Mutant Muscle LIM Protein C58G causes cardiomyopathy through protein depletion

Female Alms1-deficient mice develop echocardiographic features of adult but not infantile Alström syndrome cardiomyopathy

Alström syndrome (AS), a multisystem disorder caused by biallelic ALMS1 mutations, features major early morbidity and mortality due to cardiac complications. The latter are biphasic, including infantile dilated cardiomyopathy and distinct adult-onset cardiomyopathy, and poorly understood. We assessed cardiac function of Alms1 knockout (KO) mice by echocardiography. Cardiac function was unaltered in Alms1 global KO mice of both sexes at postnatal day 15 (P15) and 8 weeks. At 23 weeks, female - but not male - KO mice showed increased left atrial area and decreased isovolumic relaxation time, consistent with early restrictive cardiomyopathy, as well as reduced ejection fraction. No histological or transcriptional changes were seen in myocardium of 23-week-old female Alms1 global KO mice. Female mice with Pdgfra-Cre-driven Alms1 deletion in cardiac fibroblasts and in a small proportion of cardiomyocytes did not recapitulate the phenotype of global KO at 23 weeks. In conclusion, only female Alms1-deficient adult mice show echocardiographic evidence of cardiac dysfunction, consistent with the cardiomyopathy of AS. The explanation for sexual dimorphism remains unclear but might involve metabolic or endocrine differences between sexes.

Exploring Syndecan-4 and MLP and Their Interaction in Primary Cardiomyocytes and H9c2 Cells

The transmembrane proteoglycan syndecan-4 is known to be involved in the hypertrophic response to pressure overload. Although multiple downstream signaling pathways have been found to be involved in this response in a syndecan-4-dependent manner, there are likely more signaling components involved. As part of a larger syndecan-4 interactome screening, we have previously identified MLP as a binding partner to the cytoplasmic tail of syndecan-4. Interestingly, many human MLP mutations have been found in patients with hypertrophic (HCM) and dilated cardiomyopathy (DCM). To gain deeper insight into the role of the syndecan-4-MLP interaction and its potential involvement in MLP-associated cardiomyopathy, we have here investigated the syndecan-4-MLP interaction in primary adult rat cardiomyocytes and the H9c2 cell line. The binding of syndecan-4 and MLP was analyzed in total lysates and subcellular fractions of primary adult rat cardiomyocytes, and baseline and differentiated H9c2 cells by immunoprecipitation. MLP and syndecan-4 localization were determined by confocal microscopy, and MLP oligomerization was determined by immunoblotting under native conditions. Syndecan-4-MLP binding, as well as MLP self-association, were also analyzed by ELISA and peptide arrays. Our results showed that MLP-WT and syndecan-4 co-localized in many subcellular compartments; however, their binding was only detected in nuclear-enriched fractions of isolated adult cardiomyocytes. In vitro, syndecan-4 bound to MLP at three sites, and this binding was reduced in some HCM-associated MLP mutations. While MLP and syndecan-4 also co-localized in many subcellular fractions of H9c2 cells, these proteins did not bind at baseline or after differentiation into cardiomyocyte-resembling cells. Independently of syndecan-4, mutated MLP proteins had an altered subcellular localization in H9c2 cells, compared to MLP-WT. The DCM- and HCM-associated MLP mutations, W4R, L44P, C58G, R64C, Y66C, K69R, G72R, and Q91L, affected the oligomerization of MLP with an increase in monomeric at the expense of trimeric and tetrameric recombinant MLP protein. Lastly, two crucial sites for MLP self-association were identified, which were reduced in most MLP mutations. Our data indicate that the syndecan-4-MLP interaction was present in nuclear-enriched fractions of isolated adult cardiomyocytes and that this interaction was disrupted by some HCM-associated MLP mutations. MLP mutations were also linked to changes in MLP oligomerization and self-association, which may be essential for its interaction with syndecan-4 and a critical molecular mechanism of MLP-associated cardiomyopathy.

Advanced searching for hypertrophic cardiomyopathy heritability in real practice tomorrow

Hypertrophic cardiomyopathy (HCM) is the most common inherited cardiac disease associated with morbidity and mortality at any age. As studies in recent decades have shown, the genetic architecture of HCM is quite complex both in the entire population and in each patient. In the rapidly advancing era of gene therapy, we have to provide a detailed molecular diagnosis to our patients to give them the chance for better and more personalized treatment. In addition to emphasizing the importance of genetic testing in routine practice, this review aims to discuss the possibility to go a step further and create an expanded genetic panel that contains not only variants in core genes but also new candidate genes, including those located in deep intron regions, as well as structural variations. It also highlights the benefits of calculating polygenic risk scores based on a combination of rare and common genetic variants for each patient and of using non-genetic HCM markers, such as microRNAs that can enhance stratification of risk for HCM in unselected populations alongside rare genetic variants and clinical factors. While this review is focusing on HCM, the discussed issues are relevant to other cardiomyopathies.

