Myofibrillogenesis and formation of cell contacts mediate the localization of N-RAP in cultured chick cardiomyocytes

A long-read RNA-seq approach to identify novel transcripts of very large genes

RNA-seq is widely used for studying gene expression, but commonly used sequencing platforms produce short reads that only span up to two exon junctions per read. This makes it difficult to accurately determine the composition and phasing of exons within transcripts. Although long-read sequencing improves this issue, it is not amenable to precise quantitation, which limits its utility for differential expression studies. We used long-read isoform sequencing combined with a novel analysis approach to compare alternative splicing of large, repetitive structural genes in muscles. Analysis of muscle structural genes that produce medium (Nrap: 5 kb), large (Neb: 22 kb), and very large (Ttn: 106 kb) transcripts in cardiac muscle, and fast and slow skeletal muscles identified unannotated exons for each of these ubiquitous muscle genes. This also identified differential exon usage and phasing for these genes between the different muscle types. By mapping the in-phase transcript structures to known annotations, we also identified and quantified previously unannotated transcripts. Results were confirmed by endpoint PCR and Sanger sequencing, which revealed muscle-type-specific differential expression of these novel transcripts. The improved transcript identification and quantification shown by our approach removes previous impediments to studies aimed at quantitative differential expression of ultralong transcripts.

The selective NLRP3-inflammasome inhibitor MCC950 reduces myocardial fibrosis and improves cardiac remodeling in a mouse model of myocardial infarction

BACKGROUND/AIMS

Early inflammatory responses after myocardial infarction (MI) are likely to increase myocardial fibrosis and subsequent cardiac remodeling. MCC950, a specific NLRP3 inhibitor, was previously found to effectively inhibit the release of inflammatory factors IL-18 and IL-1β. In this study, we evaluated the effect of MCC950, as a potential new treatment strategy for MI, on myocardial fibrosis and cardiac remodeling using an experimental mouse model.

METHODS

Male C57BL/6 mice were subjected to left coronary artery ligation to induce MI and then treated with MCC950 (10 mg/kg) or PBS for 14 days. After 30 days, echocardiography was performed to assess cardiac function and myocardial fibrosis was evaluated using H&E- and Masson's Trichrome-stained sections. Myocardial expression of inflammatory factors and fibrosis markers was analyzed by western blotting, immunofluorescence, ELISA, and real-time quantitative PCR.

RESULTS

The ejection fraction in the 10 mg/kg group (40.7 ± 4.2%; N = 6, p = 0.0029) was statistically preserved compared to that in the control group (14.0 ± 4.4%). Myocardial fibrosis was also reduced in MCC950-treated animals (MCC950, 23.2 ± 3.0 vs PBS, 36.2 ± 3.7; p < 0.05). Moreover, myocardial NLRP3, cleaved IL-1β, and IL-18 levels were reduced in MCC950-treated animals. H&E and molecular examination revealed decreases in inflammatory cell infiltration and inflammatory factor expression in the heart. In vitro, MCC950 inhibited NLRP3, reduced caspase-1 activity, and further downregulated IL-1β and IL-18.

CONCLUSION

MCC950, as a specific NLRP3 inhibitor, can alleviate fibrosis and improve cardiac function in a mouse model by suppressing early inflammatory responses post-MI.

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.

Titin and Nebulin in Thick and Thin Filament Length Regulation

In this review we discuss the history and the current state of ideas related to the mechanism of size regulation of the thick (myosin) and thin (actin) filaments in vertebrate striated muscles. Various hypotheses have been considered during of more than half century of research, recently mostly involving titin and nebulin acting as templates or 'molecular rulers', terminating exact assembly. These two giant, single-polypeptide, filamentous proteins are bound in situ along the thick and thin filaments, respectively, with an almost perfect match in the respective lengths and structural periodicities. However, evidence still questions the possibility that the proteins function as templates, or scaffolds, on which the thin and thick filaments could be assembled. In addition, the progress in muscle research during the last decades highlighted a number of other factors that could potentially be involved in the mechanism of length regulation: molecular chaperones that may guide folding and assembly of actin and myosin; capping proteins that can influence the rates of assembly-disassembly of the myofilaments; Ca²⁺ transients that can activate or deactivate protein interactions, etc. The entire mechanism of sarcomere assembly appears complex and highly dynamic. This mechanism is also capable of producing filaments of about the correct size without titin and nebulin. What then is the role of these proteins? Evidence points to titin and nebulin stabilizing structures of the respective filaments. This stabilizing effect, based on linear proteins of a fixed size, implies that titin and nebulin are indeed molecular rulers of the filaments. Although the proteins may not function as templates in the assembly of the filaments, they measure and stabilize exactly the same size of the functionally important for the muscles segments in each of the respective filaments.

