A mutation in the human phospholamban gene, deleting arginine 14, results in lethal, hereditary cardiomyopathy

Genetics of inherited cardiomyopathies in Africa

In sub-Saharan Africa (SSA), the burden of noncommunicable diseases (NCDs) is rising disproportionately in comparison to the rest of the world, affecting urban, semi-urban and rural dwellers alike. NCDs are predicted to surpass infections like human immunodeficiency virus, tuberculosis and malaria as the leading cause of mortality in SSA over the next decade. Heart failure (HF) is the dominant form of cardiovascular disease (CVD), and a leading cause of NCD in SSA. The main causes of HF in SSA are hypertension, cardiomyopathies, rheumatic heart disease, pericardial disease, and to a lesser extent, coronary heart disease. Of these, the cardiomyopathies deserve greater attention because of the relatively poor understanding of mechanisms of disease, poor outcomes and the disproportionate impact they have on young, economically active individuals. Morphofunctionally, cardiomyopathies are classified as dilated, hypertrophic, restrictive and arrhythmogenic; regardless of classification, at least half of these are inherited forms of CVD. In this review, we summarise all studies that have investigated the incidence of cardiomyopathy across Africa, with a focus on the inherited cardiomyopathies. We also review data on the molecular genetic underpinnings of cardiomyopathy in Africa, where there is a striking lack of studies reporting on the genetics of cardiomyopathy. We highlight the impact that genetic testing, through candidate gene screening, association studies and next generation sequencing technologies such as whole exome sequencing and targeted resequencing has had on the understanding of cardiomyopathy in Africa. Finally, we emphasise the need for future studies to fill large gaps in our knowledge in relation to the genetics of inherited cardiomyopathies in Africa.

Novel Basic Science Insights to Improve the Management of Heart Failure: Review of the Working Group on Cellular and Molecular Biology of the Heart of the Italian Society of Cardiology

Despite important advances in diagnosis and treatment, heart failure (HF) remains a syndrome with substantial morbidity and dismal prognosis. Although implementation and optimization of existing technologies and drugs may lead to better management of HF, new or alternative strategies are desirable. In this regard, basic science is expected to give fundamental inputs, by expanding the knowledge of the pathways underlying HF development and progression, identifying approaches that may improve HF detection and prognostic stratification, and finding novel treatments. Here, we discuss recent basic science insights that encompass major areas of translational research in HF and have high potential clinical impact.

Stem Cells in Cardiovascular Medicine: Historical Overview and Future Prospects

Cardiovascular diseases remain the leading cause of death in the developed world, accounting for more than 30% of all deaths. In a large proportion of these patients, acute myocardial infarction is usually the first manifestation, which might further progress to heart failure. In addition, the human heart displays a low regenerative capacity, leading to a loss of cardiomyocytes and persistent tissue scaring, which entails a morbid pathologic sequela. Novel therapeutic approaches are urgently needed. Stem cells, such as induced pluripotent stem cells or embryonic stem cells, exhibit great potential for cell-replacement therapy and an excellent tool for disease modeling, as well as pharmaceutical screening of novel drugs and their cardiac side effects. This review article covers not only the origin of stem cells but tries to summarize their translational potential, as well as potential risks and clinical translation.

Viral expression of a SERCA2a-activating PLB mutant improves calcium cycling and synchronicity in dilated cardiomyopathic hiPSC-CMs

There is increasing momentum toward the development of gene therapy for heart failure (HF) that is defined by impaired calcium (Ca²⁺) transport and reduced contractility. We have used FRET (fluorescence resonance energy transfer) between fluorescently-tagged SERCA2a (the cardiac Ca²⁺ pump) and PLB (phospholamban, ventricular peptide inhibitor of SERCA) to test directly the effectiveness of loss-of-inhibition/gain-of-binding (LOI/GOB) PLB mutants (PLB_M) that were engineered to compete with the binding of inhibitory wild-type PLB (PLB_WT). Our therapeutic strategy is to relieve PLB_WT inhibition of SERCA2a by using the reserve adrenergic capacity mediated by PLB to enhance cardiac contractility. Using a FRET assay, we determined that the combination of a LOI PLB mutation (L31A) and a GOB PLB mutation (I40A) results in a novel engineered LOI/GOB PLB_M (L31A/I40A) that effectively competes with PLB_WT binding to cardiac SERCA2a in HEK293-6E cells. We demonstrated that co-expression of PLB_M enhances SERCA Ca-ATPase activity by increasing enzyme Ca²⁺ affinity (1/K_Ca) in PLB_WT-inhibited HEK293 cell homogenates. For an initial assessment of PLB_M physiological effectiveness, we used human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs) from a healthy individual. In this system, we observed that adeno-associated virus 2 (rAAV2)-driven expression of PLB_M enhances the amplitude of SR Ca²⁺ release and the rate of SR Ca²⁺ re-uptake. To assess therapeutic potential, we used a hiPSC-CM model of dilated cardiomyopathy (DCM) containing PLB mutation R14del, where we observed that rAAV2-driven expression of PLB_M rescues arrhythmic Ca²⁺ transients and alleviates decreased Ca²⁺ transport. Thus, we propose that PLB_M transgene expression is a promising gene therapy strategy that directly targets the underlying pathophysiology of abnormal Ca²⁺ transport and thus contractility in underlying systolic heart failure.

Dilated cardiomyopathy: from epidemiologic to genetic phenotypes: A translational review of current literature

Dilated cardiomyopathy (DCM) is characterized by left ventricular dilatation and, consecutively, contractile dysfunction. The causes of DCM are heterogeneous. DCM often results from myocarditis, exposure to alcohol, drugs or other toxins and metabolic or endocrine disturbances. In about 35% of patients, genetic mutations can be identified that usually involve genes responsible for cytoskeletal, sarcomere and nuclear envelope proteins. Due to its heterogeneity, a detailed diagnostic work-up is necessary to identify the specific underlying cause and exclude other conditions with phenotype overlap. Patients with DCM show typical systolic heart failure symptoms, but, with progress of the disease, diastolic dysfunction is present as well. Depending on the underlying pathology, DCM patients also become apparent through arrhythmias, thromboembolic events or cardiogenic shock. Disease progression and prognosis are mostly driven by disease severity and reverse remodelling within the heart. The worst prognosis is seen in patients with lowest ejection fractions or severe diastolic dysfunction, leading to terminal heart failure with subsequent need for left ventricular assist device implantation or heart transplantation. Guideline-based heart failure medication and device therapy reduces the frequency of heart failure hospitalizations and improves survival.

