The calpain-calpastatin system and protein degradation in fusing myoblasts

Towards Drug Repurposing in Cancer Cachexia: Potential Targets and Candidates

As a multifactorial and multiorgan syndrome, cancer cachexia is associated with decreased tolerance to antitumor treatments and increased morbidity and mortality rates. The current approaches for the treatment of this syndrome are not always effective and well established. Drug repurposing or repositioning consists of the investigation of pharmacological components that are already available or in clinical trials for certain diseases and explores if they can be used for new indications. Its advantages comparing to de novo drugs development are the reduced amount of time spent and costs. In this paper, we selected drugs already available or in clinical trials for non-cachexia indications and that are related to the pathways and molecular components involved in the different phenotypes of cancer cachexia syndrome. Thus, we introduce known drugs as possible candidates for drug repurposing in the treatment of cancer-induced cachexia.

Loss of the neurodevelopmental Joubert syndrome causing protein, Ahi1, causes motor and muscle development delays independent of central nervous system involvement

Joubert syndrome (JBTS) is a predominantly autosomal recessive neurodevelopmental disorder that presents with characteristic malformations of the cerebellar vermis, superior cerebellar peduncles and midbrain in humans. Accompanying these malformations are a heterogeneous set of clinical symptoms, which frequently include deficits in motor and muscle function, such as hypotonia (low muscle tone) and ataxia (clumsiness). These symptoms are attributed to improper development of the hindbrain, but no direct evidence has been reported linking these in JBTS. Here, we describe muscle developmental defects in a mouse with a targeted deletion of the Abelson helper integration site 1 gene, Ahi1, one of the genes known to cause JBTS in humans. While FVB/NJ Ahi1^-/- mice display no gross malformations of the cerebellum, deficits are observed in several measures of motor function, strength, and body development. Specifically, Ahi1^-/- mice show delayed physical development, delays in surface reflex righting as neonates, and reductions in grip strength and spontaneous locomotor activity as adults. Additionally, Ahi1^-/- mice showed evidence of muscle-specific contributions to this phenotype, such as reductions in 1) myoblast differentiation potential in vitro, 2) muscle desmin expression, and 3) overall muscle mass, myonuclear domain, and muscle fiber cross-sectional area. Together, these data suggest that loss of Ahi1 may cause abnormalities in the differentiation of myoblasts to mature muscle cells. Moreover, Ahi1 loss impacts muscle development directly, outside of any indirect impact of cerebellar malformations, revealing a novel myogenic cause for hypotonia in JBTS.

Efficient expression and purification of recombinant human μ-calpain using an Escherichia coli expression system

Calpains comprise a superfamily of Ca(2+) -regulated cysteine proteases that are indispensable for the regulation of various cellular functions. Of these, the mammalian μ- and m-calpains are the best characterized isoforms. They are ubiquitously expressed and form heterodimers consisting of a distinct 80-kDa catalytic subunit (CAPN1 for μ-calpain and CAPN2 for m-calpain) and a common 30-kDa regulatory subunit (CAPNS1). To date, various expression systems have been developed for producing recombinant calpains for structural and functional studies; however, no low-cost, simple and efficient bacterial expression system for μ-calpain has been available, because the protein forms aggregates. Here, we established an efficient method for producing active recombinant human μ-calpain using an Escherichia coli expression system. This was achieved by co-expressing CAPN1 and CAPNS1 lacking the N-terminal Gly-rich domain (CAPNS1ΔGR) in the SoluBL21 strain. From 1 L of E. coli culture, over 2 and 6 mg, respectively, of μ-calpain and its active-site mutant μ-calpain:C115S (CAPN1:C115S+CAPNS1ΔGR) were purified by two successive column chromatographies. Compared to the native enzyme, the purified μ-calpain showed almost identical properties, demonstrating its suitability for use in structural and functional studies. This is the first report of the bacterial expression and the simple and efficient purification of active recombinant μ-calpain.

Heat-Induced Apoptosis of Mouse Meiotic Cells Is Suppressed by Ectopic Expression of Testis-Specific Calpastatin

Calpastatin is a naturally occurring inhibitor of calpain, a protease involved in apoptotic cell death. A testis-specific isoform of calpastatin (tCAST) has been identified that is transcribed in haploid germ cells but not in spermatocytes. To investigate the possible function(s) of tCAST, we tested the hypothesis that the ectopic expression of calpastatin in spermatocytes would suppress the death of these cells in response to an apoptosis-inducing stimulus in vivo. To this end, the 5'-flanking region of the mouse ldhc gene was linked to tCAST, and transgenic mice were generated. Immunohistochemical analysis revealed that, in contrast to control sections in which the signal for tCAST was seen in round spermatids, intense staining was visualized in pachytene spermatocytes in the transgenic animals, indicating that the strategy we used to generate the transgenic animals resulted in the ectopic expression of tCAST in spermatocytes. We then tested the effect of a short period of heating on germ cell apoptosis in the testes of wild-type and transgenic mice. Pachytene spermatocytes were the major germ cell type seen to undergo apoptosis after heat treatment. There were no differences in the number of apoptotic germ cells per seminiferous tubule between wild-type and tCAST transgenic control mice; thus, there was no apparent effect of the transgene on normal apoptosis. Heating resulted in increased numbers of TUNEL-positive germ cells in both wild-type and tCAST transgenic mice, as well as increased testicular DNA fragmentation. Heating the tCAST transgenic mouse testes resulted in significantly fewer apoptotic cells per seminiferous tubule than in wild-type mice at both 8 and 24 hours after treatment. Thus, as hypothesized, the ectopic expression of tCAST in pachytene spermatocytes suppressed germ cell apoptosis.

Constant expression of mouse calpastatin isoforms during differentiation in myoblast cell line, C2C12

C2C12 is a myoblast cell line which is used to studydifferentiation into multinucleated cells in vitro. Addition of calpain inhibitors, calpeptin orE-64d, to the culture medium prevented the myoblasticfusion of C2C12 cells. Immunoblot studies usingaffinity-purified antibody, revealed that the expressedlevels of mouse calpastatin remained unaltered duringC2C12 cell fusion. The detected calpastatin migratedas a protein of 130 kDa on SDS-polyacrylamide gelelectrophoresis. The estimated molecular mass wassomewhat greater than that in mouse liver anderythrocytes, and much greater than that reported inrat myoblasts. The 130 kDa isoform may contain anadditional N-terminal region designated XL domainfound in bovine calpastatin.

Proteolysis in illness-associated skeletal muscle atrophy: from pathways to networks

Improvements in health in the past decades have resulted in increased numbers of the elderly in both developed and developing regions of the world. Advances in therapy have also increased the prevalence of patients with chronic and degenerative diseases. Muscle wasting, a feature of most chronic diseases, is prominent in the elderly and contributes to both morbidity and mortality. A major research goal has been to identify the proteolytic system(s) that is responsible for the degradation of proteins that occurs in muscle atrophy. Findings over the past 20 years have clearly confirmed an important role of the ubiquitin proteasome system in mediating muscle proteolysis, particularly that of myofibrillar proteins. However, recent observations have provided evidence that autophagy, calpains and caspases also contribute to the turnover of muscle proteins in catabolic states, and furthermore, that these diverse proteolytic systems interact with each other at various levels. Importantly, a number of intracellular signaling pathways such as the IGF1/AKT, myostatin/Smad, PGC1, cytokine/NFκB, and AMPK pathways are now known to interact and can regulate some of these proteolytic systems in a coordinated manner. A number of loss of function studies have identified promising therapeutic approaches to the prevention and treatment of wasting. However, additional biomarkers and other approaches to improve early identification of patients who would benefit from such treatment need to be developed. The current data suggests a network of interacting proteolytic and signaling pathways in muscle. Future studies are needed to improve understanding of the nature and control of these interactions and how they work to preserve muscle function under various states of growth and atrophy.

