Ca2+ homeostasis in Brody's disease. A study in skeletal muscle and cultured muscle cells and the effects of dantrolene an verapamil

Brody Disease, an Early-Onset Myopathy With Delayed Relaxation and Abnormal Gait: A Case Series of 9 Children

Brody disease is a rare autosomal recessive myopathy, caused by pathogenic variants in the ATP2A1 gene. It is characterized by an exercise-induced delay in muscle relaxation, often reported as muscle stiffness. Children may manifest with an abnormal gait and difficulty running. Delayed relaxation is commonly undetected, resulting in a long diagnostic delay. Almost all published cases so far were adults with childhood onset and adult diagnosis. With diagnostic next-generation sequencing, an increasing number of patients are diagnosed in childhood. We describe the clinical and genetic features of 9 children from 6 families with Brody disease. All presented with exercise-induced delayed relaxation, reported as difficulty running and performing sports. Muscle strength and mass was normal, and several children even had an athletic appearance. However, the walking and running patterns were abnormal. The diagnostic delay ranged between 2 and 7 years. Uniformly, a wide range of other disorders were considered before genetic testing was performed, revealing pathogenic genetic variants in ATP2A1. To conclude, this case series is expected to improve clinical recognition and timely diagnosis of Brody disease in children. We propose that ATP2A1 should be added to gene panels for congenital myopathies, developmental and movement disorders, and muscle channelopathies.

Case report: Revealing the rare-a Brody Disease patient from Turkey expanding the phenotype

Brody Disease is an exceptionally rare, autosomal recessive myopathy attributed to the pathogenic variants in the ATP2A1, which encodes the sarcoplasmic/endoplasmic reticulum Ca (2+) ATPase type 1 protein SERCA1. It was first described by Brody IA in 1969. To date, only thirty-three Brody families with forty-seven patients have been reported in the literature, and the disease prevalence is considered as 1 in 10 million, demonstrating the peculiarity of the disease. Clinical characteristics of Brody Disease include muscle stiffness after exercise, myalgia, and muscle cramps. Brody Disease patients generally have disease onset in the first decade, and genetic diagnosis is delayed as a consequence of both the rareness and the mild course of the disease. Here, we report a Turkish Brody Disease patient with a homozygous c.428G>A p.Arg143Gln (NM_004320.4) missense mutation in the ATP2A1. The male patient, whose symptoms started at the age of 14-15, is now 36 years old. His clinical manifestations are athletic appearance, exotropia, slightly elevated creatine kinase (CK), mild progressive proximal muscle weakness in the lower extremities, muscle cramps, pain and stiffness. The patient described here has a very mild progression with an onset in the second decade, expanding the Brody Disease phenotype. The study also implies that in the era of emerging genetic therapies, the routine testing of patients with myopathies is a prerequisite since not only future therapies will be designed on molecular findings, but also currently available symptomatic and palliative treatment options will be more precisely applied.

Case report: Clinical and molecular characterization of two siblings affected by Brody myopathy

Exercise-induced muscle stiffness is the hallmark of Brody disease, an autosomal recessive myopathy due to biallelic pathogenic variants in ATP2A1, encoding the sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase SERCA1. About 40 patients have been reported so far. Our knowledge about the natural history of this disorder, genotype-phenotype correlations and the effect of symptomatic treatment is partial. This results in incomplete recognition and underdiagnosis of the disease. Here, we report the clinical, instrumental, and molecular features of two siblings presenting childhood-onset exercise-induced muscle stiffness without pain. Both the probands display difficulty in climbing stairs and running, frequent falls, delayed muscle relaxation after exertion. Cold temperatures worsen these symptoms. No myotonic discharges were observed at electromyography. Whole Exome Sequencing analysis in the probands revealed the presence of two ATP2A1 variants: the previously reported frameshift microdeletion c.2464delC and the likely pathogenic novel splice-site variant c.324 + 1G > A, whose detrimental effect was demonstrated in ATP2A1 transcript analysis. The bi-allelic inheritance was verified by Sanger sequencing in the unaffected parents. This study expands the molecular defects associated with Brody myopathy.

Differential Analysis of Gly211Val and Gly286Val Mutations Affecting Sarco(endo)plasmic Reticulum Ca2+-ATPase (SERCA1) in Congenital Pseudomyotonia Romagnola Cattle

Congenital pseudomyotonia in cattle (PMT) is a rare skeletal muscle disorder, clinically characterized by stiffness and by delayed muscle relaxation after exercise. Muscle relaxation impairment is due to defective content of the Sarco(endo)plasmic Reticulum Ca²⁺ ATPase isoform 1 (SERCA1) protein, caused by missense mutations in the ATP2A1 gene. PMT represents the only mammalian model of human Brody myopathy. In the Romagnola breed, two missense variants occurring in the same allele were described, leading to Gly211Val and Gly286Val (G211V/G286V) substitutions. In this study, we analyzed the consequences of G211V and G286V mutations. Results support that the reduced amount of SERCA1 is a consequence of the G211V mutation, the G286V mutation almost being benign and the ubiquitin–proteasome system (UPS) being involved. After blocking the proteasome using a proteasome inhibitor, we found that the G211V mutant accumulates in cells at levels comparable to those of WT SERCA1. Our conclusion is that G211/286V mutations presumably originate in a folding-defective SERCA1 protein, recognized and diverted to degradation by UPS, although still catalytically functional, and that the main role is played by G211V mutation. Rescue of mutated SERCA1 to the sarcoplasmic reticulum membrane can re-establish resting cytosolic Ca²⁺ concentration and prevent the appearance of pathological signs, paving the way for a possible therapeutic approach against Brody disease.

Clinical, morphological and genetic characterization of Brody disease: an international study of 40 patients

Brody disease is an autosomal recessive myopathy characterized by exercise-induced muscle stiffness due to mutations in the ATP2A1 gene. Almost 50 years after the initial case presentation, only 18 patients have been reported and many questions regarding the clinical phenotype and results of ancillary investigations remain unanswered, likely leading to incomplete recognition and consequently under-diagnosis. Additionally, little is known about the natural history of the disorder, genotype-phenotype correlations, and the effects of symptomatic treatment. We studied the largest cohort of Brody disease patients to date (n = 40), consisting of 22 new patients (19 novel mutations) and all 18 previously published patients. This observational study shows that the main feature of Brody disease is an exercise-induced muscle stiffness of the limbs, and often of the eyelids. Onset begins in childhood and there was no or only mild progression of symptoms over time. Four patients had episodes resembling malignant hyperthermia. The key finding at physical examination was delayed relaxation after repetitive contractions. Additionally, no atrophy was seen, muscle strength was generally preserved, and some patients had a remarkable athletic build. Symptomatic treatment was mostly ineffective or produced unacceptable side effects. EMG showed silent contractures in approximately half of the patients and no myotonia. Creatine kinase was normal or mildly elevated, and muscle biopsy showed mild myopathic changes with selective type II atrophy. Sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase (SERCA) activity was reduced and western blot analysis showed decreased or absent SERCA1 protein. Based on this cohort, we conclude that Brody disease should be considered in cases of exercise-induced muscle stiffness. When physical examination shows delayed relaxation, and there are no myotonic discharges at electromyography, we recommend direct sequencing of the ATP2A1 gene or next generation sequencing with a myopathy panel. Aside from clinical features, SERCA activity measurement and SERCA1 western blot can assist in proving the pathogenicity of novel ATP2A1 mutations. Finally, patients with Brody disease may be at risk for malignant hyperthermia-like episodes, and therefore appropriate perioperative measures are recommended. This study will help improve understanding and recognition of Brody disease as a distinct myopathy in the broader field of calcium-related myopathies.