Computational analysis of the flexibility in the disordered linker region connecting LIM domains in cysteine–glycine-rich protein

One of the key proteins that are present in the Z-disc of cardiac tissues, CSRP3, has been implicated in dilated and hypertrophic cardiomyopathy leading to heart failure. Although multiple cardiomyopathy-related mutations have been reported to reside on the two LIM domains and the disordered regions connecting the domains in this protein, the exact role of the disordered linker region is not clear. The linker harbors a few post-translational modification sites and is expected to be a regulatory site. We have carried out evolutionary studies on 5614 homologs spanning across taxa. We also performed molecular dynamics simulations of full-length CSRP3 to show that the length variations and conformational flexibility of the disordered linker could provide additional levels of functional modulation. Finally, we show that the CSRP3 homologs with widely different lengths of the linker regions could display diversity in their functional specifications. The present study provides a useful perspective to our understanding of the evolution of the disordered region between CSRP3 LIM domains.

Insights into the Role of a Cardiomyopathy-Causing Genetic Variant in ACTN2

Pathogenic variants in ACTN2, coding for alpha-actinin 2, are known to be rare causes of Hypertrophic Cardiomyopathy. However, little is known about the underlying disease mechanisms. Adult heterozygous mice carrying the Actn2 p.Met228Thr variant were phenotyped by echocardiography. For homozygous mice, viable E15.5 embryonic hearts were analysed by High Resolution Episcopic Microscopy and wholemount staining, complemented by unbiased proteomics, qPCR and Western blotting. Heterozygous Actn2 p.Met228Thr mice have no overt phenotype. Only mature males show molecular parameters indicative of cardiomyopathy. By contrast, the variant is embryonically lethal in the homozygous setting and E15.5 hearts show multiple morphological abnormalities. Molecular analyses, including unbiased proteomics, identified quantitative abnormalities in sarcomeric parameters, cell-cycle defects and mitochondrial dysfunction. The mutant alpha-actinin protein is found to be destabilised, associated with increased activity of the ubiquitin-proteasomal system. This missense variant in alpha-actinin renders the protein less stable. In response, the ubiquitin-proteasomal system is activated; a mechanism that has been implicated in cardiomyopathies previously. In parallel, a lack of functional alpha-actinin is thought to cause energetic defects through mitochondrial dysfunction. This seems, together with cell-cycle defects, the likely cause of the death of the embryos. The defects also have wide-ranging morphological consequences.

Whole exome sequencing with a focus on cardiac disease-associated genes in families of sudden unexplained deaths in Yunnan, southwest of China

OBJECTIVES

To explore the causes of sudden unexpected death (SUD) and to search for high-risk people, whole exome sequencing (WES) was performed in families with SUDs.  METHODS: Whole exome sequencing of 25 people from 14 SUD families were screened based on cardiac disease-associated gene variants, and their echocardiograms and electrocardiograms (ECG) were also examined. The protein function of mutated genes was predicted by SIFT, PolyPhen2 and Mutation Assessor.

RESULTS

In the group of 25 people from 14 SUD families, 49 single nucleotide variants (SNVs) of cardiac disease-associated genes were found and verified by Sanger sequencing. 29 SNVs of 14 cardiac disorder-related genes were predicted as pathogens by software. Among them, 7 SNVs carried by two or more members were found in 5 families, including SCN5A (c.3577C > T), IRX4 (c.230A > G), LDB3 (c.2104 T > G), MYH6 (c.3G > A), MYH6 (c.3928 T > C), TTN (c.80987C > T) and TTN (c.8069C > T). 25 ECGs were collected. In summary, 4 people had J-point elevation, 2 people had long QT syndrome (LQTS), 4 people had prolonged QT interval, 3 people had T-wave changes, 3 people had sinus tachycardia, 4 people had sinus bradycardia, 4 people had left side of QRS electrical axis, and 3 people had P wave broadening. Echocardiographic results showed that 1 person had atrial septal defect, 1 person had tricuspid regurgitation, and 2 people had left ventricular diastolic dysfunction.

