Autophagy in desmin-related cardiomyopathy: thoughts at the halfway point

Accumulation of protein aggregates is a hallmark of several neurodegenerative disorders as well as for a number of protein conformation-based diseases, including those affecting muscle, liver and heart. Desminopathy or desmin-related myopathy (DRM) is a skeletal myopathy characterized by bilateral muscle weakness, but is often accompanied by cardiomyopathy as well. DRM can be caused by mutations in desmin, alphaB crystallin, myotilin, Z-band alternatively spliced PDZ-containing protein (ZASP), filamin C (FLNC) or Bcl-2-associated athanogene-3 (BAG3). The common pathological pattern in DRM is accumulation of misfolded proteins, however, clinical manifestations can differ significantly.

RAF1 mutations in childhood-onset dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is a highly heterogeneous trait with sarcomeric gene mutations predominating. The cause of a substantial percentage of DCMs remains unknown, and no gene-specific therapy is available. On the basis of resequencing of 513 DCM cases and 1,150 matched controls from various cohorts of distinct ancestry, we discovered rare, functional RAF1 mutations in 3 of the cohorts (South Indian, North Indian and Japanese). The prevalence of RAF1 mutations was ~9% in childhood-onset DCM cases in these three cohorts. Biochemical studies showed that DCM-associated RAF1 mutants had altered kinase activity, resulting in largely unaltered ERK activation but in AKT that was hyperactivated in a BRAF-dependent manner. Constitutive expression of these mutants in zebrafish embryos resulted in a heart failure phenotype with AKT hyperactivation that was rescued by treatment with rapamycin. These findings provide new mechanistic insights and potential therapeutic targets for RAF1-associated DCM and further expand the clinical spectrum of RAF1-related human disorders.

Desensitization of myofilaments to Ca2+ as a therapeutic target for hypertrophic cardiomyopathy with mutations in thin filament proteins

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is a common genetic disorder caused mainly by mutations in sarcomeric proteins and is characterized by maladaptive myocardial hypertrophy, diastolic heart failure, increased myofilament Ca(2+) sensitivity, and high susceptibility to sudden death. We tested the following hypothesis: correction of the increased myofilament sensitivity can delay or prevent the development of the HCM phenotype.

METHODS AND RESULTS

We used an HCM mouse model with an E180G mutation in α-tropomyosin (Tm180) that demonstrates increased myofilament Ca(2+) sensitivity, severe hypertrophy, and diastolic dysfunction. To test our hypothesis, we reduced myofilament Ca(2+) sensitivity in Tm180 mice by generating a double transgenic mouse line. We crossed Tm180 mice with mice expressing a pseudophosphorylated cardiac troponin I (S23D and S24D; TnI-PP). TnI-PP mice demonstrated a reduced myofilament Ca(2+) sensitivity compared with wild-type mice. The development of pathological hypertrophy did not occur in mice expressing both Tm180 and TnI-PP. Left ventricle performance was improved in double transgenic compared with their Tm180 littermates, which express wild-type cardiac troponin I. Hearts of double transgenic mice demonstrated no changes in expression of phospholamban and sarcoplasmic reticulum Ca(2+) ATPase, increased levels of phospholamban and troponin T phosphorylation, and reduced phosphorylation of TnI compared with Tm180 mice. Moreover, expression of TnI-PP in Tm180 hearts inhibited modifications in the activity of extracellular signal-regulated kinase and zinc finger-containing transcription factor GATA in Tm180 hearts.

CONCLUSIONS

Our data strongly indicate that reduction of myofilament sensitivity to Ca(2+) and associated correction of abnormal relaxation can delay or prevent development of HCM and should be considered as a therapeutic target for HCM.

Lateralized, nonepileptic convulsions in an adult with cerebral palsy: Case report and review of the literature

The authors report a case of unilateral functional neurological symptoms (nonepileptic convulsions) in a 38-year-old man with mild, motor-predominant cerebral palsy. His convulsions are all lateralized to the same side as his paretic limbs. His episodes significantly decreased after several months of weekly psychodynamic-oriented psychotherapy. Functional neurological disorders have been rarely reported in children or adults with cerebral palsy. Among patients with brain injury, right-hemispheric brain disease may be more helpful than either handedness or the side of symptoms in clinically profiling patients with suspected functional disorders. This case raises biomechanistic questions about brain injury, the development of functional disorders, and the lateralization of functional symptoms.

