The Translational Landscape of the Human Heart.

Gene expression in human tissue has primarily been studied on the transcriptional level, largely neglecting translational regulation. Here, we analyze the translatomes of 80 human hearts to identify new translation events and quantify the effect of translational regulation. We show extensive translational control of cardiac gene expression, which is orchestrated in a process-specific manner. Translation downstream of predicted disease-causing protein-truncating variants appears to be frequent, suggesting inefficient translation termination. We identify hundreds of previously undetected microproteins, expressed from lncRNAs and circRNAs, for which we validate the protein products in vivo. The translation of microproteins is not restricted to the heart and prominent in the translatomes of human kidney and liver. We associate these microproteins with diverse cellular processes and compartments and find that many locate to the mitochondria. Importantly, dozens of microproteins are translated from lncRNAs with well-characterized noncoding functions, indicating previously unrecognized biology.

S-glutathionylation of cryptic cysteines enhances titin elasticity by blocking protein folding.

The giant elastic protein titin is a determinant factor in how much blood fills the left ventricle during diastole and thus in the etiology of heart disease. Titin has been identified as a target of S-glutathionylation, an end product of the nitric-oxide-signaling cascade that increases cardiac muscle elasticity. However, it is unknown how S-glutathionylation may regulate the elasticity of titin and cardiac tissue. Here, we show that mechanical unfolding of titin immunoglobulin (Ig) domains exposes buried cysteine residues, which then can be S-glutathionylated. S-glutathionylation of cryptic cysteines greatly decreases the mechanical stability of the parent Ig domain as well as its ability to fold. Both effects favor a more extensible state of titin. Furthermore, we demonstrate that S-glutathionylation of cryptic cysteines in titin mediates mechanochemical modulation of the elasticity of human cardiomyocytes. We propose that posttranslational modification of cryptic residues is a general mechanism to regulate tissue elasticity.

Imaging of Existing and Newly Translated Proteins Elucidates Mechanisms of Sarcomere Turnover.

BACKGROUND: How the sarcomeric complex is continuously turned over in long-living cardiomyocytes is unclear. According to the prevailing model of sarcomere maintenance, sarcomeres are maintained by cytoplasmic soluble protein pools with free recycling between pools and sarcomeres. METHODS: We imaged and quantified the turnover of expressed and endogenous sarcomeric proteins, including the giant protein titin, in cardiomyocytes in culture and in vivo, at the single cell and at the single sarcomere level using pulse-chase labeling of Halo-tagged proteins with covalent ligands. RESULTS: We disprove the prevailing protein pool model and instead show an ordered mechanism in which only newly translated proteins enter the sarcomeric complex while older ones are removed and degraded. We also show that degradation is independent of protein age and that proteolytic extraction is a rate-limiting step in the turnover. We show that replacement of sarcomeric proteins occurs at a similar rate within cells and across the heart and is slower in adult cells. CONCLUSIONS: Our findings establish a unidirectional replacement model for cardiac sarcomeres subunit replacement and identify their turnover principles.

Impaired Intracellular Calcium Buffering Contributes to the Arrhythmogenic Substrate in Atrial Myocytes From Patients With Atrial Fibrillation.

BACKGROUND: Alterations in the buffering of intracellular Ca2+, for which myofilament proteins play a key role, have been shown to promote cardiac arrhythmia. It is interesting that although studies report atrial myofibrillar degradation in patients with persistent atrial fibrillation (persAF), the intracellular Ca2+ buffering profile in persAF remains obscure. Therefore, we aimed to investigate the intracellular buffering of Ca2+ and its potential arrhythmogenic role in persAF. METHODS: Transmembrane Ca2+ fluxes (patch-clamp) and intracellular Ca2+ signaling (fluo-3-acetoxymethyl ester) were recorded simultaneously in myocytes from right atrial biopsies of sinus rhythm (Ctrl) and patients with persAF, alongside human atrial subtype induced pluripotent stem cell-derived cardiac myocytes (iPSC-CMs). Protein levels were quantified by immunoblotting of human atrial tissue and induced pluripotent stem cell-derived cardiac myocytes. Mouse whole heart and atrial electrophysiology were measured on a Langendorff system. RESULTS: Cytosolic Ca2+ buffering was decreased in atrial myocytes of patients with persAF because of a depleted amount of Ca2+ buffers. In agreement, protein levels of selected Ca2+ binding myofilament proteins, including cTnC (cardiac troponin C), a major cytosolic Ca2+ buffer, were significantly lower in patients with persAF. Small interfering RNA (siRNA)-mediated knockdown of cTnC (si-cTNC) in atrial iPSC-CM phenocopied the reduced cytosolic Ca2+ buffering observed in persAF. Si-cTnC treated atrial iPSC-CM exhibited a higher predisposition to spontaneous Ca2+ release events and developed action potential alternans at low stimulation frequencies. Last, indirect reduction of cytosolic Ca2+ buffering using blebbistatin in an ex vivo mouse whole heart model increased vulnerability to tachypacing-induced atrial arrhythmia, validating the direct mechanistic link between impaired cytosolic Ca2+ buffering and atrial arrhythmogenesis. CONCLUSIONS: Our findings suggest that loss of myofilament proteins, particularly reduced cTnC protein levels, causes diminished cytosolic Ca2+ buffering in persAF, thereby potentiating the occurrence of spontaneous Ca2+ release events and atrial fibrillation susceptibility. Strategies targeting intracellular buffering may represent a promising therapeutic lead in persAF management.