CONCLUSIONS

Of the 14 heart disease-associated genes in 14 SUDs families, there are 7 possible pathological SNVS may be associated with SUDs. Our results indicate that people with ECG abnormalities, such as prolonged QT interval, ST segment changes, T-wave change and carrying the above 7 SNVs, should be the focus of prevention of sudden death.

Spatial and temporal requirement of Mlp60A isoforms during muscle development and function in Drosophila melanogaster

Many myofibrillar proteins undergo isoform switching in a spatio-temporal manner during muscle development. The biological significance of the variants of several of these myofibrillar proteins remains elusive. One such myofibrillar protein, the Muscle LIM Protein (MLP), is a vital component of the Z-discs. In this paper, we show that one of the Drosophila MLP encoding genes, Mlp60A, gives rise to two isoforms: a short (279 bp, 10 kDa) and a long (1461 bp, 54 kDa) one. The short isoform is expressed throughout development, but the long isoform is adult-specific, being the dominant of the two isoforms in the indirect flight muscles (IFMs). A concomitant, muscle-specific knockdown of both isoforms leads to partial developmental lethality, with most of the surviving flies being flight defective. A global loss of both isoforms in a Mlp60A-null background also leads to developmental lethality, with muscle defects in the individuals that survive to the third instar larval stage. This lethality could be rescued partially by a muscle-specific overexpression of the short isoform. Genetic perturbation of only the long isoform, through a P-element insertion in the long isoform-specific coding sequence, leads to defective flight, in around 90% of the flies. This phenotype was completely rescued when the P-element insertion was precisely excised from the locus. Hence, our data show that the two Mlp60A isoforms are functionally specialized: the short isoform being essential for normal embryonic muscle development and the long isoform being necessary for normal adult flight muscle function.

Structural and signaling proteins in the Z-disk and their role in cardiomyopathies

The sarcomere is the smallest functional unit of muscle contraction. It is delineated by a protein-rich structure known as the Z-disk, alternating with M-bands. The Z-disk anchors the actin-rich thin filaments and plays a crucial role in maintaining the mechanical stability of the cardiac muscle. A multitude of proteins interact with each other at the Z-disk and they regulate the mechanical properties of the thin filaments. Over the past 2 decades, the role of the Z-disk in cardiac muscle contraction has been assessed widely, however, the impact of genetic variants in Z-disk proteins has still not been fully elucidated. This review discusses the various Z-disk proteins (alpha-actinin, filamin C, titin, muscle LIM protein, telethonin, myopalladin, nebulette, and nexilin) and Z-disk-associated proteins (desmin, and obscurin) and their role in cardiac structural stability and intracellular signaling. This review further explores how genetic variants of Z-disk proteins are linked to inherited cardiac conditions termed cardiomyopathies.

LIM domain-wide comprehensive virtual mutagenesis provides structural rationale for cardiomyopathy mutations in CSRP3

Cardiomyopathies are a severe and chronic cardiovascular burden worldwide, affecting a large cohort in the general population. Cysteine and glycine-rich protein 3 (CSRP3) is one of key proteins implicated in dominant dilated cardiomyopathy (DCM) and hypertrophic cardiomyopathy (HCM). In this study, we device a rapid in silico screening protocol that creates a mutational landscape map for all possible allowed and disallowed substitutions in the protein of interest. This map provides the structural and functional insights on the stability of LIM domains of CSRP3. Further, the sequence analysis delineates the eukaryotic CSRP3 protein orthologs which complements the mutational map, but provide limited information of amino acid exchanges. Next, we also evaluated the effect of HCM/DCM mutations on these domains. One of highly destabilising mutations—L44P (also disease causing) and a neutral mutation—L44M were further subjected to molecular dynamics (MD) simulations. The results establish that L44P substitution affects the LIM domain structure by altering secondary structure and due to loss of hydrophobic interaction with Phenylananine 35. The present study provides a useful perspective to our understanding of the role of mutations in the CSRP3 LIM domains and their evolution. This study provides a novel computational screening method for quick identification of key mutation sites for specific protein structures that can reduce the burden on experimental research.

CSRP3, p.Arg122*, is responsible for hypertrophic cardiomyopathy in a Chinese family

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is a hereditary disease manifested by a thickened ventricular wall. Cysteine and glycine-rich protein 3 (CSRP3), the gene encoding muscle LIM protein, is important for initiating hypertrophic gene expression. The mutation of CSRP3 causes dilated cardiomyopathy or HCM.

