Overexpression of calpastatin inhibits L8 myoblast fusion

Novel synthetic lipopeptides derived from Mycoplasma hyorhinis upregulate calpastatin in SH-SY5Y neuroblastoma cells and induce a neuroprotective effect against amyloid-β-peptide toxicity

Previously, we showed that contamination of SH-SY5Y neuroblastoma cells by Mycoplasma hyorhinis strains NDMh and MCLD leads to increased levels of calpastatin (the endogenous, specific inhibitor of the Ca2+-dependent protease calpain), resulting in inhibition of calpain activation. We have found that the increased calpastatin level is promoted by the lipoprotein fraction (MhLpp) of the mycoplasmal membrane. Here, we present MhLpp-based novel synthetic lipopeptides that induce upregulation of calpastatin in SH-SY5Y neuroblastoma cells, leading to protection of the treated cells against Ca2+/amyloid-β-peptide toxicity. These lipopeptides present a new class of promising agents against calpain-induced cell toxicity.

Oxidative stress induces apoptosis via calpain- and caspase-3-mediated cleavage of ATM in pancreatic acinar cells

Oxidative stress-induced DNA cleavage and apoptosis in pancreatic acinar cells has been implicated in the pathogenesis of acute pancreatitis. Thus, an efficient DNA repair process is key to prevention of apoptotic pancreatic acinar cell death. Ataxia telangiectasia mutated (ATM), a sensor of DNA breaks, functions by recruiting DNA repair proteins to initiate the DNA repair process. In the present study, we investigated whether H₂O₂ produced by the action of glucose oxidase on α-D-glucose (G/GO) induces apoptosis in pancreatic acinar AR42J cells through an alteration of the level of ATM. As a result, G/GO induced apoptosis by promoting a loss of cell viability, increase in Bax, decrease in Bcl-2, cleavage of poly (ADP-ribose) polymerase (PARP) and fragmentation of DNA. In addition, ATM cleavage along with elevated levels of calpain and caspase-3 activity was induced by G/GO. By using ATM siRNA, we demonstrated that reduction in ATM levels enhanced G/GO-induced apoptosis. Moreover, inhibition of calpain activity by calpeptin or calpastatin, or by inhibition of caspase-3 with z-DEVD, suppressed G/GO-induced apoptosis and ATM cleavage. Collectively, these findings suggest that proteolysis of ATM is the underlying mechanism of apoptosis of pancreatic acinar cells caused by exposure to oxidative stress.

Comprehensive assessment of candidate genes associated with fattening performance in Holstein-Friesian bulls

The objective of this study was to determine the association of single nucleotide polymorphisms (SNPs) in selected candidate genes with fattening performance traits in a commercial cattle herd. Fifteen SNPs in 12 candidate genes (LEP, FABP4, DGAT1, TG, IGF1, IGF1R, MYF5, LGB, CAPN1, CAST, GHR, and OLR1) were evaluated in 296 purebred Holstein-Friesian bulls using PCR-RFLP (polymerase chain reaction - restriction fragment length polymorphism). Associations between each segregating SNP and genetic merit for fattening performance were quantified using linear mixed models. Traits included in the study were fattening period, final weight, dry matter intake, feed conversion rate, and average daily weight gain. Apart from the general determination of the above-mentioned traits, each trait was evaluated based on the fattening periods between five selected target body weights (W1  =  100 kg, W2  =  200 kg, W3  =  300 kg, W4  =  400 kg, W5  =  450 kg). All markers with the exception of CAPN1 530, IGF1R, TG, and DGAT1 were associated with at least one of the traits. Furthermore, novel associations were observed for LEP  ×  GHR, IGF1  ×  LEP, FABP4 3691  ×  FABP4 2834, and FAP4 3533  ×  LEP interactions. The results of this study confirm some previously reported associations. Moreover, novel associations have been identified, which may be incorporated into breeding programs to improve fattening performance.

