Alterations at the intercalated disk associated with the absence of muscle LIM protein

Developing Gene Therapy for Mitigating Multisystemic Pathology in Fabry Disease: Proof of Concept in an Aggravated Mouse Model

Fabry disease (FD) is a multisystemic lysosomal storage disorder caused by the loss of α-galactosidase A (α-Gal) function. The current standard of care, enzyme replacement therapies, while effective in reducing kidney pathology when treated early, do not fully ameliorate cardiac issues, neuropathic manifestations, and risk of cerebrovascular events. Adeno-associated virus (AAV)-based gene therapies (AAV-GT) can provide superior efficacy across multiple tissues owing to continuous, endogenous production of the therapeutic enzyme and lower treatment burden. We set out to develop a robust AAV-GT to achieve optimal efficacy with the lowest feasible dose to minimize any safety risks that are associated with high-dose AAV-GTs. In this proof-of-concept study, we evaluated the effectiveness of an rAAV9 vector expressing human GLA transgene under a strong ubiquitous promoter, combined with woodchuck hepatitis virus posttranscriptional regulatory element (rAAV9-hGLA). We tested our GT at three different doses, 5e10 vg/kg, 2.5e11 vg/kg, and 6.25e12 vg/kg in the G3Stg/GLAko Fabry mouse model that has tissue Gb3 substrate levels comparable with patients with FD and develops several early FD pathologies. After intravenous injections of rAAV9-hGLA at 11 weeks of age, we observed dose-dependent increases in α-Gal activity in the key target tissues, reaching as high as 393-fold of WT in the kidneys and 6156-fold in the heart at the highest dose. Complete or near-complete substrate clearance was observed in animals treated with the two higher dose levels tested in all tissues except for the brain. We also found dose-dependent improvements in several pathological biomarkers, as well as prevention of structural and functional organ pathology. Taken together, these results indicate that an AAV-GT under a strong ubiquitous promoter has the potential to address the unmet therapeutic needs in patients with FD at relatively low doses.

Large animal models of pressure overload-induced cardiac left ventricular hypertrophy to study remodelling of the human heart with aortic stenosis

Pathologic cardiac hypertrophy is a common consequence of many cardiovascular diseases, including aortic stenosis (AS). AS is known to increase the pressure load of the left ventricle, causing a compensative response of the cardiac muscle, which progressively will lead to dilation and heart failure. At a cellular level, this corresponds to a considerable increase in the size of cardiomyocytes, known as cardiomyocyte hypertrophy, while their proliferation capacity is attenuated upon the first developmental stages. Cardiomyocytes, in order to cope with the increased workload (overload), suffer alterations in their morphology, nuclear content, energy metabolism, intracellular homeostatic mechanisms, contractile activity, and cell death mechanisms. Moreover, modifications in the cardiomyocyte niche, involving inflammation, immune infiltration, fibrosis, and angiogenesis, contribute to the subsequent events of a pathologic hypertrophic response. Considering the emerging need for a better understanding of the condition and treatment improvement, as the only available treatment option of AS consists of surgical interventions at a late stage of the disease, when the cardiac muscle state is irreversible, large animal models have been developed to mimic the human condition, to the greatest extend. Smaller animal models lack physiological, cellular and molecular mechanisms that sufficiently resemblance humans and in vitro techniques yet fail to provide adequate complexity. Animals, such as the ferret (Mustello purtorius furo), lapine (rabbit, Oryctolagus cunigulus), feline (cat, Felis catus), canine (dog, Canis lupus familiaris), ovine (sheep, Ovis aries), and porcine (pig, Sus scrofa), have contributed to research by elucidating implicated cellular and molecular mechanisms of the condition. Essential discoveries of each model are reported and discussed briefly in this review. Results of large animal experimentation could further be interpreted aiming at prevention of the disease progress or, alternatively, at regression of the implicated pathologic mechanisms to a physiologic state. This review summarizes the important aspects of the pathophysiology of LV hypertrophy and the applied surgical large animal models that currently better mimic the condition.

A novel loss-of-function mutation in NRAP is associated with left ventricular non-compaction cardiomyopathy

Background

The nebulin-related-anchoring protein (NRAP) gene encodes actin-associated ankyrin. Few studies reported the association of the NRAP gene with cardiomyopathy. Thus, the genetic role of this gene in cardiomyopathy remains to be investigated.

Methods

The clinical data of the rare case of left ventricular non-compaction (LVNC) were collected and analyzed. Whole-exome sequencing (WES) was performed on related family members. Western blot was used to detect the effect of mutation on the NRAP protein expression. The effect of the c.259delC variant on myocardial development was further evaluated in a zebrafish model.

Results

A novel homozygous frameshift mutation c.259delC of NRAP was found in the proband with LVNC. It was found that c.259delC decreased the expression of NRAP by Western blot. In the zebrafish model, the heart development was affected while knocking out the NRAP gene, which showed pericardial edema. The pathological manifestations were uneven hypertrophy, disordered arrangement of cardiomyocytes, enlarged intercellular space, and loose muscle fibers. RNA-sequencing (RNA-seq) showed that the expression of genes related to heart development decreased significantly, and the NRAP gene mutation could participate in biological processes (BPs) such as myocardial contraction, cell adhesion, myosin coarse filament assembly of striated muscle, myosin complex composition, and muscle α-actin binding.

Conclusion

We identified a rare case of LVNC associated with a novel homozygous NRAP frameshift variant. This study further strengthened the evidence linking mutations in the NRAP gene with LVNC, providing a new clue for further study of LVNC. NRAP may be one of the pathogenic genes of cardiomyopathy.

SIRT1 activation and its effect on intercalated disc proteins as a way to reduce doxorubicin cardiotoxicity

According to the World Health Organization, the neoplasm is one of the main reasons for morbidity and mortality worldwide. At the same time, application of cytostatic drugs like an independent type of cancer treatment and in combination with surgical methods, is often associated with the development of cardiovascular complications both in the early and in the delayed period of treatment. Doxorubicin (DOX) is the most commonly used cytotoxic anthracycline antibiotic. DOX can cause both acute and delayed side effects. The problem is still not solved, as evidenced by the continued activity of researchers in terms of developing approaches for the prevention and treatment of cardiovascular complications. It is known, the heart muscle consists of cardiomyocytes connected by intercalated discs (ID), which ensure the structural, electrical, metabolic unity of the heart. Various defects in the ID proteins can lead to the development of cardiovascular diseases of various etiologies, including DOX-induced cardiomyopathy. The search for ways to influence the functioning of ID proteins of the cardiac muscle can become the basis for the creation of new therapeutic approaches to the treatment and prevention of cardiac pathologies. SIRT1 may be an interesting cardioprotective variant due to its wide functional significance. SIRT1 activation triggers nuclear transcription programs that increase the efficiency of cellular, mitochondrial metabolism, increases resistance to oxidative stress, and promotes cell survival. It can be assumed that SIRT1 can not only provide a protective effect at the cardiomyocytes level, leading to an improvement in mitochondrial and metabolic functions, reducing the effects of oxidative stress and inflammatory processes, but also have a protective effect on the functioning of IDs structures of the cardiac muscle.

