Repeated transient mRNA bursts precede increases in transcriptional and mitochondrial proteins during training in human skeletal muscle

Abstract

Exercise training induces mitochondrial biogenesis, but the time course of molecular sequelae that accompany repetitive training stimuli remains to be determined in human skeletal muscle. Therefore, throughout a seven-session, high-intensity interval training period that increased (12%), we examined the time course of responses of (a) mitochondrial biogenesis and fusion and fission proteins, and (b) selected transcriptional and mitochondrial mRNAs and proteins in human muscle. Muscle biopsies were obtained 4 and 24 h after the 1st, 3rd, 5th and 7th training session. PGC-1α mRNA was increased >10-fold 4 h after the 1st session and returned to control within 24 h. This 'saw-tooth' pattern continued until the 7th bout, with smaller increases after each bout. In contrast, PGC-1α protein was increased 24 h after the 1st bout (23%) and plateaued at +30-40% between the 3rd and 7th bout. Increases in PGC-1β mRNA and protein were more delayed and smaller, and did not persist. Distinct patterns of increases were observed in peroxisome proliferator-activated receptor (PPAR) α and γ protein (1 session), PPAR β/δ mRNA and protein (5 sessions) and nuclear respiratory factor-2 protein (3 sessions) while no changes occurred in mitochondrial transcription factor A protein. Citrate synthase (CS) and β-HAD mRNA were rapidly increased (1 session), followed 2 sessions later (session 3) by increases in CS and β-HAD activities, and mitochondrial DNA. Changes in COX-IV mRNA (session 3) and protein (session 5) were more delayed. Training also increased mitochondrial fission proteins (fission protein-1, >2-fold; dynamin-related protein-1, 47%) and the fusion protein mitofusin-1 (35%) but not mitofusin-2. This study has provided the following novel information: (a) the training-induced increases in transcriptional and mitochondrial proteins appear to result from the cumulative effects of transient bursts in their mRNAs, (b) training-induced mitochondrial biogenesis appears to involve re-modelling in addition to increased mitochondrial content, and (c) the 'transcriptional capacity' of human muscle is extremely sensitive, being activated by one training bout.

Figure 1

Study design for 2 weeks of high intensity interval training (HIIT)

Figure 2. Representative Western blots

Bands (left to right) are Pre, 1st-4 h, 1st-24 h, 3rd-24 h, 5th-24 h and 7th-24 h time points. MFN-1 and DRP-1 do not include 1st-4 h.

Figure 3. Maximal activities and protein content of mitochondrial enzymes in skeletal muscle throughout 2 weeks of high-intensity interval training

A, citrate synthase (CS). B, β-hydroxyacyl CoA dehydrogenase (β-HAD). C, COX-IV. Values are means ±s.e.m. for 9 subjects. *Significantly different from Pre, #significantly different from 1st-4 h, †significantly different from 1st-24 h, ‡significantly different from 3rd-24 h (P < 0.05).

Figure 4. Skeletal muscle PGC-1α and PGC-1β mRNA (A and B) and protein content (C and D) throughout 2 weeks of high-intensity interval training

Values are means ±s.e.m. for 8–9 subjects. *Significantly different from Pre, §significantly different from all 24 h time points, #significantly different from 1st-4 h, ^bsignificantly different from all 4 h time points, †significantly different from 1st-24 h (P < 0.05).

Figure 5. Skeletal muscle PPARα, PPARβ/δ and PPARγ mRNA (A, B and C) and protein content (D, E and F) throughout 2 weeks of high-intensity interval training

Values are means ±s.e.m. for 8–9 subjects. *Significantly different from Pre, §significantly different from all 24 h time points, †significantly different from 1st-24 h, ‡significantly different from 3rd-24 h (P < 0.05).

Figure 6. Protein content of skeletal muscle MFN-1 (A), MFN-2 (B), Fis-1 (C) and DRP-1 (D) throughout 2 weeks of high-intensity interval training

Values are means ±s.e.m. for 6–9 subjects. *Significantly different from Pre, †significantly different from 1st-24 h, ‡significantly different from 3rd-24 h (P < 0.05).

Figure 7

Representative figure demonstrating temporal responses and relationship of PGC-1α mRNA, PGC-1α protein and CS maximal activity throughout 2 weeks of high-intensity interval training