Calpain concentration is elevated although net calcium-dependent proteolysis is suppressed in dystrophin-deficient muscle

Calpain-3 not only proteolyzes calpain-1 and -2 but also is a substrate for calpain-1 and -2

Calpain is an intracellular cysteine protease that cleaves its specific substrates in a limited region to modulate cellular function. Calpain-1 (C1) and calpain-2 (C2) are ubiquitously expressed in mammalian cells, but calpain-3 (C3) is a skeletal muscle-specific type. In the course of calpain activation, the N-terminal regions of all three isoforms are clipped off in an intramolecular or intermolecular fashion. C1 proteolyzes C2 to promote further proteolysis, but C2 proteolyzes C1 to suspend C1 proteolysis, indicating the presence of C1-C2 reciprocal proteolysis. However, whether C3 is involved in the calpain proteolysis network is unclear. To address this, we examined whether GFP-tagged C3:C129S (GFP-C3:CS), an inactive protease form of C3, was a substrate for C1 or C2 in HEK cells. Intriguingly, the N-terminal region of C3:CS was cleaved by C1 and C2 at the site identical to that of the C3 autoproteolysis site. Furthermore, the N-terminal clipping of C3:CS by C1 and C2 was observed in mouse skeletal muscle lysates. Meanwhile, C3 preferentially cleaved the N-terminus of C1 over that of C2, and the sizes of these cleaved proteins were identical to their autoproteolysis forms. Our findings suggest an elaborate inter-calpain network to prime and suppress proteolysis of other calpains.

Dissociation of SH3 and cysteine rich domain 3 and junctophilin 1 from dihydropyridine receptor in dystrophin-deficient muscles

The disruption of excitation-contraction (EC) coupling and subsequent reduction in Ca²⁺ release from the sarcoplasmic reticulum (SR) have been shown to account for muscle weakness seen in patients with Duchenne muscular dystrophy (DMD). Here, we examined the mechanisms underlying EC uncoupling in skeletal muscles from mdx52 and DMD-null/NSG mice, animal models for DMD, focusing on the SH3 and cysteine rich domain 3 (STAC3) and junctophilin 1 (JP1), which link the dihydropyridine receptor (DHPR) in the transverse tubule and the ryanodine receptor 1 in the SR. The isometric plantarflexion torque normalized to muscle weight of whole plantar flexor muscles was depressed in mdx52 and DMD-null/NSG mice compared to their control mice. This was accompanied by increased autolysis of calpain-1, decreased levels of STAC3 and JP1 content, and dissociation of STAC3 and JP1 from DHPR-α_1s in gastrocnemius muscles. Moreover, in vitro mechanistic experiments demonstrated that STAC3 and JP1 underwent Ca²⁺-dependent proteolysis which was less pronounced in dystrophin-deficient muscles where calpastatin, the endogenous calpain inhibitor, was upregulated. Eccentric contractions further enhanced autolysis of calpain-1 and proteolysis of STAC3 and JP1 that were associated with severe torque depression in gastrocnemius muscles from DMD-null/NSG mice. These data suggest that Ca²⁺-dependent proteolysis of STAC3 and JP1 may be an essential factor causing muscle weakness due to EC coupling failure in dystrophin-deficient muscles.

Skeletal Muscle Mitochondria Dysfunction in Genetic Neuromuscular Disorders with Cardiac Phenotype

Mitochondrial dysfunction is considered the major contributor to skeletal muscle wasting in different conditions. Genetically determined neuromuscular disorders occur as a result of mutations in the structural proteins of striated muscle cells and therefore are often combined with cardiac phenotype, which most often manifests as a cardiomyopathy. The specific roles played by mitochondria and mitochondrial energetic metabolism in skeletal muscle under muscle-wasting conditions in cardiomyopathies have not yet been investigated in detail, and this aspect of genetic muscle diseases remains poorly characterized. This review will highlight dysregulation of mitochondrial representation and bioenergetics in specific skeletal muscle disorders caused by mutations that disrupt the structural and functional integrity of muscle cells.

Treatment with a triazole inhibitor of the mitochondrial permeability transition pore fully corrects the pathology of sapje zebrafish lacking dystrophin

High-throughput screening identified isoxazoles as potent but metabolically unstable inhibitors of the mitochondrial permeability transition pore (PTP). Here we have studied the effects of a metabolically stable triazole analog, TR001, which maintains the PTP inhibitory properties with an in vitro potency in the nanomolar range. We show that TR001 leads to recovery of muscle structure and function of sapje zebrafish, a severe model of Duchenne muscular dystrophy (DMD). PTP inhibition fully restores the otherwise defective respiration in vivo, allowing normal development of sapje individuals in spite of lack of dystrophin. About 80 % sapje zebrafish treated with TR001 are alive and normal at 18 days post fertilization (dpf), a point in time when not a single untreated sapje individual survives. Time to 50 % death of treated zebrafish increases from 5 to 28 dpf, a sizeable number of individuals becoming young adults in spite of the persistent lack of dystrophin expression. TR001 improves respiration of myoblasts and myotubes from DMD patients, suggesting that PTP-dependent dysfunction also occurs in the human disease and that mitochondrial therapy of DMD with PTP-inhibiting triazoles is a viable treatment option.

