Coupled expression of troponin T and troponin I isoforms in single skeletal muscle fibers correlates with contractility

Abstract

Striated muscle contraction is powered by actin-activated myosin ATPase. This process is regulated by Ca(2+) via the troponin complex. Slow- and fast-twitch fibers of vertebrate skeletal muscle express type I and type II myosin, respectively, and these myosin isoenzymes confer different ATPase activities, contractile velocities, and force. Skeletal muscle troponin has also diverged into fast and slow isoforms, but their functional significance is not fully understood. To investigate the expression of troponin isoforms in mammalian skeletal muscle and their functional relationship to that of the myosin isoforms, we concomitantly studied myosin, troponin T (TnT), and troponin I (TnI) isoform contents and isometric contractile properties in single fibers of rat skeletal muscle. We characterized a large number of Triton X-100-skinned single fibers from soleus, diaphragm, gastrocnemius, and extensor digitorum longus muscles and selected fibers with combinations of a single myosin isoform and a single class (slow or fast) of the TnT and TnI isoforms to investigate their role in determining contractility. Types IIa, IIx, and IIb myosin fibers produced higher isometric force than that of type I fibers. Despite the polyploidy of adult skeletal muscle fibers, the expression of fast or slow isoforms of TnT and TnI is tightly coupled. Fibers containing slow troponin had higher Ca(2+) sensitivity than that of the fast troponin fibers, whereas fibers containing fast troponin showed a higher cooperativity of Ca(2+) activation than that of the slow troponin fibers. These results demonstrate distinct but coordinated regulation of troponin and myosin isoform expression in skeletal muscle and their contribution to the contractile properties of muscle.

Deletion of a Genomic Segment Containing the Cardiac Troponin I Gene Knocks Down Expression of the Slow Troponin T Gene and Impairs Fatigue Tolerance of Diaphragm Muscle

“…Because the diaphragm is a mixed fiber muscle (13), the decreases in the slow isoform myofilament proteins could reflect decreases in the number and/or size of slow fibers as well as increases in the number and/or size of fast fibers as a compensation for the decreases in ssTnT. Our results first showed reductions of both number and size of type 1 slow muscle fibers, which would directly contribute to the atrophy of ssTnT-KD mouse diaphragm.…”

“…Except for aging studies, mice at age 3-5 months of both sexes were used for phenotype characterization. Immediately after euthanasia with pentobarbital (intraperitoneally, 100 mg/kg), diaphragm muscle strips were dissected from the 9 o'clock and 4 o'clock sectors with the dorsal side as 12 and 1 o'clock positions (13) for use in the experiments to avoid the difference between different portions of the diaphragm. All animal procedures were approved by the Institutional Animal Care and Use Committee.…”

Quantitative and evolutionary biology of alternative splicing: how changing the mix of alternative transcripts affects phenotypic plasticity and reaction norms

“…There is nearly as strong a correlation between the summed relative abundance of the same two transcripts and the power output of intact muscle (Figure 3d). Experiments in other animals have repeatedly shown that manipulation of Tnt genes and/or protein isoforms affects muscle calcium sensitivity and contractile performance (Ogut et al, 1999;Gomes et al, 2002Gomes et al, , 2004MacFarland et al, 2002;Nassar et al, 2005;Brotto et al, 2006;Chandra et al, 2006), and single amino-acid replacement mutations in human Tnt also have these effects (Hernandez et al, 2005).…”

(How) do animals know how much they weigh?

“…Alternative splicing of troponin T affects muscle performance by modulating the Ca 2+ sensitivity of muscle contraction (Brotto, 2005;Gomes et al, 2004;Ogut et al, 1999). Extrapolating variation in Ca 2+ sensitivity in muscle fiber preparations to whole muscle and animal function is non-trivial, but generally speaking, higher Ca 2+ sensitivity should cause muscle to contract sooner and remain active for longer during each neurally induced Ca 2+ wave, thereby modulating both mechanical performance and energy (i.e.…”