Stretch Harmonizes Sarcomere Strain Across the Cardiomyocyte.

BACKGROUND: Increasing cardiomyocyte contraction during myocardial stretch serves as the basis for the Frank-Starling mechanism in the heart. However, it remains unclear how this phenomenon occurs regionally within cardiomyocytes, at the level of individual sarcomeres. We investigated sarcomere contractile synchrony and how intersarcomere dynamics contribute to increasing contractility during cell lengthening. METHODS: Sarcomere strain and Ca2+ were simultaneously recorded in isolated left ventricular cardiomyocytes during 1 Hz field stimulation at 37 °C, at resting length and following stepwise stretch. RESULTS: We observed that in unstretched rat cardiomyocytes, differential sarcomere deformation occurred during each beat. Specifically, while most sarcomeres shortened during the stimulus, ≈10% to 20% of sarcomeres were stretched or remained stationary. This nonuniform strain was not traced to regional Ca2+ disparities but rather shorter resting lengths and lower force production in systolically stretched sarcomeres. Lengthening of the cell recruited additional shortening sarcomeres, which increased contractile efficiency as less negative, wasted work was performed by stretched sarcomeres. Given the known role of titin in setting sarcomere dimensions, we next hypothesized that modulating titin expression would alter intersarcomere dynamics. Indeed, in cardiomyocytes from mice with titin haploinsufficiency, we observed greater variability in resting sarcomere length, lower recruitment of shortening sarcomeres, and impaired work performance during cell lengthening. CONCLUSIONS: Graded sarcomere recruitment directs cardiomyocyte work performance, and harmonization of sarcomere strain increases contractility during cell stretch. By setting sarcomere dimensions, titin controls sarcomere recruitment, and its lowered expression in haploinsufficiency mutations impairs cardiomyocyte contractility.

Titin force in muscle cells alters lattice order, thick and thin filament protein formation.

Skeletal muscle force production is increased at longer compared to shorter muscle lengths because of length-dependent priming of thick filament proteins in the contractile unit before contraction. Using small-angle X-ray diffraction in combination with a mouse model that specifically cleaves the stretch-sensitive titin protein, we found that titin cleavage diminished the length-dependent priming of the thick filament. Strikingly, a titin-sensitive, length-dependent priming was also present in thin filaments, which seems only possible via bridge proteins between thick and thin filaments in resting muscle, potentially myosin-binding protein C. We further show that these bridges can be forcibly ruptured via high-speed stretches. Our results advance a paradigm shift to the fundamental regulation of length-dependent priming, with titin as the key driver.

Interleukin-6 drives endothelial glycocalyx damage in COVID-19 and bacterial sepsis.

