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Duchenne muscular dystrophy (DMD) is a devastating neuromuscular disease in which weakness, increased susceptibility to muscle injury, and inadequate repair underlie the pathology. While most attention has focused within the muscle fiber, we recently demonstrated significant alterations in the neuromuscular junction (NMJ) morphology and resulting neuromuscular transmission failure (NTF) 24 h after injury in mdx mice (murine model for DMD). Here we determine the contribution of NMJ morphology and NTF to the recovery of muscle contractile function post-injury. NMJ morphology and NTF rates were assessed day 0 (immediately after injury) and days 1, 7, 14 and 21 after quadriceps injury. Eccentric injury of the quadriceps resulted in a significant loss of maximal torque in both WT (39 ± 6 %) and mdx (76 ± 8 %) with a full recovery in WT by day 7 and in mdx by day 21. Post-injury alterations in NMJ morphology and NTF were found only in mdx, were limited to days 0 and 1, and were independent of changes in MuSK or AChR expression. Such early changes at the NMJ after injury are consistent with mechanical disruption rather than newly forming NMJs. Furthermore, we show that the dense microtubule network that underlies the NMJ is significantly reduced and disorganized in mdx compared to WT. These structural changes at the NMJ may play a role in the increased NMJ disruption and the exaggerated loss of nerve-evoked muscle force seen after injury to dystrophic muscles.

Duchenne muscular dystrophy (DMD) is a devastating neuromuscular disease in which weakness, increased susceptibility to muscle injury, and inadequate repair appear to underlie the pathology. While most attention has focused within the muscle fiber, we recently demonstrated in mdx mice (murine model for DMD) significant morphologic alterations at the motor endplate of the neuromuscular junction (NMJ) and corresponding NMJ transmission failure after injury. Here we extend these initial observations at the motor endplate to gain insight into the pre- vs. postsynaptic morphology, as well as the subsynaptic nuclei in healthy (WT) vs. mdx mice. We quantified the discontinuity and branching of the terminal nerve in adult mice. We report mdx- and age-dependent changes for discontinuity and an increase in branching when compared to WT. To examine mdx- and age-dependent changes in the relative localization of pre- and postsynaptic structures, we calculated NMJ occupancy, defined as the ratio of the footprint occupied by presynaptic vesicles vs. that of the underlying motor endplate. The normally congruent coupling between presynaptic and postsynaptic morphology was altered in mdx mice, independent of age. Finally we found an almost two-fold increase in the number of nuclei and an increase in density (nuclei/area) underlying the NMJ. These outcomes suggest substantial remodeling of the NMJ during dystrophic progression. This remodeling reflects plasticity in both pre- and postsynaptic contributors to NMJ structure, and thus perhaps also NM transmission and muscle function.

Duchenne muscular dystrophy (DMD) is characterized by progressive muscle wasting secondary to repeated muscle damage and inadequate repair. Elevations in intracellular free Ca2+ have been implicated in disease progression, and sarcoplasmic/endoplasmic reticulum Ca2+-ATPase 1 (SERCA1) overexpression has been shown to ameliorate the dystrophic phenotype in mdx mice. The purpose of this study was to assess the effects of SERCA1 overexpression in the more severe mdx/Utr-/- mouse model of DMD. Mice overexpressing SERCA1 were crossed with mdx/Utr+/- mice to generate mdx/Utr-/-/+SERCA1 mice and compared with wild-type (WT), WT/+SERCA1, mdx/+SERCA1, and genotype controls. Mice were assessed at ~12 wk of age for changes in Ca2+ handling, muscle mass, quadriceps torque, markers of muscle damage, and response to repeated eccentric contractions. SERCA1- overexpressing mice had a two- to threefold increase in maximal sarcoplasmic reticulum Ca2+-ATPase activity compared with WT which was associated with normalization in body mass for both mdx/+SERCA1 and mdx/Utr-/-/+SERCA1. Torque deficit in the quadriceps after eccentric injury was 2.7-fold greater in mdx/Utr-/- vs. WT mice, but only 1.5-fold greater in mdx/Utr-/-/+SERCA1 vs. WT mice, an attenuation of 44%. Markers of muscle damage (% centrally nucleated fibers, necrotic area, and serum creatine kinase levels) were higher in both mdx and mdx/Utr-/- vs. WT, and all were attenuated by overexpression of SERCA1. These data indicate that SERCA1 overexpression ameliorates functional impairments and cellular markers of damage in a more severe mouse model of DMD. These findings support targeting intracellular Ca2+ control as a therapeutic approach for DMD. [ABSTRACT FROM AUTHOR]

Duchenne muscular dystrophy ( DMD), the most common and severe muscular dystrophy, is caused by the absence of dystrophin. Muscle weakness and fragility (i.e., increased susceptibility to damage) are presumably due to structural instability of the myofiber cytoskeleton, but recent studies suggest that the increased presence of malformed/branched myofibers in dystrophic muscle may also play a role. We have previously studied myofiber morphology in healthy wild-type ( WT) and dystrophic ( MDX) skeletal muscle. Here, we examined myofiber excitability using high-speed confocal microscopy and the voltage-sensitive indicator di-8-butyl-amino-naphthyl-ethylene-pyridinium-propyl-sulfonate (di-8- ANEPPS) to assess the action potential ( AP) properties. We also examined AP-induced Ca2+ transients using high-speed confocal microscopy with rhod-2, and assessed sarcolemma fragility using elastimetry. AP recordings showed an increased width and time to peak in malformed MDX myofibers compared to normal myofibers from both WT and MDX, but no significant change in AP amplitude. Malformed MDX myofibers also exhibited reduced AP-induced Ca2+ transients, with a further Ca2+ transient reduction in the branches of malformed MDX myofibers. Mechanical studies indicated an increased sarcolemma deformability and instability in malformed MDX myofibers. The data suggest that malformed myofibers are functionally different from myofibers with normal morphology. The differences seen in AP properties and Ca2+ signals suggest changes in excitability and remodeling of the global Ca2+ signal, both of which could underlie reported weakness in dystrophic muscle. The biomechanical changes in the sarcolemma support the notion that malformed myofibers are more susceptible to damage. The high prevalence of malformed myofibers in dystrophic muscle may contribute to the progressive strength loss and fragility seen in dystrophic muscles. [ABSTRACT FROM AUTHOR]