Decreased abundance of eIF4F subunits predisposes low-birth-weight neonatal pigs to reduced muscle hypertrophy

Muscle hypertrophy is limited in low-birth-weight (LBWT) neonates, suggesting a reduction in protein synthesis and increased protein degradation. Sixteen pairs of 1-d old normal-birth-weight (NBWT) and LBWT littermates (n = 16) were euthanized and the longissimus dorsi (LD) was sampled for protein abundance and kinase phosphorylation profiles measures. Eukaryotic initiation factor (eIF) 4E and eIF4G abundance, and assembly of the active eIF4E-eIF4G complex was less for LBWT than for NBWT pig muscles. Similarly, eIF3f abundance was reduced in muscle of LBWT compared with NBWT pig and was associated with diminished ribosomal protein S6 kinase 1 (S6K1) phosphorylation. This decrease was linked to a lower phosphorylation of programmed cell death protein 4 (PDCD4) in LBWT pig muscle. By contrast, PDCD4 abundance was greater in muscle of LBWT than NBWT group, suggesting lower release and availability of eIF4A from PDCD4-eIF4A complex. Moreover, protein abundance of eIF4A was lower in LBWT muscle, which is expected to further impair the formation of eIF4F translation initiation complex. Microtubule associated light chain 3 (LC3) II to total LC3 ratio was greater in LBWT LD lysates yet P62 abundance was similar between the two groups suggesting no difference in autophagy. Muscle atrophy F-box (atrogin-1) abundance was less in LBWT LD lysates, suggesting decreased degradation through the ubiquitin-proteasome system. In conclusion, limited eIF4F subunit abundance and downregulated translation initiation are plausible mechanisms for diminished muscle growth in LBWT compared with NBWT neonatal pigs.

Heme-mediated apoptosis and fusion damage in BeWo trophoblast cells

Placental malaria (PM) is a complication associated with malaria infection during pregnancy that often leads to abortion, premature delivery, intrauterine growth restriction and low birth weight. Increased levels of circulating free heme, a by-product of Plasmodium-damaged erythrocytes, is a major contributor to inflammation, tissue damage and loss of blood brain barrier integrity associated with fatal experimental cerebral malaria. However, the role of heme in PM remains unknown. Proliferation and apoptosis of trophoblasts and fusion of the mononucleated state to the syncytial state are of major importance to a successful pregnancy. In the present study, we examined the effects of heme on the viability and fusion of a trophoblast-derived cell line (BeWo). Results indicate that heme induces apoptosis in BeWo cells by activation of the STAT3/caspase-3/PARP signaling pathway. In the presence of forskolin, which triggers trophoblast fusion, heme inhibits BeWo cell fusion through activation of STAT3. Understanding the effects of free plasma heme in pregnant women either due to malaria, sickle cell disease or other hemolytic diseases, will enable identification of high-risk women and may lead to discovery of new drug targets against associated adverse pregnancy outcome.

Loss-of-function mutations in CAST cause peeling skin, leukonychia, acral punctate keratoses, cheilitis, and knuckle pads

Calpastatin is an endogenous specific inhibitor of calpain, a calcium-dependent cysteine protease. Here we show that loss-of-function mutations in calpastatin (CAST) are the genetic causes of an autosomal-recessive condition characterized by generalized peeling skin, leukonychia, acral punctate keratoses, cheilitis, and knuckle pads, which we propose to be given the acronym PLACK syndrome. In affected individuals with PLACK syndrome from three families of different ethnicities, we identified homozygous mutations (c.607dup, c.424A>T, and c.1750delG) in CAST, all of which were predicted to encode truncated proteins (p.Ile203Asnfs∗8, p.Lys142∗, and p.Val584Trpfs∗37). Immunohistochemistry shows that staining of calpastatin is reduced in skin from affected individuals. Transmission electron microscopy revealed widening of intercellular spaces with chromatin condensation and margination in the upper stratum spinosum in lesional skin, suggesting impaired intercellular adhesion as well as keratinocyte apoptosis. A significant increase of apoptotic keratinocytes was also observed in TUNEL assays. In vitro studies utilizing siRNA-mediated CAST knockdown revealed a role for calpastatin in keratinocyte adhesion. In summary, we describe PLACK syndrome, as a clinical entity of defective epidermal adhesion, caused by loss-of-function mutations in CAST.

