Comprehensive survey of p94/calpain 3 substrates by comparative proteomics--possible regulation of protein synthesis by p94

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

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

Calpains for dummies: What you need to know about the calpain family

This review was written in memory of our late friend, Dr. Hiroyuki Sorimachi, who, following the steps of his mentor Koichi Suzuki, a pioneer in calpain research, has made tremendous contributions to the field. During his career, Hiro also wrote several reviews on calpain, the last of which, published in 2016, was comprehensive. In this manuscript, we decided to put together a review with the basic information a novice may need to know about calpains. We also tried to avoid similarities with previous reviews and reported the most significant new findings, at the same time highlighting Hiro's contributions to the field. The review will cover a short history of calpain discovery, the presentation of the family, the life of calpain from transcription to activity, human diseases caused by calpain mutations and therapeutic perspectives.

Molecular landscape of CAPN3 mutations in limb-girdle muscular dystrophy type R1: from a Chinese multicentre analysis to a worldwide perspective

BACKGROUND

Limb-girdle muscular dystrophy type R1 (LGMDR1) can be caused by recessive CAPN3 mutations accounting for the majority of LGMD. To date, no systemic evaluation has been performed to analyse the detrimental and normal mutations on CAPN3 and its hotspots.

METHODS

CAPN3 variants (n=112) from a total of 124 patients with LGMDR1 recruited in four centres in China were retrospectively analysed. Then external CAPN3 variants (n=2031) from online databases were integrated with our Chinese cohort data to achieve a worldwide perspective on CAPN3 mutations. According to their related phenotypes (LGMDR1 or normal), we analysed consequence, distribution, ethnicity and severity scores of CAPN3 mutations.

RESULTS

Two hotspot mutations were identified including c.2120A>G in Chinese population and c.550del in Europe. According to the integrated dataset, 521 mutations were classified as LGMDR1-related and converged on exons 1, 10, 5, 22 and 13 of CAPN3. The remaining 1585 variants were classified as normal-population related. The deleterious ratio of LGMDR1-relevant variants to total variants in each population was 0.26 on average with a maximum of 0.35 in Finns and a minimum of 0.21 in South Asians. Severity evaluation showed that Chinese LGMDR1-related variants exhibited a higher risk (Combined Annotation Dependent Depletion score +1.10) than that from database patients (p<0.001).

CONCLUSIONS

This study confirmed two hotspots and LGMDR1-related CAPN3 variants, highlighting the advantages in using a data-based comprehensive analysis to achieve a genetic landscape for patients with LGMDR1.

The scaffolding protein calpain-3 has multiple distributions in embryonic chick muscle cells and it is essential for the formation of muscle fibers

CAPN3 is a muscle-specific and an intrinsically disordered protein. Thus, as a scaffolding protein CAPN3 could play a role during early stages of myogenesis. To test this hypothesis, we studied the distribution and function of CAPN3 during myogenesis using embryonic chick muscle cells grown in vitro. Super-resolution microscopy showed CAPN3 distribution in (i) amorphous patches in myoblasts, (ii) a region near the nuclei of myotubes; (iii) adhesion plaques in myotubes, (iv) stress fiber-like structures in myotubes, and (v) filaments in fibroblasts. Downregulation of CAPN3 induced a decrease in the number of muscle cells and in the size of myotubes formed. These data show a diverse intracellular distribution of CAPN3, compatible with a scaffolding protein, and suggest a multitude of different interactions of CAPN3 with other partners during muscle formation.

A muscle-specific calpain, CAPN3, forms a homotrimer

Calpain-3 (CAPN3), a 94-kDa member of the calpain protease family, is abundant in skeletal muscle. Mutations in the CAPN3 gene cause limb girdle muscular dystrophy type 2A, indicating that CAPN3 plays important roles in muscle physiology. CAPN3 has several unique features. A crystallographic study revealed that its C-terminal penta-EF-hand domains form a homodimer, suggesting that CAPN3 functions as a homodimeric protease. To analyze complex formation of CAPN3 in a more convenient manner, we performed blue native polyacrylamide gel electrophoresis and found that the observed molecular weight of native CAPN3, as well as recombinant CAPN3, was larger than 240 kDa. Further analysis by cross-linking and sequential immunoprecipitation revealed that CAPN3 in fact forms a homotrimer. Trimer formation was abolished by the deletion of the PEF domain, but not the CAPN3-specific insertion sequences NS, IS1, and IS2. The PEF domain alone formed a homodimer, as reported, but addition of the adjacent CBSW domain to its N-terminus reinforced the trimer-forming property. Collectively, these results suggest that CAPN3 forms a homotrimer in which the PEF domain's dimer-forming ability is influenced by other domains.