Background: Duchenne muscular dystrophy (DMD) is characterized clinically by severe, progressive loss of skeletal muscle. The phenotype is much less severe in the mdx mouse model of DMD than that seen in patients with DMD. However, a "critical period" has been described for the mdx mouse, during which there is a peak in muscle weakness and degeneration/regeneration between the 2nd and 5th weeks of life. A number of studies have employed small animal magnetic resonance imaging (MRI) to examine skeletal muscle in various dystrophic models, but such studies represent a snapshot in time rather than a longitudinal view. Results: The in vivo cross-sectional T2-weighted image of the healthy (wild type, WT) muscles is homogeneously dark and this homogeneity does not change with time, as there is no disease. We, and others, have shown marked changes in MRI in dystrophic muscle, with multiple, unevenly distributed focal hyperintensities throughout the bulk of the muscles. Here we monitored an mdx mouse using MRI from 5 to 80 weeks of age. Temporal MRI scans show an increase in heterogeneity shortly after the critical period, at 9 and 13 weeks of age, with a decrease in heterogeneity thereafter. The 4.3-fold increase in percent heterogeneity at week 9 and 13 is consistent with the notion of an early critical period described for mdx mice. Conclusions: Age is a significant variable in quantitative MR studies of the mdx mouse. The mdx mouse is typically studied during the critical period, at a time that most closely mimics the DMD pathology, but the preliminary findings here, albeit based on imaging only one mdx mouse over time, suggest that the changes in MRI can occur shortly after this period, when the muscles are still recovering. [ABSTRACT FROM AUTHOR]

• Duchenne muscular dystrophy (DMD), the most common form of muscular dystrophy, is caused by the lack of dystrophin, a protein encoded by the DMD gene. • The neuromuscular junction (NMJ) is not a fixed, permanent structure, but instead shows plasticity in response to injury, exercise, and aging. • NMJs in the mdx mouse model of DMD show aberrant changes in both pre- and post-synaptic NMJ structure, which could influence neuromuscular transmission. • The hypothesis that the NMJ contributes to functional deficits in DMD represents a paradigm shift from more prevalent myo-centric perspectives. The focus of this review is on Duchenne muscular dystrophy (DMD), which is caused by the absence of the protein dystrophin and is characterized as a neuromuscular disease in which muscle weakness, increased susceptibility to muscle injury, and inadequate repair appear to underlie the pathology. Considerable attention has been dedicated to studying muscle fiber damage, but data show that both human patients and animal models for DMD present with fragmented neuromuscular junction (NMJ) morphology. In addition to pre- and post-synaptic abnormalities, studies indicate increased susceptibility of the NMJ to contraction-induced injury, with corresponding functional changes in neuromuscular transmission and nerve-evoked electromyographic activity. Such findings suggest that alterations in the NMJ of dystrophic muscle may play a role in muscle weakness via impairment of neuromuscular transmission. Further work is needed to fully understand the role of the NMJ in the weakness, susceptibility to injury, and progressive wasting associated with DMD. [ABSTRACT FROM AUTHOR]

Introduction: The ability to view individual myofibers is possible with many histological techniques, but not yet with standard in vivo imaging. Optical coherence tomography (OCT) is an emerging technology that can generate high resolution 1-10 μm cross-sectional imaging of tissue in vivo and in real time. Methods: We used OCT to determine architectural differences of tibialis anterior muscles in situ from healthy mice (wild-type [WT], n = 4) and dystrophic mice ( mdx, n = 4). After diffusion tensor imaging (DTI) and OCT, muscles were harvested, snap-frozen, and sectioned for staining with wheat germ agglutinin. Results: DTI suggested differences in pennation and OCT was used to confirm this supposition. OCT indicated a shorter intramuscular tendon (WT/ mdx ratio of 1.2) and an 18% higher degree of pennation in mdx. Staining confirmed these architectural changes. Conclusions: Architectural changes in mdx muscles, which could contribute to reduction of force, are detectable with OCT. Muscle Nerve, 2013 [ABSTRACT FROM AUTHOR]

Mitochondrial derangement is an important contributor to the pathophysiology of muscular dystrophies and may be among the earliest cellular deficits. We have previously shown that disruption of Mss51, a mammalian skeletal muscle protein that localizes to the mitochondria, results in enhanced muscle oxygen consumption rate, increased endurance capacity, and improved limb muscle strength in mice with wildtype background. Here, we investigate whether Mss51 deletion in the mdx murine model of Duchenne muscular dystrophy (mdx-Mss51 KO) counteracts the muscle pathology and mitochondrial irregularities observed in mdx mice. We found that mdx-Mss51 KO mice had increased myofiber oxygen consumption rates and an amelioration of muscle histopathology compared to mdx counterparts. This corresponded with greater treadmill endurance and less percent fatigue in muscle physiology, but no improvement in forelimb grip strength or limb muscle force production. These findings suggest that although Mss51 deletion ameliorates the skeletal muscle mitochondrial respiration defects in mdx and improves fatigue resistance in vivo, the lack of improvement in force production suggests that this target alone may be insufficient for a therapeutic effect. [ABSTRACT FROM AUTHOR]