Degeneration of neuromuscular junction in age and dystrophy

Abstract

Functional denervation is a hallmark of aging sarcopenia as well as of muscular dystrophy. It is thought to be a major factor reducing skeletal muscle mass, particularly in the case of sarcopenia. Neuromuscular junctions (NMJs) serve as the interface between the nervous and skeletal muscular systems, and thus they may receive pathophysiological input of both pre- and post-synaptic origin. Consequently, NMJs are good indicators of motor health on a systemic level. Indeed, upon sarcopenia and dystrophy, NMJs morphologically deteriorate and exhibit altered characteristics of primary signaling molecules, such as nicotinic acetylcholine receptor and agrin. Since a remarkable reversibility of these changes can be observed by exercise, there is significant interest in understanding the molecular mechanisms underlying synaptic deterioration upon aging and dystrophy and how synapses are reset by the aforementioned treatments. Here, we review the literature that describes the phenomena observed at the NMJ in sarcopenic and dystrophic muscle as well as to how these alterations can be reversed and to what extent. In a second part, the current information about molecular machineries underlying these processes is reported.

Keywords: aging; dystrophy; exercise therapy; neuromuscular junction; sarcopenia.

Figure 1

Pre- and post-synapse exhibit perfect complementarity in young adult NMJs. (A,B) Pre-synaptic terminal as represented by motoneuronal markers for synaptic vesicles [synaptophysin (A)] and neuronal cytoskeleton [neurofilament (B)]. (C) Post-synaptic apparatus contains large amounts of AChRs directly juxtaposed to pre-synaptic ACh filled synaptic vesicles. (D) Overlay of synaptophysin (red), neurofilament (green), and AChR stainings (blue) nicely shows the perfect complementarity of pre- and post-synaptic portions of NMJ.

Figure 2

The phenomenon of NMJ fragmentation during aging and dystrophy. Tibialis anterior muscles of wildtype (wt) or dystrophic mdx mice were injected with fluorescently labeled α-bungarotoxin to mark AChRs at the indicated ages and then imaged using in vivo confocal microscopy. Images show maximum intensity projections of several NMJs (left) and binarized pictures (right) to highlight the observation that in young wt AChRs form contiguous, “pretzel-like,” bands while they align in fragmented clusters in aged wt and middle aged dystrophic muscles.

Figure 3

Determination of post-synaptic ACh receptor cluster dispersion. Representative image of tracings made to determine total endplate area (yellow) and stained receptors clusters (blue) within that total area. To determine dispersion of AChRs within total endplate the stained area is divided by the total area and multiplied by 100.

Figure 4

Schematic representation of exercise-induced changes at post-synaptic membrane of NMJ of young adult and aged rodents. Both, exercise as well as the aging process come with augmented size of individual NMJs and increased branching complexity of AChR clusters within the NMJ organization. Aged NMJs are also often characterized by enhanced discontinuity of AChR cluster branches. Of note, exercise of aged animals partially reduces branching complexity, NMJ size, and fragmentation and, thus, leads to a morphology of synapses that resembles young adult sedentary NMJs. Relative density of AChR, indicated by gray scales, is always highest at the branch borders and does not significantly change under all conditions. Pre-synaptic branching and synaptic vesicle numbers (not shown) are largely modified in congruency with post-synaptic changes. However, upon aging partial loss of pre-to-post matching does frequently occur.

Figure 5

Scheme depicting the putative life cycle of AChR at the vertebrate NMJ. Upon assembly in the ER and glycosylation in the Golgi apparatus, AChR is delivered by exocytosis to the post-synaptic membrane where it is clustered by means of agrin/MuSK/Lrp4/rapsyn complex, to fulfill its major function, i.e., mediating neuromuscular transmission. Subsequently, AChR is endocytosed and can then be recycled using a cooperative function of myosin Va, PKA type I, and rapsyn, or degraded, presumably via autophagic decay in a MuRF1-dependent manner. Decision-making between recycling or degradation appears to be subject to manifold signals including CaMKII and PKC.