Insertion sequence 1 from calpain-3 is functional in calpain-2 as an internal propeptide

Calpains are intracellular, calcium-activated cysteine proteases. Calpain-3 is abundant in skeletal muscle, where its mutation-induced loss of function causes limb-girdle muscular dystrophy type 2A. Unlike the small subunit-containing calpain-1 and -2, the calpain-3 isoform homodimerizes through pairing of its C-terminal penta-EF-hand domain. It also has two unique insertion sequences (ISs) not found in the other calpains: IS1 within calpain-3's protease core and IS2 just prior to the penta-EF-hand domain. Production of either native or recombinant full-length calpain-3 to characterize the function of these ISs is challenging. Therefore, here we used recombinant rat calpain-2 as a stable surrogate and inserted IS1 into its equivalent position in the protease core. As it does in calpain-3, IS1 occupied the catalytic cleft and restricted the enzyme's access to substrate and inhibitors. Following activation by Ca²⁺, IS1 was rapidly cleaved by intramolecular autolysis, permitting the enzyme to freely accept substrate and inhibitors. The surrogate remained functional until extensive intermolecular autoproteolysis inactivated the enzyme, as is typical of calpain-2. Although the small-molecule inhibitors E-64 and leupeptin limited intermolecular autolysis of the surrogate, they did not block the initial intramolecular cleavage of IS1, establishing its role as a propeptide. Surprisingly, the large-molecule calpain inhibitor, calpastatin, completely blocked enzyme activity, even with IS1 intact. We suggest that calpastatin is large enough to oust IS1 from the catalytic cleft and take its place. We propose an explanation for why calpastatin can inhibit calpain-2 bearing the IS1 insertion but cannot inhibit WT calpain-3.

Predictions of Cleavability of Calpain Proteolysis by Quantitative Structure-Activity Relationship Analysis Using Newly Determined Cleavage Sites and Catalytic Efficiencies of an Oligopeptide Array

Calpains are intracellular Ca²⁺-regulated cysteine proteases that are essential for various cellular functions. Mammalian conventional calpains (calpain-1 and calpain-2) modulate the structure and function of their substrates by limited proteolysis. Thus, it is critically important to determine the site(s) in proteins at which calpains cleave. However, the calpains' substrate specificity remains unclear, because the amino acid (aa) sequences around their cleavage sites are very diverse. To clarify calpains' substrate specificities, 84 20-mer oligopeptides, corresponding to P10-P10′ of reported cleavage site sequences, were proteolyzed by calpains, and the catalytic efficiencies (k_cat/K_m) were globally determined by LC/MS. This analysis revealed 483 cleavage site sequences, including 360 novel ones. The k_cat/K_ms for 119 sites ranged from 12.5–1,710 M⁻¹s⁻¹. Although most sites were cleaved by both calpain-1 and −2 with a similar k_cat/K_m, sequence comparisons revealed distinct aa preferences at P9-P7/P2/P5′. The aa compositions of the novel sites were not statistically different from those of previously reported sites as a whole, suggesting calpains have a strict implicit rule for sequence specificity, and that the limited proteolysis of intact substrates is because of substrates' higher-order structures. Cleavage position frequencies indicated that longer sequences N-terminal to the cleavage site (P-sites) were preferred for proteolysis over C-terminal (P′-sites). Quantitative structure-activity relationship (QSAR) analyses using partial least-squares regression and >1,300 aa descriptors achieved k_cat/K_m prediction with r = 0.834, and binary-QSAR modeling attained an 87.5% positive prediction value for 132 reported calpain cleavage sites independent of our model construction. These results outperformed previous calpain cleavage predictors, and revealed the importance of the P2, P3′, and P4′ sites, and P1-P2 cooperativity. Furthermore, using our binary-QSAR model, novel cleavage sites in myoglobin were identified, verifying our predictor. This study increases our understanding of calpain substrate specificities, and opens calpains to “next-generation,” i.e. activity-related quantitative and cooperativity-dependent analyses.