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.

Circadian profiles in the embryonic chick heart: L-type voltage-gated calcium channels and signaling pathways

Circadian clocks exist in the heart tissue and modulate multiple physiological events, from cardiac metabolism to contractile function and expression of circadian oscillator and metabolic-related genes. Ample evidence has demonstrated that there are endogenous circadian oscillators in adult mammalian cardiomyocytes. However, mammalian embryos cannot be entrained independently to light-dark (LD) cycles in vivo without any maternal influence, but circadian genes are well expressed and able to oscillate in embryonic stages. The authors took advantage of using chick embryos that are independent of maternal influences to investigate whether embryonic hearts could be entrained under LD cycles in ovo. The authors found circadian regulation of L-type voltage-gated calcium channels (L-VGCCs), the ion channels responsible for the production of cardiac muscle contraction in embryonic chick hearts. The mRNA levels and protein expression of VGCCα1C and VGCCα1D are under circadian control, and the average L-VGCC current density is significantly larger when cardiomyocytes are recorded during the night than day. The phosphorylation states of several kinases involved in insulin signaling and cardiac metabolism, including extracellular signal-regulated kinase (Erk), stress-activated protein kinase (p38), protein kinase B (Akt), and glycogen synthase kinase-3β (GSK-3β), are also under circadian control. Both Erk and p38 have been implicated in regulating cardiac contractility and in the development of various pathological states, such as cardiac hypertrophy and heart failure. Even though both Erk and phosphoinositide 3-kinase (PI3K)-Akt signaling pathways participate in complex cellular processes regarding physiological or pathological states of cardiomyocytes, the circadian oscillators in the heart regulate these pathways independently, and both pathways contribute to the circadian regulation of L-VGCCs.

Scaffolds and chaperones in myofibril assembly: putting the striations in striated muscle

Sarcomere assembly in striated muscles has long been described as a series of steps leading to assembly of individual proteins into thick filaments, thin filaments and Z-lines. Decades of previous work focused on the order in which various structural proteins adopted the striated organization typical of mature myofibrils. These studies led to the view that actin and α-actinin assemble into premyofibril structures separately from myosin filaments, and that these structures are then assembled into myofibrils with centered myosin filaments and actin filaments anchored at the Z-lines. More recent studies have shown that particular scaffolding proteins and chaperone proteins are required for individual steps in assembly. Here, we review the evidence that N-RAP, a LIM domain and nebulin repeat protein, scaffolds assembly of actin and α-actinin into I-Z-I structures in the first steps of assembly; that the heat shock chaperone proteins Hsp90 & Hsc70 cooperate with UNC-45 to direct the folding of muscle myosin and its assembly into thick filaments; and that the kelch repeat protein Krp1 promotes lateral fusion of premyofibril structures to form mature striated myofibrils. The evidence shows that myofibril assembly is a complex process that requires the action of particular catalysts and scaffolds at individual steps. The scaffolds and chaperones required for assembly are potential regulators of myofibrillogenesis, and abnormal function of these proteins caused by mutation or pathological processes could in principle contribute to diseases of cardiac and skeletal muscles.

Re-expression of alpha skeletal actin as a marker for dedifferentiation in cardiac pathologies

Differentiation of foetal cardiomyocytes is accompanied by sequential actin isoform expression, i.e. down-regulation of the ‘embryonic’ alpha smooth muscle actin, followed by an up-regulation of alpha skeletal actin (αSKA) and a final predominant expression of alpha cardiac actin (αCA). Our objective was to detect whether re-expression of αSKA occurred during cardiomyocyte dedifferentiation, a phenomenon that has been observed in different pathologies characterized by myocardial dysfunction. Immunohistochemistry of αCA, αSKA and cardiotin was performed on left ventricle biopsies from human patients after coronary bypass surgery. Furthermore, actin isoform expression was investigated in left ventricle samples of rabbit hearts suffering from pressure- and volume-overload and in adult rabbit ventricular cardiomyocytes during dedifferentiation in vitro. Atrial goat samples up to 16 weeks of sustained atrial fibrillation (AF) were studied ultrastructurally and were immunostained for αCA and αSKA. Up-regulation of αSKA was observed in human ventricular cardiomyocytes showing down-regulation of αCA and cardiotin. A patchy re-expression pattern of αSKA was observed in rabbit left ventricular tissue subjected to pressure- and volume-overload. Dedifferentiating cardiomyocytes in vitro revealed a degradation of the contractile apparatus and local re-expression of αSKA. Comparable αSKA staining patterns were found in several areas of atrial goat tissue during 16 weeks of AF together with a progressive glycogen accumulation at the same time intervals. The expression of αSKA in adult dedifferentiating cardiomyocytes, in combination with PAS-positive glycogen and decreased cardiotin expression, offers an additional tool in the evaluation of myocardial dysfunction and indicates major changes in the contractile properties of these cells.

