous masses of myofilaments.

Persistence of the expression of beta-tropomyosin in dystrophic avian pectoral muscle

The isotype pattern of tropomyosin was investigated in normal and dystrophic avian pectoral muscle using two-dimensional gel electrophoresis. Previous reports have shown that adult pectoral muscle of chickens contains only the alpha-subunit of tropomyosin and a breast-type troponin-T (TN-T), whereas pectoral fetal muscle contains both alpha-and beta-tropomyosin and leg-type TN-T. The change from the fetal to the adult forms begins shortly after hatching. It has been previously reported that avian dystrophic pectoral muscle contains both the leg- and breast-type TN-T; we show that in avian dystrophic muscle there is also persistent expression of the beta-subunit of tropomyosin.

Therapeutic trial with protease inhibitor (leupeptin) in chicken muscular dystrophy. A histologic and histochemical study

For the purpose of observing the therapeutic benefit of protease inhibitors for progressive muscular dystrophy, a large quantity of doses of leupeptin of 10 mg/kg/day and 50 mg/kg/day were administered i.p. to male chickens afflicted with hereditary muscular dystrophy (line 413) for 4 months starting on the 7th day ex ovo. No clinical improvement was identified in physical ability as a result of the examination by flip test, and creatine kinase (CK) values. The number of necrotic fibers in the pectoralis superficialis (PS) muscle which is known to be preferentially damaged in dystrophic chicken, did not decrease significantly in the birds treated with 10 mg leupeptin/kg/day (number of necrotic fibers; 47.7/mm2) and 50 mg/kg/day (46.4/mm2) as compared to that of the untreated ones (43.2/mm2). A morphometric analysis of fiber diameter distribution also showed no statistical difference between the treated and untreated birds. In the second group, 10 mg leupeptin/kg and a combination of leupeptin and bestatin of 10 mg/kg each were injected directly into the left lower half of the PS muscle three times a week for 4 months. Necrotic fibers were still present in the injected site, remote area of the left upper PS muscle treated with leupeptin (52.7/mm2), leupeptin and bestatin (52.2/mm2), and contralateral right upper PS muscle (41.6 and 53.5/mm2, respectively). The number of necrotic fibers in treated muscles was again not significantly different from that in untreated dystrophic ones (39.6/mm2). In fiber diameter analysis, no statistical difference was recognized between the treated and untreated dystrophic muscles.