The Effect of Deflazacort Treatment on the Functioning of Skeletal Muscle Mitochondria in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a severe hereditary disease caused by a lack of dystrophin, a protein essential for myocyte integrity. Mitochondrial dysfunction is reportedly responsible for DMD. This study examines the effect of glucocorticoid deflazacort on the functioning of the skeletal-muscle mitochondria of dystrophin-deficient mdx mice and WT animals. Deflazacort administration was found to improve mitochondrial respiration of mdx mice due to an increase in the level of ETC complexes (complexes III and IV and ATP synthase), which may contribute to the normalization of ATP levels in the skeletal muscle of mdx animals. Deflazacort treatment improved the rate of Ca2+ uniport in the skeletal muscle mitochondria of mdx mice, presumably by affecting the subunit composition of the calcium uniporter of organelles. At the same time, deflazacort was found to reduce the resistance of skeletal mitochondria to MPT pore opening, which may be associated with a change in the level of ANT2 and CypD. In this case, deflazacort also affected the mitochondria of WT mice. The paper discusses the mechanisms underlying the effect of deflazacort on the functioning of mitochondria and contributing to the improvement of the muscular function of mdx mice.

Association between myocardial cell apoptosis and calpain-1/caspase-3 expression in rats with hypoxic-ischemic brain damage

The present study aimed to investigate the association between myocardial cell apoptosis and calpain-1/caspase-3 expression in a rat model of hypoxic-ischemic brain damage (HIBD). A total of 64 newborn rats were divided into control (n=8; sacrificed on day 7) and HIBD groups (n=56). HIBD group rats were sacrificed 2, 12 or 24 h, or 2, 3, 5 or 7 days following HIBD (n=8/group). A terminal deoxynucleotidyl transferase dUTP nick-end labeling assay was performed to detect myocardial apoptotic cells and calculate the apoptosis index (AI), reverse transcription-polymerase chain reaction was performed to detect myocardial calpain-1/caspase-3 mRNA expression levels and a western blot analysis was conducted to detect calpain‑1 protein expression levels. The correlations between calpain‑1 and caspase‑3 expression levels and AI were analyzed. The results demonstrated that apoptotic myocardial cells in the HIBD groups were markedly increased compared with the control group, with AI peaking in the day 3 group. Caspase‑3 and calpain‑1 mRNA expression levels were increased from 2 and 12 h following HIBD, respectively, with the most elevated levels in the day 2 group. Compared with the control group, calpain‑1 protein expression levels were increased from 2 h, with the greatest expression levels in the day 3 group (P<0.05). Calpain‑1 mRNA and protein (76/80 kDa) expression levels demonstrated positive linear correlations with AI (r=0.786, P=0.001; and r=0.853, P=0.001, respectively) Caspase-3 mRNA expression levels were positively correlated with AI (r=0.894; P=0.001). In conclusion, the present study demonstrated that in rats with HIBD, there is a positive correlation between increased apoptosis of myocardial cells and expression levels of calpain-1 and caspase-3.

Dysregulation of Intracellular Ca2+ in Dystrophic Cortical and Hippocampal Neurons

Duchenne muscular dystrophy (DMD) is an inherited X-linked disorder characterized by skeletal muscle wasting, cardiomyopathy, as well as cognitive impairment. Lack of dystrophin in striated muscle produces dyshomeostasis of resting intracellular Ca²⁺ ([Ca²⁺]_i), Na⁺ ([Na⁺]_i), and oxidative stress. Here, we test the hypothesis that similar to striated muscle cells, an absence of dystrophin in neurons from mdx mice (a mouse model for DMD) is also associated with dysfunction of [Ca²⁺]_i homeostasis and oxidative stress. [Ca²⁺]_i and [Na⁺]_i in pyramidal cortical and hippocampal neurons from 3 and 6 months mdx mice were elevated compared to WT in an age-dependent manner. Removal of extracellular Ca²⁺ reduced [Ca²⁺]_i in both WT and mdx neurons, but the decrease was greater and age-dependent in the latter. GsMTx-4 (a blocker of stretch-activated cation channels) significantly decreased [Ca²⁺]_i and [Na⁺]_i in an age-dependent manner in all mdx neurons. Blockade of ryanodine receptors (RyR) or inositol triphosphate receptors (IP3R) reduced [Ca²⁺]_i in mdx. Mdx neurons showed elevated and age-dependent reactive oxygen species (ROS) production and an increase in neuronal damage. In addition, mdx mice showed a spatial learning deficit compared to WT. GsMTx-4 intraperitoneal injection reduced neural [Ca²⁺]_i and improved learning deficit in mdx mice. In summary, mdx neurons show an age-dependent dysregulation in [Ca²⁺]_i and [Na⁺]_i which is mediated by plasmalemmal cation influx and by intracellular Ca²⁺ release through the RyR and IP3R. Also, mdx neurons have elevated ROS production and more extensive cell damage. Finally, a reduction of [Ca²⁺]_i improved cognitive function in mdx mice.

Absence of Dystrophin Disrupts Skeletal Muscle Signaling: Roles of Ca2+, Reactive Oxygen Species, and Nitric Oxide in the Development of Muscular Dystrophy

Dystrophin is a long rod-shaped protein that connects the subsarcolemmal cytoskeleton to a complex of proteins in the surface membrane (dystrophin protein complex, DPC), with further connections via laminin to other extracellular matrix proteins. Initially considered a structural complex that protected the sarcolemma from mechanical damage, the DPC is now known to serve as a scaffold for numerous signaling proteins. Absence or reduced expression of dystrophin or many of the DPC components cause the muscular dystrophies, a group of inherited diseases in which repeated bouts of muscle damage lead to atrophy and fibrosis, and eventually muscle degeneration. The normal function of dystrophin is poorly defined. In its absence a complex series of changes occur with multiple muscle proteins showing reduced or increased expression or being modified in various ways. In this review, we will consider the various proteins whose expression and function is changed in muscular dystrophies, focusing on Ca(2+)-permeable channels, nitric oxide synthase, NADPH oxidase, and caveolins. Excessive Ca(2+) entry, increased membrane permeability, disordered caveolar function, and increased levels of reactive oxygen species are early changes in the disease, and the hypotheses for these phenomena will be critically considered. The aim of the review is to define the early damage pathways in muscular dystrophy which might be appropriate targets for therapy designed to minimize the muscle degeneration and slow the progression of the disease.