Cast immobilization of hindlimb upregulates sarcolipin expression in atrophied skeletal muscles and increases thermogenesis in C57BL/6J mice

Mechanical unloading impairs cytosolic calcium (Ca²⁺) homeostasis in skeletal muscles. In this study, we investigated whether sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) itself or one of the regulators of the Ca²⁺ SERCA pump, sarcolipin (SLN), is altered to deregulate Ca²⁺ homeostasis in cast immobilized, atrophied muscles. Hindlimb muscles of 8-wk-old male C57BL/6J mice were subjected to bilateral cast immobilization for 2 wk. Two-week-cast immobilization induced both body weight and skeletal muscle loss. Highly phosphorylated Ca²⁺/calmodulin-dependent protein kinase II in the atrophied muscles suggested that cytosolic Ca²⁺ concentration was elevated. Extremely high expression levels of SLN mRNA and protein were observed in the atrophied muscles. Upregulation of SLN at the transcriptional level was supported by low RCAN1 expression, which is a negative regulator of SLN. We treated C₂C₁₂ cells with dexamethasone to mimic muscle atrophy in vitro and showed a direct relationship between high SLN mRNA expression and low Ca²⁺ uptake by sarcoplasmic reticulum. Since SLN reportedly plays a role in nonshivering thermogenesis, we performed a cold tolerance test of the whole body. As a result, we found that mice with cast immobilization showed high cold tolerance, suggesting that cast immobilization promoted whole body thermogenesis. Although the activity level was decreased during cast immobilization without change in food intake, adipose tissue weights also decreased significantly after cast immobilization. Concomitantly, we conclude that cast immobilization of hindlimb increased thermogenesis in C57Bl/6J mice, probably via high expression of SLN.

Disturbed Ca2+ Homeostasis in Muscle-Wasting Disorders

Ca²⁺ is essential for proper structure and function of skeletal muscle. It not only activates contraction and force development but also participates in multiple signaling pathways. Low levels of Ca²⁺ restrain muscle regeneration by limiting the fusion of satellite cells. Ironically, sustained elevations of Ca²⁺ also result in muscle degeneration as this ion promotes high rates of protein breakdown. Moreover, transforming growth factors (TGFs) which are well known for controlling muscle growth also regulate Ca²⁺ channels. Thus, therapies focused on changing levels of Ca²⁺ and TGFs are promising for treating muscle-wasting disorders. Three principal systems govern the homeostasis of Ca²⁺, namely, excitation-contraction (EC) coupling, excitation-coupled Ca²⁺ entry (ECCE), and store-operated Ca²⁺ entry (SOCE). Accordingly, alterations in these systems can lead to weakness and atrophy in many hereditary diseases, such as Brody disease, central core disease (CCD), tubular aggregate myopathy (TAM), myotonic dystrophy type 1 (MD1), oculopharyngeal muscular dystrophy (OPMD), and Duchenne muscular dystrophy (DMD). Here, the interrelationship between all these molecules and processes is reviewed.

Calcium Dynamics Mediated by the Endoplasmic/Sarcoplasmic Reticulum and Related Diseases

The flow of intracellular calcium (Ca2+) is critical for the activation and regulation of important biological events that are required in living organisms. As the major Ca2+ repositories inside the cell, the endoplasmic reticulum (ER) and the sarcoplasmic reticulum (SR) of muscle cells are central in maintaining and amplifying the intracellular Ca2+ signal. The morphology of these organelles, along with the distribution of key calcium-binding proteins (CaBPs), regulatory proteins, pumps, and receptors fundamentally impact the local and global differences in Ca2+ release kinetics. In this review, we will discuss the structural and morphological differences between the ER and SR and how they influence localized Ca2+ release, related diseases, and the need for targeted genetically encoded calcium indicators (GECIs) to study these events.

Ca2+ handling abnormalities in early-onset muscle diseases: Novel concepts and perspectives

The physiological process by which Ca²⁺ is released from the sarcoplasmic reticulum is called excitation-contraction coupling; it is initiated by an action potential which travels deep into the muscle fiber where it is sensed by the dihydropyridine receptor, a voltage sensing L-type Ca²⁺channel localized on the transverse tubules. Voltage-induced conformational changes in the dihydropyridine receptor activate the ryanodine receptor Ca²⁺ release channel of the sarcoplasmic reticulum. The released Ca²⁺ binds to troponin C, enabling contractile thick-thin filament interactions. The Ca²⁺ is subsequently transported back into the sarcoplasmic reticulum by specialized Ca²⁺ pumps (SERCA), preparing the muscle for a new cycle of contraction. Although other proteins are involved in excitation-contraction coupling, the mechanism described above emphasizes the unique role played by the two Ca²⁺ channels (the dihydropyridine receptor and the ryanodine receptor), the SERCA Ca²⁺ pumps and the exquisite spatial organization of the membrane compartments endowed with the proteins responsible for this mechanism to function rapidly and efficiently. Research over the past two decades has uncovered the fine details of excitation-contraction coupling under normal conditions while advances in genomics have helped to identify mutations in novel genes in patients with neuromuscular disorders. While it is now clear that many patients with congenital muscle diseases carry mutations in genes encoding proteins directly involved in Ca²⁺ homeostasis, it has become apparent that mutations are also present in genes encoding for proteins not thought to be directly involved in Ca²⁺ regulation. Ongoing research in the field now focuses on understanding the functional effect of individual mutations, as well as understanding the role of proteins not specifically located in the sarcoplasmic reticulum which nevertheless are involved in Ca²⁺ regulation or excitation-contraction coupling. The principal challenge for the future is the identification of drug targets that can be pharmacologically manipulated by small molecules, with the ultimate aim to improve muscle function and quality of life of patients with congenital muscle disorders. The aim of this review is to give an overview of the most recent findings concerning Ca²⁺ dysregulation and its impact on muscle function in patients with congenital muscle disorders due to mutations in proteins involved in excitation-contraction coupling and more broadly on Ca²⁺ homeostasis.

Structural study of skeletal muscle fibres in healthy and pseudomyotonia affected cattle

Cattle congenital pseudomyotonia (PMT), recognized as naturally occurring animal model of human Brody disease, is an inherited recessive autosomal muscular disorder due to missense mutations in ATP2A1 gene, encoding sarco(endo)plasmic reticulum Ca(2+)-ATPase protein, isoform 1 (SERCA1). PMT has been described in the Chianina and Romagnola italian cattle breeds and as a single case in Dutch improved Red and White cross-breed. The genetic defect turned out to be heterogeneous in different cattle breeds, even though clinical symptoms were homogeneous. Skeletal muscles of affected animals are characterized by a selective deficiency of SERCA1 in sarcoplasmic reticulum (SR) membranes. Recently, we provided evidence that in Chianina breed, the ubiquitin proteasome system is responsible for SERCA1 mutant premature disposal, even when the mutation does not affect the catalytic properties of the pump. Results presented here show that all SERCA1 mutants described until now, although expressed at low level, are correctly targeted to SR membranes. Ultrastructural studies confirm that in pathological muscle fibres, structure, as well as triads, is well preserved. All together these results suggest that a possible therapeutical approach based on the rescue of the defective protein at SR membranes could be hypothesized. Only fully functionally active missense mutants, whem located at the SR membrane could restore the efficient control of Ca(2+) homeostasis and prevent the appearance of the pathological signs. Moreover, these data demonstrate the increasing importance of domestic animals as genetic models of human pathologies.

Characterization of sarcoplasmic reticulum Ca(2+) ATPase pumps in muscle of patients with myotonic dystrophy and with hypothyroid myopathy

Sarcoplasmic/endoplasmic reticulum Ca(2+) ATPase (SERCA) pumps play the major role in lowering cytoplasmic calcium concentration in skeletal muscle by catalyzing the ATP-dependent transport of Ca(2+) from the cytosol to the lumen of the sarcoplasmic reticulum (SR). Although SERCA abnormalities have been hypothesized to contribute to the dysregulation of intracellular Ca(2+) homeostasis and signaling in muscle of patients with myotonic dystrophy (DM) and hypothyroid myopathy, the characterization of SERCA pumps remains elusive and their impairment is still unclear. We assessed the activity of SR Ca(2+)-ATPase, expression levels and fiber distribution of SERCA1 and SERCA2, and oligomerization of SERCA1 protein in muscle of patients with DM type 1 and 2, and with hypothyroid myopathy. Our data provide evidence that SR Ca(2+) ATPase activity, protein levels and muscle fiber distribution of total SERCA1 and SERCA2, and SERCA1 oligomerization pattern are similar in patients with both DM1 and DM2, hypothyroid myopathy and in control subjects. We prove that SERCA1b, the neonatal isoform of SERCA1, is expressed at protein level in muscle of patients with DM2 and, in lower amount, of patients with DM1. Our present study demonstrates that SERCA function is not altered in muscle of patients with DM and with hypothyroid myopathy.

