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

A retrospective study on the clinical and molecular outcomes of calpainopathy in a Turkish patient cohort

Background and aim

Calpainopathy, also known as limb-girdle muscular dystrophy recessive type 1, is a progressive muscle disorder that impacts the muscles around the hips and shoulders. The disease is caused by defects in the CAPN3 gene and can be inherited in both recessive and dominant forms. In this retrospective study, we aimed to evaluate the clinical and molecular results of our patients with calpainopathy and to examine the CAPN3 variants in Turkish and global populations.

Materials and methods

Molecular analyses were performed using the next-generation sequencing (NGS) method. CAPN3 variants were identified through the examination of various databases.

Results

In this retrospective study, the cohort consisted of seven patients exhibiting the CAPN3 (NM_000070.3) mutation and a phenotype compatible with calpainopathy at a single center in Türkiye. All patients displayed high CK levels and muscle weakness. We report a novel missense c.2437G>A variant that causes the autosomal dominant form of calpainopathy. Interestingly, the muscle biopsy report for the patient with the novel mutation indicated sarcoglycan deficiency. Molecular findings for the remaining individuals in the cohort included a compound heterozygous variant (frameshift and missense), one homozygous nonsense, one homozygous intronic deletion, and three homozygous missense variants. The most common variant in the Turkish population was c.550del. In both populations, pathogenic variants were most frequently located in exon 21, according to exon length. Variants were stochastically distributed based on consequences in CAPN3 domains.

Conclusion

Therefore, the NGS method proves highly effective in diagnosing rare diseases characterized by clinical heterogeneity. Assessing variants based on ethnicity holds significance in the development of precise therapies.

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.

Nuclei isolation methods fail to accurately assess the subcellular localization and behaviour of proteins in skeletal muscle

AIM

Subcellular fractionation is often used to determine the subcellular localization of proteins, including whether a protein translocates to the nucleus in response to a given stimulus. Examining nuclear proteins in skeletal muscle is difficult because myonuclear proteins are challenging to isolate unless harsh treatments are used. This study aimed to determine the most effective method for isolating and preserving proteins in their native state in skeletal muscle.

METHODS

We compared the ability of detergents, commercially available kit-based and K⁺ -based physiological methodologies for isolating myonuclear proteins from resting samples of human muscle by determining the presence of marker proteins for each fraction by western blot analyses.

RESULTS

We found that following the initial pelleting of nuclei, treatment with 1% Triton-X 100, 1% CHAPS or 0.5% Na-deoxycholate under various ionic conditions resulted in the nuclear proteins being either resistant to isolation or the proteins present behaving aberrantly. The nuclear proteins in brain tissue were also resistant to 1% Triton-X 100 isolation. Here, we demonstrate aberrant behaviour and erroneous localization of proteins using the kit-based method. The aberrant behaviour was the activation of Ca²⁺ -dependent protease calpain-3, and the erroneous localization was the presence of calpain-3 and troponin I in the nuclear fraction.

CONCLUSION

Our findings indicate that it may not be possible to reliably determine the translocation of proteins between subcellular locations and the nucleus using subcellular fractionation techniques. This study highlights the importance of validating subcellular fractionation methodologies using several subcellular-specific markers and solutions that are physiologically relevant to the intracellular milieu.

Nucleolus-localized Def-CAPN3 protein degradation pathway and its role in cell cycle control and ribosome biogenesis

The nucleolus, as the 'nucleus of the nucleus', is a prominent subcellular organelle in a eukaryocyte. The nucleolus serves as the centre for ribosome biogenesis, as well as an important site for cell-cycle regulation, cellular senescence, and stress response. The protein composition of the nucleolus changes dynamically through protein turnover to meet the needs of cellular activities or stress responses. Recent studies have identified a nucleolus-localized protein degradation pathway in zebrafish and humans, namely the Def-CAPN3 pathway, which is essential to ribosome production and cell-cycle progression, by controlling the turnover of multiple substrates (e.g., ribosomal small-subunit [SSU] processome component Mpp10, transcription factor p53, check-point proteins Chk1 and Wee1). This pathway relies on the Ca²⁺-dependent cysteine proteinase CAPN3 and is independent of the ubiquitin-mediated proteasome pathway. CAPN3 is recruited by nucleolar protein Def from cytoplasm to nucleolus, where it proteolyzes its substrates which harbor a CAPN3 recognition-motif. Def depletion leads to the exclusion of CAPN3 and accumulation of p53, Wee1, Chk1, and Mpp10 in the nucleolus that result in cell-cycle arrest and rRNA processing abnormality. Here, we summarize the discovery of the Def-CAPN3 pathway and propose its biological role in cell-cycle control and ribosome biogenesis.