Calpain dysregulation in Alzheimer's disease

Alzheimer's disease (AD) is characterized by the presence of senile plaques and neurofibrillary tangles in the neocortex and hippocampus of AD patients. In addition, a marked decrease in synaptic contacts has been detected in these affected brain areas. Due to its prevalence in the aging population, this disease has been the focus of numerous studies. The data obtained from those studies suggest that the mechanisms leading to the formation of the hallmark lesions of AD might be linked. One of such mechanisms seems to be the dysregulation of calcium homeostasis that results in the abnormal activation of calpains. Calpains are a family of Ca(2+)-dependent cysteine proteases that play a key role in multiple cell functions including cell development, differentiation and proliferation, axonal guidance, growth cone motility, and cell death, among others. In this paper, we briefly reviewed data on the structure of these proteases and their regulation under normal conditions. We also summarized data underscoring the participation of calpains in the neurodegenerative mechanisms associated with AD.

Calpastatin upregulation in Mycoplasma hyorhinis-infected cells is promoted by the mycoplasma lipoproteins via the NF-κB pathway

Mycoplasma hyorhinis frequently contaminates cultured cells, with effects on synthetic and metabolic pathways. We demonstrated for the first time that contamination of cells by a strain of M. hyorhinis (NDMh) results in increased levels of calpastatin (the endogenous inhibitor of the ubiquitous Ca(2+) -dependent protease calpain). We now show that the calpastatin upregulation by NDMh in neuroblastoma SH-SY5Y cells resides in the NDMh lipoprotein fraction (LPP), via the NF-κB transcription pathway. NF-κB activation requires dissociation of the cytoplasmic NF-κB/IκB complex followed by NF-κB translocation to the nucleus. NDMh-LPP induced translocation of the NF-κB RelA subunit to the nucleus and upregulated calpastatin. RelA translocation and calpastatin elevation were prevented when dissociation of the NF-κB/IκB complex was inhibited either by transfection with the non-phosphorylatable IκB mutant ΔNIκBα, or by using PS1145, an inhibitor of the IκB kinase (IKK complex). Increased calpastatin levels attenuate calpain-related amyloid-β-peptide and Ca(2+) -toxicity (these are central to the pathogenesis of Alzheimer's Disease). LPP-induced elevation of calpastatin provides an example of effects on non-inflammatory intracellular proteins, the outcome being significant alterations in host cell functions. Since calpastatin level is important in the control of calpain activity, mycoplasmal LPP may be of interest in treating some pathological processes involving excessive calpain activation.

Calpain mediates pulmonary vascular remodeling in rodent models of pulmonary hypertension, and its inhibition attenuates pathologic features of disease

Pulmonary hypertension is a severe and progressive disease, a key feature of which is pulmonary vascular remodeling. Several growth factors, including EGF, PDGF, and TGF-β1, are involved in pulmonary vascular remodeling during pulmonary hypertension. However, increased knowledge of the downstream signaling cascades is needed if effective clinical interventions are to be developed. In this context, calpain provides an interesting candidate therapeutic target, since it is activated by EGF and PDGF and has been reported to activate TGF-β1. Thus, in this study, we examined the role of calpain in pulmonary vascular remodeling in two rodent models of pulmonary hypertension. These data showed that attenuated calpain activity in calpain-knockout mice or rats treated with a calpain inhibitor resulted in prevention of increased right ventricular systolic pressure, right ventricular hypertrophy, as well as collagen deposition and thickening of pulmonary arterioles in models of hypoxia- and monocrotaline-induced pulmonary hypertension. Additionally, inhibition of calpain in vitro blocked intracellular activation of TGF-β1, which led to attenuated Smad2/3 phosphorylation and collagen synthesis. Finally, smooth muscle cells of pulmonary arterioles from patients with pulmonary arterial hypertension showed higher levels of calpain activation and intracellular active TGF-β. Our data provide evidence that calpain mediates EGF- and PDGF-induced collagen synthesis and proliferation of pulmonary artery smooth muscle cells via an intracrine TGF-β1 pathway in pulmonary hypertension.

Association between molecular markers for beef tenderness and growth traits in Argentinian angus cattle

Molecular markers for beef tenderness are classic examples of the contribution of genome technology to animal breeding through marker-assisted selection (MAS). Markers on the μ-calpain (CAPN1) and calpastatin (CAST) genes have been extensively evaluated for their association with tenderness. However, little is known about their potential effect on other economically important traits. In this work, the association of molecular markers for beef tenderness with growth traits was evaluated in Angus cattle of Argentina. Expected progeny differences were extracted from the 2008 Angus Sire Summary of Argentina. Information corresponding to 268 influential bulls that had been genotyped for two markers in CAPN1 and two markers in CAST was provided by the Argentine Angus Association. Genotype probabilities were assigned, by segregation analysis, to those bulls in the Sire Summary that had no marker information. Expected progeny differences of 1365 sires were regressed on the number of alleles favouring tenderness at each locus. There was a significant effect of markers on expected progeny differences of birth weight, weaning weight (direct), weight at 18 months and rib eye area. In general, there was a negative effect of alleles favouring tenderness on growth traits. These correlated responses should be taken into account when molecular markers are used in selection schemes that aim to improve beef tenderness.