Calpain 3 deficiency affects SERCA expression and function in the skeletal muscle

Limb-girdle muscular dystrophy type 2A (LGMD2A) is a form of muscular dystrophy caused by mutations in calpain 3 (CAPN3). Several studies have implicated Ca²⁺ dysregulation as an underlying event in several muscular dystrophies, including LGMD2A. In this study we used mouse and human myotube cultures, and muscle biopsies in order to determine whether dysfunction of sarco/endoplasmatic Ca²⁺-ATPase (SERCA) is involved in the pathology of this disease. In CAPN3-deficient myotubes, we found decreased levels of SERCA 1 and 2 proteins, while mRNA levels remained comparable with control myotubes. Also, we found a significant reduction in SERCA function that resulted in impairment of Ca²⁺ homeostasis, and elevated basal intracellular [Ca²⁺] in human myotubes. Furthermore, small Ankyrin 1 (sAnk1), a SERCA1-binding protein that is involved in sarcoplasmic reticulum integrity, was also diminished in CAPN3-deficient fibres. Interestingly, SERCA2 protein was patently reduced in muscles from LGMD2A patients, while it was normally expressed in other forms of muscular dystrophy. Thus, analysis of SERCA2 expression may prove useful for diagnostic purposes as a potential indicator of CAPN3 deficiency in muscle biopsies. Altogether, our results indicate that CAPN3 deficiency leads to degradation of SERCA proteins and Ca²⁺ dysregulation in the skeletal muscle. While further studies are needed in order to elucidate the specific contribution of SERCA towards muscle degeneration in LGMD2A, this study constitutes a reasonable foundation for the development of therapeutic approaches targeting SERCA1, SERCA2 or sAnk1.

Calcium-ATPases: Gene disorders and dysregulation in cancer

Ca(2+)-ATPases belonging to the superfamily of P-type pumps play an important role in maintaining low, nanomolar cytoplasmic Ca(2+) levels at rest and priming organellar stores, including the endoplasmic reticulum, Golgi, and secretory vesicles with high levels of Ca(2+) for a wide range of signaling functions. In this review, we introduce the distinct subtypes of Ca(2+)-ATPases and their isoforms and splice variants and provide an overview of their specific cellular roles as they relate to genetic disorders and cancer, with a particular emphasis on recent findings on the secretory pathway Ca(2+)-ATPases (SPCA). Mutations in human ATP2A2, ATP2C1 genes, encoding housekeeping isoforms of the endoplasmic reticulum (SERCA2) and secretory pathway (SPCA1) pumps, respectively, confer autosomal dominant disorders of the skin, whereas mutations in other isoforms underlie various muscular, neurological, or developmental disorders. Emerging evidence points to an important function of dysregulated Ca(2+)-ATPase expression in cancers of the colon, lung, and breast where they may serve as markers of differentiation or novel targets for therapeutic intervention. We review the mechanisms underlying the link between calcium homeostasis and cancer and discuss the potential clinical relevance of these observations. This article is part of a Special Issue entitled: Calcium and Cell Fate. Guest Editors: Jacques Haiech, Claus Heizmann, Joachim Krebs, Thierry Capiod and Olivier Mignen.

Calcium in health and disease

Evolution has exploited the chemical properties of Ca(2+), which facilitate its reversible binding to the sites of irregular geometry offered by biological macromolecules, to select it as a carrier of cellular signals. A number of proteins bind Ca(2+) to specific sites: those intrinsic to membranes play the most important role in the spatial and temporal regulation of the concentration and movements of Ca(2+) inside cells. Those which are soluble, or organized in non-membranous structures, also decode the Ca(2+) message to be then transmitted to the targets of its regulation. Since Ca(2+) controls the most important processes in the life of cells, it must be very carefully controlled within the cytoplasm, where most of the targets of its signaling function reside. Membrane channels (in the plasma membrane and in the organelles) mediate the entrance of Ca(2+) into the cytoplasm, ATPases, exchangers, and the mitochondrial Ca(2+) uptake system remove Ca(2+) from it. The concentration of Ca(2+) in the external spaces, which is controlled essentially by its dynamic exchanges in the bone system, is much higher than inside cells, and can, under conditions of pathology, generate a situation of dangerous internal Ca(2+) overload. When massive and persistent, the Ca(2+) overload culminates in the death of the cell. Subtle conditions of cellular Ca(2+) dyshomeostasis that affect individual systems that control Ca(2+), generate cell disease phenotypes that are particularly severe in tissues in which the signaling function of Ca(2+) has special importance, e.g., the nervous system.

Excitation–contraction coupling properties in women with work-related myalgia: a preliminary study

We investigated the potential role of selected excitation–contraction coupling processes in females with work-related myalgia (WRM) by comparing WRM with healthy controls (CON) using tissue from extensor carpi radialis brevis (ECRB) and trapezius (TRAP) muscles. For the ECRB, age (mean ± SE) was 29.6 ± 3.5 years for CON (n = 9) and 39.2 ± 2.8 years for WRM (n = 13), while for the TRAP, the values were 26.0 ± 2.1 years for CON (n = 7) and 44.6 ± 2.9 years for WRM (n = 11). For the sarcoplasmic reticulum (SR) of the ECRB, WRM displayed concentrations (nmol·(mg protein)−1·min−1) that were lower (P < 0.05) for Total (202 ± 4.4 vs 178 ± 7.1), Basal (34 ± 1.6 vs 30.1 ± 1.3), and maximal Ca2+-ATPase activity (Vmax, 168 ± 4.9 vs 149 ± 6.3), and Ca2+-uptake (5.06 ± 0.31 vs 4.13 ± 0.29), but not SERCA1a and SERCA2a isoforms, by comparison with CON. When age was incorporated as a co-variant, Total, Basal, and Ca2+-uptake remained different from CON (P < 0.05), but not Vmax (P = 0.13). For TRAP, none of the ATPase properties differed between groups (P > 0.05) either before or following adjustment for age. No differences (P > 0.05) were observed between the groups for Ca2+-release in the SR for either TRAP or ECRB. Similarly, no deficiencies, regardless of muscle, were noted for either the Na+–K+-ATPase content or the α and β subunit isoform distribution in WRM. This preliminary study provides a basis for further research, with expanded numbers, investigating the hypothesis that abnormalities in SR Ca2+-regulation are involved in the cellular etiology of WRM.

A novel bone morphogenetic protein 2 mutant mouse, nBmp2NLS(tm), displays impaired intracellular Ca2+ handling in skeletal muscle

We recently reported a novel form of BMP2, designated nBMP2, which is translated from an alternative downstream start codon and is localized to the nucleus rather than secreted from the cell. To examine the function of nBMP2 in the nucleus, we engineered a gene-targeted mutant mouse model (nBmp2NLS(tm)) in which nBMP2 cannot be translocated to the nucleus. Immunohistochemistry demonstrated the presence of nBMP2 staining in the myonuclei of wild type but not mutant skeletal muscle. The nBmp2NLS(tm) mouse exhibits altered function of skeletal muscle as demonstrated by a significant increase in the time required for relaxation following a stimulated twitch contraction. Force frequency analysis showed elevated force production in mutant muscles compared to controls from 10 to 60 Hz stimulation frequency, consistent with the mutant muscle's reduced ability to relax between rapidly stimulated contractions. Muscle relaxation after contraction is mediated by the active transport of Ca(2+) from the cytoplasm to the sarcoplasmic reticulum by sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA), and enzyme activity assays revealed that SERCA activity in skeletal muscle from nBmp2NLS(tm) mice was reduced to approximately 80% of wild type. These results suggest that nBMP2 plays a role in the establishment or maintenance of intracellular Ca(2+) transport pathways in skeletal muscle.

