Chronic paraplegia-induced muscle atrophy downregulates the mTOR/S6K1 signaling pathway

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Abstract
Ribosomal S6 kinase 1 (S6K1) is a downstream component of the mammalian target of rapamycin (mTOR) signaling pathway and plays a regulatory role in translation initiation, protein synthesis, and muscle hypertrophy. AMP-activated protein kinase (AMPK) is a cellular energy sensor, a negative regulator of mTOR, and an inhibitor of protein synthesis. The purpose of this study was to determine whether the hypertrophy/cell growth-associated mTOR pathway was downregulated during muscle atrophy associated with chronic paraplegia. Soleus muscle was collected from male Sprague-Dawley rats 10 wk following complete T4–T5 spinal cord transection (paraplegic) and from sham-operated (control) rats. We utilized immunoprecipitation and Western blotting techniques to measure upstream [AMPK, Akt/protein kinase B (PKB)] and downstream components of the mTOR signaling pathway [mTOR, S6K1, SKAR, 4E-binding protein 1 (4E-BP1), and eukaryotic initiation factor (eIF) 4G and 2α]. Paraplegia was associated with significant soleus muscle atrophy (174 ± 8 vs. 240 ± 13 mg; P < 0.05). There was a reduction in phosphorylation of mTOR, S6K1, and eIF4G ( P < 0.05) with no change in Akt/PKB or 4E-BP1 ( P > 0.05). Total protein abundance of mTOR, S6K1, eIF2α, and Akt/PKB was decreased, and increased for SKAR ( P < 0.05), whereas 4E-BP1 and eIF4G did not change ( P > 0.05). S6K1 activity was significantly reduced in the paraplegic group ( P < 0.05); however, AMPKα2 activity was not altered (3.5 ± 0.4 vs. 3.7 ± 0.5 pmol·mg−1·min−1, control vs. paraplegic rats). We conclude that paraplegia-induced muscle atrophy in rats is associated with a general downregulation of the mTOR signaling pathway. Therefore, in addition to upregulation of atrophy signaling during muscle wasting, downregulation of muscle cell growth/hypertrophy-associated signaling appears to be an important component of long-term muscle loss.