An Unbiased High-Throughput Screen to Identify Novel Effectors That Impact on Cardiomyocyte Aggregate Levels.

RATIONALE: Postmitotic cells, such as cardiomyocytes, seem to be particularly susceptible to proteotoxic stimuli, and large, proteinaceous deposits are characteristic of the desmin-related cardiomyopathies and crystallin cardiomyopathic diseases. Increased activity of protein clearance pathways in the cardiomyocyte, such as proteasomal degradation and autophagy, has proven to be beneficial in maintaining cellular and cardiac function in the face of multiple proteotoxic insults, holding open the possibility of targeting these processes for the development of effective therapeutics. OBJECTIVE: Here, we undertake an unbiased, total genome screen for RNA transcripts and their protein products that affect aggregate accumulations in the cardiomyocytes. METHODS AND RESULTS: Primary mouse cardiomyocytes that accumulate aggregates as a result of a mutant CryAB (αB-crystallin) causative for human desmin-related cardiomyopathy were used for a total genome-wide screen to identify gene products that affected aggregate formation. We infected cardiomyocytes using a short hairpin RNA lentivirus library in which the mouse genome was represented. The screen identified multiple candidates in many cell signaling pathways that were able to mediate significant decreases in aggregate levels. CONCLUSIONS: Subsequent validation of one of these candidates, Jak1 (Janus kinase 1), a tyrosine kinase of the nonreceptor type, confirmed the usefulness of this approach in identifying previously unsuspected players in proteotoxic processes.

PLEKHM2 mutation leads to abnormal localization of lysosomes, impaired autophagy flux and associates with recessive dilated cardiomyopathy and left ventricular noncompaction.

Gene mutations, mostly segregating with a dominant mode of inheritance, are important causes of dilated cardiomyopathy (DCM), a disease characterized by enlarged ventricular dimensions, impaired cardiac function, heart failure and high risk of death. Another myocardial abnormality often linked to gene mutations is left ventricular noncompaction (LVNC) characterized by a typical diffuse spongy appearance of the left ventricle. Here, we describe a large Bedouin family presenting with a severe recessive DCM and LVNC. Homozygosity mapping and exome sequencing identified a single gene variant that segregated as expected and was neither reported in databases nor in Bedouin population controls. The PLEKHM2 cDNA2156_2157delAG variant causes the frameshift p.Lys645AlafsTer12 and/or the skipping of exon 11 that results in deletion of 30 highly conserved amino acids. PLEKHM2 is known to interact with several Rabs and with kinesin-1, affecting endosomal trafficking. Accordingly, patients' primary fibroblasts exhibited abnormal subcellular distribution of endosomes marked by Rab5, Rab7 and Rab9, as well as the Golgi apparatus. In addition, lysosomes appeared to be concentrated in the perinuclear region, and autophagy flux was impaired. Transfection of wild-type PLEKHM2 cDNA into patient's fibroblasts corrected the subcellular distribution of the lysosomes, supporting the causal effect of PLEKHM2 mutation. PLEKHM2 joins LAMP-2 and BAG3 as a disease gene altering autophagy resulting in an isolated cardiac phenotype. The association of PLEKHM2 mutation with DCM and LVNC supports the importance of autophagy for normal cardiac function.

Generation of a homozygous CRYAB p.Arg120Gly mutant (UKEi001-A-1) from a human iPSC line.

Variants in CRYAB can lead to desmin-related (cardio-)myopathy (DRM), a genetic muscle disorder with no curative treatment available. We introduced a homozygous CRYAB c.358G > A (p.Arg120Gly) mutation, which is established for the study of DRM in mice, into a donor human induced pluripotent stem cell (hiPSC) line. Control and mutant hiPSCs were tested for karyotype integrity and pluripotency marker expression. HiPSCs could be differentiated into endoderm, ectoderm and cardiomyocytes as a mesodermal derivative in vitro. CRYABhom hiPSC-derived cardiomyocytes developed intracellular CRYAB aggregates, which is a hallmark of DRM. This newly created mutant can be utilized to study DRM and cardiac proteinopathy in a human context.

ACTN2 Mutant Causes Proteopathy in Human iPSC-Derived Cardiomyocytes.

