Nuclear sequestration of δ-sarcoglycan disrupts the nuclear localization of lamin A/C and emerin in cardiomyocytes

Enhanced nuclear localization of phosphorylated MLKL predicts adverse events in patients with dilated cardiomyopathy

AIMS

The role of necroptosis in dilated cardiomyopathy (DCM) remains unclear. Here, we examined whether phosphorylation of mixed lineage kinase domain-like protein (MLKL), an indispensable event for execution of necroptosis, is associated with the progression of DCM.

METHODS AND RESULTS

Patients with DCM (n = 56, 56 ± 15 years of age; 68% male) were enrolled for immunohistochemical analyses of biopsies. Adverse events were defined as a composite of death or admission for heart failure or ventricular arrhythmia. Compared with the normal myocardium, increased signals of MLKL phosphorylation were detected in the nuclei, cytoplasm, and intercalated discs of cardiomyocytes in biopsy samples from DCM patients. The phosphorylated MLKL (p-MLKL) signal was increased in enlarged nuclei or nuclei with bizarre shapes in hypertrophied cardiomyocytes. Nuclear p-MLKL level was correlated negatively with septal peak myocardial velocity during early diastole (r = -0.327, P = 0.019) and was correlated positively with tricuspid regurgitation pressure gradient (r = 0.339, P = 0.023), while p-MLKL level in intercalated discs was negatively correlated with mean left ventricular wall thickness (r = -0.360, P = 0.014). During a median follow-up period of 3.5 years, 10 patients (18%) had adverse events. To examine the difference in event rates according to p-MLKL expression levels, patients were divided into two groups by using the median value of nuclear p-MLKL or intercalated disc p-MLKL. A group with high nuclear p-MLKL level (H-nucMLKL group) had a higher adverse event rate than did a group with low nuclear p-MLKL level (L-nucMLKL group) (32% vs. 4%, P = 0.012), and Kaplan-Meier survival curves showed that the adverse event-free survival rate was lower in the H-nucMLKL group than in the L-nucMLKL group (P = 0.019 by the log-rank test). Such differences were not detected between groups divided by a median value of intercalated disc p-MLKL. In δ-sarcoglycan-deficient (Sgcd-/- ) mice, a model of DCM, total p-MLKL and nuclear p-MLKL levels were higher than in wild-type mice.

CONCLUSION

The results suggest that increased localization of nuclear p-MLKL in cardiomyocytes is associated with left ventricular diastolic dysfunction and future adverse events in DCM.

Biotinylation by antibody recognition-a method for proximity labeling

Identification of protein-protein interactions is a major goal of biological research. Despite technical advances over the last two decades, important but still largely unsolved challenges include the high-throughput detection of interactions directly from primary tissue and the identification of interactors of insoluble proteins that form higher-order structures. We have developed a novel, proximity-based labeling approach that uses antibodies to guide biotin deposition onto adjacent proteins in fixed cells and primary tissues. We showed our method to be specific and sensitive by labeling a mitochondrial matrix protein. Next, we used this method to profile the dynamic interactome of lamin A/C in multiple cell and tissue types under various treatment conditions. The ability to detect proximal proteins and putative interactors in intact tissues, and to quantify changes caused by different conditions or in the presence of disease mutations, can provide a new window into cell biology and disease pathogenesis.

Dilated cardiomyopathy mutations in δ-sarcoglycan exert a dominant-negative effect on cardiac myocyte mechanical stability

