Induction of Ankrd1 in Dilated Cardiomyopathy Correlates with the Heart Failure Progression

Ankrd1 inhibits the FAK/Rho-GTPase/F-actin pathway by downregulating ITGA6 transcriptional to regulate myoblast functions

Skeletal muscle constitutes the largest percentage of tissue in the animal body and plays a pivotal role in the development of normal life activities in the organism. However, the regulation mechanism of skeletal muscle growth and development remains largely unclear. This study investigated the effects of Ankrd1 on the proliferation and differentiation of C2C12 myoblasts. Here, we identified Ankrd1 as a potential regulator of muscle cell development, and found that Ankrd1 knockdown resulted in the proliferation ability decrease but the differentiation level increase of C2C12 cells. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyzes as well as RNA-seq results showed that Ankrd1 knockdown activated focal adhesion kinase (FAK)/F-actin signal pathway with most genes significantly enriched in this pathway upregulated. The integrin subunit Itga6 promoter activity is increased when Ankrd1 knockdown, as demonstrated by a dual-luciferase reporter assay. This study revealed the molecular mechanism by which Ankrd1 knockdown enhanced FAK phosphorylation activity through the alteration of integrin subunit levels, thus activating FAK/Rho-GTPase/F-actin signal pathway, eventually promoting myoblast differentiation. Our data suggested that Ankrd1 might serve as a potential regulator of muscle cell development. Our findings provide new insights into skeletal muscle growth and development and valuable references for further study of human muscle-related diseases.

Exploring TTN variants as genetic insights into cardiomyopathy pathogenesis and potential emerging clues to molecular mechanisms in cardiomyopathies

The giant protein titin (TTN) is a sarcomeric protein that forms the myofibrillar backbone for the components of the contractile machinery which plays a crucial role in muscle disorders and cardiomyopathies. Diagnosing TTN pathogenic variants has important implications for patient management and genetic counseling. Genetic testing for TTN variants can help identify individuals at risk for developing cardiomyopathies, allowing for early intervention and personalized treatment strategies. Furthermore, identifying TTN variants can inform prognosis and guide therapeutic decisions. Deciphering the intricate genotype-phenotype correlations between TTN variants and their pathologic traits in cardiomyopathies is imperative for gene-based diagnosis, risk assessment, and personalized clinical management. With the increasing use of next-generation sequencing (NGS), a high number of variants in the TTN gene have been detected in patients with cardiomyopathies. However, not all TTN variants detected in cardiomyopathy cohorts can be assumed to be disease-causing. The interpretation of TTN variants remains challenging due to high background population variation. This narrative review aimed to comprehensively summarize current evidence on TTN variants identified in published cardiomyopathy studies and determine which specific variants are likely pathogenic contributors to cardiomyopathy development.

VEGF-B hypertrophy predisposes to transition from diastolic to systolic heart failure in hypertensive rats

AIMS

Cardiac energy metabolism is centrally involved in heart failure (HF), although the direction of the metabolic alterations is complex and likely dependent on the particular stage of HF progression. Vascular endothelial growth factor B (VEGF-B) has been shown to modulate metabolic processes and to induce physiological cardiac hypertrophy; thus, it could be cardioprotective in the failing myocardium. This study investigates the role of VEGF-B in cardiac proteomic and metabolic adaptation in HF during aldosterone and high-salt hypertensive challenges.

METHODS AND RESULTS

Male rats overexpressing the cardiac-specific VEGF-B transgene (VEGF-B TG) were treated for 3 or 6 weeks with deoxycorticosterone-acetate combined with a high-salt (HS) diet (DOCA + HS) to induce hypertension and cardiac damage. Extensive longitudinal echocardiographic studies of HF progression were conducted, starting at baseline. Sham-treated rats served as controls. To evaluate the metabolic alterations associated with HF, cardiac proteomics by mass spectrometry was performed. Hypertrophic non-treated VEGF-B TG hearts demonstrated high oxygen and adenosine triphosphate (ATP) demand with early onset of diastolic dysfunction. Administration of DOCA + HS to VEGF-B TG rats for 6 weeks amplified the progression from cardiac hypertrophy to HF, with a drastic drop in heart ATP concentration. Dobutamine stress echocardiographic analyses uncovered a significantly impaired systolic reserve. Mechanistically, the hallmark of the failing TG heart was an abnormal energy metabolism with decreased mitochondrial ATP, preceding the attenuated cardiac performance and leading to systolic HF.