SERCA1 protein expression in muscle of patients with Brody disease and Brody syndrome and in cultured human muscle fibers

Brody disease is an inherited myopathy associated with a defective function of sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase 1 (SERCA1) protein. Mutations in the ATP2A1 gene have been reported only in some patients. Therefore it has been proposed to distinguish patients with ATP2A1 mutations, Brody disease (BD), from patients without mutations, Brody syndrome (BS). We performed a detailed study of SERCA1 protein expression in muscle of patients with BD and BS, and evaluated the alternative splicing of SERCA1 in primary cultures of normal human muscle and in infant muscle. SERCA1 reactivity was observed in type 2 muscle fibers of patients with and without ATP2A1 mutations and staining intensity was similar in patients and controls. Immunoblot analysis showed a significant reduction of SERCA1 band in muscle of BD patients. In addition we demonstrated that the wild type and mutated protein exhibits similar solubility properties and that RIPA buffer improves the recovery of the wild type and mutated SERCA1 protein. We found that SERCA1b, the SERCA1 neonatal form, is the main protein isoform expressed in cultured human muscle fibers and infant muscle. Finally, we identified two novel heterozygous mutations within exon 3 of the ATP2A1 gene from a previously described patient with BD.

Genetic architecture of gene expression in ovine skeletal muscle

Background

In livestock populations the genetic contribution to muscling is intensively monitored in the progeny of industry sires and used as a tool in selective breeding programs. The genes and pathways conferring this genetic merit are largely undefined. Genetic variation within a population has potential, amongst other mechanisms, to alter gene expression via cis- or trans-acting mechanisms in a manner that impacts the functional activities of specific pathways that contribute to muscling traits. By integrating sire-based genetic merit information for a muscling trait with progeny-based gene expression data we directly tested the hypothesis that there is genetic structure in the gene expression program in ovine skeletal muscle.

Results

The genetic performance of six sires for a well defined muscling trait, longissimus lumborum muscle depth, was measured using extensive progeny testing and expressed as an Estimated Breeding Value by comparison with contemporary sires. Microarray gene expression data were obtained for longissimus lumborum samples taken from forty progeny of the six sires (4-8 progeny/sire). Initial unsupervised hierarchical clustering analysis revealed strong genetic architecture to the gene expression data, which also discriminated the sire-based Estimated Breeding Value for the trait. An integrated systems biology approach was then used to identify the major functional pathways contributing to the genetics of enhanced muscling by using both Estimated Breeding Value weighted gene co-expression network analysis and a differential gene co-expression network analysis. The modules of genes revealed by these analyses were enriched for a number of functional terms summarised as muscle sarcomere organisation and development, protein catabolism (proteosome), RNA processing, mitochondrial function and transcriptional regulation.

Conclusions

This study has revealed strong genetic structure in the gene expression program within ovine longissimus lumborum muscle. The balance between muscle protein synthesis, at the levels of both transcription and translation control, and protein catabolism mediated by regulated proteolysis is likely to be the primary determinant of the genetic merit for the muscling trait in this sheep population. There is also evidence that high genetic merit for muscling is associated with a fibre type shift toward fast glycolytic fibres. This study provides insight into mechanisms, presumably subject to strong artificial selection, that underpin enhanced muscling in sheep populations.

Slowed relaxation and preserved maximal force in soleus muscles of mice with targeted disruption of the Serca2 gene in skeletal muscle

Sarcoplasmic reticulum Ca(2+) ATPases (SERCAs) play a major role in muscle contractility by pumping Ca(2+) from the cytosol into the sarcoplasmic reticulum (SR) Ca(2+) store, allowing muscle relaxation and refilling of the SR with releasable Ca(2+). Decreased SERCA function has been shown to result in impaired muscle function and disease in human and animal models. In this study, we present a new mouse model with targeted disruption of the Serca2 gene in skeletal muscle (skKO) to investigate the functional consequences of reduced SERCA2 expression in skeletal muscle. SkKO mice were viable and basic muscle structure was intact. SERCA2 abundance was reduced in multiple muscles, and by as much as 95% in soleus muscle, having the highest content of slow-twitch fibres (40%). The Ca(2+) uptake rate was significantly reduced in SR vesicles in total homogenates. We did not find any compensatory increase in SERCA1 or SERCA3 abundance, or altered expression of several other Ca(2+)-handling proteins. Ultrastructural analysis revealed generally well-preserved muscle morphology, but a reduced volume of the longitudinal SR. In contracting soleus muscle in vitro preparations, skKO muscles were able to fully relax, but with a significantly slowed relaxation time compared to controls. Surprisingly, the maximal force and contraction rate were preserved, suggesting that skKO slow-twitch fibres may be able to contribute to the total muscle force despite loss of SERCA2 protein. Thus it is possible that SERCA-independent mechanisms can contribute to muscle contractile function.

Life span extension via eIF4G inhibition is mediated by posttranscriptional remodeling of stress response gene expression in C. elegans

Reducing protein synthesis slows growth and development but can increase adult life span. We demonstrate that knockdown of eukaryotic translation initiation factor 4G (eIF4G), which is downregulated during starvation and dauer state, results in differential translation of genes important for growth and longevity in C. elegans. Genome-wide mRNA translation state analysis showed that inhibition of IFG-1, the C. elegans ortholog of eIF4G, results in a relative increase in ribosomal loading and translation of stress response genes. Some of these genes are required for life span extension when IFG-1 is inhibited. Furthermore, enhanced ribosomal loading of certain mRNAs upon IFG-1 inhibition was correlated with increased mRNA length. This association was supported by changes in the proteome assayed via quantitative mass spectrometry. Our results suggest that IFG-1 mediates the antagonistic effects on growth and somatic maintenance by regulating mRNA translation of particular mRNAs based, in part, on transcript length.

Pseudomyotonia, a muscle function disorder associated with an inherited ATP2A1 (SERCA1) defect in a Dutch Improved Red and White cross-breed calf

A Dutch Improved Red and White cross-breed heifer calf was evaluated for a muscular disorder resulting in exercise induced muscle stiffness. Clinical findings included generalized exercise-induced muscle spasms with normal response to muscle percussion. Electromyography showed no myotonic discharges, thus ruling out myotonia. Whereas histological examination of muscle tissue was unremarkable, Ca(2+)-ATPase activity of sarcoplasmatic reticulum membranes (SERCA1) was markedly decreased compared to control animals. Mutation analysis revealed the presence of a missense mutation in the ATP2A1 gene encoding the SERCA1 protein (p.Arg559Cys). The present case presents similarities to human Brody's disease, but also to pseudomyotonia and congenital muscular dystonia previously described in different cattle breeds.

Calcium Pumps in Health and Disease

Ca2+-ATPases (pumps) are key actors in the regulation of Ca2+ in eukaryotic cells and are thus essential to the correct functioning of the cell machinery. They have high affinity for Ca2+ and can efficiently regulate it down to very low concentration levels. Two of the pumps have been known for decades (the SERCA and PMCA pumps); one (the SPCA pump) has only become known recently. Each pump is the product of a multigene family, the number of isoforms being further increased by alternative splicing of the primary transcripts. The three pumps share the basic features of the catalytic mechanism but differ in a number of properties related to tissue distribution, regulation, and role in the cellular homeostasis of Ca2+. The molecular understanding of the function of the pumps has received great impetus from the solution of the three-dimensional structure of one of them, the SERCA pump. These spectacular advances in the structure and molecular mechanism of the pumps have been accompanied by the emergence and rapid expansion of the topic of pump malfunction, which has paralleled the rapid expansion of knowledge in the topic of Ca2+-signaling dysfunction. Most of the pump defects described so far are genetic: when they are very severe, they produce gross and global disturbances of Ca2+ homeostasis that are incompatible with cell life. However, pump defects may also be of a type that produce subtler, often tissue-specific disturbances that affect individual components of the Ca2+-controlling and/or processing machinery. They do not bring cells to immediate death but seriously compromise their normal functioning.