Leducq Transatlantic Network of Excellence to Cure Phospholamban-Induced Cardiomyopathy (CURE-PLaN)

The deletion of Arginine 14 of the phosholamban gene (PLN p.R14del) is associated with the pathogenesis of an inherited form of cardiomyopathy with prominent arrhythmias. Patients carrying the PLN R14del mutation are at risk of developing dilated cardiomyopathy or arrhythmogenic right ventricular cardiomyopathy. Although the genetic etiology is well defined, the molecular mechanism underlying the pathogenesis of PLN R14del-cardiomyopathy is unknown. Our CURE PLaN network, funded by the Foundation Leducq, will bring together leading scientists, clinicians, and patients to elucidate the genotype-phenotype relationships in R14del cardiomyopathy with the ultimate goal of developing innovative disease-specific therapeutic modalities. With the generous support of the Leducq Foundation, our Transatlantic Network of Excellence consortium to cure Phospholamban (PLN)-induced cardiomyopathy (CURE-PLaN) unites 6 leading centers to address the current challenges associated with arrhythmogenic right ventricular cardiomyopathy/dilated cardiomyopathy (DCM) with an initial focus on PLN and development of effective treatments. The Network is led by Evangelia (Litsa) Kranias (University of Cincinnati) in the United States and Pieter A. Doevendans (Netherlands Heart Institute/UMC Utrecht NL) in Europe. The other US project leaders are Kevin Costa (Cardiovascular Research Center, Icahn School of Medicine at Mount Sinai, New York) and Mark Mercola and Ioannis Karakikes (Stanford University), who are focusing on induced pluripotent stem cell (iPSC)-based disease models, tissue engineering, gene therapy, and drug discovery. On the European side, the project leaders are Despina Sanoudou (Biomedical Research Foundation of the Academy of Athens) analyzing the PLN interactome and Stephan Lehnart (University of Gottingen) addressing the subcellular and disrupted protein interactions affected in PLN-mutant cardiomyocytes. Other key members within the Netherlands Heart Institute are Peter van Tintelen on PLN genetics, Folkert Asselbergs on epigenetics and Rudolf de Boer on clinical trials. We are also privileged to get support from Arthur Wilde (University of Amsterdam), Sakthivel Sadayappan (University of Cincinnati), and Roger Hajjar (Phospholamban Foundation), who have had a long-standing interest in cardiac physiology and pathophysiology with emphasis on underlying pathways and potential therapeutic targets. The consortium is also fortunate to embrace a patient advocate, Pieter Glijnis, incorporating the voice of the patients to research in every step. Our goal is to build and share a platform of patient data coupled with in vitro and in vivo models to promote scientific discovery and advance novel treatments. Phospholamban is a small phosphoprotein in the cardiac sarcoplasmic reticulum, and it is the major regulator of SERCA2a activity and calcium (Ca)-cycling. Chronic inhibition of SERCA2a by PLN has been implicated in the aberrant Ca-cycling of failing hearts. Studies in HF models have shown that decreasing PLN activity may rescue cardiac remodeling and dysfunction. Several human PLN mutations, leading to inhibition of Ca-uptake into the sarcoplasmic reticulum, are linked to inherited DCM.

Newly Discovered Micropeptide Regulators of SERCA Form Oligomers but Bind to the Pump as Monomers

The recently-discovered single-span transmembrane proteins endoregulin (ELN), dwarf open reading frame (DWORF), myoregulin (MLN), and another-regulin (ALN) are reported to bind to the SERCA calcium pump in a manner similar to that of known regulators of SERCA activity, phospholamban (PLB) and sarcolipin (SLN). To determine how micropeptide assembly into oligomers affects the availability of the micropeptide to bind to SERCA in a regulatory complex, we used co-immunoprecipitation and fluorescence resonance energy transfer (FRET) to quantify micropeptide oligomerization and SERCA-binding. Micropeptides formed avid homo-oligomers with high-order stoichiometry (n > 2 protomers per homo-oligomer), but it was the monomeric form of all micropeptides that interacted with SERCA. In view of these two alternative binding interactions, we evaluated the possibility that oligomerization occurs at the expense of SERCA-binding. However, even the most avidly oligomeric micropeptide species still showed robust FRET with SERCA, and there was a surprising positive correlation between oligomerization affinity and SERCA-binding. This comparison of micropeptide family members suggests that the same structural determinants that support oligomerization are also important for binding to SERCA. Moreover, the unique oligomerization/SERCA-binding profile of DWORF is in harmony with its distinct role as a PLB-competing SERCA activator, in contrast to the inhibitory function of the other SERCA-binding micropeptides.

Calcium as a Key Player in Arrhythmogenic Cardiomyopathy: Adhesion Disorder or Intracellular Alteration?

Arrhythmogenic cardiomyopathy (ACM) is an inherited heart disease characterized by sudden death in young people and featured by fibro-adipose myocardium replacement, malignant arrhythmias, and heart failure. To date, no etiological therapies are available. Mutations in desmosomal genes cause abnormal mechanical coupling, trigger pro-apoptotic signaling pathways, and induce fibro-adipose replacement. Here, we discuss the hypothesis that the ACM causative mechanism involves a defect in the expression and/or activity of the cardiac Ca²⁺ handling machinery, focusing on the available data supporting this hypothesis. The Ca²⁺ toolkit is heavily remodeled in cardiomyocytes derived from a mouse model of ACM defective of the desmosomal protein plakophilin-2. Furthermore, ACM-related mutations were found in genes encoding for proteins involved in excitation‒contraction coupling, e.g., type 2 ryanodine receptor and phospholamban. As a consequence, the sarcoplasmic reticulum becomes more eager to release Ca²⁺, thereby inducing delayed afterdepolarizations and impairing cardiac contractility. These data are supported by preliminary observations from patient induced pluripotent stem-cell-derived cardiomyocytes. Assessing the involvement of Ca²⁺ signaling in the pathogenesis of ACM could be beneficial in the treatment of this life-threatening disease.

Cardiac Genetic Predisposition in Sudden Infant Death Syndrome

BACKGROUND

Sudden infant death syndrome (SIDS) is a leading cause of postneonatal mortality. Genetic heart diseases (GHDs) underlie some cases of SIDS.

OBJECTIVES

This study aimed to determine the spectrum and prevalence of GHD-associated mutations as a potential monogenic basis for SIDS.

METHODS

A cohort of 419 unrelated SIDS cases (257 male; average age 2.7 ± 1.9 months) underwent whole exome sequencing and a targeted analysis of 90 GHD-susceptibility genes. The yield of "potentially informative," ultra-rare variants (minor allele frequency <0.00005) in GHD-associated genes was assessed.

RESULTS

Overall, 53 of 419 (12.6%) SIDS cases had ≥1 "potentially informative," GHD-associated variant. The yield was 14.9% (21 of 141) for mixed-European ancestry cases and 11.5% (32 of 278) for European ancestry SIDS cases. Infants older than 4 months were more likely to host a "potentially informative" GHD-associated variant. There was significant overrepresentation of ultra-rare nonsynonymous variants in European SIDS cases (18 of 278 [6.5%]) versus European control subjects (30 of 973 [3.1%]; p = 0.013) when combining all 4 major cardiac channelopathy genes (KCNQ1, KCNH2, SCN5A, and RYR2). According to the American College of Medical Genetics guidelines, only 18 of 419 (4.3%) SIDS cases hosted a "pathogenic" or "likely pathogenic" variant.