Role of Muscle LIM Protein in Mechanotransduction Process

The induction of protein synthesis is crucial to counteract the deconditioning of neuromuscular system and its atrophy. In the past, hormones and cytokines acting as growth factors involved in the intracellular events of these processes have been identified, while the implications of signaling pathways associated with the anabolism/catabolism ratio in reference to the molecular mechanism of skeletal muscle hypertrophy have been recently identified. Among them, the mechanotransduction resulting from a mechanical stress applied to the cell appears increasingly interesting as a potential pathway for therapeutic intervention. At present, there is an open question regarding the type of stress to apply in order to induce anabolic events or the type of mechanical strain with respect to the possible mechanosensing and mechanotransduction processes involved in muscle cells protein synthesis. This review is focused on the muscle LIM protein (MLP), a structural and mechanosensing protein with a LIM domain, which is expressed in the sarcomere and costamere of striated muscle cells. It acts as a transcriptional cofactor during cell proliferation after its nuclear translocation during the anabolic process of differentiation and rebuilding. Moreover, we discuss the possible opportunity of stimulating this mechanotransduction process to counteract the muscle atrophy induced by anabolic versus catabolic disorders coming from the environment, aging or myopathies.

LMNA mutation leads to cardiac sodium channel dysfunction in the Emery-Dreifuss muscular dystrophy patient

Pathogenic variants in the LMNA gene are known to cause laminopathies, a broad range of disorders with different clinical phenotypes. LMNA genetic variants lead to tissue-specific pathologies affecting various tissues and organs. Common manifestations of laminopathies include cardiovascular system abnormalities, in particular, cardiomyopathies and conduction disorders. In the present study, we used induced pluripotent stem cells from a patient carrying LMNA p.R249Q genetic variant to create an in vitro cardiac model of laminopathy. Induced pluripotent stem cell-derived cardiomyocytes with LMNA p.R249Q genetic variant showed a decreased sodium current density and an impaired sodium current kinetics alongside with changes in transcription levels of cardiac-specific genes. Thus, we obtained compelling in vitro evidence of an association between LMNA p.R249Q genetic variant and cardiac-related abnormalities.

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.

Decrease of Pdzrn3 is required for heart maturation and protects against heart failure

Heart failure is the final common stage of most cardiopathies. Cardiomyocytes (CM) connect with others via their extremities by intercalated disk protein complexes. This planar and directional organization of myocytes is crucial for mechanical coupling and anisotropic conduction of the electric signal in the heart. One of the hallmarks of heart failure is alterations in the contact sites between CM. Yet no factor on its own is known to coordinate CM polarized organization. We have previously shown that PDZRN3, an ubiquitine ligase E3 expressed in various tissues including the heart, mediates a branch of the Planar cell polarity (PCP) signaling involved in tissue patterning, instructing cell polarity and cell polar organization within a tissue. PDZRN3 is expressed in the embryonic mouse heart then its expression dropped significantly postnatally corresponding with heart maturation and CM polarized elongation. A moderate CM overexpression of Pdzrn3 (Pdzrn3 OE) during the first week of life, induced a severe eccentric hypertrophic phenotype with heart failure. In models of pressure-overload stress heart failure, CM-specific Pdzrn3 knockout showed complete protection against degradation of heart function. We reported that Pdzrn3 signaling induced PKC ζ expression, c-Jun nuclear translocation and a reduced nuclear ß catenin level, consistent markers of the planar non-canonical Wnt signaling in CM. We then show that subcellular localization (intercalated disk) of junction proteins as Cx43, ZO1 and Desmoglein 2 was altered in Pdzrn3 OE mice, which provides a molecular explanation for impaired CM polarization in these mice. Our results reveal a novel signaling pathway that controls a genetic program essential for heart maturation and maintenance of overall geometry, as well as the contractile function of CM, and implicates PDZRN3 as a potential therapeutic target for the prevention of human heart failure.

Sarcomeres Morphology and Z-Line Arrangement Disarray Induced by Ventricular Premature Contractions through the Rac2/Cofilin Pathway

The most common ventricular premature contractions (VPCs) originate from the right ventricular outflow tract (RVOT), but the molecular mechanisms of altered cytoskeletons of VPC-induced cardiomyopathy remain unexplored. We created a RVOT bigeminy VPC pig model (n = 6 in each group). Echocardiography was performed. The histopathological alternations in the LV myocardium were analyzed, and next generation sequencing (NGS) and functional enrichment analyses were employed to identify the differentially expressed genes (DEGs) responsible for the histopathological alternations. Finally, a cell silencing model was used to confirm the key regulatory gene and pathway. VPC pigs had increased LV diameters in the 6-month follow-up period. A histological study showed more actin cytoskeleton disorganization and actin accumulation over intercalated disc, Z-line arrangement disarray, increased β-catenin expression, and cardiomyocyte enlargement in the LV myocardium of the VPC pigs compared to the control pigs. The NGS study showed actin cytoskeleton signaling, RhoGDI signaling, and signaling by Rho Family GTPases and ILK Signaling presented z-scores with same activation states. The expressions of Rac family small GTPase 2 (Rac2), the p-cofilin/cofilin ratio, and the F-actin/G-actin ratio were downregulated in the VPC group compared to the control group. Moreover, the intensity and number of actin filaments per cardiomyocyte were significantly decreased by Rac2 siRNA in the cell silencing model. Therefore, the Rac2/cofilin pathway was found to play a crucial role in the sarcomere morphology and Z-line arrangement disarray induced by RVOT bigeminy VPCs.

Demographic History, Adaptation, and NRAP Convergent Evolution at Amino Acid Residue 100 in the World Northernmost Cattle from Siberia

Native cattle breeds represent an important cultural heritage. They are a reservoir of genetic variation useful for properly responding to agriculture needs in the light of ongoing climate changes. Evolutionary processes that occur in response to extreme environmental conditions could also be better understood using adapted local populations. Herein, different evolutionary histories of the world northernmost native cattle breeds from Russia were investigated. They highlighted Kholmogory as a typical taurine cattle, whereas Yakut cattle separated from European taurines approximately 5,000 years ago and contain numerous ancestral and some novel genetic variants allowing their adaptation to harsh conditions of living above the Polar Circle. Scans for selection signatures pointed to several common gene pathways related to adaptation to harsh climates in both breeds. But genes affected by selection from these pathways were mostly different. A Yakut cattle breed-specific missense mutation in a highly conserved NRAP gene represents a unique example of a young amino acid residue convergent change shared with at least 16 species of hibernating/cold-adapted mammals from six distinct phylogenetic orders. This suggests a convergent evolution event along the mammalian phylogenetic tree and fast fixation in a single isolated cattle population exposed to a harsh climate.

Biallelic loss-of-function in NRAP is a cause of recessive dilated cardiomyopathy

BACKGROUND

Familial dilated cardiomyopathy (DCM) is typically a monogenic disorder with dominant inheritance. Although over 40 genes have been linked to DCM, more than half of the patients undergoing comprehensive genetic testing are left without molecular diagnosis. Recently, biallelic protein-truncating variants (PTVs) in the nebulin-related anchoring protein gene (NRAP) were identified in a few patients with sporadic DCM.

