Striking Denervation of Neuromuscular Junctions without Lumbar Motoneuron Loss in Geriatric Mouse Muscle

Reasons for the progressive age-related loss of skeletal muscle mass and function, namely sarcopenia, are complex. Few studies describe sarcopenia in mice, although this species is the mammalian model of choice for genetic intervention and development of pharmaceutical interventions for muscle degeneration. One factor, important to sarcopenia-associated neuromuscular change, is myofibre denervation. Here we describe the morphology of the neuromuscular compartment in young (3 month) compared to geriatric (29 month) old female C57Bl/6J mice. There was no significant difference in the size or number of motoneuron cell bodies at the lumbar level (L1–L5) of the spinal cord at 3 and 29 months. However, in geriatric mice, there was a striking increase (by ∼2.5 fold) in the percentage of fully denervated neuromuscular junctions (NMJs) and associated deterioration of Schwann cells in fast extensor digitorum longus (EDL), but not in slow soleus muscles. There were also distinct changes in myofibre composition of lower limb muscles (tibialis anterior (TA) and soleus) with a shift at 29 months to a faster phenotype in fast TA muscle and to a slower phenotype in slow soleus muscle. Overall, we demonstrate complex changes at the NMJ and muscle levels in geriatric mice that occur despite the maintenance of motoneuron cell bodies in the spinal cord. The challenge is to identify which components of the neuromuscular system are primarily responsible for the marked changes within the NMJ and muscle, in order to selectively target future interventions to reduce sarcopenia

Dlk1 Is Necessary for Proper Skeletal Muscle Development and Regeneration

Delta-like 1homolog (Dlk1) is an imprinted gene encoding a transmembrane protein whose increased expression has been associated with muscle hypertrophy in animal models. However, the mechanisms by which Dlk1 regulates skeletal muscle plasticity remain unknown. Here we combine conditional gene knockout and over-expression analyses to investigate the role of Dlk1 in mouse muscle development, regeneration and myogenic stem cells (satellite cells). Genetic ablation of Dlk1 in the myogenic lineage resulted in reduced body weight and skeletal muscle mass due to reductions in myofiber numbers and myosin heavy chain IIB gene expression. In addition, muscle-specific Dlk1 ablation led to postnatal growth retardation and impaired muscle regeneration, associated with augmented myogenic inhibitory signaling mediated by NF-κB and inflammatory cytokines. To examine the role of Dlk1 in satellite cells, we analyzed the proliferation, self-renewal and differentiation of satellite cells cultured on their native host myofibers. We showed that ablation of Dlk1 inhibits the expression of the myogenic regulatory transcription factor MyoD, and facilitated the self-renewal of activated satellite cells. Conversely, Dlk1 over-expression inhibited the proliferation and enhanced differentiation of cultured myoblasts. As Dlk1 is expressed at low levels in satellite cells but its expression rapidly increases upon myogenic differentiation in vitro and in regenerating muscles in vivo, our results suggest a model in which Dlk1 expressed by nascent or regenerating myofibers non-cell autonomously promotes the differentiation of their neighbor satellite cells and therefore leads to muscle hypertrophy

Experimental repair of the oculomotor nerve: the anatomical paradigms of functional regeneration

In adult guinea pigs, the oculomotor nerve was sectioned proximally (at the tentorial edge) or more distally (at the orbital fissure) and immediately repaired by reapproximation. During a 24-week postoperative period, extrinsic eye motility was assessed by analyzing the vestibulo-ocular reflexes. The regenerated oculomotor nerve was studied morphometrically on semi-thin histological sections at 16 and 24 weeks postinjury. The selectivity of muscle reinnervation was investigated by injection of both single (horseradish peroxidase) and double (fluorescent dyes) retrograde axonal tracers into the eye muscles. Following proximal repair of the oculomotor nerve, the degree of recovery of extraocular motility varied among different animals and remained stable over long-term observations. In animals with poor recovery, aberrant eye movements were always found, and the somatotopic map of the reinnervated eye muscles was greatly altered. Distortions of the central representation were also seen in those animals in which a good level of functional recovery was seen. However, in animals with good recovery, a topographic bias was re-established by about 65% of the original neuronal population, as opposed to 26% in the animals with poor recovery. Neurons located contralateral to the axotomized nucleus sprouted intra-axially and projected their axons to denervated eye muscles. The number and diameter of the regenerated axons, the number and soma diameter of the axotomized neurons, and the ratio of distal axonal branches to proximal supporting neurons were all related to the degree of functional recovery. Following repair of the oculomotor nerve at the orbital fissure, extraocular motility had recovered in all of the animals at 16 weeks without aberrant phenomena. Functional regeneration of the distally transected oculomotor nerve is thought to be the result of selective muscle reinnervation

Tissue specificity of mitochondrial adaptations in rats after 4 weeks of normobaric hypoxia

Purpose Exposure to hypoxia has been suggested to activate multiple adaptive pathways so that muscles are better able to maintain cellular energy homeostasis. However, there is limited research regarding the tissue specificity of this response. The aim of this study was to investigate the influence of tissue specificity on mitochondrial adaptations of rat skeletal and heart muscles after 4 weeks of normobaric hypoxia (FiO2: 0.10). Methods Twenty male Wistar rats were randomly assigned to either normobaric hypoxia or normoxia. Mitochondrial respiration was determined in permeabilised muscle fibres from left and right ventricles, soleus and extensorum digitorum longus (EDL). Citrate synthase activity and the relative abundance of proteins associated with mitochondrial biogenesis were also analysed. Results After hypoxia exposure, only the soleus and left ventricle (both predominantly oxidative) presented a greater maximal mass-specific respiration (+48 and +25%, p \u3c 0.05) and mitochondrial-specific respiration (+75 and +28%, p \u3c 0.05). Citrate synthase activity was higher in the EDL (0.63 ± 0.08 vs 0.41 ± 0.10 μmol min− 1 μg− 1) and lower in the soleus (0.65 ± 0.17 vs 0.87 ± 0.20 μmol min− 1 μg− 1) in hypoxia with respect to normoxia. There was a lower relative protein abundance of PGC-1α (−25%, p \u3c 0.05) in the right ventricle and a higher relative protein abundance of PGC-1β (+43%, p \u3c 0.05) in the left ventricle of rats exposed to hypoxia, with few differences for protein abundance in the other muscles. Conclusion Our results show a muscle-specific response to 4 weeks of normobaric hypoxia. Depending on fibre type, and the presence of ventricular hypertrophy, muscles respond differently to the same degree of environmental hypoxia