Calpain-mediated proteolysis of tropomodulin isoforms leads to thin filament elongation in dystrophic skeletal muscle

Calpain-mediated proteolysis of the thin filament pointed-end–capping protein tropomodulin results in actin subunit association onto pointed ends and increased thin filament lengths in two different murine models of Duchenne muscular dystrophy. This mechanism affects different skeletal muscles in a use- and disease severity–dependent manner.

Duchenne muscular dystrophy (DMD) induces sarcolemmal mechanical instability and rupture, hyperactivity of intracellular calpains, and proteolytic breakdown of muscle structural proteins. Here we identify the two sarcomeric tropomodulin (Tmod) isoforms, Tmod1 and Tmod4, as novel proteolytic targets of m-calpain, with Tmod1 exhibiting ∼10-fold greater sensitivity to calpain-mediated cleavage than Tmod4 in situ. In mdx mice, increased m-calpain levels in dystrophic soleus muscle are associated with loss of Tmod1 from the thin filament pointed ends, resulting in ∼11% increase in thin filament lengths. In mdx/mTR mice, a more severe model of DMD, Tmod1 disappears from the thin filament pointed ends in both tibialis anterior (TA) and soleus muscles, whereas Tmod4 additionally disappears from soleus muscle, resulting in thin filament length increases of ∼10 and ∼12% in TA and soleus muscles, respectively. In both mdx and mdx/mTR mice, both TA and soleus muscles exhibit normal localization of α-actinin, the nebulin M1M2M3 domain, Tmod3, and cytoplasmic γ-actin, indicating that m-calpain does not cause wholesale proteolysis of other sarcomeric and actin cytoskeletal proteins in dystrophic skeletal muscle. These results implicate Tmod proteolysis and resultant thin filament length misspecification as novel mechanisms that may contribute to DMD pathology, affecting muscles in a use- and disease severity–dependent manner.

The physiological response of protease inhibition in dystrophic muscle

Duchenne muscular dystrophy (DMD) is caused by the production of a non-functional dystrophin gene product and a failure to accumulate functional dystrophin protein in muscle cells. This leads to membrane instability, loss of Ca(2+) homoeostasis and widespread cellular injury. Associated with these changes are increased protease activities in a variety of proteolytic systems. As such, there have been numerous investigations directed towards determining the therapeutic potential of protease inhibition. In this review, evidence from genetic and/or pharmacological inhibition of proteases as a treatment strategy for DMD is systematically evaluated. Specifically, we review the potential roles of calpain, proteasome, caspase, matrix metalloproteinase and serine protease inhibition as therapeutic approaches for DMD. We conclude that despite early results to the contrary, inhibition of calpain proteases is unlikely to be successful. Conversely, evidence suggests that inhibition of proteasome, matrix metalloproteinases and serine proteases does appear to decrease disease severity. An important caveat to these conclusions, however, is that the fundamental cause of DMD, dystrophin deficiency, is not corrected by this strategy. Hence, this should not be viewed as a cure, but rather, protease inhibitors should be considered for inclusion in a therapeutic cocktail. Physiological Relevance. Selective modulation of protease activity has the potential to profoundly change intracellular physiology resulting in a possible treatment for DMD. However, alteration of protease activities could also lead to worsening of disease progression by promoting the accumulation of substrates in the cell. The balance of benefit and potential damage caused by protease inhibition in human DMD patients is largely unexplored.

Regulation of the calpain and ubiquitin-proteasome systems in a canine model of muscular dystrophy

INTRODUCTION

Previous studies have tested the hypothesis that calpain and/or proteasome inhibition is beneficial in Duchenne muscular dystrophy, based largely on evidence that calpain and proteasome activities are enhanced in the mdx mouse.

METHODS

mRNA expression of ubiquitin-proteasome and calpain system components were determined using real-time polymerase chain reaction in skeletal muscle and heart in the golden retriever muscular dystrophy model. Similarly, calpain 1 and 2 and proteasome activities were determined using fluorometric activity assays.

RESULTS

We found that less than half of the muscles tested had increases in proteasome activity, and only half had increased calpain activity. In addition, transcriptional regulation of the ubiquitin-proteasome system was most pronounced in the heart, where numerous components were significantly decreased.

CONCLUSION

This study illustrates the diversity of expression and activities of the ubiquitin-proteasome and calpain systems, which may lead to unexpected consequences in response to pharmacological inhibition.

Exercise and Duchenne muscular dystrophy: toward evidence-based exercise prescription

To develop a rational framework for answering questions about the role of exercise in Duchenne muscular dystrophy (DMD), we focused on five pathophysiological mechanisms and offer brief hypotheses regarding how exercise may beneficially modulate pertinent cellular and molecular pathways. We aimed to provide an integrative overview of mechanisms of DMD pathology that may improve or worsen as a result of exercise. We also sought to stimulate discussion of what outcomes/dependent variables most appropriately measure these mechanisms, with the purpose of defining criteria for well-designed, controlled studies of exercise in DMD. The five mechanisms include pathways that are both intrinsic and extrinsic to the diseased muscle cells.