CONCLUSIONS

This study shows that the VEGF-B TG accelerates metabolic maladaptation which precedes structural cardiomyopathy in experimental hypertension and ultimately leads to systolic HF.

Genetic Ablation of Ankrd1 Mitigates Cardiac Damage during Experimental Autoimmune Myocarditis in Mice

Myocarditis (MC) is an inflammatory disease of the myocardium that can cause sudden death in the acute phase, and dilated cardiomyopathy (DCM) with chronic heart failure as its major long-term outcome. However, the molecular mechanisms beyond the acute MC phase remain poorly understood. The ankyrin repeat domain 1 (ANKRD1) is a functionally pleiotropic stress/stretch-inducible protein, which can modulate cardiac stress response during various forms of pathological stimuli; however, its involvement in post-MC cardiac remodeling leading to DCM is not known. To address this, we induced experimental autoimmune myocarditis (EAM) in ANKRD1-deficient mice, and evaluated post-MC consequences at the DCM stage mice hearts. We demonstrated that ANKRD1 does not significantly modulate heart failure; nevertheless, the genetic ablation of Ankrd1 blunted the cardiac damage/remodeling and preserved heart function during post-MC DCM.

Adriamycin-Induced Podocyte Injury Disrupts the YAP-TEAD1 Axis and Downregulates Cyr61 and CTGF Expression

The most severe forms of kidney diseases are often associated with irreversible damage to the glomerular podocytes, the highly specialized epithelial cells that encase glomerular capillaries and regulate the removal of toxins and waste from the blood. Several studies revealed significant changes to podocyte cytoskeletal structure during disease onset, suggesting possible roles of cellular mechanosensing in podocyte responses to injury. Still, this topic remains underexplored partly due to the lack of appropriate in vitro models that closely recapitulate human podocyte biology. Here, we leveraged our previously established method for the derivation of mature podocytes from human induced pluripotent stem cells (hiPSCs) to help uncover the roles of yes-associated protein (YAP), a transcriptional coactivator and mechanosensor, in podocyte injury response. We found that while the total expression levels of YAP remain relatively unchanged during Adriamycin (ADR)-induced podocyte injury, the YAP target genes connective tissue growth factor (CTGF) and cysteine-rich angiogenic inducer 61 (Cyr61) are significantly downregulated. Intriguingly, TEAD1 is significantly downregulated in podocytes injured with ADR. By examining multiple independent modes of cellular injury, we found that CTGF and Cyr61 expression are downregulated only when podocytes were exposed to molecules known to disrupt the cell's mechanical integrity or cytoskeletal structure. To our knowledge, this is the first report that the YAP-TEAD1 signaling axis is disrupted when stem cell-derived human podocytes experience biomechanical injury. Together, these results could help improve the understanding of kidney disease mechanisms and highlight CTGF and Cyr61 as potential therapeutic targets or biomarkers for patient stratification.

Spatiotemporal transcriptomics reveals pathogenesis of viral myocarditis

A significant fraction of sudden death in children and young adults is due to viral myocarditis, an inflammatory disease of the heart. In this study, by using integrated single-cell and spatial transcriptomics, we created a high-resolution, spatially resolved transcriptome map of reovirus-induced myocarditis in neonatal mouse hearts. We assayed hearts collected at three timepoints after infection and studied the temporal, spatial and cellular heterogeneity of host–virus interactions. We further assayed the intestine, the primary site of reovirus infection, to establish a full chronology of molecular events that ultimately lead to myocarditis. We found that inflamed endothelial cells recruit cytotoxic T cells and undergo pyroptosis in the myocarditic tissue. Analyses of spatially restricted gene expression in myocarditic regions and the border zone identified immune-mediated cell-type-specific injury and stress responses. Overall, we observed a complex network of cellular phenotypes and spatially restricted cell–cell interactions associated with reovirus-induced myocarditis in neonatal mice.