Current and Future Therapeutic Strategies for Limb Girdle Muscular Dystrophy Type R1: Clinical and Experimental Approaches

Limb girdle muscular dystrophy type R1 disease is a progressive disease that is caused by mutations in the CAPN3 gene and involves the extremity muscles of the hip and shoulder girdle. The CAPN3 protein has proteolytic and non-proteolytic properties. The functions of the CAPN3 protein that have been determined so far can be listed as remodeling and combining contractile proteins in the sarcomere with the substrates with which it interacts, controlling the Ca²⁺ flow in and out through the sarcoplasmic reticulum, and regulation of membrane repair and muscle regeneration. Even though there are several gene therapies, cellular therapies, and drug therapies, such as glucocorticoid treatment, AAV- mediated therapy, CRISPR-Cas9, induced pluripotent stem cells, MYO-029, and AMBMP, which are either in preclinical or clinical phases, or have been completed, there is no final cure. Inhibitors and small molecules (tauroursodeoxycholic acid, salubrinal, rapamycin, CDN1163, dwarf open reading frame) targeting ER stress factors that are thought to be effective in muscle loss can be considered potential therapy strategies. At present, little can be done to treat the progressive muscle wasting, loss of function, and premature mortality of patients with LGMDR1, and there is a pressing need for more research to develop potential therapies.

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.

Developing fluorescence sensor probe to capture activated muscle-specific calpain-3 (CAPN3) in living muscle cells

Calpain-3 (CAPN3) is a muscle-specific type of calpain whose protease activity is triggered by Ca2+ Here, we developed CAPN3 sensor probes (SPs) to detect activated-CAPN3 using a fluorescence/Förster resonance energy transfer (FRET) technique. In our SPs, partial amino acid sequence of calpastatin, endogenous CAPN inhibitor but CAPN3 substrate, is inserted between two different fluorescence proteins that cause FRET. Biochemical and spectral studies revealed that CAPN3 cleaved SPs and changed emission wavelengths of SPs. Importantly, SPs were scarcely cleaved by CAPN1 and CAPN2. Furthermore, our SP successfully captured the activation of endogenous CAPN3 in living myotubes treated with ouabain. Our SPs would become a promising tool to detect the dynamics of CAPN3 protease activity in living cells.

Sodium Fluctuations in Astroglia and Their Potential Impact on Astrocyte Function

Astrocytes are the main cell type responsible for the regulation of brain homeostasis, including the maintenance of ion gradients and neurotransmitter clearance. These processes are tightly coupled to changes in the intracellular sodium (Na⁺) concentration. While activation of the sodium-potassium-ATPase (NKA) in response to an elevation of extracellular K⁺ may decrease intracellular Na⁺, the cotransport of transmitters, such as glutamate, together with Na⁺ results in an increase in astrocytic Na⁺. This increase in intracellular Na⁺ can modulate, for instance, metabolic downstream pathways. Thereby, astrocytes are capable to react on a fast time scale to surrounding neuronal activity via intracellular Na⁺ fluctuations and adjust energy production to the demand of their environment. Beside the well-documented conventional roles of Na⁺ signaling mainly mediated through changes in its electrochemical gradient, several recent studies have identified more atypical roles for Na⁺, including protein interactions leading to changes in their biochemical activity or Na⁺-dependent regulation of gene expression. In this review, we will address both the conventional as well as the atypical functions of astrocytic Na⁺ signaling, presenting the role of transporters and channels involved and their implications for physiological processes in the central nervous system (CNS). We will also discuss how these important functions are affected under pathological conditions, including stroke and migraine. We postulate that Na⁺ is an essential player not only in the maintenance of homeostatic processes but also as a messenger for the fast communication between neurons and astrocytes, adjusting the functional properties of various cellular interaction partners to the needs of the surrounding network.

Novel CAPN3 variant associated with an autosomal dominant calpainopathy

AIMS

The most common autosomal recessive limb girdle muscular dystrophy is associated with the CAPN3 gene. The exclusively recessive inheritance of this disorder has been recently challenged by the description of the recurrent variants, c.643_663del21 [p.(Ser215_Gly221del)] and c.598_612del15 [p.(Phe200_Leu204del)], associated with autosomal dominant inheritance. Our objective was to confirm the existence of autosomal dominant calpainopathies.