CONCLUSIONS

Less than 15% of more than 400 SIDS cases had a "potentially informative" variant in a GHD-susceptibility gene, predominantly in the 4- to 12-month age group. Only 4.3% of cases possessed immediately clinically actionable variants. Consistent with previous studies, ultra-rare, nonsynonymous variants within the major cardiac channelopathy-associated genes were overrepresented in SIDS cases in infants of European ethnicity. These findings have major implications for the investigation of SIDS cases and families.

AKAP6 and phospholamban colocalize and interact in HEK-293T cells and primary murine cardiomyocytes

Phospholamban (PLN) is an important Ca²⁺ modulator at the sarcoplasmic reticulum (SR) of striated muscles. It physically interacts and inhibits sarcoplasmic reticulum Ca²⁺ATPase (SERCA2) function, whereas a protein kinase A (PKA)‐dependent phosphorylation at its serine 16 reverses the inhibition. The underlying mechanism of this post‐translational modification, however, remains not fully understood. Using publicly available databases, we identified A‐kinase anchoring protein 6 (AKAP6) as a candidate that might play some roles in PLN phosphorylation. Immunofluorescence showed colocalization between GFP‐AKAP6 and PLN in transfected HEK‐293T cells and cultured mouse neonatal cardiomyocytes (CMNCs). Co‐immunoprecipitation confirmed the functional interaction between AKAP6 and PLN in HEK‐293T and isolated adult rat cardiomyocytes in response to isoproterenol stimulation. Functionally, AKAP6 promoted Ca²⁺ uptake activity of SERCA1 in cotransfected HEK‐293T cells despite the presence of PLN. These results were further confirmed in adult rat cardiomyocytes. Immunofluorescence showed colocalization of both proteins around the perinuclear region, while protein–protein interaction was corroborated by immunoprecipitation of the nucleus‐enriched fraction of rat hearts. Our findings suggest AKAP6 as a novel interacting partner to PLN in HEK‐293T and murine cardiomyocytes.

2019 HRS expert consensus statement on evaluation, risk stratification, and management of arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is an arrhythmogenic disorder of the myocardium not secondary to ischemic, hypertensive, or valvular heart disease. ACM incorporates a broad spectrum of genetic, systemic, infectious, and inflammatory disorders. This designation includes, but is not limited to, arrhythmogenic right/left ventricular cardiomyopathy, cardiac amyloidosis, sarcoidosis, Chagas disease, and left ventricular noncompaction. The ACM phenotype overlaps with other cardiomyopathies, particularly dilated cardiomyopathy with arrhythmia presentation that may be associated with ventricular dilatation and/or impaired systolic function. This expert consensus statement provides the clinician with guidance on evaluation and management of ACM and includes clinically relevant information on genetics and disease mechanisms. PICO questions were utilized to evaluate contemporary evidence and provide clinical guidance related to exercise in arrhythmogenic right ventricular cardiomyopathy. Recommendations were developed and approved by an expert writing group, after a systematic literature search with evidence tables, and discussion of their own clinical experience, to present the current knowledge in the field. Each recommendation is presented using the Class of Recommendation and Level of Evidence system formulated by the American College of Cardiology and the American Heart Association and is accompanied by references and explanatory text to provide essential context. The ongoing recognition of the genetic basis of ACM provides the opportunity to examine the diverse triggers and potential common pathway for the development of disease and arrhythmia.

Genetic Variants Are Not Rare in ICD Candidates with Dilated Cardiomyopathy: Time for Next-Generation Sequencing?

BACKGROUND

Sudden cardiac death (SCD) risk stratification in dilated cardiomyopathy (DCM) has been based on left ventricular ejection fraction (LVEF), even though SCD may occur with LVEF > 35%. Family history of unexplained SCD, especially in the young, raises concern about potential inheritable risk factors. It remains largely unknown how genetic tests can be integrated into clinical practice, particularly in the selection of implantable cardioverter defibrillator (ICD) candidates. We aimed to assess the diagnostic yield of genetic testing in DCM patients with a class I recommendation for ICD implantation, based on current guidelines.

METHODS

We included ambulatory stable adult patients with idiopathic or familial DCM with previously implanted ICD. Molecular analysis included 15 genes (LMNA, MYH7, MYBPC3, TNNT2, ACTC1, TPM1, CSRP3, TCAP, SGCD, PLN, MYL2, MYL3, TNNI3, TAZ, and LDB3) using next-generation sequencing.

RESULTS

We evaluated 21 patients, 12 (57%) males and 9 (43%) with familial DCM, including 3 (14%) with a family history of premature unexplained SCD. Mean age at DCM diagnosis was 40 ± 2 years, and mean age at ICD implantation was 50 ± 12 years. LVEF was 27 ± 9%, and LV end-diastolic diameter was 65 ± 7 mm. Genetic variants were found in six (29%) patients, occurring in 5 genes: TPM1, TNNT2, MYH7, PLN, and MYBPC3. The majority were classified as variants of uncertain significance. Family history of SCD was present in both patients with PLN variants.

CONCLUSION

In patients with DCM and ICD, genetic variants could be identified in a significant proportion of patients in several genes, highlighting the potential role of genetics in DCM SCD risk stratification.

Relevance of Titin Missense and Non-Frameshifting Insertions/Deletions Variants in Dilated Cardiomyopathy

Recent advancements in next generation sequencing (NGS) technology have led to the identification of the giant sarcomere gene, titin (TTN), as a major human disease gene. Truncating variants of TTN (TTNtv) especially in the A-band region account for 20% of dilated cardiomyopathy (DCM) cases. Much attention has been focused on assessment and interpretation of TTNtv in human disease; however, missense and non-frameshifting insertions/deletions (NFS-INDELs) are difficult to assess and interpret in clinical diagnostic workflow. Targeted sequencing covering all exons of TTN was performed on a cohort of 530 primary DCM patients from three cardiogenetic centres across Europe. Using stringent bioinformatic filtering, twenty-nine and two rare TTN missense and NFS-INDELs variants predicted deleterious were identified in 6.98% and 0.38% of DCM patients, respectively. However, when compared with those identified in the largest available reference population database, no significant enrichment of such variants was identified in DCM patients. Moreover, DCM patients and reference individuals had comparable frequencies of splice-region missense variants with predicted splicing alteration. DCM patients and reference populations had comparable frequencies of rare predicted deleterious TTN missense variants including splice-region missense variants suggesting that these variants are not independently causative for DCM. Hence, these variants should be classified as likely benign in the clinical diagnostic workflow, although a modifier effect cannot be excluded at this stage.