Trophoblast fusion

The villous trophoblast of the human placenta is the epithelial cover of the fetal chorionic villi floating in maternal blood. This epithelial cover is organized in two distinct layers, the multinucleated syncytiotrophoblast directly facing maternal blood and a second layer of mononucleated cytotrophoblasts. During pregnancy single cytotrophoblasts continuously fuse with the overlying syncytiotrophoblast to preserve this end-differentiated layer until delivery. Syncytial fusion continuously supplies the syncytiotrophoblast with compounds of fusing cytotrophoblasts such as proteins, nucleic acids and lipids as well as organelles. At the same time the input of cytotrophoblastic components is counterbalanced by a continuous release of apoptotic material from the syncytiotrophoblast into maternal blood. Fusion is an essential step in maintaining the syncytiotrophoblast. Trophoblast fusion was shown to be dependant on and regulated by multiple factors such as fusion proteins, proteases and cytoskeletal proteins as well as cytokines, hormones and transcription factors. In this chapter we focus on factors that may be involved in the fusion process of trophoblast directly or that may prepare the cytotrophoblast to fuse.

The CLIC5 (chloride intracellular channel 5) involved in C2C12 myoblasts proliferation and differentiation

CLIC5 (chloride intracellular channel 5) is a CLIC (chloride intracellular channel) with various functions. Its high expression in skeletal muscle and association with actin-based cytoskeleton suggests that it may play an important role in muscle tissue. This study was conducted to examine whether CLIC5 regulates the proliferation and differentiation of C2C12 myoblasts into myotubes. Differentiation of C2C12 myoblasts induced by switching to a differentiation culture medium was accompanied by a significant increase of CLIC5 protein expression level. Constitutive overexpression of CLIC5 was associated with reduced cell proliferation and more cells from G2/M phase into G0/G1 phase, followed by increased number and size of myotubes and up-regulation of muscle-specific proteins of myosin heavy chain, myogenin and desmin. These results demonstrate that CLIC5 is involved in C2C12 proliferation and myogenic differentiation in vitro.

Protein kinase Czeta regulates Cdk5/p25 signaling during myogenesis

Atypical protein kinase Cζ (PKCζ) is emerging as a mediator of differentiation. Here, we describe a critical role for PKCζ during myogenic differentiation. Our results identify PKCζ as a controller of myogenic differentiation by its regulation of Cdk5.

Atypical protein kinase Cζ (PKCζ) is emerging as a mediator of differentiation. Here, we describe a novel role for PKCζ in myogenic differentiation, demonstrating that PKCζ activity is indispensable for differentiation of both C2C12 and mouse primary myoblasts. PKCζ was found to be associated with and to regulate the Cdk5/p35 signaling complex, an essential factor for both neuronal and myogenic differentiation. Inhibition of PKCζ activity prevented both myotube formation and simultaneous reorganization of the nestin intermediate filament cytoskeleton, which is known to be regulated by Cdk5 during myogenesis. p35, the Cdk5 activator, was shown to be a specific phosphorylation target of PKCζ. PKCζ-mediated phosphorylation of Ser-33 on p35 promoted calpain-mediated cleavage of p35 to its more active and stable fragment, p25. Strikingly, both calpain activation and the calpain-mediated cleavage of p35 were shown to be PKCζ-dependent in differentiating myoblasts. Overall, our results identify PKCζ as a controller of myogenic differentiation by its regulation of the phosphorylation-dependent and calpain-mediated p35 cleavage, which is crucial for the amplification of the Cdk5 activity that is required during differentiation.

Differential genomic responses in old vs. young humans despite similar levels of modest muscle damage after resistance loading

Across numerous model systems, aging skeletal muscle demonstrates an impaired regenerative response when exposed to the same stimulus as young muscle. To better understand the impact of aging in a human model, we compared changes to the skeletal muscle transcriptome induced by unaccustomed high-intensity resistance loading (RL) sufficient to cause moderate muscle damage in young (37 yr) vs. older (73 yr) adults. Serum creatine kinase was elevated 46% 24 h after RL in all subjects with no age differences, indicating similar degrees of myofiber membrane wounding by age. Despite this similarity, from genomic microarrays 318 unique transcripts were differentially expressed after RL in old vs. only 87 in young subjects. Follow-up pathways analysis and functional annotation revealed among old subjects upregulation of transcripts related to stress and cellular compromise, inflammation and immune responses, necrosis, and protein degradation and changes in expression (up- and downregulation) of transcripts related to skeletal and muscular development, cell growth and proliferation, protein synthesis, fibrosis and connective tissue function, myoblast-myotube fusion and cell-cell adhesion, and structural integrity. Overall the transcript-level changes indicative of undue inflammatory and stress responses in these older adults were not mirrored in young subjects. Follow-up immunoblotting revealed higher protein expression among old subjects for NF-kappaB, heat shock protein (HSP)70, and IL-6 signaling [total and phosphorylated signal transducer and activator of transcription (STAT)3 at Tyr705]. Together, these novel findings suggest that young and old adults are equally susceptible to RL-mediated damage, yet the muscles of older adults are much more sensitive to this modest degree of damage-launching a robust transcriptome-level response that may begin to reveal key differences in the regenerative capacity of skeletal muscle with advancing age.

Interactions between genes involved in growth and muscularity in pigs: IGF-2, myostatin, ryanodine receptor 1, and melanocortin-4 receptor

In the swine breeding industry, two economical traits are of particular importance in sires, namely, muscle growth and average daily gain (ADG). These traits are quantitative, which implies that they are under the control of multiple genes. Mutations in these genes, associated with either muscularity or growth, are useful quantitative trait nucleotides (QTN) for unraveling genetic variation of these traits and can be used in marker-assisted selection. Until now, QTN involved in muscle growth and/or ADG in pigs were identified in porcine ryanodine receptor 1 (RYR1), insulin-like growth factor-2 (IGF-2), and melanocortin-4 receptor (MC4R). Recently, a fourth possible QTN was found in porcine myostatin (MSTN). All four QTN have an influence on muscle growth and/or somatic growth, so an influence of one mutation on one or more of the other mutations should not be excluded. However, although the polymorphisms in the RYR1 and the MC4R gene affect the function of the respective protein, the polymorphisms of the IGF-2 and MSTN gene influence the mRNA expression of the respective gene. Therefore, this study investigated possible interactions between the genotypes of MSTN, IGF-2, and MC4R (population 1) or the RYR1, IGF-2, and MSTN QTN (population 2) on IGF-2 and MSTN expression in different muscle types in pigs. In both skeletal muscle and heart muscle growth, the IGF-2:MSTN ratio seems to play an important role. Also, the RYR1 genotype had a significant effect on IGF-2 expression in m. longissimus dorsi. No effect of the MC4R QTN could be seen.