METHODS

Through our activity as one of the reference centres for genetic diagnosis of calpainopathies in France and the resulting collaborations through the French National Network for Rare Neuromuscular Diseases (FILNEMUS), we identified four families harbouring the same CAPN3 heterozygous variant with supposedly autosomal dominant inheritance.

RESULTS

We identified a novel dominantly inherited CAPN3 variant, c.1333G>A [p.(Gly445Arg)] in 14 affected patients from four unrelated families. The complementary phenotypic, functional and genetic findings correlate with an autosomal dominant inheritance in these families, emphasizing the existence of this novel transmission mode for calpainopathies. The mild phenotype associated with these autosomal dominant cases widens the phenotypic spectrum of calpainopathies and should therefore be considered in clinical practice.

CONCLUSIONS

We confirm the existence of autosomal dominant calpainopathies as an entity beyond the cases related to the in-frame deletions c.643_663del21 and c.598_612del15, with the identification of a novel dominantly inherited and well-documented CAPN3 missense variant, c.1333G>A [p.(Gly445Arg)]. In addition to the consequences for genetic counselling, the confirmation of an autosomal dominant transmission mode for calpainopathies underlines the importance of re-assessing other myopathies for which the inheritance is considered as strictly autosomal recessive.

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.

Divergent Features of Mitochondrial Deficiencies in LGMD2A Associated With Novel Calpain-3 Mutations

Limb girdle muscular dystrophy type 2A (LGMD2A) is an autosomal recessive disorder characterized by progressive muscle weakness and wasting. LGMD2A is caused by mutations in the calpain-3 gene (CAPN3) that encodes a Ca2+-dependent cysteine protease predominantly expressed in the skeletal muscle. Underlying pathological mechanisms have not yet been fully elucidated. Mitochondrial abnormalities have been variably reported in human subjects with LGMD2A and were more systematically evaluated in CAPN3-knocked out mouse models. We have combined histochemical, immunohistochemical, molecular, biochemical, and ultrastructural analyses in our study in order to better outline mitochondrial features in 2 LGMD2A patients with novel CAPN3-associated mutations. Both patients underwent detailed clinical evaluations, followed by muscle biopsies from the quadriceps muscles. The diagnosis of LGMD2A in both patients was first suspected on the basis of a typical clinical localization of the muscle weakness, and confirmed by molecular investigations. Two novel homozygous mutations, c.2242C>G (p.Arg748Gly) and c.291C>A (p.Phe97Leu) were identified: c.2242C>G (p.Arg748Gly) mutation was associated with a significant mitochondrial mass depletion and myofibrillar disruption in the first patient, while c.291C>A (p.Phe97Leu) mutation was accompanied by reactive mitochondrial proliferation with ragged-red fibers in the second patient. Our results delineate CAPN3 mutation-specific patterns of mitochondrial dysfunction and their ultrastructural characteristics in LGMD2A.

Calcium Mechanisms in Limb-Girdle Muscular Dystrophy with CAPN3 Mutations

Limb-girdle muscular dystrophy recessive 1 (LGMDR1), previously known as LGMD2A, is a rare disease caused by mutations in the CAPN3 gene. It is characterized by progressive weakness of shoulder, pelvic, and proximal limb muscles that usually appears in children and young adults and results in loss of ambulation within 20 years after disease onset in most patients. The pathophysiological mechanisms involved in LGMDR1 remain mostly unknown, and to date, there is no effective treatment for this disease. Here, we review clinical and experimental evidence suggesting that dysregulation of Ca2+ homeostasis in the skeletal muscle is a significant underlying event in this muscular dystrophy. We also review and discuss specific clinical features of LGMDR1, CAPN3 functions, novel putative targets for therapeutic strategies, and current approaches aiming to treat LGMDR1. These novel approaches may be clinically relevant not only for LGMDR1 but also for other muscular dystrophies with secondary calpainopathy or with abnormal Ca2+ homeostasis, such as LGMD2B/LGMDR2 or sporadic inclusion body myositis.

