Reducing Granules Without Splicing Restoration Alleviates RBM20 Cardiomyopathy.

Background: RBM20 (RNA binding motif protein 20) cardiomyopathy is a severe form of dilated cardiomyopathy (DCM). Genetic variants in the nuclear localization signal of hinder its nuclear import and promote cytoplasmic pathogenic RNP (ribonucleoprotein) granules. We aimed to investigate whether reducing RNP granules by inhibiting expression could alleviate the DCM phenotype in S639G () knock-in mice.

Importance of N2BA Titin in Maintaining Cardiac Homeostasis and Its Role in Dilated Cardiomyopathy.

Background: TTN (titin) is the third myofilament type of the cardiac sarcomere and performs important functions that include generating passive tension. Changes in TTN expression are associated with cardiac dysfunction, and TTN is one of the main genes linked to dilated cardiomyopathy (DCM). DCM is frequently associated with changes in the expression of N2BA (compliant cardiac TTN isoform), 1 of the 2 major TTN isoforms found in the heart (the other isoform being the N2B [stiff cardiac TTN isoform]).

In vivo gene editing of CAMKIID: out with the bad and in with the good.

The ability to change an organism's DNA through gene editing is of great importance for the prevention and treatment of genetic and acquired diseases. Rapid progress has been made during the last decade due to the discovery and refinement of CRISPR/Cas9 as an accurate, fast, and reliable genome editing technique. In this issue of the JCI, Lebek et al.

ZFH-2 is required for Drosophila ovarian follicle development and is expressed at the band/interband boundaries of polytene chromosomes.

The transcription factor ZFH-2 has well-documented roles in Drosophila neurogenesis and other developmental processes. Here we provide the first evidence that ZFH-2 has a role in oogenesis. We demonstrate that ZFH-2 is expressed in the wild-type ovary and that a loss of zfh-2 function produces a mutant ovary phenotype where egg chambers are reduced in number and fused.

Shortening the thick filament by partial deletion of titin's C-zone alters cardiac function by reducing the operating sarcomere length range.

Titin's C-zone is an inextensible segment in titin, comprised of 11 super-repeats and located in the cMyBP-C-containing region of the thick filament. Previously we showed that deletion of titin's super-repeats C1 and C2 (Ttn model) results in shorter thick filaments and contractile dysfunction of the left ventricular (LV) chamber but that unexpectedly LV diastolic stiffness is normal. Here we studied the contraction-relaxation kinetics from the time-varying elastance of the LV and intact cardiomyocyte, cellular work loops of intact cardiomyocytes, Ca transients, cross-bridge kinetics, and myofilament Ca sensitivity.

The number of Z-repeats and super-repeats in nebulin greatly varies across vertebrates and scales with animal size.

Nebulin is a skeletal muscle protein that associates with the sarcomeric thin filaments and has functions in regulating the length of the thin filament and the structure of the Z-disk. Here we investigated the nebulin gene in 53 species of birds, fish, amphibians, reptiles, and mammals. In all species, nebulin has a similar domain composition that mostly consists of ∼30-residue modules (or simple repeats), each containing an actin-binding site.

Nebulin and Lmod2 are critical for specifying thin-filament length in skeletal muscle.

Regulating the thin-filament length in muscle is crucial for controlling the number of myosin motors that generate power. The giant protein nebulin forms a long slender filament that associates along the length of the thin filament in skeletal muscle with functions that remain largely obscure. Here nebulin's role in thin-filament length regulation was investigated by targeting entire super-repeats in the gene; nebulin was either shortened or lengthened by 115 nm.

Triggering typical nemaline myopathy with compound heterozygous nebulin mutations reveals myofilament structural changes as pathomechanism.

Nebulin is a giant protein that winds around the actin filaments in the skeletal muscle sarcomere. Compound-heterozygous mutations in the nebulin gene (NEB) cause typical nemaline myopathy (NM), a muscle disorder characterized by muscle weakness with limited treatment options. We created a mouse model with a missense mutation p.

Phosphodiesterase 9a Inhibition in Mouse Models of Diastolic Dysfunction.