Genetic variants in α-actinin-2 (ACTN2) are associated with several forms of (cardio)myopathy. We previously reported a heterozygous missense (c.740C>T) ACTN2 gene variant, associated with hypertrophic cardiomyopathy, and characterized by an electro-mechanical phenotype in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). Here, we created with CRISPR/Cas9 genetic tools two heterozygous functional knock-out hiPSC lines with a second wild-type (ACTN2wt) and missense ACTN2 (ACTN2mut) allele, respectively. We evaluated their impact on cardiomyocyte structure and function, using a combination of different technologies, including immunofluorescence and live cell imaging, RNA-seq, and mass spectrometry. This study showed that ACTN2mut presents a higher percentage of multinucleation, protein aggregation, hypertrophy, myofibrillar disarray, and activation of both the ubiquitin-proteasome system and the autophagy-lysosomal pathway as compared to ACTN2wt in 2D-cultured hiPSC-CMs. Furthermore, the expression of ACTN2mut was associated with a marked reduction of sarcomere-associated protein levels in 2D-cultured hiPSC-CMs and force impairment in engineered heart tissues. In conclusion, our study highlights the activation of proteolytic systems in ACTN2mut hiPSC-CMs likely to cope with ACTN2 aggregation and therefore directs towards proteopathy as an additional cellular pathology caused by this ACTN2 variant, which may contribute to human ACTN2-associated cardiomyopathies.

A high-throughput screening identifies ZNF418 as a novel regulator of the ubiquitin-proteasome system and autophagy-lysosomal pathway.

The ubiquitin-proteasome system (UPS) and autophagy-lysosomal pathway (ALP) are two major protein degradation pathways in eukaryotic cells. Initially considered as two independent pathways, there is emerging evidence that they can work in concert. As alterations of UPS and ALP function can contribute to neurodegenerative disorders, cancer and cardiac disease, there is great interest in finding targets that modulate these catabolic processes. We undertook an unbiased, total genome high-throughput screen to identify novel effectors that regulate both the UPS and ALP. We generated a stable HEK293 cell line expressing a UPS reporter (UbG76V-mCherry) and an ALP reporter (GFP-LC3) and screened for genes for which knockdown increased both UbG76V-mCherry intensity and GFP-LC3 puncta. With stringent selection, we isolated 80 candidates, including the transcription factor ZNF418 (ZFP418 in rodents). After screen validation with Zfp418 overexpression in HEK293 cells, we evaluated Zfp418 knockdown and overexpression in neonatal rat ventricular myocytes (NRVMs). Endogenous and overexpressed ZFP418 were localized in the nucleus. Subsequent experiments showed that ZFP418 negatively regulates UPS and positively regulates ALP activity in NRVMs. RNA-seq from Zfp418 knockdown revealed altered gene expression of numerous ubiquitinating and deubiquitinating enzymes, decreased expression of autophagy activators and initiators and increased expression of autophagy inhibitors. We found that ZPF418 activated the promoters of Dapk2 and Fyco1, which are involved in autophagy. RNA-seq from Zfp418 knockdown revealed accumulation of several genes involved in cardiac development and/or hypertrophy. In conclusion, our study provides evidence that ZNF418 activates the ALP, inhibits the UPS and regulates genes associated with cardiomyocyte structure/function.Abbreviations: ACTN2, actinin alpha 2; ALP, autophagy-lysosomal pathway; COPB1, COPI coat complex subunit beta 1; DAPK2, death associated protein kinase 2; FYCO1, FYVE and coiled-coil domain autophagy adaptor 1; HEK293, human embryonic kidney cells 293; HTS, high-throughput screen; LC3, microtubule associated protein 1 light chain 3; NRVMs, neonatal rat ventricular myocytes; RNA-seq, RNA sequencing; RPS6, ribosomal protein S6; TNNI3, troponin I, cardiac 3; UPS, ubiquitin-proteasome system; shRNA, short hairpin RNA; SQSTM1/p62, sequestosome 1; VPS28, VPS28 subunit of ESCRT-I; ZNF418/ZFP418, zinc finger protein 418.

Autophagy in cardiomyopathies.

Autophagy (greek auto: self; phagein: eating) is a highly conserved process within eukaryotes that degrades long-lived proteins and organelles within lysosomes. Its accurate and constant operation in basal conditions ensures cellular homeostasis by degrading damaged cellular components and thereby acting not only as a quality control but as well as an energy supplier. An increasing body of evidence indicates a major role of autophagy in the regulation of cardiac homeostasis and function. In this review, we describe the different forms of mammalian autophagy, their regulations and monitoring with a specific emphasis on the heart. Furthermore, we address the role of autophagy in several forms of cardiomyopathy and the options for therapy.

Is Desmin Propensity to Aggregate Part of its Protective Function?

