Human myocytes are protected from titin aggregation-induced stiffening by small heat shock proteins

• Published in: The Journal of cell biology Vol. 204; no. 2; pp. 187 - 202
• Main Authors: Kötter, Sebastian, Unger, Andreas, Hamdani, Nazha, Lang, Patrick, Vorgerd, Matthias, Nagel-Steger, Luitgard, Linke, Wolfgang A
• Format: Journal Article
• Language: English
• Published: United States Rockefeller University Press 20-01-2014; The Rockefeller University Press
• Subjects: alpha-Crystallin B Chain - genetics; alpha-Crystallin B Chain - metabolism; alpha-Crystallin B Chain - physiology; Animals; Binding sites; Biochemistry; Cardiomyocytes; Connectin - chemistry; Connectin - metabolism; Connectin - physiology; Gene expression; Heat shock proteins; HSP27 Heat-Shock Proteins - genetics; HSP27 Heat-Shock Proteins - metabolism; HSP27 Heat-Shock Proteins - physiology; Humans; Immunoglobulins; Muscle Fibers, Skeletal - metabolism; Muscle Fibers, Skeletal - physiology; Muscle Fibers, Skeletal - ultrastructure; Myocytes, Cardiac - metabolism; Myocytes, Cardiac - physiology; Myocytes, Cardiac - ultrastructure; Myofibrils - metabolism; Protein folding; Protein Structure, Tertiary; Rats; Stress, Physiological
• ISSN: 0021-9525; 1540-8140
• DOI: 10.1083/jcb.201306077

In myocytes, small heat shock proteins (sHSPs) are preferentially translocated under stress to the sarcomeres. The functional implications of this translocation are poorly understood. We show here that HSP27 and αB-crystallin associated with immunoglobulin-like (Ig) domain-containing regions, but not the disordered PEVK domain (titin region rich in proline, glutamate, valine, and lysine), of the titin springs. In sarcomeres, sHSP binding to titin was actin filament independent and promoted by factors that increased titin Ig unfolding, including sarcomere stretch and the expression of stiff titin isoforms. Titin spring elements behaved predominantly as monomers in vitro. However, unfolded Ig segments aggregated, preferentially under acidic conditions, and αB-crystallin prevented this aggregation. Disordered regions did not aggregate. Promoting titin Ig unfolding in cardiomyocytes caused elevated stiffness under acidic stress, but HSP27 or αB-crystallin suppressed this stiffening. In diseased human muscle and heart, both sHSPs associated with the titin springs, in contrast to the cytosolic/Z-disk localization seen in healthy muscle/heart. We conclude that aggregation of unfolded titin Ig domains stiffens myocytes and that sHSPs translocate to these domains to prevent this aggregation.