Expression and alternative splicing of N-RAP during mouse skeletal muscle development

N-RAP alternative splicing and protein localization were studied in developing skeletal muscle tissue from pre- and postnatal mice and in fusing primary myotubes in culture. Messages encoding N-RAP-s and N-RAP-c, the predominant isoforms of N-RAP detected in adult skeletal muscle and heart, respectively, were present in a 5:1 ratio in skeletal muscle isolated from E16.5 embryos. N-RAP-s mRNA levels increased three-fold over the first 3 weeks of postnatal development, while N-RAP-c mRNA levels remained low. N-RAP alternative splicing during myotube differentiation in culture was similar to the pattern observed in embryonic and neonatal muscle, with N-RAP-s expression increasing and N-RAP-c mRNA levels remaining low. In both developing skeletal muscle and cultured myotubes, N-RAP protein was primarily associated with developing myofibrillar structures containing alpha-actinin, but was not present in mature myofibrils. The results establish that N-RAP-s is the predominant spliced form of N-RAP present throughout skeletal muscle development.

Myofibril assembly visualized by imaging N-RAP, alpha-actinin, and actin in living cardiomyocytes

N-RAP is a striated muscle-specific scaffolding protein that organizes alpha-actinin and actin into symmetrical I-Z-I structures in developing myofibrils. Here we determined the order of events during myofibril assembly through time-lapse confocal microscopy of cultured embryonic chick cardiomyocytes coexpressing fluorescently tagged N-RAP and either alpha-actinin or actin. During de novo myofibril assembly, N-RAP assembled in fibrillar structures within the cell, with dots of alpha-actinin subsequently organizing along these structures. The initial fibrillar structures were reminiscent of actin fibrils, and coassembly of N-RAP and actin into newly formed fibrils supported this. The alpha-actinin dots subsequently broadened to Z-lines that were wider than the underlying N-RAP fibril, and N-RAP fluorescence intensity decreased. FRAP experiments showed that most of the alpha-actinin dynamically exchanged during all stages of myofibril assembly. In contrast, less than 20% of the N-RAP in premyofibrils was exchanged during 10-20 min after photobleaching, but this value increased to 70% during myofibril maturation. The results show that N-RAP assembles into an actin containing scaffold before alpha-actinin recruitment; that the N-RAP scaffold is much more stable than the assembling structural components; that N-RAP dynamics increase as assembly progresses; and that N-RAP leaves the structure after assembly is complete.

Role of nonmuscle myosin IIB and N-RAP in cell spreading and myofibril assembly in primary mouse cardiomyocytes

We investigated the role of nonmuscle myosin heavy chain (NMHC) IIB in cultured embryonic mouse cardiomyocytes by specific knockdown using RNA interference. NMHC IIB protein levels decreased 90% compared with mock-transfected cells by 3 days post transfection. NMHC IIB knockdown resulted in a slow decrease in N-RAP protein levels over 6 days with no change in N-RAP transcript levels. N-RAP is a scaffold for alpha-actinin and actin assembly during myofibrillogenesis, and we quantitated myofibril accumulation by morphometric analysis of alpha-actinin organization. Between 3 and 6 days, NMHC IIB knockdown was accompanied by the abolishment of cardiomyocyte spreading. During this period the rate of myofibril accumulation steadily decreased, correlating with the slowly decreasing levels of N-RAP. Between 6 and 8 days NMHC IIB and N-RAP protein levels recovered, and cardiomyocyte spreading and myofibril accumulation resumed. Inhibition of proteasome function using MG132 led to accumulation of excess N-RAP, and the secondary decrease in N-RAP that otherwise accompanied NMHC IIB knockdown was abolished. The results show that NMHC IIB knockdown led to decreased N-RAP levels through proteasome-mediated degradation. Furthermore, these proteins have distinct functional roles, with NMHC IIB playing a role in cardiomyocyte spreading and N-RAP functioning in myofibril assembly.

