BackgroundMuscle hypertrophy in the mdx mouse model of Duchenne muscular dystrophy (DMD) can partially compensate for the loss of dystrophin by maintaining peak force production. Histopathology examination of the hypertrophic muscles suggests the hypertrophy primarily results from the addition of myofibers, and is accompanied by motor axon branching. However, it is unclear whether an increased number of innervated myofibers (myofiber hyperplasia) contribute to muscle hypertrophy in the mdx mice.MethodsTo better understand the cellular mechanisms of muscle hypertrophy in mdx mice, we directly compared the temporal progression of the dystrophic pathology in the extensor digitorum longus (EDL) muscle to myofiber number, myofiber branching, and innervation, from 3 to 20 weeks of age.ResultsWe found that a 28% increase in the number of fibers in transverse sections of muscle correlated with a 31% increase in myofiber branching. Notably, the largest increases in myofiber number and myofiber branching occurred after 12 weeks of age when the proportion of myofibers with central nuclei had stabilized and the mdx mouse had reached maturity. The dystrophic pathology coincided with profound changes to innervation of the muscles that included temporary denervation of necrotic fibers, fragmentation of synapses, and ultra-terminal axon sprouting. However, there was little evidence of synapse formation in the mdx mice from 3 to 20 weeks of age. Only 4.4% of neuromuscular junctions extended ultra-terminal synapses, which failed to mature, and the total number of neuromuscular junctions remained constant.ConclusionsMuscle hypertrophy in mdx mice results from myofiber branching rather than myofiber hyperplasia.

Escherichia coli glycerol kinase (EC 2.7.1.30; ATP:glycerol 3-phosphotransferase) is a key element in a signal transduction pathway that couples expression of genes required for glycerol metabolism to the relative availability of glycerol and glucose. Its catalytic activity is inhibited by protein-protein interactions with IIIglc, a phosphotransferase system protein, and by fructose 1,6-bisphosphate (FBP); each of these allosteric effectors constitutes a positive signal that glucose is available. Loss of glucose inhibition of glycerol metabolism was used to screen for regulatory mutants of glycerol kinase after hydroxylamine mutagenesis of the cloned glpK gene. Two mutant enzymes were identified and shown by DNA sequencing to contain the mutations alanine 65 to threonine (A65T) and aspartate 72 to asparagine (D72N). Initial velocity studies show the mutations do not significantly affect the catalytic properties, hence active-site structures, of the enzymes. Both mutations decrease inhibition by FBP; A65T eliminates the inhibition while D72N appears to decrease the affinity for FBP and the extent of the inhibition. However, neither mutation significantly affects inhibition by IIIglc. Gel-permeation chromatography studies show that both of the mutations alter the dimer-tetramer assembly reaction of the enzyme and the effect of FBP in increasing the molecular weight. The effects of the mutations on the assembly reaction are consistent with the locations of these two amino acid residues in the X-ray structure, which shows them to be associated with an alpha-helix that constitutes one of the two subunit-subunit interfaces within the tetramer.(ABSTRACT TRUNCATED AT 250 WORDS)

Background: Intramedullary nail (IMN) fixation of the fibula in malleolar ankle fractures has been shown to result in less wound complications then plate fixation. Therefore, IMN fibula fixation may also be associated with lower rates of wound complications when used for higher-risk pilon fractures. The purpose of this study was to compare complications of fibula IMN fixation in pilon versus malleolar ankle fractures. Methods: A retrospective cohort comparison was performed at an urban level one trauma center involving fibula fractures in 47 patients with AO/Orthopaedic Trauma Association (OTA) type 43 fractures and 48 patients with AO/OTA type 44 fractures being treated with fibula IMN fixation. Complications, fibula-specific complications, revision surgeries, and implant removals were reviewed. Results: There was no detectable difference in complications (27% vs. 23%, 95% confidence interval of the odds ratio (CIOR) 0.5 to 3.2), fibular-specific complications (6% vs. 10%, CIOR 0.1 to 3.5), revision surgeries (4% vs. 4%, CIOR 0.1 to 7.5), or symptomatic fibula implant removals (13% vs. 21%, CIOR 0.1 to 1.6) between pilon and ankle fracture groups, respectively. There was one (2%) fibular nonunion and one wound complication (2%) in each of the fracture groups. Conclusion: Fibula IMN fixation of pilon versus ankle fractures resulted in a similar number of complications. Comparative studies of fibula IMN and plate fixation are necessary to determine if the benefits of fibula IMN in ankle fractures extends to pilon fractures. Level of evidence: Level III, retrospective cohort.

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