Phospholamban cardiomyopathy: a Canadian perspective on a unique population

Introduction

Phospholamban cardiomyopathy is an inherited cardiomyopathy, characterised by a defect in regulation of the sarcoplasmic reticulum Ca²⁺ pump, often presenting with malignant arrhythmias and progressive cardiac dysfunction occurring at a young age.

Methods

Phospholamban R14del mutation carriers and family members were identified from inherited arrhythmia clinics at 13 sites across Canada. Cardiac investigations, including electrocardiograms, Holter monitoring (premature ventricular complexes, PVCs), and imaging results were summarised.

Results

Fifty patients (10 families) were identified (median age 30 years, range 3–71, 46% female). Mutation carriers were more likely to be older, have low-voltage QRS, T‑wave inversion, frequent PVCs, and cardiac dysfunction, compared to unaffected relatives. Increasing age, low-voltage QRS, T‑wave inversion, late potentials, and frequent PVCs were predictors of cardiac dysfunction (p < 0.05 for all). Older carriers (age ≥45 years) were more likely to have disease manifestations than were their younger counterparts, with disease onset occurring at an older age in Canadian patients and their Dutch counterparts.

Discussion

Among Canadian patients with phospholamban cardiomyopathy, clinical manifestations resembled those of their Dutch counterparts, with increasing age a major predictor of disease manifestation. Older mutation carriers were more likely to have electrical and structural abnormalities, and may represent variable expressivity, age-dependent penetrance, or genetic heterogeneity among Canadian patients.

Electronic supplementary material

The online version of this article (10.1007/s12471-019-1247-0) contains supplementary material, which is available to authorized users.

Prevalence and cardiac phenotype of patients with a phospholamban mutation

Pathogenic mutations in the phospholamban (PLN) gene may give rise to inherited cardiomyopathies due to its role in calcium homeostasis. Several PLN mutations have been identified, with the R14del mutation being the most prevalent cardiomyopathy-related mutation in the Netherlands. It is present in patients diagnosed with arrhythmogenic cardiomyopathy as well as dilated cardiomyopathy. Awareness of the phenotype of this PLN mutation is of great importance, since many carriers remain to be identified. Patients with the R14del mutation are characterised by older age at onset, low-voltage electrocardiograms and a high frequency of ventricular arrhythmias. Additionally, these patients have a poor prognosis often with left ventricular dysfunction and early-onset heart failure. Therefore, when there is a suspicion of a PLN mutation, cardiac and genetic screening is strongly recommended.

Proarrhythmic Remodeling of Calcium Homeostasis in Cardiac Disease; Implications for Diabetes and Obesity

A rapid growth in the incidence of diabetes and obesity has transpired to a major heath issue and economic burden in the postindustrial world, with more than 29 million patients affected in the United States alone. Cardiovascular defects have been established as the leading cause of mortality and morbidity of diabetic patients. Over the last decade, significant progress has been made in delineating mechanisms responsible for the diminished cardiac contractile function and enhanced propensity for malignant cardiac arrhythmias characteristic of diabetic disease. Rhythmic cardiac contractility relies upon the precise interplay between several cellular Ca²⁺ transport protein complexes including plasmalemmal L-type Ca²⁺ channels (LTCC), Na⁺-Ca²⁺ exchanger (NCX1), Sarco/endoplasmic Reticulum (SR) Ca²⁺-ATPase (SERCa2a) and ryanodine receptors (RyR2s), the SR Ca²⁺ release channels. Here we provide an overview of changes in Ca²⁺ homeostasis in diabetic ventricular myocytes and discuss the therapeutic potential of targeting Ca²⁺ handling proteins in the prevention of diabetes-associated cardiomyopathy and arrhythmogenesis.

Proarrhythmic Remodeling of Calcium Homeostasis in Cardiac Disease; Implications for Diabetes and Obesity

A rapid growth in the incidence of diabetes and obesity has transpired to a major heath issue and economic burden in the postindustrial world, with more than 29 million patients affected in the United States alone. Cardiovascular defects have been established as the leading cause of mortality and morbidity of diabetic patients. Over the last decade, significant progress has been made in delineating mechanisms responsible for the diminished cardiac contractile function and enhanced propensity for malignant cardiac arrhythmias characteristic of diabetic disease. Rhythmic cardiac contractility relies upon the precise interplay between several cellular Ca²⁺ transport protein complexes including plasmalemmal L-type Ca²⁺ channels (LTCC), Na⁺-Ca²⁺ exchanger (NCX1), Sarco/endoplasmic Reticulum (SR) Ca²⁺-ATPase (SERCa2a) and ryanodine receptors (RyR2s), the SR Ca²⁺ release channels. Here we provide an overview of changes in Ca²⁺ homeostasis in diabetic ventricular myocytes and discuss the therapeutic potential of targeting Ca²⁺ handling proteins in the prevention of diabetes-associated cardiomyopathy and arrhythmogenesis.

Proarrhythmic Remodeling of Calcium Homeostasis in Cardiac Disease; Implications for Diabetes and Obesity

A rapid growth in the incidence of diabetes and obesity has transpired to a major heath issue and economic burden in the postindustrial world, with more than 29 million patients affected in the United States alone. Cardiovascular defects have been established as the leading cause of mortality and morbidity of diabetic patients. Over the last decade, significant progress has been made in delineating mechanisms responsible for the diminished cardiac contractile function and enhanced propensity for malignant cardiac arrhythmias characteristic of diabetic disease. Rhythmic cardiac contractility relies upon the precise interplay between several cellular Ca²⁺ transport protein complexes including plasmalemmal L-type Ca²⁺ channels (LTCC), Na⁺-Ca²⁺ exchanger (NCX1), Sarco/endoplasmic Reticulum (SR) Ca²⁺-ATPase (SERCa2a) and ryanodine receptors (RyR2s), the SR Ca²⁺ release channels. Here we provide an overview of changes in Ca²⁺ homeostasis in diabetic ventricular myocytes and discuss the therapeutic potential of targeting Ca²⁺ handling proteins in the prevention of diabetes-associated cardiomyopathy and arrhythmogenesis.

Engineering CRISPR/Cpf1 with tRNA promotes genome editing capability in mammalian systems

CRISPR/Cpf1 features a number of properties that are distinct from CRISPR/Cas9 and provides an excellent alternative to Cas9 for genome editing. To date, genome engineering by CRISPR/Cpf1 has been reported only in human cells and mouse embryos of mammalian systems and its efficiency is ultimately lower than that of Cas9 proteins from Streptococcus pyogenes. The application of CRISPR/Cpf1 for targeted mutagenesis in other animal models has not been successfully verified. In this study, we designed and optimized a guide RNA (gRNA) transcription system by inserting a transfer RNA precursor (pre-tRNA) sequence downstream of the gRNA for Cpf1, protecting gRNA from immediate digestion by 3'-to-5' exonucleases. Using this new gRNAtRNA system, genome editing, including indels, large fragment deletion and precise point mutation, was induced in mammalian systems, showing significantly higher efficiency than the original Cpf1-gRNA system. With this system, gene-modified rabbits and pigs were generated by embryo injection or somatic cell nuclear transfer (SCNT) with an efficiency comparable to that of the Cas9 gRNA system. These results demonstrated that this refined gRNAtRNA system can boost the targeting capability of CRISPR/Cpf1 toolkits.