Krp1 (Sarcosin) promotes lateral fusion of myofibril assembly intermediates in cultured mouse cardiomyocytes

Krp1, also called sarcosin, is a cardiac and skeletal muscle kelch repeat protein hypothesized to promote the assembly of myofibrils, the contractile organelles of striated muscles, through interaction with N-RAP and actin. To elucidate its role, endogenous Krp1 was studied in primary embryonic mouse cardiomyocytes. While immunofluorescence showed punctate Krp1 distribution throughout the cell, detergent extraction revealed a significant pool of Krp1 associated with cytoskeletal elements. Reduction of Krp1 expression with siRNA resulted in specific inhibition of myofibril accumulation with no effect on cell spreading. Immunostaining analysis and electron microscopy revealed that cardiomyocytes lacking Krp1 contained sarcomeric proteins with longitudinal periodicities similar to mature myofibrils, but fibrils remained thin and separated. These thin myofibrils were degraded by a scission mechanism distinct from the myofibril disassembly pathway observed during cell division in the developing heart. The data are consistent with a model in which Krp1 promotes lateral fusion of adjacent thin fibrils into mature, wide myofibrils and contribute insight into mechanisms of myofibrillogenesis and disassembly.

Nebulette interacts with filamin C

The actin-binding proteins, nebulette, and nebulin, are comprised of a four-domain layout containing an acidic N-terminal region, a repeat domain, a serine-rich-linker region, and a Src homology-3 domain. Both proteins contain homologous N-terminal regions that are predicted to be in different environments within the sarcomere. The nebulin acidic N-terminal region is found at the distal ends of the thin filaments. Nebulette, however, is predicted to extend 150 nm from the center of the Z-line. To dissect out the functions of the N-terminal domain of nebulette, we have performed a yeast two-hybrid screen using nebulette residues 1-86 as bait. We have identified filamin-C, ZASP-1, and tropomyosin-1 as binding partners. Characterization of the nebulette-filamin interaction indicates that filamin-C predominantly interacts with the modules. These data suggest that filamin-C, a known component of striated muscle Z-lines, interacts with nebulette modules.

Baicalein protects chicken embryonic cardiomyocyte against hypoxia-reoxygenation injury via mu- and delta- but not kappa-opioid receptor signaling

Baicalein, a pure compound derived from Scutellaria baicalensis Georgi, protected cells from lethal damage in an ischemia-reperfusion model. This study was aimed to investigate the role of opioid receptors in mediating cardioprotection by baicalein against hypoxia-reoxygenation injury. By using chick cardiomyocyte as in vitro model, baicalein was added to the perfusate during 1 h-hypoxia followed by 1 h-reoxygenation. Cell viability was assessed by propidium iodide uptake, while apoptosis was assessed by TUNEL and Hoechst 33342 staining. The expression of opioid receptors mRNA in chicken embryonic myocardium was determined by RT-PCR. Opioid receptor antagonists, protein kinase C inhibitors, and KATP channel blockers were used to determine the presumed signal transduction pathways. The results showed that baicalein (0.1 approximately 5 microM) concentration dependently reduced hypoxia-reoxygenation-induced myocardial death and apoptosis. The cardioprotective effect of baicalein (1 microM) was blocked by pretreatment of nonspecific opioid receptor antagonist (naloxone), opioid mu-receptor (beta-funaltrexamine) and delta-receptor (7-Benzylidenenaltrexone) antagonists, protein kinase C inhibitors (H7 and chelerythrine), and KATP channel blockers (glibenclamide and 5-hydroxydecanoate). Finally, RT-PCR analysis successfully demonstrated the presence of opioid receptors mRNA in chicken embryonic cardiomyocytes. We conclude that the cardioprotective effect of baicalein is mediated via mu-, delta- but not kappa-opioid receptor and their related signal transduction pathways, such as protein kinase C and the KATP channel.

The area composita of adhering junctions connecting heart muscle cells of vertebrates – III: Assembly and disintegration of intercalated disks in rat cardiomyocytes growing in culture