Diminished sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) expression contributes to airway remodelling in bronchial asthma

Phenotypic modulation of airway smooth muscle (ASM) is an important feature of airway remodeling in asthma that is characterized by enhanced proliferation and secretion of pro-inflammatory chemokines. These activities are regulated by the concentration of free Ca(2+) in the cytosol ([Ca(2+)](i)). A rise in [Ca(2+)](i) is normalized by rapid reuptake of Ca(2+) into sarcoplasmic reticulum (SR) stores by the sarco/endoplasmic reticulum Ca(2+) (SERCA) pump. We examined whether increased proliferative and secretory responses of ASM from asthmatics result from reduced SERCA expression. ASM cells were cultured from subjects with and without asthma. SERCA expression was evaluated by western blot, immunohistochemistry and real-time PCR. Changes in [Ca(2+)](i), cell spreading, cellular proliferation, and eotaxin-1 release were measured. Compared with control cells from healthy subjects, SERCA2 mRNA and protein expression was reduced in ASM cells from subjects with moderately severe asthma. SERCA2 expression was similarly reduced in ASM in vivo in subjects with moderate/severe asthma. Rises in [Ca(2+)](i) following cell surface receptor-induced SR activation, or inhibition of SERCA-mediated Ca(2+) re-uptake, were attenuated in ASM cells from asthmatics. Likewise, the return to baseline of [Ca](i) after stimulation by bradykinin was delayed by approximately 50% in ASM cells from asthmatics. siRNA-mediated knockdown of SERCA2 in ASM from healthy subjects increased cell spreading, eotaxin-1 release and proliferation. Our findings implicate a deficiency in SERCA2 in ASM in asthma that contributes to its secretory and hyperproliferative phenotype in asthma, and which may play a key role in mechanisms of airway remodeling.

A defective SERCA1 protein is responsible for congenital pseudomyotonia in Chianina cattle

Recently, a muscular disorder defined as "congenital pseudomyotonia" was described in Chianina cattle, one of the most important Italian cattle breeds for quality meat and leather. The clinical phenotype of this disease is characterized by an exercise-induced muscle contracture that prevents animals from performing muscular activities. On the basis of clinical symptoms, Chianina pseudomyotonia appeared related to human Brody's disease, a rare inherited disorder of skeletal muscle function that results from a sarcoplasmic reticulum Ca(2+)-ATPase (SERCA1) deficiency caused by a defect in the ATP2A1 gene that encodes SERCA1. SERCA1 is involved in transporting calcium from the cytosol to the lumen of the sarcoplasmic reticulum. Recently, we identified the genetic defect underlying Chianina cattle pseudomyotonia. A missense mutation in exon 6 of the ATP2A1 gene, leading to an R164H substitution in the SERCA1 protein, was found. In this study, we provide biochemical evidence for a selective deficiency in SERCA1 protein levels in sarcoplasmic reticulum membranes from affected muscles, although mRNA levels are unaffected. The reduction of SERCA1 levels accounts for the reduced Ca(2+)-ATPase activity without any significant change in Ca(2+)-dependency. The loss of SERCA1 is not compensated for by the expression of the SERCA2 isoform. We believe that Chianina cattle pseudomyotonia might, therefore, be the true counterpart of human Brody's disease, and that bovine species might be used as a suitable animal model.

Identification of a missense mutation in the bovine ATP2A1 gene in congenital pseudomyotonia of Chianina cattle: an animal model of human Brody disease

Congenital pseudomyotonia in Chianina cattle is a muscle function disorder very similar to that of Brody disease in humans. Mutations in the human ATP2A1 gene, encoding SERCA1, cause Brody myopathy. The analysis of the collected Chianina pedigree data suggested monogenic autosomal recessive inheritance and revealed that all 17 affected individuals traced back to a single founder. A deficiency of SERCA1 function in skeletal muscle of pseudomyotonia affected Chianina cattle was observed as SERCA1 activity in affected animals was decreased by about 70%. Linkage analysis showed that the mutation was located in the ATP2A1 gene region on BTA25 and subsequent mutation analysis of the ATP2A1 exons revealed a perfectly associated missense mutation in exon 6 (c.491G>A) leading to a p.Arg164His substitution. Arg164 represents a functionally important and strongly conserved residue of SERCA1. This study provides a suitable large animal model for human Brody disease.

SERCA pumps and human diseases

Sarco(endo)plasmic reticulum (SER) Ca2+ ATPases represent a highly conserved family of Ca2+ pumps which actively transport Ca2+ from the cytosol to the SER against a large concentration gradient. In humans, 3 genes (ATP2A1-3) generate multiple isoforms (SERCAla,b, SERCA2a-c, SECA3a-f) by developmental or tissue-specific alternative splicing. These pumps differ by their regulatory and kinetic properties, allowing for optimized function in the tissue where they are expressed. They play a central role in calcium signalling through regenerating SER Ca2+ stores, maintaining appropriate Ca2+ levels in this organelle and shaping cytosolic and nuclear Ca2+ variations which govern cell response. Defects in ATP2A1 encoding SERCA1 cause recessive Brody myopathy, mutations in ATP2A2 coding for SERCA2 underlie a dominant skin disease, Darier disease and its clinical variants. SERCA2a expression is reduced in heart failure in human and in mice models. Gene-targeting studies in mouse confirmed the expected function of these isoforms in some cases, but also resulted in unexpected phenotypes: SERCA1 null mutants die from respiratory failure, SERCA2 heterozygous mutant mice develop skin cancer with age and SERCA3 null mice display no diabetes. These unique phenotypes have provided invaluable information on the role of these pumps in specific tissues and species, and have improved our understanding of Ca2+ regulated processes in muscles, the heart and the skin in human and in mice. Although the understanding of the pathogenesis of these diseases is still incomplete, these recent advances hold the promise of improved knowledge on the disease processes and the identification of new targets for therapeutic interventions.

Sarcoplasmic reticulum: The dynamic calcium governor of muscle

The sarcoplasmic reticulum (SR) provides feedback control required to balance the processes of calcium storage, release, and reuptake in skeletal muscle. This balance is achieved through the concerted action of three major classes of SR calcium-regulatory proteins: (1) luminal calcium-binding proteins (calsequestrin, histidine-rich calcium-binding protein, junctate, and sarcalumenin) for calcium storage; (2) SR calcium release channels (type 1 ryanodine receptor or RyR1 and IP3 receptors) for calcium release; and (3) sarco(endo)plasmic reticulum Ca2+ -ATPase (SERCA) pumps for calcium reuptake. Proper calcium storage, release, and reuptake are essential for normal skeletal muscle function. We review SR structure and function during normal skeletal muscle activity, the proteins that orchestrate calcium storage, release, and reuptake, and how phenotypically distinct muscle diseases (e.g., malignant hyperthermia, central core disease, and Brody disease) can result from subtle alterations in the activity of several key components of the SR calcium-regulatory machinery.

A mutation in serca underlies motility dysfunction in accordion zebrafish

Zebrafish acquire the ability for fast swimming early in development. The motility mutant accordion (acc) undergoes exaggerated and prolonged contractions on both sides of the body, interfering with the acquisition of patterned swimming responses. Our whole cell recordings from muscle indicate that the defect is not manifested in neuromuscular transmission. However, imaging of skeletal muscle of larval acc reveals greatly prolonged calcium transients and associated contractions in response to depolarization. Positional cloning of acc identified a serca mutation as the cause of the acc phenotype. SERCA is a sarcoplasmic reticulum transmembrane protein in skeletal muscle that mediates calcium re-uptake from the myoplasm. The mutation in SERCA, a serine to phenylalanine substitution, is likely to result in compromised protein function that accounts for the observed phenotype. Indeed, direct evidence that mutant SERCA causes the motility dysfunction was provided by the finding that wild type fish injected with an antisense morpholino directed against serca, exhibited accordion-like contractions and impaired swimming. We conclude that the motility dysfunction in embryonic and larval accordion zebrafish stems directly from defective calcium transport in skeletal muscle rather than defective CNS drive.