Proteotoxicity: an underappreciated pathology in cardiac disease

In general, in most organ systems, intracellular protein homeostasis is the sum of many factors, including chromosomal state, protein synthesis, post-translational processing and transport, folding, assembly and disassembly into macromolecular complexes, protein stability and clearance. In the heart, there has been a focus on the gene programs that are activated during pathogenic processes, but the removal of damaged proteins and organelles has been underappreciated as playing an important role in the pathogenesis of heart disease. Proteotoxicity refers to the adverse effects of damaged or misfolded proteins and even organelles on the cell. At the cellular level, the ultimate outcome of uncontrolled or severe proteotoxicity is cell death; hence, the pathogenic impact of proteotoxicity is maximally manifested in organs with no or very poor regenerative capability such as the brain and the heart. Evidence for increased cardiac proteotoxicity is rapidly mounting for a large subset of congenital and acquired human heart disease. Studies carried out in animal models and in cell culture have begun to establish both sufficiency and, in some cases, the necessity of proteotoxicity as a major pathogenic factor in the heart. This dictates rigorous testing for the efficacy of proteotoxic attenuation as a new strategy to treat heart disease. This review article highlights some recent advances in our understanding of how misfolded proteins can injure and are handled in the cell, examining the emerging evidence for targeting proteotoxicity as a new therapeutic strategy for heart disease. This article is part of a Special Issue entitled "Protein Quality Control, the Ubiquitin Proteasome System, and Autophagy."

Proteasomal and lysosomal protein degradation and heart disease

In the cell, the proteasome and lysosomes represent the most important proteolytic machineries, responsible for the protein degradation in the ubiquitin-proteasome system (UPS) and autophagy, respectively. Both the UPS and autophagy are essential to protein quality and quantity control. Alterations in cardiac proteasomal and lysosomal degradation are remarkably associated with most heart disease in humans and are implicated in the pathogenesis of congestive heart failure. Studies carried out in animal models and in cell culture have begun to establish both sufficiency and, in some cases, the necessity of proteasomal functional insufficiency or lysosomal insufficiency as a major pathogenic factor in the heart. This review article highlights some recent advances in the research into proteasome and lysosome protein degradation in relation to cardiac pathology and examines the emerging evidence for enhancing degradative capacities of the proteasome and/or lysosome as a new therapeutic strategy for heart disease. This article is part of a Special Issue entitled "Protein Quality Control, the Ubiquitin Proteasome System, and Autophagy".

Enhanced autophagy ameliorates cardiac proteinopathy

Basal autophagy is a crucial mechanism in cellular homeostasis, underlying both normal cellular recycling and the clearance of damaged or misfolded proteins, organelles and aggregates. We showed here that enhanced levels of autophagy induced by either autophagic gene overexpression or voluntary exercise ameliorated desmin-related cardiomyopathy (DRC). To increase levels of basal autophagy, we generated an inducible Tg mouse expressing autophagy-related 7 (Atg7), a critical and rate-limiting autophagy protein. Hearts from these mice had enhanced autophagy, but normal morphology and function. We crossed these mice with CryABR120G mice, a model of DRC in which autophagy is significantly attenuated in the heart, to test the functional significance of autophagy activation in a proteotoxic model of heart failure. Sustained Atg7-induced autophagy in the CryABR120G hearts decreased interstitial fibrosis, ameliorated ventricular dysfunction, decreased cardiac hypertrophy, reduced intracellular aggregates and prolonged survival. To determine whether different methods of autophagy upregulation have additive or even synergistic benefits, we subjected the autophagy-deficient CryABR120G mice and the Atg7-crossed CryABR120G mice to voluntary exercise, which also upregulates autophagy. The entire exercised Atg7-crossed CryABR120G cohort survived to 7 months. These findings suggest that activating autophagy may be a viable therapeutic strategy for improving cardiac performance under proteotoxic conditions.