Damage of the endothelial glycocalyx (eGC) plays a central role in the development of vascular hyperpermeability and organ damage during systemic inflammation. However, the specific signalling pathways for eGC damage remain poorly defined. Aim of this study was to combine sublingual video-microscopy, plasma proteomics and live cell imaging to uncover further pathways of eGC damage in patients with coronavirus disease 2019 (COVID-19) or bacterial sepsis. This secondary analysis of the prospective multicenter MICROCODE study included 22 patients with COVID-19 and 43 patients with bacterial sepsis admitted to intermediate or intensive care units and 10 healthy controls. Interleukin-6 (IL-6) was strongly associated with damaged eGC and correlated both with eGC dimensions (rs=0.36, p = 0.0015) and circulating eGC biomarkers. In vitro, IL-6 reduced eGC height and coverage, which was inhibited by blocking IL-6 signalling with the anti-IL-6 receptor antibody tocilizumab or the Janus kinase inhibitor tofacitinib. Exposure of endothelial cells to 5% serum from COVID-19 or sepsis patients resulted in a significant decrease in eGC height, which was attenuated by co-incubation with tocilizumab. In an external COVID-19 cohort of 219 patients from Massachusetts General Hospital, a previously identified proteomic eGC signature correlated with IL-6 (rs=-0.58, p < 0.0001) and predicted the combined endpoint of 28-day mortality and/or intubation (ROC-AUC: 0.86 [95% CI: 0.81-0.91], p < 0.001). The data suggest that IL-6 may significantly drive eGC damage in COVID-19 and bacterial sepsis. Our findings provide valuable insights into pathomechanisms of vascular dysfunction during systemic inflammation and highlight the need for further in vivo studies.

Regulation of titin-based cardiac stiffness by unfolded domain oxidation (UnDOx).

The relationship between oxidative stress and cardiac stiffness is thought to involve modifications to the giant muscle protein titin, which in turn can determine the progression of heart disease. In vitro studies have shown that S-glutathionylation and disulfide bonding of titin fragments could alter the elastic properties of titin; however, whether and where titin becomes oxidized in vivo is less certain. Here we demonstrate, using multiple models of oxidative stress in conjunction with mechanical loading, that immunoglobulin domains preferentially from the distal titin spring region become oxidized in vivo through the mechanism of unfolded domain oxidation (UnDOx). Via oxidation type-specific modification of titin, UnDOx modulates human cardiomyocyte passive force bidirectionally. UnDOx also enhances titin phosphorylation and, importantly, promotes nonconstitutive folding and aggregation of unfolded domains. We propose a mechanism whereby UnDOx enables the controlled homotypic interactions within the distal titin spring to stabilize this segment and regulate myocardial passive stiffness.

Virtually "muscling" in pandemic: first virtual European Muscle Conference (September 20-22, 2021).

A report on the first virtual European Muscle Conference.

Titin Gene and Protein Functions in Passive and Active Muscle.

The thin and thick filaments of muscle sarcomeres are interconnected by the giant protein titin, which is a scaffolding filament, signaling platform, and provider of passive tension and elasticity in myocytes. This review summarizes recent insight into the mechanisms behind how titin gene mutations cause hereditary cardiomyopathy and how titin protein is mechanically active in skeletal and cardiac myocytes. A main theme is the evolving role of titin as a modulator of contraction. Topics include strain-sensing via titin in the sarcomeric A-band as the basis for length-dependent activation, titin elastic recoil and refolding of titin domains as an energy source, and Ca2+-dependent stiffening of titin stretched during eccentric muscle contractions. Findings suggest that titin stiffness is a principal regulator of the contractile behavior of striated muscle. Physiological or pathological changes to titin stiffness therefore affect contractility. Taken together, titin emerges as a linker element between passive and active myocyte properties.

A Dietary Supplement Containing Fucoidan Preserves Endothelial Glycocalyx through ERK/MAPK Signaling and Protects against Damage Induced by CKD Serum.

(1) Damage to the endothelial glycocalyx (eGC), a protective layer lining the endothelial luminal surface, is associated with chronic kidney disease (CKD), which leads to a worsening of cardiovascular outcomes in these patients. Currently, there are no targeted therapeutic approaches. Whether the dietary supplement EndocalyxTM (ECX) protects against endothelial damage caused by uremic toxins is unknown. (2) We addressed this question by performing atomic force microscopy measurements on living endothelial cells. We examined the effect of ECX on eGC thickness at baseline and with pooled serum from hemodialysis patients. ECX was also successfully administered in vivo in mice, in which eGC was assessed using perfused boundary region measurements by intravital microscopy of cremasteric vessels. (3) Both ECX and fucoidan significantly improved baseline eGC thickness. Our data indicate that these effects are dependent on ERK/MAPK and PI3K signaling. After incubation with eGC damaging serum from dialysis patients, ECX increased eGC height. Intravital microscopy in mice revealed a relevant increase in baseline eGC dimensions after feeding with ECX. (4) We identified a dietary supplement containing glycocalyx substrates and fucoidan as potential mediators of eGC preservation in vitro and in vivo. Our findings suggest that fucoidan may be an essential component responsible for protecting the eGC in acute settings. Moreover, ECX might contribute to both protection and rebuilding of the eGC in the context of CKD.