METHODS AND RESULTS

We determined the frequency of rare NRAP variants in a cohort of DCM patients and control patients to further evaluate role of this gene in cardiomyopathies. A retrospective analysis of our internal variant database consisting of 31,639 individuals who underwent genetic testing (either panel or direct exome sequencing) was performed. The DCM group included 577 patients with either a confirmed or suspected DCM diagnosis. A control cohort of 31,062 individuals, including 25,912 individuals with non-cardiac (control group) and 5,150 with non-DCM cardiac indications (Non-DCM cardiac group). Biallelic (n = 6) or two (n = 5) NRAP variants (two PTVs or PTV+missense) were identified in 11 unrelated probands with DCM (1.9%) but none of the controls. None of the 11 probands had an alternative molecular diagnosis. Family member testing supports co-segregation. Biallelic or potentially biallelic NRAP variants were enriched in DCM vs. controls (OR 1052, p<0.0001). Based on the frequency of NRAP PTVs in the gnomAD reference population, and predicting full penetrance, biallelic NRAP variants could explain 0.25%-2.46% of all DCM cases.

CONCLUSION

Loss-of-function in NRAP is a cause for autosomal recessive dilated cardiomyopathy, supporting its inclusion in comprehensive genetic testing.

Electrical stimulation applied during differentiation drives the hiPSC-CMs towards a mature cardiac conduction-like cells

Human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs) resemble fetal cardiomyocytes and electrical stimulation (ES) has been explored to mature the differentiated cells. Here, we hypothesize that ES applied at the beginning of the differentiation process, triggers both differentiation of the hiPSC-CMs into a specialized conduction system (CS) phenotype and cell maturation. We applied ES for 15 days starting on day 0 of the differentiation process and found an increased expression of transcription factors and proteins associated with the development and function of CS including Irx3, Nkx2.5 and contactin 2, Hcn4 and Scn5a, respectively. We also found activation of intercalated disc proteins (Nrap and β-catenin). We detected ES-induced CM maturation as indicated by increased Tnni1 and Tnni3 expression. Confocal micrographs showed a shift towards expression of the gap junction protein connexin 40 in ES hiPSC-CM compared to the more dominant expression of connexin 43 in controls. Finally, analysis of functional parameters revealed that ES hiPSC-CMs exhibited faster action potential (AP) depolarization, longer intracellular Ca2+ transients, and slower AP duration at 90% of repolarization, resembling fast conducting fibers. Altogether, we provided evidence that ES during the differentiation of hiPSC to cardiomyocytes lead to development of cardiac conduction-like cells with more mature cytoarchitecture. Thus, hiPSC-CMs exposed to ES during differentiation can be instrumental to develop CS cells for cardiac disease modelling, screening individual drugs on a precison medicine type platform and support the development of novel therapeutics for arrhythmias.

The intercalated disc: a mechanosensing signalling node in cardiomyopathy

Cardiomyocytes, the cells generating contractile force in the heart, are connected to each other through a highly specialised structure, the intercalated disc (ID), which ensures force transmission and transduction between neighbouring cells and allows the myocardium to function in synchrony. In addition, cardiomyocytes possess an intrinsic ability to sense mechanical changes and to regulate their own contractile output accordingly. To achieve this, some of the components responsible for force transmission have evolved to sense changes in tension and to trigger a biochemical response that results in molecular and cellular changes in cardiomyocytes. This becomes of particular importance in cardiomyopathies, where the heart is exposed to increased mechanical load and needs to adapt to sustain its contractile function. In this review, we will discuss key mechanosensing elements present at the intercalated disc and provide an overview of the signalling molecules involved in mediating the responses to changes in mechanical force.

Retrospective Study of Intercalated Disk Defects Associated with Dilated Cardiomyopathy, Atrial Thrombosis, and Heart Failure in BALB/c Mice Deficient in IL4 Receptor α

An increased incidence of dilated cardiomyopathy and atrial thrombosis was noted in a breeding colony of BALB/c mice deficient in IL4 receptor α. The condition affected mice of both sexes and of various ages, and extensive testing (microbiology, serology, histopathology) failed to ascertain the cause. Transmission electron microscopy of heart samples showed structural defects in the myocardial intercalated disks, characterized by unorganized and heavily convoluted arrangement with lower density and less prominent desmosomes and adherens junctions, widening of the intercellular space, myofibrillar lysis adjacent to intercalated disks, occasional sarcomere lysis with marked myofiber degeneration, vacuolation, accumulation of cell debris, and myelin figures. The intercalated disk contains cell adhesion molecules that form cell junctions, allowing contraction coupling of cardiomyocytes and the electrical and mechanical connection between cardiac fibers. Thus, defects at this level result in poor myocardial contraction, intracardiac blood stagnation, and consequently cardiac dilation with clinical signs of heart failure. The background strain or, potentially, the Cre-loxP-mediated recombination system used to create these mice may have contributed to the elevated incidence of cardiomyopathy and atrial thrombosis in this colony. Due to the backcrossing breeding scheme used, we cannot discount the emergence and colonywide dissemination of a spontaneous mutation that affects the intercalated disk. This report underscores the importance of carefully monitoring genetically modified mice colonies for unexpected phenotypes that may result from spontaneous or unintended mutations or enhanced strain background pathology.

The M-band: The underestimated part of the sarcomere

The sarcomere is the basic unit of the myofibrils, which mediate skeletal and cardiac Muscle contraction. Two transverse structures, the Z-disc and the M-band, anchor the thin (actin and associated proteins) and thick (myosin and associated proteins) filaments to the elastic filament system composed of titin. A plethora of proteins are known to be integral or associated proteins of the Z-disc and its structural and signalling role in muscle is better understood, while the molecular constituents of the M-band and its function are less well defined. Evidence discussed here suggests that the M-band is important for managing force imbalances during active muscle contraction. Its molecular composition is fine-tuned, especially as far as the structural linkers encoded by members of the myomesin family are concerned and depends on the specific mechanical characteristics of each particular muscle fibre type. Muscle activity signals from the M-band to the nucleus and affects transcription of sarcomeric genes, especially via serum response factor (SRF). Due to its important role as shock absorber in contracting muscle, the M-band is also more and more recognised as a contributor to muscle disease.

The SLMAP/Striatin complex: An emerging regulator of normal and abnormal cardiac excitation-contraction coupling

The excitation-contraction (E-C) module involves a harmonized correspondence between the sarcolemma and the sarcoplasmic reticulum. This is provided by membrane proteins, which primarily shape the caveolae, the T-tubule/Sarcoplasmic reticulum (TT/SR) junction, and the intercalated discs (ICDs). Distortion of either one of these structures impairs myocardial contraction, and subsequently translates into cardiac failure. Thus, detailed studies on the molecular cues of the E-C module are becoming increasingly necessary to pharmacologically eradicate cardiac failure Herein we reviewed the organization of caveolae, TT/SR junctions, and the ICDs in the heart, with special attention to the Sarcolemma Membrane Associated Protein (SLMAP) and striatin (STRN) in cardiac membranes biology and cardiomyocyte contraction. We emphasized on their in vivo and in vitro signaling in cardiac function/dysfunction. SLMAP is a cardiac membrane protein that plays an important role in E-C coupling and the adrenergic response of the heart. Similarly, STRN is a dynamic protein that is also involved in cardiac E-C coupling and ICD-related cardiomyopathies. Both SLMAP and STRN are linked to cardiac conditions, including heart failure, and their role in cardiomyocyte function was elucidated in our laboratory. They interact together in a protein complex that holds therapeutic potentials for cardiac dysfunction. This review is the first of its kind to conceptualize the role of the SLMAP/STRN complex in cardiac function and failure. It provides in depth information on the signaling of these two proteins and projects their interaction as a novel therapeutic target for cardiac failure.

