Contraction-Induced Loss of Plasmalemmal Electrophysiological Function Is Dependent on the Dystrophin Glycoprotein Complex.

Cory W Baumann,Dawn A Lowe,Gordon L Warren,Sylvia R Sidky,Angus Lindsay,James M Ervasti

doi:10.3389/fphys.2021.757121

Abstract

Weakness and atrophy are key features of Duchenne muscular dystrophy (DMD). Dystrophin is one of the many proteins within the dystrophin glycoprotein complex (DGC) that maintains plasmalemmal integrity and cellular homeostasis. The dystrophin-deficient mdx mouse is also predisposed to weakness, particularly when subjected to eccentric (ECC) contractions due to electrophysiological dysfunction of the plasmalemma. Here, we determined if maintenance of plasmalemmal excitability during and after a bout of ECC contractions is dependent on intact and functional DGCs rather than, solely, dystrophin expression. Wild-type (WT) and dystrophic mice (mdx, mL172H and Sgcb−/− mimicking Duchenne, Becker and Limb-girdle Type 2E muscular dystrophies, respectively) with varying levels of dystrophin and DGC functionality performed 50 maximal ECC contractions with simultaneous torque and electromyographic measurements (M-wave root-mean-square, M-wave RMS). ECC contractions caused all mouse lines to lose torque (p<0.001); however, deficits were greater in dystrophic mouse lines compared to WT mice (p<0.001). Loss of ECC torque did not correspond to a reduction in M-wave RMS in WT mice (p=0.080), while deficits in M-wave RMS exceeded 50% in all dystrophic mouse lines (p≤0.007). Moreover, reductions in ECC torque and M-wave RMS were greater in mdx mice compared to mL172H mice (p≤0.042). No differences were observed between mdx and Sgcb−/− mice (p≥0.337). Regression analysis revealed ≥98% of the variance in ECC torque loss could be explained by the variance in M-wave RMS in dystrophic mouse lines (p<0.001) but not within WT mice (R2=0.211; p=0.155). By comparing mouse lines that had varying amounts and functionality of dystrophin and other DGC proteins, we observed that (1) when all DGCs are intact, plasmalemmal action potential generation and conduction is maintained, (2) deficiency of the DGC protein β-sarcoglycan is as disruptive to plasmalemmal excitability as is dystrophin deficiency and, (3) some functionally intact DGCs are better than none. Our results highlight the significant role of the DGC plays in maintaining plasmalemmal excitability and that a collective synergism (via each DGC protein) is required for this complex to function properly during ECC contractions.

Highlights

Duchenne muscular dystrophy (DMD), one of the most serious genetic childhood diseases, is characterized by progressive weakness and atrophy after repetitive cycles of degeneration and regeneration due to the lack of dystrophin (Bell and Conen, 1968; Kinali et al, 2011; Vohra et al, 2015)
To determine whether plasmalemmal electrophysiological function was impaired by the ECC contractions, similar to what we have previously shown in mdx muscle (Call et al, 2013; Baumann et al, 2020), mice with nerve cuffs were implanted with EMG electrodes around left TA muscle
WT between sets. (B) Dystrophin content normalized to GAPDH. (C) β-sarcoglycan content normalized to GAPDH

Summary

Introduction

Duchenne muscular dystrophy (DMD), one of the most serious genetic childhood diseases, is characterized by progressive weakness and atrophy after repetitive cycles of degeneration and regeneration due to the lack of dystrophin (Bell and Conen, 1968; Kinali et al, 2011; Vohra et al, 2015). A key feature of mdx mice is that muscles composed of predominantly fast-twitch fibers are hypersensitive to losing strength in response to eccentric (ECC) contractions (Head et al, 1992; Moens et al, 1993; Lindsay et al, 2019; Kiriaev et al, 2021). Numerous laboratories have used ex vivo, in situ, or in vivo muscle preparations to demonstrate that loss of strength during and immediately following ECC contractions is 20–60% greater in mdx compared with wild-type (WT) mice (Moens et al, 1993; Petrof et al, 1993; Brooks, 1998; Call et al, 2011; Pratt et al, 2013, 2015; Baumann et al, 2020; Lindsay et al, 2020). Despite the widespread use of ECC contraction protocols, the muscular dystrophy field remains uncertain what underlying mechanism initiates the immediate strength loss in mdx muscle. Function of several cellular and subcellular structures in muscle that are necessary for contraction and force generation have been reported to be disrupted following ECC contractions in mdx muscle, including the neuromuscular junction (NMJ) (Pratt et al, 2013, 2015), plasmalemma (Petrof et al, 1993; Call et al, 2013; Roy et al, 2016), ryanodine receptor (Bellinger et al, 2009; Lindsay et al, 2020), sarco/endoplasmic reticulum Ca2+ATPase (Mázala et al, 2015; Lindsay et al, 2020), and myofibrillar proteins (Blaauw et al, 2010)

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