Effect of the insulin-like growth factor-II and RYR1 genotype in pigs on carcass and meat quality traits

Recently, a new QTN (quantitative trait nucleotide), which is located in the regulatory sequence of the imprinted IGF-II gene was discovered in the pig and is associated with a significant increase in IGF-II mRNA expression in skeletal muscle during postnatal growth. The aim of the current study was to investigate the effect of the IGF-II paternal allele (Apat and Gpat animals that inherited, respectively, the mutant and wild type paternal allele of interest) on carcass and meat quality traits in Nn and NN RYR1 genotypes. A total of 141 animals were measured, almost equally distributed over the IGF-II and RYR1 genotypes and gender. The Apat allele increased carcass lean meat percentage with approximately 4.5% (P<0.001) as a result of decreased backfat thickness. Average live weight daily gain was not affected, hence average daily lean meat gain was significantly higher for Apat compared to Gpat animals. The IGF-II mutation had no noticeable effect on meat quality in contrast with the RYR1 mutation. No interaction effects of both mutations on meat quality were noticed.

Placenta trophoblast fusion

It has been known for more than 150 years that syncytial fusion is a normal feature in biological systems. In humans there are two larger syncytial tissues: skeletal muscles fibers and placental syncytiotrophoblast. Other fusion events take place as well from fertilization of the oocyte to infection of human cells by enveloped viruses (however, the latter does not necessarily lead to syncytium formation).Although knowledge of the fusion process is incomplete, it is clear that membranes do not fuse easily; specific proteins and other factors are required and are selectively activated. In this chapter, we describe the classic proteins, such as the syncytins, assumed to be involved in the fusion process. We also describe other factors that may play roles in the fusion process or in the preparation of the cells to fuse, such as charged phospholipids, divalent cations, and intracellular proteases. Finally, we speculate on why trophoblast cells fuse in vitro and deal with in vitro models of trophoblast fusion and how their fusion rates can be quantified.

Calpain/calpastatin activities and substrate depletion patterns during hindlimb unweighting and reweighting in skeletal muscle

Unloading of skeletal muscle by hindlimb unweighting (HU) is characterized by atrophy, protein loss, and an elevation in intracellular Ca(2+) levels that may be sufficient to activate Ca(2+)-dependent proteases (calpains). In this study, we investigated the time course of calpain activation and the depletion pattern of a specific structural protein (desmin) with unloading and subsequent reweighting. Rats underwent 12 h, 24 h, 72 h or 9 days of HU, followed by reweighting for either 0, 12 or 24 h. Total calpain-like activity was elevated with HU in skeletal muscle (P < 0.05) and was further enhanced with reweighting (P < 0.05). The increases in calpain-like activity were associated with a proportional increase in activity of the particulate fraction (P < 0.05). Activity of the mu-calpain isoform was elevated with 12 and 24 h of HU (P < 0.05) and returned to control levels thereafter. With reweighting, activities of mu-calpain were elevated above control levels for all HU groups except 9 days (P < 0.05). In contrast, minimal changes in m-calpain and calpastatin activity were observed with HU and reweighting. Although desmin depletion levels did not reach statistical significance, a significant inverse relationship was found between the mu-calpain/calpastatin ratio and the amount of desmin in isolated myofibrils (R = -0.83, P < 0.001). The results suggest that calpain activation is an early event during unloading in skeletal muscle, and that the majority of the increase in calpain activity can be attributed to the micro-isoform.

Effect of age, muscle type, and insulin-like growth factor-II genotype on muscle proteolytic and lipolytic enzyme activities in boars1

Recently, a paternally expressed quantitative trait nucleotide (QTN) in the regulatory sequence of the IGF-II gene with effects on muscle growth and fat deposition was discovered in the pig. This QTN is also known as the IGF-II intron3 G3072A mutation. The aim of the current study was to determine the effects of age, muscle type, and IGF-II genotype (Apat, mutant allele vs. Gpat, wild-type allele) on muscle proteolytic and lipolytic enzyme activities. At approximately 4, 8, 16, and 26 wk of age, boars (n = 6 to 15 per genotype x age group) were slaughtered and mu- and m-calpain (CALP), calpastatin (CAST), cathepsins (CATH) B+L and H, acid lipase, and phospholipase activities were measured in Longissimus thoracis et lumborum, Semimembranosus, and Triceps brachii muscle samples taken soon after slaughter. Activities of CATH B+L and H, mu- and m-CALP, and acid lipase were not affected by the IGF-II genotype. Activity of CAST was greater (P < 0.005) and m-CALP:CAST was less (P < 0.05) in Apat animals. Because CAST activity and m-CALP:CAST are known to be related to protein degradation, satellite cell fusion, or both, it is likely that differences in proteolytic enzyme activities are involved in the greater percentage of muscle mass in Apat animals. Age and muscle type influenced proteolytic and lipolytic enzyme activities (P < 0.05), except for mu- and m-CALP (no effect of muscle) and acid lipase (no effect of age). The same pattern in mu-CALP, CAST, and m-CALP:CAST with age was found during growth for the 3 muscles, although clear differences (P < 0.05) between muscles existed. In general, and in agreement with previous reports, greater enzyme activities were found in the more oxidative Triceps brachii muscle compared with the other 2 muscles. A remarkable increase (P < 0.05) from 16 to 26 wk of age in mu-CALP, CAST, mu-CALP:CAST, and CATH H and a large decrease (P < 0.05) in acid phospholipase and m-CALP:CAST was found. For m-CALP and CATH B+L, a gradual decrease (P < 0.05) was found with age. Although age effects on enzyme activities could only partly be interpreted biologically in relation to the muscle growth rate, this study showed that proteolytic and lipolytic enzyme activities change during growth.

Calpain 11 is unique to mouse spermatogenic cells

The calpains are a family of calcium-dependent thiol proteases involved in intracellular processing of proteins. They occur as heterodimers containing one of various large subunits and a common small subunit. Some of the large subunits are expressed ubiquitously and others are expressed in a restricted set of tissues. We have cloned the cDNA for mouse calpain 11 and demonstrated that it is expressed specifically in the mouse testis. The mRNA begins to accumulate in the testis between days 14 and 16 after birth, corresponding to the period of pachytene spermatocyte development. The protein is detected by day 18 after birth, during mid to late pachytene spermatocyte development, and is present in the acrosomal region of spermatozoa from the cauda epididymis. The expression of calpain 11 during spermatogenesis and its localization in spermatozoa suggest that it is involved in regulating calcium-dependent signal transduction events during meiosis and sperm functional processes.

C6ORF32 is upregulated during muscle cell differentiation and induces the formation of cellular filopodia

We have identified a gene by microarray analysis that is located on chromosome 6 (c6orf32), whose expression is increased during human fetal myoblast differentiation. The protein encoded by c6orf32 is expressed both in myogenic and non-myogenic primary cells isolated from 18-week old human fetal skeletal muscle. Immunofluorescent staining indicated that C6ORF32 localizes to the cellular cytoskeleton and filopodia, and often displays polarized expression within the cell. mRNA knockdown experiments in the C2C12 murine myoblast cell line demonstrated that cells lacking c6orf32 exhibit a myogenic differentiation defect, characterized by a decrease in the expression of myogenin and myosin heavy chain (MHC) proteins, whereas MyoD1 was unaltered. In contrast, overexpression of c6orf32 in C2C12 or HEK293 cells (a non-muscle cell line) promoted formation of long membrane protrusions (filopodia). Analysis of serial deletion mutants demonstrated that amino acids 55-113 of C6ORF32 are likely involved in filopodia formation. These results indicate that C6ORF32 is a novel protein likely to play multiple functions, including promoting myogenic cell differentiation, cytoskeletal rearrangement and filopodia formation.