Bowman-Birk inhibitor attenuates dystrophic pathology in mdx mice

Bowman-Birk inhibitor concentrate (BBIC), a serine protease inhibitor, has been shown to diminish disuse atrophy of skeletal muscle. Duchenne muscular dystrophy (DMD) results from a loss of dystrophin protein and involves an ongoing inflammatory response, with matrix remodeling and activation of transforming growth factor (TGF)-β(1) leading to tissue fibrosis. Inflammatory-mediated increases in extracellular protease activity may drive much of this pathological tissue remodeling. Hence, we evaluated the ability of BBIC, an extracellular serine protease inhibitor, to impact pathology in the mouse model of DMD (mdx mouse). Mdx mice fed 1% BBIC in their diet had increased skeletal muscle mass and tetanic force and improved muscle integrity (less Evans blue dye uptake). Importantly, mdx mice treated with BBIC were less susceptible to contraction-induced injury. Changes consistent with decreased degeneration/regeneration, as well as reduced TGF-β(1) and fibrosis, were observed in the BBIC-treated mdx mice. While Akt signaling was unchanged, myostatin activitation and Smad signaling were reduced. Given that BBIC treatment increases mass and strength, while decreasing fibrosis in skeletal muscles of the mdx mouse, it should be evaluated as a possible therapeutic to slow the progression of disease in human DMD patients.

In situ measurements of calpain activity in isolated muscle fibres from normal and dystrophin-lacking mdx mice

Calpains are Ca(2+)-activated proteases that are thought to be involved in muscle degenerative diseases such as Duchenne muscular dystrophy. Status and activity of calpains in adult muscle fibres are poorly documented. We report here in situ measurements of calpain activity in collagenase-isolated fibres from C57 mice and form two models of dystrophy: dystrophin-deficient mdx and calpain-3 knocked-out mice. Calpain activity was measured using a permeant, fluorogenic substrate and its Ca(2+) dependence was studied. A 30-fold change of activity was observed between the lowest and the highest steady-state Ca(2+) availability. Fast transient changes of [Ca(2+)](i) induced by electrical stimulation or KCl-dependent depolarization were ineffective in activating calpain. Slow [Ca(2+)] transients, as elicited during depletion of Ca(2+) stores, Ca(2+) store repletion and hypo-osmotic swelling were able to activate calpain. On return to resting conditions, calpain activity recovered its basal rate within 10 min. In resting intact muscle, mu-calpain was predominantly in the 80 kDa native form, with a small fraction in the 78 kDa autolysed form. The latter is thought to be responsible for the activity measured in our conditions. Calpain activity in mdx fibres showed an average 1.5-fold increase compared to activity in C57 fibres. This activity was reduced by a 10-fold lowering of [Ca(2+)](o). Calpain-3-deficient fibres showed about the same increase, thus calpain-3 did not contribute to the activity measured here and calpain activation is not specific to dystrophin deficiency. In fibres from transgenic mice over-expressing calpastatin, a 40-50% reduction of calpain activity was observed, as with synthetic drugs (Z-Leu-Leu-CHO and SNT198438). We provide novel information on the physiological factors that control calpain activity in situ, particularly the effect of intracellular Ca(2+) transients that occur in excitation-contraction coupling, Ca(2+) store depletion and refilling, and activation of mechanosensitive Ca(2+) channels.

The calpain system

The calpain system originally comprised three molecules: two Ca2+-dependent proteases, mu-calpain and m-calpain, and a third polypeptide, calpastatin, whose only known function is to inhibit the two calpains. Both mu- and m-calpain are heterodimers containing an identical 28-kDa subunit and an 80-kDa subunit that shares 55-65% sequence homology between the two proteases. The crystallographic structure of m-calpain reveals six "domains" in the 80-kDa subunit: 1). a 19-amino acid NH2-terminal sequence; 2). and 3). two domains that constitute the active site, IIa and IIb; 4). domain III; 5). an 18-amino acid extended sequence linking domain III to domain IV; and 6). domain IV, which resembles the penta EF-hand family of polypeptides. The single calpastatin gene can produce eight or more calpastatin polypeptides ranging from 17 to 85 kDa by use of different promoters and alternative splicing events. The physiological significance of these different calpastatins is unclear, although all bind to three different places on the calpain molecule; binding to at least two of the sites is Ca2+ dependent. Since 1989, cDNA cloning has identified 12 additional mRNAs in mammals that encode polypeptides homologous to domains IIa and IIb of the 80-kDa subunit of mu- and m-calpain, and calpain-like mRNAs have been identified in other organisms. The molecules encoded by these mRNAs have not been isolated, so little is known about their properties. How calpain activity is regulated in cells is still unclear, but the calpains ostensibly participate in a variety of cellular processes including remodeling of cytoskeletal/membrane attachments, different signal transduction pathways, and apoptosis. Deregulated calpain activity following loss of Ca2+ homeostasis results in tissue damage in response to events such as myocardial infarcts, stroke, and brain trauma.

