Optimal length, calcium sensitivity, and twitch characteristics of skeletal muscles from mdm mice with a deletion in N2A titin

Abstract

During isometric contractions, the optimal length of skeletal muscles increases with decreasing activation. The underlying mechanism for this phenomenon is thought to be linked to length dependence of Ca 2+ sensitivity. Muscular dystrophy with myositis (mdm), a recessive titin mutation in mice, was used as a tool to study the role of titin in activation dependence of optimal length and length dependence of Ca 2+ sensitivity. We measured the shift in optimal length between tetanic and twitch stimulation in mdm and wild-type muscles, and the length dependence of Ca 2+ sensitivity at short and long sarcomere lengths in mdm and wild-type fiber bundles. The results indicate that the mdm mutation leads to a loss of activation dependence of optimal length without the expected change in length dependence of Ca 2+ sensitivity, demonstrating that these properties are not linked, as previously suggested. Furthermore, mdm muscles produced maximum tetanic stress during sub-optimal filament overlap at lengths similar to twitch contractions in both genotypes, but the difference explains less than half of the observed reduction in active force of mdm muscles. Mdm muscles also exhibited increased electromechanical delay, contraction and relaxation times, and decreased rate of force development in twitch contractions. We conclude that the small deletion in titin associated with mdm in skeletal muscles alters force production, suggesting an important regulatory role for titin in active force production. The molecular mechanisms for titin's role in regulating muscle force production remain to be elucidated.

Non-cross Bridge Viscoelastic Elements Contribute to Muscle Force and Work During Stretch-Shortening Cycles: Evidence From Whole Muscles and Permeabilized Fibers

“…Whole muscle experiments were conducted on 11 wild type and 18 mdm soleus muscles from age-matched mice of both sexes (average wild type age = 44.4 ± 2.0 days; average mdm age = 49.6 ± 2.9 days, p = 0.12). Because wild type mice were larger in body size than mdm mice, muscle mass, optimal length (L 0 ), and maximum isometric force were significantly greater in wild type compared to mdm soleus (Table 1), as reported previously (Taylor-Burt et al, 2015;Hessel and Nishikawa, 2017;Monroy et al, 2017;Hessel et al, 2019;Tahir et al, 2020). Maximum isometric stress was also lower in mdm muscles, likely because titin transmits cross bridges forces from the A-band to the Z-disk (Horowits et al, 1986;Nishikawa, 2020).…”

“…Permeabilized ("skinned") fibers were prepared from wild type mouse psoas muscles (n = 8 mice, five male/three female, age range 3-6 months) using standard glycerol techniques (Hessel et al, 2019). Extracted muscles were permeabilized and stored in a relaxing solution [in mmol l −1 : potassium propionate (170), magnesium acetate (2.5), MOPS (20), K 2 EGTA (5), and ATP (2.5), pH 7.0] for 12 h at 4°C, then moved to a relaxing: glycerol (50:50) or rigor: glycerol solution [KCl (100), MgCl 2 (2), EGTA (5), Tris (10), DTT (1), 50% glycerol, and pH 7.0] at −20°C for a minimum of 3 weeks.…”

Effects of a titin mutation on force enhancement and force depression in mouse soleus muscles

“…Several previous studies have shown that mdm muscles and muscle fibers have normal sarcomere structure and normal active and passive tension at 24−30 days of age (Witt et al, 2004;Hessel et al, 2019). Central nuclei and fibrosis have been reported to increase with age as muscles degenerate and subsequently regenerate (Lopez et al, 2008;Heimann et al, 1996).…”

Triceps surae torque-length relationships relevant for walking activity levels with and without an ankle exoskeleton

“…Activation-dependence of optimal length, often called length-dependent activation, is an umbrella term that covers subcellular and MTU level mechanisms that are not necessarily mutually exclusive (Holt and Williams, 2018). The first mechanism is subcellular in nature and often observed in in vitro muscle preparations, where at a relative level of submaximal Ca 2+ , more force is produced than expected (Hessel et al, 2019; Stephenson and Williams, 1982; Yang et al, 1998). The exact molecular mechanism for this property is not yet clear, but seems to be connected to molecular changes of sarcomeric proteins (Hessel et al, 2019; Konhilas et al, 2002).…”

“…The first mechanism is subcellular in nature and often observed in in vitro muscle preparations, where at a relative level of submaximal Ca 2+ , more force is produced than expected (Hessel et al, 2019; Stephenson and Williams, 1982; Yang et al, 1998). The exact molecular mechanism for this property is not yet clear, but seems to be connected to molecular changes of sarcomeric proteins (Hessel et al, 2019; Konhilas et al, 2002). Outside of subcellular mechanisms, an important mechanism responsible for length-dependent activation is associated with MTU compliance (Holt and Williams, 2018; MacIntosh, 2017) and was studied by others in the human vastus lateralis (Austin et al, 2010; de Brito Fontana and Herzog, 2016; Ichinose et al, 1997; Lloyd and Besier, 2003) and in the triceps surae (Clark and Franz, 2019).…”