Increased Fatigue Resistance of Skeletal Muscle with Elevated 2-Deoxy-ATP following Ribonucleotide Reductase Overexpression

Abstract

is the most common non-dystrophic congenital myopathy, clinically characterized by muscle weakness. The disease is associated with mutations in the nebulin gene and the nebulin-based disease is referred to as NEM2. Recent work on skinned muscle fibres from NEM2 patients revealed remarkable phenotypic similarities to fibres from nebulin KO mice (Ottenheijm et al, 2012). Here we investigated mechanics and kinetics of single myofibrils from a novel NEM2 mouse model (NEB Dex55) that mimics a deletion in the nebulin gene found in a large group of NEM2 patients. We used rapid solution switching (Tesi et al.,2002) to compare maximal tension and kinetics of contraction and relaxation of myofibrils isolated from frozen skeletal muscles (tibialis cranialis of neonatal mice) of WT and NEB Dex55 mice. Myofibrils, mounted in a force recording apparatus (15 C), were maximally Ca 2þ -activated (pCa 4.5) and fully relaxed (pCa 9.0). Maximal isometric tension was markedly reduced in NEB Dex55 mouse myofibrils (49.7510.6 mN mm À2 n=11) compared to WT (135.3516.9mN mm À2 n=9).The rate constant of active tension generation following maximal Ca 2þ activation (k ACT ) was significantly reduced inNEB Dex55 mouse myofibrils (1.4650.07s À1 ) compared to WT (2.7550.27 s À1 ). Force relaxation kinetics was remarkably faster in NEB Dex55 mouse myofibrils than in WT, evidence that the apparent rate with which cross-bridges leave the force generating states is accelerated in the NEB Dex55sarcomeres. Reduction of the rate with which cross-bridges enter force generating states and of cross bridge dissociation can markedly contribute to reducing maximal tension. This is expected to increase the energetic cost of tension generation of the NEB Dex55sarcomeres. Results suggest that nebulin plays a significant role in contraction regulation and that altered cross bridge kinetics contribute to NEM2 pathogenesis