Desmin is the major protein component of the intermediate filaments (IFs) cytoskeleton in muscle cells, including cardiac. The accumulation of cleaved and misfolded desmin is a cellular hallmark of heart failure (HF). These desmin alterations are reversed by therapy, suggesting a causal role for the IFs in the development of HF. Though IFs are known to play a role in the protection from stress, a mechanistic model of how that occurs is currently lacking. On the other hand, the heart is uniquely suited to study the function of the IFs, due to its inherent, cyclic contraction. That is, HF can be used as a model to address how IFs afford protection from mechanical, and possibly redox, stress. In this review we provide a brief summary of the current views on the function of the IFs, focusing on desmin. We also propose a new model according to which the propensity of desmin to aggregate may have been selected during evolution as a way to dissipate excessive mechanical and possibly redox stress. According to this model, though desmin misfolding may afford protection from acute injury, the sustained or excessive accumulation of desmin aggregates could impair proteostasis and contribute to disease.

Activation of Autophagy Ameliorates Cardiomyopathy in Mybpc3-Targeted Knockin Mice.

BACKGROUND: Alterations in autophagy have been reported in hypertrophic cardiomyopathy (HCM) caused by Danon disease, Vici syndrome, or LEOPARD syndrome, but not in HCM caused by mutations in genes encoding sarcomeric proteins, which account for most of HCM cases. MYBPC3, encoding cMyBP-C (cardiac myosin-binding protein C), is the most frequently mutated HCM gene. METHODS AND RESULTS: We evaluated autophagy in patients with HCM carrying MYBPC3 mutations and in a Mybpc3-targeted knockin HCM mouse model, as well as the effect of autophagy modulators on the development of cardiomyopathy in knockin mice. Microtubule-associated protein 1 light chain 3 (LC3)-II protein levels were higher in HCM septal myectomies than in nonfailing control hearts and in 60-week-old knockin than in wild-type mouse hearts. In contrast to wild-type, autophagic flux was blunted and associated with accumulation of residual bodies and glycogen in hearts of 60-week-old knockin mice. We found that Akt-mTORC1 (mammalian target of rapamycin complex 1) signaling was increased, and treatment with 2.24 mg/kg·d rapamycin or 40% caloric restriction for 9 weeks partially rescued cardiomyopathy or heart failure and restored autophagic flux in knockin mice. CONCLUSIONS: Altogether, we found that (1) autophagy is altered in patients with HCM carrying MYBPC3 mutations, (2) autophagy is impaired in Mybpc3-targeted knockin mice, and (3) activation of autophagy ameliorated the cardiac disease phenotype in this mouse model. We propose that activation of autophagy might be an attractive option alone or in combination with another therapy to rescue HCM caused by MYBPC3 mutations.

Proteasome inhibition slightly improves cardiac function in mice with hypertrophic cardiomyopathy.

A growing line of evidence indicates a dysfunctional ubiquitin-proteasome system (UPS) in cardiac diseases. Anti-hypertrophic effects and improved cardiac function have been reported after treatment with proteasome inhibitors in experimental models of cardiac hypertrophy. Here we tested whether proteasome inhibition could also reverse the disease phenotype in a genetically-modified mouse model of hypertrophic cardiomyopathy (HCM), which carries a mutation in Mybpc3, encoding the myofilament protein cardiac myosin-binding protein C. At 7 weeks of age, homozygous mutant mice (KI) have 39% higher left ventricular mass-to-body-weight ratio and 29% lower fractional area shortening (FAS) than wild-type (WT) mice. Both groups were treated with epoxomicin (0.5 mg/kg/day) or vehicle for 1 week via osmotic minipumps. Epoxomicin inhibited the chymotrypsin-like activity by ~50% in both groups. All parameters of cardiac hypertrophy (including the fetal gene program) were not affected by epoxomicin treatment in both groups. In contrast, FAS was 12% and 35% higher in epoxomicin-treated than vehicle-treated WT and KI mice, respectively. To identify which genes or pathways could be involved in this positive effect, we performed a transcriptome analysis in KI and WT neonatal cardiac myocytes, treated or not with the proteasome inhibitor MG132 (1 µM, 24 h). This revealed 103 genes (four-fold difference; 5% FDR) which are commonly regulated in both KI and WT cardiac myocytes. Thus, even in genetically-modified mice with manifest HCM, proteasome inhibition showed beneficial effects, at least with regard to cardiac function. Targeting the UPS in cardiac diseases remains therefore a therapeutic option.