Muscle cramps

Muscle cramps are a common problem characterized by a sudden, painful, involuntary contraction of muscle. These true cramps, which originate from peripheral nerves, may be distinguished from other muscle pain or spasm. Medical history, physical examination, and a limited laboratory screen help to determine the various causes of muscle cramps. Despite the "benign" nature of cramps, many patients find the symptom very uncomfortable. Treatment options are guided both by experience and by a limited number of therapeutic trials. Quinine sulfate is an effective medication, but the side-effect profile is worrisome, and other membrane-stabilizing drugs are probably just as effective. Patients will benefit from further studies to better define the pathophysiology of muscle cramps and to find more effective medications with fewer side-effects.

accordion, a zebrafish behavioral mutant, has a muscle relaxation defect due to a mutation in the ATPase Ca2+ pump SERCA1

When wild-type zebrafish embryos are touched at 24 hours post-fertilization (hpf), they typically perform two rapid alternating coils of the tail. By contrast, accordion (acc) mutants fail to coil their tails normally but contract the bilateral trunk muscles simultaneously to shorten the trunk, resulting in a pronounced dorsal bend. Electrophysiological recordings from muscles showed that the output from the central nervous system is normal in mutants, suggesting a defect in muscles is responsible. In fact, relaxation in acc muscle is significantly slower than normal. In vivo imaging of muscle Ca2+ transients revealed that cytosolic Ca2+ decay was significantly slower in acc muscle. Thus, it appears that the mutant behavior is caused by a muscle relaxation defect due to the impairment of Ca2+ re-uptake. Indeed, acc mutants carry a mutation in atp2a1 gene that encodes the sarco(endo)plasmic reticulum Ca2+-ATPase 1 (SERCA1), a Ca2+ pump found in the muscle sarcoplasmic reticulum (SR) that is responsible for pumping Ca2+ from the cytosol back to the SR. As SERCA1 mutations in humans lead to Brody disease, an exercise-induced muscle relaxation disorder, zebrafish accordion mutants could be a useful animal model for this condition.

Autosomal dominant Brody disease cosegregates with a chromosomal (2;7)(p11.2;p12.1) translocation in an Italian family

Brody disease is a rare muscle disorder characterized by exercise-induced impairment in muscle relaxation, due to a markedly reduced influx of calcium ions in the sarcoplasmic reticulum. A subset of autosomal recessive families harbour mutations in the ATP2A1 gene, encoding the fast-twitch skeletal muscle sarcoplasmic reticulum Ca(2+) ATPase (SERCA1). Rare autosomal dominant families have been described, in which ATP2A1 was excluded as the causative gene, further supporting genetic heterogeneity. We report four individuals from a three-generation Italian family with a clinical phenotype of Brody disease, in which linkage analysis excluded ATP2A1 as the responsible gene. The disease cosegregates in an autosomal dominant fashion with an apparently balanced constitutional chromosome translocation (2;7)(p11.2;p12.1), suggesting a causal relationship between the rearrangement and the phenotype. FISH analysis using YAC and PAC clones as probes refined the breakpoint regions to genomic segments of about 164 and 120 kb, respectively, providing a possible clue to pinpoint the location of a novel gene responsible for this rare muscle disorder.

Targeted disruption of the ATP2A1 gene encoding the sarco(endo)plasmic reticulum Ca2+ ATPase isoform 1 (SERCA1) impairs diaphragm function and is lethal in neonatal mice

Mutations in the ATP2A1 gene, encoding isoform 1 of the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA1), are one cause of Brody disease, characterized in humans by exercise-induced contraction of fast twitch (type II) skeletal muscle fibers. In an attempt to create a model for Brody disease, the mouse ATP2A1 gene was targeted to generate a SERCA1-null mutant mouse line. In contrast to humans, term SERCA1-null mice had progressive cyanosis and gasping respiration and succumbed from respiratory failure shortly after birth. The percentage of affected homozygote SERCA1(-/-) mice was consistent with predicted Mendelian inheritance. A survey of multiple organs from 10-, 15-, and 18-day embryos revealed no morphological abnormalities, but analysis of the lungs in term mice revealed diffuse congestion and epithelial hypercellularity and studies of the diaphragm muscle revealed prominent hypercontracted regions in scattered fibers and increased fiber size variability. The V(max) of Ca(2+) transport activity in mutant diaphragm and skeletal muscle was reduced by 80% compared with wild-type muscle, and the contractile response to electrical stimulation under physiological conditions was reduced dramatically in mutant diaphragm muscle. No compensatory responses were detected in analysis of mRNAs encoding other Ca(2+) handling proteins or of protein levels. Expression of ATP2A1 is largely restricted to type II fibers, which predominate in normal mouse diaphragm. The absence of SERCA1 in type II fibers, and the absence of compensatory increases in other Ca(2+) handling proteins, coupled with the marked increase in contractile function required of the diaphragm muscle to support postnatal respiration, can account for respiratory failure in term SERCA1-null mice.

Calcium regulation and muscle disease

Changes in intracellular Ca2+-concentration play an important role in the excitation-contraction-relaxation cycle of skeletal muscle. In this review we describe various inheritable muscle diseases to highlight the role of Ca2+-regulatory mechanisms. Upon excitation the ryanodine receptor releases Ca2+ in the cytosol. During and after contraction the sarcoplasmic reticulum (SR) Ca2+ATPase (SERCA) pumps Ca2+ back in the SR resulting in relaxation. An abnormal change in the intracellular Ca2+-concentration results in defective muscle contraction and/or relaxation, which is the cause of various muscle diseases. Malignant hyperthermia (MH) and central core disease (CCD) are both caused by mutations in the ryanodine receptor but show different clinical phenotypes. In MH an acute increase of Ca2+ results in excessive muscle contraction causing rigidity, while in CCD a chronic rise of cytosolic Ca2+ is seen, leading to mitochondrial damage, disorganization of myofibrils and muscle weakness. In Brody disease and also in mitochondrial myopathies, SERCA functions sub optimal causing a prolonged physiological Ca2+-elevation leading to slowing of relaxation. Defective actin-myosin interactions, as in nemaline myopathy and also in mitochondrial myopathies due to ATP-shortage, cause Ca2+-hyposensitivity and slowness of contraction. Information of Ca2+-kinetics in these inherited muscular diseases improves our understanding of the role of calcium in the physiology and pathophysiology of the skeletal muscle cell.

Effects of fatigue and training on sarcoplasmic reticulum Ca(2+) regulation in human skeletal muscle

Little is known about fatigue and training effects on sarcoplasmic reticulum (SR) function in human muscle, and we therefore investigated this in eight untrained controls (UT), eight endurance-trained (ET), and eight resistance-trained athletes (RT). Muscle biopsies (vastus lateralis) taken at rest and after 50 maximal quadriceps contractions (180 degrees/s, 0.5 Hz) were analyzed for fiber composition, metabolites and maximal SR Ca(2+) release, Ca(2+) uptake, and Ca(2+)-ATPase activity. Fatigue reduced (P < 0.05) Ca(2+) release (42.1 +/- 3.8%, 43.4 +/- 3.9%, 31.3 +/- 6.1%), Ca(2+) uptake (43.0 +/- 5.2%, 34.1 +/- 4.6%, 28.4 +/- 2.8%), and Ca(2+)-ATPase activity (38.6 +/- 4.2%, 48.5 +/- 5.7%, 29.6 +/- 5.0%), in UT, RT, and ET, respectively. These decreases were correlated with fatigability and with type II fiber proportion (P < 0.05). Resting SR measures were correlated with type II proportion (r > or = 0.51, P < 0.05). ET had lower resting Ca(2+) release, Ca(2+) uptake, and Ca(2+)-ATPase (P < 0.05) than UT and RT (P < 0.05), probably because of their lower type II proportion; only minor effects were found in RT. Thus SR function is markedly depressed with fatigue in controls and in athletes, is dependent on fiber type, and appears to be minimally affected by chronic training status.