A novel isoform of myosin 18A (Myo18Aγ) is an essential sarcomeric protein in mouse heart.

Whereas myosin 18B (Myo18B) is known to be a critical sarcomeric protein, the function of myosin 18A (Myo18A) is unclear, although it has been implicated in cell motility and Golgi shape. Here, we show that homozygous deletion (homozygous tm1a, tm1b, or tm1d alleles) of Myo18a in mouse is embryonic lethal. Reminiscent of Myo18b, Myo18a was highly expressed in the embryo heart, and cardiac-restricted Myo18a deletion in mice was embryonic lethal. Surprisingly, using Western blot analysis, we were unable to detect the known isoforms of Myo18A, Myo18Aα and Myo18Aß, in mouse heart using a custom C-terminal antibody. However, alternative anti-Myo18A antibodies detected a larger than expected protein, and RNA-Seq analysis indicated that a novel Myo18A transcript is expressed in mouse ventricular myocytes (and human heart). Cloning and sequencing revealed that this cardiac isoform, denoted Myo18Aγ, lacks the PDZ-containing N terminus of Myo18Aα but includes an alternative N-terminal extension and a long serine-rich C terminus. EGFP-tagged Myo18Aγ expressed in ventricular myocytes localized to the level of A-bands in sarcomeres, and Myo18a knockout embryos at day 10.5 exhibited disorganized sarcomeres with wavy thick filaments. We additionally generated myeloid-restricted Myo18a knockout mice to investigate the role of Myo18A in nonmuscle cells, exemplified by macrophages, which express more Myo18Aß than Myo18Aα, but no defects in cell shape, motility, or Golgi shape were detected. In summary, we have identified a previously unrecognized sarcomere component, a large novel isoform (denoted Myo18Aγ) of Myo18A. Thus, both members of class XVIII myosins are critical components of cardiac sarcomeres.

Cronos Titin Is Expressed in Human Cardiomyocytes and Necessary for Normal Sarcomere Function.

BACKGROUND: The giant sarcomere protein titin is important in both heart health and disease. Mutations in the gene encoding for titin (TTN) are the leading known cause of familial dilated cardiomyopathy. The uneven distribution of these mutations within TTN motivated us to seek a more complete understanding of this gene and the isoforms it encodes in cardiomyocyte (CM) sarcomere formation and function. METHODS: To investigate the function of titin in human CMs, we used CRISPR/Cas9 to generate homozygous truncations in the Z disk (TTN-Z-/-) and A-band (TTN-A-/-) regions of the TTN gene in human induced pluripotent stem cells. The resulting CMs were characterized with immunostaining, engineered heart tissue mechanical measurements, and single-cell force and calcium measurements. RESULTS: After differentiation, we were surprised to find that despite the more upstream mutation, TTN-Z-/--CMs had sarcomeres and visibly contracted, whereas TTN-A-/--CMs did not. We hypothesized that sarcomere formation was caused by the expression of a recently discovered isoform of titin, Cronos, which initiates downstream of the truncation in TTN-Z-/--CMs. Using a custom Cronos antibody, we demonstrate that this isoform is expressed and integrated into myofibrils in human CMs. TTN-Z-/--CMs exclusively express Cronos titin, but these cells produce lower contractile force and have perturbed myofibril bundling compared with controls expressing both full-length and Cronos titin. Cronos titin is highly expressed in human fetal cardiac tissue, and when knocked out in human induced pluripotent stem cell derived CMs, these cells exhibit reduced contractile force and myofibrillar disarray despite the presence of full-length titin. CONCLUSIONS: We demonstrate that Cronos titin is expressed in developing human CMs and is able to support partial sarcomere formation in the absence of full-length titin. Furthermore, Cronos titin is necessary for proper sarcomere function in human induced pluripotent stem cell derived CMs. Additional investigation is necessary to understand the molecular mechanisms of this novel isoform and how it contributes to human cardiac disease.

Symmetric dimethylarginine in dysfunctional high-density lipoprotein mediates endothelial glycocalyx breakdown in chronic kidney disease.