Desmin cytoskeletal modifications after a bout of eccentric exercise in the rat

Desmin content and immunohistochemical appearance were measured in tibialis anterior muscles of rats subjected to a single bout of 30 eccentric contractions (ECs). Ankle torque was measured before EC and at various recovery times, after which immunohistochemical and immunoblot analyses were performed. Torque decreased by approximately 50% immediately after EC and fully recovered 168 h later (P < 0.001). Loss of desmin staining was maximal 12 h after EC and recovered by 72 h. Immunoblots unexpectedly demonstrated a significant increase in the desmin-to-actin ratio by 72 h after EC (P < 0.01) and was still increasing after 168 h (P < 0.0001). These data demonstrate a relatively rapid qualitative loss of desmin immunostaining immediately after a single EC bout but a tremendous quantitative increase in desmin content 72-168 h later. This dynamic restructuring of the muscle's intermediate filament system may be involved in the mechanism of EC-induced muscle injury and may provide a structural explanation for the protective effects observed in muscle after a single EC bout.

Global/temporal gene expression in diaphragm and hindlimb muscles of dystrophin-deficient (mdx) mice

The mdx mouse is a model for human Duchenne muscular dystrophy (DMD), an X-linked degenerative disease of skeletal muscle tissue characterized by the absence of the dystrophin protein. The mdx mice display a much milder phenotype than DMD patients. After the first week of life when all mdx muscles evolve like muscles of young DMD patients, mdx hindlimb muscles substantially compensate for the lack of dystrophin, whereas mdx diaphragm muscle becomes progressively affected by the disease. We used cDNA microarrays to compare the expression profile of 1,082 genes, previously selected by a subtractive method, in control and mdx hindlimb and diaphragm muscles at 12 time points over the first year of the mouse life. We determined that 1) the dystrophin gene defect induced marked expression remodeling of 112 genes encoding proteins implicated in diverse muscle cell functions and 2) two-thirds of the observed transcriptomal anomalies differed between adult mdx hindlimb and diaphragm muscles. Our results showed that neither mdx diaphram muscle nor mdx hindlimb muscles evolve entirely like the human DMD muscles. This finding should be taken under consideration for the interpretation of future experiments using mdx mice as a model for therapeutic assays.

Dissociation of the dystroglycan complex in caveolin-3-deficient limb girdle muscular dystrophy

Limb girdle muscular dystrophy is a group of clinically and genetically heterogeneous disorders inherited in an autosomal recessive or dominant mode. Caveolin-3, the muscle-specific member of the caveolin gene family, is implicated in the pathogenesis of autosomal dominant limb girdle muscular dystrophy 1C. Here we report on a 4-year-old girl presenting with myalgia and muscle cramps due to a caveolin-3 deficiency in her dystrophic skeletal muscle as a result of a heterozygous 136G-->A substitution in the caveolin-3 gene. The novel sporadic missense mutation in the caveolin signature sequence of the caveolin-3 gene changes an alanine to a threonine (A46T) and prevents the localization of caveolin-3 to the plasma membrane in a dominant negative fashion. Caveolin-3 has been suggested to interact with the dystrophin-glycoprotein complex, which in striated muscle fibers links the cytoskeleton to the extracellular matrix and with neuronal nitric oxide synthase. Similar to dystrophin-deficient Duchenne muscular dystrophy, a secondary decrease in neuronal nitric oxide synthase and alpha-dystroglycan expression was detected in the caveolin-3-deficient patient. These results implicate an important function of the caveolin signature sequence and common mechanisms in the pathogenesis of dystrophin-glycoprotein complex-associated muscular dystrophies with caveolin-3-deficient limb girdle muscular dystrophy.

Large-scale analysis of differential gene expression in the hindlimb muscles and diaphragm of mdx mouse

The mdx mouse is an animal model for Duchenne muscular dystrophy (DMD), which is caused by the absence of dystrophin. Mdx limb muscles substantially compensate for the lack of dystrophin while the diaphragm is affected like DMD skeletal muscles. To understand better the complex cascade of molecular events leading to muscle degeneration and compensatory processes in mdx muscles, we analyzed alterations of gene expression in mdx hindlimb and diaphragm muscles as compared to their normal counterparts. The strategy was based on suppression subtractive hybridization followed by reverse Northern quantitative hybridization. Four subtracted/normalized libraries, containing cDNA clones up- or downregulated in mdx hindlimb muscles or diaphragm, were constructed and a total of 1536 cDNA clones were analyzed. Ninety-three cDNAs were found to be differentially expressed in mdx hindlimb muscles and/or diaphragm. They corresponded to 54 known genes and 39 novel cDNAs. The potential role of the known genes is discussed in the context of the mdx phenotype.

Delay of muscle degeneration and necrosis in mdx mice by calpain inhibition

Inhibition of muscle degeneration by the tripeptide calpain inhibitor, leupeptin, was tested in vivo in a dystrophin-deficient mdx murine model. In a short-term control study, intramuscular administration of leupeptin for 30 days inhibited muscle degeneration as assessed by histologic analysis. Calpain inhibition could be correlated with retention of myofiber size and our results suggest that this may be a promising treatment modality in human Duchenne muscular dystrophy.