Calpain-3 impairs cell proliferation and stimulates oxidative stress-mediated cell death in melanoma cells

Calpain-3 is an intracellular cysteine protease, belonging to Calpain superfamily and predominantly expressed in skeletal muscle. In human melanoma cell lines and biopsies, we previously identified two novel splicing variants (hMp78 and hMp84) of Calpain-3 gene (CAPN3), which have a significant lower expression in vertical growth phase melanomas and, even lower, in metastases, compared to benign nevi. In the present study, in order to investigate the pathophysiological role played by the longer Calpain-3 variant, hMp84, in melanoma cells, we over-expressed it in A375 and HT-144 cells. In A375 cells, the enforced expression of hMp84 induces p53 stabilization, and modulates the expression of a few p53- and oxidative stress-related genes. Consistently, hMp84 increases the intracellular production of ROS (Reactive Oxygen Species), which lead to oxidative modification of phospholipids (formation of F₂-isoprostanes) and DNA damage. Such events culminate in an adverse cell fate, as indicated by the decrease of cell proliferation and by cell death. To a different extent, either the antioxidant N-acetyl-cysteine or the p53 inhibitor, Pifithrin-α, recover cell viability and decrease ROS formation. Similarly to A375 cells, hMp84 over-expression causes inhibition of cell proliferation, cell death, and increase of both ROS levels and F₂-isoprostanes also in HT-144 cells. However, in these cells no p53 accumulation occurs. In both cell lines, no significant change of cell proliferation and cell damage is observed in cells over-expressing the mutant hMp84^C42S devoid of its enzymatic activity, suggesting that the catalytic activity of hMp84 is required for its detrimental effects. Since a more aggressive phenotype is expected to benefit from down-regulation of mechanisms impairing cell growth and survival, we envisage that Calpain-3 down-regulation can be regarded as a novel mechanism contributing to melanoma progression.

The N- and C-terminal autolytic fragments of CAPN3/p94/calpain-3 restore proteolytic activity by intermolecular complementation

CAPN3/p94/calpain-3, a calpain protease family member predominantly expressed in skeletal muscle, possesses unusually rapid and exhaustive autolytic activity. Mutations in the human CAPN3 gene impairing its protease functions cause limb-girdle muscular dystrophy type 2A (LGMD2A); yet, the connection between CAPN3's autolytic activity and the enzyme's function in vivo remain unclear. Here, we demonstrated that CAPN3 protease activity was reconstituted by intermolecular complementation (iMOC) between its two autolytic fragments. Furthermore, the activity of full-length CAPN3 active-site mutants was surprisingly rescued through iMOC with autolytic fragments containing WT amino acid sequences. These results provide evidence that WT CAPN3 can be formed by the iMOC of two different complementary CAPN3 mutants. The finding of iMOC-mediated restoration of calpain activity indicates a novel mechanism for the genotype-phenotype links in LGMD2A.

Non-proteolytic functions of calpain-3 in sarcoplasmic reticulum in skeletal muscles

Mutations in CAPN3/Capn3, which codes for skeletal muscle-specific calpain-3/p94 protease, are responsible for limb-girdle muscular dystrophy type 2A. Using "knock-in" (referred to as Capn3(CS/CS)) mice, in which the endogenous calpain-3 is replaced with a mutant calpain-3:C129S, which is a proteolytically inactive but structurally intact calpain-3, we demonstrated in our previous studies that loss of calpain-3 protease activity causes muscular dystrophy [Ojima, K. et al. (2010) J. Clin. Invest. 120, 2672-2683]. However, compared to Capn3-null (Capn3(-/-)) mice, Capn3(CS/CS) mice showed less severe dystrophic symptoms. This suggests that calpain-3 also has a non-proteolytic function. This study aimed to elucidate the non-proteolytic functions of calpain-3 through comparison of Capn3(CS/CS) mice with Capn3(-/-) mice. We found that calpain-3 is a component of the sarcoplasmic reticulum (SR), and that calpain-3 interacts with, but does not proteolyze, typical SR components such as ryanodine receptor and calsequestrin. Furthermore, Capn3(CS/CS) mice showed that the nonenzymatic role of calpain-3 is required for proper Ca(2+) efflux from the SR to cytosol during muscle contraction. These results indicate that calpain-3 functions as a nonenzymatic element for the Ca(2+) efflux machinery in the SR, rather than as a protease. Thus, defects in the nonenzymatic function of calpain-3 must also be involved in the pathogenesis of limb-girdle muscular dystrophy type 2A.