Post-translational control of cardiac hemodynamics through myosin binding protein C

Cardiac myosin binding protein C (cMyBP-C) is an integral sarcomeric protein that associates with the thick, thin, and titin filament systems in the contractile apparatus. Three different isoforms of MyBP-C exist in mammalian muscle: slow skeletal (MyBPC1), fast skeletal (MyBP-C2, with several variants), and cardiac (cMyBP-C). Genetic screening studies show that mutations in MYBPC3 occur frequently and are responsible for as many as 30-35 % of identified cases of familial hypertrophic cardiomyopathy. The function of cMyBP-C is stringently regulated by its post-translational modification. In particular, the addition of phosphate groups occurs with high frequency on certain serine residues that are located in the cardiac-specific regulatory M domain. Phosphorylation of this domain has been extensively studied in vitro and in vivo. Phosphorylation of the M domain can regulate the manner in which actin and myosin interact, affecting the cross bridge cycle and ultimately, cardiac hemodynamics.

Functional dissection of myosin binding protein C phosphorylation

Cardiac myosin binding protein C (cMyBP-C) phosphorylation is differentially regulated in the normal heart and during disease development. Our objective was to examine in detail three phosphorylatable sites (Ser-273, Ser-282, and Ser-302) present in the protein's cardiac-specific sequences, as these residues are differentially and reversibly phosphorylated during normal and abnormal cardiac function. Three transgenic lines were generated: DAA, which expressed cMyBP-C containing Asp-273, Ala-282, and Ala-302, in which a charged amino acid was placed at residue 273 and the remaining two sites rendered nonphosphorylatable by substituting alanines for the two serines; AAD containing Ala-273, Ala-282, and Asp-302, in which aspartate was placed at residue 302 and the remaining two sites rendered nonphosphorylatable; and SDS containing Ser-273, Asp-282, and Ser-302. These mice were compared to mice constructed previously along similar lines: wild type, in which normal cMyBP-C is transgenically expressed, AllP-, in which alanines were substituted and ADA mice as well. DAA and AAD mice showed pathology that was more severe than cMyBP-C nulls. DAA and AAD animals exhibited left ventricular chamber dilation, interstitial fibrosis, irregular cardiac rhythm and sudden cardiac death. Our results define the effects of the sites' post-translational modifications on cMyBP-C functionality and together, give a comprehensive picture of the potential consequences of site-specific phosphorylation. Ser-282 is a key residue in controlling S2 interaction with the thick and thin filaments. The new DAA and AAD constructs show that phosphorylation at one site in the absence of the ability to phosphorylate the other sites, depending upon the particular residues involved, can lead to severe cardiac remodeling and dysfunction.

Bax and Bak function as the outer membrane component of the mitochondrial permeability pore in regulating necrotic cell death in mice

A critical event in ischemia-based cell death is the opening of the mitochondrial permeability transition pore (MPTP). However, the molecular identity of the components of the MPTP remains unknown. Here, we determined that the Bcl-2 family members Bax and Bak, which are central regulators of apoptotic cell death, are also required for mitochondrial pore-dependent necrotic cell death by facilitating outer membrane permeability of the MPTP. Loss of Bax/Bak reduced outer mitochondrial membrane permeability and conductance without altering inner membrane MPTP function, resulting in resistance to mitochondrial calcium overload and necrotic cell death. Reconstitution with mutants of Bax that cannot oligomerize and form apoptotic pores, but still enhance outer membrane permeability, permitted MPTP-dependent mitochondrial swelling and restored necrotic cell death. Our data predict that the MPTP is an inner membrane regulated process, although in the absence of Bax/Bak the outer membrane resists swelling and prevents organelle rupture to prevent cell death.

DOI:http://dx.doi.org/10.7554/eLife.00772.001

In all multicellular plants and animals, cells are continuously dying and being replaced. There are a number of different types of cell death, but two of the best studied are apoptosis and necrosis. Apoptosis, sometimes referred to as ‘cell suicide’, is a form of programmed cell death that is generally beneficial to the organism. Necrosis, however, occurs whenever cells are damaged—for example, due to a lack of oxygen—and can trigger harmful inflammation in surrounding tissue. Although the processes leading up to apoptosis and necrosis are very different, they both involve regulated changes in mitochondria—the organelles that supply cells with chemical energy.

Mitochondria have a distinctive appearance, being enclosed by two membranes, the innermost of which is highly folded. During apoptosis, large pores form in the outer membranes of mitochondria. These pores are generated by two proteins—Bax and Bak—and they enable the mitochondrion to release proteins that activate processes involved in apoptosis. Pores also form in the mitochondrial membrane during necrosis. However, these mitochondrial permeability transition pores (MPTPs) occur simultaneously in both the inner and outer membranes and are thought to lead to swelling and rupture of mitochondria.