Ionic signalling in astroglia beyond calcium

Astrocytes are homeostatic and protective cells of the central nervous system. Astroglial homeostatic responses are tightly coordinated with neuronal activity. Astrocytes maintain neuronal excitability through regulation of extracellular ion concentrations, as well as assisting and modulating synaptic transmission by uptake and catabolism of major neurotransmitters. Moreover, they support neuronal metabolism and detoxify ammonium and reactive oxygen species. Astroglial homeostatic actions are initiated and controlled by intercellular signalling of ions, including Ca²⁺ , Na⁺ , Cl^- , H⁺ and possibly K⁺ . This review summarises current knowledge on ionic signals mediated by the major monovalent ions, which occur in microdomains, as global events, or as propagating intercellular waves and thereby represent the substrate for astroglial excitability.

Overview of the Muscle Cytoskeleton

Cardiac and skeletal striated muscles are intricately designed machines responsible for muscle contraction. Coordination of the basic contractile unit, the sarcomere, and the complex cytoskeletal networks are critical for contractile activity. The sarcomere is comprised of precisely organized individual filament systems that include thin (actin), thick (myosin), titin, and nebulin. Connecting the sarcomere to other organelles (e.g., mitochondria and nucleus) and serving as the scaffold to maintain cellular integrity are the intermediate filaments. The costamere, on the other hand, tethers the sarcomere to the cell membrane. Unique structures like the intercalated disc in cardiac muscle and the myotendinous junction in skeletal muscle help synchronize and transmit force. Intense investigation has been done on many of the proteins that make up these cytoskeletal assemblies. Yet the details of their function and how they interconnect have just started to be elucidated. A vast number of human myopathies are contributed to mutations in muscle proteins; thus understanding their basic function provides a mechanistic understanding of muscle disorders. In this review, we highlight the components of striated muscle with respect to their interactions, signaling pathways, functions, and connections to disease. © 2017 American Physiological Society. Compr Physiol 7:891-944, 2017.

Integrative variants, haplotypes and diplotypes of the CAPN3 and FRMD5 genes and several environmental exposures associate with serum lipid variables

To determine whether the integrative variants, haplotypes and diplotypes of the calpain 3 (CAPN3) and the FERM domain containing 5 genes (FRMD5) and several environmental exposures are associated with an implication in lipid homeostasis, which are associated with cardiovascular risk. Genotyping of the CAPN3 rs4344713 and FRMD5 rs524908 was performed by Sanger sequencing in 1,640 subjects (Jing, 819 and Han, 821). Multivariate analyses of covariance models that adjusted by age, gender, body mass index (BMI), blood pressure and lifestyle (smoking and drinking), were constructed using variants, haplotypes and diplotypes of the CAPN3 rs4344713 and FRMD5 rs524908 as predictors and changes in lipid variables. Significant associations with low-density lipoprotein cholesterol and apolipoprotein (Apo) B were found. Linkage disequilibrium with each other showed the haplotype-phenotype associations with triglyceride and ApoA1. This study also suggested pleiotropic associations of the CAPN3-FRMD5 diplotypes with lipid variables. As potential confounders, diastolic blood pressure (DBP) and BMI were significantly associated with lipid variables. We conclude that integrative variants, haplotypes and diplotypes of the CAPN3 rs4344713 and FRMD5 rs524908, as well as DBP and BMI are associated with serum lipid variables in the Jing and Han populations.

Calpain research for drug discovery: challenges and potential

Calpains are a family of proteases that were scientifically recognized earlier than proteasomes and caspases, but remain enigmatic. However, they are known to participate in a multitude of physiological and pathological processes, performing 'limited proteolysis' whereby they do not destroy but rather modulate the functions of their substrates. Calpains are therefore referred to as 'modulator proteases'. Multidisciplinary research on calpains has begun to elucidate their involvement in pathophysiological mechanisms. Therapeutic strategies targeting malfunctions of calpains have been developed, driven primarily by improvements in the specificity and bioavailability of calpain inhibitors. Here, we review the calpain superfamily and calpain-related disorders, and discuss emerging calpain-targeted therapeutic strategies.

Skeletal muscle as an endocrine organ: Role of [Na+]i/[K+]i-mediated excitation-transcription coupling

During the last two decades numerous research teams demonstrated that skeletal muscles function as an exercise-dependent endocrine organ secreting dozens of myokines. Variety of physiological and pathophysiological implications of skeletal muscle myokines secretion has been described; however, upstream signals and sensing mechanisms underlying this phenomenon remain poorly understood. It is well documented that in skeletal muscles intensive exercise triggers dissipation of transmembrane gradient of monovalent cations caused by permanent activation of voltage-gated Na+ and K+ channels. Recently, we demonstrated that sustained elevation of the [Na+]i/[K+]i ratio triggers expression of dozens ubiquitous genes including several canonical myokines, such as interleukin 6 and cyclooxygenase 2, in the presence of intra- and extracellular Ca2+ chelators. These data allowed us to suggest a novel [Na+]i/[K+]i-sensitive, Ca2+i-independent mechanism of excitation-transcription coupling which triggers myokine production. This pathway exists in parallel with canonical signaling mediated by Ca2+i, AMP-activated protein kinase and hypoxia-inducible factor 1α (HIF-1α). In our mini-review we briefly summarize data supporting this hypothesis as well as unresolved issues aiming to forthcoming studies.