Calcium signaling: a tale for all seasons

An experiment performed in London nearly 120 years ago, which by today's standards would be considered unacceptably sloppy, marked the beginning of the calcium (Ca(2+)) signaling saga. Sidney Ringer [Ringer, S. (1883) J. Physiol. 4, 29-43] was studying the contraction of isolated rat hearts. In earlier experiments, Ringer had suspended them in a saline medium for which he admitted to having used London tap water, which is hard: The hearts contracted beautifully. When he proceeded to replace the tap water with distilled water, he made a startling finding: The beating of the hearts became progressively weaker, and stopped altogether after about 20 min. To maintain contraction, he found it necessary to add Ca(2+) salts to the suspension medium. Thus, Ringer had serendipitously discovered that Ca(2+), hitherto exclusively considered as a structural element, was active in a tissue that has nothing to do with bone or teeth, and performed there a completely novel function: It carried the signal that initiated heart contraction. It was a landmark observation, which should have immediately aroused wide interest. Unexpectedly, however, for decades it attracted no particular attention. Occasionally, farsighted pioneers argued forcefully for a messenger role of Ca(2+), offering compelling experimental evidence. Among them, one could quote L. V. Heilbrunn [Heilbrunn, L. V. (1940) Physiol. Zool. 13, 88-94], who contracted frog muscle fibers by applying Ca(2+) salts to their cut ends, but not to their surfaces. Heilbrunn correctly concluded that Ca(2+) had diffused from the cut ends to the internal contractile elements to elicit their contraction. One could also quote K. Bailey [Bailey, K. (1942) Biochem. J. 36, 121-139], who showed that the ATPase activity of myosin was strongly activated by Ca(2+) (but not by Mg(2+)), and concluded that the liberation of Ca(2+) in the neighborhood of the myosin controlled muscle contraction. Clearly, enough evidence was there, but only a handful of people had the vision to see it and to foresee its far-reaching implications. Perhaps no better example of clairvoyance can be offered than the quip by O. Loewy in 1959: "Ja Kalzium, das ist alles!"

Molecular genetics of Ca(2+) stores and intracellular Ca(2+) signalling

An increasing number of studies based on recombinant cells and on mouse models that express an altered repertoire of some of the key components of the intracellular Ca(2+) release stores are becoming available as a result of molecular genetics techniques. Information from these studies, together with results from studies of human diseases caused by mutations in genes that encode proteins of the intracellular Ca(2+) stores, are providing a significant advancement in understanding the interactive nature of the molecular machinery that underlies intracellular Ca(2+) signalling and how the different components of the Ca(2+) stores contribute to the regulation of cellular functions.

Ca2+ signalling and muscle disease

Transient elevations of intracellular Ca2+ play a signalling role in such complex cellular functions as contraction, secretion, fertilization, proliferation, metabolism, heartbeat and memory. However, prolonged elevation of Ca2+ above about 10 microM is deleterious to a cell and can activate apoptosis. In muscle, there is a narrow window of Ca2+ dysregulation in which abnormalities in Ca2+ regulatory proteins can lead to disease, rather than apoptosis. Key proteins in the regulation of muscle Ca2+ are the voltage-dependent, dihydropyridine-sensitive, L-type Ca2+ channels located in the transverse tubule and Ca2+ release channels in the junctional terminal cisternae of the sarcoplasmic reticulum. Abnormalities in these proteins play a key role in malignant hyperthermia (MH), a toxic response to anesthetics, and in central core disease (CCD), a muscle myopathy. Sarco(endo)plasmic reticulum Ca2+ ATPases (SERCAs) return sarcoplasmic Ca2+ to the lumen of the sarcoplasmic reticulum. Loss of SERCA1a Ca2+ pump function is one cause of exercise-induced impairment of the relaxation of skeletal muscle, in Brody disease. Phospholamban expressed in cardiac muscle and sarcolipin expressed in skeletal muscle regulate SERCA activity. Studies with knockout and transgenic mice show that gain of inhibitory function of phospholamban alters cardiac contractility and could be a causal feature in some cardiomyopathies. Calsequestrin, calreticulin, and a series of other acidic, lumenal, Ca2+ binding proteins provide a buffer for Ca2+ stored in the sarcoplasmic reticulum. Overexpression of cardiac calsequestrin leads to cardiomyopathy and ablation of calreticulin alters cardiac development.

Calcium ion in skeletal muscle: its crucial role for muscle function, plasticity, and disease

Mammalian skeletal muscle shows an enormous variability in its functional features such as rate of force production, resistance to fatigue, and energy metabolism, with a wide spectrum from slow aerobic to fast anaerobic physiology. In addition, skeletal muscle exhibits high plasticity that is based on the potential of the muscle fibers to undergo changes of their cytoarchitecture and composition of specific muscle protein isoforms. Adaptive changes of the muscle fibers occur in response to a variety of stimuli such as, e.g., growth and differentition factors, hormones, nerve signals, or exercise. Additionally, the muscle fibers are arranged in compartments that often function as largely independent muscular subunits. All muscle fibers use Ca(2+) as their main regulatory and signaling molecule. Therefore, contractile properties of muscle fibers are dependent on the variable expression of proteins involved in Ca(2+) signaling and handling. Molecular diversity of the main proteins in the Ca(2+) signaling apparatus (the calcium cycle) largely determines the contraction and relaxation properties of a muscle fiber. The Ca(2+) signaling apparatus includes 1) the ryanodine receptor that is the sarcoplasmic reticulum Ca(2+) release channel, 2) the troponin protein complex that mediates the Ca(2+) effect to the myofibrillar structures leading to contraction, 3) the Ca(2+) pump responsible for Ca(2+) reuptake into the sarcoplasmic reticulum, and 4) calsequestrin, the Ca(2+) storage protein in the sarcoplasmic reticulum. In addition, a multitude of Ca(2+)-binding proteins is present in muscle tissue including parvalbumin, calmodulin, S100 proteins, annexins, sorcin, myosin light chains, beta-actinin, calcineurin, and calpain. These Ca(2+)-binding proteins may either exert an important role in Ca(2+)-triggered muscle contraction under certain conditions or modulate other muscle activities such as protein metabolism, differentiation, and growth. Recently, several Ca(2+) signaling and handling molecules have been shown to be altered in muscle diseases. Functional alterations of Ca(2+) handling seem to be responsible for the pathophysiological conditions seen in dystrophinopathies, Brody's disease, and malignant hyperthermia. These also underline the importance of the affected molecules for correct muscle performance.

Muscle function in a patient with Brody's disease

Adductor pollicis muscle function of a 21-year-old man with genetically confirmed Brody's disease (sarcoplasmic reticulum [SR] -Ca2+ATPase deficiency) was investigated to study the possible effects of reduced SR-Ca2+ATPase activity on muscle relaxation and force production. Following maximal electrical activation of the ulnar nerve, tetanic muscle half-relaxation time was greater in the patient (246 +/- 10 ms) than control subjects (97 +/- 4 ms, n = 8). During repetitive activation, there was a similar decline in maximal shortening velocity in the patient and controls, indicating a comparable reduction in cross-bridge cycling rate. The finding that the slowing of relaxation was greater in the patient (329 ms versus 138 +/- 20 ms) suggests that there was a further reduction of SR-Ca2+ATPase activity in the patient's muscle during fatigue. Following a voluntary contraction, involuntary activity of the antagonist muscles facilitated force decline and masked the impaired relaxation in the patient. This antagonist-induced relaxation indicates that it might be difficult to establish impaired muscle relaxation with voluntary contractions.

Phenotypic variability in rippling muscle disease

OBJECTIVE

To characterize the phenotype of hereditary rippling muscle disease (RMD) and to report the results of genetic linkage studies.

BACKGROUND

RMD is a rare autosomal-dominant inherited muscle disorder. Individuals complain of muscle stiffness, exercise-induced muscle pain, and cramp-like sensations. The characteristic feature of RMD is increased mechanical muscle irritability, which is electrically silent in electromyographic examinations.

METHODS

Forty-six individuals from two unrelated German kindreds with RMD were examined. Linkage analysis to the RMD locus on chromosome 1q41-q43 was performed.