Mechanisms of resistance to pathogenesis in muscular dystrophies

A mechanistic definition of the dystrophic process is proposed, and the effects of growth factors vs. down-regulation of growth are critically analyzed. A conceptual scheme is presented to illustrate the steps leading to pathology, and various compensatory systems which ameliorate the pathology are examined, particularly in regards to the mdv mouse which is resistant to the deficiency of dystrophin, the main protein product of the Duchenne and Becker muscular dystrophy (DMD/BMD) gene. These compensatory systems are analyzed in terms of the differential resistance of fiber types to pathogenesis. The generation of a stable population of maturationally arrested centronucleated fibers which express the mature adult myosin isoforms is proposed to be the main strategy of mdx muscle to minimize apoptosis. Physiological properties of these fibers, such as utrophin expression, and high mitochondrial and endoplasmic reticulum content, together with probable increased glycerophosphorylcholine concentrations and facile access to the vascular system, are hypothesized to be instrumental in their resistance to pathogenesis. It is proposed that the major element that determines the susceptibility of most human muscles to the dystrophic process is their inability to arrest the maturation of regenerated fibers at the centronucleated stage with a concomitant expression of the adult myosins.

Microinjection of calpastatin inhibits fusion in myoblasts

Rat satellite cells (RSC) were microinjected with purified calpastatin or m-calpain, and myoblasts from a C2C12 mouse line were microinjected with purified calpastatin. Microinjection with calpastatin completely prevented fusion of myoblasts from both sources, whereas microinjection with m-calpain significantly increased the rate of fusion of cultured RSC; 44% of the nuclei of RSC cultures were in multinucleated myotubes within 48 h after microinjection with m-calpain plus labeled dextran, whereas only 15% of the nuclei were in multinucleated myotubes after microinjection with dextran alone. Western analyses indicated that neither RSC nor C2C12 myoblasts contained detectable amounts of mu-calpain before fusion. The levels of calpastatin in C2C12 myoblasts increased as cells passed from the proliferative stage to the onset of fusion, and these levels increased substantially in both the C2C12 and the RSC cells as they progressed to the late or postfusion stage. Both RSC and C2C12 myoblasts contained an 80-kDa polypeptide that was labeled with an anti-m-calpain antibody in Western blots. The results are consistent with a role of the calpain system (m-calpain in these myoblast lines) in remodeling of the cytoskeletal/plasma membrane interactions during cell fusion.

Creatine supplementation improves intracellular Ca2+ handling and survival in mdx skeletal muscle cells

Dystrophic skeletal muscle cells from Duchenne muscular dystrophy (DMD) patients and mdx mice exhibit elevated cytosolic Ca2+ concentrations ([Ca2+]c). Pretreatment of mdr myotubes for 6-12 days with creatine (20 mM) decreased the elevation in [Ca2+]c induced by either high extracellular Ca2+ concentrations or hypo-osmotic stress to control levels. 45Ca2+ influx measurements suggest that creatine lowered [Ca2+]c by stimulating sarcoplasmic reticulum Ca2+-ATPase. Creatine pretreatment increased levels of phosphocreatine but not ATP. Furthermore, myotube formation and survival were significantly enhanced by creatine pretreatment. Therefore, creatine supplementation may be useful for treatment of DMD.

E64d, a membrane-permeable cysteine protease inhibitor, attenuates the effects of parathyroid hormone on osteoblasts in vitro

Parathyroid hormone (PTH) activates calpains I and II (calcium-activated papain-like proteases) and stimulates the synthesis and secretion of cathepsin B (a lysosomal cysteine protease) in osteoblastic cells. Anabolic doses of PTH also stimulate osteoprogenitor cell proliferation and differentiation into mature, fully functional osteoblasts capable of elaborating bone matrix, whereas catabolic doses of PTH stimulate calcium mobilization and matrix turnover. Previous investigations in other cell types have demonstrated that calcium-activated calpains play a major role in regulating proliferation and differentiation by catalyzing limited regulatory proteolysis of nuclear proteins, transcription factors, and enzymes. We tested the hypothesis that inhibition of intracellular cysteine proteases such as the calpains will ablate PTH-mediated osteoblast proliferation and differentiation, two fundamental indices of bone anabolism. A brief preincubation with the membrane-permeable, irreversible cysteine protease inhibitor E64d (10 micrograms/mL) before short-term PTH treatment blunted PTH-induced cell proliferation in subconfluent cultures and also attenuated proliferation and inhibited differentiation in longer-term confluent cultures. This confirms the hypothesis that cysteine proteases such as the calpains are important in mediating the proliferative and prodifferentiating or anabolic effects of PTH on MC3T3-E1 cells in culture. Immunofluorescent localization demonstrated that calpain I, calpain II, and calpastatin (the endogenous calpain inhibitor) are abundant and widely distributed within actively proliferating MC3T3-E1 preosteoblasts. Since the calpains are active and stable at neutral intracellular pH levels in osteoblasts, whereas cathepsins are not, our results support a role for these calcium-activated regulatory proteases in mediating the anabolic effects of PTH in bone.

The calpain-calpastatin system and cellular proliferation and differentiation in rodent osteoblastic cells