Background: Low myocardial cGMP-PKG (cyclic guanosine monophosphate-protein kinase G) activity has been associated with increased cardiomyocyte diastolic stiffness in heart failure with preserved ejection fraction. Cyclic guanosine monophosphate is mainly hydrolyzed by PDE (phosphodiesterases) 5a and 9a. Importantly, PDE9a expression has been reported to be upregulated in human heart failure with preserved ejection fraction myocardium and chronic administration of a PDE9a inhibitor reverses preestablished cardiac hypertrophy and systolic dysfunction in mice subjected to transverse aortic constriction (TAC).

Correction to: Expressing a Z-disk nebulin fragment innebulin-deficient mouse muscle: effects on muscle structure and function.

Following the publication of this paper [1], it was brought to the authors' attention that one of the contributing authors was left off of the paper. The authors apologize for the unfortunate oversight. In this correction paper, they have included Dr.

Expressing a Z-disk nebulin fragment in nebulin-deficient mouse muscle: effects on muscle structure and function.

Background: Nebulin is a critical thin filament-binding protein that spans from the Z-disk of the skeletal muscle sarcomere to near the pointed end of the thin filament. Its massive size and actin-binding property allows it to provide the thin filaments with structural and regulatory support. When this protein is lost, nemaline myopathy occurs.

Titin splicing regulates cardiotoxicity associated with calpain 3 gene therapy for limb-girdle muscular dystrophy type 2A.

Limb-girdle muscular dystrophy type 2A (LGMD2A or LGMDR1) is a neuromuscular disorder caused by mutations in the calpain 3 gene (). Previous experiments using adeno-associated viral (AAV) vector-mediated calpain 3 gene transfer in mice indicated cardiac toxicity associated with the ectopic expression of the calpain 3 transgene. Here, we performed a preliminary dose study in a severe double-knockout mouse model deficient in calpain 3 and dysferlin.

Fine mapping titin's C-zone: Matching cardiac myosin-binding protein C stripes with titin's super-repeats.

Titin is largely comprised of serially-linked immunoglobulin (Ig) and fibronectin type-III (Fn3) domains. Many of these domains are arranged in an 11 domain super-repeat pattern that is repeated 11 times, forming the so-named titin C-zone in the A-band region of the sarcomere. Each super-repeat is thought to provide binding sites for thick filament proteins, such as cMyBP-C (cardiac myosin-binding protein C).

Titin mutations and muscle disease.

The introduction of next-generation sequencing technology has revealed that mutations in the gene that encodes titin (TTN) are linked to multiple skeletal and cardiac myopathies. The most prominent of these myopathies is dilated cardiomyopathy (DCM). Over 60 genes are linked to the etiology of DCM, but by far, the leading cause of DCM is mutations in TTN with truncating variants in TTN (TTNtvs) associated with familial DCM in ∼ 20% of the cases.

Congenital Titinopathy: Comprehensive characterization and pathogenic insights.

Objective: Comprehensive clinical characterization of congenital titinopathy to facilitate diagnosis and management of this important emerging disorder.

Methods: Using massively parallel sequencing we identified 30 patients from 27 families with 2 pathogenic nonsense, frameshift and/or splice site TTN mutations in trans. We then undertook a detailed analysis of the clinical, histopathological and imaging features of these patients.

The giant protein titin regulates the length of the striated muscle thick filament.

The contractile machinery of heart and skeletal muscles has as an essential component the thick filament, comprised of the molecular motor myosin. The thick filament is of a precisely controlled length, defining thereby the force level that muscles generate and how this force varies with muscle length. It has been speculated that the mechanism by which thick filament length is controlled involves the giant protein titin, but no conclusive support for this hypothesis exists.

Novex-3, the tiny titin of muscle.

The giant multi-functional striated muscle protein titin is the third most abundant muscle protein after myosin and actin. Titin plays a pivotal role in myocardial passive stiffness, structural integrity and stress-initiated signaling pathways. The complete sequence of the human titin gene contains three isoform-specific mutually exclusive exons [termed novel exons (novex)] coding for the I-band sequence, named novex-1 (exon 45), novex-2 (exon 46) and novex-3 (exon 48).