Delta-sarcoglycan is a component of the sarcoglycan subcomplex within the dystrophin-glycoprotein complex located at the plasma membrane of muscle cells. While recessive mutations in δ-sarcoglycan cause limb girdle muscular dystrophy 2F, dominant mutations in δ-sarcoglycan have been linked to inherited dilated cardiomyopathy (DCM). The purpose of this study was to investigate functional cellular defects present in adult cardiac myocytes expressing mutant δ-sarcoglycans harboring the dominant inherited DCM mutations R71T or R97Q. This study demonstrates that DCM mutant δ-sarcoglycans can be stably expressed in adult rat cardiac myocytes and traffic similarly to wild-type δ-sarcoglycan to the plasma membrane, without perturbing assembly of the dystrophin-glycoprotein complex. However, expression of DCM mutant δ-sarcoglycan in adult rat cardiac myocytes is sufficient to alter cardiac myocyte plasma membrane stability in the presence of mechanical strain. Upon cyclical cell stretching, cardiac myocytes expressing mutant δ-sarcoglycan R97Q or R71T have increased cell-impermeant dye uptake and undergo contractures at greater frequencies than myocytes expressing normal δ-sarcoglycan. Additionally, the R71T mutation creates an ectopic N-linked glycosylation site that results in aberrant glycosylation of the extracellular domain of δ-sarcoglycan. Therefore, appropriate glycosylation of δ-sarcoglycan may also be necessary for proper δ-sarcoglycan function and overall dystrophin-glycoprotein complex function. These studies demonstrate that DCM mutations in δ-sarcoglycan can exert a dominant negative effect on dystrophin-glycoprotein complex function leading to myocardial mechanical instability that may underlie the pathogenesis of δ-sarcoglycan-associated DCM.

Evolutionary diversification of the BetaM interactome acquired through co-option of the ATP1B4 gene in placental mammals

ATP1B4 genes represent a rare instance of orthologous vertebrate gene co-option that radically changed properties of the encoded BetaM proteins, which function as Na,K-ATPase subunits in lower vertebrates and birds. Eutherian BetaM has lost its ancestral function and became a muscle-specific resident of the inner nuclear membrane. Our earlier work implicated BetaM in regulation of gene expression through direct interaction with the transcriptional co-regulator SKIP. To gain insight into evolution of BetaM interactome we performed expanded screening of eutherian and avian cDNA libraries using yeast-two-hybrid and split-ubiquitin systems. The inventory of identified BetaM interactors includes lamina-associated protein LAP-1, myocyte nuclear envelope protein Syne1, BetaM itself, heme oxidases HMOX1 and HMOX2; transcription factor LZIP/CREB3, ERGIC3, PHF3, reticulocalbin-3, and β-sarcoglycan. No new interactions were found for chicken BetaM and human Na,K-ATPase β1, β2 and β3 isoforms, indicating the uniqueness of eutherian BetaM interactome. Analysis of truncated forms of BetaM indicates that residues 72-98 adjacent to the membrane in nucleoplasmic domain are important for the interaction with SKIP. These findings demonstrate that evolutionary alterations in structural and functional properties of eutherian BetaM proteins are associated with the increase in its interactome complexity.

A Haplotype of Two Novel Polymorphisms in δ-Sarcoglycan Gene Increases Risk of Dilated Cardiomyopathy in Mongoloid Population

The role of genetic abnormality of δ-sarcoglycan (δ-SG) gene in dilated (DCM) and hypertrophied (HCM) cardiomyopathy patients is still unfolding. In this study we first defined the promoter region and then searched for polymorphisms/mutations among the promoter, 5'-untranslated region, and the encoding exons in δ-SG gene in 104 Chinese patients with DCM, 145 with HCM, and 790 normal controls. Two novel polymorphisms were found, an 11 base-pair (bp) deletion (c._-100~-110; -) in the promoter region and a missense polymorphism of A848G resulting in p.Q283R in the highly conserved C-terminus. The prevalence of homozygous genotype -/- of c._-100~-110 was slightly higher in DCM (14.42%) and HCM patients (14.48%), as compared with normal controls (11.01%). The prevalence of genotype of 848A/G was significantly higher in DCM (6.73%; OR = 9.43; p = 0.0002), but not in HCM patients (1.38%; OR = 1.37; p = 0.62), as compared with controls (0.76%). Haplotype -_G consisting c._-100~-110 and A848G was associated with increased risk of DCM (OR = 17.27; 95%CI = 3.19–93.56; p = 0.001) but not associated with HCM (OR = 1.90; 95%CI = 0.38–9.55; p = 0.44). Co-occurrence of the genotypes -/- of c._-100~-110 and 848A/G was found in 5 patients with DCM (4.81%; OR = 39.85; p = 0.0001), none of HCM patients, and only 1 of the controls (0.13%). Both polymorphisms were also found in the Japanese population, but not in the Africans and Caucasians. C._-100~-110 resulted in a decrease of δ-SG promoter activity to 64±3% of the control level (p<0.01). Both co-immunoprecipitation and in vitro protein pull-down assays demonstrated that δ-SG-283R interacts normally to β- and γ-SG, but significantly decreased localization of β/δ/γ-SG on the plasma membrane. In conclusion, haplotype -_G composed of c._-100~-110 and A848G confers higher susceptibility to DCM in the Mongoloid population.