A mouse model for monitoring calpain activity under physiological and pathological conditions

Calpains are Ca(2+)-dependent cysteine proteases known to be important for the regulation of cell functions and which aberrant activation causes cell death in a number of degenerative disorders. To provide a tool for monitoring the status of calpain activity in vivo under physiological and pathological conditions, we created a mouse model that expresses ubiquitously a fluorescent reporter consisting of eCFP and eYFP separated by a linker cleavable by the ubiquitous calpains. We named this mouse CAFI for calpain activity monitored by FRET imaging. Our validation studies demonstrated that the level of calpain activity correlates with a decrease in FRET (fluorescence resonance energy transfer) between the two fluorescent proteins. Using this model, we observed a small level of activity after denervation and fasting, a high level of activity during muscle regeneration and ischemia, and local activity in damaged myofibers after exercise. Finally, we crossed the CAFI mouse with the alpha-sarcoglycan-deficient model, demonstrating an increase of calpain activity at the steady state. Altogether, our results present evidence that CAFI mice could be a valuable tool in which to follow calpain activity at physiological levels and in disease states.

Calpastatin in rat myoblasts: transient diminution and decreased phosphorylation depend on myogenin-directed myoblast differentiation

The formation of skeletal muscle fibers involves cessation of myoblast division, followed by myoblast differentiation and fusion to multinucleated myofibers. The myogenic regulatory factor myogenin appears at the onset of differentiation; it is required for muscle fiber formation, and cannot be replaced by other factors. The myogenin-dependent pathways and targets are not fully known. Previous studies, indicating an involvement of calpain-calpastatin and caspase in myoblast fusion, were based on the use of various inhibitors. The availability of myogenin deficient cell lines that are incapable of fusion, but regain the ability to differentiate when transfected with myogenin, provide a convenient means to study calpain-calpastatin and caspase in fusing and non-fusing myoblasts without the use of inhibitors. The differentiating wild type myoblasts exhibit decreased calpastatin phosphorylation, transient diminution in calpastatin mRNA, caspase-1 dependent diminution in calpastatin protein, and calpain-promoted proteolysis. In the myogenin-deficient myoblasts, calpastatin phosphorylation is not diminished, caspase-1 is not activated, calpastatin mRNA and protein are not diminished, and protein degradation does not occur. The myogenin-deficient myoblasts transfected with myogenin gene regain the ability to fuse, and exhibit the alterations in calpastatin and proteolysis observed in the wild type cells. Overall, the results demonstrate that the regulation of calpain in these myoblasts is independent of myogenin. In contrast, the regulation of calpastatin depends on myogenin function. The temporary diminution of calpastatin during myogenin-directed differentiation of myoblasts allows calpain activation and calpain-induced protein degradation, required for myoblast differentiation and fusion.

Trophoblast fusion: Fusogenic proteins, syncytins and ADAMs, and other prerequisites for syncytial fusion

Trophoblast fusion in the placenta is an event of major importance for the preservation of a healthy pregnancy. This process takes place throughout pregnancy and is crucial for the maintenance of the syncytiotrophoblast layer, the direct border between maternal blood and fetal tissues. Different regulatory proteins have been reported that are involved in trophoblast fusion. Syncytin-1 is a candidate regulator of fusion together with its receptors ASCT2 (RDR) and ASCT1. Little is known about the receptor properties and the interactions between receptor and ligand. Syncytin-2 or HERV-FRD is another strong candidate also of retroviral origin; while its actual function still remains to be explored. ADAM12 has been proposed to be a candidate regulator of trophoblast fusion since it is known to be involved in myoblast fusion, a process with a variety of similarities to trophoblast fusion. Beside these regulatory proteins, there is the necessity of a flip of phosphatidylserine from the inner to the outer leaflet of the plasma membranes of the fusing cells. Moreover, appropriate events of the early and still reversible stages of the apoptosis cascade are indispensable for trophoblast fusion. In this review, we present some details on the above events and proteins with their most important properties that could explain their roles in trophoblast fusion.

Utrophin is a calpain substrate in muscle cells

Calpains are Ca2+ -dependent cytosolic cysteine proteases that participate in the pathology of Duchenne muscular dystrophy (DMD). Utrophin is a functional homolog of dystrophin that partially compensates for dystrophin deficiency in myofibers of mdx mice. In this study, we investigated the susceptibility of utrophin to cleavage by calpain in vitro and in muscle cells. We found that utrophin is a direct in vitro substrate of purified calpain I and II. Cleavage of utrophin by calpain I or II generates specific degradation products that are also found in cultured control and DMD myotubes under conditions with elevated intracellular Ca2+ levels. In addition, we showed that activation of cellular calpains by Ca2+ ionophore treatment reduces utrophin protein levels in muscle cells and that calpain inhibition prevents this Ca2+ -induced reduction in utrophin levels. These observations suggest that, beside its known effect on general muscle protein degradation, calpain contributes to DMD pathology by specifically degrading the compensatory protein utrophin.

Differential tissue expression of a calpastatin isoform in Xenopus embryos

This study is aimed at demonstrating the role played by a calpastatin isoform (Xcalp3) in Xenopus embryos. A specific monoclonal antibody (mAb) was raised against a glutathione S-transferase (GST)-Xcalp3 fusion protein and characterized by immunoblotting and confocal fluorescence microscopy on stage 20-36 embryos. Under these conditions, calpastatin reactivity is associated with a major 110kDa protein fraction and preferentially expressed by notochord and somitic cells. In notochord cells, anti-calpastatin reactive sites were initially restricted to the luminal space of the vacuoles and later became diffused throughout the cytoplasm. In contrast, anti-calpastatin reactive sites in somitic cells were initially diffused throughout the cytoplasm and became restricted to a few intracellular granules in the later developmental stages. At the ultrastructural level, notochord cells appeared as flattened discs containing several vacuoles and numerous electron-dense granules. During transition from stages 26 to 32, electron-dense granules were gradually reduced in number as vacuoles enlarged in size and losed their calpastatin reactivity. Electron-dense granules were also present in myoblast cells and their number gradually reduced during development. To determine whether these observations bear any causal relationship to the calpain/calpastatin system, a number of Xenopus embryos were examined both ultrastructurally and histochemically following exposure to a specific calpain inhibitor (CI3). Under these conditions, Xenopus embryos exhibited an altered right-left symmetry and an abnormal axial shortening. In CI3-treated stage 32 embryos, notochord cells had a reduced vacuolar extension and exhibited at the same time an increase in granular content. The overall morphology of the somites was also distorted and myoblasts were altered both in shape and granular content. Based on these findings, it is concluded that the calpain/calpastatin may play an important role in the control of notochord elongation and somite differentiation during Xenopus embryogenesis.