Understanding the molecular basis of cardiomyopathy

Inherited cardiomyopathies are a major cause of mortality and morbidity worldwide and can be caused by mutations in a wide range of proteins located in different cellular compartments. The present review is based on Dr. Ju Chen's 2021 Robert M. Berne Distinguished Lectureship of the American Physiological Society Cardiovascular Section, in which he provided an overview of the current knowledge on the cardiomyopathy-associated proteins that have been studied in his laboratory. The review provides a general summary of the proteins in different compartments of cardiomyocytes associated with cardiomyopathies, with specific focus on the proteins that have been studied in Dr. Chen's laboratory.

Baoyuan decoction (BYD) attenuates cardiac hypertrophy through ANKRD1-ERK/GATA4 pathway in heart failure after acute myocardial infarction

BACKGROUND

The pathological cardiac functions of ankyrin repeat domain 1 (ANKRD1) in left ventricle can directly aggravate cardiac hypertrophy (CH) and fibrosis through the activation of extracellular signal-regulated kinase (ERK)/ transcription factor GATA binding protein 4 (GATA4) pathway, and subsequently contribute to heart failure (HF). Baoyuan Decoction (BYD), which is a famous classic Chinese medicinal formulation, has shown impressive cardioprotective effects clinically and experimentally. However, the knowledge is still limited in its underlying mechanisms against HF.

PURPOSE

To explore whether BYD plays a protective role against HF by attenuating CH via the ANKRD1-ERK/GATA4 pathway.

METHODS

In vivo, HF rat models were prepared using left anterior descending coronary artery (LADCA) ligation. Rats in the BYD group were administered a dosage of 2.57 g/kg of BYD for 28 days, while in the positive control group rats were given 4.67 mg/kg of Fosinopril. In vitro, a hypertrophic model was constructed by stimulating H9C2 cells with 1 uM Ang II. An ANKRD1-overexpressing cell model was established through transient transfection of ANKRD1 plasmid into H9C2 cells. Subsequently, BYD intervention was performed on the cell models to further elucidate its effects and underlying mechanism.

RESULTS

In vivo results showed that BYD significantly improved cardiac function and inhibited pathological hypertrophy and fibrosis in a rat model of HF post-acute myocardial infarction (AMI). Noticeably, label-free proteomic analysis demonstrated that BYD could reverse the CH-related biological turbulences, mainly through ANKRD1-ERK/GATA4 pathway. Further in vitro results validated that the hypertrophy was attenuated by BYD through suppression of AT1R, ANKRD1, Calpain1, p-ERK1/2 and p-GATA4. The results of in vitro model indicated that BYD could reverse the outcome of transfected over-expression of ANKRD1, including down-regulated expressions of ANKRD1, p-ERK1/2 and p-GATA4.

CONCLUSION

BYD ameliorates CH and improves HF through the ANKRD1-ERK/GATA4 pathway, providing a novel therapeutic option for the treatment of HF.