Dysfunctional high-density lipoprotein (d-HDL) in chronic kidney disease is known to have a change in composition towards an endothelial-damaging phenotype, amongst others, via the accumulation of symmetric dimethylarginine. The endothelial glycocalyx, a carbohydrate-rich layer lining the endothelial luminal surface, is a first line defense against vascular diseases including atherosclerosis. Here we conducted a translational, cross-sectional study to determine the role of symmetric dimethylarginine in d-HDL as a mediator of glycocalyx damage. Using confocal and atomic force microscopy, intact HDL from healthy donors was found to maintain the glycocalyx while isolated HDL from hemodialysis patients and exogenous symmetric dimethylarginine caused significant damage to the glycocalyx in endothelial cells in vitro in a dose-dependent manner. Symmetric dimethylarginine triggered glycocalyx deterioration via molecular pathways mediated by toll-like-receptor 2 and matrix metalloprotease-9. Corresponding intravital microscopy revealed that exogenous symmetric dimethylarginine and d-HDL from hemodialysis patients caused glycocalyx breakdown, which subsequently contributed to alterations in leukocyte rolling. Biologically effective HDL, which estimates the functionality of HDL, was calculated from circulating HDL-cholesterol and symmetric dimethylarginine, as described in the literature. Biologically effective HDL was the only parameter that could independently predict glycocalyx damage in vivo. Thus, our data suggest that symmetric dimethylarginine in d-HDL mediates glycocalyx breakdown in chronic kidney disease.

Diabetes-Induced Cardiomyocyte Passive Stiffening Is Caused by Impaired Insulin-Dependent Titin Modification and Can Be Modulated by Neuregulin-1.

RATIONALE: Increased titin-dependent cardiomyocyte tension is a hallmark of heart failure with preserved ejection fraction associated with type-2 diabetes mellitus. However, the insulin-related signaling pathways that modify titin-based cardiomyocyte tension, thereby contributing to modulation of diastolic function, are largely unknown. OBJECTIVE: We aimed to determine how impaired insulin signaling affects titin expression and phosphorylation and thus increases passive cardiomyocyte tension, and whether metformin or neuregulin-1 (NRG-1) can correct disturbed titin modifications and increased titin-based stiffness. METHODS AND RESULTS: We used cardiac biopsies from human diabetic (n=23) and nondiabetic patients (n=19), cultured rat cardiomyocytes, left ventricular tissue from apolipoprotein E-deficient mice with streptozotocin-induced diabetes mellitus (n=12-22), and ZSF1 (obese diabetic Zucker fatty/spontaneously hypertensive heart failure F1 hybrid) rats (n=5-6) and analyzed insulin-dependent signaling pathways that modulate titin phosphorylation. Titin-based passive tension was measured using permeabilized cardiomyocytes. In human diabetic hearts, we detected titin hypophosphorylation at S4099 and hyperphosphorylation at S11878, suggesting altered activity of protein kinases; cardiomyocyte passive tension was significantly increased. When applied to cultured cardiomyocytes, insulin and metformin increased titin phosphorylation at S4010, S4099, and S11878 via enhanced ERK1/2 (extracellular signal regulated kinase 1/2) and PKCα (protein kinase Cα) activity; NRG-1 application enhanced ERK1/2 activity but reduced PKCα activity. In apolipoprotein E-deficient mice, chronic treatment of streptozotocin-induced diabetes mellitus with NRG-1 corrected titin phosphorylation via increased PKG (protein kinase G) and ERK1/2 activity and reduced PKCα activity, which reversed the diabetes mellitus-associated changes in titin-based passive tension. Acute application of NRG-1 to obese ZSF1 rats with type-2 diabetes mellitus reduced end-diastolic pressure. CONCLUSIONS: Mechanistically, we found that impaired cGMP-PKG signaling and elevated PKCα activity are key modulators of titin-based cardiomyocyte stiffening in diabetic hearts. We conclude that by restoring normal kinase activities of PKG, ERK1/2, and PKCα, and by reducing cardiomyocyte passive tension, chronic NRG-1 application is a promising approach to modulate titin properties in heart failure with preserved ejection fraction associated with type-2 diabetes mellitus.

Treatments targeting inotropy.