Now, Karch et al. have shown that Bax and Bak are also involved in the formation of these permeability pores that underlie necrosis. When mouse cells that had been genetically modified to lack Bak and Bax were grown in cell culture, they were found to be resistant to substances that normally induce necrosis. Instead, their mitochondria continued to function normally, suggesting that MPTPs cannot form in the absence of Bak and Bax.

Karch et al. then generated mice with heart cells that lack Bax and Bak, and deprived their hearts of oxygen to simulate a heart attack. Compared to normal mice, the genetically modified animals experienced less damage to their heart muscle, suggesting that the absence of Bax and Bak prevents cell death due to necrosis. If Bax and Bak are involved in both apoptosis and necrosis, inhibiting them could be a powerful therapeutic approach for preventing all forms of cell death during heart attacks or in certain degenerative diseases.

DOI:http://dx.doi.org/10.7554/eLife.00772.002

Phosphorylation modulates the mechanical stability of the cardiac myosin-binding protein C motif

Cardiac myosin-binding protein C (cMyBP-C) is a thick-filament-associated protein that modulates cardiac contractility through interactions of its N-terminal immunoglobulin (Ig)-like C0-C2 domains with actin and/or myosin. These interactions are modified by the phosphorylation of at least four serines located within the motif linker between domains C1 and C2. We investigated whether motif phosphorylation alters its mechanical properties by characterizing force-extension relations using atomic force spectroscopy of expressed mouse N-terminal cMyBP-C fragments (i.e., C0-C3). Protein kinase A phosphorylation or serine replacement with aspartic acids did not affect persistence length (0.43 ± 0.04 nm), individual Ig-like domain unfolding forces (118 ± 3 pN), or Ig extension due to unfolding (30 ± 0.38 nm). However, phosphorylation did significantly decrease the C0-C3 mean contour length by 24 ± 2 nm. These results suggest that upon phosphorylation, the motif, which is freely extensible in the nonphosphorylated state, adopts a more stable and/or different structure. Circular dichroism and dynamic light scattering data for shorter expressed C1-C2 fragments with all four serines replaced by aspartic acids confirmed that the motif did adopt a more stable structure that was not apparent in the nonphosphorylated motif. These biophysical data provide both a mechanical and structural basis for cMyBP-C regulation by motif phosphorylation.

The COP9 signalosome is required for autophagy, proteasome-mediated proteolysis, and cardiomyocyte survival in adult mice

BACKGROUND

The COP9 signalosome (CSN) is an evolutionarily conserved protein complex composed of 8 unique protein subunits (CSN1 through CSN8). We have recently discovered in perinatal mouse hearts that CSN regulates not only proteasome-mediated proteolysis but also macroautophagy. However, the physiological significance of CSN in a post-mitotic organ of adult vertebrates has not been determined. We sought to study the physiological role of CSN8/CSN in adult mouse hearts.

METHODS AND RESULTS

Csn8 was conditionally ablated in the cardiomyocytes of adult mice (CSN8(CKO)) using a temporally controlled Cre-LoxP system. Loss of CSN8 accumulated the neddylated forms of cullins and noncullin proteins, increased ubiquitinated proteins, and stabilized a surrogate substrate of the proteasome in the heart. Autophagic flux was significantly decreased, whereas autophagosomes were markedly increased in CSN8(CKO) hearts, indicative of impaired autophagosome removal. Furthermore, we observed increased oxidized proteins, massive necrotic cardiomyocytes, and morphological and functional changes characteristic of dilated cardiomyopathy in CSN8(CKO) mice.

CONCLUSIONS

CSN deneddylates substrates more than cullins and is indispensable to cardiomyocyte survival in not only perinatal hearts but also adult hearts. CSN8/CSN regulates both proteasome-mediated proteolysis and the autophagic-lysosomal pathway, critical to the removal of oxidized proteins in the heart.

An endogenously produced fragment of cardiac myosin-binding protein C is pathogenic and can lead to heart failure

RATIONALE

A stable 40-kDa fragment is produced from cardiac myosin-binding protein C when the heart is stressed using a stimulus, such as ischemia-reperfusion injury. Elevated levels of the fragment can be detected in the diseased mouse and human heart, but its ability to interfere with normal cardiac function in the intact animal is unexplored.

OBJECTIVE

To understand the potential pathogenicity of the 40-kDa fragment in vivo and to investigate the molecular pathways that could be targeted for potential therapeutic intervention.