Cardiac ankyrin repeat protein contributes to dilated cardiomyopathy and heart failure

Cardiac ankyrin repeat protein (CARP) is a cardiac-specific stress-response protein which exerts diverse effects to modulate cardiac remodeling in response to pathological stimuli. We examined the role of CARP in postnatal cardiac development and function under basal conditions in mice. Transgenic mice that selectively overexpressed CARP in heart (CARP Tg) exhibited dilated cardiac chambers, impaired heart function, and cardiac fibrosis as assessed by echocardiography and histological staining. Furthermore, the mice had a shorter lifespan and reduced survival rate in response to ischemic acute myocardial infarction. Immunofluorescence demonstrated the overexpressed CARP protein was predominantly accumulated in the nuclei of cardiomyocytes. Microarray analysis revealed that the nuclear localization of CARP was associated with the suppression of calcium-handling proteins. In vitro experiments revealed that CARP overexpression resulted in decreased cell contraction and calcium transient. In post-mortem cardiac specimens from patients with dilated cardiomyopathy and end-stage heart failure, CARP was significantly increased. Taken together, our data identified CARP as a crucial contributor in dilated cardiomyopathy and heart failure which was associated with its regulation of calcium-handling proteins.

The stress responsive gene ankrd1a is dynamically regulated during skeletal muscle development and upregulated following cardiac injury in border zone cardiomyocytes in adult zebrafish

Ankyrin repeat domain 1 (ANKRD1) is a functionally pleiotropic protein found in the nuclei and sarcomeres of cardiac and skeletal muscles, with a proposed role in linking myofibrilar stress and transcriptional regulation. Rapid upregulation of its expression in response to both physiological and pathological stress supports the involvement of ANKRD1 in muscle tissue adaptation and remodeling. However, the exact role of ANKRD1 remains poorly understood. To begin to investigate its function at higher resolution, we have generated and characterized a TgBAC(ankrd1a:EGFP) zebrafish line. This reporter line displays transgene expression in slow skeletal muscle fibers during development and exercise responsiveness in adult cardiac muscle. To better understand the role of Ankrd1a in pathological conditions in adult zebrafish, we assessed ankrd1a expression after cardiac ventricle cryoinjury and observed localized upregulation in cardiomyocytes in the border zone. We show that this expression in injured hearts is recapitulated by the TgBAC(ankrd1a:EGFP) reporter. Our results identify novel expression domains of ankrd1a and suggest an important role for Ankrd1a in the early stress response and regeneration of cardiac tissue. This new reporter line will help decipher the role of Ankrd1a in striated muscle stress response, including after cardiac injury.

Atypical ALPK2 kinase is not essential for cardiac development and function

Protein kinases play an integral role in cardiac development, function, and disease. Recent experimental and clinical data have implied that protein kinases belonging to a family of atypical α-protein kinases, including α-protein kinase 2 (ALPK2), are important for regulating cardiac development and maintaining function via regulation of WNT signaling. A recent study in zebrafish reported that loss of ALPK2 leads to severe cardiac defects; however, the relevance of ALPK2 has not been studied in a mammalian animal model. To assess the role of ALPK2 in the mammalian heart, we generated two independent global Alpk2-knockout (Alpk2-gKO) mouse lines, using CRISPR/Cas9 technology. We performed physiological and biochemical analyses of Alpk2-gKO mice to determine the functional, morphological, and molecular consequences of Alpk2 deletion at the organismal level. We found that Alpk2-gKO mice exhibited normal cardiac function and morphology up to one year of age. Moreover, we did not observe altered WNT signaling in neonatal Alpk2-gKO mouse hearts. In conclusion, Alpk2 is dispensable for cardiac development and function in the murine model. Our results suggest that Alpk2 is a rapidly evolving gene that lost its essential cardiac functions in mammals.NEW & NOTEWORTHY Several studies indicated the importance of ALPK2 for cardiac function and development. A recent study in zebrafish report that loss of ALPK2 leads to severe cardiac defects. In contrast, murine Alpk2-gKO models developed in this work display no overt cardiac phenotype. Our results suggest ALPK2, as a rapidly evolving gene, lost its essential cardiac functions in mammals.