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.

Molecular determinants of survival motor neuron (SMN) protein cleavage by the calcium-activated protease, calpain

Spinal muscular atrophy (SMA) is a leading genetic cause of childhood mortality, caused by reduced levels of survival motor neuron (SMN) protein. SMN functions as part of a large complex in the biogenesis of small nuclear ribonucleoproteins (snRNPs). It is not clear if defects in snRNP biogenesis cause SMA or if loss of some tissue-specific function causes disease. We recently demonstrated that the SMN complex localizes to the Z-discs of skeletal and cardiac muscle sarcomeres, and that SMN is a proteolytic target of calpain. Calpains are implicated in muscle and neurodegenerative disorders, although their relationship to SMA is unclear. Using mass spectrometry, we identified two adjacent calpain cleavage sites in SMN, S192 and F193. Deletion of small motifs in the region surrounding these sites inhibited cleavage. Patient-derived SMA mutations within SMN reduced calpain cleavage. SMN(D44V), reported to impair Gemin2 binding and amino-terminal SMN association, drastically inhibited cleavage, suggesting a role for these interactions in regulating calpain cleavage. Deletion of A188, a residue mutated in SMA type I (A188S), abrogated calpain cleavage, highlighting the importance of this region. Conversely, SMA mutations that interfere with self-oligomerization of SMN, Y272C and SMNΔ7, had no effect on cleavage. Removal of the recently-identified SMN degron (Δ268-294) resulted in increased calpain sensitivity, suggesting that the C-terminus of SMN is important in dictating availability of the cleavage site. Investigation into the spatial determinants of SMN cleavage revealed that endogenous calpains can cleave cytosolic, but not nuclear, SMN. Collectively, the results provide insight into a novel aspect of the post-translation regulation of SMN.

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.

Caspases rather than calpains mediate remodelling of the fodrin skeleton during human placental trophoblast fusion

Fusion of cytotrophoblasts with the overlying syncytiotrophoblast is an integral step in differentiation of the human placental villous trophoblast. Multiple factors, such as growth factors, hormones, cytokines, protein kinases, transcription factors and structural membrane proteins, were described to modulate trophoblast fusion. However, the knowledge on remodelling of the membrane-associated cytoskeleton during trophoblast fusion is very limited. This study describes the link between remodelling of spectrin-like alpha-fodrin and intercellular trophoblast fusion. Experiments with primary trophoblasts isolated from term placentas and the choriocarcinoma cell line BeWo revealed a biphasic strategy of the cells to achieve reorganization of alpha-fodrin. Syncytialization of trophoblasts was accompanied by down-regulation of alpha-fodrin mRNA, whereas the full-length alpha-fodrin protein was cleaved into 120 and 150 kDa fragments. Application of calpeptin and calpain inhibitor III did not affect alpha-fodrin fragmentation in primary term trophoblasts and forskolin-treated BeWo cells, but decreased secretion of beta human chorionic gonadotropin. In contrast, inhibitors of caspases 3, 8 and 9 attenuated generation of the 120 kDa fragment and a general caspase inhibitor completely blocked fragmentation, suggesting an exclusive function of caspases in alpha-fodrin remodelling. Immunofluorescence double staining of human placenta revealed co-localization of active caspase 8 with alpha-fodrin positive vesicles in fusing villous cytotrophoblasts. These results suggest that caspase-dependent fragmentation of alpha-fodrin may be important for reorganization of the sub-membranous cytoskeleton during trophoblast fusion.