Phospholamban regulates nuclear Ca2+ stores and inositol 1,4,5-trisphosphate mediated nuclear Ca2+ cycling in cardiomyocytes

AIMS

Phospholamban (PLB) is the key regulator of the cardiac Ca²⁺ pump (SERCA2a)-mediated sarcoplasmic reticulum Ca²⁺ stores. We recently reported that PLB is highly concentrated in the nuclear envelope (NE) from where it can modulate perinuclear Ca²⁺ handling of the cardiomyocytes (CMs). Since inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R) mediates nuclear Ca²⁺ release, we examined whether the nuclear pool of PLB regulates IP₃-induced nuclear Ca²⁺ handling.

METHODS AND RESULTS

Fluo-4 based confocal Ca²⁺ imaging was performed to measure Ca²⁺ dynamics across both nucleus and cytosol in saponin-permeabilized CMs isolated from wild-type (WT) or PLB-knockout (PLB-KO) mice. At diastolic intracellular Ca²⁺ ([Ca²⁺]_i = 100 nM), the Fab fragment of the monoclonal PLB antibody (anti-PLB Fab) facilitated the formation and increased the length of spontaneous Ca²⁺ waves (SCWs) originating from the nuclear region in CMs from WT but not from PLB-KO mice. We next examined nuclear Ca²⁺ activities at basal condition and after sequential addition of IP₃, anti-PLB Fab, and the IP₃R inhibitor 2-aminoethoxydiphenyl borate (2-APB) at a series of [Ca²⁺]_i. In WT mice, at 10 nM [Ca²⁺]_i where ryanodine receptor (RyR2) based spontaneous Ca²⁺ sparks rarely occurred, IP₃ increased fluorescence amplitude (F/F₀) of overall nuclear region to 1.19 ± 0.02. Subsequent addition of anti-PLB Fab significantly decreased F/F₀ to 1.09 ± 0.02. At 50 nM [Ca²⁺]_i, anti-PLB Fab not only decreased the overall nuclear F/F₀ previously elevated by IP₃, but also increased the amplitude and duration of spark-like nuclear Ca²⁺ release events. These nuclear Ca²⁺ releases were blocked by 2-APB. At 100 nM [Ca²⁺]_i, IP₃ induced short SCWs originating from nucleus. Anti-PLB Fab transformed those short waves into long SCWs with propagation from the nucleus into the cytosol. In contrast, neither nuclear nor cytosolic Ca²⁺ dynamics was affected by anti-PLB Fab in CMs from PLB-KO mice in all these conditions. Furthermore, in WT CMs pretreated with RyR2 blocker tetracaine, IP₃ and anti-PLB Fab still increased the magnitude of nuclear Ca²⁺ release but failed to regenerate SCWs. Finally, anti-PLB Fab increased low Ca²⁺ affinity mag-fluo 4 fluorescence intensity in the lumen of NE of nuclei isolated from WT but not in PLB-KO mice.

CONCLUSION

PLB regulates nuclear Ca²⁺ handling. By increasing Ca²⁺ uptake into lumen of the NE and perhaps other perinuclear membranes, the acute reversal of PLB inhibition decreases global Ca²⁺ concentration at rest in the nucleoplasm, and increases Ca²⁺ release into the nucleus, through mechanisms involving IP₃R and RyR2 in the vicinity.

Translating emerging molecular genetic insights into clinical practice in inherited cardiomyopathies

Cardiomyopathies are primarily genetic disorders of the myocardium associated with higher risk of life-threatening cardiac arrhythmias, heart failure, and sudden cardiac death. The evolving knowledge in genomic medicine during the last decade has reshaped our understanding of cardiomyopathies as diseases of multifactorial nature and complex pathophysiology. Genetic testing in cardiomyopathies has subsequently grown from primarily a research tool into an essential clinical evaluation piece with important clinical implications for patients and their families. The purpose of this review is to provide with a contemporary insight into the implications of genetic testing in diagnosis, therapy, and prognosis of patients with inherited cardiomyopathies. Here, we summarize the contemporary knowledge on genotype-phenotype correlations in inherited cardiomyopathies and highlight the recent significant achievements in the field of translational cardiovascular genetics.

Complex roads from genotype to phenotype in dilated cardiomyopathy: scientific update from the Working Group of Myocardial Function of the European Society of Cardiology

Dilated cardiomyopathy (DCM) frequently affects relatively young, economically, and socially active adults, and is an important cause of heart failure and transplantation. DCM is a complex disease and its pathological architecture encounters many genetic determinants interacting with environmental factors. The old perspective that every pathogenic gene mutation would lead to a diseased heart, is now being replaced by the novel observation that the phenotype depends not only on the penetrance-malignancy of the mutated gene-but also on epigenetics, age, toxic factors, pregnancy, and a diversity of acquired diseases. This review discusses how gene mutations will result in mutation-specific molecular alterations in the heart including increased mitochondrial oxidation (sarcomeric gene e.g. TTN), decreased calcium sensitivity (sarcomeric genes), fibrosis (e.g. LMNA and TTN), or inflammation. Therefore, getting a complete picture of the DCM patient will include genomic data, molecular assessment by preference from cardiac samples, stratification according to co-morbidities, and phenotypic description. Those data will help to better guide the heart failure and anti-arrhythmic treatment, predict response to therapy, develop novel siRNA-based gene silencing for malignant gene mutations, or intervene with mutation-specific altered gene pathways in the heart.This article is part of the Mini Review Series from the Varenna 2017 meeting of the Working Group of Myocardial Function of the European Society of Cardiology.

Unique Ca2+-Cycling Protein Abundance and Regulation Sustains Local Ca2+ Releases and Spontaneous Firing of Rabbit Sinoatrial Node Cells

Spontaneous beating of the heart pacemaker, the sinoatrial node, is generated by sinoatrial node cells (SANC) and caused by gradual change of the membrane potential called diastolic depolarization (DD). Submembrane local Ca²⁺ releases (LCR) from sarcoplasmic reticulum (SR) occur during late DD and activate an inward Na⁺/Ca²⁺ exchange current, which accelerates the DD rate leading to earlier occurrence of an action potential. A comparison of intrinsic SR Ca²⁺ cycling revealed that, at similar physiological Ca²⁺ concentrations, LCRs are large and rhythmic in permeabilized SANC, but small and random in permeabilized ventricular myocytes (VM). Permeabilized SANC spontaneously released more Ca²⁺ from SR than VM, despite comparable SR Ca²⁺ content in both cell types. In this review we discuss specific patterns of expression and distribution of SR Ca²⁺ cycling proteins (SR Ca²⁺ ATPase (SERCA2), phospholamban (PLB) and ryanodine receptors (RyR)) in SANC and ventricular myocytes. We link ability of SANC to generate larger and rhythmic LCRs with increased abundance of SERCA2, reduced abundance of the SERCA inhibitor PLB. In addition, an increase in intracellular [Ca²⁺] increases phosphorylation of both PLB and RyR exclusively in SANC. The differences in SR Ca²⁺ cycling protein expression between SANC and VM provide insights into diverse regulation of intrinsic SR Ca²⁺ cycling that drives automaticity of SANC.