Calpain chronicle--an enzyme family under multidisciplinary characterization

Calpain is an intracellular Ca2+-dependent cysteine protease (EC 3.4.22.17; Clan CA, family C02) discovered in 1964. It was also called CANP (Ca2+-activated neutral protease) as well as CASF, CDP, KAF, etc. until 1990. Calpains are found in almost all eukaryotes and a few bacteria, but not in archaebacteria. Calpains have a limited proteolytic activity, and function to transform or modulate their substrates' structures and activities; they are therefore called, "modulator proteases." In the human genome, 15 genes--CAPN1, CAPN2, etc.--encode a calpain-like protease domain. Their products are calpain homologs with divergent structures and various combinations of functional domains, including Ca2+-binding and microtubule-interaction domains. Genetic studies have linked calpain deficiencies to a variety of defects in many different organisms, including lethality, muscular dystrophies, gastropathy, and diabetes. This review of the study of calpains focuses especially on recent findings about their structure-function relationships. These discoveries have been greatly aided by the development of 3D structural studies and genetic models.

Protein expression changes induced in murine peritoneal macrophages by Group B Streptococcus

Protein expression changes induced in thioglycolate-elicited peritoneal murine macrophages (M Phi) by infection with type III Group B Streptococcus (GBS) are described. Proteins from control M Phi and M Phi incubated 2 h with live or heat-inactivated GBS were separated by 2-DE. Proteins whose expression was significantly different in infected M Phi, as compared with control cells, were identified by MS/MS analysis. Changes in the expression level of proteins involved in both positive and negative modulation of phagocytic functions, stress response and cell death were induced in M Phi by GBS infection. In particular, expression of enzymes playing a key role in production of reactive oxygen species was lowered in GBS-infected M Phi. Significant alterations in the expression of some metabolic enzymes were also observed, most of the glycolytic and of the pentose-cycle enzymes being down-regulated in M Phi infected with live GBS. Finally, evidence was obtained that GBS infection affects the expression of enzymes or enzyme subunits involved in ATP synthesis and in adenine nucleotides interconversion processes.

Dynamic distribution of muscle-specific calpain in mice has a key role in physical-stress adaptation and is impaired in muscular dystrophy

Limb-girdle muscular dystrophy type 2A (LGMD2A) is a genetic disease that is caused by mutations in the calpain 3 gene (CAPN3), which encodes the skeletal muscle-specific calpain, calpain 3 (also known as p94). However, the precise mechanism by which p94 functions in the pathogenesis of this disease remains unclear. Here, using p94 knockin mice (termed herein p94KI mice) in which endogenous p94 was replaced with a proteolytically inactive but structurally intact p94:C129S mutant protein, we have demonstrated that stretch-dependent p94 distribution in sarcomeres plays a crucial role in the pathogenesis of LGMD2A. The p94KI mice developed a progressive muscular dystrophy, which was exacerbated by exercise. The exercise-induced muscle degeneration in p94KI mice was associated with an inefficient redistribution of p94:C129S in stretched sarcomeres. Furthermore, the p94KI mice showed impaired adaptation to physical stress, which was accompanied by compromised upregulation of muscle ankyrin-repeat protein-2 and hsp upon exercise. These findings indicate that the stretch-induced dynamic redistribution of p94 is dependent on its protease activity and essential to protect muscle from degeneration, particularly under conditions of physical stress. Furthermore, our data provide direct evidence that loss of p94 protease activity can result in LGMD2A and molecular insight into how this could occur.

Skeletal muscle-specific calpain is an intracellular Na+-dependent protease

Because intracellular [Na⁺] is kept low by Na⁺/K⁺-ATPase, Na⁺ dependence is generally considered a property of extracellular enzymes. However, we found that p94/calpain 3, a skeletal-muscle-specific member of the Ca²⁺-activated intracellular “modulator proteases” that is responsible for a limb-girdle muscular dystrophy (“calpainopathy”), underwent Na⁺-dependent, but not Cs⁺-dependent, autolysis in the absence of Ca²⁺. Furthermore, Na⁺ and Ca²⁺ complementarily activated autolysis of p94 at physiological concentrations. By blocking Na⁺/K⁺-ATPase, we confirmed intracellular autolysis of p94 in cultured cells. This was further confirmed using inactive p94:C129S knock-in (p94CS-KI) mice as negative controls. Mutagenesis studies showed that much of the p94 molecule contributed to its Na⁺/Ca²⁺-dependent autolysis, which is consistent with the scattered location of calpainopathy-associated mutations, and that a conserved Ca²⁺-binding sequence in the protease acted as a Na⁺ sensor. Proteomic analyses using Cs⁺/Mg²⁺ and p94CS-KI mice as negative controls revealed that Na⁺ and Ca²⁺ direct p94 to proteolyze different substrates. We propose three roles for Na⁺ dependence of p94; 1) to increase sensitivity of p94 to changes in physiological [Ca²⁺], 2) to regulate substrate specificity of p94, and 3) to regulate contribution of p94 as a structural component in muscle cells. Finally, this is the first example of an intracellular Na⁺-dependent enzyme.