Transcriptomic changes in Ca²⁺-depleted cells: Role of elevated intracellular [Na⁺]/[K⁺] ratio

Previously, we reported that Ca(2+) depletion increased permeability of the plasma membrane for Na(+). This study examined the relative impact of [Na(+)]i/[K(+)]i-mediated signaling on transcriptomic changes in cultured vascular smooth muscle cells from rat aorta (VSMC) subjected to Ca(2+)-depletion by extra-(EGTA) and intracellular (BAPTA-AM) Ca(2+) chelators. Na(+),K(+)-ATPase inhibition in K(+)-free medium during 3 h led to elevation of [Na(+)]i and attenuation of [K(+)]i by ∼7- and 10-fold, whereas Ca(2+)-depletion resulted in alteration of these parameters by ∼3- and 2-fold, respectively. Augmented VSMC permeability for Na(+) and elevation of the [Na(+)]i/[K(+)]i ratio was triggered by addition to Ca(2+)-free medium 50 μM EGTA and was not affected by 10 μM BAPTA-AM. Na(+),K(+)-ATPase inhibition and Ca(2+)-depletion changed expression of 3677 and 4610 mRNA transcripts, respectively. We found highly significant (p<10(-12)) positive (R(2)>0.51) correlation between levels of expression of 2071 transcripts whose expression was affected by both stimuli. Among genes whose expression in Ca(2+)-depleted cells was augmented by more than 7-fold we noted cyclic AMP-dependent transcription factor Atf3, early growth response protein Egr1 and nuclear receptor subfamily 4, group A member Nr4a1. Dissipation of transmembrane gradients of monovalent cations in high-K(+), low-Na(+)-medium abolished the increments of the [Na(+)]i/[K(+)]i ratio as well as the augmented expression of these genes triggered by incubation of VSMC in EGTA containing medium. Thus, our results demonstrate, for the first time, that robust transcriptomic changes triggered by Ca(2+)-depletion in the presence of extracellular Ca(2+)-chelators are at least partially mediated by elevation of the [Na(+)]i/[K(+)]i ratio and activation of Ca(2+)i-independent, [Na(+)]i/[K(+)]i-mediated mechanism of excitation-transcription coupling. These results shad a new light on analysis of data obtained in cells subjected to long-term exposure to Ca(2+) chelators.

Calpain-10 Activity Underlies Angiotensin II-Induced Aldosterone Production in an Adrenal Glomerulosa Cell Model

Aldosterone is a steroid hormone important in the regulation of blood pressure. Aberrant production of aldosterone results in the development and progression of diseases including hypertension and congestive heart failure; therefore, a complete understanding of aldosterone production is important for developing more effective treatments. Angiotensin II (AngII) regulates steroidogenesis, in part through its ability to increase intracellular calcium levels. Calcium can activate calpains, proteases classified as typical or atypical based on the presence or absence of penta-EF-hands, which are involved in various cellular responses. We hypothesized that calpain, in particular calpain-10, is activated by AngII in adrenal glomerulosa cells and underlies aldosterone production. Our studies showed that pan-calpain inhibitors reduced AngII-induced aldosterone production in 2 adrenal glomerulosa cell models, primary bovine zona glomerulosa and human adrenocortical carcinoma (HAC15) cells, as well as CYP11B2 expression in the HAC15 cells. Although AngII induced calpain activation in these cells, typical calpain inhibitors had no effect on AngII-elicited aldosterone production, suggesting a lack of involvement of classical calpains in this process. However, an inhibitor of the atypical calpain, calpain-10, decreased AngII-induced aldosterone production. Consistent with this result, small interfering RNA (siRNA)-mediated knockdown of calpain-10 inhibited aldosterone production and CYP11B2 expression, whereas adenovirus-mediated overexpression of calpain-10 resulted in increased AngII-induced aldosterone production. Our results indicate that AngII-induced activation of calpain-10 in glomerulosa cells underlies aldosterone production and identify calpain-10 or its downstream pathways as potential targets for the development of drug therapies for the treatment of hypertension.