Genetic variation at the delta-sarcoglycan (SGCD) locus elevates heritable sympathetic nerve activity in human twin pairs

The Syrian Cardiomyopathic Hamster (BIO-14.6/53.58 strains) model of cardiac failure, resulting from naturally occurring deletion at the SGCD (delta-sarcoglycan) locus, displays widespread disturbances in catecholamine metabolism. Rare Mendelian myopathy disorders of human SGCD occur, although common naturally occurring SGCD genetic variation has not been evaluated for effects on human norepinephrine (NE) secretion. This study investigated the effect of SGCD genetic variation on control of NE secretion in healthy twin pairs. Genetic associations profiled SNPs across the SGCD locus. Trait heritability (h(2)) and genetic covariance (pleiotropy; shared h(2)) were evaluated. Sympathochromaffin exocytosis in vivo was probed in plasma by both catecholamines and Chromogranin B (CHGB). Plasma NE is substantially heritable (p = 3.19E-16, at 65.2 ± 5.0% of trait variance), sharing significant (p < 0.05) genetic determination with circulating and urinary catecholamines, CHGB, eGFR, and several cardio-metabolic traits. Participants with higher pNE showed significant (p < 0.05) differences in several traits, including increased BP and hypertension risk factors. Peak SGCD variant rs1835919 predicted elevated systemic vascular compliance, without changes in specifically myocardial traits. We used a chimeric-regulated secretory pathway photoprotein (CHGA-EAP) to evaluate the effect of SGCD on the exocytotic pathway in transfected PC12 cells; in transfected cells, expression of SGCD augmented CHGA trafficking into the exocytotic regulated secretory pathway. Thus, our investigation determined human NE secretion to be a highly heritable trait, influenced by common genetic variation within the SGCD locus. Circulating NE aggregates with BP and hypertension risk factors. In addition, coordinate NE and CHGB elevation by rs1835919 implicates exocytosis as the mechanism of release.

S151A δ-sarcoglycan mutation causes a mild phenotype of cardiomyopathy in mice

So far, the role of mutations in the δ-sarcogylcan (Sgcd) gene in causing autosomal dominant dilated cardiomyopathy (DCM) remains inconclusive. A p.S151A missense mutation in exon 6 of the Sgcd gene was reported to cause severe isolated autosomal dominant DCM without affecting skeletal muscle. This is controversial to our previous findings in a large consanguineous family where this p.S151A mutation showed no relevance for cardiac disease. In this study, the potential of the p.S151A mutation to cause DCM was investigated by using two different approaches: (1) engineering and characterization of heterozygous knock-in (S151A-) mice carrying the p.S151A mutation and (2) evaluation of the potential of adeno-associated virus (AAV) 9-based cardiac-specific transfer of p.S151A-mutated Sgcd cDNA to rescue the cardiac phenotype in Sgcd-deficient (Sgcd-null) mice as it has been demonstrated for intact, wild-type Sgcd cDNA. Heterozygous S151A knock-in mice developed a rather mild phenotype of cardiomyopathy. Increased heart to body weight suggests cardiac enlargement in 1-year-old S151A knock-in mice. However, at this age cardiac function, assessed by echocardiography, is maintained and histopathology completely absent. Myocardial expression of p.S151A cDNA, similar to intact Sgcd cDNA, restores cardiac function, although not being able to prevent myocardial histopathology in Sgcd-null mice completely. Our results suggest that the p.S151A mutation causes a mild, subclinical phenotype of cardiomyopathy, which is prone to be overseen in patients carrying such sequence variants. Furthermore, this study shows the suitability of an AAV-mediated cardiac gene transfer approach to analyze whether a sequence variant is a disease-causing mutation.