Ankrd2 in Mechanotransduction and Oxidative Stress Response in Skeletal Muscle: New Cues for the Pathogenesis of Muscular Laminopathies

Ankrd2 (ankyrin repeats containing domain 2) or Arpp (ankyrin repeat, PEST sequence, and proline-rich region) is a member of the muscle ankyrin repeat protein family. Ankrd2 is mostly expressed in skeletal muscle, where it plays an intriguing role in the transcriptional response to stress induced by mechanical stimulation as well as by cellular reactive oxygen species. Our studies in myoblasts from Emery-Dreifuss muscular dystrophy 2, a LMNA-linked disease affecting skeletal and cardiac muscles, demonstrated that Ankrd2 is a lamin A-binding protein and that mutated lamins found in Emery-Dreifuss muscular dystrophy change the dynamics of Ankrd2 nuclear import, thus affecting oxidative stress response. In this review, besides describing the latest advances related to Ankrd2 studies, including novel discoveries on Ankrd2 isoform-specific functions, we report the main findings on the relationship of Ankrd2 with A-type lamins and discuss known and potential mechanisms involving defective Ankrd2-lamin A interplay in the pathogenesis of muscular laminopathies.

Ankyrin repeat domain 1: A novel gene for cardiac septal defects

INTRODUCTION

Cardiac septal defects account for more than 50% of congenital heart defects. Ankyrin repeat domain 1 (ANKRD1) is an important transcription factor that is mutated in multiple cardiac diseases; however, a relationship between the ANKRD1 mutation and cardiac septal defects has not been described.

METHODS

We examined genetic mutations in a large family with three cardiac septal defect patients. Whole exome sequencing, bioinformatics and conservation analysis were utilized to predict the pathogenicity of candidate mutations. Dual luciferase reporter assay and nuclear localization experiments were performed to evaluate the influence of target mutation.

RESULTS

A heterozygous, missense variant of ANKRD1 (MIM* 609599): NM_014391: exon6: c.C560T:p.S187F was identified at a highly conserved region. Sanger sequencing in extended family members demonstrated an incomplete inheritance model. When co-activated with NKX2.5, ANKRD1 repressed ANF expression as assessed by a dual-luciferase reporter assay, and p.S187F mutation enhanced the repressive effect (0.318 ± 0.018 versus 0.564 ± 0.048, p < 0.01). A real-time polymerase chain reaction confirmed that p.S187F mutation of ANKRD1 decreased the expression of endogenous ANF (0.85 ± 0.05 versus 0.61 ± 0.04, p < 0.01). Furthermore, nuclear localization experiments demonstrated that the mutation significantly decreased the nuclear distribution of ANKRD1.

CONCLUSIONS

The present study is the first to identify the p.S187F mutant of ANKRD1, which is associated with cardiac septal defects.

Osteogenesis and angiogenesis are simultaneously enhanced in BMP2-/VEGF-transfected adipose stem cells through activation of the YAP/TAZ signaling pathway

While bone has the capability to heal itself, there is a great difficulty in reconstituting large bone defects created by heavy trauma or the resection of malignant tumors.

Ankyrin Repeat Domain 1 Overexpression is Associated with Common Resistance to Afatinib and Osimertinib in EGFR-mutant Lung Cancer

Overcoming acquired resistance to epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs) is critical in combating EGFR-mutant non-small cell lung cancer (NSCLC). We tried to construct a novel therapeutic strategy to conquer the resistance to second-and third-generation EGFR-TKIs in EGFR-positive NSCLC patients. We established afatinib- and osimertinib-resistant lung adenocarcinoma cell lines. Exome sequencing, cDNA array and miRNA microarray were performed using the established cell lines to discover novel therapeutic targets associated with the resistance to second-and third-generation EGFR-TKIs. We found that ANKRD1 which is associated with the epithelial-mesenchymal transition (EMT) phenomenon and anti-apoptosis, was overexpressed in the second-and third-generation EGFR-TKIs-resistant cells at the mRNA and protein expression levels. When ANKRD1 was silenced in the EGFR-TKIs-resistant cell lines, afatinib and osimertinib could induce apoptosis of the cell lines. Imatinib could inhibit ANKRD1 expression, resulting in restoration of the sensitivity to afatinib and osimertinib of EGFR-TKI-resistant cells. In EGFR-mutant NSCLC patients, ANKRD1 was overexpressed in the tumor after the failure of EGFR-TKI therapy, especially after long-duration EGFR-TKI treatments. ANKRD1 overexpression which was associated with EMT features and anti-apoptosis, was commonly involved in resistance to second-and third-generation EGFR-TKIs. ANKRD1 inhibition could be a promising therapeutic strategy in EGFR-mutant NSCLC patients.