For cell and molecular biological studies of heart formation and function cell cultures of embryonal, neonatal or adult hearts of various vertebrates, notably rat and chicken, have been widely used. As the myocardium-specific cell-cell junctions, the intercalated disks (ID), have recently been found to be particularly sensitive to losses of - or mutations in - certain cytoskeletal proteins, resulting in cardiac damages, we have examined the ID organization in primary cultures of cardiomyocytes obtained from neonatal rats. Using immunofluorescence and immunoelectron microscopy, we have studied the major ID components for up to 2 weeks in culture, paying special attention to spontaneously beating, individual cardiomyocytes and myocardial cell colonies. While our results demonstrate the formation of some ID-like cardiomyocyte-connecting junction arrays, they also reveal a variety of structural disorders such as rather extended, junction-free ID regions, sac-like invaginations and endocytotic blebs as well as accumulations of intracytoplasmic structures suggestive of endocytosed forms of junction-derived vesicles or of junction fragments resembling fascia adhaerens elements. Moreover, we have noticed a novel type of small, obviously plaque-free cytoplasmic vesicles containing one or both of the desmosomal cadherins, desmocollin Dsc2 and desmoglein Dsg2. We conclude that cardiomyocyte cultures are useful model systems for studies of certain aspects of myocardiac differentiation and functions but, on the other hand, show progressive disintegration and deterioration. The potential value of molecular markers and reagents in studies of myocardial pathology as well as in the monitoring of myocardial differentiation of so-called stem cells is discussed.

Targeted disruption of N-RAP gene function by RNA interference: a role for N-RAP in myofibril organization

N-RAP is a muscle-specific protein concentrated in myofibril precursors during sarcomere assembly and at intercalated disks in adult heart. We used RNA interference to achieve a targeted decrease in N-RAP transcript and protein levels in primary cultures of embryonic mouse cardiomyocytes. N-RAP transcript levels were decreased by approximately 70% within 2 days following transfection with N-RAP specific siRNA. N-RAP protein levels steadily decreased over several days, reaching approximately 50% of control levels within 6 days. N-RAP protein knockdown was associated with decreased myofibril assembly, as assessed by alpha-actinin organization into mature striations. Transcripts encoding N-RAP binding proteins associated with assembling or mature myofibrils, such as alpha-actinin, Krp1, and muscle LIM protein, were expressed at normal levels during N-RAP protein knockdown, and alpha-actinin and Krp-1 protein levels were also unchanged. Transcripts encoding muscle myosin heavy chain and nonmuscle myosin heavy chain IIB were also expressed at relatively normal levels. However, decreased N-RAP protein levels were associated with dramatic changes in the encoded myosin proteins, with muscle myosin heavy chain levels increasing and nonmuscle myosin heavy chain IIB decreasing. N-RAP transcript and protein levels recovered to normal by days 6 and 7, respectively, and the changes in myofibril organization and myosin heavy chain isoform levels were reversed. Our data indicate that we can achieve transient N-RAP protein knockdown using the RNA interference technique and that alpha-actinin organization into myofibrils in cardiomyocytes is closely linked to N-RAP protein levels. Finally, N-RAP protein levels regulate the balance between nonmuscle myosin IIB and muscle myosin by post-trancriptional mechanisms.

N-RAP expression during mouse heart development

N-RAP gene expression and N-RAP localization were studied during mouse heart development using semiquantitative reverse transcriptase-polymerase chain reaction and immunofluorescence. N-RAP mRNA was detected at embryonic day (E) 10.5, significantly increased from E10.5 to E16.5, and remained essentially constant from E16.5 until 21 days after birth. In E9.5-10.5 heart tissue, N-RAP protein was primarily associated with developing premyofibril structures containing alpha-actinin, as well as with the Z-lines and M-lines of more-mature myofibrils. In contrast, N-cadherin was concentrated in patches at the periphery of the cardiomyocytes. N-RAP labeling markedly increased between E10.5 and E16.5; almost all of the up-regulated N-RAP was associated with intercalated disk structures, and the proportion of mature sarcomeres containing N-RAP decreased. In adult hearts, specific N-RAP staining was only observed at the intercalated disks and was not found in the sarcomeres. The results are consistent with N-RAP functioning as a catalytic scaffolding molecule, with low levels of the scaffold being sufficient to repetitively catalyze key steps in myofibril assembly.

Assembly and Signaling of Adhesion Complexes

Cell-extracellular matrix (ECM) adhesion is crucial for control of cell behavior. It connects the ECM to the intracellular cytoskeleton and transduces bidirectional signals between the extracellular and intracellular compartments. The subcellular machinery that mediates cell-ECM adhesion and signaling is complex. It consists of transmembrane proteins (e.g., integrins) and at least several dozens of membrane-proximal proteins that assemble into a network through multiple protein interactions. Furthermore, despite sharing certain common components, cell-ECM adhesions exhibit considerable heterogeneity in different types of cells (e.g., the cell-ECM adhesions in cardiac myocytes are considerably different from those in fibroblasts). Here, we will first briefly describe the general properties of the integrin-mediated cell-ECM adhesion and signal transduction. Next, we will focus on one of the recently discovered cell-ECM adhesion protein complexes consisting of PINCH, integrin-linked kinase (ILK), and Parvin and use it as an example to illustrate the molecular basis underlying the assembly and functions of cell-ECM adhesions. Finally, we will discuss in detail the structure and regulation of cell-ECM adhesion complexes in cardiac myocytes, which illustrate the importance and complexity of the cell-ECM adhesion structures in organogenesis and diseases.