Involvement of calpains in growth factor-mediated migration

Previous research in our laboratory has already shown the importance of the role played by ubiquitous calpains during myoblast migration. The aim of this study was to investigate calpain expression during myoblast migration and, to enhance this phenomenon via calpain stimulation. Ubiquitous calpains are members of a large family of calcium-dependent cysteine proteases. They play an important role in numerous biological and pathological phenomena, such as signal transduction, apoptosis, cell-cycle regulation, cell spreading, adhesion, invasion, myogenesis, and motility. Myoblast migration is a crucial step in myogenesis, as it is necessary for myoblast alignment and fusion to form myotubes. This study started by examining changes in calpain expression during migration, then investigated the possibility of activating myoblast migration via the stimulation of calpain expression and/or activity. The migration rate of myoblasts overexpressing mu- or milli-calpain was quantified. The results showed that calpain overexpression dramatically inhibited myoblast migration. Growth-factor treatments were then used to enhance myoblast migration. The results showed that treatment with IGF-1, TGF-beta1, or insulin induced a major increase in migration and caused a significant increase in m-calpain expression and activity. The increase in migration was totally inhibited by adding calpeptin, a calpain-specific inhibitor. These findings suggest that milli-calpain is involved in growth factor-mediated migration.

Overexpression of calpastatin inhibits L8 myoblast fusion

The formation of skeletal muscle fibers involves cessation of myoblast division, myoblast alignment, and fusion to multinucleated myofibers. Calpain is one of the factors shown to be involved in myoblast fusion. Using L8 rat myoblasts, we found that calpain levels did not change significantly during myoblast differentiation, whereas calpastatin diminished prior to myoblast fusion and reappeared after fusion. The transient diminution in calpastatin allows the Ca2+-promoted activation of calpain and calpain-induced membrane proteolysis, which is required for myoblast fusion. Here we show that calpastatin overexpression in L8 myoblasts does not inhibit cell proliferation and alignment, but prevents myoblast fusion and fusion-associated protein degradation. In addition, calpastatin appears to modulate myogenic gene expression, as indicated by the lack of myogenin (a transcription factor expressed in differentiating myoblasts) in myoblasts overexpressing calpastatin. These results suggest that, in addition to the role in membrane disorganization in the fusing myoblasts, the calpain-calpastatin system may also modulate the levels of factors required for myoblast differentiation.

Caspase-1 activity is required for neuronal differentiation of PC12 cells: cross-talk between the caspase and calpain systems

Previously, we have found that caspase-1 activity is increased during myoblast differentiation to myotubes. Here we show that caspase-1 activity is required for PC12 differentiation to neuronal-like cells. Caspase-1 is shown to be activated (by immunoblotting and by assessing activity in cell extracts) in the PC12 cells following the initial stage of differentiation. The inhibition of caspase-1 arrests PC12 cells at an intermediate stage of differentiation and prevents neurite outgrowth in these cells; the inhibition is reversed upon the removal of the inhibitor. Calpastatin (calpain endogenous specific inhibitor, and a known caspase substrate) is diminished at the later stages of PC12 cell differentiation, and diminution is prevented by caspase-1 inhibition. The degradation of fodrin (a known caspase and calpain substrate) is found in the advanced stage of differentiation. Caspase-1 has been implicated in the activation of proinflammatory cytokines, and in cell apoptosis. The involvement of caspase-1 in two distinct differentiation processes (myoblast fusion and neuronal differentiation of PC12 cells) indicates a function for this caspase in differentiation processes, and suggests some common mechanisms underlying caspase roles in such processes.

Expression and immunolocalization of the calpain-calpastatin system in the human oocyte

OBJECTIVE

To investigate the expression of the calpain-calpastatin system in the human oocyte.

DESIGN

The expression of the calpain-calpastatin system was determined by immunohistochemistry and immunoblot analysis.

SETTING

Academic research laboratory.

PATIENT(S)

Twenty Israeli women who underwent IVF for fertility problems.

INTERVENTION(S)

Oocytes that had no pronuclei 24 hours after insemination by either conventional IVF or intracytoplasmic sperm injection were retrieved for the study.

MAIN OUTCOME MEASURE(S)

Analysis of calpain isoforms (m, mu) and calpastatin distribution within the human oocyte.

RESULT(S)

Western blot analysis confirmed the expression of calpain and calpastatin. Immunohistochemistry of fixed, permeabilized oocytes exhibited localization of both calpains to the cortical region of the oocyte, as well as the cytosol. Calpastatin seemed to be distributed throughout the cytosol, with a marked accumulation in the cell membrane. We have demonstrated a negative correlation between the occurrence of cortical granule exocytosis and the stability of the metaphase plate.

CONCLUSION(S)

A complete calpain-calpastatin system is expressed in the human oocyte and might play a role in the various calcium-mediated processes occurring during activation of human oocytes.

Novel glycosaminoglycan mimetic (RGTA, RGD120) contributes to enhance skeletal muscle satellite cell fusion by increasing intracellular Ca2+ and calpain activity

Glycosaminoglycans (GAG) are classes of molecules that play an important role in cellular processes. The use of GAG mimetics called regenerating agent (RGTA) represents a tool to investigate the effect of GAG moiety on cellular behavior. A first member of the RGTA family (RG1192), a dextran polymers with defined amounts of sulfate, carboxymethyl, as well as hydrophobic groups (benzylamide), was shown to stimulate skeletal muscle repair after damage and myoblast differentiation. To obtain a comprehensive insight into the mechanism of action of GAG mimetics, we investigated the effect on myoblast differentiation of a novel RGTA, named RGD120, which was devoid of hydrophobic substitution and had ionic charge similar to heparin. Myoblasts isolated from adult rat skeletal muscles and grown in primary cultures were used in this study. We found that chronic treatment with RGD120 increased the growth of adult myoblasts and induced their precocious fusion into myotubes in vitro. It also partially overcame the inhibitory effect of the calpain inhibitor N-acetyl-leu-leu-norleucinal (ALLN) on these events. Western blot and zymography analyses revealed that milli calpain was slightly increased by RGD120 chronic treatment. In addition, using fluorescent probes (Indo-1 and Boc-leu-met-MAC), we demonstrated that RGD120 added to prefusing myoblast cultures accelerates myoblast fusion into myotubes, induced an increase of cytosolic free calcium concentration, and concomitantly an increase of intracellular calpain protease activity. Altogether, these results suggested that the efficiency of RGD120 in stimulating myogenesis might be in part explained through its effect on calcium mobilization as well as on the calpain amount and activity.

Involvement of micro-calpain (CAPN 1) in muscle cell differentiation

Several studies have already demonstrated that micro- and milli-calpains (CAPN 1-CAPN 2), calcium-dependent intracellular cysteine-proteases are involved in many biological phenomenon including muscle growth and development. More particularly, recent studies have demonstrated that milli-calpain is implicated in myoblast fusion. Moreover, in primary muscle cells, these proteases do not appear simultaneously throughout muscle cell differentiation. Because micro- and milli-calpains do not have the same intracellular localization, it appears likely that these two calcium-dependent proteases have different biological roles during muscle cell differentiation. The goal of this study is to determine the role of micro-calpain. We therefore, have developed a muscle cell line in which micro-calpain is over-expressed, using the inducible Tet Regulated Expression System. The outcome is observed by following the behavior of different proteins, considered to be potential substrates of the protease. The present study shows important decreases in the expression level of ezrin (68%), vimentin (64%) and caveolin 3 (76%) whereas many other cytoskeletal proteins remain remarkably stable. Concerning the myogenic transcription factors, only the level of myogenin decreased (59%) after the over-expression of micro-calpain. Ultra structural studies have shown that the myofibrils formed near the cell periphery are normally oriented, lying along the longitudinal axis. This regularity is lost progressively towards the cell center where the cytoskeleton presented an increasing disorganization. All these results indicate that micro-calpain is involved in regulation pathway of myogenesis via at least its action on ezrin, vimentin, caveolin 3 and myogenin, a muscle transcription factor.