Mechanical regulation of gene expression in cardiac myocytes and fibroblasts

The intact heart undergoes complex and multiscale remodelling processes in response to altered mechanical cues. Remodelling of the myocardium is regulated by a combination of myocyte and non-myocyte responses to mechanosensitive pathways, which can alter gene expression and therefore function in these cells. Cellular mechanotransduction and its downstream effects on gene expression are initially compensatory mechanisms during adaptations to the altered mechanical environment, but under prolonged and abnormal loading conditions, they can become maladaptive, leading to impaired function and cardiac pathologies. In this Review, we summarize mechanoregulated pathways in cardiac myocytes and fibroblasts that lead to altered gene expression and cell remodelling under physiological and pathophysiological conditions. Developments in systems modelling of the networks that regulate gene expression in response to mechanical stimuli should improve integrative understanding of their roles in vivo and help to discover new combinations of drugs and device therapies targeting mechanosignalling in heart disease.

Zebrafish cysteine and glycine-rich protein 3 is essential for mechanical stability in skeletal muscles

Cysteine and glycine-rich protein 3 (CSRP3) is a striated muscle-specific cytoskeleton protein which participates in cardiac stretch sensing. Mutations in CSRP3 gene cause cardiomyopathies and deregulation of CSRP3 has been found in patients with heart failure and several skeletal muscle diseases. However, the mechanism underneath these disorders still remains poorly understood. Here we generated the first csrp3 knockout zebrafish. csrp3^-/- embryos showed no gross morphological defects but csrp3 deficient skeletal muscle fibers were prone to lesions upon prolonged stretching force. Further studies revealed csrp3 cooperatively interacted with ilk to maintain skeletal muscle mechanical stability and regulated tcap activation. Thus, our work has established a zebrafish model to investigate the function of csrp3 gene, and provides novel insights towards how csrp3 defects may lead to skeletal myopathies by a mechanistic link between Csrp3 and force stimuli.

LASP2 suppressed malignancy and Wnt/β-catenin signaling pathway activation in bladder cancer

LIM and SH3 Protein 2 (LASP2), a member of the nebulin family of actin-binding proteins, is associated with the development of cancers. However, little was known about the role of LASP2 in human bladder cancer. In the current study, LASP2 expression was evaluated by reverse transcription-quantitative polymerase chain reaction analysis in bladder cancer cell lines and tissue samples. The role of LASP2 in cancer cell proliferation, migration and invasion, and angiogenesis was explored. The association between prognostic outcomes and LASP2 expression were examined using Kaplan-Meier analysis. LASP2 expression was decreased in bladder cancer cells and tissues. LASP2 expression was associated with tumor size (P=0.016) and T classification (P=0.001). Patients with lower LASP2 expression had shorter overall and recurrent-free survival times. Overexpression of LASP2 inhibited and silencing of LASP2 promoted the proliferation, migration and invasion of bladder cancer cells, and angiogenesis in bladder cancer. Furthermore, it was determined that the tumor suppressing effect of LASP2 may be associated with the inactivation of the Wnt/β-catenin signaling pathway. LASP2 may represent a novel and useful prognostic indicator, and could serve as a potential therapeutic target for bladder cancer.

PRMT1 Deficiency in Mouse Juvenile Heart Induces Dilated Cardiomyopathy and Reveals Cryptic Alternative Splicing Products

Protein arginine methyltransferase 1 (PRMT1) catalyzes the asymmetric dimethylation of arginine residues in proteins and methylation of various RNA-binding proteins and is associated with alternative splicing in vitro. Although PRMT1 has essential in vivo roles in embryonic development, CNS development, and skeletal muscle regeneration, the functional importance of PRMT1 in the heart remains to be elucidated. Here, we report that juvenile cardiomyocyte-specific PRMT1-deficient mice develop severe dilated cardiomyopathy and exhibit aberrant cardiac alternative splicing. Furthermore, we identified previously undefined cardiac alternative splicing isoforms of four genes (Asb2, Fbxo40, Nrap, and Eif4a2) in PRMT1-cKO mice and revealed that eIF4A2 protein isoforms translated from alternatively spliced mRNA were differentially ubiquitinated and degraded by the ubiquitin-proteasome system. These findings highlight the essential roles of PRMT1 in cardiac homeostasis and alternative splicing regulation.

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PRMT1 deficiency in cardiomyocytes causes dilated cardiomyopathy in juvenile mice

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PRMT1-deficient heart shows abnormal alternative splicing patterns

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Previously undefined cardiac splicing events are revealed by transcriptome analysis

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eIF4A2 isoforms are differentially ubiquitinated and degraded

Riding the waves of the intercalated disc of the heart

Cardiomyocytes interact with each other at their ends through the specialised membrane complex, the intercalated disck (ID). It is a fascinating structure. It allows cardiomyocytes to interact with several neighbouring cells, thereby allowing the complex structure of the heart to develop. It acts as tension transducer, structural prop, and multi signalling domain as well as a regulator of growth. It achieves its many functions through a number of specialised domains and intercellular junctions associated with its complex folded membrane. This review outlines the results of some 20 years of fascination with the ups and downs of the ID. These include locating the spectrin-associated membrane cytoskeleton in the ID and investigating the role of Protein 4.1R in calcium signalling; structural studies of the relationship of the ID to myofibrils, sarcoplasmic reticulum and mitochondria and, finally, consideration of the role of the ID in cardiomyocyte growth and heart disease.

At the heart of inter- and intracellular signaling: the intercalated disc

Proper cardiac function requires the synchronous mechanical and electrical coupling of individual cardiomyocytes. The intercalated disc (ID) mediates coupling of neighboring myocytes through intercellular signaling. Intercellular communication is highly regulated via intracellular signaling, and signaling pathways originating from the ID control cardiomyocyte remodeling and function. Herein, we present an overview of the inter- and intracellular signaling that occurs at and originates from the intercalated disc in normal physiology and pathophysiology. This review highlights the importance of the intercalated disc as an integrator of signaling events regulating homeostasis and stress responses in the heart and the center of several pathophysiological processes mediating the development of cardiomyopathies.

Actin-associated proteins and cardiomyopathy-the 'unknown' beyond troponin and tropomyosin

It has been known for several decades that mutations in genes that encode for proteins involved in the control of actomyosin interactions such as the troponin complex, tropomyosin and MYBP-C and thus regulate contraction can lead to hereditary hypertrophic cardiomyopathy. In recent years, it has become apparent that actin-binding proteins not directly involved in the regulation of contraction also can exhibit changed expression levels, show altered subcellular localisation or bear mutations that might lead to hereditary cardiomyopathies. The aim of this review is to look beyond the troponin/tropomyosin mechanism and to give an overview of the different types of actin-associated proteins and their potential roles in cardiomyocytes. It will then discuss recent findings relevant to their involvement in heart disease.