N-RAP scaffolds I-Z-I assembly during myofibrillogenesis in cultured chick cardiomyocytes

N-RAP is a muscle-specific protein with an N-terminal LIM domain (LIM), C-terminal actin-binding super repeats homologous to nebulin (SR) and nebulin-related simple repeats (IB) in between the two. Based on biochemical data, immunofluorescence analysis of cultured embryonic chick cardiomyocytes and the targeting and phenotypic effects of these individual GFP-tagged regions of N-RAP, we proposed a novel model for the initiation of myofibril assembly in which N-RAP organizes alpha-actinin and actin into the premyofibril I-Z-I complexes. We tested the proposed model by expressing deletion mutants of N-RAP (i.e. constructs containing two of the three regions of N-RAP) in chick cardiomyocytes and observing the effects on alpha-actinin and actin organization into mature sarcomeres. Although individually expressing either the LIM, IB, or SR regions of N-RAP inhibited alpha-actinin assembly into Z-lines, expression of either the LIM-IB fusion or the IB-SR fusion permitted normal alpha-actinin organization. In contrast, the LIM-SR fusion (LIM-SR) inhibited alpha-actinin organization into Z-lines, indicating that the IB region is critical for Z-line assembly. While permitting normal Z-line assembly, LIM-IB and IB-SR decreased sarcomeric actin staining intensity; however, the effects of LIM-IB on actin assembly were significantly more severe, as estimated both by morphological assessment and by quantitative measurement of actin staining intensity. In addition, LIM-IB was consistently retained in mature Z-lines, while mature Z-lines without significant IB-SR incorporation were often observed. We conclude that the N-RAP super repeats are essential for organizing actin filaments during myofibril assembly in cultured embryonic chick cardiomyocytes, and that they also play an important role in removal of the N-RAP scaffold from the completed myofibrillar structure. This work strongly supports the N-RAP scaffolding model of premyofibril assembly.

Genomic organization, alternative splicing, and expression of human and mouse N-RAP, a nebulin-related LIM protein of striated muscle

Linkage analysis identifies 10q24-26 as a disease locus for dilated cardiomyopathy (DCM), a region including the N-RAP gene. N-RAP is a nebulin-like LIM protein that may mediate force transmission and myofibril assembly in cardiomyocytes. We describe the sequence, genomic structure, and expression of human N-RAP, as well as an initial screen to determine whether N-RAP mutations cause cardiomyopathy. Human expressed sequence tag databases were searched with the published 3,528-bp mouse N-RAP open reading frame (ORF). Putative cDNA sequences were interrogated by direct sequencing from cardiac and skeletal muscle RNA. We identified two human N-RAP isoforms with ORFs of 5,085 bp (isoform C) and 5,190 bp (isoform S), encoding products of 193-197 kDa. Genomic database searches localize N-RAP to human chromosome 10q25.3 and match isoforms C and S to 41 and 42 exons. Only isoform C is detected in human cardiac RNA; in skeletal muscle, approximately 10% is isoform C and approximately 90% is isoform S. We investigated apparent differences between human N-RAP cDNA and mouse sequences. Two mouse N-RAP isoforms with ORFs of 5,079 and 5,184 bp were identified with approximately 85% similarity to human isoforms; published mouse sequences include cloning artifacts truncating the ORF. Murine and human isoforms have similar gene structure, tissue specificity, and size. N-RAP is especially conserved within its nebulin-like and LIM domains. We expressed both N-RAP isoforms and the previously described truncated N-RAP in embryonic chick cardiomyocytes. All constructs targeted to myofibril precursors and the cell periphery, and inhibited myofibril assembly. Several human N-RAP polymorphisms were detected, but none were unique to cardiomyopathy patients. N-RAP is highly conserved and exclusively expressed in cardiac and skeletal muscle. Genetic abnormalities remain excellent candidate causes for cardiac and skeletal myopathies.