Integration of Metabonomics and Transcriptomics Reveals the Therapeutic Effects and Mechanisms of Baoyuan Decoction for Myocardial Ischemia

Myocardial ischemia (MI) is an escalating public health care burden worldwide. Baoyuan decoction (BYD) is a traditional Chinese medicine formula with cardioprotective activity; however, its pharmacological characteristics and mechanisms are obscured. Herein, a multi-omics strategy via incorporating the metabonomics, transcriptomics, and pharmacodynamics was adopted to investigate the effects and molecular mechanisms of BYD for treating MI in a rat model of left anterior descending coronary artery (LADCA) ligation. The results indicated that BYD has a significantly cardioprotective role against MI by decreasing the infarct size, converting the echocardiographic abnormalities and myocardial enzyme markers, and reversing the serum metabolic disorders and myocardial transcriptional perturbations resulting from MI. Integrated bioinformatics analysis and literature reports constructed the interaction network based on the changes of the key MI targeted-metabolites and transcripts after BYD treatment and disclosed that the cardioprotection of BYD is mainly involved in the regulation of energy homeostasis, oxidative stress, apoptosis, inflammation, cardiac contractile dysfunction, and extracellular matrix remodeling. The results of histopathological examination, quantitative RT-PCR assay, cardiac energy synthesis, and serum antioxidant assessment complemented the multi-omics findings, and indicated the multi-pathway modulation mechanisms of BYD. Our investigation demonstrated that the multi-omics approach could achieve a complementary and verified view for the comprehensive evaluation of therapeutic effects and complex mechanisms of TCMF like BYD.

A personalized, multiomics approach identifies genes involved in cardiac hypertrophy and heart failure

A traditional approach to investigate the genetic basis of complex diseases is to identify genes with a global change in expression between diseased and healthy individuals. However, population heterogeneity may undermine the effort to uncover genes with significant but individual contribution to the spectrum of disease phenotypes within a population. Here we investigate individual changes of gene expression when inducing hypertrophy and heart failure in 100 + strains of genetically distinct mice from the Hybrid Mouse Diversity Panel (HMDP). We find that genes whose expression fold-change correlates in a statistically significant way with the severity of the disease are either up or down-regulated across strains, and therefore missed by a traditional population-wide analysis of differential gene expression. Furthermore, those "fold-change" genes are enriched in human cardiac disease genes and form a dense co-regulated module strongly interacting with the cardiac hypertrophic signaling network in the human interactome. We validate our approach by showing that the knockdown of Hes1, predicted as a strong candidate, induces a dramatic reduction of hypertrophy by 80-90% in neonatal rat ventricular myocytes. Our results demonstrate that individualized approaches are crucial to identify genes underlying complex diseases as well as to develop personalized therapies.

The Role of Serum Adiponectin for Outcome Prediction in Patients with Dilated Cardiomyopathy and Advanced Heart Failure