Calpain expression and activity during lens fiber cell differentiation

In animal models, the dysregulated activity of calcium-activated proteases, calpains, contributes directly to cataract formation. However, the physiological role of calpains in the healthy lens is not well defined. In this study, we examined the expression pattern of calpains in the mouse lens. Real time PCR and Western blotting data indicated that calpain 1, 2, 3, and 7 were expressed in lens fiber cells. Using controlled lysis, depth-dependent expression profiles for each calpain were obtained. These indicated that, unlike calpain 1, 2, and 7, which were most abundant in cells near the lens surface, calpain 3 expression was strongest in the deep cortical region of the lens. We detected calpain activities in vitro and showed that calpains were active in vivo by microinjecting fluorogenic calpain substrates into cortical fiber cells. To identify endogenous calpain substrates, membrane/cytoskeleton preparations were treated with recombinant calpain, and cleaved products were identified by two-dimensional difference electrophoresis/mass spectrometry. Among the calpain substrates identified by this approach was alphaII-spectrin. An antibody that specifically recognized calpain-cleaved spectrin was used to demonstrate that spectrin is cleaved in vivo, late in fiber cell differentiation, at or about the time that lens organelles are degraded. The generation of the calpain-specific spectrin cleavage product was not observed in lens tissue from calpain 3-null mice, indicating that calpain 3 is uniquely activated during lens fiber differentiation. Our data suggest a role for calpains in the remodeling of the membrane cytoskeleton that occurs with fiber cell maturation.

Calpastatin overexpression attenuates amyloid-beta-peptide toxicity in differentiated PC12 cells

Amyloid beta peptide (Abeta) plays a major role in the pathogenesis of Alzheimer's disease (AD). Abeta is toxic to neurons, possibly through causing initial synaptic dysfunction and neuronal membrane dystrophy, promoted by increased cellular Ca(2+). Calpain (Ca(2+)-dependent protease) and caspase have been implicated in AD. Previously, we used calpain and caspase pharmacological inhibitors to study effects of Abeta25-35 (sAbeta) on neuronal-like differentiated PC12 cells. We reported that sAbeta-treated cells exhibited calpain activation and protein degradation (due to both calpain and caspase-8). We have now found that overexpression of the calpain specific inhibitor calpastatin in differentiated PC12 cells significantly inhibited the sAbeta-induced calpain activation and decreased the protease activity. Calpastatin overexpression inhibited the sAbeta-promoted degradation of fodrin, protein kinase Cepsilon, beta-catenin (membrane structural proteins and proteins involved in signal transduction pathways), and prevented the sAbeta-induced alteration of neurite structure (manifested by varicosities). Overexpression of calpastatin also inhibited Ca(2+)-promoted calpain activation and protein degradation; this is consistent with the notion that the Abeta-induced increase in calpain activity results from a rise in cellular Ca(2+), provided the calpastatin level is not so high as to strongly inhibit calpain. Carrying out transfection without selection allowed the comparison in the same culture of calpastatin-overexpressing with non-overexpressing cells. In cultures transfected with green fluorescent protein (GFP)-calpastatin plasmid, calpastatin overexpression (indicated by GFP-labeling) led to inhibition in sAbeta-induced membrane propidium iodide (PI) permeability, whereas non-transfected, GFP-unlabeled cells exhibited PI permeability. Overall, the results demonstrate that the effects of Abeta-toxicity studied here were attenuated to a large extent by calpastatin overexpression, indicating that the protease calpain is involved in Abeta-toxicity (obviating a primary, direct role for caspases). Increased expression of calpastatin and/or decrease in calpain may serve as one of the means for ameliorating some of the early symptoms of AD.

Specific knockdown of m-calpain blocks myogenesis with cDNA deduced from the corresponding RNAi

Fusion of mononuclear myoblast to multinucleated myotubes is crucial for myogenesis. Both mu- and m-calpain are ubiquitously expressed in most cells and are particularly abundant in muscle cells. Knockout of calpain-1 (catalytic subunit of mu-calpain) induced moderate platelet dysaggregation, preserving the normal development and growth, although knockout of calpain-2 (m-calpain) is lethal in mice. Therefore, there should be muscle-specific function of m-calpain per se. Previous methods lack direct evidence for the involvement of m-calpain, because the specific inhibitor to m-calpain has not been developed yet and the inhibition was less potent. Here, we show that screened RNA interference (RNAi) specifically blocked the m-calpain expression by 95% at both the protein and the activity levels. After transfection of adenovirus vector-mediated cDNA corresponding to the RNAi-induced short hairpin RNA, m-calpain in C(2)C(12) myoblasts was knocked down with no compensatory overexpression of mu-calpain or calpain-3. The specific knockdown strongly inhibited the fusion to multinucleated myotubes. In addition, the knockdown modestly blocked ubiquitous effects, including cell migration, cell spreading, and alignment of central stress fiberlike structures. These results may indicate that m-calpain requiring millimolar Ca(2+) level for the full activation plays specific roles in myogenesis, independent of mu-calpain, and leave us challenging problems in the future.