When signalling goes wrong: pathogenic variants in structural and signalling proteins causing cardiomyopathies

Cardiomyopathies are a diverse group of cardiac disorders with distinct phenotypes, depending on the proteins and pathways affected. A substantial proportion of cardiomyopathies are inherited and those will be the focus of this review article. With the wide application of high-throughput sequencing in the practice of clinical genetics, the roles of novel genes in cardiomyopathies are recognised. Here, we focus on a subgroup of cardiomyopathy genes [TTN, FHL1, CSRP3, FLNC and PLN, coding for Titin, Four and a Half LIM domain 1, Muscle LIM Protein, Filamin C and Phospholamban, respectively], which, despite their diverse biological functions, all have important signalling functions in the heart, suggesting that disturbances in signalling networks can contribute to cardiomyopathies.

Increased Cardiac Arrhythmogenesis Associated With Gap Junction Remodeling With Upregulation of RNA-Binding Protein FXR1

BACKGROUND

Gap junction remodeling is well established as a consistent feature of human heart disease involving spontaneous ventricular arrhythmia. The mechanisms responsible for gap junction remodeling that include alterations in the distribution of, and protein expression within, gap junctions are still debated. Studies reveal that multiple transcriptional and posttranscriptional regulatory pathways are triggered in response to cardiac disease, such as those involving RNA-binding proteins. The expression levels of FXR1 (fragile X mental retardation autosomal homolog 1), an RNA-binding protein, are critical to maintain proper cardiac muscle function; however, the connection between FXR1 and disease is not clear.

METHODS

To identify the mechanisms regulating gap junction remodeling in cardiac disease, we sought to identify the functional properties of FXR1 expression, direct targets of FXR1 in human left ventricle dilated cardiomyopathy (DCM) biopsy samples and mouse models of DCM through BioID proximity assay and RNA immunoprecipitation, how FXR1 regulates its targets through RNA stability and luciferase assays, and functional consequences of altering the levels of this important RNA-binding protein through the analysis of cardiac-specific FXR1 knockout mice and mice injected with 3xMyc-FXR1 adeno-associated virus.

RESULTS

FXR1 expression is significantly increased in tissue samples from human and mouse models of DCM via Western blot analysis. FXR1 associates with intercalated discs, and integral gap junction proteins Cx43 (connexin 43), Cx45 (connexin 45), and ZO-1 (zonula occludens-1) were identified as novel mRNA targets of FXR1 by using a BioID proximity assay and RNA immunoprecipitation. Our findings show that FXR1 is a multifunctional protein involved in translational regulation and stabilization of its mRNA targets in heart muscle. In addition, introduction of 3xMyc-FXR1 via adeno-associated virus into mice leads to the redistribution of gap junctions and promotes ventricular tachycardia, showing the functional significance of FXR1 upregulation observed in DCM.

CONCLUSIONS

In DCM, increased FXR1 expression appears to play an important role in disease progression by regulating gap junction remodeling. Together this study provides a novel function of FXR1, namely, that it directly regulates major gap junction components, contributing to proper cell-cell communication in the heart.

Overview of the Muscle Cytoskeleton

Cardiac and skeletal striated muscles are intricately designed machines responsible for muscle contraction. Coordination of the basic contractile unit, the sarcomere, and the complex cytoskeletal networks are critical for contractile activity. The sarcomere is comprised of precisely organized individual filament systems that include thin (actin), thick (myosin), titin, and nebulin. Connecting the sarcomere to other organelles (e.g., mitochondria and nucleus) and serving as the scaffold to maintain cellular integrity are the intermediate filaments. The costamere, on the other hand, tethers the sarcomere to the cell membrane. Unique structures like the intercalated disc in cardiac muscle and the myotendinous junction in skeletal muscle help synchronize and transmit force. Intense investigation has been done on many of the proteins that make up these cytoskeletal assemblies. Yet the details of their function and how they interconnect have just started to be elucidated. A vast number of human myopathies are contributed to mutations in muscle proteins; thus understanding their basic function provides a mechanistic understanding of muscle disorders. In this review, we highlight the components of striated muscle with respect to their interactions, signaling pathways, functions, and connections to disease. © 2017 American Physiological Society. Compr Physiol 7:891-944, 2017.

Homozygous truncating mutation in NRAP gene identified by whole exome sequencing in a patient with dilated cardiomyopathy

The genetic background of dilated cardiomyopathy is highly heterogeneous, with close to 100 known genes and a number of candidates described to date. Nebulin-related-anchoring protein (NRAP) is an actin-binding cytoskeletal protein expressed predominantly in striated and cardiac muscles, and is involved in myofibrillar assembly in the foetal heart and in force transmission in the adult heart. The homozygous NRAP truncating variant (rs201084642), which is predicted to introduce premature stop codon into all NRAP isoforms, was revealed in the dilated cardiomyopathy patient using whole exome sequencing. The same genotype was detected in the asymptomatic proband's brother. The expression of the NRAP protein was undetectable in the patient's heart muscle by the Western blot. Genotyping for rs201084642 in the ethnically matched cohort of 231 dilated cardiomyopathy patients did not reveal any additional subjects with this variant. Our findings suggest that the biallelic loss-of-function mutation in NRAP could constitute a relatively rare, low-penetrance genetic risk factor for dilated cardiomyopathy.

Mechanotransduction and Metabolism in Cardiomyocyte Microdomains

Efficient contractions of the left ventricle are ensured by the continuous transfer of adenosine triphosphate (ATP) from energy production sites, the mitochondria, to energy utilization sites, such as ionic pumps and the force-generating sarcomeres. To minimize the impact of intracellular ATP trafficking, sarcomeres and mitochondria are closely packed together and in proximity with other ultrastructures involved in excitation-contraction coupling, such as t-tubules and sarcoplasmic reticulum junctions. This complex microdomain has been referred to as the intracellular energetic unit. Here, we review the literature in support of the notion that cardiac homeostasis and disease are emergent properties of the hierarchical organization of these units. Specifically, we will focus on pathological alterations of this microdomain that result in cardiac diseases through energy imbalance and posttranslational modifications of the cytoskeletal proteins involved in mechanosensing and transduction.

Cardiac Mechanoperception: A Life-Long Story from Early Beats to Aging and Failure

The life-long story of the heart starts concomitantly with primary differentiation events occurring in multipotent progenitors located in the so-called heart tube. This initially tubular structure starts a looping process, which leads to formation of the final four-chambered heart with a primary contribution of geometric and position-associated cell sensing. While this establishes the correct patterning of the final cardiac structure, it also provides feedbacks to fundamental cellular machineries controlling proliferation and differentiation, thus ensuring a coordinated restriction of cell growth and a myocyte terminal differentiation. Novel evidences provided by embryological and cell engineering studies have clarified the relevance of mechanics-supported position sensing for the correct recognition of cell fate inside developing embryos and multicellular aggregates. One of the main components of this pathway, the Hippo-dependent signal transduction machinery, is responsible for cell mechanics intracellular transduction with important consequences for gene transcription and cell growth control. Being the Hippo pathway also directly connected to stress responses and altered metabolism, it is tempting to speculate that permanent alterations of mechanosensing may account for modifying self-renewal control in tissue homeostasis. In the present contribution, we translate these concepts to the aging process and the failing of the human heart, two pathophysiologic conditions that are strongly affected by stress responses and altered metabolism.