The calpain-calpastatin system, which consists of calpains I and II (two ubiquitously distributed calcium-activated papain-like cysteine proteases), as well as calpastatin (the endogenous calpain inhibitor), plays an important role in cell proliferation and differentiation in many tissues. However, its contribution to the regulation of osteoprogenitor or pluripotent stem cell proliferation and differentiation into osteoblasts remains poorly defined. In these studies, rat pluripotent mesodermal cells (ROB-C26) and mouse MC3T3-E1 preosteoblasts were induced to differentiate into osteoblasts by long-term culture or in response to bone morphogenetic protein (BMP). The occurrence and distribution of calpain-calpastatin system proteins were determined by immunofluorescent microscopy, measurement of calcium-dependent proteolytic activity, and Western blotting. Treatment of intact MC3T3-E1 cells with an irreversible, membrane-permeable cysteine protease inhibitor attenuated proliferation and alkaline phosphatase upregulation under differentiation-enhancing conditions. Calpain II activity increased during differentiation of MC3T3-E1 cells in postconfluent culture. When ROB-C26 cells were maintained in long-term culture, neutral protease, calpain I, and calpain II activities increased 2- to 3-fold in the absence of BMP. In the presence of partially purified native BMP, neutral protease and calpain I activities also increased similarly, but calpain II activity increased by 10-fold in 3 days. The maximal increase in alkaline phosphatase occurred 4 to 11 days after the calpain II activity had peaked. Induction of differentiation in long-term MC3T3-E1 cultures was associated with higher calpain II and 70- and 110-kDa calpastatin protein levels and lower 17-kDa calpastatin degradation product levels. In conclusion, cysteine protease activity is essential for preosteoblastic proliferation and differentiation. The calpain-calpastatin system is regulated during osteoprogenitor proliferation and differentiation, as it is in other cells, and bone morphogenetic protein is a specific regulator of calpain II.

Calpains are activated in necrotic fibers from mdx dystrophic mice

Death of dystrophin-deficient muscle purportedly results from increases in [Ca]in that cause the activation of calpains. We have tested whether calpains play a role in this process by assaying for changes in calpain concentration and activation in peak necrotic mdx mice (4 weeks of age) and in completely regenerated mdx mice (14 weeks of age). Biochemical fractionation and immunoblotting with epitope-specific antisera allowed measurement of the concentrations of m- and mu-calpains and the extent of autoproteolytic modification. Our findings show that total calpain concentration is elevated in both 4-week and 14-week mdx mice. This increase in concentration was shown to result primarily from a significant increase in m-calpain concentration at 4 weeks. Northern analysis demonstrated that neither m- nor mu-calpain mRNA concentrations differed between mdx and controls suggesting that the increased calpain concentration results from post-translational regulation. Immunoblotting with antibodies directed against amino-terminal peptides revealed an increase in autoproteolysis of mu-calpain, indicative of increased activation. The extent of autoproteolysis of mu-calpain returns to control levels during regeneration. This is not a consequence of increased calpastatin mRNA or protein. The findings reported here support a role for calpains in both the degenerative and regenerative aspects of mdx dystrophy.

Calcium-activated proteolysis of neurofilament proteins in goldfish Mauthner axons

We have examined the proteolytic breakdown of neurofilament proteins (NFPs) in isolated Mauthner axoplasm (M-axoplasm). Documentation of proteolytic breakdown of NFPs in M-axoplasm is important because NFPs are not degraded in distal segments of severed Mauthner axons (M-axons) maintained in vivo for up to 62 days at 20 degrees C. By incubating M-axoplasm with 2 mM calcium in vitro, we have demonstrated that M-axoplasm contains an endogenous calcium-activated neutral protease that degrades NFPs. This calcium-activated proteolysis of M-axoplasm NFPs produced novel bands on silver-stained gels. These novel bands were presumed to be NFP breakdown products because they reacted with antibodies to the alpha-intermediate filament antigen (anti-IFA) on immunoblots from these gels. Incubations of M-axoplasm with 2 mM calcium plus exogenous calpain produced novel bands similar to those observed for M-axoplasm incubated with 2 mM calcium. Incubations of M-axoplasm with 2mM calcium plus calpain inhibitors did not produce these novel bands. These in vitro data indicate that M-axoplasm contains calpain that degrades NFPs and produces novel bands similar to those observed from distal segments of severed M-axons maintained in vivo longer than 62 days postseverance. Factors that affect the activity of calpain or affect the ability of calpain to degrade NFPs could account for the delayed degradation of NFPs in distal segments of severed M-axons maintained in vivo.

Talin, vinculin and DRP (utrophin) concentrations are increased at mdx myotendinous junctions following onset of necrosis

Duchenne muscular dystrophy (DMD) and the myopathy seen in the mdx mouse both result from absence of the protein dystrophin. Structural similarities between dystrophin and other cytoskeletal proteins, its enrichment at myotendinous junctions, and its indirect association with laminin mediated by a transmembrane glycoprotein complex suggest that one of dystrophin's functions in normal muscle is to form one of the links between the actin cytoskeleton and the extracellular matrix. Unlike Duchenne muscular dystrophy patients, mdx mice suffer only transient muscle necrosis, and are able to regenerate damaged muscle tissue. The present study tests the hypothesis that mdx mice partially compensate for dystrophin's absence by upregulating one or more dystrophin-independent mechanisms of cytoskeleton-membrane association. Quantitative analysis of immunoblots of adult mdx muscle samples showed an increase of approximately 200% for vinculin and talin, cytoskeletal proteins that mediate thin filament-membrane interactions at myotendinous junctions. Blots also showed an increase (143%) in the dystrophin-related protein called utrophin, another myotendinous junction constituent, which may be able to substitute for dystrophin directly. Muscle samples from 2-week-old animals, a period immediately preceding the onset of muscle necrosis, showed no significant differences in protein concentration between mdx and controls. Quantitative analyses of confocal images of myotendinous junctions from mdx and control muscles show significantly higher concentrations of talin and vinculin at the myotendinous junctions of mdx muscle. These findings indicate that mdx mice may compensate in part for the absence of dystrophin by increased expression of other molecules that subsume dystrophin's mechanical function.

