Biophys Rev DOI 10.1007/s12551-017-0263-9 REVIEW Tampering with springs: phosphorylation of titin affecting the mechanical function of cardiomyocytes Nazha Hamdani1 & Melissa Herwig1 & Wolfgang A. Linke1,2,3 Received: 2 March 2017 /Accepted: 26 March 2017 # The Author(s) 2017. This article is published with open access at Springerlink.com Abstract Reversible post-translational modifications of vari- phosphorylation affects titin-based passive tension in ous cardiac proteins regulate the mechanical properties of the cardiomyocytes. A comprehensive overview is provided of cardiomyocytes and thus modulate the contractile perfor- studies that have measured altered titin phosphorylation and mance of the heart. The giant protein titin forms a continuous titin-based passive tension in myocardial samples from human filament network in the sarcomeres of striated muscle cells, heart failure patients and animal models of heart disease. As where it determines passive tension development and modu- our understanding of the broader implications of phosphory- lates active contraction. These mechanical properties of titin lation in titin progresses, this knowledge could be used to are altered through post-translational modifications, particu- design targeted interventions aimed at reducing pathologically larly phosphorylation. Titin contains hundreds of potential increased titin stiffness in patients with stiff hearts. phosphorylation sites, the functional relevance of which is only beginning to emerge. Here, we provide a state-of-the- Keywords Cardiomyocytes . Muscle cell mechanics . art summary of the phosphorylation sites in titin, with a par- Posttranslational modification . Heart failure . Diastolic ticular focus on the elastic titin spring segment. We discuss function . Stiffness how phosphorylation at specific amino acids can reduce or increase the stretch-induced spring force of titin, depending on where the spring region is phosphorylated. We also review Introduction which protein kinases phosphorylate titin and how this Our heart continually adapts to changes in hemodynamic load and responds to neurohumoral stress, which requires the dy- namic regulation of cardiac contractile performance on a beat- This article is part of a Special Issue on ‘Titin and its Binding Proteins in Striated Muscles’ edited by Amy Li and Cristobal G. dos Remedios to-beat basis. Control of heart function is exerted predomi- nantly via signal regulation that often involves transient or Electronic supplementary material The online version of this article (doi:10.1007/s12551-017-0263-9) contains supplementary material, longer-lasting post-translational modifications (PTMs) of car- which is available to authorized users. diomyocyte proteins. A frequent and well-studied type of PTM is phosphorylation, which is important to many biolog- Nazha Hamdani and Melissa Herwig contributed equally to this work. ical processes (Pawson and Scott 2005) and plays a pivotal * Wolfgang A. Linke role in the regulation of cardiomyocyte protein function [email protected] (Hamdani et al. 2008a;Solaro2008). This phosphorylation- mediated regulation is complex and involves compartmental- 1 Department of Cardiovascular Physiology, Ruhr University Bochum, ization and cross-talk between protein kinases (PKs) and pro- MA 03/56, 44780 Bochum, Germany tein phosphatases (PPs). Alterations in PK and PP activities 2 Deutsches Zentrum für Herz-Kreislaufforschung, Partner Site are implicated in cardiomyocyte dysfunction and contribute to Göttingen, Göttingen, Germany reduced cardiac output in heart disease. Multiple other PTMs 3 Cardiac Mechanotransduction Group, Clinic for Cardiology and have been found in cardiomyocyte proteins, such as O- Pneumology, University Medical Center, Göttingen, Germany GlcNAcylation (Ramirez-Correa et al. 2008), arginylation Biophys Rev (Cornachione et al. 2014; Leite et al. 2016; Rai et al. 2008), S- disordered structures, including the unique sequence of nitrosylation (Figueiredo-Freitas et al. 2015) and oxidation the cardiac-specific ‘N2B’ element (‘N2Bus’)andthe (Alegre-Cebollada et al. 2014; Aryal et al. 2014;Balogh ‘PEVK’ segment, which contains numerous 26–28 residue et al. 2014; Beedle et al. 2016; Canton et al. 2011; Eaton motifs rich in proline, glutamate, valine and lysine (Fig. 1, et al. 2002; Grützner et al. 2009; Stathopoulou et al. 2016). top). Only repeats 27–31 in the NH2-terminal region of A myriad of functional consequences have been described for the human PEVK segment are constitutively expressed in these PTMs, which is an area of intense study. In this review, the full-length titin isoforms. When the cardiac sarcomere we focus on PTMs of titin, the largest protein in our body is stretched, the Ig-domain segments straighten out