Calpain3 is expressed in a proteolitically active form in papillomavirus-associated urothelial tumors of the urinary bladder in cattle

Background

Calpain 3 (Capn3), also named p94, is a skeletal muscle tissue-specific protein known to be responsible for limb-girdle muscular dystrophy type 2A (LGMD2A). Recent experimental studies have hypothesized a pro-apoptotic role of Capn3 in some melanoma cell lines. So far the link between calpain3 and tumors comes from in vitro studies. The objective of this study was to describe Capn3 activation in naturally occurring urothelial tumors of the urinary bladder in cattle.

Methods and Findings

Here we describe, for the first time in veterinary and comparative oncology, the activation of Capn3 in twelve urothelial tumor cells of the urinary bladder of cattle. Capn3 protein was initially identified with nanoscale liquid chromatography coupled with tandem mass spectrometry (nano LC-MS/MS) in a co-immunoprecipitation experiment on E2F3, known to be a transcription factor playing a crucial role in bladder carcinogenesis in humans. Capn3 expression was then confirmed by reverse transcription polymerase chain reaction (RT-PCR). Finally, the Ca²⁺-dependent proteolytic activity of Capn3 was assayed following ion exchange chromatography. Morphologically, Capn3 expression was documented by immunohistochemical methods. In fact numerous tumor cells showed an intracytoplasmic immunoreactivity, which was more rarely evident also at nuclear level. In urothelial tumors, bovine papillomavirus type 2 (BPV-2) DNA was amplified by PCR and the expression of E5 protein, the major oncogenic protein of BVP-2, was detected by western blotting, immunohistochemistry, and immunofluorescence. E2F3 overexpression and pRb protein downregulation were shown by western blotting.

Conclusion

The role of capn3 protein in urothelial cancer of the urinary bladder remains to be elucidated: further studies would be required to determine the precise function of this protease in tumor development and progression. However, we suggest that activated Capn3 may be involved in molecular pathways leading to the overexpression of E2F3, which in turn could be responsible for urothelial tumor cell proliferation also in cattle, though other mechanisms are likely to exist. If further studies corroborate the important role of Capn3 in urothelial tumors of the urinary bladder, cattle with urinary tumors may prove useful as animal model for bladder carcinogenesis.

A proteomic study of calpain-3 and its involvement in limb girdle muscular dystrophy type 2a

Limb-girdle muscular dystrophy type 2A is an autosomal recessive disorder generated by inactivating mutations in the gene coding for the muscle specific protease calpain-3. It is mainly expressed in skeletal muscle as a monomeric multidomain protein characterized by three unique insertion sequences (NS, IS1, IS2). It is unstable, and undergoes very rapid autolysis in solution, therefore, its heterologous expression and purification have been difficult. So far, calpain-3 substrates have been only identified in vitro and with indirect approaches. We have therefore decided to perform a comprehensive study of the substrates of the protease by comparing the 2D electrophoretic profile of myotubes from obtained from calpain-3 knockout and wild type mice. Digestion of differentially expressed spots was followed by mass spectrometry analysis. We could identify 16 proteins which differed in knockout and wild type mice. Among them: desmin, nestin, spectrin and PDLIM1 were of particular interest. In vitro experiments have then revealed that only PDLIM1 is cleaved directly by the protease, and that a fragment of about 8 kDa is released from the C-terminal portion of the protein.