Acute heart failure (HF) and in particular, cardiogenic shock are associated with high morbidity and mortality. A therapeutic dilemma is that the use of positive inotropic agents, such as catecholamines or phosphodiesterase-inhibitors, is associated with increased mortality. Newer drugs, such as levosimendan or omecamtiv mecarbil, target sarcomeres to improve systolic function putatively without elevating intracellular Ca2+. Although meta-analyses of smaller trials suggested that levosimendan is associated with a better outcome than dobutamine, larger comparative trials failed to confirm this observation. For omecamtiv mecarbil, Phase II clinical trials suggest a favourable haemodynamic profile in patients with acute and chronic HF, and a Phase III morbidity/mortality trial in patients with chronic HF has recently begun. Here, we review the pathophysiological basis of systolic dysfunction in patients with HF and the mechanisms through which different inotropic agents improve cardiac function. Since adenosine triphosphate and reactive oxygen species production in mitochondria are intimately linked to the processes of excitation-contraction coupling, we also discuss the impact of inotropic agents on mitochondrial bioenergetics and redox regulation. Therefore, this position paper should help identify novel targets for treatments that could not only safely improve systolic and diastolic function acutely, but potentially also myocardial structure and function over a longer-term.

The continuous heart failure spectrum: moving beyond an ejection fraction classification.

Randomized clinical trials initially used heart failure (HF) patients with low left ventricular ejection fraction (LVEF) to select study populations with high risk to enhance statistical power. However, this use of LVEF in clinical trials has led to oversimplification of the scientific view of a complex syndrome. Descriptive terms such as 'HFrEF' (HF with reduced LVEF), 'HFpEF' (HF with preserved LVEF), and more recently 'HFmrEF' (HF with mid-range LVEF), assigned on arbitrary LVEF cut-off points, have gradually arisen as separate diseases, implying distinct pathophysiologies. In this article, based on pathophysiological reasoning, we challenge the paradigm of classifying HF according to LVEF. Instead, we propose that HF is a heterogeneous syndrome in which disease progression is associated with a dynamic evolution of functional and structural changes leading to unique disease trajectories creating a spectrum of phenotypes with overlapping and distinct characteristics. Moreover, we argue that by recognizing the spectral nature of the disease a novel stratification will arise from new technologies and scientific insights that will shape the design of future trials based on deeper understanding beyond the LVEF construct alone.

Smyd2 controls cytoplasmic lysine methylation of Hsp90 and myofilament organization.

Protein lysine methylation is one of the most widespread post-translational modifications in the nuclei of eukaryotic cells. Methylated lysines on histones and nonhistone proteins promote the formation of protein complexes that control gene expression and DNA replication and repair. In the cytoplasm, however, the role of lysine methylation in protein complex formation is not well established. Here we report that the cytoplasmic protein chaperone Hsp90 is methylated by the lysine methyltransferase Smyd2 in various cell types. In muscle, Hsp90 methylation contributes to the formation of a protein complex containing Smyd2, Hsp90, and the sarcomeric protein titin. Deficiency in Smyd2 results in the loss of Hsp90 methylation, impaired titin stability, and altered muscle function. Collectively, our data reveal a cytoplasmic protein network that employs lysine methylation for the maintenance and function of skeletal muscle.

SPRED2 deficiency elicits cardiac arrhythmias and premature death via impaired autophagy.

Cardiac functionality is dependent on a balanced protein turnover. Accordingly, regulated protein decay is critical to maintain cardiac function. Here we demonstrate that deficiency of SPRED2, an intracellular repressor of ERK-MAPK signaling markedly expressed in human heart, resulted in impaired autophagy, heart failure, and shortened lifespan. SPRED2-/- mice showed cardiomyocyte hypertrophy, cardiac fibrosis, impaired electrical excitability, and severe arrhythmias. Mechanistically, cardiomyocyte dysfunction resulted from ERK hyperactivation and dysregulated autophagy, observed as accumulation of vesicles, vacuolar structures, and degenerated mitochondria. The diminished autophagic flux in SPRED2-/- hearts was reflected by a reduced LC3-II/LC3-I ratio and by decreased Atg7, Atg4B and Atg16L expression. Furthermore, the autophagosomal adaptors p62/SQSTM1 and NBR1 and lysosomal Cathepsin D accumulated in SPRED2-/- hearts. In wild-type hearts, SPRED2 interacted physically with p62/SQSTM1, NBR1, and Cathepsin D, indicating that SPRED2 is required for autophagolysosome formation in regular autophagy. Restored inhibition of MAPK signaling by selumetinib led to an increase in autophagic flux in vivo. Therefore, our study identifies SPRED2 as a novel, indispensable regulator of cardiac autophagy. Vice versa, SPRED2 deficiency impairs autophagy, leading to cardiac dysfunction and life-threatening arrhythmias.