Early mechanical dysfunction of the diaphragm in the muscular dystrophy with myositis (Ttn^mdm) model

Abstract

A complex rearrangement mutation in the mouse titin gene leads to an in-frame 83-amino acid deletion in the N2A region of titin. Autosomal recessive inheritance of the titin muscular dystrophy with myositis (Ttn(mdm/mdm)) mutation leads to a severe early-onset muscular dystrophy and premature death. We hypothesized that the N2A deletion would negatively impact the force-generating capacity and passive mechanical properties of the mdm diaphragm. We measured in vitro active isometric contractile and passive length-tension properties to assess muscle function at 2 and 6 wk of age. Micro-CT, myosin heavy chain Western blotting, and histology were used to assess diaphragm structure. Marked chest wall distortions began at 2 wk and progressively worsened until 5 wk. The percentage of myofibers with centrally located nuclei in mdm mice was significantly (P < 0.01) increased at 2 and 6 wk by 4% and 17%, respectively, compared with controls. At 6 wk, mdm diaphragm twitch stress was significantly (P < 0.01) reduced by 71%, time to peak twitch was significantly (P < 0.05) reduced by 52%, and half-relaxation time was significantly (P < 0.05) reduced by 57%. Isometric tetanic stress was significantly (P < 0.05) depressed in 2- and 6-wk mdm diaphragms by as much as 64%. Length-tension relationships of the 2- and 6-wk mdm diaphragms showed significantly (P < 0.05) decreased extensibility and increased stiffness. Slow myosin heavy chain expression was aberrantly favored in the mdm diaphragm at 6 wk. Our data strongly support early contractile and passive mechanical aberrations of the respiratory pump in mdm mice.

Shiver me titin! Elucidating titin's role in shivering thermogenesis

“…Passive stiffness and passive tension are higher in mutant muscle than in wild-type muscle for whole-muscle experiments (Lopez et al, 2008), although Witt et al (2004) did not observe differences in passive tension in single muscle fibers. Higher passive stiffness could result from the presence of more collagen and/or from a shorter titin spring (Lopez et al, 2008). An increase in muscle stiffness should result in an increase in tremor frequency for mdm mutants, not a decrease in tremor frequency, as we observed.…”

“…Although Lopez et al (2008) found a higher slow:fast ratio for the myosin heavy chain isoform in mutant muscle than in non-mutant control muscles, these authors reported that mutant muscle actually had a shorter time to peak force and a shorter half-relaxation time compared with non-mutant controls. Therefore, if tremor frequency were dependent on muscle contraction kinetics, we would expect that mutants would shiver at or above expected tremor frequencies, not below, as we observed.…”

“…This mutation is thought to contribute to changes in titin's structure (Lopez et al, 2008). The mdm mutants have a stiffer gait, smaller body mass and reduced lifespan (Garvey et al, 2002;Witt et al, 2004;Huebsch et al, 2005;Lopez et al, 2008).…”

“…This mutation is thought to contribute to changes in titin's structure (Lopez et al, 2008). The mdm mutants have a stiffer gait, smaller body mass and reduced lifespan (Garvey et al, 2002;Witt et al, 2004;Huebsch et al, 2005;Lopez et al, 2008). A whole-muscle study on the diaphragm reported substantial differences in the force production capabilities and elastic properties of mdm mutant muscle compared with wild-type muscle; of particular interest, mutant muscle has higher passive stiffness than wild-type muscle (Lopez et al, 2008).…”

“…Titin is a giant protein that is known to contribute to passive tension (Linke and Granzier, 1998;Huebsch et al, 2005;Lopez et al, 2008;Leonard and Herzog, 2010;Herzog et al, 2012). Historically, titin was considered to have a static contribution to muscle stiffness as this protein is too compliant to substantially contribute to active muscle stiffness.…”