Impact of titin post-translational modifications on muscle contractile function

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***Over their lifetime, cardiac muscle cells or cardiomyocytes (CMCs) must structurally, mechanically and functionally adapt to a multitude of physiological cues and biomechanical constraints from their environment. Specialized mechano-sensitive and -active proteins are central to the functional response of CMCs to their environment, as exemplified by titin, actin, myosin and others. By spanning the length of half a contractile unit of CMCs (i.e. a sarcomere), the giant protein titin (~3 MDa) acts as a passive molecular spring to limit sarcomere extension range, preventing the myosin-rich thick filaments from slipping outside the actin-rich thin filaments. The portion of titin located in the I-band region of the sarcomere plays an important role in muscle elasticity. Several isoforms of the titin elastic region exist, consisting of tandem arranged Ig domains and large unstructured domains acting as entropic springs, namely the PEVK domain, rich in proline, glutamate, valine and lysine, and the N2B, and N2A domains. Phosphorylation and dephosphorylation of the PEVK and N2B domains was demonstrated to impact the elastic properties of individual titin molecules, which in turn are believed to fine-tune the contractile activity of sarcomeres. We hypothesize that other post-translational modifications, influenced by systemic and tissue environment, could also modulate the elastic properties of titin and other muscle proteins. Such environmental modifications include the presence of dicarbonyl by-products of glycolysis, such as glyoxal and methylglyoxal which produce non-enzymatic modifications on proteins by reacting with lysine and arginine residues to produce a variety of modification including proteins crosslinks and carbonyl adducts such as carboxymethyl-lysine and carboxyethyl-lysine. In addition to the direct disruptive effects of dicarbonyl-derived protein crosslinkings, carboxymethyl-lysine and carboxyethyl-lysine modifications also have a significant disruptive potential in proteins physicochemical properties since they convert basic residues into negatively charged side chains, similarly to phosphorylation. Abundant sarcomeric contractile proteins such as myosin, actin, alpha-actinin, nebulin, myomesin and titin are prime candidates to dicarbonyl-derived modifications which could have a direct downstream impact on CMCs mechanical function. With his numerous lysine residues in its elastic regions, titin could be particularly susceptible to glycation. Interestingly, the intrinsically disordered PEVK domain, with its high density of exposed lysine residues, is expected to be particularly prone to such modifications. Therefore, the general goal of our research program is to assess the impact of dicarbonyles-derived PTM on CMCs contractile proteins. This program will focus on the mechanical properties of titin and its role in the contractile functions of CMS.**