Current perspectives on the diagnosis and management of dilated cardiomyopathy Beyond heart failure: a Cardiomyopathy Clinic Doctor's point of view

Left ventricular enlargement and dysfunction are fundamental components of dilated cardiomyopathy (DCM). DCM is a major cause of heart failure and cardiac transplantation. A wide variety of etiologies underlie acquired and familial DCM. Familial disease is reported in 20% to 35% of cases. A genetic substrate is recognized in at least 30% of familial cases. A recently proposed scheme defines DCM as a continuum of subclinical and clinical phenotypes. The evolution of classification systems permitted use of effective treatment strategies in disorders sharing the same structural and functional characteristics and common clinical expression. The major causes of death are progressive heart failure and sudden cardiac death secondary to ventricular arrhythmias or less commonly bradyarrhythmias. Remarkable progress has been made in survival owing to well-defined evidence-based therapies and appropriate guidelines for risk stratification and sudden cardiac death prevention measures. Neurohormonal antagonists and device therapy decreased all-cause mortality in adult patients with DCM. However, additional red flags in diagnosis have to be addressed in everyday practice, and cardiologists have to be aware of the subsequent effect on risk stratification and treatment plan. Genetic substrate cannot be modified, but the presence of a peculiar type of gene mutation modifies thresholds for implantable cardioverter defibrillator (ICD) implantation. DCM is part of the spectrum of heart failure which is a syndrome with certain morphological and functional characteristics. Although significant progress has been achieved in the management of patients with DCM, it seems that the future treatments of this entity will be related to the specific pathological substrate.

Functional and transcriptomic insights into pathogenesis of R9C phospholamban mutation using human induced pluripotent stem cell-derived cardiomyocytes

Dilated cardiomyopathy (DCM) can be caused by mutations in the cardiac protein phospholamban (PLN). We used CRISPR/Cas9 to insert the R9C PLN mutation at its endogenous locus into a human induced pluripotent stem cell (hiPSC) line from an individual with no cardiovascular disease. R9C PLN hiPSC-CMs display a blunted β-agonist response and defective calcium handling. In 3D human engineered cardiac tissues (hECTs), a blunted lusitropic response to β-adrenergic stimulation was observed with R9C PLN. hiPSC-CMs harboring the R9C PLN mutation showed activation of a hypertrophic phenotype, as evidenced by expression of hypertrophic markers and increased cell size and capacitance of cardiomyocytes. RNA-seq suggests that R9C PLN results in an altered metabolic state and profibrotic signaling, which was confirmed by gene expression analysis and picrosirius staining of R9C PLN hECTs. The expression of several miRNAs involved in fibrosis, hypertrophy, and cardiac metabolism were also perturbed in R9C PLN hiPSC-CMs. This study contributes to better understanding of the pathogenic mechanisms of the hereditary R9C PLN mutation in the context of human cardiomyocytes.

Insertional Mutagenesis Confounds the Mechanism of the Morbid Phenotype of a PLNR9C Transgenic Mouse Line

BACKGROUND

A mouse line with heterozygous transgenic expression of phospholamban carrying a substitution of cysteine for arginine 9 (TgPLN^R9C) under the control of α-myosin heavy chain (αMHC) promoter features dilated cardiomyopathy, heart failure, and premature death.

METHODS AND RESULTS

Determination of transgene chromosomal localization by conventional methods shows that in this line the transgenic array of 13 PLN^R9C expression cassettes, arranged in a head-to-tail tandem orientation, have integrated into the bidirectional promoter of the αMHC (Myh6) gene and the gene for the regulatory noncoding RNA Myheart (Mhrt), both of which are known to be involved in cardiac development and pathology. Expression of the noncoding RNA Mhrt in TgPLN^R9C mice exhibits profound deregulation, despite the presence of the second, intact allele.

CONCLUSIONS

The TgPLN^R9C mouse strain is, in the best case, a functionally ambiguous phenocopy of the human PLN^R9C heterozygote, because a similar constellation of genetically programmed events can not occur in a patient. Publications featuring "cardiac-specific overexpression" are focused on the phenotype and typically forgo the definition of the transgene integration site or transgene temporal expression profile, so caution should be exercised in attributing clinical relevance to pathologic phenomena observed in αMHC-driven transgenes.

The SarcoEndoplasmic Reticulum Calcium ATPase

The calcium pump (a.k.a. Ca²⁺-ATPase or SERCA) is a membrane transport protein ubiquitously found in the endoplasmic reticulum (ER) of all eukaryotic cells. As a calcium transporter, SERCA maintains the low cytosolic calcium level that enables a vast array of signaling pathways and physiological processes (e.g. synaptic transmission, muscle contraction, fertilization). In muscle cells, SERCA promotes relaxation by pumping calcium ions from the cytosol into the lumen of the sarcoplasmic reticulum (SR), the main storage compartment for intracellular calcium. X-ray crystallographic studies have provided an extensive understanding of the intermediate states that SERCA populates as it progresses through the calcium transport cycle. Historically, SERCA is also known to be regulated by small transmembrane peptides, phospholamban (PLN) and sarcolipin (SLN). PLN is expressed in cardiac muscle, whereas SLN predominates in skeletal and atrial muscle. These two regulatory subunits play critical roles in cardiac contractility. While our understanding of these regulatory mechanisms are still developing, SERCA and PLN are one of the best understood examples of peptide-transporter regulatory interactions. Nonetheless, SERCA appeared to have only two regulatory subunits, while the related sodium pump (a.k.a. Na⁺, K⁺-ATPase) has at least nine small transmembrane peptides that provide tissue specific regulation. The last few years have seen a renaissance in our understanding of SERCA regulatory subunits. First, structures of the SERCA-SLN and SERCA-PLN complexes revealed molecular details of their interactions. Second, an array of micropeptides concealed within long non-coding RNAs have been identified as new SERCA regulators. This chapter will describe our current understanding of SERCA structure, function, and regulation.