Sarcomeric lesions and remodeling proximal to intercalated disks in overload-induced cardiac hypertrophy

Pressure overload induces cardiac remodeling involving both the contractile machinery and intercalated disks (IDs). Filamin C (FlnC) and Xin actin-binding repeat-containing proteins (XIRPs) are multi-adapters localizing in IDs of higher vertebrates. Knockout of the gene encoding Xin (Xirp1) in mice leads to a mild cardiac phenotype with ID mislocalization. In order to amplify this phenotype, we performed transverse aortic constriction (TAC) on control and Xirp1-deficient mice. TAC induced similar left ventricular hypertrophy in both genotypes, suggesting that the lack of Xin does not lead to higher susceptibility to cardiac overload. However, in both genotypes, FlnC appeared in "streaming" localizations across multiple sarcomeres proximal to the IDs, suggesting a remodeling response. Furthermore, FlnC-positive areas of remodeling, reminiscent of sarcomeric lesions previously described for skeletal muscles (but so far unreported in the heart), were also observed. These adaptations reflect a similarly strong effect of the pressure induced by TAC in both genotypes. However, 2 weeks post-operation TAC-treated knockout hearts had reduced levels of connexin43 and slightly increased incidents of ventricular tachycardia compared to their wild-type (WT) counterparts. Our findings highlight the FlnC-positive sarcomeric lesions and ID-proximal streaming as general remodeling responses in cardiac overload-induced hypertrophy.

Exome sequencing identifies variants in two genes encoding the LIM-proteins NRAP and FHL1 in an Italian patient with BAG3 myofibrillar myopathy

Myofibrillar myopathies (MFMs) are genetically heterogeneous dystrophies characterized by the disintegration of Z-disks and myofibrils and are associated with mutations in genes encoding Z-disk or Z-disk-related proteins. The c.626 C > T (p.P209L) mutation in the BAG3 gene has been described as causative of a subtype of MFM. We report a sporadic case of a 26-year-old Italian woman, affected by MFM with axonal neuropathy, cardiomyopathy, rigid spine, who carries the c.626 C > T mutation in the BAG3 gene. The patient and her non-consanguineous healthy parents and brother were studied with whole exome sequencing (WES) to further investigate the genetic basis of this complex phenotype. In the patient, we found that the BAG3 mutation is associated with variants in the NRAP and FHL1 genes that encode muscle-specific, LIM domain containing proteins. Quantitative real time PCR, immunohistochemistry and Western blot analysis of the patient’s muscular biopsy showed the absence of NRAP expression and FHL1 accumulation in aggregates in the affected skeletal muscle tissue. Molecular dynamic analysis of the mutated FHL1 domain showed a modification in its surface charge, which could affect its capability to bind its target proteins. To our knowledge this is the first study reporting, in a BAG3 MFM, the simultaneous presence of genetic variants in the BAG3 and FHL1 genes (previously described as independently associated with MFMs) and linking the NRAP gene to MFM for the first time.

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.

Sarcoplasmic reticulum is an intermediary of mitochondrial and myofibrillar growth at the intercalated disc

In cardiomyocytes columns of intermyofibrillar mitochondria run up to the intercalated disc (ID); half are collinear with those in the neighbouring cell, suggesting coordinated addition of sarcomeres and mitochondria both within and between cells during cardiomyocyte growth. Recent evidence for an association between sarcoplasmic reticulum (SR) and mitochondria indicates that the SR may be an intermediary in this coordinated behaviour. For this reason we have investigated the arrangement of SR and t tubules with respect to mitochondria and myofibrils, particularly at the ID. In the body of the cardiomyocyte the mitochondrial columns are frequently intersected by transverse tubules. In addition, we find that a majority of axial tubules are sandwiched between mitochondria and myofibril. No tubules are found at the ID. SR coats mitochondrial columns and fibrils throughout their length and reaches towards the peaks of the ID membrane where it attaches in the form of junctional (j)SR. These peripheral ID couplings are often situated between mitochondria and ID membrane, suggesting an SR connection between the two. In dilated cardiomyopathy (DCM) the mitochondria are somewhat disordered and clumped. In a mouse model for DCM, the muscle LIM protein KO, we find that there is a lack of mitochondria near the ID, suggesting the uncoupling of the myofibril/mitochondria organisation during growth. SR still coats the fibrils and reaches the ID folds in a jSR coupling. Unlike in control tissue, however, loops and long fingers of ID membrane penetrate into the proximal sarcomere suggesting a possible intermediary state in cardiomyocyte growth.

Lmod2 piggyBac mutant mice exhibit dilated cardiomyopathy

BACKGROUND

Leiomodin proteins, Lmod1, Lmod2 and Lmod3, are key regulators of the thin filament length in muscles. While Lmod1 is specifically expressed in smooth muscles, both Lmod2 and Lmod3 are expressed in striated muscles including both cardiac and skeletal muscles. We and others have previously shown that Lmod3 mainly function in skeletal muscles and the mutant mice display disorganized sarcomere. Lmod2 protein has been found to act as an actin filament nucleator in both cell-free assays and in cultured rat and chicken cardiomyocytes.

RESULTS

To better understand the function of Lmod2 in vivo, we have identified and characterized a piggyBac (PB) insertional mouse mutant. Our analysis revealed that the PB transposon inserts in the first exon of the Lmod2 gene and severely disrupts its expression. We found that Lmod2 (PB/PB) mice exhibit typical dilated cardiomyopathy (DCM) with ventricular arrhythmias and postnatal lethality. Electron microscope reveals that the Lmod2 (PB/PB) hearts carry disordered sarcomere, disarrayed thin filaments, and distorted intercalated discs (ICDs). Those ICDs display not only decreased convolutions, but also reduced electron-dense staining, indicating less ICDs component proteins in Lmod2 (PB/PB) hearts. Consistent with the phenotype, the expression of the ICD component genes, β-catenin and Connexin43, are down-regulated.

CONCLUSIONS

Taken together, our data reveal that Lmod2 is required in heart thin filaments for integrity of sarcomere and ICD and deficient mice exhibit DCM with ventricular arrhythmias and postnatal lethality. The Lmod2 (PB/PB) mutant offers a valuable resource for interrogation of pathogenesis and development of therapeutics for DCM.

Animal Models of Congenital Cardiomyopathies Associated With Mutations in Z-Line Proteins

The cardiac Z-line at the boundary between sarcomeres is a multiprotein complex connecting the contractile apparatus with the cytoskeleton and the extracellular matrix. The Z-line is important for efficient force generation and transmission as well as the maintenance of structural stability and integrity. Furthermore, it is a nodal point for intracellular signaling, in particular mechanosensing and mechanotransduction. Mutations in various genes encoding Z-line proteins have been associated with different cardiomyopathies, including dilated cardiomyopathy, hypertrophic cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy, restrictive cardiomyopathy, and left ventricular noncompaction, and mutations even within the same gene can cause widely different pathologies. Animal models have contributed to a great advancement in the understanding of the physiological function of Z-line proteins and the pathways leading from mutations in Z-line proteins to cardiomyopathy, although genotype-phenotype prediction remains a great challenge. This review presents an overview of the currently available animal models for Z-line and Z-line associated proteins involved in human cardiomyopathies with special emphasis on knock-in and transgenic mouse models recapitulating the clinical phenotypes of human cardiomyopathy patients carrying mutations in Z-line proteins. Pros and cons of mouse models will be discussed and a future outlook will be given. J. Cell. Physiol. 232: 38-52, 2017. © 2016 Wiley Periodicals, Inc.

