title: Human myocytes are protected from titin aggregation-induced stiffening by small heat shock proteins.
abstract: 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.
authors.affiliation: Department of Cardiovascular Physiology and 2 Neurological University Clinic Bergmannsheil, Ruhr University Bochum, 44780 Bochum, Germany.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Kötter
authors.forename: Andreas
authors.initials: A
authors.lastname: Unger
authors.forename: Nazha
authors.initials: N
authors.lastname: Hamdani
authors.forename: Patrick
authors.initials: P
authors.lastname: Lang
authors.forename: Matthias
authors.initials: M
authors.lastname: Vorgerd
authors.forename: Luitgard
authors.initials: L
authors.lastname: Nagel-Steger
authors.forename: Wolfgang A
authors.initials: WA
authors.lastname: Linke
mesh_terms: D000818:Animals; D064211:Connectin / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D055551:HSP27 Heat-Shock Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D006360:Heat-Shock Proteins; D006801:Humans; D018832:Molecular Chaperones; D018485:Muscle Fibers, Skeletal / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure; D032383:Myocytes, Cardiac / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure; D009210:Myofibrils / Q000378:metabolism; D017434:Protein Structure, Tertiary; D051381:Rats; D013312:Stress, Physiological; D038203:alpha-Crystallin B Chain / Q000235:genetics / Q000378:metabolism / Q000502:physiology
chemical_list: D064211:Connectin; D055551:HSP27 Heat-Shock Proteins; C057725:HSPB1 protein, human; D006360:Heat-Shock Proteins; D018832:Molecular Chaperones; C577285:TTN protein, human; D038203:alpha-Crystallin B Chain