before expressed in the contractile units, the sarcomeres. the PEVK and N2Bus elements become extended (Li et al. 2002; Linke et al. 1996;Linkeetal.1999). Sarcomere stretching also increases the unfolding proba- The giant protein titin bility of the Ig domains, some of which will unfold at the low passive forces (less than 10 pN/titin molecule) present The sarcomere is composed of three main filament sys- in sarcomeres that are extended in the physiological work- tems: the myosin-based thick filament, the actin-based thin ing range (Rivas-Pardo et al. 2016). In cardiomyocytes, filament, and the titin filament (Fig. 1,top).Individualtitin titin is the primary source of passive tension within a molecules span the entire 1–2 μm distance of the half- sarcomere length (SL) range of ∼1.8–2.2 μm (Linke sarcomere from the Z-disk to the M-band. Several alterna- et al. 1994) and, thus, is the main determinant of myocar- tive splicing events, mostly in the I-band titin region which dial diastolic passive stiffness during physiological load- encompasses more than 220 of the 364 TTN exons, pro- ing (Linke and Hamdani 2014). Apart from being duce three main full-length isoforms: shorter, stiffer N2B established as the passive elastic spring of the sarcomere, (3000 kDa) and longer, more compliant N2BA isoforms titin has also been implicated in active muscle contraction (>3200 kDa) in the heart, as well as variable-length N2A (Cazorla et al. 2001; Fukuda et al. 2001;Lietal.2016; isoform in skeletal muscles (Bang et al. 2001; Neagoe et al. Rivas-Pardo et al. 2016). 2003; Prado et al. 2005). Rare isoforms of titin include the Titin-based myocardial passive stiffness can be modu- full-length variants Novex-1 and Novex-2, and the truncat- lated under physiological conditions and in heart disease, ed Novex-3 isoform (625 kDa). A novel isoform, termed including human heart failure (HF). In end-stage failing Cronos (2300 kDa), is expressed under the control of an human hearts, titin-based passive tension is lowered due alternative promoter located near the I-band/A-band junc- mainly to isoform switching towards the more compliant tion (Zou et al. 2015). Cronos was suggested to have a role N2BA isoform (Neagoe et al. 2002). Another modifier of in sarcomeric (A-band) assembly, at least in zebrafish (Zou titin stiffness is the binding of heat shock proteins to elas- et al. 2015), but a recent study found little evidence to tic I-band domains (Bullard et al. 2004; Kötter et al. support this hypothesis (Shih et al. 2016). In contrast to 2014). Moreover, titin-based elastic force is affected by the I-band part of titin, almost all exons encoding the A- PTMs, of which phosphorylation has been studied most band and M-band regions of the titin molecule are consti- intensely and will be covered in detail below. Additional tutively expressed. A-band titin is tightly associated with PTMs that alter the stiffness of titin include arginylation myosin and myosin-binding protein-C (reviewed by Linke (Leite et al. 2016) and various oxidative modifications, and Hamdani 2014). Near the M-band portion of titin is the such as disulfide bonding (Grützner et al. 2009), S- titin-kinase domain (TK), which was shown to be activated by glutathionylation (Alegre-Cebollada et al. 2014), and mechanical strain in vitro (Puchner et al. 2008). However, sulfenylation (Beedle et al. 2016). The functional role of according to recent structural data, TK may be a pseudokinase oxidative modifications in titin has been previously (Bogomolovas et al. 2014). Notwithstanding this controversy, reviewed by us (Beckendorf and Linke 2015;Breitkreuz TK has been established as a scaffold for multiple protein- and Hamdani 2015) and will not be discussed further protein interactions (Lange et al. 2005;Mayansetal.1998). here. An evolving topic over the last decade has been The function of titin as a molecular spring determines the pathologically altered titin phosphorylation in HF with the Bpassive^ elasticity of the sarcomere (Li et al. 2002; preserved or reduced ejection fraction (HFpEF or HFrEF, Linke et al. 1994; Trombitás et al. 1995). The elastic respectively), which often causes cardiomyocyte stiffen- spring region of titin is in the I-band segment of the mol- ing. Global myocardial stiffening is a hallmark of ecule. This region has a complex structure comprising two HFpEF (Gladden et al. 2014)andmaybedue,inpart, types of extensible segments (Freiburg et al. 2000): (1) to dysregulated titin phosphorylation (Linke and tandem Ig-domain regions termed ‘proximal-Ig’ (constitu- Hamdani 2014). Considering the important role of titin tively expressed), ‘middle-Ig’ (alternatively spliced), and phosphorylation for mechanical heart function under ‘distal-Ig’ (constitutively expressed);