Foxj1 regulates basal body anchoring to the cytoskeleton of ciliated pulmonary epithelial cells

The forkhead box transcription factor Foxj1 is required for cilia formation and left-right axis determination. To define the role of Foxj1 in ciliogenesis, microarray analysis was performed to identify differentially expressed genes in the pulmonary epithelium of foxj1(+/+) and foxj1(-/-) mice. In the absence of Foxj1, the expression of calpastatin, an inhibitor of the protease calpain, decreased. RNase protection confirmed the decrease in calpastatin expression and decreased calpastatin was detected in the proximal pulmonary epithelium of foxj1(-/-) mice by immunohistochemistry. No change was detected in the expression of calpain 2 in the pulmonary epithelium by western blot or immunohistochemistry. By western blot and immunofluorescence, ezrin, a substrate for calpain, was also found to decrease in the pulmonary epithelium of foxj1(-/-) mice. No change in ezrin gene expression was found by RT-PCR. A decrease in ezrin binding phosphoprotein-50 (EBP-50) was also detected by immunofluorescence in the foxj1(-/-) mouse pulmonary epithelium. Immunoelectron microscopy demonstrated ezrin associated with the basal bodies of cilia in the pulmonary epithelium. Treatment of tracheal explants from foxj1(-/-) mice with a calpain inhibitor resulted in a partial reappearance of cilia observed in these mice. Additionally, following treatment of foxj1(-/-) tracheal explants with calpain inhibitor, basal bodies were observed in an apical location along with relocalization of ezrin and EBP-50. Regulation of calpain activity by calpastatin thus provides a mechanism for regulating the anchoring of basal bodies to the apical cytoskeleton in ciliated cells. In the absence of Foxj1, decreased calpastatin expression with decreased ezrin and EBP-50 results in an inability of basal bodies to anchor to the apical cytoskeleton and subsequent failure of axonemal formation.

Cell and molecular biology of myoblast fusion

In organisms from Drosophila to mammals, the musculature is comprised of an elaborate array of distinct fibers that are generated by the fusion of committed myoblasts. These muscle fibers differ from each other in features that include location, pattern of innervation, site of attachment, and size. The sizes of the newly formed muscles of an embryo are controlled in large part by the number of cells that form the syncitial fiber. Over the past few decades, an extensive body of literature has described the process of myoblast fusion in vertebrates, relying primarily on the strengths of tissue culture model systems. More recently, genetic studies in Drosophila embryos have provided new insights into the process. Together, these studies define the steps necessary for myoblast differentiation, the acquisition of fusion competence, the recognition and adhesion between myoblasts, and the fusion of two lipid bilayers into one. In this review, we have attempted to combine insights from both Drosophila and vertebrate studies to trace the processes and molecules involved in myoblast fusion. Implicit in this approach is the assumption that fundamental aspects of myoblast fusion will be similar, independent of the organism in which it is occurring.

Degradation of sarcomeric and cytoskeletal proteins in cultured skeletal muscle cells

The goal of this research was to evaluate the roles of calpains and their interactions with the proteasome and the lysosome in degradation of individual sarcomeric and cytoskeletal proteins in cultured muscle cells. Rat L8-CID muscle cells, in which we expressed a transgene calpain inhibitor (CID), were used in the study. L8-CID cells were grown as myotubes after which the relative roles of calpain, proteasome and lysosome in total protein degradation were assessed during a period of serum withdrawal. Following this, the roles of proteases in degrading cytoskeletal proteins (desmin, dystrophin and filamin) and of sarcomeric proteins (alpha-actinin and tropomyosin) were assessed. Total protein degradation was assessed by release of radioactive tyrosine from pre-labeled myotubes in the presence and absence of protease inhibitors. Effects of protease inhibitors on concentrations of individual sarcomeric and cytoskeletal proteins were assessed by Western blotting. Inhibition of calpains, proteasome and lysosome caused 20, 62 and 40% reductions in total protein degradation (P<0.05), respectively. Therefore, these three systems account for the bulk of degradation in cultured muscle cells. Two cytoskeletal proteins were highly-sensitive to inhibition of their degradation. Specifically, desmin and dystrophin concentrations increased markedly when calpain, proteasome and lysosome activities were inhibited. Conversely, sarcomeric proteins (alpha-actinin and tropomyosin) and filamin were relatively insensitive to the addition of protease inhibitors to culture media. These data demonstrate that proteolytic systems work in tandem to degrade cytoskeletal and sarcomeric protein complexes and that the cytoskeleton is more sensitive to inhibition of degradation than the sarcomere. Mechanisms, which bring about changes in the activities of the proteases, which mediate muscle protein degradation are not known and represent the next frontier of understanding needed in muscle wasting diseases and in muscle growth biology.

Calpain regulates enterocyte brush border actin assembly and pathogenic Escherichia coli-mediated effacement

This study identifies calpain as being instrumental for brush border (BB) microvillus assembly during differentiation and effacement during bacterial pathogenesis. Calpain activity is decreased by 25-80% in Caco 2 lines stably overexpressing calpastatin, the physiological inhibitor of calpain, and the effect is proportional to the calpastatin/calpain ratio. These lines exhibit a 2.5-fold reduction in the rate of microvillus extension. Apical microvillus assembly is reduced by up to 50%, as measured by quantitative fluorometric microscopy (QFM) of ezrin, indicating that calpain recruits ezrin to BB microvilli. Calpain inhibitors ZLLYCHN2, MDL 28170, and PD 150606 block BB assembly and ezrin recruitment to the BB. The HIV protease inhibitor ritonavir, which inhibits calpain at clinically relevant concentrations, also blocks BB assembly, whereas cathepsin and proteasome inhibitors do not. Microvillus effacement is inhibited after exposure of calpastatin-overexpressing cells to enteropathogenic Escherichia coli. These results suggest that calpain regulates BB assembly as well as pathological effacement, and indicate that it is an important regulator involved in HIV protease inhibitor toxicity and host-microbial pathogen interactions.

Caspase-1-induced calpastatin degradation in myoblast differentiation and fusion: cross-talk between the caspase and calpain systems

Previously, we found that calpastatin diminished transiently prior to myoblast fusion (rat L8 myoblasts), allowing calpain-induced protein degradation, required for fusion. Here we show that the transient diminution in calpastatin is due to its degradation by caspase-1. Inhibition of caspase-1 prevents calpastatin diminution and prevents myoblast fusion. Caspase-1 activity is transiently increased during myoblast differentiation. Both calpain and caspase appear to be responsible for the fusion-associated membrane protein degradation. Caspase-1 has been implicated in the activation of proinflammatory cytokines, and in cell apoptosis. The involvement of caspase-1 in L8 myoblast fusion represents a novel function for this caspase in a non-apoptotic differentiation process, and points to cross-talk between the calpain and caspase systems in some differentiation processes.