Molecular control of mammalian myoblast fusion

The fusion of postmitotic mononucleated myoblasts to form syncytial myofibers is a critical step in the formation of skeletal muscle. Myoblast fusion occurs both during development and throughout adulthood, as skeletal muscle growth and regeneration require the accumulation of additional nuclei within myofibers. Myoblasts must undergo a complex series of molecular and morphological changes prior to fusing with one another. Although many molecules regulating myoblast fusion have been identified, the precise mechanism by which these molecules act in concert to control fusion remains to be elucidated. A comprehensive understanding of how myo-blast fusion is controlled may contribute to the treatment of various disorders associated with loss of muscle mass. In this chapter, we examine progress made toward elucidating the cellular and molecular pathways involved in mammalian myoblast fusion. Special emphasis is placed on the molecules that regulate myofiber formation without discernibly affecting biochemical differentiation.

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.

Calpastatin levels affect calpain activation and calpain proteolytic activity in APP transgenic mouse model of Alzheimer's disease

The intracellular Ca(2+)-dependent protease calpain and the specific calpain endogenous inhibitor calpastatin are widely distributed, with the calpastatin/calpain ratio varying among tissues and species. Increased Ca(2+) and calpain activation have been implicated in Alzheimer's disease (AD), with scant data available on calpastatin/calpain ratio in AD. Information is lacking on calpain activation and calpastatin levels in transgenic mice that exhibit AD-like pathology. We studied calpain and calpastatin in Tg2576 mice and in their wild type littermates (control mice). We found that in control mice calpastatin level varies among brain regions; it is significantly higher in the cerebellum than in the hippocampus, frontal and temporal cortex, whereas calpain levels are similar in all these regions. In the Tg2576 mice, calpain is activated, calpastatin is diminished, and calpain-dependent proteolysis is observed in brain regions affected in AD and in transgenic mice (especially hippocampus). In contrast, no differences are observed between the Tg2576 and the control mice in the cerebellum, which does not exhibit AD-like pathology. The results are consistent with the notion that a high level of calpastatin in the cerebellum renders the calpain in this brain region less liable to be activated; in the other brain parts, in which calpastatin is low, calpain is more easily activated in the presence of increased Ca(2+), and in turn the activated calpain leads to further diminution in calpastatin (a known calpain substrate). The results indicate that calpastatin is an important factor in the regulation of calpain-induced protein degradation in the brains of the affected mice, and imply a role for calpastatin in attenuating AD pathology. Promoting calpastatin expression may be used to ameliorate some manifestations of AD.

Calcium-dependent proteolytic system and muscle dysfunctions: a possible role of calpains in sarcopenia

The calcium-dependent proteolytic system is composed of cysteine proteases named calpains. They are ubiquitous or tissue-specific enzymes. The two best characterised isoforms are the ubiquitously expressed mu- and m-calpains. Besides its regulation by calcium, calpain activity is tightly controlled by calpastatin, the specific endogenous inhibitor, binding to phospholipids, autoproteolysis and phosphorylation. Calpains are responsible for limited proteolytic events. Among the multitude of substrates identified so far are cytoskeletal and membrane proteins, enzymes and transcription factors. Calpain activity is involved in a large number of physiological and pathological processes. In this review, we will particularly focus on the implication of the calcium-dependent proteolytic system in relation to muscle physiology. Because of their ability to remodel cytoskeletal anchorage complexes, calpains play a major role in the regulation of cell adhesion, migration and fusion, three key steps of myogenesis. Calcium-dependent proteolysis is also involved in the control of cell cycle. In muscle tissue, in particular, calpains intervene in the regeneration process. Another important class of calpain substrates belongs to apoptosis regulating factors. The proteases may thus play a role in muscle cell death, and as a consequence in muscle atrophy. The relationships between calcium-dependent proteolysis and muscle dysfunctions are being further developed in this review with a particular emphasis on sarcopenia.

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.

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.