Cardiac cytoarchitecture - why the "hardware" is important for heart function!

Cells that constitute fully differentiated tissues are characterised by an architecture that makes them perfectly suited for the job they have to do. This is especially obvious for cardiomyocytes, which have an extremely regular shape and display a paracrystalline arrangement of their cytoplasmic components. This article will focus on the two major cytoskeletal multiprotein complexes that are found in cardiomyocytes, the myofibrils, which are responsible for contraction and the intercalated disc, which mediates mechanical and electrochemical contact between individual cardiomyocytes. Recent studies have revealed that these two sites are also crucial in sensing excessive mechanical strain. Signalling processes will be triggered that## lead to changes in gene expression and eventually lead to an altered cardiac cytoarchitecture in the diseased heart, which results in a compromised function. Thus, understanding these changes and the signals that lead to them is crucial to design treatment strategies that can attenuate these processes. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

Roles of Nebulin Family Members in the Heart

The members of the nebulin protein family, including nebulin, nebulette, LASP-1, LASP-2, and N-RAP, contain various numbers of nebulin repeats and bind to actin, but are otherwise heterogeneous with regard to size, expression pattern, and function. This review focuses on the roles of nebulin family members in the heart. Nebulin is the largest member predominantly expressed in skeletal muscle, where it stretches along the thin filament. In heart, nebulin is detectable only at low levels and its absence has no apparent effects. Nebulette is similar in structure to the nebulin C-terminal Z-line region and specifically expressed in heart. Nebulette gene mutations have been identified in dilated cardiomyopathy patients and transgenic mice overexpressing nebulette mutants partially recapitulate the human pathology. In contrast, nebulette knockout mice show no functional phenotype, but exhibit Z-line widening. LASP-2 is an isoform of nebulette expressed in multiple tissues, including the heart. It is present in the Z-line and intercalated disc and able to bind and cross-link filamentous actin. LASP-1 is similar in structure to LASP-2, but expressed only in non-muscle tissue. N-RAP is present in myofibril precursors during myofibrillogenesis and thought to be involved in myofibril assembly, while it is localized at the intercalated disc in adult heart. Additional in vivo models are required to provide further insights into the functions of nebulin family members in the heart.

Muscle LIM Protein: Master regulator of cardiac and skeletal muscle functions

Muscle LIM Protein (MLP) has emerged as a key regulator of striated muscle physiology and pathophysiology. Mutations in cysteine and glycine-rich protein 3 (CSRP3), the gene encoding MLP, are causative of human cardiomyopathies, whereas altered expression patterns are observed in human failing heart and skeletal myopathies. In vitro and in vivo evidences reveal a complex and diverse functional role of MLP in striated muscle, which is determined by its multiple interacting partners and subcellular distribution. Experimental evidence suggests that MLP is implicated in both myogenic differentiation and myocyte cytoarchitecture, although the full spectrum of its intracellular roles still unfolds.

Myosin VI localization and expression in striated muscle pathology

Myosin VI (MVI) is a unique unconventional myosin translocating, unlike other myosins, towards the minus end of actin filaments. It is involved in numerous cellular processes such as endocytosis, intracellular trafficking, cell migration, and transcription. In mammalian skeletal muscles it localizes mainly to sarcoplasmic reticulum and is also present within the muscle nuclei and at the neuromuscular junction (Karolczak et al. Histochem Cell Biol 2013; 23:219-228). We have also shown that in denervated rat hindlimb muscle the MVI expression level is significantly increased and its localization is changed, indicating an important role of MVI in striated muscle pathology. Here, we addressed this problem by examining the distribution and expression levels of myosin VI in biopsies of skeletal muscles from patients with different myopathies. We found that, particularly in myopathies associated with fiber atrophy, the amount of MVI was enhanced and its localization in affected fibers was changed. Also, since a mutation within the human MVI gene was shown to be associated with cardiomyopathy, we assessed MVI localization and expression level in cardiac muscle using wild type and MLP(-/-) mice, a dilated cardiomyopathy model. No significant difference in MVI expression level was observed for both types of animals. MVI was found at intercalated discs and also at the sarcoplasmic reticulum. In the knockout mice, it was also present in ring-like structures surrounding the nuclei. The data indicate that in striated muscle MVI could be engaged in sarcoplasmic reticulum maintenance and/or functioning, vesicular transport, signal transmission and possibly in gene transcription.

The formin FHOD1 in cardiomyocytes

Members of the formin family are known to be involved in the regulation of the actin cytoskeleton. We have recently identified a muscle specific splice variant of the formin FHOD3 and demonstrated its role in the maintenance of the contractile filaments of cardiomyocytes. Here, we characterize the expression and subcellular localization of FHOD3's closest relative, FHOD1, in the heart. Confocal microscopy shows that FHOD1 is mainly located at the intercalated disc, the special type of cell-cell contact between cardiomyocytes, but also partially associated with the myofibrils. Subcellular targeting of FHOD1 is probably mediated by its N-terminal domain, since expression constructs lacking this domain show aberrant localization in primary cultures of neonatal rat cardiomyocytes. Finally, we show that in contrast to FHOD3, FHOD1 shows increased expression levels in dilated cardiomyopathy, suggesting that the two formins play distinct roles and are differentially regulated in cardiomyocytes.

Mechanotransduction in cardiac hypertrophy and failure

Cardiac muscle cells have an intrinsic ability to sense and respond to mechanical load through a process known as mechanotransduction. In the heart, this process involves the conversion of mechanical stimuli into biochemical events that induce changes in myocardial structure and function. Mechanotransduction and its downstream effects function initially as adaptive responses that serve as compensatory mechanisms during adaptation to the initial load. However, under prolonged and abnormal loading conditions, the remodeling processes can become maladaptive, leading to altered physiological function and the development of pathological cardiac hypertrophy and heart failure. Although the mechanisms underlying mechanotransduction are far from being fully elucidated, human and mouse genetic studies have highlighted various cytoskeletal and sarcolemmal structures in cardiac myocytes as the likely candidates for load transducers, based on their link to signaling molecules and architectural components important in disease pathogenesis. In this review, we summarize recent developments that have uncovered specific protein complexes linked to mechanotransduction and mechanotransmission within the sarcomere, the intercalated disc, and at the sarcolemma. The protein structures acting as mechanotransducers are the first step in the process that drives physiological and pathological cardiac hypertrophy and remodeling, as well as the transition to heart failure, and may provide better insights into mechanisms driving mechanotransduction-based diseases.