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.

Salt and gene expression: evidence for [Na+]i/[K+]i-mediated signaling pathways

Our review focuses on the recent data showing that gene transcription and translation are under the control of signaling pathways triggered by modulation of the intracellular sodium/potassium ratio ([Na+]i/[K+]i). Side-by-side with sensing of osmolality elevation by tonicity enhancer-binding protein (TonEBP, NFAT5), [Na+]i/[K+]i-mediated excitation-transcription coupling may contribute to the transcriptomic changes evoked by high salt consumption. This novel mechanism includes the sensing of heightened Na+ concentration in the plasma, interstitial, and cerebrospinal fluids via augmented Na+ influx in the endothelium, immune system cells, and the subfornical organ, respectively. In these cells, [Na+]i/[K+]i ratio elevation, triggered by augmented Na+ influx, is further potentiated by increased production of endogenous Na+,K+-ATPase inhibitors documented in salt-sensitive hypertension.

Redox state and mitochondrial respiratory chain function in skeletal muscle of LGMD2A patients

Background

Calpain-3 deficiency causes oxidative and nitrosative stress-induced damage in skeletal muscle of LGMD2A patients, but mitochondrial respiratory chain function and anti-oxidant levels have not been systematically assessed in this clinical population previously.

Methods

We identified 14 patients with phenotypes consistent with LGMD2A and performed CAPN3 gene sequencing, CAPN3 expression/autolysis measurements, and insilico predictions of pathogenicity. Oxidative damage, anti-oxidant capacity, and mitochondrial enzyme activities were determined in a subset of muscle biopsies.

Results

Twenty-one disease-causing variants were detected along the entire CAPN3 gene, five of which were novel (c.338 T>C, c.500 T>C, c.1525-1 G>T, c.2115+4 T>G, c.2366 T>A). Protein- and mRNA-based tests confirmed insilico predictions and the clinical diagnosis in 75% of patients. Reductions in antioxidant defense mechanisms (SOD-1 and NRF-2, but not SOD-2), coupled with increased lipid peroxidation and protein ubiquitination, were observed in calpain-3 deficient muscle, indicating a redox imbalance primarily affecting non-mitochondrial compartments. Although ATP synthase levels were significantly lower in LGMD2A patients, citrate synthase, cytochrome c oxidase, and complex I+III activities were not different from controls.

Conclusions

Despite significant oxidative damage and redox imbalance in cytosolic/myofibrillar compartments, mitochondrial respiratory chain function is largely maintained in skeletal muscle of LGMD2A patients.

A New Insight into the Role of Calpains in Post-mortem Meat Tenderization in Domestic Animals: A review

Tenderness is the most important meat quality trait, which is determined by intracellular environment and extracellular matrix. Particularly, specific protein degradation and protein modification can disrupt the architecture and integrity of muscle cells so that improves the meat tenderness. Endogenous proteolytic systems are responsible for modifying proteinases as well as the meat tenderization. Abundant evidence has testified that calpains (CAPNs) including calpain I (CAPN1) and calpastatin (CAST) have the closest relationship with tenderness in livestock. They are involved in a wide range of physiological processes including muscle growth and differentiation, pathological conditions and post-mortem meat aging. Whereas, Calpain3 (CAPN3) has been established as an important activating enzyme specifically expressed in livestock's skeletal muscle, but its role in domestic animals meat tenderization remains controversial. In this review, we summarize the role of CAPN1, calpain II (CAPN2) and CAST in post-mortem meat tenderization, and analyse the relationship between CAPN3 and tenderness in domestic animals. Besides, the possible mechanism affecting post-mortem meat aging and improving meat tenderization, and current possible causes responsible for divergence (whether CAPN3 contributes to animal meat tenderization or not) are inferred. Only the possible mechanism of CAPN3 in meat tenderization has been confirmed, while its exact role still needs to be studied further.

