Low dystrophin levels are insufficient to normalize the neuromuscular synaptic abnormalities of mdx mice

Abstract

Dystrophin is a sub-sarcolemmal component of skeletal muscle fibres and is enriched at the postsynaptic membrane of the neuromuscular junction (NMJ). In the mdx mouse, dystrophin absence not only causes muscle damage but also mild synaptic dysfunctions and clear morphological aberrations at NMJs. In particular, reduction of postsynaptic sensitivity for the neurotransmitter acetylcholine and extra exhaustion of presynaptic acetylcholine release during intense synaptic activity exists. Current experimental therapeutic approaches in Duchenne muscular dystrophy aim to restore dystrophin expression. An important question is what dystrophin levels are needed to improve muscle function. Recent experimental and clinical studies suggested that levels as low as a few percent of normal can be beneficial. Similarly, it is of interest to know how dystrophin levels relate to NMJ function and morphology. We investigated NMJs of a series of mdx-Xist mice, which expressed dystrophin between ~2% and 19% of normal. Most functional and morphological NMJ parameters of these mice remained comparable to mdx. On the other hand, mdx mice (expressing ~50% dystrophin) showed normal NMJ features. Thus, the minimal dystrophin level required for normal NMJ function and morphology lies between 19% and 50% of normal when expression of dystrophin is not uniform.

Natural disease history of the D2‐mdx mouse model for Duchenne muscular dystrophy

“…Importantly, however, these observations highlight the need of a single gender (preferably males) in preclinical studies using D2- mdx mice. In line with previous observations in BL10- mdx mice (29, 38), respiratory function at rest was not severely impaired in D2- mdx mice, a changed respiratory capacity as compared with WT was only statistically significant when data of both genders were combined. Although we hypothesized that the more severely diaphragm of D2- mdx mice would more significantly affect respiratory function, like in mdx-utrn −/− mice (29), this was not observed.…”

Recent insights into neuromuscular junction biology in Duchenne muscular dystrophy: Impacts, challenges, and opportunities

“…These alterations, which are also observed in ageing NMJs, may not necessarily translate into functional impairments in neuromuscular transmission [60], whereas a simplified endplate morphology can contribute to the loss of crucial postsynaptic membrane proteins [61]. These initial observations in postsynaptic alterations have since been reported numerous times in mdx mice [9À11,62À64], golden retriever muscular dystrophy dogs [65], and other animal models of DMD [66,67]. Whether these changes occur independent from dystrophy-induced muscle degeneration/regeneration is debated [68].…”

“…Perhaps the ideal resolution for the neuromuscular defects in DMD, including at the NMJ, is to restore dystrophin. Indeed, in mdx mice only 5À15% of normal dystrophin levels is sufficient to augment muscle function and prolong survival [101], whereas a higher requirement, estimated to be 20À50%, is needed to improve synaptic morphology and function [67]. However, translating this solution to the clinic is challenging.…”

The potential of utrophin and dystrophin combination therapies for Duchenne muscular dystrophy

“…Thus, the potential benefits from utrophin can only come from a specific area where the HSA promoter drives the utrophin transgene expression (47). Whereas utrophin may act as a component of the postsynaptic cytoskeleton, contributing to the development or maintenance of the postsynaptic folds (48), it was recently shown that low dystrophin levels are insufficient to normalize the neuromuscular synaptic abnormalities of mdx mice (49). As utrophin and dystrophin show different spatial expression at the NMJ (50), an essential component for force production and transmission, one hypothesis is that utrophin could perform different and complementary roles to dystrophin at the synaptic basal lamina of the NMJ.…”