Phosphorylating Titin's Cardiac N2B Element by ERK2 or CaMKIIδ Lowers the Single Molecule and Cardiac Muscle Force.

Titin is a large filamentous protein that is responsible for the passive force of the cardiac sarcomere. Titin's force is generated by its I-band region, which includes the cardiac-specific N2B element. The N2B element consists of three immunoglobulin domains, two small unique sequence insertions, and a large 575-residue unique sequence, the N2B-Us.

Role of Titin Missense Variants in Dilated Cardiomyopathy.

Background: The titin gene (TTN) encodes the largest human protein, which plays a central role in sarcomere organization and passive myocyte stiffness. TTN truncating mutations cause dilated cardiomyopathy (DCM); however, the role of TTN missense variants in DCM has been difficult to elucidate because of the presence of background TTN variation.

Methods And Results: A cohort of 147 DCM index subjects underwent DNA sequencing for 313 TTN exons covering the N2B and N2BA cardiac isoforms of TTN.

Nebulin deficiency in adult muscle causes sarcomere defects and muscle-type-dependent changes in trophicity: novel insights in nemaline myopathy.

Nebulin is a giant filamentous protein that is coextensive with the actin filaments of the skeletal muscle sarcomere. Nebulin mutations are the main cause of nemaline myopathy (NEM), with typical adult patients having low expression of nebulin, yet the roles of nebulin in adult muscle remain poorly understood. To establish nebulin's functional roles in adult muscle, we studied a novel conditional nebulin KO (Neb cKO) mouse model in which nebulin deletion was driven by the muscle creatine kinase (MCK) promotor.

Deleting titin's I-band/A-band junction reveals critical roles for titin in biomechanical sensing and cardiac function.

Titin, the largest protein known, forms a giant filament in muscle where it spans the half sarcomere from Z disk to M band. Here we genetically targeted a stretch of 14 immunoglobulin-like and fibronectin type 3 domains that comprises the I-band/A-band (IA) junction and obtained a viable mouse model. Super-resolution optical microscopy (structured illumination microscopy, SIM) and electron microscopy were used to study the thick filament length and titin's molecular elasticity.

Experimentally increasing titin compliance in a novel mouse model attenuates the Frank-Starling mechanism but has a beneficial effect on diastole.

Background: Experimentally upregulating compliant titins has been suggested as a therapeutic for lowering pathological diastolic stiffness levels. However, how increasing titin compliance impacts global cardiac function requires in-depth study. We investigate the effect of upregulating compliant titins in a novel mouse model with a genetically altered titin splicing factor; integrative approaches were used from intact cardiomyocyte mechanics to pressure-volume analysis and Doppler echocardiography.

Removal of immunoglobulin-like domains from titin's spring segment alters titin splicing in mouse skeletal muscle and causes myopathy.

Titin is a molecular spring that determines the passive stiffness of muscle cells. Changes in titin's stiffness occur in various myopathies, but whether these are a cause or an effect of the disease is unknown. We studied a novel mouse model in which titin's stiffness was slightly increased by deleting nine immunoglobulin (Ig)-like domains from titin's constitutively expressed proximal tandem Ig segment (IG KO).

Shortening of the elastic tandem immunoglobulin segment of titin leads to diastolic dysfunction.

Background: Diastolic dysfunction is a poorly understood but clinically pervasive syndrome that is characterized by increased diastolic stiffness. Titin is the main determinant of cellular passive stiffness. However, the physiological role that the tandem immunoglobulin (Ig) segment of titin plays in stiffness generation and whether shortening this segment is sufficient to cause diastolic dysfunction need to be established.

An approach to improve early detection of sternal wound infection.

We developed a reproducible and reliable method of using a rubber stamp prospectively to document appearances of the sternal wound. 395 consecutive patients undergoing median sternotomy for cardiac procedures were studied over a period of four months. Data were collected by the Senior House Officer during the ward round for all patients at Day 3 and Day 7 post-operatively.