Myogenic and nonmyogenic cells differentially express proteinases, Hsc/Hsp70, and BAG-1 during skeletal muscle regeneration

Skeletal muscle has a remarkable capacity to regenerate after injury. To determine whether changes in the expression of proteinases, 73-kDa constitutive heat shock cognate protein (Hsc70) and stress-inducible 72-kDa heat shock protein (Hsp70) (Hsc/Hsp70), and Bcl-2-associated gene product-1 (BAG-1) contribute to the remodeling response of muscle tissue, tibialis anterior muscles of male Sprague-Dawley rats were injected with 0.75% bupivacaine and removed at 3, 5, 7, 10, 14, 21, or 35 days postinjection (n = 5-7/group). The immunohistochemical analysis of desmin, alpha-actin, and developmental/neonatal myosin heavy chain expressions indicated the presence of myoblasts (days 3-7), inflammatory cells (days 3-7), degenerating myofibers (days 3-7), regenerating myofibers (days 5-10), and growing mature myofibers (days 10-21) in regenerating muscles. Our biochemical analysis documented profound adaptations in proteolytic metabolism characterized by significant increases in the enzyme activities of matrix metalloproteinases 2 and 9 and plasminogen activators (days 3-14), calpains 1 and 2 (days 3-7), cathepsins B and L(days 3-10), and proteasome (days 3-14). Proteasome activity was strongly correlated with proliferating cell nuclear antigen protein level, suggesting that proteasome played a key role in myoblast proliferation. The expression pattern of BAG-1, a regulatory cofactor of Hsc/Hsp70 at the interface between protein folding and proteasomal proteolysis, did not corroborate the changes in proteasome enzyme activity, suggesting that BAG-1 may promote other functions, such as the folding capacity of Hsc/Hsp70. Altogether, the diversity of functions attributed to proteinases in the present study was strongly supported by the relative changes in the proportion of myogenic and nonmyogenic cells over the time course of regeneration.

Transactivation of capn2 by myogenic regulatory factors during myogenesis

The calcium-activated cysteine protease m-calpain plays a pivotal role during the earlier stages of myogenesis, particularly during fusion. The enzyme is a heterodimer, encoded by the genes capn2, for the large subunit, and capn4, for the small subunit. To study the regulation of m-calpain, the DNA sequence upstream of capn2 was analyzed for promoter elements, revealing the existence of five consensus-binding sites (E-box) for several myogenic regulatory factors and one binding site for myocyte enhancer factor-2 (MEF-2). Transient transfections with reporter gene constructs containing the E-box revealed that MyoD presents a high level of transactivation of reporter constructs containing this region, in particular the sequences including the MEF-2/E4-box. In addition, over-expression of various myogenic factors demonstrated that MyoD and myogenin with much less efficiency, can up-regulate capn2, both singly and synergistically, while Myf5 has no effect on synthesis of the protease. Experiments with antisense oligonucleotides directed against each myogenic factor revealed that MyoD plays a specific and pivotal role during capn2 regulation, and cannot be replaced wholly by myogenin and Myf5.

Adverse and beneficial functions of proteolytic enzymes in skeletal muscle. An overview

Proteolytic enzymes (proteases) comprise a family of enzymes which hydrolyse protein or peptide substrates in the generalised process of intracellular protein degradation, a process essential for the normal functioning of all cells. Proteases may also have a wide range of additional functions, including metabolic control of physiologically active oligopeptides or precursor protein forms, antigen presentation/recognition by the major histocompatibility complex in the cellular immune response, as well as in digestion, blood clotting, complement activation, etc. In this article, the nomenclature and classification of proteolytic enzymes in skeletal muscle, and their role in normal muscle physiological processes have been reviewed, including exercise, muscle development and ageing. Although proteases play an important role in normal muscle functioning, in pathological situations the enzymes may themselves be regarded as 'toxic agents' in terms of their damaging effects on muscle tissue. Muscle damage resulting from inappropriate activity of proteolytic enzymes in muscle wasting associated with muscular dystrophies, denervation atrophy, inflammatory myopathies, cancer, sepsis, diabetes and alcoholism have been reviewed. In addition, evidence that the adverse effects of drugs known to induce muscle wasting, such as corticosteroids, (or beneficial effects of growth promoting drugs) may be mediated via proteolytic enzymes is also reviewed.

Characterization of the calcium-dependent proteolytic system in a mouse muscle cell line

Many studies have demonstrated that the calcium-dependent proteolytic system (calpains and calpastatin) is involved in myoblast differentiation. It is also known that myogenic differentiation can be studied in vitro. In the present experiments, using a mouse muscle cell line (C2C12) we have analyzed both the sequences of appearance and the expression profiles of calpains 1, 2, 3 and calpastatin during the course of myoblast differentiation. Our results mainly show that the expression of ubiquitous calpains (calpain 1 and 2) and muscle-specific calpain (calpain 3) at the mRNAs level as well as at the protein level do not change significantly all along this biological process. In the same time, the specific inhibitor of ubiquitous calpains, calpastatin, presents a stable expression at mRNAs level as well as protein level, all along myoblast to myotube transition. A comparison with other myogenic cells is presented.

Apoptosis and syncytial fusion in human placental trophoblast and skeletal muscle

Skeletal muscle fibers and placental villous trophoblast are the main representatives of syncytia in the human. Both syncytia are derived from fusion of mononucleated stem cells, show a high degree of differentiation, and have lost their generative potency. Consequently, for their growth both depend on fusion of additional stem cells. There is evidence that syncytial fusion is directly or indirectly related to apoptotic events: As early as in the differentiated stages of the mononucleated stem cells, initiation stages of the apoptosis cascade have been observed. After syncytial fusion progression of the cascade is retarded or blocked by a variety of mechanisms. In this review we emphasize the links between apoptosis cascade, differentiation pathways and syncytial fusion. It needs to be elucidated whether these processes simply take place in parallel, both temporally and spatially, or whether there are causal connections between apoptosis cascade and syncytial fusion. Based on recent data obtained for placental villous trophoblast, it is tempting to speculate that early molecular mechanisms of the apoptosis cascade are involved in differentiation and syncytial fusion. Data obtained in skeletal muscles support this assumption and reveal a considerable degree of homology in genesis, maintenance and turnover of both tissues.