Adaptations of cytoarchitecture in human dilated cardiomyopathy

Hypertrophic cardiomyopathy is characterised by a histological phenotype of myocyte disarray, but heart tissue samples from patients with dilated cardiomyopathy (DCM) often look comparatively similar to those from healthy individuals apart from conspicuous regions of fibrosis and necrosis. We have previously investigated subcellular alterations in the cytoarchitecture of mouse models of dilated cardiomyopathy and found that both the organisation and composition of the intercalated disc, i.e. the specialised type of cell-cell contact in the heart, is altered. There is also is a change in the composition of the M-band of the sarcomere due to an expression shift towards the more extensible embryonic heart (EH)-myomesin isoform. Analysis of human samples from the Sydney Human Heart Tissue Bank have revealed similar structural findings and also provided evidence for a dramatic change in overall cardiomyocyte size control, which has also been seen in the mouse. Together these changes in cytoarchitecture probably contribute to the decreased functional output that is seen in DCM.

Synergistic and antagonistic interplay between myostatin gene expression and physical activity levels on gene expression patterns in triceps Brachii muscles of C57/BL6 mice

Levels of myostatin expression and physical activity have both been associated with transcriptome dysregulation and skeletal muscle hypertrophy. The transcriptome of triceps brachii muscles from male C57/BL6 mice corresponding to two genotypes (wild-type and myostatin-reduced) under two conditions (high and low physical activity) was characterized using RNA-Seq. Synergistic and antagonistic interaction and ortholog modes of action of myostatin genotype and activity level on genes and gene pathways in this skeletal muscle were uncovered; 1,836, 238, and 399 genes exhibited significant (FDR-adjusted P-value < 0.005) activity-by-genotype interaction, genotype and activity effects, respectively. The most common differentially expressed profiles were (i) inactive myostatin-reduced relative to active and inactive wild-type, (ii) inactive myostatin-reduced and active wild-type, and (iii) inactive myostatin-reduced and inactive wild-type. Several remarkable genes and gene pathways were identified. The expression profile of nascent polypeptide-associated complex alpha subunit (Naca) supports a synergistic interaction between activity level and myostatin genotype, while Gremlin 2 (Grem2) displayed an antagonistic interaction. Comparison between activity levels revealed expression changes in genes encoding for structural proteins important for muscle function (including troponin, tropomyosin and myoglobin) and for fatty acid metabolism (some linked to diabetes and obesity, DNA-repair, stem cell renewal, and various forms of cancer). Conversely, comparison between genotype groups revealed changes in genes associated with G1-to-S-phase transition of the cell cycle of myoblasts and the expression of Grem2 proteins that modulate the cleavage of the myostatin propeptide. A number of myostatin-feedback regulated gene products that are primarily regulatory were uncovered, including microRNA impacting central functions and Piezo proteins that make cationic current-controlling mechanosensitive ion channels. These important findings extend hypotheses of myostatin and physical activity master regulation of genes and gene pathways, impacting medical practices and therapies associated with muscle atrophy in humans and companion animal species and genome-enabled selection practices applied to food-production animal species.

Passive ventricular remodeling in cardiac disease: focus on heterogeneity

Passive ventricular remodeling is defined by the process of molecular ventricular adaptation to different forms of cardiac pathophysiology. It includes changes in tissue architecture, such as hypertrophy, fiber disarray, alterations in cell size and fibrosis. Besides that, it also includes molecular remodeling of gap junctions, especially those composed by Connexin43 proteins (Cx43) in the ventricles that affect cell-to-cell propagation of the electrical impulse, and changes in the sodium channels that modify excitability. All those alterations appear mainly in a heterogeneous manner, creating irregular and inhomogeneous electrical and mechanical coupling throughout the heart. This can predispose to reentry arrhythmias and adds to a further deterioration into heart failure. In this review, passive ventricular remodeling is described in Hypertrophic Cardiomyopathy (HCM), Dilated Cardiomyopathy (DCM), Ischemic Cardiomyopathy (ICM), and Arrhythmogenic Cardiomyopathy (ACM), with a main focus on the heterogeneity of those alterations mentioned above.

Human muscle LIM protein dimerizes along the actin cytoskeleton and cross-links actin filaments

The muscle LIM protein (MLP) is a nucleocytoplasmic shuttling protein playing important roles in the regulation of myocyte remodeling and adaptation to hypertrophic stimuli. Missense mutations in human MLP or its ablation in transgenic mice promotes cardiomyopathy and heart failure. The exact function(s) of MLP in the cytoplasmic compartment and the underlying molecular mechanisms remain largely unknown. Here, we provide evidence that MLP autonomously binds to, stabilizes, and bundles actin filaments (AFs) independently of calcium and pH. Using total internal reflection fluorescence microscopy, we have shown how MLP cross-links actin filaments into both unipolar and mixed-polarity bundles. Quantitative analysis of the actin cytoskeleton configuration confirmed that MLP substantially promotes actin bundling in live myoblasts. In addition, bimolecular fluorescence complementation (BiFC) assays revealed MLP self-association. Remarkably, BiFC complexes mostly localize along actin filament-rich structures, such as stress fibers and sarcomeres, supporting a functional link between MLP self-association and actin cross-linking. Finally, we have demonstrated that MLP self-associates through its N-terminal LIM domain, whereas it binds to AFs through its C-terminal LIM domain. Together our data support that MLP contributes to the maintenance of cardiomyocyte cytoarchitecture by a mechanism involving its self-association and actin filament cross-linking.

Muscle lim protein isoform negatively regulates striated muscle actin dynamics and differentiation

Muscle lim protein (MLP) has emerged as a critical regulator of striated muscle physiology and pathophysiology. Mutations in cysteine and glycine-rich protein 3 (CSRP3), the gene encoding MLP, have been directly associated with human cardiomyopathies, whereas aberrant expression patterns are reported in human cardiac and skeletal muscle diseases. Increasing evidence suggests that MLP has an important role in both myogenic differentiation and myocyte cytoarchitecture, although the full spectrum of its intracellular roles has not been delineated. We report the discovery of an alternative splice variant of MLP, designated as MLP-b, showing distinct expression in neuromuscular disease and direct roles in actin dynamics and muscle differentiation. This novel isoform originates by alternative splicing of exons 3 and 4. At the protein level, it contains the N-terminus first half LIM domain of MLP and a unique sequence of 22 amino acids. Physiologically, it is expressed during early differentiation, whereas its overexpression reduces C2C12 differentiation and myotube formation. This may be mediated through its inhibition of MLP/cofilin-2-mediated F-actin dynamics. In differentiated striated muscles, MLP-b localizes to the sarcomeres and binds directly to Z-disc components, including α-actinin, T-cap and MLP. The findings of the present study unveil a novel player in muscle physiology and pathophysiology that is implicated in myogenesis as a negative regulator of myotube formation, as well as in differentiated striated muscles as a contributor to sarcomeric integrity.

From stem cells to cardiomyocytes: the role of forces in cardiac maturation, aging, and disease

Stem cell differentiation into a variety of lineages is known to involve signaling from the extracellular niche, including from the physical properties of that environment. What regulates stem cell responses to these cues is there ability to activate different mechanotransductive pathways. Here, we will review the structures and pathways that regulate stem cell commitment to a cardiomyocyte lineage, specifically examining proteins within muscle sarcomeres, costameres, and intercalated discs. Proteins within these structures stretch, inducing a change in their phosphorylated state or in their localization to initiate different signals. We will also put these changes in the context of stem cell differentiation into cardiomyocytes, their subsequent formation of the chambered heart, and explore negative signaling that occurs during disease.