METHODS AND RESULTS

We generated cardiac myocyte-specific transgenic mice using a Tet-Off inducible system to permit controlled expression of the 40-kDa fragment in cardiomyocytes. When expression of the 40-kDa protein is induced by crossing the responder animals with tetracycline transactivator mice under conditions in which substantial quantities approximating those observed in diseased hearts are reached, the double-transgenic mice subsequently experience development of sarcomere dysgenesis and altered cardiac geometry, and the heart fails between 12 and 17 weeks of age. The induced double-transgenic mice had development of cardiac hypertrophy with myofibrillar disarray and fibrosis, in addition to activation of pathogenic MEK-ERK pathways. Inhibition of MEK-ERK signaling was achieved by injection of the mitogen-activated protein kinase (MAPK)/ERK inhibitor U0126. The drug effectively improved cardiac function, normalized heart size, and increased probability of survival.

CONCLUSIONS

These results suggest that the 40-kDa cardiac myosin-binding protein C fragment, which is produced at elevated levels during human cardiac disease, is a pathogenic fragment that is sufficient to cause hypertrophic cardiomyopathy and heart failure.

Diminished autophagy limits cardiac injury in mouse models of type 1 diabetes

Cardiac autophagy is inhibited in type 1 diabetes. However, it remains unknown if the reduced autophagy contributes to the pathogenesis of diabetic cardiomyopathy. We addressed this question using mouse models with gain- and loss-of-autophagy. Autophagic flux was inhibited in diabetic hearts when measured at multiple time points after diabetes induction by streptozotocin as assessed by protein levels of microtubule-associated protein light chain 3 form 2 (LC3-II) or GFP-LC3 puncta in the absence and presence of the lysosome inhibitor bafilomycin A1. Autophagy in diabetic hearts was further reduced in beclin 1- or Atg16-deficient mice but was restored partially or completely by overexpression of beclin 1 to different levels. Surprisingly, diabetes-induced cardiac damage was substantially attenuated in beclin 1- and Atg16-deficient mice as shown by improved cardiac function as well as reduced levels of oxidative stress, interstitial fibrosis, and myocyte apoptosis. In contrast, diabetic cardiac damage was dose-dependently exacerbated by beclin 1 overexpression. The cardioprotective effects of autophagy deficiency were reproduced in OVE26 diabetic mice. These effects were associated with partially restored mitophagy and increased expression and mitochondrial localization of Rab9, an essential regulator of a non-canonical alternative autophagic pathway. Together, these findings demonstrate that the diminished autophagy is an adaptive response that limits cardiac dysfunction in type 1 diabetes, presumably through up-regulation of alternative autophagy and mitophagy.

Transgenic mouse α- and β-cardiac myosins containing the R403Q mutation show isoform-dependent transient kinetic differences

Familial hypertrophic cardiomyopathy (FHC) is a major cause of sudden cardiac death in young athletes. The discovery in 1990 that a point mutation at residue 403 (R403Q) in the β-myosin heavy chain (MHC) caused a severe form of FHC was the first of many demonstrations linking FHC to mutations in muscle proteins. A mouse model for FHC has been widely used to study the mechanochemical properties of mutated cardiac myosin, but mouse hearts express α-MHC, whereas the ventricles of larger mammals express predominantly β-MHC. To address the role of the isoform backbone on function, we generated a transgenic mouse in which the endogenous α-MHC was partially replaced with transgenically encoded β-MHC or α-MHC. A His6 tag was cloned at the N terminus, along with R403Q, to facilitate isolation of myosin subfragment 1 (S1). Stopped flow kinetics were used to measure the equilibrium constants and rates of nucleotide binding and release for the mouse S1 isoforms bound to actin. For the wild-type isoforms, we found that the affinity of MgADP for α-S1 (100 μM) is ~ 4-fold weaker than for β-S1 (25 μM). Correspondingly, the MgADP release rate for α-S1 (350 s(-1)) is ~3-fold greater than for β-S1 (120 s(-1)). Introducing the R403Q mutation caused only a minor reduction in kinetics for β-S1, but R403Q in α-S1 caused the ADP release rate to increase by 20% (430 s(-1)). These transient kinetic studies on mouse cardiac myosins provide strong evidence that the functional impact of an FHC mutation on myosin depends on the isoform backbone.