Scale-up of a myoblast culture process

The effects of different types of cell carriers, strategies for cell transfer on carriers, and of several fusion inhibitors on the growth kinetics of primary human myoblasts culture were studied in order to develop a bioprocess suitable for the treatment of Duchenne muscular dystrophy based on the transplantation of unfused cells. Our results indicate that myoblast production is larger on Cytodex 1 and 3 than on polypropylene or polyester fabrics and on a commercial porous macrocarrier. Myoblast growth conditions with Cytodex 1 were further investigated to establish the bioprocess operating conditions. It was found that microcarrier density of 3 g DW l(-1), inoculum density of 2x10(5) cells ml(-1), and continuous agitation speed of 30-rpm result in final myoblast production comparable to static cultures. However, for all the culture conditions used, myoblasts growth kinetics exhibited a lag phase that lasted a minimum of 1 week prior to growth, the end of the lag phase correlating with the appearance of microcarrier aggregates. Based on this observation, we propose that aggregation promotes cell growth by offering a network of very large inter-particular pores that protect cells from mechanical stress. We took advantage of the presence of these aggregates for the scale-up of the culture process. Indeed, using myoblast-loaded microcarrier-aggregates instead of myoblast suspension to inoculate a fresh suspension of microcarriers significantly reduced the duration of the lag phase and allowed the scale-up of the bioprocess at the 500-ml scale. In order to ensure the production of unfused myoblasts, the efficiency of five different fusion inhibitors was investigated. Only calpeptin (9.1 microg ml(-1)) significantly inhibited the fusion of the myoblasts, while TGFbeta (50 ng ml(-1)) and LPA (10 microg ml(-1)) increased myoblasts growth but did not affect fusion, sphingosine (30 microg ml(-1)) induced a 50% death and NMMA (25 microg ml(-1)) had no effect on either growth or fusion. Finally, transplantation trials on severe combined immunodeficient mice showed that microcarrier-cultured human myoblasts grown using the optimized bioprocess resulted in grafts as successful as myoblasts grown in static cultures. The bioprocess, therefore, prove to be suitable for the large-scale production of myoblasts required for muscular dystrophy treatment.

Molecular cloning and characterization of functional domains of a human testis-specific isoform of calpastatin

Human serum containing sperm-agglutinating antibodies was used to screen a testis cDNA expression library to identify the cognate antigens that may be responsible for this biological effect. The longest positive phage clone (1.9 kb) was sequenced and found to be a testis-specific isoform of calpastatin (tCAST). The testis-specific segment of tCAST is encoded by a single exon within intron 14 of the calpastatin gene. A unique protein isoform is produced that differs in domain structure from the somatic calpastatins (sCAST). Human sCAST most commonly has an N-terminal domain L plus the four functional calpain inhibitory domains. Human tCAST consists of a 40-amino-acid N-terminal T domain plus a part of domain II and all of domains III and IV from the somatic isoform. Our data show that the T domain can target cytosolic localization and membrane association of tCAST, whereas domain I of sCAST exhibits a nuclear localization function. Calpastatin is the endogenous inhibitor of calpain. The calpain/calpastatin system is involved in membrane fusion events for several cell types, and calpain has been localized to the sperm acrosome. We detected tCAST in human sperm and testes extracts by Western blotting with specific antisera. These observations suggest that tCAST may modulate calpain in the calcium-mediated acrosome reaction that is required for fertilization.

A new alternative method to quantify residual structure in 'unfolded' proteins

Pig (pCSD1) and human (hCSD1) calpastatin domain 1 proteins were studied to characterize common features of the denatured state of proteins. These proteins were chosen for the present investigation, because pCSD1 was suggested previously to be unstructured in water even at 25 degrees C (1) [T. Konno et al., Biochim. Biophys. Acta 1342 (1997) 73-82]. hCSD1 could be expected to exhibit similar features on the basis of preliminary spectroscopic studies. In the present study, the experimental grounds for the estimate of residual structure in the unfolded state were differential scanning calorimetry heat capacity and circular dichroism (CD) measurements over the temperature range 10-80 degrees C. At selected temperatures, we studied also the effect of guanidinium hydrochloride (GdnHCl) which is known to promote further unfolding of the polypeptide chain. All other measurements were performed at pH 6 in pure water. The present results support the conclusion that the comparison of the experimentally obtained heat capacity data with theoretical heat capacity values calculated on the basis of a newly established increment system gives insight into the degree of hydration of the unfolded polypeptide chain. The percentage by which the experimental heat capacity of the unfolded polypeptide chain differs from the calculated heat capacity permits a quantitative estimate of the residual structure. This estimate is in good agreement with that based on CD absorption. The heat capacity approach has the advantage of comparing fully hydrated and partially hydrated residues in the same aqueous environment, whereas for example spectroscopic measurements, such as CD, are generally referred to the fully unfolded chain in concentrated urea or GdnHCl solutions. As the unfolded chains of pCSD1 and hCSD1 exhibit a smaller heat capacity than that calculated on the new peptide-based increment system [M. Häckel et al., J. Mol. Biol. 291 (1999) 197-213], we conclude that the residues in the unfolded polypeptide chain are less hydrated than the same residues in oligopeptides. This suboptimal hydration is the result of residual structure in the chain as observed in both CD and heat capacity measurements.

Exposure to tissue culture conditions can adversely affect myoblast behavior in vivo in whole muscle grafts: implications for myoblast transfer therapy

The effects of tissue culture conditions on the viability of myoblasts in whole muscles transplanted in vivo were investigated. Whole male (SJL/J) donor muscles were exposed to various tissue culture reagents and proteolytic enzymes, and allografted into female (SJL/J) host mice. Desmin immunohistochemistry was used to assess the numbers of myogenic cells (as an index of myoblast viability and the extent of regeneration) in tissue sections of whole-muscle grafts sampled on days 7 and 14. DNA quantitation with a Y-chromosome-specific probe was used to determine the total Y-1 sequence DNA (as an index of myoblast survival and proliferation) in whole-muscle grafts sampled on days 1, 3, and 7. In grafts exposed to serum-free medium, there was a delay in myoblast fusion at 7 days that was recovered by 14 days, but exposure to serum (10% or 20%) had a prolonged adverse effect on myotube formation at 14 days. DNA quantitation demonstrated that either serum-free culture medium or 10% serum enhanced the number of male cells within whole-muscle grafts at 7 days. Proteolytic digestion (even for 5 min) of whole muscles prior to grafting was extremely detrimental to myoblast survival and viability at 7 and 14 days. The unexpected finding of adverse effects of tissue culture conditions on the regeneration of whole-muscle grafts in vivo appears to parallel the major problem of the rapid death of isolated cultured donor myoblasts after injection in myoblast transfer therapy. The use of whole-muscle grafts provides an alternative and sensitive model to analyze the crucial effects of various tissue culture components on the subsequent survival and proliferation of myogenic cells in vivo.

Calcineurin activity is required for the initiation of skeletal muscle differentiation

Differentiation of skeletal muscle myoblasts follows an ordered sequence of events: commitment, cell cycle withdrawal, phenotypic differentiation, and finally cell fusion to form multinucleated myotubes. The molecular signaling pathways that regulate the progression are not well understood. Here we investigate the potential role of calcium and the calcium-dependent phosphatase calcineurin in myogenesis. Commitment, phenotypic differentiation, and cell fusion are identified as distinct calcium-regulated steps, based on the extracellular calcium concentration required for the expression of morphological and biochemical markers specific to each of these stages. Furthermore, differentiation is inhibited at the commitment stage by either treatment with the calcineurin inhibitor cyclosporine A (CSA) or expression of CAIN, a physiological inhibitor of calcineurin. Retroviral-mediated gene transfer of a constitutively active form of calcineurin is able to induce myogenesis only in the presence of extracellular calcium, suggesting that multiple calcium-dependent pathways are required for differentiation. The mechanism by which calcineurin initiates differentiation includes transcriptional activation of myogenin, but does not require the participation of NFAT. We conclude that commitment of skeletal muscle cells to differentiation is calcium and calcineurin-dependent, but NFAT-independent.