Endoplasmic Reticulum Oxidative Stress Triggers Tgf-Beta-Dependent Muscle Dysfunction by Accelerating Ascorbic Acid Turnover

Abstract

Endoplasmic reticulum (ER) and oxidative stress are two related phenomena that have important metabolic consequences. As many skeletal muscle diseases are triggered by oxidative stress, we explored the chain of events linking a hyperoxidized ER (which causes ER and oxidative stress) with skeletal muscle dysfunction. An unbiased exon expression array showed that the combined genetic modulation of the two master ER redox proteins, selenoprotein N (SEPN1) and endoplasmic oxidoreductin 1 (ERO1), led to an SEPN1-related myopathic phenotype due to excessive signalling of transforming growth factor (TGF)-beta. The increased TGF-beta activity in the genetic mutants was caused by accelerated turnover of the ER localized (anti-oxidant) ascorbic acid that affected collagen deposition in the extracellular matrix. In a mouse mutant of SEPN1, which is dependent on exogenous ascorbic acid, a limited intake of ascorbic acid revealed a myopathic phenotype as a consequence of an altered TGF-beta signalling. Indeed, systemic antagonism of TGF-beta re-established skeletal muscle function in SEPN1 mutant mice. In conclusion, this study sheds new light on the molecular mechanism of SEPN1-related myopathies and indicates that the TGF-beta/ERO1/ascorbic acid axis offers potential for their treatment.

A maladaptive ER stress response triggers dysfunction in highly active muscles of mice with SELENON loss

“…Although we and others have shown that the leg muscles of SELENON KO mice show no gross alterations in muscle histology, physiology or in the levels of the ER stress response markers [29] , [36] , [27] , [35] , we had detected a significant increase in the time constant of leg muscle relaxation after a series of tetanic stimuli, which represent an exercise mimetic that challenges SERCA activity [42] and requires an active SERCA [43] . Such a prolonged relaxation time was completely rescued in DKO mice, despite no changes in the resistance to fatigue ( Fig.…”

“…Functional interactions between SELENON and ERO1 have been suggested by inducing a myopathic phenotype through the delivery of an ERO1-containing adeno-associated virus to otherwise normal SELENON KO limb muscles. Furthermore, the increased cell survival observed after treating SELENON KO cells with an ERO1 inhibitor is consistent with a role of SELENON in counteracting the effects of ERO1 [27] , [35] .…”

“…The injection of ERO1-AAV into the gastrocnemius of SELENON KO mice, which is spared from any sign of myopathy, elicits a myopathy with a reduction in normalised muscle force. These results suggest a functional link between SELENON and ERO1 by which SELENON counteracts ERO1 activity [27] , [35] .…”

“…5 A). Furthermore, ex-vivo force measurements on isolated strips of twenty-four-week-old old diaphragm muscle showed significant impairment in the normalised force of the SELENON KO diaphragm (not detected in leg muscles [35] ) ( Fig. 5 B), which was accompanied by a trend towards longer relaxation time and without major morphological defects or fiber type switching ( Fig.…”

“…In order to test ER stress responses in SELENON-deficient cells, we used SELENON KD C2C12 cells [27] , [35] .…”