Comparison of gene expression of 2-mo denervated, 2-mo stimulated-denervated, and control rat skeletal muscles

Persistence of regenerative myogenesis in spite of down-regulation of activity-dependent genes in long-term denervated rat muscle

Contrary to general expectation, in humans, we have recently shown that after complete conus cauda lesion, the lower motoneuron denervated myofibers may survive several years. In adult rats, the sciatectomized muscle progresses in 4-6 months from severe atrophy to a dystrophic stage and undergoes a dramatic weight loss; during this process, myofiber death/regeneration processes maintain a decreasing population of very small, but vital myofibers. At the same time, in vitro electrophysiologic recordings show that denervated fibers can maintain membrane excitability longer than they can retain contractile properties. A certain level of myofiber regeneration seems to have a role in the process, with the early re-expression of embryonic subunits of integrins and acetylcholine receptor subunits. In the present work, using the reliable real-time quantitative PCR, we confirm the long-lasting occurrence of myoblast proliferation-dependent events and their focal nature. In fact, we show here that in sciatectomized muscle, the expression of 12 selected genes was differentially regulated after 3 and 9 month denervation. At both time points, indexes of muscle activity/inactivity and tissue remodeling (proteolysis, energy usage and angiogenic factors) were down-regulated, while indexes of regenerative myogenesis (Myogenin, MyoD, MRF4 and MHCemb) were up-regulated. Immunohistochemistry with anti-MHCemb and anti-NCAM monoclonal antibodies show that such regeneration events were focally distributed. We conclude that myofiber regeneration is a non-compensatory mechanism, which prolongs the chance of reinnervation during long-lasting denervation. It may also contribute to muscle recovery in paraplegic patients, even when rehabilitation strategies based on functional electric stimulation start late after spinal cord injury (SCI).

Gene and protein expression associated with protein synthesis and breakdown in paraplegic skeletal muscle

Spinal cord injury reduces the rate of skeletal muscle protein synthesis and increases protein breakdown, resulting in rapid muscle loss. The purpose of this study was to determine whether long-term paraplegia would eventually result in a downregulation of muscle mRNA and protein expression associated with both protein synthesis and breakdown. After 10 weeks of spinal cord transection, soleus muscle from 12 rats (6 sham-control, 6 paraplegic) was studied for mRNAs and proteins associated with protein synthesis and breakdown using real-time polymerase chain reaction and immunoblotting techniques. Protein kinase B (PKB/Akt), ribosomal S6 kinase 1 (S6K1), and myogenin mRNA were downregulated, whereas muscle ring finger 1 (MuRF1) and phospho-forkhead transcription factor 4 (FoxO4) protein were increased in paraplegic rats. We conclude that gene and protein expression of pathways associated with protein synthesis are reduced, whereas some markers of protein breakdown remain elevated following chronic paraplegia. Clinical interventions designed to increase muscle protein synthesis may be helpful in preventing excessive muscle loss during long-term paraplegia.

Myogenesis in human denervated muscle biopsies

Electron microscopic studies of long-term denervated rat muscles have identified very small, immature myofibers that are believed to arise from detached satellite cells that have fused to form new fibers within the interstitial space. At present, it is unknown whether and to what extent equivalent fibers exist in denervated human muscle. Serial sections of muscle biopsies from 66 patients diagnosed with polyneuropathy or amyotrophic lateral sclerosis were immunolabeled with anti-NCAM and anti-neonatal myosin heavy chain monoclonal antibodies that are both neurally and developmentally regulated. We evaluated 200 myofibers in each section. Of the biopsy specimens, 75% contained small myofibers that showed a thin perinuclear cytoplasmic rim. Small fibers expressing neonatal myosin heavy chain (MHCn+) were found in all of these biopsies (100%) and NCAM+ fibers in 98%. The percentage of MHCn+ small fibers averaged 82% and NCAM+ small myofibers averaged 40%. The percentage of NCAM+ small fibers was significantly lower than that of MHCn+ fibers. In contrast, the percentage of MHCn+ vs. NCAM+ angular atrophic fibers did not show a significant difference. A substantial subset of neurogenic biopsies showed small fibers that differ from angular atrophic fibers both in size and expression pattern of MHCn and NCAM. Myogenesis appears to be a frequent finding in neurogenic atrophy.