Calpain activation in apoptosis

Programmed cell death is an active process wherein the cell initiates a sequence of events culminating in the fragmentation of its DNA, nuclear collapse, and disintegration of the cell into small, membrane-bound apoptotic bodies. Examination of the death program in various models has shown common themes, including a rise in cytoplasmic calcium, cytoskeletal changes, and redistribution of membrane lipids. The calcium-dependent neutral protease calpain has putative roles in cytoskeletal and membrane changes in other cellular processes; this fact led us to test the role of calpain in a well-known model of apoptotic cell death, that of thymocytes after treatment with dexamethasone. Assays for calcium-dependent proteolysis in thymocyte extracts reveal a rise in activity with a peak at about 1 hr of incubation with dexamethasone, falling to background at approximately 2 hr. Western blots indicate autolytic cleavage of the proenzyme precursor to the calpain I isozyme, providing additional evidence for calpain activation. We have also found that apoptosis in thymocytes, whether induced by dexamethasone or by low-level irradiation, is blocked by specific inhibitors of calpain. Apoptosis of metamyelocytes incubated with cycloheximide is also blocked by calpain inhibitors. These studies suggest a required role for calpain in both "induction" and "release" models of apoptotic cell death.

Dystrophin is not essential for the integrity of the cytoskeleton

Dystrophin is localized, in normal muscle fibers, on the cytoplasmic surface of the sarcolemma. The function of this protein is not known but, according to its structure and intracellular distribution, it seems likely that dystrophin interacts with other cytoskeletal proteins to form a complex linkage between myofibrils, sarcolemma and extracellular matrix. To evaluate the possibility that dystrophin deficiency induces, per se, a disarray in the cytoskeleton, we studied three components of this structure in muscle fibers of the dystrophic mdx mouse in a phase preceding the onset of necrosis. Vinculin, abundant in sarcolemmal structures called costameres, desmin, the principal component of intermediate filaments and nebulin, constituent of the so-called "third filament" within the sarcomere, were stained with the indirect immunofluorescence technique in cryostat sections. The same monoclonal antibodies were used in Western blots of proteins extracted from the same muscles. No difference was observed in the distribution or in the relative abundance of the three proteins, comparing muscles from 18 day-old mdx and control mice. Our results indicate that the lack of dystrophin does not induce, per se, alterations in the structures linking the sarcolemma to the contractile apparatus. It is likely that the structural damage in dystrophin-less muscle fibers is initially confined to limited portions of the plasma membrane. These focal lesions, impairing intracellular calcium homeostasis, can lead to muscle fiber necrosis.

PDGF stimulation induces phosphorylation of talin and cytoskeletal reorganization in skeletal muscle

Modifications in the interactions of the muscle cytoskeleton with the cell membrane occur during cell growth and adaptation, although the mechanisms regulating these interactions are unknown. We have observed that myotendinous junctions (MTJs), which are the primary sites of turnover of the thin filament-membrane associations in skeletal muscle, are greatly enriched in receptors for PDGF. The high concentration of PDGF receptors at MTJs suggested to us that receptor binding may initiate cytoskeletal remodeling in skeletal muscle. We tested this possibility by examining the organization and phosphorylation of cytoskeletal components of L6 myocytes after PDGF stimulation. We have found that 10 min after PDGF stimulation, L6 myoblasts exhibit no stress fibers discernible by phalloidin binding, and that vinculin relocates from focal contacts into a diffuse cytoplasmic distribution. After 60 min of incubation, these changes are largely reversed. Indirect immunofluorescence shows that at 10-min PDGF stimulation, there are no changes in the distribution of talin, the beta 1 subunit of integrin, pp125FAK or desmin. Phosphotyrosine distribution changes upon stimulation from focal contacts to being located both in focal contacts and granules concentrated in perinuclear regions. These granules also immunolabel with anti-PDGF receptor Immunoprecipitations with anti-phosphotyrosine show that polypeptides at 180 and 230 kD show the greatest increase in tyrosine phosphorylation after PDGF stimulation. Immunoblots of anti-phosphotyrosine precipitates show that these polypeptides are the PDGF receptor and talin. We also examined the possibility that the cytoskeletal reorganization observed may result from calpain activation caused by elevated intracellular calcium induced by PDGF stimulation. However, immunoblots of control and stimulated cells show no decrease in the inactive calpain proenzyme or increase in the proteolytic, autolyzed forms of calpain pursuant to stimulation. Furthermore, stimulation produces no increase in the proportion of the 190-kD talin fragment characteristic of calpain-mediated cleavage. The retention of talin and integrin at focal contacts after talin phosphorylation, while vinculin is redistributed, indicate that phosphorylation of talin in PDGF-stimulated cells leads to separation of talin-vinculin associations but not talin-integrin associations. We propose that PDGF binding to PDGF receptors at MTJs may provide one means of regulating myofibril associations with the muscle cell membrane.

The structural and functional diversity of dystrophin

Duchenne and Becker muscular dystrophies are caused by defects of the dystrophin gene. Expression of this large X-linked gene is under elaborate transcriptional and splicing control. At least five independent promoters specify the transcription of their respective alternative first exons in a cell-specific and developmentally controlled manner. Three promoters express full-length dystrophin, while two promoters near the C terminus express the last domains in a mutually exclusive manner. Six exons of the C terminus are alternatively spliced, giving rise to several alternative forms. Genetic, biochemical and anatomical studies of dystrophin suggest that a number of distinct functions are subserved by its great structural diversity. Extensive studies of dystrophin may lead to an understanding of the cause and perhaps a rational treatment for muscular dystrophy.