title: Deleting titin's I-band/A-band junction reveals critical roles for titin in biomechanical sensing and cardiac function.
abstract: Titin, the largest protein known, forms a giant filament in muscle where it spans the half sarcomere from Z disk to M band. Here we genetically targeted a stretch of 14 immunoglobulin-like and fibronectin type 3 domains that comprises the I-band/A-band (IA) junction and obtained a viable mouse model. Super-resolution optical microscopy (structured illumination microscopy, SIM) and electron microscopy were used to study the thick filament length and titin's molecular elasticity. SIM showed that the IA junction functionally belongs to the relatively stiff A-band region of titin. The stiffness of A-band titin was found to be high, relative to that of I-band titin (∼ 40-fold higher) but low, relative to that of the myosin-based thick filament (∼ 70-fold lower). Sarcomere stretch therefore results in movement of A-band titin with respect to the thick filament backbone, and this might constitute a novel length-sensing mechanism. Findings disproved that titin at the IA junction is crucial for thick filament length control, settling a long-standing hypothesis. SIM also showed that deleting the IA junction moves the attachment point of titin's spring region away from the Z disk, increasing the strain on titin's molecular spring elements. Functional studies from the cellular to ex vivo and in vivo left ventricular chamber levels showed that this causes diastolic dysfunction and other symptoms of heart failure with preserved ejection fraction (HFpEF). Thus, our work supports titin's important roles in diastolic function and disease of the heart.
authors.affiliation: Department of Cellular and Molecular Medicine, University of Arizona, Tucson, AZ 85724 granzier@email.arizona.edu.
authors.forename: Henk L
authors.initials: HL
authors.lastname: Granzier
authors.affiliation: Department of Cellular and Molecular Medicine, University of Arizona, Tucson, AZ 85724.
authors.forename: Kirk R
authors.initials: KR
authors.lastname: Hutchinson
authors.affiliation: Department of Cellular and Molecular Medicine, University of Arizona, Tucson, AZ 85724.
authors.forename: Paola
authors.initials: P
authors.lastname: Tonino
authors.affiliation: Department of Cellular and Molecular Medicine, University of Arizona, Tucson, AZ 85724.
authors.forename: Mei
authors.initials: M
authors.lastname: Methawasin
authors.affiliation: Department of Cellular and Molecular Medicine, University of Arizona, Tucson, AZ 85724.
authors.forename: Frank W
authors.initials: FW
authors.lastname: Li
authors.affiliation: Department of Cellular and Molecular Medicine, University of Arizona, Tucson, AZ 85724.
authors.forename: Rebecca E
authors.initials: RE
authors.lastname: Slater
authors.affiliation: Department of Cellular and Molecular Medicine, University of Arizona, Tucson, AZ 85724.
authors.forename: Mathew M
authors.initials: MM
authors.lastname: Bull
authors.affiliation: Department of Cellular and Molecular Medicine, University of Arizona, Tucson, AZ 85724.
authors.forename: Chandra
authors.initials: C
authors.lastname: Saripalli
authors.affiliation: Department of Cellular and Molecular Medicine, University of Arizona, Tucson, AZ 85724.
authors.forename: Christopher T
authors.initials: CT
authors.lastname: Pappas
authors.affiliation: Department of Cellular and Molecular Medicine, University of Arizona, Tucson, AZ 85724.
authors.forename: Carol C
authors.initials: CC
authors.lastname: Gregorio
authors.affiliation: Department of Cellular and Molecular Medicine, University of Arizona, Tucson, AZ 85724.
authors.forename: John E
authors.initials: JE
authors.lastname: Smith
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001696:Biomechanical Phenomena; D001794:Blood Pressure / Q000502:physiology; D015153:Blotting, Western; D002478:Cells, Cultured; D064211:Connectin / Q000235:genetics / Q000378:metabolism*; D004452:Echocardiography; D020869:Gene Expression Profiling; D006321:Heart / Q000502:physiology*; D016014:Linear Models; D040542:Mechanotransduction, Cellular; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D046529:Microscopy, Electron, Transmission; D016253:Microscopy, Immunoelectron; D008969:Molecular Sequence Data; D018482:Muscle, Skeletal / Q000378:metabolism / Q000502:physiology; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D032383:Myocytes, Cardiac / Q000378:metabolism / Q000502:physiology; D020411:Oligonucleotide Array Sequence Analysis; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012518:Sarcomeres / Q000378:metabolism* / Q000648:ultrastructure; D017386:Sequence Homology, Amino Acid
chemical_list: D064211:Connectin
keywords: hypertrophy; mechanosensing; molecular elasticity; passive stiffness