Understanding mechanisms underlying titin regulation in cardiac muscle function is of critical importance given recent compelling evidence that highlight titin mutations as major determinants of human cardiomyopathy. We previously identified a cardiac biomechanical stress-regulated complex at the cardiac-specific N2B region of titin that includes four-and-a-half LIM domain protein-1 (Fhl1) and components of the mitogen-activated protein signaling cascade, which impacted muscle compliance in Fhl1 knock-out cardiac muscle. However, direct regulation of these molecular components in mediating titin N2B function remained unresolved. Here we identify Fhl1 as a novel negative regulator of titin N2B levels and phosphorylation-mediated mechanics. We specifically identify titin N2B as a novel substrate of extracellular signal regulated-kinase-2 (Erk2) and demonstrate that Fhl1 directly interferes with Erk2-mediated titin-N2B phosphorylation. We highlight the critical region in titin-N2B that interacts with Fhl1 and residues that are dependent on Erk2-mediated phosphorylation in situ. We also propose a potential mechanism for a known titin-N2B cardiomyopathy-causing mutation that involves this regulatory complex. These studies shed light on a novel mechanism regulating titin-N2B mechano-signaling as well as suggest that dysfunction of these pathways could be important in cardiac disease states affecting muscle compliance.
"The N2A and N2B regions are composed of nonrepetitive sequences and Ig-like domains, and contain several protein partner-binding sites. Two members of the four and a half LIM domain proteins, DRAL/FHL2 and FHL1, bind to the restricted N2B region [Sheikh et al., 2008; Raskin et al., 2012], whereas the small heat shock proteins (sHSPs) αβ-crystallin and HSP27-binding sites expand to the N2B region and its 2 COOH-terminally located Ig domains (IIg26- IIg27) [Golenhofen et al., 2002; Bullard et al., 2004; Kötter et al., 2014]. The two sHSPs also bind to the N2A elements, as well as the lysine methyltransferase Smyd2 [Donlin et al., 2012]. "
[Show abstract][Hide abstract] ABSTRACT: The 364 exon TTN gene encodes titin (TTN), the largest known protein, which plays key structural, developmental, mechanical and regulatory roles in cardiac and skeletal muscles. Prior to next generation sequencing (NGS), routine analysis of the whole TTN gene was impossible due to its giant size and complexity. Thus, only a few TTN mutations had been reported and the general incidence and spectrum of titinopathies was significantly underestimated. In the last months, due to widespread use of NGS, TTN is emerging as a major gene in human inherited disease. So far, 127 TTN disease causing mutations have been reported in patients with at least 10 different conditions, including isolated cardiomyopathies, purely skeletal muscle phenotypes or infantile diseases affecting both types of striated muscles. However, identification of TTN variants in virtually every individual from control populations, as well as the multiplicity of TTN isoforms and reference sequences used, stress the difficulties in assessing the relevance, inheritance and correlation with the phenotype of TTN sequence changes. In this review we provide the first comprehensive update of the TTN mutations reported and discuss their distribution, molecular mechanisms, associated phenotypes, transmission pattern and phenotype-genotype correlations, alongside with their implications for basic research and for human health. This article is protected by copyright. All rights reserved.
Human Mutation 09/2014; 35(9). DOI:10.1002/humu.22611 · 5.14 Impact Factor
"Fhl1 interacts directly with the N2B region of titin to form a novel complex, and it has been proposed to act as a biomechanical sensor to myofibrillar passive tension generated upon stretch [55,56]. A dose-dependent increase in Fhl1 can reduce the titin N2B phosphorylation by interfering with the binding of Erk2 . Therefore, the increase of Fhl1 in Rbm20-/- rats is consistent with markedly reduced passive tension . "
[Show abstract][Hide abstract] ABSTRACT: Our recent study indicated that RNA binding motif 20 (Rbm20) alters splicing of titin and other genes. The current goals were to understand how the Rbm20(-/-) rat is related to physiological, structural, and molecular changes leading to heart failure. We quantitatively and qualitatively compared the expression of titin isoforms between Rbm20(-/-) and wild type rats by real time RT-PCR and SDS agarose electrophoresis. Isoform changes were linked to alterations in transcription as opposed to translation of titin messages. Reduced time to exhaustion with running in knockout rats also suggested a lower maximal cardiac output or decreased skeletal muscle performance. Electron microscopic observations of the left ventricle from knockout animals showed abnormal myofibril arrangement, Z line streaming, and lipofuscin deposits. Mutant skeletal muscle ultrastructure appeared normal. The results suggest that splicing alterations in Rbm20(-/-) rats resulted in pathogenic changes in physiology and cardiac ultrastructure. Secondary changes were observed in message levels for many genes whose splicing was not directly affected. Gene and protein expression data indicated the activation of pathophysiological and muscle stress-activated pathways. These data provide new insights on Rbm20 function and how its malfunction leads to cardiomyopathy.
PLoS ONE 12/2013; 8(12):e84281. DOI:10.1371/journal.pone.0084281 · 3.23 Impact Factor
"It is therefore not surprising that this area is involved in multiple non-sarcomeric signaling pathways that are influenced by stretch signaling. For instance, FHL-1 mediates Erk-2-regulated phosphorylation of an inter-Ig region of titin (Raskin et al., 2012). The FHL-1/titin interaction connects extracellular signals to transcription via stretch response (Sheikh et al., 2008). "
[Show abstract][Hide abstract] ABSTRACT: Giant muscle proteins (e.g., titin, nebulin, and obscurin) play a seminal role in muscle elasticity, stretch response, and sarcomeric organization. Each giant protein consists of multiple tandem structural domains, usually arranged in a modular fashion spanning 500 kDa to 4 MDa. Although many of the domains are similar in structure, subtle differences create a unique function of each domain. Recent high and low resolution structural and dynamic studies now suggest more nuanced overall protein structures than previously realized. These findings show that atomic structure, interactions between tandem domains, and intrasarcomeric environment all influence the shape, motion, and therefore function of giant proteins. In this article we will review the current understanding of titin, obscurin, and nebulin structure, from the atomic level through the molecular level.
Frontiers in Physiology 12/2013; 4:368. DOI:10.3389/fphys.2013.00368 · 3.53 Impact Factor
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