Gene and protein expression associated with protein synthesis and breakdown in paraplegic skeletal muscle

Abstract

Spinal cord injury reduces the rate of skeletal muscle protein synthesis and increases protein breakdown, resulting in rapid muscle loss. The purpose of this study was to determine whether long-term paraplegia would eventually result in a downregulation of muscle mRNA and protein expression associated with both protein synthesis and breakdown. After 10 weeks of spinal cord transection, soleus muscle from 12 rats (6 sham-control, 6 paraplegic) was studied for mRNAs and proteins associated with protein synthesis and breakdown using real-time polymerase chain reaction and immunoblotting techniques. Protein kinase B (PKB/Akt), ribosomal S6 kinase 1 (S6K1), and myogenin mRNA were downregulated, whereas muscle ring finger 1 (MuRF1) and phospho-forkhead transcription factor 4 (FoxO4) protein were increased in paraplegic rats. We conclude that gene and protein expression of pathways associated with protein synthesis are reduced, whereas some markers of protein breakdown remain elevated following chronic paraplegia. Clinical interventions designed to increase muscle protein synthesis may be helpful in preventing excessive muscle loss during long-term paraplegia.

FIGURE 1

Soleus muscle. (A) FoxO1 mRNA expression and phospho- (B) FoxO1, (C) FoxO3, and (D) FoxO4 protein expression from six control and six paraplegic rats. FoxO1 mRNA expression is labeled as fold change, FoxO1 and 4 protein are labeled as phospho/total, and FoxO3 protein is labeled in arbitrary units. All data are expressed as mean ± SE. (B-D) Representative blot images for respective phospho-FoxO protein. Protein images are loaded in duplicate for control (C) and paraplegic (P) rats. *Significantly different from controls (P < 0.05).

FIGURE 2

Soleus muscle (A) myostatin and (B) MAFbx mRNA expression, and MuRF1 (C) mRNA and (D) protein expression from six control and six paraplegic rats. Data are labeled as either fold change or arbitrary units and expressed as mean ± SE. All data expressed as mean ± SE. (D) Representative blot image for MuRF1 protein. Protein images are loaded in duplicate for control (C) and paraplegic (P) rats. *Significantly different from controls (P < 0.05).