Clinical interpretation of patients' plasma adiponectin (APN) remains challenging; its value as biomarker in dilated cardiomyopathy (DCM) is equivocal. We evaluated whether circulating APN level is an independent predictor of composite outcome: death, left ventricle assist device (LVAD) implantation, and heart transplantation (HT) in patients with nonischemic DCM. 57 patients with nonischemic DCM (average LV diastolic diameter 6.85 cm, LV ejection fraction 26.63%, and pulmonary capillary wedge pressure 22.06  mmHg) were enrolled. Patients underwent echocardiography, right heart catheterization, and endomyocardial biopsy. During a mean follow-up of 33.42 months, 15 (26%) patients died, 12 (21%) patients underwent HT, and 8 (14%) patients were implanted with LVAD. APN level was significantly higher in patients who experienced study endpoints (23.4 versus 10.9 ug/ml, p = 0.01). APN was associated with worse outcome in univariate Cox proportional hazards model (HR 1.04, CI 1.02-1.07, p = 0.001) but lost significance adjusting for other covariates. Average global strain (AGS) is an independent outcome predictor (HR 1.42, CI 1.081-1.866, p = 0.012). Increased circulating APN level was associated with higher mortality and may be an additive prognostic marker in DCM with advanced HF. Combination of serum (APN, BNP, TNF-α) and echocardiographic (AGS) markers may increase the HF predicting power for the nonischemic DCM patients.

Ankyrin Repeat Domain 1 Protein: A Functionally Pleiotropic Protein with Cardiac Biomarker Potential

The ankyrin repeat domain 1 (ANKRD1) protein is a cardiac-specific stress-response protein that is part of the muscle ankyrin repeat protein family. ANKRD1 is functionally pleiotropic, playing pivotal roles in transcriptional regulation, sarcomere assembly and mechano-sensing in the heart. Importantly, cardiac ANKRD1 has been shown to be highly induced in various cardiomyopathies and in heart failure, although it is still unclear what impact this may have on the pathophysiology of heart failure. This review aims at highlighting the known properties, functions and regulation of ANKRD1, with focus on the underlying mechanisms that may be involved. The current views on the actions of ANKRD1 in cardiovascular disease and its utility as a candidate cardiac biomarker with diagnostic and/or prognostic potential are also discussed. More studies of ANKRD1 are warranted to obtain deeper functional insights into this molecule to allow assessment of its potential clinical applications as a diagnostic or prognostic marker and/or as a possible therapeutic target.

CARP interacts with titin at a unique helical N2A sequence and at the domain Ig81 to form a structured complex

The cardiac ankyrin repeat protein (CARP) is up-regulated in the myocardium during cardiovascular disease and in response to mechanical or toxic stress. Stress-induced CARP interacts with the N2A spring region of the titin filament to modulate muscle compliance. We characterize the interaction between CARP and titin-N2A and show that the binding site in titin spans the dual domain UN2A-Ig81. We find that the unique sequence UN2A is not structurally disordered, but that it has a stable, elongated α-helical fold that possibly acts as a constant force spring. Our findings portray CARP/titin-N2A as a structured node and help to rationalize the molecular basis of CARP mechanosensing in the sarcomeric I-band.

MLP and CARP are linked to chronic PKCα signalling in dilated cardiomyopathy

MLP (muscle LIM protein)-deficient mice count among the first mouse models for dilated cardiomyopathy (DCM), yet the exact role of MLP in cardiac signalling processes is still enigmatic. Elevated PKCα signalling activity is known to be an important contributor to heart failure. Here we show that MLP directly inhibits the activity of PKCα. In end-stage DCM, PKCα is concentrated at the intercalated disc of cardiomyocytes, where it is sequestered by the adaptor protein CARP in a multiprotein complex together with PLCβ1. In mice deficient for both MLP and CARP the chronic PKCα signalling chain at the intercalated disc is broken and they remain healthy. Our results suggest that the main role of MLP in heart lies in the direct inhibition of PKCα and that chronic uninhibited PKCα activity at the intercalated disc in the absence of functional MLP leads to heart failure.

Altered function of the muscle LIM protein (MLP) causes dilated cardiomyopathy in mice and humans. Lange et al. explain the molecular role of MLP in the heart by showing that it affects the signalling complex at the intercalated discs of failing hearts that consists of PKCα, PLCβ1 and CARP by inhibiting PKCα auto-phosphorylation and function.