When signalling goes wrong: pathogenic variants in structural and signalling proteins causing cardiomyopathies

Cardiomyopathies are a diverse group of cardiac disorders with distinct phenotypes, depending on the proteins and pathways affected. A substantial proportion of cardiomyopathies are inherited and those will be the focus of this review article. With the wide application of high-throughput sequencing in the practice of clinical genetics, the roles of novel genes in cardiomyopathies are recognised. Here, we focus on a subgroup of cardiomyopathy genes [TTN, FHL1, CSRP3, FLNC and PLN, coding for Titin, Four and a Half LIM domain 1, Muscle LIM Protein, Filamin C and Phospholamban, respectively], which, despite their diverse biological functions, all have important signalling functions in the heart, suggesting that disturbances in signalling networks can contribute to cardiomyopathies.

Dilated Cardiomyopathy: Genetic Determinants and Mechanisms

Nonischemic dilated cardiomyopathy (DCM) often has a genetic pathogenesis. Because of the large number of genes and alleles attributed to DCM, comprehensive genetic testing encompasses ever-increasing gene panels. Genetic diagnosis can help predict prognosis, especially with regard to arrhythmia risk for certain subtypes. Moreover, cascade genetic testing in family members can identify those who are at risk or with early stage disease, offering the opportunity for early intervention. This review will address diagnosis and management of DCM, including the role of genetic evaluation. We will also overview distinct genetic pathways linked to DCM and their pathogenetic mechanisms. Historically, cardiac morphology has been used to classify cardiomyopathy subtypes. Determining genetic variants is emerging as an additional adjunct to help further refine subtypes of DCM, especially where arrhythmia risk is increased, and ultimately contribute to clinical management.

Tead1 is required for maintaining adult cardiomyocyte function, and its loss results in lethal dilated cardiomyopathy

Heart disease remains the leading cause of death worldwide, highlighting a pressing need to identify novel regulators of cardiomyocyte (CM) function that could be therapeutically targeted. The mammalian Hippo/Tead pathway is critical in embryonic cardiac development and perinatal CM proliferation. However, the requirement of Tead1, the transcriptional effector of this pathway, in the adult heart is unknown. Here, we show that tamoxifen-inducible adult CM-specific Tead1 ablation led to lethal acute-onset dilated cardiomyopathy, associated with impairment in excitation-contraction coupling. Mechanistically, we demonstrate Tead1 is a cell-autonomous, direct transcriptional activator of SERCA2a and SR-associated protein phosphatase 1 regulatory subunit, Inhibitor-1 (I-1). Thus, Tead1 deletion led to a decrease in SERCA2a and I-1 transcripts and protein, with a consequent increase in PP1-activity, resulting in accumulation of dephosphorylated phospholamban (Pln) and decreased SERCA2a activity. Global transcriptomal analysis in Tead1-deleted hearts revealed significant changes in mitochondrial and sarcomere-related pathways. Additional studies demonstrated there was a trend for correlation between protein levels of TEAD1 and I-1, and phosphorylation of PLN, in human nonfailing and failing hearts. Furthermore, TEAD1 activity was required to maintain PLN phosphorylation and expression of SERCA2a and I-1 in human induced pluripotent stem cell-derived (iPS-derived) CMs. To our knowledge, taken together, this demonstrates a nonredundant, novel role of Tead1 in maintaining normal adult heart function.

Additional value of screening for minor genes and copy number variants in hypertrophic cardiomyopathy

INTRODUCTION

Hypertrophic cardiomyopathy (HCM) is the most prevalent inherited heart disease. Next-generation sequencing (NGS) is the preferred genetic test, but the diagnostic value of screening for minor and candidate genes, and the role of copy number variants (CNVs) deserves further evaluation.

METHODS

Three hundred and eighty-seven consecutive unrelated patients with HCM were screened for genetic variants in the 5 most frequent genes (MYBPC3, MYH7, TNNT2, TNNI3 and TPM1) using Sanger sequencing (N = 84) or NGS (N = 303). In the NGS cohort we analyzed 20 additional minor or candidate genes, and applied a proprietary bioinformatics algorithm for detecting CNVs. Additionally, the rate and classification of TTN variants in HCM were compared with 427 patients without structural heart disease.

RESULTS

The percentage of patients with pathogenic/likely pathogenic (P/LP) variants in the main genes was 33.3%, without significant differences between the Sanger sequencing and NGS cohorts. The screening for 20 additional genes revealed LP variants in ACTC1, MYL2, MYL3, TNNC1, GLA and PRKAG2 in 12 patients. This approach resulted in more inconclusive tests (36.0% vs. 9.6%, p<0.001), mostly due to variants of unknown significance (VUS) in TTN. The detection rate of rare variants in TTN was not significantly different to that found in the group of patients without structural heart disease. In the NGS cohort, 4 patients (1.3%) had pathogenic CNVs: 2 deletions in MYBPC3 and 2 deletions involving the complete coding region of PLN.

CONCLUSIONS

A small percentage of HCM cases without point mutations in the 5 main genes are explained by P/LP variants in minor or candidate genes and CNVs. Screening for variants in TTN in HCM patients drastically increases the number of inconclusive tests, and shows a rate of VUS that is similar to patients without structural heart disease, suggesting that this gene should not be analyzed for clinical purposes in HCM.

Clinical and Mechanistic Insights Into the Genetics of Cardiomyopathy

Over the last quarter-century, there has been tremendous progress in genetics research that has defined molecular causes for cardiomyopathies. More than a thousand mutations have been identified in many genes with varying ontologies, therein indicating the diverse molecules and pathways that cause hypertrophic, dilated, restrictive, and arrhythmogenic cardiomyopathies. Translation of this research to the clinic via genetic testing can precisely group affected patients according to molecular etiology, and identify individuals without evidence of disease who are at high risk for developing cardiomyopathy. These advances provide insights into the earliest manifestations of cardiomyopathy and help to define the molecular pathophysiological basis for cardiac remodeling. Although these efforts remain incomplete, new genomic technologies and analytic strategies provide unparalleled opportunities to fully explore the genetic architecture of cardiomyopathies. Such data hold the promise that mutation-specific pathophysiology will uncover novel therapeutic targets, and herald the beginning of precision therapy for cardiomyopathy patients.

Molecular mechanisms in cardiomyopathy

Cardiomyopathies represent a heterogeneous group of diseases that negatively affect heart function. Primary cardiomyopathies specifically target the myocardium, and may arise from genetic [hypertrophic cardiomyopathy (HCM), arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D), mitochondrial cardiomyopathy] or genetic and acquired [dilated cardiomyopathy (DCM), restrictive cardiomyopathy (RCM)] etiology. Modern genomics has identified mutations that are common in these populations, while in vitro and in vivo experimentation with these mutations have provided invaluable insight into the molecular mechanisms native to these diseases. For example, increased myosin heavy chain (MHC) binding and ATP utilization lead to the hypercontractile sarcomere in HCM, while abnormal protein–protein interaction and impaired Ca2+ flux underlie the relaxed sarcomere of DCM. Furthermore, expanded access to genetic testing has facilitated identification of potential risk factors that appear through inheritance and manifest sometimes only in the advanced stages of the disease. In this review, we discuss the genetic and molecular abnormalities unique to and shared between these primary cardiomyopathies and discuss some of the important advances made using more traditional basic science experimentation.