Sub-physiological sarcoglycan expression contributes to compensatory muscle protection in mdx mice

Sarcoglycans are a group of single-pass transmembrane glycoproteins. In striated muscle, sarcoglycans interact with dystrophin and other dystrophin-associated proteins (DAPs) to form the dystrophin-associated glycoprotein complex (DGC). The DGC protects the sarcolemma from contraction-induced injury. Duchenne muscular dystrophy (DMD) is caused by dystrophin gene mutations. In the absence of dystrophin, the DGC is disassembled from the sarcolemma. This initiates a chain reaction of muscle degeneration, necrosis, inflammation and fibrosis. In contrast to human patients, dystrophin-null mdx mice are only mildly affected. Enhanced muscle regeneration and the up-regulation of utrophin and integrin are thought to protect mdx muscle. Interestingly, trace amounts of sarcoglycans and other DAPs can be detected at the mdx sarcolemma. It is currently unclear whether sub-physiological sarcoglycan expression also contributes to the mild phenotype in mdx mice. To answer this question, we generated delta-sarcoglycan/dystrophin double knockout mice (delta-Dko) in which residual sarcoglycans were completely eliminated from the sarcolemma. Interestingly, utrophin levels were further increased in these mice. However, enhanced utrophin expression did not mitigate disease. The clinical manifestation of delta-Dko mice was worse than that of mdx mice. They showed characteristic dystrophic signs, body emaciation and more macrophage infiltration. Their lifespan was reduced by 60%. Furthermore, delta-Dko muscle generated significantly less absolute muscle force and became more susceptible to contraction-induced injury. Our results suggest that sub-physiological sarcoglycan expression plays a critical role in ameliorating muscle disease in mdx mice. We speculate that low-level sarcoglycan expression may represent a useful strategy to palliate DMD.

Disease rescue and increased lifespan in a model of cardiomyopathy and muscular dystrophy by combined AAV treatments

BACKGROUND

The BIO14.6 hamster is an excellent animal model for inherited cardiomyopathy, because of its lethal and well-documented course, due to a spontaneous deletion of delta-sarcoglycan gene promoter and first exon. The muscle disease is progressive and average lifespan is 11 months, because heart slowly dilates towards heart failure.

METHODOLOGY/PRINCIPAL FINDINGS

Based on the ability of adeno-associated viral (AAV) vectors to transduce heart together with skeletal muscle following systemic administration, we delivered human delta-sarcoglycan cDNA into male BIO14.6 hamsters by testing different ages of injection, routes of administration and AAV serotypes. Body-wide restoration of delta-SG expression was associated with functional reconstitution of the sarcoglycan complex and with significant lowering of centralized nuclei and fibrosis in skeletal muscle. Motor ability and cardiac functions were completely rescued. However, BIO14.6 hamsters having less than 70% of fibers recovering sarcoglycan developed cardiomyopathy, even if the total rescued protein was normal. When we used serotype 2/8 in combination with serotype 2/1, lifespan was extended up to 22 months with sustained heart function improvement.

CONCLUSIONS/SIGNIFICANCE

Our data support multiple systemic administrations of AAV as a general therapeutic strategy for clinical trials in cardiomyopathies and muscle disorders.

Does delta-sarcoglycan-associated autosomal-dominant cardiomyopathy exist?

In this study we clinically and genetically characterize a consanguineous family with a homozygous novel missense mutation in the delta-sarcoglycan gene and a second delta-sarcoglycan mutation that has previously been reported to cause severe autosomal-dominant dilated cardiomyopathy. We identified a novel missense mutation in exon 6 (p.A131P) of the delta-sarcoglycan gene, which in a homozygous state leads to the clinical picture of a limb girdle muscular dystrophy. In four heterozygous carriers for the mutation, aged 3-64 years, a second sequence variant in exon 6 (p.S151A) of the delta-sarcoglycan gene was detected on the other allele. This second missense change had previously been reported to be responsible for fatal autosomal-dominant dilated cardiomyopathy at young age. Comprehensive clinical and cardiac investigation in all of the compound heterozygous family members revealed no signs of cardiomyopathy or limb girdle muscular dystrophy. Our findings demonstrate that, even in the presence of a second disease-causing mutation, the p.S151A mutation in the delta-sarcoglycan gene does not result in cardiomyopathy. This finding questions the pathological relevance of this sequence variant for causing familial autosomal-dominant dilated cardiomyopathy and thereby the role of the delta-sarcoglycan gene in general as a disease-causing gene for autosomal-dominant dilated cardiomyopathy.