New N-RAP-binding partners alpha-actinin, filamin and Krp1 detected by yeast two-hybrid screening: implications for myofibril assembly

N-RAP, a muscle-specific protein concentrated at myotendinous junctions in skeletal muscle and intercalated disks in cardiac muscle, has been implicated in myofibril assembly. To discover more about the role of N-RAP in myofibril assembly, we used the yeast two-hybrid system to screen a mouse skeletal muscle cDNA library for proteins capable of binding N-RAP in a eukaryotic cell. From yeast two-hybrid experiments we were able to identify three new N-RAP binding partners: alpha-actinin, filamin-2, and Krp1 (also called sarcosin). In vitro binding assays were used to verify these interactions and to identify the N-RAP domains involved. Three regions of N-RAP were expressed as His-tagged recombinant proteins, including the nebulin-like super repeat region (N-RAP-SR), the N-terminal LIM domain (N-RAP-LIM), and the region of N-RAP in between the super repeat region and the LIM domain (N-RAP-IB). We detected significant alpha-actinin binding to N-RAP-IB and N-RAP-LIM, filamin binding to N-RAP-SR, and Krp1 binding to N-RAP-SR and N-RAP-IB. During myofibril assembly in cultured chick cardiomyocytes, N-RAP and filamin appear to co-localize with alpha-actinin in the earliest myofibril precursors found near the cell periphery, as well as in the nascent myofibrils that form as these structures fuse laterally. In contrast, Krp1 is not localized until late in the assembly process, when it appears at the periphery of myofibrils that appear to be fusing laterally. The results suggest that sequential recruitment of N-RAP binding partners may serve an important role during myofibril assembly.

The complete mouse nebulin gene sequence and the identification of cardiac nebulin

Nebulin is a giant (M(r) 750-850kDa), modular sarcomeric protein proposed to regulate the assembly, and to specify the precise lengths of actin (thin) filaments in vertebrate skeletal muscles. Nebulin's potential role as a molecular template is based on its structural and biochemical properties. Its central approximately 700kDa portion associates with actin along the entire length of the thin filament, its N-terminal region extends to thin filament pointed ends, and approximately 80kDa of its C-terminal region integrates within the Z-line lattice. Here, we determined the exon/intron organization of the entire mouse nebulin gene, which contains 165 exons in a 202kb segment. We identified 16 novel exons, 15 of which encode nebulin-repeat motifs (12 from its central region and 3 from its Z-line region). One novel exon shares high sequence homology to the 20 residue repeats of the tight-junction protein, ZO-1. RT-PCR analyses revealed that all 16 novel exons are expressed in mouse skeletal muscle. Surprisingly, we also amplified mRNA transcripts from mouse and human heart cDNA using primers designed along the entire length of nebulin. The expression of cardiac-specific nebulin transcripts was confirmed by in situ hybridization in fetal rat cardiomyocytes and in embryonic Xenopus laevis (frog) heart. On the protein level, antibodies specific for skeletal muscle nebulin's N and C-terminal regions stained isolated rat cardiac myofibrils at the pointed and barbed ends of thin filaments, respectively. These data indicate a conserved molecular layout of the nebulin filament systems in both cardiac and skeletal myofibrils. We propose that thin filament length regulation in cardiac and skeletal muscles may share conserved nebulin-based mechanisms, and that nebulin isoform diversity may contribute to thin filament length differences in cardiac and skeletal muscle.

ALP and MLP distribution during myofibrillogenesis in cultured cardiomyocytes

The Z-line is a multifunctional macromolecular complex that anchors sarcomeric actin filaments, mediates interactions with intermediate filaments and costameres, and recruits signaling molecules. Antiparallel alpha-actinin homodimers, present at Z-lines, cross-link overlapping actin filaments and also bind other cytoskeletal and signaling elements. Two LIM domain containing proteins, alpha-actinin associated LIM protein (ALP) and muscle LIM protein (MLP), interact with alpha-actinin, distribute in vivo to Z-lines or costameres, respectively, and, when absent, are associated with heart disease. Here we describe the behavior of ALP and MLP during myofibrillogenesis in cultured embryonic chick cardiomyocytes. As myofibrils develop, ALP and MLP are observed in distinct distribution patterns in the cell. ALP is coincident with alpha-actinin from the first stage of myofibrillogenesis and co-distributes with alpha-actinin to Z-lines and intercalated discs in mature myofibrils. Interestingly, we also demonstrate using ALP-GFP transfection experiments and an in vitro binding assay that the ALP-alpha-actinin binding interaction is not required to target ALP to the Z-line. In contrast, MLP localization is not co-incident with that of alpha-actinin until late stages of myofibrillogenesis; however, it is present in premyofibrils and nascent myofibrils prior to the incorporation of other costameric components such as vinculin, vimentin, or desmin. Our observations support the view that ALP function is required specifically at actin anchorage sites. The subcellular distribution pattern of MLP during myofibrillogenesis suggests that it functions during differentiation prior to the establishment of costameres.