RESULTS

In kindred A, 15 individuals from four generations, and in kindred B, four individuals from three generations had clinical features of RMD. The most consistent clinical findings were percussion-induced rapid muscle contractions (PIRCs) and muscle mounding, which were present in all 19 affected individuals. Only 12 individuals exhibited muscle rippling, indicating that rippling is not always present in RMD. Twelve of 19 individuals had muscle-related complaints, primarily exertional cramps and stiffness. The mean age at the onset of complaints was 22 years (range, 5 to 54 years). Seven of 19 individuals showed only mechanical-induced muscle irritability but did not have muscular symptoms. Genetic analysis excluded linkage to the RMD locus on chromosome 1q4 in both kindreds.

CONCLUSIONS

The phenotype of RMD is variable but generalized PIRCs are the most obvious and reliable clinical feature of RMD. Diagnostic criteria of RMD should include generalized PIRCs in addition to muscle mounding, rippling, and creatine kinase elevation.

Sarcolipin regulates the activity of SERCA1, the fast-twitch skeletal muscle sarcoplasmic reticulum Ca2+-ATPase

The 31-amino acid proteolipid, sarcolipin (SLN), is associated with the fast-twitch skeletal muscle sarcoplasmic reticulum Ca2+-ATPase (SERCA1). Constructs of human and rabbit SLN and of rabbit SLN with the FLAG epitope at its N terminus (NF-SLN) or its C terminus (SLN-FC) were coexpressed with SERCA1 in HEK-293 T-cells. Immunohistochemistry was used to demonstrate colocalization of NF-SLN and SERCA1 in the endoplasmic reticulum membrane and to demonstrate the cytosolic orientation of the N terminus of SLN. Coexpression of native rabbit SLN or NF-SLN with SERCA1 decreased the apparent affinity of SERCA1 for Ca2+ but stimulated maximal Ca2+ uptake rates (Vmax). The N terminus of SLN is not well conserved among species, and the addition of an N-terminal FLAG epitope did not alter SLN function. Anti-FLAG antibody reversed both the inhibition of Ca2+ uptake by NF-SLN at low Ca2+ concentrations and the stimulatory effect of NF-SLN on Vmax. Addition of the FLAG epitope to the highly conserved C terminus decreased the apparent affinity of SERCA1 for Ca2+ relative to native SLN and decreased Vmax significantly. Mutations in the C-terminal domain showed that this sequence is critical for SLN function. Mutational analysis of the transmembrane helix, together with the additive regulatory effects of coexpression of both SLN and phospholamban (PLN) with SERCA1, provided evidence for different mechanisms of interaction of SLN and PLN with SERCA molecules. Ca2+ uptake rates in sarcoplasmic reticulum vesicles, isolated from rabbit fast-twitch muscle (tibialis anterior) subjected to chronic low frequency stimulation, were reduced by approximately 40% in 3- and 4-day stimulated muscle, with a marginal increase in apparent affinity of SERCA1 for Ca2+. SERCA1 mRNA and protein levels were unaltered after stimulation. In contrast, SLN mRNA was decreased by 15%, and SLN protein was reduced by 40%. Reduced SLN expression could explain the decrease in SERCA1 activity observed in these muscles and might represent an early functional adaptation to chronic low frequency stimulation.

Intravenous immunoglobulin preparation increases myoplasmic calcium concentration by activating the dihydropyridine-ryanodine receptor complex

A human intravenous immunoglobulin preparation (IVIg) released Ca2+ from the sarcoplasmic reticulum of cultured human skeletal muscle cells in a dose-dependent manner. Blocking the dihydropyridine-ryanodine receptor complex abrogated the IVIg-mediated Ca2+ response, whereas inhibition of the voltage-operated Na+-channels or acetylcholine receptors did not. This effect of IVIg was not mediated by its main component, the IgG molecules, and differed between preparations from different manufacturers. Heating destroyed the activity. Data shows that an unidentified serum protein present in IVIg can influence human muscle cells by an effect on the dihydropyridine receptor. This phenomenon may be important in interpreting the (side) effects of IVIg in neuromuscular diseases.

Characterization of the gene encoding human sarcolipin (SLN), a proteolipid associated with SERCA1: absence of structural mutations in five patients with Brody disease

Sarcolipin (SLN) is a low-molecular-weight protein that copurifies with the fast-twitch skeletal muscle sarcoplasmic reticulum Ca2+ ATPase (SERCA1). Genomic DNA and cDNA encoding human sarcolipin (SLN) were isolated and characterized and the SLN gene was mapped to chromosome 11q22-q23. Human, rabbit, and mouse cDNAs encode a protein of 31 amino acids. Homology of SLN with phospholamban (PLN) suggests that the first 7 hydrophilic amino acids are cytoplasmic, the next 19 hydrophobic amino acids form a single transmembrane helix, and the last 5 hydrophilic amino acids are lumenal. The cytoplasmic and transmembrane sequences are not well conserved among the three species, but the lumenal sequence is highly conserved. Like SERCA1, SLN is highly expressed in rabbit fast-twitch skeletal muscle, but it is expressed to a lower extent in slow-twitch muscle and to an even lower extent in cardiac muscle, where SERCA2a and PLN are highly expressed. It is expressed in only trace amounts in pancreas and prostate. SLN and PLN genes resemble each other in having two small exons, with their entire coding sequences lying in exon 2 and a large intron separating the two segments. Brody disease is an inherited disorder of skeletal muscle function, characterized by exercise-induced impairment of muscle relaxation. Mutations in the ATP2A1 gene encoding SERCA1 have been associated with the autosomal recessive inheritance of Brody disease in three families, but not with autosomal dominant inheritance of the disease. A search for mutations in the SLN gene in five Brody families, four of which were not linked to ATP2A1, did not reveal any alterations in coding, splice junction or promoter sequences. The homozygous deletion of C438 in the coding sequence of ATP2A1 in Brody disease family 3, leading to a frameshift and truncation following Pro147 in SERCA1, is the fourth ATP2A1 mutation to be associated with autosomal recessive Brody disease.

Myotonic dystrophy protein kinase is involved in the modulation of the Ca2+ homeostasis in skeletal muscle cells

Myotonic dystrophy (DM), the most prevalent muscular disorder in adults, is caused by (CTG)n-repeat expansion in a gene encoding a protein kinase (DM protein kinase; DMPK) and involves changes in cytoarchitecture and ion homeostasis. To obtain clues to the normal biological role of DMPK in cellular ion homeostasis, we have compared the resting [Ca2+]i, the amplitude and shape of depolarization-induced Ca2+ transients, and the content of ATP-driven ion pumps in cultured skeletal muscle cells of wild-type and DMPK[-/-] knockout mice. In vitro-differentiated DMPK[-/-] myotubes exhibit a higher resting [Ca2+]i than do wild-type myotubes because of an altered open probability of voltage-dependent l-type Ca2+ and Na+ channels. The mutant myotubes exhibit smaller and slower Ca2+ responses upon triggering by acetylcholine or high external K+. In addition, we observed that these Ca2+ transients partially result from an influx of extracellular Ca2+ through the l-type Ca2+ channel. Neither the content nor the activity of Na+/K+ ATPase and sarcoplasmic reticulum Ca2+-ATPase are affected by DMPK absence. In conclusion, our data suggest that DMPK is involved in modulating the initial events of excitation-contraction coupling in skeletal muscle.

Altered Ca2+ responses in muscles with combined mitochondrial and cytosolic creatine kinase deficiencies

We have blocked creatine kinase (CK)-mediated phosphocreatine (PCr) -->/<-- ATP transphosphorylation in skeletal muscle by combining targeted mutations in the genes encoding mitochondrial and cytosolic CK in mice. Contrary to expectation, the PCr level was only marginally affected, but the compound was rendered metabolically inert. Mutant muscles in vivo showed significantly impaired tetanic force output, increased relaxation times, altered mitochondrial volume and location, and conspicuous tubular aggregates of sarcoplasmic reticulum membranes, as seen in myopathies with electrolyte disturbances. In depolarized myotubes cultured in vitro, CK absence influenced both the release and sequestration of Ca2+. Our data point to a direct link between the CK-PCr system and Ca2+-flux regulation during the excitation and relaxation phases of muscle contraction.