Lipid metabolism and diet: Possible mechanisms of slow aging

The ability to survive to an extremely old age is a consequence of complex interactions among genes, environment, lifestyle and luck. In the last two centuries, life expectancy in western countries has doubled, increasing from 40 to 81 years (79 for males and 82 for females). The candidate factors to determine such mortality reduction are reduced exposure to infections and the subsequent reduction in inflammatory responses, and to some extent, improvement in diet and nutrition. Among the people born at the beginning of the previous century, a small portion of individuals (1 in 10,000 born) have reached 100 years, surviving approximately 20 years more than the general population. The successful longevity of these individuals shows a familial component, possibly genetic, as underlined by the centenarian sibling's increased chance of reaching 100 years of age compared to the general population. Genetic studies on long living individuals have led to the discovery of potential genetic causes of extreme longevity. These discoveries have highlighted the role of lipid metabolism as a potential key player in the ability to survive to extreme old age. Additional studies on the longevity phenotype have confirmed the role of lipids and lipid-associated cell activities in the predisposition to longevity, from lower eukaryotes to humans. The main focus of this review is the appreciation of demographic survival data and changes in recent diet with the above mentioned genetic and phenotypic biomarkers of longevity, in order to elucidate hypotheses on mechanisms of slow aging and disease resistance.

Consequences of disrupting the dystrophin-sarcoglycan complex in cardiac and skeletal myopathy

Mutations that disrupt the dystrophin glycoprotein complex lead to plasma membrane instability of cardiomyocytes and skeletal muscle myofibers. Instability of the plasma membrane leads to degeneration largely due to activation of a necrotic process in these disorders. In response to ongoing degeneration, skeletal muscle exhibits robust regeneration while in cardiac muscle regeneration is not obvious. The dystrophin complex is concentrated along the plasma membrane in costameric structures that correspond to the Z bands of sarcomeres, thus positioning the dystrophin complex to transmit force between the sarcomere and the plasma membrane to the extracellular matrix. Although it is apparent that this position is important for perpendicular force transmission, it is clear that the dystrophin complex also fulfills signaling roles. Nitric oxide synthase and stress-induced signaling cascades are activated to participate in protection but may also contribute to pathology.

Torn apart: membrane rupture in muscular dystrophies and associated cardiomyopathies

Muscular dystrophies are often caused by mutations in cytoskeletal proteins that render cells more susceptible to strain-induced injury in mechanically active tissues such as skeletal or cardiac muscle. In this issue of the JCI, Han et al. report that dysferlin participates in membrane resealing in cardiomyocytes and that exercise results in increased membrane damage and disturbed cardiac function in dysferlin-deficient mice (see the related article beginning on page 1805). Thus, in addition to repetitive membrane damage, inadequate membrane repair may participate in the pathogenesis of muscular dystrophies and cardiomyopathies.

Reduced life span with heart and muscle dysfunction in Drosophila sarcoglycan mutants

In humans, genetically diverse forms of muscular dystrophy are associated with a disrupted sarcoglycan complex. The sarcoglycan complex resides at the muscle plasma membrane where it associates with dystrophin. There are six known sarcoglycan proteins in mammals whereas there are only three in Drosophila melanogaster. Using imprecise P element excision, we generated three different alleles at the Drosophila delta-sarcoglycan locus. Each of these deletions encompassed progressively larger regions of the delta-sarcoglycan gene. Line 840 contained a large deletion of the delta-sarcoglycan gene, and this line displayed progressive impairment in locomotive ability, reduced heart tube function and a shortened life span. In line 840, deletion of the Drosophila delta-sarcoglycan gene produced disrupted flight muscles with shortened sarcomeres and disorganized M lines. Unlike mammalian muscle where degeneration is coupled with ongoing regeneration, no evidence for regeneration was seen in this Drosophila sarcoglycan mutant. In contrast, line 28 was characterized with a much smaller deletion that affected only a portion of the cytoplasmic region of the delta-sarcoglycan protein and left intact the transmembrane and extracellular domains. Line 28 had a very mild phenotype with near normal life span, intact cardiac function and normal locomotive activity. Together, these data demonstrate the essential nature of the transmembrane and extracellular domains of Drosophila delta-sarcoglycan for normal muscle structure and function.