The temporal responses of protein synthesis, gene expression and cell signalling in human quadriceps muscle and patellar tendon to disuse

Abstract

We hypothesized that rates of myofibrillar and patellar tendon collagen synthesis would fall over time during disuse, the changes being accompanied in muscle by decreases in focal adhesion kinase (FAK) phosphorylation and in gene expression for proteolytic enzymes. We studied nine men (22 +/- 4 years, BMI 24 +/- 3 kg m(-2) (means +/- s.d.) who underwent unilateral lower leg suspension for 23 days; five were studied between 0 and 10 days and four between 10 and 21 days. Muscle and tendon biopsies were taken in the postabsorptive state at days 0, 10 and 21 for measurement of protein synthesis, gene expression and protein phosphorylation. Muscle cross-sectional area decreased by 5.2% at 14 days and 10.0% (both P < 0.001), at 23 days, i.e. 0.5% day(-1), whereas tendon dimensions were constant. Rates of myofibrillar protein synthesis fell (P < 0.01) from 0.047% h(-1) at day 0 to 0.022% h(-1) at 10 days without further changes. Tendon collagen synthetic rates also fell (P < 0.01), from 0.052 to 0.023% h(-1) at 10 days and then to 0.010% h(-1) at 21 days. FAK phosphorylation decreased 30% (P < 0.01) at 10 days. No changes occurred in the amounts/phosphorylation of PKB-P70s6k-mTOR pathway components. Expression of mRNA for MuRF-1 increased approximately 3-fold at 10 days without changes in MAFbx or tripeptidyl peptidase II mRNA, but all decreased between 10 and 21 days. Thus, both myofibrillar and tendon protein synthetic rates show progressive decreases during 21 days of disuse; in muscle, this is accompanied by decreased phosphorylation of FAK, with no marked increases in genes for proteolytic enzymes.

Figure 1

Effects of unilateral lower limb suspension on human quadriceps cross-sectional area and myofibrillar fractional protein synthesis (FSR) All results are means and standard errors of the mean. Data are for n = 9 for CSA at all time points (see de Boer et al. 2007). Muscle CSA at days 14 and 23 was significantly different (repeated measures ANOVA) from each other and from baseline values (see de Boer et al. 2007). Results for myofibrillar FSR are for n = 9 at 10 days, n = 5 at day 0 and n = 4 at day 21. ^aP < 0.05 versus day 0-values.

Figure 2

Effects of unilateral lower limb suspension on human patellar tendon stiffness (N mm⁻¹) and tendon collagen synthesis (fractional synthesis rate, FSR per cent h⁻¹) All results are means and standard errors of the mean. Data are for n = 9 for tendon stiffness at all time points (see de Boer et al. 2007). Tendon stiffness values at days 14 and 23 were significantly different (repeated measures ANOVA) from each other and from baseline values (see de Boer et al. 2007). Results for tendon collagen FSR are for n = 9 at 10 days, n = 5 at day 0 and n = 4 at day 21. ^aP < 0.05 versus day 0 values; ^bP < 0.05 versus day 10 values of tendon FSR.

Figure 3

Changes in MuRF1 mRNA between days 0 and 10 and 10 and 21 in the two groups of four subjects studied twice Values are means and standard errors of the mean. *P < 0.05 for change between times indicated.

Figure 4

Changes in MAFBx mRNA between days 0 and 10 and 10 and 21 in the two groups of four subjects studied twice Values are means and standard errors of the mean. *P < 0.05 for change between times indicated.

Figure 5

Changes in tripeptidyl peptidase II mRNA between days 0 and 10 and 10 and 21 in the two groups of four subjects studied twice Values are means and standard errors. *P < 0.05 for change between times indicated.

Figure 6

Changes in FAK phosphorylation between days 0 and 10 and 10 and 21 in the two groups of four subjects studied twice Values are means and standard errors of the mean. *P < 0.05. (See Supplemental material for details of total protein amounts.)