Platelet-derived growth factor C and calpain-3 are modulators of human melanoma cell invasiveness

The molecular mechanisms responsible for the elevated metastatic potential of malignant melanoma are still not fully understood. In order to shed light on the molecules involved in the acquisition by melanoma of a highly aggressive phenotype, we compared the gene expression profiles of two cell clones derived from the human cutaneous metastatic melanoma cell line M14: a highly invasive clone (M14C2/MK18) and a clone (M14C2/C4) with low ability to invade the extracellular matrix (ECM). The highly invasive phenotype of M14C2/MK18 cells was correlated with overexpression of neuropilin-1, activation of a vascular endothelial growth factor (VEGF)-A/VEGFR-2 autocrine loop and secretion of matrix metalloprotease-2. Moreover, in an in vivo murine model, M14C2/MK18 cells displayed a higher growth rate as compared with M14C2/C4 cells, even though in vitro both clones possessed comparable proliferative potential. Microarray analysis in M14C2/MK18 cells showed a strong upregulation of platelet-derived growth factor (PDGF)-C, a cytokine that contributes to angiogenesis, and downregulation of calpain-3, a calcium-dependent thiol-protease that regulates specific signalling cascade components. Inhibition of PDGF-C with a specific antibody resulted in a significant decrease in ECM invasion by M14C2/MK18 cells, confirming the involvement of PDGF-C in melanoma cell invasiveness. Moreover, the PDGF-C transcript was found to be upregulated in a high percentage of human melanoma cell lines (17/20), whereas only low PDGF-C levels were detected in a few melanocytic cultures (2/6). By contrast, inhibition of calpain-3 activity in M14C2/C4 control cells, using a specific chemical inhibitor, markedly increased ECM invasion, strongly suggesting that downregulation of calpain-3 plays a role in the acquisition of a highly invasive phenotype. The results indicate that PDGF-C upregulation and calpain-3 downregulation are involved in the aggressiveness of malignant melanoma and suggest that modulators of these proteins or their downstream effectors may synergise with VEGF‑A therapies in combating tumour-associated angiogenesis and melanoma spread.

PLEIAD/SIMC1/C5orf25, a novel autolysis regulator for a skeletal-muscle-specific calpain, CAPN3, scaffolds a CAPN3 substrate, CTBP1

CAPN3/p94/calpain-3 is a skeletal-muscle-specific member of the calpain protease family. Multiple muscle cell functions have been reported for CAPN3, and mutations in this protease cause limb-girdle muscular dystrophy type 2A. Little is known about the molecular mechanisms that allow CAPN3 to be so multifunctional. One hypothesis is that the very rapid and exhaustive autolytic activity of CAPN3 needs to be suppressed by dynamic molecular interactions for specific periods of time. The previously identified interaction between CAPN3 and connectin/titin, a giant molecule in muscle sarcomeres, supports this assumption; however, the regulatory mechanisms of non-sarcomere-associated CAPN3 are unknown. Here, we report that a novel CAPN3-binding protein, PLEIAD [Platform element for inhibition of autolytic degradation; originally called SIMC1/C5orf25 (SUMO-interacting motif containing protein 1/chromosome 5open reading frame 25)], suppresses the protease activity of CAPN3. Database analyses showed that PLEIAD homologs, like CAPN3 homologs, are evolutionarily conserved in vertebrates. Furthermore, we found that PLEIAD also interacts with CTBP1 (C-terminal binding protein 1), a transcriptional co-regulator, and CTBP1 is proteolyzed in COS7 cells expressing CAPN3. The identified cleavage sites in CTBP1 suggested that it undergoes functional modification upon its proteolysis by CAPN3, as well as by conventional calpains. These results indicate that PLEIAD can shift its major function from CAPN3 suppression to CAPN3-substrate recruitment, depending on the cellular context. Taken together, our data suggest that PLEIAD is a novel regulatory scaffold for CAPN3, as reflected in its name.

Structure-function relationships in calpains

Calpains are a family of complex multi-domain intracellular enzymes that share a calcium-dependent cysteine protease core. These are not degradative enzymes, but instead carry out limited cleavage of target proteins in response to calcium signalling. Selective cutting of cytoskeletal proteins to facilitate cell migration is one such function. The two most abundant and extensively studied members of this family in mammals, calpains 1 and 2, are heterodimers of an isoform-specific 80 kDa large subunit and a common 28 kDa small subunit. Structures of calpain-2, both Ca2+-free and bound to calpastatin in the activated Ca2+-bound state, have provided a wealth of information about the enzyme's structure-function relationships and activation. The main association between the subunits is the pairing of their C-terminal penta-EF-hand domains through extensive intimate hydrophobic contacts. A lesser contact is made between the N-terminal anchor helix of the large subunit and the penta-EF-hand domain of the small subunit. Up to ten Ca2+ ions are co-operatively bound during activation. The anchor helix is released and individual domains change their positions relative to each other to properly align the active site. Because calpains 1 and 2 require ~30 and ~350 μM Ca2+ ions for half-maximal activation respectively, it has long been argued that autoproteolysis, subunit dissociation, post-translational modifications or auxiliary proteins are needed to activate the enzymes in the cell, where Ca2+ levels are in the nanomolar range. In the absence of robust support for these mechanisms, it is possible that under normal conditions calpains are transiently activated by high Ca2+ concentrations in the microenvironment of a Ca2+ influx, and then return to an inactive state ready for reactivation.