Effects of chronic electrical stimulation on long-term denervated muscles of the rabbit hind limb

We investigated the extent to which activity induced by chronic electrical stimulation could restore the mass and contractile function of rabbit tibialis anterior (TA) muscles that had undergone atrophy as a result of prolonged denervation. Denervation was carried out by selectively interrupting the motor nerve branches to the ankle dorsiflexors in one hind limb. Stimulators were implanted, with electrodes on the superficial and deep surfaces of the denervated TA muscle. Ten weeks later, the mass and mid-belly cross-sectional area (CSA) of TA muscles subjected to denervation alone had fallen to approximately 40% of normal. At this stage, stimulators in the other rabbits were activated for 1 h/day to deliver 20-ms rectangular bipolar constant-current pulses of 4 mA amplitude at 20 Hz with a duty cycle of 1s ON/2s OFF, a total of 24,000 impulses/day. The animals were examined after a further 2, 6 or 10 weeks. Stimulation restored the wet weight of the denervated muscles to values not significantly different to those of normal, innervated controls. It increased CSA from 39% to 66% of normal, and there was a commensurate increase in maximum isometric tetanic force from 27% to 50% of normal. Light and electron microscopic examination revealed a marked improvement in the size, packing, and internal organization of the stimulated-denervated muscle fibres, suggestive of an ongoing process of restoration. Excitability, contractile speed, power, and fatigue resistance had not, however, been restored to normal levels after 10 weeks of stimulation. Similar results were found for muscles that had been denervated for 39 weeks and then stimulated for 12 weeks. The study demonstrates worthwhile benefits of long-term electrical stimulation in the treatment of established denervation atrophy.

Denervation-induced skeletal muscle atrophy is associated with increased mitochondrial ROS production

Reactive oxygen species (ROS), especially mitochondrial ROS, are postulated to play a significant role in muscle atrophy. We report a dramatic increase in mitochondrial ROS generation in three conditions associated with muscle atrophy: in aging, in mice lacking CuZn-SOD ( Sod1−/−), and in the neurodegenerative disease, amyotrophic lateral sclerosis (ALS). ROS generation in muscle mitochondria is nearly threefold higher in 28- to 32-mo-old than in 10-mo-old mice and is associated with a 30% loss in gastrocnemius mass. In Sod1−/− mice, muscle mitochondrial ROS production is increased >100% in 20-mo compared with 5-mo-old mice along with a >50% loss in muscle mass. ALS G93A mutant mice show a 75% loss of muscle mass during disease progression and up to 12-fold higher muscle mitochondrial ROS generation. In a second ALS mutant model, H46RH48Q mice, ROS production is approximately fourfold higher than in control mice and is associated with a less dramatic loss (30%) in muscle mass. Thus ROS production is strongly correlated with the extent of muscle atrophy in these models. Because each of the models of muscle atrophy studied are associated to some degree with a loss of innervation, we were interested in determining whether denervation plays a role in ROS generation in muscle mitochondria isolated from hindlimb muscle following surgical sciatic nerve transection. Seven days postdenervation, muscle mitochondrial ROS production increased nearly 30-fold. We conclude that enhanced generation of mitochondrial ROS may be a common factor in the mechanism underlying denervation-induced atrophy.

Androgen-dependent pathology demonstrates myopathic contribution to the Kennedy disease phenotype in a mouse knock-in model

Kennedy disease, a degenerative disorder characterized by androgen-dependent neuromuscular weakness, is caused by a CAG/glutamine tract expansion in the androgen receptor (Ar) gene. We developed a mouse model of Kennedy disease, using gene targeting to convert mouse androgen receptor (AR) to human sequence while introducing 113 glutamines. AR113Q mice developed hormone and glutamine length-dependent neuromuscular weakness characterized by the early occurrence of myopathic and neurogenic skeletal muscle pathology and by the late development of neuronal intranuclear inclusions in spinal neurons. AR113Q males unexpectedly died at 2-4 months. We show that this androgen-dependent death reflects decreased expression of skeletal muscle chloride channel 1 (CLCN1) and the skeletal muscle sodium channel alpha-subunit, resulting in myotonic discharges in skeletal muscle of the lower urinary tract. AR113Q limb muscles show similar myopathic features and express decreased levels of mRNAs encoding neurotrophin-4 and glial cell line-derived neurotrophic factor. These data define an important myopathic contribution to the Kennedy disease phenotype and suggest a role for muscle in non-cell autonomous toxicity of lower motor neurons.

Electrical stimulation based on chronaxie reduces atrogin-1 and myoD gene expressions in denervated rat muscle

Denervation induces muscle fiber atrophy and changes in the gene expression rates of skeletal muscle. Electrical stimulation (ES) is a procedure generally used to treat denervated muscles in humans. This study evaluated the effect of ES based on chronaxie and rheobase on the expression of the myoD and atrogin-1 genes in denervated tibialis anterior (TA) muscle of Wistar rats. Five groups were examined: (1) denervated (D); (2) D+ES; (3) sham denervation; (4) normal (N); and (5) N+ES. Twenty muscle contractions were stimulated every 48 h using surface electrodes. After 28 days, ES significantly decreased the expression of myoD and atrogin-1 in D+ES compared to the D group. However, ES did not prevent muscle-fiber atrophy after denervation. Thus, ES based on chronaxie values and applied to denervated muscles using surface electrodes, as normally used in human rehabilitation, was able to reduce the myoD and atrogin-1 gene expressions, which are related to muscular growth and atrophy, respectively. The results of this study provide new information for the treatment of denervated skeletal muscle using surface ES.