Pre- and postsynaptic changes in the neuromuscular junction in dystrophic mice.

Stephen J P Pratt,Sameer B Shah,Gloribel K Le,Ana P Valencia,Richard M Lovering

doi:10.3389/fphys.2015.00252

Abstract

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.

Highlights

The most common and severe form of muscular dystrophy is Duchenne muscular dystrophy (DMD), a disorder caused by the absence of dystrophin, a structural protein found on the cytoplasmic surface of the sarcolemma (Watkins et al, 1988)
Dystrophin is not required for neuromuscular junction (NMJ) formation, but is thought to be required for endplate maintenance (Kong and Anderson, 1999), as the motor endplate is significantly altered in mdx muscle compared to the NMJ in wild type (WT) muscle (Figure 2, green)
Bidirectional communication between muscle fibers and motor neurons is extremely important for maintenance of the neuromuscular apparatus (Grinnell, 1995)

Summary

Introduction

The most common and severe form of muscular dystrophy is Duchenne muscular dystrophy (DMD), a disorder caused by the absence of dystrophin, a structural protein found on the cytoplasmic surface of the sarcolemma (Watkins et al, 1988). The DGC accumulates at the postsynaptic membrane (aka motor endplate) of the neuromuscular junction (NMJ), the area of synaptic contact between a motor neuron and its target muscle fiber. The absence of associated proteins can cause changes in structure, and without exception, the motor endplate is noticeably disrupted in patients with DMD and mdx mice (Kong and Anderson, 1999; Adams et al, 2000; Marques et al, 2004; Banks et al, 2009; Chipman et al, 2010; Kulakowski et al, 2011)

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Conclusion