Regulation and physiological roles of the calpain system in muscular disorders

Calpains, a family of Ca(2+)-dependent cytosolic cysteine proteases, can modulate their substrates' structure and function through limited proteolytic activity. In the human genome, there are 15 calpain genes. The most-studied calpains, referred to as conventional calpains, are ubiquitous. While genetic studies in mice have improved our understanding about the conventional calpains' physiological functions, especially those essential for mammalian life as in embryogenesis, many reports have pointed to overactivated conventional calpains as an exacerbating factor in pathophysiological conditions such as cardiovascular diseases and muscular dystrophies. For treatment of these diseases, calpain inhibitors have always been considered as drug targets. Recent studies have introduced another aspect of calpains that calpain activity is required to protect the heart and skeletal muscle against stress. This review summarizes the functions and regulation of calpains, focusing on the relevance of calpains to cardiovascular disease.

Calpains: an elaborate proteolytic system

Calpain is an intracellular Ca(2+)-dependent cysteine protease (EC 3.4.22.17; Clan CA, family C02). Recent expansion of sequence data across the species definitively shows that calpain has been present throughout evolution; calpains are found in almost all eukaryotes and some bacteria, but not in archaebacteria. Fifteen genes within the human genome encode a calpain-like protease domain. Interestingly, some human calpains, particularly those with non-classical domain structures, are very similar to calpain homologs identified in evolutionarily distant organisms. Three-dimensional structural analyses have helped to identify calpain's unique mechanism of activation; the calpain protease domain comprises two core domains that fuse to form a functional protease only when bound to Ca(2+)via well-conserved amino acids. This finding highlights the mechanistic characteristics shared by the numerous calpain homologs, despite the fact that they have divergent domain structures. In other words, calpains function through the same mechanism but are regulated independently. This article reviews the recent progress in calpain research, focusing on those studies that have helped to elucidate its mechanism of action. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Impact of genetic insights into calpain biology

Calpain has long been an enigmatic enzyme, although it is involved in a variety of biological phenomena. Recent progress in calpain genetics has highlighted numerous physiological contexts in which the functions of calpain are of great significance. This review focuses on recent findings in the field of calpain genetics and the importance of calpain function. Calpain is an intracellular Ca(2+)-dependent cysteine protease (EC 3.4.22.17; Clan CA, family C02) found in almost all eukaryotes. It is also present in a few bacteria, but not in archaebacteria. Calpain has limited proteolytic activity; rather, it transforms or modulates the structure and/or activity of its substrates. It is, therefore, referred to as a 'modulator protease'. Within the human genome, 15 genes (CAPN1-3, CAPN5-16) encode a calpain-like protease (CysPc) domain along with several different functional domains. Thus, calpains can be regarded as a distinct family of versatile enzymes that fulfil numerous tasks in vivo. Genetic studies show that a variety of defects in many different organisms, including lethality, muscular dystrophies and gastropathy, actually stem from calpain deficiencies. The cause-effect relationships identified by these studies form the basis for ongoing and future studies regarding the physiological role of calpains.

Expanding members and roles of the calpain superfamily and their genetically modified animals

Calpains are intracellular Ca²(+)-dependent cysteine proteases (Clan CA, family C02, EC 3.4.22.17) found in almost all eukaryotes and some bacteria. Calpains display limited proteolytic activity at neutral pH, proteolysing substrates to transform and modulate their structures and activities, and are therefore called "modulator proteases". The human genome has 15 genes that encode a calpain-like protease domain, generating diverse calpain homologues that possess combinations of several functional domains such as Ca²(+)-binding domains and Zn-finger domains. The importance of the physiological roles of calpains is reflected in the fact that particular defects in calpain functionality cause a variety of deficiencies in many different organisms, including lethality, muscular dystrophies, lissencephaly, and tumorigenesis. In this review, the unique characteristics of this distinctive protease superfamily are introduced in terms of genetically modified animals, some of which are animal models of calpain deficiency diseases.

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.