Ultrastructural and biochemical localization of N-RAP at the interface between myofibrils and intercalated disks in the mouse heart

N-RAP is a recently discovered muscle-specific protein found at cardiac intercalated disks. Double immunogold labeling of mouse cardiac muscle reveals that vinculin is located immediately adjacent to the fascia adherens region of the intercalated disk membrane, while N-RAP extends approximately 100 nm further toward the interior of the cell. We partially purified cardiac intercalated disks using low- and high-salt extractions followed by density gradient centrifugation. Immunoblots show that this preparation is highly enriched in desmin and junctional proteins, including N-RAP, talin, vinculin, beta1-integrin, N-cadherin, and connexin 43. Electron microscopy and immunolabeling demonstrate that N-RAP and vinculin are associated with the large fragments of intercalated disks that are present in this preparation, which also contains numerous membrane vesicles. Detergent treatment of the partially purified intercalated disks removed the membrane vesicles and extracted vinculin and beta1-integrin. Further separation on a sucrose gradient removed residual actin and myosin and yielded a fraction morphologically similar to fasciae adherentes that was highly enriched in N-RAP, N-cadherin, connexin 43, talin, desmin, and alpha-actinin. The finding that N-RAP copurifies with detergent-extracted intercalated disk fragments even though beta-integrin and vinculin have been completely removed suggests that N-RAP association with the adherens junction region is mediated by the cadherin system. Consistent with this hypothesis, we found that recombinant N-RAP fragments bind alpha-actinin in a gel overlay assay. In addition, immunofluorescence shows that N-RAP remains bound at the ends of isolated, detergent-treated cardiac myofibrils. These results demonstrate that N-RAP remains tightly bound to myofibrils and fasciae adherentes during biochemical purification and may be a key constituent in the mechanical link between these two structures.

Targeting and functional role of N-RAP, a nebulin-related LIM protein, during myofibril assembly in cultured chick cardiomyocytes

Targeting and functional effects of N-RAP domains were studied by expression as GFP-tagged fusion proteins in cultured embryonic chick cardiomyocytes. GFP-tagged N-RAP was targeted to myofibril precursors, myofibril ends and cell contacts, expression patterns that are similar to endogenous N-RAP. The GFP-tagged N-RAP LIM domain (GFP-N-RAP-LIM) was targeted to the membrane in cells with myofibril precursors and cell-cell contacts. The GFP-tagged super repeats (N-RAP-SR) and the GFP-tagged domain normally found in between the super repeats and the LIM domain (N-RAP-IB) were each observed at sites of myofibril assembly, incorporating into myofibril precursors in a manner similar to full length N-RAP. However, unlike full-length N-RAP, N-RAP-SR and N-RAP-IB were also found in mature myofibrils, associating with the sarcomeric actin filaments and the Z-lines, respectively. N-RAP-IB was also colocalized with α-actinin at cell contacts. Each of the N-RAP constructs could inhibit the formation of mature myofibrils in cultured cardiomyocytes, with the effects of N-RAP-SR and N-RAP-IB depending on the time of transfection. The results show that each region of N-RAP is crucial for myofibril assembly. Combining the targeting and functional effects of N-RAP domains with information in the literature, we propose a new model for initiation of myofibrillogenesis.

Angiotensin II stimulates alpha-skeletal actin expression in cadiomyocytes in vitro and in vivo in the absence of hypertension

Using a specific alpha-skeletal actin antibody, we have previously shown, that during hypertension-associated cardiac hypertrophy in the rat, the expression of alpha-skeletal actin in the myocardium is increased, but maintains focal distribution, compared to normotensive animals. In the present study, we have investigated whether alpha-skeletal actin expression can be induced in the absence of hypertension. For this purpose, we have examined transgenic mice overexpressing angiotensinogen exclusively in the heart. These animals are characterized by high cardiac angiotensin II levels and cardiac hypertrophy accompanied or not by high blood pressure depending on their genetic background, i.e. presence of one or two renin genes. Alpha-skeletal actin levels were highly increased in transgenic compared to wild-type myocardium independently of the number of renin genes, indicating that angiotensin II can stimulate alpha-skeletal actin expression in normotensive animals. Additional in vitro experiments using cultured mouse and rat cardiomyocytes showed that angiotension II not only increases alpha-skeletal actin expression but also induces an increase of its incorporation within II-bands compared to control cardiomyocytes. Angiotensin II increases also the expression of alpha-smooth muscle actin in sarcomeres of cardiomyocytes as well as in fibroblastic cells present within the culture.