Calcium Signaling and Cardiac Arrhythmias

There has been a significant progress in our understanding of the molecular mechanisms by which calcium (Ca2+) ions mediate various types of cardiac arrhythmias. A growing list of inherited gene defects can cause potentially lethal cardiac arrhythmia syndromes, including catecholaminergic polymorphic ventricular tachycardia, congenital long QT syndrome, and hypertrophic cardiomyopathy. In addition, acquired deficits of multiple Ca2+-handling proteins can contribute to the pathogenesis of arrhythmias in patients with various types of heart disease. In this review article, we will first review the key role of Ca2+ in normal cardiac function-in particular, excitation-contraction coupling and normal electric rhythms. The functional involvement of Ca2+ in distinct arrhythmia mechanisms will be discussed, followed by various inherited arrhythmia syndromes caused by mutations in Ca2+-handling proteins. Finally, we will discuss how changes in the expression of regulation of Ca2+ channels and transporters can cause acquired arrhythmias, and how these mechanisms might be targeted for therapeutic purposes.

Current insights into LMNA cardiomyopathies: Existing models and missing LINCs

The nuclear lamina is a critical structural domain for the maintenance of genomic stability and whole-cell mechanics. Mutations in the LMNA gene, which encodes nuclear A-type lamins lead to the disruption of these key cellular functions, resulting in a number of devastating diseases known as laminopathies. Cardiomyopathy is a common laminopathy and is highly penetrant with poor prognosis. To date, cell mechanical instability and dysregulation of gene expression have been proposed as the main mechanisms driving cardiac dysfunction, and indeed discoveries in these areas have provided some promising leads in terms of therapeutics. However, important questions remain unanswered regarding the role of lamin A dysfunction in the heart, including a potential role for the toxicity of lamin A precursors in LMNA cardiomyopathy, which has yet to be rigorously investigated.

A Sarcoplasmic Reticulum Localized Protein Phosphatase Regulates Phospholamban Phosphorylation and Promotes Ischemia Reperfusion Injury in the Heart

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PP2Ce is Ser-Thr phosphatase specifically localized on SR and expressed in cardiomyocytes.

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PP2Ce has specific phosphatase activity to dephosphorylate Thr-17 site of phospholamban.

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PP2Ce expression is induced upon pathological stress, including beta-AR stimulation and ROS.

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PP2Ce induction suppresses cardiomyocyte calcium cycling, reduces beta-AR-induced contractility, and promotes oxidative ischemia/reperfusion injury.

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PP2Ce is a new molecular component of stress-mediated cardiomyocyte calcium regulation.

Phospholamban (PLN) is a key regulator of sarcolemma calcium uptake in cardiomyocyte; its inhibitory activity to sarcolemma-endoplasmic reticulum calcium ATPase is regulated by phosphorylation. PLN hypophosphorylation is a common molecular feature in the failing heart. The current study provided evidence at the molecular, cellular, and whole-heart levels to implicate a sarcolemma membrane-targeted protein phosphatase, PP2Ce, as a specific and potent PLN phosphatase. PP2Ce expression was elevated in failing human heart and induced acutely at protein level by β-adrenergic stimulation or oxidative stress in cardiomyocytes. PP2Ce expression in mouse heart blunted β-adrenergic response and exacerbated ischemia/reperfusion injury. Therefore, PP2Ce is a new regulator for cardiac function and pathogenesis.

Potential new mechanisms of pro-arrhythmia in arrhythmogenic cardiomyopathy: focus on calcium sensitive pathways

Arrhythmogenic cardiomyopathy, or its most well-known subform arrhythmogenic right ventricular cardiomyopathy (ARVC), is a cardiac disease mainly characterised by a gradual replacement of the myocardial mass by fibrous and fatty tissue, leading to dilatation of the ventricular wall, arrhythmias and progression towards heart failure. ARVC is commonly regarded as a disease of the intercalated disk in which mutations in desmosomal proteins are an important causative factor. Interestingly, the Dutch founder mutation PLN R14Del has been identified to play an additional, and major, role in ARVC patients within the Netherlands. This is remarkable since the phospholamban (PLN) protein plays a leading role in regulation of the sarcoplasmic reticulum calcium load rather than in the establishment of intercellular integrity. In this review we outline the intracellular cardiac calcium dynamics and relate pathophysiological signalling, induced by disturbed calcium handling, with activation of calmodulin dependent kinase II (CaMKII) and calcineurin A (CnA). We postulate a thus far unrecognised role for Ca2+ sensitive signalling proteins in maladaptive remodelling of the macromolecular protein complex that forms the intercalated disk, during pro-arrhythmic remodelling of the heart.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Phospholamban is concentrated in the nuclear envelope of cardiomyocytes and involved in perinuclear/nuclear calcium handling

AIMS

Phospholamban (PLB) regulates the cardiac Ca²⁺-ATPase (SERCA2a) in sarcoplasmic reticulum (SR). However, the localization of PLB at subcellular sites outside the SR and possible contributions to Ca²⁺ cycling remain unknown. We examined the intracellular distribution of PLB and tested whether a pool of PLB exists in the nuclear envelope (NE) that might regulate perinuclear/nuclear Ca²⁺ (nCa²⁺) handling in cardiomyocytes (CMs).

METHODS AND RESULTS

Using confocal immunofluorescence microscopy and immunoblot analyses of CMs and CM nuclei, we discovered that PLB was highly concentrated in NE. Moreover, the ratio of PLB levels to SERCA levels was greater in NE than in SR. The increased levels of PLB in NE were a consistent finding using a range of antibodies, tissue samples, and species. To address a possible role in affecting Ca²⁺ handling, we used Fluo-4 based confocal Ca²⁺ imaging, with scan-lines across cytosol and nuclei, and evaluated the effects of PLB on cytosolic and nCa²⁺ uptake and release in mouse CMs. In intact CMs, isoproterenol increased amplitude and decreased the decay time of Ca²⁺ transients not only in cytosol but also in nuclear regions. In saponin-permeabilized mouse CMs ([Ca²⁺]_i=400nM), we measured spontaneous Ca²⁺ waves after specific reversal of PLB activity by addition of the Fab fragment of an anti-PLB monoclonal antibody (100μg/ml). This highly selective immunological reagent enhanced Ca²⁺ uptake (faster decay times) and Ca²⁺ release (greater intensity) in both cytosol and across the nuclear regions.

CONCLUSIONS

Besides SR, PLB is concentrated in NE of CMs, and may be involved in modulation of nCa²⁺ dynamics.