Novel variants of muscle calpain 3 identified in human melanoma cells: cisplatin-induced changes in vitro and differential expression in melanocytic lesions

Calpains are cysteine proteases comprising members ubiquitously expressed in human tissues and other tissue-specific isoforms. Alterations of calpain 3 (p94), the muscle-specific isoform that contains three peculiar sequences (NS, IS1 and IS2), are strictly associated to the limb-girdle muscular dystrophy type 2A, in which a myonuclear apoptosis has been documented. Our recent demonstration of a proapoptotic role of ubiquitous calpains in drug-induced apoptosis of melanoma cells prompted us to investigate the expression of calpain 3 in human melanoma cell lines undergoing apoptosis and in melanocytic lesions. In melanoma cell lines, we have identified two novel splicing variants of calpain 3 (hMp78 and hMp84): they have an atypical initiation exon and a putative nuclear localization signal, the shorter one lacks IS1 inset and both proteins are extremely unstable. Virtually, both isoforms (prevalently as cleavage forms) are localized in cytoplasm and in nucleoli. In cisplatin-treated preapoptotic cells, an increase of both transcription and autoproteolytic cleavage of the novel variants is observed; the latter event is prevented by the inhibitor of ubiquitous calpains, calpeptin, which is also able to protect from apoptosis. Interestingly, among melanocytic lesions, the expression of these novel variants is significantly downregulated, compared with benign nevi, in the most aggressive ones, i.e. in vertical growth phase melanoma and, even more, in metastatic melanoma cells, characterized by invasiveness properties and usually highly resistant to apoptosis. On the whole, our observations suggest that calpain 3 variants can play a proapoptotic role in melanoma cells and its downregulation, as observed in highly aggressive lesions, could contribute to melanoma progression.

Gene expression profiling in limb-girdle muscular dystrophy 2A

Limb-girdle muscular dystrophy type 2A (LGMD2A) is a recessive genetic disorder caused by mutations in calpain 3 (CAPN3). Calpain 3 plays different roles in muscular cells, but little is known about its functions or in vivo substrates. The aim of this study was to identify the genes showing an altered expression in LGMD2A patients and the possible pathways they are implicated in. Ten muscle samples from LGMD2A patients with in which molecular diagnosis was ascertained were investigated using array technology to analyze gene expression profiling as compared to ten normal muscle samples. Upregulated genes were mostly those related to extracellular matrix (different collagens), cell adhesion (fibronectin), muscle development (myosins and melusin) and signal transduction. It is therefore suggested that different proteins located or participating in the costameric region are implicated in processes regulated by calpain 3 during skeletal muscle development. Genes participating in the ubiquitin proteasome degradation pathway were found to be deregulated in LGMD2A patients, suggesting that regulation of this pathway may be under the control of calpain 3 activity. As frizzled-related protein (FRZB) is upregulated in LGMD2A muscle samples, it could be hypothesized that beta-catenin regulation is also altered at the Wnt signaling pathway, leading to an incorrect myogenesis. Conversely, expression of most transcription factor genes was downregulated (MYC, FOS and EGR1). Finally, the upregulation of IL-32 and immunoglobulin genes may induce the eosinophil chemoattraction explaining the inflammatory findings observed in presymptomatic stages. The obtained results try to shed some light on identification of novel therapeutic targets for limb-girdle muscular dystrophies.

Calpain 3, the "gatekeeper" of proper sarcomere assembly, turnover and maintenance

Calpain 3 is a member of the calpain family of calcium-dependent intracellular proteases. Thirteen years ago it was discovered that mutations in calpain 3 (CAPN3) result in an autosomal recessive and progressive form of limb girdle muscular dystrophy called limb girdle muscular dystrophy type 2A. While calpain 3 mRNA is expressed at high levels in muscle and appears to have some role in developmental processes, muscles of patients and mice lacking calpain 3 still form apparently normal muscle during prenatal development; thus, a functional calpain 3 protease is not mandatory for muscle to form in vivo but it is a pre-requisite for muscle to remain healthy. Despite intensive research in this field, the physiological substrates of the calpain 3 protein (hereafter referred to as CAPN3) and its alternatively spliced isoforms remain elusive. The existence of these multiple isoforms complicates the search for the physiological functions of CAPN3 and its pathophysiological role. In this review, we summarize the genetic and biochemical evidence that point to loss of function of the full-length isoform of CAPN3, also known as p94, as the pathogenic isoform. We also argue that its natural substrates must reside in its proximity within the sarcomere where it is stored in an inactive state anchored to titin. We further propose that CAPN3 has many attributes that make it ideally suited as a sensor of sarcomeric integrity and function, involved in its repair and maintenance. Loss of these CAPN3-mediated activities can explain the "progressive" development of muscular dystrophy.