METHODS

In the present study, we enrolled a Chinese family with HCM across three generations. Whole-exome sequencing (WES) was performed in the proband to detect the candidate genes of the family. Sanger sequencing was performed for mutational analysis and confirmation of cosegregation.

RESULTS

Through histopathological and imaging examinations, an obvious left ventricular hypertrophy was found in the proband. After WES data filtering, bioinformatic prediction and co-segregation analysis, a nonsense mutation (NM_003476.5:c.364C>T; NP_003467.1:p.Arg122*) of CSRP3 was identified in this family. This variant was predicted to be disease-causing and resulted in a truncated protein.

CONCLUSIONS

This is the first HCM family case of CSRP3 (p.Arg122*) variation in Asia. The finding here not only contributes to the genetic diagnosis and counseling of the family, but also provides a new case with detailed phenotypes that may be caused by the CSRP3 variant.

Functional analysis of a gene-edited mouse model to gain insights into the disease mechanisms of a titin missense variant

Titin truncating variants are a well-established cause of cardiomyopathy; however, the role of titin missense variants is less well understood. Here we describe the generation of a mouse model to investigate the underlying disease mechanism of a previously reported titin A178D missense variant identified in a family with non-compaction and dilated cardiomyopathy. Heterozygous and homozygous mice carrying the titin A178D missense variant were characterised in vivo by echocardiography. Heterozygous mice had no detectable phenotype at any time point investigated (up to 1 year). By contrast, homozygous mice developed dilated cardiomyopathy from 3 months. Chronic adrenergic stimulation aggravated the phenotype. Targeted transcript profiling revealed induction of the foetal gene programme and hypertrophic signalling pathways in homozygous mice, and these were confirmed at the protein level. Unsupervised proteomics identified downregulation of telethonin and four-and-a-half LIM domain 2, as well as the upregulation of heat shock proteins and myeloid leukaemia factor 1. Loss of telethonin from the cardiac Z-disc was accompanied by proteasomal degradation; however, unfolded telethonin accumulated in the cytoplasm, leading to a proteo-toxic response in the mice.We show that the titin A178D missense variant is pathogenic in homozygous mice, resulting in cardiomyopathy. We also provide evidence of the disease mechanism: because the titin A178D variant abolishes binding of telethonin, this leads to its abnormal cytoplasmic accumulation. Subsequent degradation of telethonin by the proteasome results in proteasomal overload, and activation of a proteo-toxic response. The latter appears to be a driving factor for the cardiomyopathy observed in the mouse model.

Discriminating aspects of global metabolism of neonatal cardiomyocytes from wild type and KO-CSRP3 rats using proton magnetic resonance spectroscopy of culture media samples

Knockout of multifunction gene cysteine- and glycine-rich protein 3 (CSRP3) in cardiomyocytes (CMs) of mice leads to heart dilation, severely affecting its functions. In humans, CSRP3 mutations are associated with hypertrophic (HCM) and dilated cardiomyopathy (DCM). The absence of the CSRP3 expression produces unknown effects on in vitro neonatal CMs' metabolism. The metabolome changes in culture media conditioned by CSRP3 knockout (KO-CSRP3), and wild type (WT) neonatal cardiomyocytes were investigated under untreated or after metabolic challenging conditions produced by isoproterenol (ISO) stimulation, by in vitro high-resolution proton magnetic resonance spectroscopy (¹H-MRS)-based metabolomics. Metabolic differences between neonatal KO-CSRP3 and WT rats' CMs were identified. After 72 h of culture, ISO administration was associated with increased CMs' energy requirements and increased levels of threonine, alanine, and 3-hydroxybutyrate in both neonatal KO-CSRP3 and WT CMs conditioned media. When compared with KO-CSRP3, culture media derived from WT cells presented higher lactate concentrations either under basal or ISO-stimulated conditions. The higher activity of ketogenic biochemical pathways met the elevated energy requirements of the contractile cells. Both cells are considered phenotypically indistinguishable in the neonatal period of animal lives, but the observed metabolic stress responses of KO-CSRP3 and WT CMs to ISO were different. KO-CSRP3 CMs produced less lactate than WT CMs in both basal and stimulated conditions. Mainly, ISO-stimulated conditions produced evidence for lactate overload within KO-CSRP3 CMs, while WT CMs succeeded to manage the metabolic stress. Thus, ¹H-MRS-based metabolomics was suitable to identify early inefficient energetic metabolism in neonatal KO-CSRP3 CMs. These results may reflect an apparent lower lactate transport and consumption, in association with protein catabolism.